Boosting oxidative desulfurization of model fuel by POM-grafting ZIF-8 as a novel and efficient catalyst

Article abstract of DOI:10.1016/j.poly.2019.05.058

In this article, a water-soluble Keplerate-type polyoxometalate (POM), known as {Mo₁₃₂} nanoballs, was supported over the zeolitic imidazolate framework-8 (ZIF-8) to achieve a highly efficient heterogeneous nano-structure catalyst (ZIF-8@{Mo₁₃₂}) evaluated in oxidative desulfurization (ODS) of dibenzothiophene (DBT) presenting in model fuel. The ZIF-8@{Mo₁₃₂} nano-structured catalyst was characterized using various methodologies including FT-IR, XRD, SEM, TGA, UV–Vis spectra of solid state and N₂ adsorption–desorption confirming the incorporation of the {Mo₁₃₂} nanoballs over the ZIF-8. The catalytic ODS performances of the as-synthesized catalyst revealed that high DBT removal efficiency around 87% and 92% after 6 and 12 h of reaction, respectively, in the presence of tert-butyl hydroperoxide (TBHP) as an oxidant, while they were only around 40% and 55% for pristine ZIF-8 at the same condition. The reaction mechanism suggested that the presence of Mo atoms enhances the formation of metal-peroxide formed by nucleophilic attack of TBHP and consequently enhancing DBT removal from a model fuel. Thin-layer chromatography (TLC) and gas chromatography (GC) were implemented for DBT concentration determination. In addition, the operating variables, including catalyst dosage, temperature, initial concentration of DBT, and oxidant to sulfur (O/S) ratio affecting the ODS efficiency were investigated to find the optimum condition. Finally, the remarkable heterogeneous catalyst indicated recyclability for various catalytic cycles.

Full text of DOI:10.1016/j.poly.2019.05.058

Accepted Manuscript
Boosting oxidative desulfurization of model fuel by POM-grafting ZIF-8 as a
novel and efficient catalyst
Mahboube Ghahramaninezhad, Fatemeh Pakdel, Mahdi Niknam Shahrak
PII:
S0277-5387(19)30396-1
https://doi.org/10.1016/j.poly.2019.05.058
POLY 13983
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
20 February 2019
17 May 2019
29 May 2019
Please cite this article as: M. Ghahramaninezhad, F. Pakdel, M.N. Shahrak, Boosting oxidative desulfurization of
model fuel by POM-grafting ZIF-8 as a novel and efficient catalyst, Polyhedron (2019), doi: https://doi.org/10.1016/
j.poly.2019.05.058
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Boosting oxidative desulfurization of model fuel by
POM-grafting ZIF-8 as a novel and efficient catalyst
Mahboube Ghahramaninezhad, Fatemeh Pakdel, and Mahdi Niknam Shahrak 
Department of Chemical Engineering, Faculty of Engineering, QuchanUniversity of Technology,
Quchan, Iran
Abstract
In this article, a water-soluble Keplerate-type polyoxometalate (POM), known as {Mo132
nanoballs, was supported over the Zeolitic imidazolate framework-8 (ZIF-8) to achieve a highly
efficient heterogeneous nano-structure catalyst (ZIF-8@{Mo132}) evaluated in oxidative
desulfurization (ODS) of dibenzothiophene (DBT) presenting in model fuel. The ZIF-
}
8
@{Mo132} nano-structured catalyst was characterized using various methodologies including
FT-IR, XRD, SEM, TGA, UV–Vis spectra of solid state and N adsorption-desorption
2
confirming the incorporation of the {Mo132} nanoballs over the ZIF-8. The catalytic ODS
performances of the as-synthesized catalyst revealed that high DBT removal efficiency around
8
7 and 92% after 6 and 12 hours of reaction, respectively, in the presence of tert-butyl
hydroperoxide (TBHP) as an oxidant, while they were only around 40 and 55% for pristine ZIF-
at the same condition. The reaction mechanism suggested that the presence of Mo atoms
8
enhances the formation of metal-peroxide formed by nucleophilic attack of TBHP and
consequently enhancing DBT removal from a model fuel. Thin-layer chromatography (TLC) and
gas chromatography (GC) were implemented for DBT concentration determination. In addition,
the operating variables, including catalyst dosage, temperature, initial concentration of DBT, and
oxidant to sulfur (O/S) ratio affecting the ODS efficiency were investigated to find the optimum
condition. Finally, the remarkable heterogeneous catalyst indicated recyclability for various
catalytic cycles.
Keywords: ZIF-8@{Mo132}, Oxidative desulfurization, DBT, Metal-organic framework,
Polyoxometalate.

