Aerobic photoxidation of sulfides using unique hybrid polyoxometalate under visible light

Article abstract of DOI:10.1016/j.catcom.2021.106283

A unique hybrid Polyoxometalate was synthesized, which can absorb visible light and perform photocatalytic activity. This Organic-Inorganic hybrid material was synthesized through attaching the organotin compound to the vacant polyoxometalate. Interesting

Full text of DOI:10.1016/j.catcom.2021.106283

Catalysis Communications 152 (2021) 106283
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Short communication
Aerobic photoxidation of sulfides using unique hybrid polyoxometalate
under visible light
a,
b
,
c
,
*
c
Davud Karimian
, Fatemeh Zangi
a
Department of Chemistry, Central Tehran Branch, Islamic Azad University, Tehran, Iran
Department of Biomedical Engineering, Central Tehran Branch, Islamic Azad University, Tehran, Iran
Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran
b
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
A unique hybrid Polyoxometalate was synthesized, which can absorb visible light and perform photocatalytic
activity. This Organic-Inorganic hybrid material was synthesized through attaching the organotin compound to
the vacant polyoxometalate. Interestingly, this hybridization brought an outstanding decrement in the poly-
oxometalate band gap and turned this hybrid polyoxometalate to a photocatalytic compound, which can conduct
catalytic reactions under visible light. In this line, we devised photoxidation of sulfides using this photocatalyst
Polyoxometalate
Hybrid compounds
Photocatalyst
Aerobic oxidation
Sulfide oxidation
2
and under O atmosphere as a sustainable and environmental friendly oxidant. Various kinds of sulfides were
investigated under optimum reaction conditions and the corresponding sulfoxide products were synthesized in
good to high yields.
1
. Introduction
Polyoxometalates (POMs) are a large class of metal‑oxygen clusters,
region, there is a need to expand this range of absorption and shift it to
visible region to have POMs with more efficient photonic behavior. This
improvement can be operated with making hybrid-POM structures.
Introducing an appropriate organic moiety to the POMs structure allows
us to obtain hybrid-POMs, which have the lower band gap and ability of
absorbing visible light that cause to show photonic behavior under
higher wavelengths. Attractively, this modification can provide lots of
enchanting capabilities for these compounds and impressively be uti-
lized in the POMs photocatalytic applications [11,12].
which have shown great potential to be utilized in many applications.
These nanoscale inorganic compounds have interesting structures and
unique properties, which have been put them under vast attention.
Having outstanding characteristics, bring diverse applications for these
compounds in many fields such as catalysis, materials science, life sci-
ence, solar cells, medicine, molecular magnetism and polymers [1–5].
Besides the diversity in the parent structures of POMs, these com-
pounds are capable to be attached to the various organic moieties to
make hybrid-POM structures. Hybridization of POMs with different
organic groups and structures through covalent bond can boost the
characteristics of POMs in efficient way. Following this route, it is
achieved functionalized-POMs that are more capable to be used in many
applications [6–8].
Sulfoxides are the important and vast-used fragments in organic
synthesis. These compounds can be obtained by oxidation of Sulfides
and have shown that can be utilized in various fields such as medicine,
polymer sciences, catalysis, synthesis, as well as many pharmaceuticals
and antiseptics products [13,14]. Moreover, as sulfides are considered as
one of the major air pollutants, desulfurization of fuels is a high demand
and necessary process, which is so important from environmental and
health aspects. Oxidation of sulfides proposes an effective and benign
method that desulfurizes fuels along with removal of the potential of
polluting in these compounds [15,16].
Among these characteristics, photonic behavior of POMs has been
under more consideration in recent years [9,10]. Interestingly, this
behavior can vastly be influenced by hybridization. Therefore, with
sensitizing POMs through hybridization we can improve and enhance
the photonic properties of these compounds.
For achieving this aim, one of the effective and promising methods
for desulfurization that is called oxidative desulfurization has been
vastly utilized [17]. Interestingly, Oxidation of Sulfides Using photo-
xidation pathways would be a mild and useful method for producing
One of these improvements can be monitored through the ability of
light absorption. As POMs can absorb specific wavelengths in the UV
*
Corresponding author at: Department of Chemistry, Central Tehran Branch, Islamic Azad University, Tehran, Iran.
