CsFeSiO4: a maximum iron content zeotype Paul F. Henry and Mark T. Weller* Chemistry Department, University of Southampton, Highfield, Southampton, UK SO17 1BJ. Tel and Fax No. 00 44 1703 593592. E-mail: mtw@soton.ac.uk Received (in Bath, UK) 21st September 1998, Accepted 9th November 1998 The alkali-metal iron silicate, CsFeSiO4, has been synthesised, using a gel decomposition method followed by high temperature annealing, and shown to adopt the zeolite ABW structure constructed from alternating FeO4 and SiO4 tetrahedra surrounding caesium. Substitution of iron into zeolitic frameworks using hydrothermal methods has been investigated by several groups1ndash;5 but with limited success due to the difficulty of attaining purely tetrahedral Fe(iii) in an aqueous environment.Levels of tetrahedral iron that can be incorporated into the framework, replacing aluminium, are generally restricted to a few percent and octahedral iron species often block the zeolite pores.The main motivation behind substitution of transition metal centres (particularly Fe, Co, Mn and Cr) into frameworks lies in the attempt to build selective redox catalysts by utilising the channels inherent within zeolite structures. A secondary idea is the possibility of producing new pigments by incorporating coloured species into frameworks rather than intercalating coloured species into the channels within frameworks.6ndash;9 Some complex tetrahedra based iron silcate materials have been previously reported in the literature: KFeSiO4 exists in three forms,10 a-KFeSiO4 is orthorhombic but of unknown structure, b-KFeSiO4 adopts the stuffed tridymite structure and g-KFeSiO4 has the kaliophilite structure.Studies into the K2OFe2O3- SiO2 phase field11,12 have also shown the existence of iron leucite, KFeSi2O6, and iron feldspar, KFeSi3O8, which show analogous polymorphism with their corresponding aluminium compounds. A polycrystalline sample of CsFeSiO4 was prepared as follows.Stoichiometric quantities of LUDOX (40 by weight SiO2 in water, Aldrich) and Fe(NO3)3middot;9H2O (99.9, Aldrich) were dissolved in 50 ml of 2M HNO3. A 1.5-fold excess of CsCO3 (99.9, BDH) was then added to the solution; excess was added to compensate for the high volatility of caesium salts at the intermediate temperatures used in the experimental procedure. 100 ml ethanol was added under constant stirring followed by 10 ml of .880 ammonia solution, added dropwise.A brown spongy material was seen to precipitate from the solution as the ammonia was added, which partially redissolved on addition of further ammonia solution. The mixture was heated to dryness over a period of 12 h. The resultant solid was then partially decomposed in an alumina crucible at a temperature of 250 deg;C for a further 12 h. The brown powder obtained was thoroughly ground and heated for a further 16 h at 600 deg;C, then 850 deg;C and finally 1000 deg;C. After each heat treatment a powder X-ray diffraction (PXD) pattern was collected, using a D5000 Siemens diffractometer (Cu-Ka1 radiation) operating in reflection geometry.The annealing at 1000 deg;C was repeated until there was no observable change in the powder diffraction pattern. The final product was found to be mustard yellow in colour. Data, for Rietveld analysis using the GSAS suite of programs,13 were obtained over 16 h for the 2q range 10ndash;110deg; using a step size of 0.02deg;.The initial PXD pattern collected from CsFeSiO4 after annealing at 600 deg;C was found to contain no discernible Bragg reflections. The PXD pattern collected after annealing at 850 deg;C showed a new phase to be present, although the diffraction pattern was very weak. Repeated annealing at 1000 deg;C gave a crystalline material with sharp Bragg reflections, which were indexed on an orthorhombic unit cell using the PC program TREOR90.14 Comparison of the PXD pattern with that simulated for a material adopting the zeolite ABW structure with the calculated lattice parameters gave very good agreement.No evidence of leucite or feldspar type impurities were found in the pattern. Full Rietveld analysis was then performed using the structure of the known ABW material LiAlSiO4 15 as the starting model but with iron on the aluminium position and caesium replacing lithium. In this model the silicon and aluminium positions are distinct, i.e. the framework is ordered with alternating tetrahedral ions in accordance with Loewensteinrsquo;s rule.16 The framework was refined, subject to some hard and soft constraints; these constraints are necessary due to the in- Fig. 1 Final Rietveld refinement profile of CsFeSiO4. The observed data are crosses, the calculated pattern a solid line, the tick marks show the allowed reflections and the lower line is the difference plot; Rwp = 6.85, Rp = 5.24, Re = 5.63 and RF**2 = 13.52 for 355 observations. Table 1 Atomic coordinates for