A recoverable sandwich phosphorotungstate stabilized palladium (0) catalyst for aerobic oxidation of alcohols in water

Article abstract of DOI:10.1016/j.apcata.2016.06.016

The preparation, characterization and catalytic properties of tetra metal substituted sandwich phosphorotungstate [M₄(H₂O)₂(PW₉O₃₄)₂]^10?(M₄P₂W₁₈, M²⁺?=?Fe²⁺, Co²⁺, Mn²⁺, Cu²⁺, VO²⁺) stabilized Pd nanoparticles, which were in-situ encapsulated in mesoporous aluminum phosphate (mAPO) by a one-pot method, were demonstrated for aerobic oxidation of alcohol in water. The synthesized catalysts were characterized with N₂ adsorption, XRD, TEM, FT-IR and UV–vis. TEM analysis revealed that Pd-M₄P₂W₁₈/mAPO thus obtained has a mean palladium particle size of less than 5?nm. The solid Pd-M₄P₂W₁₈/mAPO catalysts exhibit excellent activity for aerobic oxidation of benzyl alcohols in water without any base additives. It was found the introduced M₄P₂W₁₈ polyoxanions encapsulated palladium nanoparticles as both stabilizing agent and promoters, leading to enhanced activity and recyclability of palladium catalyst. The recycling test of representative Pd-(VO)₄P₂W₁₈/mAPO for oxidation of benzyl alcohol suggested no significant decrease in activity and selectivity even after twelve times of successive reactions. This novel heterogeneous catalyst composed of polyoxometalates stabilized palladium nanocatalysts and mesoporous aluminophosphate can also be extended to the selective oxidation of various alcohols.

Full text of DOI:10.1016/j.apcata.2016.06.016

Applied Catalysis A: General 523 (2016) 304–311
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
A recoverable sandwich phosphorotungstate stabilized palladium (0)
catalyst for aerobic oxidation of alcohols in water
a,b,∗
, Tao Feng , Pengfei Wang , Ziwen Chen , Riqing Yan , Bo Liaoa,b,
c
c
a
a
Lijuan Chen
a
Yujun Xiang
a
School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan, PR China
Key Laboratory of Theoretical Chemistry and Molecular Simulation of Ministry of Education, Hunan University of Science and Technology, Xiangtan, PR
b
China
c
School of Energy Source, Hunan University of Science and Technology, Xiangtan, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The preparation, characterization and catalytic properties of tetra metal substituted sandwich phos-
Received 24 March 2016
Received in revised form 18 May 2016
Accepted 10 June 2016
10−
2+
2+
2+
2+
2+
2+
phorotungstate [M4(H2O)2(PW9O34)2]
(M4P2W18, M = Fe , Co , Mn , Cu , VO ) stabilized Pd
nanoparticles, which were in-situ encapsulated in mesoporous aluminum phosphate (mAPO) by a
one-pot method, were demonstrated for aerobic oxidation of alcohol in water. The synthesized cata-
lysts were characterized with N2 adsorption, XRD, TEM, FT-IR and UV–vis. TEM analysis revealed that
Pd-M4P2W18/mAPO thus obtained has a mean palladium particle size of less than 5 nm. The solid Pd-
M4P2W18/mAPO catalysts exhibit excellent activity for aerobic oxidation of benzyl alcohols in water
without any base additives. It was found the introduced M4P2W18 polyoxanions encapsulated palladium
nanoparticles as both stabilizing agent and promoters, leading to enhanced activity and recyclability of
palladium catalyst. The recycling test of representative Pd-(VO)4P2W18/mAPO for oxidation of benzyl
alcohol suggested no significant decrease in activity and selectivity even after twelve times of succes-
sive reactions. This novel heterogeneous catalyst composed of polyoxometalates stabilized palladium
nanocatalysts and mesoporous aluminophosphate can also be extended to the selective oxidation of
various alcohols.
Available online 13 June 2016
Keywords:
Mesoporous aluminophosphate
Polyoxometalates
Alcohol oxidation
Palladium nanoparticle
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
attracted much attention [2,3]. Water is an ideal solvent for this
protocol because it can avoid explosions and the hazards associated
with the use of oxidisable organic solvents under oxygen pressure.
Among these studies, palladium-based homogeneous or heteroge-
neous catalysts show promising activity in aqueous solution under
mild conditions, a number of palladium-based catalysts in the
form of metal complexes or nanoparticles have been investigated
for this purpose [4–11]. Although the progress for palladium cat-
alyzed aerobic oxidation of alcohols is notable, palladium catalysts
often suffer from easy agglomeration and formation of palladium
black that cause their deactivation in many cases. Moreover, in
many aerobic alcohol oxidation systems using noble metal based
catalysts, base additives are often necessitated to improve conver-
sion of alcohol and selectivity for corresponding carbonyl product.
According to literature, the base additives can facilitate the dehy-
drogenation of alcohol and promote to the ␤-H elimination of a
dissociated alcohol on the Pd surface [9]. However, the alkaline
environment under oxygen atmosphere is harmful to the stability
The oxidation of alcohols is one of the most fundamental trans-
formations of great commercial and research significance because
the corresponding aldehyde/ketone products serve as important
intermediates in the fine chemicals and pharmaceutical industries.
