Construction Of High-permeability Nanofibrous Composite Pervaporation Membrane And Its Application Exploration

As a new membrane separation technology,pervaporation has a wide application prospect in the fields of clean energy production,waste water treatment,desalination and azeotropic system separation because of its advantages of low energy consumption,simple process,high single-stage separation efficiency and less environmental pollution.As the core of the separation process,pervaporation membrane determines the separation performance.At present,the separation efficiency of pervaporation membrane is low,which is difficult to meet the needs of industrial production.Nanofibrous membrane prepared by electrospinning is one of the best candidates as the substrate in preparing high-performance pervaporation composite membrane because of its high porosity,high specific surface area and interconnected porous structure,which can allow permeate molecules to pass through quickly and effectively reduce mass transfer resistance.Thin-film nanofibrous composite(TFNC)membrane has been widely used in ultrafiltration,nanofiltration and other membrane separation fields.However,it is challenging to prepare a uniform and integrated skin layer on nanofibrous substrate due to its highly porous structure and undulating surface.The current fabrication methods are complicated and difficult to achieve controllable large-scale preparation,which restricted its application in pervaporation.Therefore,the design and optimization of composite membrane structure,selection of suitable materials and membrane fabrication method for the preparation of skin layers on nanofibrous substrates have become the hotspot for researchers.Herein,focusing on different applications of pervaporation,this paper aimed to construct novel high-performance pervaporation membranes.Series of intergreted TFNC pervaporation membranes had been designed with novel membrane structures by introducing new materials and optimizing the membrane preparation process.The mechanism of membrane formation,interfacial interaction between the skin layer and the substrate,and the relationship between film-forming mechanism and separation mechanism were all systematically investigated.Under the guidance of theory,the separation performance of the nanofibrous composite membrane was optimized,the pervaporation process was been strengthened.This work provided technical support for preparing commercial high-performance pervaporation membrane.(1)In order to improve the separation efficiency of isopropanol and water,a stable and highly permeable sodium alginate(SA)nanofibrous composite membrane was prepared by introducing an intermediate layer.The polyethyleneimine modified graphene oxide(PEI-GO)interlayer with good permeability and hydrophilicity was fabricated on the surface of polyacrylonitrile(PAN)nanofibrous membrane by vacuum-assistant filtration.The PEI-GO interlayer could prevent the infiltration of casting solution and improve the interfacial interaction between the SA selective layer and nanofibrous substrate through electrostatic force and hydrogen bond interaction.The structural stability of TFNC pervaporation membrane was enhanced which could avoid the peeling of the skin layer and the collapse of the membrane structure in the pervaporation process.Moreover,the introduction of macromolecular amines expanded the d-spacing of GO sheet and endowed GO layer more hydrophilic groups,so that water molecules could pass through the membrane quickly,and the mass transfer resistance was reduced.The optimized composite membrane showed excellent separation performance:the permeation flux was 2009 g/m~2 h,and the separation factor reached1276 in dehydrating 90 wt%isopropanol aqueous solution at 70?.(2)In order to further improve the efficiency of dehydrating isopropanol and water mixture,double interfacial polymerization(double-IP)strategy was innovatively proposed to prepare the high-permeability two-tier PA layer(Bi-PA)on the PAN nanofibrous substrate.In the first IP process,relatively low concentration aqueous and organic phase solutions were selected to obtain loose and ultra-thin auxiliary PA layers(A-PA).The effects of A-PA layers with different structures on the morphology and structure of selevtive PA layers(S-PA)with high density prepared in the second IP process were investigated.The smooth and ultra-thin A-PA layer,which has close integration with the dense selective layer(S-PA layer),could effectively regulate the reaction and transfer rate of the two reaction monomers in the second IP process.High-performance TFNC pervaporation membrane could be simply prepared by double-IP method,which provided the potential of large-scale production.The separation performance in dehydrating 90 wt%isopropanol aqueous solution at 70?was as follow:permeation flux was 6075 g/m~2 h,the separation factor was 1314 and the PSI value was 7.98×10~6.Compared with the SA/PEI-GO/PAN TFNC membrane,the permeation flux was increased by 3 times.(3)For stable and high-effectively separating acetic acid and water mixture,the aciduric polysulfonamide(PSA)selective layer with high permeability was prepared onto the nanofibrous support by selecting appropriate micromolecule aliphatic amine monomer to react with 1,3-benzenesulfonyl chloride(BDSC)and optimizing the fabrication parameter of IP process.The effect of separation condition such as water concentration in feed,feed temperature and operation time were investigated.The optimized aciduric composite membrane showed a very competitive permeation flux(10557 g/m~2 h)in separating 70 wt%acetic acid aqueous solution at 30?,which laid a foundation for low energy consumption and high efficiency pervaporation.(4)In order to further expand the application of nanofibrous composite membrane,novel pervaporation composite membrane combined with catalysis was fabricated and attempted in the coupled system of pervaporation separation and reaction process.Here,PAN nanofibrous membrane containing Ui O-66-SO₃ catalyst(U-PAN)was prepared as the porous loose catalytic layer.The novel catalytic active composite pervaporation membranes with multilayer structure was successfully constructed after coating SA dense separation layer on U-PAN membrane to enhance the esterification conversion between acetic acid and isopropanol in pervaporation-assisted esterification process.The enhancement effect of SA/U-PAN composite membrane on the esterification was investigated,which laid a foundation for preparing the multifunctional TFNC pervaporation membrane.To sum up,series of novel TFNC pervaporation membranes with integrated structure were fabricated on nanofibous support by designing new composite membrane structures and selecting facile,mild and efficient membrane preparation strategies,these composite membranes could meet the demand of separating isopropanol/water and acetic acid/water mixtures,and used as catalytic membranes to enhance esterification reaction.This study is expected to provide a reference and technical support for designing and fabricating novel high-performance composite pervaporation membranes and exploring the industrial application.