Molecular Simulation Study On The Separation Performance Of Two-Dimensional Conjugated Aromatic Polymer (2D-CAP) Membrane

Desalination and CO₂ capture are effective ways to solve the shortage of freshwater resources and global warming.Membrane separation,as an efficient,energy-saving and environmentally friendly method,has been widely used in the field of water desalination and gas separation.The core of membrane separation is membrane material,which determines the separation efficiency.At present,most of the separation materials used in commerce are three-dimensional porous membranes formed by crosslinking of organic polymers,which have poor permeability and low separation ratio.Therefore,to develop new materials that can be used for efficient water desalination or CO₂ separation is significant for addressing the freshwater crisis and mitigating the greenhouse effect.In this thesis,the separation performance of a two-dimensional conjugated aromatic polymer(2D-CAP)membrane,a new two-dimensional porous membrane synthesized recently,was investigated by using molecular simulation.The feasibility of its application as a desalination membrane and CO₂ separation membrane was evaluated.The main research contents in this thesis were summarized below.First,we used molecular dynamics simulations to study the desalination performance of 2D-CAP membrane and analyze the transmembrane hydrodynamics of mono-and multilayer2D-CAP membranes as a function of layer number.The energy barriers to water and ions across these membranes were calculated to evaluate the potential of 2D-CAP to function as the ultimate reverse osmosis(RO)membrane.Our simulation results show that the bilayer CAP membrane exhibits superior ion rejection(100%)and a water flux(1172 L m^-2 h^-1 bar^-1)with a performance that is three orders of magnitude higher than the commercial reverse osmosis membrane,even is three times higher than the theoretically reported monolayer nanoporous Mo S₂ membrane(the state-of-the-art membrane reported for desalination).In addition,the 2D-CAP bilayer membrane is highly resistant to swelling even at high water flux.The effects of pore structure and functional groups around the 2D-CAP membrane on the water transport morphology and transmission rate were also analyzed,and salt rejection mechanism was revealed from two aspects:electrostatic interaction and size exclusion.The CO₂/N₂ and CO₂/CH₄ separation performances of 2D-CAP membrane were also studied by molecular dynamic simulation.The results show that the 2D-CAP membranes allow high-speed CO₂ permeation.Due to the similar size between nanopores and the molecular dynamics diameters of N₂ and CH₄,the 2D-CAP membrane cannot effectively block N₂ and CH₄.The bare 2D-CAP membrane does not show any CO₂ selectivity.To achieve effective CO₂ separation,a strategy of coating ultrathin ionic liquid(IL)onto the2D-CAP membrane was proposed to harmonize the pore size,and molecular dynamic simulations were adopted to investigate the gas separation performance of the 2D-CAP supported IL membrane(2D-CAP SILM).An ultrahigh CO₂ permeance of?10⁵ GPU,which is larger than many reported theoretically predicted results,was exhibited.Meanwhile,an excellent selectivity of CO₂/N₂ and CO₂/CH₄ beyond 40 was obtained to satisfy the industrial separation requirements.The selectivity could be ascribed to IL adsorption selectivity of CO₂over N₂/CH₄ and a fascinating gating effect that anion of IL([BF₄]^-)suspending upside the pore center allows CO₂ passage while prohibits N₂/CH₄ passage.Furthermore,the effects of different IL thicknesses and membrane layers on the separation performance of 2D-CAP SILM were examined.