Facile Synthesis of CuO–Ni/Al Composites for Catalytic Oxidation of Cyclohexene

Article abstract of DOI:10.1007/s10562-019-02719-5

CuO–Ni/Al composites were synthesized by electroless deposition and thermal oxidation and characterized by inductively coupled plasma optical emission spectrometry, X-ray powder diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy. Their performances as the heterogeneous catalysts for the solvent-free allylic oxidation of cyclohexene by oxygen were determined. It was found that the CuO contents and the amount of adsorbed oxygen species on the composites could significantly affect their catalytic performances in cyclohexene oxidation. The highest catalytic activity was achieved over CuO–Ni/Al-3 containing 18.5?wt% CuO with the highest amount of adsorbed oxygen species, which resulted in the maximum cyclohexene conversion of 39.1% and the total selectivity of 85.5% to 2-cyclohexene-1-ol, 2-cyclohexene-1-one, 2-cyclohexene-1-hydroperoxide and cyclohexene oxide. In addition, the catalyst was successfully recycled with no significant catalytic activity loss after three cycles. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-02719-5

Catalysis Letters
https://doi.org/10.1007/s10562-019-02719-5
Facile Synthesis of CuO–Ni/Al Composites for Catalytic Oxidation
of Cyclohexene
Ang Ma1 · YangYang Cheng1 · Yijia Bai1,2 · Jianmin Hao1,2 · Fumin Chu1 · Limin Han1,2 · Huacong Zhou1,2
Received: 10 September 2018 / Accepted: 19 February 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
CuO–Ni/Al composites were synthesized by electroless deposition and thermal oxidation and characterized by inductively
coupled plasma optical emission spectrometry, X-ray powder diffraction, scanning electron microscopy and X-ray photoelec-
tron spectroscopy. Their performances as the heterogeneous catalysts for the solvent-free allylic oxidation of cyclohexene by
oxygen were determined. It was found that the CuO contents and the amount of adsorbed oxygen species on the composites
could significantly affect their catalytic performances in cyclohexene oxidation. The highest catalytic activity was achieved
over CuO–Ni/Al-3 containing 18.5 wt% CuO with the highest amount of adsorbed oxygen species, which resulted in the
maximum cyclohexene conversion of 39.1% and the total selectivity of 85.5% to 2-cyclohexene-1-ol, 2-cyclohexene-1-one,
2-cyclohexene-1-hydroperoxide and cyclohexene oxide. In addition, the catalyst was successfully recycled with no significant
catalytic activity loss after three cycles.
Graphical Abstract
Keywords Electroless deposition · Thermal oxidation · Cyclohexene oxidation · Heterogeneous catalysis · Oxygen
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-019-02719-5) contains
supplementary material, which is available to authorized users.
Extended author information available on the last page of the article
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A. Ma et al.
no traditional treatments for the sensitization–activation
of SnCl2–PdCl2 required during electroless Cu deposition.
In addition, thermal oxidation may be able to strengthen
the adhesive force between the deposit and matrix metal.
Herein, CuO–Ni/Al composites were prepared by elec-
troless deposition and thermal oxidation under different
conditions and characterized by ICP-OES, XRD, SEM and
XPS. Their application as the heterogeneous catalyst for
the solvent-free allylic oxidation of cyclohexene by O2 was
explored. The optimal composite catalyst was successfully
recycled with slightly decreased catalytic activity.
1 Introduction
Copper oxide (CuO), an attractive metal oxide, has been
extensively applied in photoelectron chemistry, solar cell,
electrochemistry and catalysis [1–4]. It can be simply pre-
pared by the thermal oxidation of Cu at less cost [5–7].
For a typical preparation process, Cu metallization is often
the first step. It can be achieved by electroless deposition,
which produces a Cu deposit on the support surface via the
reduction of copper ions with a reducing agent in solution
[8]. Such metallization is a catalytic/autocatalytic redox
process requiring catalytic active sites on the support sur-
face that are usually from the support material itself or
achieved via modifications with catalytically active met-
als [9, 10]. The metalized Cu is then further oxidized to
form CuO. It has been reported that the CuO nanoparticles
supported on silicon nanowires prepared by the electro-
less deposition and thermal oxidation exhibited excellent
performances in detecting nonenzymatic H2O2 with the
detection limit as low as 1.6 µM [11]. The CuO/Co/carbon
fiber (CF) composites prepared by electroless deposition
and annealing at 673 K in air was found with strong mag-
netic loss and microwave adsorption [12]. So the electro-
less deposition and thermal oxidation could be used to
synthesize CuO composites conveniently and effectively.
