High-Performance Porous Organic Polymers for Environmental Remediation of Toxic Gases

Sulfur dioxide (SO2) is a harmful acidic gas generated from power plants and fossil fuel combustion and represents a significant health risk and threat to the environment. Benzimidazole-linked polymers (BILPs) have emerged as a promising class of porous solid adsorbents for toxic gases because of their chemical and thermal stability as well as the chemical nature of the imidazole moiety. The performance of BILPs in SO2 capture was examined by synergistic experimental and theoretical studies. BILPs exhibit a significantly high SO2 uptake of up to 8.5 mmol g-1 at 298 K and 1.0 bar. The density functional theory (DFT) calculations predict that this high SO2 uptake is due to the dipole-dipole interactions between SO2 and the functionalized polymer frames through O2S(δ+)···N(δ-)-imine and O═S═O(δ-)···H(δ+)-aryl and intermolecular attraction between SO2 molecules (O═S═O(δ-)···S(δ+)O2). Moderate isosteric heats of adsorption (Qst ≈ 38 kJ mol-1) obtained from experimental SO2 uptake studies are well supported by the DFT calculations (≈40 kJ mol-1), which suggests physisorption processes enabling rapid adsorbent regeneration for reuse. Repeated adsorption experiments with almost identical SO2 uptake confirm the easy regeneration and robustness of BILPs. Moreover, BILPs possess very high SO2 adsorption selectivity at low concentration over carbon dioxide (CO2), methane (CH4), and nitrogen (N2): SO2/CO2, 19-24; SO2/CH4, 118-113; SO2/N2, 600-674. This study highlights the potential of BILPs in the desulfurization of flue gas or other gas mixtures through capturing trace levels of SO2.