Insights into the reaction mechanism of methanol-to-olefins conversion in HSAPO-34 from first principles: Are olefins themselves the dominating hydrocarbon pool species?

Abstract

Full mechanistic understanding of methanol-to-olefins (MTO) conversion is urgently required, not least for the precise control of product selectivity and rational design of zeolite catalysts. Due to its complex nature, the MTO reaction mechanism is still a hot area of dispute. It was traditionally believed that the MTO reaction proceeds through hydrocarbon pool mechanism and methylbenzenes (MBs) are the predominant hydrocarbon pool species. However, by extensive periodic density functional theory (DFT) calculations in HSAPO-34 catalyst, this work indicates that olefins themselves other than MBs are likely to be the dominating hydrocarbon pool species. A full reaction network is established, and the routes to produce olefins, alkanes, and aromatics are formulated. We find that light olefins such as ethene and propene are mainly produced through the scission of cracking precursors (carbenium ions, alkoxides, and higher olefins), and which are formed by the methylation of lighter olefins. The distribution of these cracking precursors as the number of carbon atoms in the pore of catalysts influences the product selectivity from the reaction point of view. A decrease trend in the cracking energy barriers is observed with the carbon atom number of cracking precursors. Hydride transfer between two olefins results in the formation of alkanes and dienes and the latter are likely to be the precursors to form aromatics and subsequently leading to the deactivation of catalysts. This reaction network allows us to rationalize some experimental findings, and more importantly, provides clues on the understanding of selectivity and deactivation.