摘要:

Preparation by microwave irradiation of nanometre-sized AlPO4-5 molecular sieve Hongbin Du, Min Fang,†Wenguo Xu, Xianping Meng and Wenqin Pang Department of Chemistry, Jilin University, Changchun 130023, P. R. China The influence of the synthesis conditions on the crystallization and crystal size of AlPO4-5 molecular sieve is investigated in a (TEA)2O–Al2O3–P2O5–H2O system. The initial mixture composition and the crystallization method affect the crystallization and the crystal size of the product.Microwave heating of the synthesis mixture results in the formation of AlPO4-5 with nanometresized particles. Aluminophosphate molecular sieves are important materials dropwise to the above solution, followed by addition of the remaining water. The mixture was stirred for 6 h, and statically that are commonly used as catalysts, catalyst supports and adsorbents. Among them is the well known AlPO4-5, which aged for 12 h under ambient conditions to form a nearly transparent homogeneous gel.For the conventional hydrother- was first discovered in 1982 by Flanigen and co-workers.1,2 A large number of papers on the synthesis of AlPO4-5 have now mal synthesis of AlPO4-5, the gel was transferred into a 20 ml PTFE-lined stainless steel autoclave and heated at 333 K in been published,1–5 because of both its zeolite properties and its potential applications as advanced materials.6–9 an oven for a specified time, typically 7 h.The product was recovered by centrifugation (at 15000 rpm for 5–15 min), In the utilization of zeolites as catalysts, catalyst supports and adsorbents, the crystal size affects the performance washed repeatedly with distilled water (centrifuged and redispersed in water) and dried at ambient temperature.For the (activity, selectivity, rates of adsorption) simply by altering the diffusion path-length through the crystallites.10,11 Previous preparation under agitation, the autoclave was rotated at ca. 45 rpm in the oven. In the case of the microwave heating studies have shown that the smallest crystals are most effective as catalysts as long as the catalytic reaction proceeds in the preparation of AlPO4-5, the gel was charged into a 20 ml PTFE autoclave. The crystallization was carried out in a internal void.12,13 Currently, there is increasing interest in ultrafine particles of molecular sieves based on their potential modified domestic microwave oven operating at 2450 MHz.The reaction mixture was heated quickly at a heating rate of catalytic applications and their possible use as precursors for thin-film formation.14 However, only a few kinds of zeolites ca. 2K s-1 from room temperature to the crystallization temperature of 323–333 K and then held at the final tempera- with nanometre-sized particles have been synthesized, i.e.sodalite, A, Y, ZSM-5 and L.15–18 To our knowledge, there is very ture for 7–25 min. The products were identified by means of XRD on a Rigaku little information on the preparation of aluminophosphate molecular sieves with ultrafine particles in the literature. D/MAX-IIIA diffractometer with Cu-Ka radiation.Scanning electron images (SEM) and transmission electron images Recently, microwave heating has been applied successfully to the preparation of zeolites such as A,19 Y20 and ZSM-5,20 (TEM) were taken on Hitachi X-650 and JEM-100CXII microscopes, respectively. as well as the recently reported large AlPO4-5 crystals.21 Compared to the conventional hydrothermal crystallization, microwave heating of zeolite synthesis mixtures can drastically Results and Discussion reduce the crystallization time, often accompanied by the Effect of the P2O5 content formation of small crystals.19,20 This prompted us to explore its use as a method for the synthesis of nanometre-sized crystals The effect of the P2O5 content on the crystallization and of AlPO4-5.crystal size of AlPO4-5 is summarized in Table 1.It can be The present paper focuses on the preparation of AlPO4-5 seen that the P2O5 content in the gel plays an important role with nanometre-sized particles. The influence of the synthesis in the crystallization of AlPO4-5 molecular sieve: an excess of conditions on the crystallization and crystal size of AlPO4-5 P2O5 usually results in the formation of an unknown phase, is discussed in a (TEA)2O–Al2O3–P2O5–H2O system.while with insufficient P2O5 in the gel aluminium hydroxide contaminates the AlPO4-5 crystals. Suitable P2O5/Al2O3 molar ratios for the formation of pure AlPO4-5 range from 1.0 to 1.2. Experimental It can also be seen that the P2O5/Al2O3 ratio influences AlPO4-5 molecular sieve was synthesized using orthophos- crystal size.As shown in Fig. 1, a high P2O5/Al2O3 molar ratio phoric acid (H3PO4, 85%), aluminium hydroxide [Al(OH)3, favours the formation of AlPO4-5 composed of large aggre- 99%], tetraethylammonium hydroxide (TEAOH, 25%) and gates. At P2O5/Al2O3 ratios from 1.0 to 1.1, uniform plate-like distilled water as reactants. The chemical composition of the crystallites are obtained.initial gel was 1.0Al2O35xP2O55y(TEA)2O5zH2O, where x, y and z are changed systematically with x=1.1, y=0.7 and z= Effect of the template content and pH value in the gel 50 as the basis to study the influence of the gel composition The influence of the template content on the synthesis of on the crystallization and crystal sizes. AlPO4-5 is shown in Fig. 2. It seems that tetraethylammonium A typical synthesis procedure is described as follows. An hydroxide as a template favours the formation of small crystal- appropriate amount of aluminium hydroxide was added to the lites of AlPO4-5, in contrast to triethylamine as the template, hot orthophosphoric acid which was diluted by ca. 1/3 of the which usually favours the formation of large AlPO4-5 crystals.5 total water.After stirring for ca. 1 h, TEAOH was added In the (TEA)2O/Al2O3 ratio range from 0.6 to 1.0, pure AlPO4-5 with small crystallites is obtained. Scanning electron † Current address: Department of Chemistry, University of New Brunswick, Fredericton, NB, Canada E3B 6E2. images show that the (TEA)2O content also affects the crystal J. Mater. Chem., 1997, 7(3), 551–555 551Fig. 2 SEM images of AlPO4-5 synthesized with a (TEA)2O/Al2O3 Fig. 1 SEM images of AlPO4-5 synthesized with a P2O5/Al2O3 ratio of (a) 1.2, (b) 1.1 and (c) 1.0 ratio of (a) 0.6, (b) 0.8 and (c) 1.0 Table 1 Influence of the P2O5 /Al2O3 ratio on the crystallization gel composition no. (TEA)2O Al2O3 P2O5 H2 O product crystal habit F1600 0.7 1.0 1.4 50 unidentified — F1601 0.7 1.0 1.2 50 AlPO4-5 irregular, sphere, 10–30 mm F1602 0.7 1.0 1.1 50 AlPO4-5 uniform, 0.3 mm F1603 0.7 1.0 1.0 50 AlPO4-5 uniform, 0.8 mm F1604 0.7 1.0 0.8 50 AlPO4-5+Al(OH)3 — morphology (Fig. 2). When the (TEA)2O/Al2O3 ratio decreases similar to those caused by the (TEA)2O content, i.e. a high from 0.6 to 1.0, the average crystal size of AlPO4-5 decreases (TEA)2O content results in an increase in the pH value in the and the crystal-size distribution becomes narrower.These facts gel, thus accelerating the nucleation rate and leading to the indicate that at the highest (TEA)2O content more nuclei that formation of small crystallites with a narrow crystal-size are responsible for nucleation and subsequent crystallization distribution. are formed, and the nucleation rate to crystal growth rate ratio increases.Similar observations have been reported by Finger et al.5 Effect of water content Since an increase in the (TEA)2O content enhances the Variation of the water content results in changes to both the alkalinity, the effect of pH is investigated in a separate experi- nucleation rate and the crystal growth rate, as indicated in ment by adding hydrochloric acid (pH 5.4 and 5.8).The Fig. 4. Dilution of the reaction gel decreases the nucleation standard gel has a pH value of 6.4 [(TEA)2O/Al2O3=0.7]. As rate. The products obtained from diluted gels contain large can be seen from Fig. 3, the growth of AlPO4-5 is rather spherical agglomerates and tiny needle-shaped crystallites. On sensitive to changes in the pH value. At the lowest pH in the the other hand, concentration of the gel enhances the gel, large aggregates form.As