Tuning Task-Specific Ionic Liquids for the Extractive Desulfurization of

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Tuning Task-Specific Ionic Liquids for the Extractive Desulfurization of Liquid Fuel Hua Zhao, Gary A. Baker, Durgesh Vinod Wagle, SUDHIR RAVULA, and Qi Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.6b00972 • Publication Date (Web): 26 Jul 2016 Downloaded from http://pubs.acs.org on July 31, 2016

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ACS Sustainable Chemistry & Engineering

Tuning Task-Specific Ionic Liquids for the Extractive Desulfurization of Liquid Fuel Hua Zhao,*† Gary A. Baker,‡ Durgesh V. Wagle,‡ Sudhir Ravula,‡ and Qi Zhang† †

Department of Chemistry and Forensic Science, Savannah State University, Savannah, GA 31404, USA ‡

Department of Chemistry, University of Missouri-Columbia, Columbia, MO 65211, USA

Corresponding author: Hua Zhao Department of Chemistry and Forensic Science Savannah State University 3219 College Street Savannah, GA 31404 USA

*

Corresponding author. Email: [email protected] (or [email protected]).

1 ACS Paragon Plus Environment


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ABSTRACT: Extractive desulfurization of liquid fuel is a simple process that requires minimum energy input and can be operated via existing liquid–liquid extraction apparatuses. In particular, to achieve deep desulfurization, the conventional hydrodesulfurization (HDS) process has shown limitations in the removal of aromatic sulfur compounds. Recently, extractive desulfurization using a new type of non-volatile solvents, ionic liquids (ILs), has yielded promising results. However, there is a lack of systematic evaluation of the effect of IL structure on desulfurization efficiency, and a lack of mechanistic understanding regarding how ILs lead to the partition of aromatic sulfur compounds from fuel to the IL phase. The present study examines a total of 71 ILs and two deep eutectic solvents (DESs) with combinations representing various cations and anions. We identify a number of ILs that yield high partition coefficients [6‒15 mg(S) kg (IL)– 1

/mg(S) kg (oil)–1] for the partition of aromatic sulfur compounds between ILs and n-octane or n-

dodecane as surrogates for gasoline or diesel. We find that the high sulfur partition coefficient correlates with a high dipolarity/polarizability (π*) or a low solvent polarizability (SP) of ILs carrying the same cation and different anions, but correlates with a low dipolarity/polarizability (π*) for ILs carrying the same anion paired to cations bearing different alkyl chain lengths. We further demonstrate that a four-step extraction using ILs can achieve 99% dibenzothiophene (DBT) removal (i.e., an initial sulfur content of 500 ppm is reduced to CF3SO3‒ > PF6‒ > Tf2N‒ > FAP‒). The extraction of aromatic sulfur compounds follows a similar trend, where the extraction efficiency increases with a higher π-density and a lower degree of alkyl substituents on the sulfur compound. Anantharaj and Banerjee13 studied the interactions between thiophene with various


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ILs (including pyrrolidinium, pyridinium, and imidazolium examples) by using COSMO-RS (COnductor Like Screening MOdel for Real Solvents), and observed that the interaction energies decrease in the order of [BzMIM][BF4] > [C4MPyrr][BF4] > [C4MePy][BF4] > [C4MePy][PF6] > [C4MPyrr][PF6]

(BzMIM

=

1-benzyl-3-methylimidazolium;

C4MPyrr

=

1-butyl-1-

methylpyrrolidinium; C4MePy = 1-butyl-4-methylpyridinium). These authors indicated that the predominant interaction was the CH–π interaction rather than hydrogen bonding between the sulfur atom and the cation. The same group14 also made COSMO-RS predictions for extraction of the aromatic sulfur compounds TS, BT, and DBT from diesel oil using various ILs. Their results suggest that ILs for efficient desulfurization typically consist of non-aromatic cations (i.e., morpholiniums, pyrrolidiniums, and piperidiniums versus imidazoliums and pyridiniums) and anions without steric shielding effect, such as SCN‒, MeSO3‒, OAc‒, Cl‒, and Br‒. This contrasts with earlier COSMO-RS calculations on the liquid–liquid extraction of thiophene by various ILs reported by Kumar and Banerjee15 which suggest that ILs consisting of a smaller cation and a somewhat bulkier anion such as BF4‒ should afford higher selectivity and capacity for thiophene removal. Other groups have suggested that π-π and hydrogen bonding interactions likely play the key roles. Based on the 1H NMR chemical shifts for various molar ratios of thiophene to [BMIM][PF6], Su et al.16 determined that the aromatic ring current effect (which relates to π–π interaction) is the main factor controlling the chemical shifts of thiophene and the cation, while less important factors include CH–π interactions, weak hydrogen bonding between the acidic hydrogen(s) of BMIM+ and thiophene, and the electrostatic field effect. Using a similar approach, Revelli et al.17 suggested some possible interactions between thiophene and [BMIM][SCN], including the aromatic ring current effect, CH–π interaction between the cation and thiophene,



