Cross-linked polymer coated Pd nanocatalysts on SiO2 support: Very selective and stable catalysts for hydrogenation in supercritical CO 2

Article abstract of DOI:10.1039/b822930a

Using greener solvents, enhancing the selectivity and stability of catalysts is an important aspect of green chemistry. In this work, we developed a route to immobilize Pd nanoparticles on the surface of silica particles with cross-linked polystyrene coating by one-step copolymerization, and Pd(0) nanocatalysts supported on the silica particle supports with cross-linked polystyrene coating were successfully prepared. The catalysts were characterized by Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), plasma optical emission spectroscopy, and thermogravimetric analysis (TGA), and were used for hydrogenation of 2,4-dimethyl-1,3-pentadiene to produce 2,4-dimethyl-2-pentene and allyl alcohol to produce 1-propanol. It was found that the selectivity of the reaction was enhanced significantly by the polymer coating, and the catalysts were very stable due to the insoluble nature of the cross-linked polymers. Supercritical (sc)CO₂ can accelerate the reaction rates of the reactions catalyzed by the specially designed catalysts significantly. The excellent combination of polymer coating and scCO₂ has wide potential applications in catalysis.

Full text of DOI:10.1039/b822930a

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PAPER
www.rsc.org/greenchem | Green Chemistry
Cross-linked polymer coated Pd nanocatalysts on SiO2 support: very
selective and stable catalysts for hydrogenation in supercritical CO2
Tianbin Wu, Tao Jiang, Baoji Hu, Buxing Han,* Jinling He and Xiaosi Zhou
Received 24th December 2008, Accepted 9th February 2009
First published as an Advance Article on the web 3rd March 2009
DOI: 10.1039/b822930a
Using greener solvents, enhancing the selectivity and stability of catalysts is an important aspect
of green chemistry. In this work, we developed a route to immobilize Pd nanoparticles on the
surface of silica particles with cross-linked polystyrene coating by one-step copolymerization, and
Pd(0) nanocatalysts supported on the silica particle supports with cross-linked polystyrene coating
were successfully prepared. The catalysts were characterized by Fourier transform infrared
spectroscopy (FTIR), transmission electron microscopy (TEM), plasma optical emission
spectroscopy, and thermogravimetric analysis (TGA), and were used for hydrogenation of
2,4-dimethyl-1,3-pentadiene to produce 2,4-dimethyl-2-pentene and allyl alcohol to produce
1-propanol. It was found that the selectivity of the reaction was enhanced significantly by the
polymer coating, and the catalysts were very stable due to the insoluble nature of the cross-linked
polymers. Supercritical (sc)CO2 can accelerate the reaction rates of the reactions catalyzed by the
specially designed catalysts significantly. The excellent combination of polymer coating and scCO2
has wide potential applications in catalysis.
are not soluble in ordinary solvents or reaction substrates.
Introduction
Therefore, the nanocatalysts on the particle supports with a
It is known that metal nanoparticles are highly active and
cross-linked polymer coating should be very stable, and the
selective catalysts for many reactions. However, the naked
polymer coating may restrict access of some active bonds to
nanoparticles aggregate easily, resulting in a decrease in cat-
the active sites of the catalysts, and improve the selectivity of
alytic activity and selectivity. To solve this problem, metal
the reactions with multiple active bonds. However, how to coat
nanocatalysts have been immobilized using different supports
nanocatalysts on particle supports with cross-linked polymer
such as carbon, metal oxides, molecular sieves,1 and polymers.2
films is challenging. Besides, the coating should reduce the
Some functional polymers have also been used to stabilize
reaction rate of reactions. Therefore, acceleration of the reaction
metal nanocatalysts. For example, Jiang and Gao prepared
rate for the reactions catalyzed by this kind of specially designed
heterogeneous Pd(0) nanoparticle catalysts by encapsulating
catalyst is also important.
