Strategies for a practical and efficient use of sulfonic acid-functionalized ionic liquids as catalysts for 5-hydroxymethylfurfural production from carbohydrates

Abstract

The catalytic production of 5-hydroxymethylfurfural (5-HMF) from biomass-derived carbohydrates remains challenging, particularly in terms of efficiency and catalyst handling. Task-specific ionic liquids (TSILs) have emerged as promising acid catalysts for this process. In this work, the performance of TSILs used as both homogeneous and heterogeneous catalysts for carbohydrate dehydration to 5-HMF is systematically evaluated, and alternative strategies for improving their applicability at large scale are explored. Two TSILs based on the 1-(4-sulfonic acid)-butyl-3-methylimidazolium (bsmim) cation, with [Cl] and [HSO4] anions, were tested using fructose as a model substrate and more complex carbohydrates. Under homogeneous conditions in DMSO at relatively low temperature (353 K), bsmim[Cl] showed catalytic activity comparable to or higher than that of bsmim[HSO4], affording high 5-HMF yields (97% from fructose and starch, 88% from glucose, 78% from maltose). In addition, to facilitate catalyst recovery, both TSILs were immobilized in porous silica and evaluated as heterogeneous catalysts. The supported material bsmim@S[HSO4] maintained good activity in fructose dehydration (>85% yield) for up to three reaction cycles, whereas bsmim@S[Cl] showed much lower performance (<30%), likely due to differences in the TSIL immobilization caused by free acid encapsulated within the silica matrix during sol–gel synthesis. Additionally, to replace DMSO with a more easily separable solvent, an acetonitrile-based multiphase system was employed, enabling straightforward 5-HMF separation, with bsmim[HSO4] giving yields close to those obtained in DMSO. Overall, TSIL-based systems provide versatile and efficient catalytic platforms for 5-HMF production, whose performance can be tuned through anion selection, immobilization, and reaction medium design.