Removal of Thiophene from Mixtures with n-Heptane by Selective

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Removal of thiophene from mixtures with n-heptane by selective extraction using deep eutectic solvents Mohamed Kamel Hadj-Kali, Sarwono Mulyono, Hanee Farzana Hizaddin, Irfan Wazeer, Lahssen El-blidi, Emad Ali, Mohd Ali Hashim, and Inas Muen AlNashef Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b01654 • Publication Date (Web): 07 Jul 2016 Downloaded from http://pubs.acs.org on July 9, 2016

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Industrial & Engineering Chemistry Research

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Removal of thiophene from mixtures with n-heptane by

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selective extraction using deep eutectic solvents

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Mohamed K. Hadj-Kali*a, Sarwono Mulyonoa, Hanee F. Hizaddinb, Irfan Wazeera, Lahssen El-

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blidia, Emad Alia, Mohd. Ali Hashimb, Inas M. AlNashefc a

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Chemical Engineering Department, College of Engineering King Saud University, P.O.Box 800, Riyadh, 11421, Saudi Arabia.

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b

University of Malaya Center for Ionic Liquids (UMCiL), Department of Chemical Engineering, Faculty of Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia.

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c

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*Corresponding Author. Email address: [email protected]

Department of Chemical and Environmental Engineering, Masdar Institute of Science and Technology, Abu Dhabi, United Arab Emirates.

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Abstract

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This work investigates the use of deep eutectic solvents (DESs) to extract sulfur-based

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compound from n-heptane as model diesel compound. Four DESs were prepared by combining

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tetrabutylammonium bromide or methyltriphenylphosphonium bromide with ethylene glycol,

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triethylene glycol or sulfolane. All DESs showed good ability to extract thiophene with the best

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extraction efficiency, 35%, for the sulfolane-based DES. The extraction efficiency can be further

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enhanced to reach 98% when five extraction cycles are performed. Moreover, the DESs were

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easily regenerated using rotary evaporation. In addition,1H NMR analysis is used to elucidate the

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extraction mechanism. Finally, the COSMO-RS model was used to predict the ternary tie lines

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for the studied systems and the NRTL model allowed correlating the experimental data with an

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average root mean square deviation lower than 2% for all DESs. These models can be utilized

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for further simulation analysis of the extraction process.

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Keywords: Desulfurization, Liquid-liquid Extraction, Deep Eutectic Solvents, COSMO-RS,

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NRTL. 1 ACS Paragon Plus Environment


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1. Introduction

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The rapid growth of the world’s population accompanied by the increase in the standard of

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living over the last several decades has caused environmental pollution by the emission of SOx

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to continue increasing. Stringent legislation has been implemented all over the world to regulate

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the sulfur content of transportation fuels. Hydrodesulfurization is the conventional industrial

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process for removing sulfur containing compounds from liquid fuels.1 However, this process

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operates under severe conditions with high pressure and temperature2 and is not efficient for

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deep removal of refractory heterocyclic sulfur compounds such as thiophene, benzothiophene,

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dibenzothiophene and its derivatives.3-6 Thus, developments of innovative complementary

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technologies for fuel oil processing such as extraction, adsorption, oxidation and bio-

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desulfurization have been sought during the last few decades. Extractive desulfurization is one of

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the most promising desulfurization method because it operates under mild conditions and have

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simple separation mechanism.7 Volatile organic compounds such as sulfolane, N-methyl-2-

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pyrrolidone and dimethylformamide have been used as conventional solvents in the extraction

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process in the last few decades.8 Hence, research efforts are focused on finding the most suitable

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solvent for use in deep desulfurization process which has less impact on the environment.

