TPAOH Template Removal from High-Silica ZSM-5 by Low-Temperature Hydrocracking

Zeolite membranes, especially the MFI-type zeolite membranes, have attracted significant attention for decades because of their special properties. While organic templates such as tetrapropylammonium hydroxide (TPAOH) have typically been used for the synthesis of ZSM-5 zeolite and zeolite membranes, the templates remain trapped in the as-synthesized zeolite crystals. A common method for removing organic templates and generating porous frameworks is calcination; however, during this process, the channel structure may be affected. In particular, for ZSM-5 membranes, membrane defects may be produced and the separation efficiency therefore may decrease to some extent. In this study, the low-temperature hydrocracking of TPAOH in ZSM-5 zeolite crystals was studied under H-2/N-2, while N-2 adsorption, thermogravimetric (TG) analysis, Fourier transform infrared (FTIR) spectroscopy, temperature-programmed desorption of ammonia (NH3-TPD), and Raman spectroscopy were used to characterize zeolite samples. The results show that the organic template in the pores of ZSM-5 can be effectively removed below 350 degrees C by low-temperature hydrocracking. Characterization analyses by BET specific surface area, TG, FTIR, and Raman spectroscopy demonstrated that a reducing atmosphere containing H-2 was more conducive to template removal at low temperature than atmospheres of air or N-2. The degree of template removal increased with temperature increasing. The BET surface area of the crystal after hydrocracking at 280 degrees C reached 252 m(2).g(-1), although a small amount of organic residue remained. Furthermore, after hydrocracking at 350 degrees C, the BET surface area reached 399 m(2).g(-1), and only trace amount of inorganic carbon residue remained. In addition, the introduction of hydrogen at low temperatures could prevent coke deposits on acid sites and thus ZSM-5 zeolite crystals had greater numbers of acidic sites after low-temperature hydrocracking.