Synthesis and low-temperature detemplation of high-silica MFI zeolite membranes

Abstract

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 α-Al2O3 discs by a spin-on process. Then, based on controlling the H2O/Si molar ratio of the synthetic solution, a restricting in-plane h0h-oriented growth method with an ultra-dilute precursor was designed to prepare non-defective zeolite membranes that were as thin as possible. Finally, crosslinked and dense MFI zeolite membranes were prepared after the third synthesis step, giving a membrane layer thickness of about 8 µm, including ~5 µm dense layers and ~3 µm intermediate layers. Anovel, 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/N2 separation factor (about 5.2) and high CO2 permeance (5.8 × 10-7 mol·m-2·s-1·Pa-1) at 30 °C.