3420
Energy & Fuels 2007, 21, 3420–3424
Oxidative Desulfurization of HDS Diesel Using the Aldehyde/ Molecular Oxygen Oxidation System Tumula Venkateshwar Rao,* Bir Sain,* Sudha Kafola, Bhagat Ram Nautiyal, Yogendra Kumar Sharma, Shrikant Madhusudan Nanoti, and Madhukar Omkarnath Garg Indian Institute of Petroleum, Dehradun 248005, India ReceiVed May 15, 2007. ReVised Manuscript ReceiVed July 5, 2007
Oxidation of dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT), and 4,6-dimethyldibenzothiophene (4,6-DMDBT) was studied using the isobutyraldehyde/molecular oxygen oxidation system in the absence of metal catalyst as the sulfur present in hydrodesulfurized (HDS) diesel is mostly in the form of substituted dibenzothiophenes. This oxidation system was found to oxidize these sulfur compounds to their respective sulfones, and oxidation reactivity was found to decrease in the order 4,6-DMDBT > 4-MDBT > DBT. Further, the study was extended to oxidation of sulfur compounds present in HDS diesel in a two-phase system using the aldehyde/molecular oxygen oxidation system. The oxidized HDS diesel on extraction with polar solvents showed remarkable reduction in total sulfur. Acetonitrile was found to be the most effective among solvents studied, and isobutyraldehyde was observed to be the best among aldehydes studied. In HDS diesel, total sulfur could be reduced from 448 to 77 ppm by oxidation followed by solvent extraction, and it could be further reduced to 31 ppm by passing through a silica gel column.
Introduction Sulfur in fuels causes pollution by emitting SOx and particulate matter (PM) from internal combustion engines. It adversely affects the performance of emission control systems as well as causes corrosion of engines and process equipment. Because of the inherent benefits of diesel fuel, its consumption in automotives is increasing throughout the world. In view of environmental concerns more stringent specifications for sulfur in diesel fuel are being implemented by developed countries. The United States Environmental Protection Agency (USEPA) recommended reduction of sulfur level to 15 ppm by June 1, 20061 in on road diesel. Developed countries throughout the world put regulations to reduce sulfur to 15/10 ppm by 20092 in diesel fuel. Similar directives were followed by developing countries with later date of implementation. In India, sulfur in diesel fuel is to be reduced to 50 ppm in 11 major cities and to 350 ppm in rest of the country by 2010.3 Hydrodesulfurization has been an integral part of refinery operations. It can easily remove sulfur compounds, such as sulfides, disulfides, thiols, thiophenes, benzothiophenes, and dibenzothiophenes from middle distillates. On moderate operation with Mo/Al2O3 catalyst, hydrodesulfurization in diesel fuel reduces sulfur to the order of 350–500 ppm. However, alkylsubstituted dibenzothiophenes like 4,6-dimethyldibenzothiophene are refractive to hydrodesulfurization due to steric hindrance and are difficult to remove to a great extent.4–7 In order to * Corresponding authors: e-mail
[email protected], tumula_vrao@hotmail. com,
[email protected]; phone 91-135-2660113, 4, 5, 6; Fax 91-1352660202. (1) USEPA, Regulatory announcement: heavy-duty engine and vehicle standards and highway diesel fuel sulfur control requirements, Dec 2000. (2) Gosling, C. D.; Gembicki, V. A.; Gatan, R. M.; Cavanna, A.; Molinari, D. National Petrochemical and Refiners Association (NPRA), Annual Meeting, AM-04-48, San Antonio, TX, March 21–23, 2004. (3) The Dew Journal, May 2004, pp 10, 37. (4) Babich, I. V.; Moulijn, J. A. Fuel 2003, 82, 607–631, and references therein.
