A heterogenized vanadium oxo-aroylhydrazone catalyst for efficient and selective oxidation of hydrocarbons with hydrogen peroxide

Article abstract of DOI:10.1007/s11243-011-9561-4

A hydrazone Schiff base ligand derived from salicylaldehyde and benzhydrazide has been synthesized and reacted with vanadium(IV) leading to the corresponding vanadium(V) complex. The complex has been anchored on the surface of functionalized silica gel by N,O-coordination to the covalently Si-O bound modified salicylaldiminato ligand. The supported complex has been evaluated as a catalyst for hydrocarbon oxidation with hydrogen peroxide in acetonitrile. The heterogeneous system proved to be an efficient catalyst and was able to activate hydrogen peroxide toward the oxidation of alkenes, alkanes, benzene, and alkylaromatic compounds with more than 2,500 h^-1 activity. Springer Science+Business Media B.V. 2011.

Full text of DOI:10.1007/s11243-011-9561-4

Transition Met Chem (2012) 37:85–92
DOI 10.1007/s11243-011-9561-4
A heterogenized vanadium oxo-aroylhydrazone catalyst
for efficient and selective oxidation of hydrocarbons
with hydrogen peroxide
•
Hassan Hosseini Monfared Vahideh Abbasi
•
•
Adineh Rezaei Massomeh Ghorbanloo
•
Alireza Aghaei
Received: 9 October 2011 / Accepted: 31 October 2011 / Published online: 11 November 2011
Ó Springer Science+Business Media B.V. 2011
Abstract A hydrazone Schiff base ligand derived from
salicylaldehyde and benzhydrazide has been synthesized and
reacted with vanadium(IV) leading to the corresponding
vanadium(V) complex. The complex has been anchored on
the surface of functionalized silica gel by N,O-coordination
to the covalently Si–O bound modified salicylaldiminato
ligand. The supported complex has been evaluated as a
catalyst for hydrocarbon oxidation with hydrogen peroxide
in acetonitrile. The heterogeneous system proved to be an
efficient catalyst and was able to activate hydrogen peroxide
toward the oxidation of alkenes, alkanes, benzene, and
alkylaromatic compounds with more than 2,500 h-1
activity.
robust, efficient, and recyclable heterogeneous catalysts
have been developed [3, 4].
Many heterogeneous catalysts contain metals along
with chemically modified silica gels and active organic
components [5]. Modified silica exhibits some advantages
over modified resins such as high surface area, and high
thermal and chemical stability. Since the early 1990s and
the advent of mesoporous silicas, the development of new
solid catalysts and the heterogenization of homogeneous
systems for oxidation processes have become very attrac-
tive topics. A major drawback of transition metal catalysts
coordinated to monodentate ligands is that they usually
undergo leaching of the active species into solution,
especially in the presence of protic agents such as alcohols
or organic peroxides [6]. A simple way to minimize the
leaching of the active ingredient is to anchor polydentate
ligands onto the support surface, as this type of chelating
system offers high coordinative stability for the catalytic
metal center.
Introduction
Catalytic oxidation of hydrocarbons is of great interest in
chemical industry for the conversion petroleum-based
feedstocks to useful chemicals such as diols, epoxides,
alcohols, and carbonyl compounds [1, 2]. Economically
and environmentally, H2O2 and O2 (or air) are the most
attractive oxidants because of their high contents of active
oxygen species and coproduction of only water. Therefore,
new oxidative processes based on the activation of H2O2 by
Vanadium compounds have attracted much attention
because they have been widely used for catalytic oxidation
of hydrocarbons [7, 8], oxidative dehydrogenation [9],
oxidative coupling [10], and asymmetric oxidations [11]
with alkyl hydroperoxides, H2O2, and O2. It has been
demonstrated that vanadium(V) oxo complexes of hydra-
zone Schiff base ligands catalyze the oxidation of alkenes
by hydrogen peroxide, typically with maximum reactivity
of 390 h-1 [12]. Here, we report on a silica-supported
vanadium(V)oxo-aroylhydrazone complex that is an effi-
cient oxidation catalyst under mild conditions (80 °C),
converting alkenes and alkanes with hydrogen peroxide up
to more than 2,500 h-1 activity. Replacement of a homo-
geneous catalyst with a heterogeneous analogue provides
advantages such as easy handling and product separation,
catalyst recovery, and less waste [13].
