Deep eutectic solvent-assisted synthesis of highly efficient nanocatalyst (n-TiO2@TDI@DES (ZnCl2:urea)) for chemoselective oxidation of sulfides to sulfoxides

Article abstract of DOI:10.1002/aoc.6127

This study proposed a straightforward process to synthesize 2,4-toluene diisocyanate (TDI)-functionalized TiO₂ nanoparticles in which a cost-effective linker (TDI) with high reactivity was employed to couple nano-TiO₂ through covalent bonding to a deep eutectic solvent (DES). By this method, DES was successfully immobilized on the TiO₂@TDI surface as an adsorbent and stabilizer. The structural, morphological, and physicochemical characteristics of the synthesized nanocatalysts were evaluated using various analytical methods including Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (SEM–EDX), and elemental analysis. The heterogeneity of the catalyst was also examined by a hot filtration test. The obtained TiO₂@TDI@DES nanoparticles offered superior catalytic behavior and excellent yield as well as recyclability for the chemoselective oxidation of sulfide into sulfoxide using a green oxidant (hydrogen peroxide). This catalyst exhibited excellent reusability as it can be recovered for six successive cycles with no significant leach or reduction of catalytic efficiency.

Full text of DOI:10.1002/aoc.6127

Received: 23 October 2020
DOI: 10.1002/aoc.6127
Revised: 30 November 2020
Accepted: 7 December 2020
F U L L P A P E R
Deep eutectic solvent-assisted synthesis of highly efficient
nanocatalyst (n-TiO2@TDI@DES (ZnCl2:urea)) for
chemoselective oxidation of sulfides to sulfoxides
Shaghayegh Taghavi | Ali Amoozadeh
| Firouzeh Nemati
Department of Organic Chemistry,
This study proposed a straightforward process to synthesize 2,4-toluene
diisocyanate (TDI)-functionalized TiO2 nanoparticles in which a cost-effective
linker (TDI) with high reactivity was employed to couple nano-TiO2 through
covalent bonding to a deep eutectic solvent (DES). By this method, DES was
successfully immobilized on the TiO2@TDI surface as an adsorbent and stabi-
lizer. The structural, morphological, and physicochemical characteristics of the
synthesized nanocatalysts were evaluated using various analytical methods
including Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction
(XRD), thermogravimetric analysis (TGA), scanning electron microscopy
equipped with energy-dispersive X-ray spectroscopy (SEM–EDX), and elemen-
tal analysis. The heterogeneity of the catalyst was also examined by a hot filtra-
tion test. The obtained TiO2@TDI@DES nanoparticles offered superior
catalytic behavior and excellent yield as well as recyclability for the
chemoselective oxidation of sulfide into sulfoxide using a green oxidant
(hydrogen peroxide). This catalyst exhibited excellent reusability as it can be
recovered for six successive cycles with no significant leach or reduction of cat-
alytic efficiency.
Faculty of Chemistry, Semnan University,
Semnan, Iran
Correspondence
Ali Amoozadeh, Department of Organic
Chemistry, Faculty of Chemistry, Semnan
University, Semnan 35131-19111, Iran.
Email: aamozadeh@semnan.ac.ir
Funding information
Semnan University
K E Y W O R D S
2,4-toluene diisocyanate, deep eutectic solvent, nanocatalyst, nano-TiO2, oxidation of sulfide
1 | INTRODUCTION
encompass other benefits including cost-effectiveness,
convenient synthesis, green chemistry, and high atomic
Modern chemistry nowadays is facing with crucial chal-
lenges such as developing practical routes, reaction envi-
ronments, conditions, and materials in favor of green
chemistry to achieve an environmentally friendly proce-
dure.[1] In this regard, the chemists are now focusing on
the development of recoverable, easily separable, non-
toxic, and cost-effective catalysts.[2]
Deep eutectic solvents (DESs) have been widely
employed as the neoteric ionic solvents in industrial reac-
tions. In addition to their physical and chemical similari-
ties with the conventional ionic liquids (ILs), DESs
economy.[3,4] Due to outstanding features (e.g., broad liq-
uid range, biodegradability, superior thermal stability,
low vapor pressure, and high accessibility), DESs have
been introduced as promising candidates for catalyst and
reaction media.[5,6] During the DES synthesis, its physico-
chemical characteristics could be easily tuned by altering
the types of hydrogen-bond donors and acceptors. Lewis
acidic DESs (LADESs) are homogeneous catalysts, which
are capable of offering an efficient medium for organic
synthesis processes. Owing to their superior stability and
selectivity (compared with conventional metal halide
Appl Organomet Chem. 2020;e6127.
