Catalytic nanoreactors in continuous flow: Hydrogenation inside single-walled carbon nanotubes using supercritical CO2

Article abstract of DOI:10.1039/c3cc49247h

One nanometre wide carbon nanoreactors are utilised as the reaction vessel for catalytic chemical reactions on a preparative scale. Sub-nanometre ruthenium catalytic particles which are encapsulated solely within single-walled carbon nanotubes offering a unique reaction environment are shown to be active when embedded in a supercritical CO₂ continuous flow reactor. A range of hydrogenation reactions were tested and the catalyst displayed excellent stability over extended reaction times. the Partner Organisations 2014.

Full text of DOI:10.1039/c3cc49247h

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Catalytic nanoreactors in continuous flow:
hydrogenation inside single-walled carbon
nanotubes using supercritical CO2†
Cite this: Chem. Commun., 2014,
0, 5200
5
Received 5th December 2013,
Accepted 24th January 2014
a
a
a
Thomas W. Chamberlain,* James H. Earley, Daniel P. Anderson,
ab
c
Andrei N. Khlobystov and Richard A. Bourne*
DOI: 10.1039/c3cc49247h
www.rsc.org/chemcomm
One nanometre wide carbon nanoreactors are utilised as the reaction is the scaling down of the nanoreactor channel by one or two
vessel for catalytic chemical reactions on a preparative scale. orders of magnitude, from 10–100 nm in the case of MWNT, to
Sub-nanometre ruthenium catalytic particles which are encapsulated a size closer to that of the reactant molecules. This decrease in
solely within single-walled carbon nanotubes offering a unique reactor size to generate a more extremely confined reaction
reaction environment are shown to be active when embedded in a environment is predicted to enhance the effect that the carbon
supercritical CO
2
continuous flow reactor. A range of hydrogenation nanoreactor sidewalls have on chemical reaction pathways.
reactions were tested and the catalyst displayed excellent stability Additionally such extreme confinement should improve the
over extended reaction times.
stability of the catalyst: a narrower nanoreactor should prevent
sintering and leaching further, thus improving the reactivity
Nanoparticles of transition metals (MNPs) have been demonstrated and recyclability of the catalytic MNPs.
to be excellent candidates for catalysis due to their remarkable
chemical and physical properties. As nanoparticles are intrinsically as the ideal nanoreactor, possessing a diameter of 1–2 nm which
thermodynamically metastable the stabilisation of MNPs is vitally is commensurate in size to typical organic reactants. The
Single-walled carbon nanotubes (SWNTs) have been proposed
1
8
important to prevent leaching and sintering of the metal during diameter of SWNTs (crystallographic diameter 1.5 nm and van
catalysis. Immobilising the active nanoparticles on solid supports der Waals internal diameter B1 nm) is large enough that typical
2
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5
such as alumina, silica, zeolites, amorphous carbon and reactant molecules (van der Waals diameter B0.4–0.8 nm) can
graphene
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,7
and utilising the resultant materials as hetero- fit inside the nanotube but narrow enough that they will be
geneous catalysts is an effective method to tackle these problems. influenced by the proximity of the nanotube sidewalls (see ESI†).
Recently, carbon nanoreactors in which MNPs are located on the However significant practical transport issues exist in accessing
inside and/or the outside of multi-walled carbon nanotubes such narrow nanotube channels which have prevented SWNTs
8–12
(MWNTs) have been proposed as catalyst support materials.
from being utilised in catalytic reactions to date.
The cylindrical shape and high chemical and thermal stabilities
In this Communication, we introduced the Ru metal into the
of carbon nanotubes make them ideal catalyst supports and carbon nanotubes in the gas phase using a volatile Ru
3
(CO)12
nanoreactors, stabilising the metal nanoparticles and providing precursor following the procedure described to produce W, Re
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13
a unique local environment leading to new products. One signifi- and OsNPs inside SWNTs. Thermal treatment leads to the
cant challenge facing the development of carbon nanoreactors decomposition of the ruthenium carbonyl to generate NPs whose
size are controlled by the nanotube diameter. CO gas generated by
a
School of Chemistry, The University of Nottingham, University Park, Nottingham,
NG7 2RD, UK. E-mail: thomas.chamberlain@nottingham.ac.uk;
Fax: +44 (0)115 9513563; Tel: +44 (0)115 9513234
carbonyl decomposition inside the SWNT prevents the metal NPs
sintering to form rods. When formed the naked RuNPs are
stabilised by adhesion to the internal nanotube sidewall and the
