Highly efficient and green oxidation of alkanes and alkylaromatics with hydrogen peroxide catalysed by silver and vanadyl on mesoporous silica-coated magnetite

Article abstract of DOI:10.1002/aoc.3649

A heterogeneous catalyst (FeSi/Ag/VO) based on silver and vanadyl as active sites and mesoporous silica-coated nanospheres of magnetite (Fe₃O₄@m-SiO₂) as support was successfully prepared by deposition of Ag nanoparticles and the covalent grafting of vanadyl(IV) acetylacetonate on Fe₃O₄@m-SiO₂. The catalyst exhibited excellent activity for the oxidation of alkanes, benzene and alkylaromatics using green oxidant H₂O₂ and oxalic acid in acetonitrile at 60?°C.

Full text of DOI:10.1002/aoc.3649

Received: 25 August 2016
DOI 10.1002/aoc.3649
Accepted: 5 September 2016
F U L L PA P E R
Highly efficient and green oxidation of alkanes and alkylaromatics
with hydrogen peroxide catalysed by silver and vanadyl on
mesoporous silica‐coated magnetite
Seyed Hadi Nouri | Hassan Hosseini‐Monfared
Department of Chemistry, University of Zanjan,
A heterogeneous catalyst (FeSi/Ag/VO) based on silver and vanadyl as active sites
Zanjan, Iran
and mesoporous silica‐coated nanospheres of magnetite (Fe O @m‐SiO ) as
support was successfully prepared by deposition of Ag nanoparticles and the cova-
Correspondence
Hassan Hosseini‐Monfared, Department of
Chemistry, University of Zanjan, Zanjan 45195‐
3
4
2
lent grafting of vanadyl(IV) acetylacetonate on Fe O @m‐SiO . The catalyst
3
4
2
3
13, Iran.
exhibited excellent activity for the oxidation of alkanes, benzene and alkylaromatics
Email: monfared@znu.ac.ir
using green oxidant H O and oxalic acid in acetonitrile at 60 °C.
2
2
KEYWORDS
heterogeneous catalyst, hydrogen peroxide, hydroxylation, magnetic nanocomposite,
mesoporous silica, phenols, silver, vanadium oxide
1
|
INTRODUCTION
tremendous scientific and technological interest because
magnetic characteristics can be incorporated into mesoporous
structures with large pore volume and high surface area.
[
6]
Oxidation reactions remain challenging in the practical prep-
aration of both intermediates and fine chemicals. Effective
molecular catalysts are available for the major reaction types
such as epoxidation, alcohol oxidation or oxidation of
heteroatoms, but the major drawbacks of homogeneous cata-
lysts is the difficulty in separating the relatively expensive
catalysts from the reaction mixture at the end of the pro-
cess. The possible contamination of catalysts in products
also restricts their use in industry. Efficient anchoring of
these catalysts on supports may overcome these draw-
backs. Immobilized molecular catalysts may also allow
better control of the active site architecture and accessibility.
The active sites seem most strictly defined in covalently
anchored catalysts, and porous supports allow maximization
of the efficiency of the catalysts by ensuring optimal
Meanwhile, the silica shell can prevent the aggregation of
Fe O particles and provide numerous surface Si─OH groups
3
4
[
7]
for further modification.
As relatively inexpensive noble metal, silver
a
nanoparticles (Ag NPs) have been applied in a variety of
catalytic reactions, such as selective butadiene epoxidation,
[
1]
[8]
ethanol oxidation and selective NO reduction. Hu et al.
x
have developed a simple solution‐phase method to synthe-
size Ag‐coated Fe O @SiO composite microspheres
3
4
2
[
2]
[9]
through the Ag mirror reaction. Li and co‐workers have
reported a facile method to generate core–shell structured
Fe O @SiO ‐Ag magnetic nanocomposite with the aid of
3
4
2
[
10]
polyvinylpyrrolidone as both reductant and stabilizer.
