First Principles Study On Amination To Improve CO₂ Adorption Ability Of Covalent Triazine Skeleton

In recent years,with the rapid development of science and technology,CO₂ emissions have increased sharply with the demand for energy.The increase of CO₂concentration in the atmosphere leads to the rapid warming of the global climate,the melting of glaciers and the rise of sea level.Currently,carbon capture and storage（sequestration）（CCS）is the most mature method for reducing CO₂ emissions.The high cost of CCS makes it important to develop new CO₂ capture materials.Covalent triazine frameworks（CTFs）have great potential for CO₂ capture due to their low cost,high selectivity,high thermal,chemical,and mechanical stability.In this study,based on the density functional theory（DFT）,the structural characteristics of pym-CTF and its ammoniated structures were simulated and calculated.The ability of the materials to capture CO₂ in a dry or humid environment was judged by the adsorption simulation calculation of CO₂ or H₂CO₃.The main research contents are as follows:（1）Based on DFT,the ground-state structures,properties and CO₂adsorption capacity of pym-CTF were studied.The structural features and reactive sites of the pym-CTF were investigated by structure optimization,charge density analysis,FUKUI index and frontier orbital analysis.It is found the process of CO₂adsorbed on pym-CTF is physical adsorption,CO₂ tends to be adsorbed in the pores parallel to the framework plane,and the adsorption energy is-0.021～-0.162 e V.However,pym-CTF showed the highest adsorption energy of-0.894 e V for H₂CO₃,and a seven-membered ring structure between the pym-CTF framework and H₂CO₃ is formed by hydrogen bond,which made the adsorption configuration more stable.（2）The effect of amino doping on the structural stability of pym-CTF and the adsorption of CO₂ on the aminated backbone were investigated.It was found that the addition of-NH₂ to pym-CTF can still maintain a stable 2D porous structure,and it will increase the charge of the pyrimidine N atoms in the framework and increase its attractiveness to CO₂.The adsorption simulation calculation further proved that the adsorption capacity of the ammoniated framework for CO₂ and H₂CO₃ is greatly improved compared with the original framework,and the proportion of the adsorption energy at the same site is increased by 24.1%and 9.2%,respectively.Therefore,to improve the CO₂ adsorption effect of pym-CTF by amination is theoretically feasible.（3）Based on the DFT,the theoretical structural characteristics of pym-CTF with different amounts of amino groups and their adsorption capacities for CO₂ and H₂CO₃ were studied.The results have shown that with the increase of the number of-NH₂,the material structure can still maintain a stable 2D planar structure,but the pore size will decrease.At the same time,the adsorption effect of the material on CO₂ and H₂CO₃ showed a trend of first increasing and then decreasing with the increase of the number of-NH₂.The steric hindrance increases gradually with the decrease of pore size,which is the main reason for the decrease of adsorption ability.Comprehensively comparing the adsorption energy and adsorption structure,pym-CTF has the best CO₂ and H₂CO₃ adsorption capacity when the-NH₂concentration is between 60%and 80%.