The acid-catalyzed conversion of 5-hydroxymethylfurfural (HMF) produces levulinic and formic acids in equal amounts. Dark-colored solids, known as humins, are also formed in a parallel reaction. Aldol addition and condensation are proposed as important reactions in the acid-catalyzed growth of humins, adding HMF to 2,5-dioxo-6-hydroxy-hexanal. Consistent with this proposal, infrared (IR) spectra of humins formed from HMF indicate that the furan ring and hydroxymethyl group of HMF are present in the humins, but the carbonyl group is not. Similarly, if a mixture of HMF and benzaldehyde are processed, IR spectra of the humins indicate the additional presence of the aromatic ring from benzaldehyde but not its carbonyl group. The incorporation of the aromatic ring from benzaldehyde in the humins demonstrates the possibility of functionalizing humins to increase their value.

The IR spectra of humins formed during the acid-catalyzed conversion of glucose, fructose, and 5hydroxymethylfurfuraldehyde were compared. The spectra are quite similar except for three groups of features that can be attributed to furan rings and carbonyl groups conjugated with carbon−carbon double bonds. IR spectroscopy further revealed that benzyl groups could be added to the humins as they formed or in a separate aldol addition/condensation reaction after they had been recovered. The IR spectra are consistent with a model where each of the three reactants must first be converted to 2,5dioxo-6-hydroxyhexanal (DHH) before humins can form via subsequent aldol addition and condensation. The differences in the IR spectral features can then be explained by variations in the concentrations of other aldehydes and ketones that can react with DHH.

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