| Zeolite membranes, due to their unique properties, have been attracted a great deal of research interests worldwide. Among all of these membranes, the most intensively investigated is the MFI-type zeolite membrane. It has been targeted for potential applications in many research fields such as separation and catalysis. At present, however, MFI zeolite membranes are usually prepared on small pore (≤1.0μm) and defect-free supports. b-oriented MFI zeolite membranes should be prepared on supports with pre-coated mesoporous intermediate layer, and the seeding process is also complex. In this dissertation, macroporous alumina tubes and glass tubes are investigated as membrane supports. Novel seeding methods and synthesis strategies are proposed to facilely and effectively prepare (b-oriented) MFI zeolite membranes, and there separation performances are also investigated.(1) Preparation of zeolite MFI pervaporation membranes on low-cost macroporous alumina tube support and their separation performanceWe developed a novel seeding method of wetting assisted rub-coating technique. Low-cost, macroporous alumina tubes (average pore size:1～3μm) with many large "holes" are employed as membrane supports. With this new method, support outer surface is first wetted with a liquid agent followed by rubbing dry MFI seed crystals. Effects of many wetting agents such as H2O, n-butanol, ethanol, and n-propanol on the seed layer formation and membrane performance are investigated. Experimental results show that alcohol wetting liquid, especially n-butanol, is much more effective for preparation of a uniform and defect-free MFI seed layer on the defective alumina support. The zeolite MFI membranes synthesized at175℃for only4h on the well-seeded supports exhibit a high pervaporation performance with the ethanol/water separation factor of62and permeation flux of1.82kgm-2h-1for the5wt.%ethanol/water mixture separation at60℃. The reproducibility of MFI zeolite membranes synthesized by this novel seeding method is very high. The seeding method can be extended to prepare MFI zeolite membrane on the alumina support of20cm length. The effects of secondary hydrothermal growth conditions on the micro-structure and separation performance of as-synthesized MFI zeolite membranes are investigated. At present, the relationship between secondary growth condition and membrane micro-structure, and membrane micro-structure and separation performance are not very clear. We investigate several important parameters in secondary growth such as alkalinity, template concentration. SEM-EDX, water contact angle test, gas permeation, and single component pervaporation are used to characterize the membrane structure properties in detail. It is found that the dense degree of template-removed MFI zeolite membrane is the key factor influencing membrane pervaporation performance. When the TPA+/TEOS ratio of the synthesis solution is0.17, many twin crystals can be found in the synthesized membrane layer. These twin crystals results in the formation of cracks during calcination process. Then the membrane denseness is significantly reduced. When the TPA+/TEOS ratio is decreased to lower than0.05, twin crystals are suppressed and therefore the formation of defects during calcination is significantly eliminated. Thus the pervaporation performance of zeolite MFI membrane is further improved. The ethanol/water separation factor is higher than80when the flux is higher than1.00kg m-2h-1.(2) Preparation of b-oriented MFI zeolite membrane on porous glass supportHere b-oriented zeolite MFI membranes are prepared directly on porous glass support with pore size of～0.1μm by in situ hydrothermal synthesis method. It is difficult to obtain continuous MFI zeolite membrane if the porous glass is immersed in the synthesis solution completely. When the support is partly immersed in the synthesis solution,b-oriented zeolite MFI membrane is obtained on the un-immersed part of the support. The membrane obtained near the synthesis solution level is much denser, while the membrane obtained on the top of the un-immersed support is much looser. We therefore propose that the un-immersed support surface can be covered by the synthesis solution film through the capillary force that occurred in the support pores. Then the synthesis solution film is crystallized during the secondary synthesis process, thus the b-oriented membrane is obtained. The preparation parameters such as the synthesis solution composition and crystallization time are systematically investigated. Under the optimized synthesis condition (0.64TPAOH:1TEOS:165H2O,165℃,3h),b-oriented zeolite MFI membrane with high coverage is achieved on support surface. When the crystallization time is extended to6h, the support can be fully covered by zeolite MFI membrane, however, the membrane is randomly oriented. We also find that dense b-oriented zeolite MFI membrane with a cycle shape can be obtained on the support nearing the bubble place. These results can provide guidelines for the mechanism research of b-oriented zeolite MFI membrane formation by in situ hydrothermal synthesis.We developed a simple yet effective oriented (MFI crystal) seed printing-transfer technique to prepare continuous and dense b-oriented zeolite MFI membrane on porous glass support by secondary growth. With this technique, a monolayer b-oriented MFI seeds is prepared on the printing support (e.g., wrap film) by manual assembly. Then the outer surface of the porous glass is wrapped by this seeded wrap film, and therefore the oriented seed crystals are printed on the porous glass. During secondary growth, the synthesis solution permeated to the seed layer from the inner side of the support and then continuous b-oriented zeolite MFI membrane is obtained. Many kinds of wrap films such as PMP, PVDC, and PE are investigated as printing supports. Using these wrap films, the quality of seed layer and the dense degree of synthesized b-oriented zeolite MFI membrane are compared in detail. It is found that PMP wrap film is the most effective printing support. The secondary growth strategies and synthesis solution compositions are then optimized. We find that, by using two-step secondary growth method, the dense degree of b-oriented zeolite MFI membrane can be improved and the interaction between membrane layer and support is highly strengthened. The synthesized oriented MFI membrane exhibits certain separation performance for the p-/o-xylene mixture. This membrane manufacturing technique deserves further research. |
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