produce ultralow sulfur diesel (ULSD) fuel with this process, the deep HDS technique is to be adopted. This technique requires severe conditions and has negative effects like decrease in catalyst life, higher hydrogen consumption, and higher yield losses resulting in higher costs.4 It also causes global warming by raising CO2 emissions.8 Owing to these difficulties, alternative methods are being investigated worldwide.4,5 Among those methods, oxidative desulfurization of diesel has attracted worldwide attention due to inherent advantages like mild reaction conditions and low cost.2 In oxidative desulfurization, sulfur compounds present in diesel are oxidized to more polar sulfones/sulfoxides to facilitate their removal by solvent extraction or adsorption on solid adsorbents. Oxidation of sulfur compounds present in diesel is the key to the oxidative desulfurization, and various oxidation systems have been reported in the literature for this transformation. These include nitric acid/NO2,9 ozone,10 molecular oxygen in the presence of cobalt and manganese catalysts supported on γ-alumina,11 molecular oxygen in the presence of metal catalysts and aldehyde as sacrificial agent,12 photooxidations,13 and photooxidations along with sensitizers.14,15 Hydrogen peroxide has been widely used as oxidant by generating (5) Song, C.; Ma, X. Appl. Catal., B 2003, 41, 207–238. (6) Shafi, R.; Hutchings, G. J. Catal. Today 2000, 59, 423–442. (7) Ma, X.; Sakanishi, K.; Mochida, I. Ind. Eng. Chem. Res. 1994, 33, 218–222. (8) The Kyoto Protocol to the convention of climate change, Dec 1997. (9) Tam, P. S.; Kittrell, J. R.; Eldridge, J. W. Ind. Eng. Chem. Res. 1990, 29, 321–324. (10) Otsuki, S.; Nonaka, T.; Qian, W.; Ishihara, A.; Kabe, T. Bull. Chem. Soc. Jpn. 1998, 31, 1939–1951. (11) Sampanthar, J. T.; Xiao, H.; Dou, J.; Nah, T. Y.; Rong; X.; Kwan, W. P. Appl. Catal., B 2006, 63, 85–93. (12) Murata, S.; Murata, K.; Kidena, K.; Nomura, M. Energy Fuels 2004, 18, 116–121. (13) Shiraishi, Y.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 2001, 40, 293–303, and references therein. (14) Yazu, K.; Yamamoto, Y.; Miki, K.; Ukegawa, K. J. Oleo Sci. 2001, 50 (6), 521–525.
10.1021/ef700245g CCC: $37.00 2007 American Chemical Society Published on Web 09/01/2007
OxidatiVe Desulfurization of Diesel
Energy & Fuels, Vol. 21, No. 6, 2007 3421 Table 1. Oxidation of DBT and Alkyl DBTsa
entry
substrate
reaction time (min)
product
mp (°C)
reference mp (°C)
υmax53 (cm-1)
1 2 3
DBT 4-MDBT 4,6-DMDBT
60 45 40
DBT sulfone 4-MDBT sulfone 4,6-DMDBT sulfone
231–233.4 178–180.6 288–290.2
232–232.550 18951 29452
1167, 1289 1159, 1287 1152, 1283
a
Aldehyde: isobutyraldehyde; sulfur-to-aldehyde mole ratio 1:10.9; reaction temperature: 40 °C; solvent: dichloroethane.
