| Olefin metathesis enables the synthesis of organic chemicals by forming new carbon-carbon bonds, through the exchange of olefinic carbon atoms and their substituents. Olefin metathesis requires a catalyst to proceed, and heterogeneous catalysts are desirable for commodity chemical production. However, these catalysts are plagued by low turnover numbers, making many industrial processes involving functionalized olefins, such as the production of commodity chemicals from renewable seed oils, uneconomical. Therefore, heterogeneous metathesis catalysts were studied, in order to identify precursor/support interactions which impart catalytic activity. This was accomplished by relating catalyst performance to its structure, using kinetic studies; NMR, IR and EXAFS spectroscopies; elemental analysis; and DFT calculations.; Methyltrioxorhenium (MeReO3) grafted onto an amorphous silica-alumina is spontaneously active for the metathesis of olefins at room temperature. MeReO3 remains intact upon grafting, and forms two distinct sites at its maximum uptake on the silica-alumina surface (10 +/- 1 wt% Re). At low loadings (<1 wt% Re), MeReO3 is grafted to Al sites via two Lewis acid-base interactions: the Re=O ligand coordinates to an Al atom, and an adjacent O atom coordinates to the Re. At higher loadings, MeReO 3 is also hydrogen-bonded to surface hydroxyls. Catalyst activity is associated with the Lewis acid-base interaction at strongly Lewis acidic Al sites.; Ammonium perrhenate (NH4ReO4) supported on an amorphous silica-alumina is also spontaneously active for metathesis at room temperature. Perrhenate is anchored to silica-alumina by reaction with surface hydroxyls; however, other interactions are also present. A support O atom is coordinated to Re, and a Re=O ligand coordinates to a surface Al. The latter interaction likely activates supported perrhenate. SnMe4, a catalyst promoter for supported perrhenate, modifies the catalyst in two ways. It is grafted to surface hydroxyls on the silica-alumina support, capping sites which generate catalyst poisons. Furthermore, two equivalents of SnMe4 react with the supported perrhenate to form a dimethylrhenium(VII) oxide complex, which appears to activate more readily for olefin metathesis than supported perrhenate. |
