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Energy & Fuels 2004, 18, 116-121
A Novel Oxidative Desulfurization System for Diesel Fuels with Molecular Oxygen in the Presence of Cobalt Catalysts and Aldehydes Satoru Murata,* Kazutaka Murata, Koh Kidena, and Masakatsu Nomura† Department of Applied Chemistry, Faculty of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan Received April 25, 2003. Revised Manuscript Received July 29, 2003
Oxidative desulfurization of diesel fuels with molecular oxygen was examined by using cobalt salts and aldehydes as catalysts and sacrificial materials, respectively. At first, the authors conducted desulfurization of model oils consisting of benzene and dibenzothiophene. A mixture of benzene, dibenzothiophene, n-octanal, and an appropriate cobalt salt (acetate or chloride) was stirred at 40 °C under atmospheric pressure of oxygen to afford dibenzothiophene sulfone in almost quantitative yield within 15 min. Dibenzothiophene sulfone produced could be easily removed from the model oils by silica or alumina adsorption. Several organic sulfides including thioanisole, diphenyl sulfide, benzothiophene, and 4,6-dimethyldibenzothiophene also could be converted to the corresponding sulfones in almost quantitative yields. Then, the authors examined ultra deep desulfurization of a commercial diesel fuel, which contains 193 wt ppm of sulfur. By using the system consisting of cobalt acetate, aldehyde, and molecular oxygen, sulfur-containing compounds in the diesel fuel were oxidized, and then removed by alumina adsorption and/or solvent extraction. The resulting oil contained less than 5 wt ppm of sulfur; this corresponds to the result that more than 97% of sulfur in the oil could be removed. These results may indicate that this brand-new oxidative desulfurization process has a potential to meet a future regulation of sulfur in the diesel fuel.
Introduction From an increase of environmental concern, special interest has been paid to reduction of organosulfur compounds in transportation fuels, because these compounds are converted into sulfur oxide, i.e., SOx, during their combustion. In addition to this, SOx in flue gas of automobiles poisons the catalysts for nitrogen oxide (NOx) reduction.1 Therefore, concentration of sulfur in fuels is limited severely and its regulation level is becoming lower and lower from year to year. Regulation of sulfur concentration in diesel fuels in Japan was reduced from 0.2 wt % to 0.05 wt % in 1997, and this regulation will be decreased to 0.005 wt % (50 wt ppm) in 2005. Further reduction of the regulation to 10 or 15 wt ppm is projected in around 2008. Under these situations, many researchers are investigating developments of highly active catalysts for ultra deep hydrodesulfurization of diesel fuels to meet these sulfur regulations. Nowadays, the catalyst-making companies have developed high performance catalysts, which can reduce the sulfur level in diesel fuels to around 50 wt ppm. However, some researchers are wondering whether these high performance catalysts can reduce sulfur contents into 10-15 wt ppm or not. * Corresponding author. E-mail:
[email protected]. † Phone: +81-6-6879-7360. Fax: +81-6-6879-7362. E-mail: nomura@ ap.chem.eng.osaka-u.ac.jp. (1) Babich, I. V.; Moulijn, J. A. Fuel 2003, 82, 607.
