Direct Injection of Hydrogen Peroxide for Selective Homogeneous Conversion of Methane into Higher Hydrocarbons

The impact of direct H2O2 injection on the selective CH4 coupling reaction at high temperatures was investigated both experimentally and by kinetic modeling to provide insight into the reaction mechanism of the catalytic oxidative coupling of methane (OCM). H2O2 injection transforms CH4 into C2H6 and C2H4 at high selectivity, confirming the effectiveness of the involvement of H2O2 by generating OH radicals in OCM. For carbon oxides, there was only CO formation without CO2 at CH4 conversions at or below 10%, as expected from the pure contribution of the gas phase. These results were consistent with simulation results using kinetic modeling of gas-phase elementary reactions. Rate of production (ROP) analysis suggests that OH radicals formed from H2O2 decomposition were responsible for the high selectivity toward C-2 products. The major loss of C-2 selectivity and CH4 conversion is due to HO2 radicals, a secondary product in H2O2 decomposition. The HO2 radicals were found to both oxidize CH3 radicals and neutralize OH radicals. The kinetic model consistently overpredicted the CH4 conversion and C(2 )selectivity over the experimental results, which can be attributed to the radical quenching and overoxidation reaction on the surface of the quartz tube reactor. The findings in this work help create a better understanding of the requirements of selective C-2 formation under OCM conditions.