GPE - 3rd International Congress on Green Process Engineering
6-8 december 2011 – Kuala-Lumpur (Malaysia)
COSMO-RS Based Screening Ionic Liquids for
Separation of Benzene and Cyclohexane
G. GONFA1, M.A. BUSTAM2
1Department
of Chemical Engineering, Univeristi Teknologi PETRONAS, Tronoh-31750, Perak Malaysia
Email : kiyaagonfaa@gmail.com, Tel : +60135094924
2Department
of Chemical Engineering, Univeristi Teknologi PETRONAS, Tronoh-31750, Perak Malaysia
Email : azmibustam@petronas.com.my Tel : + 605-3687641
Abstract. Cyclohexane is mainly produced by catalytic hydrogenation of benzene and the
removal of unreacted benzene is very important process. However, the two components have
close boiling points and form azeotropic mixture which needs special separation processes, such
as extractive distillation. However, the existing conventional solvents used foe extractive
distillation has diverse limitation. Although, ionic liquids became promising solvents in
separation process, the selection of appropriate ionic liquid for specific process is impossible
without a prior screening. This paper presents the screening of potential ionic liquid entrainers
in the distillative separation of benzene and cyclohexane using COSMO-RS model. The
screening starts with the prediction of the activity coefficients at infinite dilution by tailoring 44
anions and 60 cations resulting in 2640 possible combinations. The predicted infinite activity
coefficients were compared with experimental results to check the reliability of the prediction. A
total of 424 experimental data point were used for this purpose. The root-mean-square
deviations (rmsd) of infinite dilution activity coefficient for benzene and cyclohexane are 0.12
and 0.44 ln-units respectively. The selectivity and solvent capacity at infinite dilution were
calculated from the predicted activity coefficients. It was found that structural variations of the
ionic liquid entrainer cause converse changes in selectivity and capacity. The selectivity of
benzene to cyclohexane increases with a shorter and unbranched alkyl substituent of the cation
and with anions with sterical shielding. Moreover, imidazolium and thiazolium have selectivity
higher and low solvent capacity where as higher capacities were observed for ionic liquids with
ionic liquids consisting of phosphonium and guanidinium cations.
Key-words. COSMO-RS, Extractive distillation, Entrainer, Ionic liquid
INTRODUCTION
Cyclohexane is an important bulk chemical for production of adipic acid and ε-caprolactam,
which is the a feedstock for Nylon6 and Nylon6,6 production [1]. It is also used as a solvent in the plastic
industry, in paints and varnishes, and for the extraction of essential oils[1]. Cyclohexane is mainly
produced by catalytic hydrogenation of benzene, and the removal of the unreacted benzene from the
reactor’s effluent stream is a very important process[1, 2]. However, the separation of benzene and
cyclohexane mixtures has been proved one of the most difficult tasks in the petrochemical industry
because of their close boiling points and formation of azeotropic mixture [3, 4]. Since, conventional
distillations are not practical in such cases, special distillation such as extractive distillation is widely
employed for benzene/cyclohexane separation [1]. However, the entrainers used in extractive distillation,
such as, solid salts and organic substances comprise many disadvantages. The solid salts may corrode the
column and pipeline and the organic solvents demand high energy due to their low performance [5].
Recently, ionic liquids (ILs) have been attracted attention for the replacement of conventional
solvents in separation process because of their unique merits; good performance, non volatility,
inflammability, thermal stability and dissolving ability [6, 7]. However, due to unlimited possible
combinations of cations and anions, the evaluation of ILs for extractive distillation is time consuming
and expensive if carried out experimentally. The opportunity to select new ILs may also be missed.
Hence, to effectively select the best IL with desired properties, all possible ILs must be preliminary
screened in a systematic way by tuning the cations and anions. The conductor-like screening model for
real solvents (COSMO-RS) demonstrate the rapid progress in priori prediction of thermodynamic of
liquids. An important advantage of the COSMO-RS model is that it predicts the properties of component
in a mixture without using any experimental data.
1
In this work potential ILs were screened by tuning cation and anion combinations. A quantum
chemical based “conductor-like screening model for real solvents” (COSMO-RS) is used for the
prediction of selectivity and solvent capacity of benzene and cyclohexane in ILs at infinite dilution.
BACKGROUND
COSMO-RS predicts thermodynamic properties of liquid mixtures, such as, activity coefficient,
by using the molecular structure of solute and solvent as initial inputs. The details about the COSMO-RS
model along with the expression activity coefficient in the liquid phase are described in somewhere else
[8-12]. Activity coefficients at infinite dilution (∞) are used for the preliminary selection of solvents for
extractive distillation by calculating the selectivity and solvent capacity of solutes in solvents.
