PROCESS FOR THE PREPARATION OF INORGANIC COMPOUNDS

Abstract: A process for preparing a compound of formula (1): A>Y<XM>12<O>40<, wherein A is selected from the group consisting of hydrogen, alkali metal and ammonium, X is silicon or phosphorus, M is selected from the group consisting of molybdenum, tungsten and vanadium, and Y is an integer from 1 to 8, which process comprises forming an aqueous liquor comprising the components, (a) an alkali metal salt of a compound comprising an atom denoted by X, (b) an alkali metal salt of a compound comprising an atom denoted by M, and (c) an acid having a pKa below 4, increasing the temperature of the aqueous liquor, and optionally adding more acid, having a pKa below 4, characterised in that the aqueous liquor is formed at a temperature of below 50 DEG C and i) has a pH of below 2 or ii) in which component (c) is present in an amount between 1 and 3 moles per mole of components (b). The invention provides a simple, economic process for the convenient preparation of heteropoly acids, especially silicotungstic acid with a low counter ion content in high yield and high purity and avoids the undesirable formation of silicic acid gels.

Description: Process for the Preparation of Inorganic Compounds The present invention relates to a process for the preparation of high purity heteropoly acids of tungsten, vanadium and molybdenum in high yield. In particular the invention provides a convenient and economic process for making silicotungstic acid and salts thereof. Heteropoly acids are of growing importance in the field of catalysis of commercially important processes. Heteropoly acids have been widely used in the selective transformations of organic substances, for instance oxidation of methacrolein, hydration of olefins such as propylene or butylene and polymerisation of tetrahydrofuran. Recently it has been shown that heteropoly acids can be used as catalysts for low temperature organic reactions in the liquid phase, for instance in the synthesis of fine chemicals. Such heteropoly acids are for instance those of formula (1) given below. It is known to produce heteropoly acids by reacting the corresponding alkali metal tungstate, vanadate or molybdenate salts with the corresponding alkali metal silicate or phosphate salts. Generally the yields are not high since large amounts of byproduct are formed. This is a particular problem when producing silicon containing heteropoly acids since silicic acid gels form. Generally, by-products such as silicic acid gels need to be removed from the final product as otherwise the activity of the final heteropoly acid would be impaired. Generally silicotungstic acid is produced by heating an aqueous mixture of sodium tungstate and sodium silicate followed by the addition of about 2.5 moles of hydrochloric acid per mole of sodium tungstate. Since this process results in the generation of a silicic acid gel product, this has to be removed for instance by filtration. Such a filtration step is disadvantageous since this means additional processing. The filtered product has to be treated with over 1.5 moles of hydrochloric acid per mole of sodium tungstate in order to generate the free acid form of the product. Apart from the disadvantageous formation of the silicic acid gel by-product, this process also has the disadvantage of requiring over 4 moles of hydrochloric acid per mole of sodium tungstate. It is also known to produce silicotungstic acid avoiding the formation of silica gel and avoiding hydrochloric acid contamination by the use of an ion exchange resin in the hydrogen cycle. The ion exchange resin is combined with the mixture of sodium silicate and sodium tungstate and the mixture is heated above 50°C. The process merely requires that the reaction medium is maintained at a pH between 2 and 6. Although it is possible to carry out the process at pH 6 it is generally desired to maintain a pH at less than 6 in order to prevent the silicotungstate from breaking down. At pH 6 the degradation of silicotungstic acid would severely affect the yield. Furthermore, it would be hard to generate sufficient acidity when using an ion exchange resin to efficiently drive the reaction towards the synthesis of the silicotungstate. Another disadvantage