One-pot oxidative desulfurization of fuels using dual-acidic deep eutectic solvents

Fuel 265 (2020) 116967

Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

Full Length Article

One-pot oxidative desulfurization of fuels using dual-acidic deep eutectic solvents ⁎

Wei Liu, Tenghui Li, Guojia Yu, Jiangang Wang, Zhiyong Zhou , Zhongqi Ren

T

⁎

College of Chemical Engineering, Beijing University of Chemical Technology, No. 15, North Third Ring Road East, Beijing 100029, People’s Republic of China

A R T I C LE I N FO

A B S T R A C T

Keywords: Dual-acidic deep eutectic solvent Oxidative desulfurization Aromatic sulfur compound Hydrogen bond

Four dual-acidic deep eutectic solvents (DESs) were synthesized and investigated in an oxidative desulfurization (ODS) system using hydrogen peroxide as oxidant. L-Pyroglutamic acid/trifluoracetic acid (L-Pyro/TFA) was further investigated as the extractant and catalyst owing to its excellent properties for removing aromatic sulfur compounds. Hydrogen bond formation by L-Pyro/TFA was confirmed using 1H NMR and infrared spectra. The factors affecting the desulfurization efficiency, such as the DES/model oil volume ratio (VDES/VOil), reaction temperature, and H2O2/dibenzothiophene (DBT) molar ratio (n(H2O2)/n(DBT)), were explored. The sulfur removal rates of DBT, 4,6-dimethyldibenzothiophene, and benzothiophene were up to 99.7%, 99.6%, and 99.2%, respectively, under the optimum conditions, which were VDES/Voil = 1:20, n(H2O2)/n(DBT) = 3, and T = 60 °C. The ODS reaction followed first order kinetics through dynamic analysis. Furthermore, the desulfurization efficiency reached 98% after DES regeneration, achieved by simply washing with ultrapure water.

1. Introduction Sulfur compounds found in fuel oil can be converted into sulfur oxides (SOx) by combustion, which has adverse effects on the environment and human health. For environmental protection purposes, stringent legislation has been passed worldwide to limit the maximum allowable sulfur content in fuel oil, which is 10 ppm [1]. Hydrodesulfurization (HDS) and nonhydrodesulfurization are two methods for obtaining ultralow-sulfur fuel oil [2]. HDS technology, which removes sulfides by converting them into H2S using hydrogen and a catalyst under high pressure (3.5–7.0 MPa) and temperature (300–400 °C) conditions, has been applied in industrial production, resulting in high costs [3,4]. Furthermore, aromatic sulfur compounds [5], such as benzothiophene (BT), dibenzothiophene (DBT), and 4,6dimethyldibenzothiophene (4,6-DMDBT), are difficult to remove using HDS. Recently, oxidative desulfurization (ODS), a nonhydrodesulfurization method, has attracted research attention owing to its excellent desulfurization capacity for removing thiophene sulfur under mild operating conditions. The ODS system consists of an oxidative agent, such as hydrogen peroxide [6,7], molecular oxygen [8–12], and organic peroxides [13–16], and catalyst, such as ionic liquids (ILs) [17–19], polyoxometalates [20–26], organic acids [27,28], and metal oxides [29–31]. For one-pot ODS, various acidic ILs, such as [HCPL][TFA] [17], [C4mim][TFA] [32], [CH2COOHmim]HSO4 [33],

⁎

[CH2COOHPy][HSO4] [34], [Bmim][HSO4] [35], [HDBN]Cl/nZnCl2 [18], [ODBU]Cl/nZnCl2 [36], and [Omim]FeCl4 [19], which act as both extractant and catalyst, have recently been reported for desulfurization. As deep eutectic solvents (DESs) have similar physical and chemical properties to ILs [37], they have shown potential for removing organic sulfur since first being applied to extraction desulfurization (EDS) in 2013 [38]. By constructing DESs with a hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD) through hydrogen bond interactions, their atom utilization can reach 100%, as well as being easier to synthesize and more environmentally friendly than ILs [39]. Furthermore, the raw materials for DES synthesis are inexpensive and readily available, which reduces production costs [40]. In the field of desulfurization, the use of DESs in ODS [41–44], EDS [45–50], and SO2 gas absorption [51–54], among others, has recently been widely reported. Compared with ODS, the amount of model fuel that can be treated by EDS in a single stage is limited. A series of Brønsted–Lewis acidic DESs, such as propionic acid-based (C3H6O2/X ZnCl2) and phenylpropanoic acid-based DESs (C9H10O2/X ZnCl2), have been synthesized and investigated in ODS systems [41,42]. Using hydrogen peroxide as oxidant, the desulfurization efficiencies of C3H6O2/0.5 ZnCl2 and C9H10O2/0.5 ZnCl2 for dibenzothiophene (DBT) were up to 99.42% and 99.23%, respectively. Xu et al. [55] prepared several metal-based DESs (CoCl2-ChCl/2PEG) that almost completely removed DBT using solid peroxymonosulfate (PMS) as oxidant. Li et al. [44] investigated the

Corresponding author. E-mail addresses: [email protected] (Z. Zhou), [email protected] (Z. Ren).

https://doi.org/10.1016/j.fuel.2019.116967 Received 28 July 2019; Received in revised form 15 December 2019; Accepted 27 December 2019 0016-2361/ © 2019 Published by Elsevier Ltd.

