Energy & Fuels 2006, 20, 1757-1760
1757
Silica-Catalyzed tert-Butyl Hydroperoxide Oxidation of Dibenzothiophene and Its 4,6-Dimethyl Derivative: A Route to Low-Sulfur Petroleum Feedstocks Keith J. Stanger and Robert J. Angelici* Ames Laboratory and Department of Chemistry, Iowa State UniVersity, Ames, Iowa 50011-3111 ReceiVed October 3, 2005. ReVised Manuscript ReceiVed July 13, 2006
Silica catalyzes the tert-butyl hydroperoxide (TBHP, t-BuOOH) oxidation of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-Me2DBT) to their oxides at 50-90 °C. Sulfur concentrations in simulated hydrotreated petroleum feedstock solutions containing 374 ppm of sulfur as DBT or 4,6-Me2DBT can be reduced to 1 ppm within 40 min at 90 °C. As an inexpensive, stable, and recyclable catalyst, silica has several advantages over other catalysts for the oxidation of DBTs to their easily separated oxides as a means of achieving low sulfur levels in petroleum feedstocks.
Introduction Increasingly stringent governmental restrictions on the sulfur content of petroleum feedstocks in the United States, Europe, and Japan have led to numerous studies of new approaches to the removal of dibenzothiophene (DBT) and its methylsubstituted derivatives such as 4,6-dimethyldibenzothiophene (4,6-Me2DBT) from these feedstocks.1,2 One promising approach involves the oxidation of these compounds to their sulfoxides and sulfones (eq 1), which are considerably less soluble in nonpolar hydrocarbon solvents than DBT or 4,6-Me2DBT and may be extracted into polar solvents or adsorbed onto metal oxide adsorbents.
One recent example3 of the oxidation/extraction method uses aqueous H2O2 as the oxidant, [(C18H37)2NMe2+]3[PW12O40-3] as the catalyst, and 1-methyl-2-pyrrolidinone as the polar phase that extracts the DBT and 4,6-Me2DBT sulfones from the diesel fuel. The reactions were generally conducted in the 30-90 °C temperature range and were able to reduce the sulfur content of diesel fuel from 500 to 0.1 ppm. Because of the insolubility of 30% H2O2/H2O in the diesel, the solutions were stirred at high speed and the resulting emulsions had to be demulsified after the reaction, which required time. * To whom correspondence should be addressed. Telephone: 515-2942603. Fax: 515-294-0105. E-mail:
[email protected]. (1) Song, C.; Ma, X. Appl. Catal., B 2003, 41, 207-238. (2) Song, C. Catal. Today 2003, 86, 211-263. (3) (a) Li, C.; Jiang, Z.; Gao, J.; Yang, Y.; Wang, S.; Tian, F.; Sun, F.; Sun, X.; Ying, P.; Han, C. Chem.sEur. J. 2004, 10, 2277-2280. (b) Lu¨, H.; Gao, J.; Jiang, Z.; Jing, F.; Yang, Y.; Wang, G.; Li, C. J. Catal. 2006, 239, 369-375. (c) Garcı´a-Gutie´rrez, J. L.; Fuentes, G. A.; Herna´ndez-Tera´n, M. E.; Murrieta, F.; Navarrete, J.; Jime´nez-Cruz, F. Appl. Catal., A 2006, 305, 15-20.
