Phase Equilibria upon Denitrification of Liquid Fuels Using

Article pubs.acs.org/IECR

Phase Equilibria upon Denitrification of Liquid Fuels Using Imidazolium-Based Ionic Liquids: Experiment and Quantum Chemical Calculations Anantharaj Ramalingam* and Sathishkumar Kanniayan Department of Chemical Engineering, SSN College of Engineering, Chennai-603110, Tamil Nadu, India S Supporting Information *

ABSTRACT: The interaction between pyrrole and pyridine with 1-ethyl-3-methylimidazolium ethyl sulfate ([EMIM][EtSO4]) was investigated using quantum chemical calculations, the conductor-like screening model for real solvent (COSMO-RS) approach, and experimental ternary liquid−liquid equilibrium (LLE) data. Geometry optimization was performed usign density functional theory (DFT) with B3LYP functional and 6-311+G* basis set for individual species and complexes of [EMIM][EtSO4] with pyrrole/pyridine. The amount of charge transfer in [EMIM][EtSO4] + pyrrole was found to be ∼20% greater than that in [EMIM][EtSO4] + pyridine, indicating that CH−π interaction was stronger with pyrrole. The more negative interaction energy for the complex [EMIM][EtSO4] + pyrrole implies that [EMIM][EtSO4] has a more favorable interaction with pyrrole than with pyridine. σ-profile analysis using COSMO-RS confirmed the pivotal role of the CH-π interaction between [EMIM][EtSO4] and pyrrole or pyridine. Ternary LLE experiments were performed at 298.15 K and atmospheric pressure to compare the extraction efficiency of pyrrole and pyridine from model diesel compound (n-hexadecane) using [EMIM][EtSO4]. The experimental data showed that [EMIM][EtSO4] had a higher extraction efficiency for pyrrole than for pyridine, hence validating the computational results. The experimental data were well-correlated by the NRTL model with an average root-meansquare deviation (RMSD) of 0.28%, and COSMO-RS gave excellent prediction of the LLE data, with an average RMSD of 0.99%.

1. INTRODUCTION The oil refinery sector continues to be under pressure to produce zero-emission fuels, since the limit for sulfur content in diesel fuel was decreased from 50 ppm to 15 ppm in 2006.1 Thus, an efficient method that can reduce the sulfur concentration to an absolute minimum is necessary. The conventional method for removing sulfur compounds is through the hydrodesulfurization (HDS) process, which operates under severe conditions (600− 700 K and 100−200 atm H2), where the sulfur compounds react with hydrogen and produce gaseous hydrogen sulfide compounds. It has been reported that the presence of aromatic nitrogen compounds in the diesel feedstock, even at low concentrations, suppresses the HDS process by competitive adsorption and catalyst deactivation.2,3 One way to improve the efficiency of the HDS process is to remove the aromatic nitrogen compound that is naturally found in diesel feedstock before it enters the HDS unit. Regulatory limits for nitrogen content had been proposed such that the upper limit of nitrogen concentration in diesel was reduced from >70 ppm to 2.2 Å, while the C−H···O bond angles were between 100° and 180°. Note that the origin of long-range cation liquid crystalline properties was due to the formation of the domain of “coulombic layers”.35 The cationic head groups (i.e., alkyl group) interacted with the counterions, and the “van der 12951

DOI: 10.1021/acs.iecr.5b01962 Ind. Eng. Chem. Res. 2015, 54, 12948−12959

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Industrial & Engineering Chemistry Research

