Tungstophosphoric Acid-catalyzed Oxidative Desulfurization of Light Oil with Hydrogen Peroxide in a Light Oil/Acetic Acid Biphasic System

Article abstract of DOI:10.1246/cl.2003.920

Dibenzothiophenes were oxidized effectively with H₂O ₂ in the presence of 12-tungstophosphoric acid in the tetradecane/ AcOH biphasic system to give their corresponding sulfones as the major products. The oxidation proceeded in the AcOH phase and most of the sulfones distributed there, resulting in the successive removal of the sulfur compounds from the tetradecane phase. This biphasic oxidation system can effectively reduce the sulfur content in light oil.

Full text of DOI:10.1246/cl.2003.920

920
Chemistry Letters Vol.32, No.10 (2003)
Tungstophosphoric Acid-catalyzed Oxidative Desulfurization of Light Oil
with Hydrogen Peroxide in a Light Oil/Acetic Acid Biphasic System
Kazumasa Yazu,ꢀ Takeshi Furuya, Keiji Miki, and Koji Ukegawa
Institute for Energy Utilization, National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8569
(Received July 7, 2003; CL-030605)
Dibenzothiophenes were oxidized effectively with H2O2 in
the presence of 12-tungstophosphoric acid in the tetradecane/
AcOH biphasic system to give their corresponding sulfones as
the major products. The oxidation proceeded in the AcOH phase
and most of the sulfones distributed there, resulting in the suc-
cessive removal of the sulfur compounds from the tetradecane
phase. This biphasic oxidation system can effectively reduce
the sulfur content in light oil.
In order to decrease NOx and particulates in diesel exhaust
emissions it is necessary to reduce the sulfur content in light oil,
and hence new legislation in Japan and Europe will limit the
sulfur content to 50 ppm maximum by 2005. The development
of a new desulfurization process has been expected to remove
alkylated dibenzothiophenes from light oil since they are resist-
ant to hydrodesulfurization because of their steric hindrance.1,2
We have demonstrated that the sulfur content in light oil can be
effectively reduced by oxidative desulfurization in the light oil/
MeCN biphasic system using H2O2 and 12-tungstophosphoric
acid (TPA) as oxidant and catalyst, respectively.3 We have
found that AcOH is applicable to this biphasic oxidation system
instead of MeCN. AcOH would be a more desirable solvent
than MeCN since it is nitrogen-free. Here we report a new bi-
phasic oxidative desulfurization of light oil using AcOH as a
polar solvent. Dibenzothiophenes were oxidized to the corre-
sponding sulfones with H2O2 and TPA in the tetradecane/
AcOH biphasic system. Furthermore, sulfur content in light
oil was efficiently reduced by this biphasic oxidation system.
Dibenzothiophene (1a) and 4,6-dimethyldibenzothiophene
(1b), one of the most unreactive sulfur compounds during hy-
drodesulfurization, were rapidly oxidized by H2O2 in the pres-
ence of TPA in AcOH to give the corresponding sulfones as the
major products (Scheme 1). Conversion of the substrate and
Figure 1. Pseudo-first-order plot for the oxidation of 1a (sol-
id) or 1b (open) with H2O2 and TPA at 40 ꢂC in AcOH or
ꢄ
MeCN. l, : in AcOH; n, Ã: in MeCN.
yields of the oxidation products were determined by HPLC as
reported previously.4 The rates of oxidation both in AcOH
and in MeCN were pseudo-first-order in substrate concentra-
tion, C (Figure 1). The rate constants, k, were obtained from
the equation, lnðC=C0Þ ¼ ꢁkt, where C0 is the initial concentra-
tion of a substrate. The k values for 1a and 1b were almost iden-
tical in each solvent, indicating 1a and 1b have essentially the
same oxidizability in the oxidation by H2O2 with TPA. For in-
stance, those for the oxidation of 1a (0.25 mmol) and 1b
(0.25 mmol) by 35 wt % H2O2 aqueous solution (0.5 mL,
5.8 mmol H2O2) with TPA (0.25 mmol) at 40 ꢂC in AcOH
(50 mL) were 2.2 and 2:3 ꢃ 10ꢁ3 sꢁ1, respectively, and those
in MeCN (50 mL) were 4.7 and 5:4 ꢃ 10ꢁ5 sꢁ1, respectively;
the oxidizabilities of 1a and 1b in AcOH were about 50-fold
compared with those in MeCN. Under similar conditions, both
substrates were completely consumed within 10 min with TPA
(2.5 mmol) in AcOH. They were also oxidized without TPA
due to the formation of peracetic acid by the reaction of AcOH
with H2O2. In this case 1b was oxidized faster than 1a in a sim-
ilar manner, as reported in the oxidation by H2O2 and trifu-
luoroacetic acid.5 The oxidation rates of 1a and 1b, however,
were much smaller than those in TPA-catalyzed oxidations;
the extent of conversion of 1a and 1b after 10 min oxidation
was only 1 and 3%, respectively. These results indicated that
the oxidation by peracetic acid should have little contribution
to the overall oxidation under our experimental conditions.
AcOH and tetradecane form a biphasic system since they
are immiscible. The oxidation of 1a and 1b with H2O2 and
TPA also proceeded in the tetradecane/AcOH biphasic system.
In a typical run, TPA (2.5 mmol) was dissolved in 35 wt % H2O2
Scheme 1.
Copyright ꢀ 2003 The Chemical Society of Japan

