Separation Of Phenols From Coal Tar Oil Via Forming Deep Eutectic Solvents

Phenolic compounds, which are important chemical raw materials and intermediates, are widely used in the synthesis of fiber, plastics, pesticide, pharmaceuticals, spices, dyes and other production areas. Phenolic compounds are abundant in coal tar oil, so it is valuable to extract phenolic compounds from coal tar oil. At present, the relatively mature industrial method to separate phenols from coal tar oil is alkaline washing method. However, a large amount of acid and alkali are consumed in the whole process, and it produces a lot of wastewater containing phenols which needs to be further disposed. It is necessary to find a new non-aqueous phase separation method in order to solve the above disadvantages.In the previous work of our group, we found that phenols in oils could be separated by quaternary ammonium salts via forming deep eutectic solvents (DESs). This method is of high extraction efficiency, extraction solvents can be recycled, and it avoids the consumption of inorganic acid and alkali and the generation of wastewater containing phenols.At present, the DES method has the following disadvantages, the stripping agent ether is volatile and explosive, deep eutectic solvents have entrainment of neutral oil, how different phenols are extracted during the separation of phenols from coal tar oil via forming deep eutectic solvents. This article focuses on selection of suitable stripping agents, entertainment and desorption of neutral oil, and variation of phenols and material conservation during the separation of phenols from coal tar oil via forming deep eutectic solvents. The aim of this work provides theoretical support for industrial application of the DES method to separate oil-phenol mixtures. The main research work and conclusions are shown as follows.(1) Ethers, esters, ketones and other common solvents were chosen as stripping agents in stripping experiments separately, and they were qualitatively checked whether they could be used as stripping agent. Then the appropriate stripping agents were researched in detailed. Finally ethyl acetate was selected as a suitable stripping agent, and the effects of temperature and water on the solubility of different quaternary ammonium salts in the suitable stripping agent with different concentrations of phenol were investigated. The results show that when phenol concentration is fixed, the solubility of choline chloride (ChCl) in ethyl acetate increases as the temperature increases. At room temperature, the minimum dosage of ethyl acetate should guarantee that the phenol concentration in ethyl acetate is lower than 40 g·L-1. When phenol concentration is fixed, the solubility of tetramethyl ammonium chloride (TMAC) is larger than that of ChCl in ethyl acetate, which results from that TMAC can form stronger hydrogen bonding with phenol than ChCl. The presence of water is conducive to dissolution of ChCl in ethyl acetate, which results from that water can also form DES with ChCl.(2) In this work, the effects of added ChCl amount, extraction temperature and concentration of aliphatic hydrocarbons in model oils on the separation of phenols and oil entertainment in DES phase were investigated. Toluene solution with 200 g·L-1 phenol was used as model oil and ChCl was used as extraction agent. The results showed that the separation efficiency of phenol from the model oil increased with the increase of addition of ChCl and the increase of added ChCl amount could also reduce the entrainment of toluene in DES phase. The effect of temperature on the separation of phenol was not significant. The entrainment of toluene increased with the increase of temperature at low ratios of added ChCl to phenol, while it did not change significantly at high ratios of added ChCl to phenol. The entrainment of toluene was reduced with the increase of the concentration of aliphatic hydrocarbon in the model oil. With ChCl as extraction agent, typically, the separation efficiency of phenol was 95% and the entrainment of toluene was 15% at 25℃ and mole ratios of ChCl to phenol of more than 0.6. Toluene could be removed completely within 60 min with nitrogen purging at 70℃.(3) Finally, ChCl and TMAC used as extraction agent, ethyl acetate as extraction agent, we separated phenolic compounds from coal tar oil and circulated the experiment for three times with reused extraction agent and stripping agent. The result shows that TMAC has higher extraction ability than ChCl, but the entertainment of neutral oil in TMAC based DES phase is higher and stripping agent is more difficult to remove. The kinds of chemical compounds of neutral oil in product phenols using ChCl as extractant are less that using TMAC as extractant, mainly naphthalene. TMAC-extracted phenols contains many other kinds of neutral oil besides naphthalene. The extraction rate can reach 80% by using quaternary ammonium salt to extract phenols from coal tar oil, which is lower than the extraction rate of phenol from model oils. It results from the π-π bond between phenols and aromatic compounds of neutral oil in coal tar. The properties of the quaternary ammonium salts have no obvious change before and after 3 cyclic experiments and the quaternary ammonium salts can be further recycled and reused.