the gel pH is increased from 5.4 nucleation rate, resulting in the formation of uniform small to 5.8 to 6.4 smaller crystals result (see Fig. 3). This suggests AlPO4-5 crystals. However, a further decrease in the water increased nucleation, at the expense of growth, as the pH is increased within this limited region. These phenomena are content leads to the formation of crystallites with various sizes 552 J.Mater. Chem., 1997, 7(3), 551–555Fig. 4 SEM images of AlPO4-5 synthesized with an H2O/Al2O3 ratio Fig. 3 SEM images of AlPO4-5 synthesized with a pH value in the gel of (a) 40, (b) 50 and (c) 72 of (a) 5.4, (b) 5.8 and (c) 6.4 [Fig. 4(a)], probably due to the inhomogeneity of the condensed gel.Effect of crystallization conditions The relationship between the crystallization temperature and the crystal size of the products was investigated by fixing the crystallization time. Good crystalline products are obtained at 413–453 K, and the crystal size is not distinctly dependent on the temperature. The crystallization method has an influence on the crystal Fig. 5 TEM image of AlPO4-5 synthesized under stirring size of AlPO4-5. Results from Fig. 5 show that stirring of the gel during the crystallization period is an important factor in determining the crystal size of AlPO4-5. Stirring leads to the broader reaction mixture composition range. Moreover, the AlPO4-5 crystals thus obtained usually have smaller sizes.formation of smaller crystallites than those obtained under static conditions, probably because more nucleation centres From Table 2, one can see that the P2O5/Al2O3 ratio in the reaction mixture is crucial in determining the products and are created by agitation of the gel. Moreover, it is of interest to note that microwave heating the crystal size. AlPO4-5 is formed in the P2O5/Al2O3 ratio range from 1.1 to 1.8, outside this range either Al(OH)3 coexists of the aluminophosphate gel produces AlPO4-5 crystallites with much smaller crystals in comparison with those obtained with AlPO4-5 or an unknown phase instead of AlPO4-5 is crystallized.At P2O5/Al2O3=1.1, nanocrystals of AlPO4-5 can by the conventional heating (Fig. 6). Similar results have been observed in the syntheses of zeolites A, Y and ZSM-5,19,20 be obtained when (TEA)2O/Al2O3 is fixed at 0.7 [Fig. 6(a)]. With the increase in the P2O5/Al2O3 ratio, the yields of the which are attributed to simultaneous and abundant nucleation under microwave radiation. products decrease and large crystallites are easily obtained. It seems that a relatively low P2O5/Al2O3 ratio favours the Under microwave heating conditions, the influence of the synthesis conditions on the crystal size was investigated.The formation of AlPO4-5 with small crystals. Similar results have been reported by Girnus et al.21 synthesis conditions and the crystallization products are summarized in Tables 2 and 3. Compared with the conventional The (TEA)2O/Al2O3 ratio also plays an important role in the crystallization, as shown in Table 3.Alow (TEA)2O content hydrothermal synthesis (P/Al=1.0–1.2 in the gel in this work), preparation of AlPO4-5 by microwave heating is possible in a usually results in the formation of AlPO4–C, and with an J. Mater. Chem., 1997, 7(3), 551–555 553Fig. 7 XRD patterns of AlPO4-5 samples synthesized by using (a) microwave heating (sample W126) and (b) conventional hydrothermal methods (sample F1602) microscopy shows that the specimen consists of fine particles with sizes as small as ca. 50 nm [Fig. 6(a)]. Fig. 6 TEM images of AlPO4-5 synthesized by microwave heating: (a) sample W126, (b) sample W131 and (c) sample W133 Conclusions Aluminophosphate AlPO4-5 crystals with small sizes were excess of (TEA)2O in the gel amorphous phases are obtained.Pure AlPO4-5 is crystallized in the (TEA)2O/Al2O3 ratio range synthesized using TEAOH as a template. Various synthesis parameters such as P2O5/Al2O3 , (TEA)2O/Al2O3, H2O/Al2O3 from 0.7 to 1.1. The synthesized AlPO4-5 samples usually consist of nanometre-sized crystals or loose agglomerates, as ratios and the crystallization method influence the crystallization and the crystal size of AlPO4-5.The synthesis of shown in Fig. 6. The XRD pattern of sample W126 (see also Tables 2 and 3) AlPO4-5 with uniform small crystals requires appropriate reaction mixture compositions. By microwave heating, is shown in Fig. 7(a). The peak positions are similar to those for AlPO4-5 prepared by conventional hydrothermal synthesis AlPO4-5 is synthesized successfully, and the product usually consists of smaller particles than those synthesized by the [Fig. 7(b)], but the intensities are different. The X-ray diffraction line-broadening of this sample is most probably due to conventional hydrothermal method. At (TEA)2O/Al2O3= 0.7–1.1 and P2O5/Al2O3=1.1, AlPO4-5 with nanometre-sized the size effect of the small particles. Similar observation has been reported in the case of zeolite L.18 Transmission electron is crystallized. Table 2 Influence of the P2O5/Al2O3 ratio on the crystallization of AlPO4-5 under microwave heating gel composition no.(TEA)2O Al2O3 P2O5 H2O P2O5/Al2O3 product av. size/nm W142 0.7 1.4 1.1 50 0.8 AlPO4-5+Al(OH)3 — W126 0.7 1.0 1.1 50 1.1 AlPO4-5 50 W138 0.7 0.8 1.1 50 1.4 AlPO4-5 200 W137 0.7 0.6 1.1 50 1.8 AlPO4-5 300 W136 0.7 0.5 1.1 50 2.2 unidentified — Table 3 Influence of the (TEA)2O/Al2O3 ratio on the crystallization of AlPO4-5 under microwave heating gel composition no.(TEA)2O Al2O3 P2O5 H2 O (TEA)2O/Al2O3 product av. size/nm W125 0.5 1.0 1.1 50 0.5 AlPO4-C — W126 0.7 1.0 1.1 50 0.7 AlPO4-5 50 W131 0.9 1.0 1.1 50 0.9 AlPO4-5 <50 W133 1.1 1.0 1.1 50 1.1 AlPO4-5 60 W142 1.3 1.0 1.1 50 1.3 ama — aAm=amorphous. 554 J. Mater. Chem., 1997, 7(3), 551–55511 K. Rajagopalan, A. W. Peters and G. C. Edwards, Appl. Catal., We wish to thank Prof. Jiesheng Chen for helpful discussions. 1986, 23, 69. We also acknowledge support of this work by the National 12 V. P. Shiralkar, P. M. Joshi, M. J. Eapen and B. S. Rao, Zeolites, Natural Science Foundation of China. 1991, 11, 511. 13 P. V. Verduijn, J. Mechilium, C. B. de Gruijter, W. T. Koetsier and C. W. M. van Oorschot, US Pat., 5 064 630, 1991. 14 M. Tsapatsis, M. Lovallo, T. Okubo, M. E. Davis and M. Sadakata, Chem.Mater., 1995, 7, 1734. References 15 B. J. Schoeman, J. Sterte and J. E. Otterstedt, Zeolites, 1994, 14, 208. 1 S. T. Wilson, B. M. Lok, C. A. Messina, T. R. Cannan and 16 B.J. Schoeman, J. Sterte and J. E. Otterstedt, Zeolites, 1994, 14, E. M. Flanigen, J. Am. Chem. Soc., 1992, 104, 1146. 110. 2 S. T. Wilson, B. M. Lok and E. M. Flanigen, US Pat., 4 310 17 A. E. Persson, B. J. Schoeman, J. Sterte and J. E. Otterstedt, 440, 1982. Zeolites, 1994, 14, 557. 3 U.Mu� ller and K. K. Unger, Z. Kristallogr., 1988, 182, 190. 18 X. Meng, Y. Zhang, C.Meng and W. Pang, in Proceedings of the 4 S. Qiu, W. Pang, H. Kessler and J.-L. Guth, Zeolites, 1989, 9, 440. 9th International Zeolite Conference, ed. R. von Ballmoos, 5 G. Finger, J. Richter-Mendau, M. Bu�low and J. Kornatowski, J. B. Higgens and M. M. J. Treacy, Butterworth-Heinemann, Zeolites, 1991, 11, 443. London, 1993, p. 297. 6 M. Ehrl, F. W. Deeg, C. Bra�uchle, O. Franke, A. Sobbi, G. Schulz- 19 J. C. Jansen, A. Arafat, A. K. Barakat and H. van Bekkum, in Ekloff and D.Wo� hrle, J. Phys. Chem., 1994, 98, 47. Synthesis of Microporous Materials, ed. M. L. Occelli and 7 J. Caro, G. Finger, J. Kornatowski, J. Richter-Mendau, L. Werner H. E. Robson, Van Nostrand Reinbold, New York, 1992, vol. 1, and B. Zibrowius, Adv. Mater., 1992, 4, 273. p. 507. 8 J. Caro, F. Marlow and M.Wu� bbenhorst, Adv.Mater., 1994, 6,413. 20 A. Arafat, J. C. Jansen, A. R. Ebaid and H. van Bekkum, Zeolites, 9 S. D. Cox, T. E. Gier, G. D. Stucky and J. Bierlein, J. Am. Chem. 1993, 13, 162. Soc., 1990, 110, 609. 21 I. Girnus, K. Jancke, R. Vetter, J. Richter-Mendau and J. Caro, Zeolites, 1995, 15, 33. 10 F. Fajula, in Guidelines for Mastering the Properties of Molecular Sieve, ed. D. Barthormeuf, E. G. Derouane and W. Ho�lderich, NATO Plenum Press, New York, 1989, vol. 221, p. 53. Paper 6/06132J; Received 5th September, 1996 J. Mater. Chem., 1997, 7(3), 551–555 555