In another study,4 hydrocarbon-soluble tert-butyl hydroperoxide (TBHP, t-BuOOH), was used as the oxidant with MoO3/ Al2O3 as the catalyst in the temperature range of 40-100 °C. In the light gas oil (LGO) reactant, about 50% of the sulfur was present in the form of DBT, 4,6-Me2DBT, and other substituted DBTs. After oxidation, the feed was passed through a silica adsorbent at room temperature to remove the sulfones. This oxidation/adsorption process reduced the 39 ppm sulfur content of the LGO to 5 ppm. In previous investigations,5 we used a rhenium complex tethered to the surface of silica as the catalyst for the TBHP oxidation of DBT and 4,6-Me2DBT to their sulfones. During those studies, we observed that silica alone (without the tethered rhenium complex) catalyzes the TBHP oxidation of methyl(ptolyl)sulfide to its sulfoxide and sulfone slowly even at 20-50 °C. Earlier, silica was reported6,7 to catalyze the oxidation of alkyl and aryl sulfides to sulfoxides and sulfones using TBHP. However, the oxidation of DBTs was not previously described. In the present paper, we report the silica-catalyzed oxidation of DBT and 4,6-Me2DBT by TBHP at 50-90 °C. As a catalyst for the removal of DBTs from petroleum feedstocks by oxidation/adsorption, silica has many advantages: it is heterogeneous, inexpensive, and easily regenerated. Experimental Section General Considerations. The chemicals TBHP (5-6 M solution in nonane) [labeled TBHP(nonane)], TBHP (70% aqueous solution) [labeled TBHP(aq)], DBT, 4,6-Me2DBT, hydrogen peroxide (30% aqueous solution), toluene, hexanes, and silica gel Merck grade 10184 (63-200 µm particle size, Brunauer-Emmett-Teller (BET) surface area, 300 m2 g-1; pore size of 100 Å) were purchased from commercial sources and used as received. (4) (a) Ishihara, A.; Wang, D.; Dumeignil, F.; Amano, H.; Qian, E. W.; Kabe, T. Appl. Catal., A 2005, 279, 279-287. (b) Wang, D.; Qian, E. W.; Amano, H.; Okata, K.; Ishihara, A.; Kabe, T. Appl. Catal., A 2003, 253, 91-99. (5) Stanger, K. J.; Wiench, J. W.; Pruski, M.; Espenson, J. H.; Kraus, G. A.; Angelici, R. J. J. Mol. Catal. A: Chem. 2006, 243, 158-169. (6) Breton, G. W.; Fields, J. D.; Kropp, P. J. Tetrahedron Lett. 1995, 36, 3825-3828. (7) Kropp, P. J.; Breton, G. W.; Fields, J. D.; Tung, J. C.; Loomis, B. R. J. Am. Chem. Soc. 2000, 122, 4280.
10.1021/ef050327r CCC: $33.50 © 2006 American Chemical Society Published on Web 08/16/2006
1758 Energy & Fuels, Vol. 20, No. 5, 2006
Stanger and Angelici Table 1. Effect of Temperature on Silica-Catalyzed TBHP(nonane) Oxidation of DBT in a Simulated Petroleum Feedstocka entry
temperature (°C)
half-life (min)b
1 2 3
90 80 70
6.1 (4) 26 (3) 36 (4)
entry
temperature (°C)
half-life (min)b
4 5
60 50
80 (10) NR
a Reaction conditions: 10.0 mL of a 9.45 mM (374 ppm of sulfur) solution of DBT in 45:55% toluene/hexanes, 1.00 g of silica, 94.5 mM TBHP(nonane) (10 equiv), and reaction temperature of 50-90 °C. b Errors, given in parentheses after each t1/2, were obtained from nonlinear fitting of the reaction data from 1 to 3 runs to a first-order rate equation of the form ln[A]t ) -kt + ln[A]0 and reflect the uncertainty in the last digit(s) of the half-life value.
Figure 1. Representative examples of silica-catalyzed TBHP(nonane) oxidation of DBT in a simulated petroleum feedstock. Reaction conditions: 10.0 mL of a 9.45 mM (374 ppm of sulfur) solution of DBT in 45:55% toluene/hexanes, 1.00 g of silica, 94.5 mM TBHP(nonane) (10 equiv), and reaction temperature of 80 °C (9) or 90 °C (b).
