Zeolite Surface Methoxy Groups as Key Intermediates in the Stepwise Conversion of Methane to Methanol

This contribution clarifies the overoxidation-preventing key step in the methane-to-methanol (MTM) conversion over copper mordenite zeolites. We followed the methane-to-methanol conversion over copper mordenite zeolites by NMR spectroscopy supported by DRIFTS to show that surface methoxy groups (SMGs) located at zeolite Brønsted sites are the key intermediates. The SMGs with chemical shift of 59 ppm are identical to those formed on a copper-free reference zeolite after reaction with methanol and react with water, methanol, or carbon monoxide to yield methanol, dimethyl ether, and acetate. This reactivity corroborates the location of SMGs at Brønsted sites. We find no evidence for stable SMGs directly at copper sites and explain mechanistically why H-form mordenites outperform their Na-form analogues. This finding is of interest for any future process that tries to trap the intermediate methane oxidation product towards methanol.