CsFeSiO4 Ui/Ue Atom Site x y z Occupancy 3100 Cs 4a 0.2021(4) 0.495(5) 0.5009(9) 1.013(10) 3.44(14) Si 4a 0.082(5) 20.018(19) 0.193(3) 1.0 2.30(28) Fe 4a 0.417(3) 20.021(13) 0.314(2) 1.0 2.30(28) O1 4a 0.092(3) 0.007(13) 0.014(2) 1.0 4.8(6) O2 4a 20.007(10) 20.246(9) 0.261(10) 1.0 4.8(6) O3 4a 0.019(9) 0.232(10) 0.262(10) 1.0 4.8(6) O4 4a 0.229(3) 20.077(8) 0.276(6) 1.0 4.8(6) Table 2 Cell and space group information CsFeSiO4 Space group Pc21n a/Aring; 9.5858(4) b/Aring; 5.5538(3) c/Aring; 9.0476(4) V/Aring;3 481.67(4) Z 4 Chem.Commun., 1998, 2723ndash;2724 2723sensitivity of the technique for a caesium containing material. The thermal factors of the oxygen sites were constrained to be identical, as were those of the silicon and iron sites. The Sindash;O and Fendash;O bond lengths were refined with a soft constraint (standard deviation of 0.005 Aring;) at 1.625 and 1.825 Aring; respectively. The refinement converged smoothly to give the excellent final profile fit illustrated in Fig. 1. Atomic data and cell data are given in Tables 1 and 2. Refinement was also attempted using the disordered framework ABW model of CsAlTiO4 17 in the space group Imma but this gave much poorer profile fit parameters and was discarded. The refined structure is constructed of alternating SiO4 and FeO4 vertex linked tetrahedra with the alkali-metal cation occupying the large 8-ring cavities parallel to the crystallographic b-axis. Channels also exist along the a-direction (4-rings) and c-direction (6-rings) as illustrated in Fig. 2. The stability of the product material to moisture was studied by stirring in deionised water for 3 days at 45 deg;C. Residual water content was measured by thermogravimetric analysis (TGA) using an STA1500 TGA/DSC. Elemental analysis was carried out using a JEOL JSM-6400 SEM equipped with a TRACOR series II energy dispersive X-ray analysis system. TGA showed the as-made material to contain no residual water. Attempts were made to hydrate the material by exposure to water vapour for 3 days but subsequent TGA measurements showed no weight loss up to 1000 deg;C indicating no water uptake.The material was also found to be air and moisture stable. EDAX on CsFeSiO4 showed the ratio of Cs : Fe : Si to be approximately 1 : 1 : 1 in accordance with that expected for an ABW product stoichiometry. Further, AFeTiO4 (A = Cs, Rb) have been synthesised using the same experimental technique as dark brown powders.Rietveld analysis on CsFeTiO4 using powder X-ray data show this material also adopts the zeolite ABW structure and TGA reveals no residual water within the structure. Attempts to synthesise the germanium analogues have met with limited success. Further structural characterisation of materials using powder neutron diffraction data is planned to investigate the framework ordering in detail. CsSiFeO4, with a structure based on alternating FeO4 52 and SiO4 42 tetrahedra, is the first framework material having the maximum level of iron in a zeotype.The ability to incorporate such high levels of iron is a result of templating at high temperatures using caesium rather than in aqueous solution where six coordinate iron is unavoidable and influences the nature of the zeolite product. The financial support of EPSRC is gratefully acknowledged. Notes and references 1 D. W. Lewis, C. R. A. Catlow, G. Sankar and S. W. Carr, J. Phys. Chem., 1995, 99, 2377. 2 R. B. Borade and A. Clearfield, Chem. Commun., 1996, 2267. 3 D. Mazza and M. L. Borlera, Powder Diffraction, 1997, 12, 87. 4 P. N. Joshi, S. V. Awate and V. P. Shiralkar, J. Phys. Chem., 1993, 97, 9749. 5 D. E. W. Vaughan, K. G. Strohmaier, I. J. Pickering and G. N. George, Solid State Ionics, 1992, 53ndash;56, 1282. 6 M. T. Weller and K. E. Howarth, J. Chem. Soc., Chem. Commun., 1991, 373. 7 J. B. Guimet, Bull. Soc. Enc. Ind. Nat., 1828, 27, 346. 8 C. G. Gmelin, Bull. Soc. Enc. Ind. Nat., 1828, 27, 216. 9 W. Depmeier, H. Schmid, N. Setter and M. L. Werk, Acta Crystallogr., Sect. C, 1987, 43, 2251. 10 J. J. Bentzen, J. Am. Ceramic Soc., 1983, 66, 475. 11 G. T. Faust, Am. Mineral., 1936, 21, 735. 12 G. T. Faust, Schweiz. Mineral. Petrogr. Mitt., 1963, 43, 165. 13 A. C. Larson and R. B. Von Dreele, General Structure Analysis System, Los Alomos National Laboratory LAUR86-748, 1994. 14 P. E. Werner, L. Eriksson and M. Westdahl, J. Appl. Crystallogr., 1985, 18, 367. 15 I. S. Kerr, Z. Kristallogr., 1974, 139, 186. 16 W. Loewenstein and M. Lowenstein, Am. Mineral., 1954, 39, 92. 17 B. M. Gatehouse, Acta Crystallogr. Sect. C, 1989, 45, 1674. Communication 8/07741J Fig. 2 Representations of the structure of CsFeSiO4 along (a) 100, (b) 010 and (c) 001 illustrating the channels. Dark tetrahedra represent SiO4, light tetrahedra represent FeO4 and the black spheres are caesium. 2724 Chem. Commun., 1998, 2723ndash;2724
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