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Ind. Eng. Chem. Res. 1997, 36, 530-533
Effect of Photosensitizer and Hydrogen Peroxide on Desulfurization of Light Oil by Photochemical Reaction and Liquid-Liquid Extraction Takayuki Hirai,* Yasuhiro Shiraishi, Ken Ogawa, and Isao Komasawa Department of Chemical Engineering, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560, Japan
A desulfurization process for dibenzothiophene (DBT) by a combination of photochemical reaction and liquid-liquid extraction has been investigated. The DBT dissolved in tetradecane was photodecomposed by the use of a high-pressure mercury lamp and removed into the water phase at conditions of room temperature and atmospheric pressure. The addition of benzophenone (BZP), a triplet photosensitizer, enhanced the removal of DBT from tetradecane. This reaction, however, hardly proceeded in the presence of naphthalene (NP), probably because of triplet energy transfer from photoexcited DBT or BZP to ground-state NP. The addition of hydrogen peroxide enhanced the desulfurization of commercial light oil as well as the removal of DBT from tetradecane, since H2O2 acted as a weak oxidizing agent for photoexcited DBT and interrupted the energy transfer from excited DBT to NP to some extent. In the case using a 30% H2O2 solution, the desulfurization yield of commercial light oil was 75% following 24 h of photoirradiation and the sulfur content in the light oil was reduced from 0.2 wt % to less than 0.05 wt %. Introduction There has been much recent interest in the deep desulfurization of light oil, since sulfur-containing compounds are converted to SOx by combustion and hence one of the main sources of acid rain and air pollution. In the previous paper, a novel desulfurization process for dibenzothiophene (DBT) and its derivatives by photochemical reaction and liquid-liquid extraction was proposed (Hirai et al., 1996), and this was extended for benzothiophenes and alkyl sulfides (Hirai et al., 1997). These compounds were successfully photodecomposed by the use of a high-pressure mercury lamp and removed from tetradecane into an aqueous phase as SO42-. The removal of DBT was, however, depressed markedly in the presence of naphthalene (NP). It may be a serious problem for desulfurization of commercial light oils, since they contain a considerable amount of two-ring aromatics such as NP and its derivatives. In this work, the desulfurization process for DBT by improved photochemical reaction and liquid-liquid extraction was investigated to cope with this problem. Benzophenone (BZP), a triplet photosensitizer, was used for photoexcitation of DBT with rather longer wavelength light. The effect of hydrogen peroxide for the desulfurization was also investigated, expecting to enhance the photoreaction of DBT. Experimental Section DBT, tetradecane (n-C14H30), BZP, 4-phenylbenzophenone (4-PBZP), hydrogen peroxide (30% aqueous solution), and NP were supplied by Wako Pure Chemical Industries, Ltd., and were used without further purification. The organic DBT/tetradecane solution (100 mL) was mixed vigorously with distilled water or a hydrogen peroxide solution (300 mL) using a magnetic stirrer. Commercial light oil was also used as the organic phase. The solutions were UV irradiated by immersing a highpressure mercury lamp (300 W, Eikohsha Co., Ltd., * Author to whom correspondence is addressed. E-mail:
[email protected]. Fax: +81-6-850-6273. S0888-5885(96)00576-3 CCC: $14.00
Osaka) into the solution with air bubbling (500 mL/min) and at atmospheric pressure. A Pyrex glass filter was used when only the wavelengths longer than 280 nm were required. The temperature of the solution was about 323 K under irradiation conditions. The DBT concentration in the organic phase was analyzed by gas chromatography (Shimadzu GC-14B equipped with FID). The total sulfur concentration in the light oil was measured with an inductively coupled argon plasma atomic emission spectrophotometer (Nippon Jarrell-Ash ICAP-575 Mark II). Results and Discussion Removal of DBT from Tetradecane by Photosensitizing Reaction Using BZP. Benzophenone is known as an efficient triplet photosensitizer, of which the quantum yield of triplet state formation is 1.00 and the triplet energy is 287 kJ/mol (Murov et al., 1993). The triplet energy of DBT is estimated as 285 kJ/mol. Thus, the energy transfer to DBT from the excited BZP is expected to occur efficiently. To excite BZP to the triplet state via the excited singlet state, the excitation energy of 315 kJ/mol is required, which corresponds to the light wavelength of 380 nm. Figure 1 shows the time-course variation of concentrations of DBT and BZP in tetradecane under photoirradiation with and without the glass filter. The initial DBT concentration (2 g/L) corresponds to a sulfur content of 0.05 wt %. The removal of DBT was much enhanced by the addition of BZP in both cases. In the case without the filter, however, BZP itself was photodecomposed remarkably, as shown in Figure 1a. With the filter, the photodecomposition of BZP was suppressed, while the removal rate of DBT was also decreased, as shown in Figure 1b. In this case, the effect of the addition of BZP was more noticeable; the amount of removed DBT in the presence of BZP was about 7.6 times as much as that in the absence of BZP following 10 h of photoirradiation. The removal of DBT was, however, suppressed in the presence of 10 g/L NP, as shown in Figure 2. In addition, the desulfurization of commercial light oil by © 1997 American Chemical Society
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Figure 3. Effect of the addition of BZP and 4-PBZP on timecourse variation of DBT concentration in tetradecane in the case without filter.