E-mail address: davud.karimian@gmail.com (D. Karimian).
https://doi.org/10.1016/j.catcom.2021.106283
Received 17 November 2020; Received in revised form 8 January 2021; Accepted 15 January 2021
Available online 19 January 2021
1
566-7367/© 2021 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

D. Karimian and F. Zangi
Catalysis Communications 152 (2021) 106283
desired oxidized organic products. Furthermore, in the sulfide oxidation
process, the mild photoxidation path allows us to convert the sulfide
compounds to the sulfoxides, which are important intermediates in
synthesis of many organic compounds. Applying photoxidation has been
attracted much attention recently and various efforts have been done to
improve and modify this system of desulfurization [18–20].
As an operating sustainable and useful route for photoxidation,
Utilizing visible light and molecular oxygen in the role of oxidant is so
worthwhile and important. This efficient path not only provides a
benign and cost-effective procedure for the reaction but also doesn’t
bring any harmful effect to the environment.
Table 1
Photocatalytic oxidation of thioanisol on the presence of different catalyst
a
amounts .
b
Entry
Amount of catalyst (mg)
Yield (%)
1
2
3
4
5
6
60
85
82
64
41
26
14
11
17
6
30
20
10
5
2
c
7
d
30
8
30
Incorporating of oxygen into the organic substrate can be proceeded
by two pathways: by singlet oxygen or by electron transfer mechanism
9
No catalyst
30
e
1
0
87
a
[
21]. The oxidative desulfurization process has been done using
Reaction condition: thioanisol (1 mmol), acetonitrile (5 mL), time (4 h) and
different catalysts and various procedures. However, some drawbacks
such as having environmentally harmful waste, production of byprod-
ucts, low yields, and high cost have revealed in these systems [22–24].
These systems show that there is a need to employ more environmental
friendly and useful methods for oxidation of sulfides.
photocatalyst.
b
Yields are GC yield.
c
d
e
(
2) was utilized as catalyst.
Reaction was done in dark.
Yield is achieved in acetonitrile/water (95:5, v/v).
In the following context, the novel and effective hybrid-POM was
synthesized in which the photosensitive organic moiety is covalently
attached to the POM. This hybrid POM was demonstrated the
outstanding photonic behavior and therefore was utilized in photo-
xidation reaction. Accordingly, sulfide oxidation reaction was operated
in the presence of this hybrid-POM as a photocatalyst. Aerobic Oxidation
of sulfide under visible light was carried out and the remarkable results
indicate the capability of this photocatalyst in this oxidation reaction.
2
.1.3. Synthesis of ((n-C
A the last part, for synthesizing the photocatalyst we used this pro-
cedure: 3 mmol Cl Sn(CH HCNC16 was dissolved in 60 mL of DMF
and then 1.5 mmol of K [SiW11 39] was added to the solution and the
mixture was stirred overnight at the room temprature. In the next step,
.08 mol tetrabutylammonium bromide was added to the reaction
mixture and stirred for a while. After adding Et O to the mixture, the
4 9 4 5 2 2 9
H ) N) [SiW11(Sn(CH ) HCNC16H )O39] (1)
3
2
)
2
H
9
8
O
0
2
dark orange precipitation was obtained and collected on a fine frit. The
precipitation was washed with ethanol and dried for 12 h in the vacuum
oven. IR (KBr, cm-1): 3454 (m), 2958 (m), 2932 (m), 2869 (m), 1643 (s),
2
. Experimental
1
8
551 (m), 1489 (m), 1381 (w), 1254 (w), 1036 (vw), 960 (vs), 884 (vs),
The utilized materials and chemicals were provided from Merck and
09 (vs), 745 (s), 538 (m). Calcd elemental analysis for ((n-
Sigma chemical companies. The precursor monovacant POM
[SiW11 39].13H O, which is used for the hybrid POM synthesis, was
C
4
9
H )
4
5 2 2 9
N) [SiW11(Sn(CH ) HCNC16H )O39] (%): C 27.90, H 4.59, N
K
8
O
2
1
.97. Found: C 27.86, H 4.53, N 1.95.
