Support-substrate interactions which activate heterogeneous olefin metathesis catalysts

INOR 869

Anthony Moses, moses@engineering.ucsb.edu1, Christina Raab1, Naseem Ramsahye1, Heather Leifeste1, Swarup Chattopadhyay1, Bradley F. Chmelka, bradc@engineering.ucsb.edu1, Juergen Eckert, juergen@mrl.ucsb.edu2, and Susannah L. Scott, sscott@engineering.ucsb.edu3. (1) Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, CA 93106, (2) Materials Research Laboratory, University of California, Santa Barbara, Santa Barbara, CA 93106, (3) Departments of Chemical Engineering and Chemistry, University of California, Santa Barbara, Santa Barbara, CA 93106-5080
Heterogeneous olefin metathesis catalysts are plagued by low turnover numbers, hindering their industrial application in commodity chemical processes. In order to design more efficient catalysts, it is necessary to understand which the support-substrate interactions are necessary for activity.

Therefore, we have investigated the heterogeneous olefin metathesis catalyst silica-alumina supported methyltrioxorhenium (MeReO3) by studying its structure and activity for the self-metathesis of propylene. Kinetic studies show that the majority of grafted MeReO3 fail to activate, and that the activated sites undergo severe deactivation.

Using X-ray absorption spectroscopy, NMR spectroscopy, infrared spectroscopy, and computational modeling, we have determined that MeReO3 grafts to Lewis acidic Al sites through a two-point Lewis acid-base interaction, shown below, as well as to Brønsted acidic surface silanols. However, MeReO3 is only activated by the Lewis acid sites, and not surface silanols, as previously reported.

Thus, “capping” the surface silanols with hexamethyldisilazane eliminates the Brønsted acid sites, while preserving the Lewis acid sites necessary to graft and activate MeReO3. MeReO3 on capped silica-alumina is as active as when supported on uncapped silica-alumina. Based on these results, we investigated other rhenium-based heterogeneous metathesis catalysts, and found that Lewis acidity is also necessary for olefin metathesis activity, while Brønsted acidic silanols hinder activity.

 

Inorganic Catalysts
8:30 AM-11:50 AM, Wednesday, 13 September 2006 Moscone Center -- Room 302, Oral

Division of Inorganic Chemistry

The 232nd ACS National Meeting, San Francisco, CA, September 10-14, 2006