ISSN:0959-9428    

DOI:10.1039/a606132j

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Hydrothermal treatment and strontium ion sorption properties of fibrous cerium(IV ) hydrogenphosphate Hiromichi Hayashi,*a Kazuo Toriia and Shin-ichi Nakatab aT ohoku National Industrial Research Institute, Nigatake 4-2-1,Miyagino-ku, Sendai 983, Japan bR&D Center, Chiyoda Corp., 13 Moriya-cho, Kanagawa-ku, Yokohama 221, Japan Fibrous cerium hydrogenphosphate, CeP, has been treated hydrothermally in 1 mol dm-3 phosphoric acid solution.CEP and its hydrothermally treated product, CeP(HT), have been characterized by X-ray powder diffractometry, scanning electron microscopy, solid-state 31P MAS NMR spectroscopy, FTIR spectroscopy, chemical and thermal analyses. A poorly crystalline fibrous CeP with a d-spacing of 1.1 nm converted into a highly crystalline CeP(HT) with platelet morphology by hydrothermal treatment.Solid-state 31P MAS NMR and FTIR measurements confirmed that one kind of phosphate (H2PO4) is present in CeP and two kinds of phosphate (HPO4, PO4) are present in CeP(HT), in which the integrated intensity ratio of HPO4 to PO4 is 2:1. From chemical and thermal analyses, structural formulae for CeP and CeP(HT) are assumed to be CeO(H2PO4)2 2H2O and Ce(HPO4)(PO4 )0.5(OH)0.5, respectively.The Na+ exchange capacity of CeP amounted to 4.5 mmol g-1 at pH 11 while that of CeP(HT) was less than 1.0 mmol g-1 in the pH range 2–12. The pH dependence of the metal ion distribution coefficients exhibited ideal ion-exchange behaviour on CeP while metal ion distribution coefficients on CeP(HT) scarcely depended on pH.The metal ion selectivities of CeP and CeP(HT) increased in the order: Na+<Ca2+<Sr2+<K+, and Na+<K+<Ca2+<Sr2+, respectively. The distribution coefficient for the Sr2+ ion of CeP(HT) was higher than that of CeP under hydrothermal conditions. The advent of nuclear technology initiated a search for ion- strate the effectiveness of both exchangers for Sr2+ ion uptake under hydrothermal conditions. exchange materials that would be more stable to high temperature and radiation fields than organics.Hence, inorganic ion exchangers are bright prospects for applications in nuclear Experimental technology. In many cases, however, synthetic inorganic ion exchangers are amorphous powders or fine crystals, so that Materials they must be granulated with some kind of binder for practical Cerium(IV) sulfate and phosphoric acid [as 85% m/v (ca.use. Among the various synthetic inorganic ion exchangers, 17.2 mol dm-3) solution] were GPR grade reagents (Nacalai cerium(IV) hydrogenphosphate was the first prepared fibrous tesque). ion exchanger, and it can be employed to prepare inorganic ion-exchange papers or thin layers without a binder.1–3 Preparation of cerium hydrogenphosphate However, cerium(IV) phosphates constitute a complicated system of compounds.3–7 The composition is strongly depen- Cerium(IV) hydrogenphosphate (CeP) was prepared as dent on the preparation conditions, such as temperature reported by Alberti et al.3 Cerium(IV) sulfate solution (500 cm3; and PO4/Ce molar ratio in solution.Alberti et al.obtained 0.05 mol dm-3) containing 0.5 mol dm-3 sulfuric acid was four different products: (1) amorphous cerium phosphate added dropwise to an aqueous phosphoric acid solution with PO4/Ce=1.7, (2) a microcrystalline cerium phosphate (500 cm3; 2.4 mol dm-3) with stirring at 90°C. After 16 h, the with PO4/Ce=1.15, (3) another microcrystalline product with yellow solid was filtered off, washed with doubly distilled water PO4/Ce=1.55 and (4) a fibrous crystalline solid of idealized and dried at 60°C.formula Ce(HPO4)2 H2O.3 The structure of fibrous cerium hydrogenphosphate has not yet been elucidated owing to its Hydrothermal treatment of CeP low degree of crystallinity. The resulting cerium(IV) hydrogenphosphate (1 g) and 1 mol In the nuclear reprocessing cycle, the safe disposal of 90Sr dm-3 phosphoric acid solution (200 cm3; 1 mol dm-3) were activity has always been considered a problem, owing to its transferred into a PTFE-lined pressure bottle and subjected to long half-life, high thermal neutron fission product and hazard- hydrothermal treatment at a constant temperature ranging ous reactions.8–10 Fibrous cerium hydrogenphosphate would from 100 to 175°C for 24, 72 and 168 h.The product was be applicable to the disposal of 90Sr in radioactive wastes filtered off, washed with doubly distilled water, followed by owing to its strong affinity for the Sr2+ ion.2 Despite hydrother- air-drying at 60°C. mal stability being required for a radioactive Sr2+ ion exchanger, few studies have been carried out on exchange Physical measurements properties under hydrothermal conditions.11 In this regard, hydrothermal stability is anticipated to be endowed by hydro- X-Ray powder diffraction (XRD) patterns were recorded on a thermal pretreatment of the exchanger.In the present study, Rigaku Roterflex RU-300 RAD diffractometer using Cu-Ka fibrous cerium hydrogenphosphate was treated hydrothermally radiation (35 kV and 50 mA) and a scan speed of 4° min-1 in in phosphoric acid solution with variation of temperature and 2h.The divergent and scattering slits were set at 0.5°, and the time. The objectives of this study are (1) to characterize the receiving slit was 0.3°. Scanning electron microscopy (SEM) fibrous cerium hydrogenphosphate and its hydrothermally studies were carried out with a Hitachi S-800 electron microtreated product using 31P MAS NMR spectroscopy as well as scope, operating at 15 kV.The 31P MAS NMR spectra were XRD, SEM, FTIR, chemical and thermal analyses to obtain obtained at 109.38 MHz on a Fourier-transform pulsed NMR spectrometer (JEOL JNM-GX270). All 31P NMR spectra structural information for both materials, and (2) to demon- J.Mater. Chem., 1997, 7(3), 557–562 557combined with magic angle spinning (MAS) at 4.0 kHz were pattern of CeP indicated a poorly crystalline phase with only a few lines, the smallest (1.1 nm) being much more intense measured with high-power proton decoupling during data acquisition. IR spectra of samples in KBr matrices were than the others. The layered structure of CeP was confirmed by the peak shift upon intercalation of octylamine.The inter- measured on a Perkin-Elmer Spectrum 1000 FT IR spectrometer in the range 4000–450 cm-1. layer spacing of the octylamine-intercalated CeP increased to 2.8 nm, indicating that octylamine bilayers are formed between Analysis the CeP layers. It can be seen clearly from Fig. 1(b) that CeP(HT) is highly crystalline in contrast to CeP.Characteristic Inductively coupled plasma atomic emission spectrometry peaks corresponding to higher orders of the spacing of 1.05 nm (KP–AES; Seiko SPS 1200A) was performed for cerium and were observed. The diffraction pattern of CeP(HT) matched phosphorus analyses of the samples digested with concentrated almost perfectly with that of microcrystalline cerium phosphate hydrochloric acid.Thermogravimetry experiments were carried with PO4 /Ce=1.55, which was reported previously by Alberti out on a Rigaku TG-8101D thermogravimetry–differential et al.3 This product was obtained by refluxing any type of thermal analyser (TG–DTA) referenced against recalcined tetravalent cerium phosphate in concentrated H3PO4 solution. alumina. The temperature was ramped at a rate of 10°C min-1 The crystallite size of CeP(HT) is shown in Table 1 together in air.with that of CeP. The size of CeP(HT) was larger than that of CeP. In spite of the crystallinity of the product, the structures Ion-exchange equilibria of CeP(HT) and CeP are unknown. In the case of CeP(HT), evidence for the absence of layering is based on the unchanged A 0.1 g sample of the exchanger was immersed in 10 cm3 of spacing upon intercalation of octylamine.various solutions prepared by the desired combination of 0.1 mol dm-3 NaCl and 0.1 mol dm-3 NaOH for 168 h at 25°C Scanning electron microscopy with intermittent shaking. The determination of the Na+ ion concentration was carried out by the atomic absorption Scanning electron microscopic (SEM) observations were per- method.Distribution coefficients (Kd) were determined for formed to examine the texture change of the fibrous CeP after metal ions (Na+, K+, Ca2+) at 25°C and for Sr2+ ions every its hydrothermal treatment in 1 mol dm-3 phosphoric acid 25°C in the temperature range 25–150 °C. The initial metal solution at 175 °C for 72 h. As shown in Fig. 2, the photographs ion concentration was 1×10-4 mol dm-3 and the pH was revealed that the morphology was changed markedly after the adjusted with 0.1 mol dm-3 hydrochloric acid.A 0.05 g sample hydrothermal treatment. The original CeP was observed as of the exchanger was immersed in 10cm3 of a solution with a the fibrils of diameter 20–50 nm. In contrast, CeP(HT) exhib- given pH in a sealable PTFE tube for 168 h at a constant ited polycrystalline aggregates of 2–5 mm thin plates with a temperature with intermittent shaking.The pH of the super- distinctly crystalline solid, as expected from the XRD pattern. natant solution was measured by a TOA HM-60S pH meter equipped with a GS-5015 pH combination electrode. The 31P MAS NMR spectroscopy concentration of metal ions was determined by a Hitachi Z- 31P MAS NMR spectroscopy is a powerful technique for 6000 polarized Zeeman atomic absorption spectrophotometer studying the environment of metal phosphates since the chemi- (Na+, K+) or a Seiko SPS 1200A ICP emission spectrometer cal shift of the phosphate group is remarkably sensitive to its (Ca2+, Sr2+).local environment. The 31P MAS NMR spectra for CeP and The distribution coefficient, Kd, was calculated from eqn.CeP(HT) are shown in Fig. 3. In the spectrum of CeP, there (1) is a single resonance at d -7.2 indicating the presence of only Kd=(C1 /g)/(C2/cm3) (1) one kind of phosphorus environment in the structure of this where C1 is the amount of sorbed ions per 1 g of the solid, compound. In contrast, the 31P MAS NMR spectrum of and C2 is the concentration of ions per 1 cm3 of aqueous CeP(HT) showed three different resonances at d -16.2,-27.8 solution.and -29.7 with an intensity ratio close to 15151. Many layered tetravalent metal phosphates have structures similar to those of zirconium monohydrogenphosphate, Results and Discussion Zr(HPO4 )2 ; a monohydrate, a, and dihydrate, c are known. Formation and composition The a-phase has a layer structure consisting of layers of metal atoms in a plane bridged by phosphate groups above and The conditions of hydrothermal treatment and the results below the plane of metal atoms, with the PMOH groups of the analysis of the products are summarized in Table 1.pointing between the layers.12 The 31P MAS NMR spectra of Fibrous cerium hydrogenphosphate, with the composition layered zirconium phosphates show only one resonance at d CeH2(PO4)2 3H2O, was used as the starting material.Reaction -19 for the a-phase and two resonances of equal integrated product 1, which was obtained by hydrothermal treatment at intensity at d -9 and -27 for the c-phase. The resonance at 100 °C for 24 h, had still a P5Ce atomic ratio of 251 whereas d -19 was attributed to the monohydrogenphosphate, the X-ray diffraction pattern was changed from that of the (ZrO)3POH.The resonance at d -27 was assigned to phos- original fibrous cerium hydrogenphosphate. It may be that the phorus atoms of PO4 tetrahedra.13 Clayden suggested that reaction does not reach the equilibrium state at 100°C in 24 h. substitution of zirconium by hydrogen in the local phosphate Irrespective of the hydrothermal conditions, products 2–12 group coordinate decreases the bond strength of the oxygens were equilibrium solid phases with a P5Ce atomic ratio of of the phosphate groups, causing a downfield shift in the 31P exactly 1.551.The expected composition was given as chemical shift of ca. 10 ppm. Thus the signal for c-zirconium CeH0.5(PO4)1.5 0.5H2O.Despite the hydrothermal treatment phosphates, d -9, was assigned as corresponding to being performed in 1 mol dm-3 phosphoric acid solution, a P(OZr)2(OH)2. Accordingly, c-zirconium phosphate can now quarter of the phosphate groups were released from the CeP be formulated more correctly as Zr(PO4) (H2PO4 ) 2H2O. A during the hydrothermal treatment.layer structure has been proposed