Facile synthesis of inorganic–organic Fe2W18Fe4@NiO@CTS hybrid nanocatalyst induced efficient performance in oxidative desulfurization of real fuel

Article abstract of DOI:10.1002/aoc.4895

In this work, a new nanocatalyst, Fe₂W₁₈Fe₄@NiO@CTS, was synthesized by the reaction of sandwich-type polyoxometalate (Fe₂W₁₈Fe₄), nickel oxide (NiO), and chitosan (CTS) via sol–gel method. The assembled nanocatalyst was systematically characterized by FT-IR, UV–vis, XRD, SEM, and EDX analysis. The catalytic activity of Fe₂W₁₈Fe₄@NiO@CTS was tested on oxidative desulfurization (ODS) of real gasoline and model fuels. The experimental results revealed that the levels of sulfur content and mercaptan compounds of gasoline were lowered with 97% efficiency. Also, the Fe₂W₁₈Fe₄@NiO@CTS nanocatalyst demonstrated an outstanding catalytic performance for the oxidation of dibenzothiophene (DBT) in the model fuel. The major factors that influence the desulfurization efficiency and the kinetic study of the ODS reactions were fully detailed and discussed. The probable ODS pathway was proposed via the electrophilic mechanism on the basis of the electrophilic characteristic of the metal-oxo-peroxo intermediates. The prepared nanocatalyst could be reused for 5 successive runs without any appreciable loss in its catalytic activity. As a result, the current study suggested the potential application of the Fe₂W₁₈Fe₄@NiO@CTS hybrid nanocatalyst as an ideal candidate for removal of sulfur compounds from fuel.

Full text of DOI:10.1002/aoc.4895

Received: 12 December 2018
DOI: 10.1002/aoc.4895
Revised: 5 February 2019
Accepted: 11 February 2019
F U L L P A P E R
Facile synthesis of inorganic–organic Fe2W18Fe4@NiO@CTS
hybrid nanocatalyst induced efficient performance in
oxidative desulfurization of real fuel
Mohammad Ali Rezvani
| Zeynab Maleki
Department of Chemistry, Faculty of
Science, University of Zanjan, Zanjan,
Iran
In this work, a new nanocatalyst, Fe2W18Fe4@NiO@CTS, was synthesized by
the reaction of sandwich‐type polyoxometalate (Fe2W18Fe4), nickel oxide
(NiO), and chitosan (CTS) via sol–gel method. The assembled nanocatalyst
was systematically characterized by FT‐IR, UV–vis, XRD, SEM, and EDX anal-
ysis. The catalytic activity of Fe2W18Fe4@NiO@CTS was tested on oxidative
desulfurization (ODS) of real gasoline and model fuels. The experimental
results revealed that the levels of sulfur content and mercaptan compounds
of gasoline were lowered with 97% efficiency. Also, the Fe2W18Fe4@NiO@CTS
nanocatalyst demonstrated an outstanding catalytic performance for the oxida-
tion of dibenzothiophene (DBT) in the model fuel. The major factors that influ-
ence the desulfurization efficiency and the kinetic study of the ODS reactions
were fully detailed and discussed. The probable ODS pathway was proposed
via the electrophilic mechanism on the basis of the electrophilic characteristic
of the metal‐oxo‐peroxo intermediates. The prepared nanocatalyst could be
reused for 5 successive runs without any appreciable loss in its catalytic activ-
ity. As a result, the current study suggested the potential application of the
Fe2W18Fe4@NiO@CTS hybrid nanocatalyst as an ideal candidate for removal
of sulfur compounds from fuel.
Correspondence
Mohammad Ali Rezvani, Department of
Chemistry, Faculty of Science, University
of Zanjan, Zanjan, Iran.
Email: marezvani@znu.ac.ir
KEYWORDS
chitosan, hybrid nanocatalyst, nickel oxide, oxidative desulfurization, polyoxometalate
1 | INTRODUCTION
global challenge over the last year.[4,5] Several desulfuri-
zation technologies have been developed for the produc-
Fossil fuels, including coal, oil and to a lesser extent gas,
naturally abundant organic and inorganic sulfur com-
pounds. The sulfur oxides (SOx) gases are released into
the atmosphere by combustion of hydrocarbon fuels‐
containing sulfur compounds.[1,2] These gases react with
water vapor in the atmosphere and convert to acid rain,
which causes acidification of the soil, and ultimately
leads to the destruction of forest and other types of eco-
system damages.[3] So, production of transportation fuel
with substantially lowered sulfur content has become a
tion of clean fuel such as selective adsorption
desulfurization (ADS),[6] hydrodesulfurization (HDS),[7]
biodesulfurization (BDS),[8] extractive desulfurization
(EDS)[9] and oxidative desulfurization (ODS).[10] Each of
these techniques has its specific advantages and disadvan-
tages. The HDS is applied in many industries toward sul-
fur removal. High temperature and pressure are required
for these reactions, imposes a great deal on the oil indus-
try. In addition, the efficiency of desulfurization for aro-
matic sulfurs is relatively low.[11,12] Therefore,
Appl Organometal Chem. 2019;e4895.