The allylic oxidation of cyclohexene can produce
value-added products, such as 2-cyclohexene-1-ol (Cy-
ol), 2-cyclohexene-1-one (Cy-one) and 2-cyclohexene-
1-hydroperoxide (Cy-HP), and thus has attracted consid-
erable attentions in both academic study and industrial
production [13]. Green and sustainable catalytic oxidation
processes, such as solvent-free oxidation by O2, has been
promoted for environmental protection. Such catalytic oxi-
dations are usually conducted with heterogeneous catalysts
due to the advantages of high stability, easy separation
and reutilization. For example, Zou et al. achieved 25.4%
cyclohexene conversion with 96.6% total selectivity to
Cy-ol and Cy-one over the hollow framework TS-1 (MTS-
1) containing 5 wt% manganese at 343 K using 0.4 MPa
O2 without no solvent required [14]. CuO nanoleaves with
both mesoporous structure and nanostructure were found
with excellent catalytic activity for the solvent-free oxida-
tion of cyclohexene at 353 K under 1 atm O2 [15].
2 Experimental
2.1 Synthesis of CuO–Ni/Al Composites
Certain amounts of CuSO4·5H2O (0.6 g, AR),
Na3C6H5O7·2H2O (2 g, AR), 2,2′-bipyridine (1 mg, AR),
CH3(CH2)11OSO3Na (0.4 mg, AR), NiSO4·6H2O (0.05 g,
AR) and NaH2PO2·H2O (3.8 g, AR) were added into a
round-bottomed flask containing 100 mL deionized water,
and stirred until completely dissolved. The pH of the solu-
tion was adjusted to ~ 8.5 with NaOH. A certain amount
of Al powder (20 µm, ≥ 99%) pre-coarsened in HF solu-
tion was added to the plating solution at the desired molar
ratio between Al and Cu and stirred constantly for 2.5 h at
333 K. The color of the solid particles gradually changed
from silvery white (Al) to red (Cu). The particles were then
collected by filteration, washed by deionized water, dried at
323 K for 4 h in air, milled, sintered in a muffle furnace at
773 K for 6 h and milled again to afford CuO–Ni/Al com-
posites. Other catalysts were prepared as described in the
supplementary material.
2.2 Characterization
The metal contents in the prepared CuO–Ni/Al composites
were determined with a Thermo Fisher Scientific ICAP 6300
inductively-coupled plasma optical emission spectrometer.
XRD spectra were recorded on a Bruker D8 Advance X-ray
powder diffractometer using Cu Kα in the range of 10°–90°.
The surface morphologies were imaged with a Hitachi
S-3400N scanning electron microscope. The electronic
properties of surface elements were analyzed with a Thermo
Escalab 250Xi X-ray photoelectron spectroscope. The bind-
ing energies were referenced to the C 1s line of adventitious
carbon at 284.8 eV. The Brunauer–Emmett–Teller (BET)
specific surface areas of the composites were measured
using a Quantachrome Quadrasorb SI-KR/MP gas sorption
surface area and pore size analyzer.
We reported previously the excellent catalytic perfor-
mance of CuNi-ELD/Co composite containing 6.0 wt%
amorphous Cu in the solvent-free oxidation of cyclohexene
by O2 [16]. However, the weak adhesive force between
the Cu deposit and Co powder caused the falling-off of
Cu deposit in the recycled catalyst, even after one cycle,
which was unconducive to the catalyst recycle. Meanwhile,
we found that Al powder could also provide special surface
species as the nucleation sites of Cu crystallization with
1 3

Facile Synthesis of CuO–Ni/Al Composites for Catalytic Oxidation of Cyclohexene
−
Al reacted with OH− to form AlO2 , and thus was leached
2.3 Catalytic Tests
from Al powder in the alkaline solution during electroless
Cu deposition. However, no Cu(OH)2 was formed due to the
complexation of Cu ions with sodium citrate. Ni2+ acted as
the catalyst for hypophosphite oxidation to enable the con-
tinuous deposition of Cu, resulting in the co-deposited of Ni
and Cu (Fig. S1) [17, 18]. Higher Ni2+ concentrations can
accelerate the Cu deposition. However, extremely high depo-
sition rates are unconducive to the formation of a smooth
Cu deposit coating. The optimal Ni2+/Cu2+ mole ratio was
determined to be 1:12.6. The experimental Ni2+/Cu2+ mole
ratios of CuO–Ni/Al-1 and CuO–Ni/Al-2 are higher than the
corresponding theoretical values, indicating the incomplete
reduction of Cu2+ in these composites. When increasing the
Cu/Al mole ratios, the amount of Cu deposit per unit surface
area of Al powder increased with the decrease of the amount
of Al powder, resulting in the decrease of the experimental
Ni2+/Cu2+ mole ratios.