As an alternative to hydrodesulfurization (HDS), oxidative desulfurization (ODS) was proposed in the 1990s.1,2 ODS generally consists of two processes: the first step is oxidation of organosulfur compounds in fuels, and the following step is removal of oxidized sulfur-containing compounds from the treated fuels. Compared with the traditional HDS, ODS has several advantages such as mild reaction conditions (ambient pressure and relatively low temperatures), high selectivity, no use of expensive hydrogen, and potential for desulfurization of sterically hindered sulfides such as 4,6-dimethyldibenzothiophene (DMDBT) and benzo[b]naphtho[2,1-d]thiophene (1,2-benzodiphenylene sulfide). Therefore, many papers and patents concerning oxidation of sulfides have been published and claimed, including the systems for ODS of diesel fuels by using hydrogen peroxide-formic acid,2,3 hydrogen peroxideacetic acid,4 hydrogen peroxide-polyoxometalates,5-7 (2) Aida, T. Jpn. Kokai Tokkyo Koho 1993, JP 052-86869; Aida, T. European Patent 565,324, 1993; Aida, T.; Yamamoto, D. Prepr. Pap.s Am. Chem. Soc., Div. Fuel Chem. 1994, 39, 623; Aida, T. Catalyst Catalysis 1995, 37, 243; Funakoshi, I.; Aida, T. Jpn. Kokai Tokkyo Koho 2001, JP 2001-107059. (3) Otsuki, S.; Nonaka, T.; Takashima, N.; Qian, W.; Ishihara, A.; Imai, T.; Kabe, T. Energy Fuels 2000, 14, 1232; Kabe, T. Jpn. Kokai Tokkyo Koho 1999, JP H11-140462. (4) Zannikos, F.; Lois, E.; Stournas, S. Fuel Process. Technol. 1995, 42, 35. (5) Collins, F. M.; Lucy, A. R.; Sharp, C. J. J. Mol. Catal., A 1997, 117, 397. (6) Yatsu, K.; Yamamoto, Y.; Furuya, T.; Miki, K.; Ukegawa, K. Energy Fuels 2001, 15, 1535; Yatsu, K.; Miki, K.; Ukegawa, K.; Yamamoto, N. Jpn. Kokai Tokkyo Koho 2001, JP 2001-354978.
10.1021/ef034001z CCC: $27.50 © 2004 American Chemical Society Published on Web 11/12/2003
Novel Oxidative Desulfurization System for Diesel Fuels
peracetic acid,1 ozone,8 and photooxidation with molecular oxygen in the presence of sensitizers such as cyanoarenes.9,10 Among these reactions, oxidation of dibenzothiophene (DBT) with peracids or hydrogen peroxide is very attractive because in general the reaction proceeds rapidly and selectively under very mild conditions such as ambient temperature and pressure. However, large-scale storage and use of such peroxides are somewhat dangerous. Transition metal-catalyzed co-oxidation of aldehydes and organic compounds with molecular oxygen (eq 1), which includes oxidation of aldehydes with molecular oxygen to the corresponding peroxy acids with molecular oxygen (eq 2) and oxidation of organic compounds with the peroxy acids produced (eq 3), may avoid the danger of peroxy compounds.
RCHO + substrate + O2 f RCO2H + product
(1)
RCHO + O2 f RCO3H
(2)
RCO3H + substrate f RCO2H + product
(3)
In this reaction, both aldehydes and molecular oxygen were used instead of peracids, both of which are not so dangerous reagents in general. Several authors reported that co-oxidation of aldehydes and alkenes proceeds smoothly to give the corresponding carboxylic acids and oxiranes in the presence of transition metal salts or complexes, respectively.11-13 As to co-oxidation of aldehydes and organic sulfides with molecular oxygen, a few studies were reported. Nobile et al.,14 Song et al.,15 and Venkateshwar Rao et al.16 reported co-oxidation of aliphatic aldehydes and acyclic sulfides such as dimethyl sulfide, diethyl sulfide, dibutyl sulfide, diphenyl sulfide, dibenzyl sulfide, thioanisole, etc., in the presence14,15 or absence16 of transition metal salts. However, they did not examine the cyclic sulfides such as benzothiophene, dibenzothiophene, etc.; in general, these are known to be hardly oxidized compared with acyclic (7) Mei, H.; Mei, B. W.; Yen, T. F. Fuel 2003, 82, 405. (8) Otsuki, S.; Nonaka, T.; Qian, W.; Ishihara, A.; Kabe, T. Sekiyu Gakkaishi 1999, 42, 315. (9) Hirai, T.; Ogawa, K.; Komasawa, I. Ind. Eng. Chem. Res. 1996, 35, 586; Hirai, T.; Shiraishi, Y.; Ogawa, K.; Komasawa, I. Ind. Eng. Chem. Res. 1997, 36, 530; Shiraishi, Y.; Hirai, T.; Komasawa, I. J. Chem. Eng. Jpn. 1999, 32, 158; Shiraishi, Y.; Hirai, T.; Komasawa, I. Solvent Extr. Res. Dev. Jpn. 1999, 6, 137; Shiraishi, Y.