wileyonlinelibrary.com/journal/aoc
© 2019 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4895

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REZVANI AND MALEKI
researchers have begun to focus on the development of
integrated approaches for the deep desulfurization. The
application of ODS in liquid fuels has been attracted
more interest in recent decades.[13,14] It can be carried
out under mild environmental conditions (low tempera-
ture and pressure) and economically viable. As an inter-
esting fact, ODS capable to easily remove of refractory
sulfur compounds (e.g. DBT and their alkyl‐substituted
derivatives).[15,16] High electron density on the sulfur
atom renders aromatic sulfides more susceptible to the
electrophilic attack of the oxidizing agent and their oxida-
tion to sulfones.[17] However, this method still cannot
overcome the challenge in terms of the high cost due to
the employment of a large amount of oxidants (e.g.,
H2O2). Hence, the use of catalysts has drawn increasing
attention for lowering the oxidant consumption and
increasing the reaction efficiency.[18,19]
Polyoxometalates (POMs) include metallic centers or
addenda atoms which surrounded by oxygen atoms,
hence the term “polyoxo” is used for these compounds.
Oxygen atoms are usually present as bridging and termi-
nal oxo ligands.[20] The POMs have attracted the consid-
erable attention of researchers in catalysis due to their
fascinating structures and excellent physic‐chemical char-
acteristics[21,22] Among their various structures,
sandwich‐type transition metal POMs have been devel-
oped as catalysts for the oxidation of alcohols, sulfides,
and for the epoxidation of hydrocarbons.[23] The clusters
of POMs are soluble in most of the polar solvents, causing
difficulties in the separation and recovery, which is an
important restriction for catalytic systems.[24] To confront
this limitation, many approaches were proposed to archi-
tect the POM‐based heterogeneous catalysts. A feasible
strategy is loading metal‐oxo anions on the surface of
the appropriate supports.[25,26] The chitosan (CTS), a lin-
ear polymer of 2‐amino‐2‐deoxy‐β‐D‐glucan, is widely
used as a support material due to its biocompatibility,
high availability, the adsorbent of the organic pollutants,
and mucoadhesive nature.[27] Cross‐linking can take
place between free amino and hydroxyl groups of CST
and active sides of substances such as POMs.[28] Also,
nickel oxide (NiO) particles are very popular in the coat-
ings for the catalysis because of its high specific capaci-
tance, environmental friendly inherent, practical
accessibility, and low cost.[29]
solvent. The effect of the nature of substrates, types of cat-
alyst, the dosage of catalyst, reaction temperature and
time on the ODS efficiency were evaluated. The kinetics
and mechanism of the oxidation reactions and the recy-
clability of the prepared Fe2W18Fe4@NiO@CTS
nanocatalyst were further elucidated.
2 | EXPERIMENTAL
2.1 | Materials and methods
The following chemicals and reagents were purchased
from commercial suppliers and used as received: sodium
tungstate dihydrate (Na2WO4·2H2O), iron (III) nitrate
nonahydrate (Fe (NO3)3·9H2O), chitosan (CTS, low
molecular weight with a degree of deacetylation of 75–
85%), hydrogen peroxide (H2O2, 30 vol.%), glacial acetic
acid (CH3COOH), thiophene (Th), benzothiophene (BT),
dibenzothiophene (DBT), and n‐heptane were obtained
from Sigma‐Aldrich. Also, nickel nitrate hexahydrate
(Ni (NO3)2.6H2O) and citric acid monohydrate (C6H8O7.
H2O) were obtained from Merck.
Preparation of sandwich‐type polyoxometalate
(Na9K[(FeW9O34)2Fe4(H2O)2]·32H2O).
The compound of sandwich‐type polyoxometalate was
prepared as the literature method.[30] Na2WO4·2H2O
(5.00 g) was dissolved in 15 ml of distilled water and
heated to 75 °C. A solution of Fe (NO3)3·9H2O (1.59 g)
in 5 ml of distilled water was added dropwise to the above
solution. The pH of the mixture was adjusted to 3–4 using
CH3COOH, and then the solution was stirred for 60 min.