Corresponding Author’s E-mail: M.niknam.sh@qiet.ac.ir & Mehdiniknam.sh@gmail.com
1

1
. Introduction
Sulfur-containing compounds (e.g., Mercaptans, thiophenes, benzothiophenes, and
dibenzothiophenes) are considered as one of the most harmful pollutants in various types of
liquid fuels causing air pollutions in most countries. In addition to negative environmental
effects, they also have poisoning effects over the catalysts of the chemical process [1,2]. Thus,
strict worldwide regulations for environmental protection have set the maximum total sulfur
content in diesel fuel to be less than 15 ppm [3]. Among the sulfur-containing compounds in the
liquid fuels, thiophenic compounds, and their alkyl-substituted derivatives are known as the most
persistent compounds which need stronger methods for their removal from fuels [4]. There are
several processes to remove sulfur compounds from hydrocarbon streams. Desulfurization
methods including hydro-desulfurization (HDS), bio-desulfurization, oxidative desulfurization,
adsorptive desulfurization, extractive desulfurization, and ultrasound-assisted desulfurization are
some of the well-known procedures [5, 6]. In a very conventional desulfurization process, HDS
technique, sulfur compounds are converted to hydrogen sulfide using hydrogen gas and specific
catalysts. Removal of aliphatic sulfur compounds using HDS process is very effective, but deep
desulfurization process involving the removal of sulfur aromatic compounds such as
benzothiophene (BT) and dibenzothiophene (DBT) and their derivatives requires harder
operating conditions including high temperature, pressure, and consumption of hydrogen. These
restrict operational conditions will result in the increasing of operating costs, which is not
economically affordable [7].
On the other hand, oxidative desulfurization (ODS) process has recently attracted aroused
attentions of researchers owing to mild conditions (at atmospheric pressure and below 100 °C)
required for sulfur removal in the presence of a catalyst and an oxidant agent [8-10]. In this
2

process, sulfur compounds are oxidized to their sulfones or sulfoxides, which are essentially
more polar than oil phase compounds. This allows the selective removal of sulfur compounds
from fuels by a combination process of ODS and extraction or adsorption [7]. This method is
more convenient compared to HDS from an economic and environmental point of view, and the
resistant sulfur compounds can be easily removed at ambient temperature and atmospheric
pressure. A very important point relevant to the ODS process is related to the presence of an
efficient catalyst which directly affects the ODS efficiency.
Consequently, a large variety of studies have been carried out on the evaluation of different
catalysts under different conditions. Up to now, a series of catalysts have been utilized in the
ODS processes such as mixed metal oxides, ionic liquids, alkali metal salts, and recently metal-
organic frameworks (MOFs) [11-15]. Among various catalysts that can be employed as a solid
catalyst for ODS reactions, metal-organic frameworks (MOFs) owing to several advantages
including high density of active sites, different metal centers, the ability to adjust the shape and
size of pores and high surface area and permanent porosity [16, 17] are always known as a
promising candidate for catalytic oxidation processes [18]. Although a lot of MOF materials
identified and synthesized so far, only a few works are available on using pristine MOF materials
as a catalyst in ODS reactions [18, 19].
The ODS catalytic activity of MOF materials is usually improved by functionalization,
especially with amino functional groups, and by combination particularly with polyoxometalates
(POM) [20-22]. Generally, polyoxometalates are defined as anionic molecular metals based on
transition metals or metal oxide clusters with high negative charge [23]. Among this type of
materials, water-soluble Keplerate-type {Mo132} is one of the most important polyoxometalates
[
24] acting as a catalyst for many oxidation processes [25-27].
3

In this study, for the first time, the ODS catalytic activity of a MOF material, known as zeolitic
imidazolate frameworks-8 (ZIF-8), has been improved using its surface modification with
{
Mo132}. For this purpose, after preparation of ZIF-8 [17], {Mo132} was loaded over the surface
of ZIF-8 to reach a new and green ZIF-8@{Mo132} nano-structure catalyst for successful
oxidative desulfurization of DBT in a model fuel in the presence of tert-butyl hydroperoxide
(TBHP) as an oxidant agent. It is believed that the presence of Mo atoms enhances the formation
of metal-peroxide formed by nucleophilic attack of TBHP, causing great catalytic activity of
ZIF-8@{Mo132}. Scheme 1 shows a simple schematic for the preparation of ZIF-8@{Mo132
}
nano-structure catalyst used in the ODS process. Furthermore, Scheme 2 reveals the oxidation
reaction of DBT and its corresponding sulfone and sulfoxide produced by ZIF-8@{Mo132
catalyst. Worthwhile mentioning that DBT is recognized as one of the most stable sulfur
compounds in liquid fuels so that its removal would guarantee the other types of sulfur
compounds elimination [4].
}
2
-MIM + Zn Ions
Room Temp.
DI water + {Mo132
Room Temp.
}
ZIF-8
ZIF-8
ZIF-8@{Mo132}
{Mo132}
Scheme 1. A simple schematic for the preparation of ZIF-8@{Mo132} nano-structure catalyst.
4

