An experimental and modeling study on homogeneous oxidative coupling of methane utilizing N2O as oxidant

Abstract

Experimental and kinetic modeling studies were performed to investigate the homogeneous oxidative coupling of methane utilizing nitrous oxide as the oxidant (N2O-OCM). The experiments were performed in a jet stirred reactor at atmospheric pressure and temperatures of 773–1173 K. Synchrotron vacuum ultraviolet photoionization mass spectrometry was used to achieve comprehensive and isomer-resolved identification of major products and nitrogenous, carbonyl and hydrocarbon intermediates. To interpret the experiments, a detailed kinetic model was developed by extending the widely used hydrocarbon growth mechanisms. Rate of production and sensitivity analyses were performed to provide a deep insight into the complex reaction network. The results demonstrate that N2O dissociation is the rate-controlling step, and the mild oxidation of N2O effectively reduces the production of CO2 as byproduct and meanwhile improves the selectivity of C2 species. In the presence of N2O, CH3 can actively participates in H-abstraction reactions of C2H6 and C2H4, besides the self-coupling reactions. Due to chain-propagation competition reactions (N2O+H=N2+OH and N2O+H=NH+NO), NH3 and NO exhibit low concentrations. The detailed formation pathways for NH3 and NO were elaborated, and the evolution routes of carbonyl intermediates in the presence of N2O were analyzed. Benzene formation is found mainly through C3 self-recombination and C5 ring reforming reactions. Finally, comparative analysis with O2-OCM reveals that the presence of oxygen promotes the conversion of CH3 to CH2O and deep oxidation of C2H3, explaining the lower C2 yield of O2-OCM. The inherent limitation of the C2 yield of N2O-OCM primarily arises from the reaction of C2H4 and O, as well as the subsequent reaction CH2(S)+N2O=CH2O+N2. The experiment observations confirmed the existence of C2 carbonyl intermediates such as ketene and acetaldehyde. The study highlights the notable role of O2 molecular and O atoms in the homogeneous OCM and offers kinetic support for the OCM reaction process with catalysts.