摘要:

J. MATER. CHEM., 1994, 4(11), 1715-1717 Synthesis and Mesomorphic Properties of 4-[(4-Cyanophenyl)acetylenyl]-2,3,5,6-tetrafluorophenyl 4-n-AIkoxybenzoates Jianxun Wen,* Hongbin Yu and Qi Chen Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai, 200032, China 4-[(4-Cyanophenyl)acetylenyl]-2,3,5,6-tetrafluorophenyl4-n-alkoxybenzoates, which contain a perfluorinated phenyl ring in their aromatic core system, exhibit nematic phases with a wide thermal range. In comparison with the homologues without fluorine, the introduction of the perfluorinated phenyl ring leads to the formation of a nematic phase and reduces the melting and clearing points. In recent years, laterally fluorinated aromatic liquid crystals have attracted much attention.lP5 The fluorine atom combines large electronegativity with small size, so that it significantly affects the physical properties of molecules without eliminating the possibility of mesophase formation.When one or two fluorine atoms are introduced into the aromatic core unit of liquid crystals, some valuable effects, such as a reduction in the melting point, an increase in cL6 and an increase in the width-to-length ratio,’ can be achieved. There have been some reports on liquid crystals which contain a perfluorinated phenyl These materials have high P, value^'^-'^ and low phase-transition temperature^'^ or else they favour the smectic C phasel4+l6 and thus have potential applications in message transportation and image displays.In order to pro- duce new materials with better physical properties, our group designed and synthesized new fluorinated liquid crystals. We reported some new perfluorophenyl-containing liquid crystals by coupling 4-alkoxy-2,3,5,6-tetrafluorophenylacetylenewith other materials.20P24 The liquid crystals obtained were ident- ified as having a chiral smectic C phase,24 a smectic A phase24 or an enantiotropic nematic However, for only a few of these perfluorophenyl-containing liquid crystals were their mesomorphic properties compared with their non-flu- orinated parent molecules. Most of them have the perfluoro- phenyl ring on the side of the molecular backbone. We report here a new family of compounds with a perfluorophenyl ring located at the centre of the core unit (1).Their non-fluorinated homologues (2) were reported previou~ly.~~ By comparing the mesophic properties of these two series, we can gain a deeper knowledge of perfluorophenyl-containing compounds. F\ IF F‘ ‘F 1 2 The starting material for producing 1 is 4-iodotetrafluoro- phenol. The preparation of these compounds affords a method of obtaining compounds with the perfluorophenyl ring at the centre of the core unit. The mesomorphic properties were studied by polarizing microscopy and differential scanning calorimetry (DSC). Experimental General Synthetic Procedures 4-[( 4-Cyanophenyl )acet ylen yl ]-2,3,5,6-tetrafluorophen yl 4-n- alkoxybenzoates (1) were prepared according to Scheme 1 Initially, 3 was treated with KOH in tert-butyl alcohol under reflux to yield 4.The main intermediate, 5,was obtained from a one-pot mild esterification between 4 and 4-alkoxybenzoic acid in the presence of both dicyclohexylcarbodiimide (DCC) F\ IF FeI FF 3 HO*I cop FF FF 4 5 a m=4 b m=5 c m=6 d m=7 e m=8 6 F ‘F 1 a m=4 b m=5 c m=6 d m=7 e m=8 Scheme 1 Preparation of 1. Treatments: (a) KOH, ButOH, reflux; (b) H(CH2)mo~CozH-DCC, PPY, CH,Cl,, room tempera- ture; (c) (Ph,P),PdCl,, CuI, Et,N, 60 “C. 1716 and 4-pyrrolidin-1-ylpyridine (PPY) catalyst in dried dichloromethane. 6 was synthesized following the method of ref. 26. The final products, 1 were prepared from 5 and 6 using dichlorobis( tripheny1phosphine)palladium as catalyst in triethylamine with copper(1) iodide.Characterization of Materials The final products were rigorously purified by flash chroma- tography over silica gel (200-400 mesh) using light petroleum (bp 60-90 "C)-ethyl acetate (60 : 1) as the eluent to give pale yellow solids which showed a single spot in thin-layer chroma- tography. The products were recrystallized from acetone-methanol to yield white crystals. The chemical structure of the intermediates and the final products were elucidated by 1R spectroscopy (Shimadzu IR- 440 spectrophotometer), 'H NMR spectroscopy (Varian EM390 and Varian EM360 spectrometer) with TMS as internal standard, 19F NMR spectroscopy (Varian EM3601 spectrometer) with trifluoroacetic acid as external standard and mass spectrometry (HP5989A spectrometer).The spectro- scopic data were consistent with the predicted structures. The melting points of the intermediates and the transition tempera- tures and phase assignments for the final products were determined by using an Olympus BH2 polarizing microscope in conjunction with a Mettler FP52 hot stage and FP5 control unit, while the observed textures were compared with those in the literat~re,~ to identify the mesomorphic phases. The enthalpies of transitions were investigated by DSC using a Shimadzu DSC5O calorimeter with a heating rate of 10°C min-'. Synthesis of Materials The preparation of 3,' and 626followed literature procedures. 2,3,5,6-Tetrujluoro-4-iodophenol(4) Pentafluoroiodobenzene (3) (14.7 g, 50 mmol) KOH (8.4 g, 150mmol) and tert-butyl alcohol (30ml) were refluxed at 90 "C for 6.5 h.19F NMR analysis of the reaction mixture showed that the reaction was complete. Then aqueous hydro- chloric acid (5%, 20 ml) was added and the aqueous tert- butyl alcohol (cu. 22 ml) was distilled off. The residue was acidified with aqueous hydrochloric acid (5%) and cooled. The mixture was allowed to stand for 30min and white crystals formed. The product was filtered off and washed with cold methanol. The filtrate was extracted with ether and dried over anhydrous sodium sulfate. The solvent was removed and the residue was combined with the white crystals which had been filtered off and dried under reduced pressure to give a white solid.Yield 12.8 g (87.7%), mp 46.0-46.5 "C. 'H NMR 6, (60 MHz; CCl,; TMS): 6.0 (s, OH). 19FNMR hF (60 MHz; CC1,; TFA): 48.35 (d, 2 F, J 18.8 Hz, F arom.), 83.6 (d, 2 F, J 18.8 Hz, F arom.). 2,3,5,6-Tetra~uoro-4-iodopheny~4-n-Butoxybenzoate(5a) 4-Alkoxybenzoic acids were prepared from 4-hydroxybenzoic acid and the corresponding alkyl bromide following the method of Gray and Jones.29 The general method of preparing 5 is as follows for 5a: a mixture of n-butoxybenzoic acid (1.65g, 8.5mmol), 4 (2.48g, 8.5mmol), DCC (1.93 g, 9.35 mmol) and PPY (60 mg, 0.4 mmol j in dry dichloro- methane (50 ml) was stirred at room temperature for 40 h. 19F NMR analysis revealed that complete reaction had occurred. The white precipitate was filtered off and the filtrate was washed with aqueous acetic acid (5%).The solvent was removed and the crude product was purified by flash column chromatography over silica gel (200-400 mesh) using a mix- J. MATER. CHEM., 1994, VOL. 4 ture of light petroleum (bp 60-90 "C) and ethyl acetate (30 :1) as the eluent. The solvent was removed from the collected fractions to give a white solid, 5a. Yield 1.82 g (77.8%), mp 62.9-64.4 "C. 'H NMR 6, (90 MHz; CDCl,; TMS): 8.15 (d, 2 H)/7.00 (d, 2 H) (AA'BB', J 9 Hz, H arom.), 4.09 (t, 2 H, J 6 Hz, OCH,), 0.8-2.0 [m, 7 H, (CH,),CH,:]. 19F NMR 6, (60 MHz, CDCl,, TFA): 42.7 (d, 2 F, J 18.8 Hz, F arom.), 73.0 (d, 2 F, J 18.8 Hz, F arom.). 5b-5e were prepared by the same general method and they had satisfactory 'H and 19FNMR spectra.4-(4-Cyanopheny2acetylenyl)-2,3,5,6-tetrujluorophen~l4-n-butoxybenzoate (la) The typical method of synthesis of 1 is as follows for la: a mixture of 5a (234 mg, 0.5 mmol), dichlorobis( triphenylphos- phine)palladium (20 mg, 0.03 mmol), copper(1) iodide (11.5 mg, 0.06 mmol) and 4-cyanophenylacetylene (63.5 mg, 0.5 mmol) in anhydrous triethylamine (15 ml) was stirred under an atmosphere of dry nitrogen at 60 C for 6 h. The precipitate was filtered off and washed with ether. The filtrate was washed with water and the solvent was removed. The residue was purified by flash column chromatography over silica gel (200-400 mesh) using a mixture of light petroleum (bp 69-90 "C)-ethyl acetate (60: 1) as the eluent to give a yellow solid which was recrystallized from acetone-methanol to yield white crystals of la.Yield 200 mg (85.6%). 'H NMR 6, (90 MHz; CDC1,; TMS): 8.15 (d, 2 H1'7.00 (d, 2 H) (AA'BB', J 9 Hz, ROC6H,), 7.70 (s, 4 H, CNC6H,), 4.09 (t, 2 H, J 6 Hz, OCH,), 0.8-2.0 [m, 7 H, (CH,),('H,]. 19FNMR 6, (60 MHz, CDCl,, TFA): 58.4 (d, 2 F, J 18.8 Hz, F arom.), 74.3 (d, 2 F, J 18.8 Hz, F arom.). IR (KBr) v/cm-': 2900, 2850, 2200, 1755, 1605, 1580, 1510, 1490, 1440, 1400, 1320, 1260, 1235, 1170, 1100, 1030, 990, 840. MS (willz) 468 (A4+ 1). Found: C, 66.58; H, 3.43; N, 2.90; F, 15.74%. Calc. for C2,H17F,N0,: C, 66.81; H, 3.64; N, 3.00; F, 16.27%. lb-le were prepared similarly. All of them had satisfactory elemental analyses and appropriate 'H and 19F-NMR, IR and MS spectra.Results and Discussion Optical Microscopy Studies The transition temperatures and enthalpies for compounds 1 are presented in Table 1. The reported transition tempera- ture~~~for 2a-2f are listed in Table 2. When m=4-8, com-pounds 1 had a wide thermal mesomorphic range and exhibited an enantiotropic nematic phase. The nematic ther- mal ranges were relatively stable around 114-1 18 "C. The melting points and the clearing points decreased with increas- ing the length of the alkoxy chain. The melting point decreased by 47.5 "C from the butoxy substituent (la) to the pentyloxy substituent (lb). The difference in melting points between lb and le were much smaller. A similar trend is apparent with the clearing points.Tinh et al. reported25 that compounds 2 were liquid crystal- line. The compounds with m =5 and 6 have an enantiotropic nematic phase and a monotropic S, phase. The enantiotropic S, phase appears from m=7. The nonyloxq and decyloxy derivatives have a stable re-entrant nematic phase. By compar- ing compounds 1 with compounds 2, it was found that the melting points and the clearing point reduced when the perfluorinated phenyl ring was introduced into the molecule. For m= 8, the melting point decreased from 86.0 "C to 77.3 'C and the clearing point decreased from 248'C to 193.1"C. Compounds 1 exhibited stable enantiotropic nematic phases without a smectic phase when rn =4-8. However, the smectic phase exists in compounds 2 from m=5 and re-entrant J.MATER. CHEM., 1994, VOL. 4 1717 Table 1 Transition temperatures, enthalpies and thermal ranges for compounds la-le T/”C (AH/J g-’) compound m K-N N-I recryst. temp./”C N thermal 1 ange/”C ~~ la lb lc Id le 4 5 6 7 8 146.54 74.05) 99.0( 61.93) 90.4( 81.69) 84.0( 59.09) 77.3( 85.63) 234.2( 2.74) 213.5( 2.05) 208.0( 1.44) 198.0( 2.3 1) 193.1( 1.83) 98.4 40.0 49.2 39.0 46.9 87.7 114.i 117.6 114.0 115.8 Table 2 Transition temperatures for compounds 2a-2f compound m K-S, SA-N N-S, SA-N N-I 2a 5 107 (105) --276 2b 6 113 (107) -~ 268 2c 7 102 108 --256 2d 8 86 96 --248 2e 9 90 (75.7) 141 183 239 2f 10 84 -102 208 233.5 nematic phases were observed when m=9 and 10. These two series of compounds contained the cyano group.In non-fluorinated compounds the highly polar cyano group can be attached to one end of the molecule, resulting in strong antiparrallel correlations between neighbouring molecules.30 That the fluorinated compounds exhibited only an enantio- tropic nematic phase indicated that the terminal attractive force between the molecules was dominant and the lateral attractive force was relatively weak so that the smectic phase could not form and the antiparrallel correlation between neighbouring molecules could not persist. Introduction of fluorine at the centre of the core unit disrupts the formation of the re-entrant phase when the highly polar cyano group is the terminal group. DSC Studies The phase transitions were also investigated by DSC, and the mesomorphic transition enthalpies for compounds 1 are shown in Table 1.The average value of the melting enthalpy of compounds 1 is 72.48 J 8-l. The nematic to isotropic transition enthalpies are relatively small: 1.44-2.74 J 8-l. Conclusion We synthesized a new series of liquid crystals which contain central perfluorinated phenyl ring in the aromatic core system and studied their mesomorphic properties. We have compared their mesomorphic properties with homologous non-fluori- nated compounds. Compounds 1 possessed wide nematic temperature ranges, lower melting points and lower clearing points. The authors gratefully acknowledge the Advanced Material R&D Program of China for financial support.References G. W. Gray, M. Hird, D. Lacey and K. J. Toyne, J. Chem. SOC., Perkin Trans. 2, 1989,2041. 2 G. W. Gray, M. Hird, D. Lacey and K. J. Toyne, Mol. Ctayst. Liq. Cryst., 1989, 172, 165. 3 G. W. Gray, M. Hird, D. Lacey and K. J. Toyne, Mol. Cryst. Liq. Cryst., 1991, 195, 221. 4 G. W. Gray, M. Hird, D. Lacey and K. J. Toyne, Mol. Cryst. Liq. Cryst., 1991,204,43. 5 M. Hird, G. W. Gray and K. J. Toyne, Liq. Cryst., 1992, 11, 531. 6 M. A. Osman, Mol. Cryst. Liq. Cryst., 1985,128,45. 7 K. J. Toyne, in Thermotropic Liquid Crystals, ed. G. W. Gray, Wiley, Chichester, 1987. 8 J. Goldmacher and L. A. Barton, J. Org. Chem., 1967,32 476. 9 M. M. Murza, G. P. Tataurov, L. I. Popov and Yu. V Svetkin, 2.Org. Khim., 1977,13, 1046.10 G. W. Gray, Mol. Cryst. Liq. Cryst., 1979,7, 127. 11 A. Beguin and J. C. Dubois, J. Phys. (Paris), 1979,40,9. 12 R. Sirutkaitis and P. Adomeans, in Advances in Liquid Crystal Research and Applications, ed. L. Bata, Pergamon Press, Oxford, 1980, p. 1023. 13 P. Le Barny, G. Ravaux and J. C. Dubois, Mol. Cryst. L:q. Cryst., 1985,127,413. 14 C. Baillon-Moussel, D. Broussoux, J. C. Dubois and P. I,e Barny, Eur. Pat. Appl., EP 360 683, 1989. 15 H. Takeshita and A. Mori, Jpn. Kokai Tokkyo Koho, JP 02 237 962 [90 237 9621,1990. 16 C. Baillon-Moussel, D. Broussoux, P. Le Barny and F. Stjyer,Eur. Pat. Appl., EP 418 140, 1991. 17 S. Sugawara, Jpn. Kokai Tokkyo Koho, JP 01 09 959 [8‘)09 9591, 1989. 18 S. Sugawara, Jpn. Kokai Tokkyo Koho, JP (11272552 [I89 272 5521,1989. 19 S. Sugawara, Jpn. Kokai Tokkyo Koho, JP 02 32 057 [91) 32 0571, 1990. 20 J. Wen, Q. Chen, Z. Guo, Y. Xu, M. Tian, Y. Hu, H. Yu and Y. Zhang, Chinese Pat. Appl. 92 108444.7. 21 Y. Xu, Q. Chen and J. Wen, Liq. Cryst., 1994, 15,916. 22 J. Wen, Y. Xu and Q. Chen, J. Fluorine Chem., 1994,66, 15. 23 Y. Xu, W. Wang, Q. Chen and J. Wen, Chinese J. Chdim., 1994, 12, 169. 24 J. Wen, M. Tian, H. Yu, Z. Guo and Q. Chen, J. Mattir. Chem., 1994,4, 327. 25 N. H. Tinh, A. Pourrere and C. Destrade, Mol. Cryst. L iq. Cryst., 1980,62, 125. 26 S. Takahashi, Y. Kuroyama, K. Sonogashira and N. liagihara, Synthesis, 1980, 627. 27 D. Demus and L. Richter, in Textures of Liquid Crystds, Verlag Chemie, Weinheim, 1978. 28 Y. D. Zhang and J. X. Wen, J. Fluorine Chem., 1990,47,533. 29 G. W. Gray and F. B. Jones, J. Chem. SOC., 1953,4179. 30 P. E. Cladis, R. K. Bagardus and D. Aadsen, Phys. Rei. A, 1978, 18,2292. Paper 4/02944E; Received 17th hlay, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401715

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(11), 1719-1722 X-Ray Crystal Structure and Solid-state Properties of a 1 : I Complex of Tetrathiafulvalene (TTF) and 1-Oxo-2,6-dimethyl-4-dicyanomethylenecyclohexa-2,5-diene Andrei S. Batsanov, Martin R. Bryce,* Stephen R. Davies and Judith A. K. Howard Department of Chemistry, University of Durham, Durham, UK DHI 3LE A crystalline 1 :1 complex of tetrathiafulvalene (TTF) and 1-oxo-2,6-dimethyl-4-dicyanomethylenecyclohexa-Z.5-diene has been characterised by IR, UV and EPR spectroscopy, two-probe dc conductivity data and single-crystal X-ray analysis. The complex consists of mixed donor-acceptor stacks of two crystallographically unrelated types and is an electrical insulator. In the search for new electron acceptors suitable for the formation of conducting molecular complexes, analogues of tetracyano-p-quinodimethane (TCNQ) have received con-siderable attention, and a large number of acceptor systems containing one, or more, dicyanomethylene [C(CN),] or cyanoimine (N-CN) groups have been synthesized.l-l' However, with the exception of N,N'-dicyanoquinonediimine radical anions,' there have been few structural studies on salts or complexes of new acceptor^.',^,^^ We have recently synthe- sized a series of 1-oxo-4-dicyanomethylenecyclohexa-2,5-diene derivatives la-c7 and we now describe the properties and X-ray crystal structure of a 1: 1 complex 2 formed between the 2,6-dimethyl derivative la and tetrathiafulvalene (TTF).The solution electrochemis try of acceptors la-c is discussed.Results and Discussion All the solid-state data obtained on the title complex 2 agree and show that there is no intermolecular charge transfer between TTF and acceptor molecule la. The IR spectrum of complex 2 consists of sharp peaks typical of an insulating complex; the frequency of the cyanide absorption peak in the complex (2225 cm-') is very similar to that of neutral compound la (2220cm-I). These IR data are supported by the UV spectrum of a powdered sample of 2 which does not show any absorptions characteristic of the TTF radical cation (2.2-2.8 eV) nor is a charge-transfer absorption band observed.12 Furthermore, the complex did not give an EPR signal. Consistent with these data, the single-crystal conduc- tivity (two-probe dc measurement) is that of an insulator (a,,= < S cm-').The electrochemical redox properties of the acceptors la-c have been studied by cyclic voltammetry. The compounds undergo one-electron reduction at Ered= -0.18, -0.21 and -0.30 V, respectively (data were recorded us. Ag/AgCl, Pt electrode, tetrabutylammonium perchlorate in acetonitrile at 20°C). This redox process is reversible for derivative la and irreversible for derivatives lb and lc; a second reduction wave to form the dianion was not observed. These data establish that, as expected, the acceptor ability decreases (i.e. the stability of the radical anion decreases) with successive methyl substitution in the series of compounds la-c, and these compounds are considerably weaker acceptors than TCNQ (El"'= +0.22 V; E21i2= -0.34 V, under the same electro-chemical conditions).Single crystal X-ray analysis reveals that the asymmetric unit of complex 2 comprises one TTF molecule (A) in a general position, two 'halves' of TTF molecules (B and C), each occupying a special position at an inversion centre and two molecules of 1 (D and E). TTF molecule A adopts a boat conformation, folding along the S(l)--.S(2) and S(3)...S(4) vectors by 4.3"and 10.0", respectively; molecules B and C are essentially planar. Both molecules la are slightly folded along the C(2)..-C(6) vector and twisted around the C(4)=C(9) bond, thus falling into three planar fragments, of' which [0(1)C(l)C(2)C(6)]and [C(4)C(9)(CN),] form with the 'central' [C(2)C(3)C(4)C(S)C(6)] moiety dihedral angles of 4.2 and 8.0" in molecule D, 3.8 and 11.2" in molccule E, respectively; The methyl carbon atoms deviate but slightly (0.03-0.06 A) from the central C, plane.For the atom num- bering scheme see Fig. 1. All these molecules and their symmetrical equiva Lents in the crystal are parallel to each other within 12.5" and form nearly planar sheets, approximately coinciding with the planes (xyO), (xy1/4), (xy1/2), (xy3/4) etc. [Fig. 2(u)]. There are two symmetrically independent sheets: those at z=O, l/2, 1 etc. are composed of molecules B, C and E [Fig. 2(u)], those at z= 1/4, 3/4 etc. of molecules A and D [Fig. 2(c)]. Molecular packing within sheets of either type is essentially simitar, with infinite chains of identical (translationally related) molecules running in the x direction. The shortest contacts within the sheets are between donor and acceptor molecules, viz.O(1D)**.C(2A)3.05, O(lE)*.*S(1B) 3.04, O(1E) ..C(2B) 3.02A, which ar,e shorter tgan the sums of van der Waals radii of 0 (1.52 A), C (1.70 A) or S (1.80 A).13 However, these contacts do not form infinite chains [Fig. 2(b),(c)]. On the other hand, each donor or acceptor molecule is sandwiched between two molecules of the opposite kind, belonging to the adjacent sheets. Principal planes of the adjacent molecules are p!rallel to within 2-5", with interplanar separations of 3.30-3.47 A. Thus the structure can be described alternatively as consisting of mixed donor-acceptor (1:1) stacks of two crystallographically unrelated types.Stacks of the first type that lie in the planes (xOz), (x1/2z) etc. are parallel to the direction [loll and comprise molecules in consequence ...B...D...C...D...B... [Fig. 2(d)]. Stacks of the second type (...A...E...A.. .E...) lie in parallel planes (x1/4z), (x3/4z) etc., but run in the [OOl] direction, differing from [loll by 36". The lengths of chemically equivalent bonds coincide within experimental errors. The geometry of the TTF moieties in the complex (Fig. 3) is similar to that of the neutral TTF mol- ecule,14 while differing substantially from that in TTF'+X -salts, or in molecular complexes where charge trmsfer is observed (e.g. TTF-TCNQ, TTF-DEtCNQ)." The 1-0x0-4- dicyanomethylenecyclohexa-2,5-dieneframework has not been studied structurally before, except for benzo-fused derivatives, which are non-planar, presumably due to peri-interactions between the dicyanomethylene group and the fused benzene ring.4,'6 In the present complex, compound la exhibits essen- J.MATER. CHEM., 1994, VOL. 4 S(2b') Fig. 1 Molecular overlap in the structure of the complex of TTF and acceptor la; projection on (001) plane, H atoms omitted, primed atoms are symmetry-related to the reference ones via inversion centres, double-primed by the c glide plane I= Z= y= 1/4 (e) y= 1/2 (d) Fig. 2 Crystal packing in the complex: projection along the z axis (a) and cross-sections along the planes z= 1/2 (b),z= 1;4 (c), y= 1/2 (d), z= 1/4 (e),showing short intermolecular contacts (dashes).H atoms are omitted, for molecular labelling see Fig. 1. J. MATER. CHEM., 1994,VOL. 4 [@=qSJdS Me la TTF e 1.230(6) i 1.368(3) 8 1.336(4) f 1.479(4) j 1.449(10) b 1.763(4) g 1.340(3) k 1.141(6) c 1.740(7)h 1.450(9) d 1.328(2) 1 a R'=R2=H b R' = Me; R2= H c R'=R~=M~ Fig. 3 Average bond distances in complex 2 (A),with G in parentheses (esds of individual values are 0.005-0.010A) tially quinonoid geometry, similar to that of 2,6-dimethyl-4-(a,a-diphenylmethy1ene)-1,4-benzoquinone.l7 Thus the crystal of 2 comprises neutral, rather than ionised, molecules of TTF and acceptor la. The crystallographic evidence correlates, therefore, with the spectroscopic and conductivity data in confirming the absence of any significant charge transfer from donor to acceptor within the crystal.It is known that in molecular complexes the charge-transfer stabilisation energy tends to be maximised in the observed donor-acceptor orientation (in the absence of hydrogen bonds, electrostatic interactions or other large intermolecular forces)." However, in complex 2, four crystallographically independent donor-acceptor contacts exhibit no prevailing mode of overlap (Fig. 1). Using the guidelines presented by Mayerle andTorrance et the formation of a neutral complex between TTF and acceptor la is in accord with the electrochemical data, which establish that there is a large difference (AE =0.53 V) between the first oxidation potential of TTF (Elli2= +0.34 V us.Ag/AgCl) and the reduction potential of acceptor la, reported above. Acceptor 1b also forms an insulating complex with TTF of 1: 1 stoichiometry, although single crystals could not be obtained. It is noteworthy that tetramethyl derivative lc does not form a complex with TTF, probably due to a deviation from planarity in the acceptor, by analogy with tetramethyl-TCNQ2' and tetra- methyl-N77,7-tricyanoquinomethaneimine.'f7 Experimental Preparation of Complex 2 A hot saturated solution of TTF (40 mg, 0.2 mmol) dissolved in ethanol was added to a hot saturated solution of compound la7 (36 mg, 0.2 mmol) in ethanol. The solution was slowly cooled to 20 "C and then stored at -20 "C for 168 h.Dark green plates of complex 2 (35 mg, 46%) were harvested by filtration; mp 128-132°C. Analysis, found: C, 52.1; H, 3.0; N, 6.0%: calc. for C1,H12N,0S,: C, 52.5; H, 3.1; N, 7.2%. IR vmax (KBr)/cm-': 3065, 2225 (CN), 1630, 1590, 1365, 1200, 890, 800, 780, 775, 735, 680, 660 and 440. t IUPAC-recommended name: 3-cyanoimino-6-dicyanomethylene-1,2,4,5-tetramethylcyclohexa-1,4-diene. X-Ray Structure Determination A dark-green plate-like single crystal of 2 (0.08 mm x 0.30mm x 0.40 mm) was obtained from ethanol. The X-ray diffraction experiment was carried out on a Rigaku AFC6S four-circle diffractometer at 150 K, using a Cryostream (Oxford Cryosystems) liquid-nitrogen device with an open-flow gas cryostat.21 Crystal data: CllH,N20C,H,S,2 M = 388.5.Mopoclinic, space Froup P2,/c, a=8.061(2) A, ho 33.065(7)A, c=13.746(3) A, p=105.21(2)", U=3535(1)A3 (from 24 reflections with 10"-=6'< 12"), Z =8, dcalc= 1.46g cmP3, F(000)=, 1600, graphite-monochromated 440-Ka radiation, 2 =0.7107 A, ,u =5.4 cm -'. 5998 independent reflections were measured in o-sca n mode (28< 50", no absorption correction). The structure was solved by direct methods.22 All non-hydrogen atoms were refined with anisotropic displacement parameters by full-matrix least squares (total 433 variables; all H atoms treated in riding model) against Fs of 3241 reflections having IF/>,40(1'), with w = [02(F)+ 0.0002F2]-' weights, using SHELXTLPLUS programs.23 The refinement converged at =0.045.wR = 0.046, goodness-of-fit 1.21, Apmax=0.34 e AP3. Additional material available from the Cambridge Crystallographic Data Centre comprises atomic coordinates and thermal parameters, bond lengths and angles. We thank SERC for the award of a studentship (to S.R.D.) and the Royal Society and the Nuffield Foundation for financial support (to A.S.B. and M.R.B., respectively). References 1 S. Hiinig and P. Erk, Adv. Mater., 1991,3,225. 2 K.Yui, Y. Aso, T. Otsubo and F. Ogura, J. Chem. Sol ., Chem. Commun., 1987,1816. 3 T.Czekanski, M.Hanack, J. Y. Becker, J. Bernstein, S Bittner, L. Kaufman-Orenstein and D. Peleg, J. Org. Chem., 1991, 56, 1569. 4 K. Maruyama, H.Imahori, K. Nakagawa and N. Tanaka, Bull. Chem. SOC.Jpn., 1989,1626.5 N.Martin, J. L. Seoane, A. Albert and F. H. Cano, Syrith. Met., 1993,55-57,1730. 6 T. T. Mitsuhashi, M. Goto, K. Honda, Y. Maruyama, r. Inabe, T. Sugawara and T. Watanabe, Bull. Chem. SOC.Jpn., 1988, 61, 261. 7 M. R. Bryce, S. R. Davies, A. M. Grainger, J. Hellberg, M. B. Hursthouse, M. Mazid, R. Bachmann and F. Gerson, J. Org. Chem., 1992,57, 1690. 8 A. S. Batsanov, M. R. Bryce, S. R. Davies, J. A. K. Howard, R. Whitehead and B. K. Tanner, J. Chem. SOC.,Perkin Trans. 2, 1993,313. 9 F.Iwasaki, S. Hironaka, N. Yamazaki and K. Kobayashi, Bull. Chem. SOC.Jpn., 1992,65,2180. 10 M. Yasui, M.Hirota, Y. Endo, F. Iwasaki and K. Kobayashi, Bull. Chem. SOC.Jpn., 1992,65,2187. 11 E.Torres, C. A. Panetta and R. M. Metzger, J.Org. Cht'm., 1987, 52,2944. 12 J. B. Torrance, B. A. Scott, B. Welber, F. B. Kaufman and P. E. Seiden, Phys. Rev. B, 1979,19,730. 13 A. Bondi, J. Phys. Chem., 1964,68,441. 14 W. F.Cooper, J. W. Edmonds, F. Wudl and P. Coppens, Cryst. Struct. Commun., 1974,3,23. 15 R. C. Teitelbaum, T. J. Marks and C. K. Johnson, J. Am. Chem. SOC.,1980, 102, 2986; K.Yakushi, S. Nishimura, T Sugano, H. Kuroda and I. Ikemoto, Acta Crystallogr., Sect. B, 1980, 36, 358,and references therein. 16 F. Iwasaki, Acta Crystallogr., Sect. B, 1971,27, 1360. 17 T. W.Lewis, I. C. Paul and D. Y. Curtin, Acta CrGstallogr., Sect. B, 1980,36,70. 18 B. Mayoh and C. K. Prout, J. Chem. SOC.,Faraday Trany. 2,1972, 68, 1072. 1722 J. MATER. CHEM., 1994, VOL. 4 19 J. J. Mayerle and J. B. Torrance, Bull. Chem. SOC.Jpn., 1981, 21 J. Cosier and A. M. Glaser, J. Appl. Crystallogr.. 1986, 19, 105. 54, 3 170. 22 G. M. Sheldrick, Acta Crystallogr., Sect. A, 1990,46,467. 20 B. Rosenau, C. Krieger and H. A. Staab, Tetrahedron Lett., 1985, 23 G. M. Sheldrick, SHELXTLPLUS, Gottingen & Siemens PLC, 26, 2081; see also A. Kini, M. Mays and D. 0. Cowan, J. Chem. 1990. SOC.,Chem. Commun., 1985,286. Paper 4/021161; Received 1lth April, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401719

