Separation of the mixture pyridine + methylbenzene via several acidic ionic liquids: Phase equilibrium measurement and correlation
Introduction
Nitrogenous compounds (N-compounds) is a kind of greatly considerable compounds. Usually, the N-compounds can be categorized as basic N-compounds, pyridine and quinoline, or neutral N-compounds, indole and carbazole [1], [2], [3]. Pyridine and quinoline have extensive applications in the synthesis of medicine, dyestuffs, seasonings, and can be used as solvents or preservatives. Considering the widely applications, N-compounds are of great demand in the chemical industry.
Generally, basic N-compounds such as pyridine and quinoline can be separated from coal tar, coal liquefied oil, or coking gas, which are the main source of the basic N-compounds in the industry. And numerous non-catalytic processes have already been adopted for the separation of N-compounds from coal tar, such as alkali fusion [4], sulfuric acid washing, solvent extraction [5], [6], ion-exchange resin separation [7], [8], and liquid-liquid extraction by volatile carboxylic acid [9]. Though the above methods have already been accepted, there still exist some drawbacks or limits because some processes discharge a large amount of waste water, the equipment is corroded, or the selectivity is lower. Thus, more efficient and environmental friendly methods are needed for the separation of the basic N-compounds.
Liquid-liquid extraction has been adopted in many separation fields under mild conditions and atmospheric pressure. For the purposes of developing the extraction process of N-compounds or phenolic compounds from the middle distillate feedstock or coal pyrolysis liquors, the liquid-liquid equilibrium data and excess properties have been reported by Hwang [3], [10], [11] and Venter [12]. Meanwhile, Qi [9] selected carboxylic acid as the extraction solvent for the separation of N-compounds through π-complexation. Since the organic solvents used are volatile, and the selectivity and efficiency are low, the ionic liquids (ILs) are applied in a variety of fields [13] due to their non-volatility, good thermostability, strong dissolving capacity and high selectivity. Large amounts of ILs have already been studied for the extraction processes in details. For example, some ILs have been selected as alternative solvents to separate aromatic hydrocarbons from aliphatic hydrocarbon mixtures [2], and applied in some green chemical processes [14], [15], [16], [17], [18], [19], [20], [21], [22] or used for the extractive deep desulfurization or denitrogenation of gasoline and diesel oil [23], [24], [25], [26]. High efficiency and selectivity were presented in the above researchers' work, in which Jiao [17] developed the imidazolium-based ILs as extraction agents to separate indole from wash oil with extraction efficiencies of more than 90%, Asumana [24] selected dicyanamide-based ILs for the denitrogenation of fuel oils with the extraction efficiencies from 60% to 96%, and Anugwom [25] used [C2mim][Cl] to extract the model oil containing pyridine which the extraction capacity of up to 90 wt% was achieved.
Recently, the applications of the different ILs for the separation of basic N-compounds have been reported. Wang [27] reported that H2PO4-based ILs presented high selectivity to quinoline from the simulated oil and verified that the acidity of the selected ILs contributed to the denitrification. Serban [28] selected [Bmim][HSO4], [Bmim][CH3SO4] and [Emim][EtSO4] as the denitrogenation solvents which were verified with high selectivity. Chen [29] reported that some imidazolium-based acidic ILs, such as Lewis acidic [Bmim][Cl/nZnCl2] (n = 1, 2) and Brφnsted acidic ILs [Bmim][HSO4] and [Hmim][HSO4], were capable of extracting basic N-compounds. But only few researches have reported the phase equilibrium in details. Domańska [30] investigated the separation process of pyridine from heptane by tricyanomethanide-based ILs, in which the liquid-liquid equilibrium (LLE) data were determined and correlated by NRTL model. Kȩdra-Królik [31] investigated the possible application of imidazolium-based ILs with thiocyanate, methylphosphonate anions as solvents for N-compound + aliphatic hydrocarbon separation, which is frequently encountered in petroleum industry. Królikowska [32] investigated quinolinium and iso-quinolinium based ILs, which the LLE data of [HiQuin][SCN], [OiQuin][SCN], [HiQuin][NTf2], or [OQuin][NTf2] + pyridine + heptane at 298.15 K were measured and correlated by the NRTL model. Ravilla and Banerjee [2] selected [Emim][MeSO3], [Emim][EthSO4], and [Emim][Ac] as green solvents for the separation of pyridine from n-pentane/isooctane, and the LLE data were also well correlated by the NRTL and UNIQUAC models.
