Solid sheet of anodic aluminium oxide supported palladium catalyst for Suzuki coupling reactions

Article abstract of DOI:10.1016/j.catcom.2017.06.032

A Pd(II) Schiff base complex supported on anodic aluminium oxide (with Al substrate sheet) was successfully prepared. The prepared nanocatalyst was characterized by SEM, TEM and XPS. The synthesized Pd-based catalyst showed excellent catalytic activity for Suzuki cross-coupling reactions under mild conditions. The catalytic activity did not deteriorate after five repeated cycles. Pd(II) Schiff base complex supported on solid sheets could be separated and recovered easily by taking it out of the reaction solution. The construction of solid sheet supported Pd catalyst would be expected to be a promising system to perform heterogeneous catalytic reactions.

Full text of DOI:10.1016/j.catcom.2017.06.032

Catalysis Communications 100 (2017) 139–143
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short communication
Solid sheet of anodic aluminium oxide supported palladium catalyst for
Suzuki coupling reactions
YongLing Nong, NiNa Qiao, TianHui Deng, ZiNi Pan, Ying Liang⁎
School of Chemistry and Chemical Engineering, GuangDong Pharmaceutical University, Guangzhou 510006, PR, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Pd(II) complex
Anodic aluminium oxide sheet
Suzuki reaction
A Pd(II) Schiff base complex supported on anodic aluminium oxide (with Al substrate sheet) was successfully
prepared. The prepared nanocatalyst was characterized by SEM, TEM and XPS. The synthesized Pd-based cat-
alyst showed excellent catalytic activity for Suzuki cross-coupling reactions under mild conditions. The catalytic
activity did not deteriorate after five repeated cycles. Pd(II) Schiff base complex supported on solid sheets could
be separated and recovered easily by taking it out of the reaction solution. The construction of solid sheet
supported Pd catalyst would be expected to be a promising system to perform heterogeneous catalytic reactions.
1. Introduction
recycled more than ten times with high activity. In all, the development
of Pd catalysts offering high activity, stability and easy separation is the
Palladium catalyzed carbon-carbon bond-forming reactions have
focus subject.
Nanometer-sized supports have recently attracted a great deal of
played a crucial role in synthetic organic chemistry, which have been
widely applied to diverse areas such as natural products, functional
polymer materials and pharmaceuticals. The Suzuki coupling reaction
is one of the most widely used methods for the carbon-carbon bond-
forming reactions. Traditionally, many palladium catalysts used in such
interest because of their high surface area and outstanding stability and
activity [26]. Anodized aluminium oxide (AAO) containing ordered
cylindrical tunable nanochannels with 10 nm–500 nm has been ma-
turely developed in the past two decades. These channels have been
used as template for the preparation of a wide range of nanomaterials in
the forms of nanodots, nanowires, nanotubes, etc. [27–29]. Usually,
AAO was prepared by electro-oxidization on an aluminium sheet in
inorganic acids [30,31]. The aluminium substrate and the compact
barrier layer (intercalated layer) were then removed with acids or bases
to obtain through-hole AAO membranes [32]. Then nanomaterials were
fabricated using the through-hole AAO. This process is time-consuming
and causes also environmental contamination. An important short-
coming of this method, however, is that the through-hole AAO's are
very brittle, thin, and difficult to handle during nanomaterial fabrica-
tion. Non-through-hole AAO (containing Al substrate, marked A-
AO@Al) is a solid sheet and can be cut into different sizes. Considering
the solid sheet as support, the heterogeneous catalysts can be easily
separated and recovered just by taking them out of the solution.
