Synthesis, characterization and catalytic activity of oleic acid-coated TiO2 nanoparticles carrying MoO2 (acac)2 in the oxidation of olefins and sulfides using economical peroxides

Article abstract of DOI:10.1039/c4nj00267a

Titanium oxide nanoparticles were synthesized by a modified polymerized complex derived sol-gel method followed by coating with oleic acid (OA) to modify their surface. The reaction with MoO₂ (acac)₂ yielded the TiO₂-OA-MoO₂ nanocomposite as a supported cis-dioxomolybdenum complex. The structural and morphological characterization of the as-prepared TiO₂ and TiO₂-OA-MoO₂ were carried out by different techniques such as XRD, FT-IR, TGA and TEM. The catalytic activity of the TiO₂-OA-MoO₂ nanocomplex in the epoxidation of olefins and oxidation of sulfides to sulfoxides using H ₂O₂ in ethanol as a safe solvent was investigated. The reactions also proceeded efficiently in the presence of tert-butylhydroperoxide (TBHP). Good/high yields and excellent selectivity were obtained. The recovery of the title heterogeneous nanocatalyst was easy and efficient and its catalytic activity was strikingly different from other nanooxometals such as m-ZrO ₂, MoO₃, Fe₃O₄ and TiO₂, as well as their nanocomposites such as TiO₂-OA, ZrO ₂-OA-MoO₂, MoO₃-OA-MoO₂ and Fe ₃O₄-OA-MoO₂. the Partner Organisations 2014.

Full text of DOI:10.1039/c4nj00267a

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Synthesis, characterization and catalytic activity
of oleic acid-coated TiO2 nanoparticles carrying
MoO2 (acac)2 in the oxidation of olefins and
sulfides using economical peroxides
Cite this:DOI: 10.1039/c4nj00267a
Maasoumeh Jafarpour,* Mahboube Ghahramaninezhad and Abdolreza Rezaeifard*
Titanium oxide nanoparticles were synthesized by a modified polymerized complex derived sol–gel
method followed by coating with oleic acid (OA) to modify their surface. The reaction with MoO2 (acac)2
yielded the TiO2–OA–MoO2 nanocomposite as a supported cis-dioxomolybdenum complex. The structural
and morphological characterization of the as-prepared TiO2 and TiO2–OA–MoO2 were carried out by different
techniques such as XRD, FT-IR, TGA and TEM. The catalytic activity of the TiO2–OA–MoO2 nanocomplex in
the epoxidation of olefins and oxidation of sulfides to sulfoxides using H2O2 in ethanol as a safe solvent
was investigated. The reactions also proceeded efficiently in the presence of tert-butylhydroperoxide
(TBHP). Good/high yields and excellent selectivity were obtained. The recovery of the title hetero-
geneous nanocatalyst was easy and efficient and its catalytic activity was strikingly different from other
nanooxometals such as m-ZrO2, MoO3, Fe3O4 and TiO2, as well as their nanocomposites such as TiO2–OA,
ZrO2–OA–MoO2, MoO3–OA–MoO2 and Fe3O4–OA–MoO2.
Received (in Porto Alegre, Brazil)
23rd February 2014,
Accepted 21st March 2014
DOI: 10.1039/c4nj00267a
www.rsc.org/njc
(COOH) present in the acid molecule and the hydroxyl groups
of the nanoparticle surface.7–10
1. Introduction
Nanosized metal oxides have received considerable attention
In recent years, heterogenization of homogeneous catalysts
because of their distinctive physical and chemical properties,1,2 has attracted much attention due to combining the best pro-
and the interest in the MO2 oxides, where M = Hf, Ti and Zr, perties of both homogeneous and heterogeneous catalysts.11–16
is stimulated by their wide range of industrial applications.3 Among the various approaches for immobilizing soluble cata-
However, inorganic nanoparticles tend to agglomerate and lysts, the covalent attachment has been the most frequently
aggregate,4,5 and often exhibit hydrophilic properties6 which used strategy as the resulting heterogeneous catalysts have
makes them to have low dispersal ability in non-polar solvents, good stabilities during the course of catalytic reactions. The
therefore the applications of many nanosized particles are stable covalent bonds experience no leaching from the support
largely limited. Fortunately, this problem could be solved by and thus provide an efficient approach to prepare site isolated
modifying the nanoparticle surface to avoid agglomeration catalysts.
