Quantum Chemical Evaluation of Deep Eutectic Solvents for the

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A Quantum Chemical Evaluation of Deep Eutectic Solvents for the Extractive Desulfurization of Fuel Durgesh Vinod Wagle, Hua Zhao, Carol A Deakyne, and Gary A. Baker ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00224 • Publication Date (Web): 25 Mar 2018 Downloaded from http://pubs.acs.org on March 25, 2018

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ACS Sustainable Chemistry & Engineering

A Quantum Chemical Evaluation of Deep Eutectic Solvents for the Extractive Desulfurization of Fuel Durgesh V. Wagle,† Hua Zhao,‡ Carol A. Deakyne,*† and Gary A. Baker*† †

Department of Chemistry, University of Missouri-Columbia, 601 S. College Ave., Columbia, MO, 65211, USA

‡

Department of Chemistry and Biochemistry, University of Northern Colorado, Greeley, CO, 80639

Email of corresponding authors: [email protected] (C.A.D.); [email protected] (G.A.B.) Abstract Sulfur compounds in fuels are converted to SOx during combustion, poisoning automotive catalytic converters and creating serious environmental concerns (e.g., acid rain). The efficient desulfurization of liquid fuel is thus a critical step toward minimizing SOx emissions and their associated environmental impact. To address this problem, governments worldwide have passed stringent legislation regulating the maximal sulfur levels allowable in fuels. In the petroleum refining industry, the conventional method for removing sulfur from fuel is catalytic hydrodesulfurization which, while highly efficient for removing mercaptans, thioethers and disulfides, shows limited performance in removing aromatic organosulfur compounds exemplified by dibenzothiophene. To meet these strict environmental targets, innovative strategies beyond hydrodesulfurization for the deep desulfurization of fuel are sought. One key strategy entails the oxidation of refractory organosulfur compounds in liquid fuel, coupled with

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efficient liquid/liquid extraction of the oxidized sulfur compounds using an immiscible solvent phase (i.e., oxidative desulfurization). In this study, we employ computational chemistry to gain atomistic-level insight into the specific interactions responsible for the extraction of key organosulfur compounds and their oxidation products from fuel using deep eutectic solvents (DESs). Specifically, we perform quantum chemical calculations involving the well-studied DESs reline (1:2 choline chloride/urea) and ethaline (1:2 choline chloride/ethylene glycol) to characterize the intermolecular interactions, charge transfer behavior, and thermodynamics associated with their application for organosulfur extraction. We observe that the model aromatic sulfur compounds (ASCs) benzothiophene and dibenzothiophene interact with choline and the hydrogen bond donor (HBD; i.e., urea or ethylene glycol) of the DES via a plurality of weak noncovalent interactions. However, the chloride ion is essentially non-interactive with the ASC due to retention of the conventional hydrogen bond network existing within the initial DES. Oxidation of the model ASCs to their respective sulfoxide and sulfone products was shown to enhance interactions with the DES components, particularly the HBD species due to its propensity for forming multiple hydrogen bonds. We further demonstrate that, upon oxidation, the ASCs exhibit significant and favorable free energies of solvation, suggesting that oxidation will aid in the partition of these sulfur compounds from liquid fuel to a conventional DES phase. KEYWORDS:

