CuNi/Co composites prepared by electroless deposition: Structure and catalytic activity for the oxidation of cyclohexene with oxygen

Article abstract of DOI:10.1039/c4ra13064b

CuNi/Co composites with various Cu contents have been prepared by electroless deposition (ELD) of Cu on Co powers and characterized with ICP-OES, N₂ adsorption/desorption, XRD, SEM, TEM and XPS. Their catalytic performance was examined for the allylic oxidation of cyclohexene with oxygen under solvent-free conditions. The phase structure of the Cu deposit in the composites is transformed from an amorphous phase to a crystalline phase with the increase in Cu content. The composition and microstructure of the Cu deposit in the composite significantly affect its catalytic activity for the oxidation of cyclohexene. CuNi-ELD/Co-5 composite with 6.0 wt% Cu content and amorphous Cu deposit showed the highest catalytic performance with a 46.3% conversion of cyclohexene and 88.1% total selectivity to 2-cyclohexene-1-ol (Cy-ol), 2-cyclohexene-1-one (Cy-one), 2-cyclohexene-1-hydroperoxide (Cy-HP) and cyclohexene oxide (Cy-oxide). This journal is

Full text of DOI:10.1039/c4ra13064b

RSC Advances
View Article Online
View Journal | View Issue
PAPER
CuNi/Co composites prepared by electroless
deposition: structure and catalytic activity for the
Cite this: RSC Adv., 2015, 5, 13809
oxidation of cyclohexene with oxygen†
*
Jianmin Hao, Xiaoli Jiao, Limin Han, Quanling Suo, Ang Ma, Jiansheng Liu, Xu Lian
and Linlin Zhang
CuNi/Co composites with various Cu contents have been prepared by electroless deposition (ELD) of Cu on
Co powers and characterized with ICP-OES, N2 adsorption/desorption, XRD, SEM, TEM and XPS. Their
catalytic performance was examined for the allylic oxidation of cyclohexene with oxygen under solvent-
free conditions. The phase structure of the Cu deposit in the composites is transformed from an
amorphous phase to a crystalline phase with the increase in Cu content. The composition and
microstructure of the Cu deposit in the composite significantly affect its catalytic activity for the
oxidation of cyclohexene. CuNi-ELD/Co-5 composite with 6.0 wt% Cu content and amorphous Cu
deposit showed the highest catalytic performance with a 46.3% conversion of cyclohexene and 88.1%
total selectivity to 2-cyclohexene-1-ol (Cy-ol), 2-cyclohexene-1-one (Cy-one), 2-cyclohexene-1-
hydroperoxide (Cy-HP) and cyclohexene oxide (Cy-oxide).
Received 24th October 2014
Accepted 22nd January 2015
DOI: 10.1039/c4ra13064b
www.rsc.org/advances
distribution depends on the catalysts, oxidants, and solvents
used for the reaction.18–20 Environmentally benign oxidations
1 Introduction
use heterogeneous catalyst, solvent-free and clean oxidant,
which has attracted considerable attentions.21 Compared with
other oxidant, molecular oxygen is low cost, easily accessible
and environmental benign. Various heterogeneous catalysts,
including supported metal complexes, metal–organic frame-
works, metal-containing molecular sieves and nanocatalysts
have been used in the solvent-free oxidation of cyclohexene with
molecular oxygen as the oxidant.20–23 For example, the conver-
sion of CrMCM-41 catalyzed cyclohexene oxidation with oxygen
can reach 52.2%, and the total selectivity to Cy-ol, Cy-one and
Cy-HP can be as high as 96.6% under 1 atm O2 at 343 K.24
[Cu(bpy)(H2O)2(BF4)2(bpy)] MOF exhibits promising catalytic
activity and ꢀ90% selectivity to Cy-HP in the atmospheric O2 at
318 K.25
The Cu-coated metals and metal oxide powders prepared by
electroless plating, such as W/Cu, Mo/Cu, Al2O3/Cu, and ZrW2O8/
Cu composites, show both the high thermal and electrical
conductivity of Cu and the characteristics of the support metal
materials.26–29 However, few studies have been conducted on the
Co/Cu composite. In the present work, CuNi/Co composites with
various Cu contents were prepared by Cu electroless deposition on
Co powders. Sodium hypophosphite instead of formaldehyde was
used as the reducing agent in the preparation due to its low pH,
low cost, and relative safety.30,31 The composition and structure of
CuNi/Co composites were characterized with ICP-OES, N2
adsorption/desorption, XRD, SEM, TEM and XPS. Their catalytic
activity in the oxidation of cyclohexene with molecular oxygen
under solvent-free conditions was discussed.
