Acid Regulation of Defective Sulfonic-Acid-Functionalized UiO-66 in the Esterification of Cyclohexene with Formic Acid

The esterification of cyclohexene with formic acid is the first step in synthesizing cyclohexanol via the indirect hydration of cyclohexene. In this work, we improved the efficiency of the esterification reaction by constructing a sulfonic-acid-functionalized UiO-66 solid-acid catalyst containing defect sites. By using acetic acid as the modulator, ZrCl4 as the zirconium source, and 2-sulfoterephthalic acid monosodium salt as the ligand, a sulfonic-acid-functionalized UiO-66-SO3H with defect sites was prepared using a solvothermal synthesis method. Additionally, we also prepared UiO-66 with defect sites for comparison. We then investigated the regulation effects of acetic acid addition amount on the defect structures and acidity of the as-prepared materials. The results showed that the addition of acetic acid caused lattice defects in the as-prepared materials with a missing linker, and the specific surface area, pore volume, pore size, and particle size of the material changed significantly. The number of defect sites was determined by thermogravimetric analysis. Additionally, pyridine infrared spectroscopy and X-ray photoelectron spectroscopy (XPS) analysis indicated that the ratio of Lewis acid to Brønsted acid sites, and the total acid content in the defect materials, greatly increased compared to pure UiO-66 and UiO-66-SO3H, which greatly enhanced the acid catalytic performance of the material. Among these, UiO-66-SO3H(20), which was prepared by adding 20 mL of acetic acid, exhibited the best catalytic activity. The conversion rate of cyclohexene reached 83.4%, the selectivity toward cyclohexyl formate was 99.8%, and the catalyst could be reused three times.Graphical AbstractThe purpose of this work was to improve the efficiency of the esterification reaction of cyclohexene with formic acid by constructing a sulfonic acid-functionalized UiO-66 solid acid catalyst containing defective sites. The regulation effect due to the amount of acetic acid added on the defect structure and acidity of the as-prepared materials was investigated.