| On the issue of the hydrothermal stability of NaA zeolite membranes which are applied in dehydration of organic solvent by pervaporation technology, the NaA zeolite membranes were prepared from two different synthetic solution-gel and clear liquid on macropore α-Al2O3 supports. Additionally, the hydrothermal stability of the two different kinds of membranes was investigated.(1) The influences of silica sources (silica sol, Na2SiO3-9H2O, TEOS) on the crystallinity, morphology and particle size of as-synthesized products by hydrothermal synthetic method without template were studied. The results showed that cubic shaped submicron NaA zeolite can be successfully synthesized by silica sol. Taking silica sol as silica source, the effects of aging time, synthesis time, inorganic salts on zeolite were investigated systematically. Optimized synthetic conditions were as follows:synthesis at 90℃ for 16h after aging at 30℃ for 15h. Inorganic salts also have great impact on the NaA zeolite. The addition of NaCl improved the crystallinity of the products. The more NaCl added, the higher the crystallinity. The addition of Na2SO4 produced other zeolites. It illustrates that the appearance of Na2SO4 tends to produce more stable zeolites, which reduce products purity. When the ratio of Na2SO4 was 0.05, it tends to produce NaX, while the ratio of Na2SO4 was 0.1, it tends to produce SOD.(2) Fixing the Si/Al molar ratio of synthesis hydrogel Al2O3:2SiO2:XNa2O:YH2O, the influences of alkalinity, water content, crystallization time on PV performance of membranes were investigated. The optimal molar composition of the synthesis hydrogel is Al2O3:2SiO2:2.2Na2O:240H2O, and the optimal crystallization time is 4h-4.5h. The morphology and thickness of the membranes were observed by scanning electron microscopy. SEM images showed an dense and continuous membrane with thickness of 5-7μm. The separation factor and flux were evaluated by pervaporation (PV) test at 348K using 90 wt.% ethanol in the feed mixture. The prepared zeolite NaA membrane showed excellent pervaporation performance, which obtains 99.8583 wt.% water contents in the permeate with a separation factor over 6,000 for dehydrating of the 10 wt% water/90 wt% ethanol mixture at 348K. The totally flux of the membrane was approximate 1.970 kg/(m2·h).(3) Zeolite NaA membrane (Membrane-X), with thickness of 4-5μm, was prepared from clear solution with molar ratio of Al2O3:5SiO2:50Na2O:1000H2O on macroporous α-Al2O3 supports by dynamic crystallized at 80℃ for 5h after aging at 25℃ for 6h. The hydrothermal stability of Membrane-X and Membrane-N, which was synthesized by gel above, was investigated. Successional pervaporation tests were carried out. Total flux vs. PV time, and water content in the permeate vs. PV time were made. The results showed that:(a) M-N made excellent performance at different PV temperature with various water content in alcohol/water mixture. After a long time PV test, M-N remained high separation efficiency, but its permeate flux decreased obviously, (b) M-X had totally failed after successional pervaporation tests. That is to say, a=1. Besides, the totally flux raised markedly. The higher water content and temperature, the more easily M-X failed. M-X was relatively stable at low water concentration. (c) SEM images of M-X after PV test shows cracks and pinholes, which indicate that the thermal expansion coefficient mismatch among support and zeolite NaA membrane, in addition, grain boundary have been damaged by water during the PV test. SEM images of M-N shows no cracks and pinholes. XRD patterns shows that relative intensity of zeolite NaA membranes were improved slightly. New peaks appeared after PV test, it speculated that SOD was produced during heating. |
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