Mild Process for Reductive Desulfurization of Diesel Fuel Using

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,200240China 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.1Zero-emission and zero-levels of sulfur content are previewed in the near future.2The traditional industrial process for removal of sulfur from fuels is hydrodesulfurization (HDS),which is e ?ective for aliphatic and acyclic S-compounds but less e ?ective for benzothiophene (BT),dibenzothiophene (DBT),and their derivatives.3Furthermore,it requires high investment and operating costs and su ?ers from signi ?cant loss in the octane number caused by saturation of ole ?ns.3Therefore,some non-HDS alternative technologies such as alkylation,4extraction,5,6oxidation,7,8adsorption,9?14membrane separation,1,15and biodesulfuriza-tion 16,17have been proposed.However,all of the non-HDS methods also possess their own drawbacks, e.g.,low desulfurization e ?ciency,deteriorating quality of gasoline,high cost,or di ?culty in industrial application.1?3,18Therefore,it is necessary to make further e ?orts to explore more e ?ective methods for fuel desulfurization.Sodium borohydride (NaBH 4)is an excellent reductant,which was widely used in the desulfurization of S-compounds.19,20Some studies have been carried out recently on sulfur removal from coal and gasoline using NaBH 4as reductant,and high desulfurization e ?ciency was ob-tained.21?23However,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 NaBH 4and in situ regeneration of NaBH 4via sodium metaborate (NaBO 2)electroreduction has been proposed https://www.wendangku.net/doc/dd15170114.html,ly,NaBO 2is ?rst converted into NaBH 4by electroreduction,subsequently the generated NaBH 4

is used for reductive desulfurization of fuel with NaBO 2being a byproduct,and then the byproduct NaBO 2will be converted into NaBH 4by electroreduction again.The above steps repeat again and again.As shown in Figure 1,the integrated process mainly involves four steps:24,25

(1)NaBH 4is obtained by NaBO 2electroreduction (BO 2?+6H

2O +8e ?→BH 4?+8OH ?);

(2)NaBH 4reacts with NiCl 2·6H 2O to give nickel boride

(4NaBH 4+2NiCl 2+9H 2O →Ni 2B +3H 3BO 3+4NaCl +12.5H 2);

(3)The generated nickel boride catalyzes the formation of H 2(H *)from NaBH

4and H 2O (NaBH 4+H 2O →

NaBO

2+H 2);

(4)The organosulfur compounds are transformed into

corresponding hydrocarbons and H 2S in the presence

of H 2(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.24,25However,fresh nickel boride is produced little by little with the in situ generation of NaBH 4in the integrated process,which avoid the aging e ?ect of nickel boride,accordingly desulfurization e ?ciency 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 e ?ciency greatly depends on the conversion rate of BO

2?into BH 4?;therefore,in order to improve the conversion rate of BO 2?into BH 4?,Received:April 4,2013

Revised:May 12,2013

Accepted:May 14,2013

Published:May 14,2013

NaBO 2electroreduction was ful ?lled by applying pulse voltage and using a boron-doped diamond (BDD)thin ?lm electrode as the working electrode.The NaBO 2electroreduction process was analyzed by cyclic voltammetry and 11B nuclear magnetic resonance (NMR),and the factors that in ?uence desulfuriza-tion e ?ciency 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 S-compounds were proposed.2.EXPERIMENTAL SECTION 2.1.Materials.A BDD thin ?lm electrode was prepared by a hot ?lament 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 493ppmw was prepared by dissolving 3-methylbenzothiophene (3-MBT)and dibenzothio-phene (DBT)in n -octane.Real diesel fuel with sulfur contents of 458ppmw was supplied by Sinopec Shanghai Petrochemical Company.A per ?uorosulfonic 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.NaBO 2·4H 2O (>99%,AR),and NiCl 2·6H 2O (>98%,AR)and

C 4H 6O 4Pb ·3H 2O (>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)compart-ments.The working electrode was a BD

D thin ?lm electrode (40×10mm),and the counter electrode was a graphite electrode (40×10mm).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 H 2S from the working electrode compartment to the counter electrode compartment.Pulse voltages were obtained from an electro-chemical workstation (Autolab PGSTAT30)with PC software control (GPES 4.9).All desulfurization experiments were conducted at ambient temperature (10?30°C)and pressure.The experimental procedure is as follows.First,an aqueous solution containing NaOH and a mixed aqueous solution containing NaOH and NaBO 2were 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,NiCl 2·6H 2O 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 ?ltered.The ?ltrate 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 ?ltrate was used for elements content analysis and the oil phase of the ?ltrate 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 e ?ciency was calculated by the following equation:

=?×desulfurization efficiency(wt%)TS TS TS 100%

12

1

(1)where TS 1is the S-content in original diesel fuel sample and TS 2is 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-?ame ionization detector (GC-FID,GC-2010,Shimadzu)equipped with a DB-FFAP capillary column (0.25mm ×30m).Analysis conditions were as follows:injector temperature was 340°C and detector temperature was 250°C,column temperature was programmed from 100to 250°C (8min)at 15°C/min.The injection sample was 1μL 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-5ms capillary GC column (0.25mm ×30m).The injector temperature was 250°C and oven temperature

was

Figure 1.Schematic diagram of the desulfurization experiment.

programmed from 50°C (2min)to 200°C at 5°C/min and then ramped to 280°C at 8°C/min.The injection sample was 1μL for all samples.Mass spectra conditions were as follows:ionization voltage 70eV;ion source temperature 200°C;full scan mode in m /z range 33?650with a 0.5s/scan https://www.wendangku.net/doc/dd15170114.html,pounds were identi ?ed by use of National Insititute of Standards and Technology (NIST)147and NIST 27Library of Mass Spectra.

3.RESULTS AND DISCUSSION

3.1.Analysis of the Electroreduction of NaBO 2into

NaBH

4Process.Because desulfurization e ?ciency greatly

depends on the conversion rate of BO 2?into BH 4?in the integrated process,it is crucial to improve the conversion rate of BO 2?into BH 4?.A lot of studies about the electroreduction of NaBO 2into NaBH 4have been reported.26?31The electroreduction mechanism is as follows:++→+=?????

E cathode:BO 6H O 8e BH 8OH 1.24V (vs SHE)2241/2(2)?→+↑=??E anode:4OH 4e 2H O O 1.229V (vs SHE)221/2(3)The above equations show that BO 2?is reduced to BH 4?on the cathode,but due to charge repulsion it is very di ?cult for the negatively charged BO 2?to get close to the surface of the cathode.So the conversion rate of NaBO 2into NaBH 4is 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 BO 2?will be attracted to the surface of working electrode,and then the working electrode works as the cathode,the BO 2?gathered on the surface of working electrode will be reduced immediately into BH 4?ahead of being excluded.Afterward,the directions of pulse voltage continue alternating again and again,which means that the attraction and electroreduction of BO 2?alternates on the working electrode.So the conversion rate of NaBO 2into

NaBH 4will be improved greatly by applying pulse voltage.It is generally known that the hydrogen evolution reaction (2H 2O +2e ?

→

H 2↑+2OH ?)and oxygen evolution reaction (4OH ?-4e ?→2H 2O +O 2↑)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 ?lm 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 environ-ments and its low background current is bene ?cial to improving current e ?ciency.32?39Therefore,BDD ?lm 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 NaBO 2into NaBH 4by BDD ?lm electrode.Figure 2shows the cyclic voltammogram of BDD thin ?lm electrodes in 0.1mol/L NaOH and 0.1mol/L NaOH +0.2mol/L NaBO 2aqueous solutions.No reduction peaks were observed in 0.1mol/L NaOH aqueous solution.However,a distinct reduction peak was observed in 0.1mol/L NaOH +0.2mol/L NaBO

2aqueous solution from ?1.2to ?1.8V.Therefore,it could be inferred that the observed peak from ?1.2to ?1.8V might be related to the electroreduction of NaBO 2into NaBH 4.Besides,hydrogen evolution commenced at lower than ?1.8V and oxygen evolution commenced at higher than +0.6V for the BDD ?lm electrode in 0.1mol/L NaOH +0.2mol/L NaBO

2aqueous solution.This meant that forward pulse voltage should be higher than ?1.8V and reverse

pulse voltage should be lower than +0.6V in order to avoid

hydrogen evolution reaction and oxygen evolution reaction in

the following experiments.

3.1.2.11B NMR.The electrolytes of 0.1mol/L NaOH +0.2mol/L NaBO 2before and after electroreduction were analyzed by 11B NMR.Figure 3shows the results of 11B NMR.The resonance line at near 1.6ppm can be assigned to NaBO 240and that at near 42ppm can be assigned to NaBH 441which appeared only in the electrolyte after electrochemical treatment.The result was in good agreement with the result of cyclic voltammetry,which suggested that NaBH 4was obtained via NaBO 2electroreduction with pulse voltage using a BDD

thin

Figure 2.Cyclic voltammogram of a BDD ?lm electrode in 0.1mol/L NaOH and 0.1mol/L NaOH +0.2mol/L NaBO 2aqueous solutions at a scan rate of 5

mV/s.Figure 3.11B NMR spectrogram of electrolytes before and after electroreduction.