prepared according to the literature [25]. The used light source for
photocatalytic reaction was 150 watt Superior SQP-SX1501 Xenon
lamp. Powder X-ray diffraction (XRD) patterns of the samples were
recorded on a Bruker, D8 ADVANCE, Germany, Wavelength: 1.5406 Å
2
.1.4. Synthesis of ((n-C
For comparing the result of our hybridization on the POM, we syn-
thesized a hybrid-POM without a sensitizing organic moiety. The solu-
tion of 3 mmol SnCl in the 50 mL of DMF/H O mixed solvent was
prepared. Then 1.3 mmol of K [SiW11 39] was added to the solution and
4 9 4 5
H ) N) [SiW11(SnCl)O39] (2)
(
Cu K
α), Voltage: 40 kV, Current: 40 mA in the 2θ range from 4 to 80.Gas
4
2
chromatography (GC) analysis was performed on a Varian CP 3800 in-
strument with a flame-ionization detector using silicon DC-200 or car-
bowax 20-M columns. Infrared spectra were recorded on a JASCO 6300
spectrophotometer. UV–vis and Diffuse Reflectance Spectroscopy (DRS)
spectra were recorded on JASCO V-670 spectrophotometer. HPLC
analysis was based on Ecom system quipped with UV–Vis detector. NMR
spectra were recorded using a Bruker Avance III 400 MHz spectrometer.
8
O
the mixture was stirred overnight in the room temprature. In the next
step, 0.075 mol tetrabutylammonium bromide was added to the reaction
mixture and stirred for a couple of minutes. After adding 100 mL of
ethanol to the mixture, the white precipitation was obtained and
filtered. The precipitation was washed with water and dried overnight in
the vacuum oven. IR (KBr, cm-1): 3451 (w), 2964 (m), 2934 (m), 2873
(
m), 1471 (m), 1371 (w), 1108 (w), 964 (vs), 886 (vs), 805 (vs), 743 (s),
34 (s). Calcd elemental analysis for ((n-C N) [SiW11(Sn
CH CHO)O39] (%): C 23.78, H 4.49, N 1.73. Found: C 23.75, H 4.47,
5
4
H
9
)
4
5
2
2
.1. Synthesis of photocatalyst
(
2 2
)
N 1.71.
.1.1. Synthesis of Cl
For the synthesis of first part of photocatalyst, we followed a litera-
ture procedure [26]: 0.12 mol of Acrolein was added dropwise and
slowly to the flask of prepared-solution of HSnCl .2C O (0.1 mol).
3 2 2
Sn(CH ) CHO
2.2. General oxidation procedure
3
2 5
H
After stirring of the reaction mixture for 12 h, the rotary evaporated was
used to remove the by-product and some excess reactants. The residue
was dissolved in chloroform and filtered. After evaporation of filtrate,
the white solid product was obtained and collected.
A quartz reaction tube with 10 mm diameter, containing solution of
sulfide (1 mmol) in 5 mL of mixture of solvents acetonitrile/water (95:5,
v/v) was prepared. Then, 0.007 mmol (0.030 g) of catalyst were added
2
and the O molecules purged to the solution before irradiation. The
power of the lamp was set and the lamp equipped with UV filter to
remove any irradiation in UV region. The light intensity at the sample
position was determined utilizing a genetic power meter and photocell
2
.1.2. Synthesis of Cl
For synthesizing the organic moiety of the hybrid compound, we
followed the imine formation reaction using Aminopyrene. Accordingly,
3 2 2 9
Sn(CH ) HCNC16H
◦
technology. The reaction temperature was kept around 25 C by circu-
0
.8 mmol Cl
3
Sn(CH
2
)
2
CHO was dissolved in 25 mL of ethanol and next,
lation of water through an external cooling coil. After stirring the re-
action for couple of hours, the progress of the reaction was monitored by
GC. At the end of reaction, the product was extracted by chloroform
from reaction mixture, and sulfoxide products were obtained by flash
1
mmol of 1-aminopyrene added to this solution. The reaction mixture
was stirred for 6 h at room temperature. Then the solvent was evapo-
rated under vacuum and the residue was collected for the next part of
synthesis.
