Title of Invention

A PROCESS FOR THE PREPARATION OF ARSENIC SILICATES

Abstract

This invention relates to a process for the preparation of arsenic silicates having formula in terms of mole ratios of oxides in the anhydrous state: mM20 x AS2O5 : (1-x) Si02 where x = 0.001 to 0.05 and m = 0.0 to 0.5 and M is a monovalent cation such as Li, Na,K,Cs, NH4, H or mixture thereof, which comprises mixing a source oxide, a source of arsenic and quaternary ammonium salt, heating the resultant reaction mixture at atmospheric pressure and temperature in the range of 60 - 200°C for 2 or more hours, quenching, filtering, washing, drying and then calcining the resultant solid material at a temperature in the range of 400 - 600°C to obtain porous crystalline arsenic containing silicates.

Full Text

This invention relates to a process for the preparation of arsenic silicates. More particularly, the invention relates to a process for the preparation of microporous crystalline, arsenic silicates with characteristic x-ray powder diffraction pattern, having Arsenic as a part of the structure. The crystalline, microporous metallosilicate molecular sieves prepared by the process of the present invention are constituted of oxides of Si and As and optionally of oxides of other element (s) such as H, Li, Na, K or mixtures thereof having well ordered porous structure, having the following chemical composition in terms of oxide mole ratios: m H2O : x AS2O5 : 1-x Si02 where x = 0.001 to 0.05 and m = 0.0 to 0.5 and M is a monovalent cation such as Li, Na, K, Cs, NH4, H or mixture thereof. The so obtained molecular sieves are useful in different catalytic hydrocarbon conversion processes. T.E. Gier and G.D. Stucky have reported the synthesis of berylloarsenate molecular sieve (see Nature (London) vol. 349, 1991 p. 508.). J.F. Joly et al. (Eur. Pat. Appl. EP: 507675, 1992, Chem. Absr. No.: 118:138466f) have claimed the preparation of arsenic containing gallium phosphate in the presence of Fions, with AFI topology. However, microporous, crystalline arseno-silicates or silico-arsenates are not known in the prior art. The present invention, for the first time, describes the synthesis of As-containing crystalline microporous silicas through hydrothermal crystallization from reaction mixture, arising from the sources of silica, arsenate and optinally one or more alkali metal salts described hereinbefore and an organic template (T) such as monoquatemary ammonium salt of the formula R4N+X" (where R is an alkyl group with 2-5 carbons and X = CI, Br, I, OH or mixtures thereof, used for As-silicate-1 and As-silicate-2 or diquaternary ammonium salt of the formula [{ (CH3)2C6H5-CH2N+-(CH2)n-N+C6H5-CH2(CH3)2} {X2] where n = 5 or 6 and X = CI, Br, I, OH mixtures thereof) used for As-siiicate-3 and silicate-4, heating the reaction mixture in the range of 60-200°C, lasting 2 or more hours, followed by separation of the solid from the mother liquor, washing of the crystalline material so obtained with water and calcination at temperature in the range of 450 - 600°C. Accordingly, the present invention provides a process for the preparation of arsenic silicates having formula in terms of mole ratios of oxides in the anhydrous state: m M20 x As205: (1-x) Si02 where x = 0.001 to 0.05 and m = 0.0 to 0.5 and M is a monovalent cation such as Li, Na, K, Cs, NH4, H or mixture thereof, which comprises mixing a source of silicon oxide, a source of arsenic and quaternary ammonium salt, heating the resultant reaction mixture at atmospheric pressure and temperature in the range of 60 - 200°C for 2 or more hours, quenching, filtering, washing, drying and then calcining the resultant solid material at a temperature in the range of 400 - 600°C to obtain arsenic silicates. (Table Removed) Table 1 : XRD Patterns of As-Silicate-1 (Table Removed) Table-2 X-ray powder diffraction pattern of As-silicate-2. (Table Removed) Relative intensity: vs=80-100; s=60-80; m=40-60; w=20-40; vw Table 3 X-ray powder diffraction pattern of As-silicate-3 (Table Removed) Relative intensity: vs=80-100; s=60-80' m=40-60' w=20-40; vw (Table Removed) Table 4 X-ray powder diffraction pattern of As-silicate-4. (Table Removed) oxide molecular ratios : Si02/As2O5 =15-3000 Si02/T = 1.5-40 H20/Si02 = 10-100 OH~/Si02 = 0.4-0.7 Na+/Si02 = 0.0-0.4 where, T is an organic template such as such as monoquaternary ammonium salt of the formula R4N+X" (where R is an alkyl group with 2-5 carbons and X = CI, Br, I, OH or mixtures therof) used for As-silicate-1 and