(180d) Dramatic Enhancement of Olefin Metathesis Activity for CH3ReO3 By Chlorination of ?-Al2O3

Olefin metathesis is important in large-scale industrial processes, such as SHOP (Shell Higher Olefins Process) and OCT (Olefin Conversion Technology). It is also relevant for the transformation of biomass-derived FAMEs into lubricants and fuels. The development of more active, robust and functional group-tolerant catalysts requires insight into the activation of the catalytically competent sites, which is facilitated by catalysts with a high fraction of active sites. MTO (methytrioxorhenium) shows excellent activity for olefin metathesis when deposited on silica-alumina, where the active site is generated by interaction of MTO with strong Lewis acid sites [1]. MTO supported on γ-alumina is much less active. Recently, ZnCl₂ was reported to have a promoting effect for MTO supported on γ-Al₂O₃,resulting in a dramatic increase in its olefin metathesis activity [2]. However, Zn²⁺ does not contribute to the promotion effect. The catalytic activity of MTO on γ-alumina can be enhanced simply by chlorinating the support. The activities of MTO supported on γ-Al₂O₃ and on chlorinated γ-Al₂O₃ were compared in propylene metathesis, and the role of chloride in the promotion effect was investigated. Propylene metathesis activity is ca. 35 times higher for MTO supported on chlorinated γ-Al₂O₃, compared to MTO on unmodified γ-Al₂O₃. Active site counting [4] reveals a large increase in the fraction of active sites in the presence of chloride. In the Re L₃-edge EXAFS, an R-space feature at 2.3 Ǻ attributed to a Re-Cl single scattering path appears only for MTO on the chlorinated support. The ¹³C CP-MAS NMR of ¹³C-labeled MTO/Cl-γ-Al₂O₃ shows signals at 30 and 37 ppm. The former was reported previously for MTO interacting with Lewis acidic Al sites [1], while the latter, which is not present in the absence of Cl^-, may be associated with the active sites or their immediate precursors.

1. Moses, A. W., Raab, C., Nelson, R. C., Leifeste, H. D., Ramsahye, N. A., Chattopadhyay, S., Eckert, J., Chmelka, B. F., and Scott, S. L. J. Am. Chem. Soc. 129, 8912 (2007).
2. Oikawa, T., Masui, Y., Tanaka, T., Chujo, Y., and Onaka, M. J. Organomet. Chem. 692, 554 (2007).
3. Tovar, T. M., Stewart, M. S., and Scott, S. L. Top. Catal. 55, 530 (2012).
4. Chauvin, Y., and Commereuc, D. J. Chem. Soc., Chem. Commun. 6 462 (1992).