Energy & Fuels 2007, 21, 7-10
7
Catalytic Oxidation of Dibenzothiophene Using Cyclohexanone Peroxide Xinrui Zhou,* Caixia Zhao, Jinzong Yang, and Shufen Zhang State Key Laboratory of Fine Chemicals, Dalian UniVersity of Technology, Dalian 116012, China ReceiVed August 31, 2006. ReVised Manuscript ReceiVed October 29, 2006
The catalytic oxidative desulfurization (ODS) of dibenzothiophene (DBT) in decahydronaphthalene (decalin) was performed using the oil-soluble oxidant cyclohexanone peroxide (CYHPO) with a molybdenum oxide (MoO3) catalyst supported on macroporous polyacrylic cationic exchange resin D113 of weak acid series. The influence of the reaction temperature, reaction time, the molar ratio of CYHPO/DBT, and catalyst reuse was investigated in detail. In the presence of the catalyst MoO3/D113, the conversion of DBT to DBT sulfone was up to 100% at 100 °C in 40 min. The ODS was performed using other oil-soluble alkyl peroxides, i.e., tertbutyl hydroperoxide (TBHP) and tert-amyl hydroperoxide (TAHP), in comparison with the oxidative activity of CYHPO. The results showed that the activity of alkyl peroxides decreases in the order CYHPO > TAHP > TBHP, reversing the order of the peroxy oxygen electronic density. The oxidation mechanism is then discussed.
1. Introduction Deep desulfurization of fuel has become an important research subject because of upcoming legislative regulations to reduce sulfur content. The hydrodesulfurization (HDS) process has been employed by refineries to remove organic sulfur from fuels for several decades, and the lowest sulfur content achieved by such a process in the fuels is around 500 ppm. However, to produce ultraclean fuels with a sulfur content lower than 15 ppm by HDS, both capital investment and operational costs would be rather high because of more severe operating conditions. The oxidative desulfurization (ODS) is considered to be one of the promising new methods for super deep desulfurization of fuel. In comparison to conventional HDS, ODS can be carried out under very mild conditions such as near room temperature and atmospheric pressure.1,2 In oxidation, the divalent sulfur of dibenzothiophenes (DBTs), which are the most unreactive sulfur compounds during HDS,3 can be oxidized by the electrophilic addition reaction of oxygen atoms to the hexavalent of sulfones. The chemical and physical properties of sulfones are significantly different from those of hydrocarbons in fuel. Therefore, they can easily be removed by such separation operations as distillation, solvent extraction, adsorption, and decomposition. Various studies on the ODS process have been reported.4,5 Aida and Yamamoto6 reported that peroxyacids, such as performic acid, pertrifluoroacetic acid, and a mixture of formic acid or trifluoroacetic acid and hydrogen perioxide, were some of the most positive oxidants for selective oxidation of * To whom correspondence should be addressed. Telephone: +8613352289600. Fax: +86-411-84667653. E-mail:
[email protected]. (1) Otsuki, S.; Nonaka, T.; Takashima, N.; Qian, W., Ishihara, A.; Imai, T., Kabe, T. Energy Fuels 2000, 14, 1232-1239. (2) Yazu, K.; Yamamoto, Y.; Furuya, T.; Miki, K.; Ukegawa, K. Energy Fuels 2001, 15, 1535-1536. (3) Li, J. Y.; Zhou, X.; Zhao, D. F. Chem. Online 2005, 68, w055. (4) Alkis, S. Rappas USP, 6402940, 06/11/2002. (5) Patrick, S. T.; James, R. K.; Eldridge, J. W. Ind. Eng. Chem. Res. 1990, 29, 321-324. (6) Aida, T.; Yamamoto, D. Am. Chem. Soc., DiV. Fuel Chem. 1994, 39, 623.
