Sodalite-type Na8Mg,Si90m (OH) and Na8Mg3Si9Om (OH,CI), novel framework magnesiosilica tes John G. Thompson,* Julieanne Dougherty Alexandra Melnitchenko Charlene Lob0 and Ray L. Withers Research School of Chemistry Australian National University Canberra AC T0200 Australia Na8Mg3Si,024(0H)2 and Na,Mg3Si902,(OH,Cl)2 have been prepared by solid state reaction at ca. 700 "C in air. These new materials have the sodalite structure type with I-centred cubic unit cells of dimensions a =9.059(2) and 8.989(1)A,respectively. The magnesium and silicon atoms in the sodalite framework are disordered with the interstitial sodium ions being partially exchangeable at 85 "C. These materials represent only the second type of magnesiosilicate framework structure reported. Materials with the sodalite structure while not generally considered as belonging to the family of zeolites are nonethe- less very closely related.The building block of the sodalite structure is a cuboctahedron of corner-connected tetrahedra or P-cage (see Fig. l),which is also a component of zeolites A X and Y. For this reason and because many of the sodalites display base exchange and/or interstitial water they are treated together with zeolites.' Sodalite itself has the chemical composition Na8Al6Si6024C12 which Can be rewritten as NagC12(A16Si6O2,) to represent the composition of the cuboctahedral interstices and framework respectively. The sodalite structure is chemically highly adaptable in terms of substitution into both the framework and the cuboc- tahedral interstices.Sodalite-like structures have been reported related with compositions at or close to Na2MgSi0,23-25 or K2MgSi04,26 or tridymite-related with composition K,MgSi308 .26 The proposed magnesium analogue of the framework aluminosilicate analcime namely doranite has recently been di~credited.~~ Zeolite and molecular sieve research has only produced a handful of magnesium-substituted materials where a significant level of magnesium is demonstrably located in the tetrahedral framework. All of these involve substitution of magnesium for aluminium into aluminophosphate frameworks. They have been variously labelled as and MAPOS.~' Notably approximately one third seems to be the maximum level of substitution of magnesium for aluminium.31P MAS NMR spectroscopy has been used to confirm that magnesium occurs in the framework and not in the interstices. A similar with framework compositions (A112024),2 (Alo.03Gao~97Sil~28),3 level of substitution was (A16P6024 (A12Be2Si8024 ),5 ( 12024 [Be6(AS,P)60241,7 (Be6Ge6024),8 (Be$i6024),9'10 (Ga6Si6024)," (si12024),12313and [Zn,( P,AS~)O~~].'~ A wide variety of species can occupy the cuboctahedral interstices including halides chalcogenides and oxyanions,' which together with cations such as Li+ Na+ K+ Rb+ Ag+ Ca2+ Zn2+ Mn2+ Co2+ and Cd2+ reported for the large-pore mag- nesium-containing aluminophosphate DAF- l.31 Substitution of magnesium for aluminium in alumino-phosphate framework structures compared with aluminosilic- ate structures is considered more likely for two reasons.First a magnesiosilicate tetrahedral framework would possess a much higher negative charge than magnesiophosphate requir- provide charge balance for the negatively charged framew~rk.~ ing a very high positive charge density in the interstice for In the pure silica sodalite frameworks the interstices may be charge balance. However this should not be an absolute filled by charge neutral templates such as ethylene glyc01,~~*'~ impediment as pure aluminate sodalites e~ist.~,~~ Secondly the 1,3,5-trio~ane'~.'~and dioxolane,16 or in highly siliceous alumi- nosilicate sodalite frameworks by low charge density templates such as TMA+.17-22 Among the many reported sodalite analogues to date there have been no magnesium silicates.This is not surprising as magnesium has a strong preference for octahedral coordination in oxide structures. The only magnesium silicate structures where magnesium is tetrahedrally coordinated are cristobalite-aluminium 0 silicon Fig. 1 Schematic representation of the cuboctahedron of corner-connected tetrahedra or P-cage for sodalite Na,Al,Si,O,,Cl showing the exact Si:Al ordering. The oxygen atoms linking the aluminium and silicon atoms are omitted for clarity. metal-oxygen bond length in an idealised MgO tetrahedron is closer to that in AlO than that in SiO,; the idealised metal- oxygen bond 1:ngths are derive! from bond valFnces