H. Hosseini Monfared (&) ꢀ V. Abbasi ꢀ A. Rezaei ꢀ
M. Ghorbanloo
Department of Chemistry, University of Zanjan,
45195-313 Zanjan, Islamic Republic of Iran
e-mail: monfared@znu.ac.ir; monfared_2@yahoo.com
URL: http://www.znu.ac.ir/members/hosseini_hassan
A. Aghaei
Materials and Energy Research Center, 14155-4777 Tehran,
Islamic Republic of Iran
123

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Transition Met Chem (2012) 37:85–92
Results and discussion
a
Synthesis and characterization of the catalyst
b
Salicylaldiminepropyltriethoxysilane HL1 was synthesized
by the condensation of aminopropyltriethoxysilane and
salicylaldehyde in methanol. This was grafted onto silica
gel by reacting the two together in refluxing toluene. This
involves the condensation of silanol and ethoxy groups
(Scheme 1a). The IR spectrum of the functionalized silica
is shown in Fig. 1b. The organosilane and imine group
characteristic peaks (2,938 and 1,658 cm-1, respectively)
in the FTIR spectrum of the grafted material confirmed the
presence of the salicylaldimine group on the surface of the
silica. The results of carbon and nitrogen analyses of the
functionalized silica are compiled in Table 1. The observed
C to N ratio, 9.1, is close to the theoretical value of 10.3.
Furthermore, the loading of the salicylaldimine function on
the silica gel can be calculated from quantitative elemental
analysis, which confirmed a loading of approximately
0.59 mmol imine/g of silica gel (Table 1).
c
1100
3400
2400
1400
400
Wavenumber / cm-1
Fig. 1 FTIR spectra of a dried silica gel, b (silica gel–O)2(EtO)Si–
L1H, and c (silica gel–O)2(EtO)Si–L1–VO(L2)
H2L2, and (silica gel–O)2(EtO)Si–L1H in dichloromethane
(Scheme 1b). During the reaction, V(IV) was oxidized to
V(V) by air. H2L2 acts as a dianionic tridentate ligand and
coordinates through the deprotonated hydroxyl group,
azomethine nitrogen, and amide oxygen groups to VVO as
The hydrazide Schiff base complex of V(V) was
immobilized on silica by refluxing together VO(acac)2,
Scheme 1 a Modification of
the silica gel surface by
organosilane to (silica
gel–O2)(EtO)Si–L1H and
b immobilization of vanadium
oxide on the surface of the
functionalized silica gel to give
the catalyst
(a)
MeOH
O
N
Si(OEt)3
H2N
Si(OEt)3
+
Reflux
OH
OH
HL1
O
O
Toluene
Reflux, 24 h
HL1 + silica gel
silica
gel
Si
N
O
HO
(silica gel-O2)(EtO)Si-L1H
(b)
N
N
O–
(L2)2–
(silica gel-O2)(EtO)Si-L1H + [VO(acac)2] +
O–
CH2Cl2
Reflux 19 h
O
O
silica
gel
N
Si
O
O
O
V
O
O
N
N
(silica gel-O2)(EtO)Si-L1-VO(L2)
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Transition Met Chem (2012) 37:85–92
87
Table 1 Elemental analyses of functionalized silica gel (fsg) before and after vanadium loading
Material
C (%)
H (%)
N (%)
V (%)
mmol imine/g fsg
mmol V/g fsg
mol V/mol imine
fsg
1
7.53
6.15
0.92
0.99
0.83
1.29
–
0.59
0.59
–
–
0.04
0.0079
0.013
30
25
20
15
10
5
101
99
97
95
93
91
89
87
0.90
0.60
0.30
0.00
0
-5
50
250
450
650
850
-10
Wavelength / nm
39
139
239
339
439
539
639
Fig. 2 DR-UV/Vis spectrum of (silica gel–O)2(EtO)Si–L1–VO(L2)
Temperature / °C
Fig. 3 TGA and DTA spectra of (silica gel–O)2(EtO)Si–L1–VO(L2)
VO(L2) [14]. The immobilization of VO(L2) on the surface
of the (silica gel)–O2(EtO)Si–L1H occurs by means of
chemical bonding with the imine nitrogen and deproto-
nated oxygen of the phenol groups of (silica gel–
O)2(EtO)Si–L1H as shown in Scheme 1b. The prepared
heterogeneous (silica gel–O)2(EtO)Si–L1–VO(L2) catalyst
was thoroughly washed with methanol in order to eliminate
VO(L2) species adsorbed on the support. The IR spectrum
of the heterogeneous catalyst (Fig. 1c) was the same as that
of (silica gel–O)2(EtO)Si–L1H (Fig. 1b). The microanaly-
ses confirmed that the anchored VO(L2) was too little to be
seen by FTIR. In addition to the characteristic bands of
silica gel at 3,454 (Si–OH), 1,080 (antisymmetric vibration
of Si–O–Si), and 462 (the bending vibration of O–Si–O
bond [15]), both (silica gel)–O2(EtO)Si–L1H and the cat-
alyst show the stretching vibrations of the CH2 groups
of HL1 at 2,938 and 2,867 cm-1 and imine moieties of
L1 and L2 at 1,658 cm-1 (Fig. 1). The diffuse reflectance
UV–Vis spectrum of the catalyst shows charge transfer
bands around 280 and 400 nm (Fig. 2). Microanalyses of
the catalyst (Table 1) confirm the presence of the sup-
porting of VO(L2) group on the functionalized silica gel
and also show that only 1.3% of the imine on the func-
tionalized silica gel is complexed with VO(L2).
