Synthesis of polymeric ionic liquid microsphere/Pt nanoparticle hybrids for electrocatalytic oxidation of methanol and catalytic oxidation of benzyl alcohol

Article abstract of DOI:10.1002/pola.24895

Herein, we present a facile approach for the synthesis of polymeric ionic liquids (PILs) microspheres for metal scavenging and catalysis. Crosslinked poly(1-butyl-3-vinylimidazolium bromide) microspheres with the diameter of about 200 nm were synthesized via miniemulsion polymerization, in which 1,4-di(vinylimidazolium) butane bisbromide was added as the crosslinker. Anion exchange of PIL microspheres with Pt precursor and followed by the reduction of Pt ions produced PIL microsphere supported Pt nanoparticle hybrids. The synthesized Pt nanoparticles with a diameter of about 2 nm are uniformly dispersed and strongly bound to the surface of PIL microspheres. The catalytic performances of PIL/Pt nanoparticle hybrids were evaluated for both the electrocatalytic oxidation of methanol and oxidation of benzyl alcohol. The PIL/Pt nanoparticle hybrids show better electrocatalytic activity towards the electrooxidation of methanol than pure Pt nanoparticles. Furthermore, they are effective and easily reusable catalysts for the selective oxidation of benzyl alcohol in aqueous reaction media, demonstrating that the synthesized PIL microspheres are suitable scaffolds for heterogeneous catalysts Pt. Copyright

Full text of DOI:10.1002/pola.24895

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ARTICLE
Synthesis of Polymeric Ionic Liquid Microsphere/Pt Nanoparticle Hybrids
for Electrocatalytic Oxidation of Methanol and Catalytic Oxidation of
Benzyl Alcohol
1
1
1
1
1
2
Jianhu Yang, Lihua Qiu, Baoqiang Liu, Yingjing Peng, Feng Yan, Songmin Shang
1
Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Department of Polymer Science and
Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou, 215123, China
2
Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hong Kong, China
Correspondence to: F. Yan (E-mail: fyan@suda.edu.cn)
Received 23 April 2011; accepted 18 July 2011; published online 9 August 2011
DOI: 10.1002/pola.24895
ABSTRACT: Herein, we present a facile approach for the synthesis
of polymeric ionic liquids (PILs) microspheres for metal scaveng-
ing and catalysis. Crosslinked poly(1-butyl-3-vinylimidazolium
bromide) microspheres with the diameter of about 200 nm were
synthesized via miniemulsion polymerization, in which 1,4-
di(vinylimidazolium) butane bisbromide was added as the cross-
linker. Anion exchange of PIL microspheres with Pt precursor
and followed by the reduction of Pt ions produced PIL micro-
sphere supported Pt nanoparticle hybrids. The synthesized Pt
nanoparticles with a diameter of about 2 nm are uniformly dis-
persed and strongly bound to the surface of PIL microspheres.
The catalytic performances of PIL/Pt nanoparticle hybrids were
evaluated for both the electrocatalytic oxidation of methanol
and oxidation of benzyl alcohol. The PIL/Pt nanoparticle hybrids
show better electrocatalytic activity towards the electrooxidation
of methanol than pure Pt nanoparticles. Furthermore, they are
effective and easily reusable catalysts for the selective oxidation
of benzyl alcohol in aqueous reaction media, demonstrating that
the synthesized PIL microspheres are suitable scaffolds for het-
erogeneous catalysts Pt. VC 2011 Wiley Periodicals, Inc. J Polym
Sci Part A: Polym Chem 49: 4531–4538, 2011
KEYWORDS: electrocatalysis; heterogeneous catalysis; ionic
liquids; miniemulsion polymerization
INTRODUCTION Metal nanoparticles are attracting a great
The deposited metal nanoparticles were strongly bound to
the microsphere surface and could not be removed by either
vigorous stirring or ultrasonication. Akashi and coworkers de-
deal of attention because of their potential applications in
1
8
electronic, photonic, and magnetic devices. However, direct
application of metal nanoparticles is usually difficult because
of their ultrasmall size and high tendency of agglomeration
into large clusters due to the van der Waals interactions. It
posited well-dispersed Pd nanoparticles on the surface of poly-
styrene-co-poly(N-isopropylacrylamide) microspheres, and the
catalytic properties of the resulting hybrids were investigated.
