- Phosphotungstic acid acidification-ether extraction method
In 1939, Booth et al. reported the synthesis method of 12-phosphotungstic heteropolyacid. In 1952, Tourkey et al. improved the synthesis method of Booth. The method of Booth and Tourkey to synthesize phosphotungstic heteropolyacid is still in use today. Acidification-ether extraction is a classic method for preparing 12-phosphotungstic acid. Usually, the heteroatom oxyacid and coordination atom oxyacid or coordination atom oxide are mixed evenly in a certain proportion, and after heating and refluxing for a long time, the heteropolyacid is obtained through acidification. Under acidic conditions, heteropolyacids form oily matter with ether, and solid heteropolyacids can be obtained after ether volatilizes. Care must be taken when using this method:
(a) Different reaction temperatures result in different structures of heteropolyacids. At the respective appropriate pH, the lower temperature mainly generates 1:12 series heteropolyacids, and 2:18 series heteropolyacids are generated when reflux boiling.
(b) Heteropoly compounds are very sensitive to pH. In the preparation of polyacids, the degree of acidification is very important. Sometimes the difference of 0.01 results in completely different product structures.
(c) The sequence of introduction of heteroatoms is different, the ratio of heteroatoms to coordination atoms in the prepared heteropolyacid will change, and the structure of the product will also change.
(d) Diethyl ether has a very low boiling point, is extremely volatile, poisonous, and flammable, so extra care must be taken when handling it.
- Phosphotungstic heteropolyacid ion exchange method
There are many unfavorable factors in the traditional acidification-ether extraction method, and the ion exchange method is developed on this basis. Ion exchange method For those heteropolyacids that are not easy to be extracted, the corresponding amine salt can be obtained first, and the heteropolyacid can be obtained by hydrogen ion exchange resin. The ion-exchange method avoids the use of ether, and the production safety is improved, but the production cycle is long and the production capacity is limited, which has become the bottleneck of industrial production.
US 2503991, US 3288562, and US 3361518 successively used H-type cation exchange resin method to prepare phosphotungstic acid, avoiding the use of diethyl ether, but the process is complicated, a large amount of solvent needs to be evaporated, and the energy consumption is huge. CN101417793 also discloses a preparation of 12-phosphotungstic acid by ion exchange resin method. This method also has low production efficiency and great energy consumption, so it is not suitable for industrial production.
- Two-step preparation technology of phosphotungstic heteropoly acid
In 2012, Chen Qiyuan, Li Jie, and Li Dongwei from the Institute of Metallurgy and Applied Physical Chemistry of Central South University announced a new method for preparing 12-phosphotungstic heteropolyacid (patent publication number: CN102659181A), which uses sodium tungstate as The raw material is 12-phosphotungstic acid prepared by acidification-hydrothermal method, and crystallized by salting out method, which successfully avoids the use of ether extractant and greatly enhances the safety of 12-phosphotungstic heteropoly acid preparation. Li Dongwei also explained in detail the influencing factors and principles of the preparation of 12-phosphopolyacid by this method.
- Other preparation methods of phosphotungstic heteropoly acid
Qin Yunan mentioned that electrodialysis was used to prepare phosphotungstic acid. Using tungstate and phosphoric acid as starting materials, direct current was applied to the two poles, and compounds containing coordination atoms and heteroatoms interacted in the anode chamber to form heteropolyacids. The yield of the heteropolyacid is nearly 100%, and no waste is generated.
In recent years, many patents at home and abroad have disclosed the preparation process of 12-phosphotungstic acid. CN 1978327, CN 1990387 discloses a preparation method of phosphotungstic heteropoly acid. The method uses sodium tungstate as a starting material, prepares an active tungstic acid intermediate through acidification of sodium tungstate, and then reacts with phosphoric acid to obtain phosphotungstic acid. Finally, phosphotungstic acid is precipitated by adding inorganic acid, and the yield is about 78%. This method avoids the use of flammable ether extractant, but this process uses a large amount of strong acid as the acid precipitant, which seriously corrodes the equipment and requires high equipment. , high production cost. CN 1301592 discloses a method for preparing phosphotungstic acid. The method uses sodium tungstate as a raw material, adds an oxidizing agent and an inorganic acid into the sodium tungstate solution, and then adds a reducing agent after reacting at -5~30°C to obtain a Active tungstic acid, 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 large amount of phosphate groups. CN 1544483 A uses an organic solvent C2-C20 ether for complex extraction to reduce the phosphate radical in phosphotungstic acid, but the amount of organic solvent is large, which reduces the safety of large-scale industrial production.
US 3446577 discloses the preparation of Keggin-type phosphotungstic acid by using tungsten hexachloride as raw material. Tungsten hexachloride is expensive, which greatly increases the preparation cost and is not suitable for industrial production. GB 1311849 discloses a method for synthesizing phosphotungstic acid. In this method, after mixing calcium tungstate and phosphoric acid solution stoichiometrically, adding inorganic acid, heating for a long time to generate calcium phosphotungstate, and separating calcium phosphotungstate Phosphotungstic acid is obtained by adding sulfuric acid. This method needs to use a large amount of concentrated sulfuric acid to corrode equipment, and the impurity content in phosphotungstic acid is high.
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