Toward an Understanding of the Forces Behind Extractive

Subscriber access provided by TULANE UNIVERSITY

Article

Towards an understanding of the forces behind extractive desulfurization of fuels with ionic liquids Lisa Courtney Player, Bun Chan, Matthew Yuk-Yu Lui, Anthony F. Masters, and Thomas Maschmeyer ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05585 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 22, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 20 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

ACS Sustainable Chemistry & Engineering

Towards an understanding of the forces behind extractive desulfurization of fuels with ionic liquids Lisa C. Player,† Bun Chan,‡ Matthew Y. Lui,† Anthony F. Masters† and Thomas Maschmeyer†* † Laboratory

of Advanced Catalysis for Sustainability, School of Chemistry, F11, The University of Sydney, Sydney, 2006, Australia ‡ Chemistry of Dynamic Molecules Laboratory, Graduate School of Engineering, Nagasaki University, Bunkyo 1-14, Nagasaki 852-8521, Japan Corresponding author*: [email protected]

Abstract In this study, the extractive desulfurization of model fuel oil with ionic liquids (ILs) has been studied in an attempt to gain insights into the dominant forces controlling the extraction efficiencies of aromatic sulfur compounds, thiophene and dibenzothiophene. This work investigates the intrinsic properties of a series of common ILs based on a constant (molar) amount of IL to directly draw insights into the intrinsic properties of each IL’s extraction capability. Experimentally both the cation and anion size influenced the efficiency of extraction, following the trend, pyridinium > imidazolium > pyrrolidinium for the cation, and [NTf2]- > [OTf]- > [PF6]- > [BF4]-, for the anion. Similar trends are observed for both thiophene and dibenzothiophene. Density functional theory modelling, using the APFD method, was employed to quantify the complexation energies and corresponding dispersion contributions between thiophene and the cations as well as between thiophene and the anions used in this work, showing similar trend to the experimental results. Through a combination of experimental and computational analyses it is suggested that the dominant force in extraction is dispersion-driven binding between the ions and S-compounds. Keywords: desulfurization, ionic liquid, extraction, density functional theory, dispersiondriven binding

Introduction The production of low sulfur content fuels is a major technical challenge currently facing refineries worldwide. During combustion, sulfur compounds (S–compounds) in fuel form sulfur oxides, SOx, potentially leading to acid rain, equipment corrosion, as well as deactivation

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 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

of catalytic converters in vehicles, which are optimised to reduce CO and NOx emissions. In response to the health and environmental concerns surrounding S-compounds in fuel oils, stringent regulations on the sulfur content permissible in fuel oils have been implemented. Currently, reduction of the S-compound content in fuels is achieved through hydrodesulfurization (HDS), which involves conversion of organic sulfur compounds to H2S at elevated temperatures (300 – 400 oC) and pressures (20 – 100 atm of H2) using catalysts, such as Co–Mo/Al2O3 and Ni–Mo/Al2O3.1 However, HDS is only efficient in the removal of paraffinic sulfur compounds, whereas aromatic sulfur compounds such as thiophene and dibenzothiophene (DBT), are significantly more difficult to remove through HDS.2, 3 Consequently, alternative approaches are necessary to remove aromatic sulfur compounds. Extractive desulfurization (EDS) has emerged as a potential alternative, removing S– compounds based on the preferable solvation in a solvent, over that in hydrocarbons. Large volumes of volatile organic solvents (such as acetonitrile) are typically used in the EDS process, posing both health and environmental concerns. Accordingly, alternative solvents for EDS have been studied, one promising class of solvents being ionic liquids. Common ionic liquids (ILs) are composed of large asymmetric cations, and inorganic or organic anions, and have several advantages over traditional solvents such as low volatility, non-flammability, thermal and chemical stability, high recyclability and low miscibility with hydrocarbons, making them ideal candidates for EDS. Since the first report of desulfurization using ILs by Bosmann et al. in 2001,4 several reviews have been published,1,5,6 including numerous proposed mechanisms on the understanding and effectiveness of ILs in the extraction of a range of S–compounds. Zhang and co-workers,7 reported that the extraction efficiency of S–compounds was dependent upon size and structure of both the cation and anion, as well as the aromaticity of the S-compound being extracted. Similarly, Holbery et al.8 studied the extraction of dibenzothiophene from n-dodecane for a range of cations and anions. Through a combination of liquid-liquid partition studies and quantitative structure–activity relationship analysis were able to demonstrate that the aromatic character of the cation, as well as the ion shape plays a role in the extraction efficiency. It was reported that polyaromatic quinolinium-based ILs, such as, 1-butyl-6-methylquinolinium bis(trifluoromethylsulfonyl)imide showed greater extraction efficiencies than ILs with


