Catalyst Activation and Kinetics for Propylene Metathesis by Supported WO_<i>x</i>/SiO₂ Catalysts

The activation and kinetics of propylene metathesis by well-defined supported WO_<i>x</i>/SiO₂ catalysts were investigated with transient temperature-programmed surface reaction (TPSR) spectroscopy and steady-state olefin metathesis. The TPSR measurements revealed for the first time that catalyst activation with olefins creates three distinct activated sites [highly active (<i>T</i>_p ∼ 160 °C), modestly active (<i>T</i>_p ∼ 450 °C), and sluggishly active (<i>T</i>_p ∼ 600–750 °C)]. The number and reactivity of the activated surface WO_<i>x</i> sites increase with activation temperature, olefin partial pressure, and olefin size (C₂⁼ ≪ C₃⁼ ∼ C₄⁼). Only propylene and 2-butene are able to generate the highly active sites because of the presence of CHCH₃ groups in these olefins, while ethylene is able to create only the sluggish active sites. The specific activity (TOF) of the surface WO_<i>x</i> sites increases with surface coverage because of the increasingly strained configuration (larger bridging O–W–O bond angles) at higher levels of surface WO_<i>x</i> coverage on the SiO₂ support. Steady-state propylene metathesis exhibits a first-order dependence on propylene partial pressure, and the rate-determining step is propylene adsorption on activated sites. Propylene metathesis by supported WO_<i>x</i>/SiO₂ catalysts can be represented by the simple Langmuir-type kinetics of rate = <i>kK</i>_ads[C₃⁼]/(1 + <i>K</i>_ads[C₃⁼]), where <i>k</i> is an Arrhenius rate constant and <i>K</i>_ads is the propylene equilibrium adsorption constant, that during typical high-temperature metathesis reduces to rate = <i>kK</i>_ads[C₃⁼] because the <i>K</i>_ads is a small value