Efficient synthesis of smaller crystal ZSM-5 zeolite in low TPAOH-to-silica ratio dry-gel hydrothermal system

The present study is related to the synthesis of nano-sized ZSM-5 zeolite with a dry-gel hydrothermal (DHT) method which is different from the known dry-gel conversion (DGC) method, for it allows the crystallization of the dry-gel in water or n-butylamine solution and the use of common autoclaves. The effects of TPAOH/SiO₂ ratio on the crystal size of ZSM-5 zeolite product, and the viability of synthesizing nano-sized ZSM-5 zeolite using the new method from a TPAOH-poor dry-gel with the aid of co-template n-butylamine are emphasized. Results show that, under the same TPAOH/SiO₂ ratio conditions, DHT synthesis is more beneficial for obtaining smaller crystal size ZSM-5 zeolite than the conventional hydrothermal (CHT) synthesis. The DHT synthesis can be implemented with a TPAOH-rich dry-gel (for example, TPAOH/SiO₂≥0.15) by using simply water as crystallizing solution, to yield discrete nano-sized ZSM-5 zeolite (crystal size 70 nm), or with a TPAOH-poor dry-gel (for example, TPAOH/SiO₂ = 0.075) by using a concentrated NBA aqueous solution (for example, a solution contains 75 % NBA) as crystallizing solution, to yield aggregates of nano-sized ZSM-5 zeolite (primary particle size 200 nm). The nano-sized ZSM-5 zeolites synthesized by the DHT method are inclined to have larger mesoporous and macroporous volume. After being doped with zinc ions, they become promising catalysts for the aromatization of n-hexane (n-h).