The stoichiometric oxidants like KMnO4 or CrO3 are convention-
ally used to accomplish this transformation [1]. However, these
processes usually produce large amount of toxic inorganic salts
waste and the yields of the desired product aldehyde/ketone are
often low. With the ever-increasing environmental concerns, the
development of heterogeneous transition-metal based catalysts for
aerobic oxidation of alcohols using air or oxygen as oxidant has
∗ Corresponding author at: School of Chemistry and Chemical Engineering, Hunan
University of Science and Technology, Xiangtan, PR China.
E-mail address: ljchen11@163.com (L. Chen).
http://dx.doi.org/10.1016/j.apcata.2016.06.016
0
926-860X/© 2016 Elsevier B.V. All rights reserved.

L. Chen et al. / Applied Catalysis A: General 523 (2016) 304–311
305
of palladium catalyst, leading to an easier deactivation of palladium
2. Experimental
catalyst due to over-oxidation and poisoning [6]. In this context,
the development of palladium-based heterogeneous catalysts with
improved recyclability for base-free aerobic oxidation of alcohols
in water is desirable in recent years.
2.1. Chemicals and methods
Powder X-ray diffractions of samples were recorded on a
Brucker AXS D8 Advance diffractometer using Cu K␣ radiation. TEM
images were recorded on a JEM 3010 TEM operated at 200 kV. More
than 100 particles for each sample were randomly counted to deter-
mine the particle size distributions. N2 adsorption and desorption
isotherms were obtained on a micromeritics ASAP 2010 instrument
For supported Pd nanoparticles catalyst, the morphology and
particle size control of Pd nanoparticles is an important factor
affecting the recyclability of catalysts. The previous studies indicate
that the catalytic activity of nano-sized metallic Pd for aerobic oxi-
dation of alcohols is dependent on the size and morphology of the
surface Pd nanoparticles, and the Pd nanoparticles with particle size
of a few nanometers show the highest activity [12,13]. However, it
is well known that the nanoparticles of small particle size are very
mobile and thermodynamically prone to agglomerate into larger
inactive particles [14]. Thus the improved recyclability of hetero-
geneous palladium catalysts is dependent on the better control and
maintenance of the particle size of Pd nanoparticles in desired range
during the catalyst preparation and the sequent catalytic reactions.
The strategies preventing the growth of Pd nanoparticles that are
widely used in heterogeneous palladium catalysts includes com-
binations with the promoter metal such as Bi, Pb or Au, surface
functionalization of support in order to strengthen the bonding of
Pd nanoparticles and confinement of Pd nanopartices in nano-sized
channels of supports [15–22]. However, these methods often suffer
from some disadvantages, for example, some promoters such as Bi
and Pb are not green agents, Pd and Au needed to form a desired
alloy phase to generate the enhanced activity, the procedures for
surface functionalization of support and post-treatment are com-
plicated and part of active species that located in small mesopores
is difficultly contactable by substrate molecules.
◦
at 77 K, samples were degassed at 120 C for 5 h under high vac-
uum prior to measurements. Surface areas and pore distribution
were calculated by BET and BJH methods, respectively. Elemental
analysis of samples was performed with an inductively coupled
plasma-optical emission spectrophotometer Shimadzu ICPs-7500
to determine the amount of Pd and polyoxometalates in heteroge-
neous catalyst. IR spectra were recorded on KBr pellets by a Nicolet
Niclet 6700 spectrophotometer. UV–vis spectra of samples were
obtained with Perkin-Elmer Lambda-35 spectrophotometer.
K PdCl4 was used as Pd precursor. Sodium tungstate dihydrate,
2
phosphoric acid (85%), cobalt nitrite hexahydrate, manganese
acetate tetrahydrate, vanadium sulfate heptahydrate and ferrous
sulfate pentahydrate were used as the sources of W, P, Co, Mn, V
and Fe for the synthesis of the tetra metal substituted tungstophos-
phates. To prepare mesoporous aluminophosphate, aluminium
isopropoxide and phosphoric acid were used as the sources of alu-
minum and phosphorous, respectively, and hexadecylamine (HDA)
was used as template. All the chemicals were analytical grade and
used as received without further treatment.
Recently, one of the important applications of polyoxometa-
lates emerged in metal clusters stabilization [23]. D’souza and
coworkers reported a simple synthesis of Krebs type polyoxoanions
2.2. Preparation of sandwich tungstophosphates
The
potassium
salts
of
sandwich
M
polyoxoanions
10−
2+
= Co2+
, Fe , Mn ,
2+
2+
[
(TBA) H M (H O) (XW O33)2] stabilized Pd, Au and Ag metal
[M (H O) (PW O ) ]
(M P W18,
4
4
4
2
10
9
4
2
2
2
9
34
2
4 2
+
colloidal systems, the metal particle size in these systems were
well controlled in a narrow range and remained unchanged for
approximately three/six months in solution media [24]. On the
other hand, polyoxometalates are a kind of efficient catalyst for
selective oxidation of organic chemicals, in palladium catalyzed
oxidation reactions, the combinations with polyoxometalates can
effectively promote to the activity of palladium catalysts [24–31].
VO ) were synthesized by the reaction of Na PW O with the
9
9
34
corresponding soluble metal salts in water and precipitation
with solid potassium chloride, as described by the published
literature [33]. The purity of the samples was tested by IR and NMR
spectroscopy.
