oxidation of hydrogen peroxide

1. Full Length Article
Oxidation by H2O2 of bezothiophene and dibenzothiophene over
different polyoxometalate catalysts in the frame of ultrasound
and mixing assisted oxidative desulfurization
Angelo Earvin Sy Choi a
, Susan Roces a
, Nathaniel Dugos a
, Meng-Wei Wan b,⇑
a
Chemical Engineering Department, De La Salle University, 2401 Taft Ave, Manila 0922, Philippines
b
Department of Environmental Resource Management, Chia-Nan University of Pharmacy and Science, Tainan 71710, Taiwan
h i g h l i g h t s
Oxidation process follows the
pseudo-first order kinetics and
Arrhenius equation.
NaPW/H2O2 system best oxidizes
benzothiophene and
dibenzothiophene.
Utilizing both ultrasonicator and high
shear mixer showed comparable
performance.
Results showed better oxidation rate
and activation energy than
conventional ODS.
g r a p h i c a l a b s t r a c t
a r t i c l e i n f o
Article history:
Received 7 September 2015
Received in revised form 7 March 2016
Accepted 5 April 2016
Keywords:
Benzothiophene
Dibenzothiophene
Mixing-assisted oxidative desulfurization
Polyoxometalate catalysts
Ultrasound-assisted oxidative
desulfurization
a b s t r a c t
Desulfurization involves the removal of refractory sulfur compounds in fossil-fuel derived oils. In this
study, an ultrasound and mixing assisted oxidative desulfurization of synthetic oil containing sulfur com-
pounds of benzothiophene and dibenzothiophene were carried out using different polyoxometalate cat-
alysts, H2O2 oxidant and a phase transfer agent. The effects of reaction time (2–30 min) and temperature
(30–70 °C) were examined in the oxidation of benzothiophene and dibenzothiophene. Results showed
high correlation to the pseudo first-order reaction kinetics (R2
0.97) and Arrhenius equation
(R2
0.99) that draws out the rate constant and activation energy of each catalyst tested in the oxidation
process. Oxidation of benzothiophene and dibenzothiophene using different polyoxometalate catalysts
showed a catalytic activity trend of Na3PW12O40 H3PW12O40 H3PM12O40 H4SiW12O40. Furthermore,
ultrasound and mixing assisted oxidative desulfurization showed comparable results (5% difference)
in oxidation efficiency and better performance in the kinetic reaction rate and activation energy as com-
pared to conventional oxidation step in the oxidative desulfurization technique.
Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction
The consumption of fossil fuel derived-oil has shown a marked
increase in the last three decades due to rapid technological
advancements, increasing world population, and heat and power
http://dx.doi.org/10.1016/j.fuel.2016.04.014
0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.
⇑ Corresponding author.
E-mail address: peterwan@mail.cnu.edu.tw (M.-W. Wan).
Fuel 180 (2016) 127–136
Contents lists available at ScienceDirect
Fuel
journal homepage: www.elsevier.com/locate/fuel

2. generation [1–3]. Petroleum and its fractions contain high concen-
trations of organic sulfur compounds (OSCs) which are a major
environmental concern [4]. OSCs are mainly composed of alkylated
benzothiophene (BT) and dibenzothiophene (DBT) [4]. Upon com-
bustion of oil, OSCs forms sulfur dioxides (SOx) and sulfate partic-
ulate matter that endangers public health and the environment
[5,6]. OSCs in fuel oils can poison catalytic converters, corrode
parts of internal combustion, and contribute to air pollution
[2,4,6,7]. Desulfurization prior to combustion not only protects cat-
alytic convertors from poisoning and engine parts from corrosion,
but also minimizes the formation of SOx, which is one of the key
precursors to acid rain. Increasing stringent emission control stan-
dards are implemented in recent years in order to lessen environ-
mental and health impacts. The European Union (EU) has set
maximum sulfur concentrations to 10 ppm in diesel oil [8]. The
United States Environmental Protection Agency (USEPA) set the
sulfur content in gasoline and diesel oil at 30 ppm and 15 ppm,
respectively [9,10].
