V-doped titanium mixed oxides as efficient catalysts for oxidation of alcohols and olefins

Article abstract of DOI:10.1039/c4nj00066h

V/TiO₂ nanoparticles were synthesized by a one-pot solution combustion method using ammonium metavanadate and titanyl nitrate as a source of vanadium and titanium, respectively. Atomic absorption spectroscopy (AAS), FT-IR, X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to characterize the physicochemical properties of this compound. This compound shows efficient catalytic reactivity for epoxidation of olefins and oxidation of alcohols in the presence of H₂O₂ as an oxidant.

Full text of DOI:10.1039/c4nj00066h

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V-doped titanium mixed oxides as efficient
catalysts for oxidation of alcohols and olefins
Mojtaba Amini,*a Hadi Naslhajianb and S. Morteza F. Farnia*b
Cite this: New J. Chem., 2014,
38, 1581
V/TiO2 nanoparticles were synthesized by a one-pot solution combustion method using ammonium
metavanadate and titanyl nitrate as a source of vanadium and titanium, respectively. Atomic absorption
spectroscopy (AAS), FT-IR, X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) were employed to characterize the physicochemical properties of this
compound. This compound shows efficient catalytic reactivity for epoxidation of olefins and oxidation of
alcohols in the presence of H2O2 as an oxidant.
Received (in Montpellier, France)
14th January 2014,
Accepted 4th February 2014
DOI: 10.1039/c4nj00066h
www.rsc.org/njc
to sulfoxides in the presence of H2O2 as an oxidant.27 In these
reactions, activity mainly depends on the type of vanadia species
Introduction
In the past few years there has been a growing demand for solid and the nature of the support used for the reaction. V doping was
catalysts efficient in partial oxidation of alcohols and olefins responsible for an increase of superficial hydroxyl groups and
for the production of fine and specialty chemicals.1–5 In the electron transfer, resulting in faster interactions between the sub-
majority of industrial organic oxidation reactions, the use of strate and the catalyst.28 Also, many early attempts have been made
stoichiometric inorganic reagents, such as persulfate, bromate, to characterize the surface structure of supported VOx species using
and particularly metal reagents such as chromic acid and computational methods.29–31
permanganate, is still widespread and therefore entails cumber-
We recently reported the synthesis of nano-sized manganese
some and demanding isolation and purification procedures.6,7 oxide, as a low-cost, easily synthesized and environmentally
The present stringent ecological standards increase the pressure to friendly compound, by a very simple method at moderate
develop new, environmentally benign methods.8,9 Consequently, temperature and without using any organic compounds for
safer and simple protocols are required, towards selective pro- epoxidation of olefins.32,33 In continuation of these works, here
ducts, such as the efficient conversion of alcohols to aldehydes and we report for the first time the preparation of V/TiO2 nano-
olefins to epoxides.10,11 In many instances, homogeneous catalysis particles as an efficient catalyst for oxidation of alcohols and
provides powerful solutions, but on an industrial scale the pro- olefins using H2O2 as an oxidant.
blems related to corrosion and plating out on the reactor wall,
handling, recovery, and reuse of the catalyst represent limitations
of these processes.12–15
TiO2 is considered to be an environmentally friendly, excellent
and inexpensive heterogeneous catalyst-support material with good
Experimental
Characterization
dispersion properties under most conditions.16,17 Therefore, Transmission electron microscopy (TEM) was conducted on
recently the synthesis of TiO2 or supported TiO2 on other substrates carbon-coated copper grids using a FEI Technai G2 F20 Field
has been vigorously studied.18–20 Supported vanadium oxide cata- Emission scanning transmission electron microscope (STEM)
lysts are widely studied for selective oxidation reactions.21,22 Among at 200 kV (point-to-point resolution o0.25 nm, line-to-line
all catalysts, VOx/TiO2 is a promising catalyst which shows high resolution o0.10 nm). TEM samples were prepared by placing
activity for oxidation of o-xylene, toluene, methanol, hydrogen 2–3 drops of dilute ethanol solutions of the nanomaterials onto
sulfide and lactic acid.23–26 Recently Gopinath et al. reported VOx/ carbon coated copper grids. Composition was characterized by
TiO2 as a highly effective catalyst for ambient oxidation of sulfides energy dispersive spectroscopy (EDS) line scans in STEM mode,
and by energy-filtered (EF) imaging spectroscopy (EF-TEM).