for the c-phase based on the increased spacing of the d002 reflection in the X-ray X-Ray powder diffractometry diffraction pattern upon ion exchange; the layers are made up of two ideal planes containing the metal atoms bridged by The X-ray powder diffraction patterns of CeP and CeP(HT) were measured by using Ni-filtered Cu-Ka radiation. Fig. 1 PO4 groups in which the dihydrogenphosphate moieties point towards the interlayer region.14 shows the XRD patterns of CeP and CeP(HT). The XRD 558 J. Mater. Chem., 1997, 7(3), 557–562Table 1 Hydrothermal treatment and analysis of cerium hydrogenphosphate treatment conditions X-ray analysis no. T /°C t/h CeO2 (%) P2O5 (%) H2O (%) product P/Ce (atomic ratio) d/nm D/nma 1 100 24 39.8 34.5 25.7 2.00 1.11 66 2 100 72 54.8 33.9 11.3 1.50 1.05 204 3 100 168 53.7 33.0 13.3 1.49 1.06 204 4 125 24 54.2 33.5 12.3 1.50 1.06 204 5 125 72 53.5 33.7 12.8 1.53 1.07 295 6 125 168 53.7 33.2 13.1 1.50 1.05 231 7 150 24 53.6 33.2 13.2 1.50 1.05 241 8 150 72 53.0 33.3 13.7 1.52 1.05 241 9 150 168 53.3 33.6 13.1 1.52 1.06 253 10 175 24 53.0 33.1 13.9 1.51 1.05 253 11 175 72 53.7 33.2 13.1 1.50 1.06 312 12 175 168 57.8 35.2 7.0 1.48 1.04 200 starting material 43.2 36.2 20.6 2.01 1.13 10 aCrystallite size calculated using the Scherrer equation.Fig. 1 X-Ray powder diffraction patterns for the fibrous cerium hydrogenphosphate (a) and its hydrothermally treated product (b) Fig. 2 Scanning electron micrographs illustrating the fibrous texture Fig. 3 Solid-state 31P MAS NMR spectra for the fibrous cerium of the original cerium hydrogenphosphate (a) and the platelet texture hydrogen phosphate (a) and its hydrothermally treated product (b).of its hydrothermally treated product (b) Peaks marked with an asterisk are spinning sidebands. In the present spectra, only one resonance is seen at d -7.2 for CeP, indicating that one kind of phosphorus environment gave three resonances at d -16.2, -27.8 and -29.7.The first resonance can be assigned to P(OCe)3(OH). The resonances is present, as in a-zirconium phosphate, whereas the chemical shift is close to d -9 as observed in c-zirconium phosphates at d -27.8 and -29.7 are very similar to the 31P resonances in HZr2(PO4)3 where the 31P chemical shifts are d -28.4 and rather than d -19 in a-zirconium phosphate.Thus the resonance at d -7.2 can be assigned as corresponding to -29.4.15 The resonance at d -27.8 can be ascribed to phosphorus atoms of PO4 tetrahedra.16 The other resonance at d P(OCe)2(OH)2 . The 31P MAS NMR spectrum of CeP(HT) J. Mater. Chem., 1997, 7(3), 557–562 559-29.7 corresponds to POH terminal groups as well as the CeP(HT) were postulated as CeO(H2PO4)2 2H2O and Ce(HPO4)(PO4)0.5(OH)0.5.CeO(H2PO4)2 2H2O requires: resonance at d -16.17 This is supported by the fact that two kinds of resonance at d -27 and -29 are observed in heat- CeO2, 44.5; P2O5, 36.9; H2O, 18.6% and Ce(HPO4)- (PO4 )0.5(OH)0.5 requires: CeO2 , 54.1; P2O5, 33.5; H2O, 12.4%. treated c-zirconium phosphate where the resonance at d -29 was enhanced as a function of the cross-polarization time The agreement between the calculated and experimental values was good.between the 1H and 31P spin systems corresponding to the protonic characteristics of POH group.18 The ratio of inte- Alberti et al. pointed out the possible formula for CeP; CeO(H2PO4)2 from thermal analyses.3 The structural formula grated intensities at d -16.2, -27.8 and -29.7 was 15151.Therefore, the structural formula for CeP(HT) can be postu- for CeP(HT) was compatible with Ce(HPO4)1.1(PO4)0.45- (OH)0.45 0.33H2O that was proposed by Herman and lated to be Ce(HPO4)(PO4)0.5(OH)0.5. Clearfield.7 IR spectra Na exchange capacity The 31P MASNMRresults were supported by IR spectroscopy. The IR spectra of the original CeP and CeP(HT) are shown Fig. 5 shows the Na+ uptake of CeP and CeP(HT), respectively. In the case of CeP, the uptake increased linearly with in Fig. 4. The spectrum of CeP shows few bands in the region of 4000–450 cm-1; the broad bands with absorption maxima pH indicating a monofunctional exchange site for the phosphate groups. The titration curve of CeP does not have distinct at 3402 and 1632 cm-1 were assigned to the OMH stretching and bending vibrations of interlayer water.The bands at plateaux, as in the case of layered materials, but a sloping curve is usually obtained as in the amorphous inorganic ion 1150–1000 cm-1 represent PO3 vibrations. The original fibrous CeP showed a broad absorption band at 1061 cm-1 due to exchangers. From the Na+ uptake curve of fibrous CeP an experimental ion-exchange capacity of 4.6 mequiv.g-1 is n(PMO).18 In contrast, in the spectrum of CeP(HT), the band in the region of 1200–900 cm-1 split to give bands at 1233, obtained at pH 11. The CeP had an exchange capacity close to half of the expected value (10.4 mequiv. g-1) based upon 1094, 1035 and 944 cm-1, which revealed that several kinds of phosphate groups exist.The band at 1233 cm-1 indicates the the assumption that protons of the dihydrogenphosphate groups are exchangeable. Only one hydrogen of each dihydro- presence of monohydrogenphosphate groups.7 The band at 944 cm-1 can be attributed to the bending mode of PMOMP. genphosphate group would be exchanged at pH 11. The second dissociation of dihydrogenphosphate should be difficult owing The bands at 3534 and 1611 cm-1 were attributed to the OMH stretching and bending modes of structural water.to the electrostatic attraction by the negative charges of the phosphate groups. Thus, only half of the hydrogen ions were exchanged with Na+ ions in the pH range 1–11 on the pH Structural formulae for CeP and CeP(HT) titration curve. From the chemical and thermal analyses as well as 31P MAS The titration curve for CeP(HT) shows that this material NMR and FTIR results, the structural formulae for CeP and does not exhibit appreciable exchange in the pH range 1–12.In the wide pH range Na+ uptakes are less than 1 mequiv. g-1, which is less than theoretical exchange capacity of CeP(HT) (1.7 mequiv. g-1). As expected from XRD results, CeP(HT) does not possess a layered structure; it seems to have a smaller pore size and thus exhibits a screening effect for ions that diffuse without difficulty in the CeP.Distribution coefficient In order to examine the selectivities of CeP and CeP(HT) for metal ions, distribution coefficients, Kd, were measured as a function of pH. Measurements were carried out with some Fig. 5 Na+ uptakes of (a) the fibrous cerium hydrogenphosphate and (b) its hydrothermally treated product as a function of pH.Exchanger Fig. 4 FTIR spectra for the fibrous cerium hydrogenphosphate (a) 0.1 g; initial concentration of Na+ ions 0.1 mol dm-3; vol. 10 cm3; T 25 °C. and its hydrothermally treated product (b) 560 J. Mater. Chem., 1997, 7(3), 557–562Fig. 7 Distribution coefficients for Sr2+ ion of the fibrous cerium Fig. 6 Distribution coefficients for metal ions (Na+, K+, Ca2+, Sr2+) hydrogenphosphate (a) and its hydrothermally treated product (b) as of the fibrous cerium hydrogenphosphate (a) and its hydrothermally a function of pH. Exchanger 0.05 g; initial concentration of metal ions treated product (b) as a function of pH. Exchanger 0.05 g; initial 1.0×10-4 mol dm-3; vol. 10 cm3; T : #, 25; $, 50; ', 75; +, 100; %, concentration of metal ions 1.0×10-4 mol dm-3; vol. 10 cm3; T 25 °C. 125; &, 150°C. alkali-metal (Na+ and K+) and some alkaline-earth-metal (Ca2+ and Sr2+) ions. Fig. 6(a) gives the plots of log Kd vs. pH for Na+, K+, Ca2+, Sr2+ ions on CeP. The results show groups provides suitable sites for divalent metal ions. In the that the selectivity exhibited by CeP increases in the sequence case of CeP(HT), Kd values were enhanced markedly at 100 °C Na+<Ca2+<Sr2+<K+.The slopes of the plots for Na+ and or higher, whereas the Kd values were lower than those of the K+ ions are approximately unity and those for Ca2+ and Sr2+ CeP below 100 °C. Dehydration of Sr2+ ions and acceleration are two, indicating that ideality for ion-exchange reaction of the dissociation of hydrogenphosphate groups seem to be between metal ions and hydrogen ions can be expected to hold the reasons for the enhanced Sr2+ sorption activity on for CeP.CeP(HT) under hydrothermal conditions. Powder X-ray Plots of Kd vs. pH for CeP(HT) are shown in Fig. 6(b). diffraction patterns of the products after reacting in Sr2+ Strangely, Kd values scarcely depend on the pH.The reason solution at 150 °C were determined. The XRD pattern of for this exchange behaviour is not clear, but may be attributed CeP(HT) was unchanged after the reaction suggesting that it to the fact that only surface sites are available for the exchange is hydrothermally stable, while fibrous CeP was collapsed at reaction due to the screening effect mentioned above.It was 100°C or higher. observed that the Kd value increases in the sequence Na+<K+<Ca2+<Sr2+. CeP(HT) exhibits an unexpected selectivity for Sr2+ whereas the Kd values for each of the metal Conclusions ions were lower than those of CeP. Hydrothermal treatment of fibrous cerium hydrogenphosphate leads to the condensation of phosphate groups; subsequently Temperature dependence of Sr2+ uptake the layers are firmly cross-linked to give a non-expandable structure.The condensation of phosphate groups affects the The temperature dependence of the distribution coefficients for Sr2+ ions on CeP and CeP(HT) as a function of pH is selectivity for metal ions as well as the ion-exchange capacity. Under hydrothermal conditions, Sr2+ sorption behaviour on shown in Fig. 7. In the case of CeP, plots of Kd vs. pH shifted to lower pH as the temperature was increased to 100°C, the hydrothermally treated product is more useful than that of the original cerium hydrogenphosphate owing to the hydro- suggesting that ion exchange between Sr2+ ions and H+ ions is an endothermic reaction.19 Above 100 °C, although the Kd thermal stability in addition to the selectivity for Sr2+ ion.Since Sr2+ uptake can be achieved by cerium hydrogenphos- value decreased slightly at 125°C, it increased subsequently at 150 °C to retain the sorption ability under hydrothermal phate and its hydrothermal product under hydrothermal conditions, both materials are of great promise as an adsorbent conditions. The dissociation constant increased with the temperature owing to the decreasing dissociation constant of for 90Sr.Further studies are being conducted to elucidate the Sr2+ sorption mechanism under hydrothermal conditions. water.20 The second dissociation of dihydrogenphosphate J. Mater. Chem., 1997, 7(3), 557–562 56111 S. Ahrland and G. Carleson, J. Inorg. Nucl. Chem., 1977, 33, 2229.References 12 A. Clearfield and G. D. Smith, Inorg. Chem., 1969, 8, 431. 1 G. A. Alberti, M. A. Massucci and E. Torracca, J. Chromatogr., 13 N. J. Clayden, J. Chem. Soc., Dalton T rans., 1987, 1877. 1967, 30, 579. 14 A. N. Christensen, E. K. Andersen, I. G. K. Andersen, G. Alberti, 2 G. A. Alberti, M. Casciola, U. Costantino and M. L. Luciani, M. Nielsen and M. S. Lehmann, Acta Chem. Scand., 1990, 44, 865. J. Chromatogr., 1976, 128, 289. 15 N. J. Clayden, Solid State Ionics, 1987, 24, 117. 3 G. A. Alberti, U. Costantino, F. Di Gregorio, P. Galli and 16 K. Nakashiro, Y. Ono, S. Nakata and Y. Morimura, Zeolites, E. Torracca, J. Inorg. Nucl. Chem., 1968, 30, 295. 1993, 561. 4 G. A. Alberti, U. Costantino and L. Zsinka, J. Inorg. Nucl. Chem., 17 L. Maistriau, Z. Gabelica, E. G. Derouane, E. T. C. Vogt and J. van 1972, 34, 3549. Oene, Zeolites, 1991, 583. 5 K. H. Konig and E. Meyn, J. Inorg. Nucl. Chem., 1967, 29, 1153. 18 K. Segawa, S. Nakata and S. Asaoka, Mater. Chem. Phys., 1987, 6 E.M. Larsen and W. A. Cilley, J. Inorg. Nucl. Chem., 1968, 30, 287. 17, 181. 7 R. G. Herman and A. Clearfield, J. Inorg. Nucl. Chem., 1975, 37, 19 H. Hayashi, T. Iwasaki, T. Nagase, Y. Onodera and K. Torii, 1697. Solvent Extr. Ion Exch., 1995, 13, 1145. 8 T. Sasaki, Y. Komatsu and Y. Fujiki, Chem. L ett., 1981, 957. 20 F. H. Sweeton, R. E. Mesmer and C. F. Baes, Jr, J. Solution Chem., 9 N. Yamasaki, K. Yanagisawa, S. Kanahara, M. Nishioka, 1974, 3, 191. K. Matsuoka and J. Yamazaki, J. Nucl. Sci. T echnol., 1984, 21, 71. 10 D. K. Bhattacharyya and N. C. Dutta, J. Mater. Sci., 1995, 30, Paper 6/06397G; Received 17th September, 1996 2248. 562 J. Mater. Chem., 1997, 7(3), 557–562