the nanocatalysts in polyamidoamine dendrimers on SBA-
It is known that supercritical (sc)CO2 is a cleaner solvent and a
15.3 The organic-inorganic hybrid composites showed highly
promising alternative to organic solvents. The interfacial tension
catalytic activity and selectivity for the hydrogenation of allyl
of scCO2 is zero and its diffusivity is much larger than liquids,
alcohol. Bruening and coworkers embedded Pd nanoparticles in
which can enhance the rates of many reactions.6 Therefore, it
multilayer polyelectrolyte films around alumina particles, which
is possible to increase the reaction rate of the polymer-coated
were prepared by the layer-by-layer deposition method.4 The
metal nanoparticles using scCO2.
supported catalysts were used to catalyze hydrogenation of allyl
In this work, we developed a route to immobilize Pd
alcohol, 1-penten-3-ol and 3-methyl-1-penten-3-ol, which differ
nanoparticles on the surface of silica particles with cross-
only in the substituents at the a-carbon of the double bond, and
it was demonstrated that the polyelectrolytes limited aggregation
linked polystyrene coating by one-step copolymerization. The
supported Pd catalysts were used to catalyze the hydrogenation
of the metal nanoparticles, and reaction rates of the alcohols
of 2,4-dimethyl-1,3-pentadiene and allyl alcohol (Scheme 1). It
were very different due to restricting access to active sites of the
was found that the selectivity of the reaction was enhanced
branched alcohols.
Hydrogenation is among the most important processes in the
chemical industry. In many cases it is very important to control
the selectivity of hydrogenation reactions.3,4 The design and
preparation of special catalytic materials with steric effects is an
effective way to do this.5 It is known that cross-linked polymers
Scheme 1 Illustration of Pd(0) nanocatalysts on SiO2 particles with a
cross-linked polystyrene coating and hydrogenation of 2,4-dimethyl-1,3-
pentadiene and allyl alcohol.
Beijing National Laboratory for Molecular Sciences, Institute of
Chemistry, Chinese Academy of Sciences, Beijing, 100190, China.
E-mail: hanbx@iccas.ac.cn; Fax: (+86) 10-62562821
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significantly by the polymer coating, and the catalysts were
very stable due to the insoluble nature of the cross-linked
polymers. As far as we know, this is the first work to coat the
nanocatalysts on particle supports with cross-linked polymers.
This method can be potentially used to prepare the cross-
linked polymer-coated nanocatalysts of some other metals. Our
work demonstrated that supercritical (sc)CO2 can accelerate
the reaction rates significantly. We believe that the idea to
combine cross-linked polymer-coated nanocatalysts on supports
and scCO2 can be applied to enhance the selectivity of many
reactions with two or more active bonds effectively.
Results and discussion
Synthesis of the polymer-coated catalyst
Scheme 2 shows the route to fabricate the cross-linked
polystyrene-coated catalysts. The detailed procedures to prepare
the catalysts are described in the Experimental section, and
the chemical structures of styrene (St), divinylbenzene (DVB),
potassium persulfate (KPS), 3-aminopropyltriethoxysilane
(APS), poly(N-vinyl pyrrolidone) (PVP), sodium styrene sul-
fonate (NaSS) and allylamine (AA) are given in Scheme 3. The
monodispersed SiO2 particles with the average size of about
230 nm were prepared by the method in the literature.7
Scheme 3 The chemical structures of some chemicals used.
Fig. 1 FTIR spectra of (a) original SiO2, (b) amino-functionalized
SiO2, (c) catalyst B and (d) catalyst C. The inset shows magnified FTIR
spectra.
Scheme 2 General scheme of synthesis of polystyrene-coated Pd(0)
nanoparticles on the surface of SiO2.
