Mild Process for Reductive Desulfurization of Diesel Fuel Using

Article pubs.acs.org/IECR

Mild Process for Reductive Desulfurization of Diesel Fuel Using Sodium Borohydride in Situ Generated via Sodium Metaborate Electroreduction Chenhua Shu, Tonghua Sun,* Jinping Jia, and Ziyang Lou School of Environmental Science and Engineering, Shanghai Jiaotong University, Dongchuan Road 800, Shanghai, 200240 China ABSTRACT: A novel integrated process was proposed for reductive desulfurization of diesel fuel, in which the reductant sodium borohydride (NaBH4) was in situ generated via sodium metaborate (NaBO2) electroreduction. In order to improve the conversion rate of BO2− into BH4−, NaBO2 electroreduction was fulfilled by applying pulse voltage and using a boron-doped diamond (BDD) thin film electrode. The NaBO2 electroreduction process was analyzed by cyclic voltammetry and 11boron nuclear magnetic resonance (11B NMR) and the factors that influenced desulfurization efficiency were investigated. Under the optimal conditions desulfurization efficiency reached more than 93% for model diesel fuel. The components of model diesel fuel after desulfurization were analyzed by gas chromatography/mass spectrometry (GC/MS) and the electrolyte and digestion solution of precipitate were analyzed by inductively coupled plasma (ICP). Results indicated that B recycle was realized at the same time as desulfurization. Finally, the desulfurization of real diesel fuel was investigated. This work showed that the integrated process may be a new option for desulfurization of diesel fuel due to its mild conditions.

1. INTRODUCTION Many countries have implemented stringent legislation to regulate the sulfur content of transportation fuels such as diesel fuel because it has an impact on the environmental pollution and it spoils the low-temperature activity of automotive catalytic converters.1 Zero-emission and zero-levels of sulfur content are previewed in the near future.2 The traditional industrial process for removal of sulfur from fuels is hydrodesulfurization (HDS), which is effective for aliphatic and acyclic S-compounds but less effective for benzothiophene (BT), dibenzothiophene (DBT), and their derivatives.3 Furthermore, it requires high investment and operating costs and suffers from significant loss in the octane number caused by saturation of olefins.3 Therefore, some non-HDS alternative technologies such as alkylation,4 extraction,5,6 oxidation,7,8 adsorption,9−14 membrane separation,1,15 and biodesulfurization16,17 have been proposed. However, all of the non-HDS methods also possess their own drawbacks, e.g., low desulfurization efficiency, deteriorating quality of gasoline, high cost, or difficulty in industrial application.1−3,18 Therefore, it is necessary to make further efforts to explore more effective methods for fuel desulfurization. Sodium borohydride (NaBH4) is an excellent reductant, which was widely used in the desulfurization of Scompounds.19,20 Some studies have been carried out recently on sulfur removal from coal and gasoline using NaBH4 as reductant, and high desulfurization efficiency was obtained.21−23 However, its industrial application in fuel desulfurization has been hindered for its high price and wet instability. To solve this problem, an integrated process of reductive desulfurization by NaBH4 and in situ regeneration of NaBH4 via sodium metaborate (NaBO2) electroreduction has been proposed here. Namely, NaBO2 is first converted into NaBH4 by electroreduction, subsequently the generated NaBH4 is used for reductive desulfurization of fuel with NaBO2 being a © 2013 American Chemical Society

byproduct, and then the byproduct NaBO2 will be converted into NaBH4 by electroreduction again. The above steps repeat again and again. As shown in Figure 1, the integrated process mainly involves four steps:24,25 (1) NaBH4 is obtained by NaBO2 electroreduction (BO2− + 6 H2O + 8 e− → BH4− + 8 OH−); (2) NaBH4 reacts with NiCl2·6H2O to give nickel boride (4NaBH4 + 2NiCl2 + 9H2O→Ni2B + 3H3BO3 + 4NaCl + 12.5H2); (3) The generated nickel boride catalyzes the formation of H2 (H*) from NaBH4 and H2O (NaBH4 + H2O → NaBO2 + H2); (4) The organosulfur compounds are transformed into corresponding hydrocarbons and H2S in the presence of H2 (H*) and nickel boride. Consequently, B recycle can be realized at the same time as fuel desulfurization and the cost of desulfurization can be drastically reduced. On the other hand, the reactivity of nickel boride is gradually lost as it ages, which degrades its desulfurization performance.24,25 However, fresh nickel boride is produced little by little with the in situ generation of NaBH4 in the integrated process, which avoid the aging effect of nickel boride, accordingly desulfurization efficiency can be greatly improved. In this work, the integrated process was applied to the desulfurization of model and real diesel fuels. It should be especially pointed out that desulfurization efficiency greatly depends on the conversion rate of BO2− into BH4−; therefore, in order to improve the conversion rate of BO2− into BH4−, Received: Revised: Accepted: Published: 7660