2
9
(a)
has demonstrated that such drawbacks could be partially
overcome by use of suitable carriers or supporting materials,
Zhang and coworkers
reported the synthesis of noble metal
nanoparticles embedded in the shell layer of core-shell poly
(styrene-co-4-vinylpyridine) microspheres. The resultant shell-
embedded Pd nanoparticle hybrids are an efficient and easily
reusable catalyst for Suzuki reactions. Recently, ionic liquid-
grafted rigid poly(p-phenylene) microspheres have been devel-
oped for metal scavenging and deemed to be an effective
3
4
5
such as dendrimers, polymers, polymeric microgels, and
6
polymeric or inorganic spheres. Among the various carriers,
polymeric microspheres have received particular attention
because the loading of metal nanoparticles onto the poly-
meric microsphere surface facilitates the high amount of ac-
cessible surface active sites of metal nanoparticles, and
thereby raises the activity of the catalysts. Furthermore,
polymeric microspheres can be easily synthesized, purified,
and surface modified with functional groups.
9
(b)
heterogeneous catalyst.
Moreover, polyelectrolyte-coated
1
0
polymeric microspheres prepared by surface grafting or
1
1
layer-by-layer deposition have also been used as scaffolds for
the immobilizing of metal nanoparticles. However, preparation
of the polymeric sphere templates mentioned above generally
needs a multistep synthesis, which is relatively time-consuming.
A variety of polymeric microspheres with or without surface
modification have been synthesized and used as templates for
7
–11
ꢀ
the immobilization of metal nanoparticles.
For instance,
Ionic liquids are organic salts being liquid below 100 C. They
7
Thompson and coworkers loaded Pt and Au nanoparticles
onto the amino-functionalized polystyrene microsphere surface.
are attracting wide attentions because of their negligible vapor
pressure, high ionic conductivity, and wide electrochemical
V
C
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SCHEME 1 Reaction Scheme for the Preparation of PIL/Pt Hybrids.
window. More recently, growing attention has been paid to poly-
meric ionic liquids [(PILs), polymers formed from IL monomers],
because they are a new class of polymers that combine both the
novel properties of ILs and improved mechanical durability and
dimensional control. Most studies on PILs are related to the de-
velopment of advanced polymer gels or electrolytes for electro-
steps: the synthesis of PIL microspheres via miniemulsion
ꢁ
polymerization, and the Br ions of ILs moiety were then
exchanged with PtCl2 and followed by the reduction of
ꢁ
4
metal ions with NaBH to zerovalent metal nanoparticles to
4
produce the PIL/Pt nanoparticle hybrids. In the first stage,
copolymerization of [Bvim][Br] and [C (vim) ][Br] in a
4 2 2
water/oil miniemulsion stabilized with Span-80 produced
highly cross-linked PIL microspheres with a diameter of
about 200 nm although some smaller spheres are observed
as well (Fig. 1). Thermogravimetric analysis (TGA) revealed
12
chemical devices. Furthermore, the excellent ion-exchange
capability of PILs enables the preparation of PIL microspheres
with a variety of counter anions by polymerization of only one IL
13,14
monomer and followed by anion-exchange reactions.
that the produced PIL microspheres decomposed at about
Herein, we propose a facile strategy for the synthesis of PIL
microspheres and their application as potential scaffolds for
the immobilizing of metal nanoparticles for electrocatalysis
and heterogeneous catalysis. The approach involves the syn-
thesis of PIL microspheres via miniemulsion polymerization,
the anion-exchange of PIL microspheres with metal precursor
and followed by the reduction of metal ions to zerovalent
metal nanoparticles. Taking the advantage of the high ionic
conductivity of PILs, the electrocatalytic activities of produced
PIL microsphere/Pt nanoparticle (denoted as PIL/Pt) hybrids
toward the methanol oxidation reaction were evaluated. The
catalytic performances of the resultant PIL/Pt hybrids were
evaluated using the oxidation of aerobic benzyl alcohol as the
model reaction. The activity and durability of the heterogene-
ous catalysts were systematically investigated as well.
