082015pe

http://http://www.chemistrymag.org//cji/2006/082015pe.htm	Feb. 20, 2006 Vol.8 No.2 P.15 Copyright

Oxidative desulfurization of simulated gasoline over metal loaded m olecular sieve

Chen Lanju^1,2 Zhao Dishun², Guo Shaohui¹
(¹ State Key Lab of Heavy Oil Processing, China University of Petroleum, Beijing 102249; ²Department of Chemistry, Hebei University of Science and Technology, Shijiazhuang 050018, China)

Abstract A simulated gasoline consisting of model sulfur compounds of thiophene(C₄H₄S) and 3-methythiophene(3-MC₄H₄S) and n-heptane as solvent was employed for the oxidative desulfurization test in hydrogen peroxide (H₂O₂) and formic acid oxidative system over cerium- loaded molecular sieve. The effects of oxidative system, loaded metals, phase transfer catalyst, the addition of olefin and aromatics on sulfur removal were investigated in detail. The results showed that the sulfur removal rate of simulated gasoline in H₂O₂/ formic acid system was higher than the other oxidative system. The cerium-loaded molecular sieve was very active catalyst for oxidation of simulated gasoline in H₂O₂/ formic acid system, while the copper- and nickel-loaded molecular sieve was less active. The cobalt-loaded molecular sieve was the least active catalyst for the oxidation reaction. The sulfur removal rates of C₄H₄S and 3-MC₄H₄S were enhanced when phase transfer catalyst emulsifier OP or tetrabutylammonium bromide (TBAB) was added. However, the sulfur removal rate of simulated gasoline was reduced with the addition of olefin and aromatics.
Keywords Oxidative desulfurization; simulated gasoline; thiophene; 3-methythiophene; molecular sieve

1 INTRDUCTION
From an increase of environmental concern, special interest has been paid to reduction of organic sulfur-containing compounds in light fuels. Therefore, sulfur content in light fuels is limited severely and its regulation level is becoming lower and lower from year to year. Faced with continuing fuel quality challenges, the conventional method of catalytic hydrodesulfurization(HDS) under severe conditions for reducing sulfur content in light fuel is unavoidable. The necessity of producing low sulfur fuels to meet new regulation mandates will require new desulfurization technique. Under these situations, many researchers are engaged in the development of highly active desulfurization technique. There has been much interest in oxidative desulfurization(ODS) process under low reaction temperature and pressure.
The ODS process generally consists of two stages: the first stage is the oxidation of organic sulfur-containing compounds in fuel, the following step is the removal of oxidized sulfur-containing compounds by extraction. In the previous papers^[1-10], ODS process for sulfur-containing compounds in fuels employing oxidants and liquid-liquid extraction have been proposed. The extraction of oxidized sulfur-containing compounds is considered to be a useful method for removal of sulfur compounds^[3,4,7]. Otsuki et al.^[7]have reported the thiophene and thiophene derivatives with lower electron densities on the sulfur atoms could not be a 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.
In the present work, the oxidative desulfurization of simulated gasoline was studied in H₂O₂/ formic acid system, particularly, the influence of oxidative system, metal-loaded molecular sieve, phase transfer catalyst and the addition of olefin and aromatics to the oxidation of C₄H₄S and 3-MC₄H₄S. The research was conducted on simulated gasoline consisted of model sulfur compounds of C₄H₄S and 3-MC₄H₄S, which were selected from the most representative of those contained in commercial gasoline, and n-heptane as solvent. Sulfur-containing compounds in commercial gasoline are given in Table 1.

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

sulfur-containing compounds	content
thiophene	14.69
methylthiophene	50.57
dimethylthiophene	21.86
trimethylthiophene	4.04
tetramethylthiophene	0.19
tetrahydro-thiophene	1.72
mercaptan	0.75
sulfide	0.67
benzothiophene	5.45

2 EXPERIMENTAL
2.1 Materials
n-Heptane, cyclohexene and xylene(isomers) were chosen as representatives of the most important hydrocarbons classes constituting the matrixes of commercial gasoline. The organic solvents used in this study were formic acid, acetic acid, oxalic acid，benzoic acid, N,N-dimethylformamide(DMF). 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 and 3-MC₄H₄S that was among those found more frequently in the light distillates from which commercial gasoline pools are produced. Hydrogen peroxide (30%), molecular sieve, H₂SO₄, 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 and 3-MC₄H₄S was dissolved into n-heptane solvent to make a simulated gasoline solution with a sulfur content of 500mg/mL. 50 mL of the stock solution, 5 mL acid and 0.1g molecular sieve loaded with metal 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 and 3-MC₄H₄S 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; over temperature, 180 °C; injector temperature, 200 °C; detector temperature, 230 °C; split ratio, 1/00.