ISSN:0959-9428    

DOI:10.1039/a606397g

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Atomistic simulation of the surface structure of the TiO2 polymorphs rutile and anatase Peter M. Oliver,*† Graeme W. Watson, E. Toby Kelsey and Stephen C. Parker Computational Solid State Chemistry Group, School of Chemistry, University of Bath, Claverton Down, Bath, UK BA2 7AY Atomistic simulation has been used to calculate the surface structures and stability of the rutile and anatase polymorphs of TiO2.The surface and attachment energies were used to evaluate the equilibrium and pseudo-kinetic morphologies. The surfaces expressed in rutile were {011}, {110}, {100} and {221} with surface energies of 1.85, 1.78, 2.08 and 2.02 J m-2 respectively. For anatase the {011} and {001} surfaces were dominant in the morphology with relaxed surface energies of 1.40 and 1.28 J m-2. The predicted equilibrium forms were largely in good agreement with the reported experimental morphologies of rutile and anatase and showed the importance of surface relaxation.Rutile is the most stable form of TiO2 at ambient conditions anatase were obtained using the computer code METADISE17 developed to model dislocations, interfaces and surfaces. with anatase metastable with respect to rutile.The surface properties of rutile have been the subject of numerous studies The energy of the crystal is described via interatomic potentials. The potential is comprised of parametrised analytical since the early 1970s when the decomposition of H2O into hydrogen and oxygen in a photo-electrolysis cell was first expressions describing the interactions between atoms.The parameters used were developed by Matsui and Akoagi18 reported.1 This is particularly true of the TiO2 {100} surface which has been studied experimentally2–5 and theoretically.6–10 (Table 1) and include electrostatic terms with short-range (9.6 A° ) Buckingham potentials of the form shown in eqn. (1) Chung et al.2 reported that argon-ion bombardment and annealing of the {100} surface at ca. 600, 800 and 1200°C gave rise to three distinct structures, i.e.(1×3), (1×5) and (1×7) Urij =.¾ ij qiqj rij +Aij exp(-rij/rij)-Cij r6ij (1) reconstructions. We have previously11 modelled the (1×3) reconstruction of the {100} surface of TiO2 and concluded where the charges of ions i and j, separated by a distance rij, that it was comprised of {110} facets in agreement with the are qi and qj , and Aij, rij and Cij are variable parameters fitted LEED evidence.4,5 to the lattice properties such as elastic constants of rutile.The surface properties of anatase have been studied less The Parry method,19,20 a special case of the Ewald method,21 extensively. The surface dehydration of the {001} and {111} was used to sum the electrostatic interactions by considering surfaces of anatase has been studied by Cordoba and Luque,12 the crystal to be composed of a series of charged planes, of and they concluded that the {111} surface was most likely to be infinite size which terminate at a surface.This leads to three exposed. The morphological characteristics of authigenic rutile types of surfaces, as identified by Tasker.22 In type I surfaces and anatase have been investigated by Morad13 and these the stacking plane is neutral and is composed of both cations morphologies and those from other workers14–16 will be used for and anionsin astoichiometric ratio with no dipole perpendicular comparison with the calculated morphologies later in this paper.to the surface. Type II surfaces contain a series of charged The aim of this paper is to provide reliable models for the planes making a repeat unit which has no dipole perpendicular surfaces of the low index planes of TiO2 and to develop a to the surface. Type III surfaces are composed of alternately strategy for ascertaining whether the models are reliable.The charged planes that produce a dipole perpendicular to the latter requires some independent check of the surface properties surface if cut between any plane.The Coulombic sum for such such as the crystal habit. Thus we are also concerned with a surface cannot be evaluated as it is divergent.23 If such surfaces using the energies from the final relaxed surfaces to determine are to be studied then the surface must be reconstructed such whether the crystal growth of the polymorphs of TiO2 is that the dipole is cancelled.24 For example, the {111} surface of kinetically or thermodynamically controlled.This is achieved MgO is a type III surface and the simplest reconstruction is to by considering the merits of evaluating the pseudo-kinetic and remove half the surface plane making the surface layer 50% equilibrium forms and comparing them with available experi- vacant.This cancels the dipole, enabling it to be simulated. The ment. This is discussed later on, but first the methodologies surface energy (ci) of a particularMiller index plane is calculated used to calculate the equilibrium and pseudo-kinetic energies from the difference between the surface block simulation energy are described, followed by the generation of morphologies.(Usurf) and the energy of the same number of bulk ions (Ubulk), i.e. surface excess energy, per unit area. Care is necessary to ensure that a sufficient number of layers are modelled so that Methodology the energy of the block has converged. The calculation of the The crystal is considered to be a series of charged planes lying parallel to the surface and periodic in two dimensions.The block is divided into two regions, region I and region II. The ions Table 1 Potential parameters for TiO2 close to the surface in region I are explicitly relaxed whereas the interaction A/eV r/A° C/eV A° 6 ions in region II are held fixed. Thus during the minimisation process the ions in region I are allowed to relax relative to Ti2.196+–O1.098- 16957.53 0.194 12.59 region II.The surface energies and structures of rutile and Ti2.196+–Ti2.196+ 31120.2 0.154 5.25 O1.098-–O1.098- 11782.76 0.234 30.22 † Email: P.M. Oliver: p.m.oliver@bath.ac.uk J. Mater. Chem., 1997, 7(3), 563–568 563surface energy is given in eqn. (2) where ci and wi are the specific surface energy and surface area of the ith crystallographic face.The crystal morphology is the shape which describes a minimum volume from a polar plot ci=AUsurf-Ubulk area B (2) of surface energy as a function of orientation, i.e. the height of a face is proportional to its surface energy. This was later In the calculation of the surface energy for a particular proved using both geometric27–29 and thermodynamic argu- Miller index there may be multiple unique repeat units. If this ments.30,31 This approach works best for small crystals, but is the case then each cut must be considered separately and for large crystals the morphology is best described using a the lowest energy cut used in the generation of the equilibrium kinetic treatment.Two straightforward phenomenological morphology. This is further complicated by the presence of models which attempt to incorporate these kinetic factors are asymmetric cuts.A symmetric repeat unit (hkl )s has the same the Donnay Harker scheme32 and that of Hartman and surface at the top and bottom. An asymmetric repeat unit Bennema.33 These are briefly described below. (hkl )as has a different surface at the top and bottom. If the The Donnay Harker methodology,32 which attempts to crystal space group has a centre of inversion, as is the case for model the rate of growth, applies the relation shown in eqn.(4) rutile and anatase, then each asymmetric surface will have its own inverse (hkl)asi and the asymmetric cut is treated indepen- height3 1 dhkl (4) dently (Fig. 1). where dhkl is the repeat distance for Miller index {hkl} and the Crystal morphology height is the length of the normal to the face in the Wulff plot.The attachment energy used in the Hartman and Bennema The equilibrium shape of a crystal is that which minimises the model33 is the energy per atom released for face (hkl ) when a total surface free energy, which at 0 K is approximated by the new slice of thickness dhkl crystallises on it and is used to internal lattice energy.The relationship between equilibrium model the rate of growth. The thickness dhkl is the minimum morphology and surface energy is most conveniently obtained slice which will repeat the same surface configuration. When from Wulff ’s theorem,25 which is a corollary of an earlier evaluatingthe pseudo-kinetic morphology from the attachment theorem by Gibbs.26 Gibbs proposed that the equilibrium form energies the lowest attachment energy is used.The length of of a crystal should possess a minimum (free) energy for a given the normal is proportional to the attachment energy34 thus volume and is shown in eqn. (3) height3Uattach(hkl) (5) Esurf=. i ciwi=minimum for constant volume (3) where Uattach is the attachment energy for surface (hkl).In the next section the morphologies generated using the above methodologies are compared and contrasted. Structure and morphology of rutile and anatase Rutile is tetragonal (a=bc, P42 /mnm)35 with Ti4+ surrounded by six O2- at the corners of a slightly distorted octahedron and each O2- surrounded by three Ti4+ lying in a plane at the corners of an equilateral triangle. Anatase is also tetragonal (a=bc, I4/amd)35 and has the same coordination as rutile.The structural difference between these two polymorphs is in how the octahedra are connected. Rutile is comprised of linear chains of edge-sharing octahedra where the chains themselves are connected by the octahedra corners. The anatase structure comprises of two interpenetrating zigzag chains of edge-shared octahedra which are linked to form a three-dimensional network of edge-shared octahedra.Fig. 1 Stacking sequence illustrating asymmetric surface (as) and All surface planes up to index 2 were considered for rutile asymmetric surface-inverse (asi) for the (a) {111a}-as and (b) {111b}- asi of rutile and anatase. The low index planes were chosen as they are in Table 2 Surface characteristics and energies (before and after relaxation) for rutile showing the area, repeat distance, unrelaxed and relaxed surface energies, attachment energy, symmetry and type of surface {hkl} area/A°2 drepa/A° Eunrelb/J m-2 Erelc/J m-2 Eattach/eV symmetryd typee {011b} 38.79 3.51 2.71 2.18 -0.54 s II {011a} 1.60 1.40 -0.32 s II {001} 14.22 2.39 1.28 1.28 -0.37 s II {112} 58.42 2.33 2.05 1.81 -0.63 s II {100} 36.09 1.89 2.26 1.68 -1.02 s II {121b} 81.94 1.66 2.42 1.94 -2.21 s II {121a} 4.96 3.05 -5.35 s II {012} 45.94 1.48 4.76 2.42 -3.10 s II {110} 51.03 1.34 2.65 2.19 -1.57 s II {021b} 73.56 0.93 2.36 1.81 -1.12 s II {021a} 5.91 2.97 -3.91 s II {120} 80.69 0.84 2.54 1.98 -1.61 s II {122} 85.56 0.79 4.98 2.05 -4.50 s II {111} 52.98 0.64 5.75 2.87 -5.20 s II {221} 103.05 0.33 4.89 2.69 -8.10 s II aRepeat distance.bUnrelaxed, crelaxed surface energy. ds=Symmetric surface. eTasker classification. 