Preparation And Pervaporation Application Of Zeolite Membranes And Composite Membranes By Wet Gel Conversion

Compared with distillation,absorption,extraction and other separation methods,membrane separation technology shows the advantages of low energy consumption,high efficiency and no secondary pollution,attracting growing attentions to both scientific and industrial communities.High separation performance,high stability and low cost are required for development of membrane materials and technology.The separation performance of membrane is jointly determined by the structural properties of membrane materials and physicochemical properties of separation system.LTA and FAU zeolite membranes have regular pore structure,high mechanical strength,good thermal stability,excellent anti-pollution performance and hydrophilicity,which make them be ideal membrane materials for pervaporation technology,especially suitable for the removal of water in organic mixture.In this paper,LTA zeolite membrane with small pore size as a main model system was investigated,attemting to build a new route for synthesis of high performance LTA zeolite membrane for organic dehydration.At the same time,industrial scale-up preparation of LTA zeolite membrane by the new route was carried out.Considering the degradation of separation performance of LTA zeolite membrane under acid condition,a post modification method was developed to prepare bis（triethoxysilyl）ethane（BTESE）/LTA composite membrane to effectively enhance the acid stability,so as to broaden its application field.Finally,FAU zeolite membrane with large pore size was prepared using new synthesis method to meet the separation requirement of large molecular size or high flux system.The main contents and achievements are as follows:（1）In order to overcome the problems of low utilization rate of synthesis solution,waste of raw materials and environmental pollution caused by traditional hydrothermal synthesis of LTA zeolite membrane,wet gel conversion was used to prepare LTA zeolite membrane without adding any synthesis solution or water in the bottom of the autoclave.Firstly,the 5 cmα-Al₂O₃tube was selected as the support to prepare LTA zeolite membrane.The effects of crystallization time,water content in the gel and water dipping treatment on the growth and separation performance of LTA zeolite membranes were investigated in detail.Under optimal condition,the synthesized LTA zeolite membrane showed excellent permeation flux of 2.56 kg/（m²·h）with prominent separation factor of near 9000 for the dehydration of 90 wt.%ethanol/water mixture at 75℃by pervaporation,indicating the LTA zeolite membrane can effectively separate ethanol/water system.Based on the wet gel conversion synthesis of 5 cm LTA zeolite membrane,industrial scale LTA zeolite membrane was prepared on the 80 cmα-Al₂O₃support by this route.It was found that the as-synthesized LTA zeolite membranes showed high permeation flux of 1.78±0.06 kg/（m²·h）with excellent selectivity of 7296±65 for dehydration of 90 wt.%ethanol/water mixture at 75℃by pervaporation.The results showed that industrial scale LTA zeolite membrane can still efficiently separate ethanol/water system,and is expected to be applied in industry.Furthermore,the gel weight for membrane growth using wet gel conversion and traditional hydrothermal synthesis method was compared.The result showed that gel weight was saved by more than 96%and the cost of membrane preparation can be significantly reduced by wet gel conversion.（2）The dealuminization easily occurs on LTA zeolite membrane in acid condition,resulting in the decrease of separation performance.In order to improve the separation characteristics of LTA zeolite membrane,the surface modification was proposed.Organosilica BTESE shows high acid resistance,excellent molecular permselectivity and good binding force with zeolite material.If it is used for surface modification of defective LTA zeolite membrane,the acid resistance of LTA zeolite membrane is improved while the intergranular defects of LTA zeolite membrane are modified.The obtained membrane can maintain good selectivity and the potential high permeation flux is also developed.Defective LTA zeolite membrane was selected as the pristine membrane,and the organosilica BTESE layer was coated on the surface of LTA zeolite membrane by post-modification method.By optimizing the coating cycle and concentration of BTESE sol,BTESE/LTA composite membrane can be prepared.The composite membrane was applied for the separation of acidic（p H=3）90 wt.%ethanol/water mixture at 75℃by pervaporization.During the stability test of 4 h,the separation performance remained stable,and the permeation flux was 1.91 kg/（m²·h）with separation factor of 1013.Compared with pure LTA zeolite membrane,the acid resistance of the composite membrane was enhanced and the separation efficiency was high.（3）The wet gel conversion approach was extended to prepare high performance FAU zeolite membrane with large pore size in order to meet the separation requirement of large molecular size or high flux system.The effects of crystallization time,small seeds concentration,small seeds size and aluminum source in the synthesis gel on the growth and separation performance of the FAU zeolite membranes were investigated in detail.The dehydration performances of FAU zeolite membrane and LTA zeolite membrane for ethanol,isopropyl alcohol and propylene glycol monomethyl ether（PGME）were compared.It was found that LTA zeolite membrane was more advantageous for the separation of small molecules such as ethanol and isopropanol solutions,while FAU zeolite membrane was more advantageous for the separation of large molecules PGME solution.FAU zeolite membrane synthesized under optimal condition exhibited a permeation flux of 5.61 kg/（m²·h）and separation factor>10000for the separation of 90 wt.%PGME/water mixture at 75℃by pervaporation.On this basis,defect FAU zeolite membrane and dense FAU zeolite membrane were used as the pristine membranes respectively,and their surfaces were modified by BTESE to prepare BTESE/FAU composite membranes.It was found that the BTESE/FAU composite membrane prepared by using defective FAU zeolite membrane as pristine membrane showed higher permeation flux and equivalent separation factor,indicating that the combination of defective pristine membrane and BTESE material had synergistic effect on improving the separation performance of BTESE/FAU composite membrane...