Y-Faujasite Encapsulated Co Clusters: Synthesis, Characterization and Theoretical Model as Probe of the Methane Homologation

Abstract

The adsorption of Co₂(CO)₈ onto the dehydrated Y-faujasite powder under an N₂ atmosphere and onto the tetrahydrofuran slurry of Y-faujasite under a mixed CO and H₂ atmosphere predominately yielded supported Co₄(CO)₁₂ and supported Co₆(CO)₁₆, respectively. The molecular cobalt-carbonyl clusters and their decarbonylated products have been structurally characterized by extended X-ray absorption fine structure (EXAFS). The decarbonylated sample a possesses a cluster of two Co atoms and the decarbonylated sample b has a cluster phase of three Co atoms. The decarbonylated sample a exhibited higher CH₄ conversion and C₂₊ selectivity (C₂₊ selectivity = ∑nC _n(n = 2–5)/∑nC _n (n = 1–5) * 100%) in comparison with the decarbonylated sample b in methane homologation. A density functional theory (DFT) model was employed to calculate Co clusters adsorbed on a silica substrate which simulates Y-faujasite encapsulated Co clusters. The structural geometries, net spin electronic charge densities, energies of the metal–silica and metal–metal interactions in stable geometries are discussed and used to interpret the cluster size dependence of the catalytic activity and selectivity to C ₂₊ hydrocarbons in the methane homologation.