Reaction conditions:?1.5V forward pulse voltage,0.3V reverse pulse voltage,1.5s forward pulse duration,0.5s reverse

pulse

duration,0.2mol/L NaBO 2concentration,0.1mol/L NaOH

concentration,and 1.5h electrolytic time.

?lm electrode.Furthermore,faradaic e ?ciency of 84.96%was obtained for the process of NaBO 2electroreduction.3.2.E ?ect of Pulse Parameters on Desulfurization E ?ciency.3.2.1.Forward Pulse Voltage.The role of forward pulse voltage is to convert BO 2?into BH 4?by electroreduction.According to the cyclic voltammogram of BDD thin ?lm electrode (Figure 2),the reduction peak of NaBO 2into NaBH 4was observed from ?1.2to ?1.8V.Although reducing the forward pulse voltage probably improved desulfurization e ?ciency,too low forward pulse voltage would lead to hydrogen evolution reaction.Figure 4a shows the e ?ect of forward pulse voltage on desulfurization e ?ciency,compared with constant voltage.When constant voltage was applied desulfurization e ?ciency initially increased with the reduction of constant voltage and then reached its maximum value (about 40%)at ?1.8V.However,the maximum value of desulfurization e ?ciency soared to 93.3%when forward pulse voltage was ?1.5V.The results suggested that the e ?ectiveness of pulse voltage on electroreduction NaBO 2into NaBH 4was signi ?cantly better than that of constant voltage.When forward pulse voltage continued reducing desulfurization e ?ciency decreased instead of increasing.The possible reason was that hydrogen evolution reaction took place,which reduced the conversion rate of NaBO 2into NaBH 4.The result was consistent with the result of cyclic voltammetry (Figure 2).So ?1.5V was used in the following experiments.3.2.2.Reverse Pulse Voltage.The role of reverse pulse voltage was to attract BO 2?to the surface of BDD thin ?lm electrode.A high reverse pulse voltage bene ?tted the attraction of BO 2?to the electrode surface.But oxygen evolution reaction (Figure 2)and conversion of BO 2?to borax 40might take place when reverse pulse voltage was too high.Furthermore,the electrooxidation of the generated BH 4?would intensify with the increasing of reverse pulse voltage.On the contrary,a low reverse pulse voltage was not enough to attract BO 2?to the surface of BDD thin ?lm electrode.As shown in Figure 4b,desulfurization e ?ciency initially increased with the increasing of reverse pulse voltage,and then decreased.The maximum value of desulfurization e ?ciency was obtained when reverse pulse voltage was +0.3V.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 e ?ciency in certain range due to the increase of conversion rate of BO 2?into BH 4?,if forward pulse duration was too long,desulfurization e ?ciency 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 e ?ciency was obtained when forward pulse duration was 1.5s.3.2.4.Reverse Pulse Duration.The electrooxidation of BH 4?into BO 2?(BH 4?+8OH ?→BO 2?+6H 2O +8e ?)took place when reverse pulse voltage was applied,which was in competition with the hydrolysis of BH 4?.If reverse pulse

duration was too short,it would not be enough to transfer BO 2?to the surface of BDD thin ?lm electrode,which would lead to the decrease of desulfurization e ?ciency.Conversely,desulfurization e ?ciency would also decrease because the electrooxidation of BH 4?into BO 2?would increase.Figure 4d

shows the e ?ect of reverse pulse duration on desulfurization e ?ciency,desulfurization e ?ciency initially increased with the increasing of reverse pulse duration and then decreased.The maximum value of desulfurization e ?ciency was obtained when reverse pulse duration was 0.5

s.

Figure 4.E

?ect of (a)forward

pulse voltage,(b)reverse pulse voltage,(c)forward pulse duration,and (d)reverse pulse duration on desulfurization

e ?ciency.Other reaction conditions:0.2mol/L NaBO 2concentration,1.2mmol/L NiCl 2concentration,1/3volume ratio o

f oil to electrolyte,and

1.5h electrolytic time.

