【Original Contribution】 Dehydration Performance of Sodalite Membranes

Abstract

Zeolites are crystalline aluminosilicates with welldefined pores and channels of molecular size. Intensive research efforts have been made in the last decades to shape various kinds of zeolite in thin films. Zeolite membranes have been examined in separating liquid mixtures and gaseous mixtures, including dehydration 1) , CO 2 /CH 4 2, 3) , NH 3 /H 2 4) , isomer separations 5) and other separations 6, 7). Successful commercial application of zeolite membranes is reported with LTA-type and T-type zeolite membranes in dehydration 8). One of the unique properties of zeolite is the variety of zeolitic pore size. Sodalite has one of the smallest zeolitic pore size of 0.28 nm 9) , allowing only small molecules, such as water, helium and hydrogen, penetrate into. Accordingly, defect-free sodalite membranes are expected to show significantly high selectivity of small molecules. Several attempts have been made to prepare sodalite membranes 10, 11). However, single gas permeations of helium and nitrogen through sodalite membranes followed Knudsen diffusion mechanism, suggesting a large contribution of inter-crystalline pathways 10). Recently, hydroxyl sodalite membranes are reported to show dehydration peformance 12, 13). Surface-polished flat supports used in these studies may facilitate oriented nucleation and growth of sodalite crystals, and enhanced intergrowth. From the point of scaling up the membrane preparation process, we focused on preparing sodalite membranes on symmetric tubular ceramic supports. Effects of synthesis solution composition, synthesis temperature and crystallization time on the formation of sodalite membranes were studied. Membrane performance was evaluated in pervaporation with ethanol and iso-propanol solutions at 348 K, 0.1 MPa. Influence of feed compositions was also examined. 2. Experimental 2.1 Synthesis of sodalite seed crystals Sodalite crystals were prepared by hydrothermal synthesis with solution having molar composition of 1.00 SiO2: 0.50 Al2O3: 0.96 Na2O: 0.65 NaCl: 20.50 H2O. Alumina hydroxide (Al(OH)3, 100%, Wako), sodium hydroxide (NaOH, 97%, Wako) and distilled water were mixed and heated until the solution became transparent. After cooling the solution to room temperature, sodium chloride (NaCl, 99.99%, Wako) and colloidal silica (Ludox HS-40, Aldrich) were added