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 1998, 37, 203; Shiraishi, Y.; Hara, H.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 1998, 38, 1589; Shiraishi, Y.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 1999, 38, 3300; Shiraishi, Y.; Taki, Y.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 1999, 38, 3310; Shiraishi, Y.; Taki, Y.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 1999, 38, 4538; Shiraishi, Y.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 2001, 40, 293; Komasawa, I.; Hirai, T. Jpn. Kokai Tokkyo Koho 2000, JP 2000-096068 (10) Yazu, K.; Yamamoto, Y.; Miki, K.; Ukegawa, K. J. Oleo Sci. 2000, 50, 521; Yatsu, K.; Miki, K.; Ukegawa, K.; Yamamoto, N. Jpn. Kokai Tokkyo Koho 2001, JP 2001-151748. (11) Kholdeeva, O. A.; Grigoriev, V. A.; Maksimov, G. M.; Fedotov, M. A.; Golovin, A. V.; Zamaraev, K. I. J. Mol. Catal. A 1996, 114, 123. (12) Nam, W.; Kim, H. J.; Kim, S. H.; Ho, R. Y. N.; Valentine, J. S. Inorg. Chem. 1996, 35, 1045. (13) Komiya, N.; Naota, T.; Oda, Y.; Murahashi, S.-I. J. Mol. Catal. A 1997, 117, 21. (14) Mastrorilli, P.; Nobile, C. F. Tetrahedron Lett. 1994, 35, 4193; Giannandrea, R.; Mastrorilli, P.; Nobile, C. F.; Suranna, G. P. J. Mol. Catal. A 1995, 103, 17; Dell’Anna, M. M.; Mastrorilli, P.; Nobile, C. F. J. Mol. Catal. A 1996, 108, 57. (15) Song, G.; Wang, F.; Zhang, H.; Lu, X.; Wang, C. Synth. Commun. 1998, 28, 2783. (16) Venkateshwar Rao, T.; Sain, B.; Kumar, K.; Murthy, P. S.; Prasada Rao, T. S. R.; Joshi, G. C. Synth. Commun. 1998, 28, 319.
Energy & Fuels, Vol. 18, No. 1, 2004 117
sulfides. In addition to this, these reaction systems required polar or halogen-containing solvents such as 1,2-dichloroethane. Kaneda et al. reported the noncatalytic reaction of DBT in the presence of acetaldehyde or benzaldehyde in an autoclave at elevated temperatures such as 100-150 °C.17 The reaction afforded the corresponding sulfone in almost quantitative yield; however, the completion of the reaction required higher temperatures. Paybarah et al. reported noncatalytic photooxidation of DBT and benzaldehyde under molecular oxygen, but completion of the reaction required somewhat longer reaction duration.18 In the present paper, the authors examined cooxidation of DBT and its derivatives, and aldehydes with molecular oxygen in the presence of transition metal salts in nonpolar organic solvent such as benzene. It was found that the reaction proceeds very rapidly and selectively to give the corresponding sulfones in almost quantitative yields under very mild conditions (ambient pressure and 40 °C). The authors also examined the applicability of the method to ultra deep desulfurization of diesel fuels. The results were reported briefly. Experimental Section Samples and Apparatus. All the reagents except aldehydes employed in this study (metal salts, sulfur-containing compounds, and solvents) were commercially available and used without further purification. Aldehydes used were commercially available and were distilled just before an experiment to remove the peracids contained therein. A diesel fuel was purchased from a gas station and used as obtained. Gas chromatographic and gas chromatography-mass spectrometric analyses were conducted on a Shimadzu GC-14B and a Shimadzu QP-5050A apparatus, respectively. Sulfur contents of original and treated diesel fuels were measured by using a PHILIPS MagiXPRO apparatus by the XRF method, whose limitation of sulfur detection is 5 wt ppm. Oxidation of DBT. A