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(11), 1723-1730 X-Ray Absorption Spectroscopic Study of the AIPO,-5 :Ferrocene Inclusion Compound and its Thermally Decomposed Products Astrid Lund,*a David G. Nicholson," Geraldine Lambleb and Brian Beagley" a Department of Chemistry, University of Trondheim, AVH, N-7055 Dragvoll, Norway Brookhaven National Laboratory, Upton, NY 11973, USA Department of Chemistry, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester, UK M60 7QD The synthesis and decomposition of an inclusion compound, AIPO, :Fe(C,H& is reported. This, and the decomposition products have been characterised by extended X-ray absorption fine structure spectroscopy. The clustering of iron atoms either through Fe-Fe or Fe-0-Fe interactions is shown to be absent.Instead, Fe"' is sited within the double 6-rings of the 12-ring system. A model for distortions of the local lattice about Fe"' is proposed. There is intense interest in the microporous materials collec- tively known as zeotypes, a class of materials that include aluminium phosphorus oxygen frameworks AlPOs (Al, P), silicon aluminium phosphorus oxygen frameworks, SAPOs (Si, P), metal-substituted aluminium phosphorus oxygen frameworks, MeAPOs (Me, Al, P) and metal-substituted aluminium phosphorus silicon oxygen frameworks, MeAPSOs (Me, Si, P). Interest in these materials has been increased by their structural relationships to the zeolites, which enjoy wide industrial applications as ion exchangers, molecular sieves and A1P04 frameworks are built up from strictly alternating A1-0 and P-0 tetrahedra whose formal charges are mutu- ally compensated.Thus, these materials have neutral frame- works without any ion-exchange properties or strong acid sites (with the possible exception of some metal- and silicon- substituted A~POS~,~). In the specific case of A1P04-5 there appears to be some weak acidity, which has been attributed to lattice This property is of consequence in the present study. Materials of this type are attractive hosts for catalytic species because of their excellent thermal and hydrothermal stabilities (e.g. A1PO4-5 withstands calcination at 1000"C9). The thermal stability of A1P04-5 makes the material a promising candidate for some industrial appli- cations and is one of the reasons that the material was chosen in the present study.In addition, A1PO4-5 also has features that commend it as a host in the study of channel inclusion compounds, namely channel or cavity size. The structure (Fig. 1) is well defined, has a large internal surface area, and eP 00 oAl OFe Fig. 1 View of the A1Po4-5 12-ring system that consists of alternating 4-rings and 6-rings with an Fe"' atom sited in the 6-ring. The A1(0), and P(O), tetrahedra that build up the framework are shown schematically. has the potential to impose size and shape selectivity on the product distribution arising from the molecular sieving effect.lO," To date, a number of organometallic guest molecules, including ferrocene, have been adsorbed on several zeolites and some of the properties of the resulting materials have been studied.12-18 Loading an organometallic complex such as ferrocene into dehydrated AlPOs may lead to materials in which the chemical and physical properties of the guest and host are modified.Interest in encapsulating organoinetallic complexes into AlPOs stems from work carried out on zeolites. The zeolite inclusion materials contain guests ranging from atoms, ions, metal clusters and salts to coordination complexes and organometallics. Extended X-ray absorption fine structure (EXAFS) spec- troscopy using a synchrotron radiation source is ir useful method for probing the local environment of a selected atom as the EXAFS contains information about the near neighbours surrounding the target atom.The parameters extracted include interatomic distances, coordination numbers and the degree of disorder (static and dynamic). We report here the results of an EXAFS study on the A1PO4-5:ferrocene inclusion compound and the fate of the iron atoms in the thermally decomposed products. Experimental Gel Preparation A1P04-5 was prepared according to procedures previously described." The synthesis gel was prepared using pseudo- boehmite (BA Chemicals Ltd., a subsidiary of British Alcan Aluminium plc), orthophosphoric acid (Merck, 85 wt.% solu- tion in H,O), triethylamine, (TEA; Merck, <0.2% H,O) and water. The molar gel composition was 1.0 TEA: 1.0 A120,: 1.0 P2O5:4OH20.Pseudoboehmite was added to a solution of orthophosphoric acid and water simultaneously with vigorous stirring until the gel was homogeneous (typically after 2 h). Upon combining the alumina slurry with the acid solution, the pH of the precursor mixture rose with ageing as the orthophosphoric acid slowly reacted with alumina. The pH stabilised to 1.4-1.5 after approximately 3.5 h. After ,iddition of TEA to the precursor mixture (still under stirring) the pH immediately increased to over 3 and gradually climbed to its final value of 3.5. The ageing of the gel is important because of the rise in pH during this period. Homogeneity was attained after ageing for 3 h when the pH stabilised at 3.5. Crystallisat ion The gel was transferred to a stainless-steel bomb lined with a Teflon cup with cover, pre-treated at 90 "C for 24 h and heated to 200°C at autogeneous pressure for 24 hZo after which time the crystallisation was completed, and the pH increased to 7.The colourless crystalline product was recovered by filtration, washed with distilled water and dried at room temperature. Routine X-ray powder diffraction (XRD) was used to check the crystallinity and the identity of the product. Template Removal The template in the as-synthesized AlPO, was removed by calcination at 600°C for 24 h. The final product consisted of colourless crystals. X-Ray diffraction confirmed that this material was A1PO4-5. Furthermore, AlPO,-5 retained its structure after calcination at 800 "C for 24 h.Activation ofA1P04-5 It is known17 that water molecules block the pores in the non- or partially-dehydrated zeolite NaY thereby excluding ferrocene from the non-or partially-dehydrated zeolite. Therefore dehydrating the A1PO4-5 molecular sieve is a crucial step. Activation was carried out using a similar procedure as described by Ozin and Godber.'* This involved three 2h cycles of evacuation and heating to 300 "C in a sealed system. After 6 h the sample was allowed to cool to room temperature. Adsorption of Ferrocene on A1PO4-5 Ferrocene, Fe( Cp), was diffused into the microporous AlP04- 5 framework by contact with a saturated solution of ferrocene in pentane, and allowed to react for 2 h. According to the literature" the rates of diffusion of the metallocenes into the pores of zeolite Y (faujasite-like structure) are fast, approxi- mately 90% being taken up from solution within 3 min.The saturated ferrocene solution was prepared by dissolving ferro- cene (Aldrich-Chemie, 98%, 1g) in pentane (Fluka Chemica, >99.5%, 50 ml). The green A1P04-5 :ferrocene product was filtered and washed with pentane until the excess ferrocene was washed out and the washings were colourless. The preparation was carried out in a glovebox under a nitrogen atmosphere, the flow of nitrogen being dried by means of liquid nitrogen. Heated In Vacuo at 200 "C Heating A1P04-5:ferrocene in uucuo at 200°C for 24 hours produces a change in colour from green to grey. Bearing in mind that ferrocene itself decomposes at a much higher temperature (470 "C), the structural integrity of the guest molecules in A1PO4-5 is of interest.Thermal Decomposition The thermal decomposition of the A1Po4-5:ferrocene inclusion compound was carried out by heating samples in a sealed evacuated quartz ampoule and also by heating in open air. The in vacuo decomposition was carried out by placing the ampoule in a stainless-steel vessel and heating to 600°C for 15 min. The product was black; this can be attributed to high-carbon content products (this material is discussed later). The second sample was decomposed at 600°C in air for 24 h in order to remove the high-carbon materials by oxidation, and to examine the effect of oxidising the organic residues at elevated temperatures on the iron environments. This product was orange-brown.The samples were analysed by atomic absorption spec- troscopy (AAS) using a Perkin-Elmer, Zeeman 5100 instru- ment. The analysis yielded ca. 1-2 wt.% iron in the AlP04- 5 :ferrocene compound and both its decomposed A1P04-5 :Fe products. This was confirmed by X-ray fluorescence analysis. J. MATER. CHEM., 1994, VOL. 4 The AlPO, samples were structurally characterised by XRD in order to confirm the microporous AlPO,-5 structures. Model Compounds Ferrocene, Fe(Cp), and iron (111j phosphate, FePO, were chosen as model (reference) compounds. The structures ar? knownZ1q2' and the Fe-C bond values are 2.064 and 2.045 A in the gas and solid phase, respectively (Table 1 ).The structure of FePO,, which is isotopic with the condensed AlPO, phase berlir~ite,'~is thus also related to that of a-quartz but with a nearly doubled c-axis. The Fe and P atoms in FePO, are tetrahedrally coordinated to oxygen atoms. X-Ray Powder Diffraction Data Collection XRD data were collected at the University of Trondheim, NTH using a Philips X-ray generator, a diffractometer employing a Cu tube as X-ray source and a graphite crystal monochromator or at the University of Manchester Institute of Science and Technology using a SCINTAG powder diffractometer. Extended X-ray Absorption Fine Structure Preparation of Samples and Data ColEection EXAFS data collection was carried out at the Daresbury Synchrotron Radiation Source (SRS), and the National Synchrotron Light Source (NSLS), Brookhaven.Data were obtained on stations 9.2 (Daresbury) and X11A (Brookhaven) using the iron K-edge (A=1.74334 A; energy =71 12 eV). Double-crystal silicon (220)and order-sorting silicon crystal (111) monochromators were used to scan the X-ray spectra on stations 9.2 (Daresbury) and X11A (Brookhaven), respect- ively. At Daresbury, ion chambers were used for detecting the intensities of the incident and transmitted X-rays. The first ion chamber was filled with 20% Ar (24.1 Torrj and the second ion chamber with 80% Ar (174.2Torr) and both chambers were filled to atmospheric pressure with He. At Brookhaven fluorescence data were collected using a Lytle dete~tor.'~ The experiments were performed with electron beam ener- gies of 2.0 and 2.5 GeV, and maximum stored currents of 205 and 21 1mA for Daresbury and Brookhaven, respectively.The harmonic rejection for the data collections was set to 70%. The materials that had been decomposed in vacuo were prepared in the form of sealed samples for EXAFS experiments immediately after opening the evacuated ampoules. One of the ampoules was opened under a dry nitrogen atmosphere and the EXAFS sample sealed in that atmosphere in order to compare its EXAFS spectrum with those that had been exposed to the atmosphere. The amount of material used to prepare the samples for transmission XAS was calculated from element mass fractions and the absorption coefficients of the elements25 just above the absorption edge to give log(Io/I) of almost unity.The well powdered samples were mixed with boron nitride so as to obtain a sample thickness of ca. 1 mm and placed in aluminium sample holders and Table 1 Fe-C bonds and root mean square vibrational amplitudes of ferrocene according to the literature sample method ref. rF€& %& solid X-ray 21 2.045 (k0.0l)a -gas phase GEDb 22 2.064 ( f0.003p 0.062 (k0.001)" 'The standard deviations in parentheses include the effect of errors in the electron wavelength. bGas-phase electron diffraction. J. MATER. CHEM., 1994, VOL. 4 held in place by Sellotape. Spectra of the inclusion compound and its thermally decomposed products (i.e. unknowns) were collected in the fluorescence mode, because they are relatively dilute in Fe content (1-2%). Samples (100 mg) were placed on a surface area of 1.2 cm2.Results X-Ray Powder Diffraction The X-ray powder diffractogram confirms that the product of r,! 2the synthesis is hFxagonal A1Po4-5 with the cell dimensions: -O.4 4 6 8 10 12 14 a =b = 13.697(1) A, c =8.375(1) A and y = 120" which are clo5e to the paiameters found in the literature26: a =b =13.726 A, c =8.484 A and y =120". The A1P04-5:ferrocene material gives an XRD pattern that closely matches the A1PO4-5 diffractogdam. The cell parameters a= b =13.753(2) A, c= 8.410(3)A and y= 120" for the latter are consistent with the A1P04-5 framework being essentially unmodified by the guest molecules, and the small changes in cell parameters reflect only minor adjustments of the framework. The decomposed black sample matched the AlPO,-5 XRD.A decrease in weight was not found upon further heating to 900°C indicating that the AlPO heated to 600°C is C-free. XRD patterns of the brown A1P04-5 :Fe sample also match the A1P04-5 diffractogram. Extended X-ray Absorption Fine Structure The EXAFS data were analysed using standard procedures within the EXCURV90 program based on a curved-wave formalism as described in detail in the literat~re.~~.~~ Data were measured, summed, corrected for dark currents and converted into k-space by using the EXCALTB29 routine. The EXBACK29 program was used to normalise the data and extract the EXAFS functions Xybs (E-&) by background subtraction.In the preliminary stages of refinement, Fourier filtering (FF) using various ranges was carried out in order to isolate the contributions to the EXAFS from the different distances. A Gaussian window function was used. Fig. 2 shows the unweighted raw spectra of AlPO4-5:Fe(Cp), and its two decomposed products. The k3 weighted experimental spectra of the three synthesised samples and their Fourier transforms are given in Figs. 3-5. Model Compounds The model (reference) compounds chosen were Fe(Cp), (Daresbury and Brookhaven) and FePO, (Brookhaven). EXCURV90 calculates phases for their individual scatterers, Fe and the C, P and 0 atoms by ab initio methods. The reference compounds were used to calibrate the parameter sets, which were used in calculating the EXAFS of unknown samples.In Fe(Cp), and FePO,, the actual number of each kind of atom (C, 0 or P) within the shells was kept constant at the crystal!ographic values. The Debye-Waller-type factor, A of 0.007 A2 obtained for the Fe(Cp), sample is comparable to the vibrational amplitude, (u =0.0077 A2) obtained from gas- phase electron diffraction (Table 1).22 A1PO4-5:Fe(Cp), (Daresbury) The parameters obtained from the Fe(Cp), model compound were transferred to the A1PO4-5 :Fe(Cp), sample. The refined structural parameters for the latter confirm that the structural integrity of the ferrocene molecule in the host framework is maintained (Table 2). Fig.3 shows the calculated data fitted to the experimental EXAFS. 412t I11 2 4 6 8 10 12 14 0.2 0.0 -0.2 -0.4 Is2 4 6 8 10 12 14 k IA-' Fig. 2 Raw spectra (unweighted) for (a)AlPO,-5 :Fe(Cp),, (b AlP0,-1 5 :Fe (black) product and (c) AlPO,-5 :Fe (brown) product The present work describes two decomposition routes for the A1PO4-5:Fe(Cp), inclusion compounds. The EXAFS results of these decompositions are discussed below. Material (1) A1P04-5:Fe-decomposed in vacuo (Brookhaven). The EXAFS spectra of two samples of this material were collected; one being the EXAFS sample pre- pared in a dry nitrogen atmosphere. The two spectra were similar thereby showing that the iron atoms are locked into the structure and therefore not readily available to atmos- pheric reactions under the conditions prevailing here.Clearly the possibility of the existence of iron-carbon containing materials in this sample has to be considered. As noted above, we find that the iron atoms are dispersed within the lattice of the AlPO. We suggest that the blackening of this material is due to tar-like decomposition products of cyclopen tadiene that adhere to the external surfaces of the AlPO. Accordingly, we have not used a model that incorporates coordination to carbon and the EXAFS of the A1P04-5 :Fe (black) sample was fitted using the transferable parameters of the model compound FePO,. The model parameter set is given in Table 2. A region of 1-4A was isolated from the EXAFS spectrum by means of ff.The short-distance region includes the nearest neigh!ours, 0, P and A1 (Fig. 4). Coordination shells beyond 4A contribute only weakly to the EXAFS. Refined parameters are given in Table 2. Two different Fe-0 J. MATER. CHEM., 1994, VOL. 4 -1o.o+ I I I I I I I I 4.0 6.0 8.0 10.0 1; kfA-' 2-o/ (b) 2.0 3.0 4.0 5.0 6.0 RIA Fig.3 (a) Background subtracttd k3 EXAFS spectrum and (b) its Fourier transform (window 1-5 A) for the A1P04-5 :Fe(Cp), material. Experimental (-) and theoretical (---). -8.0 6.0 8.0 10.0 12.0 RIA Fig.4 Background subtrFcted k3 EXAFS spectrum and its Fourier transform (window 1-4 A) for the A1P04-5 :Fe (black) product (1). Experimental (-) and theoretical (---). I I I I -+-8.01 ! 6.0 8.0 10.0! k IA-' m0 (b) o) 0.8 D3 c.-c 0.6 E EZ 0.4 c v)c -2 0.2 0.0 R/A Fig.5 (a) Background subtracted ).' EXAFS spectrum and (b) its Fourier transform (window 1-4 A) for the A1P04-5:Fe (brown) product (2).Experimental (-) and theoretical (---). distances (1.95 and 2.59 A) within the host are discernible with multiplicities of 4 and 2 for the first and second oxygen shell, respectively. There are no clusters present, because Fe-Fe flistances are absent. The composite Fe-0 distance of 1.95 A indicates bonding of iron to oxygen atoms of the framework in the host channel. Statistical tests3' were used to determine whether addition of the shell made a significant improvement to the fit. All the shells added were significant at the 99% probability level, and therefore included. Material (2) AlPO,-5 :Fe-decomposed in uir (Brookhaven).The EXAFS of the AlPO,-5 :Fe (brown) sample was Fourier filtered and the data analysis carried out in the same manner as for the black AlPO,-5 :Fe unknown as described above. The results of the refinement are presented in Table 2. Fig. 5 shows the calculated data fitted to EXAFS data. The first peak in the Fourjer transform (Fig. 5) was fitted to an Fe-0 distance of 1.91A with a multiplicity 4. The second and Jhird peak both include 2 Fe-0 distances of 2.46 and 2.64A. As we see this is a very narrow range and the seFond peak consists more or less of a mean distance of 2.6 A. The last peak included in the Fpurier transform corresponds to an Fe-P distance of 3.22 A with a multiplicity of 3.Statistical tests3' were carried out and all the shells added were significant at the 99% probability level and therefore included. Discussion The ferrocene guest molecules of the inclusion compound A1P04-5:ferrocene occupy positions within the host channels. It is known that the A1P04-5 structure is stable up to 1000 0C,9and the XRD of the present materials confirm that the structure is maintained after decomposing the inclusion material at 600 "C.Ferrocene itself is a very stable molecule, J. MATER. CHEM., 1994, VOL. 4 Table2 Refined structural parameters for Fe K-edge EXAFS data for the Fe(Cp), and FePO, model compounds and A1P04-5:Fe(Cp), and its decomposed product samples back Re sample scatterer EdeV NU rb/A A'lA2 FId ("/I Fe(CP)2f Fe (CP)zg FePO,g A1PO4-5:Fe(Cp), A1P04-5:Feh A1P04-5:Fe' C C 0 P 0 C 0 0 P A1 0 0 P 0 17.30 12.88 22.76 2 1.23 26.83 26.85 10 10 4 4 4 10 4.2(1) 2.96( 10) 2.86(22) 2.81 (30) 2.44( 15) 2.33( 14) 3.22(20) 4.09(9) 2.047( 1) 2.043( 1) 1.850( 1) 3.118(5) 3.498(9) 2.058(1) 1.952( 1) 2.594( 2) 3.052(5) 3.285(8) 1.9 14( 2) 2.465(5) 2.640( 5) 3.098(5) 0.0067( 2) 0.0062( 2) 0.007 1 (2) 0.0213( 12) 0.0147( 21) 0.009 1 ( 3) 0.01 74 ( 2) 0.0155( 6) 0.2089(17) 0.0330( 3 1 ) 0.0154( 4) 0.0097( 11) 0.0063(10) 0.0219( 12) 1.60 1.94 1.33 1.40 0.16 0.74 15.33 19.29 19.59 14.90 11.98 22.45 a Multiplicity. Distances, r, estimated standard deviations in parentheses.Note that systematic errors in bond distances arising from data collection and analysis are ca. ?0.02-0.03 A for well defined shells. Debye-Waller-type factor. dFit index = NPT [Resi kWTI2, where Res, i=l =Residual =j(obs -xcalc, NPT =number of data points and WT =integral weighting. Residual =R-factor, R= {c[(slobs -j(,,kc)k~]2}/ {I[(j(obs)k3]2}. Fe(Cp), Daresbury data. Fe(Cp), and FePO, Brookhaven data. 'A1P04-5: Fe after decomposition of ke(Cp), at 600T in V~CUUM. A1PO4-5:Fe after decomposition of Fe(Cp), at 600 "C in air. decomposing at ca. 3470 "C3' The decomposition of ferrocene sandwich. We note that heating this material to 200°C results guest molecules within the AlPO raises the question as to the in its colour changing from green to grey (see belob).This manner in which iron is incorporated into the AlPO. This is also distinguishes A1PO4-5:ferrocene from the zeolite one of the questions addressed below. inclusion material. Also of particular interest, and central to Of the AlPO/SAPO :ferrocene systems in~estigated~~,~~ this work, is whether decomposition leads to clustering ofthe AlP04-5:ferrocene material stands out because it is green iron either through direct Fe -Fe bonding or through whilst the others exhibit the yellow-brown colour of ferrocene. Fe-0-Fe bridging. This question is addressed below. Clearly, something in the behaviour of the A1P04-5 framework towards ferrocene is different from the others. It has already been remarked (see above) that A1PO4-5 exhibits weak acidity, EXAFS Data possibly due to lattice imperfections.It is also known that under acidic conditions ferrocene is oxidised to the ferricenium The parameters extracted from the EXAFS data lor the which has an intense blue colour for dilute A1PO4-5:ferrocene and its thermally decomposed PIoductscation F~(CP)~+, samples, while more concentrated samples are red.31 Our are listed in Table 2. Before discussing the details of the supposition is that the green of A1P04-5 :ferrocene arises from decomposed material we turn first to the precursor, the a mixture of ferrocene and ferricenium. Only minor amounts inclusion compound itself. of ferricenium are needed to produce a green material because the colour of ferricenium is much more intense than the colour A1PO4-5:Fe(Cp), of ferrocene. Support for the view that ferricenium is present Moller et d3' have studied the intracavity chemistry of is given by Mossbauer studies34 on the materials (see below) organometallic fragments including ferrocene in different acid and also from literature studies on zeolite reactions with forms zeolite Y by EXAFS.They obtained Fe-C distances ferro~ene.'~~'~~'~~~~ of 2.01 A for a half-sandwich fragment attached to the zeolite. From this it would seem that the environment around These workers did not collect EXAFS data for the zeolite :fer-ferrocene in the A1PO4-5 channels significantly modifies the rocene inclusion compound but instead took up data on a chemical stability of that molecule and chemical reaction can sample that had been heated to 200°C.In the present study take place. Mossbauer spectroscopy indicates that ca. 25YO EXAFS data pertaining to the A1P04-5:ferrocene and the ferricenium is present.34 This would mean that certain pos- A1P04-5:ferrocene heated at 200 "C in vucuo were collected itions within the channels constitute different environments and it was found that complete ferrocen: molecules are present than those occupied by the majority of the ferrocene molecules. with Fe-C distances of 2.06 and 2.05 A for the two samples, If the supposition that A1Po4-5 does actually contain respectively. The small amount of ferricenium was not Bronsted acid sites is correct it does not seem unreasonable detected. Related studies36 on A1PO4-5 :cobaltocene channel to couple the ferricenium sites with Bronsted acidity.It is inclusion compounds also show that the metallocene is present known from zeolite chemistry35 that the inner surface of the as such. acid forms are highly reactive. Moller et have studied the The Debye-Waller-type factor, A, from the EXAFS data chemistry of ferrocene in zeolite Y cavities. They conclude analysis (EXCURV90) can be compared with the root mean that thermally decomposing ferrocene at 200 "C in partially square vibrational amplitude, u, obtained from gas-phase acidic zeolite Y results in the formation of zeolite-attached electron diffraction (where u,= u of EXAFS). This calculation half-sandwich fragments, Fe+(Cp)(OZ),- . These workers sug-gives an A value of 0.0077 A2 for the gas phase.FOFAlP04-gest that the half-sandwich FeCp is bonded to two zeolite 5:Fe(Cp), the Debye-Waller-type factor is 0.0091 A2. These oxygen atoms thereby producing FeCp(OZ), . Although this values are similar within experimental error, and they show material is also green, our EXAFS data clearly show that that on going from the gas phase to the solid state the more A1PO4-5:ferrocene contains ferrocene and not a bound half- distant atoms of the lattice play only a minor role. Moreover, J. MATER. CHEM., 1994, VOL. 4 the gas phase value would seem to indicate that this effect is indeed negligible. The room temperature Mossbauer results3, reveal that molecular axes of the ferrocene guest molecules reorientate ca. > lop8 s-') rapidly within the channel of the A1PO4-5 host.This is reminiscent of the thiourea :ferrocene ~ystem.~~,~~ The same experiment is also consistent with the presence of ferricenium cations at ca. 25% of the ferrocene concentration. This lends support to the suggestion (see above) that the intense green colour indicates the presence of ferricenium in the sample. One of the aims in preparing A1P04-5:ferrocene was to study the decomposition of ferrocene within the A1PO4-5 molecular sieve. This is discussed in some detail below. Calcined A1P04-5:Fe(Cp), Fe-Fe bond distances in a material depend on the nature qf the metal environments. Thus, in a-Fe the distance is 2.48 A but in !morphous Fe metallic glasses a typical Fe-Fe distance is 1.8 A.39 This large decrease in bond length is consistent with increased covalent character of the bonds between Fe and the nonmetallic impurities in the glass.Clearly, we have to examine the possibility that formation of small metal clusters have been formed within the channels, and that interactions between them and the channel atoms also could produce short Fe-Fe distances. Hence, we have to examine the EXAFS for evidence of short and long Fe-Fe interactions. The first shell contribution to the EXAFS can only be fitted to oxygen-backscatters. From the parameters for the two decomposed materials (Table 2) it is clear that iron clusters are not formed on decomposing the inclusion compounds since Fe-Fe bond distances are absent. The absence of Fe---Fe interactions shows that the metal atoms are well dispersed in the structure and accordingly significant aggre- gation through oxygen-bridged Fe ---Fe clusters [for example in the form of FeO(0H)-based species] is excluded. We therefore conclude that the majority of the iron atoms are distributed within the structures at considerable distances from each other.Material (1 ) A1P04-5:Fe-decomposed in vacuo. The EXAFS analysis shows that iron is bonded to four oxygen atoms with longer range interactions to two additional oxygen atoms. Although, EXAFS (neglecting multiple scattering) does not contain information on the spatial distribution of the oxygen atoms about the central atom, there is information contained on this aspect in the pre-edge region.,' For some iron environ- ments there is a pre-edge feature that is assigned to Fe ls+3d transition^.^^ Although these transitions are forbidden by dipole selection rules they become allowed when the Fe 3d and 0 2p states mix.It has been shown that the position of the pre-edge peak42.43 is sensitive to the iron valence state and that its intensity is sensitive to the number of ligands and to the symmetry of the metal environment. For example, the intensity of the pre-edge feature is several times larger for tetrahedral Fe"' compounds than for octahedral Fe"' com-.~~pounds. Roe et ~ 1 have measured the areas of the pre-edge peak for 28 Fe"' complexes of varying coordination. The largest areas are shown consistently for tetrahedral environ- ments and the smallest for six and seven coordination.The decrease in area in going from four coordination to six coordination reflects the ligand field of the various symmetries. The pre-edge peak can therefore be used as a diagnostic test for tetrahedral Fe"' environments. A key example (FePO,, berlinite, a-quartz structure) is shown in Fig. 6 for a tetrahedral Fe"' environment. In berlinite the average 0-Fe -0 angle is tetrahedral within experimental error, although the individual I I I II II I I I 7100 7110 . 7120 7130 7140 energyleV Fig. 6 Normalised pre-edge intensity of (a) FePO, and (b) AIPO,: Fe (black) product and (c) AlPO, :Fe (brown) product angles range from 105.5 to 114.8'. The actual point symmetry is C2 (e.g.mixing of Fe 3d and 0 2p states, see above), but is close to Td.The distortion from the ideal angle allows mixing of orbitals that now have the same irreducible representation under the operations of the point group.Also shown is the pre-edge region for the A1P04-5 :Fe material ( 1). The edge energy (7124 eV) is similar to that for berlinite (7123 eV) and therefore defines the valence state as Fe"'. Comparison of the pre-edge regions of Fig. 6 shows that the Fe(O), environment in A1P04-5 is not tetrahedrally coordinated Fe"' and is consistent with the 4Fe-0 and 2Fe-0 distances derived from the EXAFS. The high temperature decomposition of ferrocene in the A1PO4-5:ferrocene takes place in an environment made chemically aggressive by the elevated temperature. From the EXAFS and from chemical analysis it is clear that ferrocene decomposed within the channels and was not sublimed out of the AlPO,.The availability of both Bronsted (see above) and Lewis sites makes possible the attachment of Fe"' to the channels. By using molecular modelling (INSIGHTII/ DISCOVER software package45) it is possible to visualise iron atoms coordinated to four framework oxygen atoms at Fe-0 distances that ace consistent with the observed com- posite distance of 1.95 A. These distances, which are longer than those in berlinite (Table 2),23 are characteristic of higher coordination numbers as indicated by the pre-edge data (Fig. 6). The distances derived from EXAFS show that iron is positioned within the double six-ring of the A1P04-5 structure.This is illustrated in Fig. 7 which shows details of the iron coordination. The iron atom is displaced from the centre of the ring thereby distorting the local lattice. The interaction between iron and A1P04-5 can be understood in terms of coordination from oxygen atoms of the P-0-A1 bridges to Fe3+, the formal charge being compensated by the anionic framework charges caused by the lattice imperfections (see above). A similar picture has been suggested by Lee et al. for CUSAPO,-~.~~ The two longer Fe---0 distances of 2.59 A reflect weak interactions within Fe -0-P and/or Fe -0-A1 bridging. Consistent with this interpretation are the Fe ---A1 and Fe ---P distances listed in Table 2.Interactions of this type have been described as secondary bonds by Alco~k.,~ These bonds are defined as those interatomic distances, often regarded as non- J. MATER. CHEM., 1994, VOL. 4 Fig. 7 Schematic model illustrating the displacement of the Fe"' atom from the symmetry axis of the six-ring as the ring system locally distorts from the symmetrical arrangement (left) to the unsymmetrical environment (right). Bonds between the atoms are not shown. Note that only every other P---A1 distance is bridged by an 0 atom. bonded distances, which are longer than normal bonds but shorter than van der Waals distances. Examples of other long distances in zeotypes include the Co-0- (Si,AI) bridges ~ [2.36( 3) A] found in cobalt(1r) exchanged zeolite A.48 Material (2) A1P04-5:Fe-decomposed in air.The AlP04- 5 :ferrocene decomposed jn air also yields four composite Fe-0 distances at 1.91 A that are similar to those in the above material (1). However, the iron environments differ somewhat because sample (2) contains four secondary Fe--- 0 interactions (Table 2). The pre-edge feature has also been investigated for the AIP04-5:Fe (2) material and Fig. 6 shows the pre-edge region. The edge energy (7127eV) is close to that for berlinite (7123 eV) and consistent with Fe"'. Furthermore, the pre-edge regions of Fig. 6 shows that the Fe(0)4 environment in the AlPO is not close to tetrahedral Fe"' but has a higher coordination. This lends support to the 4Fe-0 and 3Fe-0 interactions derived from the EXAFS (see above).The differ- ences between the two samples are small and can be attributed to adjustments of the local lattice about iron during the much longer period of annealing. Conclusions It is clear from this study that decomposition of ferrocene included in an A1P04-5 lattice both in the absence and presence of oxygen results in Fe3+ moving into the double six-rings of the framework. Bridging to six or more oxygens of the surroundings leads to irregular coordinations about Fe3+. The formal charge is balanced by anionic framework charges introduced via lattice defects. It is also clear that iron clusters are not formed within the molecular sieve when the guest molecule contains a single metal atom. A major factor that probably prevents clustering is the spacing of the ferrocene molecules along the channel axes.T>ese spacings are dictated by the periodic variation (8.410 A, c-direction) of the potential. Hence, if clustering is required, the most likely route for producing small clusters of metal atoms within the AlPO would seem to be to incorporate polymetal compounds into the host channels prior to calcination. Support of this work by the Norwegian Research Council for Humanities and Science (NAVF), VISTA (STATOIL) and the Nansen Foundation is gratefully acknowledged. We also appreciate the constructive comments of the referees. Part of this work was performed at X-11A Beamline at the National Synchrotron Light Source and is supported by the US Department of Energy under Contract numbers DE-AC02- 76CH00016 and DE-FG05-89ER45384.References 1 R. M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular Sieves, Academic Press, London, 1978. 2 Y. Murakami, A. Lijima and J. W. Ward, New Developments in Zeolite Science and Technology, Kodansha, Tokyo, 1986. 3 Studies in Surface Science and Catalysis, ed. P. J. Grobet, W. J. Mortier, E. F. Vansant and G. Schulz-Ekloff, Elsevier, Amsterdam, 1988, p. 37. 4 E. M. Flanigen, B. M. Lok, R. L. Patton and S. T. Wilson, Pure Appl. Chem., 1986,58,1351. 5 C. W. Lee, X. Chen and L. Kevan, Catal. Lett., 1992,15,75. 6 G. Dworezkov, G. Rumplmayr, H. Mayer and J. A. Lercher, Adsorption and Catalysis on Oxide Surfaces, Proceedings of a Symposium, Uxbridge, June 28-29, 1984, ed.M. Che and G. C. Bond, Elsevier, Amsterdam, 1985, p.163. 7 A. Bhattacharya, J. Das, S. Mitra and S. K. Roy, J. Chem. Tech. Biotechnol. 1992,54, 399. 8 V. R. Choudhary and D. B. Akolekar, J. Catal., 1987,103,115. 9 S. T. Wilson, B. M. Lok, C. A. Messina, T. R. Cannan and E. M. Flanigen, J. Am. Chem. Soc., 1982,104, 1146. 10 D. W. Breck, Zeolite Molecular Sieves, Wiley, New York, 1974. 11 R. Szostak, Molecular Sieves Principles of Synthesis and Zdentijcation, Van Nostrand Reinhold, New York, 1989. 12 P. A. Jacobs, Stud. Surf. Sci. Catal., 1982, 12, 71. 13 T. Bein and P. A. Jacobs, J. Chem. Soc. Faraday Trans. I, 1983, 79, 1819. 14 T. Bein and P. A. Jacobs, J. Chem. Soc. Faraday Trans.1, 1994, 80, 1391. 15 G. A. Ozin and J. Godber, ACS Symp. Ser., 1986,307,212. 16 G. A. Ozin, A. Kuperman and A. Stein, Angew. Chem., Int. Ed. Engl., 1989,28, 359. 17 G. A. Ozin and C. Gil, Chem. Rev., 1989,89,1749. 18 G. A. Ozin and J. Godber, J. Phys. Chem., 1989,93,878. 19 A. F. Ojo, J. Dwyer, J. Dewing, P. J. O'Malley and A. Nabhan, J. Chem. SOC., Faraday Trans, 1992,88,105. 20 N. J. Tapp, N. B. Milestone and D. M. Bibby, Zeolites, 1988, 8, 183. 21 J. D. Dunitz, L. E. Orgel and A. Rich, Acta Crystallogr., 1956, 9, 373. 22 A. Haaland and J. E. Nilsson, Acta Chem. Scand., 1968,22,2653. 23 H. N. Ng and C. Calvo, Can J. Chem., 1975,53,2064. 24 F. W. Lytle, Nucl. Instr. Methods Phys. Res., 1984,226, 542. 25 International Tables for X-Ray Crystallography, The Kynoch Press, Birmingham, UK, 1962, p.175.1730 J. MATER. CHEM., 1994, VOL. 4 26 27 28 29 30 31 32 33 34 35 J. M. Bennett, J. P. Cohen, E. M. Flanigen, J. J. Pluth and J. V. Smith, ACS Symp. Ser., 1983,218, 109. S. J. Gurman, N. Binsted and I. Ross, J.Phys. C: Solid State Phys., 1984, 17, 143. S. J. Gurman, N. Binsted and I. Ross, J. Phys. C: Solid State Phys., 1986,19,1845. N. Binsted, S. J. Gurman and P. C. Stephenson, SERC Daresbury Laboratory Programs: EXCALIB, EXBACK and EXCURV90. R. W. Joyner, K. J. Martin and P. Meehan, J. Phys. C: Solid State Phys., 1987, 20, 4005. M. Rosenblum, in The Chemistry of Organometallic Compounds, A Series of Monographs, ed. D. Seyferth, John Wiley, New York, 1965. A. Moen, Ph.D. Thesis, The University of Trondheim, AVH, to be submitted.A. Lund, Ph.D. Thesis, The University of Trondheim, AVH, sub-mitted February 1994. A. Lund, D. G. Nicholson, R. V. Parish and J. P. Wright, Acta Chem. Scund., in press. K. Moller, A. Borvornwattananont and T. Bein, J. Phys. Chem., 1989,93,4562. 37 38 39 40 41 42 43 44 45 46 47 48 M. D. Lowery, R. J. Witterbort, M. Sorai and D. N. Hedrickson, J. Am. Chem. SOC., 1990,112,4214. S. J. Heyes, N. J. Clayden and C. M. Dobson. J. Phys. Chem., 1991,95,1547. S. G. Saxena and K. B. Garg, Phys. Status Solid1 A, 1985,87, K25. D. Sunil, J. Sokolov, M. H. Rafailovich, R. Kotyuzhanskii, H. D. Gafney, B. J. Wilkens and A. L. Hanson, J. Appl. Phys., 1993,74,3768. A. Bianconi, in EXAFS for Inorganic Systems, ed. C. D. Garner and S. S. Hasman, Science and Engineering Research Council of Daresbury Laboratory, Daresbury, 1981, 13. G. A. Waychunas, M. J. Apted and G. E. Brown, Phys. Chem. Miner., 1983, 10, 1. N. Motta, M. De Crescenzi and A. Balzarotti, Phys. Rev: B, 1983, 27,4712. A. L. Roe, D. J. Schneider, R. J. Mayer, J. W. Pjrz, J. Widom and L. Que, Jr., J. Am. Chem. SOC., 1984,106,1676. INSIGHT11 version 2.1.O, DISCOVER version 2.8, INSIGHTIIi DISCOVER, Biosym Technologies, San Diego, CA, USA, 1992. C. W. Lee, X. Chen and L. Kevan, Catal. Lett., 1992,15,75. N. W. Alcock, Adv. Inorg. Chem. Radiochem., 1972,15,2. P. E. Riley and K. Seff, J. Phys. Chem., 1975,79. 1594. 36 M. Endregard, Ph.D. Thesis, The University of Trondheim, AVH, to be submitted. Paper 4/01 105H;Received 23rd February,1994

