Magnetic Core–Shell Nanostructured Palladium Catalysts for Green Oxidation of Benzyl Alcohol

Article abstract of DOI:10.1007/s10562-016-1756-z

Abstract: Multifunctional catalysts are highly attractive for green oxidation process integrating excellent conversion and selectivity with high separability and recyclability. In this regards, a facile method is developed to fabricate yolk–shell microsph

Full text of DOI:10.1007/s10562-016-1756-z

Catal Lett
DOI 10.1007/s10562-016-1756-z
Magnetic Core–Shell Nanostructured Palladium Catalysts
for Green Oxidation of Benzyl Alcohol
Liping Kong1 Chengcheng Wang1 Feilong Gong2 Weidong Zhu1
•
•
•
•
Yijun Zhong1 Xiangrong Ye1 Feng Li3
•
•
Received: 7 April 2016 / Accepted: 21 April 2016
Ó Springer Science+Business Media New York 2016
Abstract Multifunctional catalysts are highly attractive for
green oxidation process integrating excellent conversion
and selectivity with high separability and recyclability. In
this regards, a facile method is developed to fabricate yolk–
shell microspheres consisting of Fe3O4 cores, mesoporous
CeO2 shells and loaded Pd nanoparticles. Microspheres
with 2.6 % Pd loading exemplify excellent catalytic
activity for solvent-free, selective oxidation of benzyl
alcohol to benzaldehyde by molecular oxygen at 373 K and
atmospheric pressure, resulting in 80.5 % conversion of
benzyl alcohol with 94.8 % selectivity for benzaldehyde.
The catalyst can be magnetically separated and recycled
without significant loss of catalytic efficiency even after
seven catalytic runs.
Keywords Multifunctional catalyst Á Core–shell
nanostructure Á Green oxidation Á Benzyl alcohol
1 Introduction
Catalytic partial oxidation of benzyl alcohol is one of the
main routes to benzaldehyde, an indispensable bulk inter-
mediate for perfumes, pharmaceuticals, dyestuffs and
agrochemicals. In a typical homogeneous catalytic process,
costly and hazardous organic solvents are often heavily
used [1–3]. Therefore, a green reaction with water as by-
product, e.g., solvent-free, liquid-phase oxidation of benzyl
alcohol to benzaldehyde by molecular oxygen over sup-
ported noble metal catalysts, has triggered enormous
interests [4, 5], and the mostly used catalysts consist of Pd
or Au [6, 7] as primary active component, the performance
of which highly depends on its chemical status and the
support materials [8, 9]. Particularly, Pd or Au nanoparti-
cles highly dispersed on reductive (CeO2 [10], TiO2 [11],
etc.) supports show significantly promoted activity com-
pared to those on non-reductive (C, SiO2, Al2O3, etc.)
substrates because of the synergistic metal–support inter-
action [12]. It has been reported that, due to the reversible
valence exchange of Ce4?/Ce3? and high oxygen transport
capability [13], CeO2 can supply reactive oxygen in the
form of surface superoxide and peroxide at one-electron
defect site to the supported active species of metal for
oxidation [14]. With surface oxygen vacancies, typical
reductive support material such as CeO2 serves as catalyst
promoter to activate the O2 molecules through support or
metal–support interface [15]. Synergistic metal–support
interaction has also been reported for visible-light-driven
oxidation of benzyl alcohol by O2 at room temperature
and ambient pressure using Pd core@ CeO2 shell
Graphical Abstract
& Xiangrong Ye
yxr@zjnu.cn
1
College of Chemistry and Life Science, Zhejiang Normal
University, Jinhua 321004, Zhejiang, China
2
College of Materials and Chemical Engineering, Zhengzhou
University of Light Industry, Zhengzhou 450002, Henan,
China
3
American Advanced Nanotechnology, Houston, TX 77459,
USA
123

