Green Synthesis of Zr-CAU-28: Structure and Properties of the First Zr-MOF Based on 2,5-Furandicarboxylic Acid

A new Zr-based metal-organic framework denoted as Zr-CAU-28 with framework composition [Zr₆O₄(OH)₄(FDC)₄(OH)₄(H₂O)₄] (H₂FDC = 2,5-furandicarboxylic acid, CAU = Christian-Albrechts-University) was obtained under green synthesis conditions from a mixture of H₂O and acetic acid and employing microwave-assisted heating. Zr-CAU-28 is the first Zr-MOF based on H₂FDC, which is often considered a promising renewable alternative to terephthalic acid. The crystal structure was determined from powder X-ray diffraction data using a combination of direct methods, force field calculations, and Rietveld refinement. The compound crystallizes in the hexagonal crystal system (space group P6₃/mmc) with the cell parameters a = 24.9919(9) and c = 24.7688(9) Å. The framework structure adopts a kagome-like topology and hence contains large hexagonal channels with a pore diameter of approximately 16 Å and small trigonal channels with a size of 3 Å. Nitrogen sorption measurements were carried out at -196 °C and gave a specific surface area of S_BET = 1006 m²/g and a micropore volume of 0.42 cm³/g. Thermogravimetric analyses showed a stability up to 270 °C although temperature dependent PXRD measurements revealed a decrease in long-range order already above 150 °C. Furthermore, the Ce⁴⁺ based analogue Ce-CAU-28 could be obtained employing dimethylformamide/water mixtures as solvent. The structure and framework composition of this MOF are very similar to the ones of the Zr-based compound, but its thermal stability is clearly inferior. Thus, Ce-CAU-28 cannot be fully desolvated and exhibits a specific surface area of only S_BET = 360 m²/g and a micropore volume of 0.15 cm³/g.