Catalytic activity studies
The activity of the catalyst was first tested in the oxidation
of cis-cyclooctene. In this regard, we were very interested
to combine the advantages of hydrogen peroxide as a ter-
minal oxidant with the possibility to recover and recycle
the vanadium oxo catalyst. The catalyzed oxidation of cis-
cyclooctene was carried out with hydrogen peroxide to
give cis-cyclooctene oxide as the sole product. Control
experiments revealed that the presence of oxidant, sodium
bicarbonate, and catalyst were all essential for the oxida-
tion (Table 2, entries 1–3). While the individual reaction
components (NaHCO3, silica gel, functionalized silica gel)
did not catalyze cis-cyclooctene oxidation at room tem-
perature even in the presence of excess H2O2 (10 equiva-
lent), they catalyzed the oxidation in the presence of
NaHCO3 at 80 °C (Table 2, entries 4–6). The immobilized
vanadium oxo complex catalyzed the conversion of cis-
cyclooctene to the epoxide with NaHCO3/H2O2 in 95%
yield (Table 2, entry 7). The addition of sodium bicar-
bonate is absolutely required in order to produce an effi-
cient catalytic system. Hydrogen peroxide is a widely used
oxidant with high active oxygen content [16], but it is
rather slow in the absence of activators. Alkaline NaHCO3
improves the oxidation rate [17, 18].The reaction showed
maximum conversion within 5 h.
Further investigations using TGA (Fig. 3) showed that
the pre-catalyst (silica gel–O)2(EtO)Si–L1–VO(L2) lost
10.5% of its weight up to 700 °C, where the organic sali-
cylaldimine and hydrazide Schiff base were removed,
leaving SiO2. The DTA peaks are exothermic and can be
attributed to the decomposition of salicylaldimine and the
Schiff base ligands.
To determine the optimal experimental conditions, the
effects of reaction temperature, NaHCO3 concentration,
and H2O2/cis-cyclooctene molar ratio were studied.
Increasing the reaction temperature resulted in significantly
higher yields of cis-cyclooctene oxide. A yield of 90% was
123

88
Transition Met Chem (2012) 37:85–92
Table 2 Control experiments for catalytic oxidation of cis-cyclooctene
No
Catalyst
Cocatalyst
Oxidant
Conversion (%)
1
2
3
4
5
6
7
(silica gel–O)2(EtO)Si–L1–VO(L2)
NaHCO3
None
None
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
0
0
SiO2
(silica gel–O)2(EtO)Si–L1H
None
0
None
NaHCO3
NaHCO3
NaHCO3
NaHCO3
24
10
40
95
SiO2
(silica gel–O)2(EtO)Si–L1H
(silica gel–O)2(EtO)Si–L1–VO(L2)
Reaction conditions: catalyst 10 mg, cis-cyclooctene 1.0 mmol, H2O2 3 mmol, acetonitrile 3.0 mL, NaHCO3 1.0 mmol, time 5 h, and tem-
perature 80 °C
100
90
80
70
60
50
40
30
20
10
0
100
80
60
40
20
0
1.5
0.5
1.0
NaHCO3 / mmol
80 °C
60 °C
0.25
40 °C
25 °C
0
1
2
3
4
5
6
0
1
2
3
4
5
6
Time / h
Time / h
Fig. 5 Effect of the amount of NaHCO3 on the catalyzed oxidation of
cis-cyclooctene. Reaction conditions: catalyst 10 mg (0.079 lmol
vanadium), cis-cyclooctene 1.0 mmol, MeCN 3.0 mL, and H2O2
3.0 mmol at 80 °C
Fig. 4 Temperature effect on the oxidation of cis-cyclooctene
with H2O2. Conditions: catalyst 10 mg (0.079 lmol vanadium), cis-
cyclooctene 1.0 mmol, NaHCO3 1.0 mmol, H2O2 3.0 mmol, and
MeCN 3.0 mL. Conversion was based on the starting alkene
achieved at 90 °C after 3 h. Furthermore, an induction
period that was required for the reactions at 25, 40, and
60 °C was not observed at 80 °C (Fig. 4). This induction
period may indicate catalyst modification to an active form
within the first few hours.