Catalytic Oxidation Reactions. All reactions were run in a 50 mL jacketed flask equipped with a magnetic stir bar and a septum for the addition and removal of the solvent. Silica (0.00-5.00 g) and oxidant [TBHP(nonane), TBHP(aq), or H2O2] were placed in the air-filled reaction vessel, which was sealed with the septum. After the reaction temperature was increased (50-90 °C), 10.0 mL of a 9.45 mM (374 ppm of sulfur) solution of DBT or 4,6-Me2DBT in 45:55% (vol/vol) toluene/hexanes containing an internal standard of diphenylmethane was added by syringe. The reaction progress was monitored by the periodic removal of gas chromatography (GC) samples. Gas chromatographic analyses were performed on a Hewlett-Packard HP 6890 GC using a 25 m HP-5 capillary column at 190 °C and a flame ionization detector (FID). Because of the poor solubility of the oxides of DBT and 4,6-Me2DBT and the tendency of the products to precipitate out of solution8,9 or remain adsorbed to silica,8,10 the reactions were monitored by GC following the disappearance of DBT or 4,6-Me2DBT relative to the internal standard diphenylmethane (see Figure 1 for example). The reactions were first-order in DBT or 4,6-Me2DBT, and data from one to three runs for each reaction were fit to the first-order rate law: ln[A]t ) -kt + ln[A]0. From a nonlinear fitting of the data, rate constants, half-lives, and errors were obtained for each oxidation reaction. For reactions with half-lives < 2 h, data were taken to at least 95% conversion (19 ppm of remaining DBT or 4,6-Me2DBT); longer reactions were monitored for 24 h, as noted in the text. Most of the reactions were followed until the DBT or 4,6-Me2DBT concentrations were reduced below 5 ppm. When the silica catalyst was reused for additional oxidation reactions, the catalyst was filtered in air after each reaction, washed with methanol, and vacuum-dried before being reweighed and reused.
Results and Discussion Effect of Temperature on the Oxidation of DBT. It is known that silica is able to catalyze the oxidation of simple alkyl and aryl sulfides (RSR′) to sulfoxides and sulfones at 25 °C with TBHP or OXONE.6,7 However, we observe that DBT and 4,6-Me2DBT are much more resistant to TBHP oxidation because no measurable oxidation occurs in reactions of 9.45 mM (374 ppm of sulfur) DBT in 10 mL of 45:55% (vol/vol) toluene/hexanes over 1.00 g of silica with 10 equiv of TBHP in nonane at temperatures of 50 °C or below even after 24 h (entry 5 in Table 1). However, at temperatures above 50 °C, (8) Collins, F. M.; Lucy, A. R.; Sharp, C. J. Mol. Catal. A: Chem. 1997, 117, 397. (9) Wang, Y.; Lente, G.; Espenson, J. H. Inorg. Chem. 2002, 41, 1272. (10) Murata, S.; Murata, K.; Kidena, K.; Nomura, M. Energy Fuels 2004, 18, 116.
Table 2. Effect of the Amount of Silica on the TBHP(nonane) Oxidation of DBT in a Simulated Petroleum Feedstocka entry
temperature (°C)
SiO2 (g)
half-life (min)b
1 2 3 4 5 6 7
80 80 80 80 80 50 50
3.00 2.00 1.00 0.10 0.00 5.00 1.00
8.2 (4) 11 (2) 26 (3) 238 (14) NR 58 (2) NR
a Reaction conditions: 10.0 mL of a 9.45 mM (374 ppm of sulfur) solution of DBT in 45:55% toluene/hexanes, 0.00-5.00 g of silica, 94.5 mM TBHP(nonane) (10 equiv), and reaction temperature of 50 or 80 °C. b Errors, given in parentheses after each t , were obtained from nonlinear 1/2 fitting of the reaction data from 1 to 3 runs to a first-order rate equation of the form ln[A]t ) -kt + ln[A]0 and reflect the uncertainty in the last digit(s) of the half-life value.
DBT oxidation is observed. At 60 °C (entry 4 in Table 1, t1/2 ) 80 min), a slow but significant oxidation occurs and increases further as the temperature is raised to 90 °C (entry 1 in Table 1, t1/2 ) 6.1 min). Thus, the rate of the reaction increases substantially with increasing temperature and roughly doubles with every 10 °C increase in temperature. At 90 °C, >95% of the DBT is oxidized within 25 min; i.e.,
