Separation of phenolic compounds from coal liquefaction oil by choline chloride-glycerol deep eutectic solvents

Abstract

Conventional extraction techniques for phenolic compounds are always associated with large amount of wastewater production or high energy consumption. To establish an environmentally benign method for the phenolic compounds separation from coal liquefaction oil, deep eutectic solvents (DES) as a green alternative to traditional solvent have been considered. Initially, four kinds of choline chloride (ChCl)-based DES containing carboxylic acid, alcohol amine, polyol, or glycol as a hydrogen bond donor (HBD) were screened for the proposed extraction. The FT-IR spectra demonstrated that it is hydrogen bonding between phenols and DESs that promotes the extraction process to achieve superior phenols removal efficiency. Among the investigated DESs, the ChCl-glycerol (1:1, molar ratio) eutectic mixture (DES-1) exhibited the best extractive performance, showing high phenols removal efficiency and low neutral oil entrainment. Then the extraction of phenolic compounds by this DES at various mole ratios of DES to phenols and the extractive temperature ranged from 25 to 50 °C were investigated. In an optimal condition, the phenols removal efficiency can be as high as 98.5%, and the neutral oil entrainment can be as low as 3.4%. Moreover, the extracted phenolic compounds can be easily back-extraction and the good property of high phenols removal efficiency remained during four consecutives extraction/back-extraction cycles. Last, the extraction of phenolic compounds from real coal liquefaction oil using the DES was also explored, which obtained better extractive performances compared to that of single ChCl or glycerol aqueous solution extraction. The knowledge acquired in this study will present a new approach to remove phenolic compounds from coal-based liquid oil.