sulfur compounds in fuel. However, in these reactions, the reactor is limited to a batch type because of the need for sufficient mixing of two phases. If the selective catalytic oxidation of sulfur compounds in the liquid fuel using a flow reactor could be possible, the amount of fuel treated would increase remarkably. To develop ODS by means of a flow reaction process, it is necessary to find an oil-soluble oxidant. Wang et al.7 reported that DBT could be effectively oxidized to DBT sulfone by the oil-soluble oxidant tert-butyl hydroperoxide (TBHP) but no oxidation was observed without a catalyst. Otsuki et al. reported that the oxidation of DBT was conducted using tert-butyl hypochlorite in the presence of several catalysts but no oxidation of DBT was observed in the absence of a catalyst as well.8 In our recent study, the oxidation of DBT in decahydronaphthalene (decalin) was conducted using the oil-soluble oxidant cyclohexanone peroxide (CYHPO).9 The results showed that, under the molar ratio of CYHPO/DBT of 2.5:1, at the reaction temperature of 100 °C, and in the reaction time of 3 h, the desulfurization rate was up to 87% without any catalyst. CYHPO is more attractive as an oil-soluble oxidant because it is comparatively cheap compared to other oil-soluble oxidants and large-scale industrial supply, although the conversion of DBT oxidation is still not enough to meet the need of deep desulfurization without a catalyst and the reaction time is too long to develop a flow reaction system. The aim of the present work is to obtain basic information about CYHPO oxidation using the catalyst MoO3/D113 to increase the conversion of DBT, shorten the reaction period, as well as investigate the oxidative activity of CYHPO compared with other oil-soluble oxidants and the CYHPO oxidative reaction mechanism. (7) Wang, D.; Qian, W.; Amano, H.; Okata, K.; Ishihara, A.; Kabe, T. Appl. Catal., A 2003, 253, 91-99. (8) Otsuki, S.; Nonaka, T.; Ewihua, Q.; Atsushi, I. Sekiyu Gakkaishi 2001, 44, 18-24. (9) Li, J. Y.; Zhou, X. R.; Zhao, D. F.; Zhao, C. X. J. Fuel Chem. Technol. 2006, 34, 249-251.
10.1021/ef060441p CCC: $37.00 © 2007 American Chemical Society Published on Web 12/29/2006
8 Energy & Fuels, Vol. 21, No. 1, 2007
Zhou et al.
Scheme 1. Simplified Reactions of Cyclohexanone to CYHPO
2. Experimental Section 2.1. Materials. DBT (analytical-grade reagent, AR) was obtained from Acros; n-hexadecane (chromatographic-grade reagent) was obtained from Merck-Schuchardt; decalin (AR), cyclohexanone (AR), hydrogen peroxide (30 wt %, AR), sulfuric acid (AR), and D113 resin were purchased from Beijing Chemicals Co. 2.2. Instruments. The reaction products were identified and quantified by infrared spectroscopy (IR, Nicolet-20DXB), nuclear magnetic resonance (NMR, Varian INOVA 400 MHz), gas chromatography-mass spectrometry (GC-MS, HP6890GC/5793MS), and GC (HP6890). GC was equipped with a flame photometric detector and HP-5 capillary column (30 m × 0.32 mm × 0.25 µm), using highly purified nitrogen as the carrying gas. 2.3. Experiments. 2.3.1. Catalyst Preparation. Catalyst MoO3/ D113 was prepared by an incipient wetness impregnation;10 10 wt % MoO3 content was calculated after ether washing and vacuum drying. 2.3.2. CYHPO Synthesis. CYHPO was synthesized generally using cyclohexanone and hydroperoxide. It was confirmed11 that treatment of cyclohexanone with hydrogen peroxide in neutral solution yields only the 1,1′- dihydroxydicyclohexyl peroxide (2), which is a comparatively stable member of CYHPOs. In acidic solution, the more highly peroxygenated compound 3 is formed (Scheme 1). The component of commercial CYHPO is mainly compound 3, which gives high oxidative activity and therefore was chose as an oxidant in the oxidations of DBT. CYHPO (3) was prepared according to ref 12, and its structure was identified by IR (KBr) spectra. After oxidation, the characteristic infrared carbonyl stretching peak (1713 cm-1) disappeared and was displaced by the characteristic peaks of CYHPO, at 3323 cm-1 for the O-H bond, 1067 and 1053 cm-1 for C-O bonds, and 926 and 914 cm-1 for O-O bonds. Then, the 65 wt % CYHPO concentration was determined by the iodometric titration method. 2.3.3. Procedure. The oxidation was conducted in a 100 mL twoneck glass flask with a reflux column, temperature meter, and magnetic stirrer. Certain amounts of catalyst and CYHPO were added in 0.5 wt % DBT decalin solution. The mixture was stirred continuously and kept the temperature in the range of 40-120 °C; for a certain time, some white precipitate appeared during the reaction. The following step was the identification of the precipitate by IR, NMR, GC-MS, and the sulfur content analysis by GC-flame ionization detector (FID). The peak corresponding to DBT was identified by a comparison of their retention times with reference compounds, while the peak of the corresponding sulfone was identified using GC-MS (HP6890/5973MS). After DBT was oxidized with CYHPO, the peak of DBT shifted to a higher retention time, i.e., for a heavier molecular mass. The peak of DBT almost completely disappeared after oxidation with the catalyst. While the peak of the corresponding DBT sulfone appeared after oxidation, confirming that the oxidations of DBT lead to the formation of DBT sulfone.