ꢀ
250 C. Such a thermal stability of PIL microspheres makes
it possible to be used for medium temperature catalytic
reactions.
ꢁ
The PIL microspheres bearing Br counter anions were
anion exchanged with PtCl2 anions and subsequently
ꢁ
4
reduced by NaBH4 to produce PIL/Pt nanoparticle hybrids.
Figure 2 shows the representative transmission electron mi-
croscopy (TEM) images of the synthesized PIL/Pt nanopar-
ticle hybrids. It can be clearly seen that high-density Pt
nanoparticles were well dispersed on the surface of PIL
microspheres. The inset of Figure 2(B) shows the size distri-
bution of Pt nanoparticles deposited on PIL microspheres. It
is observed that the Pt nanoparticles show a relatively nar-
row size distribution and have a size of about 2.1 nm with
the deviation of 0.2 nm. The formation of high-density and
well-dispersed Pt nanoparticles on the surface of PIL micro-
spheres is probably due to the excellent anion exchange abil-
RESULTS AND DISCUSSION
Synthesis of PIL-Metal Nanoparticle Hybrids
Scheme 1 shows the reaction route for the preparation of
PIL/Pt hybrids. The synthesis procedure involves following
ity of PILs that enables PtCl2 anions to be enriched on the
ꢁ
4
surface of PIL microspheres, which therefore facilitates the
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FIGURE 3 XRD patterns of (A) PIL microspheres and (B) PIL/Pt
nanoparticle hybrids. [Color figure can be viewed in the online
issue, which is available at wileyonlinelibrary.com.]
FIGURE 1 TEM image of the PIL microspheres synthesized via
ꢀ
miniemulsion polymerization.
23.4 corresponding to the diffraction of amorphous nature
of PIL microspheres. Such a weak peak at the same location
can be also observed in Figure 3(B). The XRD pattern of
PIL/Pt nanoparticle hybrids is shown in Figure 3(B). Typical
1
5
preparation of high-density Pt nanoparticles. Moreover, the
low interfacial tensions of the imidazolium groups on the
surface of PILs microspheres promote the nucleation rate of
Pt nanoparticles, and thus allowing the formation of small
ꢀ
ꢀ
ꢀ
ꢀ
peaks at 39.6 , 46.3 , 67.4 and 81.4 can be assigned to
111), (200), (220), and (311) crystalline plane diffraction
(
peaks of Pt nanoparticles, respectively. The broad peaks in
the XRD pattern indicate that the synthesized Pt nanopar-
ticles are small. Here, the peak width of the (111) Bragg
reflection was chosen to calculate the mean size of the Pt
1
6
nanoparticles because of the low rates of Ostwald ripening.
TEM images also demonstrate that the Pt nanoparticles are
strongly bound to the surface of PIL microspheres and could
not be removed by either vigorous stirring or even ultrasonica-
tion. The strong interaction between PIL microsphere and metal
nanoparticles is probably due to the electrostatic interaction
1
3
nanoparticles by using the Debye-Scherrer equation, which
gives a mean size of 2.4 nm. This value agrees well with the
TEM images (Fig. 2).
1
5,16
between imidazolium cations and metal nanoparticles.
These results suggest that the PIL microspheres are one of the
suitable scaffolds for the metal nanoparticle synthesis.
Catalysts for Electrocatalytic Oxidation of Methanol
Direct methanol fuel cells (DMFCs) are promising energy
conversion devices that convert the chemical energy stored
in the fuel directly into electricity. However, high cost and
Figure 3 shows the X-ray diffraction (XRD) spectra of PIL/Pt
nanoparticle hybrids. Figure 3(A) shows a broad peak at
FIGURE 2 TEM images of PIL/Pt nanoparticle hybrids (insets of B show the corresponding Pt particle size distribution).
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modified glassy carbon (GC) electrode (denoted as Pt/GC)
was applied for the electrooxidation of methanol under the
same experimental condition for comparison.
