Performance evaluation of the carbon nanotubes supported Cs2.5H0.5PW12O40 as efficient and recoverable catalyst for the oxidative removal of dibenzothiophene

Article abstract of DOI:10.1016/j.cattod.2009.10.001

With the issues of increasingly stringent legislations on sulfur level in transportation fuels, clean fuels research including desulfurization has become a more important subject of environmental catalysis studies. Oxidative desulfurization (ODS) combined with extraction is one of the most promising desulfurization processes. With the loading of Cs_2.5H_0.5PW₁₂O₄₀, a new multi-walled carbon nanotube (MWNT) supported catalyst (Cs_2.5H_0.5PW₁₂O₄₀/MWNT) has been developed in this study. Through experimental evaluations, Cs_2.5H_0.5PW₁₂O₄₀/MWNT was found to be very effective for the oxidative removal of DBT, with a desulfurization efficiency of up to 100%. Factors affecting the process, including reaction temperature, O/S molar ratio, initial sulfur content, catalyst dosage, and pre-immersion time of the catalyst in H₂O₂ solution, were evaluated, and the favourable operating conditions were determined. Sulfone species was confirmed by GC-MS analysis to be the only product from DBT oxidation by H₂O₂ in the presence of Cs_2.5H_0.5PW₁₂O₄₀/MWNT after 160 min. Moreover, the new catalyst developed is recoverable. The recovered Cs_2.5H_0.5PW₁₂O₄₀/MWNT demonstrates quite close catalytic activity to that of the fresh. As a whole, Cs_2.5H_0.5PW₁₂O₄₀/MWNT has a high potential for using as an effective catalyst for deep desulfurization of diesel.

Full text of DOI:10.1016/j.cattod.2009.10.001

Catalysis Today 150 (2010) 37–41
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Performance evaluation of the carbon nanotubes supported Cs H PW O as
2
.5 0.5
12 40
efficient and recoverable catalyst for the oxidative removal of dibenzothiophene
Rui Wang *, Fengli Yu, Gaofei Zhang, Haixia Zhao
School of Environmental Science & Engineering, Shandong University, No. 27, Shanda South Road, Jinan, Shandong, 250100, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
With the issues of increasingly stringent legislations on sulfur level in transportation fuels, clean fuels
research including desulfurization has become a more important subject of environmental catalysis
studies. Oxidative desulfurization (ODS) combined with extraction is one of the most promising
desulfurization processes. With the loading of Cs2.5H0.5PW12O40, a new multi-walled carbon nanotube
Available online 7 November 2009
Keywords:
Oxidative desulfurization
Hydrogen peroxide
Carbon nanotubes
Heteropoly compound
Dibenzothiophene
Diesel oil
(
MWNT) supported catalyst (Cs2.5
H
0.5PW12
O
40/MWNT) has been developed in this study. Through
experimental evaluations, Cs2.5
H
0.5PW12
O40/MWNT was found to be very effective for the oxidative
removal of DBT, with a desulfurization efficiency of up to 100%. Factors affecting the process, including
reaction temperature, O/S molar ratio, initial sulfur content, catalyst dosage, and pre-immersion time of
the catalyst in H
Sulfone species was confirmed by GC–MS analysis to be the only product from DBT oxidation by H
the presence of Cs2.5 40/MWNT after 160 min. Moreover, the new catalyst developed is
recoverable. The recovered Cs2.5 40/MWNT demonstrates quite close catalytic activity to that
of the fresh. As a whole, Cs2.5
for deep desulfurization of diesel.
2
O
2
solution, were evaluated, and the favourable operating conditions were determined.
2 2
O
in
H
0.5PW12O
H
0.5PW12
O
H
0.5PW12
O
40/MWNT has a high potential for using as an effective catalyst
ß 2009 Elsevier B.V. All rights reserved.
