Understanding the effects of acid strength of active center and local confinement environment on the conversion of methanol to olefins in H-RUB-50

Abstract

Brønsted acid (BA) strength and confinement effect of the cavity-type zeolites play an important role in the formation of products during methanol to olefins (MTO) conversion. In the present study, olefin-based cycle using 2,3-dimethyl-2-butene (iso-C6) as the hydrocarbon-pool (HCP) species was systematically investigated in H-RUB-50 zeolite with 8-membered ring (MR) pore opening and small LEV cavity by using periodic density functional theory calculations combined with microkinetic modeling, so as to evaluate the effects of different local confined space, active acid sites and acid strength caused by different Al sites on the catalytic performance of MTO reaction. The free energy profiles at four different BA sites (Al1O2, Al1O4, Al2O2 and Al2O5) generated by Al sitting at T1 and T2 sites in 8-MR window show that the olefin-based cycle is conducive to the formation of ethene at the four BA sites in H-RUB-50 zeolite, and Al1O2 BA site with the strongest acid strength is the most active site for MTO reaction. The change trend of TOF of ethene and propene shows that the acid strength is closely related to the formation of ethene, but for the formation of propene, not only the acid strength, but also the local confined space of the active center plays a more important role. In H-RUB-50 zeolite, the selectivity to ethene based on the olefin-based cycle is higher than to propene, which is consistent with the previous theoretical and experimental results. It is shown that the overall catalytic activity and ethene selectivity of H-RUB-50 at T1 site is much better than that at T2 site. This study provides a useful guidance for the experimental synthesis and adjustment of Al sites to improve the catalytic performance of MTO.