It
has been found that Fe O @SiO ‐Ag nanospheres exhibit
3
4
2
[
1]
accessibility.
enhanced catalytic reduction efficiency for rhodamine B
[
11]
Recently, magnetic particles have been extensively
employed as alternative catalyst supports, in view of their
convenient catalyst recycling, high surface area resulting in
high catalyst loading capacity, high dispersion and outstand-
and eosin Y compared with pure Ag or Fe O NPs.
It
3
4
has been proved that supported silver catalysts possess a
[
12]
stable activity for CO oxidation at low temperatures.
Our previous studies also showed that Ag NP‐decorated
[
3,4]
ing stability.
It has been reported that silica as a protecting
graphene oxide catalyses aerobic oxidation of benzyl
[
13]
shell can be utilized to coat Fe O particles to form a core–
shell (Fe O @SiO ) structure.
silica (Fe O @m‐SiO ) nanocomposites have attracted
alcohol.
However, silver has rarely been considered as a
3
4
5]
[
Magnetite–mesoporous
catalyst for the selective oxidation of saturated
hydrocarbons.
3
4
2
3
4
2
Appl Organometal Chem 2016; 1–9
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
NOURI AND HOSSEINI‐MONFARED
Over the past decade, metal acetylacetonate complexes
have emerged as versatile catalysts for a broad range of indus-
(phenylmethylsiloxane, 30 m × 320 μm × 0.25 μm) with a
flame‐ionization detector. GC–MS analysis of the oxidation
products was conducted with a PerkinElmer Clarus SQ8S
apparatus.
[14]
trially and academically interesting reactions.
Vanadyl
(
IV) acetylacetonate ([VO(acac) ]) is a typical efficient cata-
2
1
lyst extensively employed for various oxidative transforma-
H NMR spectra of reaction mixtures (without purifica-
[
15]
tions.
Immobilization of [VO(acac) ] on mesoporous
tion) were recorded with a Bruker 250 MHz spectrometer.
Fourier transform infrared (FT‐IR) spectra were obtained
using a PerkinElmer 597 spectrophotometer after making
pellets with KBr powder. Powder X‐ray diffraction (XRD)
patterns were collected using a D8ADVANCE instrument
(Bruker, Germany), with a wavelength of 1.5406 Å (Cu
Kα), voltage of 40 kV and current of 40 mA. The size and
morphology of solid compounds were recorded using a
Hitachi F4160 scanning electron microscopy (SEM)
instrument operated at an accelerating voltage of 10 kV.
Magnetization measurements were performed at room
temperature using a vibrating sample magnetometer, in the
Development Center of the University of Kashan (Iran).
2
[
15,16]
[17,18]
silicas,
amine‐functionalized clay,
[20]
carbon mate-
via reactions
[
19]
rials
or mesoporous carbon nitride,
between the carbonyl group of the acetylacetonate ligand
and the amino groups of the solid surfaces, provides an
effective strategy to utilize [VO(acac) ]. Supported metal
2
acetylacetonate complexes have been found to be more effec-
[
21]
tive and selective than their homogeneous counterparts.
Co‐catalysts or additives which are in reality the ligands
for catalysts can improve the catalyst impact. It has been
reported that oxidation of cyclohexane with hydrogen perox-
ide and [VO(acac) ] is achieved efficiently with oxalic acid
2
[22]
and glyoxal added as additives.
Carboxylic acid takes an
[23]
active role in the oxidation of polypropylene.
co‐workers
White and
demonstrated that by acting as ligands for the
metal, carboxylic acids overcome a range of substrate biases
electronic, steric and stereoelectronic) in C─H hydroxyl-
[
24]
2
.2 | Preparation of hydrophobic magnetite (Fe O )
3 4
NPs
(
ation reactions which are catalysed by a non‐heme iron
complex. The partial oxidation of toluene by hydrogen perox-
Hydrophobic magnetite NPs were synthesized based on a
modification of a published one‐pot chemical co‐precipita-
[
28]
ide was also achieved with catalytic amounts of [VO(acac) ]
tion method.
