Sonochemical syntheses of nano-sized dioxomolybdenum complexes: An efficient, selective and reusable heterogeneous nanocatalyst for oxidation of alkenes

Article abstract of DOI:10.1016/j.apcata.2013.03.003

Two Schiff base ligands derived from 2-hydroxy-4-methoxyacetophenone and 2,2′-dimethylpropylenediamine (H₂L¹) or 1,2-cyclohexanediamine (H₂L²) have been synthesized and characterized by physico-chemical and spectroscopic methods. Then dioxomolybdenum complexes have been prepared by reaction of Schiff base ligands and MoO₂(acac)₂ under ultrasonic irradiation to give nanoparticles of MoL¹ and MoL². FT-IR, UV-vis, ¹H NMR and ¹³C NMR spectroscopy, elemental analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), were used to characterize and investigate the nanocatalysts. A practical catalytic method for efficient and highly selective oxidation of wide range of olefins with anhydrous tert-butyl hydroperoxide over the prepared molybdenum nanocatalysts was investigated. Under mild reaction conditions, oxidation of cyclooctene led to the formation of corresponding epoxide with a yield of more than 90%. The catalysts were reused five consecutive times without detectable catalyst leaching or significant loss of activity.

Full text of DOI:10.1016/j.apcata.2013.03.003

Applied Catalysis A: General 456 (2013) 240–248
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Sonochemical syntheses of nano-sized dioxomolybdenum complexes:
An efficient, selective and reusable heterogeneous nanocatalyst for
oxidation of alkenes
Saeed Rayati∗, Payam Abdolalian
Department of Chemistry, K.N. Toosi University of Technology, P.O. Box 16315-1618, Tehran 15418, Iran
a r t i c l e i n f o
a b s t r a c t
Two Schiff base ligands derived from 2-hydroxy-4-methoxyacetophenone and 2,2ꢀ-
dimethylpropylenediamine (H2L1) or 1,2-cyclohexanediamine (H2L2) have been synthesized and
characterized by physico-chemical and spectroscopic methods. Then dioxomolybdenum complexes
have been prepared by reaction of Schiff base ligands and MoO2(acac)2 under ultrasonic irradiation to
give nanoparticles of MoL1 and MoL2. FT-IR, UV–vis, 1H NMR and 13C NMR spectroscopy, elemental
analysis, scanning electron microscopy (SEM) and transmission electron microscopy (TEM), were used
to characterize and investigate the nanocatalysts. A practical catalytic method for efficient and highly
selective oxidation of wide range of olefins with anhydrous tert-butyl hydroperoxide over the prepared
molybdenum nanocatalysts was investigated. Under mild reaction conditions, oxidation of cyclooctene
led to the formation of corresponding epoxide with a yield of more than 90%. The catalysts were reused
five consecutive times without detectable catalyst leaching or significant loss of activity.
© 2013 Elsevier B.V. All rights reserved.
