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 SBET = 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 SBET = 360 m2/g and a micropore volume of 0.15 cm3/g.

一种新型的锆基金属-有机框架材料，命名为Zr-CAU-28，其框架组成为[Zr6O4(OH)4(FDC)4(OH)4(H2O)4]（H2FDC为2,5-呋喃二羧酸，CAU代表克里斯蒂安-阿尔布雷希特斯大学），在绿色合成条件下，通过水和乙酸混合物以及微波辅助加热的方法获得。Zr-CAU-28是首个基于H2FDC的Zr-MOF，H2FDC常被视为对苯二甲酸具有潜力的可再生替代品。其晶体结构通过结合直接方法、力场计算和Rietveld精修，从粉末X射线衍射数据中确定。该化合物以六方晶系（空间群P63/mmc）结晶，晶胞参数为a = 24.9919(9) Å和c = 24.7688(9) Å。框架结构采用类似于kagome的拓扑结构，因而包含了大型的六边形通道，孔径约为16 Å，以及小型三角通道，尺寸为3 Å。在-196 °C下进行的氮气吸附测量给出了SBET = 1006 m2/g的比表面积和0.42 cm3/g的微孔体积。热重分析表明，材料在270 °C以下具有稳定性，尽管温度依赖性的PXRD测量显示，在150 °C以上就已经出现长程有序度的降低。此外，通过使用二甲基甲酰胺/水混合物作为溶剂，可以获得基于Ce4+的类似物Ce-CAU-28。该MOF的结构和框架组成与锆基化合物非常相似，但其热稳定性明显较差。因此，Ce-CAU-28无法完全脱溶剂，其比表面积仅为SBET = 360 m2/g，微孔体积为0.15 cm3/g。