Article pubs.acs.org/jced
Solubility of Benzothiazolium Ionic Liquids in Water and in Furfural Chunmei Jia,† Yu Cao,‡ Tao Zuo,† Rong Hu,† Tian Yao,† and Hang Song*,† †
School of Chemical Engineering, Sichuan University, Chengdu 610065, P. R. China School of Life Science and Technology, Mianyang Normal University, Mianyang 621000, P. R. China
‡
S Supporting Information *
ABSTRACT: The solubility of benzothiazolium ionic liquids in water and in furfural was determined by static analytical and dynamic methods, respectively. The solid−liquid equilibrium data were correlated by using the nonrandom two-liquid (NRTL), Wilson, modified Apelblat, and λh models, respectively. Among the models, the modified Apelblat gave the best correlation results with the average relative deviation (ARD) of 2.02%. The effect of alkyl chain length on the solubility was discussed. The (liquid + liquid) phase equilibrium was observed in the system of benzothiazolium ionic liquids and water below the melting temperature of the ionic liquid. The solubility of benzothiazolium ionic liquids in furfural was much higher than in water, indicating the potential of application in extraction of furfural from aqueous solution.
1. INTRODUCTION Furfural is a versatile and key derivative produced from agricultural raw materials, which is rich in pentosan, e.g., corncobs, oat hulls, bagasse, etc.1 Furfural has drawn lots of attention as a rich source of derivatives such as 2-methylfuran (MF), dimethylfuran (DMF), methyltetrahydrofuran (MTHF), and so forth.2 With the development of biochemicals and biofuel, the demand for furfural as a major intermediate continues to increase.3 Importantly, the potential of the furfural platform relies heavily on the cost-competitive production of furfural from lignocellulosic feedstock. In conventional production process, furfural is recovered and purified by vapor distillation from very diluted aqueous furfural feed. The distillation costs high energy, and the operation is complicated with the existence of heterogeneous azeotrope.2 In terms of higher efficiency of furfural recovery, organic solvent extraction,4−6 supercritical CO2 extraction,7 polymer sorbents adsorption,8 membrane separation,9,10 biphasic hydrolysis system,11,12 nitrogen stripping,13 and ionic liquid (IL) extraction14 have been applied. However, most of these methods stop on benchtop, due to some considerations such as the costs of separation medium, equipment and the environmental protection. Thus, more efficient and less costly processes for the recovery of furfural need to be developed. It was demonstrated that some benzothiazolium ILs were characterized by sensitive solubility to temperature variation and so can be easily separated from organic substance near at room temperature.15,16 A novel recovery of furfural was proposed with the application of the temperature-sensitive ILs in our recent studies.17 In the recovery, furfural was concentrated by liquid IL−furfural aqueous solution extraction at certain temperature. After two liquid phase separation, the IL precipitated in the furfural-rich phase during cooling down and was separated by solid−liquid filtration. Benzothiazolium © XXXX American Chemical Society
ILs revealed potential in furfural separation. Obviously, the solubility of the benzothiazolium ILs in water and in furfural is essential for the residual of benzothiazolium IL in water (raffinate) and in furfural (product). Research on the relationship of chemical structure and solubility is also important for the design of new ILs with a better performance.
2. EXPERIMENTAL SECTION 2.1. Materials. The ILs presented in Figure 1 were synthesized ([C4Bth]PF6: N-butyl-benzothiazolium hexafluorophosphate; [C5Bth][PF6]: N-pentyl-benzothiazolium hexafluorophosphate; [C6Bth][PF6]: N-hexyl-benzothiazolium hexafluorophosphate; [C7Bth][PF6]: N-heptyl-benzothiazolium hexafluorophosphate; [C8Bth][PF6]: N-octyl-benzothiazolium hexafluorophosphate). Reagents including benzothiazole, n-butyl bromide, n-pentyl bromide, n-hexane bromide, n-heptyl bromide, n-octyl bromide, and potassium hexafluorophosphate (KPF6) were of analytical grade and purchased from Hanhong Chemical Technology Co. Ltd. (Shanghai, China). Furfural provided by Kelong Chemical Reagents Factory (Chengdu, China) was of analytical grade and distillated under vacuum before use. The water was purified by an ultra pure water system (Youpu Ultrapure Technology Co., Ltd., Chengdu, China). The synthetic routes of [C4Bth]PF6, [C5Bth]PF6, and [C6Bth]PF6 were elaborated in the literature.16 [C7Bth]PF6 and [C8Bth]PF6 were synthesized in a similar way, in which the temperature of reaction were 120 and 130 °C, respectively. All the IL structures were confirmed by 1H-NMR or 13C-NMR and FT-IR. The purity Received: August 15, 2014 Accepted: February 12, 2015
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DOI: 10.1021/je500762n J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Journal of Chemical & Engineering Data
Article
Figure 1. Structures of benzothiazolium ILs with different lengths of carbon chains.
