Catalytic Transfer Hydrogenation of Biomass-Derived Levulinic Acid and Its Esters to γ-Valerolactone over Sulfonic Acid-Functionalized UiO-66

Production of gamma-valerolactone (GVL) from biomass-derived levulinic acid and its esters via a catalytic transfer hydrogenation (CTH) process over sulfonic acid-functionalized UiO-66, a microporous zirconium-based metalorganic framework (Zr-MOF), is reported herein. On the basis of comprehensive structural analyses by means of XRD, N-2 physisorption, IR, TG, and Zr K-edge XAFS, we show that free sulfonic acid (-SO3H) groups can uniformly be tethered on a UiO-66 framework without affecting the coordination state of Zr atoms, while crystallinity and surface area decrease along with the functionalization. As a consequence, UiO-66 bearing a 60 mol % fraction of sulfonic acid-containing benzene dicarboxylate (BDC) linker and retaining a high surface area exhibits the highest catalytic activity in the CTH reaction of levulinic acid and its esters to give GVL with the maximum GVL yield of up to 85% at 140 degrees C. Comparative experiments, together with characterization results, reveal that the high catalytic activity is provided by the cooperative effect between Lewis-basic Zr6O4(OH)4 clusters and Bronsted-acidic -SO3H sites arranged in a confined nanospace adjacently with each other, which catalyze the CTH reaction of levulinic acid and its esters and facilitate successive intramolecular dealcoholization to afford GVL, respectively. The catalyst is reusable during repeated cycles without appreciable loss of activity and selectivity, shows broad scopes toward substrates and alcohols, and also allows direct synthesis of GVL from furfural, making this material a promising candidate for efficient GVL production from biomass resources