Molecular dynamics simulation for desulphurization of hydrocarbon ...

Journal of Molecular Liquids 264 (2018) 490–498

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Molecular dynamics simulation for desulphurization of hydrocarbon fuel using ionic liquids Meena B. Singh a,b,⁎, Ameya U. Harmalkar a, Saina S. Prabhu c, Neha R. Pai c, Shashank K. Bhangde c, Vilas G. Gaikar a a b c

Department of Chemical Engineering, Institute of Chemical Technology, Matunga, Mumbai 400019, India Department of Chemical Engineering, Indian Institute of Technology Bombay, Mumbai 400076, India Department of Pharmaceutical Sciences and Technology, Institute of Chemical Technology, Matunga, Mumbai 400019, India

a r t i c l e

i n f o

Article history: Received 3 December 2017 Received in revised form 15 May 2018 Accepted 20 May 2018 Available online 22 May 2018 Keywords: Ionic liquids Green solvent Molecular dynamics (MD) simulation Desulphurization Molecular interactions Thiophene Dibenzothiophene (DBT)

a b s t r a c t Ionic liquids, forming a newer class of green solvents, have shown potential in solvent extraction due to their remarkable physical and chemical properties, which can be tailored for specific tasks. Ionic liquids are one of the potential extractants for desulphurization of crude oil to produce ultra-low sulphur fuels. However, due to the constraints of economic feasibility and the broad domain of Ionic liquids, their varying affinity towards sulphur impurities along with the overall expensive methods of production, limit the selection of an Ionic Liquid that provides optimum extraction rates. We investigated the biphasic extraction of two sulphur compounds, thiophene and dibenzothiophene (DBT) from dodecane by using ionic liquid systems of ethyl methyl imidazolium (EMI) and butyl methyl imidazolium (BMI) cations; and tetrafluoroborate (BF4) and thiocyanate (SCN) anions. The molecular arrangement and extraction mechanism of both sulphur compounds at the interface of dodecane-ionic liquid system are studied by molecular dynamics (MD) simulations. We observed that electrostatic interactions between the aromatic plane of sulphur compounds and cations of ionic liquids are the major forces responsible for the extraction of sulphur compounds. © 2018 Elsevier B.V. All rights reserved.

1. Introduction A petroleum refinery refines crude oil into more useful petroleum products such as gasoline, diesel, kerosene, heating oil, liquefied petroleum gas, etc. The crude oil contains a mixture of hydrocarbon compounds; both aromatic and aliphatic, and relatively small quantities of oxygen, nitrogen and sulphur containing organic compounds. The refinery needs to separate these impurities from various process streams since their presence adversely affects catalytic processes in the refinery. The combustion products of such impurities also add to environmental pollution. The presence of sulphur compounds in automobile fuel is a major concern worldwide, because on combustion they generate acidic gases such as SOx which are responsible for respiratory diseases, health disorders, environmental pollution and acid rains. Consequently, the regulations are increasingly made more stringent to produce sulphur-free fuels. The permissible sulphur content in the fuels is maximum 10 ppm as proposed by the European Union in 2009 [1]. The catalytic hydrodesulphurization (HDS), conventionally practiced for removing sulphur impurities like sulfides, disulfides, thiols, thiophenes, ⁎ Corresponding author. E-mail address: [email protected] (M.B. Singh).

https://doi.org/10.1016/j.molliq.2018.05.088 0167-7322/© 2018 Elsevier B.V. All rights reserved.

dibenzothiophene (DBT) and their mono- and di-alkylated derivatives, is a highly efficient process. But few complex aromatic sulphur compounds such as DBT and methyl substituted dibenzothiophenes because of their complex structure and multiple aromatic rings show poor HDS efficiency, thus making it hard to obtain ultra-low-sulphur-fuels [1,2]. Although HDS is the current state-of-the-art technology in the petroleum refineries for removing sulphur compounds, to develop further its sulphur conversion efficiency will demand increasingly extreme conditions which will also escalate the cost severely. [3,4] The use of greener and more efficient solvents, in addition to of HDS, is the need of the hour, not only with environmental concerns into consideration, but also as potential method for separation [5]. Ionic liquids, a range of such green solvents, supposed to be more efficient extractive solvents due to their very low vapor-pressures at room temperature, higher affinity for polar compounds, and a broad range of applications in catalysis and separation technology, offer an alternative to the conventional HDS process. Ionic liquids are organic salts which exist in liquid phase for the wide range of temperature ranging from somewhere around −96 °C to 400 °C [3,6]. Ionic liquids are non-ionizing, generally highly viscous liquids (with some exceptions such as 1-ethyl-3methylimidazolium dicyanamide, 1-butyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide, etc. with relatively lower viscosity) and exhibit very low vapor pressure at room temperature [3]. Most

