Separation of sulfur compounds from alkanes with 1-alkylcyanopyridinium-based ionic liquids

J. Chem. Thermodynamics 69 (2014) 27–35

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Separation of sulfur compounds from alkanes with 1-alkylcyanopyridinium-based ionic liquids Urszula Doman´ska a,b,⇑, Klaudia Walczak a, Maciej Zawadzki a a b

Department of Physical Chemistry, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland Thermodynamic Research Unit, School of Engineering, University of KwaZulu-Natal, Howard College Campus, King George V Avenue, Durban 4041, South Africa

a r t i c l e

i n f o

Article history: Received 5 August 2013 Received in revised form 19 September 2013 Accepted 22 September 2013 Available online 5 October 2013 Keywords: Ionic liquids 1-Butyl-3-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide 1-Butyl-4-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide 1-Hexyl-4-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide Ternary (liquid + liquid) phase diagrams Selectivity Solute distribution ratio NRTL correlation

a b s t r a c t In this work, we studied the applicability of new 1-alkyl-cyanopyridinium-based ionic liquids (ILs) in the extraction of aromatic-sulfur compounds from aliphatic mixtures. The synthesis of new 1-alkyl-cyanopyridinium-based ionic liquids is presented. Experimental data for (liquid + liquid) equilibrium (LLE) were obtained for various binary and ternary mixtures of {IL (1) + thiophene, or benzothiophene (2) + heptane (3)} at T = 308.15 K and ambient pressure. Three ILs have been studied: 1-butyl-3-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide [BCN3Py][NTf2], 1-butyl-4-cyanopyridinium bis{(trifluoro4 methyl)sulfonyl}imide [BCN Py][NTf2] and 1-hexyl-4-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide, [HCN4Py][NTf2]. One of the IL showed excellent results in terms of sulfur compounds selectivity and solute distribution ratio compared to what is currently used in industry. Chromatography analysis showed that the IL used as an entrainer was not present in the alkane layer. This eliminates the step needed for the separation of the solvent. A comparative study of the separation effectiveness of [BCN4Py][NTf2] in this work in comparison with other reported ILs was performed for choosing the best solvent for the intended separation. Density and viscosity of [BCN4Py][NTf2] were measured over a range of temperature from (278.15 to 358.15) K. The non-random two liquid NRTL model was used successfully to correlate the experimental tie-lines and to calculate the phase composition error in mole fraction in the ternary systems. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Some petroleum processes need the removal of low level aromatic-sulfur compounds for many benefits such as product purity and cost, which is extremely important to meet new strict environmental regulations to reduce sulfur content compounds in liquid fuels. The allowed limits of sulfur containing compounds in fuels, in order to reduce atmospheric pollution and eliminate acid rain, are (10 to 15)  106 according to new law enacted in the USA and Europe [1,2]. During the hydrodesulfurization process, the aromatic-sulfur compounds are not fully converted to olefins. In addition, this process requires high pressure and temperature. From the industrial point of view and according to the previous argument, it is beneficial to remove the aromatic-sulfur compounds totally from the feed stream. The removal of sulfur compounds is a challenging process since these compounds are converted to H2S during the hydrodesulfurization process. Commercial separation methods used for the separation of organic compounds are (liquid + liquid) ⇑ Corresponding author at: Department of Physical Chemistry, Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland. Tel.: +48 22 6213115; fax: +48 22 6282741. E-mail address: [email protected] (U. Doman´ska). 0021-9614/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jct.2013.09.032

extraction, which is suitable for the range of (20 to 65)% (wt/wt) aromatic, or extractive distillation, or azeotropic distillation with higher efficiency [3]. The (liquid + liquid) extraction (LLE) was widely used in industry for the aromatic hydrocarbon separation and purification because of usually low temperature, mild operating conditions and due to its simplicity. Industrially used chemicals such as entrainers are mostly conventional polar organic solvents, like sulfolane, and N-methylpyrrolidinone [4,5]. In order to meet the more restrictive regulations for content of sulfur compounds, new alternative techniques and entrainers have been proposed [6–19]. Among them, it is crucial to develop new extractants as ILs with a high solute distribution coefficient of entrainer, high selectivity of sulfur-compounds to alkanes, and with little entrainer loss. The challenger of petroleum industry is to diminish the typical sulfur compounds that are found in fuels as thiophene, benzothiophene, methyldibenzothiophenes, 4,6-dibenzothiophenethiols, thioethers and disulfides. The (liquid + liquid) extraction with ILs is probably the most studied technique and entrainers during recent years [20–40]. The interest for using ILs to replace volatile solvents in future technologies has increased significantly recently. Development of new ILs that are stable when contacted with air and water increased the potential for their industrial application [20,23,26–

