Oxidation of alcohols with molecular oxygen promoted by Nafion ionomer anchored pyrochlore composite at room temperature

Article abstract of DOI:10.1016/j.tetlet.2008.05.046

Nafion ionomer anchored ruthenium oxide pyrochlore composite has been demonstrated for selective oxidation of alcohols to aldehydes and ketones in good yields. In the absence of any additives, the reaction was achieved by atmospheric air or molecular oxygen at room temperature.

Full text of DOI:10.1016/j.tetlet.2008.05.046

Tetrahedron Letters 49 (2008) 4339–4341
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Oxidation of alcohols with molecular oxygen promoted by Nafion ionomer
anchored pyrochlore composite at room temperature
S. Venkatesan a, A. Senthil Kumar b, J.-M. Zen a,
*
a Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan
b Department of Chemistry, Vellore Institute of Technology University, Vellore 632 014, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 28 February 2008
Revised 2 May 2008
Accepted 9 May 2008
Available online 13 May 2008
Nafion ionomer anchored ruthenium oxide pyrochlore composite has been demonstrated for selective
oxidation of alcohols to aldehydes and ketones in good yields. In the absence of any additives, the reac-
tion was achieved by atmospheric air or molecular oxygen at room temperature.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Nafion
Catalyst
Lead ruthenate pyrochlore
Alcohols
Oxidation
Selective oxidation of alcohols to aldehydes and ketones is a key
reaction in organic synthesis. These transformations are of signifi-
cant importance, both for fundamental research and industrial
manufacturing.1 The development of new procedures that can
use air or molecular oxygen as oxidant is certainly more attractive
than other traditional methods which are environmentally unde-
sirable.2 In this point of view, much of research recently described
the use of solid supported heterogeneous catalyst notably ruthe-
nium-based catalysts for the aerobic oxidation of alcohols.3 Among
these, tetra-n-propyl ammonium perruthenate (TPAP)–O2
catalyzed systems were highly active for the preparation of wide
range of carbonyl compounds under mild conditions.4 However,
it requires high temperature to regenerate the active site (i.e.,
RuO2 to TPAP) and suffers from catalytic deactivation as well as
oxidative degradation of polymers. Meanwhile, slow addition of
H2O2 on organically modified silica gels doped TPAP has been suc-
cessfully worked out for the selective oxidation reactions at room
temperature with good yields.5 Herein, first time we report ruthe-
nium oxide pyrochlore (Pyc) modified Nafion membrane catalytic
system (jNPycj), which has chemically generated perruthenate/
ruthenate active site similar to the TPAP active site for aerobic
alcohol oxidation at room temperature (rt).
oligomerization, and isomerization,6 whereas catalytic active site
modified membrane is rarely developed for synthetic applications.
Recently, we developed the Pyc active site-modified Nafion
membrane, jNPycj catalyst for some organic oxidations at elevated
temperature with conventional co-oxidants.7 For instance, the
membrane catalyst was effective for selective benzyl alcohol oxi-
dation to benzaldehyde with H2O2 or NaOCl co-oxidant at ꢀ80 °C
in a biphasic medium (CH2Cl2//pH 11).7b,c In this Letter, we suc-
cessfully demonstrate the jNPycj catalyst for selective aerobic alco-
hol oxidations at rt using CH3COCH3 as a solvent system.
Initially, oxidation of benzyl alcohol was monitored (Scheme 1)
by using an in situ attenuated total reflection infrared (ATR-IR)
spectroscopy as described elsewhere.8 The reaction was checked
at rt during contact of the benzyl alcohol adsorbed Nafion compos-
ite membrane on ZnSe crystal under oxygen atmosphere.9 The time
dependence of the intensity of the C@O signal at 1747 cmꢁ1
O
HO
H
| NPyc |
O2, Acteone, RT
Nafion, a perfluorinated sulfonate ionomeric membrane has
many practical applications in catalysis. For instance, it is used as
a solid-acid catalyst for alkylation, esterification, acylation, olefin
R1
R2
R1
R2
(1b)
(1a)
R
R
1 = aryl, allyl, alkyl
2 = H, alkyl
* Corresponding author. Tel.: +886 4 22850864; fax: +886 4 22854007.
E-mail address: jmzen@dragon.nchu.edu.tw (J.-M. Zen).
Scheme 1.
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.05.046

