http://www.chemistrymag.org/cji/2006/082015pe.htm |
Feb. 20, 2006 Vol.8 No.2 P.15 Copyright |
(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 H2O2 under
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.
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 H2O2
was 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
EaΘ（H2O2/H2O）= 1.763V showed that H2O2 has 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 H2O2 at 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.
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 H2O2 solution, and its Ka 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 BaCl2 is added. This phenomenon indicated that the sulfur of
C4H4S and 3-MC4H4S have been converted to SO42-
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 H2O2 which 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).
REFERENCES
[1] Tam P S, Kittrell J R, Eldridge J W. Ind. Eng. Chem. Res.1990,29, 321-324.
[2] Tam P S, Kittrell J R, Eldridge J W. Ind. Eng. Chem. Res.1990, 29,324－329.
[3] Shiraishi Y, Hara H, Hirai T et al. Ind. Eng. Chem. Res., 1999,38, 1589-1595
[4] Shiraishi Y, Hirai T. Energy Fuels, 2004, 18, 37-40.
[5] Collins F M, Lucy A R, Sharp C J. Mol. Catal. A, 1997, 117, 397-403.
[6] Mei H, Mei B W, Yen T F. Fuel , 2003, 82: 405–414.
[7] Otsuki S, Nonaka T, Takashima N et al. Energy Fuels 2000, 14, 1232-1239.
[8] Shiraishi Y, Tachibana K, Hirai T et al. Ind. Eng. Chem. Res., 2002,41, 4362-4375
[9] Anisimov A V, Fedorova E V, Lesnugin A Z et al. Catalysis Today. 2003, 78, 319-325.
[10] Murata S, Murata K, Kidena K et al. Energy Fuels 2004, 18, 116-121.
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
（
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