when the particles are filled into a hydrophobic matrix.6,7
The enthusiasm shown in the coordination chemistry of
Functional groups like NH2, SH, COOH, SO2OH, and PO(OH)2 molybdenum followed the discovery of molybdenum in a number
are capable of interacting with inorganic nanoparticles by coordi- of redox enzymes, such as aldehyde oxidase, sulphite oxidase,
native, electrostatic or hydrogen bonding and act as reactive xanthine oxidase, nitrate reductase and nitrogenase.17 Dioxo-
anchors.6
molybdenum(VI)-complexes are important catalysts as well as
Oleic acid as an amphiphilic molecule is often used as a catalyst-precursors for oxygen-transfer-reactions in chemical
surfactant to chemically modify the surface of metal oxide and biological systems.18–21 Considerable efforts devoted to
nanoparticles, such as SiO2, ZnO, Fe3O4, and Al2O3, which have the preparation and investigation of catalytic properties of
sufficient hydroxyl groups (OH) on their surface, allowing us to dioxomolybdenum complexes under both homogeneous and
conduct an esterification reaction between the carboxyl group heterogeneous conditions are a testimony to show that they are
valuable catalysts.22–27 In continuation of our ongoing research
on the catalytic activity of Mo catalysts,28–35 in this work, we
incorporated simple and easily available MoO2 (acac)2 to oleic
Catalysis Research Laboratory, Department of Chemistry, Faculty of Science,
University of Birjand, Birjand, 97179-414, Iran. E-mail: mjafarpour@birjand.ac.ir,
acid-coated TiO2 nanoparticles to obtain a novel heterogeneous
molybdenum nanocatalyst for the oxidation of olefins and sulfides.
rrezaeifard@birjand.ac.ir, rrezaeifard@gmail.com; Fax: +98 561 2502515;
Tel: +98 561 2502516
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Scheme 1 Catalytic activity of the TiO2–OA–MoO2 nanocomposite in
the oxidation of olefins and sulfides.
The catalyst showed high thermal and oxidation stability and
desired activity and selectivity in the oxidation of olefins and
sulfides using hydrogen peroxide and tert-butylhydroperoxide
(TBHP) as industrially and environmentally important oxidants
(Scheme 1). The additional advantage of this catalytic system is
the facile and efficient reusability of the solid catalyst at the end
of reaction.
Fig. 1 The flow chart for preparation of nano TiO2 by the sol–gel process.
In the final step of the sol–gel process, the wet gel was fully
dried by direct heating on a hot plate at 150 1C for 6 h. The
resultant product was a black powder. Then it was calcined in a
furnace at 600 1C for 3 h at a rate of 5 1C minÀ1. Finally TiO2
nanoparticles with a white color were obtained (Fig. 1).
2. Experimental
2.1. General remarks
2.3. Preparation of TiO2–OA nanocomposite particles
All chemicals were purchased from Merck and Fluka Chemical
Companies. Powder X-ray diffraction (XRD) was performed
on a Bruker D8-advance X-ray diffractometer with Cu Ka (l =
1.54178 Å) radiation. The FT-IR spectra were recorded on a
NICOLET system. Raman spectra were recorded on a Bruker
senterra (2009) model with spectral range 200–3500 cmÀ1 and
laser wavenumber 785 nm. Thermogravimetric analysis (TGA)
of nanopowders was performed in air using a Shimadzu 50.
TEM images were obtained using TEM instrumentation
(Philips CM 10). Progresses of the reactions were monitored
by TLC using silica-gel SIL G/UV 254 plates and also by GC
on a Shimadzu GC-16A instrument using a 25 m CBP1-S25
(0.32 mm ID, 0.5 mm coating) capillary column. NMR spectra
were recorded on a Brucker Avance DPX 250 and 400 MHz
instrument.
To 1.0 g TiO2 particles 10.0 mL of 0.01 M oleic acid in methanol
was gradually added over a period of 20 min at room temperature
under ultrasonic agitation. Then, the reaction mixture was stirred
at room temperature for 8 h. Afterwards, the product was centri-
fuged and washed with methanol. Finally, TiO2–OA nano-
composite particles were obtained after drying for 4 h at 80 1C.
2.4. Preparation of the TiO2–OA–MoO2 nanocomplex
To 0.2 g of TiO2–OA nanocomposites 1.0 mmol MoO2 (acac)2
dissolved in ethanol was gradually added over a period of
20 min at room temperature under ultrasonic agitation. Then,
the as-obtained mixture was refluxed for 12 h. Afterwards, the
product was centrifuged and washed with ethanol. Finally, the
TiO2–OA–MoO2 nanocomplex was obtained after drying for 4 h
at 100 1C (Scheme 2).