percarboxylic acids in situ with formic acid16–20 and acetic acid.21–23 It has also been used in the presence of catalysts like polyoxometallates,24–27 amphiphilic catalysts of polyoxometallates associated with long-chain-containing quaternary ammonium ions in emulsions,28,29 polymolybdates supported on alumina,30 Ti molecular sieves,31 WOx/ZrO2,32 and vanadium oxide.33 Other oxidants like tert-butyl hypochlorite34 and tertbutyl hydroperoxide2,35 have also been reported in the literature for oxidation of sulfur compounds. Among these oxidants, peracids produced in situ from organic acids and H2O2 are reported to be very effective for rapid oxidation of sulfur compounds present in diesel under mild conditions. Molecular oxygen is an environment-friendly and abundantly available cheap oxidant. The peracids generated in situ from aldehyde and molecular oxygen form a powerful oxidation system which has been used for a variety of oxygenation reactions.36–41 Venkateshwar Rao et al.42 reported oxidation of (15) Shiraishi, Y.; Taki, Y.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 1999, 38, 3310–3318. (16) Rappas, A. S. Unipure Corporation, US 6402940, 2002. (17) Aida, T. EU 0565324, 1993. (18) Aida, T.; Yamamoto, D. Prepr. Pap.—Am. Chem. Soc. DiV. Fuel Chem. 1994, 39, 623–626. (19) Otsuki, S.; Nonaka, T.; Takashima, N.; Qian, W.; Ishihara, A.; ImaiT.; Kabe, T. Energy Fuels 2000, 14, 1232–1239. (20) Yu, G.; Lu, S.; Chen, H.; Zhu, Z. Carbon 2005, 43, 2285–2294. (21) Bonde, S. E.; Chapados, D.; Gore, W. L.; Dolbear, G. E.; Skov, E. National Petrochemical and Refiners Association (NPRA), Annual Meeting, A M-00-25, San Antonio, TX, March 26–28, 2000. (22) Zannikos, F.; Lois, E.; Stournas, S. Fuel Process. Technol. 1995, 42, 35–45. (23) Ramírez-Verduzco, L. F.; Murrieta-Guevara, F.; Garcia-Gutierrez, J. L.; Martin-Castanon, R. S.; Martinez-Guerrero, M. C.; Montiel-Pacheco, M. C.; Manta-Diaz, R. Pet. Sci. Technol. 2004, 22, 129–139. (24) Collins, F. M.; Lucy, A. R.; Sharp, C. J. Mol. Catal. A: Chem. 1997, 117, 397–403. (25) Yazu, K.; Yamamoto, Y.; Furuya, T.; Miki, K.; Ukegawa, K. Energy Fuels 2001, 15, 1535–1536. (26) Mei, H.; Mei, B. W.; Yen, T. F. Fuel 2003, 82, 405–414. (27) Te, M.; Fairbridge, C.; Ring, Z. Appl. Catal., A 2001, 219, 267– 280. (28) Lu, H.; Gao, J.; Jiang, Z.; Jing, F.; Yang, Y.; Wang, G.; Li, C. J. Catal. 2006, 239, 369–375. (29) Huang, D.; Wang, Y. J.; Yang, L. M.; Luo, G. S. Ind. Eng. Chem. Res. 2006, 45, 1880–1885. (30) Garcia-Gutierrez, J. L.; Fuentes, G. A.; Hernandez-Teran, M. E.; Murrieta, F.; Navarrete, J.; Jimenez-Cruz, F. Appl. Catal., A 2006, 305, 15–20. (31) Hulea, V.; Fajula, F.; Bousquet, J. J. Catal. 2001, 198, 179–186. (32) Ramírez-Verduzco, L. F.; Torres-García, E.; Gómez-Quintana, R.; González-Peña, V.; Murrieta-Guevara, F. Catal. Today 2004, 98, 289–294. (33) Caero, L. C.; Hernández, E.; Pedraza, F.; Murrieta, F. Catal. Today 2005, 107–108, 564–569. (34) Kabe, T.; Otsuki, S.; Nonaka, T.; Qian, W.; Ishihara, A. J. Jpn. Pet. Inst. 2001, 44, 18–24. (35) Wang, D.; Qian, E. W.; Amano, H.; Okata, K.; Ishihara, A.; Kabe, T. Appl. Catal., A 2003, 253, 91–99. (36) Mukaiyama, T.; Yamada, T. Bull. Chem. Soc. Jpn. 1995, 68, 17– 35. (37) 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– 130. (38) Nam, W.; Kim, H. J.; Kim, S. H.; Ho, R. Y. N.; Valentine, J. S. Inorg. Chem. 1996, 35, 1045–1049. (39) Dell’Anna, M. M.; Mastrorilli, P.; Nobile, C. F.; Suranna, G. P. J. Mol. Catal. A 1995, 103, 17–22. (40) Bolm, C.; Schlingloff, G.; Weickhardt, K. Tetrahedron Lett. 1993, 34, 3405–3408.
