Title of Invention

"A PROCESS FOR THE PREPARATION OF TIN SILICATE MOLECULAR SIEVE "

Abstract

A process for the preparation of tin silicate molecular sieve by mixing aqueous solutions of sources of tin, silicon and a mixture of cetyl trimethyl ammonium hydroxide/chloride and heating the resultant gel in autoclave at autogeneous pressure, at temperatures in the range of 80-140°C for a period in the range of 3-10 days till crystallization is complete, filtering, washing with deionized water and drying the resultant composite material at a temperature in the range of 80-110JC for a period in the range of 12 to 20 hrs., calcining at a temperature in the range of 550 to 600°C for a period ranging between 2-8 hours in an inert atmosphere followed by calcining at the same temperature for a period range of 3-10 hours in air to obtain catalytically active crystalline stannosilicate.

Full Text

This invention relates to a process for the preparation of tin silicate molecular sieve. More particularly, the invention relates to the preparation of a crystalline, mesoporous tin silicate molecular sieve having tin as a part of the mesoporous structure, belonging to the M-41S family, useful as a catalyst for the transformation of organic compounds. Amorphous and paracrystalline materials (porous inorganic solids) have been used for many years in industrial applications. Typical examples of these materials are the amorphous silicas commonly used in catalyst formulations and the paracrystalline transitional aluminas used as solid acid catalysts and petroleum reforming catalyst supports. The size of the pores in amorphous and paracrystalline materials falls into a regime called the mesoporous range (1.3 to 20 nm). Recently a new class of crystalline silicate materials, called M-41S, with pores in the range of 14-100A have been reported (US Pat.). It was observed that the most regular preparations of silicate analogs of the above materials (called MCM-41) exhibit a hexagonal X-ray diffraction pattern with a few distinct maxima in the extreme low angle region. The x-ray diffraction pattern, however, is not always a sufficient indicator of the presence of these materials, as the degree of regularity in the microstructure and the extent of repetition of the structure within individual particles affect the number of peaks observed. Indeed, preparations with only one distinct peak in the low angle region of the x-ray diffraction pattern have been found to contain substantial amounts of the material of the invention. In this preferred arrangement, the porosity of the crystalline material of the invention is provided by a hexagonal arrangement of pore channels, a property that can be readily observed by electron diffraction and transmission electron microscopy. In the prior art, tin (Sn) substituted zeolites have been prepared by post-synthesis methods. For example, via treatment of a parent aluminosilicate zeolite with ammoniurr fluoride salt of tin or chromium as described in US Patent Appl. No. 133, 372 (Dec. 1987) and European Patent application no. 321,177 (Dec. 1988) (both to UOP, Inc). Extraction of aluminium from the framework and insertion of Cr or Sn by secondary synthesis process are described in these patents. Another US Patent No. 4, 933, 161 (to Exxon R & E Co) describes the preparation of faujasite type and other aluminosilicate zeolites containing tin in their tetrahedral framework by treatment of the respective zeolites with SnC12 or SnCU or SnCU/HCl depending on the Si/Al ratio in the parent zeolites at higher temperatures which is again a post-synthesis procedure. N.J. Tapp. et al. describe a method of preparation of a tin containing AlPO4-5 molecular sieve (SnAPO-5) and its characterization (Zeolites. Vol.10, 1990, P. 680), wherein the substitution of Sn (IV) for P (V) within the A1PO4-5 structure in tetrahedral coordination is demonstrated. Other Sn-containing molecular sieves designated as Sn-A and Sn-B and a novel tin sulfide molecular sieve named as SnS-1 are mentioned in the literature (see for example, Hand Book of molecular sieves. Ed. R. Szostak. Van Nostrand Reinhold, New York 1992. p. 434) but are not described in detail. Various attempts have been made to substitute tin into a zeolite framework via primary synthesis methods. Attempts to synthesize zeolites particularly of the pentasil family (ZSM-5 like) with a number of ions other than aluminium have been made. Dwyer et ?!. in US Patent No. 3,941,871 