ZIF-8@{Mo132}
TBHP
+
S
S
S
O
O
O
Scheme 2. A schematic of the oxidation DBT to its corresponding sulfone and sulfoxide.
2
. Experimental
2
.1. Materials
All chemicals and reagents were used as received without any further purification. 2-
Methylimidazole (2-MIm) (purity 99%) and zinc nitrate hexahydrate (purity 99%) were obtained
from Sigma-Aldrich and were used for the synthesis of the ZIF-8. Ammonium heptamolybdate,
hydrazine sulfate, ammonium acetate were obtained from Sigma-Aldrich and were used for the
synthesis of the {Mo132} nanoballs. Dibenzothiophene (DBT, 98%), toluene (99.9%), acetonitrile
(99.9%), N,N-dimethylformamide (99%), n-octane (99%), hexane (99%), dichloromethane
(99.9%), TBHP (aqueous solution, 70%), acetone (99%), acetic acid (99.9%), ammonia solution
(25%) and ethanol (98%) were purchased from Merck Company (Germany).
2
.2. Instruments
Physico-chemical characteristics of the catalysts were determined by several methods including
powder X-ray diffraction (XRD), Scanning Electron Microscope (SEM), Fourier transform
infrared spectroscopy (FT-IR), thermo-gravimetric analysis (TGA), UV–Vis spectroscopy, and
Brunauer- Emmett-Teller (BET) surface area. XRD has performed on a Bruker D8-Advance X-
ray diffractometer with Cu-Kα (λ=1.54178 Å) radiation. SEM images were taken using
instrument model LEO1450 VP (Variable Pressure) equipped with an Energy Dispersive
−
1
Spectrometer (EDS). The FT-IR spectra (400–4000 cm ) of the adsorbent dispersed in KBr
pellets were recorded on a NICOLET system. TGA of the catalyst was performed in a nitrogen
5

atmosphere by Perkin Elmer, Pyris. UV–Vis spectra of the solid state were obtained with a
Lambda-35 spectrometer (Perkin-Elmer). Finally, N adsorption and desorption isotherms at 77
2
K were measured by using a Micromeritics ASAP 2020 adsorption instrument to determine the
textural properties of ZIF-8 and ZIF-8@{Mo132} catalysts including the specific surface area and
pore volume. The progress of the reactions and sulfur content of samples was monitored by thin
layer chromatography (TLC) using silica-gel SIL G/UV 254 plates also by an Agilent-6890 gas
chromatograph equipped with an FID using an HP-5ms capillary column (30 m ×0.25 mm i.d.,
0
.25 μm).
2
.3. Preparation of ZIF-8@{Mo132} catalyst
Preparation of ZIF-8@{Mo132} nano-structure catalyst was followed according to our recently
published procedure [28]. In brief, after successful synthesis of ZIF-8 [17] and {Mo132} [29]
compounds (see supporting information, Fig. S1 to Fig. S9), 0.001 mmol of {Mo132} was
carefully weighted and solved in 50 mL deionized water, then it was added to 1 mmol ZIF-8
sample at room temperature. The reaction mixture kept stirred for 14 h at 25 ⁰C. Afterward, the
product was separated using 14000 rpm centrifuging and followed by washing with deionized
water for several times. Finally, the pale brown color ZIF-8@{Mo132} nano-structure was
obtained after drying for 6 h at 100 ⁰C in a vacuum oven.
2
.4. General procedure for desulfurization of the model fuel
First, the model fuel with the initial sulfur concentration of 500 ppm was prepared using
dissolving of 500 mg DBT in 1 liter of toluene. Then, the required amount of catalyst is added to
the appropriate amount of TBHP, and the mixture is stirred in the air for a few minutes. In a
typical run, the model fuel solution is added to this mixture (15 mL) in a two-necked glass flask
6

equipped with a recyclable water condenser. The glass flask is then put inside a paraffin bath
located over a magnetic stirrer to heat and stabilize the specified temperatures during all runs
(Scheme 3). The reaction mixture finally kept under continuous stirring at a constant speed to
reach the required efficiency. The reaction progress was monitored by TLC and GC. After the
reaction, the residual sulfur content was removed by a polar extractant (acetonitrile) and
monitored via GC analysis equipped with an FID. To find the optimum reaction conditions, the
main factors influencing the process that may impact the conversion enhancement of DBT
oxidation were investigated.
During all experiments, DBT removal efficiency (%) or DBT conversion to its corresponding
sulfone compound is calculated as follows:
DBT Conversion (%) = [(C
0
-C
t
0
)/C ]*100
(1)
where, C
0
and C are the initial concentration of DBT in the model oil and DBT concentration of
t
the treated oil after any certain reaction time (t), respectively.
Scheme 3. A simple schematic for reactor setup used in ODS process catalyzed by ZIF-8 and ZIF-8@{Mo132} nano-
structure (1. Magnetic stirrer, 2. Paraffin bath, 3. Cooling water recycling system, 4. Condenser, 5.Two-necked
flask, 6. Model fuel)
7

