Synthesis of MFI-type zeolite membranes on porous α-alumina supports by wet gel crystallization in the vapor phase

A new method has been developed for synthesis of MFI-type zeolite membranes on porous α-alumina supports. Using this method, a thin layer of wet-gel precursor precoated from a synthesis sol containing SiO₂, tetrapropylammonium hydroxide (TPAOH), NaOH, and H₂O was converted to an MFI zeolite film by vapor-phase treatment at elevated temperatures. MFI zeolite films were formed on the α-alumina substrates, respectively, in the vapor phases of TPAOH solution and an ethylenediamine(EDA)/triethylamine (TEA)/water mixture, but an amorphous material was obtained in the vapor phase of pure water. Scanning electron microscopy and helium permeation examinations before calcination showed that the MFI membrane obtained in the vapor phase of the TPAOH solution was of higher quality than that synthesized in the vapor phase of the EDA/TEA/water mixture in terms of zeolite film integrity and compactness. It was also found that the existence of the template TPA⁺ in the parental synthesis sol was critical to the formation of MFI zeolite under the investigated conditions. When precursor layers coated from a template-free colloidal silica system were used, the vapor-phase treatment resulted in formation of Na-P1 and ANA zeolite films, respectively, in the vapors of TPAOH solution and the EDA/TEA/water mixture. The method developed in this work has the advantages of improved controllability, minimal waste generation, and reduced chemical consumption that are desirable for large-scale production of zeolite membranes.