Fuel 265 (2020) 116967

W. Liu, et al.

where C0 and Ct are the initial sulfur concentration and transient sulfur concentration at any time, respectively, in the model fuels.

desulfurization effect of choline chloride/p-toluenesulfonic acid (ChCl/ p-TsOH) and tetrabutylammonium chloride/p-toluenesulfonic acid (TBAC/p-TsOH), with both achieving sulfur removal rates of up to 99.99%. Notably, ChCl/p-TsOH and TBAC/p-TsOH achieved 97.25% and 95.90% sulfur removal rates, respectively, in a real oil desulfurization system. Amino acids are used as HBAs in DES synthesis [56] owing to their nontoxic and inexpensive properties. Lü et al. [43] proposed using L-proline-based DESs for sulfur removal, obtaining 99% desulfurization efficiency for DBT, 4,6-DMDBT, and BT. However, despite the high desulfurization rate, introducing metal into DESs is not environmentally friendly. Furthermore, the lower volume ratio of DESs to fuel oils means that the amount treated in one-stage desulfurization is low, limiting its industrial application. Therefore, the development of an efficient and eco-friendly catalyst for one-pot ODS is needed. In this work, several dual-acidic DESs, including L-pyroglutamic acid-based DESs (L-Pyro/TFA, L-Pyro/Formic acid) and L-proline-based DESs (L-Pro/TFA, L-Pro/Formic acid), were synthesized and studied in the ODS system with hydrogen peroxide as oxidant. The effects of reaction temperature, time, DES/model oil volume ratio, O/S molar ratio, additives, and different sulfur compounds on the desulfurization efficiency were investigated. To demonstrate the catalytic ability of the DESs, reaction kinetic analysis was conducted. The DES recyclability and regeneration method were also investigated.

2.4. Analytical methods The sulfur content in the upper model fuel after each ODS procedure was analyzed by gas chromatography (GC-FID, HP-5 column, 30 m × 0.32 mm × 0.25 μm) using tetradecane as the internal standard. The GC column flow rate was 1.5 mL/min with nitrogen as the carrier gas. 2.5. Characterization The structures of DESs used in this work were characterized by 1H NMR spectroscopy (Bruker AV400MHz, Germany) using deuterated DMSO as solvent and FT-IR spectroscopy (Thermo Electron, NEXUS8700, USA). The white solid obtained in the ODS process was also characterized by FT-IR spectroscopy. The components of the upper model fuel after ODS and other treatments were detected by gas chromatography–mass spectrometry (GC–MS, Agilent 7890B-5977A, USA). 3. Results and discussion

2. Experimental

3.1. Effect of different ODS systems on desulfurization efficiency

2.1. Materials

The synthetic procedure for L-Pyro/TFA is provided as an example, where L-pyroglutamic acid and TFA were selected as the HBA and HBD, respectively. In a typical process, L-Pyro and TFA were mixed in a 1:1 M ratio in a 100-mL round-bottomed flask at room temperature. The mixture was then heated at 50 °C in an oil bath with vigorous magnetic stirring for 2 h. Finally, a dark yellow homogeneous liquid was obtained after rotary evaporation under vacuum conditions to remove possibly remaining TFA and water.

DES acidity plays an important role in the ODS process, directly affecting the sulfur removal efficiency. Therefore, in this study, four dual-acidic DESs, namely, L-Pyro/TFA, L-Pyro/For, L-Pro/TFA, and LPro/For, were prepared from L-Pyro and L-Pro as HBAs, and formic acid (For) and TFA as HBDs. Different desulfurization systems were tested. As shown in Table 1, the desulfurization efficiency of DESs with TFA as the HBD (L-Pyro/TFA and L-Pro/TFA) are higher than that with formic acid (L-Pyro/For and L-Pro/For) as the HBD, reaching 99% in model fuels containing different aromatic sulfur compounds. By comparing the desulfurization ability of the four acidic DESs, L-Pyro/TFA was selected as the extractant and catalyst for this ODS procedure and further studied in different desulfurization systems, as shown in Table 2. Notably, the DBT removal efficiency was 18.2% without adding any acid catalyst. Furthermore, the sulfur removal efficiency in the DES/oil EDS system was very low. Organic acids are highly volatile and can cause equipment corrosion. Therefore, the TFA itself was not a good choice as catalyst, despite the desulfurization rate reaching 98.1%. However, LPyro/TFA synthesized based on the hydrogen bond interactions showed good catalytic and extraction properties with low vapor pressure. Therefore, L-Pyro/TFA clearly showed a better desulfurization capacity, serving not only as the catalyst, but also as the extractant.

2.3. Oxidative desulfurization process

3.2. Characterization of DESs

The model oil containing different aromatic sulfur compounds (DBT, BT, and 4,6-DMDBT) was prepared by dissolving in n-octane with an initial sulfur content of 500 ppm. ODS experiments on model fuel were conducted as follows. 1 mL DES, 20 mL model oil and 30 wt% H2O2 with the O/S molar ratio of 3 were mixed in a 100-mL roundbottomed flask and reacted at 60 °C for 3 h in an oil bath with continuous stirring. A white solid was observed to precipitate throughout the ODS procedure when the model oil contained DBT or 4,6-DMDBT. However, no white solid was observed in the BT oxidative desulfurization system. After allowing to settle, about 3 mL of the upper oil phase was taken as the sample for gas chromatography analysis. The organic sulfur removal efficiency was defined using the following equation.

