Single bilayered organic nanotubes: Anchors for production of a reusable catalyst with nickel ions

Article abstract of DOI:10.1039/c1gc00005e

Organic nanotubes self-assembled from lipid compounds facilitate the synthesis of a new nano-catalyst with nickel ions. The nickel ions were fixed on the surface of the organic nanotubes through coordination. The nanotubes catalyzed oxidation of a wide range of organic compounds with hydrogen peroxide without organic solvent, and are reusable in at least five cycles without loss of activity.

Full text of DOI:10.1039/c1gc00005e

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Green Chemistry
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COMMUNICATION
Single bilayered organic nanotubes: anchors for production of a reusable
catalyst with nickel ions†
a,b
b
b
c
b
Tanmay Chattopadhyay, Masaki Kogiso, Masaru Aoyagi,* Hiroharu Yui, Masumi Asakawa and
Toshimi Shimizu
b
Received 2nd January 2011, Accepted 14th February 2011
DOI: 10.1039/c1gc00005e
Organic nanotubes self-assembled from lipid compounds
facilitate the synthesis of a new nano-catalyst with nickel
ions. The nickel ions were fixed on the surface of the
organic nanotubes through coordination. The nanotubes
catalyzed oxidation of a wide range of organic compounds
with hydrogen peroxide without organic solvent, and are
reusable in at least five cycles without loss of activity.
To diminish these difficulties with nano-catalysts, we have
utilized organic nanotubes (ONTs) with well-defined morpholo-
gies and dimensions, which were self-assembled from lipid
compounds. These lipids were synthesized from low-cost natural
16–20
materials under mild conditions.
The C-terminated peptide
lipids are interesting due to the presence of carboxylate groups
21,22
that can coordinate to metal ions.
lipids were treated with metal ions, the metal-coordinated ONTs
M-ONTs) self-assembled, making the ONTs potential solid
Indeed, when the peptide
(
‘
Green chemistry’ is an issue of current interest for chemical
1–4
supports for metal ion catalysts. The metal ions are present
at the surface of the M-ONTs, and are easily accessible during
the reaction. The M-ONTs have outer diameters of 30–100 nm
and are micrometre-scale in length. This high axial-ratio tubular
structure has a large surface area and good separability from
reaction media by filtration.
synthesis in both academia and industry. Although many
existing oxidation processes are acceptable from with regard to
the products generated, production of waste and unsuccessful
5,6
recovery of solvents are harmful for the environment. Thus,
the development of an environmentally friendly oxidation
technology is desired. Usually, the product yields in homogenous
catalytic reactions are very high because all the catalytic active
sites are accessible, but the difficulty in separation of the catalyst
andreactionproducts limits their scope. Heterogeneous catalysts
We have already reported the catalytic activity of Cu-ONTs
23
for the oxidation of various organic compounds. The wall of
the Cu-ONTs comprises 4–5 lipid-metal coordination bilayers.
Therefore, only 10–20% of the copper ions are on the inner and
outer surfaces of the nanotube, and thus able to participate in
the catalytic cycle. Some of the Cu-ONT-catalyzed oxidation
7–9
appear to be a solution to this issue, but the reactivity of these
catalytic systems is generally lower than that of homogenous
catalysts due to the lower availability of active sites during
the catalytic reactions. Catalysts based on nanoparticles show
promise as a ‘bridge’ between homogenous and heterogeneous
catalysts, because they should combine the advantages of
reactions did not proceed using H
hydroperoxide (TBHP) as an oxidant. This was unfortunate
because H is a particularly attractive oxidant, because water
is the only side product of oxidation.
2
O
2
, and required tert-butyl
2
O
2
24–27
10–14
M-ONTs catalysts
both.
The high surface area of these nano-catalysts dramat-
made from single-bilayer membranes are expected to show
higher catalytic activity because all active sites should be
available during the reactions. In this work, we report that
self-assembly of nickel ions and a peptide lipid gave Ni-ONTs
comprising a single bilayer (Scheme 1), meaning that all the
nickel ions can participate in the catalytic cycle. Nickel ions are
ically enhances the contact between reactants and the catalysts,
whereas their insolubility in reaction media facilitates separation
from the reaction mixture. However, unsupported nanoparticles
are frequently unstable, and coagulation during the reactions is
15
often unavoidable.
