Energy & Fuels 2001, 15, 1535-1536
1535
Communications Oxidation of Dibenzothiophenes in an Organic Biphasic System and Its Application to Oxidative Desulfurization of Light Oil Kazumasa Yazu,*,† Yorihiro Yamamoto,‡ Takeshi Furuya,† Keiji Miki,† and Koji Ukegawa† Institute for Energy Utilization, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8569, Japan, and Department of Chemistry and Biotechnology Graduate School of Engineering, The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan Received June 26, 2001. Revised Manuscript Received September 5, 2001 Dibenzothiophenes are oxidized effectively with hydrogen peroxide in the presence of 12-tungstophosphoric acid (TPA) in n-octane/acetonitrile (MeCN) biphasic system to give their corresponding sulfones as the major product. The oxidation proceeds in the MeCN phase and sulfones do not migrate into the n-octane phase. 4,6-Dimethyldibenzothiophene, one of the most unreactive sulfur compounds during hydrodesulfurization, was also oxidized and removed effectively from the n-octane phase. The sulfur content of a light oil was effectively reduced by using this biphasic oxidation system. Air pollution due to exhaust from diesel engines has been a major concern of the public. Although an exhaust after-treatment device can reduce NOx and particulates in diesel exhaust emission, sulfur compounds in light oil damage the device. It is therefore desirable to reduce the sulfur content in light oil. Organic sulfur in light oil is usually removed by hydrodesulfurization, but the remaining sulfur content is still high (about 500 ppm in the United States and Japan). New legislation in Japan and Europe will limit the sulfur content in light oil to 50 ppm maximum by 2005. The United States will also limit that for highway use to 15 ppm by the middle of 2006. To achieve this requirement, alkylated dibenzothiophenes substituted at 4 and/or 6 positions should be removed since they are resistant to hydrodesulfurization because of their steric hindrance.1,2 Oxidative desulfurization has received much attention in recent years because hindered dibenzothiophenes can be oxidized selectively to their corresponding sulfoxides and sulfones, and these products can be removed by extraction and adsorption. High concentrations of peroxyacids3,4 and hydrogen peroxide5 were necessary to oxidize sulfur compounds in light oil. However, the use of these * Corresponding author. Tel: +81-298-61-8433. Fax: +81-298-618433. E-mail:
[email protected]. † National Institute of Advanced Industrial Science and Technology. ‡ The University of Tokyo. (1) Kabe, T.; Ishihara, A.; Tajima, H. Ind. Eng. Chem. Res. 1992, 31, 1577. (2) Ma, X.; Sakanishi, K.; Mochida, I. Ind. Eng. Chem. Res. 1994, 33, 218. (3) Aida, T.; Yamamoto, D. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1994, 39, 623. (4) Bonde, S. E.; Gore, W.; Dolbear, G. E.; Skov, E. R. Prepr. Pap.s Am. Chem. Soc., Div. Pet. Chem. 2000, 45, 364.
Scheme 1
oxidants at high concentrations should be avoided in terms of safety and the loss of oil quality. Shiraishi et al. reported the oxidative desulfurization of light oil using 9,10-dicyanoanthracene-sensitized photooxidation.6-8 They used a light oil-MeCN biphasic system that permits dibenzothiophenes to be photooxidized in MeCN phase and successively extracted from light oil. We found anthraquinone is more useful in this photooxidative desulfurization system since it is less expensive than 9,10dicyanoanthracene.9 Here we report a more practical MeCN biphasic system using hydrogen peroxide and TPA as oxidant and catalyst, respectively. In this n-octane/ acetonitrile biphasic system, dibenzothiophenes were oxidized to the corresponding sulfones. Moreover, sulfur content in light oil was efficiently reduced. Dibenzothiophene (1a) is oxidized rapidly by hydrogen peroxide in the presence of TPA in n-octane-MeCN to give the corresponding sulfone, dibenzothiophene 5, 5-dioxide (2a), as the major product (Scheme 1). For a (5) Collins, F. M.; Lucy, A. R.; Sharp, C. J. Mol. Catal. A 1997, 117, 397. (6) Shiraishi, Y.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 1998, 37, 203. (7) Shiraishi, Y.; Taki, Y.; Hirai, T.; Komasawa, I. Chem. Commun. 1998, 2601. (8) Shiraishi, Y.; Taki, Y.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 1999, 38, 3310. (9) Yazu, K.; Yamamoto, Y.; Miki, K.; Ukegawa, K. J. Oleo Sci. 2000, 50, 521.