To determine whether a hydrogen bond had formed between L-Pyro and TFA, 1H NMR and FT-IR spectra of L-Pyro and L-Pyro/TFA, were recorded, as shown in Fig. 1(a) and (b). The H signal at 12.72 ppm was

C 0 − Ct × 100% C0

Experimental conditions: V(DES) = 1 mL, V(model oil) = 10 mL, O/S = 3, T = 60 °C, t = 180 min, model oil (S, 500 ppm).

L-Pyroglutamic acid (L-Pyro, 99%), L-proline (L-Pro, 99%), and noctane (95%) were purchased from Shanghai Macklin Reagent. Dibenzothiophene (DBT, 99%), benzothiophene (BT, 99%), and 4,6dimethyldibenzothiophene (4,6-DMDBT, 99%) were purchased from Adamas Reagent. Formic acid (For, 99%) and trifluoroacetic acid (TFA, 99%) were supplied by Shanghai Aladdin Reagent. Hydrogen peroxide (H2O2, 30 wt% aqueous solution) was purchased from Beijing Chemical Works. All chemicals above were used directly without purification.

2.2. Preparation of DESs

S − removal efficiency (%) =

Table 1 Effect of different DESs. DESs

L-Pyro/TFA L-Pyro/For L-Pro/TFA L-Pro/For

(1) 2

S-removal efficiency/% DBT

4,6-DMDBT

BT

99.7 97 99.6 4.7

100 97.7 99.5 1.2

99.9 83.3 99.8 0

Fuel 265 (2020) 116967

W. Liu, et al.

3.6. Effect of different sulfur compounds on desulfurization efficiency

Table 2 Effect of different ODS systems.a Entry

S-removal systems

S-removal efficiency/%

1 2b 3c 4d 5

H2O2/Oil H2O2/TFA/Oil H2O2/L-Pyro/Oil DES/Oil H2O2/DES/Oil

18.2 98.1 38.3 34.1 99.7

From the above experiments, optimum conditions (VDES/ VOil = 1:20, n(H2O2)/n(DBT) = 3, T = 60 °C) for DBT removal were obtained. As sulfides in real oil systems are complex and diverse, the desulfurization efficiency of L-Pyro/TFA in different thiophenic sulfur compounds, such as BT and 4,6-DMDBT, was investigated under the optimum experimental conditions. As shown in Fig. 5, desulfurization efficiencies of 99.7%, 99.6%, and 96.6% for DBT, 4,6-DMDBT, and BT, respectively, were achieved in 45 min, clearly demonstrating that both DBT and 4,6-DMDBT were more easily oxidized than BT in this ODS system. Notably, the BT desulfurization efficiency reached 99.2% after reacting for 3 h. As shown in Table 3, all the desulfurization efficiencies obtained in this work are higher than those reported previously. In addition, both the DES/oil volume ratio and O/S molar ratio used in this study are much lower than that used in previous literature [41,42]. Moreover, the equilibrium time is shorter than that reported previously too [18,36,43]. Steric hindrance from the methyl group and the electron density of the three sulfur compounds were the two main factors that might explain this result. The methyl groups in DBT and 4,6DMDBT would hinder ODS progressing, resulting in a lower desulfurization efficiency, while a higher electron density would result in higher oxidation activity. As reported previously [27], the electron density of the three substrates decreased in the order 4,6-DMDBT (5.760) > DBT (5.758) > BT (5.739). Therefore, the desulfurization efficiencies of DBT, 4,6-DMDBT and BT were determined by the synergistic effect of steric hindrance and electron density.

Experimental conditions: a V(DES) = 1 mL, V(model oil) = 10 mL, O/S = 3, T = 60 °C, t = 180 min, model oil (DBT, 500 ppm), b TFA (0.6939 g), c L-Pyro (0.7806 g), d T = 25 °C, t = 30 min, V(DESs) = 1 mL, V(model oil) = 1 mL.

clearly attributed to the –COOH group of L-Pyro, with the downfield shift to 14.40 ppm indicating DES formation. Based on the chemical shift principle, the electrophilicity of the O atoms in –COOH caused a decreased electron density at the H atoms. This result was further confirmed by the FT-IR spectra in Fig. 1(b). Compared with L-Pyro, the –COOH peak of L-Pyro/TFA was redshifted (from 3402 to 3334 cm−1) and wider, while the C=O peak (shifted from 1649 to 1644 cm−1) was slightly wider, indicating that hydrogen bonds had been formed. 3.3. Effect of DES/oil volume ratio on desulfurization efficiency As the catalyst and extractant, the amount of L-Pyro/TFA in the ODS system was a vital factor affecting the desulfurization efficiency. Therefore, the effect of the DES/oil volume ratio on the sulfur removal efficiency was studied. As shown in Fig. 2, when the volume ratio decreased from 1:5 to 1:30, the reaction equilibrium time increased. However, DBT was almost completely removed in 1 h at 60 °C, even when the volume ratio was 1:30, indicating that L-Pyro/TFA had a significant effect on desulfurization. From the model oil treatment capacity and ODS equilibrium time results, a volume ratio of 1:20 was selected for subsequent experiments.