partial charge of −0.6798, was not delocalized into the aromatic π-system, because of its basic nature. For an efficient CH−π interaction, the aromatic π density would need to be increased. A decrease in the partial charge of the N atom resulted in the increase in the density of the π-electron system, thereby promoting cation−π interaction. The partial charge of the N atom of pyrrole in the [EMIM][EtSO4] + pyrrole complex (−0.1069) was less negative than that of the N atom of pyridine in the [EMIM][EtSO4] pyridine complex (−0.5320). In the complex with pyrrole, the partial charge transfer for the N atom of pyrrole was 0.1992 (i.e., −0.3061 − (−0.1069)) from an individual pyrrole to the pyrrole in the complex [EMIM][EtSO4] + pyrrole. Meanwhile, the partial charge transfer from the N atom in the individual pyridine to the N atom of pyridine in the [EMIM][EtSO4] + pyridine complex was 0.1478 (i.e., −0.6798 − (−0.5320)). Comparing the partial charge transfer in the nitrogen atoms of pyrrole/pyridine between the two complexes, the partial charge transfer for the N atom in the complex with pyrrole was ∼20% more than that in the complex with pyridine. In other words, there was a greater decrease in the partial charge of the N atom of the aromatic nitrogen compound in the complex with pyrrole, indicating that a more-efficient cation−π interaction was taking place in the complex with pyrrole than that with pyridine. The extent of delocalization between the complexes [EMIM][EtSO4] + pyrrole and [EMIM][EtSO4] + pyridine could also be compared by comparing the partial charge transfer in the two N atoms in the imidazolium cation (i.e., N1 and N2), and the total positive charge of H atoms of the cation in a neat IL complex, before and after interaction with pyrrole and pyridine. Before interaction with pyrrole and pyridine, the partial charges of N1 and N2 atoms in the [EMIM] cation were 0.8324 and 0.4194, respectively, and the total positive charge of H atoms in the cation was 1.1883. When in the complex with pyrrole ([EMIM][EtSO4] + pyrrole), the partial charges of N1 and N2 reduced to 0.1676 and 0.0597, respectively, while the total positive charge for H atoms in the cation increased to 1.7398. On the other hand, the partial charges of N1 and N2 in the [EMIM] cation were reduced to 0.2533 and 0.0208, and the total positive

([EMIM])−π (pyridine) interaction, H(pyridine)−O([EtSO4]) interaction, and N (pyridine)−H ([EtSO4]) (see Figure 7).

Figure 7. Possible molecular interaction between [EMIM][EtSO4] and pyrrole/pyridine at molecular level.

4.2. Partial Charge Transfer. The electronic properties of the substituents on the π-system influenced the strength of the attraction. The electron-withdrawing groups decreased the amount of negative charge in the π-system and, thus, weakened the interaction, while the electron-donating groups, such as −NH2, increased the amount of negative charge in the π-system. Therefore, the partial charge on the N atom of pyrrole/pyridine played an important role in increasing or decreasing the negative charge in the π-system. Hence, this section focuses on the partial charge of the N atom and the extent of delocalization when they were in a complex that contained an IL. Table 2 shows the partial charges for each element in both the individual species and clusters studied in this work. The partial charge of the N atom in pyrrole was −0.3061. The “electron pairs” on the N atom in pyrrole were significantly delocalized in the π-electron system, because of its neutral nature. However, the lone pair electron on the N atom in pyridine, which carried a

Table 2. Net and Grand Partial Charges of H, N, and O Elements in Studied Systemsa Net Partial Charge of the N Atom No.

a

name

net + partial charges of H atoms

1 2 3

PYRR PY [EMIM]

0.7639 0.3130 1.2993

4 5

[EtSO4] [EMIM][EtSO4]

0.1814 1.1883

6

[EMIM] + PYRR

2.2274

7

[EMIM] + PY

2.0974

8 9 10

[EtSO4] + PYRR [EtSO4] + PY [EMIM][EtSO4] + PYRR

1.2478 0.9520 1.7398

11

[EMIM][EtSO4] + PY

1.3717

from [EMIM]a

from aromatic nitrogen

net partial charges of O atom from [EtSO4]

−0.3061 −0.6798 N1 = 0.2077 N2 = 0.0609 −2.7787 −2.5744

N1 = 0.8324 N2 = 0.4194 N1 = 0.2026 N2 = −0.1195 N1 = −0.0387 N2 = 0.2178

N1 = 0.1676 N2 = 0.0597 N1 = 0.2533 N2 = 0.0208

−0.4703 −0.0491 −0.1069

−1.9121 −1.8861 −2.5244

−0.5320

−2.5566

N1 and N2 refers to the partial charge of the two N atoms of the imidazolium cation. 12952