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TAGHAVI ET AL.
catalysts), LADESs have received extensive attention of
academic communities.[7–9] Despite their high potency,
industrialization of LADESs has been hindered by several
issues including the application of relatively large
amounts of LADESs, their homogeneity and difficulty in
their separation, and poor reusability, which can result in
serious economical and environmental consequences.
Considering the mentioned problems along with the
industrial preference for employing heterogeneous
catalysts, DESs have been immobilized onto solid
supports.[10,11]
Over the last decade, the green chemistry concerns
have preferred to perform the reactions with recyclable
catalyst and under solvent-free conditions. For these
reasons, wide attentions were paid to the use of heteroge-
neous catalysts due to their recyclability and ease of sepa-
ration.[12,13] On the other hand, nanoparticles (NPs) have
more active sites due to their high surface/volume ratio.
Therefore, the study of heterogeneous nanocatalysts is
considered as a progressive research that leads to the
introduction of many modified catalysts.[14]
The insoluble nature of solid acids has resulted in
their proper stability, superior reusability, moderate reac-
tion condition, environmental friendliness, ease of sepa-
ration, and facile operation.[15] A well-defined grafted
solid catalyst can improve the dispersion status and pre-
vent the formation of aggregates in the solvent through
steric hindrance.[16–18] In this regard, transition metal
oxide nanocatalysts undoubtedly play a remarkable role
in the organic reactions due to their superior aspect ratio
and coordination sites, which lead to their excellent cata-
lytic activities.[19,20] Heterogeneous catalysts were supe-
rior to homogeneous catalysts due to their effortless
isolation, easy handling, and reusability. To increase the
efficiency and stability of homogeneous catalysts, several
approaches of immobilization have been proposed on
various heterogeneous supports.[21–23] Nanocatalysts can
be considered as a mediator between the homogeneous
and heterogeneous catalysts. They can provide efficient,
accessible, and extraordinary stable heterogeneous sup-
ports with high catalyst loading capacity. Owing to their
availability, nontoxicity, superior activity, recoverability,
strong oxidizing behavior, and long-term stability, tita-
nium dioxide NPs (nano-TiO2) have attracted the atten-
tion of scientists in this field. Nano-TiO2 has been widely
employed as the photocatalyst for environmental decon-
tamination and technological purposes due to its remark-
able features. Furthermore, it has been long employed in
photo-electrochemical cells, water or air purification, and
degradation of organic pollutants.[24–27]
chemistry.[28,29] Organic sulfoxides are also significant
intermediates in the asymmetric synthesis of bioactive
substances.[30] They also play a pivotal role in the
pharmaceutical industry or other delicate chemical
procedures.[31–33] Omeprazole and fipronil pesticides are
two typical instances of the widespread use of the men-
tioned intermediates.[34] The sulfides oxidation into their
corresponding sulfoxides and sulfones has been exten-
sively explored, and plenty of synthetic routes have been
already proposed. For this purpose, some oxidants (such
as concentrated HNO3, high-valence metal salts,[35]
sodium metaperiodate, m-chloroperoxybenzoic acid,[36]
halogens, and nitrogen pentoxide[37,38]) have been widely
used. The majorities of these procedures are complex pro-