describe the presence of Sn in place of 01 as a part of the organic template in a ZSM-5 type of a structure but not as a part of the ZSM-5 framework structure itself. In US Patent No. 4, 329, 328 (McAnespic et al.) the synthesis of stannosilicate is suggested but no example of such a synthesis are given nor are any properties of such materials suggested. It was presumed that tin is not a part of the zeolite framework in primaiy synthesis products because at high pH conditions required for such synthesis, it is probable that tin or such metals precipitate as oxides and/or hydrous oxides much before their crystallization linking with Si-O species to form micr;>porous material wherein tin or such metals become a part of the zeolite framework The precipitation of metals such as tin, iron, chromium, etc., in alkaline solution can be prevented by complexing it with oxo-anions like the oxalates, citrates, EDTA and tartarates. It is feasible to prevent the polymerization and precipitation of these ions (as hydroxides, oxo-hydroxides or oxides) in basic media, provided these ,ons are suitably complexed with appropriate ligands. This factor is important in the primaiy synthesis procedure which is earned out in aqueous, alkaline medium under hydrothermal conditions. It is crucial to prevent the precipitation of the oxo/hydroxide complexes under such conditions and facilitate the incorporation of ions such as Sn possibly in the framework of the zeolite. Mesoporous molecular sieves (MCM-41) reported in the prior-art were prepared using aluminium (R.B. Borade and A. Clear-field, Catal. Lett., 1995, 31, 267), iron (Zhong Yuan el al. J. Chem. Soc., Chem. Commun., 1995, 973), titanium (A. Corma et al J. Chem. Soc., Chem. Commun., 1994, 147) and vanadium (K. M. Reddy et al. J. Chem. Soc., Chem. Commun., 1994, 1059) ions. Accordingly, the present invention provides a process for the preparation of tin silicate molecular sieve having composition in terms of mole ratios of oxides by the formula : (Formula Removed) wherein Ri is selected from dodecyl trimethyl ammonium bromide, myristyl trimethyl ammonium bromide, cetyl trimethyl ammonium bromide, cetyl trimethyl ammonium chloride, the preferred ones are cetyl trimethyl ammonium chloride and tetramethyl ammonium hydroxide and R2 is selected from tetraethyl ammonium hydroxide, tetrabutyl ammonium hydroxide, tetramethyl ammonium hydroxide preferably tetramethyl ammonium hydroxide, w is between 0.002 to 0.05 and z is from 5-100 which comprises mixing aqueous solutions of sources of tin, silicon and a mixture of cetyl trimethyl ammonium hydroxide/chloride in the molar compositions in terms of ratios of oxides as under: H2O/ SiO2 = 5-100, H2O/ SnO2 = 1000-2000, H2O/(Ri)2O = 125-550, H2O/(R2)2O = 40-550. mixing and heating the resultant gel in autoclave at autogeneous pressure, at temperatures in the range of 80-140°C for a period in the range of 3-10 days till crystallization is complete, filtering, washing with deionized water and drying the resultant composite material at a temperature in the range of 80-110°C for a period in the range of 12 to 20 hrs., calcining at a temperature in the range of 550 to 600°C for a period ranging between 2-8 hours in an inert atmosphere followed by calcining at the same temperature for a period range of 3-10 hours in air to obtain catalytically active crystalline stannosilicate. In a preferred embodiment of the present invention, the catalyst composite material produced has the chemical composition in terms of mole ratios of oxides as follows : x [(R1)2O]: y [(R2)2O]: w SnO2: SiO2: z H2O where w is between 0.003 to 0.04, x and y below 0.3 and z is between 5-80. According to a preferred feature of the present invention the starting gel has a composition in terms of mole ratios as follows : 0.089 (R1)2O : 0.155 (R2)2O : 0.02 SnO2 : SiO2 : 40 H2O where R1and R2 are organic templates In one of the embodiments of the present invention the template RI used in the reaction mixture in the preparation of the material of present invention can be dodecyl trimethyl ammonium bromide (DTMABr), myristyl trimethyl ammonium bromide (MTMABr), cetyl trimethyl ammonium bromide (CTMABr), cetyl trimethyl ammonium chloride (CTMAC1), In one of the embodimetits of the present invention the template R2 is tetraethyl ammonium hydroxide.