564 J. Mater. Chem., 1997, 7(3), 563–568Table 3 Surface characteristics and energies (before and after minimisation) for anatase showing the area, repeat distance, unrelaxed and relaxed surface energies, attachment energy, symmetry and type of surface {hkl} area/A°2 dhkl/A° Eunrela/J m-2 Erelb/J m-2 Eattach/eV symmetryc typed {110} 19.12 3.18 2.05 1.78 -0.42 s II {011} 24.30 2.50 2.06 1.85 -0.50 s II {100} 13.52 2.25 2.40 2.08 -0.64 s II {111b} 27.81 2.18 3.95 2.60 -1.22 asi (a) II {111a} 3.95 2.91 -1.22 as II {120b} 30.23 2.01 6.13 2.62 -2.45 asi (a) II {120a} 6.13 3.66 -2.45 as II {121} 36.35 1.67 2.67 2.16 -1.11 s II {001} 20.19 1.50 2.81 2.40 -1.40 s I {221a} 43.24 1.4 3.83 2.02 -1.78 as II {221b} 3.83 3.29 -1.78 asi (a) II {112} 44.68 1.36 4.88 4.01 -3.04 s II {122b} 50.44 1.20 4.01 2.52 -2.36 asi (a) II {122a} 4.01 2.82 -2.36 as II {021} 33.75 0.90 2.85 2.28 -1.61 s II {012} 42.58 0.72 6.06 2.95 -5.10 s I aUnrelaxed, brelaxed surface energy.cs=Symmetric surface, as=asymmetric surface, asi(a)=asymmetric inverse of the asymmetric surface a.dTasker classification. general the most stable and to provide some limits to the agreement. However, the best agreement is obtained by using the relaxed surface energies generating a morphology express- computational search. The surface energies are summarised in Tables 2 and 3. ing the {011}, {110}, {100} and {221} faces [Fig. 2(d)]. For all of the calculated morphologies the {011} surface is too The equilibrium morphology is compared with experiment, the Donnay Harker scheme and the attachment energy scheme stable resulting in a squashed habit. These results indicate that surface relaxation plays an important role in determining in Fig. 2 and 3 for rutile and anatase, respectively.The pseudokinetic morphologies of rutile calculated using the Donnay Harker (DH) [Fig. 2(a)] and attachment energy (AE) [Fig. 2(b)] schemes are very similar with only the {011} and {110} surfaces expressed. This is contrary to the observed morphology14,15 [Fig. 2(e)]. The unrelaxed equilibrium morphology [Fig. 2(c)] includes the {100} surface and shows closer Fig. 3 The calculated and experimental morphology of anatase Fig. 2 The calculated and experimental morphology of rutile (a) Donnay Harker, (b) attachment energy, (c) before minimisation, (d) after minimisation, (e) experimental {011},15 (f ) experimental (a) Donnay Harker, (b) attachment energy, (c) before minimisation, (d) after minimisation, (e) experimental15 {111}16 J. Mater. Chem., 1997, 7(3), 563–568 565which faces are expressed and thus schemes that omit surface titanium ions in the [1�10] direction.This can be envisaged as cleaving between the chains of octahedra. Upon relaxation the relaxation are disadvantaged. These results compare well to the electronic structure calculations of Ramamoorthy et al.9 surface energy decreased from 2.05 to 1.78 J m-2. On this surface the first layer surface bridging oxygen ions relaxed Their calculations incorrectly predicted the inclusion of the {001} surface leading to a capped morphology.In addition 0.08 A° into the surface. On the second layer the six- and fivecoordinate titanium ions moved outward and inward by 0.25 A° the atomistic approach allowed us to investigate index 2 surfaces which were beyond the scope of the electronic structure and the second layer oxygens moved outward slightly by 0.02 A° .In comparison to electronic structure calculations8,9 calculations, and the {221} surface was identified as being in the calculated morphology. This is apparently a discrepancy the agreement is good, but in their study the amplitudes of the relaxations are less with the second layer six- and five-coordi- as the {111} face is commonly identified in this position. However, both are first-order dipyramids of the tetragonal nate titanium ions moving 0.1 A° .This discrepancy (as the authors point out) could be because their five-layer slab is not system and thus would appear very similar, especially since the faces are present with very small surface areas. This leads completely converged.The {011} surface is comprised of rows of oxygen ions in to the prediction that the previously identified {111} surfaces may in fact be {221}. the [0 10 4] direction bonded to five-coordinate titanium ions and can be envisaged as cleaving through the linear chains of On simulating anatase the unrelaxed equilibrium, DH and AE morphologies all expressed the {001} surface, contrary to octahedra.The surface energy of the {011}surface of 2.06 J m-2 before relaxation is very similar to that of the {110} surface.the experimentally reported morphology, resulting in a capped {011} octahedral habit (Fig. 3). Upon relaxation the mor- However, after relaxation the surface is slightly less stable than the {011} by 0.07 J m-2 with a surface energy of 1.85 J m-2.phology hardly changes with the slight expression of the {112} face. Both experimental morphologies show an octahedral For this surface the relaxation is small which agrees with previous electronic structure calculations.10 The surface oxygen habit but there is some ambiguity as to whether this is due to {011}15 [Fig. 3(e)] or {111} [Fig. 3(f )] faces.16 This study ions relaxed 0.02 A° out of the surface and the second layer of oxygen ions relaxed 0.01 A° into the surface. Similarly the suggests that the octahedrare formed from {011} faces and that they appear as {011} octahedra capped with {001} faces.relaxationsof the titanium ions is small, with their displacement being 0.01 A° out of to the surface on the first layer and in Although the {001} surface is not present in the experimental morphology15 the {001} is a major cleavage plane for anatase, subsequent layers they did not move.The relaxations for the two stable surfaces of anatase, {001} identifying it as a stable surface which is reflected in its low surface energy. and {011}, with relaxed surface energies of 1.28 and 1.40 J m-2 respectively, are given in Plate 2(a) and (b).The {001} surface is composed of a surface layer of oxygen ions which are two- Relaxed surface structure of rutile and anatase coordinate in the [010] direction. The five-coordinate titanium ions bonded to these oxygen ions are themselves bonded to One very important advantage that energy minimisation has over the DH and AE schemes is that the atomic structure and three-coordinate oxygen ions in the [100] direction.The surface energies before and after relaxation for the {001} relaxations of ions at the surface can be examined. The surface structures after minimisation for the two most stable surfaces surface of anatase are identical, indicating that this is a very stable surface. From the perspective of the linked octahedra of rutile, the {110} and {011}, are given in Plate 1(a) and (b).The {110} surface [Plate 1(a)] is comprised of two-coordi- this surface is generated by cleaving between the interpenetrating octahedral chains, which may account for its stability. This nate oxygen ions in the [001] direction bonded to the sixcoordinate titanium ions which alternate with five-coordinate is reflected in the relaxed positions of the ions.The first layer (a) (b) [110] [110] [001] [001] [110] [011] [100] [100] [0104] [0104] Plate 1 The relaxed surface structure (top and side views) of rutile (a) {110} and (b) {011}. The surface oxygen ions are yellow, bulk oxygens red and titanium ions blue. 566 J. Mater. Chem., 1997, 7(3), 563–568(a) (b) [001] [010] [010] [011] [016] [016] [100] [100] [100] [100] Plate 2 The relaxed surface structure (top and side views) of anatase (a) {001} and (b) {011}.The surface oxygen ions are yellow, second layer oxygen ion green, bulk oxygens red and titanium ions blue. oxygen ions moved 0.04 A° into the surface and the five- found at the surface, which in part may be the source of the different reactivities and surface properties. coordinate titanium ions moved by 0.01 A° out of the surface.This is in contrast to the {001} surface of rutile where the surface contains four-coordinate titanium ions and is corre- We would like to thank the EPSRC for funding and Molecular spondingly less stable with a surface energy of 2.40 J m-2. Simulations Inc. for the provision of INSIGHT II. The {011} surface anatase is composed of rows of oxygen ions in the [100] direction two-coordinated to five-coordinate titanium ions in the second layer in the [016�] direction.These References titanium ions are themselves bonded to three-coordinate 1 A. Fujishima and K. Honda, Nature (L ondon), 1972, 238, 37. oxygen ions in the [100] direction. The {011} surface can be 2 Y. W. Chung, W.J. Lo and G.A. Somorjai, Surf. Sci., 1977, 64, 588. envisaged as cleaving through the interpenetrating chains 3 C. A. Muryn, P. J. Hardman, J. J. Crouch, G. N. Raiker, which are inclined to the surface normal. In contrast to the G. Thornton and D. S. L. Law, Surf. Sci., 1991, 251/252, 747. 4 P. Zschack, J. B. Cohen and Y. W. Chung, Surf. Sci., 1992, 262, 395. {001} surface the relaxations on the {011} surface were larger, 5 P.W.Murray, F.M. Leibsle, H. J. Fisher,C. F. Flipse, C. A. Muryn the first layer oxygen ions moved 0.08 A° into of the surface and G. Thornton, Phys. Rev. B, 1992, 46, 12877. and the second layer oxygen ions 0.15 A° out of the surface. 6 R. V. Kasowski and R. H. Tait, Phys. Rev. B, 1992, 20, 5168. The titanium ions moved relatively little with displacements 7 M.Tsukada, C. Satoko and H. Adachi, J. Phys. Soc. Jpn., 1979, of 0.02 A° for the five-coordinate titanium into the surface and 47, 1610. 0.03 A° out of the surface for the six-coordinate surface 8 M. Ramamoorthy, R. D. King-Smith and D. Vanderbilt, Phys. Rev. B, 1994, 49, 7709. titanium ions. 9 M. Ramamoorthy, D. Vanderbilt and R. D. King-Smith, Phys. Rev. B, 1994, 49, 16723.Conclusions 10 S. Munnix and M. Schmeits, Phys. Rev. B, 1984, 30, 2202. 11 P. M. Oliver, S. C. Parker, J. Purton and D. W. Bullett, Surf. Sci., The morphologies of rutile and anatase have been calculated 1994, 307–309, 1200. using equilibrium and pseudo-kinetic methodologies and in 12 A. Cordoba and J. J. Luque, Phys. Rev. B, 1985, 31, 8111. 13 S. Morad, Sedimentary Geology, 1986, 46, 77.general agree with those observed experimentally. The four 14 W. E. Ford and E. S. Dana, A T extbook of Mineralogy, Wiley, surfaces expressed in the relaxed equilibrium morphology of Chichester, 1958, 4th edn. rutile, {011}, {110}, {100} and {221} with surface energies of 15 I. Kostov,Mineralogy, Nauka i Izkustvo, Sofia, 1968. 1.85, 1.78, 2.08 and 2.02 J m-2, are in good agreement with 16 G.Munuera, F. Moreno and F. Gonzalez, Reactivity of Solids, ed. experiment and with electronic structure calculations. This J. S. Anderson, M. W. Roberts and F. S. Stone, Chapman and Hall, indicates that surface relaxation is important for determining London, 1972. 17 G. W. Watson, E. T. Kelsey, N. H. de Leeuw, D. J. Harris and morphology because neither the Donnay Harker nor the S.C. Parker, J. Chem. Soc., Faraday T rans., 1996, 92, 433. attachment energy schemes achieved this level of agreement. 18 M. Matsui and M. Akoagi, Molecular Simulation, 1991, 6, 239. For anatase the {011} and {001} surfaces were dominant in 19 D. E. Parry, Surf. Sci., 1975, 49, 433. the morphology with relaxed surface energies of 1.40 and 20 D. E. Parry, Surf. Sci., 1976, 54, 195. 1.28 J m2. The most stable surface of rutile, the {110}, contains 21 P. P. Ewald, Ann. Phys., 1921, 64, 253. five- and six-coordinate species at the surface and for the most 22 P. W. Tasker, J. Phys. C: Solid State Phys., 1979, 12, 4977. 23 F. Bertaut, Compt. Rend., 1958, 246, 3447. stable surface of anatase, the {001}, five-coordinate species are J. Mater. Chem., 1997, 7(3), 563–568 56724 P. M. Oliver, S. C. Parker and W. C. Mackrodt, Modelling Simul. 31 M. Volmer, Kinetik der Phasenbildung, Steinkopff, Leipzig, 1939. 32 J. D. Donnay and G. Harker, Am. Mineral., 1937, 22, 446. Mater. Sci. Eng., 1993, 1, 755. 25 G. Wulff, Z. Kristallogr. Kristallgeom., 1901, 39, 449. 33 P. Hartman and F. Bennema, J. Crystal Growth, 1980, 49, 145. 34 C. F.Woensdregt, Faraday Discuss., 1993, 95, 97. 26 J. W. Gibbs, Collected Works, Longman, New York, 1928. 27 H. Hilton,Mathematical Crystallography, Oxford, 1903. 35 W. A. Deer, R. A. Howie and J. Zussman, Rock Forming Minerals, Longmans, London, 1962, 1st edn., vol. 5. 28 H. Leibman, Z. Kristallogr., 1914, 53, 171. 29 M. von Laue, Z. Kristallogr., 1943, 105, 124. 30 I. N. Stranski, Z. Phys. Chem. B, 1938, 38, 451. Paper 6/06353E; Received 16th September, 1996 568 J. Mater. Chem., 1997, 7(3), 563–5

