Aerobic oxidative desulfurization of benzothiophene, dibenzothiophene and 4,6-dimethyldibenzothiophene using an Anderson-type catalyst [(C 18H37)2N(CH3)2] 5[IMo6O24]

Article abstract of DOI:10.1039/c0gc00271b

Benzothiophene (BT), dibenzothiophene (DBT) and 4,6- dimethyldibenzothiophene (4,6-DMDBT) are oxidized to their corresponding sulfones by an Anderson-type catalyst [(C₁₈H₃₇) ₂N(CH₃)₂]₅IMo₆O ₂₄ using molecular oxygen as the oxidant under mild reaction conditions. These refractory sulfur-containing compounds can be oxidized completely in the absence of any sacrificial agent. Solvents such as acetonitrile and water play a negative effect on the oxidative desulfurization system. The catalytic activities of the amphiphilic Anderson catalysts depend on the quaternary ammonium cations. The reactivity of the sulfur-containing compounds follows the order 4,6-DMDBT > DBT > BT. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/c0gc00271b

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www.rsc.org/greenchem | Green Chemistry
Aerobic oxidative desulfurization of benzothiophene, dibenzothiophene and
4,6-dimethyldibenzothiophene using an Anderson-type catalyst
[(C18H37)2N(CH3)2]5[IMo6O24]
Hongying Lu¨,*a Yongna Zhang,b Zongxuan Jiangb and Can Li*b
Received 28th June 2010, Accepted 26th August 2010
DOI: 10.1039/c0gc00271b
Benzothiophene (BT), dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene
(4,6-DMDBT) are oxidized to their corresponding sulfones by an Anderson-type catalyst
[(C18H37)2N(CH3)2]5IMo6O24 using molecular oxygen as the oxidant under mild reaction
conditions. These refractory sulfur-containing compounds can be oxidized completely in the
absence of any sacrificial agent. Solvents such as acetonitrile and water play a negative effect on
the oxidative desulfurization system. The catalytic activities of the amphiphilic Anderson catalysts
depend on the quaternary ammonium cations. The reactivity of the sulfur-containing compounds
follows the order 4,6-DMDBT > DBT > BT.
as the oxidizing agent under mild conditions has long been
Introduction
desired due to its low cost and green chemistry advantages.16
Sulfur in transportation fuels, particularly in gasoline and
Nevertheless, it is difficult to oxidize these refractory sulfur-
diesel, is a major source of air pollution. Hydrodesulfurization
containing compounds present in diesel using molecular oxygen
(HDS) is highly efficient in removing thiols, sulfides and
as the oxidant under mild conditions. The oxidation of these
disulfides from fuels, however is less effective for refractory
refractory compounds by molecular oxygen has been achieved
sulfur-containing compounds, such as dibenzothiophene (DBT)
at high temperatures or in the presence of a sacrificial agent in a
and its derivatives.1 Developing non-HDS technologies for the
few reports.17–19 On the other hand, molecular oxygen takes part
production of clean diesel with extremely low concentrations
in preferably non-selective radical reactions, which results in
of sulfur-containing compounds is still a challenge for both
the autooxidation of a large quantity of hydrocarbons present
academia and industry.2
One of the most promising alternative processes is the
in diesel at high temperatures.20 Another disadvantage is that
using a large amount of sacrificial agent increases the operating
oxidative desulfurization (ODS) process, which includes the
cost and leads to difficulties in separation.
oxidation of sulfur-containing compounds to corresponding
Therefore, selective oxidation of refractory sulfur-containing
sulfones and their removal from diesel by extraction or ad-
compounds using molecular oxygen as the oxidant under mild
sorption. The ODS process offers several advantages over
conditions is still a highly challenging subject. Here, we report
the HDS process. For example, refractory compounds, pre-
that an Anderson-type catalyst, [(C18H37)2N(CH3)2]5IMo6O24,
dominantly 4,6-dibenzothiophenes (4,6-DMDBT), can be ox-
can oxidize these refractory sulfur-containing compounds
idized under mild conditions (ambient temperature and pres-
present in diesel to their corresponding sulfones without any
sure) and are very difficult to remove through conventional
sacrificial agent using molecular oxygen as the oxidant under
HDS.3–6 Up to now, many types of oxidative systems have
mild conditions.
