Identification of the active sites and mechanism for partial methane oxidation to methanol over copper-exchanged CHA zeolites

To make methane a suitable energy carrier and transport less costly, it is an urgent and challenging task for us to convert methane to liquid under mild conditions efficiently. In this study, we explored partial methane oxidation to methanol by density functional theory(DFT) calculations using a hybrid functional(HSE06) with van der Waals(vdW) interactions. The stabilities of different active sites over SSZ-13 and SAPO-34, two CHA type zeolites, are thoroughly investigated by ab initio molecular dynamics(AIMD) simulations and ab initio thermodynamics analyses. Four possible active sites, namely CuOHCu]~(2+), Cu(OH)_2Cu]~(2+),CuOCu]~(2+) and CuOH]~+, are identified stable. Methane-to-methanol reaction mechanisms are further studied upon these most stable active sites, among which CuOCu]~(2+) and CuOH]~+ are proved to be reactive. The migration of species among zeolite pores are also discussed, which accounts for the activity on CuOH]~+ sites. This concept may represent a more complete picture of catalytic reactions over zeolites in general.