One step microwave-assisted synthesis of nanocrystalline WOx–ZrO2 acid catalysts

Abstract

Nanocrystalline ZrO₂ and WO_x–ZrO₂ catalysts with good control over the zirconia phase and the nature of the W species were synthesized by a microwave-assisted aqueous route. The monoclinic ZrO₂ (m-ZrO₂) and tetragonal ZrO₂ (t-ZrO_z) polymorphs were isolated through accurate control of the synthetic conditions, in acidic and basic media, respectively. The evolution of the zirconia and W species under annealing of the WO_x–ZrO₂ nanocomposites at 500 °C and 800 °C was studied, and the resulting materials were tested as catalysts for aldoxime dehydration and hydration of alkynes for the production of nitriles and carbonyls, respectively. In the dehydration reaction, the most active species are W(VI) in tetrahedral coordination, independently of the ZrO₂ polymorph, while in the hydration reaction the zirconia phase plays a key role, as the tungsten doped t-ZrO₂ appears to be the most active catalyst.