As-silicate-2 or diquaternary ammonium salt of the formula [ { (CH3)2C6H5-CH2N+-(CH2)n-N (C6H5-CH2 (CH3)2) (X2)] (where n * 5 or 6 and X = CI, Br, I, OH or mixtures thereof) used for As- silicate-3 and As-silicate-4 In a preferred embodiment of the present invention, the starting gel has the following molar composition : Si02/As2O5 = 20-500 Si02/T = 2-20 H20/Si02 =15-50 QH~/Si02 =0.4-0.7 Na+/Si02 = 0.0-0.4 where T is an organic additive such as monoquaternary ammonium salt of the formula R4N+X" (where R is an alkyl group with 2-5 carbons and X = Ci, Br, I, OH or mixtures thereof) used for As-silicate-1 and As-silicate-2 or diquatemary ammonium salt of the formula [((CH3)2C6H5-CH2N+-(CH2)n-N+(C6H5-CH2(CH3)2)(X2)] (where n = 5 or 6 and X = CI, Br, I, OH or mixtures therof) used for As-silicate-3 and As-silicate-4. Some physicochemical characteristic properties of As-Silicate 1-4 are recorded in table 5. Table 5. Composition and physico chemical characteristics of As-silicate samples. (Table Removed) The practice of the present invention is described in the following examples which are for illustrative purposes only and are not to be construed to limit the scope of this invention. Example-1 In this example, a detailed method of preparation of As-containing molecular sieve-1 (As-Silicate-1) is described. In a typical preparation, to 35.0 g tetraethylorthosilicate , Si(OC2H5)4 (TEOS) (98%, Aldrich) 56.4 g tetrapropyl ammoniumhydroxide (TPAOH, 20% aqueous solution, Aldrich) was added with continuous stirring. After about one hour a solution of 2.6 g disodium hydrogen arsenate heptahydrate (Na2HAsO4.7H20) in 10 g water was added slowly to the above mixture under stirring. The clear gel so obtained was filled in a stainless steel autoclave and placed in an oven for crystallization at 160 °C for 20 hours. The autoclave was then removed from oven and quenched with cold water. The contents were filtered, washed thoroughly with deionised water and dried at 120° C. Calcination of the as synthesized crystalline AS-Silicate-1 was done at 500°C for 16 hours. The yield was obtained 85 %. The X-ray diffraction data of the calcined material correspond to those of As-Silicate-1 are given in Table 1. The chemical composition of the calcined material on anhydrous basis was found to be : 40 Si02:As202. Example-2 In this example, a detailed method of preparation of As-containing molecular sieve-2 (As-Silicate-2) is described. In a typical preparation, to 21.0 g tetraethylorthosilicate, Si(OC2H5)4 (TEOS) (98%, Aldrich) 16.2 g tetrabutylammonium hydroxide (TBAOH, 40% aqueous solution, Aldrich) was added with continuous stirring. After about one hour a solution of 1.3 g disodium hydrogen arsenate heptahydrate (Na2HAsO4.7H20) in 10 g water was added slowly to the above mixture under stirring. The clear gel so obtained was filled in a stainless steel autoclave and placed in an oven for crystallization at 160° C for 24 hours. The autoclave was then removed from oven and quenched with cold water. The contents were filtered, washed thoroughly with deionised water and dried at 120° C. Calcination of the synthesized crystalline As-Silicate-2 was done at 500° C for 16 hours. The yield was obtained 85 %. The X-ray diffraction data of the calcined material correspond to those of As-Silicate-2 are given in Table 2. The chemical composition of the calcined material on anhydrous basis was found to be: 48 Si02 AS205. Example-3 In this example, a detailed method of preparation of As-containing molecular sieve-3 (As-Silicate-3) material is described. In a typical preparation, to 12 g fumed silica, a solution containing 0.46 g NaOH, 20 g water and 16 g hexametylene bis (benzyl dimethyl ammonium hydroxide) (15% aqueous solution) was added under stirring. This mixture was stirred for about two hours at room temperature before adding to it a solution of 0.4 g Na2HAS04 in 10 g water slowly under stirring . The gel so obtained was filled in a stainless steel autoclave and placed in an oven for crystallization at 160° C for 48 hours. The autoclave was then removed from oven and quenched with cold water. The contents were filtered, washed thoroughly with deionised water and dried at 120° C. Calcination of the synthesized crystalline As-Silicate-3 was done at 500° C for 16 hours. The yield was obtained 85 %. The X-ray diffraction data of the calcined material correspond to those of AsSiiicate-3 are given in Table 3. The chemical composition of the calcined material on anhydrous basis was found to be: 150 SiC>2: AS2O5. Example-4 