Theoretical Study On CO₂ Adsorption And Microcosmic Mechanism Of The M₂?dobpdc? Modified By Ethylenediamine And Its Variants

Metal organic frameworks?MOFs? assembled by metal cation and polydentate organic ligands,high specific surface areas,channel size adjustable,along with the strong controllable functionality and chemistry,making them great candidates for a variety of application,including catalysis,gas storage,sensors and so on.The introduction of nitrogen-containing organic functional groups near the metal center of MOFs can improve the CO₂ adsorption performance of MOFs materials.However,it is difficult to study the mechanism of CO₂ adsorption by experimental chemistry,especially the search for transition state of reaction.In contrasts,computational chemistry can break through the boundedness of traditional research.The method has superiority on structure-activity relationship study of complex systems,which can explain the mircoscopic mechanism.Based on computational chemistry,this thesis systematically studied the adsorption properties of CO₂ in a series of amine functionalized MOFs by using density functional theory?DFT?.The transition state of the reaction was searched as well as the mircoscopic mechanism was revealed.In order to provide practical reference value and guiding significance for selecting a suitable system in the experiment.We studied the adsorption model and adsorption energy of CO₂ molecules by ethylenediamine or methylethylenediamine functionalized M₂?dobpdc??M=Mg,Sc,Ti,V,Cr,Mn,Fe,Co,Ni,Cu,Zn?.For en-M₂?dobpdc?,the adsorption energy of CO₂ under the chain model is 76.23 kJ/mol,and the adsorption energy of CO₂ under the pair model is37.46 kJ/mol.The results show us the chain model is more stable.In contrast,men-M₂?dobpdc?has higher CO₂ adsorption capacity,as well as the Mg₂?dobpdc?,Sc₂?dobpdc?,Mn₂?dobpdc? and Ti₂?dobpdc? shows better performance in CO₂ adsorption.By multi-scale simulation calculation,we explored the microscopic mechanism of amine-functionalized M₂?dobpdc? adsorbing CO₂ under chain model.The complete reaction path and transition state structure were obtained for the first time by quantum chemical calculation.Firstly,the CO₂ is converted from a physical adsorption to a chemisorption,and forms a C-N bond with the N of the amine.This process energy barrier is about 0.9¹.7 eV.Secondly,molecular rearrangement forms a four-membered ring transition state,during the structure changes from N-coordination to O-coordination.The energy barrier of this step is about 0.1⁰.6 eV.Finally,a more stable ammonium carbamate product is obtained.