Table 1 Influence of the nature and proportion of the CD on the
catalytic hydrogenation of furfural with PVP-stabilized Ru(0) NPs
Finally the effect of CD on the hydrogenating performances
of Ru(0) NPs has also been examined at different PVP : CD
ratios by using RaMe-b-CD (entries 6–9). Our results establish
that only a small amount of CD (one equiv. per ruthenium) is
required to induce a significant increase in activity without
causing any decrease in furfuryl alcohol selectivity (entry 8).
Moreover, the fact that the conversion can be maintained
at high levels with a wide range of PVP : CD ratios, i.e.
from 8 : 1 to 8 : 4, is a clear indication of the stability of these
CD-involving catalytic systems.
Selectivity (%)
Entry CD
PVP : CD Conversion (%) FA
THFA
1
2
3
4
5
6
7
8
9
/
8 : 0
8 : 2
8 : 2
30
30
38
34
61
53
37
52
52
94
95
94
97
90
90
97
93
90
6
5
6
a-CD
g-CD
RaMe-a-CD 8 : 2
RaMe-g-CD 8 : 2
RaMe-b-CD 8 : 2
RaMe-b-CD 8 : 0.5
RaMe-b-CD 8 : 1
RaMe-b-CD 8 : 4
3
10
10
3
7
10
In conclusion, we have reported that cyclodextrins could be
successfully used as growth controlling agents of polymer-
stabilized Ru(0) NPs. The most impressive effects were
obtained with b- and g-methylated CDs, allowing the stabilization
of catalytically active NPs with a narrow size-distribution centered
at around 2.3 nm, while retaining an excellent stability under
reaction conditions. We believe that this finding will give new
opportunities to expand the scope of aqueous supramolecular
assembly studies in the fields of nanochemistry and nanocatalysis.
Reaction conditions: Ru(0) (3.8 Â 10À5 mol), PVP-K30 (3.0 Â
10À4 mol), substrate/Ru(0) (mol/mol)
= 50, H2O (12 mL),
H2 (1.0 MPa), stirring rate (750 rpm), 30 1C, 1.5 h.
at 30 1C under a dihydrogen pressure of 1 MPa (detailed procedure
in ESIw). Note that, under these conditions, control experiments,
performed with PVP-stabilized Ru(0) NPs and no CD, have
allowed us to show that the ratio of PVP to Ru of 8 was the best
compromise between stability and reactivity (Table S2 in ESIw).
When adding cyclodextrin with a ratio of PVP : CD of 8 : 2, it
appears that the efficiency of the cyclodextrins mixed with the PVP
depends on the number and modification of the glucose units.
When comparing the conversions after 1.5 hours, the reaction run
using the control PVP-stabilized Ru(0) gives a low 30% conversion
(entry 1) whereas the addition of native g-CD to PVP leads under
the same conditions to an 8% increase (entry 3). However, it is
readily apparent that RaMe-g-CD and RaMe-b-CD are by far the
most efficient promoters of PVP with levels of conversion of 61 and
53%, respectively (entries 5 and 6). These activities are of the same
order of magnitude as those reported by Liaw et al. on NiB NP
catalysts, but under more severe conditions (T = 80 1C, P(H2) =
1.8 MPa).16 Note also that RaMe-a-CD does not appear as a
valuable additive since a precipitate is observed at the end of the
reaction, probably explaining the moderate activity (entry 4). The
product distribution shows in all cases the preferential formation of
furfuryl alcohol with selectivity ranging from 90 to 97%.
Notes and references
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¨
H.-H. Strehblow, P. Keller, E. Rentschler and W. Klaui, Angew.
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7 (a) Z. Khan, S. A. Al-Tnabaiti, E. H. El-Mossalamy and A. Y. Obaid,
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The above results can be rationalized in terms of size and
morphology control of Ru(0) NPs. As previously described,
the mean size of the metallic NPs was shown to decrease in the
following order: no CD (B2.9 nm) > native CD (B2.5 nm) >
RaMe-CD (B2.3 nm). In line with what is generally observed
in nanocatalysis, the decrease in the particle size results in the
increase in the number of available surface active sites, and
consequently increases the catalytic efficiency. The reusability
of the catalytic system prepared from mixtures of PVP and
RaMe-CD (b and g) has also been evaluated. Notably, no
significant loss of activity is observed after three consecutive
runs. More impressively, TEM examination of the catalytic
suspensions recovered after the first run has proved that the
particles remain stable against aggregation with practically
unchanged particle size distributions: 2.12 and 2.23 nm for
RaMe-b-CD and RaMe-g-CD, respectively (Fig. S5 in ESIw).
8 L. Leclercq, H. Bricout, S. Tilloy and E. Monflier, J. Colloid
Interface Sci., 2007, 307, 481.
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(b) M. J. Climent, A. Corma and S. Iborra, Green Chem., 2011,
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11 It is worth noting that the synthesis method of PVP-stabilized
Ru(0) NPs in the presence of native b-CD could not be applied due
to its low solubility in water (18.5 g LÀ1 at 20 1C).
12 RaMe-CDs (a, b and g) are native CDs partially O methylated
with statistically 1.8 OH groups modified per glucopyranose unit.
13 A. Denicourt-Nowicki, A. Ponchel, E. Monflier and A. Roucoux,
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c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3451–3453 3453