typical procedure was as follows. A mixture of cobalt(II) acetate (12 mg, 0.05 mmol), n-octanal (512 mg, 4 mmol), DBT (184 mg, 1 mmol), n-undecane (100 mg, an internal standard for gas chromatographic analysis), and benzene (10 mL) was stirred at 40 °C under an ambient pressure of oxygen (with a balloon). An aliquot of the reaction mixture was analyzed by a Shimadzu GC-14B gas chromatograph every 15 min. After the completion of the reaction, about 50 mL of hexane was added to the reaction mixture, and then cooled to 0 °C in an ice-water bath. The product, dibenzothiophene-5,5-dioxide (dibenzothiophene sulfone) was obtained as white precipitates (205 mg, 95 mol %). Measurement of Oxygen Uptake. Benzene solution (5 mL) containing metal salts (0.05 mmol) and DBT (0.5 mmol) was stirred at 30 °C for at least 30 min under static pressure of oxygen. By introducing the benzene solution (5 mL) of n-octanal (2 mmol), the reaction started. The time profile of oxygen uptake was monitored at 15 s intervals by a gas buret equipped with the flask. Oxidative Desulfurization of Diesel Fuels. A typical procedure was as follows. A mixture of a commercial gas oil (100 mL), cobalt(II) acetate (24 mg, 0.1 mmol), and n-octanal (2.05 g, 16 mmol) was stirred in a 200 mL-round-bottom flask at 40 °C for 16 h under an ambient pressure of oxygen. After the completion of the reaction, the resulting oil was extracted with sodium carbonate aqueous solution (5 wt %, 100 mL × (17) Kaneda, T.; Daimon, H. Jpn. Kokai Tokkyo Koho 1979, JP 54016465. (18) Paybarah, A.; Bone, R. L.; Corcoran, W. H. Ind. Eng. Chem. Process Res. Dev. 1982, 21, 426.
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Murata et al.
2) to remove n-octanoic and octaneperoxoic acids produced. After being dried over sodium sulfate, the treated oil was passed through a glass column (2 cm id × 30 cm length) containing alumina (aluminum oxide 90 active neutral, Merck & Co., Inc., 60 g) or silica (Wakogel C-200, Wako Pure Chemical Industries, Ltd., 20 g) to remove oxidized sulfur species. Extraction with acetonitrile (50 mL, twice) was also examined. Concentration of the remaining sulfur compounds was determined by the XRF method.
Results and Discussion Oxidation of Sulfur-Containing Model Compounds. At first, the authors examined the oxidation of DBT, which is a typical model compound for sulfurcontaining compounds in diesel fuels. A benzene solution (10 mL) containing cobalt(II) acetate (0.05 mmol), n-octanal (4 mmol), and DBT (1 mmol) was stirred at 40 °C under an ambient pressure of oxygen. Sulfur content of this model oil is calculated to be 0.33 wt %. The reaction mixture became clouded within 5 min. Consumption of DBT was monitored by a gas chromatograph, indicating that almost all DBT was consumed within 15 min to give dibenzothiophene sulfone. Aldehydes are known to be oxidized easily to the corresponding peracids by molecular oxygen in the presence of transition metal salts. Therefore, oxidation of DBT with the oxygen-cobalt catalyst-aldehyde system might proceed via the following two steps: The first step is oxidation of aldehydes to peracids (eq 4):
C7H15CHO + O2 f C7H15CO3H
(4)
and the next step is oxidation of DBT to sulfone by the peracids produced (eq 5):
To check the stoichiometry of the reactions shown in eqs 4 and 5, the following reaction was conducted. A benzene solution (10 mL) containing cobalt acetate (0.05 mmol), DBT (1 mmol), and n-octanal (1 mmol) was stirred at 40 °C for 1 h under an ambient pressure of oxygen. The resulting mixture was treated with diazomethane and analyzed by a gas chromatograph, this indicating the formation of methyl n-octanoate (95%) and conversion of DBT (51%). The result indicates that stoichiometry of the reaction obeys eqs 4 and 5 and the role of aldehyde is sacrificial materials for oxidation of sulfides. Some cobalt(II) salts such as chloride, acetylacetonate, and bromide were examined as a catalyst for the reaction, the results being summarized in Table 1. Cobalt(II) chloride also showed a high activity for oxidation of DBT, while two other salts had modest activities. The authors also examined applicability of other metal salts such as manganese(II), nickel(II), and copper(I), the results being also shown in Table 1. On the basis of conversion of DBT in 15 min, the order of