In this example, a detailed method of preparation of As-containing molecular sieve-4 (As-Silicate-4) material is described. In a typical preparation, to 21.0 g tetraethyl orthosilicate, Si(OC2H5)4 (TEOS) (98%, Aldrich) 45 g hexametylene bis (benzyl dimethyl ammonium hydroxide) (15% aqueous solution) was added under stirring. This mixture was stirred for about two hours at room temperature before adding to it a solution of 0.40 g (NhU^HASCUJhbO in 10 g water slowly under stirring. The clear gel so obtained was filled in a stainless steel autoclave and placed in an oven for crystallization at 160° C for 56 hours. The autoclave was then removed from oven and quenched with cold water. The contents were filtered, washed thoroughly with deionised water and dried at 120° C. Calcination of the as synthesized crystalline As-Silicate-4 was done at 500° C for 16 hours. The yield was obtained 85 %. The X-ray diffraction data of the calcined material correspond to those of As-Silicate-4 are given in Table 4. The chemical composition of the calcined material on anhydrous basis was found to be: 120 Si02: AS205 Example-5 In this example, a higher amount of As is taken for the synthesis of As-Silicate-1 . In a typical preparation, to 35.0 g tetraethylorthosilicaie, Si(OC2H5)4 (TEOS) (98%, Aldrich) 56.4 g tetrapropyl ammoniumhydroxide (TPAOH, 20% aqueous solution, Aldrich) was added with continuous stirring. After about one hour a solution of 5.2 g disodium hydrogen arsenate heptahydrate (Na2HAs04.7H2O) in 10 g water was added slowly to the above mixture under stirring. The clear gel so obtained was filled in a stainless steel autoclave and placed in an oven for crvstallization at 160° C for 20 hours. The autoclave was then removed from oven and quenched with cold water. The contents were filtered, washed thoroughly with deionised water and dried at 120° C. Calcination of the as synthesized crystalline AS-Silicate-1 was done at 500° C for 16 hours. The yield was obtained 85%. The X-ray diffraction data of the calcined material correspond to those of As-Silicate-1 are given in Table 1. The chemical composition of the calcined material on anhydrous basis was found to be: 20 Si02: AS2O5. Example-6 This example illustrates the effect of temperature required for hydrothermal crystallization of As-Silicate-1. In a typical preparation, to 140 g tetraethylorthosillcate, Si (OC2H5)4 (TEOS) (98%, Aldrich) 225.6 g tetrapropyl ammoniumhydroxide (TPAOH, 20% aqueous solution, Aldrich) was added with continuous stirring. After about one hour a solution of 10.4 g disodium hydrogen arsenate heptahydrate (Na2HAS04.7H20) in 40 g water was added slowly to the above mixture under stirring. The clear reaction mixture so obtained was filled in five stainless steel autoclave and placed in different ovens for crystallization at 70°C, 85 °C, 130 °C, 160 °C and 190 °C. The time (in hours) required for complete crytallization was 66, 18, 8, 4 and 2 at 70°C, 85°C, 130°C, 160°C and 190X, respectively. These observations are consistant with the conventional knowledge that the crystallization becomes faster with the increase in temperature. The autoclave was then removed from oven and quenched with cold water. The contents were filtered, washed thoroughly with deionised water and dried at 120° C. Calcination of the as synthesized crystalline As-Silicate-1 material was done at 500 °C for 16 hours. The yield was obtained 85 %. The X-ray diffraction data of the calcined material correspond to those of As-Silicate-i are given in Table 1. Example-7 In this example, the effect of Si02 / AS2O5 molar ratio in the reaction mixture on the crystallization time for As-Silicate-1 samples is illustrated. As-Silicate-1. In a typical preparation four sets of synthesis mixture with different Si02 / AS2O5 molar ratio of 20 (reaction mixture A), 40 (reaction mixture B), 80 (reaction mixture C) and 400 (reaction mixture D) were prepared according the procedure given in example 1 and in table 5. The reaction mixture so obtained was filled in stainless steel autoclaves and placed in an oven for crystallization at 85 °C till fully crystalline As-Silicate-1 materials are obtained as shown in table 6. The autoclave was then removed from oven and quenched with cold water. The contents were filtered, washed thoroughly with deionised water and dried at 120° C. Calcination of the as synthesized crystalline As-Silicate-1 was done at 500°C for 16 hours. The X-ray diffraction data of the calcined material correspond to those of As-Silicate-1 