Green oxidation of olefins and methyl phenyl sulfide with hydrogen peroxide catalyzed by an oxovanadium(IV) Schiff base complex encapsulated in the nanopores of zeolite-Y

Article abstract of DOI:10.1007/s13738-014-0485-8

Oxovanadium(IV) complex of a Schiff base ligand derived from 2,4-dihydroxyacetophenone and 2,2′-dimethylpropanediamine has been encapsulated in the nanopores of zeolite-Y by flexible ligand method and characterized by metal analysis, IR spectroscopic studies and X-ray diffraction patterns. The encapsulated complex [VOL-Y] catalyzes the oxidation of various olefins and methyl phenyl sulfide using hydrogen peroxide as a green oxidant in good yield. Under the optimized reaction conditions, in the presence of VOL-Y, 86 % conversion of cyclooctene with 100 % selectivity for epoxide and 51 % conversion for methyl phenyl sulfide with 92 % selectivity for sulfone were obtained.

Full text of DOI:10.1007/s13738-014-0485-8

J IRAN CHEM SOC
DOI 10.1007/s13738-014-0485-8
ORIGINAL PAPER
Green oxidation of olefins and methyl phenyl sulfide
with hydrogen peroxide catalyzed by an oxovanadium(IV) Schiff
base complex encapsulated in the nanopores of zeolite-Y
•
Saeed Rayati Fatemeh Salehi
Received: 26 December 2013 / Accepted: 3 June 2014
Ó Iranian Chemical Society 2014
Abstract Oxovanadium(IV) complex of a Schiff base
ligand derived from 2,4-dihydroxyacetophenone and 2,20-
dimethylpropanediamine has been encapsulated in the
nanopores of zeolite-Y by flexible ligand method and
characterized by metal analysis, IR spectroscopic studies
and X-ray diffraction patterns. The encapsulated complex
[VOL-Y] catalyzes the oxidation of various olefins and
methyl phenyl sulfide using hydrogen peroxide as a green
oxidant in good yield. Under the optimized reaction con-
ditions, in the presence of VOL-Y, 86 % conversion of
cyclooctene with 100 % selectivity for epoxide and 51 %
conversion for methyl phenyl sulfide with 92 % selectivity
for sulfone were obtained.
In the catalytic reactions, heterogenization of the
homogeneous catalysts is a suitable method to achieve the
higher activity, selectivity, and reusability of the catalysts.
Therefore, heterogenization may be done by immobiliza-
tion through covalent bonding on the functionalized MCM-
41, SBA-15, etc. [11–18] or encapsulation in the nano-
cavity of, e.g. zeolites [19–21]. Recently, metal complexes
encapsulated in the nanocavity of zeolite-Y have been
widely used for many catalytic reactions such as the
selective oxidation, alkylation, dehydrogenation, cycliza-
tion, amination, acylation, isomerization and rearrange-
ment of various substrates and are able to produce
intermediates as well as most industrial products [22–25].
In this work, we have encapsulated the dioxovanadi-
um(IV) complex of a Schiff base ligand in the nanocavity
of zeolite-Y and the catalytic activity of the heterogeneous
catalyst in the oxidation of various olefins and methyl
phenyl sulfide with hydrogen peroxide as a green oxidant
has been investigated.
Keywords Heterogeneous catalyst ꢀ Zeolite ꢀ Schiff
base ꢀ Oxidation ꢀ Alkene ꢀ Sulfide
Introduction
Vanadium has been found to play a number of roles in
biological systems. For example vanadium complexes are
present in the haloperoxidases [1–3] where these com-
plexes catalyze the oxidative halogenation of aromatic
substrates using H2O2 as oxidant [4–7] or vanadium-based
drugs that lower glucose [8–10] by enhancing the effects of
insulin. Therefore, the study of vanadium compounds has
long been of interest to chemists.