Copper can be metalized in many ways, such as precipitation,
physical vapor deposition (PVD), chemical vapor deposition
(CVD), electro deposition (ED), electroless deposition (ELD) and
so on.1–5 Due to its low cost, fast deposition, uniform products,
and simplicity, electroless copper deposition has been widely
used in electromagnetic interference (EMI) shielding, through-
hole plating in printed circuit boards and copper matrix
composites.6–9 In addition, uniform and homogeneous metal
catalysts can be deposited on the surface of supports with
electroless deposition. Suitable catalysts for various reactions
can be prepared by changing the metallic components such as
Co, Ag and Ni, in the plating solution.10–12 For example, the
plate-type copper-based catalyst prepared with electroless
deposition exhibits much higher catalytic performance in
methanol steam reforming than the commercial copper-based
catalysts.13–15
The oxidation of cyclohexene is a very important chemical
process in the chemical industry, which can produce the
chemical intermediates, such as alcohols, ketones, epoxides,
and acids for the synthesis of polymers, drugs, agrochemicals
and surfactants.16 Both C]C double bond and allylic C–H bond
in cyclohexene are active sites for its oxygenation.17 The product
Chemical Engineering College, Inner Mongolia University of Technology, Hohhot,
010051, P. R. China. E-mail: haojmin@foxmail.com; Fax: +86 471 6575675; Tel:
+86 471 6575675
† Electronic supplementary information (ESI) available: Additional catalyst
preparation method and characterization results. See DOI: 10.1039/c4ra13064b
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 13809–13817 | 13809

View Article Online
RSC Advances
Paper
2 Experimental
3 Results and discussion
2.1 Preparation and characterization of CuNi-ELD/Co
3.1 Characterization of CuNi-ELD/Co
catalyst
The chemical compositions of the catalyst samples are listed in
All reagents were obtained from commercial sources and used Table 1. Composites CuNi-ELD/Co-1–7 were prepared with a Ni/
as received. In a typical synthesis, Co powder (300 meshes, Cu mole ratio of 1 : 2.5 and various Cu/Co mole ratios ranging
99.5%), 0.192 g CuSO4$5H2O ($99%), 0.08 g NiSO4$6H2O from 1 : 62 to 1 : 5. Composites CuNi-ELD/Co-8–11 were
($98.5%), 1.8 g NaH2PO2$H2O ($99.0%), 1.4 g Na3C6H5O7- prepared with a Cu/Co mole ratio of 1 : 16 and various Ni/Cu
$2H2O ($99.0%) and 80 mL distilled water were added into a ratios ranging from 1 : 1 to 1 : 5. It is clear that the chemical
250 mL round bottom ask and the pH value was adjusted to 9 compositions of the catalyst prepared with Cu electroless
with NaOH ($96%). The amounts of Co powder were adjusted deposition under different conditions are consistent with the
to obtain desired ratios between Co and Cu. The reaction was theoretical values, indicating the complete reduction of Cu2+
allowed to continue for 2 h at 333 K under stirring. The ions. Ni2+ is used as the catalyst for the oxidation of hypo-
precipitate was ltered, washed with distilled water three times phosphite to obtain continuous Cu deposition. Small portion of
and air-dried at 373 K for 3 h. The color of precipitate was Ni2+ can be co-deposited Ni in the Cu deposits during the
changed from Co gray to Cu red of the product, CuNi-ELD/Co. deposition (Scheme S1†).31,34 The Cu deposition rate decreases
The chemical composition of CuNi-ELD/Co composite with the increase of reaction time and eventually stopped when
including the metal content of Cu, Co and Ni was determined the molar ration of Ni2+/Cu2+ decrease to a limit. Thus, a certain
with ICP-OES (Thermo Fisher SCIENTIFIC ICAP 6000 SERIES). amount of Ni2+ in the reaction substrate is necessary to main-
The phase structure of the catalyst was determined with X-ray tain the Cu deposition rate.35 A initial Ni2+/Cu2+ mole ratio of
diffraction (XRD) on a Bruker-AXS D8 ADVANCE with 2q Cu 1/2.5 can ensure the completion of the whole Cu deposition
Ka in the range of 30–100ꢁ. A scanning electron microscope within the reaction time (Table 1 entries 2–8). The mole ratios of
(SEM, Hitachi S4800) was used to examine the surface Ni/Cu in the produced deposit are in the range of 1/10 to 1/5
morphology of the catalysts. Transmission electron microscopy (Table 1 entries 2–8) and decreases with the decrease of
(TEM) images were obtained on a JEM-2010. An X-ray photo- Ni2+/Cu2+ mole ratio in the reaction substrate (Table 1 entries
electron spectroscope (XPS, VG Microtech 3000 Multilab) was 9–12), indicating that the deposit on Co surface is a Cu–Ni
used to determine the electronic properties of the catalyst. The alloy.30,36 No signicant change was found in the Cu/Co mole
binding energy values were referenced to the C 1s peak of ratios. NaH2PO2$H2O was used as a reducing agent and small
contaminant carbon at 284.6 ꢂ 0.2 eV. The N2 adsorption– amount of phosphorus could also be co-deposited in the deposit
desorption isotherms of the catalyst at 77 K were determined to form Cu–Ni–P alloy during the copper electroless deposi-
with a Micromeritics' ASAP 2020 Accelerated Surface Area and tion.31,36 Phosphorus was not detected in all the CuNi/Co-ELD
Porosimetry analyzer aer it was automatically in situ degassed composite samples by the energy dispersive X-ray analysis
at 473 K.
(EDX), due to its low content. The presence of phosphorus in the
deposits was conrmed by the XPS analysis.
Fig. 1 shows the XRD patterns of CuNi-ELD/Co composite
samples. The Co powder consists of a cubic phase (JCPDS, no.
15-0806) and a hexagonal phase (JCPDS, no. 05-0727). All
diffraction peaks of the composites with Cu contents ranging
from 1.7 wt% to 4.3 wt% (CuNi-ELD/Co-1–4) are from the Co
powder, and no trace of a crystalline Cu phase is detected
(Fig. 1A).
This indicates that these samples with low Cu contents may
be amorphous Cu phase or the amount of Cu crystal may be
blow the detection limit of XRD. To conrm this, Co powder was
mixed with same amounts of commercial Cu nanoparticles (10–
30 nm, 99.9%) as those in CuNi-ELD/Co-1–4 samples. The XRD
patterns of the mixtures of Cu nanoparticles and Co powder
showed two weak peaks at 2q ¼ 43.3ꢁ and 50.5ꢁ in Fig. S1,†
which were ascribed to the (111) and (200) planes of the cubic
Cu phase (JCPDS, no. 04-0836). This indicates that XRD can
detect the Cu crystal with contents ranging from 1.7 wt% to 4.3
wt% in Co powder. It has also been reported that the XRD can
identify Cu crystal in the composites with contents as low as 1.5
wt%.37 Therefore, the Cu phase in the composites of CuNi-ELD/
Co-1–4 is amorphous.38 Further increasing the Cu content to 6.0
wt% in CuNi-ELD/Co-5 led to a weak and broad diffraction peak
2.2 Oxidation of cyclohexene
Cyclohexene (Aladdin 99%) and high-purity oxygen (99.999%)
were used as received. In a typical reaction, certain amounts of
substrate and catalyst were charged into a 50 mL stainless steel
reactor with a Teon inner liner and heated to the desired
temperature in an oil bath. A certain amount of O2 gas was then
introduced into the reactor. The reaction was conducted under
stirring with a magnetic stirrer. The reactor was allowed to cool
to room temperature and depressurized. The catalyst was
removed by ltering the mixture through a lter. The ltrate
was collected and diluted with ethanol. The composition of the
reaction mixture and major oxidation products including Cy-ol,
Cy-one, and Cy-oxide were analyzed with gas chromatograph
(GC, SHIMADZU GC-14C, column RTX-50). Product Cy-HP was
analyzed by triphenylphosphine reduction.25 The by-products
were quantied with GC-MS. The conversion was dened as
the total moles of all products divided by the moles of initial
cyclohexene reactant. The selectivity was dened as the moles of
a specic product divided by the total moles of products
formed.32,33 Note: the use of compressed O2 in the presence of
organic substrates requires appropriate safety precautions and
must be performed in the suitable equipment.
