Oxidative desulfurization of coal tar pitch using a urea-hydrogen peroxide complex/carboxylic anhydride system in THF

Article abstract of DOI:10.1246/cl.140902

Oxidative desulfurization of coal tar pitch (CTP) using a urea-hydrogen peroxide complex with a carboxylic anhydride in THF was investigated. The sulfur content in CTP was reduced by more than 15% through the process.

Full text of DOI:10.1246/cl.140902

Received: September 26, 2014 | Accepted: November 9, 2014 | Web Released: November 22, 2014
CL-140902
Oxidative Desulfurization of Coal Tar Pitch Using a Urea-Hydrogen Peroxide
Complex/Carboxylic Anhydride System in THF
Kazumasa Yazu* and Toshimasa Takanohashi
Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology,
Tsukuba, Ibaraki 305-8569
(E-mail: yazu-k@aist.go.jp)
UHP/PA
Oxidative desulfurization of coal tar pitch (CTP) using a
+
urea-hydrogen peroxide complex with a carboxylic anhydride in
THF, 313K
THF was investigated. The sulfur content in CTP was reduced
by more than 15% through the process.
S
S
S
O
O
O
DBT
Scheme 1.
Coal tar pitch (CTP) is the residue left by the distillation of
coal tar, which is a by-product of coal when it is carbonized to
make coke. CTP is a widely used precursor for many carbon
materials such as graphite electrodes1,2 and carbon fibers.3,4
Recently, the use of high-sulfur coal has become increasingly
prevalent, and this trend will result in the increase of the sulfur
content in CTP. For the effective use of high-sulfur CTP, it is
necessary to reduce the levels of sulfur. However, it is difficult
to remove organosulfur compounds in CTP with conventional
hydrodesulfurization, since CTP is chiefly composed of aromatic
compounds, which would be susceptible to reduction under the
reaction conditions. Reducing these aromatics would increase the
processing costs by consuming additional hydrogen and further
change the characteristics of the CTP. Therefore, the develop-
ment of a novel desulfurization process for CTP is essential.
Oxidative desulfurization would be a potential candidate since
it does not require hydrogen. In this process, organosulfur
compounds would be oxidized to more polar oxidation products,
such as sulfoxides and sulfones, and could be removed by
extraction or adsorption.5-11 However, there have been few
reports of the oxidative desulfurization of CTP. Jin et al. reported
the oxidative desulfurization of CTP with hydrogen peroxide,
catalyzed by trichloroacetic acid and ultrasonic waves: the sulfur
content was reduced by more than 90%, although a significant
increase in chloride content was observed.12
It is known that the system comprising the urea-hydrogen
peroxide complex (UHP) with carboxylic anhydrides acts as an
oxidizing agent for organosulfur compounds in MeCN.13,14 We
found that this system can also oxidize organosulfur compounds
in THF. This would be useful for the oxidative treatment of CTP,
which is soluble in THF but not in MeCN. In addition, the
oxidation system would be favorable because the reaction can
be carried out under anhydrous conditions, thus avoiding a
reduction in the solubility of CTP due to the presence of water.
Here, we report the oxidative desulfurization of CTP using the
UHP/carboxylic anhydride system in THF.
5
4
3
2
1
0
0
6
12
18
24
Time /h
Figure 1. The disappearance of 5 mM DBT in MeCN ( ) or
THF ( ) during the oxidation with 40 mM UHP and 80 mM PA
at 313 K.
sulfone (Scheme 1). Oxidations of 5 mM DBT in MeCN at
313 K were carried out with 20 mM UHP and 80 mM of a
carboxylic anhydride such as PA, maleic anhydride (MA),
succinic anhydride (SA), glutaric anhydride (GA), and acetic
anhydride (AA). The activities decrease in the order PA >
MA > SA > GA > AA. PA and MA had the highest activities
and hence were employed in further reactions. Figure 1 shows
the oxidation of DBT with UHP and PA in MeCN or THF. The
oxidation proceeded in THF as well as in MeCN, although the
oxidation rate in THF was slower. The oxidation in THF was
promoted by increasing the reaction temperature. Similar results
were obtained using MA. These results suggested that the UHP/
carboxylic anhydride system would be an effective oxidant for
the oxidative desulfurization of CTP.
The oxidation system was applied to the oxidative desul-
furization of CTP containing 0.35 wt % sulfur. A solution of
CTP (1 g) and UHP (2 mmol) in THF (50 mL) was warmed to
313 K with stirring and then PA (4 mmol) was added. The ratio
of UHP to sulfur in CTP is about 20 times greater than the
stoichiometric amount. The mixture was stirred at 313 K for
Dibenzothiophene (DBT) was used as a model organosulfur
compound, since most of the organosulfur compounds in CTP
would be polycyclic aromatic sulfur heterocycles.15 For a typical
run, a solution of DBT (0.25 mmol) and UHP (2 mmol) in THF
(50 mL) was warmed to 313 K with stirring, and then phthalic
anhydride (PA; 4 mmol) was added. The disappearance of DBT
was monitored using a previously reported HPLC method.7 The
major oxidation products were the corresponding sulfoxide and
Chem. Lett. 2015, 44, 169–170 | doi:10.1246/cl.140902
© 2015 The Chemical Society of Japan | 169

Table 1. Oxidative desulfurization of CTP at 313 K for 24 ha
Carboxylic anhydride
/mmol
Sulfur removalc
/%
Element/wt %
UHP
/mmol
Entry
PA
MA
S
C
N
H
Total
1
2
3
4b
2
2
2
®
4
®
4
®
®
0.28
0.28
0.34
0.35
90.6
89.0
91.5
93.2
0.90
0.82
0.88
0.99
4.34
4.23
4.60
4.17
96.1
94.3
97.3
98.7
18
16
1
®
®
®
®
b
c
aReaction conditions: CTP, 1 g; THF, 50 mL. CTP. Calculated based on the change in the S/C ratio.
Their removal from the oxidized petroleum fuels is rather
facile.5-11,16,17 However, it would be much more difficult to
remove the oxidized organosulfur compounds from the oxidized
CTP by extraction and adsorption due to their polarities. The
oxidized CTP, as well as CTP itself, is sufficiently soluble only
in polar solvents such as THF, pyridine, and 1-methyl-2-
pyrrolidinone.18 Additional investigations of the separation of
the oxidized organosulfur compounds from the oxidized CTP
are in progress.
References
Figure 2. IR spectra of CTP and oxidized CTP.
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170 | Chem. Lett. 2015, 44, 169–170 | doi:10.1246/cl.140902
© 2015 The Chemical Society of Japan