Abstract
Microporous silica membranes have silica polymer network voids smaller than 3 Å where only small gas molecules such as helium (2.6 Å) and hydrogen (2.89 Å) can be transported. These silica membranes are highly expected to be available for H2 separation. In order to examine gas permeation mechanisms in the silica polymer network voids, factors such as membrane porous structures, gas diffusivity, and gas permeability were studied via membrane permeation molecular dynamics simulation. The thermal motions of silica membrane constituent atoms were examined according to classic harmonic oscillation potential using a suitable amorphous silica structure and non-equilibrium molecular dynamics (NEMD) simulations of gas permeation. The dynamic model successfully simulated the gas permeation characteristics in an amorphous silica membrane with a suitable Hooke’s potential parameter. The introduction of the oscillative thermal motion of the membrane atoms enhanced gas diffusivity. Helium and hydrogen diffusivity and permeability were analyzed using gas translation (GT) and solid vibration (SV) models. The diffusion distance of gas molecules between adsorption sites was around 5.5–7 Å. The solid-type vibration frequencies of gas molecules in the site were on the order of 1013 and were reasonably smaller for heavier helium than for hydrogen. Both the GT and SV models could explain the temperature dependency of helium and hydrogen gas diffusivities, but the SV model provided a more realistic geometrical representation of the silica membrane. The SV model also successfully explained gas permeability in an actual silica membrane as well as the virtual amorphous silica membrane.
Highlights
IntroductionThey reported that the size of an oxygen-membered ring determined the activation energy for gas permeation through silica network voids, and that silica glass had mainly five to six oxygen-membered rings, while six to seven oxygen-membered rings were dominant in silica membranes [19]
Microporous amorphous silica membranes have small network voids that are effective for the permeation of small molecules such as helium, neon (K.D. = 0.275 nm), and hydrogen (K.D. = 0.289 nm), which has led to their anticipated application to high-temperature hydrogen separation processes [1]
Harmonic oscillation potential was successfully applied to simulate the thermal motion of atoms that constitute an amorphous silica polymer network
Summary
They reported that the size of an oxygen-membered ring determined the activation energy for gas permeation through silica network voids, and that silica glass had mainly five to six oxygen-membered rings, while six to seven oxygen-membered rings were dominant in silica membranes [19] These types of quantum chemical calculations involved a relatively small system composed of several tens of atoms, whereas molecular simulation provides data on a molecular scale that are difficult to obtain from actual experiments. Parameters in the model, such as effective void space and vibration frequency of permeating molecules, were difficult to measure directly in order to confirm the validity of the estimated values This statistical dynamics model has not been definitively compared with the empirical activated diffusion/permeation models for subnano-scale microporous materials [7,22]. Gas diffusion and permeation mechanisms through amorphous silica network voids, for gas translation (GT) and solid vibration (SV) models, were clarified quantitatively via non-equilibrium molecular dynamic simulation technique
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