Mechanism of n-Butane Hydrogenolysis Promoted by Ta-Hydrides Supported on Silica

The mechanism of hydrogenolysis of alkanes, promoted by Ta-hydrides supported on silica via 2 Si-O-bonds, has been studied with a density functional theory (DFT) approach. Our study suggests that the initial monohydride ( Si-O-)(2)(TaH)-H-(III) is rapidly trapped by molecular hydrogen to form the more stable tris-hydride ( Si-O-)(2)(TaH3)-H-(v). Loading of n-butane to the Ta-center occurs through C-H. activation concerted with elimination of molecular hydrogen (sigma-bond metathesis). Once the Ta-alkyl species is formed, the C C activation. step corresponds to a beta-alkyl transfer transfer to the metal with elimination of an olefin. According to these calculations, an alpha-alkyl transfer to the metal to form a Ta-carbene species is of higher energy. The olefins formed during the C-C activation step can be rapidly hydrogenated by both mono- and tris-Ta-hydride species, making the overall process of alkane cracking thermodynamically favored.