SEM was carried out using Philips CM120 and LEO 1430VP
instruments. The X-ray powder patterns were recorded using a
Bruker D8 ADVANCE (Germany) diffractometer (Cu-Ka radia-
a Department of Chemistry, Faculty of Science, University of Maragheh, Golshahr,
P.O. Box 55181-83111731, Maragheh, Iran. E-mail: mamini@maragheh.ac.ir
b Department of Chemistry, University of Tehran, Tehran, Iran.
E-mail: mfarnia@khayam.ut.ac
tion). Manganese content in bulk and on the catalyst surface
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was determined by atomic absorption spectroscopy (AAS) and
energy-dispersive X-ray (EDX). AAS was performed on a Varian
Atomic Absorbtion Spectrometer AA 110. Prior to analysis, the
oxide (1.0 mg) was added to 1 mL of concentrated nitric acid
and H2O2, left at room temperature for at least 2 h to ensure
that the oxides were completely dissolved. The solution was
then diluted to 10.0 mL and analyzed by AAS. The products of
oxidation of olefins were determined and analysed using an HP
Agilent 6890 gas chromatograph equipped with a HP-5 capillary
column (phenyl methyl siloxane 30 m  320 mm  0.25 mm) and
a flame-ionization detector. The FT-IR spectrum was obtained
by using a Unicam Matson 1000 FT-IR spectrophotometer using
KBr disks at room temperature.
Fig. 1 XRD pattern of Ti1ÀxVxO2 (x = 0, 0.05, 0.1, 0.15 and 0.3).
Synthesis of Ti1ÀxVxO2
V-content to 30%. Although a small amount of V2O5 phase is
observed with high V-content (30%), there is no vanadium oxide
peak observed even at 15% of vanadium loading. This observation
hints that vanadium ions are likely to occupy the lattice position of
titania. This observation also rules out the homogeneous distribu-
tion of vanadium oxide on titania.27 Nonetheless, it is to be noted
that Feng et al. observed the V2O5 phase even at 7% loading itself.34
Fig. 2 shows the FT-IR spectra of pure titania and TVx materials.
Pure TiO2 shows a broad band in the region of 400–900 cmÀ1 due to
unresolved stretching vibrations of Ti–O–Ti. The weak peak at
1620 cmÀ1 corresponds to the bending vibrations of O–H, and the
broad bands at around 3430 and 2840 cmÀ1 are attributed to the
surface adsorbed water and hydroxyl groups. Bulk V2O5 displays well
resolved stretching and bending vibrations of VQO at 1025 and
831 cmÀ1, respectively. However, there are no V2O5 features observed
even with TV15. This confirms the absence of separate V2O5 phase in
all TVx (x r 15) materials. Along with XRD, FT-IR also supports the
lattice incorporation of vanadium in titania. With the increase in V
content to 30%, a new peak appeared at 1626 cmÀ1 which is
attributed to a VQO bond and is characteristic of the V2O5 phase.
Particle morphology and textural properties of the V/TiO2 catalyst
were also studied carefully by SEM and TEM. Representative SEM
and TEM images recorded for Ti1ÀxVxO2 materials (x = 0.0 and 0.15)
are shown respectively in Fig. 3 and 4. Aggregated nanoparticles
with spherical shapes are observed with a particle diameter in the
range of 60–90 nm.
All the chemicals employed were of analytical grade and used as
such without any further purification. Titanyl nitrate [TiO(NO3)2] as
a Ti precursor, ammonium metavanadate (Sigma-Aldrich) as a
vanadium precursor and urea (Merck) as a fuel were used as such.