CO gas is vented from the nanotube leaving the internal channel
ready to accommodate reactant molecules. TEM shows very small
RuNPs with a narrow diameter distribution (dNP = 0.92 Æ 0.13 nm).
The internal channel of the SWNT acts as a template to nano-
particle formation enabling precise control of the NP size. As the
b
c
Nottingham Nanotechnology and Nanoscience Centre, University of Nottingham,
University Park, Nottingham, NG7 2RD, UK.
E-mail: andrei.khlobystov@nottingham.ac.uk; Fax: +44 (0)115 9513563;
Tel: +44 (0)115 9513917
Institute for Process Research and Development, School of Process,
Environmental & Materials Engineering, University of Leeds, Leeds, LS2 9JT, UK.
E-mail: R.A.Bourne@Leeds.ac.uk; Fax: +44 (0)113 3436565;
Tel: +44 (0)113 3430109
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4
NP size is closely linked to the catalytic activity, varying the
diameter of the SWNT to template NPs of different sizes could be
utilized to modulate the catalytic properties of the NPs.
†
Electronic supplementary information (ESI) available: Experimental and TOF
calculation details and TEM characterisation. See DOI: 10.1039/c3cc49247h
5
200 | Chem. Commun., 2014, 50, 5200--5202
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Fig. 1 (a) Schematic diagram of the hydrogenation of cyclohexene (blue)
to cyclohexane (pink) using bundles of RuNPs@SWNT catalytic nanoreac-
2
tors (shown in the expanded box) in a continuous flow scCO milliscale
reactor. TEM images of (b) a bundle of RuNPs@SWNTs and (c) an individual
RuNPs@SWNT both showing that the Ru metal is located solely within the
internal nanotube cavities. (d) EDX spectroscopy confirms the presence of
Ru metal in the nanotubes (Cu and Ni peaks are due to the specimen
holder and metal catalyst used for nanotube fabrication respectively).
(
e) RuNP and SWNT diameters were measured by TEM.
The RuNPs@SWNT catalyst was immobilised in a high pressure
milliscale reactor packed with a mixture of the catalyst and sand
Fig. 1a). Reaction performed over the reaction bed using a gaseous
mixture of cycloalkene and H
(
2
led to negligible (o5%) conversion.
This is in contrast to previous work reported for MWNT based
nanoreactor catalysts reporting high yields for a wide variety of gas
Fig. 2 Reaction kinetics for the reduction of (a) cyclooctene (’) to
cyclooctane (J) and other products (m), (b) butyraldehyde (’) to butanol
8
phase reactions. The reduced conversion in SWNT is most likely as
(J) and small amounts of other products (m) and then changing the
a result of mass transfer limitations due to the much narrower
feedstock solution for the reduction of (c) cinnamaldehyde (’) to hydro-
diameter of SWNTs (dNT = 1.39 Æ 0.05 nm). However the conversion cinnamaldehyde (J) and cinnamyl alcohol (m). The reactor internal
temperature is shown as the solid line in all plots. Flow rates were as
of reactions using RuNPs@SWNT increased significantly when
À1
À1
À1
follows; 1 mL min CO
under 100 bar pressure.
2
, 0.03 mL min organic substrate, 0.06 mL min
supercritical carbon dioxide (scCO ) was added as a reaction solvent.
2
H
2
This is most likely due to the negligible surface tension and low
viscosity of scCO
2
enabling it to deliver molecules into very narrow
nanotubes. In addition, scCO is a highly effective medium for the temperature of the fixed catalyst bed was cycled and showed
hydrogenation as H is fully miscible with scCO It has also be good stability over a series of temperature cycles at constant flow
15
2
16–18
2
2
.
shown to be a suitable solvent for non-polar organic molecules being rates for a number of hours (Fig. 2a). Following this process the
utilized in a number of continuous flow reactions over a variety of catalyst bed was held at fixed temperature of 110 1C for 11 hours
19,20
heterogeneous supported metal catalysts.
for the hydrogenation of cyclooctene with no loss in activity
For example, the catalyst efficiently converted cyclooctene observed. TEM of the RuNPs@SWNT catalyst after 24 h of reaction
to cyclooctane in yields up to 80% at temperatures above 160 1C revealed that the structure remained unaltered confirming the
despite low amounts of available catalyst (1 mg of Ru metal in stability of the catalyst (ESI†). Unconfined RuNPs would be
2
0 mg of Ru@SWNT) relative to the flow rate of organic substrate unstable under these conditions and undergo fast coalescence
À1
(0.03 mL min ), Fig. 2a. For the hydrogenation of cyclooctene and Ostwald ripening drastically reducing their activity.
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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.
À1
À1
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
2
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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202 | Chem. Commun., 2014, 50, 5200--5202
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