Sodium Tungstate Preparing Pyrochlore Type Tungsten Oxide

The chemical formula of pyrochlore type tungsten oxide is  H₂W₂O₇. Pyrochlore type tungsten oxide has recently found its wide applications in many different areas such as photoelectron-chemistry, functional material and sensing. In addition, pyrochlore type tungsten oxide can be used as intermediate product for the industrial production of WO₃. There are many kinds of synthesis methods of the pyrochlore type tungsten oxide. And there are two main types: Aurivillius type compound as precursor, pyrochlore type tungsten oxide is obtained by acid treatment. The second method is hydrothermal synthesis method, namely sodium tungstate solution acidification to a certain pH range and it can be obtained by hydrothermal reaction for a certain time. The hydrothermal method that has overwhelming superiorities than others also has its own sets of flaws such as high cost, long reaction time and low quality product. Therefore, how to intensify the preparation process of H₂W₂O₇ to obtain high quality product with low cost has become an urgent problem.

In this article, XRD, TGA-DSC, and SEM were used to characterize the products. The sodium content in the H₂W₂O₇ is

determined by chemical analysis and most sodium could be removed by ion exchange method. 

The main research results are listed as follows.

(I)The seeded precipitation and salting-out precipitation methods are not appropriate to prepare H₂W₂O₇ but salting-out precipitation method suggests a new way to prepare Na₂WO₄ and Na₂W₂O₇.

(2)Under hydrothermal conditions, the addition of oxalic acid has its optimum value. When C (Na₂WO₄) =150g/L, T=126℃, t=11h, adding oxalic acid 65g/L, the reaction ratio reaches its peak value 91%. Additive C plays a part in reaction ratio and seeds of H₂W₂O₇ could accelerate reaction rate. The W₇O₂₄^6- ions may be the precursor in precipitating H₂W₂O₇, but the Fe²⁺ ions seem to change the precipitation mechanism.            

(3)The TGA-DSC curve shows a continuous water loss from ambient to 450℃, and the H₂W₂O₇ remains stable under 340℃. The SEM pictures of theH₂W₂O₇ exhibit cubic morphology which can be changed by organic surfactant.

(4)Sodium is an inevitable impurity with its content varying from 3.3% to 4.9%, and ion exchange is a valid way to remove sodium content to about 0.3%.

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