Template-free Synthesis Of Mfi-type Zeolite Membranes

The silicalite-1 (pure silica MFI-type zeolite) membrane is one of the most thermally and chemically stable inorganic microporous materials. It has high Si/Al ratio and hydrophobic property, which has shown good prospect for separating ethanol/water. The traditional synthetic method is usually performed in the presence of organic templates, which needs to be removed by calcinating to open up the zeolitic channels. During the template removal step, intercrystalline gaps tend to form in the membranes due to the difference in thermal expansion between the zeolite layer and the support or by changes in lattice parameters of zeolite crystals, which can affect the performance of the membranes. This work is to prepare continuous MFI zeolite membranes onα-Al₂O₃ substrates by the template-free secondary growth method, and apply the synthesized membranes in ethanol/water separation with low ethanol concentration.Here, the silicalite-1 zeolite membranes were prepared on porousα-Al₂O₃ substrates using the template-free secondary growth method. Prior to the synthesis, two different sizes of seeds were successively deposited on the surface of substrates by spin-coating, and then the seeded supports were placed in autoclaves for hydrothermal synthesis with the template-free secondary synthesis solution after growing in the organic template synthesis solution first. The obtained zeolite membranes were tested using scanning electron microscopy (SEM), X-ray diffraction (XRD), infrared ray (IR), CO₂/N₂ binary gas separation and ethanol/water pervaporation.For 600 nm-sized zeolite seeds, after hydrothermal synthesis in the organic template synthesis solution at 150°C, whose molar ratio was 1 TEOS: 0.2 TPAOH: 800 H₂O, the template-free secondary synthesis solution with the molar composition of 1 TEOS: 0.40 NaOH: 270 H₂O was used for the subsequent membrane synthesis at 180°C. A synthesis procedure of 6+16*+16* h was found to be the best compromise between duration and temperature crystallization. The membrane prepared in this condition is highly crystalline, the gas separation factor was 1.227, and ethanol/water separation factor was 2.115.For the 200 nm-sized zeolite seeds, with the same steps to form the zeolite membrane, the permeation flux was found to be smaller. A synthesis procedure of 8+24* h crystallization was found to be the best with the molar composition of 1 TEOS: 0.25 NaOH: 180 H₂O. The separation factor was 1.264, and the permeation flux was 2.383 kg·m^-2·h^-1.It is indicated that the seed layer prepared with 200 nm-sized zeolite seeds was found to be better than that of 600 nm-sized ones, for the smaller seed can be more closely arranged on the support surface and the seed layer was relatively flat. However, the latter one was found less amorphous materials, and the effect of ethanol/water separation was better.