Oxidative desulfurization of dibenzothiophene with molecular oxygen using emulsion catalysis

Article abstract of DOI:10.1039/b610504a

Dibenzothiophene (DBT) is oxidized to the corresponding sulfoxide and sulfone in an emulsion system (W/O) composed of polyoxometalate anion [C ₁₈H₃₇N(CH₃)₃]₅[PV ₂Mo₁₀O₄₀] as both the surfactant and catalyst, using molecular oxygen as the oxidant and aldehyde as the sacrificial agent under mild conditions. The Royal Society of Chemistry.

Full text of DOI:10.1039/b610504a

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COMMUNICATION
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Oxidative desulfurization of dibenzothiophene with molecular oxygen
using emulsion catalysis{
Hongying L u¨ , Jinbo Gao, Zongxuan Jiang, Yongxing Yang, Bo Song and Can Li*
Received (in Cambridge, UK) 24th July 2006, Accepted 29th September 2006
First published as an Advance Article on the web 26th October 2006
DOI: 10.1039/b610504a
Dibenzothiophene (DBT) is oxidized to the corresponding
sulfoxide and sulfone in an emulsion system (W/O) composed
emulsion catalytic system where an amphiphilic catalyst
37N(CH [PV 40] acts as the surfactant. It is found
[C18H
3
)
3
]
5
2
Mo10O
that the mixture of MeCN and sulfur-containing molecules present
in decalin can form an emulsion under stirring. The aldehyde and
molecular oxygen dissolve in acetonitrile, inside the emulsion
droplets, while the polyoxometalate anion [C H N(CH ) ] -
3 3 5 2
of polyoxometalate anion [C18H37N(CH ) ] [PV Mo10O40] as
both the surfactant and catalyst, using molecular oxygen as the
oxidant and aldehyde as the sacrificial agent under mild
conditions.
18
37
3 3 5
2
[PV Mo10O40] as both the surfactant and catalyst assembles on the
In the last decade, much attention has been paid to the deep
desulfurization of fuel oils due to more stringent environmental
regulations. Although hydrodesulfurization (HDS) is highly
efficient in removing thiols, sulfides, and disulfides, it is difficult
to reduce refractory sulfur-containing compounds such as
dibenzothiophene (DBT) and its derivatives, especially 4,6-
dimethydibenzothiophene (4,6-DMDBT), to an ultra-low level.
Therefore, it is highly desirable to develop non-HDS methods to
interface of emulsion droplets. As a result, the acetonitrile,
including isobutyl aldehydes, and molecular oxygen dissolved in
acetonitrile is a disperse phase, and the decalin is thought of as a
continuous phase in the emulsion system. DBT can be oxidized to
the corresponding sulfoxide and sulfone in this emulsion system
under mild conditions.
The oxidation of DBT present in MeCN was investigated firstly
3 3 5 2
with amphiphilic catalyst [C18H37N(CH ) ] [PV Mo10O40] using
1
molecular oxygen as oxidant. Fig. 1 shows the sulfur-specific gas
chromatography (GC) analyses before and after the catalytic
oxidation of DBT in MeCN. DBT present in MeCN can be
completely oxidized to sulfoxide and sulfones at 60 uC in 2 h, but
the DBT present in decalin can not be oxidized at all (see ESI{).
Thus, MeCN is considered to be one of the most suitable solvents
meet the demands of producing clean diesel with an extremely low
concentration of sulfur-containing compounds. Among them,
oxidative desulfurization combined with extraction is considered to
be one of the most promising processes. Our previous work found
that amphiphilic catalysts, Q [PW O ] and Q [H NaPW O ],
3
12 40
4
2
10 36
assembled in an emulsion in diesel, could selectively oxidize sulfur-
containing compounds into their corresponding sulfones by using
an approximately stoichiometric amount of H O as the oxidant.
4
for the oxidative desulfurization of diesel. In order to oxidize
DBT present in model diesel, MeCN was added to the reaction
system. It was found that the emulsion droplets, which were
2
2
The sulfones can be readily separated from the diesel using an
extractant and ultra-deep desulfurization of diesel can be
2
achieved.
However, the oxidation of these refractory sulfur-containing
compounds using molecular oxygen instead of hydrogen peroxide
as oxidizing agent under mild conditions has long been desired due
to its low cost and green chemistry advantage. However, there are
only a few reports on the oxidation of those refractory sulfur-
containing compounds with molecular oxygen. Recently, cobalt
salt and some metal oxide catalysts (including manganese and
2 3
cobalt oxide) supported on c-Al O have been used as catalysts for
the oxidation of sulfur-containing compounds with molecular
3
oxygen. In these processes, the sulfur compounds present in diesel
can be oxidized to corresponding sulfones, which can be removed
by subsequent adsorption or solvent extraction.
In this communication, we report the oxidation of dibenzothio-
phene (DBT) with molecular oxygen as the oxidant using an
State Key Laboratory of Catalysis, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences, 457 Zhongshan Road,
Dalian 116023, China. E-mail: canli@dicp.ac.cn;
Web: http://www.canli.dicp.ac.cn; Fax: +86 411-84694447;
