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(43e) Investigating the Genesis of Catalytic Promotion for Silica-Supported Molybdenum Oxide during Propylene Metathesis

Zhu, R. - Presenter, Massachusetts Institute of Technology
Adamji, H., Massachusetts Institute of Technology
Zhu, J., Massachusetts Institute of Technology
Head, A., Brookhaven National Laboratory
Copéret, C., ETH Zurich
Roman, Y., MIT
Olefin metathesis is a robust strategy for various applications ranging from fine chemical syntheses to large-scale production of petroleum commodities. Recently, we showed that co-feeding substituted olefins (2,3-dimethyl-1-butene, i4ME) during propylene self-metathesis over silica-supported W and Mo catalysts increases reaction rates by orders of magnitude. We hypothesized the co-fed promoter (i4ME) facilitates the active site formation through a 1,2-proton shift mechanism resulting in a drastic increase in the number of active metal centers. Here, we present this kinetic and spectroscopic study to investigate further this promotion effect using silica-supported Mo catalysts. Compared with WOx/SiO2 catalysts, we observed similar kinetic trends with the MoOx/SiO2 catalysts: i) the co-fed i4ME decreases the apparent activation energy of propylene self-metathesis and ii) the apparent reaction order of propylene with a positive reaction order of i4ME. Exposing the pretreated catalyst to 1 mol.% i4ME and 10 mol.% propylene separately in transmissive in-situ Fourier-transform infrared (FTIR) spectrometer, i4ME perturbs more Si-OH groups than propylene at 50 °C, suggesting that co-feeding i4ME provides more accessible protons involved in the 1,2-proton shift mechanism. We identify a structure-function relationship in which co-fed i4ME influences the reactivity of atomically dispersed molybdate more efficiently than the reactivity of molybdate clusters. With i4ME co-feeding, the Mo loading normalized turnover frequency of catalysts with low Mo loading (1.4-3.6 wt.%) is higher than high Mo loading (6.6-14.2 wt.%). Our results from in-situ ultraviolet-visible (UV-Vis) spectroscopy and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) show the co-fed i4ME further reduces surface Mo ions (V/VI) to Mo (IV), which has been proposed either as the active site or a key intermediate formed during olefin metathesis. These observations have reinforced our hypothesis that i4ME can facilitate active site formation during propylene self-metathesis and shown that only atomically dispersed molybdate on the catalyst surface is active for propylene self-metathesis.