Preparation and characterization of nanosized copper (II) oxide embedded in hyper-cross-linked polystyrene: Highly efficient catalyst for aqueous-phase oxidation of aldehydes to carboxylic acids

Article abstract of DOI:10.1016/j.catcom.2015.12.016

Preparation and catalytic properties of nanosized copper (II) oxide embedded in hypercrosslinked polystyrene (HPS) were investigated in this article. The CuO@HPS nanocomposite was characterized by inductively coupled plasma optical emission spectrometry (ICP-OES), Fourier transform infrared spectroscopy (FT-IR), N₂-sorption analysis, X-ray powder diffraction (XRD), energy dispersive X-ray spectroscopy analysis (EDS), and transmission electron microscopy (TEM). The TEM analysis showed that the mean diameter of the resulted particles is ~ 4 nm. The nanocomposite was found to be efficient and durable catalyst in the oxidation of aldehydes to the corresponding carboxylic acids in water. The catalyst can be recycled and reused in 4 reaction runs.

Full text of DOI:10.1016/j.catcom.2015.12.016

Preparation and characterization of nanosized copper (II) oxide embedded in
hyper-cross-linked polystyrene: Highly efficient catalyst for aqueous-phase
oxidation of aldehydes to carboxylic acids
Fariba Saadati, Neda Khani, Mohammad Rahmani, Farideh Piri
PII:
DOI:
Reference:
S1566-7367(15)30168-0
doi: 10.1016/j.catcom.2015.12.016
CATCOM 4533
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
1 September 2015
21 November 2015
15 December 2015
Please cite this article as: Fariba Saadati, Neda Khani, Mohammad Rahmani,
Farideh Piri, Preparation and characterization of nanosized copper (II) oxide em-
bedded in hyper-cross-linked polystyrene: Highly efficient catalyst for aqueous-phase
oxidation of aldehydes to carboxylic acids, Catalysis Communications (2016), doi:
1
0.1016/j.catcom.2015.12.016
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ACCEPTED MANUSCRIPT
Preparation and characterization of nanosized Copper (II) oxide embedded in
Hyper-cross-linked polystyrene: highly efficient catalyst for aqueous-phase
oxidation of aldehydes to carboxylic acids
*
Fariba Saadati, Neda Khani, Mohammad Rahmani, Farideh Piri
Department of Chemistry, Faculty of Science, University of Zanjan, P.O. Box 45195-313, Zanjan, Iran
Received:
Abstract: Preparation and catalytic properties of nanosized copper (II) oxide embedded in hypercrosslinked polystyrene
(
HPS) was investigated in this article. The CuO@HPS nanocomposite was characterized by inductively coupled plasma
optical emission spectrometry (ICP-OES), Fourier transform infrared spectroscopy (FT-IR), N -sorption analysis, X-ray
2
powder diffraction (XRD), energy dispersive X-ray spectroscopy analysis (EDS), and transmission electron microscopy
(
TEM). The TEM analysis showed that the mean diameter of the resulted particles is  4 nm.
The nanocomposite was found to be efficient and durable catalyst in the oxidation of aldehydes to the corresponding
carboxylic acids in water. The catalyst can be recycled and reused in 4 reaction runs.
Keywords: Copper (II) oxide; Hypercrosslinked polystyrene; Nanocomposite; Oxidation; Aldehyde; Carboxylic acid.
1
. Introduction
Nano-sized metal and metal oxide particles have found their pertinent from industrial and
economic points of view. Catalysts containing metal nanoparticles have been revealed a subject of
interest due to their enhanced the efficiency, selectivity, and stability. This impressive success
would have been possible considering to the factors that severely control the size, size distribution
and morphology of nanoparticles, whose understanding control the catalyst properties [1].
However, small size particles are susceptible to agglomeration and the formation of larger particles
that lead to unstable catalytic properties. In addition, the application of the nanoparticles is limited
by further obstacles, such as simple separation and regeneration of the nanocatalysts from the
reaction mixture. In this pursuit, the development of supports for the preparation of
*
Corresponding author.
E-mail address: piri@znu.ac.ir (F. Piri)