ISSN:0959-9428    

DOI:10.1039/a606353e

出版商:RSC    

年代:1997

数据来源: RSC

摘要:

Routine ab initio structure determination of chlorothiazide by X-ray powder diffraction using optimised data collection and analysis strategies Kenneth Shankland,* William I. F. David and Devinderjit S. Sivia ISIS Facility, Rutherford Appleton L aboratory, Chilton, Didcot, Oxon, UK OX11 0QX The likelihood of solving crystal structures from powder diffraction data is greatly enhanced if data collection and analysis strategies can be designed to effectively remove Bragg peak overlap.In this way, accurate normalised structure factors of essentially single-crystal quality are obtained. Such strategies are illustrated here with the ab initio solution, from powder diffraction data using traditional direct methods, of the clinically used diuretic compound chlorothiazide.The structure solution is outstanding in that, despite the non-centrosymmetric, triclinic symmetry, all 17 non-hydrogen atom positions are clearly visible in the E-map generated from the top direct methods solution. The structure solution of moderately complex crystal structures the diffraction pattern. This leads to the familiar fall-off in intensity with increasing angle, that can amount to more than is generally straightforward and routine using single-crystal diffraction data.1 This is not the case with powder diffraction an order of magnitude between low and high angles.Bragg intensities at low angles are measured with great precision data, principally because of the inevitable overlap of Bragg reflections that occurs with the compression of the three whilst high-angle reflections may be scarcely distinguishable from the background.As direct methods utilise |E| values, dimensions of diffraction space onto the one dimension of a powder diffraction pattern. The demand for structural infor- these should ideally be measured with equal precision across the diffraction pattern. |E| values are related to structure factor mation obtained from powder diffraction data comes from the large number of materials that are only easily synthesised or magnitudes by crystallised in powder form; for example, many zeolites and drug polymorphs.This situation has spurred a significant |E(h)|2=|F(h)|2/e(h) .n j=1 fj2(h)exp(-2Bj sin2 h/l2) theoretical2–6 and experimental effort, leading over the past decade to an increasing catalogue of successes using powder where e(h) is the enhancement (epsilon) factor, fj (h) and Bj are diffraction methods.7 Many of these have been the crystal the atomic scattering factor and Debye–Waller factor, respectstructures of small molecules,8,9 simple inorganic systems10 or ively, for the jth of the n atoms in the unit cell, and l is the systems in high-symmetry space groups,11 though a smaller X-ray wavelength. The measured number of photons, N(h), is number of much larger systems, which indicate the potential proportional to the structure factor magnitudes, through the of the powder diffraction technique, have been reported.12,13 relationship The solution of an inorganic structure with 50 non-H atoms N(h)3t(h)L p(h)m(h)|F(h)|2 in the asymmetric unit from laboratory X-ray data alone is particularly impressive.14 However, these more complex sys- where t(h) is the dwell time, L p(h) the Lorentz–polarisation tems possess a structural hierarchy of heavy and light atoms correction and m(h) is the reflection multiplicity.The absence that leads to a customised, sequential structure solution process of a polarisation correction in a synchrotron radiation in which the first step involves the detection of part of the measurement gives L p(h)=1/sin h sin 2h. overall crystal structure (principally the heavy atoms) by direct Combining these equations, and noting that e(h)m(h) is the methods, with the remainder of the structure being found via order of the space group, leads to a series of difference Fourier map operations.Such sequential techniques are less appropriate for the solution of compounds N(h)3 Ct(h)|E(h)|2 .n j=1 fj2 (h)exp(-2Bj sin2h/l2 )D in which heavy atoms are absent and so more reliance is placed upon direct methods to find the bulk of the structure /(sin hsin 2h) in the initial structure solution. We show here that a combination of optimised data collec- Thus N(h)/|E(h)|2 should be constant in order to collect the tion strategies and Bayesian data analysis procedures can turn same number of photons for a given E magnitude irrespective a complex powder diffraction pattern into effectively a single- of its position in the diffraction pattern.Within the levels of crystal data set where the Bragg peak overlap is essentially approximation in this analysis, the atomic scattering factors and Debye–Waller factors may be represented by average removed.All of the direct methods algorithms developed for values. This leads to the time spent at a given diffraction angle the analysis of single-crystal diffraction data thus become conforming to the equation applicable to powder diffraction data without modification or approximation. t(h)3(sin h sin 2h)/[ fav2(h)exp(-2Bav sin2 h/l2)] The approach is straightforward; conventional direct methods have sufficient power to solve moderately complex where fav is a representative atomic scattering factor (carbon crystal structures if accurate structure factor magnitudes, |F|, for a typical organic compound) and Bav is an estimated are available.Thus reliable |F| values should be determined overall Debye–Waller factor.The dwell times obtained from whilst recognising that, for direct methods, it is desirable to this calculation can be scaled to the duration of the experiment, collect normalised structure factors, |E|, with a precision that subject to constraints such as a minimum count time below is independent of the position of the associated Bragg peaks which the overhead incurred in moving a detector makes the in the powder diffraction pattern.Diffraction data are usually counting scheme inefficient. Related protocols15,16 have been proposed for improving the quality of Rietveld refinements of collected in a step-by-step manner at a constant rate across J. Mater. Chem., 1997, 7(3), 569–572 569structural models, but have not previously been applied to the issue of structure determination.Even with such a count scheme, the reliability of the |E| estimates for the crucial high-resolution reflections is generally much worse than that of isolated lower resolution reflections, as the increasing extent of reflection overlap with increasing 2h makes accurate intensity extraction by whole pattern fitting problematic.6 However, in general, molecular structures undergo anisotropic thermal expansion. Therefore, reflections which are completely overlapped at one temperature are likely to be separated at another (sufficiently different) temperature, and this separation is generally enough to allow direct determination of the reflection intensities in the pattern fitting process.More reliable intensity estimates can thus be obtained by collecting diffraction data at more than one temperature, and combining the data sets to give effectively a single-crystal quality data set, without resort to computational schemes such as equipartitioning or permutation of intensities. The methods described above were used in the collection of synchrotron X-ray diffraction data from a powder sample of the drug substance chlorothiazide I for which only the unitcell dimensions and space group were previously known.17 N S H N Cl SO2NH2 O O I Fig. 1 (a) The variable count time scheme used in the collection of diffraction data from chlorothiazide. (b) Raw powder diffraction data Experimental obtained on station 9.1 at the Daresbury Synchrotron Radiation Source from a powder sample of chlorothiazide at 130 K.A sample of chlorothiazide powder obtained from the Sigma Chemical Co. was recrystallised from ethanol to ensure that only a single powder phase was present. The sample was Fig. 2 shows the effect over a small region of the diffraction loaded into a 1 mm capillary and placed inside a cryostat on pattern of the anisotropic thermal expansion experienced by station 9.1 at the Daresbury Synchrotron Radiation Source.chlorothiazide upon changing the temperature from 130 to Using an incident wavelength of 1.0985 A° and a temperature 160 K, whilst Table 1 lists the positions and extracted intensities of 130 K, diffraction data were collected using the variable for the corresponding reflections. At 130 K, the 21� 0 and 1� 21 count time scheme shown in Fig. 1(a). The scheme was calcu- reflections are so closely spaced in 2h that they are treated as lated for the range 10–60° 2h in 0.01° steps using l=1.1 A° , a single reflection in the extraction process, and the resultant B=1.0, a total count time of 6 h and a minimum count time intensity is equipartioned between the two contributors.At of 1.0 s. To simplify implementation within the confines of the 160 K, the doublet is sufficiently resolved to allow the reflec- instrument control software, the original continuously variable tions to be treated as separate entities in the extraction. This scheme was approximated by 33 ranges of constant count time behaviour is reflected in other overlapped Bragg peaks with a 0.5 s difference between each adjacent range.The raw throughout the diffraction pattern, i.e. 22 out of the 30 overlap powder diffraction data obtained are shown in Fig. 1(b). The sets present at 130 K are split at 160 K. The estimated intensity pattern represents the summation of two 6 h data sets collected ratio of ca. 1.251 for the 21� 0/-1� 21 pair is in good agreement using the count scheme described.In addition, but unrelated with the calculated ratio of ca. 1.351 obtained from the refined to the count scheme, data were collected in the 130 K structural model. direction 60°�10° 2h so that the fall in incident synchrotron The data collection strategies described only partly address flux with time after beam injection was somewhat offset by the the problem of collecting precise |E| values.The reflection increasingly strong Bragg reflections encountered at lower intensities