Microkinetic Modeling of Non-Oxidative Methane Coupling to Ethylene over Iron/Silica Catalyst

The production of large amounts of shale gas, lower overall costs, and sustainability concerns have generated interest for the production of chemicals directly from methane. Currently, methane is converted into fuels and chemicals through syngas, an energy intensive and large capital investment process. A recent study has reported that isolated iron atoms embedded in a silica matrix (Fe@SiO₂) enables direct conversion of methane to ethylene under non-oxidative conditions at high temperatures (1223 – 1363 K). It has been suggested that methane conversion is initiated by catalytic generation of methyl radicals and hydrogen followed by subsequent gas-phase reactions.¹ Although the concept was demonstrated experimentally, fundamental understanding of the reactions is missing. In this study, we model this process by combining detailed catalytic and gas-phase reaction mechanisms. The latter includes up to 600 species and 10,000 reactions. For the former, we investigate the C-H activation of methane and C-C coupling reactions using density functional theory (DFT) calculations. The combined catalytic and gas-phase reaction mechanisms allow not only to capture the trends in terms of conversion and selectivity but also allow to assess the predictive capability of the model. Microkinetic simulations show that ethylene is the only primary product among the major species including acetylene and higher molecular weight aromatics. Overall, the present study exemplifies the interplay between catalytic and gas phase reactions that controls selectivity.

1. Guo, X.; Fang, G.; Li, G.; Ma, H.; Fan, H.; Yu, L.; Ma, C.; Wu, X.; Deng, D.; Wei, M.et al. Direct, nonoxidative conversion of methane to ethylene, aromatics, and hydrogen. Science 2014, 344 (6184), 616.