Combining the ordered porous structure and the advantage of nan-
ometer-sized solid sheet supports, nanomaterials supported on AAO@Al
is a potential research subject. Recently Li et al. [33] have synthesized
successfully Pd nanowires confined on AAO@Al. The catalysts showed
great activity, stability and reusability in Suzuki carbon-carbon cou-
pling reactions. They reported that the nanoarrays are confined in the
pores of AAO and were not easily agglomerated. To the best of our
reactions are homogeneous, such as [Pd(PPh
3 4 2
) ], [Pd(OAc) ] and
[
PdCl (PPh ] [1–5]. Although these catalysts present excellent ac-
2
3 2
)
tivity, there remain problems that need to be addressed. On the one
side, most of the ligands are of high cost, sensitive to air oxidation and
thus require air-free conditions, which pose significant inconvenience
for applications [6,7]. On the other side, the product of extraction is
time-consuming and impossible to be recovered in consecutive reac-
tions. As a potential solution, immobilization of Pd nanoparticles on
solid support, such as polymers [8,9], zeolites [10], silica spheres
[
11,12], magnetic particles [13,14] and carbon nanotubes [15–20]
have recently received considerable attention as a new generation of
heterogeneous catalysts. However, supported Pd catalysts have some
disadvantages. For example, a loss of catalyst could occur after re-
covered by filtration [21]. Also, leaching and aggregation of Pd nano-
particles from the support might occur. For example, Pd nanomaterials
supported on functionalized silica were slowly deactivated after three
runs in Suzuki coupling reactions [22]. Recently, M. Gómez and co-
workers [23–25] presented a new system of palladium nanoparticles
immobilized in a glycerol phase. The innovative dual homogeneous/
heterogeneous catalytic colloidal solution had been proved by its effi-
cient application in carbon-carbon bond-forming reactions and could be
⁎
Corresponding author.
E-mail address: l_xyf@gdpu.edu.cn (Y. Liang).
http://dx.doi.org/10.1016/j.catcom.2017.06.032
Received 19 March 2017; Received in revised form 19 June 2017; Accepted 20 June 2017
Available online 29 June 2017
1566-7367/ © 2017 Elsevier B.V. All rights reserved.

Y. Nong et al.
Catalysis Communications 100 (2017) 139–143
knowledge, AAO@Al supported metal catalysts have been rarely re-
ported. Here, we prepared a novel Pd (II)-AAO@Al catalyst through the
incorporation of schiff bases. The application of the solid sheets sup-
ported Pd-based catalyst as a heterogeneous catalyst in Suzuki reactions
is reported herein. The results showed that Pd(II) complex anchored on
the functionalized AAO presents great activity, stability and reusability,
which demonstrates a promising strategic approach in the field of
heterogeneous nanocatalysis.
2.4. General procedure for the Suzuki cross-coupling reactions
In a typical Suzuki cross-coupling reaction, a piece of Pd(II)/
AAO@Al sheet was immersed into a mixture solution, which included
1 mmol aryl halides, 1.2 mmol arylboronic acid, 1.5 mmol NaHCO and
3
5 mL solvent. The reaction was conducted at different temperatures for
different times. After completion of reaction, the Al sheet was taken out
and the residual was extracted with ethyl acetate. The organic layer was
4
dried with anhydrous MgSO , filtered and concentrated to get the de-
2
. Experimental
sired product. The conversions and yields were analyzed by gas chro-
matography, based on the peak area normalization method. For the
recycling test, after completing the Suzuki reaction, the solid sheet was
recovered just by taking it out of solution, washed and dried, and then
reused for the next run.
2.1. Materials and physical measurements
Palladium chloride, ethylene glycol, oxalic acid, phosphoric acid,
chromium trioxide, acetone and ethanol were purchased from
Guangzhou Chemical Reagent Factory in the highest available purity
and used without further purification. The high purity Aluminium
sheets (99.99%) were purchased from NanChang Mat-Cn. The
morphologies of the nanomaterials were examined by scanning electron
microscopy (SEM, Zeiss ULTRA 55). Transmission electron microscopy
3. Results and discussion
3.1. Synthesis and characterization of Pd(II)/AAO@Al
Pd-schiff base complex supported on AAO@Al was synthesized by
the functionalization method. AAO@Al substrate with hydroxyl groups
was aminated by (3-aminopropyl) triethoxysilane in ethanol. Then,
amino-AAO@Al was functionalized with 3,5-di-tert-butyl salicylalde-
hyde to produce schiff base groups. Lastly, Pd(II) ions were coordinated
with schiff base anchored on the surface of AAO@Al. The schematic
diagram is shown in Scheme 1.