April 4, 2013 May 12, 2013 May 14, 2013 May 14, 2013 dx.doi.org/10.1021/ie401073c | Ind. Eng. Chem. Res. 2013, 52, 7660−7667

Industrial & Engineering Chemistry Research

Article

Figure 1. Schematic diagram of the desulfurization experiment.

The experimental procedure is as follows. First, an aqueous solution containing NaOH and a mixed aqueous solution containing NaOH and NaBO2 were added into the counter electrode and working electrode compartments, respectively. Meanwhile, model or real diesel fuel was added into the working electrode compartment. Then, the magnetic stirrer and power were turned on in sequence, and the reaction began immediately. A few minutes later, NiCl2·6H2O was added into the working electrode compartment in small portions. Second, after the end of the reaction, the mixture in the working electrode compartment was filtered. The filtrate which included two liquid phases was separated by a separatory funnel and the precipitate was digested by phosphoric acid. Finally, the aqueous phase of the filtrate was used for elements content analysis and the oil phase of the filtrate was used for S-content analysis and components analysis. Meanwhile, the digestion solution of precipitate and the electrolytes in the counter compartment were also used for elements content analysis. The desulfurization efficiency was calculated by the following equation:

NaBO2 electroreduction was fulfilled by applying pulse voltage and using a boron-doped diamond (BDD) thin film electrode as the working electrode. The NaBO2 electroreduction process was analyzed by cyclic voltammetry and 11B nuclear magnetic resonance (NMR), and the factors that influence desulfurization efficiency were investigated. The components of model diesel fuel after desulfurization were analyzed, and the element content of electrolytes and the digestion solution of precipitate were determined. Possible reaction routes of model Scompounds were proposed.

2. EXPERIMENTAL SECTION 2.1. Materials. A BDD thin film electrode was prepared by a hot filament chemical vapor deposition (HFCVD) technique on tantalum substrate from acetone and hydrogen mixtures. Trimethyl borate was served as the boron source. Model diesel fuel with sulfur contents of 493 ppmw was prepared by dissolving 3-methylbenzothiophene (3-MBT) and dibenzothiophene (DBT) in n-octane. Real diesel fuel with sulfur contents of 458 ppmw was supplied by Sinopec Shanghai Petrochemical Company. A perfluorosulfonic acid ionic exchange membrane was purchased from Best Industrial & Trade Co. Ltd. (Beijing, China). 3-MBT (96%), DBT (98%) and n-octane (AR) were purchased from Shanghai Aladdin reagent Co. Ltd. NaBO2·4H2O (>99%, AR), and NiCl2·6H2O (>98%, AR) and C4H6O4Pb·3H2O (>99%, AR) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). 2.2. Desulfurization Experiment. As shown in Figure 1, a typical desulfurization experiment was carried out in an electrolytic cell with a cation exchange membrane separating working electrode (WE) and counter electrode (CE) compartments. The working electrode was a BDD thin film electrode (40 × 10 mm), and the counter electrode was a graphite electrode (40 × 10 mm). The reference electrode (RE) was a saturated calomel electrode, and all experimental potentials reported were normalized to this reference electrode. A gas pipeline was used to transfer the generated H2S from the working electrode compartment to the counter electrode compartment. Pulse voltages were obtained from an electrochemical workstation (Autolab PGSTAT30) with PC software control (GPES 4.9). All desulfurization experiments were conducted at ambient temperature (10−30 °C) and pressure.

