Methane Activation by Transition-Metal Oxides, MO<i>_x</i> (M = Cr, Mo, W; <i>x</i> = 1, 2, 3)

Abstract

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, MOx (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 MOx, the strength of the M−O π bond, and the bonding properties of MOx to methyl or hydrogen as M and x are varied. It is found that σ-bond metathesis to the metal hydride product (H−MOx-1−OCH3) occurs preferentially over the high oxidation state form (MO3) of the heavier metals, as well as all chromium oxides (CrOx). Instead, oxidative addition of MOx leading to methyl metal hydride (H−M(Ox)−CH3) is more favorable over the low oxidation state of MOx (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.