Theoretical Study On Microscopic Mechanism Of CO₂ Capture By M₂?dobpdc? Materials With N,n-dialkylethylenediamine Modified

Since the industrial revolution,the sharp increase of human activities has made the concentration of CO₂ in the atmosphere keep climbing,which has caused a huge impact on the earth's environment.Therefore,the technology of CO₂ capture and storage emerges at the historic moment.Metal organic framework materials?MOFs?are a class of periodic crystalline porous materials formed by self-assembly of metal central nodes and organic ligand links,with diversity in the structure,functional designability,high porosity,etc.These characteristics make it widely used in many fields such as gas adsorption and storage,catalyst and drug transportation.Moreover,by functionally designing MOFs materials,it is possible to exhibit more abundant performance.In terms of CO₂ adsorption,amine-functionalized materials obtained by riveting organic amine molecules on the open metal sites of MOFs materials combine with the Lewis alkalinity of nitrogen-containing functional groups and the porosity of MOFs materials to further enhance the CO₂ adsorption performance.In recent years,researchers have conducted in-depth experimental studies on the adsorption of CO₂ by amine-functionalized M₂?dobpdc?.In addition to excellent adsorption performance,a unique adsorption curve has been discovered,which has aroused great interest.However no confirmed adsorption mechanism has been reported yet.In this paper,density functional theory method was used to study the performance and microscopic mechanism of M₂?dobpdc?adsorption of CO₂ functionalized by N,N-dialkylethylenediamine.The structure-activity relationship was studied by comparing different coordination modes,different alkyl substituent sizes,and different types of metal central ions.Specific research work includes:?1?For N,N-dimethylethylenediamine?dmen?functionalized M₂?dobpdc?materials?M=Mg,Sc,Ti,V,Cr,Mn,Fe,Co,Ni,Cu,Zn?,the structural stability of the binding of primary amine with metal central ions is better than that of tertiary amine.When CO₂ is adsorbed,the corresponding chain model structure is more stable than that of the pair model structure.Taking Mg₂?dobpdc?as an example,the amine binding energy of the primary amine is 142.6 KJ/mol,and that of tertiary amine is 110.3 KJ/mol,which is 32 KJ/mol lower.After insertion of CO₂,the CO₂ adsorption energy of the chain structure corresponding to the coordination of the primary amine is 90.8 KJ/mol,and that of the pair model structure corresponding to the coordination of the tertiary amine is 58.6 KJ/mol.The same is true for most of the other metal ion species studied.Based on the chain model structure,we use the quantum chemical transition state search method combining the Berny method of Gaussian 09 and the CI-NEB method of VASP to obtain the reaction path and microscopic mechanism of the CO₂ adsorption process.The whole reaction process is divided into two steps.The first step is the activation of CO₂ molecules,accompanied by the transfer of H atoms,so that the interaction between organic amine molecules and CO₂ from physical adsorption to chemical adsorption.Due to the stable structure of CO₂,it is very difficult to activate,and the reaction energy barrier of this process is also very high.Taking Mg₂?dobpdc?as an example,it reaches 1.39 eV.The second step is the rearrangement of the molecular structure,and the coordination environment change of metal center ions.For Mg₂?dobpdc?,the reaction barrier of this process is 0.14 eV,which is significantly lower than the energy barrier of the first step.The same is true for most of the other metal ion species studied.?2?Moreover,we further studied the M₂?dobpdc??M=Mg,Ti,Fe,Co,Ni,Zn?materials functionalized by N,N-diethylethylenediamine?deen?and N,N-diisopropylethylenediamine?dien?materials to explore the influence of the alkyl substituents of the tertiary amine on the CO₂ adsorption process.The results show that,with Mg₂?dobpdc?as an example,the amine binding energy of deen is 168.5 KJ/mol,that of dien is 182.0 KJ/mol.The CO₂ adsorption energy of deen is 81.2 KJ/mol,and that of dien is 94.5 KJ/mol.Compared with dmen,the amine binding energy is dmen<deen<dien while CO₂ adsorbs is dien>dmen>deen.Through the analysis of the structure,we can see that the size of the alkyl substituents on the tertiary amine will affect the stability of the structure and restrict each other from both the steric hindrance effect and the interaction with the frame atom.Deen and dmen exhibit a similar process for the reaction path and the microscopic mechanism.Since the first step is mainly the reaction between CO₂ and primary amine moiety,it is not affected by alkyl substituents.Taking Mg₂?dobpdc-deen?as an example,the energy barrier is 1.43 eV,which is close to dmen.However,the second step of molecular structure rearrangement involves the movement of the organic amine molecules.Therefore deen,which has a large alkyl substituent,exhibits a higher energy barrier?0.24 eV?.Through the computational chemistry simulation research,we have for the first time fully revealed the adsorption properties of M₂?dobpdc?functionalized by N,N-dialkylethylenediamine and the microscopic mechanism of the reaction process.Visualizing the microscopic world can provide theoretical guidance for experiments and save Time and material costs,increase efficiency,and a more comprehensive and deeper understanding of the underlying of the nature of the series of reaction mechanism.