Figure 1. Time-course variation of concentrations of DBT and BZP in tetradecane in the cases without filter (a) and with filter (b).
Figure 2. Time-course variation of concentrations of NP (a) and DBT and BZP (b) in tetradecane in the case without filter.
Figure 4. Effect of the addition of hydrogen peroxide on timecourse variation of DBT concentration in tetradecane in the case without filter. Initial DBT concentration: 2 g/L (a) and 10 g/L (b).
the present method was found to be hardly effected by the addition of BZP. Naphthalene seems to suppress the photochemical reaction of DBT even in the presence of BZP. This result can be attributed to triplet energy of NP, 253 kJ/mol, which is lower than that of DBT and BZP. Thus, the energy transfer from photoexcited DBT or BZP to ground-state NP probably occurs during the photoirradiation. This was confirmed by the experiment using 4-PBZP, whose triplet energy was 254 kJ/mol. As shown in Figure 3, the photoreaction of DBT was also suppressed by the addition of 4-PBZP. Therefore, the addition of triplet photosensitizer is not effective for the desulfurization of commercial light oil containing tworing aromatics such as NP. Removal of DBT from Tetradecane and Desulfurization of Commercial Light Oil in the Presence of Hydrogen Peroxide. It was reported that DBT was oxidized by 30% hydrogen peroxide only in the presence of glacial acetic acid to form dibenzothiophene-5-oxide and -5-dioxide (Gilman and Esmay, 1952; Heimlich and Wallace, 1966). In the tetradecane/ water system, oxidation, decomposition, or removal of DBT by addition of H2O2 without photoirradiation was not observed. However, hydrogen peroxide may act as an oxidizing agent for a photoexcited DBT molecule. In
addition, H2O2 is easily photodecomposed into hydroxy radicals under UV irradiation, which acts as a strong oxidizing agent for organic compounds. If these reactions occur also in the present two-phase system, the photodecomposition of DBT is expected to be enhanced. The removal of DBT from tetradecane was enhanced by the addition of hydrogen peroxide into the aqueous phase, as shown in Figure 4. The addition of hydrogen peroxide also decreases the aqueous phase pH. The pH value of a 30% H2O2 solution is about 2.5. However, the pH effect for the photochemical removal of DBT can be negligible, since the reaction was found not to be enhanced by decreasing the pH value by the addition of a HCl solution. As in the case without H2O2 (Hirai et al., 1996), a third (emulsion) phase first appeared and then disappeared during the course of photoirradiation, indicating the formation of intermediates having surface activity. The sulfur compounds were thought to be finally converted to SO42- to be removed into the water phase as well. This was confirmed by addition of BaCl2 into the resulting water phase, which made precipitates with the SO42- ion. Addition of H2O2 may have two effects on the photoreaction of DBT. One is the effect of hydroxy radical produced by photodecomposition of H2O2. The other is
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Figure 6. Relationship between the concentration of hydrogen peroxide and the removal rate of DBT from tetradecane in the cases with and without filter.
Figure 5. Effect of the addition of hydrogen peroxide on timecourse variation of DBT concentration in tetradecane in the case using a filter (λ > 280 nm). Initial DBT concentration: 2 g/L (a) and 10 g/L (b).
the effect as a weak oxidizing agent, which oxidizes the photoexcited DBT molecule before deactivation. As shown in Figure 5, the addition of H2O2 also enhanced the removal of DBT under UV irradiation with the glass filter. This indicates that the latter effect of hydrogen peroxide is dominant in the present system, since H2O2 is hardly photodecomposed into hydroxy radicals at λ > 280 nm. On the other hand, no photoreaction of DBT was observed under photoirradiation at λ > 325 nm regardless of the presence of hydrogen peroxide, since the wavelength corresponding to the excited singlet energy of DBT (367 kJ/mol) was 325.9 nm and thus DBT was hardly photoexcited. These findings support the proposed mechanism; H2O2 itself does not directly oxidize DBT but helps the photoexcited DBT to be oxidized. A similar photoreaction mechanism was proposed by Omura and Matsuura (1968, 1970) for the case of hydroxylation of phenols by the light of 280 nm in the presence of H2O2. In addition, they also proposed that the photoexcited phenol may transfer its excited energy to a H2O2 molecule to give hydroxy radicals. This kind of reaction may occur in the present two-phase system. The removal rate of DBT from tetradecane was increased with H2O2 concentration as shown in Figure 6, since the greater concentration enhanced the contact of photoexcited DBT with H2O2. However, the degree of increase in the case using the filter is slightly smaller than the case without filter. This difference between the two cases can be attributed to the reaction of DBT with hydroxy radical generated by direct photodecomposition of H2O2. On the other hand, the removal rate was found to be much faster when the initial DBT concentration was increased to 10 g/L. In this case, the amount of DBT molecule photoexcited by UV irradiation is much greater than that in the case [DBT]initial ) 2 g/L, except for the case using the filter. The removal of the photoexcited DBT is enhanced by contacting with a H2O2 molecule in the water phase; otherwise, a certain amount of the excited DBT is deactivated. The removal rate at [DBT]initial ) 10 g/L increased with H2O2
Figure 7. Time-course variation of concentrations of NP (a) and DBT (b) in tetradecane in the cases without filter ([H2O2] ) 0% or 30%) and with filter (λ > 280 nm, [H2O2] ) 30%).
