Stability and reactivity of copper oxo-clusters in ZSM-5 zeolite for selective methane oxidation to methanol

A periodic density functional theory study complemented by ab initio thermodynamic analysis was carried out to identify the active sites and mechanism of selective oxidation of methane to methanol in Cu/ZSM-5 catalysts. We systematically analyzed structure and stability of a wide range of potential extra-framework Cu complexes in ZSM-5 to address Cu speciation in realistic zeolite materials. We demonstrate that depending on the conditions of catalyst activation, binuclear [Cu( μ -O)Cu] 2+ species and trinuclear oxygenated [Cu 3 ( μ -O) 3 ] 2+ clusters can preferentially be stabilized in ZSM-5. The trinuclear Cu sites are the most stable extra-framework Cu species in Cu/ZSM-5 activated by calcination, whereas the formation of the binuclear complexes is favored under O 2 -poor atmosphere. Although both types of Cu complexes contain extra-framework O −, radical species necessary for the homolytic C–H bond cleavage, the reaction paths for methane conversion that they provide are drastically different. Binuclear Cu sites react with CH 4 stoichiometrically to yield methoxy groups strongly bound in the zeolite micropores. In contrast, the trinuclear [Cu 3 ( μ -O) 3 ] 2+ cluster favors the direct conversion of CH 4 to CH 3 OH coordinated with the partially reduced Cu complex. These computational findings point to the trinuclear Cu-oxo clusters in ZSM-5 as the potential candidates for promotion of the low temperature direct conversion of CH 4 to CH 3 OH.