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heterogeneously supported nanocatalysts permits the formation of nanosized and highly
monodispers particles while prevent agglomeration.
As a consequence, the various types of support, such as zeolites, silicas, and carbon compounds
have been extensively employed [2-5]. In recent years, efforts have been made to replace inorganic
materials with porous organic supports [6].
Inorganic nanoparticles incorporated into well-defined organic polymer matrix represent a very
favorable method for the preparation of nanoparticles. Metalated polymer nanocomposites
combine the particular properties of both inorganic nanoparticles and demanded processing
characteristics of polymers. These materials have potential applications in various fields especially
as efficient catalysts [7]. Hypercrosslinked polystyrene (HPS) with unique porosity is one of the
few examples of hydrophobic porous polymers. It possesses several advantages like high porosity,
nanoscale rigid pores, the ability to swell in wide variety of liquid mediums, large internal surface
2
-1
area (1000-1500 m g ), high diffusivity, low viscosity, and low cost. Similar to the other supports,
metal compounds can be incorporated into the porous polymers. According to the reports, metal
nanoparticles were introduced into the polymer matrix of HPS represent a peculiar possibility for
the modification of catalytic properties along with catalyst stability [8, 9].
Copper and copper based compound gained considerable attention due to several properties and
consequences of its application [10, 11]. In comparison with expensive and air-sensitive catalyst,
the copper (II) oxide is easily handled and remarkably stable under acidic and basic conditions.
According to the low toxicity, high stability and recyclability, beside the ease of production, copper
(
II) oxide nanoparticles have been of intense interest in semiconductors [12], sensors [13] and
catalysis. On this basis, CuO was utilized as an effective nanocatalyst for various organic
transformations [14, 15], cross coupling reactions[16], and thermal decomposition [17].

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Recent work demonstrated that CuO in combination with ZrO [18], CeO [19], Ce Ti O [20],
2
2
0.3 0.7
2
and Ag O [21] were found to be the effective catalysts for organic compound synthesis, CO and
2
NO oxidation as well as for the oxidation of organic chemicals by molecular oxygen.
In this work, we attempt to ascertain if HPS as porous polymer support can serve to immobilization
of copper (II) oxide particles. Herein, for the first time, we show that the resulting nanocomposite
can be used as a heterogeneous catalyst for oxidation of aldehydes to carboxylic acids. In this
clean, mild and simple procedure, the transformation can be performed under air atmosphere in
water as green and abundant resources. The optimized aerobic oxidation is attractive, since as
opposed to most methodologies, this environmentally benign method does not require ligands and
stoichiometric amounts of oxidant.
2
2
. Experimental
.1.
Material
HPS Macronet- MN270 (Purolite Int., United Kingdom) was washed with water and acetone
twice and dried under vacuum for 24 hours. All chemicals were obtained from commercial
suppliers. Sodium hydroxide and sulfuric acid were used as received but other chemicals were
distilled or recrystallized before use. Deionized water was used for washing and solution
preparation.
2
.2.
Instruments and measurements
The IR spectra were obtained using FT-IR Brucker-Vector in KBr/Nujol mull in the range of
-1
1
13
4
00-4000 cm . NMR spectra ( H NMR and C NMR) were recorded using a 250 Bruker
Avance instrument. Merck silica gel 60 F254 plates were used to monitor the reaction progress
in thin-layer chromatography (TLC).
X-ray powder diffraction patterns were collected on a Bruker D8 advance X-ray powder
diffractometer with the 2θ range from 5 to 90° and Cu electrode as anode. The step size of each
run was maintained at 0.07 2.