(
TEM) studies were conducted using a PHILIPS TECNAI 10 equipment
in conjunction with energy dispersive x-ray spectroscopy (EDX). X-ray
photoelectron spectroscopy (XPS) analysis was conducted using a
ESCALAB 250 (Thermo Scientific, USA). The binding energy was cali-
brated with respect to C (1 s) at 284.6 eV. Peak deconvolution and
fitting procedures were performed using XPSPEAK Version 4.1 soft-
ware. The Pd content of catalysts was determined by inductively cou-
pled plasma-atomic emission spectroscopy (ICP-AES) using a 5300DV
SEM and TEM measurements were carried out to observe the mor-
phology and distribution of the samples. Fig. 1a and b show the SEM
images of AAO@Al and Pd(II)/AAO@Al. The average pores size of AAO
is 50 nm with rather uniform pores (Fig. 1a). After the Pd was deposited
on the AAO, the morphology of AAO was slightly different (Fig. 1b). It
can be seen that the original structure had not been destroyed after pure
AAO was treated with functionalization and metal deposition. Com-
pared with Fig. 1a and b, the existence of some materials on the
channels of AAO is clearly distinguishable. To further research the
materials, after dissolving the AAO and Al, the Pd complex was char-
acterized by TEM (Fig. 1c). The insert figure (Fig. 1c) is the EDX ana-
lysis, which identify the presence of palladium element, whereas the
existence of Fe element may be due to the impurity of aluminium sheet.
It is obvious that the distribution of Pd complex was abundant and some
of nanoparticles aggregated to form some clusters (Fig. 1c). After the
Suzuki reaction, the Pd(II)/AAO@Al catalyst was characterized by SEM
(Fig. 1d). It can be seen that the surface of Pd(II)/AAO@Al was de-
stroyed and flower-like micrometer materials appeared. The reason is
due to the fact that AAO was eroded under the basic catalytic conditions
of the Suzuki reaction, which was identified by placing pure AAO@Al
(
PE, USA).
The yield of Suzuki reaction was analyzed via gas chromatography.
The measurements were performed on an Agilent GC-6820 chromato-
graph with a 30 m (column height) × 0.32 mm (column diame-
ter) × 0.5 mm (column coating thickness) OV-17 capillary column with
a flame ionization detector.
2.2. Preparation of AAO@Al
The AAO used in this work was prepared according to the procedure
previously reported [33]. Briefly, the high purity Aluminium sheet
0.5 mm thickness) was annealed at 500 °C for 4 h and rinsed thor-
(
oughly by acetone and ethanol. Then, the sheet was electrochemically
polished by a mixture of ethanol and perchloric acid (volume ratio of
4
:1). After that, the sheet was first anodized in 0.3 M oxalic acid solu-
tion at 0 °C and 40 V for 2 h. The formed alumina film was then re-
moved in a mixture of 6 wt% H PO and 1.8 wt% H CrO at 60 °C for
h, while the second anodization was conducted under the same con-
3
4
2
4
2
ditions as the first anodization but for 8 h. The synthesized AAO
membrane formed on the surface of Aluminium sheet was labeled as
AAO@Al.
3
sheet in the 1.5 mmol NaHCO ethanol/water solution at 60 °C for 2 h.
After reaction, SEM images of AAO@Al (Fig. S1) revealed that the
surface of AAO@Al was also eroded, result little similar to that shown
in Fig. 1d.
2
.3. Preparation of AAO@Al – supported Pd catalyst
XPS survey spectrum of Pd(II)/AAO@Al catalyst is shown in Fig. S2.
The signals of Al2p, C1s, O1s, Pd3d were detected, confirming the
presence of Pd in the complex materials. Except the above elements, the
N1s and Si2p signals were also detected, which arose from 3-amino-
propyltriethoxysilane during synthesis. The spectrum of Pd 3d core
level is shown in Fig. S3 which was fitted into a main doublet peaks by
constraining the spin-orbit separation of 5.2 eV and the ratio of doublet
intensities at 3:2. The binding energy at 337.3and 342.5 eV is attributed
to Pd3d5/2 and Pd3d3/2, respectively, which is in accordance with those
reported for Pd (II) state [34]. Thus, XPS analysis reveals that almost all
Pd element in the catalyst exists as a Pd(II) complex.