Article history:
Received 11 October 2012
Received in revised form 22 February 2013
Accepted 3 March 2013
Available online xxx
Keywords:
Epoxidation
Ultrasonic irradiation
Olefins
Heterogeneous nanocatalyst
Molybdenum Schiff base complexes
1. Introduction
many attempts have been made to synthesis nanocatalysts as effi-
cient alternatives for the traditional heterogeneous catalysts. The
Since epoxides are very important chemicals in organic syn-
thetic chemistry [1–5] and can serve as versatile precursors
for the synthesis of many natural products and drug molecules
[6,7], therefore, the development of new methods for selective
oxidation of olefins to epoxides is of great interest. Various oxy-
gen donors such as sodium hypochlorite [8], sodium peroxide
[9], m-chloroperbenzoic acid [10], sodium chlorite [11], sodium
perborate tetrahydrate [12], oxone [13], idosylbenzene [14],
tert-butyl hydroperoxide [15–22], hydrogen peroxide [23–25],
tetra-n-butylammonium hydrogen monopersulfate [26–29] and
urea hydrogen peroxide [30–33] have been used in the catalytic
epoxidation of olefins. However, there were always some prob-
lems due to long reaction time, the cost of expensive catalyst,
involving toxic solvents or difficult experiment operation. There-
fore, many improvements such as the use of ultrasonic irradiation
[34–39] or use of heterogeneous catalysts [40–45] instead of homo-
geneous ones were investigated. Compared to the homogeneous
catalysts, the heterogenized catalysts offer the opportunity of easy
catalyst handling and recycling and result in higher turnover num-
bers. However, the latter are generally less active and therefore
use of ultrasonic irradiation for the preparation of nanomaterials
[46] is one of the most promising techniques to achieve such a
goal. The advantages of ultrasound procedures such as good yields,
short reaction times and mild reaction conditions are due to the
reduced particle size, increased surface area and the improvement
of mass transfer between the liquid and the catalyst surface due
to large solid–liquid interfacial area [47]. In the present investiga-
tion, two nanostructured dioxomolybdenum Schiff base complexes
have been synthesized under ultrasonic irradiation and used as cat-
alyst for oxidation of different olefins with tert-butyl hydroperoxide
(TBHP).
2. Experimental
2.1. Instruments and reagents
Infrared spectra were recorded as KBr pellets using Unicam Mat-
son 1000 FT-IR. A Bruker FT-NMR 500 (500 MHz) spectrometer
was utilized to obtain NMR spectra. Melting points were measured
on an Electrothermal 9100 apparatus. The electronic absorption
spectra were recorded on a single beam spectrophotometer (Cam
Spec-M330). The high-power ultrasonic cleaning unit Bandelin
Super Sonorex RK-100H with ultrasonic peak output 320 W and HF
power 80 Weff has been used. Scanning electron microscopy (SEM)
∗
Corresponding author. Tel.: +98 21 22850266; fax: +98 21 22853650.
E-mail addresses: rayati@kntu.ac.ir, srayati@yahoo.com (S. Rayati).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.03.003

S. Rayati, P. Abdolalian / Applied Catalysis A: General 456 (2013) 240–248
241
OMe
MeO
MeO
OMe
O
O
O
N
O
O
N
O
Mo
O
Mo
O
N
N
Me
Me
Me
Me
Me
Me
MoL1
MoL2
Fig. 1. The Mo(VI) complexes used in this study.
1H NMR (ı): 1.10 (s, 6H, NCH2C(CH3)2CH2N), 2.37 (s, 6H, N CCH3),
3.42–3.52 (4H, NCH2C(CH3)2CH2N), 3.75 (s, 6H, OMe), 6.26–7.56
images were obtained on a Hitachi S-1460 field emission scanning
electron microscope using Acc voltage of 15 kV. The oxidation prod-
ucts were analyzed with a gas chromatograph (Shimadzu, GC-14B)
equipped with a SAB-5 capillary column (phenyl methyl siloxane
30 m × 320 mm × 0.25 mm) and a flame ionization detector.
2,2ꢀ-Dimethylpropylenediamine,1,2-cyclohexanediamine,
1
(m, 6H, ArH). 13C{ H} NMR (ı): 15.03 (NCH2C(CH3)2CH2N), 23.92
(N CCH3), 36.18 (NCH2C(CH3)2CH2N), 55.58 (OMe), 101.18–173.52
(aromatic C), 177.36 (C N).
MoL2-bulk: 80%, 0.261 g, D.p.: >238, Selected FT-IR data, ꢀ
(cm−1): 2932 (C H), 1602 (C N), 1540 (C C), 842 and 903 (Mo O).
1H NMR (ı): 1.47–1.52, 1.71–1.80, 1.91–1.97, 2.02–2.10 (complex
m, 8H, CH2 of cyclohexyl), 2.36 (s, 6H, N CCH3), 3.82, 3.86 (d,
2H, CH of cyclohexyl), 3.80 (s, 6H, OMe), 6.28–7.42 (m, 6H, ArH).
