Selective activation of methane C[sbnd]H bond in the presence of methanol

Abstract

Direct methane to methanol (MTM) conversion over heterogeneous catalysts is a promising route for valorization of methane. The methane CH bond activation is considered as the key step of the MTM and is the focus of considerable research activity. However, the formed methanol typically suffers from overoxidation largely due to the cleavage of a methanol CH bond, whose bond dissociation energy is ca. 0.5 eV lower than that of the methane CH bond, which usually translates to a transition state energy of the methanol CH bond cleavage that is ca. 0.55 eV lower than that of methane whenever the reactions proceed through a radical mechanism. Here, we propose a general approach for decreasing the transition state energy difference between the CH4 and CH3OH CH bond dissociation. When a metal-oxide supported cationic transition metal atom and a neighboring oxygen on the oxide surface serve as the active site, the transition state energy difference through a surface-stabilized pathway can be noticeably narrowed as compared with that of a radical pathway. For Ir, Pt and Rh-doped anatase TiO2(1 0 1), the CH4 CH bond activation can be preferred over that of CH3OH at significant methanol mole fraction. Also, for PdAu alloys containing adsorbed oxygen and positively charged Pd, calculations suggest in agreement with recent experiments (Science367 (6474), 193–197) that the CH4 CH bond can be selectively activated in the presence of CH3OH.