Deep

eutectic

solvent,

Desulfurization,

Organosulfur

Introduction


Extraction,

Dibenzothiophene,

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Organosulfur compounds in fuel oils are highly undesirable because they can poison precious metal catalysts (in fuel cells, for example) and lead to pollution and acid rain due to sulfur oxide (SOx) emissions when burnt in internal combustion engines. Petroleum-based fuels with very low sulfur contents are thus critical in order to minimize SOx emissions from the burning of fossil fuels. Accordingly, petroleum-refining industries are now required to comply with strict government regulations specifying the sulfur contents allowable in transportation fuels. For instance, the U.S. Environmental Protection Agency (EPA) mandates diesel and gasoline levels of 15 and 30 ppm, respectively, while European Union (EU) legislation allows only 10 ppm for both diesel and gasoline.1 However, in spite of environmental concerns and legal requirements, it remains a technical challenge to efficiently remove sulfur compounds (particularly aromatic ones) to meet these ultra-low-sulfur targets with current approaches built around traditional refining processes. For example, conventional hydrodesulfurization (HDS) processes present several disadvantages, including the requirement for high temperatures (300–400 °C) and pressures (20–100 atm. of H2), a reduced octane/cetane number,2 and the notorious difficulty in removing aromatic sulfur compounds.3,4 Representative refractory organosulfur species of interest include the aromatic sulfur-containing species benzothiophene and dibenzothiophene. A number of non-HDS desulfurization methods are currently being actively explored, such as oxidative desulfurization (ODS), microbial bio-desulfurization, adsorptive desulfurization, precipitative desulfurization, extractive desulfurization (EDS), and desulfurization by polymer membranes, among others. However, each of these methods also presents its own particular hurdles and limitations.1 Widely considered the most promising among these alternate approaches, ODS uniquely combines oxidation chemistry with an extraction process, making it attractive for practical, 3 ACS Paragon Plus Environment


industrial-scale applications owing to its low energy cost, elimination of molecular hydrogen usage, relatively mild operating conditions, and retention of the chemical integrity of the fuel, all without requiring highly specialized equipment.5,6 A current shortcoming of ODS is the requirement for large quantities of volatile organic solvent (VOC) as the extractant phase, a requirement that in itself imposes certain health, environmental, and economic concerns. A possible resolution to this limitation is the use of deep eutectic solvents as sustainable and lowvolatility alternatives to conventional VOCs. Deep eutectic solvents (DESs) have gained in popularity as inexpensive and biofriendly alternatives to VOCs and ionic liquids (ILs), with attractive characteristics such as nonflammability, low vapor pressure, and good recyclability.7-10 Abbott et al. first reported that mixing an appropriate stoichiometric amount of choline chloride (a quaternary ammonium salt with a melting point (m.p.) of 302 °C) with urea (m.p. = 133 °C) produces a viscous, freeflowing fluid at room temperature.8 Several combinations of DES mixtures comprising various hydrogen bond donor (HBD) species (typically, amides, carboxylic acids, glycols, or phenols) paired with an ammonium salt (most prevalently, choline chloride)have emerged to offer a range of solvent features.10,11 The flexible nature of DESs coupled with their ecologically friendly nature and cost advantages over ILs have broadened their domain to include nanochemistry,12 micelle/emulsion chemistry,13 electrochemistry,14,15 catalysis,16 polymer synthesis,17 gas separation,18,19 (meso)porous carbon generation,20,21 and biomass treatment.21,22 Recently, there has also been exploration of DESs as alternative solvents for the liquidliquid extraction of organosulfur compounds from liquid fuels.23 Such DESs include those based on choline chloride21,24 and those derived from other quaternary ammonium halide salts, amino acids, and metal halides (e.g., ZnCl2, FeCl3).25-30 4 ACS Paragon Plus Environment

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In parallel with the expansion of DESs into diverse areas, there has been a considerable increase in the use of computational tools to better understand the structure, dynamics, and interactions present within DES systems,31-35 although currently little is known about the effects of DES molecular structure and composition on interactions with aromatic sulfur compounds of utmost significance as refractory targets in the desulfurization of fossil fuel. In this work, we employ quantum mechanical (QM) calculations to gain a molecular-level understanding of how specific components within conventional DESs interact with model aromatic sulfur compounds (ASCs) to control the extraction of these target compounds from sulfur-contaminated liquid fuels. Likewise, due to the relevance of ODS as a desulfurization strategy, we also consider the effects of oxidation of these model ASCs on the interaction with the DES components. Particularly, we systematically studied the structure, charge transfer processes, and thermochemistry associated with interaction between the well-studied, archetypal DESs comprising 1:2 molar ratio mixtures of choline chloride/urea (reline) and choline chloride/ethylene glycol (ethaline) with the model ASCs benzothiophene (BT) and dibenzothiophene (DBT), as well as their sulfoxide/sulfone oxidation products benzothiophene oxide