2.2. Fabrication of TiO2 nanoparticles
2.5. General procedure for catalytic oxidation of olefins
To a solution of TiCl3 (0.05 mol) in a mixture of double-distilled To a mixture of olefin (1.0 mmol) and TiO2–OA–MoO2 nano-
water and absolute ethanol (1 : 1) citric acid (0.15 mol) and complex (0.001 g) in ethanol (1.0 mL) H2O2 (2.0 mmol) was
ethylene glycol (0.15 mol) were added subsequently. The result- added, and the reaction mixture was stirred in air at 70 1C for
ing mixture was dissolved at 45 1C under ultrasound for 15 min the required time. The reaction progress was monitored by GC,
to give a clear violet solution. The solution was refluxed at and the yields of the products were determined by GC and
120 1C for 8 h which turned into a metal–citrate homogeneous 1H NMR analysis. After the completion of the reaction, the
complex with a little color change from clear violet to black- TiO2–OA–MoO2 nanocomplex (solid phase) was separated by
violet. After cooling down, in order to bring about the required centrifugation followed by decantation (3 Â 5 mL ethanol).
chemical reactions for the development of polymerization and Then, the desired product (liquid phase) was extracted by plate
evaporation of the solvent, the sol was further slowly heated at chromatography eluted with n-hexane–EtOAc (10/1).
90 1C for 6 h in an open bath until a beige wet gel was obtained.
Assignments of products were made by IR, 1H NMR and MS
During continuous heating at this temperature, the polymeri- spectral data in comparison with authentic samples. [Note:
zation between citric acid, ethylene glycol and complexes is oxidation of olefins using TBHP in dichloroethane (DCE) was
developed and ultimately the sol became more viscous as a wet gel. performed using the same procedure.]
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Scheme 2 Schematic representation of the procedure for the fabrication of the TiO2–OA–MoO2 nanocomplex.
2.6. General procedure for oxidation of sulfides
To a mixture of sulfide (1.0 mmol) and TiO2–OA–MoO2 nano-
complex (0.001 g) in ethanol (1.0 mL) H2O2 (2.0 mmol) was
added, and the reaction mixture was stirred in air at 70 1C
for 30 min. The reaction progress was monitored by TLC,
and the yields of the products were determined by GC analysis.
After the completion of the reaction, the TiO2–OA–MoO2 nano-
complex (solid phase) was separated by centrifugation followed
by decantation (3 Â 5 mL ethanol). Then the desired product
(liquid phase) was extracted by plate chromatography and eluted
with n-hexane–EtOAc (10/3).
Assignments of products were made by IR, 1H NMR and MS
spectral data in comparison with authentic samples. [Note:
oxidation of sulfides using TBHP in DCE was performed using
the same procedure.]
Fig. 2 XRD pattern of TiO2 nanoparticles;
(anatase),
(rutile).
2.7. Reusability of catalyst
Mo was 3.63 wt%. Therefore, each gram of heterogeneous catalyst
contains 38 mmol Mo or MoO2 (acac)2 complex.
To a mixture of cyclooctene (10.0 mmol) and TiO2–OA–MoO2
nanocomplex (0.01 g) in ethanol (10.0 mL) H2O2 (20.0 mmol)
was added, and the reaction mixture was stirred in air at 70 1C
for 4 h. After completion of the reaction, the nanocomplex was
separated by centrifugation followed by decantation (3 Â 5 mL
ethanol). The isolated solid phase (TiO2–OA–MoO2 nanocomplex)
was dried under reduced pressure and reused for subsequent
runs. Catalyst recovery was also investigated in the oxidation of
olefins with TBHP in DCE as well as in the oxidation of sulfides
with both H2O2 and TBHP according to the above-mentioned
procedures.
3. Results and discussion
3.1. Synthesis and characterization of the catalyst
In the first step of this investigation, TiO2 nanoparticles of
approximately 18–20 nm diameters were prepared by the PC
sol–gel method.33 The particles were subsequently coated with
an oleic acid layer followed by complexation with MoO2 (acac)2
to yield the corresponding supported cis-dioxomolybdenum
nanocomplex (Scheme 2). Inductively coupled plasma atomic
Fig. 3 FT-IR spectra of (a) TiO2 nanoparticles, (b) TiO2–OA nanocomposite
emission (ICP) analysis revealed that the content of supported and (c) TiO2–OA–MoO2 nanocomplex.
