Evaluating the Performance of Deep Eutectic Solvents for Use in

Article pubs.acs.org/jced

Evaluating the Performance of Deep Eutectic Solvents for Use in Extractive Denitrification of Liquid Fuels by the Conductor-like Screening Model for Real Solvents Hanee F. Hizaddin,† Anantharaj Ramalingam,*,† Mohd Ali Hashim,† and Mohamed K.O. Hadj-Kali‡ †

University of Malaya Center for Ionic Liquids (UMCiL), Department of Chemical Engineering, Faculty of Engineering, University of Malaya, Kuala Lumpur, 50603, Malaysia ‡ Chemical Engineering Department, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia S Supporting Information *

ABSTRACT: A total of 94 deep eutectic solvents (DESs) based on different combinations of salt cation, anion, hydrogen-bond donor (HBD) and salt:HBD molar ratio are screened via the conductor-like screening model for real solvents for potential use in the extractive denitrification of diesel. Five nonbasic and six basic nitrogen compounds were included in this study. The activity coefficient at infinite dilution, γ∞, of each nitrogen compound in the DESs was predicted; and the values are used to screen the DESs on the basis of selectivity, capacity, and performance index at infinite dilution (S∞, C∞, and PI). The extraction of nitrogen compounds using DES is driven by hydrogen-bonding interaction. It was found that nonbasic compounds report higher S∞ and C∞ than basic compounds. Ammonium-based DESs give higher S∞ but phosphonium-based DESs report higher C∞. DESs combined with Cl− anion give higher S∞, but those with Br− anion report higher C∞. DESs with alcohol- and amide-based HBDs give higher S∞ but HBDs with carboxylic acid group report high C∞. Molar ratio has little effect toward S∞ and C∞. DESs with high values of S∞ generally have high PI.

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INTRODUCTION In recent years, desulfurization of liquid fuels has become even more challenging than ever, because of the rising average concentration of sulfur in crude feedstock, while the limit of sulfur concentration in fuel oil is getting lower. The latest regulation currently requires less than 10 ppm of sulfur for the production of ultralow sulfur diesel (ULSD); targeting toward ultimately zero-sulfur fuels.1 Besides the motivation from a regulatory requirement, the presence of sulfur in liquid fuels is also not desirable as it causes corrosion which leads to faster equipment deterioration. The conventional process for sulfur removal in most refineries is the hydrodesulfurization (HDS) process, which is energy intensive and costly since HDS operates at severe conditions (600−700 K and 100−200 atm H2).2 Additionally, the presence of aromatic nitrogen compounds in crude oil inhibits the efficiency of the HDS process through competitive adsorption and catalyst deactivation. It is reported that the aromatic nitrogen compounds are present in crude oil as basic (six-membered ring) and nonbasic (five-membered ring) compounds. Nonbasic aromatic nitrogen compounds which constitute about 70 % of the total nitrogen content in crude oil3 are more difficult to remove because of their low reactivity compared to the basic ones. Therefore, to improve the efficiency of the HDS process, it is inevitable that attention © XXXX American Chemical Society

must also be paid to the removal of aromatic nitrogen compounds in crude oil before it goes into the HDS unit. Similar to that of sulfur removal, the conventional method for removal of nitrogen compounds is through hydrotreatment, that is, hydrodenitrification (HDN), in which the C−N bond cleavage in the nitrogen-containing compounds is induced to convert the nitrogen to NH3.4 Because of the severe operating conditions of hydrodenitrification, extractive denitrification appeals as an alternative due to its mild operating conditions (can be operated at room temperature and pressure), while maintaining the chemical properties and structure of the species involved.5 The efficiency of extractive denitrification strongly depends on the choice of solvent. Ideally, the solvent must have high selectivity, capacity, and distribution ratio toward the aromatic nitrogen compound, as well as having favorable thermophysical properties and being environmentally benign. Compared to conventional solvents which are typically toxic and volatile, ionic liquids (ILs) have been reported as attractive alternative solvents because of their negligible vapor pressure; hence, they are nonvolatile and tailorable to desired Received: May 14, 2014 Accepted: August 29, 2014

A

dx.doi.org/10.1021/je5004302 | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Table 1. List of DESs Involved in This Studya no. 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**

salt

HBD

choline-based DESs Choline chloride Urea choline chloride ethylene glycol choline chloride glycerol choline chloride 2,2,2-trifluoroacetamide choline chloride phenylpropionic acid choline chloride levulinic acid choline chloride xylitol choline chloride D-sorbitol choline chloride D-isosorbide choline chloride malonic acid choline fluoride urea ethylcholine chloride urea acetylcholine chloride urea chlorocholine chloride urea N-benzyl-2-hydroxy-N,N-dimethylethanaminium chloride urea N,N -diethyl ethanol ammonium chloride glycerol N,N -diethyl ethanol ammonium chloride glycerol N,N -diethyl ethanol ammonium chloride glycerol N,N -diethyl ethanol ammonium chloride ethylene glycol N,N diethyl ethanol ammonium chloride ethylene glycol N,N -diethyl ethanol ammonium chloride ethylene glycol Tetramethyl -Based DESs tetramethylammonium chloride glycerol tetramethylammonium chloride ethylene glycol tetramethylammonium chloride phenylacetic acid tetramethylammonium chloride malonic acid tetramethylammonium chloride tetraethylene glycol tetramethylammonium chloride caproic acid tetramethylammonium chloride acetic acid tetramethylphosphonium chloride glycerol tetramethylphosphonium chloride ethylene glycol tetramethylphosphonium chloride phenylacetic acid tetramethylphosphonium chloride malonic acid tetramethylphosphonium chloride tetraethylene glycol tetramethylphosphonium chloride caproic acid tetramethylphosphonium chloride acetic acid tetramethylammonium bromide glycerol tetramethylammonium bromide ethylene glycol tetramethylammonium bromide phenylacetic acid tetramethylammonium bromide malonic acid tetramethylammonium bromide tetraethylene glycol tetramethylammonium bromide caproic acid tetramethylammonium bromide acetic acid tetramethylphosphonium bromide glycerol tetramethylphosphonium bromide ethylene glycol tetramethylphosphonium bromide phenylacetic acid tetramethylphosphonium bromide malonic acid tetramethylphosphonium bromide tetraethylene glycol tetramethylphosphonium bromide caproic acid tetramethylphosphonium bromide acetic acid Tetrabutyl-Based DESs tetrabutylammonium chloride malonic acid tetrabutylammonium chloride glycerol tetrabutylammonium chloride tetraethylene glycol tetrabutylammonium chloride ethylene glycol tetrabutylammonium chloride phenylacetic acid tetrabutylammonium chloride caproic acid tetrabutylammonium chloride acetic acid tetrabutylphosphonium chloride malonic acid tetrabutylphosphonium chloride glycerol B

salt:HBD molar ratio

abbreviation

1:2 1:2 1:2 1:2 1:1 1:2 1:1 1:1 1:2 1:1 1:2 1:2 1:2 1:2 1:2 1:2 1:3 1:4 1:2 1:3 1:4

ChCl/Ur (1:2) ChCl/EG (1:2) ChCl/Gly (1:2) ChCl/TFA (1:2) ChCl/PPA (1:1) ChCl/LA (1:2) ChCl/Xy (1:1) ChCl/DS (1:1) ChCl/DI (1:2) ChCl/MA (1:2) ChF/Ur (1:2) EChC/Ur (1:2) ATCC/Ur (1:2) CChC/Ur (1:2) BHDAC/Ur (1:2) DEAC/Gly (1:2) DEAC/Gly (1:3) DEAC/Gly (1:4) DEAC/EG (1:2) DEAC/EG (1:3) DEAC/EG (1:4)

1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2

TMAC/Gly (1:2) TMAC/EG (1:2) TMAC/PA (1:2) TMAC/MA (1:2) TMAC/TEG (1:2) TMAC/CA (1:2) TMAC/AA (1:2) TMPC/Gly (1:2) TMPC/EG (1:2) TMPC/PA (1:2) TMPC/MA (1:2) TMPC/TEG (1:2) TMPC/CA (1:2) TMPC/AA (1:2) TMAB/Gly (1:2) TMAB/EG (1:2) TMAB/PA (1:2) TMAB/MA (1:2) TMAB/TEG (1:2) TMAB/CA (1:2) TMAB/AA (1:2) TMPB/Gly (1:2) TMPB/EG (1:2) TMPB/PA (1:2) TMPB/MA (1:2) TMPB/TEG (1:2) TMPB/CA (1:2) TMPB/AA (1:2)

1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2

TBAC/MA (1:2) TBAC/Gly (1:2) TBAC/TEG (1:2) TBAC/EG (1:2) TBAC/PA (1:2) TBAC/CA (1:2) TBAC/AA (1:2) TBPC/MA (1:2) TBPC/Gly (1:2)