acyclic sulfides with the aldehyde/molecular oxygen system. Kaneda et al.43 reported oxidation of dibenzothiophene with the aldehyde/molecular oxygen system at elevated temperature. Nobile et al.,44,45 Song et al.,46 and Venkateshwar Rao et al.47 studied acyclic sulfides oxidation with aldehyde/molecular oxygen in the presence of metal catalysts. Murata et al.12 studied oxidation of DBT and alkyl DBTs with aldehyde/molecular oxygen in the presence of various catalysts and extended these studies for oxidative desulfurization of diesel. We herein report for the first time our results on oxidation of dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT), 4,6-dimethyldibenzothiophene (4,6-DMDBT), and sulfur compounds present in HDS diesel using the aldehyde/molecular oxygen system in the absence of metal catalyst and removal of oxidized sulfur compounds from HDS diesel by solvent extraction/adsorption. Experimental Section Oxidation of DBT and Alkyl DBTs. Dichloroethane (10 mL) and isobutyraldehyde (IBA; 0.5 mL, 5.45 mmol) were taken in a specially designed reaction assembly. The assembly consists of 50 mL round-bottom flask fused with a condenser and having a provision to attach oxygen balloon. Contents of assembly were stirred using a magnetic stirrer for 30 min at 40 °C under an oxygen atmosphere provided by a balloon. After that, stirring and supply of oxygen from the balloon were stopped, and DBT (92 mg, 0.5 mmol) was introduced into the system. Stirring was further continued in an oxygen atmosphere. Conversion of sulfide to sulfone was monitored with thin-layer chromatography (TLC). In regular intervals, reaction contents were spotted on TLC plates (TLC grade silica gel coated 7.5 cm long glass plates) and run the TLC using 10% ethyl acetate in benzene. After reaching solvent to top of the plate, solvent on the plate was dried, and the plate was exposed to iodine vapors to make different spots visible. With this solvent system approximate retention factors (Rf) of sulfides, sulfoxides, and sulfones are 0.9, 0.45, and 0.73, respectively. Complete disappearance of sulfide and sulfoxide indicates complete conversion of sulfide to sulfone. After complete conversion of sulfide to sulfone, contents were transferred to a separating funnel, thoroughly washed with distilled water/sodium bicarbonate solution, and dried on anhydrous sodium sulfate. Evaporation of the solvent yielded DBT sulfones (101 mg). Similarly, oxidation of 4-MDBT and 4,6DMDBT to their sulfones was carried out. Melting points of sulfones were determined using a Buchi B-540 instrument; IR spectra were recorded on a Perkin-Elmer FTIR 1760X instrument. Results are reported in Table 1. Oxidation of Sulfur Compounds Present in HDS Diesel and Their Removal. Similar oxidation assembly as described above with 150 mL capacity of round-bottom flask was used for oxidation (41) Murahashi, S.; Oda, Y.; Naota, T. J. Am. Chem. Soc. 1992, 114, 7913–7914. (42) Venkateshwar Rao, T.; Sain, B.; Kumar, K.; Murthy, P. S.; Prasada Rao, T. S. R.; Joshi, G. C. Synth. Commun. 1998, 28, 319–326. (43) Kaneda, T.; Daimon, H. Jpn. Kokai. Tokkyo Koho 1979,JP 540 16465. (44) Mastrorilli, P.; Nobile, C. F. Tetrahedron Lett. 1994, 35, 4193– 4196. (45) Dell’Anna, M. M.; Mastrorilli, P.; Nobile, C. F. J. Mol. Catal. A: Chem. 1996, 108, 57–62. (46) Song, G.; Wang, F.; Zang, H.; Lu, X.; Wang, C. Synth. Commun. 1998, 28, 2783–2787. (47) Venkateshwar Rao, T.; Sain, B.; Murty, P. S. N.; Joshi, G. C.; Prasada Rao, T. S. R. Sut. Sur. Sci. Catal. 1998, 113, 921–926.
3422 Energy & Fuels, Vol. 21, No. 6, 2007
Venkateshwar Rao et al.
Table 2. Oxidative Desulfurization of HDS Diesela entry
sulfur:aldehyde mole ratio
reaction time (min)
sulfur (ppm)d
1 2 3 4 5 6 7 8
1:64 1:64 1:64 1:64 1:32 1:0 1:64b 1:64c
10 30 90 180 180 10 180 180
122 115 82 77 84 313 284 165
a Aldehyde: isobutyraldehyde; HDS diesel: 40 g; acetonitrile: 50 mL; reaction temperature: 40 °C. b Without induction period (30 min stirring under oxygen was not done before introducing diesel). c Reaction temperature 25 °C. d Sulfur in ppm after oxidation and extraction.