1
. Introduction
worldwide researchers as a challenging subject. The conventional
process for the removal of organosulfur is known as catalytic
hydrodesulfurization (HDS). HDS is highly efficient in removing
thiols, sulfides and disulfides, but less effective for dibenzothio-
phene (DBT) and its derivatives with steric hindrance on the sulfur
atom (refractory organosulfur compounds) [3,4]. Severe operating
conditions of high temperature, high pressure and high hydrogen
consumption, as well as the use of more active catalyst or longer
residence time are inevitably required for HDS to produce ultra-
low sulfur fuel. However, the implementation of these alternatives
requires huge capital investment. Hence, it is desirable to develop
alternative more energy-efficient deep desulfurization processes.
Potential deep desulfurization processes other than HDS
include, but are not limited to, adsorption [2,5], extraction [1],
oxidation [6–16], and bioprocesses [6]. Oxidative desulfurization
(ODS) combined with extraction is one of the most promising
desulfurization processes [1,6,13]. Compared with conventional
HDS, ODS requires very mild conditions as ambient temperature
and atmospheric pressure. In the ODS process, the refractory
organosulfur compounds can be easily oxidized to their corre-
sponding sulfoxides and sulfones, which can be removed by
extraction or adsorption. The commonly used oxidant in ODS is
The presence of sulfur compounds in transportation fuels has
x
been recognized as a major source of SO which contributes to air
pollution, acid rain and damage of exhaust after-treatment
devices. The global need for clean fuels requires the minimization
of the sulfur level in transportation fuels. In the past decade, the
specifications for sulfur in diesel have been updated dramatically
in many countries. According to the legislations in Japan and
Europe, the sulfur content in diesel was limited to a maximum of
5
0 ppmw by 2005, and further to 10 ppmw by 2007 [1]. Actually,
diesel fuel and gasoline with less than 10 ppmw of sulfur content
were available commercially by the end of 2005 in Japan. Whereas
in the US, the EPA regulations had cut the highway diesel fuel
sulfur from 500 ppmw down to 15 ppmw by June 2006 [2]; the
level of 300 ppmw in automotive gasoline starting in 2004 was
reduced to 15 ppmw in 2006 [1]. In the long run, these restrictions
are only a milestone before the society at large stepping on the
road of zero sulfur fuel.
To meet the requirements of more and more stringent
legislations, clean fuels research including ultra-deep desulfuriza-
tion of diesel and gasoline is currently being conducted by
H
2
O
2
, owing to its low cost, environmental compatibility, and
commercial availability. In very recent years, polyoxometalates
POMs), renowned as green catalysts in many processes, have won
particular attention for their effectiveness in ODS with H
(
*
Corresponding author. Fax: +86 531 88364513.
E-mail address: ree_wong@hotmail.com (R. Wang).
2 2
O
0920-5861/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2009.10.001

3
8
R. Wang et al. / Catalysis Today 150 (2010) 37–41
[
11,13,17–21]. POMs belong to a large class of nanosized metal–
neutral pH value was attained. The resultant solid was dried in
oxygen cluster anions, formed by a self-assembly process typically
in an acidic aqueous solution [22]. Among numerous applications
of POMs, catalysis is by far the most important. Heteropoly
compounds, affiliated to POMs as a branch, are remarkably
important due to the significant opportunities they offered in
fundamental and applied catalysis. However, reported ODS
processes catalyzed by POMs are quite limited to date, and
heteropoly compounds were used in almost all these reports.
Although single heteropoly compound catalyst showed good
activity in the ODS process coupled with extraction, however,
the used catalyst was very difficult to recover. In view of this
drawback, a superior and recoverable ODS catalyst, i.e., multi-
vacuum at 80 8C and added into an aqueous solution containing
desired amount of Cs
temperature for 3 h, dried in vacuum at 120 8C for 12 h, and
calcinated at 300 8C in N atmosphere for 3 h. The solid was
impregnated stoichiometrically with H 40 solution under
2 3
CO . The mixture was stirred at room
2
3
PW12O
agitation for 2 h and kept overnight. After the same procedures of
drying and calcination, the target catalyst was gained.
