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Energy & Fuels 2003, 17, 1452-1455
Oxidative Desulfurization: Oxidation Reactivity of Sulfur Compounds in Different Organic Matrixes Paolo De Filippis and Marco Scarsella* Universita` degli Studi di Roma, “La Sapienza”, Dipartimento di Ingegneria Chimica, dei Materiali, delle Materie Prime e Metallurgia, Via Eudossiana 18, 00184 Roma, Italy Received October 27, 2002. Revised Manuscript Received July 22, 2003
Sulfur oxidation appears a very promising route for obtaining ultralow-sulfur fuels requested worldwide by the new regulation mandates. In this work, the oxidizing system constituted by hydrogen peroxide and formic acid was used to study the influence of the solvent on the oxidation rate of sulfur compounds in the organic phase. The experimentation was performed on organic sulfur compounds selected as representative of those contained in crude distillates. The organic solvent seems to play an important role in the reaction kinetics, indicating a strong influence of its aromaticity on the oxidation rate. A different kinetic was found for heterocyclic sulfur compounds such as benzo- and dibenzothiophene when compared with thiols and sulfides, indicating for the last ones a more complex reaction path.
Introduction Due to the dramatic environmental impact of sulfur oxides contained in engine exhaust emissions, sulfur content specifications in both gasoline and diesel pools are becoming more and more stringent worldwide.1,2 The necessity of producing ultralow-sulfur fuels to meet the new regulation mandates will require new desulfurization strategies: the revamping of the traditional catalytic hydrodesulfurization units or the development of new processes capable of higher desulfurization efficiency. Among these new processes, the one generally known as “oxidative desulfurization” appears particularly promising and is currently receiving a growing attention. This process is based on the wellknown propensity of organic sulfur compounds to be oxidized; it consists of an oxidation followed by the extraction of the oxidized products. One of the main advantages of the oxidative desulfurization process is that some sulfur compounds among the most resistant to hydrodesulfurization, owing to their steric hindrance3-5 (e.g., dibenzothiophene and alkylated dibenzothiophenes), show a high reactivity toward oxidation. This behavior seems to be related to the high electron density on the sulfur atom.6 Potential oxidative routes to produce ultralow-sulfur fuels include the use of various oxidizing agents such * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Avidan, A.; Klein, B.; Ragsdale, R. Hydrocarbon Process. 2001, February, 47. (2) Frederick, C. Hydrocarbon Process. 2002, February, 45. (3) Ma, X.; Sakanishi, K.; Mochida, I. Ind. Eng. Chem. Res. 1994, 33, 218. (4) Kabe, T.; Ishihara A.; Qian, W. Hydrodesulfurization and Hydrodenitrogenation; Kodansha Scientific/Wiley-VCH: Tokyo/New York, 1999. (5) Whitehurst, D. D.; Isoda, T.; Mochida, I. Adv. Catal. 1998, 42, 345. (6) Otsuki, S.; Nonaka, T.; Takashima, N.; Qian, W.; Ishihara, A.; Imai, T.; Kabe, T. Energy Fuels 2000, 14, 1232.
as nitric acid,7-9 nitrogen oxides,7,10,11 organic hydroperoxides12 and peroxyacids,13,14 hydrogen peroxide,6,15-18 ozone.19 Actually the industrial applications of such oxidative desulfurization do not appear easily feasible, due to some drawbacks related to product separation and process economics. In this respect, the most promising oxidation systems in terms of selectivity, product quality, safety, environmental impact, and cost-effectiveness are those using hydrogen peroxide as oxidizing agent. In previous studies,6,15-18 hydrogen peroxide was found to be effective in the oxidation of organic sulfur compounds normally contained in fuels and it appears as a promising oxidizing agent for the industrial desulfurization of gasolines and diesel oils. Nevertheless, it has to be considered that the efficiency of this process could be influenced both by the complexity and compositional diversity of the hydrocarbon mixture constituting such distillates other than by the chemical nature of the sulfur compounds contained within. In the present work, we specifically studied the (7) Tam, P. S.; Kittrell, J. R.; Eldridge, J. W. Ind. Eng. Chem. Res. 1990, 29, 321. (8) Tam, P. S.; Kittrell, J. R.; Eldridge, J. W. Ind. Eng. Chem. Res. 1990, 29, 324. (9) Baxendale, J. H.; Evans, M. G.; Park, G. S. Trans. Faraday Soc. 1946, 42, 155. (10) Venturiello, C.; Alneri, E.; Ricci, M. J. Org. Chem. 1983, 48, 3831. (11) Tam, P. S.; Kittrell, J. R. U.S. Patent 4,485,007, 1984. (12) Drushel, H. V.; Miller, J. F. Anal. Chem. 1958, 30, 1271. (13) Abonde, S. E.; Gore, W.; Dolbear, G. E.; Skov, E. R. Prepr. Pap.s Am. Chem. Soc., Div. Pet. Chem. 2000, 45, 364. (14) Aida, T.; Yamamoto, D. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1994, 39, 623. (15) Collins, F. M.; Lucy, A. R.; Sharp, C. J. Mol. Catal. A 1997, 117, 397. (16) Aida, T. Catalysts Catalysis 1995, 37, 243. (17) Yazu, K.; Yamamoto, Y.; Furuya, T.; Miki, K.; Ukegawa, K. Energy Fuels 2001, 15, 1535. (18) Te, M.; Fairbridge, C.; Ring, Z. Appl. Catal. A 2001, 219, 267. (19) Paybarah, A.; Bone, R. L.; Corcoran, W. H. Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 426.