After completing the reaction, the mixture was cooled
down to room temperature and precipitated by addition
of KCl (6 g). The light brown precipitate of
Na9K[(FeW9O34)2Fe4(H2O)2]·32H2O (abbreviated as
Fe2W18Fe4) was filtrated, washed thoroughly with water
and dried at 80 °C for 3 hr.
2.2 | Preparation of NiO nanoparticles
For the preparation of NiO particles, 2.90 g of Ni
(NO3)2.6H2O and 1.90 g of C6H8O7.H2O were dissolved
completely in 50 mL of distilled water, separately. Then,
the Ni (NO3)2.6H2O solution was dripped in the
C6H8O7.H2O solution. The mixed solution was heated to
80 °C with stirring for 1 hr. After the removal of water
through evaporation, a green gel was formed. The
obtained gel was aged and dried at 90 °C for 2 hr. Then,
the dried gel was calcined at 400 °C for 4 hr.
In consideration for application of the inorganic–
organic hybrid nanocatalysts, in this work, the
Fe2W18Fe4@NiO@CTS nanocatalyst was synthesized
and found to be a high‐performance catalyst for ODS pro-
cess. The oxidation reactions were performed on the
model fuels and real gasoline. The sulfur‐containing mol-
ecules were oxidized using in situ production of
peroxyacetic acid and then extracted by acetonitrile

REZVANI AND MALEKI
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SCHEME 1 Schematic illustration of the preparation of Fe2W18Fe4@NiO@CTS nanocatalyst
ultrasonic conditions using an ultrasonic water bath
(frequency 35 kHz mains connection 230 V) for 30 min.
The resultant product (Fe2W18Fe4@NiO@CTS) was sepa-
rated by centrifugation (2500 rpm for 40 min) and dried
at 70 °C for 4 hr (Scheme 1).
2.3 | Preparation of Fe2W18Fe4@NiO@CTS
nanocatalyst
In the same manner as described above, the mixed solu-
tion of Ni (NO3)2.6H2O and C6H8O7.H2O was prepared
and heated to 80 °C under stirring. Then, 0.03 g of the syn-
thesized Fe4W18Fe2 was dissolved in 5 ml boiling distilled
water and added to the mixture. The solution was heated
until the formation of a green gel. The gel was dried at
90 °C for 2 hr and calcined at 400 °C for 4 hr to obtain a
black powder. Also, 0.10 g of CTS was dissolved in 25 ml
CH3COOH (2%) solution, separately, and stirred for half
an hour to get a clear solution. Then, 0.05 g of the prepared
black powder was added to the CTS solution under
2.3.1 | Oxidative desulfurization of model
fuels
In a typical reaction run, the catalysis system was pre-
pared as follows: the different model thiophenic com-
pounds (MTCs) such as Th, BT, and DBT as a source of
sulfur were dissolved separately in n‐heptane to
FIGURE 1 FT‐IR spectra of (a) Fe2W18Fe4, (b) NiO, (c) CTS, and (d) Fe2W18Fe4@NiO@CTS nanocatalyst

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REZVANI AND MALEKI
represent model fuels. The initial concentration of each
MTC in the model fuels was 500 ppm by weight.
Then, prepared model fuel (50 ml) was placed in a two‐
necked round‐bottom flask coupled in a temperature‐
controlled oil bath. The appropriate amounts of Fe2W18
Fe4@NiO@CTS nanocatalyst and the aqueous solution
of peroxyacetic acid (3 ml of CH3COOH/H2O2 in a vol-
ume ratio of 1/2) were added to the flask. The reaction
was proceeded under vigorous stirring (700 rpm) at vari-
ous temperatures and times to evaluate the catalytic
activity of the nanocatalyst. After completion of the reac-
tion, the solution was cooled to room temperature,
followed by addition of 10 mL of acetonitrile to extract
the oxidation products. The formed immiscible mixture
of n‐heptane and water was separated by a separation
funnel. The sulfur content in both phases was analyzed
by GC–MS (Varian cp‐1200 quadrupole MS), HP5890
Series II with 5972 Series MS detector. The results were
indicated that a perfect match of the mass spectrum of
the product with the standard Th, BT, and DBT sulfone.