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Table 1. continued no. 59** 60** 61** 62** 63** 64** 65** 66** 67** 68** 69** 70** 71* 72** 73** 74** 75** 76** 77** 78* 79* 80* 81* 82* 83* 84* 85* 86* 87* 88* 89** 90** 91** 92** 93** 94** a

salt

HBD

salt:HBD molar ratio

Tetrabutyl-Based DESs tetrabutylphosphonium chloride tetraethylene glycol tetrabutylphosphonium chloride ethylene glycol tetrabutylphosphonium chloride phenylacetic acid tetrabutylphosphonium chloride caproic acid tetrabutylphosphonium chloride acetic acid tetrabutylammonium bromide malonic acid tetrabutylammonium bromide glycerol tetrabutylammonium bromide tetraethylene glycol tetrabutylammonium bromide ethylene glycol tetrabutylammonium bromide phenylacetic acid tetrabutylammonium bromide caproic acid tetrabutylammonium bromide acetic acid tetrabutylphosphonium bromide ethylene glycol tetrabutylphosphonium bromide malonic acid tetrabutylphosphonium bromide glycerol tetrabutylphosphonium bromide tetraethylene glycol tetrabutylphosphonium bromide ethylene glycol tetrabutylphosphonium bromide phenylacetic acid tetrabutylphosphonium bromide caproic acid Phenylammonium- and Phenylphosphonium-Based DESs methyltriphenylphosphonium bromide glycerol methyltriphenylphosphonium bromide glycerol methyltriphenylphosphonium bromide glycerol methyltriphenylphosphonium bromide ethylene glycol methyltriphenylphosphonium bromide ethylene glycol methyltriphenylphosphonium bromide ethylene glycol methyltriphenylphosphonium bromide ethylene glycol methyltriphenylphosphonium bromide triethylene glycol methyltriphenylphosphonium bromide triethylene glycol methyltriphenylphosphonium bromide triethylene glycol ethyltriphenylphosphonium bromide ethylene glycol phenyltrimethylammonium chloride glycerol phenyltrimethylammonium chloride ethylene glycol phenyltrimethylammonium chloride triethylene glycol N,N,N-trimethyl-1-phenylethanaminium bromide glycerol N,N,N-trimethyl-1-phenylethanaminium bromide ethylene glycol N,N,N-trimethyl-1-phenylethanaminium bromide triethylene glycol

abbreviation

1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2 1:2

TBPC/TEG (1:2) TBPC/EG (1:2) TBPC/PA (1:2) TBPC/CA (1:2) TBPC/AA (1:2) TBAB/MA (1:2) TBAB/Gly (1:2) TBAB/TEG (1:2) TBAB/EG (1:2) TBAB/PA (1:2) TBAB/CA (1:2) TBAB/AA (1:2) TBPB/MA (1:2) TBPB/Gly (1:2) TBPB/TEG (1:2) TBPB/EG (1:2) TBPB/PA (1:2) TBPB/CA (1:2) TBPB/AA (1:2)

1:2 1:3 1:4 1:2 1:3 1:4 1:5 1:3 1:4 1:5 1:2 1:2 1:2 1:2 1:2 1:2 1:2

MTPPB/Gly (1:2) MTPPB/Gly (1:3) MTPPB/Gly (1:4) MTPPB/EG (1:2) MTPPB/EG (1:3) MTPPB/EG (1:4) MTPPB/EG (1:5) MTPPB/TrEG (1:3) MTPPB/TrEG (1:4) MTPPB/TrEG (1:5) ETPPB/EG (1:2) PTMAC/Gly (1:2) PTMAC/EG (1:2) PTMAC/TrEG (1:2) TPENB/Gly (1:2) TPENB/EG (1:2) TPENB/TrEG (1:2)

The asterisk (∗) denotes a real DES reported in the literature, and the double asterisk (∗∗) denotes a hypothetical DES.

to those of ILs, particularly in having negligible vapor pressure and being tailorable to desired application. With an appropriate selection of DESs’ constituents, the advantages of DESs over ILs include (1) a simple synthesis process, the materials can be easily mixed and ready to be used without further purification; (2) low cost, due to the simple synthesis process; and (3) some DESs are expected to have good biocompatibility due to their constituents being organic compounds such as choline chloride and urea.11,12 Some quarternary salts are in fact ILs; however, they should not be confused with DESs because ILs are salt in liquid form at room temperature which consist of only the cation and anion, whereas DESs are combinations of salt and HBD. Application of DESs in separation processes include the separation of aromatic/aliphatic mixtures such as toluene/nheptane13,14 and benzene/n-hexane,15 separation of the ethanol/heptane azeotropic mixture,16 and also extractive desulfurization of benzothiophene from model oils.17 Kareem et al. reported the performance of DESs to be at par or even better than conventional organic solvents and ILs.13−15 Li et

applications, as well as having high thermal and chemical stability. ILs have been reported as highly potential solvents for extractive denitrification in the past few years, with published experimental data reporting a high selectivity and distribution ratio for the removal of aromatic nitrogen compounds from model fuels.5−9 In the reports, most of the ILs are imidazoliumbased cations, with a few reported uses of pyridinium cations, combined with anions from the likes of acetate, alkylsulfate, methanesulfonate, thiocyanate, dicyanamide, methylphosphonate, triflate, bis(trifluoromethanesulfonyl)imide, and chloride. A new class of solvents named “deep eutectic solvents” (DESs) has gained increasing attention as an alternative to ILs. A DES is typically made of a salt combined with hydrogenbond donor (HBD) or a complexing agent. A typical salt for a DES is a combination of a large quarternary ammonium or phosphonium cation with a halide anion. Mixing the salt with a HBD at a certain ratio will lead to lowering of the freezing point of the produced eutectic mixture far from that of its individual constituent due to a complex formed between the HBD with the halide anion.10 This eutectic mixture has properties similar C



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Table 2. Equations and Constants Involved in the COSMO-RS Mathematical Expressions:

α′ (σ + σ ′)2 2

electrostatic misfit energy

Emisfit(σ , σ ′) = aeff

hydrogen bond interaction energy

2 E hb = aeff chb min(0, σσ ′ + σhb )

van der Waals interaction energy

′ ) EvdW = aeff (τvdW + τvdW

chemical potential of a segment

⎡ μS (σ ) = − RT ln⎢ ⎣

activity coefficient of a segment

⎡ ln γS(σ ) = − ln⎢ ⎣

activity coefficient of a solute i in an ensemble S

∫ dσ′pS (σ ) × exp{− 12 α′(σ+σ′)

2

⎤ − μS′(σ ′)/RT ⎥ ⎦

}

⎧ − aeff e(σ , σ ′) ⎫⎤ ⎬⎥ ⎭⎦ RT

∫ dσ′pS (σ )γS(σ′) × exp⎨⎩

res ln γi /S(σ ) = ln γi /S + ln γicomb /S comb γres i/S = residual activity coefficient γi/S = combinatorial activity coefficient

residual activity coefficient

∫ dσ′pi (σ ){ln γS(σ′) − γi(σ′)}] + ln γicomb /S

res ln γi /S (σ ) = − ni[

ln γicomb = ln /S combinatorial activity coefficient

θi =

xiqi ∑j xjqj

⌀i θ ⌀ z + qi ln i + li − i ∑ xjl j 2 ⌀i xi xi j xiri z ⌀i = li = ((ri − qi) − (ri − 1)) ∑j xjrj 2

∑ xip Xi (σ )

σ-profile of a mixture

pS (σ ) =

Constants: α′ aeff chb σhb τvdW

general constant effective contact area interaction strength coefficient polarization charge density threshold for hydrogen bond element-specific parameter for dispersion coefficient

i∈S

al.17 reported the use of various ammonium-based DESs for extractive desulfurization with very high efficiency for removal of benzothiophene from model oil. In line with these potential applications of DESs in liquid−liquid extraction, we strongly believe that DESs have good prospect for extractive denitrification as well. However, similar to ILs, there is no trivial answer to choosing the right DES for a specific application because of the tremendous number of potentially existing DESs based on different combinations of the parameters mentioned above. Hence, having to conduct experiments to screen the potential DESs based on these parameters is time-consuming and not practical. Previously, the conductor-like screening model for real solvents (COSMO-RS) has been used to screen ILs for various separation processes, including desulfurization,18 denitrification,19 separation of monoethylene glycol from water,20 and gas separation processes.21,22 Through COSMORS, we can make use of the prediction of the activity coefficient at infinite dilution (γ∞) for aromatic nitrogen compounds and model diesel in DES to assess the performance of DES for this particular separation process. The activity coefficient at infinite dilution is an essential and highly valuable parameter because it indicates the extent of molecular interaction between the solute and solvent at miniscule concentration of the solute. In the separation of aromatic nitrogen compounds from diesel, the most difficult bit is the removal of the last traces of the nitrogen compounds present. Thus, γ∞ can be exploited to quantify the ability of DES for the separation of aromatic nitrogen compounds from diesel. The COSMO-RS method has been used to screen ILs for extractive denitrification as was reported by Anantharaj and

Banerjee19 where 168 ILs were screened by predicting the activity coefficient at an infinite dilution of nitrogen compounds in each IL and in the simulated diesel. In other reports, COSMO-RS was used to predict the liquid−liquid equilibrium data for systems containing ionic liquids and aromatic nitrogen compounds with satisfactory accuracy, that is, less than 10 % in the root-mean-square deviation (RMSD).23−25 Till date, there is not yet a published work regarding the use of the COSMORS method to screen potential DESs for any applications. In this work, the objective is to screen potential DESs for the removal of aromatic nitrogen compounds from diesel fuel by studying the effect of (1) the choice of salt cation, (2) the choice of salt anion, (3) the choice of HBD, and (4) the ratio between salt and HBD in the DES mixture. This is done by quantifying the performance of DES using selectivity, capacity, and performance index at infinite dilution for the separation of aromatic nitrogen compounds from model diesel. The structure of heterocyclic nitrogen compounds along with the structure of the DES constituents (i.e., salt cation, salt anion, and HBD) investigated in this work are provided in the Supporting Information (Table S1). Three aromatic nitrogen compounds studied are basic (six-membered) compounds, that is, pyridine (PY), quinoline (QU), and benzoquinoline (BQU), and five are nonbasic (five-membered) compounds, that is, pyrrole (PYR), indole (INDO), indoline (INDL), carbazole (CAR), and benzocarbazole (BCAR). A total of 94 DESs consisting of various combinations of salt and HBD with different molar ratios are screened in this study. For the sake of comparison, the DESs examined consist of both real and hypothetical mixtures. The real DESs are those which are liquid at room temperature as reported in the literature.10,16,17 The hypoD