3 RESULTS AND DISCUSSION
3.1 The influence of oxidative system to the oxidation of simulated gasoline
E_a^Θ（H₂O₂/H₂O）= 1.763V showed that H₂O₂has intensive oxidative ability in acid condition. Formic acid, acetic acid, oxalic acid，benzoic acid and H₂SO₄，selected as acid system, were added into simulated gasoline solution by v/v=1:1 with H₂O₂at 50°C. The molecular sieve loaded with cerium was used as a catalyst in the oxidation reaction. The influence of oxidative system to oxidation of C₄H₄S and 3-MC₄H₄S in different acid conditions was shown in Table 2.

Table 2 Influence of oxidative system to oxidation of C₄H₄S and 3-MC₄H₄S

acid condition	formic acid	acetic acid	oxalic acid	benzoic acid	H₂SO₄
C₄H₄S removal rate (%)	78.4	69.7	56.9	66.2	32.3
3-MC₄H₄S removal rate (%)	82.3	72.5	65.8	73.9	36.8

The results of Table 2 showed that the sulfur removal rate of C₄H₄S and 3-MC₄H₄S in H₂O₂/ formic acid system was higher than the other oxidative system. The high sulfur removal rate could be caused by small formic acid molecule which can dissolve in both simulated gasoline and H₂O₂solution, and its K_a was higher than acetic acid. Due to inorganic acids H₂SO₄ cannot dissolve in simulated gasoline solution, the sulfur removal rate of simulated gasoline in H₂O₂/ inorganic acid was lower than in H₂O₂/ organic acid.
3.2 Evaluation of various molecular sieves loaded with metal for oxidation of thiophene
A series of experiments were performed to compare the activity of copper-, cobalt-, nickel- and cerium-loaded molecular sieve as a catalyst for oxidation of C₄H₄S and 3-MC₄H₄S. The mixture of n-heptane solution of sulfur-containing compounds and H₂O₂/formic acid became two layers after oxidation: oil layer (top), aqueous layer(bottom). The sulfur removal rates of C₄H₄S and 3-MC₄H₄S in oil layer are shown as functions of reaction time in Fig.1 and Fig.2.

Fig.1 Oxidation of C₄H₄S over various molecular sieves

Fig.2 Oxidation of 3-MC₄H₄S over various molecular sieve

In H₂O₂/fomic systems, it is clear that metal-loaded molecular sieves are much better catalyst compared to molecular sieve. The cerium-loaded molecular sieve was very active catalyst for the oxidation of C₄H₄S and 3-MC₄H₄S with 78.4% and 82.3% sulfur removal rate, while the copper- and nickel-loaded molecular sieve were less active, the sulfur removal rate were 59.5, 62.2% and 54.3, 55.2% respectively. The cobalt-loaded molecular sieve was the least active catalyst for the oxidation reaction with 40.3% and 45.9% 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 in GC-FPD analysis in oil layer after oxidation. This indicated that new organic sulfur compounds did not come into being. And deposition occurs obviously in aqueous layer when BaCl₂is added. This phenomenon indicated that the sulfur of C₄H₄S and 3-MC₄H₄S have been converted to SO₄^2-in the process of oxidation.
3.3 Influence of phase transfer catalyst on the oxidation of simulated gasoline
Since the reaction system was heterogeneous with three phases, the oxidation reaction should be improved by phase transfer catalyst(PTC). The oxidation of n-heptane solution of C₄H₄S and 3-MC₄H₄S was studied over molecular sieve loaded with cerium in H₂O₂/formic systems when PTC was added. Table 3. showed the influence of PTC on the oxidation of C₄H₄S and 3-MC₄H₄S.

