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

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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