087046pe

http://http://www.chemistrymag.org//cji/2006/087046pe.htm	Jul. 1, 2006 Vol.8 No.7 P.46 Copyright

Oxidation of thiophene over metal-loaded alumina and phase transfer catalyst

Lanju Chen ¹,2 Shaohui Guo ¹Dishun Zhao ²
(¹ State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249; ²Department of Chemistry, Hebei University of Science and Technology, Shijiazhuang, 050018, China)

Received Jan.18, 2006.

Abstract Thiophene(C₄H₄S) are typical thiophenenic sulfur compounds that existed in flow catalytic cracking (FCC) gasoline. Oxidation reactions of C₄H₄S were conducted with hydrogen peroxide (H₂O₂) and formic acid over a series of metal-loaded alumina. The effects of loaded metals, temperature, solvent and phase transfer catalyst on sulfur removal were investigated in detail. The results showed that the copper-loaded alumina was very active catalyst for oxidation of C₄H₄S in H₂O₂/ formic acid system. The oxidation of C₄H₄S was performed under mild reaction conditions and it was easy to achieve high oxidation conversions by increasing reaction temperature or reaction time. The sulfur removal rate of C₄H₄S was enhanced when phase transfer catalyst emulsifier OP or tetrabutylammonium bromide (TBAB) was added. Interestingly, in a H₂O₂ and formic acid system, with the addition of TBAB, a bromine substitution trend appeared in the oxidation of C₄H₄S, suggesting the influence of TBAB to the oxidation of C₄H₄S.
Keywords Oxidative desulfurization; thiophene; alumina; phase transfer catalyst

1. INTRODUCTION
The presence of sulfur compounds in commercial gasoline, for which more than 90% formed from flow catalytic cracking (FCC) gasoline in China, is highly undesirable since they result in device corrosion and environmental contamination. 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]. Faced with continuing fuel quality challenges, the conventional method of catalytic hydrodesulfurization(HDS) under service conditions for reducing sulfur content in FCC gasoline is unavoidable. The necessity of producing low sulfur fuels to meet new regulation mandates will require new desulfurization technique. Recently, there has been much interest in oxidative desulfurization(ODS) process under low reaction temperature and pressure.
The ODS process is composed of two stages: oxidation, followed by liquid extraction. Oxidants can convert sulfur-containing compounds in light oils to much more polar oxidized species. Such oxidants include nitric acid^[3-4],nitrogen oxides^[5], O₃^[6], H₂O₂^[7-12]et al. After oxidation, the sulfur compounds are transformed to sulfones. The extraction of sulfones is considered to be useful method for removal of sulfur compounds^[3-4]. The reactivity of sulfur compounds for oxidation is increased with electron density on sulfur atom. Otsuki et al.^[7]have reported the thiophene and thiophene derivatives with lower electron densities on the sulfur atoms could not be oxidized at 50 °C, while dibenzothiophenes with higher electron densities could be oxidized. This is in accordance with the conventional thinking that thiophene cannot be oxidized by H₂O₂under mild conditions owing to its aromaticity.
Sulfur-containing compounds of FCC gasoline are given in Table 1. The content of thiophenic sulfur compounds was more than 80% of sulfur-containing compounds that existed in flow catalytic cracking(FCC) gasoline. Specifically, the chosen sulfur compounds were C₄H₄S which could not be oxidized at 50 °C according to Otsuki^[7]. In the present work, the oxidative desulfurization of C₄H₄S was studied in H₂O₂/ formic acid system, particularly, the influence of the metal-loaded alumina and catalyst to the oxidation of C₄H₄S. The research was conducted on simplified model systems of C₄H₄S selected from the most representative of those contained in FCC gasoline, dissolved in different organic solvents.

Table 1 Sulfur-containing compounds of FCC gasoline from Shijiazhuang Refinery, China

sulfur-containing compounds	content
thiophene	10.77
methylthiophene	45.93
dimethylthiophene	25.17
trimethylthiophene	4.63
tetramethylthiophene	0.25
tetrahydro-thiophene	3.18
mercaptan	0.79
sulfide	1.18
benzothiophene	7.98

