One-Pot Extraction and Oxidative Desulfurization of Fuels with

Article pubs.acs.org/EF

One-Pot Extraction and Oxidative Desulfurization of Fuels with Molecular Oxygen in Low-Cost Metal-Based Ionic Liquids Chao Wang,† Zhigang Chen,*,† Wangqin Zhu,‡ Peiwen Wu,‡ Wei Jiang,§ Ming Zhang,§ Hongping Li,§ Wenshuai Zhu,*,‡ and Huaming Li*,§ †

School of the Environment and Safety Engineering, ‡School of Chemistry and Chemical Engineering, and §Institute for Energy Research, Jiangsu University, 301 Xuefu Road, Zhenjiang, Jiangsu 212013, People’s Republic of China S Supporting Information *

ABSTRACT: Four economic and easily prepared metal-based ionic liquids (MILs) were synthesized based on triethylamine hydrochloride (Et3NHCl) and a series of anhydrous metal chlorides (MClx). The stiochemistry of the as-prepared MILs is characterized systematically. Extraction and catalytic oxidative desulfurization (ECODS) of thiophenic sulfides is then achieved using the as-prepared MILs and molecular oxygen (O2) in air as the extractant and oxidant, respectively. The factors that could influence sulfur removal are investigated systematically, and deep desulfurization of oils with different S concentrations and substrates are achieved under optimized conditions simultaneously. Dibenzothiophene (DBT) removal reaches up to 99.4% under the optimized conditions. Sulfur removal with different sulfides decreases in the order of DBT > 4,6dimethyldibenzothiophene (4,6-DMDBT) > benzothiophene (BT). The influence of the co-existence of alkenes and aromatics is also investigated. Moreover, the used amount of low-cost [Et3NH]FeCl4 is relatively low [V(MIL)/V(oil) = 1:15], and it could be recycled 5 times without a significant decrease in sulfur removal. Besides, it is verified that sulfur compounds are converted to their corresponding sulfones in the reaction process with the help of gas chromatography−mass spectrometry (GC−MS) measurement.

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temperature (usually around 120 °C).41−44 Recently, some breakthroughs have been made, Lü et al. find that the aerobic ODS could be realized at 80 °C by the Anderson-type catalyst.42,45 However, reports about aerobic ODS at room temperature and atmospheric pressure are still scarce. In addition, isobutyraldehyde (IBA) is found to be used as an additive agent to activate O2 at room temperature and atmospheric pressure under the irradiation of ultraviolet (UV) light. The existence of an additive agent and its byproduct, as hydrocarbons, could not only avoid the contamination of liquid fuels but also contribute heat in the combustion process. However, sulfur removal in this aerobic ODS reaction system is still unsatisfied. Thus, a liquid−liquid extraction and catalytic oxidative desulfurization (ECODS) reaction system, as a special and effective ODS method, had been developed in recent years.46−51 In this process, ionic liquids (ILs) are often used as an extractant to extract sulfur compounds to the IL phase and then the extracted sulfur compounds were oxidized to sulfone.52 However, the high costs of these ILs make them hard to be used in industrial-scale application.53,54 To overcome the above drawbacks, metal-based ionic liquids (MILs) synthesized by coupling economic quaternary ammonium salts (e.g., Me3NCH2C6H5Cl55) and metal halide (e.g., FeCl3,56,57 CuCl2,58,59 AlCl3,60−62 and ZnCl255,63,64) have been exploited recently. The powerful extraction ability made MILs promising extractants in the ECODS process. Besides,

