Silica-Supported, Single-Site Sc and Y Alkyls for Catalytic Hydrogenation of Propylene

Single-site Sc and Y on silica catalysts have been prepared by aqueous and organometallic grafting methods. The former yields Y(III) ions with five bonds at an average bond distance of 2.31 angstrom by X-ray absorption spectroscopy. Although the aqueous synthesis gave single-site Y with low coordination number, these were not catalytic for alkane dehydrogenation or olefin hydrogenation. Single-site Sc(III) and Y(III) species were also prepared by grafting Sc(CH2Si(CH3)(3))(3)(THF)(2) and Y(CH2Si(CH3)(3))(3)(THF)(2), respectively, and these are catalysts for olefin hydrogenation at temperatures from about 60 to 100 degrees C; however, they were thermally unstable at higher temperatures necessary for alkane dehydrogenation. The structure of the grafted Y complex was determined by X-ray absorption spectroscopy, IR, and NMR. Grafting led to protonolysis of two of the three CH2Si(CH3)(3) ligands. Additionally, there was loss of one THE ligand. The EXAFS indicated that there were four Y ligand bonds in the surface species, two at 2.16 angstrom and two at 2.39 angstrom. The metal alkyl ligand was thought to be necessary for catalytic activity and likely proceeds through a sigma bond metathesis mechanism. In the single-site centers without alkyl bonds, Sc and Y ions cannot generate metal alkyl or metal hydride moieties in situ. We conclude that this is likely due to the very high M-O-Si bond strengths, which must be broken through heterolytic dissociation of C-H bonds during alkane activation for either alkane dehydrogenation or olefin hydrogenation reactions. This study demonstrates the importance of precatalyst choice versus in situ formation of reactive intermediates to produce active catalysts for alkane bond activation.