Table 3. Effect of Solvent on Oxidative Desulfurization of HDS Diesela entry
solvent
sulfur (ppm)b
1 2 3 4
acetonitrile dimethylformamide 2-ethoxyethanol methanol
77 100 304 304
a Aldehyde: isobutyraldehyde; reaction time: 180 min; reaction temperature: 40 °C; sulfur-to-aldehyde mole ratio: 1:64. b Sulfur in ppm after oxidation and extraction.
Table 4. Effect of Aldehyde on Oxidative Desulfurization of HDS Diesela entry
aldehyde
sulfur (ppm)b
1 2 3 4
isobutyraldehyde n-butyraldehyde benzaldehyde n-octaldehyde
77 222 205 210
a Reaction time: 180 min; reaction temperature: 40 °C; sulfur-toaldehyde mole ratio: 1:64; solvent: acetonitrile. b Sulfur in ppm after oxidation and extraction.
of sulfur compounds present in HDS diesel. Acetonitrile (50 mL) and the required amount of aldehyde (Table 2) were taken in the reaction assembly. Contents were stirred for 30 min at 40 °C under an oxygen atmosphere. The diesel (40 g) was then introduced, and the contents were further stirred for a specified period as mentioned in Table 2. Acetonitrile and diesel layers were separated using a separating funnel, and oxidation of sulfur compounds was confirmed by analyzing both layers by a gas chromatograph with pulsed flame photometric detector (GC PFPD) of Varian Star 3600 Cx model and capillary column (DB-1, VA-123103-50, 30 m × 0.32 mm, 5 µm). Diesel layer was further extracted with 50 mL of acetonitrile and washed with distilled water, sodium bicarbonate solution, and finally with distilled water and dried on anhydrous sodium sulfate. The concentrations of sulfur compounds were determined by an Oxford Laboratory X 3000 X-ray fluoresecence sulfur analyzer and are mentioned in Tables 2, 3, and 4. The sample with maximum sulfur reduction (Table 2, entry 4) obtained by oxidation and extraction was passed through a silica gel column for further reduction of sulfur.
Results and Discussion Oxidation of DBT and Alkyl DBTs. Studies on oxidation of DBT and alkyl DBTs has direct relevance to oxidative desulfurization as these types of sulfur compounds are difficult to remove by the HDS process. Oxidation of DBT, 4-MDBT, and 4,6-DMDBT was studied with the isobutyraldehyde/ molecular oxygen oxidation system using dichloroethane as solvent. The reaction time required for oxidation of DBT (60 min) was higher as compared to 4-MDBT (45 min) and 4,6DMDBT (40 min), confirming the oxidation reactivity order to be 4,6-DMDBT > 4-MDBT > DBT (Table 1). This reactivity
Scheme 1. Peracid Formation
order is reverse in the hydrodesulfurization process where steric hindrance due to substituents in sulfur compounds limits their interaction with catalysts. It indicates that steric hindrance is not playing much of a role here as the interacting peracid molecule is small and flexible, unlike the solid catalyst site, and therefore reactivity is mainly governed by electronic effects.19 The presence of alkyl substituents in DBT increases the electron density on sulfur, resulting in more reactivity of the molecule for oxidation. The observed order of reactivity for oxidation of DBT and alkyl DBTs with aldehyde/molecular oxygen is reverse to the conventional HDS reactivity and is likely to be advantageous for oxidative desulfurization of HDS diesel as the sulfur compounds which are difficult to remove in the hydrodesulfurization process can be easily oxidized and removed by extraction/adsorption. Oxidation reactions using aldehyde/molecular oxygen proceed through autooxidation of aldehyde, forming an acyl radical. Acyl radical on reaction with molecular oxygen forms a peroxy radical. Peroxy radical abstracts a hydrogen atom from the other aldehyde molecule, forming peracid and another acyl radical in the chain propagation step48 (Scheme 1). Peracid converts sulfur compound to sulfoxide by transferring an oxygen atom. As a result, the peracid molecule itself transforms to the corresponding carboxylic acid. Another peracid molecule converts sulfoxide to sulfones on transfer of oxygen atom (Scheme 2).42 Therefore, stoichiometrically two molecules of aldehyde and two molecules of oxygen are required to oxidize a sulfide molecule to its sulfone and forms two molecules of the corresponding acid. Peracid also reacts with aldehyde and forms two corresponding acid molecules without participating in oxidation of sulfide. Because of this, more than a stoichiometric amount of aldehyde is required. Oxidation of Sulfur Compounds Present in HDS Diesel and Their Removal. The above experiments