Preparation of Zeolite NaA Membranes on Macroporous Alumina Supports by Secondary Growth of Gel Layers

Zeolite NaA membranes are synthesized on tubular alpha-Al(2)o(3) supports by the secondary growth of gel layers. The gel layer is prepared by wetting rubbing hydrogel with a composition similar to the secondary synthesis solution xNa(2)O:2SiO2:Al2O3:150H(2)O. The hydrogel's good uniformity makes it easier to gain a uniform gel layer. Zeolite NaA membranes are characterized by SEM and pervaporation separation of a 90 wt % ethanol/water mixture at 75 degrees C. The effects of the pretreatment time, pretreatment temperature, gel loading, and Na2O/Al2O3 ratio of the hydrogel on properties of zeolite NaA membranes are investigated. It is found that zeolite NaA membranes with high separation factors up to 10 000 can be obtained by secondary growth of gel layers. The optimal pretreatment conditions of the hydrogel are as follows: pretreatment temperature, 50 degrees C; pretreatment time, >= 18 h; and gel loading, 0.6-0.9 mg/cm(2). The optimal composition of the hydrogel is 2.2Na(2)O:2SiO(2):Al2O3:150H(2)O, which is the same as the synthesis solution. Like crystal seeds, the gel layer improves the zeolite membrane formation on the support surface, and the role of the gel layer becomes more significant with greater amounts of crystals.