giving &-o= 1.949 A dAl-o= 1.757 A &-o= 1.624 A.33 There are other reasons why framework magnesiosilicates might not be prepared.We have already mentioned the crystal chemical preference of magnesium for higher coordination. Another reason is the relative insolubility of magnesium under alkaline conditions which precludes conventional preparative routes for zeolites. Therefore it is not surprising that the new materials Na8Mg,Si902,(OH) and Na,Mg3Si90,,(0H,C1) which are the subject of this paper could only be prepared by solid-state reaction. The syntheses of these new members of the sodalite family their characterisation and cation exchange properties are described below. Experimental Synthesis Magnesiosilicate analogues of both sodalite and hydroxysoda- lite have been prepared.Both solid-state and hydrothermal reaction conditions were investigated though only the solid- state processes were successful in yielding the desired products. AR grade reagents [NaNO NaOH MgCl Mg(NO,),] were reacted with various sources of silica under a range of con- ditions The sources of silica used were sodium silicate solution (Aldrich ca 14 mass% NaOH ca 27 mass% SiO,) colloidal silica (Ludox AM du Pont ca 30 mass% SiO,) talc and a commercial magnesium silicate gel (Florisil U S Silica 15 5 mass% MgO 840 mass% SiO,) Detailed descnptions of successful syntheses are given below as well as a brief summary of unsuccessful strategies Magnesiosilicate analogue of hydroxysodalite Na8Mg,Si,0zlr(OH)2To a solution of 2 82868 NaNO and 42662g Mg(NO3),*6H,O in 10ml H,O log of colloidal silica (Ludox AM) was added dropwise with stirring resulting in the formation of a gel A solution of 0448 g of NaOH in 4ml of H20 was added to the stirred gel causing the gel to thicken Stirring was continued for ca 20 min the gel was then dehydrated at 130 "C overnight The powdery product was ground divided into three portions of 2 7 g and pelleted using 10 tonnes uniaxial pressure on a 12mm die Apart from a small proportion of crystalline NaNO the material prior to pressing was XRD-amorphous.The pellets were placed in a furnace at 250"C then ramped to 650°C over a period of 2 h and held at this temperature for 1 2 and 3 days respectively XRD patterns of pulverised pellets showed that the sodalite-type material was the predomi- nant phase with crystallinity increasing with heating time A small quantity of amorphous material was also observed in the XRD patterns but the proportion decreased with longer heating A small amount of a P-cristobalite-type phase was also produced and was minimised in the sample heated for 2 days The XRD profile of this matenal which is discussed in more detail later is given as Fig 2(a).The ,'Si MAS NMR 2Bl degrees (Cu-Ka) Fig.2 X-Ray powder diffraction patterns of (a) Na,Mg,Si902,(0H)2 and (b) Na,Mg,Si,O,,(OH,Cl) whose syntheses are described in detail m the Experimental section 1934 J Muter Chern 1996 6(12) 1933-1937 spectrum and ion exchange data for this sample are also described later.Further heating of repelleted material at higher temperatures (665 "C) for short times resulted in a slight improvement in crystallinity and reduction in P-cristobalite-type phase content but at higher temperatures (2675 "C) the magnesiosilicate sodalite material began to decompose in favour of the amorph- ous material. Attempts to prepare the magnesiosilicate sodalite by solid state reaction using other reactive forms of silicate (eg gel prepared from sodium silicate solution) or magnesiosilicate (Florisil delaminated talc) were all unsuccessful though the reason for this is not understood The main crystalline products in these cases were P-cristobalite-type phases34 and sodium silicate Na,SiO Attempts to synthesise the magnesiosilicate sodalite material from colloidal silica without pelleting the reaction mixture before firing were also unsuccessful resulting in similar unwanted products Magnesiosilicate analogue of sodalite 'Na8Mg&,0&1,' To a stirred solution of 3 772 g NaNO 1 128 g MgC1 *6H,O and 2 844 g Mg(NO,) *6H,O in 8 5 ml H,O 10 g of colloidal silica solution (Ludox AM) was added slowly forming a gel A further 15 ml of H20 was used to rinse the beaker Stirring of the gel was continued for ca 20 min the gel was then dehydrated at 110 "C overnight The reaction mixture was pelleted as above and fired at 650°C for 4 days The product at this stage contained some cristobalite-related impurities which