Comparative Study of Diverse Copper Zeolites for the Conversion of Methane into Methanol

The characterization and reactive properties of copper zeolites with twelve framework topologies (MOR, EON, MAZ, MEI, BPH, FAU, LTL, MFI, HEU, FER, SZR, and CHA) are compared in the stepwise partial oxidation of methane into methanol. Cu2+ ion-exchanged zeolite omega, a MAZ-type material, reveals the highest yield (86 mu mol g(cat.)(-1)) among these materials after high-temperature activation and liquid methanol extraction. The high yield is ascribed to the relatively high density of copper-oxo active species, which form in its three-dimensional 8-membered (MB) ring channels. In situ UV/Vis studies show that diverse copper species form in different zeolites after high-temperature activation, suggesting that there are no universally active species. Nonetheless, there are some dominant factors required for achieving high methanol yields: 1) highly dispersed copper-oxo species; 2) large amount of exchanged copper in small-pore zeolites; 3) moderately high temperature of activation; and 4) use of proton form zeolite precursors. Cu-omega and Cu-mordenite, with the proton form of mordenite as the precursor, yield methanol after activation in oxygen and reaction with methane at only 200 degrees C, that is, under isothermal conditions.