Fig. 1 shows FTIR spectra of pure SiO2 and amino-
functionalized SiO2 particles. The bands at 3425 and 1630 cm-1
are attributed to the stretching and bending vibrations of
water molecules adsorbed on the surface of SiO2. After the
functionalization of SiO2 with APS, the intensity of the band at
967 cm-1 (Si-OH bending vibration peak) obviously decreases
and a new band appears at 2980 cm-1 due to C-H stretching
vibration, suggesting that SiO2 particles were functionalized by
APS.8 Pd2+ ions were then anchored onto the surface of amino-
functionalized SiO2 particles. The Pd2+ on the functionalized-
SiO2 particles was reduced by H2 at 150 ◦C for 5 hours to obtain
dark brown Pd(0)-SiO2 particles (catalyst A), as shown in the
TEM image in Fig. 2a. It can be seen that Pd(0) nanoparticles
with the size of 2–5 nm were uniformly decorated on the surface
Fig. 2 TEM images of (a) catalyst A without the polymer coating,
(b) catalyst A after used 4 times, (c) catalyst B with incomplete
polystyrene coating, (d) catalyst C with complete polystyrene coating,
(e) catalyst C after used 4 times. The inserts of (b), (c), and (d) are
corresponding SEM images.
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of SiO2 particles. Coating of the cross-linked polystyrene
on the catalyst A was achieved by emulsion polymerization
using St as the monomer, DVB as a crosslinker, PVP as a
stabilizer, KPS as an initiator, NaSS as an emulsifier and AA
as a copolymeric monomer. It can be observed that incomplete
polystyrene coating was formed without the addition of AA
(Fig. 2c), while complete coating was formed with the addition
of AA (Fig. 2d). The reason may be that the surface of
SiO2 was functionalized by APS containing richly cationic
amines.9 On the other hand, polystyrene particles prepared
by emulsion polymerization using KPS as an initiator and
NaSS as an emulsifier exhibited a negatively charged surface.10
Without addition of AA, incomplete polystyrene coating
on Pd(0) nanoparticles (catalyst B) was therefore formed
by electrostatic adsorption of polystyrene particles on the
surface of SiO2 particles. In the presence of AA, however,
AA firstly interacted with Pd nanoparticles on the surface of
SiO2 particles by means of terminal amines of AA, and then
Fig. 3 Thermogravimetric curves for catalyst B (a) and catalyst C (b).
to 500 ◦C because of the thermal decomposition of polystyrene.
The final weights (assumed to be Pd(0)-SiO2) were 81.4 and
69.5% for catalysts B and C, respectively. This indicates that
the content of polystyrene in catalyst C was higher than that in
catalyst B. The two-step decomposition of catalyst C may result
from the existence of AA in the polymer films. The thickness of
catalyst C calculated from the densities and mass percents of the
silica and polystyrene in the catalyst was about 24 nm, which
agrees with that estimated from the diameters of the coated and
uncoated catalyst particles.
=
copolymerized with monomer St via a C C double bond of AA,
resulting in the formation of complete polystyrene coating on
Pd(0) nanoparticles (catalyst C) with the average thickness of
polystyrene coating about 25 nm, which was estimated from the
diameters of the coated and uncoated particles. This indicates
that the existence of AA can greatly affect the morphology of
polystyrene coating. FTIR spectra of catalysts B and C (Fig. 1,
Spectra c and d) further confirm the presence of the cross-linked
polystyrene on the surface of SiO2 particles according to the
typical absorption bands of polystyrene at 1452, 1492, 2928 and
3028 cm-1.11 The contents of Pd(0) in catalyst A, B and C were
0.47%, 0.35% and 0.28%, respectively, which was determined
by inductively coupled plasma optical emission spectroscopy
(ICP-AES, Vista-MPX).