3
. Results and discussion
3
.1. Characterization of ZIF-8@{Mo132}
The successful synthesis of ZIF-8@{Mo132} catalyst was firstly confirmed by XRD and FTIR
analyses as illustrated in Figure 1 and 2 where the good crystallinity and chemical composition
of the catalyst was well demonstrated, respectively. The significant peaks of ZIF-8 are observed
in the XRD pattern of ZIF-8@{Mo132}denoting appreciable structure stability after supporting of
{
Mo132} over the ZIF-8 structure. The prominent peaks revealed at 2θ= 7.28, 10.24, 12.74 are
related to the ZIF-8 structure [17, 28]. The other observed peaks around 2θ= 14, 20.49, 21.68,
4.86, 27.16, 28.80, 29.84, 32.40, 34.16, 35.80 were assigned to the presence of {Mo132} cluster
2
in the surface of ZIF-8@{Mo132} framework [28, 29]. Figure 1 shows the XRD pattern of the as-
prepared ZIF-8@{Mo132} catalyst in comparison with the as-synthesized ZIF-8 and its simulated
(
obtained from Mercury Software) patterns and pristine {Mo132}. On the other hand, Figure 2
illustrates the FTIR patterns of the as-synthesized ZIF-8@{Mo132} in comparison with ZIF-8 and
Mo132} confirming the trueness of catalyst synthesis [17, 28]. The Observed a major peak at
{
-1
around 3455 cm corresponds to hydroxyl groups [14] owing to Zn−OH active sites generated by
the dissociative adsorption of water. In addition, the appeared strong peaks around at 878, and
-1
6
74 cm correspond to the stretching vibrations of {Mo132} cluster [17, 28]. As it can be
-1
observed, a slight change in position of the observed band of Mo-O from 854 to 878 cm has
occurred in ZIF-8@{Mo132} structure. This shift confirms an interaction between {Mo132
clusters and ZIF-8.
}
8

Figure 1. XRD patterns of simulated ZIF-8 (down), as-synthesized ZIF-8 (middle), {Mo132} (middle) and ZIF-
@{Mo132} (top).
8
8
54
Zn-OH
Mo-O
Figure 2. FT-IR spectra of {Mo132} (top), ZIF-8 (middle) and ZIF-8@{Mo132} (down).
The successful incorporation of {Mo132} clusters with the surface of ZIF-8 was also confirmed
by the dispersive energy spectrum (EDS) analysis (Fig. S10). This figure clearly illustrates the
presence of Mo element in the ZIF-8@{Mo132} nano-structure.
9

The surface morphology of the ZIF-8@{Mo132} in comparison with ZIF-8 crystals are depicted
in Figure 3. The images clearly show the rhombic dodecahedral and cubic morphology of
crystals for ZIF-8 and ZIF-8@{Mo132} structures with the average particle sizes around 600 and
7
50 nm, respectively. This change in the surface morphology can be attributed to the increasing
of H O concentration [30] when an aqueous solution of {Mo132} is added to ZIF-8 during ZIF-
@{Mo132} synthesis. Moreover, the presence of {Mo132} on the surface of ZIF-8 is observed in
2
8
the picture of the as-prepared ZIF-8@{Mo132} (red circles in Figure 3) where the {Mo132
nanoballs are fairly distributed on the surface of ZIF-8.
}
Figure 3. SEM images of ZIF-8 (left) and ZIF-8@{Mo132} (right).
Red circles demote the presence of {Mo132} nano-balls over the ZIF-8 surface.
Furthermore, Fig. S.11 and S.12 give TGA and UV curves of the ZIF-8@{Mo132} catalyst,
respectively. High thermal stability around 450 °C was observed for ZIF-8@{Mo132} nano-
structure, which is near to that of pristine ZIF-8 [14]. On the other hand, UV-Vis spectra of ZIF-
8
@{Mo132} reveal absorption bands at around 406, 253, and 333 nm which confirm the
interactions between {Mo132} and ZIF-8 structure [28].
Nitrogen adsorption-desorption isotherms in ZIF-8@{Mo132} at 77 K were also performed to
characterize the textural properties of ZIF-8 after supporting {Mo132} nanoballs over its surface.
As it was expected (Figure 4), due to the presence of {Mo132} nanoballs the pore volume and the
1
0

surface area of ZIF-8@{Mo132} dropped significantly in comparison with pristine ZIF-8 (Table
). This indicates that {Mo132} atoms have blocked some of the pores existing in ZIF-8 structure.
1
500
400
300
200
100
0
100
80
ZIF-8
60
ZIF-8@{Mo132
}
40
20
0
0
0.2
0.4
0.6
0.8
1
P/P0
Figure 4. N
2
adsorption-desorption isotherms for ZIF-8 (square) and ZIF-8@{Mo132} (Circle) at 77 K
(
Full symbols: adsorption, open symbols: desorption).
Table 1. Textural properties of the as-synthesized ZIF-8 and ZIF-8@{Mo132} nano-structure.
Langmuir
BET
Average pore
size
(A)
Pore volume
(cc/g)
Sample
surface area
(m /g)
surface area
2
2
(
m /g)
ZIF-8
1281
591
1875
777
21.2
19.4
0.68
0.29
ZIF-8@{Mo132
}
3
.2. Evaluation of ZIF-8@{Mo132} catalyst
To evaluate the catalytic performances of ZIF-8@{Mo132} in DBT removal from a model fuel, a
certain amount of the as-prepared nano-structure catalyst was employed in a batch reactor system
at the specified temperature, time and O/S ratio (Scheme 2). To demonstrate the effect of
mentioned operating parameters affecting the removal efficiency, each parameter including
temperature, catalyst dosage, O/S ratio, were systematically investigated in different reaction
times from 1 to 12 hours. In addition, the performances of ZIF-8@{Mo132} catalyst is compared
1
1