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Recently, the use of ionic liquids (ILs) to replace conventional solvents have been reported in

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various research work. ILs are gaining increasing attention owing to their favorable chemical and

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physical properties5 including high thermal stability, non-volatility and recyclability.9-13 A

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comprehensive review on the application of ILs on desulfurization of fuel oils can be referred in

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a review done by Francisco et al.14

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In the past decade, ILs analogues called deep eutectic solvents (DESs) have emerged as

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alternatives to conventional solvents and ILs.15,16 A DES is a mixture of two or more components

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that has a melting point less than that of any of its individual constituent. DESs are usually 2 ACS Paragon Plus Environment

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composed of salts combined with a hydrogen-bond donor (HBD) or a complexing agent. DESs

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have many advantages over ILs as they can be easily synthesized with high purity and they are

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generally cheaper than ILs due to the simple synthesis process and their cost depend on the cost

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of the constituent components. By carefully choosing the components of the DES, it is possible

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to obtain non-toxic and biodegradable solvents. In addition, most DESs do not react with water.

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DESs have been used extensively in various applications including material preparation,17

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electrochemistry,18 purification of palm oil,19 enzyme catalysis20 and separation processes.21

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DESs have good dissolution capabilities for different substances because they can form hydrogen

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bond by receiving or giving electrons or protons. However, reports on the applications of DESs

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in separation processes are not as widespread as those using ILs. Among the earliest report on

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the use of DESs in separation processes is by Abbott et al. who used a mixture of quaternary

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ammonium salts with glycerol to extract excess glycerol from soybean oil-based biodiesel.21

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Later, Guo et al. used DESs to extract phenol from oil22 and achieved high phenol extraction

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efficiencies by using quaternary ammonium salts based DESs which could reach up to 99.9%.

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They also reported that the extraction efficiency was not sensitive to temperature. Shahbaz et al.

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investigated eighteen DESs for the removal of residual catalyst content from palm oil-based

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biodiesel23 and reported that the removal efficiency of the residual catalyst increased with the

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increase in DES:biodiesel molar ratio.

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More recently, sixteen DESs were experimentally screened by Li et al. to extract

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benzothiophene from n-octane for use in desulfurization of liquid fuels.1 They stated that choline

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chloride/propionate (1:2), tetrabutyl ammonium chloride/propionate (1:2) and tetrabutyl

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ammonium chloride / polyethylene glycol (1:2) DESs reported the highest extraction efficiency

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and that the extraction of benzothiophene from n-octane can be conducted up to five times

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without significantly compromising the extraction efficiency. However, after six times of

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repeated use, the DESs became saturated and lost their extraction capacity.

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Previously, our group has reported liquid-liquid equilibrium data on the use of imidazolium

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and pyridinium-based ILs and ammonium- and phosphonium-based DESs to remove nitrogen

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compounds from n-hexadecane as model diesel fuel.24,25 The DESs used in our previous work

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are selected based on COSMO-RS screening and it was hypothesized that the DESs selected are

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highly potential for use in removal of aromatic sulfur compounds as well.26 This work, thus, aims

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at studying the effectiveness of using similar DESs to remove thiophene from liquid fuel. We

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showed that low cost DESs can be used for the removal of sulfur containing compounds using

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liquid-liquid extraction method. Four DESs have been tested where ethylene glycol (EG),

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triethylene glycol (TrEG) and sulfolane (Sul) were used as complexing agents, while

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tetrabutylammonium bromide (TBAB) and methyltriphenylphosphonium bromide (MeTPPBr)

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were chosen as typical salts. The extraction effectiveness was assessed experimentally and also

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numerically using two models: COSMO-RS and NRTL. Furthermore, the extraction mechanism

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with the best DES was investigated using 1H NMR analysis.