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Transmission electron microscopy was performed with a Philips CM120 electron spectroscopic
microscope operated at 120 kV.
JEOL JSM-7100 field-emission scanning electron microscopy (FESEM) and energy dispersive
X-ray analyses (EDS) were used to study the morphology and elemental composition.
Nitrogen adsorption–desorption isotherms was investigated at 77 K with a BELSORB-max
system (SIP). The surface area and pore size distribution were calculated with the BET and
BJH methods.
The initial and final concentrations of metal in the nanocomposite were analyzed by
inductively coupled plasma- optical emission spectrometry (ICP-OES, Spectro-Genesis).
.3. Incorporation of copper(II) oxide in HPS
2
In a typical procedure, 0.5 g HPS and 3 mL of ethanol were placed in a Schlenk tube and the
HPS sample was allowed to swell for 30 min in the solution. After forward, the solution of 0.2
g Copper (II) acetate monohydrate in 12 mL of ethanol and 0.15 mL acetic acid were added
separately. When the temperature reached 78 C, the NaOH aq. (1M, 5 mL) was subsequently
dropped into the flask under continuous stirring. The mixture was refluxed for 1 hour more.
The catalyst (CuO@HPS) was then recovered by filtration, washed with water and ethanol and
dried in vacuum overnight.
2
.4.
Oxidation of aldehydes catalyzed by CuO@HPS
In a two-necked round bottom flask equipped with a condenser, the aldehyde (1 mmol), was
dropped into the mixture of catalyst (4 mol %) and NaOH (1.5 mmol) in deionized water (3
mL). The resulting mixture was stirred at room temperature under air atmosphere for 15 min
and then the temperature reached 75 C. The progress of the reaction was monitored by TLC.
After completion of the reaction, the reaction mixture was filtered off and the catalyst rinsed
with deionized water and ethanol. The filtrate was treated with H SO (30% wt.). The
2
4
precipitated carboxylic acid was filtered, washed with water and dried in vacuum. (For liquid
products after addition of H SO , the product was extracted using diethyl ether (4×5 mL).
2
4

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3
. Result and discussion
The preparation of nanocomposite is quite straightforward. To do this, briefly, hypercrosslinked
polystyrene (HPS) sample was swelled in ethanol. The resulting support was then subjected to
mixing with an appropriate concentration of Cu(CH COO) .H O in the presence of acetic acid
3
2
2
in ethanol. Next, the obtained slurry were allowed to react with a freshly prepared aqueous
solution of NaOH (1M) to obtain the corresponding CuO nanoparticles supported in HPS
(CuO@PMO). The Cu content is 3.7 wt. % in the resulting nanocomposite, a value which was
confirmed by inductively coupled plasma analysis (ICP-OES).
The porosity of parent HPS and HPS– CuO samples has been evaluated using liquid nitrogen
physisorption analysis. The shape of nitrogen adsorption -desorption isotherms (Figs. A1 and
A2, ESI†) can be signify that HPS-based samples encompass both mesopores and micropores
[
22]. The analysis demonstrated that the HPS prior to copper oxide addition has the BET
2
−1
3 -1
surface area of 1537 m g and primary pore volume of 1.22 cm g . Both surface area and the
volume fraction of the HPS-CuO nanocomposite significantly decrease, due to copper oxide
nanoparticles deposition within the pores and on the surface. This is in agreement with
3
-1
porosimetry results, in which a reduction of the pore volume 0.75 cm g and surface area 1171
2
-1
m g was observed. The fraction of pores decreases by 38%.
Fig. 1 showes the FT-IR spectra of investigated HPS (a) and CuO@HPS (b). In the present
-1
investigation, the low frequency region 400– 700 cm assigned the vibrational properties of
-1
-1
CuO nanoparticles. The peaks at 531 cm and 579 cm correspond to CuO bond stretching
−
1
modes. In addition, observed broad absorption band at 3420 cm might be due to vibrations of
the O-H group that adsorbed on the surface.

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Fig. 1. FT- IR spectra of HPS (a) and CuO@HPS nanocomposite (b)
As shown in Fig. 2, all of the diffraction peaks of the XRD pattern of the CuO at 2θ values of
3.1°, 35.2, 38.4°, 42.2°, 48.5°, 53.3°, 58.3°, 61.3°, 65.8°, 67.9°, 72.1°, and 74.9° are observed
3
and are consistent with the JCPDS data (card no: 044-0706). This indicated that CuO
nanoparticles were obtained.
Fig. 2. X-ray diffraction pattern of CuO @HPS
The EDS measurement has been used to confirm the presence of copper oxide components on
the surface of nanocomposites. The EDS analysis, presented in Fig. 3, is supporting the CuO
stoichiometry.

