C O M M U N I C A T I O N S
Table 1. Catalytic Hydrogenation of Anthracene Using
CNT-Supported Nanocatalysts
starting material anthracene was detected by TLC, NMR, and HPLC
after 1 h of reaction under the same conditions.
An interesting observation is that when a mixture of Pd2+ and
Rh3+ ions was used as the starting material in the water core of the
microemulsion for the synthesis, the resulting Pd/Rh nanoparticles
deposited on the fMWCNTs (Figure 1c) are more active than the
single metal nanoparticles prepared in the same way for catalytic
hydrogenation of anthracene. Using CNT-supported Pd or Rh
monometallic nanoparticles, the conversion yield of anthracene to
1,2,3,4,5,6,7,8-octahydroanthracene was less than 10%. The cor-
responding product was obtained at 92.7% with CNT-supported
Pd/Rh nanoparticles under the same conditions (Table 1). The CNT-
supported Pd/Rh catalyst was found to contain 6.3 ( 0.1 wt % Pd
and 6.9 ( 0.4 wt % Rh in EDX and XPS analyses. The XRD data
showed a single peak for the Pd/Rh nanoparticles deposited on the
fMWCNTs with a 2θ value between that of the Pd (2θ ) 40.20°)
and Rh (2θ ) 41.15°) metal nanoparticles. XPS spectra of the Pd/
Rh catalyst revealed the presence of Pd(3d5/2) and Pd(3d3/2) peaks
a 0.0300 g of 5 wt % Rh. b 0.0100 g of the CNT-supported metal
nanoparticles. c 6.3 wt % Pd and 6.9 wt % Rh. All catalysts were used
after drying in the oven for 1 h at 100 °C, and stirring speed was optimized
at 600 rpm. Anthracene (0.0178 g, 0.1 mmol) with the nanocatalyst was
stirred for 30 min under 10 atm of H2 pressure in methanol at 25 °C. The
conversion yield was determined by NMR and HPLC (C18 column, MeOH/
H2O ) 4:1). ND ) not detectable.
To test activities of the CNT-supported Pd nanoparticles,
iodobenzene and styrene were selected for the Heck coupling
reaction. The coupling product was obtained with 94% of the
isolated yield after 3 h of the reaction shown in eq 1. A conventional
palladium/carbon catalyst (10 wt % Pd from Aldrich) was also used
for this coupling reaction under the same conditions, and the Heck
coupling product (trans-stilbene) with 53% conversion yield was
detected after 24 h. The particle size of the commercial Pd catalyst
showed a very large variation, typically about 2 orders of magnitude
larger than the CNT-supported nanoparticles. It was reported that
the pore size, acid-base properties, and impurities of the conven-
tional carbon supports could influence the metal particle distribu-
tions.8 The conversion of trans-stilbene to 1,2-diphenylethane was
about 99% after 10 min of reaction using 1 atm of H2 gas at 25 °C.
Using the commercial Pd/C catalyst, it would require 1 h to achieve
the same degree of hydrogenation of trans-stilbene under the same
conditions. The high catalytic activity of the CNT-supported Pd
nanoparticles in forming a new carbon-carbon bond via the Heck
coupling is potentially significant for organic synthesis applications.
The debenzylation of the protected group is conventionally
carried out with Pd as a catalyst. To test the effectiveness of the
Pd/CNT catalyst, we synthesized O-alkylated 5,5-dimethyl-1,3-
cyclohexanedione derivatives as shown in eq 2.9 After 3 h using
the Pd/CNT catalyst, the debenzylation products were obtained with
95% of yield (eq 2). In case of the commercial Pd/C catalyst, less
than 10% of the starting materials were converted to the deben-
zylated products after 9 h of reaction under the same conditions.
The CNT-supported Pd nanoparticles are obviously much more
active than the commercial Pd/C catalyst for the debenzylation
process.
at 335.4 and 340.7 eV, respectively, and Rh(3d5/2) and Rh(3d3/2
)
peaks at 307.2 and 311.9 eV, respectively.
According to a recent report, bimetallic nanoparticles may have
a number of different morphologies that would not show changes
in XPS spectra.10 The enhanced catalytic activity of the CNT-
supported Pd/Rh bimetallic system could be due to certain arrange-
ments of the Pd and Rh nanoclusters on the nanoparticles’ surfaces.
The Pd/Rh nanoparticles synthesized by the microemulsion method
described in this communication are probably bimetallic in nature
with an unknown morphology.
In conclusion, the fMWCNT-supported Pd, Rh, and Rh/Pd
nanoparticle catalysts can be prepared easily with high yields using
water-in-oil microemulsions containing metal ions as starting
materials. Metal nanoparticles are formed by hydrogen reduction
of metal ions in the water core with 1 atm H2 gas at 25 °C. The
CNT-supported Rh nanoparticles are active catalysts for hydrogena-
tion of arenes, and the CNT-supported bimetallic Pd/Rh nanopar-
ticles show an unusually high catalytic activity for hydrogenation
of anthracene. This simple and novel synthetic technique for making
CNT-supported monometallic and bimetallic nanoparticles may
have a wide range of catalytic applications for chemical syntheses.
Acknowledgment. This work was supported by a grant from
DOD-AFOSR (F49620-03-1-0361).
Supporting Information Available: Experimental details and XRD
data. This material is available free of charge via the Internet at http://
pubs.acs.org.
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