INTRODUCTION Air pollution caused by SOx emission from vehicle engines has been one of the theorist problems in the past few decades.1,2 To address these issues, many regulations have been formulated to reduce the sulfur content in gasoline and diesel oil around the world.3,4 In modern refining industry, hydrodesulfurization (HDS) is a conventional and effective method to remove aliphatic sulfur compounds.5,6 However, its harsh reaction environment (high temperature, high pressure, etc.) combined with low activity to thiophenic sulfur compounds compel us to search for an alternative desulfurization method.7,8 Therefore, many other non-HDS methods, such as adsorptive desulfurization (ADS),9−11 extractive desulfurization (EDS),12−15 and oxidative desulfurization (ODS),16−20 have been developed in the latest few decades. Among all of these non-HDS methods, ODS is considered to be one of the most promising desulfurization methods.21,22 In this process, sulfur compounds are oxidized to their corresponding sulfones by oxidants,23,24 such as hydrogen peroxide (H2O2),25−30 ozone (O3),31,32 tert-butyl hydroperoxide (TBHP),33−35 hydrogen peroxide cumene,36 molecule oxygen (O2),37−39 etc. Among which, H2O2, as an environmentally friendly and green oxidant, is often used in the ODS process.40 However, its potential explosion in industrial transportation and production limits its application. Thus, the ODS reaction with using O2 as an oxidant, known as aerobic ODS, has drawn much attention these years.37−39,41−44 Although aerobic ODS could be realized by boron nitride (BN),41 Anderson-type catalysts,42 metal organic frameworks (MOFs),38 and many other catalysts,43,44 one of the toughest problems that needs to be confronted is its high reaction © 2017 American Chemical Society

Received: October 31, 2016 Revised: January 7, 2017 Published: January 9, 2017 1376

DOI: 10.1021/acs.energyfuels.6b02624 Energy Fuels 2017, 31, 1376−1382

Article

Energy & Fuels reports about ECODS using O2 as an oxidant are still scarce. Thus, in this paper, ECODS has been developed using low-cost MILs and O2 as the extractant and oxidant, respectively. Here, MILs were synthesized by economic triethylamine hydrochloride (Et3NHCl) and a serials of hydrous metal halide (FeCl3, ZnCl2, FeCl3, and SnCl2) by a simple method. In the reaction process, thiophenic sulfur compounds are extracted to the MIL phase and then oxidized to their corresponding sulfones by active oxygen species. To optimize the ECODS process, main factors that would affect the desulfurization of fuels are investigated systematically. The obtained data illustrate that such MILs are potential solvents to be used in the ECODS reaction system to produce clean fuels.

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EXPERIMENTAL SECTION

Photochemical Oxidative Desulfurization. A total of 1 mL of MIL and 15 mL of model oil were added to a 30 mL homemade twonecked quartz flask. Fresh air was introduced to the system by an air pump at the airflow velocity of 5 mL/min. Here, a super thermostatic water bath was used to maintain the reaction temperature. Then, a certain amount of aldehyde was added to the reaction system after extraction for 20 min. The mixture was stirred for 2.5 h under UV irradiation (with a 250 W high-pressure Hg lamp). The reaction solution samples were collected every 0.5 h. In the experiment, the upper clear oil was collected and injected into gas chromatography (GC, Agilent 7890A, HP-5 column, 30 m long × 0.32 mm inner diameter × 0.25 μm film thickness) with a flame ionization detector (FID) by microinjector to evaluate the sulfur content. The conversion of dibenzothiophene (DBT), benzothiophene (BT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT) in the model oil was used to indicate the removal of sulfur compounds. The temperature of the GC process started at 100 °C and rose to 200 °C at 15 °C/min. The injector temperature was 300 °C, and the detector temperature was 250 °C. The sulfur removal efficiency was calculated as shown in eq 1, where C0 (ppm) was the initial sulfur concentration in the model oil and Ct (ppm) was the transient sulfur concentration at time t (h). This calculated method was accepted by almost all groups. In combination with the above two reasons, this conversion rate could provide the sulfur removal efficiency.

sulfur removal (%) = (1 − Ct /C0) × 100

Figure 1. FTIR spectra of (a) Et3NHCl, (b) [Et3NH]FeCl4, (c) [Et3NH]2CuCl4, (d) [Et3NH]SnCl3, and (e) [Et3NH]2ZnCl4.

