Nature of WOx Sites on SiO2 and Their Molecular Structure− Reactivity/Selectivity Relationships for Propylene Metathesis

Supported WOx/SiO2 catalysts were investigated for propylene metathesis as a function of tungsten oxide loading and temperature. The catalysts were synthesized by incipient-wetness impregnation of an aqueous ammonium metatungstate solution onto the silica support and calcined at elevated temperatures to form the supported tungsten oxide phase. In situ Raman spectroscopy under dehydrated conditions revealed that below 8% WOx/SiO2, only surface WOx sites are present on the silica support: dioxo (O)2WO2 and mono-oxo OWO4. The in situ XANES analysis showed that dioxo surface WO4 sites were the dominant surface WOx sites on SiO2 (>90%). The isolated nature of the surface WOx sites was confirmed with in situ UV−vis spectroscopy. The surface WOx sites are activated by exposure to propylene at elevated temperature that removes oxygen from these sites. The activation process produces a highly active surface WOx site that can perform olefin metathesis at ∼150−250 °C. For 8% WOx/SiO2 and higher tungsten oxide loading, crystalline WO3 nanoparticles (NPs) are also present, and their amount increases with greater tungsten oxide loading. WO3 NPs, however, are not active for propylene metathesis. The acid character of the surface WOx sites (Lewis) and WO3 NPs (Brønsted) is responsible for formation of undesirable reaction products (C4−C6 alkanes and dimerization of C2 to C4). This study represents the f irst time that molecular level structure−activity/selectivity relationships have been established for propylene metathesis by conventionally impregnated supported WOx/SiO2 catalysts.