The catalytic cyclohexene oxidation was conducted in a
magnetically stirred 50-mL PTFE-lined autoclave. Briefly,
5 mL cyclohexene and a certain amount of CuO–Ni/Al
composite were added into the reactor, heated at 351 K in
2 MPa O2 atmosphere in an oil bath for 6 h under constant
stirring and cooled in an ice-bath for 1 h. The residual gas
was released slowly and the reaction solution was filtered.
The filtrate was analyzed qualitatively by GC–MS and quan-
titatively analyzed using a SHIMADZU GC-14C quipped
with a RTX-50 column. The GC column temperature was
programmed as: initial temperature 313 K, held for 5 min,
increased to 493 K at the heating rate of 10 K/min, and held
for 5 min. The conversion and selectivity were calculated by
the area normalization method using the relative correction
factors as follows:
(
)
Acfc
Figure 1 shows the XRD patterns of CuO–Ni/Al com-
posites prepared under different conditions. Pure Al pow-
der exhibited five strong peaks at 2θ=38.7°, 44.9°, 65.3°,
78.4° and 82.8°, which were respectively ascribed to the
Conversion (%) = 1 −
× 100%
Acfc + A1f1 + A2f2 + A3f3 + ⋯ Aifi
(
)
Aifi
Selectivity (%) =
× 100%
A1f1 + A2f2 + A3f3 + ⋯ Aifi
where Ac is the GC peak area of residual cyclohexene, fc is
the relative correction factor of cyclohexene, Ai is the GC
peak area of product i, fc is the relative correction factor of
product i. The relative correction factor was calculated based
on the difference between the substance amounts with the
same GC peak area.
3 Results and Discussion
3.1 Characterization of CuO–Ni/Al Composites
A certain amount of CuO–Ni/Al was dissolved in
HNO3–HCl and analyzed by ICP-OES for metal contents as
shown in Table 1. The theoretical mole ratio of Cu/Al in the
composite ranged from 1:30 to 1:5 with a fixed Ni/Cu mole
ratio of 1:12.6. The experimental Al contents were lower
than the corresponding theoretical values because partial
Fig. 1 XRD patterns of pure Al powder (a), CuO–Ni/Al-1 (b), CuO–
Ni/Al-2 (c), CuO–Ni/Al-3 (d), CuO–Ni/Al-4 (e), CuO–Ni/Al-5 (f)
and CuO–Ni/Al-3 recycled after one cyclohexene oxidation cycle (g)
Table 1 Chemical compositions
Entry Sample
nCu/nAl
theoreti-
cal
n
Cu/nAl actual nNi/nCu theoretical nNi/nCu actual wt%
of CuO–Ni/Al composites
CuO Al
Ni
1
2
3
4
5
CuO–Ni/Al-1 1:30
CuO–Ni/Al-2 1:20
CuO–Ni/Al-3 1:15
CuO–Ni/Al-4 1:10
CuO–Ni/Al-5 1:5
1:24
1:12.6
1:12.6
1:12.6
1:12.6
1:12.6
1:7
10.8 88.1 1.1
13.6 85.1 1.3
18.5 80.5 1.0
25.7 73.3 1.0
49.7 48.9 1.4
1:18.5
1:12.8
1:8.4
1:2.9
1:7.7
1:13.5
1:19.1
1:25.5
1 3

A. Ma et al.
(111), (200), (220), (311) and (222) planes of cubic Al phase
(JCPDS, No. 04-0787) (Fig. 1a). The thermal oxidation did
not change the cubic Al phase in the composites signifi-
cantly and no aluminum oxide was detected. The 2θ peak of
CuO–Ni/Al-1 with a low Cu content at 35.5° was indexed
to the preferred orientation (11-1) plane of monoclinic CuO
phase (JCPDS, No. 48-1548) (Fig. 1b). Increasing the Cu
content resulted in more diffraction peaks at 2θ = 32.5°,
38.7°, 48.7°, 58.2°, 61.5°, 66.2° and 68.1° that were assigned
to the (110), (111), (20-2), (202), (11-3), (31-1) and (220)
planes of monoclinic CuO phase, respectively (Fig. 1c–f).