First, deionized water was purged with
2
[
25]
in acetic acid.
nitrogen gas for 10 min. Then, 4.80 g of FeCl ⋅6H O,
3
2
Although there have been substantial advances in the
oxidation of hydrocarbons, the development of effective and
selective methods for the catalytic functionalization of
inactive carbon–hydrogen bonds in saturated hydrocarbons
2.00 g of FeCl ⋅4H O and 0.85 ml of oleic acid were added
2
2
to 30 ml of deionized water under nitrogen atmosphere with
vigorous stirring. The mixture solution was heated to 90 °C.
Then, 20 ml of ammonium hydroxide (14 wt%) was added
rapidly to the solution, and it immediately turned black. The
reaction was kept at 90 °C for 2.5 h and then allowed to cool
to room temperature. The black precipitate was collected
using an external magnet.
[
26]
still remains a major challenge in oxidation chemistry.
In
the study reported here, we used the synergic effect of Ag
NPs and [VO(acac) ] supported on mesoporous silica‐coated
2
Fe O NPs for the oxidation of various hydrocarbons includ-
3
4
ing alkanes, benzene and alkylaromatics with hydrogen
peroxide. Hydrogen peroxide is probably the best terminal
oxidant after dioxygen with respect to environmental and
2
.3 | Mesoporous silica‐coated magnetite NPs
(Fe @m‐SiO
As‐synthesized Fe O NPs were coated with mesoporous
[27]
3
O
4
)
2
economic considerations.
3
4
silica by employing the modified Stöber method and using
[
29]
CTAB as a template.
CTAB also serves as a stabilizing
2
2
|
EXPERIMENTAL
surfactant for the transfer of hydrophobic Fe O nanocrystals
3
4
to the aqueous phase. In a typical procedure, 0.350 g of oleic
acid‐stabilized magnetite NPs dispersed in 30 ml of chloro-
form was added to 100 ml of an aqueous CTAB (2.0 g)
solution and the resulting solution stirred vigorously for
30 min. The formation of an oil‐in‐water microemulsion
resulted in a turbid brown solution. Heating the brown
solution at 60 °C for 20 min induced evaporation of the
chloroform, and generating an aqueous dispersion of NPs.
The resulting mixture solution was added to a mixture of
200 ml of water and 5 ml of 2 M NaOH solution, and the
mixture was heated to 70 °C. Then ethyl acetate (10 ml)
was successively added to the diluted aqueous solution
containing the magnetite NPs, and, after 10 min, TEOS
.1 | Materials and instrumentation
Iron (III) chloride hexahydrate (VMR, >99%), iron (II)
chloride tetrahydrate (Merck, >99%), oleic acid (VMR,
9
lammonium bromide (CTAB; Merck, 97%), (3‐aminopropyl)
triethoxysilane (Fluka, 98%), cyclohexane, [VO(acac)2]
6%), tetraethoxysilane (TEOS; Merck, 99%), cetyltrimethy-
(Merck, 98%) and all other chemicals were obtained
commercially from Merck or Fluka and used without further
purification. Deionized water was used for all experiments.
The reaction products of the oxidation were determined
and analysed using an HP Agilent 6890 gas chro-
matograph equipped with an HP‐5 capillary column

NOURI AND HOSSEINI‐MONFARED
3
(
3 ml) was added dropwise. The resulting mixture was stirred
In order to determine the content of cyclohexanol, cyclo-
hexanone and cyclohexyl hydroperoxide, each sample was
for 3 h and the precipitate magnetically separated and
thoroughly washed with ethanol to remove unreacted species
and surfactant, then oven‐dried in air at 60 °C for 24 h.
analysed by GC twice, before and after addition of PPh (in
3
the presence of PPh , cyclohexyl hydroperoxide is quantita-
3
tively transformed into cyclohexanol; then, its true content
can be calculated as the difference between the cyclohexanol
2
.4 | Synthesis of Fe O @m‐SiO /Ag
[31]
3
4
2
concentration before and after PPh3 addition).
revealed that the yield of peroxide was negligible.
