摘要:

J O U R N A L O F C H E M I S T R Y Materials Preparation method for organic–inorganic layered compounds including fibrous materials by the reaction of Zn(OH)2 with organic compounds Sumikazu Ogata,a Izumi Miyazaki,a Yoshiharu Tasaka,b Hideyuki Tagaya,*a Jun-ichi Kadokawaa and Koji Chibaa aDepartment of Materials Science and Engineering, Yamagata University, Yonezawa, Yamagata 992, Japan. E-mail: tc021@dip.yz.yamagata-u.ac.jp bNational Institute of Materials and Chemical Research, Higashi, Tsukuba, Ibaraki 305, Japan Received 16th June 1998, Accepted 17th September 1998 A preparation method of surface modified inorganic layered compounds including fibrous materials was established by the reaction of amorphous Zn(OH)2 with organic oxychlorides.The resultant layer structures were similar to those of the layered double hydroxides (LDHs).Although the LDH layers have positive charges the layers of the reaction products obtained here are uncharged. IR spectra indicate that hydroxyl groups reacted with the organic oxychlorides, and new peaks assigned to RCO2–Zn bonds appeared around 1400 and 1550 cm-1 and 40–98% of hydroxyl groups were reacted.Interlayer spacings of the reaction products of Zn(OH)2 with dioxychlorides were 0.71–1.48 nm, and those of Zn(OH)2 with monooxychlorides were 1.19–2.67 nm and depended on the length of organic compounds. SEM images showed that the reaction products of Zn(OH)2 with organic oxychlorides had plate-like morphology similar to LDHs. However, the reaction product of Zn(OH)2 with benzoyl chloride was fibrous. Recently, the preparation of various organic–inorganic hybrid reacted with Zn(OH)2 giving surface modified layered compounds.The products diVer from the surface modified Zn/Al materials has been reported.1 Incorporation of a molecule into LDHs since their layers are uncharged and inclusion of anionic a crystalline inorganic host lattice to form an intercalation compounds between the layers is not required.compound can lead to ordered materials.2,3 For example, it In this study, we have prepared and characterized layered has been extensively reported that metal phosphonates are organic–inorganic compounds including fibrous materials useful for organizing molecules into lamellar structures.4 by the reaction of amorphous Zn(OH)2 with organic Their structures are very similar to those formed by oxychlorides.Langmuir–Blodgett (LB) techniques, but have higher thermal stabilities than LB films.5 Recently, attempts on the preparation of new organic derivative nanocomposites have been Experimental reported.6,7 It is possible to bond organic molecules on the surface of metal phosphonate covalently by the reaction of Reagents phosphate hydroxyl groups with organic compounds.Thus, All reactions were carried out using commercial reagents. the reaction of hydroxyl groups with organic compounds leads n-Butyryl chloride [CH3(CH2)2COCl], hexanoyl chloride to organic derivatived hybrid materials. These chemical modi- [CH3(CH2)4COCl], glutaryl dichloride [ClCO(CH2)3COCl] fications included reactions of inorganic compounds with and adipoyl chloride [ClCO(CH2)4COCl] were purchased alcohols.from Tokyo Chemical Industry Co. Zn(OH)2, suberoyl We have already reported the preparation method for chloride [ClCO(CH2)6COCl], sebacoyl chloride [ClCO- inorganic layered compounds the surfaces of which are modi- (CH2)8COCl ] and dodecanedioyl dichloride [ClCO(CH12)10COCl ] fied by organic compounds.In this reaction, a water-treated were purchased from Aldrich Chemical Co. Phenylacetyl Zn/Al layered double hydroxide (LDH) was prepared by chloride [C6H5CH2COCl], benzoyl chloride (C6H5COCl), placing the calcined Zn/Al LDH in degassed water and reacting Mg(OH)2, Cu(OH)2 and Ni(OH)2 were purchased from with an organic oxychloride in organic solvent. The resultant Wako Chemical Co.Al(OH)3 was purchased from Kanto compound is a well organized organic–inorganic hybrid mate- Chemical Co. rial, and the interlayer space is hydrophobic. The LDH is an inorganic layered compound8 and many organic intercalates Surface modification reactions of metal hydroxides into LDHs are known.9–13 Since the LDH layer has a positive charge, the LDH undergoes anion exchange intercalation Zn(OH)2 (0.2 g; 2.01 mmol) and other metal hydroxides such reactions with guests such as organic acids and inorganic as Cu(OH)2, Ca(OH)2 , Ni(OH)2 , Mg(OH)2 and Al(OH)3 anions.The surface modified LDH also has a positive charge were reacted with 0.4 equivalents of organic oxychlorides in 3 and includes anionic compounds between the layers.14,15 or 5 ml of acetonitrile or diethyl ether for 5 h under stirring On the other hand, chemical surface modification of at 333 K.For benzoyl chloride the reaction product of Zn(OH)2 was reported in which Zn(OH)2 was reacted with Zn(OH)2 with 0.4 equivalents of benzoyl chloride dissolved methanol at 588 K. In this reaction hydroxy ions were substi- in the solvent. Therefore, the reaction was repeated using 0.2 tuted by methoxy ions.16 Recently, we have described a method equivalents of benzoyl chloride.After being filtered oV, the to prepare new surface modified inorganic layered compounds reaction products were washed with the same solvent to by the reaction of Zn(OH)2 with organic oxychlorides at remove unreacted organic oxychlorides and impurities, and dried under reduced pressure at room temperature. 333 K.17 Not only monooxychlorides but also dioxychlorides J. Mater. Chem., 1998, 8, 2813–2817 2813Characterization Powder X-ray diVraction (XRD) spectra were recorded on a Rigaku powder diVractometer unit, using Cu-Ka (filtered) radiation (l=0.154 nm) at 40 kV and 20 mA between 1.8 and 50°. FTIR spectra (KBr disc method) were recorded on a Horiba FT-200 apparatus. Thermogravimetric analysis (TG) and diVerential thermal analysis (DTA) of powdered samples up to 873 K were carried out at a heating rate of 10 K min-1 under flowing N2 using a Seiko SSC5000 thermal analysis system.Scanning electron microscopy (SEM) was performed with a JEOL JSM-5300 instrument, operating at 20 kV. Results and discussion Thermal characteristics of the reaction products of Zn(OH)2 with organic oxychlorides Thermal characteristics of the reaction products of Zn(OH)2 with organic oxychlorides were determined by TG analysis. For Zn(OH)2, weight loss was observed up to 343 K as shown in Fig. 1(a) and corresponded to dehydration of Zn(OH)2. For benzoyl chloride weight loss was observed up to 470 K as shown in Fig. 1(c). For the reaction product of Zn(OH)2 with Fig. 2 XRD patterns of the reaction products of Zn(OH)2 with benzoyl chloride, two weight loss regions were observed (a) hexanoyl chloride, (b) suberoyl chloride, (c) benzoyl chloride, and between 450 and 720 K as shown in Fig. 1(b). The first weight (d) Zn(OH)2 alone. loss near 450 K corresponded to the dehydration of OH groups of Zn(OH)2 and desorption of organic groups reacted on the outer surface of Zn(OH)2.The second weight loss at between similar crystalline products were obtained by the reaction with 570 and 720 K corresponded to desorption of organic groups suberoyl chloride or benzoyl chloride in acetonitrile or diethyl reacted on the inner surface of Zn(OH)2. Certainly, the ether. Interlayer spacings of these reaction products were 1.67, reaction product of Zn(OH)2 with benzoyl chloride was more 1.08 and 1.48 nm as shown in Fig. 2(a), (b) and (c), respectstable thermally than benzoyl chloride itself. Similar thermal ively. The XRD peaks of these reaction products were diVerent stabilities were observed for other reaction products of from those of the individual organic oxychlorides and car- Zn(OH)2 with organic oxychlorides.boxylic acids. We have already reported that water-treated Zn/Al LDHs reacted with various organic oxychlorides to give X-Ray powder diVraction patterns of the reaction products surface modified LDHs.14,15 These were diVerent from those of intercalation compounds of organic carboxylate anions.18,19 No clear peaks were observed in the XRD pattern of Zn(OH)2 Upon reaction of sebacic acid with calcined Zn/Al LDH in alone as shown in Fig. 2(d). Upon reaction of this amorphous water, an intercalation compound was obtained, and the Zn(OH)2 with 0.4 equivalents of hexanoyl chloride in acetointerlayer spacing increased to 1.88 nm (Table 1). This value nitrile, a crystalline product was obtained, although the reacis larger than the spacing of 1.28 nm for the surface modified tion product dissolved in acetonitrile in the reaction of LDH.XRD peak patterns of all reaction products except that Zn(OH)2 with 1.0 equivalents of hexanoyl chloride. Crystalline of Zn(OH)2 with benzoyl chloride were similar to those of Zn(OH)2 was prepared by the reaction of Zn(NO3)2 with the surface modified Zn/Al LDH in which the water-treated NaOH, and was reacted with organic oxychlorides. The reac- Zn/Al LDH was reacted with oxychlorides.In the reaction tivity of crystalline Zn(OH)2 with organic compounds is fairly products of Zn(OH)2 with straight chain oxychlorides such as low, and peaks of Zn(OH)2 remained even after reaction with hexanoyl chloride and suberoyl chloride, the interlayer spac- organic oxychlorides. In the reaction of amorphous Zn(OH)2, ings of the reaction products increased with an increase in Table 1 XRD profiles of the reaction products of Zn(OH)2 with organic oxychlorides d-value/nm Oxychloride Length/nm OH-LDHa LDHb Zn(OH)2 n-Butyryl chloride 0.47 1.18 1.19 Hexanoyl chloride 0.72 1.61 1.67 Benzoyl chloride 0.54 1.12 1.54 1.48 Phenylacetyl chloride 0.61 1.54 1.53 p-Phenylazobenzoyl 1.16 2.67 2.28 2.67 chloride Glutaryl dichloride 0.51 0.71 Adipoyl chloride 0.64 0.77 1.48 0.83 Suberoyl chloride 0.89 1.07 1.08 Sebacoyl chloride 1.14 1.28 1.88 1.28 Dodecanedioyl 1.40 1.52 2.28 1.48 dichloride aThe reaction products of the water-treated LDHs with various organic oxychlorides.14 bThe intercalation compounds of correspond- Fig. 1 Thermal analysis of (a) Zn(OH)2, (b) the reaction product of ing organic anions into the Zn/Al LDHs.Zn(OH)2 with benzoyl chloride and (c) benzoyl chloride. 2814 J. Mater. Chem., 1998, 8, 2813–2817products, no absorption peaks corresponded to carboxylic acids or dimers of the organic reactants. In the IR spectra of the reaction products of Zn(OH)2 with straight chain type oxychlorides such as hexanoyl chloride and suberoyl chloride, n(CKH) at ca. 2900 and 2800 cm-1 as well as d(CKH) at ca. 1470 cm-1 were observed. In the IR spectrum of the reaction product of Zn(OH)2 with benzoyl chloride a new peak at 1600 cm-1 was observed and assigned to n(CLC) of the benzene ring. n(OH) absorptions at ca. 3500 cm-1 in the IR spectrum of Zn(OH)2 decreased upon reaction with organic oxychlorides and new peaks at ca. 1540 and 1400 cm-1 appeared. The absorption peak at 1540 cm-1 is assigned to the asymmetric stretching vibration of carboxylate, and the absorption at 1400 cm-1 assigned to the symmetric stretching vibration of Fig. 3 Interlayer spacings of the intercalation compound of Zn/Al LDH (&), the reaction products of Zn(OH)2 with ClCO(CH2)nCOCl carboxylate. The presence of these two peaks suggest the (n=4, 6, 8 and 10) (%), and CH3(CH2)nCOCl (n=2 and 4) (#).formation of RCO2–Zn bonds by reaction of Zn(OH)2 with organic oxychlorides. These results indicate the formation of surface modified layered compounds as shown in Fig. 5. methylene chain length as shown in Fig. 3. However, the To clarify the mechanism of the formation of the layered interlayer spacings of the reaction product of Zn(OH)2 with compounds, Zn(OH)2 was reacted with an excess of oxychlor- monooxychlorides were larger than those of Zn(OH)2 with ide and the product was dissolved in an organic solvent such dioxychlorides. The interlayer spacing and the length of oxyas acetonitrile or diethyl ether.Upon evaporation of aceto- chlorides suggested that the reaction products from nitrile or ether, powders were obtained the IR spectrum of monooxychlorides were bilayer structures and the reaction which indicated the presence of RCO2–Zn bonds while XRD products from dioxychlorides were bridging structures. showed the formation of carboxylic acid.The interlayer spacing of the reaction product of Zn(OH)2 It is proposed that the first step of the reaction of Zn(OH)2 with phenylacetyl chloride was 1.53 nm.The value was similar with monooxychlorides (RCOCl) is a dehydration reaction to that (1.54 nm) of the reaction product of the water-treated between OH groups of Zn(OH)2 and RCOCl to give Zn/Al LDH with phenylacetyl chloride. However, in the RCO2–Zn–OH or RCO2–Zn–OCOR. The cross-sectional area reaction product of Zn(OH)2 with benzoyl chloride, the of one HO–Zn–OH unit22 is calculated to be 0.96 nm2.It is interlayer spacing was 1.48 nm and larger than that of the proposed that the layered structure is assembled by interacting reaction product of the water-treated LDH with benzoyl RCO2–Zn–OH or RCO2–Zn–OCOR units as shown in Fig. 5. chloride (1.12 nm). The d-spacing of zinc benzoate However, the cross-sectional area of RCO2–Zn–OCOR is [Zn(C6H5CO2)2]20 was 1.10 nm.21 The XRD pattern of the >0.96 nm2 and therefore, an excess amount of reaction product of Zn(OH)2 with benzoyl chloride was RCO2–Zn–OCOR might interfere with the assembly reaction diVerent to that of zinc benzoate.owing to steric repulsion. IR spectroscopy The IR spectra of Zn(OH)2 and its reaction products with organic oxychlorides are shown in Fig. 4. In all reaction Fig. 4 IR spectra of (a) Zn(OH)2, the reaction product of Zn(OH)2 Fig. 5 Reaction of Zn(OH)2 with organic oxychlorides to give with (b) benzoyl chloride, (c) suberoyl chloride, and (d) hexanoyl chloride. 2: RCO2–Zn band. organic–inorganic hybrid layered compounds. J. Mater. Chem., 1998, 8, 2813–2817 2815Table 2 Elemental analysis of the reaction products of Zn(OH)2 with organic oxychlorides, Zn(OH)x(OKG)y or Zn(OH)x(OKGKO)z Oxychloride C(%) H(%) x y z n-Butyryl chloride 32.94 4.80 0.31 1.69 — Hexanoyl chloride 42.80 6.49 0.58 1.42 — Benzoyl chloridea 41.26 2.83 1.00 1.00 — Phenylacetyl chloride 52.57 3.88 0.46 1.54 — p-Phenylazobenzoyl chloride 47.23 3.07 1.19 0.81 — Glutaryl dichloride 23.68 2.48 0.74 — 0.63 Adipoyl chloride 34.10 3.83 0.04 — 0.98 Suberoyl chloride 34.89 4.29 0.54 — 0.73 Sebacoyl chloride 40.50 5.41 0.50 — 0.75 Dodecanedioyl dichloride 43.82 6.04 0.52 — 0.74 aFibrous. Composition of organic–inorganic layered compounds Elemental analyses of the reaction products of Zn(OH)2 with organic oxychlorides are shown in Table 2.About 85% of the OH groups of Zn(OH)2 reacted with hexanoyl chloride to assemble a layered structure and for cases of straight chain oxychlorides and phenylacetyl chloride, 63–98% of OH groups reacted.However, for bulky oxychlorides, such as benzoyl chlorides and p-phenylazobenzoyl chloride (C6H5NNC6H4COCl), only ca. 50 and 40%, respectively, of OH groups react. The results indicate that the extent of reaction of OH groups in Zn(OH)2 depends on the structure Fig. 6 SEM images (5000 ×magnification) of (a) Zn(OH)2 and the reaction products of Zn(OH)2 with (b) hexanoyl chloride, (c) suberoyl of the oxychloride. chloride and (d) benzoyl chloride. Reaction of Cu(OH)2, Mg(OH)2, Ni(OH)2 and Al(OH)3 with oxychloride We have already reported that water-treated Zn/Al LDHs react with organic oxychlorides to give surface modified LDHs.14,15 However, we could not obtain a water-treated Mg/Al LDH, and Mg/Al LDH scarcely reacted with oxychlorides. Furthermore, Al(OH)3, Cu(OH)2 and Ni(OH)2 did not react with oxychlorides. We confirmed reaction of Mg(OH)2 with oxychlorides, however, the amounts of reacted OH groups were small, and XRD peaks of the products were not clear. Morphology of the reaction product of Zn(OH)2 with organic compounds Zn(OH)2 has a plate-like morphology as shown in Fig. 6(a). Upon reaction of Zn(OH)2 with hexanoyl chloride and suberoyl chloride, clear plate morphologies were obtained as shown in Fig. 6(b) and (c). Similar plate-like crystals were also obtained by the reactions of Zn(OH)2 with other straight chain organic oxychlorides. The plate-like morphologies were quite similar to those of LDHs.23,24 By contrast, the reaction product of Zn(OH)2 with benzoyl chloride had a fibrous morphology as shown in Fig. 6(d). A TEM image of the reaction product of Zn(OH)2 with benzoyl chloride indicates that the fibrous compound was a layered compound as shown in Fig. 7. The reaction products of Zn(OH)2 with phenylacetyl chloride and p-phenylazoben- Fig. 7 TEM images (500 000 ×magnification) of the reaction product zoyl chloride were not fibrous although their structures are of Zn(OH)2 with benzoyl chloride.similar to that of benzoyl chloride. The distance between Zn centres of the inorganic layer and the benzene ring in Conclusions C6H5CH2CO2–Zn bond was larger than that in C6H5CO2–Zn. However, the distance between Zn of the inorganic layer and We have established a preparation method of organic– inorganic layered compounds by the reaction of Zn(OH)2 the benzene ring of C6H5NNC6H4CO2–Zn was the same as that for C6H5CO2–Zn.The molecular length of p-phenylazo- with various organic oxychlorides. The IR spectra of the reaction products of Zn(OH)2 with organic oxychlorides indi- benzoyl chloride (1.16 nm) is about twice as large as that of benzoyl chloride (0.54 nm). These results suggest that steric cated the formation of CO2–Zn bonds. 40–98% of OH groups reacted with the organic compounds. SEM images showed repulsion between organic moieties between layers is important in determining the morphology. that the structures of the reaction products were plate-like, 2816 J. Mater. Chem., 1998, 8, 2813–28177 M.Inoue, H. Kominami, Y. Kondo and T. Inui, Chem. Mater., whereas that of the reaction product with benzoyl chloride 1997, 9, 1614. was fibrous. These results suggest that the structure of the 8 F. Cavani, F. Trifiro and A. Vaccari, Catal. Today, 1991, 11, 173. nanomaterial depends on the structures of the organic com- 9 H. Tagaya, S. Sato, H. Morioka, M. Karasu, J. Kadokawa and pounds incorporated.Chemical surface modification of inor- K. Chiba, Chem.Mater., 1993, 5, 1431. ganic compounds has been studied extensively to change 10 H. Tagaya, S. Sato, T. Kuwahara, M. Karasu, J. Kadokawa and K. Chiba, J. Mater. Chem., 1994, 4, 1907. their chemical and/or physical properties in a controlled 11 H. Tagaya, T. Kuwahara, S. Sato, M. Karasu, J. Kadokawa and manner.25,26 The present work describes a viable preparation K.Chiba, J. Mater. Chem., 1993, 3, 317. method of surface-modified inorganic layered compounds. 12 H. Tagaya, A. Ogata, T. Kuwahara, S. Ogata, M. Karasu, Organic intercalation compounds of smectite clays are known J. Kadokawa and K. Chiba, Microporous Mater., 1996, 7, 151. as organic sorbents and since the interlayer space of surface 13 T.Kuwahara, H. Tagaya and K. Chiba, Microporous Mater., modified Zn(OH)2 is hydrophobic the hybrid materials have 1995, 4, 247. 14 H. Morioka, H. Tagaya, M. Karasu, J. Kadokawa and K. Chiba, potential as shape-selective sorbents and catalysts. The method J. Solid State Chem., 1995, 117, 337. is also useful for controlling the organization of organic 15 H. Tagaya, H.Morioka, M. Karasu, J. Kadokawa and K. Chiba, molecules in the solid state, and preparation of microporous Supramol. Sci., 1995, 2, 33. materials which have controlled morphology. 16 T. Kubo, K. Uchida, K. Tsubosaki and F. Hashimi, Kogyo Kagaku Zasshi, 1970, 73, 75; Chem. Abstr., 1970, 73, 41407p. 17 H. Tagaya, S. Ogata, H. Morioka, J. Kadokawa, M. Karasu and Acknowledgments K.Chiba, J. Mater. Chem., 1996, 6, 1235. 18 M. Meyn, K. Beneke and G. Lagaly, Inorg. Chem., 1990, 29, 5201. The work was supported by a Grant-in-Aid for Scientific 19 S. Miyata and T. Kumura, Chem. Lett., 1973, 843. Research from the Ministry of Education, Science, Sports and 20 G. A. Guseinov, F. N. Musaev, B. T. Usubaliev, Culture, Japan and Iketani Science and Technology I.R. Amiraslanov and Kh. S. Mamedov, Koord. Khim., 1984, 10, 117. Foundation. The authors are grateful to K. Fujita and 21 J. D. Hanawalt, H. W. Rinn, and L. K. Frevel, Ind. Eng. Chem., H. Morioka for their technical assistance. 1938, 10, 457. 22 R. Allmamm, Acta Crystallogr., Sect. B, 1968, 24, 972. 23 S. Kannan and C. S. Swamy, J. Mater. Sci. Lett., 1992, 11, 1585. References 24 J. M. Fernandez, C. Barriga, M. Ulibrarri, F. Labajos and V. Rives, J. Mater. Chem., 1994, 4, 1117. 1 P. Judeinstein and C. Sanchez, J. Mater. Chem., 1996, 6, 511. 25 J. E. Mark, Y-C. Lee, and P. A. Bianconi, Hybrid 2 D. O’Hare, in Inorganic Materials, ed. D. W. Bruce and Organic–Inorganic Composites, ACS Symp. Ser. 585, American D. O’Hare, Wiley, Chichester, 2nd edn., 1996, p. 171. Chemical Society, Washington, 1995. 3 A. Clearfield, in Progress in Intercalation Research, ed. 26 E. F. Vansant, P. Voort, and K. C. Vrancken, Characterization and W. M. Warmuth and R. Scho� llhorn, Kluwer, Dordrecht, 1994, Chemical Modification of the Silica Surface, Elsevier, New York, p. 223. 1995. 4 H. E. Katz, Chem.Mater., 1994, 6, 2227. 5 M. E. Thompson, Chem. Mater., 1994, 6, 1168. 6 J. J. Tunney and C. Detellier, J. Mater. Chem., 1996, 6, 1679. Paper 8/04557G J. Mater. Chem., 1998, 8, 2813–2817 28

