Molecular dynamics simulation of diffusion and separation of CO₂/CH₄/N₂ on MER zeolites

The adsorption behaviors of CO₂, CH₄ and N₂ on MER zeolites were investigated by Grand Canonical Monte Carlo (GCMC) simulation method. The calculated pure gas uptake agreed well with the experimental data, which proved that the current simulation model and COMPASS force field are reliable. On this basis, molecular dynamics (MD) simulations were carried out for diffusion and separation of CO₂, CH₄ and N₂ on K-MER zeolites, with the silicon MER zeolite as the reference. The results show that mean squared displacement (MSD) versus simulation time is sublinear. In MER zeolites, the configuration diffusion regime is dominant resulted from the tight fit of the gas molecules and the zeolite pore size. The diffusion of CO₂, CH₄ and N₂ in MER zeolites with three-dimensional cage structures is anisotropic. The gas molecules diffuse preferentially along with the direction of x axis in K-MER zeolites. Extra-framework cations in zeolite plays important influence on the gas diffusion. In K-MER zeolites, the self-diffusion coefficients of CO₂ and N₂ are negative correlated with loading, whereas for the self-diffusion coefficient of CH₄, it firstly increases and then decreases with the increase of loading. All of the self-diffusion coefficients of CO₂, CH₄ and N₂ increase with the elevation of temperature. The order of diffusion activation energy is N₂ (16.51 kJ/mol)﹥CH₄ (8.39 kJ/mol)﹥CO₂ (4.38 kJ/mol). K-MER zeolite membrane has good separation selectivity for gas mixture system of CO₂/CH₄, CO₂/N₂ and N₂/CH₄. The permeance of CO₂ and N₂ through K-MER zeolite membrane is as high as 10⁴ GPU (1 GPU= 3.35×10⁻¹⁰ mol/(s·m²·Pa)).