Role of vanadium and pyridine in heteropolycompounds for selective oxidation of alcohols with hydrogen peroxide

Article abstract of DOI:10.1007/s12039-013-0523-6

This study describes the application of heteropolyacids H3PMo12O40,H4SiMo12O40, H4PMo11VO40, H5PMo10V2O40, H9PMo6V6O40, and a hybrid pyridine-modified heteropolyacid with Keggin structure for selective oxidation of alcohols to ketones or aldehydes using aqueous hydrogen peroxide and acetonitrile as solvent. Performance of these different catalysts in 1-phenylethanol oxidation was studied. Influence of reaction temperature, amount of catalyst and hydrogen peroxide and reaction time on the yield of acetophenone was investigated to obtain optimal reaction conditions. Oxidation ability of the catalyst depended on the number of vanadium atoms present in the Keggin ion and to a lesser extent on pyridine substitution in the Keggin secondary structure. In order to explore the applicability of the method for selective oxidation of alcohols to ketones or aldehydes, various alcohols were investigated according to the general procedure using hybrid pyridine-modified heteropolyacid. Indian Academy of Sciences.

Full text of DOI:10.1007/s12039-013-0523-6

J. Chem. Sci. Vol. 125, No. 6, November 2013, pp. 1375–1383. ꢀc Indian Academy of Sciences.
Role of vanadium and pyridine in heteropolycompounds for selective
oxidation of alcohols with hydrogen peroxide
a
a,b
a
VALERIA PALERMO , PAULA I VILLABRILLE , PATRICIA G VÁZQUEZ ,
a
c
a,b,∗
CARMEN V CÁCERES , PIETRO TUNDO and GUSTAVO P ROMANELLI
a
Centro de Investigación y Desarrollo en Ciencias Aplicadas “Dr. Jorge J. Ronco” (CINDECA),
Departamento de Química, Facultad de Ciencias Exactas, Universidad Nacional de La Plata-CCT-CONICET.
Calle 47 Nro 257, B1900AJK La Plata, Argentina
b
Facultad de Ciencias Agrarias y Forestales, Universidad Nacional de La Plata. Calles 60 y 119 s/n,
B1904AAN La Plata, Argentina
c
Department of Environmental Science, Cà Foscari University, Dorsoduro 2137, 30123 Venice, Italy
e-mail: gpr@quimica.unlp.edu.ar
MS received 22 April 2013; revised 27 August 2013; accepted 30 August 2013
Abstract. This study describes the application of heteropolyacids H3PMo12O40,H4SiMo12O40,
H4PMo11VO40, H5PMo10V2O40, H9PMo6V6O40, and a hybrid pyridine-modified heteropolyacid with Keggin
structure for selective oxidation of alcohols to ketones or aldehydes using aqueous hydrogen peroxide and
acetonitrile as solvent. Performance of these different catalysts in 1-phenylethanol oxidation was studied.
Influence of reaction temperature, amount of catalyst and hydrogen peroxide and reaction time on the
yield of acetophenone was investigated to obtain optimal reaction conditions. Oxidation ability of the catalyst
depended on the number of vanadium atoms present in the Keggin ion and to a lesser extent on pyridine
substitution in the Keggin secondary structure. In order to explore the applicability of the method for selective
oxidation of alcohols to ketones or aldehydes, various alcohols were investigated according to the general
procedure using hybrid pyridine-modified heteropolyacid.
Keywords. Oxidation reaction; alcohol oxidation; vanadium-substituted heteropolyacids; pyridinium
heteropolyacid salt.
1
. Introduction
catalytic oxygen transfer reactions. The use of aqueous
hydrogen peroxide as an oxidant is very attractive,
since it is environmentally friendly and easy to handle.5
A variety of different catalytic systems for the hydro-
gen peroxide-promoted oxidation of alcohols have
been developed, e.g., tungsten-based polyoxometalate
Selective oxidation of organic substrates is an important
transformation in organic synthesis and several methods
are known for this particular conversion. Aldehydes
and ketones are essential intermediates for many drugs,
vitamins, fragrances and food additives, and as inter-
mediates in organic syntheses such as Aldol, Michael,
Cannizaro and Perkins reactions.2
1
6,7
8
catalyst, Lewis acid iron aluminum chloride, di-
9
nuclear Mn (IV) complexes, metal–organic gel-based
10
11
on Fe(III) and CuCl bifunctional ionic liquid.
