Polymerization of glucose during acid-catalyzed pyrolysis at low temperatures

This study reports the importance of polymerization reactions during the pyrolysis of acid-impregnated glucose at 60-150 °C. The experimental results demonstrate that polymerization reactions are the dominant reactions to produce oligosaccharides of various linkages and degrees of polymerization (DP) during acid-catalyzed glucose pyrolysis at low temperatures. The maximal DP of the oligosaccharide products depends on pyrolysis temperature, i.e., from ∼4 at 60 °C to ∼18 at 120 °C. As the major primary products, the disaccharides of various α and β linkages (including 1,6-, 1,4-, 1,3-, 1,2- and 1,1-glycosidic bond) have been successfully identified, indicating that mutarotation reactions also play important roles in acid-catalyzed glucose pyrolysis. Due to the high reactivity of the hydroxyl group on C6, disaccharides with 1,6-glycosidic bond (i.e., gentiobiose and isomaltose) are more favorably formed with high initial selectivities, i.e., ∼27 and ∼20% for gentiobiose and isomaltose at an acid loading of 0.5 mmol/g, respectively. The formation of α-linkage disaccharides follows an order of 1,6-glycosidic bond > 1,4-glycosidic bond > 1,3-glycosidic bond > 1,2-glycosidic bond > 1,1-glycosidic bond. An increase in acid loading enhances the formation of disaccharides, especially those with 1,6-glycosidic bond. The experimental results reported in the study provide new insights into acid-catalyzed glucose pyrolysis mechanism for the development of advanced pyrolysis technologies to produce biofuels and biochemicals.