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.

Recent Advances on Gallium-Modified ZSM-5 for Conversion of Light Hydrocarbons

“…Metal precursors and preparation procedures are vital for the successful fabrication of metal modified zeolites. Conventionally, Ga-containing salts [ 45 , 46 , 47 , 48 , 49 , 50 ] (gallium nitrate, etc. ), trimethyl gallium [ 51 ], gallium chloride [ 52 ], gallium oxide [ 53 , 54 ], etc.…”

“…Isomorphous substitution refers to the incorporation of trivalent Ga into the zeolite framework without changing the framework structure [ 55 ]. Generally, the incorporation of Ga into zeolite materials can be achieved during the crystallization process, normally in the presence of a suitable template, such as TPAOH [ 46 , 55 , 56 ], TBAOH [ 57 ], TBPOH [ 58 ], etc. In the product, Ga cations take the place of Si or Al in the framework of zeolite, which are highly dispersed and stable.…”

Light Paraffinic Naphtha to BTX Aromatics over Metal‐Modified Pt/ZSM‐5

“…Xin et al achieved a BTX yield of 66.1 wt % in n-heptane aromatization over Ga modified ZSM-5 (Si/Al 2 = 30) prepared by in-situ hydrothermal synthesis due to high Lewis acid positions and mesopore volumes produced by the Ga species system. [23] In another paper, Zhao et al investigated the aromatization of heptane over Zn supported alkali-treated HZSM-5 zeolite. The results showed a yield of 43 % aromatics due to suitable pore structure and coordination of proportion sites between Lewis and Brønsted acid.…”

Ethylene Dehydroaromatization over Ga‐ZSM‐5 Catalysts: Nature and Role of Gallium Speciation

“…[11c,14] Jones and co-workers [7b] developed amethod using synthesis mixtures with 3-mercaptopropyl-trimethoxysilane (MPS) to generate gallosilicates with ah igh percentage of Ga Lewis acid sites occluded as extra-framework species into zeolite pores without directly forming covalent bonds with framework oxygens.T he resulting gallosilicate catalyst (i.e.G a-MFI in the absence of Al) enhanced rates of propane dehydrogenation compared to gallosilicates prepared by conventional methods.Shu and coworkers evaluated several Ga-ZSM-5 catalysts obtained via different methods (i.e.d irect synthesis,i on exchange,i mpregnation, and physical mixing) on n-heptane aromatization. [15] They reported the Ga-ZSM-5 catalyst prepared by direct synthesis exhibited the highest BTX selectivity,w hich they attribute to enhanced Lewis acidity and the presumed mesoporosity incurred by the incorporation of framework Ga. Moreover,t hey hypothesized that both liquid-phase ion exchange and direct synthesis lead to Ga substitution in framework sites.O nt he contrary,H sieh et al [16] argued that Ga is less prone to be incorporated into the zeolite framework (compared to Al) based on their analysis of [Ga, Al]-ZSM-5 catalysts obtained via seeded synthesis.…”

“…Shu and co‐workers evaluated several Ga‐ZSM‐5 catalysts obtained via different methods (i.e. direct synthesis, ion exchange, impregnation, and physical mixing) on n‐heptane aromatization . They reported the Ga‐ZSM‐5 catalyst prepared by direct synthesis exhibited the highest BTX selectivity, which they attribute to enhanced Lewis acidity and the presumed mesoporosity incurred by the incorporation of framework Ga.…”