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29,39,40]. The use of the imidazolium-based ILs with the bis{(trifluomethyl)sulfonyl}imide anion in the process of separation of sulfur compounds from alkanes has been reported by group of Arce [31–39] with satisfactory results. Marciniak and Królikowski [28,29] compared the suitability of using four different, more specific ILs as 4-(2-methoxyethyl)-4-methylmorpholinium trisfluorotris(perfluoroethylphosphate [COC2MMOR][FAP], 4-(2methoxyethyl)-4-methylmorpholinium bis{(trifluomethyl)sulfonyl}imide [COC2MMOR][NTf2], 1-(2-methoxyethyl)-1-methylpyrrolidinium trisfluorotris(perfluoroethylphosphate, [COC2MPYR][FAP] and 1-(2-methoxyethyl)-1-methylpyrrolidinium bis{(trifluomethyl)sulfonyl}imide [COC2MPYR][NTf2] as solvents in (liquid + liquid) extraction at T = 298.15 K. The results of that research showed that out of these four ILs, the [COC2MMOR][NTf2] was the best solvent for the sulfur compounds/aliphatic separation. Three other pyrrolidinium-based ILs with  tricyanomethanide, [TCM], {CðCNÞ 3 }, tetracyanoborate, [TCB] and tris(pentafluoroethyl)trifluorophosphate, [FAP] anions have been studied recently in ternary LLE [26]. The [TCM] anion revealed high selectivity and an acceptable solute distribution ratio for extraction of thiophene from heptane at T = 298.15 K [26]. Even better results for the extraction of thiophene and benzothiophene than those described in reference [26] were obtained with 1-ethyl3-methylimidazolium tricyanomethanide, [EMIM][TCM] IL [27]. Recently, very promising results of the extraction of sulfur compounds from gasoline and diesel fuels were obtained with tricyanomethanide-based, [TCM] and dicyanamide-based [N(CN)2] ILs [7]. The ILs investigated in this work revealed extraction of thiophene and benzothiophene at levels high as (0.78 and 0.87) mass fraction respectively. The selectivity is in the order [M3 BPY][N(CN)2 ]  [BMIM][TCM] > [M4 BPY][N(CN) 2] > [M4 BPY] [SCN] > [BMIM][N(CN)2] > [BMIM][SCN] [7]. This is not the first time it has been shown that pyridinium-based ILs are superior to imidazolium-based ILs with the same anion [7]. Thus, the pyridinium-based ILs were found to be good solvents to extract sulfur compounds [7,21,22,26]. Polymethylpyridinium bis{(trifluoromethyl)sulfonyl imides [NTf2] were used as entrainers for the sulfur compounds with good selectivity [22]. Many works reported that there is great potential for using ILs with the cyano group, CN– in the cation, or in the anion as a good extractant for the separation of sulfur compounds from aliphatic hydrocarbons due to their remarkable selectivity towards sulfur compounds [7,23–25,30]. These ILs could reduce the complexity and high energy cost of existing purification processes for fuels. Having this in mind we synthesized the series of alkylcyanopyridinium-based ILs, with bis{(trifluoromethyl)sulfonyl}imide anion as a possible entrainer, for the extraction of the sulfur compounds from alkanes [40]. Careful analysis of the results of density, dynamic viscosity and surface tension revealed that the lowest values of density and viscosity, which is the most convenient for new technologies, may be expected for 1-butyl-4-cyanopyridinium bis{(trifluoromethyl)sylfonyl}imide, [BCN4Py][NTf2]. Thus the aim of this work was to synthesise the new IL, namely [BCN4Py][NTf2], and to investigate the ternary LLE using this particular IL. The current work represents the first known study using cyanopyridinium-based ILs. The experimental tie lines for three ternary mixtures of {1-butyl-3-cyanopyridinium bis{(trifluoromethyl)sylfonyl}imide [BCN3Py][NTf2], 1-butyl-4-cyanopyridinium bis{(trifluoromethyl)sylfonyl}imide [BCN4Py][NTf2] and 1hexyl-4-cyanopyridinium bis{(trifluoromethyl)sylfonyl}imide, [HCN4Py][NTf2] (1) + thiophene (2) + heptane (3)} and two ternary mixtures for {[BCN4Py][NTf2], or [HCN4Py][NTf2] (1) + benzothiophene (2) + heptane (3)} have been determined at T = 308.15 K and ambient pressure. From the experimental results, the extraction selectivity and solute distribution ratio were determined and discussed.

2. Experimental 2.1. Chemicals and materials The synthesis and the purification method for the ILs studied, viz.: 1-butyl-3-cyanopyridinium bis{(trifluoromethyl)sylfonyl}imide [BCN3Py][NTf2], and 1-hexyl-4-cyanopyridinium bis{(trifluoromethyl)sylfonyl}imide, [HCN4Py][NTf2], was presented in our previous work [40]. The 1-butyl-4-cyanopyridinium bis{(trifluoromethyl)sylfonyl}imide [BCN4Py][NTf2] was synthesized in this work. The N-butyl-4-cyanopyridine bis{(trifuoromethyl)sulfonyl}imide was prepared using the procedure as reported for similar ILs previously [40]. First N-butyl-4-cyanopyridine bromide was synthesized using 31.05 g 4-cyanopyridine dissolved in 100 cm3 of acetonitrile and 45.36 g butyl bromide (11% excess). This mixture was stirred in reflux for 24 h. After crystallization from acetonitrile, 64.53 g of N-butyl-4-cyanopyridine bromide was obtained with yield 90%. Afterwards, 84.40 g of lithium bis{(trifuoromethyl)sulfonyl}imide and 200 cm3 of dichloromethane were added to a solution of 64.53 g of N-butyl-4-cyanopyridine bromide in 100 cm3 of water. The mixture was stirred for 4 h and next two phases were separated. The organic phase was further extracted 10 times with 10 cm3 of water to remove residual salts. Solvents were removed by rotary evaporator. As a result, 99.54 g of yellow oil were obtained. The Yield of the synthesis was 84.3%. Relevant information is as follows. 1

