Synthesis of heteropolyacids

Details Synthesis of heteropolyacids Synthesis of heteropolyacids

2001-06-05

2002-12-24

Nazario-Gonzalez, Porfirio (Department: 1621)

Chemistry of inorganic compounds

Treating mixture to obtain metal containing compound

Group vib metal

C423S307000, C423S326000, C423S658500, C502S204000, C502S206000, C502S255000, C502S256000

Reexamination Certificate

active

06497849

ABSTRACT:

The present invention relates to a process for the purification of a heteropolyacid, and in particular, to a process for the purification of a silicotungstic acid.
It is well known that heteropolyacids substantially free of extraneous cations (hereafter “free heteropolyacids”) are valuable chemical compounds which can be used as acid catalysts in organic reactions such as the addition reaction of lower aliphatic carboxylic acids to olefins to form the corresponding esters. One such method is described in our EP-A-0757027 which describes the use of free heteropolyacids as catalysts for the production of aliphatic esters such as ethyl acetate in an addition reaction as described above. Methods of synthesising heteropolyacids are also well known. For instance, such processes are described e.g. by North, E O in “Organic Synthesis”, 1, page 129 (1978), Editor H S Booth, Robert E Krelger publishing company Huntington, N.Y.) and by Tatsuhiko, H et al in Kogyo Kagaku Zasshi, 72 (9), 1945-48 (1969).
Known methods for the preparation of heteropolyacids generally involve three stages, namely:
a. A reaction stage in which the tungstosilicic acid is formed by the addition of hydrochloric acid to a refluxing aqueous solution of sodium tungstate and sodium silicate. Upon neutralisation and hydrolysis, the reaction results in the formation of a brine solution containing a tungstosilicic acid Keggin structural unit. Overall, the reaction can be summarized as follows:
12Na
2
WO
4
+Na
2
SiO
3
+HCl→H
4
SiW
12
O
40
.n
H
2
O+NaCl+H
2
O
The product is filtered, acidified and cooled prior to purification.
b. A product recovery and purification stage in which the cooled product solution from (a) is purified by at least one liquid/liquid extraction step using an organic solvent. The solvent is such that it is capable of forming a solvent/heteropolyacid complex which is largely insoluble in the acidified aqueous brine. The resulting solvent/complex phase enables the disengagement of the contaminating inorganic salts from the brine solution.
c. A product regeneration stage in which an aqueous solution of the heteropolyacid is regenerated from the solvent/complex phase produced in step b). This regeneration is achieved by adding water to displace the solvent involved in the formation organic/complex phrase. The organic solvent is removed from the mixture by distillation. The organic solvent is conveniently removed as a solvent/water azeotrope. This azeotrope may form a two phase mixture on cooling. The denser of these two phases is an aqueous phase, which may be recycled to the reboiler. The less dense phase is an organic phase, which may be recovered, for example, for reuse. The amount of water added and removed can be adjusted to yield a 20-80%w/w aqueous solution of heteropoly acid.
The resultant concentrated aqueous solution of the heteropolyacid product can then be used to impregnate an inorganic support.
The known synthetic methods referred to above mention the use of diethyl ether, methyl ether ketone and ethyl acetate as possible solvents for purification of the product. We have found that all these materials have inherent disadvantages. For instance, diethyl ether has a high volatility and low auto-ignition point;
methyl ethyl ketone gives rise to colour formation due to desired acid catalysing an aldol type reaction with the solvent ketone; and esters such as ethyl acetate have a propensity to undergo hydrolysis in the presence of the heteropolyacid being synthesised and any neutralising acids present therein.
It has now been found that these disadvantages can be mitigated if a suitable solvent is used during the purification stages.
According to a first aspect of the present invention, there is provided a process for purifying a heteropolyacid which comprises:
subjecting an aqueous solution comprising (i) the heteropolyacid and (ii) salt impurities to at least one liquid/liquid extraction step with an organic solvent,
characterised in that the organic solvent comprises a dihydrocarbyl ether having at least 5 carbon atoms.
In other words, the dihydrocarbyl ether is such that the total number carbon atoms in the two hydrocarbyl units, one on either side of the ethereal oxygen atom, is at least 5.
The liquid/liquid extraction step enables the heteropolyacid to form a complex in situ with the organic solvent. This allows the extract to separate into two or more phases including inter alia a dense organic phase, which generally settles at or towards the bottom of the separation vessel. This phase comprises the dissolved complex of the heteropolyacid.
Preferably, the dense organic phase is recovered. This may be mixed with water to form a separate layer of a dilute aqueous mixture, from which a concentrated 20-80% w/w solution of the heteropolyacid substantially free of organic impurities may be isolated.
The starting aqueous solution of heteropolyacid may be prepared by reacting (i) an aqueous solution of one or more alkali or alkaline earth metal salts selected from a tungstate and a molybdate with (ii) an alkali or an alkaline earth metal silicate or an alkali or an alkaline earth metal phosphate. Preferably, this reaction is carried out under reflux. The ensuing reaction produces an impure aqueous solution comprising the heteropolyacid of the chosen salt and any other salt formed in situ or used as a reactant. This solution may then be filtered, cooled and subjected to the liquid/liquid extraction step of the first aspect of the present invention.
Preferably, the heteropolyacid in the aqueous solution is a silicotungstic acid.
According to a second aspect of the present invention, there is provided a process for the synthesis of a heteropolyacid substantially free of extraneous cations, said process comprising:
a. reacting (i) an aqueous solution of one or more alkali or alkaline earth metal salts selected from a tungstate and a molybdate under reflux with (ii) an alkali or an alkaline earth metal silicate or an alkali or an alkaline earth metal phosphate in the presence of an acid to form in an impure aqueous solution comprising the heteropolyacid of the chosen salt and any other salt formed in situ or used as a reactant;
b. filtering and cooling the impure aqueous solution comprising the heteropolyacid;
c. purifying the cooled, impure aqueous solution from (b) by at least one liquid/liquid extraction with an organic solvent to enable the heteropolyacid to form a complex in situ with the organic solvent and allowing the extract to separate into two or more phases including inter alia a dense organic phase which comprises the dissolved complex of the heteropolyacid and which dense organic phase is below the aqueous phase;
d. recovering the dense organic phase from (c) and mixing it thoroughly with water to form a separate layer of a dilute aqueous mixture and
e. recovering a concentrated 20-80%w/w aqueous solution of the heteropolyacid substantially free of organic impurities from the dilute aqueous mixture,
wherein the organic solvent used in the liquid/liquid extraction step (c) comprises a dihydrocarbyl ether having at least 5 carbon atoms.
Preferably, the organic solvent used in the liquid/liquid extraction step (c) is a dihydrocarbyl ether having at least 5 carbon atoms.
The dihydrocarbyl ether may have 5-20 carbon atoms, and preferably, has 5-10 carbon atoms. The dihydrocarbyl ether may be such that at least one of the hydrocarbyl groups is a branched chain alkyl group. Preferably, the branched chain alkyl group is t-butyl or amyl. At least one of the hydrocarbyl groups in the ether may be a methyl, ethyl or propyl group. Preferably, this hydrocarbyl group is a methyl or ethyl group, and most preferably, it is a methyl group. It may be advantageous to employ ethers which have different hydrocarbyl groups on either side of the ethereal oxygen. For example, the ether may have a methyl group on one side of the ethereal oxygen, and a butyl or amyl group on the other.
The ether used should be substantially immiscible with water. In other words, the ether should be subs

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