of using ion exchange resins is that these would need to be separated from the final product and regenerated which would require additional processing steps. In GB-A-1520726 a process for making heteropolyacids is described, which process involves first forming sodium germanate solution by combining germanium dioxide with sodium hydroxide and then separately combining hydrochloric acid with sodium tungstate solution at a molar ratio of acid to tungstate of 0.68:1. The germanate and acidified tungstate solutions are then mixed together and heated followed by further acidification, then boiling, followed by further acidification steps. The final product is said to have been extracted and filtered. Other heteropolyacids based on silicon, phosphorus, vanadium, molybdenum, tungsten, including silicotungstic acid are also said to be made by the method. However, this process which uses less than 1 mole of acid per mole of tungstate prior to heating the acidified germanate/tungstate mixture does not overcome the problem of producing by-products, with the inevitable poor yield. The tungstate solution is said to have a pH of 6.8 to 7.0 prior to combining with the alkaline sodium germanate. In addition the process requires several acidification steps subsequent to heating, such that the total amount of acid used in the process is more than 4 moles per mole of tungstate. It would therefore be desired to provide a process for the convenient preparation of heteropoly acids in high purity and yield which avoids the formation of by-products. Furthermore it would be particularly desired to conveniently prepare silicotungstic acid in high purity and yield by a process which avoids the formation of silicic acid gel and the excessive total use of hydrochloric acid and also avoids the use of ion exchange resin. Thus the invention provides a process for preparing a compound of formula (1) A Y XM 12 O 40 (1), wherein A is selected from the group consisting of hydrogen, alkali metal and ammonium, X is silicon or phosphorus, M is selected from the group consisting of molybdenum, tungsten and vanadium, and Y is an integer from 1 to 8. which process comprises forming an aqueous liquor comprising the components, (a) an alkali metal salt of a compound comprising an atom denoted by X (b) an alkali metal salt of a compound comprising an atom denoted by M and, (c) an acid having a pKa below 4. increasing the temperature of the aqueous liquor, and optionally adding more acid having a pKa below 4, characterised in that the aqueous liquor is formed at a temperature of below 50°C and i) has a pH of below 2 or ii) in which component (c) is present in an amount between 1 and 3 moles per mole of component (b). Typically the process can be used to prepare sodium silicotungstate Na SiWι 2 0 4 o, potassium silicotungstate K4SiW 12 O 4 o , ammonium silicotungstate (NH ) 4 SiW 12 0 4 o, silicotungstic acid H SiWι 2 0 4u , sodium phosphorotungstate Na 3 PW 12 0 4 o, potassium phosphorotungstate K 3 PW l2 0 4 o, ammonium phosphorotungstate (NH 4 ) 3 PWι 2 0 4 o, phosphorotungstic acid H 3 PW 12 0 4 o, sodium silicovanadate Na 4 SiVι 2 0 4 o, potassium silicovanadate (NH 4 )4SiW 12 O 0 , ammonium silicovanadate (NH 4 ) 4 SiVι 2 0 4 o, silicovanadic acid H SiV 12 0 4 o, sodium phosphorovanadate Na 3 PV 2 0 4 o, potassium phosphorovanadate K 3 PV l2 0 4 o, ammonium phosphorovanadate (NH ) 3 PVι 2 0 o, phosphorovanadic acid H 3 PVι 2 O 40 , sodium silicomolybdenate Na 4 SiMo-ι 2 0 4 o, potassium silicomolybdenate K4SiMθι 2 0 4 o, ammonium silicomolybdenate (NH ) 4 SiMo 12 0 4 o > silicomolybdenic acid H 4 SiMθι 2 0 o, sodium phosphoromolybdenate Na 3 PMθι 2 O 0 , potassium phosphoromolybdenate K 3 PMθι 2 0 4 o, ammonium phosphoromolybdenate (NH 4 ) 4 SiWι 2 0 4 o and phosphoromolybdenic acid H 3 PMθι 2 O 0 . Compounds that can be used as component (a) in the process include suitable alkali metal salts of compounds containing an atom denoted by X, for instance sodium silicate, trisodium phosphate, disodium hydrogen phosphate or sodium dihydrogen phosphate. Preferably X is silicon and component (a) is an alkali metal salt of a silicon compound, for instance silicic acid, wherein the preferred alkali metal salt is sodium silicate. M is preferably tungsten