H NMR (400 MHz, D6-DMSO) d (106): 0.915 (t, 3H, 3JHH = 7.4 Hz), 1.335 (hex, 2H, 3JHH = 7.4 Hz), 1.935 (pent, 2H, 3JHH = 7.4 Hz), 4,683 (t, 2H, 3JHH = 7.6 Hz), 8.633 (d, 2H, 3JHH = 6.0 Hz), 9.362 (d, 2H, 3JHH = 6.8 Hz). 13 C NMR (100 MHz, D6-DMSO) d (106): 13.045, 18.815, 32.793, 61.978, 114.568, 119.487 (q, 1JCF = 319 Hz), 127.218, 131.022, 146.165. Elementary microanalysis: found: C 32.77%, N 9.48%, H 3.08%, S 14.48%; theory: C 32.65%, N 9.52%, H 2.97%, S 14.53%. All ILs investigated in this work with structure, name, abbreviation of name, molar mass (M) and purity are presented in table 1. The Differential Scanning Calorimetry, DSC, 1H NMR and 13C NMR spectra of [BCN4Py][NTf2] are presented in figures 1S–3S, respectively in the Supplementary Material (SM). The density and viscosity of [BCN4Py][NTf2] were measured in a range of temperature (278.15 to 358.15) K. The apparatus and method of measurements of DSC, density and viscosity were described previously [40]. Standard uncertainties are: u(q) = ±0.00005 g  cm3; u(T) = ±0.02 K; u(p) = ±0.03 kPa. The samples of ILs were dried for 24 h at T = 300 K under reduced pressure to remove volatile impurities and trace water. The origins of the chemicals were as follows: heptane, thiophene, benzothiophene, propan-1-ol and acetone (all Sigma Aldrich Chemie GmbH). Thiophene and benzothiophene were stored over freshly activated molecular sieves of type 4  108 m (Union Carbide). The densities and the refractive index for solvents at T = 298.15 and for benzothiophene within the range of temperature (308.15 to 363.15) K were measured at 101.33 kPa (see tables 1S and 2S and figure 4S in the Supplementary Material (SM)). The refractive index nD at the sodium line (kD = 589.3 nm) was measured with an Abbe refractometer (Carl Zeiss Jena). The temperature of the sample was controlled with a laboratory thermostat within ± 0.1 K and measured by a mercury thermometer with an uncertainty of ±0.5 K. The refractometer was calibrated with water, and its expanded uncertainty in the measurement of the refractive

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TABLE 1 List of investigated ionic liquids: structure, name, abbreviation of name, molar mass (M) and purity. Structure

CN

N

S O

C6H13

1-Hexyl-4-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide [HCN4Py][NTf2]

469.42

>0.99

S

O

O

O

O N

S F 3C

>0.99

-

CN

+

440.47

CF3

F 3C

N

1-Butyl-4-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide [BCN4Py][NTf2]

O

O N

S

C4H9

>0.99

S

O

+

440.47

-

CN

N

Mass fraction purity

1-Butyl-3-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide [BCN3Py][NTf2]

CF3

F 3C

C4H9

M/(g  mol1)

O

O

+

N

Name, abbreviation

-

S CF3

O

O

index is estimated to be less than ±0.00002. The list of ILs and solvents investigated, source, purity, CAS number, water content, density and refractive index are listed in table 1S in the SM. 2.2. Water content The water content was analysed by the Karl-Fischer titration technique (method TitroLine KF). The sample of IL or solvent was dissolved in methanol and titrated in steps of 0.0025 cm3. The error in water content was ±10  106 for the 3 cm3 of IL injected. The water content in ILs and in solvents used was less than 320  106. 2.3. Procedure in ternary system For the determination of the experimental LLE tie-lines, mixtures with compositions within the immiscible region of the systems were introduced into a jacketed glass cell with a volume of 10  103 cm3. The solution was mixed with a coated magnetic stirring bar. The vessel was properly closed to avoid losses by evaporation or to absorb of moisture from the atmosphere. The jackets were connected to a thermostatic water bath (LAUDA Alpha) to maintain a constant temperature of T = (308.15 ± 0.05) K. The mixtures were stirred for 6 h to reach the thermodynamic equilibrium and after minimum of 12 h were analysed. After the phase separation, the samples of about (0.1 to 0.3)  103 cm3 were taken from both phases using glass syringes with coupled stainless steel needles. The sample of the phase was placed in an ampoule with a capacity of 2  103 cm3. The ampoule was closed with a septum cap. Next, acetone (1.0 cm3) was added to the samples to avoid phase splitting and to maintain a homogeneous mixture. Propan1-ol was used as the internal standard for the GC-analysis. Because of the low vapour pressure, the ILs used in this work cannot be

analysed by GC. Thus only thiophene, or benzothiophene and heptane were analysed; the mass fraction of the third component, the IL, was determined by subtracting the mole fractions of the two other components from one. The composition was analysed by gas chromatography (PerkinElmer Clarus 580 GC equipped with auto sampler and FID and TCD detectors). The capillary column of the chromatograph was protected with a pre-column to avoid the non-volatile ionic liquid reaching the column in case of leak from the glass wool in the liner. The TotalChrom Workstation software was used to obtain the chromatographic areas for the thiophene, or benzothiophene, heptane and internal standard, propan-1-ol. Samples were injected three times, and the average value was calculated. Details of the operational conditions of the apparatus are reported in table 2. The estimated uncertainty in the determination of mole fraction compositions is ±0.003 for compositions of the hydrocarbon-rich phase and ±0.005 for compositions of IL-rich phase.