2
Several methods are reported in literature for the
selective oxidation of alcohols and this transformation
is of great significance to the synthetic organic com-
Recently, Maity et al. described an efficient aqueous
system to perform selective oxidation of primary and
secondary alcohols using platinum nanoparticules as
1
12
munity. Classical methodologies for this transforma-
catalyst.
tion use conventional mineral oxidants in stoichiometric
On the other hand, the design of a new catalyst which
gives excellent conversion with maximum selectivity
for organic transformation is one of the challenges in
the field of catalysis. In well-known that materials based
on vanadium are excellent catalysts in different oxida-
tion procedures. Among the various transition metals,
vanadium, an early member of the 3d transition metal
series, exists on the surface of the earth more abun-
dantly and vanadium-base oxidant are effectively used
3,4
amounts.
The described method generates considerable
amount of inorganic waste and is not environmentally
attractive. Consequently, one of the preferred solutions
is to use hydrogen peroxide as an oxygen donor in
∗
For correspondence
1375

1376
Valeria Palermo et al.
O
for various oxidation reactions such as methanol oxi-
dation, carbon monoxide oxidation, partial oxidation of
OH
HPA catalyst
R
13
R
hydrocarbons and many organic transformations. For
example, a recent report by Gopinath and co-workers
describes the use of nanocrystalline Ti V O as effi-
7
0°C, acetonitrile
R'
R'
1.X
X
2
cient catalyst in the oxidative dehydrogenation of ethyl
Scheme 1. Catalytic oxidation of benzylic alcohols in li-
quid phase.
benzene, using oxygen as friendly oxidant.14
Heteropolyacids (HPAs) are transition metal-oxygen
anion clusters that exhibit a wide range of molecu-
oxidation,25 oxidation of benzoins to benzyls, aldehy-
lar sizes, compositions and architectures.15 There is an
28
des and esters by dioxygen, and liquid-phase oxida-
increasing interest in the area of heteropolycompound-
induced organic transformations. HPAs have been used
in a variety of acid-catalysed reactions such as esterifi-
cation, etherification, olefin hydration and dehydration
of alcohols and are also attractive as catalysts for oxida-
29
tion of methane with hydrogen peroxide. Recently,
30
Leng and co-workers reported the use of a pyridine-
modified molybdovanadophosphate hybrid catalyst for
direct hydroxylation of benzene by hydrogen perox-
ide in acetic acid and acetonitrile mixed solvent. They
demonstrated that substituted vanadium atoms in HPAs
are essentially active sites with high performance for
hydroxylation of benzene to phenol.
16
tion processes. Recently, we have applied the use of
Keggin HPAs in a wide range of processes, i.e., prepa-
1
7,18
ration of heterocycles,
protection/deprotection of
and oxidation processes,
organic functional groups1
9,20
In this study, vanadium-substituted HPAs were pre-
pared. The H PMo VO was transferred to pyridinium
as well as substituted phenol, alcohol and amine oxi-
4
11
40
dation, and the selective oxidation of sulphides to
sulphoxides and sulphones.21–24
salt to get the hybrid material HPyH PMo VO . Then,
3 11 40
we used these catalysts for the liquid-phase selective
Vanadium-incorporated molybdophosphoric acid
catalysts show unique catalytic features for oxida-
tions due to their bifunctional character, which arises
because of the redox nature of vanadium and the
oxidation/acidic character of molybdophosphoric acid
catalysts by replacing Mo atoms with correspond-
ing V atoms.2 Several researchers have investigated
vanadium-incorporated molybdophosphoric acid cata-
lysts by replacing 1, 2 and 3 molybdenum atoms with
oxidation of alcohols with 35% (w/v) aqueous hydro-
◦
gen peroxide (H O ) in acetonitrile as solvent at 70 C
2
2
(scheme 1).
2. Experimental
5
2.1 Catalytic preparation
corresponding number of vanadium atoms. Acidic, Commercially available H PMo O (PMo ) and
3
12 40
12
redox and thermal stability properties of vanadium H SiM O (SiMo ) were purchased from Aldrich.
4
12 40
12
catalysts are influenced by the degree of vanadium HPAs containing vanadium in the Keggin primary
25
substitution. In solutions, reduction potentials of het- structure [H PMo VO (PMo V), H PMo V O
4
11
40
11
5
10
2
40
eropolyanions containing Mo and V are high as these (PMo V ) and H PMo V (PMo V )] were prepared
10
2
9
6
6
6
6
ions are easily reduced. Oxidative ability decreases and fully characterized according to the procedure
26
generally in the order: V- > Mo- > W-containing het- already published in the literature. Then, starting
eropolyanions, which means that vanadium-containing from PMo V, a pyridinium salt, HPyH PMo VO
11
3
11
40
26
heteropoly compounds are the strongest oxidants.
(Py-PMo V), was obtained. An ethanolic solution of
11
As reported by Song et al.,15 metal substitution may aniline was added to an ethanolic solution of PMo V.
