Journal Pre-proof Multipolar/polarizable molecular dynamics simulations of Liquid–Liquid extraction of benzene from hydrocarbons using ionic liquids Erik A. Vázquez-Montelongo, G. Andrés Cisneros, Hugo M. Flores-Ruiz PII:
To appear in:
Journal of Molecular Liquids
Received Date: 5 June 2019 Revised Date:
22 August 2019
Accepted Date: 28 September 2019
Please cite this article as: E.A. Vázquez-Montelongo, G.André. Cisneros, H.M. Flores-Ruiz, Multipolar/ polarizable molecular dynamics simulations of Liquid–Liquid extraction of benzene from hydrocarbons using ionic liquids, Journal of Molecular Liquids (2019), doi: https://doi.org/10.1016/j.molliq.2019.111846. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Multipolar/Polarizable Molecular Dynamics Simulations of Liquid–Liquid Extraction of Benzene from Hydrocarbons Using Ionic Liquids Erik A. V´ azquez-Montelongoa, G. Andr´es Cisnerosa , Hugo M. Flores-Ruizb,∗ a Department
of Chemistry, University of North Texas, Denton, TX 76201 de Ciencias Naturales y Exactas, CUValles, Universidad de Guadalajara, Carr. Guadalajara-Ameca km 45.5, 46600, Ameca, Jalisco. M´ exico
Abstract Separation of aromatic compounds from aromatic/aliphatic mixtures using various solvents has been a field of intense studies. One possibility for this separation is liquid–liquid extraction using room temperature ionic liquids (ILs) as solvent. Computational simulations using classical molecular dynamics (MD) calculations provide a viable approach to investigate these processes. Due to the highly charged nature of the solvent, an accurate force field is needed to accurately describe the inter–molecular interactions between the different mixture components. Here, we present the use of the multipolar/polarizable force field AMOEBA to explore the capacity of 1,3-dimethylimidazolium tetrafluorobrorate, [DMIM][BF4 ], and ethyl-methylimidazolium tetrafluorobrorate,[EMIM][BF4], to extract benzene from a mixture of benzene-dodecane. Our results indicate that [DMIM][BF4 ] exhibits a better capacity of extracting benzene than [EMIM][BF4 ]. Detailed structural and selectivity/distribution ratio analysis are provided based on our simulations to gain further insights on the different systems. Keywords: extraction, benzene, ionic liquids, multipolar/polarizable force field
author Email address: [email protected]
(Hugo M. Flores-Ruiz)
Preprint submitted to Journal of Molecular Liquids
October 9, 2019
1. Introduction The use of petroleum derivatives has been a major source of pollution for several decades. One of these derivatives is gasoline, which is widely used in vehicle engines. The use of gasoline with impurities and/or inefficient combustion 5
from the vehicle engines can lead to the production of various contaminants such as carbon oxide (CO), nitrogen oxides (NOx ), unburned hydrocarbons, etc. . Aromatic and linear unburned hydrocarbons like benzene (PhH), toluene, xylene, naphthalene, octane, etc., are a source of pollution and present a danger to human health [2, 3, 4]. For instance, short–term exposure to PhH can
cause vomiting, eye irritation and allergies, while long–term exposure can lead to various cancers . Therefore, the reduction of aromatic hydrocarbons in gasoline can aid in decreasing the level of pollution due to unburned aromatic hydrocarbons, while alternative energy sources come online. Ionic liquids (ILs) provide a possible vehicle for the liquid– liquid extraction
of aromacic compounds from gasoline. ILs are attractive solvents due to their unique properties such as low vapor pressure, reusability, thermal and chemical stability (liquids at room temperature), among others . Experimental evidence of the used of ILs to extract PhH and other aromatic compounds from hydrocarbon mixtures like gasoline is wide [6, 7, 8, 9, 10, 11].