ISSN:0959-9428    

DOI:10.1039/JM9940401723

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(11), 1731-1735 Properties of the Guest Molecules in the 1,lo-Dibromodecane/Urea Inclusion Compound A Molecular Dynamics Simulation Study Ashley R. Georgeayband Kenneth D. M. Harris*a a Department of Chemistry, University College London, 20 Gordon Street, London, UK WCI H OAJ The Royal Institution, 21 Albemarle Street, London, UK W1X 4BS A molecular dynamics (MD) simulation of the 1 ,lo-dibromodecane/urea inclusion compound has been carried out at a temperature of 300 K to investigate various local structural properties of the 1,lO-dibromodecane guest molecules within the urea tunnel structure. The bromine radial distribution function determined from the MD simulation indicates a broad distribution for the intermolecular Br.. -Br distance, but considerably narrower distributions for intramolecular Br.-.C distances.This supports information determined experimentally via bromine K-edge EXAFS spectroscopy. The MD simulation provides evidence that a small proportion of the 1,lO-dibromodecane molecules contain a gauche end-group, and provides direct evidence for the interconversion between gauche and trans end-group conformations on a timescale of the order of picoseconds. The MD simulation indicates that there is substantial dynamic disorder of the 1,lO-dibromodecane guest molecules within the urea tunnel structure. These local structural properties and dynamic properties of the 1,lO-dibromodecane guest molecules, determined from the MD simulation, are compared directly with corresponding information established via experimental approaches.Urea inclusion compounds have been investigated widely from the viewpoint of their fundamental physico-chemical proper tie^.'-^ The ‘host’ substructure in the ‘conventional’ urea inclusion compounds comprises an extensively hydrogen- bonded arrangement of urea molecules, and this structure contains a regular arrangement of one-dimensional, parallel ‘Guest’ molecules of appropriate dimensions can be accommodated within these tunnels. The cross-section of the tunnels (defined by the van der Waals s;rface of the tunnel wall) is comparatively narrow (ca. 5.5-5.8 A) and, as a conse- quence, only guest molecules based on a sufficiently long n-alkane chain and with an appropriately limited degree of substitution (e.g.a, m-dibromoalkanes [Br(CH,),Br)] can fit within these tunnels. Within the urea tunnel structure, these guest molecules must adopt a linear, extended conformation, and inclusion within the urea tunnel structure has often been used as a means of constraining molecules in such confor- mations. Importantly, these conformations may differ substan- tially from the preferred conformations of the same molecules in other solid-state environments or in dispersed phases. The urea inclusion compounds have thus been exploited as proto- typical materials for understanding the structural, dynamic and spectroscopic properties of molecules in linear, extended conformations. The physico-chemical properties of urea inclusion com-pounds containing alkane guest molecules have been studied extensively2 via a wide range of experimental techniques, although, at present, much less is known about the corre- sponding properties of urea inclusion compounds containing functionalized alkane guests.In this paper, we consider the application of MD simulation techniques to study local structural properties and dynamic properties of the 1,lO-dibromodecane [Br(CH,),,Br] guest molecules in the Br(CH2 ),,Br/urea inclusion compound, and we first review relevant experimental information on this material. The periodic repeat distance of the guest molecules along the urea tunnel (c,) is, in general, incommensurate with the periodic repeat distance of the host substructure along the tunnel; a detailed discussion of incommensurate us.commen-surate behaviour in one-dimensional inclusion compounds (typified by the urea inclusion compounds) has been given else~here.~?~The periodic repeat distance, c,, of guest mol- ecules in the :conventional’ urea inclusion compounds is always ca. 0.5 A shorter than the ‘van der Waals length’ of the guest molecule in its ‘all-trans’ conformation.’ This arises from the fact that, in the energetically most stable state of the inclusion compound, there is a repulsive interaction between adjacent guest molecules in the same t~nnel.~,~ Recent incoherent quasielastic neutron scattering studies’ of the dynamic properties of the guest molecules in Br(CH,),Br/urea inclusion compounds (n=8-10) have shown that both reorientational motions of the guest molecules about the tunnel axis and translational motions of the guest mol- ecules along the tunnel axis are effective on the picosecond timescale.The translation olength depends critically upon temperature, and is ca. 2.3 A for Br(CH,),Br/urea at 280 K. The possibility to determine dynamic properties of the guest molecules from the MD simulations carried out in this work is discussed further below. A recent Br K-edge EXAFS investigation” assessed the feasibility of determining local structural information for the guest substructures of ‘conventional’ urea inclusio11 com-pounds containing Br(CH,),Br (n=7-1 1) guest molecules. One major aim of these Br EXAFS studies was to determine the Br...Br distance between adjacent guest molecules in the urea tunnel.(This is particularly important in vieu of the theoretical prediction that the interaction between adjacent guest molecules in the urea tunnel is repulsive, corresponding to a short intermolecular Br--.Br distance.) However, the Br..-Br distance could not be determined accurately from the Br EXAFS data collected between room temperature and 77 K [and also at 9 K for Br(CH,),,Br/urea]; this can be attributed to dynamic disorder in the guest substructure at high temperature and static positional disorder at lcjw tem-perature. In these Br EXAFS studies, backscattering from the first three intramolecular C neighbours was detected. As a consequence of the incommensurate structural relationship between the host and guest substructures along the tunnel axis, no well defined features arising from backscattering by atoms of the host substructure were observed in the Br EXAFS data.Raman spectroscopic investigations of urea inclusion com- pounds containing Br(CH,),Br (n=7-1 1) guest molecules have been reported,” with the C-Br stretching vibration J. MATER. CHEM., 1994, VOL. 4 studied as a function of the length (n) of the guest molecule, temperature and pressure. From these results, trends in the relative amounts of gauche and trans end-groups for the Br(CH,),Br guest molecules within the urea tunnel structure were assessed. For Br(CH,),,Br/urea, both trans and gauche end-group conformations were detected, with ca.11.5% of end-groups in the gauche conformation at room temperature. Computational Details In the MD simulation, a single tunnel of the Br(CH,),,Br/urea inclusion compound was considered. Despite the approxi- mations used here, our model for the Br(CH,),,Br/urea inclusion compound is nevertheless more sophisticated than that used in recent simulations12 of similar inclusion com- pounds, in which only a single guest molecule was considered. The urea tunnel structure determined previously5 for Br(CH,),,Br/urea from room-temperature single-crystal X- ray diffraction data was used; this is the hexagonal tunnel structure of the conventional urea inclusion compounds. In the MD simulation, the length of the tunnel corresponded to 18 unit ce!ls of the host substructure (periodic repeat distance, ch =11.0 A), and 11 guest molecules [Br(CH,),,Br] were considered.Because both host and guest substructures are confined within the simulation cell, a commensurate relation- ship between the host and guest substructures is effectively imposed upon the system in the MD simulation; nevertheless, the host :guest ratio for the simulation cell considered here is acceptably close to the experimental host :guest ratio (the experimental periodic repeat dist?nce of the guest molecules along the tunnel axis is cg=18.0A at room temperature). In the MD simulation, the host substructure was held rigid and the position of the Br atom at each end of the tunnel was fixed (in their fixed positions, these two Br atoms were on the 61 symmetry axis of the host structure).These constraints represent an approximation to a constant-volume simulation. Apart from the restriction that the positions of two Br atoms in the structure were fixed, the Br(CH,),,Br molecules were allowed to move freely in the MD simulation. Although these constraints limit the ability of the simulation to provide meaningful information on certain properties (e.g. diffusion parameters) of the guest molecules, they nevertheless allow reliable information to be derived for local structural proper- ties of the guest molecules provided only those guest molecules close to the centre of the simulation cell are considered. The MD simulation was carried out within the DISCOVER program package,13 and the potential-energy parameterization embodied within this package was used.The Coulombic energy contribution of a molecular system of the type con- sidered here is comparatively small, and thus the use of an Ewald summation is not essential (unlike the situation for simulations of ionic systems). The MD simulation was run for 20 x s for equilibration, and then for 200 x s during which the structural properties were determined every 1OOx lo-’’ s. The thermal energy was equivalent to a tem- perature of 300K and a numerical integration step of 1 x s was used in the Verlet a1g0rithm.l~ The evolution of the structure was monitored during the MD simulation, and selected properties were calculated sub- sequently for those guest molecules close to the centre of the simulation cell.The Br radial distribution function (RDF) was measured for the nearest intramolecular C neighbours and for the nearest intermolecular Br neighbour. The end- group conformations were probed by monitoring the intra- molecular Br-C( l)-C(2)-C(3) torsion angles; plots show- ing the variation of this angle as a function of time yield information on the dynamics of the conformational changes in the guest molecule. Results and Discussion Fig. l(u) shows the structure at the beginning of the MD simulation, with all Br(CH,),,Br molecules taken to be in the ‘all-trans’ conformation. Fig. l(b) shows a snapshot of the structure, viewed along the direction of the urea tunnel axis, at the end of the MD simulation.It is clear that there is substantial positional and orientational disorder of guest molecules within the host tunnel. The extent of disorder of the guest molecules will result in a rapid diminution in the intensity of X-ray scattering from the guest substructure with increasing diffraction angle, as observed experimentally.* The occurrence of substantial motion of the guest molecules in the Br(CH,),,Br/urea inclusion compound is in agreement with conclusions from incoherent quasielastic neutron scattering’ and ,H NMR” studies of this material. Fig.2(u) shows the Br RDF determined during the MD simulation. This RDF contains sharp, well defined peaks for the first four intramolecular Br...C distances (Table 1); the linewidth increases for C neighbours further away from the Br atom, in the manner expected for a flexible molecule.The very broad peak at 3.7A in the Br RDF [expanded in Fig. 2(b)] represents the intermolecular Br-.. Br distance for neighbouring Br(CH,),,Br molecules in the urea tunnel. The P 9 P Fig. 1 (a) Structure of Br(CH,),,Br/urea (both host and guest substructures shown) at the start of the MD simulation, viewed along the urea tunnel axis. Note that all guest molecules are in the all-trans conformation, although the exact orientations of the guest molecules differ slightly. (b) Structure of Br(CH,),,Br/urea at the end of the MD simulation, illustrating the extent of disorder within the guest substructure. J.MATER. CHEM., 1994, VOL. 4 1 (I I1 I' I' I' c I' I! I distance/A Fig.2 Bromine RDF for Br(CH,),,Br/urea determined for the five guest molecules towards the centre of the simulation cell in the MD simulation: (a) the full Br RDF; (b) the Br RDF expanded in the region of the Br...Br peak. (-) Br..-Br; (---) Br...C(l); (---) Br...C(2); (-.-) Br..C(3);(--.-) Br-..C(4). Table 1 Intramolecular Br-C distances (r) and root-mean-squared displacements (CT)in these distances for Br(CH,),,Br/urea r/A CT/A experimental calculated experimental calculated Br-C( 1) 1.96 1.921 0.083 0.036 Br--C( 2) 2.83 2.824 0.124 0.056 Br--C( 3) 3.91 4.232 0.114 0.074 Br--C( 4) -5.373 -0.082 Calculated data were obtained from the MD simulation (at 300 K) reported here, and experimental data were obtained from the Br EXAFS studies (at 296 K) reported in ref.10. large width of this peak, in comparison to the widths of the peaks for intramolecular Br.. .C distances, is particularly significant in view of the fact that the Br.-.Br distance could not be determined from Br EXAFS data on Br(CH,),Br/urea inclusion compounds. Specifically, the width of the distri- bution for the Br..-Br distance (corresponding to substantial local structural disorder and/or dynamic disorder) will render backscattering due to the intermolecular Br neighbour essen- tially insignificant in the Br EXAFS experiment. The asym- metry of the Br-..Br peak in the Br RDF is noteworthy [see Fig.2(b)],There is a well defined low-distance cut-off just above 3 A, whereas at large tistances the RDF tails off gradually, extending beyond 6 A. Note that the sum ,of the van der Waals radii for a pair of Br atoms is ca. 3.9 A. The neighbouring Br atoms are apparently located mainly in regions corresponding to a repulsive Br- .Br interaction (in agreement with theoretical predictions6) although local con- figurations with an attractive Br..-Br interaction are also sampled. The Br-..C distances are sufficiently well defined to allow these distances to be extracted from the Br EXAFS data; this is vindicated by the presence of sharp peaks for Br...C distances in the Br RDF determined from the MD simulation. The Br.-.C distances determined from the MD siniulation and from the Br EXAFS data are compared in Table 1.For the third and fourth neighbouring C atoms, the Br-..C distance is represented by a pair of peaks in the Br RDF: a main peak and a small broad peak on the low-distance side of the main peak. The small broad peak can be attributed to confor- mations [containing gauche end-groups (vide infra)] that differ substantially from the major (all-trans) conformation as dis- cussed in more detail below. In principle, it should be feasible to correlate roo t-mean- squared displacements in the Br.--C distances extracted from the appropriate peaks in the calculated Br RDF with the experimental root-mean-squared displacements determined from Debye-Waller factors obtained in fitting the Br EXAFS spectra.However, acceptable agreement between these calcu- lated and experimental root-mean-squared displacemmts was not obtained (Table l), and this failure is attributed to a combination of factors arising, in part, from the ;tpproxi- mations embodied within our MD simulation (particularly the fact that the terminal Br atoms in the simulation cell were fixed in position), and, in part, from the fact that Debye- Waller factors extracted from the Br EXAFS spectra can often subsume substantial errors in the fitting procedure. €or these reasons, it is not appropriate here to attempt a detailed comparison of calculated and experimental root-mean-squared displacements, but rather it is prudent to recognize the potential difficulties in attempting to make comparisons of this type.The existence of a proportion of gauche end-group.; for the guest molecules in the Br(CH,),,Br/urea inclusion compound is in agreement with our findings from Raman spectroscopy." From the conformational trajectory, the variation in the torsion angles in the guest molecules has been determined as a function of time; Fig. 3 reports the variation of the terminal torsion angles [Br-C( 1)-C(2)-C(3)] for two different mol- ecules located towards the centre of the simulation cell. The molecule probed in Fig. 3(a) has remained in the trans confor-mation during the simulation, with fluctuations (up to ca.+25") in this torsion angle attributed to thermal vibration." For the molecule probed in Fig.3(b), on the other hand, a trans-gauche-trans conformational charige has occurred during the simulation. The lifetime of thc gauche conformation is ca. 4.8x s, and is appreciabl? shorter than the lifetime of the trans conformation (as sampled for other molecules). Fig.4 shows snapshots of sections of the guest substructure illustrating the conformations ot interest (for clarity, the host substructure is not shown). In Fig. 4(a) all the molecules shown have trans end-groups, whereas in Fig. 4(b) the central molecule has a gauche end-group. The proportion of gauche end-groups, averaged over the simu- lation, is ca. 1.9% (the corresponding value determined exper- imentally" is ca. 11.5%). Thus, our MD simulation has verified the existence of gauche end-groups for Br((:H,),,Br guest molecules within the urea tunnel structure, although it is important to emphasize that good quantitative agreement between the results from the MD simulation and the exper- imental results is not necessarily expected, in view of the approximations embodied within our simulation, and in view of the fact that the number of trans-gauche interconversions observed in the MD simulation was insufficient to give an accurate time average of the conformational populations.Concluding Remarks As discussed above, computer simulation of incommensurate materials requires the use of large simulation cells, often J. MATER. CHEM., 1994, VOL. 4 t -80.-12 J I I I I I 0 -40 -80 -1 20 trans end-group \'gauche end-group -1 60 I I I I I I I I I I 8000 16000 24000 32000 40000 time/fs Fig.3 Conformational trajectories, as a function of time, for the Br-C( l)-C(2)-C(3) torsion angles of two Br(CH,),,Br molecules towards the centre of the simulation cell. [The value of angle reported on the graphs represents the deviation of the Br-C( 1 )-C(ZtC( 3) torsion angle from the value (180 "C) for a trans end-group.] For (a)the end-group has remained trans during the simulation, whereas for (bl the end-group has undergone a trans-gauche-trans conformational change during the simulation. gauche end-group \ Fig. 4 Snapshots of the structure of Br(CH,),,Br/urea (with the host substructure omitted for clarity) illustrating: (a) a section of the set of guest molecules in which all end-groups are trans; (b)a section of the set of guest molecules in which one molecule has a gauche end-group.prohibitively large. Nevertheless, if valid simplifying assump- tions can be made, the extent of the problem can often be reduced to manageable proportions. In the present case, the approximation of considering a single tunnel of the inclusion compound, without the application of periodic boundary conditions, has been shown to produce results on local structural properties that agree well, at least qualitatively (and in some cases quantitatively), with information derived from experimental approaches. Specifically, in the present case: (i) the Br RDF determined from the MD simulation is consistent with information determined experimentally uia Br K-edge EXAFS spectroscopy; (ii) the MD simulation has demonstrated the existence of gauche end-group confor- mations of the guest molecules and has provided direct J.MATER. CHEM., 1994, VOL. 4 evidence for the interconversion between trans and gauche end-group conformations on a timescale of the order of picoseconds; (iii) the MD simulation implies that there is substantial dynamic disorder of the Br(CH,),,Br guest mol- ecules within the urea tunnel structure. The present work has illustrated the usefulness of computer simulation as an aid to the interpretation of experimental data and the rationalization of experimental observations. For example, the MD simulation of Br(CH,),,Br/urea reported here has provided a clear understanding of the absence of information from Br EXAFS data on the intermol- ecular Br..-Br distance, and has provided direct evidence for interconversion between trans and gauche end-group confor- mations and an estimate of the lifetime of the gauche con-formation. Nevertheless, in order to derive a deeper understanding of the dynamic properties and the long-range structural properties of the Br( CH,),,Br/urea inclusion com-pound, MD simulations are required in which some of the simplifying approximations embodied within the present work have been eliminated. We are grateful to SERC for financial support (studentship to A.R.G.), and to BIOSYM Technologies Inc. for providing the INSIGHT/DISCOVER suite of programs.Professor Sir John Meurig Thomas, Professor C. R. A. Catlow and Dr. G. Sankar are thanked for useful discussions in connection with this work. References 1 K. Takemoto and N. Sonoda, in Inclusion Compoiinds, ed. J. L. Atwood, J. E. D. Davies and D. D. MacNicol, Academic Press, London, 1984, vol. 2, p. 47. 2 K. D. M. Harris, J. Solid State Chem., 1993, 106, 83. 3 F. Guillaume, A. El Baghdadi and A-J. Dianoux, Phyx. Scr. T, 1993, 49,691. 4 A. E. Smith, Acta Crystallogr., 1952, 5,224. 5 K. D. M. Harris, and J. M. Thomas, J. Chem. Soc., Faraday Trans., 1990, 86, 2985. 6 A. J. 0.Rennie, and K. D. M. Harris, Proc. R. Soc. LCndon, A, 1990,430,615. 7 A. J. 0. Rennie, and K. D. M. Harris, J. Chem. Ph.:ts., 1992, 96, 7117. 8 K. D. M. Harris, S. P. Smart and M. D. Hollingsworth, J. Chem. Soc., Faraday Trans., 1991,87, 3423. 9 F. Guillaume, S. P. Smart, K. D. M. Harris and A-J. Ilianoux, J. Phys.: Condensed Matter, 1994,6,2169. 10 I. J. Shannon, K. D. M. Harris, A. Mahdyarfar, P. Johnston and R. W. Joyner, J. Chem. SOC.,Faraday Trans., 1993,89,3[199. 11 S. P. Smart, A. El Baghdadi, F. Guillaume and K. D. hl. Harris, J. Chem. SOC., Faraday Trans., 1994,90, 1313. 12 K. J. Lee, W. L. Mattice and R. G. Snyder, J. Chem. Phys., 1992, 96,9138. 13 DISCOVER, BIOSYM Technologies Inc., San Diego, ('A, USA, 1993. 14 L. Verlet, Phys. Rev., 1967,159,98. 15 A. E. Aliev, S. P. Smart, I. J. Shannon and K. D. hi. Harris, manuscript in preparation. 16 B. D. Hudson, A. R. George, M. G. Ford and D. J. Lixingstone, J. Comput. Aided Mol. Des., 1992,6, 191. Paper 4/02385D; Received 22nd April, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401731

