Physiochemical Properties of Hydrodenitrification andHydrodesulphurization Inhibiting Compounds with 1-Ethyl-3-Methylimidazolium Ethylsulphate at T = ( 298 . 15 to 323 . 15 ) K and p = 1

This work investigates the ability of 1-ethyl-3-methylimidazolium ethylsulphate ([emim][EtSO4]) as a green and tuneable solvent for denitrification and desulphurization of diesel oil. Experimental density, surface tension, and refractive index data have been measured for the following systems: [emim][EtSO4](1) + pyridine(2), [emim][EtSO4](1)+ pyrrole(2), [emim][EtSO4](1) + quinoline(2), [emim][EtSO4](1) + indoline(2), [emim][EtSO4](1) + thiophene(2), and [emim][EtSO4](1) + water(2) over the entire mole fraction of [emim][EtSO4] at temperatures of (298.15 to 323.15) K and at atmospheric pressure. Further, from experimental density values, coefficient of thermal expansivity and excess molar volume were also calculated. It was found that the heteroaromatic nitrogen/sulphur compounds and water are completely miscible in the [emim][EtSO4] ionic liquid. The surface tension values were found to increase while the refractive index decrease with increasing mole fraction of [emim][EtSO4]. On the other hand, dissimilar molecule such as water showed mobility of ions on mixing resulting in lower surface tension. The experimental values of surface tension increased in the order: thiophene > pyridine > pyrrole > indoline > quinoline and for refractive index: quinoline > indoline > pyrrole > pyridine > thiophene > water. It was found that the composition of [emim][EtSO4] has a greater influence than temperature in deciding the densities, surface, optical, and thermodynamic properties.


Introduction
Aromatic nitrogen, sulphur, and its derivatives in diesel oil cause severe environmental problems.Nitrogen and sulphur are present in diesel oil exhaust in the form of NO x [1], SO x [2], and particulate matter (PM) [1].These are the major contributors for air pollution and also reduce the efficiency of the pollution control equipment in vehicles [2].Therefore, the removal of aromatic nitrogen, sulphur, and its derivatives from diesel oil has become a challenge for petroleum industries.The growing environmental regulations and strict regulatory environments compel the industries to reduce nitrogen and sulphur level in diesel oil to less than 0.1 ppm [3] and 10 ppm [4,5], respectively.The separation of aromatic nitrogen species and sulphur from diesel oil is not feasible via conventional hydrodesulphurization (HDS) process, due to the presence of aromatic nitrogen species.Aromatic nitrogen species strongly act as inhibitors for HDS process in the form of catalyst poisoning or deactivation.Additionally, HDS cannot meet current environmental regulation in aromatic nitrogen and sulphur level in diesel oil.On the other hand, HDS process requires high active catalyst at high operating temperature or pressure [6].It also requires high reactor volume for the removal of limited amount of saturated nitrogen and sulphur species from diesel oil.
Currently, ionic liquids (ILs) are getting more attraction for aromatic/aliphatic [7], thiophene [8][9][10][11], and nitrogen [12] compound separation.Liquid-liquid extraction (LLE) is a well-established process; it can operate at moderate temperature and atmospheric pressure without the requirement of extensive energy.Further, it does not change the chemical structure of the components due to their unique feature as tunable or designer solvents [12,13].
Having negligible vapor pressure, they do not evaporate like volatile organic component (VOC) to the environment.They posses high chemical and thermal stability and have a good solubility for organic, inorganic, and polymeric compounds.These properties justify them as a solvent for HDS and hydrodenitrification (HDN) process.
To design new desulphurization and denitrification process involving ionic liquid on an industrial scale, it is necessary to know the physiochemical properties with aromatic sulphur and nitrogen compounds.There are limited literature data with experimental data involving binary systems consisting of ionic liquid and aromatic sulphur/nitrogen compound.The physiochemical properties are very important for process optimization.This further helps in regeneration and subsequent recycling of the ILs.The presence of water and other impurities like residual halide in ILs has an important role in tailoring their physiochemical properties [14,15].The fluorinated anionbased ILs are harmful since they produces HF, when in contact with water [16] even at 100 ppm [15].Thus, ILs with halide anion are corrosive in nature and, therefore, cannot be used for industrial applications.On the other hand, alkylsulphates, organoborates, and alkylsulfunates [16] anions are easily available at low cost with required physical properties which includes low viscosity and low melting point [14].
One of the most important ionic liquid is based on the ethylsulphate anion, that is, 1-ethyl-3-methylimidazolium ethylsulphate ([emim][EtSO 4 ]).The solubility of aliphatic and aromatic hydrocarbons in the ionic liquid, [emim][EtSO 4 ], is already available [17].The physical properties including density, viscosity, refractive index and speed of sound were measured in several alcohols [15,17,18], water [18], and in 2-ethoxy-2-methylpropane [15].Excess molar volume, deviation of refractive index, dynamic viscosity, and speed of sound were also studied at several temperatures and at atmospheric pressure [15,17,18].Heat capacity and excess enthalpy of 1-ethyl-3-methyl imidazolium based ionic liquid with water has been recently reported by Ficke et al. [19].Pereiro et al. [20] has used the alkylsufate-based ionic liquids to separate azeotropic mixtures such as hexane or heptane from ethanol via liquid-liquid extraction [20].However, till date the thermophysical properties of ethyl sulphate based ionic liquid with aromatic nitrogen and sulphur components are not available in the literature, so a systematic study needs to be carried out.
The aim of this work is to measure the physiochemical properties including density, refractive index and surface tension for pure 1-ethyl-3-methylimidazolium ethylsulphate ([emim][EtSO 4 ]) (Table 1) 2).Further excess molar volume are calculated from experimental data.Thus, the binary mixture data will be helpful for the simultaneous desulphurization and denitrification of diesel oil as well as the regeneration of [emim][EtSO 4 ].

