image: Researchers demonstrated 5 wt.% Na2WO4/SiO2 as the first heterogeneous coupling catalyst for oxidative coupling of methane. The bifunctional system generated CH3* radicals over La2O3 and selectively coupled CH3* on the surface of 5 wt.% Na2WO4/SiO2. As a result, it boosts the C2 selectivity of La2O3 by three times and achieves a C2 yield as high as 10.9% at bed temperature of only 570 ºC. view more
Credit: Chinese Journal of Catalysis
Oxidative coupling of methane (OCM) is an economically valuable and promising reaction because it can directly convert the abundant feedstock natural gas into valuable C2 hydrocarbons. The commercialization of OCM, however, is hampered by the low yield of C2 products, which is associated with OCM reaction mechanism. It is generally accepted that traditional OCM reaction follows a heterogeneous-homogeneous mechanism, which consists of a heterogeneous activation of methane at the surface of catalysts to generate methyl radicals (CH3• ) and a subsequent homogeneous coupling of methyl radicals in the gas phase to produce C2 species (C2H6 and C2H4). Because high temperature favors the desorption of CH3• into gas phase, the role of the traditional OCM catalysts is limited mainly to the generation of CH3• . The uncontrollable homogeneous reaction of CH3• in the presence of O2 thermodynamically favors the overoxidation to CO2, resulting in an upper bound of ca. 25 - 28% for C2 yield which is economically infeasible for industrial application. In terms of conventional fixed-bed reactor, the possible exceeding of this limit can be obtained only if catalyst plays significant role not only in heterogeneous generation of CH3• but also in their subsequent transformations. Unfortunately, there is no such catalyst that has been ever reported to have activity of coupling methyl radicals.
Recently, a research team led by Prof. Jie Fan from Zhejiang University, China demonstrated 5 wt.% Na2WO4/SiO2 as the first heterogeneous coupling catalyst for OCM reaction and systematically investigated its structure-property relationship. The activation of methane takes place over a La2O3 surface at relative low temperatures, which allows the coupling of CH3• proceed in a controlled manner on the surface of Na2WO4/SiO2 catalyst. The controllable surface coupling against overoxidation barely changes the activity of La2O3 but boosts the C2 selectivity by three times and achieves a C2 yield as high as 10.9% at bed temperature of only 570 ºC. The selective coupling of CH3• over Na2WO4/SiO2 is experimentally confirmed by synchrotron-based vacuum ultraviolet photoionization mass spectrometry, a validated technique for in-situ CH3• monitoring. Structure-property studies suggest that Na2WO4 nanoclusters are the active sites for methyl radical coupling. The strong CH3• affinity of these sites can even endow some methane combustion catalysts with OCM activity. The results were published in Chinese Journal of Catalysis.
Prof. Jie Fan stated: "The findings of the surface coupling of methyl radicals open a new direction to develop OCM catalyst. The bifunctional OCM catalyst system, which composes of a methane activation center and a CH3• coupling center, may deliver promising OCM performance at reaction temperatures below the ignition temperature of C2H6 and C2H4 (~600 °C) and is therefore more controllable, safer, and certainly more attractive as an actual process."
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This work was supported by the Natural Science Foundation of China (92045301, 91845203, 21802122, 21703050), and Key Program of Research and Development of Hefei Science Center, CAS (2018HSC-KPRD002).
About the Journal
Chinese Journal of Catalysis is co-sponsored by Dalian Institute of Chemical Physics, Chinese Academy of Sciences and Chinese Chemical Society, and it is currently published by Elsevier group. This monthly journal publishes in English timely contributions of original and rigorously reviewed manuscripts covering all areas of catalysis. The journal publishes Reviews, Accounts, Communications, Articles, Highlights, Perspectives, and Viewpoints of highly scientific values that help understanding and defining of new concepts in both fundamental issues and practical applications of catalysis. Chinese Journal of Catalysis ranks among the top six journals in Applied Chemistry with a current SCI impact factor of 6.146. The Editors-in-Chief are Profs. Can Li and Tao Zhang.
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Chinese Journal of Catalysis