Hydrothermal Reactions of Biomass-Derived Platform Molecules: Catalytic Decomposition Mechanisms of Glucose and Fructose over a Solid Acid Catalyst

Solid acid catalysts have been widely used to enhance the formation of 5-hydroxymethylfurfural (5-HMF) during hydrothermal sugar decomposition. However, the catalytic decomposition mechanisms of sugars are complex since decomposition reactions can occur both in the aqueous phase and on the catalyst surface. This study aims to elucidate the fundamental mechanisms of hydrothermal decomposition of glucose and fructose using sulfonated biochar as a solid acid catalyst, providing new insights into primary sugar decomposition reactions occurring in different phases. During catalytic hydrothermal decomposition, isomerization reactions to fructose and mannose and dehydration reactions to levoglucosan (LGA) and 5-HMF are identified as the key primary reactions for glucose, while retro-aldol condensation reaction to glyceraldehyde (GLA) is also an important primary reaction for fructose in addition to isomerization reactions to glucose and mannose and dehydration reaction to 5-HMF. The selectivities of dehydration reactions increase with catalyst loading for both glucose and fructose, at the expense of other primary reactions. For example, the selectivity of the dehydration reaction to 5-HMF during hydrothermal decomposition of glucose (at 200 °C) and fructose (at 175 °C) increases from ∼15% and ∼22% at a catalyst-to-substrate (C/S) ratio of 0.5 to ∼50% and ∼66% at a C/S ratio of 8, respectively. Further kinetic analyses show that the rate constants of dehydration reactions to 5-HMF and LGA increase almost linearly with the amount of active sites (i.e., −SO3H) on the catalyst surface, while the rate constants of isomerization and retro-aldol condensation reactions almost remain unchanged under all catalytic conditions. Our results clearly indicate that the solid acid catalyst strongly enhances dehydration reactions but has negligible effects on isomerization and retro-aldol condensation reactions, which occur only in the aqueous phase. Interestingly, dehydration reaction of glucose to LGA is found to take place both in the aqueous phase and on the catalyst surface.