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The spectral and analysis data confirmed successful synthesis
of the title nanocatalyst. The XRD pattern of TiO2 nanoparticles
exhibited two different phases of TiO2 such as anatase (tetragonal,
a = b = 3.782 Å, c = 9.502 Å) and rutile (tetragonal, a = b = 4.584 Å,
c = 2.953 Å)36,37 (Fig. 2). The strong diffraction peaks observed at
2y = 25, 38, 48, 54, 55, 63, 69, 70, 75 were assigned to the (101),
(004), (200), (105), (211), (204), (116), (220), (215) reflection planes
of the tetragonal crystal of anatase TiO2, respectively (JCPDs
No. 21-1272).38 The other diffraction peaks observed at 2y = 27,
35, 41, 56 were assigned to the (110), (101), (111), (220) reflection
planes of the tetragonal crystal of rutile TiO2 respectively (JCPDS
No. 21-1276).39 The size of particles was estimated to be 20 nm
according to the Debye–Scherrer formula (D = Kl/bcos y). It is well
known that TiO2 containing both the rutile and anatase phases
exhibits higher photocatalytic activity under visible light than
either pure phase alone.40
Fig. 4 TEM images of TiO2 (A) and TiO2–OA–MoO2 nanoparticles (B).
The FT-IR spectrum of TiO2 nanoparticles (Fig. 3a) reveals the
presence of major bands at 450–775 cmÀ1 which are attributed to
the stretching vibrations of Ti–O groups. Broad peaks at 3400 and
1630 cmÀ1 correspond to the surface adsorbed water and hydroxyl
groups.41,42 Fig. 3b shows the successful fabrication of the TiO2–
OA composite. The peaks at 2917 and 2851 cmÀ1 are rationalized
to CH3 and CH2 in the molecular structure; the presence of –COOÀ
is proved by two peaks at 1552 and 1450 cmÀ1. The disappearance
of the peak at 1711 cmÀ1 demonstrated that the OA layer on TiO2
is formed mainly by a chemical reaction instead of physical
adsorption. The changes in intensity of nOH of TiO2–OA are
distinctly lower than that of TiO2 nanoparticles.43–46 Apparently,
OA coating on the surface of TiO2 particles contributes to the
decrease of the number of –OH on the TiO2 surface. The peak at
1643 cmÀ1 can be assigned to the CQC stretching mode for oleic
acid as well as the remaining hydroxyl groups on the TiO2 surface.
The vibration absorption at low frequencies, such as the
peak at 722 cmÀ1, shows the existence of the TiO2 core.
Comparison of the FT-IR spectra of the TiO2–OA–MoO2 nano-
complex (Fig. 3c) with those of TiO2 and the TiO2–OA nano-
composite demonstrates significant spectral changes supporting
the formation of the respective complex. It reveals the presence
of major bands at 500–750 cmÀ1 and 800–950 cmÀ1 which are
attributed to the stretching vibrations of Ti–O groups and the
symmetric and asymmetric stretching vibrations of the cis-MoO2
moiety, respectively (Fig. 3).
Fig. 5 The TGA curve for the TiO2 powder.
Fig. 6 The TGA curve for the TiO2–OA–MoO2 nanocomplex.
Fig. 7 (left) The screening of the solvent nature at 70 1C except for solvents having lower b.p. which was performed at reflux temperature. (right) The screening
of temperature for the epoxidation of cyclooctene (1 mmol) in ethanol using H2O2 (2 mmol) catalyzed by the TiO2–OA–MoO2 nanocomplex (0.001 g).
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Fig. 8 The screening of catalyst amount for the epoxidation of cyclo-
octene (1 mmol) using H2O2 (2 mmol) in EtOH (1 mL) catalyzed by the
TiO2–OA–MoO2 nanocomplex at 70 1C after 4 h.
Fig. 10 Comparison of the catalytic activity of the TiO2–OA–MoO2
nanocomplex with other nanooxometals and nanocomposites in the
epoxidation of cyclooctene (1 mmol) in EtOH (1 mL) at 70 1C after 4 h.
Transmission electron microscopy (TEM) clearly revealed
spherical morphology for TiO2 and TiO2–OA–MoO2 nanoparticles
with sizes ranging between 18–20 nm and 20–28, respectively
(Fig. 4).