11
◦
occur by modifying the energy and composition of the The mixture was stirred at 80 C for 1.5 h. The pre-
lowest unoccupied molecular orbital (LUMO) and con- cipitate was filtered, washed with ethanol and dried
◦
sequently its redox properties. Substitution of vana- at 20 C in vacuum. The resulting fine orange powder
dium ions in the molybdenum framework stabilizes the was then dissolved in water to recrystallize for further
LUMOs because these orbitals derive in part from vana- purification.
dium d-orbitals, which have been assumed to be more
stable than those of molybdenum and tungsten.
2.2 Catalytic characterization
These heteropolycompounds have recently been
studied due to their importance in catalytic oxida- 2.2a Fourier transform infrared spectroscopy: Thermo
tion reactions, for example, hydroxylation of benzene, Bruker IFS 66 IR equipment was used to obtain the
oxidation of toluene, nitrobenzene and norbornene Fourier transform-Infrared (FT-IR) spectra of the solid
using aqueous hydrogen peroxide,27 benzyl alcohol samples. Samples were run in transmission mode with

Heteropolycompounds as catalysts for alcohol oxidation
1377
−1
◦
a wavenumber resolution of 4 cm . The solids were reactor at 70 C. Reactions were performed by adding
ground and mixed with BrK, previously dried in a stove, H O 35% (w/v) in excess to a solution of 0.7 mmol of
2
2
and pellets were formed. The measuring range was the corresponding alcohols and the catalyst (1% mmol)
−1
4
00–4000 cm . Measurements were made in air and in acetonitrile (5 mL). The reaction was followed by
the effect of adsorbed moisture or carbon dioxide was thin-layer chromatography (TLC). The reaction mixture
eliminated using air as reference in the measurements.
was diluted with 20 mL of distilled water and extracted
twice with 10 mL of dichloromethane (JT Baker).
Extract was dried over anhydrous sodium sulphate.
Crude product was purified by column chromatogra-
phy on silica gel using a mixture of hexane–toluene
2
.2b Potentiometric titration: An amount of 0.05 mL
of 0.1 N n-butylamine (Carlo Erba) in acetonitrile
JT Baker) was added to a mass of solid (0.05 g)
(
(1:1) (Cicarelli) as eluent. Products were identified by
using acetonitrile as solvent, and stirred for 3 h. Later,
the suspension was titrated with the base solution at
comparison of the mass spectra of standard samples.
Selectivity of reaction was calculated as the molar ratio
between the obtained amount of ketone or aldehyde
and the total amount of the corresponding alcohols.
0.05 mL/min. Electrode potential variation was mea-
sured with an Instrumentalia SRL digital pH meter,
using a double junction electrode. This technique
enables the evaluation of the total number of acid sites
and their acid strength. In order to interpret the results,
it is suggested that the initial electrode potential (E)
indicates the maximum acid strength of the surface
sites, and the values (meq/g solid) where the plateau is
reached indicates the total number of acid sites. Acid
strength of surface sites can be assigned according to
the following ranges: very strong site, E > 100 mV;
strong site, 0 < E < 100 mV; weak site, −100 <
2
.3c Quadrupole mass spectra of synthesized com-
pounds: Acetophenone: m/z (%) 120 (32), 105 (100),
7 (87), 51 (38), 43 (18); 4-Methylacetophenone: m/z
%) 134 (32), 119 (100), 91 (83), 55 (28), 43 (10),
7
(
3
1
9 (11); 4-Methoxyacetophenone: m/z (%) 150 (38),
35 (100), 107 (12), 92 (14), 77 (25), 43 (10); 4-
Chloroacetophenone: m/z (%) 156 (8), 154 (27), 141
33), 139 (100), 113 (14), 111 (42), 75 (17), 50 (9),
(
31
E < 0 mV and very weak site, E < −100 mV.