and component (b) is an alkali metal salt of a tungsten compound, for instance sodium tungstate. In one preferred aspect of the invention the compound of formula (1) is an alkali metal salt of silico tungstic acid, more preferably sodium silicotungstate Na SiW 12 O 40 . It is possible to combine the components (a), (b) and (c) in any order as long as the temperature is maintained below 50°C during the formation of the liquor. Preferably the aqueous liquor can be formed by first forming an aqueous solution of both component (a) and component (b) and then adding component (c). One way of ensuring that the temperature remains below 50°C is by adding component (c) slowly so as to avoid an excessive increase in temperature due to the exotherm. Another way of maintaining the temperature below 50°C is to apply cooling to the aqueous solution during the addition of component (c). Once the aqueous liquor has been formed the temperature may be allowed to increase to above 50°C and further component (c) may be added. Preferably the aqueous liquor is formed at a temperature in the range 30°C to below 50°C, more preferably 35°C to 45°C. Although it is possible for the process to be conducted by employing component (a) and component (b) in approximately equimolar proportions it is generally preferred to use greater proportions of (b). For instance the process may conveniently proceed if molar ratio of component (a) to (b) is between 1 :1 and 1 :12. If the ratio exceeds 1 :12 the reaction will still proceed though the amount of unreacted component (b) will start to become significant and this would affect the economical viability of the process. If the molar ratio component (a) to component (b) is below 1 :1 the reaction would proceed more slowly and furthermore would result in the significant quantities of unreacted component (a). In order to maximise the yield of heteropoly acid the molar ratio of component (a) to (b) is preferably between about 1 :10 and 1 :12. It is important for the reaction to proceed for component (c) to be a strong acid, with a pKa below 4. Ideally the strong acid is an acid with a pKa below 3. The acid is for instance a mineral acid selected from the group consisting of hydrochloric acid, sulphuric acid and nitric acid. Generally hydrochloric acid is preferred. The process proceeds more effectively if the acid is concentrated. Therefore it is highly desired if the strength of the acid is in excess of 10 wt. %, preferably at least 30 wt. % most preferably around 36.5 wt. %. Preferably the aqueous liquor is formed from 1 to 3 moles of component (c), for instance 1 to 2.5 moles of component (c) per mole of component (b). More preferably though the amount of component (c) present in the aqueous liquor will be for instance between 1.5 and 2.25 moles, especially around 2 moles per mole of component (b). In one aspect of the invention the aqueous liquor that is formed has a pH of below 2. More preferably the aqueous liquor has a pH in the range of from 1 to below 2. The compound of formula (1) is formed by subjecting the aqueous liquor to reaction conditions at an elevated temperature. The elevated temperature should be at least 50°C. Desirably the aqueous liquor is heated to a temperature in excess of 60°C or 70°C, preferably above 80°C, more preferably above 95°C. Most preferably the aqueous liquor is refluxed for example for at least 15 minutes, preferably at least 30 minutes and more preferably around 60 minutes. As described earlier it is possible for additional amounts of component (c) to be added to the aqueous liquor once the temperature has reached 50°C or more. When component (c) is added to the aqueous liquor at a temperature of at least 50°C, the total amount of component (c) incorporated in the aqueous liquor is preferably not more than 3.5 moles per mole of component (b). Thus according to the invention the total amount of component (c) used in the process is between 1 and 3.5 moles per mole of component (b). For instance it would be possible to form an aqueous liquor comprising component (a), component (b) and 1 to 2.5 moles of component (c) per mole of component (b), at a temperature below 50°C. Once the temperature of the aqueous liquor has been elevated to at least 50°C, up to