(BTO)/benzothiophene

dioxide

(BTO2)

and

dibenzothiophene

oxide

(DBTO)/dibenzothiophene dioxide (DBTO2), respectively (Figure 1). Our results offer valuable atomistic insight into the interactions between the DESs and ASCs (compared with their oxidation products) vital to the effective extraction of these pervasive and stubborn aromatic sulfur compounds from sulfur-containing liquid fuels using DESs as the extraction medium in an EDS or ODS method. Computational Methods



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Optimization of aromatic sulfur compound–deep eutectic solvent (ASC–DES) complexes was carried out from various starting orientations of ASCs around the DES. In one instance, the ASC was placed closer to the choline cation whereas it was placed closer to the HBD or the chloride ion in other starting orientations. Optimization of the ASC–DES complexes was carried out at the M06-2X/6-31++G(d,p) level of theory for singlet ground state structures with no symmetry restrictions. The counterpoise procedure was used to account for basis set superposition error (BSSE).36 Lack of imaginary frequencies in the vibrational spectrum of the optimized systems indicated minimum energy structures. Population analysis was performed using the CHELPG method based on electrostatic potential charges. All calculations reported were carried out using the Gaussian 09 D.01 program package.37 A scaling factor of 0.986 was used for the correction of vibrational frequencies calculated at the M06-2X/6-31++G(d,p) level of theory.38 The dispersion correction in the M06-2X calculations was taken into account for the thermochemistry of ASC–DES complex formation using the method developed by Grimme.39 Solvation free energies were computed at the M06-2X/6-31++G(d,p) level of theory using the SMD solvation model developed by Truhlar and co-workers (see Supporting Information).40 Thermochemical values for ASC–DES interactions were obtained using eq 1, where X is the BSSE-corrected enthalpy (H) or free energy (G). ∆XASC–DES = XASC – XDES

(1)

Differences in the free energies of solvation (∆G°solv) were obtained using eq 2 ∆G°solv = GDES – Goctane

(2)

where GDES and Goctane represent the free energies of solvation for an ASC in a DES and octane, respectively. 6 ACS Paragon Plus Environment

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Results and Discussion Optimized Geometry In a previous study, we effectively calibrated the M06-2X/6-31++G(d,p) level of calculation by comparing thermochemical parameters from its use with results obtained using the MP2 method in conjunction with the aug-cc-pVDZ and 6-31++G(d,p) basis sets for model choline chloride-based DES clusters to validate its reproducibility and reliability.41 It is, moreover, important to note that the M06-2X functional has been demonstrated to provide accurate insights into hydrogen-bonding interactions, charge transfer processes, dipole interactions, and noncovalent interactions, including dispersion interactions.38,42 A primary focus of the current study is to compare geometries of representative DES clusters before and after the introduction of model ASCs or their oxidation products as putative targets for desulfurization by an EDS or ODS process, respectively. As shown for reline in Figure 2a, there is a strong interaction between urea and Cl– via N–H···Cl– hydrogen bonds, with shortest H···Cl– interaction distances of 2.284 Å. The DES is also stabilized by two weaker cis-N–H···O═C hydrogen bonds, with a smallest H···O distance of 2.060 Å. Finally, a strong interaction between the C═O on urea and C–H bonds on the methyl groups of the choline cation is observed, with a smallest C═O···H–C distance of 2.206 Å, an indication of C–H···lone pair interactions (Figure 2a). Likewise, the oxygen atoms of the ethaline HBD ethylene glycol (EG) interact with the methyl protons of the choline cation via C– H···O unconventional hydrogen bonds, with a shortest distance of 2.440 Å (Figure 2b). Similarly to our previously reported structure, ethaline also exhibits multiple hydrogen bonds between EG and Cl– (Figure 2b).41



