Controlled Assembly And Performance Of Polyoxometalate-based Separation Membrane Materials

Novel membrane materials,such as two-dimensional（2D）lamellar membranes and mixed matrix membranes（MMMs）,have attracted much attention for their efficient separation performance in the field of liquid and gas separation.However,these new membrane materials still face many challenges in the process of application,such as the precise construction of interlayer channels and the regulation of the chemical environment of the channels.Breakthroughs have been made in regulating the geometry and chemical microenvironment of interlayer channels through intercalation strategies,but the controllable assembly of intercalated molecules with 2D nanosheets is still a concern.Uncontrollable assembly may lead to uneven distribution of the intercalation molecules in the 2D interlayer channels,which in turn generates non-selective defects and reduces the selectivity of molecular separation;meanwhile,2D lamellar membranes also face membrane fouling problems during water treatment,and the accumulation of contaminants on the surface of the membranes and within the pore channels can cause membrane pore blockage,reducing the permeability of the membrane material.MMMs,on the other hand,face the problem of agglomeration and sedimentation of functional fillers in the polymer solution,which can easily lead to uneven distribution of fillers inside the membrane,serious aggregation,poor compatibility at the polymer-filler interface,resulting in non-selective defects;in addition,the effect of filler incorporation on the polymer structure and the mechanism of mass transfer diffusion of gas molecules inside the membrane has not yet been given sufficient attention.Polyoxometalates（POMs）are unique nanoscale excimer clusters with well-defined crystal structures and flexible tunability in composition,charge,and counterbalancing cations.In addition,POMs exhibit excellent photocatalytic activity and high hydrophilicity,and some of the molybdenum-based high-nucleation clusters have a pore structure that allows flexible access to molecules/ions.The strong polarizability of metal-oxygen bond in POMs generates abundant intermolecular interaction sites such as electrostatic,hydrogen bonding,ionic-dipoles,which promote strong interactions between POMs and other functional groups.In this thesis,these features of POMs were utilized to construct POM-intercalated layered double hydroxides（LDHs）2D lamellar membranes and POM-based MMMs by controllable assembly of POMs with LDHs nanosheets and polymers to achieve the angstrom-level size tuning of mass transfer channels.The microstructures of the composite membranes and their mass-transfer mechanisms were also thoroughly investigated by combining advanced characterization techniques and molecular dynamics simulations,as described below:（1）To address the problem of 2D lamellar membranes with mutual constraints on permeability and selectivity and accompanying membrane fouling,2D lamellar membrane(Mg Al-Si W₁₂)of POMs-intercalated LDHs were successfully constructed by combining electrostatic-induced assembly and vacuum-assisted pumping filtration by utilizing the uniform nanoscale Keggin-type POMs with spherical shape,high hydrophilicity,and photocatalytic activity.The LDHs laminates had a uniform positive charge density,and thus the negatively charged Si W₁₂ and LDHs were assembled by electrostatic interaction,where the POM clusters combined both columnar bracing and photocatalytic effects on the 2D laminates.The interlayer channel height of the Mg Al-Si W₁₂membrane was precisely controlled at 0.89 nm,which was 0.19 nm enlarged than that of the pristine Mg Al-NO₃ membrane.The water permeance was up to～130 L m^-2h^-1bar^-1,which was four times higher than that of the Mg Al-NO₃membrane while maintaining a high retention rate of>99%for dye molecules.Molecular dynamics simulations showed that the strong interaction between water molecules and the 2D channels induced an orderly arrangement of water molecules within the confined channels,reduced the collision frequency between water molecules,and formed a water transport layer parallel to the laminate.Meanwhile,the POMs-intercalated LDHs can inhibit the compounding of photogenerated carriers,and the generated·OH and·O₂^-radicals accelerate the degradation of pollutants.The water flux of the fouled membranes was restored to more than 95%by UV-vis irradiation,realizing cyclic regeneration.（2）To address the problems of non-selective defects arising from the inhomogeneous stacking of nanosheets in 2D lamellar membrane and the lack of specific affinity sites between the layers,the liquid mobility of POMs ionic liquids and the affinity of the high charged lacunary POMs for CO₂ were utilized to construct a POMs-based ionic liquid domain-limited LDHs membrane(Mg Al-（THA）₈Si W₁₁)using a layer-by-layer assembly technique.The negatively charged POMs induced an orderly and controllable layer-by-layer assembly of positively charged LDHs nanosheets.The viscous ionic liquid sealed the non-selective defects of the LDHs membranes,and the high charge of the POMs enhanced their Lewis acid-base interactions with CO₂.The Mg Al-（THA）₈Si W₁₁ membranes exhibited a permeance of 99 GPU for CO₂,a selectivity of 99.5 for CO₂/N₂,and a selectivity of 35.5 for CO₂/CH₄.Molecular dynamics simulations showed that CO₂ molecules had stronger interaction energy with the anions and cations of（THA）₈Si W₁₁ compared to N₂ molecules.This was mainly due to the basicity of Si W₁₁ and THA itself,which can generate Lewis acid-base interactions with acidic CO₂,thus promoting the preferential adsorption of CO₂.（3）To address the problems of poor polymer-filler interfacial interactions and a single mass transfer mechanism in MMMs,the Keplerate-type POM cluster{Mo₁₃₂}was successfully assembled at the molecular level with block copolymers using a hydrogen bonding-induced assembly strategy.MMMs with synergistic multiple mass transfer mechanisms were constructed by utilizing the ultra-small nanoscale of{Mo₁₃₂},abundant surface metal-oxygen bonding,suitable pore size,and carbonic anhydrase-mimicking properties.The POMs were uniformly distributed in the polymer matrix in a monodisperse state,and the introduction of POMs not only reduced the crystallinity of the polymers,but also accelerated the dynamics of their molecular chains,thus increasing the free volume within the membrane.In addition,the presence of 0.35 nm sub-nanopores on the POMs also contributed the size-sieving ability of the membranes.The{Mo₂^V}linker was able to catalyze the reversible hydration reaction of CO₂ in a carbonic anhydrase-mimicking mechanism to generate HCO₃^-that can be rapidly transported,thus enhancing the membrane facilitated delivery mechanism.The optimal MMMs achieved a CO₂ permeability of～384Barrer and a CO₂/N₂ selectivity of up to～244.