(676j) Olefin Metathesis By Supported WOx/SiO2 Catalysts

The kinetics of propylene metathesis by supported WO_x/SiO₂ catalysts were studied using in situ XAS (XANES/EXAFS), temperature programmed surface reaction (TPSR) and steady-state experiments. The in situ XAS experiments with well-defined catalysts provide information about the reducibility of different WO_x phases (surface dioxo and mono-oxo WO_x species and WO₃ crystalline nanoparticles). Various TPSR experiments and steady-state partial pressure experiments allow for derivation of kinetic equations and determination of important kinetic parameters. Although WO₃ crystals are more reducible upon contact with olefins, and thus reduce desorption activation energy (E_d), they do not contribute to metathesis reactivity and increase apparent activation energy (E_app). The steady-state propylene partial pressure experiments show the first-order dependence of reactivity on propylene partial pressure. This is also confirmed by C₃= or C₂=/C₄ TPSR experiments which provide the same peak temperature (T_p) value. Ethylene (C₂=) cannot create sites unlike propylene (C₃=) or 2-butene (C₄=).