Formation of Surface Oxygen Species and the Conversion of Methane to Value-Added Products with N2O as Oxidant over Fe-Ferrierite Catalysts

Formation of active oxygen species from N2O over Fe-ferrierite (Fe-FER) catalyst prepared by solid-state ion-exchange method at moderate temperatures was studied using spectroscopic and solid characterization techniques including H2 temperature-programmed reduction (H2-TPR), N2O temperature-programmed desorption (N2O-TPD), and in situ Fourier transform infrared (FTIR). The utilization of active oxygen species for the direct conversion of methane to value-added products at moderate temperatures was investigated. The active oxygen sites for the selective conversion of methane were identified by a TPR feature at 220 °C. This site is also characterized by an infrared band observed at 1872 and 1892 cm–1 upon adsorption of NO. These bands are NO stretching vibrations of NO adsorbed on iron oxygen monomeric species, present in the zeolite cages and responsible for selective oxidation. We show that these oxidized species react with methane to form oxygenates but at higher temperatures form molecular oxygen. IR bands of surface methoxy groups were observed in significant concentration in the FTIR spectra and are suggested to be intermediate species of the selective oxidation of methane. Studies using continuous reactors demonstrated that cofeeding of methane and N2O-promoted generation of desired products from methane conversion by N2O over Fe-FER catalyst can be enhanced by optimizing the feed ratio of CH4/N2O. Furthermore, N2, O2 and NO were detected as the products of N2O decomposition over Fe-FER catalysts.