Page 2 of 20

Page 3 of 20 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


monocyclic cations (90 % extraction efficiency at 60 °C). More recently,9 a correlation between the sulfur partition co-efficient and the Kamlet-Taft parameter π* (representative of the dipolarity and polarizability of the IL) was published for [C4C1im]+ based ILs, exploring variations of the anion, as well as for ILs of similar anions, simply differing in the length of the alkyl chain. However, other properties of ILs (such as hydrophobicity (log P), polarity, hydrogen bond basicity of anions (β), and hydrogen bond acidity of anions (α)) showed no correlation with the experimental S-extraction efficiencies reported. In addition to experimental based studies, several computational studies have been reported studying the interaction between sulfur compounds and ILs. Bayat et al.10 recently studied the type and nature of interactions between aromatic sulfur compounds and phosphonium based ILs. In particular the addition of carboxylic acid side chains on the cation moiety, such as tributyl(carboxymethyl)phosphonium bromide, was shown through DFT modelling to have increased

interaction

strength

between

DBT

and

the

cation,

compared

to

tributylethylphosphonium bromide. This finding was consistent with the experimental work showing an increased extraction efficiency of DBT with incorporation of carboxylic side chains.

Figure 1: Structures of aromatic sulfur compounds studied, left: dibenzothiophene (DBT) and right: thiophene (TP). Although several noteworthy studies have been completed on the extraction of S–containing compounds from fuel oils, a comprehensive understanding of how the structure and properties of ILs influence the removal of aromatic sulfur compounds remains elusive. Most, previous desulfurization work has drawn conclusions based on a simple mass ratio of IL to oil, which provides useful information for the removal of sulfur compounds from fuel oils on a large scale. However, we believe that for a comprehensive exploration of the intrinsic properties of the ILs tested, results should be compared per mole of IL. Therefore, in this work, we explore the intrinsic properties of room temperature ILs (Figure 2) regarding the extraction efficiency of thiophene and dibenzothiophene (Figure 1) based on the mole ratio of IL to oil. A range of



common

cations

(pyridinium,

bis(trifluoromethylsulfonyl)imide,

imidazolium,

pyrrolidinium)

tetrafluoroborate,

Page 4 of 20

and

hexafluorophosphate

anions and

trifluoromethanesulfonate is explored, with the effect of cation and anion size (in terms of ionic volume) as well as the electronic configuration studied both experimentally and through DFT modelling.

Figure 2: Ionic liquids (ILs) cations and anions used throughout this work. Experimental Ionic Liquid synthesis N,N-dialkylpyrrolidinium bromide, N,N-dialkylimidazolium bromide and N-alkylpyridinium bromide salts [CnC1im]Br (n = 2, 4, 6, 8, 10), [C4C1Pyrr]Br, [C4Py]Br, [C4C1C1im]Br were synthesized using a modified method from Burrell et al.10 Bromoalkane (0.16 mol) was heated at 30 °C with the desired amine (0.16 mol) under nitrogen for 24 h. The products were then washed with diethyl ether (3 x 20 mL). Solid products were recrystallized from a minimum amount of acetonitrile and filtered. The solid was then dried in vacuo to yield the desired quaternary ammonium bromide salts.


Page 5 of 20 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


[CnC1im][NTf2], [C4C1pyrr][NTf2], [C4py][NTf2], [C4C1C1im][NTf2] and [C44C1py][NTf2] were synthesized with the corresponding quaternary ammonium bromide salts using a similar method to that of Weber et al.11 The relevant ammonium bromide salt (91 mmol) and lithium bis(trifluoromethylsulfonyl)imide (96 mmol) were separately dissolved in distilled water (200 mL total) and then the solutions combined, resulting in the formation of a separate liquid phase. To this solution dichloromethane (200 mL) was added and the organic phase was collected and washed successively with several aliquots of water until no halide could be detected by treatment of the aqueous extract with concentrated aqueous silver nitrate. Removal of solvent in vacuo afforded the bis(trifluoromethylsulfonyl)imide IL product. For the synthesis of [C4C1im][BF4] and [C4C1im][PF6], a modified procedure to that of Zhao et al.12 was used. A mixture of [C4C1im]Br (70 mmol) and sodium tetrafluoroborate or sodium hexafluorophosphate (76 mmol) was stirred in acetone (500 mL) for 24 h at room temperature. The resultant white solid was separated by filtration, and the solvent removed in vacuo. Then the dried solid was re-dissolved in dichloromethane and the solution passed through neutral alumina. The dichloromethane eluent was collected and washed successively with several aliquots of water until no bromide could be detected by treatment of the aqueous extract with concentrated aqueous silver nitrate. Removal of solvent in vacuo afforded [C4C1im][BF4] or [C4C1im][PF6]. Extractive desulfurization procedure The model oil used was composed of n-decane (85 wt %) and toluene (15 wt %) with thiophene (4.2 mmol) or dibenzothiophene (2.1 mmol) added. Extractive experiments were performed in a similar method to that previously outlined.13 The IL (3.4 mmol) and model oil (2 mL) were mixed in a test-tube at 500 rpm at 25 °C for 20 mins. The samples were then allowed to equilibrate with no stirring for 5 mins prior to analysis. The oil phase was removed, avoiding disturbance of the IL phase. An aliquot of the oil phase (1 mL) was taken and 0.1 mL external standard added. These standards were guaiacol (16.1 mmol) for quantification of thiophene, or anisole (16.2 mmol) for the quantification of dibenzothiophene. Analyses were performed on a LC-CTO-20A (Shimadzu) liquid chromatograph equipped with a reversed phase Luna C18 column (150 x 2.00 mm; 5 µm) and a UV detector. The mobile phase for both S-compounds was 80 % acetonitrile in water (v/v %) with a 1.0 mL.min-1 flow rate. All experiments were performed in triplicate, with the error in efficiencies calculated as the standard deviation between repeat experiments.



