Fuel Processing Technology 92 (2011) 1656–1661

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Fuel Processing Technology

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Oxidation of dibenzothiophene by hydrogen peroxide in the presenceof bis(acetylacetonato)oxovanadium(IV)

Goldie Silva, Samantha Voth, Paul Szymanski, Ernest M. Prokopchuk ⁎Department of Chemistry, University of Winnipeg, 515 Portage Ave, Winnipeg, Manitoba, R3L Canada 0Z5

⁎ Corresponding author. Tel.: +1 204 786 9730; fax:E-mail address: e.prokopchuk@uwinnipeg.ca (E.M. P

0378-3820/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.fuproc.2011.04.014

a b s t r a c t

a r t i c l e i n f o

Article history:Received 30 August 2010Received in revised form 18 March 2011Accepted 10 April 2011Available online 2 May 2011

Keywords:Oxidative desulfurizationHydrogen peroxideDibenzothiopheneVanadiumCatalysis

In the presence of bis(acetylacetonato)oxovanadium(IV), dibenzothiophene was oxidized by hydrogenperoxide to form sulfoxide and sulfone products. The percentage of dibenzothiophene remaining after theoxidation and the ratio of these oxidation products, as determined by 1H NMR spectroscopy, was largelydependent on the solvent used. Reactions in acetonitrile consumedmore than 99% of the dibenzothiophene inthe reaction and yielded the sulfone as the major product. The efficacy of biphasic oxidations performed inheptane and acetonitrile were dependent on the ratio of acetonitrile to heptane with a higher proportion ofacetonitrile resulting in greater consumption of dibenzothiophene. After multiple oxidations, all of thedibenzothiophene was removed from the heptane solution.

+1 204 775 2114.rokopchuk).

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Around the world, stricter regulations limiting the sulfur content indiesel fuel are being adopted. In North America, the maximum sulfurcontent for on-road applicationshasbeen set at 15 ppmsince 2006,whileEuropean Union regulations have required limits of 10 ppm for on-roaduse since2009 [1]. These tighter limitswill not only reduce thequantity ofsulfur-containing emissions, which contribute to air pollution and acidrain, but will also allow the use of more advanced emission controlsystems in automobiles. Hydrodesulfurization (HDS), the usual methodfor removing organosulfur compounds from fuels requires hightemperature and pressure to remove dibenzothiophene (DBT) andoften has difficulties with substituted dibenzothiophenes [2,3].

Oxidative desulfurization (ODS) has received attention as a methodfor achieving lower sulfur content in fuels [4–10]. The oxidation of sulfurcompounds results in sulfoxides and sulfones which can then beextracted out of the non-polar petroleum stream by a polar solvent[9,10]. A variety of catalyst and oxidant combinations have beeninvestigated including hydrogen peroxidewith oxides of metals such astungsten, molybdenum, and titanium [4–6]. Oxygen in the presence ofMn, Ni, and Co-based catalysts [7] and iron phthalocyanine [8] has beeninvestigated, as have superoxides [11]. Ionic liquidmedia have also beenused in ODS studies involving various oxidizers, such as H2O2 catalyzedby phosphotungstic acid [12] or ozone with H2O2 [13].

Vanadium complexes are known to act as catalysts for a variety ofoxidation reactions [14] such as the oxidation of styrene, cyclohexene,

cumene [15], and phenol [16] by salen-type complexes, lignin usingdipicolinate complexes [17], and organic sulfides by vanadiumperoxocomplexes [18] and vanadium oxo-monoperoxo complexes[19]. A variety of vanadium complexes have also been investigated fortheir use as catalysts for ODS. Examples include peroxocomplexes inbiphasic systems [18], V–Mo oxide catalysts [20], V2O5 in ionic liquids[21], and polymer-bound oxidovanadium(IV) and dioxidovanadium(V)complexes [22]. The last examples [22] were synthesized from bis(acetylacetonato)oxovanadium(IV), abbreviated VO(acac)2, which it-self has been demonstrated to catalyze the oxidation of organicmolecules such as anthracene [23]. A polystyrene bound VO(acac)2has been used in the oxidation of phenyl sulfides [24] by hydrogenperoxide. Hereinwe report the results of our investigation into usingVO(acac)2 (Fig. 1) and hydrogen peroxide to oxidize dibenzothiophene.