ISSN:0959-9428    

DOI:10.1039/a804557g

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials The influence of template extraction on the properties of primary amine templated aluminosilicate mesoporous molecular sieves Robert Mokaya* and William Jones Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, UK CB2 1EW. E-mail: rm140@cus.cam.ac.uk Received 3rd August 1998, Accepted 19th October 1998 Aluminosilicate mesoporous molecular sieves (Al-MMS) prepared at room temperature using hexadecylamine as template have been subjected to template extraction prior to calcination. Extraction in ethanol alone removes only that part of the template (in neutral form) which is not associated with framework Al while the presence of a cation (Na+ or NH4+) ensures total template removal.Template extraction has no eVect on elemental composition and for dry (non-calcined) samples results in an improvement in structural ordering.The eVect of calcination depends on the mode of extraction; samples extracted in ethanol or ethanol/NH4+ are structurally stable to calcination and possess surface area and pore volume similar to directly calcined samples while ethanol/Na+ extracted samples are relatively unstable and undergo considerable structural degradation resulting in lower surface area and pore volume.Dealumination results from calcination of amine or ammonium ion containing samples while Na containing samples do not undergo any dealumination. The acid content of calcined ethanol and ethanol/NH4+ extracted samples is comparable to that of the directly calcined samples but the extracted samples exhibit higher catalytic activity for the cracking of cumene. Calcined ethanol/Na extracted samples possess very low acidity and exhibit no catalytic activity.soluble silicon/aluminium species and also as a source of Introduction charge balancing protons (during calcination of the as- Recent advances in the synthesis of mesoporous molecular synthesised material ).This is advantageous because Brønsted sieves which possess uniform and sharply distributed pores of acid sites are generated by simple calcination. We and others diameter 20–100 A° have increased the range of well ordered have shown that in contrast to the purely siliceous analogue, solid acid catalysts (previously the domain of microporous where all the amine is in neutral form, a part of the templating zeolites) into the mesoporous regime.1 Synthesis of such silica amine in the Al-MMS samples exists in a protonated form based materials involves the use of surfactants to assemble where it is electrostatically bound to the inorganic framework inorganic species from solution into a solid framework in and cannot therefore be removed by simple solvent extracwhich the organic surfactant template is occluded.2–4 The tion.9,12 The proportion of such protonated amine increases removal of the template to generate the molecular sieve with with the amount of aluminium in the solid framework.9 We regular void spaces is therefore an integral part of the synthetic also found that high amounts of aluminium in the Al-MMS process.For M41S materials which are synthesised using a framework increased the attraction between the templating cationic surfactant and anionic inorganic species, template amine micelles and the framework and thus removal of the removal may be achieved either by calcination or by solvent template by direct calcination resulted in greater structural extraction followed by calcination.5,6 It has been shown that (framework) collapse due to the increase in local heating more of the template is removed during the extraction if the eVects.11 It is thus of interest to investigate other methods of solvent system contains a cation donor such as an acid or template removal. Here we report a detailed comparison of salt.5–7 For Al-containing MCM-41, template removal by the properties of Al-MMS catalysts prepared at room temperasolvent extraction results in an increase in the amount of non- ture using hexadecylamine as template and subjected to various framework octahedral Al (due to partial dealumination).methods of template removal. Directly calcined samples are Furthermore solvent extraction on its own does not readily compared with samples in which template removal is achieved remove any template associated with tetrahedrally coordinated by extraction using ethanol alone followed by calcination or framework Al and therefore calcination at elevated tempera- by solvent extraction using ethanol in the presence of an acid ture is required to ensure complete removal of the template.7 generating (NH4+) or a non-acidic (Na+) cation prior to Tanev and Pinnavaia8 have, however, shown that in the calcination.We have investigated the eVects of the extraction absence of strong electrostatic interactions between the surfac- procedure on the structural integrity of the Al-MMS materials tant and framework (e.g. when neutral primary amine micelles and also on the nature of the Al they contain. We also report are used to direct the assembly of neutral silica inorganic on the influence of the template extraction procedure on the species) it is possible to achieve complete template removal by acidity and catalytic performance for cumene cracking. solvent extraction alone.Particular emphasis is given to the eVect of cations during the We have recently reported the synthesis, acidity and catalytic extraction.properties of Al-containing mesoporous molecular sieves (Al- MMS) prepared using primary amines as the templates.9–11 We have shown that when compared to equivalent Al- Experimental containing MCM-41 materials the Al-MMS materials possess significantly higher Brønsted acid content and consequently Synthesis of materials exhibit higher catalytic activity for Brønsted acid catalysed The as-synthesised aluminosilicate materials were prepared as reactions such as the cracking of cumene.10,11 In the synthesis follows: aluminiumisopropoxide [Al(i-C3H7O)3 dissolved in 35 ml of Al-MMS materials the primary amine surfactant micelles act both as structure directing agents during the assembly of isopropanol ] was mixed with 0.2 mol tetraethylorthosilicate J.Mater. Chem., 1998, 8, 2819–2826 2819(TEOS, in 80 ml ethanol ) at Si/Al molar ratios in the range Catalytic testing 40–5/1 and vigorously stirred at room temperature for 15 The conversion of cumene was performed at 300 °C and a minutes. The template solution was separately prepared by WHSV of 4.0 using a tubular stainless steel, continuous flow dissolving 0.05 mol hexadecylamine in a mixture of 80 ml fixed-bed microreactor (of internal diameter 10 mm) with water and 120 ml ethanol.The TEOS–Al(i-C3H7O)3 mixture helium (25 ml min-1) as carrier gas. The catalyst bed (100 mg; was added to the template solution under vigorous stirring at 30–60 mesh) was first activated for 1.5 h at 500 °C under room temperature. The pH of the synthetic mixtures was close helium (25 ml min-1). For the reaction a stream of cumene to 9.5.The resulting gel mixture was allowed to react at room vapour in helium was generated using a saturator at room temperature for 20 hours following which the solid product temperature. The reaction products were separated and ana- was obtained by filtration and air dried at room temperature. lysed using a Carlo Erba HRGC 5300 gas chromatograph on Template removal was achieved either by direct calcination in line with the microreactor.Gas chromatographs were obtained air at 650 °C for 4 hours or by solvent extraction prior to automatically on samples of the product stream which were calcination. Solvent extraction was performed as follows: 1.5 g collected at regular intervals using a Valco 6-port valve.The of the dry (as synthesised) material was added to 150 ml of gas chromatographs were used to calculate the percent overall extraction media and stirred at 65 °C for 1 h followed by cumene conversion. filtration and washing with ethanol. This procedure was repeated once to give the ‘dry extracted’ samples. Three types Results and discussion of extraction media were used: (1) 150 ml ethanol alone; (2) 1.5 g of sodium acetate in 150 ml ethanol; and (3) 1.5 g Chemical composition and thermal analysis ammonium acetate in 150 ml ethanol.All the ‘dry extracted’ samples were then calcined in air at 650 °C for 4 hours. The The bulk Si/Al molar ratios of the parent (directly calcined) Al-MMS samples, shown in Table 1, indicate that Si and Al directly calcined materials are designated Al-MMSX (where X is the Si/Al ratio used in the synthesis gel ).The extracted are incorporated into the solid framework in proportions which are largely in line with the synthesis gel composition. samples are named with a prefix indicating the extraction media (Et for ethanol alone; NH for ammonium acetate and The mode of template extraction does not have any significant or systematic eVect on the Si/Al ratio except that for samples Na for sodium acetate) derived from Al-MMS20 there is some preferential leaching Characterisation of a small amount of silica (see Table 1) reflected in lower Si/Al ratios.However samples derived from Al-MMS10 (data Elemental composition (via EDA analysis) was obtained using not shown) had very similar Si/Al ratios (in the range a Camscan S4 scanning electron microscope at 20 kV.The 12.8±0.3) which implies that silica leaching is not necessarily data were processed through a ZAF4 program running on a a characteristic of solvent mediated template extraction. Link 860 series 2 processor. TGA and DTG curves were Fig. 1 shows the TGA and DTG curves obtained for the obtained using a Polymer Laboratories TG analyser with a as-synthesised purely siliceous sample (prepared according to heating rate of 20 °Cmin-1 under nitrogen flow of ref. 9) and the Al-MMS samples. The first mass loss between 25 ml min-1. Powder X-ray diVraction (XRD) patterns were 25 and 100 °C is attributed to the release of water and/or recorded using a Philips 1710 powder diVractometer with Cuethanol.All samples show a further mass loss centred at ca. Ka radiation (40 kV, 40 mA), 0.02° step size and 1 s step time. 300 °C which is due to amine desorption. In addition the Al- Textural properties (surface area, pore volume and pore size) MMS samples show a third mass loss centred at 450 °C were determined at -196 °C using nitrogen in a conventional which increases as the (synthetic gel ) Si/Al ratio reduces and volumetric technique by a Micromeritics ASAP 2400 sorptodevelops at the expense of the mass loss centred at 300 °C.meter. Before measurement each sample was oven dried at We have previously shown that for primary amine templated 280 °C and evacuated overnight at 200 °C under vacuum. 27Al aluminosilicate samples such as those described here, the magic-angle-spinning (MAS) NMR spectra were recorded at occluded amine exists in two forms, i.e., neutral ‘low 9.4 T using a Chemagnetics CMX-400 spectrometer with zirtemperature’ amine similar to that present in the purely conia rotors 4 mm in diameter spun at 8 kHz.The spectra siliceous material and electrostatically bound ‘high tempera- were measured at 104.3 MHz with 0.3 s recycle delays and ture’ amine.9 During thermogravimetric analysis the former corrected by subtracting the spectrum of the empty MAS less strongly bound template is desorbed and decomposed rotor.External Al(H2O)63+ was used as a reference. To ensure between 100 and 350 °C while the electrostatically bound quantitative reliability all calcined samples were fully hydrated template is removed between 350 and 500 °C.However, and equilibrated with room air prior to the measurements.13 despite the varying amounts of neutral and charged amine, it is clear from Fig. 1 that the total amine occluded remains Acidity measurements more or less the same regardless of the Si/Al ratio. This implies that both the neutral and charged amine are occluded The acid content of the materials was measured by using TPD of cyclohexylamine.The samples were exposed to liquid cyclo- in micellar arrangements which are encased by the inorganic framework. As more Al is incorporated into the framework hexylamine at room temperature after which they were kept overnight (at room temperature) and then in an oven at 80 °C more of the occluded amine is protonated so as to balance the resulting negative charge.for 2 hours so as to allow the base to permeate the samples. The oven temperature was then raised to 250 °C and main- Fig. 2 compares the TGA and DTG curves of the dry parent (as-synthesised) Al-MMS10 sample before and after tained at that temperature for a further 2 hours. The samples were then cooled to room temperature under dry nitrogen being subjected to various modes of template extraction.The mass loss between 150 and 800 °C, which excludes adsorbed following which they were subjected to thermogravimetric analysis using a Polymer Laboratories TG analyser with a water or ethanol, was 36% for the parent as-synthesised Al- MMS10 sample and 26, 11 and 10% for Et-, NH- and Na- heating rate of 20 °Cmin-1 under nitrogen flow of 25 ml min-1. The weight loss associated with desorption of Al-MMS10 samples respectively. Unlike the parent sample, the ethanol-extracted sample (Et-Al-MMS10) shows only one the base from acid sites occurred between 300 and 450 °C, with a maximum at ca. 370 °C. This weight loss was used to mass loss due to the desorption of electrostatically bound amine.Thus extraction in ethanol alone removes the neutral quantify the acid content (in mmol of cyclohexylamine per gram of sample) assuming that each mole of cyclohexylamine amine but has no noticeable eVect on the electrostatically bound amine. On the other hand the TGA and DTG curves corresponds to one mole of protons.11,14 2820 J. Mater. Chem., 1998, 8, 2819–2826Table 1 Elemental composition, d spacing and textural properties of calcined Al-MMS samples subjected to various modes of template removal Surface area/ Pore volume/ Wall Sample Si/Al ratio d100/A° m2 g-1 cm3 g-1 APDa/A° a0 b/A° thickness/A° Directly calcined Al-MMS40 44.1 35.1 1165 0.67 25.1 40.5 15.4 Al-MMS20 26.3 34.2 (35.8)c 1050 0.58 23.7 39.5 15.8 Al-MMS10 12.8 32.2 1104 0.47 21.2 37.2 16.0 Al-MMS5 7.6 31.6 901 0.40 20.1 36.5 16.4 Extracted prior to calcination Et-Al-MMS20 22.7 36.0 (37.0)c 941 0.81 26.4 41.6 15.2 NH-Al-MMS20 24.1 36.2 (38.4)c 894 0.71 25.7 41.8 16.1 Na-Al-MMS20 24.6 32.5 (36.0)c 731 0.53 27.8 37.5 9.7 aAPD=Average pore diameter (determined using BJH analysis). ba0=Lattice parameter, from the XRD data using the formula a0=2d100/Ó3.cValues in parentheses are d spacing before calcination.of NH- and Na-Al-MMS10 indicate that extraction in the presence of a cation removes all the occluded amine. Indeed FTIR spectroscopy confirmed the total removal of amine from samples subjected to template extraction in the presence of NH4+ and Na+ ions, i.e., we did not observe any peaks attributable to hexadecylamine. This means that the events in the TGA curve of the NH- and Na-Al-MMS10 samples are new and are not attributable to amine desorption.For Na- Al-MMS10 we attribute the mass loss centred at ca. 550 °C to dehydroxylation. The dehydroxylation starts at 200 °C and is complete at 700 °C. The DTG of the NH-Al-MMS10 sample shows two mass losses in the temperature range 200–500 °C which we attribute to the decomposition of ammonium ions.The presence of two distinct mass losses suggests two types of ammonium ions, one more stable than the other. If we consider the NH-Al-MMS10 sample as being similar to an ammonia saturated solid Brønsted acid material, with the base adsorbed as ammonium ions on Brønsted acid sites, it is then possible to ascribe the mass loss centred at 350 °C to diamination from weaker acid sites while ammonium ions held on stronger acid sites are desorbed at the higher (420 °C) temperature.This explanation is supported by Fig. 3 which shows TGA and DTG curves of all the Al-MMS samples after template extraction in the presence Fig. 1 Thermogravimetric analysis (TGA) and diVerential thermo- of NH4 ions. As the Si/Al reduces, the mass loss at 420 °C gravimetric (DTG) analysis curves for as-synthesised purely siliceous increases which is agreement with higher population of strong (MMS) and Al-MMS samples.acid sites due to increasing Al incorporation. Fig. 2 (A) Thermogravimetric analysis (TGA) and (B) diVerential thermogravimetric (DTG) analysis curves for an Al-MMS sample prepared at Si/Al=10 before and after template extraction; (a) as-synthesised, (b) extracted in ethanol alone, (c) extracted in ethanol/NH4+, (d) extracted in ethanol/Na+.J. Mater. Chem., 1998, 8, 2819–2826 2821which again emphasises the absence of long range order. As expected for such mesoporous materials,15 the intensity of the basal peak reduces with increase in Al incorporation indicating that Al has a deleterious eVect on structural ordering. Furthermore as the amount of Al incorporated increases the d spacing reduces (see Table 1).Fig. 5 compares the XRD patterns of the parent assynthesised (dry) and calcined Al-MMS20 samples with equivalent samples after being subjected to template extraction (dry samples) and subsequent calcination. For the dry samples template extraction has the eVect of increasing the intensity of the basal (100) peak.This may be related to better ordering occasioned by the benign removal of all or some of the template and formation of cross-linking siloxane or Si–OH–Al bonds. Upon calcination the intensity of the basal peak for Et-Al-MMS20 and NH-Al-MMS20 is maintained suggesting that structural ordering is retained in the calcined forms of these samples.For Na-Al-MMS20 the decrease in intensity of the basal peak is indicative of some structural collapse which in turn suggests that template free Na-containing Al-MMS materials are unstable towards calcination (see interpretation of porosity data). Similar behaviour has been previously reported for aluminosilicate Na-MCM-41 materials from which most of the template had been extracted.7 The instability of template free Na-containing Al-MMS materials is further Fig. 3 Thermogravimetric analysis (TGA) and diVerential thermogravimetric (DTG) analysis curves for Al-MMS samples after template highlighted by the extent of lattice contraction due to calciextraction in ethanol/NH4+. nation; the basal spacing of Na-Al-MMS20 reduces by 9.7% compared to 4.5, 2.7 and 5.7% for the parent (as-synthesised), Et-Al-MMS20 and NH-Al-MMS20 samples respectively (see Physical characterisation Table 1).The higher contraction for Na-Al-MMS20 suggests The XRD patterns of the as-synthesised and (directly) calcined extensive dehydroxylation which supports our interpretation parent Al-MMS materials are shown in Fig. 4. The samples of the TGA results described above.We note that calcined Etmainly exhibit a single basal (100) peak which is characteristic Al-MMS20 and NH-Al-MMS20 samples have similar d100 of materials possessing short range hexagonal ordering.8 As- spacings despite the larger lattice contraction in the latter. synthesised materials prepared at Si/Al>10 show an additional The textural parameters of the parent (directly calcined) Alweak and diVuse peak at ca. 1.8 nm which may be an indication MMS materials are given in Table 1 while Fig. 6 shows the of slightly better long range ordering in these silica-rich corresponding nitrogen sorption isotherms. The isotherms materials. On calcination the scattering intensity of the basal exhibit a mesopore filling step in the relative pressure (p/po) peak increases.The increase in scattering intensity observed range 0.05 to 0.4 which is characteristic of such materials.16 here may be due to better ordering of the inorganic framework The height and steepness of the step which are an indication (as a result template removal and formation of cross-linking of the extent and uniformity of the mesopores reduce at lower siloxane bonds) or increase in scattering domain size.However Si/Al ratios. This is due to increasing amount of framework despite the increase in the intensity of the basal peak (especially for Al-MMS40), the diVuse peak at ca. 1.8 nm disappears Fig. 5 Powder XRD patterns of dry and calcined Al-MMS samples Fig. 4 Powder XRD patterns of as-synthesised and calcined Al-MMS prepared at Si/Al=20; (a) as-synthesised, (b) extracted in ethanol alone, (c) extracted in ethanol/NH4+, (d) extracted in ethanol/Na+.samples prepared at Si/Al of (a) 5, (b) 10, (c) 20 and (d) 40. 2822 J. Mater. Chem., 1998, 8, 2819–2826Fig. 7 Nitrogen sorption isotherms of the directly calcined Al-MMS20 Fig. 6 Nitrogen sorption isotherms of directly calcined Al-MMS sample compared to equivalent samples subjected to template extracsamples.tion prior to calcination. Inset is the pore size distribution of the samples. Al which results in lower structural ordering and therefore less well defined framework confined mesoporosity. Na-Al-MMS sample is indicative of a material with lower Furthermore the mesopore filling range generally shifts to structural ordering which is in agreement with the XRD and lower partial pressures as Si/Al reduces which is an indication TGA results described earlier.The picture that emerges is that of decrease in pore size as shown in Table 1. There is no template free Na-AlMMS materials are unstable to calcination systematic variation of interparticle or ‘textural’ mesoporosity during which they undergo extensive dehydroxylation and (which is indicated by the presence of high pressure hyster- significant structural collapse. esis)16 with Si/Al ratio.The surface area, pore volume and average pore diameter (APD) given in Table 1 are consistent Nature of Al nuclei (solid state 27Al NMR) with those previously reported for similar mesoporous materials such as MCM-41.17,18 In general the pore volume and The 27Al NMR spectra of the parent as-synthesised and directly calcined samples is shown in Fig. 8. The as-synthesised pore sizes decrease as the amount of aluminium incorporated increases while the surface area does not change in any samples show a relatively sharp resonance at 53 ppm due to tetrahedrally coordinated Al. As expected this resonance systematic way except for a significant reduction for the most aluminous (Al-MMS5) sample.The decrease in pore volume increases with increasing incorporation of Al into the inorganic framework. Most if not all the Al is in tetrahedral coordination and pore size is probably due to some collapse of the structure (during calcination to remove the template) caused by local for samples prepared at Si/Al10.The sample prepared at Si/Al ratio of 5 (Al-MMS5) exhibits a broad low intensity heating eVects associated with the presence of increasing amounts of framework aluminium.12,15 The wall thickness peak at ca. 0 ppm indicating that some of the Al is in octahedral coordination, i.e., extra-framework Al (EFAL). values obtained by subtracting the average pore diameter (APD) from the lattice parameter (a0), are consistent with Calcination in air results in some dealumination in all the samples and the amount of EFAL generally increases with those previously reported for similar materials.16 The wall thickness increases slightly with increasing amounts of incor- lowering Si/Al ratio.In Fig. 9 we compare the 27Al NMR spectra of calcined template extracted samples (prepared at porated Al which is consistent with the presence of increasing amounts of Al in the framework; the Al–O bond being longer Si/Al=10) with the directly calcined sample (Al-MMS10).The spectra of the dry extracted samples (not shown) were than the Si–O bond. Fig. 7 compares the sorption isotherms of directly calcined similar to that of as-synthesised Al-MMS10 shown in Fig. 8 implying that extraction on its own had no eVect on the Al-MMS20 with equivalent materials subjected to template extraction prior to calcination. The isotherms of Et-Al-MMS20 environment of Al nuclei. The spectra of the calcined Et- and NH-Al-MMS10 samples indicate the presence of EFAL while and NH-Al-MMS20 are similar to that of the parent material indicating that these three samples have a similar extent of that of Na-Al-MMS10 exhibits only one resonance at 53 ppm due to framework Al.We may therefore infer that on calci- mesopore uniformity. The height and steepness of the mesopore filling step for Na-Al-MMS20 is lower indicating a less nation Et- and NH-Al-MMS samples undergo some dealumination while Na-Al-MMS samples apparently retain all the Al well defined mesopore structure.The inferior mesopore structure of Na-Al-MMS20 compared to the other samples is in tetrahedral coordination. In this respect the Et- and NHAl- MMS samples are similar to the directly calcined sample. confirmed by the pore size distribution curves shown in Fig. 7 (inset). Furthermore as shown in Table 1, Et- and NH-Al- The factor that distinguishes these three samples from Na-Al- MMS samples is that during calcination they undergo diamin- MMS20 samples have surface area and pore volume comparable or higher than those of the parent (directly calcined) Al- ation.In the directly calcined and Et-Al-MMS samples the decomposing species is protonated amine while for the NH- MMS20 while Na-Al-MMS20 on the other hand exhibits lower values. In addition the wall thickness of Na-Al-MMS20 Al-MMS sample the diaminating species are ammonium (NH4+) ions.Since diamination is an exothermic process it is at 9.7 A° , is much lower than those of the other three samples which have walls of thickness in the range 15.2 to 16.1 A° ; the likely that it causes local heating in the vicinity of the (framework) Al sites on which the diaminating species are attached thinner wall for the Na-AlMMS sample is consistent with structural collapse during calcination.The textural data of the resulting in the extraction of some of the Al. No such local J. Mater. Chem., 1998, 8, 2819–2826 2823Fig. 8 27Al MAS NMR spectra of as-synthesised and calcined Al-MMS samples prepared at Si/Al ratio of (from top to bottom) 5, 10, 20, 40.present during the calcination of dry relatively template free samples.7 Acidity and catalytic activity The acid content of the samples was obtained using TPD of cyclohexylamine.14,15 The results are given in Table 2. As expected the acid contents of the directly calcined materials is dependent on the Si/Al ratio and increases with the Al content. The acid contents of Et- and NH-Al-MMS samples prepared at Si/Al=10 or 20 are similar to those of the equivalent directly calcined samples. This is in agreement with the elemental composition (see Table 1) and 27Al NMR results.We may therefore assume that for Et- and NH-Al-MMS materials, template extraction prior to calcination does not have any significant eVect on the population of acid sites (mainly of the Brønsted type)14 which are strong enough to retain adsorbed cyclohexylamine after thermal treatment at 250 °C. Na-Al- MMS samples on the other hand exhibited very low acid content.This is due to the blocking of potential Brønsted acid Fig. 9 27Al MAS NMR spectra of a directly calcined Al-MMS10 sites by Na+ ions which for these samples are the charge sample compared to equivalent samples subjected to template extrac- balancing cations.Na-Al-MMS samples therefore require an tion prior to calcination. extra ion exchange (with ammonium ions) and calcination step to generate Brønsted acid sites. The conversion of cumene over directly calcined Al-MMS heating eVects exist for the Na-sample and therefore no samples as a function of time is shown in Fig. 10. The curves dealumination occurs. Furthermore it seems that amine of some reference materials are included for comparison. The decomposition causes greater dealumination than the conversion achieved over the Al-MMS samples is dependent ammonium ion; from the deconvoluted NMR spectra we on the Si/Al ratio while the rate of deactivation is comparable estimate that ca. 68% of the Al in the directly calcined and for all the samples.From Fig. 10 we may conclude that the Et-Al-MMS10 samples is in four coordination while for the activity of hexadecylamine templated Al-MMS materials is NH-Al-MMS10 sample the value is close to 78% (see Fig. 9). higher than that of aluminosilicate MCM-41 and amorphous Presumably the diamination of the larger amine molecules aluminosilicates but lower than that of ultra-stable Y (USY) causes more local heating than the decomposition of the zeolite.The activity per acid site for Al-MMS samples is given smaller ammonium ion. Our interpretation of the NMR results in Table 2 as the turnover frequency (TOF). The TOF values suggests therefore that despite lower structural stability were obtained by dividing the rate of cumene conversion towards calcination, dry template free Na-Al-MMS materials (mmol g-1 h-1) after 10 minutes time on stream with the acid are able to retain most if not all Al in tetrahedral coordination.content (mmol g-1). The TOF increases as the Al content This in turn implies that structural disintegration of Al-MMS reduces implying that the catalytic eYciency of the acid sites materials is not always linked to dealumination and for on Al-MMS materials reduces as their density increases.calcined Na-Al-MMS materials the tetrahedral Al may exist Similar trends have been observed for zeolites.19 Fig. 11 com- in a disordered (or amorphous) phase as in the case of pares the activity of the directly calcined (Al-MMS20) sample amorphous aluminosilicates.Indeed it has previously been with equivalent calcined extracted samples. The Na-Al-MMS reported that the formation of amorphous (or even crystalline samples had no observable catalytic activity under the reaction alkali metal–silicate phases) can be favoured over the formation of the sodium form of AlMCM-41 if alkali cations are conditions used. The initial conversion achieved by the Et- 2824 J.Mater. Chem., 1998, 8, 2819–2826Table 2 Acidity and catalytic activity of the study materials Cumene conversion Sample Acidity/mmol H+ g-1 Initial ratea TOFb Directly calcined Al-MMS5 650 1830 2.82 Al-MMS10 535 1780 3.31 Al-MMS20 330 1493 4.52 Al-MMS40 210 1210 5.76 Extracted prior to calcination Et-Al-MMS10 520 1936 3.72 Et-Al-MMS20 320 1760 5.50 NH-Al-MMS10 550 2002 3.64 NH-Al-MMS20 330 1905 5.77 Na-Al-MMS10 80 No catalytic activity Na-Al-MMS20 50 No catalytic activity aObtained after 10 min time on stream in mmol (g cat h)-1. bObtained by dividing initial rate by acid content.Fig. 11 EVect of template extraction on the catalytic activity and Fig. 10 Catalytic activity and deactivation behaviour of Al-MMS samples and reference materials compared at 300 °C and WHSV of deactivation behaviour of Al-MMS samples prepared at Si/Al=20 compared at 300 °C and WHSV of 4.0; Al-MMS20 (+); Et-Al- 4.0.Al-MMS5 (#), Al-MMS10 (%), Al-MMS20 (6), Al-MMS40 ((), USY (CBV740) Si/Al=21 ($), H+-MCM-41-20, Si/Al=20 (+) MMS20 (&) and NH-Al-MMS20 ($). and amorphous silica–alumina (ASA12) Si/Al=12 (&). and NH-Al-MMS extracted samples is higher than that of be achieved by direct calcination, extraction in ethanol followed by calcination or extraction in a cation-containing their directly calcined analogue; the order of initial conversion is Et-Al-MMS20>NH-Al-MMS20>Al-MMS20.This order ethanol solution. Extraction in ethanol alone removes only that part of the template which is not associated with frame- of activity is reflected by the TOF values for the samples given in Table 2.Fig. 11 also shows that the rate of deactivation is work Al while extraction in the presence of a cation results in complete template removal. Template extraction has no eVect higher for the extracted samples; this may be an indication that extracted samples possess stronger acid sites (which on elemental composition and for dry (non-calcined) samples results in an improvement in structural ordering.The eVect of therefore deactivate more rapidly) than those on the directly calcined sample. The lack of catalytic activity for the Na-Al- calcination depends on the mode of extraction. Samples subjected to template extraction in ethanol or ethanol/NH4+ are MMS20 sample is due to the absence of Brønsted acid sites which as mentioned earlier are blocked by the charge balancing structurally stable to calcination and possess textural parameters (surface area and pore volume) similar or higher than Na+ ions.for directly calcined samples. Samples subjected to template extraction in ethanol/Na+ are, on the other hand, relatively Conclusions unstable to calcination and undergo considerable structural degradation resulting in lower surface area and pore volume.Aluminosilicate mesoporous molecular sieves (Al-MMS) exhibiting short range hexagonal order have been prepared at Dealumination results from calcination of samples containing amine or ammonium ions (i.e., directly calcined, ethanol and room temperature using the primary amine hexadecylamine as structure director.The templating amine is occluded in the ethanol/NH4+ extracted samples) while Na containing (i.e., ethanol/Na+ extracted) samples do not appear to undergo materials either in neutral or charged (protonated) form. The relative amounts of neutral and protonated amine depend on any dealumination. We conclude therefore that removal of aluminium from framework positions is due to local heating the Si/Al ratio.The proportion of protonated amine increases with increasing framework Al content. Template removal may eVects arising from diamination of protonated amine or J. Mater. Chem., 1998, 8, 2819–2826 2825P. Sieger, R. Leon, P. M. PetroV, F. Schuth and G. D. Stucky, ammonium ions. The acid content of calcined ethanol and Nature, 1994, 368, 317.ethanol/NH4+ extracted samples is comparable to that of 4 J. S. Beck and J. C. Vartuli, Curr. Opin. Solid State Mater. Sci., directly calcined samples but the former exhibit higher catalytic 1996, 1, 76. activity for the cracking of cumene. Calcined ethanol/Na+ 5 C.-Y. Chen, H.-X. Li and M. E. Davis, Microporous Mater., 1993, extracted samples possess very low acidity and exhibit no 2, 17. 6 R. Schmidt, D. Akporiaye, M. Stocker and O. H. Ellestad, Stud. catalytic activity. Surf. Sci. Catal., 1994, 84, 677. 7 S. Hitz and R. Prins, J. Catal., 1997, 168, 194. 8 P. T. Tanev and T. J. Pinnavaia, Science, 1995, 267, 865. 9 R. Mokaya and W. Jones, Chem. Commun., 1996, 981. Aknowledgements 10 R. Mokaya and W. Jones, Chem. Commun., 1996, 983. 11 R. Mokaya and W. Jones, J. Catal., 1997, 172, 211. R.M. is grateful to the EPSRC for an Advanced Fellowship 12 A. Tuel and R. Gontier, Chem. Mater., 1996, 8, 114. and Trinity College, Cambridge for a Research Fellowship. 13 R. J. Ray, B. L. Meyes and C. L. Marshall, Zeolites, 1987, 7, 307. The assistance of Dr H.Y. He with the NMR measurements 14 C. Breen, Clay Miner., 1991, 26, 487. and Laporte Adsorbents with surface area determinations is 15 R. Mokaya, W. Jones, Z. Luan, M. D. Alba and J. Klinowski, Catal. Lett., 1996, 37, 113. appreciated. 16 P. T. Tanev and T. J. Pinnavaia, Chem.Mater., 1996, 8, 2068. 17 P. J. Branton, P. G. Hall, K. S. W. Sing, H. Reichert, F. Scuth and K. K. Unger, J. Chem. Soc., Faraday Trans., 1994, 90, 2965. 18 J. Rathousky. A. Zukal, O. Franke and G. J. ShulzekloV, J. Chem. References Soc., Faraday Trans., 1994, 90, 2821. 19 M. J. Remy, D. Stanica, G. Poncelet, E. J. P. Feijen, P. J. Grobet, 1 A. Corma, Chem. Rev., 1997, 97, 2373. J. A. Martens and P. A. Jacobs, J. Phys. Chem., 1996, 100, 12440. 2 C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 1992, 359, 710. 3 Q. S. Hue, D. I. Margolese, U. Ciesla, P. Y. Feng, T. E. Gier, Paper 8/06049E 2826 J. Mater. Chem., 1998, 8, 2819–2826

