Vanadium Containing Metal-organic Frameworks as Highly Efficient Catalysts for the Oxidation of Refractory Aromatic Sulfur Compounds

Article abstract of DOI:10.1002/cctc.202001327

The wet impregnation method was applied to prepare vanadium species on the chromium(III) terephthalate metal-organic framework (MIL-101) and on the aluminum fumarate metal-organic framework (A520), two highly efficient catalysts, as well as vanadium species on the zinc terephthalate metal-organic framework (MOF-5), using vanadium(V) oxytributoxide as a precursor. The prepared catalysts were characterized by powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDX), N₂ sorption measurement (BET), thermal analysis (TGA-DTA), and X-ray photoelectron spectroscopy (XPS). The prepared materials were utilized as catalysts for the oxidation of the refractory aromatic sulfur compound, dibenzothiophene (DBT), in a model diesel fuel. The catalytic oxidation of DBT to DBT-sulfone was performed in the presence of tert-butyl hydroperoxide (TBHP) under mild operating conditions. The heterogeneous catalysts, OV(O^tBu)_3?x(OH)_x@MIL-101(Cr), OV(O^tBu)_3?x(OH)_x@A520, and OV(O^tBu)_3?x(OH)_x@MOF-5 demonstrated activities of 98, 98, and 10 %, respectively, in the oxidative desulfurization (ODS) process at 60 °C. The catalysts were recycled easily and reused without a significant decrease in their activity. The oxidation of DBT to DBT-sulfone was investigated using gas chromatography, and the formation of DBT-sulfone was confirmed by FT-IR and ¹H NMR spectroscopies.

Full text of DOI:10.1002/cctc.202001327

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Vanadium Containing Metal-organic Frameworks as Highly
Efficient Catalysts for the Oxidation of Refractory Aromatic Sulfur
Compounds
Authors: Marjan Abdi Jamali, Alireza Arvani, and Mostafa Amini
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To be cited as: ChemCatChem 10.1002/cctc.202001327
Link to VoR: https://doi.org/10.1002/cctc.202001327
01/2020

10.1002/cctc.202001327
ChemCatChem
Vanadium Containing Metal-organic Frameworks as Highly Efficient
Catalysts for the Oxidation of Refractory Aromatic Sulfur
Compounds
Marjan Abdi Jamali, Alireza Arvani, Prof. Dr. Mostafa M. Amini*
Department of Chemistry, Shahid Beheshti University, Tehran, 1983963113, Iran
*Corresponding author: Tel.: (+98) 212 990 3109; fax: (+98) 212 243 1663;
E-mail: m-pouramini@sbu.ac.ir (M.M. Amini)
Abstract
The wet impregnation method was applied to prepare vanadium species on the
chromium(III) terephthalate metal-organic framework (MIL-101) and on the aluminum
fumarate metal-organic framework (A520), two highly efficient catalysts, as well as
vanadium species on the zinc terephthalate metal-organic framework (MOF-5), using
vanadium(V) oxytributoxide as a precursor. The prepared catalysts were characterized by
powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FT-IR), scanning
electron microscopy (SEM), energy-dispersive spectroscopy (EDX), N2 sorption measurement
(BET), thermal analysis (TGA-DTA), and X-ray photoelectron spectroscopy (XPS). The
prepared materials were utilized as catalysts for the oxidation of the refractory aromatic
sulfur compound, dibenzothiophene (DBT), in a model diesel fuel. The catalytic oxidation of
DBT to DBT-sulfone was performed in the presence of tert-butyl hydroperoxide (TBHP)
under mild operating conditions. The heterogeneous catalysts, OV(OtBu)3-x(OH)x@MIL-
101(Cr), OV(OtBu)3-x(OH)x@A520, and OV(OtBu)3-x(OH)x@MOF-5 demonstrated activities of
98, 98, and 10%, respectively, in the oxidative desulfurization (ODS) process at 60 C. The
catalysts were recycled easily and reused without a significant decrease in their activity. The
oxidation of DBT to DBT-sulfone was investigated using gas chromatography, and the
formation of DBT-sulfone was confirmed by FT-IR and 1H NMR spectroscopies.
