Separation Of Phenol From Oils With Quaternary Ammonium Salts And Separation Mechanism

Phenols, such as phenol, cresol, dimethyl phenol and so on, are one of the major industrial organic chemicals and chemical intermediates, which are currently used in processes to produce synthetic fibers, engineering plastics, phenolic resins, caprolactam, alkylphenols, pesticides, pharmaceuticals, explosives, plasticizer, preservatives, spices, dye intermediates and other products. Phenolic compounds are mainly derived from coal liquefaction oil and coal tar via pyrolysis as documented in recent reports. Up to now, the phenols could be separated from liquefaction oil and coal tar by alkaline solutions in industry. The method would produce plenty of water with phenols and require a great deal of inorganic alkalis. Consequently, it is of great significance to search a process to-extract phenols from real oils without the use of alkalis.In our previous work, we demonstrated that phenols in oil mixtures could be separated by ChCl by forming DES. When DES method is employed for separations of phenols from oils, it is very important to understand the properties of DES phases for their significant influence on mass transfer, phase separation efficiency, and so forth. However, data on the DES phase properties during separations of phenols from oils have not been reported in the literature. Therefore, this work focused on the synthesis of DES based on three different hydrogen-bond donors (HBD), namely phenol, o-cresol and2,3-xylenol, and choline chloride (ChCl). The melting temperature and glass transition temperatures have been measured. Compared with an ideal mixture of the two components, the freezing temperature of the DES depresses greatly from (120to127) K. The physical properties, such as density, viscosity and conductivity, of phenol-based and o-cresol-based DES were determined at atmospheric pressure and temperatures from (293.2to318.2) K at an interval of5K. The results show that the type of HBD, the mole ratio of HBD to ChCl, and temperature have great influences on the physical properties of DES. Densities and viscosities of DES formed by phenol and ChCl decrease with increases of temperature and phenol content. The conductivities of the DES are from (1.40to7.06) mS·cm-1, similar to that of room temperature ionic liquids. The conductivities of the DES increase with an increase of temperature, and reach highest values at phenol to ChCl mole ratios of4.00to5.00. The temperature dependence of densities and conductivities for these DES was correlated by an empirical second-order polynomial with relative deviations less than0.91%, and the viscosities were fitted to the VTF equation with relative deviations less than0.52%.However, the phenol-removal efficiency for ChCl was relatively low. More importantly, the mechanism of quaternary ammonium salt separation and the effect of the structure of quaternary ammonium salts on the removal efficiency remain unclear. It is necessary to study the mechanism of DES separation in order to design a suitable quaternary ammonium salt. In this work, a variety of quaternary ammonium salts including ammonium chloride (NH4Cl), tetramethylammonium chloride (TMAC), tetraethylammonium chloride (TEAC), tetrapropylammonium chloride (TPAC), tetrabutylammonium chloride (TBAC), methyltriethyl-ammonium chloride (MTAC), tetramethylammonium bromide (TMAB), tetraethylammonium bromide (TEAB) and choline bromide (ChBr) were used to separate phenols from model oils.The results show that quaternary ammonium salts that are composed of cations with appropriate chain lengths as well as high symmetry and anions with higher electronegativity were conducive to separate phenols from oil mixtures. A maximum removal efficiency of phenol was attained by TEAC at a mole ratio of0.8:1(TEAC:phenol), with99.9%removal. Moreover, the extraction equilibrium is very fast and the extraction separation efficiency is not sensitive to temperature. Fourier-transform infrared spectroscopies of DES based on the phenol and quaternary ammonium salts suggested the existing of hydrogen-bondings between phenol and quaternary ammonium salts when the DES were formed. TMAC can separate efficiently phenols, especially for the low-level phenols from real oils.