Tel: +86 411-84379070
Fig. 1 Sulfur specific GC-FPD chromatograms for the oxidation of
DBT with molecular oxygen/aldehyde in MeCN. (Conditions:
[
C
18
H
37N(CH
0.8 mmol) in 50 ml MeCN, isobutyraldehyde 0.72 ml (8 mmol), reaction
temperature 60 uC.)
3 3 5 2
) ] PV Mo10O40 33 mg (0.01 mmol), DBT 147 mg
{
Electronic supplementary information (ESI) available: Synthesis and
characterization of catalysts, oxidation of DBT, analysis of sulfur content
and spectra. See DOI: 10.1039/b610504a
(
1
50 | Chem. Commun., 2007, 150–152
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catalytic activity, as it can form relatively uniform emulsion
droplets suspended in decalin phase. However, the catalyst
37 2 3 2 5 2
[(C18H ] N(CH ) ] PV Mo10O40 shows low reaction activity
despite the emulsion droplets being formed, possibly because the
small emulsion droplets cannot supply enough oxidative sacrificial
agent.
Aldehydes were known to be oxidized easily to corresponding
2
5 5
peracids by molecular oxygen in the presence PV
Therefore, the oxidation of DBT to sulfoxide and sulfone with
the dioxygen-[C18 37N(CH PV 40-aldehydes might
proceed via the following two steps. First, the catalyst
37N(CH PV 40 can continuously catalytically
2
Mo10
O
40
.
H
3
)
3
]
5
2
Mo10O
[C H
18
3
)
3
]
5
2
Mo10O
oxidize aldehyde to peracids with molecular oxygen on the
interface of the emulsion droplets:
Fig. 2 Removal of DBT vs. reaction time in emulsion at 60 uC.
Conditions: Q PV Mo10 40 (0.01 mmol), DBT 147 mg (0. 8 mmol) in
(
5
2
O
decalin, MeCN 50 ml, isobutyraldehyde 0.72 ml (8 mmol), reaction
temperature 60 uC.)
Then, the DBT was oxidized to sulfoxide and sulfone by
isobutyl peracid on the emulsion droplets:
composed of MeCN, decalin and amphiphilic catalyst
18 3 3 5 2
[C H37N(CH ) ] [PV Mo10O40], could be formed under the
conditions used, and the volume of emulsion was increased with
increase of the volume of MeCN within a certain range.
Three kinds of catalysts with different quaternary ammonium
cations were synthesized to investigate the effect of the amphiphilic
catalyst on the property and stability of the emulsion system. Fig. 2
shows the removal of DBT vs. reaction time ith three kinds of
catalyst. Before reaction, about 40% DBT was transferred into the
emulsion droplets (MeCN phase) from the decalin phase. During
the process of oxidative reaction, the catalyst [C H N(CH ) ] -
The amount of MeCN has a strong influence on the
emulsion system. The ratio between MeCN and model diesel
was tuned to optimize a suitable emulsion condition (see ESI{).
DBT is not oxidized to sulfoxide or sulfone without MeCN;
the conversion of DBT is increased with increasing volume of
MeCN in emulsion, so the volume of emulsion was increased by
18
37
3 3 5
6
[
PV Mo O ] exhibits more activity than the catalyst
2
reducing the ratio of oil to water (MeCN) in a certain range. In
10 40
[
C
12
H
25N(CH
and efficiently removed from decalin phase in 4 h with the catalyst
37N(CH [PV 40]. When the oxidation reaction was
3
)
3
]
5
[PV
2
Mo10
O
40]. DBT was oxidized completely
addition, the sulfones can be separated directly from diesel to
MeCN, and the oxidation and extraction can be finished
simultaneously.
[C
18
H
3
)
3
]
5
2
Mo10O
over, all DBT in MeCN and decalin had oxidized into DBT’s
sulfone, detected by GC-FPD. However, the catalyst
10 40
In summary, the catalyst [C H N(CH ) ] PV Mo O
1
8
37
3 3 5
2
assembled in emulsion droplets can oxidize DBT present in
decalin to sulfoxide and sulfone using molecular oxygen as the
oxidant and isobutyl aldehydes as the sacrificial agent, under mild
conditions. The sulfones can be removed directly from diesel to
MeCN and the oxidation and extraction can proceed simulta-
neously. These results demonstrate that the desulfurization of
diesel oils via an oxidative desulfurization process with molecular
oxygen as the oxidant is a possible process for ultra-deep
desulfurization.
37 2 3 2 5 2
[(C18H ] N(CH ) ] PV Mo10O40 shows low reaction activity,
and after reaction the removal of DBT from model diesel was
only 70%, even for 8 h.
The different catalytic performances of the three catalysts
indicate that the quaternary ammonium cations in the catalysts
play a vital role in the formation of the emulsion system. As is
3 3 5 2
shown in Fig. 3, the catalyst [C12H25N(CH ) ] [PV Mo10O40] with
a shorter carbon chain has difficulty forming an emulsion,
resulting in low conversion of DBT. The catalyst
The authors are grateful for financial supported by the National
Science Foundations of China (20503031 and 20673114).
[C H N(CH ) ] [PV Mo O ] with a longer chain has high
18 37 3 3 5 2 10 40
3 3 5 2 3 3 5 2
Fig. 3 Optical micrographs of the three emulsion systems. a: [C12H25N(CH ) ] [PV Mo10O40]; b: [C18H37N(CH ) ] [PV Mo10O40]; c:
[
(C18 N(CH [PV Mo10 40].
H
37
]
2
3
)
2
]
5
2
O
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1
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52 | Chem. Commun., 2007, 150–152
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