Dispersion of Asphaltenes in Petroleum with Ionic Liquids: Evaluation

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Dispersion of Asphaltenes in Petroleum with Ionic Liquids: Evaluation of Molecular Interactions in the Binary Mixture Adeniyi S. Ogunlaja,* Eric Hosten, and Zenixole R. Tshentu Department of Chemistry, Nelson Mandela Metropolitan University, P.O. Box 77000, Port-Elizabeth 6031, South Africa

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S Supporting Information *

ABSTRACT: Ionic liquids containing imidazolium cations 1-butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium nitrate, and 1-methyl-1H-imidazol-3-ium-2-carboxybenzoate were successfully synthesized and employed for the dispersion of asphaltenes in heavy oil, which is becoming a necessity during oil recovery and transportation. Molecular interaction studies indicated that 1-butyl-3-methylimidazolium chloride displayed a small HOMO−LUMO energy gap, which best explains its higher polarizability and reactivity as compared with 1-butyl-3-methylimidazolium nitrate and 1-methyl-1H-imidazol-3-ium-2carboxybenzoate. Dispersion indices obtained from the experiments were in agreement with the modeling studies. Maximum asphaltene dispersion indices (%) of 78, 70, and 53 were obtained for 1-butyl-3-methylimidazolium chloride,1-butyl-3methylimidazolium nitrate, and 1-methyl-1H-imidazol-3-ium-2-carboxybenzoate, respectively, when using an ionic liquid-toasphaltenes molarity ratio of 9:1. The excellent performance displayed by 1-butyl-3-methylimidazolium chloride is based on the thermodynamic interaction energy and HOMO−LUMO energy it holds.

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INTRODUCTION Asphaltenes are defined as the precipitated fraction in heavy oil upon addition of hydrocarbon solvents such as n-pentane or in heavy oil recovery and transportation, forming aggregates of between 3 and 15 nm in diameter.1 Asphaltene polar moieties, along with π-bonding among aromatic moieties, are responsible for asphaltenes’ self-association (aggregation).1−6 Asphaltenes possess a high degree of polynuclear aromatic rings that have alkyl side chains and incorporate heteroatoms (such as O, S, and N) and are soluble in aromatic solvents (e.g., benzene/ toluene). The polar functional groups (heteroatoms) confer a limited molecular solubility of asphaltenes in hydrocarbon solvents that are partially or fully aliphatic. During heavy oil recovery and transportation, asphaltenes precipitate once the thermodynamic stability of the colloidal solution (oil) is disturbed by changes in pressure, temperature, and composition.7−21 The precipitate causes severe problems in crude oil exploitation because pipeline blockages are frequently observed, hence reducing daily oil production output as a result of frequent removal of asphaltene aggregates.22−24 This has led to several studies by researchers in an attempt to understand the chemistry of asphaltenes.24−27 Mutelet et al.25 reported that petroleum asphaltenes are strong hydrogen bond acceptors and weak hydrogen bond donors. Furthermore, Wu et al.26,27 modeled a simplified asphaltenes-resin-oil interaction system by considering asphaltenes and resin as solutes dispersed in the oil. Thermodynamic parameters such as changes in pressure and temperature were also simulated in the aforementioned system. From the reported findings, the prevention of asphaltenes aggregation in heavy oils can be carried out in the following ways: (a) determination of what causes the precipitation (change in pressure or temperature) and optimization of asphaltene transportation conditions to prevent precipitations27 and (b) use of aromatic solvents and dispersants such as ionic liquids.28 Ionic liquids possess unique properties, such as a low © 2014 American Chemical Society

melting point, nonflammability, poor conductance of electricity, thermal stability, and solvating properties for diverse substances, and they frequently exhibit low vapor pressure.29 Ionic liquids are widely used as novel solvents in the fields of synthesis, separation, and catalysis30−34 and for simultaneous desulfurization and denitrogenation of diesel oil35 and are also applied as extractants and dispersants for crude oil upgrading.36−39 The commonly used ionic liquids for heavy oil recovery are those containing [PF6]− and [BF4]−; however, these liquids produce hydrogen fluoride (HF) gas when applied. This gas, although inert in nature, reacts with some organic compound found in heavy oil, thus limiting their applications.39 In this paper, the synthesis and characterization of imidazonium ionic liquids 1-butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium nitrate, and 1-methyl-1H-imidazol-3-ium-2-carboxybenzoate is shown. The imidazolium ionic liquid dispersion properties were investigated on asphaltenes owing to their hydrophilic and hydrophobic moieties. Molecular modeling of the individual ionic liquid was employed to discuss the local forces acting within the individual synthesized ionic liquid molecules and also explain the possible interactions between asphaltenes and the employed ionic liquids.

