Insights into the Kinetics and Reaction Network of Aluminum Chloride-Catalyzed Conversion of Glucose in NaCl–H₂O/THF Biphasic System

We performed a systematic experimental kinetics study on AlCl₃-catalyzed conversion of glucose to 5-hydroxymethylfurfural (HMF) in NaCl–H₂O/tetrahydrofuran (THF) biphasic solvent. The kinetics model covers an extensive reaction network including the parallel and tandem reactions of isomerization, dehydration, decomposition, and polymerization from glucose. The accuracy of the model was verified by a parity plot and statistical significance analysis of the kinetic parameters. A deliberate insight into the intrinsic kinetic properties (reaction rate constant and apparent activation energy) of each subreaction elaborates the regulatory role of THF and NaCl on reaction pathways within the network. That is, THF suppresses the rehydration, degradation, and polymerization of HMF to unwanted byproducts, inhibits fructose-to-HMF dehydration and fructose-to-humins polymerization, but promotes the generation of formic acid (FA) from the direct degradation of both glucose and fructose by facilitating the generation of [Glc/Fru + H–H₂O–FA]⁺ species without formation of levulinic acid (LA); while NaCl promotes the dehydration and polymerization of fructose, decelerates the glucose-to-fructose isomerization, and effectively suppresses glucose-to-humins polymerization. The suppression role of NaCl on glucose conversion may come from the inhibition on mutarotation and ring opening from glucose due to the existence of a hydrogen bond between (C6)­O–H on glucose and Cl^– ion. The Brønsted acid (HCl) from the hydrolysis of AlCl₃ is responsible for direct glucose/fructose-to-FA degradation, HMF-to-humins polymerization, and HMF-to-FA/LA rehydration. The Lewis acidic [Al­(OH)₂(aq)]⁺ species is active for the reversible glucose-to-fructose isomerization and direct HMF-to-FA degradation, whereas glucose/fructose-to-humins polymerization and fructose-to-HMF dehydration are both Brønsted and Lewis acid-catalyzed. This work highlights a deep understanding of the complicated reaction network in the acid-catalyzed conversion of glucose to HMF in a biphasic solvent