revealed that oxidation of DBT and alkyl-substituted DBTs is possible with the aldehyde/molecular oxygen system. Therefore, it was planned to oxidize sulfur compounds present in HDS diesel in a two-phase system. Keeping in view the miscibility of dichloroethane with diesel, acetonitrile was chosen as solvent. Several experiments were carried out to observe the effect of reaction time, induction period, quantity of aldehyde, and temperature. The results are reported in Table 2. It was observed that on increase of the reaction time sulfur removal in HDS diesel increases due to increase in oxidation of sulfur compounds. Sulfur in HDS diesel could be reduced to 122 ppm within 10 min of reaction time. Upon further increase in reaction time to 30, 90, and 180 min, sulfur could be reduced to 115, 82, and 77 ppm, respectively. It showed that initially oxidation of sulfur compounds is fast and becomes slow as time progresses. Even 50% reduction in the sulfur-to-aldehyde mole (48) Larkin, D. R. J. Org. Chem 1990, 55, 1563–1568.
OxidatiVe Desulfurization of Diesel
Energy & Fuels, Vol. 21, No. 6, 2007 3423 Scheme 2. Oxidation of DBT
ratio did not affect sulfur removal much (Table 2, entry 5). Therefore, it was inferred that the sulfur-to-aldehyde mole ratio 1:32 could be preferred, although the above studies were carried out on the sulfur-to-aldehyde mole ratio 1:64 for achieving higher sulfur reduction. A blank experiment in the absence of aldehyde showed that only extraction by acetonitrile could reduce sulfur from 448 to 313 ppm (35.4% removal). Therefore, only extraction is also effective to some extent to remove sulfur compounds from HDS diesel but not to the extent of oxidation followed by extraction. In comparison with the blank experiment, sulfur reduction is significant in the presence of aldehyde (Table 2, entry 1), where oxidation of sulfur compounds occurs. It showed that after the oxidation the extractability of sulfur compounds becomes easier. It is known that the presence of sulfides inhibit chain reactions by scavenging free radicals, and because of this property, sulfur compounds are used as antioxidants in free-radical-initiated autooxidations.49 The inhibition effect of sulfur compounds in chain reaction was also observed in experiment carried out without induction period of 30 min (Table 2, entry 7). In this, the extent of sulfur removal was more or less similar to the experiment without aldehyde. To study the effect of temperature, oxidative desulfurization of HDS diesel was carried out at 25 °C. The observed sulfur removal was found to be less than that at 40 °C and yielded the diesel with sulfur content 164 ppm (Table 2, entry 8). It showed that with increase in temperature oxidation of sulfides increases; therefore, at 40 °C, sulfur removal was more. However, further increase in temperature is not preferred due to aldehyde losses. Figure 1a–c shows the GC PFPD chromatograms of HDS diesel before oxidation (a) and after oxidation, diesel layer (b), and solvent layer (c). After oxidation, most of the sulfur compounds present in HDS diesel (a) decreased drastically and appeared at higher retention times in diesel layer (b) and solvent layer (c). It reveals that most of the sulfur compounds present in HDS diesel were oxidized to sulfones/sulfoxides. Figure 1 also shows that 4,6-DMDBT was more prone to oxidation, which also supports our findings. To study the effect of solvent, oxidative desulfurization of HDS diesel was carried using different solvents, namely acetonitrile, dimethylformamide, 2-ethoxyethanol, and methanol, and the results are presented in Table 3. Among the solvents studied, acetonitrile was found to be the most suitable solvent followed by dimethylformamide, 2-ethoxyethanol, and methanol. However, acetonitrile and dimethylformamide showed very close effectiveness. Even though solubility of sulfones is more in dimethylformamide,32 acetonitrile is more effective in overall (49) Scott, G. Amospheric Oxidation and Antioxidants; Elsevier: Amsterdam, 1993; Vols. 1–3. (50) Sato, K.; Hyodo, M.; Aoki, M.; Zheng, X.-Q.; Noyori, R. Tetrahedron 2001, 57, 2469–2476. (51) Boberg, F.; Bruns, W.; Musshoff, D. Phosphorus, Sulfur Silicon Relat. Elem. 1992, 72 (1–4), 13–32. (52) Gerdil, R.; Luken, E. A. C. J. Am. Chem. Soc. 1965, 87, 213–217. (53) Mondal, S.; Hangun-Balkir, Y.; Alexandrova, L.; Link, D.; Howard, B.; Zandhuis, P.; Cugini, A.; Horwitz, C. P.; Collins, T. J. Catal. Today 2006, 116, 554–561.
