Catalytic dehydrocondensation of methane with CO and CO2 toward benzene and naphthalene on Mo/HZSM-5 and Fe/Co-modified Mo/HZSM-5

The catalytic dehydroaromatization of methane was performed by the addition of CO and CO2 at 1 atm and 973 K on Mo/HZSM-5 and Fe/Co-modified Mo/HZSM-5. With pure methane as feed gas, ca. 10% of methane is initially converted to mainly benzene at a formation rate of 600 nmol/g cat/s (on carbon base) on Mo-supported catalysts, which are drastically deactivated owing to significant coke formation (35-40%). In contrast, addition of a few percent of CO and CO2 to methane feed promotes benzene production and significantly improves the stability of the Mo/HZSM-5 catalyst at prolonged times-on-stream. For Fe- and Co-modified Mo/HZSM-5 catalysts the methane reaction with CO yields higher activities (950-1000 nmol/g cat/s in carbon base) of benzene production with good catalytic stability for more than 100 h owing to minimization of the coke formation to less than 20%. It is demonstrated that added CO2 is converted to 2 mol of CO by the reforming process (CO2 + CH4 = 2CO + 2H(2)) or by the reverse Boudart reaction (CO2 + C = 2CO), which similarly promotes the catalytic stability. TPO experiments revealed that the amount of coke formed on the catalyst surface was greatly reduced by adding a few percent of CO or CO2 to the methane feed gas. TPO experiments also showed that, in contrast to CO, addition of more than 4% CO2 to the methane feed reduces not only the inert coke but also the reactive coke, which can be converted to aromatics such as benzene and naphthalene. (CO)-C-13 isotopic labeling tracer studies showed that C-13 from (CO)-C-13 is easily incorporated into methane and products such as benzene and ethene. From the studies mentioned above, it may be suggested that the unique role of CO addition to methane feed is based on the formation of minute amounts of CO2 and C by the Boudart reaction, where C is hydrogenated to a common active carbon species [CHx] involving methane conversion toward aromatic products such as benzene and naphthalene, while CO2 reacts with the surface inert carbon species (coke) to regenerate CO, resulting in improved catalyst stability due to efficient suppression of coke formation on the catalyst. (C) 1999 Academic Press.