3.7. Effect of cycloparaffin/aromatic/olefin addition on desulfurization efficiency Owing to the complexity of hydrocarbons in diesel, cycloparaffin, aromatics, and olefins of different mass fractions were added to the model oil to test their influence on the L-Pyro/TFA desulfurization system. As shown in Fig. 6, adding cyclohexane and p-xylene had little effect on the sulfur removal rate, while adding olefins clearly affected ODS. As the mass fraction of olefins added increased, the desulfurization efficiency of the 1-octene system decreased only slightly, from 99.7% to 94.6%. However, the sulfur removal rate sharply declined to 35.2% when 10 wt% cyclohexene added to the model oil, which might be due to the ring-opening reaction of cyclohexene [8].

3.4. Effect of reaction temperature on desulfurization efficiency In the ODS system, reaction temperature is an important factor affecting the catalysis of hydrogen peroxide by L-Pyro/TFA. The effect of reaction temperature on the desulfurization efficiency is shown in Fig. 3. The DBT removal efficiencies all approached 99.7% at temperatures of 40–70 °C. Furthermore, the reaction equilibrium was reached in a shorter time at higher temperatures. Therefore, within a certain temperature range, increasing the reaction temperature was beneficial to sulfur removal. However, hydrogen peroxide decomposes to water and oxygen at high reaction temperatures, causing a decline in the desulfurization efficiency. Considering these factors and energy costs, the reaction temperature used in this study was 60 °C.

3.8. Dynamics analysis and apparent activation energy To further understand the ODS reaction and evaluate the catalytic capacity of L-Pyro/TFA for removing DBT, BT, and 4,6-DMDBT in the ODS system, a reaction kinetics method was applied. Based on previous studies [42,57], the ODS reaction was confirmed to follow first order kinetics. The reaction kinetic constants were obtained by calculation from the experimental data for the effect of different sulfur compounds on the desulfurization efficiency using Eqs. (2)–(4), where K is the kinetic constant, R2 is the correlation coefficient, and X is the desulfurization rate.

3.5. Effect of O/S molar ratio on desulfurization efficiency As reported in many studies [2], hydrogen peroxide is an optimum oxidative agent among molecular oxygen, organic peroxide, oxyacid salts such as potassium permanganate, and others because its byproduct is water and it is not harmful to the environment. In the ODS system, at least 2 equiv. of hydrogen peroxide was required to oxidize 1 equiv. of sulfur compound to the corresponding sulfone. However, as shown in Fig. 4, a desulfurization efficiency of only 96.2% was achieved in 3 h when the O/S molar ratio was 2, which was mainly due to the slight decomposition of hydrogen peroxide in the reaction process. DBT in the model oil was almost completely removed when the O/S molar ratio was higher than 2 in 3 h. However, the amount of hydrogen peroxide added and the resulting water produced also affected the viscosity of LPyro/TFA, further influencing its catalytic and desulfurization ability. Therefore, an O/S molar ratio of 3 was confirmed as optimal for the ODS experiment.

−d ct = KCt dt

ln

(2)

C0 = Kt Ct

(3)

ln(1 − X ) = −Kt

(4) 2

As shown in Fig. 7, the R values for DBT, 4,6-DMDBT, and BT were 0.9787, 0.9839, and 0.9848, respectively, which indicated that the reaction in the L-Pyro/TFA ODS system had first order kinetics, which was consistent with literature reports. The half-life of the first order reaction is defined as t 1 2 = ln 2/ K . Therefore, the half-lives for DBT, 4,6-DMDBT, and BT were 4.95, 5.10, and 8.65 min, respectively, 3

Fuel 265 (2020) 116967

W. Liu, et al.

Fig. 1. Spectroscopic characterization of DESs L-Pyro/TFA (I) and L-Pyro (II): (a) 1H NMR spectra; (b) FT-IR spectra.

According to the Arrhenius equation ln K = −Ea/(RT ) + ln A , where Ea is the apparent activation energy and A is the Arrhenius constant, the Ea for this DES/H2O2/oil reaction system was 44.37 kJ/mol, as shown in Fig. 8(b).

indicating that the ODS reaction rate was very fast. Furthermore, the K value of the different sulfides decreased in the order DBT > 4,6DMDBT > BT, which was in agreement with the results in Fig. 5. From the analysis above, the ODS reaction in this study followed first order kinetics. Dynamics analysis was performed at different reaction temperatures, as shown in Fig. 8(a). The K values were 0.0390, 0.0806, 0.1082 min−1 at temperatures of 40, 50, and 60 °C, respectively, indicating that the reaction rate was faster with increasing temperature. This result was consistent with previous reports.

3.9. Recycling and regeneration of DES Regarding sustainable development and economic benefits, the recyclability and regeneration of DES are crucial aspects of the 4

Fuel 265 (2020) 116967

W. Liu, et al.

Fig. 2. Effect of DES/oil volume ratio on desulfurization efficiency. Experimental conditions: V(DES) = 1 mL, O/S = 3, T = 60 °C, model oil (DBT, 500 ppm).