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Industrial & Engineering Chemistry Research charge of H atoms in the cation increased to 1.3717 when in the complex with pyridine ([EMIM][EtSO4] + pyridine). Thus, it can be deduced that the extent of delocalization was slightly higher for [EMIM][EtSO4] + pyrrole complex, as compared to the [EMIM][EtSO4] + pyridine complex. Quantitatively, the extent of delocalization was ∼42% higher in complexes with pyrrole than with pyridine. This is consistent with the results obtained from the previous work, where it was observed that the interaction was found to be preferentially through N (heteroaromatic)−H (imidazolium) hydrogen bonds.5,7,34 Thus, looking both via charge transfer and the extent of delocalization, the CH−π bond interaction was determined to be weaker for pyridine than for pyrrole. The total number of positively charged H atoms in [EMIM][EtSO4] was 16, but the maximum hydrogen contribution was 11 from cation, and 5 from anion. Note that it was unlikely for the rear H atoms of the cation to form CH−π interactions with pyrrole/pyridine.5,7,30 Meanwhile, the two H atoms of the “CH 2” group could not be occupied at the same time, because of Coulombic repulsion. Thus, there was a limit on the maximum number of sites that could be occupied at a time. The electrostatic field effect within the IL was also important when the electrons surrounding the resonating nucleus of the cation were displaced with a chemically bonded polar atom, such as oxygen (O−H interaction), which was present in the [EtSO4] anion. As for favorable interaction, the pyrrole/pyridine molecule should be able to overcome the O−H interaction and form cation−π bonds, hydrogen bonds, and π−π interactions. However, the change in partial charge of the O atom (i.e., from −2.5744 in the neat [EMIM][EtSO4] complex to −2.5244 in the [EMIM][EtSO4] + pyrrole complex and −2.5566 in the [EMIM][EtSO4] + pyridine complex) could be considered to be negligible. This showed that the O−H interaction was not important in the systems. 4.3. Interaction Energy. In this section, the interactions of pyrrole and pyridine with cation, anion, and the neat IL are discussed. The total energy for each individual species and the complexes involved are presented in Table 3. The interaction

Table 4. Calculated Net Interaction Energy and Activity Coefficient at Infinite Dilution Values for Studied Aromatic Nitrogen Species in [EMIM][EtSO4]

name

total energy (hartrees)

1 2 3 4 5 6 7 8 9 10 11

PYRR PY [EMIM] [EtSO4] [EMIM][EtSO4] [EMIM] + PYRR [EMIM] + PY [EtSO4] + PYRR [EtSO4] + PY [EMIM][EtSO4] + PYRR [EMIM][EtSO4] + PY

−208.7198 −246.6003 −342.1316 −775.5019 −1117.7710 −550.8119 −588.7429 −984.2449 −1022.1145 −1326.5108 −1364.3835

name

12 13 14 15 16

[EMIM] + PYRR [EMIM] + PY [EtSO4] + PYRR [EtSO4] + PY [EMIM][EtSO4] + PYRR [EMIM][EtSO4] + PY

17 a

net interaction energya (hartrees)

COSMO-RS IDAC predictions

0.0398 −0.0120 −0.0232 −0.0156 −0.0192

0.0200

−0.0122

1.65112

As determined using eq 1.