(TEAOH), tetrabutyl ammonium hydroxide (TBAOH), tetramethyl ammonium hydroxide (TMAOH). The preferred templates used in the synthesis were cetyl trimethyl ammonium chloride and tetramethyl ammonium hydroxide In another embodiment of the present invention the stannosilicate catalyst composite material is prepared by crystallizing an aqueous mixture of source of tin, source of silicon and cetyl trimethyl ammonium chloride/hydroxide and tetramethyl ammonium hydroxide (TMAOH). Typically the mole ratios of the various reactants can be varied to produce stannosilicates of this invention. By regulation of the quantity of the tin ions in the reaction mixture, it is possible to vary the Si/Sn molar rat os in the final product in range from about 50 to 200. In yet another embodiment of the present invention the source of silica in the reaction mixture used in the preparation of the stannosilicate of this invention is selected from silicon dioxide, silica hydrosol, silica gel, silicic acid, alkoxides of silicon, alkali metal silicates preferably tetramethyl ammonium silicate (TMA-silicate). In another embodiment of the present invention the source of tin in the reaction mixture used in the preparation of the material of the present invention can be tin (IV) acetate, tin (IV bromide, tin (IV) chloride, tin (IV) fluoride. A preferred source of tin is stannous tetrachloride (SnCU. 5 HbO). The stannosilicate of the present invention can be prepared by mixing in any order the source of silicon, tin, cetyl trimethyl ammonium hydroxide and water. The resulting mixture is then heated in an autoclave and maintained at a temperature between 80~140°C for 3-10 days. After crystallization is over, the contents of the autoch.ve are quenched in cold water and filtered, washed thoroughly by deionized water and dried at 120°C for 12 hours. Then this material so obtained, is calcined preferably at 550°C for 2-10 hours in nitrogen atmosphere followed by calcining in air at the same temperature for 3-15 hours. Even though the catalyse composite material thus obtained may be directly used as a catalyst, it is desirable in many large scale applications to enhance its mechanical strength and ease of handling by mixing it with a suitable binder material and converting into a suitable shape such as cylindrical extrudates, spheres etc. Examples :of suitable material which impart improved mechanical properties of the catalyst material of this invention include acetic acid, stearic acid, silica, alumina, silica-alumina, clay minerals such as bentonite, kaolinite, kiesulghur and mixtures thereof. The structure of the prepared stannosilicate by the process of the present invention can be characterized by different techniques, the x-ray powder diffraction pattern is set forth as shown in Table 1 and the framework infra red spectrum set forth as shown in Table 2. The calcined form of the stannosilicate manifests substantially the same pattern with some minor shifts in the interplanar spacing and variation in relative intensity. Other minor variations can occur depending on the Si/ Sn ratio of the particular sample and its thermal history. The stannosilicate of the present invention is seen to have an x-ray diffraction pattern similar to that of MCM-41 molecular sieves. The novel stannosilicate catalyst composite material prepared by the process of this invention has molecular sieve preparation property analogous to other known molecular sieve. The stannosilicate may be characterized by its adsorption capacity for molecules of various sizes. Typical results are shown in Table 3. The uptake of water, benzene, cyclohexane and n-hexane indicate that the stannosilicate has significantly hydrophilic voids. Table 3 : Sorption capacity of Sn-MCM-41 sample at 25°C and p/p0 = 0.5 (Table Removed) * = Leonard Jones kinetic diameter The process of the present invention will be further described with reference to the following examples which are illustrative only and should not to be construed to limit the scope of the present invention in any manner. Example 1 This example illustrates the preparation of silicalite MCM-41 using the hydrothermal gel with the following molar composition : SiO2 : 0.089 (CTMA)2O : 0.155 (TMA)2O : 0.09 (NH4)2O : 40 H2O 3.6 g ammonium hydroxide (25% solution) diluted with water (25 g) was added to 32.8 g solution of cetyl trimethyl ammonium chloride (25% solution, Aldrich) with stirring. To this mixture, 4.16 g tetramethyl ammonium hydroxide, TMAOH. 5H2O (99% Aldrich) was added followed by the addition of 34.6 g tetramethyl ammonium silicate (10% SiO2, SACHEM Inc.). This thick gel was allowed to stir for 20 minutes. 