Communications
about 80% CD from solution after 3 minutes (Figure S14).
rate and enantioselectivity of Pt/CNTs(in) relative to those of
Pt/CNTs(out).
Based on the adsorption data, the CD concentration in the
channels of Pt/CNTs(in) is about 2700 times higher than that
in solution (for calculation details see the Supporting
Information S1.4). This data suggests that the CD can be
enriched in the channels of CNTs irrespective of the location
of Pt particles inside or outside of the CNT channels. Pt/AC
can also adsorb CD (Figures S13 and S15), but the amount of
CD adsorbed is much less than that of Pt/CNTs(in) and Pt/
CNTs(out).
Figure 4B gives the competitive adsorption of the reac-
tant, ETPY, and the hydrogenation product, (R)-Et-lactate by
Pt/CNTs(in). Both the reactant and the product are adsorbed
very quickly, and reach the adsorption equilibrium in a few
minutes. However, the equilibrium amount of the reactant
adsorbed is about 40–50% higher than that of product
adsorbed within Pt/CNTs(in). These facts indicate that the
channels of CNTs have the ability to discriminate between the
reactants and products having slightly different hydrophilic-
ity/hydrophobicity. As a result, the CNT channels are kineti-
cally favorable for the asymmetric hydrogenation of a-
ketoesters.
Figure 4A shows the adsorption concentration of ETPY
(
reactant) onto the Pt/CNTs(in), Pt/CNTs(out), and Pt/AC
catalysts. The adsorption of ETPY onto Pt/CNTs(in) and Pt/
CNTs(out) is very fast and the adsorption equilibrium was
To evaluate the stability of the catalyst, the recycling of Pt/
CNTs(in) was performed for the asymmetric hydrogenation
of ETPY. The solid catalyst was recovered by centrifugation
(or filtration), washed with acetic acid in air, and re-modified
by CD in acetic acid. A constant conversion of 99.9% with up
to 96% enantioselectivity was obtained even after nine
consecutive cycles of the reaction. The result indicates that
the Pt/CNTs(in) catalyst is quite stable and can be recycled
without deterioration in activity or enantioselectivity
(Table S3).
In summary, we found that when a Pt nanocatalyst
modified by CD was encapsulated within CNTs it is very
active and leads to the highly enantioselective hydrogenation
of a-ketoesters. The high activity and enantioselectivity are
attributed mainly to the unique properties of the nano-
channels of the CNTs as they can readily enrich both CD and
the reactants. This work shows the unique effect of the
nanochannels of CNTs as nanoreactors for asymmetric
catalysis. The encapsulation of a nanocatalyst and chiral
modifier within CNTs as described in this work opens an
opportunity for developing novel, highly active and enantio-
selective heterogeneous chiral catalysts.
Figure 4. Adsorption kinetics of the reactant (ETPY) and product ((R)-
ethyl lactate). A) Comparison of ETPY adsorption concentration as a
function of time on Pt/CNTs(in) (&), Pt/CNTs(out) (*), and commer-
cial Pt/AC (!). The adsorption concentration of (R)-ethyl lactate as a
function of time on the Pt/CNTs(in) (~)catalyst. B) Co-adsorption of a
mixture of ETPY (&) and (R)-ethyl lactate (*; the mole ratio of ETPY/
(R)-Et-lactate=1.1) as a function of time on Pt/CNTs(in). The adsorp-
tion amount was calculated based on GC analysis.
achieved in less than 3 minutes; more than 35% ETPY in the
solution can be adsorbed (Figures S16–S21). However, Pt/AC
shows an obviously lower adsorption concentration and less
than 15% ETPY from the solution is adsorbed. The ETPY
concentration in the channels of CNTs is about 400 times
higher than ETPY in solution based on the adsorption data
(
see the Supporting Information S1.4). This result shows that
Experimental Section
the reactant can be also enriched in the channels of CNTs.
The adsorption concentration of CD or ETPY for Pt/AC
is much lower than that of Pt/CNTs(in) or Pt/CNTs(out) as
shown in Figures 3 and 4A. These data indicates that the
capillary adsorption effect of the opened nanochannels of
CNTs has a vital impact on the adsorption uptake. The
adsorption results suggest that open CNT channels have a
unique enrichment effect on both CD and the reactant,
ETPY, and is probably due to the intensified capillary effect
of CNT channels when their diameters are in the nanoscale
range. Therefore, the ultrahigh enrichment of both CD and
ETPY in CNT channels is proposed to be a reason for the
dramatically different catalytic performances of Pt/CNTs(in)
and Pt/CNTs(out). The channels of the CNTs with ultrahigh
enrichment of CD should favor the strong and high density
adsorptions of CD and ETPY on Pt nanoparticles inside the
channels of CNTs, thus contributing to the higher reaction
Preparation of Pt/CNTs(in) and Pt/CNTs(out): The Pt/CNTs(in)
catalyst was prepared by introducing the platinum precursor (5% wt)
into the CNTs channels using an improved wet-chemistry
[
31,32]
method.
The CNTs (2.0 g) with open ends (for the preparation
see the Supporting Information S1.2) were immersed in an aqueous
À1
solution of H PtCl (50 mL, 10.25 mmolL ) at room temperature.
2
6
After ultrasonic treatment for 3 h, the mixture was stirred for 48 h at
room temperature. Then the mixture was heated to 1108C with a
À1
heating rate of 18Cmin and held at 1108C for 24 h. By this slow
drying method, the catalyst precursor was introduced into the CNTs
channels. The dried sample was reduced with a sodium formate
À1
solution (42 mgmL ) at 1008C for 1 h, and then the solid product was
filtered, washed with deionized water, and dried at 608C for 18 h. The
process for the synthesis of Pt/CNTs(out) was described in the
Supporting Information. For the TEM characterization and asym-
metric hydrogenation see the Supporting Information.
Received: November 2, 2010
Revised: January 25, 2011
Published online: March 2, 2011
4
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 4913 –4917
Chen, Zhijian
