Counterions Determine if Metal-Organic Cages Convert SO₂ to Sulfate or Reversibly Adsorb It

The continuous emission of harmful gases into the atmosphere damages the environment, air quality, and public health worldwide. To mitigate their impact, materials that capture and chemically inactivate gases are required; however, integrating and precisely controlling both abilities within a single material remains challenging. Herein, we demonstrate for the first time that switching between SO₂-physisorption and chemisorption is possible for porous materials by using different counterions, as illustrated with a series of Pd₆L₈ Metal-Organic Cages (MOCs). Pd-MOCs bearing BF₄ ^-, PF₆ ^-, or SO₄ ^2- expressed reversible adsorption (up to 3.6 mmol g^-1), cyclability, and SO₂/CO₂ selectivity. NO₃ ^- promoted conversion of SO₂ to sulfate, as corroborated with M₆L₈ cages built on Co(II), Cu(II), and Zn(II) ions. Noteworthy, the nitrate derivative of Pd₆L₈ captures 6.0 mmol g^-1 of SO₂, cleanly transforms it to SO₄ ^2- within its cavity in 94 % yield at room temperature, it is quantitatively regenerated, and tolerates humid SO₂; such qualities are unprecedented for SO₂ adsorbents. The deliberate use of counterions for modulating adsorption could be applied to charged MOFs, COFs, or POCs, potentially leading to the development of new reactivity or catalysis pathways for advanced applications against contaminant gases.