For instance Requejo et al.
showed that 1-butyl-1-methylpyrrolidinium di-
cyanamide, [BMpyr][DCA], is an excellent candidate for extraction of PhH from a mixture of octane, decane and PhH . Babak et al. have shown that 1-butyl 3-methylimidazolium nitrate ([BMIM][NO3 ]) is also a good candidate for extraction of PhH and p-xylene from a mixture of hexane and PhH or octane and 25
p-xylene . Toluene has been shown to be able to be extracted from a mixture of heptane and tolune using 1-ethyl-3-methylimidazolium tricyanomethanide ([EMIM][TCM]) . Previously, some of us have studied PhH extraction by polar solvents and ILs , employing classical molecular dynamics (MD) with the GROMOS
54A force field and showed that 1-butyl-1-methylpyrrolidinium dicyanamide,
[BMpyr][DCA], is a good candidate, which is in agreement with experimental results . In this contribution, we present the use of the advanced multipolar/polarizable AMOEBA force field [14, 15] to explore PhH extraction from a gasoline model, 35
by means of room temperature ionic liquids. Our entire system consist of gasoline model, which is a 1:1 mixture of n-dodecane (NC12) and PhH, and a combination of 1,3-dimethylimidazolium tetrafluorobrorate, [DMIM][BF4 ], and ethyl-methylimidazolium tetrafluorobrorate, [EMIM][BF4 ] as extracting agents [16, 17]. The remainder of the paper is organized as follows: In section 2, we
briefly describe the AMOEBA force field, followed by simulation details. Subsequently the results for the simulations of the ternary systems (benzene, NC12, IL pair) for the extraction of benzene from the mixture using the two IL pairs, as well as radial and spatial distribution analyses for both ternary and binary (IL pairs + one of the solutes) systems are presented in section 3, followed by
concluding remarks in section 4.
2. Computational Methods This section presents a brief description of the details of the AMOEBA polarizable potential followed by details of the classical MD simulations, construction of systems under consideration, and radial and spatial distribution 50
function analyses. 2.1. AMOEBA force field The chosen force field used for this work is AMOEBA to provide an accurate representation of the inter–molecular interactions for these complicated systems. This force field was initially developed for water  and it has been extended to
biomolecules, organic molecules [15, 18, 19], and ionic liquids [16, 17, 20, 21, 22]. AMOEBA employs several terms in the interaction potential, perm ind V = Vbond + Vangle + Vbθ + Voop + Vtorsion + VvdW + Vele + Vele ,
where Vbond , Vangle , Vbθ , Voop and Vtorsion represent bond stretching, angle bending, bond-angle cross term, out-of-plane bending and torsional rotation interacperm ind tions, respectively. The remaining non–bonded terms, VvdW , Vele and Vele 60
account for the van der Waals, permanent and induced electrostatic interacperm tions, respectively. The Vele terms involves atom–centereed point–multipoles ind up to quadrupoles, and the Vele term is calculated by means of inducible atomic
point–dipoles. 2.2. Molecular Dynamics details 65
The AMOEBA force field was used to describe both hydrocarbons and ILs, but with a different atomic multipoles description. For the hydrocarbons, Stone’s distributed multipole analysis  (DMA) is the default option in the latest AMOEBA force field, while for the ILs, we use the Gaussian electrostatic model distributed multipoles (GEM-DM); GEM-DM has been shown to provide
accurate atomic multipoles for several systems, including ILs [24, 25, 26, 27, 28]. All MD simulations were performed in the NPT ensemble at 300 K and 1 bar of pressure with periodic boundary conditions in all directions using smooth particle mesh Ewald (PME) for the long–range electrostatcic interactions . A cutoff of 10 ˚ A was employed for the short–range non–bonded interactions,
with a switching function for the Van der Waals interactions. The Beeman integrator was used for all simulations with an integration time step was 2 fs as implemented in TINKER-OpenMM . 2.2.1. Ternary Systems A mixture of 500 NC12 and 500 PhH molecules was used to create our gaso-
line model (Figure 1a). For the ILs, we used 500 IL pairs (Figures 1b,c). First, both ILs and NC12-PhH mixture were simulated separately, then [DMIM][BF4 ] +NC12-PhH and [EMIM][BF4 ]+NC12-PhH mixtures were run for 150 ns and 300 ns, respectively. The average simulation size box was x = y = 4.5 nm and z = 18.7 nm for [DMIM][BF4 ]+NC12-PhH and x = y = 4.5 nm and z = 19.0
nm for [EMIM][BF4 ]+NC12-PhH. The density profiles along the z direction
(the largest box size) were calculated to measure the PhH extracting capabilities. The density profiles were obtained by evaluating the density along 200 equi–spaced sections along the z–direction.
Figure 1: Molecules employed for the various simulation systems. a) Benzene (PhH) and dodecane (NC12), b) [DMIM][BF4 ], and c) [EMIM][BF4 ].