The first application of COSMO-RS to the prediction of activity coefficients at infinite dilution
of solutes was done by Klamt et al.[13] . The predictions obtained exhibit the same accuracy with that is
observed for normal organic solvents with largest a root mean square deviation of 0.52 ln-units. Banerjee
and Khanna used COSMO-RS, originally fitted for the LLE prediction of neutral compounds, for the
prediction of activity coefficients at infinite dilution in trihexyl-tetradecylphosphonium cation based ILs
with the anions chloride, tetrafluoroborate, and bis (trifluoromethane sulfonylimide). The maximum
average absolute deviation (AAD) of the ∞ values is about 16%.
Thermodynamic optimization of ILs as solvent in separation based on COSMO-RS predictions
has been reported by Jork et al.[14]. The same procedure has been used for extractive distillation of 1hexene/hexane [15], denitrification of diesel oil [16, 17], desulfurization diesel oil [17, 18] , extraction
of drugs from urine[19], and carbon dioxide capture [20]. From these works it can be understood that
COSMO-RS is a powerful tool for a prior screening of ILs for specific purpose. In this work, COSMORS predictions of the selectivity and solvent capacity will be reported. The effect of structure of ILs on the
selectivity and solvent capacity will be discussed using the predicted selectivity and solvent capacity and
by comparing the sigma profile of solutes and ILs. .
COMPUTATIONAL DETAILS
COSMO-RS calculations were performed using the COSMOtherm software. The BP-TZVP
C21_0110 parameterization has been used. The data base used calculation were performed on the density
functional theory (DFT) level, utilizing the BP functional [21-23] with resolution of identity (RI)
approximation and a triple-ξ valence polarized basis set (TZVP) [24, 25]. The structures are fully
optimized and the quantum chemical calculations have to be performed for each molecule with
TURBOMOLE program package. For the prediction involving ILs, electroneutral mixture approach
(complete dissociation of ILs into cations and anions) were assumed. Thus, the sigma profile is simply the
linear addition of the sigma profiles of the cation and anion. COSMO-RS calculations were performed
using the COSMOthermX program.
PIL ( )  Panion ( )  Pcation ( )
(1)
Where, Pcation (σ) and Panion (σ) are the sigma profiles for cation and anion, respectively.
For comparison sigma profile of solutes and ILs the sigma profile of cation and anion were added
up by simple algebraic addition and normalized. Thus, this profile will behave as if it is a profile of a
single molecule. This is equivalent to calculating the sigma profile of a mixture of cation and anion. The
selectivity and solvent capacity are calculated using equation (2) and 3).
Sij 
Cj 
 i
 j
1
 j
(2)
(3)
Where, γi∞ and, γj∞ are the activity coefficients of cyclohexane and benzene in ILs respectively.
RESULT AND DISCUSSION
To validate the result, the predicted infinite dilution activity coefficients of benzene and
cyclohexane were compared with experimental results reported in literature. For comparison, 424 data
points over 298.15 to 368.15 K temperatures were used. The root-mean-square deviations (rmsd) of
infinite dilution activity coefficient for benzene and cyclohexane are 0.12 and 0.44 ln-units respectively.
This agrees with the expected ( maximum deviation of 1.0 ln-units) infinite dilution activity coefficient of
normal organic solutes in ILs [13, 26].
After benchmarking, the selectivity at infinite dilution was predicted for various classes of
cations. Selectivity values of different families of cations (imidazolium, pyridinium, pyrrolidinium,
piperidinium, pyrazolium, thiazolium, guanidinium, ammonium and phosphonium based cations) coupled
2
with 44 anions were investigated. The basic structures of the cations are shown in Table 1. Effects of
variations in alkyl chain length, branching, and presence of electronegative groups in anions are discussed
in the subsequent sections.
Table 1 Basic cations investigated in this study
No
Cation
Structure
No No
1.
Cation
R3
2.
Imidazolium
R1
+
N
3.
N
N
+
R1
R3
+
Ammonium
N
R2
S
4.
+
Pyridinium
Thiazolium
R1
Structure
R4 N
R2
R1
R
5.
R3
6.
Pyrrolidinium
R
1
N
+
R
Phosphonium
2
+
R4 P
7.
+
8.
Peperidinium
R2
R1
NH2
Guanidinium
N
H2N
NH2
R
R2
N
9.