with those obtained from pristine ZIF-8 to better show the effect of {Mo132} on DBT removal
efficiency.
3
.2.1. The effect of temperature
The effect of reaction temperature on the DBT oxidation catalyzed by ZIF-8@{Mo132} is
indicated in Figure 5. As it's been shown in this figure, the reaction time required for removal of
DBT is reduced by increasing the temperature. It was found that the DBT removal efficiency
o
could be achieved around 55% at 100 C after 8 h. However, the conversion of DBT removal
o
was around 45% at 80 C, at the same time. This little difference in removal efficiency convinced
o
us to choose 80 C for reaction temperature in all experiments because of the importance of
energy economy and the mild reaction conditions for the removal of DBT. On the other hand, the
same behavior was found for DBT removal using pristine ZIF-8. Although 48% conversion of
o
DBT was obtained at 100 C after 8 h of reaction time, the conversion of 40% was found at 100
o
o
C at the same time. So, for all experiments, the reaction temperature was fixed at 80 C for DBT
removal using pristine ZIF-8. Figure 5 also illustrates the effect of temperature on the DBT
removal efficiency using ZIF-8 catalyst.
1
2

1
8
00°C
0°C
60
50
40
30
20
10
0
ZIF-8@{Mo132
}
6
5
4
3
0
0
0
0
1
8
6
00 °C
0 °C
0 °C
ZIF-8
60°C
5°C
2
25 °C
20
10
0
0
2
4
6
8
10
12
0
2
4
6
8
10
12
Time (h)
Time (h)
Figure 5. The effect of temperature on oxidation of DBT model oil (500 ppm in toluene) with O/S = 3 (TBHP =
L) catalyzed by ZIF-8@{Mo132} (left) and ZIF-8 (right) (100mg of each catalyst).
4
Enhancement of DBT removal efficiency by increasing reaction temperature is also reported for
other catalysts [29]. It seems that at elevated temperatures, the molecular movement of sulfur
compounds in the oil phase is facilitated, resulting in an increase in the presence of DBT and
oxidant substances on the catalyst surface and consequently higher efficiency.
3
.2.2. The effect of catalyst dosage and O/S ratio
The reaction was further optimized by varying the catalyst dosage and molar ratio of TBHP to
DBT (O/S) parameters. Figure 6 reveals the effect of ZIF-8@{Mo132} catalyst dosage on the
oxidative removal of DBT in comparison with performances of pristine ZIF-8. It was observed
under the same reaction condition, only 10% DBT conversion was obtained from the model fuel
in the absence of catalyst after 10 h of reaction and an increase in the reaction time did not show
any remarkable effect on the reaction rate. Therefore, the presence of the catalyst has a profound
effect on the oxidative removal of DBT.
The results shown in Figure 6 represent the effective presence of ZIF-8@{Mo132} as a catalyst
on DBT oxidation so that by increasing the amount of catalyst, DBT removal conversion also
1
3

increases. From this figure, the optimum amount of ZIF-8@{Mo132} was found to be 150 mg to
o
reach the highest efficiency around 95% after 8 h of reaction at 80 C. In addition, the optimum
amount of ZIF-8 catalyst is found 50 mg to reach 55% of DBT conversion. Note that increasing
the amount of ZIF-8 from 50 to 150 mg will result in lower efficiency due to surface
agglomeration of ZIF-8 catalyst. The presence of negative charges on the surface of {Mo132
17] prevents the catalyst agglomeration of ZIF-8@{Mo132} and consequently, higher efficiency
by increasing the amount of catalyst can be observed.
}
[
1
00
60
ZIF-8@{Mo132
}
50 mg
ZIF-8
1
1
00 mg
50 mg
50
40
30
20
10
0
80
60
40
20
0
0 mg
1
1
5
50 mg
00 mg
0 mg
0 g rm g
0
2
4
6
8
10
12
0
2
4
6
8
10
12
Time (h)
Time (h)
Figure 6. The effect of quantity of catalyst on oxidation of DBT model oil (500 ppm in toluene) with O/S=3 (TBHP
4 L) at 80 ⁰C in the presence of ZIF-8@{Mo132} (left) and ZIF-8 (right) catalysts.
=
As the dosage of oxidant (TBHP) plays a vital role in ODS process, the effect of oxidant to
sulfur content (O/S) mole ratio was performed by various O/S molar ratios to investigate the
effect of oxidant dosage on the oxidation removal of DBT. Figure 7 shows the effect of the
amount of oxidant (TBHP) to the total amount of sulfur in the initial solution of DBT catalyzed
by ZIF-8 and ZIF-8@{Mo132}. Worthwhile mentioning that use of TBHP as a good oxidizing
agent has been previously reported by many researchers [31-33].
1
4

