Optimal synthesis of nanosize seeds for secondary growth of high performance hydroxy-sodalite (H-SOD) zeolite membranes for small gas and water separations

Nanosize H-SOD particles were hydrothermally synthesized as seeds for the secondary growth of hydroxy-sodalite (H-SOD) zeolite membrane. The size and phase purity of H-SOD zeolite particles were carefully controlled by changing several hydrothermal conditions. Especially, NaOH concentration in the hydrothermal solution made a significant effect on the phase purity and particle size. The high phase purity particles with a mean diameter of 80 nm were successfully synthesized at a 3 M NaOH concentration and applied for secondary growth. Overall, uniform H-SOD zeolite membranes with various thicknesses (3, 7, and 9.5 μm) were prepared by varying secondary growth time. The 9.5 μm thick H-SOD zeolite membrane exhibited high H2/N2 permselectivity of 95 at 473 K and a transmembrane pressure of 0.3 MPa. And, the pervaporation performance was evaluated for 90 wt% methanol-water mixture at 423 K, an elevated temperature. It showed a high water/methanol separation performance; its water/methanol separation factor was 2726 and the total flux, 1.02 kg/m2h. The high small gases and water separation performances were because interparticular voids between the nano size seeds were small and the surface of seed layer was flat. Therefore, the secondary growth remained a continuous zeolite layer with few non-zeolitic pores. In the present work, it was well proven experimentally that application of nanosize seed is an effective option to prepare high performance zeolite membrane in H-SOD zeolite system.