Methane Activation by Transition-Metal Oxides, MOx (M = Cr, Mo, W; x = 1, 2, 3)

Recent experiments on the dehydrogenation−aromatization of methane (DHAM) to form benzene using a MoO 3 /HZSM-5 catalyst stimulated us to examine methane activation by the transition-metal oxide molecules, MO x (M = Cr, Mo, W; x = 1, 2, 3). The present studies use hybrid density functional theory (B3LYP). The reactivity trend is rationalized in terms of changes in the electrophilicity of MO x, the strength of the M−O π bond, and the bonding properties of MO x to methyl or hydrogen as M and x are varied. It is found that σ-bond metathesis to the metal hydride product (H−MO x - 1 −OCH 3 ) occurs preferentially over the high oxidation state form (MO 3 ) of the heavier metals, as well as all chromium oxides (CrO x ). Instead, oxidative addition of MO x leading to methyl metal hydride (H−M(O x )−CH 3 ) is more favorable over the low oxidation state of MO x (M = Mo, W, x = 2, 1). In particular, it is found that WO 2 can undergo oxidative addition with negligible activation barrier and is predicted to be the most reactive compound of this class toward methane activation. Our finding that MO 2 (M = W, Mo) is the best oxidation state for this class of metal oxides toward methane activation suggests that the MO 3 /HZSM-5 catalysts active in the DHAM reaction may be W and Mo oxycarbides (MO 2 C 2 ). The formation of such intermediates may be the reason that the experiments find an induction period before the catalyst is active for the desired reaction.