6.2 g fumed silica (99% SiO2. Sigma) was added slowly to the above gel under stirring and the mixture was stirred for 1 hour. The pH of the mixture was 11.5. The gel was then transferred to a stainless steel autoclave and crystallization was carried out at 400K for 7 days to complete the crystallization. After the crystallization, the products were filtered, \vashed with deionized water, dried at 373K for 5 hours The product was then calcined at 823K for 1 hour in nitrogen, followed by 6 hours in air. The X-ray diffraction pattern of the calcined products of this example is given in Table 1. Example 2 This example illustrates the preparation of aluminium containing MCM-41 with the following molar composition : SiO2 : x A12O3 : 0.089 (OTMA)2O : 0.155 (TMA)2O : 40 H2O where x 0.174 g sodium aluminate dissolved in water (20 g.) was added to 33.4 g of 24.6% solution of cetyl trimethyl ammonium chloride/hydroxide [prepared by partial exchange of CTMAC1 (Aldrich) over Amberlite IRA-400(OH) (Aldrich) ion-exchange resin] with stirring. 0.08 g NaOH pellet dissolved in 10 g of water and added to the above mixture. To this mixture, 4.16 g tetramethyl ammonium hydroxide, TMAOH. 5H2O (99% Aldrich) was added followed by the addition of 34.6 g tetramethyl ammonium silicate (10% SiO2, SACHEM Inc.). This thick gel was allowed to stir for 20 minutes. 6.2 g fumed silica (Sigma, 99% SiO2) wa;. added slowly to the above gel under stirring and the mixture was stirred for I hour. The pH of the mixture was 11.8. The gel was then transferred to a stainless steel autoclave and crystallization was earned out at 380K for 6 days. After the crystallization, the products were filtered, washed with deionized water, dried at 373K for 5 hours. The product was then calcined at 823K for 1 hour in nitrogen, followed by 6 hours in air. The X-ray diffraction pattern of the calcined products of this example is given in Table 1. Example 3 This example illustrates the preparation of Sn-MCM-41 with the following molar composition : SiO2 : x SnO2 : 0.089 (CTMA)2O : 0.155 (TMA)2O : 40 H2O where x Tin tetrachloride 0.52 g SnC14.5 H2O (Loba Chemie, India) dissolved in water (30 g.) was added to 3.5.4 g. of 24.6% solution of cetyl trimethyl ammonium chloride/hydroxide [prepared by partial exchange of CTMAC1 (Aldrich) over Amberlite IRA-400(OH) (Aldrich) ion-exchange resin] with stirring. To this mixture, 4.16 g tetramethyl ammonium hydroxide, TMAOH. 5H2O (99% Aldrich) was added followed by the addition of 34.6 g tetramethyl ammonium silicate (10% SiO2, SACHEM Inc.). This thick gel was allowed to stir for 20 minutes. 6.2 g fumed silica (Sigma, 99% SiO2) was added slowly to the above gel under stirring and the mixture was stirred for 1 hour. The pH of the mixture was 12.0. The gel was then transferred to a stainless steel autoclave and crystallization was earned out at 410K for 10 days. After the crystallization, the products were filtered, washed with deionized water, dried at 373K for 5 hours. The product waj then calcined at 823K for 1 hour in nitrogen, followed by 6 hours in air. The X-ray diffraction pattern of the calcined products of this example is given in Table 1. The surface area ,of th;: calcined product is 1031 m2/g and the equilibrium sorption capacities are shown in Table 3. Example 4 This example illustrate-, the preparation of Sn-MCM-41 with the following molar composition : SiO2 : x SnO2 : 0.089 (CTMA)2O : 0.155 (TMA)2O : 40 H2O where x 4.16 g tetramethy! ammonium hydroxide, TMAOH. 5H2O (99% Aldrich) was added to 33.4 g of 24.6% solution of ceryl trimethyl ammonium chloride/hydroxide [prepared by partial exchange of CTMACl (Aldrich) over Amberlite IRA-400(OH) (Aldrich) ion-exchange resin] with stirring followed by the addition of 34.6 g tetramethyl ammonium silicate (10% SiO2, SACHEM Inc.). This thick gel was allowed to stir for 20 minutes. 6.2 g fumed silica (99% SiO2, Sigma) was added slowly to the above gel under stirring and the mixture was stirred for 1 hour. Finally 0.26 g tin tetrachloride, SnC14. 