Various desulfurization technologies have drawn attention
around the world such as hydrodesulfurization (HDS), extractive
desulfurization, biodesulfurization and oxidative desulfurization
to produce low-sulfur fuels [2,4–6]. HDS is the conventional tech-
nology that is used at large-scale chemical process industries to
produce low-sulfur petroleum feedstock and fuel oils [11]. This
hydrotreatment process desulfurizes fossil fuels by the use of
hydrogen at high pressure and Co–Mo or Ni–Mo catalyst. It
typically operates at high temperatures between 300 and 400 °C
[12–14]. However, this process has low reactivity toward aromatic
sulfur compounds such as BT, DBT and its alkylated derivatives
[4,8,15–17]. The disadvantages of using HDS process are as fol-
lows: it requires severe reaction conditions such as high hydrogen
pressure (30–75 bars), high reaction temperature (300–400 °C),
use of large reactors, large amounts of catalysts and a long resi-
dence time that leads to high operating cost [13,15,18]. Alternative
desulfurization processes are searched with the aim to improve
efficiency and reduce costs while complying with the environmen-
tal standards.
The oxidative desulfurization (ODS) is considered as one of the
most promising technologies for deep desulfurization due to the
mild operating conditions it applies. It also does not require the
use of expensive hydrogen and has superior selectivity toward sul-
fur removal as compared to the HDS [2,19]. OSCs in the ODS pro-
cess are selectively oxidized to their corresponding sulfones (1-
oxides) and/or sulfoxides (1,1-dioxide). This method prevents a
C–C cleavage due to the electrophilic addition of oxygen that has
a strong affinity toward sulfur [20,21]. The oxidized sulfur com-
pounds are highly polar as compared to the hydrocarbons which
can be easily removed through distillation, liquid/liquid extraction
or adsorption to complete the desulfurization process [6,22]. To
date, various oxidants such as Fenton’s reagent [19], ferrate [22–
24], hydrogen peroxide [4,11,25–32], ozone [33,34] and superox-
ides [35] have been reported. Hydrogen peroxide (H2O2) is pre-
ferred and widely used over other oxidants due to its high
oxidation capacity, low cost, and lack of toxic by-product [36].
The utilization of H2O2 in fuel oil gives a biphasic system which
is an essential consideration to take note. This requires attention
in order to improve desulfurization performances and achieve
low-sulfur fuels.
An innovative technology called ultrasound-assisted oxidative
desulfurization (UAOD) is introduced to improve desulfurization
efficiencies in ODS. The use of an ultrasound probe in UAOD has
a higher desulfurization rate caused by smoother dispersion
[4,15,37,38]. The chemical mechanism for ultrasonication involves
the acceleration of reaction due to the production of radicals
through the transient collapse of cavitation bubbles, and the phys-
ical mechanism is the formation of fine emulsion due to ultrasonic
cavitation which increases mass transfer [39,40]. For ultrasonica-
tion reaction, the oxidation of sulfur compounds such as BT and
DBT is physical in nature which implies that UAOD improves oxi-
dation due to mass transfer through increasing the interfacial area
between the oil and oxidant phase [41,42]. The difficulties to use
UAOD in industrial scale are its high energy utilization and high
capital cost due to additional sono-reactor, amplifier and function
generator [25]. It is important, therefore, to find alternatives to
ultrasonication to up-scale the ODS process more effectively.
Mixing-assisted oxidative desulfurization (MAOD) was intro-
duced recently and it showed a promising oxidation reaction per-
formance [25,43,44]. Using H2O2 as the oxidant requires a good
dispersion between the organic and aqueous phase to accelerate
oxidation of OSCs. The utilization of a high-shear mixer in the
MAOD process provides an enhanced fluid/fluid interfacial area
through molecular diffusion. The organic phase containing the
OSCs can easily breakdown into smaller droplets that improves
mass transfer to the aqueous phase which contains the H2O2 oxi-
dant [45]. Rapid mixing essentially provides an adequate and effec-
tive contact between the biphasic system which improves
oxidation performances in the ODS system. Polyoxometalate cata-
lysts contains early transition metal–oxygen anion clusters and
have shown to attract attention due its effective combination with
the H2O2 oxidant and being a green catalyst in ODS
[4,11,20,25,43,46,47]. Catalysis is considered to be the most impor-
tant role among the various applications and uses of polyoxometa-
lates [47]. To the best of our knowledge no reports have shown the
application and mechanism of different polyoxometalate catalysts
on both the UAOD and MAOD processes. It can be noted that there
is limited information in relation to the comparative assessment of
the oxidation kinetics for BT and DBT in both UAOD and MAOD.