Experimental
Materials and physical measurements
Infrared spectra were recorded as KBr pellets using Uni-
cam Matson 1000 FT-IR. A Varian (AA240) atomic
absorption spectrometer was used for vanadium determi-
1
nation. H NMR spectrum was obtained in CDCl3 with a
Bruker FT-NMR 500 (500 MHz) spectrometer. The
residual CHCl3 in conventional 99.8 % CDCl3 gives a
signal at d 7.26 ppm and was used for calibration of the
chemical shift scale. Gas chromatography (GC) analyses
were conducted on a Shimadzu chromatograph (model GC-
14B) equipped with flame ionization detector and capillary
S. Rayati (&) ꢀ F. Salehi
Department of Chemistry, K.N. Toosi University of Technology,
P.O. Box 16315-1618, 15418 Tehran, Iran
e-mail: rayati@kntu.ac.ir; srayati@yahoo.com
123

J IRAN CHEM SOC
column SAB-5 (phenyl methyl siloxane 30 m 9 320 mm 9
0.25 mm). Elemental analysis was performed using a CHN
Perkin Elmer 2400 Series II.
5.73; N, 6.56 %. Selected FT-IR data, t (cm-1): 2,962 (C–
H), 1,605 (C=N), 1,540 (C=C), 1,057 (C–O).
2,20-Dimethylpropylenediamine, 2,4-dihydroxyaceto-
phenone, Na-Y zeolite, vanadyl acetylacetonate, methyl
phenyl sulfide and hydrogen peroxide (30 % in H2O) were
used as received from commercial suppliers. The solvents
were dried and distilled using standard methods before
being used. Other chemicals were purchased from Merck
or Fluka chemical companies.
Incorporation of oxovanadium(IV) in Na-Y (metal
exchanged Y-zeolite)
5 g Na-Y zeolite was suspended in 100 mL distilled water
containing vanadyl acetylacetonate (12 mmol). The mix-
ture was then stirrer for 24 h. The solid was filtered and
washed with deionized water and dried at room tempera-
ture to give the light-green powder of VO-Y (Scheme 1, I).
Preparation of the ligands (H2L)
Immobilization of H2L in VO-Y
To a stirred ethanolic solution (30 mL) of 2,20-dimethylp-
ropelenediamin (0.102 g, 1 mmol), 2,4-dihydroxyaceto-
phenone (0.304 g, 2 mmol) was added. The solution was
stirred and heated to reflux for 1 h. An orange precipitate
was obtained that was filtered off and washed with warm
ethanol. Yield (90 %), Mp 120 °C. Anal. Calc. for
C21H26N2O4 (370.44): C, 68.08; H, 7.07; N, 7.56. Found:
C, 68.72; H, 7.53; N, 7.22 %. Selected FT-IR data, t
(cm-1): 3,445(O–H), 2,972 (C–H), 1,612 (C=N), 1,504
(C=C), 1,057 (C–O). 1H NMR (d/ppm):1.04 (s, 6H,
NCH2C(CH3)2CH2N), 2.26 (s, 6H, CH3C=N), 3.42 (s, 4H,
NCH2C(CH3)2CH2N), 6.05–7.39(m, 6H, ArH), 9.70 (br,
2H, ArOH), 17.01(s, 2H, ArOH).
An amount of 1.0 g VO(IV)-Y and 2.5 g of ligand H2L
were mixed in 100 mL of methanol and the reaction mix-
ture was refluxed for 15 h in an oil bath with stirring. The
resulting material was separated by filtration and then
extracted with methanol using Soxhlet extractor to remove
unreacted ligand from the cavities of the zeolite as well as
those located on the surface of the zeolite along with neat
complexes, if any. The unreacted oxovanadium(IV) ions
present in the zeolite was removed by stirring with aqueous
0.01 M NaCl solution. The resulting solid was filtered and
washed with distilled water until free from chloride ions.
Finally, it was dried at 120 °C in an air oven for several
hours.
Preparation of oxovanadium(IV) complex
General oxidation procedure
The complex was prepared by a general procedure: the
ligand, H2L (0.37 g, 1 mmol), was dissolved in 20 mL of
ethanol. An ethanolic solution of vanadyl acetylacetonate
(0.265 g, 1 mmol) was added to above solution and the
reaction mixture was refluxed for 1 h. The colored solution
was concentrated to yield colored powders. The products
were washed with warm ethanol. General structure of
oxovanadium(IV) complexes is shown in Fig. 1. Yield
(72 %), Dp 290 °C. Anal. Calc. for C21H24N2O5V
(435.37): C, 57.93; H, 5.55; N, 6.43. Found: C, 57.65; H,
Catalytic experiments were carried out in a 10-mL glass
reaction flask fitted with a water condenser. In a typical
reaction, alkene (0.2 mmol) and catalyst (1 mg) were
mixed in 1 mL of CH3CN and hydrogen peroxide (1 mL)
was added to it and the reaction mixture was heated in an
oil bath for 4 h. The reaction products were analyzed using
a gas chromatograph after a specific interval of time by
withdrawing a small aliquot of reaction mixture.