4
7
3 (10); Benzophenone: m/z (%) 182 (42), 105 (100),
7 (60), 51 (23); 4,4 -Dimethylbenzophenone: m/z (%)
ꢁ
2.3 Catalytic test
210 (33), 195 (17), 119 (100), 91 (43), 76 (28), 39
ꢁ
(
10); 4,4 -Dichlorobenzophenone: m/z (%) 254 (4),
2.3a 1-Phenylethanol oxidation: In all the reactions, 253 (3), 252 (23), 251 (5), 250 (40), 215 (10), 131
a 25 mL three-necked round-bottomed flask heated at (33), 129 (100), 75 (22), 50 (6); Benzaldehyde: m/z
the reaction temperature and connected with a water- (%) 106 (92), 105 (89), 77 (100), 51 (50), 50 (28); 4-
jacketed condenser, was used. The reactor was loaded Chlorobenzaldehyde: m/z (%) 142 (23), 141 (42), 140
with a mixture of 0.7 mmol of 1-phenylethanol (71), 139 (100); 2-Octanone: m/z (%) 128 (6), 113 (4),
(Aldrich), 5 mL of acetonitrile, 1 mL of aqueous hydro- 85 (7), 71 (19), 58 (86), 43 (100), 29 87); 2-Decanone:
gen peroxide 35% (w/v) (Analquim), n-dodecane (Sigma- m/z (%) 156 (5), 141 (2), 127 (2), 113 (2), 98 (5), 96
Aldrich) as internal standard (0.056 g, 0.39 mmol) and (5), 85 (5), 71 (38), 58 (100), 43 (85), 41 (27); Decanal:
1
% mmol of the HPAs with respect to the substrate. The m/z (%) 156 (1), 138 (2), 128 (84), 112 (21), 96 (15),
mixture was stirred at 700 rpm.
95 (18), 84 (16), 83 (28), 82 (34), 81 (23), 71 (33), 70
Samples were withdrawn from the organic phase (47), 69 (29), 68 (27), 67 (25), 57 (62), 56 (43), 55 (62),
at different intervals (15, 30, 60, 90 and 120 min). 44 (54), 43 (100), 41 (80), 29 (50), 27 (33).
Each sample volume was approximately 20 μL and
was diluted with 2 mL of acetonitrile. Concentrations
3
. Results and discussion
of substrates were calculated with an internal stan-
dard method, using a Varian gas chromatography 3400
instrument (Chromopack CP Sil 8 CB, 30 m × 0.32 mm
ID). Identification of products was performed with gas
chromatography-mass spectrometry (GC-MS) (detector
HP 5971).
3.1 Catalysis characterization
Vibration spectra of bulk heteropolyacids with Keggin
structure are modified as a function of the nature of
the elements introduced in their structure. The main
changes observed when Mo atoms are substituted by
V
+
6
+5
2.3b General procedure for alcohol oxidation:
atoms in the structure of PMo12 are a frequency
Alcohols were purchased from Aldrich. Alcohol oxida- decrease of the Mo=O band, and a possible splitting
d
26
tions were performed under vigorous stirring in a glass of the P-O band. In the FT-IR spectrum of PMo V
a
11

1378
Valeria Palermo et al.
(
(
figure 1), bands at 1062 and 1081 (P–O ), 960
1200
1000
a
Mo=O ), 866 (Mo-O -Mo), and 776 (Mo-O -Mo)
d
b
c
−1
cm were observed. These values, compared with
those corresponding to PMo , agree with the indicated
behaviour. Similar changes can be seen for vibrational
spectra of PMo V and PMo V . This indicates that
indeed V has been incorporated into the Keggin units.
The FT-IR spectra of pyridinium salt (Py-PMo V)
PMo12
PMo11
12
800
600
400
V
10
2
6
6
Py-PMo V
11
11
showed the four peaks assigned to the Keggin-structure
heteropolyacid (PMo V) and a characteristic band for
11
200
0
−1
pyridinium ion at 1483 cm , in good agreement with
30
a previous report. This behaviour indicates that pyri-
dine acts as countercation and does not modify the
Keggin units.
0
1
2
3
4
5
Base/catalyst (meq/g)
Potentiometric titration with n-butylamine of the
synthesized HPAs was carried out. According to these
Figure 2. Potentiometric titration curves for PMo12,
results, a relative order of maximum acid strength PMo11V and Py-PMo11V.
can be assigned as follows: Py-PMo V > PMo V >
11
11
PMo12 ꢂ PMo V > PMo V . PMo V shows very
10
2
6
6
11
strong sites, with a maximum acid strength, correspond-
ing to an initial electrode potential of 978 mV, which is
higher than that obtained for the V-free sample, 600 mV
charge of the anion is higher. Consequently, the anion–
proton interaction increases and acidity of the com-
pounds decreases.
(
figure 2). When PMo V was transformed into hybrid
11
material (Py-PMo V), acid strength increased from
11
3.2 Catalytic performance
9
78 to 1100 mV, and potentiometric titration curve was
very similar to that of the corresponding HPA. In the
case of PMo V and PMo V acids, acid strength
decreased. When the number of V atoms increases,
This study describes the application of a homogeneous
system for the oxidation of alcohols to ketones or
aldehydes with aqueous hydrogen peroxide in the
presence of Keggin HPAs as catalyst. Influence of
vanadium-doped atoms in the primary structure and
pyridine in the secondary structure of different synthe-
sized catalysts was also considered. Figure 3 shows the
results obtained for 1-phenylethanol conversion using
different catalysts. A high selectivity of benzophenone
was obtained (96–100%) with traces of a secondary
product, 4-hydroxyacetophenone.