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meaningful and accurate values,28 while the 6-31G* basis set accounts for polarization effects of the species in the complexes. From the optimized geometry for each individual species, a single point calculation was performed with activation of the .cosmo file generation using density functional theory with Becke−Perdew functional and triple-ζ valence potential (TZVP) basis set. All of these jobs were performed with Turbomole Software Package.29 Thereafter, the .cosmo f iles were imported into the COSMOthermX software package with parametrization file BP_TZVP_C30_1301.ctd.30 Representation of DES in COSMOthermX. Representation of ILs in COSMOthermX is adapted to suit the representation of DESs due to their similar nature. In COSMO-RS, there are three approaches to represent ILs; (i) the electro-neutral approach, (ii) the ion pair approach, and (iii) the meta-file approach. Approach (i) is considered to be closer to the real nature of ILs because it describes the two ions as separate species in a liquid mixture.30 Hence, this approach was implemented in this work to describe the DES in COSMORS. Indeed, it is reported that in liquid form, the HBD forms a complex with the halide anion of the salt and thereby lowering the interaction energy between the salt cation and the salt anion.10 Therefore, in liquid form, we can safely assume that there are three distinct species constituting the DES, which are the cation, anion and HBD. For example, let us consider a DES with a salt/HBD molar ratio of 1:n. In a laboratory situation, this means that the DES consists of 1 mol of cation, 1 mol of anion, and n mole of HBD. A comparison of the representation of IL in COSMOthermX as 1:1 for its cation/anion ratio shows that DES is represented as 1:1:n for its cation/anion/HBD ratio. However, similar to using approach (i) for representing ILs in COSMOthermX, the mole fractions obtained by a COSMORS calculation by COSMOthermX need to be converted to reflect the actual experimental definition of the mole fraction. This can be done as follows: The experimental definition of mole fraction of solute in a mixture of solute, carrier, and DES is given by ni xiEXP = ni + nj + nDES (1)

thetical ones are based on real DESs by interchanging the cation, halide anion, or HBD in order to investigate the different effects that each substituent has toward the selectivity, capacity, and performance index at infinite dilution. The list of all DESs studied is provided in Table 1 where the real and hypothetical DESs are indicated.

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COSMO-RS The COSMO-RS is the most progressive kind of a dielectric model, COSMO (conductor-like screening model) where molecules are placed in a conductor as the reference state. COSMO-RS (COSMO for real solvents) is the extension of COSMO to statistical thermodynamics. In COSMO-RS, the interaction energy is represented in terms of its polarization charge density, σ and σ′. The molecular interactions represented in COSMO-RS include the electrostatic misfit energy (Emisfit), hydrogen bond interaction (Ehb), and van der Waals interaction (EvdW).26 The local screening charge density σ is the most important descriptor in the COSMO-RS as it is the only descriptor determining the interaction energies and thus all other statistical thermodynamic properties such as chemical potential and activity coefficient. This descriptor can be calculated using any quantum chemical program in the form of a .cosmo file output and it only needs to be done once for a specific molecule. In the .cosmo f ile, the surface of the species is divided into segments with a certain surface charge density. By applying a local averaging algorithm on the surface charge densities over effective contact segments, a probability function (σ-profile) can be plotted.26 Details on the local averaging algorithm can be found in the reference.27 As discussed by Klamt and coworkers,26 the σ-profile and σ-potential may help to understand the properties and behavior of the compounds and their mixtures in terms of charge interactions. The σ-profile represents the probability distribution of finding a surface segment with specific screening charge density. It should be noted that in σ-profile, negative σ represents positive polarities, and vice versa. Meanwhile, σ-potential (μ(σ)) describes the affinity of a solvent for a molecular surface of polarity σ. A lower μ(σ) value at a particular σ implies better affinity for that polarity, and vice versa. μ(σ) is determined from a selfconsistent equation derived by Klamt27 for the chemical potential of segments with charge density σ in the ensemble S. From the chemical potential of a segment, the activity coefficient of the segment and further the activity coefficient of a solute i in an ensemble S can be calculated. Table 2 shows the mathematical representation of the COSMO-RS used for quantifying the performance of DESs in this work. Further details on the derivation and background of these equations can be found in the original work of Klamt.23

Meanwhile in COSMOthermX, the definition of mole fraction in COSMO-RS by using the approach (i) is given by ni xiCOSMO ‐ RS = ni + nj + ncation + nanion + nHBD (2) where xEXP = mole fraction of solute i in the experimental i definition. xCOSMO‑RS = mole fraction of solute i in COSMO-RS i usingthe electroneutral approach, ni = number of moles of solute i, nj = number of moles of carrier j, nDES = number of moles of DES as a whole, ncation = number of moles of cation in DES, nanion = number of moles of anion in DES, nHBD = number of moles of HBD in DES From eqs 1 and 2, we can obtain the conversion from calculated mole fraction in COSMOthermX (xCOSMO‑RS ) to the i experimental framework (xEXP ): i

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COMPUTATIONAL DETAILS Geometry Optimization and .cosmo f ile generation of Heterocyclic Nitrogen Compounds and DES constituents. The first step in COSMO-RS is to generate a .cosmo file from the optimized geometry of each species involved. Geometry optimization was performed for the cation, anion, HBD, heterocyclic nitrogen compounds, and model diesel studied in this work. Initial structures for each compound were drawn, and the geometry optimization was performed at Hartree−Fock level and 6-31G* basis set. Calculation of geometry optimization at the Hartree−Fock level gives more

xiEXP =

xiCOSMO ‐ RS/υi ∑k xkCOSMO ‐ RS/υk

(3)

where υi = stoichiometric coefficient of each species in the mixture (i.e., solute, carrier, cation, anion, HBD) E



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The conversion of activity coefficient in COSMOthermX uses the same approach: γi LAB

⎛ x COSMO − RS ⎞ ⎟ = γiCOSMO − RS × ∑ ⎜ k υk ⎠ k ⎝

Table 3. Composition of Artificially Simulated Diesel

(4)

At infinite dilution, ⎛ x COSMO ‐ RS ⎞ k ⎟= υk ⎝ ⎠

∑⎜ k

∴ γi∞ ,LAB =

∑ k

∑ k

1 υkDES

1 ∞ ,COSMO − RS γ υkDES i

(5)

⎛ 1 ⎞ C1∞ = ⎜⎜ ∞ ⎟⎟ ⎝ γ1 ⎠DES phase

(8)

(9)

The performance index can be defined as the product of maximum selectivity and maximum capacity, that is, the ∞ product of S∞ 12 and C1 as shown in eq 10. ∞ PI = S12 × C1∞ ⎛ γ∞ ⎞ ⎛ 1 ⎞ = ⎜⎜ 2∞ ⎟⎟ × ⎜⎜ ∞ ⎟⎟ ⎝ γ1 ⎠DES phase ⎝ γ1 ⎠DES phase

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⎛ γ∞ ⎞ 2 ⎟ = ⎜⎜ ∞ 2⎟ γ ( ⎝ 1 ) ⎠DES phase

(10)

RESULTS AND DISCUSSION Benchmarking LLE Prediction with Experimental Results. The accuracy of COSMO-RS for nitrogen containing systems is ascertained by comparing the ternary LLE data predicted by the COSMO-RS method to those reported in the literature. For this purpose, the LLE data reported by KedraKrolik et al.7,8 were used where the authors report LLE data at 298.15 K for two ternary systems, which are pyridine (1) + nheptane (2) + 1-ethyl-3-methylimidazolium thiocyanate ([EMIM][SCN]) (3),7 and pyridine (1) + n-heptane (2) + 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM][Otf]) (3). 8 The root-mean-square deviation (RMSD) between the experimental and predicted tie lines is calculated as shown in eq 11, where m is the number of tie lines, c is the number of components, and the number of phases

i

i∈S

42.11 16.84 24.21 10.53 5.26

Capacity at infinite dilution can be used as an estimate for the amount of DES needed for removing the nitrogen compounds. Operationally, it defines the maximum amount of nitrogen compounds that can be dissolved in the DES. The maximum capacity, C1,max can be approximated by the capacity at infinite dilution for nitrogen compound, C∞ 1 where it is estimated as the inverse of the activity coefficient at infinite dilution of the nitrogen compound in the DES; as shown in eq 9.

(6)

∑ xip X (σ )

wt %

C16H34 C12H26 C15H24 C10H20 C12H24

⎛ γ∞ ⎞ ∞ S12,max = S12 = ⎜⎜ 2∞ ⎟⎟ ⎝ γ1 ⎠DES phase

The simulated diesel oil is defined with the composition given in Table 3. In COSMO-RS, the simulated diesel oil is treated as one single compound where the sigma profile is added linearly according to the composition as expressed in eq 7, where xi and pXi represent the composition and sigma profile of each diesel constituent, respectively.

pS (σ ) =

molecular formula

hexadecane isododecane phenylnonane butylcyclohexane 1-dodecene

Selectivity of nitrogen compounds can be defined as the ratio of concentration of nitrogen compounds in the extract phase to its concentration in the raffinate phase. At infinite dilution, we can estimate the maximum selectivity, S12,max which can be defined as the ratio between the activity coefficients at infinite dilution for diesel to that of nitrogen compound in the DES. This can be expressed by eq 8, where S∞ 12 is the selectivity at infinite dilution, while subscripts 1 and 2 refer to the nitrogen compound and simulated diesel, respectively.