2. Experimental

Dibenzothiophene (98%) was purchased from Alfa Aesar, vanadylsulfate from Fisher, hydrogen peroxide (50%) from Hach, acetonitrilefromAldrich, and all other solvents were purchased from JT Baker. NMRsolventswere purchased fromCDN Isotopes. All chemicalswere used asreceived without any further purification. VO(acac)2 was preparedaccording to a modified literature procedure [25]. NMR spectra wereobtained on a Bruker Avance II 400 MHz spectrometer and the chemicalshifts were referenced to the residual peak of acetone-d6.

2.1. General oxidation procedure

See Table 1 for the specific conditions used in each case. In a 25 mlround bottom flask was placed VO(acac)2 and DBT. Solvent was added

Fig. 1. VO(acac)2.

1657G. Silva et al. / Fuel Processing Technology 92 (2011) 1656–1661

and the mixture stirred for 1–2 min. Hydrogen peroxide is then addedand the solution turns red. Immediately the flask is placed in an oilbath at the desired temperature. The reaction is stirred for theallocated time and then 20 ml of water is added to stop the reaction.Solid product was isolated by filtration and collected while theremaining solution is then extracted with 3×10 ml of CHCl3. Theorganic extracts were then evaporated and the solid productcollected. The oxidation products were then analyzed by 1H NMRspectroscopy and GC-MS.

2.2. Catalyst recycling

The initial reaction setup was performed as outlined in section 2.1.VO(acac)2 (10 mg) and DBT (100 mg) were dissolved in acetonitrile(4 ml) and 0.2 ml of 50% H2O2 was added. The reaction was allowed toproceed for 1 h at 40 °C at which time the reaction was filtered and theisolated solid was allowed to air dry before being collected. The filtrate

Table 1Oxidation of DBT in the presence of VO(acac)2 in ethylacetate or acetonitrile.

VO(acac)2 H2O2a DBT Volume

(mmol) (mmol) (mmol) (ml)

Reactions in ethyl acetate1 0 2.9 0.81 82 0.070 2.9 0.82 83 0.038 5.9 0.81 84 0.040 2.9 0.81 85 0.040 2.9 0.84 86 0.039 2.9 0.83 87 0.040 2.9 0.41 88 0.041 2.9 1.6 89 0.038 2.9 0.81 810 0.039 2.9 0.82 811 0.039 1.5 0.81 8

Reactions in acetonitrile12 0.038 2.9 0.81 813 0.038 2.9 0.83 814 0.039 2.9 0.82 815 0.039 2.9 0.82 816 0.038 2.9 0.81 417 0.037 1.5 0.82 418 0.039 2.9 0.82 219 0.037 2.9b 0.82 820 0.038 2.9b 0.81 421 0.038 2.9 1.6 822 0.038 8.9c 1.6c 4

Biphasic reactions in heptane/acetonitriled

23 0.038 2.9 0.82 8/424 0.038 2.9 0.82 8/425 0.038 2.9 0.82 8/426 0.038 2.9 0.82 8/427 0.038 2.9 0.82 8/828 0.038 2.9 0.81 16/429 0.038 2.9 0.43 8/430 0.037 2.9 0.22 8/431 0.037 5.9e 0.43 8/4

a 50% aqueous H2O2 unless otherwise stated.b 30% aqueous H2O2.c Hydrogen peroxide added in three 0.2 ml portions and DBT added in three 100 mg pord Solvent volume given as heptane/acetonitrile.e Second portion of 0.2 ml of H2O2 added after 1 h.

