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.