3.3.E ?ect of NaBO 2Concentration on Desulfurization

E ?ciency.NaBO 2is the source of NaBH 4,so NaBO 2

concentration is a key factor of producing NaBH 4.As shown in Figure 5,desulfurization e ?ciency increased with the

increasing of NaBO

2concentration when NaBO

2concentration

was less than 0.2mol/L and then remained almost constant even though NaBO 2concentration continued increasing.The reason was that the BH 4?concentration

in the reaction system mainly depended on the conversion rate of BO 2?into BH 4?and the hydrolysis rate of BH 4?.The conversion rate of BO 2?

into BH 4?increased with the increasing of NaBO 2concen-tration.When the conversion rate of BO 2?into BH 4?exceeded

the hydrolysis rate of BH 4?,the BH 4?concentration in the reaction system would increase gradually.However,the increase of BH 4?concentration would accelerate its electro-oxidation when reverse pulse voltage was applied.Therefore,excess NaBO 2practically made no contribution to desulfuriza-

tion e ?https://www.wendangku.net/doc/dd15170114.html,ly,desulfurization e ?ciency would remain almost constant while increasing to a certain value.Con-sequently,0.2mol/L was chosen as the optimum NaBO 2concentration.3.4.E ?ect of NiCl 2Concentration on Desulfurization E ?ciency.It is well-known that metal borides (e.g.,Ni 2B,Co 2B)are highly active catalysts which can be prepared readily from metal halides (e.g.,NiCl 2,CoCl 2)and boron hydrides

(e.g.,NaBH 4)in protic conditions.42Especially,nickel boride has been employed as an e ?cient reagent for reductive desulfurization.19,20In this work,NiCl 2was also added in the

reaction system to produce nickel boride.As shown in Figure 6,desulfurization e ?ciency (50%)was still very low after

electrolyzing for 3.5h without NiCl 2.However,desulfurization e ?ciency increased and the treatment time required to reach

the maximum value of desulfurization e ?ciency decreased with the increasing of NiCl

2concentration.When NiCl

2concen-

tration increased to 1.2mmol/L desulfurization e ?ciency and the treatment time required to reach the maximum value of desulfurization e ?ciency remained almost stable.The desul-furization e ?ciency was above 93%after electrolyzing for 1.5h.Obviously,the addition of NiCl 2greatly improved desulfuriza-tion e ?ciency.On the other hand,based on the elements content of digestion solution of precipitate (Table 1),most of the Ni from NiCl 2·6H 2O was precipitated as nickel boride.It could be inferred that nickel boride actually play a role in

improving desulfurization e ?ciency.According to previous reports,24,25a possible mechanism of desulfurization with nickel boride was proposed.As shown in Scheme 1,the prepared nickel boride from NaBH 4and NiCl 2·6H 2O 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

Figure 5.E ?ect of NaBO 2concentration on desulfurization e ?ciency.Reaction conditions:?1.5V forward pulse voltage,0.3V reverse pulse voltage,1.5s forward pulse duration,0.5s reverse pulse duration,1.2mmol/L NiCl 2concentration,1/3volume ratio of oil to electrolyte,and 1.5h electrolytic

time.Figure 6.E ?ect of NiCl 2concentration on desulfurization e ?ciency.Reaction conditions:?1.5V forward pulse voltage,0.3V reverse pulse voltage,1.5s forward pulse duration,0.5s reverse pulse duration,0.2

mol/L NaBO 2concentration,1/3volume ratio of oil to electrolyte,and 1.5h electrolytic time.

Table 1.ICP Analysis of Electrolytes and Digestion Solution of Precipitate a

elements before

reaction (mg)b after reaction

(mg)working electrode

compartment S 4.12

Ni 7.04

B 216.2213.37(98.6%)

counter electrode

compartment S 3.82

Ni 0.02

digestion solution of

precipitate Ni 6.99

B 2.49S 0.02

a

Reaction

condition:?1.5V forward pulse

voltage,0.3

V reverse pulse voltage,1.5s forward pulse duration,0.5s reverse pulse duration,0.2

mol/L NaBO 2concentration,1.2mmol/L NiCl 2concentration,1/3

volume ratio of oil to electrolyte,and 1.5h electrolytic time.b Theoretical value.Scheme 1.Possible Mechanism of Desulfurization with Nickel

Boride

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.E ?ect of Volume Ratio of Oil to Electrolyte on

