Energy Fuels 2010, 24, 3443–3445 Published on Web 05/12/2010
: DOI:10.1021/ef1002205
Oxidation of Thiophene over Modified Alumina Catalyst under Mild Conditions Lan-ju Chen* and Fa-tang Li College of Science, Hebei University of Science and Technology, Shijiazhuang 050018, P. R. China Received March 2, 2010. Revised Manuscript Received April 28, 2010
Thiophene is a typical thiophenenic sulfur compound that exists in flow catalytic cracking gasoline. Oxidation desulfurization of thiophene was conducted with hydrogen peroxide and formic acid over a series of modified alumina catalysts. The copper oxide modified alumina catalyst was an active catalyst for thiophene oxidation with a 91.2% sulfur removal rate in the oxidation system, while the other oxides less actively modified alumina. The oxidation of thiophene was achieved under mild reaction conditions, and the high conversion of thiophene was easy to be achieved by increasing the reaction temperature or reaction time. The sulfur removal rate of thiophene was enhanced when a phase transfer agent was added in the oxidation system but was reduced with the addition of aromatics.
dance with the conventional concept that thiophene, including alkylderivatives, are highly stable heterocyclic compounds and cannot be oxidized by hydrogen peroxide due to its aromaticity under mild conditions. Chen et al.4 reported that thiophene could be oxidized over silica gel loaded with oxide by hydrogen peroxide in the formic acid system. Kong et al.9 reported that thiophene could be oxidized over the TS-1 catalyst slowly by hydrogen peroxide in water or tert-butanol, but it could not be oxidized in methanol or acetonitrile as the solvent. In this work, the oxidative desulfurization of thiophene was studied in the hydrogen peroxide/formic acid system, particularly, the influence of the modified alumina catalyst, reaction temperature, reaction time, phase transfer agent, or aromatics.
The organosulfur compounds that exist in commercial gasoline, which are mainly from flow catalytic cracking (FCC) gasoline, are highly undesirable since they could result in device corrosion and environmental contamination. Because of increasing environmental concern, special interest has been paid to the reduction of organosulfur compounds in commercial gasoline. Faced with such challenges, the conventional method of catalytic hydrodesulfurization under severe conditions for reducing sulfur content in FCC gasoline demands further development.1 The necessity of producing low sulfur fuels to meet new regulation will require a new desulfurization technique. Much attention has been paid to the oxidative desulfurization (ODS) process under mild conditions.2-13,15 The ODS process was very efficient in reducing benzothiophene, dibenzothiophene, and their corresponding alkylderivatives, and the extraction of oxidized sulfur-containing compounds was considered to be a useful method for removal of sulfur compounds.2,3,7,8 However, Otsuki et al.3 reported that thiophene and thiophene derivatives with lower electron densities on the sulfur atoms could not be oxidized in the hydrogen peroxide/formic acid system at 50 °C, while dibenzothiophenes with higher electron densities were oxidized. This is in accor-
Experimental Section Materials. Thiophene(C4H4S, analytical grade, China) was used without further treatment. Hydrogen peroxide(H2O2, aqueous solution 30%, China) and formic acid (HCOOH, analytical grade, China) were used as the oxidation system. Xylene and n-heptane (analytical grade, China) were chosen as representative substance of the most important hydrocarbon classes constituting the matrixes of gasoline. The phase transfer agent (PTA), emulsifier OP, tetraoctyl ammonium bromide (TOAB), cetyl trimethyl ammonium bromide (CTAB), tetrabutyl ammonium bromide (TBAB), and extractant N,N-dimethylformamide (DMF) were also used in this work, which are analytical grade from China. Catalysts. Modified alumina, prepared using aluminum isopropoxide, ammonia solution, TBAB, and metal salts by the sol-gel method14 and method of impregnation,10 was used as the catalyst. The alumina modified by metal oxide was, respectively, prepared with 0.79% copper oxide, 0.80% iron oxide, 1.64% cerium oxide, 0.87% manganese oxide, and 0.80% zinc oxide. Typical Reaction Run. In the typical reaction run, the water bath was first heated up and stabilized to the desired reaction temperature (293-333 K). Then, 10 mL of the model compound,
