Oxidative coupling of methane: MOx-modified (M = Ti, Mg, Ga, Zr) Mn2O3-Na2WO4/SiO2 catalysts and effect of MOx modification

Mn 2 O 3 -Na 2 WO 4 /SiO 2 is considered as the most promising catalyst for the oxidative coupling of methane (OCM) process; however, it only has a better catalytic performance over 800 °C. To improve its low-temperature performance, an attempt has been made to modify the Mn 2 O 3 -Na 2 WO 4 /SiO 2 catalyst using TiO 2 , MgO, Ga 2 O 3 , and ZrO 2 . Among the synthesized catalysts, the TiO 2 -modified Mn 2 O 3 -Na 2 WO 4 /SiO 2 catalyst shows markedly improved low-temperature OCM performance, achieving a high CH 4 conversion of ~23% and a good C 2 -C 3 selectivity of ~73% at 700 °C (the catalyst bed temperature), along with promising stability for at least 300 h without signs of deactivation. In comparison with the unmodified Mn 2 O 3 -Na 2 WO 4 /SiO 2 catalyst, the TiO 2 modification results in significant improvement in the low-temperature activity/selectivity, whereas the MgO modification has almost no impact and the Ga 2 O 3 and ZrO 2 modifications have a negative effect. The X-ray diffraction (XRD) and Raman results reveal that the formation of a MnTiO 3 phase and a MnTiO 3 -dominated catalyst surface is crucial for the improvement of the low-temperature activity/selectivity in the OCM process.