Results and discussion
OH
HO
Characterization of oxovanadium complex
encapsulated in the zeolite-Y
O
O
N
O
N
IR spectral studies
V
A sharp band appearing at 1,612 cm-1 due to m(C=N) in
the H2L (azomethine) shifts to lower wave numbers and
appears at 1,605 cm-1 in the VOL. This indicates the
involvement of azomethine nitrogen in coordination to the
vanadium center. Also, m(C=N) of VOL-Y appears at fre-
quencies shifted 6 cm-1 (1,599 cm-1) from those of the
Me
Me
Me
Me
Fig. 1 General structure of oxovanadium(IV) complex
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J IRAN CHEM SOC
Scheme 1 Encapsulation of the
oxovanadium(IV) complex in
the nanopores of zeolite-Y
O
O
O
O
OH
ONa
ONa
ONa
ONa
HO
Z
E
O
L
I
T
E
Z
E
O
L
I
T
E
VO
VO
VO
VO(acac)2
OH
N
HO
N
H2O, R.T., 24 h
Me
Me
Me
Me
O
O
ONa
ONa
O
O
O
O
O
Z
E
O
L
I
T
E
Z
E
O
L
I
T
E
OH
HO
OH
HO
O
O
V
O
N
O
N
O
V
ONa
ONa
O
N
O
VO
NaCl
N
Me
Me
Me
Me
ONa
ONa
O
O
Me
Me
Me
VO
Me
and a slight change in the intensity of the bands shows that
the zeolite crystallinity is retained on encapsulating com-
plex. This observation confirms that the framework of the
zeolite has not undergone any significant structural change
during encapsulation.
The catalytic oxidation of alkenes
The catalytic performance of VOL-Y in the oxidation of
cyclooctene, as a model substrate, and hydrogen peroxide
as the source of oxygen was investigated. A series of blank
experiments (Table 1) show that the presence of catalyst
and oxidant is essential for an effective catalytic reaction.
The results in Table 1 show that oxidation of cyclooc-
tene needed 1 mL of H2O2 for completion.
Fig. 2 XRD patterns of VO-Y (a) and VOL-Y (b)
To find the optimum reaction conditions, the effect of
various reaction parameters that may affect the conversion
and selectivity of the reaction was investigated. Solvent,
catalyst concentration, reaction time and amount of the
oxidant are the factors that have been evaluated.
The influence of the solvent nature in the catalytic
activity of the VOL-Y for epoxidation of cyclooctene has
been studied. Therefore, chloroform, acetonitrile, dichlo-
romethane, methanol and ethanol were used and the highest
conversion was obtained in acetonitrile (Table 2).
The higher conversions in acetonitrile (86 %) relative to
the others possibly may be due to the higher boiling point
of acetonitrile.
free complex [VOL]. On the other hand, neat complex
[VOL] exhibits a sharp band at 846 cm-1 due to the
m(V=O) stretch [26], while in zeolite encapsulated vana-
dium complex m(V=O) stretch appears at 846 cm-1. These
observations not only confirm the presence of [VOL] in the
zeolite, but also suggest that its structure is almost identical
to that of the free complex. The spectra of Na-Y and VO-Y
show a broad band in the range 3300–3700 cm-1 due to
surface hydroxylic groups and lattice vibrations at ca.
1,000 cm-1
.
Powder X-ray diffraction studies
The effect of reaction time in the catalytic epoxidation
of cyclooctene with H2O2 in the presence of VOL-Y is
shown in the Fig. 3 and the highest conversion (100 %)
was obtained after 5 h.
The powder X-ray diffraction patterns of VO-Y and VOL-
Y (Fig. 2) were recorded at 2h values between 5° and 90°.