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Fig. 3. EDS analysis of CuO@HPS
Fig. 4 shows TEM images of HPS– CuO sample prepared by impregnation of HPS with the
copper(II) oxide. Diameter of the CuO compound NPs is  4 nm.
Fig. 4. Transmission electron microscopy (TEM) image of CuO@HPS

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It is worth mentioning that since the catalyst are prepared in the presence of sodium hydroxide,
the copper (II) oxide and copper (II) hydroxide embedded in hyper-cross-linked polystyrene
(HPS) is believed to be the active sites. The catalytic performance of CuO@HPS was
investigated in the transformation of aldehydes to carboxylic acids. Several reagents such as
chromium compounds [23], potassium permanganate [24] , sodium perborate [25], chlorite
species [26], hydrogen peroxide [27] and oxone [28], also several transition metal, for example,
copper [29, 30], vanadium [31], gold [32], silver [33], have been developed the oxidation of
aldehydes. In addition, Ag O/CuO [21], , Fe O nanoparticles [34], and iron oxalate capped
2
3
4
iron-copper nanomaterial [35] catalyzed oxidation of aldehyde has been reported. Using of
stoichiometric amount of reagent, limited substrate scope, high catalyst loading, high cost and
environmental contamination are particular drawbacks of previous methods. These were
reasoned that a revision in reaction conditions to introduce efficient heterogeneous catalyst
systems in green solvents such as water under air atmosphere.
To study the nanocomposite performance for aldehydes oxidation, the temperature, base,
atmospheric conditions and catalyst loading were screened as the main factors affecting the
transformation. For this purpose, a model oxidation of benzalehyde was initially carried out by
using 2 mol% of CuO@HPS in the presence of NaOH under air atmosphere in water at room
temperature for 24 h. A study of the same reaction identified that increase in the reaction
temperature to 75 C greatly enhanced the reaction efficiency from 10 % to 70% yield (Table1,
entries 1, 3). Next, four variations of the catalyst loading (1, 2, 3, 4 mol %) were examined
(Table1, entries 2-5). It was found that by a slight increase in the catalyst loading to 4 mol%
greatly enhanced the reaction efficiency. Furthermore, the reaction was also carried out at
atmospheric pressure of O and Ar when the product yield was decreased to 75 and 50%
2
respectively (Table1, entries 6, 7). To further determine the co catalyst effect, K CO was also
2
3
studied (Table 1, entry 8) and in comparison, NaOH was employed as base to obtain

ACCEPTED MANUSCRIPT
satisfactory results. Notably, the present reaction is sluggish in the absence of NaOH at 75 C
(Table 1, entry 9).
Table 1. Optimization of the oxidation of benzaldehyde using CuO@HPS catalyst in water
Having established the optimal reaction conditions we further examined the scope of oxidation
reaction in the presence of 4 mol% nanocomposite at 75 C in water for various types of
aromatic aldehydes (Table 2).
a
Table 2 Oxidation of aldehydes using CuO@HPS nanocatalyst
Table 2 shows that oxidation is completed under desired reaction conditions with good to
excellent yields. It is worth noting that alcohol byproduct wasn’t detected in this
transformation.
To demonstrate the ability of HPS to stabilize nanoparticles, the recyclability of nanocatalyst
was examined by isolating the catalyst from the reaction mixture of oxidation of 4-methyl
benzaldehyde under optimal reaction conditions. In each run, after the completion of the
reaction, the used nanocatalyst was filtered off and then washed with ethanol and large amounts
of water to remove any remaining impurity. The recovered catalyst experiments were
performed for four times. The results are shown in Fig. 5. These results indicate that catalyst
activity slowly decreases over the consecutive runs. Therefore, this might be due to leaching of
copper oxide particles and also the partial loss of the catalyst during the recovery and washing
stages. However, copper leaching of the catalyst after the four reaction run was determined by
inductively coupled plasma (ICP- OES) to be less than 1.5 ppm.