10
2
6
6
PMo12
PMo V
11
In all cases, the catalyst reaches certain conversion
value in the first 30 min and no further changes occur
afterwards. This is a consequence of the hydrogen
peroxide decomposition. In a blank experiment, 1 mL
of 35% p/v of hydrogen peroxide in 4 mL of ace-
tonitrile, in the presence of catalyst (1 mmol%), was
stirred at 70 C for 30 min. Titration of hydrogen per-
oxide remainder showed a 100% of hydrogen peroxide
decomposition.
PMo V
10
2
6
PMo V6
◦
Py-PMo V
11
3.2a Effects of vanadium content in the heteropoly-
acid on catalytic oxidation of 1-phenylethanol:
Table 1 shows the results of 1-phenylethanol oxidation
to acetophenone with aqueous hydrogen peroxide using
different catalysts. Blank experiment was performed
in the absence of catalyst. Under these conditions,
1
400
1200
1000
800
-
600
400
1
Wavenumber (cm )
Figure 1. FT-IR spectra of PMo12, PMo11V, PMo10V2,
PMo6V6 and Py-PMo11V samples.

Heteropolycompounds as catalysts for alcohol oxidation
1379
higher yields of acetophenone were reached, indicat-
ing that the presence of vanadium atoms is essential
for performing oxidation reaction. This is in accor-
Py-PMo11
PMo11
V
5
4
3
2
1
0
0
0
0
0
0
V
25,26,29
dance with previous reports.
However, the con-
version of 1-phenylethanol and the turnover number
(TON) decreases with the content of substituted vana-
dium atoms in the HPA, i.e., catalytic activities of
the three catalysts increased in the following order:
PMoV ∼ PMoV < PMo V. PMo V showed the best
PMo6V6
PMo V
10
2
2
6
11
11
performance (table 1, entry 4). A conversion of 48%
was observed at 2 h, with 97.9% of acetophenone selec-
tivity. In this case, traces of a secondary product,
4-hydroxyacetophenone, were detected.
PMo12
SiMo12
none
Improvement in the catalytic activity of vanadium-
HPAs is in agreement with redox potential values
0
.0
0.5
1.0
1.5
2.0
2.5
33
reported in the literature, 0.52 V for PMo
and 0.65 V
12
Time (h)
26
for PMo V.
11
Figure 3. Effect of reaction time on conversion of 1-
phenylethanol (X) to acetophenone (Y) using different
synthesized catalysts.
Maximum acid strength also decreased when the
number of V atoms in the HPAs increased. There-
fore, it could be assumed that the higher yield of
acetophenone from 1-phenylethanol resulted from two
factors: the oxidative ability and acidic properties of
HPA. Brønsted acidity is an important catalytic pro-
◦
reaction conversion was very low (2% in 2 h) at 70 C perty since protons could be involved in the mechanism
34
(table 1, entry 1).
of oxidation reaction.
On the other hand, a previous study reported that
We performed oxidation reaction of 1-phenylethanol
using two commercial Keggin HPAs (PMo12 and pyridine incorporation in the secondary structure of
30
SiMo ). Conversion was very low, 5% and 4% for HPAs can increase catalytic activity. For this reason,
12
PMo and SiMo , respectively (table 1, entries 2 and we prepared a hybrid vanadium-substituted HPA cata-
12
12
3
). Only a trace amount of acetophenone was observed. lyst (Py-PMo V), which was tested in 1-phenylethanol
11
Results of 1-phenylethanol oxidation using the oxidation under aforementioned conditions. A slight
prepared catalysts (PMo V, PMo V and PMo V ) effect of pyridine on the yield of ketones or aldehy-
11
10
2
6
6
are listed in table 1, entries 4–6. In these cases, des was observed. A conversion of 49% was observed
Table 1. Catalytic performance of different HPAs in 1-phenylethanol oxidation.
