Solvent and Chloride Ion Effects on the Acid-Catalyzed Conversion of Glucose to 5-Hydroxymethylfurfural

Catalytic systems that can selectively convert glucose to 5-hydroxymethylfurfural (HMF), a valuable platform chemical, and allow for the cheap separation of HMF would aid in the development of green routes for the production of different chemicals and fuels. Glucose can be converted to HMF either by direct dehydration or via a sequential path involving the isomerization of glucose to fructose and the subsequent dehydration of fructose. Regardless of the path, the yields of HMF are often limited by unselective acid-catalyzed C-C bond-forming reactions that lead to the production of humins. Recent experimental results show that the overall yield of HMF can be significantly influenced by the solvents used and the presence of halide anions. Herein, we examine the role of acetone-water mixtures and chloride ions on the elementary acid-catalyzed isomerization and dehydration steps involved in the conversion of glucose to 5-hydroxymethylfurfural. Ab initio molecular dynamic simulations were carried out to examine the elementary reactions involved in glucose isomerization, fructose dehydration, glucose dehydration, and humin formation paths in water as well as acetone-water solvent mixtures and to explore the influence of chloride ions. The results indicate that the catalytic reactivities for glucose isomerization using a Sn-β catalyst are independent of the solvent choice due to the weak interactions between the solvent and the transition state structure. The Brønsted acid-catalyzed dehydrations of glucose and fructose are much more sensitive to the solvent and exhibit lower activation free energies in an acetone-water system than in water as a result of the localization of water molecules, a proton and chloride ion in the vicinity of hydrophilic hydroxyl groups by the confinement from the acetone cosolvent. Such effects are not observed in the aldol addition reactions involving C-C bond formation between relatively hydrophobic HMF and acetone molecules. The destabilization of the acetone reactant in its enol form in the acetone solvent and the unfavorable protonation of the carbonyl group of HMF lead to high activation free energies for aldol addition, which hinders the formation of humins and results in high HMF selectivity. The addition or presence of chloride ions stabilizes the transition states for both fructose and glucose dehydration but does not affect humin formation as C-C addition lacks hydrophilic, positively charged binding sites at the reactive centers. As a result, chloride ions act to enhance the conversion with little effect on the selectivity.