Distinctive size effects of Pt nanoparticles immobilized on Fe3O4@PPy used as an efficient recyclable catalyst for benzylic alcohol aerobic oxidation and hydrogenation reduction of nitroaromatics

Article abstract of DOI:10.1039/c4nj01869a

Fe₃O₄@PPy composite microspheres have been synthesized using Fe₃O₄ microspheres as a chemical template in an ultrasonic treatment process. Pt nanoparticles (NPs) were immobilized on Fe₃O₄@PPy by using ethylene glycol (EG) and NaBH₄ as reducing agents. The information on the morphologies, sizes, and dispersion of Pt NPs of the as-prepared catalysts was verified by TEM, XRD, FTIR and XPS. As expected, the chemical reduction methods remarkably affected the size of Pt NPs (～2.5 nm and ～5.5 nm) and the prepared catalysts exhibited high catalytic activities as well as awesome stabilities for aerobic oxidation of benzylic alcohols and hydrogenation reduction of nitroaromatics. It was highlighted that size effects for the catalytic properties of the two reactions were found to be quite different. Fe₃O₄@PPy-Pt (2.5 nm) afforded a higher conversion for benzylic alcohol aerobic oxidation, while the selectivities toward benzaldehyde over these two catalysts were similar. However, they showed almost the same catalytic performance for hydrogenation reduction of a majority of nitroaromatics. What is more, Fe₃O₄@PPy-Pt (5.5 nm) gave better activities than several nitroaromatics, which were relatively difficult to be hydrotreated under the same conditions. In addition, the EG reduced Fe₃O₄@PPy-Pt catalyst exhibited slightly poorer stability than the NaBH₄ reduced Fe₃O₄@PPy-Pt catalyst in the recycle tests, which might be due to the agglomeration of small Pt NPs.

Full text of DOI:10.1039/c4nj01869a

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Size effects of supported Pt nanoparticles were entirely different for benzylic alcohols
aerobic oxidation and hydrogenation reduction of nitroaromatics

New Journal of Chemistry
Page 2 of 8
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DOI: 10.1039/C4NJ01869A
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ARTICLE TYPE
Distinctive size effects of Pt nanoparticles immobilized on Fe O @PPy
3
4
used as an efficient recyclable catalyst for benzylic alcohols aerobic
oxidation and hydrogenation reduction of nitroaromatics
Yu Long, Bing Yuan, Jianrui Niu, Xin Tong, Jiantai Ma*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Fe O @PPy composite microspheres have been synthesized using Fe O microspheres as a chemical
3
4
3
4
template under an ultrasonic treatment process. Pt nanoparticles (NPs) were immobilized onto
Fe O @PPy by using ethylene glycol (EG) and NaBH as reducing agent. The information of the
3
4
4
1
1
2
0
5
0
morphologies, sizes, and dispersion of Pt NPs of the asꢀprepared catalysts was verified by TEM, XRD,
FTIR and XPS. As expected, the chemical reduction methods remarkably affected the size of Pt NPs
(~2.5nm and ~5.5nm, respectively) and the prepared catalysts exhibited high catalytic activities as well as
awsome stabilities for aerobic oxidation of benzylic alcohols and hydrogenation reduction of
nitroaromatics. It was highlighted that size effects for the catalytic properties of the two reactions were
found to be quite different. Fe O @PPyꢀPt (2.5nm) afforded a higher conversion for benzylic alcohols
3
4
aerobic oxidation, while the selectivities toward benzaldehyde over these two catalysts were similar.
However, they showed almost same catalytic performance for hydrogenation reduction of a majority of
nitroaromatics. What’s more, Fe O @PPyꢀPt (5.5nm) gave better activities of several nitroaromatics
3
4
which were relatively difficult to be hydrotreated under the same conditions. In addition, the EG reduced
Fe O @PPyꢀPt catalyst exhibited slightly poorer stability than the NaBH reduced Fe O @PPyꢀPt
3
4
4
3
4
catalyst in the recycle tests, which might due to the agglomeration of small Pt NPs.