desulfurization efficiency(wt%) =

TS1 − TS2 × 100% TS1 (1)

where TS1 is the S-content in original diesel fuel sample and TS2 is the S-content in treated diesel fuel sample. 2.3. Analysis Methods. The S-content in model diesel fuel was determined by using a gas chromatography-flame ionization detector (GC-FID, GC-2010, Shimadzu) equipped with a DB-FFAP capillary column (0.25 mm × 30 m). Analysis conditions were as follows: injector temperature was 340 °C and detector temperature was 250 °C, column temperature was programmed from 100 to 250 °C (8 min) at 15 °C/min. The injection sample was 1 μL for all samples. The S-content in real diesel fuel was determined by sulfur−nitrogen analyzer (Antek 9000, Antek). The element content of aqueous solutions was determined by inductively coupled plasma (ICP, 7500a, Agilent). The components of model diesel fuel after desulfurization was analyzed by gas chromatography/mass spectrometer (GC/MS-QP2010, Shimadzu) equipped with a RTX-5 ms capillary GC column (0.25 mm × 30 m). The injector temperature was 250 °C and oven temperature was 7661

dx.doi.org/10.1021/ie401073c | Ind. Eng. Chem. Res. 2013, 52, 7660−7667


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programmed from 50 °C (2 min) to 200 °C at 5 °C/min and then ramped to 280 °C at 8 °C/min. The injection sample was 1 μL for all samples. Mass spectra conditions were as follows: ionization voltage 70 eV; ion source temperature 200 °C; full scan mode in m/z range 33−650 with a 0.5 s/scan velocity. Compounds were identified by use of National Insititute of Standards and Technology (NIST) 147 and NIST 27 Library of Mass Spectra.

3. RESULTS AND DISCUSSION 3.1. Analysis of the Electroreduction of NaBO2 into NaBH4 Process. Because desulfurization efficiency greatly depends on the conversion rate of BO2− into BH4− in the integrated process, it is crucial to improve the conversion rate of BO2− into BH4−. A lot of studies about the electroreduction of NaBO2 into NaBH4 have been reported.26−31 The electroreduction mechanism is as follows: cathode:

BO2− + 6H 2O + 8e− → BH4 − + 8OH−

E1/2 = − 1.24 V anode:

Figure 2. Cyclic voltammogram of a BDD film electrode in 0.1 mol/L NaOH and 0.1 mol/L NaOH + 0.2 mol/L NaBO2 aqueous solutions at a scan rate of 5 mV/s.

(vs SHE)

mol/L NaBO2 aqueous solution from −1.2 to −1.8 V. Therefore, it could be inferred that the observed peak from −1.2 to −1.8 V might be related to the electroreduction of NaBO2 into NaBH4. Besides, hydrogen evolution commenced at lower than −1.8 V and oxygen evolution commenced at higher than +0.6 V for the BDD film electrode in 0.1 mol/L NaOH + 0.2 mol/L NaBO2 aqueous solution. This meant that forward pulse voltage should be higher than −1.8 V and reverse pulse voltage should be lower than +0.6 V in order to avoid hydrogen evolution reaction and oxygen evolution reaction in the following experiments. 3.1.2. 11B NMR. The electrolytes of 0.1 mol/L NaOH + 0.2 mol/L NaBO2 before and after electroreduction were analyzed by 11B NMR. Figure 3 shows the results of 11B NMR. The

(2)

4OH− − 4e− → 2H 2O + O2 ↑

E1/2 = 1.229 V

(vs SHE)

(3)

