Formation and Growth of Humins via Aldol Addition and Condensation during Acid-Catalyzed Conversion of 5-Hydroxymethylfurfural

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.