Olefin methylation and cracking reactions in H-SSZ-13 investigated with ab initio and DFT calculations†
The olefin cycle of the methanol-to-olefins process is investigated for the zeolite H-SSZ-13 using periodic, van-der-Waals corrected DFT calculations, together with MP2 corrections derived from cluster models, which are essential for accurate barriers. The two main reactions, olefin methylation and cracking are systematically investigated for different olefin isomers up to C9. The barrier for cracking depends sensitively on the involved cationic intermediates. The most favorable cracking reactions involve tertiary cations, in particular the t-butyl cation that leads to the formation of isobutene along with another olefin. The transition state for olefin methylation is mainly influenced by van-der-Waals interactions and is therefore stabilized for larger olefins.