【作者】 郑鑫；

【导师】 杨利明；

【作者基本信息】 华中科技大学 ， 高分子化学与物理， 2019， 硕士

【摘要】 从工业革命以来,急剧增加的人类活动使大气中的CO₂浓度不断攀升,对地球环境造成了巨大影响,对CO₂的捕获与封存技术应运而生。金属有机框架材料（Metal-Organic Frameworks,MOFs）是一类由金属中心结点和有机多齿配体链节通过自组装而形成的周期性多孔材料,具有结构的多样性、功能的可设计性、多孔性、高比表面积与高孔隙率等特点,使其在气体吸附与储存、催化剂、药物运输等多个领域受到广泛应用。并且,通过对MOFs材料进行功能化设计,能够使其展现出更为丰富的性能。在CO₂的捕获方面,利用有机胺分子铆接在MOFs材料的开放金属位点上得到的胺修饰材料,结合了含氮官能团的Lewis碱性和MOFs材料的多孔性,进一步提高了对CO₂的吸附性能。近年来,研究者们对胺修饰的M₂（dobpdc）（dobpdc=4,4’-二羟基联苯二甲酸）捕获CO₂进行了深入的实验研究,在优异的吸附性能的同时,发现了独特的吸附曲线,这引起人们极大的兴趣,但目前尚未有被证实的吸附机理报道出来。本文采用密度泛函理论方法,对N,N-二烷基乙二胺修饰的M₂（dobpdc）捕获CO₂的性能和微观机理进行了深入研究。通过对比不同配位方式、不同烷基取代基大小、不同金属中心离子种类以研究其构效关系规律。具体研究工作包括:（1）对于N,N-二甲基乙二胺（dmen）修饰的M₂（dobpdc）（M=Mg、Sc、Ti、V、Cr、Mn、Fe、Co、Ni、Cu、Zn）材料,一级胺N原子与金属中心离子结合的结构稳定性相比三级胺N原子更好,进而捕获CO₂时,对应形成的链模型结构比对模型结构更为稳定。以Mg₂（dobpdc）为例,一级胺N原子配位的胺结合能为142.6 KJ/mol,三级胺N原子配位的胺结合能为110.3 KJ/mol,相比之下低了32 KJ/mol。捕获CO₂之后,一级胺N原子配位对应链模型结构的CO₂吸附能为90.8 KJ/mol,三级胺N原子配位对应对模型结构的CO₂吸附能为58.6 KJ/mol。其他所研究的大部分金属离子种类也是如此。以链模型结构为基础,我们采用量子化学过渡态搜索的方法结合了Gaussian 09的Berny方法和VASP的CI-NEB方法,得到了CO₂捕获过程的反应路径与微观机理。整个反应过程分为两步,第一步为CO₂分子的活化,伴随着H原子的转移,使有机胺分子与CO₂之间的作用从物理吸附到化学吸附。由于CO₂分子结构稳定,活化难度很大,该过程的反应能垒也很高,以Mg₂（dobpdc）为例,达到了1.39 eV。第二步为分子结构重排,金属中心离子的配位环境变化。对Mg₂（dobpdc）来说,该过程的反应能垒为0.14 eV,显著低于第一步的能垒。其他所研究的大部分金属离子种类也是如此。（2）此外,我们进一步研究了N,N-二乙基乙二胺（deen）和N,N-二异丙基乙二胺（dien）修饰的M₂（dobpdc）（M=Mg、Ti、Fe、Co、Ni、Zn）材料,以探寻三级胺N原子上的烷基取代基大小对CO₂捕获过程的影响。结果表明,以Mg₂（dobpdc）为例,胺结合能deen为168.5 KJ/mol,dien为182.0 KJ/mol,CO₂吸附能deen为81.2 KJ/mol,dien为94.5 KJ/mol。结合dmen比较可得,胺结合能dmen<deen<dien。而CO₂吸附能dien>dmen>deen。通过对结构的分析我们可以看出,三级胺N原子上的烷基取代基的大小会从位阻效应和与框架原子的相互作用这两方面影响结构的稳定性,并相互制约。对于反应路径和微观机理,deen与dmen表现出相似的过程。由于第一步主要为CO₂与一级胺部分的反应,受烷基取代基的影响不大,以Mg₂（dobpdc）-deen为例,能垒为1.43 eV,与dmen接近。但第二步分子结构重排的过程中,涉及到有机胺分子的移动,因此具有较大烷基取代基的deen表现出更高的能垒（0.24 eV）。通过计算化学模拟研究,我们首次完整揭示了N,N-二烷基乙二胺修饰的M₂（dobpdc）对CO₂的捕获性能和反应过程的微观机理,将微观世界可视化,能够为实验提供理论指导,以节约时间和材料成本,提高效率,并且能够更全面、更深入地了解该系列反应的本质机理。更多还原

【Abstract】 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.更多还原