3. Results and Discussion 3.1. Influence of Reaction Conditions. 3.1.1. Effect of the Reaction Temperature. Figure 1 shows the effect of the reaction (10) Huan, Q. Y.; Xu, R. Q. J. Peking UniV. 1989, 25, 427-430. (11) Antonovski, V. L.; Nesterov, A. F.; Lyashenko, O. K.; Khim, Z. P. J. Appl. Chem. 1967, 40, 2443. (12) Story, P. R.; Lee, B.; Bishop, C. E.; Denson, D. D.; Busch, P. J. Org. Chem. 1970, 35, 3059-3063.
Figure 1. Effect of temperature on the conversion of DBT at a CYHPO/DBT molar ratio of 2.5, in 40 min. Initial concentrations of DBT in decalin were 0.5 wt %, and MO3/D113 concentrations were 1 wt %.
Figure 2. Influence of the molar ratio of CYHPO/DBT on the conversion of DBT at 100 °C. Initial concentrations of DBT in decalin were 0.5 wt %, and MO3/D113 concentrations were 1 wt %.
temperature on the conversion rate of DBT. When the initial concentration of DBT was 0.5 wt %, the molar ratio of CYHPO/ DBT was 2.5; an increase of the reaction temperature led to a remarkable increase in the conversion of DBT. At 100 °C, the conversion of DBT was up to 100% in 40 min, compared with 87.2% at 80 °C, 57.8% at 60 °C, and 16.5% at 40 °C. The conversions were kept at the maximum value of 100% during 100-110 °C and then dropped sharply over 110 °C, probably because of the occurrence of the thermolysis of CYHPO at higher temperatures. 3.1.2. Effect of the Molar Ratio. We carried out the oxidation reaction of DBT in decalin (0.5 wt %), under various molar ratios of CYHPO/DBT, at 100 °C, with 1 wt % MO3/D113 catalysts, to investigate the effect of the amount of oxidation agent on the oxidation activity. The O/S molar ratio was defined as the molar ratio of the molar amount of the peroxy bond of peroxide to the molar amount of sulfur in DBT. A previous study performed on model molecules suggested that the stoichiometric O/S molar ratio is 2 for achieving complete conversion of DBT to DBT sulfone. Because there are two peroxy bonds in one molecule of CYHPO, 1 mol of CYHPO is stoichiometrically required for the oxidation of 1 mol of DBT to DBT sulfone. As shown in Figure 2, the oxidation conversion increased with the molar ratio of CYHPO/DBT, at the 2.5 point, and the conversion was up to 100%. The oxidation of 2.5 of the CYHPO/DBT molar ratio was used for maximum conversion at about 2.5 times more than the theoretical stoichiometric molar ratio; this is may be due to the self-decomposition of CYHPO during the oxidative reaction.
Catalytic Oxidation of DBT Using CYHPO
Energy & Fuels, Vol. 21, No. 1, 2007 9
Table 1. Results of Reuse of the MoO3/D113 Catalyst number
1
2
3
4
5
6
7
8
9
10
conversiona 87.0 87.1 87.2 87.3 87.1 87.3 87.5 87.6 86.9 87.2 conversionb 100 100 100 100 100 100 100 100 100 100 a The conversion of DBT (in percent) at 80 °C. b The conversion of DBT (in percent) at 100 °C.
Table 2. Maximum Conversion (wt %) of DBT with Oil-Soluble Oxidantsa oxidants
noncatalytic
catalytic
TBHP TAHP CYHPO
0 0 87
84 91 100
a At 100 °C, the CYHPO/DBT molar ratio is 2.5, with the 1 wt % catalyst MoO3/D113.