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and anion-cation interaction. They proposed a pseudo-tetrahedral geometric arrangement of four thiophene molecules neighboring the BMIM+ cation with the sulfur atom facing toward the SCN‒ anion. The Zhang group18 reported that an increase in the alkyl chain length appended to the imidazolium cation led to an almost doubling in the absorption capability for thiophene, with a larger anion (e.g. PF6‒) showing a stronger extraction capability than a smaller one (e.g. BF4‒). These authors suggested that the favorable electronic interaction of charged ion pairs of ILs and aromatic sulfur compounds with highly polarizable π-electron density drives the molecular insertion of solute with the IL. Lü et al.19 examined the interactions between dibenzothiophene and [MMIM][MeSO4] by density functional theory (DFT) calculations and pointed out some major interactions such as π–π and CH‒π bond interactions. Nie and Yuan20 optimized ion pairs of N,N-dialkylimidazolium dialkylphosphate ILs to achieve the most stable geometries at the Becke3LYP level of DFT. Their calculations confirm the existence of hydrogen bonding between IL and aromatic sulfur compounds; indeed, stronger interactions exist between IL and sulfur compound than between IL and benzene. In addition, studies also suggested that the specific volume and shape of ILs could be important. Holbrey et al.21 found that the partition ratio of DBT between IL and dodecane is strongly dependent on the choice of cation (dimethylpyridinium > methylpyridinium > pyridinium > imidazolium > pyrrolidinium), but is weakly dependent on the IL anion (SCN‒ > OAc‒ > C8H17SO4‒ > PF6‒ > OTf‒ > Tf2N‒ > BF4‒). They also suggested that a higher aromatic character of the cation and the ion shape play important roles in sulfur compound extraction. Wilfred et al.22 evaluated 18 ILs for the desulfurization of DBT from n-dodecane and identified the four top ILs for this task, [BMIM][SCN], [BMIM][dca], [BMIM][C8H17SO4], and [N4441][CH3CO3] which affording 66.1%, 66.1%, 63.6%, and 61.9% extraction efficiency,


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respectively. These authors suggested that π–π interaction between the aromatic rings of DBT and the IL was not the key extraction mechanism, an assertion supported by the relatively high extraction efficiency seen with the non-aromatic IL [N4441][CH3CO3]. This group further identified a loose correlation between specific volume and desulfurization efficiency, with exceptions. Nie et al.23 evaluated a series of N,N-dialkylimidazolium dialkylphosphate based ILs for the removal of aromatic sulfur compounds from gasoline, reporting that a longer alkyl group on the imidazolium ring afforded a higher partition coefficient (KN) value. A possible explanation is that a larger cation size could lead to a lower Coulombic interaction between the cation and the anion, but a higher π–π-interaction between unsaturated bonds of sulfur compounds and the imidazolium ring. The KN value of DBT partitioning between [BMIM][Me2PO4] and gasoline was reportedly 1.57 at 25 ºC. Currently, a complete understanding of how IL structure influences the desulfurization of aromatic sulfur compounds from liquid fuel remains elusive. In this study, we systematically screen various ILs containing different types of cations and anions for their effectiveness in desulfurizing n-octane and n-dodecane. Our results offer a useful insight into how IL molecular structure can be tailored to achieve efficient sulfur removal. EXPERIMENTAL METHOD Materials. Thiophene (TS) (>98.0%) was purchased from TCI America. Benzothiophene (BT) (97%) was obtained from Acros Organics. Dibenzothiophene (DBT) (98%), 4,6dimethyldibenzothiophene (DMDBT) (97%), n-octane (98+%), n-dodecane (99+%) were acquired from Alfa Aesar. The commercial sources and preparation methods for the ILs studied are provided in Table S1 of the Supporting Information.