were removed by heating the specimen at 795°C for 1 h The principal product observed by XRD [Fig 2(b)] was sodium magnesiosilicate sodalite with a small impurity of NaCl and some XRD-amorphous material contributing to the background.Attempts at hydrothermal synthesis. Attempts to synthesise sodium magnesiosilicate sodalite using hydrothermal con-ditions were made using reaction conditions similar to those reported for the hydrothermal synthesis of hydroxysodalite 3s Reactions were carried out in a Teflon-lined stainless-steel autoclave fitted with an internal Teflon-coated thermocouple Various reaction mixtures were heated at 100-200 "C under ambient pressure for periods of 1-12 days (up to 14 days) In all hydrothermal preparations a five-fold excess of Na' over that required by the desired stoichiometry was used as for many standard sodalite preparations The tetrapropylam-monium cation (TMA') commonly used as a template for the formation of the sodalite structure was included in some of the hydrothermal preparations In those preparations using TMA' where temperatures above 100 "C were used glycerol was added TMA+ had been used successfully in the synthesis of MAPO-20 a partly magnesium substituted aluminophosph- ate with the sodalite-type structure.Reactions using colloidal silica and sodium silicate solution produced an amorphous product whereas those using ball milled talc and Florisil gave a poorly ordered vermiculite-like reaction product In none of these experiments were there any signs of framework magnesiosilicate formation.Characterisation Specimens were examined initially by X-ray powder diffraction using a Siemens D5000 diffractometer with Cu-Ka radiation (A= 15418 A) To obtain accurate unit-cell dimensions selected specimens were further examined using a Guinier-Hagg camera with monochromated Cu-Ka radiation (A= 1 5406 A) with Si (NBS No 640) as an internal standard. Selected area electron diffraction patterns (SAEDPs) from sodalite-type containing specimens were obtained in a JEOL 1OOCX transmission electron microscope (TEM). Finely ground specimens were dispersed onto a holey carbon grid for examination in the TEM. Close to single-phase specimens of the sodalite-type material were also studied using solid-state ,'Si NMR spectroscopy.29Si MAS NMR spectra were obtained using a Bruker MSL400 spectrometer operating at 79.488 MHz. Samples were spun at the magic angle at a frequency of 4.2 kHz in Bruker double- air bearing rotors and the spectra collected using the single- pulse excitation technique with various recycle times. The compositions of the reaction products were analysed quantitatively using energy dispersive X-ray spectroscopy (EDS) in a JEOL 6400 scanning electron microscope (SEM). Analyses were made at 15 kV and 1 nA using a Link ATW detector (138 eV resolution) and data processed using the Link ISIS system. ZAF corrections were made using the SEMQUANT software package. Ion exchange Small quantities of Na8Mg3Si902,(OH) were treated with solutions containing a 100-fold excess of Li+ ,K+,Rb' Cs+ Ag+ and T1+ at 85°C to assess the exchangeability of the interstitial Na+ ions.For K+ ,exchange was also attempted at 40 "C. Following exchange the specimens were rinsed thor- oughly with distilled water dried at ca. 100°C then analysed using EDS in the SEM. Results Synthesis Sodium magnesiosilicate sodalite (Na-MgSOD) was first observed by solid-state synthesis using colloidal silica as reagent but all attempts to synthesise the material using other sources of silica were unsuccessful. The sodalite-type material could only be prepared over a narrow range of conditions; 650-700°C with heating times of 1-4 days or 700-800°C with heating times of several hours. Higher temperatures or longer heating times led to the decomposition of the Na- MgSOD.We were not able to prepare the sodalite-type material or any other framework magnesiosilicate for that matter using hydrothermal conditions. XRD The XRD profiles (Fig. 2)t of the Na-MgSOD specimens whose synthesis is described above in detail both showed great similarity to the XRD data reported for sodalite and hydroxy- sodalite. In each case the observed peaks could be fitted to a body-cenped cubic (bcc) unit cell with a=9.059(2) and 8.989( 1) A for Fig. 2(a) and (b) respectively. The cubic dimen- sions were-slightly larger than those reported for sedalite [a = 8.8784(4) A]36 and hydroxysodalite [a =8.750(1) A].36 There was no evidence from the XRD profiles of violation of the body-centring condition h +k +1=2n.The Na-MgSOD speci- men shown in Fig. 2(a) contains two small peaks corresponding to a small impurityoof a P-cristobalite-like phase of cubic cell dimension a =7.32 A. The specimen in Fig. 2( b) was unrinsed and contained a small amount of NaCl byproduct. Inspection of the XRD profiles in Fig. 2 also reve5ls a broad background "hump centred on 28~28 (ca. 3.2 A) indicative of some remnant glassy material being present in each case though relatively less in Fig. 2(a). 