5H2O (Loba Chemie, India) dissolved in water (30 g), was added to the above gel and stirred it for another 1 hour. The pH of the mixture was 12.0. This was then transferred to a stainless steel autoclave and crystallization was carried out at 390K for 5 days to ensure complete the crystallization. After the crystallization, the products were filtered, washed with deionized water, dried at 373K for 5 hours. The product was then calcined at 823K for 1 hour in nitrogen, followed by 6 hours in air. The X-ray diffraction pattern of the calcined products of this example is given in Table 1. The surface area of the calcined product is 990 m2/g and the equilibrium sorption capacities are shown in Table 3. Example 5 This example illustrates the preparation of Sn-MCM-41 with the following molar composition : SiO2 : x SnO2 : 0.089 (CTMA)2O : 0.155 (TMA)2O : 40 H2O where x Tin tetrachloride 0.17 g SnCl4.5 H2O (Loba Chemie, India) dissolved in water (30 g.) was added to a mixture 32.8 g cetyl trimethyl ammonium chloride (25% solution, Aldrich) and 3.6 g NH4OH (25% solution) with stirring. To this mixture, 4.16 g tetramethyl ammonium hydroxide, TMAOH. 5H2O (99% Aldrich) was added followed by the addition of 34.6 g tetramethyl ammonium silicate (10% SiO2, SACHEM Inc.). This thick gel was allowed to stir for 20 minutes. 6.2 g fumed silica (Sigma, 99% SiO2) was added slowly to the above gel under stirring and the mixture was stirred for 1 hour. The pH of the mixture was 12.3. The gel was then transferred to a stainless steel autoclave and crystallization was carried out at 370K for 7 days to complete the crystallization. After the crystallization, the products were filtered, washed with deionized water, dried at 373K for 5 hours. The product was then calcined at 823K for 1 hour in nitrogen, followed by 6 hours in air. The X-ray diffraction pattern of the calcined products of this example is given in Table 1. The surface area of the calcined product is 975 m2/g and the equilibrium sorption capacities are shown in Table 3. Example 6 This example illustrate; the preparation of Sn-MCM-41 with the following molar composition ; SiO2 : x SnO2 : 0.089 (C TMA)2O : 0.155 (TMA)2O : 40 H2O where x 3.6 g ammonium hydrcxide (25% solution) in 10 g of water was added to a miture of 9.93 g of cetyl trimethyl ammonium bromide (99% Aldrich) and 24.6 g water with stirring. To this mixture, 4.16 g tetramethyl ammonium hydroxide, TMAOH. 5H2O (99% Aldrich) was added followed by the addition of 34.6 g tetramethyl ammonium silicate (10% SiO2, SACHEM Inc.). This thick gel was allowed to stir for 20 minutes. 6.2 g fumed silica (Sigma, 99% SiO2) was added slowly to the above gel under stirring and the mixture was stirred for 1 hour. Finally 0.52 g lin tetrachloride, SnCl4. 5H2O (Loba Chemie, India) dissolved in water (20 g) was added to the above gel and stirred it for another 1 hour. The pH of the mixture was 11.8. The gel was then transferred to a stainless steel autoclave and crystallization was earned out at 400K for 8 days. After the crystallization, the products were filtered, washed with deionized water, dried at 373K for 5 hours. The product was then calcined at 823K for 1 hour in nitrogen, followed by 6 hours in air. The X-ray diffraction pattern of the calcined products of this example is given in Table 1. The surface area of the calcined product is 978 m2/g and the equilibrium sorption capacities are shown in Table 3. Example 7 In this example, the catalytic activity of these novel Sn-MCM-41 samples synthesized according to procedures described in example 3-5in a selective oxidation reaction, namely the hydroxylation of phenol using H2O2 as the oxidant is described. The catalytic runs were carried out batchwise in a two-necked flask (100 ml capacity) fitted with a magnetic stirrer, a condenser, feed pump and a septum. The temperature of the reaction vassel was maintained using an oil bath. In a standard run 1 g of phenol, 0.1 g of catalyst and 10 g water were placed in a reaction vassel. After the required temperature of 553K was attained. 0.5 ml H;C>2 (25% aqueous solution) was added through the feed pump. The aliquots were taken out periodically for analysis in a capillary GC (HP 5880). Details of conversion of phenol to products like hydroquinone, catechol and benzoquinone under the reaction conditions are given in Table 4. For comperison, the catalytic behaviours of Sn-free MCM-41 and of a Sn-impregnated MCM-41 samples are also included in the Table 4. Under similar reaction conditions, the presently described Sn-MCM-41 samples prepared by hydrothermal synthesis as described in the present invention is more active in this reaction. Table 4 : Activity of the samples in the hydroxylation of phenol" (Table Removed) "Reaction condition : Catalyst = 100 mg, solvent (water) = 10 g, phenol/H2O2 (mol.) = 3, Temp. = 553K, Reaction time = 24 h, Substrate = 1 g. bSi/Sn molar ratio in paientheses, sample synthesized according to example 3. °Si/Sn molar ratio in parentheses, sample synthesized according to example 4. dSi/Sn molar ratio in paientheses, sample synthesized according to example 5. eSynthesized according '.o example 1. fBQ = benzoquinone; CAT = catechol; HQ = hydroquinone. Example 8 In this example, the catalytic activity of these novel Sn-MCM-41 samples with different Si/Sn ratios prepared according to examples 3 to 5 in a selective oxidation reaction, namely, the hydroxylation of 1-naphthol using H2O2 as the oxidant is described. The catalytic runs were earned out batchwise in a two-necked flask (100 ml capacity) fitted with a magnetic stirrer, a condenser, feed pump and a septum. The temperature of the reaction vassel was maintained using an oil bath. In a standard run 1 g of 1 -naphthol, 0.1 g of catalyst and 10 g acetonitrile were placed in a reaction vassel. After the required temperature of 553K was attained. O.'S ml H2O2 (25% aqueous solution) was added through the feed pump. The product aliquots were taken out periodically for analysis in a capillary GC (HP 5880). Details of conversion of 1-naphthol to products like 1,4-naphthoquinone, 1,4-dihydroxy naphthalene and 1,2-dihydroxy naphthalene under the reaction conditions are given in Table 4. For comperison, the catalytic behaviours of Sn-free MCM-41 and of a Sn- impregnated MCM-41 samples are also included in the Table 4. Under similar reaction conditions, the presently described Sn-MCM-41 samples prepared by hydrothermal synthesis as described in the present invention is more active in this reaction. Table 5 : Activity of the samples in the hydroxylation of l-Naphthola (Table Removed) "Reaction conditiop : Catalyst =100 mg, solvent (acetonitrile) = 10 g, 1-naphthol/H2O2 (mql.) = 3, Temp. = 553K, Reaction time = 24 h, Substrate = 1 g bSi/Sn molar ratio ih paientheses, sample synthesized according to example 3. °Si/Sn molar ratio in parentheses, sample synthesized according to example 4. dSi/Sn molar ratio in paientheses, sample synthesized according to example 5. eSynthesized according :o example 1. Si/Sn molar ratio in paientheses. We claim : 1. A process for the preparation of tin silicate molecular sieve having composition in terms of mole ratios of oxides by the formula : (Formula Removed) wherein R-\ is selected from dodecyl trimethyl ammonium bromide, myristyl trimethyl ammonium bromide, cetyl trimethyl ammonium bromide, cetyl trimethyl ammonium chloride, the preferred ones are cetyl trimethyl ammonium chloride and tetramethyl ammonium hydroxide and R2 is selected from tetraethyl ammonium hydroxide, tetrabutyl ammonium hydroxide, tetramethyl ammonium hydroxide preferably tetramethyl ammonium hydroxide, w is between 0.002 to 0.05 and z is from 5-100 which comprises mixing aqueous solutions of sources of tin, silicon and a mixture of cetyl trimethyl ammonium hydroxide/chloride in the molar compositions in terms of ratios of oxides as under: H2O/ SiO2 = 5-100, H2O/ SnO2 = 1000-2000, H2O/(R1)2O = 125-550, H2O/(R2)2O = 40-550. mixing and heating the resultant gel in autoclave at autogeneous pressure, at temperatures in the range of 80-140°C for a period in the range of 3-10 days till crystallization is complete, filtering, washing with deionized water and drying the resultant composite material at a temperature in the range of 80-110°C for a period in the range of 12 to 20 hrs., calcining at a temperature in the range of 550 to 600°C for a period ranging between 2-8 hours in an inert atmosphere followed by calcining at the same temperature for a period range of 3-10 hours in air to obtain catalytically active crystalline stannosilicate. 2. A process as claimed in claim 1 wherein the source of silica is selected from silicon dioxide, silica hydrosol, silica gel, silicic acid, alkoxides of silicon, alkali metal silicates preferably tetramethyl ammonium silicate (TMA-silicate). 3. A process as claimed in claims 1 to 2 wherein the source of tin is selected from tin (IV) acetate, tin (IV) bromide, tin (IV) chloride, tin (IV) fluoride, the preferred source of tin being stannous tetrachloride (SnC14 5H2O). 4. A process as claimed in claims 1 to 3 wherein the inert gas used is selected form Nitrogen, helium and argon. 5. A process for the preparation of tin silicate molecular sieve substantially as herein described with reference to the examples.

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