Sustainable One-Pot Synthesis of 5-(Hydroxymethyl)furfural from C6-Sugars by Enhanced H+ Exchange Heterogeneous Catalysis

This study focuses on the characterization of NaCl-induced enhancement of H+ exchange catalysis and preparation of the renewable C6 platform chemical, 5-(hydroxymethyl)furfural (5-HMF), through dehydration of fructose and glucose, C6-sugars, utilizing a heterogeneous catalyst. The effect of a catalytic amount of NaCl was explored to enhance the H+ exchange of cation exchangers, characterized on the distortion of tetrahedral sulfonates due to the formation of direct ionic bonds with Na+, as observed by FT-IR. Dehydration of high concentrations of C6-sugars (24%, w/v) was carried out as a one-pot multistep reaction to synthesize 5-HMF in DMSO/MIBK (0.35/0.65) at temperatures ranging from 110 to 120 °C under ambient pressure. Cation exchangers (hydrogen form) were employed as catalysts in amounts ranging from 2.5% to 10% (w/w). The addition of NaCl resulted in a decrease in the pH of both water and organic solvents, leading to improved H+ exchange on cation exchangers. Consequently, this facilitated a reduction in catalyst quantity and significantly enhanced the dehydration of fructose and glucose to 5-HMF. At 110 °C, fructose achieved a 99% conversion rate with an 81% 5-HMF yield within 1 h, while at 120 °C, glucose conversion reached 92.2% with a 36.1% 5-HMF yield over 11 h. Furthermore, the catalyst exhibited consistent activity across at least five consecutive fructose dehydration batches. The NaCl-induced enhancement of H+ exchange catalysis was first proposed and successfully demonstrated for the dehydration of C6-sugars in the organic phase. This concept can be extended to various organic synthesis reactions using cation exchangers in both aqueous and organic systems, with the aim of reducing the catalyst amount and facilitating the reaction rate.