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( ll), 1737-1744 Ion-exchange Properties of Lithium Aluminium Layered Double Hydroxides Ian C. Chisem and William Jones* Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, UK CB2 I EW The synthesis of layered lithium aluminium hydrotalcite-like materials is described along with different anion exchange procedures for the preparation of materials intercalated with chloride, nitrate and vanadate. The products have been characterised using elemental chemical analysis, powder X-ray diffraction, Fourier-transform infrared spectroscopy and thermogravimetric analysis. The matrices are found to be reasonably stable to acid treatment at pH 4.5 for periods of up to 72 h, with anion exchange taking place. Results indicate that total exchange of interlayer carbonate for chloride, nitrate and vanadate may be accomplished.The thermal properties of the materials have been studied: they demonstrate interesting differences in thermal behaviour compared with hydrotalcite. Layered double hydroxides (LDHs) are mixed-metal hydroxides with the general formula [MZ+1-xM3+x(OH)2]A+Xm-Alm*nH20,forwhere A=x z=2 and A=2x-1 for z=l.l A recent review describes their synthesis, properties and applications.2 They are re-lated to the naturally occurring mineral hydrotalcite [Mg6Al,(OH),6(CO,). 4H20] whose structure consists of a positively charged framework in which the cations occupy the octahedral sites between the sheets of close-packed anions. The layers are thus similar to those of brucite [Mg(OH),], but with a proportion of the divalent metal cations replaced by trivalent cations.The resulting excess positive charge is balanced by anions incorporated in the interlayer region. Such solids are comparable to the well known and widely studied cationic clays,, and thus hydrotalcite-like materials are some- times referred to as 'anionic clays'. Here, we examine the particular layered lithium alum- inate hydroxide system represented by the formula [LiAl,(OH),] +X-.nH,O, which consists of sheets of alu- minium octahedra with vacancies filled by lithium The symmetry of the cationic sheets is hexagonal, with AB-BA-AB-BA stacking. The sheets are neutralised by exchangeable anions, X-, which are located between the layers. A major difference in the structure of this material in comparison with M"/M"' LDHs, however, is that the latter Experimental Synthesis Details Analytical-grade reagents were used for all preparations described in this work.The various synthetic routes used are outlined in Scheme 1. LiAlCO, (Parent) [(LiA12(0H)6+)2]C032- -nH,O was prepared by a method similar to that used by Serna et al.' and based on a synthesis first described by Feitknecht.', AIC1,.6H20 (250ml of a 0.4mol 1-' solution) was added dropwise to 600ml of a mixture of 1.5 moll-' LiOH*H,O and 0.08 moll-' Na2C03 with vigorous stirring. The addition took 1 h. The pH varied from 12.8 (initially) to 10.2 (finally). The gel-type precipitate was then crystallised at 65°C in a thermal bath with gentle stirring for 18 h.When the product had been coolcd it was filtered off and washed several times with hot distilled water. The material was dried for 18 h at 100°C in air. Acid Exchange ofLiAlC0, LiAlCO, (1.0 g) was added to 150 ml distilled, deioniced water and the pH adjusted to 4.5 by the addition of 0.1 mol 1-' HNO, or HC1. The addition was accompanied by rapid expulsion of CO,. The mixture was stirred whilst maintaining The lithium aluminium system has, in general, been much less widely studied than its MI', M"' analogues. In particular, comparatively little is known about the exchange properties of these materials. Meyn et al., have examined the anion- exchange properties of a variety of LDHs including LiAl materials by exchanging the nitrate-intercalated LDH with aqueous solutions of organic acid salts, and Borja and Dutta7 have exchanged lithiurn aluminium chloride LDH with long- chain fatty acids.Exchange of chloride for 4-nitrohippuric acid was performed by Cooper and Dutta.8 Here, we demon- strate for the first time the anion exchange of carbonate for chloride and nitrate by exchange with acid by a method similar to that used by Bi~h','~ for the M" MI1'systems. A recent paper'' examining the properties of vanadate- exchanged MgAl LDHs has concluded that the decavanadate anion (V100,,6~) may be readily incorporated by a variety of synthetic routes. However, it has previously been reported that V,00286-may not be ion-exchanged into lithium aluminium LDHs.', Here we examine routes to vanadate-exchanged LiAl LDHs and attempt to compare the properties of the LiAl system with the MgAl system.show little evidence for cation ordering (i.e. M" and MIr1 the pH at 4.5 by further addition of acid. The reaction was occupy the same set of octahedral sites), whereas X-ray studies carried out over varying periods of time, of between 30min of the lithium compound show evidence of cation ~rdering.~*~ and 72 h. The resulting product was isolated by fillration or centrifugation and washed several times with hot distilled water. LiA1N03N(72) was prepared by reaction of ,.i mixture of 1.0 g LiA1CO3 in 150 ml distilled, deionised water contain- ing 0.1 mol 1-' NaNO, with the pH adjusted to pH 4.5 by addition of 0.1 mol I-' HNO,.The mixture was stirred whilst maintaining the pH at 4.5 by further addition of acid for a period of 72 h. The resulting product was isolated bj filtration or centrifugation and washed several times with hot distilled water. Direct Synthesis of LiAlCl (D) The preparation was similar to that used by Twu and Dutta', and Serna et a!.' Aluminium (0.05mol) was dissolved in 100ml of 2.0 mol 1-' NaOH solution. LiCl(0.25 moll-') was dissolved in this solution, and the mixture was heated for 48 h at 90°C under a nitrogen atmosphere. The products were washed with hot distilled water followed by 0.1 mol 1-' NaCl. Vanadate Intercalates A 150 ml solution of 0.1 mol 1-' NaVO, was adjusted to pH 4.5 by addition of 0.5 mol 1-' HCl (for LiAlCO, and LiAlCl J.MATER. CHEM., 1994, VOL. 4 heal to 150 "C for 18 hLiAlC03 direct synthesis b 1 treat with HCI LiAICl(0.5) I4h LiAlCl(4)a 124th -LiAICl(24) LiAIV(CI) treat with NaZC03 LiAIC03(CI) LiAK=O,(C) & # Itreatwith inpresence of glycerol LiAIV(CO3)G treat with HN03 LiAIN03(0.5) I4h LiAINO3(4)QI24h LiAIN03(24)1 LiAIN03(72)Qtreat with NaVOT I18hr-lLiAIV(N03) treat with HN03-NaN03I ,ltrrwilhiJdQ4Cl LiAIV(NO3N) treat with NaVOdCI I18h LiAIV(CID)U Scheme 1 The various routes followed for the synthesis of the carbonate, nitrate, chloride and vanadate materials precursors) or 0.5 mol 1-' HNO, (for LiAlNO, precursors).LiAlCO,, LiAlC1(24), LiAlN0,(72) or LiA1N03N(72) (1.0 g) was added and the pH was adjusted to 4.5 by addition of 0.1 mol 1-' acid. The mixture was stirred and the pH was maintained at 4.5 throughout the course of the reaction by further addition of 0.1 mol 1-' acid. The reaction was carried out for periods of between 4 and 24 h. The resulting product was isolated by filtration or centrifugation and washed with hot distilled water. Vanadate Exchange of LiAlCO, in the Presence of Glycerol To a suspension of 0.5 g of LiAlCO, in 50 ml distilled water and 100ml glycerol was added, with stirring, a solution of 0.45 g of NaV0, in 25 ml water acidified with 2.0 mol 1-1 HC1 to a pH of 4.5. The pH was maintained at this value for a further 5 h.During the reaction the system was purged with N, to remove evolved CO,. All processes were carried out at room temperature. The resulting product was isolated by centrifugation and washed with hot distilled water. Techniques Elemental chemical analysis for Li, A1 and V was performed using a Perkin-Elmer 3100 atomic absorption spectrometer. C and H were determined using a Perkin-Elmer 2400C instrument. Powder X-ray diffraction (PXRD) patterns were recorded using a Philips APD 1710 instrument with Ni- filtered Cu-Kn radiation. A step scan of 0.05" (28) at a rate of 0.05" s-' was used. The FTIR spectra of the solids were recorded at room temperature using a Nicolet 205 FTIR spectrometer using the KBr pellet technique, with a resolution of 2 cm-' and 60 scans averaged.Thermogravimetric analysis was carried out using a Polymer Laboratories TGA 1500 with a heating rate of 10°C min-' in all cases. Results and Discussion Elemental Analysis Results for chemical analysis of the samples are given in Table 1. The A1:Li ratio is found to be close to 2 for the parent LiAlCO, sample. Despite the fact that the A1 :Li ratio is rather high (2.5) for LiAlC1(24), there appear to be no systematic trends which might suggest preferential leaching of lithium or aluminium in these samples. For the vanadate- exchanged samples with chloride or carbonate precursors the ratio is significantly less than 2, being around 1.7&0.2. Where LiAlNO, is used as the precursor, the A1 :Li ratio is found to be close to 2.The vanadium content is found to remain fairly constant for samples with chloride or carbonate precursors (ca. 22 2%). In the case of vanadate-exchanged LiAlNO, mate- rials the vanadium content is significantly higher (ca. 27%). The carbon and hydrogen content were determined in selected samples (Table 2). The values are consistent with the presence of carbonate in the interlayer of LiAlCO, but its absence in LiAlNO,, LiAlCl and LiAlV (since a C content of 0.2-0.3O/0 is close to the sensitivity of the instrument). On the basis of the above data it can be assumed that the oxovanadate species V,O,"- are the only anions present in the interlayer of the vanadate-intercalated samples. The excess positive charge on the layers must therefore be balanced by J.MATER. CHEM., 1994, VOL. 4 1739 Table 1 Elemental analysis data for the samples synthesized sample Li(%) Al(Y0) V(%) (1-x) x A1:Li z/ua LiAlCO, 2.9 20.9 -0.35 0.65 1.87 -LiAlCO,(Cl) 2.5 20.7 -0.32 0.68 2.15 -LiAlCl(0.5) 2.8 21.3 -0.34 0.66 1.97 -LiAlCl(4) 2.6 20.8 -0.32 0.68 2.07 -LiAlCl(24) 2.4 23.1 -0.29 0.71 2.50 -LiAlCl(D) 3.1 23.2 -0.35 0.65 1.94 -LiAlNO,( 0.5) 2.8 21.3 -0.34 0.66 1.97 -~LiAlNO,( 4) 2.7 22.4 -0.32 0.68 2.15 LiAlNO, (24) 2.8 21.2 -0.34 0.66 1.96 -LiAlNO,( 72) 2.6 21.5 -0.32 0.68 2.14 -LiAlNO,N( 72) 2.7 22.2 -0.32 0.68 2.13 -LiAlV(C0,) 2.0 13.0 20.3 0.37 0.63 1.69 0.50 LiAlV( C0,)G 1.8 12.9 20.2 0.35 0.65 1.86 0.56 LiAlV( C1) 2.1 12.6 22.5 0.39 0.61 1.56 0.42 60 80 LiAlV (ClD) 2.1 14.3 23.2 0.36 0.64 1.77 0.51 2Bldegrees LiAlV( NO,) 2.0 15.3 26.3 0.33 0.67 1.98 0.56 LiAlV( NO,N) 2.1 15.8 27.2 0.34 0.66 1.95 0.53 Fig.1 Powder X-ray diffraction patterns for: (a) LiAlCO,, (b) " Assuming a formula [Li,~,A~,(OH),~(V,Ob)"~nH2OV-LiAlCl(O.5), (c)LiAlCl(4). (d) LiAlCl(24) and (e) LiAlCl(D Ifor the containing samples. higher values of 28. The basal spacing of 7.!7 A is slightly Table 2 Carbon and hydrogen content for a selection of the samples lowe! than those reported by Masco10'~ (7.60 A), Serna et al.l synthesized (7.6 A) and Sissoko et aL5 (7.56 A). It is well known that the basal spacing for hydrotalcite is influenced strongly by the sample c(Yo) H(%) drying treatment of the material, and hence small differences LiAICO, 2.5 4.0 in basal spacings are not significaat.The value oblained is LiAlCI(24) 0.2 3.7 indicative of a gallery height of 2.7 A, which !pproximates to LiAlNO,( 72) 0.2 3.3 the thickness of .a carbonate anion (2.84A). assuming a LiAlV(C0,) 0.3 2.7 thickness of 4.77 A for the cationic sheets. The material is relatively crystalline and has a well ordered sheet structure. as observed elsewhere. Indexing of the powder pattern based the negative charge on the interlayer anion, with the ratio of on the basis of an or4ered supercell' is given in Table 4.Thez/a balancing the aluminium content. The ratio is 0.53k0.03 reflection at d=4.38 A cannot be indexed on the basis of a for all samples except LiAlV(C1) which has a z/a ratio of 0.42.random cell, providing evidence for cation ordering (i.e. onlyThis sample also exhibits an unusually low Al: Li ratio of on the basis of an ordered cell can the pattern be indexed). 1.56. We can thus describe the anion as a single species with An alternative indexing based on a monoclinic cell5 is also the formula [V10027.5]s-,which may, within the grounds of given.experimental error, be reasonably assumed to be [Vlo02,]6-. (Assuming the vanadium to be present as V5+,an assumption LiAlCl Samplesgiven the inherent instability of V3+ in air.) The validity of Irrespective of the duration of acid treatment therc is little this analysis is somewhat open to question; it is highly likely variation in the observed c parameter when compared with that no single vanadium species is present in the interlayer, LiAlCO, [Fig.ol(a)-(d)].The observed values of c lie between but in fact a number of species are present. 14.9 and 15.2 A, whic! is slightly smaller than that reported by Mascolo14 (15.40 A) and Twu and Dutta12 (15.6 A) for Powder X-Ray Diffraction chloride-treated lithium aluminate. The values are similar to that obtained for LiA1C03, reflecting the similar size of theThe values for the c dimension of the various materials chloride and carbonate anions. As a result, PXRD gives little synthesized are given in Table 3. LiAlCO, Samples Table 4 Indexing of powder pattern for LiAlCO, The PXRD pattern of LiAlCO, [Fig. l(a)] is typical of that for an LDH, with sharp, symmetric basal (001) reflections at 20/ hexagonal cell" monoclinic cell' -~ lower values of 28 and relatively broad, weak reflections at degrees I/Io(%) dobs/A dcalc/A hkl dcalc/A hkl Table 3 c parameters for samples obtained from PXRD data 11.91 100.0 7.43 7.48 002 7.55 002 20.27 10.9 4.38 4.38 101 4.38 110 sample c/A sample CIA 23.72 67.9 3.75 3.74 004 3.78 004 ~ 35.85 42.4 2.50 2.49 006 2.52 006 LiAlCO, 15.0" LiAlNO, (72) 17.7" 35.85 42.4 2.50 2.49 112 LiAICO,(Cl) 15.1" LiAlNO,N( 72) 17.8" 40.02 7.2 2.25 2.19 202 2.25 016 LiAlCl(0.5) 15.1" LiAlV(C03) 22.9b 40.02 7.2 2.25 2.19 106 LiAlClI4) 15.2" LiAlV(C03)G 23.7b 46.92 5.1 1.94 1.98 115 1.99 017 LiAlCl(24) 15.0" LiAlV( C1) 23.5' 63.26 16.8 1.47 1.46 303 1.46 330 LiAICl(D) 14.9" LiAlV( C1D) 23.5b 64.70 13.1 1.44 1.42 10,lO 1.44 600 LiAlNO,(O.5) 15.1" LiAlV(NO3) 23.1 68.51 5.2 1.37 1.36 305 LiAlNO,( 4) 15.1" LiAlV( N0,N) 23.4' LiAlNO,( 24) 15.2" "AssignmFnt based on a hexagonal unit cell with a=5.29 A, C= 14.95 A." c =( 1/3) [2d( 002) +4d( 004) +6d(006)]. 'c =( 113) [44004) + bAssignment bayd on a monoclinic unit cell with a=8.68 A,b= 6d( 006 I]. 5.07 A, C= 15.12 A, /?=92"56'. 1740 useful information about the intercalation of chloride, though it can be concluded that the LDH structure is stable at this pH since the PXRD confirms that the layered structure remains intact even after 24 h. A gradual weakening in the intensity of both the basal and non-basal reflections is observed upon increasing exposure to acid. The PXRD pattern of the directly synthesized chloride [LiAlCl(D), Fig.1 (e)] is similar to that ,Of the other materials and exhibits a basal spacing of 14.9 A. Treatment of LiAlCI( 24) with 0.1 mol 1-Na2C03 solution yields a material [LiAlCO,(Cl)] with a PXRD pattern, IR spectrum and TG curve identical to that of LiAlCO,. We thus conclude that the ion-exchange process is essentially reversible. LiAINO, Samples There is little change in the observed c parameter for treatment times of up to 24 h, but after 72 h of acid treatment a significant increase in the c parameter occurs (Fig. 2). The values of c for samples LiAlNO,(O.S), LiAlN0,(4) and LiAIN0,(24) are similar to those observed for LiAlCO,, suggesting that little nitrate has been intercalated.After 72 treatments, materials with c values of 17.7 and 17.8 A [LiAlNO,( 72) and LiAlNO,N( 72), respectively] are prod-duced. These values are consistent with that of 17.60A reported by Masc01o.l~ It can be thus deduced that nitrate is incorporated, but at a rate which is significantly slower than that of chloride. Again, the structure is found to be stable to the acid treatment, although some weakening in the intensity of the basal reflections is observed. Results indicate that a treatment time of 48 h gives rise to a material wjth two phases corresponding to values of c of 14.9 and 17.7 A, respectively, suggesting co-intercalation of nitrate and carbonate [Fig. 2(e)]. LiAl Vunadates The PXRD patterns for the vanadate intercalated materials are similar in appearance (Fig.3). The first peak is veryobroad and its position is variable between ca. 8.8 and 10.4A. The second and third peaks are relatively sharp, and more similar to those recorded for the carbonate-, chloride- and nitrate- intercalated derivatives. In a similar way to the MgAlV materials" the position of the first reflection is incorrect for (002) if the second and third reflections are indexed as (004) and (006), respectively. It is possible that the material is in fact biphasic, with the broad reflection hiding the (002) of the 1 1 (a4 20 40 60 80 2Bldeg rees Fig. 2 Powder X-ray diffraction patterns for: (a) LiAlCO,, (b) LiAlNO,(O.S), (c) LiAlN0,(4), (d) LiAIN0,(24), (e) LiAIN0,(48),(f)LiAlN0,(72) and (g) LiAlNO,N(72) J.h4ATER. CHEM., 1994, VOL. 4 0 20 40 60 80 2eldegrees Fig. 3 Powder X-ray diffraction patterns for: (a) LiAIV(CO,), (b) LiAlV(CO,)G, (c) LiAlV(Cl), (d) LiAlV(ClD), (e) L,iAIV( NO,) and (f) LiAIV(N0,N) intercalated phase. The reason why the (002) peak should be so weak as to be hidden by this broad reflection is not known, however. Similar broad peaks have been reported for MgAl vanadates'l and ZnAl hydrotalcites intercalated with [a-SiW11039]8- and [a-1,2,3-SiV,W,0,0]7- .I5 ,4s a result of this, the interlayer spacings for the vanadate-intercalated samples have been calculated from the positions of the (004) and (006) reflections only. A gallery height of between 6.7 and 7.0 A is obtained for vanadate intercalatedoLiAl LDHs.Twu and Dutta observed gallery heights of 6.0 A for LiAl vanadate LDHs prepared by a route similar to that used to prepare LiA1V(CID).l2 They proposed that V2074-was the vanadate species at pH 8-11, and V40124- at pH between 3 and 8. They did not report the presence of a broad peak in the X-ray powder pattern. For MgAl vanadate LDHs, Twu and Dutta,I6 Pinnavaia and co- worker~:~and Ulibarri et d." all observed gallery heights of 7.0-7.1 A. In this case, V,002,6- was proposed as the interca- lated species. Pinnavaia and co-workers found that ion exchange of V4OlZ4- into transition-metal hydrotalcite~l~ (at pH 5.5-10.0) led to a gallery height of 4.7 A. 'Twu and Dutta account for the difference in spacing for the V,0,24-in LiAl and MgAl systems by suggesting that there is a stronger electrostatic interaction in the transition-metal LDH due to the higher charge density.It is clear that the value we have obtained for the gallery height of our vanadate samples from PXRD analysis is close to that of MgAl decavanadates and it thus appears that we have Vl,02,6-intercalated LiAl LDHs. Assignment of Monoclinic or Hexagonal Cell Two alternative cells may be used to index the powder patterns of these materials; a monoclinic cell or a heFagonal cell (with cation ordering). If the reflection at d =4.38 A moves upon intercalation, then it cannot be the (1 10) reflection of a monoclinic cell; conversely, if its position does not change upon intercalation then it cannot be the (101) reflection of the hexagonal cell.Fig. 4 shows the change in the position of the reflection in the various samples synthesked, and com- pares the observed value of d for this reflection with the calculated values for a monoclinic and hexagonal cell. Experimentally the data are not a good fit to either line, but it appears that there is no evidence for an increase in dobsas the c parameter increases. Thus it is reasonable to suggest that the reflection does indeed correspond to (110) and a monoclinic cell is more appropriate. J. MATER. CHEM., 1994, VOL. 4 014.3 ! I I I 20 25 30 35 c value Fig. 4 Variation of d spacing for (110), monoclinic, and (101), hexagonal. dabs, 0;dale (hex), broken line; and d,,,,(mono), solid line. Infrared Spectroscopy LiA1C03Samples The infrared spectrum of LiA1CO3 is shown in Fig.5(a), and assignments are given in Table 5. The main bands to note are those due to carbonate at 1370, 1050, 875 and 675 cm-' and the H-bonding between water and carbonate in the interlayer at 3000 cm-'. These bands should therefore be lost on inter- calation with chloride, nitrate or polyvanadate. The presence of the band at 1050cm-' provides some evidence for a lowering of symmetry of the carbonate species, witnessed also by the shoulder at 1400cm-'.18 LiAlCl Samples A gradual loss in the intensity of the carbonate bands is observed with increasing acid treatment time [Fig. 5(b)-(d)]. The H-bonding vibration between H20 and C032- is also lost with increasing treatment time.After 24 h treatment there is little remaining residual carbonate visible. The presence of bands attributable to hydroxy groups at ca. 940 and 600 cm-' but the lack of vibrations due to interlayer anions would seem to indicate the presence of chloride in the interlayer. LiAlCl(D) [Fig. 5(e)] shows a weak carbonate band, suggesting that I I4000 3200 2000 1600 1200 800 400 wavenum berkm-' Fig. 5 Infrared spectra of: (a)LiAlCO,, (b)LiAlCl(OS), (c) LiAlC1(4), (d) LiAlCl(24) and (e)LiAlCl(D) 1741 Table 5 IR data for LiAlCO, wavenumber /cm -assignment 3450 H-bonding stretching vibrations of OH group in layer 3000 H-bonding between the H20 and C0,2-in the interlayer 1650 H20 bending vibration 1400 lowering of symmetry of C0,2- (C2") 1370 CO,'-absorption band (v,) 1050 lowering of symmetry of C0,2-causes v1 mode to become active (usually Raman-active only for D,, co32-1 1030 OH bending vibration in layer 875 CO,'-absorption band (v2) 725 A1-0 (A2u)675 C0,2-absorption band (v4) 550 A1-0 (E,) 455 A1-0 (E,) some carbonate was incorporated by the direct synthesis method, despite attempts to exclude air from the system.LiAlNO, Samples For acid treatment times of up to 24 h there is little change in the appearance of the carbonate bands in the spectrum [Fig. 6(b)-(d)]. Note, however, the appearance of a nitrate band at 1385 cm-' in all samples treated with HN03.It may be concluded that only a small quantity of nitrate is incorpor-ated even after a 24 h treatment.The spectrum of LiA1N03( 72) [Fig. 601 is quite different, however. The complete loss of n 4 30 3200 2000 1600 1200 800 400 wavenumberkm-' Fig. 6 Infrared spectra of (a) LiA1CO3, (b) LiAlN0,t OS), (c) LiA1N03(4), (d) LiA1N03(24), (e) LiAlN0,(48), (f) LiAIN0,(72) and (g) LiAlNO,N(72) 1742 the carbonate bands is observed and there is a strong v3 NO,-absorption at 1385cm-' and a weak v2 band at 825 cm-', as observed by Hernandez-Moreno et a!.'' The v1 band is just visible at around 1050cm-' in some samples. There is evidence for loss of symmetry of the nitrate anion upon intercalation, witnessed by the splitting of the v, band (1425, 1385 cm-'). LiA1N03N(72) has a similar IR spectrum [Fig.6(g)], but the v3 band appears to be more symmetric. The IR evidence thus confirms the observations made by PXRD that little nitrate intercalation occurs for treatment times of up to 24 h, but major conversion is achieved after 72 h. LiAl Vanadates The IR spectra of the LiAl vanadates are shown in Fig. 7. There are several points to note. The shoulder at ca. 3000 cm-' is absent, as are the other bands due to the presence of carbonate anions. The bands associated with hydroxy groups and molecular interlayer water are present, however (ca. 3520 and 1630 cm-'). The main diagnostic bands in these spectra are the medium-intensity bands at around 967 and 951 cm-'. It has been shown that crystalline polyvanadates give rise to absorption bands in the 950-1000 cm-' regionlg attributable to the V=O terminal stretching mode.The nature of the vanadate anion influences the number and position of the bands, with decavanadates giving rise to a single band and hexavanadates giving two bands in this region. Here, the presence of a double band suggests that decavanadate is not the only interlayer species. Other bands at ca. 827, 729 and 668 cm-' are in similar positions to those reported for vana- 1OOr I 40 20 g{" Lc 20 0 !.. :. 4 1100 900 700 500 wavenum berkm-' Fig. 7 Infrared spectra over the regon 1100-400 cm-' for: (a) LiA1V(C03), (b) LiAlV(CO,)G, (c) LiAlV(Cl), (d) LiAlV(ClD), (e) LiA1V(N03),(f) LiA1V(NO3N)and (8)a typical infrared spectrum over the region 4000-400cm-' for an LiAl vanadate LDH [LiAlV(NO,) shownJ J.MATER. CHEM., 1994, VOL. 4 date-pillared MgAl LDHs.~' A comparison of the observed bands in the region 500-1000cm-' is given in Table 6. Metavanadates (i.e. MVO,) give rise to IR bands at 850-863 cm-', 920-935 cm-' for terminal V=O stretching and 475-495 and 685-693cm-' for V-0 stretching in bridging V-0-V bonds. In view of the fact that no bands are seen in these regions, the presence of these species would seem unlikely. Thermal Treatment All thermogravimetric analysis was performed in a nitrogen atmosphere. LiAlCO, Samples The TG curve [Fig. 8(a)] shows an overall weight loss of 43.3%, which corresponds to the loss of interlamellar water (nH,O), dehydroxylation and loss of C02.The weight loss occurs over a wide range of temperatures between 150 and 350°C and is centred around 225-245 "C. It is not possible therefore to distinguish the three transitions, unlike the case of hydrotalcite,21 where a weight loss at 216°C had been identified as due to the loss of interlamellar water and weight losses at 330 and 370°C attributed to simultaneous loss of CO, and dehydroxylation. On the basis of the observed weight loss and chemical Table 6 Bands observed in the 1000-500 cm-' region for MgAl vanadates and LiAl vanadates v( MgAl )/cm- v( LiAl)/cm-' - 967 970 951 825 827 748 729 675 668 605 602 555 i79 10%I I I I I I I I 1 4 100 200 300 400 500 TI'C Fig.8 TG curves for: (a) LiAICO,, (b) LiAlCl(O.S),(c) LiAIC1(4), (d) LiAlCI(24) and (e) LiAICl(D) J. MATER. CHEM., 1994, VOL. 4 analysis, the number of moles of interlamellar water, n=0.26, giving the formula [Lio.3sAl,~,s(OH)2](C03)o~ls-0.26H20. Heating a sample of LiAlCO, to 150°C for 18 h leadsoto a material (LiAlC0,C) with a gallery height of 2.02 A as compared with a height of 2.7 A for the precursor. There is also a significant loss in the intensity of the basal reflections, to such an extent that the (004) reflection is no longer visible. It would appear that prolonged treatment at this temperature is enough to result in significant loss of carbonate and water from the galleries. The TG curve of this sample cooled to room temperature exhibits a weight loss of 24.8% at tempera- tures up to 500°C. This compares with a theoretical weight loss of 31.9% if the sample contained no interlayer carbonate or water but there was no dehydroxylation.This suggests that partial dehydroxylation also occurs upon prolonged treatment at 150"C. Heating the sample beyond 500 and up to 800°C results in virtually no further weight loss. It has been proposed that at temperatures >450 "C the framework is destroyed, leading to the formation of Li20 and y-A120,.'2 LiAlCl Samples When this material is treated with HCl at pH 4.5 significant changes are seen in the TG curve [Fig. 8(b)-(d)]. In all samples, the first transition corresponds to the loss of physi-sorbed surface water at relatively low temperature.After 30min treatment the broad weight loss which is seen in the TG curve of LiAlCO, is resolved into three weight losses centred at 200, 250 and 320°C. After 4 h treatment the transitions may be more clearly resolved and the positions have shifted to 190, 250 and 320°C. After 24 h only two transitions are then visible at 250 and 315°C. This sequence suggests that the weight loss below 200°C must be due to the loss of residual carbonate from the interlayer, since it is not present in the sample treated for 24 h. Indeed, the IR spectrum of this sample heated to 210°C shows no bands in the 1350-1450 cm-' region, proving that carbonate has been lost. We may thus conclude that carbonate becomes increasingly unstable in the interlayer as the amount of intercalated chloride increases.The second and third transitions are due to the loss of interlamellar water and dehydroxylation, respectively. For LiAlCl( 24) calculations show that the second weight loss, centred at 250"C, corresponds to the loss of 0.31 mol of interlamellar water. On the basis of this analysis, the molecular formula for LiAlCl(24) is [Lio,,,Al,,,,(OH)2]C1,~42 .0.31H20. The total observed weight loss is 31.6Y0, which compares with a theoretical value of 31.2% based on the above formula, and is therefore within experimental error. It can be seen that dehydroxylation occurs at a higher temperature in this com- pound than in LiAlCO,, which possibly reflects differences in the H-bonding stability of the hydroxy groups.The TG curve for LiAlCl prepared by direct synthesis [LiAlCl( D), Fig. 8(e)] is similar to that of LiAlCl(24). LiAlNO, Samples With HNO, and for treatment times up to 24 h the TG curves appear to be similar to that of the carbonate material, but with a shift in the weight loss towards lower temperature [Fig. 9(b)-(d)]. PXRD and elemental analysis show these materials to contain predominantly carbonate in the inter- layer. It appears that, as for the LiAlCl samples the carbonate is destabilised as the acid-treatment time increases. The TG curve of LiA1N03(72) is quite different [Fig. 9(f)] and two transitions are seen. The first is associated with the loss of physisorbed water (at low temperature), the second transition, centred at 310 "C, corresponding to dehydroxylation of the layers.There is no weight loss close to 200"C, indicating the I I 1 I I I I I 1-1 100 200 300 400 500 TI'C Fig. 9 TG curves for: (a) LiAlCO,, (b)LiAlNO,(O.S), (c) LiAlY03(4), (d) LiA1N03(24), (e) LiA1N03(48), (f) LiAlN0,(72) and (g) LiAlNO,N( 72) absence of carbonate in the interlayer, consistent ~ith the data obtained by other techniques. There is in addition no observed weight loss associated with the loss of interlayer water in this compound. It is possible that the galleries are so tightly packed with nitrate anions that there rs little remaining space for interlayer water. This would suggest that the H20 bending vibrations seen in the IR spectrum of this compound are in fact due to physisorbed surface water and not interlamellar water.The IR spectrum of this sample heated to a temperature of 250°C shows little change with respect to the precursor, confirming there is no dehydroxyl- ation or nitrate decomposition up to this temperature. The theoretical weight loss based on the formula Li0.32A10.68(OH)2(N03)0.36is 23.4%, and the observed weight loss of 23.7% up to 350°C is thus in agreement. At higher temperatures a further gradual weight loss is observed which may correspond to the loss of the nitrate anion as NO2. A weight loss of 45.2% is observed up to a temperature of 610 "C, whereupon there is no further decomposition. The theoretical weight loss based on the assumption that NO2 is lost is 45.2%, which appears to be within experimental error.A similar TG curve is seen for LiA1N03N(72) [Fig. 9(g)], with a 23.0% weight loss observed at temperatures up to 350 "C. LiAl Vanadates Two transitions are seen in the TG scans of these materials (Fig. 10). The first weight loss occurs at temperatures of <145"C and has previously been ascribed to the loss of interlayer water.12 The second weight loss at around 325 "C may be attributed to the dehydroxylation of the layers. This weight loss is around 13.5% of the initial weight. On the basis of the formula [Lio.36Alo,64(oH)2(V100286-)0.04-] the expected weight loss due to dehydroxylation would be 18.2%. The reason for this discrepancy is as yet unknown. It is possible that this sample contains some co-inter calated glycerol which decomposes upon thermal treatment.f) 1 I 1 I I If I I I 100 200 300 400 500 TI'C Fig. 10 TG curves for: (a) LiAlV(CO,), (b) LiAlV(CO,)G, (c) LiAlV(Cl), (d) LiAlV(ClD), (e)LiAlV(N0,) and (f) LiAlV(N0,N) Conclusions It is clear from this study that LiAlCO, sheets are stable to acid treatment at pH 4.5 with prolonged treatment, resulting in complete anion exchange of the carbonate for the anion of the acid. Exposure of these acid-treated samples to sodium carbonate solution results in the regeneration of the precursor carbonate-containing material. It is also clear that vanadate-intercalated LiAl LDHs may be readily prepared via carbonate, chloride or nitrate precur- sors and the structure and properties of the products appear to be largely independent of the precursor.Thermal treatment of LiAlCO, leads to the simultaneous loss of interlayer water, dehydroxylation of the layers and decomposition of the carbonate anions over an extended temperature range between 150 and 375°C. Treatment of J. MATER. CHEM., 1994, VOL. 4 LiAlCl leads to loss of interlayer water between 235 and 260 "C and dehydroxylation at temperatures between 260 and 365 "C. For LiAlNO,, dehydroxylation of the layers occurs between 285 and 350°C and, at temperatures of >450"C, decomposition of the nitrate anions. There is no evidence for the existence of interlayer water in this compound. For LiAlV there is a loss of interlayer water at temperatures below 145"C.Dehydroxylation of the layers occurs at temperatures above 250°C and is complete by 350°C. We are grateful to the EPSRC for support (studentship to I.C.C.).Discussions with V. Rives are appreciated. This work was performed under the Concerted European Action on Pillared Layered Solids (CEA-PLS). Support via Accion Integrada is appreciated. References 1 C. J. Serna, J. L. Rendon and J. E. Iglesias, Ckiys Clay Mineral., * 1982,30, 180. 2 F. Cavani, F. Trifiro and A. Vaccari, Catal. Today, 1991,11, 173. 3 Pillared Clays, in Catal. Today, ed. R. Burch, 19S8, vol. 2. 4 W. T. Reichle, CHEMTECH 1986, 86. 5 I. Sissoko, T. Iyagaba, R. Sahai and P. Bilnen, J. Solid State Chem., 1985,60,283. 6 M. Meyn, K. Beneke and G. Lagaly, Inorg. Chcm., 1990,29,5201. 7 M. Borja and P. K. Dutta, J. Phys. Chem., 1992,96,5434. 8 S. Cooper and P. K. Dutta, J. Phys. Chem., 1990,94, 114. 9 D. L. Bish and G. W. Brindley, Am. Mineral., 1977,62,458. 10 D. L. Bish, Bull. Mineral., 1980, 103, 170. 11 M. A. Ulibarri, F. M. Labajos, V. Rives, R. Trujillano, W. Kagunya and W. Jones, Inorg. Chem., 1994,33,2592. 12 J. Twu and P. K. Dutta, J. Phys. Chem., 1989,93,7863. 13 W. Feitknecht and M. Gerber, Helv. Chim. Actu. 1942,25, 131. 14 G.Mascolo, Thermochim. Acta, 1986,102,67. 15 E. Narita, P. Kaviratna and T. J. Pinnavaia, Chem. Lett., 1991, 805. 16 J. Twu and P. K. Dutta, J. Catal., 1990, 124, 503. 17 T. Kwon, G. A. Tsigdinos and T. J. Pinnavaia, J. Am. Chem. Soc., 1988,110,3653. 18 M. J. Hernandez-Moreno, M. ,4. Ulibarri, J. L. Rendon and C. J. Serna, Phys. Chem. Mineral.. 1985, 12, 34. 19 L. D. Frederickson Jr. and D. M. Hausen, Anal. Chem., 1978, 23,93. 20 E. Lopez Salinas and Y. Ono, Bull. Chem. Soc. Jpn., 1992,65,2465. 21 L. Pesic, S. Salipurovic, V. Markovic, D. Vucelic, W. Kagunya and W. Jones, J. Muter. Chem., 1992,2, 1069. Paper 4/02607A; Receivtd 3rd May, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401737