Fig. 1. UAOD apparatus.
128 A.E.S. Choi et al. / Fuel 180 (2016) 127–136

3. In this study, sulfur oxidation of model sulfur compounds such
as BT and DBT in both UAOD and MAOD using polyoxometalate/
H2O2 systems with an ammonium ion phase transfer agent (PTA)
was investigated. The main objective of this study aims to deter-
mine the oxidation performances for BT and DBT at varying reac-
tion temperature and time. A comparative assessment of utilizing
polyoxometalate catalysts namely sodium phosphotungstate
(NaPW), phosphotungstic acid (PW), phosphomolybdic acid
(PMo) and silicotungstic acid (SiW) is to be studied. The oxidation
kinetic rate constant and activation energy for both UAOD and
MAOD using different polyoxometalate catalysts is to be also
determined and compared to assess its oxidation performances
associated to conventional ODS technique.
2. Experimental
2.1. Materials
Benzothiophene (C8H6S, 97% purity) was obtained from Acros
Organics (Taiwan). Dibenzothiophene (C12H8S, 99% purity) and
sodium phosphotungstate hydrate (Na3PW12O40ÁxH2O) were pro-
cured from Alfa Aesar (Taiwan). Tetraoctylammonium bromide
(C32H68BrN, 98% purity) was acquired from Hungyao (Taiwan)
and toluene (C7H8, 0.99 mass fractions) was purchased from Merck
Chemical Company (USA). Hydrogen peroxide (H2O2, 50% concen-
tration) and phosphotungstic acid hydrate (H3PW12O40ÁxH2O, 98%
purity) were supplied by G-Watt Co., Ltd. (Taiwan). Phospho-
molybdic acid hydrate (H3PM12O40ÁxH2O) was purchased from
Ferak Berlin GmbH (Germany). Silicotungstic acid hydrate
(H4SiW12O40ÁxH2O, 99.9% purity) was obtained from Sigma–
Aldrich (Wisconsin, USA). All chemicals used were analytical grade
without further purification.
2.2. Instrumental analysis
The presence of sulfur compounds before and after oxidation
were analyzed using a gas chromatograph (GC, Agilent 7890A
Gas Chromatograph, California, USA) with fused-silica capillary
column HP-5 ms having 0.25 mm film thickness (J W Scientific,
USA) and equipped with a sulfur chemiluminescence detector
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30
BTOxidation(%)
Sonication Time (min)
(a)NaPW
PW
PMo
SiW
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30
DBTOxidation(%)
Sonication Time (min)
(d)NaPW
PW
PMo
SiW
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30
BTOxidation(%)
Sonication Time (min)
(b)NaPW
PW
PMo
SiW
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30
DBTOxidation(%)
Sonication Time (min)
(e)
NaPW
PW
PMo
SiW
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30
BTOxidation(%)
Sonication Time (min)
(c)
NaPW
PW
PMo
SiW
0
10
20
30
40
50
60
70
80
90
100
0 10 20 30
DBTOxidation(%)
Sonication Time (min)
(f)
NaPW
PW
PMo
SiW
Fig. 2. Comparison of various polyoxometalate catalysts in the oxidation of BT at (a) 30 °C, (b) 50 °C and (c) 70 °C and DBT at (d) 30 °C, (e) 50 °C and (f) 70 °C under UAOD.
A.E.S. Choi et al. / Fuel 180 (2016) 127–136 129

4. (SCD, Agilent 355). Two temperature programs were used in order
to identify and quantify sulfur compounds: (1) the column temper-
ature setting for the analysis of BT was set at an initial GC oven
temperature of 150 °C for 1 min, heated at a rate of 20 °C/min to
220 °C and retained for 1 min; (2) the initial column temperature
settings for DBT was set at 200 °C for 1 min, heated at an increasing
rate of 20 °C/min to 280 °C and kept constant at 1 min.
2.3. Sulfur oxidation
An initial sulfur concentration of 500 ppm for BT and DBT was
separately prepared by dissolving each sulfur compound in toluene
to produce a synthetic oil. The percentage sulfur oxidation (%St)
was calculated from the gathered experimental data using Eq. (1).
