Direct Synthesis of Hierarchically Porous Metal–Organic Frameworks with High Stability and Strong Brønsted Acidity: The Decisive Role of Hafnium in Efficient and Selective Fructose Dehydration

The direct synthesis of metal–organic frameworks (MOFs) with strong Brønsted acidity is challenging because the functional groups exhibiting Brønsted acidity (e.g., sulfonic acid groups) often jeopardize the framework integrity. Herein, we report the direct synthesis of two hierarchically porous MOFs named NUS-6 composed of either zirconium (Zr) or hafnium (Hf) clusters with high stability and strong Brønsted acidity. Via the modulated hydrothermal (MHT) synthesis, these two MOFs can be easily synthesized at a low temperature (80 °C) with high throughput. They exhibit BET surface areas of 550 and 530 m2 g–1 for Zr and Hf one, respectively, and a unique hierarchically porous structure of coexisting micropores (∼0.5, ∼0.7, and ∼1.4 nm) and mesopores (∼4.0 nm) with dangling sulfonic acid groups. Structural analysis reveals that the hierarchical porosity of NUS-6 is a result of missing linkers and clusters of the parental UiO-66 framework. These unique features make NUS-6 highly efficient and selective solid acid catalysts for dehydration of fructose to 5-hydroxymethylfurfural (HMF), in which NUS-6(Hf) demonstrates a superior performance versus that of NUS-6(Zr) because of the stronger Brønsted acidity contributed from Hf-μ3-OH groups as well as smaller pore sizes suitable for the restriction of unwanted side reactions. Our results have demonstrated for the first time the unique attributes of Hf-MOFs featured by superior stability and Brønsted acidity that can be applied as heterogeneous catalysts in biobased chemical synthesis.