2.3. Experimental method
The solution of DBT in normal octane was used as simulated
diesel, in which the sulfur content was set by fixing the dosage of
DBT. The oxidation reactions were carried out in a three-necked
250 mL round-bottomed flask immersed in a thermostatically
controlled water bath. The middle neck connected with a water
condenser tube; the other two side necks were closed with glass
stoppers. The liquid in the flask was continually and vigorously
stirred at a constant speed by a magnetic agitator. The catalyst was
walled carbon nanotube (MWNT) supported Cs2.5
H0.5PW12O
40
(Cs2.5H0.5PW12O40/MWNT), has been developed in this study. Since
the discovery of carbon nanotubes (CNTs), they have triggered
intensive research due to the wide applications they demon-
strated. As a good adsorbent for many species, MWNT may
promote the ODS activity of the heteropoly compound catalyst
supported, hence it was chosen as support. Bulk Cs2.5
H
0.5PW12
O
40
,
2 2
pre-immersed with needed amount of H O solution for a fixed
because of its high surface area, has been found to catalyze many
organic reactions more effectively than the bulk parent acid
time, then mixed with 60 mL of acetonitrile. The oxidation reaction
was started by further addition of the above mixture into 60 mL of
the simulated diesel. During the reactions, test samples were
withdrawn from the n-octane phase of the reaction mixture and
the sulfur content were analysed by a GC-FID equipped with a
capillary column. The temperatures for GC-FID tests were set as
334 8C for both the injector and the detector, and 280 8C for the
oven. The sulfur content was quantified by external standard
H
3
PW12
O
40
.
Moreover, owing to its insolubility in water,
40 is superior to H 40 in that it can be
reclaimed after use. However, used for ODS process, single
40 is not active enough to reach the goal of deep
desulfurization. The composite material of Cs2.5
Cs2.5 0.5PW12
H
O
3
PW12O
Cs2.5H0.5PW12O
40
H0.5PW12O /
MWNT would be of interest as catalyst to researchers in the field
of fuel oil deep desulfurization once its performance is found to be
2 2
method. The product derived from DBT oxidation by H O was
high. Through experimental evaluations, Cs2.5
H
0.5PW12
O
40/MWNT
identified using Agilent HP-6890N/MS-5793 GC–MS analyzer.
TEM observation of the catalysts was conducted using a JEM-
100CXII transmission electron microscope at an acceleration
voltage of 200 kV. The specific surface areas of the catalysts were
determined by a Quadrasorb SI specific surface area analyzer.
was found to be very effective for the oxidative removal of DBT,
with a desulfurization efficiency of up to 100%. Moreover, the
catalyst is recoverable. The recovered catalyst demonstrates quite
close catalytic activity to that of the fresh.
Of all factors governing the activity of the above catalyst, we
found that pre-immersion of the catalyst in H
2
O
2
is a key step to get
3. Results and discussion
high activity. This is due to the formation of the so-called peroxo-
heteropoly compound which has been reported to possess high
activity for the oxidation of many organic compounds in the
3.1. Dispersion of Cs2.5H0.5PW12O40 on MWNT
presence of H
2
O
2
[22].
TEM observation of Cs2.5
H
0.5PW12
O
40 supported on MWNT was
40 was
As to the knowledge of the authors, the results derived from
performance evaluation of this new catalyst were reported herein
with no literature precedent.
carried out at a magnification of 36,000. Cs2.5H0.5PW12O
found to be in the form of irregular aggregates immobilized on
MWNT and dispersed heterogeneously all around MWNT. The fact
that Cs2.5H0.5PW12O40 aggregates are not well dispersed could
2
. Experimental
depend on the impregnating process itself but could also be the
result of a mobility of the species under the TEM beam and the
resulting increase of temperature.
2.1. Chemicals
MWNT with outside diameters of 30–50 nm, inside diameters
3.2. Main factors affecting the process
of 5–12 nm, lengths of 10–20
1
m
m, and specific surface area of
34.8 m /g was purchased from Chengdu Organic Chemicals Co.
