In situ single-crystal X-ray diffraction studies of physisorption and chemisorption of SO₂ within a metal-organic framework and its competitive adsorption with water

Living on an increasingly polluted planet, the removal of toxic pollutants such as sulfur dioxide (SO₂₎ from the troposphere and power station flue gas is becoming more and more important. The CPO-27/MOF-74 family of metal–organic frameworks (MOFs) with their high densities of open metal sites is well suited for the selective adsorption of gases that, like SO₂, bind well to metals and have been extensively researched both practically and through computer simulations. However, until now, focus has centered upon the binding of SO₂ to the open metal sites in this MOF (called chemisorption, where the adsorbent–adsorbate interaction is through a chemical bond). The possibility of physisorption (where the adsorbent–adsorbate interaction is only through weak intermolecular forces) has not been identified experimentally. This work presents an in situ single-crystal X-ray diffraction (scXRD) study that identifies discrete adsorption sites within Ni-MOF-74/Ni-CPO-27, where SO₂ is both chemisorbed and physisorbed while also probing competitive adsorption of SO₂ of these sites when water is present. Further features of this site have been confirmed by variable SO₂ pressure scXRD studies, DFT calculations, and IR studies.