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The cause of carbon deposition-induced deactivation in methanol-to-olefins conversion identified / Author/Source: Dalian Institute of Chemical Physics, Chinese Academy of Sciences / Date: 2020-04-16 / Clicks: 11. Recently, a team led by Academician Liu Zhongmin and Researcher Wei Yingxu from the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, made new progress in understanding the mechanism of carbon deposition-induced deactivation in methanol-to-olefins reactions; they discovered the mechanism by which carbon deposits grow across the pores of cage-shaped molecular sieves used as catalysts for methanol conversion. In this study, the research team combined the reaction of methanol conversion to olefins with high-resolution mass spectrometry and isotope labeling techniques, along with theoretical calculations, to successfully decipher the structures of complex carbon deposits with high relative molecular masses that had previously been unknown. They conducted a systematic investigation of the cross-linking process of the initial carbon deposit precursor species within the molecular sieve cages during the growth of these carbon deposits, and provided a relatively complete overview of the evolution pathway of carbon deposits in the methanol-to-olefins reaction: Active hydrocarbon species within the SAPO-34 molecular sieve cages gradually undergo ring expansion and ring fusion to form carbon deposit precursors with 3 to 4 rings; subsequently, these carbon structure units cross-link across the cages through covalent bonds, giving rise to poly nucleus, nano-graphene-like structured polycyclic aromatic compounds. Such cross-cage cross-linked polycyclic aromatic species severely hinder reaction mass transfer, leading to catalyst deactivation. Subsequent extended studies found that the carbon deposition-induced deactivation behavior across cages is also prevalent in catalytic systems using molecular sieves with other cage structures. Industrial processes such as petrochemical operations catalyzed by molecular sieves (catalytic cracking, isomerization, etc.) and coal chemical processes (methanol to olefins, syngas conversion, etc.) hold an important position in the national economy. Due to their acid-catalysis nature, it is common for catalysts to become deactivated as a result of carbon deposition. The catalyst needs to be continuously carbonized and regenerated to maintain its activity, thereby enabling long-term operation of the device. Although the problem of carbon deposition-induced deactivation of molecular sieve catalysts has received widespread attention, finding ways to prevent carbon deposition on these catalysts remains a long-term scientific challenge. To solve this problem, it is necessary to have a thorough understanding of the mechanism by which catalyst carbon deposition leads to deactivation. Currently, the understanding of carbon deposition on molecular sieves in the literature is summarized as either simple polycyclic aromatic species within the molecular sieve or graphitized carbon deposits formed on its outer surface, making it difficult to determine the exact pathway of carbon deposition. Performing full-spectrum analysis of the structure of carbon deposition species at the molecular level and tracking their evolution hold great practical significance for the processes of carbon deposition deactivation and catalyst regeneration.