Table 1. Oxidation of DBT with Molecular Oxygen in the Presence of Several Transition Metal Salts and Aldehydesa conv. of DBT (mol %)b metal salt
aldehyde (mmol)
15 min
Co(OAc)2 Co(OAc)2 Co(OAc)2 Co(OAc)2 CoCl2 CoBr2 Co(acac)2 Mn(OAc)2 Mn(acac)2 Ni(OAc)2 CuCl none Co(OAc)2 Co(OAc)2 Co(OAc)2 Co(OAc)2
n-octanal (4) none n-octanal (2) n-octanal (3) n-octanal (4) n-octanal (4) n-octanal (4) n-octanal (4) n-octanal (4) n-octanal (4) n-octanal (4) n-octanal (4) n-hexanal (4) n-decanal (4) benzaldehyde (4) cinnamaldehyde (4)
> 99 0 70 98 > 99 82c 86c > 99 81c 54 35 0 > 99 96 93 0
45 min
90 min
72 >99
93 68
97 84
> 99 > 99
a Oxidation of DBT (1 mmol) with molecular oxygen (1 atm, with a balloon) was conducted at 40 °C in the presence of metal salts (0.05 mmol) and aldehydes. b Determined by GLC analysis. c Extension of reaction duration to 120 min did not lead to increase of DBT conversion.
the catalytic activity of the metal salts employed was found to obey the following order: cobalt(II) chloride, cobalt(II) acetate, and manganese(II) acetate > cobalt(II) bromide and cobalt(II) acetylacetonate > manganese(II) acetylacetonate > nickel(II) acetate > copper(I) chloride. Without metal salts, the reaction did not proceed and DBT was recovered quantitatively. In the absence of n-octanal, the reaction did not proceed and DBT was recovered quantitatively. Decrease of n-octanal to 2 or 3 mmol (1.0 or 1.5 molar equivalent to DBT) resulted in lower conversion and slower oxidation rate. These results suggest that higher conversion and faster reaction rate require 4 mmol of n-octanal. Several aliphatic and aromatic aldehydes could be employed instead of n-octanal (Table 1). Using n-hexanal, n-decanal, or benzaldehyde, DBT could be converted to the sulfone in almost quantitative yields. However no DBT was consumed in the reaction with cinnamaldehyde. A flow of air was applicable instead of static oxygen; a benzene solution (10 mL) of cobalt acetate or chloride (0.05 mmol), DBT (1 mmol), and n-octanal (4 mmol) was stirred at 30 °C for 75 min under air bubbling (20 mL/min), almost all DBT being consumed (conversion of DBT, 97-99%). Oxidation of the other sulfur-containing compounds was also examined by using cobalt(II) chloride, cobalt(II) acetate, and manganese(II) acetate, the results being summarized in Table 2. Both diphenylsulfide and thioanisole seemed to be as highly active as DBT and were oxidized to the corresponding sulfones within 15 min under reaction conditions similar to those of DBT. In the case of DMDBT, slightly severe conditions were required for the completion of the oxidation; reaction temperature should be raised or a larger amount of aldehyde should be employed. For the complete conversion of benzothiophene, which has the lowest reactivity among the substrates employed,3 both extension of reaction duration and increase of the amount of noctanal employed were required. In addition to this, it should be noted that cobalt(II) catalysts are slightly
Novel Oxidative Desulfurization System for Diesel Fuels
Energy & Fuels, Vol. 18, No. 1, 2004 119
Table 2. Oxidation of Organic Sulfides with Molecular Oxygen in the Presence of Cobalt Salts and n-Octanala
a Oxidation of organic sulfide with molecular oxygen (1 atm, with a balloon) was conducted in the presence of metal salt and n-octanal. b Determined by GLC analysis.