13810 | RSC Adv., 2015, 5, 13809–13817
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Table 1 Chemical compositions and BET surface areas of CuNi-ELD/Co catalyst samples
Wt%
nCu2+/nCo theoretical nCu/nCo actual nNi2+/nCu2+ theoretical nNi/nCu actual Cu Co
Entry Catalyst
Ni BET surface area (m2 gꢃ1
— 0.9
)
1
Co powder
—
—
—
—
—
—
2
3
4
5
6
7
8
9
CuNi-ELD/Co-1 1 : 62
CuNi-ELD/Co-2 1 : 47
CuNi-ELD/Co-3 1 : 31
CuNi-ELD/Co-4 1 : 23
CuNi-ELD/Co-5 1 : 16
CuNi-ELD/Co-6 1 : 10
CuNi-ELD/Co-7 1 : 5
CuNi-ELD/Co-8 1 : 16
CuNi-ELD/Co-9 1 : 16
CuNi-ELD/Co-10 1 : 16
CuNi-ELD/Co-11 1 : 16
1 : 61
1 : 49
1 : 31
1 : 24
1 : 17
1 : 9
1 : 2.5
1 : 2.5
1 : 2.5
1 : 2.5
1 : 2.5
1 : 2.5
1 : 2.5
1 : 1
1 : 5
1 : 7
1 : 6
1 : 10
1 : 7
1 : 10
1 : 7
1 : 11
1 : 9
1 : 17
1 : 19
1.7 98.0 0.3 1.8
2.2 97.5 0.3 1.5
3.3 96.2 0.5 1.6
4.3 95.3 0.4 1.8
6.0 93.2 0.8 1.5
10.7 88.3 1.0 1.0
17.5 80.2 2.3 0.9
1 : 5
1 : 14
1 : 15
1 : 14
1 : 15
7.0 92.4 0.6
6.9 92.5 0.6
7.3 92.3 0.4
6.6 93.1 0.3
—
—
—
—
10
11
12
1 : 2
1 : 4
1 : 5
detected in the deposits. These results suggest that the forma-
tion of the phase structure of Cu deposit is attributed to the
content of copper, the deposition rate, and the presence of
nickel. Various amount of Co powder were added to the reactor
to produce different composite with various Cu contents. Large
amount of Co powder results in a low relative content of Cu in
the reaction substrate and large surface areas with more
nucleation sites available for Cu deposition. The high Cu
deposition rate leads to a rapid copper nucleation rate, which
favoured the formation of the amorphous Cu phase. Whereas
small amount of Co powder results in a high relative content of
Cu and less nucleation sites available. The crystal growth
becomes dominant, which promoted the formation of crystal-
line phase. However, a single metal can be rarely converted into
amorphous form.39 The Nickel atoms in the copper lattice can
increase the defects in the copper deposit, which is necessary for
the formation of amorphous Cu structure.30 The XRD patterns of
CuNi-ELD/Co-8–11 with different Ni2+/Cu2+ mole ratio are
similar to that of CuNi-ELD/Co-5, suggesting that the initial Ni2+/
Cu2+ mole ratio does not affect the structure of deposits (Fig. 1B).
SEM was used to examine the morphologies of the produced
composite. The Co particles have irregular shapes with smooth
surfaces (Fig. 2a). Some of the Co particles were coated with Cu
nanoparticles and some were not during the copper electroless
deposition (Fig. 2b), indicating the ununiform Cu distribution
on the Co powders. This might be caused by the absence of the
preliminary treatment for the sensitization-activation before the
deposition process. The occurrence of Cu electroless deposition
suggests that there are special species on the surface of Co
particles acting as Cu nucleation sites. Thus, the inadequate
nucleation sites may also cause the uneven Cu distribution. The
composition and acting mechanism of nucleation sites are not
clear.
Fig. 1 XRD patterns of catalyst samples: (A) (a) Co powder; (b) CuNi-
ELD/Co-1; (c) CuNi-ELD/Co-2; (d) CuNi-ELD/Co-3; (e) CuNi-ELD/
Co-4; (f) CuNi-ELD/Co-5; (g) CuNi-ELD/Co-6; (h) CuNi-ELD/Co-7; (B)
(a) CuNi-ELD/Co-8; (b) CuNi-ELD/Co-9; (c) CuNi-ELD/Co-5; (d)
CuNi-ELD/Co-10; (e) CuNi-ELD/Co-11.
at 2q ¼ 43.3ꢁ, which was ascribed to a preferred orientation
(111) plane of the cubic Cu phase (JCPDS, no. 04-0836) (Fig. 1A).
