Ind. Eng. Chem. Res. 2006, 45, 1853-1856
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Oxidative Desulfurization Using Polyoxometalates C. Komintarachat and W. Trakarnpruk* Petrochemistry and Polymer Science, Faculty of Science, Chulalongkorn UniVersity, Bangkok 10330, Thailand
Oxidative desulfurization of model compounds (benzothiophene, dibenzothiophene, 4,6-dimethyldibenzothiophene) with hydrogen peroxide/acetic acid using polyoxometalates as catalysts has been studied. The tetrabutylammonium salts of [W6O19]2-, [V(VW11)O40]4-, [PVW11O40]4-, and [PV2Mo10O40]4- were prepared, and their activities were compared with (VO)2P2O7. The experimental results show that the highest active catalyst is [V(VW11)O40]4-. The method was also used for the treatment of gas oil. The combination of solvent extraction and alumina adsorption can efficiently separate sulfone products. The resulting oil contained less than 0.055% sulfur, and this corresponds to 90 % removal. Introduction Sulfur compounds in fuels are a major source of pollution. Sulfur containing compounds such as thiophenes can poison catalysts used to remove the residue of hydrocarbons and nitrogen oxides derived from combustion reactions. The requirement to produce diesel fuels with very low levels of sulfur has stimulated much work in the area of hydrodesulfurization (HDS). This usually requires high temperatures and the presence of hydrogen. Oxidative desulfurization (ODS), an alternative or complementary technology to HDS for deep desulfurization, has several advantages such as mild reaction conditions (ambient pressure and relative low temperatures), high selectivity, and the potential for desulfurization of sterically hindered sulfur compounds. Benzothiophene and dibenzothiophenes are hardly oxidized. ODS generally consists of two processes: the first step is oxidation, and the following step is the removal of oxidized compounds. Different oxidizing agents were used, such as H2O2 in combination with acetic acid, formic acid,1 NO2,2 ozone,3 and t-butyl-hydroperoxide.4 Phosphotungstic acid5 and polyoxometalates/hydrogen peroxide were found to increase the conversion of DBT to sulfone.6 The Keggin ion, PV2Mo10O405-, was found to be a very active and selective oxidation catalyst. The oxidation reactivities of the thioethers in the polyoxometalate/peroxide system do not correlate with their redox potentials, where steric effects play a significant role. A European patent claims a method of removing organic sulfur compounds from liquid oil using oxidizing agents, followed by distillation, and solvent extraction or adsorption.7 The aim of the present work was to carry out a comparative study of the benzothiophene and dibenzothiophene (model compounds) oxidations using a series of polyoxometalates as catalyst precursors. Hydrogen peroxide was chosen as an oxidant as it does not adversely affect the product or cause environmental problems. Additionally, results on the chemical oxidation and extraction of organosulfur compounds from gas oil are presented. Experimental Procedures Materials and Apparatus. Phosphotungstic acid hydrate (99%), dibenzothiophene (C12H8S, DBT, 98%), 4,6-dmethyldibenzothiophene (C14H12S, 4,6-DMDBT, 97%), and benzothiophene (C12H8S, BT 98%) were purchased from Fluka. Gas * Author to whom correspondence should be addressed. E-mail:
[email protected].