oxidation and extraction process. It is probably due to the combined effect of solvent in the oxidation and extraction process. Less efficiency of 2-ethoxyethanol is probably due to low polarity and of methanol is due to its protic nature. To compare the effectiveness of various aldehydes, studies were carried out on oxidative desulfurization of HDS diesel under similar conditions by using different aldehydes, namely isobutyraldehyde, n-butyraldehyde, benzaldehyde, and n-octaldehyde, and these results are presented in Table 4. Among the aldehydes studied, isobutyraldehyde was the most suitable aldehyde followed by n-butyraldehyde, benzaldehyde, and n-octaldehyde. It is due to more stability of acyl radical formed from isobutyraldehyde with tertiary R-carbon. The HDS diesel with sulfur content 77 ppm (Table 2, entry 4)) obtained after oxidation followed by solvent extraction on passing through a silica gel (80–120 mesh) column yielded diesel with a total sulfur content of 31 ppm. The characteristics of diesel obtained by oxidation followed by extraction (Table 2, entry 4) and diesel sample obtained after passing this oxidized and extracted diesel through silica gel column are compared with HDS diesel (Table 5). It was found that in oxidized and extracted diesel, apart from significant reduction in total sulfur, there was reduction in aromatics which can also improve cetane number in diesel. After passing oxidized
Figure 1. GC PFPD chromatograms: (a) diesel layer, (b) diesel after oxidation, and (c) solvent layer.
3424 Energy & Fuels, Vol. 21, No. 6, 2007
Venkateshwar Rao et al. Table 5. Characteristics of Diesel Samples
entry
characteristics
HDS diesel
HDS diesel after oxidation and extraction
1 2 3 4 5 6 7 8
total sulfur, ppm refractive index at 20 °C density at 20 °C, g/L nonaromatics, wt % total aromatics, wt % monoaromatics, wt % polyaromatics, wt % acid value, mg KOH/g
448 1.4658 0.8334 75.5 24.5 20.5 4.0 0.059
77 1.4601 0.8277 81.8 18.2 16.0 2.2 0.101
and extracted diesel through a silica gel column, sulfur content and aromatics were further reduced. Conclusions It can be concluded that the isobutyraldehyde/molecular oxygen system in the absence of catalyst is very efficient for oxidation of DBT and alkyl DBTs to their sulfones with the reactivity order 4,6-DMDBT > 4-MDBT > DBT, which is reverse to the reactivity order observed in conventional HDS. The system was also found to be very effective for oxidation of sulfur compounds present in HDS diesel to sulfones/ sulfoxides. HDS diesel with initial total sulfur content 448 ppm
HDS diesel after oxidation, extraction, and adsorption 31 1.4569 0.8211 84.5 15.5 14.2 1.3 0.03
after oxidation followed by solvent extraction with acetonitrile yielded diesel with total sulfur content of 77 ppm and when it was further passed through a column of silica gel yielded diesel sample with sulfur content 31 ppm. These results clearly indicate that the aldehyde/molecular oxygen system is highly efficient in oxidizing hindered dibenzothiophenes and their removal from diesel which are difficult to remove in the hydrodesulfurization process. Acknowledgment. We thank the Center for High Technology, New Delhi, India, for financial support. EF700245G
