Clay-supported acidic ionic liquid as an efficient catalyst for conversion of carbohydrates to 5-hydroxymethylfurfural

Abstract

Halloysite nanoclay has been used as a natural support for the immobilization of an acidic ionic liquid with multiple acidic sites. To graft the ionic liquid on the halloysite, amino-functionalized halloysite was reacted with 2,4,6-trichloro-1,3,5-triazine and 2-aminopyrimidine successively to form heterocyclic ligand on halloysite, which was then converted to acidic ionic liquids through reaction with chlorosulfuric acid. The resultant catalyst was then characterized via SEM, EDS, and elemental mapping analysis, FTIR, XRD and TGA. Noteworthy, acid-base titration method confirmed that introduction of acidic ionic liquid significantly improved the Brønsted acidity of halloysite. The catalytic activity of the catalyst was investigated for conversion of various carbohydrates to 5-hydroxymethylfurfural. Gratifyingly, the results showed that the catalyst could promote the reaction of fructose, sucrose, glucose and galactose under mild reaction conditions (catalyst (0.03 g), at 85 °C in 70 min) to give the product in high to excellent yields. Computational DFT studies unveiled the reaction mechanism, pointing out the limiting steps kinetically, and in addition how important are the non-covalent interactions. Furthermore, the recycling tests confirmed high recyclability of the catalyst up to six runs.