clearly observed in comparison with Et3NHCl, indicating the formation of a more stable state.53 To better understand the anionic structure of MILs, Raman spectra of different MILs were investigated. A characteristic peak of 333 cm−1 is observed in Figure 2a, which is attributed to Fe−Cl stretching of FeCl4−. Characteristic peaks of CuCl42−, SnCl3−, and ZnCl42− are observed in panels b, c, and d of Figure 2, respectively. Besides, UV−vis spectra of the asprepared MILs were collected and displayed in Figure S1 of the Supporting Information. When Fe-based IL is selected, characteristic peaks of FeCl4− are observed at 246, 316, and 346 nm according to the previous reports.65,66 Characteristic peaks at 258, 311, and 459 nm are detected in Cu-based IL, which is assigned to CuCl42− according to the literature published previously.67 The only one band observed at 226 nm in Figure S1c of the Supporting Information indicates the existence of tetrahedral Sn species, i.e., SnCl3−.68,69 The absorption curve in Figure S1d of the Supporting Information is consistent with that reported in the previous literature,70 indicating the structure of the ZnCl42− anion in MIL. In combination with the data from FTIR and Raman obtained above, the chemical formula of four different MILs can be confirmed as [Et3NH]FeCl4, [Et3NH]2CuCl4, [Et3NH]SnCl3 and [Et3NH]2ZnCl4. Effect of Different Desulfurization Systems on the Sulfur Removal of DBT. The results of different desulfurization systems were investigated systematically and shown in Figure 3. Experimental conditions that might affect desulfurization were studied systematically. Here, DBT was selected as a representative sulfur compound in fuel because it was one of the main refractory sulfur compounds in the HDS treatment. First of all, experiments were carried out with only MIL extraction (14.5%), UV light irradiation (2.2%), and IBA addition (3.3%). Although sulfur removal for these reaction systems is very low, the relatively higher extraction efficiency indicates that Fe-based IL has extraction ability of DBT to some extent. Secondly, sulfur removal of UV light irradiation and IBA addition combined with MIL extraction was researched, respectively. However, the sulfur removal increased slightly (16.1 and 15.8%, respectively). Thirdly, sulfur removal was studied in conditions of UV light irradiation and IBA addition. The increased sulfur removal (54.1%) shows that active oxide species are generated from IBA and air introduced under the

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After reaction, the MIL phase was separated and collected in a 10 mL centrifugal tube and then 2 mL of CCl4 was added to the centrifugal tube to anti-extract oxidized sulfur compounds. Then, the CCl4 phase was collected and analyzed by gas chromatography−mass spectrometry (GC−MS, Agilent 7890/5975C GC/mass selective detector (MSD); HP-5 MS column, 30 m long × 250 mm inner diameter × 0.25 μm film thickness). The temperature program of the GC−MS process started at 100 °C and rose to 200 °C at 15 °C/min. The injector temperature was 250 °C. The temperature of the quadrupole, ion source, and AUX interface was 150, 230, and 280 °C, respectively. Solubility of MIL in n-Octane. High-performance liquid chromatography [HPLC, Agilent Technologies 1200 series equipped with an ultraviolet−visible (UV−vis) detector under 316 nm wavelength; column, Agilent TC-C18(2), 4.6 × 150 mm, 5 μm; column temperature, 35 °C; mobile phase, 85:15 methanol/water; and flow rate, 1 mL/min] was used to analyze solubility of MIL in noctane.

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RESULTS AND DISCUSSION Characterization of MILs. To confirm the structure of the as-prepared MILs, Fourier transform infrared (FTIR) spectra of four different MILs were collected and displayed in Figure 1. The analogous spectrum of each compound indicates a similar structure. However, a red shift of peaks of C−H (from 1476 to 1469−1466 cm−1, from 1398 to 1290−1288 cm−1, from 1173 to 1163−1157 cm−1, and from 1038 to 1030−1025 cm−1) is 1377

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Figure 2. Raman spectra of (a) [Et3NH]FeCl4, (b) [Et3NH]2CuCl4, (c) [Et3NH]SnCl3, and (d) [Et3NH]2ZnCl4.

