Microscopic Mechanism Study Of Gas Separation And Adsorption On Two-dimensional Porous Carbon-nitrogen Material

With the rapid development of industry and economy,the demand for He,CO₂and other gases as important chemical raw materials is increasing.At present,He is widely used in military industry,medical treatment,superconducting experiment and other fields,and it is an indispensable rare strategic resource for the development of high-tech industry and national defense and military industry.CO₂can be used as a chemical raw material in alkali industry,as a foaming agent in plastic industry and as a shielding gas in welding field,etc.For gas separation and capture,the prospect of membrane separation technology is very promising.The core lies in the selection of membrane materials.As a new two-dimensional material,two-dimensional porous carbon-nitrogen material has excellent electrical,optical,mechanical and thermal properties,and has a wide application prospect in surface engineering,energy and environment.Based on molecular dynamics,density functional theory and Giant Canonical Monte Carlo method,this thesis discusses the separation and adsorption of He and CO₂by two-dimensional porous carbon-nitrogen material g-C₉N₇through stress,interlayer spacing or charge control,and understands the microscopic mechanism of interaction between two-dimensional carbon-nitrogen material g-C₉N₇and gas,which provides some guidance for two-dimensional porous carbon-nitrogen material in gas separation and adsorption.The main conclusions of this paper are as follows:（1）Combining MD and DFT simulation methods,the microscopic mechanism of separating He from natural gas by new g-C₉N₇membrane was studied.The research shows that at room temperature,the He permeability of g-C₉N₇membrane is as high as 1.48×10⁷GPU.With the increase of temperature,g-C₉N₇membrane has incomparable He separation performance in the mixed gas of He,CH₄,CO₂,N₂,H₂S,Ne and Ar.This is also consistent with the separation performance of g-C₉N₇membrane for six gas groups of He/Ne,He/Ar,He/CH₄,He/CO₂,He/N₂and He/H₂S at 298 K.With the introduction of stress regulation,the separation performance of g-C₉N₇membrane for He/Ne was significantly improved.According to DFT calculation,under the condition of 7.5%compressive strain,the He selectivity of g-C₉N₇membrane reached the highest value of 9.41×10².It is higher than the conventional He.（2）Based on study content one,we found that the g-C₉N₇membrane showed a unique adsorption properties for CO₂.The adsorption and separation performance of CO₂on g-C₉N₇slit was studied by GCMC and DFT method.The research shows that g-C₉N₇slits with different widths have stronger adsorption capacity for CO₂than CH₄and N₂.At 1 bar and 298K,the adsorption capacity reaches 7.06 mmol/g.At the same time,it is found that the width of g-C₉N₇slit of 0.7 nm shows better separation performance.At 298 K and 1 bar,the selectivity of CO₂/N₂and CO₂/CH₄reached 41.43 and 18.67 respectively.In addition,the adsorbent also showed good water stability.According to DFT calculation,the interaction energy of g-C₉N₇slit and CO₂（0.280e V）is stronger than CH₄（0.167e V）and N₂（0.134e V）,which proves the ultra-high CO₂adsorption performance of g-C₉N₇membrane from a microscopic perspective.（3）Based on study content 2,in order to further improve the CO₂adsorption performance of the g-C₉N₇slit,we propose a strategy for charge regulation.Using GCMC simulation,the CO₂adsorption of 30e g-C₉N₇slit at 1 bar and 298 K reach 19 mmol/g,which is more than twice that of intrinsic g-C₉N₇slit.And at 298 K,the selectivity of 30e g-C₉N₇slit to CO₂/N₂reaches 54-32,which is nearly twice that of intrinsic g-C₉N₇slit（25-17）.In addition,it is found that at room temperature and high pressure,charge regulation will make g-C₉N₇slit to adsorb more CO₂and obtain ultra-high CO₂/N₂separation performance.