7 XRD data for Na,Mg,Si,O,,(OI.f) [Fig. 2(a)] and Na,Mg,Si,O,,(OH,C1) [Fig. 2( b)] are available as supplementary data (SUP. No. 57176) from the British Library. Details are available from the Editorial Office. TEM As the sodium magnesiosilicate sodalite materials decompose rapidly in the electron beam and the crystal domain size is relatively small (< 1pm) it is not possible to obtain high quality electron diffraction patterns.The recording of conver- gent beam electron diffraction patterns to circumvent the small domain size was not possible owing to the rapid beam damage. Fig. 3 presents SAEDPs for the three principal zone axes of Na8Mg3Sig0,,(OH),~ .All diffraction patterns could be indexed to a bcc unit cell of dimension a z9.05 A. No further extinction conditions were observed consistent with both 143m and lm3m which are possible for sodalite-type structure^,^^ in agreement with the XRD data. ,'Si NMR spectroscopy Fig.4 shows the 29Si MAS NMR spectra for the Na,Mg,Si,O,,(OH) specimen shown in Fig. 2(a).The spec- trum consists of a broad composite signal centred on 6 ca. -88 (relative to Me,Si) which can be simulated by a series of Gaussian peaks as shown. The positions half-widths and relative intensities are listed in Table 1. As there are no published 29Si NMR data for framework magnesiosilicates due mainly to there being almost no reported examples it is only possible to interpret the spectra by analogy with framework aluminosilicates which display a stepwise change in chemical shift with change in c~ordination,~~.~' and which to a first approximation is independent of the type of framework. The deconvolution presented in Fig. 4 and Table 1 is pro- posed on the basis that the spectra are dominated by the various Si(nMg) (n=0-4) environments with a 6 ca.7 increase in chemical shift with each substitution compared to 6 ca. 6 for Si(nA1) in framework aluminosilicates.39~40 The assignment is made on the basis that there is a silica-rich XRD amorphous component at low frequency. Further argument to support this assignment is presented in the Discussion section. SEM/EDS Characterisation of the products of successful syntheses in the SEM revealed a glass-like mass with no obvious crystal formation. Quantitative microanalysis of the various specimens Fig. 3 Selected area electron diffraction pstterns (SAEDPs) for Na8Mg,Si,0,,(OH) along (a) (Ool) (b) (110) and (c) (111) type zone axes. Owing to the small single-crystal domain size and rapid damage in the electron beam the SAEDPs are unavoidably contami- nated with diffraction spots from neighbouring crystal domains.J. Muter. Chem. 1996,6( 12) 1933-1937 1935 -88 0h9 calculated residual ... ... ... ... .. -60 -iO -40 -1io 3 s(29sl) Fig.4 29S1 MAS NMR spectrum for Na,Mg,Si,O,,(OH) as shown in Fig 2(a) collected on a Bruker MSL400 at 79 468 MHz A sample spinning speed of 4 2 kHz was used with a recycle time of 20 s A simulated spectrum comprising five Gaussian peaks is Juxtaposed together with the residual spectrum when the simulated spectrum is subtracted from the observed Details of the simulated spectrum are presented in Table 1 Table 1 Details of simulated 29S1 MAS NMR spectrum for Na,Mg,Si,O,,(OH) in Fig 4 6 FWHM observed' (%) calculatedb (YO) Sl(4Mg) -00 04 Si(3Mg) -740 6 5 26 47 Si(2Mg) -81 5 65 21 9 21 1 Si(1Mg) -882 65 43 8 42 2 Si(0Mg) -954 65 31 7 31 6 -'Silica' -101 0 100 27 7 'Only Si(nMg) (n=0-4) signals included in summation Calculated using binomial theorem for random distnbution of Si and Mg confirmed within error the proposed magnesium silicon ratio ze 1 3 for both the sodalite and hydroxysodalite analogue specimens However the analytical volume of spot analyses (several pm3) was almost certainly greater than the crystallite size and therefore could not alone provide conclusive evidence for the composition of these new materials gven the presence