L. Kong et al.
nanocomposite as photocatalyst. Under visible light irra-
diation, electron–hole pairs are produced by CeO2 shell,and
the generated electrons are trapped by the Pd cores and the
adsorbed benzyl alcohol interacts with holes to form the
corresponding radical cation. Further reaction with dioxy-
gen or superoxide radical species will lead to the formation
of the corresponding aldehydes [16]. Nevertheless, further
studies are inexorable integrating CeO2 shell with other
materials to develop advanced, multifunctional heteroge-
neous catalysts, e.g., magnetically responsive catalyst as
addressed bellow.
2 Experimental
2.1 Preparation of Pd/Fe3O4@mCeO2
The magnetite microspheres were prepared according to
the method described below. Typically, 1.35 g of FeCl3Á
6H2O, 1.0 g of polyethylene glycol and 3.6 g of NaAc
were dissolved in 40 mL of ethylene glycol with stirring
applied for 30 min to form a clear solution. The solution
was then transferred into a 50 ml Teflon-lined stainless
steel autoclave and heated at 200 °C for 8 h. The resultant
precipitate was collected by magnetic separation and then
washed with ethanol and deionized water for several times.
The mCeO2 outer layer was deposited onto Fe3O4 cores
through chemical precipitation to form Fe3O4@mCeO2
nanocomposite. 0.7 g of polyethylene glycol monododecyl
ether (Brij) and 0.05 g of cetyltrimethylammonium bro-
mide (CTAB) were blended in mixed-liquor of 20 mL
water and 20 mL ethanol to form solution A. 0.1 g of
Fe3O4 particles were ultrasonicated in 0.1 M nitric acid
solution for 15 min in order to obtain hydroxyl-rich sur-
face, and then dispersed in solution A. (NH4)2Ce(NO3)6
was dissolved in 20 mL of H2O to form a yellow solution
B, which was added dropwise to solution A and kept at
room temperature under vigorous stirring for 2 h. The
mixture was then titrated with ammonium hydroxide until
pH 9.0, and stirred at room temperature for 5 h. After-
wards, the particles were separated from the mixture by a
magnet, washed, dried, and then re-dispersed in 100 mL of
ethanol and refluxed at 95 °C for 24 h to remove the CTAB
and Brij template. The ethanol extraction was repeated
twice and the resultant powder was washed with water and
dried at 60 °C overnight.
Conventional filtration-based recovery of expensive Pd
or Au catalyst from the liquid reaction system is quite
time and energy consuming [17, 18]. Hybridization of
magnetically responsive phase with the catalyst support
offers a solution to efficient recovery of catalyst from the
reaction system under a magnetic field, and re-dispersion
of the recovered catalyst for new run of reaction upon
removal of the applied magnetic field [19, 20]. Although
magnetically responsive catalyst has been intensively
investigated [21, 22], there is dearth of work dedicated to
its combination with reductive support for the selective
oxidation of benzyl alcohol. From both the magnetic and
catalytic point of view, Fe3O4@mesoporous CeO2 (Fe3
O4@mCeO2) core–shell nanostructured composite could
be a satisfactory support to realize convenient catalyst
recycling in addition to excellent conversion and selec-
tivity. The Fe3O4 core enables magnetically driven sep-
aration and recycle of catalyst. The CeO2 shell not only
protects the Fe3O4 core from external harsh conditions
and provides extra adsorption sites for nanocatalyst
loading besides the interior space, but also promotes the
performance of the loaded nanocatalyst in catalyzing
liquid-phase oxidation of benzyl alcohol. In the present
Pd nanoparticles were loaded onto Fe3O4@mCeO2
work,
a layer-by-layer assembly method has been
nanocomposite through sol-immobilization.
A certain
developed to fabricate Pd/Fe3O4@mCeO2 core–shell
nanostructured catalyst. Mesoporous CeO2 shell was
chemically deposited onto the surface of Fe3O4 magnetic
cores, followed by one pot loading of Pd nanoparticles
using NaBH4 as reductant. The composition and structure
of the resultant catalysts were examined by X-ray
diffraction (XRD), scanning electron microscope (SEM),
transmission electron microscope (TEM) and X-ray
photoelectron spectroscopy (XPS) characterization, and
their catalytic performance as well as separability and
recyclability were probed in solvent-free, aerobic oxida-
tion of benzyl alcohol to explore the correlation with
composition and structure for future development of
multifunctional heterogeneous nanocatalyst.
amount of H2PdCl6Á6H2O and 0.05 mL of 1 wt% polyvinyl
alcohol (PVA) solution were diluted in 40 mL of water.
After 30 min of stirring, 0.29 mL of 0.1 M NaBH4 solution
was added to form a dark brown mixture. 0.1 g of Fe3-
O4@mCeO2 was then ultrasonically dispersed in the mixture
under vigorous agitation. After 3 h, particles were collected
by a magnet, and washed with deionized water and absolute
alcohol to remove the surface adsorbed PVA. Finally, the
particle samples were dried in vacuum at 60 °C for 8 h.
2.2 Reaction Process
The catalytic performance of the materials was tested in the
selective oxidation of benzyl alcohol to benzaldehyde at
123