The performance of the PIL/Pt nanoparticle hybrids toward
the electrocatalytic activity was firstly evaluated by cyclic
voltammetry (CV). Figure 4 shows the CVs for PIL/Pt/GC
and Pt/GC in 0.5 M H SO solution. The preoxidation/reduc-
2
4
tion peaks of Pt indicate that Pt nanoparticles have a very
clean active surface on the electrode. A pair of current peaks
between ꢁ0.20 and þ0.10 V are attributed to the hydrogen
adsorption and desorption at Pt nanoparticles. The electro-
chemical surface areas (ECSA) was measured from the area
of the hydrogen adsorption–desorption peaks after correct-
1
8
ing for the double layer charging current from the CV. The
2
ECSA value of the PIL/Pt/GC is calculated to be 27.04 m
g
2
FIGURE 4 CVs of PIL/Pt/GC and Pt/GC in N saturated 0.5 M
ꢁ
1
2
ꢁ1
ꢁ1
of Pt, which is higher than that of Pt/GC (4.29 m g of
2 4
H SO solution at a scan rate of 50 mV s . [Color figure can
Pt). It has demonstrated that the higher ECSA value of a cat-
alyst, the higher the electrocatalytic activity. Therefore, it is
expected that the PIL/Pt nanoparticle hybrids will have a
higher electrocatalytic activity than pure Pt catalyst depos-
ited on the GC electrode.
be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
carbon monoxide poisoning of Pt catalysts are still the bar-
riers that impede widescale commercialization of DMFCs
1
7
Figure 5(A) shows the CVs of PIL/Pt/GC and Pt/GC in a 0.5
technologies. Recently, Pt and Pt-based alloys deposited on
the supporting materials have been investigated as the cata-
lyst for electrooxidation of methanol. To overcome this obsta-
cle and lower the cost of DMFCs, lots of supporting materials
for Pt nanoparticles have been tested. It has already demon-
strated that a suitable supporting material for dispersing me-
tallic nanoparticles for electrocatalysis should have a high
conductivity to provide a facile flow of electronic charges
through the supporting material in electrochemical processes
and reduce the inner electrical resistance of the catalytic
layer. Conductive supporting materials such as carbon nano-
tubes, graphene sheets, and conducting polymers have
been extensively investigated as catalyst support materials
for electrooxidation of methanol. As the PIL microspheres
prepared in this work are high ionic conducting polymers,
they are first used as the nanoelectrocatalysts for the elec-
trooxidation of methanol. Pure Pt nanoparticles (ꢂ4 nm)
M H SO4 solution containing 0.5 M methanol in a potential
2
ꢁ
1
window of 0–1.0 V at a scan rate of 50 mV s . It can be
clearly seen that the voltammogram of methanol oxidation at
PIL/Pt/GC and Pt/GC was very similar. In the forward scan,
the onset potential and the prominent anodic peak were
observed at about 0.18 and 0.65 V, respectively. An anodic
peak appeared at around 0.46 V which was assigned to the
removal of incompletely oxidized carbonaceous species gen-
erated in the forward scan is observed in the reverse scan.
As can be seen from Figure 5(A) that the peak current of
PIL/Pt/GC for the electrooxidation of methanol is 120 mA
per mg Pt, which is about 1.45 times higher than that of Pt/
GC (83 mA per mg Pt). The results indicate that the PIL/Pt
nanoparticle hybrids have much better electrocatalytic activ-
ity toward the electrooxidation of methanol than pure Pt
nanoparticles. The high efficiency of PIL/Pt nanoparticle
1
8
19
20
ꢁ1
FIGURE 5 (A) CVs of PIL/Pt/GC and Pt/GC at a scan rate of 50 mV s ; and (B) effect of forward CV scan rate on PIL/Pt/GC in N
2
sat-
urated 0.5 M H SO OH electrolyte. Inset of Figure 5(B) shows the anodic peak currents versus square root of the scan
2
4
þ 0.5 M CH
3
1
/2
rates (v ). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
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FIGURE 6 (A) Potential-dependent steady-state current of PIL/Pt/GC and Pt/GC (recorded at 120 s), and (B) long-term stability of
ꢁ1
PIL/Pt/GC and Pt/GC in N
2
saturated 0.5 M H
2
SO
4
þ 0.5 M CH
3
OH electrolyte at a scan rate of 50 mV s .
