Water in the Micropores of CPO-27 Metal-Organic Frameworks: A Comprehensive Study

The metal-organic framework CPO-27 exhibits free coordination sites (open metal sites) and can be prepared with a wide range of metals that influence its properties. It is therefore an intriguing structure to study sorption phenomena. We analyze the water resistance and sorption behavior of these frameworks, with particular attention to the sorption mechanism in detail and the structure of the confined water molecules. For this purpose, we use manometric water vapor sorption analysis and FTIR spectroscopy. The respective metal center orchestrates both the adsorption behavior and the arrangement of the water molecules in the micropores of the framework. The extent to which water molecules form hydrogen bonds (with each other and with framework oxygen atoms) plays a crucial role in the stability of the framework towards water. Water adsorption is governed by the coordination of water molecules to the open metal sites (except for CPO-27-Cu) and subsequent H-bonding. A stepwise adsorption of water is observed, with significant differences depending on the choice of metal.