Preliminary studies were completed with [C4C1im][OTf] (3.4 mmol) and model oil (2 mL) containing thiophene (5.00 x10-3 mol L-1) or dibenzothiophene (5.00 x10-3 mol L-1) stirred for 10, 15, 20, 30 minutes at 25 °C, prior to analysis, as outlined above. The results indicated maximum extraction efficiency had been reached with 15 mins stirring time. Accordingly, the S-compound extraction in different ILs was performed with stirring for 20 minutes allowing for a comparison of the ILs when extraction equilibria has been reached. Density Functional Theory (DFT) modelling Standard DFT computations were carried out with Gaussian 16.14 Our modelling focused on the binding of thiophene with various IL components because, as discussed below, the general trends in experimental results for thiophene are representative of those for dibenzothiophene. ILs contain large molecular species, and dispersion interactions between them might be important. To elucidate this factor, we used the APFD method.15 It is a dispersion-corrected functional in which the DFT component is designed to be “dispersionless”, allowing the dispersion component to be properly quantified. For geometry optimization and vibrational frequency calculations the 6-31G(d) basis set was used. Refined single-point energies were obtained with the 6-311+G(3df,2p) basis set. In these DFT computations, the PCM16 continuum solvation model was used to account for the effect of the medium. Typically, ILs have dielectric constants of ~10 - 20,17 to closely mimic such a characteristic, among the range of available solvents that can be used in conjunction with the PCM model, 1-pentanol for which the dielectric constant is 15.1 was selected. Inspection of the calculated vibrational frequencies was performed to ensure minima were obtained, and unscaled vibrational frequencies were used to obtain zero-point vibrational frequencies and thermal corrections to 298 K enthalpies. For some representative species, we have also briefly explored the conformational space to obtain a low-energy structure. Unless otherwise noted, relative energies quoted in the present study are 298 K enthalpies in kJ mol-1. Results and discussion We believe, for the reasons outlined above, that extraction results should be compared and reported based on a constant amount (mole %) of IL to allow for comparisons of the intrinsic molecular properties of the ILs examined to be made. This is in contrast to a large portion of published literature where extractions are compared based on a mass or volume ratio of IL to oil. In the following we examine whether this approach is, indeed, meritorious and report the


Page 6 of 20

Page 7 of 20 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


S-extraction efficiencies in mol %, calculated as outlined below (Equation 1). With the S-compound concentration in the oil phase is determined through HPLC.

(

S ― compound in oil phase final (mol)

)

S ― extraction efficiency (mol%) = 1 ― S ― compound in oil phase initial (mol) × 100

(1)

Additionally, previous work using quantitative 1H-NMR we determined the mutual solubility of the IL in the oil phase, which alters the final volume of the oil phase.13 Based on these observations a correction parameter was introduced, as outlined in Equation 2, allowing the calculation of the final volume of the oil phase. Volume of oil phase final (dm ―3) = 0.002 ― oil content in ionic liquid phase

(2)

The final number of mole of S-compound in the oil phase is then calculated as outlined in Equation 3, using the concentrations of S-compounds in oil phase determined as per above. S ― compound in oil phase final (mol) = [S ― compound in oil phase] (mol.dm3) × volume of oil phase final (dm ―3)

(3)

Anion variation with common cation Initially, the S–compound extraction efficiency was examined over a range of anions with 1-butyl-3-methylimidazolium as a common cation. The S–compound extraction efficiency for thiophene

and

DBT

was

bis(trifluoromethylsulfonyl)imide trifluoromethanesulfonate

studied

in

1-butyl-3-methylimidazolium

([C4C1im][NTf2]),


([C4C1im][OTf]),


hexafluorophosphate ([C4C1im][PF6]), and 1-butyl-3-methylimidazolium tetrafluoroborate ([C4C1im][BF4]). In Table 1 a clear dependence on the anion type and the extraction efficiency can be observed for both thiophene and DBT. For both S–compounds analysed the extraction efficiency follows the trend, [C4C1im][NTf2] > [C4C1im][OTf] > [C4C1im][PF6] > [C4C1im][BF4]. In comparison, the DFT-calculated energies of complexation between thiophene and the anions, [NTf2]-, [OTf]-, [PF6]- and [BF4]-, yield values of -24, -12, -17 and 3 kJ mol-1 (Table 5), respectively, consistent with the experimental trend observed. These trends do not suggest a strong correlation between the S–compound extraction efficiency and hydrogen bonding strength (in terms of Kamlet-Taft parameters α and β),18 echoing results previously published.9 Indeed, inspection of the optimized structures of the thiophene-anion