Ltd. AC with diameters of 550–600 m and specific surface area of
60.3 m /g, and phosphotungstic acid (H O) of
2
First of all, a comparison of the effects of MWNT and AC as
alternative supports for Cs2.5H0.5PW12O40 was made, concerning
the oxidative removal of DBT. The experiments were carried out at
60 8C for 120 min, using the foregoing biphasic system with an
initial sulfur content of 500 ppmw and an O/S molar ratio
m
2
3
3
PW12
O
40Á12H
2
analytical reagent grade, were purchased from National Drug &
Chemical Group Co. Ltd. Hydrochloric acid (HCl, 37 wt%), hydrogen
peroxide (H
CH CN, AR), and dibenzothiophene (C12
purchased from suppliers and used without further purification.
2
O
2
, 37 wt%), normal octane (C
8
H
18, AR), acetonitrile
(equivalent to 2[H
otherwise specified, the amount of catalyst used was 1 wt% of
normal octane, and the catalysts were pre-immersed in H
solution (containing needed amount of H with minimum
water) for 30 min before fed into the reaction mixture. As a
2 2
O ]/[S]) of 20. For all the experiments, unless
(
3
H S, AR), were all
8
2 2
O
2 2
O
2.2. Catalysts preparation
result, Cs2.5
H
0.5PW12
O
40/MWNT was found to be superior to
The catalysts used in this study comprise Cs2.5
H
0.5PW12
O
40
/
Cs2.5 0.5PW12
H
O
40/AC, although the specific surface areas of them
2
2
MWNT and Cs2.5
H0.5PW12
O
40/AC, and were prepared by impreg-
correspond to 91.6 m /g and 349.6 m /g respectively. Apparently,
the adsorption effect due to high surface area is not a predominant
factor. The desulfurization efficiency of Cs2.5
nation according to literature method [23]. The pristine MWNT or
AC was immersed in 20 wt% hydrochloric acid for 12 h, and washed
with deionized water and separated by centrifuging/washing till a
H0.5PW12O40/MWNT
(30 wt% loading) is 100%, evidently higher than that of

R. Wang et al. / Catalysis Today 150 (2010) 37–41
39
Fig. 1. Comparison of the effects of Cs2.5
H0.5PW12
O
40/MWNT (CsPWM),
Cs2.5 40/AC (CsPWA), Cs2.5H0.5PW12O40 (CsPW), MWNT and AC.
H
0.5PW12
O
Fig. 3. The effect of O/S molar ratio. Conditions: temperature, 60 8C; initial sulfur
content, 500 ppmw; catalyst dosage, 1 wt% of n-octane.
Cs2.5H0.5PW12
O
40/AC (30 wt% loading) being 90.8% (Fig. 1). Since
3.2.2. O/S molar ratio
both MWNT and AC can adsorb DBT in the above experiments and
hence contribute positively to the results, further experiments on
the net effects of them were conducted, yielding close desulfur-
ization efficiencies of 71.5% and 72.8% for MWNT and AC
As a common principle, to a given chemical reaction with
multiple reactants, the increase in concentration of any reactant
will lead to the increase of the conversion of the rest reactants as
well as increasing the reaction rate. This is the case as observed in
respectively (Fig. 1). Whereas, using single Cs2.5
desulfurization efficiency of 90.2% was gained (Fig. 1). Hence,
loading of Cs2.5 40 onto MWNT is essential to deep
desulfurization of the simulated diesel. Moreover, using MWNT as
support, 30 wt% of Cs2.5 40 was confirmed to be the
H
0.5PW12
O
40, a
2 2
Fig. 3 when the O/S molar ratio (equivalent to 2[H O ]/[S]) was
increased from 10 to 28. Same desulfurization efficiency of 100%
was achieved in 160 min under O/S molar ratios of 20 and 28,
except that a quicker reaction speed can be observed in the latter
case. From the results, the favourable O/S can be chosen as 20.
H
0.5PW12O
H
0.5PW12O
optimum loading for deep desulfurization and therefore used for
the following evaluations.