Pd-Schiff base complex supported on the AAO@Al carrier was
synthesized as follows. Firstly, AAO@Al was modified by putting a
piece of AAO@Al (1 × 2 cm) into a solution of 3-aminopropyl-
triethoxysilane (1 mL), glacial acetic acid (0.5 mL) and 20 mL absolute
ethanol. The reaction mixtures were refluxed for 3 h in nitrogen at-
mosphere. The sheet was taken out of solution, washed with ethanol
and dried in vacuum. Then, the modified AAO@Al sheet was added into
a solution of 3.5-di-tert-butyl-salicylaldehyde (1:5 mol ration to 3-
aminopropyltriethoxysilane) and 20 mL ethanol followed by refluxing
at 70 °C for 5 h. Lastly, a round bottomed flask was charged with a
certain amount of palladium chloride and 20 mL N,N-dimethyllforma-
mide and modified AAO@Al sheet above and refluxed at 80 °C for 1 h,
The sheet was then washed with ethanol, dried in vacuum for use and
labeled as Pd(II)/AAO@Al.
3.2. Catalytic performance for Suzuki reaction
The Suzuki cross-coupling reaction of 4-bromobenzaldehyde with
phenylboronic acid was chosen as a model reaction to find out the
140

Y. Nong et al.
Catalysis Communications 100 (2017) 139–143
Scheme 1. Synthesis route of Pd(II)/AAO@Al materials.
optimum reaction conditions. The effect of different reaction para-
60 °C the reaction temperature. The Pd content of Pd(II)/AAO@Al
meters such as solvent, base, catalyst loading and temperature were
(0.25 mg−PdCl
2
precursor) was determined by ICP-AES to be
mmol.
4
evaluated (Table 1). The solvents H
and the best result was found to be EtOH/H
bases evaluated, NaHCO was the best choice. Increasing the tem-
2
O, EtOH, DMF were investigated
4.7 × 10
2
O (v:v = 4:1). Among the
To survey the generality of the catalyst, we investigated the reac-
tions using various aryl halides coupled with arylboronic acid (Table 2).
3
perature from 25 °C to higher temperature (60 or 80 °C) had a sub-
stantial reaction rate acceleration. At 60 °C for 1 h, the reaction resulted
in a 98% yield. Considering the mildness and low risk of Pd leaching,
the temperature of 60 °C was selected as the best reaction temperature.
2
Pure AAO@Al and the same Pd amount of PdCl were also used as
catalyst for comparison. As expected, pure AAO@Al can hardly catalyze
the Suzuki coupling reaction of bromobenzene with phenylboronic acid
(Table 2, entry 1). From Table 2, the results show that aryl bromides
with either electron-withdrawing or electron-donating groups react
with arylboronic acid rather rapidly and generate the carbon-carbon
coupled products with good yields. For entries 5, 7 and 9, we had not
For the catalyst loading, 0.25 mg PdCl
best choice. Thus, the optimum conditions selected are: NaHCO
base, EtOH/H O (v:v = 4:1) as solvent, 0.25 mg PdCl precursor and
2
precursor was found to be the
3
as
2
2
Fig. 1. (a) SEM image of pure AAO@Al, (b) SEM image of
Pd(II)/AAO@Al, (c) TEM image of Pd complex after resol-
ving AAO and Al, insert is the EDX analysis, (d) SEM image
of Pd(II)/AAO@Al after Suzuki reaction.
141

Y. Nong et al.
Catalysis Communications 100 (2017) 139–143
Table 1
a
Optimization of the conditions for Suzuki coupling reactions of 4-bromobenzaldehyde with phenylboronic acid.
