Studies On Precise Construction And Performance Enhancement For Layered Double Hydroxide-Based Two-Dimensional Gas Separation Membranes

Membrane separation exhibits the characteristics of high efficiency,low energy consumption,environmental friendliness,simple operation and easy intensification.It is of great significance in the fields of the petrochemical industry,chemical industry,food processing,water treatment and medical technology.Realize the foundation and core of gas separation technology.Due to the advantages of easy film formation,good mechanical stability,adjustable structure,and low cost,two-dimensional material membranes have achieved high-efficiency separation in the field of gas separation,and are expected to break through the upper limit of the performance of traditional organic polymer materials.However,the current research on two-dimensional material films is still in its infancy.How to realize the precise construction and structural control of two-dimensional material films to meet the ideal separation effect under different separation systems and separation scales still faces challenges:First,for two-dimensional nanosheets assembled membrane,it is difficult to realize the precise construction and modulation of the size of the two-dimensional channel,the channel structure,and how to construct a membrane material with uniform channel size.The combined effect of the physical and chemical environment and other factors complicates the gas separation mechanism.How to exert the interaction between the host and the guest to realize the synergistic enhancement of the performance of membrane materials lacks systematic experimental and theoretical basis.Therefore,designing and obtaining high-performance gas separation membrane materials,deeply revealing the transport mechanism of gas molecules,and providing new ideas and methods for the structural design and performance enhancement of membrane materials are crucial for gas separation.Aiming at how to improve the gas permeability,separation selectivity and membrane stability of LDH-based two-dimensional material films to expand their theoretical significance and application value,this paper proposes a research idea for the structural design and performance enhancement of LDH-based two-dimensional gas separation membranes:First,through layer-by-layer self-assembly and spraying methods,combined with the solution,diffusion and reaction selectivity of each building block of the membrane,the directional synthesis of gas separation membranes is realized,and the influencing factors affecting the flux and selectivity of LDH-based separation membranes are revealed.Furthermore,through precise control of the interlayer channel size of LDH-based two-dimensional gas separation membranes,the effective separations of CO₂/H₂,CO₂/N₂ and CO₂/CH₄ were achieved,and the relationship between each structural unit and separation performance was revealed.Finally,based on the improvement of the membrane construction method,the construction of two-dimensional ordered channels in the membrane,and the synergistic strengthening of the physical and chemical microenvironment in the confined mass transfer channel by structural design,the performance of LDH-based gas separation membranes can be enhanced,thereby developing a new type of membrane with high permeselectivity and stability,providing new ideas and new approaches for chemical gas separation processes.The detailed research content and results are as follows:1.Precise construction of hybrid LDH-polymer membranes for high-performance CO₂ separationMembrane-based gas separation exhibits many advantages over other conventional techniques;however,the construction of membranes with simultaneous high selectivity and permeability remains a major challenge.Herein,（LDH/FAS）_n-PDMS hybrid membranes,containing two-dimensional sub-nanometer channels were fabricated via self-assembly of high aspect ratio,unilamellar layered double hydroxide（LDH）nanosheets and formamidine sulfinic acid（FAS）,followed by spray-coating with a poly（dimethylsiloxane）（PDMS）layer.The permeance of individual gases through the（LDH/FAS）_n-PDMS membranes with different bilayer numbers（n=5-25）was investigated.With n=5,gas transmission rates were in the order H₂>CO₂>N₂>CH₄（27k Pa,298 K）.A CO₂ transmission rate（CO₂TR）for（LDH/FAS）₂₅-PDMS of7748 GPU was observed at 27 k Pa,298 K.Interestingly,the CO₂ transmission rate（CO₂TR）for these membranes decreases at a lower rate than those of other gases when the bilayer number（n）exceeds 10.An n=25 membrane provides the best trade-off between CO₂ permeability and selectivity.We observe CO₂selectivity factors（SF）in 1:1 binary mixtures,SF（CO₂/H₂）,SF（CO₂/N₂）and SF（CO₂/CH₄）of 43,86 and 62 respectively.The CO₂ permselectivity for（LDH/FAS）₂₅-PDMS outperforms most reported systems and is higher than the Robeson or Freeman upper bound limits.