oil (density at 15 °C, 0.8565 g/mL, 0.575 wt % sulfur) was supplied from the Thai Oil Company. Hydrogen peroxide (30 wt % H2O2, Fluka) was used as an oxidant. DMSO, DMF, and acetonitrile were used as solvents. The silica gel and aluminum oxide adsorbent were Merck Silica Gel 60. IR spectra were measured using an infrared spectrophotometer Nicolet FT-IR Impact 410 on KBr disks. The XRD patterns of catalysts were obtained on Rigaku, DMAX 2002 Ultima Plus X-ray powder diffractometer equipped with a monochromator and a Cu-target X-ray tube (40 kV, 30 mA) and angles of 2θ ranged from 2 to 60˚. UV-vis spectra were recorded on Milton Roy Spectronic 3000 Array. The sulfur content of original and treated oils was determined using a SISONS X-ray fluorescence spectrometer ARL 8410 (ASTM D4294 method), whose limitation of sulfur detection is 5 wt ppm. The test method is based on the ASTM D-4294 standards. Model compounds and oil samples were analyzed with a gas chromatograph (Shimadzu GC-14B equipped with a flame ionization detector and a 30 m (0.25 mm i.d., 0.25 µm film thickness) capillary column). Preparation of Polyoxometalates. Polyoxometalates were synthesized using the following procedures. (A) Polyoxotungstate Complex, [W6O19]2-.8 A 3.3 g quantity of sodium tungstate dihydrate (Na2WO4‚2H2O) was dissolved in 250 mL of deionized water, followed by the addition of 250 mL of acetonitrile. After the addition of 1.5 mL of 10 M hydrochloric acid, 3 g of tetrabutylammonium bromide (n-Bu4NBr) was added to precipitate a white salt, which was filtered off and washed with deionized water and ethanol. The crude salt was further purified by recrystallization from a mixture of an acetonitrile/ethanol solution (3:1 v/v). (B) 11-Tungstovanadate, [V(VW11)O40]4-.9 To a solution of 4.9 g of sodium tungstate dihydrate (Na2WO4‚2H2O) and 1.7 g of ammonium m-vanadate (NH4VO3) in 45 mL of warm water was added 50 mL of acetonitrile. A total of 27 mL of 9 M perchloric acid was added with vigorous stirring. The resultant orange solution was heated at 70 °C for 24 h, and after cooling to room temperature, the precipitate was filtered off. To the filtrate was added 3 g of tetrabutylammonium bromide to precipitate an orange salt. The salt was filtered and washed with deionized water and ethanol and air-dried. It was recrystallized from a mixture of an acetonitrile/ethanol solution (3:1 v/v). (C) 11-Tungsto-1-vanadophosphate, [PVW11O40]4-.10 A 0.4 g quantity of sodium dihydrogen orthophosphate (NaH2PO4‚ 2H2O) was added to a 400 mL aqueous solution of 8.2 g of
10.1021/ie051199x CCC: $33.50 © 2006 American Chemical Society Published on Web 02/21/2006
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Table 1. Characterization of Catalysts catalyst [W6O19]2[V(VW11)O40]4[PVW11O40]4[PV2Mo10O40]4(VO)2P2O7
mp (°C)
λmax (nm)
FT-IR (cm-1)
XRD (2θ, deg)
168-170 332-335 235-238 178-182 332-335
275 265 261 308 267
979, 889, 813, 586 967, 894, 777, 518 1095, 1071, 963, 890 1075, 1057, 941, 873 1246, 1135, 1081, 962
8.9, 9.5, 10.8,14.0, 25.9 6.6, 7.0, 8.0, 8.5, 29.9 6.6, 7.6, 12.1, 23.5, 29.9 6.7, 7.7, 12.1, 23.5, 30.0 23.0, 28.4, 29.9
sodium tungstate dihydrate (Na2WO4‚2H2O), followed by the addition of concentrated nitric acid. After stirring, 5 mL of aqueous solution containing 0.2 g of ammonium m-vanadate (NH4VO3) and 0.2 g of sodium hydroxide was added, and then the mixed solution was refluxed for 24 h. After cooling to room temperature, the precipitate was filtered off. To the filtrate was added 3 g of tetrabutylammonium bromide to precipitate a yellow salt. The salt was filtered and washed with deionized water and ethanol and dried. It was recrystallized from acetonitrile. (D) Molybdophosphate Complex, [PV2Mo10O40]4-.11 A 0.5 g quantity of ammonium m-vanadate (NH4VO3) was dissolved in 10 mL of 1 M sodium hydroxide. This solution was added to a solution of 6.0 g of sodium molybdate (Na2MoO4‚2H2O) and 0.4 g of sodium dihydrogen orthophosphate (NaH2PO4‚ 2H2O). Concentrated nitric acid was slowly added until the solution had a pH of 4.0. The brown solution was refluxed for 24 h. After cooling to room temperature, 3.5 g of tetrabutylammonium bromide was added to precipitate an orange salt. The salt was isolated, washed with water and ethanol, and airdried. It was recrystallized from acetonitrile/ethanol (1:1 v/v). (E) Vanadyl Pyrophosphate, (VO)2P2O7.12 A 3.0 g quantity of vanadium oxide (V2O5) was refluxed in a mixture of 2-butanol (18 mL) and benzyl alcohol (12 mL) for 14 h followed by the addition of 85% phosphoric acid (H3PO4) (P/V ratio of 1.1) and refluxed for a further 6 h to form a light green precipitate, which was filtered off, dried, and calcined in air at 400 °C for 4 h. Typical Pprocedure