Increasing the H2O2/cis-cyclooctene ratio to 3:1 increased
the conversion from 0 to 49% after 1 h. Further increase in
the ratio gave only a small additional increase in the con-
version. Hence, a H2O2/cis-cyclooctene ratio of 3:1 was
selected for further studies. Oxidant requirements of the
order of 10:1 epoxidation reactions have been documented
in the literature [19, 20]. Notably, no induction period was
observed in the reactions with H2O2/cis-cyclooctene at
ratios of 3:1 or 5:1.
Epoxidation of cis-cyclooctene at 80 °C by the catalyst
with H2O2 in acetonitrile with different concentrations of
NaHCO3 (0.25, 0.50, 1.0, and 1.5 mmol) confirmed that
1.0 mmol of sodium bicarbonate is suitable as H2O2 pro-
moter (Fig. 5). A conversion of 96% was achieved with
1.0 mmol NaHCO3 after 5 h.
The catalytic efficiency of the catalyst was studied in a
variety of solvents. No catalytic activity was observed in n-
hexane, chloroform, dichloromethane, methanol, or ethanol
at a temperature of 60 °C. The best performance of the
catalyst was observed in acetonitrile, possibly due to its
dielectric constant (e/e0 = 37.5) that is the highest of all
the solvents used. The optimum polarity of acetonitrile
that dissolves both alkene and H2O2 might be facilitating
the epoxidation reaction in this case. Furthermore, the
large improvement in the cis-cyclooctene conversion
with changing the solvent from methanol (e/e0 = 32.7) to
acetonitrile suggests the possible involvement of
The catalyst decomposes hydrogen peroxide to some
extent, but the decomposition rate is much smaller than
H2O2 activation for the oxidation of hydrocarbons. We
have monitored the progress of cis-cyclooctene epoxida-
tion for different molar ratios of oxidant and substrate
(Fig. 6). Molar ratios of H2O2/cis-cyclooctene of 2, 3, and
5 were considered while keeping a fixed amount of cis-
cyclooctene (1.0 mmol) and catalyst (10 mg) in 3 mL of
acetonitrile at 80 °C. At a ratio of oxidant/alkene of 2:1, no
conversion occurred in the first hour of the reaction.
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Transition Met Chem (2012) 37:85–92
89
100
system probably proceeds through a radical intermediate.
Oxidation of 1,3-cyclooctadiene resulted in the monoepox-
ide product (Z)-9-oxabicyclo[6.1.0]non-2-ene. Apparently,
the presence of an electron-withdrawing epoxide group does
not allow the oxidation to proceed further. The system was
able to oxidize linear 1-octene to the corresponding epoxide
with good yield (74%) and excellent selectivity. Remark-
ably, a-methyl styrene was oxidized to give a-methyl styrene
oxide in 45% yield, without the formation of benzaldehyde.
Norbornene gave exo- and endo-norbornene oxide with
61 and 39% selectivity, respectively. The low yields
of 1-decene, norbornene, and allylbenzene were probably
due to their poor solubility acetonitrile.