a
b
c
TONd
TON-Ve
Entry
Catalyst
1-PHE conv. (%)
APH yield (%)
APH select. (%)
1
2
3
4
5
6
7
None
PMo12
SiMo12
PMo11V
PMo10V2
PMo6V6
Py-PMo11V
2
5
4
48
28
32
49
2
5
4
47
27
31
48
100
100
100
97.9
96.4
96.8
97.9
1.4
3.5
2.8
32.9
18.9
21.7
34.3
-
-
-
32.9
9.45
3.61
34.3
Experimental conditions: 0.01 mmol catalyst, 0.7 mmol of 1-phenylethanol, 1 mL of H2O2 35% (w/v) and
◦
5
mL of acetonitrile; reaction for 2 h at 70 C. 1-PHE = 1 phenylethanol; APH = acetophenone; 4OHAPH =
4
-hydroxyacetophenone (a secondary product)
a
Conversion (%) of 1-PHE calculated as mmol 1-PHE/mmol of initial 1-PHE ×100
Yield (%) of APH calculated as mmol APH/mmol of initial 1-PHE ×100
Selectivity (%) of APH calculated as mmol APH/(mmol of AHP + mmol 4OHAPH)
Turnover number calculated as mmol APH/mmol catalyst
b
c
d
e
32
Turnover number calculated as mmol APH/mmol V atom in polyoxometalates

1380
Valeria Palermo et al.
1
00
100
80
100
80
60
40
20
0
1
00
100
80
60
40
20
0
100
80
60
40
20
0
8
0
8
6
4
2
0
0
0
0
0
60
40
20
0
60
40
20
0
20
30 40 50 60 70 80
Temperature (ºC)
0.5
1.0
Time (h)
1.5
2.0
2.5
0
2
4
6
8
10
(a)
(b)
(c)
Amount of catalyst (% mmol)
Figure 4. (a) Effect of reaction temperature on the conversion of 1-phenylethanol (X) and selectivity to
acetophenone (Y) in 1-phenylethanol oxidation using Py-PMo11V; (b) effect of reaction time on conversion
of 1-phenylethanol (X) and selectivity to acetophenone (Y) using Py-PMo11V catalyst; (c) effect of amount
of catalyst on conversion of 1-phenylethanol (X) and selectivity to acetophenone (Y) using Py-PMo11V
catalyst.
◦
at 2 h, with 97.9% of acetophenone selectivity (table 1, and 80 C, respectively). This could be due to further
entry 7) compared to the PMo V (48% and 97.9%, oxidation of acetophenone to 4-hydroxyacetophenone.
11
◦
respectively, table 1, entry 4).
For this reason, 70 C was employed as optimum
Pyridine in the secondary structure of HPA can pro- temperature.
mote catalytic activities of the hybrid catalyst due
3
.2c Effect of reaction time on catalytic oxidation of
to electronic interaction between pyridine and HPA
along with the more remarkable pseudo-liquid-phase
behaviour, as reported by Leng and co-workers.
1-phenylethanol: Figure 4b shows the result for selec-
30
tive oxidation of 1-phenylethanol as a function of reac-
tion time using Py-PMo V catalyst at a reaction tem-
11
◦
3
.2b Effect of reaction temperature on catalytic oxi- perature of 70 C. It can be observed that conversion of
dation of 1-phenylethanol: Influence of reaction tem- 1-phenylethanol increases with reaction time up to 0.5 h
perature on reactivity of 1-phenylethanol oxidation and then it does not change. Selectivity of acetophenone
was investigated using Py-PMo V, and the results are decreases slightly with reaction time.
11
illustrated in figure 4a. Five temperatures were tested
◦
◦
◦
◦
◦
(
20 , 30 , 50 , 70 and 80 C). No 1-phenylethanol con- 3.2d Effect of amount of Py-PMo V on catalytic
11
◦
version was observed at 20 C. A higher temperature oxidation of 1-phenylethanol: Figure 4c displays the
leads to a higher 1-phenylethanol conversion. This is effect of amount of Py-PMo V on conversion of 1-
1
1
◦
◦
only 15% at 50 C, after 1 h, whereas at 70 C, conver- phenylethanol and selectivity of acetophenone. It can be
sion is 48%, which is three times higher. An increase seen that conversion of 1-phenylethanol increases from
of only 1% is observed when the temperature is raised 28% to 49% when the amount of Py-PMo V increases
11
◦
◦
from 70 to 80 C.
from 0.5% to 1% mmol. No significant change in
As regards acetophenone selectivity, a slight decrease the conversion of 1-phenylethanol was observed when
◦
with temperature was observed (97.9% to 94%, at 70
the amount of Py-PMo V was between 1% and
11
1
00
100
80
60
40
20
0
100
80
60
40
20
0
8
6
4
2
0
0
0
0
0
1
2 2
portion of 1 mL H O
4 portions of 0.25 mL H
at 0, 0.5, 1 and 1.5 h
2 2
O
0
1
2
3
4
0.0
0.5
1.0
1.5
2.0
2.5
(
a)
(b)
Amount of H O (mL)
Time (h)
2
2
Figure 5. (a) Effect of amount of aqueous hydrogen peroxide 35% (w/v) on
conversion of 1-phenylethanol (X) and selectivity to acetophenone (Y) using Py-
PMo11V catalyst; (b) effect of addition of hydrogen peroxide 35% (w/v) on
conversion of 1-phenylethanol (X) and selectivity to acetophenone (Y) using
Py-PMo11V catalyst.