M.B. Singh et al. / Journal of Molecular Liquids 264 (2018) 490–498

ionic liquids are composed of heterocyclic organic cations, such as 1alkyl-3-methylimidazolium [CnMI], n-alkylpyridinium, tetraalkyl ammonium or tetraalkyl phosphonium cations, and various anions − such as hexafluorophosphate [PF− 6 ]; tetrafluoroborate [BF4 ]; − trifluoromethylsulfonate [CF3SO3 ]; bis[(trifluoromethyl)sulfonyl] − amide [(CF3SO2)2N−]; trifluoroethanoate [CF3CO− 2 ]; thiocynate[SCN ]; acetate [CH3CO− 2 ]; nitrate, and halide [3]. They are non-volatile and non-flammable thus facilitating maximum solvent recovery [3,6]. The properties of the ionic liquids can be altered to suit a specific need by changing the constitution of the ionic liquids [6]. The use of ionic liquid as an extraction medium for desulphurization is one of the promising, technically feasible, and economical option, if recycled without any loss. The process is expected to require less energy consumption and lesser number of steps [6]. Solvent recovery is also optimum because of its non-volatile nature and by a known established process such as flash-distillation or stripping of the extracted sulphur compounds [6]. The reason for the increased inclination towards the use of ionic liquids as an extraction medium for aromatic sulphur compounds lies in the fundamentals of organic chemistry. Aromatic compounds have a cloud of delocalized electrons (pi electrons) over the entire ring, resulting in a strong electrostatic field around the aromatic molecule [6]. It is expected that strongly electrostatically charged sulphur containing molecules will be attracted towards the bipolar ionic liquids, and thus separating out of their initial medium (crude oil). The maximum selectivity can be achieved by choosing a suitable solvent with optimum solubility for a particular heterocyclic aromatic impurity in the fuel [6]. The selection of an appropriate ionic liquid for desulphurization from vast numbers of ionic liquids (due to combination of different cations and anions) is a great challenge, and requires a great deal of experimentation, which is not always feasible, owing to the expensive costs of the ionic liquid solutions. Molecular simulation, being one of the crucial ways to understand and analyze the molecular interactions and behavior, can be used to screen and to design newer and high efficiency extractive solvents. Sulphur heteroaromatic compounds like thiophene and DBT with their substitutes are the most common type of sulphur compound impurities in the transport fuels. The use of Molecular Dynamic (MD) simulation as a tool to design and select an optimum ionic liquid for extraction of sulphur compounds is very beneficial. Prado and co-workers [7] reported ionic liquids as a designer solvent based on their potential to act as extraction medium. The ionic liquid system comprising of BMI and BF4 was studied and radial distribution functions (RDF) and other thermodynamic parameters were determined by them. Another study by Feng and Voth [8] elaborated the effect of alkyl side chain length of imidazolium based ionic liquid systems and anions effects on the overall extraction efficiency of the system on sulphur compounds. Although, the diffusion characteristics were studied with water as the diffusing mixture, the physical properties can be extrapolated for any polar species diffusing into the ionic liquid system. Another study reported the self-diffusion of ionic liquids and several factors which affect the diffusion of any species into the ionic liquid systems thus, providing pre-requisites for ideal extraction modeling [9]. The simulation reported by Liu et al. [10] on desulphurization gives a preliminary idea about the extraction systems. None of these studies have reported the extraction mechanism for the sulphur compounds and neither compared the extraction efficiencies of ionic liquids for sulphur compounds. The present work involves extensive MD simulations for the use of ionic liquids as an extractive medium to extraction heterocyclic sulphur aromatic compounds, thiophene and DBT from dodecane. A biphasic system comprising of two bulk phases, i.e., dodecane (fuel phase) with sulphur compounds (DBT or thiophene) and an ionic liquid, has been simulated to approximate the dynamic behavior of the extraction system. The study is expected to provide an idea about the efficiency of various ionic liquid as well as molecular changes taking place at oil ionic liquid interface before the transfer process as a whole. The density,