Also, the oil phase (n‐heptane) was analyzed using the
XRF spectrometer according to D‐4294 and D‐3227
ASTM analysis methods.
at the predefined temperature for 60 min. After cooling
the solution, 10 ml of polar acetonitrile extraction solvent
was utilized to remove sulfoxide and sulfone molecules.
The gasoline phase was separated from the aqua phase
and analyzed for the determination of the sulfur amount.
The percentage of sulfur removal efficiency was expressed
by the following equation:
Perecentage of sulfur removal efficiency
ꢀ
ꢁ
Sf
¼ 1 −
× 100
(1)
Si
where Si and Sf represent the initial concentration and
the final concentration of sulfur compounds, respectively.
2.5 | Characterization methods
The Fourier transform infrared (FT‐IR) spectra of the
samples were recorded using a Thermo‐Nicolet‐iS10
spectrometer. Ultraviolet–visible (UV–vis) spectra were
measured with a double beam Thermo‐Heylos spectrom-
eter. The powder X‐ray diffraction (XRD) of the mate-
rials was performed by a Bruker D8 Advance X‐ray
diffractometer with the wavelength of 1.5406 Å of CuKα
radiation in the 2θ range of 5–80°. The surface morphol-
ogy of the samples was examined by scanning electron
microscopy (SEM) on LEO 1455 VP equipped with an
energy dispersive X‐ray (EDX) spectroscopy apparatus.
The EDX analysis was performed to further confirm
the composition of the prepared nanocatalyst. The ultra-
sonic water bath was used a Bandelin Sonorex Digitec
frequency 35 kHz mains connection 230 V. The content
of total sulfur in gasoline and model fuel were
2.4 | Oxidative desulfurization of real
gasoline
The ODS test of gasoline was conducted in the same man-
ner as in the way described above. In brief, gasoline
(50 ml) was added to the flask and heated to 35 °C. The
prepared Fe2W18Fe4@NiO@CTS nanocatalyst (0.10 g)
and peroxyacetic acid oxidant (3 ml) were poured into
the reaction vessel. The mixture was stirred magnetically
FIGURE 2 UV–vis spectra of (a) Fe2W18Fe4, (b) NiO, (c) CTS,
FIGURE 3 XRD patterns of (a) Fe2W18Fe4, (b) NiO, (c) CTS, and
and (d) Fe2W18Fe4@NiO@CTS nanocatalyst
(d) Fe2W18Fe4@NiO@CTS nanocatalyst

REZVANI AND MALEKI
5 of 12
FIGURE 4 SEM image of (a) Fe2W18Fe4, (b) NiO, (c) CTS, and (d) Fe2W18Fe4@NiO@CTS nanocatalyst
FIGURE 5 EDX analysis of Fe2W18Fe4@NiO@CTS nanocatalyst

6 of 12
REZVANI AND MALEKI
determined using X‐ray fluorescence (XRF) with a
TANAKA X‐ray fluorescence spectrometer RX‐360 SH.
500–4000 cm−1 wavenumber range. According to
Figure 1(a), the absorption peaks of the synthesized
Fe2W18Fe4 are shown bands at 939, 866, 782 and
667 cm−1, which are assigned to the stretching vibra-
tions of W‐Ot, W‐Ob‐W, W‐Oc‐W, and also Fe‐Oa,
respectively.[30,31] These peaks suggest that the sandwich
structure of synthesized Fe2W18Fe4 is preserved. In
Figure 1(b), the absorption band at 800 cm−1 is assigned
to Ni‐O‐H stretching bond. A broad peak at 571 cm−1 is
caused by the vibration band of Ni‐O in.[32] The infrared
spectrum of CTS is shown in Figure 1. (c). The
stretching vibrations of ‐OH bond is found at
3409 cm−1. The absorption band at 2905 cm−1 is
3 | RESULTS AND DISCUSSION
3.1 | Characterization of materials
The FT‐IR spectroscopic analysis was carried out to
demonstrate the successful preparation of the
Fe2W18Fe4@NiO@CTS nanocatalyst. Figure 1 displays
the sequence FT‐IR spectra of (a) Fe2W18Fe4, (b)
NiO, (c) CTS, and (d) Fe2W18Fe4@NiO@CTS in the
TABLE 1 ODS results of real gasoline by Fe2W18Fe4@NiO@CTS nanocatalyst
a
Entry
Properties of gasoline
Unit
Method
Before ODS
After ODS
1
2
3
4
5
6
Total Sulfur content by X‐Ray
Mercaptans
wt.%
ppmw
g/ml
ptb
ASTM D 4294
ASTM D 3227
ASTM D 1298
ASTM D 3230
ASTM D 4006
ASTM D 86
0.4997
97
0.0146
3
Density by hydrometer @ 15 °C
Salt
0.7995
15
0.7994
15
Water content by distillation
vol.%
°C
Nil.