This indicates that the Cu deposits in CuNi/Co-ELD-5 are
composed of amorphous and crystalline phases. The peaks at
2q ¼ 43.3ꢁ, 50.5ꢁ, 74.1ꢁ and 89.9ꢁ of CuNi-ELD/Co-6 and
CuNi-ELD/Co-7 can be assigned to the (111), (200), (220), and
(311) planes of cubic Cu phase respectively, indicating the
intensive formation of the high crystallized Cu phase with
further increased Cu content. No copper oxide phase was
The amorphous Cu phase has honeycomb-like porous
morphology (Fig. 2c) and the crystalline Cu phase exhibits
sphere-like morphology (Fig. 2d), which is consistent with the
results of XRD analysis. The size of spherical particles in the
crystalline Cu phase is not uniform and they are agglomerated
tightly (Fig. 2e), which is similar to those in the Cu-coated Al2O3
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 13809–13817 | 13811

View Article Online
RSC Advances
Paper
electron diffraction pattern of the Cu particle displays the blurry
ring characteristics of the amorphous structure, which is
consistent with the results of XRD and SEM analysis.
The XPS spectra of CuNi-ELD/Co catalyst samples are pre-
sented in Fig. 4. Two major Co 2p peaks at binding energies of
781.1 eV (Co 2p3/2) and 797.1 eV (Co 2p1/2) are found in the
spectra of all samples. The Co 2p doublet separation energy is
16.0 eV. This indicates that the surface Co of the particle is
mainly in oxidation state. A thin oxide lm on the surface of Co
powders can form in alkaline solutions, containing Co(OH)2
and a certain amount of CoO from the dehydration of Co(OH)2,
while Co remained in the metal state in the bulk of particles.40–42
The Cu 2p3/2 and 2p1/2 peaks are found at binding energies of
932.7 eV and 952.3 eV, respectively. The Cu 2p doublet separa-
tion energy is 19.6 eV. These demonstrate that the deposit is
metallic Cu.43,44 Although trace amount of Ni was detected in the
deposits by EDX as discussed above, no signicant change of
the binding energy of metallic Cu was observed with the change
of Ni amount. This might be explained that the amount of Ni in
the deposits is not high enough to affect the properties of
metallic Cu. The Ni 2p3/2 and 2p1/2 show at binding energies of
856.0 eV and 873.6 eV. In addition, a shake-up satellite peak was
found at 861.5 eV. The Ni 2p doublet separation energy is 17.6
eV. These peaks can be assigned to the trivalent Ni cation in
N2O3,45,46 indicating that the Ni in the deposit is in oxidation
state. However, more recent work argued that the Ni might
present as Ni(OH)2,47,48 and NiOH+ was detected on the surface
of the NiP alloy electroless plated on Al substrate.49 A major
peak at 133.3 eV is found in the P 2p spectra of all the CuNi-ELD/
Co samples except CuNi-ELD/Co-7 composite, which could be
assigned to the oxidized P species. This indicates the presence
of phosphorus in the deposit.
Theꢃoxidized P species produced from the hydrolysis of
H2PO2 are hard to be removed, even though the sample was
thoroughly washed (Scheme S1†).45,47,48,50–52 A minor peak at
130.0 eV is found in the P spectra of CuNi-ELD/Co-5 and CuNi-
ELD/Co-6 samples, which could be assigned to the elemental P
species (Scheme S1†).45,47,48,50,52 The XPS peak for metal Ni was
also detected at 853.0 eV in CuNi-ELD/Co-5 and CuNi-ELD/Co-
6.45–47,50 These indicate the covalent interaction between Ni0 and
P0 and trace amount of Cu–Ni–P alloy in CuNi-ELD/Co-5 and
CuNi-ELD/Co-6 catalyst samples. The positive shi of the
binding energy of Ni0 in the alloy can be attributed to the
electron donation of Ni0.
Fig. 2 SEM images of catalyst samples: (a) Co powder; (b)–(f) CuNi-
ELD/Co-5.
composites produced with electroless plating at the pH 11.5.28
This indicates initiation of the Cu coating layer formed on the
surface of Co particles. In addition, some spherical Cu crystal-
line particles are embedded in the amorphous Cu phase
(Fig. 2f), indicating the change of Cu phase from amorphous
phase to crystalline phase with the increase of Cu content.
These observations indicate that the chemical composition of
CuNi-ELD/Co composite is closely related to the surface
morphology of Cu deposit (Table 1). N2 adsorption–desorption
was used to determine the BET surface areas of the CuNi-ELD/
Co composites (data not shown here). The uncoated Co powder
sample has low surface area, which can be attributed to the
large particle size (300 meshes). The porous structure and high
dispersion of amorphous Cu layer in CuNi-ELD/Co-1–4
composites signicantly increased their surface areas. The
crystalline Cu spherical particles grew and agglomerated to
form coating layer and the Cu dispersion is decreased with the
increase of Cu content. Therefore, the surface areas of CuNi-
ELD/Co-5–7 samples are gradually decreased with the increase
of Cu content.
3.2 Catalytic performances in the oxidation of cyclohexene
Fig. 3 illustrates the TEM images, selected electron diffrac-
tion patterns and EDS analysis of CuNi-ELD/Co-5 catalyst
sample where the amorphous phase and crystalline phase co-
exist in the Cu coating layer. A ꢀ2 mm particle was observed
and Co and Cu co-existed in this particle (Fig. 3a). The selected
electron diffraction pattern of this sample is ascribed to the
(111), (220) and (222) orientations of a cubic Co phase (Fig. 3a).
A ꢀ250 nm Cu particle with a porous structure was observed in
this particle (Fig. 3b). The EDX analysis of the Cu particle
indicates that it is the segregation of free copper. The selected
The major products of the oxidation of cyclohexene are Cy-ol,
Cy-one, Cy-oxide and intermediate Cy-HP. Meanwhile, several
by-products including 1,2-cyclohexanediol, hexanediol, adipic
acid and cyclohexanone are also produced (Scheme 1). The
possible reaction pathways of the oxidation of cyclohexene over
CuNi-ELD/Co catalyst are shown in Scheme 2.20–22 Cyclohexene
subjects to a complex radical-chain reaction and cyclohexenyl
peroxyl radical (Cy-OOc) is the main chain propagator. This is
evidenced by the fact that the addition of a free-radical scav-
enger (hydroquinone) essentially stops the reaction and
13812 | RSC Adv., 2015, 5, 13809–13817
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
Fig. 3 TEM images, the selected electron diffraction patterns and EDS analysis of CuNi-ELD/Co-5 catalyst sample.