%St ¼
Co À Ct
Co
 100 ð1Þ
where Co is the initial concentration of the model sulfur compound
present in the toluene solution and Ct denotes the concentration of
model sulfur compound left after oxidation reaction at a given time
(t).
2.4. Batch UAOD methodology
The batch UAOD experiments were carried out using a 20 kHz
frequency ultrasound apparatus (Sonic VCX-500, USA) equipped
with a titanium probe tip (25 mm diameter and 122 mm length)
as shown in Fig. 1. The amplification for ultrasonication was
adjusted to 40% (200 W power output) without pulse. Equal vol-
ume of model sulfur compound and H2O2 was prepared in a glass
reactor mixed with a respective polyoxometalate catalyst and PTA.
A total of 160 mL working solution was used for the oxidation
experiments. The glass reactor was immersed in a water bath in
order to adjust the reaction temperature (30–70 °C). The mixture
was irradiated using ultrasound at a varying time interval (2–
30 min). The cooled mixture was centrifuged for 10 min to sepa-
rate the synthetic oil from the oxidant. The sulfur content for the
treated synthetic oil was subjected for analysis.
2.5. Batch MAOD methodology
The batch MAOD experiments were conducted using a high-
shear mixer (IKA T-25 Ultra-Turrax digital homogenizer) with agi-
tation speed of 10,000 rpm. A glass reactor was used with an elec-
tronic control unit in order to monitor and maintain the
temperature. An equal amount of organic and aqueous solution
was mixed together with sodium phosphotungstate catalyst and
tetraoctylammonium bromide as the PTA. The mixture was agi-
tated at a pre-determined time interval and temperature. The
organic phase was separated and drawn out for further analysis
of its sulfur content using a GC-SCD.
3. Results and discussion
3.1. UAOD and MAOD oxidation reaction mechanism
UAOD and MAOD techniques are a biphasic system that com-
prises of two immiscible phases. One is the organic phase which
contains the synthetic oil and the other is an aqueous phase that
contains the H2O2 oxidant. A proposed conceptual model for the
catalytic oxidation of the UAOD and MAOD system has five
stages. For this study, the polyoxometalate anions represents
[XM12O40]xÀ8
where X is the central atom (P5+
or Si4+
), M is the
metal ion (W6+
or Mo6+
), and x is the oxidation state [48].
The first stage is the key precursor to the UAOD and MAOD pro-
cess; wherein the presence of excess amount of H2O2 reacting with
the polyoxometalate anion is peroxidized to form a polyperox-
ometalate, {XO4[MO(O2)2]4}xÀ8
. The polyoxometalate catalysts
such as NaPW or PW, PMo and SiW were peroxidized and disaggre-
gated and formed {PO4[WO(O2)2]4}3À
, {PO4[MoO(O2)2]4}3À
and
{SiO4[WO(O2)2]4}4À
, respectively [4,48,49]. The anionic perox-
ometal complex compound bears active oxygen that is an effective
species for epoxidation. The quaternary ammonium cation (Q+
)
reacts as the PTA in the second stage which binds with the perox-
ometal complex compound that is subsequently transferred to the
organic phase. Due to the presence of the peroxometal complex in
the organic phase, the sulfur compounds such as BT and DBT are
selectively and efficiently oxidized in the third stage to form prod-
ucts of benzothiophene sulfone (BT-O) and dibenzothiophene sul-
fone (DBT-O), respectively. After the oxidation reaction, the
peroxometal complex in the fourth stage is reduced and dissociates
with the PTA which is brought back to the aqueous phase [4,50].
Ultrasound or high-shear mixing enhances the oxidation efficiency
in the fifth stage due to effective mass transfer in the biphasic sys-
tem [4,23,44,50,51].