Table 1. Effect of MILs on Sulfur Removal Efficiencya S removal efficiency of different ILs (%) entry

types of MILs

only extraction

S removal

1 2 3 4 5

[Et3NH]FeCl4 [Et3NH]2CuCl4 [Et3NH]SnCl3 [Et3NH]2ZnCl4

14.5 9.5 10.2 3.4

99.4 63.8 69.3 62.7

S removalb

54.1

Reaction conditions: T = 40 °C, t = 2.5 h, V(model oil) = 15 mL, V(MILs) = 1 mL, V(IBA) = 150 μL, and v(air) = 5 mL/min. bS removal without MILs.

a

from the results, all MILs show their extraction ability of DBT to some extent. Among all of these MILs, [Et3NH]FeCl4 shows the best extraction ability, which led to the highest sulfur removal directly after reaction for 2.5 h. All of these data indicated that the structure and the species of anions would impact the sulfur removal to a large extent. As mentioned above, [Et3NH]FeCl4 exhibits excellent performance in extraction and catalytic oxidative processes and, thus, selected in the following investigation finally. The amount of MIL is also found to be an important factor for the extraction process, which may influence the final distribution of sulfur compounds in the MIL and oil phase significantly. It can be seen in Figure 4 that the desulfurization efficiency increased with the increased MIL volume in the same reaction time. The increased MIL volume could enhance the transfer of sulfur compounds from the oil phase to the MIL phase, which might be a reasonable explanation for the experimental results mentioned before. When the MIL amount increases from 0.8 to 1.0 mL, the desulfurization efficiency increases from 95.8 to 99.4%. However, when 1.2 mL of MIL is used, the desulfurization efficiency could reach up to 99.7%,

Figure 3. Sulfur removal efficiency with different reaction systems. Reaction conditions: T = 40 °C, t = 2.5 h, V(model oil) = 15 mL, V([Et3NH]FeCl4) = 1 mL, V(IBA) = 150 μL, and v(air) = 5 mL/min.

irradiation of UV light. Finally, deep sulfur removal (99.3%) achieved after Fe-based IL is added, forming a liquid−liquid extraction and catalytic oxidative desulfurization system. A blank experiment with high pure nitrogen (N2) introduced instead of air is also conducted to confirm the important role that air played in this desulfurization system. All of these data demonstrate that MIL extraction, UV light irradiation, IBA addition, and fresh air introduction all play important roles in this desulfurization system. Optimization of Desulfurization Parameters. Sulfur removal with four as-prepared MILs is investigated at the volume of 1:15 and shown in Table 1. Using such a small amount of exractant has been rarely reported before. Both the EDS and ECODS of model oil are investigated. As can be seen 1378

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Figure 6. Sulfur removal of DBT with different S concentrations. Reaction conditions: T = 40 °C, V(model oil) = 15 mL, V(IBA) = 150 μL, V([Et3NH]FeCl4) = 1 mL, and v(air) = 5 mL/min.

Figure 4. Influence of the amount of [Et3NH]FeCl4 on sulfur removal. Reaction conditions: T = 40 °C, V(model oil) = 15 mL, V(IBA) = 150 μL, and v(air) = 5 mL/min.

Effect of Different Substrates on the Sulfur Removal. The removal of DBT in model oil is proven effective from the data that we have obtained previously. Considering the existence of various sulfur compounds in refined oil, the desulfurization efficiency of different substrates is investigated and shown in Table 2. The sulfur removal of three different

which is slightly higher than 99.4%. Finally, 1 mL of MIL is selected as an optimized volume in the following studies. The amount of IBA added to the reaction system, as another important factor, was also investigated, as shown in Figure 5.

Table 2. Effect of Different Substrates on Sulfur Removala S removal (%) entry

substrate

EDS

ECODS

1 2 3

DBT BT 4,6-DMDBT

14.5 14.7 9.7

99.4 87.3 82.1

Reaction conditions: T = 40 °C, V(model oil) = 15 mL, V(IBA) = 150 μL, V([Et3NH]FeCl4) = 1 mL, v(air) = 5 mL/min, and t = 2.5 h. a