profoundly affects the product distribution (Table 2 entry 10).53 2 step 1 and 2). Cy-HP can also be converted to Cy-ol and
Cy-HP is initially formed as the key primary product, and Cy-oxide through the epoxidation of cyclohexene, and the
decomposes to Cy-ol and Cy-one or to Cy-one and water (Scheme Cy-oxide can further react with water to produce 1,2-
Fig. 4 XPS spectra of CuNi-ELD/Co catalyst samples: (a) CuNi-ELD/Co-1; (b) CuNi-ELD/Co-2; (c) CuNi-ELD/Co-3; (d) CuNi-ELD/Co-4; (e)
CuNi-ELD/Co-5; (f) CuNi-ELD/Co-6; (g) CuNi-ELD/Co-7.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 13809–13817 | 13813

View Article Online
RSC Advances
Paper
in Table 2. The conversion of the oxidation of cyclohexene in the
absence of catalyst was 7.5% (Table 2 Entry 1). The conversion
was increased to 17.9% with Co powders only as catalyst (Table
2 entry 2). A conversion of 23.0% was obtained over CuNi-ELD/
Co-1 with 1.7 wt% Cu content (Table 2 entry 3). The conversions
varied between 30% and 40% over CuNi-ELD/Co-2–4 catalysts
with the Cu contents increasing from 2.2 wt% to 4.3 wt% (Table
2 entries 4–6). CuNi-ELD/Co-5 catalyst with 6.0 wt% Cu content
exhibited the highest conversion of 46.3% among all compos-
ites (Table 2 entry 7). Further increasing Cu wt% in CuNi-ELD/
Co-6 and CuNi-ELD/Co-7 decreased the conversions to less than
30% (Table 2 entries 8 and 9). The variation of Cy-HP selectivity
is contrary to that of cyclohexene conversion, suggesting that
the decomposition of Cy-HP depends on the catalytic activity of
the catalyst and plays an aforementioned dominant role in the
total oxidation rate. These results indicate that the Cu deposits
on Co powder substrate can signicantly improve the catalytic
activity. Both the Cu content and the microstructure of Cu
deposits can affect its catalytic activity for the oxidation of
cyclohexene. The Cu deposits in the CuNi-ELD/Co-1–5 catalyst
samples are mainly composed of amorphous phase, and the
active sites increase with the increase of Cu content. Thus,
reaction rate and cyclohexene conversion are increased with the
increase of Cu content from 1.7 wt% to 6.0 wt%. The crystallized
Cu phase in CuNi-ELD/Co-6 and CuNi-ELD/Co-7 catalysts leads
to lower Cu dispersion and less active sites, which reduces the
reaction rate and decreases cyclohexene conversion. Further-
more, amorphous alloy possesses unique short-range order and
long-range disordered atomic arrangement, which results in an
isotropic and homogeneous structure and affords a uniform
Scheme 1 The oxidation of cyclohexene over CuNi-ELD/Co catalyst.
Scheme
2 The possible reaction pathways of the oxidation of
cyclohexene over CuNi-ELD/Co catalyst.
cyclohexanediol (Scheme 2 step 3 and 4). Small fractions of Cy-
ol, Cy-one, Cy-oxide and 1,2-cyclohexanediol products can be
oxidized to form deep oxygenated by-products.
The catalytic performances of the CuNi-ELD/Co catalysts
were examined for the oxidation of cyclohexene with oxygen
under solvent-free conditions, and the results are summarized
Table 2 Catalytic performances in the oxidation of cyclohexene over CuNi-ELD/Co catalystsa
Selectivity (%)
Entry
Catalyst
Conversion (%)
Cy-ol
Cy-one
Cy-HP
Cy-oxide
Others
1
2
3
4
5
6
7
8
—
7.5
17.9
23.0
33.9
39.3
35.3
46.3
26.0
23.6
1.3
19.7
40.7
43.6
45.1
49.4
47.6
17.7
27.2
34.1
8.6
9.0
9.3
22.9
23.4
25.0
26.7
29.2
28.7
28.1
20.0
24.0
25.2
26.9
29.5
27.9
27.8
33.9
34.1
20.6
24.2
28.8
64.8
58.9
57.4
51.5
45.0
48.1
46.5
62.1
59.0
25.1
54.1
43.4
45.9
46.1
24.2
30.1
66.0
55.2
45.1
1.7
2.7
3.1
4.1
4.4
3.8
4.5
4.1
3.0
25.7
2.6
4.5
4.6
4.7
5.1
4.7
3.1
3.7
3.1
2.0
6.0
5.2
8.0
10.9
9.1
11.9
7.9
5.6
0.4
8.2
13.6
12.5
12.7
20.6
13.7
4.0
Co powder
CuNi-ELD/Co-1
CuNi-ELD/Co-2
CuNi-ELD/Co-3
CuNi-ELD/Co-4
CuNi-ELD/Co-5
CuNi-ELD/Co-6
CuNi-ELD/Co-7
CuNi-ELD/Co-5
Ni-ELD/Co-5
CuCo-ELD/Co-5
CuNi-ELD/Co-8
CuNi-ELD/Co-9
CuNi-ELD/Co-10
CuNi-ELD/Co-11
Al powder
9.7
10.5
10.3
9.0
5.9
8.4
23.6
8.2
9.0
9.1
8.7
16.2
17.4
6.3
8.3
9.7
9
10b
11c
12c
13
14
15
16
17
18c
19c
CuNi-ELD/Al-5
Cu-ELD/Co-5
8.6
13.3
a
Reaction conditions: cyclohexene 5 mL, molar ratio of CuNi-ELD/Co and cyclohexene 1 : 250, oxygen 2 MPa, temperature 349 K, time 6 h. Side
products-1,2-cyclohexanediol, hexanediol, adipic acid and cyclohexanone were identied by comparison with standard samples (retention time
b
in GC) and other products were detected by GC-MS. Reactions were carried out in presence of small amount (3 wt% of cyclohexene) of
c
hydroquinone. The preparation method was described in ESI.