491

diffusion coefficients and RDFs for different species in the system are correlated to provide a broad view on the mechanism of the entire extraction process [11]. As per our knowledge we are reporting the molecular dynamic study for the biphasic extraction and interfacial behavior of thiophene and DBT in oil and ionic liquid system for the first time. This study will be helpful in shortlisting the suitable ionic liquids from the vast number of available ionic liquids for the desulphurization of fuels before synthesizing random ionic liquid and testing it by performing experiments in laboratory. It will also be beneficial in understanding the interaction between various ionic liquids and sulphur compounds (thiophene and DBT) at the molecular level, which in turn can be beneficial in designing of more efficient ionic liquids for the desulphurization of fuel. 2. Methodology Molecular dynamics (MD) simulations were performed with the standard periodic boundary conditions using the GROMACS program version 5.0.7 [12–15]. Simulations are performed in four categories, (i) pure ionic liquid systems, (ii) sulphur compound in ionic liquids, (iii) sulphur compound in dodecane system and (iv) sulphur compound in biphasic system of dodecane and ionic liquid (with initial positions of sulphur compounds in bulk dodecane (oil) phase). Details of all systems are given in Table 1. The ionic liquids, consists of cations ethyl methyl imidazolium [EMI] and butyl methyl imidazolium, [BMI] and anions tetrafluoroborate [BF4] and thiocynate [SCN] forming four ionic liquids [EMI][BF4], [EMI][SCN], [BMI][BF4] and [BMI][SCN]; have been explore to study the effect of anion and alkyl side chain length on the structure and dynamics of the ionic liquid mixtures. The number of sulphur compounds (DBT and thiophene molecules) in the system was kept around 1%. The initial positions of sulphur compound molecules were generated randomly and in biphasic system, sulphur compounds are placed at random position in dodecane phase. The forcefield parameters for DBT [10], thiophene [16], [EMI] [9], [BMI] [7], [BF4] [7], [SCN] [17] and n-dodecane [18] are taken from literature which are based on OPLS-AA forcefield. Each system was initially equilibrated (NVT and NPT) for 40 ns and sulphur compounds are restrained during equilibration. Later restrain was relaxed and system was analyzed for further 100 ns (5 runs of 20 ns each) at particular temperature and pressure. Velocity-rescaling temperature coupling is used during NVT equilibration of the system which is Berendsen thermostat with additional term for correcting the kinetic energy distribution [19]. Nose-Hoover thermostat [20,21] and Parrinello-Rahman pressure coupling [22,23] methods are used to maintain the temperature and pressure of the system during NPT equilibration and further analysis simulations. MD simulations were carried out for the temperature ranging from 300 to 425 K. A cut off distance of 1.5 nm was applied for the Lennard Jones and short range columbic interactions and long range interactions were treated by particle mesh Ewald (PME) method [24]. Neighbouring list was updated after every 10 ns and a Verlet type cut off scheme was used. The diffusion coefficients of various species were estimated from mean square displacement (MSD) function [11]. The VMD (Visual Molecular Dynamics) [25] has been used to analyze the trajectory of the compounds from the dodecane phase to the ionic liquid phase and in other systems for the current simulations. 3. Results and discussion 3.1. Extraction of thiophene and DBT in biphasic system of dodecane and ionic liquids The MD simulations are carried out for biphasic system consisting of n-dodecane with thiophene and DBT as heteroaromatic sulphur impurities and four ionic liquid extraction systems over a broad range of temperature (300 K–425 K). The structural properties, dynamic properties, interaction-spheres of the compounds in a particular extraction system

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Table 1 Details of the periodic box used for MD calculations. Sr. no

Size of periodic box in nm

Ionic liquid 1 4.95 × 4.95 × 4.95 2 5.06 × 5.06 × 5.11 3 5.07 × 5.07 × 5.07 4 5.26 × 5.26 × 5.26 Dibenzothiophene (DBT) Compound in ionic liquid 5 5.09 × 5.15 × 5.13 6 5.02 × 5.01 × 4.86 7 5.24 × 5.24 × 5.24 8 5.08 × 5.08 × 5.08 Compound in dodecane 9 4.94 × 4.94 × 4.94 Biphasic system 10 5.05 × 5.05 × 10.22 11 5.08 × 5.07 × 10.21 12 5.19 × 5.19 × 10.39 13 5.09 × 5.09 × 10.18 Thiophene Compound in ionic liquid 14 5.04 × 5.12 × 5.09 15 5.07 × 5.06 × 4.91 16 5.26 × 5.26 × 5.25 17 5.41 × 5.38 × 5.45 Compound in dodecane 18 4.93 × 4.93 × 4.91 Biphasic system 19 5.05 × 5.05 × 10.22 20 5.04 × 5.05 × 10.05 21 5.19 × 5.19 × 10.39 22 5.07 × 5.07 × 10.14

Dodecane (no.)

Compound (no.)

[EMI] (no.)

[BMI] (no.)

[BF4] (no.)

[SCN], (no.)

– – – –

– – – –

485 500 – –

– – 400 411

485 – 400 –

– 500 – 411

– – – –

4 5 4 4

485 500 – –

– – 400 410

485 – 400 –

– 500 – 410

350

4

–

–

–

–

350 350 350 350

5 4 4 4

485 500 – –

– – 400 410

485 – 400 –

– 500 – 410

– – – –

5 4 4 4

485 500 – –

– – 400 410

485 – 400 –

– 500 – 410

350

4

–

–

–

–

350 350 350 350

5 4 4 4

485 500 – –

– – 400 410

485 – 400 –

– 500 – 410

a) DBT in Dodecane Phase

b) DBT at the interface

c) DBT extracted in the ionic liquid [EMI][SCN] Fig. 1. S-aromatic compound extraction mechanism in a biphasic model of dodecane and ionic liquid [EMI] [SCN].