Interestingly, it was observed that the two DESs reline and ethaline largely retain their initial geometries and orientations in the presence of the unoxidized (non-polar) sulfur species benzothiophene and dibenzothiophene, as summarized in the optimized geometries presented in Figures 2–4. In particular, the optimized geometries reveal the absence of interaction between chloride ion (Cl–) and BT or DBT, whereas choline and the HBD species (i.e., urea and EG in reline and ethaline, respectively) interact with the aromatic rings of BT or DBT via weak noncovalent (C–H···π, C=O···π, O–H···π, and N–H···π) interactions. The DES clusters largely retain the original intermolecular hydrogen-bonding network after introduction of a non-oxidized ASC, indicating a preference for preserving the hydrogen-bonding interactions within the DES cluster over formation of new interactions with a non-polar, unoxidized aromatic species (BT or DBT). As illustrated in panels a and d of Figures 3 and 4, the choline cation and HBD species preferentially interact with the plane of the unoxidized ASC from the top. This behavior is reminiscent of observations from QM analysis of IL/polyaromatic hydrocarbon (PAH) interactions, wherein the IL cation prefers to engage with the negative electrostatic surface on the top or bottom of the PAH.43 However, in contrast to our previous study wherein the IL anion predominantly interacts with the PAH edge possessing a positive electrostatic surface, here we see no evidence for interaction between the chloride anion and the ASC species. This difference arises from retention of the strong, preexisting hydrogen-bonding network in the DES cluster, which effectively ties up the chloride, preventing its thermodynamically unfavorable interaction with BT or DBT. In the case of the monoxide (BTO and DBTO) and dioxide (BTO2 and DBTO2) forms of the ASCs, the presence of an oxygen atom on the sulfur atom of the aromatic rings (denoted Os) allows for formation of conventional hydrogen bonds between the ASCs and choline or HDB 8 ACS Paragon Plus Environment

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components of the DES. Although the presence of Os facilitates additional interactions (such as C–H···Os, N–H···Os, and O–H···Os), it also significantly weakens the choline–π and HBD–π interactions with the aromatic ring, as can be seen by the increased interaction distances between the DES components and the aromatic rings (see panels b, e, and f in Figures 3 and panels b, c, e, and f in Figure 4). The only exception to this pattern is exhibited by the reline-BTO2 system, where one of the urea molecules is close to the BTO2 as a result of the lack of interaction of that urea with Os. Despite the ability of Os to hydrogen bond with the choline and HBD species, oxidation of BT or DBT results in only minor alterations in the intermolecular hydrogen-bonding motif observed in the DES system of the BT–DES and DBT–DES complexes. Overall, analysis of the optimized geometries of the ASC–DES complexes reveals that oxidation of the ASCs only moderately improves the interaction with the DESs despite the additional anchoring point for interaction between the oxidized ASC and DES. Charge Transfer Behavior In this work, charge transfer (CT) processes between a DES and an ASC were investigated using the CHarges from Electrostatic Potentials using a Grid-based method (CHELPG) developed by Breneman and Wiberg, an approach in which the atomic charges are assigned to reproduce the molecular electrostatic potential at a number of points around the molecule.44 The CHELPG analysis has been shown to provide reliable estimates for atomic charges for systems involving ionic and aromatic compounds.43 To decipher the magnitude and direction of the CT process between specific parts of the ASC–DES complexes, the CHELPG atomic charges on individual components of the DES and ASC were evaluated before and after formation of the