sulfur compounds decreases in the order of DBT > BT > 4,6DMDBT. The process of ECODS could be divided into two processes. The sulfur compounds in the oil phase would be extracted into the MIL phase first. In this process, the EDS of different substrates decreases in the order of DBT > BT > 4,6DMDBT, which might be caused by the electron affinity and electrophilicity of different sulfides.71 However, after reaction for 2.5 h, the sulfur removal of different sulfides changes as DBT > 4,6-DMDBT > BT. This is consistent with the f(r) Fukui function, a function that describes the electron density in a frontier orbital, of different sulfides.71 After reaction for 5 h, DBT could be totally removed and the sulfur removal of 4,6DMDBT and BT could reach up to 87.3 and 82.1%, respectively. After reaction, the MIL phase is collected to detect the oxidized sulfur compounds. An appropriate amount of CCl4 is added to the collected MIL phase to re-extract the oxidized sulfur compounds. Then, the CCl4 phase is separated and analyzed by GC−MS procedures. The total ion chromatogram (TIC) of CCl4 is collected and displayed in Figure S3 of the Supporting Information. The mass spectrum shows that the molecular ion peak at m/z 216, 166, and 244 was corresponded to the products of DBT, BT, and 4,6-DMDBT, respectively. Now, the following conclusions can be drawn as: the thiophenic sulfur compounds are extracted and oxidized to their corresponding sulfones.

Figure 5. Influence of the amount of IBA on the removal of DBT. Reaction conditions: T = 40 °C, V(model oil) = 15 mL, V([Et3NH]FeCl4) = 1 mL, and v(air) = 5 mL/min.

The oxidization process happened slightly when IBA is absent in the reaction system. As the adding amount of IBA increased from 50 to 150 μL, the desulfurization efficiency increased from 64.2 to 99.4% accordingly, achieving deep desulfurization. Finally, the used amount of IBA (150 μL) is confirmed in the following research process. Sulfur Removal of DBT with Oils of Different S Concentrations. As the concentration of sulfur compounds is varied in the refined oil, the study of desulfurization efficiency with different S concentrations is necessary and important. As displayed in Figure 6, the desulfurization efficiency decreases slightly (>90%), even if the initial S concentration increases to 1000 ppm. After reaction for 4 h, the final desulfurization efficiency is 100% (500 ppm), 98.7% (600 ppm), 95.4% (800 ppm), and 91.3% (1000 ppm), respectively. All of these data shown above indicated the super sulfur removal ability of this ECODS system, which is rarely reported previously. 1379

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Energy & Fuels Effect of the Co-existence of Olefins and Aromatics on Sulfur Removal. The complicated components in the refined oil might influence the desulfurization efficiency. Thus, the actual impacts of other components are needed to be investigated. Olefins and aromatics are two main components that co-existed in the refined oil, besides alkanes. They are the two main hydrocarbons that might affect the desulfurization efficiency. Cyclohexene and 1-octene, here, are selected as representative substances (cyclic olefin and linear olefin, respectively) in the following experiments. The influence of the addition of the above two olefins are displayed in Figures 7 and 8, respectively.

wt % addition of 1-octene, respectively. Here, it can be concluded that the existence of cyclohexene has an obvious influence in this oxidative desulfurization system. Thus, some pretreatments need to be performed to remove cyclic olefins before the oxidative desulfurization process. Besides, it can also be concluded that this desulfurization method is more suitable for the oils with a lower cyclic olefin concentration. Paraxylene is selected as a representative of aromatics in the following experiments. The influence of final desulfurization efficiency with the co-existence of paraxylene is collected and displayed in Figure 9. The desulfurization efficiency decreases

Figure 9. Effect of additional paraxylene at various concentrations (wt %) in the model oil. Reaction conditions: T = 40 °C, V(model oil) = 15 mL, V(IBA) = 150 μL, V([Et3NH]FeCl4) = 1 mL, and v(air) = 5 mL/min.

Figure 7. Effect of additional cyclohexene at various concentrations (wt %) in the model oil. Reaction conditions: T = 40 °C, V(model oil) = 15 mL, V(IBA) = 150 μL, V([Et3NH]FeCl4) = 1 mL, and v(air) = 5 mL/min.

from 99.4 to 98.6 and 96.1% with 1 and 5 wt % addition of paraxylene, respectively. Although hard to be oxidized, the added aromatic could absorb UV light, which might be negative to the oxidative reactions. Here, we can draw the following conclusion: the co-existence of aromatics would not influence the final desulfurization efficiency too much in this reaction system. Recycling Performance of MILs. From an industrial point of view, the recycling performance of the extractant is an important factor that needs to be investigated. After reaction, this reaction system still exhibits a biphasic system, including oil phase (upper phase) and MIL phase (lower phase). The immiscibility of MIL and oil is confirmed by HPLC, as shown in Figure S4 of the Supporting Information. Thus, the model oil is simply separated by a simple decantation method from the MIL phase after each run. Then, fresh model oil and IBA are added directly for the next run. As shown in Figure 10, sulfur removal changed slightly after 5 times recycling using such a small amount. All of these data indicate that [Et3NH]FeCl4 is a good candidate for industrial application as a remarkably reusable and low-cost regeneration method.

