Catalyst Activation and Kinetics for Propylene Metathesis by Supported WOx/SiO2 Catalysts
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- 作者：Soe LwinIsrael E. Wachs
- 刊名：ACS Catalysis
- 出版年：2017
- 出版时间：January 6, 2017
- 年：2017
- 卷：7
- 期：1
- 页码：573-580
- 全文大小：384K
- ISSN：2155-5435
文摘
The activation and kinetics of propylene metathesis by well-defined supported WOx/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 (Tp ∼ 160 °C), modestly active (Tp ∼ 450 °C), and sluggishly active (Tp ∼ 600–750 °C)]. The number and reactivity of the activated surface WOx 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 WOx sites increases with surface coverage because of the increasingly strained configuration (larger bridging O–W–O bond angles) at higher levels of surface WOx 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 WOx/SiO2 catalysts can be represented by the simple Langmuir-type kinetics of rate = kKads[C3=]/(1 + Kads[C3=]), where k is an Arrhenius rate constant and Kads is the propylene equilibrium adsorption constant, that during typical high-temperature metathesis reduces to rate = kKads[C3=] because the Kads is a small value.