was then returned to theflask, another 100 mg of DBT and 0.2 ml of 50%H2O2 was added and the reaction stirred for another hour at 40 °C. Thereaction was then filtered again and the isolated precipitate allowed toair dry. The filtrate was returned to the round bottom flask and 100 mgof DBT and 0.2 ml of 50%H2O2was added and the reaction stirred for 1 hat 40 °C. The reaction was then terminated by the addition of 20 ml ofwater, the precipitatewas isolated byfiltration and thefiltrate extractedwith three 10 mlportions of CHCl3. The organic solutionwas evaporatedunder vacuumand the resulting residuewas collected. All of the isolatedproducts were then analyzed by 1H NMR spectroscopy.

2.3. Biphasic reactions in heptane

Other than the choice of solvent, the biphasic oxidation reactionswere carried out following the same procedure outlined in section 2.1.Heptane was used to dissolve DBT, while the catalyst was dissolved inacetonitrile. The solutions were mixed together before the hydrogenperoxide was added. See Tables 1 and 2 for the specific conditionsused in each case.

2.4. NMR characterisation

For DBTO and DBTO2, longer range couplings can often be observedthough not clearly enough to calculate accurate coupling constants, soonly the three bond coupling is reported in the following data.

T Time % DBTconsumed

Product distribution

(°C) (h) %DBTO %DBTO2

40 2 0 – –

40 2 87 49 5140 2 79 47 5340 1 74 62 3840 2 85 53 4740 4 86 51 4940 2 81 56 4440 2 72 61 3922 2 59 69 3170 2 73 63 3740 2 74 66 34

40 0.5 N99 24 7640 1 N99 5.1 94.940 2 N99 4.6 95.422 2 N99 25 7540 2 N99 4.5 95.540 2 99 29 7140 2 N99 10 9040 2 99 11 8940 2 N99 4.7 95.340 2 99 26 7340 3 100 10 90

40 2 96 15 8522 2 96 15 8540 1 96 15 8570 1 92 37 6340 2 98 12 8840 2 90 13 8740 2 95 6.0 9440 2 95 5.3 94.740 2 N99 1.6 98.4

tions, 1 h apart. Precipitated DBTO2 filtered off after each hour.

Table 2Removal of DBT from heptane.

VO(acac)2 H2O2a DBT Heptane MeCN T Time DBT DBTO DBTO2

(mmol) (mmol) (mmol) (ml) (ml) (°C) (h) % % %

1b 0.038 2.9 0.41 8 4 40 2 0.6c 14.9c 84.5c

0.038 2.9 4 40 2 0c 0c 100c

0.038 2.9 4 40 2 0c 0c 100c

2 0.038 2.9 0.41 8 4 40 2 N0d 0d N0d

0.038 2.9 4 40 2 0e 0e 0e

a 50% aqueous H2O2.b After each of the first two reaction periods, the heptane layer was removed and added to a fresh solution of catalyst and peroxide in acetonitrile. After the third oxidation, no DBT

was observed in the heptane layer.c Products isolated from acetonitrile solution at the end of each run and analyzed by 1H NMR spectroscopy.d After oxidation, heptane washed with 3×10 ml of water. Only heptanes layer was analyzed by GC-MS to confirm non-quantitative presence of compounds. After washings,

another oxidation was performed.

1658 G. Silva et al. / Fuel Processing Technology 92 (2011) 1656–1661

DBT 1HNMR (400 MHz, acetone-d6): δ 8.33 (m, 2 H), 7.98 (m, 2 H),7.52 (m, 4 H, DBT).DBTO1HNMR(400MHz, acetone-d6): δ8.08 (d, 3J=7.6 Hz, 2 H), 8.04(d, 3J=7.6 Hz, 2 H), 7.72 (t, 3J=7.6 Hz, 2 H), 7.61 (t, 3J=7.6 Hz, 2 H).DBTO2

1H NMR (400MHz, acetone-d6): δ 8.14 (d, 3J=7.6 Hz, 2 H),7.90 (d, 3J=7.6 Hz, 2 H), 7.80 (t, 3J=7.6, 2 H), 7.69 (t, 3J=7.6 Hz, 2 H).

