Homolytic versus Heterolytic Methane Hydroxylation in Copper Zeolites

Abstract

Oxygen-activated copper zeolites are capable of selectively converting methane to methanol at mild conditions, using a mono-oxygen bridged Cu(II) site [CuOCu]²⁺ as the active core. Based on previous DFT reports on the [CuOCu]²⁺ + CH₄ reaction a general consensus was reached concerning the methane oxidation mechanism, where the rate-limiting step involves homolytic C–H bond cleavage to form [Cu(OH)Cu]²⁺ with a physisorbed •CH₃. An alternative possibility, i.e. heterolytic H-abstraction passing through a four-center transition state to give an intermediate with a Cu–CH₃ bond, was given consideration only in a few recent DFT studies, but was found less favorable than radical C–H activation. In this contribution methane-to-methanol conversion by Cu–CHA is investigated using large cluster models and employing either DFT, with an extensive list of 97 functionals, or the high-level correlated DMRG/cu(4)-CASPT2 method. In all cases homolytic C–H dissociation most favorably proceeds via a (S = 1) transition state TS1r, whereas the transition state of heterolytic H-abstraction, TS1n, has an (S = 0) ground state. The DMRG/cu(4)-CASPT2 results convincingly point to the heterolytic route, with a calculated activation enthalpy of 12.3 kcal/mol, as compared to 21.1 kcal/mol for homolytic C–H dissociation. In contrast, the results obtained with DFT are strongly functional dependent. Conform with previous DFT studies, homolytic H-abstraction is preferred by the B3LYP functional (almost exclusively used in previous cluster model studies). However, many other functionals, hybrid meta-GGA functionals in particular, agree with DMRG/cu(4)-CASPT2 on heterolytic C–H activation. The present results reopen the debate on the general validity of the radical rebound mechanism for methane hydroxylation by a [CuOCu]²⁺ core in copper zeolites and also highlight the need for caution when relying on a specific DFT functional to elucidate oxidation reaction mechanisms in metal-based catalytic systems.