3. Results and discussion The LLE data for the experimental tie-line ends of the three ternary systems {IL (1) + thiophene (2) + heptane (3)} at T = 308.15 K and ambient pressure are reported in table 3 and for two ternary systems {IL (1) + benzothiophene (2) + heptane (3)} at T = 308.15 K and ambient pressure in table 4. The ternary LLE determined for each system (including results in binary systems) are plotted in a form of the Gibbs triangle with the experimental tie-lines in figures 1–3 and in figures 4 and 5 for thiophene and benzothiophene, respectively. Figures 1–5 are the ternary diagrams of all systems at T = 308.15 K. They show clearly how a good separation was achieved which further supports the

´ ska et al. / J. Chem. Thermodynamics 69 (2014) 27–35 U. Doman

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TABLE 2 Operational conditions in the gas chromatograph for the compositional analysis of the phases in equilibrium. Element

Characteristic

Description

Columns

Type

Flow Carrier gas

Elit-5 PerkinElmer DB-5 (5% diphenyl/95% Dimethyl polysiloxane), length 30 m, inner diameter 0.53 mm, film thickness: 1.5 lm Elite-Wax PerkinElmer, length 30 m, inner diameter 0.53 mm, film thickness: 1.0 lm 5 mL  min1 Helium

Oven Injector

Temperature Injection volume Split ratio Temperature

343.15 K 0.1 lL 10:1 423 K

Detector

Type Temperature

Flame ionisation detector (FID) 493 K

argument of this work. These figures show that the two-phase region is smaller for benzothiophene. Because of the highest solubility of heptane in [HCN4Py][NTf2], the two phase region was also the smallest in [HCN4Py][NTf2] between three ILs used. In the binary {IL (1) + heptane (3)} system, the complete liquid miscibility occurs from mole fraction x1 = 0.975, x1 = 0.975, x1 = 0.928 for [BCN3Py][NTf2], [BCN4Py][NTf2], and [HCN4Py][NTf2], respectively. Similar solubility of heptane in the IL is observed in [BCN3Py][NTf2] and [BCN4Py][NTf2]. The solubility of heptane in [HCN4Py][NTf2] is greater than those in [BCN3Py][NTf2], [BCN4Py][NTf2] (see figures 1–3). This can be attributed to the longer alkane chain at the pyridinium ring. This effect is observed since there is an increase in the van der Waals interactions between the lipophilic hydrocarbon chain of the cation and the heptane. In the binary {IL (1) + thiophene (2)} system, the complete miscibility in the liquid phase occurs from mole fraction x2 = 0.763, x2 = 0.787, x2 = 0.856 for [BCN3Py][NTf2], [BCN4Py][NTf2], and [HCN4Py][NTf2], respectively. The greatest solubility of thiophene in the IL is observed for [HCN4Py][NTf2] (see figures 1–3). In the binary {IL (1) + benzothiophene (2)} system, the complete miscibility in the liquid phase occurs from mole fraction x2 = 0.838, x2 = 0.898 for [BCN4Py][NTf2] and [HCN4Py][NTf2], respectively. As above, the greatest solubility of benzothiophene is observed in [HCN4Py][NTf2] (see figures 4 and 5). The immiscibility gap is observed in the {thiophene (2) + heptane (3)} binary mixture, as was published previously [26–29]. In this work, solute distribution ratios (b) and selectivities (S) for all these systems were tested for the potential application in the separation of sulfur compounds from a mixture of (sulfur compounds + heptane). These parameters are defined by the following expressions:

b¼

xII2 ; xI2

ð1Þ

S¼

xII2  xI3 ; xI2  xII3

ð2Þ

where x is the mole fraction; superscripts I and II refer to heptanerich phase and IL-rich phase, respectively. Subscripts 2 and 3 refer to the sulfur compound and heptane, respectively. The values of b and S, calculated from the experimental data, are shown in tables 3 and 4, for thiophene and benzothiophene, respectively. The effect of the position of the nitrile group and the alkane chain length in the cation of the IL was examined with respect to the selectivity of thiophene, or benzothiophene/heptane phase separation. The concentration of the sulfur compounds in the thiophene, or benzothiophene/heptane mixtures used in this study are listed in tables 3 and 4. These mixtures were considered as tie-lines samples. Analysis of samples of two phases with gas chromatogra-