2.5. Gas chromatography

GC-MS data was obtained using an HP5890/5970B gas chromato-graph/mass spectrometer equipped with a Varian CP8907 column(15 m×0.25 mm×0.25 μm) and using helium as the carrier gas. Theprocedure used was based on a literature method using a similar,though longer, column [1]. Initial column temperaturewas set at 100 °Cand held for 3 min. Temperature was increased at a rate of 6 °C/min to afinal temperature of 275 °C which was maintained for 10 min.

3. Results and discussion

The oxidation reactions (Scheme 1) were performed under variousconditions with the intent of determining the optimal reactionconditions for the complete oxidation of DBT. Temperature, time,stoichiometry, and solvent all had an influence on the outcome of thereaction. Tables 1 and 2 list the results of the oxidation reactions.

Scheme 1. Oxidation of DBT by H2O2 in the presence of VO(acac)2.

3.1. Analysis of oxidation products

The composition of the oxidation products was determined by 1HNMR spectroscopy. Choice of NMR solvent was critical, with CDCl3resulting in a spectrum with too much overlap to permit anydetermination of composition. Using acetone-d6 provided a spectrumwhere the DBT resonances were separate from those of the oxides, asillustrated in Fig. 2. All but one set of peaks from each of the oxideswere sufficiently separated to allow the composition to be assessed byintegration.

Analyzing the products of DBT oxidations by GC-MS is fairlycommon [1,2,6–8,24,26–28] andwe did try to analyze our products byGC-MS, using a modified literature method for the GC [1], but theresults were less informative than the NMR analyses. While DBT(13 min) and DBTO2 (19 min) could be detected clearly by GC-MS,DBTOwas not observed as a separate peak. In cases where there was asignificant amount of DBTO produced, the ratio of DBT to DBTO2 asdetermined by the GC TIC did not match that from the NMR spectrum.Instead, the GC data seemed to indicate that the DBTO in the producthad been divided between the DBT and DBTO2 peaks in the GC. Forexample, in a sample that by NMR was determined to have a DBT/DBTO/DBTO2 ratio of 41/41/18, the GC data gave a ratio of 60/0/40.Another sample that had a composition of 4/44/52 by NMR resulted inGC data showing a ratio of 28/0/72. Though significant overlapbetween DBTO and DBTO2 has been reported by other researchers[26,27], it is also known that DBT oxides are thermolabile and candecompose in the injection port to formDBT, although this conversionis inconsistent in the degree to which it occurs [28]. A possibleexplanation for our observed compositions, as determined by GC-MS,is that both decomposition and co-elution are occurring. This appears

Fig. 2. 1H NMR spectrum in acetone-d6 of a mixture of DBT, DBTO, and DBTO2.

1659G. Silva et al. / Fuel Processing Technology 92 (2011) 1656–1661

to be supported by the observation that part of the peak observed at19 min in the TIC of the GC displayed peaks at m/z=216 and 200 inthe mass spectrum. Furthermore, we did not find any evidence in theliterature, or experimentally in our work, to suggest that DBTOdisproportionates at 200 °C to form DBT and DBTO2.

3.2. Oxidation reactions

In ethyl acetate, both DBT and VO(acac)2 were soluble with the VO(acac)2 imparting a green color to the solution. Upon addition of thehydrogen peroxide, red aqueous droplets were present in the yelloworganic solution, which suggests that the catalyst is at least partiallyextracted into the aqueous layer upon oxidation by hydrogen peroxide.

