Single-crystal X-ray Diffraction Studies of Physisorption and Chemisorption of SO 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 (SO) 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 SO, 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 SO 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 single-crystal X-ray diffraction (scXRD) study that identifies discrete adsorption sites within Ni-MOF-74/Ni-CPO-27, where SO is both chemisorbed and physisorbed while also probing competitive adsorption of SO of these sites when water is present. Further features of this site have been confirmed by variable SO pressure scXRD studies, DFT calculations, and IR studies.