Postsynthetic acid modification of amino-tagged metal-organic frameworks: Structure-function relationship for catalytic 5-hydroxymethylfurfural synthesis

Developing an efficient and selective catalyst for the dehydration of fructose to 5-hydroxymethylfurfural (HMF) is significant for biomass conversion. Herein, a metal-organic framework (MOF) with acidity and strong hydrophobicity is first reported by the condensation of amino-tagged MOFs with mercapto carboxylic acids and subsequent oxidation. The hydrophobic acidic MOFs possess acid densities ranging from 0.2−1.0 mmol·g⁻¹, H₂O contact angles of 114°−125°, and specific surface areas above 260 m²·g⁻¹. Compared to the methyl sulfo-functionalized MOF, the benzene sulfo-functionalized MOF with a strong hydrophobicity shows much higher activity and selectivity for the conversion of fructose to 5-hydroxymethylfurfural. In particular, 2.99% (mass) UiO-PhSO₃H shows the best catalytic performance with a 90.4% HMF yield due to its suitable hydrophobicity and abundant acidic sites. Moreover, the catalyst shows great stability after recycling for 5 runs. This work provides an interesting design strategy for the preparation of hydrophobic acidic MOFs and shows the powerful synergistic effect of acidity and hydrophobicity.