It was
A simple electroless plating process was carried out to
deposit Ag NPs on the surface of Fe O @m‐SiO . AgNO3
[11]
3
4
2
For recycling experiments, after completion of the
reaction, the nanocatalyst was recovered using a magnet,
washed with acetonitrile, dried and reused without further
purification.
(0.034 g) was dissolved in ethanol (150 ml) by
ultrasonication in a polypropylene flask and then a solution
of Fe O @m‐SiO NPs (0.12 g) in ethanol (50 ml) was
3
4
2
added. The mixture was stirred for 50 min at 50 °C with a
magnetic stirrer and then a solution of dibutylamine
−
1
+
(
0.015 g ml , 1.0 ml) in ethanol was added to reduce Ag
3
3
|
RESULTS AND DISCUSSION
to Ag NPs on the surface of Fe O @m‐SiO NPs. The ratio
3
4
2
of dibutylamine to AgNO3 was maintained at 1:1. The
mixture was stirred for 3 h at 50 °C. Finally, the products
were magnetically separated, washed several times with etha-
nol and dried at 60 °C under vacuum. Fe O @m‐SiO /Ag
.1 | Synthesis and characterization
Synthesis of the nanocomposite was performed as shown in
Scheme 1. The magnetite NPs were synthesized and
stabilized with oleic acid using a co‐precipitation method.
3
4
2
[
28]
was obtained by calcination at 550 °C for 6 h to remove the
organic materials.
Then, mesoporous silica was used to coat the synthesized
Fe O NPs by employing the modified Stöber method with
3
4
[
29]
CTAB used as a template.
Ag NPs were deposited on
Fe O @m‐SiO by a simple electroless plating process.
2
.5 | Synthesis of [VO(acac) ] covalently anchored onto
[11]
2
3
4
2
amine‐functionalized Fe O @m‐SiO /Ag
3
4
2
Finally, [VO(acac) ] was covalently anchored onto amine‐
2
[30]
[30]
Following
a
reported procedure,
as‐synthesized
functionalized Fe
Figure 1 shows FT‐IR spectra of Fe
Ag and FeSi/Ag/VO. A strong absorption band at 595 cm
is related to the vibrations of the Fe─O functional group,
which shows that the phase of as‐prepared Fe particles
/Ag and FeSi/
3
O
4
@m‐SiO
2
/Ag to afford FeSi/Ag/VO.
, Fe @m‐SiO /
2
Fe O @m‐SiO /Ag (0.6 g) was added to a solution of
O
3
4
O
3
4
3
4
2
−
1
(
3‐aminopropyl)triethoxysilane (0.6 ml, 5 mmol) in dry
toluene (100 ml) and refluxed for 20 h. Then [VO(acac)2]
0.099 g, 0.375 mmol) was added into the solution, which
(
O
3 4
[
32]
was refluxed for another 20 h. The catalyst was filtered off
and washed successively with water, ethanol and acetone
until the washings were colourless, and dried in an oven at
is mainly magnetite.
For Fe
3
O
@m‐SiO
4
2
−
1
Ag/VO, the shoulder at 638 cm might be assigned to the
formation of some amount of oxidized maghemite on the
magnetite surface after calcination at 550 °C for 6 h. In the
Fe─O range, maghemite shows broad FT‐IR bands at 668,
1
10 °C for 24 h. The prepared Fe O @m‐SiO /Ag/VO
3
4
2
(acac) composite is denoted as FeSi/Ag/VO.
2
−
1 [33,34]
6
3
30 and 442 cm .
The broad characteristic band at
424 cm− (not shown) could be assigned to O─H stretching
1
2
.6 | Catalytic oxidation of cyclohexane with H O2
2
vibration arising from the Fe─OH groups on Fe O NPs and
3
4
[
35]
The catalytic experiments were carried out at 60 °C in a
round‐bottom 25 ml glass flask reactor equipped with a
reflux condenser and a magnetic stirrer. In a typical experi-
ment, the flask was charged with a suspension of substrate
coating oleic acid and adsorbed water.