1
0
Introduction
synthesis and biomedical sciences . Moreover, interest in
preparation of shell coated magnetic Fe O4 microparticles is
increasing dramatically. So far, different kinds of shell have been
reported to coat magnetic Fe O microparticles . Our team also
has reported many works about synthesis and application of coreꢀ
3
The function of a heterogeneous catalyst with supported active
component is determined by the complicated interplay of many
factors. The size of the active component is one of the most
important factors in controlling the catalytic performance in many
11
5
5
6
6
7
0
5
0
5
0
3
4
2
3
3
4
4
5
0
5
0
5
7b, 12
shell Fe O4 supported nanoparticle catalysts
. The catalyst
with magnetic particle core allows it to be facilely recovered and
reused using an external magnet. And the shell coating Fe O
1
3
systems . Nanosized particles of noble metals as active
components have attracted much interest in recent years because
of their sizeꢀdependent catalytic properties . Among noble
2
3
4
could forcefully fasten metal NPs by strong coordination effect,
thus, avoiding metal NPs splitting from the support during the
catalytic reaction.
In previous report, reduction method or reducing agent of
platinum mainly include photodeposition, radiationꢀinduced,
ethylene glycol, ethanol, sodium borohydride, hydrogen, sodium
phosphinate, argon plasma and so on . And size effects of
supported Pt NPs have been used in nanoparticlesꢀelectrocalytic
oxidation reaction. In this work, Fe O @PPy composite
immobilize different sizes of Pt NPs using ethylene glycol and
sodium borohydride as classical reducing agent. And the
information of the morphologies, sizes, and dispersion of Pt NPs
for the asꢀprepared catalysts is investigated. Meanwhile, the size
effects of Pt NPs are tested in aerobic oxidation of benzylic
alcohols and hydrogenation reduction of nitroaromatics. It is
conceivable that catalytic activities of the two completely
opposite reactions would give a relevant conclusion about sizes
metals, Pt NPs, with good resistance to corrosion and chemical
3
attack , play a major role in many applications such as
4
5
6
electrocatalysis , photocatalysis , biosensors
and other
7
heterogeneous catalysis . It is expected that, the size of Pt NPs
may affect the electronic structure and the coordination structure
of the active site, and thus may influence the activation of
reactant molecules and the active species, leading to differences
13
8
in activity and/or selectivity . Therefore, the deep understanding
3
4
of the size effect of supported Pt NPs in various kinds of
reactions would be very helpful for the rational design of highly
efficient catalysts.
However, Pt is not common and cheap enough for widespread
application. So it is a big challenge to improve the efficiency,
save resource and increase recycle rate. In recent years, there has
been an increasing trend toward the use of magnetically
9
retrievable materials in a variety of areas . Fe O as a superior
magnetic material, has been used in efficient green chemical
3
4
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[journal], [year], [vol], 00–00 | 1

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DOI: 10.1039/C4NJ01869A
effects of the supported Pt NPs catalysts. To our best knowledge,
an external magnet and washed with deionized water and ethanol
size effects of Pt NPs supported on magnetic coreꢀshell material
for different kinds of reaction have not been reported.
several times and finally dried in a vacuum at 60℃for 12 h.
Characterizations of Fe O @PPy-Pt catalyst
3
4
60
These magnetic microꢀmaterials were characterized inductively
coupled plasma (ICP), powder Xꢀray diffraction (XRD),
transmission electron microscopy (TEM), Xꢀray photoelectron
spectroscopy (XPS), fourier transformꢀinfrared (FTꢀIR) and
vibrating sample magnetometry (VSM). XRD measurements
Experimental
5
Materials
Iron (III) chloride hydrate, sodium acetate, PolyꢀNꢀvinylpyrrolleꢀ
2
ꢀone (PVP, Kꢀ30), ethylene glycol, pyrrole, potassium
chloroplatinite, hexachloroplatinic acid, ethanol were purchased 65 were performed on a Rigaku D/maxꢀ2400diffractometer using
from Sinopharm Chemical Reagent Co., Ltd. Various reaction
reagents, such as 4ꢀmthylbenzyl alcohol, 4ꢀchlorobenzyl alcohol,
Cu–Ka radiation as the Xꢀray source in the 2θ range of 5–80°.
