Isoreticular Synthesis of Ionic Covalent Organic Frameworks for Enhanced SO2 Adsorption and Separation

Achieving selective SO2 capture at low pressures is pivotal and challenging for possible flue gas desulfurization and air pollution control. In this study, we synthesized a series of ionic covalent organic frameworks (iCOFs) with β-ketoenamine linkages and sulfonic acid groups using a solvothermal method. TpPa-SO3H and TpBD-(SO3H)2 show a higher SO2 uptake of 4.46 and 5.24 mmol g−1 than TpPa-1 (4.24 mmol g−1) at 1 bar and 298 K, respectively, due to the combination of the good SO2 affinity of the polar sulfonic acid groups, higher pore volumes, and the good stability of β-ketoenamine COFs. TpBD-(SO3H)2 captured 2.83 mmol g−1 of SO2 at 0.1 bar and 298 K, which is 1.6 times higher than TpPa-1 (1.82 mmol g−1) under the same conditions. Notably, the IAST SO2/CO2 selectivity of TpBD-(SO3H)2 and TpPa-1 are 61 and 51, respectively, reflecting the impact of the incorporated SO3H groups’ higher affinity toward SO2. Notably, the multicomponent gas mixture breakthrough experiments confirm that TpBD-(SO3H)2 displays longer breakthrough time than TpPa-1 (987 vs. 311 min g−1). These β-ketoenamine iCOFs demonstrate nearly complete retention of crystallinity and porosity after exposure to dry or humid SO2. This work demonstrates that iCOFs are promising adsorbents for SO2 capture due to their high capacity, stability, and affinity for SO2 at low pressure.