ISSN:0959-9428    

DOI:10.1039/a806049e

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Synthesis and structure of a 2-D aluminophosphate Al3P4O16·3CH3CH2CH2NH3 N. Togashi,a J. Yu,*a,b,c S. Zheng,c K. Sugiyama,d K. Hiraga,d O. Terasaki,*a,b W. Yan,c S. Qiuc and R. Xuc aDepartment of Physics, Graduate School of Science and CIR, Tohoku University, Sendai 980–8578, Japan bCREST, Japan Science and Technology Corporation (JST), Japan cKey Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun, 130023, P.R. China dInstitute for Materials Research, Tohoku University, Sendai 980–8577, Japan Received 13th July 1998, Accepted 25th September 1998 Using n-propylamine as a template, a new two-dimensional aluminophosphate Al3P4O16·3CH3CH2CH2NH3, 1 [space group P21/n (no. 14), a=11.310(1) A° , b=14.854(1) A° , c=14.796(1) A° , b=93.64(1)°, Z=4] is synthesised in an alcoholic system and the structure is solved using single-crystal X-ray diVraction data.The structure consists of tetrahedral AlO4 and PO3(LO) which are linked alternatively to form macroanionic layers parallel to the (1019) plane. The organic cations (C3H7NH3+) are located in the interlayer regions and are connected to the oxgens of the layers by hydrogen bonding.The 2-D nets of compound 1 are constructed from 4.6.8-nets which resemble the 2-D nets (4.6.8)1(6.8.8)1 in microporous AlPO4-21.under autogeneous pressure. The resulting product was filtered Introduction oV and dried in air at 70 °C. Following the discovery of microporous aluminophosphates Powder X-ray diVraction (XRD) patterns were obtained on (AlPO4-n) whose formation is promoted by the presence of a Philips PW3050 X-ray diVractometer using Cu-Ka radiation organic templates,1–3 a number of low-dimensional materials, (l=1.5418 A° ).The powder X-ray diVraction pattern of comi. e., one-dimensional (1-D) chains and 2-D layers, have been pound 1 shows good agreement with the simulated one based successfully synthesized in non-aqueous systems.4 These mate- on single-crystal XRD structure analysis, establishing that the rials exhibit diverse stoichiometries with Al/P ratios lower product is a single phase.than unity, in contrast to 3-D aluminophosphates with an Al/P ratio of 1/1 (except for JDF-20: [Al5P6O24H]2-5 and Single-crystal X-ray diVraction analysis AlPO-HDA: [Al4P5O20H]2- 6).Among the 2-D layer compounds, three diVerent stoichiometries have been observed, A colourless plate-like crystal of dimensions i.e., Al3P4O163-,7–14 Al2P3O123-,15,16 and AlP2O83-.14,17,18 In 0.16×0.80×0.03 mm was selected and mounted on a thin the case of Al3P4O163- compounds, except for Al3P4O16- glass capillary using cyanoacrylate adhesive. The intensity data 2C3N2H5,14 all the anionic inorganic layers are constructed were measured on a Rigaku R-AXIS IV imaging-plate detector from tetrahedral AlO4 and PO3(LO) alternately linked to form using graphite-monochromated Mo-Ka radiation (l= 4.6-,7,8 4.6.8-,9–11 and 4.6.12-nets.12,13 The topologies of these 0.710 69 A° ) generated by a rotating anode X-ray tube. Details 2-D nets show a resemblance to some of the three-connected of data collection are listed in Table 1.The lattice constants 2-D nets in 3-D microporous compounds. For example, the were determined by a least-squares procedure applied to the 4.6.12-nets resemble the 2-D nets in AlPO4-5.19 Here we report measurement of 25 well centered reflections (2h range a new 2-D aluminophosphate with 4.6.8-nets. The 4.6.8-nets 24.2–31.8°, Mo-Ka) on a Rigaku AFC7R diVractometer. resemble the 2-D nets in 3-D AlPO4-2119,20 that can be The structure was solved by direct methods using the structurally directed by the same template, n-propylamine, as program SIR9222 and refined by the least-squares program for the title compound.SHELXL97.23 In the final cycles of each refinement, nonhydrogen atoms except for C atoms were refined with anisotropic thermal parameters. Some C atoms have large thermal motions because the organic cations are located in large spaces Synthesis Compound 1 was synthesised solvothermally in a reaction Table 1 Summary of data collection details for Al3P4O16· mixture of 1.0 Al(OPri)352.4 H3PO455.0 CH3CH2CH2NH2550 3CH3CH2CH2NH3 butan-2-ol at 180 °C for 9 days.AlPO4-21 was prepared Resolution range/A° 14.80–0.76 hydrothermally in a reaction mixture of 1.0 Al(OPri)351.0 Total measured reflections 19 097 H3PO45(0.5–1.0) CH3CH2CH2NH25100 H2O at 180–220 °C Unique reflections 5228 for 9–12 days.2,21 The synthesis procedure is identical for both Possible reflections 6053 compounds, except for the solvent. Aluminium triisopropoxide Completeness(%) 86.3 was first dispersed into weighed amounts of butan-2-ol or Rmerge=.|I -Ii|/. Ii (%) 6.22 H2O.Phosphoric acid (85%) was then added dropwise and Oscillation angle per exposure 4.8 Total oscillation range 144.3 the mixture was stirred continuously. Finally, n-propylamine Camera length/mm 100 was added to the mixture with further stirring to form a Exposure time per frame/min 30 homogeneous gel.The reaction mixture was placed in a Teflon- Overlapped angle/° 0.3 lined stainless steel autoclave and heated at 180 °C for 9 days J. Mater. Chem., 1998, 8, 2827–2830 2827Table 2 Crystal data and structure refinement for Al3P4O16· Table 3 Atomic coordinates (×104) and equivalent isotropic displacement parameters (A° 2×103) of compound 1 3CH3CH2CH2NH3 Compound Al3P4O16·3CH3CH2CH2NH3 Atom x y z Ueq a Empirical formula C9H30Al3N3O16P4 Formula weight 641.18 P(1) 9162(2) -2406(2) 4392(2) 12(1) P(2) 8774(2) -2608(2) 1286(2) 12(1) Temperature/K 293(2) Wavelength/A° 0.71069 P(3) 5862(3) -923(1) 2402(2) 12(1) P(4) 5867(3) -4068(1) 2643(2) 13(1) Crystal system Monoclinic Space group P21/n Al(1) 8668(3) -1113(2) 2742(2) 12(1) Al(2) 5963(2) -2578(2) 1122(2) 10(1) Unit cell dimensions a/A° 11.310(1) Al(3) 8662(3) -3897(2) 2955(2) 11(1) O(1) 10252(5) -2327(3) 5057(4) 18(2) b/A° 14.854(1) c/A° 14.796(1) O(2) 9231(7) -1612(4) 3740(5) 18(2) O(3) 8025(5) -2393(4) 4853(5) 26(2) b/° 93.64(1) Volume/A° 3 2480.7(4) O(4) 9317(7) -3276(4) 3844(5) 22(2) O(5) 4555(5) -2315(4) 5514(4) 21(2) Z 4 Dc/g cm-3 1.717 O(6) 8964(7) -3402(4) 1928(5) 22(2) O(7) 9072(7) -1738(4) 1812(5) 22(2) Absorption coeYcient/mm-1 0.488 F(000) 1328 O(8) 7470(5) -2567(4) 955(4) 23(2) O(9) 9223(7) -4979(4) 3004(5) 23(2) Crystal size/mm 0.160×0.080×0.030 h range for data collection/° 2.20–27.44 O(10) 5550(7) -1584(4) 1623(5) 23(2) O(11) 5069(8) -1021(4) 3160(5) 29(2) Index ranges 0h14, 0k17, -19l18 Independent reflections (I>0) 2620 O(12) 7144(7) -1088(4) 2754(5) 24(2) O(13) 7155(7) -3907(4) 3013(5) 25(2) No.obs. data [I>2s(I )] 1948 Adsorption correction — O(14) 5617(7) -3476(4) 1813(5) 20(2) O(15) 5028(7) -3884(4) 3362(5) 26(2) Refinement method Full-matrix least-squares on F2 No. of parameters refined 263 O(16) 9237(7) -44(3) 2679(5) 21(2) N(11) 6318(8) -1079(4) 4822(6) 30(2) Goodness-of-fit on F2 0.934 Final R indices [I>2s(I )] R1=0.0668, wR2=0.1146 C(12) 6479(13) -104(8) 5088(11) 68(4) C(13) 7722(15) 215(9) 4953(11) 75(5) R indices (all data) R1=0.1111, wR2=0.1266 Largest diV.peak and hole/e A° -3 0.655, -0.467 C(14) 8457(16) -118(9) 5699(12) 89(6) N(21) 6232(8) -3625(4) 5093(6) 24(2) C(22) 6347(12) -4429(6) 5686(9) 50(3) C(23) 7080(15) -5193(9) 5186(12) 80(5) between the layers and while one end of the cation is connected C(24) 8339(18) -4990(11) 5361(15) 117(7) by hydrogen bonds to the oxygens of the layers (as will be N(31) 3674(6) -2415(5) 3694(5) 27(2) discussed in the next section) the other end is free.The C(32) 2351(11) -2337(8) 3488(9) 61(4) hydrogen atoms were geometrically placed. The isotropic C(33) 2098(16) -2536(12) 2448(13) 120(6) thermal parameters of all the hydrogen atoms were constrained C(34) 2160(3) -3517(14) 2280(2) 275(18) to be equal. A summary of the crystallographic data is given aUeq is defined as one third of the trace of the orthogonalized Uij tensor.in Table 2. Full crystallographic details, excluding structure factors, have been deposited at the Cambridge Crystallographic Data of these capped-6-MRs along [101] and [010], respectively, to Centre (CCDC).See Information for Authors, J. Mater. form the 4.6.8-nets. It should be noted that the eight- Chem., 1998, Issue 1. Any request to the CCDC for this membered-rings (8-MRs) possess both circular and elliptical material should quote the full literature citation and the shapes, as is the case in [Al3P4O16]3-[NH3(CH2)5NH3]2+- reference number 1145/126.[C5H10NH2]+,10 in which the distortion of the 8-MRs was thought to be due to the diVerential templating eVect of the Results and discussion two organic cations in the structure. However, in compound 1, only one template species exists in the structure. The factors The atomic coordinates, and selected bond lengths and bond causing the distortion of 8-MRs are worth considering.angles of compound 1 are given in Table 3 and 4, respectively. The organic ammonium cations (CH3CH2CH2NH3+) reside The Al3P4O163- anionic layers are constructed from alterin the interlayer regions between the inorganic layers, and the nating tetrahedral Al and P units [Fig. 1(a)] and the labeling layers are stacked in AAAA sequence along the c-axis (Fig. 2). scheme is shown in Fig. 1(b). There are three crystallograph- There are extensive H-bonding networks between the organic ically distinct Al sites. Each aluminium atom is coordinated ammonium cations and the framework oxygens. The H- to four phosphate groups. The AlKO bond lengths and bonding information is summarised in Table 5. Each RNH3+ OKAlKO bond angles are in the range 1.711(9)–1.746(7) A° supplies three H atoms to form H-bonds with three diVerent and 106.0(3)–111.3(3)°, which are typical for aluminophosterminal oxygens attached to the phosphate groups.The O(5) phates.7–18 All the four crystallographically distinct P atoms atom attached to the capped phosphate P(2) group accepts share three oxygens with adjacent Al atoms and the PKO bond three H atoms from three crystallographically distinct organic lengths are in the range 1.522 (6)–1.543(8) A° .The fourth PKO ammonium cations, whereas other terminal oxygens accept bond lengths for each P atom are P(1)KO(3) 1.493(6) A° , two H atoms from two crystallographically distinct templates P(2)KO(5) 1.492(6) A° , P(3)KO(11) 1.488(8) A° and to form H-bonds. P(4)KO(15) 1.496(8) A° , which are the shortest among the 4.6.8-nets have been observed in several 2-D aluminophos- others.These distances suggest that each crystallographically phates, such as [Al3P4O16]3-[NH3(CH2)5NH3]2+- distinct P atom possesses one PLO group.7–18 [C5H10NH2]+ 2,10 [Al3P4O16]3-[NH3(CH2)2NH3]2+- The inorganic layer consists of a series of capped six- [OH(CH2)2OH2]+[OH(CH2)2(OH)] 39 and [Al3P4O16]3-· membered-rings (capped-6-MRs)† [Fig. 1(a)]. All the tetra- 3[CH3CH2NH3]2+.23 However, all 2-D structures with 4.6.8- hedral P(2) atoms cap the 6-MRs with the terminal oxygens nets possess the same topology, which is schematically shown protruding into the interlayer region above or below the sheet. in Fig. 3(a). The direction of the PLO in the capped phosphate The layer is constructed from corner sharing and edge sharing groups is opposite to that of the other three PLO in the 6- MR.It is found that there are two typical stacking sequences †n-MR represents a loop, where n is the number of T(Al or P)-atoms or O-atoms forming the loop. of the inorganic layers, i.e., AAAA (e.g. compound 210) and 2828 J. Mater. Chem., 1998, 8, 2827–2830Table 4 Selected bond lengths (A° ) and angles (°) of compound 1 P(1)KO(3) 1.493(6) P(1)KO(2) 1.530(6) P(1)KO(1) 1.532(6) P(1)KO(4) 1.542(6) P(2)KO(5)a 1.492(6) P(2)KO(6) 1.521(6) P(2)KO(8) 1.525(6) P(2)KO(7) 1.534(6) P(3)KO(11) 1.487(8) P(3)KO(9)b 1.527(6) P(3)KO(12) 1.528(8) P(3)KO(10) 1.538(7) P(4)KO(15) 1.496(8) P(4)KO(14) 1.522(7) P(4)KO(16)c 1.528(6) P(4)KO(13) 1.541(8) Al(1)KO(16) 1.718(6) Al(1)KO(12) 1.725(9) Al(1)KO(2) 1.736(7) Al(1)KO(7) 1.745(7) Al(2)KO(10) 1.728(6) Al(2)KO(1)d 1.728(6) Al(2)KO(8) 1.738(6) Al(2)KO(14) 1.740(7) Al(3)KO(13) 1.713(9) Al(3)KO(9) 1.727(6) Al(3)KO(4) 1.734(7) Al(3)KO(6) 1.742(7) N(11)KC(12) 1.51(1) C(12)KC(13) 1.51(2) C(13)KC(14) 1.43(2) N(21)KC(22) 1.48(1) C(22)KC(23) 1.61(2) C(23)KC(24) 1.46(2) N(31)KC(32) 1.51(1) C(32)KC(33) 1.57(2) C(33)KC(34) 1.48(2) O(3)KP(1)KO(2) 111.0(4) O(3)KP(1)KO(1) 112.8(4) O(2)KP(1)KO(1) 106.0(4) O(3)KP(1)KO(4) 112.4(4) O(2)KP(1)KO(4) 107.6(3) O(1)KP(1)KO(4) 106.7(4) O(5)aKP(2)KO(6) 110.5(4) O(5)aKP(2)KO(8) 111.4(4) O(6)KP(2)KO(8) 109.1(4) O(5)aKP(2)KO(7) 109.3(4) O(6)KP(2)KO(7) 108.6(3) O(8)KP(2)KO(7) 107.9(4) O(11)KP(3)KO(9)b 111.1(4) O(11)KP(3)KO(12) 109.4(5) O(9)bKP(3)KO(12) 108.4(4) O(11)KP(3)KO(10) 112.5(4) O(9)bKP(3)KO(10) 106.6(4) O(12)KP(3)KO(10) 108.7(4) O(15)KP(4)KO(14) 112.0(4) O(15)KP(4)KO(16)c 110.8(4) O(14)KP(4)KO(16)c 106.9(4) O(15)KP(4)KO(13) 110.2(4) O(14)KP(4)KO(13) 108.7(4) O(16)cKP(4)KO(13) 108.1(4) O(16)KAl(1)KO(12) 111.0(4) O(16)KAl(1)KO(2) 108.8(4) O(12)KAl(1)KO(2) 108.3(4) O(16)KAl(1)KO(7) 109.4(4) O(12)KAl(1)KO(7) 109.4(4) O(2)KAl(1)KO(7) 110.1(4) O(10)KAl(2)KO(1)d 109.7(3) O(10)KAl(2)KO(8) 110.3(4) O(1)dKAl(2)KO(8) 105.9(3) O(10)KAl(2)KO(14) 109.0(3) O(1)dKAl(2)KO(14) 111.3(3) O(8)KAl(2)KO(14) 110.5(4) O(13)KAl(3)KO(9) 110.7(4) O(13)KAl(3)KO(4) 110.2(4) O(9)KAl(3)KO(4) 109.0(3) O(13)KAl(3)KO(6) 107.3(4) O(9)KAl(3)KO(6) 109.8(4) O(4)KAl(3)KO(6) 109.8(4) P(1)KO(1)KAl(2)e 152.6(4) P(1)KO(2)KAl(1) 146.6(5) P(1)KO(4)KAl(3) 142.0(5) P(2)KO(6)KAl(3) 147.6(5) P(2)KO(7)KAl(1) 141.9(5) P(2)KO(8)KAl(2) 153.0(4) P(3)cKO(9)KAl(3) 145.5(5) P(3)KO(10)KAl(2) 144.2(5) P(3)KO(12)KAl(1) 157.9(5) P(4)KO(13)KAl(3) 154.7(5) Fig. 1 (a) The inorganic sheet parallel to the (1019) plane and P(4)KO(14)KAl(2) 151.6(5) P(4)bKO(16)KAl(1) 150.7(5) (b) capped-6-MR showing the labeling scheme. C(13)KC(12)KN(11) 111.(1) C(14)KC(13)KC(12) 107.(1) N(21)KC(22)KC(23) 109.(1) C(24)KC(23)KC(22) 107.(1) N(31)KC(32)KC(33) 107.(1) C(34)KC(33)KC(32) 110.(2) Symmetry transformations used to generate equivalent atoms: ax+1/2, -y-1/2, z-1/2 b-x+3/2, y+1/2, -z+1/2; c-x+3/2, y-1/2, -z+1/2; dx-1/2, -y-1/2, z-1/2; ex+1/2, -y-1/2, z+1/2; fx-1/2, -y-1/2, z+1/2.ABAB (e.g. compound 39). Taking account of the 8-MR shape, two types of configurations are observed. In the case of compound 1 and 2 mentioned above, the inorganic layers contain two types of 8-MR systems with both circular and elliptical shape, whereas all the other layers contain regular elliptical 8-MRs.It is worth mentioning that the 4.6.8-nets in the 2-D aluminophosphates resemble the (4.6.8)1(6.8.8)1 2-D net (node 401) in 3-D microporous AlPO4-21 as shown in Fig. 3(b) (its 3-D structure can be represented by the addition of up–down linkages to the 2-D nets).19 An obvious diVerence of the 2-D nets in 2-D layer compounds from that in 3-D AlPO4-21 lies Fig. 2 View of the stacking of the layers with the interlamellar organic in the presence of capped phosphate groups in the 6-MRs . It cations; H-bondings are indicated by dashed lines.can be seen that if the capped phosphate groups are removed from the 6-MRs in the 2-D layer network, all the tetrahedral Al and P become three-connected, like the 2-D net in AlPO4-21. non-aqueous systems favour the formation of low-dimensional materials. It should be noted that most of the low-dimensional Interestingly, the 3-D microporous AlPO4-21 can be synthesized in an aqueous system using the same template, n- materials were synthesized in non-aqueous systems.The existence of triply, doubly, or singly-bridging phosphate groups propylamine, as for compound 1. However, compound 1 can only be prepared in non-aqueous systems. It is known that with terminal oxygens (PLO /or PKOH) in the low-dimensional J. Mater. Chem., 1998, 8, 2827–2830 2829Table 5 Hydrogen-bonding distances (A° ) present in Al3P4O16· structure consists of alternately linked tetrahedral AlO4 and 3CH3CH2CH2NH3 PO3(LO) to give Al3P4O163- stoichiometry.The 2-D inorganic nets are constructed from 4.6.8-nets which resemble the Distance NKH,O (4.6.8)1(6.8.8)1 2-D nets in 3-D microporous AlPO4-21, which can be also synthesized using the same template (n-propyl- N11KH113,O3 2.74 (1) N11KH111,O11 2.76 (1) amine) but in aqueous systems.Further investigation of the N11KH112,O5 2.94 (1) formation mechanism in diVerent solvent systems might prove N21KH212,O5 2.81 (1) to be a valuable contribution towards achieving a rational N21KH213,O15 2.85 (1) design of target materials. N21KH211,O3 2.77 (1) N31KH312,O5 2.82 (1) N31KH313,O11 2.75 (1) Acknowledgments N31KH311,O15 2.73 (1) We are grateful to CREST (Japan Science and Technology Corporation), NNSF (China) and the Key Lab of ISPC (China) for their support.References 1 S. T. Wilson, B. M. Lok, C. A. Messian, T. R. Cannon and E. M. Flanigen, J. Am. Chem. Soc., 1982, 104, 1146. 2 E.M. Flanigen, B. M. Lok, R. L. Patton and S. T. Wilson, in New Developments in Catalysis, ed.Y. Murakami, A. Ijima and J. W.Ward, Elsevier, Amsterdam, 1986, p. 103. 3 J.M. Bennett, W. J. Dytrych, J. J. Pluth, J. W. Richardson, Jr and J. V. Smith, Zeolites, 1986, 6, 349. 4 R. Xu, Q. Huo and W. Pang, in Proceedings in the Ninth International Zeolite Conference, ed. R. V. Ballmoos, J. B. Higgins and M. M. J. Treacy, Montreal, 1992, p.271. 5 Q. Huo, R. Xu, S. Li, Z.Ma, J. M. Thomas, R. H. Jones and A. M. Chippindale, J. Chem. Soc., Chem. Commun., 1992, 875. 6 J. Yu, K. Sugiyama, S. Zheng, S. Qiu, J. Chen, R. Xu, Y. Sakamoto, O. Terasaki, K. Hiraga, M. Light, M. B. Hursthouse and J. M. Thomas, Chem. Mater., 1998, 10, 1208. 7 K.Morgan, G. Gainsford and N. Milestone, J. Chem. Soc., Chem. Commun., 1995, 425. 8 I. D. Williams, Q. Gao, J. Chen, L-Y.Ngai, Z. Lin and R. Xu, Chem. Commun., 1996, 1781. 9 R. H. Jones, J. M. Thomas, R. Xu, Q. Huo, A. K. Cheetham and A. V. Powell, J. Chem. Soc., Chem. Commun., 1991, 1266. 10 R. H. Jones, A. M. Chippindale, S. Natarajan and J. M. Thomas, J. Chem. Soc., Chem. Commun., 1994, 565. 11 Q. Gao, B. Li, J. Chen, S. Li, R. Xu, I. D. Williams, J. Zheng and D. Barber, J. Solid State Chem., 1997, 129, 37. 12 J. M. Thomas, R. H. Jones, R. Xu, J. Chen, A. M. Chippindale, S. Natarajan and A. K. Cheetham, J. Chem. Soc., Chem. Commun., 1992, 929. 13 P. A. Barrett and R. H. Jones, J. Chem. Soc., Chem. Commun., 1995, 1979. 14 J. Yu and I. D. Williams, J. Solid State Chem., 1998, 136, 141; J. Yu, I. D. Williams, S. Qiu, O. Terasaki and R. Xu, Supermol. Fig. 3 (a) Topology of 4.6.8-nets in aluminophosphate with Sci., 1998, 5, 297.Al3P4O163- stoichiometry. The open and filled circles represent the 15 A. M. Chippindale, A. V. Powell, L. M. Bull, R. H. Jones, positions of P atoms and correspond to the diVerent directions of A. K. Cheetham, J. M. Thomas and R. Xu, J. Solid State Chem., PLO bonds relative to the inorganic sheet, that is, face up and down, 1992, 96, 199.respectively. The nodes without circles represent those of Al atoms. 16 J. Yu, K. Sugiyama, K. Hiraga, N. Togashi, O. Terasaki, S. Qiu Oxygen positions are not shown but are located about half-way and R. Xu, Chem. Mater., in press. between P and Al atoms. (b) (4.6.8)1(6.8.8)1 2-D net in AlPO4-2119. 17 K. R. Morgan, G. J. Gainsford and N. B. Milestone, Chem. Commun., 1997, 61. materials impedes the formation of a 3-D structure. Therefore, 18 M. A. Leech, A. R. Cowley, K. Prout and A. M. Chippindale, such terminal oxygen groups are suggested to be more stabil- Chem. Mater., 1998, 20, 451. 19 J. V. Smith, Stud. Surf. Sci. Catal., 1989, 49, p. 29. ized in non-aqueous systems compared to aqueous systems. 20 J. B. Parise and C. S. Day, Acta. Crystallogr., Sect. C, 1985, 41, 515. Conclusion 21 Z. Liu and R. Xu, Stud. Surf. Sci. Catal., 1997, 105, 405. 22 A. Altomare, G. Cascarano, G. Giacovazzo, A. Guagliardi, The utilization of a non-aqueous synthesis technique has M. C. Burla, G. Polidori and M. Camalli, J. Appl. Crystallogr., greatly extended the aluminophosphate family upon continu- 1994, 27, 453. 23 SHELXL97, G. M. Sheldrick, Program for the Refinement of ing synthesis of a number of low-dimensional materials. A Crystal Structure, University of Go� ttingen, Germany, 1997. new compound Al3P4O16·3CH3CH2CH2NH3 was prepared in an alcoholic system in the presence of n-propylamine. Its Paper 8/05423A 2830 J. Mater. Chem., 1998, 8, 2827–28

ISSN:0959-9428    

DOI:10.1039/a805423a

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Solvothermal synthesis and characterization of silica-pillared titanium phosphate Xiuling Jiao,a Dairong Chen,bWenqin Pang,a* Ruren Xua and Yong Yuec aDepartment of Chemistry, Jilin University, Changchun 130023, PR China bDepartment of Chemistry, Shandong University, Jinan 250100, PR China cWuhan Institute of Physics, The Chinese Academy of Science, Wuhan 430071, PR China Received 15th April 1998, Accepted 4th August 1998 Silica-pillared titanium phosphate has been synthesized from Ti(OC4H9)4–H3PO4– H2N(CH2)3Si(OC2H5)3–C2H5OH and characterized by XRD, SEM, IR and TGA.The synthesis conditions were investigated and the optimal conditions are reported: crystallization at 180 °C for about 7 days with a batch composition of 1.0 Ti(OC4H9)453.6 H3PO451.0 H2N(CH2)3Si(OC2H5)3533 C2H5OH.XRD analysis gives the lattice parameters of the monoclinic cell as a=1.99952 nm, b=0.41803 nm, c=0.90062 nm, b=97.462°, V= 0.74642 nm3, and Z=2. N2 adsorption–desorption test shows that the pillared compound has a BET surface area of 51 m2 g-1. 2. Characterization Introduction XRD patterns of the products were recorded with a Rigaku Pillaring of layered compounds such as clay minerals, oxides D/MAX-III diVractometer using Cu-Ka radiation (l= and phosphates has been extensively studied for the last 0.15418 nm) at room temperature over the range 3–60°.decade.1–5 Since the first preparative studies by Clearfield Infrared spectra were measured on a Nicolet 5DX FT-IR et al.6 and Alberti et al.7 of a-ZrP, many works have focused instrument using the KBr pellet technique. 13C, 29Si, 31P MAS on the syntheses and characterization of pillared layered NMR spectra were recorded on a Bruker MSL-400 spec- metal(IV) phosphates,8,9 and a review discussed the preptrometer. Inductive coupled plasma analyses (ICP) for the Ti, aration, characterization and properties of pillared layered P, Si contents in the product were obtained from a Leeman metal(IV) phosphates.10 Generally, the pillared compounds are ICP-AES instrument, and elemental analyses for C, H, N prepared by ion exchange of polynuclear species or by the contents were done on a Perkin-Elmer 240C elemental ana- hydrolysis of organometallic precursors, such as lyzer.Thermogravimetric analyses (TGA) were performed on [Al13O4(OH)24(H2O)12]7+, RxSi(OR¾)4-x etc., using the a Perkin-Elmer TGA7 with increasing temperature rate of sol–gel method; several pillared metal phosphates such as 10 °Cmin-1.Water adsorption measurements were carried out silica-pillared a-ZrP,11 silica-pillared a-SnP,12 Al-pillared aon a Cahn 2000 vacuum electron balance at room temperature, ZrP13 etc.14,15 have been obtained.However, intercalation of while N2 adsorption–desorption and BET surface area on the large cations into the interlayer space often results in a low calcined sample (77 K) were measured on an ASAP 2010 yield and low crystallinity of the pillared compounds. For micromeritics apparatus. example, well crystallized silica-pillared titanium phosphate can not be obtained by this method.11,12 Here, we report a new route for the synthesis of well Results and discussion crystallized silica-pillared layered titanium phosphate.The 1. Synthesis pillared compound is hydrothermally synthesized in Ti(OC4H9)4–H3PO4–H2N(CH2)3Si(OC2H5)3–C2H5OH, and The crystalline pillared compound was synthesized from the the product obtained by this method has a high crystallinity.batch composition 1.0 TBOT5(2.0–5.0) H3PO451.0 APTEOS533 EA at 140–240 °C for several days. Hydrolysis may occur in the mixed gel as follows: Experimental H2N(CH2)3Si(OEt)3+nH2O 1. Synthesis �H2N(CH2)3Si(OEt)3-n(OH)n+nEtOH (n3) (1) All the reagents were of analytical grade and were not further The hydrolysized APTEOS then polymerizes continuously. To purified before utilization.adjust the rate of hydrolysis and polymerization, ethanol Tetrabutyltitanate (analytical reagent, TBOT), phosphoric (99.7%) was used as the solvent. The title compound could acid (analytical reagent, 85 wt.%) and 3-aminopropyltriethoxy- not be obtained with ethanol (95%) or water as solvent. silane (APTEOS, Aldrich) were used as titanium, phosphorus Further experiments indicated that the crystallization reaction and silica sources.To adjust the hydrolysis and polymerization accelerated upon increasing the H3PO4 content in gel mixture, rate of APTEOS in the hydrothermal process, ethanol (EA, and an unknown phase formed when the reaction time was analytical reagent, 99.7%) was selected as the solvent. In a prolonged to 15 days. Crystallization at 180 °C for about 7 typical synthesis, phosphoric acid was added to a homo- days with the batch composition of 1.0TBOT53.6 H3PO451.0 geneously mixed solution of TBOT and EA under stirring.APTEOS533 EA was optimal. APTEOS was added dropwise to this mixture and a white gel formed. The gel was placed into a Teflon-lined stainless steel 2. Characterization autoclave and heated to 140–240 °C for about 7 days.It was then cooled to room temperature and the product was reco- Fig. 1 shows the SEM of the as-prepared crystals. The crystals are homogeneous with sheet-like morphology indicating that vered by filtration, thoroughly washed with deionized water, and dried at room temperature. the solid is phase pure. The average particle size is about 2 mm. J. Mater. Chem., 1998, 8, 2831–2834 2831Table 1 X-Ray powder diVraction data and indexing results of silicapillared layered titanium phosphate h k l dobs/A° dcal/A° 2hobs/ ° 2hcalc/ ° 1 0 0 19.87 19.83 4.443 4.453 2 0 0 9.94 9.91 8.889 8.913 3 0 0 6.60 6.61 13.405 13.387 2 0 1 6.25 6.24 14.159 14.173 3 0 1 5.71 5.67 15.506 15.599 4 0 0 4.95 4.96 17.905 17.881 1 0 2 4.50 4.48 19.713 19.791 0 1 0 4.20 4.18 21.136 21.237 1 1 0 4.07 4.09 21.820 21.709 5 0 0 3.97 3.97 22.376 22.404 0 1 1 3.79 3.79 23.454 23.479 3 1 0 3.53 3.53 25.208 25.188 5 0 2 3.17 3.18 28.127 28.066 4 0 2 3.12 3.12 28.587 28.569 2 0 3 2.96 2.96 30.168 30.182 2 1 2 2.85 2.85 31.362 31.403 3 1 2 2.68 2.68 33.408 33.390 5 1 2 2.53 2.53 35.352 35.463 Fig. 1 Scanning electron micrograph of as-synthesized powder. 8 0 1 2.47 2.47 36.343 36.310 Fig. 2 gives the XRD pattern of the title compound. The product recovered is of high crystallinity. It can be seen from V=0.74642 nm3. From the unit cell parameters and the PMO Fig. 2 that the XRD pattern of the as-prepared product is and TiMO bond lengths and the structure of layered titanium similar to that of the c-phase structure,14,16 although the peak phosphate, the result of Z=2 can be obtained.positions and intensities are diVerent. Because of the replace- Further experiments shows that no significant change of the ment of H3O+ in the interlayer space of c-TiP by interlayer spacing is observed when the calcination temperature NH3+(CH2)3(O)SiOSi(O)(CH2)3NH3+ in the title compound, is lower than 300 °C, and the significant change of the basal the basal spacing of the as-prepared product is much larger spacing takes place between 300 and 500 °C, decreasing from than that of c-TiP.This replacement further leads to a change 1.93 nm to 1.42 nm. Upon increasing the calcination temperaof b value and an increase of the unit cell volume. The ture, the basal spacing decreases to 1.40 nm. The pillared similarity of the layer structures of c-TiP and the title com- compound is stable up to 700 °C (Fig. 3). pound can also be seen from the XRD patterns, although the The 13C MAS NMR spectrum (Fig. 4) exhibits three peaks basal space reflections are diVerent. We therefore assume that at 42.96, 20.99, and 10.21 ppm, which are attributed to Ca, Cb the layer structure of the as-synthesized product is similar to and Cc in H3N+CaH2CbH2CcH2Si, respectively,17 which indithat of c-TiP.The diVerence between the XRD patterns can cates that EA and other organic materials do not exist in the be attributed to the influence of the silica pillar to the product. The disappearance of these resonances after calciphosphate layer, and even to the lattice structure of the cryst. nation at 600 °C reveals that the organic material is lost The X-ray powder diVraction data were indexed with the at 600 °C.TREOR program (Table 1). The cell is monoclinic with a= 1.99952 nm, b=0.41803 nm, c=0.90062 nm, b=97.462° and Fig. 3 XRD patterns of as-prepared samples (a), and of samples Fig. 2 XRD pattern of the title compound. calcined at 200 (b), 300 (c), 400 (d), 500 (e), 600 (f ), 700 °C (g). 2832 J. Mater. Chem., 1998, 8, 2831–2834Fig. 4 13C MAS NMR spectrum of as-prepared product (from TMS). 29Si CP MAS NMR spectra (Fig. 5) for as-synthesized and calcined products have two resonances at -66.80 and -103.85 ppm, which are assigned to the silicon atoms as shown in Fig. 6.18 This result confirms that APTEOS is thoroughly hydrolyzed and polymerized. 31P MAS NMR spectra (Fig. 7) of the powder give two Fig. 7 31P MAS NMR spectra of as-prepared (a) and calcined (b) resonances at -12.95, -25.22 ppm with an intensity ratio of samples (from H3PO4). about 152, which shift to -14.14 and -29.45 ppm after calcination at 500 °C for 3 h with a peak area ratio of 151. The 31P MAS NMR spectrum of intercalated a-phase structure metal(IV) phosphates either have a single resonance or have several similar shifts,1,10,12,13,19 although there are two crystallographically inequivalent P sites in the structure, and only after thermal treatment are two 31P shifts obtained.1 Thus from the two 31P resonances of the as-prepared compound and the shape of the shifts, these peaks should be assigned to two diVerent phosphorus atoms—P(OTi)2(OH)2 and P(OTi)4.20 This indicates that the structure of the phosphate layer is similar to that of c-TiP, which also has these two diVerent phosphorus atoms.After calcination, the two resonances shift to -14.14 and -29.45 ppm and the area ratio changes to about 151, these resonances can be attributed to P(OTi)2(OH) (OSi) and P(OTi)4. The replacement of OH with OSi causes the resonances to shift from -12.95 and -25.22 ppm to -14.14 and -29.45 ppm, and the area ratio of these resonances for the calcined sample is consistent with the P/Si ratio in the product, i.e.only half of the P are linked Fig. 5 29Si CP MAS NMR spectra for as-prepared (a) and calcined to OSi—the molar ratio of P(OTi)2(OH)(OSi) to P(OTi)4 (b) product (from TMS). is 151. Fig. 8 shows the IR spectra of as-synthesized samples and those heated to 200, 300, 400, 500, 600, 700 °C, respectively.The bands for the as-synthesized sample are assigned as follows: 3479, hydrogen bound OH; 3261, NH3+ stretching; 2917, CH2 stretching; 1623, NH3+ asymmetric deformation H2O bending; 1525, NH3+; 1475, CH2 bending; 1398, PMO, 1215, CMCMCMN; 1152, PMO; 1004, PMO; 962, SiMOMSi; 798, NH3+ in-plane rocking; 701, NH3+ out-of-plane deformation; 646, TiMO, 549, 512, 470, TiMO.No significant changes of the spectra were observed below 300 °C except for the shift of the bands around 1000–900 cm-1, indicating the distortion of the structure. Significant changes of the spectra above 300 °C result from the decomposition and loss of organic materials. The weight loss from room temperature to 300 °C (about 3%, Fig. 9) is assigned to the loss of water in the interlayer space, while that from 300–600 °C (about 19%) is attributed Fig. 6 Schematic representation of the product. to the loss of the organic material. The weight loss between J. Mater. Chem., 1998, 8, 2831–2834 2833644 and 732 °C is attributed to the loss of water from the silanol groups. N2 adsorption–desorption tests indicate a BET surface area of ca.51 m2 g-1 and the amount of N2 adsorption is small.From the basal spacing of the calcined sample, the interlayer space is about 7 A° , which is large enough to adsorb the N2 molecules. The interlayer region is occupied by the silica pillar for each P(OTi)2(OH)2 is connected to one silica pillar, which prevents the N2 molecules from entering.The results further indicate that there are no mesopores in the calcined product, as there is little hysteresis on the N2 adsorption–desorption curve when P/P0 is higher than 0.5, which shows that the pillared phosphate is a cross-linked compound rather than a porous one. However, adsorption of water shows that the pillared compound (600 °C, 0.5 h) has the capacity to adsorb small polar molecules.The uptake of water at P/P0=0.3 is as high as about 10% by weight. Fig. 10 shows the water adsorption isotherm of the sample. We thank the National Natural Scientific Foundation and the Key Laboratory of Synthesis and Preparative Chemistry, Jilin University for financial support. References 1 L. Li, X. Liu, Y. Ge, L. Li and J. Klinowski, J. Phys. Chem., 1991, Fig. 8 IR spectra of as-prepared samples (a), and of samples calcined 95, 5910. at 200 (b), 300 (c), 400 (d), 500 (e), 600 (f ), 700 °C (g). 2 W. L. Ijdo, T. Lee and T. J. Pinnavaia, Adv. Mater., 1996, 8, 79. 3 A. Kudo and T. Sakata, J. Mater. Chem., 1993, 3, 1081. 4 F. J. Perez-Reina, P. Olivera-Pastor, E. Rodriguez-Castellon and A. Jimenez-Lopez, J. Solid State Chem., 1996, 122, 231. 5 A. Espina, J. B. Parra, J. R. Garcý�a, J. A. Pajares and J. Rodrý�guez, Mater. Chem. Phys., 1993, 35, 250. 6 A. Clearfield and B. D. Roberts, Inorg. Chem., 1988, 27, 3237. 7 G. Alberti, U. Costantino, R. Vivani and P. Zappelli, Mater. Res. Soc. Symp. Proc., 1991, 233, 95. 8 A. Guerrero-Ruiz, I. Rodrý�guez-Ramos, J. L. G. Fierro, A. Jime�nez-Lo� pez, P. Olivera-Pastor and P. Maireles-Torres, Appl.Catal. A, 1992, 92, 81. 9 G. Alberti, F. Marmottini, S. Murcia-Mascaro� s and R. Vivani, Angew. Chem., Int. Ed. Engl., 1994, 33, 1594. 10 P. Olivera-Pastor, P. Maireles-Torres, E. Rodrý�guez-Castello� n, A. Jime�nez-Lo�pez, T. Cassagneau, D. J. Jones, and J. Rozie`re, Chem. Mater., 1996, 8, 1758. Fig. 9 TGA curve of the title compound. 11 J. Rozie`re, D. J. Jones and T.Cassagneau, J. Mater. Chem., 1991, 1, 1081. 12 P. Sylvester, R. Cahill and A. Clearfield, Chem. Mater., 1994, 6, 1890. 13 J. M. Me�rida-Robles, P. Olivera-Pastor, A. Jime�nez-Lo�pez and E. Rodrý�guez-Castello� n, J. Phys. Chem., 1996, 100, 14726. 14 T. Cassagneau, D. J. Jones and J. Rozie`re, J. Phys. Chem., 1993, 97, 8678. 15 M. Alca�ntara-Rodrý�guez, P. Olivera-Pastor, E. Rodrý�guez- Castello� n, and A. Jime�nez-Lo�pez, J. Mater. Chem., 1996, 6, 247. 16 A. N. Christensen, E. K. Andersen, I. G. K. Andersen, G. Alberti, M. Nielsen and M. S. Lehmann, Acta Chem. Scand., 1990, 44, 865. 17 E. Bayer, K. Albert, J. Reiners, M. Nieder and D. Mu� ller, J. Chromatogr., 1983, 264, 197. 18 G. E. Maciel, P. W. Sindorf and V. J. Bartuska, J. Chromatogr., 1981, 205, 438. 19 D. J. MacLachlan and K. R. Morgan, J. Phys. Chem., 1990, 94, 7656. 20 N. J. Clayden, J. Chem. Soc., Dalton Trans., 1987, 1877. Fig. 10 Water adsorption isotherm of the sample calcined at 600 °C . Paper 8/02838I 28

