Insight into the direct conversion of methane to methanol on modified ZIF-204 from the perspective of DFT-based calculations

Abstract

Direct oxidation of methane over oxo-doped ZIF-204, a bio-mimetic metal–organic framework, is investigated under first-principles calculations based on density functional theory. In the pristine ZIF-204, the tetrahedral methane molecule anchors to an open monocopper site via the so-called η² configuration with a physisorption energy of 0.24 eV. This weak binding arises from an electrostatic interaction between the negative charge of carbon in the methane molecule and the positive Cu²⁺ cation in the framework. In the modified ZIF-204, the doped oxo species is stabilized at the axial position of a CuN₄-base square pyramid at a distance of 2.06 Å. The dative covalent bond between Cu and oxo is responsible for the formation energy of 1.06 eV. With the presence of the oxo group, the presenting of electrons in the O_p_z orbital accounts for the adsorption of methane via hydrogen bonding with an adsorption energy of 0.30 eV. The methane oxidation can occur via either a concerted direct oxo insertion mechanism or a hydrogen-atom abstraction radical rebound mechanism. Calculations on transition-state barriers show that reactions via the concerted direct oxo insertion mechanism can happen without energy barriers. Concerning the hydrogen-atom abstraction radical rebound mechanism, the C–H bond dissociation of the CH₄ molecule is barrierless, but the C–O bond recombination to form the CH₃OH molecule occurs through a low barrier of 0.16 eV. These predictions suggest the modified ZIF-204 is a promising catalyst for methane oxidization.