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

Abstract

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.

Efficient utilization of potash alum as a green catalyst for production of furfural, 5-hydroxymethylfurfural and levulinic acid from mono-sugars

“…20 However, due to the presence of Al 3+ and Brønsted acidity in the investigated reaction, an aluminate-mediated rearrangement of glucose to fructose occurs through formation of a furanoid fructose-aluminate complex, where Al 3+ catalyze the mutarotation of a-and b-glucopyranose isomers. 40 A similar mechanism has been suggested by other prominent research groups working in the area of biomass conversion and related catalysis. 41 Thus, we propose a similar mechanism for experiments performed in the presence of Al containing PA catalyst.…”

Acid‐Catalyzed Conversion of Carbohydrates into Value‐Added Small Molecules in Aqueous Media and Ionic Liquids

“…The immiscibility of such systems is often maintained by the addition of inorganic electrolytes (NaCl, KCl, KBr, etc. ), which additionally support the dehydration of fructose …”

“…It is evident in Scheme that rehydration of HMF into levulinic acid also leads to the formation of formic acid, which has a wide use in applied chemistry and agricultural, fuel, and pharmaceutical industries . There is also evidence that formic acid, a useful small molecule, can be obtained directly from glucose and fructose …”

Catalytic Dehydration of Fructose into 5‐Hydroxymethylfurfural by a DMSO‐like Polymeric Solid Organocatalyst

“…Significantly, no LA was observed to form by catalysis with the eight Au@PTh–PThO x catalysts, revealing a prominent inhibiting effect similar to DMSO for HMF rehydration. Therefore, we inferred that FA was mainly derived from the direct degradation of fructose or HMF rather than the fructose‐to‐FA/FF decarburization or HMF‐to‐FA/LA rehydration under the catalysis of the polymeric organocatalyst. A possible reaction pathway for FA formation is shown in Scheme S1 (in the Supporting Information).…”