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EXPERIMENTAL SECTION Materials. 1-Chlorobutane (Sigma-Aldrich), phythalic anhydrides (Sigma-Aldrich), silver nitrate (Saarchem), and 1methylimidazole (Sigma-Aldrich) were purchased in South Africa. Asphaltenes were obtained as crude oil fractionates (Nigeria heavy crude oil), Anal. Found for asphaltenes (%): C, Received: Revised: Accepted: Published: 18390

July 7, 2014 November 8, 2014 November 8, 2014 November 9, 2014 dx.doi.org/10.1021/ie502672q | Ind. Eng. Chem. Res. 2014, 53, 18390−18401

Industrial & Engineering Chemistry Research

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of toluene), and this gives an indication of the concentration of asphaltenes precipitate produced. This criterion provides an estimate of the asphaltenes that would have been precipitated or suspended in the solution. Theoretical Calculations. Molecular Modeling for the Proposed Adsorption Study. Orbital Energies. Interaction between atoms or molecules happens most likely between the HOMO of one molecule and the LUMO of the other molecules. The amount of energy required to add or remove electrons in a molecule can be obtained from the HOMO and LUMO energy values. HOMO characterizes the nucleophilicity of a species, that is, its tendency to donate an electron, whereas LUMO characterizes the electrophilicity of a species, that is, its tendency to receive an electron.41 Orbital Energy Gap. From the HOMO and LUMO energy values, the HOMO−LUMO energy gap can be determined. A large HOMO−LUMO gap indicates high stability and resistance to charge transfer and changes in electron number and distribution. Therefore, hard molecules have a large HOMO−LUMO gap. Meanwhile, a small HOMO−LUMO gap indicates high polarizability because they require only a small amount of energy to get them to the excited states. A small HOMO−LUMO gap is indicative of soft molecules.42 Partial Charge Transfer. In a partial charge transfer process, a fraction of the electronic charge is transferred either within a molecule or through intermolecular interaction between species of an adduct. The charge transfers in adducts are between different regions in which one acts as electron donor and another acts as electron acceptor. Charge transfer occurs through boundary orbitals and usually proceeds in the direction from HOMO to LUMO.41 In this study, natural bond orbital (NBO) analysis for partial charges and charge transfer, which also gives chemical insights, were calculated for each ionic liquid before and after interaction with the asphaltenes by using DFT molecular modeling.43 Interaction Energies. Density functional theory (DFT) for molecular modeling was employed to understand the mode of interactions between asphaltenes and ionic liquids.44,45 Geometry optimizations and vibrational analyses of asphaltenes−ionic liquid adducts were performed using the Gaussian09 software. The B3LYP functional was employed with a 6-31G(d) basis set.46 The enthalpies of formation (ΔΔHadduct), Gibb’s free energies (ΔΔGadduct), and entropies of formation (ΔΔSadduct) for each adduct formed were calculated by using eqs 2 and 3.

83.83; H, 9.67; N, 0.99; S, 3.27. Hydrochloric acid (HCl) (analytical reagent), n-hexane, toluene, and acetonitrile (HPLC) were purchased from Merck, South Africa. Instrumentation. FT-IR spectra (4000−400 cm−1) were run on a Bruker, Tensor 27 platinum ATR-FTIR spectrometer. The solution electronic spectra were recorded on a PerkinElmer Lambda 35 UV−vis spectrophotometer using 1 cm quartz cells. The spectra were recorded over the wavelength range of 2000−250 nm, and the scans were conducted at a medium speed using a 20 nm slit width. Melting point determination was conducted using Stuart melting point SMP30 at a heating ramp of 10 °C/min. Conductivity of ionic liquids was measured with HANNA Instrument (HI 2300) pH, potentiometry/conductivity meter. The 1H NMR spectra of the ionic liquids were recorded on a Bruker 400 MHz spectrometer in DMSO-d6. Thermogravimetric analysis was performed using a PerkinElmer TGA 7 thermogravimetric analyzer (TGA). Typically, the samples were heated at a rate of 10 °C min−1 under a constant stream of nitrogen gas. Asphaltene levels in ionic liquids were determined by employing an Agilent 7890A gas chromatograph (GC) fitted with a flame ionization detector (FID) after dispersion. Asphaltene Precipitation in Crude Oil. A weighed amount of crude sample (0.5 g) was dissolved in 10 mL of pentane and allowed to stand for 4 h, then centrifuged at 5000 rpm for 30 min to separate the pentane soluble compounds from the solid residue known as asphaltenes.40 Dispersion of Asphaltenes with Ionic Liquids. A 1000 ppm (1000 mg/L) amount of asphaltenes dissolved in toluene was employed for the dispersion study. The asphaltene dispersion was carried out by varying the molarity ratio of the ionic liquids to asphaltenes (ionic liquids equivalent molar ratios: 1, 3, 5, 7, and 9), and 5 mL of the asphaltene solution was employed for each dispersion study. Prior to UV−vis analysis, a sample mixture (asphaltene solutions and ionic liquid) in a flask was stirred at 500 rpm for 5 h. After the dispersion contact time, the mixture was allowed to stand for an hour, after which it was decanted, leaving some asphaltene precipitate (undispersed asphaltenes) at the bottom of the flask. The resulting precipitate was dissolved in 5 mL of toluene, and its absorbance was measured at 346 nm by using a UV−vis spectrophotometer. Dispersion studies were recorded at various temperatures (25, 40, 55, and 70 °C). Influence of Ionic Liquids on Asphaltenes Precipitation in Crude Oil. A series of vials containing 0.1 g of crude oil with various ionic liquid ratios was added to a fixed volume of pentane (2 mL), and the mixture was allowed to stand for 4 h and then centrifuged at 5000 rpm for 30 min to separate the asphaltenes from the pentane soluble fraction. The amounts of asphaltene precipitate formed in the absence and presence of the various ionic liquids were evaluated by employing the GC− FID. Optimal Dispersion Calculations. The dispersion yields were measured with a UV spectrophotometer (PerkinElmer Lambda 35 UV−vis) at a wavelength of 346 nm. The percent dispersion index is defined by eq 1: dispersion index =