The size and morphology of the magnetic microparticles were
observed by a Tecnai G2 F30 transmission electron microscopy
and samples were obtained by placing a drop of a colloidal
4ꢀ 70 solution onto a copper grid and evaporating the solvent in air at
room temperature. Pt content of the catalyst was measured by
ICP on IRIS Advantage analyzer. Magnetic measurements of
1
0
4
ꢀmethoxybenzyl
bromonitrobenzene, 4ꢀnitroanisole, 4ꢀmethylnitrobenzene, 2, 5ꢀ
dichloronitrobenzene, 1ꢀnitronaphthalene and
alcohol,
4ꢀnitrobenzyl
alcohol,
4ꢀ
chloromethylnitrobenzene, were purchased from Alfa Aesar. All
chemicals were of analytical grade and used as received without
further purification. Deionized water was used throughout the
experiments
15
Fe O @PPyꢀPt were investigated with a Quantum Design VSM
3 4
at room temperature in an applied magnetic field sweeping from ꢀ
8 to 8 kOe. XPS was recorded on a PHIꢀ5702 instrument and the
C1s line at 284.8 eV was used as the binding energy reference.
75
Preparations of Fe O @PPy-Pt catalysts
3
4
Fe O microparticles were synthesized with the solvothermal
3
4
Typical procedure for aerobic oxidation of benzylic alcohols
7
b
20
25
30
35
40
45
50
55
method according to our previous Work . Firstly, 1.5 g
FeCl ·6H O, 1.0 g PVP and 2.0 g NaAc were added into 200 mL
In a typical process, benzylic alcohols and Pt of Fe
with a mole ratio of 50 were dissolved in 5 mL deionized water.
make all materials dissolve completely. Then the mixture was 80 Then the reactions were carried out under oxygen atmosphere
O @PPyꢀPt
3
2
3
4
ethylene glycol. The mixture was stirred violently for 2 hours to
transferred to a Teflonꢀlined stainless steel autoclave and sealed
to heat at 200 °C for 8 h. The precipitated black product was
collected from the solution by an external magnet and washed
with deionized water and ethanol several times. Finally, the black
product was dried in a vacuum for 24 h at 60°C.
with electroꢀmagnetic stirring at 40℃for a certain time. After the
reaction, the aqueous phase was extracted five times with diethyl
ether. Then the combined organic extracts were dried over
MgSO
and analyzed by characterizing the reaction mixture using
4
85 Gas ChromatographꢀMass Spectrometer (GCꢀMS).
1
2c
Typically , Fe O microparticles (0.30 g) were dispersed in 70
3
4
Typical procedure for hydrogenation reduction
of
mL H O under sonication, and then pyrrole (4 mL) in ethanol (15
2
nitroaromatics
mL) and HCl solution (15 mL, 6 M) were added into the above
solution in turn under sonication for 1 h. Finally, the black
product was collected with the help of a magnet, washed with
deionized water and ethanol repeatedly to remove the residual
pyrrole monomers and HCl acid, and then dried in a vacuum at
In a typical process, nitroaromatics and Pt of Fe O @PPyꢀPt with
3
4
a mole ratio of 200 were dissolved in 5 mL ethanol. Then the
reactions were carried out under hydrogen atmosphere with
electroꢀmagnetic stirring conditions at room temperature for a
certain time. The reaction was monitored and analyzed by
characterizing the reaction mixture using GCꢀMS.
90
6
0℃for 12 h to get magnetic Fe O @PPy composite.
3 4
Syntheses of Fe O @PPyꢀPt(EG). An aqueous solution of
3
4
Fe O @PPy (100 mg) in deionized water (100 mL) was
3
4
Results and Discussion
ultrasonicated for
1 h to form a uniform suspension.
Subsequently, K PtCl ꢀEG (0.005 M, 10 mL) solution was added
2
4
95 Preparation and characterizations of Fe O @PPy composite
3 4
to the suspension and sonicated for 2 h. The pH of the solution
was adjusted to 10 using potassium hydroxideꢀEG (KOHꢀEG)
solution (0.1 M), and then the solution was stirred under argon at
and Fe O @PPy-Pt catalyst
3 4
1
30℃ for 3h. The precipitated black product was collected from
the solution by an external magnet and washed with deionized
water and ethanol. The step was repeated several times before
drying in vacuum at 60℃ for 24h.