Figure 8. Effect of additional 1-octene at various concentrations (wt %) in the model oil. Reaction conditions: T = 40 °C, V(model oil) = 15 mL, V(IBA) = 150 μL, V([Et3NH]FeCl4) = 1 mL, and v(air) = 5 mL/min.

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When cyclohexene is selected, the final desulfurization efficiency decreases sharply after reaction for 2.5 h. This might be caused by its ring-opening reaction as the existence of a frangible double bond. Finally, desulfurization efficiency decreases from 99.4 to 93.8 and 68.4% with 1 and 5 wt % addition of cyclohexene, respectively. When 1-octene is selected, the final desulfurization efficiency also decreases but not too much after reaction for 2.5 h. Finally, desulfurization efficiency decreases from 99.4 to 98.2 and 95.1% with 1 and 5

CONCLUSION In this work, MILs are synthesized successfully and used as an extractant for the ECODS process of model oil using O2 as an oxidant under mild conditions. After the selection of proper MIL, the desulfurization process is optimized through the study of the main parameters. Model oils with different S concentrations and substrates all show excellent sulfur removal performance, indicating the powerful sulfur removal ability in 1380

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ACKNOWLEDGMENTS

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REFERENCES

This work was financially supported by the National Natural Science Foundation of China (21376111, 21506080, and 21576122), the Natural Science Foundation of Jiangsu Province (BK2012717 and BK20150485), the Postdoctoral Foundation of China (2015M570412), the Graduate Education Innovation Project of Jiangsu Province (KYLX16_0911) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Figure 10. Investigation of the recycling performance of MIL. Reaction conditions: T = 40 °C, V(model oil) = 15 mL, V(IBA) = 150 μL, V([Et3NH]FeCl4) = 1 mL, and v(air) = 5 mL/min.

this reaction system. The existence of other hydrocarbons, especially the co-existence of cyclohexene, could influence the sulfur removal to some extent, indicating that this reaction system is more suitable for actual diesel fuel and gasoline with a lower olefin concentration. The sulfur compounds existing in model oil are confirmed to convert to their corresponding sulfones by GC−MS measurements. The above study shows many advantages, such as deep desulfurization, low-cost of extractant and oxidant, use of a small amount of extractant, moderate reaction environment, and providing a new route for the deep desulfurization process of fuels.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b02624. Experimental section, elemental analysis of C, H, and N in the as-prepared MILs (Table S1), UV−vis spectra of (a) [Et3NH]FeCl4, (b) [Et3NH]2CuCl4, (c) [Et3NH]SnCl3, and (d) [Et3NH]2ZnCl4 (Figure S1), (a) cation and (b) anion electrospray ionization mass spectrometry (ESI−MS) spectra of (A) [Et 3 NH]FeCl 4 , (B) [Et 3 NH] 2 CuCl 4 , (C) [Et 3 NH]SnCl 3 , and (D) [Et3NH]2ZnCl4 (Figure S2), GC−MS of the oxidation products of different substrates: (a) BTO2, (b) DBTO2, and (c) 4,6-DMDBTO2 (Figure S3), and HPLC chromatograms of fresh oil, oil after reaction, and MIL ([Et3NH]FeCl4) (Figure S4) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhigang Chen: 0000-0003-4350-1000 Huaming Li: 0000-0002-9538-5358 Notes

The authors declare no competing financial interest. 1381

DOI: 10.1021/acs.energyfuels.6b02624 Energy Fuels 2017, 31, 1376−1382

Article

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DOI: 10.1021/acs.energyfuels.6b02624 Energy Fuels 2017, 31, 1376−1382