ISSN:0959-9428    

DOI:10.1039/a802838i

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Synthesis and photocatalytic properties of titania pillared H4Nb6O17 using titanyl acylate precursor M. Yanagisawa, S. Uchida, Y. Fujishiro and T. Sato* Institute for Chemical Reaction Science, Tohoku University, Sendai 980–8577, Japan. E-mail: tsusato@icrs.tohoku.ac.jp Received 27th July 1998, Accepted 14th September 1998 Titania pillared H4Nb6O17 has been synthesised by the intercalation of [Ti(OH)x(CH3CO2)y]z+ followed by photodecomposition with UV light irradiation.The incorporation of TiO2 in the interlayer of H4Nb6O17 has been confirmed by powder X-ray diVraction, DTA, UV-VIS reflectance and BET measurements. The incorporation of TiO2 in the interlayer of H4Nb6O17 results in enhanced water cleavage by band gap irradiation. The photocatalytic activity of TiO2 pillared H4Nb6O17 prepared using [Ti(OH)x(CH3CO2)y]z+ was higher than that of a sample prepared using a TiO2 sol solution.prepared by stepwise incorporation of TiO2 as follows. Introduction [Ti(OH)x(CH3CO2)y]z+ was prepared by modifying Kao and The pillaring of layered compounds by inorganic compounds Yang’s method10 starting with titanium isopropoxide, acetic is a promising method for fabricating unique porous materials acid and water in a 1516520 volume ratio.In a typical that possess some of the properties of zeolites. Semiconductor experiment, acetic acid (400 cm3) was mixed first with titanium pillars have attracted special attention from researchers isopropoxide (25 cm3) for 30 min, followed by the addition of because of their excellent photocatalytic activity.The photo- water (500 cm3), resulting in the formation of a white precipicatalytic activity of semiconductor pillars such as CdS–ZnS, tate, which was dissolved to give a clear solution upon Fe2O3 and TiO2 was much higher than those of unsupported continuous stirring for 5 h at room temperature. H4Nb6O17 catalysts.1–7 In the previous studies,5,7 we found that the (5 g) was converted to (C3H7NH3)4Nb6O17 by stirring in a 50 photoactivities of semiconductor pillars were dependent on vol.% C3H7NH2 aqueous solution (1 dm3) at 60°C for 3 days.the physico-chemical properties of the host layer (e.g. CdS–ZnS After separation by filtration, (C3H7NH3)4Nb6O17 (1 g) was pillars which are constructed in the interlayer of semi- added to a clear aqueous solution of [Ti(OH)x(CH3CO2)y]z+ conductors, such as H4Nb6O17 and H2Ti4O9) showed much and allowed to stand for 120 h at room temperature to allow higher photoactivities than those of insulators (such as intercalation of [Ti(OH)x(CH3CO2)y]z+. The obtained montmorillonite and layered double hydroxide), since electron sample, [Ti(OH)x(CH3CO2)y]z/4Nb6O17 after being filtered oV and hole recombination was eVectively depressed due and washed with water, was dispersed in water, and irradiated to charge transfer from the guest semiconductor to the host with UV light from a 450 W high pressure mercury lamp semiconductor layer.at room temperature for 12 h in order to decompose In general, semiconductor pillars are constructed by the [Ti(OH)x(CH3CO2)y]z+ in the interlayer. The resulting intercalation of soluble inorganic ion precursors, followed by material is designated H4Nb6O17/TiO2(c).the precipitation of the intercalated ions by chemical, thermal, and/or photochemical reactions. TiO2 pillars, however, have Synthesis of titania pillared H4Nb6O17 using a titania sol been made using TiO2 sol solutions because of the lack of solution water soluble titanium complex ions.8 By using TiO2 sols, TiO2 pillars were successfully constructed in clay minerals A titania pillar was constructed in the interlayer of H4Nb6O17 which swell in water8 such as smectite.However, it is not easy using titania sol, as reported previously.7 Titanium tetraisoto construct TiO2 pillars in semiconductor layer compounds propoxide (25 cm3) was added dropwise to vigorously stirred such as H4Nb6O17 and H2Ti4O9, since they do not easily swell 1 M HCl (250 cm3) so as to give a final molar ratio of alkoxide in water.A recent study found that a water soluble ionic to HCl of 0.25. The resulting slurry was peptized by further molecular precursor [the titanyl acylate complex, stirring for 3 h so as to give a clear TiO2 sol solution.Then Ti(OH)x(CH3CO2)y z+] can be obtained by the reaction of (C3H7NH3)4Nb6O17 (1 g) was added to the TiO2 sol solution, titanium isopropoxide, glacial acetic acid and water,9 and was and the suspension was continuously stirred for the desired used as a source of TiO2 in the preparation of a strontium time at room temperature in order to incorporate TiO2.After titanate ceramic.10 In the continuation of our studies on the being filtered oV and washed with water, the specimen was photocleavage of water, a series of tests were performed to dispersed in water and irradiated with UV light from a 450W evaluate the synthesis of titania pillared H4Nb6O17 by using high pressure mercury lamp at 60 °C for 12 h in order to titanyl acylate precursors.decompose the C3H7NH3+ remaining in the interlayer. The sample thus obtained is designated as H4Nb6O17/TiO2(s). Experimental Analysis Synthesis of titania pillared H4Nb6O17 using a titanyl acylate The crystalline phases of the products were identified by X-ray complex diVraction analysis (Rigaku Denki Geiger-flex 2013) using graphite-monochromatized Cu-Ka radiation. The chemical H4Nb6O17 was prepared by the ion-exchange reaction of K4Nb6O17 in 5 M HCl at 60 °C for 5 h, K4Nb6O17 being compositions of the products were determined by inductively coupled plasma–atomic emission spectroscopy (Seiko SPS- prepared by the calcination of K2CO3 and Nb2O3 in 253 molar ratio at 1200 °C for 20 min.H4Nb6O17/TiO2 was 1200A) after alkali fusion with Na2CO3 followed by dissolving J.Mater. Chem., 1998, 8, 2835–2838 2835the samples in 6 M HCl-15 wt.% H2O2. The band gap energies of the products were determined from the onset of the diVuse reflectance spectra of the powders measured using a Shimadzu Model UV-2000 UV–VIS spectrophotometer. The specific surface area was determined by the nitrogen gas adsorption method (Shibata SA-1000).Photocatalytic reactions Photocatalytic reactions were performed in a Pyrex reactor with a capacity of 1250 cm3 which was attached to an inner radiation type 450W high-pressure mercury lamp. The inner cell had thermostated water flowing through a jacket between the mercury lamp and the reaction chamber, and was constructed of quartz glass. The photoactivity of the catalyst was determined by measuring the total gas volume of hydrogen and oxygen evolved during the irradiation of the catalyst Fig. 2 XRD patterns of (A) H4Nb6O17, (B) suspensions in water with a gas burette after confirming the [Ti(OH)x(CH3CO2)y]z/4Nb6O17 prepared by the reaction of production of both hydrogen and oxygen by gas chromatogra- (C3H7NH3)4Nb6O17 and the titanyl acylate complex for 120 h at room temperature and (C) H4Nb6O17/TiO2(c) prepared by phy (Yanagimoto G2800) using a Molecular Sieve 13X (30–60 photodecomposition of (B).mesh) column. the Nb6O174- layer thicknesses of 0.56 nm) were 0.40, 0.62 Results and discussion and 0.52 nm, respectively. Synthesis of H4Nb6O17/TiO2 UV–VIS reflection spectra of (A) H4Nb6O17, (B) H4Nb6O17/TiO2(c) and (C) unsupported TiO2 are shown in DTA curves of (A) [ Ti(OH)x(CH3CO2)y]z/4Nb6O17 and (B) Fig. 3. From the onset of the spectra, the band gap energies H4Nb6O17/TiO2(c) (measured in air) are shown in Fig. 1. A of TiO2 gel, H4Nb6O17/TiO2(c) and H4Nb6O17 were deter- sharp exothermic peak at 291 °C (which corresponds to the mined as 3.0, 3.26 and 3.34 eV, respectively. Although the combustion of the titanyl acylate complex) was observed spectrum showed a red shift upon the incorporation of TiO2, for [Ti(OH)x(CH3CO2)y]z/4Nb6O17, but was absent in the band gap energy of the TiO2 pillar could not be determined H4Nb6O17/TiO2(c), indicating that it was photochemically by UV–VIS reflection spectra since H4Nb6O17/TiO2(c) did decomposed by UV-light irradiation.not show separate onsets corresponding to H4Nb6O17 and Fig. 2 depicts the powder X-ray diVraction patterns of (A) incorporated TiO2. H4Nb6O17, (B) [Ti(OH)x (CH3CO2)y]z/4Nb6O17 and (C) The time dependence of the amount of TiO2 incorporated H4Nb6O17/TiO2. The main peaks (which correspond to (040) using both (A) titanyl acylate complex and (B) TiO2 sol of H4Nb6O17 of samples (B) and (C)) shifted significantly solution is shown in Fig. 4. The amount of TiO2 pillar increased to lower 2h angles as compared to sample (A), indicating rapidly with time up to 50 h, then increased more gradually the expansion of the interlayer by the incorporation of and was almost constant after 120 h. The amount of TiO2 Ti(OH)x(CH3CO2)y z+ and TiO2. These results indicate that incorporated using the TiO2 sol was slightly larger than that the layer structure was still retained after the intercalation of using the titanyl acylate complex after 120 h.Ti(OH)x(CH3CO2)y z+ and after photochemical decompo- As seen in Fig. 2, H4Nb6O17/TiO2(c) (prepared using the sition of Ti(OH)x(CH3CO2)y z+ to TiO2 in the interlayer. The titanyl acylate precursor for 120 h) showed XRD diVraction gallery heights of H4Nb6O17, [Ti(OH)x(CH3CO2)y]z/4Nb6O17 peaks corresponding only to H4Nb6O17, indicating that TiO2 and H4Nb6O17/TiO2(c) (determined by XRD by subtracting was incorporated in the interlayer.On the other hand, diVraction peaks corresponding not only to H4Nb6O17 but also to rutile were observed for H4Nb6O17/TiO2 (s) as shown in Fig. 5: the diVraction peaks corresponding to rutile were not observed up to 6 h, but became noticeable after 24 h and increased with time.In general, TiO2 sols transform into rutile via anatase above 500 °C. The unusual formation of rutile at such a low temperature as 60 °C indicates that the H4Nb6O17 acts as nuclei to form rutile. Therefore, it appears that, when a TiO2 sol is used as a precursor, TiO2 is incorporated mainly in the Fig. 1 DTA patterns of [Ti(OH)x(CH3CO2)y]z/4Nb6O17 before (A) and after (B) UV irradiation, where [Ti(OH)x(CH3CO2)y]z/4Nb6O17 Fig. 3 DiVuse reflectance spectra of (A) H4Nb6O17, was prepared by the reaction of (C3H7NH3)4Nb6O17 and the titanyl acylate complex for 120 h at room temperature. (B) H4Nb6O17/TiO2(c) and (C) unsupported TiO2 sol. 2836 J. Mater. Chem., 1998, 8, 2835–2838Fig. 6 Cumulative amounts of hydrogen and oxygen gas produced from 1250 cm3 of water containing 1 g of dispersed catalysts at 60 °C exposed to irradiation from a 450 W mercury arc.(A) H4Nb6O17/TiO2(c), (B) H4Nb6O17/TiO2(s) intercalated for 6 h, (C) H4Nb6O17/TiO2(s) intercalated for 144 h and (D) mixture of Fig. 4 Time dependence of the amounts of TiO2 incorporated using H4Nb6O17 and TiO2 sol.(A) the titanyl acylate complex (&) and (B) a TiO2 sol ($). H4Nb6O17/TiO2(s) possessed gallery heights of 0.52 and 0.48 nm, indicating that the gallery height of the titania pillared H4Nb6O17 increased slightly when the titania pillar was prepared using the titanyl acylate precursor. Photocatalytic water cleavage The amounts of gas produced from 1250 cm3 of water containing 1 g of dispersed H4Nb6O17/TiO2(c), H4Nb6O17/ TiO2(6s), H4Nb6O17/TiO2(144s) and a mixture of 75 wt.% H4Nb6O17 and 25 wt.% TiO2 sol at 60 °C exposed to irradiation from a 450W mercury arc were measured, where H4Nb6O17/TiO2(6s) and H4Nb6O17/TiO2(144s) were prepared using TiO2 sol for 6 and 144 h, respectively. Significant gas evolution was observed in the presence of H4Nb6O17/TiO2(c), H4Nb6O17/TiO2(6s) and H4Nb6O17/TiO2(144s), but no noticeable gas was evolved for a mixture of H4Nb6O17 and TiO2 sol.Therefore, the titanium oxide pillar which was incorporated in the interlayer plays an important part in photocatalytic water cleavage, whereas TiO2 at the outerlayer does not possess photocatalytic activity for water cleavage. Previously7 we found that the charge injection from an excited TiO2 pillar Fig. 5 XRD diVraction patterns of H4Nb6O17/TiO2(s) prepared by into the conduction band of H4Nb6O17 occurs at a rate of reacting (C3H7NH3)4Nb6O17 and TiO2 sol solutions for (A) 6, (B) 24, 0.12×109 s-1; therefore, photogenerated electrons can quickly (C) 71 and (D) 120 h. be transferred from a TiO2 pillar into a H4Nb6O17 layer while the holes remain in the TiO2 pillar.Consequently, the recombi- interlayer of H4Nb6O17 in the initial stage, but a significant nation between the photoinduced charge carriers was eVec- amount of TiO2 is precipitated on the outerlayer after 24 h. tively depressed and the photocatalytic water cleavage was Consequently, the amount of TiO2 pillars constructed in the enhanced. Taking into acount a saturated water vapour press- interlayer can be increased using the titanyl acylate complex ure of 20.0 kPa at 60 °C and the molar ratio of hydro- as precursor.gen5oxygen evolved as 251, from the slope of the straight The gallery height, the amounts of TiO2 incorporated, and lines in Fig. 6 the rates of hydrogen evolution in the presence the specific surface area of H4Nb6O17, H4Nb6O17/TiO2(c) of H4Nb6O17/TiO2(c), H4Nb6O17/TiO2(6s) and H4Nb6O17/ and H4Nb6O17/TiO2(s) are listed in Table 1, where TiO2(144s) were determined as 0.0417, 0.0241 and H4Nb6O17/TiO2(c) and H4Nb6O17/TiO2(s) were prepared 0.0142 mmol h-1, i.e., the photocatalytic activity of using the titanyl acylate complex and TiO2 sol solution for H4Nb6O17/TiO2(c) was 1.7 times larger than that of 120 and 6 h, respectively.The amounts of TiO2 incorporated H4Nb6O17/TiO2(6s). This may be due to an increase in the in H4Nb6O17/TiO2(c) and H4Nb6O17/TiO2(s) were 26.7 and amount of TiO2 pillars. On the other hand, the photoactivity 7.3 wt.%, respectively. The specific surface area increased of H4Nb6O17/TiO2(144s) was about half that of greatly with increase in the amount of TiO2, indicating the H4Nb6O17/TiO2(6s), although the TiO2 content was 3.7 times construction of titania pillars.H4Nb6O17/TiO2(c) and larger. This may be attributed to the precipitation of TiO2 on the outerlayer, because such TiO2 does not possess photo- Table 1 Amounts of TiO2 incorporated, gallery heights and specific catalytic activity for water cleavage and may cut oV the light surface areas of the samples required to excite TiO2 pillars in the interlayer.Gallery TiO2 Specific height/ content surface Conclusion Sample nm (wt.%) area/m2 g-1 The conclusions from this study are: (i) titania pillared H4Nb6O17 0.40 0 16.1 H4Nb6O17 is first fabricated by the reactions of H4Nb6O17 H4Nb6O17/TiO2 (c) 0.52 26.7 125.6 with a titanyl acylate complex followed by UV light irradiation.H4Nb6O17/TiO2(s) 0.48 7.3 38.6 (ii) The amount of the titania pillar and the photocatalytic J. Mater. Chem., 1998, 8, 2835–2838 28373 H. Yoneyama, S. Haga and S. Yamanaka, J. Phys. Chem., 1989, activity of H4Nb6O17/TiO2 prepared using the titanyl acylate 93, 4833. precursor were larger than those fabricated using a TiO2 4 T. Sato, H. Okuyama, T. Endo and M. Shimada, React. Solids, sol solution. 1990, 8, 63. 5 T. Sato, K. Masaki, T. Yoshioka and A. Okuwaki, J. Chem. Tech. Biotechnol., 1993, 58, 315. Acknowledgements 6 T. Sato, Y. Yamamoto, Y. Fujishiro and S. Uchida, J. Chem. Soc., Faraday Trans., 1996, 92, 5089. This work was supported in part by a Grant-in-Aid for 7 S. Uchida, Y. Yamamoto, Y. Fujishiro, A. Watanabe, O. Ito and Scientific Research from the Ministry of Education, Science T. Sato, J. Chem. Soc., Faraday Trans., 1997, 93, 3229. and Culture. 8 S. Yamanaka, T. Nishihara and M. Hattori, Mater. Chem. Phys., 1987, 17, 87. 9 S. DoeuV, M. Henry, C. Sanchez and J. Livage, J. Non-cryst. References Solids, 1987, 89, 206. 10 C. Kao and W. Yang, Ceram. Int., 1996, 22, 57. 1 O. Enea and A. J. Bard, J. Phys. Chem., 1986, 90, 301. 2 H. Miyoshi and H. Yoneyama, J. Chem. Soc., Faraday Trans. 1., 1989, 85, 1873. Paper 8/05836I 2838 J. Mater. Chem., 1998, 8, 2835–2838

ISSN:0959-9428    

DOI:10.1039/a805836i

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Structural characterisations of the NaxSi136 and Na8Si46 silicon clathrates using the Rietveld method Edouard Reny,* Pierre Gravereau, Christian Cros and Michel Pouchard† Institut de Chimie de la Matie`re Condense�e de Bordeaux, UPR CNRS 9048, 87, Avenue du Docteur Albert Schweitzer, 33608 Pessac Cedex, France. E-mail: Reny@chimsol.icmcb.u-bordeaux.fr Received 16th June 1998, Accepted 16th September 1998 The crystal structure of the non-stoichiometric NaxSi136 silicon clathrate has been refined using the Rietveld method, in order to determine accurately the distribution of the sodium atoms within the two available sites.In agreement with the previous data, it was found that for x8, the alkali atoms occupy exclusively, and not only preferentially the eight larger Si28 sites.For 8<x<24, the filling of the sixteen smaller Si20 cages occurs gradually with increasing x, and a slight increase of the unit cell parameter is then observed. The crystal structure of the stoichiometric Na8Si46 clathrate, which is present as impurity in the studied samples, has also been refined. alkali metal is sodium or potassium, the silicon cages seem to Introduction be fully occupied leading to the stoichiometric compound Thermal decomposition of the Zintl phase MSi (M=Na, K, M8Si46.Rb, Cs) under vacuum or inert atmosphere leads to the The silicon host lattice of the MxSi136 structure (Fig. 2) is formation of clathrate type alkali metal silicides.1–6 Depending composed of 16 pentagonal dodecahedra and 8 hexakaý�deon the alkali metal and the experimental conditions, two types cahedra (12 pentagonal and 4 hexagonal faces), Si28.The unit of structures are formed, corresponding to the formula MxSi46 cell is also cubic (a#14.62 A° ) with the space group Fd39m. The (x#8 for M=Na and K, x#6 for M=Rb) and MxSi136 (M= silicon lattice oVers 16 sites with a 39m symmetry and 8 sites Na, Cs).The two structures were found to be respectively with a 439m symmetry located respectively at the (0 0 0) and isostructural to the clathrate hydrates of type I [or gas hydrate, (3/8 3/8 3/8) positions, consequently, the maximum authorised such as (Cl2)8(H2O)46] and type II [or liquid hydrate, such as value for x is 24. Unlike the Na8Si46 compound, MxSi136 is a (CHCl3)8(H2O)136 or (H2S)16(CCl4)8(H2O)136].2,3,7 In both non-stoichiometric phase, all the cages are not necessarily structures, the silicon host lattice is formed by a combination occupied. NaxSi136 can be obtained within a very large range of two types of polyhedra of fullerene type, i.e.having only of compositions depending on the experimental conditions: pentagonal and hexagonal faces.The basic polyhedron, which 1<x<23. is common to the two structures, is the pentagonal dodeca- The presence of alkali metals trapped in open host lattices hedra (12 pentagonal faces), Si20; it is the smallest possible made of tetrahedrally bonded covalent silicon atoms induces fullerene type cage. interesting physical properties for these compounds.3,8,9 The silicon host lattice of the MxSi46 structure (Fig. 1) is Following the discovery of the fullerene forms of carbon and composed of two pentagonal dodecahedra, Si20 and six tetraka- the superconducting behaviour of the intercalation compounds ý� decahedra (12 pentagonal and 2 hexagonal faces), Si24.The M3C60 or MM¾2C60 (M, M¾=Na, K, Rb, Cs), the silicon corresponding unit cell is cubic (a#10.19 A° ) with the space clathrates have been intensively reinvestigated on the group Pm39n.This silicon lattice oVers two sites with a 39m theoretical and experimental viewpoints.10–22 symmetry and six sites with a 4m2 symmetry located respectively at the (0 0 0) and (1/4 1/2 0) positions. When the trapped †Member of the Institut Universitaire de France. Fig. 2 Representation of the MxSi136 structure.Fig. 1 Representation of the M8Si46 structure. J. Mater. Chem., 1998, 8, 2839–2844 2839One of the major new results was the observation of General information on the Rietveld refinements superconductivity (Tc#4K) in the two clathrates Na2Ba6Si46 Rietveld refinements have been performed on each XRD and K2Ba6Si46, where the six larger Si24 cages are occupied pattern using the FULLPROF program.24 For every by barium instead of sodium and potassium respectively.14,15,18 diVractogram, the following parameters have been refined: the Other results concerning the non-stoichiometric NaxSi136 phase zero point, one asymmetry parameter, the six background have been obtained.For instance, a noticeable variation of polynomial parameters, the three full width at half maximum the electric properties has been measured depending on the (Hk) parameters of the Cagliotti law: Hk2=U composition, from semiconductor, NaxSi136 becomes progresstan2 h+V tan h+W, the g parameter of the pseudo–Voigt ively metallic when x increases.7–9,19 Moreover theoretical fonction, i.e.representing the combination of a Lorentzian calculation on pure Si136 (no alkali atoms trapped in the silicon and a Gaussian type of peak [PV=gL+(1-g)G], the scale cages) shows that the band gap opens by 0.7 eV comparatively factor, the atomic positions, the isotropic thermal agitation to diamond silicon.10–13,20 The 1.9 eV broad bandgap of this factors of the silicon and sodium atoms.compound is very close to that of porous silicon.This could For every diVractogram, the observed peaks are very close be of great interest in new electronic applications. More to the Lorentzian type (0.8<g<0.9). Consequently, the contri- recently, a 23Na NMR study performed on various NaxSi136 bution of a peak at 2hp is taken into account between samples revealed interesting information about the ionisation 2hp-20Hk and 2hp+20Hk.degree of the Na atoms encapsulated in the silicon cages. Due to correlation between the rate of occupancy and the According to this technique, the electronic state of the trapped thermal agitation parameters of the sodium atoms, it was atoms is intermediate between metallic and atomic.21–23 necessary to fix the sodium concentration x to the values The crystal structures of NaxSi46 and NaxSi136 have been found in analysis.The reliability factors used are defined in already investigated. In the first refinement of the structure of Table 2. Standard deviation was calculated taking into account NaxSi136 with x=9.5, a preferential but not exclusive occuthe Berar factor to correct local correlations.25 pancy of the eight large Si28 cages was observed, the occupancy rates being respectively 0.79 and 0.21 for the large Si28 and Determination of the sodium ratio in NaxSi136 the small Si20 sites.2 Another calculation performed by Cros on two compositions x=3 and x=10 led to more equilibrated The global sodium content for each sample has been occupancy rates.7 In a more recent investigation by Sim of a determined by X-ray microprobe using an E.P.M.A.Cameca series of ten compositions, it was observed that the sodium SX-100 apparatus. The values obtained have been confirmed atoms occupy almost exclusively the large Si28 cages for x8, by flame emission technique with a Perkin Elmer 306 double and for 8x24, the smaller Si20 sites are progressively beam spectrometer. occupied with increasing x.9 All the above reported structural To obtain a reliable value of x in NaxSi136, it is necessary studies by XRD were performed without the use of the most to determine the relative amount of the Na8Si46 impurity advanced refinement methods, which enable to get more phase.The mass ratio wj of each phase has been approached accurate data than previously. Furthermore, the preferential by quantitative phase analysis using the Rietveld method: site occupancy of the large Si28 cages for x8 was called into question in recent work.22 wj (%)=100× SjZjMjVj .i n (SiZiMiVi) These two reasons led us to undertake a careful investigation of the crystal structure of a series of samples of NaxSi136, with well characterised x values, by the Rietveld method.As far as where Sj is the scale factor for phase j, Mj the mass of the the Na8Si46 clathrs present as an impurity in our formula unit, Zj the number of formula units per unit cell and samples, this latter compound was also investigated. The Vj the volume of the unit cell.This allowed a correction on results of this study are reported in the present paper. the experimental value of x which was fixed in the final Rietveld refinements (Table 4).The similar isotropic thermal parameters found for the Na atoms in the diVerent Rietveld Experimental refinements constitute an indirect confirmation of the x fixed values. Preparation The clathrates NaxSi136 with x<14 are synthesised by thermal Structural characterisation of the Na8Si46 compound decomposition of NaSi under vacuum (10-4 Pa) at tempera- The XRD pattern of Na8Si46 (Fig. 3) revealed the existence tures between 340 and 440 °C. Those with larger values of x of a minor impurity phase NaxSi136 (ca. 7.7% in weight). A can be prepared in a closed steel reactor, according to the 23Na NMR spectrum obtained on this sample showed that the reaction: NaxSi136+Na (vapour)�Nax¾Si136 (x¾>x), in the impurity compound was very rich in sodium: thus x has been temperature range 370–400 °C.Six samples of NaxSi136 were considered equal to 24.21 Due to a very weak quantity of prepared, the concentration in sodium depending on the amorphous phase in the range 10–40° range (2h), a fifth order pyrolysis temperature. Samples have been numbered with polynomial could not fit well the background of all the XRD increasing values of x (Table 4).The concentration in sodium pattern. Consequently a background file was generated in the of sample VI has been raised to x=20.5 by a thermal treatment following way: in the 10–40° (2h) angular range, background under sodium vapour. All these samples contained some points were manually determined from the XRD pattern; for amounts of the second phase, Na8Si46. 2h>40°, the corresponding background points file was The clathrate Na8Si46 has been synthesised by thermal generated from the fifth order polynomial refined for this decomposition of NaSi under argon at 410 °C. It contained range, with the ‘pattern-matching’ option of the small amounts of the other phase, NaxSi136 with x>20. FULLPROF program. In a 10–120° (2h) angular range, 161 reflections were X-Ray diVraction patterns acquisition obtained for the major compound Na8Si46 and 141 for the minor Na24Si136 phase.The powder diVraction data extracted The powder diVraction patterns were collected on a X’PERT MPD (h-h) Philips diVractometer (Cu-Ka, graphite mono- for Na8Si46 are listed in Table 1. The final results (Table 2) obtained from the refinement of 29 parameters led to atomic chromator, 40 kV, 40 mA, receiving slit: 50 mm, angular range: 10–120° (2h), counting time: 30 s by steps of 0.02° (2h), positions of the silicon atoms with similar thermal parameters.These values are very close to those obtained previously by sample rotation, room temperature). 2840 J. Mater. Chem., 1998, 8, 2839–2844some of us.7 No preferred orientation correction was applied.One asymmetry correction parameter has been refined for values of 2h<40°. A list of the interatomic distances and angles is presented in Table 3 and visualised in Fig. 4. The average interatomic SiKSi distance is 2.369 A° and is close to the value in diamond-type silicon (2.352 A° ). The observed SiKSiKSi bond angles range from ca. 105 to ca. 125° and the average value is close to 109.54°, which is characteristic of an sp3 hybridisation. The calculated ‘free radius’ of the Si20 cages, based on the eight shortest Na(1)KSi(2) distances (r146= dNa(1)Si(2)-rSi), is 2.08 A° . The ‘free radius’ of the Si24 larger cages (r246) is 2.241 A° . The volume per formula unit of the clathrate type silicon host lattice is V/Z=23.058 A° 3, cf. 20.023 A° 3 in diamond-type silicon. Consequently, the clathrate type silicon network is 15.2% more open. Structural characterisation of the NaxSi136 compound Fourier diVerence functions have been calculated using the SHELXL 93 program.26 The diVerence between Fourier trans- Fig. 3 Final Rietveld plot to the X-ray diVraction for Na8Si46. The formation of the structural factors observed for NaxSi136 crosses represent the experimental data points and the upper continuous line the calculated spectra.The upper tick marks indicate (‘Fobs’) obtained via FULLPROF and the calculated structural the calculated reflection positions for the minor impurity phase factors of the empty silicon lattice Si136, (Fcalc), provides us NaxSi136 and the lower tick marks the calculated reflection position with a map of electronic densities attributed to the sodium of Na8Si46.The lower continuous line represents the diVerence. atoms. This study, performed with the diVraction pattern of sample II (x=3) revealed the two important following points: (i) there is no sodium in the pentagonal dodecahedric sites for Table 1 Powder diVraction data of Na8Si46 (Cu-Ka; l=1.540 60 A° ) x8 and (ii) the residual electronic density appears clearly in h k l dcalc lcalc h k l dcalc lcalc the centre of the Si28 cage (3/8 3/8 3/8).This rules out the hypothesis of a decentering of the sodium atoms in the silicon 1 1 0 7.211 30.9 5 1 0 2.000 10.6 cages, that could have been envisaged considering the relatively 2 0 0 5.099 27.2 4 3 1 2.000 <1 high value of the isotropic atomic displacement parameter 2 1 0 4.561 152.3 5 2 0 1.894 14.8 (B#8 A° 2). 2 1 1 4.163 85.7 4 3 2 1.894 47.2 In the case of sample V, where the value of x in NaxSi136 is 2 2 0 3.606 <1 5 2 1 1.862 <1 3 1 0 3.225 11.0 4 4 0 1.803 4.2 higher than the number of available Si28 sites (x=13.6), this 2 2 2 2.944 231.9 5 3 0 1.749 250.7 study shows a full occupation of the Si28 cages, the remaining 3 2 0 2.829 239.3 4 3 3 1.749 110.0 sodium atoms being perfectly centred in the pentagonal 3 2 1 2.726 692.3 5 3 1 1.724 124.1 dodecahedric cages (16c sites). 4 0 0 2.550 64.1 6 0 0 1.700 89.7 It is now possible to fix the atomic positions of the sodium 4 1 0 2.473 157.5 4 4 2 1.700 45.5 atoms and, knowing the global composition of NaxSi136, to 3 3 0 2.404 79.8 6 1 0 1.677 28.0 4 1 1 2.404 1.1 6 1 1 1.654 34.0 define the rate of occupancy of the two types of sodium sites 4 2 0 2.280 8.9 5 3 2 1.654 255.1 in all samples.In sample VI, a weak amorphous contribution, 4 2 1 2.225 52.4 6 2 0 1.613 42.8 probably linked to a high concentration in Na8Si46 (#13%) 3 3 2 2.174 23.4 5 4 0 1.593 28.9 compound, led to a background determination in two steps, 4 2 2 2.082 7.8 6 2 1 1.593 12.1 as seen previously for Na8Si46 and allowed us to release atomic 4 3 0 2.040 21.5 5 4 1 1.574 1.2 Table 2 Atomic parameters and R factorsa for Na8Si46 in space group Pm39n Atom Site x y z Biso/A° 2 Si(1) 6c 0.25 0 0.5 1.13(11) Si(2) 16i 0.1847(2) 0.1847(2) 0.1847(2) 1.08(6) Si(3) 24k 0 0.3088(2) 0.1173(2) 1.03(5) Na(1) 2a 0 0 0 2.5(3) Na(2) 6d 0.25 0.5 0 3.6(2) Cell parameter/A° 10.1983(2) Volume/A° 3 1060.67(5) Dx/g cm-3 2.311 g 0.43(1) Profile parameters U1=0.000(1) V1=0.013(2) W1=0.040(5) Rietveld reliability cRp=0.145 cRwp=0.177 x2=1.56 factors: Rp=0.0889 Rwp=0.127 RI=0.0426 Rf=0.0438 The R factors are defined as cRp=.i |yio-yic|/.i |yio-yib|, cRwp=(.i wi( yio-yic)2/.i wi( yio-yib)2)1/2, x2=.i wi( yio-yic)2/ (N-P+C), Rp=. i |yio-yic|/.i yio, Rwp=(.i wi( yio-yic)2/.i wiyio2)1/2, Fig. 4 Representation of two connected cages in the Na8Si46 structure. RI=. k |Iko-Ikc|/.k Iko, RF=. k |Fko-Fkc|/.k Fko. The eight non-equivalent bonding angles and the four SiKSi distances are indicated. J. Mater. Chem., 1998, 8, 2839–2844 2841Table 3 List of refined interatomic distances (A° ) and angles (°) for Na8Si46 SiKSi d1=Si(1)KSi(3) 2.373(2) NaKSi Na(1)KSi(2) 3.263(2) d2=Si(2)KSi(3) 2.371(2) Na(1)KSi(3) 3.369(2) d3=Si(3)KSi(3) 2.393(3) Na(2)KSi(1) 3.606(1) d4=Si(2)KSi(2) 2.306(2) Na(2)KSi(2) 3.786(2) Na(2)KSi(3) 3.425(2), 3.948(2) angles on Si(1) c=Si(3)KSi(1)KSi(3) 110.5(1) ales on Si(3) Q=Si(1)KSi(3)KSi(2) 105.9(1) e=Si(3)KSi(1)KSi(3) 109.0 d=Si(1)KSi(3)KSi(3) 124.8(1) angles on Si(2) h=Si(2)KSi(2)KSi(3) 108.5(1) j=Si(2)KSi(3)KSi(2) 105.2(1) b=Si(3)KSi(2)KSi(3) 110.4 a=Si(3)KSi(3)KSi(2) 106.8(1) positions and average isotropic thermal factors for this work.2,7,9 They confirm that the free radius of the Si20 cages is slightly smaller in the Si136 clathrate than in the Si46 one compound.Results of the Rietveld refinements for all samples are (r1136#2.00 A° instead of r146#2.08 A° ).No noticeable structural evolution, i.e. variation in the presented in Table 5 and 6. Fig. 5(a) and (b) present two examples of the refined XRD patterns (sample II and VI). lattice parameter and the atomic positions, occurs in sample I, II and III. Sample IV–VI see their lattice parameters Interatomic distances and bond angles for these two samples are presented in Table 7.They are visualised on Fig. 6. increasing slightly but significantly with the sodium concentration, i.e. when the Si20 cages start to fill up (Fig. 7) and is In the highly non-stoichiometric Na3Si136 compound, the average SiKSi distance is 2.360 A° (2.352 A° in diamond-type another point that is in favour of the preferential occupancy of the Si28 sites. 23Na NMR spectra acquired on various silicon). The SiKSiKSi bonding angles range from ca. 105.7 to ca. 120°. The calculated ‘free radius’ of the Si28 cage (r2136= NaxSi136 samples showed that the trapped Na atoms tend to conserve their 3s electron density and consequently can be dNa(1)Si(3)-rSi), based on the shortest Na(1)KSi(3) distance, is 2.722 A° . The ‘free radius’ of the empty Si20 cages is r1136= described as in a state between metallic and neutral, i.e.their radius is situated between 1.54 and 2.30 A° .27 As the free radius 1.990 A° (r1136=dNa(2)Si(1)-rSi). In the almost stoichiometric Na20.5Si136 clathrate, the average SiKSi distance is 2.371 A° and of the Si20 cages (r1136) is ca. 1.99 A° , sodium atoms would be less likely to intercalate in these cages than in the wider Si28 the values of r2136 and r1136 are 2.724 and 1.998 A° respectively. All these data are consistent with the results of previous cages (r2136#2.72 A° ).When the pentagonal dodecahedra start Table 4 List of the NaxSi136 samples studied Sample I II III IV V VI Pyrolysis temp. °C 440 400 370 340 340 Navap a x in NaxSi136 1.0±0.5 3.0±0.8 3.8±0.4 10.5±0.5 13.6±1.2 20.5±1.5 Si20 occupancy 0.0 0.0 0.0 0.15 0.35 0.78 Si28 occupancy 0.125 0.375 0.475 1.0 1.0 1.0 Weight ratio in 1.8 4.7 9.5 2.9 6.9 12.8 Na8Si46 (%) aSample VI has been obtained by submitting Na6Si136 (previously synthesised ) under a sodium vapor atmosphere for 30 h at 320 °C.Table 5 Refined atomic positions of NaxSi136 in samples I–VI Atom [site] x y z Biso/A° 2 Sample I Si(1) [2a] 0.125 0.125 0.125 0.49(18) (Na1Si136) Si(2) [32c] 0.2173(2) 0.2173(2) 0.2173(2) 0.45(10) Si(3) [96g] 0.1831(1) 0.1831(1) 0.3712(2) 0.50(6) Na(1) [8b] 0.375 0.375 0.375 6.9(2.7) Sample II Si(1) [2a] 0.125 0.125 0.125 0.29(16) (Na3Si136) Si(2) [32c] 0.2174(2) 0.2174(2) 0.2174(2) 0.26(9) Si(3) [96g] 0.1830(1) 0.1830(1) 0.3714(1) 0.35(5) Na(1) [8b] 0.375 0.375 0.375 5.6(1.1) Sample III Si(1) [2a] 0.125 0.125 0.125 0.27(12) (Na3.8Si136) Si(2) [32c] 0.2174(1) 0.2174(1) 0.2174(1) 0.28(7) Si(3) [96g] 0.1830(1) 0.1830(1) 0.3712(1) 0.49(4) Na(1) [8b] 0.375 0.375 0.375 8.2(9) Sample IV Si(1) [2a] 0.125 0.125 0.125 0.49(16) (Na10.4Si136) Si(2) [32c] 0.2175(1) 0.2175(1) 0.2175(1) 0.53(10) Si(3) [96g] 0.1831(1) 0.1831(1) 0.3712(2) 0.59(5) Na(1) [8b] 0.375 0.375 0.375 2.2(1.3) Na(2) [16c] 0 0 0 9.6(5) Sample V Si(1) [2a] 0.125 0.125 0.125 0.41(12) (Na13.6Si136) Si(2) [32c] 0.2178(1) 0.2178(1) 0.2178(1) 0.38(7) Si(3) [96g] 0.1831(1) 0.1831(1) 0.3715(1) 0.39(4) Na(1) [8b] 0.375 0.375 0.375 1.9(4) Na(2) [16c] 0 0 0 8.9(4) Sample VI Si(1) [2a] 0.125 0.125 0.125 0.35(18) (Na20.5Si136) Si(2) [32c] 0.2186(2) 0.2186(2) 0.2186(2) 0.43(10) Si(3) [96g] 0.1832(1) 0.1832(1) 0.1722(2) 0.45(5) Na(1) [8b] 0.375 0.375 0.375 0.7(2) Na(2) [16c] 0 0 0 7.8(6) 2842 J.Mater. Chem., 1998, 8, 2839–2844Table 6 Results from the refinements of NaxSi136 in samples I–VI Sample I Sample II Sample III Sample IV Sample V Sample VI Formula Na1Si136 Na3Si136 Na3.8Si136 Na10.4Si136 Na13.6Si136 Na20.5Si136 a/A° 14.6428(8) 14.6410(6) 14.6426(5) 14.6449(8) 14.6607(6) 14.7030(5) Cell volume/A° 3 3139.5(2) 3138.4(1) 3139.4(1) 3140.9(2) 3151.1(1) 3178.5(1) Dx/g cm-3 2.032 2.057 2.066 2.146 2.178 2.242 g 0.81(3) 0.89(2) 0.94(2) 0.87(2) 0.76(2) 0.84(3) Caglioti coeV.U 0.13(2) 0.106(10) 0.119(10) 0.136(16) 0.20(2) 0.19(1) V -0.033(12) -0.035(7) -0.033(7) -0.031(12) -0.035(11) -0.022(8) W 0.024(2) 0.016(1) 0.020(1) 0.021(2) 0.022(2) 0.015(2) Rietveld factors cRp 0.115 0.110 0.108 0.117 0.099 0.107 cRwp 0.148 0.146 0.138 0.148 0.127 0.127 x2 2.43 2.54 1.85 1.97 1.85 3.31 Rp 0.0883 0.0872 0.0820 0.0832 0.0764 0.0712 Rwp 0.120 0.123 0.111 0.114 0.104 0.0941 RI 0.0488 0.0484 0.0385 0.0398 0.0328 0.0562 RF 0.0296 0.0287 0.0241 0.0250 0.0220 0.0355 Fig. 6 Representation of two connected cages in the NaxSi136 structure.The seven non-equivalent bonding angles and the four SiKSi distances are indicated. occupied by sodium atoms, involving the value x=8 and the formulation Na8Si46. In the case of NaxSi136, our results show unambiguously that the sodium atoms are exclusively, and not only preferentially, located in the eight large Si28 sites for Fig. 5 Final Rietveld plot of the X-ray diVraction data for (a) sample x8, and that for 8<x24, the smaller Si20 sites are progress- II and (b) sample VI.The crosses represent the experimental data ively occupied with increasing x. These results are consistent points and the upper continuous line the calculated spectra. The upper tick marks indicate the calculated reflection positions for the with those of our study by 23Na NMR spectroscopy of the major phase NaxSi136 and the lower ticks marks the calculated two clathrates.21 In Na8Si46, two sharp lines with a shift of reflection position of the impurity phase Na8Si46.The lower continuous 1766 and 2019 ppm are observed, which have been identified line represents the diVerence. to correspond to sodium atoms in the Si20 and Si24 cages, respectively.In the case of NaxSi136, a broad line, centred at ca. 1800 ppm is observed in the composition range x8. Then, to fill up, strain caused by the guest atoms seems to have a steric influence on the host atomic positions and extends the with increasing x, this broad line exhibits two components which are finally resolved into two sharp lines located at 1608 lattice parameter.and 1812 ppm for x>20. These two sharp lines, which have been related to the appearance of metallic-like conductivity, Conclusion have been attributed to sodium atoms in the eight Si28 and sixteen Si20 sites, respectively. The present study confirms the previously reported data on the crystal structure of the two clathrates NaxSi46 and NaxSi136. Similar results, as to the position of the lines for the two clathrates have been recently reported by other authors.23 In NaxSi46, both the two Si20 and six Si24 sites are fully J.Mater. Chem., 1998, 8, 2839–2844 2843Table 7 List of refined interatomic distances (A° ) and angles (°) for NaxSi136 (sample II and VI) Sample II SiKSi d1=Si(1)KSi(2) 2.343(2) NaKSi Na(1)KSi(2) 3.997(2) d2=Si(2)KSi(3) 2.365(3) Na(1)KSi(3) 3.975(2) d3=Si(3)KSi(3) 2.339(3) Na(1)KSi(3) 3.902(3) d4=Si(3)KSi(3) 2.403(2) angles on Si(1) a=Si(2)KSi(1)KSi(2) 109.5(1) angles on Si(3) e=Si(2)KSi(3)KSi(3) 105.7(2) h=Si(2)KSi(3)KSi(3) 107.5(1) angles on Si(2) b=Si(1)KSi(2)KSi(3) 107.8(1) g=Si(3)KSi(3)KSi(3) 119.9(1) d=Si(3)KSi(2)KSi(3) 111.1(2) w=Si(3)KSi(3)KSi(3) 108.7(1) Sample VI SiKSi d1=Si(1)KSi(2) 2.383(3) NaKSi Na(1)KSi(2) 3.983(2) d2=Si(2)KSi(3) 2.376(3) Na(1)KSi(3) 3.989(2) d3=Si(3)KSi(3) 2.338(3) Na(1)KSi(3) 3.909(3) d4=Si(3)KSi(3) 2.420(3) Na(2)KSi(1) 3.183(1) Na(2)KSi(2) 3.280(3) Na(2)KSi(3) 3.384(3) angles on Si(1) a=Si(2)KSi(1)KSi(2) 109.5(2) angles on Si(3) e=Si(2)KSi(3)KSi(3) 105.3(2) h=Si(2)KSi(3)KSi(3) 108.1(2) angles on Si(2) b=Si(1)KSi(2)KSi(3) 107.2 g=Si(3)KSi(3)KSi(3) 119.8(2) d=Si(3)KSi(2)KSi(3) 111.6(2) w=Si(3)KSi(3)KSi(3) 108.8(2) 4 C.Cros and J. C. Benejat, Bull. Soc. Chim. Fr., 1972, 5, 1739. 5 J. Gallmeier, H. Scha�fer and A. Weiss, Z. Naturforsch. Teil B, 1967, 22, 1080. 6 J. Gallmeier, H. Scha�fer and A. Weiss, Z. Naturforsch. Teil B, 1969, 24, 665. 7 C. Cros, PhD Thesis, 1970, Univ. Bordeaux I, no. 291. 8 N. F. Mott, J.Solid State Chem., 1973, 6, 348. 9 K. E. Sim, PhD Thesis, 1983, Imperial College of Science and Technology, London. 10 G. B. Adams, M. O’Keefe, A. A. Demkov, O. F. Sankey and Y. M. Huang, Phys. Rev. B, 1994, 49, 8048. 11 A. A. Demkov, O. F. Sankey, K. E. Schmidt, G. B. Adams and M. O’Keefe, Phys. Rev. B, 1994, 50, 17001. 12 S. Saito and A. Oshiyama, Phys. Rev. B, 1995, 51, 2628. 13 V.I. Smelyanski and J. S. Tse, Chem. Phys. Lett., 1997, 264, 459. 14 H. Kawaji, H. Horie, S. Yamanaka and M. Ishikawa, Phys. Rev. Lett., 1995, 74, 1427. 15 S. Yamanaka, H. Horie, H. Kawaji and M. Ishikawa, Eur. J. Solid State Inorg. Chem., 1995, 32, 799. 16 P. Me�linon, P. Ke�ghe�lian, X. Blase, J. Le Brusc, A. Perez, E. Reny, C. Cros and M. Pouchard, Phys. Rev. Lett., to be published. 17 Y.Guyot, B. Champagnon, E. Reny, C. Cros, M. Pouchard, P. Melinon, A. Perez and I. Gregora, Phys. Rev. B, 1998, 57, 9475. 18 F. Shimizu, Y. Maniwa, K. Kume, H. Kawaji, S. Yamanaka and M. Ishikawa, Synth. Met., 1997, 86, 2141. Fig. 7 Plot of the lattice parameter of NaxSi136 vs. x. 19 S. Roy, K. E. Sim and A. D. Caplin, Philos. Mag. B, 1992, 65, 1445. 20 M. Menon, E. Richter and K. R. Subbaswamy, Phys. Rev. B, However, these authors observe the two sharp lines of NaxSi136 1997, 56, 1290. for a value of x as low as 9 (determined from a density 21 E. Reny, M. Me�ne�trier, C. Cros, M. Pouchard and J. Se�ne�gas, C.R. Acad. Sci., Ser. IIc, 1998, 1, 129. measurement), a result which implies that the two available 22 J. Gryko, P. F. McMillan and O. F. Sankey, Phys. Rev. B, 1996, sites are almost equally occupied by sodium atoms. The 54, 3037. departure of these data from our own data seems to be due 23 J. Gryko, P. F. McMillan, R. F. Marzke, A. P. Dodokin, to a diVerence in the value of x, since we only observe the A. A. Demkov and O. F. Sankey, Phys. Rev. B, 1998, 57, 4172. two sharp lines for the highest values of x, i.e. x>20. 24 J. Rodriguez-Carvajal, A program for Rietveld refinement and pattern matching analysis, Collected Abstracts of Powder DiVraction Meeting, 1990, Toulouse, p. 127. References 25 J. F. Berar and P. Lelann, J. Appl. Crystallogr., 1991, 24, 1. 26 G. M. Sheldrick, SHELXL 93, a program for the refinement of 1 C. Cros, M. Pouchard and P. Hagenmuller, C.R. Acad. Sci., 1965, crystal structure, Univ. Go� ttingen, 1993. 260, 4764. 27 J. E. Huheey, E. A. Keiter and R. L. Keiter, Chimie Inorganique, 2 J. S. Kasper, P. Hagenmuller, M. Pouchard and C. Cros, Science, De Boeck, University, Bruxelles, 1996. 1965, 150, 1713. 3 C. Cros, M. Pouchard and P. Hagenmuller, J. Solid State Chem., Paper 8/04565H 1970, 2, 570. 2844 J. Mater. Chem., 1998