hybrids on electrocatalytic oxidation methanol could be due
to the small size and highly distributed Pt nanoparticles on
the PIL microsphere surface, as well as high electron transfer
at the electrochemical interface.
cycle CVs in 0.5 M H SO solution containing 0.5 M metha-
2 4
nol, which shows the variation of peak current density with
respect to cycle number. It can be seen that the peak current
density decreases gradually with the successive scans and
about 63% of the catalytic activity is kept after 200 succes-
sive scans. However, the catalytic activity of Pt/GC decreased
rapidly and about 10% was retained after 200 cycles. The
loss of catalytic activity is probably due to the consumption
of methanol during the CV scans or catalyst poisoning by
adsorbing of CO produced in methanol electrooxidation proc-
During the methanol oxidation process, the adsorption of in-
termediate carbonaceous species on the catalyst’s surface
would lead to ‘‘catalyst poisoning.’’ The ratio of the forward
oxidation current peak (I ) to the reverse current peak (I ),
f
b
I /I , is an important index of the catalyst tolerance to the
f
b
carbonaceous species accumulation. A higher ratio indicates
2
0(a),22
ess.
The results demonstrate that the activity and sta-
that methanol is efficiently oxidized to CO and a little accu-
2
2
1
bility of PIL/Pt hybrids are higher than pure Pt deposited on
GC electrode.
mulation of carbonaceous residues at the catalyst surface.
Here, the I /I value is calculated to be 1.94 for PIL/Pt and
.78 for pure Pt nanoparticles, respectively. The high I /I
f
b
1
Catalysts for Oxidation of Benzyl Alcohol
f
b
value of PIL/Pt catalyst indicates more effective oxidation of
methanol occurs and most of the intermediate carbonaceous
The polymeric microsphere-supported catalysts have
attracted much attention because they facilitate the separa-
tion process by precipitation, ultracentrifugation, and ultrafil-
tration. Furthermore, the high dispersion of the polymeric
microspheres in the reaction medium allows the reactant
molecules to easily access to the active site of nanoparticle
catalysts.
species have been oxidized to CO during the forward poten-
2
2
3
tial scan. The enhanced stability for methanol oxidation of
PIL/Pt hybrids is probably due to the strong interactions
between the PIL microsphere surface and Pt nanoparticles
which inhibit the formation of strongly bond chemisorbed
2
0(a)
carbonaceous species.
The development of oxidation process is a topic of current
interests. The selective oxidation of alcohols into carbonyl
compounds is one of the most important fundamental reac-
tions both at the laboratory and industrial level. Most
researches are commonly referred to the selectivity for alde-
To further evaluate the transport characteristics of methanol
in the PIL/Pt hybrids-modified GC electrode, the effect of dif-
ferent scan rate on CVs of PIL/Pt/GC has also been investi-
gated [Fig. 5(B)]. The anodic peak currents are linearly pro-
portional to the square root of the scan rates [inset of Figure
6
(f),24
hydes,
while the direct conversion of primary alcohols
2
1
5(B)], suggesting that the electrocatalytic oxidation of metha-
to the corresponding carboxylic acids is still a challenge.
nol at PIL/Pt nanoparticle hybrids is a diffusion-controlled
surface process.
Here, we expand the use of the produced PIL/Pt nanopar-
ticle hybrids as heterogeneous catalysts for the oxidation of
benzyl alcohol to carboxylic acid using oxygen as the oxidant.
Considering of the green-chemistry processes and low cost
for the clean catalytic synthesis under mild conditions water
was chosen as the reaction media instead of organic solvent
2
2
Figure 6(A) shows that the steady-state polarization curve
for PIL/Pt hybrids is about 25 times as high as that for Pt
nanoparticles during the whole testing process, indicating
that the PIL/Pt nanoparticle hybrids have excellent electroca-
talytic activity toward the electrooxidation of methanol.
From the viewpoint of practical applications, the long-term
stability of the catalyst modified electrode is very important.
The stability of the PIL/Pt/GC was characterized by multi-
2
5
in this work.