Page 8 of 20

complexes (Figure 3) reveals there is no structural evidence to suggest strong hydrogen-bond interactions between the components. Table 1: S-extraction efficiencies for thiophene and dibenzothiophene with change in anion composition. S-extraction efficiency (%)

Ionic Liquid

Thiophene

Dibenzothiophene

[C4C1im][NTf2]

43.75  0.07

42.59  0.22

[C4C1im][PF6]

31.24  0.26

25.87  0.15

[C4C1im][OTf]

32.07  0.20

31.76  0.20

[C4C1im][BF4]

24.80  0.25

19.51  0.14

[NTf2]-

[PF6]-

[OTf]-

[BF4]-

Figure 3: DFT-optimized structures for complexes between thiophene and (from left to right) the anions [NTf2]-, [OTf]-, [PF6]- and [BF4]-. However, similar to recent work on denitrogenation of fuel oils using ILs,13 the extraction efficiency for both S-compounds appears to correlate with the volume of the anion (Table 2) with [NTf2]- (0.232 nm3) > [OTf]- (0.131 nm3) > [PF6]- (0.109 nm3) > [BF4]- (0.073 nm3). As


Page 9 of 20 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


highlighted in Figure 4, the positive correlation between anion volume and extraction efficiency suggests that the higher extraction efficiency is potentially due to an increased surface area for absorption through van der Waals interactions with larger anions. Additionally, it is likely that other factors, such as interaction energies between the ions of the IL and anion shape, are also contributing to the extraction efficiencies, however to a smaller extent.19 Table 2: Volume of anions used throughout this study.20 Ion

Anionic volume (nm3)

[NTf2]−

0.232

[PF6]−

0.109

[OTf]−

0.131

[BF4]−

0.073

Our present findings contradict those reported by Holbrey and co-workers,8 who showed the ordering of DBT extraction efficiency with anion type [PF6]- > [OTf]- = [NTf2]- > [BF4]-. This difference may result from the methodology used by Holbrey et al.,8 where the anionic effects were studied using a constant volume of IL, rather than a constant mole fraction of IL, as used in this work. Since the anions studied have significantly different molar volumes, the quantity of extractants used in the experiments are different from one IL to another. For example, for a fixed volume the amount of [C4C1im][PF6] is 1.4 times larger than that of [C4C1im][NTf2]. Therefore, the extraction efficiencies being measured were overstated/understated depending on the molar volume of ILs and are not a true reflection of the intrinsic extraction capabilities of each IL.



Figure 4: Dependence of S-extraction efficiency with anion volume (nm3). Cation variance with common anion Having established that the anion plays a significant role in the extraction efficiency of both S–compounds tested, the influence of cation type was studied by varying the cations of bis(trifluoromethylsulfonyl)imide ([NTf2]-) based ILs (Table 3). Comparatively, DBT showed a higher extraction efficiency than thiophene for most ILs tested, consistent with literature reports of favourable absorption of S–compounds with a higher aromatic π-electron density.7, 21

Table 3: S–extraction efficiencies for thiophene and dibenzothiophene with change in cation composition.


Page 10 of 20

Page 11 of 20 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


S-extraction efficiency (mol %)

Ionic Liquid

Thiophene

Dibenzothiophene

[C2C1im][NTf2]

40.39  0.26

28.44  0.16

[C4C1im][NTf2]

43.75  0.28

42.59  0.22

[C6C1im][NTf2]

47.73  0.11

53.08  0.14

[C8C1im][NTf2]

51.76  0.19

56.61  0.09

[C10C1im][NTf2]

53.85  0.33

62.53  0.28

[C4C1C1im][NTf2]

45.02  0.20

49.95  0.23

[C4py][NTf2]

52.66  0.16

53.51  0.12

[C4C1pyrr][NTf2]