ISSN:0959-9428    

DOI:10.1039/a804565h

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Correlation of friction, adhesion, wettability and surface chemistry after argon plasma treatment of poly(ethylene terephthalate) Ben D. Beake,a John S. G. Lingb and Graham J. Leggett*a aDepartment of Chemistry, The University of Manchester Institute of Science and Technology, PO Box 88, Manchester, UK M60 1QD bBritish Steel, Welsh Technology Centre, Port Talbot, West Glamorgan, UK SA13 2NG Received 17th September 1998, Accepted 12th October 1998 The combination of wettability, X-ray photoelectron spectroscopy and scanning force microscopy has been used to analyse the changes to the surface after plasma treatment of poly(ethylene terephthalate) film. Calculations on contact angle data with a combination of polar and non-polar liquids have shown that argon plasma treatment considerably enhances the work of solid–(polar) liquid adhesion and the surface free energy of the films due to the creation of acidic and basic functions on the polymer surface.In contrast, Lifshitz–van derWaals (apolar) interactions decrease slightly as a consequence of plasma-induced chain-scission. We present the first study of a plasma-treated polymer by chemical force microscopy.Plasma-modified surfaces exhibit substantially higher friction than untreated material and are more easily disrupted by the movement of the tip during scanning. Friction is reduced when methyl-functionalised tips are employed. There is a correlation, on plasma treatment, between the rapid increases in surface friction probed by lateral force microscopy and surface free energy probed by wettability and X-ray photoelectron spectroscopy.The modified mechanical properties and polar group incorporation both result from scission of polymer chains and contribute to the lateral force contrast. terephthalate) [PET] and the thermodynamic work of adhesion Introduction between the surface and polar liquids, and have determined The control of the chemical, mechanical and topographical the eVect of diVerent plasma treatment conditions on these properties of surfaces is relevant in numerous applications of quantities.X-Ray photoelectron spectroscopy has been used polymers in the textiles, adhesives, composites and coatings to investigate the compositional changes in the surface/nearindustries. 1–6 Many of these applications require good surface region that are responsible for the improvement in adhesion between the polymer and a surface coating. Plasma wettability after exposure to argon plasma. The level of oxygen treatment is an eVective method for improving the bondability incorporation with increasing time of argon plasma treatment and wettability of polymer surfaces whilst leaving bulk proper- provides a means to monitor the extent of surface functies unaltered.In a plasma the surface is exposed to a broad tionalisation. spectrum of ions, electrons, excited neutrals, radicals, UV and Recently, scanning force microscopy has been used to VUV radiation.2,7 The predominant reactive species in an examine the changes to surface topography which occur on inductively coupled radio frequency argon plasma are thought plasma treatment of polymers such as polytetrafluoroto be argon ions and VUV photons which produce excited ethylene,12 polypropylene,13,14 polymethylmethacrylate15 and states at the surface, the decay of which leads to the formation poly(ethylene terephthalate) (PET).8 In addition to the topoof radicals.2,6,7 graphical information, by operating the scanning force micro- We have previously investigated the relationship between scope in lateral (or frictional ) force mode the eVects of the changing wettability and surface morphology under diVerent plasma modification on the nanoscale surface properties of plasma conditions.8 Completely hydrophilic surfaces were not the polymer can be investigated.In lateral force microscopy obtained even after several hours treatment. Wettability is (LFM), the torsional or twisting motions of the cantilever are thought to be limited by competition between etching of the recorded with high contrast, and are regarded as being indicasurface (via chain scission) and chemical functionalisation tive of frictional interactions between tip and sample.16 The (incorporation of polar groups).8–10 Scanning force diVerence in the lateral force signal between forward and microscopy (SFM), imaging in an intermittent contact mode, reverse scans is proportional to the friction force during where the tip taps the surface, showed that the formation of imaging.16,17 This friction is thought to correlate with adhesion orientated, ridged surface structures occurred over an extended since on the molecular scale both processes involve bond time scale.8 breaking and formation.18 In order to investigate the eVect of In this paper we aim to analyse the changes to surface tip–sample chemical interactions on the friction force, we have chemistry further and investigate their role in surface friction.used chemical force microscopy (CFM), in which the tip Our objective is the development of SFM-based technologies chemistry is controlled by the deposition of a self-assembled for exploring the nanoscale properties of polymer surfaces monolayer (SAM).To our knowledge this is the first study of that have been modified in plasma. We were interested to a plasma treated polymer by CFM. We hypothesised that if know whether there was a correlation between changes in the increase in friction on plasma-treatment is predominantly wettability and the frictional properties of the modified poly- the result of increased acid–base interactions between tip and mer surface.It is known that acid–base (electron donor– sample then the measured friction force should vary with acceptor) chemistry plays an important role in the interfacial tip chemistry in the following order: acid-terminated interactions of polymers, significantly improving their mixing, tip>uncoated silicon nitride tip>methyl-terminated tip and adhesion, adsorption on fillers and fibres and their solubility the magnitude of the increase in friction on plasma treatin organic liquids.11 Specifically, we have shown how it is ment should be significantly lower when the apolar, methyl-functionalised tips are used.possible to calculate the surface free energy of poly(ethylene J. Mater. Chem., 1998, 8, 2845–2854 2845of Cr and 20–25 nm of Au on the front face, and (b) 18–20 nm Experimental of Au on the back-face of the cantilever. The evaporation rate Melinex ‘O’, an additive-free PET with low surface roughness, for the gold was always below 0.03 nm s-1 to ensure that the was obtained from ICI ( Wilton, UK).Mylar D (manufactured cantilevers did not bend during heating.18,22 Once cool, by Du Pont, USA), a PET film treated to incorporate a the cantilevers were immersed in 1 mM solutions of dodecaneparticulate silicate surface additive, was obtained from thiol (from Fluka) or mercaptoundecanoic acid in degassed Goodfellow Advanced Materials (Cambridge, UK).Both ethanol for at least 18 h for the self-assembly process. The materials were biaxially orientated and were used as received. mercaptoundecanoic acid was synthesised according a pro- Plasma treatments were carried out in an inductively-coupled cedure adapted from the literature.23 All glassware was cleaned radio frequency (13.56 MHz) reactor with a base pressure of with ‘Piranha’ solution (357 mixture of 30% hydrogen peroxide 4×10-2 mbar, constructed following a design by Dr R.D. and concentrated sulfuric acid) before use. (Great care should Short of the Department of Engineering Materials at the be exercised in handling Piranha solution; it is an extremely University of SheYeld. Argon (BOC, special gases, UK) was strong oxidising agent and has been known to detonate flowed through the reactor for 15 min before treatment.Plasma spontaneously on contact with organic material.) The functreatment was carried out at 0.1 or 1.0 mbar argon pressure tionalised tips were kept in the alkanethiol solutions until use.and 10W power. After treatment, the reactor was evacuated down to base pressure before exposing the sample to laboratory Results atmosphere. Static advancing contact angles were measured within 30 Contact angle goniometry minutes of plasma treatment on a Rame–Hart model 100-00 The observed contact angles of water, ethylene glycol, formam- goniometer. Water was triply distilled before being passed ide, diiodomethane and glycerol on Melinex ‘O’ treated in 0.1 through a Millipore ‘Milli-Q’ purification system.and 1.0 mbar argon plasmas are shown in Fig. 1 and 2 Diiodomethane (>99%), 1-bromonaphthalene (>97%), forrespectively. Increasing plasma treatment led to increasing mamide (>99.5%), glycerol (>99.5%), and ethylene glycol wettability of the polymer by all the polar liquids.Near- (>99.5%) were all from Merck (Darmstadt, Germany) and limiting values at 0.1 and 1.0 mbar were obtained after 120 used as supplied. Recorded angles are averages of at least six and 300 s respectively. In contrast, the contact angle of the measurements. apolar liquid diiodomethane was found to increase on short X-Ray photoelectron spectra of PET samples treated with time exposure to plasma.Diiodomethane has a slight c+ argon plasma at 0.1 mbar were recorded using a Vacuum component, but as a first approximation this liquid may be Generators ESCALAB instrument with a base pressure of considered apolar.24 We have found the inclusion or exclusion 1×10-8 mbar. The system was equipped with an unmonochroof this small polar term makes no diVerence to the surface mated twin anode X-ray source and 100 mm radius hemispherical electron energy analyser.The sample area analysed by this system was approximately 9 mm in diameter with a take-oV angle of 60°. Al-Ka radiation (1486.6 eV) was used throughout. Survey scans (at 50 eV analyser pass energy) and C 1s and O 1s scans (at 10 eV pass energy) were recorded. The areas under C 1s and O 1s curves were calculated and the O5C ratios were determined using empirically derived sensitivity factors reported by Briggs and Seah.19 Topographic scanning force microscopy (SFM) images were obtained in ambient conditions with a TopoMetrix Explorer scanning probe microscope (TopoMetrix Corp., SaVron Walden, UK).Contact mode imaging was generally performed using silicon nitride cantilevers (nominal force constant 0.064 N m-1) supplied by the microscope manufacturer.The only exception was for the CFM data where Nanoprobes (nominal force constant 0.12 N m-1, from Digital Instruments) were used. The applied load was thought to be Fig. 1 Contact angles of water (&), ethylene glycol ($), formamide (+), diiodomethane (,) and glycerol (2) on PET plasma modified <10 nN for the contact mode imaging in constant force mode.in 0.1 mbar argon. Lateral force imaging was performed simultaneously with the topographical imaging. The eVect of scan velocity on the observed lateral force contrast has been documented.20,21 Since we are only interested in relative changes to the frictional force signal, this complication has been avoided by acquiring all scans in Fig. 7–10 at a constant scan rate (21 mm s-1). To allow meaningful comparison of the frictional responses of diVerent materials, the same tip was used throughout, and the alignment of the laser on the cantilever was not altered for all of the samples tested. The lateral force signals on line profiles from the forward and reverse lateral force images were compared to produce friction loops.A Nanoscope IIIa MultiMode AFM (Digital Instruments, UK) was used for the chemical force microscopy study. The scope mode of the microscope was utilised to provide friction loops. ‘Nanoprobe’ SFM tips were modified18 with alkanethiol SAMs of the same carbon chain length terminated with either hydrophilic (carboxylic acid) groups or hydrophobic (methyl ) Fig. 2 Contact angles of water (&), ethylene glycol ($), formamide groups. A General Engineering bell jar vacuum system was (+), diiodomethane (,) and glycerol (2) on PET plasma modified in 1.0 mbar argon.used to coat the tip-cantilever assemblies, as follows: (a) 2 nm 2846 J. Mater. Chem., 1998, 8, 2845–2854energy calculations. Similar behaviour was also observed using another apolar liquid, 1-bromonaphthalene.There is close similarity between water and glycerol angles. The contact angles of the five liquids shown in Fig. 1 and 2 have been used in the determination of the surface free energy change on argon plasma treatment using computer programs25 following the method of van Oss et al., using their values24,26,29 for the surface tension parameters of the five liquids.Details of the calculation of surface free energy from contact angle data are given in Appendix 1. The change in surface free energy after plasma treatment at 0.1 and 1.0 mbar argon is shown in Fig. 3 and 4. A significant increase in the polar component can be clearly seen, with changes occurring faster at the lower pressure studied.Fig. 3 and 4 also show that at both argon pressures, the apolar component initially decreases on exposure to the plasma before recovering to a value near Fig. 5 Thermodynamic work of polymer–water (&) and polymer– that on untreated Melinex ‘O’. The increase in basic component formamide (+) adhesion after plasma treatment at 0.1 mbar. The acid–base components to the total work of adhesion are also shown; to the surface free energy after plasma treatment is larger at water ($) and formamide (,).either pressure studied than the increase in acidic component. The variation in the thermodynamic work of solid–liquid adhesion with treatment time has also been calculated. Fig. 5 illustrates the changes at 0.1 mbar for water and formamide. X-Ray photoelectron spectroscopy An O5C ratio of 0.39±0.02 was determined for untreated Melinex ‘O’ in good agreement with the theoretical value of 0.40. Fig. 6 shows C 1s spectra (corrected for charging eVects) for untreated and plasma treated Melinex ‘O’. The data show broadening of the line widths of the C 1s peaks. Similar behaviour has been reported in the XP C 1s spectra of several polymers, including PET, after argon plasma treatment, and is regarded as evidence of an increased variety of carbon species on plasma treatment.7 Notably, a new peak has Fig. 3 Surface free energy change on plasma treatment at 0.1 mbar argon; csLW (&), csAB (,), c+ (+) and c- ($). Fig. 6 Fitted C 1s spectra of PET, untreated (a) and plasma-treated Fig. 4 Surface free energy change on plasma treatment at 1.0 mbar argon; csLW (&), csAB (,), c+ (+) and c- ($).for 10 min (b) and 2 h (c). J. Mater. Chem., 1998, 8, 2845–2854 2847appeared in the C 1s spectrum. Argon plasma treatment at either pressure studied increased the O5C ratio by a similar amount. Table 1 shows the O5C ratio as a function of treatment time for 0.1 mbar argon. Near-limiting values were obtained after 1 min treatment, after which only small changes to the lineshapes occurred and it reached a steady state by about 10 min.Survey scans showed that no nitrogen was present after plasma treatment. Lateral force microscopy The plasma-modified Melinex ‘O’ and Mylar D surfaces were more easily disrupted by the motion of the tip during scanning (at the same applied load) than the untreated polymer.Lateral force imaging of untreated Melinex ‘O’ and Mylar D films showed only small frictional contrast on scanning in forward and reverse directions. After treatment with argon plasma there was a significant enhancement in the observed lateral force contrast. Lateral force images revealed greater detail of the structure of the surface additives on the plasma treated Mylar D surface (Fig. 7) although we have observed that the quality of images of the polymeric regions is poor in contact mode. Illustrative topography, lateral force images and friction loops (see below) of Mylar D and Melinex ‘O’ both plasma treated for 2 min are shown in Fig. 8 and 9. Friction loops have been constructed from line profiles and are shown for plasma-modified Melinex ‘O’ and Mylar D in Fig. 8(d) and 9(d) respectively. Fig. 10 shows the dependence of the frictional force on plasma treatment time for Melinex ‘O’ and over the additive features and the polymer surface for Mylar D. The frictional forces are larger over the polymer than the silicate additives in Mylar D. The diVerence in friction is much greater than on the untreated film. The maximum LFM signal is about seven times greater than that on the untreated Melinex ‘O’ and about five times that on the untreated Mylar D.Chemical force microscopy Imaging plasma treated surfaces with AFM tips functionalised with a hydrophobic methyl-terminated alkanethiol monolayer led to some reduction in sample damage, which was more pronounced on surfaces which were treated for longer periods.Friction loops were constructed from images of plasma-treated Mylar D surfaces (or taken directly from the scope mode of Fig. 7 LFM images of Mylar D plasma treated for 10 s at 0.1 mbar. Images are forward (a) and reverse (b) directions. Z-scale ranges: (a) the Nanoscope) with three chemically distinct types of ‘Nano- 1.66 to -2.55 nA; (b) 6.58–3.17 nA. probe’ AFM tips; (i) unmodified silicon nitride, (ii) methylterminated, (iii) carboxylic acid-terminated.It was found that 0.064 N m-1 tips, as expected since the nanoprobes used to the observed lateral forces did depend on the chemistry of the study the eVect of tip chemistry were much stiVer laterally. AFM tip, for both untreated and plasma-treated Mylar D surfaces, with the unmodified and carboxylic acid-modified tips having greater frictional interaction with the sample than Discussion the hydrophobic tips (Fig. 11). Table 2 shows relative frictional The main mechanisms currently thought responsible for the coeYcients calculated from the slopes of friction vs. applied bondability improvement of plasma treated polymer surfaces load plots. The change in lateral force signal on plasma are interfacial diVusion (aided by an increased molecular treatment was much smaller than that obtained with the mobility caused by chain scission) and increased wettability.30 The increased molecular mobility has been inferred from the observation that plasma treated polymers are bondable below Table 1 XP data, O5C ratio as a function of treatment time (0.1 mbar their melting points.30 The incorporation of polar functionaliargon) ties should result in an improvement in the polymer wettability, and the data in Fig. 1 and 2 show much lower contact angles Treatment time/min O5Ca are obtainable for all the polar test liquids used following plasma treatment. 0 0.39 1 0.50 Surface free energy calculations for untreated Melinex ‘O’ 5 0.52 by the method of van Oss and coworkers revealed a similar, 10 0.48 but slightly higher, value (cs#47 mN m-1) than has been 15 0.52 obtained in recent studies (cs#44 mN m-1).29 Although the 30 0.50 PET was obtained directly from sheets, nominally with clean 60 0.50 sides facing inwards, it is possible that it may pick up some 120 0.51 oleophilic impurities on exposure to atmosphere.31 To test for aTypical error in O5C ratio is ±0.03.contamination the polymer was sonicated in diethyl ether for 2848 J. Mater. Chem., 1998, 8, 2845–2854Fig. 8 LFM images of Melinex ‘O’ plasma treated for 2 min at 0.1 mbar. Topographic (a) and lateral force images in forward (b) and reverse (c) directions. Friction loop (d) constructed from lateral force line profiles. Z-scale ranges: (a) 0–2 nm; (b) -1.74 to-9.28 nA; (c) 14.54–8.85 nA. 10 min and dried in high-purity nitrogen immediately before been reported for corona discharge-treated PET by Briggs et al.33 and these probably contribute to the increase in acidic the contact angle measurements. The angles obtained were the same as those without the cleaning procedure; we conclude interactions reported here. The significant increase in basic interactions on the plasma- that the slight diVerences in wettability between our samples and others more likely reflect details of the polymer manufac- treated surface is notable.We suggest that the increase is due predominantly to the formation of carbonyl groups. XP ture (such as surface roughness) rather than the presence of contamination. XPS showed the ratio of oxygen to carbon in spectra (see later) reveal a new peak which is attributed to the creation of carbonyl functions on the plasma-modified sur- the untreated film to be close (0.39) to the expected value (0.40), and a lack of obvious contamination.face.8,34 Carbonyl groups are thought to exhibit basic character32 and CueV et al. have reported XPS data showing that, Calculations of the variation in surface free energy after plasma treatment at 0.1 and 1.0 mbar argon are shown in for argon plasma treatment under their experimental conditions, the only carbon–oxygen functions to increase in Fig. 3 and 4 respectively. The total surface free energy obtained in this study is 64±2 mNm-1, after a period very much intensity were carbonyl groups.34 Nitrogen and oxygen plasma treatments of polypropylene have both been reported to pro- shorter than that required for the formation of the oriented, ridged structures reported in a previous study.8 As expected duce a predominantly basic surface.35,36 Whilst in the former case incorporation of basic N-containing functionalities is from the contact angles, a significant increase in the polar component can be clearly seen.On plasma treatment the expected, in the latter a similar incorporation of carbonyl functions may be occurring. surface has acquired a pronounced acidic and more notably, basic character. Acidic interactions through formation of van Oss, Chaudhury and Good have noted24 that the occurrence of large basic components of the polar free energy hydroxy and carboxyl groups on plasma treatment were expected,7 and an enhancement in the acidic component of (together with a much smaller c+ value) is not uncommon in polymers and natural compounds such as proteins.These are about 2 mN m-1, very similar in magnitude to that calculated in the present study, has been reported in the argon plasma termed24 ‘monopolar surfaces’, and PET is considered27 to be a moderate c- monopole.Strong hydrogen bonding in the treatment of polycarbonate.32 This was explained by the formation of phenolic hydrogen species, formed by photo- depolymerised surface may increase the pKa of the acid groups formed on plasma treatment, leading to a decrease in the acid Fries rearrangements. The formation of phenolic species has J. Mater. Chem., 1998, 8, 2845–2854 2849Fig. 9 LFM images of Myler D plasma treated for 2 min at 0.1 mbar. Topographic (a) and lateral force images in forward (b) and reverse (c) directions. Friction loop (d) constructed from lateral force line profiles. Z-scale ranges: (a) 0–40 nm; (b) -1.33 to -11.11 nA; (c) 9.46–2.44 nA. Fig. 11 Variation of friction force with AFM tip chemistry for Fig. 10 Variation in friction signal with treatment time.Melinex ‘O’ (+), on polymer background of Mylar D (&), over additives on untreated Mylar D and plasma treated for 20 min at 0.1 mbar. Tipsample combinations: unmodified-plasma treated (+), unmodified- Mylar D ($). untreated (6), methyl-plasma treated ($), methyl-untreated (#). component of the polar free energy. It should also be noted that the exact acidic and basic parameters are dependent on set cw+=cw-=25.5 mN m-1).The authors have suggested that the modified values may allow the acidic properties of the values of the test liquid acid and base components chosen. A revised scale of the acid–base parameters of common polymers to be more correctly expressed.37 The similarity of water and glycerol contact angles on solvents with cw+=65 mN m-1 and cw-=10 mN m-1 has recently been proposed (previously van Oss and co-workers plasma-modified surfaces is notable.It has been reported that 2850 J. Mater. Chem., 1998, 8, 2845–2854Table 2 Dependence of the relative friction coeYcient over polymeric regions of Mylar D on the chemistry of the AFM tip Relative friction coeYcient Tip chemistry Untreated polymer After plasma treatmenta Silicon nitride 0.20±0.02 0.36±0.05 Methyl-terminated 0.07±0.01 0.23±0.05 Carboxylic acid-terminated 0.21±0.04 0.32±0.03 aPlasma treatment for 20 min at 0.1 mbar.Errors shown are standard deviations of values from 3–10 separate determinations. the contact angle of glycerol is virtually identical to that of treatment in agreement with the rapid increase in basic interaction revealed by the contact angle data.Small changes in water for a wide range of biological systems.27 DiVerences in the relative acid–base parameters of the two liquids can the C 1s lineshape do occur beyond this point, notably an increase in the carbonyl peak intensity; however the changes account for the unexpected closeness of angles on two liquids whose surface tensions diVer by 12%.27 PET does not become are smaller than those that occur in the first min of treatment and steady state is reached after 10 min.An exact correlation completely wetted by either water or glycerol on argon plasma treatment. This is indicative of a maximum in surface energy. between the time-scales of the variations in contact angle and XP spectra is not expected due to the diVerent sampling depths As suggested previously,8–10 this is probably due to a steadystate being reached between functionalisation (polar group and hence diVering surface selectivities of the two techniques: wettability is sensitive to the outer 0.5–1.0 nm of the surface40 incorporation) and surface etching.Comparison with data for diiodomethane shows that, sur- whilst XPS data contain contributions from a greater depth, ca. 5–10 nm.19 prisingly, the contact angle of the apolar liquid increases on exposure to plasma. We have previously shown8 that plasma Fig. 7 shows lateral force images of the Mylar D surface after only 10 s exposure to plasma. While we have previously treated material is much more susceptible to tip-induced damage during scanning.We suggest that the observed increase reported that plasma treated PET is disrupted during contact mode SFM,8 the additive particles are imaged here with clarity.in the contact angles with apolar liquids (indicative of a decrease in dispersive interactions between surface and test Some of the additive features are in fact aggregates of several smaller particles.After longer exposure this delineation of the liquid) is related to surface disorder caused by plasma-induced chain-scission. The increase in surface mobility (disorder) is additives became less clear, presumably because they were damaged by the plasma. discussed further below in connection with the SFM data. The contact angle measurements of test liquids on a solid The contrast in LFM arises from twisting motions of the cantilever as it transverses the surface.These twisting motions polymer surface have also been used to calculate the thermodynamic work of solid–liquid adhesion, as illustrated in Fig. 5. arise from forces acting parallel to the plane of the sample surface. It is clear that frictional forces contribute to the LFM The acid–base contribution to the work of adhesion increases on plasma treatment, as expected from the increase in surface signal; however, when the local topography of the surface changes then the LFM signal may also contain contributions free energy.38 The increase in the acid–base component of the work of water–PET adhesion is greater than the corresponding from normal ( load) forces.Appendix 2 shows how the eVects of normal forces on the frictional signal can be eliminated by increase in the work of formamide–PET adhesion.Since water is a much stronger acid than formamide the importance of scanning in forward and reverse directions. The images in Fig. 7 and 9 appear to indicate that an inversion of contrast basic interactions on the plasma-modified surface is clear. As shown in Table 1, XPS reveals a substantial increase in occurs over the additives on reversing the scan direction, implying significant frictional interaction.However, examin- the O5C ratio after plasma treatment and subsequent exposure to atmosphere. The O5C ratio is 0.50±0.03 after 1 min ation of the friction loops of Mylar D plasma-treated for 2 min [Fig. 9(d)] reveals that the contrast inversion in the treatment at 0.1 mbar argon and does not rise significantly thereafter.The magnitude of this increase is in agreement with LFM image is illusory. Careful consideration of the line profiles shows that the lateral force over the additives changes a recent determination (O5C#0.51–0.56) by France and Short7 at 10 W and 2.5×10-2 mbar argon using a similar little, while a large change is seen over the polymeric regions.The magnitude of the change is such that the relative contrast reactor configuration. It has been considered7–10 that plasma attack on the ester functionally would be likely to lead to over the silicates changes in the image; however, the largest frictional interaction, according to the analysis of Grafstro�m chain scission, leading to etching and a relatively low saturation level for oxygen incorporation and a large amount of low et al.17 (see Appendix 2) is over the polymer.Friction loops have shown that the lateral force is constant over an image, molecular weight material.7,9 By comparing the etching rates of several organic materials, Prat et al. have concluded that suggesting that although the plasma-modified surface can be worn by the SFM tip during scanning the tip-induced topo- polymers containing functions such as ester groups are more susceptible to degradation since it can more easily occur by graphic changes (which, in principle, could aVect the friction measurement by making the subtraction inexact) are small.In initial chain scission at the functional groups.10 Indeed, there are reports of PET surface modification by argon plasma an earlier paper41 we observed a similar (although smaller) contrast inversion over the additive particles in untreated which show a small decrease in C–O and ester peak intensity. 34,39 It has been suggested that the breaking of ester bonds Mylar D. However a re-examination of the untreated material suggests that the inversion is illusory there too, and that, for can lead to radicals that are resonance-stabilised over those formed in C–C bond breaking.9 Under conditions where the untreated Mylar D, the largest friction interaction is on the polymer surface.Obviously, great care must be taken in the authors reported a loss of ester oxygen, the only peak found to increase in intensity was a new species of ca. 3.0 eV higher interpretation of image contrast in LFM. For Mylar D, the contrast over the polymer background binding energy than hydrocarbon.34 These authors, and others,7 have assigned this to the creation of isolated car- and over the additive surface both show a sharp initial increase with time of plasma treatment as shown in Fig. 10. The bonyl groups. Peak fitting to the C 1s lineshape, after plasma treatment diVerence in friction between the additives and the polymeric background is much larger than on the untreated film.has also revealed a new peak 3 eV from hydrocarbon in the present study (Fig. 6). The peak appears after only 1 min Illustrative topography, lateral force images and friction loops J. Mater. Chem., 1998, 8, 2845–2854 2851of Mylar D and Melinex ‘O’ both plasma treated for 2 min If acid–base interactions were solely responsible for the increase in friction following plasma treatment, the magnitude are shown in Fig. 8 and 9. Comparison of the lateral force line profiles with the topographic image provides evidence that of the change would be significantly smaller when the apolar, methyl-functionalised tips were used.However, the magnitude the lateral forces are aVected by the local sample slope; the friction signal is clearly altered as the tip encounters the surface of the frictional increase on plasma treatment was similar for all the tip chemistries used. Therefore it is clear that while additives. However, the friction over the central gions of the additives and over the polymeric background is invariant chemical interactions contribute to friction following treatment, there is a substantial additional contribution, which we across the image; the method of Grafstro�m et al.17 is applicable for surfaces of this roughness.attribute to an increase in the tip–sample contact area as a consequence of the mechanical softening of the surface. The The data shown in Fig. 10 have all been recorded using the same SFM tip. Thus, although we do not know the exact observed correlation between surface free energy, probed by wettability and XPS, and surface friction, probed by LFM, is lateral forces (as both the lateral spring constants of our tips and the sensitivity of our microscope to lateral displacements a consequence of the fact that chain scission ( leading to mechanical weakness in the surface layer) and polar group are not accurately known), the relative frictional forces are accurately (±10%) determined.After plasma treatment incorporation ( leading to increasing surface energy) occur on a similar time scale, and are complementary aspects of the Melinex ‘O’ surface has a frictional response up to seven times greater than the virgin material.The surface friction measured same physical process. by LFM reaches limiting values on closely similar time-scales to the wettability and XPS data. If acid–base interactions between tip and sample are import- Conclusion ant in determining the frictional interaction then the measured friction force should vary with tip chemistry in the follow- The combination of lateral force microscopy, wettability, and ing order: acid-terminated tipµunmodified silicon nitride X-ray photoelectron spectroscopy has been used to analyse tip>methyl-terminated tip.Although the bulk composition of changes at the film surface after plasma treatment of polythe unmodified tip is silicon nitride, it is thought that oxides (ethylene terephthalate).Calculations on contact angle data and silanols are present at the surface yielding a polar tip.42 with a combination of polar and non-polar liquids have shown Unmodified and COOH-terminated tips are known to show that argon plasma treatment considerably enhances the work similar frictional characteristics when imaging SAMs in air.43 of solid–(polar) liquid adhesion and the surface free energy of Fig. 11 shows the variation in friction force with load for the films.This is shown to be due to the creation of acidic methyl-terminated and unmodified tips before and after plasma and basic functions on the polymer surface. This is confirmed treatment for 20 min. It can be seen that for a given load, the by XP spectra which show an increased oxygen:carbon ratio friction force measured with the methyl terminated tip for the after plasma treatment.In contrast, the Lifshitz–van derWaals treated polymer is significantly higher than that measured with (apolar) interactions decrease as a consequence of plasmathe same tip for the untreated polymer, but similar to the induced chain-scission. Friction force microscopy has shown force measured for the treated polymer with a bare tip.From that plasma-modified surfaces exhibit substantially higher fricthe slopes of such plots it is possible to determine a relative tion than untreated material and are more easily disrupted by coeYcient of friction for a specific tip–sample combination, the movement of the tip during scanning. Typically, modified and these data are plotted in Table 2 along with measurements surfaces show a maximum frictional response which is about for acid-terminated tips. Both polar (unmodified and car- seven times higher than untreated material when imaging with boxylic acid-modified) tips have greater frictional interaction the less stiV cantilevers.Friction forces on plasma-treated and with the polymeric samples than the hydrophobic (apolar) tips unmodified Mylar D depend on the surface chemistry of the do.There was a noticeable improvement in resolution on AFM tip. There is a correlation, on plasma treatment, between imaging the plasma-treated samples with hydrophobic methyl- the rapid increases in surface friction probed by lateral force coated tips. These results indicate that polar interactions microscopy and surface free energy probed by wettability and between tip and sample make a significant contribution to the X-ray photoelectron spectroscopy.Increased surface disorder friction force measured by LFM. and polar group incorporation both result from scission of The frictional force of an adhesive contact is a function of polymer chains and contribute to the increase in friction.the contact load (here kept constant), the area of contact and the surface free energies of the two surfaces. On plasma treated surfaces both an increase in surface free energy (shown by the improved wettability) and an increase in tip–sample contact Acknowledgements area (treated surfaces are more easily disrupted by the motion The authors are grateful to the EPSRC (grant GR/K/88071), of the tip during contact mode scanning) occur and are the Royal Society and the Society of Chemical Industry for mediated via chain scission processes.Studies by Bar et al. on financial support. J.S.G.L. thanks the EPSRC for a research LFM and force modulation microscopy of self-assembled studentship. G.J.L. thanks the NuYeld Foundation for a monolayers have shown the importance of packing density Science Research Fellowship.The authors would like to thank (and hence tip–sample contact area) in determining the conthe Department of Materials Engineering and Materials trast in LFM images on chemically identical regions, with Design, University of Nottingham, where some of this work sharp contrast observed between heptanethiol and octadecanewas carried out, and acknowledge the assistance of J.C. thiol monolayers.44 We have used LFM to probe the surface Bussey (University of Nottingham) in obtaining some of the friction on regions of diVerent hydrophobicity on photopat- XPS spectra. The authors are most grateful to Dr J. H. Clint terned PET films and photopatterned SAMs.45 Imaging under (Surfactant Science Group, School of Chemistry, University similar conditions led to considerably smaller observed LFM of Hull ) for supplying copies of his programs to obtain surface contrast between hydrophilic and hydrophobic regions on free energy data from contact angles and for useful discussions, these surfaces than was observed here between treated and and to Dr R.D. Short (University of SheYeld) for his untreated polymers. This observation, together with measureassistance in designing the plasma reactor.The authors would ments of frictional forces between single-component selfalso like to thank Professor D. Briggs (ICI, Wilton) for assembled monolayers,45 suggests that the greater frictional supplying Melinex ‘O’ with known orientation and for helpful contrast observed after plasma treatment is not entirely due to diVerences in wettability.and stimulating discussion. 2852 J. Mater. Chem., 1998, 8, 2845–2854contribution to the LFM image may be removed by sub- Appendix 1: Theory and calculation of surface free tracting images recorded in the forward and reverse directions. energies from contact angles These comparisons of forward and reverse scans are often called friction loops and should reflect the frictional force Contact angle measurements of liquids on a solid surface can be used to calculate the thermodynamic work of solid–liquid acting between tip and sample.16 adhesion according to the Young–Dupre� equation, [Wsl= cl(1+cosh)].The acid–base contribution to the work of adhesion can be separated30,46 from the Lifshitz–van derWaals References interactions: 1 E.M. Liston, L. Martinu and M. R. Wertheimer, J. Adhes. Sci. Wsltotal=WslLW+WslAB Technol., 1993, 7, 1091. 2 E. M. Liston, in The Interfacial Interactions in Polymer LWrepresents the sum of the three electrodynamic interactions Composites, ed. G. Akovali, Kluwer Academic, London, 1993, which decay with distance at the same rate (primarily the p. 223.don [dispersion] force, with small contributions from the 3 A. M.Mayes and S. K. Kumar, MRS Bull., 1997, 22, 43. 4 F. D. Egitto and L. J. Matienzo, IBM J. Res. Dev., 1994, 38, 423. Keesom dipole–dipole [orientation] and Debye dipole–induced 5 K. Harth and H. Hibst, Surf. Coat. Technol., 1993, 59, 350. dipole [induction] force), collectively designated24,26 as 6 F. Denes, TRIP, 1997, 5, 23.Lifshitz–van der Waals (‘apolar’) interactions. AB represents 7 R. M. France and R. D. Short, J. Chem. Soc., Faraday Trans., the polar or acid–base interactions. Similarly, it has recently 1997, 93, 3173. been established24,26–29 that the surface free energy of the solid 8 B. D. Beake, J. S. G. Ling and G. J. Leggett, J. Mater. Chem., can be separated into two terms: 1998, 8, 1735. 9 F. Clouet and M. K. Shi, J. Appl. Polym. Sci., 1992, 46, 1955. cstotal=csLW+csAB 10 R. Prat, M. K. Shi and F. Clouet, J. Macromol. Sci–Pure Appl. Chem. A, 1997, 34, 471. A geometric mean approach can be used to determine the 11 Acid–Base Interactions, Relevance to Adhesion Science and apolar component of the solid free energy from the contact Technology, ed.K. L. Mittel and H. R. Anderson, Jr, VSP, angle of an apolar test liquid providing that the liquid is of Utrecht, 1991. 12 J. A. McLaughlin, D. Macken, B. J. Meenan, E. T. McAdams and high enough surface tension that it does not spread completely P. D. Maguire, Key Eng. Mater., 1995, 99–100, 331. over the solid surface.24,26–29 However, such an approach is 13 J. F. Friedrich, W. Unger, A.Lippitz, T. Gross, P. Rohrer, flawed when considering the polar component.29,38 Instead, W. Saur, J. Erdmann and H-V. Gorsler, J. Adhes. Sci. Technol., several authors35,36,38 have suggested the approach of van Oss, 1995, 9, 575. Chaudhury and Good24,26–29 is the most useful in the investi- 14 A. Ringenbach, Y. Jugnet and Tran Minh Duc, J. Adhes. Sci. gation of acid–base properties of plasma-modified polymer Technol., 1995, 9, 1209. 15 P. Gro�ning, M. Collaud, G. Dietler and L. Schlapbach, Vide surfaces. Van Oss et al. have introduced the concept of surface Couches Minces, 1994, 272SS, 140. tension parameters, c+ and c-, due to acidic and basic 16 R. Overney and E. Meyer, MRS Bull., 1993, 18, 26. functionalities respectively.24 These comprise (non-additively) 17 S.Grafstro�m, M. Neitzert, T. Hagen, J. Ackermann, the polar surface free energy of a material i, (ciAB), thus: R. Neumann, O. Probst and M. Wo� rtge, Nanotechnology, 1993, 4, 143. ciAB=2(ci+ci-)1/2 18 A. Noy, C. D. Frisbie, L. F. Rozsnyai, M. S. Wrighton and C. M. Lieber, J. Am. Chem. Soc., 1995, 117, 7943. The key to the modification of the Young’s equation (below) 19 Practical Surface Analysis, ed.D. Briggs and M. Seah, 2nd edn. proposed by these authors is the realisation that acidic func- Wiley, 1990, vol. 1, p. 635. tions in the test liquid interact with only basic functionalities 20 G. Haugstad, W. L. Gladfelter, E. B. Weberg, R. T. Weberg and on the solid surface and vice versa. R. R. Jones, Langmuir, 1995, 11, 3473. 21 J. A. Hammershmidt, B. Moasser, W.L. Gladfelter, G. Haugstad (1+cosh)cl=2[(csLWclLW)1/2+(cs+cl-)1/2+(cs-cl+)1/2] and R. R. Jones, Macromolecules, 1996, 29, 8996. 22 E. W. van der Vegte and G. Hadziioannou, Langmuir, 1997, 13, In this Lifshitz–van der Waals/acid–base approach there are 4357. three unknown values for the surface tension components. 23 C. D. Bain, E. B. Troughton, Y.-T. Tao, J. Evall, G.M. Whitesides When contact angle measurements are carried out with three and R. G. Nuzzo, J. Am. Chem. Soc., 1989, 111, 321. or more liquids (at least two of which are polar) with known 24 C. J. van Oss, M. J. Chaudhury and R. J. Good, Adv. Colloid cLW, c+ and c- values, a standard least-squares method can Interface Sci., 1987, 28, 35. 25 The computer programs used to obtain surface free energy data be used to solve for the three unknown parameters.29 from contact angles were from Dr J.H. Clint ( University of Hull ). 26 C. J. van Oss, M. J. Chaudhury and R. J. Good, Chem. Rev., 1988, 88, 927. Appendix 2: Construction of friction loops to 27 C. J. van Oss, M. J. Chaudhury and R. J. Good, Langmuir, 1988, determine surface friction 4, 884. 28 C. J. van Oss, Colloids Surf.A: Physico. Eng. Asp., 1993, 78, 1. A line profile of the forward and reverse scans, a friction loop, 29 W.Wu, R. F. Giese, Jr. and C. J. van Oss, Langmuir, 1995, 11, 379. can be used to determine the frictional interaction between tip 30 Polymer Interface and Adhesion, ed. S. Wu, Marcel Dekker, New and sample according to the treatment of Grafstro�m et al.17 York, 1982, pp. 298–321. These authors have shown that for a force (FN) normal to the 31 Prof. D. Briggs, personal communication. 32 S. Vallon, B. Dre�villon, F. Poncin-Epalliard, J. E. Klemberg- sample due to the application of a load during imaging, the Sapieha and L. Martinu, J. Vac. Sci. Technol. A, 1996, 14, 3194. lateral force (Fy) in a direction y orthogonal to both the 33 D. Briggs, D. G. Rance, C. R. Kendall and A. R. Blythe, Polymer, cantilever and surface normal is given by 1980, 21, 895. 34 R. CueV, G. Baud, J. P. Besse, M. Jacquet and M. Benmalek, Fy=Fty-syFN J. Adhes., 1993, 42, 249. 35 J. Behnisch, A. Holla�nder and H. Zimmermann, Int. J. Polym. where sy is the slope in the y-direction and Fty is the force Mater., 1994, 23, 215. tangential to the surface normal in the y-direction. When there 36 J. Behnisch, A. Holla�nder and H. Zimmermann, J. Appl. Polym. are substantial variations in surface topography the syFN Sci., 1993, 49, 117. component can become significant. Since the lateral force is 37 C. Della Volpe and S. Siboni, J. Colloid Interface Sci., 1997, 195, determined from the diVerence between signals reaching the 121. left and right halves of a four-segment photodetector, in 38 N. Shahidzadeh-Ahmadi, F. Arefi-Khonsari and J. Amouroux, J. Mater. Chem., 1995, 5, 229. principle ignoring piezoelectric drift) the topographic J. Mater. Chem., 1998, 8, 2845–2854 285339 Q. T. Le, J. J. Pireaux and J. J. Verbist, Surf. Interface Anal., 1994, 44 G. Bar, S. Rubin, A. N. Parikh, B. I. Swanson, T. A. Zawodzinski, 22, 224. Jr. and M.-H. Whangbo, Langmuir, 1997, 13, 373. 40 P. E. Laibinis, C. D. Bain, R. G. Nuzzo and G. M. Whitesides, 45 B. D. Beake and G. J. Leggett, unpublished work. J. Phys. Chem., 1995, 99, 7663. 46 X. Qin and W. V. Chang, J. Adhes. Sci. Technol., 1996, 10, 963. 41 J. S. G. Ling and G. J. Leggett, Polymer, 1997, 38, 2617. 42 J. M. Williams, T. Han and T. P. Beebe, Jr., Langmuir, 1996, 12, 1291. 43 J.-B. D. Green, M. T. McDermott, M. D. Porter and L. M. Siperko, J. Phys. Chem., 1995, 99, 10 960. Paper 8/07261B 2854 J. Mater. Chem., 1998, 8, 2

