Precise Construction Of Nanoporous Membranes For Selective Ion Transport And Separation

The studies of theoretical and experimental show that mass transport in confined nanoporous space is intrinsic different from the bulk solution.The ordering,regularity,and chemistry of nanoporous membranes will significantly affect the transport behavior of mass.Herein,the thesis is mainly focused on the precise construction of nanoporous membranes and ion transport in the confined space of these constructed membranes.By characterizing the pore size,micromorphology,and chemistry of nanoporous membranes,the transport behavior of ions were investigated.The nanoporous membranes with high-efficient cation separation properties were constructed.The mechanism of ion transport in nanoporous membranes were revealed in molecular level.The main research is as follows:1)The ultra-thin polyamide nanoporous layer was constructed on the surface of hydrolyzed polyacrylonitrile(HPAN)ultrafiltration membrane by interfacial polymerization.The surface of prepared membrane was then quaternized to form positively charged nanochannels,and the quaternization degree was modulated by varying the reaction time of quaterization.The porous structure of HPAN membrane and the carboxylic acid group formed by hydrolysis enhance the transfer of cations.The ultra-thin polyamide nanoporous layer can realize the selective transport of ions with different size.Furthermore,the positively charged nanochannels has s stronger repulsion effect on divalent cations,which endow the excellent separation properties between monovalent and divalent cations.2)The ZSM-5 zeolite with regular subnanometer pores was amalgamated with the PVA to construct ZSM-5/PVA nanoporous membranes.The influence of doping amount of ZSM-5 zeolite on the properties of prepared nanoporous membranes were systematic investigated.The highest content of ZSM-5 in the mixed matrix membranes(MMMs)can be extended up to 60 wt%,while the MMMs with optimized content(50 wt%)achieved high perm-selectivity of 3.67 for Li+/Mg2+and 34.4 for H+/Zn2+,which are over two times and almost 10 times,respectively,compared to CSO membranes.The presence of cation exchange sites in the ZSM-5 facilated the fast transport of proton,while the microporous crystalline morphology restricted the transport of larger hydrated cations.Moreover,the participating sites and porosity of ZSM-5 halted the accumulation of ions on the membrane surface and granted high limiting current density to the MMMs.3)An interfacial growth strategy was used to construct pure phase covalent organic framework(COF)nanoporous membranes with vertically aligned 1D nanochannels.By characterizations,a defect-free COF nanoporous thin layer(-20 nm)can be observed on the surface of AAO substrate.The COF nanoporous layer has a pore size of～1.4 nm and abundant hydrogen bonding sites which can form a strong hydrogen bonding interaction with water molecules.The COF nanoporous membranes have high monovalent cation permeation rates and extremely low multivalent cation permeabilities,leading to selectivity for K+/Mg2+of-765.According to the MD simulation and DFT calculations,the mechanism of ion transport in the confined space of COF nanoporous membrane were revealed at the molecular level.For cations with high charge density,the active hydrogen atom in the "hydration sphere" can form a stronger interaction with the hydrogen bonding sites in the COF nanochannel,leading to a high energy barrier and low permeability in the COF membrane nanochannels,so the transport rate is extremely low.4)A continuous cationic COF membrane with the pore size of～1.4 nm was constructed by interfacial growth strategy and transferred onto the surface of HPAN porous substrate.The prepared cationic COF nanoporous membranes exhibit high rejection of cationic dyes and extremely low rejection for salts,which means a high selectivity for dye/salt separation.The mechanism of dye rejection was investigated by controlling the molecular size and charge ability of dyes.The high rejection of methylene blue is mainly governed by the electrostatic interaction between cationic COF and the dye with positive charges.In addition,when the dye of a higher size than the pore size,the sieving effect will dominate the rejection of dyes.