M.B. Singh et al. / Journal of Molecular Liquids 264 (2018) 490–498

were studied and detailed mechanism of extraction pathway of sulphur compounds (DBT and thiophene) are described. 3.1.1. Analysis of the extraction pathway The extraction of the thiophene or DBT molecule from dodecane to the interfacial layer, followed by its translation into the ionic liquid is shown in Fig. 1. The molecular dynamic simulations were performed at varying temperatures. The extraction mechanism of the S-aromatic compounds can be predicted from Fig. 1(a–c). The initial stages of simulations depict the sulphur aromatic molecules in the bulk of the dodecane. As the simulation progresses, the sulphur aromatic molecules start moving towards the ionic liquid phase. The Imidazolium cation's affinity towards negative charged aromatic carbons (due to delocalized electrons) of the sulphur compound and the overall strong polarizing effect of the ionic liquid proves to be the driving force behind the translation. A representation of the coordination sphere of the compounds at the interface is shown in Fig. 2 and this leads to a perturbation at the interfacial layer. The changes at the interface of ionic liquid and dodecane exhibited by the biphasic system are shown in Fig. 3. It is clearly evident from the representation that when the sulphur compound shifts towards the interface from the bulk of dodecane phase, perturbation occurs at the interphase (Fig. 3a). Each sulphur compound at the interphase is surrounded by a sphere of dodecane and ionic liquid molecules. Once, this particular stage is reached, the ionic liquid forms pockets or crest and troughs to engulf the sulphur compound in the ionic liquid phase. The ionic liquid thus initiates the formation of clathrates around the compound (Fig. 3b). Orientation of ‘S’ atom of the DBT molecules at the interface may not be always towards ionic liquid or towards dodecane phase, it can orient towards any one of the phase (ionic liquid or dodecane). As the partial charge on sulphur atom is negligible (−0.0841) due to delocalization of lone pair of electrons on the sulphur atom to the aromatic ring. The aromatic п-п interactions of the Imidazolium cation and the aromatic core of heteroaromatic species assist in the absorption of the molecules into the ionic liquid. This inference was taken from the work of Su and co-workers [26] where they studied the interaction of thiophene with the ionic liquids by multinuclear NMR spectroscopy and stated that C– H–pi interactions and pi-pi interactions are responsible for the extraction of thiophene by ionic liquids. The incoming heteroaromatic sulphur compound species can thus be incorporated in the ionic liquid ensemble owing to the high affinity of the ionic liquids towards aromatic species (Fig. 3c). The temperature dependence of the simulations has been studied. The elevation in temperature brings about a drastic fall in the extraction capacity, stating that extraction is optimum at temperatures around 300–350 K. Both the sulphur compounds exhibit similar spatial

a) Interaction of thiophene at the interphase

493

orientations in the biphasic model system. All the ionic liquid systems show similar results with each ionic liquid extraction system exhibiting extraction at optimum temperatures. 3.1.2. Effect of size of cation and anion of ionic liquid With BF4 or SCN anions, absorption of the incoming compound was observed to be more pronounced in the system with BMI rather than EMI as cation. The interactions of thiophene and DBT in four extraction systems with ionic liquids namely: [EMI][BF4]; [EMI][SCN]; [BMI][BF4]; [BMI][SCN], at the interface show that the size of alkyl chain on Imidazolium cation and the structure of the anion greatly affect the packing and geometrical arrangements of the incoming heteroaromatic sulphur compound molecule with respect to the cations and anions. The simulations also conclude that the packing structure intimacy of the given ionic liquid systems has the following order: [EMI] [BF4] N [EMI] [SCN] N [BMI] [BF4] N [BMI] [SCN]. The NMR spectra observations for sulphur compound (thiophene) by Su et al. [26] illustrating shielding and deshielding of NMR chemical shifts depending upon the type and size of cation and anion are in total agreement with that of the simulation results. Zhang and co-workers [27] also reported the similar extraction behavior for DBT by the ionic liquids. 3.1.3. Effect of the sulphur aromatic compound on the extraction rate of the ionic liquid The structural and dynamic properties of thiophene and DBT, and their interactions with the proposed ionic liquid systems depict the extraction capacity of each ionic liquid system used as extractants. The extraction of heteroaromatic compounds from the model unit of dodecane by ionic liquids extraction system studied by Xie and coworkers [28]. The variation in the extraction capacity for DBT and thiophene can be attributed mainly due to the п-п interactions between the planar heteroaromatic ring of the compound and the heteroaromatic alkyl imidazolium cation in the ionic liquid. Both thiophene and DBT interactions with imidazolium cation at the interface suggest that both the compounds exhibit a high affinity towards the ionic liquid systems. As simulation proceeds, it is observed that the sulphur compounds move towards the interphase. The electrostatic interaction of the sulphur compound molecules, with the dynamic interface of the dodecane ionic liquid system, acts as the driving force for the displacement of these sulphur compounds towards the ionic liquid phase. At the interface, each incoming molecule of the heteroaromatic sulphur compounds is surrounded by the molecules of dodecane and the respective ionic liquid. The incoming heteroaromatic sulphur compound exhibits a C–H–п interaction between the acidic hydrogen of the cation and the delocalized ring of the heteroaromatic sulphur compound (thiophene or DBT). A more crucial interaction however, is the п–п interaction between the planar and delocalized aromatic rings of

b) Interaction of DBT at the interphase

Fig. 2. Primary interaction sphere of S-aromatic compounds at the interface of the biphasic extraction system.

