Ga Substitution during Modification of ZSM-5 and Its Influences on Catalytic Aromatization Performance

Abstract

ZSM-5 zeolites, Ga modified via different methods (in situ hydrothermal synthesis, mechanical mixing, incipient wetness impregnation, solid-state ion exchange, and liquid phase ion exchange), were systematically investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), 29Si, 27Al, and 71Ga magic-angle spinning (MAS) NMR, and H2 temperature-programmed reduction (H2-TPR). It is important to prove that both impregnation and liquid phase ion exchange could facilitate the incorporation of Ga species into the framework in addition to in situ hydrothermal synthesis. The liquid phase ion exchange method drove part of the Ga species into the framework, and it was further migrated into the framework by drying, while the incipient wetness impregnation method promoted part of the Ga species into the framework only during the calcination process. In the n-heptane catalytic aromatization procedure on the fixed bed, Ga modified ZSM-5 by in situ hydrothermal synthesis showed the highest benzene, toluene, ethylbenzene and xylene (BTEX) selectivity, owing to the increased strong Lewis acidic sites and mesopore volumes induced by the framework Ga species.