Magnetic Core–Shell Nanostructured Palladium Catalysts for …..
Scheme 1 Schematic
illustration of preparation
procedure for Pd/
Fe3O4@mCeO2
(NH4)2Ce(NO3)6
H2PdCl6
CTAB+Brij
Pd/Fe3O4@mCeO2
Fe3O4@mCeO2
Fe3O4
normal pressure. Briefly, the oxidation was carried out in a
three-neck flask connected to O2. Added to the reactor was
5 mL of benzyl alcohol and Pd/Fe3O4@mCeO2 catalyst
with the molar ratio of substrate to metal at about 2500.
Before the reaction, the device was purged for half an hour
with O2 to get rid of the air. Magnetic stirring (1000 rpm)
was initiated when the reaction mixture was heated up to
the desired temperature. After the reaction, the catalyst was
separated via magnetic field, and the liquid mixture
accompanied by N,N-dimethylformamide as an internal
standard was analyzed by a gas chromatograph (Agilent
6820) equipped with an FID detector and a capillary col-
umn (DB-5, 30 m 9 0.45 mm 9 0.42 lm). In order to test
the catalytic reusability, the catalyst collected through
magnetic separation was washed with ethanol and dried in
a vacuum oven at 333 K for 8 h. The recovered catalyst
was then used in the next run of reaction.
card (JCPDS No. 72-2303) of Fe3O4 with cubic spinel
structure. After being coated with CeO2, the intensity of
Fe3O4 phase decreased, and some new diffraction peaks
appeared at 28.6°, 33.1°, 47.6°, 56.5°, and 59.3°, which can
be assigned to CeO2 phase. In the case of Pd/Fe3O4@-
mCeO2, a weak diffraction peak corresponding to the (111)
reflection of the Pd crystal was observed, conforming the
successful loading of highly dispersed Pd nanoparticles.
Figure 2 displays the N2 adsorption–desorption iso-
therms and BJH measurements of the pure CeO2 materials,
Fe3O4@mCeO2 spheres and 2.6 wt% Pd/Fe3O4@mCeO2
nanocatalyst, and some results are summarized in Table 1.
The adsorption–desorption isotherm of CeO2 was type IV-
like with an apparent H2 hysteresis loop at relatively high
P/P0, signifying the presence of mesopores, and the Bru-
nauer–Emmett–Teller (BET) surface area for CeO2 was
195.41 m2 g-1. For Fe3O4@mCeO2 spheres, a type IV-like
isotherm combined with an H3 hysteresis loop was
observed, also indicating the presence of mesoporous
structure. The BET surface area of the Fe3O4@mCeO2
nanocomposite was 125.57 m2 g-1 with an average pore
size of 5.07 nm. The surface area and average pore size
both reduced for 2.6 wt% Pd/Fe3O4@mCeO2, implying the
loading of Pd into mesoporous structure.
3 Results and Discussion
The preparation of Pd/Fe3O4@mCeO2 core–shell struc-
tured nanocatalyst mainly included three steps (Scheme 1),
and Fig. 1 represents the XRD patterns of the products at
different synthetic steps. As shown in Fig. 1a, the XRD
pattern of the magnetic core matches well with the standard
The SEM images of the representative products from
each synthetic step are exhibited in Fig. 3. The Fe3O4
magnetic cores clearly display a spherical shape as shown
in Fig. 3a,b. After CeO2 coating, the size of core–shell
spheres increased and the surface became rougher. No
obvious aggregation occurred after Pd loading. As verified
by the TEM images in Fig. 4, the as-prepared Fe3O4@-
mCeO2 microsphere is composed of a magnetite core and a
mesoporous CeO2 shell (ca. 20 nm) with a relatively rough
surface. HR-TEM image in Fig. 4d further indicated that
the shell is porous and consists of a large number of
assembled ultrafine nanocrystallites with an average size of
approximately 5 nm. The lattice fringe with d-spacing of
0.32 nm can be readily indexed to the (111) lattice planes
of CeO2, being consistent with the XRD results. After sol-
immobilization, Pd NPs with average diameter of about
4 nm are remarkably well-dispersed on the surface of the
Fe3O4@mCeO2 microspheres without obvious aggrega-
tion, and the lattice fringes are clearly visible with spacing
of about 0.23 nm which corresponds to that of the (111)
lattice planes of Pd.
Fig. 1 X-ray diffractograms of (a) Fe3O4, (b) Fe3O4@mCeO2 and
(c) 2.6 wt% Pd/Fe3O4@mCeO2
123