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a) Deformation of interphase due to presence of compound at the interphase

b) Ionic liquid phase accomodating compound by forming clathrate

c) 90% molecular structure of compound engulfed by ionic liquid phase Fig. 3. Molecular changes at the interphase during the extraction of compound.

liquid. This indicates that affinity of ionic liquid for the DBT is more as compared to thiophene due to stronger interactions between DBT and cation of ionic liquid. The presence three conjugated aromatic rings of DBT increases the overall electron density on DBT when compared with thiophene, which has only one 5-membered aromatic ring. Hence DBT migrate faster towards the ionic liquid phase (higher diffusivity) as compare to thiophene. Diffusion coefficients of DBT in pure ionic liquid systems is lesser as compared to thiophene which indicates that higher density of DBT results in stronger interaction between DBT and cation of ionic liquid and hence formation of stable complex between cation of ionic liquid and DBT (Section 3.2). So, in spite of DBT being a comparatively bulky group, it has higher extraction rates than

2.5

Thiophene DBT

2 D X 10-9 m2/s

thiophene and DBT, with that of the cation. This conclusion is based on the experimental work of Su and co-workers [26]. They have also observed that the C–H–п interactions are less significant in comparison with the п–п interactions and the aromatic delocalization plays a dominant role in varying the extraction capacity of the ionic liquid system [26]. The phenomenon of п–п interactions thus attribute to the fact that the sulphur atom in thiophene and DBT may or may not oriented themselves towards ionic liquid phase. The Su and coworkers [26] carried out multinuclear NMR spectroscopy of thiophene in a set of ionic liquids and stated that due to delocalization of lone pair of electron from sulphur atom to the aromatic ring, there is possibility of sulphur atom might carry a partial positive and aromatic ring partial negative charge and hence sulphur atom might not be always oriented ionic liquid phase. Fig. 2 shows the primary interaction sphere of DBT in the [EMI] [BF4] extraction system at the interfacial stage. It can be observed that the DBT molecule has its plane facing that of the [EMI] cation and the sulphur atom directed towards the ionic liquid phase at an instance which is not the case always. This indicates the pi-pi stacking of two aromatic rings (one sulphur compound and other imidazolim cation). Quite few interactions are seen with the fuel phase i.e. n-dodecane. The diffusion coefficients of DBT and thiophene compared with each other in the four biphasic system of ionic liquid and dodecane, it was found that DBT has the higher diffusion coefficient as compare to thiophene (Fig. 4). For current work it has been assumed that higher the diffusion coefficient of sulphur compounds in the biphasic system, higher is rate of extraction. The diffusion coefficients of sulphur aromatic compounds are higher for DBT than that of thiophene for the migration of DBT and thiophene from bulk dodecane phase to interface of dodecane-ionic

1.5 1 0.5 0

EMI BF4

EMI SCN

BMI BF4

BMI SCN

Fig. 4. Comparison of diffusion coefficient of DBT and thiophene in four ionic liquids.

M.B. Singh et al. / Journal of Molecular Liquids 264 (2018) 490–498

thiophene. Experimental extraction rates reported in the literature follow the similar order [27,29]. Gao et al. [30] have also reported the experimental extraction efficiencies of S-aromatic compounds in the order of DBT N BT N thiophene N 4,6-dimethyl dibenzothiophene. The size of the cations and anions also play a vital role in determining the extraction efficiencies. A cation with an increased alkyl chain length shows more absorption rates than its comparatively smaller alkyl chain length counterpart. Similarly, the increment in the size of the anions increases the extraction rate [27]. It has been observed the experimental extraction rates of DBT show higher extraction in systems with [SCN] rather than [BF4], independent of the cation used [4]. The extraction efficiency from the plot is as follows: [EMI] [BF4] b [EMI] [SCN] b [BMI] [BF4] b [BMI] [SCN]; which follows a reverse trend in comparison with the packing intimacy. This implies that the more intimate the packing amongst the cations and anions of the ionic liquids is, the less is its efficiency for the absorption of an incoming sulphur heteroaromatic compound. 3.1.4. Effect of extraction temperature on the extraction of the compound The biphasic extraction systems were simulated at a broad range of temperatures ranging from 300 K to 425 K, with a temperature interval of 25 K. Although, no particular comment can be made on the relation of extraction temperature with the extraction efficiency. The factors on which extraction efficiency depends as discussed above are favourable for the extraction of the heteroaromatic sulphur compound to occur at room temperature. In spite of the increase in diffusion coefficients

495

with increasing temperature, the enhancement of the extraction rate is not assured because even though the temperature increases the interaction between the sulphur compound and cation decreases. Optimum temperature of 300–350 K shows good results for the extraction of sulphur compound. A similar suggestion is provided by the RDF plots as the peaks decrease with increase in temperature. The simulations are also carried out for the single phase of ndodecane with the heteroaromatic impurities of thiophene or DBT in it. Molecular simulations were carried out over a wide temperature range from 300 to 425 K and the corresponding interactions were studied. The interactions between n-dodecane and the sulphur compounds are not substantial enough to indicate strong interactions between them. As the concentration of sulphur compound in dodecane is very less (b1%), the simulated densities of sulphur compound – dodecane mixture have been compared with experimental densities of pure dodecane. The density of mixture of S-compound with n-dodecane (765–640 kg/m3) is in accordance with the experimental density of ndodecane (746–650 kg/m3) [31] as depicted in supplementary data Fig. S4. The density of S-compound – dodecane mixture decreases with increase in temperature which is in good agreement with the experimental densities [31] (Fig. S4). Similarly, the single phase pure ionic liquid systems were also simulated at various temperatures (300–425 K). The interaction between cation and anions of ionic liquids are also shown in supplementary data Fig. S1. The simulated densities are in good agreement with the experimental densities from the literature [17,32–34] (Fig. S2). Densities

a) DBT in [BMI][BF4]

b) DBT in [BMI][SCN]

c) thiophene in [BMI][BF4]

d) thiophene in [BMI][SCN]

Fig. 5. Interactions of S-aromatic compounds with cations of various ionic liquids.

