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

Supported WO_x/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% WO_x/SiO₂, only surface WO_x 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 WO_x sites on SiO₂ (>90%). The isolated nature of the surface WO_x sites was confirmed with in situ UV-vis spectroscopy. The surface WO_x sites are activated by exposure to propylene at elevated temperature that removes oxygen from these sites. The activation process produces a highly active surface WO_x site that can perform olefin metathesis at 150-250 °C. For 8% WO_x/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 WO_x 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 first time that molecular level structure-activity/selectivity relationships have been established for propylene metathesis by conventionally impregnated supported WO_x/SiO₂ catalysts.