Syntheses of Fe O @PPyꢀPt(NaBH ). An aqueous solution of
3
4
4
Fe O @PPy (100 mg) in deionized water (100 mL) was
3
4
Scheme 1 Preparation process of catalysts
The process of preparation of the Fe O @PPyꢀPt catalyst is
ultrasonicated for 1 h to form a uniform suspension. Then, an
aqueous solution of H PtCl (0.01 M, 5.6 mL) was added into the
3
4
2
6
1
00 schematically described in Scheme 1. Fe O microparticles were
suspension under stirring and the mixture was stirred for 30 min
3 4
synthesized with the solvothermal method according to our
at room temperature. Subsequently, the NaBH solution (0.01 M,
4
7b
3+
previous Work . Secondly, in the strong acidic solution, the Fe
5
0 mL) was delivered by drops into the above suspension under
released from Fe O4 microspheres and the polymerization of
continuous stirring. After reacting for 3 h at room temperature,
the precipitated black product was collected from the solution by
3
pyrrole monomers occurred, thus a layer of polypyrrole was
2
|
Journal Name, [year], [vol], 00–00This journal is © The Royal Society of Chemistry [year]

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1
2c
coated on the surface of Fe O . Thirdly, under magnetic stirrers,
3
4
K PtCl and H PtCl were reduced onto the surface of
2
4
2
6
polypyrrole by EG and NaBH to obtain Fe O @PPyꢀPt catalysts.
4
3
4
5
Fig. 1 TEM images of (a) Fe
3
O
4
, (b) Fe
3
O
4
@PPy
3 4 3 4
Fig. 3 FTꢀIR patterns of (a) Fe O , (b) Fe O @PPy
Fig. 1a shows the typical TEM image of Fe O microspheres
The compositions of Fe
O @PPy composite microsphere were
3 4
3
4
prepared by the versatile solvothermal reaction. As can be seen
from the image, the average diameter of the asꢀsynthesized 30 the lowꢀfrequency region, the peak at 586.3 cm (Fig. 3a), 582.5
confirmed by FTꢀIR (Fig. 3). As was also shown in the picture, in
ꢀ
1
ꢀ
1
spherical particles was about 400 nm. The TEM image in Fig. 1b
showed that a continuous layer of PPy could be observed on the
outer shell of the Fe O microsphere cores and the thickness was
cm (Fig. 3b) corresponded to the stretching vibration peak of
ꢀ
1
1
0
FeꢀO. From the spectrogram, the peaks around at 1562 cm (C=C
stretching vibrations), 1209 cm (CꢀN stretching vibrations), and
931 cm (CꢀH stretching vibrations) revealed the presence of
ꢀ
1
3
4
ꢀ
1
about 35 nm. The resultant Fe O @PPy composites had good
3
4
dispersibility and spherical morphology.
35 PPy.
1
5
3 4 3 4
Fig. 2 XRD patterns of (a) Fe O , (b) Fe O @PPy
The XRD pattern of Fe O (Fig. 2a) and Fe O @PPy (Fig. 2b)
3
4
3
4
showed characteristic peaks of magnetite microparticles and the
sharp. The main peaks of the Fe O @PPy composite were similar
3
4
to the Fe O , exhibiting that the coating process under acidic
3
4
2
0
conditions did not affect the structure of Fe O . Both of the two
3 4
patterns showed strong peaks which confirmed the products were
wellꢀcrystallized and detected diffraction peaks in every pattern
could be indexed as cubic Fe O (JCPDS card No. 82ꢀ1533). A
3
4
broad peak was observed at 2θ = 10°ꢀ25°, which suggested
1
4
2
5
newly generating a layer of PPy .