phy showed that the IL has not been found in the heptane layer after the LLE experiments. This achievement is very important from an industrial point of view. Currently, different ionic liquids are proposed as a LLE solvent for the same process (see table 5). However, the selectivity decreases at higher concentrations of sulfur compounds in the hydrocarbon layer, a behaviour similar to what is known for other solvents [30–39]. The analysis of presented ILs shows that the most suitable IL for the separation of thiophene, or benzotiophene is [BCN4Py][NTf2] because of a high solubility of thiophene in the IL and a low solubility of heptane in the IL. The second condition is the more important for the selectivity and the solute distribution ratio. Figures 6 and 7 present values of b and S for all measured ILs for thiophene and benzothiophene. The results for thiophene show that the best parameters were obtained for [BCN4Py][NTf2]. The solute distribution ratio is from 0.8 to 1.93 and selectivity for the best tie-line is 62.2. The results for benzothiophene were similar, with the best selectivity and high solute distribution ratio for [BCN4Py][NTf2]. For a longer alkane chain at the pyridinium ring, the values of b are as usual higher, but the selectivity is lower. The values of b are expected to be higher than one and that of S as high as possible. The values of b in our ternary systems studied were from 0.8 to 2.4 and from 0.8 to 5.5 for thiophene and benzothiophene, respectively. It can be seen from figures 6 and 7 that b and S decrease as a mole fraction of the solute (thiophene, or benzothiophene) in a heptane phase increases for all systems when going through the tie-line end compositions. For the sake of comparison with previous published data for LLE-ternary system with thiophene and benzothiophene, table 5 is presented in this work. Table 5 shows a comparison between the solute distribution ratio and the selectivity achieved by one of the systems studied in this work to those of other authors reported in the open literature. These experimental findings support the argument on which this paper is based, namely that it is still possible to find solvents able to compete with the other solvents studied in the literature. Focus will be directed to low sulfur compound concentrations as they represent a practical separation problem as discussed above. In a previous work [27], we showed that 1-ethyl-3-methylimidazolium tricyanomethanide IL, [EMIM][TCM] achieve good performances in fuels desulfurization with b = 1.94 and S = 164.7 for thiophene and b = 3.25 and S = 321.1 for benzothiophene at T = 308 K. The cost of the imidazolium solvents, such as [EMIM][TCM] [27] or [DMIM][MP] [20] is now much lower than that of [COC2MMOR][FAP] [28], or [COC2MMOR][NTf2] [29] ILs. However, all of these ILs show very promising parameters b and S. The [BCN4Py][NTf2] is superior to other imidazolium-based ILs [31–33,39] or polysubstituted pyridium-based ILs [34,36,37] in both the solute distribution ratio and selectivity (see table 5). The substitution of the cyano-group at position 3 on pyridinium

´ ska et al. / J. Chem. Thermodynamics 69 (2014) 27–35 U. Doman TABLE 3 Compositions of experimental tie lines, solute distribution ratios, b and selectivity, S for ternary systems {[BCN3Py][NTf2], or [BCN4Py][NTf2], or [HCN4Py][NTf2] (1) + thiophene (2) + heptane (3)} at T = 308.15 K, p = 101.33 kPa. Hydrocarbon – rich phase

IL – rich phase

xI1

xI2

xII1

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

0.000 0.188 0.327 0.415 0.554 0.656 0.806 0.866 0.932 1.000

0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000

B

S

31

TABLE 4 Compositions of experimental tie lines, solute distribution ratios, b and selectivity, S for ternary systems {[BCN4Py][NTf2], or [HCN4Py][NTf2] (1) + benzothiophene (2) + heptane (3)} at T = 308.15 K, p = 101.33 kPa. Hydrocarbon – rich phase

IL – rich phase

xII2

xI1

xI2

xII1

B

S

[BCN3Py][NTf2] 0.975 0.682 0.547 0.496 0.420 0.374 0.317 0.295 0.266 0.237

0.000 0.294 0.429 0.482 0.558 0.604 0.665 0.690 0.724 0.763

1.56 1.31 1.16 1.01 0.92 0.82 0.80 0.78 0.76

52.9 36.8 30.9 20.4 14.4 8.9 7.1 5.3

0.000 0.062 0.142 0.223 0.375 0.454 0.571 0.734 0.829 0.917 0.999

[BCN4Py][NTf2] 0.975 0.755 0.599 0.545 0.471 0.368 0.312 0.263 0.232 0.189 0.162

0.000 0.130 0.296 0.459 0.619 0.700 0.784 0.848 0.933 1.000

[BCN4Py][NTf2] 0.975 0.722 0.562 0.448 0.362 0.329 0.300 0.275 0.241 0.213

0.000 0.001 0.002 0.001 0.000 0.000 0.000 0.000 0.000 0.000 0.001

0.000 0.217 0.371 0.426 0.498 0.589 0.644 0.691 0.725 0.780 0.838

3.50 2.61 1.91 1.33 1.30 1.13 0.94 0.87 0.85 0.84

117.1 74.5 51.1 25.9 16.5 11.0 5.6 3.5 2.3

0.000 0.251 0.415 0.525 0.613 0.647 0.678 0.706 0.747 0.787

1.93 1.40 1.14 0.99 0.92 0.86 0.83 0.80 0.79

62.2 42.9 21.4 15.1 11.5 8.5 6.7 4.5

0.000 0.088 0.226 0.340 0.366 0.485 0.594 0.688 0.783 0.853 0.933 1.000

[HCN4Py][NTf2] 0.928 0.720 0.528 0.428 0.412 0.344 0.296 0.261 0.223 0.200 0.170 0.144

0.000 0.213 0.411 0.514 0.531 0.599 0.649 0.688 0.731 0.762 0.807 0.856

2.42 1.82 1.51 1.45 1.24 1.09 1.00 0.93 0.89 0.87 0.86

32.9 23.1 17.2 16.1 11.2 8.1 6.1 4.4 3.4 2.5

0.000 0.000 0.000 0.002 0.000 0.000 0.000 0.000 0.000 0.001 0.000 0.005

0.000 0.012 0.099 0.161 0.200 0.319 0.433 0.526 0.730 0.806 0.903 0.995

[HCN4Py][NTf2] 0.928 0.865 0.575 0.478 0.430 0.347 0.296 0.261 0.192 0.164 0.125 0.102

0.000 0.064 0.356 0.454 0.501 0.576 0.623 0.656 0.721 0.751 0.813 0.898

5.34 3.59 2.82 2.50 1.80 1.44 1.25 0.99 0.93 0.90 0.90

74.2 46.3 34.2 29.0 16.0 9.9 7.1 3.1 2.1 1.4

xII2

Standard uncertainties are: u(x) < 0.003, u(T) = 0.05 K.