2.3. In-situ encapsulation of M P W stabilized Pd
4
2
18
II
Neumann and co-workers reported a Pd -polyoxometalate catalyst
nanoparticles in mesoporous aluminophosphate
by attaching a sandwich type [WZn (H O) ][(ZnW O34)2] poly-
3
2
2
9
II
oxoanion to a Pd center for aerobic oxidation of primary aliphatic
alcohols, this group also reported a polyoxometalate appended
with alkylthiol tethers [SiW11O40(SiCH CH CH SH) ] stabilized
The pre-synthesized M P W stabilized Pd nanoparti-
4
2
18
cles was incorporated in mesoporous aluminophosphate by a
hydrothermal method from a gel composition of 0.0057 K PdCl :
2
2
2
2
2
4
Pd nanoparticles catalysts for liquid aerobic oxydehydrogena-
tion of vinylcyclohexene and vinylcyclohexane to styrene [29,30].
Zhang and coworkers reported the enhanced electrocatalytic
properties of Pd and Pt nanoparticles/polyoxometalates/graphene
tri-component nanohybrids for methanol and formic acid oxida-
tion [31]. In these systems, the synergic catalysis of Pd species and
polyoxometalates lead to improved conversion and selectivity, the
promotion effect of polyoxometalates is most likely related to their
electron-accepter properties and oxygen-activating capability [32].
Inspired by the above studies, here we reported a inte-
grated catalytic system based on sandwich type polyoxometalates
0.0011 M P W : Al O : P O5: 1.2HDA: 65ethanol: 300H O. In a
4 2 18 2 3 2 2
typical synthesis, M P W stabilized Pd nanoparticles colloidal
4
2
18
solution were obtained by the following procedures: the mixture
of 2.5 mL of M P W (10 mM) and 12.6 mL of K PdCl (10 mM)
4
2
18
2
4
II
aqueous solution was stirred for 5 min, then Pd was reduced to
Pd with 3 mL of freshly prepared 0.1 M NaBH4 by ultrasonication
0
for 10 min. The M P W stabilized Pd nanoparticles colloidal
4
2
18
solution was then added to the precursor solution composed of
2.24 g of aluminium isopropoxide, 1.06 g of orthophosphoric acid
and 1.32 g of hexadecylamine in 11.5 g of water and 27.8 g of
ethanol. The resulting gel of the above composition was stirred for
3 h at room temperature and then transferred into a Teflon lined
1
0−
(M P W18, M = Fe , Co , Mn , Cu ,
4 2
2+
2+
2+
2+
2+
[
M (H O) (PW O ) ]
4
2
2
2
9
34
2
+
◦
VO ) and palladium nanoparticles through in situ encapsulation
of pre-synthesized M P W stabilized palladium nanoparticles
autoclave in an oven at 120 C for 24 h. After cooling, the solid in
gray color was filtered, washed thoroughly with distilled water
and dried in air. The template was removed by a combined solvent
extraction and calcination method. For extraction procedures,1 g
of the above dried solid was stirred in 40 mL of 0.05 M HCl/ethanol
at room temperature for 15 min and washed with ethanol, the
process repeated for five times. The solid product after extraction
4
2
18
in mesoporous aluminophosphate by a sol-gel method. The syn-
thesized catalyst showed high catalytic performance for aerobic
oxidation of alcohols in water. The catalyst also showed enhanced
stability and recyclability compared with other heterogeneous pal-
ladium catalysts without modification of polyoxometalates.
◦
◦
was dried at 80 C overnight, and then calcinated at 400 C for

3
06
L. Chen et al. / Applied Catalysis A: General 523 (2016) 304–311
Fig. 1. N2 absorption-desorption isotherms and insert pore size distribution of (a) mAPO and (b) Pd-Fe4P2W18/mAPO.
◦
Table 1
5
h with a temperature ramp rate of 1 C/min. The ICP results of
sample after calcination indicates that the Pd amount is near to
wt% (0.97%) based on aluminophosphate and the molar ratio of
palladium to M P W is near to 5:1. The catalysts prepared by
Properties of mesoporous aluminophosphate and various M4P2W18-Pd/MAPO
samples.
1
sample
SBET
2
Pore volume
pore diameter
(DP/nm)
4
2
18
3
−1
)
(
m /g)
(VP/cm
g
this process are denoted as Pd-M P W18/mAPO.
4
2
In order to make comparative studies, two heterogeneous
palladium catalyst, Pd/mAPO (1 wt%) and M P W + Pd/mAPO
were prepared. Pd/mAPO was prepared through adsorption of
mAPO
204
216
210
214
216
218
0.43
0.50
0.45
0.48
0.50
0.52
3.8
Pd-Fe4P2W18/mAPO
Pd-Mn4P2W18/mAPO
Pd-Co4P2W18/mAPO
Pd-Cu P W18/mAPO
3.8 +12.6
3.9 + 12.4
3.9 + 12.7
4.0 + 12.8
4.0 + 12.6
4
2
18
K PdCl4 solution on mesoporous aluminophosphate and reduc-
2
4
2
tion. M P W + Pd/mAPO was prepared through adsorption of
Pd-(VO)4P2W18/mAPO
4
2
18
M P W
solution on Pd/mAPO (1 wt%). The polyoxometalates
4
2
18
and palladium content in these two heterogeneous catalysts were
adjusted to be same as that in Pd-M P W18/mAPO.