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994,4(11), 1745-1748 Formation and Decomposition of LaBa,Cu,O, -& J. M. S. Skakle and A. R. West Department of Chemistry, University of Aberdeen, Meston Walk, Aberdeen, UK AB9 2UE The formation and decomposition of LaBa,Cu,O, -,has been studied by X-ray diffraction and thermogravimetric analysis. The attempted preparation of LaBa,Cu,O,-, in air or oxygen gives a mixture of a solid solution of composition La, +xBa,-,Cu,07-d (xz0.2),and BaCuO, (J. M. S. Skakle and A. R. West, Physica C, 1994, 220, 187). However, heating this mixture in a less oxidising atmosphere such as flowing argon at temperatures above 850°C for a short time, ca. 16 h, yields a single-phase tetragonal product, which, when annealed in oxygen at lower temperature, gives an orthorhombic superconductor with T, (onset)z94 K.The orthorhombic and tetragonal structures have been confirmed by Rietveld refinement of X-ray powder diffraction data; an orthorhombic-tetragonal transition occurs at 6 =0.2. The stability of the single-phase material under air and oxygen has been studied by thermogravimetry. It is stable up to 800 "C in air; at higher temperatures, decomposition to La, +xBa2--xC~307-,(x %0.2), and BaCuO, occurs. On prolonged heating of single-phase tetragonal LaBa,Cu,O,-, at high temperatures, e.g. 875 "C for 64 h under argon, decomposition to give La,-,Ba,+,Cu,-,O,,-, (xz0.2),BaCuO, and BaCu,O, occurs. This is similar to the behaviour of YBCO under low oxygen pressure. Thus, under all conditions studied so far, LaBa,Cu,O,-, may be considered to be a non-equilibrium phase.In orthorhombic samples of LaBa,Cu,O,-, prepared by quenching, T, varies linearly with 6; samples cooled slowly show evidence of a plateau at ca. 94 K for 6%-0.05 to -0.15. The 93 K superconductor YBa,Cu,O, may be readily pre- pared by sintering in air or oxygen at ca. 950°C followed by a post-sinter anneal at 400 "C in oxygen. Rare-earth-metal analogues, REBa,Cu,O, (RE=Lu, Yb, Tm, Er, Ho, Dy, Gd, Eu and Sm) may easily be synthesized under similar con- ditions, but for the larger rare-earth metals, e.g. La, prep- aration of a single-phase 123 material in air or oxygen is not possible. This difficulty in preparation has been attributed to the ready formation of the tetragonal solid solutions La, +,Ba2-,Cu30, (x#O),l,, in which the similarity in the sizes of La and Ba permits cross-substitution on the cation sites.Accurate structure determination on La-123 has also proved difficult, since La and Ba have similar X-ray scattering factors,, and several refinements of powder diffraction data have apparently failed to account for the presence of BaCuO, as a second Some studies have reported the La-123 phase as tetragonal with T,<90 K (e.g.ref. 3 and 6), consistent with the formation of a solid solution member, x>O, rather than 123, whilst others have shown that preparation in air gives a mixture of a tetragonal solid solution composition, La, +xBa2-xCu30z and BaCuO, .7-9 More recently, it has been shown that under less oxidising conditions, such as heating under pure nitrogen or argon, a phase-pure 123 material can be synthesized, which, after low-temperature oxygenation, is orthorhombic with a T,of 93 K.lo," Another study has shown that in oxygen atmospheres richer than 50/002/N2, La-123 does not form.', In single-phase orthorhombic La-123, struc- tural studies have shown that the oxygen sites in the basal plane exhibit more disorder than in YBCO.' In particular, some oxygen is present in the (!LOO) sites while the (O%O) sites may not be fully occupied.Several studies have shown that excess oxygen (above 7) may be accommodated by increasing the occupancy of the (%OO) In an attempt to clarify the conditions of synthesis, we have studied the effects of different atmospheres on the formation and decomposition of La-123, together with the structure, stability, and superconducting properties of the single-phase material, and we report the results here.Experimental The starting materials were La,O, (Aldrich 99.99%) BaC0, (Aldrich 99.9%) and CuO (BDH 99.5%); the La,O, was dried at 1000°C prior to use, the CuO at 700°C and the BaCO, at 300°C. These were weighed out to give ca. 5 g rcaction mixtures and mixed together with acetone in an agate mortar and pestle. The mixture was pressed into 13 mm pellets, placed on pre-seasoned partially yttria-stabilized zirconia discs, heated in a muffle furnace at 900°C for 12 h to decarbonate it, then heated at 950°C for 24 h. Heat treatments under different atmospheres were carried out in a horizontd tube furnace.For quenching experiments, ca. 0.2g of the sample was placed in a Pt foil envelope and suspended by Pt thread in a vertical tube furnace; after 30min the sample was dropped into mercury, and measurements were carried out immediately. Phase identification was carried out using a Hagg (hinier camera with Cu-Ka, radiation, and data for unit-cell determi- nation and Rietveld refinements were collected using a Stoe STADI/P automated powder diffractometer in transmission mode with linear position-sensitive detector. Ac susceptibility measurements were carried out on powder samples itsing a Lakeshore AC7000 susceptometer. A Stanton Redcroft STA 1500 simultaneous TG-DTA instrument was used for t hermo- gravimetric measurements under different atmospheres, using a heating rate of 10 "C min-'.Results and Discussion Formation of LaBa,Cu,O, -The initial reaction in air or oxygen at temperatures ranging from 900 to 1000°C always gave a mixture of BaCuO, and a 123-related solid solution La, +,Ba2 -,Cu3OZ [Fig. 1(a)]whose composition was determined as x z0.2 from its lattice param- eter~.~The effect of heating this mixture under flowing argon was studied; heat treatment at 875 "C for 4-16 h gave single- phase La-123, as determined by X-ray diffraction. At 850 "C it was necessary to heat the same mixture for 25-40 h to give a phase-pure material, but at 825 "C stoichiometric La-123 did not begin to form even after 7 days.Structural Studies The oxygen content of a phase-pure sample heated at 875 "C for 16 h was determined as 6.49f0.03 by reduction in 10% H,-N, on the thermobalance. The structure was determined I A I, 20 30 40 50 60 2fldegrees Fig. 1 X-Ray diffraction patterns of LaBa,Cu,O,-d under different preparative conditions. (a) Air at 950 "C for 24 h, (6) Ar at 875 "C for 16 h, (c) Ar at 875 "C for 16 h followed by 0, at 400 "C for 4 h, (d) Ar at 875 "C for 32 h, (P) Ar at 875 "C for 48 h and (f)Ar at 875 "C for 64 h. 0,BaCuO,; V,422; 3,BaCu,O,; V,Cu,O. as the tetragonal YBCO structure by Rietveld refinement [Fig. 1(b)];starting parameters were taken from David et a1.,I5 and a squared Lorentzian function was used to model the peak shape.The final structural parameters are shqwn in Table 1 (a), with unit-cell parameters a =3.9250( 2) A, c = Table 1 (a) Results of Rietveld refinement on composition LaBa,Cu,O,.,, position ~ 0 X Y Z uiso La Id 1.O 0.5 0.5 0.5 0.015(3) Ba 2h 1.O 0.5 0.5 0.1796(5) 0.020(2) Cu(1) Cu(2) O(1) O(2) O(3) la 2g 4i 2g 2f 1.o 1.0 1.o 1.0 0.245 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.5 0.0 0.5 0.0 0.350(1) 0.375(2) 0.157(4) 0.0 0.025( 6) 0.041(4) 0.03(1) 0.03(1) 0.03 (2) space group P4/mmm, u =3.9250( 2) A,c = 11.9263( 10) A,R, =5.05%, R,, =6.50Y0, R, =4.26%. (h)Results of Rietveld refinement on composition LaBa,Cu,O,,,, position 0 z uiso Ih 1.O 0.5 0.5 0.5 0.016(3) 2t 1.o 0.5 0.5 0.1799(5) 0.018(2) la 1.o 0.0 0.0 0.0 0.023( 6) 2q 1.o 0.0 0.0 0.350( 1) 0.040(3) 2s 1.O 0.5 0.0 0.387( 6) 0.03 (2) 2r 1.o 0.0 0.5 0.355 (6) 0.04( 1) 2q 1.o 0.0 0.0 0.157( 3) 0.03( 1) le 1.0 0.0 0.5 0.0 0.041 (7) Ib 0.14 0.5 0.0 0.0 0.039(6) space group Pmmm, a= 3.8779(4)A, b =3.9432(4) A, c = 11.7601(15)A, R,=4.71%, R,,=6.12%, R,=3.85%0.J. MATER. CHEM., 1994, VOL. 4 11.9263( 10) A R,, =6.50%. The difference plot from the Rietveld refinement is also shown in Fig. l(bl. Annealing tetragonal L~B~,CU,O~.,~ in oxygen at 400 "C for 4 h gave the orthorhombic YBCO structure with 6= -0.14+0.02. There are two possible explanations for an oxygen content of 7.14. First, as with YBCO, it may indicate the presence of the '124' phase, LaBa2Cu,0,, in the ratio 14% 124, 86% 123.16 This appears not to be the case in the present materials; X-ray data indicate the La- 123 to be phase- pure.In addition, magnetic susceptibility results show no evidence for a second, superconducting phase (Fig. 2), and there have been no reports on the La analogue of YBa,Cu,O, to date. Secondly, the excess oxygen may be present in the 123 structure in the O(5)sites of the basal plane, as indicated by previous ~tudies.'~,~~ Hence, attempts were made to refine the structure of L~B~,CU,O,~~,in both the orthorhombic space group Pmmm and tetragonal P4/mrnm; the excess oxygen in both cases was assumed to be in the 0(4)/0(5) sites. R,, for the tetragonal model was 11.87%, with the thermal parameters of O(4) diverging. In addition, it was not possible fully to index all of the diffraction lines adequately. Using the orthorhFmbic model all oparameters conve:ged, with u =3.8779(4) A, b = 3.9432(4) A, c= 11.7601(15)A, RW,=6.l2% [Fig.l(c)], with the difference plot for the Rietveld refinement shown beneath the X-ray diffraction pattern: final parameters are given in Table l(b). It is concluded, therefore, that LaBa,Cu,O,,,, is a single-phase, orthorhombic material, with oxygen occupying both sites in the basal plane. Oxygen Content of La-123 The stability of the single-phase material was studied by thermogravimetry and by annealing experiments followed by quenching into Hg. 80mg samples of the single-phase, 6= -0.14 material were heated in steps to 900 'C (a) in air and (b) in oxygen on the thermobalance; samples were held isothermally for 30 min at 50 "C intervals to achieve thermal equilibrium; typical TG traces are shown in Fig.3. These experiments showed that LaBa,Cu,O,,,, is stable in air to ca. 300°C and in oxygen up to ca. 400°C after which weight loss commences, corresponding to an increase in (j. The quenching experiments showed that the orthorhombic structure is retained up to 600°C in air, above which temperature the structure becomes tetragonal. The tetragonal structure is preserved up to ca. 800°C in air and 750°C in oxygen, above which decomposition occurs to give La,,,Bal,,Cu,O, and BaCuO,. This seems to suggest that decomposition is not associated with an absolute oxygen content.but rather that 0.5 94K 1 0 0 -IY0) E -0.5. 0 m :I I 't 0 02 -Q 0 0 -1.0. 0 0 0 Fig. 2 Ac susceptibility (x)trace for LaBa,Cu,O,,,, J. MATER. CHEM., 1994, VOL. 4 1000 0 0 0 800 O oono [I 0 0 600 0 9i= 400 200 01 n]' ' 6.2 6.4 6.6 6.8 7.0 7.2 nominal oxygen content Fig. 3 TG trace for LaBa,Cu,O,-a heated in air (0)and oxygen (0). The oxygen contents refer to single phase 123 for the range 6.65 to 7.14 in air and 6.85 to 7.14 in oxygen; lower oxygen contents refer to multiphase mixtures. at a specific temperature and atmosphere, it is more favourable for the Lal.2Bal.8Cu30Z composition to form than the stoi- chiometric, tetragonal La-123 phase. The decomposition observed in the quenched samples also appears to correspond to a change of gradient in the weight loss traces shown in Fig.3; this implies that the Lal.2Bal&~30Z composition loses oxygen at a different rate than the La-123 phase. Unit-cell Data X-Ray diffraction data for a number of samples quenched from temperatures in the range 400-800°C were measured over the range 7-90" 28; the unit-cell results for 6 =0.49 and -0.14 were taken from the Rietveld refinements. It is essential when studying the orthorhombic-tetragonal phase transition in these triple perovskite materials to consider the ratio R= c// % (a +b). For the ideal triple perovskite, this ratio is equal to 3; the structure is pseudo-cubic. Deviations from the ideal value lead to line splitting, e.g.a pseudo-cubic (100) line splits to (100) and (001);however, the line-splitting does not imply that the structure is orthorhombic. For instance, in the XRD powder pattern of the tetragonal LaBa2Cu306,49 [Fig. 1(b)], R =3.038 and significant line splitting is observed. In the case of orthorhombic LaBa2Cu,07~,, [Fig. 1(c)], the strongest line shows no splitting, and R =3.007. For intermediate values of 6, attempts were made to index the data using both the orthorhombic and tetragonal unit cells. For values of 6 <0.2, the tetragonal model did not index all lines; samples with 6>0.2 could be accurately indexed on a tetragonal unit cell. The results shown in Fig. 4 indicate that an orthorhombic-tetragonal phase transition occurs at 6~0.2.This is in clear contrast with YBCO where the orthorhombic-tetragonal phase transition occurs at 6 z 0.4. The c parameter appears to show a slight change in gradient at the phase transition, again in contrast to YBCO where the change in c is more pron~unced.'~,~' T,Data Critical temperatures of the quenched samples were measured from ac susceptibility data; a plot of T,against oxygen content shows linear variation (Fig. 5), as is also seen in quenched samples of YBa2Cu307 Samples were also prepared by annealing in air at temperatures up to 750°C, followed by slow cooling to room temperature; this did not give as wide a variation in 6 as the quenching method, but clearly shows 11.951 3.9501 oxygen content Fig.4 Variation of unit-cell parameters with oxygen content for samples prepared by quenching loo r t 2o t t 01 \\-i 7.2 7.1 7.0 6.9 6.8 6.7 oxygen content Fig.5 Variation of T, with oxygen content for quenched (0)and slowly cooled (m) samples evidence for a plateau in T, us. oxygen content over the range ca. 7.05-7.15. A similar plateau is seen in YBCO, but at significantly lower oxygen contents of ca. 6.85-7 (e.g. ref. 18 and 20). Decomposition of LaBa,Cu,O, -The results for heating at 875 "C in flowing argon are summar- ised in Fig. l(d)-(f). The tetragonal phase remains phase- pure for heating times of 16 and 32 h, but after 48 h, lines corresponding to La,~2xBa2+2xCu2~,010~,,,x ~0.2,(422), BaCuO, and Cu20 are evident.After 64h, extra lines of La, -2xBa2+ 2xCu2-,Ole-2x, x M0.2, (422), BaCuOz and BaCu202 are present. This behaviour is similar to the gradual decomposition of YBCO under low oxygen partial press- ure~.~'-~~Subsequent reduction of these heat-treated samples in 10% H2-N, on the thermobalance gave the overall oxygen content at each of these stages. A decrease in the total oxygen content was observed for the increased heating times, corre- 1748 sponding to the processes La1.2Ba1.8C~20z+BaCuO, U L~B~,CU,O~,~ U Conclusions LaBa,Cu,O, -can be prepared as a single-phase tetragonal material by heating at 875 "C for 16 h under flowing argon which, on subsequent annealing in oxygen at 400°C gives an orthorhombic superconductor with T, (onset)=94 K.An orthorhombic-tetragonal phase transition occurs at 6 =0.2. Under the conditions described here, single-phase LaBa,Cu,O, -is not an equilibrium phase; when it is heated in air or oxygen at 800°C decomposition occurs to give La,.,Ba,,,Cu,O,~, and BaCuO,, and under flowing argon at temperatures of 850 "C and above, the material gradually decomposes to give La,.6Ba2,,Cul,80,,, +BaCuO, + BaCu,O,. On lowering the temperature to 825 "C, however, La-123 does not form in argon. It is possible that at some intermediate oxygen partial pressure and temperature, the stoichiometric single-phase material is thermodynamically stable. We thank SERC for a research grant (A.R.W.) and the International Centre for Diffraction Data for a scholarship (J.M.S.S.).References 1 M. Izumi, T. Yabe, T. Wada, A. Maeda, K. Uchinokura, S. Tanaka and H. Asano, Phys. Rev. B, 1989,40,6771. 2 A. Maeda, T. Noda, H. Matsumota, T. Wada, M. Izumi, T. Yabe, K. Uchinokura and S. Tanaka, J. Appl. Phys., 1988,64,4095. J. MATER. CHEM., 1994, VOL. 4 3 I. Nakai, K. Imai, T. Kawashima and R. Yoshizaki, Jpn. J. Appl. Phys., 1987,26, L1244. 4 M. Izumi, K. Uchinokura, A. Maeda and S. Tanaka, Jpn. J. Appf. Phys., 1987, 26, L1555. 5 R. Yoshizaki, H. Sawada, T. Iwazumi, Y. Saito. Y. Abe, H. Ikeda, K. Imai and I. Nakai, Jpn. J. Appl. Phys., 1987, 26, L1703. 6 M. Hikita, S. Tsurumi, K. Semba, T. Iwata and S. Kurihara, Jpn. J. Appl. Phys., 1987,26, L615. 7 J. M. S. Skakle and A. R. West, Physica C, 1994.220, 187.8 C. U. Segre, B. Dabrowski, D. G. Hinks, K. Zhang, J. D. Jorgensen, M. A. Beno and I. K. Schuller, Nature (London), 1987, 329,227. 9 E. Takayama-Muromachi, Y. Uchida, A. Fujimori and K. Kato, Jpn. J. Appl. Phys., 1987,26, L1546. 10 T. Wada, N. Suzuki, T. Maeda, A. Maeda, S. Uchida, K. Uchinokura and S. Tanaka, Appl. Phys. Letr., 1988,52,1989. 11 T. Wada, N. Suzuki, A. Maeda, T. Yabe, K. Uchinokura, S. Uchida and S. Tanaka, Phys. Rev. B, 1989,39.9126. 12 T-T. Fang, J-W. Huang and M-S. Wu, J. Muter. Res., 1994, 9, 1369. 13 L. Ganapathi, A. K. Ganguli, R. A. Mohan Ram and C. N. R. Rao, J. Solid State Chem., 1988,73, 593. 14 A. Sequeira, H. Rajagopal, L. Ganapathi and C. N. R. Rao, J. Solid State Chem., 1988,76,235.15 W. I. F. David, W. T. A. Harrison, J. M. F. Gunn, 0. Moze, A. K. Soper, P. Day, J. D. Jorgensen, D. G. Hinks, M. A. Beno, L. Soderholm, D. W. Capone 11, I. K. Schuller, C. U. Segre, K. Zhang and J. D. Grace, Nature (London), 1987,327,310. 16 D. M. Pooke, J. L. Tallon, R. G. Buckley and J. Loram, Physica C, 1991,185-189,563. 17 P. K. Gallagher, H. M. O'Bryan, S. A. Sunshine and D. W. Murphy, Muter. Res. Bull., 1987,22,995. 18 R. J. Cava, B. Batlogg, C. H. Chen, E. A. Rietman, S. M. Zahurak and D. Werder, Nature (London), 1987,329,423. 19 C. Namgung, J. T. S. Irvine and A. R. West, Physica C, 1990, 168, 346. 20 R. Beyers, B. T. Ahn, G. Gorman, V. Y. Lee. S. S. P. Parkin, M. L. Ramirez, K. P. Roche, J. E. Vazquez. T. M. Gur and R. A. Huggins, Nature (London), 1989,340,619. 21 J. S. Kim and D. R. Gaskell, Muter. Lett., 1993. 16, 327. 22 B. T. Ahn, T. M. Gur, R. A. Huggins, R. Beyers, E. M. Engler, P. M. Grant, S. S. P. Parkin, G. Lim, M. L. Ramirez, K. P. Roche, J. E. Vazquez, V. Y. Lee and R. D. Jacowitz. Physica C, 1988, 153-155,590. 23 T. Lada, A. Paszewin, R. Molinski, A. Morawski and W. Pachla, Appf.Supercon., 1993, 1, 591. Paper 4/04119D; Received 6th July, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401745