2. EXPERIMENTAL
2.1. Materials
Xylene(isomers)and n-heptane were chosen as a representative of the most important hydrocarbons classes constituting the matrixes of light distillates. The organic solvents used in this study were formic acid, N,N-dimethylformamide. The phase transfer catalysts(PTC) used were emulsifier OP, sodium dodecyl benzene sulfonate (SDBS), tetrabutyl ammonium bromide(TBAB), polyglycol-400.
    The sulfur compound selected was C₄H₄S that was among those found more frequently in the light distillates from which commercial gasoline pools are produced. Hydrogen peroxide (30%), alumina, Cu(Ac)₂ , Co(Ac)₂, Ni(NO)₃, Ce(NO)₃ and BaCl₂ were supplied by Tianji Reagent Company. Before use, the concentration of H₂O₂was determined by iodometry. All the products were commercial reagent grade.
2.2. Procedure
C₄H₄S was dissolved into xylene(isomers) or n-heptane to make a stock solution with a sulfur content of 500μg/mL. 50 mL the stock solution, 5 mL formic acid and 0.1g metal-loaded alumina were put in a 100 mL three-necked flask equipped with a magnetic stirrer and reflux condenser. The system was heated in a thermostatic bath under stirring with a magnetic stirrer at about 1500 rpm. When the mixture reached the selected reaction temperature (50°C), 5 mL of H₂O₂ and PTC was then added and the reaction was started. Since the mixture was a heterogeneous system of three phases (an organic phase, an aqueous phase and solid phase), efficient mixing was necessary to ensure a homogeneous composition of the bulk liquids.
    To determine the initial and residual concentration of C₄H₄S 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 HP 6890 gas chromatograph (GC) equipped with a flame photometric detector (FPD) and a flame ionic detector (FID) using a 30m, i.d. 0.32 mm SE-30 column. The main parameters were the following: carrier gas, nitrogen with a flow of 2 mL/min; oven temperature, 180 °C; injector temperature, 200 °C; detector temperature, 230 °C; split ratio, 1/00.

3. RESULTS AND DISCUSSION
3.1. Evaluation of various alumina loaded with metal for oxidation of thiophene
A series of experiments were performed to compare the activity of copper-, cobalt-, nickel- and cerium-loaded alumina as a catalyst for oxidation of C₄H₄S. The mixture of n-heptane solution of sulfur compounds and H₂O₂/formic acid became two layers after oxidation: oil layer (top), aqueous layer (bottom). The sulfur removal rates of C₄H₄S in oil layer are shown as functions of reaction time in Fig.1.

Fig.1 Oxidation of C₄H₄S over various alumina loaded with metal

    In H₂O₂/fomic systems, it is clear that metal-loaded alumina is much better compared to alumina. The copper-loaded alumina was very active for the oxidation of C₄H₄S with 70.1% sulfur removal rate, while the nickel- and cerium-loaded alumina were less active, the sulfur removal rate were 59.3% and 57.1% respectively. The cobalt-loaded alumina was the least active for the oxidation reaction with 45.2% sulfur removal rate. The sulfur removal rate of the oxidized oil layer was the same when N,N-dimethylformamide was used as the extraction solvent. There were no new peaks of the productin GC-FPD analysis in oil layer after oxidation. And deposition occurs obviously in aqueous layer when BaCl₂is added. This phenomenon indicated that the sulfur of C₄H₄S has been converted to SO₄^2-in the process of oxidation.
3.2 Influence of reaction temperature to the oxidation of thiophene
The oxidation of n-heptane solution of C₄H₄S was studied in H₂O₂/fomic systems as the reaction temperature varied from 293K to 393K. The copper-loaded alumina was used as a catalyst in the oxidation. Fig. 2 showed the influence of reaction temperature to oxidation of C₄H₄S.
    The result indicated that lower reaction temperature (293K) was unfit for oxidation of C₄H₄S. The sulfur removal rate of C₄H₄S was enhanced with the increase of reaction temperature. The sulfur removal rate of C₄H₄S reached 70.1% when reaction temperature was 323K. When the reaction temperature exceeded 323K, the conversion of C₄H₄S fell due to solvent evaporation.

Fig.2 Influence of reaction temperature on oxidation of C₄H₄S

3.3 Influence of Solvent to the oxidation of thiophene
Xylene(isomers) and n-heptane were chosen as the organic solvents in the oxidation of C₄H₄S in H₂O₂/fomic systems. The copper-alumina loaded was used as a catalyst in the oxidation. The oxidation behavior in different solvents was shown in Fig.3.