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Extractive desulfurization of model fuel by ILs. n-Octane was used as a model fuel simulating gasoline and n-dodecane was used as a surrogate of diesel. A typical extraction procedure is described as follows. First, 200 µL of alkane (n-octane or n-dodecane) containing DBT at a level of 500 ppm sulfur (by weight) was placed in a 1.5 mL microcentrifuge tube, followed by the addition of 200 µL of IL. After closing the lid, the biphasic mixture was inverted 5 times before being placed on a Grant-bio mini rocker-shaker. The mixture tube was gently rocked (fixed tilt angle 7º) at room temperature at 30 oscillations/min for 15 min (our experiments suggest that equilibrium had been reached by this time, which was confirmed by earlier studies9, 18, 23). The microcentrifuge tube was centrifuged for 5 min to allow for complete layer separation. Next, 50 µL of each layer was carefully withdrawn and diluted by 450 µL of methanol. Note that the n-octane became soluble during this dilution; n-dodecane was only partially soluble in methanol, however, most of the sulfur compound was extracted into methanol. The volume of undissolved n-dodecane was factored in the calculation of sulfur compound concentrations in methanol. The clear methanol solution was injected into a LC-20AT Shimadzu HPLC equipped with a SPD-20A UV-visible dual-wavelength detector. The injection-loop volume was 20 µL. The column employed was a Phenomenex® Kinetex C18 column (100 mm × 4.6 mm, particle size 2.6 µm). The isocratic eluent consisted of 80/20 (v/v) methanol/water. The same eluent was also used for thiophene while 70/30 (v/v) methanol/water was used for BT and 85/15 (v/v) methanol/water was used for DMDBT. The flow rate was 1.0 mL min–1. The sulfur compounds were identified by a UV-visible detector at 280 nm for DBT, BT, and DMDBT; 210 nm was used for thiophene. The peak areas of sulfur compounds (S compounds) in both phases were used to determine the partition coefficient, KN [mg(S) kg (IL)-1/mg(S) kg (oil)–1] following


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Eq (1), which is also known as the Nernst partition coefficient.9 The densities of ILs are listed in Table S1. =

( ℎ)⁄( ℎ ∗ ) (1) ( ℎ)⁄( ℎ ∗ )

RESULTS AND DISCUSSION Several common aromatic sulfur compounds (i.e. TS, BT, DBT and DMDBT, see Scheme 1) are evaluated at 500 ppm concentration (sulfur wt) in n-octane (a model fuel of gasoline) or n-dodecane (a model fuel of diesel). We systematically evaluated a total of 71 ILs and two deep eutectic solvents (DES) (Table S1 and Table 1), consisting of various types of cations (such as imidazoliums, pyridiniums, ammoniums, pyrrolidiniums, phosphoniums, alkoxy- or hydroxy-functionalized cations and others), and different anions (such as Tf2N‒, beti‒, PF6‒, BF4‒, dca‒, OTf‒, SCN‒, NO3‒, MeSO4‒, Me2PO4‒, CF3COO‒, OAc‒, and HCOO‒, etc.). It is interesting to notice the miscibility of ILs with model fuels. n-Octane forms biphasic layers with most ILs except becoming miscible with several phosphonium-based ILs with long alkyl chains (i.e. #49 [P14,666][beti], #50 [P14,666][L-Lact], and #51 [P14,666][C4F9CO2]). nDodecane seems miscible with more ILs including those phosphoniums (49‒51), pyrrolidiniums (40‒46), ammoniums (29‒33), silver-based ammoniums (38 and 39), as shown in Table S2. The miscibility of ILs and fuel oils is dependent on the structures of both cations and anions. For example, not all pyrrolidinium-based ILs are miscible with n-dodecane; both [C4MPyrr][B(CN)4] (47) and [ChxmMPyrr][Tf2N] (48) form biphasic phases with n-dodecane. IL hydrophobicity could be an important factor, but the exact solvation thermodynamics is unknown and there should be other contributing factors. For example, [C12Py][Tf2N] (26) is highly hydrophobic but it forms biphasic phases with n-dodecane, while [C10MPyrr][Tf2N] (45) is miscible with ndodecane. The mutual solubility between ILs and liquid fuels is a critical factor when 9 ACS Paragon Plus Environment