The peaks around
2927 and 2852 cm− are assignable to asymmetric/symmetric
1
[
36]
vibrations of C─H in ─CH ─ of oleic acid
lecular hydrogen bond derived from OH.
and intramo-
The bands at
2
[
37,38]
1623 and 1441 cm− can be ascribed to the asymmetric and
1
(
2 mmol), catalyst (10.0 mg) and oxalic acid (0.10 mmol)
as co‐catalyst in 5 ml of acetonitrile. After heating for
min under stirring, the oxidation reaction was started with
the addition of 4.5 mmol aqueous hydrogen peroxide
−
symmetric stretching vibrations of CO , respectively. Based
2
5
on the FT‐IR spectra, it is concluded that oleic acid is coating
−
1
(
12.5 mol l ). Usually, the reaction was carried out for 8 h
under atmospheric pressure. Aliquots of the reaction mixture
0.2 ml) were withdrawn periodically during the course of the
(
reaction using a syringe. Each aliquot was analysed by GC
after addition of toluene as internal standard. The identifica-
tion of the oxidation products was performed by comparison
with the retention times of commercial cyclohexanol and
1
cyclohexanone, H NMR spectroscopy or GC–MS.
SCHEME 1 Synthesis of FeSi/Ag/VO nanocomposite

4
NOURI AND HOSSEINI‐MONFARED
FIGURE 1 FT‐IR spectra of (a) Fe
3
O
4
, (b) Fe
3
O
4
2
@m‐SiO /Ag and (c)
FeSi/Ag/VO
[39]
the surface of the magnetite NPs.
The spectrum of
Fe O @m‐SiO /Ag shows broad characteristic antisymmet-
3
4
2
[40]
ric (ν ) Si─O─Si vibration bands
observed around 1038–1097 cm , suggesting the successful
of the [SiO ] unit
4
as
−1
coating of silica layer on the Fe O surface. The V═O
3
4
−1
stretching band of FeSi/Ag/VO at about 1000 cm is not
seen due to the overlap by the very strong and broad band
of silica. In addition to the CH and CH stretching bands
3
−
1
around 3000 cm , the bands appearing in the range
1
1
[
554–1587 cm− are assigned to ν(
VO(acac) ] in FeSi/Ag/VO.
) of the supported
The field‐emission SEM images of Fe O @SiO and
[41]
2
3
4
2
FeSi/Ag/VO nanocomposites show that the particles are
non‐aggregated and almost monodispersed spheres with a
smooth surface (Figure 2a–d). The morphological features
such as the spherical shape and non‐aggregation are found
to be retained in the case of FeSi/Ag/VO, similar to
Fe O @SiO . After applying FeSi/Ag/VO five times in the
3
4
2
catalytic oxidation of cyclohexane (Section 3.2), its size and
morphology remain intact, which shows its stability in the
oxidation medium (Figure 2e and f). However, the surfaces
of the particles are not perfectly smooth after five recycles.
The Fe O NPs and FeSi/Ag/VO show no hysteresis
3
4
curves at room temperature (Figure 3), confirming their para-
magnetic nature. The magnetic saturation values of Fe O
3
4
−
1
and FeSi/Ag/VO are 49.2 and 34.6 emu g , respectively.
After surface modification of magnetite there is some
decrease in the saturation magnetization, which is attributed
to the presence of the non‐magnetic materials. Nevertheless,
the nanocomposite can be enriched in 30 s upon the applica-
tion of an external magnetic force (Figure 3, inset).