a further 1 mole of component (c) per mole of component (b) could be added to the aqueous liquor. In one preferred aspect of the invention up to 2.5 moles of component (c) per mole of component (b) is used to form the aqueous liquor at a temperature of below 50°C. Preferably the aqueous liquor is formed at a temperature from 30°C to below 50°C, most preferably in the range 35°C to 45°C. In one further aspect of the invention the heteropoly acid is prepared in essentially free acid form. After the reaction of components (a), (b) and (c) is complete, the aqueous medium, comprising the heteropoly acid as a fully or partially neutralised alkali metal or ammonium salt, is further treated using up to 1 mole of acid per mole of component (b) used in the aqueous liquor. Preferably between 0.1 and 0.5 moles of acid per mole of component (b) are used to treat the heteropoly acid salt. The heteropoly acid can be conveniently extracted from the aqueous medium by the use of a suitable non-polar solvent. Suitable non-polar solvents include oxygen containing non-polar organic solvents, for instance ethers or ketones. Preferred organic solvents are C . 8 ethers, especially tertiary butyl methyl ether, methyl ethyl ketone or diethyl ether. Ideally sufficient organic solvent is added to the aqueous medium to enable the heteropoly acid contained therein to be extracted. One way of achieving this is by adding sufficient organic solvent to the aqueous solution and stirring vigorously, for example between 5 and 15 minutes, for instance 10 minutes. On settling the mixture will form an organic phase into which the heteropoly acid has been extracted. Thus the phase containing the extracted heteropoly acid is separated and the heteropoly acid contained therein recovered. One way of achieving this is to solvent swap the organic phase containing the extracted heteropoly acid with water. The organic phase can then be removed resulting in an aqueous solution of the heteropoly acid. It may also be desirable to treat the heteropoly acid in solvent by washing it with acid. This ensures that traces of residual counter-ions are removed, thus providing the heteropoly acid in essentially high purity free acid form. Desirably the solvent extracted heteropoly acid is washed at least once, preferably between two and four times, more preferably three times prior to solvent swap with water. Typically the acid is the same acid as component (c) used to form the aqueous liquor. Preferably the acid is hydrochloric acid of between 5 weight % and 37 weight %, preferably at least 15 weight %. The solvent extracted heteropoly acid which has been washed is then essentially in free acid form, containing no or virtually no alkali metal or ammonium ions. The process can be used to provide heteropoly acid of purity greater than 95%, preferably at least 96%, more preferably at least 97% or 98%, and most preferably at least 99%. The process of the invention also provides a means for obtaining heteropoly acids and their alkali metal or ammonium salts in yields of at least 95%, preferably at least 96%, . The process is of particular importance in the synthesis of a compound of formula 1 wherein X is silicon, M is tungsten, and A and Y have the same meaning as above, by reacting at an elevated temperature, an aqueous liquor comprising, (a) an alkali metal salt of a silicon compound, (b) an alkali metal salt of a tungsten compound, (c) an acid having a pKa below 4, increasing the temperature of the aqueous liquor, and optionally adding more acid, having a pKa of below 4, characterised in that the aqueous liquor is formed at a temperature of below 50°C. Compounds that can be used as component (a) in the process include suitable alkali metal salts of compounds containing silicon. Desirably component (a) is an alkali metal silicate, for instance sodium silicate. Typically component (b) is sodium tungstate. In one preferred aspect of the invention the compound of formula 1 is sodium silicotungstate Na 4 SiW l2 0 4 o or silicotungstic acid H 4 SiW-| 2 0 o. Component (a) is an alkali metal salt of a silicon compound, for instance silicic acid, wherein the preferred alkali metal salt is sodium silicate. Component (b) is an alkali metal salt of a tungsten compound, for