2-hydroxy-4-methoxyacetophenone,
molybdenyl
acetyl-
acetonate and tert-butyl hydroperoxide (solution 80% in
di-tert-butylperoxide) were used as received from commer-
cial suppliers. Solvents were dried and distilled by standard
methods before use. Other chemicals were purchased from Merck
or Fluka chemical companies.
1
13C{ H} NMR (ı): 24.96 ((CH3)C N), 54.24 (OCH3), 22.82, 46.96,
54.72 (cyclohexyl carbons), 100.33–168.38 (aromatic C), 203.16
(C N).
2.2. Preparation of the Schiff base ligands (H2L1 and H2L2)
The dioxo-molybdenum(VI) complexes have been shown in
Fig. 1.
The Schiff base ligands were prepared by standard methods
[20]. The solution of 2-hydroxy-4-methoxyacetophenone (0.332 g,
2 mmol) were mixed with 2,2ꢀ-dimethylpropylenediamine
(0.102 g, 1 mmol) or 1,2-cyclohexanediamine (0.114 g, 1 mmol) for
preparation of H2L1 and H2L2, respectively in ethanol (20 mL). The
bright yellow solutions were stirred and heated to reflux for 1 h.
The desired dark yellow solutions were precipitated by mixing
with diethyl ether and filtered off and finally dried in air.
2.4. Preparation of dioxo-molybdenum(VI) complexes at
nano-size under ultrasonic irradiation
A chloroform solution (50 mL) of the Schiff base ligands, H2L1
(0.200 g, 0.50 mmol) and H2L2 (0.205 g, 0.50 mmol), was poured
dropwise under ultrasonic irradiation into to a 50 mL molybdenyl
acetylacetonate (0.163 g, 0.50 mmol) in tetrahydrofuran (THF) dur-
ing 40 min. After the end of the titration the solution was kept in
the ultrasonic bath for a period of 40 min. The obtained precipitates
were filtered and dried. Yield (MoL1-nano: 88%, 0.21 g; MoL2-nano:
92%, 0.23 g).
H2L1: Yield: 84%, 0.335 g, M.p. = 155, Selected FT-IR data, ꢀ
(cm−1): 3440 (O H), 2962 (C H), 1611 (C N), 1550 (C C).
1H NMR (ı): 1.10 (s, 6H, NCH2C(CH3)2CH2N), 2.34 (s, 6H,
N
CCH3), 3.33 and 3.62 (4H, NCH2C(CH3)2CH2N), 3.74 (s, 6H,
1
OMe), 6.24–7.54 (m, 6H, ArH), 17.17 (s, 2H, OH). 13C{ H}
NMR (ı): 14.37 (NCH2C(CH3)2CH2N), 24.45 ((CH3)C N), 35.80
(NCH2C(CH3)2CH2N), 55.20 (OCH3), 55.44 (NCH2C(CH3)2CH2N),
102.22–170.63 (aromatic C), 172.00 (C N).
2.5. General heterogeneous oxidation procedure
Catalytic experiments were carried out in a 5 mL test tube.
In a typical procedure, to a chloroform solution of cyclooctene
(3.3 mmol), 0.033 mmol dioxo-molybdenum(VI) complex and
9.9 mmol TBHP was added. The molybdenum complex is com-
pletely insoluble in chloroform and therefore the catalytic system is
heterogeneous in nature. The reaction mixture was stirred for 6 h at
room temperature (RT). The reaction products were monitored at
periodic time intervals using gas chromatography. The oxidation
products were identified by comparison with authentic samples
(retention times in GC).
