Green synthesis of PbCrO4 nanostructures using gum of ferula assa-foetida for enhancement of visible-light photocatalytic activity

Article abstract of DOI:10.1039/c8ra06910g

Photocatalytic selective oxidation has attracted considerable attention as an environmentally friendly strategy for organic transformations. Some methods have been reported for the photocatalytic oxidation of sulfides into sulfoxides in recent years. Howe

Full text of DOI:10.1039/c8ra06910g

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Green synthesis of PbCrO4 nanostructures using
gum of ferula assa-foetida for enhancement of
visible-light photocatalytic activity
ab
Cite this: RSC Adv., 2018, 8, 40934
Rahele Zhiani,ab Ali Es-haghi,c Seyed Mohsen Sadeghzadeh
*
and Farzaneh Shamsaab
Photocatalytic selective oxidation has attracted considerable attention as an environmentally friendly
strategy for organic transformations. Some methods have been reported for the photocatalytic oxidation
of sulfides into sulfoxides in recent years. However, the practical application of these processes is
undermined by several challenges, such as low selectivity, sluggish reaction rates, requirement of UV-
light irradiation, use of additives, and instability of the photocatalyst. Pure monoclinic lead chromate
nanoparticles were prepared via a new simple way as Pb and Cr sources. PbCrO4 NPs were synthesized
via a green method in the presence of gum of ferula assa-foetida from Pb(NO3)2 and CrCl3 as lead and
chromium resources, respectively. The structural analysis of the samples confirmed the formation of
PbCrO4 nanostructures in the range of 30 ꢀ 5 nm. The PbCrO4 nanocatalyst was thoroughly
characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission
electron microscopy (TEM), and energy dispersive X-ray spectroscopy (EDX) study. Considering the large
ionic internal character and high mechanical and thermal stability as well as long-term colloidal stability,
this system can be considered as a perfect nanocatalyst by using the host–guest approach. A green and
ecofriendly method for oxidation of sulfides to sulfones in the presence of O2 as an oxidant was
examined for the synthesised PbCrO4 NPs. The easy and applied reusability of the catalyst was observed
after the completion of the reaction under visible-light irradiation.
Received 18th August 2018
Accepted 12th November 2018
DOI: 10.1039/c8ra06910g
rsc.li/rsc-advances
et al. reported photocatalytic systems that exhibited aerobic
oxidation of suldes into sulfoxides by using titanium(IV) oxide
as the catalyst.24–26 These methods used methanol as the
solvent, and it was also oxidized to formaldehyde. Also, Li et al.
conducted photocatalytic oxidation of suldes on Pt/BiVO4 in
water under visible-light illumination.27
Introduction
The oxidation reaction is one of the most vital protocols for
changing raw materials into products.1–4 Oxidation reactions
have some desired features of a ‘green’ procedure in terms of
reduced waste production and play key roles in the industry.5–9
There are many methods for conventional oxidants for oxida-
tion of sulde.10–19 O2 is clean, inexpensive, easily accessible and
eco-friendly as its side product is only water. The over-oxidation
of suldes resulting in sulfone formation is a common problem.
In this regard, intense efforts have been assigned to the
expansion of oxidation reactions using green methods and
recyclable catalysts that co-produce safe waste.20,21 Photo-
catalytic developments, which use visible light as a great and
easily accessible source of energy for catalytic reactions, have
been considered to be signicant for a sustainable future.22,23
There are some reports on the use of molecular oxygen for
the photocatalytic oxidation of suldes into sulfoxides. Chen
In the past decades, inorganic nanomaterials have attracted
signicant interest because of their exclusive properties such as
optical, electronic and magnetic properties and potential
applications in nanodevices.28–32 There are four types of re-
ported oxides in Pb–Cr–O ternary system: PbCrO4, Pb2CrO5,
Pb5CrO8 and Pb11CrO16. Three compounds with Pb(PbO)nCrO4
(n ¼ 0, 1, 4) composition are known to have optoelectronic
properties. PbCrO4 crystallizes in two structures such as stable
monoclinic and unstable orthorhombic.33 PbCrO4 is an impor-
tant solid material that is applied as a photosensitizer; it has
a stable monoclinic structure and is yellow pigment because of
its thermal stability and electrical properties.34 Lead chromate is
a signicant photocatalyst material. It has been widely applied
in decorative and protective systems and mass coloration of
bers, plastics and elastomers.35,36 Also, lead chromate has been
applied as a host material for humidity-sensing resistors,
photosensitizers and other devices.37
aNew Materials Technology and Processing Research Center, Department of Chemistry,
Neyshabur Branch, Islamic Azad University, Neyshabur, Iran
bYoung Researchers and Elite Club, Neyshabur Branch, Islamic Azad University,
Neyshabur, Iran. E-mail: seyedmohsen_sadeghzadeh@yahoo.com
cDepartment of Biology, Mashhad Branch, Islamic Azad University, Mashhad, Iran
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Given our continued interest in nanocatalysis and catalyst
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Compound 2d. White solid, mp 154–157 ꢁC. 1H NMR;
d, ppm: 8.34 (d, J ¼ 8.9 Hz, 2H), 7.88 (d, J ¼ 8.9 Hz, 2H), 2.81 (s,
3H); 13C NMR; d, ppm: 153.1, 149.3, 124.9, 124.3, 44.1.
