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, Ili 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 Its and 5HMF were mainly ascribed to the strong hydrogen bonds of 5-HMF with anions of the Its, and the formation abilities of hydrogen bonds of the anions with 0-H group of 5-HMF were found to decrease in the order [CH(3)C00](-), [C(2)H(5)C00](-) > [HSO4](-) > [CF(3)C00](-) > [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 5I -IMF between 1,4-dioxane and the ILs phases were determined experimentally to support the conclusion.