Fig. 4 TEM images of (a) Fe
Pt(NaBH ); HRTEM images of (b) Fe
@PPyꢀPt(NaBH ); particle size distribution of (c) Fe
Pt(EG) and (f) Fe @PPyꢀPt(NaBH
3
O
4
@PPyꢀPt(EG) and (d) Fe
@PPyꢀPt(EG) and (e)
@PPyꢀ
3 4
O @PPyꢀ
4
3 4
O
Fe
3
O
4
4
3 4
O
40
3
O
4
4
)
The morphologies, the sizes, and the dispersion of Pt NPs
deposited on Fe O @PPy had been investigated by TEM (Fig. 4).
3
4
As exhibited in Fig. 4d, e and f, the Pt NPs reduced by NaBH4
were randomly dispersed on Fe O @PPy with a wide size
3
4
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distribution affording an average diameter of about 5.5 nm (Fig
f). On the other hand, the Pt NPs got by using EG as reducing
agent, which were observed with a narrow size distribution with a
average diameter of about 2.5 nm (Fig 4c), were equably attached
Since the ability of recoverability was important for this catalyst,
magnetic measurements were performed using a VSM at room
temperature. As shown in Fig. 6, Fe O @PPy (Fig. 6b),
4
3
4
Fe O @PPyꢀPt(EG) (Fig. 6c) and Fe O @PPyꢀPt(NaBH ) (Fig.
3
4
3
4
4
5
on the surface of Fe O @PPy (Fig. 4a, b). However, it possessed 30 6d) showed small reduction of the saturation magnetization. This
3 4
a more inferior crystal structure than that of the Pt/NaBH4
HRTEM in the inset image in Fig. 4b, e), which indicated
slightly reduction of the saturation magnetization for the material
loaded solid was expected. The measured saturation
(
ꢀ
1
NaBH was a better reducing agent to forming perfect crystal
magnetizations were 36.0 emug for Fe O @PPyꢀPt(EG) (Fig.
3 4
4
ꢀ
1
structure of Pt NPs. Therefore, it was significant to control the
6c) and 34.4 emug for Fe O @PPyꢀPt(NaBH ) (Fig. 6d). The
3 4 4
10
reducing agents and reducing conditions for the design of the 35 decrease of the saturation magnetization suggested the presence
supported nanoparticle catalyst.
of platinum nanoparticles on the surface of the magnetic support.
Even with this reduction in the saturation magnetization, the
catalyst could still be efficiently separated from solution with an
external magnet as shown in the inset image in Fig. 6.
40
Catalyst testing for the aerobic oxidation of benzylic alcohols
and hydrogenation reduction of nitroaromatics
For compare of the catalytic activity of the two Fe O @PPyꢀPt
3
4
catalysts, the benzylic alcohols aerobic oxidation and
hydrogenation reduction of nitroaromatics were carried out under
atmospheric pressure of O /H as model reactions.
45
2
2
3 4 3 4
Fig. 5 XPS spectra for (a) Fe O @PPyꢀPt(EG) and (b) Fe O @PPyꢀ
Pt(EG) showing Pd 3d5/2 and Pd 3d3/2 binding energies.
15
The XPS elemental survey scan of the surface of the
Fe O @PPyꢀPt(EG) was showed in Fig. 5a. Peaks corresponding
3
4
to iron, oxygen, nitrogen, palladium and carbon were clearly
observed. To ascertain the oxidation state of the Pt, XPS studies
was carried out. The XPS analysis of the Pt (0) catalyst showed in
Fig. 5b. As expected, the spectrum of the Pt 4f region confirmed
the presence of Pt (0) with peak binding energy of 71.2 and 75.2
eV, which were assigned to Pt 4f5/2 and Pt 4f7/2, respectively.