A − Ae × 100% A

ΔΔHadduct = ΔHadduct − (mΔHasphaltenes + t ΔHionic liquid) (2)

where m and t are the stoichiometric amounts of asphaltenes and ionic liquid involved in adduct formation. ΔΔGadduct = ΔΔHadduct − TΔΔSadduct

(3)

ΔΔG, T, and ΔΔS are the Gibbs free energy for the adduct formation, temperature (298 K), and entropy for adduct formation at standard conditions (i.e., 1 molar concentration for solvents and 1 atm pressure), respectively. The Gibbs free energy (ΔG0) gives information about the feasibility of the dispersion process, entropy (ΔS0) describes the spontaneous nature of dispersion (degree of randomness), and the sign of ΔH0 reflects the endo- or exothermic nature of the process. Ionic Liquid Synthesis. 1-Butyl-3-methylimidazolium Chloride. A solution of 1-methylimidazole (25 mL, 0.31 mol) was added to 1-chlorobutane (36 mL, 0.35 mol) in 50 mL of

(1)

where A represents the absorbance of asphaltenes in toluene without the addition of any dispersant and Ae represents the absorbance of asphaltenes precipitate left after the dispersion and the decantation process (precipitate was dissolved in 5 mL 18391

dx.doi.org/10.1021/ie502672q | Ind. Eng. Chem. Res. 2014, 53, 18390−18401


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Scheme 1. Synthesis of 1-Butyl-3-methylimidazolium Chloride

Scheme 2. Synthesis of 1-Butyl-3-methylimidazolium Nitrate

Scheme 3. Synthesis of 1-Methyl-1H-imidazol-3-ium-2-carboxybenzoate

Figure 1. An ORTEP view of 1-methyl-1H-imidazol-3-ium-2-carboxybenzoate with ellipsoids drawn at the 50% probability level (CCDC: 1004231).

[C8H15N2]+[Cl]− (%): C, 55.01 (54.87); H, 8.66 (8.43); N, 16.04 (15.76). FT-IR (cm−1): 3465 ν(>N−), 2858 ν(C−H), 1320 ν(C−N), 854 ν(NO3−); λmax = 289 nm. 1-Butyl-3-methylimidazolium Nitrate. AgNO3 (0.32 mol) was dissolved in 10 mL of water, and this solution was added to a solution of 1-butyl-3-methylimidazolium chloride (0.29 mol) in dichloromethane and stirred for 24 h (Scheme 2). The white suspended precipitate of AgCl was removed from the solution by filtering over Celite. The complete replacement of Cl− with NO3− was established by further addition of a concentrated AgNO3 solution to the repeatedly washed ionic liquid until no precipitation of AgCl occurred in the aqueous phase. The ionic

toluene at room temperature. The solution mixture was heated to reflux at 110 °C under vigorous stirring for 24 h, after which the resulting brown viscous oil was placed in an ice bath for 24 h (Scheme 1). The toluene was decanted, and the remaining semisolid was recrystallized from acetonitrile and then ethyl acetate to yield a white crystalline solid. The resulting solution of 1-butyl-3-methylimidazolium chloride gives a yield of ∼89%. 1 H NMR (δ, ppm in DMSO): 9.67 (1H, s, NCHN), 7.99 (2H, d, CH 3 NCHCHN, CH 3 NCHCHN), 4.22 (2H, t, NCH2(CH2)2CH3), 3.98 (3H, s, NCH3), 1.69 (2H, m, NCH2CH2CH2CH3), 1.09 (2H, m, N(CH2)2CH2CH3), 0.72 (3H, t, N(CH 2 ) 3 CH 3 ). Anal. Calcd. (found) for 18392



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H(22) = 0.8800 Å, N(21)−C(24) = 1.4631(2) Å, O(11)− C(17) = 1.3113(1) Å, O(12)−C(17) = 1.2162(1) Å, O(13)− C(18) = 1.2426(1) Å, O(14)−C(18) = 1.2647(1) Å. The short bond length of N(22)−H(22) implies a stronger bond as compared with other nitrogen atom bond lengths. Some bond angles displaying electron pair arrangements are provided as follows: O(13)−C(18)−O(14) = 124.59(9)°, O(11)−C(17)− O(12) = 124.09(9)°, O(14)−C(18)−C(11) = 116.85(9)°, and O(11)−C(17)−C(12) = 113.47(9)°. The structure of 1methyl-1H-imidazol-3-ium-2-carboxybenzoate showed that the imidazolium cation and carboxylate anion planes lie at 75.40° to each other with respect to the two rings (Supporting Information Figure S2). Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre (CCDC-1004231). Crystal data and details for the structure determination are provided in Table 1.