cesses depending on dangerous and high-toxicity
reagents[39] or are accompanied by overoxidation of sulf-
oxides[40] resulting in the formation of unpredicted sul-
fones or unwanted by-products.[41,42] In addition to
molecular oxygen, hydrogen peroxide is one of the most
powerful green oxidants. It is a cost-effective and accessi-
ble oxidant with high atom efficiency with water as the
only by-product.[43–45] In this context, numerous studies
have proposed a method for oxidizing sulfides into their
corresponding sulfoxides by the use of H2O2 at the pres-
ence of NiFe2O4,[46] Ti(IV) complex,[47] V/TiO2,[48]
cerium(IV) triflate,[49] molybdate-based catalyst,[50] met-
alloporphyrins immobilized into montmorillonite,[51] Zn
complex,[52] Cu salen–Fe3O4,[53] Cu(II) Schiff base
complex,[54]
preformed
manganese
complex,[55]
tantalum(V) chloride,[56] zirconium tetrachloride,[57]
polymer-supported catalysts,[58,59] protic acid,[60] and
organocatalysts.[61,62]
The latest development in the surface immobilization
strategies involves the covalent linkage of an organic
component to a solid support. The coapplication of
nanosized inorganic moieties along with organic sub-
stances could significantly contribute to the development
of multifunctional materials, which are known as
inorganic–organic hybrids with a wide diversity of aca-
demic or technical applications. Such hybrid materials
can simultaneously offer the advantages of both organic
and inorganic compounds.[63,64]
Therefore, the present research is designed to intro-
duce a new approach for the DESs immobilization onto
the surface of TiO2 NPs using 2,4-toluene diisocyanate
(TDI) as a bifunctional covalently bound linker. In com-
parison with the costly traditional coupling agents
(e.g., titanate and silane coupling) used for organic sur-
face modification of TiO2 NPs, TDI has several prefer-
ences including high availability, lower cost, and high
activity thanks to its highly unsaturated bonds. Moreover,
the TDI–TiO2 NPs reaction will be rapid and feasible
with no need for a catalyst to bridge between nano-TiO2
The organic sulfides oxidation into their corres-
ponding sulfoxides has been recognized as one of the
prominent functional group transformations in organic

TAGHAVI ET AL.
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and DESs for enhancing the catalyst's lifetime and recy-
clability. To this end, nano-TiO2@TDI@DES were suc-
cessfully synthesized and applied to improve sulfides
oxidation into their corresponding sulfoxides with the
help of aqueous H2O2 (30%) oxidant. Upon the reaction
termination, the catalyst can be easily separated and
reused with no DES leaching or significant drop in its
activity. Ultimately, these catalysts were employed for the
preparation of several derivatives under the optimized
condition, which showed high efficiency and simple
work-up.
suspension was left at room temperature for a day
followed by filtration and washing by NH4OAc–HOAc to
eliminate all Cl−. The final precipitates were centrifuged,
washed by ethanol, and vacuum dried. TiO2 NPs were
attained by 2-h calcination at 650ꢀC.
2.3 | TiO2 NP functionalization by TDI
(n-TiO2@NCO)
n-TiO2@NCO was synthesized by the previously reported
procedures.[66] In summary, a dispersion of n-TiO2 (1.0 g)
and TDI (1.40 g) was prepared in 50-ml dried toluene by
ultrasonication for 10–15 min. The reaction mixture
underwent 6 h of stirring under the N2 atmosphere. The
products were centrifuged and washed by dry toluene to
eliminate the physisorbed and unreacted TDI molecules
followed by 24 h of vacuum drying at 80ꢀC.