Pyrazolium
R1
N
O
2+
O
-
Selectivity was found to vary with cation and anion alkyl chain length. Figure 1 shows the
influence of alkyl chain length of the pyridinium based cations on the selectivity of benzene to
cyclohexane at infinite dilution at 288.15 K. The alkyl chain varies from ethyl group to octyl group. It can
be seen that longer cation alkyl chain is unfavorable for selectivity, suggesting less preferential affinity for
benzene. Generally, for all imidazolium, pyridinium, pyrrolidinium, thiazolium, ammonium and
phosphonium based cations the same trend was observed. The exception to this trend is imidazolium
based ILs containing [Cl]-, [(CF3SO2)3N]-, [BMB]-, [(C4F9)3PF3]-, and [BPB]- anions. Ammonium based
ILs containing [Cl]-, [NO3]-, [SCN] - and [CH3COO] - are also different from the general trends. The
exceptional cases were also observed for [Cl]- and [CH3SO3]− by Lei et al.[15] while screening ILs for
separation of 1-hexene and n-hexane. For ammonium and phosphonium based ILs the selectivity widely
varies with the number of alkyl chain attached to the central atoms, nitrogen and phosphorus, respectively.
Besides, under a given cation, the shorter anion alkyl chain length favours higher the selectivity of
benzene to cyclohexane. The selectivity decreases according to the sequence of [HSO4]-, [CH3SO4]-,
[C2H5SO4]-, [C8H17SO4]- and [H2PO4]- [(CH3)2PO4]-, [(C2H5)2PO4]- , [(C4H9)2PO4]- for sulfur and
phosphate based anions.
The influence of group substitution on the selectivity of benzene to cyclohexane at infinite
dilution has also been investigated. As it can be seen from Figure 2 the selectivity of benzene to
cyclohexane decreases as the degree of substitution on the pyridinium cation increases. Thus, for given
anion the selectivity of 1- ethyl > 1-ethyl-3-methyl > 1-ethyl-2, 6-dimethyl imidazolium pyridinium. The
same trends were observed for imidazolium based IL. If 2-hydrogen of 1, 3- dimethyl and 1-butyl-3methyl imidazolium substituted by methyl group the selectivity of benzene to cyclohexane under a given
anion decreases. This indicates that group substitution decreases the selectivity. Introduction of long alkyl
chain increases the van der Waals volume of cation and anion, which leads to a decrease in cation- anion
interaction and increase volume of the channel formed between cation and anion as reported by Hanke et
al.[27]. This facilitates the accommodation of benzene molecules in the cations. This is consistent with
the lower infinite dilution of benzene in longer alkyl chain ILs. However, as the longer alkyl chain also
favours the van der Waals interaction between cyclohexane and IL which reduces the selectivity of
cyclohexane to benzene.
Moreover, anions with sterical shielding effect around their charge centers give high selectivity
than those with no sterical shielding effect around their charge centers. Thus, [PF 6]-, [BOB]- , [B (CN)4]-,
[BTA]-, [CF3SO3]-, [BMB]- are favourable for higher selectivity than [(CH3)2PO4]-, [CH3SO3]-, [OAc]and [Cl]-. This is consistent with the result reported by Lei et al.[15] for hexane/hexene system. .
3
20
18
16
14
12
10
8
6
4
2
0
EMPy
BEPy
OtMPy
[Cl][NO3]
[SCN]
[CH3COO][BF4][H2PO4][ClO4][HSO4 ][NCN]
[CH3SO3 ]
[CF3COO]
[MeSO4]
[PF6]
[(CN)3C][CF3SO3]
[(CH3SO2)2N][BTP][EtSO4]
[DMP]
[MDEGSO4 ][(F2PO)N][ba][B(CN)4]
[Sal]
[BOB] [BtSO4][HB][DEP]
[EOESO4][TOS]
[BMB][Tf2N]
[(C2F5SO2)2CH][F9C4SO3][DEC]
[(C2F5)2PO2][OtSO4]
[DBP]
[BSB][(C2F5SO2)2N][(CF3SO2)3N][TPfEPF3]
[BPB][(C4F9)3PF3]-
benzene to cyclohexane selectvity
However, the opposite holds true for polar systems as reported by Jork et al.[14], Mohanty et al. [28] and
Lei et al.[19]. The anion without sterical shielding effect would more approach the positive charge center
of cation due to its naked negative charge, and thus inhibit the interaction between positive charge center
and benzene π – electron. Therefore, the ILs with sterical shielding effect around the anion charge center
are favorable for the separation of benzene and cyclohexane. Moreover, for some anions without sterical
shielding effect, such as [Cl]-, the longer cation alkyl chain may inhibit the naked negative charge of
anions from approaching the positive charge center of cation and enhance the selectivity as observed in
this study. Therefore, the suitable ILs for the separation of benzene and cyclohexane are of small
molecular volume, unbranched group and sterical shielding effect around anion charge center.
