Mechanism insight into α-D-galactopyranose conversion to 5-hydroxymethyl furfural by combining Py-GC–MS experiments and theoretical calculations

Pyrolysis of biomass is a promising technique for producing various chemicals. During the fast pyrolysis of biomass, 5-hydroxymethyl furfural (5-HMF) is a significant product derived from holocellulose, which consists of cellulose and hemicellulose. In this study, the formation mechanism of 5-HMF was systematically revealed by combining fast pyrolysis experiments of C-labeled and unlabeled D-galactose and density functional theory calculations that α-D-galactopyranose was selected as the model compound for hemicellulose. Experimental results show the yield and concentration of 5-HMF gradually decrease as the temperature rises (≥400 °C). In addition, the origin of the C atoms in 5-HMF was determined. The aldehyde group and hydroxymethyl are primarily derived from C1 and C6 of D-galactose, respectively. Based on computational results, α-D-galactopyranose prefers to undergo a ring-opening reaction to form acyclic D-galactose with an energy barrier of 166.5 kJ/mol. (2R,3S,E)-2,3,5,6-tetrahydroxyhex-4-enal (C1-i2) and (2R,3S)-2,3,6-trihydroxy-5-oxohexanal (C1-i3) generated by D-galactose are important intermediates for 5-HMF formation. Because C1-i3 can readily form 5-HMF through successive dehydration at 3-OH+2-H site, cyclization, and dehydration at 5-OH+4-H site. In favorable paths, the aldehyde group of 5-HMF all comes from C1 of α-D-galactopyranose, which reasonably explains the experimental results. Overall, this study will be helpful to improve the formation mechanism of 5-HMF and to develop relevant pyrolysis techniques to prepare it.