6 6
chromatography on a silica-gel plate with n-C H /EtOAc as eluent. After
2

D. Karimian and F. Zangi
Catalysis Communications 152 (2021) 106283
Table 2
Table 3
a
Using different kinds of solvent in photocatalytic oxidation of thioanisol .
Aerobic oxidation of sulfides in the presence of hybrid POM (1)
a
b
photocatalyst .
Entry
Solvent
Yields (%)
c
1
2
3
4
5
6
CH
CH
3
CN / H
CN
2
O (95:5, v/v)
87
3
82
31
51
48
24
2 5
C H OH
2
H O
CH
CH
3
OH
Cl
2
2
a
Reaction condition: thioanisol (1 mmol), photocatalyst (30 mg), time (4 h)
and solvent (5 mL).
b
Yields are GC yield.
c
The yield is achieved in 3 h.
gaining the products, they were qualified by IR and HNMR spectroscopy
methods.
For recycling the catalyst, after extraction of the product by silica-gel
plate, the remaining of the reaction mixture was evaporated under the
vacuum and the catalyst was collected at the end of the flask. The
catalyst was washed three times with methanol and then dried under
vacuum.
Tables 1–3 show the results for optimized reaction conditions and the
oxidation of various sulfides under optimum conditions.
3
. Results and discussion
Hybridization of POM is a unique and useful way to synthesize more
efficient polyoxometalates, which can be exploited in many fields like
photochemical applications. Utilizing hybridization, the highly capable
polyoxometalates containing new and modified characteristics are
achieved. It is known that photoactivity is one of the most important
characteristics of POMs. However, the activity range of simple poly-
oxometalates is in UV region, which limits the applicability and effi-
ciency of these valuable compounds. Interestingly, hybridization of the
POMs can be useful arsenal to make more efficient and valuable poly-
oxometalates and enhance the applications of these compounds. In this
regard, we devise a hybridization system, which can work greatly as a
photocatalyst for oxidation of sulfides. During this hybridization, the
band gap of the final compound is decreased more than 1 ev and the
absorption range of polyoxometalate is spread from UV to visible region.
For this synthesis, we started from [SiW11O39] lacunary poly-
oxometalate that followed by reaction with organotin derivatives and
aminopyrene to conclude the hybrid compound (1).
Following the synthesis route, first, we synthesized lacunary poly-
oxometalate [SiW11O39]. Then the synthesis of complete organic part is
proceeded through forming the imine bond between photosensitizer
Pyrene molecule and 3-(trichlorostannyl)propanal. In the last part, the
synthesized organotin molecule was attached to monovacant POM in
order to gain the final hybrid product (1).
For the aim of showing the formation of new bonds in the hybrid
compound, the FT-IR spectroscopy was utilized to monitoring the bonds
before and after hybridization.
As Fig. 1 shows, besides the characteristic peaks of polyoxometalate
ꢀ
1
that have appeared around 960, 984, 809 cm , the spectrum of hybrid
ꢀ 1
compound (1) show the strong peak at 1634 cm , which is related to
the formation of imine bond. Moreover, the characteristic peaks of
polyoxometalate are observable in both spectra of (2,1) that indicate the
intact remaining of POM structure during hybridization.
Employing XRD patterns also revealed that POM structure has not
changed in the hybridization process. According to the fig. S2 (Sup-
◦
porting Information), the peak around 7.5 shows the (111) plane of
Keggin structure crystalline phase. As both spectra depict this orienta-
tion along (111) direction, it indicates intact structure of POM after
hybridization. This comparison has done on the basis of reported XRD
pattern analysis [27].
Observing the HNMR and 13CNMR spectrums of compound (1) was
1
3

D. Karimian and F. Zangi
Catalysis Communications 152 (2021) 106283
a
Reaction conditions: sulfide (1 mmol), photocatalyst (30 mg),
b
acetonitrile/water (95:5, v/v) (5 mL). GC yield.