5
3
80
60
40
2
[H2O2]/[Cyclooctene] ratio
20
0
0
1
2
3
4
5
6
Time / h
The activity of the catalyst was also tested for the oxi-
dation of benzene, alkylaromatics, cyclooctane, and a lin-
ear alkane (Table 4). The reaction was carried at 80 °C in
acetonitrile as solvent. The catalyst oxidized benzene to
phenol using hydrogen peroxide in the absence of any acid
with 100% selectivity and 82% conversion. In this respect,
the activity and/or selectivity of this catalyst is much better
than the pyridine-modified molybdovanadophosphates in
acetic acid–acetonitrile mixed solvent, which require an
acid for the oxidation of benzene with H2O2 [23]. Toluene
and ethylbenzene were also oxidized with 100% selectivity
to benzyl alcohol and acetophenone, respectively. How-
ever, the main product of the oxidation of mesitylene was
3,5-dimethylbenzaldehyde with 47% selectivity. The cat-
alytic reactions involve only selective oxidation of the side
chain. In the oxidation of cyclooctane, an overall conver-
sion of 78% was observed, with 67% selectivity to cyclo-
octanol and 33% to cyclohexanone. This catalyst can be
considered very efficient compared to the previously
reported oxovanadium(V)-salicyl-hydroximate/H2O2 [24],
zeolite-encapsulated vanadium picolinate peroxo com-
plexes of V(IV)/H2O2 [25], or oxovanadium(V)-amine-
bis(phenolate) complexes/H2O2 [26]. Interestingly, the
present catalyst efficiently and almost quantitatively oxi-
dized the very stable linear alkanes n-octane and n-heptane
to the corresponding alcohol isomers (Table 4).
Fig. 6 Effect of H2O2 concentration on the oxidation of cis-
cyclooctene by the catalyst. Reaction conditions: catalyst 10 mg
(0.079 lmol V), cis-cyclooctene 1.0 mmol, NaHCO3 1.0 mmol, and
MeCN 3.0 mL at 80 °C
acetonitrile in the oxidation. This idea was supported by
the detection of acetamide in the GC analyses and 1H NMR
spectra of all the alkene oxidation products. Hence, per-
oxycarboximidic acid is probably formed in situ by the
reaction of hydrogen peroxide and acetonitrile in the
presence of NaHCO3. Bases such as NaOH and KOH are
known to catalyze the epoxidation of alkenes using
hydrogen peroxide in the presence of nitriles [21].
To suggest a detailed mechanism for the catalytic oxi-
dation of hydrocarbons by the present system will need
further studies. However, it seems that peroxycarboximidic
acid interaction with the catalyst makes the active species
for hydrocarbon oxidation, and the presence of NaHCO3 is
essential for peroxycarboximidic acid formation from
MeCN and H2O2.
Oxidation of various hydrocarbons
With the optimal reaction conditions for this catalyst to hand,
we proceeded to investigate the scope for other hydrocar-
bons. The molar ratio of catalyst/H2O2/NaHCO3/hydrocar-
bon for these experiments was 1:38,000:13,000:13,000. The
results for the catalytic epoxidation of different alkenes are
summarized in Table 3. Various alkenes including cyclic
(cis-cyclooctene, 1,3-cyclooctadiene, indene, cyclohexene,
norbornene), acyclic (allylbenzene, a-methylstyrene), and
linear terminal alkenes (1-octene, 1-decene) were oxidized
to the corresponding epoxides with 100% selectivity, except
for cyclohexene. Cyclohexene gave the corresponding
epoxide as the minor product with 26% selectivity, while the
main product was 2-cyclohex-1-one. Cyclohexene is more
prone to both epoxidation and allylic oxidation [22]. To
provide evidence for or against a non-radical mechanism and
to evaluate the catalyst selectivity, oxygenation of cyclo-
hexene is a good probe; thus, oxygenation by the present
Catalyst recycling
We checked the recyclability of the present catalyst for the
oxidation of cis-cyclooctene with hydrogen peroxide in a
molar ratio 1:3 under similar reaction conditions. For this
study, the solid catalyst was recovered from the reaction
mixture by centrifugation after 5 h. The supernatant was
decanted, and then the solid catalyst was washed twice
