Hydrothermal Reactions of Biomass-Derived Platform Molecules: Mechanistic Insights into 5-Hydroxymethylfurfural (5-HMF) Formation during Glucose and Fructose Decomposition

The study investigates the primary reaction mechanisms of hydrothermal decomposition of glucose and fructose at 150–225 °C under various initial pH conditions (i.e., with an initial pH of 3–7) to reveal the formation mechanism of 5-hydroxymethylfurfural (5-HMF) from glucose and fructose. The results clearly show that dehydration to produce 5-HMF is not a primary reaction during hydrothermal decomposition of both glucose and fructose under noncatalytic conditions, indicating that 5-HMF cannot be directly produced from either glucose or fructose in water (with an initial pH of ∼7). In the presence of formic acid, dehydration to produce 5-HMF becomes a primary reaction during the hydrothermal decomposition of both glucose and fructose. The selectivity of dehydration reaction to produce 5-HMF increases with reducing the initial pH, i.e., up to ∼40% for glucose decomposition at 175 °C and ∼49% for fructose decomposition at 150 °C when the initial pH reduces to 3. Further kinetic analyses show that the rate constants of isomerization and retro-aldol condensation reactions almost remain unchanged under all initial pH conditions, while the rate constants of dehydration reactions to produce 5-HMF and levoglucosan increase almost linearly with the hydrogen ion concentration at the reaction temperature. This leads to increases in the selectivities of dehydration reactions, at the expense of other primary reactions. Our data also clearly indicate that dehydration reactions are acid-catalyzed reactions, while organic acid has almost no effect on isomerization and retro-aldol condensation reactions. Since organic acids can be formed at the early stage of sugar decomposition, this study provides the first experimental evidence in the field to clarify the formation mechanism of 5-HMF during sugar hydrothermal decomposition.