Standard uncertainties are: u(x) < 0.003, u(T) = 0.05 K.

ring ([BCN3Py][NTf2]) gives slightly worse results of extraction in comparison with position 4. The selectivity for [BCN4Py][NTf2] from this work is almost three times lower than that for [EMIM][TCM] [27], and is comparable to [EMIM][NTf2] at lower temperature [35]. The selectivity for [BCN4Py][NTf2] is lower than those previously published for [COC2MMOR][FAP], [COC2MMOR][NTf2] and [COC2MPIP][NTf2] [28,29] as well as [BMPYR][TCM] [26]. Moreover, the [EMIM][OAc] has higher values of parameters than that of [BCN4Py][NTf2] (parameters b = 1.49 and S = 127 at T = 298 K) [39]. Interestingly, table 5 shows different findings. The [BMPYR][TCM] [26], [COC2MPYR][FAP] [28], polysubstituted pyridium-based ILs [34,36,37] and [BMPYR][FAP] [26] were better than [BCN4Py][NTf2] in terms of solute distribution ratio. The best is [BMPYR][FAP] [26] with b = 5.46, in comparison with b = 3.50 in this work. It is necessary to mention that the values of b and S for [BCN4Py] [NTf2] from this work for benzothiophene are high, however lower than that for [EMIM][TCM] [27]. As a conclusion from all the data presented in tables 3–5, it is clear that the [BCN4Py][NTf2] IL achieved good separation efficiency for the sulfur compounds/heptane separation at T = 308 K. This particular IL can be considered as the solvent that can be adopted for further process investigation and analysis. The additional important information about the density and viscosity must be noted here. The density and viscosity of [BCN4Py][NTf2] at T = 298.15 K are q = 1.47423 g  cm3 and

FIGURE 1. Plot of the experimental (d, grey solid lines) vs. calculated with NRTL equation (h, black dotted lines) for the composition tie lines of the ternary system {[BCN3Py][NTf2] (1) + thiophene (2) + heptane (3)} at T = 308.15 K.

g = 250.45 mPa  s (308.15 K) (see table 2S in SM), which are unfortunately higher than those for [EMIM][TCM] [27] with q = 1.08146 g  cm3 and g = 15.02 mPa  s at T = 298.15 K. The sulfur compounds can be separated from heptane by LLE extraction with better transport phenomena and selectivity with [EMIM][TCM], or with slightly better solute distribution ratio with [BCN4Py][NTf2]. 4. Data correlation The ternary LLE values determined in this study were correlated (the tie-line correlation) using the non-random liquid equation, (NRTL) [41]. The equations and algorithms used for the calculation

32

´ ska et al. / J. Chem. Thermodynamics 69 (2014) 27–35 U. Doman

FIGURE 2. Plot of the experimental (d, grey solid lines) vs. calculated with the NRTL equation (h, black dotted lines) for the composition tie lines of the ternary system {[BCN4Py][NTf2] (1) + thiophene (2) + heptane (3)} at T = 308.15 K.

FIGURE 5. Plot of the experimental (d, grey solid lines) vs. calculated with NRTL equation (h, black dotted lines) for the composition tie lines of the ternary system {[HCN4Py][NTf2] (1) + benzothiophene (2) + heptane (3)} at T = 308.15 K.

6 5 4

β 3 2 1 FIGURE 3. Plot of the experimental (d, grey solid lines) vs. calculated with NRTL equation (h, black dotted lines) for the composition tie lines of the ternary system {[HCN4Py][NTf2] (1) + thiophene (2) + heptane (3)} at T = 308.15 K.

0 0.0

0.2

0.4

0.6

x2

0.8

1.0

HC

FIGURE 6. Plot of the solute distribution ratio, (b) as a function of the mole fraction of solute in the heptane – rich phase for the ternary systems: d, {[BCN3Py][NTf2] (1) + thiophene (2) + heptane (3)}; N, {[BCN4Py][NTf2] (1) + thiophene (2) + heptane (3)}; j, {[HCN4Py][NTf2] (1) + thiophene (2) + heptane (3)}; s, {[BCN4Py][NTf2] (1) + benzothiophene (2) + heptane (3)}; D, {[HCN4Py][NTf2] (1) + benzothiophene (2) + heptane (3)} at T = 308.15 K.