20
Fig. 6 VSM of (a) Fe
3
O
4
3 4 3 4
, (b) Fe O @PPy, (c) Fe O @PPyꢀPt(EG) and (d)
25
Fe @PPyꢀPt(NaBH )
3
O
4
4
4
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Table 1 Aerobic oxidation of various benzylic alcohols in water
25 and 4ꢀmethoxybenzyl alcohol showed a higher conversion, while
ꢀnitrobenzyl alcohol showed a lower conversion over both
Fe O @PPyꢀPt(EG) and Fe O @PPyꢀPt(NaBH ) catalysts. Both
4
3
4
3
4
4
catalysts showed higher catalytic activity for substituted aromatic
alcohols containing electronꢀdonating group (ꢀCH , ꢀOCH and ꢀ
3
3
Time A Yield B Yield
30 Cl) than those containing electroꢀwithdrawing group (ꢀNO2),
implying that the electronic effects and intrinsic properties of Pt
Entry
Substrate
Catalyst
(
h)
(%)
81
72
75
70
(%)
13f
a
NPs over these two catalysts were the same . On the other hand,
while the conversions over Fe O @PPyꢀPt(EG) were constantly
1
2
3
4
Pt/EG
8
13
3
4
b
Pt/NaBH
4
8
8
8
10
higher than Fe O @PPyꢀPt(NaBH ) for different aromatic
3
4
4
c
Pt/EG
12
35 alcohols, the selectivities toward benzaldehyde over these two
catalysts were similar. This illustrated the EG reduced small Pt
NPs was only beneficial to improve the conversion of benzylic
alcohols aerobic oxidation not the selectivity of benzaldehyde. In
addition, both Fe O @PPyꢀPt(EG) and Fe O @PPyꢀPt(NaBH )
d
Pt/NaBH
4
10
Pt/CNTꢀ
plasma
e
f
5
6
7
2
2
18
3
4
3
4
4
Pt/CNTꢀ
IMP
40 exhibited better catalytic properties than those already reported as
13
13f, 15
shown in Table 1, entry 5ꢀ8, 11ꢀ13.
g
nPt@hC
24
24
46
44
h
8
9
Pt/AC
Pt/EG
6
6
93
85
80
60
0
4
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
Pt/NaBH
4
4
i
PSꢀPtNPs
10
10
10
6
16
10
0
j
Pt/C
k
2 4
K PtCl
Pt/EG
Pt/NaBH
Pt/EG
96
87
82
77
<1
<1
15
12
4
6
10
10
Pt/NaBH
4
1
1
8
9
Pt/EG
12
12
c, d
74
59
9
7
Pt/NaBH
4
a
b
Fe
3
O
4
@PPyꢀPt(EG); Fe
3
O
4
@PPyꢀPt(NaBH
4
); Yield after 5 runs;
e,f
[
ref 13f] conversion of benzyl alcohol. Reaction conditions: alcohol 1
mmol, deionized water 50 ml, catalyst 4mg (0.001 mmol Pt), O flow rate
5 ml/min, T=75℃, t=2 h; [ref 15a] Reaction conditions: catalyst 0.1
5
2
g, h
2
i, j, k
mol% of Pt, O
2
balloon, T=60 ℃, t=24 h;
[ref 15b] Reaction
conditions: alcohol 0.5 mmol, deionized water 1 ml, catalyst 2.0 mol%
of Pt, O balloon, T=30 ℃, t=10 h;
2
1
1
2
0
Table 1 listed the catalytic performances of Fe O @PPyꢀPt(EG)
3 4
and Fe O @PPyꢀPt(NaBH ) in aerobic oxidation of benzyl
3
4
4
alcohol in aqueous solution. The reactions with Fe O @PPyꢀ
3
4
Pt(EG) took place at 40℃for 8 h in the absence of base to gave a
mixture of benzaldehyde and benzoic acid in 81% and 13%
yields, respectively (Table 1, entry 1). In contrast, Fe O @PPyꢀ
5
3
4
Pt(NaBH ) exhibited slightly lower catalytic activity, probably
4
due to their large particle size (entry 2). To further address this
issue, alcohol substrates with different electron densities were
introduced as probes to examine the catalytic activities of
Fe O @PPyꢀPt(EG) and Fe O @PPyꢀPt(NaBH ), and the results
0
3
4
3
4
4
were shown in Table 1 (entry 9ꢀ19). When the aerobic oxidation
was performed with paraꢀsubstituted benzyl alcohols, the
electronꢀdonating group facilitated the reaction. Compared to
benzyl alcohol, 4ꢀmthylbenzyl alcohol, 4ꢀchlorobenzyl alcohol
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Table 2 Hydrogenation reduction of various nitroaromatics in ethanol
