Active Site Formation in WO<sub><i>x</i></sub> Supported on Spherical Silica Catalysts for Lewis Acid Transformation to Brønsted Acid Activity
Sirawat Boonpai, Sippakorn Wannakao, Joongjai Panpranot, Bunjerd Jongsomjit, Piyasan Praserthdam
Abstract
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 NH3. 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 N2 pretreatment compared with H2 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 W5+ species is found to facilitate the change of this acidity. In addition, the performance of Lewis acid transformation was correlated with tungstate W5+ active species, which could be more generated by activating process of H2 and N2 pretreatments. The impregnated W/SSP needs H2 pretreatment to activate active site formation, while the incorporated W-SSP can be activated in both of H2 and N2 pretreatments because the latter prefers the formation of tungstate W5+ species in catalyst, instead of W dispersed on SSP surface to form W surface structure characteristic of the W6+ species.