BO2−

BH4−

The above equations show that is reduced to on the cathode, but due to charge repulsion it is very difficult for the negatively charged BO2− to get close to the surface of the cathode. So the conversion rate of NaBO2 into NaBH4 is very low when constant voltage is applied. Pulse voltage is that a forward pulse (cathodic pulse) followed by a reverse pulse (anodic pulse), which means that the working electrode will turn into the anode after working for a period of time as the cathode. As shown in Figure 1, when the working electrode works as the anode, the negative charged BO2− will be attracted to the surface of working electrode, and then the working electrode works as the cathode, the BO2− gathered on the surface of working electrode will be reduced immediately into BH4− ahead of being excluded. Afterward, the directions of pulse voltage continue alternating again and again, which means that the attraction and electroreduction of BO2− alternates on the working electrode. So the conversion rate of NaBO2 into NaBH4 will be improved greatly by applying pulse voltage. It is generally known that the hydrogen evolution reaction (2H2O + 2e− → H2↑ + 2OH−) and oxygen evolution reaction (4OH− - 4e− → 2H2O + O2↑) may occur when the working electrode works as the cathode and anode, respectively. So an electrode with wide potential window is necessary to avoid the two undesirable reactions. BDD film electrode is a typical electrode with wide potential window. Furthermore, its corrosion resistance makes it stable in strong alkaline solution, its antifouling property makes it useful in complex environments and its low background current is beneficial to improving current efficiency.32−39 Therefore, BDD film electrode was used as the working electrode in this work. 3.1.1. Cyclic Voltammetry. Cyclic voltammetry analysis was conducted in order to determine the voltage range for electroreduction of NaBO2 into NaBH4 by BDD film electrode. Figure 2 shows the cyclic voltammogram of BDD thin film electrodes in 0.1 mol/L NaOH and 0.1 mol/L NaOH + 0.2 mol/L NaBO2 aqueous solutions. No reduction peaks were observed in 0.1 mol/L NaOH aqueous solution. However, a distinct reduction peak was observed in 0.1 mol/L NaOH + 0.2

Figure 3. 11B NMR spectrogram of electrolytes before and after electroreduction. Reaction conditions: −1.5 V forward pulse voltage, 0.3 V reverse pulse voltage, 1.5 s forward pulse duration, 0.5 s reverse pulse duration, 0.2 mol/L NaBO2 concentration, 0.1 mol/L NaOH concentration, and 1.5 h electrolytic time.

resonance line at near 1.6 ppm can be assigned to NaBO240 and that at near 42 ppm can be assigned to NaBH441 which appeared only in the electrolyte after electrochemical treatment. The result was in good agreement with the result of cyclic voltammetry, which suggested that NaBH4 was obtained via NaBO2 electroreduction with pulse voltage using a BDD thin 7662



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Figure 4. Effect of (a) forward pulse voltage, (b) reverse pulse voltage, (c) forward pulse duration, and (d) reverse pulse duration on desulfurization efficiency. Other reaction conditions: 0.2 mol/L NaBO2 concentration, 1.2 mmol/L NiCl2 concentration, 1/3 volume ratio of oil to electrolyte, and 1.5 h electrolytic time.

film electrode. Furthermore, faradaic efficiency of 84.96% was obtained for the process of NaBO2 electroreduction. 3.2. Effect of Pulse Parameters on Desulfurization Efficiency. 3.2.1. Forward Pulse Voltage. The role of forward pulse voltage is to convert BO2− into BH4− by electroreduction. According to the cyclic voltammogram of BDD thin film electrode (Figure 2), the reduction peak of NaBO2 into NaBH4 was observed from −1.2 to −1.8 V. Although reducing the forward pulse voltage probably improved desulfurization efficiency, too low forward pulse voltage would lead to hydrogen evolution reaction. Figure 4a shows the effect of forward pulse voltage on desulfurization efficiency, compared with constant voltage. When constant voltage was applied desulfurization efficiency initially increased with the reduction of constant voltage and then reached its maximum value (about 40%) at −1.8 V. However, the maximum value of desulfurization efficiency soared to 93.3% when forward pulse voltage was −1.5 V. The results suggested that the effectiveness of pulse voltage on electroreduction NaBO2 into NaBH4 was significantly better than that of constant voltage. When forward pulse voltage continued reducing desulfurization efficiency decreased instead of increasing. The possible reason was that hydrogen evolution reaction took place, which reduced the conversion rate of NaBO2 into NaBH4. The result was consistent with the result of cyclic voltammetry (Figure 2). So −1.5 V was used in the following experiments. 3.2.2. Reverse Pulse Voltage. The role of reverse pulse voltage was to attract BO2− to the surface of BDD thin film electrode. A high reverse pulse voltage benefitted the attraction of BO2− to the electrode surface. But oxygen evolution reaction (Figure 2) and conversion of BO2− to borax40 might take place when reverse pulse voltage was too high. Furthermore, the electrooxidation of the generated BH4− would intensify with

