Mechanism of 1-(1-propylsulfonic)-3-methylimidazolium chloride catalyzed transformation of d-glucose to 5-hydroxymethylfurfural in DMSO: an NMR study

Abstract

The conversion of d-glucose to 5-hydroxymethylfurfural (HMF) in the presence of 5.48mol% 1-(1-propylsulfonic)-3-methylimidazolium chloride acidic ionic liquid catalyst in DMSO at 150°C was studied using 1H, 13C NMR, and visible spectroscopy. The HMF yield rapidly increases in the first 100min of reaction, however yield drops beyond 100min and levels off to a maximum yield of about 15.7% around 600min. The visible spectroscopy study of the reaction mixture suggests that rate of HMF formation slows down after 100min due to increase in the rate of humin formation after first 100min. A mechanism has been proposed and key intermediates in the pathway could be identified by studying the 13C NMR spectra of acidic ionic liquid catalyzed transformations of C-1 and C-2 13C labeled d-glucose under identical conditions. The proposed mechanism involves the isomerization of d-glucose to d-fructose via the complexation of the open chain sugar with the imidazolium cation of the acidic ionic liquid catalyst.