Tuning the Catalytic Activity of UiO-66 via Modulated Synthesis: Esterification of Levulinic Acid as a Test Reaction

Graphical Esterification of levulinic acid with ethanol was catalyzed by the cooperative effects between the Lewis acid sites offered by unsaturated Zr 6 nodes and Brønsted acid sites supplied by OH in defective UiO-66 MOFs with high defect density, which was tuned by precursor ratios and modulators. Due to their high stabilities and tunable defects, zirconium terephthalate UiO-66 type metal organic frameworks (MOFs) have been promising catalysts in many catalytic reactions. The catalytic activity of UiO-66 MOFs is significantly affected by the density of defects and hydroxyl groups bonded to Zr 6 nodes, which can be tuned by modulators. In this work, a series of defective UiO-66 were synthesized via tuning the ratio of H 2 BDC/Zr and adding monocarboxylic acids with different chain length as modulators. The synthesized UiO-66 MOFs were characterized by powder X-ray diffraction, N 2 physisorption, IR and 1 H NMR spectroscopy, scanning and transmission electron microscopy. Esterification of levulinic acid with ethanol was selected to reveal the influence of modulators, defects and hydroxyl groups. The results showed that UiO-66 with 0.5 of H 2 BDC/Zr ratio and lauric acid as the modulator owned excellent catalytic activity. The cooperative effects, between the Lewis acid sites offered by unsaturated Zr 6 nodes and Brønsted acid sites supplied by OH, has been revealed to have a significant influence on the catalytic activity.