Expressions of Selectivity, Capacity and Performance Index at Infinite Dilution. Important criteria in choosing a solvent for a separation process include its selectivity, distribution ratio or capacity, its insolubility in the raffinate phase, its recovery, safety, and environmental factors such as flammability and toxicity, and finally its cost and availability. Similar to ILs, DESs are insoluble in the raffinate phase as proven by previously reported experimental data of ternary liquid−liquid equilibria for systems with DES.13−15 It has also been proven that DES can be regenerated relatively easily and without any degradation in the quality and performance of the DES.17 Having negligible vapor pressure and high thermal stability, DESs are nonflammable and nonvolatile, therefore minimizing its environmental impact through zero volatile compounds emission. On the other hand, DESs are said to be less toxic than ILs, although this subject is yet to be validated through further studies. Finally, the cost of DESs is generally much lower than ILs, due to their abundant raw materials and simpler synthesis route than that of ILs. Thus, it can safely be concluded that DESs fulfill most of the important criteria mentioned above. Hence, it is reasonable to screen the potential DES as solvent on the basis of its selectivity and capacity. It is desirable to have a DES with high selectivity toward the nitrogen compounds as this means less number of stages needed to remove them and thus lower capital cost. Higher selectivity means that the DES has a more favorable interaction toward the nitrogen compounds as compared to diesel. Besides, a high capacity is also desirable as this can lower the solvent-tofeed ratio, and also requires a smaller diameter for the extraction column and thus lower operating cost. The activity coefficient at infinite dilution γ∞ i can be used to estimate the maximum selectivity and capacity of the DES studied. γi∞ is very important because it describes the interaction between the solvent and solute at a very miniscule concentration of solute present. γ∞ i is defined as the limit of activity coefficient of a solute i as the concentration of i approaches zero. The mathematical expression is given in eq 6.

γi∞ = lim xi → 0 γi

diesel constituent

(7) F



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= 2. Figure 1 panels a and b show the ternary tie-lines for the systems pyridine (1) + n-heptane (2) + [EMIM][SCN] (3) and pyridine (1) + n-heptane (2) + [BMIM][Otf] (3), respectively. It was observed that the COSMO-RS predicted tie-lines were in good agreement with the experimental tie lines, with RMSD values of 2.08 % and 4.98 %, respectively.

m

RMSD(%) = 100

c

2

∑∑∑ k=1 i=1 j=1

(xikj − xik̂ j )2 2mc

(11)

Additionally, to validate our calculation method for systems containing DES, ternary liquid−liquid equilibria data are predicted using the COSMO-RS and the results are compared with experimental data reported in the literature. The ternary system toluene (1) + heptane (2) + (tetrabutylphosphonium bromide/ethylene glycol 1:2) DES (3) at T = 313.15 K reported by Kareem et al.13 was used for this benchmarking purpose. Figure 1c shows the comparison between experimental and COSMO-RS predicted tie-lines where it is observed that the predicted tie-lines are in good qualitative agreement with the experimental tie-lines, with a calculated RMSD of 4.91 %. For all benchmarking calculations with experimental LLE data, the reported values of the COSMO-RS prediction against the experimental tie lines were provided in the Supporting Information (Tables S2 to S4). The prediction of ternary tie lines by COSMO-RS is proven to be sufficient for systems containing nitrogen compounds and DES, considering COSMO-RS to be a priori method which does not require any experimental data as input at all. Further, these results can be used as a basis for our calculation methods to be acceptable for use in further predictions. Benchmarking the Prediction of Infinite Dilution Activity Coefficient. Recently, Domanska et al.31 reported the activity coefficients at infinite dilution for several organic solutes in the IL 1-butyl-1-methylpyrrolidinium tricyanomethanide at various temperatures. Among the reported organic solutes is pyridine. Thus, this is used to compare the predicted γ∞ against the reported data so as to further validate our computation method. The result of this is shown in Table 4, Table 4. Experimental and COSMO-RS Predicted Activity Coefficient at Infinite Dilution of Pyridine in 1-Butyl-1methylpyrrolidinium Tricyanomethanide T/K 318.15 328.15 338.15 348.15 358.15 368.15 RMSD = 0.166 %

expt γ∞

COSMO-RS predicted γ∞

0.567 0.58 0.592 0.603 0.617 0.627

0.640 0.633 0.626 0.620 0.614 0.608

∞ 2 RMSD (%) = 1/N ΣNi (γ∞ i,expt − γi,pred) , where N is the number of data points

where the reported RMSD between experimental and COSMO-RS predicted γ∞ is 0.166 %, which is again very good considering COSMO-RS does not use any experimental data for correlation. Calculated Selectivity, Capacity, and Performance Index at Infinite Dilution. In this section, we will discuss the effects of the heterocyclic structure of nitrogen compounds, salt cations, salt anions, HBD choice, and salt/HBD molar ratio toward the selectivity, capacity, and performance index at infinite dilution. Sigma profile and sigma potential will be used to facilitate understanding of intermolecular interaction between the species involved.

Figure 1. Experimental and COSMO-RS predicted tie-lines for the ternary systems of (a) pyridine (1) + n-heptane (2) and [EMIM][SCN] (3) at T = 298.15 K, (b) pyridine (1) + n-heptane (2) and [EMIM][Otf] (3) at T = 298.15 K, and (c) toluene (1) + n-heptane (2) and (tetrabutylphosphonium bromide/ethylene glycol 1:2) DES (3) at T = 313.15 K. Solid lines and full circles denote experimental tie-lines, whereas dashed lines and empty circles denote COSMO-RS predicted tie-lines. G



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Li et al.17 reported that the extraction of benzothiophene from n-octane using ammonium-based DES is mainly driven by the hydrogen-bond interaction between the active hydrogen in the DES and the sulfur atom in benzothiophene, in which the presence of benzothiophene weakens or destroys the hydrogen bonding between the halide anion and the active hydrogen in the DESs’ HBD. The sigma profile and sigma potentials of all species involved are available in the Supporting Information (Figures S4 to S11). If we consider the interaction between DES and nitrogen compounds to be analogous to that between DES and sulfur compounds, we can illustrate the interaction as shown in Figure 2, where the intermolecular interactions are dominated by

discuss the different effects each of the parameters has with regards to the selectivity, capacity, and therefore performance index of the DESs toward the nitrogen compounds. The selectivity, capacity, and performance index at infinite dilution for all aromatic nitrogen compounds in each DES are provided in the Supporting Information (Figures S1 to S3 and Tables S5 to S8). The DESs are divided into six categories, which are (a) choline-based DESs in which the salt cations are different forms of choline cation, (b) tetramethyl−based DES in which the salt cations are tetramethylammonium and tetramethylphosphonium cations with chloride anions, (c) tetramethyl-based DES in which the salt cations are tetramethylammonium and tetramethylphosphonium cations with bromide anions (d) tetrabutyl-based DES in which the salt cations are tetrabutylammonium and tetrabutylphosphonium cations with chloride anion, (e) tetrabutyl-based DES in which the salt cations are tetrabutylammonium and tetrabutylphosphonium cations with bromide anion, and (f) DESs based on salts with phenylammonium and phenylphosphonium cations, both with chloride or bromide anion. Effect of Heterocyclic Structure of Nitrogen Compounds. In general, nonbasic nitrogen compounds are weak hydrogen-bond donors, whereas basic nitrogen compounds are weak hydrogen-bond acceptors. This can be proven by looking at the sigma profiles and sigma potentials of each nitrogen heterocycle as given by Figures 3 and 4. The dashed vertical lines in the sigma profile and potential diagrams represent the threshold value for the hydrogen bond interaction, σhb = ±

Figure 2. Depiction of molecular interaction between salt cation, anion, HBD, and nitrogen compound. The DES TMAC/EG (1:2) is taken as model DES while pyrrole (a) and pyridine (b) are taken as the representative five-membered and six-membered nitrogen compounds, respectively. Thick dashes represent hydrogen bonding while small dots represent electrostatic interaction.

hydrogen bonding. In Figure 2, tetramethylammonium chloride + ethylene glycol is taken as a model DES while pyrrole and pyridine are taken as model basic and nonbasic nitrogen compounds. The interaction between tetramethyl cation and chloride is via electrostatic interaction, which is somehow weakened by the presence of ethylene glycol by forming a hydrogen bond between the free hydrogen and the chloride anion. When pyrrole is present, pyrrole being a weak hydrogenbond donor interacts with chlorine ion to form another hydrogen bond in the mixture, weakening the existing hydrogen bonding between ethylene glycol and the chloride ion. In the case where pyridine is present, pyridine being a weak hydrogen-bond acceptor creates a hydrogen bonding with ethylene glycol, which also results in the weakened hydrogen bonding between ethylene glycol and chloride ion. We will

Figure 3. σ-profiles of selected DESs and simulated diesel with (a) five-membered compounds and (b) six-membered compounds. Vertical dashed lines represent the threshold value for the hydrogen bond interaction, σhb = 0.0084 eA−2. H



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affinity toward the DESs’ side of hydrogen bond acceptor. Meanwhile, Figures 3b and 4b elucidate the interaction between DESs, diesel, and six-membered nitrogen compounds, and it can be observed from Figure 4b that the six-membered nitrogen compounds interact with DES only by having affinity toward the hydrogen-bond donor side of the DESs. Figure 5 panels a and b show the values of selectivity and capacity at infinite dilution for nonbasic and basic nitrogen

Figure 4. σ-Potentials of selected DESs and simulated diesel with (a) five-membered compounds and (b) six-membered compounds. Vertical dashed lines represent the threshold value for the hydrogen bond interaction, σhb = 0.0084 eA−2.