Heteropolycompounds as catalysts for alcohol oxidation
1381
10% mmol. Selectivity of benzophenone remained con- 49% at 1 mL. A slight decrease in the conversion
stant, 97–98%. It is independent of the amount of cata- of 1-phenylethanol and in the selectivity of ace-
lyst. Thus, 1% mmol of Py-PMo V is a suitable tophenone was observed with further increase on the
11
amount for this reaction condition.
amount of hydrogen peroxide. Selectivity decrease may
result from further oxidation of acetophenone to 4-
hydroxyacetophenone.
3.2e Effect of amount of hydrogen peroxide on cata-
Results suggest that an excess of hydrogen peroxide
lytic oxidation of 1-phenylethanol: Figure 5a shows is needed for improving substrate conversion although
the influence of the amount of hydrogen peroxide 35% the theoretical molar ratio of hydrogen peroxide to 1-
(
w/v) on conversion of 1-phenylethanol and selecti- phenylethanol for acetophenone production is 1:1. This
vity of acetophenone using Py-PMo V. It can be seen is a consequence of hydrogen peroxide decomposi-
11
30
that the conversion was dependent on the amount of tion as reported by Leng and co-workers for benzene
hydrogen peroxide and reached a maximum value of oxidation using a similar catalyst.
Table 2. Oxidation of alcohols to ketones and aldehydes using Py-PMo11V catalyst.
Entry
Substrate
Product
Time (h)
Conv (%)
93
Select (%)
96
1
2
2
3
2
2
91
90
99
97
4
5
2
2
93
96
99
98
6
7
2
2
97
93
99
99
8
9
1
1
100
100
81
83
Experimental conditions: 0.01 mmol catalyst, 0.7 mmol of 1-phenylethanol, 1 mL of H2O2 35 % (w/v), 5 mL of acetonitrile,
◦
7
0 C

1382
Valeria Palermo et al.
In order to avoid hydrogen peroxide decomposi- by-products were detected. However, for benzylic pri-
tion, addition of consecutive portions of 35% (w/v) mary alcohols, selectivity is low due to the formation of
hydrogen peroxide was studied. Figure 5b shows the carboxylic acids as by-products.
effect of oxidant addition on 1-phenylethanol conver-
Further investigations on the preparation of new
sion. Addition of 1 mL of aqueous hydrogen peroxide hybrid insoluble heteropolyacids for a more ecofriendly
at the beginning gives a conversion of 1-phenylethanol oxidation process (ethanol–water as solvent) are in
of 49% after 1 h and remain constant. However, when progress in our laboratory.
portions of aqueous hydrogen peroxide are added to the
system (0.25 mL at 0, 0.5, 1 and 1.5 h) 1-phenylethanol Acknowledgements
conversion increases notably, reaching 93% at 1.5 h.
The authors thank Graciela Valle for the FTIR measure-
Acetophenone selectivity was 97%.
ments and INCA, CONICET, ANPCyT and UNLP for
their financial support.
3.2f Selective oxidation of alcohols to ketones or
aldehydes: Oxidation of other alcohols was then
References
investigated using Py-PMo V as catalyst in optimum
11
reaction conditions for 1-phenylethanol (0.7 mmol
1
. Ley S V and Madin A 1991 In: Comprehensive organic
synthesis (eds) B M Trost and I Fleming (Oxford:
Pergamon) Vol. 7
of substrate, 1 mL 35% (w/v) hydrogen peroxide,
◦
1
% mmol of catalyst, 5 mL of acetonitrile, 70 C).
Results are listed in table 2.
High conversion and selectivity were obtained for
aromatic secondary alcohol oxidations (table 2, entries
2. Rahman A, Pullabhotla V S R R and Jonnalagadda S B
008 Catal. Commun. 9 2417
. Lee D G and Spitzer U A 1970 J. Org. Chem. 35
589
. Menger F M and Lee C 1981 Tetrahedron Lett. 22
655
2
3
4
3
1
–7). In the case of aromatic primary alcohols, forma-
tion of small amounts of benzoic and 4-chlorobenzoic
acid was observed (table 2, entries 8 and 9).
On the other hand, Py-PMo V was employed in
aliphatic alcohol oxidations, using the above condi-
tions. For secondary alcohols such as 2-octanol and
1
5. Manyar H G, Chaure G S and Kumar A 2006 J. Mol.
Catal. A: Chem. 243 244
11
6
7
8
9
. Venturello C and Gambero M 1991 J. Org. Chem. 56
5924
. Zhao W, Zhang Y, Ma B, Ding Y and Qiu W 2010 Catal.