As can be seen in Figure 7, DBT removal was progressed with an increase in O/S molar ratio up
to 3, the conversion of DBT reached to 95 and 55% after 8 h of reaction at 80 °C for ZIF-
8
@{Mo132} and ZIF-8, respectively. It is noteworthy that when the O/S molar ratio increases to
a higher value than the amount of 3, the DBT conversion rate decreased. This can be attributed to
the production of tert-butanol from the thermal decomposition of TBHP and catalytic oxidation
of DBT, which prevented the ODS process [11, 14, 29]. The ascending trend of DBT removal
catalyzed by ZIF-8@{Mo132} at both O/S values of 1 and 3 is relatively similar; therefore, the
amount of O/S = 1 was chosen due to less use of oxidant. In the case of ZIF-8 (Figure 7), the O/S
ratio equals to 3 is more appropriate where the highest removal rate of DBT is achieved (ca.
5
5%).
6
5
4
3
2
1
0
0
0
0
0
0
0
1
00
O/S=3
O/S=1
O/S=5
ZIF-8@{Mo132
}
O/S=3
O/S=1
O/S=5
ZIF-8
8
6
4
2
0
0
0
0
0
0
2
4
6
8
10
12
0
2
4
6
8
10
12
Time (h)
Time (h)
Figure 7. The effect of TBHP/DBT molar ratio on the removal of DBT from model oil (500 ppm in
toluene) catalyzed by ZIF-8@{Mo132} (150 mg, left) and ZIF-8 (50 mg, right) at 80 ⁰C.
3
.2.3. The effect of initial concentration
The importance of the initial concentration of DBT is related to practical situations where the
sulfur content can be varied from low to high contents from a specified fuel to another one. To
study the effect of the initial concentration of DBT on catalytic performances of ZIF-8 and ZIF-
1
5

8
@{Mo132}, five different initial concentrations of 500, 1000, 1500, 2000 and 4000 ppm of DBT
in the model fuel were prepared and utilized in our experiments and the results are illustrated in
Figure 8.
60
50
40
30
20
10
0
1
00
ZIF-8
ZIF-8@{Mo132
}
8
6
4
2
0
0
0
0
0
5
1
00ppm
500 ppm
1000 ppm
2000 ppm
000ppm
2000ppm
4
000 ppm
4
000ppm
0
2
4
6
8
10
12
0
2
4
6
8
10
12
Time (h)
Time (h)
Figure 8. The effect of initial DBT concentration on removal efficiency from the model oil (500 ppm in toluene)
catalyzed by ZIF-8@{Mo132} (150 mg, left) and ZIF-8 (50 mg, right) at 80 ⁰C.
As it is illustrated in Figure 8, removal efficiency is reduced by increasing of the initial
concentration of DBT in the model fuel for both pristine and nano-structure catalysts at a
specified time. This may be attributed to the fact that the increase of DBT sulfone and sulfoxide
concentration in a higher concentration of DBT and consequently occupying more active sites
over the surface of catalysts by DBT sulfone and sulfoxide [34]. Thus, for actual situations, it is
suggested that the use of two sequential ODS process catalyzed by ZIF-8@{Mo132} to reach
complete DBT removal.
Table 2 summarizes the optimal operating variables reaching to the highest efficiency of DBT
removal using both ZIF-8 and ZIF-8@{Mo132} catalysts. As it is shown, the efficiency of the
ODS process has increased by approximately twice after surface modification of ZIF-8 catalyst
1
6

using {Mo132} polyoxometalate. This is very interesting results that ODS catalytic performances
of a commercial MOF catalyst (ZIF-8) can be surprising improved into twice using a surface
modification with {Mo132}. The actual effect of {Mo132} over DBT removal will be completely
explained in the next sections.
Table 2. Optimal operating conditions of DBT removal using ODS process catalyzed by ZIF-8@{Mo132} in
comparison with pristine ZIF-8.
DBT Removal
Catalyst
Temp. (°C)
Dosage (mg) O/S ration Initial conc. (ppm) Solvent
efficiency
Toluene 55
Toluene 92
ZIF-8
80
80
50
3
1
500
500
ZIF-8@{Mo132
}
150
Furthermore, Figure 9 illustrates the efficiency of DBT removal using pristine ZIF-8 and
Mo132} at the optimal operating conditions of ZIF-8@{Mo132}. The interpretation of this figure
confirms the higher activity of ZIF-8@{Mo132} in comparison with pristine ZIF-8 and {Mo132}.
{
1
00
ZIF-8@{Mo132}
ZIF-8
{
Mo132}
80
60
40
20
0
2
4
6
8
10
12
Reaction time (h)
Figure 9. DBT removal efficiency from the model oil (500 ppm in toluene) catalyzed by ZIF-8@{Mo132} in
comparison with pristine ZIF-8 and {Mo132} at the optimal operating conditions of ZIF-8@{Mo132
150 mg, 80 ⁰C, and O/S=1).
}
(
1
7

Furthermore, to understand the capability of the as-prepared nano-structure catalyst in real
situations, the process was carried out under optimal operating condition for a real fuel obtained
from ABADAN's refinery in Iran. The obtained results revealed that the catalyst could efficiently
reduce the total sulfur content of the fuel by around 80%.
3
.3. Kinetics of sulfur removal
To determine the kinetic of DBT removal in model oil catalyzed by ZIF-8@{Mo132}, different
types of kinetics orders including the zero-order (Equation 2), pseudo-first-order (Equation 3)
and pseudo-second-order (Equation 4) [35, 36] are applied as shown in Table 3.
C  C  Kt
(2)
3)
t
0
(
ln(q q )  lnq  K t
e
t
e
1
t
1
t
(4)