more active than a manganese(II) catalyst as seen in the results of the reaction of benzothiophene (Table 2). As to the relative reactivity of DMDBT to DBT, the former showed slight less reactivity than the latter under the reaction conditions employed. Kabe et al. reported that using relatively small oxidant such as performic acid (prepared in situ from formic acid and hydrogen peroxide), DMDBT reacted more rapidly than DBT because electron density of the sulfur atom in the former compound is higher than that in the latter.3 On the other hand, Te et al. reported the opposite results, DBT reacted more rapidly than DMDBT by using hydrogen peroxide-polyoxometalate system and they concluded that the different results between two systems originated from the difference of the molecular size of the oxidants.19 In our system, octaneperoxoic acid seems to be a plausible oxidant. This has a larger molecular size than performic acid, so this may induce smaller reactivity of DMDBT in our system. Reaction Mechanism. The authors investigated the mechanism of this oxidation reaction. A chain mechanism was proposed for transition metal-catalyzed autoxidation of aldehydes20 and co-oxidation of aldehydes and alkenes.11-13 At first, aldehydes are oxidized by metal salts to give a proton and the corresponding acyl radical (eq 7). Acyl radicals formed react easily with oxygen to form acylperoxy radicals (eq 8). Acylperoxy radicals react with aldehydes to give peracids and to regenerate acyl radicals (eq 9). It was reported that peroxy acids could oxidize sulfides to sulfones.21-23 (19) Te, M.; Fairbridge, C.; Ring, Z. Appl. Catal. A: General 2001, 219, 267-280. (20) Crich, D.; Komatsu, M.; Ryu, I. Chem. Rev. 1999, 99, 1991. (21) Hemlich, B. N.; Wallace, T. J. Tetrahedron 1966, 22, 3571. (22) Drago, R. S.; Mateus, A. L. M. L.; Patton, D. J. Org. Chem. 1996, 61, 5693.
Figure 1. Oxygen uptake during the reaction of DBT (0.5 mmol) with n-octanal (2 mmol) in the presence of transition metal salts (0.025 mmol) in benzene (10 mL) at 30 °C. Metal salts: Co(acac)2 (×), Mn(acac)2 (+), Mn(OAc)2 (0), CoBr2 (9), CoCl2 (4), Co(OAc)2 (2), CuCl (b), and Ni(OAc)2 (O).
Therefore, peracids produced might be real active species for oxidation of dibenzothiophene (eqs 10-11).
Mn+ + O2 f M(n+1)+ + O2•-
(6)
RCHO + M(n+1)+ f RCO• + H+ + Mn+
(7)
RCO• + O2 f RCO3•
(8)
RCO3• + RCHO f RCO3H + RCO•
(9)
RCO3H + R′SR′ f RCO2H + R′SOR′
(10)
RCO3H + R′SOR′ f RCO2H +R′SO2R′
(11)
where Mn+and M(n+1)+ represent a metal ion, R is alkyl or aryl group, R′SR′ represents sulfides, R′SOR′ is sulfoxides, and R′SO2R′ is sulfones To obtain detailed information concerning the role of the metal salts, the authors conducted the measurement of the rate for oxygen uptake, the results being summarized in Figure 1. After a rather short induction period (usually, 0-1 min), oxygen uptake started and about 1.3-1.8 mmol of oxygen was absorbed, this corresponding to 65-90% for stoichiometric amounts (eq 4). Both amounts and rates of oxygen uptake changed drastically, depending on the metal salts employed. On the basis of the results shown in Figure 1 and Table 1, these metal salts could be classified into three groups: (a) fast rate of oxygen uptake but moderate DBT conversion: cobalt(II) acetylacetonate, cobalt(II) bromide, and manganese(II) acetylacetonate; (23) Thenraja, D.; Subramaniam, P.; Srinivasan C. Tetrahedron 2002, 58, 4283.