*To whom correspondence should be addressed. Telephone: þ86311-81668532. E-mail:
[email protected]. (1) Song, C. S. Catal. Today 2003, 86, 211. (2) Shiraishi, Y.; Hirai, T. Energy Fuels 2004, 18, 37. (3) Otsuki, S.; Nonaka, T.; Takashima, N.; Qian, W.; Ishihara, A.; Imai, T.; Kabe, T. Energy Fuels 2000, 14, 1232. (4) Chen, L. J.; Guo, S. H.; Zhao, D. S.; Wang, J. L.; Mou, T. Energy Sources, Part A 2008, 30 (4), 370. (5) Collins, F. M.; Lucy, A. R.; Sharp, C. J. Mol. Catal. A 1997, 117, 397. (6) Wan, M. W.; Yen, T. F. Appl. Catal., A: General 2007, 319, 237. (7) Te, M.; Fairbridge, C.; Ring, Z. Appl. Catal., A: General 2001, 219, 267. (8) Mei, H.; Mei, B. W.; Yen, T. F. Fuel 2003, 82, 405. (9) Kong, L. Y.; Li, G.; Wang, X. S. Catal. Lett. 2004, 92, 163. (10) Chen, L. J.; Guo, S. H.; Zhao, D. S. J. Chem. Ind. Eng. (China) 2007, 58 (3), 652. (11) Zhao, D. S.; Li, F. T.; Zhou, E. P.; Sun, Z. M. Chem. Res. Chin. Univ. 2008, 24 (1), 96. (12) Shiraishi, Y.; Tachibana, K.; Hirai, T.; Komasawa, I. Ind. Eng. Chem. Res. 2002, 41, 4362. (13) Murata, S.; Murata, K.; Kidena, K.; Nomura, M. Energy Fuels 2004, 18 (1), 116. r 2010 American Chemical Society
(14) Li, Z. P.; Zhao, R. H.; Guo, F.; Chen, J. F.; Wang, G. Chem. J. Chin. Univ. 2008, 29 (1), 13. (15) Chen, L. J.; Guo, S. H.; Zhao, D. S. Chin. J. Chem. Eng. 2007, 15 (4), 520.
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pubs.acs.org/EF
Energy Fuels 2010, 24, 3443–3445
: DOI:10.1021/ef1002205
Chen and Li Scheme 1. Oxidation Process of Thiophene
Figure 1. Oxidation of thiophene over various forms of alumina modified by a metal oxide.
prepared by 0.6 mL of thiophene dissolved in 500 mL of n-heptane, was added to three-necked flask equipped with a magnetic stirrer and reflux condenser. Then 0.1 g of catalyst and 2 mL of oxidant containing H2O2 at a H2O2/HCOOH volume ratio of 1:1 was added to the reactor. The resulting mixture was stirred for 10-120 min at the reaction temperature and analyzed periodically. Catalysts were centrifuged off, and the organic phase was delivered to the microcoulometer (WK-2D, China) and gas chromatograph (GC, SE-30 column of i.d. 0.32 mm 30 m) equipped with a FPD detector (6890II, China). The oxidation products were identified through gas chromatography-mass spectrometry (GC-MS) analysis (American, 5973N).
Figure 2. Influence of solvent on the oxidation of thiophene.
Results and Discussion Evaluation of Various Modified Alumina for Oxidation of Thiophene. Experiments were performed to compare the activity of alumina modified by copper oxide, iron oxide, cerium oxide, manganese oxide, and zinc oxide as a catalyst for the oxidation of thiophene. The mixture of the n-heptane solution of thiophene and H2O2/HCOOH became three phases after the completion of the oxidation reaction: organic phase (top), aqueous phase (middle), and solid phase (bottom). The conversion of thiophene in the organic phase was shown as a function of reaction time in Figure 1. Figure 1 showed that the conversion of thiophene increased with the reaction time increasing; however, it increased relatively less obvious if the reaction time exceeded 60 min due to the restrictions of oxidant, catalyst, solvent evaporation, and other factors. In H2O2/HCOOH systems, it was clear that the catalytic activity of alumina modified by metal oxide was much better compared to no modified alumina. The copper oxide modified alumina was a very active catalyst for the oxidation of thiophene with 91.2% sulfur removal rate at 60 min, while alumina modified by iron oxide, cerium oxide, or manganese oxide was a less active catalyst, and zinc oxide modified alumina was the least active catalyst for thiophene oxidation. Though the oxidized organic phase was extracted three times with DMF, the conversion of thiophene did not change. No new peaks appeared in the gas chromatography-flame photometric detection (GC-FPD) analysis of the oxidized organic phase. In the aqueous phase, white deposition occurs obviously if BaCl2 was added. This phenomenon indicated that the sulfur of thiophene had been converted to SO42in the process of oxidation, which is shown in Scheme 1.15
Figure 3. Influence of reaction temperature to the oxidation of thiophene.