3.2. Comparison of polyoxometalate catalysts in the oxidation of BT
and DBT
Four commercially available polyoxometalate catalysts were
compared and analyzed under the UAOD process for the sulfur oxi-
dation of BT and DBT. Results indicate the dominance of the phos-
phorus based polyoxometalate over the silicon based counterpart
as shown in Fig. 2. This is attributed to the unstable generation
of the peroxometal complex, {SiO4[WO(O2)2]4}4À
, in SiW/H2O2 sys-
tem [30]. Weak sulfur oxidation occurs without the formation of
the active polyperoxo species. The formation of {PO4[WO
(O2)2]4}3À
and {PO4[MoO(O2)2]4}3À
are well-known active anions
for sulfur oxidation in the PW/H2O2 and PMo/H2O2 systems,
0
20
40
60
80
100
0 10 20 30
BTOxidation(%)
Reaction Time (min)
(a)
30
40
50
60
70
80
90
100
0 10 20 30
DBTOxidation(%)
Reaction Time (min)
(b)
Fig. 3. Oxidation performances through an ultrasonicator and high shear mixer of
(a) benzothiophene and (b) dibenzothiophene using NaPW/H2O2 system.
130 A.E.S. Choi et al. / Fuel 180 (2016) 127–136

5. respectively [48,49]. Molybdic compounds have a higher oxidation
potential as compared to tungstic compounds [48]. However, the
PMo catalyst displayed a lower catalytic activity in comparison
to PW catalyst for both BT and DBT oxidation. This implies that oxi-
dation activities depend on the catalytic species after being perox-
idized and not on the original catalyst [30]. NaPW catalyst showed
the best oxidation performance for both BT and DBT oxidation.
Moreover, the sodium salt catalyst has a higher catalytic perfor-
mance over the PW catalyst which indicates that the acidic func-
tion does not play an important role for the oxidation reaction of
sulfur compounds.
3.3. Evaluation of UAOD and MAOD process
The NaPW/H2O2 system was used to evaluate the sulfur oxida-
tion of BT and DBT under UAOD and MAOD as shown in Fig. 3. In
the UAOD and MAOD system, emulsions produced by sonication
and high-shear mixing are finer and more stable than the ODS sys-
tem due to promotion of increased interfacial area and mass trans-
fer for oxidation reactions to occur [4,23,44,50]. The reaction time
is referred to as the contact time of the organic and aqueous phase.
Increasing the reaction time from 2 to 30 min improves the trans-
fer of the NaPW/H2O2 system to the model sulfur compounds
through the electrophilic addition of oxygen to sulfur. Varying
reaction temperature from 30 to 70 °C improves sulfur oxidation
of BT and DBT. This is due to the increase in formation of perox-
ometal complexes in the system which improves the rate of sulfur
to sulfone conversion efficiently [26].
UAOD showed a lower oxidation performance than MAOD. BT
oxidation started at 4.0% (UAOD) and 10.0% (MAOD) while DBT oxi-
dation started at 31.7% (UAOD) and 59.2% (MAOD) when the tem-
perature was set at 30 °C. The large difference in the oxidation of
BT or DBT is attributed to a better mixing performance in the
MAOD system due to adequate emulsification at the start of the
oxidation reaction. However, the oxidation efficiencies for both
BT (Fig. 3a) and DBT (Fig. 3b) in UAOD are only slightly lower than
the MAOD process after 30 min in all its respective reaction tem-
perature. This implies that UAOD takes a little more time to ade-
quately emulsify the organic and aqueous phase than MAOD. At
the end of reaction, comparable oxidation efficiency is observed
R² = 0.9908
R² = 0.9958
R² = 0.9968
0
1
2
3
4
5
6
7
0 10 20 30
ln(Co/Ct)
Mixing Time (min)
(a)30°C
50°C
70°C
R² = 0.9912
R² = 0.9920
R² = 0.9953
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 10 20 30
ln(Co/Ct)
Sonication Time (min)
(d)
R² = 0.9962
R² = 0.9984
R² = 0.9971
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
0 10 20 30
ln(Co/Ct)
Sonication Time (min)
(b)
R² = 0.9746
R² = 0.9918
R² = 0.9938
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0 10 20 30
ln(Co/Ct)
Sonication Time (min)
(e)
R² = 0.9958
R² = 0.9974
R² = 0.9913
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
0 10 20 30
ln(Co/Ct)
Sonication Time (min)
(c)
30°C
50°C
70°C
30°C
50°C
70°C
30°C
50°C
70°C
30°C
50°C
70°C
Fig. 4. Oxidation kinetics of BT under MAOD using (a) NaPW and under UAOD using (b) NaPW, (c) PW, (d) PMo and (e) SiW.
