Molecular Simulation of Capture of Sulfur-Containing Gases by Porous Aromatic Frameworks

The adsorption of pure SO₂ and H₂S and their selective adsorption from various gas mixtures by porous aromatic frameworks (PAFs) are investigated using grand canonical Monte Carlo (GCMC) simulations and first-principles density functional theory calculations. The influence of functional groups including -CH₃, -CN, -COOH, -COOCH₃, -OH, -OCH₃, -NH₂, and -NO₂ on the adsorption of pure SO₂ and H₂S as well as selective capture of SO₂ and H₂S from SO₂/N₂, SO₂/CO₂, H₂S/CO₂, and H₂S/CH₄ mixtures is explored. Our calculations indicate that PAFs exhibit high loadings for pure SO₂ and H₂S gas adsorption at 298 K up to 40 bar compare to other gases such as CH₄ and CO₂. Additional functional groups enhance gas uptake at low pressures because of stronger interaction with the gas molecules while reducing gas uptake at high pressures because of a decrease in pore volume. The contributions of electrostatic interactions to gas adsorption loadings are analyzed in GCMC simulations. Ideal adsorbed solution theory calculations generally overestimate SO₂ and H₂S adsorption selectivity in gas mixtures but qualitatively predict the trends seen in GCMC simulations for these systems. The GCMC simulations further show that the inclusion of any of the functional groups we considered increases the selectivity of SO₂/N₂, SO₂/CO₂, H₂S/CO₂, and H₂S/CH₄ relative to unfunctionalized materials. Electron-withdrawing groups such as -CN, -COOH, -COOCH₃, and -NO₂ are more effective at enhancing adsorption selectivity in this work. The highest selectivity in the PAFs functionalized by these groups is predicted at the lowest temperature we considered (273 K), whereas it occurs at 298 K for PAFs with other functional groups.