0.0084eA−2. In the sigma profiles, peaks at σ < − σhb (left-hand side) show capability of hydrogen-bond donor while peaks on the right-hand side at σ > + σhb show capability for hydrogenbond acceptor. The sigma profiles for the five-membered compounds generally show peaks at the left-hand side of the dashed line, which indicates the presence of a hydrogen-bond donor while in their sigma potentials, the values of μ(σ) are negative at σ > + σhb = 0.0084 eA−2 which indicates affinity for a hydrogen-bond acceptor. On the other hand, sigma profiles for the six-membered compounds show peaks at σ > + σhb indicating the presence of a hydrogen-bond acceptor, and similarly their sigma potentials have negative values of μ(σ) at σ < − σhb = 0.0084 eA−2 indicating affinity for ahydrogen-bond donor. An exception to this generalization is indoline, a fivemembered compound which exhibits a sigma potential like those of six-membered compounds, that is, being a weak hydrogen-bond acceptor. Figures 3 and 4 also qualitatively demonstrate the interaction between the DESs, diesel, and aromatic nitrogen compounds. Figure 3 panels a and b show the sigma profiles of some representative DESs, diesel, and five- and six-membered compounds, respectively, whereas Figure 4 panels a and b show the sigma potentials of the said compounds. In these figures the sigma profile and sigma potential of the DESs are represented as a whole DES instead of its individual constituents (cation, anion, HBD). As shown in the figures, the DESs have a hydrogen bond donor side and a hydrogen bond acceptor side. It can be observed from Figures 3a and 4 a that the five-membered nitrogen compounds show slight

Figure 5. Selectivity (a) and capacity (b) at infinite dilution for representative DESs toward nitrogen compounds in comparison of the effect of nitrogen heterocyclic structure. Nitrogen compounds: 1-PYR, 2-INDO, 3-INDL, 4-CAR, 5-BCAR, 6-PY, 7-QU, 8-BQU.

compounds in several representative DESs, that is, ChCl/Ur (1:2), TMAC/MA (1:2), TMPB/MA (1:2), TBAC/MA (1:2), TBPB/MA (1:2), MTPPB/EG (1:2), and PTMAC/Gly (1:2). It was observed that DESs report higher values for selectivity and capacity at infinite dilution and hence performance index (S∞, C∞, and PI, respectively) for pyrrolic nitrogen compounds compared to the pyridinic ones. A possible explanation for this observation is due to pyrrolic compounds being weak hydrogen-bond donors, and when present in the DES mixture, they will increase the hydrogen bonding in the mixture through bonding of the halide anion and the pyrrolic compounds’ −NH atoms. Conversely, pyridinic compounds as a weak hydrogenbond acceptor when present in a DES mixture will cause the hydrogen bonding between the halide anion and the HBD to be weakened, because of the need to share the same hydrogenbond donor to two species now, that is, the halide anion and the pyridinic compounds. Table 5 shows the summary of results according to the maximum and minimum values of S∞, C∞, and PI for the aromatic nitrogen heterocycles. Indoline reports the lowest selectivity and capacity compared to the rest of the fivemembered nitrogen compounds. This result is similar to the findings by Anantharaj and Banerjee,19 and it is explained as due to indoline’s lack of aromatic rings in its structure, which I



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Table 5. Maximum and Minimum Values for S∞, C∞, and PI Reported by DESs for each Nitrogen Heterocycles N cmpd PYR INDO INDL CAR BCAR PY QU BQU

value

S∞

DES

C∞

DES

max min max min max min max min max min max min max min max min

980 618.5 207.0 502 663.7 248.1 24 081.6 32.2 154 693.8 165.2 82 588.4 130.85 96 702.9 15.2 20 568.7 11.1 8 781.8 9.4

CChC/Ur TBAB/CA CChC/Ur TBAB/CA CChC/Ur TBAB/CA CChC/Ur TBAB/CA CChC/Ur MTPPB/TrEG TBAB/MA TBAC/CA CChC/Ur TBAB/CA CChC/Ur TBAB/CA

96.6 4.2 146.6 0.6 4.8 0.2 121.3 0.35 173.8 0.1 6.0 0.3 3.6 0.1 4.5 0.03

ChF/Ur TMAB/MA ChF/Ur PTMAC/EG ChF/Ur TMAB/MA ChF/Ur TMAB/MA ChF/Ur TMAB/MA MTPPB/TrEG ChCl/Xy ChF/Ur ChCl/Xy ChF/Ur ChCl/Xy

PI 9 036 5 2 374 1 5 640 1 1 315 75 3

159.4 097.1 316.8 419.1 449.5 123.4 260.0 184.4 198.7 139.2 330.2 30.1 975.5 16.1 873.9 8.2

DES CChC/Ur TBAB/CA CChC/Ur PTMAC/EG CChC/Ur TBAC/CA ChF/Ur DEAC/Gly ChF/Ur DEAC/Gly TMAB/MA TBAC/CA CChC/Ur TBAC/CA ChF/Ur TBAC/Gly

Figure 6. Selectivity at infinite dilution for nitrogen compounds according to different types of DESs in comparison of the effect of cation choice: (a) choline-based DESs, (b) tetramethyl-based DESs, (c) tetrabutyl-based DESs, (d) phenylammonium and phenylphosphonium-based DESs. Nitrogen compounds: 1-PYR, 2-INDO, 3-INDL, 4-CAR, 5-BCAR, 6-PY, 7-QU, 8-BQU.

C∞ value is given by the DES MTPPB/TrEG. Mixed results are observed for the PI for each nitrogen compound where there is no clear trend on which DES excels in the value of PI for all of the aromatic nitrogen compounds. However, generally DESs with high S∞ will give high PI as well because PI is the product of S∞ and C∞. Effect of Cation Choice. The effect of cation choice on selectivity, capacity, and performance index at infinite dilution was also explored. Choline-based cations, tetramethylammonium, tetramethylphosphonium, tetrabutylammonium, tetrabutylphosphonium, phenylammonium, and phenylphosphonium cations were compared. Figure 6a shows the values of S∞ for

inhibits the delocalization of π-electrons inside the rings. This leads to the lack of interaction between the halide anion and the −NH atoms of indoline, thus resulting in lower selectivity and capacity. Unlike indoline, other five-membered compounds have a similar order of magnitude in maximum selectivity and capacity due to their similarity in the π-electron density. For the six-membered compounds, it is observed that S∞ and C∞ values decrease with increasing benzene ring attached to the pyridinic compounds. Overall, in terms of S∞, the DES CChC/Ur (1:2) gives the highest value for all nitrogen compounds. The DES ChF/Ur (1:2) gives the highest values of C∞ for all of the nitrogen compounds except for pyridine, of which the highest J



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Figure 7. The capacity at infinite dilution for nitrogen compounds according to different types of DESs in comparison of the effect of cation choice: (a) choline-based DESs, (b) tetramethyl-based DESs, (c) tetrabutyl-based DESs, (d) phenylammonium and phenylphosphonium-based DESs. Nitrogen compounds: 1-PYR, 2-INDO, 3-INDL, 4-CAR, 5-BCAR, 6-PY, 7-QU, 8-BQU.

choline-based DESs toward all nitrogen compounds. For the same anion (Cl), HBD (urea), and salt/HBD molar ratio (1:2), it was observed that DESs with chlorocholine cation give the highest S∞ most probably due to the presence of the Cl atom attached to the choline cation, thereby increasing the polarity of the DES as a whole and thus enabling it to be more selective toward the nitrogen compounds than toward diesel compounds. To simplify the comparison, pyrrole and pyridine are taken as the benchmark for five-membered and six-membered compounds, respectively. The values of S∞ for pyrrole decreases for DESs in the following cations order: chlorocholine > acetylcholine > choline > N-benzyl-2-hydroxy-N,Ndimethylethanaminium > ethylcholine, whereas, for pyridine as representative of six-membered compounds, the S∞ values decreases for DESs with cations as follows: chlorocholine > acetylcholine > N-benzyl-2-hydroxy-N,N-dimethylethanaminium > choline > ethylcholine. Similar comparison can be made for DESs with tetramethylammonium (TMA) and tetramethylphosphonium (TMP) salts, by keeping the anion and HBD constant for consistent comparison. This is illustrated in Figure 6b which presents the values of S∞ by TMA- and TMP-based DESs toward nitrogen compounds. A comparison of DESs with tetramethyl-based salts with chloride anions and malonic acid as the HBD for example, shows that DESs with a TMA cation give significantly higher selectivity than DESs with a TMP cation for both fivemembered and six-membered nitrogen compounds. A similar trend was observed for tetramethyl-based salt DESs with bromide anion and the same HBD, whereas DESs with TMA cation give higher selectivity for both types of nitrogen compounds (Figure S1c) in the Supporting Information. The trend is further confirmed when comparing the DESs of TMA and TMP cations with different HBDs, for example, glycerol

and ethylene glycol, in which in all cases DESs with a TMA cation report higher selectivity. Sigma profiles of TMA and TMP cations show that TMA is slightly more polar than TMP owing to TMA having more surface segments which are positively charged (negative σ values) than TMP. The sigma profile of TMA is skewed to the left compared to the sigma profile of TMP which is closer to the center (σ = 0) indicating that TMP has more surface segments closer to a neutral charge. Another possible explanation for TMA having higher selectivity toward nitrogen compounds is that the N atom attached to the tetramethyl chain in tetramethylammonium cation is more electronegative than the P atom attached to tetramethylphosphonium cation. For DESs with tetrabutyl-based salts, it was generally observed that DESs with tetrabutylphosphonium (TBP) cations give higher values of S∞ for both basic and nonbasic nitrogen compounds than those with tetrabutylammonium (TBA) cations as shown in Figure 6c). However, the difference in values is not as big as compared to the difference in S∞ values between DESs with TMA and TMP cations. When the sigma profile between TBA and TBP cations are compared, TBA is slightly less polar than TBP such that TBA has more surface segments with charges close to neutral (σ = 0) than TBP. However, both the sigma profile and sigma potential of TBA and TBP look very similar and this explains the close values of S∞ obtained by the DESs with those cations. DESs with TBA and TBP cations have lower selectivity than those with choline-based or tetramethyl-based cations because the sigma profiles of TBA and TBP resemble that of diesel compounds, which means that DESs with TBA and TBP is also entropically miscible with the diesel compounds. Figure 6d compares the values of S∞ for phenylammonium and phenylphosphonium-based DESs toward nitrogen comK



