Dispersing palladium nanoparticles using a water-in-oil microemulsion - Homogenization of heterogeneous catalysis

Article abstract of DOI:10.1039/b211836j

Palladium nanoparticles dispersed by a water-in-oil microemulsion are very effective catalysts for hydrogenation of olefins in an organic solvent.

Full text of DOI:10.1039/b211836j

Dispersing palladium nanoparticles using a water-in-oil
microemulsion—homogenization of heterogeneous catalysis
Byunghoon Yoon, Hakwon Kim and Chien M. Wai*
Department of Chemistry, University of Idaho, Moscow, Idaho, 83844, USA. E-mail: cwai@uidaho.edu
Received (in Corvallis, USA) 27th November 2002, Accepted 5th March 2003
First published as an Advance Article on the web 2nd April 2003
Palladium nanoparticles dispersed by a water-in-oil micro-
emulsion are very effective catalysts for hydrogenation of
olefins in an organic solvent.
samples were examined by TEM. Figure 1(b) shows the
presence of spherical palladium metallic particles with diame-
ters in the range 4–10 nm.
Three olefins (1-phenyl-1-cyclohexene, methyl trans-cinna-
mate, and trans-stilbene) were selected for this investigation
and their structures are given in Table 1. A selected olefin (e.g.
1-phenyl-1-cyclohexene 23.7 mg or 0.15 mmol) was added to
the hexane solution containing the microemulsion with
Catalytic hydrogenation using metallic palladium particles is an
important process in organic synthesis. Commercially available
palladium catalyst is stabilized in activated carbon (Pd/C) that is
traditionally used for heterogeneous catalysis including hydro-
genation of olefins.1 A recent report shows that palladium
nanoparticles can be stabilized and dispersed in a water-in-CO2
microemulsion for catalytic hydrogenation of olefins and nitro
compounds with very fast reaction rates.2 Efficient hydro-
genation of olefins is most likely due to the fact that palladium
nanoparticles in the microemulsion are uniformly dispersed and
mixed with reactants resembling a homogeneous catalytic
system. This concept of homogenization of heterogeneous
catalysis utilizing microemulsion as a medium for dispersing
metal catalysts should have important applications in conven-
tional heterogeneous catalysis carried out in organic solvents.
Sato et al. recently prepared ultrafine Pd particles in reverse
micells using KBH4 as a reducing agent and observed their
catalytic activity for hydrogenation of allyl alchohol and styrene
in isooctane.3 The authors noted that the hydrogenation activity
was inhibited by the AOT, bis(2-ethylhexyl) sulfosuccinate,
surfactant. In another paper, de Jesus and Spiro showed the
oxidation of TMPPD (N,N,NA,NA-tetramethyl-p-phenylenedia-
mine) by Co(NH3)5Cl2+ catalyzed by Pd nanoparticles in an
aqueous/AOT/n-heptane microemulsion.4 In this paper, we
report the dispersion of palladium nanoparticles in a water/
AOT/n-hexane microemulsion by hydrogen gas reduction of
22
PdCl4
and its efficiency for hydrogenation of olefins in
organic solvents.
Sodium tetrachloropalladate (Na2PdCl4) was purchased from
Alfa. AOT was obtained from Aldrich and used as received. A
Pd/C catalyst containing 10% Pd by weight was obtained from
Aldrich. An Ocean Optics (OOI CHEM2000) UV-Vis spec-
trometer and a 300 MHz NMR (Bruker AMX300) were used for
spectroscopic measurements. Transmission electron micro-
scope (TEM) pictures were obtained using a Jeol (JEM-
1200EXP) instrument. The procedure of preparation of the
microemulsion is described as follows. Ten mL of hexane was
first placed in a 25 mL round bottomed flask with a magnetic
stirrer. After that, 40.5 mL of a 0.04 M aqueous Na2PdCl4
solution and 66.7 mg (0.15 mmol) of AOT were added to the
flask that gave a W value ([H2O]/[surfactant]) of 15. The system
was stirred for 1 hour and the solution was optically transparent
with a light yellowish color. To make Pd nanoparticles, the
system was flushed with nitrogen gas under stirring followed by
passing hydrogen gas (1 atm) through the system. Hydrogen gas
is capable of reducing Pd2+ to metallic Pd according to a
previous study.2 The UV/Vis spectra of the solution before and
after addition of hydrogen gas are given in Figure 1(a).