2 | EXPERIMENTAL
2.1 | Materials and instruments
Chemicals were of commercial reagent grade and pur-
chased from Aldrich and Merck. TDI was in industrial
grade with 80:20 mixture of 2,4-isomer and 2,6-isomer,
and it was used as received. The purification of solvents
was achieved by conventional methods. The purity of
the synthesized compounds and progress of reactions
were assayed by thin-layer chromatography (TLC) on
commercial plates coated with silica gel 60 F254. Fou-
rier transform infrared (FT-IR) spectroscopy was
implemented on a Shimadzu FT-IR 8400 equipment
using KBr pressed powder plates in the range of
400–4000 cm−1. Thermogravimetric analyses (TGAs)
were performed by applying a DuPont 2000 thermal
analysis apparatus under air atmosphere heated from
30ꢀC to 800ꢀC at a ramp rate of 5ꢀC min−1. Field emis-
sion scanning electron microscope (FE-SEM) images
were taken on a TESCAN MIRA3 scanning electron
microscope instrument operating at an accelerating
voltage of 10 kV. X-ray diffraction (XRD) patterns were
conducted at room temperature on a Siemens D5000
diffractometer (Siemens AG, Munich, Germany) using
Cu Kα radiation of wavelength 1.54ꢀ A . Mira 3-XMU
scanning electron microscope was used for energy-
dispersive X-ray spectroscopy (EDX) spectra of the cata-
lyst. Particles were fixed on a double-sided adhesive car-
bon disk and sputter coated with a thin layer of gold to
evade sample charging problems.
2.4 | DES preparation
For DES preparation, urea (20.0 mmol, 1.200 g) and
zinc chloride (5.0 mmol, 0.680 g) were mixed at 100ꢀC
to reach a clear, transparent, homogeneous liquid
[urea]4[ZnCl2]. The obtained DES was then employed
to the synthesis processes with no further
purification.[67,68]
2.5 | Synthesis of DES-functionalized n-
TiO2 (n-TiO2@TDI@DES)
To synthesis of DES-functionalized n-TiO2, a dispersion
containing 0.5-g TiO2@TDI NPs was prepared in which
0.5-g DES was added. The solution was then stirred at
100ꢀC for 18 h. The resulting products were washed by
ethanol (3 × 20 ml) and dried at 60ꢀC under reduced
pressure for 6 h.
2.6 | General procedure for the oxidation
of sulfides
A mixture of sulfide (1 mmol) and 30% H2O2 (2 equiv)
with 40-mg catalyst was magnetically stirred at ambient
temperature for the appropriate time (monitored by
TLC). After the reaction was completed, the mixture was
eluted with ethanol (10 ml) and centrifuged to separate
the catalyst. Then, the combined organic layer was dried
over anhydrous Na2SO4, and the pure products were
obtained in 87–99% yields by evaporation of diethyl ether
under reduced pressure.[69]
2.2 | Preparation of nano-TiO2
The hydrothermal method was utilized for the synthesis
of nano-TiO2.[65] In a typical procedure, aqueous NH3
was initially added dropwise to a TiCl4 solution to set its
pH at 1.8. Then, it was heated to 70ꢀC for 2 h under stir-
ring to the balance final pH at 6. The obtained

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TAGHAVI ET AL.
3 | RESULTS AND DISCUSSION
To develop our current programs[70–73] on the use of
support-immobilized DES in different organic reactions,
the preparation of LADESs was addressed on nano-TiO2.
In this reaction, nano-TiO2 served as a novel, high-effi-
ciency, and recyclable nanocatalyst. The nanocatalyst
preparation procedure is illustrated in Scheme 1. The
hydrothermal approach was exploited for the preparation
of nano-TiO2. The NPs surface was then modified by TDI
such that their surface hydroxyls were preferentially
reacted with a para-isocyanate group of TDI giving rise to
urethane bond leaving unreacted ortho-isocyanate group.
Such selectivity can be assigned to the different reactivity
of these two isocyanate groups as well as the steric hin-
drance of TDI molecules. Subsequently, the unreacted
isocyanate groups were transformed into amid through
the treating with urea molecules of LADESs. The struc-
tural properties of the nanocatalyst were assessed by FT-
IR, FE-SEM, XRD, TGA, and EDX techniques.
