The Application Of Mesoporous Zeolite In The Synthesis Of NaA Zeolite Membranes

Zeolite NaA membranes are widely used in dehydration of organics because of excellent hydrophilicity. Although there have been many zeolite NaA membrane pervaporation industrial devices in both China and overseas, it is important to reduce the cost of zeolite NaA membranes, which can be achieved by using hollow fibers and large-pore a-alumina tubes as supports. High flux membranes can be synthesized by using hollow fiber supports, but the nanocrystals as seeds are difficult to be obtained. Inexpensive large-pore a-alumina tubes can reduce the cost but the as-synthesized membranes on them have relatively low flux.Based on the above fact, we do research in two aspects:(1) Nanocrystasls are produced from mesoporous zeolite micro-sized crystals and be used as seeds to synthesize zeolite NaA membranes on the external surface of hollow fibers. (2) Mesoporous zeolite NaA membranes are synthesized to reduce the resistance of membrane. The main experimental results and conclusions are summarized as follows.(1) Zeolite NaA membranes prepared by the nano-sized particles exfoliated from mesoporous zeolite NaA crystalsMesoporous zeolite NaA crystals are synthesized and denoted as meso-NaA. It is proved that the mechanical strength of meso-NaA crystals is lower than that of conventional NaA crystals. About 10% nanocrystals can be obtained by simply grinding of micro-sized meso-NaA crystals for 10 min, but almost nothing can be got from conventional NaA crystals. Using the grinded crystals in water as a seed suspension, meso-NaA crystals can induce dense membranes on the external surface of hollow fibers at 100℃ for only 2 h. After removing micro-sized crystals from the seed suspension by centrifugation, nano-sized meso-NaA crystals can induce membranes with separation factors above 10,000 for a 3 h synthesis. But conventional NaA crystals can not induce good membranes in these two ways because of lacking of small crystals after grinding. Therefore, nanocyrstals can be collected by simple grinding of mesoporous zeolite NaA micro-sized crystals followed by low-rate centrifugation. Moreover, this kind of nanocrystals is facile to be collected with high yield, which may be used for membranes, catalysts and so on.(2) Zeolite NaA membrane prepared by mesoporous zeolite NaA crystals as seeds directlyNaA membranes are prepared by mesoporous zeolite NaA crystals as seeds directly using the seeding method of dip-coating+rubbing. We investigate the influence of NaA-1* NaA-3 and NaA-5 crystals, which have different mesopore volumes, on the performance of membranes synthesized on the external surface of hollow fibers, showing out that the higher lever of mesopores, the more easily dense membrane layers are obtained. Using mesoporous zeolite NaA-5 as seeds, ultrathin membranes (600 nm) with separation factors above 10,000 and flux 9.32 kg m-2 h-1 can be obtained after synthesize for 1 h. Compared with the membrane synthesized using 180 nm nanocrystals,120 nm nanoparticles are proved to be stripped off from NaA-5 crystals adhered on the support and small particles are beneficial for formation of membranes on hollow fibers.(3) The synthesis of mesoporous zeolite NaA membranesWe attempt to synthsize mesoporous zeolite NaA membranes on the α-Al2O3 tubes, and investigate the influence of synthesis method, seed type, seeding method, alkalinity of solution, amount of mesopore generating agent TPHAC (3-(trimethoxysilyl)propyl-hexadecyldimethylammonium chloride) and Si source on the morphology and performance of membranes. The TPHAC is removed by calcination, and finally the calcined membrane is repaired by re-synthesis. The result shows that the dynamic synthesis can guarantee the dispersion of TPHAC on solution, and conventional NaA crystals are more suitable to be seeds than mesoporous NaA crystals. Moreover, dense zeolite membranes can be prepared using relatively low crystallization rate and moderate TPHAC amount. For example,2.5 Na2O:SiO2:0.67 Al2O3:200 H2O:0.03 TPHAC with sodium metasilicate nonahydrate as Si source, and 2.2 Na2O:2 SiO2:Al2O3:150 H2O:0.03 TPHAC with sodium silicate as Si source. Theoretically, calcination at 350℃ can remove TPHAC, but the membrane structure is destroyed. The re-synthesis can repair the membrane to some extent by lightly improving the separation factor.