interaction of pyrrole with [EMIM][EtSO4] occurred through CH (heteroaromatic)−π (cation) hydrogen bonds, while the interaction of pyridine with [EMIM][EtSO4] occurred mainly through N (heteroaromatic)−H (cation) hydrogen bonds. Note that more negative interaction energy indicated more favorable interaction between the two species in the complex, since it meant that an extra amount of energy (which is positive in nature) was required for disassembling the complex. On this basis, the interaction between the [EMIM] cation and pyridine was stronger than that between [EMIM] and pyrrole, as the interaction energy for [EMIM]−pyridine was more negative (−0.0120 hartree) than that of [EMIM]−pyrrole (0.0398 hartree). However, as for the [EtSO4] anion, the interaction was stronger with pyrrole (−0.0232 hartree) than with pyridine (−0.0156 hartree). Next, for complexes with neat IL, the interaction in the complex [EMIM][EtSO4] + pyrrole (−0.0192 hartree) was stronger than that in the complex [EMIM][EtSO4] + pyridine (−0.0122 hartree). It was also observed that a qualitative relationship existed between the partial charges and the interaction energies. In the previous section, we have discussed that the partial charge transfer in the N atom of pyrrole in the [EMIM][EtSO4] + pyrrole complex was higher than the partial charge transfer in the N atom of pyridine in the [EMIM][EtSO4] + pyridine complex, thus indicating that the complex with pyrrole had a stronger cation−π interaction than that with pyridine. This shows that the extent of delocalization indicated by the partial charge transfer was coherent with the strength of interaction demonstrated by the calculated net interaction energy of a complex. Up to this point, we have discussed the interaction energies/ partial charges, which cannot be measured experimentally. However, an attempt was made where the interactions between the energies were correlated to experimental measurements. The infinite dilution activity coefficient (IDAC) can be measured via gas−liquid chromatography and gives us an important descriptor for the effectiveness of a solvent to remove the last trace of impurity (i.e., pyrrole or pyridine in our case). For this purpose, a COSMO-RS-based approach was used to determine the IDAC values for pyrrole and pyridine, respectively, in [EMIM][EtSO4]. In our earlier work, we have successfully benchmarked the IDAC predictions of thiophene,37 pyridine,5,7 and other solutes23 in ILs. Table 4 shows that the interaction energy of [EMIM][EtSO4] + pyridine was less negative than that of [EMIM][EtSO4] + pyrrole. On the other hand, the activity coefficient at an infinite dilution of pyridine in [EMIM][EtSO4] (1.6511) was higher than that of pyrrole in [EMIM][EtSO4] (0.0200). This was completely consistent with each other, because less negative

Table 3. Total Energy for Studied Aromatic Nitrogen Species No.

No.

energy for each complex was calculated from the total energy of the individual species, as mentioned in the previous section and eq 1, and the results are presented in Table 4. On a quantum scale, the interactions between the IL and pyrrole/pyridine occurred through π−π interactions, CH−π interactions, and hydrogen bonding interactions.5,7 The amount of pyrrole or pyridine adsorbed to [EMIM][EtSO4] increased with the increase of the π-electron cloud density, as well as the availability of hydrogen sites, either from [EMIM] or [EtSO4]. The 12953


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offered the hydrogen-bond interaction potential with pyrrole and pyridine, because of its negative charge. The strength of the interactions between cations and pyrrole/pyridine was dependent on the hydrogen-bond donor or acceptor availability. The cations had better donor ability, since part of the profile resided to the left of the cut-off zone, i.e., σ hb < −0.0082 e/A2, while the acceptor side was almost nonexistent, i.e., to the right of the cutoff zone, i.e., σhb > + 0.0082 e/A2. This had been expected because cations carry positive charge. From the sigma profiles too, it was clear that the sigma profiles of cations and pyrrole/ pyridine were complementary, indicating favorable interaction, which confirmed the presence of CH−π bonding. As for the [EtSO4] anion, the peaks resided on the right side of the cut-off zone for hydrogen bonding, which was due to the inherent negative charges of the O atom of the anion. However, the pyrrole and pyridine showed peaks at the outermost position in the negative direction. These negative positions of the screening charges were due to the positive charge residing outside the pyrrole and pyridine molecules. The complementary profiles confirmed that the attraction of anions with the aromatic compounds was through the O atom, i.e., O−H interaction. The profiles of the cation and anion were then added to obtain the profile for the neat IL, as was done in the literature.5,7,8,37 The added profile reflected the same information as that of individual profiles of cations and anion, where the profile showed a complementary shape between the cation and the anion. In other words, very few polar surface segments of pyrrole and pyridine could make energetically acceptable pairs with a nonpolar cation on the IL surface. This implies that the O−H interaction was negligible, compared to the CH−π interaction, in agreement with our discussion in section 4.3. Thus, it can be safely concluded that the CH−π interaction dominated over the hydrogen-bonding effect of the N/O atom with the cation. 4.5. Consistency of the Liquid−Liquid Equilibrium (LLE) Data. The Othmer−Tobias correlation38 and the Hand correlation39 were used to ensure the quality of the obtained experimental tie-line data. The correlations are obtained using eqs 3 and 4, respectively:

interaction energy indicated weaker interaction strength, and higher values of IDAC indicated that the solute had less favorable interaction with the solvent. Thus, an inverse relationship between IDAC and interaction energies was obtained. This agrees well with our previous work on the simultaneous interaction with nitrogen compounds and ILs,5,7 since it was reported that complexes with less-negative values of interaction energies had higher values for activity coefficient and, therefore, lesser miscibility between the solute and the solvent. 4.4. Sigma Profile by COSMO-RS Calculation. In the COSMO-RS scheme, the screening charge density or sigma (σ) is the only descriptor5,7,8 that describes the local polarity of a molecular surface and determines the interaction energies, replacing the empirical interaction parameters usually used in conventional chemical engineering models such as UNIQUAC and UNIFAC. The σ parameter further indicates the polarity (or lack thereof) of the components in its pure state or in a mixture. The sigma profile for the combination of cation and anions of the IL ([EMIM][EtSO4]), along with pyrrole and pyridine, is discussed in this section. Figure 8 shows the sigma profiles for the combination of [EMIM] cation and [EtSO4] anion of the IL [EMIM][EtSO4], and the two solute compounds (i.e., pyrrole and pyridine).

Figure 8. Sigma profile for [EMIM], [EtSO4], [EMIM][EtSO4], pyrrole, and pyridine. (The dashed lines indicate the cut-off surface charge density for hydrogen bonding, i.e., σhb = ±0.0082 e/Å2.)

Othmer−Tobias: ⎛1 − xI ⎞ ⎛ 1 − x II ⎞ HC IL ⎟ ⎜ ⎟ = a + b ln⎜ ln I II ⎝ x IL ⎠ ⎝ x HC ⎠

The two vertical dashed lines in Figure 8 are the locations of the cut-off values for the hydrogen bond donor (σhb < −0.0082 e/ A2) and acceptor (σhb > 0.0082 e/A2). The importance of this cut-off value lies in the fact that the profile lying in the left side of σhb = −0.0082 e/A2 would have high donor ability and the profile lying in the right side of σhb = +0.0082 e/A2 would have high acceptor ability. Profiles lying in the negative region were due to inherent positive charge of the atom/molecule and vice versa for the positive region of the profile. The sigma profile of [EMIM] cation showed a symmetrical nature, while the prominent peaks of pyrrole and pyridine lied on the positive side of the sigma profile, which was due to the negative charge on the N atom. Overlapping of the sigma profiles of pyrrole and pyridine indicates high immiscibility, which proves that they do not like each other.5,7 The negative screening charge of the cation was due to the positive charge residing inside the aromatic ring of the cations. [EMIM] showed a peak at the outermost position in the negative direction. It could be seen that, for [EMIM] and pyrrole/pyridine, a very small fraction of the profile resided in the donor or acceptor region. Thus, weak hydrogen bonding was favored between the acidic hydrogen of [EMIM] cation with pyrrole/pyridine. Also note that the aromatic ring on the cation

(3)

Hand: ⎛xI ⎞ ⎛ x II ⎞ ⎟ ⎜ NC ⎟ ln⎜ NC = c + d ln I II x ⎝ IL ⎠ ⎝ x IL ⎠

(4)

where a and b are the correlation parameters for the Othmer− Tobias correlation and c and d are the correlation parameters for the Hand correlation. The correlation parameters (a and b in eq 3, c and d in eq 4) and correlation factor (R2) were determined via partial squares regression. The correlated results are reported in Table 5. The closeness of the correlation factor R2 to unity and the linearity of the plot reveal the high degree of consistency of measured data in this study. 4.6. Ternary LLE Data. The extraction performance of [EMIM][EtSO 4 ] toward pyrrole and pyridine from nhexadecane as the model diesel compound was evaluated by determining the distribution coefficient and selectivity of the IL toward pyrrole and pyridine. 12954


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concentration of pyrrole of 50 wt %. Meanwhile, from Table 7 and Figure 10, the concentration of solute (pyridine) in the raffinate phase was higher, ranging from 0.0319 to 0.2534, which was indicative that larger solvent:feed ratios were required to remove higher concentrations of pyridine. Based on the results shown in Tables 6 and 7, it is observed that [EMIM][EtSO4] gave higher values of D and S for pyrrole compared to pyridine. The highest value of D for pyrrole was 349.65, whereas for pyridine, the highest value for D was 3.17. In terms of selectivity, the highest value for pyrrole was 465 607, whereas for pyridine, it was 3161. Again, the higher values of D and S were obtained for the system with pyrrole reaffirmed that the IL [EMIM][EtSO4] had a stronger interaction with pyrrole than with pyridine. 4.7. NRTL Correlation. In the nonrandom two-liquid (NRTL) model,40 the nonideal liquid phase activity coefficient (γ) of component i is given by eq 7:

Table 5. Parameters for the Othmer−Tobias and Hand Correlations for the Ternary Systems Containing Pyrrole and Pyridine Othmer−Tobias

Hand

nitrogen compound

a

b

R2

c

d

R2

pyrrole pyridine

1.197 0.965

−5.233 −1.301

0.981 0.995

1.186 0.943

−5.232 −1.319

0.979 0.996

The distribution coefficient (D) describes how the solute (pyrrole/pyridine) was distributed between the two phases (extract or IL-rich phase and raffinate or hexadecane-rich phase). It is defined as the ratio of solute mole fraction in the extract phase (xIPYR/PY) to that of in the raffinate phase (xIIPYR/PY), as shown in eq 5: DPYR/PY =

I x PYR/PY II x PYR/PY

(5)

c

Selectivity (S) was used to measure the affinity of the solute (pyrrole/pyridine) in the fuel mixture toward the solvent ([EMIM][EtSO4]) quantitatively. It is the ratio of the distribution coefficient of pyrrole/pyridine (DPYR/PY) to the distribution coefficient of n-hexadecane (DHEX), as shown in eq 6, where xIHEX and xIIHEX are the mole fraction of n-hexadecane in the extract and raffinate phases, respectively. SPYR/PY =

ln γi =

c ∑k = 1 Gkixk

c

+

⎡

⎛

Gijxj ⎜τij c ∑ G x ⎜ j = 1 ⎣ k = 1 kj k ⎝

∑ ⎢⎢

∑i = 1 τijGijxi ⎞⎤ ⎟⎥ c ∑k = 1 Gkjxk ⎟⎠⎥⎦ c

−

(7)

where Gji = exp( −αjiτji)

DPYR/PY DHEX

∑ j = 1 τjiGjixj

Here, τij and τji are binary interaction parameters and αij is the nonrandomness parameter. Nonrandomness parameters of αij = α ji = 0.2 were used in the NRTL calculations. The NRTL interaction parameters were obtained (Table 8) by minimizing the objective function, as defined by eq 8, which was defined as the sum of the square of errors between experimental and calculated compositions of all three components for the entire set of tie lines. Details on the methodology and application are presented in an earlier work.41 The population size (npop = 100) and the number of generations (ngen = 200) were used for the genetic algorithm (GA) program. Since the GA toolbox42 in MATLAB is used for maximization, the objective function (F), which is used for minimizing the total error between the experimental and calculated mole fractions, was defined using eq 8. The modified Rachford−Rice algorithm43 was used to compute the tie lines:

(6)

The numerical values of experimental tie lines are provided in Tables 6 and 7. In both ternary diagrams, the extract phase is rich in [EMIM][EtSO4], while the raffinate phase is rich in nhexadecane. The concentration of [EMIM][EtSO4] in the raffinate phase was below the detection limit of NMR, and thus, was considered to be zero. Figures 9 and 10 show the ternary diagrams for the [EMIM][EtSO4] (1) + pyrrole (2) + nhexadecane (3) and [EMIM][EtSO4] (1) + pyridine (2) + nhexadecane systems, respectively. These figures show that the slopes of the tie lines were positive for both ternary LLE diagrams. For the [EMIM][EtSO4] (1) + pyrrole (2) + nhexadecane (3) system, the concentrations of solute (pyrrole) in the raffinate phase were very small, ranging between 0.0004 and 0.0079 (see Table 6). This indicated high separation efficiency and was indicative that a solvent:feed ratio of 1:1 was sufficient for efficient removal of pyrrole from n-hexadecane, up to a feed