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(b) fast or medium rate of oxygen uptake and quantitative conversion of DBT: cobalt(II) chloride, cobalt(II) acetate, and manganese(II) acetate; (c) slow rate of oxygen uptake and low DBT conversion: nickel(II) acetate and copper(I) chloride. The slow rate of oxygen uptake with group (c) salts agrees well with low conversion of DBT in oxidation reaction (Table 1). These salts may have lower activity for oxidation of aldehydes (eq 7) than the other ones. In the case of group (a) salts, oxidation of DBT stopped around 80% conversion and complete conversion could not be attained by extension of reaction duration as shown in Table 1, though oxygen uptake proceeded rapidly. This may indicate that aldehydes were consumed by some side reactions or these salts lost their catalytic activities during the reaction.24 Consequently, group (b) salts seems to be the best catalysts for the reaction. The authors also conducted experiments on oxygen uptake with several aldehydes and cobalt(II) acetate. By using aliphatic aldehydes such as n-hexanal, noctanal, and n-decanal, oxygen uptake was rather rapid and about 1.6 mmol of oxygen was consumed within 30 min, this corresponding to 80% of the stoichiometric amount (eq 4). In the reaction with benzaldehyde, oxygen uptake was slow, but 1.6 mmol of oxygen was consumed within 60 min. In the case using cinnamaldehyde, only 0.3 mmol of oxygen was consumed within 60 min, this indicating that oxidation of the aldehyde proceeds very slowly under the conditions examined. These results agree well with the results of oxidation of DBT shown in Table 1. Among the aliphatic aldehydes, acetaldehyde is available less expensively in petroleum refinery, because it can be prepared easily and in large scale, by palladiumcatalyzed air-oxidation of ethylene (so-called, Wacker process).25 Therefore, it will become the prevailing candidate for the sacrificial material in the practical application of this process. Desulfurization of Diesel Fuels. In the previous section, the authors found that DBT in benzene (sulfur content, ca. 0.33 wt %) could be oxidized to the sulfone by using an oxygen-cobalt catalyst-aldehyde system in almost quantitative yield. If 99% of the sulfone produced can be removed, the sulfur content in the model oil should be decresed to around 33 wt ppm. Accordingly, the authors examined how to remove the sulfone from the solution treated with this oxidation system. Several researchers examined the method for this treatment and reported that extraction with polar solvents, adsorption with silica or alumina, and distillation were useful for this purpose.2-5 The authors examined an adsorption method. In our system, signifi(24) To examine the origin of lower conversion of DBT with group (a) salts, the following experiment was conducted. A benzene solution (10 mL) containing cobalt(II) bromide (0.05 mmol), dibenzothiophene (1 mmol), and n-octanal (4 mmol) was stirred at 40 °C for 1 h under static oxygen, then the additional octanal (2 mmol) was introduced. Before addition of octanal, about 80% of DBT was converted as shown in Table 1, while introduction of 2 mmol of octanal lead to almost complete conversion of DBT. These results may indicate that the origin of lower conversion of DBT with group (a) salts is not deactivation of the catalyst, but consumption of aldehydes by side reaction. (25) Because of very high volatility, the authors did not use acetaldehyde in lab. experiments. However, Murahashi et al. reported that acetaldehyde is applicable in epoxidation of olefins by O2-metal catalyst-aldehyde system.13
Murata et al. Table 3. Sulfur Removal from Oxidized Model Oil method for sulfur removal adsorption with SiO2 (10 g) adsorption with SiO2 (5 g) adsorption with Al2O3 (15 g) adsorption with Al2O3 (10 g) adsorption with Al2O3 (5 g)
degree of removal recovery of of sulfone (mol %) benzene (wt %) 100 100 100 100 50
68 74 75 80 83
a Oxidation of DBT (1 mmol) with molecular oxygen (1 atm, with a balloon) was conducted at 40 °C in the presence of cobalt acetate (0.05 mmol) and n-octanal (4 mmol) for 15 min.
Figure 2. Oxygen uptake during the reaction of DBT (0.5 mmol) with aldehydes (2 mmol) in the presence of cobalt(II) acetate (0.025 mmol) in benzene (10 mL) at 30 °C. Aldehyde: n-hexanal (b), n-decanal (O), n-octanal (9), benzaldehyde (0), and cinnamaldehyde (2).
