摘 要

多孔芳香骨架材料（PAFs）因其具有高比表面积、高孔隙率和高物理、化学稳定性，被广泛应用于气体储存、分离和分子催化等领域。PAFs材料的结构多样性、化学可裁剪性为多孔材料定向设计带来机遇的同时，也带来巨大的挑战。利用传统实验方法对大量的结构进行筛选，时间成本、物质成本投入都非常巨大，而通过理论计算不仅可以突破传统实验方法的局限性，而且还可为CO₂的吸附和选择性吸附定向设计提供理论指导，实现从以经验为主向定量、定向制备的转变。本文主要通过分子模拟手段分析CO₂在PAFs材料中的吸附特性和吸附机理，为筛选具有最佳CO₂吸附性能的PAFs材料奠定理论基础。

通过巨正则蒙特卡罗（GCMC）方法模拟了不同温度下CO₂在四种新型PAFs材料表面的吸附性能。结果发现PAF-322PC、PAF-324PC、PAF-332PC、PAF-334PC对CO₂在298K、40bar下超额吸附量分别可达到306wt%、313wt%、232wt%和212wt%，远远高于PAF-1的吸附量(56.5wt%)，归因于该PAFs材料具有非常大的自由体积与比表面积以及其结构中所含的芳香基团、碳碳三键对客体分子较强的静电吸附作用。研究发现合适孔径的选取能提高CO₂的吸附量，孔径过大会导致力场作用减弱，孔径过小则容纳的CO₂分子数会减少；由吸附等温模型的拟合结果可知，298K下PAFs材料的CO₂吸附数据符合Langmuir模型，而350K和400K下符合Langmuir-Freundlich模型；通过计算亨利系数和等量吸附热发现四种PAFs材料与CO₂之间的相互作用强度大小顺序为：PAF-322PCgt;PAF-324PCgt; PAF-332PCgt;PAF-334PC；从径向分布函数和质心密度分布两个微观方面研究吸附机理发现CO₂与PAFs材料之间的相互作用主要为范德华力，即分子之间的作用力，多重峰的存在表明静电作用也是CO₂与PAFs材料之间的相互作用之一；PAFs对CO₂的主要吸附位置是SP³杂化碳原子的苯环附近，成球状分布，在碳碳键间增加插入基团的苯环数和富电子基团碳碳三键，可以增加吸附位，但碳碳三键对CO₂的优先吸附贡献度不大。另外，低温能减小CO₂的分子运动的动能，增强力场对CO₂的约束作用，提高CO₂的吸附量；电荷的存在和较大的原子部分电荷的绝对值也能大幅度提高CO₂的吸附量。

关键词：多孔芳香骨架材料；二氧化碳；分子模拟；质心密度分布；径向分布函数

Abstract

Porous Aromatic Frameworks (PAFs) is widely applied in the field of gas storage, gas separation and molecular catalysis because of their high specific surface area, high porosity, physical amp; chemical stability. Their structural diversity and chemical tenability have not only brought opportunity for the design of PAF materials, but also brought a huge challenge for their application. Screening a large number of structures with the traditional experimental methods face a problem of time-consuming and material-costing. However, the theoretical calculations method can not only break through the limitations of traditional experimental methods, but also provide theoretical guidance to the oriented design of high-performance materials for CO₂ adsorption and selective CO₂ adsorption, which will help us realize the transition from empirical production to quantitative and directional production of materials. In this paper, the CO₂ adsorption properties and mechanism in PAF materials were computed and analyzed using molecular simulations, which laid the theoretical foundation of screening high-performance PAF materials for CO₂ absorption.

CO₂ adsorption properties in four new PAFs materials at different temperatures were calculated using the Grand Canonical Monte Carlo (GCMC) simulations. The results indicate that the amount of excess adsorption of CO₂ at 298K and 40 bar were 306wt%、313wt%、232wt and 212wt%, respectively, higher than PAF-1(56.5wt%), due to their very large free volumes and specific surface areas as well as the huge electrostatic interactions between guest molecules and the aromatic groups and carbon-carbon triple bonds of PAFs materials. In addition, the radial distribution function (RDF) between different molecular pairs were also computed by self-made code. It was found that the interaction between CO₂ and PAFs mainly come from van der Waals forces (also known as intermolecular forces). The research has showed that selecting a proper aperture can improve the amount of adsorption obviously, too large aperture can weaken the force field and too small aperture cannot contain much more CO₂. By fitting the traditional adsorption isotherm models, it was found that the CO₂ adsorption isotherms on PAFs at 298K fit the Langmuir model well, while those at 350K and 400K fit the Langmuir-Freundlich model well. The calculation of Henry coefficient (K_H) and isosteric heat of adsorption (Q_st⁰) shows that the interaction energy between CO₂ and PAFs follows the order as PAF-322PCgt;PAF-324PCgt; PAF-332PCgt;PAF-334PC. The multiple peaks of RDF indicate that there are electrostatic interactions between CO₂ and PAFs. The centroid density distributions were also added up according to the output configurations of GCMC. It was found that the preferential adsorption sites with a spherical distribution of CO₂ in PAFs locate near the benzene ring closed to a sp³ hybridized carbon atoms. As the number of benzene rings in the linkers of PAFs increase, the CO₂ adsorption sites increase. But it takes no effect for the number of electron-rich group carbon-carbon triple bonds in the linkers of PAFs on the CO₂ adsorption sites in the materials. In addition, low temperature facilitates to improve the amount of CO₂ adsorption on PAFs, due to the kinetic energy of CO₂ molecules decreasing and the strong restraint to object molecules from framework atoms. The high absolute values of partial atomic charges also facilitate to improve obviously the amount of CO₂ adsorption on PAFs.

Key words: Porous aromatic frameworks, carbon dioxide, molecular simulation, radial distribution function, centroid density distribution

目录

摘 要 I

Abstract II

第1章 绪论 1

1.1概述 1

1.2 多孔芳香骨架材料（PAFs）简介 2

1.2.1 结构特点 2

1.2.2 研究现状及其应用领域 3

1.3 选题的依据和意义 4

第2章 理论计算原理及材料设计与优化 6

2.1 理论计算原理 6

2.2 PAFs材料结构的设计 6

2.3 PAFs材料结构的优化 7

第3章 PAFs材料CO₂吸附的分子模拟研究 10

3.1 引言 10