10.1021/ef0101412 CCC: $20.00 © 2001 American Chemical Society Published on Web 10/17/2001
1536
Energy & Fuels, Vol. 15, No. 6, 2001
Communications Table 1. Oxidative Desulfurization of Light Oila for 3 h at 60 °C entry
TPA/30%H2O2
light oil:MeCN (mL/mL)
sulfur content (ppm) mean ( S. D. (n ) 3)
1 2 3 4
0 µmol/0 mL 2.5 µmol/0.5 mL 2.5 µmol/0.5 mL 2.5 µmol/0.5 mL
50/50 50/50 50/100 50/200
281 ( 0.2 44 ( 0.3 23 ( 0.3 12 ( 0.1
a
Figure 1. The disappearance of 1a (b) or 1b (O) in n-octane during the oxidation with hydrogen peroxide in the presence of TPA at 60 °C in an n-octane-MeCN biphasic system.
typical run, TPA (2.5 µmol) was dissolved in 0.5 mL of 30% aqueous hydrogen peroxide and mixed with 50 mL of MeCN. Then, 50 mL of a 10 mM n-octane solution of 1a was added and this biphasic mixture was heated to 60 °C with stirring. Concentration of 1a in n-octane was determined by HPLC as reported previously.9 Briefly, the reaction solution was injected on a Wakosil-II 5C18 RS column (4.6 × 150 mm) eluting with a linear gradient of MeCN/H2O ) 30/70 to 90/10 (v/v) over 20 min at a flow rate of 1.0 mL min-1 and detected at 320 nm. Figure 1 shows the rapid decrease of 1a during the oxidation. Without oxidation, 57% of 1a was transferred into the MeCN phase from the n-octane phase in our conditions. It has been demonstrated that MeCN extracts sulfur compounds such as benzothiophenes and dibenzothiophenes from light distillates.6 The oxidation of 1a to 2a proceeded in the MeCN phase in the presence of hydrogen peroxide and TPA because no oxidation of 1a was observed without the addition of MeCN. It is easy to remove 2a from the n-octane phase since 99% of 2a resides in the MeCN phase due to its polarity. 4,6Dimethyldibenzothiophene (1b) is one of the most unreactive sulfur compounds during hydrodesulfurization.1,2 Because of substitution with methyl groups, a lesser amount (40%) of 1b was transferred into the MeCN phase from the n-octane phase without oxidation when compared with 1a, but 1b was oxidized smoothly and ef-
Initial sulfur content of light oil was 330 ppm.
ficiently removed from the n-octane phase (Figure 1). Most of 2b (96%) was also found in the MeCN phase. It is noteworthy that the oxidation products such as 2a and 2b did not interfere with the oxidation as observed in the photooxidation.7-9 Next, we applied this oxidation system to light oil containing 330 ppm sulfur. TPA (2.5 µmol) was dissolved in 0.5 mL of 30% aqueous hydrogen peroxide and mixed with 50-200 mL of MeCN and 50 mL of light oil. The biphasic mixture was heated to 60 °C with stirring. After the oxidation, the light oil phase was separated, washed with water, dehydrated, and then the sulfur content was measured as reported previously.9 Briefly, the sulfur content was measured by pyro-fluorescence method with Antek 7000B analyzer using a tetradecane solution of dibenzothiophene as a standard solution. Table 1 shows that a small reduction in sulfur content was observed without TPA and hydrogen peroxide (entry 1) since organic sulfur compounds in light oil migrate into the MeCN phase from light oil phase because of solubility difference. The oxidation initiated with TPA and hydrogen peroxide further reduced sulfur content in light oil to 44 ppm (entry 2). An increase in MeCN volume from 50 mL to 200 mL enhanced the removal of sulfur compounds to 12 ppm (entry 4). When this oxidized light oil containing 12 ppm sulfur was treated with an equal volume of MeCN, the sulfur content further decreased to 3 ppm. We are currently optimizing the process. In summary, we have demonstrated effective oxidation of dibenzothiophenes using hydrogen peroxide and TPA in an organic biphasic system. This oxidation system was found to be useful for the oxidative desulfurization of light oil. EF0101412
