Highly efficient separation of 5-hydroxymethylfurfural from imidazolium-based ionic liquids

Ionic liquids (ILs) have been widely studied as effective reaction media or catalysts for the conversion of renewable resources to 5-hydroxymethhylfurfural (5-HMF). However, the separation of 5-HMF from ILs remains a significant challenge. Thus, the aim of this work is to develop a useful guideline for the selection and design of greener solvents for efficient separation of 5-HMF from ILs by a simple solvent extraction approach. To this end, 9 kinds of low-toxic and medium boiling point organic solvents and 10 kinds of ILs typically used in biomass conversion have been evaluated for the separation of 5-HMF. The effects of the solvent hydrogen bond acceptor capacity, anion type and cationic structure of the ILs on the separation of 5-HMF from the ILs are systematically investigated and the molecular interactions among 5-HMF, ILs and the solvent contained in the mixtures have been revealed by FTIR and 1H NMR measurements. It has been found that besides a low mutual solubility with ionic liquids, an ideal solvent should have strong hydrogen bond acceptor properties. At the same time, the interactions between the anion and cation of the ILs and the hydrogen bonding interaction of the anion with 5-HMF are also very important for the separation of 5-HMF. A weak interaction of the anion with 5-HMF and a strong interaction between the anion and cation of the ILs all result in the high extraction efficiency. Under the optimized conditions, the extraction efficiency of 5-HMF from the ILs is up to 61.0%, which is a record-breaking value at the current time. This finding provides a useful guide for the selection of greener solvents for the separation of 5-HMF from ILs.