3.1.3. Catalyst Recycle and Reuse. Pure 200 mesh MoO3 powder was applied once directly to the oxidative reaction, but it was difficult to disperse in the organic phase and hardly reused. The catalyst efficiency was prompted when MoO3 was supported on the D113 resin and when the catalyst recovery was simplified. To determine the catalyst lifetime, DBT oxidations at 80 and 100 °C, with a CYHPO/DBT molar ratio of 2.5, in 40 min in decalin were performed in 10 consecutive batch reactions, where the catalyst was recycled and reused; the results show that the catalyst activity was not decreased (Table 1). 3.2. Comparison of the Oxidative Reactivity of Peroxides. To investigate the difference of the oil-soluble peroxide oxidant activity, we carried out the oxidation of DBT using TBHP and tert-amyl hydroperoxide (TAHP) in the same way as using CYHPO. As shown in the Table 2, the oxidation of DBT using TBHP was no reaction without the catalyst nor was ODS using TAHP compared with 87% conversion of DBT using CYHPO. On the other hand, the conversion of the catalytic oxidation of DBT using CYHPO was reached at 100%, significantly higher than 84% using TBHP and 91% using TAHP. These results indicated that the activity of alkyl oil-soluble peroxides decreases in the order CYHPO > TAHP > TBHP. The reason can be attributed to the different electronic density of the peroxy bond; the lower value is the electron density of peroxy bond, and the higher value is the oxidative activity of the oxidant. From the electron donation of the methyl group, the order of the peroxy bond electronic density decreases in the order TBHP > TAHP > CYHPO; this is just reversing the order of the electronic density of the oxidant peroxy bond. 3.3. Comparison between Catalytic and Noncatalytic Oxidations. To improve the removal of DBT using CYHPO, some catalysts were used in the oxidation. MoO3 shows excellent efficiency for the oxidation. As Figure 3 shows, the conversion of DBT with the catalyst MoO3/D113 increased more rapidly than without the catalyst. When the CYHPO/DBT molar ratio was 2.5 and the initial concentration of DBT was 0.5 wt %, 87% of DBT was removed from the model oil phase with MoO3/D113 merely in 25 min, compared to more than 3 h without the catalyst. The highest conversion with the catalyst was up to 100%, much higher than 87% without the catalyst (Figure 4). Results indicated that MoO3/D113 accelerated the removal of DBT and increased the maximum conversion of DBT effectively. 3.4. Oxidative Reaction Mechanism of DBT by CYHPO. A detail oxidative reaction mechanism with peroxides has been published by Chen et al.13 The oxidation of DBT is considered to be a consecutive reaction. Sulfoxide was detected first in the samples of our experiments at the beginning of the reaction by
Figure 3. Influence of the reaction time on the conversion of DBT, at 100 °C, at a CYHPO/DBT molar ratio of 2.5.
Figure 4. GC of the unoxidized DBT liquid phase and oxidized liquid phase.
thin-layer chromatography (TLC) and GC but disappeared with the reaction time. This agrees well with the previous results that sulfoxide formation is considered to dominate the reaction and the rate-determining step is the sulfide to sulfoxide. DBT reacts with CYHPO to produce DBT sulfone, and then CYHPO was transformed back to cyclohexanone11 (Scheme 2). Sheldon14 reported that the mechanism involved a metal peroxide intermediate formed by the reaction of an oxometal group with alkyl hydroperoxide. According to the results of the crystal structure,15 the distance between the Mo atom and the oxygen atom of MoO3/Al2O3 is 1.96 Å and the distance between the oxygen atom and the hydrogen atom of t-BuOOH is 1.93 Å; therefore, a good agreement is observed. Wang et al. suggested that the coordination of hydroperoxide to Mo-O was prompted by the polarization of the Mo-O bond when MoO3 dispersed (13) Chen, L.-j.; Guo, S.-h.; Zhao, D.-s. J. Fuel Chem. Technol. 2005, 33, 247-252. (14) Sheldon, R. A. J. Mol. Catal. 1980, 7, 107-126. (15) Busing, W. R.; Levy, H. J. Chem. Phys. 1965, 42, 3054-3059.
10 Energy & Fuels, Vol. 21, No. 1, 2007
Zhou et al.
Scheme 2. Simplified Reactions of DBT in the Catalytic Oxidation
Scheme 3. Oxidation Mechanism of DBT with CYHPO
on Al2O3.7 On the basis of these results, we propose a peroxidic oxidation mechanism of DBT on the MoO3 catalyst with CYHPO as shown in Scheme 3. According to this activation mechanism of the peroxy oxygen, the peroxide reacts with DBT to produce DBT sulfoxide and further oxidation produces the corresponding sulfone. The oxidation of the sulfoxide to the sulfone occurs by the same mechanism. The coordination of hydroperoxide to the Mo-O bond was prompted by MoO3 dispersed on macroporous polyacrylic cationic exchange resin; therefore, the electrophilicity of peroxy oxygen is increased.
4. Conclusion The oxidations of DBT in decalin using CYHPO in the presence of the catalyst MoO3/D113 were conducted, and the following results were obtained: (1) The conversion of DBT in decalin into DBT sulfone reached 100% at 100 °C, and the CYHPO/DBT molar ratio of 2.5:1 was reached in 40 min. (2) CYHPO oxidative activity was significantly higher than TBHP and TAHP. The activity of alkyl peroxides decreases in the order CYHPO > TAHP > TBHP, reversing the order of the peroxy bond electronic density. (3) The catalyst efficiency was prompted when MoO3 was supported on the D113 resin and when the catalyst recovery was simplified. DBT oxidations at 100 °C in decalin were performed in 10 consecutive reactions, where the catalyst was recycled and reused; the results show that the catalyst activity was not decreased. As an oil-soluble oxidant, CYHPO has a good solubility in the oil phase containing sulfur compounds; the ODS using CYHPO does not need to mix violently the oil and aqueous two phases nor does it need to recover the aqueous phase compared with the hydrogen peroxide system. These advantages may facilitate ODS industrialization by means of a flow reaction system. EF060441P
