Characterization and reactivity of γ-Al2O3 supported Pd-Cu bimetallic nanocatalysts for the selective oxygenization of cyclopentene

Article abstract of DOI:10.1016/j.cclet.2016.02.026

In this work, Pd-Cu/γ-Al₂O₃ is prepared by the impregnation method and investigated for selective oxygenization of cyclopentene to cyclopentanone. A series of bimetallic Pd-Cu/γ-Al₂O₃ nanocatalysts were prepared and the structures characterized by XRD, XPS and TEM. We determined that the obtained Pd-Cu/γ-Al₂O₃ (molar ratio Pd:Cu = 5:1) was an efficient catalyst for the oxygenization of cyclopentene to cyclopentanone with >95% selectivity and >85% conversion (100 °C, 1 MPa initial O₂ pressure, 7 h).

Full text of DOI:10.1016/j.cclet.2016.02.026

Accepted Manuscript
Title: Characterization and reactivity of γ-Al2O3 supported
Pd-Cu bimetallic nanocatalysts for the selective oxygenization
of cyclopentene
Author: Wei-Wei Liu Yi-Si Feng Guang-Yu Wang Wei-Wei
Jiang Hua-Jian Xu
PII:
S1001-8417(16)30014-6
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.cclet.2016.02.026
CCLET 3614
To appear in:
Chinese Chemical Letters
Received date:
Revised date:
Accepted date:
16-12-2015
29-1-2016
25-2-2016
Please cite this article as: W.-W. Liu, Y.-S. Feng, G.-Y. Wang, W.-W. Jiang, H.-J.
Xu, Characterization and reactivity of rmgamma-Al2O3 supported Pd-Cu bimetallic
nanocatalystsfortheselectiveoxygenizationofcyclopentene, ChineseChemicalLetters
(2016), http://dx.doi.org/10.1016/j.cclet.2016.02.026
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Graphical Abstract
Characterization and reactivity of γ-Al2O3 supported Pd-Cu bimetallic
nanocatalysts for the selective oxygenization of cyclopentene
Wei-Wei Liua, Yi-Si Fenga,b,c*, Guang-Yu Wanga, Wei-Wei Jianga, Hua-Jian Xua*
a School of Chemistry and Chemical Engineering, School of Medical Engineering, Hefei University of Technology, Hefei 230009, China
b Anhui Key Laboratory of Controllable Chemical Reaction & Material Chemical Engineering, Hefei 230009, China
c Anhui Provincial Laboratory of Heterocyclic Chemistry, Maanshan 243110, China
A series of γ-Al2O3 supported Pd-Cu bimetallic nanocatalysts are investigated for the selective oxygenization of cyclopentene to
cyclopentanone.
Page 1 of 8