extracted by the above refinement were transformed angles. A 10 h data set was also collected at 160 K. to an optimal estimate of the structure factor amplitudes using a Bayesian procedure which entails a formal method of propagating errors that circumvents the problems associated with Results negative intensities and minimises the effects of Bragg peak The first 20 lines of the 130 K diffraction pattern [Fig. 1(b)] overlap, enabling the best statistical decorrelation of neigh- were indexed using the DICVOL9118 program which returned a unit cell with dimensions a=6.372 A° , b=8.915 A° , c=4.853 A° , Table 1 Extracted structure factor intensities in the range 23.6–24° 2h a=96.12°, b=99.47°, c=74.40°, V=261.3 A° 3 (implying a space for chlorothiazide at 130 and 160 K group of P1, given the estimated molecular volume), in good agreement with the previous cell determination.The unit-cell 130 K 160 K dimensions, reflection intensities, zero point and peak shape hk l 2h/degrees I s(I) 2h/degrees I s(I) (Voigt, with an asymmetry correction for axial divergence19,20) were refined using the SR15LS21 program to give a 130 K cell 2� 2� 1 23.834 4.30 0.28 23.793 4.04 0.20 of dimensions a=6.3720(1) A° , b=8.9159(1) A° , c=4.8554(1) A° , 21� 0 23.865 9.39 0.18 23.850 9.80 0.97 a=96.1321(3)°, b=99.4760(4)°, c=74.4119(5)°, with a x2 of 1� 2 1 23.869 9.39 0.18 23.859 8.40 0.93 3.71 and an Rp of 3.38% for the fit. 570 J. Mater. Chem., 1997, 7(3), 569–572factor was varied gave a reasonable fit to the data, showing the determined structure to be substantially correct. Hydrogen atoms were added in calculated positions (CxMH=0.95 A° , NxMH=0.9 A° ) with fixed Biso values of 3.0 and allowed to ride on their parent atoms. The final cycle of least squares had 65 variables and gave agreement factors of RB=6.33%, Rp= 5.10%, Rwp=5.46%, Re=1.77% and x2=9.54.Refined atomic coordinates are listed in Table 2 and the corresponding profile fit is shown in Fig. 3. Fig. 4 shows four adjacent unit cells of the refined structure in projection down the c axis. The intermolecular hydrogen-bond distances shown are all within the ranges observed for organic molecular crystals containing these functional groups.23 Fig. 2 Cell and intensity least squares fits to a selected data region at (a) 130 K and (b) 160 K. The unit cell at 160 K is a=6.3802(1) A° , b= 8.9270(1) A° , c=4.8594(1) A° , a=96.1818(3)°, b=99.4932(4)°, c= 74.3422(5)°. The tick marks, corrected for zero-point error, show the positions (in order of increasing 2h) of the 2� 2� 1, 2 1� 0, and 1� 2 1 reflections in both cases.bouring |E| value estimates.6 Using only the 130 K data, a total of 420 structure factors were obtained and used as input to the MITHRIL9422 direct methods package. 1542 Triplets and 1605 negative quartets were generated from the top 157 and top 140 |E| values respectively; 15 possible solutions were subsequently developed by tangent refinement. The solution with the highest combined figure of merit (CFOM=2.48, next highest 2.23, then 1.77) produced an E-map in which the Fig. 3 Final observed, calculated and (yobs-ycalc)/s(yobs) plots for positions of all the non-H atoms in the structure were found chlorothiazide at 130 K. The fitted data are those shown in Fig. 1(b), by an automatic peak search (Table 2). An initial Rietveld normalised to take account of the variation in dwell times across the pattern.refinement against the 130 K pattern in which only the scale Table 2 Peak numbers, heights and atom assignments for the E-map corresponding to the top MITHRIL94 solution. Refined fractional atomic coordinates and Biso values for the assigned atoms are also given. Atom S(2) was used to fix the location of the molecule in the unit cell.Dref is the distance between an initial atomic position determined from the E-map and a corresponding final refined position peak height atom x/a y/b z/c Biso/A°2 Dref/A° 1 4418 S(2) 0.4164 0.2481 0.0214 0.13(7) — 2 3323 S(1) 0.2559(7) 0.8019(4) 0.6364(7) 0.56(8) 0.159 3 2719 Cl(1) 0.7575(5) 0.6876(4) 0.9701(8) 1.55(8) 0.115 4 1840 C(6) 0.383(2) 0.522(1) 0.346(3) 0.1(3) 0.344 5 1518 N(3) 0.336(2) 0.938(1) 0.545(2) 1.4(2) 0.366 6 1284 C(2) 0.735(2) 0.336(1) 0.403(2) 0.4(2) 0.260 7 1272 C(1) 0.820(2) 0.102(1) 0.117(2) 0.5(2) 0.381 8 1267 N(2) 0.880(1) 0.202(1) 0.345(2) 0.5(2) 0.336 9 1245 C(3) 0.808(3) 0.433(2) 0.639(3) 2.7(4) 0.429 10 1215 C(5) 0.446(2) 0.631(1) 0.569(2) 1.1(3) 0.222 11 1107 C(4) 0.653(2) 0.575(2) 0.700(3) 1.8(3) 0.110 13 1004 O(3) 0.294(2) 0.177(1) 0.138(2) 2.2(2) 0.255 16 974 O(2) 0.262(1) 0.8213(8) 0.942(2) 1.6(2) 0.289 17 937 C(7) 0.514(2) 0.382(1) 0.266(3) 0.4(2) 0.121 20 878 O(4) 0.309(1) 0.3297(8) -0.222(2) 1.0(1) 0.313 21 856 O(1) 0.0467(1) 0.7991(8) 0.468(2) 0.9(2) 0.094 23 739 N(1) 0.638(1) 0.107(1) -0.044(2) 1.9(2) 0.322 H(1) 0.237 0.547 0.250 3.00 — H(4) 0.957 0.393 0.715 3.00 — H(2) 0.934 0.019 0.056 3.00 — H(3) 1.006 0.173 0.464 3.00 — H(5A) 0.470 0.939 0.635 3.00 — H(5B) 0.340 0.924 0.359 3.00 — J.Mater. Chem., 1997, 7(3), 569–572 571down to include the top 32 peaks in the E-map, cf. Table 2, which shows a solution with fewer significant spurious peaks. Conclusions The approach to structure solution from powder data presented here permits accurate structure factor magnitudes to be extracted across a complete diffraction pattern, a crucial step if conventional direct methods are to be employed effectively in solving large structures.Applicable to both synchrotron and laboratory X-ray powder diffraction experiments, the approach also enhances the widely used sequential method of structure solution, by allowing larger initial fragments to be located with greater accuracy for subsequent use in difference Fourier Fig. 4 The refined structure of chlorothiazide at 130 K, shown as a calculations. Other methods, based on fitting trial models to projection dacent unit cells. Intermolecular hydrogen bonds, including a weak CMH,O interaction, are shown diffraction data24,25 and using an agreement factor (such as by the finely dotted lines.An additional2.17 A° intermolecular hydrogen Rwp ) for discrimination, will also benefit from the improved bond running from N(3)MH(5B),O(2) (in molecule at x, y, z-1) is signal to background ratio at high 2h obtained when an not shown. optimised count time scheme is used. We thank G. Bushnell-Wye for access to, and calibration of, station 9.1.We also thank S. W. Love for his help during the data collection. References 1 G. H. Stout and L. H. Jensen, X-ray Structure Determination, Wiley Interscience, New York, 1989. 2 W. I. F. David, J. Appl. Crystallogr., 1987, 20, 316. 3 G. Bricogne, Acta Crystallogr., Sect. A, 1991, 47, 803. 4 J. Jansen, R. Peschar and H. Schenk, J. Appl. Crystallogr., 1992, 25, 231. 5 M. A. Estermann and V.Gramlich, J. Appl. Crystallogr., 1993, 26, 396. 6 D. S. Sivia and W. I. F. David, Acta Crystallogr., Sect. A, 1994, 50, 703. 7 J. I. Langford and D. Loue�r, Rep. Prog. Phys., 1996, 59, 131. 8 P. Lightfoot, M. Tremayne, C. Glidewell and K. D. M. Harris, J. Chem. Soc., Perkin T rans. 2, 1993, 1625. Fig. 5 Distribution of E magnitudes for the 420 reflections extracted 9 R.G. Delaplane, W. I. F. David, R. M. Ibberson and C. C. Wilson, from the 130 K data set Chem. Phys. L ett., 1993, 201, 75. 10 H. Fjellvag and P. Karen, Inorg. Chem., 1992, 31, 3260. 11 D. M. Poojary, R. B. Borade, F. L. Campbell and A. Clearfield, J. Solid State Chem., 1994, 112, 106. Discussion 12 R. E. Morris, W. T. A. Harrison, J. M. Nicol, A. P. Wilkinson and A. K. Cheetham, Nature (L ondon), 1992, 359, 519.It is clear that the combination of an improved data collection 13 R. E. Morris, J. J. Owen, J. K. Stalick and A. K. Cheetham, J. Solid strategy and an optimal structure factor extraction procedure State Chem., 1994, 111, 52. has resulted in a very high quality, routine structure solution, 14 D. M. Poojary, A. Cabeza, M. A. G. Aranda, S. Bruque and A.Clearfield, Inorg. Chem., 1996, 35, 1468. both in terms of the ease with which it was identified from its 15 W. I. F. David, Accuracy in Powder Diffraction, NIST Special CFOM and the proximity of the initial atomic positions to Publication no. 846, 1992, 210. their final refined positions. It is significant that 80% of the 16 I. C. Madsen and R. J. Hill, J. Appl. Crystallogr., 1994, 27, 385.data collection time was spent in the range 40–60° and that 17 P. L. Dupont and O. Dideberg, Acta Crystallogr., Sect. B, 1970, 75% of strong |E| magnitudes (|E|>1.5) lie in this range 26, 1884. (Fig. 5). This underlines the fundamental point that the strategy 18 A. Boultif and D. Louer, J. Appl. Crystallogr., 1991, 24, 987. 19 M. M. Eddy, A. K. Cheetham and W.I. F. David, Zeolites, 1986, has ensured accurate estimates of these crucial strong |E| 6, 449. magnitudes. Furthermore, the superiority of the Bayesian 20 L. W. Finger, D. E. Cox and A. P. Jephcoat, J. Appl. Crystallogr., method of extracting of structure factors over the traditional 1994, 27, 892. method of cell and intensity least squares (CAILS, commonly 21 W. I. F. David, R. M. Ibberson and J. C. Matthewman, Rutherford known as the Pawley method) is demonstrated when structure Appleton Laboratory Report RAL-92-032, 1992. solutions that neglect the Bayesian analysis are attempted. All 22 C. J. Gilmore and S. R. Brown, J. Appl. Crystallogr., 1988, 21, 571. 23 G. A. Jeffrey and W. Saenger, Hydrogen Bonding in Biological positive intensities extracted by the CAILS procedure (about Structures, Springer-Verlag, Berlin, 1991. 90% of the total) were used, and structure factor magnitudes 24 M. Tremayne, B. M. Kariuki and K. D. M. Harris,J.Mater. Chem., calculated using |F|=ÓF2 and s(F)=s(F2)/2|F|. The E-map 1996, 6, 1601. generated from the top MITHRIL94 solution (CFOM=2.56) 25 Y. G. Andreev, P. Lightfoot and P. G. Bruce, Chem. Commun., did not contain the correct structure. The second and fifth 1996, 2169. ranked solutions did generate E-maps with almost the entire structure visible, but only when peak connectivity was extended Paper 6/06998C; Received 14th October, 1996 572 J. Mater. Chem., 1997, 7(3), 569–5

ISSN:0959-9428    

DOI:10.1039/a606998c

出版商:RSC    

年代:1997

数据来源: RSC