H2L2: Yield: 88%, 0.362 g, M.p.: >151, Selected FT-IR data, ꢀ
(cm−1): 3460 (O H), 2931 (C H), 1617 (C N), 1540 (C C). 1H NMR
(ı): 1.47–1.51, 1.68–1.76, 1.91–1.93, 2.02–2.09 (complex m, 8H,
CH2 of cyclohexyl), 2.25 (s, 6H, N CCH3), 3.81, 3.84 (d, 2H, CH
of cyclohexyl), 3.79 (s, 6H, OMe), 6.21–7.26 (m, 6H, ArH), 12.78
1
(s, 2H, OH). 13C{ H} NMR (ı): 24.19 ((CH3)C N), 55.21 (OCH3),
14.20, 32.48, 61.36 (cyclohexyl carbons), 102.12–169.77 (aromatic
C), 170.73 (C N).
2.3. Preparation of molybdenum(VI) complexes
3. Results and discussion
The molybdenum complexes were prepared as follows: the
Schiff base ligand, H2L1 (0.399 g, 1 mmol) or H2L2 (0.410 g, 1 mmol)
was dissolved in 20 ml of ethanol. An ethanolic solution of molyb-
denyl acetylacetonate (0.326 g, 1 mmol) was added and the reaction
mixture was refluxed for 1 h. The colored solution was concentrated
to yield colored powders. The products washed with warm ethanol.
MoL1-bulk: Yield: 77%, 0.252 g, D.p.: >219, Selected FT-IR data, ꢀ
(cm−1): 2957 (C H), 1600 (C N), 1527 (C C), 847 and 908 (Mo O).
3.1. Characterization of the ligands and dioxo-molybdenum(VI)
complexes
3.1.1. IR spectral studies
A practical list of IR spectral data is presented in Table 1.
Comparison of the spectra of the complexes with the lig-
ands provides evidence for the coordination mode of ligand in

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S. Rayati, P. Abdolalian / Applied Catalysis A: General 456 (2013) 240–248
Table 1
IR spectral data of ligands and molybdenum complexes.
Compound
Selective IR bands (cm−1
)
Mo
O
C N
H2L1
H2L2
MoL1
MoL2
–
–
1612
1617
1600
1602
847, 908
842, 903
catalysts. A sharp band appearing at 1612–1617 cm−1 due to
ꢀ(C N) (azomethine), shifts to lower wave number by 12–15 cm−1
and appears at 1600–1602 cm−1. This indicates the involvement
of azomethine nitrogen in coordination to the metal center [48].
IR spectra of the Mo complexes show two characteristic strong
bands in the regions 847–842 and 908–903 cm−1, which could be
assigned to the symmetric and asymmetric stretching vibrations
of cis-dioxo (MoO2) respectively [49–52]. The infrared spectra of
the molybdenum complexes as bulk or nano size are identical
(Fig. 2).
Fig. 2. IR spectra of MoL1-nano (A) and MoL1-bulk (B).
Table 2
UV–vis data for molybdenum complexes in methanol.
3.1.2. Electronic spectral studies
Compound
MoL1
ꢁmax (nm) (ε, M−1 cm−1
)
Assignment
Table 2 provides electronic spectral data of the molybdenum(VI)
complexes along with their assignments. The electronic absorp-
tion spectrum (in methanol) show three absorption bands due to
the ␲→␲*, n→␲* transitions and LMCT respectively. The absorp-
tion band observed at 231 nm or 222 nm is attributed to the ␲→␲*
transition of the imino group [53]. The spectra of the complexes
show a band at 299 or 280 nm, which is due to the n to ␲* tran-
sition and another band at 373 nm assignable to a ligand to metal
charge transfer (LMCT) due to the promotion of an electron from
the highest occupied molecular orbital (HOMO) of the ligand to the
lowest unoccupied molecular orbital (LUMO) of molybdenum atom
[54].