38.66  0.21

40.03  0.16

The influence of cation size of the IL on the S–extraction efficiency for both thiophene and DBT over a series of 1-alkylimidazolium, [CnC1im][NTf2] (n = 2, 4, 6, 8, 10) ILs was tested (Figure 4) As outlined Figure 6 increasing the alkyl chain length resulted in an increase in the S–extraction efficiency for both thiophene and DBT. This increase in S–extraction efficiency as the alkyl chain size on the cation is elongated is consistent with published work.7, 9, 22 This may be resulting from an increase in the number of CH-π interactions with elongation of the alkyl chain.23 To investigate such a hypothesis, DFT was used to calculate the binding energies between thiophene and the cations [C2C1im]+, [C4C1im]+ and [C6C1im]+. The reaction energies for complex formation with the three cations are -47, -57 and -61 kJ mol-1, respectively. The more exothermic reaction energies for longer alkyl chains are consistent with the experimental observations. Further examination of the optimized structures of these complexes (Figure 5), reveals that the thiophene ring is positioned above the imidazolium ring but no “direct” stacking between the two is observed. Another interesting observation is that the sulfur atom in thiophene is not directed at the acidic C2 hydrogen of the imidazolium ring, preferentially interacting with the protons on the alkyl chain.

[C2C1im]+

[C4C1im]+


[C6C1im]+


Page 12 of 20

Figure 5: DFT-optimized structures for complexes between thiophene and (from left to right) the cations [C2C1im]+, [C4C1im]+ and [C6C1im]+, where the blue spheres represent nitrogen, the yellow sulfur, dark grey carbon and light grey hydrogen. Table 4: Cationic volumes used throughout this study.20 Ion

Cationic

volume

(nm3)

Cationic

Ion

(nm3)

[C2C1im]+

0.156

[C4C1C1im]+

0.229

[C4C1im]+

0.196

[C4py]+

0.198

[C6C1im]+

0.242

[C44C1py]+

0.240

[C8C1im]+

0.288

[C4C1pyrr]+

0.221

[C10C1im]+

0.346

C [C 2

] Tf 2 [N ] m 1i

C [C 4

] Tf 2 [N ] m 1i

C [C 6

] Tf 2 [N ] m 1i

volume

C [C 8

] Tf 2 [N ] m 1i

C

0

[C 1

] Tf 2 [N ] im 1

Figure 6: Cation variation on S–extraction efficiency of thiophene and dibenzothiophene with various ionic liquids with an [NTf2]- anion.


Page 13 of 20 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


Hydrogen bonding between the IL cation and the S–compounds through the acidic proton has been suggested as a driving force for the extraction of S–compounds.24,

25

However, as

previously mentioned (Figure 5) our DFT calculations suggest that this particular interaction might not be a prominent factor, if it is involved in the binding at all. Thus, the role of hydrogen bonding in the extraction of thiophene and DBT was investigated with 1-butyl-2,3dimethylimidazolium bis(trifluoromethylsulfonyl)imide, ([C4C1C1im][NTf2]), in which the C2 position

of

[C4C1im][NTf2]

is

methylated,

and

1-butyl-3-dimethylimidazolium

bis(trifluoromethylsulfonyl)imide ([C4C1im][NTf2]) where the proton on the C2 position is highly acidic.26 Slightly higher extraction efficiencies were obtained for both thiophene and DBT extraction in [C4C1C1im][NTf2] (45.02 mol % for thiophene, and 49.95 mol % for DBT), compared those in [C4C1im][NTf2] (43.75 mol % for thiophene, and 42.59 mol % for DBT). This suggests the proton on the C2 position of the imidazolium ring indeed does not play a crucial role in the S-compound extraction, and any hydrogen bonding effect is negligible as opposed to the extraction of Lewis basic N-compounds from fuel oils.13 However, it should be noted that [C4C1C1im][NTf2] has a larger cationic volume (0.229 nm3) in comparison to [C4C1im][NTf2] (0.196 nm3), and this larger size of the cation may be contributing to the increased extraction efficiency observed with [C4C1C1im][NTf2].

Figure 7: Dependence of cation volume (nm3) of ILs tested in this work with the corresponding S–extraction efficiencies.



Page 14 of 20

Further exploring the role of the cation moiety on extraction, 1-butyl-pyridinium bis(trifluoromethylsulfonyl)imide

([C4py][NTf2])

and

1-butyl-1methylpyrrolidinium

bis(trifluoromethylsulfonyl)imide ([C4C1pyrr][NTf2]) were tested in conjunction with [C4C1im][NTf2]. Holbrey et al.8 suggested the cation head group did influence the extraction efficiency following the order pyridinium > imidazolium ≈ pyrrolidinium for monocyclic cations. In a similar trend, the present work shows that [C4C1pyrr][NTf2], [C4C1im][NTf2], and [C4C1C1im][NTf2] extracted 38.66, 43.75 and 45.02 mol % of thiophene and 40.03, 42.59, 49.95 mol % of DBT, respectively. The highest extraction efficiency for both S–compounds was shown by [C4py][NTf2] (52.66 mol% for thiophene, and 53.51 mol% for DBT). We have also calculated complexation energies of thiophene with [C4C1pyrr]+, [C4C1im]+, and [C4py]+. The values are respectively -52, -57 and -59 kJ mol-1, which follow the experimentally observed trend of extraction efficiencies for these cations. The optimized structures shown in Figure 8, show that in all cases, thiophene is situated directly over the ring moiety of the cation.

