Cellobiulose as a Key Intermediate during Biomass Hydrothermal Conversion into Biofuels and Biochemicals: Fundamental Decomposition Mechanisms

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Cellobiulose (glucosyl-fructose, GF), as a dominant primary product from cellobiose isomerization during hydrothermal conversion, plays a key role in the production of biofuels and biochemicals from biomass or cellulose, but the reaction mechanism of GF hydrothermal decomposition remains largely unclear. For the first time in the field, this study reports a systematic investigation into the fundamental reaction mechanism of GF hydrothermal decomposition using a continuous reactor at 200–250 °C and 10 MPa. The GF conversion increases with reaction temperature and residence time, with a high conversion of 91% achieved at 250 °C and ∼52 s. The experimental results demonstrate that GF hydrothermal decomposition proceeds with several primary reactions: (1) hydrolysis to produce glucose and fructose, (2) isomerization to produce cellobiose and glucosyl-mannose (GM), and (3) retro-aldol condensation to produce glucosyl-erythrose (GE) and glycolaldehyde. Among all primary reactions, the hydrolysis reaction plays a key role during GF decomposition, contributing to ∼60% of the primary decomposition reactions of GF at 200 °C, while isomerization and retro-aldol condensation reactions only contribute to ∼31 and ∼9% of the primary decomposition reactions of GF at 200 °C. Those primary products are further decomposed to various secondary products (mainly furans, aldehydes, and organic acids). Particularly, various organic acids (i.e., saccharinic acid, formic acid, lactic acid) produced at the early stage of GF decomposition can further catalyze the primary reactions of GF and the secondary reactions of primary products. This study clearly demonstrates that isomerization of cellobiose to GF plays a key role in the breakage of the glycosidic bond during noncatalytic hydrothermal decomposition of cellobiose, providing new insights into the fundamental reaction mechanism of sugar oligomer hydrothermal decomposition for producing renewable biofuels and biochemicals.