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( 11), 1749-1 753 1749 170Nuclear Magnetic Resonance Spectroscopy of the Structural Evolution of Vanadium Pentaoxide Gels G. A. Pozarnsky and A. V. McCormick* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455, USA Recent studies of the synthesis of V205gels by acidifying metavanadate salt solutions suggest that VO,' polymerizes into chains of vanadate octahedra. In this study we characterize the growth of vanadate polymers by both solution and magic angle spinning 170nuclear magnetic resonance (NMR) spectroscopy. The spectra are consistent with the formation of a chain polymer with a repeat unit of V02(0H)(OH,),. The 170 NMR spectra also suggest that the chains might connect to each other by hydrogen bonding.Although the chemistry of V205 gels has been of recent interest,'-" the reason that acidification of vanadate solutions produces a characteristic two-dimensional (20) ribbon micro- str~cture'.~,~,~remains unclear. Previous workers have pro- posed that a 2D fragment of V205 crystal is formed,*g9 but recent work using electron paramagnetic resonance (EPR) spectroscopy and 51V nuclear magnetic resonance (NMR) spectroscopy has instead suggested chain polymerization of V02+ .7 In this work, we investigated whether the 170NMR spectra are consistent with the chain growth mechanism by labelling the vanadate solution species and the intermediate polymers with 170 early in the synthesis. By using 170NMR chemical shift assignments from several studies of vanadate structures in solution,'2p20 we were also able to speculate as to whether hydrogen bonding occurs among the polymer chains in a way that might serve to build 2D structures recently observed by cryogenic transmission electron microscopy (cryo-TEM).""' Experimental Samples were prepared by dissolving sodium metavanadate in 1 g of 10 atom% I7O enriched water (Aldrich).A further 1 g of unenriched water was then added to yield a stable 1.0mol dmP3 sodium metavanadate solution (pH 8). By adding the 170enriched water first the extent of 170 enrichment into the vanadate species over the time period studied was increased. A column of Dowex 50W X-2 50-100 mesh ion-exchange beads was charged with hydrochloric acid, then washed with deionized, distilled water and finally used to acidify the solutions.NMR spectra were acquired using a GE 500 MHz NMR spectrometer. 51V NMR solution spectra were acquired at 131.487 MHz using a 90" pulse width of 12 ps, a spectral width of 60 kHz, a relaxation delay of 0.5 s and with 256 transients. 170NMR solution spectra were acquired at 67.8087 MHz using a 90" pulse width of 61 ps, a spectral width of 111 kHz, a relaxation delay of 0.1-0.2 s and with 10000 transients. 51V and I7O chemical shifts were calculated with reference to external samples of VOCl, and water, respectively. Magic angle spinning (MAS) spectra were acquired using a 5 mm Doty probe at a spinning speed of 10 kHz with Si,N, sample rotors.Prior to spectral acquisition, excess solution was removed from the gelled sample by filtration in order to maximize the proportion of 170enriched gel in the solid and -f Present address: Centre for Advanced Materials Processing (CAMP), Clarkson University, Potsdam NY, USA. to remove any dissolved species still present. Spect rometer parameters were the same as for solution spectra except that 1000 and 50000 transients were used for 51Vand I7O MAS NMR, respectively. Results and Discussion The 51V NMR spectrum of the sol immediately after ion exchange is shown in Fig. 1. Detailed discussion of the assign- ments can be found el~ewhere.~ The peaks at -420, -513 and -532 pprn correspond to the fast proton exchange limit of the di- and tri-protonated forms of the decavanadat e anion, and the peak at -545 ppm corresponds to V02 -.7 The remaining peaks at -523 and -537 ppm were assigned to the triprotonated form of the decavanadate anion.7,21,22 The progression of the 51V NMR spectra as gelation occurs has been presented and discussed el~ewhere.~ I7O NMR is used here only to identify the intermediate and final structures of the V205 gel.Owing to the low enrichment and unknown isotope exchange rate used, J kinetic analysis is not expected, and such an analysis has already been performed using 51V NMR and EPR.7 The 170 NMR spectrum immediately after ion exchange is shown in Fig. 2 and the progression of the "0 NMR spectra as gelation occurs is shown in Fig.3. The peaks at 80 ppm (V60); 348 ppm (V30); 705, 843 and 931 ppm (V,O); and 1199 and 1210 ppm (VO) have all been previously iissigned to the oxygen sites indicated in the decavanadate species at I I -280 -360 -440 -520 -600 -680 6 Fig. 1 'lV solution NMR spectrum of reacting solution, t =0 J. MATER. CHEM., 1994, VOL. 4 containing a high concentration of V02+, as demonstrated by the 51V spectrum (Fig. 4). The 170 NMR spectrum of the V60(decavanadate) V -0-V(polymer) \VO(decavanadat e) I I 1400 1000 600 200 6 Fig. 2 170 solution NMR spectrum of reacting solution, t =0 A V-0-V(polymer) 1400 1000 600 200 6 Fig. 3 "0 NMR spectrum of reacting solution; (a) t= 2 h, (b)t=4 h, (c) t=S h pH 2.12-" Each of these peaks represents both protonated and deprotonated forms of the decavanadate anion under- going rapid exchange.This fast exchange limit can depend on the solvent and solution c~ncentration.~~ Apparently at this high concentration some decavanadate anion can be formed that does not exchange quickly enough to cause peak coalesc- en~e.~Hence, the 170 NMR peak at 453 ppm matches a peak observed for an H3V100283- oxygen site undergoing slow exchange.14 It was assigned to the triprotonated decavanadate anion since this peak was present throughout gelation in a fashion mimicking the corresponding 51V NMR peaks at -523 and -537 ppm.7,21,22 The appearance of the triproton- ated form of the decavanadate anion suggested that the decomposition of the decavanadate anion into V02+ might involve protonation of the diprotonated decavanadate anion (decavanadic e.g., H+ 3-13Hf H,V,002,4--H,V,o028 -1OVO2++8H20 (1) In this study, the triprotonated decavanadate anion was present for somewhat longer periods of time (Fig. 3) than with the lower concentration sols studied previ~usly.~ Having accounted for all possible 170decavanadate peaks, we then associated the small 170peaks observed at 220, 580, 1050 and 1380 ppm with non-decavanadate species. The peak at 220 ppm was present throughout gelation.To assign this peak, we compared it with the spectrum of a vanadate solution V02+ solution in Fig. 4 showed the same peak at 220 ppm. Since, according to Howarth and coworkers, the V=O bond of V02+ is not observable in aqueous we assigned this peak to water molecules that are coordinated to the vanadium.This is a triply coordinated oxygen site (V-OH,), so it is reasonable that it should be near the peaks of the V,O groups of the decavanadate anion.12-15 This assignment is also consistent with the chemical shift of the coordinated water in V'"O( H2OI5,+,which appears at ca. 180 ppm in aqueous vanadyl sulfate solution^.'^ The 170NMR peaks at 580, 1050 and 1380ppm clearly disappeared from the solution spectra after the initial polymer growth OCCU~S.~ Previously reported 51V NMR solution spec- tra7 and the 170NMR spectra (Figs. 2 and 3) showed that no new mobile vanadate species were formed in solution during gelation; so it was deduced that these three 170 peaks were associated with the growing vanadate polymer whereas the corresponding 51Vpolymer peaks were not well re~olved.~ The three 170peaks were well resolved, perhaps because the anisotropy of the 170sites in the polymer was less severe than of the 5'V sites.The 580 ppm peak was near to the V,O peaks from decavanadates,12-15 and chemical shift correlations of V,O bond angles in aqueous salt solutions containing dimers and cyclic species show that it might correspond to a H3V10°28* I -400 -440 -480 -520 -560 V20(decavanadate) V30(decavanadate) V-OHOH, 700 600 500 400 300 201 6 Fig. 4 (a) 51VNMR and (b)170 NMR spectra of solution containingvo2 (H20)4+ J. MATER. CHEM., 1994, VOL. 4 V20 bridge at ca.160-180°.1s~'6~'8 I7O NMR studies of vanadate peroxy compounds and of the decavanadate anion suggested that the peak at 1050ppm corresponded to a terminal V-0 group.12p16 This peak probably shows the exchange-averaged shift for a V-OH site on the polymer undergoing the fast reaction;" I/ I/ fVIOH t fVIO-+H+ (2)/I /I The 1380 ppm peak may be associated with the V=O group on the vanadate polymer. Although the V=O site can not be observed for V02+,17,18 I7O NMR studies on VO(NO,), and VOCl, showed that short V=O bonds have a chemical shift of ca. 1400 ppm. The suggestion from previous 51V NMR kinetic studies7 that the polymer is built by chain polymerization of V02+,26 was supported by the appearance of the linear oxygen bridge at 580 ppm in the 170NMR solution spectra.Further evidence was obtained by examining the gel with MAS NMR. The 51V MAS NMR spectrum of the undried gel (Fig. 5) showed only one peak at -547 ppm. This was consistent with spectra reported earlier and corresponds to an octahedral environ- ment.7,27,28 All peaks observed in the corresponding I7OMAS NMR spectrum (Fig. 6) should be associated with this sole 51V environment. The I7OMAS NMR of the wet gel showed -400 -480 . -560 -640 -720' 6 1751 peaks at 223, 688, 1150, 1380, 1396, 1407 and 1430 ppm. The peak at 223 pprn was associated with coordinated water molecules as above. The peak at 688 ppm was near the chemical shift range of V20 environments for decavanad- ates.12-15 The peak at 1150 ppm was near that for terminal VO sites in decavanadates.12-16 The peaks at 1380, 1396, 1407 and 1430ppm were all consistent with V=O bonds17; the formation of multiple peaks (cf.the single solution I7C) NMR peak observed at 1380 ppm in Fig. 2) may result from different degrees of hydrogen bonding in the same way that 170 chemical shifts were affected by hydrogen bonding between water and C =0 gro~ps.~~-~' These peak assignments suggest the identity of the repeat unit of the polymer. Since microscopic observations showed linear polymer growth, the two bridging oxygens should be on opposite sides of the vanadium centre."," Moreovtbr, EPR infrared (IR) and Raman studies showed that one of rhe two water molecules was opposite the V=O bond.3,32-34 A possible structure is shown in Fig.7. There are no reports of optical isomerism, so the equatorially coordinated water and hydroxy groups can presumably switch positions on the vanadium centre. However, it was noted that while the expected ratio of these oxygen sites from Fig. 7 was Obr:0,:OH :OH, = 1: 1 : 1 :2, the ratio of the 170peak intensities was approxi- mately 1 :1:0.5 :1. This discrepancy might be due to the exchange of I7O isotope between the enriched -0112 sites and the less enriched bulk water. This process has been confirmed and studied in vanadyl sulfate solutions.25 Although there is a possibility of short-lived and or low concentration intermediates that are undetectable bj NMR occurring in the polymerization process, the similarity of the repeat unit in Fig.7 to the structure of V02+ suggests that the polymerization process might require no other intermedi- ates. The hydroxy group on the repeat unit could be the result of hydrolysis of a coordinated water on the just-polymerized VO2 .'+ Hydrogen bonding between these chains3' was suggested by the downfield shift of the V20 and VOH peaks of the polymer compared with the signals of these sites in stdution. Since no dissociation of the polymer formed was observed by either I7O or 51V NMR during gelation,' the mobt likely explanation for the disappearance of the solution "0 NMR peaks at 580 and 1050ppm was the broadening caused by continued polymer growth and hydrogen bonding.This might also account for the reappearance of these peaks (shifted to 688 and 1150 ppm) in the I7O MAS NMR. A hypothetical Fig. 5 51VMAS NMR spectrum of V,O, gel structure is shown schematically in Fig. 8 and is consistent with the electron diffraction pattern from cryo-TEM studies on the wet Note that since the 170 peak of the coordinated water shows no downfield shift, it might not interact with any hydroxy groups. Previous cryo-TEM observations have shown that the gel bu'kHvI ribbons are ca. 25 nm in width, so the transverse assembly of V-0-V v--2 1 linear polymers shown in Fig. 8 must cease at some point or \ else sheets would be produced. This cessation might be caused by the presence of negatively charged ligands in the equatorial positions of the repeat unit shown in Fig.7. The reaction mixture remains at a constant pH of 2, and since neither mobile vanadate anions nor counter-ions (e.g. Cl-) are present in the sols at this concentration, only the 1400 1200 1000 800 600 400 200 6 Fig. 6 I7O MAS NMR spectrum of V,O, gel Fig. 7 Repeat unit in vanadia polymer J. MATER. CHEM., 1994, VOL. 4 top view of ribbon planar view of ribbon 0 OH2 0 III I OH2 II HO-V--H-O-V-O-HH-O-V--H-0-V-OH I II It I OH2 0 0 OH2 1 2 3 4 Fig. 8 Wet ribbon structure: proposed assembly of hydrolysed linear polymers into ribbon structure. Individual chains are numbered for identification. linear vanadate polymers can provide such buffering capacity. These negative charges could only be the result of further deprotonation of the hydroxy group on the octahedral repeat unit. VO,(OH,),(OH) + VO,(OH,),O-+Hf (3) We might not expect to see the 0-site in the 170 NMR spectra because of its low concentration and fast exchange.At the concentration where the ribbons were observed with cryo-TEM (0.5 mol dmP3 vanadate), one of every 50 vanadia repeat units should be negatively charged to maintain the pH. If this change were responsible for halting the ribbon's trans- verse growth (by interfering with the hydrogen-bonding mech- anism), then using literature-cited bond and the hypothetical configuration shown in Fig. 8 we might expect the width of a 50 chain ribbon to be in good agreement with that observed with cryo-TEM (ca.25 nm)." The ribbon structure shown in Fig. 8 should be stable upon drying at room temperature since X-ray diffraction, IR and Raman studies have indicated that the short V=O bond and the coordinated water opposite it are retained even after drying.1,32-34,37 Th e same characteristic ribbon structure and electron diffraction pattern are also retained.5,6 1R and NMR studies have shown that the V-OH groups are largely retained upon drying, so apparently there are no condensation reactions between V-OH gro~ps.~~,~'-~'If the linear bridges between vanadia octahedra remain ~nchanged,~'-~~ the only change in structure we might expect with drying would be the loss of the equatorial coordinated water to allow associ- 320°C1-3H20 v2°5 Fig.9 V,O, unit in dried gel ation with the oxygen of the V-0-V bridge of an adjacent chain. This would suggest the structure shown schemati- cally in Fig. 9 with the formula, V,05 -3H,O. Although we note that this indicates somewhat more water than the usual hydrated gel formula of V,05*1.6H,0obtained by thermogravimetry.1,33 Conclusions The I7O NMR spectra obtained was consistent with two earlier points suggested by 51VNMR: (i) that VO,' forms a chain polymer; and (ii) that this polymer can undergo hydroly- sis to form V-OH sites.7 Evidence from "0 NMR also suggested that the hydrolysed, linear polymers may assemble into a ribbon structure by aligning hydroxy groups, bridging oxygens and equatorially coordinated waters to achieve hydrogen bonding.This work was supported by the Office of Naval Research and by a fellowship for G.A.P. from the University of Minnesota Center for Interfacial Engineering, an NSF Engineering Research Center. The authors are grateful for helpful discussions with Professor Martha McCartney (UC Irvine) and Drs. Eric Morrison and Joseph Bailey (3M). References 1 J. Livage, Chem. Mater., 1991,3, 578. 2 N. Gharbi, C. Sanchez, J. Livage, J. Lemerle and J. Lefebvre, fnorg. Chern. 1982 21,2758. 3 J. Lemerle, L. Nejem and J. Lefebvre, J. Inorg. Vucl. Chern., 1980, 42, 17. 4 J. Lemerle, L. Nejem and J. Lefebvre, J. Inorg. ,Vucl. Chem., 1981, 43,2683. 5 J. Livage and J. J Legendre, J. Coll. fnt. Sci., 19h2,94, 75. 6 J. Legendre, P.Aldebert, N. Baffier and J. Livage, J. Coll. Int., 1982,94,84. 7 G. A. Pozarnsky and A. V. McCormick, Chrm. Muter., 1994, 6, 380. 8 J. Livage, M. Henry and C. Sanchez, Prog. Solid Stute Chem., 1988, 18,259. 9 M. Henry, J. P. Jolivet and J. Livage, Structure Bonding, 1990, 77, 154. 10 J. K. Bailey, T. N. Nagase, G. A. Pozarnsky and M. L. Mecartney, in Better Ceramics through Chemistry IV, MRS Proceedings 180, eds. C. J. Brinker, D. E. Clark and D. L. Ulrich, Materials Research Society, Pittsburgh, 1990, p. 759. 11 J. K. Bailey, G. A. Pozarnsky and M. L. Mecartney, J. Muter. Sci., 1992,7, 2530. 12 W. G. Klemperer and W. Shum, J. Am. Chem. Soc., 1977,99,3544. 13 C. J. Besecker, W. G. Klemperer, D. J. Maltbie and D. A. Wright, Inorg.Chem., 1985,24, 1027. J. MATER. CHEM., 1994, VOL. 4 1753 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 V. W. Day, W. G. Klemperer and D. J. Maltbie, J. Am. Chem. SOC.,1987, 109,2991. A. T. Harrison and 0.W. Howarth, J. Chem. SOC., Dalton Trans., 1985,1953. A. T. Harrison and 0.W. Howarth, J. Chem. SOC., Dalton Trans., 1985,1173. R. C. Hibbert, N. Logan and 0. W. Howarth, J. Chem. Soc., Dalton Trans., 1986,369. E. Heath and 0.W. Howarth, J. Chem. SOC., Dalton Trans., 1981, 1105. W. G. Klemperer, in The Multinuclear Approach to NMR Spectroscopy, eds. J. B. Lambert and F. G. Riddell, Reidel Press, Boston, 1983, 671. W. G.Klemperer, Angew. Chem., Int. Ed. Engl., 1978, 17,246. L. Pettersson, B. Hedman, I. Anderson and N. Ingri, Chem. Scr., 1983,22, 254.L. Pettersson, B. Hedman and I. Anderson, Chem. Scr., 1985 25, 309. R. E. Mesmer and C. F. Baes, The Hydrolysis of Cations, John Wiley, New York, 1976,p. 195. C. Detellier, in Modern NMR Techniques and Their Application in Chemistry, eds. A. I. Popov and K. Hallenga, Marcel Dekker, New York, 1991, p. 521. J. Reuben and D. Fiat, Inorg. Chem., 1967,6, 579. C. Madic, G. M. Begun, R. L. Hahn, J. P. Launay and W. E. Thiessen, Inorg. Chem. 1984,23,469. D. Rehder, Bull. Magn. Reson., 1982,4, 82. 0.W. Howarth, Prog. NMR Spectrosc., 1990,22,453. J. Reuben, J. Am. Chem. SOC., 1969,91,5725. 31 32 33 34 35 36 37 38 39 40 41 42 43 44 H. M. Schwartz, M. MacCoss and S. Danyluk, Tetrahedron Lett., 1980,21,3837. C. Sanchez, J. Livage and G. Lucazeau, J. Raman Spectrosc., 1982, 12, 68.P. Aldebert, N. Baffier, N. Gharbi and J. Livage, Muter. Hes. Bull., 1981, 16, 669. L. Abello and G. Lucazeau, J. Chim. Phys., 1984,81,539. G. Pimental and G. L. McClellan, The Hydrogen Bond, lreeman, San Francisco, CA, 1969. H. T. Evans, Inorg. Chem., 1966,5,967. L. Abello, E. Husson, Y. Repelin and G. Lucazeau, J. Solid State Chem., 1985,56,379. R. C. T. Slade, J. Barker and T. K. Halstead, Solid Sta e lonics, 1987,27,221. M. T. Vandenborre, R.Prost, E. Huard and J. Livage, Mtiter. Res. Bull., 1983, 18, 1133. J. Livage, P. Barboux, J. C. Badot and N. Baffier, Better Ceramics Through Chemistry 111, MRS Proceedings 121, eds. C. J. Brinker, D. E. Clark and D. L. Ulrich, Materials Research Society, Pittsburgh, 1988, p. 167. M. Nabavi, F. Taulelle, C. Sanchez and M. Verdaguer, J. Phys. Chem. Solids, 1990,51, 1375. S. Stizza, M. Benfatto, A. Bianconi, J. Garcia, G. Mancini and C. R. Natoli, J. Phys. (Paris), 1986,47-C8,691. S. Stizza, I. Davoli and M. Benfatto, J. Non-Cryst. Solids, 1987, 96, 327. S. Stizza, G. Mancini, M. Benfatto, C. R. Natoli, J. Garcia and A. Bianconi, Phys. Rev. B, 1989,4229. 30 M. Burgar, T. E. St. Amour and D. Fiat, J. Phys. Chem., 1981, 85, 502. Paper 4/02409E; Received 25th April, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401749