the difficulty of the hydrogenation reaction, the gap between the
yields got by the two catalysts was widening in the same reaction
time. For instance, the yield of 2, 5ꢀdichloroaniline catalyzed by
Fe O @PPyꢀPt(NaBH ) achieved 37.2% (entry 24) — more than
20
3
4
4
twice the yield by Fe O @PPyꢀPt(EG) (entry 22). And even if the
3
4
Entry
Substrate
Catalyst
Time (min) Yield (%)
reactions time catalyzed by Fe O @PPyꢀPt(EG) had been
3
4
a
1
2
3
4
3
4
5
6
7
8
Pt/EG
40
40
40
40
40
40
40
40
40
40
>99
>99
25 doubled, the yields of phenylamines were still lower than yields
catalyzed by Fe O @PPyꢀPt(NaBH ), indicating these
b
3
4
4
Pt/NaBH
4
hydrogenation reactions had achieved the final stages of reactions
in the corresponding time. This might be due to the better crystal
c
Pt/EG
92.5
95.1
d
structure of NaBH
reduced Pt NPs, which probably be in favour
4
Pt/NaBH
4
30
of improving catalytic activities.
Pt/EG
98.7
98.2
98.4
98.0
97.7
97.5
Pt/NaBH
4
Pt/EG
Pt/NaBH
Pt/EG
4
Pt/NaBH
4
9
Pt/EG
Pt/NaBH
Pt/EG
40
40
40
40
>99
>99
95.2
95.6
1
0
1
2
4
1
1
Pt/NaBH
4
1
1
3
4
60
120
85.5
88.6
Pt/EG
Pt/NaBH
Pt/EG
1
5
4
60
88.8
1
1
6
7
90
180
90.3
92.9
Fig. 7 TEM images of (a) Fe
Pt(NaBH ); HRTEM images of (b) Fe
@PPyꢀPt(NaBH ) after 5 runs of nitroaromatics hydrogenation
3
O
4
@PPyꢀPt(EG) and (c) Fe
3 4
O @PPyꢀ
4
3 4
O
@PPyꢀPt(EG) and (d)
1
8
Pt/NaBH
4
90
95.5
Fe
3
O
4
4
1
2
9
0
240
480
67.3
71.6
3
5
The recycling experiment of the catalyst was further investigated
because the recyclability of the heterogeneous catalyst was one of
the most important issues for practical applications. Both
catalysts could be recycled 5 times for the two reactions, while
less than 10% loss of conversion after recycle reactions was
observed (Table 1. entry 3, 4 and Table 2. entry 3, 4).
Interestingly, after 5 times recycle reactions, Fe O @PPyꢀ
Pt/EG
Pt/NaBH
Pt/EG
2
1
4
4
240
81.9
2
2
2
3
240
480
18.4
20.7
40
2
4
Pt/NaBH
240
37.2
3
4
Pt(NaBH ) showed a lesser loss of activities for both aerobic
4
a
b
c, d
Fe
3
O
4
@PPyꢀPt(EG); Fe
3
O
4
@PPyꢀPt(NaBH
4
); Yield after 5 runs
oxidation of benzylic alcohols and hydrogenation reduction of
nitroaromatics than Fe O @PPyꢀPt(EG). Therefore, the EG
In order to give a relevant conclusion about sizes effects of the
3
4
5
0
5
supported Pt NPs catalysts, the catalytic activities were also 45 reduced Fe O @PPyꢀPt catalyst exhibited slightly poorer stability
3 4
tested for the hydrogenation of a variety of nitroaromatics to their
corresponding products. The reactions were carried out in ethanol
than the NaBH reduced Fe O @PPyꢀPt catalyst in the recycle
4 3 4
tests, which might due to the agglomeration of small Pt NPs. In
order to certify this assumption, TEM and HRTEM were used to
observe the morphology of Pt NPs after 5 runs of nitroaromatics
at room temperature and under 1 atm pressure of H . Detailed
2
observations of all the reactions were given in Table 2. In contrast
1
to aerobic oxidation reactions, catalytic properties of 50 hydrogenation, as shown in Fig. 7. Compared with Pt NPs of
Fe O @PPyꢀPt(EG) and Fe O @PPyꢀPt(NaBH ) for
Fe O @PPyꢀPt(NaBH ) before the reactions (Fig. 4d, e), it could
hydrogenation of most nitroaromatics were almost the same
entry 1, 2, 5ꢀ12), being very active for the hydrogenation
reaction under such mild conditions and affording over 95% yield
3
4