cant amounts of acids and peracids from oxidation of aldehydes were contained in the oils treated. At first, the reaction mixture was passed through a glass column filled with silica or alumina without any pretreatment, this leading to retaining of almost all sulfur-containing compounds in benzene solution. This may be due to the fact that polar compounds such as acids or peracids adsorbed more strongly on silica or alumina than sulfurcontaining compounds. Therefore, the reaction mixture was extracted with sodium carbonate aqueous solution in order to remove acidic products, dried over sodium sulfate, and then submitted to adsorption experiments. The results of adsorption experiments are shown in Table 3. More than 5 g of silica or more than 10 g of alumina could adsorb whole sulfone, and around 7080% of benzene employed was recovered. Then, the authors investigated the method for removing sulfur-containing compounds from the diesel fuels. The oil (193 wtppm sulfur) was oxidized by molecular oxygen, n-octanal, and cobalt(II) acetate,26 washed with sodium carbonate solution, and dried over sodium (26) In oxidation of DBT, both cobalt(II) acetate and chloride showed similar activity. However, in practical application, halide ligand is not appropriate in view of green chemistry. Therefore, the authors examined the applicability of cobalt acetate for oxidation of sulfurcontaining compounds in diesel fuels.
Novel Oxidative Desulfurization System for Diesel Fuels Table 4. Oxidative Desulfurization of a Commercial Diesel Fuela
method for sulfur removal
sulfur contents in the product (wt ppm)
degree of desulfurization (%)
recovery of oil (wt %)
extraction with acetonitrile adsorption with SiO2 (20 g) adsorption with Al2O3 (60 g)
36 19 97
80 47 46
extraction with acetonitrile followed by adsorption with Al2O3 (20 g)
97
58
a A mixture of a commercial diesel fuel (100 mL), cobalt(II) acetate (0.1 mmol), n-octanal (16 mmol) was stirred at 40 °C for 16 h under static oxygen (1 atm, with a balloon).
sulfate. Three methods for removing sulfur-containing compounds from the oils were employed; adsorption, extraction with a polar solvent, and combination use of them. The results are summarized in Table 4. When the treated oils were passed through a glass column filled with alumina or silica, more than 90% of sulfurcontaining compounds could be removed. Extraction of the treated oil with acetonitrile also resulted in removal of 80% of sulfur from the oil. Combination use of extraction with acetonitrile and adsorption with alumina was also effective; more than 97% of sulfur could be removed from the oil. Under the best conditions, the remaining sulfur was less than 5 wt ppm (detection limit of the measurement of sulfur concentration is 5 wt ppm in our apparatus), the value is less than a future regulation of sulfur in diesel fuels, 10 wt ppm. Compared with hydrogen peroxide-carboxylic acid or hydrogen peroxide-polyoxometalate systems,3-5 the present system has some advantages; the reaction proceeds rapidly and requires smaller amounts of
Energy & Fuels, Vol. 18, No. 1, 2004 121
sacrificial materials because of the homogeneous nature of the system. Hydrogen peroxide-carboxylic acid and hydrogen peroxide-polyoxometalate systems generally requires 10 to 60 vol % of hydrogen peroxide solution3-5 and 10 to 100 vol % of carboxylic acid3,4 to gas oil. Besides, the process with polyoxometalate requires surfactants such as tetraoctylammonium bromide and methyltrioctylammonium bromide.5,7 On the basis of the above facts, the system examined in this study seems to be more cost-effective than hydrogen peroxide-based system. Now the authors are conducting research concerning optimization of our system including fixation of cobalt catalyst, reduction of the amounts of aldehydes used, and so on. Conclusion The authors tried to develop a novel oxidative desulfurization (ODS) process using molecular oxygen. Dibenzothiophene, a typical model for sulfur-containing compounds in diesel fuels, was found to be oxidized easily to the corresponding sulfone in excellent yield by using molecular oxygen-cobalt catalyst-aldehyde system. Other organic sulfides also could be oxidized by this system very rapidly. The authors also examined the desulfurization of a commercial diesel fuel with 193 wt ppm of sulfur. The sulfur content could be reduced to less than 5 wt ppm by using this system. These results may indicate that this brand-new ODS process has a potential to meet a future regulation of sulfur content in diesel fuels. Acknowledgment. The authors thank Dr. Ikushima (Petroleum Energy Center, Japan) and Mr. Nagai (Taiyo Oil Co., Ltd.) for their help concerning analysis of sulfur content in the oils. EF034001Z