The crystal structures and the phase purity of Fe O @m‐
3
4
SiO , Fe O @m‐SiO /Ag and FeSi/Ag/VO were determined
2
3
4
2
using XRD. Figure 4(a) shows the wide‐angle XRD pattern
of Fe O @m‐SiO NPs. A broad reflection at 2θ = 20–25°
3
4
2
suggests an amorphous structure of the mesoporous silica
matrix in Fe O @m‐SiO , and all the reflection peaks of
the product can be easily indexed to a crystalline cubic spinel
3
4
2
FIGURE 2 Field‐emission SEM images of (a, b) Fe O @SiO , (c, d) FeSi/
Ag/VO and (e and f) FeSi/Ag/VO after five recycles
3
4
2

NOURI AND HOSSEINI‐MONFARED
5
additive and support was examined (Table 1). It is found
that the presence of oxidant, catalyst and oxalic acid (as
additive) is essential for the oxidation (Table1, entries
1–3). Oxalic acid is a kind of weak acid, and, in this cata-
lytic system, it may provide protons to active species. To
check this proposal, we studied the oxidation of cyclohexane
by H O in a CH CN–oxalic acid mixture with varying
2
2
3
amounts of oxalic acid ranging from 0.10 to 1.0 mmol.
From Table 1, we observe that there is a maximum
conversion as a function of amount of oxalic acid (entries
4–6), as is the case for the yields of cyclohexanol and
cyclohexanone. The presence of protons has accelerated
the oxidation, but there is no substantial increase of activ-
ity with further increase of oxalic acid concentration. It
has been reported that carboxylic acid (stearic acid) takes
3 4
FIGURE 3 Magnetization curves of (a) Fe O and (b) FeSi/Ag/VO. Inset:
(a) dispersion of FeSi/Ag/VO NP in the reaction solution and (b) FeSi/Ag/
VO NPs adsorbed on the wall of sample tube using an external magnet
[
23]
an active role in oxidation of polypropylene
with the
possibility of involvement in an increase of the initiation
rate by enhancement of bimolecular decomposition of
hydroperoxides due to changing of the polarity of the
reaction environment or increasing the initiation rate by
direct catalysis of hydroperoxide decomposition. Alterna-
tively, it is reasonable to propose that acetic acid may
take part in cyclohexane oxidation in other ways, such
as complexing with the vanadium catalytic active centre
at high acetic acid concentration or forming peroxyacetic
structure of Fe O . In the case of Fe O @m‐SiO /Ag,
3
4
3
4
2
besides the characteristic diffraction of Fe O microspheres,
3
4
the obvious diffraction peaks at 2θ = 37.9°, 44.1°, 64.3°
and 77.2° can be readily indexed to face‐centred cubic
[11]
structure of Ag (Figure 4b).
The XRD patterns of
Fe O @m‐SiO /Ag and FeSi/Ag/VO (Figure 4c) are very
3
4
2
similar.
The isotherms of FeSi/Ag/VO are type IV BET isotherms
with a hysteresis loop (Figure S1 in the supporting informa-
tion), demonstrating the mesoporous characteristics of FeSi/
Ag/VO. The specific areas and pore sizes were calculated
using the BJH method. As a result, for FeSi/Ag/VO, the
average pore size and BET surface area are 6.87 nm and
[
25]
acid.
The reaction temperature changes markedly the catalyst
activity in the oxidation of cyclohexane. The catalyst shows
activity at temperatures higher than 45 °C and the conversion
increases on increasing the temperature (entries 6 and 7). The
lowest temperature of 60 °C was selected in order to manage
the H O decomposition. While FeSi/Ag/VO could catalyse
2
−1
6
3 m g , respectively.
2
2
the oxidation of cyclohexane up to 99% conversion, the
3
.2 | Catalytic studies
support Fe O @m‐SiO and Fe O @m‐SiO /Ag could catal-
3
4
2
3
4
2
Oxidation of cyclohexane with H O was chosen as a
yse it up to 35 and 50%, respectively, for the same conditions
2
2
representative reaction and the effect of catalyst, oxidant,
(entries 6, 8 and 9).