Hydrogenation reactions
We studied the hydrogenation of 2,4-dimethyl-1,3-pentadiene
and allyl alcohol (Scheme 1) using the three catalysts, and the
results are listed in Tables 1 and 2, respectively. The naked Pd(0)
nanoparticles in catalyst A were much more active than the
polymer-coated Pd(0) nanoparticles in catalyst B and catalyst
C. This is easy to understand because the polymer coating
Fig. 3 shows the TGA results of catalyst B and catalyst C. The
major weight losses occurred in the temperature range from 250
Table 1 The conversion, selectivity and turnover frequency (TOF) of hydrogenation of 2,4-dimethyl-1,3-pentadiene using different catalysts at
40 ◦C a
Selectivity [%]
2,4-dimethyl-2-
pentene
2,4-dimethyl-1-
pentene
2,4-dimethyl-
pentane
Entry
Catalyst
PCO2 [MPa]
Time [h]
Conversion [%]
TOFb
1
A
A
A
A
A
B
C
C
C
C
C
C
C
C
0
0
0
0
0
0
0
2
5
8
8
8
8
0.2
0.3
0.5
0.7
1.0
>99.7
>99.7
>99.7
>99.7
>99.7
>99.7
>99.7
27.5
28000
18667
14000
8000
5600
2604
57
96
192
350
350
350
350
350
302
277
22400
51.2
51.5
51.1
51.2
51.3
65.0
75.2
75.5
77.2
80.1
80.2
79.9
80.0
79.5
77.1
77.0
50.5
48.4
48.2
48.3
48.5
48.2
34.5
24.5
24.4
22.7
19.8
19.6
20.0
19.9
20.4
22.8
22.9
48.9
0.4
0.3
0.6
0.3
0.5
0.5
0.2
0.1
0.1
0.1
0.2
0.1
0.1
0.1
0.1
0.1
0.6
1-1c
1-2c
1-3c
1-4c
2
2.5
3
4
5
6
99.0
16.0
16.0
16.0
16.0
16.0
16.0
16.0
16.0
16.0
0.25
54.8
>99.7
>99.7
>99.7
>99.7
>99.7
86.2
6-1d
6-2d
6-3d
6-4d
7
8
C
11
14
0
8
9
C
79.1
>99.7
Pd/Ce
a Reaction conditions: H2, 3.0 MPa; the molar ratio of 2,4-dimethyl-1,3-pentadiene to Pd(0) is 5600. b The turnover frequencies (TOFs) are moles
of reacted substrate per mole of Pd(0) per hour. c Hydrogenation reaction over catalyst A after being used 4 times. d Hydrogenation reaction over
catalyst C after being used 4 times. e Commercial Pd/C with 5% Pd on carbon powder from Baoji Rock Co. Ltd, China.
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Table 2 The conversion, selectivity, and turnover frequency (TOF) of hydrogenation of allyl alcohol using different catalysts at 40 ◦C a
Entry
Catalyst
PCO2 [MPa]
Time [h]
Conversion [%]
TOFb
Selectivity [%]c
1
A
A
A
A
A
B
B
C
C
C
C
C
0
0
0
0
0
0
8
0
8
8
8
8
8
0
1.0
1.1
1.2
1.3
1.5
5.0
3.5
9.0
5.0
5.0
5.0
5.0
5.0
1.5
>99.7
>99.7
>99.7
>99.7
>99.7
>99.7
>99.7
>99.7
>99.7
>99.7
>99.7
>99.7
>99.7
>99.7
7500
6818
6250
5769
5000
1500
2140
833
1500
1500
1500
1500
1500
5000
75.3
75.6
75.4
75.3
75.7
83.1
86.4
92.4
93.1
92.9
93.0
92.6
92.4
74.0
1-1d
1-2d
1-3d
1-4d
2
3
4
5
5–1e
5–2e
5–3e
5–4e
6
C
Pd/Cf
a Reaction conditions: H2, 2.0 MPa, the molar ratio of allyl alcohol to Pd(0) is 7500. b The turnover frequencies (TOFs) are moles of reacted substrate
per mol of Pd(0) per hour. c The selectivity to 1-propanol (product). d Hydrogenation reaction over catalyst A after being used 4 times. e Hydrogenation
over Catalyst C after being used 4 times. f Commercial Pd/C, 5% Pd on carbon powder from Baoji Rock Co. Ltd, China.
restricts the access of the reactants to the active sites of the
catalysts. At the same time, this provides further evidence that
Pd nanocatalysts were coated by the polymer.
Table 1 shows that the selectivity of the reaction to the two
products, 2,4-dimethyl-1-pentene and 2,4-dimethyl-2-pentene,
was nearly the same when catalyst A was used (entry 1), while the
selectivity to 2,4-dimethyl-2-pentene was much higher than that
to 2,4-dimethyl-1-pentene when catalyst C was used (entry 3).
