Swelled plastics in supercritical CO2 as media for stabilization of metal
nanoparticles and for catalytic hydrogenation
Hiroyuki Ohde, Mariko Ohde and Chien M. Wai
Department of Chemistry, University of Idaho, Moscow, Idaho 83844, USA. E-mail: cwai@uidaho.edu;
Fax: (+1)208-885-6173; Tel: (+1)208-885-6787
Received (in Corvallis, OR, USA) 19th September 2003, Accepted 27th February 2004
First published as an Advance Article on the web 16th March 2004
Swelled plastics in supercritical carbon dioxide provide unique
environments for stabilizing palladium and rhodium nano-
particles and for catalytic hydrogenation. Complete hydro-
genation of benzene to cyclohexane can be achieved in 10
minutes using the plastic stabilized Rh nanoparticles at 50 °C in
supercritical CO2. High efficiency, reusability, and rapid
separation of products are some advantages of the plastic
stabilized metal nanoparticles for catalytic hydrogenation in
supercritical CO2.
pressure cell together with 250 mg of the metal precursor [Pd(hfa)2
or Rh(acac)3]. The 50 mL cell was then pressurized by 100 atm CO2
to dissolve the Pd or Rh precursor in supercritical CO2. Hydrogen
gas (10 atm) was introduced into a separate injection vessel (10 mL
volume) followed by pressurizing the injection vessel with 200 atm
CO2. By opening an interconnecting valve between the 50 mL high-
pressure cell and the 10 mL injection vessel, hydrogen was injected
into the 50 mL high pressure cell with the aid of the pressure
difference between the two cells. The hydrogen reduction of
Pd(hfa)2 was performed at 200 atm and 50 ± 5 °C. After about 2
hours, Pd was successfully deposited into HDPE granules or PFA
rings. The yellowish color caused by Pd(hfa)2 became dark gray
due to Pd metal particle formation as seen in Fig. 1a. A piece of the
PFA with Pd nanoparticles was cut in half for observation. The
whole cross section (width 1 mm) was also dark gray in color
indicating that the Pd precursor penetrated deep into the whole PFA
ring in supercritical CO2 and deposited uniformly in the plastic as
metal nanoparticles. Fig. 1b shows a TEM micrograph of the Pd
nanoparticles deposited in HDPE. The method for TEM sample
preparation was identical to that reported in the literature.9 From the
TEM micrograph, the size distribution of the metal nanoparticles
was estimated to vary from 2 to 10 nm with the largest fraction in
the range of 4–5 nm diameter.
In the synthesis of Rh nanoparticles, 3.5 mL chloroform was
added to enhance the solubility of Rh(acac)3 in supercritical CO2.
In addition, 10 mg of Pd(hfa)2 was also added as a catalyst for
hydrogen reduction of Rh(acac)3. The Rh nanoparticle depositions
in HDPE granule and PFA ring were performed at 110 and 150 °C,
respectively. The plastic supported metal nanoparticles are very
stable in air. No obvious agglomeration of the metal nanoparticles
in the plastic materials was observed even after 3 months of storage
in our laboratory based on TEM micrographs.
It should be noted that newly prepared plastic stabilized Pd and
Rh catalysts must be washed thoroughly in supercritical CO2 to
remove possible byproducts produced from the reduction process.
We recommend washing of the plastic catalysts with neat
supercritical CO2 at 100 atm and 50 °C repeatedly until the
byproducts [hexafluoroacetylacetone from Pd(hfa)2 and acet-
ylacetone from Rh(acac)3] are no longer detectable from the
trapped solution. Hydrogenations of olefins, arenes and nitro
compounds were studied using the plastic supported Pd or Rh
nanoparticles prepared by the procedure given above. The plastic
catalyst (4.5 g PFA or 3.0 g HDPE) and a starting material (the
amounts are shown in Table 1) were placed in a 10 mL high-
There has been much interest recently in synthesizing nanometer-
sized metal particles because of their potential applications as new
catalysts for organic reactions.1–3 The effect of size of metal
nanoparticles on catalytic activities is one of the interesting aspects
of the current research in this area.4 However, nanometer-sized
metal particles are unstable and tend to agglomerate without a
suitable support. Different stabilizing approaches for nanoparticles
have been reported in the literature including the use of chemical
stabilizers,3 dendrimers,2 polymers5 and microemulsions.6
In this communication, we report a novel approach of catalytic
hydrogenation in supercritical CO2 using palladium and rhodium
nanoparticles stabilized in plastics. Since plastics swell in super-
critical CO2,7,8 metal precursors dissolved in the fluid phase can
penetrate into plastic structures. After hydrogen reduction of the
metal precursors, the resulting metals are trapped as nanometer-
sized particles and stabilized in the plastic structures. During
catalytic hydrogenation, starting materials dissolved in CO2 can
penetrate into the swelled plastic structures and diffuse into the
interior of the plastic containing metal nanoparticles for catalytic
hydrogenation to take place effectively. Because the hydrodynamic
diameters of metal nanaoparticles trapped in the plastic are much
larger than that of starting materials (such as benzene and phenol),
the plastic structures allow the reactants to diffuse into the interior
but forbid metal nanoparticles to diffuse and to agglomerate.
Therefore, not only do the metal nanoparticles exist on the surface
of the plastic but they are also present in the interior of the plastic
and are available for catalysis in supercritical CO2. Consequently,
the amount of metal nanoparticles per volume of the supporting
plastic material can be extremely high compared with those of
conventional active carbon and alumina supported metal catalysts.
Thus, the plastic stabilized Pd and Rh nanoparticles can be used
repeatedly without losing their catalytic capabilities. After reaction,
the products of the catalytic hydrogenation diffuse away from the
plastic structure. Finally the products can be easily separated from
the plastic catalysts by rapid expansion of CO2.
The plastic supported palladium and rhodium nanoparticles were
prepared by hydrogen reduction of Pd(II) hexafluoroacetylaceto-
nate [Pd(hfa)2] and Rh(III) acetylacetonate [Rh(acac)3] in super-
critical CO2. Watkins and McCarthy previously reported that
nanometer-sized platinum particles could be uniformly deposited
into poly(4-methyl-1-pentene) and poly(tetrafluoroethylene) in
supercritical CO2 by hydrogen reduction of a platinum precursor.9
In our experiments, a 50 mL high-pressure stainless flat-bottom cell
was used for the synthesis of the metal nanoparticles. High density
polyethylene (HDPE) granules (3 mm diameter) and fluoropolymer
(PFA) tube (6 mm diameter) obtained from Aldrich were used as
supporting plastics. The PFA tubing was sliced into rings with a 1
mm width. The plastic materials were placed in a 50 mL high-
Fig. 1 (a) Optical image of HDPE, (1) original, (2) with Pd(hfa)2, (3) with
Pd nanoparticles. (b) TEM micrograph of Pd nanoparticles deposited in
HDPE; scale = 50 nm.
930
C h e m . C o m m u n . , 2 0 0 4 , 9 3 0 – 9 3 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4