Nil.
Distillation
IBP
FBP
10
48.7
209.3
68.9
48.6
209.1
68.8
vol.%
50
90
95
119.8
187.8
207.4
119.5
187.6
207.3
aCondition of ODS process: 50 ml of gasoline, 0.10 g of nanocatalyst, 3 ml of oxidant, 10 ml of acetonitrile, reaction temperature = 35 °C, and reaction
time = 60 min.
a
TABLE 2 Effect of different catalysts on the ODS of real gasoline and model fuels
ODS efficiency (%)
Entry
Catalyst
Th
BT
DBT
Gasoline
Ref.
1
Fe2W18Fe4@NiO@CTS
TBA‐PW11Ni@NiO
CTAB‐PTA@CS
PMo@HKUST‐1
K5PMo11CuO39
K3PMo12O40
K3PW12O40
97
96
92
90
61
60
58
56
68
48
38
98
97
93
‐
99
98
95
95
64
61
60
58
71
45
35
97
97
94
‐
In this work
[29]
2
[35]
[14]
[41]
[41]
[41]
[41]
3
4
5
62
60
59
56
69
46
34
60
59
58
55
67
49
34
6
7
8
K4SiW12O40
9
Fe2W18Fe4
In this work
In this work
In this work
10
11
NiO
CTS
aCondition of ODS process: 50 ml of gasoline, 0.10 g of nanocatalyst, 3 ml of oxidant, 10 ml of acetonitrile, reaction temperature = 35 °C, and reaction
time = 60 min.

REZVANI AND MALEKI
7 of 12
corresponded to C‐H stretching vibration of ‐CH2‐
groups. The band at 1661 cm−1 is assigned to the
stretching vibrations of C=O in the aldehyde on the ter-
minal anhydroglucose unit. Also, the peak at 1589 cm−1
is related to the bending vibrations of the N‐H (N‐acety-
lated residues, amide band).[33] In the IR spectrum of
Fe2W18Fe4@NiO@CTS nanocatalyst, the observed bands
at 660,873, 1012 cm−1 are indicated that the structure of
sandwich‐type POM retained after immobilization on
supports (Figure 1 (d)). Further, the peaks of bulk NiO
at 571 and 800 cm−1 are discernable maybe by covering
with the bands of Fe2W18Fe4. Nevertheless, some shifts
that are seen in the FT‐IR spectrum of the
Fe2W18Fe4@NiO@CTS nanocatalyst compared to the
bulk samples confirmed their successful composition.
The UV–vis absorption spectra of (a) Fe2W18Fe4, (b)
NiO, (c) CTS, and (d) Fe2W18Fe4@NiO@CTS are repre-
sented in Figure 2. The two strong peaks are observed
at 238 and 294 nm are due to charge transfer transition
The powder X‐ray diffraction patterns of (a)
Fe2W18Fe4, (b) NiO, (c) CTS, and (c) Fe2W18
Fe4@NiO@CTS nanocatalyst was comparatively exam-
ined by XRD analysis (Figure 2). As shown in Figure 3
(a), the intense diffraction peaks at 2θ values of 17.8,
24.3, 27.2, 29.4, 31.7, 35.3, 45.5, 56.5, and 75.3° are related
2 −
6 +
of O
→ W
in the Fe2W18Fe4 polyoxoanion, where
W atoms are located in W–Ob/c–W.[34] The absorption
band at 239 nm in Figure 2(c) is related to the formation
of NiO particles.[32] The absorbance spectrum of
prepared Fe2W18Fe4@NiO@CTS nanocatalyst is reported
in Figure 2 (d). In this curve, the intensity of peaks is
decreased and the slight shift is observed, which
indicates the interactions between Fe2W18Fe4, NiO,
and CTS.