The effect of catalyst amount on the alcohol conversion was
first investigated and the data were summarized in Table 1.
Under the same experimental conditions, the blank
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a
TABLE 1 Benzyl Alcohol Oxidation Catalyzed by PIL/Pt Nanoparticle Hybrids with Different Amounts of Catalysts
Selectivity (%)
Entries
PIL/Pt Hybrids (g)
Benzyl Alcohol Conversion (%)
Benzaldehyde
Benzoic Acid
1
2
3
4
5
a
0
–
–
–
0.002
0.005
0.01
0.02
55
61
92
92
87
71
0
13
29
100
100
ꢀ
0
Benzyl alcohol (0.432 g, 4 mmol), K
4 h.
2
CO
3
(0.552 g, 4 mmol), PIL/Pt hybrids (containing 19.5 wt % of Pt), H
2
O (50 mL), 90 C, O
2
,
2
experiment in the absence of catalyst did not show any cata-
lytic activity even after 24 h reaction (Table 1, entry 1). At
low catalyst amount, a low alcohol conversion was achieved,
and the products were mainly benzaldehyde plus small
amount of benzoic acid. The increasing catalyst amount
improved the alcohol conversion and so does the selectivity
of benzoic acid. Under the catalyst of 0.01 g, the highest
alcohol conversion (92%) and acid selectivity (100%) can be
acquired. However, a further increase of catalyst amount will
not lead to higher alcohol conversion.
vinyl-2-pyrrolidone) (PVP, M
n
¼ 55,000), diethyl ether, and
ethyl acetate were used as purchased. 1-Vinylimidazole was
made inhibitor-free by passing the liquid through a column
filled with base Al O . Deionized water was used for all
2
3
experiments.
Characterization
1
The H NMR measurement was carried out on a Varian 400
MHz spectrometer. The particle size of the PIL microspheres
was determined by TecnaiG 220 TEM. The powder XRD pat-
tern was recorded on a Rigaku D/max 2500 X-ray diffrac-
tometer with parallel Cu Ka radiation (k ¼ 1.54178 Å). The
One of the advantages of PIL/Pt nanoparticle hybrids is that
the catalysts can be easily separated from the product and
recycled. After the reaction was quenched, the product was
extracted from the reaction mixture with ethyl acetate and
the hybrid catalysts remaining in the water phase were col-
lected by using centrifugation, washed with water and meth-
anol, and dried before further using in the next run.
ꢀ
samples were dried under vacuum at 50 C and then be
placed on a glass slide. The weight percentage of metal
nanoparticles on the surface of PIL microspheres was deter-
mined using a TGA at an increasing temperature rate of
ꢀ
ꢁ1
ꢀ
1
5 C min from 50 to 900 C under a continuous nitrogen
gas flow. The composition of the samples was determined by
inductively coupled plasma-mass spectroscopy (ICP-MS, Var-
ian 710-ES).
As can be seen from Table 2 that the catalytic activity of pre-
formed PIL/Pt catalysts was almost constant, and only a
slight decrease in conversion were found after six consecu-
tive runs. The decrease may be caused by the slight leaching
of hybrid catalysts on recycling, and the diminishing activity
of catalysts during catalytic oxidation. These results indicate
that PIL/Pt hybrids can be used as efficient and durable cat-
alysts for direct conversion of benzyl alcohol to the corre-
sponding carboxylic acid under relatively mild conditions,
which suggests their potential applications in other alcohol
catalytic reactions.
Synthesis of 1-Butyl-3-vinylimidazolium Bromide
Under a nitrogen atmosphere, 1-butyl-3-vinylimidazolium
bromide ([Bvim][Br]) was synthesized by stirring the mix-
ture containing 1-vinylimidazole (9.4 g, 100 mmol) and
equivalent molar amount of 1-bromobutane (13.7 g, 100
mmol) at room temperature for 24 h. The product was
washed three times with diethyl ether and ethyl acetate and
dried in vacuum. Colorless liquid. Yield: 95%.