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Effect of Anion Choice. In this section, the influence of anion choice for the salt making up the DESs will be discussed. We will discuss the effect of chloride (Cl−) and bromide (Br−) anions based on the performance of DESs with the salts tetramethylammonium chloride (TMAC), tetramethylammonium bromide (TMAB), tetramethylphosphonium chloride (TMPC), tetramethylphosphonium bromide (TMPB), tetrabutylammonium chloride (TBAC), tetrabutylammonium bromide (TMAB), tetrabutylphosphonium chloride (TBPC), and tetrabutylphosphonium bromide (TBPB); combined with a few HBDs such as malonic acid (MA), glycerol (Gly), and ethylene glycol (EG) with a salt/HBD ratio of 1:2. This is to enable consistent comparison on the effects on anion alone where the cation, HBD, and salt/HBD ratio are made constant. Figure 8 panels a and b are referred for observation toward the selectivity at infinite dilution. In terms of selectivity, DESs

pounds. In general phenylammonium salt DESs report higher S∞ than phenylphosphonium DESs. A comparison of the sigma profiles of methyltriphenylphosphonium (MTPP) and N,N,Ntrimethyl-1-phenylethanaminium (TPEA) cations shows that MTPP is less polar than TPEA, as it is observed that TPEA has a broader profile and less surface segments with neutral charge; whereas the sigma profile for MTPP is narrow and has more surface segments close to neutral charge. In summary, generally the selectivity at infinite dilution of the nitrogen compounds in decreasing order based on the influence of the cation is as follows: choline-based > tetramethylammonium > phenylammonium > tetramethylphosphonium > phenylphosphonium > tetrabutylphosphonium > tetrabutylammonium, where DESs with the last two cations report almost similar values of S∞. DESs with more polar cations will generally have higher values of S∞ compared to those with less polar cations. A similar approach of comparison is taken to investigate the effect of cation choice toward the capacity at infinite dilution, C∞. Overall, the DES choline fluoride/urea (ChF/Ur (1:2)) reports the highest value of C∞ for all nitrogen compounds except for pyridine. A comparison of the other choline-based DESs with the Cl anion and urea as the HBD with salt/HBD ratio 1:2 as shown in Figure 7a shows that the trend for C∞ of pyrrole in the DES follows the order: ethylcholine > acetylcholine > choline > N-benzyl-2-hydroxy-N,N-dimethylethanaminium > chlorocholine. Meanwhile for pyridine, the values of C∞ follows the order chlorocholine > acetylcholine > choline > ethylcholine > N-benzyl-2-hydroxy-N,N-dimethylethanaminium. The values of C∞ by choline-based DESs are less than unity for pyridine, and the difference between them is not very significant. The values of C∞ for DESs with TMA and TMP cations are also compared by keeping the anion and HBD constant for consistent comparison, and this is shown in Figure 7b. It is observed that DESs with a TMP cation give higher values of C∞ for both five-membered and six-membered nitrogen compounds. Similarly for tetrabutyl cations, Figure 7c shows that DESs with TBP cations have a higher capacity for nitrogen compounds than those with TBA cations. Conversely, it is observed from Figure 7d that DESs with phenylphosphonium cations have slightly higher values of C∞ for five-membered compounds than those with phenylammonium cations by about 4 % to 10 %. However, for six-membered compounds, DESs with phenylphosphonium cation report significantly higher values of C∞ by about twice the values of C∞ reported by DESs with phenylammonium cations. As the definition of C∞ is the inverse of γ∞ of nitrogen compound in the DES, higher values of C∞ indicate that the solute has smaller values of γ∞ in the solvent, thereby indicating a more favorable interaction. Thus, based on the higher values of C∞ of nitrogen compounds in DESs with phosphonium-based cations, the nitrogen compounds have better interaction with DESs with phosphonium cations than those with ammonium cations. Overall, in terms of capacity, the DESs with the following cations are arranged in the order of decreasing values of C∞: tetrabutylphosphonium ≥ tetrabutylammonium > tetramethylphosphonium > tetramethylammonium > phenylphosphonium > phenylammonium > choline-based cations. The trend in performance index (PI) generally follows the trend of selectivity because DESs with high values of S∞ usually will have a high performance index; because PI is the product of S∞ and C∞ as mentioned earlier.

Figure 8. Selectivity at infinite dilution in comparison of anion choice: (a) tetramethyl-based DESs; (b) tetrabutyl-based DESs. Nitrogen compounds: 1-PYR, 2-INDO, 3-INDL, 4-CAR, 5-BCAR, 6-PY, 7-QU, 8-BQU.

with TMA/TMP salts with a Br− anion give higher values of S∞ compared to those with a Cl− anion for both five-membered and six-membered nitrogen compounds. For example, with MA kept constant as the HBD, the DES TMAC/MA give higher values of S∞ for pyrrole and pyridine compared to the DES TMAB/MA. The values of S∞ are also higher for DES TMAB/ MA toward other five-membered (i.e., indole, indoline, carbazole, benzocarbazole) and six-membered compounds (i.e., quinoline, benzoquinoline). This trend is also true for DESs with TMP salts, where the values of S∞ for pyrrole and pyridine are higher for the DES TMPC/MA compared to the DES TMPB/MA, which is also true for all other five-membered and six-membered compounds. For DESs with TBA/TBP salts, mixed trends were obtained for the values of S∞ for five-membered and six-membered compounds. DESs with a TBA/TBP cation report higher values L



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of S∞ for five-membered compounds when combined with Cl− anion vs those with Br− anion. However, the difference in values is very small, that is, around 2 % to 10 %. The only exception from this trend is indoline, which as discussed in a previous section shows behavior similar to six-membered compounds instead. DESs with TBA/TBP cations report higher values of S∞ toward six-membered nitrogen compounds when combined with Br− anions, but lower values of S∞ are reported when they are combined with Cl− anions. The difference in values of S∞ for DESs with TBA/TBP cations with Cl− or Br− anions toward six-membered compounds is more significant than that toward five-membered compounds. In terms of capacity (Figures 9a,b), there is a mixed trend between DESs with Cl− and Br− anion. With the same

dilution. Observation of the trend on the effect of halide anion toward the performance index (PI) of the DESs indicates that the DESs with Cl− anions give higher values of PI for fivemembered nitrogen compounds while higher PI values were reported for six-membered nitrogen compounds by DESs with Br− anions. This is expected since PI is more influenced by the values of selectivity than the values of capacity; and we have observed that in general DESs with Cl− anions give higher selectivity for five-membered compounds and DESs with Br− anions give higher selectivity for six-membered compounds. From the sigma profile and sigma potential of the anions (Figures S8 and S17 in Supporting Information), Cl− is a stronger hydrogen-bond acceptor and it has a better affinity toward a hydrogen-bond donor compared to Br−. This can be observed from its sigma potential, where the values of μ(σ) of Cl− become more negative at σ ← 0.0084 eÅ−2. From the sigma profile, Br− has more surface segments with σ > + 0.0084 eÅ−2. Cl− is a smaller ion than Br− and it is more electronegative. The values of activity coefficient at infinite dilution (γ∞) of nitrogen compounds are smaller in DESs with Cl− anions. Conversely, γ∞ of a diesel compound is also smaller in DESs with Cl− anions than in those with Br− anions. Smaller values of γ∞ indicate that both nitrogen compounds and diesel compounds have favorable interaction with DESs with Cl− anions than with those with Br− anions. This explains why DESs with Cl− anions report greater values of capacity but lower values of selectivity, especially for five-membered nitrogen compounds. The higher values of C∞ toward sixmembered compounds reported by DESs with Br− anions indicate that six-membered compounds have a better affinity with those DESs. A possible explanation based on the analysis of the sigma profile and potential is that the six-membered compounds are weak hydrogen-bond acceptors, and have affinity toward hydrogen-bond donors. Compared to Br−, Cl− is a stronger hydrogen-bond acceptor and has a better affinity toward HBD species. Thus, six-membered compounds would rather mix with Br− compounds than with Cl− compounds to reduce the competition of creating hydrogen-bonding interaction with an available hydrogen-bond donor species. Effect of HBD. In this section, we will discuss the influence of the choice of HBD toward the selectivity and capacity of the DESs for the nitrogen compounds at infinite dilution. The DESs can be divided into two categories: (1) ChCl-based DESs with different HBDs, and (2) tetraalkyl salt-based DESs with different HBDs. Both categories of DESs are compared at the same salt/HBD molar ratio. It was observed that the choice of HBD has a significant effect toward the values of S∞, C∞, and thus the PI of the DESs for the nitrogen compounds. For ChCl-based DESs, there are DESs with a salt/HBD molar ratio of 1:1 and 1:2. To enable a consistent comparison, we will discuss the effect of HBDs based on a constant salt/ HBD ratio. Figure 10a is referred to for this discussion. ChClbased DESs with a salt/HBD molar ratio of 1:1 are ChCl/MA, ChCl/Xy, ChCl/DS, and ChCl/PPA; while those with a 1:2 salt to HBD molar ratio are ChCl/Ur, ChCl/TFA, ChCl/Gly, ChCl/EG, ChCl/DI, and ChCl/LA. The reason behind the choice of such DESs at specified molar ratios is because these are all real DESs which have been reported in the literature as liquid at room temperature. For ChCl-based DESs with a salt/ HBD ratio of 1:1, the values of S∞ for five- and six-membered nitrogen compounds according to HBD is as follows: MA > Xy > DS > PPA. For ChCl-based DESs with a salt/HBD ratio of 1:2, the DESs which reports the highest value of S∞ are ChCl/

Figure 9. Capacity at infinite dilution in comparison of anion choice: (a) tetramethyl-based DESs; (b) tetrabutyl-based DESs. Nitrogen compounds: 1-PYR, 2-INDO, 3-INDL, 4-CAR, 5-BCAR, 6-PY, 7-QU, 8-BQU.

tetramethyl-based salt cation and HBD, DESs with Cl− anion generally give higher C∞ for five-membered compounds but DESs with Br− anion give higher C∞ for six-membered compounds. However, for six-membered compounds, the values of C∞ are small (≤ 1 and the difference in the values of C∞ reported by DESs with Br− and Cl− anion is also very small; in some cases similar values of C∞ were obtained for the DESs. A similar trend was observed for DESs with tetrabutylbased salts with the same HBD, where DESs with Cl− anions report higher values of C∞ compared to those with Br− anions. When observing the effect of halide anion toward the selectivity and capacity at infinite dilution of the DESs for aromatic nitrogen compounds, it should be noted that the difference in values of S∞ for DESs with Cl− and Br− is more significant than the difference in values of C∞. In other words, the choice of halide anion has a greater impact toward the selectivity at infinite dilution than it has for capacity at infinite M



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Figure 10. Selectivity at infinite dilution in comparison of HBD choice: (a) ChCl-based DES; (b) TMAC-based DESs. Nitrogen compounds: 1-PYR, 2-INDO, 3-INDL, 4-CAR, 5-BCAR, 6-PY, 7-QU, 8-BQU.

