Oxidative desulfurization of diesel oil with hydrogen peroxide in the presence of acid catalyst in diesel oil/acetic acid biphasic system

Article abstract of DOI:10.1246/cl.2004.1306

Dibenzothiophenes are oxidized effectively with H₂O₂ in the presence of acid catalysts in a tetradecane/AcOH biphasic system. The oxidation proceeds in the AcOH phase and most of the oxidation products stay there, resulting in the successive removal of the dibenzothiophenes from the tetradecane phase. Using this biphasic oxidation system can effectively reduce the sulfur content in diesel oil.

Full text of DOI:10.1246/cl.2004.1306

1306
Chemistry Letters Vol.33, No.10 (2004)
Oxidative Desulfurization of Diesel Oil with Hydrogen Peroxide in the Presence
of Acid Catalyst in Diesel Oil/Acetic Acid Biphasic System
Kazumasa Yazu,Ã Mitsunori Makino, and Koji Ukegawa
Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology,
Tsukuba, Ibaraki 305-8569
(Received July 21, 2004; CL-040859)
Dibenzothiophenes are oxidized effectively with H2O2 in
slowly oxidized by peracetic acid (AcOOH) formed from
H2O2 and AcOH. Recently, the addition of a strong acid, e.g.
H2SO4, in the biphasic system has been found to accelerate
the oxidation of dibenzothiophenes. Here we report a new oil/
AcOH biphasic oxidation system of dibenzothiophenes using
H2O2 and strong acids as an oxidant and acid catalysts, respec-
tively. This biphasic oxidation was applied to the oxidative
desulfurization of diesel oil.
the presence of acid catalysts in a tetradecane/AcOH biphasic
system. The oxidation proceeds in the AcOH phase and most
of the oxidation products stay there, resulting in the successive
removal of the dibenzothiophenes from the tetradecane phase.
Using this biphasic oxidation system can effectively reduce the
sulfur content in diesel oil.
Figure 1a shows the oxidation of dibenzothiophene (1a) and
4,6-dimethyldibenzothiophene (1b) with H2O2 in AcOH in the
presence or absence of acid catalysts. For a typical run, 50 mL
of 5 mM 1a in AcOH containing 10 mM H2SO4 (1 mequiv.)
was heated to 50 ꢀC with stirring, and then 0.5 mL of 35 wt %
H2O2 (5.8 mmol) aqueous solution was added. Concentrations
of the substrate and the oxidation products were determined by
HPLC as reported previously.9 Major products are the corre-
sponding sulfoxides and sulfones (Scheme 1). The addition of
H2SO4 promotes the oxidations of 1a and 1b, since the rate of
AcOOH formation can be accelerated by the addition of a strong
acid.10 In addition, 1b underwent faster oxidation than 1a under
the same conditions, suggesting that 1b would be more oxidiza-
ble by AcOOH than 1a. The oxidation of 1b by using H2O2–for-
mic acid3 and H2O2–trifluoroacetic acid11 has been reported to
proceed faster than that of 1a. The oxidation rates increase with
increasing reaction temperature and H2SO4 concentration, possi-
bly owing to the increase of the rates of AcOOH formation.
Furthermore, a strong acid cation exchange resin, Amberlyst
15DRY (Organo Co., Ltd., 3.0 meq/g-dry resin), is applicable
as an acid catalyst, although the catalytic activity is lower
than that of H2SO4 under similar conditions (Figure 1a). Ion
exchange resins also promote the formation of AcOOH.12
Tetradecane solutions of 1a and 1b were used as model
diesel oils. Tetradecane and AcOH are immiscible under the
Air pollution due to diesel exhaust gas has been a major
concern of the public. It is necessary to decrease NOx and par-
ticulates in the exhaust gas in order to reduce the sulfur content
in diesel oil since the sulfur causes the damage to exhaust after-
treatment devices (e.g., exhaust catalysts and diesel particulate
filters). Thus new regulations in Japan and Europe will limit
the sulfur content to 10 ppm maximum from 2007 and 2009, re-
spectively. The US EPA will also regulate the sulfur in on-road
diesel oil to below 15 ppm from mid-2006. The development of
a novel desulfurization process, therefore, has been expected to
remove alkylated dibenzothiophenes from diesel oil since they
are resistant to conventional hydrodesulfurization because of
their steric hindrance.1,2 Oxidative desulfurization would be
the most likely candidate since the dibenzothiophenes in fuel
oil have been effectively oxidized by using H2O2–formic acid,3
H2O2–Ti-containing molecular sieve,4 H2O2–tungstophosphoric
acid (TPA)–ultrasonication,5 t-BuOOH-Mo/Al2O3,6 and O2–Co
salts–aldehydes,7 and the oxidation products would be easily
removed from the oil by extraction and adsorption. We have
demonstrated that dibenzothiophenes are effectively oxidized
with H2O2 catalyzed by TPA in an organic biphasic system
using AcOH as a polar solvent and that this oxidation system