2
qt K2qe qe
In these equations, K
1
is pseudo-first order rate constant, and its value was calculated from the
slope of the linear plot of ln (q
e
- q
t
) vs. time. K
2
is the second-order rate constant. The plot of t/q
2. Catalyst conversion
t
vs. t gives a straight-line with a slope of 1/q
e
and an intercept of 1/k
q
2 e
-1
capacity values at equilibrium and specified time (q
following equations:
e
and q , mg.g ) were calculated using the
t
(
C C )
(5)
(6)
0
e
qe 
qt 
V
m
(C C )
0
t
V
m
1
8

-1
Where C
0
, C
e
, and C (mol.L ) are the DBT concentrations at initial, equilibrium (after 8 hours
t
of reaction) and specified time, respectively. Moreover, the solution volume in liter and the mass
of catalyst in gram are denoted as V and m, respectively. Table 3 exhibits the calculated rate
2
constants of each order as well as corresponding regression coefficient values (r ). As it can be
observed, the DBT oxidation in toluene solution suspended on ZIF-8@{Mo132} catalyst obeys
from pseudo-second-order reaction.
Table 3. Kinetic parameters of DBT oxidation over the ZIF-8@{Mo132} catalyst using different kinetic models
-
1
-1
r2
Reaction order
Zero-order
Equation
1 2
K, K , K ,(mol/L) .h
30.179
0.1823
0.0016
0.6299
Y = -30.179x + 328.21
Pseudo-first-order
0.8412
0.9634
Y = 0.1823x + 0.5061
Y = 0.0016x + 0.0028
Pseudo-second-order
Reaction conditions: O/S = 1, DBT model oil (500 ppm of S), m (catalyst) = 150 mg, T =80 °C.
In addition, the activation energy (Ea) for DBT removal catalyzed by ZIF-8@{Mo132} was
obtained using the Arrhenius equation at two different temperatures as follows [19].
K=Ae-Ea/RT
(7)
(8)
K1
Log ( ) =
K2
−퐸푎
푇2−푇1
푇1푇2
×
2.303푅
Where R is the universal gas constant, A is defined as the frequency factor for the reaction, and T
is the temperature (in Kelvin). Moreover, K is the rate constant with the subscripts of 1 and 2
denoting to temperatures of 60 and 80 °C, respectively. The activation energy (E
a
) was
calculated 194.50 kJ.mol for ZIF-8@{Mo132} catalyst. Two different temperatures of 60 and 80
C and their corresponding rate constant of 0.00003 and 0.0016 were utilized for the E
calculation.
-1
°
a
1
9

3
.4. Postulated reaction mechanism
As it was concluded, the reaction mechanism of DBT oxidation is obeyed from pseudo-second-
order reaction; however, the real mechanism is very complex. Recently, some mechanisms have
been proposed for oxidation of DBT to the corresponding sulfone and sulfoxide as it is
speculated in Figure 10 [32].
III
I
II
Figure 10. Suggested mechanism for the oxidation of DBT by TBHP in the presence of ZIF-8@{Mo132} catalyst.
Initially, the mechanism starts with the nucleophilic attack of TBHP on ZIF-8@{Mo132} surface.
The result of this attack is the reaction of TBHP with Mo or Zn active species and consequently,
the formation of complex species or metal-peroxide. After that, electrons are withdrawn from the
peroxyl compound; so that the electrophilic characteristics of the peroxidic oxygen rapidly
increases and becomes more and more reactive. Therefore, dibenzothiophene/sulfoxide (I) is
formed by oxidation of a sulfur atom in DBT compound via nucleophilic attack of electron
2
0

density rich center in sulfur atoms to complex species. Tert-butanol (II) is also produced as a by-
product in this reaction. Afterward, the dibenzothiophene/sulfoxide undergoes further oxidation
by complex species to form dibenzothiophene sulfones compounds (III). After the ODS reaction,
GC-MS analysis was carried out on the toluene phase and proved the presence of DBT sulfone
during the oxidation reaction. The GC-MS analysis registered at m/z = 216, confirming the
presence of the sulfone of DBT (Fig. S11).
3
.5. Stability and reusability of ZIF-8@{Mo132} catalyst
Stability and reusability of catalysts are very important parameters, and they act as a vital role
from the industry point of view. For this purposes, at the end of reaction with fresh ZIF-
8
@{Mo132}, the catalyst is separated from the reaction mixture, washed three times by ethanol
and then dried at 80 °C in a vacuum oven. Then, the collected catalyst was used for five runs
under the identical reaction conditions (optimal, as shown in Table 2) to evaluate the catalyst
reusability. As it can be observed in Figure 11, DBT removal efficiency is very slightly reduced
from 92 to 81 after five cycles of ODS reaction. Furthermore, FT-IR and XRD patterns of the
spent catalyst confirmed the stability of ZIF-8@{Mo132} after ODS reaction (Fig. 12).
ZIF-8@{Mo132
}
92
91
1
00
90
8
5
81
8
6
4
2
0
0
0
0
0
1
2
3
4
5
Reaction cycles
Figure11. Cycling runs of sulfur removal in the ODS process catalyzed by ZIF-8@{Mo132
150 mg, 500 ppm DBT in toluene, 80 ⁰C and O/S=1).
}
(
2
1

