Mn-Na2WO4/SiO2-tridymite with improved stability and selectivity for high-pressure oxidative coupling of methane

Abstract

Elevated pressures are essential for industrial chemical processes. Oxidative coupling of methane (OCM) is also preferred to be operated at high pressures. This study investigates high-pressure OCM with operando temperature visualization over the Mn-Na2WO4/SiO2 and Mn-K2WO4/SiO2 catalysts (initially with cristobalite SiO2), which have potential applicability in the selective conversion of CH4 to olefins and paraffins frequently measured at ambient pressure. At high pressure condition of 0.9 MPa, we observed the decrease in C2-4 selectivity and stability with time. During the deactivation, the hotspot generated in the catalyst bed was found to shift towards the end of the bed and sintered the catalyst in the process. High-pressure OCM condition was necessary to cause a phase transformation from SiO2-cristobalite to SiO2-tridymite, resulting from high temperatures in the presence of high H2O pressure (>100 kPa) and was found to play a major part in loss of surface area of active site (Na and K). The hotspot’s influence was reduced by increasing the CH4/O2 ratio, and treatment to form SiO2-tridymite prior to the start of the reaction was found to significantly increase the high-pressure stability. After optimizing the reaction conditions and catalyst, the C2-4 selectivity decreased only from 77.0 % to 76.5 % with a CH4 conversion of 12.7 % over 100 h at a total pressure of 901 kPa and a furnace temperature of 775 °C with Mn(2 wt%)-Na2WO4(5 wt%)/SiO2-tridymite catalyst. In addition, a practical method is proposed to design SiO2-tridymite supported alkali metal catalyst. The insights obtained could be used to design new catalysts and minimize stability issues during high-pressure OCM reactions and in other catalyst systems.