Influences of the confinement effect and acid strength of zeolite on the mechanisms of Methanol-to-Olefins conversion over H-ZSM-5: A theoretical study of alkenes-based cycle

摘要

Methanol-to-Olefins (MTO) conversion over acidic zeolite catalysts has become the most important non petrochemical route for the production of light olefins. The 'dual-cycle' mechanism (i.e., alkenes-based cycle and aromatics-based cycle) over H-ZSM-5 zeolite has been generally accepted for olefins generation from methanol conversion. However, the relationship between the catalytic performance and the confinement effect/acid strength of the catalyst is still unclear. Herein, the methylation, isomerization and cracking processes involved in the alkenes-based cycle are discussed in-depth by density functional theory (DFT) calculations. The calculation results predicted that the transition states can be considerably stabilized by the van der Waals (vdW) interactions from the zeolite framework, resulting in the reduction of the activation barriers. And acid strength can also enhance the reaction activities. However, the catalytic reactivity of all elementary steps in the alkenes-based cycle can be improved at a different degree with increasing the acid strength. In addition, the ethene formation, transformation and the precursor of ethene formation need higher energy. And increasing acid strength can sharply decrease the activation barriers of ethene formation of cracking reaction, indicating that ethene formation may need strong acid strength. (C) 2016 Elsevier Inc. All rights reserved.