For catalyst A, the two double bonds of the pentadiene with
similar activity have similar opportunity to contact the naked
Pd(0) nanoparticles on SiO2 particles, leading to approximately
the same selectivity for the two pentenes. For catalyst C, however,
the selectivity to 2,4-dimethyl-2-pentene was 75.2%, which is
much higher than that catalyzed by silica-supported PAMAM-
Pd complexes with 2,4-dimethyl-2-pentene selectivity of 59.0%.5c
In other words, more terminal double bonds were hydrogenated.
This is understandable because the reactant is a linear molecule,
it prefers to approach the Pd particles perpendicularly through
the coating surface. Therefore, the terminal double bond has
more opportunity for access to Pd(0) nanoparticles than the
internal double bond. As catalyst B was used, the selectivities to
2,4-dimethyl-2-pentene and 2,4-dimethyl-1-pentene lie between
those of catalyst A and catalyst C because of the incomplete
coating (entry 2).
In this work, we also studied the reaction in scCO2 catalyzed
by catalyst C because it was the most selective. The results
Fig. 4 The selectivity of products versus conversion of 2,4-dimethyl-1,3-
pentadiene over catalyst A under solvent-free conditions (a), catalyst C
at different pressures are also listed in Table 1 (from entry 3
to entry 8). Obviously, scCO2 can enhance the reaction rate
significantly. Moreover, under scCO2 conditions, the selectivity
to 2,4-dimethyl-2-pentene can reach 80.1% using catalyst C
(entry 6), which is higher than that under solvent-free conditions
(entry 3). This may result from the higher diffusivity of the
molecules in scCO2. After the hydrogenation of the terminal
double bond, the product diffuses out more quickly than the
case without scCO2.
The selectivity at different conversions using catalysts A and C
are shown in Fig. 4. It can be seen that for the two catalysts used
the selectivities to 2,4-dimethyl-2-pentene and 2,4-dimethyl-1-
pentene are almost constant with increasing conversion. It is
under solvent-free conditions (b), and catalyst C under scCO2 (8.5 MPa)
condition (c). (ꢀ) 2,4-dimethylpentane; (᭹) 2,4-dimethyl-1-pentene; (ꢀ)
2,4-dimethyl-2-pentene. Reaction conditions: H2 3.0 MPa, temperature
40 ◦C, the molar ratio of substrate and Pd(0) was 5600.
also worth noticing that the formation of pentane is almost
negligible prior to complete consumption of the pentadiene for
both catalyst A and catalyst C.
For the hydrogenation of allyl alcohol to produce 1-propanol
in Table 2, the selectivity was also greatly improved by the
polymer coating on Pd(0) nanoparticles. Especially, the selec-
tivity to 1-propanol approached 92.4% when catalyst C was
used (entry 4), which was much higher than that of catalyst
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A (75.3%, entry 1) and commercial Pd/C (74.0%, entry 6),
suggesting that the selectivity to 1-propanol was enhanced by the
polystyrene coating. The mechanism to enhance the selectivity
of this reaction is different from that of the hydrogenation of 2,4-
dimethyl-1,3-pentadiene. During hydrogenation of allyl alcohol
performed with Pd(0) nanoparticles, allyl alcohol also easily
isomerizes into byproduct propanal or acetone.3,4,12 As catalyst
C was used, the polymer coating in the catalysts suppressed
isomerization of allyl alcohol during hydrogenation because it
modified both the environment of active sites and access to
these active sites, similar to the function of dendrimers3 and
polyelectrolyte multilayers4 The reaction catalyzed by catalyst
C was also conducted in scCO2. As can be seen in Table 2,
the activity of catalyst C under scCO2 condition (entry 5)
is much higher than in the absence of CO2 (entry 4), which
further demonstrated that scCO2 can accelerate the reaction
rates significantly. The main reason for this is that scCO2 has
high diffusion and low viscosity, which is favourable to the mass
transfer of the reactant and the product.