The solution was allowed to stir for the allotted time at thespecified temperature as indicated in Table 1. The reaction was thenterminated by the addition of water, which typically resulted in ayellow biphasic mixture. Separation of the organic layer andextraction of the aqueous layer resulted in a yellow organic solutionwhichwas evaporated to yield amixture of DBT, sulfoxide (DBTO) andsulfone (DBTO2). Consumption of DBT approached 90% to producemixtures of oxides. Given the data presented in Table 1, it appears thatthe amount of catalyst used can influence the amount of DBT oxidized.A 75% increase in the amount of catalyst (entries 2 and 4) resulted in13% more DBT being consumed. This is a larger increase than may beexpected considering reports such as that by Xu [21] which show adoubling of the amount of V2O5 catalyst resulting in up to a 2.5%increase in the amount of DBT reacted. In the absence of VO(acac)2,hydrogen peroxide did not oxidize any detectible amount of DBTunder the conditions and reaction times that we investigated. Othershave shown that DBT can be oxidized by H2O2 without the presence ofa catalyst, though not as quickly. For example, in refluxing ethanolDBT is completely oxidized by hydrogen peroxide after 49h [29].

The amount of peroxide, with respect to DBT, appears to impactthe total consumption of DBT as well as affecting the oxide ratio. Usinga peroxide to DBT ratio of 1.9 (entries 8 and 11) resulted in 72 and 74%DBT being oxidized with 61 and 66% of the product being sulfoxide. Inentry 8, the relative amount of catalyst present is about half of what itis in entry 11, and it is likely that this explains the difference betweenthe two reactions. As discussed above, the increased proportion ofcatalyst results in higher conversion with a greater proportion ofsulfoxide product.

Increasing the ratio of H2O2 to DBT to 3.5 (entry 5) resulted in agreater conversion of DBT (85%) and produced a 53/47 mixture ofDBTO and DBTO2. The greater amount of peroxide present would beexpected to allow for more oxidation to occur, but if the ratio ofperoxide to DBT is increased further (entries 3 and 7) then theconversion of DBT decreases to 79 and 81% with the sulfoxide makingup 47 and 56% of the product. Once again, the reaction with a greaterrelative proportion of catalyst does result in the higher conversion.

While sulfone is typically reported as the product of DBT oxidation[4,20,22,30,31], other DBT oxidation reactions have produced mix-tures of the sulfoxide and sulfone with the composition of theproducts being controlled by factors such as reaction time, pH, andsolvent [32,33]. It has been shown that the oxidation of DBT byperoxybenzoic acid first produces the sulfoxide, which is thenoxidized further to produce the sulfone [32]. The sulfoxide is actuallyoxidized more quickly than DBT itself. Chaudhuri et al. [33] reportedoxidation reactions using borax as a catalyst, where changes in pHresulted in differing proportions of the sulfoxide and sulfone product.For the oxidation of DBT, they found that sulfone production wasfavored at basic pH.

Acetonitrile has been used as solvent for peroxide oxidations of DBTin other studies [30,31] and has been found to allow for higherconversion compared to protic solvents [4]. The VO(acac)2, DBT, andaqueousH2O2 solutionwere all soluble in acetonitrile. The color changesobserved during the reactionwere similar to those in ethyl acetate with

the VO(acac)2 dissolving in acetonitrile to produce a green solution thatturns red upon addition of the hydrogen peroxide. The work up of thereaction differed from that done in ethyl acetate as the addition ofwaterproduced a single phase in solution as well as a large amount ofprecipitate, whichwas determined to be over 99%DBTO2. Afterfiltering,the solution was extracted with chloroform and the organic solutionwas then evaporated to isolate the remaining reaction product whichwould be mostly DBTO with some DBT and DBTO2. No product or DBTremained in the aqueous layer after the extractions. The overall resultsof these oxidations are based on the total isolated product of thesereactions.