A.E.S. Choi et al. / Fuel 180 (2016) 127–136 131

6. for both UAOD and MAOD. The oxidation of DBT reached 94.8% and
97.4% for UAOD and MAOD process, respectively.
3.4. Oxidation reaction kinetic study
Oxidation kinetics can be determined using the sulfur concen-
tration before and after oxidation as a function of time. Diffusional
resistance is negligible due to effective mixing using an ultrasoni-
cator and high shear mixer under the UAOD and MAOD system,
respectively [25,50]. The kinetic model for the oxidation of sulfur
compounds can be determined as:
À
d½CSulfurŠ
dt
¼ k½CH2O2
Ša
½CSulfurŠb
ð2Þ
where ½CH2O2
Š and [CSulfur] represents the concentration of H2O2 and
sulfur compound (BT or DBT) subjected for oxidation reaction,
respectively, while a and b signifies the order of reaction to the con-
centration of H2O2 and sulfur compound, respectively.
The oxidation of aromatic sulfur compounds using solid cata-
lysts follows pseudo-first order reaction kinetics as reported in
previous works [25,30,50,52–54]. This is due to the presence of
excess H2O2 oxidant. It was assumed that the H2O2 concentration
remains constant throughout the reaction. Therefore, Eq. (2) is
reduced as follows:
À
d½CSulfurŠ
dt
¼ k
0
½CSulfurŠ ð3Þ
where the apparent rate constant (k0
) is described as:
k
0
¼ k½CH2O2
Ša
ð4Þ
The linearized form of the pseudo-first-order catalytic oxidation
reaction for BT or DBT is shown in Eq. (5).
ln
½CŠo
½CŠt
¼ k
0
t ð5Þ
where [C]o and [C]t denotes the concentration of model sulfur com-
pound (ppm) at time 0 min and a given time t min, respectively, and
k0
represents the oxidation kinetic rate constant for the catalytic
reaction of BT or DBT.
R² = 0.9918
R² = 0.9905
R² = 0.9997
0
2
4
6
8
10
12
14
16
18
0 10 20 30
ln(Co/Ct)
Mixing Time (min)
(a)
R² = 0.9976
R² = 0.9954
R² = 0.9978
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
0 10 20 30
ln(Co/Ct)
Sonication Time (min)
(d)
R² = 0.9987
R² = 0.9965
R² = 0.9977
0
2
4
6
8
10
12
14
16
0 10 20 30
ln(Co/Ct)
Sonication Time (min)
(b)
R² = 0.9914
R² = 0.9990
R² = 0.9971
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0 10 20 30
ln(Co/Ct)
Sonication Time (min)
(e)
R² = 0.9928
R² = 0.9929
R² = 0.9963
0
2
4
6
8
10
12
14
0 10 20 30
ln(Co/Ct)
Sonication Time (min)
(c)
30°C
50°C
70°C
30°C
50°C
70°C
30°C
50°C
70°C
30°C
50°C
70°C
30°C
50°C
70°C
Fig. 5. Oxidation kinetics of DBT under MAOD using (a) NaPW and under UAOD using (b) NaPW, (c) PW, (d) PMo and (e) SiW.
132 A.E.S. Choi et al. / Fuel 180 (2016) 127–136

7. The plots of ln[C]o/ln[C]t vs t at different reaction temperature
conditions for BT and DBT is graphically illustrated in Figs. 4 and
5, respectively. The plots observed were straight lines in all cases.
The coefficient of correlation (R2
) of BT and DBT oxidation ranges
from 0.9746 to 0.9984 and from 0.9905 to 0.9997, respectively.
The pseudo-first-order reaction kinetics is highly suitable to
describe the oxidation activity of BT and DBT under the UAOD
and MAOD system due to its high coefficient of correlation.
Table 1 summarizes the catalytic kinetic constant of BT and DBT
oxidation in UAOD and MAOD using 21.32 M of H2O2. The best cat-
alyst for sulfur oxidation proves to be the NaPW catalyst due to its
highest oxidation kinetic rate constant at different temperatures as
compared to other polyoxometalate catalyst. The kinetic rate con-
stant of 0.1953 minÀ1
and 0.4008 minÀ1
at 70 °C of MAOD for BT
and DBT, respectively, exhibits higher catalytic oxidation than
UAOD for BT of 0.1652 minÀ1
and 0.3802 minÀ1
, respectively. In
comparison to the study of Te et al. [30], conventional ODS showed
kinetic rate constant of 0.278 minÀ1
for DBT oxidation at 70 °C.