10.1021/ef0202539 CCC: $25.00 © 2003 American Chemical Society Published on Web 09/09/2003
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Table 1. Initial Rate (evaluated as disappearance rate) for Sulfur Compound Oxidation in Various Organic Matrixes initial rate (mol L-1 s-1) sulfur compound
2,2,4-trimethylpentane
decahydronaphthalene
toluene
butanethiol thiophenol diphenyl sulfide dibenzothiophene 1-benzothiophene
-1.20 × 10-3 -7.32 × 10-4 -6.68 × 10-4 -3.33 × 10-4 -1.21 × 10-4
-3.08 × 10-3 -1.97 × 10-3 -1.68 × 10-3 -3.80 × 10-4 -1.27 × 10-4
-7.28 × 10-3 -2.67 × 10-3 -1.87 × 10-3 -1.45 × 10-3 -4.93 × 10-4
oxidation step of the oxidative desulfurizationsparticularly the influence of the organic medium toward the oxidation of organic sulfur compounds. The study was conducted on simplified model systems of single sulfur compounds, selected from among the most representative of those contained in fuels, dissolved in different organic solvents. The oxidizing system used was constituted by hydrogen peroxide and formic acid. Experimental Section Materials. The organic solvents used in this study were isooctane (2,2,4-trimethylpentane), methylcyclohexane, xylenes (isomers), toluene, and decahydronaphthalene (cis and trans mixture). These solvents were chosen as representative of the most important hydrocarbons classes constituting the matrixes of light and medium distillates. The sulfur compounds selected were among those found more frequently in the light and medium distillates from which commercial gasoline and diesel oil pools are produced. Specifically, the chosen sulfur compounds were the following: thiophenol, diphenyl sulfide, 1-benzothiophene, dibenzothiophene, and butanethiol. All the products were commercial reagent grade, supplied by Aldrich and Fluka and used as received. Hydrogen peroxide (40%) and formic acid were supplied by Carlo Erba. Before use, the concentration of H2O2 was determined by iodometric titration. Procedure. The reaction mixtures were prepared adding a weighted amount of the selected sulfur compound in a known volume of the chosen solvent. The amount of sulfur compound added to each solvent was calculated to give for all the solutions an elemental sulfur concentration of 5000 ppm. Depending on the solvent density, the corresponding molar concentration ranged from 0.107 for isooctane to 0.139 for decahydronaphthalene. The experimental procedure was as follows. A 50 mL quantity of the selected reaction mixture and 2.5 mL of formic acid were put in a 100 mL two-necked flask equipped with a magnetic stirrer and reflux condenser. The system was heated in a thermostatic bath under stirring at about 750 rpm; when the mixture reached the selected reaction temperature (50 °C), 5 mL of H2O2 was then added and the reaction was started. Since the mixture was a heterogeneous system of two phases (an organic phase and an aqueous phase), efficient mixing was necessary to ensure a homogeneous composition of the bulk liquids. To determine the initial and residual concentration of the selected sulfur compound in the organic phase, approximately 0.5 mL aliquots of liquid samples were withdrawn from the reactor at fixed time intervals and after phase separation the organic phase was analyzed by gas chromatography. GLC analyses were performed with an HP 5890 GLC gas chromatograph equipped with a flame ionization detector, using a 25 m, i.d. 0.32 mm HP-5 column. The main parameters were the following: carrier gas, helium with a flow of 2 mL/min; injector temperature, 250 °C; detector temperature, 250 °C; split ratio, 1/100; temperature program, 40 °C for the first 5 min, 40100 °C at 10 °C/min, 100 °C for 2 min, 100-200 °C at 15 °C/ min, 200 °C for 5 min, 200-250 °C at 10 °C/min, 250 °C for 15 min.
