Wells-Dawson Phosphotungstic Acid Preparation

Heteropoly acids are heteroatom (such as P, Si, Co, etc.) oxyacids and polyatomic (such as Mo, W, V, etc.) oxyacids which are formed by condensation and dehydration according to a certain structure. They have strong acidity and redox properties, and are widely used in the field of catalysts. Phosphotungstic acid is a kind of common heteropoly acid, which has strong acidity, excellent conductivity and strong oxidation performance. It has been widely used in the field of photocatalytic degradation of organic wastewater.

Phosphotungstic acid has two structures: type Keggin and type Wells-Dawson. Among them, Keggin-type phosphotungstic acid is widely used and related research is more, but Wells-Dawson-type phosphotungstic acid is less. In fact, Wells-Dawson also has rich acids and better thermal stability, and its catalytic performance for some acids is better. Generally speaking, the preparation of high purity Wells-Dawson phosphotungstic acid requires the following steps:

1. The phosphoric acid was heated to 80-100 ~℃, the tungstic acid was divided into 6-10 parts, and mixed into deionized water at 80-100 ~℃ in batches within 1-5 hours to obtain tungstic acid aqueous solution. The molar concentration of tungstic acid in tungstic acid aqueous solution was 1.5-3 mol/L.
2. The aqueous solution of tungstic acid was added to phosphoric acid at 80-100 ℃ and heated to 100-140 ℃ by water bath or microwave for 12-24 hours. The molar ratio of phosphoric acid to tungstic acid was 7.5-15:1.

Phosphotungstic acid was obtained by mixing phosphotungstic acid solution with inorganic acid for 3-5 hours and then extracting 5-8 times with ether.

The preparation method of high purity Wells-Dawson phosphotungstic acid using tungstic acid instead of sodium tungstate as raw material can effectively avoid introducing Na ions into the reaction to reduce the purity and yield of finished phosphotungstic acid.

In this method, phosphoric acid was heated to the critical temperature of 80-100 ℃ to keep its activity, and tungstic acid with low solubility was dissolved in deionized water at 80-100 ℃ to obtain tungstic acid aqueous solution. Then the tungstic acid solution was added to phosphoric acid to make the tungstic acid molecule and phosphoric acid molecule mix evenly and keep dispersed state. The phosphotungstic acid solution was obtained by reaction at 100-140 ℃ for 12-24 hours. During the reaction, the tungstic acid molecule and phosphoric acid molecule were dehydrated and condensed to form Wells-Dawson phosphotungstic acid with P:W=2:18. The purity of Wells-Dawson type phosphotungstic acid has the advantages of high purity, less impurities and strong catalytic performance.

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