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.