3.2.3. Initial sulfur content
To a given sulfur containing diesel to be desulfurized, initial
sulfur content embodies the task to be undertaken. In order to
understand the desulfurization efficiencies at different sulfur
levels, the effect of initial sulfur content was investigated. From the
results in Fig. 4, it can be seen that, within the initial sulfur content
range from 100 to 500 ppmw, the same final desulfurization
efficiency of 100% was achieved in 160 min. As the initial sulfur
contents increase to 800 and 1100 ppmw respectively, the final
desulfurization efficiencies were found to be 98.5% and 97.5%
accordingly.
3
.2.1. Temperature
Temperature plays an important role in the oxidation of DBT
2 2
with H O . From the results shown in Fig. 2, it is evident that the
desulfurization efficiency after 160 min’ reaction increases with
the increase of temperature till reaching a maximum of 100% at
6
and 50 8C, efficiencies of 96.6% and 98.2% can be obtained
respectively. From the viewpoint of chemical kinetics, increasing
temperature leads to the improvement of the reaction rate. This
accounts for the effect of temperature. As a whole, an optimal
temperature of 60 8C can be recommended.
0 8C and more quickly at 70 8C. Below 60 8C, corresponding to 40
The increase in initial sulfur content can speed up the oxidation
2 2
of DBT by H O and meanwhile deepens the consumption of both
Fig. 2. Temperature dependence of the process. Conditions: initial sulfur content,
Fig. 4. The effect of initial sulfur content. Conditions: temperature, 60 8C; O/S molar
ratio, 20; catalyst dosage, 1 wt% of n-octane.
5
00 ppmw; O/S molar ratio, 20; catalyst dosage, 1 wt% of n-octane.

4
0
R. Wang et al. / Catalysis Today 150 (2010) 37–41
species. Accordingly, the desulfurization efficiency will rise when
the initial sulfur content is not too high. However, after a certain
high content of initial sulfur, owing to the large amount of
excessive DBT in the reaction mixture, the conversion of DBT will
definitely decline. From the results demonstrated, agreeable effect
of deep desulfurization has been achieved within the initial sulfur
content below 1100 ppmw.
3.2.4. Catalyst dosage
Another factor that should be concerned is the catalyst dosage.
It was found that the catalyst dosage does have a marked influence
on the process efficiency (Fig. 5). Under otherwise identical
conditions, without catalyst, 66% of the DBT is removed from the n-
octane phase in 160 min by the joint action of extraction and direct
oxidation; the efficiencies of DBT removal in the presence of
Cs2.5H0.5PW12O40/MWNT were found to be 83%, 97.5% and 100%,
corresponding to catalyst dosages of 0.25, 0.5 and 1.0 wt%
respectively. Compared to the direct oxidation of DBT with
H
2
O
2
, the presence of catalyst provides an efficient reaction
pathway. Since Cs2.5 40 can react with H , producing
active intermediates known as peroxo-heteropoly compound
22,24,25] which can oxidize DBT more easily. In light of literatures
reports [22,24,25] and phenomena observed from the immersion
of Cs2.5 40 in 30% H solution, the active intermediate
can be the insoluble Cs2.5 [WO(O immobilized
on MWNT. Since, on one hand, {PO [WO(O
confirmed to be the active intermediate in oxidation with H
in the presence of H 40 [22,24,25]; on the other hand, we
found Cs2.5 40 is insoluble in 30% solution.
2 2
Fig. 6. Effect of pre-immersion of the catalyst in H O solution. Conditions:
temperature, 60 8C; initial sulfur content, 500 ppmw; O/S molar ratio, 20; catalyst
dosage, 1 wt% of n-octane.
H
0.5PW12
O
2 2
O
[
H
0.5PW12
O
2
O
H
2
reaction. Since no further enhancement can be found when the
catalyst dosage exceeds 1.0 wt%, the optimal catalyst dosage can be
recommended as 1.0 wt%.
