An in situ approach to synthesize pure phase FAU-type zeolite membranes: effect of aging and formation mechanism

FAU-type zeolite membranes were prepared on porous α-Al₂O₃ tubes by a two-stage in situ synthesis method. The synthesis was carried out in clear solutions without the aid of seeds or organic templates. The first stage synthesis was devoted to the in situ nucleation of FAU-type zeolite, aiming to induce an evenly covered zeolite layer on the support. In situ aging played an important role at this stage as it suppressed LTA phase and promoted the formation of a continuous zeolite layer. The second stage synthesis was performed in a dilute solution, where the pre-formed zeolite layer went on to grow and became compact. Scanning electron microscopy (SEM) observations show that the membranes were made up of well-shaped intergrown crystals, with the thickness around 7 μm. Pervaporation (a membrane-based separation process in which the liquid components diffuse through a membrane and are subsequently vaporized due to the low-pressure exerted on the permeate side of the membrane) measurements were carried out in a water/ethanol (10/90, w/w) mixture to evaluate the membrane performance. The structural and morphological evolutions of FAU-type zeolite membranes during synthesis were tracked down by X-ray diffraction and SEM. The formation mechanism was illuminated based on the experimental results.