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2. Experimental Details

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2.1.Chemicals and experimental protocol

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All chemicals used in this work are listed in Table S1 in the Supporting Information and their

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chemical structures are listed in Table 1. They were of high purity (>98 wt.%) and were used

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without any further purification. The DESs were prepared according to the method described by

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Abbott et al.16 The different combinations tested are listed in Table S2 in Supporting Information

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along with their specific molar ratio. The mixtures were put in screw-capped bottles and then

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stirred in an incubating-shaker equipped with temperature and speed control at a temperature of 4 ACS Paragon Plus Environment

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100°C (±0.1°C) with a rotational speed of 200 rpm until a clear liquid was formed. For the

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mixture TBAB/sulfolane, the minimum molar ratio giving a liquid mixture at room temperature

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was 1:7. However, mixing MeTPPBr with either sulfolane or TrEG led to a solid mixture at

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room temperature even for higher molar ratios.

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The thermal stability of three TBAB-based DESs investigated in this work has been checked

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using TGA-50 thermogravimetric analyzer (Shimadzu, Kyoto, Japan) under air atmosphere with

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flux of 100 mL/min and a heating rate of 10 °C/min. approximately 3 mg of each DES sample

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were subjected to a temperature range of 25 to 400 °C. The analysis results are shown in fig. S1

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(Supporting Information). TGA analysis confirms that all three DESs are thermally stable and

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have similar behavior to their individual constituents. Moreover, it can be concluded from this

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analysis that the hydrogen bond interactions are stronger between TBAB and EG compared with

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TrEG which is also stronger than that between TBAB and sulfolane.

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On the other hand, feed mixture containing 10 wt% of thiophene in n-heptane was prepared

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by mixing weighed amounts of the chemicals using an analytical balance (±0.0001 g).The feed

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was then mixed with the DESs in a mass ratio of 1:1. Each set of experiments was conducted at

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25°C. The vials were placed in the incubator shaker. The shaking time was six hours followed by

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a settling time of about 12 hours to guarantee that the equilibrium state was completely attained.

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Samples were then taken from the top and bottom layers and analyzed using HPLC.

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2.2. Analysis

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Samples from the top and bottom layers were withdrawn using a syringe and then diluted using

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acetonitrile. The samples were analyzed using a HPLC Agilent 1100 series (USA) with a zorbax

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eclipse xdb-c8 column (Agilent, USA). The temperature of the column oven was set to 30°C.

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The mobile phase was acetonitrile and distilled water with a volume ratio of 3:1. The flow rate of

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the mobile phase was 1.4 ml min-1 with a pressure of 120 bars. Each sample was analyzed three 5 ACS Paragon Plus Environment


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times and the average is reported. The maximum uncertainty is estimated to be ±0.008 in mole

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fraction, while the average uncertainty doesn't exceed ±0.002. Details about uncertainty

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estimation and error propagation, as well as the calibration of thiophene are shown in the

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supporting information.

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Moreover, in order to highlight the extraction mechanism, both nuclear magnetic resonance

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spectroscopy (NMR) and FT-IR were used. 1H NMR spectra were recorded using a JEOL

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RESONANCE spectrome-ter ECX-500 II. Dimethyl sulfoxide (DMSO) was used as solvent. The

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infrared spectra were recorded on a IRTracer-100 FT-IR spectrometer interfaced (Shimadzu,

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Kyoto, Japan). The spectrum was obtained over 64 scans with a spectrum resolution of 4 cm−1 in

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the region 4000 to 400 cm−1.

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3. Results and discussion

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3.1. Consistency tests

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The Othmer-Tobias27 and Hand28 correlations were used to perform the consistency tests of

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the experimental results as shown by Eq. 1 and 2, respectively.

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Thio Thio

= +

hep

= +

DES

(Eq. 1)

(Eq. 2)

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Here xhep, xDES and xThio refer to the concentrations of n-heptane, DES and thiophene,

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respectively. Fitting parameters of the Othmer-Tobias correlation are a and b, while c and d are

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the fitting parameters of the Hand correlation, whereas ' and '' refers to the extract and raffinate

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phases, respectively. The value of R2 close to unity (the linearity of each plot) shows the

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excellent degree of consistency of the data. The parameters of the Othmer-Tobias and Hand

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correlations are listed in Table 2.


