The understanding of catalytic mechanism is of great significance for the rational design of advanced catalysts and the optimization of catalytic processes.
Recently, a research team led by Prof. HOU Guangjin and Prof. BAO Xinhe from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) made breakthrough in the mechanistic study of syngas conversion over oxide–zeolite (OXZEO) bifunctional catalysts.
This study was published in Nature Catalysis on June 23. https://www.nature.com/articles/s41929-022-00806-2
Since its proposal by Prof. BAO Xinhe and Prof. PAN Xiulian in 2016, OXZEO catalysis has developed into a booming platform for efficient utilization of coal and other carbon resources. Compared to the rapid development in applied research, the reaction mechanism study in OXZEO catalysis has been extremely challenging, considering the complex reaction network and the diversity of multi-carbon products in such a special bifunctional catalysis system. Researchers generally draw the lessons from the hydrocarbon pool (HCP) mechanism, which has been developed for decades in the field of methanol-to-hydrocarbon (MTH) reaction, but the direct conversion of syngas over OXZEO bifunctional catalysts has been observed differently from the reaction of methanol conversion over the zeolite component.
In this study, syngas conversion over ZnAlOx (a typical metal oxide for syngas to methanol process) / H-ZSM-5 (a typical zeolite for MTH reaction) bifunctional catalyst is chosen as a model system to highlight the mechanistic difference in the OXZEO-based syngas direct conversion. By the aid of the quasi-in situ ssNMR-GC analysis strategy, the dynamic evolution of abundant critical and/or transient intermediates, including multi-carbon carboxylates, alkoxyls, acid-bounded methyl-cyclopentenones and methyl-cyclopentenyl carbocations, have been revealed from the very early induction period to the steady-state conversion under high-pressure flow-reaction conditions. Oxygenate-based routes have been well proved to contribute to the outlet olefins and aromatics, where the feed, i.e., CO and H2, is also vigorous participant in these secondary reactions. In addition to the ZnAlOx/H-ZSM-5 catalyst, the researchers also discovered that the key intermediates exist in multiple OXZEO catalysts, proving the universality of oxygenate-based routes in OXZEO-based syngas conversion. These findings are also expected to help the mechanistic understanding in CO2 and biomass conversion.
This work was financially supported by the National Key R&D Programme of China, National Natural Science Foundation of China, LiaoNing Revitalization Talents Programme, China National Postdoctoral Programme for Innovative Talents and the China Postdoctoral Science Foundation. (Text/image by JI Yi and GAO Pan)