Aromatization of methane by using propane as co-reactant over cobalt and zinc-impregnated HZSM-5 catalysts

The activities of the cobalt and zinc-impregnated HZSM-5 catalysts to the non-oxidative conversion of propane (C3) and methane (C1) into aromatic hydrocarbons were evaluated using a fixed-bed microreactor. C1 conversion reached 36.7% and the selectivity of aromatic products reached above 88.7% at atmospheric pressure, weight (hourly) space velocity (WHSV) 1.6 g h−1/(g cat)−1 and 873 K. The influence of the acidity and the ratio of cobalt in the catalyst on the conversion of methane and propane was evaluated. C1 incorporation was conclusively confirmed by the mass spectral analyses of aromatic products produced in a run with 13CH4 which shows a significant 13C enrichment in the C6H6+, C7H8+ and C8H10+ fragments. The methane activation could result from its hydrogen-transfer reaction with alkenes. These catalysts were thoroughly characterized using XRD, N2 adsorption measurements, TPD of NH3, and FT-IR. The results showed that the activation of methane in low temperature was due to existence of propane. The acidic changes and micropore area of the catalyst severely affected aromatization, and resulted in drastic modifications in product distribution. From this work, we found that only a small fraction of tetrahedral framework aluminum, which corresponds to the Bronsted acid sites, is sufficient to accomplish the aromatization of the intermediates in methane and propane aromatic reaction, while the superfluous strong Bronsted acid sites, which can be decreased by adding Co and Zn, are showed to be related with the aromatic carbonaceous deposits on the catalysts. The density of acidic site and the strength of strong acid decreased when the concentration of Co and Zn in the catalyst increased. Therefore, a much higher benzene yield and a longer durability of the catalysts are obtained when compared with the conventional HZSM-5 catalysts.

The TPD–NH3 spectrum shows that the progressive shift to higher peak temperatures for low-temperature peak as cobalt concentration in the catalysts increased. The IR spectra in the pyridine region (1410–1575 cm−1) shows that as Co and Zn concentration in the catalyst increased the intensity of the band at 1550 cm−1 (Bronsted acid sites) decreased a little and an increase of the intensity of the band at 1460 and 1616 cm−1 (Lewis acid sites). Figure optionsDownload as PowerPoint slide