Methanol-to-hydrocarbons conversion: The alkene methylation pathway

Research on zeolite-catalyzed methanol-to-hydrocarbons (MTH) conversion has long been concerned with the mechanism of the reaction between methanol and alkenes. Two pathways have been debated: (1) the stepwise, proceeding through a surface-methoxy intermediate and (2) the concerted, in which the alkenes react directly with methanol. This work addresses the debate through micro-kinetic modeling based on density functional theory calculations of both pathways, as well as experiments employing tem- poral analysis of products to investigate the kinetics of the stepwise pathway for a series of alkenes in H- ZSM-22 zeolite. The model predicts the stepwise pathway to prevail at typical MTH reaction tempera- tures, due to a higher entropy loss in the concerted as compared to the stepwise pathway. The entropy difference results from intermediate release of water in the stepwise pathway. These findings lead us to suggest that the stepwise pathway should also be considered when modeling MTH conversion in other zeolites.