FIGURE 1 The Fourier transform infrared (FT-IR) spectra of
(a) n-TiO2, (b) 2,4-toluene diisocyanate (TDI), (c) n-TiO2@TDI,
(d) deep eutectic solvent (DES), and (e) n-TiO2@TDI@DES
3.1 | Characterization of n-
TiO2@TDI@DES
3.1.1 | FT-IR spectra
reaction through the formation of a urethane bond
(OCONH). The peak at 1533 cm−1 is also assigned to the
phenyl ring in the TDI configuration.[76] These results
show the successful anchoring of TDI on the nano-TiO2
surface. Figure 1d verifies the formation of DES. Accord-
ingly, the adsorption bands emerged at 1664, 1332, 3360,
and 3473 cm−1 can be attributed to the vibrations of
C O, C N, and N H bond in amide groups. Further-
more, the one at 485 cm−1 represents the Zn Cl
stretching vibration.[77] Upon the DES immobilization on
n-TiO2@TDI, the formation of the amino group can be
verified through the fading of the NCO peak at
2266 cm−1 and the emergence of the absorption bands
related to the NH stretching vibration at 3473 and
3360 cm−1 (overlapping with N H band of urethane).
These shifts can be assigned to the direct interaction of
DESs with the NCO groups of the functionalized nano-
TiO2 (Figure 1e). Therefore, the IR results indicate that
the DESs were successfully immobilized on nano-TiO2.
FT-IR spectroscopy was employed to study the DES
immobilization on the nano-TiO2 surface. According to
Figure 1a, the broad strong band below 1200 cm−1 is
indicative of Ti O Ti vibration. The absorptions at 1623
and 3355 cm−1 could be also attributed to the hydroxyl
groups.[74] Based on Figure 1b, which illustrates the FT-
IR results of TDI, the major adsorption at 2243 cm−1 rep-
resents the vibration of the NCO group.[75] Moreover, the
absorption band at 1550 cm−1 can be assigned to the
presence of phenyl ring in the TDI structure. Figure 1c
represents the results of TiO2@TDI. Some new peaks
emerged at 2266, 1649, 1595, and 1533 cm−1. The one at
2266 cm−1 can be ascribed to the unreacted ortho-
isocyanate groups of TDI in the attachment with the n-
TiO2 surface, whereas those that appeared at 1649 and
1595 cm−1 are related to the C O and C N stretching
vibrations reflecting the occurrence of TDI–titania
SCHEME 1 Synthetic diagram of n-
TiO2@TDI@DES catalyst

TAGHAVI ET AL.
5 of 12
3.1.2 | XRD spectra
The X-ray powder diffraction technique was utilized to
evaluate the crystalline structure of the synthesized NPs.
As suggested by Figure 2a, diffraction peaks
corresponding to (101), (103), (104), (112), (200), (105),
(211), (204), (116), (220), (215), and (224) planes clearly
show the formation of the anatase TiO2 NPs (JCPD
89-4921). The diffraction pattern and the location of the
characteristic peaks were also detected in the case
of n-TiO2@TDI@DES (Figure 2b). This figure also
suggests the crystalline stability of TiO2 NPs during
DES immobilization. Its higher noise/signal ratio of
n-TiO2@TDI@DES reflects the successful surface func-
tionalization of n-TiO2. An ultraweak broadband at
2θ = 10–30ꢀ is indicative of the amorphous DESs and the
presence of ZnCl2 in the structure.[78–80] The successful
immobilization of DESs on n-TiO2 NPs can be confirmed
due to the emergence of the other peaks, which are prop-
erly in line with the anatase phase of n-TiO2.[81] The pres-
ence of ZnCl2 was further assessed by the EDX results.
3.1.3 | FE-SEM analysis
FE-SEM analysis was employed to describe the surface
morphology of the NPs upon their functionalization.
Figure 3 presents the surface morphology and particle
shape and size of the samples. Based on Figure 3b, nano-
TiO2@TDI possesses spherical particles that resemble the
morphology of pristine nano-TiO2 (Figure 3a). It can be
FIGURE 3 Field emission scanning electron microscope (FE-
SEM) images of (a) n-TiO2, (b) n-TiO2@TDI, and (c) n-
TiO2@TDI@DES. DES, deep eutectic solvent; TDI, 2,4-toluene
diisocyanate
concluded that nano-TiO2 did not exhibit significant
deformation upon the grafting of organic molecules. Such
a spherical-shaped structure is highly favorable as it
FIGURE 2 The X-ray diffraction patterns of (a) n-TiO2 and
(b) n-TiO2@TDI@DES. DES, deep eutectic solvent; TDI,
2,4-toluene diisocyanate

6 of 12
TAGHAVI ET AL.
offers plenty of active sites for anchoring other species.
