Solvent-Free Synthesis of a Hierarchical Catechol-Functionalized UiO-66(Zr) for Efficient Esterification of Levulinic Acid to Alkyl Levulinates

Abstract

Catechol-functionalized UiO-66(Zr) holds considerable promise for targeted catalytic transformations owing to its distinctive electronic and structural properties. However, the direct synthesis of such frameworks has remained a persistent synthetic challenge. Herein, we report a rational, solvent-free, one-pot synthetic strategy enabling precise control over catechol functionalization in UiO-66(Zr) (UiO-66(Zr)-(OH)2) with well-defined hierarchical porosity. The optimized UiO-66(Zr)-0.25(OH)2 displays markedly improved catalytic performance in the esterification of levulinic acid (LA) with primary alcohols to yield alkyl levulinates. This superior activity arises synergistically from three key features: (i) increased Brønsted acidity imparted by the ortho-dihydroxyphenyl groups; (ii) a hierarchical pore architecture that enhances mass transport and substrate accessibility; and (iii) strong adsorptive affinity for both LA and alcohol substrates. Under standardized 12 h batch reactions with an alcohol-to-LA molar ratio of 20 and 50 mg of catalyst, UiO-66(Zr)-0.25(OH)2 achieves LA conversions of 98.9, 97.9, 95.6, and 94.3% with methanol, ethanol, 1-propanol, and 1-butanol, respectively, while maintaining >99.9% selectivity for the corresponding products of alkyl levulinates at optimally tuned reaction temperatures (60, 70, 80, and 100 °C). This work establishes a practical and environmentally sustainable synthetic strategy for the rational design of functionalized metal–organic frameworks, enabling high-efficiency, heterogeneous catalysis in targeted reactions.