The weak and broad diffraction peaks of the monoclinic
CuO phase in the composites indicate their low crystallinity
due to more crystal defects and poor crystal growth quality
caused by the Ni atoms doped in the Cu lattice [19]. The
characteristic XRD diffraction peaks of cubic Al and mono-
clinic CuO of CuO–Ni/Al-3 remained unchanged after one
reaction cycle, indicating that the integrate structure of the
composite was maintained during the oxidation.
Figure 2 shows the XPS spectra of CuO–Ni/Al com-
posites prepared under different conditions. All compos-
ites exhibited the Cu 2p3/2 and Cu 2p1/2 peaks at 933.8 eV
and 953.7 eV, respectively, with the doublet separation of
19.9 eV, and satellite features at 943 eV and 962.5 eV of
CuO. The peaks of pure Al powders at 71.9 eV and 74.4 eV
were respectively attributed to the metallic Al and oxidized
Al, suggesting that the Al particle surface was partially cov-
ered with a passive Al2O3 film. The Al-ELD powders that
was treated with the same electroless deposition process,
but in the absence of CuSO4·5H2O, exhibited the charac-
teristic XPS peaks of AlOOH or Al(OH)3. The aluminum
hydroxide(s) was completely oxidized to Al2O3 during the
thermal oxidation, affording Al-ELD-TO powder. Both CuO
and Al2O3 phases were found in all CuO–Ni/Al composites
with overlapped Cu 3p and Al 2p peaks that were resolved
to be at 78.4 eV and 75.8 eV, respectively [20, 21]. The
detailed peak fitting results for the relative atomic ratios of
Cu and Al are shown in Table 2. The highest surface Cu
content was found on CuO–Ni/Al-3. The O 1s spectra of
pure Al powder, Al-ELD powder and Al-ELD-TO powder
suggested the presence of aluminum oxide. The O 1s curves
of all CuO–Ni/Al composites were deconvoluted into three
peaks at ~529.8 eV, ~531.7 eV and ~533.1 eV, respectively,
that were denoted as OI, OII and OIII. The OI and OII peaks
were attributed to Cu–O and Al–O, respectively [22]. The
introduction of CuO shifted the Al 2p peak to the high bond-
ing energy region and O 1s peak to the low bonding energy
region, because Cu with a higher electronegativity enhanced
the polarization of Al–O [23]. The OIII peak was due to the
adsorbed and dissociated surface oxygen species, such as O2,
O2− and O−, which were formed during the thermal oxida-
tion in the oxidizing atmosphere [24]. It has been reported
that the oxygen adsorption and vacancy coexist and interact
Fig. 2 a Cu 2p XPS spectra of CuO–Ni/Al-1 (a), CuO–Ni/Al-2 (b), CuO–Ni/
Al-3 (c), CuO–Ni/Al-4 (d) and CuO–Ni/Al-5 (e); b Al 2p-Cu 3p XPS spectra
of pure Al powder (a), Al-ELD powder (b), Al-ELD-TO powder (c), CuO–Ni/
Al-1 (d), CuO–Ni/Al-2 (e), CuO–Ni/Al-3 (f), CuO–Ni/Al-4 (g) and CuO–Ni/
Al-5 (h); c O 1s XPS spectra of pure Al powder (a), Al-ELD powder (b), Al-
ELD-TO powder (c), CuO–Ni/Al-1 (d), CuO–Ni/Al-2 (e), CuO–Ni/Al-3 (f),
CuO–Ni/Al-4 (g) and CuO–Ni/Al-5 (h)
1 3

Facile Synthesis of CuO–Ni/Al Composites for Catalytic Oxidation of Cyclohexene
Table 2 Peak fitting results of the Al 2p-Cu 3p and O 1s XPS spectra for CuO–Ni/Al composites
CuO–Ni/Al composites
Al 2p-Cu 3p
Al 2p
O 1s
Cu 3p
OI
OII
OIII
B.E. (eV)
Area%
B.E. (eV)
Area%
B.E. (eV)
Area%
B.E. (eV)
Area%
B.E. (eV)
Area%
CuO–Ni/Al-1
CuO–Ni/Al-2
CuO–Ni/Al-3
CuO–Ni/Al-4
CuO–Ni/Al-5
75.8/74.5
75.8
75.8
75.8
75.8
72.5
70.4
61.7
63.3
71.4
78.4
78.5
78.3
78.4
78.5
27.5
29.6
38.3
36.7
28.6
529.8
529.8
529.9
529.9
529.7
25.3
28.1
28.2
28.6
39.9
531.5
531.7
531.7
531.9
531.6
57.5
47.6
46.9
58.5
48.1
533.0
533.0
533.1
533.4
533.1
17.2
24.3
24.9
12.9
12.0
with each other in p-type CuO, specifically, the adsorbed
oxygen increases the oxidation states of Cu, which provides
more oxygen vacancies to induce stronger oxygen adsorption
at high temperatures in the oxidizing atmosphere [25, 26].
