Cu-ZSM-5 Zeolites for the Formation of Methanol from Methane and Oxygen: Probing the Active Sites and Spectator Species

A series of Cu-ZSM-5 zeolites was prepared by varying nature of the charge compensating cation, copper precursor, copper loading, and pH. The materials were tested for the oxidation of methane to methanol using oxygen. A linear relationship between the amount of methanol produced over Cu-ZSM-5 zeolites from methane and oxygen and a UV-Vis-NIR DRS charge transfer band at 22,700 cm(-1) is reported irrespective of the synthesis route used. The absolute intensity of the 22,700 cm(-1) band is always low, indicating a low number of active sites in the samples. In all studied Cu-ZSM-5 zeolites at least two copper species were present: (a) Cu-O clusters dispersed on the outer surface of ZSM-5 and (b) highly dispersed copper-oxo species inside the channels, a minority fraction in the sample. By relating catalytic activity to FT-IR data of adsorbed pivalonitrile, visualizing Cu-O particles on the outer surface of the zeolite, and subsequently adsorbed NO, indicative of the Cu-O species inside the zeolite channel, it was concluded that Cu-O species on the outer surface are not involved in the oxidation reaction, while copper inside the channels are responsible for the selective conversion of methane to methanol.