Dynamic Evolution from Propane to Ethylene over Mordenite Zeolite for Syngas Conversion: Transition from Hydrocarbon Pool to Carbonylation Route

Understanding the topology-dependent evolution of reaction intermediates during syngas conversion over an oxide-zeolite (OX-ZEO) bifunctional catalytic system is essential to the development of novel catalysts for regulating product distribution. In this study, we observed a dynamic shift in ethylene selectivity from 14.3% to 80% over time on stream using an unmodified commercial mordenite (MOR) zeolite combined with ZnAlO_x. Concurrently, the propane selectivity gradually decreased from 46.9% to 0.3%. We confirmed that this alternative change in reaction time-dependent product distribution was closely linked to the evolution of intermediates, which relied on the accessibility of the 12-ring (R) and 8R pore channels varying with the degree of in situ coke deposition. The methanol-to-propylene (MTP) reaction route within 12R pores predominated the initial reaction phase with methanol as the intermediate, resulting in the formation of propane. Once the 12R pores were filled with MTP-formed coke, the carbonylation route within the 8R pores took over the reaction process. The coke-deposited ZnAlO_x–MOR bifunctional system demonstrated stable activity, achieving a high ethylene selectivity of 80% and an ethylene/propylene ratio of 12.7. This study illustrates that in situ coke deposition into the 12R pores of MOR and molecular diffusion behavior are effective methods to enhance ethylene production during syngas conversion over the OX-ZEO catalyst, but there is no need to selectively deactivate the 12R acid sites through the introduction of pyridine.