Molecular Origin for the Difficulty in Separation of 5-Hydroxymethylfurfural from Imidazolium Based Ionic Liquids

Ionic liquids (ILs) have shown superior performance in the conversion of biomass to 5-hydroxymethylfurfural (5-HMF) as reaction medium and/or catalyst, which is a green platform compound with a wide range of applications in manufacturing fine chemicals and biofuels. Nevertheless, the separation of 5-HMF from ILs is very difficult and becomes a technical bottleneck for IL application in the preparation of 5-HMF. To resolve this problem, understanding the interactions between ILs and 5-HMF is essential. In this work, attenuated total reflectance Fourier transform infrared, 1H nuclear magnetic resonance, and quantum chemistry calculations were combined to investigate the interaction between 5-HMF and each of the eight ILs over the whole composition range. The studied ILs have the same 1-butyl-3-methylimidazolium cation [C4mim]+ but different anions. It was found that interactions between the ILs and 5-HMF were mainly ascribed to the strong hydrogen bonds of 5-HMF with anions of the ILs, and the formation abilities of hydrogen bonds of the anions with O–H group of 5-HMF were found to decrease in the order [CH3COO]−, [C2H5COO]− > [HSO4]− > [CF3COO]− > [N(CN)2]− > [NO3]− > [CH3OSO3]− > [BF4]−. These results suggest that the anions with stronger hydrogen bond accepting ability have stronger interaction with 5-HMF and the separation of 5-HMF from the ILs is mainly governed by the hydrogen bonding interactions between anion of the ILs and 5-HMF. In addition, partition coefficients of 5-HMF between 1,4-dioxane and the ILs phases were determined experimentally to support the conclusion.