instance sodium tungstate. In one preferred aspect of the invention the compound of formula (1) is an alkali metal salt of silicotungstic acid, more preferably sodium silicotungstate, most preferably silicotungstic acid in essentially free acid form. The process can be used to provide silicotungstic acid H 4 SiWι 2 0 4u at a purity of greater than 95%, preferably at least 96%, more preferably at least 97% or 98%, and most preferably at least 99%. The process of the invention also provides a means for obtaining silico tungstic acid H 4 SiWι 2 O 40 at yields of at least 95%, preferably at least 96%. The general process requirements described previously also apply to the specific process of making sodium silicotungstate and silicotungstic acid in essentially free acid form, where component (a) an alkali metal or ammonium salt of a silicon compound, component (b) is an alkali metal or ammonium salt of a tungsten compound and (c) an acid with a pKa below 4. Preferably the aqueous liquor comprises as component (a) an alkali metal silicate, for example sodium silicate, and as component (b) an alkali metal tungstate, for example sodium tungstate. Silicotungstic acid H 4 SiW-| 2 0 4u is desirably prepared comprises the steps of: (i) forming an aqueous liquor by combining an alkali metal silicate with an alkali metal tungstate in a molar ratio of silicate to tungstate of between 1 :10 to 1 :12, (ii) adding to the aqueous liquor between 1 and 2.5 moles of hydrochloric acid per mole of tungstate used in step (a), (iii) heating the resulting aqueous liquor that forms to a temperature in excess of 95°C for at least 15 minutes, (iv) adding between 0.1 and 0.5 mole of hydrochloric acid, (v) adding to the aqueous liquor a non-polar organic solvent selected from the group consisting of C 2 -s ketones and C 2 .s ethers, to form a separate organic phase into which the silicotungstate is extracted, (vi) washing the extracted silicotungstate in acid to provide silicotungstic acid. More preferably in the process the molar ratio of silicate to tungstate to silicate is around 1 :12. In a still preferred aspect of the process the aqueous liquor in step (iii) is formed at a temperature below 50°C, preferably between 35°C and 45°C, before being refluxed between 30 and 70 minutes, preferably around 60 minutes. In a yet more preferred aspect of the process the organic solvent in step (e) is tertiary butyl methyl ether, diethyl ether or methyl ethyl ketone. The following examples demonstrate the invention. Example 1 1800g of a commercial grade sodium tungstate is added to 1800g water at ambient temperature (about 15°C) and allowed to stir for approximately 10 minutes. 98.1g sodium metasilicate is then added to the aqueous solution and stirring continues for a further 10 minutes. 1080ml concentrated hydrochloric acid (36.5wt.%) is then added drop-wise over 1.5 to 2 hours and the aqueous solution is allowed to exotherm to between 35-45°C. Once all of the hydrochloric acid has been added the aqueous liquor that is formed is heated to reflux and maintained at reflux for 1 hour. The solution is them cooled to ambient temperature and stirred for approximately 10 minutes. The resultant aqueous solution contained no silica gel by-product. 82ml concentrated hydrochloric acid (36.5wt.%) is then added drop-wise to the aqueous solution over 30-60 min. The aqueous solution is then cooled to ambient temperature. 972 ml diethyl ether is then added to the solution and the mixture is agitated thoroughly for 10 minutes. The resulting mixture has three layers. The bottom layer is separated and combined with 1035 ml water. The resulting mixture is then subjected to evaporative distillation to remove the ether from the aqueous mixture, initially at about 35°C and finally allowing the temperature of the resultant aqueous liquid rise to 80°C to remove residual ether. The resultant aqueous liquid is a solution of silicotungstic acid (about 50wt. %). The yield of the silicotungstic acid is 99.32wt.%. Example 2 Example 1 is repeated using tertiary butyl methyl ether in place of diethyl ether. The yield of silicotungstic acid is 98.7wt.% Example 3 Example 2 is repeated except using 700 ml tertiary butyl methyl ether. The yield of silicotungstic acid is 98.3wt.%.