prior to sampling. The achievement of equilibrium was verified by repetitive measurements after 2 or more hours. When equilibrium was reached, upper clear solution was withdrawn and measured with electronic balance (type ESJ200-4A, Longteng Electronics Ltd., Shenyang, China) with an accuracy of ± 0.0001 g. The IL concentration in the sample was quantified using UV afterward.19 All measurements were run in triplicate. 2.2.2. Differential Scanning Calorimetry (DSC) Measurement. The thermodynamic parameters of the ILs, i.e., temperature of fusion (Tfus), enthalpy of fusion (ΔfusH), and change of heat capacity at melting temperature, ΔCp, were measured using DSC. Applied scan rate was 10 K·min−1 with a recorder sensitivity of 0.2 μW. The apparatus (Thermal Analysis Q200, USA with Refrigerated Cooling System 90) was calibrated with a 0.999999 mole fraction purity indium sample.
of the ILs was evaluated by HPLC (LC-20A, Shimadzu Corporation, Japan). [C7Bth][PF6]: 1H NMR (600 MHz, DMSO-d6) δppm: 0.85 (t, J = 7.0 Hz, 3H, −CH 3 ), 1.18−1.40 (m, 8H, −CH2CH2CH2CH2), 1.90−2.01 (m, 2H, −CH2), 4.84 (t, J = 7.5 Hz, 2H, N−CH2), 7.84−7.90 (m, 1H, Ar−H), 7.95 (ddd, J = 8.5, 7.2, 1.1 Hz, 1H, Ar−H), 8.43 (d, J = 8.5 Hz, 1H, Ar−H), 8.52 (dd, J = 8.3, 0.8 Hz, 1H, Ar−H), 10.60 (s, 1H, S−CH−N). 13 C NMR (150 MHz, DMSO-d6) δppm: 14.35, 22.41, 26.13, 28.53, 29.02, 31.49, 52.93, 117.66, 125.74, 128.95, 130.09, 140.65, 132.09, 164.76. FT-IR: υmax(KBr): 3461, 3109 (ν CH− H), 2956 (νas CH3−H), 2930 (νas CH2−H), 2860 (νs CH3− H), 1583 (Ar), 1515 (Ar), 1465 (δ CH2−H), 1434 (Ar), 1282 (ν C−N), 1087 (ν C−S), 833 (ν P−F), 769 (δ Ar−H), 557. [C8Bth][PF6]: 1H NMR (600 MHz, DMSO-d6) δppm: 0.85 (t, J = 7.0 Hz, 3H, -CH 3 ), 1.20−1.41 (m, 10H, −CH2CH2CH2CH2CH2), 1.89−2.00 (m, 2H, −CH2), 4.84 (t, J = 7.5 Hz, 2H, N−CH2), 7.84−7.90 (m, 1H, Ar−H), 7.95 (ddd, J = 8.6, 7.2, 1.2 Hz, 1H, Ar−H), 8.43 (d, J = 8.5 Hz, 1H, Ar−H), 8.52 (d, J = 8.2 Hz, 1H, Ar−H), 10.60 (s, 1H, S−CH− N). 13C NMR (150 MHz, DMSO-d6) δppm: 14.38, 22.49, 26.16, 28.82, 28.91, 29.00, 31.58, 52.94, 117.67, 125.74, 128.94, 130.08, 132.09, 140.65, 164.77. FT-IR: υmax(KBr): 3459, 3108 (ν CH−H), 2957 (νas CH3−H), 2930 (νas CH2−H), 2859 (νs CH3−H), 1583 (Ar), 1515 (Ar), 1464 (δCH2−H), 1434 (Ar), 1282(ν C−N), 1087 (ν C−S), 833 (ν P−F), 767 (δ Ar− H), 557. 1 H NMR, 13C NMR, and FTIR spectra of [C7Bth][PF6] and [C8Bth][PF6] are provided as Figures 1S to 6S in the Supporting Information. 2.2. Experimental Operation. 2.2.1. Determination of Solubility of Benzothiazolium ILs in Furfural. A dynamic method for measuring the solubility of ILs in furfural was used in the present work. The (IL + furfural) mixture was prepared by weighing the pure components with an accuracy of ± 0.0001 g (type ESJ200-4A, Longteng Electronics Ltd., Shenyang, China); errors did not exceed 5·10−4 in mole fraction. The sample was heated very slowly (