the increasing of reverse pulse voltage. On the contrary, a low reverse pulse voltage was not enough to attract BO2− to the surface of BDD thin film electrode. As shown in Figure 4b, desulfurization efficiency initially increased with the increasing of reverse pulse voltage, and then decreased. The maximum value of desulfurization efficiency was obtained when reverse pulse voltage was +0.3 V. 3.2.3. Forward Pulse Duration. It is generally known that total electrolytic time consists of total forward pulse time and total reverse pulse time (i.e., Total electrolytic time = forward pulse duration × pulse numbers + reverse pulse duration × pulse numbers). Although increasing forward pulse duration could improve desulfurization efficiency in certain range due to the increase of conversion rate of BO2− into BH4−, if forward pulse duration was too long, desulfurization efficiency would also decrease because pulse numbers would reduce when both total electrolytic time and reverse pulse duration were constant. As shown in Figure 4c, the maximum value of desulfurization efficiency was obtained when forward pulse duration was 1.5 s. 3.2.4. Reverse Pulse Duration. The electrooxidation of BH4− into BO2− (BH4− + 8 OH− → BO2− + 6 H2O + 8 e−) took place when reverse pulse voltage was applied, which was in competition with the hydrolysis of BH4−. If reverse pulse duration was too short, it would not be enough to transfer BO2− to the surface of BDD thin film electrode, which would lead to the decrease of desulfurization efficiency. Conversely, desulfurization efficiency would also decrease because the electrooxidation of BH4− into BO2− would increase. Figure 4d shows the effect of reverse pulse duration on desulfurization efficiency, desulfurization efficiency initially increased with the increasing of reverse pulse duration and then decreased. The maximum value of desulfurization efficiency was obtained when reverse pulse duration was 0.5 s. 7663



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3.3. Effect of NaBO2 Concentration on Desulfurization Efficiency. NaBO2 is the source of NaBH4, so NaBO2 concentration is a key factor of producing NaBH4. As shown in Figure 5, desulfurization efficiency increased with the

Figure 6. Effect of NiCl2 concentration on desulfurization efficiency. Reaction conditions: −1.5 V forward pulse voltage, 0.3 V reverse pulse voltage, 1.5 s forward pulse duration, 0.5 s reverse pulse duration, 0.2 mol/L NaBO2 concentration, 1/3 volume ratio of oil to electrolyte, and 1.5 h electrolytic time. Figure 5. Effect of NaBO2 concentration on desulfurization efficiency. Reaction conditions: −1.5 V forward pulse voltage, 0.3 V reverse pulse voltage, 1.5 s forward pulse duration, 0.5 s reverse pulse duration, 1.2 mmol/L NiCl2 concentration, 1/3 volume ratio of oil to electrolyte, and 1.5 h electrolytic time.

content of digestion solution of precipitate (Table 1), most of the Ni from NiCl2·6H2O was precipitated as nickel boride. It Table 1. ICP Analysis of Electrolytes and Digestion Solution of Precipitatea

increasing of NaBO2 concentration when NaBO2 concentration was less than 0.2 mol/L and then remained almost constant even though NaBO2 concentration continued increasing. The reason was that the BH4− concentration in the reaction system mainly depended on the conversion rate of BO2− into BH4− and the hydrolysis rate of BH4−. The conversion rate of BO2− into BH4− increased with the increasing of NaBO2 concentration. When the conversion rate of BO2− into BH4− exceeded the hydrolysis rate of BH4−, the BH4− concentration in the reaction system would increase gradually. However, the increase of BH4− concentration would accelerate its electrooxidation when reverse pulse voltage was applied. Therefore, excess NaBO2 practically made no contribution to desulfurization efficiency. Namely, desulfurization efficiency would remain almost constant while increasing to a certain value. Consequently, 0.2 mol/L was chosen as the optimum NaBO2 concentration. 3.4. Effect of NiCl2 Concentration on Desulfurization Efficiency. It is well-known that metal borides (e.g., Ni2B, Co2B) are highly active catalysts which can be prepared readily from metal halides (e.g., NiCl2, CoCl2) and boron hydrides (e.g., NaBH4) in protic conditions.42 Especially, nickel boride has been employed as an efficient reagent for reductive desulfurization.19,20 In this work, NiCl2 was also added in the reaction system to produce nickel boride. As shown in Figure 6, desulfurization efficiency (50%) was still very low after electrolyzing for 3.5 h without NiCl2. However, desulfurization efficiency increased and the treatment time required to reach the maximum value of desulfurization efficiency decreased with the increasing of NiCl2 concentration. When NiCl2 concentration increased to 1.2 mmol/L desulfurization efficiency and the treatment time required to reach the maximum value of desulfurization efficiency remained almost stable. The desulfurization efficiency was above 93% after electrolyzing for 1.5 h. Obviously, the addition of NiCl2 greatly improved desulfurization efficiency. On the other hand, based on the elements

before reaction (mg)b

elements working electrode compartment counter electrode compartment digestion solution of precipitate