concentration linearly at H2O2 concentrations up to 10%. At [H2O2] ) 30%, however, the removal rate was not so enhanced compared with that at [H2O2] ) 10%. Under this condition, the photoexcitation of DBT can be the rate-determining step, while the concentration of H2O2 is great enough. At [H2O2] ) 0%, in contrast, the possible rate-determining step is the contact of excited DBT with O2 dissolved in the organic and water phase. The finding that the removal rate is independent of the DBT concentration at [H2O2] ) 0% probably shows that the O2 concentration governs the removal rate of DBT in this case. The addition of 30% H2O2 enhanced the removal of DBT from tetradecane even in the presence of NP, as shown in Figure 7. Although the energy transfer from photoexcited DBT to ground-state NP was found to occur since the removal rate of DBT was decreased as compared to that in the absence of NP, the energy transfer was interrupted by H2O2 to some extent. Photodecomposition of NP was also enhanced by the addition of H2O2, indicating that H2O2 had a similar effect for NP as for DBT. In the case with the filter, the removal of DBT was only slightly enhanced by 30% H2O2, while the removal of NP was much suppressed. The direct photodecomposition of NP, as well as photoexcitation of DBT, by UV irradiation was suppressed by using the filter. The desulfurization of commercial light oil was also enhanced by the addition of H2O2, as shown in Figure 8. In the case using a 30% H2O2 solution, about 75% of sulfur was removed and the sulfur concentration was lowered to less than 0.05 wt % following 24 h of
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the desulfurization yield of the light oil was 75% following 24 h of photoirradiation and the sulfur content in the light oil was reduced from 0.2 wt % to less than 0.05 wt %. Acknowledgment I.K. acknowledges The Sumitomo Foundation for financial support. Literature Cited Figure 8. Effect of the addition of hydrogen peroxide on desulfurization of commercial light oil in the case without filter.
irradiation. The regulation of the sulfur content in light oil will be tightened in Japan, from 0.2 to 0.05 wt % in 1997. The proposed desulfurization method is, therefore, effective for deep desulfurization of light oil to meet the newer regulation. Conclusion The removal of dibenzothiophene (DBT) from tetradecane and desulfurization of commercial light oil by photochemical reaction and liquid-liquid extraction was investigated by the use of a high-pressure mercury lamp, with the following results. 1. The addition of benzophenone (BZP), a triplet photosensitizer, enhanced the removal of DBT from tetradecane. This reaction, however, hardly proceeded in the presence of naphthalene (NP), probably because of energy transfer from photoexcited DBT or BZP to ground-state NP. 2. The addition of hydrogen peroxide enhanced the desulfurization of commercial light oil as well as the removal of DBT from tetradecane, since H2O2 acted as a weak oxidizing agent for photoexcited DBT and interrupted the energy transfer from excited DBT to NP to some extent. In the case using a 30% H2O2 solution,
Gilman, H.; Esmay, D. L. The Oxidation of Dibenzothiophene and Phenoxathiin with Hydrogen Peroxide. J. Am. Chem. Soc. 1952, 74, 2021. Heimlich, B. N.; Wallace, T. J. Kinetics and Mechanism of the Oxidation of Dibenzothiophene in Hydrocarbon Solution. Oxidation by Aqueous Hydrogen Peroxide-Acetic Acid Mixtures. Tetrahedron 1966, 22, 3571. Hirai, T.; Ogawa, K.; Komasawa, I. Desulfurization Process for Dibenzothiophenes from Light Oil by Photochemical Reaction and Liquid-Liquid Extraction. Ind. Eng. Chem. Res. 1996, 35, 586. Hirai, T.; Shiraishi, Y.; Komasawa, I. Desulfurization Process for Light Oil by Photochemical Reaction and Liquid-Liquid Extraction: Removal of Benzothiophenes and Alkyl Sulfides. J. Chem. Eng. Jpn. 1997, 30, in press. Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry, 2nd ed.; Marcel Dekker: New York, 1993. Omura, K.; Matsuura, T. Photoinduced Reactions-IX. The Hydroxylation of Phenols by the Photodecomposition of Hydrogen Peroxide in Aqueous Media. Tetrahedron 1968, 24, 3475. Omura, K.; Matsuura, T. Photoinduced Reactions-XXXV. The Hydroxylation of Phenols by the Photodecomposition of Hydrogen Peroxide in Acetonitrile. Tetrahedron 1970, 26, 255.
Received for review September 20, 1996 Revised manuscript received December 10, 1996 Accepted December 16, 1996X IE960576Q
X Abstract published in Advance ACS Abstracts, January 15, 1997.
