Reaction mechanism of methanol-to-hydrocarbons conversion: Fundamental and application

As a significant reaction in heterogeneous catalysis, methanol-to-hydrocarbon (MTH) conversion supplies a sustainable way for the production of important platform chemicals, e.g., olefins, gasoline and aromatics. Recently, the commercial operation of MTO plants has aroused interest in the reaction mechanism again from both the academic and industrial fields, and extensive attentions have been attracted in the formation of direct C-C bond.

Several experimental and/or theoretical evidences have been provided to support the direct mechanism of MTH conversion, including the developed methane-FA mechanism, methyleneoxy mechanism and carbonylation mechanism. Nevertheless, due to the complexity of the MTH conversion, the formation of the first C-C bond is still controversial.

Although many commercial MTO plants have already been operated in China recently, the catalytic efficiency, i.e., the olefins selectivity and the catalyst lifetime still need to be improved. Consequently, extensive attentions have been attracted to the fundamental insights into the reaction mechanism to provide the guidance for catalyst design and optimization.

Until now, several other works focused on the reaction mechanism of MTH reaction, including catalytic materials, first principle chemical kinetics, solid-state NMR perspectives, deactivation behaviors, and several recent trends and fundamental insights, have been reviewed. However, the very recent breakthroughs in the mechanism of the MTH conversion, especially for the first C-C bond formation and its evolution in the MTH conversion including the bridge connecting the direct and indirect mechanisms, are still missing in the recent reviews. Additionally, the catalyst design and optimization for enhancing the product selectivity and promoting the catalyst lifetime based on the reaction mechanism, are also missed in the recent reviews.

Recently, a research team led by Prof. Weili Dai from Nankai University, China summarized recent progress in the reaction mechanism of MTH reaction. Whole roadmap of the MTH conversion has also been highlighted in the present review. Additionally, the catalyst design and optimization for enhancing the product selectivity and promoting the catalyst lifetime based on the reaction mechanism are also highlighted. The dual cycle, including two competing cycles, has been well understood, and could be utilized as an efficient tool to control the catalytic performance of the MTH conversion. With the aid of sophisticated spectroscopic approaches, the detailed structure and role of the active HCP species in the dual cycle have already been clarified. The predominant cycle and the active HCP species are strongly dependent on zeolite structures: pore/cage dimensions could lead steric hindrance preventing specific intermediates from being involved in dual-cycle mechanism. According to tuning the reaction condition, feed composition, zeolite structure and acidity, the target product selectivity and catalyst lifetime could be well altered. The present review provide a theoretical reference for understanding the reaction mechanism of MTO reaction and shed a light on the development of highly efficient MTO catalysts. The review was published in Chinese Journal of Catalysis (doi.org/10.1016/S1872-2067(22)64209-8).

Provided by Chinese Academy of Sciences