liquid was further purified by dissolving in methanol in the presence of a small amount of activated charcoal, and the mixture was heated under reflux for 6 h. The solvent and activated charcoal were removed by rotary evaporation and filtering over Celite, respectively. The resulting solution of 1butyl-3-methylimidazolium nitrate give an approximate yield of 82%. 1H NMR (δ, ppm in DMSO): 9.64 (1H, s, NCHN), 7.95 (2H, d, CH3NCHCHN, CH3NCHCHN), 4.23 (2H, t, NCH2(CH2)2CH3), 3.92 (3H, s, NCH3), 1.70 (2H, m, NCH2CH2CH2CH3), 1.14 (2H, m, N(CH2)2CH2CH3), 0.71 (3H, t, N(CH 2 ) 3 CH 3 ). Anal. Calcd. (found) for [C8H15N2]+[NO3]− (%): C, 47.75 (47.55); H, 7.51 (8.04); N, 20.88 (19.53). FT-IR (cm−1): 3483 ν(>N−), 2826 ν(C−H), 1302 ν(C−N), 854 ν(NO3−); λmax = 280 nm. 1-Methyl-1H-imidazol-3-ium-2-carboxybenzoate. 1-Methyl-1H-imidazol-3-ium-2-carboxybenzoate was synthesized by reacting phthalic anhydride (14.8 g, 0.1 mol) with 1-methyl imidazole (8 mL, 0.1 mol) in 10 mL of dry acetonitrile. The reaction was allowed to proceed for 60 min at 50 °C, after which the precipitates were collected by filtration. To 1 g of the white precipitate, 10 mL of 0.005 M of HCl was added, and the mixture was stirred for 1 h, after which the colorless ionic liquid obtained was dried under vacuum for 6 h (Scheme 3). An overall yield of 94% was recorded. 1H NMR (δ, ppm in DMSO): 8.31 (1H, s, NCHN+), 7.87 (2H, d, CH3NCHCHN+, CH3NCHCHN+), 7.53 (2H, t, CHCHCHCH, CHCHCHCH aromatic), 7.39 (1H, s, HN+CHNCH 3), 7.33 (2H, s, CHCHCHCH, CHCHCHCH aromatic), 5.02 (1H, s, OH), 2.53 (3H, s, HN+CHNCH3). Anal. Calcd (found) for [C4H7N2]+[C8H5O4]− (%): C, 58.06 (57.14); H, 4.87 (4.82); N, 11.26 (11.31). FT-IR (cm−1): 1324 ν(C−N), 1535 ν(COO−), 1691 ν(COOH), 3147 ν(C−H), 3376 ν(N−H); λmax = 305 nm. Structural Determination and Refinement. Single crystals of 1-methyl-1H-imidazol-3-ium-2-carboxybenzoate suitable for X-ray analysis were grown by placing the vacuum-dried liquid ionic solution in a refrigerator for over 10 h to produce a colorless crystalline salt (Figure 1). All attempts to grow the single crystal structure of 1-butyl-3-methylimidazolium nitrate and 1-butyl-3-methylimidazolium chloride were unsuccessful. Intensity data were collected on a Bruker APEX II CCD diffractometer with graphite monochromated Mo Kα radiation using the APEX 2 data collection software.47 The structure was solved by direct methods applying SHELXS-201348 and refined by least-squares procedures using SHELXL-2013.48 The crystal structure diagram was drawn with ORTEP-3 for Windows.49

Table 1. Crystal Data and Details of the Structure Determination formula formula weight crystal system space group a, b, c (Å) V (Å3) Z D (calcd, g/mL) μ (Mo Ka, mm ) F(000) crystal size (mm)

C8 H5 O4, C4 H7 N2 248.24 orthorhombic Pbca (no. 61) 8.3767(4), 12.0709(5), 23.2943(10) 2355.39(18) 8 1.400 0.107 1040 0.22 × 0.31 × 0.63 Data Collection

temp (K) radiation (Å) θ, min/max (°) data set tot., unique data, R(int) obs data (I > 2.0 σ (I))

200 Mo Kα, 0.71073 3.0, 28.4 −11:11; −16:15; −20:31 21035, 2938, 0.018 2551 Refinement

Nref, Npar R, wR2, S max, av shift/error min, max resd. dens. (e/Å)

2938, 165 0.0372, 0.1015, 1.04 0.00, 0.00 −0.27, 0.27

TG Profile of 1-Methyl-1H-imidazol-3-ium-2-carboxybenzoate. Imidazonium ionic salt thermogram showed that the ionic compound remained stable up to a temperature of 90 °C, after which the compound structure began to collapse. A total weight loss of 96% was observed at a temperature between 97 and 154 °C (Figure 2). TGA analysis on 1-butyl-3methylimidazolium chloride and 1-butyl-3-methylimidazolium nitrate were not carried out because the ionic liquids occur in liquid state. FT-IR and Proposed Structure of Asphaltenes. The existence of one or more aromatic rings in the asphaltene matrix is confirmed by C−H and CC−C vibrations between 2700 and 3000 cm−1 (Figure 3).40 The FT-IR spectrum also gave characteristic bands with frequencies similar to most nitrogen-containing compounds, confirming the study reported by Jewell et al.50 that nitrogen-containing compounds are enriched in asphaltenes. The bands at 1570, 1370, and 780 cm−1 are attributed to N−H bending, v (C−N) and v (C−S) vibrations, respectively.

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RESULTS AND DISCUSSION The reaction between 1-methylimidazole with phythalic anhydride and butyl chloride produced ionic liquids; 1methyl-1H-imidazol-3-ium-2-carboxybenzoate and 1-butyl-3methylimidazolium chloride, respectively. 1-Butyl-3-methylimidazolium nitrate, on the other hand, was produced by replacing the chloride anion with the nitrate anion from silver nitrate. 1-Methyl-1H-imidazol-3-ium-2-carboxybenzoate Crystal Structure. The crystral structure of 1-methyl-1Himidazol-3-ium-2-carboxybenzoate (Figure 1) confirms the formation of an ionic compound. The structure of the compound showed an imidazonium cation and carboxylate anion close to each other. The closest contact distance between the imidazonium cation and carboxylate anion, N(22)− H(22)···O(14), is 1.83 Å (Supporting Information Figure S1). The molecule possess the following bond lengths: N(22)− 18393



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Figure 2. TG profile curve for 1-methyl-1H-imidazol-3-ium-2carboxybenzoate.