Importantly, the surface oxygen species and oxygen vacan-
cies can be reactively involved in the oxidation process and
contribute to the high catalytic activity significantly [27, 28].
The relative proportions of OI, OII and OIII peak areas listed
in Table 2 suggest that CuO–Ni/Al-3 surface possessed
the highest amount of adsorbed oxygen species. The high
amount of surface oxygen species can improve the redox
properties of the composite, and is thus conducive to the
catalytic oxidation of cyclohexene.
reaction mechanism. At beginning the homolytic cleavage of
the α-C–H bond of cyclohexene produces allylic cyclohex-
enyl radical, which reacts with O2 to form the dominant
radical species cyclohexenyl peroxyl radical (Cy-OO·). The
generated Cy-HP behaves as the principal source of main
products. It directly decomposes to Cy-one and Cy-ol or to
Cy-one and water in the presence of CuO–Ni/Al composites
through redox cycling between Cu(II) and Cu(I) oxidation
states. It is also converted to Cy-ol and Cy-oxide through the
epoxidation of cyclohexene, and Cy-oxide is hydrolyzed to
1,2-cyclohexanediol.
The cyclohexene conversion was only 7.9% in the
absence of catalyst (Table 3 entry 1), and was only
increased to 11.1% over pure Al powder catalyst (Table 3
entry 2), similar to those over Al-ELD and Al-ELD-TO
catalysts (Table 3 entries 3 and 4), suggesting the low cata-
lytic activities of these catalyst supports. The cyclohexene
conversion was further slightly increased to 13.8% and
15.1% over CuO–Ni/Al-1 and CuO–Ni/Al-2 with low CuO
contents, respectively (Table 3 entries 5 and 6). CuO–Ni/
Al-3 catalyst containing 18.5 wt% CuO gave the highest
cyclohexene conversion of 39.1% (Table 3 entry 7). Fur-
ther increasing the CuO contents resulted in lower con-
versions of cyclohexene (Table 3 entries 8 and 9). These
results indicate that the CuO–Ni deposit on the CuO–Ni/Al
is the active catalytic species and the CuO content affects its
catalytic activity evidently. The appropriate CuO content in
CuO–Ni/Al composite can provide the maximum dispersion
of CuO–Ni species and the higher amount of adsorbed oxy-
gen species on Al support, and more CuO content cannot
further improve the dispersion of CuO–Ni species because
of the high crystallinity.
The surface morphologies of CuO–Ni/Al composites
were imaged by SEM. The Al particles in CuO–Ni/Al-1
with a low Cu content were partially coated with the CuO
deposit (Fig. 3a). For CuO–Ni/Al-2 with a higher Cu con-
tent, the surface coverage of Al particles with CuO deposit
was increased and the CuO crystals agglomerated into differ-
ent sizes of aggregates, indicating a CuO coating layer was
almost formed (Fig. 3b). The CuO deposit became tight and
smooth on the surface of CuO–Ni/Al-3 with a few pores that
provided contact sites between CuO and Al2O3 (Fig. 3c).