Desulfurization E ?ciency.Model diesel fuel is nonpolar and

hardly soluble in aqueous solution.So the volume ratio of oil to electrolyte can a ?ect the mass transfer.As shown in Figure 7,

desulfurization e ?ciency 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 BO 2?to the surface of BDD thin ?lm electrode di ?cult,which reduced the conversion rate of BO 2?into BH 4?,accordingly the desulfurization e ?ciency also reduced.Furthermore,the internal resistance of electrolyte increased with the increasing of oil volume,which decreased current density,accordingly the desulfurization e ?ciency also reduced.The maximum value of desulfurization e ?ciency 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 8shows 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?46the 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 di ?cult,which was in agreement with the previous reports.44?46 3.7.Mass Balance.Table 1presents 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 e ?ciency (wt %)×total sulfur content (ppmw)×model diesel fuel mass (mg)=93.3%×493×10?6×17500mg =8.05mg;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+0.02mg =7.96mg.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,C 4H 6O 4Pb aqueous solutions were added into the electrolytes in the working and counter compartments after desulfurization.Black precipitation of lead sul ?de (PbS)was obtained immediately and the mass of precipitation in the working and counter compartments was 30.63mg and 28.1mg after washing and drying,respectively.So the S 2?mass in the working and counter compartments was 4.1and 3.77mg,respectively.According to Table 1,the TS mass in the working and counter compartments was almost in accordance with their S 2?mass,which indicated that most of the S in the working and counter electrode compartments was present in the form of S 2?.Most of the Ni from NiCl 2·6H 2O (7.04mg)was precipitated as nickel boride (6.99mg).A large proportion of B (98.6%)still remained in the electrolyte in the working electrode compartment after reaction except for

a

Figure 7.E ?ect of volume ratio of oil to electrolyte on desulfurization e ?ciency.Reaction conditions:?1.5V forward pulse voltage,0.3V reverse pulse voltage,1.5s forward pulse duration,0.5s reverse pulse

duration,0.2mol/L NaBO 2concentration, 1.2mmol/L NiCl 2

concentration,and 1.5h electrolytic

time.Figure 8.GC/MS spectra of model diesel fuel after desulfurization.Reaction conditions:?1.5V forward pulse voltage,0.3V reverse pulse

voltage,1.5s forward pulse duration,0.5s reverse pulse duration,0.2

mol/L NaBO 2concentration,1.2mmol/L NiCl 2concentration,1/3volume ratio of oil to electrolyte,and 1.5h electrolytic time.

Scheme 2.Reaction Routes of 3-Methylbenzothiophene and

Dibenzothiophene

small part precipitated as nickel boride (2.49mg),which indicated that B recycle was realized at the same time as desulfurization.3.8.Desulfurization of Real Diesel Fuel.The desulfur-ization performance of the integrated process for real diesel fuel was also investigated.Figure 9shows the e ?ect of electrolytic time on desulfurization e ?ciency for real diesel fuel.The desulfurization e ?ciency increased with the increasing of electrolytic time when electrolytic time was less than 1.5h and then remained almost constant even though electrolytic time continued increasing.The e ?ect of electrolytic time on desulfurization e ?ciency for real diesel fuel was similar to that for model diesel fuel (as shown in Figure 6).However,desulfurization e ?ciency 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 NaBH 4was in situ generated via NaBO 2electroreduction.In order to improve the conversion rate of BO 2?into BH 4?,NaBO 2electroreduction was full ?led by applying pulse voltage and using a BDD thin ?lm electrode.The results of cyclic voltammetry and 11B NMR con ?rmed that NaBO 2was converted into NaBH 4by electroreduction and the electro-reduction voltage ranged from ?1.2to ?1.8V.The factors that in ?uenced desulfurization e ?ciency were investigated.Under the conditions of ?1.5V forward pulse voltage,0.3V reverse pulse voltage,1.5s forward pulse duration,0.5s reverse pulse duration,0.2mol/L NaBO 2concentration,1.2mmol/L NiCl 2concentration,1/3volume ratio of oil to electrolyte,1.5h electrolytic time,desulfurization e ?ciency 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 e ?ciency of 86.3%was obtained.In summary,the integrated process may be a new option for desulfurization of diesel fuel due to its mild conditions.Furthermore,this process is environment-friendly because fuel desulfurization was ful ?lled only using low-voltage electricity and recyclable electrolyte.■AUTHOR INFORMATION

Corresponding Author *Tel.:+862154742817.E-mail:sunth@https://www.wendangku.net/doc/dd15170114.html,.Notes

The authors declare no competing ?nancial interest.

■ACKNOWLEDGMENTS

This work is ?nancially 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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