2.2.2. Binary Systems 90
Binary mixtures containing [DMIM][BF4 ]+PhH and [EMIM] [BF4 ]+PhH were also simulated, to study the affinity between ILs and PhH. A mixture of 200 PhH molecules with 500 cations and 500 anions was simulated in both cases. The [DMIM] [BF4 ]+PhH system was run for 180 ns and the [EMIM][BF4 ]+PhH for 220 ns. The average simulation box size was x = y = 3.46 nm and z = 11.55
nm for [DMIM][BF4 ]+PhH, meanwhile for [EMIM][BF4 ]+PhH the average box size was x = y = 3.36 nm and z = 11.19 nm. In the case of the analysis of the affinity between PhH and NC12, a mixture of PhH+NC12 was performed with 250 molecules for the solute and each ion, resulting in an average simulation box size of x = y = 4.66 nm, z = 6.21 nm. The PhH+NC12 mixture was run
for 10 ns.
2.3. Radial and Spatial Distribution Functions Radial and spatial distribution functions were calculated using TINKER 8  and TRAVIS  software packages, respectively, to gain further insights on the intermolecular interactions within the mixtures studied. To obtain the spa105
cial distribution function, we use the NTP ensemble option in TRAVIS, and we choose a radius for the calculation sphere rc of 1700 pm for [DMIM][BF4 ]+NC12PhH, and 1600 pm for [EMIM][BF4 ]+NC12-PhH.
3. Results 3.1. Calculated Density Profiles for the Ternary Mixtures 110
The density profile generated for [DMIM][BF4 ]+PhH-NC12 is shown in figure 2. Our results indicate that that a fraction of PhH goes to the IL rich region, however the NC12 remains in the hydrocarbon rich region. This behavior suggests that the NC12 molecules do not exhibit affinity for [DMIM][BF4 ], whereas significant affinity of the PhH for the IL region is observed. This behavior is
consistent with experimental and computational results using other ILs [7, 12]. Figure 3 shows the density profile for the [EMIM][BF4 ]+PhH-NC12 system. In this case, it is observed that the PhH also goes to the IL rich region. However, the region between 5 nm and 6 nm shows no PhH population. Similarly to [DMIM][BF4 ], NC12 does not have affinity with [EMIM][BF4 ]. Additionally,
high density peaks for PhH at the IL/hydrocarbon interphase region, 2 nm and 9 nm, in the [EMIM][BF4 ] system are observed. These peaks are smaller in the [DMIM][BF4 ] RDF with respect to the [EMIM][BF4 ] RDF by 8%. The solute distribution ratio (β) and the selectivity (S) parameters have been calculated to evaluate the feasibility of PhH extraction by these two IL
combinations. Both parameters are evaluated as a function of the molar fraction of NC12 (x1 ), and PhH (x2 ) in the hydrocarbon (I) and IL (II) rich regions as follows: II xII 2 /x1 , S= xI2 /xI1 6
ρ (kg/m )
Benzene Dodecane DMIM BF4
250 200 150 100 50 0
Box length in z (nm) Figure 2: Density profile along the z direction for the mixture [DMIM][BF4 ]+PhH-NC12. Black, red, green and blue lines represent the density of PhH, NC12, DMIM and BF4 along z axis, respectively. This profile is obtained with last 50 ns out of 150 ns of the complete MD simulation.
Benzene Dodecane EMIM BF4
ρ (Kg/m )
300 250 200 150 100 50 0
Box length in z (nm) Figure 3: Density profile along the z direction for the mixture [EMIM][BF4 ]+PhH-NC12. Black, red, green and blue lines represent the density of PhH, NC12, EMIM and BF4 along z axis, respectively. This profile is obtained with last 50 ns out of 300 ns of the complete MD simulation.
β = xII / xI2 . 2
Table 1: NC12 and PhH molar fraction values in the hydrocarbon and IL rich regions, solute distribution ratio (β), and selectivity (S) for the PhH-NC12+[DMIM][BF4] and PhHNC12+[EMIM][BF4 ].