Phase Equilibria Measurement.
Samples were prepared by transferring known mass of the pure liquids into stoppered bottles via syringe.The stoppered bottles were closed with screw caps to seal and prevent evaporation.All weighing was carried out in a balance (Mettler Toledo AT 261) with an accuracy of ±10 −4 ) gm.Previous experiments [11] showed that equilibrium was recognized after 6 hours of stirring at 100 rpm at a temperature of 298.15 K, using circulating water bath along with automatic controller.In our study, the equilibrium was attained by keeping the mixture settled for 12 hours during which good contact was obtained between two pure components (Figure 1).It is clear from Figure 1 that all the compounds form a single homogenous phase after equilibrium.Thus, our study mainly deals with the physiochemical aspects of the mixtures.Samples from the homogeneous mixture were withdrawn using syringes for their physiochemical studies.All the samples were prepared immediately prior to performing density, surface tension, and refractive index measurements so as to prevent variation in composition due to water/air retention via the hygroscopic IL.

Density Measurement.
Densities of the pure components and binary mixture were measured at atmospheric pressure with Anton Paar DSA-4500MA digital vibrating U-tube densimeter.The densitometer has a well-defined thermoelectric temperature control system.The uncertainty in the density measurement is ±0.0011 g•cm −3 .The instrument automatically corrects the influence of the viscosity on the measured density.The apparatus was calibrated by measuring the density of Millipore quality water and ambient air.

Surface Tension Measurement.
The surface tension of the pure components and binary mixtures were measured with a tensiometer by plate type method (Hanging drop tensiometer method, KRUSS K9, Germany) with a precision of 0.01 mN/m.The apparatus was calibrated by measuring the surface tension of Millipore quality water at ambient temperature.

Refractive Index Measurement.
Refractive index of the pure components and binary mixtures were determined at ambient temperature using an automatic refractometer AD-13 model (ABBEMAT-WR Dr.Kernchen), with an uncertainty of ± 0.00004.From these measurements, the coefficient of thermal expansion "α" of [EMIM][EtSO 4 ] ionic liquid is calculated via (1) Here, V is the molar volume of the pure fluid, ρ is the density of the pure fluid, and subscript P indicates constant pressure.The excess molar volume of the binary mixture V E m is calculated from the density of binary mixture ρ mix , and density of the pure components ρ 1 and ρ 2 according to (2) [21][22][23][24][25][26]] where x 1 and x 2 are the mole fraction of component 1 and 2, respectively.M 1 , M 2 are the molecular weight of the component 1 and 2, respectively.3 and 4).Table 3 shows the comparison of measured densities for pure [emim][EtSO 4 ] with temperature.Beside the effect of temperature and experimental method, the presence of trace amount of impurities such as water or ions can have a remarkable effect on thermodynamic properties [29].Due to this very reason, the % deviation in the density values is 6% when compared to the literature values.Table 4 also shows the comparison between experimental and literature data of the pure aromatic nitrogen/sulphur and water at 298.15 K.The density ρ of pure ionic liquid        nitrogen/sulphur and water.A smaller size of IL molecule was found to have a significant influence on the densities of the mixture with increasing temperature.These behaviors strongly agree with the studies carried out be Rodríguez and Brennecke [21] and Doma ńska et al. [34,35].A similar trend was observed by Gonzalez et al. [17] and Gomez et al.   [34], weak hydrogen bond interaction [6,36], van der waals interaction [6,36], CHπ bond interaction [37,38] and π-π interaction [39]  ] have stronger interaction with similar molecules like pyrrole than dissimilar molecules such as pyridine, indoline, quinoline, and water.Similar and dissimilar compounds imply a similarity in their structure.For example, pyrrole or thiophene has a similar five member ring structure as compared to the imidazolium ring.Thus, IL and pyrrole/thiophene possessing different electronegative atom within their structure is known to play a significant role in enhancing the solubility.