a
2 2
TABLE 1 Catalytic oxidation of cyclohexane with H O
Entry
Catalyst
None
Oxidant/additive
Conversion (%)
1
2
3
4
5
6
7
8
9
H
2
O
2
/H
C
2 2
O
4
0
0
FeSi/Ag/VO
FeSi/Ag/VO
FeSi/Ag/VO
FeSi/Ag/VO
FeSi/Ag/VO
FeSi/Ag/VO
None/H C O
2 2
4
H
H
H
H
H
H
H
2
2
2
2
2
2
2
O
O
O
O
O
O
O
2
2
2
2
2
2
2
/none
0
b
c
/H
/H
/H
/H
/H
/H
C
2 2
C
2 2
C
2 2
C
2 2
C
2 2
C
2 2
O
O
O
O
O
O
4
4
4
4
4
4
77
92
99
0
d
Fe
Fe
3
O
O
4
@SiO
@SiO
2
2
35
50
3
4
/Ag
a
Reaction conditions: catalyst, 0.010 g; co‐catalyst (oxalic acid, H
2 2 4
C O ),
1
.0 mmol; cyclohexane, 2.0 mmol; CH CN, 5 ml; aqueous 30% H , 4.5 mmol;
3
2 2
O
temperature, 60 °C; reaction time, 8 h.
b
0.1 mmol oxalic acid.
c
0
.26 mmol oxalic acid.
FIGURE 4 XRD patterns of (a) Fe
and (c) FeSi/Ag/VO
3
O
4
@m‐SiO
2
, (b) Fe
3
O
4
2
@m‐SiO /Ag
d
20°C.

6
NOURI AND HOSSEINI‐MONFARED
a
TABLE 2 Oxidation of various hydrocarbons with H
2
O
2
catalysed by FeSi/Ag/VO
Conversion (%)
100
Entry
Substrate
Products (yield, %)
1
n‐Hexane
b
2
99
b
3
87
66
4
5
61
46
c
6
7
8
31
14
9
45
23
72
1
1
0
1
a
Reaction conditions: FeSi/Ag/VO, 0.010 g; oxalic acid, 1 mmol; CH
3
CN, 5 ml; aqueous 30% H
2
O
2
, 4.5 mmol; temperature, 60 °C; reaction time, 8 h. Naphthalene,
1
mmol; others, 2 mmol.
b
Other products were the corresponding ‐diol, −dione and (−ol and ‐one) isomers.
c
After 6 h.

NOURI AND HOSSEINI‐MONFARED
7
[
31]
corresponding alkyl hydroperoxides are formed. A similar
result (no cyclohexyl hydroperoxide after 12 h reaction time)
was reported for the oxidation of cyclohexane by H O over
2
2
[
48]
Co‐substituted heteropolytungstate catalysts.
Our result
illustrates that selectivity and/or activity of FeSi/Ag/VO is
[
49]
higher than that of Cr‐MCM‐41,
[51]
[Mn(salophen)]‐
catalysts in the
[
50]
NaY
and V‐MCM‐41/acetic acid
oxidation of cyclohexane using hydrogen peroxide as
oxidant.
To our delight, benzene is oxidized successfully to phenol
in 66% yield (entry 4). One‐step synthesis of benzene
oxygenates with H O as green oxidant is a great challenge
2
2
[
52]
in modern synthetic chemistry.
Phenol is one of the most
important intermediates of the chemical industry with a
global production estimated at around 8 Mt per year, almost
completely based upon the cumene process which also co‐
FIGURE 5 Recycling of FeSi/Ag/VO catalyst for the oxidation of cyclohex-
ane. For reaction conditions, see the footnote to Table 2. Averages of dupli-
cate experiments and error bars represent the standard deviations
[
53]
produces acetone.
In the oxidation of alkylaromatic substrates, the catalytic
reactions involve side‐chain oxidation (entries 5–9) and the
hydroxylation of the aromatic ring does not occur. No over‐
oxidation of products is observed. Weakly electron‐donating
In order to expand the hydrocarbon scope, a linear alkane,
benzene and alkylaromatics were also tested and the results
are presented in Table 2. We find that n‐hexane is oxidized
at a relatively low temperature (60 °C) in the presence of
oxalic acid to give a mixture of isomeric ketones and
acids with an unusual distribution of regioisomers.