L. Kong et al.
Fig. 2 N2 adsorption–desorption isotherms and BJH measurements of a mCeO2, b Fe3O4@mCeO2 and c 2.6 wt% Pd/Fe3O4@CeO2
Table 1 BET surface area,
Compound
Surface area (m2 g-1
)
Pore volume (cm3 g-1
)
Pore diameter (nm)
pore volume and average pore
size of mCeO2, Fe3O4@mCeO2
and 2.6 wt% Pd/Fe3O4@mCeO2
CeO2
195.41
125.57
108.18
0.13
0.15
0.09
2.92
5.07
3.87
Fe3O4@mCeO2
2.6 wt % Pd/Fe3O4@mCeO2
To investigate the composition and chemical state of
especially the surface of the core–shell Pd/Fe3O4@CeO2
material, XPS analysis was carried out and the results are
shown in Fig. 5. Fe, Ce, O, Pd and C were detected dis-
tinctly in the survey scan XPS spectrum. The regional
spectrum of Pd exhibits two peaks centered at 340.7 and
335.4 eV, which are assigned to 3d3/2 and 3d5/2 of Pd(0)
[23], respectively. The spectrum for the O1 s ionization
feature in Fig. 5d concluded two primary peaks, the one
located at around 532.3 eV was denoted as oxygen coming
from the H2O and CO2 molecules chemisorbed on the
surface, and the other at 530.5 eV was mainly ascribed to
lattice oxygen (Olat) of Fe3O4 and ceria support. Regarding
the typical Ce3d spectrum shown in Fig. 5c, 3d3/2 peaks at
917.5, 907.8 and 901.4 eV and 3d5/2 peaks at 898.9, 888.2
and 883.2 eV correspond to Ce4? [24] while 3d3/2 at
coating and then Pd loading, Fe3O4@mCeO2 and 2.6 wt%
Pd/Fe3O4@mCeO2 nanocomposites exhibit slightly but
gradually decreased Ms values as compared with Fe3O4,
which is attributed to the slight increase in both the mass
and size of the resultant materials. However, 2.6 wt% Pd/
Fe3O4@mCeO2 still can be separated from the reaction
mixture under external magnetic field, as showed in Fig. 6
(inset).
Shown in Table 2 is the activity of Pd loaded by various
magnetic supports on the oxidation of benzyl alcohol under
identical condition. Pd loaded on CeO2 or TiO2 support
exhibited higher activity than the counterpart on SiO2 or C.
As has been widely reported [28, 29] and demonstrated by
the above-mentioned XPS result,there usually exist
numerous oxygen defects in CeO2 and TiO2,which not only
enhance the synergistic interaction between the loaded
metals and support, but also enable more efficient activa-
tion of O2 for oxidation of benzyl alcohol.
904.3 eV and 3d5/2 at 885.5 eV are characteristic of Ce3?
.
The coexistence of some Ce3? with Ce4? in the fresh Pd/
Fe3O4@mCeO2 catalyst might be resulted from the
reduction of Ce4? to Ce3? by ethanol during synthesis, and
creates more oxygen defects in the CeO2 supports which
have been speculated to benefit the lattice oxygen transfer
in the catalytic oxidation process [25–27].
Table 3 summarizes the influence of Pd content on
benzyl alcohol oxidation catalyzed by Pd/Fe3O4@mCeO2.
At a same reaction temperature of 373 K, the conversion of
benzyl alcohol dramatically increased from 25.5 to 88.4 %
along with the increase of Pd loading from 0.3 to 4.3 %,
whilst the selectivity towards benzaldehyde slightly
increased and reached a value more than 94.5 %. For Pd/
Fe3O4@mCeO2 with 2.6 % of Pd loading, the turnover
frequency (TOF), an indicator of the average reaction rate
over the catalyst, showed a maximal value of 443.5 h-1
Magnetic properties of pure Fe3O4, Fe3O4@mCeO2 and
Pd/Fe3O4@mCeO2 were investigated via a vibrating sam-
ple magnetometer at room temperature. The materials have
magnetization saturation (Ms) values of 85.45, 27.56,
18.63 emu g-1, respectively. Due to the stepwise mCeO2
123