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Introduction
Recently, environmental concerns have let to significant attempts to attain the deep
desulfurization of diesel to optimize sulfur levels in fuels.[1] Sulfur compounds have harmful
effects on the environment because of the production of sulfur oxides after ignition, which
is the primary source of environmental contamination and the greenhouse effect.[2] The
elimination of polycyclic aromatic sulfur heterocycles (PASH) in fuel, such as
dibenzothiophene (DBT) and their derivatives, by the conventional hydrodesulfurization
procedure is difficult,[3] and to ensure highly effective elimination, harsh conditions, such as
high temperatures, are essential.[4] Alternative processes have been considered to
overcome these issues,[5] including adsorptive desulfurization (ADS),[6] oxidative
desulfurization (ODS),[7] and extractive desulfurization for the effective removal of resistant
sulfurs have been considered.[8] Developing effective sorbents that are highly selective of
PASHs is a key in the ADS process. Until now, several commercial sorbents have been used
in ADS, such as metal ion-exchanged zeolites,[9] carbon-based materials,[10] metal-organic
frameworks (MOFs),[11] and molecular sieves.[1b] In particular, ODS is considered a promising
process due to the advantage of eliminating aromatic sulfur compounds under mild
operating conditions.[12] In the ODS method, sulfur compounds are converted to sulfoxides
and sulfones by an oxidizing agent. These oxidized compounds have higher polarities than
preliminary compounds, and they can be eliminated by diverse separation methods, such as
solvent extraction. Previously, several oxidants were investigated for the ODS method, such
as hydrogen peroxide,[13] tert-butyl hydroperoxide (TBHP),[14] molecular oxygen[15], and
superoxides.[16] Hydrogen peroxide has been extensively used in many catalysts due to its
low cost and remarkable oxidation feature.[17] However, since hydrogen peroxide is not oil-
soluble, it leads to a biphasic system, which restricts the mass transfer and lowers the
oxidation process rate.[15] The utilization of organic oxidizing agents such as TBHP has been
suggested to avoid such problems.[18]
Metal-organic frameworks have been of great interest in recent years because of their
prominent features and broad application potential.[19] MOFs with high surface area and
pore volumes are a new class of crystalline microporous or mesoporous materials composed
of metal ions and multi-topic organic linkers that are connected by coordinate bonds to
form a 3D network.[20] Because of their tunable pore size and pore shape, which can be
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tailored to the specific application by the proper combination of metal/ligand, MOFs are
excellent porous materials. Hence, they have been used in many applications, such as gas
separations,[21] sensing,[22] and catalysis.[23]
During the last two decades, many MOFs have been developed for multiple applications.
Among them, MIL-101(Cr) has numerous properties, such as high chemical and thermal
stability, rigid architecture with MTN topology (ZSM-Thirty-Nine), two kinds of quasi-
spherical cages, high surface area, and many unsaturated chromium sites.[24] The aluminum
fumarate MOF, A520, is another porous material which is promising support in catalytic
applications. This compound is especially noteworthy because of its high stability and low-
production expense.[25] Moreover, MOF-5,
a
zinc-based MOF, Zn4O(1,4-
benzenedicarboxylate)3, with a surface area of 3800 m2g-1, is also promising support for
catalysts. [26]
Despite the vast utilization of vanadium and MOFs as catalysts in oxidative
desulfurization of refractory sulfur compounds,[21, 27] to the best of our knowledge, there is
no report on the employment of MOFs as support for vanadium species and use of them as
catalysts in the ODS of DBT. In this study, three catalysts, OV(OtBu)3-x(OH)x@MIL-101(Cr),
OV(OtBu)3-x(OH)x@A520, and OV(OtBu)3-x(OH)x@MOF-5, were prepared by the infiltration
method using vanadium(V) oxytributoxide as the vanadium source with different loadings.
The prepared materials were employed as catalysts for oxidation of DBT in a model diesel
fuel, using TBHP as an oxidant. The catalytic oxidative desulfurization of DBT was conducted
under mild operating conditions at 60 C under atmospheric pressure.
Results and Discussion
Characterization of OV(OtBu)3-x(OH)x@MIL-101(Cr)
The OV(OtBu)3-x(OH)x@MIL-101(Cr) catalyst was prepared using MIL-101(Cr) as a support.
MIL-101(Cr) has high thermal and chemical stability. Its large pores of around 3.5 nm,
surface area of 1835 m2g-1, and pore of volume around 0.87 cm3g-1 offer appropriate
support for various catalysts.[28] In addition, it can mass transfer through its hexagonal
windows (1.5 nm). For these reasons, MIl-101(Cr) was used as the support in the present
study.
