An in Situ NMR Study of the Mechanism for the Catalytic Conversion of Fructose to 5-Hydroxymethylfurfural and then to Levulinic Acid Using ¹³C Labeled d-Fructose

Abstract

The pathways for the formation of 5-hydroxymethylfurfural (HMF) by dehydration of D-fructose and for the formation of levulinic acid and formic acid from HMF by rehydration were investigated by in situ 13 C and 1 H NMR using both unlabeled and 13 C-labeled fructose. Water or DMSO was used as the solvent with Amberlyst 70, PO 4 3− / niobic acid, or sulfuric acid as catalysts. Only HMF is observed using NMR for fructose dehydration in DMSO with any of the three catalysts or without a catalyst. For each system, results with 13 C-labeled fructose indicate that the first carbon (C-1) or sixth carbon (C-6) of fructose maps onto the corresponding carbons of HMF. For fructose dehydration in H 2 O with a PO 4 3− / niobic acid catalyst, in addition to HMF, furfural was observed as a product. However, we show that furfural is not a reaction product deriving from HMF under our conditions. Rather our data indicate that there is a parallel reaction pathway open to fructose when the reaction takes place in H 2 O with a PO 4 3− /niobic acid catalyst. The corresponding 13 C-labeled results show that the first carbon in fructose maps onto the first carbon (aldehyde carbon) in furfural. Using 13 C-enriched HMF formed from dehydration of 13 C-labeled fructose in DMSO or H 2 O, we investigated the pathway for HMF rehydration to levulinic and formic acid. The data in different solvents and with different catalysts are consistent with a common mechanism for HMF rehydration, which results in the C-1 and C-6 carbon of HMF being transformed to the carbon of formic acid and methyl carbon (C-5) of levulinic acid, respectively.

The role of water in catalytic biomass-based technologies to produce chemicals and fuels

“…A similar study reported for DMSO and H 2 O has verified that the C 1 -and C 6 -carbons of D-fructose appears at the corresponding position in 5-HMF [144], which was transferred into the formic acid and the carboxyl group of the levulinic acid, as expected (Scheme 16).…”

Comparison of Homogeneous and Heterogeneous Catalysts for Glucose‐to‐Fructose Isomerization in Aqueous Media

“…Notably, the deuterium atom from HMF was transferred onto formic acid and not onto levulinic acid, which indicated that the rehydration of HMF involved 1,2-CÀC bond breaking to form formic and levulinic acids, as also indicated in a recent study. [23] As demonstrated herein, the mapping of hydrogen atoms from the reactants to the products or intermediates can assist in the development of mechanisms for the dehydration and rehydration reactions in biomass-derived molecules.…”

Catalytic Conversion of Lignocellulosic Biomass to Value-Added Organic Acids in Aqueous Media

“…The optimum temperature was found to be 200-220°C. [31,46,47,160]. The amount of water solvent can affect the hydrolysis process and the optimal concentration was found to be *90 wt%.…”