Compound 2e. Yellow oil, 1H NMR; d, ppm: 7.61 (d, J ¼
8.6 Hz, 2H), 7.50 (d, J ¼ 8.6 Hz, 2H), 2.68 (s, 3H); 13C NMR;
d, ppm: 144.4, 137.3, 129.4, 125.0, 43.9.
development for organic reactions,38–40 we offer a new, low-
temperature and facile route to synthesize nanostructured
PbCrO4 utilizing gum of ferula assa-foetida. To the best of our
knowledge, it is our rst effort to exploit gum of ferula assa-
foetida as a novel and renewable catalyst to produce nano-
structured PbCrO4. The fuel exploited for the fabrication of the
nanostructured PbCrO4 in this research is environmentally
friendly and non-toxic. Gum of ferula assa-foetida is rich in
ferulic acid as well as disulte that can play the role of reduc-
tant; also, it may be responsible for control over shape and size
due to the formation of favorable steric hindrance effect in the
synthesis process. The reported method can be easily applied
for large-scale production of nanostructured PbCrO4. Therefore,
herein, we have used the advantages of the properties of PbCrO4
NPs for the photocatalytic oxidation of sulde under visible-
light irradiation.
Compound 2f. Yellow oil, 1H NMR; d, ppm: 7.65 (d, J ¼
8.4 Hz, 2H), 7.48 (d, J ¼ 8.4 Hz, 2H), 2.70 (s, 3H); 13C NMR;
d, ppm: 145.0, 132.8, 125.3, 124.9, 43.8.
Results and discussion
Crystallite size and respective phase composition of the samples
were evaluated by XRD. The crystalline structures of the as-
synthesized PbCrO4 NPs were detected by XRD analysis. Fig. 1
shows the XRD pattern of PbCrO4 NPs synthesized with
Pb(NO3)2 and CrCl3. The presence of peaks in the XRD pattern
of PbCrO4 is consistent with the monoclinic structure, which is
in agreement with JCPDS standard cards no. 08-0210. PbCrO4
belongs to the space group P21/n and could be identied in
some cases by the (011), (120), and (200) peaks, of which the
(011) is most distinct from those of other phases. Also, (120) is
the most intense line for PbCrO4. To prove the correct prepa-
ration of PbCrO4, EDX pattern was obtained for the sample
(Fig. 2). As seen in the spectrum, the sample is composed of Cr,
Pb, and O. SEM was used to study the size and morphology of
PbCrO4. The SEM image of PbCrO4 showed that the prepared
catalyst is nanometer quasi-spherical with an average diameter
of 30 ꢀ 5 nm (Fig. 3a). Also, the TEM image of PbCrO4 is pre-
sented in Fig. 3b. It clearly shows well dispersion of the
prepared catalyst and several larger structures, which can be
ascribed to the aggregation/coalescence of each nanoparticle.
Table 1 shows the values of BET surface area, pore size, and pore
volume for PbCrO4. As for PbCrO4, the BET surface area, total
pore volume, and pore diameter (nm) are obtained as 719 m2
gꢂ1, 2.73 cm3 gꢂ1, and 24.82 nm, respectively.
Experimental
General procedure for the preparation of PbCrO4 NPs
Here, 0.5 g of lead nitrate and 1.21 g of chromium chloride were
dissolved in distilled water. Then, 2 mmol of citric acid was
added to the mixed solution and the pH of the above solution
was adjusted to 9 by adding tetraethylenepentamine (TEPA)
drop-wise; in the next step, 2 mmol of ethylene glycol as the
esterication agent was added to the resultant solution.