been attempted, such as H2O2/inorganic acids,7 H2O2/organic
acids,8,9 H2O2/heteropolyacid,10 H2O2/Ti-containing zeolites,11
H2O2/ionic liquid12,13 and other non-hydrogen peroxide systems
Results and discussion
(e.g., t-butyl hydroperoxide, etc.).14,15
Although catalytic ODS of these refractory sulfur-containing
compounds can show high activity, most of these processes use
hydrogen peroxide as the oxidant. The oxidation of these re-
fractory sulfur-containing compounds using molecular oxygen
Structure of the Anderson-type polyoxometalate catalyst
Polyoxoanions, such as Keggin-type polyoxometalates, have
been utilized widely as catalysts for both homogeneous and het-
erogeneous reactions.21,22 However, Anderson-type compounds
have not received much attention as catalysts.23–25 This is the first
time that this kind catalyst has been utilized for the selective
aerobic oxidation of sulfur-containing compounds present in
diesel. As shown in Fig. 1, the Anderson structure contains an
IO6 octahedron that is surrounding by six MoO6 groups. The
six Mo atoms form hexagons around the heteroatom, resulting
in a structure with an approximate D3d symmetry structure. The
Anderson structure may be described as an isopolyoxometalate
aScience and Engineering College of Chemistry and Biology, Yantai
University, 32 Qingquan Road, Yantai, 264005, China.
E-mail: lhrye@yahoo.com.cn
bState 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
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Fig. 1 The Anderson-type [IMo6O24]5- polyoxometalate.
Fig. 2 (a) Sulfur-specific GC-FPD chromatograms of the oxidation
of DBT in decalin and (b) spectroscopic characterization of pro-
duction (DBTO2) after the aerobic oxidation of DBT. Conditions:
[(C18H37)2N(CH3)2]5[IMo6O24] (0.01 mmol), DBT (S: 500 ppm) in 50 mL
decalin, reaction temperature 80 ◦C, oxidant O2 (1 atm).
containing a crown of six octahedrons sharing an edge. In the
Anderson structure, the oxygen atoms are classified into three
groups, and the positions of these types are depicted in Fig. 1. Oa
(I–Oa–Mo2) is a bridge oxygen between the I octahedron and two
Mo groups, Ob (Mo–Ob–Mo) is another bridge oxygen between
two edge-sharing Mo groups and Ot is the terminal oxygen that
is bonded to each group.
Effect of solvents on the aerobic oxidation of DBT
The solvent effect was also investigated for the oxidation of DBT
using O2 as the oxidant. As shown in Table 1, DBT can be oxi-
dized completely within 8 h using [(C18H37)2N(CH3)2]5[IMo6O24]
as the catalyst. When acetonitrile was used, the activity
of [(C18H37)2N(CH3)2]5[IMo6O24] decreased sharply, and only
23% conversion of DBT could be obtained after 10 h. A
similar tendency was also observed for the combination of
Na5[IMo6O24] and surfactant. If water was added to the
reaction system, the conversion of DBT was less than 5%
over [(C18H37)2N(CH3)2]5[IMo6O24] after 10 h. This does not
agree with our previous work in the emulsion system,19 where
acetonitrile was considered to be one of the most suitable
solvents for ODS, probably owing to a different reaction
mechanism. Intuitively, there is a strong interaction between the
Anderson-type catalyst and acetonitrile or water, which occupies
lots of the active sites of this polyoxometalate, leading to a
decreased conversion of DBT.
The aerobic oxidation of DBT using different Anderson species
The catalytic activities of Anderson catalysts for the desulfu-
rization of DBT using molecular oxygen the as oxidant are
listed in Table 1. DBT is not oxidized at all without the catalyst.
Also, the DBT is hardly oxidized when using Na5[IMo6O24] as
the catalyst without any surfactant because of the insolubility
of the catalyst in the reaction system and the mass-diffusion
limitations of the liquid–solid–gas three-phase system. It is
worthwhile to note that the conversion of DBT can reach to
84% with the addition of the surfactant octadecyl trimethyl
ammonium chloride (OTAC), suggesting that the surfactant
plays an important role in this reaction. When the amphiphilic
catalyst [(C18H37)N(CH3)3]5[IMo6O24] was used, the conversion
of DBT increased to 100% in 10 h, which is a little higher than
the combination of Na5[IMo6O24] and surfactant. This result
indicates that the amphiphilic catalyst is very active for the
oxidation of DBT using O2 as the oxidant. The oxidation of DBT
was further confirmed by sulfur-specific gas chromatography
(GC) analyses and the IR spectrum of the product after the
aerobic oxidation of DBT (the infrared absorptions at 1165 and
1288 cm-1 are attributed to sulfones8), as shown in Fig. 2.