13814 | RSC Adv., 2015, 5, 13809–13817
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
dispersion of active sites in an identical chemical environment synergistic effect between Cu and Co oxides plays a key role in
and high concentration of coordinatively unsaturated sites. the signicantly enhanced activity of the Cu–Co catalyst.57–59 In
These are essential for the bonding and activation of the reac- addition, the conversion of cyclohexene over the Cu-ELD/Co-5
tants and are benecial for high catalytic activity.54–56
composite prepared with formaldehyde as the reducing agent
As discussed above, the Cu–Ni deposits can signicantly and a Cu/Co mole ratio of 1 : 16 is lower than that over CuNi-
improve the catalytic activity, suggesting that the Cu–Ni alloy may ELD/Co-5 (Table 2 entry 19), indicating that environmental
be the catalytic active center. To prove it, Ni-ELD/Co-5 was benign Cu electroless deposition with sodium hypophosphite
prepared with a Ni/Co mole ratio of 1 : 16, which is same as that of as reducing agent is more effective.
CuNi-ELD/Co-5 composite. The conversion of cyclohexene over the
The effect of reaction temperature and time on the catalytic
Ni-ELD/Co-5 is 19.7% (Table 2 entry 11), which is close to that over activity was also examined and the results are listed Table 3.
Co powder, suggesting that the Ni deposits alone cannot show any When the temperature was increased from 339 to 349 K, the
catalytic activity and the Cu species in Cu–Ni alloy are the main conversion of cyclohexene increased from 9.8% to 46.3%, and
active sites. CuCo-ELD/Co-5 with a Cu/Co mole ratio of 1 : 16 was the total selectivity to Cy-ol, Cy-one, Cy-HP and Cy-oxide
also prepared. Co2+ instead of Ni2+ was used as the catalyst for the decreased from 96.5% to 88.1% (Table 3 entries 1–4). This is
oxidation of hypophosphite. The conversion of cyclohexene over caused by the formation of by-products such as 1,2-cyclo-
CuCo-ELD/Co-5 is 40.7% (Table 2 entry 12), which is slightly lower hexanediol, hexanediol and adipic acid at high temperature.
than that over CuNi-ELD/Co-5 composite. To further investigate The selectivity to Cy-HP decreased from 62.5% to 46.5%,
the effects of Ni species on the catalytic activity, CuNi-ELD/Co-8–11 whereas the selectivity to Cy-one increased from 23.4% to 28.1%
composites with different Ni contents were prepared and their with the increase of reaction temperature, indicating that Cy-HP
catalytic activities in the oxidation of cyclohexene were deter- rapidly decomposed into Cy-ol and Cy-one at high reaction
mined. The conversion was increased slightly from 43.6% to 49.4% temperatures. The opposite changes of selectivity to Cy-HP and
(Table 2 entries 13–16) with the decrease of the Ni content from 0.6 to Cy-ol and Cy-one and higher selectivity to Cy-one than to Cy-
wt% to 0.3 wt% (Table 1 entries 9–12), indicating that the appro- ol conrm the aforementioned reaction pathways. Suitable
priate amount of Ni species in Cu–Ni deposits can improve cata- reaction temperature is required in the oxidations of cyclo-
lytic activity. In all, Cu–Ni alloy in the CuNi-ELD/Co composite is hexene over CuNi-ELD/Co catalyst. The carbonization occurs
the catalytic active center, where Cu species are the major catalytic through deep oxidations at higher temperature, whereas no
active component and Ni species are a promoter.
product is produced at lower temperature.
To investigate the effect of the support metal on the catalytic
Reaction time also affects the conversion of cyclohexene and
activity, CuNi-ELD/Al-5 composite with a Cu/Al mole ratio of selectivity of the reaction. The conversion increased from 3.2%
1 : 16 was prepared and its catalytic performance in the oxida- to 46.3% when the reaction time was extended from 2 h to 6 h
tion of cyclohexene was determined. The conversion of cyclo- (Table 3 entries 4–7). The selectivity to Cy-HP of a 2 h reaction
hexene over Al powder is 17.7% (Table 2 entry 17), which is close was 63.3% and that of a 6 h reaction was decreased to 46.5%,
to that over Co powder. However, but CuNi-ELD/Al-5 composite suggesting that Cy-HP is the primary product at the initial stage
exhibited lower conversion of 27.2% compared with CuNi-ELD/ of reaction. Meanwhile, the selectivity to Cy-one increased from
Co-5 composite (Table 2 entry 18). These results indicate that 24.7% to 28.1% with the increase of reaction time from 2 h to 6
the Co powder with the cobalt oxide formed on the surface in h. The free radicals accumulate as the reaction proceeds, which
alkaline solutions and Cu deposits have a synergistic effect on gradually increases the reaction rate, Cy-HP decomposition rate
the oxidation of cyclohexene. It has been reported that the and the formation rates of Cy-ol and Cy-one. The total selectivity
Table 3 Catalytic oxidation of cyclohexene with oxygen in different reaction conditions
Selectivity (%)
Entry
Catalyst
T (K)
Time (h)
Conversion (%)
Cy-ol
Cy-one
Cy-HP
Cy-oxide
Others
1a
2a
3a
4a
5a
6a
7a
8b
9c
CuNi-ELD/Co-5
CuNi-ELD/Co-5
CuNi-ELD/Co-5
CuNi-ELD/Co-5
CuNi-ELD/Co-5
CuNi-ELD/Co-5
CuNi-ELD/Co-5
[Co2(DOBDC) (H2O)2]$8H2O
MG@NSal-Co
339
344
346
349
349
349
349
353
343
373
6
6
6
6
5
4
2
20
12
8
9.8
12.9
27.6
46.3
39.2
14.2
3.2
32.8
46.8
61.0
9.1
9.2
9.1
9.0
8.6
8.8
9.6
39.3
8.7
4.0
23.4
24.6
26.7
28.1
27.3
25.1
24.7
51.2
77.2
63.0
62.5
59.2
52.6
46.5
49.4
57.7
63.3
—
1.5
2.0
3.8
4.5
3.9
2.2
1.4
—
3.5
5.0
7.8
11.9
10.8
6.2
1.0
—
8.8
19.0
—
—
5.3
14.0
10d
MSS-SH-Au0
a
Reaction conditions: cyclohexene 5 mL, molar ratio of CuNi-ELD/Co-5 and cyclohexene 1 : 250, oxygen 2 MPa. b Reaction conditions: cyclohexene
5 mL, catalyst 50 mg, oxygen balloon, in ref. 21. c Reaction conditions: cyclohexene 19.7 mmol, catalyst 3.0 mg, oxygen 1 atm, in ref. 60. d Reaction
conditions: cyclohexene 6 mmol, 10 mL toluene, catalyst 60 mg, oxygen 1 atm, in ref. 61.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 13809–13817 | 13815

View Article Online
RSC Advances
Paper
to Cy-ol, Cy-one, Cy-HP and Cy-oxide decreased linearly because
of the occurrence of peroxidation as the reaction time was
increased. Therefore, the oxidation was conducted at 6 h to
obtain desired conversion and selectivity under optimized
reaction conditions.