[C4C1pyrr]+

[C4C1im]+

[C4py]+

Figure 8: DFT-optimized structures for complexes between thiophene and (from left to right) the cations [C4C1pyrr]+, [C4C1im]+ and [C4py]+, where the blue sphere represents nitrogen, the yellow sulfur, dark grey carbon and light grey hydrogen. The dependence of S–extraction efficiency on cation volume is depicted in Figure 4, showing an overall positive correlation. However, the two cations of similar volume, [C4C1im]+ (0.196 nm3) and [C4py]+ (0.198 nm3), show quite different extraction efficiencies, with [C4py][NTf2] extracting 8.91 mol% more thiophene, and 10.92 mol % more DBT than does [C4C1im][NTf2]. The higher extraction efficiencies reported with the pyridinium based ILs is consistent with previous work.8, 27 Nie and Yuan,19 suggested the higher extraction efficiency with aromatic ILs is resultant from increased - interactions and hydrogen bonding between the IL and aromatic sulfur compounds.


Page 15 of 20 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


A brief note on the contribution of dispersion interactions The present study uses the APFD method that consists of the “dispersionless” APF density functional and a complementary dispersion component. This allows quantification of the dispersion components in the complexation energies calculated. Table 5 shows the complexation energies obtained with the APFD method and the corresponding dispersion contributions. We can see that the dispersion contributions are in fact more negative than the total binding energies. In addition (and perhaps more importantly), the trend in the dispersion contributions correlate well with the trend in the total binding energies, with an R2 value of 0.964. These two observations from the DFT results suggest a dispersion-driven binding between thiophene and the various ions. Nonetheless, there are minor differences between the two trends, such as that between [C4C1im]+ and [C4py]+ (total binding energies of -57 and -59 kJ mol-1, respectively, but with corresponding dispersion contributions of -88 and -84 kJ mol-1). This confirms the presence of other minor factors alluded to above, which in some cases might rise to greater prominence to play an important role in determining the extraction efficiencies. In that context, the present theoretical results enable the qualitative rationalization of the experimentally observed correlation between extraction efficiency and ionic volume. Dispersion interactions in general grow with the size (and hence volume) of the species. Therefore, as a general rule for dispersion-driven binding, the larger the components the stronger the binding, though the exact geometries of the species involved should also be considered for a more detailed quantitative treatment. Table 5: DFT energies of complexation (kJ mol-1) for the binding between thiophene and various ions, and the corresponding dispersion contribution. Total

Dispersion

binding

contribution

(kJ mol-1)

(kJ mol-1)

[NTf2]-

-24

-50

[OTf]-

-17

-32

[PF6]-

-12

-31

[BF4]-

-3

-15

[C2C1im]+

-47

-72

Ion



[C4C1im]+

-57

-88

[C6C1im]+

-61

-93

-52

-67

-59

-84

[C4C1pyrr ]+ [C4py]+


Page 16 of 20

Page 17 of 20 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


Conclusions The determination of the intrinsic properties of a range of common room temperature ILs has lead to a deeper understanding of the fundamental interactions driving the extraction of aromatic sulfur compound. Variation of the cation significantly altered the extraction efficiencies following the order pyridinium > imidazolium > pyrrolidinium, which is positively correlated with the cation volume. This experimental trend was then verified through DFT modelling of the binding energies between the cations and thiophene, which followed the same trend. Additionally, the anion was shown to influence extraction efficiency following the trend [NTf2]- > [OTf]- > [PF6]- > [BF4]-, which was similarly verified through DFT modelling. The correlation between ion volume and extraction efficiency reported in this work, suggests that the higher extraction efficiency is due to an increased surface area for absorption through van der Waals interactions. It has been shown that ionic liquids composed of pyridinium cations and anions of large volume, such as [NTf2]- are most efficient in extraction of sulfur compounds from fuel oils. This combination of experimental and theoretical studies highlights the importance of reporting and comparison of experimental results with a constant molar amount of IL for a greater understanding of the fundamental driving forces in the extraction of sulfur compounds from fuels. Acknowledgements LCP thanks support from the Henry Bertie and Florence Mabel Research Scholarship. BC thanks the Japan Society for the Promotion of Science for financial support for this research. Computational resources were provided by RIKEN Advanced Center for Computing and Communication.



References 1. 2. 3. 4. 5. 6. 7. 8.

9. 10. 11. 12.

13.

14.