In the present work, three acidic imidazolium-based ILs, 1-butyl-3-methylimidazolium hydrogen sulfate, [Bmim][HSO4], 1-butyl-3-methylimidazolium dihydrogen phosphate, [Bmim][H2PO4], and 1-butyl-3-methylimidazolium perchlorate, [Bmim][ClO4], were selected as the solvents to separate basic N-compound pyridine. The structures of the selected ILs are shown in Table 1. The liquid-liquid equilibria data of the three ILs + pyridine + methylbenzene were measured at 298.15 k under 101.3 kPa. The interaction energies between the ILs, and pyridine and methylbenzene respectively were calculated to verify the extraction ability. Moreover, the distribution ratio and extraction selectivity were calculated from the experimental data and discussed in detail. The experimental LLE data were correlated by the NRTL and UNIQUAC activity coefficient models, and the binary interaction parameters were regressed. And those obtained parameters are of importance for the optimization and design of the extraction process.
Section snippets
Chemicals and materials
The chemicals used in this work were pyridine, methylbenzene, [Bmim][HSO4], [Bmim][H2PO4], and [Bmim][ClO4]. All chemicals used in this work were commercially obtained with analytical pure reagents grade. Pyridine, with the purity of 99.5% (mass fraction), was supplied by Tianjin Fuyu Fine Chemical Co., Ltd.. Methylbenzene, with the purity of 99.5% (mass fraction), was supplied by Tianjin Kemiou Chemical Reagent Co., Ltd.. The purities of those two reagents were checked and confirmed by gas
Thermodynamic modeling
The NRTL [35] and UNIQUAC [36] thermodynamic models were adopted to correlate the measured LLE data, which were presented and used in the ternary systems including ILs [2], [30], [31], [32], [33], [37], [38], [39], [40], [41]. Meanwhile, the molecular volume structure parameter r and the molecular surface area parameters q, q' of ILs in UNIQUAC model must be calculated before the correlation. For the ILs, r and q were calculated by the polarizable continuum model (PCM) [42] after optimizing the
Experimental tie-line data
The experimental tie-line data for the three ternary systems ILs (1) + pyridine (2) + methylbenzene (3) were measured at 298.15 K under 101.3 kPa. The experimental tie-lines for the determined ternary systems are all expressed in mole fraction and listed in Table 4. The subscript 1, 2 and 3 represent the selected ILs, pyridine and methylbenzene; the superscript Ⅰ and Ⅱ refer to the organic phase and the ILs phase, respectively. Meanwhile, the corresponding triangular diagrams with the measured
Conclusion
In order to select the potential extraction solvents for pyridine separation from model coal tar, the LLE tie-line data of [Bmim][HSO4] (1) + pyridine (2) + methylbenzene (3), [Bmim][H2PO4] (1) + pyridine (2) + methylbenzene (3) and [Bmim][ClO4] (1) + pyridine (2) + methylbenzene (3) were measured at 298.15 K under 101.3 kPa. The ternary phase diagrams of the three systems were classified as Treybal's type 1 phase diagrams. The distribution coefficient and selectivity were calculated, and the
Acknowledgement
The authors are grateful to the support of National Natural Science Foundation of China [Grant NO. 21306093] and Shenzhen Supercomputer Center (DMol3 and Reflex modules of Materials Studio 7.0).
The authors also thank Prof. Jianfu Song (Shandong university of science and technology) for English revision.
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