is useful for the oxidative desulfurization of diesel oil.8 We have
also observed that even without TPA dibenzothiophenes are
[Oil phase]
(a)
(b)
5
4
3
2
1
0
5
4
3
2
1
0
S
R2
R1
[AcOH phase]
H2O2 + AcOH
acid catal.
AcOOH
0
20
40
60
0
20
40
60
+
S
S
R2
R2
S
Time /min
Time /min
R2
R1
R1
O
O
R1
O
Figure 1. The disappearance of 1a (solid) or 1b (open) during
the oxidation with H2O2 in the presence of an acid catalyst
at 50 ꢀC in (a) AcOH and (b) tetradecane in tetradecane/AcOH
1
2
3
a, R1 = R2 = H; b, R1 = R2 = Me
biphasic system. Acid catalyst: , H2SO4;
15DRY; , none.
, Amberlyst
Scheme 1.
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.10 (2004)
1307
conditions employed; they form a biphasic system. The oxida-
tion of 1a or 1b in a tetradecane/AcOH biphasic system
was done as follows: 50 mL of 5 mM 1a in tetradecane
was mixed with 50 mL of AcOH containing 10 mM H2SO4
(1 mequiv.) This biphasic mixture was heated to 50 ꢀC with stir-
ring, and then 0.5 mL of 35 wt % H2O2 (5.8 mmol) aqueous solu-
tion was added. Concentration of 1a in tetradecane was similarly
determined by HPLC. Figure 1b shows that 1a and 1b are also
consumed by the oxidation in this biphasic system; 45% of 1a
or 34% of 1b is extracted from the tetradecane to the AcOH
and the oxidation of 1a or 1b proceeds in the latter, resulting
in the successive removal of 1a or 1b from the tetradecane phase
(Scheme 1). This oxidation mechanism is similar to those in
other biphasic oxidation systems.8,13 Further, most of the oxida-
tion products resides in the AcOH phase owing to their polarity
(e.g., the oxidation of 1b for 1h gave 2b and 3b in 18 and 82%
yield, respectively, and 97% of 2b and 92% of 3b resided in the
AcOH phase), indicating that most sulfur compounds can be re-
moved from model diesel oils only by separation of the model
oils and the AcOH after oxidation.
This oxidation treatment was applied to diesel oil containing
333 ppm sulfur as follows: 50 mL of diesel oil was mixed with
50 mL of AcOH containing 20 mM H2SO4 or 667 mg of Amber-
lyst 15DRY (each 2 mequiv.) This biphasic mixture was heated
to 60 ꢀC with stirring, and then 0.5 mL of 35 wt % H2O2
(5.8 mmol) aqueous solution was added. After 1 h of oxidation,
the oil phase was separated, washed with water, and dehydrated
with CaCl2. Then its sulfur content was measured using a Horiba
SLFA-UV21 analyzer. It is insufficient for desulfurization of
diesel oil to simply extract sulfur compounds with AcOH (entry
4, Table 1). The oxidation with H2O2 in the presence of H2SO4
and Amberlyst 15DRY, however, can reduce the sulfur contents
to 63 and 64 ppm, respectively (entries 1 and 2, Table 1). The
complete oxidation of the sulfur compounds has been confirmed
by using sulfur-specific GC analysis as reported previously:8 the
sulfur compounds in an unoxidized oil disappeared owing to the
oxidation and they were replaced by new sulfur-containing prod-
ucts having higher boiling points in an oxidized oil (Figure 2).
Thus the sulfur contents can be decreased to below 10 ppm by
extraction with an equal volume of MeOH two times. The oxida-
tion without acid catalysts also lowers the sulfur content to
85 ppm (entry 3, Table 1), but the oxidation of the sulfur com-
pounds is incomplete since unoxidized sulfur compounds remain
(Figure 2). The sulfur content is consequently lowered only to
about 40 ppm by the same extraction mentioned above. These re-
sults have indicated that the addition of acid catalysts is effective
Figure 2. Sulfur-specific GC of (a) unoxidized, (b) oxidized in
the presence of H2SO4, and (c) oxidized diesel oil in the absence
of H2SO4.
for the complete oxidation of sulfur compounds in diesel oils
and that the complete oxidation is essential for an oxidative
desulfurization. Furthermore the availability of a heterogeneous
acid catalyst would provide some advantages, e.g., ease of
recovery.
In conclusion, dibenzothiophenes are effectively oxidized
with H2O2 in the presence of acid catalysts in an oil/AcOH bi-
phasic system. This oxidation system is useful for the oxidative
desulfurization of diesel oil.
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7
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9
Table 1. Oxidative desulfurization of diesel oila for 1 h at 50 ꢀC
Sulfur Content / ppm
35%H2O2
/ mL
Acid
Entry
After Extraction with MeOH
After
Oxidation
Catalyst
One time
18
Two times
6
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1
2
0.5
0.5
H2SO4
Amberlyst
15DRY
—
63
64
16
6
3
4
0.5
0
85
284
55
267
36
244
—
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aInitial sulfur content of diesel oil was 333 ppm.
Published on the web (Advance View) September 11, 2004; DOI 10.1246/cl.2004.1306