From table 1 we can see, in general, higher values of the molar concentration of PhH and NC12 for [DMIM][BF4 ] than [EMIM][BF4 ]. These results indicate
that [DMIM][BF4 ] allows a better distribution of PhH and NC12 in the ternary mixture. Additionally, it is observed that the calculated molar fraction of PhH in the IL rich region, xII 2 , in the [DMIM][BF4 ] system is larger that in the [EMIM][BF4 ], which is consistent with the calculated density profiles 2 and 3 in the ionic liquid region respectively. On the other hand, higher values of β and
S of [DMIM][BF4 ] with respect to [EMIM][BF4 ] are obtained. This means that [DMIM][BF4 ] is significantly more efficient at extracting PhH. 3.2. Radial distribution function analysis For the binary systems, we computed the radial distribution function, g(r), between one PhH carbon (C) atom and the boron (B) atom of [BF4 ] (figure 4), and between one PhH carbon atom and the nitrogen (N) atom in [DMIM]/[EMIM] (figure 5) to analyze the IL/PhH interactions. 1.4 g1(r) C of Benzene - B of [DMIM][BF4] g2(r) C of Benzene - B of [EMIM][BF4]
r (Å) Figure 4: Radial distribution functions, g1 (r) and g2 (r) for the PhH/ILs systems.
Figure 4 suggests there are no significant differences along the radial direction r between g1 (r) and g2 (r); for both g(r) the first three peaks are at the 9
g3(r) C of Benceno - N of [DMIM][BF4] g4(r) C of Benceno - N of [EMIM][BF4]
r [Å] Figure 5: Radial distribution functions, g3 (r) and g4 (r) for the PhH/ILs systems.
same distance, around 4.1, 5.2 and 6.3 ˚ A, respectively. The major difference 145
between g1 (r) and g2 (r) is the observed magnitude, in particular for the first three peaks. Higher intensity of g1 (r) with respect to g2 (r) indicates that PhH has more probability to interact with [BF4 ] neighbors in [DMIM][BF4 ] than in [EMIM][BF4 ]. For the PhH/cation interactions differences are observed for the first peak
between g3 (r) and g4 (r) along the radial direction r (figure 5). The first peaks in g3 (r) and g4 (r) are located around 4 and 5 ˚ A, respectively. The fact that the first peak in g3 (4˚ A) is shorter in radial direction and larger in intensity with respect to g4 (4˚ A), means that PhH has more probability to interact with [DMIM] neighbors at short distances than with [EMIM]. These resulsts suggest
that the PhH molecules exhibit stronger interactions with [DMIM] than [EMIM], which again is consistent with the above results showing that [DMIM][BF4 ] shows better extraction capabilities than [EMIM][BF4 ].
3.3. Spatial distribution function analysis The spatial distribution function (SDF) for the PhH/NC12 binary mixtures 160
is shown in figure 6. The SDF for this system is observed to be mostly continuous, indicating that there is a great affinity between NC12 and PhH. A similar behavior is observed from the corresponding density profile (figure 2). The difference in SDFs for the binary and ternary systems, i.e. PhH+NC12+IL − PhH+NC12, are shown in figures 6b, and 6c. In both cases, the similarity
in the SDF difference plots indicates that the interactions between PhH and DC12 are similar regardless of whether the system is neat (PhH+DC12) or in a mixture with either IL. That is, the presence of the ions in the ternary systems does not show a significant effect in the interactions between PhH and DC12 in the hydrocarbon rich phase.
Figure 6: a) SDF of PhH and NC12 in the binary mixture, b) difference between the SDF of PhH+NC12 in the binary mixture and the SDF of PhH and NC12 in the ternary mixture (PhH + NC12 + [DMIM][BF4 ]), c) difference between the SDF of PhH+NC12 in the binary mixture and the SDF of PhH and NC12 in the ternary mixture (PhH + NC12 + [EMIM][BF4 ]).
Figures 7 to 10 show the SDF analyses for PhH with the cations and anions for the [DMIM][BF4 ] and [EMIM][BF4 ] systems, as well as the difference between the ternary and binary systems. The spatial distribution of both ions with the solute in the PhH+[DMIM][BF4 ] binary systems, figures 7a and 8a, share some similarities, suggesting that both ions interact similarly with PhH.
Conversely for the structural distribution difference between the ternary and binary systems (DC12+PhH+[DMIM][BF4 ] − PhH+[DMIM][BF4 ]), 7b and 8b 11
Figure 7: a) SDF of PhH and one nitrogen atom in the ring of [DMIM] in a binary mixture (PhH+[DMIM][BF4 ]), b) difference between the SDF of PhH+[DMIM] (panel a) in the binary mixture and the SDF of PhH and one nitrogen atom in the ring of [DMIM] in the ternary system (PhH+NC12+[DMIM][BF4 ]).
