Unraveling the Reaction Mechanism and Active Sites of Metal–Organic Frameworks for Glucose Transformations in Water: Experimental and Theoretical Studies

The catalytic performance of two different MOFs, UiO-66 and MOF-808, containing Lewis acid active sites has been evaluated for the transformation of glucose in water and compared with that of analogous Lewis acid Zr-β zeolite. While fructose is the main product obtained on Zr-β, mannose production increases when using Zr-MOFs as catalysts. Kinetic studies reveal a lower activation energy barrier for glucose epimerization to mannose when using Zr-MOF catalysts (∼83–88 and ∼100 kJ/mol for glucose epimerization and isomerization, respectively). A 13C NMR study using 13C1-labeled glucose allows confirming that on Zr-MOF catalysts, mannose is exclusively formed following the glucose epimerization route through a 1,2-intramolecular carbon shift, whereas the two-step glucose → fructose → mannose isomerization via 1,2-intramolecular proton shifts is the preferred pathway on Zr-β. A computational study reveals a different mode of adsorption of deprotonated glucose on Zr-MOFs that allows decreasing the activation barrier for the 1,2-intramolecular carbon shift. The combination of spectroscopic, kinetic, and theoretical studies allows unraveling the nature of the metal sites in Zr-MOFs and Zr-β catalysts and to propose a structure–activity relationship between the different Lewis acid sites and the glucose transformation reactions. The results presented here could permit new rationalized MOF catalyst designs with the specific active sites to facilitate particular reaction mechanisms.