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The prepared catalyst, OV(OtBu)3-x(OH)x@MIL-101(Cr), was characterized by powder X-ray
diffraction (PXRD), and Fourier transform infrared spectroscopy (FT-IR). Powder XRD
patterns of MIL-101(Cr) before and after OV(OtBu)3 loading are illustrated in Figure 1. A
comparison of the PXRD patterns showed that MIL-101(Cr) maintained its structure after
loading.[29] Notably, the absence of diffraction peaks related to vanadium species in the
PXRD pattern of OV(OtBu)3-x(OH)x@MIL-101(Cr) demonstrated only a slight degree of
crystallinity or amorphous vanadium species loaded on MIL-101(Cr).
Figure 1. PXRD patterns of MIL-101 (a) and OV(OtBu)3-X(OH)X@MIL-101 (b).
As shown in Figure S1 (Supporting Information, SI), the FT-IR spectra of MIL-101(Cr) and
OV(OtBu)3-x(OH)x@MIL-101(Cr) confirmed the formation of the catalyst. In both spectra,
stretching and bending vibrations of adsorbed water were observed at 3428 and 1631 cm-1,
respectively, and the main stretching vibrational band of dimethylformamide (DMF) in the
pores of MIL-101(Cr) appeared at 1666 cm-1. As the spectra indicated, all MIL-101(Cr) bands,
including those at 1403 and 1542 cm-1, which were associated with the symmetrical and
asymmetrical stretching vibration modes of CO, respectively, remained intact. Finally, the
bands observed at about 2890−2950 and 989 cm-1 were assigned to the aliphatic CH and
V=O stretching vibrations, respectively, of OV(OtBu)3-x(OH)x species in MIL-101(Cr).[29-30]
The thermal stability of OV(OtBu)3-x(OH)x@MIL-101(Cr) was investigated by thermal
analysis (TGA-DTA). As Figure 2 shows, three events occurred by sweeping temperature
from 25 C to 800 C. The first mass loss, which occurred around 100 C in the TGA curve
and was accompanied by an endothermic peak in DTA, was associated with the desorption
of physically adsorbed water. The second mass loss at around 200 C was related to the
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release of DMF from the pores of OV(OtBu)3-x(OH)x@MIL-101(Cr). Finally, the distinct mass
loss from 250 C to about 480 C, which was accompanied by a large exothermic peak, was
attributed to combustion of organic parts, including the MIL-101(Cr) ligand and the butyl
groups of OV(OtBu)3-x(OH)x. No further mass loss was observed after 500 C, which indicated
that residues were oxides.
Figure 2. TGA-DTA diagram of OV(OtBu)3-x(OH)x@MIL-101(Cr).
Figure 3 shows scanning electron microscopy (SEM) images of the OV(OtBu)3-x(OH)x@MIL-
101(Cr) catalyst. As shown in the micrographs, the morphology of MIL-101(Cr) was
maintained constant[28b] after vanadium(V) oxytributoxide loading, and the particles were in
the range of 230360 nm.
Figure 3. Low (a) and high (b) magnified SEM images of OV(OtBu)3-x(OH)x@MIL-101(Cr).
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The energy-dispersive spectroscopy (EDX) analysis confirmed the presence of vanadium
in MIL-101(Cr) (Figure S2), and the EDX elemental mapping showed a homogenous
distribution of vanadium in MIL-101(Cr) (Figure 4). The purple dots in the mapping image
represent vanadium.
Figure 4. EDX elemental mapping images of OV(OtBu)3-x(OH)x@MIL-101(Cr).
Catalytic performance and optimization of OV(OtBu)3-x(OH)x@MIL-101(Cr) in the ODS
process
Catalysts with different amounts of OV(OtBu)3 loading were prepared and utilized in the ODS
reaction. The performances of OV(OtBu)3-x(OH)x@MIL-101(Cr) catalysts with different
loadings on sulfur removal in the ODS process are illustrated in Figure 5. As the
desulfurization diagram shows, the removal efficiency of the sulfur was enhanced by
increasing the amount of OV(OtBu)3 loading. It is notable that two catalysts with different
loadings of 0.1 mmol/g and 0.15 mmol/g had the same performance in sulfur removal.
