Comparison of SiO2-supported molybdena, tungsta and rhenia catalysts for olefin metathesis†

Abstract

The characteristics of supported MO_x/SiO₂ catalysts (M = Re, Mo, W) for olefin metathesis were compared side-by-side to understand the differences among these catalyst systems. The series of SiO₂-supported metal oxide (ReO_x, MoO_x, WO_x) catalysts, with maximum achievable dispersion of surface MO_x sites, was synthesized by incipient-wetness impregnation of the corresponding aqueous precursors, molecularly characterized (in situ Raman and DRIFTS), chemically probed with transient studies (C₃H₆-TPSR-MS, C₃H₆-TPSR-IR, and C₂H₄/2-C₄H₈ titration with C₂H₄-TPSR-MS) and steady state activity for C₃H₆ metathesis. Under dehydrated conditions, the initial SiO₂-supported surface metal oxide sites are fully dispersed as isolated sites on SiO₂ and present as isolated tri-oxo (O)₃ReO sites, isolated di-oxo (O)₂MoO₂ sites and a mixture of isolated di-oxo (O)₂WO₂ and mono-oxo (O)WO₄ sites (dioxo ≫ mono-oxo) anchored at the isolated Si–OH surface hydroxyls of SiO₂ support. High temperature propylene pre-treatments were required to activate the SiO₂-supported MO_x catalysts, but also resulted in MO_x volatilization (ReO_x > MoO_x ≫ WO_x). The number of activated surface MO_x sites and TOF values varied with the specific MO_x site, activation temperatures and reaction temperatures, which are related to the stability of the surface MO oxo and Mcarbene bonds. The higher thermal stability, activity and TOF of the surface WO_x sites on SiO₂ at elevated temperatures relative to the supported ReO_x/SiO₂ and MoO_x/SiO₂ catalysts account for adoption of the supported WO_x/SiO₂ catalyst as the industrial olefin metathesis catalyst among these supported MO_x/SiO₂ catalysts.