S Ni B S Ni Ni B S

after reaction (mg) 4.12

7.04 216.2

213.37 (98.6%) 3.82 0.02 6.99 2.49 0.02

Reaction condition: −1.5 V forward pulse voltage, 0.3 V reverse pulse voltage, 1.5 s forward pulse duration, 0.5 s reverse pulse duration, 0.2 mol/L NaBO2 concentration, 1.2 mmol/L NiCl2 concentration, 1/3 volume ratio of oil to electrolyte, and 1.5 h electrolytic time. b Theoretical value. a

could be inferred that nickel boride actually play a role in improving desulfurization efficiency. According to previous reports,24,25 a possible mechanism of desulfurization with nickel boride was proposed. As shown in Scheme 1, the prepared Scheme 1. Possible Mechanism of Desulfurization with Nickel Boride

nickel boride from NaBH4 and NiCl2·6H2O could strongly absorb the generated hydrogen (or active hydrogen) and the sulfur of organosulfur compounds because a part of electrons of boron were transferred to nickel, leading to an electron-rich state of nickel. Then the activated hydrogen adsorbed on the surface of nickel boride and nickel would form into a kind of 7664



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nickel hydride intermediate. Finally, the oxidative addition of the C−S bond of organosulfur compounds to the nickel atom of nickel hydride intermediate was followed by the reductive elimination of C−H, which resulted in the C−S bond cleavage. 3.5. Effect of Volume Ratio of Oil to Electrolyte on Desulfurization Efficiency. Model diesel fuel is nonpolar and hardly soluble in aqueous solution. So the volume ratio of oil to electrolyte can affect the mass transfer. As shown in Figure 7,

Figure 8. GC/MS spectra of model diesel fuel after desulfurization. Reaction conditions: −1.5 V forward pulse voltage, 0.3 V reverse pulse voltage, 1.5 s forward pulse duration, 0.5 s reverse pulse duration, 0.2 mol/L NaBO2 concentration, 1.2 mmol/L NiCl2 concentration, 1/3 volume ratio of oil to electrolyte, and 1.5 h electrolytic time.

Scheme 2. Reaction Routes of 3-Methylbenzothiophene and Dibenzothiophene Figure 7. Effect of volume ratio of oil to electrolyte on desulfurization efficiency. Reaction conditions: −1.5 V forward pulse voltage, 0.3 V reverse pulse voltage, 1.5 s forward pulse duration, 0.5 s reverse pulse duration, 0.2 mol/L NaBO2 concentration, 1.2 mmol/L NiCl2 concentration, and 1.5 h electrolytic time.

desulfurization efficiency increased initially and then decreased with the increasing of volume ratio of oil to electrolyte. The reason was that the increase of oil volume could make the transfer of BO2− to the surface of BDD thin film electrode difficult, which reduced the conversion rate of BO2− into BH4−, accordingly the desulfurization efficiency also reduced. Furthermore, the internal resistance of electrolyte increased with the increasing of oil volume, which decreased current density, accordingly the desulfurization efficiency also reduced. The maximum value of desulfurization efficiency was obtained when the volume ratio of oil to electrolyte was 1/3. 3.6. GC/MS Analysis of Model Diesel Fuel after Desulfurization. Figure 8 shows the components of model diesel fuel after desulfurization by GC/MS. There were mainly isopropylbenzene (IPB), biphenyl (BP), cyclohexylbenzene (CHB), traces of tetrahydrodibenzothiophene (THDBT) and hexahydrodibenzothiophene (HHDBT), unreacted 3-MBT and DBT. According to previous reports,43−46 the reaction route was shown in Scheme 2. The IPB should be obtained by direct desulfurization (DDS) of 3-MBT. The other products should be obtained by DDS and hydrogenation (HYD) of DBT. The DDS route proceeded over direct cleavage of C−S bond of DBT to BP, followed by hydrogenation to CHB. The HYD route proceeded over hydrogenation of DBT to THDBT and HHDBT, followed by cleavage of C−S bond to CHB. According to Figure 8, BP and CHB were the only primary reaction products of DBT and BP was far more than CHB, so it could be inferred that the DDS of DBT should be faster than the HYD of DBT and the hydrogenation of BP to CHB should be difficult, which was in agreement with the previous reports.44−46