The controlled hydrolysis of titanium isopropoxide (Sigma-Aldrich)
under ice-cold (0 1C) conditions with vigorous stirring gives white
precipitate of TiO(OH)2. The precipitate was washed several times
in distilled water and then dissolved in nitric acid to get a clear,
transparent titanyl nitrate [TiO(NO3)2] solution.18 This solution was
used as the precursor for the synthesis of Ti1ÀxVxO2 (TVx). All TVx
materials were prepared with a 1 : 1 molar ratio of urea to metal
ions (Ti + V); x in TVx indicating a nominal vanadium atom
percentage used for TVx preparation. Aqueous solution of the
desired amount of titanyl nitrate, ammonium metavanadate and
urea was taken in a 250 mL beaker and introduced into a muffle
furnace maintained at 673 K. Solid products were obtained within
2 h of total preparation time. A series of catalysts were prepared by
changing the nominal vanadium content from 5 to 30 at%.
Results and discussion
Catalyst characterization
Titanyl nitrate and ammonium metavanadate were used respec-
tively as a source of titanium and vanadium for the synthesis of
V/TiO2 nanoparticles by a one-pot solution combustion method.
When the molar percentage of vanadium was increased, the color
of the solid samples changed from white to dark due to the color of
V in the doped TiO2. The amount of the V on TiO2 was determined
by AAS analysis. The catalyst was stirred in HNO3 at room tem-
perature for 5 h and the solution was subjected to AAS analysis.
XRD measurements were carried out in order to explore the
structural features of the vanadium incorporated titania Ti1ÀxVxO2
(TVx) materials. In TVx, x stands for nominal V atom%, which is
higher than the actual V-content due to the evaporation loss of some
V during solution combustion synthesis. Fig. 1 shows XRD patterns
of as prepared TVx materials from a low angle to a wide angle. All
TVx materials were prepared with a 1 : 1 urea to oxidizer ratio. XRD
patterns of TVx materials could be indexed to the rutile phase of
titania. It is clear that all the samples can be indexed to the rutile
Catalytic performance of Ti1ÀxVxO2
In order to evaluate the catalytic activities of Ti1ÀxVxO2 for
oxidation reactions, the reactions were optimized according to
the oxidation of benzylalcohol (BA) through the investigation of
phase of TiO2 with some anatase phase of TiO2 upon increasing Fig. 2 IR spectra of Ti1ÀxVxO2 (x = 0, 0.05, 0.1, 0.15 and 0.3).
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Fig. 5 The effect of reaction time on the BA oxidation. Reaction condi-
tions: catalyst (4.0 mg), CH3OH (1 mL), BA (0.8 mmol), H2O2 (0.8 mmol), at
room temperature.
In order to understand the effect of the V dopant on the
catalytic reactivity, first catalyst screening of the reaction was
carried out. BA conversion and benzaldehyde yield with all TVx
catalysts are recorded in Table 1. This shows a volcano type
activity pattern with V-content and the maximum activity has
been observed using the TV15 catalyst. Pure TiO2 gives very
low benzaldehyde yield. BA conversion and benzaldehyde
yield increase with increasing V incorporation and attain the
maximum with the TV15 catalyst. Among all TVx catalysts, TV30
gives low yield of benzaldehyde. Various oxidants including
TBHP, H2O2, NaClO and molecular oxygen were tested for the
oxidation of BA over TV15 at room temperature. Compared with
TBHP, NaClO and molecular oxygen, H2O2 was much more
efficient for the oxidation of BA. The amount of H2O2 could also
significantly affect the conversion and benzaldehyde selectivity
(Table 1). When the amount of H2O2 was increased from 0.6 to
0.8 mmol, the conversion of BA increased drastically from 88 to
100%. With a further increase of H2O2 to 1.0 mmol, the
selectivity to benzaldehyde decreases from 88 to 69%. Then,
a series of reactions was investigated by performing the
model reaction in different solvents such as dichloromethane,
methanol, acetonitrile, benzene, acetone and n-hexane. Among
the solvents examined, methanol was found to be the best for
this protocol (Table 1).