with acetonitrile and centrifuged. The recovered catalyst
was used again in a fresh reaction, being recycled seven
times for cis-cyclooctene epoxidation. In general, a slight
loss in the activity of the catalyst was observed (conversion
85% after seven recycles) compared with that of a fresh
123

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Transition Met Chem (2012) 37:85–92
Table 3 Catalytic epoxidation of alkenes by (silica gel–O)2(EtO)Si–L1–VO(L2)/NaHCO3/H2O2/MeCN
Alkene
Conv. (%)a
Epoxide yield (%)a
Epoxide selectivity (%)b
TOF (h-1 c
)
cis-Cyclooctene
1,3-Cyclooctadiene
1-Octene
96
75
96
75
74
58
12
45
33
28
100
100
100
100
26
2,430
1,899
1,874
1,469
1,190
1,139
836
74
Indene
58
47d
Cyclohexene
a-Methylstyrene
1-Decene
45
100
100
100e
33
28e
Norbornene
709
Reaction conditions: catalyst 10 mg (0.079 lmol), substrate 1.0 mmol, chlorobenzene (internal standard) 0.10 g, MeCN 3.0 mL, H2O2
3.0 mmol, NaHCO3 1.0 mmol, time 5 h at 80 °C
a
Conversions and yields are based on the starting alkene and determined by GC analysis
b
Epoxide selectivity = ([epoxide]/[conversion]) 9 100
c
TOF: turnover frequency = number of moles of product formed per mole of V in the catalyst per unit time = (Conv. 9 1.0 mmol)/
(0.000079 mmol 9 5 h)
d
Other product yields: 2-cyclohexen-1-ol 14% and 2-cyclohex-1-one 21%
e
Mixture of exo- (61%) and endo-norbornene oxide (39%). Products determined and quantified by 1H NMR (d = 2.84 ppm for exo- and
3.40 ppm for endo-epoxide)
Table 4 Catalytic C–H oxidation by (silica gel–O)2(EtO)Si–L1–VO(L2)/NaHCO3/H2O2/MeCN
Hydrocarbon
Product(s)
Conv. (%)
Yield (%)
Selectivity (%)
TOF (h-1
)
Benzene
Phenol
82
76
68
45
78
99
99
82
36
68
45
52
71
55
100
47
2,076
1,924
1,722
1,139
1,975
2,507
2,507
Mesitylene
Ethylbenzene
Toluene
3,5-Dimethylbenzaldehydea
Acetophenone
Benzyl alcohol
Cyclooctanolb
2-Octanolc
100
100
67
Cyclooctane
n-Octane
72
n-Heptane
1-Heptanold
56
H2O2 5 mmol, other conditions like Table 3
a
Other products (yield): 3,5-dimethylbenzyl alcohol (7%), 3,5-dimethylbenzoic acid (8%), and 2,4,6-trimethylphenol (25%)
Other products (yield): cyclooctanone (26%)
b
c
d
Other products (yield): 3-octanol (17%) and 4-octanol (11%)
Other products (yield): 2-heptanol (10%), 3-heptanol (7%), and 4-heptanol (27%)
sample (conversion 95%), as shown in Table 5. Therefore,
This system could be useful for a variety of base-catalyzed
selective oxidations. Our experimental data revealed that
NaHCO3 can work very efficiently as a cocatalyst in the
presence of the immobilized heterogeneous catalyst.
recycling of this catalyst is possible, with only slight cat-
alyst leaching to the solution.
Conclusion
Experimental
We have developed an efficient heterogeneous catalyst
system utilizing (silica gel–O)2(EtO)Si–L1–VO(L2) for
hydrocarbon oxidation with a combined oxidant of aqueous
hydrogen peroxide and acetonitrile. Good to excellent
conversions with excellent selectivities were achieved for
both alkenes and alkanes. The solid catalyst was easily
separated from the reaction mixture via filtration, which
made the work-up procedure simple. The catalyst was
reused for several catalytic runs with slight loss of activity.
Silica gel (0.063–0.200 mm) and all other reagents were
obtained from commercial sources (Merck or Fluka) and
were used as received without further purification. Aque-
ous 30% hydrogen peroxide (8.12 mol/L) was used, and its
exact concentration was determined before use by titration
with standard KMnO4. The linker N-(triethoxysilylpro-
pyl)salicylaldimine [(EtO)3Si–L1H] [27] and (E)-N0-(2-
hydroxybenzylidene)benzohydrazide (H2L2) [28] were
synthesized according to reported procedures. 1H NMR
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Transition Met Chem (2012) 37:85–92
91
solution and the mixture stirred under reflux for 19 h fol-
lowed by Soxhlet extraction with MeOH for 24 h. The
green material was dried under vacuum overnight, yielding
740 mg. CHN analyses showed the presence of both L1
and L2.