Therefore, OV(OtBu)3-x(OH)x@MIL-101(Cr) (0.1 mmol/g) was selected as the catalyst by
balancing the loading of OV(OtBu)3 and reaction time, and the reaction conditions were
optimized accordingly.
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Figure 5. Effect of OV(OtBu)3 loading on sulfur removal as a function of time; Reaction conditions:
0.04 mL of 70% wt. TBHP, model diesel fuel (500 ppm), 20 mg catalyst and temperature 60 C.
The kind and amount of utilized oxidizing agents are important and effective factors in
oxidative reactions. In this study, TBHP was elected as the oxidizing agent for dispersion in
the organic phase of the substrate, thus enhancing reaction efficiency.[18b, 31] As shown in
Figure S3, the oxidation of DBT was completed in an hour at O/S=5 (O/S was defined as the
molar ratio of TBHP to the total amount of sulfur in the initial solution) in the presence of 20
mg OV(OtBu)3-x(OH)x@MIL-101(Cr) (0.1 mmol/g) catalyst at 60 C. Moreover, the ODS
process was carried out in the absence of TBHP under the same condition to investigate the
ability of the catalyst to adsorb of DBT. According to the results, the OV(OtBu)3-x(OH)x@MIL-
101 (0.1 mmol/g) catalyst was neither an effective adsorbent nor an active material for the
removal of DBT in the absence of oxidant. Therefore, O/S=5 was selected as the optimized
ratio.
The effect of the amount of catalyst in the ODS reaction of DBT in the model diesel fuel
was investigated by using 10, 20, and 30 mg of catalyst under the optimized condition. The
results indicated that desulfurization efficiency was enhanced by increasing the amount of
the catalyst up to 20 mg. However, its further increase did not show considerable
enhancement of sulfur removal. The probable reason is that in higher amounts of the
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catalyst, the agglomeration of catalyst particles prevented further improvement of the ODS
reaction. Therefore, 20 mg of the catalyst was selected as the appropriate amount.
Figure 6 shows the effects of reaction time and temperature on DBT removal in the ODS
process in the presence of the OV(OtBu)3-x(OH)x@MIL-101 (0.1 mmol/g) catalyst. The
desulfurization diagram demonstrated that sulfur removal increased during the reaction
time. After 60 min, the amount of DBT in the model diesel fuel decreased to 10 ppm at 60
C. According to Torres-Garcia et al., the Gibbs free energy in the oxidative reaction of DBT
is positive,[32] so by decreasing the temperature, desulfurization was declined, as expected.
As a result, the oxidation yield of DBT to the corresponding sulfone (DBTO2) after 60 min
reached 71% and 98% at 30 and 60 C, respectively. Therefore, the catalytic desulfurization
tests were conducted at the optimized temperature 60 C.
Figure 6. ODS of DBT as a function of time at 30 and 60 C in the presence of 20 mg OV(OtBu)3-
x(OH)x@MIL-101(Cr) (0.1 mmol/g) catalyst.
Oxidative desulfurization of DBT
The oxidative desulfurization of DBT in the presence of TBHP and OV(OtBu)3-x(OH)x@MIL-
101(Cr) catalyst occurred through two sequential stages at the solid liquid phase interface, as
illustrated in Scheme 1.[32-33] In the first stage of reaction, DBT was converted to DBT-
sulfoxide and the resulting DBTO was rapidly transformed to DBT-sulfone at the second
stage. The results of the GC analysis showed that the intensity of the DBT peak decreased
during the reaction time. Moreover, no peak in the chromatogram could be attributed to
DBTO because sulfoxide is rapidly transformed to sulfone. At higher retention times, a weak
peak was observed in the chromatogram, which was associated with the main product of
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DBT oxidation, DBTO2.[34] At the end of the reaction, the product was collected by filtration,
and the formation of DBT-sulfone was confirmed by FT-IR and 1H NMR spectroscopies.[35]
Scheme 1. ODS of DBT occurs through two sequential stages.