Therefore, the UAOD and MAOD processes increased the kinetic
rate for oxidation in DBT by 1.37 and 1.44 times, respectively. This
proves to show that oxidation performances were enhanced upon
using either ultrasonicator or high-shear mixer. Results showed
higher kinetic rate constant for the oxidation of DBT than BT. This
can be explained by the lower electron density exhibited in BT [55].
This means that the electrons of BT are more compact that hinders
oxidation performance as compared to DBT.
Activation energy for the oxidation of model sulfur compounds
can be obtained through the use of the Arrhenius equation in Eq.
(4).
ln k ¼ À
Ea
R
1
T
þ ln A ð4Þ
where Ea, R, T and A are the activation energy (kJ/mol), universal gas
constant (8.314 Â 10À3
kJ/mol K), temperature (K) and the pre-
exponential factor, respectively.
The Arrhenius plot of Àln k vs 1/T for the pseudo-first order
reaction of BT and DBT oxidation is shown in Fig. 6. High coefficient
of correlation is observed in the range between 0.9940–0.9999 and
0.9911–0.9945 for BT and DBT, respectively. The linear plot
strongly suggests that the Arrhenius equation is well-suitable to
calculate the activation energy of BT and DBT oxidation.
Table 2 summarizes the activation energy of different polyox-
ometalate/H2O2 systems upon BT and DBT oxidation. Results under
the NaPW/H2O2 system indicate higher activation energy in BT
than DBT at 58.0 kJ/mol and 30.9 kJ/mol for MAOD, respectively,
and 57.8 kJ/mol and 29.9 kJ/mol for UAOD, respectively. This
proves to show the ease of oxidation for DBT than BT as it needs
a lower energy to activate oxidation reaction in the system. The
conventional ODS showed activation energy of 53.8 kJ/mol in
Table 1
Oxidation rate constants of BT and DBT for various polyoxometalate/H2O2 systems in UAOD and MAOD.
Polyoxometalate catalysts Temperature (°C) BT rate constant (minÀ1
) DBT rate constant (minÀ1
)
UAOD MAOD UAOD MAOD
Sodium phosphotungstate (NaPW) 30 0.0114 0.0133 0.0948 0.0954
50 0.0505 0.0545 0.1781 0.1819
70 0.1652 0.1953 0.3802 0.4008
Phosphotungstic acid (PW) 30 0.0076 0.0078 [54] 0.0448 0.0450 [54]
50 0.0411 0.0446 [54] 0.1546 0.1655 [54]
70 0.1435 0.1641 [54] 0.3585 0.3727 [54]
Phosphomolybdic acid (PMo) 30 0.0040 0.0042 [54] 0.0141 0.0148 [54]
50 0.0109 0.0116 [54] 0.0253 0.0279 [54]
70 0.0230 0.0273 [54] 0.0520 0.0538 [54]
Silicotungstic acid (SiW) 30 0.0012 0.0013 [54] 0.0025 0.0029 [54]
50 0.0032 0.0034 [54] 0.0054 0.0066 [54]
70 0.0063 0.0073 [54] 0.0090 0.0108 [54]
y = 6979.2x - 18.699
R² = 0.9999
y = 6954.6x - 18.49
R² = 0.9991
y = 7648.2x - 20.391
R² = 0.9976
y = 4590.5x - 9.6341
R² = 0.9974
y = 4415.3x - 7.8446
R² = 0.9940
0
1
2
3
4
5
6
7
8
0.0028 0.0029 0.003 0.0031 0.0032 0.0033 0.0034
1/T (1/K)
-ln(k)
(a)
MAOD (NaPW)
UAOD (NaPW)
UAOD (PW)
UAOD (PMo)
UAOD (SiW)
y = 3720.6x - 9.887
R² = 0.9913
y = 3600.8x - 9.4885
R² = 0.9922
y = 5425.5x - 14.833
R² = 0.9945
y = 3386x - 6.8721
R² = 0.9911
y = 3321.3x - 4.9984
R² = 0.9945
0
1
2
3
4
5
6
7
0.0028 0.0029 0.003 0.0031 0.0032 0.0033 0.0034
1/T (1/K)
-ln(k)
(b)
MAOD (NaPW)
UAOD (NaPW)
UAOD (PW)
UAOD (PMo)
UAOD (SiW)
Fig. 6. Arrhenius plot for the oxidation of (a) BT and (b) DBT.