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determining ILs as extractants due to these reasons: (1) if a nitrogen-containing IL dissolves in fuel, it contaminates the fuel and lead to NOx pollution; (2) if the fuel has a considerable solubility in an IL, this will increase the separation cost; (3) the mutual solubility also alters the volume ratio of phases and their densities, complicating the partition coefficient of sulfur compounds. Therefore, a systematic study of the mutual solubility is much needed,23, 24 which is an ongoing project in our laboratory. To quantify the extractive capability of ILs toward aromatic sulfur compounds, we use the partition coefficient KN defined in Eq (1). To determine the KN value, most current methods only determined the aromatic sulfur compounds in the oil phase (such as n-octane or n-dodecane) before and after partitioning; since the oil phase is very volatile, inaccuracy could occur if the sample is not handled quickly and properly. However, our method measured the contents of sulfur compounds in both oil and IL phases after partitioning using HPLC, and the ratio between these two phases is used in Eq (1). Systematic Evaluation of Extraction of DBT from n-Octane As shown in Table 1, partition coefficients of DBT in IL/n-octane biphasic systems range from 0.36‒14.85. In general, pyridinium- and pyrrolidinium-based ILs seem most effective for desulfurization (which is consistent with previous work on pyridiniums25), followed by the imidazolium type; tetraalkylammonium-based ILs are less effective and phosphoniums with long alkyl chains (49‒51) are even miscible with the oil phase. Two silver-based ammoniums, [Ag(nPrNH2)2][Tf2N] (38) and [Ag(i-PrNH2)2][Tf2N] (39), showed relatively high desulfurization capability (KN = 9.19 and 5.21 respectively); similar silver-based ILs were previously reported for the extraction of C5−C8 linear α-olefins from olefin/paraffin mixtures,26 and Huang et al.27 suggested that thiophene derivatives could strongly bind to silver (I) sites. However, one


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drawback of these silver-based ILs is their light sensitivity leading to IL decomposition and fine particles formation; this could negatively affect the desulfurization process and chromatography analysis. The structures of ILs play an essential role in the desulfurization efficiency. An increase in alkyl chain size of cations could improve the desulfurization (e.g. 17 > 11 > 16; 19 > 3; and 45 > 44 > 43 > 42 > 41 > 40); however, with a further increase in the alkyl chain length, KN values begin to decline (e.g. pyridiniums 22, 25 and 26). The influence of anions (such as ILs 114, 47 and 68 in Table 1) on DBT removal efficiency seems much more complicated than the cation scenario. Generally, ILs carrying anions of dca‒, B(CN)4‒, Br‒, SCN‒, Tf2N‒, CF3COO‒, beti‒, MeSO4‒ and BF4‒ are efficient for desulfurization of aromatic sulfur compounds, which is in line with some literatures.14, 21, 22, 28 Yu et al28 pointed out that a low-viscosity dca‒‒based imidazolium IL ([EMIM][dca], 56.5% DBT removal from n-dodecane) is more effective for extractive desulfurization than imidazolium ILs based on other anions (e.g. Me2PO4‒, EtSO4‒, BF4‒, CF4SO3‒, PF6‒, Tf2N‒, MeSO3‒, and MeSO4‒). However, this IL was not effective for oxidative extraction of DBT due to the strong interaction between this IL and CH3COOH or CH3COOOH. Initially, we suspected the physical properties of ILs (such as hydrophobicity in term of log P, IL polarity, hydrogen-bond basicity (β) of anions, or hydrogen-bond acidity (α) of anions) may be relevant factors. However, plots in Figures 1 and 2 indicate that none of these parameters could produce meaningful correlations with KN values. As discussed in Introduction, previous studies have examined several mechanisms of the partition of aromatic sulfur compounds between ILs and fuel oils: CH‒π bond interaction, π–π interaction, hydrogen bonding, and specific volume and shape of ILs. As discussed earlier on Figure 2, hydrogen bonding is unlikely the major interaction between ILs and aromatic sulfur compounds. Although



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the π–π interaction is likely to exist between aromatic cations (such as pyridinium and imidazolium) and the aromatic DBT ring,29 such an interaction may not be the key factor since non-aromatic pyrrolidiniums (40‒45, 47) and functionalized ammoniums (38, 39, 67, 68 and 73) exhibited equally high DBT extracting capability. Wilfred et al.22 found a loose correlation of a higher specific volume leading to higher desulfurization efficiency with some exceptions. However, our correlation in Figure S1 (Supporting Information) could not establish such a correlation. As suggested by Anantharaj and Banerjee13 who studied the interactions between thiophene with various ILs (such as pyrrolidiniums, pyridiniums and imidazolium) by the COSMO-RS method, the predominated interaction was the CH–π interaction not hydrogen bonding between sulfur atom and cation. Using the COSMO-RS model, Anantharaj and Banerjee14 predicted that efficient ILs for desulfurization typically consist of nonaromatic cations (such as morpholinium, pyrrolidinium and piperidinium vs imidazolium and pyridinium) and anions without steric shielding effect (such as SCN‒, MeSO3‒, OAc‒, Cl‒, and Br‒); anions with steric shielding effect (such as PF6‒ and B(CN)4‒) are not desirable. This prediction is not always consistent with the experimental findings: as discussed earlier, a number of pyridinium-based ILs are known very efficient for desulfurization, and B(CN)4‒ based 47 is known as one of the best ILs for sulfur removal (Table 1). Gao et al.30 used the COSMO-RS method to screen 1860 potential ILs for desulfurization of TS and DBT, and reported that a higher non-polarity and vdW energies of cations, a longer alkyl chain on anion, and a higher polarity and hydrogenbonding energies of anions contribute to an effective desulfurization. The trend of cations seems in agreement with our data (Table 1) although the trend of polarity and hydrogen-bonding properties of anions is not always supported by our experimental data (Figure 1 and 2).