Table 5 Recycling studies of the catalyst in the oxidation of cis-
cyclooctene by hydrogen peroxide after 5 h of reaction time
Entry
No. of recycle
Conversion (%)b
Selectivity (%)
1
2
3
4
5
6
7
8
0
1
2
3
4
5
6
7
95
90
89
88
88
87
86
85
[99
[99
[99
[99
[99
[99
[99
[99
General procedure for the oxidation of hydrocarbons
The hydrocarbon oxidations with hydrogen peroxide were
performed in a 25-mL round-bottom flask equipped with a
condenser. In a typical experiment, the flask was charged
with 3.0 mL of MeCN, 1.0 mmol hydrocarbon, 1.0 mmol
NaHCO3, 0.10 g chlorobenzene as internal standard and
10 mg (silica gel–O)2(EtO)Si–L1–VO(L2) catalyst. To this
mixture, 2–5 mmol of an 8.12-mol/L aqueous solution of
hydrogen peroxide was added. The reactant mixture was
stirred vigorously for 5 h at the desired temperature. At
appropriate intervals, aliquots were removed and analyzed
immediately by GC. The activities of bare silica gel and
functionalized silica gel were also measured in the absence
of V(V). Conversions with (silica gel–O)2(EtO)Si–L1H
were much lower than the conversion in the presence of the
V-containing catalyst. The reaction products were quanti-
fied by GC and identified by comparison with the retention
time and spectroscopic data with those of an authentic
sample. Hydrocarbon conversion was calculated from the
obtained hydrocarbon to internal standard peak area ratio at
the beginning of the experiment and at the desired reaction
times. In some instances, reactions were carried out on a
larger scale, and the products were isolated for further
confirmation of identity and yield by NMR.
Reaction conditions: catalyst 10 mg (0.079 lmol), substrate
1.0 mmol, chlorobenzene (internal standard) 0.10 g, MeCN 3.0 mL,
H2O2 3.0 mmol, NaHCO3 1.0 mmol, time 5 h at 80 °C
spectra were recorded by the use of a Bruker 250 MHz
spectrometer. Elemental analyses were determined on a
CHN Perkin-Elmer 2400 analyzer. IR spectra were recor-
ded on a Perkin-Elmer 597 spectrometer. The vanadium
content of the final materials was determined using an ICP
model ARL 3410. The reaction products of the oxidation
were analyzed using an HP Agilent 6890 gas chromato-
graph equipped with a HP-5 capillary column (phenyl
methyl siloxane 30 m 9 320 lm 9 0.25 lm) with flame-
ionization detector and gas chromatograph–mass spec-
trometry (Hewlett-Packard 5973 Series MS-HP gas chro-
matograph with a mass-selective detector).
Preparation of (silica gel–O)2(EtO)Si–L1H
The N-(triethoxysilylpropyl)salicylaldimine linker was
anchored onto the silica surface via a grafting method. The
synthesis was similar to the reported procedure with some
modifications [29]. Amorphous silica gel was first heated at
500 °C for 24 h to remove water. A 5.0-g sample of the
pretreated silica gel powder was suspended in 50 mL of
dry toluene under N2. (EtO)3Si–L1H (1.2 g, 3.7 mmol) was
then added, and the suspension was stirred for 24 h under
reflux under N2. The resulting suspension was filtered off
and washed with Et2O (3 9 15 mL). The recovered pow-
der was extracted in a Soxhlet using refluxing MeOH
(750 mL) for 24 h. The functionalized silica gel, desig-
nated as (silica gel)-O2(EtO)Si–L1H, was dried under
vacuum at room temperature for 18 h. A yellow powder
was recovered. IR (KBr, cm-1): 3,444 (m, br), 2,948 (w),
2,864 (w), 1,647 (m), 1,101 (vs, br), 813 (m), 473 (s).
For vanadium leaching tests in the oxidation of cis-
cyclooctene, after 4 h, the solid catalyst was removed from
the reaction mixture by centrifugation. The supernatant was
decanted, 2 mL concentrated H2SO4 was added, and the
mixture was refluxed for 2 h. The resulting solution was
analyzed by atomic absorption spectrometry for traces of
vanadium.
Acknowledgments Authors are thankful to the University of Zanjan
for financial support of this study.
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