231 (27,780)
299 (18,732)
373 (8090)
␲→␲*
n→␲*
LMCT
MoL2
222 (110,870)
280 (30,890)
373 (10,955)
␲→␲*
n→␲*
LMCT
3.1.3. Scanning electron microscopy (SEM) images of the
complexes
Scanning electron micrographs (SEMs) were recorded to obtain
the shape and diameter of the MoL1-nano and MoL2-nano
Fig. 3. SEM images of (A) MoL1-nano, (B) MoL2-nano nanoparticles and (C) MoL1-bulk.

S. Rayati, P. Abdolalian / Applied Catalysis A: General 456 (2013) 240–248
243
Table 3
Experiments on catalytic oxidation of cyclooctene at room temperature.a
No.
Catalyst
Oxidant
Conversionb (%)
Time (h)
1
2
3
4
5
6
None
TBHP
None
TBHP
TBHP
TBHP
TBHP
0
0
67
20
54
19
6
6
6
6
6
6
MoL1-nano
MoL1-nano
MoL1-bulk
MoL2-nano
MoL2-bulk
a
Reaction conditions: the molar ratio of catalyst:alkene:TBHP was 1:100:300.
Solvent: chloroform.
b
GC yields based on the starting olefin.
diameter of 40–60 nm (MoL1-nano) or 40–60 nm (MoL1-nano)
that is in agreement with other results.
3.2. Heterogeneous catalytic epoxidation of olefins
The catalytic performance of MoL1-nano and MoL2-nano was
investigated in the epoxidation of olefins with tert-butyl hydroper-
oxide (TBHP) as oxygen donor. A series of blank experiments
(Table 3) revealed that the presence of catalyst and oxidant are
essential for an effective catalytic reaction. In the absence of catalyst
(entry 1 in Table 3) or oxidant (entry 2 in Table 3), the reactions did
not proceed even under reflux. Nano-sized catalysts (MoL1 or MoL2)
show better catalytic activity in the epoxidation of cyclooctene with
respect to the bulk ones.
In order to find the suitable reaction conditions, the effect of
various reaction parameters that may affect the conversion and
selectivity of the reaction were studied. Solvent, catalyst concen-
tration, temperature, the nature and concentration of oxidant are
the factors that have been evaluated.
The influence of different solvents on the oxidation of
cyclooctene was studied using bulk and nanoparticle of MoL1 and
MoL2 as catalyst and results are presented in Fig. 6. Chloroform,
1,2-dichloroethane (DCE), dichloromethane (DCM) and acetonitrile
were used as solvent and the highest conversion (67% in MoL1-nano
and 54% in MoL2-nano) was obtained in chloroform. The higher
conversions in chloroform with respect to the other solvents pos-
sibly are due to the non-coordinating behavior of the chloroform.
Coordinative solvent such as acetonitrile compete with TBHP to
bind to the molybdenum center and inhibited the reaction [55,56].
Fig. 7 shows that the highest cyclooctene conversion was
obtained after 6 h and further oxidation of cyclooctene did not occur
after 6 h.
In order to investigate the effect of oxidizing agent in the oxi-
dation reaction, two other peroxides (hydrogen peroxide and urea
hydrogen peroxide (UHP)) were used, and results show that no sig-
nificant product formation was observed in the case of UHP and
hydrogen peroxide. Therefore, TBHP is a more efficient oxidant with
respect to the others (Table 4).
Different catalyst concentrations have been used in the oxi-
dation of cyclooctene (Fig. 8). It was observed that oxidation of
cyclooctene required 0.033 mmol of catalyst for completion.
Fig. 4. The particle size distribution for MoL1 (A) and MoL2 (B) after sonication and
MoL1-bulk (C).
Table 4
complexes (Fig. 3). The results showed that spherical particles were
well distributed and the average particle size was from 50 to 80 nm
for MoL1-nano and 40 to 70 nm for MoL2-nano (Fig. 4A and B) while
average particle size for MoL1-bulk is 250–300 nm (Fig. 4C). This
confirms that sonication causes a significant decrease in the particle
size.