Increasing temperature and evaporation of the gel-like solution
led to the preparation of a porous solid mass. One g of gum of
ferula assa-foetꢁida was mixed with prepared powders and
calcined at 700 C for 3 h.
General procedure for the oxidation of sulde
Here, 0.1 mg of PbCrO4 NPs, 3 mmol of thioanisole and
0.1 mmol of triethylamine were added to 5 mL of CH3OH. Aer
the reaction mixture was stirred for 30 min in dark to reach the
adsorption equilibrium, O2 was purged into the vessel to raise
the initial pressure to 0.1 MPa. The reaction mixture was
magnetically stirred and illuminated with l > 400 nm visible
light irradiation at room temperature. At the end of reaction,
the PbCrO4 photocatalyst particles were separated from the
reaction mixture by ltration and the products were quantita-
tively analyzed by gas chromatography (GC) equipped with
a ame ionization detector (FID) using chlorobenzene as the
internal standard.
Fig. 4 shows the effect of solvent on the catalytic function.
The reaction did not proceed in THF or in solvent-free condition
and showed low conversion in ethanol and water. Acetonitrile
was the best solvent for photocatalytic reactions in certain
reports,41,42 and it also produced a high conversion of 83.9%.
Compound 2a. Yellow oil, 1H NMR; d, ppm: 7.68–7.66 (m,
2H), 7.51–7.40 (m, 3H), 2.67 (s, 3H); 13C NMR; d, ppm: 145.6,
131.0, 129.3, 123.4, 44.1.
Compound 2b. Yellow oil, 1H NMR; d, ppm: 7.48 (d, J ¼
8.2 Hz, 2H), 7.30 (d, J ¼ 8.2 Hz, 2H), 2.67 (s, 3H), 2.38 (s, 3H); 13
NMR; d, ppm: 142.5, 141.1, 130.0, 123.5, 44.1, 21.4.
C
Compound 2c. Yellow oil, 1H NMR; d, ppm: 7.55 (d, J ¼
8.7 Hz, 2H), 7.00 (d, J ¼ 8.7 Hz, 2H), 3.76 (s, 3H), 2.65 (s, 3H); 13
NMR; d, ppm: 161.9, 136.6, 125.2, 114.1, 55.4, 43.8.
C
Fig. 1 XRD analysis of PbCrO4 NPs.
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Fig. 5 The influence of amine on the selective aerobic oxidation of
thioanisole under visible-light irradiation.
Fig. 2 The EDX spectra of PbCrO4 NPs.
Fig. 3 (a) SEM images of PbCrO4 NPs; (b) TEM images of PbCrO4 NPs.
Fig. 6 The screening of the oxidant nature.
Table 1 Structural parameters of PbCrO4 NP materials determined
from nitrogen sorption experiments
Pore diameter
(nm)
SBET (m2 gꢂ1
)
Va (cm3 gꢂ1
)
719
2.73
24.82
Fig. 7 Time course of the product distribution for the photocatalytic
oxidation of methyl phenyl sulfide into methyl phenyl sulfoxide.
In the next step, we conducted aerobic sulde oxidation with
a variety of amines, which could be easily separated from the
sulfoxide product (Fig. 5). The conversion of thioanisole without
amine was very low under visible-light irradiation. Interestingly,
with the introduction of amine into the reaction system, the
progression of the reaction was signicantly boosted. Primary
amines can initiate the oxidation, and a decrease in the amount
of amine leads to a decrease in the conversion of the product. As
expected, the tertiary amines TMA and TEA remained stable
Fig. 4 Photocatalytic oxidation of sulfides in different solvents.
Methanol resulted in 99.9% conversion. Interestingly, the
solvent exhibited a great effect on the photocatalytic activity of
PbCrO4 NPs towards the oxidation of thioanisole. Thus, meth-
anol was selected as a safe solvent.
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Table 3 Substrate scope of the oxidation of sulfides into sulfoxides
Conversion
Entry
1
Substrate
Product
[%]
99.9
2
99.9
Fig. 8 Control experiment for the synergistic photocatalytic oxidation
of thioanisole.
3
4
99.9
87.2
5
6
83.9
86.1
Fig. 9 The effect of catalyst amount.
during the oxidation process, ensuring higher conversions of
thioanisole. Based on the data in Fig. 6, common oxidants such
as H2O2, TBHP, UHP and oxone were weaker oxidants than O2
for this transformation. During the reaction process, the
concentration of methyl phenyl sulde gradually decreased,
whereas the concentration of methyl phenyl sulfoxide gradually
increased (Fig. 7). No intermediates or other by-products were
observed during the reaction process. Aer light irradiation for
2 h, methyl phenyl sulde was quantitatively converted into the
target product of methyl phenyl sulfoxide.
