Ga-Substituted Nanoscale HZSM-5 in Methanol Aromatization: The Cooperative Action of the Brønsted Acid and the Extra-Framework Ga Species

A series of nanosized [Al]-, [Ga, Al]-, and [Ga]-HZSM-5 catalysts with a fixed Si-to-M 3+ ratio (M = Al or Ga) was synthesized by a seed-induced crystallization method. In order to reveal the catalytic nature of the extra-framework Ga species, an acid treatment was applied to selectively extract Lewis acidic amorphous Ga cations of as-synthesized catalysts. A comparative evaluation of freshly prepared and acid-treated catalysts in methanol conversion to aromatics showed the dehydrogenative nature of the extra-framework Ga species, which is essential in the enhancement of aromatics. Among tested catalysts, [Ga, Al]-HZSM-5 with a low extra-framework Ga-to-Brønsted acid ratio (0.06) was the most effective. The efficacy is due to the contact synergy of the extra-framework Ga species and the Brønsted acid, by which aromatics originated from the dual-cycle mechanism of methanol conversion can be accelerated.