for the Oxidation. Model Sulfur Compounds. Model sulfur compounds (benzothiophene, BT; dibenzothiophene, DBT; or 4,6-dimethyldibenzothiophene, 4,6DMDBT) in the amount of 0.1 mmol, which dissolved in 5 mL of hexane, were added to the catalyst and a 30% solution of hydrogen peroxide (as an oxidant). It was stirred for 3 h at 60 °C. After that, 5 mL of extraction solvent was added to the mixture and stirred for 10 min. The biphasic mixture was separated by decantation. After being dried over anhydrous sodium sulfate, a 1 mL sample was withdrawn and worked-up, added to an internal standard, and subjected to GC analysis. The GC program started at 100 °C for 2 min, and the temperature was raised to 290 °C at 8 °C/min and was held for 10 min. Gas Oil. The mixture of 50 mL of gas oil (containing 0.575 wt % sulfur), catalyst (0.18 mmol), hydrogen peroxide, and acetic acid (hydrogen peroxide/acetic acid molar ratio of 1) was stirred at 60 °C for 5 h. The oil was extracted with acetonitrile 3 times (total volume 50 mL) and passed through a glass column (1 × 25 cm) containing 10 g of alumina to remove oxidized sulfur. The oxidized oil was washed with water and dried over anhydrous sodium sulfate. The concentration of the remaining sulfur compounds was determined by XRF. Results and Discussion Characterization of Catalysts. The FT-IR, UV-vis spectroscopy, XRD analyses, and the melting temperatures of the prepared catalysts are collected in Table 1.
Table 2. Oxidation of DBT Using [W6O19]2-a DBT/ oxidant/ entry catalyst DBT 1 2 3 4 5 6 7 8 9b 10b
100 100 100 50 25 100 100 100 100 25
5 5 5 5 5 10 20 5 5 20
extraction solvent EtOH/H2O2 MeCN DMSO DMSO DMSO DMSO DMSO DMF DMF DMF
conversion DBT in % extraction solvent % 50 56 58 78 83 80 86 61 70 93
22 28 20 18 9 11 7 0 0 0
a Conditions: DBT 0.1 mmol, catalyst 0.01 mmol, hexane 5 mL, temperature 60 ˚C, reaction time 3 h, extraction solvent 5 mL. b Added acetic acid (1 equiv to oxidant).
The absorption of the tetrabutylammonium salts of polyoxometalates in acetonitrile were measured, which showed a maximum at about 260 nm.13 The IR spectra showed four characteristic bands at around 1070-1080 cm-1, (P-O); 967976 cm-1, (M-Oterminal); 875-894 cm-1, (M-O-M, octahedral corner-sharing); and 810-813 cm-1 (M-O-M, octahedral edge-sharing).13 Oxidation of Model Compounds. Dibenzothiophene (DBT) was selected as a sulfur compound representative of those present in gas oils. Oxidation of DBT was performed with a [W6O19]2- catalyst to study the effect of different extraction solvents, as well as the amounts of catalyst and oxidant; the results are tabulated in Table 2. After oxidation, the gas chromatographic analysis shows remaining dibenzothiophene and dibenzothiophene sulfone as the only product. The oxidation reaction was monitored with time (0-3 h), and sulfone was still the only product detected. Generally, the oxidation of sulfur compounds is considered to be a consecutive reaction (i.e., sulfur-containing compoundss sulfoxides and sulfones). Therefore, in this study, no sulfoxide was detected, and the sulfoxide formation was considered to dominate the reaction rate. From Table 2, comparing among several solvents tested, it was found that DMSO and DMF yielded a higher % conversion than MeCN or EtOH/H2O2. This agrees well with that reported.14 The disadvantage of the former two solvents is that they have a high boiling point at 300 °C, which is close to the boiling point of the sulfone. Acetonitrile (MeCN) has a relatively low boiling point (82 °C) and is easily separated from the sulfone by distillation. It is known that an aprotic solvent, like DMF, and MeCN enhance water and hydrogen peroxide dissociations to obtain OH- and HO2-, respectively. The perhydroxyl ion (HO2-) is quite stable and interacts with the hydroxyl groups formed during water dissociation to produce the superoxide radical O2-. It should be noted that when using DMF as an extraction solvent, no DBT was detected in this solvent phase. Therefore, it is suitable for the complete separation between substrate and sulfone product. The percent conversion increases when the amount of oxidant and catalyst is increased. The addition of acetic acid enhanced the conversion. The reaction proceeds via oxidation by peracetic
Ind. Eng. Chem. Res., Vol. 45, No. 6, 2006 1855 Table 3. Oxidation of DBT Using Different Catalystsa entry
catalyst
DBT/catalyst
oxidant/DBT
conversion %
1 2 3 4 5 6 7 8 9 10
]2-
100 25 100 25 100 25 100 25 100 25
5 20 5 20 5 20 5 20 5 20
70 91 74 94 62 90 64 85 28 40
[W6O19 [W6O19]2[V(VW11)O40]4[V(VW11)O40]4[PVW11O40]4[PVW11O40]4[PV2Mo10O40]4[PV2Mo10O40]4(VO)2P2O7 (VO)2P2O7
Figure 1. Pseudo-first-order oxidation of BT at 50-70 ˚C using [V(VW11)O40]4-.
