Synthesis of a hierarchical ZSM-5 zeolite from iron-ore tailings by a two-step method

Iron-ore tailings (IOT) are mineral waste obtained via the iron-ore mining process. This has become a very critical issue in industrial solid waste management. The stacking of IOT occupies a large area and causes serious pollution, which can harm human life. This has become a serious matter of concern for society. IOT are usually rich in SiO₂ and Al₂O₃ and can be used as raw materials in producing zeolitic materials. They have substantial benefits from both economic and environmental perspectives. However, few studies have been reported on the synthesis of zeolite or zeolite-like materials using IOT. As a type of zeolite, ZSM-5 has a regular pore structure and high thermal and hydrothermal stability and is widely used in catalytic materials. However, the application of ZSM-5 is greatly limited in many catalytic reactions owing to the large molecules associated with its inherently small pore sizes (<1.5 nm). Introduction of a hierarchically porous structure into conventional ZSM-5 maintains the crystal structure, acidic active center, and high thermal and hydrothermal stability and accelerates the diffusion/transfer of large molecules and greatly reduces the formation of carbon residue. This prolongs the service life of the catalyst used, which is extremely desirable in catalysis. In this study, hierarchically porous ZSM-5 was prepared via a two-step process using IOT as a silica source instead of pure chemical reagents. First, mesoporous MCM-41 was synthesized using cetyltrimethylammonium bromide as a mesoporous template. Then, hierarchically porous ZSM-5 was fabricated by impregnating a structure-directing agent into the as-synthesized MCM-41, followed by a solid-phase conversion method to transform amorphous silica into a zeolite crystal. To evaluate the textural properties of the zeolites, the as-synthesized samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and nitrogen adsorption. The results show that phase separation between the surfactant and zeolite crystals is avoided because of the absence of the liquid-water phase during the solid-phase conversion. Therefore, the synthetic route presented herein provides a novel method for the synthesis of hierarchically porous ZSM-5 from IOT.