28–31
known to catalyze organic oxidation reactions.
Therefore,
a
this method, using single-bilayer Ni-ONTs and H O without
organic solvents or heating, is a simple, effective and practical
oxidation method.
TEM observation of the Ni-ONTs showed that their average
outer and inner diameters were 30 and 20 nm, respectively
(Fig. 1a, inset). Therefore, the thickness of the membrane was
estimated to be ca. 5 nm. X-ray diffraction (XRD) indicated
that the Ni-ONTs were composed of a lipid-metal bilayer
membrane with a d-spacing of 4.01 nm (Fig. S1†). The similarity
of the membrane thicknesses obtained from TEM and XRD
Department of Chemistry, Panchakot Mahavidyalya, Sarbari, Neturia,
Purulia, 723 121, India
2
2
b
Nanotube Research Centre (NTRC), National Institute of Advanced
Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi,
Tsukuba, Ibaraki, 305-8565, Japan. E-mail: masaru-aoyagi@aist.go.jp;
Fax: + 81 29-861-4676; Tel: + 81 29-861-4676
c
Department of Chemistry, Faculty of Science, Tokyo University of
Science (TUS), Funagawara-machi 12, Ichigaya, Shinjuku-ku, Tokyo,
1
62-0826, Japan
†
Electronic supplementary information (ESI) available: UV-Vis,
FTIR and XRD spectra, and catalyst reuse properties. See DOI:
0.1039/c1gc00005e
1
1
138 | Green Chem., 2011, 13, 1138–1140
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Table 1 Oxidation of various organic substrates with H
2
O
2
, catalyzed by Ni-ONTs
c
c
d
-1
Substrates
Products
Conversion (%)
Selectivity (%)
TOF (s )
a
-2
Benzyl alcohol
Benzaldehyde
Acetophenone
1-Octanal
2-Octanone
Cinnamaldehyde
Cinnamic acid
TMQ
58
57
48
60
41
34
57
52
59
35
40
89
99
86
99
90
84
91
99
99
87
88
14.3 ¥ 10
a
-2
1
1
2
-Phenylethanol
15.7 ¥ 10
a
-2
-Octanola
11.5 ¥ 10
-2
-Octanol
16.5 ¥ 10
b
a
-2
Cinnamyl alcohol
Cinnamaldehyde
10.3 ¥ 10
b
-2
7.9 ¥ 10
a
-2
TMP
14.4 ¥ 10
a
-2
Tetraline
Tetralone
14.3 ¥ 10
-2
Diphenylmethane
Benzophenone
a-Pinene oxide
Styrene oxide
16.2 ¥ 10
a
-2
a-Pinene
8.5 ¥ 10
9.8 ¥ 10
a
-2
Styrene
a
◦
b
Reaction conditions: catalyst (0.02 mmol), substrate (2 mmol), 30% H
2
O
2
c
(2 mmol), stirring for 5 h at 25 C; Reaction conditions: catalyst (0.02
◦
mmol), substrate (2 mmol), 10% H
as an internal standard. TOF = Turnover number (TON)/s after 1 h.
2
O
2
(2 mmol), stirring for 5 h at 25 C. Determined by GC analysis on the basis of the substrate, with biphenyl
d
unsaturated aldehyde, a phenol, alkyl benzenes, a terpene and
an alkene. No reactions take place without the catalyst.
In these reactions, the amount and concentration of H
2
O
2
were crucial. One equivalent of 30% H solution was necessary
2
O
2
for the oxidation of alcohols, phenol, alkyl benzenes, the terpene
and the alkene, whereas selective oxidation of a,b-unsaturated
alcohols and aldehydes required one equivalent of 10% H
2
O
2
solution. In contrast, use of a large excess of H
many by-products.