Table 3 Influence of PTC to oxidation of C₄H₄S and 3-MC₄H₄S

PTC	Emulsifier OP	TBAB	SDBS	Polyglycol-400	Without PTC
C₄H₄S removal rate (%)	94.5	91.3	84.8	80.2	78.4
3-MC₄H₄S removal rate (%)	96.2	93.6	85.1	83.5	82.3

From Table 3, it can be seen that emulsifier OP was the most effective among four PTC with 94.5% and 96.2% sulfur removal rate. The sulfur removal rate of C₄H₄S and 3-MC₄H₄S in the oxidized oil layer was the same as N,N-dimethylformamide was used as the extraction solvent. There were no new peaks in GC-FPD analysis in oil layer after oxidation. TBAB was the second effective PTC with 91.3% and 93.6%. The analysis of GC-FPD indicated bromine substituted reactions on C₄H₄S and 3-MC₄H₄S. However, there was no bromine substituted reactions on n-heptane from the analysis of GC-FID. Fig.3 showed the influence of TBAB on the oxidation of C₄H₄S and 3-MC₄H₄S.

Fig .3 GC-FPD chromatogram of simulated gasoline (a- before oxidation; b- after oxidation with addition of emulsifier OP; c- after oxidation with addition of TBAB)

Fig.3 indicated the bromine substitution increases as the concentration of TBAB increases. A part of C₄H₄S and 3-MC₄H₄S was oxidized, and the others were reacted to form bromine substituted C₄H₄S and 3-MC₄H₄S when the added TBAB was over 0.2g. If 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.
3.4 Influence of olefin and aromatics on the oxidation of simulated gasoline
Cyclohexene and xylene chosen as representative olefin and aromatics were added into simulated gasoline. In H₂O₂/fomic oxidative systems, the sulfur removal rate of C₄H₄S and 3-MC₄H₄S with addition of cyclohexene and xylene was showed in Fig. 4 and Fig. 5.

Fig. 4 Influence of cyclohexene on oxidation of C₄H₄S and 3-MC₄H₄S

Fig. 4 indicated that the sulfur removal rate of C₄H₄S and 3-MC₄H₄S were reduced with the addition of cyclohexene. Low sulfur removal rate of simulated gasoline could be caused the reduction of H₂O₂which could be induced by oxidation of cyclohexene.

Fig. 5 Influence of xylene on oxidation of C₄H₄S and 3-MC₄H₄S

Fig. 5 indicated that the sulfur removal rate of simulated gasoline was reduced with the addition of xylene. Low sulfur removal rate of C₄H₄S and 3-MC₄H₄S could be led by the competition of solvent xylene and sulfur-containing compounds on catalyst.

4 CONCLUSIONS
(1) The sulfur removal rate of simulated gasoline was higher in H₂O₂/organic acid condition than in H₂O₂/inorganic acid condition.
(2) In H₂O₂ /formic acid system, the cerium-loaded molecular sieve was very active catalyst for the oxidation of C₄H₄S and 3-MC₄H₄S with 78.4% and 82.3% sulfur removal rate, respectivly while the copper- and nickel-loaded molecular sieve were less active, the cobalt-loaded molecular sieve was the least active catalyst.
(3) PTC improved the sulfur removal rate of C₄H₄S and 3-MC₄H₄S in the oxidation reaction system. Emulsifier OP was the most effective among four PTC with 94.5% and 96.2% sulfur removal rate. The bromine substitution of C₄H₄S and 3-MC₄H₄S occurs when TBAB added in the H₂O₂/formic acid system.
(4) The sulfur removal rate of C₄H₄S and 3-MC₄H₄S was reduced with the addition of cyclohexene and xylene.

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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负载金属分子筛催化氧化模拟汽油的研究
陈兰菊^1，2赵地顺²郭绍辉¹
（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）
摘要以负载金属分子筛为催化剂，在H₂O₂-HCOOH体系中，对模拟汽油噻吩（C₄H₄S）和3-甲基噻吩（3-MC₄H₄S）的正庚烷溶液进行了氧化脱硫研究。考察了氧化体系、负载金属种类、相转移催化剂、烯烃和芳烃的存在等因素对噻吩和3-甲基噻吩脱除的影响。实验结果表明：H₂O₂-HCOOH体系中模拟汽油中硫的脱除率较其他氧化体系高；在H₂O₂-HCOOH体系中，负载铈分子筛的催化活性较负载铜、镍的活性高，负载钴分子筛的活性最差；相转移催化剂乳化剂OP和四丁基溴化胺（TBAB）的加入可提高模拟汽油中硫的脱除率，但烯烃和芳烃的加入降低了硫的脱除率。
关键词 氧化脱硫；模拟汽油；噻吩；3-甲基噻吩；分子筛

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