Results and Discussion The experimentation was conducted by using a series of model molecules representative of the different classes of sulfur compounds present in crude oil distillates. These compounds, either reactive or resistant to hydrodesulfurization, are all susceptible, with different reaction rates, to oxidation. It is known from literature6 that only sulfides with an electron density on the sulfur atom higher than 5.73 can be oxidized in the conditions adopted in this work. As a consequence, thiophene and alkylated thiophenes were not considered in this study. To make a comparison with literature,6 the reaction rates determined as initial rate (calculated below 40% conversion following the reactivity of the single sulfur compounds in the different solvents), considering the oxidation of each sulfur compound as a pseudo-firstorder reaction, are reported in Table 1. The data obtained are in accordance with literature,6 where the discrepancies could probably be due to different stirring conditions. The reaction rate for the different sulfur compounds follows the same order reported in the literature:6 butanethiol > thiophenol > diphenyl sulfide > dibenzothiophene > benzothiophene. This trend is the same in the tested solvents (isooctane, toluene, and decahydronaphthalene), where the oxidation rate increases with the increase of the electron density on the sulfur atom. In Figure 1 is reported the time-sequence of the GLC chromatograms for the oxidation of thiophenol in isooctane. The advancement of the oxidation reaction was evaluated considering the conversion of the selected sulfur compound, calculated as (C0 - C)/C0, where C0 is its initial concentration, and C its concentration after t minutes of reaction. As an example, the plot relative to the conversion of sulfur compounds in isooctane is reported in Figure 2. From a kinetic point of view, the behavior of benzoand dibenzothiophene is completely different from the other compounds. This is better pointed out in Figure 3, where the ln(C/C0) vs time is reported. In fact, hypothesizing that the reactions follow a pseudo-firstorder reaction kinetics, the rate constant k can be calculated from the following equation:
ln(C/C0) ) -kt As clearly evidenced from the plot of Figure 3, in the adopted reaction conditions the kinetic of benzo- and dibenzothiophene can be well approximated with a pseudo-first-order reaction rate, while this is not true for the oxidation of thiophenol, diphenyl sulfide, and butanethiol. This behavior is confirmed in all the considered organic solvents. In Table 2 are summarized the pseudo-first order rate constants of benzo- and dibenzothiophene in the used organic solvents. The
1454 Energy & Fuels, Vol. 17, No. 6, 2003
De Filippis and Scarsella
Figure 3. Ln(C/C0) vs time for the selected sulfur compounds (solvent: isooctane).
Figure 2. Typical plot of sulfur compound conversion in isooctane.
disulfides can easily lead to sulfonic acids.22 Moreover, in the case of aliphatic and aromatic thiols and sulfides, the sulfur oxidation path could be more complex due to the concurrent oxygen transfer from sulfoxide obtained as intermediate oxidation product to the original sulfur compound.23,24 The organic solvent does not influence the relative reactivity of the sulfur compounds toward oxidation (in the solvents used the reaction rates follow the order: butanethiol > thiophenol > diphenyl sulfide > dibenzothiophene > benzothiophene, as shown in Table 1), meaning that the organic solvent does not influence the oxidation reaction mechanism but only the reaction rate. The degree of conversion, determined experimentally as residual concentration after 1 and 2 h, is reported in Table 3. As shown, the organic matrix strongly influences the yield of conversion, while after 2 h in almost all cases the conversion is higher than 99%. The desulfurization degree obtained for each sulfur compound is reported in Table 4. Owing to the absence of the necessary extraction step after oxidation, the desulfurization depends only on the partition ratio between organic and aqueous phases of the oxidized sulfur compounds and on their solubility in the reaction mixture. The data show that in the case of butanethiol and thiophenol the desulfurization is almost complete, in accordance with the high solubility in the aqueous phase of the produced sulfonic acids. On the contrary, for benzothiophene, dibenzothiophene, and diphenyl sulfide the desulfurization degree will mainly depend on the very low solubility of the corresponding sulfones, both in organic and in aqueus phase, proved by the presence of such sulfones as a solid residue in the reaction mixture at the end of all the oxidation reactions performed. For such compounds the obtained desulfurization degree will be dependent on the initial sulfur
evidence that in the case of thiophenol, diphenyl sulfide, and butanethiol the oxidation kinetic cannot be approximated as a pseudo-first-order points out a different reaction path when compared with heterocyclic sulfur compounds. In fact, the oxidation of thiols with peroxidic compounds is described by a typical reaction path where the initially formed product could be the corresponding disulfides.20,21 It is also demonstrated that oxidation of pure sulfur compounds such as thiols, sulfides, and
(20) Kesavan, V.; Bonnet-Deplon, D.; Begue, J. P. Synthesis 2000, 223. (21) Organic Chemistry of Sulfur, ‘Disulfides and Polysulfides’; Oae, S., Ed.; Plenum Press: New York, 1977. (22) Borah, D.; Baruah, M. K.; Haque, I. Fuel 2001, 80, 1475. (23) The Chemistry of the Thiol Group, Part 2; Patai, S., Ed.; Wiley: New York, 1974. (24) The Synthesis of Sulphones, Sulphoxides and Cyclic Sulphides; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1994. (25) The Chemistry of Ethers, Crown Ethers, Hydroxyl Groups and Their Sulphur Analogues, Part 1; Patai, S., Ed.; Wiley: New York, 1980.
