MTO is activated by alumina for olefin metathesis under mild conditions, presumably by support-induced tautomerization of the methyl group, but the role of the alumina surface and the activation mechanism are still not understood. We synthesized supported MTO catalysts on two alumina supports with similar textural properties but different degrees of crystallinity. MTO grafted onto a largely amorphous mesoporous alumina is ca. 5 times more active in propene homo-metathesis than MTO grafted onto γ-Al2O3. The amorphous alumina is more Lewis acidic and has a lower surface hydroxyl content relative to γ-Al2O3. The number of active sites remains constant as the MTO loading increases, suggesting that activation occurs on specific Lewis acid sites whose abundance limits the number of sites that eventually participate in metathesis. Even higher metathesis activities were found for MTO grafted onto supports with higher Lewis acidities, such as SiO2-Al2O3, Cl-Al2O3 and F-Al2O3. However, catalysts made with these supports vary greatly in their stability, and thus their total productivity. The specific role of Lewis acid sites in promoting active site formation, and the possible role of Brønsted acid sites in their subsequent deactivation, will be discussed.

Role of Lewis and Brønsted acid sites of alumina in the activation of methyltrioxorhenium (MTO) for olefin metathesis

254th National Meeting (2017)

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