Fig.3 Influence of solvent on the oxidation of C₄H₄S

It can be seen from Fig. 3, the sulfur removal rate was lower in solvent xylene than in solvent n-heptane. Low sulfur removal rate of C₄H₄S could be resulted by the competition of solvent xylene and C₄H₄S on catalyst.
3.4. Influence of phase transfer catalyst to the oxidation of thiophene
Since the reaction system was heterogeneous with three phases, the oxidation reaction should be improved by PTC. The oxidation of n-heptane solution of C₄H₄S was studied over copper-loaded alumina in H₂O₂/fomic systems when PTC was added. Table 2 showed the influence of PTC on oxidation of C₄H₄S.

Table 2 Influence of PTC on oxidation of C₄H₄S

PTC	Emulsifier OP	SDBS	TBAB	Polyglycol-400	Without PTC
Sulfur removal rate(%)	91.3	74.8	86.5	70.2	70.1

From Table 2, it can be seen that emulsifier OP was the most effective among four PTC. The sulfur removal rate of C₄H₄S in the oxidized oil layer was the same when N,N-dimethylformamide was used as the extraction solvent. There were no new peaks of in GC-FPD analysis in oil layer after oxidation. TBAB was the second effective PTC with 86.5%. The analysis of GC-FPD indicated bromine substituted reactions on C₄H₄S. However, there was not bromine substituted reactions on xylene or n-heptane from the analysis of GC-FID. Fig.4(a,b,c,d,e) showed the influence of TBAB to oxidation of C₄H₄S.

Fig .4 GC-FPD chromatogram of thiophene solution ( a- before oxidation; b- after oxidation (without TBAB); c- after oxidation (0.02gTBAB added); d- after oxidation (0.2gTBAB added); e- after oxidation (0.5gTBAB added) )

Fig.(4c,d,e) indicate that the bromine substitution increases as the concentration of TBAB increases. A part of C₄H₄S was oxidized, and the other was reacted to form bromine substituted C₄H₄S when added TBAB was over 0.2g. Sulfur-containing compounds in the oil layer after oxidation was extracted with N,N-dimethylformamide. The sulfur removal rate was 100% in the oil layer after extraction.

4. CONCLUSIONS
(1) C₄H₄S was oxidized in H₂O₂ /formic acid over a series of catalysts of metal-loaded alumina. The copper-loaded alumina was most active for oxidation of C₄H₄S in H₂O₂/ formic acid system, while the nickel- and cerium-loaded alumina was less active. The cobalt-loaded alumina was the least active for the oxidation reaction.
(2) The lower reaction temperature (293K) was unfit for oxidation of C₄H₄S. The sulfur removal rate of C₄H₄S was enhanced with the increase of reaction temperature.
(3) The conversions of C₄H₄S are lower in solvent xylene than in solvent n-heptane due to the competition of solvent xylene and thiophene on catalyst.
(4) Emulsifier OP was the most effective PTC with 91.3% sulfur removal rate in the oxidized oil layer. The bromine substitution of C₄H₄S occurs when TBAB added in the H₂O₂/formic acid system. The sulfur removal rate of C₄H₄S was 100% in the oil layer after extraction with N,N-dimethylformamide.

Acknowledgment Authors are grateful for the financial support from national natural science foundation of china (20276015) and natural science foundation of Hebei Province(203364).

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负载金属氧化铝和相转移催化剂氧化噻吩的研究
陈兰菊^1，2郭绍辉¹赵地顺²
（¹石油大学（北京）重质油加工国家重点实验室，北京昌平 100022; ²河北科技大学化学系，河北石家庄 050018）
摘要以负载金属的氧化铝为催化剂，在H₂O₂-HCOOH体系中，对催化裂化汽油中特征含硫化合物噻吩的正己烷溶液进行了氧化脱硫研究。考察了负载金属种类、氧化温度、溶剂和相转移催化剂等因素对噻吩脱硫的影响。实验结果表明：在H₂O₂-HCOOH体系中，负载铜的氧化铝催化活性最好；提高反应温度或延长反应时间可提高噻吩的转化率；相转移催化剂乳化剂OP和四丁基溴化胺（TBAB）的加入可提高噻吩硫的脱除率。值得提出的是，随着TBAB加入量的增多，氧化过程出现了噻吩的溴代产物，这说明TBAB对噻吩氧化的影响随其加入量的增加而增大。
关键词 氧化脱硫；噻吩；氧化铝；相转移催化剂

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