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3.2. Distribution ratio and selectivity

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Table S3 in the Supporting Information shows the experimental tie lines with the values of

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distribution coefficient D, and selectivity S, for the four ternary systems (thiophene (1) + n-

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heptane (2) + DES (3)) where the compositions are expressed in mole fractions. Figure 1 depicts

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the corresponding triangular phase diagrams for each system. The values of distribution

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coefficient obtained in this work are all less than unity, which indicates that a relatively large

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quantity of solvent is required in order to achieve high separation effectiveness. Nevertheless, the

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values of selectivity reported here are encouraging. Figures. 2 and 3 show the plot of distribution

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coefficient and selectivity as a function of thiophene concentration in the raffinate phase for

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different DESs. TBAB/Sul (1:7) gave the highest values of distribution coefficient followed by

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TBAB/TrEG (1:4), MeTPPBr/EG (1:4) and TBAB/EG (1:4). This can be attributed to the higher

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molar ratio of the HBD in TBAB/Sul (1:7) DES compared to other DESs. For selectivity, at low

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concentrations the TBAB/TrEG (1:4) gave the highest values followed by TBAB/Sul (1:7),

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TBAB/EG (1:4) and MeTPPBr/EG (1:4). However, as the concentration of thiophene in the

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raffinate increases, the TBAB/TrEG (1:4) gave lower value of S compared to the other DESs.

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Besides, for the best DES (TBAB/Sul (1:7)), the 1H NMR of the raffinate phase has shown that

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no sulfolane is detected in the top layer (see fig. S2 in the supporting material).

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Moreover, we have compared the selectivity and distribution ratio results obtained in this

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work with those available in the literature for more than 20 ionic liquids and pure sulfolane. This

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comparison is provided in table S4 as supporting material. In overall, it is obvious that many

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ionic liquids show better extraction performances than all DESs investigated in this work.

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However, if we consider the relative high price and the complicated synthesis procedures of

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ionic liquids compared to DESs, we can conclude that DESs could be considered as potential

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alternatives especially for industrial scale applications. When compared to pure sulfolane, we can 7 ACS Paragon Plus Environment


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see that the salt has almost no effect on the extraction performance of thiophene (selectivity and

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distribution ratio values are almost the same). However, the addition of TBAB has other

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advantages compared to the pure sulfolane, since (i) no cross contamination is observed between

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the two layers with TBAB/Sul DES and (ii) the operating temperature is lower than in the

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classical process. So, low energy consumption is required when using the DES.

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3.3. Thiophene extraction efficiency

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The following equation is used to calculate the extraction efficiency.

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Extraction efficiency '%) =

*1 −*2 *1

× 100

(Eq. 3)

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Where C1 is the initial concentration of thiophene in fuel and C2 is the final concentration of

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thiophene in the raffinate phase after extraction with the DES. The thiophene extraction

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efficiency of each DES is given in Table 3. As expected the highest extraction efficiency is for

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TBAB/Sul (1:7). It can be noted that both the salt and HBD affect the extraction efficiency.

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However, in most cases the difference is not significant. The results obtained in this work are

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somewhat comparable with those reported in the literature for the same type of oil mixtures. For

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example, Jian-long et al.29 studied the extraction of thiophene from thiophene, n-heptane, xylol

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mixture using five different ionic liquids. The reported sulfur removal percentage, with IL to oil

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mass ratio of 1:1, ranges from 21.8% to 45.5% depending on the type of IL used. This means that

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the DES investigated in this work has slightly lower extraction ability (ranging from 16% to

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35%). However, we used binary oil mixture while ternary oil mixture was utilized in the

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aforementioned work.29 Alonso et al. showed that using [C8mim][BF4] IL, thiophene recovery

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can reach 79 wt% when three cycles are used.30,31 However, they used different oil mixtures

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which contain thiophene in i-octane, in n–hexane, or in toluene.


