Mechanistic Insights in Catalytic Conversion of Sugars to Platform Furan Derivatives

An approach to utilize biomass requires its conversion to various furan derivatives, which can potentially substitute petroleum precursors in producing chemicals, polymers and fuels. The objective of this work is to gain mechanistic understanding into the aldose conversion to platform furan derivatives using a Lewis acid with a Brønsted acid. For the first time, we employed kinetics experiments in conjunction with speciation modeling of the metal salts (CrCl₃ and AlCl₃) in aqueous media and revealed that among the various metal ions generated in aqueous media, partially hydrolyzed ions, such as CrOH²⁺ or AlOH²⁺, are the most active for the metal salt catalyzed aldose-to-ketose isomerization. Additionally, complex interactions between the two catalysts were explained: Brønsted acidity retards aldose-to-ketose isomerization by decreasing the equilibrium concentration of [Cr(H₂O)₅OH]²⁺ or [Al(H₂O)₅OH]²⁺. In contrast, Lewis acidity promotes side reactions during fructose dehydration and HMF rehydration reactions. Thus, an optimization between the Lewis and Brønsted acidities along with reaction conditions is required. Additionally, spectroscopic studies (NMR and EXAFS) revealed strong catalytic and mechanistic similarities among CrCl₃, AlCl₃ and Sn-beta catalyzed aldose-ketose isomerization.