Original article
Characterization and reactivity of γ-Al2O3 supported Pd-Cu bimetallic
nanocatalysts for the selective oxygenization of cyclopentene
Wei-Wei Liua, Yi-Si Fenga,b,c*, Guang-Yu Wanga, Wei-Wei Jianga, Hua-Jian Xua
aSchool of Chemistry and Chemical Engineering, School of Medical Engineering, Hefei University of Technology, Hefei 230009, China
bAnhui Key Laboratory of Controllable Chemical Reaction & Material Chemical Engineering, Hefei 230009, China
cAnhui Provincial Laboratory of Heterocyclic Chemistry, Maanshan 243110, China
ARTICLE INFO
ABSTRACT
Article history:
In this work, Pd-Cu/γ-Al2O3 is prepared by the impregnation method and
investigated for selective oxygenization of cyclopentene to cyclopentanone. A
series of bimetallic Pd-Cu/γ-Al2O3 nanocatalysts were prepared and the
structures characterized by XRD, XPS and TEM. We determined that the
obtained Pd-Cu/γ-Al2O3 (molar ratio Pd:Cu = 5:1) was an efficient catalyst for
the oxygenization of cyclopentene to cyclopentanone with >95% selectivity and
>85% conversion (100° C, 1 MPa initial O2 pressure, 7 h).
Received 16 December 2015
Received in revised form 29 January 2016
Accepted 25 February 2016
Available online
Keywords:
Impregnation method,
Oxygenization,
Cyclopentanone,
Cyclopentene,
Pd-Cu /γ-Al2O3.
1. Introduction
Cyclopentanone is an important fine chemical intermediate for the preparation of new spices, a variety of anti-
inflammatory, anti-cancer drugs, pesticides and herbicides [1]. Currently, the main method of preparation of cyclopentanone in
industrialized production is by intramolecular decarboxylation of adipic acid at high temperature with barium hydroxide as
catalyst. But this method has disadvantages due to the shortage and high cost of raw materials, and is not in conformity with
green chemistry principles. With the rapid development of the petrochemical industry, cyclopentene is abundant and the method
to synthesize cyclopentanone via cyclopentene has received wide attention. The early literature reported that cyclopentene was
oxidized by N2O with high yield and the absence of a catalyst. But there were shortcomings, for example, the reaction pressure
and temperature were high, N2O damaged the equipment and caused environmental pollution [2]. Takehira and others used
homogeneous catalyst PdCl2-CuCl2, PdCl2-FeCl3 to catalyze the oxygenization of cyclopentene to generate cyclopentanone,
however, this method had a low conversion rate and the reusability of catalyst was unsatisfactory [3, 4].
According to the mechanism of oxygenization of cyclopentene to cyclopentanone, a solid catalyst consisting of metal would be
useful for this reaction. Due to good mechanical strength and thermal stability, γ-Al2O3 has been widely applied as the carrier in
industry. According to literature, Pd is active to the fracture of C=C and shows good selectivity in the catalytic oxygenization of
cyclopentene, but has relatively low catalytic efficiency. Nickel and copper are non-noble metals with great catalytic ability. In
the study of the Wacker method to synthesize cyclopentanone, PdCl2/CuCl2 performed good catalytic activity [3, 4],
consequently we speculated that Cu may be beneficial in the synthesis of C=O. Mao used Pd-Cu bimetallic catalysts for the
oxygenization of styrene, showed good catalytic ability and selectivity [5]. Therefore, in our study, Pd-Cu/γ-Al2O3 was chosen as
the catalyst to oxidize cyclopentene.
The aim of this paper is to report the preparation, characterization and catalytic performance of Pd-Cu/γ-Al2O3 (with difference
mole ratios of Pd:Cu). Moreover, the parameters influencing the reaction of cyclopentene to cyclopentanone are intensely
studied.
2. Experimental
Preparation of Pd-Cu/γ-Al2O3 catalysts: The Pd-Cu/γ-Al2O3 catalysts were prepared by the impregnation method
[6-10]. In a typical Pd-Cu (5:1)/γ-Al2O3 catalyst synthesis process, PdCl2 (131.9 mg, 0.75 mmol) and CuCl2·2H2O (25.1 mg, 0.15
mmol) were dissolved in 10 mL acetic acid solutions (0.0225 mol/L) with constant agitation (500 rpm/min) at r.t. To this clear
solution, then was slowly added γ-Al2O3 (1.5 g) and uniformly stirred at 300 rpm/min for 5 h. After impregnated, the solution was
heated at 100 °C for removal of the redundant H2O, forming a sticky substance which was dried in vacuum oven at 80 °C for 12
h. Subsequently, the resulting catalytic forerunners were ground to powders and calcined in nitrogen (0.3 L/min) heating from
———
 Corresponding author.
E-mail address: hjxu@hfut.edu.cn
Page 2 of 8