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of all four ionic liquids are at various temperatures are calculated by simulations and compared with the experimental values from the literature in the Supporting information (SI), Fig. S2. The densities are in order of [BMI][SCN] N [EMI][SCN] N [BMI][BF4] N [EMI][BF4]. Highest density of [EMI][BF4] and lowest density of [BMI][SCN] can be attributed to the stronger packing efficiency between the cation and the anion of [EMI][BF4] and vice versa for [BMI][SCN]. This in turn is related to the size of the corresponding ions. Both [EMI] and [BF4] which are smaller in size in comparison to their counterpart ions are intimately packed and thus possess a lower volume. Due to strongly packed cations and anions it's difficult for sulphur compounds to move in [EMI][BF4] as compared to loosely packed [BMI][SCN]. Holbrey and co-workers [4] also concluded that amongst [SCN] and [BF4], the packing is more compact in the [BF4] system. Physiological behavior in terms of densities and molecular interactions for all four ionic liquids obtained from the simulations are in good agreement with the literature [17,32–34]. RDF of cation-cation, cation-anion and anion are calculated plotted in Supplementary data (Fig. S3). 3.2. Sulphur compounds in ionic liquid systems To get a deeper insight about the molecular interactions between of the ionic liquids the sulphur compounds (DBT and thiophene), four bulk ionic liquid systems with 1% of heteroaromatic sulphur compounds were simulated by MD simulations. The various interactions of the sulphur compounds with the ionic liquid system are shown in Fig. 5. The sulphur atom bearing a partial negative charge interacts with the aromatic hydrogen atom of cations in the imidazolium ring. The aromatic C-atoms interact with both the alkyl and aromatic H-atoms of the ionic liquid. Since, the N-atom of the imidazolium ion bears a partial positive charge; it interacts with the aromatic rings of DBT and thiophene. The aromatic hydrogen's, being less electronegative than C, possess a partial positive charge and hence interacts with the anion present in the ionic liquid. All the fluorine atoms in [BF4] and the S-atom and Natom in [SCN] are negatively charged and are thus likely to interact with the aromatic Hydrogen atoms. The ionic liquids exhibit a clathrate formation around the sulphur compounds. They involve π-π interactions between the unsaturated bonds of the aromatic ring of the sulphur compounds and the imidazolium ring of the ionic liquid. As the side alkyl chain length on imidazolium cation increases, more interspacial spaces created between the cations and anions of the ionic liquid (explained previously based on the densities of ionic liquids) to accommodate sulphur compounds easily. Conclusions can be derived asserting that both thiophene and DBT exhibited better interactions with [BMI] than [EMI] cation.

3.5

Table 2 Diffusion coefficients of DBT in various ionic liquids. Ionic liquid

D of DBT (simulation) in m2/s

D of DBT (literature) [35] in m2/s

[EMI][BF4] [EMI][SCN] [BMI][BF4] [BMI][SCN] Dodecane

6.89 (±2.33) × 10−11 8.11 (±3.51) × 10−11 1.114 (± 0.561) × 10−10 1.256 (± 0.394) × 10−10 1.1582 (± 0.728) × 10−9

~10−11

The ionic liquids and the heteroaromatics mostly rely on the π-π interactions, CH-π interactions and cation-π interactions [26]. A heteroaromatic ring with a moderately electronegative atom such as sulphur possesses higher delocalized π-electron density than any of its aromatic counterparts. The partial charges on sulphur decreases which leads to the increase in the density of the π system, which promotes cation-π interactions. The diffusion coefficients of thiophene and DBT in the ionic liquid systems have been analyzed for various temperature domains. The diffusion coefficients (D) of the DBT in various ionic liquids are compared at various temperatures in Fig. 6. The maximum value of diffusion coefficient of DBT was observed for the [BMI] [BF4] ionic liquid system followed by that in [EMI] [BF4] system. The ionic liquids with [SCN] counterparts follow a similar trend. Hence, DBT has the highest values of diffusivity in [BMI] [SCN] and the lowest in [EMI] [BF4]. This can be attributed to the structure of the ions of the ionic liquid. [BMI] has a longer alkyl side chain as compared to that of [EMI]. Also, the ionic size of [SCN] is greater than that of [BF4]. Thus, [EMI] [BF4] molecules are the most tightly packed. Hence, the compound can easily diffuse through the ionic liquid comprising of [BMI] and [SCN] whilst it can diffuse at the least rate through [EMI] [BF4]. A comparison of simulated diffusivity of DBT in ionic liquids and as well as in dodecane at 300 K with experimental data [35] is presented in Table 2. The diffusivity of DBT in ionic liquid and in dodecane determined by simulation are in the range of 7–12 × 10−11 m2/s and 12.6 × 10−9 m2/s respectively. These values are in good agreement with experimental values reported by [35] in the range of ~10−11 m2/s and ~10−9 m2/s for DBT in ionic liquids and dodecane, respectively. The diffusion coefficients of thiophene were also analyzed with a broad temperature domain similar to that of DBT and are shown in Table 3. Thiophene, being a comparatively smaller species, has diffusion coefficient in the range of 2.97–31.01 × 10−10 m2/s and 6.85–37.35 × 10−10 m2/s in EMI and BMI systems, respectively. Thiophene diffuses easily through the [BMI] [SCN] system and follows an almost similar trend as that of DBT.