ACCEPTED MANUSCRIPT
Figure 5. Recyclability chart of the oxidation of 4-methylbenzaldeyde using CuO@HPS.
4
. Conclusions
In conclusion, a novel heterogeneous nanocatalyst was successfully prepared through the
incorporation of copper (II) oxide in porous polymeric support. The hypercrosslinked
polystyrene was used as a stable and effective support to the preparation and stabilization of
copper oxide nanoparticles. In the present approach, the resulting nanocomposite is utilized as
an efficient catalyst for oxidation of aldehydes in water as the environmentally benign
conditions. Besides the heterogeneous catalysis advantages such as the easy separation,
reusability and minimization of the metal contamination in the product, the easy set up and the
absence of any organic solvent make this catalyst system a peculiar candidate for green
chemistry processes.
Acknowledgments
The authors acknowledge the Research Council of University of Zanjan for the financial
support of this work.
Appendix A. Supplementary data

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Table 1. Optimization of the oxidation of benzaldehyde using CuO@HPS catalyst in water
O
CuO@HPS (X mol%)
Base (1.5 equiv)
O
H
OH
Atmosphere
H O, t °C
2
Time (h)
Catalyst
Base Atmosphere
(mmol)
t
[C]
rt
Time
[h]
Yield
(%)
a
b
(
mol %)
1
2
2.0
NaOH
Air
Air
Air
Air
Air
24
20
20
20
20
20
20
20
24
10
45
7
7
5
5
1.0
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
3
4
2.0
3.0
70
75
75
75
75
75
75
75
4
.0
5
6
7
8
9
80
75
50
57
47
4.0
4.0
4.0
4.0
O
2
Ar
Air
Air
2 3
K CO
-
a
The reaction was carried out in the presence of the benzaldehyde (1 mmol); base (1.5 mmol); water (3 mL) as solvent
Isolated yield
b

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a
Table 2 Oxidation of aldehydes using CuO@HPS nanocatalyst
Time
h]
Yield
Entry Aldehyde
Carboxylic acid
b
[
[%]
80
90
90
91
97
93
85
87
85
87
90
85
92
71
60
50
75
C
6
H
5
CHO
-BrC
-ClC
6 5
C H COOH
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
20
20
20
18
18
18
20
20
20
20
20
20
20
24
24
24
24
4
4
4
4
4
4
3
3
3
3
2
2
2
4
6
H
4
CHO
4-BrC
6
H
4
COOH
6
H
4
CHO
6 4
4-ClC H
COOH
-CH
3
OC
6
H
4
CHO
CHO
CHC
CHO
CHO
CHO
CHO
4-CH
3
OC
6
H
4
COOH
COOH
CHC COOH
COOH
COOH
COOH
COOH
-CH
3
C
6
H
4
4-CH
3
C
6
H
4
-(CH
)
3 2
6
H
4
CHO
4-(CH
4-CH
)
3 2
6 4
H
-CH
3
SC
6
H
4
3 6 4
SC H
-ClC
6
H
4
6 4
3-ClC H
-CH
3
C
6
H
4
3 6 4
3-CH C H
,5-(CH
3
)
2
C
6
H
3
3 2 6 3
3,5-(CH ) C H
0
1
2
3
4
5
6
7
-NO
2
2
C
6
6
H
4
CHO
CHO
3-NO
2
2
C
6
6
H
4
4
COOH
COOH
-NO
C
H
4
2-NO
C
H
-ClC
6
H
4
CHO
CHO
CHO
Nicotinaldehyde
-Naphthaldehyde
2-ClC
6
H
4
COOH
,4-Cl
C
2 6
H
3
2,4-Cl
C H COOH
2 6 3
-CHOC
6
H
4
6 4
4-COOHC H COOH
Nicotinic acid
2
2-Naphthoic acid
a
Conditions: Substrate (1 mmol); NaOH (1.5 mmol); Catalyst (4 mol %); Water (3 mL) as solvent; 75 C
b
Isolated yield.

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Graphical abstract

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Highlights
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Copper (II) oxide nanoparticle was incorporated in crosslinked polystyrene matrix.
The CuO@HPS nanocomposite was fully characterized.
The nanocomposite is utilized as an efficient catalyst for oxidation of aldehydes.
The reaction was done in water as the environmentally benign conditions.
The nanocatalyst can be successfully recycled and reused in 4 reaction runs.