Palladium nanoparticles are known to absorb in the UV region
with virtually no structure.2,5 About 1 mL of the hexane solution
after hydrogen reduction was taken for TEM study. The solution
was shaken with a methanol–chloroform(1+1) mixture and
samples were taken from the methanol phase and placed on
copper grids. After evaporation to dryness, the copper grid
Fig. 1 (a) UV-Vis spectra of [PdCl4]22 and Pd nanoparticls in a water-in-oil
microemulsion (0.04 M aq. Na2PdCl4, [AOT] = 0.15 mmol in 10 mL of n-
hexane, W = 15) (b) TEM micrograph of Pd nanoparticles collected from
the water-in-oil microemulsion system. (scale = 20 nm)
Table 1 Catalytic hydrogenation of olefins with Pd nanoparticles in a water-
in-hexane microemulsion
Reaction
time
Conversion
%
Olefin
Catalyst
Product
Pd/MEa
Pd/Cb
Pd/Cb
5 min
20 min
40 min
> 97
50
> 97
Pd/MEa
Pd/Cb
Pd/Cb
6 min
25 min
45 min
> 97
60
> 97
Pd/MEa
Pd/Cb
Pd/Cb
7 min
40 min
60 min
> 97
68
> 97
a Molar ratio of olefin/AOT/Na2PdCl4 = 1/1/0.01 and 10 mL of n-hexane
used as solvent (W = 15) at 30 °C. b 1.7 mg of Pd/C (0.17 mg or 1.6 µmol
of Pd) in 10 ml of n-hexane.
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CHEM. COMMUN., 2003, 1040–1041
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Na2PdCl4 in the water core. This is followed by bubbling
hydrogen through the system. The system was stirred at 30 °C
and samples were taken every minute to monitor the amounts of
the reactant and the product using TLC and NMR spectroscopy.
The amount of the olefin added to the solution was about two
orders of magnitude greater than that of the total palladium
present in the system. The hydrogen gas serves a dual purpose
of reducing Pd ions as well as the starting material for the
hydrogenation process as shown in Scheme 1.
less than 45% after 1 hour of reaction under the same reaction
condition. In the case of the palladium nanoparticles dispersed
by the water-in-oil microemulsion, however, the product,
10-propylanthracene,8 was detected with more than 50% by
NMR after 9 minutes of reaction.
The formation of palladium nanoparticles in the AOT
microemulsion was also studied using different solvents
including ethyl acetate, dimethyl formamide, toluene, chloro-
form and dichloromethane. Chloroform and dichloromethane
are not able to form a stable water-in-oil microemulsion with
AOT and a Na2PdCl4 solution. Ethyl acetate, dimethyl
formamide and toluene are able to form water-in-oil micro-
emulsion with AOT and a solution of Na2PdCl4. Other factors
such as the W value,9 temperature,10 amount and nature of
surfactant11 can also affect the stability of the microemulsion
and consequently the catalytic efficiency of the metal nano-
particles. Extensive studies of these parameters are currently in
progress.
The Pd nanoparticles formed in the water/AOT/n-hexane
microemulsion at W = 15 was stable for about 15–20 minutes.
After that aggregation of nanoparticles would occur and black
Pd particles became visible in the solution. The Pd particles
could be separated from the hexane solution by filtration. We
could also recover AOT by adding an acid solution (e.g. 6 M
HCl) to hexane and AOT would partition favorably in the acid
solution.12
In conclusion, we have demonstrated that palladium nano-
particles dispersed by a water-in-oil microemulsion are very
efficient catalysts for hydrogenation of olefins in an organic
solvent. This approach can be applied to other metal catalyst
systems including platinum, rhodium, nickel, etc. The concept
of homogenation of heterogeneous catalysis by dispersing
nanoparticles in solutions described in this paper may have a
wide range of applications in organic synthesis and in industrial
manufacturing processes.
The results of hydrogenation of the three olefins catalyzed by
the Pd nanoparticles dispersed in the hexane solution by the
microemulsion are summarized in Table 1. In the case of
1-phenyl-1-cyclohexene, the reactant was not detectable by
NMR after 5 minutes of reaction. Only the product 1-phenyl-
1-cyclohexane shown in Scheme 1 was detected. From the
detection limit of the NMR peak of the reactant, we estimated
the conversion yield to be > 97%. A control experiment was
also done without Na2PdCl4 in the water core of the micro-
emulsion. In this case, only the reactant 1-phenyl-1-cyclohex-
ene was detected under the specified experimental conditions
indicating Pd nanoparticles were responsible for the catalytic
hydrogenation. The hydrogenation reaction was also carried out
using commercial Pd/C catalyst under the same conditions
except in the absence of the microemulsion. About 1.7 mg of the
Pd/C (0.17 mg, 1.6 mmol of Pd) was used in this experiment.