Fig. 5. Effect of different sulfur compounds on desulfurization efficiency. Experimental conditions: V(DES) = 1 mL, V(oil) = 20 mL, O/S = 3, T = 60 °C.

desulfurization system. After each ODS reaction, the upper oil phase was separated by decantation, and the lower DES layer was mixed with the aforementioned white solid (DBT oxidation product). After removing with a pipette, DES was separated from the mixture by highspeed centrifugation and used for the next ODS reaction under the same experimental conditions after vacuum rotary evaporation to remove water and residual hydrogen peroxide without further treatment. The sulfur removal efficiency slightly decreased after the DES was reused three times, but maintained a desulfurization rate of 84% in subsequent recycling experiments, owing to the accumulation of oxidation product in DES. By adding ultrapure water to reused DES, the insoluble white solid was removed by filtration, and regenerated DES was obtained after removing water from the filtrate by rotary evaporation. As shown in Fig. 9, the sulfur removal rate increased to 98% in the sixth ODS cycle experiment. The reason for regenerated DES not reaching a 99.7% removal rate was probably incomplete removal of water. 3.10. Possible reaction process Fig. 3. Effect of reaction temperature on desulfurization efficiency. Experimental conditions: V(DES) = 1 mL, V(oil) = 20 mL, O/S = 3, model oil (DBT, 500 ppm).

The upper oil was separated and allowed to stand for a while after the ODS reaction, then some white crystals could be observed at the bottom of the centrifuge tube. FT-IR spectra of the precipitated white crystals (Ι) and oxidation products (ΙΙ) are shown in Fig. 10. These two solids were the same substance, DBTO2, because they showed the same infrared absorption peaks for O=S=O at 1288, 1167, and 1047 cm−1 [42]. Furthermore, GC–MS analysis showed that DBTO2 in the upper oil phase was completely removed after filtering the precipitated white crystals, as shown in Fig. 11. A possible reaction process for L-Pyro/TFA as extractant and catalyst in ODS is shown in Fig. 12. According to the similarity–compatibility theory, DBT in the oil phase is first extracted into the DES phase. When H2O2 is added, TFA tends to exist in the form of peroxytrifluoroacetic acid, which is attributed to the high oxidative desulfurization effect according to previous reports [17]. DBT in the lower liquid phase is then oxidized to the corresponding sulfones. DBTO2 partially dissolved in oil due to the high reaction temperature is separated completely from the oil phase through static settlement. Finally, the sulfur-free fuel oils are obtained. 4. Conclusions

Fig. 4. Effect of O/S molar ratio on desulfurization efficiency. Experimental conditions: V(DES) = 1 mL, V(oil) = 20 mL, T = 60 °C, model oil (DBT, 500 ppm).

Four kinds of dual-acidic DESs were synthesized using a one-step method, with L-Pyro/TFA, in which hydrogen bond formation was confirmed by 1H NMR and infrared spectroscopy, selected for further investigation owing to its excellent catalytic ability in ODS systems. The 5

Fuel 265 (2020) 116967

W. Liu, et al.

Table 3 Comparison of different ODS systems. catalysts

Model oil

L-Pyro/TFA

n-Octane, n-Octane, n-Octane, n-Octane, n-Octane, n-Octane, n-Octane, n-Octane, n-Octane, n-Octane, n-Octane, n-Octane,

L-Pro/p-TsOH

[Bmim][HSO4]

C9H10O2/0.5 ZnCl2 C3H6O2/0.5 ZnCl2 C5H9NO·0.3FeCl3

DBT (500 ppm) BT (500 ppm) 4,6-DMDBT (500 ppm) DBT (500 ppm) BT (500 ppm) 4,6-DMDBT (500 ppm) DBT (1000 ppm) BT (1000 ppm) 4,6-DMDBT (500 ppm) DBT (500 ppm) DBT (500 ppm) DBT (500 ppm)

V (cat)/V (oil)

n (O)/n (S)

T

t

S-removal rate/%

refs

1:20

3

60 °C

5

60 °C

1:2

5

25 °C

1:4 0.15:1 1:3

6 4 12

50 °C 30 °C 30 °C

99.7 99.6 99.2 99 99 99 99.8 94.2 85.2 99.23 99.42 97

This work

1:5

45 min 45 min 180 min 120 min 120 min 180 min 120 min 90 min 90 min 80 min 180 min 180 min

[43]

[35]

[42] [41] [58]

Fig. 6. Effect of cycloparaffin/aromatic/olefin addition on desulfurization efficiency. Experimental conditions: V(DES) = 1 mL, V(oil) = 20 mL, O/S = 3, T = 60 °C, t = 45 min. Fig. 8. (a) Dynamics analysis at different reaction times; (b) apparent activation energy of DBT.

Fig. 7. Dynamics analysis with different sulfur compounds. Fig. 8. (continued)

optimum experimental conditions, namely, VDES/VOil = 1:20, n(H2O2)/ n(DBT) = 3, and T = 60 °C, were determined using a series of ODS experiments. The desulfurization efficiencies of DBT, 4,6-DMDBT, and BT were up to 99.7%, 99.6%, and 99.2%, respectively. The reaction kinetic constants of the different sulfides, obtained by dynamics calculations, decreased in the order DBT > 4,6-DMDBT > BT, which

was in agreement with the experiment results. Adding cyclohexane, pxylene, and 1-octene to the model oil slightly affected the desulfurization efficiency, while adding cyclohexene caused the sulfur removal rate to decrease sharply. L-Pyro/TFA served as both catalyst and extractant in the ODS system. In the recycling experiment, the 6

Fuel 265 (2020) 116967

W. Liu, et al.

Fig. 9. Effect of recycling and regeneration of DES on desulfurization efficiency. Experimental conditions: VDES/Voil = 1:20, O/S = 3, T = 60 °C, t = 45 min.