Fig. 1. The FT-IR of (2) (red line) and (1) (blue line). (For interpretation of the
references to colour in this figure legend, the reader is referred to the web
version of this article.)
Fig. 2. UV–vis spectra for (2) (green line) and (1) (dark red line). (For inter-
pretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)
also indicated that during functionalization, the organic moiety remains
intact and attachment to the POM was properly carried on (supporting
information, Fig. S4 and S5).
The unique changes in POM properties that come from hybridization
are in optical properties. The UV–vis study of both (2, 1) demonstrate
that after hybridization, two strong peaks at 399 and 562 nm appear in
the spectrum (Fig. 2). These new peaks arise from expanding the ab-
sorption range of hybrid compound to visible region. This expansion
provides the remarkable photoactive behavior for hybrid POM in com-
parison to bare POM.
Employing the Kubelka-Munk equation, F(R) =
the percentage of reflected light and is absorption coefficient) and
transformed Kubelka-Munk function, [F(R)h – E ) where h is
]p = A(h
incident photon energy, E is optical band gap energy, A is the related
α = (1-R)2/2R, (R is
α
ν
ν
g
ν
g
constant to transition probability and p is the power index according to
optical absorption process [28], the band gap study was carried out.
Regarding to DRS analysis and mentioned calculations, the band gap
4

D. Karimian and F. Zangi
Catalysis Communications 152 (2021) 106283
and the results recorded. Among these solvents, acetonitrile exhibits the
better yield for this oxidation reaction (Table 2). Since acetonitrile have
shown photoinert behavior [32], it would be predictable to observe this
result. Moreover, it brings in mind that the reaction probably following
the electron transfer (ET) mechanism.
We found that adding a little amount of water as a co-solvent can
affect on the speed of the reaction. For shedding more lights on this
issue, the reaction in acetonitrile and mixed solvents (acetonitrile/
water) was operated. Using different ratios of acetonitrile and water, the
photoxidation reaction was done and the results are monitored in fig. S3.
According to results, the best ratio as a solvent is 95:5 for acetonitrile
and water respectively. Applying this ratio, the yield of the reaction is
nearly the same in comparison with the solely acetonitrile, but the re-
action is expedited and proceed in the lower time. However, adding
more amount of water as a co-solvent cause to decrease the yield of the
reaction.
Regarding to the obtained optimum conditions for oxidation reac-
tion, the various kinds of sulfides such as aliphatic and aromatic ones
was employed in the aerobic oxidation reaction in the presence of hybrid
POM photocatalyst (Table 3). It is shown that all of the sulfides were
oxidized to the corresponding sulfoxide products in good to high yields.
As Table 3 monitors, various kinds of sulfides were examined in the
photocatalytic oxidation reaction. Interestingly, these sulfides are
oxidized effectively up to the corresponding sulfoxide compounds and
the substrates didn’t overoxidized to the sulfone products, although, in
some cases the sulfones were detected in very low yields as a side
product. The thioanisol and its derivatives were converted to the sulf-
oxide. The results show that for thioanisol derivatives, both kinds
include electron-releasing and electron-withdrawing ones have shown
good conversion yields under the reaction condition (Table 3, entries
Fig. 3. Transformed UV–vis spectra for (2) (green line) and (1) (dark red line),
based on Kubelka-Munk equation. (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of
this article.)
energy for (2) and (1) was obtained. Comparison between these two
evaluated band gaps, clearly demonstrate that after hybridization the
POM shows the significant decrease in band gap from 3.78 eV to 2.35 eV
(
Fig. 3).
This decrement in band gap interestingly depicts that through hy-
bridization, POM is sensitized and absorb higher wavelength in visible
region. This attracting characteristic allows the hybrid POM to behave as
a good photocatalyst, which can catalyze substrates to desired products
using visible light. Also, many organic reactions can be photocatalyzed
utilizing this hybrid POM. In this regard, in this paper we investigated
the aerobic oxidation of sulfides to corresponding oxidized compounds
using hybrid POM photocatalyst and under visible light.
1
–3). However, the derivatives with high electron deficiency didn’t
sufficiently progress the reaction and demonstrated lower yields even in
the longer reaction time (Table 3, entry 4).
