Breaking the dilemma of low selectivity in the oxidative coupling of methane over Mn-Na2WO4/SiO2 at low temperatures

[Display omitted] •Mn-Na2WO4/SiO2 oxidized CH4 with 87.3% C2+-hydrocarbons selectivity at 675 °C.•H2O reduces the reaction order of C2H6 formation with respect to O2 near to zero.•Low oxygen partial pressure intensifies the enhancing H2O effect on C2H6 formation.•The strength of this effect also increases with a decrease in reaction temperature.•Breaking the dilemma of low selectivity in the oxidative coupling of methane over Mn-Na2WO4/SiO2 at low temperatures. Oxidative coupling of methane allows to directly produce higher hydrocarbons (C2-hydrocarbons, mainly C2H6 and C2H4) from cheap and available methane. Although various catalysts have been developed, they operate selectively above 750 °C that is detrimental in terms of reactor construction materials and safe operation. The present study demonstrates that the supported Mn-Na2WO4/SiO2 catalyst achieves the selectivity to C2+-hydrocarbons of 87.3% at methane conversion of 6.6% at 675 °C only. With the aid of sophisticated kinetic measurements, the origins of this unexpectedly high performance have been elucidated. When operating with water-containing feeds, the reaction order of the rate of C2H6 formation with respect to O2 at 675 °C is practically zero in the presence of water, while the corresponding values for CO and CO2 are 0.9 and 0.7. Moreover, O2 partial pressure is also decisive for the strength of the enhancing water effect. The water-induced increase in the rate of CH4 conversion into C2-hydrocarbons can be about 120 times using a feed with the ratio of CH4/O2 of 24. The corresponding enhancement factors for the formation of CO and CO2 are only 20 and 6, respectively. These new fundamentals provide hints for optimizing reactor operation for efficient conversion of methane into C2+-hydrocarbons at low temperatures.