r.t.to 500 °C at a rate of 5 °C/min, then kept at 500 °C for 4 h. Subsequently, the sample precursors were reduced by hydrogen
(0.15 L/min) in the nitrogen at 500 °C for 2 h. Finally, the Pd-Cu (5:1)/γ-Al2O3 catalyst was obtained. In order to research the
effect of the loading of metals, different catalysts of Pd/Cu mole ratios were synthesized for use in the oxygenization reaction.
Characterization: The XRD of different samples were recorded on a Rigaku D/MAX 2500 (V) Diffractometer with Cu Kα
radiation and scanned at a rate of 10o/min over the range 20o~90o. The XPS analysis was confirmed on Thermo Fisher Scientific
ESCALAB 250 using a monochromatic Al Kα gun with photonic energy of 1486.6 eV as the X-ray source under reduced vacuum
conditions. The binding energies (B.E.) acquired in the XPS analysis were compensated by using the adventitious C 1s 284.6 eV
signal for any charging effects. The TEM images of catalysts were obtained on a JEOL JEM-2100F Electron Microscope
equipped with EDS at an accelerating voltage of 120 kV. Samples were prepared by ultrasonic dispersion in ethanol and
deposited on a copper grid for analysis.
Catalytic reaction: In a typical procedure, 15 mL cyclopentene (0.17 mol), 60 mL ethyl alcohol (1.03 mol) and Pd-Cu (5:1)/γ-
Al2O3 catalyst (0.24 g, 2.0 wt% of cyclopentene) were added into 0.1 L closed autoclave. The mixture was stirred (400 rpm/min)
under 1.0 MPa oxygen pressure at 100 °C for 7 h. Specific parameters are provided in Supporting Information.
3. Results and discussion
3.1. Characterization of catalysts
3.1.1. X-ray diffraction
The XRD patterns of Pd/γ-Al2O3, Cu/γ-Al2O3 and diverse Pd-Cu/γ-Al2O3 catalysts are exhibited in Fig. 1. The results show a
low intensity level for all characteristic diffraction planes, except for the planes associated with the γ-Al2O3 support [10]. For γ-
Al2O3, four apparent signals (Fig. 1A) at 2θ = 19.6o, 21.2o, 27.5o, 57.2o were detected, corresponding to the (110), (102), (211)
and (305) facets (JCPDS 50-1496), respectively [11]. Shown in Fig. 1B is the XRD patterns of Cu/γ-Al2O3 catalysts with
apparent signals at 2θ = 43.3o and 74.1o corresponding to the (111) and (220) facets (JCPDS 04-0836). In Fig. 1C, there are four
signals at 2θ = 27.3o (PdO, JCPDS 50-1496) and 40.1o, 46.7o, 68.2o (Pd, JCPDS 46-1043) and the Cu peak found at 2θ = 43.3o
facets (JCPDS 04-0836). Three apparent signals at 2θ = 30.580o, 35.121o and 63.106o facets (JCPDS 48-0587) are due to Pd-Cu
bimetallic alloys peaks (Fig. 1C, D, E). The XRD spectra of Pd-Cu/γ-Al2O3 with different mole ratios of Pd/Cu show that the
diffraction peaks became wider and the positions of the diffraction peaks were shifted to lower angle with the addition of Pd
content. Shifting to lower angle is probably due to the formation of a nano-alloy and the wider diffraction signal indicated that
the crystal grain became smaller and the Pd content would be helpful into the reduction of Cu. No signals of the Pd species could
be observed in the XRD spectra of Pd-Cu/γ-Al2O3, which may be attributed to the good dispersion of Pd [12]. In addition, the
calculative results indicated that the size of Pd-Cu crystalline grain was around 13.4, 15.1 and 16.3 nm by the Scherrer formula.
In conclusion, we need further data to confirm the presence of hypothetical Pd-Cu bimetallic nanoparticles in catalysts by XPS
and TEM analysis.
3.1.2. X-ray photoelectron spectroscopy
The Pd-Cu/γ-Al2O3 XPS spectra of different Pd-Cu molar ratios are shown in Fig. 2. The results show that there were two
oxygenization states in XPS Cu 2p spectra. The binding energies in Fig. 2. centered at 953.1 eV (2p1/2) and 932.7 eV (2p3/2) were
attributed to Cu0 and CuI [13]. The differences of Cu0 and CuI were not obvious at the 2p signals because the binding energy only
exhibit 0.1 eV differences [14]. The Cu0 and CuI species, usually appear below 933 eV and CuII typically appears with binding
energies higher than 933 eV. The XPS spectra of Cu/γ-Al2O3 showed that oxidized CuII existed, which was indicated by both
second sets of spin-orbit split peaks shifted by 1.3±0.2 eV to higher binding energy. It could be observed that the satellite peak at
942 eV for CuII which became weaker with the addition of Pd and might indicate that CuII transforms to CuI/Cu from the XPS
spectra of the Pd-Cu/γ-Al2O3 [12].
In Fig. 3, the Pd 3d5/2 appeared at 335±0.1 and 336.1 ±0.1 eV in all the catalysts, which correspond to Pd0 and PdII, respectively
[15, 6]. The Pd primary state in the Pd/γ-Al2O3 catalyst was Pd0 and PdII, while Pd0 was the main valence state in diverse Pd-
Cu/γ-Al2O3 samples. In Fig.3, the peaks of PdII were shifted to weaker B.E. with added Cu, which might indicate that PdII
transformed into Pd0 [6, 16].
Furthermore, the surface atom percentages of Pd and Cu shown in Table 1 did not correspond to the preparation. According to
the XPS spectra, the metals were introduced into the core of γ-Al2O3, or covered by the visible metal. The above results
illustrated that appropriate ratio (1:1) of the surface atom percentages of Pd and Cu led to the simultaneous distribution of the two
metals on the surface of the catalysts [8, 17].
3.1.3. Transmission electron microscopy
Page 3 of 8