4
2
12.6 nm. The chain alkyl amine surfactant and polyoxometalates
stabilized Pd0 clusters can be combined through formation of
2.4. Catalytic application
electrostatic bonding NH3+. . .polyoxoanion, the subsequent con-
densation with the sol-gel precursor aluminium isopropoxide and
orthophosphoric acid leads to a more disordered mesostructure
with wide pore distribution. After the condensation, the M P W
The reactions were performed in a two-neck round bottom flask,
provided with a reflux condenser and an electrically controlled
magnetic stirrer. In typical conditions, 30 mg of solid catalyst,
4
2
18
stabilized Pd0 clusters are uniformly dispersed in the framework
1
mmol of alcohol in 10 mL of distilled water was mixed in the flask,
of mesoporous aluminophosphate. The present of large mesopores
in Pd-Fe P W18/mAPO is advantageous to the substrate molecule
and oxygen gas was introduced into the flask from an O2 balloon
under atmospheric pressure. The reaction was started by placing
the flask into an oil bath preheated to the reaction temperature
and starting magnetic stirring. After the given reaction time, the
catalyst was separated from the solution by filtration, the reaction
solution was extracted with diethyl ether twice, 5 mL for each time,
4
2
entering into the pores and contacting with active centers inside the
mesopores. The Brunauer-Emmett-Teller specific area (SBET), total
pore volume and mesopore diameter of other Pd-M P W18/mAPO
4
2
and mAPO are summarized in Table 1.
Fig. 2 is the UV–vis spectra of samples. Fe P W was selected
4
2
18
and the combined organic layer was dried with MgSO , analyzed
as an example to reflect the chemical transformation during the
preparation of Pd-M P W18/mAPO. The plots a–c are UV–vis spec-
4
by GC to determine the conversion and selectivity.
4
2
tra of Fe P W , K PdCl4 and Fe P W protected Pd colloid
4
2
18
2
4
2
18
0
3
. Results and discussion
(Fe P W -Pd ) in diluted aqueous solution, respectively. A strong
4 2 18
adsorption band at 206 nm is observed in the UV–vis spectra of
K PdCl , which was attributed to ligand-to-metal charge transi-
3
.1. Structural features of catalysts
2
4
2+
tion of Pd ion in tetrahedral coordination [34]. This adsorption
0
The N2 adsorption-desorption isotherms and the correspond-
band disappears in UV–vis spectra of Fe P W -Pd , indicating
4
2
18
0
ing pore diameter distribution of mesoporous aluminophosphate
the complete reduction of Pd2+ to Pd . An absorption band at
0
0
(
mAPO) and the representative Fe P W - Pd /MAPO supported
about 250 nm in the UV–vis spectra of Fe P W and Fe P W18-Pd
4
2
18
4 2 18 2 2
catalyst were presented in Fig. 1. Both are IV type isotherms with a
pronounced H3 hysteresis loop evidenced of its mesoporous struc-
ture. A sharp step increase in the relative pressure range of 0.5-0.8
was observed in the isotherm of mAPO due to the capillary con-
densation of N2 inside the mesopores. The increase step shifted
is attributed to the charge transfer associated with heteropoly-
9−
tungstate anion PW O
. A weak absorbance in the range of
9
34
300–400 nm, shoulder at 350 nm in the UV–vis spectra of Fe P W
4
2
18
is probably assigned to the d-d transition of iron (II) ions. The plots
d and e in Fig. 2 represent the UV–vis spectra of pure mAPO and Pd-
to higher P/P0 for Pd-Fe P W18/mAPO, suggesting formation of
Fe P W18/mAPO, respectively. Both show similar and very strong
4 2
4
2
larger pore sizes in Fe P W18-Pd/mAPO composite material. The
absorption bands in the 200–350 nm wavelength region, which was
attributed to the Al in tetrahedral coordination with Al P bonds,
4
2
Barrett-Joyner-Halenda (BJH) plot of Fe P W18-Pd/MAPO reveals a
O
4
2
wider pore distribution with two types of mesopores, the smaller
mesopores centered at 3.8 nm and larger mesopores centered at
[Al(OP) ]. A small hump after 350 nm can be observed in the UV–vis
4
spectra of Pd-Fe P W18/mAPO, which may arise due to the pres-
4
2

L. Chen et al. / Applied Catalysis A: General 523 (2016) 304–311
307
0
Fig. 2. UV–vis spectra of aqueous solution of (a) K2PdCl4, (b) Fe4P2W18-Pd , (c) Fe4P2W18 and solid samples: (d) mesoporous aluminophosphate (mAPO) and Pd-
Fe4P2W18/mAPO(e).
Pd-Fe P W /mAPO after calcination
4
2
18
Pd-Co
4
P
2
W
18/mAPO
3
Pd-Fe P W /mAPO extracted
1
2
4
2
18
2
theta/degree
mAPO
Pd-Fe P W /mAPO as synthesized
4
2
18
Pd-Co P W18/mAPO
4
2
Fe P W
4
2
18
Co P W
4 2
18
20
30
40
50
60
70
80
2
theta/degree
4000
3500
3000
2500
2000 -
1500
1000
500
1
wavenumbers(cm )
Fig. 4. XRD patterns of the samples in wide-angel region. (a) Co P W (b) Pd-
4
2
18
Co4P2W18/MAPO (c) mAPO. (the insert is the low-angel XRD of (b)).
Fig. 3. IR spectra of samples(a) Fe4P2W18; (b) Pd-Fe4P2W18/mAPO-CTAB; (c) Pd-
Fe4P2W18/mAPO-CTAB(extracted); (d) calcinated Pd-Fe4P2W18/mAPO.
stretching of surfactant, which undergo a sharp decrease in inten-
ence of heteropolyanion, its absorbance shifts to higher wavelength
compared to free Fe P W in water solution, the shift is related to
sity in the FT-IR of HCl/ethanol extracted Pd −Fe P W18/mAPO and
4
2
disappear in the FT-IR of Pd-Fe P W /mAPO after calcinations.