2
2
O produced
Primary and secondary alcohols, such as benzyl alcohol, 1-
octanol, 1-phenylethanol and 2-octanol, were oxidized to the
corresponding aldehydes or ketones in 48–60% conversion with
high selectivity. a,b-Unsaturated alcohols and aldehydes such
as cinnamyl alcohol and cinnamaldehyde have two oxidizable
groups (olefin and hydroxyl or aldehyde). Nevertheless, cinnamyl
alcohol was oxidized to cinnamaldehyde in 57% conversion
with 90% selectivity, and cinnamaldehyde was oxidized to
cinnamic acid in 34% conversion with 84% selectivity. 2,3,6-
Trimethylphenol (TMP) was examined for phenol oxidation,
as it was expected that this would be oxidized to trimethyl-
Scheme 1 Self-assembly of Ni-ONTs from the peptide lipid.
1,4-benzoquinone (TMQ), which is a key intermediate for the
production of vitamin E. TMP was oxidized to TMQ in 57%
conversion with 91% selectivity. Alkylbenzenes, tetralin and
diphenylmethane were oxidized to the corresponding ketones
in 52 and 59% conversion respectively. Alkenes, such as styrene
and a-pinene, were oxidized to the corresponding epoxides in
3
5 and 40% conversion. All the results are shown in Table 1.
Cu-ONTs required an organic solvent, and high temperature
◦
(
60 C) was necessary for oxidation of the substrates. Moreover,
decomposition of hydrogen peroxide (via formation of a catalyt-
ically inactive peroxo bridge in the Cu-ONTs complex) reduced
the efficiency of the oxidation reactions. In contrast, the TOFs
of the oxidation reactions with Ni-ONTs at room temperature
were more than 120 times greater than those with Cu-ONTs
Fig. 1 The morphology of Ni-ONTs (a) before and (b) after oxidation
reactions. The inset in (a) is a phase-contrast TEM image.
confirmed that the tubular membrane of the Ni-ONTs com-
prised a single bilayer, and thus that all the nickel cations were
located on the nanotube surfaces.
◦
23
at 60 C (Table S1†). Furthermore, the oxidation of TMP,
tetralin, and diphenylmethane by H proceeded with Ni-
2
O
2
Catalyzed reactions with the Ni-ONTs were performed by
ONTs, but not with Cu-ONTs. No color change or bubbling were
observed during the Ni-ONTs-catalyzed reactions, in contrast to
mixing of substrates, aqueous solutions of H
2
2
O and the Ni-
13
ONTs in 100 : 100 : 1 molar ratio and stirring for 5 h at room
temperature. From the results, it was clear that the Ni-ONTs
were an efficient and reusable oxidation catalyst for primary
and secondary alcohols, an a,b-unsaturated alcohol, an a,b-
the Cu-ONTs-catalyzed reactions. According to the literature,
the catalytic activity between Ni and Cu is not that different
32–34
when the ligands are same.
Here, the difference between Ni-
ONTs and Cu-ONTs are the metal ions and the number of layers.
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Therefore, we propose that the number of the layers were impor-
tant to diminish the peroxo-complex formation. Thus, Ni-ONTs
the cavities of the catalyst before sampling. Then the Ni-ONTs
were washed with water several times and dried under vacuum.
In each case, we followed the catalytic experimental methods
stated above.
favored the catalytic oxidation reactions in the presence of H
2
2
O .
The chemical structure and morphology of the Ni-ONTs
before and after the reactions were checked by UV-Vis and
Fourier-transform infrared (FT-IR) spectroscopy, and field
emission scanning electron microscopy (FE-SEM). The UV-
Vis study demonstrated that no absorption band assignable
to the nickel ion was observed in the reaction medium (Fig.
S3†), showing that there was no leaching of the Ni ions on the
nanotubes into the medium. The FT-IR spectrum of the Ni-
Acknowledgements
This work was financed by JST-SORST and partly financed by
JSPS-KAKENHI 21710085.
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140 | Green Chem., 2011, 13, 1138–1140
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