Solvent capacity is also an important parameter to select solvent for separation process because it
determines the amount entrainer required which in turn affect the size of the equipment. Generally, solvent
with high selectivity have low solvent capacity in separation of non polar components. However, a few
ILs exhibit both a relative high selectivity and good capacity. Fig. 3 shows the effect of cation on the
selectivity and solvent capacity of benzene and cyclohexane. Imidazolium and thiazolium have higher
selectivity and low solvent capacity. Higher capacities are realized with ionic liquids consisting of
phosphonium and guanidinium cations.
Anions
Figure 1 Selectivity values at infinite dilution for 1-ethyl-3-methyl, 1-butyl-3-methyl and 1-octyl-3-methy imidazolium at 298.15 K
Benzene to cyclohexane selectvity
35
Epy
EMPy
EMMPy
30
25
20
15
10
5
[PF6]
[B(CN)4]
[BOB] [ClO4][BSB][(CN)3C][BF4][BPB][NCN]
[CF3SO3]
[(F2PO)N][SCN]
[CF3COO]
[Sal]
[(CH3SO2)2N][HSO4 ][MeSO4]
[EtSO4]
[MDEGSO4 ][Tf2N]
[(C2F5SO2)2CH][NO3]
[EOESO4][TOS]
[BMB][BtSO4][ba][HB][F9C4SO3][(CF3SO2)3N][(C2F5SO2)2N][H2PO4][OtSO4]
[(C2F5)2PO2][CH3SO3 ]
[DEP]
[DMP]
[TPfEPF3]
[CH3COO][DBP]
[DEC]
[BTP][Cl][(C4F9)3PF3]-
0
Anion
Figure 2 Influence of group substitution on the selectivity of benzene to cyclohexane at infinite dilution at 298.15 K 1ethylpyridinium (Epy), 1-ethyl-3-methyl pyridinium (EMPY) and 1-ethyl-2, 6-dimethylimidazolium pyridinium (EMMPY) based
ILs.
4
Benzene to Cyclohexane Selectvity
50
40
30
20
10
0
0
1.5
3
4.5
6
7.5
Solvent capcity
Figure 3 Influences different cation on the selectivity and solvent capacity of benzene and cyclohexane cations at 298.15 K. The
Cations consist of a) ■ 1-ethyl-3 methyl, ▼ 1-octyl-3-methyl, ♦ 1-(2-hydroxy) methyl imidazolium b) ▼1-ethyl-3 methyl, ■ 1octyl-3-methylpyridinium c) ■1-ethyl-3 methyl, ▼ 1-octyl-3-methyl pyrrodinium d ) ■ 1-methyl-2-(phenyl), ▼1-methyl-2-propyl
pyrazolium e) ) ■ 1-ethyl-3-methyl, ▼ 1-butyl-3- methyl thiazolium f) ▲ trimethylethylammoinium g) ● trisobutylmethylphosphonium h) ♦ hexamethyl-guanidinium with selected 44 anions.
CONCLUSION
A systematic screening of 2640 possible cation–anion combinations was done using COSMO-RS
for separation of benzene and cyclohexane. It was found that structural variations of the ILs liquid
entrainer cause converse changes in selectivity and capacity. The selectivity of benzene to cyclohexane
increases with a shorter and unbranched alkyl substituent of the cation and with anions with sterical
shielding. Moreover, imidazolium and thiazolium have selectivity higher and low solvent capacity where
as higher capacities were observed for ionic liquids with ionic liquids consisting of phosphonium and
quinolinium cations.
Refrences
[1] J. P. Garcia Villaluenga and A. Tabe-Mohammadi, "A review on the separation of
benzene/cyclohexane mixtures by pervaporation processes," Journal of Membrane Science, vol. 169,
pp. 159-174, 2000.
[2] Y. Bai, et al., "Compatibility of PTET-60/CA blends and separation performance of their membranes
for benzene/cyclohexane mixture by pervaporation," Journal of Applied Polymer Science, vol. 102,
pp. 2832-2838, 2006.
[3] F. Peng, et al., "Significant increase of permeation flux and selectivity of poly (vinyl alcohol)
membranes by incorporation of crystalline flake graphite," Journal of Membrane Science, vol. 259,
pp. 65-73, 2005.
[4] S. J. Lue, et al., "Pervaporation of benzene/cyclohexane mixtures using ion-exchange membrane
containing copper ions," Journal of Membrane Science, vol. 240, pp. 149-158, 2004.
[5] Q. Li, et al., "Selection of ionic liquids as entrainers for the separation of ethyl acetate and ethanol,"
Industrial & Engineering Chemistry Research, vol. 48, pp. 9006-9012, 2009.