TABLE 3 Effect of catalyst dosage on the ODS of real gasoline
and model fuels by Fe2W18Fe4@NiO@CTS nanocatalysta
ODS efficiency (%)
Amount of
Entry
catalyst (g)
Th
BT
DBT
Gasoline
1
0 (blank test)
0.01
13
19
27
37
48
60
71
77
87
92
97
97
97
15
20
28
37
48
59
70
79
88
92
98
98
98
16
20
31
38
49
64
74
83
90
95
99
99
99
14
18
26
37
49
58
70
78
86
90
97
97
97
2
3
0.02
4
0.03
5
0.04
6
0.05
7
0.06
8
0.07
9
0.08
10
11
12
13
0.09
0.10
0.11
0.12
FIGURE 6 Effect of reaction temperature and reaction time on
ODS of (a) Th, (b) BT, and (c) DBT. (Conditions of ODS: 50 ml of
model fuel, 0.10 g of Fe2W18Fe4@NiO@CTS nanocatalyst, 3 ml of
oxidant, 10 ml of acetonitrile)
aCondition of ODS process: 50 ml of gasoline, 0.10 g of nanocatalyst, 3 ml of
oxidant, 10 ml of acetonitrile, reaction temperature = 35 °C, and reaction
time = 60 min.

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REZVANI AND MALEKI
to the pattern of synthesized sandwich‐type Fe2W18Fe4,
indicating the presence of WO3 species according to
JCPDS card No. 00–054‐0508. According to Figure 3(b),
the XRD diffraction peaks of NiO at 37.5, 43.3, 62.8,
75.5, and 79.2° are attributed to 101, 012, 110, 113, and
202 crystal planes, respectively (JCPDS, No. 04–0835).
The diffraction pattern of pure CTS that is shown in
Figure 3(c) has a sharp peak at 19.8° which demonstrates
the semi‐crystalline nature of polymer.[35] The
Fe2W18Fe4@NiO@CTS nanocatalyst was exhibited simi-
lar reflections to the pattern of NiO and CTS support
and the peaks of Fe2W18Fe4 did not observe clearly
(Figure 3(d)). This phenomenon reflected due to the
excellent dispersion of Fe2W18Fe4 particles on the surface
of the solid supports (NiO and CTS).[36] The average crys-
tallite size of the Fe2W18Fe4@NiO@CTS nanocatalyst
was estimated to be about 46.50 nm using Debye–
Scherrer formula.
SEM as a surface imaging method was accomplished
to provide morphological details of the materials. The
irregular and agglomerated particles of Fe2W18Fe4 with
different sizes are depicted in Figure 4(a). Regarding
the electronic microscopy studies, the NiO sample has a
spherical morphology and the pure CTS shows the flat
surface (Figure 4(b) and 4(c)). Compared to the morphol-
ogy of the bulk materials, the Fe2W18Fe4@NiO@CTS
composite implies the well‐dispersed nanoscale spherical
particles (Figure 4(d)). According to the EDX analysis of
Fe2W18Fe4@NiO@CTS, the successful preparation of
nanocatalyst is confirmed by the existence of Fe, W, Ni,
C, and O elements with an approximated percentage
quantity of 0.86, 5.70, 23.20, 44.01, and 26.23%, respec-
tively (Figure 5).
3.2 | Oxidative desulfurization results of
real gasoline and model fuels
The catalytic activity of Fe2W18Fe4@NiO@CTS heteroge-
neous nanocatalyst was assessed by the removal of
sulfur compounds using peroxyacetic acid as the oxidant.
The results of the ODS technique were listed in Table 1.
The total sulfur content (wt.%) of gasoline was reduced
drastically from 0.4997 to 0.0146 wt.% with the removal
efficiency of 97% at 35 °C after 60 min. Also, the finest
removal of mercaptan sulfur compounds occurred from
97 to 3 ppm under the same conditions. It was worth
pointing out that the other specifications of gasoline
(e.g. density or water content) were remained
largely unchanged after ODS treatment compared to their
corresponding initial values. On the other hand, the
removal of each MTC was evaluated in order to examine
the performance of Fe2W18Fe4@NiO@CTS nanocatalyst.
