Novel halogen-free deep eutectic solvents for efficient extraction of phenolic compounds from real coal tar

Abstract

The separation of phenolic compounds from coal tar is conducive to the manufacture of high value-added phenolic products and the further processing of oil. Herein, three novel halogen-free choline dihydrogen citrate (CDC)-alcohols deep eutectic solvents (DESs) were prepared to extract phenolic compounds from real coal tar. The effects of the critical parameters including components and constituent mole ratio of DESs, mass ratio of DESs to coal tar, temperature, and time on extraction efficiency were systematically investigated. It was demonstrated that the extraction efficiency decreased with the extension of alkyl chain of alcohols. Hence, CDC-ethylene glycol (EG) was selected as the appropriate extractant, and the extraction efficiency reached 89.3% under the optimal conditions. Fourier transform infrared spectra revealed that the hydrogen bond interaction between CDC and EG was the main reason for their formation of DES. Phenolic compounds also had hydrogen bond interactions with DES, which was the main driving force for the separation of phenolic compounds from coal tar by DES. Differential scanning calorimeter results displayed that the CDC-EG had no definite melting point, and its glass transition temperature was significantly lower than the melting point of individual components. The CDC was recovered by using ethyl acetate as the back extraction agent. Moreover, to avoid back extraction operation, phenolic resin gel was prepared by one-pot method, which was expected to be suitable candidate as the precursor for functional carbon materials.