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( ll), 1755-1761 Characterization of Silicated Anatase Powders Li Yi,+ Gianguido Rarnis, Guido Busca and Vincenzo Lorenzelli lstituto di Chimica, Facolta di Ingegneria, Universita, P. le Kennedy, I- 16129 Genova, Italy Silicated titanias have been prepared by reaction of commercial anatase-based Ti0, powders with tetraethylorthosilicate, followed by calcination. At low loadings orthosilicate species are found, that interact mainly with surface defects of anatase where surface hydroxy groups are located. At higher loadings polysilicate species are found, while amorphous silica is observed if the nominal loading is comparable to that allowing the coverage of the entire TiO, surface with a silicate layer. Surface silicate species progressively mask the TiO, surface, and do not generate Bransted acidity that is sufficiently strong to protonate pyridine.Silication strongly hinders anatase crystal growth and loss of surface area upon heating, as well as the anatase-to-rutile phase transformation. Thus, supported silica in small amounts can act as an efficient morphology and structure promoter of anatase Ti0,-based catalysts, although it also perturbs the TiO, surface chemistry significantly. Anatase is largely used in heterogeneous catalysis as the support of vanadia catalysts for the selective catalytic reduction of NO, by ammonia (SCR process') and for the selective oxidation of o-xylene to phthalic anhydride.2 Moreover, it is used as the support of molybdena-based catalysts for hydrotreatments and for the Claus proce~s.~ Anatase is also a good photocatalyst, and is better than r~tile.~However, anatase is a metastable form of TiO,, the stable polymorph being rutile at all temperatures at 1 atm.5 Anatase tends to transform into rutile and this phase trans- formation is accelerated by some catalytically active oxides like vanadium o~ide.~,~ The anatase-to-rutile phase transform- ation is much faster for high-area anatase than for low-area anatase',' and is accompanied by a dramatic decrease of surface area.Furthermore, rutile-based catalysts are generally less active than anatase-based catalyst~.~,~* The anatase-to- rutile phase transformation is therefore the main cause of deactivation of vanadia-titania catalysts." This is probably why the supports of SCR catalysts (characterized by high surface areas, 50-loom2 g-') also contain W03, which strongly inhibits both anatase sintering and the anatase-to- rutile phase transf~rmation,~"~~'~ besides having other favour- able effects.Most industrial SCR catalysts also contain sil- iceous materials. Much has been reported on SCR catalysts constituted by vanadia supported on silica-titania mixed o~ides.~~'~~Bulk silica-titanias prepared by coprecipitation techniques have also been investigated in detail in relation to their own catalytic activity in acid-type reaction^'^,'^ and as catalysts for the oxidation of halogenated organic com-pounds." Interest has also been devoted to titania-on-while little data has been reported, to our knowledge, on silica-on-titania or silicated titania, i.e.anatase with small amounts of silica deposited on its surface.22 This paper reports our data on a study of the solid-state chemistry of anatase-based TiO, commercial preparations doped with small amounts of silica, and on the effect of silica on the surface properties of commercial anatase. Experimental Commercial Ti02 powders, P25 from Degussa (Hanau, Germany) and Eurotitania from Tioxide (Billingham, UK) were used. A home-made anatase sample and amorphous silica (Aerosil 130 from Degussa) were also used for comparison. t Permanent address: MMEI Tianjin Institute of Reprographic Technology, Tianjin 300131, Peoples' Republic of China.Silicated samples were prepared by a method similar to that proposed for the preparation of silicated al~minas.~~ The support was immersed in solutions of tetraethylorthosilicate [TEOS, Si(OC,H,),] in butan-2-01. After impregnation of the support the solvent was evaporated and the solid was dried. The dried samples were then calcined in air at 773 or 1023 K for 3 h. Samples were also characterized after differential thermal analysis-thermogravimetry (DTA-TG) runs up to 1240K. The samples are hereinafter denoted by E (Eur otitania support) or D (Degussa support) followed by the nominal percentage of SiO, (e.g. E2 means 2% SiO,-TiO, with Eurotitania support). Table 1 summarizes some characteristics of the Ti02 supports and of the silicated samples.Blank experiments with the supports treated with butan-2-01 have been performed. No other detectable differences we1 e found in the behaviour of the supports. FTIR spectra were recorded using a Nicolet SZDX instru- ment equipped with conventional gas manipulation 'outgas- sing lines. For IR measurements, samples precalcined at 773 K were pressed into self-supporting discs of appropriate thick- nesses and heated in air at 620K in the IR cell for 30min, and finally evacuated at the same temperature for 1 h. I'yridine (1 Torr, 5 min) and CO (50 Torr, 5 min) were adsorbed from the gas phase. The outgassing time at the given temperatures was 5 min. FT Raman spectra were recorded using a Brucker RFSlOO (Nd:YAG laser). Simultaneous TG-DTA experiments were performed with a Setaram TGA92, while XRD patterns were recorded using a Philips PW1130-1049/10, with a Co-Kcr source.Crystal sizes were measured by the Scherrer and the anatase-to-rutile ratios were calculated u\ing the Spurr and Myers formula.25 Adsorbates and reagents were hyperpure commercial prod- ucts from SIO and Carlo Erba (Milano, Italy). Results Solid-state Characterization In Table 1 the crystal phase composition of our samples, deduced by XRD analyses, is reported. The Eurotitaniii sample as obtained is rutile-free, but its XRD pattern shows, together with the peaks typical of anatase (JCPDS Table no. 2 1-1272), also weak diffractions from brookite (JCPDS Table no. 29-1360). Instead, the Degussa sample as received is a mixture of anatase and rutile (JCPDS Table no.21-1276). As shown in Table 1, EO is still stable in the anatase form (with brookite) after calcination at 773 K but is completely converted to rutile by heating at 1023 K. The anatase-to- J. MATER. CHEM., 1994, VOL. 4 Table 1 XRD phase composition, surface areas (S/m2 g-') and crystal sizes (r,/A)of titania and silicated titania samples" sample Yo SiO, T, XRD S r €0 0 300 Ab 125 86 770 Ab 78 236 1020 R 2 452 1240' R >500 E2 2 770 Ab 119 86 1020 Ab 67 157 1240' A 91% 248 R 9% 21 1 El0 10 770 Ab 99 82 1020 Ab 102 82 1240' Ab 128 DO 0 300 A 68% 55 230 R 32% 317 770' A 64% 40 225 R 36% 328 1020 A 12% 15 315 R 88% 413 1240' R 365 D0.5 0.5 7 70 A 66% 49 215 R 34% 297 1020 A 68% 45 248 R 32% 339 1240' A 32% 295 R 68% 328 D1.3 1.3 770 A 68% 51 201 R 32% 288 1020 A 68% 48 210 R 32% 297 1240' A 61% 255 R 39% 306 D2.5 2.5 770 A 68% 51 205 R 32% 288 1020 A 68% 47 210 R 32% 279 1240' A 66% 220 R 34% 297 D5 5 770 A 69% 50 205 R 31% 288 1020' A 68% 48 205 R 32% 288 a T,=calcination temperature; A =anatase; R =rutile.Traces of brookite. Samples after DTA-TG runs. rutile phase transition during DTA runs is evident at 1030 K from the detection of a distinct exothermic peak (Fig. 1). This peak is not observed at all in the case of the silicated Eurotitania samples. Accordingly, XRD analyses show that the rutile phase is not present in sample El0 either after DTA runs or after calcination at 1023 K for 3 h.Sample E2 is still rutile-free (with brookite impurities) after calcination at 1020K, but after DTA runs it contains 9% rutile and no brookite. It is known that brookite is converted thermally into rutile, not into anatase26 and faster than anata~e,~~ so rutile should mostly be formed by brookite transformation, while anatase is stable. This allows us to conclude that the brookite content in Eurotitania TiOz is 69%. Analysis of the behaviour of the E samples suggests that silica stabilizes both anatase and brookite with respect to their phase transformation to rutile.However, in the case of E2, brookite transforms to rutile rather than anatase. The pure Degussa sample DO shows almost no conversion upon calcination at 770 K, while it is only partly converted to rutile after calcination at 1020 K. As shown previ~usly,~ the Degussa sample is more resistant to phase transformation with respect to other materials, in spite of the presence of rutile crystals, due to its relatively low surface area and small porosity. This is also evident from Fig. 1, which shows the r 673 873 1073 1273 TIK Fig. 1 DTA traces for samples EO (a),E2 (b),DO (c)and D2.5 (d). Heating rate 10 K min-' in air. DTA peak associated with the anatase-to-rutile phase trans- ition near 1173 K with respect to 1030 K for EO.D0.5 is partly converted at 1240 K (DTA runs) while none of the other silicated Degussa samples are converted signifi- cantly even at 1240 K. Thus, the data on D samples confirm that silica also strongly inhibits the anatase-to-rutile phase transformation in the presence of rutile. To obtain more information on the state of supported silica species we also studied the skeletal IR spectra with KBr pressed disks. All the spectra present the typical absorption of TiOz in the region 1000-400 cm-', as discussed previ~usly.~~ However, silica-containing samples show additional absorption in the region 1300-400 cm- '. These absorptions, after subtraction of the spectrum of the pure TiO, support, are compared in Fig. 2 with the spectrum of amorphous silica for the D samples.The spectra observed are analogous with those of the E samples. D5 shows a strong band centred at 1065 cm-' with a definite shoulder near 1200 cm-'. Additional shoulders are very evident at 940 and 895cm-' together with a very broad band centred near 740cm-'. The spectra of D2.5 and D1.3 no longer show the shoulder near 1200 cm-', while the components in the region 1000-900 cm-I are more intense 1400 1000 600 wavenurnberIcm-' Fig.2 FTIR spectra of silicate species in D0.5 (a), D1.3 (b),D2.5 (c) and D5 (d)samples (KBr pressed disks, anatase spectrum subtracted) (e)skeletal spectrum of amorphous silica J. MATER. CHEM., 1994, VOL. 4 with respect to the highest-frequency maximum that also shifts to 1030 cm-l.Moreover, the broad band near 740 cm-' decreases for D2.5 and is absent for D1.3 and D0.5. The features in the 1300-1000cm-' region observed for D5 correspond rather well to those of the envelope due to the Si -0-Si asymmetric stretching modes of amorphous silica.28 However, amorphous silica does not present shoulders between 1000 and 850cm-' and shows a sharp band at 800 cm-' due to the Si-0-Si symmetric stretching/bending modes. Thus, while amorphous silica is probably present as a separate phase in D5 (as well as in DlO), it is absent in the samples of lower silica content. D2.5 the persistence of the broad band at 740 cm-', associated with the symmetric stretching of Si- 0-Si bridges, indicates that polysilicate anions are present.The position and broadness of this band allow us to exclude pyrosilicates as predominant species29 while suggesting pyroxene-like chains3' or layer silicate-type sheets31 as predominant in D2.5 but present also in D5. For D1.3 and D0.5 the absence of the band in the 800-700 cm-' region strongly supports the idea that only orthosilicate species are present,32 the band with components at 1030,920 and 880 cm-' being associated with the stretching of terminal Si-0 bonds of isolated sio44-. In conclusion, FTIR spectra suggest that on D0.5 and D1.3 isolated orthosilicate anions exist, on D2.5 polysilicates pre- dominate, while on D5 and D10 together with polysilicates, bulk silica particles are also present. These data can be discussed taking into account the amount of silica added to titania in the samples, in relation to the surface areas.The area available for each Si atom (assuming that they ade homogeneou!ly distributed a! the surface) is aboyt 100 A2 for D0.5, 38 A2 for D1.3, 20 A2 for D2.5 and 10A2 for D5. These areas can be compared with the areas occupied by isolated and polymeric silicate anions. While the arca occupied by an isolated orthosilicate anion is less than 3 A2, in the sheets of layer silicateso like kaolinite there is no more than one silicon atom per 11 A2.33Previously, ImFmura et a[.'' evaluated the area of a silicate unit as 25.4 A2, by comparison with that occupied in the cristobalite structure. However, the density of the most stable silica polymorphs is far lower than that of the silicate sheets of layer silicates.The area available for silicate species in D5 and D10 necessarily implies the formation of polymeric species. On the contrary, the area available for each silicate species in D0.5 and D1.3 allows their dispersion as isolated ions. Obviously, their dispersion implies that the interaction of silicate species with the titania surface is stronger than the mutual interaction between silicate species. For D2.5, the area theoretically available is only twice that of layer silicates, in accord with the presence of both isolated and polymeric silicate anions. Fig. 3 shows the Raman spectra of the Si0,-Ti02 samples and of the corresponding Ti02 supports.As discussed else- where,27 the Raman spectrum of anatase shows strong peaks at 639, 516,397 and 144 cm-' and a weak peak at 197 cm-'. Additional peaks are observed at 590, 550 (broad shoulder), 452, 366, 322, 290, 246 and 210cm-' in the pattern of the Eurotitania support, due to brookite. For the Degussa sample the peak at 447 cm-' and a shoulder at 610 cm-' are indica- tive of the presence of rutile. The spectra of the Si02-Ti02 samples do not differ qualitatively from those of the corre- sponding support. No new bands associated with silicate species are found. Correspondingly, amorphous silica gives an extremely weak and broad Raman pattern.21 However, the intensity of the Ti02 patterns is significantly reduced by the addition of silica.The intensity of the peak at 144cm-' for El0 is 75% of that of EO, while for D10 it is about 50% of that for DO. This weakening appears to be roughly pro- portional to the amount of loaded silica. According to previous 6 3 0 800 600 400 200 wavenumber/cm-' Fig. 3 FT Raman spectra of EO (a),El0 (b),DO (c), D1.3 (d I, D2.5 (e) and D5 (f)[spectra (c) and cf) are repeated using an expantled scale] data2' this could be associated with increased disorder at the surface. Morphology Characterization Table 1 shows the surface areas and the crystal sizes of the SO2-Ti02 samples and of the corresponding Ti02 hupports, as prepared or after calcination. The surface area of the Eurotitania sample is decreased to less than 30"/0 upon calcination at 773 K, when the anatase and brookile phases are still stable, but the anatase crystal size has gropn signifi- cantly.The surface area drops to 2 m2 g-' after calcination at 1020 K, when the sample is completely converted to rutile. The addition of silica strongly hinders the surface area loss, together with the anatase-to-rutile phase transition. For El0 the surface area remains near 100 m2 g-' even after calcination at 1020 K. Correspondingly, the sintering of anatahe is also inhibited. El0 retains the same crystal size of the starting support even after calcination at 1020 K. The area available for each silicate anion on the El0 s?mple, which appears to be morphologically stable, is just 10 A2, which is very similar to the area per silicate ion in the sheets of layer silicates. The IR spectra show that silica particles can be present together with polysilicates in this sample, whose loading nearly corre- sponds to that of D5.As discussed elsewhere, the Degussa sample, although it already contains rutile, is converted to rutile more slowly than Eurotitania owing to its much lower surface area arid higher crystal size.7 Correspondingly, it also retains a highcr surface area after calcination at 1020K (15 m2 g-'). Also, in this case, the addition of silica strongly hinders the surface-area loss and anatase crystal growth. D1.3 and D2.5 appear to be almost morphologically stable upon calcination at 1020K, although the silica loa4ing relative to the surface area is nearly half fp-D2.5 (20 A2 per Si atom) than that of the El0 sample (10 A2 per Si atom).J. MATER. CHEM., 1994, VOL. 4 Surface Characterization Surface Hydroxy Groups Fig. 4 shows the spectra of the surface hydroxy groups of DO and of the corresponding SO,-TiO, samples, after activation at 623 K in vacuum. The spectrum of DO exhibits several components: 3734, shoulder, 3713,3690,3658 and 3641 cm-'. These components are due to the different coordination states of different OH groups on anatase (and on rutile), as discussed previou~ly.~~We previously showed that anatase samples with smaller surface areas give much simpler OH ~pectra,~,~~ showing that most OH groups of high-area anatase are located on the corners, edges or surface defects.The spectrum of D0.5 is very different from that of DO, showing one band only, centred at 3738 cm-', with a shoulder near 3745 cm-' and a tail at lower frequency. This band does not correspond to that of low-area anatase samples, observed at 3680 cm-'.7*34 By increasing the silica loading, the intensity of this band increases progressively, while the maximum also progressively shifts towards 3744 cm-' as measured for D10. The same frequency was observed by us under the same conditions for the silanol groups of pure amorphous silica. This allows us to assign the main band in all silica-containing samples to OH-stretching of surface silanol groups. The shift towards lower frequency by decreasing the silica loading agrees with the previous data reported for coprecipitated ~ilica-titanias.~~This shift is likely to be associated with the higher electron densities at silicon atoms in orthosilicate anions with respect to polysilicates and framework silica (because of the more ionic nature of SiO-Ti bonds with respect to SiO-Si bonds), although the dependence of the OH stretching frequency on the electron densities on atoms and on its own acidity is a complex matter, as already remarked upon.36 Note that in D0.5 the hydroxy groups of titania appear to be completely removed, so a very complex TiOH pattern is substituted by a single SiOH band.This apparently points to the complete dispersion of the orthosilicate anions, probably on surface defects, and to the role of the surface hydroxy groups of TiO, as reaction sites with respect to TEOS, giving rise to silicate species.Adsorption of Carbon Monoxide In Fig. 5 the spectra of carbon monoxide adsorbed at room temperature on the supports DO and EO, and on a third TiO, anatase sample that, according to XRD and Raman analyses, appears to be both rutile-free and brookite-free are reported. The samples were outgassed at 623 K before adsorption. The 3800 3750 3700 3650 3600 wavenurnbedcm-' Fig.4 FTIR spectra of the surface hydroxy groups of DO (a), D0.5 (b),D1.3 (c), D2.5 (d)and D5 (e)samples, all outgassed at 623 K 2300 2200 2100 2000 wavenurnber/cm-' Fig. 5 FTIR spectra of the surface carbonyl species arising from CO adsorption on DO (a),a home-made pure anatase sample (b)and EO (c) spectra show in all cases a band split between a main component at 2188 cm-' and a minor one at 2206 cm-'.This split band is typical of CO adsorbed on anata~e,~~.~~,~~in contrast to rutile that shows only one band near 2185 cm-'. The comparison of the three samples, one of which is pure anatase, with the others containing rutile (DO) or brookite (EO), seem to suggest that brookite does not have a significant effect on the nature of the active sites. On the contrary, the stronger intensity of the lower-frequency component with respect to the higher-frequency one on DO related to the other samples could be associated with the additional effect of rutile surface in this sample. A third band is observed in the three cases near 2105 cm-'.As discussed elsewhere, this band is associated with CO adsorbed on reduced Ti3 centre^?^,^' This band is not reported by several authors reporting the CO adsorption on titania~~',~~ because they pretreated the sample in oxygen and, consequently, worked with nearly stoichiometric TiO,. Its intensity cannot be compared with that of the higher-frequency band: in fact it is well known that the C-0 stretching of coordinated C(.) on reduced centres gains much intensity when n-type back-donation occurs from d electrons of the metal centre, also causing a lowering of the C-0 stretching frequency, with respect to the gas-phase value (2143cm-I). So, this band is probably associated with a very small number of reduced centres, in spite of its remarkable intensity.In Fig. 6 the spectra of CO adsorbed on DO, D0.5, D1.3 and D2.5 previously outgassed at 623 K are compared. On all three silica-containing samples the higher-frequency com- ponent is present as a poorly resolved shoulder, probably because its maximum is slightly shifted downwards, to 2202 cm-', while the band at 2188 cm-' decreases sharply in intensity, almost disappearing in D2.5. The band at 2105 cm-' has a different behaviour: it grows in intensity and shifts significantly upwards in DO, D0.5 and D1.3, but it almost disappears in D2.5. On D5 and D10 no bands of adsorbed CO are observed. In Fig. 7 the spectra of CO adsorbed on silicated Degussa samples outgassed at 773 K are reported.After this pretreat- ment a band at 2205 cm-' is observed together with the main one, always observed at 2188 cm-', but it is much weaker compared with the relative intensities observed on the pure support. Moreover, the intensity of the band at lower frequen- cies, associated with CO adsorbed on Ti3+ centres, is stronger, as expected for a stronger outgassing treatment that can cause partial oxygen depletion with increasing non-stoichiometry of the semiconducting TiO,-, phase. However, in this case the J. MATER. CHEM., 1994, VOL. 4 1.05 wavenum ber/cm-' Fig. 6 FTIR spectra of the surface carbonyl species arising from CO adsorption on DO (a), D0.5 (b), D1.3 (c) and D2.5 (d); samples previously outgassed at 623 K a, t 95: a I I I I 1 I 2300 2200 2100 2000 wavenum ber/crn-' Fig.7 FTIR spectra of the surface carbonyl species arising from CO adsorption on D0.5 (a), D1.3 (b) and D2.5 (c); samples previously outgassed at 773 K intensity of this band is not as dependent on the silica loading, but its position is dependent on the loading (it progressively shifts from 2105 to 2125 going from DO to D2.5, see Fig. 6 and 7). These reduced species were also observed on outgassed mixed silica-titanias, and were characterized by a split band at 2118 and 2130 cm-'.35 The upwards shift of this frequency with increasing silica loading may be due to a higher cationic charge on these reduced sites when they are surrounded by silicate ions than when they are surrounded by the more basic oxide ions.These data can be interpreted as follows: (i) silication causes the progressive disappearance of Ti4+ sites active in CO adsorption; (ii) the stronger sites, responsible for the band above 2200 cm-', are less affected quantitatively by silication than the weaker sites, responsible for the band at 2188 cm-l, but the corresponding band is slightly shifted downwards; and (iii) silication causes a decreased electron densit!. on the Ti3+ reduced sites produced by outgassing, with a consequent shift of the corresponding band from 2105 to 2125 crr-'. Adsorption of Pyridine Fig. 8 shows the spectra of pyridine adsorbed on the Degussa TiO, support, on the corresponding Si0,-TiO, samples and on pure amorphous SiOz after activation at 623 K in vacuum.In all cases the absence of bands in the regions near 1630 and 1550cm-' point to the lack of sufficient Brsnsted ataidity to protonate pyridine. On the contrary, the positions of the 8a (shifting from 1603 to near 1610 cm-' during outgassmg) and 19b (1443-1447 crn-') components provide evidence (or pyri- dine species coordinated on Lewis-acid sites. However, the intensities of these bands strongly decrease with increasing silica loading without evident shifts, being very weak already for the sample D2.5. However, another 8a component grows, close to that near 1610 cm-', with increasing silica loading, at 1595 cm-l. This species is also associated with the forma- tion of strong absorption in the region 2500-4000~m-~ at the expense of the sharp OH band near 3740 cm-', .issigned above to silanol groups (Fig.9). It is evident that the silanol groups associated with the surface silicate species interact with pyridine via hydrogen bonding. Their Brsnsted strength is sufficient to interact rather strongly with pyridine but is not sufficient to cause proton transfer to the adsorbed base. The OH stretching band of the silanol-pyridine H-bonded complexes is observed near 3100 cm-' on silicated tit;mia and near 2900 cm-' on silica (Fig. 9). This should indicate that the interaction is stronger on silica than on silicated titania, thus providing evidence of a lower Brsnsted acid strength of the surface silanols of silicated titania with respect to ,imorph- ous silica.It is interesting to compare these results with those reported on coprecipitated silica-titania with silica contents higher than 50%17 that show that H-bonding oi surface silanols with pyridine result in the formation of very strong H-bonds, with the OH stretching band being split due to its Fermi resonance with the first overtone of the in-plane Si- OH deformation, at 2700 and 2300 cm-l. This indicates that coprecipitated silica-titanias are stronger Brsnsted solid acids than silicated titania and silica. Upon interaction of the sample with pyridine, the spectrum of the adsorbant in the region 1300-1000 cm-' is pcrturbed, as shown by the negative bands appearing in the suhtraction spectra in the region 1300-1000 cm-' (Fig.10).These negative bands correspond to absorptions in the spectrum of the activated sample that disappear (or are strongly displaced) r wavenurnbe r/cm-' Fig.8 FTIR spectra of adsorbed pyridine on DO (a) (-1, D0.5 (b) (---), D2.5 (c) (---) and amorphous silica (d) (-.-), all previously outgassed at 623 K 4000 3200 2400 wavenum berkm-' Fig. 9 FTIR spectra of DO (a), D0.5 (b),D1.3 (c), D2.5 (d) and silica (e), outgassed at 623 K (-) and after pyridine adsorption (---). The transmittance scale for the spectra in (a) is twice that of the other spectra. upon pyridine adsorption. For D0.5 this negative band is broad, centred near 1100 cm-', while for D1 it is centred near 1150 cm- '.For D2.5 this band is split, with components at 1173 and 1034 cm-'.These features are still present when the band at 1595cm-' has disappeared. This indicates that they are due to species that are perturbed upon Lewis-type coordinative interaction on Ti4+ sites. Accordingly, these bands can be assigned neither to the Si-0 stretching nor to the Si--OH in-plane deformation of surface silanol groups. In fact, it seems that these modes fall at 980cm-' and near 800 cm-' for the silanol groups of silica.39 It is unlikely that these modes can be strongly shifted in the case of silicated titania with respect to bulk amorphous silica. Thus, these bands are associated with the stretchings of terminal Si- O-Ti4+ bonds in orthosilicate or polysilicate species. Pyridine coordination on Ti cations increases their electron densities and allows the Si-O(Ti) bonds to relax, so their stretching frequency shifts down.Conclusions (i) Silication strongly stabilizes anatase phase with respect to its conversion to rutile. (ii) Silication inhibits anatase sintering and loss of surface area. (iii) At small loadings silication of anatase results in the formation of isolated orthosilicate species. At higher loadings J. MATER. CHEM., 1994, VOL. 4 1300 1100 wavenum berkm-' Fig. 10 Perturbations, arising from pyridine coordination, of the FTIR spectra of D0.5 (a), D1.3 (b)and D2.5 (c), prebiously outgassed at 623 K. (-) Outgassing at room temperature, after pyridine adsorption; (---) outgassing at 423 K; (-*-) outgassing at 623 K.The upwards sharp bands near 1235,1219,1150,1070 and 1040 cm-' are due to coordinated pyridine, while the downwards broad bands are due to perturbed Si-0 stretching modes. polysilicate species, possibly pyroxene-like chains or layer silicate-type sheets are formed. (iv) When the amount of loaded silica is increased to a value similar to that corresponding to coverag: of the whole titania surface with a layer silicate (about 10 A2 per silicon atom) amorphous silica also appears. (v) The surface silicate species cause a decrease in the intensity of the anatase Raman pattern, possibly associated with increased disorder at the surface. (vi) Isolated orthosilicate species contain surface silanol groups whose OH stretching is observed at lower frequency than those of polysilicate species and of silica (3738 us.3744 cm-') (vii) Silicate species are coordinated to Lewis-acidic Ti4+ sites and are perturbed by pyridine coordination on them. (viii) Silication causes the progressive destruction of the active sites for CO and pyridine adsorption on Ti02. (ix) Silicated titanias do not have sufficiently strong Brsnsted acidity to protonate pyridine. Surface silanol groups of silicated titanias are more active in hydrogen bonding than TiOH groups of titania, but have less Brarnsted acidity than silanols of amorphous silica and of coprecipitated silica- titania. (x) The characteristics of TiOz surface are almost complete!y destroyed when the surface silicon density approaches 20 A2 per Si atom, corresponding to half the density of layer silicate sheets.These data indicate that silica deposited on the anatase surface acts as a strong morphology and structure stabilizing agent. However, silica also modifies significantly the surface chemical properties of anatase. This work was supported by MURST, Rome, Italy. L.Y. acknowledges the governments of Italy and of the Peoples' Republic of China for an exchange grant. The authors are indebted to Dr. G. Oliveri for the surface area measurements. J. MATER. CHEM., 1994, VOL. 4 1761 References 20 A. Fernandez, J. Leyrer, A. R. Gonzalez-Elipe, G. Munuera and H. Knozinger, J. Catal., 1988,112,489. 1 H. Bosch and F. Janssen, Catal. Today, 1988,2, 369.21 S. Srinivasan, A. K. Datye, M. Hampden-Smith, I. E. Wachs, 2 M. S. Wainwright and N. R. Foster, Catal. Rev., 1979,19, 211. G. Deo, J. M. Jehng, A. M. Turek and C. H. F. Peden, J. Catal., 3 S. Matsuda and A. Kato, Appl. Catal., 1983,8, 149. 1991,131,260. 4 J. Augustynski, Electrochim. Acta, 1993,38,43. 22 S. Sato, Langmuir, 1988,4, 1156. 5 I. Barin, Thermochemical Data of Pure Substances, VCH, Berlin, 23 F. Buonomo, V. Fattore and B. Notari, US Pat. 401.;589 and 1990, part 11, pp.1546, 1547. 4013590, 1977; G. Manara, V. Fattore and B. Notari, US Pat. 6 D. J. Cole, C. F. Cullis and D. J. Hucknall, J. Chem. Soc., Faraday 4038337,1977. Trans. I, 1976,72,2185. 24 H. P. Klug and L. E. Alexander, X-Ray DiBraction Piocedures, 7 G. Oliveri, G.Busca, G. Ramis and V. S. Escribano, J. Muter. Wiley, New York, 1970. Chern., 1993, 3, 1239. 25 R. A. Spurr and H. Myers, Anal. Chern., 1957,29,760. 8 S. R. Yoganarasimhan and C. N. R. Rao, Trans. Faraday Soc., 26 C. N. R. Rao, S. R Yoganarasimhan and P. A. Faetti, Trans. 1962,58,1579. Faraday Soc., 1961,57,504. 9 1. Gasior, M. Gasior, B. Grzybowska, R. Kozlowski and 27 G. Busca, J. M. Gallardo Amores, P. Piaggio, G. R'rmis and J. Sloczynski,Bull. Acad. Pol. Sci., 1970,27, 829. V. Sanchez Escribano, J. Chem. Soc., Faraday Trans., in the press. 10 I. M. Pearson, H. Ryu, W. C. Wong and K. Nobe, Ind. Eng. Chem. 28 H. H. W. Moenke, in The Infrared Spectra of Minrmls, ed. Prod. Res. Det.., 1983,22,381. V. C. Farmer, The Mineralogical Society, London, 1974.p. 365. 11 V. A. Nikolov and A. I. Anastasov, Ind. Eng. Chem. Rex, 1992, 29 M. Gabelica-Robert and P. Tarte, Spectrochim. Acta Part A, 1979,35A, 649. 31, 80. 30 M. S. Bilton, T. R. Gilson and M. Webster, Spectrochim. Actu, 12 G. Ramis, G. Busca, G. Cristiani, L. Lietti, P. Forzatti and Part A, 1972,28A, 21 13. F. Bregani, Langrnuir, 1992,8, 1744. 31 V. C. Farmer and J. D. Russel, Spectrochim. Acta 1964,20, 1149. 13 L. Lietti, P. Forzatti, G. Busca and E. Giamello, submitted for 32 P. Tarte, Spectrochirn. Acta 1963,19, 25. publication. 33 D. T. Griffen, Silicate Crystal Chemistry, Oxford I rniversity14 C. U. I. Odenbrand, S. T. Lundin and L. A. H. Andersson, Appl. Press, 1992.Catal., 1985,18, 335. 34 G. Busca, H. Saussey, 0.Saur, J. C. Lavalley and V. I orenzelli,15 A. Baiker, P. Dollenmeier, M. Glinski and A. Reller, Appl. Catal., Appl. Catal., 1985,14, 245. 1987,35, 365. 35 C. U. I. Odenbrand, S. L. T. Andersson, L. A. H. Andersson,16 K. Shibata, T. Kiyourd, T. Kitagawa, T. Sumiyoshi and J. G. M. Brandin and G. Busca, J. Catal., 1990,125,541L. Tanabe, Bull. Chem. Soc. Jpn., 1973,46,2985. 36 K. Hadjiivanov, D. Klissurski, G. Busca and V. I orenzelli, 17 C. U. I. Odenbrand, J. G. M. Brandin and G. Busca, J. Catal., J. Chem. Soc., Furaday Trans., 1991,87,175. 1992,135,505. 31 D. J. T. Yates, J. Phys. Chem., 1961,65,746. 18 S. Imamura, S. Ishida, H. Tarumoto and Y. Saito, J. Chem. Soc., 38 E. Garrone, V. Bolis, B. Fubini and C. Morterra, Langniuir, 1989, Faraday Truns., 1993,89,757; S. Imamura and H. Tarumoto, Ind. 5,892. Eng. Chem. Rex, 1989,28, 1949. 39 B. A. Morrow and A. J. McFarlan, J. Phys. Chem., 1992.96,1395. 19 M. G. Reichman and A. T. Bell, Langmuir, 1987, 3, 111; M. G. Reichmann and A. T. Bell, Appl. Catal., 1987,32, 315. Paper 4/02606C; Received 3rd May, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401755