The sulfur‐containing species consisting of Th, BT, and
DBT were removed from n‐heptane phase with 97, 98,
and 99% efficiency, respectively. The oxidation reactivity
of the MTCs by Fe2W18Fe4@NiO@CTS nanocatalyst
was in the order of Th < BT < DBT. This is the same
order that obtained in peroxyacetic acid/POM catalytic
oxidation systems.[37,38] The lowest reactivity of Th in
SCHEME 2 Schematic illustration of the probable mechanism for the oxidation of DBT as a model sulfur compound by formation of
peroxo‐tungstate complex

REZVANI AND MALEKI
9 of 12
the ODS can be explained by its low electron density
(5.696). Compared to Th, the high reactivity of DBT is
due to the electronic donation effect of the alkyl
groups surrounded the sulfur atom that improves the
electron density (5.758).[39,40]
The results revealed that changing the temperature and
time affect the rate of the ODS reactions. Despite the ana-
lyzing experimental data, when the reaction was carried
out at 30 °C, the desulfurization percentage was approxi-
mately 85% after 60 min. The highest sulfur removal effi-
ciency was recorded at the temperature of 35 °C. The
reaction time was the most important design parameter
3.3 | Effect of different catalysts
In order to investigate the effect of different catalysts, a
series of ODS reactions were carried out. The comparative
results of catalytic activity of Fe2W18Fe4, NiO, CTS,
Fe2W18Fe4@NiO@CTS, and some different types of cata-
lysts were listed in Table 2. The results indicated clearly
that in the blank test (without catalyst) the removal effi-
ciencies of Th, BT, and DBT were examined 13, 15, and
16%, respectively (Table 2, Entry 8). Also, 14% of the sulfur
content of real gasoline was lowered. In this work, it was
reflected that the Fe2W18Fe4@NiO@CTS was the most
effective catalyst among the tested ones. The NiO and
CTS did not show significant catalytic performance com-
pared with the Fe2W18Fe4@NiO@CTS nanocatalyst. The
results suggested that the combination of the
beneficial properties of POM, NiO, and CTS compounds
could be a worthwhile response to the need for an efficient
inorganic–organic hybrid catalyst for ODS. According to
the Entry 1, nearly complete ODS efficiency (up to 96%)
was
attained
using
only
0.10
g
of
the
Fe2W18Fe4@NiO@CTS nanocatalyst within 60 min of
the reaction at 35 °C.
3.4 | Effect of nanocatalyst dosage
The optimum dosage of the Fe2W18Fe4@NiO@CTS
nanocatalyst also was determined for the ODS of real gas-
oline and MTCs present in model fuels with the concen-
tration of 500 ppm (Table 3). It could be deduced that
the desulfurization rate increased with increasing
nanocatalyst dosage, although no significant change
observed when the catalyst dosage increased. No increase
in the ODS efficiency at high dosage (>0.10 g) might be
associated with the accumulation of the metal‐oxo‐peroxo
intermediates that hinders the adsorption and oxidation
of sulfides. The optimum nanocatalyst dosage was found
to be 0.10 g for the forthcoming experiments.
3.5 | Effect of reaction temperature and
time
As shown in Figure 6, the catalytic oxidation perfor-
mance of Fe2W18Fe4@NiO@CTS nanocatalyst was com-
pared at different temperatures 25, 30, 35, and 40 °C.
FIGURE 7 Plots of S f /Si against time for the ODS of (a) Th, (b)
BT, and (c) DBT

10 of 12
REZVANI AND MALEKI
that affects the performance of the ODS system. The
removal efficiencies of MTCs from the model fuels at
35 °C rapidly were increased from 35% in the 10 min of
reaction up to 95% as the stirring time increased to
60 min. Thus, reaction temperature and time were set to
30 °C and 60 min, respectively, as optimum conditions
of ODS systems.[15,35]
first‐order kinetic model. The variations of MTCs concen-
tration [Sf /Si] with t were shown in Figure 7. In this
study, the correlation coefficients were found close to
unity (Table 4). It was clearly observed that ODS systems
followed pseudo‐first‐order kinetics in all cases. Also, the
apparent activation energies (Ea) for the ODS of sulfur
molecules were calculated from Arrhenius plots
3.6 | Proposed catalytic pathway of
oxidative desulfurization process
According to the experimental results and discussion
above, the ODS process of sulfur‐containing compounds
catalyzed by the heterogeneous Fe2W18Fe4@NiO@CTS
nanocatalyst was proposed as follows: before the ODS,
the MTCs were in the non‐polar oil phase (n‐heptane or
gasoline). After the addition of the nanocatalyst and oxi-
dant, the oxidation reactions were performed in the inter-
face of non‐polar/catalyst phase (Scheme 2). The catalytic
sulfur oxidation mechanism was well studied using POM‐
based catalysts.[41–43] In all cases, the metal‐oxo‐peroxo
intermediate complexes were introduced as responsible
species for oxidation of sulfides. These electrophilic inter-
mediate species (abbreviated M(O2)) were formed by the
reaction of the peroxide oxygen in CH3COOH with termi-
nal metal‐oxygen groups (M = Ot) in the structure of
sandwich‐type POM. The sulfur atom of organic com-
pounds, which is quite nucleophilic, can attack on
M(O2) species to produce the corresponding sulfoxide
(RSO) and sulfones (RSO2).[44] The reaction products
accumulate in the water phase due to their nature of
polarity, which can be considered to be a benefit for
extracting by acetonitrile solvent.[45]
3.7 | Kinetic studies of oxidative
desulfurization process
In order to evaluate the kinetics of the oxidation of
MTCs, the values of the apparent reaction rate constant
k (min−1) were determined on the basis of the pseudo‐
TABLE 4 Pseudo‐first‐order rate constants and correlation fac-
tors of the ODS of MTCs
Rate constant k
Th BT DBT Th
Correlation factor R2
BT DBT
Temperature
(°C)
25
30
35
40
0.019 0.021 0.021 0.9125 0.9187 0.9342
0.024 0.025 0.029 0.8777 0.8892 0.9576
0.048 0.045 0.052 0.9305 0.9174 0.8725
0.048 0.050 0.058 0.9305 0.9477 0.9057
FIGURE 8 Arrhenius plots for the ODS of (a) Th, (b) BT, and (c)
DBT

REZVANI AND MALEKI
11 of 12
(Equation 6 and Figure 8). The Ea values were 49.69,
51.84 and 57.54 kJ/mol for Th, BT, and DBT, respectively.
