Hydrothermal Synthesis of Hexagonal WO3 Microspheres with APT

Hexagonal WO3 (h-WO3) has caught a great attention due to its unique tunnel structure. In h-WO3, six-membered rings composed of WO6 octahedra are arranged along the (001) plane by sharing corner oxygens, with formation of open tunnels that have a theoretical intracrystalline tunnel size of 0.39 nm, along the c-axis throughout the entire unit cell.

Since h-WO3 is generally a metastable phase, Hydrothermal synthesis is a low-temperature, simple, low-energy process, and is therefore one of the most promising among various methods for h-WO3 preparation.

h-WO3 microspheres were hydrothermally synthesized without the use of any organic or inorganic structure-directing agents. The effects of the hydrothermal parameters on the formation of impurity phases were investigated in detail. Pure-phase h-WO3 microspheres were used as the support for a Pd catalyst for the hydrogenation of styrene; a high catalytic activity was obtained.

The hydrothermal synthesis of hexagonal WO3 (h-WO3) microspheres with ammonium paratungstate (APT) is as follows:

Analytical-grade ammonium paratungstate (APT), Na2WO4, hydrochloric acid, palladium chloride, ascorbic acid, styrene, ethylbenzene, biphenyl, and ethanol were used as received.

Firstly, APT (2.5355 g) and deionized (DI) water (65 mL) were added to a conical flask. HCl solution (4 mol/L, 2 mL) was added dropwise under vigorous stirring for 1.0 h. The resultant solution was transferred to a Teflon-lined stainless-steel autoclave of capacity 100 mL. The autoclave was sealed, heated to 100 °C, kept in an isothermal state for 12.0 h, and then cooled to room temperature. The product was collected by filtration, washed three times with DI water, and dried at 80 °C for 24.0 h. Finally hexagonal WO3 microspheres were prepared.

In summary, hexagonal WO3 microspheres (h-WO3) were synthesized by a hydrothermal method, with ammonium paratungstate (APT) and hydrochloric acid as the raw materials. Pure-phase h-WO3 microspheres were synthesized at a molar ratio of APT:HCl = 1:9.6 and a hydrothermal temperature of 100 °C. They were used as a support for a Pd catalyst, which gave a high catalytic performance in the hydrogenation of styrene. The TOF value of the prepared Pd/h-WO3 reached 3050 h−1, and the recycling performance of the catalyst proved that it had excellent recyclability.

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