Confinement effect of zeolite cavities on methanol-to-olefin conversion: A density functional theory study

The confinement effect of zeolite cavities on the methanol-to-olefin (MTO) conversion is investigated through density functional theory calculations. According to the side-chain mechanism, we select several hydrocarbon pool (HP) intermediates that may exist during the MTO conversion process and optimize their structures within the cluster models of zeolite cavities cha, lev, and lta, respectively. The transition states during methylation, deprotonation, methyl shift, and olefin production are also located within these cavities. According to our results, all of the HP intermediates are stabilized in zeolite cavities, especially in cha and lta. Moreover, the cha cavity displays the lowest intrinsic free-energy barriers for all of the methylation and olefin-production steps, indicating its high MTO catalytic activity. We find that the differences in reaction barriers and reaction energies are highly related to the different confinement effects of zeolite cavities. In comparison with lev and lta, the cha cavity matches the dimensions of HP species very well, so it is able to provide the most suitable confinement to HP species. Our discovery will provide further understanding of the side-chain mechanism, which is important for finding new catalysts for MTO conversion.