Kulkarni, P. S. and Afonso, C. M., Deep Desulfurization of Diesel Fuel using Ionic Liquids: Current Status and Future Challenges. Green Chemistry, 2010. 12(7): p. 1139-49. DOI 10.1039/C002113J Ma, X., Sakanishi, K. and Mochida, I., Hydrodesulfurization Reactivities of Various Sulfur Compounds in Diesel Fuel. Industrial & Engineering Chemistry Research, 1994. 33(2): p. 218-22. DOI 10.1021/ie00026a007 Kwak, C., Lee, J. J., Bae, J. S., Choi, K. and Moon, S. H., Hydrodesulfurization of DBT, 4MDBT, and 4,6-DMDBT on Fluorinated CoMoS/Al2O3 Catalysts. Applied Catalysis A: General, 2000. 200(1): p. 233-42. DOI 10.1016/S0926-860X(00)00635-9 Bosmann, A., Datsevich, L., Jess, A., Lauter, A., Schmitz, C. and Wasserscheid, P., Deep Desulfurization of Diesel Fuel by Extraction with Ionic Liquids. Chemical Communications, 2001. 23: p. 2494-5. DOI 10.1039/B108411A Ibrahim, M. H., Hayyan, M., Hashim, M. A. and Hayyan, A., The Role of Ionic Liquids in Desulfurization of Fuels: A Review. Renewable and Sustainable Energy Reviews, 2017. 76: p. 1534-49. DOI 10.1016/j.rser.2016.11.194 Francisco, M., Arce, A. and Soto, A., Ionic liquids on Desulfurization of Fuel Oils. Fluid Phase Equilibria, 2010. 294(1): p. 39-48. DOI 10.1016/j.fluid.2009.12.020 Zhang, S., Zhang, Q. and Zhang, Z. C., Extractive Desulfurization and Denitrogenation of Fuels Using Ionic Liquids. Industrial & Engineering Chemistry Research, 2004. 43(2): p. 614-22. DOI 10.1021/ie030561 Holbrey, J. D., Lopez-Martin, I., Rothenberg, G., Seddon, K. R., Silvero, G. and Zheng, X., Desulfurisation of Oils using Ionic Liquids: Selection of Cationic and Anionic Components to Enhance Extraction Efficiency. Green Chemistry, 2008. 10(1): p. 87-92. DOI 10.1039/B710651C Zhao, H., Baker, G. A., Wagle, D. V., Ravula, S. and Zhang, Q., Tuning Task-Specific Ionic Liquids for the Extractive Desulfurization of Liquid Fuel. ACS Sustainable Chemistry & Engineering, 2016. 4(9): p. 4771-80. DOI 10.1021/acssuschemeng.6b00972 Burrell, A. K., Sesto, R. E. D., Baker, S. N., Mccleskey, T. M. and Baker, G. A., The Large Scale Synthesis of Pure Imidazolium and Pyrrolidinium Ionic Liquids. Green Chemistry, 2007. 9(5): p. 449-54. DOI 10.1039/B615950H Weber, C. C., Masters, A. F. and Maschmeyer, T., Pseudo-Encapsulation-Nanodomains for Enhanced Reactivity in Ionic Liquids. Angewandte Chemie International Edition, 2012. 51(46): p. 11483-6. DOI 10.1002/anie.201206113 Zhao, Y., Long, J., Deng, F., Liu, X., Li, Z., Xia, C. and Peng, J., Catalytic Amounts of Brønsted Acidic Ionic Liquids Promoted Esterification: Study of Acidity–Activity Relationship. Catalysis Communications, 2009. 10(5): p. 732-6. DOI 10.1016/j.catcom.2008.11.030 Lui, M. Y., Cattelan, L., Player, L. C., Masters, A. F., Perosa, A., Selva, M. and Maschmeyer, T., Extractive Denitrogenation of Fuel Oils with Ionic Liquids: A Systematic Study. Energy & Fuels, 2017. 31(3): p. 2183-9. DOI 10.1021/acs.energyfuels.6b02157 Frisch, M. J., Trucks, G. W., Schlegel, H. B., Scuseria, G. E., Robb, M. A., Cheeseman, J. R., Scalmani, G., Barone, V., Petersson, G. A., Nakatsuji, H., Li, X., Caricato, M., Marenich, A. V., Bloino, J., Janesko, B. G., Gomperts, R., Mennucci, B., Hratchian, H. P., Ortiz, J. V., Izmaylov, A. F., Sonnenberg, J. L., Williams, Ding, F., Lipparini, F., Egidi, F.,


Page 18 of 20

Page 19 of 20 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


15. 16. 17. 18.

19. 20. 21. 22. 23. 24. 25. 26.