Figure 4. Proposed structure of asphaltenes (1 and 2).52

respectively, at 21.5 °C. The change in conductivity was attributed mainly to the different anions on the liquids.53,54 The magnitude of conductivities of the individual ionic liquids as discussed helps in explaining the mobility of the ions (anions and cations) within their environment. From the observed conductivity values, the order of charge carrier ions for charge transfer is 1-butyl-3-methylimidazolium chloride > 1-butyl-3methylimidazolium nitrate > 1-methyl-1H-imidazol-3-ium-2carboxybenzoate. Asphaltenes Concentration Calibration. Various standards of asphaltenes ranging from 10 to 100 ppm were prepared. UV−vis measurements were employed for this calibration study because the absorbance of light depends on the concentration of the asphaltenes in the toluene solutions. The spectrophotometer (PerkinElmer Lambda 35 UV−vis) was operated at a wavelength of 346 nm to measure the asphaltene concentrations in the toluene solutions, and this gave a steady absorbance value for the asphaltenes concentration. The linear plot of the absorbance against the concentrations of the asphaltenes is presented in Figure 5. The limit of detection (LOD) and limit of quantification (LOQ) of the asphaltenes were found to be 12.5 and 37.8 ppm, respectively. The LOD represents the lowest concentration of asphaltenes in the sample, which can be detected but not necessarily quantified as a precise value, and the LOQ is the lowest concentration of asphaltenes in a sample that can be quantitatively determined with a high degree of confidence.55 The slope and standard deviation values were obtained from the graph and data, respectively. LOD = 3.3 (SD/S) and LOQ = 10 (SD/S), where SD is standard deviation of the response and S is the slope of the graph.

Figure 3. FT-IR spectrum of asphaltenes.

Asphaltenes are known to be enriched with nitrogen-, oxygen-, and sulfur-containing compounds, and the molecular structure of asphaltenes is difficult to determine because the molecules tend to stick together;51 however, several structures of apshaltenes have been proposed with the molecular masses distribution in the range of 400−1500 g/mol.51 From the FTIR spectrum, the broad peak assigned to hydroxyl {v(−OH)} was absent; hence, a unit of the proposed structure of asphaltenes (1) reported by Suzuki et al.52 (Figure 4) was adopted for the study and also employed for the molecular modeling studies. Molecular modeling data (theoretical data) were also generated for the proposed structures of asphaltenes (2) containing oxygen atoms in the hydroxyl (−OH) form, as reported by Suzuki et al.52 The obtained results are available in the Supporting Informmation. Conductivity Analysis. Conductivity is often used as a measure of the characteristics of electrolyte solutions. In this case, the conductivity of the ionic liquid was analyzed. It is generally known that ionic conductivity is proportional to the number of charge carrier ions and their mobility. 1-Methyl-1Himidazol-3-ium-2-carboxybenzoate, 1-butyl-3-methylimidazolium chloride, and 1-butyl-3-methylimidazolium nitrate gave conductivity values of 3.70, 4.95, and 4.54 mS·cm−1 , 18394



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steady increase in the amount of asphaltenes dispersed (Figure 7). An increase in the amount of ionic liquids brings about an

Figure 5. Calibration using various asphaltene concentrations.

Dispersion Studies. Effect of Varying Temperature. Temperature is one of the most important factors influencing asphaltene deposition in heavy oils, and asphaltenes are known to precipitate out of heavy oils at higher temperatures.24 A constant molarity ratio of ionic liquid-to-asphaltenes (5:1) was maintained at each temperature. An increase in dispersion temperature resulted in an increase in asphaltenes dispersion (Figure 6). At a temperature above 55 °C, the asphaltene

Figure 7. Effect of varying ionic liquid concentrations in asphaltenes: (■) 1-methyl-1H-imidazol-3-ium-2-carboxybenzoate, (▲) 1-butyl-3methylimidazolium nitrate, and (●) 1-butyl-3-methylimidazolium chloride.

increase in the bonding interactions between asphaltenes and the ionic liquids, consequently assisting the solvation of asphaltenes within the ionic liquids (1-methyl-1H-imidazol-3ium-2-carboxybenzoate, 1-butyl-3-methylimidazolium nitrate and 1-butyl-3-methylimidazolium chloride). From the obtained result at an ionic liquid-to-asphaltenes molarity ratio of 9:1 (Figure 7), 1-butyl-3-methylimidazolium chloride gave a better dispersion, leaving fewer amounts of asphaltenes in the toluene as compared with 1-methyl-1H-imidazol-3-ium-2-carboxybenzoate and 1-butyl-3-methylimidazolium nitrate. From the trend, a further increase in the molarity ratio of the ionic liquids to asphaltenes will result in a successive increase in asphaltene dispersion. Influence of 1-Butyl-3-methylimidazolium Chloride on Asphaltenes Precipitation in Crude Oil. The prevention of the precipitation of asphaltenes in crude oils was monitored by the addition of various ratios of ionic liquids into the crude oil system as described in the experimental methodology. The mass of asphaltene precipitate formed in the absence of ionic liquid was 0.04 g (100 wt %). The amount of asphalthenes deposited after the addition of the various ionic liquids is presented in Table 2. 1-Butyl-3-methylimidazolium chloride presented less asphaltene precipitate as compared with the other ionic liquids. The GC chromatograms of the dissolved asphaltenes show a relative decrease in peak abundance as the