4
3
2
1
000
000
000
000
0
1
1
1
40
20
00
Spent
Fresh
80
60
40
20
0
Spent
Fresh
3
600
2800
2000
1200
400
5
10
15
20
25
30
35
40
-1
Wavenumbers (cm )
2-Theta (deg)
Figure. 12. Structural stability of the ZIF-8@{Mo132} catalyst after ODS reaction confirmed by FT-IR (left) and
XRD pattern (right) (150 mg, 500 ppm DBT in toluene, 80 ⁰C and O/S=1).
Furthermore, for the sake of a better investigation on the stability of the ZIF-8@{Mo132} catalyst
and confirming ZIF-8 and {Mo132} interaction after its exposure in the reaction media, the
leaching test was performed. To this purpose, 150 mg of the catalyst was soaked in 50 ml
distilled water. Then, an equal volume of the solution was separated after 1, 2, 4, and 8 h and
analyzed using UV/Vis spectrophotometry. As it has been depicted in Figure 13, the obtained
results compared to pristine {Mo132} which was soaked in distilled water for 8 h, indicated good
stability for the catalyst. Furthermore, these results adequately confirm that there is a good
interaction between POM content and MOF support owing to a strong reduction in the prominent
peak of {Mo132} around 450 nm at a different time of soaking.
2
2

Figure. 13. UV-Vis leaching test of the ZIF-8@{Mo132} catalyst. Aqueous solution of {Mo132} (black), aqueous
solution after soaking of the ZIF-8@{Mo132} catalyst for 1 h (red), 2 h (green), 4 h (purple), and 8 h (blue).
4
. Conclusion
In this study, the ODS catalytic activity of ZIF-8 on the removal of DBT was greatly improved
using a surface modification of the ZIF-8 using {Mo132} nanoballs. The as-synthesized new
catalyst named as ZIF-8@{Mo132} indicated an excellent catalytic activity on the removal of
DBT, which could reach a high DBT removal of up to 92% under the mild conditions of 80 °C
temperature, O/S molar ratio of 1, catalyst dosage of 150 mg and initial concentration of 500
ppm of DBT in toluene. Besides this, obtained results showed that the new nano-structure
catalyst in the presence of TBHP exhibits higher catalytic oxidation activity compared to the
pristine ZIF-8 and also pure {Mo132} at the same condition. Table 4 demonstrates the catalytic
efficiency of ZIF-8@{Mo132} catalyst in comparison with the other POM and MOF catalysts
reported in the literature [37-40]. As it can be seen, the ODS efficiency of the as-prepared ZIF-
8
@{Mo132} nano-structure catalyst is much better than TMU catalysts and is slight lower than
pristine and metal oxide-modified POMs such as Q . Nevertheless, the
3
H
3
V
10
O28 and HPA/Al
O
2 3
2
3

ODS efficiency of ZIF-8 has been properly increased and reached to these POM catalysts by using
Mo132}. Worthwhile mentioning that although Tb(PW11 @MIL-101, an another example of POM-
{
)
2
modified MOF catalysts, reveals better performances in comparison with our catalyst, ZF-8
indicates higher stability compared to MIL-101 (Cr), which is very important in practical
situations [41]. Moreover, the reusability tests revealed that the catalyst could be reused five
times without a significant decline in the removal performance. This result introduces ZIF-
8
@{Mo132} as one of the most promising candidates for high resistant sulfur compounds
removal from fossil fuels.
Table 4. Comparison of the ODS catalytic performance of ZIF-8@{Mo132} by TBHP as oxidant with
other catalysts reported in the literature.
Catalyst
Temp. Conc. Catalyst
Solvent
Conversion Ref.
(%)
(
°C)
(ppm)
500
dosage (gr)
0.15
ZIF-8
80
80
80
60
60
Toluene
Toluene
Toluene
n-Hexane
n-Hexane
55
Present work
ZIF-8@{Mo132
}
500
500
500
500
0.15
0.15
0.1
92
Present work
{
Mo132
}
80
Present work
TMU-10
TMU-12
40.5
75.2
19
19
0.1
Toluene–
hexane
IV
V O (sal-HBPD)
40
---
---
0.0146
0.0047
88
87
37
37
(
1 : 4)
IV
Toluene–
hexane
P[V O(sal-
AHBPD)]
4
0
(
1 : 4)
HPA/Al
2
O
3
60
500
500
0.1
Toluene
98.8
100
38
39
3 3
Q H V
10
O
28
100
0.04
Decalin
Tb(PW11) @MIL-
2
5
0
500
0.061
n-Octane
96.6
40
101
2
4

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2
8

Graphical Abstract
ZIF-8
Model Fuel
Oxidant
DBT Sulfone
ZIF-8@{Mo132
}
The presence of {Mo132} over the surface of ZIF-8 greatly promotes the DBT removal
efficiency of ZIF-8 by about twice.
2
9