Lang Lin

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.

Synthesis and Low-Temperature Detemplation of High-Silica MFI Zeolite Membranes

High-silica MFI zeolite membranes supported on porous a-alumina discs were prepared by a seeded secondary growth method, using tetrapropylammonium hydroxide (TPAOH) as organic template. First, nanocrystals were deposited on rough alpha-Al2O3 discs by a spin-on process. Then, based on controlling the H2O/Si molar ratio of the synthetic solution, a restricting in-plane hOh-oriented growth method with an ultra-dilute precursor was designed to prepare non-defective zeolite membranes that were as thin as possible. Finally, cross linked and dense MR zeolite membranes were prepared after the third synthesis step, giving a membrane layer thickness of about 8 mu m, including similar to 5 mu m dense layers and 3 pm intermediate layers. A novel, two-step method, coupling by low-temperature hydrocracking and oxidation, is proposed for efficient removal of the template from zeolite membranes. Compared with traditional high-temperature calcination, template removal by the two-step method could eliminate the grain boundary defects formed in response to stresses induced by heat treatment. As a result, the membranes treated by the two-step detemplation method displayed a preferable CO2/N-2 separation factor (about 5.2) and high CO2 permeance (5.8 x 10(-7) mol.m(-2).s(-1.)Pa(-1)) at 30 degrees C.