|
http://http://www.chemistrymag.org//cji/2006/087046pe.htm |
Jul. 1, 2006 Vol.8 No.7 P.46 Copyright |
|
| |
(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 H2O2 under 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.
|
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 H2O2 was
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. 
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 BaCl2 is added. This phenomenon indicated that the sulfur of C4H4S has been converted to SO42- 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.
|
PTC |
Emulsifier OP |
SDBS |
TBAB |
Polyglycol-400 |
Without PTC |
|
Sulfur removal rate(%) |
91.3 |
74.8 |
86.5 |
70.2 |
70.1 |
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).
REFERENCES
[1] Avidan, A.;
Klein, B.; Ragsdale, R. Hydrocarbon Process. 2001,February, 47-48.
[2]
Frederick, C. Hydrocarbon Process. 2002, February, 45-46.
[3] Tam,
P.S.; Kittrell, J.R.; Eldridge, J.W. Ind. Eng. Chem.
Res.1990,29,321-324.
[4] Tam, P.S.; Kittrell, J.R.; Eldridge, J.W. Ind.
Eng. Chem. Res.1990,29,324－329.
[5]
Tam, P.S.; Kittrell, J.R.. U. S. Patent 4,485,007,1984.
[6] Paybarah
A.; Bone R. L.; Corcoran W. H. Ind. Eng. Chem. Process. Res. Dev.
1982,21,426-428.
[7] Otsuki, S.; Nonaka, T.; Takashima, N.; Qian, W.;
Ishihara, A.;Imai, T.; Kabe, T. Energy Fuels 2000, 14, 1232-1239.
[8]
Te, M.; Fairbridge, C.; Ring, Z. Appl. Catal. A: General 2001, 219,
267-280.
[9] Collins, F. M.; Lucy, A. R.; Sharp, C. J. Mol. Catal. A
1997, 117, 397-403.
[10] Mei, H.; Mei, B. W. and Yen, T. F.. Fuel ,
2003,82: 405–414.
[11] Shiraishi,
Y.; Tachibana, K.; Hirai, T. and Komasawa, I.. Ind. Eng. Chem. Res.,
2002,41, 4362-4375.
[12] Shiraishi, Y. and Hirai, T.. Energy Fuels
2004, 18, 37-40.
負(fù)載金屬氧化鋁和相轉(zhuǎn)移催化劑氧化噻吩的研究
陳蘭菊1，2 郭紹輝1
趙地順2
（1石油大學(xué)（北京）重質(zhì)油加工國(guó)家重點(diǎn)實(shí)驗(yàn)室，北京 昌平 100022;
2河北科技大學(xué)化學(xué)系，河北 石家莊 050018）
摘要
以負(fù)載金屬的氧化鋁為催化劑，在H2O2-HCOOH體系中，對(duì)催化裂化汽油中特征含硫化合物噻吩的正己烷溶液進(jìn)行了氧化脫硫研究�？疾炝素�(fù)載金屬種類、氧化溫度、溶劑和相轉(zhuǎn)移催化劑等因素對(duì)噻吩脫硫的影響。實(shí)驗(yàn)結(jié)果表明：在H2O2-HCOOH體系中，負(fù)載銅的氧化鋁催化活性最好；提高反應(yīng)溫度或延長(zhǎng)反應(yīng)時(shí)間可提高噻吩的轉(zhuǎn)化率；相轉(zhuǎn)移催化劑乳化劑OP和四丁基溴化胺（TBAB）的加入可提高噻吩硫的脫除率。值得提出的是，隨著TBAB加入量的增多，氧化過(guò)程出現(xiàn)了噻吩的溴代產(chǎn)物，這說(shuō)明TBAB對(duì)噻吩氧化的影響隨其加入量的增加而增大。
關(guān)鍵詞
氧化脫硫；噻吩；氧化鋁；相轉(zhuǎn)移催化劑