FðPÞ ¼

n h X

xI2exp  xIcalc ðPTÞ 2

i2

h i2 þ xI3exp  xIcalc ðPTÞ 3

i¼1

h i2 h i2 þ xII2 exp  xIIcalc ðPTÞ þ xII3 exp  xIIcalc ðPTÞ ; 2 3

FIGURE 4. Plot of the experimental (d, grey solid lines) vs. calculated with NRTL equation (h, black dotted lines) for the composition tie lines of the ternary system {[BCN4Py][NTf2] (1) + benzothiophene (2) + heptane (3)} at T = 308.15 K.

of the compositions in both phases followed the method described by Walas [42]. The objective function F(P), was used to minimise the difference between the experimental and calculated compositions:

ð3Þ

where P is the set of parameters vector, n is the number of experiIcalc mental points, xI2exp , xI3exp and xIcalc ðPTÞ are the experimen2i ðPTÞ, x3 tal and calculated mole fractions of one phase and xII2 exp , xII3 exp and xIIcalc ðPTÞ, xIIcalc ðPTÞ are the experimental and calculated mole frac2 3 tions of the second phase. The binary parameters of each constituent were regressed by minimising the square of the differences between the experimental and calculated mole fractions of each component of both liquid phases for each ternary system. These binary parameters were obtained for the all data points together (binaries and ternaries).

´ ska et al. / J. Chem. Thermodynamics 69 (2014) 27–35 U. Doman

33

TABLE 5 A summary of previous related research results reported in the literature for LLE with sulfur compounds: b and S are the solute distribution ratio of sulfur compound and selectivity respectively. IL

LLE system

T/K

bmax

Smax

Refs.

[BCN4Py][NTf2]a [BCN4Py][NTf2]a [EMIM][SCN]b [EMIM][SCN]b [DMIM][MP]c [TEMA][MeSO4]d [EMIM][TCM]e [EMIM][TCM]e [BMPYR][FAP]f [BMPYR][TCB]g [BMPYR][TCM]h [COC2MMOR][FAP]i [COC2MPIP][FAP]j [COC2MPYR][FAP]k [COC2MMOR][NTf2]l [COC2MPIP][NTf2]m [COC2MPYR][NTf2]n [EMIM][OAc]o [EMIM][DEP]p [H2,4MMPy][NTf2]r [H3,5MMPy][NTf2]s [H3,5MMPy][NTf2]s [OMIM][NTf2]t [OMIM][NTf2]t [OMIM][NTf2]t [OMIM][NTf2]t [OMIM][BF4]u [EMIM][NTf2]w [EMIM][NTf2]w

IL + thiophene + heptane IL + benzothiophene + heptane IL + thiophene + heptane IL + thiophene + heptane IL + thiophene + heptane IL + thiophene + heptane IL + thiophene + heptane IL + benzothiophene + heptane IL + thiophene + heptane IL + thiophene + heptane IL + thiophene + heptane IL + thiophene + heptane IL + thiophene + heptane IL + thiophene + heptane IL + thiophene + heptane IL + thiophene + heptane IL + thiophene + heptane IL + thiophene + hexane IL + thiophene + hexane IL + thiophene + hexane IL + thiophene + hexane IL + thiophene + heptane IL + thiophene + hexane IL + thiophene + heptane IL + thiophene + n-dodecane IL + thiophene + cyclohexane IL + thiophene + heptane IL + thiophene + heptane IL + thiophene + hexane

308 308 298 303 298 298 308 308 298 298 298 298 298 298 298 298 298 298 298 298 298 298 298 298 298 298 298 298 298

1.93 3.50 0.68 0.60 0.42 0.08 1.94 3.25 5.46 3.31 3.47 2.7 4.0 3.1 1.76 2.64 2.39 1.49 2.88 3.59 3.50 2.57 2.18 2.30 1.39 2.24 1.61 1.96 1.80

62.2 117.1 1598 497 1756 27.5 164.7 321.1 57.9 74.9 133.4 109.0 56.8 56.6 104.3 62.9 70.6 127 48.8 22.1 17.20 15.77 7.55 9.62 14.23 6.44 10.48 74.6 43.8

This work This work [20] [20] [20] [20] [27] [27] [26] [26] [26] [28] [28] [28] [29] [29] [29] [39] [39] [37] [34] [36] [31] [31] [32] [32] [33] [35] [38]

a

1-Butyl-4-cyanopyridinium bis{(trifluoromethyl)sulfonyl}imide. 1-Ethyl-3-methylimidazolium thiocyanate. c 1,3-Dimethylimidazolium methylphosphonate. d Tris-(2-hydroxyethyl)-methylammonium methylsulfate. e 1-Ethyl-3-methylimidazolium tricyanomethanide. f 1-Butyl-1-methylpyrrolidinium trifluorotris(perfluoroethyl)phosphate. g 1-Butyl-1-methylpyrrolidinium tricyanoborate. h 1-Butyl-1-methylpyrrolidinium tricyanomethanide. i 4-(2-Methoxyethyl)-4-methylmorpholinium trifluorotris(perfluoroethyl)phosphate. j 1-(2-Methoxyethyl)-1-methylpiperidinium trifluorotris(perfluoroethyl)phosphate. k 1-(2-Methoxyethyl)-1-methylpyrrolidinium trifluorotris(perfluoroethyl)phosphate. l 4-(2-Methoxyethyl)-4-methylmorpholinium bis{(trifluoromethyl)sulfonyl}imide. m 1-(2-Methoxyethyl)-1-methylpiperidinium bis{(trifluoromethyl)sulfonyl}imide. n 1-(2-Methoxyethyl)-1-methylpyrrolidinium bis{(trifluoromethyl)sulfonyl}imide. o 1-Ethyl-3-methylimidazolium acetate. p 1-Ethyl-3-methylimidazolium diethylphosphate. r 1-Hexyl-2,4-dimethylpyridinium bis{(trifluoromethyl)sulfonyl}imide. s 1-Hexyl-3,5-dimethylpyridinium bis{(trifluoromethyl)sulfonyl}imide. t 1-Octyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide. u 1-Octyl-3-methylimidazolium tetrafluoroborate. w 1-Ethyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide. b