Fig. 10. Infrared characterization of oxidation products. White crystals (Ι), white solid (ΙΙ).

Fig. 11. GC–MS characterization of upper oil phase.

desulfurization efficiency decreased with DBTO2 accumulation in the DES, but returned to 98% after regeneration treatment with water.

interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement Acknowledgments Wei Liu: Conceptualization, Writing - original draft. Tenghui Li: Investigation, Writing - original draft. Guojia Yu: Resources, Validation. Jiangang Wang: Visualization, Data curation. Zhiyong Zhou: Conceptualization, Supervision, Writing - review & editing. Zhongqi Ren: Conceptualization, Supervision, Writing - review & editing.

We thank Simon Partridge, PhD, from Liwen Bianji, Edanz Editing China (www.liwenbianji.cn/ac), for editing the English text of a draft of this manuscript. This work was supported by the National Natural Science Foundation of China (21576010, 21606009, U1607107 and U1862113), Beijing Natural Science Foundation (2172043) and Big Science Project from BUCT (XK180301). The authors gratefully acknowledge these grants.

Declaration of Competing Interest The authors declare that they have no known competing financial 7

Fuel 265 (2020) 116967

W. Liu, et al.

Fig. 12. Possible reaction process for L-Pyro/TFA in ODS.

References

type emulsion catalyst. Ind Eng Chem Res 2017;56:3839–52. [23] Banisharif F, Dehghani MR, Campos-Martin JM. Oxidative desulfurization of diesel using vanadium-substituted dawson-type emulsion catalysts. Energy Fuels 2017;31:5419–27. [24] Lu HY, Zhang YN, Jiang ZX, Li C. Aerobic oxidative desulfurization of benzothiophene, dibenzothiophene and 4,6-dimethyldibenzothiophene using an Andersontype catalyst [(C18H37)2N(CH3)2]5[IMo6O24]. Green Chem 2010;12:1954–8. [25] Lu HY, Deng CL, Ren WZ, Yang X. Oxidative desulfurization of model diesel using [(C4H9)4N]6Mo7O24 as a catalyst in ionic liquids. Fuel Process Technol 2014;119:87–91. [26] Lu HY, Ren WZ, Wang HY, Wang Y, Chen W, Suo ZH. Deep desulfurization of diesel by ionic liquid extraction coupled with catalytic oxidation using an Anderson-type catalyst [(C4H9)4N]4NiMo6O24H6. Appl Catal A Gen 2013;453:376–82. [27] Li MX, Zhou ZY, Zhang F, Chai WS, Zhang LL, Ren ZQ. Deep oxidative-extractive desulfurization of fuels using benzyl-based ionic liquid. AIChE J 2016;62:4023–34. [28] Ren ZQ, Zhou ZY, Li MX, Zhang F, Wei L, Liu WD. Deep desulfurization of fuels using imidazole anion-based ionic liquids. ACS Sustain Chem Eng 2019;7:1890–900. [29] Hou LP, Zhao RX, Li XP, Gao XH. Preparation of MoO2/g-C3N4 composites with a high surface area and its application in deep desulfurization from model oil. Appl Surf Sci 2018;434:1200–9. [30] He J, Wu PW, Wu YC, Li HP, Jiang W, Xun SH, et al. Taming interfacial oxygen vacancies of amphiphilic tungsten oxide for enhanced catalysis in oxidative desulfurization. ACS Sustain Chem Eng 2017;5:8930–8. [31] Yao XY, Wang C, Liu H, Li HP, Wu PW, Fan L, et al. Immobilizing highly catalytically molybdenum oxide nanoparticles on graphene-analogous BN: Stable heterogeneous catalysts with enhanced aerobic oxidative desulfurization performance. Ind Eng Chem Res 2019;58:863–71. [32] Fang DW, Wang Q, Liu Y, Xia LX, Zang SL. High-efficient oxidation-extraction desulfurization process by ionic liquid 1-butyl-3-methyl-imidazolium trifluoroacetic acid. Energy Fuels 2014;28:6677–82. [33] Yu GR, Zhao JJ, Song DD, Asumana C, Zhang XY, Chen XC. Deep oxidative desulfurization of diesel fuels by acidic ionic liquids. Ind Eng Chem Res 2011;50:11690–7. [34] Zhang C, Pan XY, Wang F, Liu XQ. Extraction-oxidation desulfurization by pyridinium-based task-specific ionic liquids. Fuel 2012;102:580–4. [35] Gao HS, Guo C, Xing JM, Zhao JM, Liu HZ. Extraction and oxidative desulfurization of diesel fuel catalyzed by a Bronsted acidic ionic liquid at room temperature. Green Chem 2010;12:1220–4. [36] Wang JY, Zhang LH, Sun YL, Jiang B, Chen Y, Gao X, et al. Deep catalytic oxidative desulfurization of fuels by novel Lewis acidic ionic liquids. Fuel Process Technol 2018;177:81–8. [37] Chandran D, Khalid M, Walvekar R, Mubarak NM, Dharaskar S, Wong WY, et al. Deep eutectic solvents for extraction-desulphurization: a review. J Mol Liq 2019;275:312–22. [38] Li CP, Li D, Zou SS, Li Z, Yin JM, Wang AL, et al. Extraction desulfurization process of fuels with ammonium-based deep eutectic solvents. Green Chem 2013;15:2793–9. [39] Wang X, Jiang W, Zhu WS, Li HP, Yin S, Chang YH, et al. A simple and cost-effective extractive desulfurization process with novel deep eutectic solvents. RSC Adv 2016;6:30345–52. [40] Cheng HY, Liu CY, Zhang JJ, Chen LF, Zhang BJ, Qi ZW. Screening deep eutectic solvents for extractive desulfurization of fuel based on COSMO-RS model. Chem Eng Process 2018;125:246–52. [41] Mao CF, Zhao RX, Li XP. Propionic acid-based deep eutectic solvents: synthesis and ultra-deep oxidative desulfurization activity. RSC Adv 2017;7:42590–6. [42] Mao CF, Zhao RX, Li XP. Phenylpropanoic acid-based DESs as efficient extractants and catalysts for the removal of sulfur compounds from oil. Fuel 2017;189:400–7. [43] Hao LW, Wang MR, Shan WJ, Deng CL, Ren WZ, Shi ZZ, et al. L-proline-based deep eutectic solvents (DESs) for deep catalytic oxidative desulfurization (ODS) of diesel. J Hazard Mater 2017;339:216–22. [44] Yin JM, Wang JP, Li Z, Li D, Yang G, Cui YN, et al. Deep desulfurization of fuels based on an oxidation/extraction process with acidic deep eutectic solvents. Green Chem 2015;17:4552–9. [45] Li CP, Zhang JJ, Li Z, Yin JM, Cui YN, Liu Y, et al. Extraction desulfurization of fuels