The XRD patterns of VO-Y is similar to that of VOL-Y
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J IRAN CHEM SOC
Table 1 Catalytic oxidation of 0.2 mmol cyclooctene with H2O2 in
the presence of VOL-Y in acetonitrile under reflux condition
Table 3 The influence of catalyst VOL-Y concentration on the cat-
alytic epoxidation of cyclooctene with H2O2 under reflux condition
Catalyst H2O2
Conversion (%) Selectivity (%) (Epoxide)
VOL-
Y(mg)
Catalyst:substrate
(mol % cat. = 4.1)
Conversion
(%)
Selectivity (%)
(Epoxide)
VOL-Y
None
1 mg
–
0
0
–
0.8
1
1:300
1:250
1:200
46
86
26
100
100
100
0.6 mmol
0.6 mmol
1 mL
–
9
100
100
1.2
1 mg
86
Reaction conditions: (1 mL) CH3CN, (0.2 mmol) cyclooctene,
(1 mL) H2O2, 4 h (time)
Table 2 The catalytic epoxidation of cyclooctene in different
solvents
vanadium complex cannot be recovered even once.
Therefore, reusability of the heterogeneous catalyst VOL-
Y was studied and results showed that the VOL-Y can be
separated from reaction mixture, washed with solvent and
dried before being used. The catalyst was consecutively
reused six times without a detectable catalyst leaching or a
significant loss of its activity.
Solvent
Boiling
point(°C)
Dielectric
constants
Conversion
(%)
Dichloromethane
Chloroform
Methanol
40
61
64
78
81
8.9
4.9
0
0
32.7
26.6
37.5
20
29
86
Ethanol
Acetonitrile
In a proposed catalytic cycle, hydrogen peroxide will be
activated by coordination to the oxovanadium(IV) center
and then nucleophilic attack of olefin on the electrophilic
oxygen atom covalently bonded to the metal leading to the
formation of epoxide product (Scheme 2) [27–29].
Reaction conditions: (0.2 mmol) cyclooctene, (1 mL) H2O2, (4 h)
reaction time, (1 mg) VOL-Y
Different catalyst concentrations have been used in the
oxidation of cyclooctene (Table 3). It was observed that
oxidation of cyclooctene required 0.015 g of catalyst for
completion.
Catalytic oxidation of methyl phenyl sulfide
To establish the general applicability of the method,
under the optimized conditions, oxidation of various olefins
was subjected in the presence of the catalytic amount of
VOL-Y and the results are presented in Table 4.
Comparison of the catalytic behavior of the oxovana-
dium Schiff base complex encapsulated onto nanopores of
zeolite-Y and free VOL (Table 5) showed the higher cat-
alytic activity of the heterogeneous catalyst with respect to
the homogenous one.
Oxidation of methyl phenyl sulfide with hydrogen peroxide
catalyzed by VOL-Y gave methyl phenyl sulfone as the
major product. In a search for suitable reaction conditions
to achieve the maximum conversion, the effect of different
parameters including solvent, temperature, amount of oxi-
dant and the presence of acetic acid (HOAc) as oxidant
activator was studied.
The results for the oxidation of methyl phenyl sulfide
show that after 120 min (Table 6), only 25 % methyl phenyl
sulfide conversion was obtained. Therefore, hydrogen per-
oxide can be activated by addition of acetic acid (HOAc).
