The olefin metathesis reaction was commercialized in the late 1960s to produce ethylene and butane from propylene in the Phillips Triolefin Process. However, due to a global propylene shortage caused by the shift to lighter feedstocks derived from shale gas fracking, currently, the reverse reaction is desired. Heterogenous supported WOx/SiO2 catalysts are one type of commercial catalyst employed. They are used in the Phillips Triolefin Process and operate between 400-600°C. To probe the molecular details of the supported WOx/SiO2 catalysts, modern in situ spectroscopy was applied. In situ Raman spectroscopy revealed that at loadings less than 8%WOx/SiO2, two surface WOx structures are present: the dioxo (O=)2WO2 and mono-oxo O=WO4. In situ XANES determined that the dioxo WO4 dominates the surface structures and are present in greater than 90% abundance. In situ UV-vis was used to confirm the presence of only isolated surface WOx sites. Activation of these catalysts at elevated temperatures produced highly active surface WOx sites, determined to be capable of performing olefin metathesis at temperatures as low as ~150-200°C. At loadings equal to or greater than 8%WOx/SiO2, in situ Raman spectroscopy indicated the presence of crystalline WO3 nanoparticles as well, which are inactive for propylene metathesis. The kinetics of propylene metathesis were probed using transient TPSR spectroscopy and steady-state propylene metathesis. The TPSR studies revealed for the first time that catalyst activation with olefins creates three distinct activated sites – highly active (TP ~160°C), moderately active (TP ~450°C), and sluggishly active (TP ~600-750°C). Additionally, the number and reactivity of the activated surface WOx sites was determined to increase with activation temperature, olefin partial pressure, and olefin size (C2= &lt;&lt; C3= ~ C4=). The highly active surface WOx sites were only produced by propylene and 2-butene because of the presence of the =CHCH3 group in these olefins. The TOF of the WOx surface sites increased with surface coverage due to the increasing strained configuration (larger O-W-O bond angles) at higher surface WOx coverage on SiO2. The steady-state propylene metathesis studies indicated a first order dependence on propylene partial pressure. The rate-determining step is therefore propylene adsorption on the activated WOx surface sites. This presentation will address the kinetics of olefin metathesis by supported WOx/SiO2 catalysts.

Olefin metathesis by supported WOx/SiO2 catalysts: Structure-Reactivity relationships, number of active sites, and kinetics

253rd National Meeting (2017)

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