Compared with pristine n-TiO2, the SEM image of n-
TiO2@TDI@DES displayed no evident alternation in its
crystallite size. Besides, aggregation and altered morphol-
ogy of n-TiO2@TDI@DES are observable; however, the
final NPs maintained their spherical shapes at a larger
size (22–34 nm). These findings indicate the modification
and grafting of organic compounds on nano-TiO2.
well as hydroxyl group removal.[82] The n-TiO2@TDI
exhibited three major weight losses (Figure 4b). The first
one occurred below 170ꢀC, which can be assigned to the
elimination of trapped water. The weight losses (14 wt.%)
at 170–250ꢀC can be assigned to the TDI-modified titania,
whereas the next step at T > 250ꢀC can be related to the
thermal degradation of TDI groups.[81] Upon DES immo-
bilization on n-TiO2@TDI surface, the weight loss in the
n-TiO2@TDI@DES showed an enhancement. Similarly,
the TGA findings of TiO2@TDI@DES (Figure 4c)
manifested three prominent weight losses. Although the
first weight (20 wt.%; T < 170ꢀC) loss corresponded to the
adsorbed moisture and elimination of the DES moieties
contributing to the ionic structure of DES and the emer-
gence of urea, the other two ones (at 190–250ꢀC [9 wt.%]
and T > 250ꢀC [5 wt.%]) can be attributed to the degrada-
tion of grafted TDI. Therefore, TGA results confirmed the
proper grafting of TDI and DES on the n-TiO2 surface.
3.1.4 | Thermogravimetric analysis
TGA can be employed to investigate the characteristic
decomposition pattern of the organic compounds as well
as their degradation temperature, bond formation, and
thermal stability. Figure 4 depicts the TGA results in the
temperature range of 30ꢀC to 800ꢀC. According to
Figure 4a, the weight loss (7 wt.%) at T < 170ꢀC corre-
sponds to the evaporation of the physisorbed moisture as
3.1.5 | Energy-dispersive X-ray
EDX analysis can efficiently investigate the elemental sta-
tus of the samples (Figure 5). The EDX results of n-
TiO2@TDI@DES in Figure 5b suggest the existence of
the anticipated elements (C, O, Zn, and Ti), hence con-
firming the nanocatalyst structure.
A comparison
between EDX results of n-TiO2@TDI@DES and n-
TiO2@TDI verified that DES was successfully
immobilized on the TiO2 NPs.
Table 1 lists the elemental status of n-TiO2@TDI and
n-TiO2@TDI@DES. Comparing CHNS analysis of n-
TiO2@TDI with that of n-TiO2@TDI@DES and incre-
ment was observed in the content of C, H, and N atoms
of n-TiO2@TDI@DES reflecting successful coating of
n-TiO2@TDI by DES. An enhancement in Zn and Cl
contents in n-TiO2@TDI@DES can be assigned to the
immobilization of ZnCl2.
FIGURE 4 The thermogravimetric analysis (TGA) patterns of
(a) n-TiO2, (b) n-TiO2@TDI, and (c) n-TiO2@TDI@DES. DES, deep
eutectic solvent; TDI, 2,4-toluene diisocyanate
FIGURE 5 The energy-dispersive
X-ray spectroscopy (EDX) patterns of
(a) n-TiO2@TDI and (b) n-
TiO2@TDI@DES. DES, deep eutectic
solvent; TDI, 2,4-toluene diisocyanate

TAGHAVI ET AL.
7 of 12
TABLE 1 Elemental analysis for the catalyst
using 30% H2O2 at ambient temperature. Initial tests
were conducted to optimize the reaction condition in
which the methyl phenyl sulfide oxidation was consid-
ered as a model and the impacts of time, product yield,
oxidant, and catalyst were explored (Table 2).
Amount (wt.%) n-
Element TiO2@TDI@DES
Amount (wt.%) n-
TiO2@TDI
C
19.02
45.13
6.53
7.62
O
42.71
4.96
44.71
—
According to Table 2, the proper yield oxidation reac-
tion is achievable without catalyst after 24 h (Entry 1).