Thermal behavior of the synthesized amorphous powder
dried at 150 1C was studied through thermogravimetric analysis
(TGA) from ambient temperature to 800 1C. The results showed
that 600 1C was an optimum calcination temperature for the
synthesis of TiO2 nanopowders (Fig. 5).
The TGA curve of the TiO2–OA–MoO2 nanocomplex (Fig. 6)
demonstrates its degradation at 658 1C, which indicates the
high thermostability of the catalyst. The organic parts decom-
posed completely at 877 1C.
Fig. 9 The screening of H2O2 amount for the epoxidation of cyclooctene
(1 mmol) in EtOH (1 mL) catalyzed by the TiO2–OA–MoO2 nanocomplex
(0.001 g) at 70 1C after 4 h.
Table 1 The screening of the photocatalytic activity of the TiO2–OA–MoO2 nanocomplex in the oxidation of a-methyl styrene and diphenyl sulfidea
Conversion (%) (time)
Substrate
Conditions
Product (selectivity (%))
H2O2
TBHP
Room light lamps
a-Methyl styrene oxide (83)
100 (8 h)
100 (5 h)
UV light
a-Methyl styrene oxide (85)
100 (5.30 h)
100 (3 h)
Darkness
a-Methyl styrene oxide (60)
100 (11 h)
100 (8 h)
Room light lamps
Diphenyl sulfoxide (90)
80 (30 min)
80 (15 min)
UV light
Diphenyl sulfoxide (90)
Diphenyl sulfoxide (90)
80 (20 min)
50 (45 min)
80 (5 min)
Darkness
50 (30 min)
a
The molar ratio of substrate/oxidant/catalyst was 10 000 : 20 000 : 4. The reactions were run under air at 70 1C.
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3.2. Catalytic activity of the TiO2–OA–MoO2 nanocomplex
us to evaluate the epoxidation activity of the title nanocatalyst
under these conditions. Different olefins were oxidized smoothly
with 2 equiv. of TBHP by using 0.04 mol% of TiO2–OA–MoO2
3.2.1. Epoxidation of olefins with H2O2. Preliminary experi-
ments were initiated with oxidation of cyclooctene (1 mmol)
using H2O2 (2 mmol) as a model reaction under neat conditions
which did not proceed in the absence of a catalyst under
different conditions. To find optimum reaction conditions, the
influence of different factors that may affect the conversion and
selectivity of the cyclooctene epoxidation was investigated.
The screening of the solvent nature and temperature was
investigated. The epoxidation of cyclooctene proceeded with
high performance in ethanol at 70 1C in the presence of 0.04 mol%
of TiO2–OA–MoO2 nanocomposite catalyst (Fig. 7).
The reaction was further optimized with respect to catalyst
(Fig. 8) and oxidant amounts (Fig. 9). Using 0.04 mol% nano-
catalyst and two equiv. of H2O2 cyclooctene consumed within
4 h and increasing any of these amounts did not affect the
reaction rate noticeably. It should be noted that when H2O2 was
replaced by TBHP and NaIO4 cyclooctene oxide was produced
in 20 and 65% yield, respectively, at the same time. Under these
conditions no activity was observed for Oxones.
Moreover, a prolonged reaction resulted for cyclooctene
oxidation when the TiO2–OA–MoO2 nanocomplex was replaced
by other nanooxometals such as MoO3,34 m-ZrO2, Fe3O4 and
TiO2, as well as their nanocomposites such as TiO2–OA, ZrO2–
OA–MoO2, MoO3–OA–MoO2 and Fe3O4–OA–MoO2 under the
same conditions. The photocatalytic properties of TiO2 at the
core of the title nanocomplex may be an acceptable reason for
the great improvement in the catalytic activity (Fig. 10). To
support this claim oxidation of a-methyl styrene and diphenyl
sulfide under UV light, room light lamps as well as in the dark
was investigated (Table 1). Accelerated reactions under light
radiation particularly UV light demonstrated the photocatalytic
activity of the TiO2 core in the oxidation of the TiO2–OA–MoO2
nanocomplex.