4
2
18
4
2
18
the interaction of Fe P W18with aluminophosphate.
The FT-IR spectra of other Pd-M P W18/mAPO samples synthesized
4
2
4 2
The success in incorporating M P W stabilized Pd nanopar-
ticles in mAPO was confirmed with FT-IR spectroscopy. The
with different polyoxometalates shows similar patterns to that of
Pd-Fe P W18/mAPO.
4
2
18
4
2
FT-IR spectra of Fe P W and Pd −Fe P W18/mAPO samples (as-
The XRD patterns of Co P W , mAPO and Pd-Co P W18/mAPO
are shown in Fig. 4. The wide angle powder XRD patterns
4
2
18
4
2
4 2 18 4 2
prepared, HCl/ethanol extracted and calcined) are shown in Fig. 3.
−
1
The bands at 3500 and 1640 cm are attributed to the asymmetric
OH stretching vibration and bending vibration of water, respec-
of mAPO and Pd-Co P W18/mAPO exhibit a broad peak in 2␪
4
2
◦
ranges of 10–30 , indicating their amorphous nature. In Pd-
Co P W /mAPO, no individual crystalline peaks of Co P W and
−
1
tively. The bands in wavenumber range of 1250–700 cm in FT-IR
spectra of Fe P W are attributed to the vibration frequencies of
4
2
18
4
2
18
metallic Pd are observed, indicating Co P W and Pd nanopar-
4
2
18
4
2
18
−1
−1
P-O(1020 cm ), W = O(926 cm ), W
O
W and W
O
Fe bands
ticles remain high dispersion on mAPO. The small angle XRD of
Pd-Co P W /mAPO (Fig. 4, insert) exhibit an intense reflection at
(
876, 813 and 725 cm 1), respectively, in agree with previously
published values [35]. However, in the FT-IR spectra of Pd-
Fe P W18/mAPO samples, these bands are not clearly visible due to
−
4
2
18
◦
◦
0.82 and a very weak reflection at 1.17 , corresponding to a d-
spacing of 10.7 nm. These low angle diffraction peaks signify the
formation of mesopores, the single intense peak in low angle range
suggests the pores are arranged with large irregularity in these
materials.
4
2
overlapping with the bands of mAPO in the wavenumber range of
−
100–400 cm . In the FT-IR of as-prepared Pd-Fe P W18/mAPO,
4 2
1
1
−1
−1
bands at 1512 cm and 2800–3000 cm are assigned to the C
H

3
08
L. Chen et al. / Applied Catalysis A: General 523 (2016) 304–311
Fig. 5. TEM image of mesoporous Pd-(VO)4P2W18/mAPO using hexadecylamine
as templating agent after extraction and then calcinated at 400 C for 5 h (scale
bar = 100 nm).
◦
Fig. 7. High-resolution TEM image of Pd-(VO)4P2W18/mAPO showing the crystal
structure of Pd nanoparticles (scale bar = 5 nm).
TEM images of representative Pd-(VO) P W18/mAPO of dif-
4
2
ferent magnification are shown in Figs. 5–7 As seen in Fig. 5,
the mesopores of Pd-(VO) P W18/mAPO, seen as low electron
3.0 nm, the Pd-(VO) P W18/mAPO used for four times exhibits Pd
4
2
particle size distribution from 2.3 to 5.4 nm with mean Pd particle
size 3.7 nm. The comparison between the fresh and recovered Pd-
(VO) P W18/mAPO indicates not obvious variation in particle size
4
2
density spots in specimen, are distributed randomly to form a
wormhole-like disordered mesostructure. As seen in Fig. 6, the
freshly prepared Pd-(VO) P W18/mAPO exhibits Pd particle size
4
2
and dispersion state before and after reaction. The high-resolution
TEM image of Pd-(VO) P W18/mAPO in Fig. 7 confirms the crys-
4
2
distribution from 1.8 to 4.5 nm with the mean Pd particle size
4
2
Fig. 6. Typical TEM images and Pd nanoparticle size distribution of (A) freshly prepared Pd-(VO)4P2W18/mAPO and (B) Pd-(VO)4P2W18/mAPO used for four successive runs
in the oxidation of benzyl alcohol (scale bar = 20 nm).

L. Chen et al. / Applied Catalysis A: General 523 (2016) 304–311
309
Table 2
conversion
selectivity
aerobic oxidation of benzyl alcohol using various catalysts.a
100
90
Entry
Catalysts
mol%
Pd
mol%
M4P2W18
Conv.
(%)
Select.
(%)
b
8
0
1
2
3
4
5
6
7
8
9
none
–
–
–
4.8
51.6
10.5
8.0
–
mAPO
–
–
99
96
70
c
Pd/mAPO
0.2
–
–
–
–
–
6
5
0
0
Fe4P2W18/mAPO
Mn4P2W18/mAPO
Cu4P2W18/mAPO
Co4P2W18/mAPO
(VO)4P2W18/mAPO
Pd-Fe4P2W18/mAPO
Pd-Mn4P2W18/mAPO
Pd-Cu4P2W18/mAPO
Pd-Co4P2W18/mAPO
Pd-(VO)4P2W18/mAPO
Fe4P2W18 + Pd/mAPO
Mn4P2W18 + Pd/mAPO
Cu4P2W18 + Pd/mAPO
Co4P2W18 + Pd/mAPO
(VO)4P2W18 + Pd/mAPO
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
0.04
98
98
98
98
98
99
99
99
99
99
99
99
99
99
99
9.2
10.7
11.2
66.9
63.3
65.3
67.3
68.6
62.2
60.4
61.1
62.4
63.2
40
30
–
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
10
11
12
13
14
15
16
17
18
2
1
0
0
0
a
Fig. 8. Catalytic activities of Pd-M4P2W18/mAPO with different substituent metal
ion in M4P2W18 for aerobic oxidation of benzyl alcohol.