of some XRD amorphous material Microanalysis of the sodalite analogue whose XRD profile is shown in Fig 2(b) after rinsing to remove the NaCl byproduct confirmed the presence of chlorine in the structure but only about half of that required for charge balance suggesting an actual composi- tion closer to Na,Mg3Si9O2,C1(OH) Owing to variability in the C1 content we represent the formula for this material as N%Mg3S19024 (C1,OH )Z Ion exchange At 40 "C Na,Mg,Si,O,,(OH) showed negligible Na+-exchange capacity in the 100-fold excess K+ solution However at 85"C partial exchange (up to cu 50%) was observed for Li' K+ Rb' Cs' Ag+ and T1+ With none of the exchange solutions was complete exchange observed In hydroxysodalite complete exchange was observed at 85°C for Li' Na+ and Ag' ,and partial exchange for K+,T1+,Rb' and Cs' Our results indicate that the interstitial sodium is less exchangeable in Na8Mg3Si902,(OH) than in hydroxysodalite Discussion The combined XRD and electron diffraction data for these new materials provide strong evidence for the formation of magnesiosilicate analogues of hydroxysodalite and sodalite given the composition of the reaction mixture The only other possibility in the absence of aluminate would be the formation of a pure silica or highly siliceous sodalite While this would be highly implausible in the absence of an organic template the unit-cebl dimensions further preclude tkis as the observed 9 05-9 00 A are much larger than the 8 83 A reported for pure silica sodalite 42 The increased unit-cell dimensions of the Na- MgSOD materials over hydroxysodalite and sodalite are quite consistent with the presence of 25% MgO tetrahedra in the sodalite framework Depmeier3 has presented the structural relationships between the various symmetries observed for members of the sodalite family The highest possible symmetry is lmh but this is rarely observed as the ideal tetrahedral framework usually distorts by means of concerted rotation of the tetra- hedra about their 4 axes The resultant space-group symmetry is I43m Ordering of the tetrahedral framework atoms for the I43m structure preserves the cubic symmetry but destroys the body centring The resultant space-group symmetry is then P43n Such lowering of symmetry would occur in Na-MgSOD if the composition of the tetrahedra alternated strictly between (Mg 5Sio 5)0 and SiO making the two sites inequivalent However our XRD and electron diffraction data demonstrate that our Na-MgSOD materials conform to bcc symmetry which requires that there is no Mg Si ordering in the frame- work Givtn that the cubic cell dimension is significantly less than 9 50 A which is the theoretical dimension for an uncol- lapsed framework of composition Mg,2sSio75 it is most probable that the correct symmetry for Na-MgSOD is 143m This is further supported by the close similarity in unit-cell dimensions and relative intensities of XRD peaks between the two Na-MgSOD materials and their aluminosilicate ana-logue~,~~37 indicative of similar degrees of collapse Having established the probable space group for Na-MgSOD we can consider further the 29S1 NMR data If we assume a random distribution of Mg and Si within the framework we can calculate the expected distribution of Si(nMg) (n=0-4) environments using the binomial theorem for MgxSil -where Si( OMg) =(1-x) Si( 1 Mg) =4x( 1-x)~ Si(2Mg)=6~~(1-~)~,Si(3Mg)=4x3(1-x) Si(4Mg)=x4 A framework composition of Mg 25Sio 75 would give a distri-bution of intensities as follows Si(OMg)=31 6% Si( 1Mg)= 42 2% Si(2Mg)=21 1% Si(3Mg)=4 7% Si(4Mg)=O4% Deconvolution of the spectrum of Na,Mg,Si,O,,(OH) shown in Fig 4 allowing for the presence of some silica-rich XRD- amorphous matenal gave relative intensities of 31 7% 43 8% 21 9% 26% and 0% if our proposed peak assignment is correct While there is always an element of subjectivity in spectrum deconvolution it would be fair to propose that the 29S1 NMR data were consistent with a random distribution of Mg and Si on the tetrahedral framework cation sites in Na8Mg3S19024(0H 12 A number of reasons were proposed in the introduction as to why framework magnesiosilicates might not be prepared Our inability to prepare Na-MgSOD or any other framework magnesiosilicates by hydrothermal methods up to 200 "C agreed with these propositions This does not preclude the possibility that Na-MgSOD might be prepared hydrothermally under more rigorous conditions Hydroxysodalite can be