Figure 11. Capacity at infinite dilution in comparison of HBD choice: (a) ChCl-based DES; (b) TMAC-based DESs. Nitrogen compounds: 1-PYR, 2-INDO, 3-INDL, 4-CAR, 5-BCAR, 6-PY, 7-QU, 8-BQU.

Ur and ChCl/TFA, followed by ChCl/Gly, ChCl/EG, ChCl/ DI, and ChCl/LA in decreasing order of S∞ values. In terms of C∞ values, Figure 11a shows an opposite order of the DESs. For ChCl-based DESs with 1:1 salt/HBD ratio, ChCl/PPA reports the highest value of C∞ for both five- and six-membered nitrogen compounds, followed by ChCl/MA, ChCl/DS, and finally ChCl/Xy. ChCl-based DESs with a 1:2 salt/HBD molar ratio reports the highest values of C∞ for all nitrogen compounds by ChCl/LA, followed by ChCl/Ur, ChCl/DI, ChCl/TFA, ChCl/EG, and last ChCl/Gly. From the trends, it was observed that ChCl-based DESs which give high selectivity, report low values of capacity, and those which report low selectivity, have high values of capacity. An exception to this observation is ChCl/Ur which reports high values of S∞ and C∞ for all aromatic nitrogen compounds. Oliveria et al.16 reported the use of ChCl-based DESs for the separation of azeotropic mixtures of ethanol and heptane. The DESs used in their study are ChCl/Gly, ChCl/EG, and ChCl/ LA with a salt/HBD ratio of 1:2. They reported that a high selectivity with a low distribution coefficient of ethanol was obtained with the DESs ChCl/Gly and ChCl/EG, whereas high distribution coefficients and low selectivity of ethanol were obtained with the DES ChCl/LA. The differences among the three HBDs (Gly, EG and LA) are in their functional groups, in which Gly and EG contains three and two −OH groups, respectively, while LA has a carboxylic group (−COOH) and a carbonyl group (CO). The presence of the carboxylic and carbonyl groups in LA enhances the polarization of the −OH group present in LA rather than those of alcohols in Gly and EG. This enhances the dipole strength and allows for a higher number and stronger dipoles which lead to a larger number and stronger hydrogen-bonding interaction to occur in the mixture of solute−DES. Furthermore, the total energy of hydrogen

bonding interaction for carboxylic acid is greater than that observed for other organic compounds containing −OH and/ or CO dipoles, thus ChCl with LA can form stronger hydrogen bonds with ethanol which leads to higher solutecarrying capacity and thus higher distribution coefficient. Conversely, for ChCl/Gly and ChCl/EG, the number of −OH groups in them present more groups capable of establishing hydrogen bonding with the −OH group of ethanol, thus it can extract ethanol more easily, and this is reflected in the higher values of selectivity. Analogous to our application in denitrification of diesel oil, where the main driving force is also based on a hydrogenbonding interaction as discussed in Figure 2, a similar explanation can be adopted to elucidate the behavior of ChCl-based DESs with different HBDs toward the selectivity and capacity for nitrogen compounds. For ChCl-based DES with 1:1 molar ratio, ChCl with phenylpropionic acid (PPA) reports the highest capacity due to the presence of −COOH and a phenyl group which increases the dipole strength and enhances the hydrogen-bonding interaction with the −N and −NH groups in the six- and five-membered nitrogen compounds, respectively. However, because of the presence of seven −CH groups in PPA, ChCl/PPA has a relatively favorable interaction with the diesel compounds compared to other HBDs with less −CH groups. This is reflected in the smaller value of γ∞ of diesel compounds in ChCl/PPA than in other ChCl-based DESs with 1:1 ratio, which then resulted in the low value of selectivity toward aromatic nitrogen compounds exhibited by ChCl/PPA. Meanwhile, ChCl with malonic acid reports high selectivity and high capacity for nitrogen compounds because of the presence of two −COOH groups in its dicarboxylic acid functional group. Not only does N



Article

(MA), tetraethylene glycol (TEG), caproic acid (CA), and acetic acid (AA) at a molar ratio of 1:2 will be discussed in this section. From Figure 10b the highest value of S∞ is reported when TMAC was combined with MA, followed by TMAC combined with Gly ≥ EG > AA ≥ PA > TEG > CA. In terms of capacity, Figure 11b shows that DESs combined with AA reports the highest value of C∞ followed by DESs combined with CA > PA > TEG > EG > Gly > MA for five-membered compounds. For six-membered compounds, the values of S∞ follows the order of DESs combined with AA > CA > PA > TEG > EG > MA > Gly. Although in this section only TMACbased DESs are discuessed for the sake of simplicity, the same trend was observed for other tetraalkyl-salt-based DESs, that is, DESs based on TMAB, TBAB, TMPC, TMPB, TBPC, and TBPB. As was explained in the discussion regarding ChCl-based DESs, we can observe that the same trend is also exhibited by the tetraalkyl salt-based DESs. DESs which give high values of S∞ report low values of S∞. TMAC and TBAC-based DESs combined with MA, Gly, or EG give high values of S∞ but low values of C∞ for both basic and nonbasic compounds, whereas, DESs with AA, CA, or PA give high values of C∞ and low values of S∞. DES with TEG sits somewhere in the middle but compared to EG, DES with TEG give lower S∞ and higher C∞. MA, Gly, and EG, as previously discussed with regards to ChClbased DESs, have two or three −OH groups in the molecule, which enables a higher number of groups for HB interaction with the nitrogen compounds and can thus extract them more easily, resulting in higher values of selectivity. Meanwhile, HBDs with carboxylic acid functional groups (AA, PA, and CA) give higher values of C∞ for the same reason as discussed previously in ChCl-based DESs (i.e., enhanced dipole strength due to −COOH group enabling stronger HB interaction with aromatic nitrogen compounds). Among these three carboxylic acids, the order of S∞ is AA > PA > CA; while the order of C∞ is AA > CA > PA for nitrogen compounds in all tetraalkyl-saltbased DESs. Overall, tetraalkyl salt-based DESs with AA as the HBD have the highest value of S∞ and C∞. This is because in acetic acid, there is a −COOH group enhancing the HB interaction while at the same time acetic acid has relatively unfavorable interaction with diesel compounds, unlike its phenyl derivative (PA) and caproic acid. Between PA and CA, CA has a higher capacity for aromatic nitrogen compound but it also has a more favorable interaction with diesel compounds due to the longer alkyl chain compared to the aromatic phenyl ring present in PA. It should also be noted that particularly for pyridine, DESs with PA have higher values of C∞ due to the similarity between the 6-membered pyridine ring and the phenyl ring. There is no explicit trend observed in the values of PI of the DESs on the effect of HBD choice. Similar to the observation in the previous sections, DESs with high values of S∞ tend to have high values of PI too due to PI being the product of S∞ and C∞. Overall, it can be deduced that DESs with HBDs from alcohol (ethylene glycol, glycerol) and an amide functional group (urea, trifluoroacetamide) will have high values of S∞ but with low values of C∞. An exception is DES with urea, where ChCl/urea reports high values both in S∞ and C∞. DESs with HBDs from the carboxylic acid functional group (acetic acid, phenylpropanoic acid, phenylacetic acid, caproic acid, levulinic acid) generally have high values of C∞ but low values of S∞. DESs with sugar alcohols as the HBD (D-sorbitol, xylitol, Disosorbide) generally do not give good values of either S∞ and

the presence of these carbonyl groups enhance the hydrogen bonding interaction with nitrogen compounds, but also it facilitates the extraction of nitrogen compounds via hydrogen bonding more easily. Besides, there are only three C atoms and only one of them is part of the −CH group, thus it does not have a good interaction with a diesel compound. For ChCl-based DESs with 1:2 molar ratio, ChCl with TFA and urea reports a high value of S∞ due to the presence of −CF3, CO, and −NH2 groups, which can all form hydrogen bonds with the nitrogen compounds. ChCl combined with TFA or Ur have more sites for hydrogen bonding, especially with nonbasic nitrogen compounds. In TFA, the −CF3 group acts as a hydrogen-bond acceptor while the −NH2 group can be both HB acceptor and HB donor. In urea, there are two −NH2 groups, enabling more sites for HB interaction. The presence of the CO group both in TFA and urea enhances the dipole moment of molecules for stronger HB interaction. Thus, they can extract the nitrogen compounds more easily and therefore the high values of S∞. Similarly, MA and EG have two −OH groups while Gly have three −OH groups, which also increases the number of sites for HB interactions, thus high values of S∞ are reported. Compared to ChCl/LA which report the highest value of C∞ but lowest value of S∞, LA is a keto acid (carboxylic acid + ketone). There are one CO group and one −COOH group in LA, which are strong enough to induce high dipole moment for the −OH site for enhanced HB interaction with the nitrogen compounds. However, LA has five carbon atoms, and three of these are −CH groups. Diesel compounds have a relatively more favorable interaction with compounds with longer carbon chain, and thus diesel compounds have smaller values of γ∞ in the DES ChCl/LA compared to other DESs where the HBDs have shorter carbon chain length. Similarly, ChCl/PPA also reports high value of C∞ for both basic and nonbasic compounds, but low values of S∞ are reported. Same explanation on the HB interaction strength due to the presence of −COOH group can be used and the presence of a phenyl group in the PPA molecule adds up the total carbon number to 8, making ChCl/PPA have good interaction with diesel compounds, which contributes to the low value of S∞. ChCl-based DESs with sugar alcohols as HBD (DI, DS, and Xy) do not have a good value of S∞ and C∞ as the hydrogen bonding formed with the −OH groups attached to the sugar alcohol is not as strong as those formed with the −OH groups attached to the other HBDs with different functional groups (e.g., carboxylic acids, amides, short-chained alcohols, and keto acid). Meanwhile, it was observed that ChCl/Ur has high values of both S∞ and C∞ toward the nitrogen compounds. Urea has many sites for HB interaction with both basic and nonbasic compounds. There are two −NH2 groups in urea which can both act as HB acceptor and donor due to the presence of a lone pair of electrons at the N atoms. Urea also has only one carbon atom, which makes the interaction with diesel compounds unfavorable. Thus, ChCl combined with urea can form strong and a large number of HB interactions, while at the same time not having favorable interaction with the diesel compounds. This is ideal if we were looking for a DES which can extract nitrogen compounds easily (high selectivity) and have large solute-carrying capacity (high capacity). For DESs based on tetraalkyl salts, we will compare the DESs with constant cation, anion, and molar ratio. Comparison among the DESs with tetramethylammonium chloride (TMAC) salt combined with the HBDs glycerol (Gly), ethylene glycol (EG), phenylacetic acid (PA), malonic acid O