a Condition: DBT 0.1 mmol, hexane 5 mL, extraction solvent DMF 5 mL, [AcOH]/[H2O2] ) 1, temperature 60 °C, reaction time 3 h.
Table 4. Oxidation of Different Sulfur Compoundsa
Table 5. Rate Constants at Different Temperaturesa substrates
conversion %
BT
entry
catalyst
DBT
4,6-DMDBT
BT
1 2 3 4 5
[W6O19]2[V(VW11)O40]4[PVW11O40]4[PV2Mo10O40]4(VO)2P2O7
87 99 88 86 50
74 80 70 70 39
50 55 40 42 20
a Condition: substrate 0.1 mmol, hexane 5 mL, extraction solvent DMF 5 mL, substrate/catalyst ) 100, oxidant/substrate ) 30, [AcOH]/[H2O2] ) 1, temperature 60 °C, reaction time 5 h.
acid, which was formed in situ from a mixture of hydrogen peroxide and acetic acid. To compare the reactivity between the different catalysts, the oxidation of DBT was carried out under the same reaction conditions. The results are shown in Table 3. The oxidation reactivity order of the catalyst in the presence of acetic acid is [V(VW11)O40]4- > [W6O19]2- ∼ [PVW11O40]4- > [PV2Mo10O40]4- > (VO)2P2O7. Comparison of the Oxidation Reactivity of the Sulfur Compounds. To investigate the difference of reactivity between the different sulfur compounds, the oxidation of three model sulfur compounds (DBT, 4,6-DMDBT, and BT) was carried out under the same reaction condition. The results are shown in Table 4. The oxidation reactivity decreased in the order of DBT > 4,6-DMDBT > BT. BT exhibited the lowest reactivity, and this was attributed to the significantly lower electron density on the sulfur atom on BT. For DBT and 4,6-DMDBT, the difference in the electron density on the sulfur is very small (5.756 and 5.760, respectively), calculated by the semiempirical MO method.15 Therefore, reactivity was governed by the steric hindrance of the methyl groups, which become an obstacle for the approach of the sulfur atom to the catalytic active species. The results obtained in this work correspond to that reported for a formic acid/H2O2 system.16 In the oxidation of sulfur compounds, the formation of corresponding sulfones can be explained as follows: the sulfur compound reacts with peracetic acid, which resulted from a combination of H2O2 and acetic acid. In the presence of a catalyst, the mechanism involved a metal-peroxo intermediate formed by the reaction of the catalyst with the oxidant. This intermediate was detected by NMR.17 In this work, it was found that polyoxometalate containing vanadium has the highest reactivity, and this corresponds to the reported results on the effect of substituted transition metal in the polyoxometalate compounds.18 The substitution of Mo(VI) with V(V) would result in the generation of more reactive lattice oxygen associated with the Mo-O-V species. Kinetics. Since H2O2 was present in large excess, the reaction data were fitted to a first-order rate equation. The rate constant
DBT 4,6-DMDBT
reaction temperature (°C)
rate constants (min-1)
correlation factor R2
50 60 70 50 60 70 50 60 70
0.0057 0.0103 0.0223 0.0109 0.0171 0.0344 0.0061 0.0152 0.0223
0.9845 0.9916 0.9856 0.9971 0.9897 0.9801 0.9870 0.9820 0.9925
a Condition: substrate 0.01 M, [V(VW )O ]4- 1.25 × 10-4 M, oxidant 11 40 1.0 M. substrate/catalyst molar ratio ) 80, oxidant/substrate molar ratio ) 100, hexane 5 mL, extraction solvent DMF 5 mL, [AcOH]/[H2O2] ) 1.