Reaction conditions: 1 mmol benzyl alcohol in 10 mL water, 1 atm of O2, 30 mg
◦
of catalyst, 80 C, 600 rpm, 3 h.
b
Determined by GC.
Toluene was detected as byproduct.
c
100
conversion
selectivity
9
0
talline nature of Pd nanoparticles, it also reveals that the particles
are spherical in shape.
80
70
60
50
3
.2. Catalytic activity results
3.2.1. Aerobic oxidation of benzyl alcohol in water
The catalytic activities of different heterogeneous catalysts such
as mAPO, Pd/mAPO, Pd-M P W /mAPO and M P W + Pd/mAPO
40
4
2
18
4
2
18
were evaluated for aerobic oxidation of benzyl alcohol in water,
and the results are summarized in Table 2. As shown in Table 2,
the reaction can’t proceed in the absence of catalyst, and mAPO
affords 4.8% of conversion (Table 2, entries 1 and 2). The activity of
mAPO for aerobic oxidation of benzyl alcohol was probably associ-
ated with its solid bifunctional acid-base properties [36]. With 0.2
mol% Pd to benzyl alcohol, Pd/mAPO (1 wt%) afforded a benzyl alco-
hol conversion of 51.6% within 3 h (Table 2, entry 3). Compared with
Pd/mAPO, the palladium catalysts combined with polyoxometa-
lates exhibited enhanced activity, the enhanced activity is related to
the contribution of polyoxometalates component in catalytic sys-
tem, the previous studies have demonstrated that the multinuclear
3
0
20
10
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15
cycle times
Fig. 9. The recyclability test of Pd-(VO) P W18/mAPO for aerobic oxidation of benzyl
alcohol.
4
2
ion substituted M P W18. The catalytic activity increased with
4
2
9−
sandwich complexes based on trivacant Keggin anion [PW O34]
the order: Pd-(VO) P W /mAPO > Pd-Co P W /mAPO ≈ Pd-
9
4
2
18
4
2
18
is active to O or H O -based oxidations [37,38]. Under the reaction
Fe P W /mAPO > Pd-Cu P W /mAPO > Pd-Mn P W /mAPO.
4 2 18 4 2 18 4 2 18
2
2
2
conditions described in Table 2, the contribution of polyoxometa-
lates component in catalyst to the conversion of benzyl alcohol is
around 10% (Table 2, entries 4–8).
Further experiments were performed to test the recyclability
of the present catalyst. The consecutive oxidations of benzyl alco-
hol over the representative Pd-(VO) P W18/mAPO catalyst were
4
2
Under the same reaction conditions, Pd-M P W18/mAPO is
carried out under the same conditions as before. After the first reac-
tion, the catalyst was filtered and the catalyst-free solution was
kept under the same reaction conditions for further 12 h. No fur-
ther increase in conversion was detected and atomic absorption
4
2
more active than M P W + Pd/MAPO. The higher catalytic activity
4
2
8
of Pd-M P W18/mAPO is related to its more efficient synergic
4
2
catalysis of M P W and Pd nanoparticles. The pre-synthesis of
4
2
18
M P W stabilized Pd nanoparticles and in-situ incorporation
analysis of the remaining solution indicated no Pd and (VO) P W
4
2
18
4 2 18
of M P W stabilized Pd nanoparticles in the mesoporous alu-
species had leached into the reaction, this clearly suggest the
actual catalytic activity of the recovered catalyst is come from
the immobilized active centers. The recovered catalyst was used
in 15 successive reactions. For each recycle run, the catalyst was
recovered by a centrifugation, washed by acetone and dried for
the next cycles. The results of conversion and selectivity after
each recycling were reflected in Fig. 9. It is found that the cata-
lyst shows excellent recyclability and exhibits the same activity in
15 subsequent reactions. The amount of Pd and polyoxometalates
in the recovered Pd-(VO) P W18/mAPO were further determined
4
2
18
minophosphate by hydrothermal method lead to a closer contact of
M P W and Pd nanoparticles in Pd-M P W18/mAPO, the synergic
4
2
18
4 2
catalysis derived from the interaction of polyoxometalates and pal-
ladium metal can produce an enhanced activity and selectivity [5].