Diffusivity of DBT in Ionic liquid

3

D X 10-9 m2/s

2.5 2 EMI+BF4 1.5

EMI+SCN BMI+BF4

1

BMI+SCN

0.5 0 300

320

340

360

~10−9

380

400

420

T in K

Fig. 6. Diffusion coefficients of DBT as function of temperature in ionic liquids.

440

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group shows a peak at 0.4 nm. Thus, these RDF plots incontrovertibly state that intermolecular interaction between C5 and sulphur is higher in the given ionic liquid and thus has highest interaction, suggesting that DBT prefers to interact with tail C-atom of cation's alkyl group due to C–H–π interaction between cation and S-compounds. The RDF plot between the Nitrogen atoms; N1 and N2, and the sulphur atom of the DBT shows a broad maximum peak at 1.0 nm. The peaks for the H atoms are observed in a range of about 0.6–0.9 nm which suggest that the interaction between DBT's ‘S’ and imidazolium ring's ‘H’ is very weak. Similar results are obtained with the ionic liquids with [EMI] cation derivatives, although the steric hindrances of the ionic liquids do play a major role in determining the RDFs. The RDF plots obtained are in agreement with the experimental and simulation results mentioned in literature [10]. The interactions of ‘S’ atom of thiophene with various atoms of cation in all four ionic liquids are plotted in Fig. 7c–d. The sharp peak appears at 0.4 nm and 0.9 nm for C5 and C6 of terminal ‘C’ in alkyl group of [EMI] cation while for inner C of ethyl shows a broad peak between 0.4 and 0.5 nm indicating the weaker interaction with sulphur atom of thiophene. For both the ‘N’ atoms a sharp peak appears at 0.5 nm and second broad peak appears at 0.9 nm. As far as aromatic ‘H’ atoms of [EMI] are concerned, they also exhibit peaks at 0.3 nm and 0.7 nm. The comparison of the RDF plots of thiophene and DBT shows that the sharpness of the peak decreases with the increase in the size of sulphur aromatic compounds. The peaks of aromatic N and C tend to be sharper for thiophene than that for DBT. This can be attributed to the hindrance in DBT because of its greater size than thiophene. As the number of aromatic rings increases, the hindrance around the S-atom also elevates and its availability for interaction comes down. A similar behavior is observed for H-atoms of thiophene and DBT, as the RDF peaks for H-atoms from thiophene are more prominent as compared

Table 3 Diffusion coefficients of thiophene in various ionic liquids. Ionic liquid

D of thiophene (simulation) in m2/s

[EMI][BF4] [EMI][SCN] [BMI][BF4] [BMI][SCN]

2.97 (±0.975) × 10−10 4.11 (±1.211) × 10−10 6.85 (±0.419) × 10−10 11.312 (±0.795) × 10−10

In bulk ionic liquid phase diffusion coefficients of DBT is lesser as compared to thiophene which indicates that DBT forms stronger interactions with the ionic liquids as compared to thiophene. Higher mass density of DBT makes it bulkier to move inside ionic liquid phase and hence slower diffusion of DBT while thiophene has lesser mass density and doesn't interact as strongly as DBT can diffuse at faster rate. The comparison of the Radial Distribution Functions (RDFs) for ionic liquids for different temperature zones was done to analyze the interactions amongst the molecules. The radial distribution functions in the ionic liquids in presence of the heteroaromatic sulphur compound were calculated and shown in Fig. 7. The interaction between the heteroaromatic sulphur compound and C atoms of cation of ionic liquid is well spaced with two peaks at 0.4 nm and 1.0 nm respectively. Similarly, two peaks are also observed in the cation-heteroaromatic interactions with N atoms at 0.6 and 1.0 nm for N1 and show a sharp peak at 0.5 nm for N2. Ionic liquid phase with the heteroaromatics has an organized structure with strong ionic interactions. These two peaks represent the 1st and 2nd coordination spheres of cation around the sulphur compounds (DBT and thiophene). The distribution function for the C4 and C5 atoms of cation's ethyl group around DBT's ‘S’ show sharp peaks at around 0.5 nm and 0.4 nm, respectively (Fig. 7a–b). Similarly, C6 atom of cation's methyl 1.4

1.6

SDBT-EMI-BF4 ( 300 K )

1.2

1

1

0.6 0.4 0.2 0 0

0.2

0.4

0.6

0.8 r in nm

1

1.2

g(r )