Figure 6. Effect of varying temperature in asphaltenes dispersion: (●) 1-methyl-1H-imidazol-3-ium-2-carboxybenzoate, (▲) 1-butyl-3-methylimidazolium chloride, and (■) 1-butyl-3-methylimidazolium nitrate.

dispersion yield decreases for 1-methyl-1H-imidazol-3-ium-2carboxybenzoate, and this was probably due to a disruption of the ionic liquid properties at higher temperature, thus altering the interactions between ionic liquid and asphaltenes. However, 1-butyl-3-methylimidazolium nitrate and 1-butyl-3-methylimidazolium chloride maintained their dispersion properties at elevated temperatures of up to 90 °C. The temperature studies were halted at 90 °C owing to the possible evaporation of toluene at elevated temperatures, thereby deterring proper analysis. Effect of Varying the Amount of Ionic Liquids. The concentration ratio of ionic liquid to asphaltenes at 55 °C for the dispersion of asphaltenes depends on the amount of ionic liquid. An increase in the amount of ionic liquid resulted in a

Table 2. Effect of Varying Ionic Liquid Ratios in the Precipitation of Asphaltenes from Crude Oils ionic liquids

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molarity ratio of ionic liquids-tocrude oil (wt/wt)

1-methyl-1Himidazol-3-ium2carboxybenzoate (wt % asphaltenes)

1-butyl-3methylimidazolium chloride (wt % asphaltenes)

1-butyl-3methylimidazolium nitrate (wt % asphaltenes)

2:1 5:1 9:1

95 ± 2 73 ± 4 42 ± 3

76 ± 6 41 ± 3 23 ± 4

84 ± 4 47 ± 8 29 ± 7



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recovered 1-butyl-3-methylimidazolium chloride, the following were observed: first, a new band around 1379 cm−1, which is attributed to the C−N band of asphaltene, was observed (highlighted in red); and second, an increase in the intensity of −CC−C− bands of the recovered 1-butyl-3-methylimidazolium chloride around 1574 cm−1 (blue line), confirming the presence of a more aromatic structure. It is also to be noted that the structure of 1-butyl-3-methylimidazolium chloride was preserved after recovery. Molecular Modeling Studies. Molecular interactions between asphaltene compounds with ionic liquids were modeled, and the interactions occurred through both the cation and the anion. The interactions between the ionic liquid and the proposed asphaltene compound in any system depend on orbital interactions (with interaction between the HOMO and LUMO orbitals), π−π interaction, hydrogen bond interaction, and van der Waals forces observed among the alkyl groups. These interactions are discussed in terms of their orbital energies (HOMO and LUMO energies), orbital energy gap, and partial charge transfer, as explained in the Experimental Section. A high ionization potential is indicated by low HOMO energy (better electron donor), whereas a high electron affinity is indicated by a high LUMO energy (better electron acceptor). Prior to interactions, geometry optimizations of ionic liquids were carried out for 1-butyl-3methylimidazolium chloride, 1-butyl-3-methylimidazolium nitrate, 1-methyl-1H-imidazol-3-ium-2-carboxybenzoate, and proposed asphaltenes (structure 1 in Figure 4). The HOMO and LUMO energy locations are presented in Figures 9, 10, 11, and 12, respectively. With ionic liquids, HOMO energies come from the anion heteroatoms, and the LUMO energies originate from the cation heteroatom, with the bulk of the energy coming from the N1− C2−N3 atoms40,56−58 (Figures 9 and 10). With 1-methyl-1Himidazol-3-ium-2-carboxybenzoate, HOMO energies originated from the cationic ring (imidazolium N1−C2−N3 atoms), and the LUMO energies came from the carboxylate anion (Figures 11). The hydrogen bond between one of the carboxylate’s oxygen atoms and C−H of its cationic ring was observed in 1-methyl1H-imidazol-3-ium-2-carboxybenzoate (Supporting Information Figure S5); however, hydrogen-bonding interactions

ionic liquid (1-butyl-3-methylimidazolium chloride) molarity ratio increased (Supporting Information Figure S3). Reusability Studies of 1-Butyl-3-methylimidazolium Chloride. In this study, 1-butyl-3-methylimidazolium chloride (9 equiv) in asphaltenes was employed and the same procedure as explained in the Experimental Section was employed. The ionic liquid was recovered after use by pouring 10 mL of an asphaltenes/1-butyl-3-methylimidazolium chloride mixture into excess methanol, and the methanolic solution was collected via filtration to leave the asphaltenes behind. The ionic liquids were recovered by rotary evaporation at 55 °C and then stored for use. The recycled 1-butyl-3-methylimidazolium chloride was used to disperse asphaltenes for two further cycles. From Figure 8, it can been seen that the dispersion yield decreased for the

Figure 8. Reusability studies using 1-butyl-3-methylimidazolium chloride.

second and third cycles in comparison with the yield obtained by the fresh 1-butyl-3-methylimidazolium chloride in the first cycle. The loss of performance can be attributed to the presence of some asphaltenes recycled alongside the 1-butyl-3methylimidazolium chloride (see Supporting Information Figure S4 for the FT-IR spectrum of the recovered 1-butyl-3methylimidazolium chloride). From the FT-IR spectrum of the

Figure 9. HOMO and LUMO locations of 1-butyl-3-methylimidazolium nitrate: (a) HOMO and (b) LUMO. Red, blue, and gray represent oxygen, nitrogen, and carbon atoms, respectively. 18396



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Figure 10. HOMO and LUMO locations of 1-butyl-3-methylimidazolium chloride: (a) HOMO and (b) LUMO. Green, blue, and gray represent chlorine, nitrogen and carbon atoms, respectively.