In the same line, various kinds of aliphatic sulfides were treated
under this photoxidation reaction. The obtained results exhibited that
yield of the reaction for these sulfides are good and near to the aryl
sulfides, however, reaction for cyclic aliphatic sulfides were shown
lower yields in some measure, and there was need to run the reaction in
longer times for these sulfides (Table 3, entries 9–11). Conversion of
benzyl phenyl sulfide to corresponding sulfoxide was proceeded in
appropriate time and good yields, however, around 14% of benzalde-
hyde which is a known side product of photoxidation of benzyl sulfides
In the second part of this paper, we investigated the photocatalytic
activity of this hybrid POM on various sulfides in aerobic oxidation re-
action. For modeling the oxidation reaction and find optimum reaction
condition, we utilized thioanisol through following typical route: 1
2
mmol of sulfide was dissolved in 5 mL solvent, the O purged to the
reaction tube, the photocatalyst added and irradiated at room temper-
ature. Using various amount of photocatalyst, we gained the optimum
usage of hybrid POM (1) (Table 1). As it is shown in this table, the
conversion yields increase with higher amount of photocatalyst. But for
the reactions using more than 30 mg of photocatalyst, no significant
increasing in yield was observed. Therefore, the 30 mg of photocatalyst
selected as an optimum amount that should be used in oxidation re-
actions (Table 1, entry 2). In order to monitor the capability of photo-
catalyst in oxidation of sulfides, the reaction was done with (2) as a
catalyst (Table 1, entry 7). As the results show, it is observable that
running the reaction in the presence of (1) as a catalyst cause to get
much more yield than using (2), and powerfully shows the effect of
hybridization on increasing the photoactivity of POM. Also, the oxida-
tion reaction, in the presence of (1) and in dark, proceeds only by 17% of
sulfide conversion (Table 1, entry 8).
[
33] was detected (Table 3, entry 6). Also, it is observed that conversion
of sulfides to sulfoxide was proceeded without affecting and over-
–
C double bond side functional group (Table 3, entries 5,
1). Moreover, thiophene and benzothiophene were efficiently oxidized
oxidation of C
–
1
into the corresponding sufoxide products (Table 3, entries 7, 8).
In order to examine the recyclability of this hybrid POM photo-
catalyst, the oxidation reaction of thioanisol was repeatedly operated at
the same condition. The results showed that after four times running the
reaction and reusing the photocatalyst, good yield is still observable for
the reaction (55%). This capability of catalytic activity shows that the
photocatalyst didn’t have critical changes during recycling and reusing,
which is also observed in characterization investigation for reused
catalyst.
In the same line, the optimization for catalyst was repeated in the
mixed solvent (acetonitrile/water 95:5, v/v) and the results indicated
that the 30 mg of catalyst is the efficient amount of catalyst for pro-
ceeding the reaction with good yield (Table 1, entry 10).
In order to investigate the quantum yield for this photoxidation re-
action, the thioanisol as a representative of sulfide compounds was
selected. According to the presented equation [34], the quantum yield
was calculated using the equation Φ = Δn / Δl. where Δn is number of
converted substrate molecules and Δl is number of absorbed photons by
the reaction mixture. In this regard, the Δn was obtained from the HPLC
analysis of reaction mixture and the Δl was calculated through standard
ferrioxalate actinometer. After determination of incident light intensity,
calculation was carried on at the wavelength of 562 nm as a selected
wavelength, in which the catalyst has intense absorption. Accordingly,
the quantum yield was obtained from the linear kinetic part of the
The next optimization was carried out on the solvent. As the studies
have depicted, the photoxidation reactions of sulfides are affected by the
nature of solvent [29,30]. Depending on the mechanism of the reaction,
various kinds of protic and aprotic solvents can demonstrate their po-
tency and efficiency by influencing on the yield and time of the reaction
[
31].
In this regard, different kinds of solvent were operated in the reaction
5

D. Karimian and F. Zangi
Catalysis Communications 152 (2021) 106283
University-Central Tehran Branch for supporting this Article.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.catcom.2021.106283.
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The Authors are thankful of University of Isfahan and Islamic Azad
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