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3.4. Extraction mechanism

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1

H NMR and FT-IR analysis were used to elucidate the mechanism of extracting thiophene

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using DESs. Fig. 4 shows the 1H NMR analysis of pure TBAB, TBAB with sulfolane in DES

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form, and different molar ratios of DES/thiophene. First, the comparison of free TBAB and DES

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showed that the chemical shifts of all peaks of TBAB decreased, this shielding effect is due to

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interactions between oxygen atoms of sulfolane and the ammonium cation in TBAB. These

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interactions lead to the decrease of ammonium cation electron withdrawing effect on all TBAB

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hydrogen atoms, especially atoms adjacent to it such as 1 (moves from 3.360 ppm to 2.966 ppm).

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The addition of thiophene to DES causes an opposite effect, that is to say, all peaks of TBAB

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return to values very close to that in pure TBAB. This shows that the bromide anion in TBAB

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interacts with thiophene aromatic ring. Moreover, it can be noted that oxygen atoms in sulfolane

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created new interactions with the sulfur atom in thiophene. The result of these interactions is the

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decreasing of the electron withdrawing power of sulfur atom over hydrogen atoms in thiophene

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ring and increasing the electronic density. These effects are reflected in the values of chemical

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shifts of hydrogen atoms of thiophene and sulfolane. Fig. 4 shows that the hydrogen atoms in

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thiophene moved to higher field intensities with increasing of thiophene:DES ratio from 0.4:1to

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1:1. For example, H atom in position 1 (7.437 ppm, 7.420, 7.255, 7.242) and for position 2

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(7.232 ppm, 7.188, 7.034, 7.020). On the other hand, for sulfolane, these interactions resulted in

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the increasing of electron withdrawing power of sulfur atom, and its hydrogen atoms moved in

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lower field intensities (from 1.923 ppm, 2.700 in DES/thiophene: 1/0.4 to 2.050 ppm, 2.860 in

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DES/thiophene: 1/1 respectively). Therefore, the results of 1H NMR analysis showed the

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existence of interactions between DES and thiophene, and it can be concluded that these

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interactions contributed to the extraction efficiency.



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The FT-IR analysis confirms the extraction mechanism. Three samples have been analyzed:

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(i) pure thiophene, (ii) thiophene – sulfolane mixture and (iii) thiophene – (TBAB – Sulfolane)

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DES mixture, as shown in fig. 5. As shown in this figure, FT-IR spectra confirms the existence

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of interactions between thiophene and pure sulfolane (the green line) and also between thiophene

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and DES system (the blue line). The main modifications due to the addition of sulfolane alone or

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the DES mixture are observed on the signals relative to the symmetric (1408 cm-1) and the anti-

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symmetric (1590 cm-1) carbon double bond (C=C) stretching vibration bands of the thiophene

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rings. These bands were moved to higher wavenumbers (from 1408 to 1415 and from 1590 to

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1604 (with sulfolane) then to 1625 cm-1 (with the DES)) and became less stretched and stretched.

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These effects could be attributed to the interactions between thiophene ring and oxygen atoms of

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sulfolane and these interactions were conserved by the addition of the salt in the DES mixture.

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3.5. Number of extraction stages

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In order to determine the number of cycles (stages) necessary to extract all thiophene from

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thiophene/n-heptane mixture using TBAB:Sulfolane DES with molar ratio 1:7, we have

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separated the top layer (n-heptane rich phase) from the bottom layer (DES rich phase) and we

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have added the same amount of DES to the raffinate phase. This procedure was repeated many

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times and the thiophene concentration for each step was determined using HPLC analysis.

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Fig. 6 shows the HPLC chromatograms of thiophene in the raffinate phase after each

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extraction step. As can be seen, the peak representing thiophene decreased gradually until

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reaching a total removal of about 98% after six extraction cycles. Therefore, after only five steps,

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it is possible to reduce the sulfur content of the model fuel (n-heptane) to less than 0.02 of its

235

initial value.