Furthermore, under the stabilization of polyoxometalates and pro-
tection of mesoporous aluminophosphate, Pd nanoparticles with
small size in Pd-M P W18/mAPO remain high dispersion on meso-
4
2
porous alumnophosphate, showing a better activity for aerobic
oxidation of alcohols. As presented in Fig. 8, the substituted metal in
4
2
0
2+
M P W affected the activity of M P W -Pd /mAPO, VO substi-
to be 0.97 wt% of mAPO and 16.7 ␮mol/g of mAPO using ICP-AES,
4
2
18
4
2
8
tuted M P W showed relatively higher activity than other metal
respectively, almost remain unchanged compared with fresh Pd-
4
2
18

3
10
L. Chen et al. / Applied Catalysis A: General 523 (2016) 304–311
Table 3
Aerobic oxidation of various alchols using Pd-(VO)4P2W18/mAPO.
a
◦
Conv. (%)b
55.6
Select. (%)b
98
Entry
substrates
product
mol%Pd
0.5
T( C)
Time(h)
6
1
80
80
80
2
3
4
0.5
0.5
0.5
8
53.3
51.5
47.3
98
98
97
12
12
90
5
6
0.5
0.5
90
80
12
12
53.0
49.8
99
98
7
0.5
0.8
0.8
80
12
20
20
43.4
10.6
34.5
98
99
99
8
100
100
9c
1
1
0
0.8
0.8
100
100
20
20
9.3
99
99
1c
31.0
1
1
2
0.8
0.8
90
90
20
20
13.3
38.0
99
99
3c
a
b
c
Reaction conditions: 1 mmol of alcohol in 10 mL of water, 1 atm of O2.
Based on GC.
Polyethylene glycol/water (4 mL/6 mL) mixture as solvent.
(
VO) P W18/mAPO (the contents of Pd and polyoxometalates in
alcohols. The allyl alcohol substrates such as 3-phenyl-2-propene-
1-ol and 3-methyl-2-butene-1-ol afforded the conversions of 68.2%
and 66.9%, respectively (Table 3, entries 5 and 6). The high reac-
tivity of allyl alcohol can be ascribed to its structural factor. The
allyl alcohols have C C double bond in their structure, the coor-
dination of C C double bond to the Pd species can enhance the
interaction of hydroxyl group with Pd species [9]. Comparatively,
secondary aliphatic alcohols are less reactive than the benzoic ones,
2-hexanol, 2-octanol and cyclohexanol were converted to the cor-
responding ketones in conversions of 10.6%, 9.3% and 13.3% at
higher temperature within 20 h, respectively (entries 4–6, Table 2).
But the conversions of these secondary aliphatic alcohols can be
enhanced to 34.5%, 31% and 38%, respectively by using polyethylene
glycol/water mixed solvent (entries 7–9, Table 3). The improve-
ment of conversion may be ascribed to the increase in solubility of
the substrates by adding polyethylene glycol as co-solvent.
4
2
fresh catalyst is 0.97 wt% and 16.8 ␮mol/g of mAPO, respectively).
The excellent recyclability of Pd-M P W18/mAPO may be
4
2
associated with the size and morphology controlling of Pd
nanoparticles. TEM images of fresh Pd-(VO) P W18/mAPO and Pd-
(
in Pd particle size and Pd dispersion, this indicates the stabilization
of polyoxometalates and support protection successfully prevent
Pd nanoparticles from aggregating into larger particles of Pd black,
thus the catalyst can remain its catalytic activity even after twelve
times of cycle.
4
2
VO) P W18/mAPO used for four times have not obvious difference
4 2
3
.2.2. Aerobic oxidation of other alcohols in water
The present Pd-M P W18/mAPO was also active for the aer-
4
2
obic oxidation of a wide range of alcohols, the representative
results using Pd-(VO) P W18/mAPO as catalyst were summa-
4
2
rized in Table 3. Under the investigated conditions, the secondary
aromatic alcohol, 1-phenylethanol, 1-phenyl-1-propanol and 1-
4. Conclusion
(
4-methoxyphenyl)-1-propanol were oxidized selectivity into the
corresponding ketone in high conversion (Table 3, entries 1 and
In this present work, we described a hydrothermal synthe-
sis of a new recyclable and efficient palladium-based catalyst
for the aerobic oxidation of alcohols in water by in situ encap-
sulation of polyoxometalates stabilized palladium nanoparticles
in mesoporous aluminophosphate. We also demonstrated the
2
4
). The substituted benzylic alcohol, 2-methoxybenzyl alcohol and
-methoxybenzyl alcohol also exhibit good catalytic productivity,
reached conversion of 47.3% and 53%, respectively (Table 3, entries
–4). Pd-(VO) P W18/mAPO also showed good activity for allyl
3
4
2

L. Chen et al. / Applied Catalysis A: General 523 (2016) 304–311
[12] F. Li, Q.H.Y. Wang, Appl. Catal A: Gen. 334 (2008) 217–226.
311
dual function of polyoxometalates M P W in the obtained Pd-
4
2
18
[
[
13] J. Chen, Q.H. Zhang, Y. Wang, H.L. Wan, Adv. Synth. Catal. 350 (2008) 453–464.
14] D.R. Rolison, Science 299 (2003) 1698–1701.
M P W18/mAPO as both stabilizing agent and promoter for palla-
4
2
dium nanoparticles. Comparison of activity of Pd-M P W18/mAPO
4
2
[15] C. Mondeli, D. Ferri, J.D. Grunwaldt, F. Krumeich, S. Mangold, R. Psaro, A.
Baiker, J. Catal. 252 (2007) 77–87.
[
[
[
and Pd + M P W18/mAPO revealed the stronger interaction of poly-
4
2
16] F. Alardin, B. Delmon, P. Ruiz, M. Devillers, Catal. Today 61 (2000) 255–262.
17] A.B. Powell, S.S. Stahl, Org. Lett. 15 (2013) 5072–5075.