complex. This analysis not only identified the components of the DES that are specifically involved in interaction with a target ASC but also aided in quantifying the strength of these interactions. We observe (Table 1) that interaction with the ASCs leads to significant perturbation of the CT process occurring within an initial (ASC-free) DES cluster. In the case of reline, the CT process exhibits opposing trends for the benzothiophene and dibenzothiophene derivatives. The benzothiophene derivatives (BT, BTO, and BTO2) gain electron density, whereas the dibenzothiophene derivatives (DBT, DBTO, and DBTO2) lose electron density upon interaction with reline. Furthermore, in the case of ethaline, the ASCs lose electron density upon interaction with the DES, with the exception of the ethaline–BTO2 system in which the ASC gains electron density. The gain or loss in electron density of the ASC appears to be loosely correlated to the distance between the choline cation and the ASC (Figures 2 and 3). The closer the cation is to the ASC, the greater the loss of electron density from the ASC. Overall, the choline cation gains electron density upon the formation of most of the ASC–DES complexes (Table 2).The ASC-tocholine cation CT is accompanied by a weakening of the interaction between the choline cation and Cl– or HBD species, concomitant with an increased number of ASC-HBD and ASC-choline interactions (Figures 3 and 4). Fascinatingly, the Cl– ion gains electron density in ASC–DES complexes involving reline but loses electron density in ethaline-based complexes, a difference in CT behavior dictated by the disparity in the ability of the respective HBD species (urea and EG) to donate or accept electron density (Table 2). Furthermore, from the viewpoint of the HBD species, there are opposing trends in CT behavior wherein the HBDs predominantly lose electron density in ASC–DES complexes containing BT, BTO, or BTO2, whereas the HBD gains electron density in DES clusters containing DBT, DBTO, or DBTO2. We attribute these distinctly 10 ACS Paragon Plus Environment

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contrasting trends in CT behavior for BT and DBT to the presence of the additional benzene ring in DBT and its oxygenated derivatives, a feature that provides an extended π-electron system (negative electrostatic surface) and a larger molecular surface for interaction with DES components. Overall, full CHELPG analysis reveals that the presence of the ASC significantly alters the CT interactions between the constituents of the DES in a complex manner that depends on the choice of HBD in the DES (reline vs. ethaline) as well as the identity of the aromatic sulfur compound and its degree of oxidation. Thermochemistry Gas-Phase Thermodynamics. The changes in enthalpy and free energy associated with the formation of an ASC-DES complex were determined to help understand the extent to which the interaction between a given DES and ASC is thermodynamically favorable, a key factor influencing the extractive desulfurization of fuels. These thermochemical parameters were obtained using eq 1. The ∆G values calculated for the interactions between the DESs and ASCs are positive (Table 3) except for BTO, whereas the ∆H values are negative for all of the ASC–DES complexes (Table 4). With a similar value for the T∆S term for all of the complexes of ca. 11–12 kJ/mol, only the intermolecular interactions within the BTO-DES complexes are sufficiently strong to exhibit favorable thermodynamic behavior in a gas-phase environment. This favorable interaction between BTO and the DESs is attributed to the smaller size and greater polarity of BTO compared to the other ASCs. Although the gas-phase thermodynamic results on the formation of the ASC–DES complexes are not favorable, the relatively low positive values for ∆G and the negative values for ∆H are 11 ACS Paragon Plus Environment


encouraging to test the liquid-phase thermodynamics of ASC extraction. Therefore, we carried out a liquid-phase study that involves determination of the change in the free energy of salvation (∆G°solv) when a solute molecule (ASC) is transferred from one phase (octane) to another (DES). Liquid-Phase Thermodynamics. The difference in the standard free energy of solvation was determined to elucidate the efficacy of transfer of an ASC from the liquid-fuel phase to the DES. Octane was used as a model solvent to mimic liquid fuel. That is, the ∆G°solv was obtained as a difference in free energy when an ASC is transferred from the octane to DES phase (Figure 5 and eq 2). The SMD solvation model developed by the Truhlar group was used for determination of ∆G°solv.40 This solvation model has been successfully applied in the calculation of ∆G°solv for various solutes in ionic liquids due to its ability to uniquely describe the solvent environment (solvent cavity) in which the solute is placed. To this end, the SMD model allows for the use of experimentally derived solvent parameters such as hydrogen-bond acidity (α), hydrogen-bond basicity (β), surface tension (γ), dielectric permittivity (ε), and refractive index (n) and mathematically calculated parameters such as carbon aromaticity (ϕ) and electronegative halogenicity (ψ).The values of the experimentally derived solvent parameters for reline and ethaline were obtained from the literature, whereas the mathematically derived parameters were calculated from the molecular structureof the various components of the DES (Table5).40,45-48 The values of ∆G°solv for benzothiophene and dibenzothiophene (Table 6) are positive, whereas those for the mono and dioxides are negative, thus indicating an increase in the propensity of an ASC to transfer from the octane to DES phase upon oxidation of the sulfur atom in the aromatic ring (Table 6). This increased preference for the DES phase is attributed to the increased ability of the ASC to hydrogen bond with the DES through Os upon oxidation of the sulfur atom (Figures 3 and 4). 12 ACS Paragon Plus Environment

