Optimal Size of a Cylindrical Pore for Post-Combustion CO₂ Capture

Pore size is an essential factor in controlling gas sorption in porous separation media. The overlap of the potential energy surface (PES) of CO₂ interacting with a cylindrical pore wall can be used to tune gas sorption inside a porous material, but how such overlap can benefit post-combustion CO₂ capture has not been fully addressed from a computational perspective. Here we use van der Waals density functional (vdW-DF) theory to assess the overlap of PES of CO₂ inside cylindrical pores as represented by carbon nanotubes (CNTs) of different diameters. Then we employ grand-canonical Monte Carlo simulations to obtain the adsorption capacity and selectivity of a CO₂/N₂ mixture with a CO₂ partial pressure of 0.15 bar at room temperature. We find that the maximum PES overlap and maximum amount of CO₂ adsorbed are both achieved at a CNT diameter or cylindrical pore size of 7.8 Å, which corresponds to an accessible pore size of 4.4 Å. A further investigation of N₂ adsorption corroborates the idea of PES overlap. GCMC simulations reveal that a maximum CO₂/N₂ selectivity of ∼33 is reached at a CNT diameter of 7.05 Å for the gas mixture. This work suggests that a cylindrical pore size between 7 and 8 Å would be most beneficial for post-combustion CO₂ capture from overlap of PES. (Graph Presented).