The microstructure of Pd-Cu(5:1)/γ-Al2O3 catalyst was observed by TEM. Fig. 4A and B exhibited that similar spherical
particles are well-dispersed on support and clearly show that the metal particles average size was about 10 nm. Electron mapping
image analysis (Fig. 4C and D) showed the distribution of oxygen (O), aluminum (Al), copper (Cu) and palladium (Pd) atoms
indicated with red, green, blue and white colors, respectively, further confirmed the homogeneous distribution of Pd-Cu
bimetallic nanoparticles on the surface of the γ-Al2O3 support.
3.2. Activity measurements
The catalytic performance of catalysts was measured for selective oxygenization of cyclopentene under different conditions
(Table 2). The results showed that all catalysts had the ability of catalytic oxygenization except for Cu/γ-Al2O3, which illustrated
palladium is an essential composition of catalysts. Meanwhile, Pd-Cu(5:1)/γ-Al2O3 catalyst exhibited the best catalytic ability
during the reaction. Compared with Pd-Cu(5:1)/γ-Al2O3, the catalytic activity of Pd-Cu(3:1)/γ-Al2O3 and Pd-Cu(7:1)/γ-Al2O3
was slightly lower. It was apparent that appropriate proportion of Pd and Cu of Pd-Cu/γ-Al2O3 played an important role in
oxygenization of cyclopentene. According to the XPS data of catalysts in Table 1 and Table 2, the Pd-Cu/γ-Al2O3 (5:1) catalyst
showed the highest activity with 88% conversion of cyclopentene and it could be speculated that the surface atom percentage of
Pd and Cu was close to 1:1. The inferior catalytic activity of the Pd/γ-Al2O3 sample was probably due to that plentiful PdO
(according to XPS measurement) dispersed on the surface of catalyst. In addition, the catalytic activity of mixed-(Pd/γ-Al2O3 +
Cu/γ-Al2O3) was not as good as the Pd-Cu/γ-Al2O3 catalyst, which illustrated that the Pd-Cu bimetal was the crucial factor in the
oxygenization of cyclopentene. Simultaneously, the XPS and TEM measurements for Pd-Cu(5:1)/γ-Al2O3 catalyst demonstrated
the homogeneous distribution of Pd-Cu bimetallic nanoparticles on the sample surface. We suggest that the above reasons led to
Pd-Cu(5:1)/γ-Al2O3 showing the best catalytic performance. The temperature, pressure, speed (rate) and amounts of catalyst were
all factors in the selective oxygenization reaction. With the changing of these factors, catalytic results were also transformed.
When the values of those factors were neither too high nor too low, an optimal condition was produced. Subsequently, the impact
of solvents such as 1,2-dichloroethane, ethanol, and methanol) were also studied for the reaction and we ascertained ethanol as
o
the solvent, and 100 C, 1 Mpa oxygen pressure, 400 rpm/min and Pd-Cu(5:1)/γ-Al2O3 catalyst (2.0 wt% of cyclopentene) were
the optimal reaction conditions.
4. Conclusion
In conclusion, we developed the γ-Al2O3 supported mono- (Cu and Pd), mixed- (Pd + Cu) and bi-metallic Pd-Cu catalysts for
the oxygenization reaction of cyclopentene to cyclopentanone. The best results showed that the selective conversion of
cyclopentene to cyclopentanone could be simultaneously up to more than 85% by employing Pd-Cu(5:1)/γ-Al2O3 catalyst under
mild reaction conditions. The excellent performance of Pd-Cu(5:1)/γ-Al2O3 catalyst was attributed to the homogeneous
distribution of Pd-Cu bimetallic nanoparticles on the surface of support. Our protocol provides a potential method of producing
cyclopentanone in commercial production.
Acknowledgment
We are grateful to the financial assistance from the National Natural Science Foundation of China (Nos. 21272050, 21371044,
21472033 and 21571047).
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4588-4591.
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(2013) 952-955.
a
b
c
Al2O3
Cu
c
Pd-Cu
c
a
a
c
a
b
a
F
E
D
C
B
A
0
20
40
60
80
100
2 (degree)
Page 5 of 8

