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Communication
We have successfully combined the concept of carbon
nanotube nanoreactors with supercritical continuous flow tech-
nology, which demonstrate the potential for the utilisation of
fixed bed carbon nanoreactors. This is the first example of
catalysis within SWNT, which not only provide precise control
of catalyst size but also preventing sintering of the RuNPs
leading to enhanced stability. Due to the extreme spatial
Fig. 3 (a) TEM image of two nanotubes of the RuNPs@SWNT catalyst after
exposure to C60. (b) Structural diagram showing the positions of the C60 confinement the RuNP@SWNT catalyst showed a lower TOF
molecules (grey) and the RuNP (blue) inside the carbon nanotubes.
for the reduction of cyclic alkenes in comparison to a Ru/C
catalyst, but no drop in activity or change in structure of the
RuNPs embedded in SWNT observed over 24 hours at 110 1C.
The development of a methodology to utilise nanoreactors
of dimensions commensurate with molecular reactants and
products provides the potential for the formation of new
species which are impossible to form without this unique
reaction environment.
The extreme confinement imposed by the narrow nanotubes
efficiently stabilises the nanoparticle catalyst and also provides a
unique reaction environment. Interestingly, the activity of the
confined catalyst is reduced as compared to that of the traditionally
used Ru/C catalyst due to the spatial restriction in SWNT nano-
reactor: the turnover frequency (TOF, number of product mole-
cules formed per active Ru atom, see ESI† for details) of the
RuNPs@SWNT and Ru/C (20 mg of the commercially available
catalyst, 5% by wt Ru) catalysts for the hydrogenation of cyclooctene
2
We thank Prof. M. Poliakoff for access to the scCO equip-
ment and scientific support. This work was supported by the
European Research Council (ERC), Johnson Matthey and the
Engineering and Physical Sciences Research Council (EPSRC).
We also thank M. Dellar, M. Guyler, D. Litchfield, R. Wilson
and P. Fields for support.
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at 50 1C were measured to be 32 min and 103 min respectively.
The reduction in catalyst activity in nanoreactors is less severe than
expected under the conditions of extreme spatial confinement in
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RuNPs@SWNT due to the fact that scCO is an ideal solvent able to
access the catalyst in SWNT narrow channel.
Notes and references
In addition to olefins, the catalyst also exhibited good activity
towards carbonyl hydrogenation, RuNPs@SWNT successfully
reduced butyraldehyde to butanol (ca. 50% conversion at 200 1C,
see Fig. 2b). It was also possible to selectively reduce cinnamalde-
hyde to hydrocinnamaldehyde (ca. 70% conversion, >99%
selectivity, Fig. 2c). Interestingly at 60 1C, the major product
was cinnamyl alcohol, but at relatively low conversion (o6%). In the
case of cinnamaldehyde no doubly reduced products were observed
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7
8
A. Roucoux, J. Schulz and H. Patin, Chem. Rev., 2002, 102,
757–3778.
K. Okitsu, A. Yue, S. Tanabe and H. Matsumoto, Chem. Mater., 2000,
12, 3006–3011.
I. Lee, M. A. Albiter, Q. Zhang, J. P. Ge, Y. D. Yin and F. Zaera, Phys.
Chem. Chem. Phys., 2011, 13, 2449–2456.
D. K. Mishra, A. A. Dabbawala and J. S. Hwang, J. Mol. Catal. A:
Chem., 2013, 376, 63–70.
3
N. A. Dhas, H. Cohen and A. Gedanken, J. Phys. Chem. B, 1997, 101,
6834–6838.
(i.e. phenylpropanol). In both cases the reduction proceeded
B. S. Kong, J. X. Geng and H. T. Jung, Chem. Commun., 2009,
2174–2176.
G. M. Scheuermann, L. Rumi, P. Steurer, W. Bannwarth and
R. Mulhaupt, J. Am. Chem. Soc., 2009, 131, 8262–8270.
A. N. Khlobystov, ACS Nano, 2011, 5, 9306–9312.
smoothly with negligible reduction in yields when held at the
upper reduction temperature of 200 1C.
To illustrate that the size and shape of the RuNPs themselves do
not impede reactants from being able to reach a portion of the
RuNPs within these structures and to confirm that hydrogenation
takes place solely inside the nanoreactors, the RuNPs@SWNT
9 J. P. O’Byrne, R. E. Owen, D. R. Minett, S. I. Pascu, P. K. Plucinski,
M. D. Jones and D. Mattia, Catal. Sci. Technol., 2013, 3, 1202–1207.
0 X. L. Pan and X. H. Bao, Acc. Chem. Res., 2011, 44, 553–562.
1 X. L. Pan and X. H. Bao, Chem. Commun., 2008, 6271–6281.
1
1
catalyst was exposed to C60 vapour. TEM was subsequently utilised 12 P. Serp and E. Castillejos, ChemCatChem, 2010, 2, 41–47.
1
1
3 T. W. Chamberlain, T. Zoberbier, J. Biskupek, A. Botos, U. Kaiser
and A. N. Khlobystov, Chem. Sci., 2012, 3, 1919–1924.
4 E. Castillejos, P. J. Debouttiere, L. Roiban, A. Solhy, V. Martinez,
Y. Kihn, O. Ersen, K. Philippot, B. Chaudret and P. Serp, Angew.
Chem., Int. Ed., 2009, 48, 2529–2533.
to confirm the encapsulation of the fullerene molecules and
the extent to which they could penetrate the internal channels of
the catalyst (Fig. 3). As the 1 nm sized C60 cages fit snuggly into the
nanotube channel they prevent access of any substrate molecules to
15 A. N. Khlobystov, D. A. Britz, J. W. Wang, S. A. O’Neil, M. Poliakoff
the internal RuNPs. In a control experiment the resultant (C60
+
and G. A. D. Briggs, J. Mater. Chem., 2004, 14, 2852–2857.
RuNPs)@SWNT material was used in a test hydrogenation reaction 16 W. Leitner, Acc. Chem. Res., 2002, 35, 746–756.
1
1
1
2
7 R. A. Bourne, J. G. Stevens, J. Ke and M. Poliakoff, Chem. Commun.,
of cyclooctene and showed no activity confirming that the catalysis
occurs solely via RuNPs located inside the nanotubes. The fact
that the C60 molecules can penetrate through to RuNPs within
the nanotube (Fig. 3a) visually demonstrates the accessibility of
the metal centres in RuNPs@SWNT to organic reactants during
2007, 4632–4634.
8 J. G. Stevens, R. A. Bourne, M. V. Twigg and M. Poliakoff, Angew.
Chem., Int. Ed., 2010, 49, 8856–8859.
9 A. C. Frisch, P. B. Webb, G. Zhao, M. J. Muldoon and D. J. Cole-
Hamilton, Dalton Trans., 2007, 5531–5538.
0 P. B. Webb, T. E. Kunene and D. J. Cole-Hamilton, Green Chem.,
2005, 7, 373–379.
2
preparative hydrogenation in a flow of scCO .
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