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3.6. Deep eutectic solvent regeneration

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The regeneration of the extraction solvent and its reuse has economic and environmental

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benefits. For this purpose, the back extraction and rotary evaporator process were used. In our

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case, the regeneration of TBAB/Sul (1:7) DES was performed by using a rotary evaporator under

240

100 mbar vacuum and 40°C. After the first extraction, the two phases were separated using a

241

separating funnel and thiophene was removed from the bottom layer (DES-rich phase) using a

242

rotary evaporator. Then, the recovered DES was used for new extraction cycle. As can be seen

243

from Fig. 7, even after four cycles, the performance of recycled DES was as effective as that of a

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fresh one. In this figure, the extraction performance reflects the quantity of thiophene trapped

245

and is expressed in (%) based on the first extraction cycle. Therefore, these advantages, i.e. easy

246

recycling and maintained extraction efficiency, make this DES an attractive solvent for

247

desulfurization.

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On the other hand, based on the normal boiling point data of thiophene and different

249

hydrogen bond donors (listed in table S5), it is clear that the same technique (rotary evaporation)

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can also be used for all DESs investigated in this work.

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4. LLE Modeling

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4.1. COSMO-RS predictions

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The Conductor-like Screening Model for Real Solvents is useful to describe the

254

thermodynamic behavior of pure compounds and mixture of compounds. The only input required

255

in COSMO-RS calculation is the chemical structure of the species involved in order to predict

256

the properties and behavior of the compounds and mixture of compounds. In COSMO-RS,

257

molecules are placed in a virtual conductor environment and divided into segments. Interaction

258

energies are calculated based on the energetic cost of removing the virtual conductor 11 ACS Paragon Plus Environment


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environment which is described in terms of the screening charge density, σ. Klamt and co-

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workers derived the mathematical expressions for the interaction energies, chemical potential,

261

and activity coefficient and these can be referred in their work32,33 and are also summarized in

262

our previous works. 26,34,35

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In order to do predictions using COSMO-RS, a .cosmo file must be generated for all

264

compounds i.e. cations, anions, HBD, thiophene, and n-heptane. First, geometry optimization

265

was performed at Hartree-Fock level and 6-31G* basis set. This was followed by a single point

266

calculation using density functional theory combined with Becke-Perdew functional and triple

267

zeta valence potential (TZVP) basis set to generate the .cosmo files. All computational work

268

were performed using TURBOMOLE software package.36 Thereafter, ternary LLE tie lines

269

based on the experimental feed composition of each ternary system were generated using

270

COSMOthermX software package with the parameterization file BP_TZVP_C30_1401.ctd.37

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Description of ILs in COSMO-RS approaches are i) the electro-neutral approach, ii) the ion

272

pair approach, and iii) the meta-file approach. The first approach i.e. the electroneutral approach

273

is widely accepted to be the closest representation of ILs in COSMO-RS because it describes the

274

cation and anion as separate species in a liquid mixture. In this work, the same approach is

275

considered to represent DESs in COSMO-RS. The mathematical adaptation can be referred to in

276

our previous works. 26,34,35

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Figs. 1 (a) – (d) show the ternary tie lines calculated using COSMO-RS along with the

278

experimental tie lines. The goodness of fit between the COSMO-RS predicted tie lines and the

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experimental tie lines was evaluated using the root mean square deviation (RMSD) as defined in

280

Equation 4, where x is the concentration of species in mole fraction, and the subscripts i, j, and k


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designate the component, phase, and tie line, respectively, and m is the number of tie lines, c the

282

number of components, and j refers to the phases. 678 :78 ; ? = 0123 '%) = 1004∑> CA ∑BA ∑@A 9

283

9

=>?

Removal of Thiophene from Mixtures with n-Heptane by Selective

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