Besides the difference in the workup, using acetonitrile as thesolvent instead of ethyl acetate had a significant effect on the outcomeof the oxidation reactions. The major product of these reactions wasalways the sulfone, DBTO2. As illustrated in entries 12–22 of Table 1, atleast 99% of the DBT was consumed in the oxidation reactions whilethe distribution of oxidation products was dictated by the reactionconditions. Many of the reactions show conversions above 99%. Thedifference in DBT consumed between the 30 minute (entry 12) andtwo hour (entry 14) reactions was not significant, though the longerreaction did result in far more DBTO2 being formed. Reaction timesgreater than 2 h did not show any significant increase in the amountof DBT consumed.

It has been shown [34] that peroxide oxidations in acetonitrile caninvolve the formation of peroxycarboximidic acid which then transfersoxygen to the sulfur atom, but the extent to which such a mechanismmay be involved here is uncertain given that VO(acac)2 was able tooxidize DBT in ethyl acetate, demonstrating that the catalyst itself iscapable of transferring oxygen to DBT. It is possible that the availability ofadifferentmechanism inacetonitrilemaybeat leastpartially responsible,along with the miscibility of aqueous H2O2 in acetonitrile, for thedifference in theproduct distribution. Themechanismof oxidationsusinghydrogen peroxide in the presence of vanadium complex catalysts hasbeen reported [14,20–22] to involve the formation of a peroxo orhydroperoxo vanadium(V) complex as the active catalystwhich can thentransfer the oxygens of the peroxo group to the substrate.

Reducing the ratio ofH2O2 toDBT (entries 16, 17, and21) reduced thepercent consumption of DBT by less than 1%,which is in linewith resultsreported by others [21]. A more significant difference was observed inthe productdistributionwith the reduced amount of peroxide (entry 17)producinga29/71mixture of sulfoxide andsulfone, compared to the4.5/95.5mixture produced in entry 16. Lowering the proportion of peroxideby increasing the amount of DBT present (entry 21) also resulted in aproduct with an increased proportion of sulfoxide. These data, alongwith the 30 minute reaction, are in agreement with the mechanismsproposed in the literature [21,32] indicating that that oxidation of DBTfirst produces DBTOwhich is then oxidized further to DBTO2. A decreasein the relative amount of peroxide present will reduce the proportion ofsulfone produced.

These homogeneous reactions in acetonitrile did show someinfluence of solvent volume. Using 50% hydrogen peroxide (entries14, 16, and 18), there was no significant difference in DBT consumptionbetween the reactions performed in 8, 4, or 2 ml of solvent, although theproduct distribution did show an increase in the amount of sulfoxideproduced in the2 ml reaction (entry18). This difference is likely due toagreater degree of precipitation of sulfoxide product because of the smallamount of solvent present. Using 30% hydrogen peroxide solution,reducing the volume of acetonitrile resulted in slightly higherconversions and a greater proportion of DBTO2. These volume effectsare likely due to the solubility of the oxidation products in acetonitrile/water mixtures. If the volume of acetonitrile is decreased sufficientlyrelative to the amount of water present from the peroxide solution,more of the oxidation products precipitate out during the reactionwhich shifts the equilibrium of the reaction towards the products.

Unlike the reactions in ethyl acetate, running the reactions at roomtemperature (entry 15) or at 40 °C (entry 14) does not have a significant

1660 G. Silva et al. / Fuel Processing Technology 92 (2011) 1656–1661

impact on the amount of DBT consumed. There is however a largerproportion of the sulfoxide produced in the lower temperature reaction.The results of a 30 minute oxidation at 40 °C (entry 12) were verysimilar to those obtained after 2 h at 22 °C (entry 15), with the sameamount of DBT being consumed but only a 1% difference in theproportion of sulfone present in the product.