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S. Venkatesan et al. / Tetrahedron Letters 49 (2008) 4339–4341
ble and can be reused for a series of alcohols oxidation regardless
of the substrate.10 The results are summarized in Table 1. Alcohols
studied in this work were all selectively oxidized to aldehydes (i.e.,
primary alcohols) or ketones (i.e., secondary alcohols) in good
yields. Aromatic alcohols were converted into corresponding alde-
hydes or ketones in high yields since they have electron-donating
groups (entries 2–7, 17, 18 and 21).
Similarly, allylic alcohols were oxidized to their respective alde-
hydes without any cleavage of carbon–carbon double bond (entries
8 and 9). This system was also applicable to heterocyclic alcohols
having sulfur and nitrogen atom (entries 10 and 11). Aliphatic
and cyclic alcohols (cyclohexanol and menthol) were oxidized in
moderate yields without any over-oxidation to carboxylic acids
(entries 12–15). The jNPycj catalyst also shows appreciable chem-
ioselectivity. For example, catalysis of equimolar concentration of
benzyl alcohol and 1-phenyl ethanol mixture resulted correspond-
ing benzaldehyde and acetophenone yields of 85% and 15%, respec-
tively, at room temperature (5 h run). The catalytic results of
benzylalcohol oxidation between the first run and last run as in
the entries 1 and 22 (Table 1) clearly indicate that there is no dif-
ference in either reactivity and selectivity of the system. The cata-
lytic performance of the jNPycj in rt is reasonably closer to the
TPAP encapsulated ORMOSIL systems at elevated temperature.
Nevertheless, reusable ability of the membrane catalyst is remark-
able over the existing catalysts. Regardless of run time and sub-
strate, single piece of jNPycj could be recycled for 22 times
(Table 1) without loss in the active site, while the TPAP-ORMOSIL
catalyst lost ꢀ40% of its active sites within five times of recyclic
runs evidence enhanced performance of the new membrane
catalyst.5
Figure 1. ATR-IR spectra of benzyl alcohol oxidation at two different intervals of (A)
1 h and (B) 9 h.
observed (Fig. 1) together with those signals at 3250 cmꢁ1 (OH)
and 2917–2848 cmꢁ1 (C–H), was all measured to ensure jNPycj
reactive template/air interphase assisted alcohol oxidation reac-
tion. In addition, the reaction mixture was isolated from catalyst
and analyzed by GC and GC–MS to confirm its selectivity. The sys-
tem showed only two-peak responses in GC with retention timings
of 5 and 10 min, respectively, corresponding to 1a and 1b (Scheme
1), where 1a ꢂ 1b due to the jNPycj/air interphase assisted cata-
lytic conversion. These results indicate that ionic cluster network
of Nafion is a convenient template for both stable pyrochlore for-
mation and oxygen adsorption. Nafion pyrochlore composite cata-
lyst can incorporate higher concentration of alcohols in the active
hydrophilic site followed by oxidation of alcohols in the presence
of air at room temperature. Figure 2 shows variation in run time,
reactant and catalyst amount against yield for alcohol 1a-adsorbed
oxidation reaction at room temperature. As can be seen, the in-
crease of run time and catalyst amount leads to the increase of
the reaction up to 99% yield of 1b (Fig. 2A and B). On the other
hand, the increase of the concentration (1a) on a fixed surface leads
to a decrease in reaction yield (Fig. 2C). This may reflect that some
kinetic restriction for the alcohol adsorption on the composite sur-
face and hence less catalyst/air interface area available for the alco-
hol oxidation. Accordingly, batch reactions were carried out in
acetone in the presence of molecular oxygen or argon. It was found
that molecular oxygen-assisted reaction was more efficient than
air oxidation reaction (50% conversion), and no marked yield was
observed in an argon atmosphere which clearly indicates the
specific role of oxygen in catalytic reaction. Recycled catalyst is sta-
Possible mechanism for the selective catalytic alcohol oxidation
is due to high-valent oxo-ruthenium active site within the jNPycj
(Pyc–RuO4ꢁ, perruthenate active site).7b,c In the recycling process
Table 1
Oxidation of alcohols to aldehydes and ketones with the membrane catalysta
Entry Substrate
Time Yieldb Sel.c
Membrane
recycle
(h)
(%)
(%)
numberd
1
2
3
4
5
6
7
Benzyl alcohol
5
4
99
98
95
91
98
96
95
>99
>99
>99
>99
>99
>99
>99
1
2
3
4
5
6
7
p-Cl Benzyl alcohol
2-Br Benzyl alcohol
p-Methyl benzyl alcohol
p-Methoxybenzyl alcohol
2-Methoxybenzyl alcohol
2,3,4-Trimethoxbenzyl
alcohol
13.5
2.5
3.5
4.5
7.5
8
9
trans-Cinnamyl alcohol
Geraniol
2-Thiophenemethanol
2-Pyridinemethanol
n-Octyl alcohol
n-Decyl alcohol
Cyclohexanol
Norborneol
Benzyl alcohol
1-Phenyl ethanol
1-(4-Methoxyphenyl) ethanol
1-(4-Nitrophenyl) ethanol
1-(4-tert-Butyl phenyl)
ethanol
6
8.5
3
88
96
90
98
38
36
40
98
99
90
95
58
40
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
8
9
10
11
12
13
14
15
16
17
18
19
20
10
11
12
13
14
15
16
17
18
19
20
6
15
15
16
12
5
13
9.5
16
15
21
22
Diphenyl methanol
Benzyl alcohol
15
5
88
99
>99
>99
21
22
a
Reaction conditions: 0.05 mmol alcohol, small pieces of membrane composite
catalyst (250 mg, 0.007 mmol ruthenium species) in acetone (1.5 mL) was stirred
under oxygen atmosphere at room temperature.
b
Determined by GC analysis.
Selectivity (Sel.) calculated by GC and NMR.
Single run reaction regardless of substrate. Other working conditions are the
c
Figure 2. The influence of various parameters: (A) reaction time, (B) catalyst am-
ount, (C) alcohol concentration versus yield on the membrane catalyzed alcohol
oxidation reaction at rt.
d
same as in this table.