The oxidation of n‐hexane gives mainly 1‐hexanal
─CH3 and ─CH CH3 substituents slightly decrease the
2
aromatic ring oxidation rate (entries 5–7). Strong electron‐
donating substituents (like ─OH, entry 8) or electron‐with-
drawing substituents (like ─NO , not shown in Table 2)
2
(47%), 3‐hexanone (28%) and 2‐hexanone (12%) with
make the aromatic ring unsusceptible to oxidation by the
minor products including hexanoic acid (5%) and 2,5‐
hexandione (9%) (Table 2, entry 1). The interesting
feature of the reaction is that oxidation of both methyl
and methylene groups is observed, whereas the reactivity
of the more inert methyl groups is higher (the CH2
groups are 80–100 times more reactive than the CH3
groups). This observation, which is not in accordance
H O –FeSi/Ag/VO system. Oxidation of nitrobenzene by
2
2
this system was not successful. An electron‐withdrawing aryl
[
54]
substituent
slightly decreases the conversion of naphtha-
lene (23% conversion; entry 10). Benzaldehyde has the
highest conversion, 72%. The high conversion of benzalde-
hyde is probably due to the coordination of benzaldehyde
through oxygen to supported vanadyl to accelerate the
oxidation.
[42]
with other findings,
clearly testifies that the alkane
oxidation involves free hydroxyl radicals or, possibly,
metal–oxo species. For the reaction of n‐heptane with
hydroxyl radical generators (the ‘vanadate ion–pyrazine‐
The oxidation of aromatic C─H bonds via hydrogen atom
abstraction by the active oxidizing intermediate is not possi-
ble because aromatic C─H bonds are stronger than aliphatic
[
55]
2
‐carboxylic acid–H O ’ reagent, hν–H O , FeSO4–
C─H bonds.
In oxidation reactions, the cleavage of the
2
2
2 2
H O ), the normalized selectivity parameter C :C :C :C
4
2
2
1
2
3
[
56]
[57]
for the relative reactivities of the hydrogen atoms in posi-
tions 1, 2, 3 and 4 of the hydrocarbon chain was found to
[
43,44]
be 6.7:60:47:47.
For oxidation with the H O –
2
2
2+
[
(LMe ) Mn (O) ] –CH COOH system (LMe = 1,4,7‐
3
2
2
3
3
3
trimethyl‐1,4,7‐triazacyclononane), believed to proceed
via abstraction of hydrogen atoms from the alkane by
the Mn═O fragment, this parameter was found to be
3
.3 | Catalyst reuse
[
45,46]
1.3:60:46:46.
Thus, it is reasonable to assume that
The FeSi/Ag/VO nanocomposite could be recycled at least
five times without loss of catalytic properties (Figure 5).
Importantly, no catalyst regeneration/reactivation is needed
before reuse. The retention of the FeSi/Ag/VO structure was
confirmed by FT‐IR (Figures S3–S5) and SEM (Figure 2c
and d) techniques. At the end of the reaction, the catalyst
was separated by magnetic filtration, washed by adding
acetonitrile and ultrasound for 1 min, then died and used in
a fresh oxidation for five successive runs. The FeSi/Ag/VO
alkane oxygenation with the H O –FeSi/Ag/VO system
2
2
occurs with participation of certain hydroxyl radical
[
22,47]
species.
Cyclohexane and cyclooctane are oxidized by H O and
2
2
FeSi/Ag/VO mainly to the corresponding cyclic alcohol and
ketone (Figure S2; Table 2, entries 2 and 3). We analysed
the reaction solutions both before and after addition of PPh3
and demonstrated that only small amounts of the

8
NOURI AND HOSSEINI‐MONFARED
catalyst is truly heterogeneous and only a trace amount of
vanadium (0.03%) was determined in the filtrate using induc-
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The FeSi/Ag/VO–H O –oxalic acid–CH CN system is a
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How to cite this article: Nouri, S. H., and Hosseini‐
Monfared, H. (2016), Highly efficient and green
oxidation of alkanes and alkylaromatics with
hydrogen peroxide catalysed by silver and vanadyl
on mesoporous silica‐coated magnetite, Appl.
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