Then, the experiments were continued to determine the
required catalyst load. The best yield was achieved using
40 mg of the catalyst, giving rise to the lowest reaction
time and maximum product yield (Entry 5). Table 2 also
suggests that the optimized H2O2 content was 2 equiv,
which maximized the product yield (Entries 5, 7, and 8).
Finally, the influence of the solvent (EtOH, H2O, CH3CN,
phCH3, and solvent free) on the reaction was explored
(Entries 5 and 9–12). Based on the results, the highest
yield in short reaction time (40 min) can be achieved
under the solvent-free condition, which is in line with
the principles of green chemistry. The optimal condition
resulting in the best reaction yield was the solvent-free
condition at room temperature using 2 equiv of H2O2 and
40 mg of nano-TiO2@TDI@DES.
N
Ti
17.82
3.82
Zn
Cl
7.68
—
Total
100.00
100.00
Abbreviations: DES, deep eutectic solvent; TDI, 2,4-toluene diisocyanate.
3.2 | Catalytic activity of nano-
TiO2@TDI@DES for selective oxidation of
sulfides to sulfoxides
For further evaluation of the environmentally friendly
chemical procedures,[83–85] the synthesized TiO2@-
TDI@DES was employed as an interphase catalyst in the
chemoselective oxidation of various structures of sulfides
to their corresponding sulfoxides at solvent-free condition
Motivated by these interesting findings, the generality
of this approach was further examined through the facile
TABLE 2 Optimization of conditions for synthesis of methyl phenyl sulfoxidea
Entry
Catalyst (mg)
H2O2 (equiv)
Solvent
—
Time (min)
Yield (%)b
1
n-TiO2@TDI@DES (0)
n-TiO2@TDI@DES (10)
n-TiO2@TDI@DES (20)
n-TiO2@TDI@DES (30)
n-TiO2@TDI@DES (40)
n-TiO2@TDI@DES (50)
n-TiO2@TDI@DES (40)
n-TiO2@TDI@DES (40)
n-TiO2@TDI@DES (40)
n-TiO2@TDI@DES (40)
n-TiO2@TDI@DES (40)
n-TiO2@TDI@DES (40)
DES (40)
2
1440 (24 h)
90
75
80
86
98
91
89
95
68
73
62
73
70
2
2
—
55
3
2
—
55
4
2
—
50
5
2
—
40
6
2
—
45
7
1.5
2.5
2
—
60
8
—
45
9
H2O
ETOH
Toluene
CH3CN
—
120
130
155
190
600
10
11
12
13
2
2
2
2
Abbreviations: DES, deep eutectic solvent; TDI, 2,4-toluene diisocyanate.
aReaction conditions: methyl sulfide (1 mmol), room temperature.
bIsolated yield.

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TAGHAVI ET AL.
TABLE 3 Oxidation of sulfides to sulfoxides in the presence of nano-TiO2@TDI@DESa
Entry
Sulfide
Sulfoxide
Time (min)
Conversion (%)
Yield (%)b
1
60
100
97
2
45
100
99
3
4
5
35
40
40
100
100
100
98
91
98
6
80
100
99
7
8
35
100
100
92
120
98
(Continues)

TAGHAVI ET AL.
9 of 12
TABLE 3 (Continued)
Entry
Sulfide
Sulfoxide
Time (min)
Conversion (%)
Yield (%)b
9
55
100
93
10
70
100
87
11
12
25
65
100
100
89
94
Abbreviations: DES, deep eutectic solvent; TDI, 2,4-toluene diisocyanate.
aReaction conditions: sulfide (1 mmol), nano-TiO2@TDI@DES (40 mg), H2O2 (2 equiv), solvent free at room temperature.
bIsolated yield.