Figure 11. HOMO and LUMO locations of 1-methyl-1H-imidazol-3-ium-2-carboxybenzoate: (a) HOMO and (b) LUMO. Red, blue, and gray represent oxygen, nitrogen, and carbon atoms, respectively.

Figure 12. HOMO and LUMO locations of the proposed asphaltenes (structure 1 in Figure 4): (a) HOMO and (b) LUMO. Blue, yellow, and gray represent nitrogen, sulfur, and carbon atoms, respectively.

were absent in 1-butyl-3-methylimidazolium chloride and 1butyl-3-methylimidazolium nitrate. HOMO−LUMO Gap. The HOMO−LUMO gap describes the stability and resistance of molecules, and it also predicts the reactivity between species by providing the electrical transport properties as well as electron carrier and mobility in molecules.41 From the three ionic liquids, 1-butyl-3-methylimidazolium chloride displayed the smallest HOMO−LUMO energy gap, which is indicative of soft molecules, and this small gap leads to better polarizability and reactivity as observed in the dispersion studies. 1-Methyl-1H-imidazol-3-ium-2-carboxybenzoate, on the other hand, gave the highest HOMO− LUMO energy gap, and this indicated a hard molecule with low reactivity (Table 3). The HOMO−LUMO gap energies of 1butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazo-

Table 3. HOMO (EH) and LUMO (EL) Energies for Asphaltenes and the Various Imidazolium Compounds compd 1-butyl-3-methylimidazolium chloride 1-methyl-1H-imidazol-3-ium-2carboxybenzoate 1-butyl-3-methylimidazolium nitrate asphaltenes (structure 1 in Figure 4)

EH (eV)

EL (eV)

orbital energy gap (EG) (eV)

−4.40

−1.35

3.05

−6.42

−1.13

5.29

−5.02

−0.76

4.26

−4.92

−2.02

2.90

lium nitrate, 1-methyl-1H-imidazol-3-ium-2-carboxybenzoate are 3.05, 4.26, and 5.29 eV, respectively (Table 3). 18397



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Figure 13. HOMO and LUMO location of adduct between 1-butyl-3-methylimidazolium nitrate and asphaltenes (structure 1 in Figure 4), (a) HOMO and (b) LUMO. Hydrogen was removed for clarity of the structure. Yellow, blue, and gray represent sulfur, nitrogen, and carbon atoms, respectively.

Figure 14. HOMO and LUMO location of adduct between 1-methyl-1H-imidazol-3-ium-2-carboxybenzoate and asphaltenes (structure 1 in Figure 4), (a) HOMO and (b) LUMO. Hydrogen was removed for clarity of the structure. Yellow, blue, and gray represent sulfur, nitrogen, and carbon atoms, respectively.

Figure 15. HOMO and LUMO location of adduct between 1-butyl-3-methylimidazolium chloride and asphaltenes (structure 1 in Figure 4), (a) HOMO and (b) LUMO. Hydrogen was removed for clarity of the structure. Yellow, blue, and gray represent sulfur, nitrogen, and carbon atoms, respectively.

Adducts formed when 1-butyl-3-methylimidazolium nitrate was employed, indicating that the HOMO comes from the ionic liquid’s anionic center and the LUMO originates from the asphaltenes (Figure 13). This clearly indicated that the interaction between the ionic liquid and asphaltenes is through electron donation from the HOMO to the LUMO. Hence, possible interactions resulting in adduct formation are controlled by hydrogen bonding between the ionic liquids and the π face of the asphaltenes. With 1-methyl-1H-imidazol3-ium-2-carboxybenzoate, the LUMO comes from the cationic center of the ionic liquid and the HOMO centers around the asphaltenes, and this indicates that the interactions are controlled by a CH-π bond between 1-methyl-1H-imidazol-3ium-2-carboxybenzoate and the asphaltenes (Figure 14). 1Butyl-3-methylimidazolium chloride showed that the HOMO and LUMO centers around the asphaltenes, giving rise to π−π

interaction (π−π stacking) between asphaltenes (structure 1 in Figure 4) and 1-butyl-3-methylimidazolium chloride (Figure 15). In determining favorable interaction between ionic liquids and asphaltenes, LUMO energies with higher negative values are more important because the LUMO sites lead to the overlapping of the incoming HOMO orbitals. The LUMO energy values of adducts formed between asphaltenes (structure 1 in Figure 4) with 1-butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium nitrate, and 1-methyl1H-imidazol-3-ium-2-carboxybenzoate are −3.34, −2.64, and −3.07 eV, respectively. On the basis of the reported LUMO energy values of adducts, 1-butyl-3-methylimidazolium chloride is the most favorable ionic liquid for the dispersion of asphaltenes. Interaction Energies. The interaction energies of the various ionic liquids relate to the compactness between the cation and 18398



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0.105 and −0.365 to −0.062, respectively, on forming the adduct. The atomic charge of the free oxygen atom on the carboxylate end of 1-methyl-1H-imidazol-3-ium-2-carboxybenzoate became more negative after adduct formation (from −0.177 to −0.214), confirming that charge transfer is from asphaltenes to 1-methyl-1H-imidazol-3-ium-2-carboxybenzoate. The conductivity studies conducted earlier confirm the presence of in charge carrier ions in all ionic liquids. NBO analysis has further helped in showing the atoms where significant charge transfer/mobility took place.

