(500d) Microkinetic Modeling of Direct, Non-Oxidative Conversion of Methane to Value-Added Chemicals over Iron/Silica Catalyst

The abundance of shale gas has generated a lot of interest for production of chemicals. Currently, methane is converted into fuels and chemicals through syngas, an energy intensive and large capital investment process. Direct methane conversion routes have received considerable interest for their potential to provide more efficient conversion routes. Recently, it has been reported that isolated iron atoms embedded in a silica matrix (Fe@SiO₂) can catalyze methane conversion under non-oxidative conditions to produce olefins, aromatics and hydrogen at high temperatures (1223 – 1363 K). It has been proposed that methane conversion is initiated by catalytic generation of methyl radicals followed by subsequent gas-phase reactions.¹ Although the concept was demonstrated experimentally, fundamental understanding of the reactions is missing. Herein, 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 to assess the predictive capability of the model. The interplay of catalytic and gas-phase reactions is delineated. Simulations show that ethylene is a primary product. Under differential conditions, an increase in selectivity towards acetylene and higher molecular weight cyclic species (benzene, toluene and naphthalene) at the expense of ethylene and ethane with increasing temperature is observed.

1. Guo X, Fang G, Li G, et al. Direct, nonoxidative conversion of methane to ethylene, aromatics, and hydrogen. Science. May 09 2014;344(6184):616-619.