| Zeolite membranes,due to its regular pore structures,good thermal stability and chemical stability,have been widely used in many research fields,such as separation,catalysis and anti-corrosion.MFI-type zeolite membranes have strong hydrophobicity because its skeleton structure does not contain aluminum elements,which is one of the most ideal materials for separating low concentration alcohol aqueous solution.To date,although zeolite membranes have attracted much attention,the preparation of membranes with high separation performance is still challenging.In this thesis,it is mainly focused on simplifying the means of fabricating zeolite membranes,developing the preparation strategy of high-flux zeolite membranes,and successfully preparing b-oriented MFI zeolite membranes on the surface of porous support by designing a new preparation method of oriented seed layer.(1)Preparation of MFI zeolite membranes on solution-modified hollow fiber supports by in-situ crystallization:High separation performance MFI-type zeolite membranes were prepared by in-situ crystallization on the hollow fiber supports coated with immersion solution with appropriate composition.The effects of the composition of the immersion solution(water content,template content,aging time)and immersion time of the support on the membrane properties were investigated in detail.The results showed that when the composition of the immersion solution was TEOS:TPAOH:H2O=1:0.3:100,the aging time was 4 h,and the immersion time of the support was controlled at about 15 s,the membrane prepared by in-situ crystallization method had high separation performance,that is the separation factor for pervaporation of 5 wt%ethanol aqueous solution at 60℃ can reach 72 with a flux of 6.5 kg m-2 h-1.On the one hand,the dissolution of the species dried from the immersion solution changes the composition of synthetic solution near the support,so that more nucleation sites can be formed quickly on the support surface.On the other hand,the species dried from the immersion solution acts as nutrients inside the support to promote membrane growth.By comparing with the membrane structure prepared on the surface of pure alumina support in different synthesis periods,it is found that a large number of crystal nucleus can be quickly formed in the early stage on the solution-modified hollow fiber support,and the growth rate of the membrane in the early stage is significantly higher than that on pure alumina support.In addition,by directly dipping an alkali solution,it was found that the appropriate alkali content on the surface of the support was conducive to the formation of enough nucleation sites for membrane growth.(2)Induction of zeolite membrane formation by implanting zeolite crystals into the precursor of ceramic supports:By adding zeolite to the alumina spinning slurry,a new type of hollow fiber support containing zeolite calcination transformant(Zeolite-Cal/Al2O3 support)was designed after calcination.It was found that MFI-type zeolite membranes with high separation performance could be obtained by in-situ crystallization on the surface of this special support.The effects of zeolite content and types of zeolite on the compactness of the membrane were investigated in detail.Moreover,the behavior characteristics of the zeolite calcination transformant in the synthetic solution were explored,and the differences of the MFI zeolite membrane prepared by in-situ crystallization between pure alumina support and Zeolite-Cal/Al2O3 support in different time periods were compared in detail.It was found that the MFI zeolite membrane prepared on the 10 wt%NaA-Cal/Al2O3 support showed good separation performance,that is,the separation factor for pervaporation of 5 wt%ethanol aqueous solution at 60℃ can reach 74 with a flux of 5.1 kg m-2 h-1.Although the crystal structure of NaA zeolite collapsed after high temperature sintering of ceramic support,the versatile support can effectively form nucleation sites on the surface due to the larger pores and the NaA-Cal in the support can be dissolved in the alkali synthesis solution to continuously provide active species for the membrane formation.In addition,in order to further reduce the use of zeolite and reduce the manufacturing cost of the support,a doublelayer inorganic hollow fiber support was prepared through a triple-orifice spinneret by optimizing the spinning parameters.Only the outer layer of the hollow fiber support contained zeolite calcination transformant and the thickness is about 15 μm.The design of this support greatly reduces the manufacturing cost,and conducive to large-scale application.(3)Regulation of the membrane surface microstructure and the effect on pervaporation flux:The MFI zeolite membrane with high separation performance was prepared by adjusting the coating method of crystal seed,system alkalinity,system alkaline cation content,type of alkaline cation and double alkalis.It was found that the flux of the membrane was closely related to the concentration polarization of the feed solution,the thickness of the membrane and the mass transfer resistance of the support.In addition,ammonia was introduced into the synthetic system to adjust the alkalinity of the system for the first time,and the synthetic solution composition was optimized to obtain the membrane with high separation performance.It was found that when the composition of the synthetic solution was TPAOH:NH4OH:TEOS:H2O=0.05:0.15:1:165,the as-prepared membrane showed high separation performance,where the separation factor for pervaporation of 5 wt%ethanol aqueous solution at 60℃ can reach 84 with a flux of 7.0 kg m-2 h-1.The separation performance is higher than the membrane prepared in the conventional synthetic solution containing NaOH mineral agent.(4)Fabrication of b-oriented MFI zeolite membrane by a novel printing-transfer seed layer procedure:The hand-rubbed b-oriented MFI seed layer on the surface of plastic wrap(e.g.,PMP,PE)was successfully transferred to the surface of porous alumina disc by printing-transfer seed layer technique,and thin b-oriented zeolite membrane was prepared by introducing poly(hexamethylene biguanide)hydrochloride as a twin crystal inhibitor during seeded growth.The effects of the printing-transfer seed layer technique(e.g.,the direction of the oriented seed layer and the existence of the binder polyvinyl alcohol)were investigated in detail.Moreover,it is found that the oriented seed layer can be prepared not only on the surface of the disk but also on the surface of the tubular support,and the surface roughness of the support is not limited.The introduction of PHMB in the synthetic solution was demonstrated to tailor the crystallization kinetics via controlling the pH of the synthetic solution and preferablly adsorbing on the a-c plane of the MFI crystals via their amino groups to promote the uniform epitaxial growth of the original b-oriented seed layer.The b-oriented MFI zeolite membrane obtained on porous Al2O3 support show a flux of 2.8 kg m-2 h-1 with a separation factor as high as 71 for pervaporation of 5 wt%ethanol aqueous solution at 60℃.The flux of this membrane is much higher than that of the random-oriented membrane prepared on the same support surface. |
PDF Full Text Request