C4 C5 C6 H1 H3 H4 N1 N2

0.8

C4 C6 H3 N1

0.4 0.2

C5 H1 H4 N2

0 0

1.4

a) RDF of DBT’s ‘S’ and EMI in [EMI][BF4] 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

0.8 0.6

0.2

0.4

0.6

0.8 r in nm

1

1.2

1.4

b) RDF of DBT’s ‘S’ and EMI in [EMI][SCN] 2

Sthiophene-EMI-BF4

Sthiophene-EMI-SCN

1.8 1.6 1.4 1.2

C4 C6 H3 N1

C5 H1 H4 N2

g( r)

g( r)

SDBT-EMI-SCN( 300 K )

1.4

1.2

g(r )

497

1 0.8 C4 C6 H3 N1

0.6 0.4 0.2 0

0

0.2

c) RDF of [EMI][BF4]

0.4

0.6

0.8 r in nm

1

1.2

1.4

0

0.2

0.4

0.6

0.8 r in nm

1

1.2

C5 H1 H4 N2 1.4

thiophene’s ‘S’ and EMI in d) RDF of thiophene’s ‘S’ and EMI in [EMI][SCN]

Fig. 7. Radial distribution function of ‘S’ atom of sulphur aromatic compound with alkyl ‘C’, imidazolium ‘N’ and ‘H’ atoms of cations of ionic liquids.

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to that of DBT. The interaction between BMI is weaker than those in EMI which is stated by the broadening of peak in BMI system. The study of the second coordination sphere portrays the effects of anions and the interactions are stronger in BF4 system as compared to the SCN system. The tetrahedral geometry of the BF4 system aids the interaction, as the packing becomes more intimate between the cation and anion as compared to the loosely bound packing in the SCN system. 4. Conclusion Amongst the ionic liquids used for the study of extractive solvents for desulphurization, an indisputable trend has been observed, indicating efficiency of the extractive solvents. Imidazolium based ionic liquids were found to be effective for the extraction of the heteroaromatic impurities of thiophene and DBT from a dodecane (model liquid fuel) phase. The results from the MD simulations indicate that the extraction efficiency follows the following trend: [EMI] [BF4] b [EMI] [SCN] b [BMI] [BF4] b [BMI] [SCN]. DBT, in spite of being a larger molecule in comparison with thiophene, exhibits higher extraction tendency in the simulations. A complete trajectory was traced in order to study the extraction pathway of these compounds. RDF peaks are sharper for thiophene's sulphur as compared to DBT because two aromatic rings cause stearic crowding around sulphur atom of DBT. We believe this will not affect the extraction rates for DBT and thiophene because major interaction between sulphur compounds and ionic liquids is pi-pi interactions between aromatic rings of sulphur compound and cation of ionic liquid and not due to single sulphur atom of sulphur compounds. This study proves to be of great help in selecting an appropriate extractant and improving the process conditions. Acknowledgement Authors would like to acknowledge Department of Atomic Energy, Government of India (DAE/ICT/2015/6) for providing funding for current work. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2018.05.088. References [1] C.D. Wilfred, C.F. Kiat, Z. Man, M.A. Bustam, M.I.M. Mutalib, C.Z. Phak, Extraction of dibenzothiophene from dodecane using ionic liquids, Fuel Process. Technol. 93 (2012) 85–89, https://doi.org/10.1016/j.fuproc.2011.09.018. [2] F. Li, C. Kou, Z. Sun, Y. Hao, R. Liu, D. Zhao, Deep extractive and oxidative desulfurization of dibenzothiophene with C5H9NO·SnCl2 coordinated ionic liquid, J. Hazard. Mater. 205–206 (2012) 164–170, https://doi.org/10.1016/j.jhazmat.2011.12.054. [3] S.A. Dharaskar, Ionic liquids (a review): the green solvents for petroleum and hydrocarbon industries, Res. J. Chem. Sci. 2 (2012) 80–85. [4] J.D. Holbrey, I. López-Martin, G. Rothenberg, K.R. Seddon, G. Silveroc, X. Zheng, Desulfurisation of oils using ionic liquids: selection of cationic and anionic components to enhance extraction efficiency, Green Chem. 10 (2008) 87–92 doi: 10.1039/b710651c. [5] G. Parkinson, Diesel desulfurization puts refiners in a quandary, Chem. Eng. 108 (2001) 37. [6] G.W. Meindersma, A.J.G. Podt, A.B. de Haan, Selection of ionic liquids for the extraction of aromatic hydrocarbons from aromatic/aliphatic mixtures, Fuel Process. Technol. 87 (2005) 59–70, https://doi.org/10.1016/j.fuproc.2005.06.002. [7] C.E.R. Prado, L.C.G. Freitas, Molecular dynamics simulation of the room-temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate, J. Mol. Struct. THEOCHEM 847 (2007) 93–100, https://doi.org/10.1016/j.theochem.2007.09.009. [8] S. Feng, G.A. Voth, Molecular dynamics simulations of imidazolium-based ionic liquid/water mixtures: alkyl side chain length and anion effects, Fluid Phase Equilib. 294 (2010) 148–156, https://doi.org/10.1016/j.fluid.2010.02.034.

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