synthesised hydrothermally at 450-750 "C and 1000 kg cmP2 pressure 43 Also Shannon24 succeeded in growing hydrother- mally crystals of the cristobalite-related framework magnesios- ilicate Na2MgSi04 at 700 "C and 3000 atm pressure That Na-MgSOD forms at all by our solid-state synthesis is almost certainly due to the pelleting of the reaction mixture This assists in the retention of the water required in the structures of both NasMg3Sig024(OH)2 and Na8Mg3Si9024 (OH,Cl) Without pelleting the phases do not form While other sodalites can be synthesised by solid-state none of these require water in the structure We attnbute the 10 11 12 13 14 15 16 J J Glass R H Jahns and R E Stevens Am Mineral 1944 29,163 L B McCusker W M Meier and K Suzuki Zeolites 1986,6,388 D M Bibby N I Baxter D Grant-Taylor and L M Parker in Zeolite synthesis ed M L Occelli and H E Robson ACS Symp Ser 398 ACS Washington DC 1989 ch 15 p 329 D M Bibby and M P Dale Nature (London) 1985,317,157 T M Nenoff W T A Harrison T E Gier and G D Stucky J Am Chem SOC 1991,113,378 J Keijsper C J J den Ouden and M F M Post in Zeolites Facts Figures Future ed P A Jacobs and R A van Santen Elsevier The Netherlands 1989 p 237 K Futterer W Depmeier F Altorfer P Behrens and J Felsche ready decomposition of Na-MgSOD above 750-800 "C to water loss Structural water loss has been reported to occur at 860 "C in hydroxysodalite 43 17 18 Z Kristallogr 1994,209 517 R H Jarman J Chem SOC Chem Commun 1983,512 B J Schoeman J Sterte and J-E Otterstedt Zeolites 1994 14 208 19 C J J den Ouden K P Datema F Visser M Mackay and Conclusion 20 M F M Post Zeolites 1991,11,418 P D Hopkins in Molecular Sieues Vol 1 ed M L Occelli and Our lack of success in reproducing the synthesis of Na-MgSOD using synthesis strategies other than by solid-state reaction from colloidal silica reagent descnbed in the present work 21 22 H Robson Van Nostrand Reinhold New York 1992 ch 11 p129 R M Barrer and P J Denny J Chem SOC 1961,971 C Baerlocher and W M Meier Helv Chim Acta 1969,52 1853 suggests that the preparation of other magnesiosilicate zeolite analogues will be difficult The temperatures necessary to promote solid-state reaction preclude the use of organic tem- plates.This limitation requires that the framework is built around thermally stable inorganic species such as those which 23 24 25 26 C M Foris F C Zumsteg and R D Shannon J Appl Crystallogr ,1979 12,405 R D Shannon Phys Chem Miner 1979,4,139 W H Baur T Ohta and R D Shannon Acta Crystallogr Sect B 1981,37,1483 E W Roeder Am J Sci ,1951,249,224 have been used in other solid-state syntheses of sodalite-type materials inevitably restricting the possible structures to small cage frameworks with non-exchangeable positively charged moieties occupying the interstices Also it is clear that for hydrothermal synthesis to succeed it will require much higher temperatures and pressures than were available to us again 27 28 29 30 D K Teertstra and A Dyer Zeolites 1994,14,411 P J Barrie and J Klinowski J Phys Chem ,1989,93,5972 F Deng Y Yue T Xiao Y Du C Ye L An and H Wang J Phys Chem ,1995,99,6029 S T Wilson and E M Flanigen in Zeolite synthesis ed M L Occelli and H E Robson ACS Symp Ser 398 ACS Washington DC 1989 p 329 preventing the use of conventional organic templates Nevertheless our synthesis of a zeolitic magnesiosilicate frame- work demonstrates that such structures are possible and further work in this area may well reveal further zeolite analogues 31 32 33 P A Wright R H Jones S Natarajan R G Bell J Chen M B Hursthouse and J M Thomas J Chem SOC Chem Commun 1993,633 W Depmeier Phys Chem Miner 1988,15,419 N E Brese and M O'Keeffe Acta Crystallogr Sect B 1991 47,192 References 34 R L Withers C Lobo J G Thompson and S Schmid Acta Crystallogr Sect B in press R M Barrer Hydrothermal chemistry of zeolites Academic Press 35 36 R M Barrer and E A White J Chem SOC,1952,1561 JCPDS-ICDD file no 37-476 London 1982 p 46 37 JCPDS-ICDD file no 40-100.4 V I Ponomarev D M Kheiker and N V Belov Sou Phys Crystallogr ,1971,15,799 D E W Vaughan M T Melchior and A J Jacobson in Intrazeolite Chemistry ed G D Stucky and F G Dwyer ACS Symp Ser 218 ACS Washington DC 1983,ch 14 p 231 S T Wilson B M Lok C A Messina T R Cannon and E M 38 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New York 1971 9 S E Dann and M T Weller Inorg Chem ,1996,35,555 Paper 6/04946J Received 15th July 1996 J Mater Chern 1996 6(12) 1933-1937 1937