The total amount of Pd in this case was the same equivalent for
the microemulsion experiment. The system was stirred and
samples were taken at different times and analyzed by the
procedure described above. The conversion was about 50%
after 20 minutes of reaction, After 40 minutes of reaction, the
conversion was more than 97%. The palladium nanoparticles
dispersed by the microemulsion in the organic solvent is
obviously more efficient than the conventional Pd/C catalyst.
This work was supported by a DEPSCoR grant (DAAD19-
01-0458) and by the Idaho NSF-EPSCoR Program (EPS-
0132626)
Scheme 1 Hydrogenation of 1-phenyl-1-cyclohexene
The results of hydrogenation of the other two olefins, methyl
trans-cinnamate and trans-stilbene, catalyzed by microemul-
sion dispersed palladium nanoparticles (Table 1) are similar to
that of 1-phenyl-1-cyclohexene described above. Virtually all of
the starting olefins were converted to the saturated hydro-
carbons in 6–7 minutes. Using the conventional Pd/C catalyst,
the conversion was 60% after 20 min for methyl trans-
cinnamate and 68% after 40 min for trans-stilbene. Quantitative
comparison of hydrogenation activity of the Pd nanoparticles in
microemulsion versus conventional Pd/C catalysts requires
detail knowledge about mass transport processes from the bulk
solvent to the particle surface and the effective surface area that
is not available at the present time. Research along this direction
is currently in progress.
Notes and references
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2 H. Ohde, C. M. Wai, H. Kim, J. Kim and M. Ohde, J. Am. Chem. Soc.,
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3 H. Sato, T. Ohtsu and I. Komasawa, J. Chem. Eng. Jpn., 2002, 35,
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4 D. M. de Jesus and M. Spiro, Langmuir, 2000, 16, 4896.
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7 B. Yoon, Master’s thesis, Department of Chemistry, Kyung Hee
University, Seoul, Korea, 1998.
Another challenging test was the catalytic hydrogenation of
monoalkylated anthracene, 10-(3-propenyl)anthracene as
shown in Scheme 2. This molecule was synthesized from a
monoalkylated anthrone derivative.6 The monoalkylated an-
thracene derivative, 10-(3-propenyl)anthracene,7 showed rela-
tively poor hydrogenation in Pd/C catalyst. The conversion was
8 TLC (Rf: 0.59, EA+Hex = 1+20) and NMR (300 MHz, CDCl3) d 8.32(s,
1H), d 8.27(d, J = 5.25, 2H), d 7.97(d, J = 8.04, 2H), d 7.39–7.46(m,
4H), d 3.53(t, J = 8.05, 2H), 1.82(m, 2H), d 1.09(t, J = 7.35, 3H).
9 A. Maitra, J. Phys. Chem., 1984, 88, 5122; A. Hanmouda, Th. Gulik and
M. P. Pileni, Langmuir, 1995, 11, 3635; J. Yano, H. Furedi-Milhofer, E.
Wachtel and N. Garti, Langmuir, 2000, 16, 9996.
10 Q. Li, T. Li and J. Wu, J. Phys. Chem. B., 2000, 104, 9011; S. Burauer,
T. Sottmann and R. Strey, Phys. Chem. Chem. Phys., 1999, 1, 4299.
11 S. Ahuja and J. Cohen, Anal. Profiles Drug Subst., 1983, 12, 713; K. N.
Bakeer, S. A. Chugunov, I. Teraoka, W. J. Macknight, A. B. Zezin and
V. A. Kabanov, Macromolecules., 1994, 27, 3926; M. Bujan, M. Sikiric,
N. Filipovic-Vincekovic, N. Vdovic, N. Garti and H. Furedi-Milhofer,
Langmuir, 2001, 17, 646.
12 I. Casero, D. Sicilia, S. Rubio and D. Perez-Bendito, Anal. Chem, 1999,
71, 4519.
Scheme 2 Hydrogenation of 10-(3-propenyl)anthracene
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