7 L. Li, G. Yan, Z. Cheng, J. Wu, X. Yu and Q. Guo, Surf.
Interface Anal., 2009, 41, 69.
8 R. H. Guo, S. Q. Jiang, C. W. M. Yuen and M. C. F. Ng, J.
Mater. Sci.: Mater. Electron., 2009, 20, 33.
9 H. Wang, J. Jia, H. Song, X. Hu, H. Sun and D. Yang, Ceram.
Int., 2011, 37, 2181.
In full, the CuNi-ELD/Co-5 composite shows compatible
catalytic activity as the catalysts previously reported for the 10 F. Cai, X. Shen, M. Dai, M. Gao, Z. Wang, B. Zhao and
oxidation of cyclohexene (Table 3 entries 8–10). High conver- W. Ding, Chin. J. Inorg. Chem., 2013, 29, 689.
sion and selectivity of the oxidation of cyclohexene can be 11 B. Çelen, D. Ekiz, E. Pi¸skin and G. Demirel, J. Mol. Catal. A:
obtained over CuNi-ELD/Co catalyst under solvent-free
conditions.
Chem., 2011, 350, 97.
12 H. Cao, Z. Wang, G. Hou and G. Zheng, Surf. Coat. Technol.,
2010, 205, 885.
13 C. Fukuhara, H. Ohkura, Y. Kamata, Y. Murakami and
A. Igarashi, Appl. Catal., A, 2004, 273, 125.
4 Conclusions
In summary, CuNi-ELD/Co composites with different compo- 14 C. Fukuhara, Y. Kamata and A. Igarashi, Appl. Catal., A, 2005,
sitions and microstructures were prepared by electroless 296, 100.
deposition and used for the rst time as a catalyst for the 15 C. Fukuhara and H. Ohkura, Appl. Catal., A, 2008, 344, 158.
oxidation of cyclohexene. The amorphous phase can be formed 16 R. A. Sheldon and J. K. Kochi, Metal Catalyzed Oxidations of
as a dominating phase in Cu deposits with low Cu content and
possesses a honeycomb-like porous morphology. The micro- 17 K. C. Gupta, A. K. Sutar and C. Lin, Coord. Chem. Rev., 2009,
structure of the Cu deposit changes from amorphous phase to 253, 1926.
cubic sphere-like crystalline phase with the increase of Cu 18 N. V. Maksimchuk, M. N. Timofeeva, M. S. Melgunov,
Organic Compounds, Academic Press, New York, 1981.
content. Both Cu content and microstructure of the CuNi-ELD/
A. N. Shmakov, Y. A. Chesalov, D. N. Dybstev, V. P. Fedin
Co composite can signicantly affect its catalytic activity for the
and O. A. Kholdeeva, J. Catal., 2008, 257, 315.
oxidation of cyclohexene. CuNi-ELD/Co-5 catalyst with 6.0 wt% 19 S. Ganji, P. Bukya, V. Vakati, K. S. R. Rao and D. R. Burri,
Cu content and amorphous phase shows the best catalytic
Catal. Sci. Technol., 2013, 3, 409.
performance in the oxidation of cyclohexene under solvent-free 20 J. Tong, Y. Zhang, Z. Li and C. Xia, J. Mol. Catal. A: Chem.,
conditions. Compared with the traditional catalysts, CuNi-ELD/ 2006, 249, 47.
Co composite prepared by electroless deposition is low coast 21 Y. Fu, D. Sun, M. Qin, R. Huang and Z. Li, RSC Adv., 2012, 2,
and environmentally benign with compatible catalytic activity. 3309.
It is a promising catalyst for the oxidation of hydrocarbon 22 Z. Cai, M. Zhu, J. Chen, Y. Shen, J. Zhao, Y. Tang and
compounds.
X. Chen, Catal. Commun., 2010, 12, 197.
23 H. Huang, H. Zhang, Z. Ma, Y. Liu, H. Ming, H. Li and
Z. Kang, Nanoscale, 2012, 4, 4964.
24 S. E. Dapurkar, H. Kawanami, K. Komura, T. Yokoyama and
Y. Ikushima, Appl. Catal., A, 2008, 346, 112.
Acknowledgements
The authors gratefully acknowledge the nancial support from
the National Natural Science Foundation of China (NSFC 25 D. Jiang, T. Mallat, D. M. Meier, A. Urakawa and A. Baiker, J.
21163011), the Natural Science Foundation of Inner Mongolia Catal., 2010, 270, 26.
(2013MS0208), and the Science Research Projects of Inner 26 A. Ibrahim, M. Abdallah, S. F. Mostafa and A. A. Hegazy,
Mongolia University (NJ10072). Thanks are also due to Zhanli Mater. Des., 2009, 30, 1398.
