Zirconium-Formate Macrocycles and Supercage: Molecular Packing versus MOF-like Network for Water Vapor Sorption

In systematic efforts toward a new type of molecule-based porous materials, facile and efficient synthetic methods have been established to obtain macrocyclic [Zr₆]₆ and supercage-like {[Zr₆]₆}₈, where [Zr₆] represents [Zr₆O₄(OH)₄(CO₂) _n] building unit commonly found in Zr-based metal-organic frameworks. The reactions involve in situ hydrolysis of DMF solvent to produce formate linkers and thus do not require any organic ligand. A minor variation in the composition of two cyclic hexamers thus obtained results in dramatic differences in crystal packing which in turn lead to distinctive and selective sorption behavior for water vapor. It is shown that the high heat of water adsorption and unrestricted uptake under high humidity are consequences of the highly polar surface and flexible crystal packing. The reversibility of water adsorption is demonstrated by cyclic measurements of uptake and regeneration under dynamic flow conditions.