are given in Table I.The results shown in Table 6 are quite interesting and contrary to the commonly observed phenomenon in the case of other crystalline microporous metallo silicates like aluminosilicates, ferrisilicate, titaniumsilicates etc. that the crystallization becomes slower with the increase in heterometal content in metallosilicate reaction mixture and concequently metal free pure silica containing reaction mixture crystallizes most quickly. However, in the present case of As-Silicates it has been observed that the crystallization becomes faster with the increase of As content in the reaction mixture, pure silica analogue being the slowest to crystallize. This fact can be due the reason that higher the charge to ionic radius ratio of an element taster its incorporation in the crystalline network. The values of the charge / radius ratio of As, Si, Ti and Al ions are 10.6, 9.7, 6.9 and 6.0 respectively, suggesting the faster incorporation of As5+ (vis-a-vis Si4+ ) in the crystalline network. Table 6: Effect of Si02/As205 molar ratio in the reaction mixture on the crystallization time for As-silicate-1 materials. (Table Removed) a: The values in parentheses are Si02/As205 molar ratio in the reaction mixture. b: TEOS= tetraethyl orthosilicate, TPAOH= tetrapropyl ammonium hydroxide (20 wt.% aqueous solution), As5+=Na2HAso4.7H20. Example-8 In this example, the catalytic activity of these As-Silicate samples, described in example 1-4 in an oxidation reaction is demonstrated. For this, hydroxylation of phenol using H2O2 as the oxidant in the presence of As-Silicate catalysts synthesised according to the procedure described in this invention is described. The catalytic runs were carried out batchwise in a four-necked glass flask (200 ml capacity) fitted with a mechanical stirrer, a condenser, a feed pump and a septum. The temperature of the reaction vessel was maintained using an oil bath, in a standard run, 10 g phenol, 1.0 g of the catalyst (particle size (Table Removed) samples prepared by hydrothermal synthesis as described in the present invention is far more active in this reaction. Table 6. Hvdroxylation of phenola aReaction conditions: Catalyst/Phenol = 10 g mol ; Solvent (Water) = 20 g; Phenol/H202 (mole) » 3; Temp. =» 348 K; Reaction time 24 h; reaction carried out in a batch reactor. bFor all products. cFor the formation of benzoquinone, catechol and hydroquinone excluding tar. dBreak up (wt,%) of products excluding tar. We claim : 1. A process for the preparation of arsenic silicates having formula in terms of mole ratios of oxides in the anhydrous state: m M20 x As205 : (1-x) Si02 where x = 0.001 to 0.05 and m = 0.0 to 0.5 and M is a monovalent cation such as Li, Na, K, Cs, NHA, H or mixture thereof, which comprises mixing a source of silicon oxide, a source of arsenic and quaternary ammonium salt, heating the resultant reaction mixture at atmospheric pressure and temperature in the range of 60 - 200°C for 2 or more hours, quenching, filtering, washing, drying and then calcining the resultant solid material at a temperature in the range of 400 - 600°C to obtain porous crystalline arsenic containing silicates. 2. A process as claimed in claim 1 where the molar composition of the starting reaction mixture used is in the range of: SiO2/As205 =15- 3000 Si02/T = 1.5-40 H20/Si02 = 10-100 OH7Si02 = 0.4-0.7 Na7Si02 = 0.0-0.4 where T is a quaternary ammonium salt. 3. A process as claimed in claims 1 and 2 wherein the quaternary ammonium salt is a monoquaternary ammonium salt of the formula R4NX" (where R is an alkyl group with 2-5 carbons and X is CI, Br, I, OH or mixtures thereof). 4. A process as claimed in claims 1 and 2 wherein the quaternary ammonium salt is a diquaternary ammonium salt of the formula [{(CH3)2C6H5-CH2N+-(CH2)n-N+(C6H5-CH2(CH3)2)(X2)] (where-n5 or 6 and X is CI, Br, I, OH or mixtures thereof). 5. A process as claimed in claims 1 and 2 wherein the source of arsenic is M2HAs04.7H20 where M is H, Li, Na, K or M |-fy 6. A process as claimed in claims 1 to 4 wherein the quaternary ammonium salt is selected from tetrapropylammonium hydroxide, tetrabutylammoinum hydroxide or hexamethylene bis (benzyl dimethyl ammonium hydroxide). 7. A process as claimed in claims 1 and 2 wherein the source of silicon oxide is tetraethylorthosilicate, colloidal silica or fumed silica. 8. A process for the preparation of arsenic silicates substantially as herein described with reference to the examples.

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