Claims: Claims 1. A process for preparing a compound of formula (1) A Y XM 12 O 40 (1), wherein A is selected from the group consisting of hydrogen, alkali metal and ammonium X is silicon or phosphorus, M is selected from the group consisting of molybdenum, tungsten and vanadium, and Y is an integer from 1 to 8, which process comprises forming an aqueous liquor comprising the components, a) an alkali metal salt of a compound comprising an atom denoted by X b) an alkali metal salt of a compound comprising an atom denoted by M, and c) an acid having a pKa below 4. increasing the temperature of the aqueous liquor, and optionally adding more acid, having a pKa below 4, characterised in that the aqueous liquor is formed at a temperature of below 50°C and i) has a pH of below 2 or ii) in which component (c) is present in an amount between 1 and 3 moles per mole of component (b). 2. A process according to claim 1 in which the aqueous liquor is formed at a pH of below 2. 3. A process according to claim 1 or claim 2 in which the aqueous liquor is formed comprising between 1 and 3 moles of component (c) per mole of component (b). 4. A process according to any of claims 1 to 3 in which X is silicon and component (a) is an alkali metal salt of silicic acid, M is tungsten and component (b) is an alkali metal salt of tungstic acid. 5. A process according to any one of claims 1 to 4 in which the compound of formula (1) is Na 4 SiW 12 O 0 or H 4 SiW 12 O 40 . 6. A process according to any of claims 1 to 5 in which the aqueous liquor is formed by first combining components (a) and (b) followed by combining component (c). 7. A process according to any of claims 1 to 6 in which the molar ratio of component (a) to component (b) is between 1 :1 and 1 :15. 8. A process according to any of claims 1 to 7 in which the molar ratio of component (a) to component (b) is between about 1 :10 and 1 :12. 9. A process according to any of claims 1 to 8 in which component (c) is a mineral acid having a pKa below 3. 10. A process according to any of claims 1 to 9 in which component (c) is hydrochloric acid. 11. A process according to claims 1 to 10 in which component (c) is present in the aqueous liquor in an amount of between 1 and 2.5 moles per mole of component (b). 12. A process according to any of claims 1 to 11 in which the aqueous liquor is formed at a temperature in the range of 35°C to 45°C. 13. A process according to any of claims 1 to 12 in which the temperature of the aqueous liquor after formation is increased to above 50°C. 14. A process according to any of claims 1 to 13 in which the aqueous liquor after formation is heated to a temperature above 80°C, preferably above 95°C. 15. A process according to any of claims 1 to 14 in which the aqueous liquor after formation is refluxed for at least 15 minutes, preferably at least 30 minutes. 16. A process according to any of claims 1 to 15 in which up to 1 mole of acid per mole of component (b) is added after the temperature of the aqueous liquor has been increased above 50°C. 17. A process according to any of claims 1 to 16 in which the total amount of component (c) used in the process is between 1 and 3.5 moles per mole of component (b). 18. A process according to any of claims 1 to 17 in which the compound of formula (1) is extracted into an oxygen-containing non-polar organic solvent. 19. A process according to claim 18 in which the organic solvent is selected from the group consisting of C 4 .s ethers and C .s ketones. 20. A process according to claim 18 or claim 19 in which the organic solvent is diethyl ether, tertiary butyl methyl ether or methyl ethyl ketone. 21. A process according to any of claims 18 to 20 in which the solvent extracted compound of formula (1) is washed at least once with an acid to provide the compound of formula (1) in essentially free acid form. 22. A process according to any of claims 1 to 21 in which the compound of formula (1) is H 4 SiW 12 O 40 of purity at least 99 wt.%.

Inventors: BELLERBY DARREN KEITH, COLLIER SIMON ANDREW, KENNING COLIN ROY, WOOD SCOTT ALLEN, BELLERBY, DARREN, KEITH, COLLIER, SIMON, ANDREW, KENNING, COLIN, ROY, WOOD, SCOTT, ALLEN
Applicants: BELLERBY DARREN KEITH, CIBA SPEC CHEM WATER TREAT LTD, COLLIER SIMON ANDREW, KENNING COLIN ROY, WOOD SCOTT ALLEN, BELLERBY, DARREN, KEITH, CIBA SPECIALTY CHEMICALS WATER TREATMENTS LIMITED, COLLIER, SIMON, ANDREW, KENNING, COLIN, ROY, WOOD, SCOTT, ALLEN
Application year: 2000
Main classification: C01B33
IPC codes: C01B33