Table 2 Oxidation of alkenes using H2O2 catalyzed by the TiO2–OA–
MoO2 nanocomposite in EtOHa
Conversion
(%) (iso-
Selectivityb Time
Entry Alkene
1
lated yield) Productb
(%)
(h)
100 (92)
100 (95)
100 (94)
100
4
2
3
100
100
4
4
4
5
100 (93)
100
75
8
100
14
25
83
6
100 (94)
8
17
75
To establish the general applicability of the method, various
olefins were subjected to the oxidation protocol under the
catalytic influence of the TiO2–OA–MoO2 nanocomplex (Table 2).
Several useful features of this catalytic method can be seen in
Table 2. Different olefins were generally good substrates for this
catalyst. It led to complete conversion of cyclooctene, norbornene
and cyclohexenes with the formation of the corresponding
epoxides as sole products (entries 1–4). When the terminal
CQC bond was conjugated with the phenyl ring, some oxiran
ring-opening reaction occurred and the related carbonyl com-
pound was achieved alongside the corresponding epoxide as a
major product (entries 5–7). However, a cyclic olefin conjugated
with a phenyl ring produced the pertinent epoxide exclusively,
albeit with low yield (entry 8). It should be noted that when
2-cyclohxene-1-ol as an allylic alcohol was subjected to the
oxidation procedure excellent chemoselectivity was observed
albeit formation of the related a,b-unsaturated ketone and no
epoxide product was observed (entry 9).
7
8
80
18
25
30
100
24
10
9
100
100
a
The molar ratio of substrate/H2O2/catalyst was 10 000 : 20 000 : 4. The
3.2.2. Epoxidation of olefins with TBHP. The recent success-
ful results obtained for olefin epoxidation catalyzed by cis-dioxo
Mo–Schiff base complexes using TBHP in DCE28–31,34 encouraged
b
reactions were run under air at 70 1C. The products were identified by
1H NMR or by comparison with authentic samples retention times of
GC analysis.47–51
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nanocomposite in DCE at 70 1C (Table 3). Although, the TiO2– the yields and selectivity remained almost the same, except for
OA–MoO2 nanocomplex displayed a higher epoxidation activity 2-cyclohexene-1-ol which produced mainly the pertinent epoxide
with TBHP than that with the H2O2–ethanol system (this work), (85%) along with 15% of the a,b-unsaturated ketone (entry 9).
Table 3 Oxidation of olefins using TBHP catalyzed by the TiO2–OA–MoO2 nanocomplex in DCEa
Entry
Alkene
Conversion (%) (isolated yield)
Productb
Selectivityb (%)
100
Time (h)
2
1
100 (92)
2
3
100 (95)
100 (95)
100
100
2
2
4
5
100 (93)
100
100
75
4
50
24
6
7
100
8
5
25
84
100 (96)
16
72
8
80
12
28
85
9
100
4
15
a
b
The molar ratio of substrate/TBHP/catalyst was 10 000 : 20 000 : 4. The reactions were run under air at 70 1C. The products were identified by
1H NMR or by comparison with authentic sample retention times of GC analysis.47–51
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3.2.3. Oxidation of sulfides to sulfoxides. To extend the presence of the title nanocatalyst (Table 4). The results given in
generality and scope of the present methodology, oxidation of Table 4 illustrate the high efficiency of this protocol for the
sulfides was investigated. Oxidation of thioanisole using 2 equiv. oxidation of structurally different sulfides. All substrates could
of H2O2 in the presence of 0.04 mol% of TiO2–OA–MoO2 nano- be smoothly converted into sulfoxides with excellent conver-
complex in ethanol at 70 1C produced the related sulfoxide as the sion rates, and excellent selectivities were obtained under mild
sole product in excellent yield within 30 min. Accordingly, conditions.
different sulfides were subjected to the reaction system in the
The chemoselectivity of the method is noteworthy, as exem-
plified by sulfides containing a hydroxyl group (Table 4, entry 3)
or a CQC bond (entries 2 and 6). While the sulfide oxidized
completely, the alcohol and olefin moieties remained intact.
In addition, dibenzyl sulfide (entry 4) was selectively oxidized to
its corresponding sulfoxide without formation of any benzylic
oxidation byproducts.
As observed in olefin epoxidation, accelerated oxidation of
sulfide occurred when H2O2 was replaced with TBHP (in DCE),
albeit with the same yields and selectivity (Table 4).