Epoxidation of cyclooctene with different oxidants catalyzed by MoL1 and MoL2.a
Entry
Oxidant
MoL1-bulk
MoL1-nano
MoL2-bulk
MoL2-nano
1
2
3
H2O2
UHP
TBHP
0
0
20.0
1.5
1.5
67.0
0
0
19.0
1.0
1.5
54
Fig. 5 shows TEM images of MoL1-nano and MoL2-nano. As
can be seen, the products are formed from particles with mean
a
Reaction conditions: the molar ratios for using cat.:cyclooctene:oxidant are
1:100:300. The reactions were run for 6 h at 25 ◦C.

244
S. Rayati, P. Abdolalian / Applied Catalysis A: General 456 (2013) 240–248
Fig. 5. TEM images of (A) MoL1-nano and (B) MoL2-nano.
70
80
70
60
50
40
30
20
10
0
MoL1 (Bulk)
MoL1 (Nano)
60
MoL2 (Bulk)
MoL1-bulk
50
MoL2 (Nano)
MoL1-nano
40
MoL2-bulk
30
20
10
0
MoL2-nano
1
2
3
4
0.011 mmol
0.022 mmol
0.033 mmol
0.044 mmol
Dichloromethane
Chloroform
Dichloroethane
Acetonitrile
Fig. 8. The influence of catalyst concentration on the oxidation of cyclooctene using
TBHP catalyzed by MoL1-nano and MoL2-nano. The reactions were run under air in
chloroform and the molar ratio of cyclooctene:TBHP:catalyst was 3.3:9.9:X.
Fig. 6. The effect of various solvents on the epoxidation of cyclooctene with TBHP
using molybdenum complexes. Reaction conditions: cyclooctene (100 mmol), cat-
alyst (1 mmol), solvent (3 mL), TBHP (300 mmol). The reactions were run for 6 h at
room temperature.
The effect of ultrasound irradiation on the epoxidation of
cyclooctene with TBHP in the presence of nano or bulk molybde-
num catalysts was also investigated. As shown in Fig. 11 application
of ultrasonic waves has different effects in the presence of nano or
bulk catalysts. For example, if the reaction runs with MoL1-nano,
the reaction was completed after 50 min of ultrasonic irradiation,
while during this time 38.5% conversion was obtained with bulk
catalyst (Fig. 11).
In order to establish the general applicabilityof this catalytic sys-
tem, under the optimized conditions, oxidation of different olefins
were subjected in the presence of the catalytic amount of MoL1-
nano and MoL2-nano and the results are presented in Table 5. Since
temperature of the reaction also has influence on the performance
of the catalyst for olefin conversion, therefore, catalytic activity of
Different oxidant (TBHP) concentrations have been studied in
the oxidation of cyclooctene (Fig. 9). It was observed that oxidation
of cyclooctene required 300 mmol of TBHP for completion.
In order to investigate the effect of temperature on the activity
of the catalyst, four different temperatures (15, 25, 35 and 45 ◦C)
were used while keeping other parameters fixed (i.e. 3.3 mmol
cyclooctene, 9.9 mmol TBHP and 0.033 mmol catalyst). Effect of
change in temperature on the rate of reaction in the presence of
nano-catalysts is more than bulk ones. The results presented in
Fig. 10 show that in 45 ◦C reaction temperature gives the maxi-
mum percentage conversion of cyclooctene (ca. 100%) after 1 h in
the presence of MoL1/MoL2-nano whereas increase in temperature
has lesser effect on the conversion of cyclooctene in the presence
of bulk catalysts.
