In Situ Single-Crystal X-ray Diffraction Studies of an Anomalous Nitric Oxide Adsorption in a Partially Activated Metal–Organic Framework

Metal-organic frameworks (MOFs), with their high porosities and surface areas, show great utility in the field of gas adsorption. To unlock this porosity, MOFs are generally fully activated by removing all adsorbed guests using high temperatures and low pressures. However, this is energy intensive and can be unfeasible if the MOF is part of a composite, where the maximum temperature of the composite is below the activation temperature. To investigate the effect of activation temperature on adsorption, a series of in situ single-crystal X-ray diffraction (scXRD) studies were performed on Ni-MOF-74 loaded with the gas nitric oxide (NO) under different conditions. These experiments uncovered anomalous adsorption results where partially activated samples adsorb ∼14% more NO per framework material than did the fully activated sample. The scXRD experiments revealed a new NO binding site that is only present if the open metal sites are partially occupied by water molecules. To shed more light on the respective binding of the two different NO sites in Ni-MOF-74, these were studied in situ under different treatment conditions, such as the exposure to vacuum at different temperatures. This study yields insights into the nature of binding sites in MOFs, how these are affected by activation, and helps to pave the way for the improved design of processing conditions.