Formation of humins during degradation of carbohydrates and furfural derivatives in various solvents

Humins are undesired solids formed during the hydrothermal degradation of carbohydrates. In order to reveal the mechanism of formation of humins, we studied the degradation behavior of 11 model compounds including carbohydrates and furfural derivatives, within water and various pure organic solvents as reaction media. All the studied carbohydrates could generate solid humins in both water and studied organic solvents except ethanol, while the furfural derivatives could generate solid humins in only water. The results could be explained by regarding the formed α-carbonyl aldehydes and α,β-unsaturated aldehydes as primary precursors for formation of humins. Furfural derivatives could generate chain α-carbonyl aldehydes (for example, 6-hydroxy-2,5-dioxohexanal from 5-hydroxymethylfurfural and 2-oxopentanedial from furfural) through hydrolytic ring opening reaction; thus, water is essential for these furfural derivatives to generate humins. As for carbohydrates, they could generate α-carbonyl aldehydes and α,β-unsaturated aldehydes through simple step of β-elimination in all solvents; thus, they could form humins in both water and studied organic solvents except ethanol. Ethanol could react with α-carbonyl aldehydes by acetalization; thus, the condensation between α-carbonyl aldehydes was suppressed in ethanol, leading to few humins formation from carbohydrates. Based on the above analysis, we proposed that the formed α-carbonyl aldehydes and α,β-unsaturated aldehydes should be the primary precursor of humins.