Figure 1. Time-sequence of the GLC chromatograms for the oxidation of dibenzothiophene in isooctane. Retention times: dibenzothiophene, 25.82 min; dibenzothiophene sulfone, 32.91 min.
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Energy & Fuels, Vol. 17, No. 6, 2003 1455
Table 2. Influence of the Organic Matrix on Dibenzothiophene and Benzothiophene Rate Constants sulfur compound
organic matrix
rate constant (s-1)
correlation factor (R2)
dibenzothiophene
2,2,4-trimethylpentane methylcyclohexane decahydronaphthalene xylenes toluene
3.34 × 10-4 4.05 × 10-4 3.798 × 10-4 1.377 × 10-3 1.453 × 10-3
0.996 0.926 0.928 0.887 0.890
1-benzothiophene
2,2,4-trimethylpentane methylcyclohexane decahydronaphthalene xylenes toluene
1.209 × 10-4 1.212 × 10-4. 1.273 × 10-4 4.502 × 10-4 4.925 × 10-4
0.989 0.938 0.990 0.891 0.996
Table 3. Conversion of the Sulfur Compounds (determined after 1 and 2 h reaction) as a Function of the Organic Matrix solvent
conversion after 1 h (%)
conversion after 2 h (%)
2,2,4-trimethylpentane decahydronaphthalene toluene 2,2,4-trimethylpentane decahydronaphthalene toluene 2,2,4-trimethylpentane decahydronaphthalene toluene 2,2,4-trimethylpentane methylcyclohexane decahydronaphthalene xylenes toluene 2,2,4-trimethylpentane methylcyclohexane decahydronaphthalene xylenes toluene
99.97 99.99 n.d 99.63 99.76 99.77 99.97 99.71 99.98 69.88 76.73 74.54 99.30 99.94 35.29 35.35 36.77 80.21 83.02
99.98 n.d. n.d. 99.88 99.88 99.86 99.97 99.92 n.d. 90.93 94.59 93.52 99.99 99.99 58.12 58.21 60.01 96.08 97.12
sulfur compound butanethiol thiophenol diphenyl sulfide dibenzothiophene
1-benzothiophene
of oxidized sulfur in the organic phase will be constant and dependent only on the solubility in the organic solvent. The influence of the different organic matrixes on the oxidation rate was analyzed for each sulfur compound. From data obtained in toluene, decahydronaphthalene, and isooctane, the oxidation rate appears to be higher in the aromatic matrix than in the aliphatic ones. In the three aliphatic matrixes used in this work, benzothiophene and dibenzothiophene do not show significant differences, while for thiophenol and diphenyl sulfide the reactivity could be directly related to the C/H ratio of the matrix, as pointed out in Figure 4. Figure 4. Influence of the aromaticity of the organic matrix on the initial oxidation rate, evaluated as disappearance rate of selected sulfur compounds. Table 4. Desulfurization Degree Calculated as Residual Concentration of the Sulfur Compounds in the Organic Phase sulfur compound
desulfurization after 2 h (%)
butanethiol thiophenol diphenyl sulfide dibenzothiophene 1-benzothiophene
>99 >99 >94 >56 >31
concentration in the organic solution, that will determine the reaching of the saturation conditions for the oxidized products. In saturation conditions (i.e., in the presence of a precipitated sulfone), the residual amount
Conclusions The reactivity toward oxidation of various classes of sulfur compounds generally present in petroleum distillates was investigated. The results of this study confirm and extend the literature data and point out the more complex oxidation path of thiols and sulfides, as highlighted by kinetic evidences, when compared with heterocyclic sulfur compounds such as benzo- and dibenzothiophene. The aromaticity of the organic matrix plays an important role in the reaction kinetics. The reaction rates, in fact, increase when the C/H ratio of the solvent increases, indicating that an oxidative desulfurization process based on the system hydrogen peroxide/formic acid could be more active when applied to aromatic petroleum fractions. EF0202539