3.7. Mass Balance. Table 1 presents the ICP analytical results of the electrolytes from working electrode and counter electrode compartments and the digestion solution of precipitate. The mass of total sulfur (TS) removal: calculated value (mg) = desulfurization efficiency (wt %) × total sulfur content (ppmw) × model diesel fuel mass (mg) = 93.3% × 493 × 10−6 × 17 500 mg = 8.05 mg; measured value (mg) = S in the working electrode compartment after reaction + S in the counter electrode compartment after reaction + S in the precipitate = 4.12 + 3.82 + 0.02 mg = 7.96 mg. Obviously, the measured value of TS removal was almost in accordance with its corresponding calculated value. Furthermore, it could be inferred that most of the removed S was transferred to the electrolytes. On the other hand, in order to determine the valence of S in the electrolytes, C4H6O4Pb aqueous solutions were added into the electrolytes in the working and counter compartments after desulfurization. Black precipitation of lead sulfide (PbS) was obtained immediately and the mass of precipitation in the working and counter compartments was 30.63 mg and 28.1 mg after washing and drying, respectively. So the S2‑mass in the working and counter compartments was 4.1 and 3.77 mg, respectively. According to Table 1, the TS mass in the working and counter compartments was almost in accordance with their S2‑ mass, which indicated that most of the S in the working and counter electrode compartments was present in the form of S2‑. Most of the Ni from NiCl2·6H2O (7.04 mg) was precipitated as nickel boride (6.99 mg). A large proportion of B (98.6%) still remained in the electrolyte in the working electrode compartment after reaction except for a 7665



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desulfurization of diesel fuel due to its mild conditions. Furthermore, this process is environment-friendly because fuel desulfurization was fulfilled only using low-voltage electricity and recyclable electrolyte.

small part precipitated as nickel boride (2.49 mg), which indicated that B recycle was realized at the same time as desulfurization. 3.8. Desulfurization of Real Diesel Fuel. The desulfurization performance of the integrated process for real diesel fuel was also investigated. Figure 9 shows the effect of electrolytic

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 21 54742817. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National Key Technology R&D Program (No. 2010BAK69B24) and the National Natural Science Foundation of China (No. 41173108).