Entry 22 in Table 1 describes an experiment aimed at investigatingthe recyclability of this catalyst by simply filtering to remove theoxidation products, followed by the addition of more DBT and H2O2.After each hour of reaction, some DBTO2 precipitated out and wasremoved by filtration before the addition of more peroxide and DBT.Because the oxidation products are somewhat soluble in acetonitrile,the actual yield for the first isolation was quite low (33.6 mg), but theproduct was pure DBTO2. After adding the second portion of DBT andperoxide and allowing the reaction to proceed for an hour there wasmore precipitate (95.6 mg) which was also pure DBTO2. After thethird portion of DBT and peroxide was added, the reaction wasallowed to proceed for 1 h and then it was terminated and worked upin the samemanner used for the single batch oxidation reactions. Thisproduced another 208.2 mg of precipitate that was a 90/10 mixture ofDBTO2 and DBTO. Extraction of the remaining filtrate with chloroformresulted in the isolation of 16.2 mg of product that was a 6/94mixtureof DBTO2 and DBTO. This complete consumption of 300 mg of DBT toform a mixture of sulfone and sulfoxide product does illustrate thecapacity of the catalyst to handle larger amounts of DBT. It alsosuggests that in order to consume 100% of the DBT, it is necessary toperiodically remove oxidation products from the reaction.

3.3. DBT oxidation and removal from heptane

Biphasic reactions (Table 1, entries 23–31) were carried out bydissolving DBT in heptane and adding an acetonitrile solution of thecatalyst and H2O2. The reactions described in entries 23–31 wereperformed on a single-batch basis, as had been donewith the previousreactions in acetonitrile. In general, the biphasic reactions resulted ina lower consumption of DBT and a greater amount of sulfoxideproduct compared to the reactions in acetonitrile. The outcome ofthese biphasic reactions was dependent on the ratio of acetonitrile toheptane, with a higher proportion of acetonitrile resulting in a greaterconversion of DBT. Reactions performed without acetonitrile resultedin no detectible amount of DBT being oxidized. Due to the insolubilityof the hydrogen peroxide and the catalyst in heptane, the oxidationreaction is taking place in the acetonitrile layer, which is consistentwith other biphasic oxidation reactions [4,21]. A greater proportion ofacetonitrile would allow for a greater amount of DBT to be extractedout of the heptane and thus be oxidized in the acetonitrile layer.

Given the decreased conversion of DBT observed in the biphasicreactions, it was decided to attempt reactions at different temperatures.As indicated in entries 23 and 24, there is no observable differencebetween the reaction performed at room temperature and at 40 °C,witheach reaction consuming 96% of the DBT to produce 15/85 mixtures ofDBTO andDBTO2. Increasing the temperature to 70 °C (entry 26) resultsin a decrease in the conversion of DBT and an increase in the relativeproportionof sulfoxidepresent in theproduct compared to the reactionsat lower temperatures (entry 25) . The increased temperature likelyincreases the decomposition of hydrogen peroxide during the reaction,thus resulting in the reduced degree of oxidation.

Reducing the concentration of DBT in heptane did not allow thereaction to proceed to 100% oxidation of DBT. Rather, a 1% and 0.5% w/vsolutionofDBT inheptane (entries29and30) resulted inonly95%of theDBT being consumed,which is close to the 96% consumption in entry 23where almost twice as much DBT was used. A greater proportionof sulfone was observed in the products of these reactions (entries 29and 30) with a lower DBT concentration.

Adding a second portion of peroxide after 1 h of reaction did allowfor a slight increase in the amount of DBT oxidized. Comparing entries

29 and 31 shows an improvement of over 3% in the total amount ofDBT oxidized, and a higher percentage of DBTO2 being present in theproduct. Interestingly, this extra peroxide did not allow the reactionto consume 100% of the DBT.