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Another important but often neglected factor for interpreting the interaction between ILs and aromatic sulfur compounds is the cation–π interaction. At a matter of fact, the cation–π interaction has been known as one of the most important noncovalent forces (in addition to hydrophobic effect and hydrogen bonding) for molecular recognition, and has received tremendous attention in many biological systems, material science and chemical catalysis.31, 32 Mecozzi et al.33 suggested furan forms much weaker cation‒π interaction than benzene based on the computational results of electrostatic potential surfaces; however, the cation‒π interaction involving the thiophene-type of compounds was not discussed. Through high-level ab initio calculations, Tsuzuki et al.34 noted that the major interaction for benzene complexes with pyridinium and N-methylpyridinium is the cation‒π interaction although the benzene-pyridine complex is mainly formed by the π‒π interaction. Based on NMR measurements of [C12MIM][Tf2N] in benzene solutions, the Takamuku group35 indicated that the imidazolium ring is sandwiched between benzene molecules via the cation‒π interaction while the anion Tf2N‒ interacts with the cation even at a high mole fraction of benzene of 0.9 or more. The impact of the anion on the cation–π interaction is mainly determined by the anion’s charge dispersion: a higher anion’s charge density leads to a stronger cation’s charge polarization, which makes the cation’s charge less available for interaction with the host causing a weaker binding. This is supported by Bartoli and Roelens’36 investigation on the interaction of cyclophane with several acetylcholine and tetramethylammonium salts in deuteriochloroform. A computational study by Carrazana-García et al.37 further confirms that in addition to the electrostatic cation−anion attraction, the influence of the anion on the cation−π interaction is mostly a result of polarization which could be explained by the changes in the anion−π and the non-additive (three-body) terms of the interaction. The charge dispersion can be indirectly reflected by a parameter known as the



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IL dipolarity/polarizability (π*). Kamlet and Taft38-41 developed three solvatochromic parameters, known as the hydrogen bond acidity (α), hydrogen bond basicity (β), and dipolarity/polarizability

(π*). The π* reflects the exoergic effects of solute/solvent, dipole/dipole, and dipole/induced dipole interactions, and thus represents a combination of dipolarity and polarizability of the solvent.42 For ILs with the same BMIM+ cation but different anions, Figure 3 (solid triangles) shows a correlation between the partition coefficient and the π* value: a higher π* leads to a higher partition coefficient of DBT. The Spange group43 suggests to separate the solvent polarizability (SP) and solvent dipolarity (SdP), and then determined their values by the solvatochromic method. Based on a limited set of SP and SdP data available, the SP values increase in the order of [BMIM][Tf2N] (1) (SP = 0.793) < [BMIM][BF4] (3) (0.800) < [BMIM][OTf] (5) (0.808) < [BMIM][NO3] (7) (0.868), which correlated with an opposite decreasing trend of partition coefficients of these ILs (Table 1). Another example is the SP values of pyridinium ILs increasing in the order of [C6Py][Tf2N] (22) (0.805) < [C6Py][dca] (24) (0.878) while the sulfur partition coefficients decrease (Table 1). When the charge on the IL anions is highly dispersed over more atoms, the solvent polarizability decreases as a result of the decrease in Coulombic interactions.44 Therefore, the IL polarizability may reflect the charge dispersion of the anion: a lower polarizability indicates a higher anion’s charge dispersion. The Kochi group45 investigated the effect of different monovalent cations on the cation‒π interaction in the decreasing order of Cs+ ≈ Rb+ > K+ > Na+ (due to the formation of σ-complexes requiring sufficient electron redistribution), which is a reverse order of that in the gas phase and that by theoretical predictions. For organic cations, the hydrophobic interaction between ILs and aromatic sulfur compounds is favored by more hydrophobic cations, and thus a decreasing partition coefficient can be observed for a lower hydrophobicity of the cation (such as 17 > 11 >