Fig. 3 SEM images of Ti1ÀxVxO2; (a) x = 0, (b) x = 0.15.
Fig. 4 TEM images of Ti1ÀxVxO2 (x = 0.15).
the influence factors of oxidation, such as the effect of the V dopant,
The reaction time could also significantly affect the BA con-
various oxidants, the amount of the oxidant and the solvent. version and benzaldehyde selectivity (Fig. 5). When the time of
Table 1 The effect of various conditions on the oxidation of BA by Ti1ÀxVxO2/H2O2
Entry
Ti1ÀxVxO2 (TVx)
Oxidant
Oxidant amount (mmol)
Solvent
Conversiona (%)
Selectivityb (%)
1
2
3
4
5
6
7
8
—
TiO2
TV5
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
TBHP
NaClO
O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
H2O2
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
0.8
1.0
0.6
0.8
0.8
0.8
0.8
0.8
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3OH
CH3CN
CH2Cl2
C6H6
Trace
27
51
84
97
49
54
45
23
499
88
75
30
21
47
Trace
—
100
100
98
88
97
73
87
100
69
97
93
99
100
100
—
TV10
TV15
TV30
TV15
TV15
TV15
TV15
TV15
TV15
TV15
TV15
TV15
TV15
9
10
11
12
13
14
15
16
CH3COCH3
n-Hexane
a
b
The GC conversion (%) is measured relative to the starting substrate. Selectivity to benzaldehyde = (aldehyde%/(aldehyde% + carboxylic acid%)) Â 100.
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Table 2 Oxidation reactions catalyzed by Ti1ÀxVxO2
Entry
Oxidation reaction
Conditionsa
Time (h)
Conversionb (%)
Selectivityc (%)
1
2
A
A
2
2
97
81
88
93
3
A
2
85
93
4
5
6
A
A
A
2
2
2
89
91
77
90
91
95
7
8
A
A
2
2
99
93
100
100
9
A
A
4
4
75
69
100
100
10
11
B
2
97
72
12
13
B
B
B
2
4
4
99
85
94
98
100
100
14
15
B
B
4
4
47
45
100
100
16
a
Reaction conditions; A: catalyst (4.0 mg), CH3OH (1 mL), alcohols (0.8 mmol), H2O2 (0.8 mmol), at room temperature; B: catalyst (4.0 mg), CH3CN
b
(1 mL), olefins (0.8 mmol), H2O2 (0.8 mmol), at room temperature. The GC conversion (%) are measured relative to the starting substrate.
c
Selectivity to benzaldehyde = (aldehyde%/(aldehyde% + carboxylic acid%)) Â 100; selectivity to epoxide = (epoxide%/(epoxide% + aldehyde%)) Â 100.
reaction was increased from 0 to 2 h, the conversion of BA regular trends in the conversion. The catalytic oxidation system
increased drastically from 0 to 97%. With a further increase of can over-oxidize the aldehyde product to carboxylic acid.
reaction time to 3 h, the selectivity towards benzaldehyde Secondary alcohols, e.g., diphenylmethanol and 1-indanole, could
decreases from 88 to 64%, where the conversion of BA was be converted to the corresponding ketones in 93–99% conversion
also increased from 97 to 100%.
(entries 7 and 8). Aliphatic alcohols were less reactive in com-
All of these experiments demonstrate the key role of parison to aromatic alcohols (entry 10). The oxidation of 2-hexanol
Ti1ÀxVxO2 in the oxidation of BA. The diversity of oxidation and cyclohexanol required a longer reaction time (4 h) to afford
reaction catalyzed by Ti1ÀxVxO2 was extended and oxidation the corresponding ketones (entries 9 and 10).
of various alcohols and alkenes was investigated (Table 2).
