A Mechanistic Study of Methanol-to-Aromatics Reaction over Ga-Modified ZSM-5 Zeolites: Understanding the Dehydrogenation Process

The methanol-to-aromatics (MTA) reaction was investigated on Ga-modified ZSM-5 zeolites (Ga/ZSM-5). As revealed by 71Ga and 1H solid-state NMR and FT-IR of pyridine adsorption measurements, cationic Ga species are formed as Lewis acid sites by substitution of Brønsted acid sites on H-ZSM-5. Further experimental studies show that C5- and C6-cycloalkenes are generated during the MTA reaction, which lead to the formation of cyclic carbocations as precursors to aromatics. Isotope exchange experiments demonstrate that the reactivity of the cyclic carbocations on Ga/ZSM-5 is much higher than that on H-ZSM-5 and they play an intermediate role in the formation of aromatics. In addition to the traditional bimolecular hydrogen transfer (HT) reaction, the dehydrogenation of alkenes with the release of H2 (DeaH2 process) was identified to significantly contribute to the formation of aromatics. The transformation of cycloalkenes to aromatics is favored by promotion of the DeaH2 process and competes with the HT route on Ga/ZSM-5, while these cycloalkenes tend to crack back to alkenes, and the dominating HT route results in lower aromatic selectivity on H-ZSM-5. A DeaH2-aromatization route mediated by the cooperation of cationic Ga species and Brønsted acid sites was proposed for the enhanced formation of aromatics on Ga/ZSM-5 zeolite.