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