[6] K. Marsh, et al., "Room temperature ionic liquids and their mixtures--a review," Fluid Phase
Equilibria, vol. 219, pp. 93-98, 2004.
[7] J. Zhu, et al., "Study on the separation of 1-hexene and trans-3-hexene using ionic liquids," Fluid
Phase Equilib, vol. 247, pp. 102-106, 2006.
5
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
A. Klamt and G. Schüürmann, "COSMO: a new approach to dielectric screening in solvents with
explicit expressions for the screening energy and its gradient," Journal of the Chemical Society,
Perkin Transactions 2, vol. 1993, pp. 799-805, 1993.
A. Klamt, et al., "Refinement and parametrization of COSMO-RS," J. Phys. Chem. A, vol. 102, pp.
5074-5085, 1998.
A. Klamt and F. Eckert, "COSMO-RS: a novel and efficient method for the a priori prediction of
thermophysical data of liquids," Fluid Phase Equilibria, vol. 172, pp. 43-72, 2000.
A. Klamt. (2005). COSMO-RS: From Quantum Chemistry to Fluid Phase Thermodynamics and
Drug Design (1st ed ed.).
M. Diedenhofen and A. Klamt, "COSMO-RS as a tool for property prediction of IL mixtures--A
review," Fluid Phase Equilibria, vol. 294, pp. 31-38, 2003.
M. Diedenhofen, et al., "Prediction of Infinite Dilution Activity Coefficients of Organic Compounds
in Ionic Liquids Using COSMO-RS†," J. Chem. Eng. Data, vol. 48, pp. 475-479, 2003.
C. Jork, et al., "Tailor-made ionic liquids," The Journal of Chemical Thermodynamics, vol. 37, pp.
537-558, 2005.
Z. Lei, et al., "Separation of 1-hexene and n-hexane with ionic liquids," Fluid Phase Equilibria, vol.
241, pp. 290-299, 2006.
R. Anantharaj and T. Banerjee, "COSMO-RS-Based Screening of Ionic Liquids as Green Solvents
in Denitrification Studies," Industrial & Engineering Chemistry Research, 2010.
R. Anantharaj and T. Banerjee, "COSMO-RS based predictions for the desulphurization of diesel oil
using ionic liquids: Effect of cation and anion combination," Fuel Processing Technology, 2010.
A. Kumar and T. Banerjee, "Thiophene separation with ionic liquids for desulphurization: A
quantum chemical approach," Fluid Phase Equilibria, vol. 278, pp. 1-8, 2009.
Z. Lei, et al., "COSMO-RS modeling on the extraction of stimulant drugs from urine sample by the
double actions of supercritical carbon dioxide and ionic liquid," Chemical engineering science, vol.
62, pp. 3940-3950, 2007.
X. Zhang, et al., "Screening of ionic liquids to capture CO2 by COSMO RS and experiments,"
AIChE Journal, vol. 54, pp. 2717-2728, 2008.
A. D. Becke, "Density-functional exchange-energy approximation with correct asymptotic
behavior," Physical Review A, vol. 38, pp. 3098-3100, 1988.
S. H. Vosko, et al., "Accurate spin-dependent electron liquid correlation energies for local spin
density calculations: a critical analysis," Canadian Journal of Physics, vol. 58, pp. 1200-1211,
1980.
J. P. Perdew, "Density-functional approximation for the correlation energy of the inhomogeneous
electron gas," Physical Review B, vol. 33, pp. 8822-8824, 1986.
A. Schäfer, et al., "Fully optimized contracted Gaussian basis sets of triple zeta valence quality for
atoms Li to Kr," The Journal of Chemical Physics, vol. 100, p. 5829, 1994.
K. Eichkorn, et al., "Auxiliary basis sets for main row atoms and transition metals and their use to
approximate Coulomb potentials," Theoretical Chemistry Accounts: Theory, Computation, and
Modeling (Theoretica Chimica Acta), vol. 97, pp. 119-124, 1997.
F. Eckert and A. Klamt, "Fast solvent screening via quantum chemistry: COSMO RS approach,"
AIChE Journal, vol. 48, pp. 369-385, 2002.
C. Hanke, et al., "Solvation of small molecules in imidazolium ionic liquids: a simulation study,"
Green Chemistry, vol. 4, pp. 107-111, 2002.
S. Mohanty, et al., "Quantum Chemical Based Screening of Ionic Liquids for the Extraction of
Phenol from Aqueous Solution," Industrial & Engineering Chemistry Research, vol. 49, pp. 29162925, 2010.
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