Theoretical Overview of Methane Hydroxylation by Copper-Oxygen Species in Enzymatic and Zeolitic Catalysts.

As fossil-based energy sources become more depleted and with renewable-energy technologies still in a very early stage of development, the utilization of highly abundant methane as a transitional solution for current energy demands is highly important despite difficulties in transport and storage. Technologies enabling the conversion of methane to liquid/condensable energy carriers that can be easily transported and integrated into the existing chemical infrastructures are therefore essential. Although there commercially exists a two-step gas-to-liquid process involving syngas production, a novel route of methane conversion that can circumvent the high-cost production of syngas should be developed. Among all of the conceptually possible methods for converting methane to methanol, methane hydroxylation (CH4 + 1/2O2 → CH3OH) at low temperature seems to be the most viable since it provides a direct route of conversion and allows a much lower operational cost. However, it is hampered by the fact that the complete oxidation to CO2 is thermodynamically more favored. To overcome this, an effective catalyst that is able to "mildly" oxidize methane and stabilize the resultant methyl radical toward methanol formation is required. Particulate methane monooxygenase (pMMO) and copper-exchanged zeolites are two catalysts known to hydroxylate methane into methanol at low temperature with high selectivity. Having been studied for more than 30 years, these copper-cored catalysts are still relevant topics of discussion since the actual structure of the active sites has not been agreed upon, and thus, the reaction mechanism and factors influencing their reactivity and productivity are yet to be understood. Density functional theory (DFT) has provided us with a powerful computational tool for accomplishing these tasks. This Account presents an overview of the recent progress in the computational elucidation of the catalytic mechanism of methane hydroxylation by mono-, di-, and trinuclear copper sites in pMMO and Cu-exchanged zeolites as well as its correlations to the influencing factors that must be controlled to achieve higher reactivity. First, we briefly introduce the catalytic mechanism of a bare CuO+ cation as the simplest copper-oxo system in methane hydroxylation. The system is then extended to the copper-oxo species in pMMO and zeolites, and the radical and nonradical mechanisms are examined. Investigations of the reactivities of mononuclear and dinuclear copper-oxo species in the pMMO active site suggest that the bis(μ-oxo)CuIICuIII, (μ-oxo)(μ-hydroxo)CuIICuIII, and CuIIIO species are important for the catalytic activity of pMMO. In the case of Cu-exchanged zeolites, as the mono(μ-oxo)CuIICuII and tris(μ-oxo)CuIICuIIICuIII active sites have been fully characterized in experiments, here we discuss the effects of zeolite structures on the geometry and reactivity of the active sites.