Effect of the quaternary ammonium cations on the aerobic
oxidation of DBT
As is known, the catalytic activity of the amphiphilic cat-
alysts depends on the quaternary ammonium cation and
polyoxometalate anion. Three kinds of catalysts with different
quaternary ammonium cations were synthesized in order to
investigate the effect of the quaternary ammonium cations
Table 1 Oxidation of DBT by molecular oxygen with Anderson-type catalystsa
Entry
Catalyst
OTAC/mmol
Solvent/mL
Time/h
Conversion of DBT (%)
TOF/h-1
1
2
3
4
5
6
7
8
9
—
0
0
0.05
0
0
0
0.05
0
0
0
0
0
0
0
0
10
24
10
10
10
8
10
10
10
0
<5
84
—
—
—
1.8
3.7
12.8
—
—
—
Na5IMo6O24
Na5IMo6O24
Q1 IMo6O24
74
5
Q2 IMo6O24
100
100
14
23
<5
5
Q3 IMo6O24
5
Na5IMo6O24
CH3CN (50)
CH3CN (50)
H2O (5)
Q3 IMo6O24
5
Q3 IMo6O24
5
a The conversion of DBT was calculated as follows: Q1: dodecyl trimethyl ammonium chloride (DTAC), Q2: octadecyl trimethyl ammonium chloride
(OTAC), Q3: dioctadecyl dimethyl ammonium chloride (DODMAC). Conditions: catalyst (0.01 mmol), DBT (147 mg, 0.8 mmol) in 50 mL decalin,
reaction temperature 80 ◦C and O2 (1 atm).
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on the performance of the amphiphilic catalysts (Fig. 3).
From the TOF of the three catalysts, the reaction activity
decreases in the order [(C18H37)2N(CH3)2]5[IMo6O24] > [(C18H37)
N(CH3)3]5[IMo6O24] > [(C12H25)N(CH3)3]5[IMo6O24], as shown
in Table 1. [(C18H37)2N(CH3)2]5[IMo6O24], with two C18 carbon
chains, exhibits the highest activity among the three catalysts
investigated. The catalyst [(C12H25)N(CH3)3]5[IMo6O24], with the
shortest carbon chain, exhibits the lowest DBT conversion.
This may be ascribed to the effect of the nature of the
quaternary ammonium cation, as surfactants were believed to
activate oxygen in early literature.26–28 The charge of the central
atom is increased by electrons in-flowing from electron donor
substituents, which has a great effect on the catalytic activity of
the amphiphilic catalysts.26 Detailed investigations on this result
are in progress.
Fig. 4 The conversion of sulfur-containing compounds vs. reaction
time at 80 ◦C. Conditions: [(C18H37)2N(CH3)2]5[IMo6O24] (0.01 mmol),
sulfur-containing compound (S: 500 ppm) in 50 mL decalin, reaction
temperature 80 ◦C, oxidant O2 (1 atm).
Fig. 3 The conversion of dibenzothiophene (S: 500 ppm) vs. reaction
time. Conditions: Q5 [IMo6O24] (0.01 mmol), DBT (S: 500 ppm) in 50 mL
decalin, 80 ◦C, oxidant O2 (1 atm).
Fig.
5 Sulfur-specific GC-FPD chromatograms of the oxida-
tion of sulfur-containing compounds in decalin. Conditions:
[(C18H37)2N(CH3)2]5[IMo6O24] (0.01 mmol), sulfur-containing com-
pound (S: 500 ppm) in 50 mL decalin, reaction temperature 80 ◦C,
oxidant O2 (1 atm).
The ODS of different sulfur-containing compounds
The performance of catalyst [(C18H37)2N(CH3)2]5[IMo6O24] for
different sulfur-containing compounds, including benzothio-
phene (BT), DBT and 4,6-dimethyldibenzothiophene (4,6-
DMDBT), was evaluated using molecular oxygen as the oxidant
(Fig. 4). All the sulfur-containing compounds mentioned above
can be oxidized to their corresponding sulfones. The catalytic
activity for the oxidation of sulfur-containing compounds
decreases in the order 4,6-DMDBT > DBT > BT. As calculated
by Kabe et al.,8,9 the electron density on the sulfur atoms
is 5.739 for BT, 5.758 for DBT and 5.760 for 4,6-DMDBT.
The result indicates that the reaction rates of these sulfur-
containing compounds increases with the election density on
the sulfur atom. Therefore, the reactivity trend reflects the
intrinsic properties of the sulfur-containing compound. Gas
chromatography (GC) analyses before and after the aerobic
catalytic oxidation of sulfur-containing compounds in a model
of diesel (decalin) was used to confirm that all the sulfur-
containing compounds had been completely oxidized to sulfones
(Fig. 5).
The activation energy data again showed the following reactivity
order: 4,6-DMDBT > DBT > BT.