Experimental
Materials
Tetraethyl orthosilicate (TEOS) from Beijing Beihua Fine
Chemical Company was distilled prior to use. Aqueous NH3
solution with a NH3 content of 25% was supplied by Beijing Yili
Fine Chemical Company. 3-Aminopropyltriethoxysilane (APS,
analytical grade) provided by Beijing Shenda Fine Chemical
Company was used as received. Potassium persulfate (KPS),
allylamine (AA), poly(N-vinyl pyrrolidone) (PVP), sodium
styrene sulfonate (NaSS) and palladium chloride (PdCl2) were
all analytical grade and purchased from the Beijing Chemical
Reagent Company. Styrene (St, analytical grade) from Beijing
Chemical Reagent Company and divinylbenzene (DVB) from
Tokyo Kasei Kogyo Company were distilled under reduced
pressure before use. 2,4-Dimethyl-1,3-pentadiene and allyl al-
cohol were provided by Aldrich. Hydrogen (99.99%) and CO2
(99.995%) were supplied by the Beijing Analytical Instrument
Factory.
Reusability of the catalysts
Preparation of SiO2 particles
The recyclability of catalysts A and C was tested in this work.
After each cycle, the catalyst was reused after washing with
ethanol and drying. The results are also presented in Table 1
(entries 1-1, 1-2, 1-3 and 1-4 for catalyst A and entries 6-1,
6-2, 6-3 and 6-4 for catalyst C) and Table 2 (entries 1-1, 1-2,
1-3 and 1-4 for catalyst A and entries 5–1, 5–2, 5–3 and 5–4
for catalyst C). Clearly, the reduction in activity and selectivity
of catalyst C was negligible after being reused 4 times, while
the activity of catalyst A reduced significantly. The reason is
that the stable polymer coating can protect the Pd nanoparticles
from aggregation and falling off the support. Figs. 2b and 2e
show the TEM images of catalyst A and catalyst C after being
used 4 times for hydrogenation of 2,4-dimethyl-1,3-pentadiene,
respectively. Apparently, the number of Pd particles of the used
catalyst A is much less than that of the original one, as can be
seen from Figs. 1a and 1b. However, the morphology of catalyst
C was not changed noticeably after being used 4 times (Fig. 2e,
the Pd particles cannot be seen because of the coating), providing
further evidence for the excellent stability.
The monodisperse SiO2 particles were synthesized according to
the well-known Sto¨ber method.7 Typically, the ethanol solution
of TEOS was quickly added into the other ethanol solution of
NH3 and H2O in a 500 mL Erlenmeyer flask. The molar ratio
of NH3:H2O:TEOS was 1:11:0.22 in the final gel mixture that
was stirred at 30 ◦C for 4 hours. The resulting SiO2 particles
were centrifugally separated from the suspension, ultrasonically
washed with ethanol three times, and finally dried at 100 ◦C.
Preparation of amino-functionalized SiO2 particles
The monodisperse SiO2 particles were functionalized with APS.8
In a typical experiment, SiO2 particles (1 g) and APS (1.2 g)
were added into a round-bottomed flask containing 15 mL of
toluene. After the mixture was stirred under refluxing at 110 ◦C
for 24 hours, the obtained amino-functionalized SiO2 particles
were centrifuged, washed three times with toluene and twice
with ethanol, respectively, and dried in air at 100 ◦C.
Preparation of Pd(0) nanoparticles on amino-functionalized
SiO2 particles
Conclusions
In summary, Pd(0) nanocatalysts on a silica support with a
cross-linked polystyrene coating were successfully prepared.
Highly selective hydrogenation of 2,4-dimethyl-1,3-pentadiene
to 2,4-dimethyl-2-pentene and allyl alcohol to 1-propanol can
be achieved using the catalysts fabricated, and the catalysts are
very stable. The polymer coating provides a steric effect for the
high selectivity of the reactions and enhances the stability of the
catalysts. scCO2 can accelerate the reaction rates considerably.
We believe that this method can also be used to prepare some
other nanocatalysts on the particle supports with a very stable
polymer coating, and this class of catalysts may also be used
to enhance the selectivity of some other reactions with more
than one active chemical bond of similar activity, where high
selectivity is difficult to reach by other catalysts.