Table 6. Experimental Tie Lines for the System [EMIM][EtSO4] (1) + Pyrrole (2) + n-Hexadecane (3) at 298.15 Ka IL-Rich Phase

a

Hexadecane-Rich Phase

x1′

x2′

x3′

x1″

x2″

x3″

DPYRR

SPYRR/HEX

0.8450 0.7397 0.6678 0.5890 0.5395 0.4903 0.4518 0.4187 0.3898 0.3656

0.1543 0.2582 0.3297 0.4074 0.4577 0.5085 0.5470 0.5802 0.6083 0.6339

0.0008 0.0021 0.0025 0.0036 0.0028 0.0011 0.0012 0.0010 0.0020 0.0004

0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

0.0004 0.0014 0.0022 0.0025 0.0031 0.0041 0.0050 0.0053 0.0070 0.0079

0.9996 0.9986 0.9978 0.9975 0.9969 0.9959 0.9950 0.9947 0.9930 0.9921

349.65 188.11 149.51 160.42 146.17 124.56 108.95 110.10 86.79 80.72

465 607 90 135 58 670 44 677 51 447 110 496 88 364 105 493 43 593 181 597

Values given in terms of mole fraction. 12955


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Industrial & Engineering Chemistry Research Table 7. Experimental Tie Lines for the System [EMIM][EtSO4] (1) + Pyridine (2) + n-Hexadecane (3) at 298.15 Ka IL-Rich Phase

a

Hexadecane-Rich Phase

x1′

x2′

x3′

x1″

x2″

x3″

DPY

SPY/HEX

0.8959 0.8078 0.7439 0.6776 0.6215 0.5665 0.5353 0.4938 0.4601 0.4257

0.1011 0.1907 0.2550 0.3211 0.3768 0.4299 0.4622 0.5047 0.5389 0.5737

0.0030 0.0015 0.0011 0.0013 0.0017 0.0036 0.0025 0.0015 0.0009 0.0005

0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

0.0319 0.0661 0.0966 0.1184 0.1461 0.1729 0.2025 0.2189 0.2351 0.2534

0.9681 0.9339 0.9034 0.8816 0.8539 0.8271 0.7975 0.7811 0.7649 0.7466

3.17 2.89 2.64 2.71 2.58 2.49 2.28 2.31 2.29 2.26

1016 1838 2230 1881 1260 573 724 1215 1848 3161

Values given in terms of mole fraction.

Figure 9. Ternary diagram for the system [EMIM][EtSO4] (1) + pyrrole (2) + n-hexadecane (3) at 298.15 K. Filled circles and solid lines indicate experimental tie lines, empty circles and dashed lines indicate NRTL tie lines and filled triangle and dotted lines indicate COSMO-RS predicted tie lines. m

Maximize

F

⎛ with respect to A ij ⎞ ⎜ ⎟ ⎜ ⎟ ⎝ where i , j = 1,2,3 and j ≠ i ⎠

= −∑

II

c

The tie lines calculated using the NRTL model were compared against experimental data, as shown in Figures 9 and 10. It was observed that the calculated tie lines were in excellent agreement with experimental tie lines (in fact, they overlap each other). The RMSDs were 0.058% and 0.51% for systems containing pyrrole and pyridine, respectively. 4.8. Prediction of Ternary LLE Tie Lines with COSMORS. COSMOthermX software package44 was used to predict the ternary LLE data for the [EMIM][EtSO4] + pyrrole/pyridine + n-hexadecane system at 298.15 K, based on the computed COSMO files. The ternary LLE data were predicted at the BPTZVP level with parametrization file BP_TZVP_C30_1401.ctd. COSMO-RS gave excellent prediction with an RMSD of 0.85% for the [EMIM][EtSO4] (1) + pyrrole (2) + n-hexadecane (3) system, and 1.12% for the [EMIM][EtSO4] (1) + pyridine (2) + n-hexadecane (3) system, as shown in Figures 9 and 10.

∑ ∑ wikl(xikl − xik̂ l )2

k=1 l=I i=1

(for wikl = 1)

(8)

where xikl and x̂̂ikl are, respectively, the experimental and predicted values of composition (mole fraction) for component i for the kth tie line in phase l. The NRTL model parameters are shown in Table 5. The goodness of fit between experimental and NRTL calculated tie lines were evaluated using the root-mean-square deviation (RMSD), as shown in eq 9, ⎡m RMSD (%) = ⎢∑ ⎢⎣ k=1

c

2

∑∑ i=1 j=1

⎤1/2 (xikj − xik̂ j )2 ⎥ × 100 ⎥⎦ 2mc

(9)