ISSN:0959-9428    

DOI:10.1039/a807261b

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials N,N-Dialkylcarbamato complexes as precursors for the chemical implantation of metal cations on a silica support. Part 3† Palladium Luigi Abis,a Daniela Belli Dell’ Amico,b Carlo Busetto,a Fausto Calderazzo,*b Ruggero Caminiti,c Fabio Garbassia and Alessandra Tomeib,d aEnichem S.p.A., Centro Ricerche Novara ‘Guido Donegani’, Via G. Fauser 4, I-28100 Novara, Italy bUniversita` di Pisa, Dipartimento di Chimica e Chimica Industriale, Sezione di Chimica Inorganica, Via Risorgimento 35, I-56126 Pisa, Italy cUniversita` di Roma ‘La Sapienza’, Dipartimento di Chimica and Istituto Nazionale per la Fisica della Materia (I.N.F.M.), P.le Aldo Moro 5, I-00185 Roma, Italy dScuola Normale Superiore, Piazza dei Cavalieri 7, I-56100 Pisa, Italy Received 12th June 1998; Accepted 8th September 1998 Chemical implantation of palladium(II) has been carried out under mild conditions by reacting trans- Pd(O2CNEt2)2(NHEt2)2 with the silanol groups of amorphous silica, carbon dioxide and secondary amine being released in the process.The palladium-containing silica has been characterized and the coordination environment of the implanted cation has been defined by 13C CP MAS NMR, DRIFT and XPS spectra, and by WAXS measurements.Silica-bonded palladium(II) was reduced thermally in vacuo or with dihydrogen at room temperature. Catalytic activity in the hydrogenation of cyclohexene was found for all samples containing the silicasupported reduced palladium; the best results, with rates independent of olefin concentration, were found for the samples treated thermally (200 °C) under reduced pressure. Earlier papers from these laboratories have pointed out that dehydrated silica has been reported in the literature, with release of propylene upon reaction with the silanol groups.6h cationic implantation on silica can be carried out with tin(IV)1a and platinum(II),1b by using the corresponding N,N-dialkylcarbamato complexes, of general formula M(O2CNR2)n, as pre- Experimental cursors. As the silanol groups are the reactive sites, it was anticipated that cations would be homogeneously distributed Materials and reagents on the surface.This method therefore appears to be an useful All operations were carried out in conventional Schlenk tubes alternative to more traditional ones,2 and to the methodology under a dry dinitrogen or argon atmosphere, unless otherwise based on organometallics.3 The use of N,N-dialkylcarbamates specified.Solvents were dried according to conventional presents several advantages, which have been pointed out methods. Carbon dioxide was dried over calcium chloride. earlier;1 implanted species and their reduction products can The secondary amine was distilled from sodium prior to use.be investigated by conventional surface methods. The compound trans-Pd(O2CNEt2)2(NHEt2)2 was prepared The availability of the N,N-diethylcarbamato derivative of according to a method reported earlier.4 Commercial silica palladium(II)4 has urged us to use this compound as starting (Grace SD 3217/50; surface area, 318 m2 g-1; pore volume, material for the implantation of palladium(II) on silica; its 2.22 cm3 g-1) was heated at 160 °C at ca. 10-2 mmHg for 16 h further reduction was predicted to occur easily (E0=0.951 V5), to eliminate most of the physisorbed and chemisorbed water, both thermally and chemically. The use of reduced palladium cooled down to room temperature, and flame-sealed in vials in the hydrogenation of cyclohexene is also reported.under carbon dioxide ( label: SiO2-160). The silanol content Supported palladium represents an important technical probwas assumed to correspond to the weight loss after calcination lem;6a this metal has been implanted on inorganic supports by at 850 °C. The silanol content (expressed as mmol of OH per ion exchange,6b by metal evaporation,6c by solvent extraction gram of silica) was thus estimated to be 2.8.followed by reduction,6d by organometallic chemical vapor deposition (OMCVD) using allyl derivatives,6e and by cluster generation in the presence of an inorganic support.6f In spite Gas-volumetric and elemental analyses of this intense activity in the field, we are not aware of any The carbon dioxide content of the silica-supported palladium prior use of a non-organometallic compound of palladium for species and the carbon dioxide evolved in the course of the a chemoselective reaction with an oxide support under mild implantation reaction were determined in a thermostatted gas conditions. On the other hand, palladium on copper has been burette,7 using liquid media previously saturated with carbon obtained by chemical vapour deposition of volatile paldioxide at the temperature of the experiment.ladium(II) coordination compounds.6g Also, the chemical Elemental (C, H, N) analyses were carried out with a C. interaction of p-allyl derivatives of palladium(II) with partially Erba mod. 1106 elemental analyzer at the Microanalytical Laboratory of the University of Pisa (Faculty of Pharmacy) or in the house.Other elemental analyses were carried out by inductively coupled plasma-atomic emission spectrometry †In partial fulfilment of the requirements for the PhD Thesis of A.T., Scuola Normale Superiore of Pisa. Part 2: ref. 1(b). (ICP-AES) with a Perkin-Elmer Plasma II instrument (Pd). J. Mater. Chem., 1998, 8, 2855–2861 2855Instrumental analysis absorption, polarization and inelastic scattering of the incident X-ray radiation; thus, the total intensity, observed by an 1H and 13C NMR solution spectra were measured with a energy dispersive detector, was corrected accordingly and for Varian Gemini 200 BB instrument, and chemical shifts are the escape peak suppression as well.Experimental conditions expressed in ppm with respect to SiMe4.IR spectra were were: voltage, 45 kV; current, 35 mA; total power, 1.575 kW; measured with a FTIR Perkin-Elmer mod. 1725X instrument energy interval, 16.0–38.0 keV; h values, 26.0, 21.0, 15.5, 10.5, equipped with a KBr beam splitter and a TGS detector, in the 8.0, 5.0, 3.5, 3.0, 2.0, 1.5, 1.0, 0.5 and 0.4°; scattering parameter 4000–400 cm-1 range, using CaF2 windows for liquid samples range (q), 0.16–15.64 A° -1.Normalization to a stoichiometric and KBr or CaF2 plates for Nujol mulls. DiVusion reflectance unit volume containing one palladium atom was performed. infrared Fourier transform (DRIFT) spectra were measured The static structure function i(q) was obtained from the with the same spectrophotometer by mixing the sample with observed intensity I (E, h), and expressed as qi(q)M(q), where dry KBr under an inert atmosphere and by rapid transfer to M(q) is a sharpening factor for a given atom (silicon in this the cell (Spectra Tech).case), defined (c=0.01) as for eqn. (1). The cross polarization magic angle spinning (CP MAS) 13C M(q)=fSi2(0)/fSi2(q) exp(-cq2) (1) NMR spectra were measured at room temperature with a MSL 200 Brucker instrument operating at 50.321 MHz, The Fourier transformation of the experimental static structure chemical shifts being referred to external TMS.functions gives the radial distribution function D(r): XPS spectra were measured with a Perkin-Elmer PHI 5500 ESCA spectrometer equipped with a monochromatic X-ray D(r)=4pr2r0+ 2r p Pqmax qmin qi(q)M(q)sin(qr) dq (2) source and an aluminium anode (Al-Ka radiation, hn= 1483.6 eV), the source being maintained at 14 kV, with a In this equation, r0 is the average electronic density of the power of 200 W.Powdered samples pressed on clean indium sample [r0=.i nifi(0))2V-1], V is the stoichiometric unit foils were used and the diameter of the analyzed sample area volume, ni is the number of atoms i per unit volume and fi is was ca. 400 mm while the background pressure in the analysis the scattering factor for atom i. chamber was 10-8 Pa. For each sample, a preliminary general The EDXD measurements were carried out on both the analysis was performed, in order to detect the presence of parent silica and the palladium-containing silica. The static possible contaminants; the relevant photoemission peaks structure functions are similar, see Fig. 1, thus showing that (Pd 3d , O 1s, C 1s, Si 2p) were recorded under high resolution the presence of palladium(II) does not appreciably modify the conditions. From the photoemission peak intensity, the surface structure of the matrix. atomic concentrations were estimated, using the elemental This experimental observation made the application of the sensitivity factors method.8 Electrostatic charge was attenuated diVerence or isomorphous substitution method possible.12 In by using a low-energy flow electron gun: generally, peaks free the diVerence curve of the radial distribution functions from the typical deformations due to this phenomenon were (SiO2/PdII-SiO2), which contains the contributions due to obtained.the implanted atoms only, the area of each peak is proportional Transmission electron microscopy (TEM) images were to the number of scattering atoms and to their scattering obtained with a JEOL TEM 2010 instrument operating at factors. 200 kV. The material was ground in a mortar until a very fine The coordination environment of palladium was established powder was obtained, which was deposited on a lacy carbon by a curve fitting procedure of the experimental diVerence film supported on a standard copper grid.To avoid deteriorradial distribution function. The experimental static structure ation or contamination, the time required for sample prepfunction i(q) can be interpreted as a weighted sum of partial aration, carried out under dinitrogen, was reduced to a structure functions due to pairs of interacting atoms, by using minimum (ca. 10 min). Bright field images were used in order the Debye function i(q)13 [eqn. (3)] and by adjusting the sij to obtain the size distribution of the palladium particles. and the rij parameters, sij being the rms variation of the Wide-angle X-ray spectroscopy (WAXS) and small angle interatomic distance rij [starting parameters are those obtained X-ray spectroscopy (SAXS) data were collected with a nonin the parent compound4 trans-Pd(O2CNEt2)2(Et2NH)2, commercial energy scanning diVractometer9a equipped with an namely Pd–O 2.022(3), Pd–N 2.058(3) A° .For the non-bond- X-ray generator (water-cooled, tungsten target, 3.0 kW maxiing Pd,Si distance, reference has been made to several mum power), a germanium solid-state detector (SSD) connecmolecular complexes of transition metal cations with alkyl- ted to a multichannel analyzer by means of an electronic and aryl-silanolato ligands which have recently appeared in chain, a collimator system, step motors and sample holder.the literature,14 in addition to the structural data of an iron(III ) The X-ray tube and the detector can rotate in the vertical plane around a common centre in order to reach the appropriate 2h scattering angle.A schematic drawing of the diVactometer has been published earlier.9b WAXS, as applied to liquid10 and amorphous11 systems, allows the static structure function i(q) to be derived, the scattering parameter being q=(4p/l) sinh; 2h is the scattering angle, and l is the radiation wavelength.Since q depends on both E and h, an angular scanning with a monochromatic Xray radiation (ADXD technique) or an energetic scanning with a white X-ray beam at a fixed value of h (EDXD technique) can be performed. In the present case, the latter procedure was used. The EDXD technique presents several advantages, namely: (a) the time of measurement is strongly reduced at approximately constant statistical accuracy; (b) measurements are independent of the intensity fluctuation of the primary beam; (c) the instrument is static during the measurement, which simplifies the instrumental geometry and reduces the errors due to misalignment.On the other hand, Fig. 1 Structure function qi(q)M(q) (e.u. A° -1) vs.q (A° -1) of SiO2-160 (,) and palladium-containing silica (———). energy-dependent phenomena have to be considered, such as 2856 J. Mater. Chem., 1998, 8, 2855–2861silicate14a]. i(q)=.fi(q)fj(q) sin(qrij ) qrij exp(-1/2sij2q2) (3) The palladium containing silica samples were reduced both thermally and chemically (with dihydrogen), vide infra, and were subjected to both WAXS and SAXS measurements. In the latter case, the intensity of the scattered X-ray radiation I(E, h), with h<1°, is related to the size and shape of the scattering centres, as for the Guinier law expressed by eqn.(4),15 where Rg is the gyration radius of the scattering particle, which depends on both its shape and size. The gyration radius can be determined from the slope of the plot of lnI(q) vs.q2. Fig. 2 13C CP MAS NMR spectrum of palladium(II ) supported on SiO2 resulting from the reaction of trans-Pd(O2CNEt2)2(NHEt2)2 I(q)=I(0) exp (-1/3Rg2q2) (4) with the silanol groups (sample AT-323). Irradiating field, 50 kHz; spinning rate, 5 kHz; contact time, 5 ms; sequence recycle time, 4 s; For a given shape of the particle, the gyration radius is related number of transients, 16 640; spectral width, 20 kHz; time domain to the particle size by simple equations [for a cube, Rg2=l2/4; points, 2048; chemical shifts are referred to SiMe4.for a sphere, Rg2=(3/5) R2]. For the small-angle X-ray scattering (SAXS) measurements, the scattering angle h was 0.3, 0.4, and 0.5°, with a slit width sponded to a CO2/Pd molar ratio of 0.8. Addition of excess of about 60 mm in order to reduce the X-ray angular diver- acetic acid caused the evolution of 1.2 mol of carbon dioxide gence.A blank measurement showed that no intensity was per palladium. In another experiment (AT-203), the suspension monitored by the detector. after the reaction with the silica was filtered under carbon dioxide and the filtrate was treated with excess acetic acid: no Chemical implantation carbon dioxide was evolved.Palladium implantation on silica was carried out by the Reduction of palladium following procedure. Silica SiO2-160 (DB-14–214, 5.6 g, corresponding to 15.7 mmol of silanol groups) was added to a With dihydrogen. In a preparative experiment, the palladiumtoluene (100 cm3) solution of trans-Pd(O2CNEt2)2(NHEt2)2 containing silica was reduced at room temperature under (1.01 g, 2.08 mmol; OH/Pd molar ratio, 7.5) in a 500 cm3 flask dihydrogen at atmospheric pressure in benzene (AT-166, and the mixture was stirred at room temperature for 2 h, Found: C, 4.2; H, 1.0; N, 0.6; Pd, 2.2%), see Table 2.WAXS occasionally reducing the partial pressure of carbon dioxide and SAXS measurements were carried out on this sample.released in the process. After being recovered by filtration, the Reduction was also achieved by treating silica-supported palyellow palladium-containing silica was dried in vacuo for 20 h ladium(II) with dihydrogen at room temperature for 6 d, in (5.75 g) and the following analytical results were obtained: the absence of any solvent. AT-19, C, 5.5; H, 1.3; N, 1.5; Pd, 4.0; CO, 1.5%, corresponding In a gas volumetric experiment, the palladium-containing to a substantially quantitative yield (see Table 1) of the silica (AT-323; 0.92 g; Pd, 3.4%; 0.29 mmol of palladium) was implantation reaction and to the following molar ratios: added to cyclohexane (25 cm3) presaturated with dihydrogen CO2/Pd, 0.9; N/Pd, 2.8.The 13C CP MAS NMR spectrum and the volume of dihydrogen was measured at constant showed resonances (d, ppm from SiMe4) at 12.0 (CH3), 41.1 temperature (24.0±0.1 °C) and pressure (1 atm).The orig- (CH2), and 164.0 (O2C ), see Fig. 2. The 13C NMR spectrum inally yellow silica became brown and finally black after the of the parent compound trans-Pd(O2CNEt2)2(NHEt2)2 completion of the gas absorption (0.43 mmol, corresponding has bands ([2H8]toluene, d, ppm from SiMe4) at 14.1 to a H2/Pd molar ratio of 1.5).In a blank experiment (AT- [NH(CH2CH3)2], 14.5 [O2CN(CH2CH3)2], 41.6 [NH(CH2CH3)2], 395) carried out with SiO2-160, noH2 was found to be absorbed 46.0 [O2CN(CH2CH3)2], 164.9 (O2C). The DRIFT spectrum under the same experimental conditions. showed bands at 3259, 2978, 2939, 2882, 1598, 1549, 1483, 1463, 1426, 1382 and 1303 cm-1.The parent compound has Thermal reduction. A sample of the palladium-containing IR bands (PCTFE mull ) at 3060, 2985, 2935, 2870, 1590, silica (AT-323; 2.1 g; Pd, 3.4%; 0.67 mmol of palladium) was 1555, 1475, 1455, 1440, 1410, 1375 and 1325 cm-1. For the heated at 100 °C in vacuo (3 mmHg) for ca. 7 h; the brown XPS data and other details on the implantation reactions, product was stored under dinitrogen (AT-381, Found: C, 3.6; see Table 1.H, 0.9; N, 0.7%). The implantation reaction was monitored gas volu- Another sample (AT-323; 1.7 g; Pd, 3.4%, corresponding to metrically, using a large excess of silica. A toluene (25 cm3) 0.54 mmol of palladium) was heated at 200 °C in vacuo suspension of SiO2-160 (5.4 g, 15.1 mmol of silanol groups), (5×10-2 mmHg) for 6 h; at the end of the treatment the presaturated with carbon dioxide, was treated with the palladium( II) complex (0.20 g, 0.41 mmol, for a OH/Pd molar Table 2 Reduction of palladium ratio of 37) at 20±0.1 °C: the evolved carbon dioxide corre- Reduction XPSc TEMd Sample Precursor methoda,b Eb/eV d/nm Table 1 Implantation of palladium(II) on silica AT-166e AT-90 a¾ 335.3 n.d.DB-16–9 AT-323 a n.d. 6–8 Molar ratio Sample OH/Pd Yield (%) Pd (%) XPSaEb/eV AT-381e AT-323 b¾ 335.7 3–6 AT-408e AT-323 b 335.5 2–5 AT-19 7.5 quant. 4.0 336.3 aReduction with H2 at room temperature: a¾, in benzene; a, without AT-323b 6.9 83 3.4 336.3 solvent. bThermal reduction: b¾, T=100 °C; b, T=200 °C. cPd 3d5/2 AT-355 20.5 88 1.8 336.1 binding energy (eV) (±0.2 eV); for reference Eb values see Table 1, footnote a.dPalladium particle, mean diameter. eSamples used for aPd 3d5/2 binding energy. Reference data (eV ) (±0.2 eV): Pd(s), 335.1; PdO, 336.1.8 bSample used for WAXS experiments. WAXS experiments. J. Mater. Chem., 1998, 8, 2855–2861 2857Fig. 4 WAXS data: radial distribution function D(r) (×10-3 e2 A° -1) Fig. 3 Catalytic hydrogenation of cyclohexene. Moles of dihydrogen absorbed as a function of time ($). Solvent, cyclohexane; temperature, vs. r for SiO2-160 (,) and palladium-containing silica (———), AT-323. 24.1±0.1 °C; cyclohexene, 2.3 mmol. sample became brown (AT-408, Found: C, 0.6; H, 0.0; N, The gas volumetric data, see Experimental section, show that 0.0%). For further data concerning the reduced samples, at least 80% of palladium is chemically bonded to the silica see Table 2.surface. The XPS binding energies (336.1–336.3 eV) are to be compared with the value of 336.1 for PdO.8a,b Moreover, the Catalysis 13C CP MAS NMR data, see Fig. 2, show the presence of Palladium-containing silica (AT-408, 0.23 g; Pd, 3.4%; residual carbamato groups (resonances around 160 ppm), thus 0.073 mmol of palladium), thermally pre-treated at 200 °C was confirming that the silica-bonded palladium still maintains suspended in cyclohexane (25 cm3); the suspension was satu- part of the original coordination environment.As far as other rated with dihydrogen at 24.1±0.1 °C for 3 h and then added resonances are concerned (amine or carbamato ethyl groups), of cyclohexene (0.25 cm3, 2.4 mmol) under vigorous magnetic the intrinsic low resolution of the solid-state NMR spectrum stirring. The olefin hydrogenation was independent of the does not allow any specific assignment to be made.The IR cyclohexene concentration and substantially complete in ca. reflectance spectra show bands in the 1600–1300 cm-1 region, 5 min, corresponding to a H2/Pd molar ratio of 33.The plot which closely resemble those of the parent palladium(II) of dihydrogen absorption as a function of time is shown in compound. Fig. 3. A catalytic activity of 70 mmol of H2 min-1 gPd-1 was WAXS measurements allowed the radial distribution estimated from the plot of Fig. 3; this corresponds to an function, see Fig. 4, to be calculated, in comparison with the apparent turnover frequency of 0.12, expressed as mols of data for the parent silica.dihydrogen absorbed per mol of palladium per second. The diVerence radial distribution function was calculated A commercially available sample of palladium supported (see Experimental section) and a curve fitting procedure of on a silico-aluminate matrix (AT-416, 0.11 g; Pd, 2%; this function was carried out by Fourier transformation of the 0.021 mmol of palladium) suspended in cyclohexane (25 cm3) intensities obtained by the Debye formula of eqn.(3), using and cyclohexene added (0.3 cm3, 2.9 mmol) absorbed the the same M(q) and qmax as for the experimental data. Starting expected amount of dihydrogen for complete hydrogenation bonding and non-bonding parameters are from the literain about 6 min at 24.2±0.1 °C (for a H2/Pd molar ratio of ture:4,14 particularly relevant in this connection are the results 138).The rate of cyclohexene hydrogenation was substantially of a crystallographic study14f on a silanolato complex of independent of olefin concentration: a catalytic activity of palladium(I) showing Pd,Si non bonding distances of 2.981 250 mmol of H2 min-1 gPd-1 was estimated, corresponding to and 3.520 A° .The Debye–Waller factors (sij, A° ) are considered an apparent turnover frequency (as defined above) of 0.44 s-1. as parameters in the curve fitting procedure (sij=0.04 A° for Samples of silica-supported palladium, prereduced with rij1.50 A° ; sij=0.08 A° for 1.50<rij2.20 A° ; sij=0.13 A° for dihydrogen at room temperature or thermally at 100 °C were 2.20<rij3.50 A° ; sij=0.20 A° for rij>3.50 A° ).The experfound to catalyze cyclohexene hydrogenation, the gas absorp- imental and calculated diVerence radial distribution functions tion being dependent on olefin concentration (initial rates, are shown in Fig. 5; in the model, see Fig. 6, which gave the respectively: 2.9 and 22 mmol of H2 min-1 gPd-1).best fit of the experimental curve, each palladium atom is coordinated to one silanolato and two diethylamine ligands, Results and discussion and to a residual carbamato group as well. Implantation Reduction The N,N-diethylcarbamato complex of palladium(II), trans- The reduction of the silica-supported palladium was carried Pd(O2CNEt2)2(NHEt2)2, reacts with the silanol groups of out both with dihydrogen at room temperature or thermally partially dehydroxylated silica in toluene at room temperature.(at 100 or 200 °C) under reduced pressure. On the basis of the experimentally determined volume of carbon dioxide evolved in the reaction, see eqn. (5), the Reduction with dihydrogen. The reduction process was implantation was found to involve the evolution of about one followed gas volumetrically and found to require 1.4 mol of mol of carbon dioxide per palladium.H2 per palladium. Eqn. (6) represents the idealized reduction process for a palladium(II ) centre containing both carbamato and silanolato groups. The most eYcient reduction method, trans-Pd(O2CNEt2)2(NHEt2)2+n :Si–OHA A(:Si–O)nPd(O2CNEt2)2-n(NHEt2)2+n CO2+n NHEt2 (n1) as judged from the XPS binding energies approaching that of palladium bulk, is the treatment with dihydrogen, see Table 2.