Amino-functionalized SiO2 particles (1.2 g) were ultrasonically
dispersed with 40 mL of water in a 100 mL round-bottomed
flask, followed by the addition of 32 mL of PdCl2 acidic
solution (2 mmol L-1, pH = 3). The mixture was stirred
with a magnetic stirrer at room temperature for 30 min. Pd2+
ions formed a complex with the amine group on the surface
of amino-functionalized SiO2 particles. Separated centrifugally
from the suspension and washed ultrasonically with deionized
water, Pd2+-adsorbed-amino-functionalized-SiO2 particles were
obtained and dried in air at 100 ◦C for 4 hours. The dried Pd2+-
adsorbed-amino-functionalized-SiO2 particles were reduced us-
◦
ing H2 (3 MPa) at 150 C for 5 hours in a 6 mL high-pressure
stainless steel reactor, leading to the formation of dark brown
Pd(0)-SiO2 particles.
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Preparation of polymer-coated Pd nanoparticles on
APS-functionalized SiO2 particles
were obtained in the region of 400–4000 cm-1 by using a Bruker
Tensor 27 spectrometer. The contents of Pd(0) in catalyst A,
B and C were 0.47%, 0.35% and 0.28%, respectively, which
was determined by inductively coupled plasma optical emission
spectroscopy (ICP-AES, Vista-MPX).
The dark brown Pd(0)-SiO2 particles (0.3 g) were ultrasonically
dispersed with 35 mL of water in a 250 mL round-bottomed
flask equipped with a condenser. Allylamine (AA, 0.25 mL) and
poly(N-vinyl pyrrolidone) (PVP, 0.5 g) were added into the flask
and agitated with a magnetic stirrer at room temperature for
1 hour. Then, 0.25 g of sodium styrene sulfonate (NaSS), 0.15 g
of potassium persulfate (KPS) and 40 mL of water were added
into the flask and allowed to mix for 30 min. 2 mL of styrene (St)
and 0.2 mL of divinylbenzene (DVB) were then added. Finally,
the polymerization was carried out in a nitrogen atmosphere
under stirring at 80 ◦C for 12 hours. The resulting polystyrene-
coatedPd(0) nanoparticles on APS-functionalized SiO2 particles
were centrifugally separated from the suspension, ultrasonically
washed three times with water and twice with ethanol, and dried
under vacuum at 60 ◦C for 24 hours.
Acknowledgements
The authors are very grateful to the National Natural Science
Foundation of China (20773144) and National Key Basic
Research Project of China (2006CB202504).
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Hydrogenation reaction
Hydrogenation of 2,4-dimethyl-1,3-pentadiene and allyl alcohol
was carried out in a 6 mL high-pressure stainless steel reactor
with a magnetic stirrer with or without scCO2. In the experiment,
2 mmol of substrate and the desired amount of catalyst were
introduced into the reactor, and then the reactor was sealed.
The air in the reactor◦was replaced by CO2 and the reactor
was then heated to 40 C. After the introduction of H2 to the
desired pressure, CO2 was charged into the reactor with a high-
pressure pump (DB-80) to the desired pressure. The reaction
was performed under stirring for a desired time. The reactor
was then cooled in an ice bath and the gases were vented slowly.
After the catalyst was centrifuged from the suspension, the liquid
phase was analyzed by GC (Agilent 4890 D) equipped with a
flame-ionized detector. The retention time of the products was
compared with available authentic standards. The purity and
structure of the products obtained at some typical experimental
conditions were also checked by GC-MS methods. In the
catalyst recycling experiments, the product and the catalyst were
centrifuged at 16000 rpm for 5 min and the catalyst was reused
after washing with ethanol and drying.
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Characterization
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Thermogravimetric analysis (TGA) was performed using a
Perkin-Elmer 7 Series Thermal Analysis System. Dried samples
◦
◦
were heated to 750 C in air at a heating rate of 10 C min-1,
and the observed mass loss was attributed to the quantitative
degradation of the polystyrene. The morphologies of the samples
were characterized using scanning electron microscopy (SEM)
and transmission electron microscopy (TEM), which were
performed on a JEOL JSM-6700F field emission microscope
and on a JEOL 2010 transmission electron microscope operating
at 200 kV, respectively. The Fourier transform infrared (FT-IR)
spectra of the samples through compressing with KBr powder
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