CAU-52: An Iron Metal–Organic Framework Containing Furandicarboxylate Linker Molecules

The V-shaped linker molecule 2,5-furandicarboxylic acid (H₂FDC), which can be derived from lignocellulosic biomass, was used in a systematic screening with various iron salts and led to the discovery of a new iron-based metal-organic framework (Fe-MOF) with the composition [Fe₃(μ₃-O)(FDC)₃(OH)(H₂O)₂]·5H₂O·H₂FDC, designated as CAU-52 (CAU = Christian-Albrechts-Universität zu Kiel). The crystal structure of CAU-52 was determined using 3D electron diffraction (3D ED) and further refined by Rietveld refinement against powder X-ray diffraction (PXRD) data. CAU-52 contains the well-known trinuclear [Fe₃(μ₃-O)]⁷⁺ cluster as the inorganic building unit (IBU) that is six-connected by FDC^2- ions to form the pcu net. The connectivity leads to two types of cubic cages, similar to the ones observed in soc-MOFs. Comprehensive characterization of the title compound, including N₂ and water vapor sorption measurements, confirmed its chemical composition. CAU-52 exhibits microporosity toward nitrogen with a type-I isotherm (77 K), yielding a specific surface area of a_s,BET = 1077 m²/g. The H₂O sorption measurement at 298 K leads to an isotherm that exhibits three steps. The water sorption capacity was determined to be 390 mg/g, and it decreases slightly in subsequent sorption cycles. The MOF is stable up to 250 °C in air and chemically resistant in various solvents.