In this work, the NRTL model was used with the non-randomness parameter, aij , set to an equal value for all three binary pairs. The value for aij was optimised in order to obtained the best fit model. The correlated parameters are given in table 6 along with the root mean square deviations (RMSD). The RMSD values, which can be taken as a measure of the precision of the correlation, were calculated according to the equation:

RMSD ¼

XXX exp 2 xilm  xcalc =6k ilm i

l

!1=2 ;

ð4Þ

m

where x is the mole fraction and the subscripts i, l, and m designate the component, phase, and tie-line, respectively. The Rosenbrock simplex method was used in an effort to minimise the objective function. The compositions calculated from the correlations are included in figures 1–5. The correlation results, obtained for the five

systems under study, were satisfactory. The experimental and calculated LLE values agreed relatively well.

5. Conclusions A novel class of ionic liquids, 1-alkyl-cyanopyridinium-based, were synthesized. Liquid + liquid equilibria experiments were carried out to investigate the ability of these ILs to extract selectively sulfur compounds from aromatic sulfur compounds/aliphatic mixtures. Five ternary systems (IL + thiophene, or benzothiophene + heptane) were analytically determined using GC for the composition analysis at temperature T = 308.15 K at ambient pressure. The 1-alkyl-cyanopyridinium-based ILs were investigated as a function of alkyl-chain length (butyl, or hexyl) and position of the pyridinium ring (3 or 4). The experimental results revealed that the solubility of thiophene, or benzothiophene in the IL increases as the alkyl chain length increases. Similar observations were made

´ ska et al. / J. Chem. Thermodynamics 69 (2014) 27–35 U. Doman

34

120

decrease as the mole fraction of thiophene, or benzothiophene in the heptane-rich phase increases. These solute distribution ratios are larger than two at low and medium concentration of solute in most cases. This achievement is very important from an industrial point of view. The experimental results in this work were regressed using the NRTL activity coefficient model and binary interaction parameters. The non-randomness parameter was also determined through the reduction of the experimental results. The model exhibited an excellent fit to the data indicated the average RMSD equal to 0.0064 with aij = 0.2.

100 80

S

60 40

Acknowledgement

20

This work has been supported by the project of National Science Center 011/01/B/ST5/00800.

0 0.0

0.2

0.4

0.6

0.8

1.0

x2HC

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jct.2013.09.032.

FIGURE 7. Plot of the selectivity, (S) as a function of the mole fraction of solute in the heptane – rich phase for the ternary systems: d, {[BCN3Py][NTf2] (1) + thiophene (2) + heptane (3)}; N, {[BCN4Py][NTf2] (1) + thiophene (2) + heptane (3)}; j, {[HCN4Py][NTf2] (1) + thiophene (2) + heptane (3)}; s, {[BCN4Py][NTf2] (1) + benzothiophene (2) + heptane (3)}; D, {[HCN4Py][NTf2] (1) + benzothiophene (2) + heptane (3)} at T = 308.15 K.

for the solubility of the IL in heptane. The solvent capacity, described in terms of selectivity and the solute distribution ratio coefficients were computed for all ILs in this study and compared to the published data used in similar separation problems. Particularly, the [BCN4Py][NTf2] gave best separation performance compared to many pyridinium-based ILs [34,36,37]. Based on the values obtained, the [BCN4Py][NTf2] IL was found interesting, however not as good as [EMIM][TCM] measured by us previously [27] and many other ILs by various researchers [20,26,28,35,39]. The selectivity and the solute distribution ratio

TABLE 6 Binary interaction parameters and root mean square deviation (rx) for the NRTL equation for the ternary systems {IL (1) + thiophene, or benzothiophene (2) + heptane (3)} at T = 308.15 K.a ij

a

gij/(J  mol1)

gji/(J  mol1)

12 13 23

{[BCN3Py][NTf2] (1) + thiophene (2) + heptane (3)} 960.72 53385.28 5551.55 16037.05 4085.52 1095.15

12 13 23

{[BCN4Py][NTf2] (1) + thiophene (2) + heptane (3)} 7256.50 35831.97 6357.53 17865.30 248.96 1861.48

12 13 23

{[HCN4Py][NTf2] (1) + thiophene (2) + heptane (3)} 7519.17 18408.51 2445.34 11867.81 5946.57 2437.89

RMSD rx 0.002

0.006

0.006

12 13 23

{[BCN4Py][NTf2] (1) + benzothiophene (2) + heptane (3)} 6305.68 15712.21 0.011 5300.54 24851.56 5287.10 953.64

12 13 23

{[HCN4Py][NTf2] (1) + benzothiophene (2) + heptane (3)} 8240.01 19015.69 0.007 2915.12 16439.27 1333.06 1600.33

Parameter aij = 0.2.

Appendix A. Supplementary data

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JCT 13-470