Table 6. Apparent Activation Energies for Oxidation of Sulfur Compounds substrates
Ea (kJ/mol)
correlation factor R2
BT DBT 4,6-DMDBT
62.73 52.83 57.35
0.9849 0.9943 0.9963
(k) and reaction time (t) can be described using the following equation: ln(Ct/Co) ) -kt, where k ) A exp(-Ea/RT). A is the preexponential factor, Ea is the apparent activation energy, and R and T are the gas constant and reaction temperature (K), respectively. A plot of ln(Ct/Co) versus reaction time (t) displayed a linear relationship that confirmed the pseudo-firstorder reaction kinetics, as shown in Figure 1. The rate constants (k) were determined and collected in Table 5. A plot of ln k versus 1/T allows us to calculate the activation energy Ea (slope ) -Ea/R). The apparent activation energies are listed in Table 6. The apparent activation energy of pseudo-first-order reaction for BT, 4,6-DMDBT, and DBT were determined to be 62.73, 57.35, and 52.83 kJ/mol, respectively. It should be noted that the apparent activation energy obtained from this work is in good agreement with those reported. For the polyoxometalate/ H2O2 system, the apparent activation energy of 4,6-DMDBT and DBT were 57.41 and 53.8 kJ/mol, respectively.16 Oxidative Desulfurization of Gas Oil. To investigate whether the oxidative desulfurization system is effective for diesel fuels, the oxidative desulfurization of a commercial gas oil (containing 0.575 wt % of sulfur) was carried out. It was previously reported that the most effective solvent for the removal of the sulfone product was DMF, but oil recovery was low.19 In regards to oil recovery, acetronitrile is better than DMF.20 Therefore, in this work, acetonitrile was chosen as an extraction solvent. The oxidative reaction was followed by the adsorption of oxidized product using alumina as an adsorbent. The results are shown in Table 7. Acetonitrile has a relatively low boiling point and can be easily separated by distillation, and it can be reused for further extraction. Sulfone is more polar and is likely to bond strongly
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Table 7. Oxidative Desulfurization of Gas Oila entry
catalyst
sulfur content remained (wt %)
1 2 3 4 5
[W6O19]2[V(VW11)O40]4[PVW11O40]4[PV2Mo10O40]4(VO)2P2O7
0.142 0.055 0.098 0.129 0.208
conversion %
recovery of oil (%)
75 90 83 78 64
87 87 86 87 86
a Condition: gas oil (0.575 wt % S) 50 mL, catalyst 0.18 mmol, extraction solvent acetonitrile 50 mL, Al2O3 10 g, substrate/catalyst molar ratio ) 50, oxidant/substrate molar ratio ) 100, [AcOH]/[H2O2] ) 1, temp 60 °C, time 5 h.
to an alumina adsorbent without decomposition of H2O2 taking place.21 From the results obtained in this work, it was demonstrated that the catalyst can catalyze the oxidation reaction in 5 h and can reduce the sulfur content of diesel oil from 0.575 wt % S to 0.055 wt % S. For comparison, it should be mentioned that in the oxidation of straight run light gas oil (1.35 wt % S) in a hydrogen peroxide/formic acid system, the removal of sulfur was 99% in 46 h.19 Conclusions A desulfurization process of light oil using hydrogen peroxide and a catalyst has been investigated. It was found that the oxidation reaction proceeds very rapidly and selectively to give the corresponding sulfones under mild conditions. The oxidation with solvent extraction and alumina adsorption to further remove the oxidized product reduces the sulfur content of oil successfully up to 90%, from 0.575 wt % S to 0.055 wt % S. Acknowledgment The authors are grateful for the financial support from the Graduate School, Chulalongkorn University. Literature Cited (1) Rappas, A. S.; Nero, V. P.; DeCanio, S. J. Process for removing low amounts of organic sulfur from hydrocarbon fuels. United States Patent 6,406,616, 2002. (2) Tam, P. S.; Kittrel, J. R.; Eldridge, J. W. Desulfurization of fuel oil by oxidation and extraction. 1. Enhancement of extraction oil yield. Ind. Eng. Chem. Res. 1990, 29, 321. (3) Otsuki, S.; Nonaka, T.; Qian, W.; Ishihara, A.; Kabe, T. Oxidative desulfurization of middle distillate using ozone. Sekiyu Gakkaishi 1999, 42, 315. (4) Koch, T. A.; Krause, K. R.; Manzer, L. E.; Mehdizadeh, M.; Odom, J. M.; Senupta, S. K. Environmental challenges facing the chemical industry. New J. Chem. 1996, 20, 163. (5) (a) Yazu, K.; Yamamoto, Y.; Furuya, T.; Miki, K.; Ukeawa, K. Oxidation of dibenzothiophenes in an organic biphasic system and its application to oxidative desulfurization of light oil. Energy Fuels 2001, 15, 1535. (b) Yazu, K.; Furuya, T.; Miki, K.; Ukegawa, K. Tungstophos-
