Molecular Simulation Studies on Membrane Permeation and Molecular Design of Membranes

本工作的目标是双重的：分子模拟研究的气体渗透机制通过无机膜和表面改性方法的尝试，使用从头算量子化学计算。主要结果如下：（1）采用γ VT-NEMD方法对纯气体和混合气体在碳膜中的渗透进行了有效的模拟，发现混合气体在由带状势垒异质表面组成的碳膜中的分离机理随着势垒的增加由吸附控制转变为扩散控制。(2)有趣的观察结果之一是一种阻塞现象，其中强烈吸附在碳表面上的乙烷分子覆盖狭缝孔的外部开口，从而阻止小甲烷分子进入。(3)对氦气、氮气、二氧化碳、甲烷、乙烷等气体在纯硅ZSM-5膜中的渗透进行了模拟计算， ...更多信息 发现气体的密度分布提供了通过膜的入口、内部和出口的渗透阻力的定量估计。(4)首先提出了一种波包分析方法。引入量子化学从头计算方法，研究了分子穿过狭缝状壁面时的统计行为，研究了垂直于壁面的动能对气体从内向外渗透的影响，提出了一种通过从头计算对表面原子进行修饰的策略：GAUSSIAN用于优化模拟表面的分子和簇的几何形状，GAMESS用于将分子相互作用能分解为各种能量项：静电、交换、极化、电荷转移等。研究发现，在上述相互作用项中，CO^2与MgO团簇之间的相互作用的电荷转移项最大（电子电荷从表面转移到CO^2分子），因此，选择电负性比Mg原子小的Ca原子来增强从表面到CO^2原子的电荷转移效应。Mg原子与Ca原子的部分交换导致CO^2在改性表面上吸附的势能降低，这表明所提出的方法将是评估待改性原子的合理工具。少

The objectives of the present work are twofold : molecular simulation studies of gas permeation mechanisms through inorganic membranes and an attempt of surface modification method by use of ab initio quantum chemical calculations. Followings are important results : (1) The gammaVT-NEMD method has been effectively used to simulate permeation of pure and mixed gases through carbon membranes ; it is found that the separation mechanism of mixed gases through a membrane composed of heterogeneous surfaces of belt-like potential barrier changes from adsorption-control to diffusion-control as the potential barrier increases. (2) One of interesting observations is a blocking phenomenon where ethane molecules, which are strongly adsorbed on carbon surfaces, cover outer mouths of slit pores, resulting in preventing small methane molecules to enter. (3) Simulations of permeation of helium, nitrogen, carbon dioxide, methane, ethane through the pure silica ZSM-5 membrane have been carried out and d … More ensity profiles of gases are found to provide quantitative estimations of permeation-resistance through the entrance, inside and exit of the membrane. (4) A wavepacket analysis method was firstly. introduced to investigate the statistical behavior of molecules that penetrate a slit-shaped micropore ; the effect of kinetic energy vertical to the wall surface on gas permeation from inside to outside was investigated by the new method.A strategy has been suggested to modify surface atoms through ab initio quantum chemical calculations : the GAUSSIAN is used to optimize the geometry of a molecule and a cluster imitating a surface and the GAMESS is used to decompose the molecular interaction energy into various energy terms : electrostatic, exchange, polarization, charge-transfer and others. It is found that the charge-transfer term is the largest in the above interaction terms for molecular interactions between CO^2 and a cluster of MgO (electron charge is transferred from the surface to the CO^2 molecule) ; therefore, Ca atoms, which have smaller electronegativity than Mg atoms, are selected to enhance the charge-transfer effect from surface to the CO^2 atom. A partial exchange of Mg atoms with Ca atoms resulted in lower potential energy for adsorption of CO^2 on the modified surface as expected, which suggests that the proposed method will be a rational tool for the evaluation of atoms to be modified. Less