5. CONCLUSION Ab initio calculations were performed to investigate the interactions between [EMIM][EtSO4] and pyrrole/pyridine

where m is the number of tie lines, c the number of components, and j is the number of phases (i.e., 2). 12956


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Figure 10. Ternary diagram for the system [EMIM][EtSO4] (1) + pyridine (2) + n-hexadecane (3) at 298.15 K. Solid lines and dashed lines indicate experimental and COSMO-RS predicted tie lines, respectively.

to describe the interaction further using the sigma profiles of [EMIM], [EtSO4], [EMIM][EtSO4], pyrrole, and pyridine. It was observed that a very small fraction of either profile lay in the donor or acceptor region, implying that the CH−π interaction was more dominant than the hydrogen bonding interaction between the O and N atoms of the anion and the aromatic nitrogen compound of the cation. Finally, ternary LLE experiments were conducted for the [EMIM][EtSO4] + pyrrole/pyridine + n-hexadecane system to validate the calculation results. The experimental results reported that [EMIM][EtSO4] gave higher values of distribution coefficient and selectivity for pyrrole than for pyridine, confirming that the IL had a stronger interaction with pyrrole than with pyridine. The experimental results were well-correlated by NRTL model with a root-mean-square deviation (RMSD) of 0.058% for pyrrole and 0.51% for pyridine. Ther COSMO-RS model was also used to predict the ternary liquid−liquid extraction (LLE) data and the predictions were in excellent agreement with experimental data, with RMSD of 0.85% and 1.12% for pyrrole and pyridine, respectively.

Table 8. Nonrandom Two-Liquid (NRTL) Model Parameters for the Two Ternary Systems at T = 298.15 K i−j

τij

τji

F

root-mean-square deviation, RMSD (%)

[EMIM][EtSO4] (1) + Pyrrole (2) + n-Hexadecane (3) System −355.51 4769.1 2.02 × 10−5 1179.6 1964.2 0.058 −266.49 1072.6 [EMIM][EtSO4] (1) + Pyridine (2) + n-Hexadecane (3) System 1−2 2814 4249 1.56 × 10−3 1−3 4920.4 1729.1 0.51 2−3 14153 5000

1−2 1−3 2−3

for a potential denitrification process using ionic liquids (ILs). The interactions between the individual cation ([EMIM]) and pyrrole/pyridine, between the anion ([EtSO4]) and pyrrole/ pyridine, as well as between the neat IL ([EMIM][EtSO4]) and pyrrole/pyridine, were studied in detail. The partial charge transfer in the N atom of pyrrole was more significant than the partial charge transfer in the N atom of pyridine before and after interactions with [EMIM][EtSO4], indicating greater delocalization for pyrrole than for pyridine when in contact with [EMIM][EtSO4]. The calculated interaction energy as an attempt to quantify the CH−π and O−H interaction in complexes of [EMIM][EtSO4] with pyrrole/pyridine indicated that the interaction between [EMIM][EtSO4] and pyrrole was stronger than that with pyridine, based on the more-negative value of the interaction energy that the [EMIM][EtSO4] + pyrrole complex had. The infinite dilution activity coefficient (IDAC) values for pyrrole and pyridine in [EMIM][EtSO4] were calculated using the conductor-like screening model for real solvent (COSMO-RS) methodology. There was an inverse relationship between the predicted IDAC and the interaction energy of the complex, i.e., a higher value of IDAC was reported for systems with less-negative interaction energy (i.e., [EMIM][EtSO4] + pyridine). This is completely consistent, since a higher value of IDAC indicates less favorable interaction between the solute and solvent. The COSMO-RS approach was also applied

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.5b01962. COSMO-RS and NRTL model LLE data for the ternary system of {[EMIM][EtSO4] (1) + pyrrole/pyridine (2) + n-hexadecane (3) at 298.15 K (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 044-32909138-263. Fax: 044-32909138. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 12957


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NOTATION

List of Symbols

[EMIM] = 1-ethyl-3-methylimidazolium [EtSO4] = ethyl sulfate [EMIM][EtSO4] = 1-ethyl-3-methylimidazolium ethyl sulfate PYR = pyrrole PY = pyridine Greek Symbols

σ = screening charge density (e/Å2) σ hb = cut-off screening charge density for hydrogen bonding (e/Å2)

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