phoric acid-catalyzed oxidative desulfurization of light oil with hydrogen peroxide in a light oil/acetic acid biphasic system. Chem. Lett. 2003, 32, 920. (6) Collins, F. M.; Lucy, A. R.; Sharp, C. Oxidative desulphurization of oils via hydrogen peroxide and heteropolyanion catalysis. J. Mol. Catal., A 1997, 117, 397. (7) Funakoshi, T.; Aida, T. Solvent effects during oxidation-extraction desulfurization process of aromatic sulfur compounds from fuels. European Patent 0565324A1, 1993. (8) Himeno, S.; Yoshihara, M.; Maekawa, M. Formation of voltammetrically active isopolyoxotungstate complexes in aqueous CH3CN media. Inorg. Chim. Acta 2000, 298, 165. (9) Himeno, S.; Takamoto, M.; Higuchi, A.; Maekawa, M. Preparation and voltammetric characterization of keggin-type tungstovanadate [VW12O40]3and [V(VW11)O40]4- Complexes. Inorg. Chim. Acta 2004, 57, 87. (10) Ueda, T.; Komatsu, M.; Hojo, M. Spectroscopic and voltammetric studies on the formation of Keggin type V(V)-substituted tungstoarsenate(V) and phosphate(V) complexes in aqueous and aqueous-organic solutions. Inorg. Chim. Acta 2003, 344, 77. (11) Himeno, S.; Ishiro, N. A voltammetric study on the formation of V(V) and V(IV) substituted molybdophosphate(V) complexes in aqueous solution. J. Electroanal. Chem. 1998, 451, 203. (12) Pillai, U. R.; Sahle-Demessie, E.; Varma, R. S. Alternative routes for catalyst preparation: use of ultrasound and microwave irradiation for the vanadium phosphorus oxide catalyst and their activity for hydrocarbon oxidation. Appl. Catal., A 2003, 252, 1. (13) Himeno, S.; Takamoto, M.; Ueda, T. Synthesis, characterization, and voltammetric study of a Keggin-type [PWO]3- complex. J. Electroanal. Chem. 1999, 456, 129. (14) Otsuki, S.; Nonaka, T.; Takashima, N.; Qian, W.; Ishihara, A.; Imai, T.; Kabe, T. Oxidative desulfurization of light gas oil and vacuum gas oil by oxidation and solvent extraction. Energy Fuels 2000, 14, 1232. (15) Wang, X.; Xu, L.; Chen, X.; Ji, W.; Qijie, T.; Chen, Y. Novel modifications in preparing vanadium phosphorus oxides and their applications for partial oxidation of n-butane. J. Mol. Catal., A 2003, 206, 261. (16) Te, M.; Fairbride, C.; Ring, Z. Oxidation reactivities of dibenzothiophenes in polyoxometalate/H2O2 and formic acid/H2O2 systems. Appl. Catal., A 2001, 219, 267. (17) Kala Raj, N. K.; Ramaswamy, A. V.; Manikandan, P. Oxidation of norbornene over vanadium-substituted phosphomolybdic acid catalysts and spectroscopic investigations. J. Mol. Catal., A 2005, 227, 37. (18) Liu, Y.; Murata, K.; Inaba, M. Liquid-phase oxidation of benzene to phenol by molecular oxygen over transition metal substituted polyoxometalate compounds. Catal. Commun. 2005, 6, 679. (19) Zannikos, E.; Lous, E.; Stournas, S. Desulfurization of petroleum fractions by oxidation and solvent extraction. Fuel Proc. Technol. 1995, 42, 35-45. (20) Murata, S.; Murata, K.; Kidena, K.; Nomura, M. A novel oxidative desulfurization system for diesel fuels with molecular oxygen in the presence of cobalt catalysts and aldehydes. Energy Fuels 2004, 18, 116. (21) Shiraishi, Y.; Hara, H.; Hirai, T.; Komasawa, I. A deep desulfurization process for light oil by photosensitized oxidation using a triplet photosensitizer and hydrogen peroxide in an oil/water two-phase liquidliquid extraction system. Ind. Eng. Chem. Res. 1999, 38, 1589.
ReceiVed for reView October 28, 2005 ReVised manuscript receiVed January 23, 2006 Accepted January 26, 2006 IE051199X