Table 4 Oxidation of sulfides using H2O2–ethanola and TBHP–DCEb
catalyzed by the TiO2–OA–MoO2 nanocomplex
Conversion (%)
(isolated yield)
Sulfoxide
selectivity
(%)
Entry Substrate
H2O2
TBHPb
a
3.2.4. Recovery of the catalyst. The good/excellent yields of
epoxides and sulfoxides obtained using these new catalytic
methods demonstrated the high stability and catalytic activity
of the prepared TiO2–OA–MoO2 nanocomposite. This was further
supported by evaluation of the recovery potential of the catalyst in
the oxidation of both olefins and sulfides under different condi-
tions mentioned in Tables 1–4.
Recovery of the TiO2–OA–MoO2 nanocomplex catalyst was
easy and efficient. The catalyst was recovered by centrifugation
and decantation of the reaction mixture. It was then washed
with ethanol as the safe solvent, dried under vacuum, and used
directly for the next round of reaction without further purification.
The ease of recovery, combined with the intrinsic stability of the
TiO2–OA–MoO2 nanocomplex, allows the catalyst to be recovered
efficiently over at least four times in the oxidation of olefins and
sulfides under different conditions used in this study (Fig. 11).
The comparison of FT-IR and TEM images of the used TiO2–
OA–MoO2 nanocomplex (Fig. 12) with fresh one showed that
the structure, size and morphology of the catalyst remained
almost intact after recovery four times.
1
100 (96) 100 (96) 100
2
3
100 (95) 100 (94)
90
100 (96) 100 (96) 100
4
5
80 (72)
80 (73)
90 (85)
80 (75)
95
90
6
7
100 (96) 100 (94) 100
90 (80) 95 (89) 95
Therefore, title methodologies are cost effective and indust-
rially important because of reuse of the catalyst and using H2O2
and TBHP as environmentally oxidants especially in ethanol as
8
100 (96) 100 (95) 100
a
The reactions were run at 70 1C and the molar ratio of the sulfide : safe reaction media. These advantages of high yielding oxida-
H2O2 : catalyst was 10 000 : 20 000 : 4 in 1 mL ethanol within 30 min.
tion methods offered ready scalability. For example the use of a
semi scale-up procedure (10 mmol) for epoxidation of cyclooctene
and oxidation of thioanisole in the presence of the TiO2–OA–MoO2
b
The reactions were run at 70 1C and the molar ratio of the sulfide :
TBHP : catalyst was 10 000 : 20 000 : 4 in 1 mL DCE within 15 min. The
products were identified by comparison with authentic samples.52–55
Fig. 11 Recycling of the catalytic system for oxidation of cyclooctene (left) and thioanisole (right) using H2O2 and TBHP by the TiO2–OA–MoO2
nanocomplex at 70 1C according to procedures mentioned in the Experimental section.
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Fig. 12 (left) FT-IR spectra of a fresh TiO2–OA–MoO2 nanocomplex (A) and after reuse 4 times (B) in the oxidation of cyclooctene. (right) TEM image of
the TiO2–OA–MoO2 nanocomplex after reuse 4 times in the oxidation of cyclooctene.
suspensions for SERS, in Nanoparticles: Synthesis, Stabilization,
Passivation, and Functionalization, ed. R. Nagarajan and
T. Alan Hatton, ACS Symposium Series, Washington, DC,
2008, pp. 16–30.
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related epoxide and sulfoxide in 86–91% and 90–94% yields,
respectively.
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ogy, in Nanoparticles: Synthesis, Stabilization, Passivation, and
Functionalization, ed. R. Nagarajan and T. Alan Hatton, ACS
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4. Conclusions
In conclusion, the TiO2–OA–MoO2 nanocomplex prepared by
interaction of MoO2 (acac)2 with TiO2 coated with oleic acid with
size ranging between 20 and 28 nm efficiently catalyzed epoxida-
tion of olefins and oxygenation of sulfides to sulfoxides using H2O2
and TBHP in good/high yields and selectivity. It was observed that
the catalytic activity of the title heterogeneous nanocatalyst was
strikingly different from other nanooxometals as well as nano-
composites. Our results demonstrated well the oxidative stability
of nanocatalyst which makes it amenable to effective reusability.
Additionally, the use of ethanol as a ‘‘green’’ solvent and H2O2 and
TBHP as eco-friendly and economic oxidants as well as easy and
safe workup procedures are cost-effective, environmentally benign,
and possess high generality, which makes title methodologies
suitable for industrial goals.
Acknowledgements
Support for this work by Research Council of University of
Birjand is highly appreciated.
16 M. Masteri-Farahani, F. Farzaneh and M. Ghandi, J. Mol.
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