Synthesis of gallium-containing ZSM-5 molecular sieves and their catalytic performance in methanol aromatization

Gallium species were introduced into ZSM-5 molecular sieves through three different measures, i.e. hydrothermal synthesis, impregnation, and ion exchange. The gallium-containing ZSM-5 molecular sieves were characterized by XRD, SEM, FT-IR, NH₃-TPD, ICP, XPS and N₂ sorption. The effects of silica source and the content and state of gallium species on their structure, acid property, and catalytic performance in the aromatization of methanol were investigated. The results indicated that the silica source (silica white, tetraethyl orthosilicate, and silica sol) exhibits significant effects on the crystal size and the state of gallium species, and thus influences the catalytic activity and stability of the gallium-containing ZSM-5 molecular sieves in methanol aromatization considerably. There are three kinds of gallium species, i.e. framework Ga, Ga₂O₃ species on the crystal surface, and non-framework Ga species with strong interaction with the framework oxygen. The framework gallium species in the tetrahedral structure offers the acidic sites of the molecular sieves, while the non-framework gallium acts as the active sites for methanol aromatization. The proper amounts of non-framework gallium species matched with the acid sites can promote the formation of aromatics.