Proposed mechanism for catalytic desulfurization
Despite numerous experimental and theoretical studies undertaken to clarify the role of
vanadium center in the oxidation of organic compounds by hydrogen peroxide,[36] there are
only a few reports on oxidation in the presence of TBHP.[37] The oxidation mechanisms of
vanadium compounds in catalytic applications have been extensively investigated, and two
mechanistic pathways, radical and non-radical, were postulated.[36c, 38] To realize which
mechanism in the literature could be adapted in the present study, a scavenger (TEMPO =
(2,2,6,6-tetramethylpiperidine-1-yl)oxyl) was added to potentially trap radicals and to
observe its effects in desulfurization. Interestingly, when TEMPO was introduced in the
reaction media, the reaction continued, and the yield increased. This result indicated that
the non-radical pathway was highly possible for the present catalyst in the ODS of DBT. As
illustrated in scheme 2, a plausible mechanism was proposed in which the high activity was
attributed to the interaction of peroxide with vanadium and the formation of monomeric
vanadium complex (B).[39] First, active peroxo species intermediate (B) was formed by a
nucleophilic attack of TBHP on the V=O bonds of the OV(OtBu)3-x(OH)x@MIL-101(Cr) catalyst
(A) and the elimination of tert-butyl alcohol. Then, because of its high electron density on
peroxo species (C), the nucleophilic attack of the sulfur atom caused the oxidation of sulfur
in DBT, the formation of DBTO, and the recovery of catalyst (A). Subsequently, DBTO2 was
generated by further the oxidation of DBTO in the presence of another TBHP.
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Scheme 2. Proposed mechanism for ODS of DBT by OV(OtBu)3-x(OH)x@MIL-101(Cr) catalyst in the
presence of TBHP.
Recycling of the OV(OtBu)3-x(OH)x@MIL-101(Cr) catalyst
Because of the importance of the recycling capability and reusability of catalysts in the ODS
process, after 20 mins, the spent catalyst was collected from the reaction mixture and
reused for desulfurization in consecutive runs. As shown in Figure S4, the catalyst’s
reusability was indicated in the 0.1 mmol/g vanadium(V) oxytributoxide loaded MIL-101
catalyst, which showed a slight decrease in DBT removal during the second and third runs.
However, the catalytic activity remained more or less constant in the fourth and fifth runs.
Eventually, the recycled catalyst was characterized by PXRD and FT-IR analyses, the results
of which showed that the structure of the catalyst was retained after the fifth run (Figures
S5−S6).
Characterization of OV(OtBu)3-x(OH)x@A520
The OV(OtBu)3-x(OH)x@A520 catalyst was prepared using aluminum fumarate MOF (A520) as
the support. This MOF was prepared under mild operating conditions with a yield of 98%.
The surface area and pore volume of MOF A520 were 1010 m2g-1 and 0.45 cm3g-1,
respectively, which were in accordance with the reported values.[40]
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The prepared catalyst, OV(OtBu)3-x(OH)x@A520, was characterized by PXRD and FT-IR
analyses. Figure 7 shows the powder XRD patterns of A520 before and after OV(OtBu)3
loading. The comparison of the PXRD patterns indicated that the structure of A520 was
maintained after loading.[41] The absence of vanadium species diffraction peaks in the PXRD
pattern of OV(OtBu)3-x(OH)x@A520 demonstrated a slight degree of crystallinity or the
amorphous nature of vanadium species in MOF A520.
Figure 7. PXRD patterns of A520 (a) and OV(OtBu)3-X(OH)X@A520 (b).
Figure S7 shows the FT-IR spectra of A520 and vanadium(V) oxytributoxide loaded A520.
The spectrum of MOF A520 was in accordance with the reported data.[41] The FT-IR
spectrum of OV(OtBu)3-x(OH)x@A520 was similar to that of A520. Noticeable in the FT-IR
spectra was the pronounced difference in the intensity of the band at 3500 cm-1. The
collapse of the broad band at 3500 cm-1 of MOF A520 after vanadium(V) oxytributoxide
loading was attributed to the reaction of the endowed hydroxyl-rich surface of the support
with the highly reactive butoxy groups of vanadium(V) oxytributoxide, which led to the
formation of butanol, and, consequently, the dehydroxylation of MOF A520.
Thermal analysis (TGA-DTA) was performed to investigate the thermal behavior of
OV(OtBu)3-x(OH)x@A520. As the TGA curve in Figure 8 shows, there were three stages of
mass loss. The first stage was related to the release of the adsorbed water molecules at 120
C. The second stage, which occurred between 200 and 300 C, was due to the combustion
of the organic moiety of vanadium(V) oxytributoxide. The third stage, which occurred
between 430 and 510 C, was accompanied by an exothermic peak at 496 C in the DTA
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curve associated with the decomposition and combustion of the organic linker of A520.[42]
The small peak in the DTA curve at 710 C without mass loss was attributed to the
[43]
crystallization of AlVO4.