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Interestingly, there was only marginal improvement in the ∆G°solv for the ASCs when the sulfur atom is oxidized from the monoxide to dioxide. This result suggests that the monoxidation of sulfur is sufficient for extraction of ASCs from the octane to DES. Moreover, the benzothiophene derivatives exhibit more favorable ∆G°solv values than do the dibenzothiophene derivatives. This trend is due to the reduced hydrophobic character of the benzothiophene derivatives compared to the dibenzothiophene derivatives, making the former more compatible with the hydrophilic DESs. Conclusions A quantum chemical investigation of the interaction between aromatic sulfur compounds (ASCs) and the two most popularly studied deep eutectic solvents (DESs) was performed to probe the utility of these conventional DESs as potential alternative solvents for the extractive desulfurization of fossil fuels. The aromatic sulfur compounds interact with the choline cation and hydrogen bond donor (HBD) components of the deep eutectic solvent through multiple weak noncovalent interactions (e.g., C–H···π, C=O···π, O–H···π, and N–H···π) but do not interact directly with the chloride ion. The absence of ASC–Cl– interactions results from the retention of multiple, strong hydrogen bonds between the Cl– and the choline and HBD species upon formation of the ASC–DES complexes. In fact, perhaps surprisingly, all of the intermolecular interactions within the DES cluster are retained upon complex formation. Oxidation of the sulfur atom enhances the interaction of the ASCs with the DES components, as the oxygen atom acts as an additional electron donor for ASC–choline and ASC–HBD hydrogen bonding. Charge transfer analysis reveals a significant amount of charge transfer occurring from the ASC to the DES, an indication of strong interaction. Finally, the ASCs exhibit a significant improvement in the change in the free energy of solvation upon oxidation, a result that suggests that oxidation 13 ACS Paragon Plus Environment


should enhance the selective extraction of ASCs from a liquid fuel phase to a DES phase in desulfurization efforts. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: http://pubs.acs.org. Computational details, examples of Gaussian 09 input files for SMD calculations (e.g., benzothiazole in reline)

AUTHOR INFORMATION Corresponding Authors Email: [email protected] (C.A.D.); [email protected] (G.A.B.)

Notes The authors declare no competing ﬁnancial interest.

ORCID Durgesh V. Wagle: 0000-0002-2522-0670 Hua Zhao: 0000-0002-5761-2089 Gary A. Baker: 0000-0002-3052-7730

Acknowledgement This research was performed with financial support provided by a Research Council grant at the University of Missouri. We also gratefully acknowledge the University of Missouri Division of Information Technology (DOIT) for providing computing resources. 14 ACS Paragon Plus Environment

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Table 1. Charges on Model Aromatic Sulfur Compounds and their Sulfoxide and Sulfone Oxidation Products in the Absence and Presence of Representative DES Components No DES reline ethaline