Magnetic Core–Shell Nanostructured Palladium Catalysts for …..
Fig. 3 SEM images of a,b Fe3O4, c,d Fe3O4@mCeO2 and e,f 2.6 wt% Pd/Fe3O4@mCeO2
along with relatively high benzyl alcohol conversion
around 80.5 % and benzaldehyde selectivity around
94.8 %. Such a high performance might be resulted from
the collaborative effect between the CeO2 and Pd
particles.
alcohol conversion occurred. At 333 K, the conversion was
10.1 % only. In the reaction process, the surface of Pd
catalyst might be deactivated due to the oxidation by the
activated oxygen to form PdO. The reduction of PdO back
to Pd by the substrate alcohol could recover the active sites
of Pd to keep the reaction proceed continuously. However,
such reduction would be slow at a temperature as low as
333 K, resulting in a low conversion of benzyl alcohol.
When the reaction temperature increased to 353 K, the
conversion of benzyl alcohol increased significantly to
58.9 %.
The influence of temperature on benzyl alcohol oxida-
tion is shown in Table 4. At an elevated temperature of
413 K, the reaction rate was significantly enhanced, lead-
ing to a very high TOF value of 501.3 h-1. Also the con-
version of benzyl alcohol increased to 88.2 %. These
results indicate that high temperature is beneficial to oxi-
dation reaction, one of possible reasons being that the small
amount of water produced in the reaction process could be
more easily removed from the activate sites of catalyst at
high temperature. However, the selectivity to benzaldehyde
almost unchanged. If the reaction was carried out at a
temperature lower than 373 K, a sharp decline of benzyl
The catalytic activity of Pd/Fe3O4@CeO2 on oxidation
of benzyl alcohols with different substituent groups was
also evaluated and the results are compared in Table 5. The
conversion of alcohol decreases according to the
descending order of substituent groups: –C(CH3)3 [
–OCH3 [ –CH3 [ –Cl [ –NO2. Most likely, the electron
123

L. Kong et al.
Fig. 4 TEM and HR-TEM images of a,b Fe3O4, c,d Fe3O4@mCeO2 and e,f 2.6 wt% Pd/Fe3O4@mCeO2
cloud density of benzene ring increases with the electron
donating ability of substituent group, leading to easier
oxidation of aromatic alcohol to aromatic aldehyde.
Figure 7 describes the results of catalytic stability test of
the fabricated 2.6 wt% Pd/Fe3O4@mCeO2 nanocomposite.
After seven cycles, the conversion of benzyl alcohol
decrease to 74.5 %, and the selectivity for benzyl aldehyde
reduce to 89.8 %. ICP analysis indicated a negligible loss
of palladium from catalyst to the reaction system, less than
1 ppm.
123