anion.59 Ionic liquids with high interaction energy have higher compactness (closer distance between cation and anion), thus making it difficult for incoming molecules to reorganize around the cation and anion, although ionic liquids with smaller interaction energy have a lower compactness and a larger distance between the cation and anion, thus allowing for the facile organization of incoming molecules. The interaction energies of 1-butyl-3-methylimidazolium chloride, 1-butyl-3methylimidazolium nitrate and 1-methyl-1H-imidazol-3-ium-2carboxybenzoate were −55.4 × 104, −44.1 × 104, and −54.8 × 104 kcal/mol, respectively. 1-Butyl-3-methylimidazolium chloride presented the smallest interaction energies among the three ionic liquids, which makes it a better candidate in the dispersion of asphaltenes, as observed in the presented experimental results. However, when looking at the various adducts, it is preferable to have a higher interaction energy, since this indicates that an extra amount of energy is required to separate the formed adducts.41 The adduct interaction energies generated between asphaltenes and the various ionic liquids1-butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium nitrate, and 1-methyl-1H-imidazol-3-ium-2-carboxybenzoatewere −24.9 × 105, −23.8 × 105, and −24.8 × 105 kcal/mol, respectively. The magnitudes of the various adduct energies are relatively close to one another. Thermodynamic parameters such as enthalpy (ΔΔH), entropy (ΔΔS), and free energies (ΔΔG) are presented in Table 4. Adduct formations among asphaltenes

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CONCLUSIONS Ionic liquids containing imidazolium cations, 1-butyl-3methylimidazolium chloride, 1-butyl-3-methylimidazolium nitrate, and 1-methyl-1H-imidazol-3-ium-2-carboxybenzoate were successfully synthesized and employed for the dispersion of asphaltenes, thereby showing potential for reducing asphaltene precipitation during petroleum recovery and transportation. Asphaltenes (polyaromatic compounds) possess attraction with both the cationic and anionic ends of the ionic liquids owing to stronger quadruple moments of polyaromatics compounds as compared with the monoaromatics.60,61 Computationally optimized interactions between ionic liquids and asphaltenes confirmed that interaction between asphaltenes and ionic liquids took place through π−π interaction between cation and asphaltenes and via hydrogen bonding. The HOMO−LUMO gap energies, partial charge transfer and optimization energies were calculated for each individual species, and interaction energies were calculated for the adducts. From the calculations based on interaction energies, the order of reactivity is 1-butyl3-methylimidazolium chloride > 1-butyl-3-methylimidazolium nitrate > 1-methyl-1H-imidazol-3-ium-2-carboxybenzoate. Adduct formation between 1-butyl-3-methylimidazolium chloride and asphaltenes (structure 2 of Figure 4), (also see the Supporting Information) also suggested π−π interaction (π−π stacking), and charge transfer were observed. The calculated results are in agreement with the experimental data (i.e., with dispersion indices).

Table 4. DFT Molecular Modelling Thermodynamic Data on the Formation of Adduct between Ionic Liquids (1-butyl-3methylimidazolium chloride, 1-butyl-3-methylimidazolium nitrate, 1-methyl-1H-imidazol-3-ium-2-carboxybenzoate) and Asphaltenesa

1-butyl-3-methylimidazolium chloride 1-butyl-3-methylimidazolium nitrate 1-methyl-1H-imidazol-3-ium-2carboxybenzoate a

ΔΔH (kcal mol−1)

ΔΔG (kcal mol−1)

ΔΔS (cal mol−1)

−14.2

−1.0

−48.4

−10.4

−0.3

−36.9

−5.5

−3.8

−6.2

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ASSOCIATED CONTENT

S Supporting Information *

See structure 1 in Figure 4.

Nine additional figures and six additional tables of data. This material is available free of charge via the Internet at http:// pubs.acs.org.

and the various ionic liquids are favorable at low temperature. From thermodynamic parameters, Gibbs free energy is a most useful thermochemical process at constant temperature and pressure because ΔG < 0 indicates that a reaction is possibly spontaneous, whereas ΔG > 0 shows that a reaction is possibly nonspontaneous. Electrostatic Charge Transfer in Ionic Liquids. Electrostatic (Coulombic) interaction is the main form of interaction in ionic liquids. The charge transfer was investigated by employing natural bond orbital (NBO) analysis by looking at the partial charge difference in both asphaltenes and ionic liquids before and after the interaction. Some atomic partial charges of 1butyl-3-methylimidazolium chloride and 1-butyl-3-methylimidazolium nitrate were observed to be less negative after forming adducts with asphaltenes; hence, charge transfer occurs from the ionic liquids to asphaltenes. Partial charge on the chloride ion of 1-butyl-3-methylimidazolium chloride changed from −0.239 to −0.139 on interaction with asphaltenes to form te adduct. Nitrogen and an oxygen atom on the nitrate ion of 1butyl-3-methylimidazolium nitrate decreased from 0.215 to

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +27 73 194 3554. Fax: +27 41 504 4236. E-mail: slaja1@ yahoo.com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Center for High Performance Computing (CHPC), Cape Town, South Africa for providing the platform in carrying out the molecular modelling studies on the Gaussian09 software. The authors are also grateful to SASOL (Pty) Ltd for funding and to Nigeria National Petroleum Co-orporation (NNPC) in Nigeria for supplying the crude oil. 18399



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Dispersion of Asphaltenes in Petroleum with Ionic Liquids: Evaluation

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