Influence of Reaction Solvent to the Oxidation of Thiophene. Xylene and n-heptane were chosen as the typical organic solvents in the oxidation of thiophene in H2O2/HCOOH systems. The oxidation behavior in different solvents was shown in Figure 2. The thiophene conversion was lower in the solvent xylene than in the solvent n-heptane. A low sulfur removal rate of thiophene could result from the competition of the solvents xylene and thiophene on the catalyst. Because of the acidity of the catalyst surface, the oxidation of aromatic sulfur compounds is initiated with the activation of conjugated electrons. The sequence of thiophene oxidation began with the activation of the electrons on the thiophene ring under the action of a catalyst, followed by the sulfur oxidation once the aromaticity had been destroyed. The activation of the electrons on the thiophene ring was affected by xylene because of the conjugated electrons on the solvent xylene. 3444
Energy Fuels 2010, 24, 3443–3445
: DOI:10.1021/ef1002205
Chen and Li
Table 1. Influence of PTAs to Oxidation of Thiophene PTA
emulsifier OP
TOAB
CTAB
TBAB
without PTA
thiophene conversion (%)
98.5
94.7
96.2
97.8
91.2
Scheme 2. Process of Catalytic Oxidation of Thiophene under the Action of TBAB
conversion of thiophene did not change if the oxidized organic phase was extracted with DMF. It is known that the cationic structure of the QAS not only influences its ability to transfer the anion from the aqueous to organic phase but also strongly affects the rate of the organic phase reaction. The catalytic activities of the three QASs followed the order: TBAB > CTAB > TOAB. The GC-FPD analysis indicated bromine substitution reactions on thiophene in oxidized organic phase when one of the QASs was added in the oxidation system. However, there was no bromine substitution reaction on xylene or n-heptane from the analysis by GC-FID. Figure 4 showed the GC-FPD analysis of the oxidation of thiophene under the action of PTA, and Scheme 2 showed the process of catalytic oxidation of thiophene under the action of TBAB. The process of catalytic oxidation of thiophene under the action of TBAB consisted of six basic steps. Step 1, HCOOOH was formed in the oxidation system. Step 2, in the presence of TBAB, HCOOOH was substituted to form (C4H9)4NOOOCH, the effective species for oxidation. Step 3, (C4H9)4NOOOCH exchanged in aqueous phase and organic phase. Step 4, thiophene was oxidized to a sulfate ion by (C4H9)4NOOOCH with high efficiency and high selectivity. Step 5, the reduced (C4H9)4NOOOCH dissociated with HBr and returned to the aqueous phase. Step 6, the bromine substitution reactions on thiophene could occur in the presence of excess TBAB.
Figure 4. GC-FPD analysis of oxidized organic phase under the action of PTA (emulsifier OP, TOAB, CTAB, TBAB).
Influence of Reaction Temperature to the Oxidation of Thiophene. The oxidation of the n-heptane solution of thiophene was studied in H2O2/HCOOH systems when the reaction temperature varied from 293 to 333 K. The copper oxide modified alumina was used as a catalyst in the oxidation reaction. Figure 3 showed the influence of reaction temperature to the oxidation of thiophene. The result indicated that a lower reaction temperature (293 K) was unfit for oxidation of thiophene. The conversion of thiophene was enhanced with the reaction temperature increasing. However, it increased relatively less obvious, and even reduced slightly if reaction temperature exceeded 323 K due to the restrictions of solvent evaporation, oxidant decomposition, and other factors. Influence of Phase Transfer Agent to the Oxidation of Thiophene. Since the reaction system was heterogeneous with three phases, the oxidation reaction should be improved by PTA. In this study, emulsifier OP and quaternary ammonium salts (QAS) with different cationic structures were employed to improve the catalytic activity of thiophene oxidation. The PTAs could deliver the oxidant anion into the organic phase or interfacial region, thus facilitating the oxidation of organic sulfur compounds. In H2O2/HCOOH systems, the oxidation of thiophene was studied over copper oxide modified alumina with PTAs added in the reaction system. With slightly increased amounts of PTAs, there was no increased efficiency of the sulfur reduction. However, when the amount of QAS added was over 0.02 mol/L, there were obvious bromine substitution reactions on thiophene, which was similar to the literature.4 Table 1 showed the influence of 0.01 mol/L PTAs to the oxidation of thiophene. From Table 1, it can be seen that emulsifier OP was the most effective among four PTAs. There were no new peaks in the GC-FPD analysis of the oxidized organic phase. The
Conclusions The oxidation of a typical sulfur compound in FCC gasoline was conducted in the H2O2/HCOOH system over a series of modified alumina catalysts. The copper oxide modified alumina catalyst was a more active catalyst for the oxidation of thiophene than the other oxide modified alumina. The conversion of thiophene was easily enhanced by increasing the reaction temperature or reaction time. The conversion of thiophene was higher in the solvent n-heptane than in the solvent xylene due to the competition of xylene and thiophene on the catalyst. Thiophene conversion was enhanced when PTA was added in the oxidation system; however, the bromine substitution reactions on thiophene occurred when excess QAS was added in the oxidative system. Acknowledgment. The authors would like to thank the financial support from the National Natural Science Foundation of China (Grant No. 20806021), Scientific and Technological Research and Development of Hebei Province (Grant No. 09215137), and Research Projects of Department of Education of Hebei Province (Grant No. 2009432). 3445