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16, 19 > 3, 25 > 22, and 45 > 44 > 43 > 42 > 41 > 40). As illustrated in Figure 3 (empty diamonds), for pyrrolidinium ILs with different alkyl chain chains (same Tf2N‒ anion), a higher π* correlates with a lower partition coefficient in general (a higher π* for a longer alkyl chain is due to a lower polarizability43). This could be explained by the cation’s impact being primarily the hydrophobicity effect as discussed earlier: the hydrophobicity of cations decrease in the order of 44 > 43 > 42 > 41 > 40, and a more hydrophobic cation is expected to favorably interact with DBT via hydrophobic interaction. Partition coefficients for different types of cations decrease in the order of [C6Py][Tf2N] (22) > [C4MPyrr][Tf2N] (41) > [BMIM][Tf2N] (1) > [C4MPip][Tf2N] (52) > [N4441][Tf2N] (30) whilst the SP values43 also decrease as 22 (SP = 0.805) > 1 (0.793) > 30 (0.786). High KN values (> 6.0) are highlighted in Table 1, which include 25, 47, 22, 26, 4, 68, 44, 23, 6, 38, 64, 43, 73, 42, 67, 41, and 39, etc. In practical extractive desulfurization processes, two other important factors (i.e. viscosity and cost) should be considered. For example, pyridinium ILs (22‒26) afforded high sulfur partition coefficients, but they usually have relatively high viscosities (> 80 mPa·s, see Table 1). In addition, both imidazolium and pyridinium ILs are typically very expensive, especially for large-scale operations. To balance the desulfurization performance, viscosity and cost of ILs, two ammonium based ILs, [Me(OCH2CH2)3-Et3N]Br (68) and [(CH3OCH2CH2)2NH2][OAc] (73), could be very promising for large-scale extraction processes. Extraction of Other Aromatic Sulfur Compounds To evaluate the desulfurization capability of ILs toward other aromatic sulfur compounds (such as TS, BT and DMDBT in n-octane), we selected some high-performing ILs from Table 1 and reported their KN values in Table 2. The desulfurization efficiencies of these ILs are very



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high for BT, moderate for DMDBT and relatively low for TS; this is in line with earlier studies on extractive desulfurization.46 The extraction efficiencies of DBT from n-dodecane are very high in these ILs, only slightly lower than those of DBT from n-octane in most cases. This indicates that ILs are efficient for extractive desulfurization of both gasoline and diesel. Multi-Step Extraction of DBT by ILs Based on the sulfur partition coefficients in Table 1, one-step extraction by ILs at room temperature could result in up to 60‒70% DBT removal. To further improve the desulfurization efficiency, we conducted the consecutive multi-step extractions by several ILs (4, 25, 47, 68 and 73), and found (Figure 4) about 90% or more sulfur removal (< 50 ppm sulfur remaining) at 2nd extraction using 25, 68 and 73, above 95% sulfur removal (< 25 ppm sulfur remaining) at 3rd extraction using 4, 25, 68 and 73, and about 99% sulfur removal (< 5 ppm sulfur remaining) using 4, 25, 68 and 73. These encouraging results suggest that four-step extraction using ILs could be an efficient and low-energy consumption process for deep desulfurization of aromatic sulfur compounds. One concern of employing ILs as extractants is their toxicity and biodegradability comparing with common organic solvents. In general, the IL toxicity increases with the alkyl chain in the cation, and the cation usually has more influence on the toxicity than the anion.47-50 For example, one study evaluated the IL toxicity towards two aquatic organisms (Vibrio fischeri and Daphnia magna) and suggested an increasing order of the cation toxicity as: ammonium < pyridinium < imidazolium < triazolium < tetrazolium.47 In addition, the toxicity of ILs can be tuned with the manipulation of structures of cations and anions. It is known that hydroxyl/etherfunctionalized quaternary ammoniums (such as cholinium salts47, 51-54) and other cations become less toxic/cytotoxic or nontoxic,55-57 and non-toxic anions include alkylsulfates, docusates,


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alkanoates (carboxylates), lactate, and dimethylphosphate.55 The incorporation of oxygenbearing functionality (as ether or ester groups) to the IL substituents could make ILs more susceptible to biodegradation and hydrolysis.58 In the present study, a number of ILs (such as 68, 72 and 29) are hydroxyl/ether-functionalized ammonium salts, and are likely to have a low toxicity and a high biodegradability. Therefore, task-specific ILs could be designed not only to achieve high desulfurization efficiency, but also to have controllable toxicity and biodegradability. CONCLUSIONS Extractive desulfurization of DBT from n-octane was systematically examined using 73 ILs and deep eutectic solvents consisting of various types of cations and anions. The structures of ILs have significant impact on the sulfur partition between IL phase and oil phase. The main partition mechanisms could be the hydrophobic interaction of cations and sulfur compounds, and the cation‒π interaction. In general, the combination of a more hydrophobic cation and a chargedispersed anion results in a high partition of aromatic compounds into the IL phase. A more charge-dispersed anion often leads to a lower solvent polarizability, which enhances the cation–π interaction; on the other hand, a more hydrophobic cation results in a stronger hydrophobic interaction between ILs and sulfur compounds. We further demonstrated that extractive desulfurization using ILs could be adopted for removal of BT and DMDBT, and a four-step extraction by ILs could lead to 99% sulfur removal. ACKNOWLEDGEMENTS HZ acknowledges support from the ACS Petroleum Research Fund (PRF# 54875-UR9) and the Henry Dreyfus Teacher-Scholar Award (2012‒2017). Supporting Information