Figure 8. TGA-DTA diagram of OV(OtBu)3-x(OH)x@A520.
The textural properties of the prepared catalysts and supports were investigated using N2
sorption measurement (BET). The results are provided in Table 1, which shows that by
loading OV(OtBu)3 on the MOFs, the surface area and pore volume significantly dropped,
which indicated that the active catalytic ingredient was accommodated inside the pores of
the MOFs.
Table 1. Textural properties of MIL-101(Cr), A520, OV(OtBu)3-x(OH)x@MIL-101(Cr)
(0.1 mmol/g) (1), and OV(OtBu)3-x(OH)x@A520 (0.1 mmol/g) (2)
Sample
SBET
m2 g-1
1835
672
1010
56
Pore volume
mL g-1
0.87
MIL-101(Cr)
Catalyst (1)
MOF A520
Catalyst (2)
0.23
0.45
0.17
Figure 9 shows SEM micrographs of the prepared OV(OtBu)3-x(OH)x@A520 catalyst. As
shown in the figure, the particles were in the range of 5080 nm. In addition, the
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morphology of the catalyst was similar to that reported for MOF A520.[40]
Figure 9. SEM images of OV(OtBu)3-X(OH)X@A520.
An X-ray photoelectron spectroscopy (XPS) analysis was conducted to determine the
oxidation state of vanadium in the catalyst. The binding energies of C 1s, O 1s, and Al 2p
obtained from the XPS survey spectrum of OV(OtBu)3-x(OH)x@A520 (Figure S8) were
consistent with A520. The deconvoluted vanadium(V) 2p peak is illustrated in Figure 10. The
peaks observed in the vanadium(V) 2p region of the spectrum with binding energies of
516.2 and 522.7 eV were attributed to vanadium(V) 2p3/2 and vanadium(V) 2p1/2,
respectively, which indicated the oxidation state of (V) for the vanadium in the catalyst. The
peak appearing at 520.8 eV was associated with the O 1s´ satellite.[44]
Figure 10. High-resolution XPS spectra of vanadium(V) 2p in OV(OtBu)3-x(OH)x@A520 catalyst.
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Catalytic performance of OV(OtBu)3-x(OH)x@A520 in the ODS process
The OV(OtBu)3-x(OH)x@A520 catalyst was employed to oxidize DBT to DBT-sulfone under
mild operating conditions. The conversion rate of 94% was observed at O/S=5 in the model
diesel fuel (500 ppm) at 60 C after 60 min. As Figure 11 shows, the desulfurization yield
reached 98% after 80 min.
Figure 11. ODS of DBT as a function of time in the presence of OV(OtBu)3- x(OH)x@A520 (0.1 mmol/g);
Reaction conditions: O/S=5, model diesel fuel (500 ppm), temperature 60 C.
Recycling of OV(OtBu)3- x(OH)x@A520 catalyst
The spent catalyst was collected from the reaction mixture after 20 min and reused for
desulfurization. The result of catalyst reusability for 0.1 mmol/g vanadium(V) oxytributoxide
loaded A520 catalyst is illustrated in Figure S9, which shows that desulfurization decreased
during the second and third runs. However, the catalytic activity remained more or less
constant during the fourth and fifth runs. Eventually, the recycled catalyst was characterized
by PXRD and FT-IR analyses. The results showed that the structure of the catalyst was
retained after the fifth run (Figures S10−S11).
Comparison of catalysts
As shown in Figure 12, the OV(OtBu)3-x(OH)x@MIL-101(Cr) (0.1 mmol/g) and OV(OtBu)3-
x(OH)x@A520 (0.1 mmol/g) catalysts efficiently oxidized DBT to DBTO2 in the model diesel
fuel under mild operating conditions. Potential reasons for this result are the kind and
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nature of the utilized support. The structure of MIL-101(Cr) and A520 MOFs possess OH
groups, which cause the hydrolysis of the loaded vanadium(V) oxytributoxide and the
subsequently creation of active vanadium sites. To clarify the role of OH groups on the
surface of MOF, a similar catalyst was prepared using MOF-5 with a surface area of 3800
m2g-1.[26] The powder XRD patterns of MOF-5[26] and OV(OtBu)3-x(OH)x@MOF-5 (Figure S12)
indicated that MOF-5 maintained its structure after loading. The catalyst prepared with
MOF-5, which lacked OH functional groups on the surface of MOF, showed only 10% activity
in the ODS reaction of DBT in 60 min (Figure 12). However, vanadium(V) oxytributoxide was
not hydrolyzed because of the lack of OH groups, so active vanadium sites were not
created. Interestingly, when MOF-5 was used as the support, the FT-IR spectrum of the
prepared catalyst (Figure S13) did not show a broad stretching vibration of OH at 3500 cm-
1, which was in contrast to MIL-101(Cr) and A520.
