Investigation Of The CO₂/CH₄ Adsorption Separation Property Of Nitrogen-rich Metal-organic Frameworks

Metal-Organic Frameworks（MOFs）have strong structural adjustability and designability,which make them exhibit enormous application potential in gas storage and separation.Therefore,it is very valuable to further improve the storage and separation performance of MOFs by introducing functional groups.In this thesis,the density functional theory（DFT）and the giant regular Monte Carlo simulation（GCMC）method were used to study the adsorption separation performance of the single-component CO₂ and CH₄ and their binary mixed components in the nitrogen-rich MOFs materials modified by functional groups.The main tasks as follows:1.The dual-ligand nitrogen-rich CPM-35-M（M=H,F,CF₃,NH₂,OH,N）materials can improve the adsorption of CO₂ and CH₄ significantly after modified by functional group in single-component adsorption.The improvement of gas adsorption performance after functional group modification is mainly due to the fact that polar functional groups can significantly enhance the adsorption energy of CO₂ with ligands and the heat of CO₂ adsorption in the framework.Furthermore,for the adsorption separation performances of the two-component CO₂/CH₄ gas mixture,the CO₂ adsorption amount of CPM-35-M exceeds largely the adsorption amount of CH₄after modified-F and-OH groups,and thus the selectivity of CO₂/CH₄ is significantly improved.2.The double ligand nitrogen-rich JLU-38-M（M=H,F,Cl,NH₂,OH）materials have a reverse trend in the single component adsorption of CO₂ and CH₄ after modified functional groups at low pressures.The-OH and-NH₂ groups can significantly improve the adsorption performance of CO₂,but has a negative effect on the adsorption of CH₄,while-F and-Cl can increase the adsorption amount of CH₄,but show a negative effect on the adsorption of CO₂.In the CO₂ adsorption process at low pressures,the non-electrostatic interaction between CO₂ and the skeleton plays a major role,which is the main factor of the difference in gas adsorption.For the CO₂/CH₄ two-component adsorption separation,the JLU-38-M materials modified by-OH and-NH₂ significantly show the improved selectivity of CO₂/CH₄,and the selectivity hardly changes with the increase of pressure,which is beneficial to practical applications in industry.3.For the nitrogen-rich rht-MOF materials NU-125-M（NU-125,NU-125-N1 and NU-125-N2）,it can be seen from the radial distribution function,heat of adsorption,and adsorption erergy that the substitute of triazole by tetrazole in the ligands（NU-125-N2）can increase the CO₂ adsorption capacity;while the substitution of N on the benzene ring（NU-125-N1）reduces the CO₂adsorption capacity.In the CO₂ adsorption process,the non-electrostatic interaction between CO₂ and the skeleton is the main factor of the difference in gas adsorption amount.The effect of functional group modification on the gas adsorption performance of MOF materials is related to the physicochemical properties of MOF frameworks.Not all functional group modifications can further improve the gas adsorption performance of any MOFs.It is necessary to make targeted modifications according to the specific properties of the skeleton.