Methane activation by transition-metal oxides, MO_x (M = Cr, Mo, W; x = 1, 2, 3)

Recent experiments on the dehydrogenation-aromatization of methane (DHAM) to form benzene using a MoO₃/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-OCH₃) occurs preferentially over the high oxidation state form (MO₃) 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)-CH₃) is more favorable over the low oxidation state of MO_x (M = Mo, W, x = 2, 1). In particular, it is found that WO₂ 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 MO₂ (M = W, Mo) is the best oxidation state for this class of metal oxides toward methane activation suggests that the MO₃/HZSM-5 catalysts active in the DHAM reaction may be W and Mo oxycarbides (MO₂C₂). 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.