Separation of CO2 and N2 by Adsorption in C168 Schwarzite: A Combination of Quantum Mechanics and Molecular Simulation Study

Abstract

Using a hierarchical multiscale approach combining quantum mechanics and molecular simulation, we have investigated the adsorption of pure CO₂ and N₂ and their mixture at room temperature in C₁₆₈ schwarzite, as a model for nanoporous carbons. First, the adsorbate−adsorbent interaction potential is determined using ab initio quantum mechanics computations, and then the adsorption is predicted using full atomistic Monte Carlo simulations. The extents of adsorption, adsorption energies, and isosteric heats of pure CO₂ and N₂ simulated with the ab initio potential are found to be higher than those with the empirical Steele potential that had been developed from gas adsorption on planar graphite. The inclusion of the electric quadrupole moment of adsorbate in simulation has no discernible effect on N₂ adsorption but results in a larger extent of CO₂ adsorption at high coverages. The selectivity of CO₂ over N₂ in the C₁₆₈ schwarzite from a model flue gas is predicted to be significantly larger with the ab initio potential than with the Steele potential. This illustrates the importance of an accurate adsorbate−adsorbent interaction potential in determining gas adsorption and suggests that nanoporous carbons might be useful for the separation of flue gases. As a comparison, the adsorption and selectivity of CO₂ and N₂ in ZSM-5 zeolites are also simulated with the experimentally validated potential parameters. The selectivity in the C₁₆₈ schwarzite predicted with the ab initio or Steele potential is found to be larger than the selectivity in all-silica ZSM-5, but less than that in Na-exchanged ZSM-5 zeolites.