Active Site Formation in WO_x Supported on Spherical Silica Catalysts for Lewis Acid Transformation to Brønsted Acid Activity

The transformation of Lewis acid to new Brønsted acid upon hydrogen exposure of tungsten oxide supported on mesoporous spherical silica nanoparticles (SSP) catalysts has been investigated by in situ diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) with adsorbed NH 3 . The principal goal of this work is to explore the influence of W species on the support and the activating processes involved in the formation of catalytically active sites for Lewis acid transformation. The samples were prepared by two preparation methods including incipient wetness impregnation and incorporation of W species to SSP support with the sol–gel method. Then, they were activated by N 2 pretreatment compared with H 2 before investigation. From the results of in situ DRIFTS experiments, the Lewis acid sites were changed to the Brønsted acid sites upon hydrogen exposure over activated catalysts. The abundance of tungstate W 5+ species is found to facilitate the change of this acidity. In addition, the performance of Lewis acid transformation was correlated with tungstate W 5+ active species, which could be more generated by activating process of H 2 and N 2 pretreatments. The impregnated W/SSP needs H 2 pretreatment to activate active site formation, while the incorporated W-SSP can be activated in both of H 2 and N 2 pretreatments because the latter prefers the formation of tungstate W 5+ species in catalyst, instead of W dispersed on SSP surface to form W surface structure characteristic of the W 6+ species.