Molecular Simulation Studies on Membrane Permeation and Molecular Design of Membranes

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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