Insights into the Interplay of Lewis and Brønsted Acid Catalysts in Glucose and Fructose Conversion to 5-(Hydroxymethyl)furfural and Levulinic Acid in Aqueous Media

Abstract

5-(Hydroxymethyl)furfural (HMF) and levulinic acid production from glucose in a cascade of reactions using a Lewis acid (CrCl₃) catalyst together with a Brønsted acid (HCl) catalyst in aqueous media is investigated. It is shown that CrCl₃ is an active Lewis acid catalyst in glucose isomerization to fructose, and the combined Lewis and Brønsted acid catalysts perform the isomerization and dehydration/rehydration reactions. A CrCl₃ speciation model in conjunction with kinetics results indicates that the hydrolyzed Cr(III) complex [Cr(H₂O)₅OH]²⁺ is the most active Cr species in glucose isomerization and probably acts as a Lewis acid–Brønsted base bifunctional site. Extended X-ray absorption fine structure spectroscopy and Car–Parrinello molecular dynamics simulations indicate a strong interaction between the Cr cation and the glucose molecule whereby some water molecules are displaced from the first coordination sphere of Cr by the glucose to enable ring-opening and isomerization of glucose. Additionally, complex interactions between the two catalysts are revealed: Brønsted acidity retards aldose-to-ketose isomerization by decreasing the equilibrium concentration of [Cr(H₂O)₅OH]²⁺. In contrast, Lewis acidity increases the overall rate of consumption of fructose and HMF compared to Brønsted acid catalysis by promoting side reactions. Even in the absence of HCl, hydrolysis of Cr(III) decreases the solution pH, and this intrinsic Brønsted acidity drives the dehydration and rehydration reactions. Yields of 46% levulinic acid in a single phase and 59% HMF in a biphasic system have been achieved at moderate temperatures by combining CrCl₃ and HCl.