[1] Wagle DV, Zhao H, Deakyne CA, Baker GA. Quantum chemical evaluation of deep eutectic solvents for the extractive desulfurization of fuel. ACS Sustain Chem Eng 2018;6:7525–31. [2] Dai CN, Zhang J, Huang CP, Lei ZG. Ionic liquids in selective oxidation: catalysts and solvents. Chem Rev 2017;117:6929–83. [3] Ren ZQ, Wei L, Zhou ZY, Zhang F, Liu W. Extractive desulfurization of model oil with protic ionic liquids. Energy Fuels 2018;32:9172–81. [4] Shang H, Du W, Liu ZC, Zhang HC. Development of microwave induced hydrodesulfurization of petroleum streams: a review. J Ind Eng Chem 2013;19:1061–8. [5] Srivastava VC. An evaluation of desulfurization technologies for sulfur removal from liquid fuels. RSC Adv 2012;2:759–83. [6] Palomeque-Santiago JF, Lopez-Medina R, Oviedo-Roa R, Navarrete-Bolanos J, Mora-Vallejo R, Montoya-de La Fuente J, et al. Deep oxidative desulfurization with simultaneous oxidative denitrogenation of diesel fuel and straight run gas oil. Appl Catal B-Environ 2018;236:326–37. [7] Jiang W, Zheng D, Xun SH, Qin YJ, Lu QQ, Zhu WS, et al. Polyoxometalate-based ionic liquid supported on graphite carbon induced solvent-free ultra-deep oxidative desulfurization of model fuels. Fuel 2017;190:1–9. [8] Wang C, Chen ZG, Yao XY, Chao YH, Xun SH, Xiong J, et al. Decavanadates anchored into micropores of graphene-like boron nitride: efficient heterogeneous catalysts for aerobic oxidative desulfurization. Fuel 2018;230:104–12. [9] Zeng XY, Xiao XY, Li Y, Chen JY, Wang HL. Deep desulfurization of liquid fuels with molecular oxygen through graphene photocatalytic oxidation. Appl Catal B-Environ 2017;209:98–109. [10] Bertleff B, Claussnitzer J, Korth W, Wasserscheid P, Jess A, Albert J. Catalyst activation and influence of the oil matrix on extractive oxidative desulfurization using aqueous polyoxometalate solutions and molecular oxygen. Energy Fuels 2018;32:8683–8. [11] Li SW, Yang Z, Gao RM, Zhang G, Zhao JS. Direct synthesis of mesoporous SRLPOM@MOF-199@MCM-41 and its highly catalytic performance for the oxidesulfurization of DBT. Appl Catal B-Environ 2018;221:574–83. [12] Yang C, Ji HW, Chen CC, Ma WH, Zhao JC. Desulfurization of thiophenes in oils into H2SO4 using molecular oxygen. Appl Catal B-Environ 2018;235:207–13. [13] Kang L, Liu HY, He HJ, Yang CP. Oxidative desulfurization of dibenzothiophene using molybdenum catalyst supported on Ti-pillared montmorillonite and separation of sulfones by filtration. Fuel 2018;234:1229–37. [14] Chen K, Zhang XM, Yang XF, Jiao MG, Zhou Z, Zhang MH, et al. Electronic structure of heterojunction MoO2/g-C3N4 catalyst for oxidative desulfurization. Appl Catal BEnviron 2018;238:263–73. [15] Wei SN, He HJ, Cheng Y, Yang CP, Zeng GM, Kang L, et al. Preparation, characterization, and catalytic performances of cobalt catalysts supported on KIT-6 silicas in oxidative desulfurization of dibenzothiophene. Fuel 2017;200:11–21. [16] Du Q, Guo YP, Duan HN, Li H, Chen YJ, Liu HZ. Synthesis of hierarchical TS-1 zeolite via a novel three-step crystallization method and its excellent catalytic performance in oxidative desulfurization. Fuel 2017;188:232–8. [17] Jiang B, Yang HW, Zhang LH, Zhang RY, Sun YL, Huang Y. Efficient oxidative desulfurization of diesel fuel using amide-based ionic liquids. Chem Eng J 2016;283:89–96. [18] Zhang LH, Wang JY, Sun YL, Jiang B, Yang HW. Deep oxidative desulfurization of fuels by superbase-derived Lewis acidic ionic liquids. Chem Eng J 2017;328:445–53. [19] Andevary HH, Akbari A, Omidkhah M. High efficient and selective oxidative desulfurization of diesel fuel using dual-function [Omim]FeCl4 as catalyst/extractant. Fuel Process Technol 2019;185:8–17. [20] Ji HY, Sun J, Wu PW, Wu YC, He J, Chao YH, et al. Silicotungstic acid immobilized on lamellar hexagonal boron nitride for oxidative desulfurization of fuel components. Fuel 2018;213:12–21. [21] Lu HY, Li PC, Liu YM, Hao LW, Ren WZ, Zhu WJ, et al. Synthesis of a hybrid Anderson-type polyoxometalate in deep eutectic solvents (DESs) for deep desulphurization of model diesel in ionic liquids (ILs). Chem Eng J 2017;313:1004–9. [22] Banisharif F, Dehghani MR, Capel-Sanchez M, Campos-Martin JM. Desulfurization of fuel by extraction and catalytic oxidation using a vanadium substituted dawson-