Pd(II)/AAO@Al
OHC
Br
+
OHC
B(OH)
2
Base, Solvent
Entry
Solvent
DMF/H
Base
T (°C)
PdCl
2
(mg)
T (h)
Convb (%)
Yieldc (%)
1
2
3
4
5
6
7
8
9
2
O
NaHCO
NaHCO
NaHCO
3
3
3
60
60
60
60
60
25
80
60
60
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.1
0.25
1
1
1
1
92
90
99
52
99
79
99
70
98
90
84
98
49
98
73
98
66
96
H
2
O
EtOH/H
EtOH/H
EtOH/H
EtOH/H
EtOH/H
EtOH/H
EtOH/H
2
O
O
O
O
O
O
O
2
2
2
2
2
2
Et
3
N
K
2
CO
3
1
NaHCO
NaHCO
NaHCO
NaHCO
3
3
3
3
12
0.5
1
1
a
Reaction conditions: a piece of AAO@Al sheet with Pd nanocatalysts (1 × 2 cm), 4-bromobenzaldehyde (1 mmol), phenylboronic acid (1.2 mmol), base (1.5 mmol), solvent (5 mL).
Conversion was determined by GC, using (trifluoromethyl)benzene as an internal standard.
Isolated yield.
b
c
Table 2
Palladium-catalyzed Suzuki cross-coupling reactions.a
Pd(II)/AAO@Al
Ar1
X
+
Ar2
Ar1
Ar2
B(OH)2
NaHCO , EtOH/H O
3
2
Ar1X
Ar B(OH)
2
Product
T (°C)
80
t (h)
2
Cov (%)b
3
Yield (%)c
1.4
Entry
2
1d
2e
3
60
80
60
60
2
18
92
99
90
15
90
98
86
2
4
0.5
3
5
6
7
60
60
0.5
2
99
95
98
93
8
9
60
2
86
83
60
60
60
2
6
2
82
16
82
80
10
76
1
1
0
1
1
2
60
2
65
59
1
1
3
4
60
60
2
2
98
98
95
96
a
Reaction conditions: a piece of AAO@Al sheet with Pd nanocatalysts (1 × 2 cm), aryl halides (1 mmol), arylboronic acid (1.2 mmol), NaHCO
precursor.
Conversion was determined by GC.
Isolated yield.
3
(1.5 mmol), 4:1 mixture of ethanol and
water, 0.25 mg PdCl
2
b
c
d
e
Pure AAO@Al as catalyst.
−4
PdCl
2
(4.7 × 10
mmol) as catalyst.
142

Y. Nong et al.
Catalysis Communications 100 (2017) 139–143
observed homo-coupling or hydrodehalogenated products. It can be
observed that electron-withdrawing groups,
good activity for Suzuki reactions at mild conditions and was reused for
five consecutive runs without significant loss of its catalytic activity. It
is important that the catalyst can be easily separated and reused just by
taking it out of the reaction solution. The excellent catalytic perfor-
mance and easy separation make it a good heterogeneous system and
can be a promising alternative approach in the application of organic
reactions.
Such as aldehyde group (entries 4, 5, 13 and 14), nitryl group
(
entries 6, 7, 11 and 12), are more electronically activated and lead to a
higher activity than electron-donating groups (entries 8 and 9). It was
also found that the yield of meta-position aryl bromide was lower than
those of para-position (entries 4–9). The results are consistent with
others reported [4,15,17]. Compared to entries 2 and 9, the conclusion
that Pd supported on AAO@Al provides higher catalytic activity than
Acknowledgements
2
pure PdCl , which may be attributed to the larger surface area of Pd
immobilized on AAO@Al can be made. In addition, as expected, the
coupling reaction of aryl chlorides with phenyboronic acid under ex-
tended reaction time gave the lower yield (entry 10). Furthermore,
coupling reaction of two electron-rich arylboronic acids with aryl-
bromides were examined (entries 11–14), which provided desired
yields, especially aldehyde group substituted bromobenzene with high
yields.
The work was supported by the National Natural Science
Foundation of China (Project No. 21201043).
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2017.06.032.
To investigate the catalyst's synthesis reproducibility, five Pd(II)/
AAO@Al catalytsts prepared independently were examined for the
Suzuki reaction of phenyboronic acid with 4-Bromobenzaldehyde. Five
catalysts had almost the same high yield (98%), which indicates that Pd
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In summary, a Pd(II)-schiff base complex anchored on AAO@Al
solid sheet was synthesized successfully. AAO had ordered pores with
average size of 50 nm and Pd(II) complex entered into the pores with
some of nanoparticles aggregated together. The catalyst demonstrates
(
143