Figure 1. XRD patterns of (A) γ-Al2O3 (JCPDS 50-1496), (B) Cu/γ-Al2O3, (C) Pd/γ-Al2O3, (D) Pd:Cu =3:1, (E) Pd:Cu =5:1 , (F) Pd:Cu =7:1
2p3/2
2p1/2
Cu(2)
Cu(0,1)
Cu(0,1)
A
B
C
D
925
930
935
940
945
950
955
960
965
Binding energy (eV)
Figure 2. XPS spectra of all catalysts. (A) Pd:Cu =3:1, (B) Pd:Cu =5:1 , (C) Pd:Cu =7:1, (D) Cu
Pd2
Pd0
A
B
C
D
329
336
343
Binding energy (eV)
Figure 3. (A) Pd 3d peaks of Pd, (B) Pd:Cu =3:1,(C) Pd:Cu =5:1, (D) Pd:Cu =7:1
Page 6 of 8

Figure 4. (A) and (B) TEM images of Pd-Cu/γ-Al2O3=5:1, (C) and (D) electron mapping images
Table 1. XPS data of catalysts
Catalyst
Binding energy (eV)
Pd:Cu (atom%) a
Pd 3d5/2
Cu 2p3/2
Pd
336.1
335.4
335.2
335.4
—
—
—
Pd:Cu -3:1
Pd:Cu -5:1
Pd:Cu -7:1
Cu
852.2
852.5
852.1
855.5
0.43:0.56
0.47:0.52
0.55:0.44
—
a The proportion of atomic percentage of Pd and Cu
Table 2. The Oxygenization of cyclopentene
Entry
1
Catalyst
Pd
Solvent
Ethanol
Temp. (oC)
Time (h)
Conv. (%)
Sele. (%)
100
7
71
＞95
2
Pd:Cu = 3:1
Pd:Cu = 7:1
Pd:Cu = 5:1
Pd:Cu = 5:1
Pd:Cu = 5:1
Pd:Cu = 5:1
Pd:Cu = 5:1
Pd:Cu = 5:1
Ethanol
100
100
100
100
100
120
150
80
7
80
83
88
67
—
81
67
80
＞95
＞95
＞95
＞95
—
3
Ethanol
7
4
Ethanol
7
5b
6c
7
Methanol
1,2-Dichloroethane
Ethanol
7
7
6
＞95
＞90
＞95
8
Ethanol
4
9
Ethanol
10
Page 7 of 8

10d
11e
12f
13g
14h
15
Pd:Cu = 5:1
Pd:Cu = 5:1
Pd:Cu = 5:1
Pd:Cu = 5:1
Pd+Cu
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
100
100
100
100
100
100
100
100
7
7
7
7
7
7
7
7
55
85
80
79
55
—
75
77
＞95
＞95
＞95
＞95
＞95
—
Cu
16i
17j
Pd:Cu = 5:1
Pd: Cu =5:1
＞95
＞95
Reaction conditions: 0.17 mol cyclopentene, 1.03 mol ethanol, 400 rpm/min, catalyst (0.24 g, 2.0 wt% of cyclopentene) and 1.0 MPa oxygen
pressure.
a Reaction rate=△c/△t, △c=ccyclopentene (before reaction) - ccyclopentene (after reaction), △t=reaction time (from Table 2); b 1.03 mol methanol; c 1.03
mol 1,2-dichloroethane; d 0.5 MPa oxygen pressure; e 1.5 MPa oxygen pressure; f 0.18 g 5:1 catalyst (1.5 wt% of cyclopentene); g 0. 30 g 5:1
catalyst (2.5 wt% of cyclopentene); h Mixture of 0.16 g Pd/Al2O3 and 0.16 g Cu/Al2O3; i 300 rpm/min; j 500 rpm/min.
Page 8 of 8