As the Cu content further increased, Al particles were com-
pletely coated with a CuO film, and the CuO coating layer
became rough and loose due to more CuO crystalline grains
of different morphologies and sizes formed on the surface
(Fig. 3d, e).
3.2 Catalytic Oxidation of Cyclohexene Over CuO–
Ni/Al Composites by Oxygen
The catalytic activities of CuO–Ni/Al composites for the
oxidation of cyclohexene by oxygen were then evaluated.
The reaction mainly produced Cy-ol, Cy-one, Cy-HP and
cyclohexene oxide (Cy-oxide) (Fig. 4) as identified by
GC–MS (Fig. S2). The potential reaction pathways were also
presumed for the catalytic oxidation of cyclohexene over
CuO–Ni/Al composites by oxygen according to the litera-
ture [29, 30] (Fig. S3). It subjects to a typical radical-chain
To further investigate the role of Ni in the catalytic
activity of CuO–Ni/Al composite, Ni/Al-3-TO composite
was synthesized at the Ni/Al mole ratio of 1:15, same as
that in CuO–Ni/Al-3. The cyclohexene conversion over Ni/
Al-3-TO was found to be 12.2% (Table 3 entry 10), similar
to that over the catalyst supporter. CuO/Al-3 with no Ni
species prepared by formaldehyde reducing agent resulted
in the cyclohexene conversion of 34.3% (Table 3 entry
1 3

A. Ma et al.
Fig. 3 SEM images of CuO–Ni/Al-1 (a), CuO–Ni/Al-2 (b), CuO–Ni/Al-3 (c), CuO–Ni/Al-4 (d) and CuO–Ni/Al-5 (e)
Fig. 4 Catalytic oxidation of
cyclohexene over CuO–Ni/Al
composites by oxygen
11), lower than that over CuO–Ni/Al-3. These results indi-
cate the Ni deposit alone has no catalytic activity, but can
promote the catalytic activity of CuO–Ni/Al composite.
And hypophosphite was used as the reducing agent for the
electroless Cu deposition, which made the process more
environmentally benign, as compared with conventional
methods using formaldehyde as the reducing agent.
The cyclohexene conversion over the mixture of CuO and
Al-ELD-TO powder was determined to be 22.0% (Table 3
entry 12), much lower than that over CuO–Ni/Al-3 catalyst.
1 3

Facile Synthesis of CuO–Ni/Al Composites for Catalytic Oxidation of Cyclohexene
Table 3 Catalytic performance
Entry
Catalyst
Conv.
(%)
Selectivity (%)
of different catalysts in the
oxidation of cyclohexene
Cy-ol
Cy-one
Cy-HP
Cy-oxide
Others
1
2
3
4
5
6
7
8
–
7.9
11.1
10.9
11.9
13.8
15.1
39.1
29.2
16.7
12.2
34.3
22.0
25.9
31.9
24.9
13.2
6.7
6.2
14.8
9.2
9.6
10.4
10.4
18.1
9.3
59.1
22.7
69.5
39.0
28.2
27.5
31.8
32.3
41.2
28.4
29.8
32.5
23.1
23.1
26.5
24.9
64.8
20.7
35.8
54.2
51.3
41.0
41.2
32.0
58.4
45.1
38.3
50.6
59.7
55.8
1.1
2.2
2.2
2.0
3.1
2.1
2.3
5.3
1.3
2.7
3.1
2.5
4.2
5.1
3.3
1.7
3.6
1.4
8.4
5.3
Pure Al powder
Al-ELD powder
Al-ELD-TO powder
CuO–Ni/Al-1
CuO–Ni/Al-2
CuO–Ni/Al-3
CuO–Ni/Al-4
CuO–Ni/Al-5
Ni/Al-3-TO
9.5
14.5
10.8
7.4
9
10
11
12
13
14
15
1.2
CuO/Al-3
9.7
12.3
16.0
10.2
4.9
CuO+Al-ELD-TO powder
CuO–Ni/Al2O3-3
Cu-Ni/Al-3
10.7
11.9
7.2
Cu-Ni/Al-32nd
7.9
6.5
Reaction conditions: cyclohexene 5 mL, O2 2 MPa, reaction temperature 351 K, reaction time 6 h, mole
ratio of Cu to cyclohexene 1:200
In addition, CuO–Ni/Al2O3-3 with the Cu/Al mole ratio of
1:15, where Al2O3 instead of Al was used as catalyst support,
also resulted in a lower conversion of 25.9% (Table 3 entry
13), as compared with that over CuO–Ni/Al-3. These results
indicate that the Al support matrix and CuO deposit exhibited
a synergistic effect on the catalytic activity. The Al support
matrix provided free electrons to accelerate the conversion of
the adsorbed oxygen species to active oxygen species, which
enhanced the abstraction of the weakly bonded allyic hydrogen
atom of cyclohexene to form cyclohexenyl peroxyl radical (Cy-
OO·), the dominant radical species in cyclohexene oxidation
radical-chain reaction [16, 29, 30]. Therefore, CuO–Ni/Al-3
composite was able to provide more active CuO sites and a
higher amount of adsorbed oxygen species according to XPS
results, and thus exhibited the excellent catalytic performance
in cyclohexene oxidation.
