Theoretical insight into the minor role of paring mechanism in the methanol-to-olefins conversion within HSAPO-34 catalyst

Insight into the structure of hydrocarbon pool species and its effect on the catalytic activity and selectivity are urgently required in methanol-to-olefins (MTO) conversion. The fundamental issue is the understanding of its reaction mechanism. Previously, we have elucidated a complete catalytic cycle of side chain hydrocarbon pool mechanism. In this paper, paring hydrocarbon pool mechanism for different methylbenzenes (MBs) in HSAPO-34 zeotype catalyst is comprehensively investigated by periodic density functional theory calculations. The complete catalytic cycle involves a sequence of elementary steps that include methylation, ring contraction, shift of proton or methyl group, elimination of side alkyl groups, and regeneration of MBs. The major bottleneck is identified as the regeneration of MBs from five-membered ring cations. The intermediate cations having five-membered ring structure and the transition states featuring primary carbocations are unstable in the paring route. The overall energy barriers of different MBs depend strongly on the number of methyl groups. By comparing the kinetics of the paring route and the side chain route, we demonstrate that the full paring mechanism exhibits a higher barrier, and which is a minor route in the MTO conversion. (C) 2012 Elsevier Inc. All rights reserved.