Theoretical Study On Methane Activation By Metallic Clusters

The catalytic reactions mediated by transition-metal clusters have attracted considerable attention in recent years. The heteronuclear metal dimers PtM⁺ (M = Cu, Ag, Au) are capable of efficiently dehydrogenating methane, and they exhibit relatively high reactivities with respect to the metal ions. The resulting metallic species [PtMCH2]⁺ should be taken as the important precursors to C-N bond coupling for the synthesis of HCN. Compared with the comprehensive studies of the reaction of metal ion with methane, there are little theoretical reports about the C-H bond activation by metal dimmers. However, the dehydrogenation reaction which catalyzed by metal clusters PtM⁺ (M = Cu, Ag, Au,Pt) have been studied in theoretical investigation at the ZORA level, but there is no theoretical report about the reaction mechanism of PdM⁺(M = Cu,Ag) with methane.In this paper, we chose several typical reactions that have been carefully studied using quantum methods, obtained some interesting results. On the basis of the molecular orbital theory, the tradition transition state theory as well as quantum chemistry theory, the systems (bimetallic clusters) chose have been investigated using Density Functional Theory (DFT), the coupled cluster CCSD(T) calculations and the National Orbital analysis. The structures of the reagents, the reaction products and transition states along the reaction paths have been obtained, and then obtained the reaction surfaces, the spectrum datum, thermodynamic datum as well as the information of orbitals. The reaction mechanism has been argued deeply using these data.The whole paper consists of four chapters. Chapter 1 mainly reviews the evolution of bimetallic clusters with methane. The second chapter summarizes the theory of quantum chemistry and calculation methods of this paper. The contents of two chapters were the basis and background of our studies and offer us with userful and reliable quantum methods.In chapter 3, a computational study of the bimetallic clusters dehydrogenation reaction mechanism with methane is presented. The results shows the reactants give molecule–ion complex [PdMCH₄]⁺, [PdMCH₄]⁺ could undergo oxidative addition, cleaving a C-H bond and yielding the insertion product [MPd(CH3)H]⁺, which is the stable structure on potential energy surfaces . The third step is a reductive elimination, leading to a molecule–ion complex [H₂-Pd(μ-CH₂)M]⁺, and this step is the rate-determination step in the whole reaction path. Predicted high activation energies are understood through the electronic configurations of their reactive precursors. The results of these calculations are compared with those of PtM⁺ clusters. The calculated results indicate that bimetallic clusters PdM⁺ (M = Cu, Ag) exhibit relatively high activity toward CH₄ with respect to Pd⁺, but it is still lower than PtM⁺ (M = Cu, Ag) clusters.Chapter 4, in this paper we have carried out a theoretical investigation at the DFT (B3LYP) level of the mechanism of the C-H insertion reaction catalyzed by heteronuclear NiM⁺(M = Cu, Ag, Au) cluster. The results indicate that the reaction of NiM⁺(M = Cu, Ag) cluster is completely same with the heteronuclear cluster PdM⁺(M = Cu, Ag) in the activation of methane C-H bond. However, the NiAu⁺ shows more complex reaction mechanism. Different from NiM⁺(M = Cu, Ag), the first H atom transfer rather than to the Ni atom but to the Au atom in the activation of a methane C-H bond, and the whole dehydrogenation reaction is exothermic.