70
80
70
60
50
40
30
20
10
0
60
50
40
30
20
10
0
MoL1 (Bulk)
MoL1 (Nano)
MoL2 (Bulk)
MoL2 (Nano)
0
1
2
3
4
5
6
3.3 mmol
6.6 mmol
9.9 mmol
13.2 mmol
Time (h)
Fig. 9. The influence of oxidant concentration on the oxidation of cyclooctene
catalyzed by MoL1-nano and MoL2-nano. The reactions were run under air in chlo-
roform and the molar ratio of cyclooctene:TBHP:catalyst was 3.3:X:0.033.
Fig. 7. Effect of reaction time in the oxidation of cyclooctene with TBHP in the
presence of MoL1-nano in chloroform.

S. Rayati, P. Abdolalian / Applied Catalysis A: General 456 (2013) 240–248
245
Table 5
Epoxidation of olefins using TBHP catalyzed by MoL1-nano or MoL2-nano in chloroform.a
Entry
Alkene
Product
MoL1
MoL2
Time in 25 ◦C (min)
Conversion
Time in 25 ◦
C
C
Conversion
(selectivity) %
(selectivity) %b
Time in 45 ◦C (min)
Time in 45 ◦
1
2
3
4
5
360
60
67 (100)
360
60
54 (100)
100 (100)
100 (100)
O
360
60
54 (100)
360
60
52 (100)
100 (100)
100 (100)
O
360
60
57 (100)
360
60
50 (100)
97 (100)
100 (100)
O
O
360
60
62 (66)c
80 (35)
360
60
54 (77)
75 (38)
360
60
52 (42)d
76 (30)
360
60
46 (65)
70 (29)
O
Cl
Cl
6
7
360
60
45 (40)e
73 (21)
360
60
41 (41)
69 (28)
O
Me
MeO
Me
360
60
32 (35)f
50 (32)
360
60
34 (50)
52 (46)
O
MeO
8
9
360
60
51 (69)g
57 (48)
360
60
49 (60)
52 (49)
Me
Me
O
360
60
28 (100)
44 (100)
360
60
24 (100)
40 (100)
O
O
10
360
60
43 (100)
84 (100)
360
60
41 (100)
76 (100)
5
5
a
The molar ratio of catalyst:alkene:TBHP was 1:100:300.
b
c
d
e
f
Conversions and selectivities were determined by GC based on the starting alkene.
Benzaldehyde is the by-product.
4-Chlorobenzaldehyde is the by-product.
4-Methylbenzaldehyde is the by-product.
4-Methoxybenzaldehyde is the by-product.
Acetophenone is the by-product.
g
Table 6
Comparison of the activity of Mo(VI) complexes in the oxidation of cyclooctene.
Alkene
Mo complex
Conversion (%)
Reaction time (h)
Oxidant
Ref.
Cyclohexene
Cyclooctene
Cyclohexene
Cyclooctene
Cyclooctene
Cyclooctene
Cyclooctene
MO2Ar/ZAPS-PVPA
98
87
86
98
92
8
9
9
8
4
6
1
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
[59]
[60]
[60]
[61]
[62]
[63]
MoO2alaacacAmpMCM-41
MoO2leuacacAmpMCM-41
Immobilized Schiff base MoO2(VI) catalysts
MoO2pycaAmpMCM-41
[Mo(O)2-(salen)–POM]
MoL1
100
100
Present work

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S. Rayati, P. Abdolalian / Applied Catalysis A: General 456 (2013) 240–248
100
80
60
40
20
0
A
B
100
80
60
40
20
0
45°c
35°c
25°c
15°c
45°c
35°c
25°c
15°c
0
2
4
6
0
2
4
6
100
D
C
100
80
60
40
20
0
80
60
40
20
0
45°c
45°c
35°c
25°c
15°c
35°c
25°c
15°c
0
2
4
6
0
2
4
6
Fig. 10. Effect of temperature on the oxidation of cyclooctene in the presence of MoL1-nano (A), MoL2-nano (B), MoL1-bulk (C) and MoL2-bulk (D).