1
H NMR (400 MHz, CDCl ):10.89 (s, 1H, NCHN), 7.86 (s, 1H,
3
EXPERIMENTAL
NCHC), 7.58 (s, 1H, CCHN), 7.52–7.44 (dd, 1H, CH CHN),
2
Materials
6.01–5.95 (dd, 1H, HCHCHN), 5.40–5.36 (dd, 1H, HCHCHN),
0
1-Vinylimidazole, 1-bromobutane, 1,4-dibromobutane, 2,2 -
4.43–4.38 (t, 2H, NCH
CH CH CH ), 1.44–1.32 (sextet, 2H, CH
(t, 3H,CH CH ).
2
C), 1.97–1.87 (quintet, 2H,
azobis(2-methylpropionami-dine)-dihydrochloride
cyclohexane, Span80, K PtCl , sodium borohydride, poly(N-
(V-50),
2
2
2
2
CH CH ), 0.97–0.92
2
3
2
4
2
3
a
TABLE 2 Recovery and Reuse of PIL/Pt Hybrids for Benzyl Alcohol Oxidation
Runs
First
Second
92 (100)
Third
Fourth
90 (98)
Fifth
88 (97)
Sixth
b
Conversion (%) [selectivity(%)]
92 (100)
90 (98)
87 (99)
a
ꢀ
Benzyl alcohol (0.432 g, 4 mmol), K
, 24 h.
2
CO
3
(0.552 g, 4 mmol), PIL/Pt hybrids (0.01 g, containing 0.002 g Pt ), H
2
O (50 mL), 90 C,
O
b
2
Refers to the main product of benzoic acid.
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Synthesis of 1,4-Di(vinylimidazolium)butane Bisbromide
GC electrode was coated with 5 lL of PIL/Pt hybrid solution
and then dried in air. The surface of modified GC electrode
was further coated with 5 lL of Nafion ethanol solution (0.2
wt %) and then dried in air before the electrochemical
Under a nitrogen atmosphere, 1,4-di(vinylimidazolium)bu-
2
6
tane bisbromide [C (vim) ][Br]
2
was synthesized by stir-
4
2
ring a mixture containing 1-vinylimidazole (10.4 g, 110
mmol) and 1,4-dibromobutane (10.8 g, 50 mmol) at room
temperature for 24 h. The resultant white solids were eluted
three times with ether and then dried in vacuo. Yield: 87%.
1
experiments. A solution containing 0.5 M CH OH and 0.5 M
3
H SO was used as an electrolyte solution and degassed by
2
4
high purity nitrogen gas before the measurement. The cur-
ꢁ
1
rent (mA mg ) per unit of the loading mass of Pt nanopar-
ticles was used throughout. All the electrochemical charac-
terizations were performed at room temperature.
H NMR (400 MHz, DCCl ): 7.58–7.57 (d, 2H, NCHC), 7.37–
3
7
.36 (d, 2H, CCHN), 6.96–6.88 (dd, 2H, CH CHN), 5.62–5.55
2
(dd, 2H, HCHCHN), 5.23–5.19 (dd, 2H, HCHCHN), 4.09 (s, 4H,
NCH CH ), 1.75 (s, 4H, NCH CH CH ).
2
2
2
2
2
Procedure for Alcohol Catalytic Oxidation
Synthesis of PIL Microspheres via Inverse Miniemulsion
Polymerization
A typical procedure for the synthesis of PIL microspheres is
described as below: a mixture containing [Bvim][Br] (1.0 g,
Oxygen gas was bubbled into the reaction mixture containing
benzyl alcohol (0.432 g, 4 mmol), K CO (0.552 g, 4 mmol),
2
3
5
0 mL H O and various amount of PIL/Pt hybrids (contain-
2
ing 19.5 wt % of Pt nanoparticles). The reaction mixture
4 2 2
4.3 mmol), [C (vim) ][Br] (0.2 g, 0.5 mmol) and V50 (0.01
ꢀ
was stirred vigorously at 90 C for 24 h. Then the reaction
g, 0.037 mmol) in water (1.4 mL) was stirred at room tem-
perature under a nitrogen atmosphere. The resultant clear
solution was mixed with Span 80 (1.0 g) and cyclohexane
was cooled to room temperature and quenched with 1.0 M
HCl. The organic product was extracted with ethyl acetate
three times and dried over anhydrous MgSO . The substrate
4
(
20 g). Then the mixture was ultrasonicated for 10 min in
1
conversion and product yield were determined by H NMR
an ice bath to obtain a stable inverse miniemulsion. The min-
iemulsion was degassed via nitrogen gas bubbling and then
and gas chromatography. The PIL/Pt hybrids were separated
by centrifugation and washed thoroughly with ethanol and
ꢀ
kept at 60 C for 6 h. The reaction was cooled to room tem-
ꢀ
distilled water, and then dried at 80 C under vacuum before
perature and poured into acetone. The precipitated PIL
microspheres were isolated by decantation, and then sepa-
rated by centrifugation. The remaining residues were washed
with acetone three times to remove unreacted materials and
the next recycle run.