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S TS

S BT

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S

S

DBT

DMDBT

Scheme 1 Structures of aromatic sulfur compounds.


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Table 1 Partition coefficients of DBT between ILs and n-octane #

IL

Imidazolium-based ILs 1 [BMIM][Tf2N] 2 [BMIM][beti] 3 [BMIM][BF4] 4 [BMIM][dca] 5 [BMIM][OTf] 6 [BMIM][SCN] 7 [BMIM][NO3] 8 [BMIM][MeSO4] 9 [BMIM][Me2PO4] 10 [BMIM][CF3COO] 11 [BMIM][OAc] 12 [BMIM][HCOO] 13 [BMIM][BuSO3] 14 [BMIM][Me(OCH2CH2)2SO4] 15 [EMIM][HSO4] 16 [EMIM][OAc] 17 [HMIM][OAc] 18 [HMIM[PF6] 19 [OMIM][BF4] 20 [BzMIM][Tf2N] 21 [ChxmMIm][Tf2N] Pyridinium-based ILs 22 [C6Py][Tf2N] 23 [C6Py][SCN] 24 [C6Py][dca] 25 [C8Py][Tf2N] 26 [C12Py][Tf2N] 27 [BzPy][Tf2N] 28 [ChxmPy][Tf2N] Ammonium-based ILs 29 [Choline][Tf2N] 30 [N4441][Tf2N] 31 [N6111][Tf2N] 32 [N8881][Tf2N] 33 [N10,111][Tf2N] 34 [C3Me2iPrN][Tf2N] 35 [N11iPrC3Br][Tf2N] 36 [C6Me2iPrN][Tf2N] 37 [3Cl2OHPropN111][Tf2N]

DBT partition coefficient

Viscosity (mPa·s, 25 ºC, 1 atm)

5.43 4.14 3.37 10.72 2.97 9.42 2.33 3.86 1.11 4.29 2.03 2.81 1.05 4.11 0.11 0.55 3.07 4.54 6.16 2.22 2.50

50.959 11260 10861 30.0562 89.762 51.163 165.364 213.265 58566 76.960 44867 138.5 (30 ºC)68 11169 103370

11.75 10.28 6.71 14.94 11.52 2.58 2.04

84.574 viscous viscous 114.375 200.475

0.30 3.31 2.26 6.23 6.31 2.22 0.74 2.94 0.26

13971 49659 33472 11373

53976 15377



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

38 [Ag(n-PrNH2)2][Tf2N] 39 [Ag(i-PrNH2)2][Tf2N] Pyrrolidinium-based ILs 40 [C3MPyrr][Tf2N] 41 [C4MPyrr][Tf2N] 42 [C5MPyrr][Tf2N] 43 [C6MPyrr][Tf2N] 44 [C8MPyrr][Tf2N] 45 [C10MPyrr][Tf2N] 46 [C12MPyrr][beti] 47 [C4MPyrr][B(CN)4] 48 [ChxmMPyrr][Tf2N] Phosphonium-based ILs 49 [P14,666][beti] 50 [P14,666][L-Lact] 51 [P14,666][C4F9CO2] Other types of ILs 52 [C4MPip][Tf2N] 53 [Li(G3)][Tf2N] 54 [DAB8][Tf2N] 55 [DBNH][OAc] 56 [DBNH][CF3COO] 57 [Me4Etguan][FAP] Deep eutectic solvents (DES) 58 Choline chloride/glycerol (1:2) 59 Choline acetate/glycerol (1:1.5) Alkoxy- or hydroxy-functionalized ILs 60 [HO-EMIM][Tf2N] 61 [MeOCH2CH2-MIM][Tf2N] 62 [Me(OCH2CH2)3-Et-Im][Tf2N] 63 [Me(OCH2CH2)3-Bu-Im][OAc] 64 [Me(OPr)3-Et-Im][OAc] 65 [Me(OCH2CH2)3-Et3N][OAc] 66 [Me(OCH2CH2)3-Et3N][Tf2N] 67 [Me(OCH2CH2)3-Et3N][HCOO] 68 [Me(OCH2CH2)3-Et3N]Br 69 [Me(OCH2CH2)3-Et-Pip][OAc] 70 [Me(OCH2CH2)3-Et-Pip][Tf2N] 71 [Me(OCH2CH2)3-Et-Pip]Br 72 [Me2NH(CH2CH2OH)][OAc] 73 [(CH3OCH2CH2)2NH2][OAc]