Chemistry Letters Vol.32, No.10 (2003)
921
Figure 3. Sulfur-specific GC of (A) untreated and (B) treated
light oil. 1b, 4,6-Dimethyldibenzothiophene; 2b, 4,6-dime-
thyldibenzothiophene 5,5-dioxide.
ꢄ
Figure 2. The disappearance of 1a (l) or 1b ( ) in tetrade-
cane during the oxidation with H2O2 and TPA at 40 ꢂC in tet-
radecane/AcOH biphasic system.
of sulfur-free aromatics from the AcOH phase. Figure 3 shows
the sulfur-specific GC of untreated and treated light oil. Sulfur-
specific GC analysis was carried out using a DB-5MS capillary
column (J&W Scientific) on a Shimadzu GC-2010 with a flame
photometric detector. The major peaks in the GC of untreated
and treated oil were identified as 1b and 2b, respectively, by
comparison of their retention times. Sulfur compounds in un-
treated oil disappeared by the oxidation, and they were replaced
by new sulfur-containing products having higher boiling points
in treated oil; these products should be the oxidized ones of the
sulfur compounds in untreated oil. Figure 3 also shows small
amounts of oxidized sulfur products remained in treated oil, in-
dicating that additional refining is necessary for further reduc-
tion of the sulfur content. For example, the sulfur content in
the treated oil decreased to 17 and 2ppm by extraction with
an equal volume of MeOH one and three times, respectively.
These results indicated that oxidized sulfur compounds can be
readily removed from light oil.
aqueous solution (0.5 mL, 5.8 mmol H2O2) and mixed with
AcOH (50 mL). Then, tetradecane solution (50 mL) of 1a
(0.25 mmol) was added and this biphasic mixture was heated
to 40 ꢂC with stirring. Concentration of 1a in tetradecane was
determined by HPLC. Figure 2shows that 1a and 1b were
smoothly removed from tetradecane phase by the oxidation.
At a reaction time of zero, 45% of 1a or 34% of 1b was extract-
ed from tetradecane with AcOH, and the oxidation proceeded in
AcOH (Scheme 1) in the same way as in the biphasic one using
MeCN.3 Most oxidation products, 2a (97%) and 2b (92%), dis-
tributed in the AcOH phase, resulting in the successive removal
of the sulfur compounds from the tetradecane phase. According-
ly, most of the oxidized sulfur compounds can be eliminated
from tetradecane only by separating the two phases. In the tet-
radecane/MeCN system, 52% of 1a or 31% of 1b distributed in
MeCN; the extractability of 1b with AcOH is a little higher than
that with MeCN, in contrast to that of 1a. The rates of removal
of 1a and 1b in the tetradecane/AcOH system were larger than
those in the tetradecane/MeCN one as expected; e.g., 1b was
completely consumed within 40 min in the former (Figure 2),
while the conversion was incomplete (ca. 57%) after 1 h oxida-
tion in the latter. These results suggested that AcOH would be a
more favorable polar solvent than MeCN.
In conclusion, we have demonstrated effective oxidation of
dibenzothiophenes with H2O2 and TPA in an organic biphasic
system using AcOH as a polar solvent. This oxidation system
was found to be effective for the oxidative desulfurization of
light oil. We are currently undertaking a more detailed investi-
gation to organize an overall process.
This oxidation process was applied to light oil containing
318 ppm sulfur. Light oil (50 mL) was treated for 1 h at
40 ꢂC in the acetic acid (50 mL) biphasic system with 35 wt %
H2O2 aqueous solution (0.5 mL, 5.8 mmol H2O2) and TPA
(2.5 mmol). The oil phase was separated, washed with water,
and dehydrated with CaCl2. Then its sulfur content was meas-
ured using a Horiba SLFA-UV21 analyzer. The sulfur content
in the treated oil was effectively reduced to 58 ppm, while that
without H2O2 and TPA was decreased to only 273 ppm; thus,
extraction alone is not sufficient for desulfurization. MeCN ex-
tracts aromatics as well as sulfur-containing compounds from
light oil in a light oil/MeCN system.6,7 The light oil used con-
tained 21 vol% aromatics and AcOH also extracted the aromat-
ics in this biphasic system, indicating the need of the recovery
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Published on the web (Advance View) September 8, 2003; DOI 10.1246/cl.2003.920