The ease of separation is one of the greatest advanta-
ges of heterogeneous catalyst while the homogeneous
Fig. 3 The effect of reaction
time in the oxidation of
cyclooctene with H2O2 in the
100
100
86
80
60
40
20
0
presence of VOL-Y. Reaction
conditions: (1 mL) CH3CN,
(0.2 mmol) cyclooctene, (1 mL)
H2O2, (1 mg) VOL-Y
43
20
12
0
0
1
2
3
4
5
Time (h)
123

J IRAN CHEM SOC
Conversion (%)a Product
20
Selectivity (%) TONe (total)
Table 4 Oxidation of olefins
using H2O2 catalyzed by VOL-
Y in CH3CN
Alkene
75b
50
CHO
O
26
100
64
Cl
Cl
O
5
100
90c
13
MeO
MeO
42
192
O
100
63
57d
100
250
157
(0.2 mmol) alkene, (1 mL)
H2O2, (1 mL) CH3CN, (4 h)
time
O
a
Conversions and selectivities
were determined by GC based
on the starting alkene
b, c, d
86
6
100
100
215
14
Epoxide was formed as
O
by-product
e
TON: (total turnover number)
the ratio of the number of moles
of product to the number of
moles of catalyst
Table 5 Comparison of homogeneous and heterogeneous systems
In the absence of catalyst, the oxidation of methyl
phenyl sulfide with hydrogen peroxide did not proceed
even after 45 min. Also, in the presence of acetic acid a
conversion of 14 % was obtained. These observations
suggest a catalytic cycle containing heterogeneous catalyst
and peracetic acid formed by the reaction of hydrogen
peroxide and HOAc [30] as oxidant. Increasing the rate of
the reaction by addition of HOAc suggests the formation of
a peracetic acid species that will be activated under surface
of the catalyst (Scheme 3).
Catalyst
Catalyst:alkene
(mol % cat. = 4.1)
Conversion
(%)
Selectivity
epoxide (%)
VOL-Y
VOL
1:250
1:250
86
61
100
100
Reaction conditions: (1 mL) solvent, (0.2 mmol) cyclooctene, (1 mL)
H2O2
The amount of HOAc has been optimized and results pre-
sented in Table 7 show that the 51 % methyl phenyl sulfide
conversion was obtained by addition of 80 lL HOAC.
To achieve the best reaction solvent, the reaction was
performed in methanol, ethanol and acetonitrile and the
highest conversion was achieved in ethanol as a more
convenient and environmentally friendly solvent (Table 8).
Different catalyst concentrations have been used in the
oxidation of methyl phenyl sulfide (Fig. 4) and the highest
conversion was achieved with 6 mg of heterogenous catalyst.
Conclusion
In conclusion we have reported the synthesis and charac-
terization of an oxovanadium(IV) Schiff base complex
encapsulated in the nanopores of zeolite-Y. Also, this
heterogeneous catalyst has been used for oxidation of dif-
ferent alkenes and methyl phenyl sulfide with hydrogen
123

J IRAN CHEM SOC
Scheme 2 Proposed catalytic
cycle for oxidation of alkenes
OH
V
O
V
H2O2
O
N
O
N
O
N
O
N
O
O
H
OH
O
N
O
N
V
O
O
O
+
H2O
H
Table 6 Catalytic oxidation of methyl phenyl sulfide in the presence
of 6 mg of VOL-Y in ethanol at room temperature
60
50
40
30
20
10
0
Conversion (%)
Selectivity (%) Sulfone
Time (min)
12
25
100
100
30
120
Table 7 The effect of acetic acid concentration on the oxidation of
methyl phenyl sulfide catalyzed by VOL-Y in ethanol at room
temperature
Acetic acid (lL)
Conversion (%)
Selectivity (%)
Sulfone
0
2
4
6
8
10
12
14
40
26
51
5
100
92
VOL-Y (mg)
80
100
100
Fig. 4 The effect of catalyst concentrations on the oxidation of
methyl phenyl sulfide
Reaction conditions: (1 mL) solvent, (0.2 mmol) cyclooctene, (1 mL)
H2O2
Table 8 The effect of solvent on the oxidation of methyl phenyl
sulfide
peroxide. Different reaction parameters were investigated
and optimized in the oxidation reaction. The results showed
that the vanadyl complex was active and highly selective in
the oxidation of olefins. In addition, catalytic oxidation of
methyl phenyl sulfide with activated hydrogen peroxide by
acetic acid gave methyl phenyl sulfone as the major
product.
Solvent
Conversion (%)
Selectivity (%)
(Sulfone)
Methanol
Acetonitrile
Ethanol
23
5
70
100
92
51
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J IRAN CHEM SOC
Scheme 3 Proposed catalytic
cycle for oxidation of methyl
phenyl sulfide
O
O
V
CH3COO2H
O
N
O
N
O
N
O
N
O
V
I
C
O
H3C
O
H
Sulfone
Sulfoxide
Sulfide
Sulfoxide
Acknowledgments This work has been supported by the Iran
National Science Foundation (INSF) (Grant No. 88001216).
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