desulfurization yield for DBT using the recovered catalyst
is only slightly lower (about 4%) than the fresh catalyst
after the 5 times. The decrease in the removal efficiency
of DBT from 99 to 95% may be attributed to the adsorp-
tion of DBTO and DBTO2 molecules on the surface of
the Fe2W18Fe4@NiO@CTS catalyst. On the basis of the
above evidence, it can be concluded that the designed
Fe2W18Fe4@NiO@CTS nanocatalyst is a promising het-
erogeneous catalyst in terms of catalytic activity and
reusability.
d½Sꢀ
rs ¼
¼ −k½Sꢀ
(3)
(4)
dt
S
∫
t
f d½Sꢀ
¼ −∫0kdt
Si
½Sꢀ
ꢀ
ꢁ
ꢀ
ꢁ
Â
Ã
Sf
Si
Sf
Si
ln Sf − ln½Siꢀ ¼ −kt ⇒ ln
¼ e−kt
¼ −kt ⇒
(5)
(6)
4 | CONCLUSIONS
ꢂ ꢃ
−E
RT
−Ea
1
T
a
In this work, the Fe2W18Fe4@NiO@CTS hybrid material
was prepared successfully and developed to catalyze the
ODS reactions. The Fe2W18Fe4@NiO@CTS nanocatalyst
exhibited outstanding performance for the oxidation of
organic sulfur‐containing molecules of gasoline and
model fuels under mild conditions. The removal effi-
ciency of the sulfur content and the mercaptan com-
pounds of gasoline were achieved to 97% within 60 min
at 35 °C. Further, the aromatic sulfurs such as Th, BT,
and DBT could be removed from n‐heptane with the yield
of 97, 98, and 99%, respectively. The report indicated that
the kinetics of oxidative processes followed the pseudo‐
first‐order model. Moreover, the heterogeneous
nanocatalyst showed excellent reusability properties for
5 oxidation runs. Despite the results attended in this
research, it is suggested that the Fe2W18Fe4@NiO@CTS
can be a high‐performance inorganic–organic hybrid
nanocatalyst for deep ODS treatments.
k ¼ Ae ⇒ lnk ¼
þ lnA
R
where Si and S f were the initial and final concentrations
of the sulfur compound (ppmw) in the n‐heptane phase
and t was reaction time (min.).[35,41,44]
3.8 | Reusability performance of
Fe2W18Fe4@NiO@CTS nanocatalyst
The reusability performance of heterogeneous
Fe2W18Fe4@NiO@CTS nanocatalyst was carried out
under optimized conditions for the ODS of DBT as the
model component. The catalyst was separated from the
reaction media at the end of the oxidation process by sim-
ple filtration, washed with the CH2Cl2 solvent, and then
dried at 70 °C for 3 hr. The recycled nanocatalyst was
processed for the consequent reaction using the oxidant
and extraction solvents. After each catalytic run, the oil
phase was analyzed to determine the DBT concentration
in n‐heptane. The results are shown in Figure 9. The
ORCID
Mohammad Ali Rezvani
https://orcid.org/0000-0001-
6155-5147
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How to cite this article: Rezvani MA, Maleki Z.
Facile synthesis of inorganic–organic
Fe2W18Fe4@NiO@CTS hybrid nanocatalyst
induced efficient performance in oxidative
desulfurization of real fuel. Appl Organometal
Chem. 2019;e4895. https://doi.org/10.1002/aoc.4895
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