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REFERENCES

(1) Mortaheb, H. R.; Ghaemmaghami, F.; Mokhtarani, B. A review on removal of sulfur components from gasoline by pervaporation. Chem. Eng. Res. Des. 2012, 90, 409. (2) Ito, E.; Rob van Veen, J. A. On novel processes for removing sulphur from refinery streams. Catal. Today 2006, 116, 446. (3) Song, C. S. An overview of new approaches to deep desulfurization for ultra-clean gasoline, diesel fuel and jet fuel. Catal. Today 2003, 86, 211. (4) Wang, R.; Li, Y. H.; Guo, B. S.; Sun, H. W. Catalytic mechanism of MCM-41 supported phosphoric acid catalyst for FCC gasoline desulfurization by alkylation: experimental and theoretical investigation. Energy Fuels 2011, 25, 3940. (5) KedraKrolik, K.; Fabrice, M.; Jaubert, J. Extraction of thiophene or pyridine from n-heptane using ionic liquids. Gasoline and diesel desulfurization. Ind. Eng. Chem. Res. 2011, 50, 2296. (6) Nie, Y.; Li, C. X.; Wang, Z. Extractive desulfurization of fuel oil using alkylimidazole and its mixture with dialkylphosphate ionic liquids. Ind. Eng. Chem. Res. 2007, 46, 5108. (7) Zhang, H. X.; Gao, J. J.; Meng, H.; Li, C. X. Removal of thiophenic sulfurs using an extractive oxidative desulfurization process with three new phosphotungstate catalysts. Ind. Eng. Chem. Res. 2012, 51, 6658. (8) Zhang, H. X.; Gao, J. J.; Meng, H.; Lu, Y. Z; Li, C. X. Catalytic oxidative desulfurization of fuel by H2O2 in Situ produced via oxidation of 2-propanol. Ind. Eng. Chem. Res. 2012, 51, 4868. (9) Hernández-Maldonado, A. J.; Yang, R. T. Desulfurization of diesel fuels via π-complexation with nickel (II)-exchanged X- and Yzeolites. Ind. Eng. Chem. Res. 2004, 43, 1081. (10) Wang, Y. H.; Yang, R. T. Desulfurization of jet fuel JP-5 light fraction by MCM-41 and SBA-15 supported cuprous oxide for fuel cell applications. Ind. Eng. Chem. Res. 2009, 48, 142. (11) Wang, Y. H.; Yang, F. H.; Yang, R. T. Desulfurization of highsulfur jet fuel by π-complexation with copper and palladium halide sorbents. Ind. Eng. Chem. Res. 2006, 45, 7649. (12) Seredych, M.; Bandosz, T. J. Removal of dibenzothiophenes from model diesel fuel on sulfur rich activated carbons. Appl. Catal. BEnviron. 2011, 106, 133. (13) Seredych, M.; Bandosz, T. J. Adsorption of dibenzothiophenes on nanoporous carbons: identification of specific adsorption sites governing capacity and selectivity. Energy Fuels 2010, 24, 3352. (14) Seredych, M.; Wub, C. T.; Brenderc, P.; Aniac, C. O.; VixGuterlc, C.; Bandosz, T. J. Role of phosphorus in carbon matrix in desulfurization of diesel fuel using adsorption process. Fuel 2012, 92, 318. (15) Lin, L. G.; Zhang, Y. Z.; Kong, Y. Recent advances in sulfur removal from gasoline by pervaporation. Fuel 2009, 88, 1799. (16) Hou, Y. F.; Kong, Y.; Yang, J. R.; Zhang, J. H.; Shi, D. Q.; Xin, W. Biodesulfurization of dibenzothiophene by immobilized cells of Pseudomonas stutzeri UP-1. Fuel 2005, 84, 1975.

Figure 9. Effect of electrolytic time on desulfurization efficiency for real diesel fuel. Other reaction conditions: −1.5 V forward pulse voltage, 0.3 V reverse pulse voltage, 1.5 s forward pulse duration, 0.5 s reverse pulse duration, 0.2 mol/L NaBO2 concentration, 1.2 mmol/L NiCl2 concentration, and 1/3 volume ratio of oil to electrolyte.

time on desulfurization efficiency for real diesel fuel. The desulfurization efficiency increased with the increasing of electrolytic time when electrolytic time was less than 1.5 h and then remained almost constant even though electrolytic time continued increasing. The effect of electrolytic time on desulfurization efficiency for real diesel fuel was similar to that for model diesel fuel (as shown in Figure 6). However, desulfurization efficiency of 86.3% was obtained for real diesel fuel, which was somewhat lower than that for model diesel fuel. The reason was probably that real diesel fuel contained more complex components.

4. CONCLUSIONS In this work, a novel integrated process was presented for reductive desulfurization of diesel fuel, in which the reductant NaBH4 was in situ generated via NaBO2 electroreduction. In order to improve the conversion rate of BO2− into BH4−, NaBO2 electroreduction was fullfiled by applying pulse voltage and using a BDD thin film electrode. The results of cyclic voltammetry and 11B NMR confirmed that NaBO2 was converted into NaBH4 by electroreduction and the electroreduction voltage ranged from −1.2 to −1.8 V. The factors that influenced desulfurization efficiency were investigated. Under the conditions of −1.5 V forward pulse voltage, 0.3 V reverse pulse voltage, 1.5 s forward pulse duration, 0.5 s reverse pulse duration, 0.2 mol/L NaBO2 concentration, 1.2 mmol/L NiCl2 concentration, 1/3 volume ratio of oil to electrolyte, 1.5 h electrolytic time, desulfurization efficiency reached more than 93% for model diesel fuel. The primary reaction products of model S-compounds 3-MBT and DBT were IPB, BP, and CHB. B recycle was realized at the same time as desulfurization. Finally, the desulfurization of real diesel fuel was carried out and desulfurization efficiency of 86.3% was obtained. In summary, the integrated process may be a new option for 7666



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Mild Process for Reductive Desulfurization of Diesel Fuel Using

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