18] D. Wang, A. Villa, P. Spontoni, D.S. Su, L. Prati, Chem. Eur. J. 16 (2010)
10007–10013.
oxometalates and palladium nanoparticles resulted in a more
efficient synergic effect that led to enhanced activity. It is found
the combinations of polyoxometalates and mesoporous channels
of support effectively suppressed the agglomeration of palladium
nanoparticles and produced a durable catalyst. This approach may
find potential applications in other reactions that involving other
noble metal nanocatalyst. Efforts for further applications of other
nanocatalyst by this approach are now in progress.
[19] S. Marx, A. Baiker, J. Phys. Chem. C. 113 (2009) 6191–6201.
[
[
20] Y.T. Chen, Z. Guo, T. Chen, Y. Yang, J. Catal. 275 (2010) 11–24.
21] T. Harada, S. Ikeda, F. Hashimoto, T. Sakata, K. Ikeue, T. Torimoto, M.
Matsumura, Langmuir 26 (2010) 17720–17725.
[22] B. Karimi, S. Abedi, J.H. Clark, V. Budarin, Angew. Chem. Int. Ed. 45 (2006)
776–4779.
4
[
23] F.F. Bamoharram, A. Ahmadpour, M.M. Heravi, A. Ayati, H. Rashidi, B. Tanhaei,
Synth. React. Inorg. Met. Org. Nano Met. Chem. 42 (2012) 209–230.
Acknowledgement
[24] L. D’souza, M. Noeske, R.M. Richards, U. Kortz, Appl. Catal. A: Gen. 453 (2013)
62–271.
2
[
[
25] J. Ettedgui, R. Neumann, J. Am. Chem. Soc. 131 (2009) 4–5.
26] V. Kogan, Z. Aizenshtat, R. Popovitz-Biro, R. Neumann, Org. Lett. 4 (2002)
3529–3532.
27] A. Corma, S. Iborra, F.X. Llabrés i Xamena, R. Montón, J.J. Calvino, C. Prestipino,
J. Phys. Chem. C 114 (2010) 8828–8836.
28] P.P. Zhao, Y. Leng, M.Q. Zhang, J. Wang, Y.J. Wu, J. Huang, Chem. Commun. 48
(2012) 5721–5723.
29] D. Barats, R. Neumann, Adv. Synth. Catal. 352 (2010) 293–298.
30] M. De bruyn, R. Neumann, Adv. Synth. Catal. 349 (2007) 1624–1628.
31] R. Liu, S. Li, X. Yu, G. Zhang, S. Zhang, J. Yao, L. Zhi, J. Mater. Chem. 22 (2012)
This research was supported by grants from the natural Sci-
ence Foundation of China (no. 2140605 and no.51306047), Hunan
Provincial Science and Technology department of China (no.
[
[
2
014NK3060), Hunan Natural Science Foundation (no. 13JJB009)
and state key lab for powder metallurgy.
[
[
[
References
3
319–3322.
[
[
[
[
1] M. Hudlick y´ , Oxidation in Organic Chemistry, ACS, Washington DC, 1990.
2] C. Parmeqqiani, F. Cardona, Green Chem. 14 (2012) 547–564.
3] M.J. Schultz, M.S. Sigman, Tetrahedron 62 (2006) 8227–8241.
4] I.W.C.E. Arends, G.J. Brink, R.A. Sheldon, J. Mol. Catal. A: Chem. 251 (2006)
[32] R. Neumann, Inorg. Chem. 49 (2010) 3594–3601.
[33] R.G. Finke, M.W. Droege, P.J. Domaille, Inorg. Chem. 26 (1987) 3886–3896.
[34] C. Ornelas, J. Ruiz, L. Salmon, D. Astruca, Adv. Synth. Catal. 350 (2008)
837–845.
2
46–254.
5] Z.C. Ma, H.Q. Yang, Y. Qin, Y.J. Hao, G. Li, J. Mol. Catal. A: Chem. 331 (2010)
8–85.
[35] M.S. Balula, J.A. Gamelas, H.M. Carapu c¸ a, A.M.V. Cavaleiro, W. Schlindwein,
Eur. J. Inorg. Chem. 3 (2004) 619–628.
[36] J.M. Campelo, M. Jaraba, D. Luna, R. Luque, J.M. Marinas, A.A. Romero, Chem.
Mater. 15 (2003) 3352–3364.
[37] I.C.M.S. Santos, J.A.F. Gamelas, M.S.S. Balula, M.M.Q. Simões, M.G.P.M.S. Neves,
J.A.S. Caveleiro, A.M.V. Cavaleiro, J. Mol. Catal. A: Chem. 262 (2007) 41–47.
[38] N.M. Okun, T.M. Anderson, C.L. Hill, J. Am. Chem. Soc. 125 (2003) 3194–3195.
[
[
7
6] T. Harada, S. Ikeda, F. Hashimoto, T. Sakata, K. Ikeue, T. Torimoto, M.
Matsumura, Langmuir 26 (2010) 17720–17725.
7] Y. Uozumi, R. Nakao, Angew. Chem. Int. Ed. 42 (2003) 194–197.
8] A. Biffis, L. Minati, J. Catal. 236 (2005) 405–409.
[
[
[
9] B. Feng, Z.S. Hou, H.M. Yang, X.R. Wang, Y. Hu, H. Li, Y.X. Qiao, X.G. Zhao, Q.F.
Huang, Langmuir 26 (2010) 2505–2513.
[
[
10] N. Jamwal, M. Gupta, S. Paul, Green Chem. 10 (2008) 999–1003.
11] M. Mifsud, K.V. Parkhomenko, I.W.C.E. Arends, R.A. Sheldon, Tetrahedron 66
(
2010) 1040–1044.