8

Fuel 265 (2020) 116967

W. Liu, et al.

[46]

[47] [48]

[49]

[50]

[51]

[52]

deep eutectic solvents. ACS Sustain Chem Eng 2017;5:6382–6. [53] Sun SY, Niu YX, Xu Q, Sun ZC, Wei XH. Efficient SO2 absorptions by four kinds of deep eutectic solvents based on choline chloride. Ind Eng Chem Res 2015;54:8019–24. [54] Yang DZ, Hou MQ, Ning H, Zhang JL, Ma J, Yang GY, et al. Efficient SO2 absorption by renewable choline chloride-glycerol deep eutectic solvents. Green Chem 2013;15:2261–5. [55] Xu H, Zhang DD, Wu FM, Wei XF, Zhang J. Deep desulfurization of fuels with cobalt chloride-choline chloride/polyethylene glycol metal deep eutectic solvents. Fuel 2018;225:104–10. [56] Mjalli FS, Al-Hajri R, Al-Muhtaseb A, Ahmed O, Nagaraju M. Novel amino acidbased ionic liquid analogues: neutral hydroxylic and sulfur-containing amino acids. Asia-Pac J Chem Eng 2016;11:683–94. [57] Zhu MY, Luo GQ, Kang LH, Dai B. Novel catalyst by immobilizing a phosphotungstic acid on polymer brushes and its application in oxidative desulfurization. RSC Adv 2014;4:16769–76. [58] Li FT, Wu B, Liu RH, Wang XJ, Chen LJ, Zhao DS. An inexpensive N-methyl-2pyrrolidone-based ionic liquid as efficient extractant and catalyst for desulfurization of dibenzothiophene. Chem Eng J 2015;274:192–9.

with 'metal ions' based deep eutectic solvents (MDESs). Green Chem 2016;18:3789–95. Jiang W, Li HP, Wang C, Liu W, Guo T, Liu H, et al. Synthesis of ionic liquid-based deep eutectic solvents for extractive desulfurization of fuel. Energy Fuels 2016;30:8164–70. Li JJ, Xiao H, Tang XD, Zhou M. Green carboxylic acid-based deep eutectic solvents as solvents for extractive desulfurization. Energy Fuels 2016;30:5411–8. Zaid HFM, Kait CF, Mutalib MIA. Extractive deep desulfurization of diesel using choline chloride-glycerol eutectic-based ionic liquid as a green solvent. Fuel 2017;192:10–7. Almashjary KH, Khalid M, Dharaskar S, Jagadish P, Walvekar R, Gupta TCSM. Optimisation of extractive desulfurization using choline chloride-based deep eutectic solvents. Fuel 2018;234:1388–400. Tang XD, Zhang YF, Li JJ, Zhu YQ, Qing DY, Deng YX. Deep extractive desulfurization with arenium ion deep eutectic solvents. Ind Eng Chem Res 2015;54:4625–32. Chen Y, Jiang B, Dou HZ, Zhang LH, Tantai XW, Sun YL, et al. Highly efficient and reversible capture of low partial pressure SO2 by functional deep eutectic solvents. Energy Fuels 2018;32:10737–44. Yang DZ, Han YL, Qi HB, Wang YB, Dai S. Efficient absorption of SO2 by emimCI-EG

9