Besides simply demonstrating that DBT can be oxidized in a biphasicsystem, we wished to test the efficacy of VO(acac)2 with respect tooxidative desulfurization using heptane as a model fuel. Table 2describes a pair of experiments that were performed to demonstratethat multiple oxidation steps could be used to completely oxidize andremove DBT from a 1% w/v solution of DBT in heptane. The productcomposition described in entry 1 is only describing the compoundsfound in the acetonitrile solutions after each oxidation. This shows thatthe oxidation products were readily extracted from the heptane layerinto the acetonitrile during the course of the oxidation reaction.Comparing the analysis to the previous entries in the table, it is clearthat most of the unreacted DBT remains in the heptane solution whilethe oxidation products remain in the acetonitrile. After the thirdoxidation, the heptane layer was found to contain no DBT, sulfoxide, orsulfone.

As observed in the single-batchbiphasic reactions, themajority of theDBT is indeed oxidized during a single oxidation reaction and is thusreadily extracted from the heptane. For comparison, an attempt wasmade to simply extract DBT from heptane using acetonitrile and thesame reaction conditions butwithout thepresenceofhydrogenperoxideand VO(acac)2. This resulted in up to 50% removal of the DBT from theheptane which is nowhere near the removal that is observed when theDBT is oxidized before extraction. This is in agreement with the work ofRamírez-Verduzco [4,35] which demonstrated that acetonitrile is muchmore effective at extracting organosulfur compounds, including DBT,from heptane after they have been oxidized. Similarly, using ionicliquids, Xu et al. demonstrated that up to 98.7% of sulfur can be removedby oxidizing and extracting, as opposed to 16.5% sulfur removal byextraction only [21].

The secondentry in Table 2 is a similar reactionwith the addition of awater washing step after each oxidation. In this reaction the heptanelayer was analyzed after each oxidation. The first oxidation wasperformed as usual, followed by the separation of the heptane andacetonitrile layers. The heptane layer was washed three times with10 ml of water and then the heptane solution was analyzed by GC-MSwhich indicated the presence of sulfur-containing compounds, appar-ently DBT and DBTO2. As discussed in section 3.1, observing DBT in theGC may be an indication of DBT or DBTO being present, but in thisexperiment the specific identities of the compounds were not asimportant as simply determining if any sulfur-containing compoundswere still present in the heptane. A second oxidation reaction was thenperformed on the heptane layer followed by water washings of theheptane. After these final washings, the GC-MS showed that there wereno sulfur-containing compounds remaining in the heptane. Polarorganic solvents are known [9,10] to be effective for extracting DBTOand DBTO2 from non-polar solvents which agrees with our observationthat acetonitrile used in the oxidations does indeed extract most of theoxidation products from the heptane. Our data suggest that water canassist, to a lesser degree, in the extraction of the oxidation products fromheptane, though as we saw in theworkup of ourmonophasic reactions,water is not as effective a solvent for these compounds as are polarorganic solvents such as acetonitrile and chloroform.

While VO(acac)2 has been used as a precursor to the synthesis ofcatalysts such as the polymer-bound vanadium complex reported byMaurya, which was capable of removing 72% of DBT from a model fuel[22] our results illustrate that the simple VO(acac)2 is itself an effectivecatalyst for removing DBT via an oxidation/extraction process.

4. Conclusions

The complex VO(acac)2 is an effective catalyst for the oxidation ofDBT by hydrogen peroxide, producing a mixture of the sulfoxide and

1661G. Silva et al. / Fuel Processing Technology 92 (2011) 1656–1661

sulfone. Carried out in acetonitrile solution, over 99% of the DBT isoxidized at 40 °C with sulfone as the main product. In biphasicheptane/acetonitrile reactions, increasing the proportion of acetoni-trile improves the conversion of DBT. Multiple oxidation reactionscan be performed to achieve complete oxidation of DBT. In as few astwo oxidation steps, using acetonitrile as a polar solvent, DBT canbe completely removed from a 1% w/v solution of DBT in heptane,illustrating the potential application of this catalyst in an oxidativedesulfurization process.

Acknowledgments

Thank you to the University of Winnipeg and the Department ofChemistry for financial support, facilities, and supplies. Thank you tothe referees for their helpful suggestions.

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