S. Venkatesan et al. / Tetrahedron Letters 49 (2008) 4339–4341
4341
ꢁ
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2ꢁ
In conclusion, a Nafion polymer anchored ruthenium oxide
pyrochlore composite has been utilized as a reactive template for
selective alcohol oxidation with molecular oxygen at room temper-
ature. A series of alcohols were cleanly oxidized to corresponding
aldehydes and ketones in good yields without additives. The cata-
lyst can be recovered and reused several times without any loss of
activity.
M.; Aquino, F.; Bonrath, W. Appl. Catal.
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Hoelderich, W. F. Chem. Commun. 2002, 16, 1684.
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Acknowledgments
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9. IR spectroscopy measurements for alcohol oxidation on pyrochlore composite
were performed by using FTIR spectrometer (Jasco FT/IR–460 PLUS) equipped
with ATR cell. The composite membrane was cut into rectangle (5 cm ꢃ 1.5 cm)
and immersed in the solution of alcohol in acetone (0.03 mM) for 5–10 min.
Then, the alcohol adsorbed membrane was placed on the (ZnSe) ATR crystal
and kept into the cell at room temperature. The spectrum was collected
using mercury–cadmium–telluride detector with 32 scans per spectrum at a
The authors gratefully acknowledge financial support from
National Science Council of Taiwan. This work was supported in
part by the Ministry of Education, Taiwan under the ATU plan.
References and notes
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.
10. Alcohol oxidation reaction was typically carried out as follows: A mixture of
benzyl alcohol (5 mg, 0.05 mmol) and several pieces of the membrane catalyst
(250 mg, 0.007 mmol Ru species) in acetone (1.5 mL) was stirred under oxygen
atmosphere (1 atm, O2 ballone) at room temperature. After 5 h, the yield of
benzaldehyde (99%) was determined by GC (China chromatography 9800,
Taiwan, stainless steel column consists of 10% SP-2100 on a Chromosorb W HP
(80/100 mesh) support) using commercially available authentic samples. After
separation, the composite catalyst was sequentially washed with an aqueous
solution of NaOH (1.0 M), deionized water, acetone, and dried in vacuum prior
to recycling.
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