Article

C∞. The ability to establish a strong network of hydrogen bonding and interaction with the diesel compounds are the main contributing factor toward the values of S∞ and C∞ for nitrogen compounds. Effect of Salt/HBD Molar Ratio. In this section, the effect of salt/HBD molar ratio toward the values of S∞ and C∞ and its PI will be discussed by comparing the DESs of the same cation, anion, and HBD. The DESs studied in this section are methyltriphenylphosphonium bromide (MTPPB) combined with glycerol (Gly), ethylene glycol (EG), or triethylene glycol (TrEG) at molar ratios between 1:2 and 1:5. Figure 12 shows the values of S∞ for nitrogen compounds by the DESs with varied molar ratio. In terms of selectivity at

compounds. An insignificant effect when changing the molar ratio is observed for DES MTPPB/TrEG, where the values of S∞ are very close for all the nitrogen compounds. For basic compounds, DESs with higher molar ratios report higher values of C∞. Similarly for selectivity, the values of C∞ for quinoline and benzoquinoline are at a plateau for DESs at different molar ratio. In summary, increasing the salt/HBD molar ratio will lower the values of S∞ and C∞ for nonbasic compounds while for basic compounds, the increase in molar ratio will lead to higher values of S∞ and C∞. This indicates that for nonbasic compounds, there is less favorable interaction when there is more HBD in the DES as the value of γ∞ increases with increasing HBD ratio. This is in concurrence with the nature of nonbasic compounds which have a better affinity toward a hydrogen-bond acceptor than a hydrogen-bond donor due to themselves being a weak HBD. Meanwhile for basic compounds, there is more favorable interaction when there is more HBD, and this is reflected in the decreasing values of γ∞ of basic compounds in the DES with higher ratios of HBD. Also as has been discussed in section 4.3.1, basic compounds are hydrogen-bond acceptor and have better affinity for hydrogenbond donor than nonbasic compounds due to the presence of −NH in their structure. Thus, having more HBDs will strengthen the HB interaction with the DESs, which subsequently gives higher values of S∞ and C∞. However, it was observed from the trend that the salt/HBD molar ratio does not have a significant effect on the values of S∞ and C∞. Although there seems to be some increase or decrease in the values of S∞ and C∞ when the molar ratio is varied, the change is not substantial in impacting the operating or capital cost when such DESs are used as solvent for extractive denitrification. The same observation follows for the values of PI of the DESs, where because of the insignificant effect that the salt/HBD molar ratio has on the values of S∞ and C∞, there is not much difference in the values of PI of the DESs when the salt:HBD molar ratio is varied.

Figure 12. Selectivity at infinite dilution in comparison of a salt/HBD molar ratio. Nitrogen compounds: 1-PYR, 2-INDO, 3-INDL, 4-CAR, 5-BCAR, 6-PY, 7-QU, 8-BQU.

infinite dilution, S∞, in general for nonbasic compounds, DESs with lower salt:HBD molar ratio give higher values of S∞ than those with a higher salt/HBD molar ratio. An exception for this trend is indoline, which exhibits a trend similar to the basic compounds instead. For basic compounds, DESs with a higher salt/HBD molar ratio give higher values of S∞. However, the difference is very small especially for the DES MTPPB/TrEG where varying the molar ratio does not have a significant effect toward the values of S∞, especially for multiaromatic basic nitrogen compounds (i.e., quinoline and benzoquinoline). In terms of capacity at infinite dilution, C∞, as presented in Figure 13, for nonbasic compounds, the DESs with lower molar ratios report higher values of C∞ for all DESs and all nonbasic

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CONCLUSION A total of 94 DESs based on different combinations of salt cation, anion, HBD, and salt/HBD molar ratios has been screened via the COSMO-RS approach for possible use in extractive denitrification of diesel. This is done by predicting the selectivity, capacity, and performance index at infinite dilution (S∞, C∞, and PI, respectively) using the COSMO-RS calculated values of the activity coefficient at infinite dilution, γ∞ of the nitrogen compounds in each DES. The nitrogen compounds studied include basic or five-membered compounds (pyrrole, indole, indoline, carbazole, and benzocarbazole) and nonbasic or six-membered compounds (pyridine, quinoline, and benzoquinoline). Hydrogen-bonding interaction is the main driving force in removal of nitrogen compounds from diesel using DES as the extraction solvent. The heterocyclic structure of nitrogen compounds influence the values of S∞ and C∞ such than the nonbasic compounds have higher values of both S∞ and C∞ than the nonbasic compounds. The values of S∞ and C∞ also decrease with increased aromatic rings in their structure. In general, DESs with ammonium-based cations give higher values of S∞ but lower values of C∞ while phosphonium-based DESs give higher values of C∞ and lower values of S∞. For the effect of cation choice toward selectivity, the selectivity ranking of the DES cations is as follows: choline-based > tetramethylammonium >

Figure 13. Capacity at infinite dilution in comparison of the salt/HBD molar ratio. Nitrogen compounds: 1-PYR, 2-INDO, 3-INDL, 4-CAR, 5-BCAR, 6-PY, 7-QU, 8-BQU. P



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phenylammonium > tetramethylphosphonium > phenylphosphonium > tetrabutylphosphonium > tetrabutylammonium. Meanwhile, the capacity ranking of the DES follows the decreasing order of tetrabutylphosphonium ≥ tetrabutylammonium > tetramethylphosphonium > tetramethylammonium > phenylphosphonium > phenylammonium > choline-based cations. A mixed trend is observed for the effect of halide anions toward the values of S∞ and C∞. DESs with halide salt show differing values of S∞ and C∞ toward basic and nonbasic compounds when combined with different cations. Tetramethyl-based DESs (both ammonium and phosphonium-based) give high values of S∞ for all basic and nonbasic compounds when the anion is combined with Br− anions versus when combined with Cl− anions. However, the same types of DESs give higher values of C∞ when combined with Br− anions for all nitrogen compounds. Tetrabutyl-based DESs (both ammonium and phosphonium-based) give higher values of S∞ for nonbasic compounds when combined with Cl− anions, while higher values of S∞ are reported for basic compounds when the DESs are combined with Br− anions. In terms of C∞, tetrabutyl-based DESs give high values of C∞ for nonbasic compounds when combined with Br− anions and high values of C∞ are reported for basic compounds when combined with Cl− anions. The choice of HBD has a noteworthy effect toward the values of S∞ and C∞, which is essentially due to the functional group making up the HBD structure. It was observed that DESs combined with HBD from alcohol and amide functional groups give high values of S∞ but low values of C∞. An exception to this trend is urea, a HBD with amide groups, where ChCl combined with urea give high values both in S∞ and C∞. The DESs with a HBD from the carboxylic acid group give high values of C∞ but low values of S∞. HBDs from sugar alcohol groups (xylitol, D-sorbitol, D-isosorbide) are not good candidates to make up DESs for the extractive denitrification process. Varying the salt/HBD molar ratio does not have a significant effect toward the values of S∞ and C∞, although some increase and decrease S∞ and C∞ was observed when the molar ratio was changed. In conclusion, the choice of cation, anion, HBD, and salt/ HBD molar ratio needs to be put into consideration when selecting the appropriate DESs for extractive denitrification process. In general there is no ideal DES which gives high values of both S∞ and C∞. From an economics point of view, the operator may decide the choice of DES on the basis of its ability or on his willingness to spend either on capital or operating cost, because selectivity has a direct impact toward the capital cost (especially the number of stages for extraction process), while the capacity or distribution ratio has a direct impact on the operating cost (in terms of the flow rate of the solvent used for the extraction process). Apart from the criteria of selectivity and capacity, the selection of DESs as solvent must also take into account the physical and transport properties of the DESs for industrial application. It should also be noted that the hypothetical DESs included in the screening process in this work may or may not exist at the specified ratio and be liquid at room temperature. Nevertheless, to select DESs for an application of interest, a systematic approach needs to be taken and one of the approaches that may be used is the predictive quantum chemical model COSMORS, keeping in mind the limitations of the COSMO-RS such that the results are appropriate only for preliminary screening rather than design calculation.

Article

ASSOCIATED CONTENT

S Supporting Information *

Detailed results on the selectivity, capacity, and performance index at infinite dilution, sigma profiles, and sigma potentials. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 603-7967-5295. Fax: 603-7967-5319. Funding

This research was carried out with funding from University of Malaya under the HIR Grant No. HIR-MOHE (D00000316001) in collaboration with the Deanship of Scientific Research at King Saud University through the international research group number IRG14-13. Notes

The authors declare no competing financial interest.

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