(5) 2858 J. Mater. Chem., 1998, 8, 2855–2861Fig. 7 WAXS data. Radial distribution functions D(r) (×10-3) for Fig. 5 WAXS data. Experimental (——) and calculated (,) diVerence palladium containing silica after reduction with dihydrogen at room radial distributions of silica-coordinated palladium (sample AT-323).temperature (———), sample AT-166 and for SiO2-160 (,). D(r) values (×10-3) were calculated by using the Debye formula and the interatomic parameters specified in text. 166 and the silica support were obtained, see Fig. 8. These data show that reduced palladium gives metallic crystallites, with some long-range ordered structure.Fig. 8 shows peaks at ca. 2.7, 3.9 and 4.8 A° , corresponding, respectively, to the palladium–palladium distances of the twelve nearest- (2.751 A° ), the six second-nearest- (3.891 A° ), and the twentyfour third-nearest (4.765 A° ) neighbours of the fcc crystal lattice of bulk palladium17 [a=3.8900(7) A° ], the twelve fourthnearest neighbours being at 5.502 A° .An approximate evaluation of the mean dimension of the metal particles was carried out by SAXS measurements: a value of 19±1 A° was obtained for the gyration radius, corresponding to a radius of 24±2 A° for a spherical shape and to an edge of 38±2 A° for cubic shape. The theoretical structure function qi(q)M(q) was calculated for metal particles containing 2457 palladium atoms (d= 2.750 A° ), corresponding to cubic particles with l=31 A° , the Debye–Waller factors being s=0.15 A° for 0d5.60 A° and s=0.20 A° for d>5.60 A° .The curve of the structure function superimposed on the experimental one is shown in Fig. 9. Fig. 6 Suggested model of the coordination shell of silica-bonded For the palladium-containing samples subjected to thermal palladium(II), see text; bond distances (A° ): Pd–N(1) 2.06(8), Pd–N(2) treatment under reduced pressure (at 100 or 200 °C), reduction 2.06(8), Pd–O(1) 2.02(8), Pd–O(2) 2.02(8), the Pd,Si non-bonding to palladium(0) was anticipated by the darkening of the distance is 3.08(0.13) A° .substance and by the slight decrease of the XPS binding energy values with respect to PdO (336.1 eV).On the other hand, the WAXS structure functions for both samples, see Fig. 10, are :Si–O–Pd(O2CNEt2) (NHEt2)2+H2A CO2+3 NHEt2+:Si–OH+Pd (6) similar to that of the unreduced substance, thus showing that only partial reduction had occurred (the analytical nitrogen The excess of dihydrogen absorbed with respect to the content decreasing with increasing temperature is suggestive stoichiometry of reaction (6) is presumably used to form Pd–H bonds (palladium is reported 16 to react with dihydrogen forming a so-called b phase characterized by a H/Pd molar ratio of 0.6 at room temperature, with a dihydrogen equilibrium pressure of ca. 10 mmHg; other palladium hydride phases are known16c–g) or to reduce part of carbon dioxide formed in the process. The palladium-containing silica was also treated thermally under reduced pressure; the sample showed only partial reduction to palladium(0), as suggested by the XPS data, with an Eb (335.7 eV for the sample thermally treated at 100 °C and 335.5 eV for that treated at 200 °C), slightly but signifi- cantly higher than those pertaining to bulk palladium (335.1 eV) and to the sample reduced with dihydrogen at room temperature (335.3 eV).The WAXS and SAXS data of the samples reduced with dihydrogen will be discussed first and then compared with those obtained by thermal reduction. The radial distribution functions for the reduced palladiumcontaining silica and for the parent silica are shown in Fig. 7. Fig. 8 WAXS data. Experimental DiV(r) (×10-3) of AT-166 after After correction for the contribution by the average bulk reduction with dihydrogen at room temperature.The silica contribution was subtracted. electron density (4pr2r0), the DiV(r) functions of sample ATJ. Mater. Chem., 1998, 8, 2855–2861 2859Fig. 9 Calculated structure function qi(q)M(q) vs. q for palladium metal particles (clusters of 2457 atoms, l=31 A° ) with face-centred cubic unit cell (———) compared with the experimental curve (,), AT-166.Fig. 11 TEM image (1.7 nm cm-1; ×6 000 000) of one of the palladium particles; sample DB-16.1, obtained by thermal reduction at 200 °C under reduced pressure. the TEM image of the thermally (200 °C) treated sample, see Fig. 11; the picture shows two diVerent orientations of the atomic planes within the same particle.Catalytic hydrogenations The highest catalytic activity was observed with the palladium catalyst thermally pretreated under reduced pressure at 200 °C, see Experimental section and Fig. 3. In this case, the plot of the dihydrogen absorbed vs. time shows a zero-order dependence with respect to olefin concentration (under the conditions of the experiment, the dihydrogen concentration is constant).Heterogeneous catalytic hydrogenations of unsaturated substrates have been extensively studied.18 Rates have frequently been found to be zero-order with respect to olefin concen- Fig. 10 Structure functions qi(q)M(q) (e.u. A° -1) vs. q (A° -1) of tration, and this has been attributed to the dissociative adsorp- SiO2-160 (,) and palladium-containing silica (———): (A) after tion of dihydrogen being rate determining.19 On the other thermal treatment at 100 °C (AT-381); (B) after thermal treatment at hand, poisoning may lead to a decreased adsorption rate of 200 °C (AT-408).These curves should be compared with those shown the olefin by the catalyst, thus possibly leading the overall rate in Fig. 1. to become dependent on substrate concentration. Some of the catalysts prepared in the course of this study retain secondary of palladium(II) persisting in the thermally treated samples at amine, after thermal (100 °C under reduced pressure) or chemilower temperature).cal (by dihydrogen at room temperature) activation. The Reduction to palladium(0) in all three cases (room thermal treatment at 200 °C under reduced pressure shows no temperature with dihydrogen, 100 or 200 °C) has been con- residual amine and the rate of hydrogenation is independent firmed by transmission electron microscopy (TEM) measure- of substrate concentration, in agreement with similar findings ments which have evidenced the formation of metallic particles. for other supported palladium catalysts in hydrogenation The particle size appears to increase as the temperature of the reactions.19b,20 Although the time required for completing the reduction decreases, the larger distribution being comprised reaction is ca. 5 min, a diVusion-controlled process is unlikely between 6 and 8 nm for the sample reduced with dihydrogen because of the very eYcient stirring. Similar observations were at room temperature. The apparent disagreement between the made for the silica-supported catalysts based on platinum.1b SAXS and TEM measurements concerning the particle size may be reconciled by considering that TEM measurements Conclusions may overestimate the particle size due to the fact that the contribution by the smaller sizes may become almost negligible.This paper has shown that a palladium silicate can be prepared on a partially hydroxylated amorphous silica by protonation An enlargement of one of the palladium particles is shown in 2860 J.Mater. Chem., 1998, 8, 2855–286146, 197; (c) Y. Takasu, R. Unwin, B. Tesche, A. M. Bradshaw and (the silanol groups are the reactive sites on the surface) of a M. Grunze, Surf. Sci., 1978, 77, 219; (d) G. L. Haller and hydrocarbon-soluble N,N-dialkylcarbamato derivative of pal- D.E. Resasco, Adv. Catal., 1989, 36, 173. ladium(II). The implantation reaction depends on the OH/Pd 9 (a) R. Caminiti, C. Sadun, V. Rossi, F. Cilloco and R. Felici, 25th molar ratio used; in the present case, when such a ratio is &7, Italian Congress of Physical Chemistry, Cagliari, Italy, June 17–21, the implantation reaction is substantially quantitative.This 1991; Ital.Pat., 01261484, June 23rd 1993; (b) M. Carbone, R. Caminiti and C. Sadun, J. Mater. Chem., 1996, 6, 1709. third paper of the series shows that a readily available molecu- 10 (a) R. Caminiti, R. Cucca and T. Radnai, J. Phys. Chem., 1984, lar compound of palladium(II) can be used to carry out the 88, 2382; (b) G. Paschina, G. Piccaluga and M.Magini, J. Chem. chemical implantation on the silica surface, with a presumably Phys., 1984, 81, 6201; (c) R. Caminiti, D. Atzei, P. Cucca, uniform distribution of the palladium(II ) centres. As the A. Anedda and G. Bongiovanni, J. Phys. Chem., 1986, 90, 238; palladium(II) precursor is easily prepared from the cationic (d) R. Caminiti, C. Sadun, M. Basanisi and M. Carbone, J. Mol.acetonitrile complex of palladium(II) [Pd(MeCN)4]2+ through Liq., 1996, 20, 55. 11 (a) R. Caminiti, C. Munoz Roca, D. Beltran-Porter and the oxidation of palladium metal by NO+, this paper discloses A. Z. Rossi, Z. Naturforsch., Teil A, 1988, 43, 591; (b) A. Musinu, a facile method of ultimately obtaining palladium particles on G. Piccaluga and G. Pinna, J. Non-Cryst. Solids, 1990, 122, 52; silica based on a simple and selective chemical methodology (c) A.Capobianchi, A. M. Paoletti, G. Pennesi, G. Rossi, in the preliminary step. It is easy to predict that this method- R. Caminiti and C. Ercolani, Inorg. Chem., 1994, 33, 4635; (d) ology can be extended to implant palladium(II) and thus to D. Atzei, R. Caminiti, C. Sadun, R. Bucci and A. Corrias, produce palladium particles on other inorganic, partially Phosphorus Sulfur Silicon, 1993, 79, 13. 12 (a) W. Bol, G. J. H. Gerrit and C. van Panthaleon, J. 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ISSN:0959-9428    

DOI:10.1039/a804456b

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials Gas adsorption properties of mesoporous c-alumina prepared by a selective leaching method Kiyoshi Okada,* Takahiro Tomita and Atsuo Yasumori Department of Inorganic Materials, Tokyo Institute of Technology, O-okayama, Meguro, Tokyo 152-8552, Japan. E-mail: kokada@o.cc.titech.ac.jp Received 7th August 1998, Accepted 27th September 1998 Gas adsorption properties of mesoporous c-Al2O3 prepared by the selective leaching method were investigated using various adsorbate gases.The mesoporous c-Al2O3 was prepared by calcining kaolinite to form a microtexture of fine c-Al2O3 grains dispersed in a matrix of amorphous SiO2 followed by selective leaching of amorphous SiO2 from the microtexture. The mesopores were formed inside the pseudomorphic hexagonal platy particles of kaolinite.The specific surface area, total pore volume and pore size measured at 77 K using N2 gas were ca. 240 m2 g-1, 0.7 ml g-1 and 6 nm, respectively. The gas adsorption isotherms of polar molecules such as water, methanol and butan-1-ol showed type IV isotherms (IUPAC classification) while those of non-polar molecules such as cyclohexane showed type V isotherms.The c-Al2O3 was therefore recognized as having hydrophilic surface characteristics. The onset of adsorption of these gases in the low relative pressure (P/P0) region was methanol>water>butan-1-ol>cyclohexane, this order corresponding to the aYnity between the adsorbate gas and surface of the c-Al2O3 and also reflecting the steric eVect of adsorbate gas.The fractal dimension obtained from the BET monolayer adsorption capacity plot was 2.1, indicating a smooth surface. The gas adsorption properties of the c-Al2O3 are discussed and compared with commercial c-Al2O3 and Al2O3–SiO2 gel. H2O=13.95 mass%. A small amount of TiO2 impurity was Introduction present as anatase and rutile in the sample. The starting Since c-Al2O3 has a number of superior properties as a porous material was calcined at 950 °C for 24 h using heating and material, it has been widely used in industry as a catalyst, cooling rates of 15 and 20 °Cmin-1, respectively.The calcined catalyst support and adsorbent. In order to improve the porous material (3 g) was dispersed in 250 ml of aqueous 4 M KOH properties of c-Al2O3, many preparation methods1–5 have been at 90 °C and stirred for 1 h.After leaching, the sample was reported which achieve enhancement of the properties by washed with 0.5 M KOH and then washed a further three controlling the primary particle size. Various methods have times with deionized water to remove dissolved ions. The also been used to control the microtexture of c-Al2O3 aggre- suspension was then centrifuged and the resulting powder gates.We have reported a new preparation method of c-Al2O3 dried at 110 °C overnight. For comparison, commercially from the clay mineral kaolin [Al2(OH)4Si2O5] by a selective available high purity c-Al2O3 (AKP-G015, Sumitomo leaching method,6 in which kaolin is calcined at ca. 1000 °C Chemicals) and Al2O3–SiO2 gel (Neobead DS-5, Mizusawa and converted into a mixture of c-Al2O3 and amorphous silica.Chemicals) were used as reference materials. Since the calcined kaolin retains pseudomorphic particles of The phases formed in the samples were studied by powder the original kaolin with a microstructure consisting of a X-ray diVraction (XRD) using a Rigaku Geigerflex nanocomposite of very fine c-Al2O3 grains several nm in size diVractometer with monochromated Cu-Ka radiation.The uniformly dispersed in a matrix of amorphous SiO2, meso- chemical compositions were determined by X-ray fluorescence porous c-Al2O3 can be prepared by selective leaching of the (XRF) using Rigaku RIX2000 and RIX3000 spectrometers. amorphous SiO2 from the pseudomorph particles. As a result, The microtexture of the samples was observed by field-emission the fine c-Al2O3 grains which remain in the microtexture are scanning electron microscopy (FE-SEM) using a JEOL JCMinter- connected by residual amorphous SiO2 and form meso- 890S instrument.The specific surface area was measured by pores in the pseudomorph particles. Since the c-Al2O3 prepared the BET method9 using nitrogen gas as adsorbate at -196 °C by the selective leaching method has such a unique microtex- with a Quanta Chrome Autosorb-1 instrument.The pore size ture, it retains a high surface area at high temperatures, i.e., distribution was calculated in the radius range from 1 to it has high thermal stability7 and shows unique and interesting 100 nm by the BJH method10 using the desorption isotherm. water vapor adsorption properties.8 Gas adsorption–desorption isotherms were measured at 25 °C The present paper examines the gas adsorption properties by automatic gas adsorption apparatus using a Japan Bel of c-Al2O3 prepared by the selective leaching method using Belsorp 18.The adsorbates used were deionized water, a adsorbate gases with various molecular shapes and diVerent special grade of methanol (Shuzui Kitaro Co.), butan-1-ol polarities.The results are compared with commercial c-Al2O3 (Wako Pure Chemicals) and cyclohexane (Wako Pure and Al2O3–SiO2 gel to elucidate surface properties of the Chemicals). They were pre-treated using zeolite 3A (Wako c-Al2O3. Pure Chemicals) to remove any contaminating water. The alumina samples were evacuated at 300 °C for >4 h until the pressure in the vessel dropped below 5×10-3 Torr.The dur- Experimental ation of each measurement point was set at 1000 s. The measured relative pressure (P/P0) range was 0–0.9 in the Kaolinite from Georgia, USA, was used as the starting adsorption and 0.9–0.1 in the desorption experiments except material. As listed in Table 1, the chemical composition of this for the methanol runs, in which the measurements were made starting material is close to the ideal composition of kaolinite [2SiO2·Al2O3·2H2O], i.e.SiO2=46.51, Al2O3=39.54 and only to P/P0=0.7 because of limitations of the apparatus. J. Mater. Chem., 1998, 8, 2863–2867 2863Table 1 Chemical composition and porous properties of samples Chemical composition (mass%) Sample Al2O3 SiO2 TiO2 K2O Fe2 O3 Surface area/m2 g-1 Pore volume/ml g-1 Original kaolinite 38.3 45.2 1.44 0.11 0.76 9.9 0.09 Selectively leached c-Al2O3 84.0 6.8 5.5 1.4 1.0 244 0.72 c-Al2O3 (AKP-G015) 100 0 0 0 0 139 1.22 Al2O3–SiO2 gel (DS-5) 70.5 24.5 0 0 0 313 0.51 Results and discussion Characterization of the samples As reported in previous papers,5–7 the calcined and KOH leached sample has mesopores of uniform pore size of ca. 3 nm radius. The porous properties and chemical composition of the sample prepared in the present study are listed in Table 1 and are in good agreement with those reported elsewhere. 5–7 Fig. 1 shows the pore size of this sample lies in a narrow distribution around 3 nm radius. The data for the reference materials of commercially available c-Al2O3 and Al2O3–SiO2 gel are also given in Table 1 and Fig. 1, and show diVerent pore sizes from the present c-Al2O3. The pore size of the reference c-Al2O3 (AKP-G015) is larger than those of the other two samples and the pore size distribution is relatively broad. The pore size of the Al2O3–SiO2 gel (DS-5) is smaller than those of the other samples, but with a rather broad bimodal size distribution consisting of a relatively sharp peak at ca. 1.9 nm radius and very broad peak at ca. 3–4 nm. The order of specific surface areas of these samples was AKPG015< present c-Al2O3<DS-5, this order being correlated with the pore size. On the other hand, the total pore volumes of these samples showed the opposite trend with specific surface area. The XRD pattern of the calcined sample showed broad peaks assigned to c-Al2O3 and a halo arising from amorphous SiO2 which disappeared after leaching.Corresponding to this XRD pattern change, the chemical composition revealed a reduction of SiO2 upon leaching (Table 1). The microtextures of the original and leached samples were observed by FESEM (Fig. 2). Photographs show that the sample preserves the hexagonal platy particle shape of the original kaolinite even after calcination and leaching, but with apparent changes in the surface of pseudomorphic particles.Very fine grains of c-Al2O3 were observed on the surface of the particles, shown Fig. 2 FE-SEM photographs of the selectively leached c-Al2O3: (a) original sample, (b) leached sample and (c) leached sample with high magnification.in Fig. 2(b) and (c) to be several nm in size. The spaces between the fine c-Al2O3 grains formed by selective leaching of amorphous SiO2 from the matrix correspond to the meso- Fig. 1 Pore size distributions of the present c-Al2O3 prepared by pores in this sample. The fine c-Al2O3 grains are of uniform selective leaching, commercial c-Al2O3 (AKP-G015) and Al2O3–SiO2 gel (DS-5).size and are distributed uniformly in the pseudomorphic 2864 J. Mater. Chem., 1998, 8, 2863–2867bonds with surface OH groups of the c-Al2O3. As depicted schematically in Fig. 4, the first layer of adsorbed water vapor is also able to form hydrogen bonds with further water vapor molecules to form second and third adsorbed layers, successively leading to multi-layer adsorption with increasing P/P0.On the other hand, alcohol molecules adsorb on the surface of the c-Al2O3 by forming hydrogen bonds between their OH groups and the surface OH groups of the c-Al2O3. In the case of methanol, this adsorption occurs steeply and starts from very low P/P0 because the aYnity of methanol for c-Al2O3 is strong and the steric hindrance of the methanol molecule is small.The isotherm of methanol starts at lower P/P0 than that of water vapor, suggesting that the adsorption energy of methanol is higher than that of water vapor. On the other hand, the isotherm of butan-1-ol shows an induction region Fig. 3 Adsorption isotherms of various adsorbates on the selectively of adsorption at very low P/P0 but with increasing P/P0 it leached c-Al2O3. increases steeply, behaving similarly to methanol above a certain P/P0. This induction region is considered to reflect the particles, yielding uniformly sized mesopores with a narrow steric hindrance of butan-1-ol, which has a molecular length pore size distribution.The 29Si MAS NMR spectra of this (0.8–0.9 nm) estimated to be about twice that of methanol. sample showed the Q4 structure unit of SiO4 tetrahedra, as The adsorption of the first layer of alcohol molecules occurs well as a framework structure and a Q0 structure unit rep- in such a way that the CH3 groups face away from the surface resenting SiO4 tetrahedra incorporated in the c-Al2O3.11 Since as depicted in Fig. 4. This changes the surface property from amorphous SiO2, which has the Q4 structure, is present as the hydrophilic to hydrophobic after adsorption of the first layer, matrix of the microtexture of the calcined sample and the explaining the plateau in the alcohol isotherm.Since the steric pseudomorphic particle shape is retained after the leaching hindrance of butan-1-ol is larger than that of methanol, the treatment, these fine c-Al2O3 grains are thought to be plateau in the isotherm of butan-1-ol is flatter than for connected by residual amorphous SiO2.methanol. In contrast to the hydrogen bond adsorption mechanism in water vapor and alcohols, the adsorption of cyclohex- Gas adsorption properties of the samples ane is considered to occur by physical adsorption, explaining the linear correlation with P/P0. Fig. 3 shows gas adsorption isotherms (0<P/P00.3) of the Generally, the surface roughness of porous materials is present c-Al2O3 for the polar adsorbates water, methanol, evaluated in terms of its fractal dimension.Pfeifer and Avnir12 butan-1-ol and the non-polar molecule cyclohexane. The give the following formula to calculate the fractal dimension adsorption isotherms of polar and non-polar molecules are of porous materials from adsorption data for diVerent diVerent; all the polar molecules show convex isotherms indimolecular size adsorbates cating the strong aYnity of these molecules for the surface of the c-Al2O3, i.e.hydrophilic behavior. On the other hand, the log(vm)=-(D/2) log(am)+C adsorption of cyclohexane shows an almost linear relationship with P/P0, thus, its aYnity for c-Al2O3 is weak.DiVerences where, vm, D, am are the BET monolayer adsorption capacity, surface fractal dimension and cross-sectional area of the can also be seen in the isotherms of the three polar molecules. The isotherms of the two alcohols showed a steep increase of adsorbed molecules, and C is a constant. Fig. 5 shows a plot of log(vm) vs. log(am) for the present c-Al2O3 prepared by the adsorption in the low P/P0 range, reaching a plateau in the medium P/P0 range, however the P/P0 value at which this selective leaching method and for commercial c-Al2O3 (AKPG015). Both data sets show a good linear correlation, from increase commenced was much lower for methanol than for butan-1-ol.By contrast, the water vapor adsorption isotherm which the fractal dimensions were calculated from the slopes as 2.1 and 2.2 for the present c-Al2O3 and commercial c- was a diVerent shape to those of the alcohols, increasing continuously with increasing P/P0 up to the medium P/P0 Al2O3, respectively.Since rough and smooth surface states correspond to fractal dimensions of 3 and 2, respectively, the range and without a plateau. The diVerence in the adsorption of these four diVerent molecules is shown schematically in present c-Al2O3 has a rather smooth surface.Fig. 6 shows a comparison of the adsorption–desorption Fig. 4. The hydrophilic surface of the present c-Al2O3 contains OH groups which absorb water vapor by forming hydrogen isotherms of various adsorbates on the present c-Al2O3 and Fig. 4 Schematic illustrations of adsorption mechanisms for various adsorbates.J. Mater. Chem., 1998, 8, 2863–2867 2865in the high P/P0 range correspond to the capillary condensation, the slope ratio is related to the pore size distribution. We therefore conclude that the steep change in the isotherms of the present c-Al2O3 is due to its narrow pore size distribution. The present c-Al2O3 contains a small amount of K2O contamination from the leaching treatment.This impurity may have some influence on the adsorption property. Previously it was found to show only little influence in lowering the starting value of P/P0 for the capillary condensation.8 Conclusions The gas adsorption properties of c-Al2O3 prepared by a selective leaching method were investigated using four adsorbates with diVerent molecular sizes and polarities (water, methanol, butan-1-ol and cyclohexane).The following results Fig. 5 BET monolayer adsorption capacity (vm) of various adsorbates were obtained. on the present c-Al2O3 and c-Al2O3 (AKP-G015) as a function of (1) Since the leached c-Al2O3 showed a strong aYnity for cross section (am) of adsorbate. polar molecules but a weak aYnity for non-polar molecules, the surface appears to have a hydrophilic character. the reference samples.Similar trends are observed in the (2) The convex shape of the isotherms diVered for the isotherms of all the adsorbates on corresponding samples. diVerent adsorbates in a manner which could be related to the Adsorption at low P/P0 increases in the order c-Al2O3 (AKP- adsorption mechanism. G015)<the present c-Al2O3<Al2O3–SiO2 gel (DS-5).This (3) The fractal dimension of the present c-Al2O3 was tendency is suggested to correlate with the specific surface calculated to be 2.1, corresponding to a rather smooth surface. area, pore size and also the hydrophilicity of the samples. We (4) The present c-Al2O3 showed a very steep increase and discussed the hydrophilicity of these samples in a previous decrease of the isotherms in the high P/P0 range due to paper8 and concluded that the hydrophilicity shows a tendency capillary condensation, consistent with a narrow pore size to decrease with increase of SiO2 content in the samples.On distribution in this sample. the other hand, the maximum adsorption of these samples increases in the order c-Al2O3 (AKP-G015)<Al2O3–SiO2 gel Acknowledgments (DS-5)<the present c-Al2O3.This should correspond to the sample pore volume giving rise to capillary condensation up We are grateful to Dr Katsunori Kosuge of National Institute for Resources and Environment for fruitful suggestions on to P/P0=0.9. Since the changes in the adsorption isotherms Fig. 6 Adsorption–desorption isotherms of various adsorbates in the present c-Al2O3, c-Al2O3 (AKP-G015) and Al2O3–SiO2 gel (DS-5). 2866 J.Mater. Chem., 1998, 8, 2863–28674 D. L. Trimm and A. Stainislaus, Appl. Catal., 1986, 21, 215. adsorption experiments and also grateful to Dr K.J.D. 5 R. F. Vogel, G. Marcelin and W. L. Kehl, Appl. Catal., 1984, MacKenzie of New Zealand Institute for Industrial Research 12, 237.and Development for proof reading and editing of this paper. 6 K. Okada, H. Kawashima, Y. Saito, S. Hayashi and A. Yasumori, A part of this work was supported by a Grant-in-Aid for J. Mater. Chem., 1995, 5, 1241. Scientific Research (B) (No.09450240) by the Ministry of 7 Y. Saito, T. Motohashi, S. Hayashi, A. Yasumori and K. Okada, J. Mater. Chem., 1997, 7, 1615. Education, Science, Culture and Sports, Japan and also by 8 K. Okada, Y. Saito, M. Hiroki, T. Tomita and S. Tomura, TOSTEM Foundation. J. Porous Mater., 1997, 4, 253. 9 S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc., 1938, 60, 309. References 10 E. P. Barrett, L. G. Joyner and P. P. Halenda, J. Am. Chem. Soc., 1 N. G. Papayannakos, A. M. Thomas and V. E. Kaloidas, 1951, 73, 373. Microporous Mater., 1993, 1, 413. 11 K. J. D. MacKenzie, J. S. Hartman and K. Okada, J. Am. Ceram. 2 T. Ono, Y. Oguchi and O. Togari, in Preparation of Catalysis III, Soc., 1996, 79, 2980. ed. G. Poncelet, P. Grange and P. A. Jacobs, Elsevier, Amsterdam, 12 P. Pfeifer and D. Avnir, J. Chem. Phys., 1983, 79, 3558. 1983, p. 631. 3 Y. Mizushima and M. Hori, in EUROGEL’91, ed. S. Vilminot, R. Nass and H. Schmidt, Elsevier, Amsterdam, 1992, p. 195. Paper 8/06240D J. Mater. Chem., 1998, 8, 2863–2867 2867