BT 0.0000 –0.0073 0.1175

BTO 0.0000 –0.0198 0.0597

BTO2 0.0000 –0.0096 –0.0495

DBT 0.0000 0.0430 0.1281

DBTO 0.0000 0.1746 0.0591

DBTO2 0.0000 0.0279 0.1309

Table 2. Charges on the Components of Reline and Ethaline in the Presence and Absence of Model Aromatic Sulfur Compounds Charge on the Choline Cation in ASC–DES Complexes and in the Neat DES BT BTO BTO2 DBT DBTO DBTO2 Neat DES reline 0.8018 0.7186 0.7930 0.8857 0.6842 0.8256 0.8248 ethaline 0.7381 0.8609 0.8812 0.7343 0.7937 0.7360 0.8598 Charge on the Chloride Anion in ASC–DES Complexes and in the Neat DES BT BTO BTO2 DBT DBTO DBTO2 Neat DES reline –0.7969 –0.7923 –0.7848 –0.7939 –0.7809 –0.7941 –0.7801 ethaline –0.6923 –0.6875 –0.6889 –0.6944 –0.6756 –0.6912 –0.6918 Charge on the HBD in ASC–DES Complexes and in the Neat DES DBT DBTO DBTO2 Neat DES BT BTO BTO2 reline –0.0024 0.0935 0.0014 –0.1349 –0.0779 –0.0595 –0.0448 ethaline –0.1633 –0.2331 –0.1428 –0.1680 –0.1772 –0.1756 –0.1679

Table 3. Free Energy Changes (∆G/kcal mol–1) Associated with Formation of the Various DES–ASC Complexes BT BTO BTO2 DBT DBTO DBTO2 reline 8.58 –20.68 4.12 5.44 5.85 2.92 ethaline 5.28 –19.53 5.80 3.15 3.89 3.24

Table 4. Enthalpy Changes (∆H/kcal mol–1) Associated with Formation of the Various DES–ASC Complexes BT BTO BTO2 DBT DBTO DBTO2 reline –3.48 –32.13 –8.39 –6.87 –6.64 –10.03 ethaline –6.85 –30.82 –6.26 –6.46 –7.57 –7.73



Table 5. Solvent Parameters Employed in the SMD Solvation Model to Describe the Solvent Cavities in Reline, Ethaline, and Octane ε n2 α β γ ϕ ψ reline 12.0 2.26 0.27 0.47 52.0 0.00 0.0625 ethaline 32.0 1.77 0.35 0.59 48.0 0.00 0.0625 octane 2.0 1.918 0.00 0.00 21.14 0.00 0.00

Table 6. Changes in the Standard Free Energy of Solvation (∆G°solv) for Transfer of an ASC from the Octane Phase to a Reline or Ethaline DES phase BT BTO BTO2 DBT DBTO DBTO2 reline 0.62 –3.12 –3.50 0.59 –2.12 –2.03 ethaline 0.60 –4.42 –4.74 0.91 –3.76 –3.74


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Figure 1. a) Deep eutectic solvents (DESs) studied in this work. b) Representative aromatic sulfur compounds (ASCs) of interest for oxidative and extractive desulfurization. Shown are benzothiophene (BT), dibenzothiophene (DBT), and their respective sulfoxide and sulfone products.



Figure 2. M06–2X/6-31++G(d,p) optimized geometries of a) reline and b) ethaline DES clusters. Dark gray is used to color carbon, light gray for hydrogen, red for oxygen, green for chlorine, and blue for nitrogen. Ch+ denotes choline, U1 and U2 denote urea, and EG1 and EG2 represent ethylene glycol.


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Figure 3. M06–2X/6-31++G(d,p) optimized geometries of reline and aromatic sulfur compound complexes. Dark gray is used to color carbon, light gray for hydrogen, red for oxygen, yellow for sulfur, green for chlorine and blue for nitrogen.



Figure 4. M06–2X/6-31++G(d,p) optimized geometries of ethaline and aromatic sulfur compound complexes. Dark gray is used to color carbon, light gray for hydrogen, red for oxygen, yellow for sulfur, green for chlorine and blue for nitrogen.


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Figure 5. Schematic representation of the determination of the free energy of solvation for aromatic sulfur compounds for transfer from an octane phase to a DES phase.



TOC graphic:

Synopsis: Quantum chemical calculations expose interactions controlling removal of aromatic sulfur compounds and their oxidation products by deep eutectic solvent extractants.


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Quantum Chemical Evaluation of Deep Eutectic Solvents for the

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