Effect of Composition on Surface Tension.
Table 12 presents the variation of surface tension with mole fraction for all the studied binary systems.The values of surface tension exhibit a linear increase with increasing mole fraction of [emim][EtSO 4 ] except for water.The fusion of benzene ring or addition of benzene ring with nitrogen species has significant influence on the surface tension.Thus, quinoline and indoline (Table 12) poss higher surface tension as compared to those compounds without additional benzene ring such as pyrrole, thiophene, pyridine, and water.For ).Although the nature of alkyl substitution at [emim][EtSO 4 ] dominates the surface tension and can define the trend of surface tension, still the ratio of van der Waals interaction [33] and columbic interaction [6,36,40] plays an important part upon mixing.The observed trend and values are consistent with those reported by Gomez et al. [18] and Wandschneider et al. [23].
The surface tension of aromatic nitrogen and sulphur are lower than [EMIM][EtSO 4 ] however; a comparison could not be done because of the scarcity of literature data.
[emim][EtSO 4 ] with aromatic nitrogen/sulphur are highly governed by the nature of their structure as well as the electronegative atom located on the studied compound and   gives information over the net destruction of interactions and packing phenomena that appears in the mixing process [43,44].The excess molar volume V E m was calculated from experimental density data for all studied systems according to (2).These calculated values are given in Tables 6 to 11 when in contact with water molecules due to the high dissociation of ions [45,46].
Thus, the Ionic Liquid in a mixture can be explained by two different types of interactions: (1) if the sign and magnitude of the excess molar volume is positive implies physical interaction mainly via dispersion forces or weak dipole-dipole interaction; (2) negative values refers to the chemical or specific interaction which includes charges transfer, CH-π bond interaction n-π interactions, formation of hydrogen bond, and so forth.The sign and magnitude of the excess molar volume is negative upon mixing with two similar aromatic structure of molecules indicates a strong ππ stacking [47,48].

Combined Effect of Temperature and Composition on
Transport Properties.The trend in the performance of experimental data for density (Tables 6 to 11) of the x In general for all the systems other than water, densities were found to decrease with increasing temperature.It can be seen that the variation of the thermodynamic property is much dependent on composition as compared to temperature.This can be explained by the composition of [emim][EtSO 4 ], which plays a significant role upon mixing than temperature over the transport properties of mixtures.Water molecules are influenced by hydrogen bonding upon mixing, which is more temperature dependent; therefore, the columbic force [36] and other interaction parameters [6,34,35,[37][38][39] are negligible.The aromatic nitrogen/sulphur with [emim][EtSO 4 ] is influenced by composition and by nature of species which are strongly recognized with several possible interaction parameters as discussed earlier and also due to the effective structural orientation of similar molecules.This behaviour is more consistent with information available in the literature [41].Thus, it is observed that the mole fraction of [emim][EtSO 4 ] has significant influence on the separation of aromatic nitrogen and sulphur.

Combined Effect of Temperature and Composition on
Thermodynamic Properties.The combined effect of temperature and composition gives the information about the net destruction of interaction and packing effect for all the studied systems.The excess molar volume is negative for [emim][EtSO 4 ](1) + PY(2) (Table 6) mixture over the entire mole fraction of [emim][EtSO 4 ] as the temperature is raised; however, the maximum interaction occurs between 0.4 < x IL < 0.5.For the system [emim][EtSO 4 ](1) + TS(2) (Table 10), a negative excess molar volume are seen when x IL > 0.5 irrespective of temperature.The excess molar volume and density is seen to vary linearly with respect to temperature and mole fraction for all the systems.It was thus observed that the size and shape of the components in mixture and the nature of electro negative atom located on the structure of the compounds greatly affect the thermodynamic properties.
[emim][EtSO 4 ]-water mixture, the surface tension linearly decreases with an increasing mole fraction due to the decreasing strength of hydrogen bond coupled with the high difference in their surface tensions (72.1 mN•m −1 for water and 48.4 mN•m −1 for [emim][EtSO 4 ]

Table 2 :
List of studied aromatic nitrogen and sulphur compound used in this work.

Table 3 :
Comparison of measured density ρ of Pure [emim][EtSO 4 ] data with literature data at the temperatures (298.15 to 323.15) K.

Table 4 :
Comparison of measured density ρ as a function of temperature with literature data for of pure compounds.

Table 5 :
Surface tension σ and refractive index nD for pure compounds of the binary system studied in this work at 298.15 K and atmospheric pressure.
emim][EtSO 4 ].It was found that as compared to temperature, concentration [emim][EtSO 4 ] has a greater influence on the interaction and packing effect of [emim][EtSO 4 ] with aromatic nitrogen/sulphur and water.This is mainly due to the stronger interaction between similar structure molecules ([emim][EtSO 4 ] + Thiophene/Pyrrole) than that between dissimilar structure molecules (i.e., [emim][EtSO 4 ] + water mixture).The surface tension increases with increasing mole fraction of [emim][EtSO 4 ] in the order of TS > PY > PYRR > INDO > QU.The refractive index decreases in the order: QU < INDO < PYRR < PY < TS with mole fraction.Furthermore, the excess molar properties for all the systems are negative especially at room temperature, which suggest effective packing between studied molecules.Thus, the study reveals that [emim][EtSO 4 ] might be a alternative solvent or the desulphurization and denitrification of diesel oil.