show significant differences. It is observed that DMIM interacts with PhH exclusively on the plane of the PhH ring (figure 7b), whereas BF4 interacts both along the plane and on the edges (figure 8b). The isochore was 4.19 for all cases. 180
And in the case of figures 6 and 7, the reference atoms for PhH was C1, C2 and H4. In the case of figure 6, the reference atom form NC12 was C6. In the case of figure 7, the reference atom for [DMIM] was the nitrogen atom N2. By contrast, for the [EMIM][BF4 ] system, significant differences are observed for the cation and anion spatial distributions in the binary PhH+[EMIM][BF4 ]
systems, figures 9a and 10a. In this case, the solute is observed to interact with EMIM only along the plane of the PhH molecule, whereas BF4 interacts with PhH both along the edge and on the plane. For the SDF difference between the ternary and binary mixtures, (DC12+PhH+[EMIM][BF4] − PhH+[EMIM][BF4 ]) a similar trend is observed, where EMIM interacts exclu-
sively along the plane of PhH, and BF4 only on the edges. Comparing the structural distribution function differences for the cations (figures 7b and 9b) shows only slight differences between the [DMIM][BF4 ] and [EMIM][BF4 ] systems. This indicates that the interactions between the PhH 12
a) SDF of PhH and the boron atom in [BF4 ], in a binary mixture
(PhH+[DMIM][BF4 ]), b) difference between the SDF of PhH and [BF4 ] in the binary mixture (panel a), and the SDF of PhH and boron atom in [BF4 ] in the ternary system (PhH+NC12+[DMIM][BF4 ]).
Figure 9: a) SDF of PhH and one nitrogen atom in the ring of [EMIM] in a binary mixture (PhH+[EMIM][BF4 ]), b) difference between the SDF of PhH+[EMIM] (panel a) in the binary mixture and the SDF of PhH and one nitrogen atom in the ring of [EMIM] in the ternary system (PhH+NC12+[EMIM][BF4 ]).
and the cations is similar in both cases. Conversely, the SDF differences for the 195
anions between both IL systems, figures 8b and 10b show significant differences, with much larger surfaces for the [DMIM][BF4 ]. Moreover, it can be observed
that the anion can interact with PhH along the edge and the plane in the [DMIM][BF4 ] system, whereas only interactions along the plane are observed for the [EMIM][BF4 ] system.
a) SDF of PhH and the boron atom in [BF4 ], in a binary mixture
(PhH+[EMIM][BF4 ]), b) difference between the SDF of PhH and [BF4 ] in the binary mixture (panel a), and the SDF of PhH and boron atom in [BF4 ] in the ternary system (PhH+NC12+[EMIM][BF4 ]).
These results suggest that the reason for the increased selectivity for PhH by [DMIM][BF4 ] is driven, at least in part, by the increased interaction with the anion, not just with the cation. This may be due to the fact that the symmetry of the DMIM cation allows for slightly improved ordering when paired with BF4 , as observed from the radial distribution functions for both the cation and
anion (figures 4 and 5).
4. Conclusions Molecular dynamics simulations have been performed on mixtures of PhHNC12 with [DMIM][BF4 ] and [EMIM][BF4 ] to explore the feasibility of these two IL pairs to extract PhH from the hydrocarbon rich phase. The results 210
suggest that [DMIM][BF4 ] is a good candidate for benzene extraction, showing significant selectivity for the aromatic compound. Structural analysis based on 14
radial and structural distribution functions indicates that [DMIM][BF4 ] shows significantly more interactions with PhH than [EMIM][BF4 ]. Interestingly, the BF4 anion is shown to interact with benzene along both the edge and the plane 215
of the ring, in the [DMIM][BF4 ] system, whereas the interactions are reduced in the [EMIM][BF4 ] system only to contacts along the plane. This suggests that the selectivity of [DMIM][BF4 ] is driven, in part, by the increased interactions of the anion to PhH, which may be due to the increased structural ordering in the [DMIM][BF4 ] mixture.
Acknowledgments The authors thank the UNT Department of Chemistry for the use of the HPC Cluster CRUNTCh3 and NSF Grant No. CHE–1531468. E.A.V.-M. thanks CONACyT for funding. H.M.F-R gratefully acknowledges the computing time granted by LANCAD and CONACyT on the supercomputer Yoltla/Miztli/Xiuhcoatl
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Separation of aromatic from hydrocarbon mixtures using solvents is a field of studies
We use AMOEBA to extract benzene from a mixture of benzene dodecane
Our results indicate that [DMIM][BF4] is better extracting benzene than [EMIM][BF4]