Ternary phase behavior of phenol + toluene + zwitterionic alkaloids for separating phenols from oil mixtures via forming deep eutectic solvents
Introduction
Phenols are one of the major industrial organic chemicals and chemical intermediates, which are currently used to produce synthetic fibers, engineering plastics, dye intermediates phenolic resins and other products [1]. They are mainly derived from coal liquefaction oil, coal tar, petroleum and biomass pyrolysis oil. For instance, low-temperature pyrolized, coal tar oil typically consists of 20–30% of phenols [2], [3], [4]. The separation of phenols becomes increasingly necessary for the deep processing of the oil mixtures. The present industrial method for separating phenols from oil mixtures uses the principle of acid-base neutralization. In the method, strong alkaline aqueous solutions (such as NaOH) react with phenols to form phenolate, then mineral acid (such as H2SO4) is used to acidify the phenolate to recover phenols [5], [6], [7]. The disadvantages of the method include the uses of large amounts of both alkalis and acid aqueous solutions, the production of a large quantity of wastewater containing phenols and serious corrosion to the equipment due to the use of strong alkaline and acid. Therefore, we seek to find an environmentally friendly method to separate phenols from oil mixtures.
In recent years, deep eutectic solvents (DESs) have drawn much attention as a type of environmental solvents. DESs are formed by hydrogen bond acceptors (HBAs, such as choline chloride) and hydrogen bond donors (HBD, such as phenols, acids, alcohols) [8], [9]. DESs have widespread applications in a variety of areas, such as spanning electrochemistry [10], biocatalysis [11], material chemistry [12], purification of biodiesel [13]. Our group have found that phenols could be efficiently separated from oil mixtures by quaternary ammonium salts via forming DESs [14], [15] with high selectivity and distribution coefficients [16], which avoids the problems mentioned above in the traditional method, promising for industrial application.
In this work, we found that two kinds of efficient, biodegradable, environmentally friendly and halogen free extractants, zwitterionic alkaloids, betaine (Bet) and L-carnitine (L-car), could form DESs with phenol from oils. Their molecular structures are shown in Scheme 1. The result indicates that it is feasible to use zwitterionic alkaloids to separate phenol from oil.
It is well known that phase equilibria is very important for designing separation processes as well as their simulation and optimization. We measured the phase equilibria of separation of phenols from model oil with betaine and L-carnitine via forming DESs at temperatures from 25.0 °C to 65.0 °C. The phase behavior diagrams of the ternary systems and distribution coefficient (D) and selectivity coefficient (S) of phenol were also studied.
Section snippets
Chemical materials
The chemicals used in this work include phenol, toluene, o-nitrotoluene, betaine and L-carnitine. Their information is shown in Table 1. All the chemicals were analytical reagent grade and used without further purification.
Experimental procedures
In this experiment, the mixtures of phenol and toluene were used to model oils. The content of phenol in the model oil is 0.2140 in mass fraction. The experiment procedure was performed as follows. Taking L-carnitine as an example, L-carnitine was added into model oils with
LLE data for toluene + phenol + zwitterionic alkaloids systems
Liquid-liquid equilibria (LLE) of two different systems (toluene + phenol + L-carnitine and toluene + phenol + betaine) were measured at temperatures of 25.0 °C, 45.0 °C and 65.0 °C under atmospheric pressure. The LLE data of two systems are listed in Table 2, Table 3.
When the mole ratio of L-carnitine to phenol is increased to 0.15 and more, the content of phenol in oil phase decreases quickly and a DES is formed by phenol and L-carnitine, which is the lower phase. Such as phenol content in
Conclusions
In this work, phase equilibria of toluene + phenol + L-carnitine ternary system and toluene + phenol + betaine ternary system at temperatures of 25.0 °C, 45.0 °C and 65.0 °C were measured. The phase behaviors of the two ternary systems were illustrated and analysed. The effect of zwitterionic alkaloids and temperature were studied. The results show that the two kinds of zwitterionic alkaloids have similar phase behavior and there are three kinds of phase regions: a liquid phase, liquid-liquid
Acknowledgements
We thank Professors Zhenyu Liu and Qingya Liu for their help. This work is financially supported by the National Basic Research Program of China (2011CB201303), Specialized Research Fund for the Doctoral Program of Higher Education (20120010110005) and the long-term subsidy mechanism from the Ministry of Finance and the Ministry of Education of PRC (BUCT).
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