As shown in Table 2, the performance of the prepared catalysts in the ODS reaction of
DBT were compared with other reported catalysts. In this study, 20 mg of OV(OtBu)3-
x(OH)x@MIL-101(Cr) (0.1 mmol/g) or OV(OtBu)3-x(OH)x@A520 (0.1 mmol/g) catalyst were
used to desulfurize the model diesel fuel at the efficiency of 98% in comparison with 95100
mg of other reported catalysts at a temperature as low as 60 C in 60 min, which was an
excellent performance in the ODS process. Moreover, from an economic perspective, the
results indicate that the preparation of catalyst from A520 is more affordable than MIL-
101(Cr) because A520 can be prepared readily from inexpensive precursors under mild
operating conditions. Due to easy preparation and modest costs, these catalysts show
potential not only for academic study, but also for future industrial implementation.
15
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10.1002/cctc.202001327
ChemCatChem
Figure 12. The efficiency comparison of prepared catalysts in ODS of DBT.
Table 2. Comparison of performance of prepared catalysts in ODS of DBT with several reported catalysts
Sulfur
Oxidant removal
(%)
Catalyst
amount
(mg)
Time
(min)
Catalyst
O/S
Ref.
[45]
MoV/Zr/SBA
CsxMo8W4/SBA-15
Mo5V15
5
5
6
5
100
100
100
95
TBHP
98.5
100
75
80
90
75
[46]
[47]
[48]
TBHP
H2O2
H2O2
99.0
99.1
20% MoO3/20% V2O5/60% MCM-41
OV(OtBu)3-x(OH)@MIL-101(0.1 mmol/g)
OV(OtBu)3-x(OH)x@A520 (0.1 mmol/g)
5
5
20
20
TBHP
TBHP
98.0
98.0
60
80
This work
This work
Temperature: 60 C; model compound concentration: 500 ppm.
Conclusions
In this work, two efficient catalysts were prepared using the wet impregnation method. The
effective parameters in the ODS process, such as temperature, time, amount of catalyst, and
the O/S ratio, were investigated. Catalytic oxidative reactions were conducted under
16
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10.1002/cctc.202001327
ChemCatChem
optimized conditions. The experimental results indicated that OV(OtBu)3-x(OH)x@MIL-
101(Cr) (0.1 mmol/g) is a highly efficient catalyst in the ODS reaction of DBT in the model
diesel fuel (500 ppm) at 60 min. The catalysts utilized in the experimental process were
easily recycled, and they were reused several times without a significant decrease in their
activity. Finally, it should be mentioned that ODS with the prepared catalysts can be
extended to other refectory aromatic sulfur compounds such as benzothiophene,
thiophene, and 4,6-dimethyldibenzothiophene. Moreover, since the MIL-101 (Cr) and A520
hydrolytically are stable, the corresponding catalysts, OV(OtBu)3-x(OH)x@MIL-101(Cr) and
OV(OtBu)3-x(OH)x@A520, can be used in the desulfurization process of water containing
fuels.
Acknowledgments
The authors gratefully acknowledge the Vice-President’s Office for Research Affairs of
Shahid Beheshti University for supporting this study.
Conflicts of Interest
There are no conflicts of interest to declare.
Keywords: Desulfurization . Heterogeneous catalysis . MOF . Supported catalysts . Vanadium
oxytributoxide
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Table of Contents Entry
Two highly efficient catalysts for the oxidative desulfurization of the refectory aromatic
sulfur species in a model were prepared by the wet impregnation of vanadium(V)
oxytributoxide on the chromium(III) terephthalate (MIL-101) and on the aluminum fumarate
(A520) metal-organic frameworks. The prepared catalysts showed good recyclability and
reusability without a significant decrease in their activity.
20
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