Table 2
Activation energy of BT and DBT for various polyoxometalate/H2O2 systems in UAOD
and MAOD.
Polyoxometalate catalysts Activation energy
for BT (kJ/mol)
Activation energy
for DBT (kJ/mol)
UAOD MAOD UAOD MAOD
Sodium phosphotungstate –
(NaPW)
57.8 58.0 29.9 30.9
Phosphotungstic acid (PW) 63.6 65.8 [54] 45.1 45.9 [54]
Phosphomolydic acid (PMo) 38.2 40.6 [54] 28.2 29.0 [54]
Silicotungstic acid (SiW) 36.7 37.4 [54] 27.6 28.3 [54]
A.E.S. Choi et al. / Fuel 180 (2016) 127–136 133

8. DBT. The activation energy of ODS is 1.80 and 1.74 times lower in
UAOD and MAOD, respectively. This means that using the UAOD
and MAOD system can effectively minimize the minimum energy
required for sulfur oxidation reaction.
3.5. Product identification analysis using various polyoxometalate
catalysts
The model sulfur compounds were analyzed before and after
oxidation using the GC-SCD. The products and retention time of
BT and DBT using the different polyoxometalate/H2O2 systems at
30 min and 70 °C were identified in Fig. 7. Results showed that
BT (1.6 min) formed BT-O (3.4 min) as its major product and 2-
bromothianapthene 1,1-dioxide (2.6 min) and benzothiophene, 3-
bromo (4.3 min) as its minor by-product. Meanwhile, DBT
(2.2 min) formed only DBT-O (3.7 min) as its product after oxida-
tion reaction. The formation of brominated by-products in the oxi-
dation of BT is caused by the bromide used in the PTA. Both NaPW/
H2O2 and PW/H2O2 systems only formed a benzothiophene, 3-
bromo by-product. The PMo/H2O2 system formed two brominated
products, while the SiW/H2O2 system did not form a BT-O but only
a 2-bromothianapthene 1,1-dioxide by-product due to its weak
oxidation capacity toward BT. It can also be consistently observed
in Fig. 7b that the SiW/H2O2 system has a low oxidation perfor-
mance on DBT due to having a low peak area in DBT-O.
4. Conclusions
In this study, oxidation reaction is done using the UAOD and
MAOD technique together with a PTA and various polyoxometa-
late/H2O2 systems to oxidize BT and DBT. Results showed that
the oxidation of BT and DBT is highest in the NaPW/H2O2 system
Fig. 7. Product identification for the oxidation of (a) BT and (b) DBT.
134 A.E.S. Choi et al. / Fuel 180 (2016) 127–136

9. and lowest in the SiW/H2O2 system. It was found that the sulfur
oxidation efficiency increases with reaction time and temperature.
UAOD showed a lower oxidation performance at the start of oxida-
tion but comparable oxidation efficiency with the MAOD process
was achieved at the end of reaction. Oxidation performance
showed a trend of ODS UAOD 6 MAOD based on the results
obtained kinetic rate constant and activation energy which was
obtained in the kinetic study. The model sulfur compound of DBT
proves to be easier to oxidize than that of BT. Oxidation of DBT
showed only DBT-O as its product while BT formed a major pro-
duct of BT-O and a minor product of benzothiophene, 3-bromo in
the NaPW/H2O2 system. Furthermore, this research demonstrated
high oxidation performances for BT and DBT at mild operating con-
ditions, under normal atmospheric pressure and low temperature
(30–70 °C) in the UAOD and MAOD process. Therefore, both UAOD
and MAOD showed a promising oxidation performance to achieve
low-sulfur fuel oil after further extraction step is done.
Acknowledgments
The authors would like to acknowledge the Ministry of Science
and Technology, Taiwan (MOST 104-2221-E-041-002 and MOST
101-2221-E-041-010-MY3) and the Engineering Research and
Development for Technology – Department of Science and Tech-
nology (ERDT-DOST), Philippines for the financial support in this
research.
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