Goings, J., Peng, B., Petrone, A., Henderson, T., Ranasinghe, D., Zakrzewski, V. G., Gao, J., Rega, N., Zheng, G., Liang, W., Hada, M., Ehara, M., Toyota, K., Fukuda, R., Hasegawa, J., Ishida, M., Nakajima, T., Honda, Y., Kitao, O., Nakai, H., Vreven, T., Throssell, K., Montgomery Jr., J. A., Peralta, J. E., Ogliaro, F., Bearpark, M. J., Heyd, J. J., Brothers, E. N., Kudin, K. N., Staroverov, V. N., Keith, T. A., Kobayashi, R., Normand, J., Raghavachari, K., Rendell, A. P., Burant, J. C., Iyengar, S. S., Tomasi, J., Cossi, M., Millam, J. M., Klene, M., Adamo, C., Cammi, R., Ochterski, J. W., Martin, R. L., Morokuma, K., Farkas, O., Foresman, J. B. and Fox, D. J., Gaussian 16 Rev. B.01. 2016: Wallingford, CT. Austin, A., Petersson, G. A., Frisch, M. J., Dobek, F. J., Scalmani, G. and Throssell, K., A Density Functional with Spherical Atom Dispersion Terms. Journal of Chemical Theory and Computation, 2012. 8(12): p. 4989-5007. DOI 10.1021/ct300778e Tomasi, J., Mennucci, B. and Cammi, R., Quantum Mechanical Continuum Solvation Models. Chemical Reviews, 2005. 105(8): p. 2999-3094. DOI 10.1021/cr9904009 Weingärtner, H., The Static Dielectric Constant of Ionic Liquids, Journal of Molecular Liquids, 2014. 192: p. 185-90. DOI 10.1016/j.molliq.2013.07.020 Ab Rani, M. A., Brant, A., Crowhurst, L., Dolan, A., Lui, M., Hassan, N. H., Hallett, J. P., Hunt, P. A., Niedermeyer, H., Perez-Arlandis, J. M., Schrems, M., Welton, T. and Wilding, R., Understanding the Polarity of Ionic Liquids. Physical Chemistry Chemical Physics, 2011. 13(37): p. 16831-40. DOI 10.1039/C1CP21262A Nie, Y. and Yuan, X., Theoretical Study on Interaction between Ionic Liquids and Aromatic Sulfur Compounds. Journal of Theoretical and Computational Chemistry, 2011. 10(01): p. 31-40. DOI 10.1016/S1872-5813(13)60007-9 Slattery, J, M., Daguenet, C., Dyson, P. J., Schubert, T. J. and Krossing, I., How to Predict the Physical Properties of Ionic Liquids: A Volume-Based Approach. Angewandte Chemie International Edition, 2007. 46(28): p. 5384-8. DOI 10.1002/anie.200700941 Gao, H., Luo, M., Xing, J., Wu, Y., Li, Y., Li, W., Liu, Q. and Liu, H., Desulfurization of Fuel by Extraction with Pyridinium-Based Ionic Liquids. Industrial & Engineering Chemistry Research, 2008. 47(21): p. 8384-8. DOI 10.1021/ie800739w Mochizuki, Y. and Sugawara, K., Removal of Organic Sulfur from Hydrocarbon Resources Using Ionic Liquids. Energy & Fuels, 2008. 22(5): p. 3303-7. DOI 10.1021/ef800400k Anantharaj, R. and Banerjee, T., Quantum Chemical Studies on the Simultaneous Interaction of Thiophene and Pyridine with Ionic Liquid. AIChE Journal, 2011. 57(3): p. 749-64. DOI 10.1002/aic.12281 Su, B. M., Zhang, S. and Zhang, Z. C., Structural Elucidation of Thiophene Interaction with Ionic Liquids by Multinuclear NMR Spectroscopy. The Journal of Physical Chemistry B, 2004. 108(50): p. 19510-7. DOI 10.1021/jp049027l Headley, A., and Jackson, N. M., The Effect of the Anion on the Chemical Shifts of the Aromatic Hydrogen Atoms of Liquid 1-Butyl-3-methylimidazolium Salts. Journal of Physical Organic Chemistry, 2002. 15(1): p. 52-5. DOI 10.1002/poc.449 Avent, A. G., Chaloner, P. A., Day, M. P., Seddon, K. R. and Welton, T., Evidence for Hydrogen Bonding in Solutions of 1-Ethyl-3-methylimidazolium Halides, and its Implications for Room-Temperature Halogenoaluminate(III) Ionic Liquids. Journal of the Chemical Society, Dalton Transactions, 1994(23): p. 3405-13. DOI 10.1039/DT9940003405



27.

Holbrey, J. D., Reichert, W. M., Nieuwenhuyzen, M., Sheppard, O., Hardacre, C. and Rogers, R. D., Liquid Clathrate Formation in Ionic Liquid-Aromatic Mixtures. Chemical Communications, 2003(4): p. 476-7. DOI 10.1039/B212726A

Synopsis: Through a combination of both experimental and computational studies, this work investigates the extraction efficiencies of two sulfur-compounds from a series of ionic liquids. TOC:


Page 20 of 20

Toward an Understanding of the Forces Behind Extractive

Recommend Documents