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

• Stability was greatly improved at high-pressure OCM reactions. • Instability of catalyst bed was directly observed via operando thermal visualization. • Support’s phase transformation caused the performance’s instability. • Mn-Na 2 WO4/SiO 2 -tridymite catalyst was synthesized. • C 2-4 selectivity maintained ∼77 % at a CH 4 conversion of ∼12.7 % over 100 h at 901 kPa. 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-Na 2 WO 4 /SiO 2 and Mn-K 2 WO 4 /SiO 2 catalysts (initially with cristobalite SiO 2 ), which have potential applicability in the selective conversion of CH 4 to olefins and paraffins frequently measured at ambient pressure. At high pressure condition of 0.9 MPa, we observed the decrease in C 2-4 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 SiO 2 -cristobalite to SiO 2 -tridymite, resulting from high temperatures in the presence of high H 2 O 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 CH 4 /O 2 ratio, and treatment to form SiO 2 -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 C 2-4 selectivity decreased only from 77.0 % to 76.5 % with a CH 4 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%)-Na 2 WO 4 (5 wt%)/SiO 2 -tridymite catalyst. In addition, a practical method is proposed to design SiO 2 -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.