Impact of Al sites on the methane co-aromatization with alkanes over Zn/HZSM-5

Abstract

The presented work investigates the co-aromatization of methane and propane over Zn/HZSM-5 catalysts. The impact of Al sites, such as framework and extra-framework aluminum (EFAL) sites, on the methane participation pathway during the co-aromatization reaction is explored. The Al sites in the catalysts are manipulated by using ZSM-5 support materials with variable SiO2/Al2O3 molar ratios. Post-synthetic treatments, such as the ammonium hexafluorosilicate (AHFS) treatment to remove EFAL and the steam treatment to convert some of the framework Al sites to EFAL sites, are also employed to prepare catalysts and evaluate the contribution of these Al sites to the methane participation pathway. A higher concentration of Al in the catalysts would facilitate the methane participation in the phenyl ring formation. An average carbon number increment of 0.93, a substitution index reduction of 0.028, and an enhanced aromatic carbon fraction by 0.036 among the product molecules are witnessed over the Zn/HZSM-5 (23:1) catalyst during the co-aromatization reaction. This phenomenon is further enhanced when some of the framework Al atoms are converted to the EFAL sites by the steam treatment. The changes of substitution index and aromatic carbon fraction are expanded to 0.045 and 0.055. When the total Al concentration is low in the catalyst, however, the increased EFAL site number at the cost of framework Al sites does not improve methane participation, indicating that the framework Al sites are also crucial for the methane participation. The chemical environment of the Al sites in the catalysts is probed by 27Al solid-state NMR and XAS studies. The NH3-TPD and DRIFTS investigations of the catalysts demonstrate that the framework Al sites and the EFAL sites are closely related to the Brönsted and Lewis acid sites, respectively. The TEM and XRD studies reveal the intact crystalline structure of the catalysts upon the AHFS and steam treatments. The impact of the Al environments on the methane participation pathway in the co-aromatization reaction may be due to the acid properties of the catalysts.