Magnetic Core–Shell Nanostructured Palladium Catalysts for …..
Fig. 5 XPS spectra of the 2.6 wt% Pd/Fe3O4@mCeO2 microspheres catalyst: a full spectrum, b Pd 3d, c Ce 3d and d O 1s
Table 2 Support effect on oxidation of benzyl alcohol
Pd/support
Conversion (%) Selectivity (%) TOF(h-1 a
)
Benzyldehyde
Pd/Fe3O4
60.0
72.0
74.5
70.1
83.5
93.2
94.8
93.1
303.5
353.3
369.7
443.5
428.9
Pd/Fe3O4@C
Pd/Fe3O4@SiO2
Pd/Fe3O4@CeO2 80.5
Pd/Fe3O4@TiO2 80.1
Reaction condition: benzyl alcohol (5 ml), O2 (20 ml/min),
w = 2.6 %, molar ratio of substrate:metal at about 2500, t = 7 h,
T = 373 K
a
TOFs were calculated on the basis of practical loading of Pd
Fig. 6 Magnetization curves of samples measured at 298 K:
(a) Fe3O4, (b) Fe3O4@mCeO2, (c) 2.6 wt% Pd/Fe3O4@mCeO2.
Photographs (inset) of 2.6 wt% Pd/Fe3O4@mCeO2 dispersed in or
collected from aqueous solution with or without external magnetic
field
It is widely accepted that oxidative dehydrogenation is
responsible for the mechanism of alcohol oxidation shown
in Fig. 8, although the precise pathway is still under debate
because the nature and concentration of adsorbed species
123

L. Kong et al.
Table 3 Pd content effect on oxidation of benzyl alcohol
Actual content (%)a
Conversion (%)
Selectivity (%)
Benzaldehyde
TOF (h-1 b
)
Toluene
Benzoic acid
Benzyl
benzoate
0.3
0.8
1.5
2.6
3.4
4.3
25.5
40.5
55.3
80.5
87.8
88.4
88.3
89.5
90.1
94.8
94.6
93.2
8.6
8.3
7.8
3.4
3.9
4.4
2.1
1.3
1.3
0.9
1.1
1.4
1.0
0.9
0.8
0.9
0.4
1.0
348.4
358.5
308.3
443.5
289.2
258.1
Reaction condition: benzyl alcohol (5 ml), O2 (20 ml/min), m = 0.05 g, t = 7 h, T = 373 K
a
Determined by ICP-AES
b
TOFs were calculated on the basis of practical loading of Pd
Table 4 The influence of
Temperature Conversion Selectivity
temperature to oxidation of
benzyl alcohol
TOF (h-1 a
)
Benzaldehyde Toluene Benzoic acid Benzyl benzoate
333
353
373
403
413
10.1
58.9
80.5
87.5
88.2
89.8
91.2
94.8
94.5
93.9
5.9
5.8
3.4
3.0
3.2
2.1
1.4
0.9
1.1
0.8
2.2
1.6
0.9
1.4
2.1
35.2
253.7
443.5
454.8
501.3
Reaction condition: benzyl alcohol (5 ml), O2 (20 ml/min), w = 2.6 %, molar ratio of substrate:metal at
about 2500, t = 7 h
a
TOFs were calculated on the basis of practical loading of Pd
Table 5 Catalytic activity of Pd/Fe3O4@CeO2 on oxidation of different alcohols
Substrate
Conversion (%)
60.8
Selectivity (%)
87.8
TOF (h-1
)
304.1
65.2
81.3
81.5
85.6
93.9
92.9
371.3
444.2
444.5
84.6
95.4
457.1
Reaction condition: benzyl alcohol (5 ml), O2 (20 ml/min), w = 2.6 %, molar ratio of substrate:metal at about 2500, t = 7 h
123

Magnetic Core–Shell Nanostructured Palladium Catalysts for …..
4 Conclusion
For low cost, environmentally friendly and selective oxi-
dation of benzyl alcohol to benzaldehyde, a layer-by-layer
strategy has been established to prepare core–shell struc-
tured multifunctional Pd/Fe3O4@mCeO2 nanocatalyst
uniquely integrating excellent conversion and selectivity
with efficient separability and recyclability. The Fe3O4
core allows for magnetically driven separation and recycle
of catalyst, and the defect-rich CeO2 shell enables pro-
moted catalytic performance of the loaded Pd nanoparti-
cles. Being superior to other reported heterogeneous
nanocatalysts, the synthesized Pd(2.6 %)/Fe3O4@mCeO2
composite results in 80.5 % benzyl alcohol conversion and
94.8 % selectivity for benzaldehyde in a short reaction
period of 7 h at 100 °C, and no significant loss of effi-
ciency was observed after seven catalytic runs.
Fig. 7 Catalytic stability tests over 2.6 wt% Pd/Fe3O4@mCeO2
Acknowledgments This work was financially supported by Key
Discipline of Materials Physics and Chemistry of Zhejiang Province.
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