Simulation Study On The Mass Transfer Mechanism Of CO₂ Separation Using Ionic Liquid-PVDF Membran

Carbon dioxide（CO₂）,as one of the major greenhouse gases,it is crucial to reduce its emission.Carbon capture,utilization and storage technologies are one of the effective ways to reduce CO₂ emissions,where CO₂ capture is a key step in the whole process.Currently,ionic liquids（ILs）/polymeric hybrid membranes membranes are effective in separating CO₂,and researchers have conducted a lot of experimental studies,but the current problems are the unclear microstructure and the lack of mass transfer theory.By revealing the mass transfer mechanism of CO₂ in hybrid membranes membranes,we can provide a theoretical basis for the development of efficient separation membranes.Therefore,in this paper,the microscopic mechanism of ILs hybrid membranes membranes and their interaction with CO₂are investigated by molecular dynamics method.The research contents and conclusions are twofold as follows.（1）Molecular dynamics simulations of[Bmim][PF₆]/PVDF and[Bmim][Tf₂N]/PVDF hybrid membranes membrane systems with different ionic liquids were carried out.Structural analysis showed that PVDF chains tended to aggregate on the surface and ILs were confined within the membrane,and ILs could greatly weaken the crystallinity of PVDF and improve the flexibility of PVDF chains,thus facilitating gas permeation and diffusion.In addition,calculations of permeation selectivity,dissolution selectivity and diffusion selectivity confirm that[Bmim][Tf₂N]/PVDF membranes perform better than[Bmim][PF₆]/PVDF membranes for CO₂/CH₄ separation,and dissolution selectivity plays a dominant role.The high selectivity in the hybrid membranes membranes is due to the stronger interaction between CO₂ and ILs/PVDF than that of CH₄and the rapid diffusion of ILs.Detailed analysis of the CO₂ transport process shows that the driving force of CO₂ permeation to the interface is about-4.11 kJ-mol-1 and most of the CO₂ at the interface is almost parallel to the membrane.（2）Molecular dynamics simulations were carried out for systems with hybrid membranes membranes of different thicknesses（3 nm,4 nm,5 nm and 6 nm）.The simulation results showed that the interactions between PVDF and anions and between CO₂ and anions increased with decreasing membrane thickness,that strong interactions between PVDF and anions could promote CO₂ uptake and dissolution,and that decreasing the thickness of ILs/PVDF membranes could improve CO₂/CH₄ selectivity.Moreover,the selectivity of CO₂/CH₄ in[Bmim][Tf₂N]/PVDF membranes was higher than that in[Bmim][PF₆]/PVDF membranes at all thicknesses.（3）Molecular dynamics simulations were performed for different concentrations of[Bmim][PF₆]ionic liquids（0 wt%,10 wt%,20 wt%,30 wt%,35 wt%,40 wt%and50 wt%）with PVDF hybrid membranes membrane membranes.The simulation results showed that with the increase of ILs content,the interaction between PVDF chains gradually weakened and the polymer crystalline regions in the composite membrane gradually decreased,so ILs can weaken the crystallinity of PVDF and facilitate the formation of porous morphology.