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4(11), 1763-1764 MATERIALS CHEMISTRY COMMUNICATIONS Preliminary Crystal Structure of Mixed-valency Sr4Ni309, the Actual Formula of the so-called Sr5Ni401 Francis Abraham,* Sylvie Minaud and Catherine Renard Laboratoire de Cristallochimie et Physicochimie du Solide, URA CNRS 0452, ENSCL, Universite des Sciences et Technologies de Me, B.P. 708, 59652 Villeneuve d'Ascq Cedex, France A crystal structure investigation of the so-called Sr,Ni,O,, from single-crystal X-ray data has shown that the composition of this oxide is in fact close to Sr,Ni,O,. The structure has been solved in the trigonal P321 space group. The final refinement gave an R factor of 0.045 for 51 2 independent reflections. The structure contains NiO, chains with two NiO, octahedra and one NiOG trigonal prism alternating and sharing faces. The chains run along the three-fold axis and are connected by Sr ions.The Ni-0 distances seem to indicate a possible repartition of Ni'" in the octahedral sites and Ni" in the trigonal prisms. In the Sr-Ni-0 system two compounds have been charac- terized for Sr :Ni = 1. SrNiO,, which contains tetravalent nickel, has been synthesized under a high pressure of oxygen (50-2000 atm, 600°C) from Sr(OH),-8H20 and NiO. It adopts a pervoskite-like stzucture with towo-layer hexagonal close-packing [a =5.355( 1)A, c =4.86( 1)A]. Sr,Ni,O,, which contains trivalent nickel, has been obtained above 600 "C and at 1 atm of wet oxygen gas flow;' the >exagonal unit-cell parameters [a = 5.465(1)A, c =4.137(1)A at 600 "C; 1atm) vary with the synthesis conditions. In the course of a study of the Ba, -xSrxNi03-y system, Marcizak and oKatz2 foun$, for x= 1, a hexagonal nine-layer phase (a =5.47 A, c =19.734)accomp?nied by a 'compressed' two-layer phase (a =5.47 A, c=4.05 A) close to Sr,Ni,O,. In fact, Sr,Ni,O, is unstable and different oxygen-deficient compounds are obtained depending on the conditions.Recently, Lee and Holland3 reported the preparation of a new strontium nickel oxide. From microprobe analysis and thermal gravimetric studies under a reducing atmosphere, they deduced an idealized empirical formula Sr,Ni,O which corresponds to trivalent nickel. Tbe hexagonal unit-cell~ parameters [u=9.480(1) A=5.4743 A; c=7.815(4) A] do not seem to correspond to a close-packing layer.Recently, James and Attfield, also obtained Sr,Ni,O,, as an impurity during the investigation of the Yb,-xSr, NiO,-, system. The behaviour of Ni"' in the oxide correlated to its atomic environment and to the linkage of the nickel polyhedra which governs the interactions between metal centres. Evidently, a knowledge of the crystal structures and the relationship between them is of interest. To date no crystal structure of these strontium nickel oxides has been published. This paper deals with a preliminary investigation of the crystal structure of Sr,Ni,O,, which allows us to question the real composition of this compound and the oxidation state of Ni. A single crystal of Sr,Ni401, was obtained following the method of Lee and H01land.~ The X-ray powder spectra of the crushed single crystals obtained is in accord with that of Sr,Ni,O,, (JCPDS No.42:521). The unit-cell parameters were refined to a =9.477( 1) A, c =7.826(4) A. A single crystal was mounted with the greatest dimension of the needle as the rotation axis. Preliminary rotation and Weissenberg photographs indicated 3ml or 31m Laiie symmetry. The intensity data were collected with a Philips PW1100 automated diffractometer (Mo-Ka radiation, ,I= 0.7107 A, 0.~26' scans). A total of 3774 measured reflections (0 range 2-30", -13<1.1<13, -13<k<13,0<1<10) yielded 512 independent reflections with I >341) used in the structure determination (merging R factor =0.055 on I).From extinc- tion conditions, the possible space groups are P3rn1, P31m, P31m, Phi, P321 and P312. Analysis of our single crystals by EDS [Sr, 56.6(5); Ni, 43.2(5); K, 0.2(2)%] gave an Sr :Ni ratio >5 :4 and ~ose -1 to 5.25 :4 or 4 :3. Then the weight loss for the reduction process (obs. loss, 11.8%) suggests the idealized formula Sr,Ni,O, (calc. loss, 11.9%); this formula was confirmed by a successful structural determination. The density measured from a few single crystals [p =5.4(1)g cm-,] is in fair agreemcnt with the theoretical density (5.49 g cmP3) assuming three formulae units in the unit cell. Then absorption corrections were applied using the analytical method of De Meulenaer and rompa5 (p=343 cm-l, transmission factor range 0.020-0.064 1.The structure was successfully solved in the space group P321. The positional parameters for the strontium and nickel atoms were determined from the SHELXS program, with the oxygen atoms being found from a difference Fourier map. Refinement was carried out by the full-matrix least-squares method. In a preliminary stage the positional parameters and isotropic factors were refined. Then a new difference Fourier synthesis showed maxima at the vertices of triangles centred on Ni(4) and Ni(5); these atoms were delocalized onto the centres [Ni(4) and Ni(5) sites in Table 11 and the apices [Ni(4)' and Ni(5)' sites in Table 11 of the triangles. Owing to the high correlations between occupancy and temperature Table 1 Atomic coordinates for Sr,Ni,O, atom site x Y Z B/A2 occupancy 0.0233( 3) 0.3276(4) 0.6918(3) 0 0.2476( 3) 112 0.63 (4) 0.97(5) 1 1 0.3603( 3) 113 0 213 0 0.1086(9) 0.77 (6) 0.93(9) 1 1 113 213 0.4217( 6) 0.58(9) 1 0 0 0.3383( 7) 0.36(8) 1 213 113 0.237( 1) 0.3(2) 0.58 0 0 0 1.5(3) 0.655 0.610(2) 0.924( 4) 0.819( 2) 0.1 58 (3) 0.172( 3) 0.671 (2) 0.846( 3) 0.273(2) 0 O.SOO( 2) 0.007 (2) 0.519(3) 0.177( 2) 0 0.241 (2) 0 0.038(2) 0.190( 3) 0.263(2) 0.445( 2) 112 0.6( 3) 1.4( 7) 0.3(2) 1.8(3) 1.0( 3) 1.8(3) 0.6(4) 0.14 0.115 1 1 1 1 1 J.MATER. CHEM., 1994, VOL. 4 The structure of Sr,Ni,09 consists of NiO, chains running along the three-fold axis and linked together by Sr-0 bonds \ a Fig.1 Projection of the Sr,Ni,O, structure along the [OOl] direction +-NiIr tNiJv Fig. 2 The NiO, chain in Sr,Ni,O, Tableo2 Nickel-oxygen distances in A (esds are between 0.01 and 0.02 A) Ni(l)O(l)(3x) 1.90 Ni(4) 0(1)(3x)2.17 Ni(4)’ 0(1)(1x) 2.06 0(3)(3x) 1.90 0(4)(3x) 2.22 O(1)(1 x) 2.08 O(1)( 1 x) 2.61 Ni(2)0(3)(3x) 1.92 Ni(5) 0(2)(6x) 2.09 0(4)( 1 x) 2.06 0(4)(3x) 1.83 0(4)( 1 x) 2.08 Ni(5)’ 0(2)(2x) 1.94 0(4)( 1 x) 2.60 Ni(3) 0(2)(3 x) 1.87 0(2)(2x) 1.97 0(5)(3x) 1.93 0(2)(2x) 2.64 factors, simultaneous refinement was difficult and occupancy rates were varied step by step to minimize reliability factors. Finally, in the last cycles, the refinement of anisotropic displacement parameters for strontium and non-delocalized nickel atoms led to R=0.045 and Ro=0.045 with o=l for all reflections.The better results are reported in Table 1. The principal interatomic distances are given in Table 2. The structure determination confirmed a chemical formula close to Sr4Ni,09, which corresponds to a mean oxidation state of nickel equal to 3.33. Iodometric titration, using the method described by James and Attfield,4 gave, assuming the formula Sr4Ni,0g, an experimental average oxidation state of 3.28. (Fig. 1). The Ni03 chains are built from alternating face- sharing Ni06 octahedra and NiOs trigonal prisms with the sequence two octahedra-one trigonal prism (Fig. 2). The Ni06 octahedra are quit? regular with Ni-0 distances rangiag from 1.83 to 1.93 A; the mean values [1.90, 1.88 and 1.90A for Ni(l), Ni(2) and Ni(3)] are in agreement with Ni4+ located in these sites.6 Within the trigonal prisms, the actual location of the nickel atoms is still doubtful.It is unusual for nickel atoms to occupy an oxygen trigonal prismatic site; moreover, the distances between the centre of the prism and the six vertices are too long, even for the Ni2+ cation. The nickel atoms are certainly displaced from the centre towards one of the rectangular faces of the trigonal prism, giving rise to a pseudo-planar coordi!ation with four Ni-0 distances being short [average: 2.07 A for Ni(4)’ and 1.96 A for Ni(5)’] and the remain@ two Ni-0 distances considerably longer C2.60 and 2.64 A for Ni(4)’ and Ni( 5)’].The electron density at the centre of the trigonal prism would result from the overlapping of thermal vibration ellipsoids elongated perpendicularly to the pseudo-square. The Ni- 0 distances and the chemical formula are in favour of Ni2+ in these sites. However, the combination of Ni4+ and square Ni2+ would be diamagnetic, yet the compound is paramag-neti~.~in view of the disorder in the structure and the large errors on the Ni-0 distances, the possibility that Ni3+ is present on the octahedral site cannot be totally excluded. It is interesting to compare this mixed-valency oxide to Sr,C~Pt0~,~9’which contains chains of alternating Pt4+06 octahedra and Cu2+04 ‘squares’. In this compound the dis- placement of the copper atom from the centre of the trigonal prism generates a slight deformation of the unit cell, which becomes monoclinic.For Sr4Ni,09, such a distortion and a twinning of the crystals would explain the difficulties of the location of the nickel atoms in the trigonal prism. In Sr4Ni,09 there are two kinds of NiO, chains with slightly different nickel-nickel distances: Ni (3)-Ni( 3)-Ni( 5) located at (0,O) and Ni(l)-Ni(2)-Ni(4) at (2/3,1/3) and (1/3,2/3) in the ab plane. The results of this preliminary crystal structure determi- nation which leads to a different chemical formula from that reported previously and indicates a mixed-valency oxide should allow the interpretation of the electrical and magnetic behaviour of this compound. It would be also interesting to study the substitution, for example, of Cu2+ for Ni2+. References Y. Takeda, T. Hashino, H. Miyamoto, F. Kanamuru, S. Kume and M. Koizumi, J. Inorg. Nucl. Chem., 1972,34, 1599. R. J. Marcizak and L. Katz, J. Solid State Chem., 1978,34, 295. J. Lee and G. F. Holland, J. Solid State Chem., 1991,93,267. M. James and J. P. Attfield, J. Muter. Chem., 1994.4, 575. J. De Meulenaer and H. Tompa, Acfa Crystallogr., 1965, 19, 1014. R. D. Shannon, Acta Crystallogr., Sect. A, 1976,32, 721. A. P. Wilkinson, A. K. Cheetham, W. Kunnman and A. Kvick, Eur. J. Solid State Inorg. Chem., 1991,28,453. J. L. Hodeau, H. Y. Tu, P. Bordet, T. Fournier, P. Strobel, M. Marezio and G. V. Chandrashekhar, Acta Crj.stallogr., Sect. B, 1992,48,1. Communication 4/04929B; Received 1 1th August, 1994

ISSN:0959-9428    

DOI:10.1039/JM9940401763

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( ll), 1765 CORRIGENDA Corrigendum to Self-consistent Interatomic Potentials for the Simulation of Binary and Ternary Oxides Timothy S. Bush,*" Julian D. Gale: C. Richard A. Catlow" and Peter D. Battle" a Davy-Faraday Laboratory, The Royal Institution of Great Britain, 21 Albemarie St., London, UK W1X 4BS b Department of Chemistry, Imperial College, South Kensington, London, UK SW7 2AY " Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, UK OX1 3QR J. Mater. Chern., 1994, 4, 831. The potential parameters for Yb3+-02-in the penultimate line of Table 1 were misprinted. The correct parameters are listed below. +M" O2-q (core) q (shell) q (core) q (shell) AleV pjA c/evk6 le Ie Ie Ie k,/eV A-' k/eV k' yb3 +-02 -991.029 0.3515 0.0 -0.278 3.278 0.513 -2.513 308.91 20 53 Corrigendum to Examination of the Structural Features necessary for Mesophase Formation with Aroylhydrazinato=nickel(ii)and -copper(ii) Complexes Mohammed N. Abser: Martin Bellwood," Christina M. Buckley," Michael C. Holmesb and Richard W. McCabe*" " Department of Chemistry, University of Central Lancashire, Preston, UK PR1 2HE Department of Physics and Astronomy, University of Central Lancashire, Preston, UK PRI 2HE J. Mater. Chern., 1994, 4, 1173. The caption to Plate 2 on p. 1177 should read as follows: Plate 2 Schlieren texture of the S, phase of 11 (R = OCI4H9),138 "C,on cooling from the isotropic phase (x 5 magnification)

ISSN:0959-9428    

DOI:10.1039/JM9940401765

出版商:RSC    

年代:1994

数据来源: RSC