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

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 ¹³C and ¹H NMR using both unlabeled and ¹³C-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 ¹³C-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 ₂O 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 ₂O with a PO ₄ ^3-/niobic acid catalyst. The corresponding ¹³C-labeled results show that the first carbon in fructose maps onto the first carbon (aldehyde carbon) in furfural. Using ¹³C-enriched HMF formed from dehydration of ¹³C-labeled fructose in DMSO or H ₂O, 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.