Synthesis of Hierarchical Hollow ZSM-5 Zeolite Crystals by the Synergistic Effect of Dual Etching Agents for Dye Adsorption

Efficient adsorptive removal of dyes from aqueous solutions by solid porous materials depends on the number of adsorption sites and rapid mass transfer in the pore channel networks. In this work, alkaline coetching on conventional bulky ZSM-5 zeolite by tetrapropylammonium hydroxide and sodium metaaluminate was developed to produce hierarchical ZSM-5 zeolites with a hollow structure. The synergistic etching effect was systematically investigated by adjusting the etchant amount and etching conditions. The dissolution-protection reconciled etching mechanism was demonstrated, and a map illustrating the synergistic relationship was proposed. The etched zeolite exhibited a hierarchical pore structure with a high mesopore volume of 0.68 cm³ g^-1 and a large pore size centered around 30 nm, which are beneficial for the mass transfer of large dye molecules. Besides, it had more silanol groups and a lower Si/Al ratio (11-15) with evenly distributed Al element, thus significantly increasing the active adsorption sites. The hierarchical hollow ZSM-5 zeolites presented a great adsorption capacity of 120.95 mg g^-1 for methylene blue at a pH of 12 and a high rhodamine B uptake (23.84 mg g^-1 under specified conditions). The synergistic etching method is expected to finely manipulate the microstructure of zeolite materials, which would expand their applications in many fields, such as adsorption and catalysis.