0.5{PO
4
2 2 4
) ] }
3
À
has been
4
2
)
2
]
4
}
2 2
O
3
PW12
O
3.2.5. Pre-immersion of the catalyst in H
As an important factor, the pre-immersion time of
40/MWNT in H solution was investigated. It
can be found from Fig. 6 that, while the pre-immersion time of
40/MWNT in H solution varies from 5 to 20 min
2 2
O solution
H
0.5PW12
O
2 2
H O
Although the emphasis has been focused on the performance
evaluation of the catalyst, as a further study, confirmation of the
exact composition of the active intermediate is still need.
According to literature [22], the presence of the peroxo complex
can be confirmed by isolation of the peroxo species by the addition
of methyltrioctylammonium chloride in dichloromethane. Separ-
Cs2.5
H
0.5PW12
O
2 2
O
Cs2.5
H
0.5PW12
O
2 2
O
and further to 30 min, the final desulfurization efficiency increases
considerably from 89.4% to 99.7% and 100% accordingly. In light of
the foregoing analyses, the reaction between H
solid Cs2.5 40 supported on MWNT for the formation of
active intermediate is responsible for the above results. Since it
takes time to reconstruct Cs2.5 40 into catalytically active
2 2
O solution and
ating out the formed [(C
the IR data as the proof of the presence of the peroxo complex are:
8
H
17
)
3
NCH
3
]
3
{PO
4
[WO(O
2
)
2
]
4
} for IR test,
H0.5PW12O
1
088, 1058 and 1035 (P–O), 975 (W 55 O), 856 and 846 (O–O), 591
H0.5PW12O
À1
and 523 (W–O–O) cm
.
species over MWNT through solid–liquid reaction, corresponding
to a reaction time of 5 min, the amount of the produced active
species is rather limited and hence demonstrates lower desulfur-
ization efficiency compared to the case of longer reaction times.
From the results, the optimal pre-immersion time can be
determined as 30 min.
The role of MWNT in the ODS process is to enhance the transfer
of DBT onto the catalyst surface by adsorption, and hence
facilitates the oxidation of DBT by the peroxo-heteropoly
compound on MWNT.
Increasing the dosage of catalyst, more peroxo-heteropoly
compound species can be formed and therefore deepens the
3.3. Reaction product
2 2
With the aid of GC–MS, the product from DBT oxidation by H O
in the presence of Cs2.5H0.5PW12O40/MWNT after 160 min was
confirmed to contain the corresponding sulfone species only.
Hence, the overall reaction equation can be expressed as follows.
3.4. Catalyst reuse
When the reaction was finished, the reaction mixture was kept
still for 30 min. Two layers of n-octane phase and acetonitrile
phase emerge. Cs2.5 40/MWNT catalyst can be recovered
from acetonitrile phase by filtration. After a further treatment of
calcination at 350 8C and N atmosphere (0.3 L/min) for 4 h, the
H
0.5PW12O
2
catalyst can be reused. Evaluated through two recovery-reusing
runs, the recovered catalyst was found to demonstrate an excellent
catalytic performance, showing only a slight decline compared to
Fig. 5. The effect of catalyst dosage. Conditions: temperature, 60 8C; initial sulfur
content, 500 ppmw; O/S molar ratio, 20; reaction time, 160 min.

R. Wang et al. / Catalysis Today 150 (2010) 37–41
41
strates quite close catalytic activity to that of the fresh. As a whole,
40/MWNT has a high potential for using as an
Cs2.5H0.5PW12O
effective catalyst for deep desulfurization of diesel.
Acknowledgements
Financial support from the National Natural Science Foundation
of China (20976097) is gratefully acknowledged. Thanks are also
given to Dr. Sarayute in Queen’s University of Belfast for his kind
assistance in catalyst characterization.
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O
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2 2
immersion time of the catalyst in H O solution, were evaluated.
Resultantly, the favourable operating conditions were determined
as: reaction temperature, 60 8C; O/S molar ratio, 20; initial sulfur
content, below 1100 ppmw; pre-immersion time of the catalyst in
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H
2
O
2
solution, 30 min; catalyst dosage, 1 wt% of n-octane. With the
aid of GC-MS, sulfone species was confirmed to be the only product
from DBT oxidation by H in the presence of Cs2.5
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2
O
2
40
H0.5PW12O /
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