（LDH/FAS）₂₅-PDMS also exhibited excellent CO₂ permselectivity using a 1:1:1:1 mixture of H₂,N₂,CH₄ and CO₂.Using（LDH/FAS）₂₅-PDMS at 27 k Pa,298 K the proportion of CO₂ in the filtered gas was 94.9%with a CO₂ permeance of 1938 GPU.These（LDH/FAS）₂₅-PDMS membranes are both thermally and mechanically robust maintaining their high selective CO₂ separation performance during long-term operational testing.We believe this highly-efficient CO₂ separation performance is based on the synergy of enhanced solubility,diffusivity and chemical affinity for CO₂ in the sub-nanometer channels.2.Precise modulation of two-dimensional LDH nanochannels within sub-nanometer for customizable gas separationMembranes with precise molecular sieving channels that break the permeability-selectivity trade-off are desirable for energy-efficient gas separation.Two-dimensional（2D）membranes sieve gas through their special interlayer channels between neighboring nanosheets.However,the regulation and precise control of the nanochannels that match well with the size of the gas molecules remains a big challenge.Herein,accurate tuning of the interlayer spacing of layered double hydroxide（LDH）membranes at sub-nanometer level was achieved by intercalation of Cl^-,Br^-,I^-,and NO₃^-ions.Such high-precision control allows customizable gas separation by selecting specific LDH membranes with appropriate channels according to the size of the gas molecules.Two membranes were used for demonstration:Cl-LDH membrane shows high H₂ permeance of～1870 GPU and desirable selectivities for H₂/CO₂（81）,H₂/N₂（197）,H₂/CH₄（320）,and H₂/C₃H₈（603）;while I-LDH membrane displays CO₂ permeance of～1780 GPU and CO₂/N₂,CO₂/CH₄ selectivities of 182 and297,respectively.The simultaneously high permeabilities and selectivities surpass the 2008 Robeson upper bounds.Molecular dynamics simulations quantitatively support the experiment results,further confirming the significant role of interlayer anions in the regulation of gas-sieving channels.Given the rich variability of layered spacing and interlayer micro-environment for LDH materials,this work provides a platform membrane for various molecular sieving,including gas separation,solvent purification,seawater desalination,etc.3.Ultrathin two-dimensional membrane with tunable gas sieving channel towards synergistic enhancement for gas separationBased on the above understanding of the structure-activity relationship of gas separation membranes,we report the construction of ultrathin membranes with layered double hydroxides（LDHs）nanosheets,polyacrylic acid（PAA）and polyethyleneimine（PEI）alternately deposited on porous substrates based on layer-by-layer assembly,in order to further improve the permeability of CO₂while achieving high selectivity.The chemical tuning of PEI leads to an accurate regulation of interlayer spacing in 0.1?scale,resulting in selective nanochannels for CO₂ permeation.The laminar membranes with CO₂ transport-facilitated channels exhibit excellent gas separation performance and exceed the limit of the state-of-the-art membranes with CO₂ permeance of 1068 GPU,CO₂/N₂ and CO₂/CH₄ selectivity of 126 and 330 respectively.The strategy demonstrated in this work would open up new avenues for effective CO₂separation and capture in the recycling of carbon resources.4.Intelligent response gas separation thin films were constructed based on two-dimensional channel stress engineeringTraditional energy-intensive separation processes can be effectively alleviated by modulating the permeability and selectivity in membrane systems.This chapter reports the reversed temperature-induced molecular-gating ultrathin membranes（～100 nm）fabricated by compositing flexible two-dimensional Mesh Adjustable Molecular Sieve,Ni₈（5-bbdc）₆（μ-OH）₄（MAMS-1）nanosheets with LDH nanosheets for H₂/CO₂ separation.By controlling the gas permeation direction to leverage the thermal-switching phase transition of the MAMS-1,thermal-switching behaviors are observed in the resultant membranes,which are accompanied by the sharp increase of H₂ permeance of620 GPU as well as H₂/CO₂ selectivity of 314.This work extends the possibility of LDH-based smart films as molecular gated switches.