Moreover, the reaction shows light-wavelength-dependent
behavior. Thus, a wide range of visible-light spectra can be
used for this reaction, representing a giant step forward
compared to the results obtained in previous reports, where
only visible light of approximately 400 nm could be used (Fig. 8).
To further prove the catalytic nature of PbCrO4, we varied the
amount of PbCrO4 used in the reaction to study its effect on the
selective aerobic oxidation of thioanisole under visible-light
irradiation; more details and results are summarized in Fig. 9.
We discovered that decreasing the amount of PbCrO4 led to
a drop in conversion to some extent in comparison with the
starting result. Increasing PbCrO4 did not lead to an apparent
increase in conversion. These combined results prove that
PbCrO4 truly acts as a catalyst for the photoredox process.
In comparison to previously reported methods, our method
is milder, greener from viewpoint of oxidant, support, and
Table 2 An overview of some methods for oxidation of thioanisole
Entry
Catalyst
Solvent
Oxidant
T (ꢁC)
t (h)
1.0
9.0
0.03
0.5
Yield (%)
1
2
3
4
5
6
Na2WO4
H2O
CCl4
MeOH
DCM/MeOH
DCM
H2O2
TBHP
Oxone
H2O2
CHP
O2
50
25
0
97 (ref. 43)
28 (ref. 44)
84 (ref. 45)
82 (ref. 46)
79 (ref. 47)
99
Ti4+
—
[WO4][SiO2/PrNH3]2
VO(salen)
PbCrO4
25
ꢂ20
720.0
2.0
MeOH
25
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Fig. 10 The reusability of catalysts for the oxidation of thioanisole.
Scheme 1 Possible mechanism for the visible-light-induced oxidation
of sulfides with O2.
reaction results are summarized in Table 3. The suldes were
successfully oxidized into the corresponding sulfoxides with
good to excellent yields. The electronic properties of the
substituents in the aryl suldes played an important role in the
substrate activity. Generally, the substrates with electron-
donating groups demonstrate higher activity than the
substrates with electron-withdrawing groups; the latter require
longer reaction times to give high yields of the corresponding
sulfoxides. The substrates with electron-donating groups more
easily donate electrons to sulfur atoms than the substrates with
electron-withdrawing groups, which can be benecial for the
oxidation reactions.
One of the most important merits of heterogeneous catalysts
is that they can be recycled and reused. Thus, the recyclability of
the PbCrO4 catalyst was investigated for the oxidation of thio-
anisole. Aer the rst catalytic run, the catalyst was separated by
centrifugation, leached and washed with water and methanol,
and dried at 80 ꢁC under vacuum. The used PbCrO4 catalyst was
subjected to a next run under the same conditions. As shown in
Fig. 10, the PbCrO4 catalyst demonstrated excellent recycla-
bility. It was recycled six consecutive times with almost unal-
tered catalytic activity.
The catalytic activity of nano-PbCrO4 depends on the size of
the particles, and we observed that PbCrO4 nanoparticles with
sizes are effective in catalyzing reactions. However, these
nanoparticles generally lose their activity slowly during every
reaction cycle. This loss in activity is observed because the
nanoparticles grow in size through the Ostwald ripening
process, in which small nanoparticles merge together to form
large nanoparticles. In the case of the PbCrO4 catalyst system,
we observed no appreciable Ostwald ripening, and the particle
size and distribution remained the same even aer the reaction,
which in turn maintained the catalytic activity. Therefore, even
aer several reaction cycles, the size and distribution of PbCrO4
nanoparticles remain nearly the same (Fig. 11) and thus, the
system maintains nearly the same catalytic activity.
Fig. 11 TEM images of PbCrO4 nano-catalysts after reaction (a).
Particle size distribution before reaction (b) and after reaction (c).
metal; it is also cost-effective with higher conversion and
selectivity towards production of sulfoxide. This comparison
clearly exhibits that our proposed catalyst is excellent (Table 2).
Aer the formulation of tentative reaction mechanism, we
nally seek to establish the scope of the substrates for this
visible-light photoredox catalysis, which is important for
demonstrating the generality of this reaction scheme. The
The tentative mechanism of visible-light photoredox catal-
ysis by PbCrO4 with triethylamine as a redox mediator is ratio-
nalized in Scheme 1. PbCrO4 can absorb a wide range of visible
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