Encouraged by these results, several cyclic and linear
Oxidation of two kinds of secondary alcohols was also investi- alkenes were subjected to oxidation reactions. It was obvious
gated and good conversion was obtained. Benzylalcohols were that the Ti1ÀxVxO2 catalyst could efficiently convert aromatic
oxidized to give the corresponding aldehydes with 77–97% alkenes (styrene and indene) to their corresponding epoxides
conversion (entries 1–6). The substrates having electron-donating with 97–99% conversion. However, the catalytic epoxidation of
and -withdrawing substituents in the aromatic ring were com- carbocyclic alkenes like cyclohexene and cyclooctene and the
patible with this protocol. The change of the nature and site of terminal linear alkenes (e.g., 1-hexene and 1-octene) was found
substituents (methyl, methoxy and nitro) in the aromatic ring to be less efficient. The low p-electron density of linear terminal
of benzylalcohol to probe electronic effects displayed almost no alkene decreased the ability of electrophilic cycloaddition
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Fig. 7 Recycling studies of the catalyst Ti1ÀxVxO2 for the oxidation of BA.
Fig. 6 Influence of the radical scavenger addition on the conversion of BA.
Scheme 1 Proposed mechanism of BA oxidation catalyzed by Ti1ÀxVxO2.
resulting in the epoxidation. So, terminal alkenes are less
reactive toward electrophiles than internal alkenes due to the
electron-donating effect of the alkyl substituents.
Fig. 8 Leaching experiment; blue line: reaction with Ti1ÀxVxO2 and red
line: reaction without a catalyst.
The efficiency of the H2O2/TV15 system was checked by two
tests (one without catalyst and the other without H2O2). In both
tests the BA conversion was very low after 2 h of reaction in
methanol, at room temperature. Then this demonstrates that
the activation of the hydrogen peroxide by the catalyst is
necessary. Also it has been reported that the catalytic oxidation
in the presence of H2O2 and metal oxides follows a radical
oxidation pathway.35 This pathway may be taken due to hydrogen
peroxide being a source of OH radicals when decomposed
homolytically. Therefore, to further verify the role of OH radicals
in the oxidation system of the Ti1ÀxVxO2 (x = 0.15), we examined
the role of 2,6-di-tert-butyl-p-cresol as an radical scavenging
agent in this process. When 2,6-di-tert-butyl-p-cresol (0.01 mmol)
was added as a radical scavenger, the oxidation was strongly
suppressed (Fig. 6).
the reaction mixture was analyzed by AAS and no vanadium was
detected. These results showed that Ti1ÀxVxO2 is truly hetero-
geneous and catalyst leaching is negligible under these conditions.
Conclusion
In conclusion, catalyst Ti1ÀxVxO2 was synthesized and applied
for oxidation of alcohols and epoxidation of alkenes using H2O2
as an oxidant at room temperature. The effects of the V dopant,
various oxidants, the amount of the oxidant and the solvent as
the influencing factors of oxidation were also studied. Finally,
a further set of experiment was carried out to check the leaching
and reusability of the catalyst Ti1ÀxVxO2 for the oxidation of BA
as a model reaction.
This reveals that the oxidation of BA to benzaldehyde is a
free radical mechanism as shown in Scheme 1.
Acknowledgements
Recycling studies
Partial funding support was provided by the Research Council
of University of Maragheh (M.A.) and Tehran University (M.F.).
A further set of experiments were carried out to check the
reusability of the Ti1ÀxVxO2 (x = 0.15) catalyst for the oxidation
of BA at room temperature for 2 h and results have been shown
in Fig. 6. For each recycle, the catalyst was recovered from
reaction mixture by filtration, washed 2–3 times with distilled
water and ethyl ether repeatedly, dried at 50 1C for half an hour,
and reused for the next run. As shown in Fig. 7 the catalyst was
reused 5 times and no significant loss of activity was observed.
In order to investigate catalyst leaching in the oxidation of
BA as a model reaction, the reaction was stopped at half the
reaction time (1 h) and the catalyst was completely separated
from solution. The rest of the reaction mixture (without catalyst)
was allowed to stir for another period of half the reaction time.
As seen in Fig. 8, a trace amount of benzaldehyde was pro-
duced after catalyst separation. After separation of the catalyst,
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