Possible mechanism of the aerobic oxidation of sulfur-containing
compounds
The ODS reaction mechanism was also investigated. Fig. 8
shows the UV-vis spectra of [(C18H37)2N(CH3)2]5[IMo6O24] in
decalin with and without the introduction of molecular oxygen;
similar absorption bands at 201 and 254 nm are observed. Upon
the addition of DBT, the band at 254 nm disappears and a
new band appears at 233 nm. These results suggest that the
Anderson-type polyoxometalate is easily coordinated by sulfur-
containing compounds such as DBT, but not with molecular
oxygen. This suggests a proposed mechanism, as shown in
Scheme 1. The active sites of the Anderson-type polyoxometalate
react with DBT, a transfer state is formed, and then the activated
DBT is oxidized to the corresponding sulfone and the catalyst
reduced. Subsequently, the reduced catalyst is recycled in pres-
ence of dioxygen. During the process of the oxidation of sulfur-
containing compounds, the Anderson-type polyoxometalates
are reduced. However, the reduced polyoxometalates are often
The conversion of different sulfur-containing compounds
increased with increasing temperature, as shown in Fig. 6. From
the reaction rates determined at various temperatures, the appar-
ent activation energies for the oxidation of the sulfur-containing
compounds were derived from the Arrhenius equation (Fig. 7).
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Fig. 6 The conversion of sulfur-containing compounds vs. reaction time at different temperature. Conditions: [(C18H37)2N(CH3)2]5[IMo6O24] (0.01
mmol); sulfur-containing compound (S: 500 ppm) in 50 mL decalin; oxidant, O2 (1 atm).
Scheme 1 The proposed mechanism for dioxygen activation and
aerobic ODS.
known to undergo re-oxidation in the presence of molecular
oxygen to complete the catalytic cycle.
Fig. 7 Arrhenius activation energies for sulfur-containing compound
oxidation with Anderson catalyst [(C18H37)2N(CH3)2]5[IMo6O24] using
dioxygen as the oxidant. Conditions: [(C18H37)2N(CH3)2]5[IMo6O24]
(0.01 mmol), sulfur-containing compound (S: 500 ppm) in 50 mL
decalin, reaction temperature 80, 85, 90 and 100 ◦C, oxidant O2 (1 atm).
Conclusion
Benzothiophene, dibenzothiophene and 4,6-dimethyldibenzo-
thiophene can be oxidized to their corresponding sulfones
with an Anderson-type polyoxometalate, [(C18H37)2N(CH3)2]5-
[IMo6O24], as a catalyst under mild conditions. The catalytic
activities of the amphiphilic Anderson catalysts depends on the
nature of the quaternary ammonium cation. It provides a new
possible method for the ODS of diesel with molecular oxygen as
the oxidant.
Experimental
Synthesis of catalysts
All chemicals were used as received. Na5IMo6O24 was prepared
according to procedures described elsewhere.23,29 An aqueous
solution was prepared by dissolving 13.5 g of Na2MoO4·2H2O
in 50 mL water and then heating it to 95 ◦C. 6.2 mL of HCl was
added slowly to the solution, followed by the dropwise addition
of a hot solution of periodic acid (1.759 g in 10 mL water). Half
of the reaction solution was evaporated. Upon cooling, white
platelet type crystals precipitated from the solution.
Q5IMo6O24 was prepared as follows: a 20 mL ethanolic
solution with 5 mmol of quaternary ammonium was added
dropwise into 40 mL of an aqueous solution of Na5IMo6O24
(1 mmol) under stirring at room temperature. A snow-white
precipitate was immediately formed. After continuously stirring
for 4 h, the resulting mixture was filtered and dried at 60 ◦C in a
Fig. 8 UV-vis spectra of [(C18H37)2N(CH3)2]5[IMo6O24]. All spec-
tra were collected from 0.02 mmol L-1 solutions (including
[(C18H37)2N(CH3)2]5 [IMo6O24] and DBT). (a) [(C18H37)2N(CH3)2]5-
[IMo6O24] dissolved in decalin at 80 ◦C for 0.5 h, (b) [(C18H37)2N(CH3)2]5-
[IMo6O24] in decalin after treatment with 1 atm of O2 at 80 ◦C for 4 h,
(c) [(C18H37)2N(CH3)2]5[IMo6O24] in decalin after the addition of DBT
at 80 ◦C for 4 h under highly pure nitrogen and (d) DBT dissolved in
decalin at 80 ◦C for 0.5 h.
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vacuum for 24 h to obtain the catalyst. There were three kinds
of quaternary ammonium, including [(C12H25)N(CH3)3]Cl,
[(C18H37)2N(CH3)2]Cl and [(C18H37)N(CH3)3]Cl.
The 127I chemical shifts were referenced to solid NaI. d = 3092,
3116 ppm, IR: n = 942, 910, 720, 691, 629 cm-1
supported by the SKLC cooperation project (N-08-08) and the
PhD fund of Yaitai University (HY07B33).
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©