CONCLUSIONS
ꢀ
then dried in vacuum at 50 C.
In summary, we have developed a facile strategy for the syn-
thesis of PIL microspheres supported Pt nanoparticle
hybrids. Crosslinked PIL microspheres with the diameter of
about 200 nm were synthesized via miniemulsion polymer-
ization. Anion exchange of PIL microspheres with metal pre-
cursor and followed by the reduction of metal ions produced
PIL/Pt nanocatalysts. The resultant Pt nanoparticles are uni-
formly dispersed and strongly bound to the surface of PIL
microspheres. The catalytic performances of the produced
PIL/Pt hybrids were evaluated for the electrocatalytic oxida-
tion of methanol and oxidation of benzyl alcohol. The results
revealed that PIL/Pt nanoparticle hybrids show better elec-
trocatalytic activity toward the electrooxidation of methanol
than pure Pt nanoparticles. They are effective and easily
reusable catalysts for the selective oxidation of benzyl alco-
hol in aqueous reaction media, demonstrating that the syn-
thesized PIL microspheres are suitable scaffolds for hetero-
geneous catalysts. Further investigations to develop other
noble metals or their alloys nanoparticles decorated on the
surface of PIL microspheres for heterogeneous catalysis as
well as for electrocatalysis and electrochemical sensors are
currently underway.
Synthesis of Metal Nanoparticles Immobilized on the
Surface of PIL Microspheres
In a typical experiment, 6 mL of K PtCl solution (7.5 mmol/
2
4
L) was added into 4 mL of water in which PIL nanoparticles
0.01 g) and PVP (0.26 g) were dispersed. The mixture was
stirred at room temperature for 1 h under a nitrogen atmos-
(
ꢁ
2ꢁ
phere to convert Br anions to PtCl4 . The anion-exchanged
PIL microspheres were separated by centrifugation and
washed thoroughly with distilled water to remove the excess
PVP and K PtCl . A sixfold excess of NaBH (0.017 g) solu-
2
4
4
tion was added dropwise to the suspension with vigorous
stirring, then the mixture was kept at room temperature for
1
h. The resultant PIL/Pt hybrids was separated by centrifu-
gation and washed with water to remove the excess salts
and free metal nanoparticles not firmly bound to PIL micro-
sphere surface. The produced hybrids were washed with ace-
tone and then vacuum dried. The amount of Pt nanoparticles
calculated by ICP is about 19.5 wt %.
Electrocatalysis of Methanol
Electrochemical performance of PIL/Pt nanoparticle hybrids
was evaluated by CV and chronoamperometry and recorded
on a CHI660 electrochemical workstation with a conven-
tional three-electrode electrochemical cell. A modified GC
electrode with a diameter of 3 mm and platinum foil were
used as working and counter electrode, respectively. A satu-
rated calomel electrode was used as the reference electrode.
The PIL/Pt hybrids were ultrasonically dispersed in water to
form a homogeneous dispersion (1 mg/mL). The surface of
This work was supported by Natural Science Foundation of
China (No. 20874071, 20974072), Research Fund for the Dec-
toral Program of Higher Education (20103201110003), The
Program for New Century Excellent Talents in University (Grant
NCET-07-0593), Fok Ying Tung Education Foundation
(114022) and a Project Funded by the Priority Academic Pro-
gram Development of Jiangsu Higher Education Institutions.
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4537

ARTICLE
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