Fig. 5 Recycle of CuO–Ni/Al-3 catalyst. Reaction conditions:
cyclohexene 5 mL, O2 2 MPa, reaction temperature 351 K, reaction
time 6 h, mole ratio of Cu to cyclohexene 1:200
The cyclohexene conversion over Cu-Ni/Al-3 composite
prepared by electroless deposition was 31.9% (Table 3 entry
14), slightly lower than that over CuO–Ni/Al-3, but was sig-
nificantly decreased to 24.9% in the second oxidation cycle
(Table 3 entry 15), possibly due to the falling-off of Cu deposit
from the composite (Fig. S4). In contrast, the cyclohexene
conversion over CuO–Ni/Al-3 was only slightly decreased
after three cycles (Fig. 5), suggesting that the thermal oxida-
tion strengthened the adhesion and interaction between the
CuO–Ni deposit and Al support. In addition, no metal loss was
found in the product solution after the reaction, indicating that
the oxidation occurred on the CuO–Ni/Al-3 catalyst surface
heterogeneously.
4 Conclusions
In summary, CuO–Ni/Al composites containing cubic Al
and monoclinic CuO phases were synthesized by electro-
less deposition and thermal oxidation, and their appli-
cation as a heterogeneous catalyst for the solvent-free
allylic oxidation of cyclohexene by oxygen was explored.
The results suggest that their catalytic performance was
strongly dependent on the CuO content and the amount of
adsorbed oxygen species. In addition, the Ni species also
contributed to the high catalytic activity of the composite
and the Al support exhibited a synergistic effect with CuO
1 3

A. Ma et al.
6. Horak P, Bejsovec V, Vacik J et al (2016) Appl Surf Sci 389:751
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deposit on the catalytic performance of the composite in
the oxidation of cyclohexene. Among all prepared com-
posites, CuO–Ni/Al-3 containing 18.5 wt% CuO exhibited
the highest catalytic activity and the highest amount of
adsorbed oxygen species. In addition, it was successfully
recycled with no significant catalytic activity losses after
three cycles due to the strong adhesion and interaction
between the CuO–Ni deposit and Al support achieved by
thermal oxidation.
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Acknowledgements This work was financially supported by the
National Natural Science Foundation of China (No. 21163011),
the Natural Science Foundation of Inner Mongolia (Grant No.
2017MS(LH)0205) and the Research Program of Science and Tech-
nology at Universities of Inner Mongolia Autonomous Region (Grant
No. NJZZ17081).
Compliance with Ethical Standards
21. Li F, Zhang L, Evans DG et al (2004) Colloids Surf A 244:169
22. Kim KM, Kwak BS, Park NK et al (2017) J Ind Eng Chem 46:324
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27. Menon U, Galvita VV, Marin GB (2011) J Catal 283:1
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Conflict of interest The authors declare no conflict of interest.
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jurisdictional claims in published maps and institutional affiliations.
Affiliations
Ang Ma1 · YangYang Cheng1 · Yijia Bai1,2 · Jianmin Hao1,2 · Fumin Chu1 · Limin Han1,2 · Huacong Zhou1,2
2
* Jianmin Hao
haojmin@foxmail.com
Inner Mongolia Engineering Research Center for CO2
Capture and Utilization, Hohhot 010051, China
1
College of Chemical Engineering, Inner Mongolia
University of Technology, Hohhot 010051, China
1 3