A density functional theory study of a silica-supported zirconium monohydride catalyst for depolymerization of polyethylene

A silica-supported zirconium hydride catalyst for depolymerization of polye thylene is studied using density functional theory (DFT) together with a ge neralized gradient approximation (GGA) for the exchange and correlation ene rgy. The (100) and (111) surfaces of beta-cristobalite are used as two poss ible models of a silica surface. Based on the experimental surface structur e determined by J. Corker et al. (Science 1996, 271, 966), we propose a det ailed atomic model of the zirconium monohydride that is believed to be the active site for depolymerization of polyolefins. Our model of the zirconium monohydride on the (100) surface is found to be very stable and the struct ure is in good agreement with extended X-ray absorption fine structure (EXA FS) measurements. We have carried out depolymerization of a small polyolefi n chain (C3H8) to give CH4 and C2H6 by addition of H-2 The rate-limiting st ep is a beta-methyl transfer to the zirconium atom, and the activation ener gy is 29 kcal/mol on the (100) surface.