Comparison of Structural Features of Humins Formed Catalytically from Glucose, Fructose, and 5-Hydroxymethylfurfuraldehyde

The IR spectra of humins formed during the acid-catalyzed conversion of glucose, fructose, and 5-hydroxymethylfurfuraldehyde 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 2,5-dioxo-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.