Direct Conversion of Methane to Methanol by Metal-Exchanged ZSM-5 Zeolite (Metal = Fe, Co, Ni, Cu)

Metal-exchanged zeolites are known to exhibit catalytic activity in the direct conversion of methane to methanol. The influence of different metals on this reaction has been theoretically investigated by using density functional theory (DFT) calculations on a periodic system of MO+-ZSM-5 zeolite (M = Fe, Co, Ni, Cu). The results indicate a high dependence of the reaction on the metals, where the reactivity toward C-H bond dissociation is predicted to increase in the order CoO+-ZSM-5 < NiO+-ZSM-5 < FeO+-ZSM-5 < CuO+-ZSM-5 and the selectivity of methanol is predicted to increase in the order FeO+-ZSM-5 < CoO+-ZSM-5 < NiO+-ZSM-5 < CuO+-ZSM-5. The role of ZSM-5 zeolite in the catalytic activity is also investigated by comparing our calculation results with those reported for the reaction by bare MO+ species in the gas phase. We found that the nanopores of ZSM-5 zeolite exert a confinement effect which destabilizes the adsorption of methane and lowers the activation energy for the C-H bond dissociation. In addition to the conversion of methane, we investigated the direct conversion of ethane to ethanol by FeO+-ZSM-5 and found that this reaction proceeds with a lower C-H bond activation energy and a higher product selectivity in comparison to the conversion of methane to methanol by the same catalyst.