Page 26 of 31

9.19 5.21

low viscosity low viscosity

5.50 6.65 8.18 8.86 10.42 11.40 3.34 14.85 2.91

61.578 77.679

viscous very viscous

miscible with n-octane miscible with n-octane miscible with n-octane 4.43 3.98 12.20 1.14 2.05 1.81 0.36 0.40 1.35 4.03 7.58 1.46 9.12 1.77 3.62 7.84 10.64 4.12 4.99 6.35 3.57 8.82

180.280 viscous

472.97 (20 ºC)81

viscous low viscosity viscous

low viscosity


Page 27 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Table 2 Partition coefficient between ILs and model oils #

IL

TS from n-octane 0.33 0.19 0.30 0.16 0.39 0.34 0.23 1.13 0.24 0.02

BT from n-octane 13.50 6.87 10.31 7.77 8.56 ‒a ‒a 10.43 9.30 4.58

DMDBT from n-octane 3.02 1.53 4.66 1.88 5.18 ‒a ‒a 4.28 2.64 1.66

[BMIM][dca] [BMIM][SCN] [C6Py][Tf2N] [C6Py][SCN] [C8Py][Tf2N] [Ag(n-PrNH2)2][Tf2N] [Ag(i-PrNH2)2][Tf2N] [C4MPyrr][B(CN)4] [Me(OPr)3-Et-Im][OAc] [Me(OCH2CH2)3-Et3N][HCOO], viscous 0.16 10.35 2.48 68 [Me(OCH2CH2)3-Et3N]Br 0.92 9.11 4.27 73 [(CH3OCH2CH2)2NH2][OAc] a Note: not determined due to the decomposition of silver-based ILs into fine particles causing HPLC problem. 4 6 22 23 25 38 39 47 64 67

DBT from n-dodecane 8.92 10.89 10.60 8.45 12.93 miscible miscible 12.63 8.11 8.73 9.03 9.50



12 4

4

10 Log P


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

Polarity

8

19

6

19 1

18

4

10

3

3

5 7

7

2

1 18

11

16

0 -4

-3

-2

-1

0

1

Log P or polarity of ILs Figure 1 Correlations of DBT partition coefficient with log P (data from Refs82, 83), and polarity (selected data from Ref84, 85) of ILs.


Page 29 of 31

12 4

4

10 6

H-bond basicity


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H-bond acidity

8

19

6

1

19 1

18 18

4

8

3

10

5 12

2

11

10

8 3 5

12

7 11

9 16

9 16

0 0.00

0.20

0.40

0.60

0.80

1.00

1.20

H-bond basicity or H-bond acidity of ILs Figure 2 Correlations of DBT partition coefficient with H-bond basicity (data from Ref86 except [BMIM][HCOO] (12) from Ref87), and H-bond acidity (selected data from Refs44, 87) of ILs.



14 12 DBT partition coefficient

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 31

4

44

10

43 42

8 41 40

6

1

4

8 3 12

5

2

11 9

0 0.5

0.7

0.9

1.1

1.3

IL dipolarity/polarizability (π*) Figure 3 Correlations of DBT partition coefficient with IL polarizability π* (selected data from Ref87): the solid triangles represent BMIM+ type of ILs with different anions, the empty diamonds represent pyrrolidinium type of ILs with different alkyl chain chains (same Tf2N‒ anion), and the dashed lines represent the general trends of these two series.


Page 31 of 31

100.0 DBT removal from n-octane (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80.0 IL #4

60.0

IL #25 IL #47

40.0

IL #68 IL #73

20.0 0.0 1st

2nd

3rd

4th

Extraction times Figure 4 Multi-step extractions of DBT (500 ppm) from n-octane by ILs (1st batch of extraction began with 500 µL n-octane containing 500 ppm DBT and 500 µL IL; after exaction at r.t. for 15 min, each layer was withdrawn (50 µL each) for HPLC analysis, and 350 µL of n-octane layer was withdrawn and mixed with 350 µL of fresh IL; after 2nd extraction, 250 µL of n-octane layer was mixed with 250 µL of fresh IL; after 3rd extraction, 150 µL of n-octane layer was mixed with 150 µL of fresh IL).


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