the catalysts has been tested for two temperatures. The activity of
the catalyst was found to increase as the temperature was raised
from 25 to 45 ◦C. It is observed that the time required to achieve the
maximum conversion is around 1 h at 45 ◦C, whereas, the same was
achieved at 6 h at 25 ◦C. Oxidation of cyclooctene, cyclohexene, 1-
methylcyclohexene, 1-octene and indene lead to the corresponding
epoxide as the sole product, while in the oxidation of styrene some
benzaldehyde (Table 5, entry 4), in the oxidation of substituted
styrene, some substituted benzaldehyde (Table 5, entries 5–7) and
also in the oxidation of ␣-methyl styrene, acetophenone (Table 5,
entry 8) were detected as by product. Production of benzaldehyde
in the oxidation of styrene is due to the over-oxidation of styrene
oxide with TBHP. The lower catalytic activity of ␣-methyl styrene
with respect to the styrene is due to steric hindrance of the methyl
substituent in the ␣-methyl styrene. Epoxidation of terminal alkene
(oct-1-ene) proceeds in moderate yield (43% in the case of MoL1-
nano and 41% for MoL2-nano). Much reactivity observed in the case
of MoL1-nano in comparison with MoL2-nano seems to be due to
its lower steric hindrance around molybdenum center.
in the presence of various heterogeneous Mo(VI) complexes H2O2
or TBHP are summarized in Table 6 [59–63]. Comparison of this
catalytic system with previously reported systems shows that
higher conversion of cyclooctene in the shorter reaction time was
achieved.
3.3. Catalyst reuse and stability
The reusability of a heterogeneous catalyst is of prime impor-
tance not only from the economic point of view but also due
to the easy work-up procedure. The homogeneous molybdenum
complexes cannot be recovered even once, in contrast the het-
erogeneous catalysts can be filtered and reused multiple times
without significant loss of catalytic activity. The stability of the
molybdenum complexes was monitored using multiple sequential
epoxidation reaction. For each of the repeated reactions, the cata-
lyst was filtered, washed with chloroform and methanol and dried
before being used. The catalyst was consecutively reused five times
(Table 7) without a detectable catalyst leaching or a significant loss
of its activity. No detectable leaching (tested by atomic absorption
spectroscopy) was observed after each catalytic reaction. Also the IR
spectra of the catalysts remained intact during the reaction. These
observations show that the catalysts are stable during the oxidation
reaction.
In the proposed catalytic cycle TBHP will be activated by coordi-
nation to the molybdenum center and formed a hepta-coordinated
molybdenum intermediate (I, Scheme 1) [57,58]. Then olefin as a
nucleophile will attack to the oxygen atom of the coordinated TBHP.
Many molybdenum Schiff base complexes have been reported
for alkene epoxidation. The results of the oxidation of cyclooctene,
MoL1, Nano
100
100
MoL2, Nano
MoL1, Bulk
80
MoL2, Bulk
80
60
40
20
0
60
40
20
0
10
20
30
40
50
10
20
30
40
50
Time of ultrasonic irradiation (min)
Time or ultrasonic irradiation (min)
Fig. 11. Effect of ultrasonic waves on the oxidation of cyclooctene with TBHP in the presence of nano and bulk catalysts.

S. Rayati, P. Abdolalian / Applied Catalysis A: General 456 (2013) 240–248
247
Me
Me
Me
OMe
OMe
MeO
MeO
+TBHP
C
O
H
O
O
O
O
O
O
N
O
O
O
Mo
Mo
TBOH
N
N
N
R
R
Me
Me
Me
Me
I
Me
Me
Me
Me
Me
Me
OMe
MeO
OMe
MeO
C
H
O
C
O
H
O
O
O
O
O
O
O
N
O
Mo
O
Mo
N
N
N
R
R
Me
Me
Me
Me
O
Scheme 1. Proposed mechanism.
Table 7
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The results obtained from catalyst reuse in the oxidation of cyclooctene with TBHP
by molybdenum complexes.
Run
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