Studies on Membrane Permeation Mechanisms and Molecular Design of Inorganic Membranes by Using Molecular Simulations and Quantum Chemical Calculations

本工作的目的有两个：一是通过分子模拟研究气体透过无机膜的机理，二是利用从头算量子化学方法研究无机膜的表面修饰方法，以实现无机膜的分子设计。在分子模拟中，选取了三种不同孔径的碳膜(菱形、锯齿形和光滑缝型)作为模型膜，计算了甲烷/乙烷混合物的渗透系数。主要结果如下：(1)气体的分子渗透和分离机理可以用三个因素来解释：选择性吸附、孔内相对扩散阻力和膜进出口的相对渗透阻力。(2)对于金刚石膜和锯齿形膜，扩散阻力和出口阻力占主导地位，而对于狭缝形膜，只有出口阻力占主导地位。(3)提高渗透…的膜结构。分子被强吸附的速率越高可能越好。此外，还开发了一些柔性分子的渗透程序，并对丁烷异构体通过沸石膜的渗透进行了模拟，但这些程序都需要很长的计算时间。因此，我们已经努力重写并行处理程序，并期望有一个新的并行计算系统。通过从头算量子化学计算，提出了一种表面原子修饰策略：即选择一个候选原子，以增强分子与表面(团簇)之间的稳定能量因子。NH_3-TiO_2之间的分子间相互作用较大，未经修饰可归类为化学吸附。对于CO_2-MgO和CO_2-TiO_2体系，分子间势为物理吸附，最大的稳定贡献是表面原子向CO_2分子的电荷转移。因此，选择了电负性比镁和钛原子小的钙原子来增强电荷转移，从而导致CO2的吸附能如选择规则所预测的那样增加。在分子极化是相互作用的主要因素的情况下，我们可以选择比原始表面原子更软的原子，尽管我们最近的计算表明，极化项对分子间稳定性的敏感性似乎很弱。较少

The objectives of the present work are twofold : molecular simulation studies on gas permeation mechanisms through inorganic membranes and studies on surface modification method by using ab initio quantum chemical calculations, toward molecular design of inorganic membranes.1. In molecular simulations, three carbon membranes different in pore shape (diamond, zigzag, and smooth slit types) were proposed as model membranes and the permeation of methane/ethane mixtures were calculated. Followings are important results : (1) Molecular permeation and separation mechanisms of gases may be explained by three factors : selective adsorption, relative diffusion resistance in pores, and relative permeation resistance at the inlet and outlet of membranes.(2) For the diamond and zigzag shape membranes, the diffusion resistance and the outlet resistance are predominant while only the outlet resistance is predominant for the slit-shaped membrane.(3) The membrane structure that enhances the permeation … More rate of molecules strongly adsorbed may be the best. Furthermore, some programs for permeation of flexible molecules have been developed and simulations of permeation of butane isomers through a zeolite membrane have been carried out ; however, they are found to take a lot of computation time. Therefore, we have taken efforts to rewrite the programs for parallel processing, with expecting to have a new parallel computing system.2. A strategy for modification of surface atoms has been proposed by means of ab initio quantum chemical calculations : i.e. selecting a candidate atom so as to enhance the factors of stabilization energy between a molecule and the surface (a cluster). The intermolecular interaction between NH_3-TiO_2 was found to be large and to be classified into chemical adsorption without modification. For the CO_2-MgO and CO_2-TiO_2 systems, the intermolecular potentials were found to be physical adsorption and the largest contribution for the stabilization was the charge transfer from the surface atoms to a CO_2 molecule. Therefore, Ca atoms, which have smaller electronegativity than Mg and Ti atoms, were selected to enhance the charge-transfer, which resulted in the increase in adsorption energy of CO_2 as the selection rule predicted. In the case when the molecular polarization is the major factor for the interaction, we can choose softer atoms than the original surface atoms, though the sensitivity of the polarization term on the intermolecular stabilization seems to be weak, suggested by our recent calculations. Less