oxidation of diverse sulfides (e.g., dialkyl sulfides [Entries
1–4], alkylary l sulfides [Entries 5–7], and sulfides with
other functional groups) under the optimal reaction
conditions (Table 3). Table 3 showed the high rate
production of diverse sulfoxides at superior selectivity
with no overoxidation by-products like sulfones. Besides,
SCHEME 2 Proposed mechanism for the oxidation
of sulfides using H2O2 catalyzed by n-TiO2@TDI@DES
nanoparticles

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TAGHAVI ET AL.
the steric effect was observed (Entries 10–12) and some
of the products were also evaluated by 1H-NMR
(Figures S1–S4).
catalytic activity (Figure 6). In Figure 7, the FT-IR spec-
trum of the catalyst after six runs with that of the fresh
catalyst has been compared and showed the intact
structure of nanocatalyst after six runs.
The catalyst leaching was also examined in the oxida-
tion reaction of sulfides. For this purpose, the reaction
was conducted under optimal conditions and stopped
3.2.1 | Proposed mechanism
The proposed mechanism of the sulfides oxidation
through hydrogen peroxide is illustrated in Scheme 2.
The in situ formation of peroxy acid through the catalyst
reaction with hydrogen peroxide and subsequent oxygen
transfer to the organic substrate can be seen in Scheme 2.
In particular, the catalytic activity of TiO2@TDI@DES
can be assigned to ZnCl2 groups of DES on catalyst sur-
face, which offer efficient Lewis acidic sites.
3.3 | Test of recycling, leaching, and
heterogeneity of catalyst
Regarding environmental concerns, industries are
searching for novel approaches for pollution elimina-
tion. In this regard, the heterogeneous catalysts are
promising candidates due to their facile separation,
recoverability, reusability, and leaching. The stability
and the reusability of nano-TiO2@TDI@DES were
explored within the oxidation of the sulfide using
methyl phenyl sulfide (1 mmol), H2O2 (2 equiv), and
catalyst (40 mg) at room temperature under solvent-free
condition. Upon the reaction completion, the catalyst
can be easily removed and recovered through EtOAc
washing and vacuum drying. The recovered catalyst can
be then reused six times with no significant loss of their
FIGURE 7 The Fourier transform infrared (FT-IR) spectra of
(a) fresh n-TiO2@TDI@DES catalyst and (b) reused n-
TiO2@TDI@DES catalyst after six times. DES, deep eutectic
solvent; TDI, 2,4-toluene diisocyanate
FIGURE 6 Reusability of nano-TiO2@TDI@DES for the
synthesis of methyl phenyl sulfoxide. Reaction conditions: methyl
phenyl sulfide (1 mmol), H2O2 (2 equiv), catalyst (40 mg), reaction
time of 40 min at room temperature under solvent-free conditions.
DES, deep eutectic solvent; TDI, 2,4-toluene diisocyanate
FIGURE 8 Leaching of the n-TiO2@TDI@DES catalyst in the
oxidation reaction of thioanisole. (a) In the presence of catalyst and
(b) after separation of catalyst. DES, deep eutectic solvent; TDI,
2,4-toluene diisocyanate

TAGHAVI ET AL.
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4 | CONCLUSION
The present study introduced a novel and cost-effective
industrial approach for preparing supported LADES NPs.
The obtained samples exhibited high efficiency as the
heterogeneous inorganic–organic hybrid catalysts. This
efficient and environmentally benign Lewis acidic cata-
lyst was successfully applied in the chemoselective oxida-
tion of sulfides. The presented method offered several
advantages including high yield, moderate and cleaner
reaction conditions, and feasibility with no need for col-
umn chromatographic purification; these features offer
them as a promising alternative for the conventional
approaches. The catalysts were successfully recovered six
times with no significant drop in their activity, which
demonstrates their superior stability due to the covalent
linkage.
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ACKNOWLEDGMENTS
We gratefully acknowledge the Faculty of Chemistry of
Semnan University for supporting this work.
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AUTHOR CONTRIBUTIONS
Shaghayegh Taghavi: Conceptualization; data curation.
Ali Amoozadeh: Supervision. Firouzeh Nemati:
Supervision.
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ORCID
Ali Amoozadeh https://orcid.org/0000-0002-9405-2412
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
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