Commun. 11 527
. Ma B, Zhang Y, Ding Y and Zhao W 2010 Catal.
Commun. 11 853
2-decanol, conversion of 20% was reached after 2 h.
Corresponding ketones were obtained with 95% of
selectivity. Furthermore, a lower conversion was
achieved for 1-decanol (4% after 2 h); in this case,
decanal was obtained with 100% of selectivity.
. Lei Z and Wang R 2008 Catal. Commun. 9 740
1
1
1
1
0. Hosseini-Monfared H, Näther C, Winkler H and
Janiak C 2012 Inorg. Chim. Acta 391 75
1. Karthikeyan P, Aswar S A, Muskawar P N, Bhagat P R
and Kumar S S 2012 Catal. Commun. 26 189
2. Maity P, Gopinath C S, Bhaduri S and Lahiri G K 2009
Green Chem. 11 554
3. Zolfigol M A, Khazaei A, Safaiee M, Mokhlesi M,
Rostamian R, Bagheri M, Shiri M and Kruger H G 2013
J. Mol. Catal. A: Chem. 370 80
4. Sivaranjani K, Verma A and Gopinath C S 2012 Green
Chem. 14 461
5. Park D, Park S, Bang Y and Song I 2010 Appl. Catal.
A373 201
4
. Conclusion
Vanadium and pyridine-modified molybdovanadophos-
phate catalysts were prepared for selective oxidation of
alcohols by aqueous hydrogen peroxide.
Substituted vanadium atoms in heteropolyacids are
essentially active sites with high performance for
1
1
selective oxidation of 1-phenylethanol to acetophe- 16. Romanelli G and Autino J 2009 Mini-Rev. Org. Chem.
6
359
none. Pyridine hybrid catalysts can even promote cata-
lytic activity due to electronic interaction between
heteropolyacid and pyridine. Py-PMo V exhibits
the highest activity for selective oxidation of 1-
phenylethanol to acetophenone with 93% of conversion
and 96% of selectivity.
1
7. Bennardi D, Romanelli G, Autino J, Pizzio L, Vázquez
P, Cáceres C and Blanco M 2010 React. Kinet. Mech.
Catal. 100 165
8. Romanelli G, Autino J C, Vázquez P, Pizzio L, Blanco
M and Cáceres C 2009 Appl. Catal. A352 208
11
1
19. Villabrille P, Romanelli G, Quaranta N and Vázquez P
010 Appl. Catal. B96 379
2
Oxidation of other alcohols was then investigated
using Py-PMo V catalyst. For benzylic secondary
2
0. Sambeth J E, Romanelli G, Autino J C, Thomas H J and
Baronetti G T 2010 Appl. Catal. A378 114
1. Tundo P, Romanelli G P, Vázquez P G and Aricó F 2010
Catal. Commun. 11 1181
11
alcohols, conversion and selectivity to the correspond-
ing ketone were very high. No appreciable amounts of
2

Heteropolycompounds as catalysts for alcohol oxidation
1383
2
2
2
2
2. Tundo P, Romanelli G P, Vázquez P G, Loris A and 28. El Ali B, El-Ghanam A M and Fettouhi M 2001 J. Mol.
Aricó F 2008 Synlett 967 Catal. A: Chem. 165 283
3. Villabrille P, Romanelli G, Vázquez P and Cáceres C 29. Seki Y, Mizuno K and Misono M 2000 Appl. Catal.
008 Appl. Catal. A334 374 A194 13
4. Romanelli G, Villabrille P, Cáceres C, Vázquez P and 30. Leng Y, Ge H, Zhou C and Wang J 2008 Chem. Eng. J.
Tundo P 2011 Catal. Commun. 12 726 145 335
2
5. Rao P S N, Venkateswara Rao K T, Sai Prasad P S 31. Cid R and Pecchi G 1985 Appl. Catal. 14 15
and Lingaiah N 2010 Catal. Commun. 11 547 (and the 32. Zhang J, Tang Y, Li G and Hu C 2005 Appl. Catal. A278
references cited herein)
6. Villabrille P, Romanelli G, Vázquez P and Cáceres C 33. Eguchi K, Seiyama T, Yamazoe N, Katsuki S and Taketa
004 Appl. Catal. A270 101 H 1988 J. Catal. 111 336
7. Kende A S and Ebetino F H 1984 Tetrahedron Lett. 25 34. Seki Y, Min J S, Misono M and Mizuno N 2000 J. Phys.
23 Chem. B104 5940
251
2
2
2
9