Transformation of butanes over ZSM-5 zeolites. Part 2.—Formation of aromatic hydrocarbons over Zn-ZSM-5 and Ga-ZSM-5

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Abstract
Selectivities for aromatic hydrocarbons in the transformation of butanes are greatly enhanced by incorporating zinc or gallium cations into H-ZSM-5. This work is concerned with the activation of butanes over Zn-ZSM-5 and Ga-ZSM-5 at 773 K. The zinc and gallium cations serve as catalysts for dehydrogenation of the starting alkanes. In the case of the transformation of butane, 80–85% of butane molecules undergo dehydrogenation on the cationic centres, while the remainder (15–20%) undergo cracking on the acidic sites. In the case of isobutane, ca. 70% of isobutane molecules undergo dehydrogenation and ca. 30% undergo cracking. The mode of activation of alkanes depends on the ease of carbenium-ion formation and the acid strength of the zeolite. The metal cations act also as very efficient catalytic centres for dehydrogenating the intermediate alkenes into aromatic hydrocarbons. Because of the higher acidity of Ga-ZSM-5, the intermediate alkenes undergo hydrogen-transfer reactions to form alkanes as well as aromatics. This leads to a lower yield of aromatics over Ga-ZSM-5 in comparison with Zn-ZSM-5.