ISSN:0959-9428    

DOI:10.1039/a806240d

出版商:RSC    

年代:1998

数据来源: RSC

摘要:

J O U R N A L O F C H E M I S T R Y Materials The 63 K phase transition of ZrTe3: a neutron diVraction study Ram Seshadri,a Emmanuelle Suard,b Claudia Felser,a E.Wolfgang Finckh,a Antoine Maignanc and Wolfgang Tremel*a aInstitut fu�r Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universita�t, Becher Weg 24, D-55099, Mainz, Germany bInstitut Laue Langevin, Avenue des Martyrs, BP156, Grenoble F-38042, France cLaboratoire CRISMAT, ISMRA, 6, Boulevard Mare�chal Juin, Caen F-14050, France Received 13th July 1998, Accepted 9th September 1998 The 63 K phase transition of ZrTe3 has been followed through SQUID magnetisation and neutron diVraction studies.The transition is characterized by a small quenching of the magnetic susceptibility. Contrary to the expectation that the structural transition is associated with a charge density wave, the results of Rietveld refinements of high resolution neutron powder diVraction profiles indicate that below the phase transition the diVerent bonding Te–Te contacts are more equal rather than less.The results are examined with the help of band structure calculations on structures determined at three diVerent temperatures.The picture that emerges supports the view that anion–cation redox competition plays a crucial role in determining not only the structures of these compounds, but also the temperature dependence thereof. Before the realization of high-Tc superconductivity in the long-range coherence, there is considerable faulting along the layered copper oxides, considerable attention was paid to the other directions.physical properties of layered transition metal chalcogenides, Canadell, Mathey and Whangbo9 examined the electronic in particular to the occurrence of phase transitions in systems structure of type B ZrTe3 in considerable detail within the such as NbSe3 associated with the formation of charge density extended Hu�ckel approximation.They were interested particuwaves (CDW).1 Of these chalcogenides, ZrTe3 has had a larly in the diVerence between ZrTe3 and other group IV rather colourful history. First reported by McTaggart and trichalcogenides. They suggested that the unusual properties Wadsley,2 the structure was determined by Furuseth et al.3 of ZrTe3 including the transition at 63 K could be explained who found for MX3 (M=Ti, Zr, Hf and X=S, Se, Te) two only within the B type structure.These conclusions have since diVerent structure types within the P21/m space group. In been undermined in the light of Furuseth and Fjellva°g10 both structure types, the atoms occupy the 2(e) position (x, B, redetermining the structure of ZrTe3 and establishing that it z). The atom positions of the so-called types A and B structures is all or mostly type A and not type B.Recently, photoemission are related to one another through xA=1-xB and zA=zB for and thermopower measurements and detailed electronic strucall the atoms. ZrTe3 was determined as crystallizing in the ture calculations on type A ZrTe3 have been presented.11 type B structure. Through the use of so-called frozen-phonon calculations, the Room-temperature resistivity measurements on ZrTe3 sug- importance of short Te–Te distances (and metrical changes gested that it is metallic or semimetallic.2 This was confirmed thereof ) in determining the shape of the Fermi surface and by Bayliss and Liang who performed optical reflectivity the number of states near the Fermi energy have been estabmeasurements on single crystals4 and found that although the lished.11 Nearly simultaneously a redetermination of the type A material is anisotropic, the strengths of the optical transitions structure and detailed ab initio band structure calculations along the diVerent crystallographic directions are similar.have been presented by Sto�we and Wagner.12 The conclusions Takahashi et al.5 reported transport measurements on single reached by these authors largely coincide with those presented crystals as a function of temperature. They observed an in ref. 11 regarding the ro� le of the short Te–Te contacts in anomaly in the resistivity peaking around 55 K. The magnitude influencing the electronic properties of ZrTe3. of the Hall coeYcient was found to rise sharply at this In the absence of a description of the structure of ZrTe3 temperature.Their initial interpretation was that the anomaly below the transition, and in particular of an idea of what was caused by increased scattering of charge carriers rather happens to the diVerent distances between the atoms, the than a decrease in their density. Below 2 K they found that precise nature of the transition would remain a matter of ZrTe3 becomes superconducting. These authors extended their speculation.To this end, we have examined the evolution of first study, using elastic measurements to complement the the structure of ZrTe3 through the collection and analysis of transport studies.6 While they could confirm the presence of powder neutron diVraction (PND) profiles. We present the the transition at 63 K, they were unable to establish whether rather surprising results here.it arose from a depletion of charge carriers or from increased The occurrence of what seemed to be a CDW transition scattering. The nature of the superconducting state turns out and superconductivity in the same compound has prompted to be rather unusual, being described as not bulk, but filaour interest in ZrTe3.These two diVerent ground states are mentary from an analysis of the excess conductivity.7 The sometimes thought to arise from similar causes but are usually nature of the transition associated with the resistivity anomaly competing. Indeed, in ref. 11 it has been suggested that the was examined through electron diVraction and dark-field structural transition and superconductivity are related to inde- imaging at low temperatures by Wilson and coworkers,8 who pendent features of the band structure.An understanding of found that even at room temperature, some of the ZrTe3 the relations between crystal and electronic structure and crystals displayed structural modulation. Below # 63 K, they physical properties of systems such as ZrTe3 could yield found a phase transition characterized by the appearance of insights crucial to the preparation of new materials with superlattice spots at nearly q=( 1 14, 0, H ).Imaging in the satellite spots suggested that while along the a axis the structure has interesting properties. J. Mater. Chem., 1998, 8, 2869–2874 2869the calculated profiles in suitable reciprocal space directions. Experimental Asymmetry was handled using the Hermite polynomial Sample preparation approach of Be�rar and Baldinozzi.17 During the later stages of the refinement, some peaks were An approximately 15 g powder sample of ZrTe3 was prepared found to be fitted rather poorly.Through trial and error, a by heating well ground, stoichiometric mixtures of the elements small quantity of ZrTe518 was found to be present in the in a sealed, evacuated quartz glass ampoule at 973 K for three sample.This phase was therefore included in all the refinements days. Longer heating times were avoided because an analysis presented here. For the D20 diVractometer, the wavelength of the relevant Ellingham diagrams showed that the thermowas calibrated to be 2.4085 A° using a yttrium iron garnet dynamic product (in reaction with the walls of the quartz glass (YIG) standard, and for the D2B diVractometer the wave- ampoule) would be oxides of zirconium.This rather mild length was calibrated to be 1.5941 A° using an Si standard. heating protocol resulted in samples contaminated by small Cell volumes and their errors were calculated using standard amounts of ZrTe5 (determined from the PND profiles).The formulae.19 Other metrical information was obtained through samples also suVered some texturing. the use of the PLATON97 program.20 Correlations between the diVerent refined parameters were ignored in the calculations SQUID magnetisation of quantities such as interatomic distances and angles. Magnetisation data under a 1 T field were collected on a 0.1174 g sample of ZrTe3 on cooling between 100 and 5 K ire calculations a Quantum Design MPMS 5 magnetometer.The sample was The electronic structure of type A ZrTe3 has been discussed held in a gelatine capsule that was fixed to the end of a in detail in ref. 11 and is discussed only very briefly here. drinking straw. The magnetisation of the gelatine capsule and Tight-binding linear muYn-tin orbital (LMTO) band structure drinking straw were recorded separately and fitted with high methods21 were used within the atomic sphere approximation reliability to the analytic form M=A0+A1/T.This was then (ASA). The calculations were performed on the room- used to correct the data for the sample holder. temperature single crystal X-ray structure of Furuseth and Fjellva°g10 and the structures obtained from the Rietveld Neutron diVraction refinement of the D2B data at 100 and 1.7 K. 2774 irreducible Neutron data were collected on two diVerent diVractometers, k points were used to achieve convergence. D2B and D20 at the Institute Laue-Langevin, Grenoble France. D2B is a high resolution diVractometer and as we Results and discussion shall see the resolution of the powder profiles obtained on it were largely sample limited.Data on D2B were collected at Description of type A ZrTe3 1.7 and 100 K using a wavelength of 1.5941 A° . In order to Despite it having been discussed extensively in the litera- enhance the resolution a 10¾ primary beam collimation was ture,3,9,11,22 we present for completion a depiction of the employed and the monochromator beam divergence was limstructure of type A ZrTe3 in the panels of Fig. 1 focusing on ited by means of a system of slits positioned after the monothose aspects of the structure that we will find most important chromator. The uncertainties in the temperature are with respect to the phase transition. The Zr atoms are eight- throughout <0.2 K.Each data collection took about 12 h. D20 is a diVractometer of lower resolution but working with a fixed sample and detector. As a result, during an experiment (e.g. a temperature ramp) there are eVectively no systematic errors introduced. The special geometry of D20 allows rapid acquisition of diVraction profiles of very low noise and very high precision both in counts and in scattering angle.We also attempted to study the pressure dependence of the structure of ZrTe3 using a hydrostatic Ti–Zr (‘zero matrix’) pressure cell with He as the working fluid. Data were collected at fixed temperature while ramping the pressure up to 1000 bar. Due to certain experimental constraints, the data were of insuYcient quality13 for positional parameters to be extracted.The data were analyzed using the Rietveld method14 as implemented in the XND program.15 We have used a special feature of this implementation which requires some explanation. When several diVraction profiles are collected as a function of some external variable (temperature, pressure, composition etc.), two possibilities exist for their treatment.The prevailing method is to refine each data set independently, extracting structural parameters that are then assembled in order to follow a certain trend. The lesser used alternative is implemented in the XND Rietveld program, allowing the refinable parameters in the structural model to be expanded as a polynomial in the external variable. This results in a considerable reduction in the ratio of refinable parameters to data, providing better refinements from data sets of limited quality.This has been previously employed to examine the composition dependence of structure in some layered manganites.16 The profiles are handled in the reciprocal space as convolutions of Lorentzian and Gaussian functions with their individ- Fig. 1 Structure of type A-ZrTe3 showing (a) the nature of the Zr–Te ual angular dependencies.Preferred orientation (particularly polyhedra, (b) the complete structure projected down [010] with the for the data acquired on the D20 diVractometer under pressure) Te atoms marked and (c) the nature of the Te(2)–Te(3) contacts within the sheets. was treated using first order Legendre polynomials to weight 2870 J. Mater.Chem., 1998, 8, 2869–2874coordinated by Te as shown in Fig. 1(a). These polyhedra are arranged in double sheets stacked along the monoclinic c axis as displayed in Fig. 1(b), where the view is looking down [010]. The basal plane is then defined by rectangular sheets of Te(2) and Te(3) atoms with short and long distances between them as shown in Fig. 1(c) (with a view down [001]).The Te(1) are arranged in buckled sheets around z=0 and z=1. Sheets of Zr are then stacked between the Te sheets in a manner that yields the coordination depicted in Fig. 1(b). The motif of rectangular sheets, with short, alternating Te(2)–Te(3) contacts along the a direction is something that we return to in the discussion of the phase transition. Note that when presented in the manner employed here the structure of ZrTe3 is very much quasi-2D, comprising slabs that are stacked along the monoclinic c axis.We prefer this description over the traditional view that the structure is quasi-1D or chain-derived. Magnetic susceptibility Between 100 and 65 K, the SQUID susceptibility (corrected for Fig. 3 Temperature dependence of the unit cell volume of ZrTe3 as a sample holder contribution and for core diamagnetism23) dis- function of temperature.The filled circles are D20 data and the played in Fig. 2 is eVectively Curie–Weiss. Below this tempera- squares are D2B data. For the D2B data, the error bars are smaller than the symbols. The solid line is a linear fit to data above 63 K and ture, the susceptibility and its inverse display a small plateau, the curve is a quadratic fit to the data between 13 and 61 K.before once again reverting to nearly Curie–Weiss behaviour around 30 K. The phase transition can thus be described as a small quenching of the susceptibility. We postpone the interpretation of this behaviour to after we have discussed the structure. The CDW transition in ZrTe5 takes place above 100 K,24 so the changes observed here cannot be attributed to the small amount of the ZrTe5 impurity that is present.Thermal evolution of the structure We commence with a description of the evolution of the cell volume between 120 and 1.9 K. The D20 data were collected on a continuous temperature ramp from 1.9 to 200 K with data being binned into approximately 4 K intervals. The refinements yielded the cell volume evolution displayed in Fig. 3 as small filled circles with error bars. The squares at 1.7 and 100 K are from refinements of the high resolution D2B data discussed shortly. The error bars for these two data are smaller than the symbols (note that at 1.7 K, the points from D2B and D20 overlap). Between 120 and 65 K, the data can be fitted with high reliability to a straight line.The data point at 61 K deviates significantly from this evolution. Between 61 and 13 K, the volume evolves in a nearly quadratic manner as shown by the fitted curve. The fits allow the transition to be characterized by a very small volume contraction at 63 K of about 0.02%. The evolution of the individual lattice parameters with temperature is displayed in the diVerent panels of Fig. 4. We observe immediately that the clearest changes are along the a Fig. 4 Temperature dependence of the cell parameters of ZrTe3. The points with error bars are from individual refinements of D20 data and the lines are from coupled refinements of D20 data (13 data sets at a time) treated separately above and below the transition. axis. From the preceding discussion of the structure, this would implicate the Te(2)–Te(3) distances in the phase transition.The points with error bars are the values obtained from individual Rietveld refinements of D20 data. The lines are the results of refinement of two structural models against 13 data sets above the transition and (separately) against 13 data sets below the transition, expanding all refineable structural parameters to first order in the temperature.More explicitly, instead of refining some structural parameter p, the parameter is expanded in the temperature according to p=p0+p1(T ). p0 and p1 are then refined against N data sets acquired at the diVerent temperatures T. At the end of the refinements, one Fig. 2 Temperature dependence of the magnetic susceptibility and its obtains p0, p1, and the errors dp0 and dp1.The close correspon- inverse of ZrTe3 under a 1 T magnetic field. The lines are guides to the eye. dence between the points from individual refinements and lines J. Mater. Chem., 1998, 8, 2869–2874 2871obtained from the coupled refinements validates the procedure profile of ZrTe3 (with a small ZrTe5 admixture) collected at 1.7(1) K.The fit between the data and the refined model is not (and especially, the first order expansion). The necessity for this coupling is that the structural parameters thus obtained very good, due to problems of texturing in the sample, and what seems to be high dispersion in the structural coherence lengths are of greater reliability, possessing smaller error bars. Methods for estimating the error from such a coupled refine- in the sample.This is also manifested in the rather large values of the agreement factors; the Bragg and weighted profile R ment have been presented previously.16,25 In the present case they are approximately half as small as the error bars associ- factors. Indeed, refining two ZrTe3 phases with all structural parameters except the lattice parameters constrained, and ated with the parameters resulting from refining the data sets individually. Since the strategy of the coupled refinements allowing for diVerent peak widths corresponding to distinct particle sizes did result in better fits ( lower weighted-profile R yields a temperature dependence of structure (i.e.a state function rather than a state point), we can calculate the factors) but considerably increased the number of refined parameters, leading us to abandon this strategy.Nonetheless, the errors diVerent structural parameters at arbitrary temperatures within the refined state function.Doing so at the temperatures of 50 on the interatomic distances seem to be only slightly larger than those associated with the high-quality single-crystal results pre- and 80 K, representing the structure below and above the phase transition, we obtain the distances presented in Table 1.sented in ref. 10. From the refined Rietveld scale factors, quantitative analysis26 ignoring the Brindley factors27 (which are not The magnitudes of the error bars in this table are clearly too large to discern changes in interatomic distances that might very important for neutron refinements) suggested that the ZrTe5 impurity included in all the refinements presented here was as characterize the phase transition.We note however, that the long and short Te(2)–Te(3) distances are 2.99(5) and high as 15%. Table 2 lists the structural parameters of ZrTe3 obtained from the refinements at the two temperatures, and 2.88(5) A° at 50 K while they are 3.00(9) and 2.88(9) A° at 80 K.The D20 data thus do not support the traditional picture Table 3, the important interatomic distances extracted from the data. For comparison, interatomic distances obtained in the of a CDW transition, following which one would expect a divergence in the distances below the transition. room-temperature single-crystal study of Furuseth and Fjellva° g10 are also displayed.The most significant diVerences between the structures at the three temperatures are in the short Structures at higher resolution and long Te(2) and Te(3) distances, which show a small but Refinement of data fromtheD2B diVractometer at temperatures perceptible divergence between 1.7 and 100 K, through to the around 100 K and 1.7 K yield structures of much higher pre- room temperature.This is plotted in Fig. 6. If the structural cision. Fig. 5 displays the experimental and fitted PND (D2B) changes at 63 K were associated with a CDW-like distortion of the sheets defined by Te(2) and Te(3), the observed metrical Table 1 Interatomic distances obtained from the coupled Rietveld behaviour is precisely the opposite of what would then be refinement of D20 data, calculated at 50 and 80 K expected.Atom 1 Atom 2 D(50 K)/A° D(80 K)/A° Densities of state and the nature of the transition Te(1) Zr 3.09(4) 3.08(7) In Fig. 7 we display a comparison of the Te(2) and Te(3) p- Te(1) Zr 3.13(4) 3.14(7) orbital and Zr d-orbital derived densities of state (DOS) Te(1) Zr(×2) 2.97(3) 3.01(5) Te(2) Zr(×2) 2.98(3) 2.96(6) Table 2 Structures obtained from the Rietveld refinement of D2B data Te(3) Zr(×2) 3.01(3) 3.01(5) at 1.7 and 100 K.Space group P21/m (no. 11) Te(2) Te(3) 2.88(5) 2.88(9) Te(2) Te(3) 2.99(5) 3.00(9) Atom x y z B/A° 2 T=1.7 Ka Zr 0.2878(4) 0.25 0.6654(3) 0.84(5) Te(1) 0.7635(6) 0.25 0.5574(2) 0.73(6) Te(2) 0.4284(5) 0.25 0.1647(3) 0.08(5) Te(3) 0.9068(6) 0.25 0.1600(3) 0.24(5) T=100.0 Kb Zr 0.2882(5) 0.25 0.6648(3) 1.00(5) Te(1) 0.7643(6) 0.25 0.5579(3) 0.84(6) Te(2) 0.4297(5) 0.25 0.1650(3) 0.25(5) Te(3) 0.9068(6) 0.25 0.1601(3) 0.46(5) aa=5.8726(3), b=3.9084(2), c=10.0536(5)A° , b=97.848(3)°.RB= 9.7%, Rwp=8.5%. ba=5.8775(3), b=3.9125(2), c=10.0645(5)A° , b= 97.835(3)°. RB=9.4%, Rwp=8.2%. Table 3 Interatomic distances obtained from the Rietveld refinement of D2B data, at 1.7 and 100 K, compared with those at 298 K Atom 1 Atom 2 D(1.7 K)/A° D(100 K)/A° Da(298 K)/A° Te(1) Zr 3.132(4) 3.132(5) 3.156(2) Te(1) Zr 3.122(4) 3.120(5) 3.140(2) Te(1) Zr(×2) 2.957(3) 2.959(3) 2.956(1) Te(2) Zr(×2) 2.956(3) 2.956(3) 2.939(2) Fig. 5 Rietveld refinement of D2B data acquired at 1.7(1) K. The Te(3) Zr(×2) 2.959(3) 2.966(3) 2.961(2) Te(2) Te(3) 2.816(5) 2.811(5) 2.793(2) data (a), Rietveld fit to the ZrTe3 structure (b), Rietveld fit to the ZrTe5 structure (c), a parasitic peak due to the cryostat (d) and the Te(2) Te(3) 3.057(5) 3.067(5) 3.103(2) diVerence between observed and refined profiles (e) are displayed, as aSingle crystal data of Furuseth and Fjellva°g.10 are markers of the peak positions for the diVerent structures. 2872 J. Mater. Chem., 1998, 8, 2869–2874a rearrangement of some states arising from (or causing) changes in Te(2)–Te(3) distances. More simply, the transition is associated with charge transfer. Rouxel has argued eloquently for the importance of what he calls redox-competition28 in going from oxides to the more covalent transition metal chalcogenides, whereby the observed reduction in the energy diVerence between cation d bands and anion sp bands results in a competition between these for valence electrons.In the previous work from this group11 the importance of such redox competition in the ZrTe3 system was discussed. Access to the low-temperature structures now allows us to show that redox competition not only governs structural principles but perhaps also the temperature dependence thereof.The changes in the DOS indicate that the biggest diVerences occur between the room temperature and 100 K rather than between 100 and 1.7 K as one might expect from the temperature at which the transition is found. However, the band structure calculations are eVectively at 0 K, and the ro� le of phonons is ignored. It is possible that when temperature eVects Fig. 6 Evolution of the short and long Te(2)–Te(3) distances with are taken into account, the situation would correspond more temperature.The 298 K data are from the single crystal study of closely to what is observed. We interpret the quenching of the Furuseth and Fjellva°g.10 magnetic susceptibility as arising from a small decrease in the total density of states (not shown) at the EF on cooling below the transition temperature resulting in a decrease in the Pauli paramagnetic contribution.The sharp drop in the thermopower along the a axis at the transition and below11 [the direction being that of the Te(2)–Te(3) contacts] could be ascribed to increased mobility along this axis. The one question remaining seems to be the symmetry change at the phase transition associated with a structural modulation as observed in the electron diVraction measurements. Electron diVraction is much more sensitive to changes in symmet than is powder neutron diVraction (particularly when the symmetry change is associated with a very large supercell ) while it does not enjoy the metrical precision of neutron diVraction coupled with Rietveld refinement.Indeed, that the symmetry of the low-temperature structure could be diVerent is suggested by the behaviour of the thermal parameters. Those of Zr and Te(1) at 1.7 K are about 80% the values at 100 K, but for Te(2) and Te(3) the changes in B on cooling are larger, the values of B at 1.7 K for Te(2) being about 30% of the value at 100 K and for Te(3) about 50% of the value at 100 K.The use of the P21/m space group with its associated unit cell for the low-temperature refinement is thus only approximate, being the commensurate subcell of some lower symmetry system. We do not expect this to aVect our conclusions regarding the trends in the interatomic distances. The nature of the rearrangement of atoms within the sheets permits the classification of the transition as displacive and second order.29,30 Identifying an order parameter in the structural metric from the present study is however made diYcult by the fact that the Te–Te distances along the a axis are Fig. 7 Partial LMTO densities of state of ZrTe3 calculated using the published 298 K structure of Furuseth and Fjellva°g10 and the unequal in both the high and the low temperature phases.structures determined in this study at 100(1) and 1.7(1) K. The That the low-temperature structure [when we consider only Te(2)-p, Te(3)-p and Zr-d states are shown in a small window around the Te(2)–Te(3) sheet] is less distorted than the high-tempera- the Fermi energy. ture structure is perhaps unusual but by no means novel. Perovskite manganites of the general formula Ln1-xAxMnO3 where Ln is a rare-earth ion and A is an alkaline earth ion obtained from high level LMTO calculations on the structures at three diVerent temperatures.Without going into the details have received much recent attention because of the finding that in these systems the onset of ferromagnetism on cooling of the electronic structure (discussed extensively in ref. 11), we note that the Te(2)–Te(3) p-orbital interaction is s*, and this is accompanied by the phenomena of giant negative magnetoresistance (GMR) being displayed.31 Careful structural interaction results in a partially filled band that crosses the Fermi energy. Changing the distances between the Te(2) and investigations on such GMR manganites have revealed that the delocalization of the eg electron on MnIII below the Te(3) atoms in the sheets should result in some rearrangement in the number of states in the valence band.In fact, the ferromagnetic transition results in the MnO6 octahedra being less distorted below the transition temperature rather than photoemission data (acquired at 298 and 80 K) presented in ref. 11 support the picture of a transfer of weight from the more.32 Similar eVects have been observed as a function of temperature or pressure in some layered manganites.33 In valence band to just below EF, in keeping with what is observed here.The structural transition is thus associated with ZrTe3, the transition in the thermopower and the quenching J. Mater. Chem., 1998, 8, 2869–2874 2873References 1 J.Rouxel, in Crystal Chemistry and Properties of Materials with Quasi-One-Dimensional Structures, ed. J. Rouxel, D. Riedel, Dordrecht, 1986 pp. 1–26. 2 F. K. McTaggart and A. D. Wadsley, Aust. J. Chem., 1958, 11, 845. 3 S. Furuseth, L. Brattas and A. Kjekshus, Acta Chem. Scand. Ser. A, 1975, 29, 623. 4 S. C. Bayliss and W. Y. Liang, J. Phys. C., 1981, 14, L803. 5 S. Takahashi, T.Sambongi and S. Okada, J. Phys. (Paris) Colloq. C, 1983, 3, 1733. 6 S. Takahashi, T. Sambongi, J. W. Brill and W. Roark, Solid State Commun., 1984, 49, 1031. 7 H. Nakajima, K. Nomura and T. Sambongi, Physica B, 1986, 143, 240. 8 D. J. Eaglesham, J. W. Steeds and J. A. Wilson, J. Phys. C., 1984, 17, L697. 9 E. Canadell, Y. Mathey and M.-H. Whangbo, J. Am. Chem. Soc., 1988, 110, 104. 10 S. Furuseth and H. Fjellva°g, Acta Chem. Scand., Ser. A, 1991, 45, 694. 11 C. Felser, E. W. Finckh, H. Kleinke, F. Rocker and W. Tremel, Fig. 8 Isothermal (19 K) pressure dependence of the a, b and c lattice J. Mater. Chem., 1998, 8, 1787. parameters of ZrTe3 obtained from a coupled refinement of D20 data 12 K. Sto�we and F. Wagner, J. Solid State Chem., 1998, 138, 160. during a pressure ramp from ambient pressure to 1 kbar. 13 By quality, we refer to a combination of resolution, statistics (high signal to noise ratios) and dynamic range s=sin h/l that would allow large numbers of parameters to be refined stably and with precision. of the susceptibility, in conjunction with the calculated decrease 14 H. M. Rietveld, J. Appl. Crystallogr., 1969, 2, 5. in the DOS at the EF below the transition suggests that what 15 J.-F.Be� rar, computer code XND version 1.11, ESRF, Grenoble, is observed is similar to the situation in the manganites except France, 1996; J. F. Be�rar, Proceedings of the I.U.Cr. Satellite that the the cause is not an atomic energy level (a Jahn–Teller Meeting on Powder DiVraction, Toulouse, France, 1990; J. F. distortion) but rather the behaviour of the s* band formed Be�rar and F.Garnier, Advanced Powder DiVraction II by Te(2) and Te(3) p orbitals. Conference, N.I.S.T. Gaithersburg, Maryland, 1992; The program is freely available at the URL http://rx-crg1.polycnrsgre. fr/public/xnd/xnd.html. The eVect of pressure on lattice parameters 16 R. Seshadri, C. Martin, M. Hervieu, B. Raveau and C. N. R.Rao, Chem. Mater., 1997, 9, 270. From the discussion so far it is evident that only data of the 17 J.-F. Be� rar and G. Baldinozzi, J. Appl. Crystallogr., 1993, 26, 128. highest quality can reveal details of the phase transition in 18 H. Fjellva°g and A. Kjekshus, Solid State Commun., 1986, 60, 91. ZrTe3. From the isothermal data collected by us on the D20 19 C. Giacovazzo in Fundamentals of Crystallography, ed.diVractometer under pressure, it was not possible to extract C. Giacovazzo, IUCr-Oxford, 1992, pp. 122–124. structural parameters. However, through the use of the data- 20 A. L. Spek, computer code PLATON97, Acta Crystallogr. Sect. coupling strategy using six data sets, it was possible to obtain A, 1990, 46, C34. 21 R. W. Tank, O. Jepsen, A. Burkhardt and O.K. Andersen, The experimental compressibilities along the diVerent directions. Stuttgart TB-LMTO-ASA program, MPI fu� r Festko� rperfor- The assumption is that in the small pressure range studied (up schung, Stuttgart, Germany, 1998. to 1 kbar) the changes are linear. Fig. 8 displays the percentage 22 W. Tremel and R. HoVmann, J. Am. Chem. Soc., 1987, 109, 124. changes in the a, b and c lattice parameters of ZrTe3 at 19 K. 23 K.-H. Hellwege and A. M. Hellwege, Landolt-Bo�rnstein Tables, The points refer to the temperatures at which the data were New Series, Volume 2, Springer-Verlag, Heidelberg, 1966; Note acquired. The error bars on the line are smaller than the points that the correction for core diamagnetism is considerably aVected by how charges are assigned to the diVerent atoms, so the true representing the temperatures. The Te(2)–Te(3) interactions susceptibility might be in some error.along the a axes should be softer than the Zr–Te interactions 24 T. Sambongi, in Crystal Chemistry and Properties of Materials so one might naý�vely expect that this is the axis most sensitive with Quasi-One-Dimensional Structures, ed. J.Rouxel, D. Riedel, to pressure. We find instead that both the a and the b axes Dordrecht, 1986 pp. 281–313. have similar compressibilities but the c axis along which the 25 Briefly, the final error on the refined parameter is independent of sheets are stacked is significantly softer. This is in keeping all terms except those that are of zeroth order in the expansion. For N parameters, we then have dp=ÓN×dp0.R. S. thanks Dr. with the presence of a van der Waals gap between the double J. P. Attfield for a clarification of this point. sheets of the Zr–Te polyhedra and in keeping with the struc- 26ajal, Manual of the FullProf Rietveld Program, tural description that we have chosen. Similar results were Laboratoire Leon Brillouin, CEA Saclay, France, 1997. Available observed from the pressure isotherms at 100 K suggesting that by anonymous ftp from bali.saclay.cea.fr. the transition seems to have no significant eVect on the 27 G. W. Brindley, Philos. Mag., 1945, 36, 347. contractions of the diVerent axes within the resolution of the 28 J. Rouxel, Chem. Eur. J., 1996, 2, 1053. 29 H. D. Megaw, Crystal Structures, A Working Approach, experiment. W. B. Saunders, Philadelphia, 1973. 30 M. T. Dove, Am. Mineral., 1997, 82, 231. Acknowledgements 31 C. N. R. Rao, A. K. Cheetham and R. Mahesh, Chem. Mater., 1996, 8, 2421. It is a pleasure to thank Professor O. K. Anderson and Dr. 32 V. Caignaert, E. Suard, A. Maignan, C. Simon and B. Raveau, O. Jepsen for providing the LMTO codes, and Dr. P. Convert C. R. Acad. Sci. (Paris) Ser. IIb, 1995, 321, 515; P. G. Radaelli, M. Marezio, H. Y. Hwang, S. W. Cheong and B. Batlogg, Phys. of the ILL for help and advice on D20. During the various Rev. B, 1996, 54, 8992. stages of this work, we have received assistance from Dr. 33 J. F. Mitchell, D. N. Argyriou, J. D. Jorgensen, D. G. Hinks, V. Ksenofontov and Mr. F. Rocker (Mainz) and Messrs. C. D. Potter and S. D. Bader, Phys. Rev. B, 1997, 55, 63; J. Torregrossa, L. Melesi and P. Cross (ILL). We thank them. D. N. Argyriou, J. F. Mitchell, J. B. Goodenough, O. Chmaissem, S. Short and J. D. Jorgensen, Phys. Rev. Lett., 1997, 78, 1568. This work has been supported by the Deutsche Forschungsgemeinschaft (DFG) and the Fonds der chem- Paper 8/05427D ischen Industrie. 2874 J. Mater. Chem., 1998, 8, 2869–2874

ISSN:0959-9428    

DOI:10.1039/a805427d

出版商:RSC    

年代:1998

数据来源: RSC