Selective conversion of methane to methanol facilitated by molecular metal-methoxy complexes via a self-correcting chemical cycle.

Abstract

The controlled oxidation of methane to methanol has been an area of intense research over the past decades. Despite the efforts, the identification of an efficient catalyst with high selectivity is still elusive. Here we propose a thoroughly different strategy employing catalysts containing a metal methoxy unit. This family of catalysts has been used for the activation of C-H bonds but this is the first systematic investigation for the conversion of methane to methanol highlighting the advantages over the typically used metal oxides. Specifically, we start our investigation with an Fe(III) center coordinated by four ammonia ligands, (NH3)4FeOCH32+. Structures and energetics are reported for two mechanisms ([2+2] and proton coupled electrons transfer) and for different spin multiplicities via density functional theory, multi-reference, and coupled cluster quantum chemical calculations. The excited low-spin doublet state of this model system exhibits the best performance in terms of activation barriers and selectivity. Therefore, we then switched to the corresponding Ru(III) complex, which has a doublet ground state and manifests better performance than the doublet state of Fe(III). For both systems the activation barrier for methanol is larger than that of methane due to the interaction of the OH group of methanol with the coordinated NH3 ligands (hydrogen bonding) and/or the metal center. This observation suggests that the activation of methanol is slower, hindering its oxidation. In addition, we show that the metal-methoxy family of catalysts offers a potential mechanism that can prevent the oxidation of an activated methanol molecule (self-correcting chemical loop). This work aspires to induce experimental interest and pave the road for the development of high-performance high-selectivity methane to methanol direct conversion routes under mild conditions.