3
4
4
3
4
4
be concluded that Pt NPs of Fe O @PPyꢀPt(NaBH ) after 5 runs
3
4
4
(
had no obvious change in running process, except that its crystal
structure was unable to be easily observed (Fig. 7c, d). However,
1
of aminobenzene. It was worth noting that Fe O @PPyꢀ 55 Pt NPs of Fe O @PPyꢀPt(EG) after 5 runs exhibited an apparent
3 4
3 4
Pt(NaBH ) gave better activities to several nitroaromatics which
were relatively difficult to be hydrotreated under the same
conditions (entry 13ꢀ24). Interestingly, along with the increase of
agglomeration of Pt NPs (Fig. 7a, b), compared with Pt NPs of
Fe O @PPyꢀPt(EG) before the reactions (Fig. 4a, b).
4
3
4
6
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New Journal of Chemistry
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DOI: 10.1039/C4NJ01869A
R. Mülhaupt, O. Riant, S. Hermans, J. Barthel and C. Janiak, Carbon,
2014, 66, 285.
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Conclusion
6
6
7
7
8
8
0
5
0
5
0
5
In conclusion, we used NaBH4 and EG as classical reducing
agent to successfully prepare highly dispersed different diameters
of Pt nanoparticles supported on PPyꢀcoated Fe O . As expected,
the chemical reduction methods remarkably affected the size of
Pt nanoparticles about 5.5nm and 2.5nm, respectively. The
prepared catalysts exhibited high catalytic activity and good
stability for aerobic oxidation of benzylic alcohols and
hydrogenation reduction of nitroaromatics. It was highlighted that
7
3
4
5
0
5
0
5
0
—
7
13; (e) Y. Liu, J. Chung, Y. Jang, S. Mao, B. M. Kim, Y. Wang and
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1
1
2
2
3
Fe O @PPyꢀPt(EG) afforded a higher conversion for benzylic
3 4
alcohols aerobic oxidation, while the selectivity toward
benzaldehyde over these two catalysts was similar. However,
catalytic performances for hydrogenation reduction of a majority
of nitroaromatics of two catalysts were almost the same. More
8 Q. Zhang, W. Deng and Y. Wang, Chem. Commun., 2011, 47, 9275.
9
(a) J. M. ClementeꢀJuan, E. Coronado and A. GaitaꢀAriñ, Chem. Soc.
Rev., 2012, 41, 7464; (b) M. ClementeꢀLeón, E. Coronado, C. Martíꢀ
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interesting, Fe O @PPyꢀPt(NaBH ) gave better activities of
3
4
4
several nitroaromatics which were relatively difficult to
be hydrotreated under the same conditions. Furthermore, both
catalysts could be recycled 5 times for the two reactions, while
less than 10% loss of conversion after recycle reactions is
observed. And because of the agglomeration of small Pt
nanoparticles, the EG reduced Fe O @PPyꢀPt catalyst exhibited
3
4
slightly poorer stability than the NaBH reduced Fe O @PPyꢀPt
4
3
4
1
1
0 (a) M. A. M. Gijs, F. Lacharme and U. Lehmann, Chem. Rev., 2010,
catalyst in the recycle tests. Our work demonstrated the important
role of reducing agents for immobilizing Pt nanoparticles. It also
could be obtained that size effects might be quietly different for
different kinds of reactions. Sometimes, superior crystal structure
was probably more meaningful than smaller size diameter for a
certain kind of reaction. It is surely that preparing smaller metal
nanoparticles with prefect crystal structure and avoiding
interaction agglomeration can greatly improve catalytic activities.
1
2
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