Predicted Separation of Acid Gases from Gas Mixtures by Functionalized Porous Aromatic Frameworks

The selective adsorption of target acid gas molecules from binary gas mixtures by porous aromatic frameworks (PAFs) with two identical functional groups per aromatic ring (PAF-R₂) was computationally investigated using grand canonical Monte Carlo simulations. PAF-R₂ adsorption was considered for three binary mixtures of small molecular concentrations of acid gas and abundant nitrogen gas (CO₂/N₂, SO₂/N₂, and H₂S/N₂). The results indicate that additional functional groups enhance acid gas loadings and selectivity, compared with pristine PAF and single-functionalized PAFs. Low pressures yield linearly increasing gas loadings and constant selectivity, while high pressures yield much higher adsorption and selectivity. In particular, SO₂ loading and selectivity under high pressures are heavily influenced by the PAF's maximum adsorption limit, which can be linked back to the functional groups and their configuration. In summary, PAF-(3,5)-(COOH)₂ (nomenclature of PAFs is provided in the Appendix in the Supporting Information) and many other PAF with the same two electron-withdrawing groups are predicted to have great acid gas adsorption and selectivity from gas mixtures, while PAF-(3,5)-(OH)₂ (one of PAFs with two identical electron-donating groups) is predicted to have good adsorption and selectivity, especially under elevated pressures. The results of this work can provide insights into various types of PAFs with great selective adsorption ability and their corresponding conditions. The simulation procedures and results may inspire the exploration and screening of other types of PAFs or porous materials, for acid gas absorption.