Acidification-ether extraction is a classic method to prepare 12 phosphotungstic acid. Usually, heteropoly acid is obtained by mixing heteroatom oxygen acid with coordination atom oxygen acid or coordination atom oxide in a certain proportion, heating and refluxing for a long time, and acidifying. Heteropoly acid and ether form oil under acidic conditions, and solid heteropoly acid is obtained after ether volatilizes. When using this method, we must pay attention to:
(a) Heteropoly acids have different structures at different reaction temperatures. At their respective suitable pH, 1: 12 series heteropoly acids are mainly generated at low temperature, and 2: 18 series heteropoly acids are generated by reflux boiling.
(2) Heteropoly compounds are very sensitive to pH. In the preparation of polybasic acids, the degree of acidification is very important, and sometimes the product structures are completely different when the difference is 0.05438+0.
(c) The introduction sequence of heteroatoms is different, and the ratio of heteroatoms to coordination atoms in the prepared heteropoly acid will change, and the product structure will also change.
(d) Ether has a very low boiling point, which is highly volatile, toxic and flammable, so it must be handled with extra care.
In the 1960s and 1970s, Britain, the United States and other countries published a large number of patents for the preparation of 12- phosphotungstic acid, and U.S. Pat. No.3,425,794, U.S. Pat. No.3,446,575 and U.S. Pat. No.3,42815 disclosed improved processes based on sodium tungstate acidification and ether extraction, but the yield was less than 75%. There are many disadvantages in the traditional acidification-ether extraction method, and on this basis, the ion exchange method was developed. For those heteropolyacids that are not easy to be extracted by ion exchange, the corresponding amine salts can be obtained first, and then the heteropolyacids can be obtained by hydrogen ion exchange resin. The ion exchange method avoids the use of ether and improves the production safety, but the long production cycle and limited production capacity have become the bottleneck of industrial production.
US 250399 1, US 3288562 and US 336 15 18 successively used h-type cation exchange resin to prepare phosphotungstic acid, which avoided the use of ether, but the process was complicated, a large amount of solvent needed to be evaporated, and the energy consumption was extremely high. Cn101417793 also discloses a method for preparing 12- phosphotungstic acid with ion exchange resin, which is also low in production efficiency and high in energy consumption, and is not suitable for industrial production. Qin Yunan mentioned that phosphotungstic acid was prepared by electrodialysis. Tungstate and phosphoric acid are used as starting materials, and direct current is applied to both poles. Compounds containing coordination atoms and heteroatoms interact in the anode chamber to generate heteropoly acids. The yield of heteropoly acid in this method is close to 100%, and no waste is produced.
In recent years, many patents at home and abroad have disclosed the preparation process of 12- phosphotungstic acid. Cn 1978327 and CN 1990387 disclose a preparation method of phosphotungstic acid. In this method, sodium tungstate was used as the starting material, and the active tungstic acid intermediate was prepared by acidification of sodium tungstate, and then reacted with phosphoric acid to obtain phosphotungstic acid. Finally, inorganic acid was added to precipitate phosphotungstic acid, and the yield was about 78%. This method avoids using flammable ether extractant, but uses a lot of strong acid as acid precipitant. CN 130 1592 discloses a method for preparing phosphotungstic acid. The method takes sodium tungstate as raw material, adds oxidant and inorganic acid into the sodium tungstate solution, reacts at -5 ~ 30℃ and then adds reducing agent to obtain active tungstic acid, and the active tungstic acid reacts with dilute phosphoric acid to prepare phosphotungstic acid solution. The preparation process of this method is complicated, and the obtained phosphotungstic acid contains a lot of phosphate radicals. Cn 154483A uses organic solvent C2-C20 ether for complex extraction to reduce phosphate in phosphotungstic acid, but the amount of organic solvent is large, which reduces the safety of large-scale industrial production.
American patent. 446,577 discloses the preparation of Keggin phosphotungstic acid from tungsten hexachloride. Tungsten hexachloride is expensive, which greatly increases the preparation cost and is not suitable for industrial production. GB 13 1 1849 discloses a method for synthesizing phosphotungstic acid. The method comprises the following steps: mixing calcium tungstate and phosphoric acid solution according to stoichiometry, then adding inorganic acid, and heating for a long time to generate calcium phosphotungstate. After separating calcium phosphotungstate, sulfuric acid was added to prepare phosphotungstic acid. This method uses a lot of concentrated sulfuric acid, which corrodes equipment and has high impurity content in phosphotungstic acid.