Chai of School of Chemistry and Chemical Engineering, Inner 27 G. Wang and D. Wang, Rare Met., 2008, 27, 434.
Mongolia University, for helpful discussions.
28 H. Wang, J. Jia, H. Song, X. Hu, H. Sun and D. Yang, Ceram.
Int., 2011, 37, 2181.
29 Y. Wu, Q. Liu and H. Wang, J. Compos. Mater., 2011, 46, 1453.
30 J. Li, H. Hayden and P. A. Kohl, Electrochim. Acta, 2004, 49,
1789.
31 X. Gan, K. Zhou, W. Hu and D. Zhang, Surf. Coat. Technol.,
2012, 206, 3405.
Notes and references
1 Z. Huang, F. Cui, H. Kang, J. Chen, X. Zhang and C. Xia,
Chem. Mater., 2008, 20, 5090.
2 M. J. Grapperhaus, Z. Krivokapic and M. J. Kushner, J. Appl.
Phys., 1998, 83, 35.
32 J. Hao, B. Liu, H. Cheng, Q. Wang, J. Wang, S. Cai and
F. Zhao, Chem. Commun., 2009, 3460.
3 H. Choi and S. Park, J. Am. Chem. Soc., 2004, 126, 6248.
4 Q. Zhang, Y. Hua, Y. Wang, H. Lu and X. Zhang, 33 J. Hao, S. Li, L. Han, L. Cheng, Q. Suo, Y. Xiao, X. Jiao,
Hydrometallurgy, 2009, 98, 291.
X. Feng, W. Bai and X. Song, Inorg. Chim. Acta, 2014, 421,
5 R. K. Aithal, S. Yenamandra, R. A. Gunasekaran, P. Coane
and K. Varahramyan, Mater. Chem. Phys., 2006, 98, 95.
6 G. O. Mallory and J. B. Hadju, Electroless Plating:
246.
34 D. Cheng, W. Xu, Z. Zhang and Z. Yiao, Met. Finish., 1997, 95,
34.
Fundamentals and Applications, American Electroplates and 35 X. Gan, Y. Wu, L. Liu, B. Shen and W. Hu, Surf. Coat.
Surface Finishers Society, Orlando, FL, 1990.
Technol., 2007, 201, 7018.
13816 | RSC Adv., 2015, 5, 13809–13817
This journal is © The Royal Society of Chemistry 2015

View Article Online
Paper
RSC Advances
36 X. Gan, Y. Wu, L. Liu, B. Shen and W. Hu, J. Alloys Compd., 50 Y. Chen, Catal. Today, 1998, 44, 3.
2008, 455, 308.
51 S. M. S. I. Dulal, T. H. Kim, C. B. Shin and C. Kim, J. Alloys
Compd., 2008, 461, 382.
52 M. C. Oliveira and A. M. B. do Rego, J. Alloys Compd., 2006,
425, 64.
53 R. Raja, G. Sankar and M. Thomas, J. Am. Chem. Soc., 1999,
121, 11926.
37 S. Lambert, C. Cellier, E. M. Gaigneaux, J. Pirard and
B. Heinrichs, Catal. Commun., 2007, 8, 1244.
38 R. V. Kumar, Y. Mastai, Y. Diamant and A. Gedanken, J.
Mater. Chem., 2001, 11, 1209.
39 M. H. Bhat, V. Molinero, E. Soignard, V. C. Solomon,
S. Sastry, J. L. Yarger and C. A. Angell, Nature, 2007, 448, 787. 54 A. Baiker, Faraday Discuss. Chem. Soc., 1989, 87, 239.
´
´
´
40 T. Xue and J. Lee, J. Power Sources, 2014, 245, 194.
41 J. Chang, C. Wu and I. Sun, J. Mater. Chem., 2010, 20, 3729.
42 L. Li, H. Qian and J. Ren, Chem. Commun., 2005, 4083.
43 H. Ma, Z. Liu, L. Wu, Y. Wang and X. Wang, Thin Solid Films,
2011, 519, 7860.
44 L. He, H. Cheng, G. Liang, Y. Yu and F. Zhao, Appl. Catal., A,
2013, 452, 88.
45 J. Deng, H. Chen, X. Bao and M. Muhler, Appl. Surf. Sci.,
1994, 81, 341.
55 A. Molnar, G. V. Smith and M. Bartok, Adv. Catal., 1989, 36,
329.
56 Y. Pei, G. Zhou, N. Luan, B. Zong, M. Qiao and F. Tao, Chem.
Soc. Rev., 2012, 41, 8140.
57 S. Deng, W. Chu, H. Xu, L. Shi and L. Huang, J. Nat. Gas
Chem., 2008, 17, 369.
58 L. Jia, Y. Guo, T. P. Tran, M. Sakurai and H. Kameyama, J.
Chem. Eng. Jpn., 2012, 45, 590.
59 C. Li, J. Zhou, W. Gao, J. Zhao, J. Liu, Y. Zhao, M. Wei,
D. G. Evans and X. Duan, J. Mater. Chem. A, 2013, 1, 5370.
46 F. Xing, B. Zhao and W. Shi, Electrochim. Acta, 2013, 100, 157.
47 H. Liu, H. L. Yao, Y. Liu, G. E. Thompson and Z. Liu, Cryst. 60 X. Cai, H. Wang, Q. Zhang, J. Tong and Z. Lei, J. Mol. Catal. A:
Res. Technol., 2014, 49, 178. Chem., 2014, 383–384, 217.
48 Y. Wang, F. Shi, Y. Yang and W. Cai, J. Power Sources, 2013, 61 Y. Fang, Y. Chen, X. Li, X. Zhou, J. Li, W. Tang, J. Huang,
243, 369.
J. Jin and J. Ma, J. Mol. Catal. A: Chem., 2014, 392, 16.
49 B. C. Zhang, G. Barth, H. K. Liu and S. Chang, Appl. Surf. Sci.,
2004, 231–232, 868.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 13809–13817 | 13817