NaA zeolite/α-alumina composite membranes were hydrothermally synthesized at 100℃ for 24 hr by using seeds of 100nm in diameter and an α-alumina support of 0.1μm in pore diameter. The effect of seed coating layer on the microstructure of NaA zeolite separation layer was systematically investigated. In cases when nanosized seeds were coated with a monolayer, increased seed coverage induced small grained and thick NaA zeolite separation layer. On the other hand, when nanosized seeds were coated with a multilayer, much smaller grained and thicker separation layer was formed. It is clear that an uniform monolayer seed coating lead to hydrothermally a thin and defect-free NaA zeolite separation layer.
Also, NaA zeolite membrane of intermediate layer structure, uniformed (3-5μm) intermediate layer were prepared by gel coating process using pressurized containment. The pervaporation performances of the membranes were evaluated in water/ethanol mixtures. The membranes having the intermediate layer were thermally more stable than those having the non-intermediate layer. The membranes were stable, even at about 70℃ for 8hours, in the thermal shock pervaporation mode. The high thermal stability was due to the well-developed intermediate layer in which the NaA zeolite and support phases were three-dimensionally interconnected with each other. It is expected that in the novel intermediate layer, the contraction of the NaA zeolite phase is compensated and relaxed by the expansion of the support phase during heating. In the present study, it was experimentally proven that microstructure control of the intermediate layer is an effective tool for improving the thermal stability of zeolite membranes, especially those showing negative expansion during heating.