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 WO<sub><i>xi>sub>/SiO<sub>2sub> 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>Ti><sub>psub> ∼ 160 °C), modestly active (<i>Ti><sub>psub> ∼ 450 °C), and sluggishly active (<i>Ti><sub>psub> ∼ 600–750 °C)]. The number and reactivity of the activated surface WO<sub><i>xi>sub> sites increase with activation temperature, olefin partial pressure, and olefin size (C<sub>2sub><sup>=sup> ≪ C<sub>3sub><sup>=sup> ∼ C<sub>4sub><sup>=sup>). Only propylene and 2-butene are able to generate the highly active sites because of the presence of ═CHCH<sub>3sub> groups in these olefins, while ethylene is able to create only the sluggish active sites. The specific activity (TOF) of the surface WO<sub><i>xi>sub> sites increases with surface coverage because of the increasingly strained configuration (larger bridging O–W–O bond angles) at higher levels of surface WO<sub><i>xi>sub> coverage on the SiO<sub>2sub> 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<sub><i>xi>sub>/SiO<sub>2sub> catalysts can be represented by the simple Langmuir-type kinetics of rate = <i>kKi><sub>adssub>[C<sub>3sub><sup>=sup>]/(1 + <i>Ki><sub>adssub>[C<sub>3sub><sup>=sup>]), where <i>ki> is an Arrhenius rate constant and <i>Ki><sub>adssub> is the propylene equilibrium adsorption constant, that during typical high-temperature metathesis reduces to rate = <i>kKi><sub>adssub>[C<sub>3sub><sup>=sup>] because the <i>Ki><sub>adssub> is a small value.