Evolution of Surface Intermediate Species in the Direct N2O-Mediated Conversion of Methane to Methanol over H+-Ferrierite

The direct conversion of methane to methanol under mild conditions remains a central challenge in catalysis. In this study, we employ a series of ferrierite (FER) zeolite catalysts to investigate the surface species and acid-site characteristics associated with methane activation in the presence of N2O. In situ DRIFTS measurements reveal the formation of surface methoxy-related species on protonic ferrierite (H+-FER) at temperatures above 200 °C, with their spectral intensity increasing with reaction temperature. Variation of the parent NH4+-FER calcination temperature (450, 550 °C) and alkali-ion exchange (Na+, K+) demonstrates a correlation between methoxy formation and the presence and density of Brønsted acid sites. Importantly, H+-FER maintains methoxy stabilization up to 300 °C, exhibiting enhanced persistence of methoxy-related species at elevated temperatures, consistent with the influence of the confined FER pore environment on the stability of surface intermediates. Complementary solid-state NMR, NH3-TPD, XPS, and pyridine-desorption IR spectroscopy provide consistent evidence for the structural and acidic properties of the FER catalysts. Additional DRIFTS-MS experiments show that H2O exposure leads to the attenuation of surface methoxy-related bands and a transient methanol-related mass-spectrometric response, supporting the formation of methanol-related products from surface species generated during CH4 + N2O exposure. Overall, this study provides spectroscopic evidence linking the acid environment of H+-FER to the formation of surface methoxy-related species and highlights the potential role of zeolite confinement in stabilizing reactive surface intermediates during methane oxidation.