Enhanced stability and activity for solvent-free selective oxidation of cyclohexane over Cu2O/CuO fabricated by facile alkali etching method

Article abstract of DOI:10.1016/j.mcat.2020.111134

The easy deactivation of cuprous oxide (Cu₂O) have significantly limited their practical applications in catalysis. Formation of composites with other semiconductors including CuO is an crucial strategy for stability improvement of Cu₂O. However, this strategy developed so far has often focused on only Cu₂O thin film systems. Herein, Cu₂O/CuO composites were fabricated via a simple alkali etching method, for the first time, to stabilize the powdered Cu₂O in liquid-phase selective oxidation of cyclohexane under solvent-free conditions. The etching method presented remarkable high dispersion of copper species and an appropriate Cu⁺/Cu²⁺ ratio on catalyst surface, which leads to superior catalytic activity and stability. Hot filtration experiment confirmed that Cu₂O/CuO composite was a heterogeneous catalyst which could be reused at least five times without considerable loss of catalytic activity, whereas the pure Cu₂O suffered from severe deactivation caused by oxidative decomposition of Cu₂O and copper leaching. Moreover, the amount of Cu⁺ and Cu²⁺ sites on catalyst surface are dramatically affected by the amount of alkali during etching process and the catalytic performance can be further tuned by regulating the Cu⁺/Cu²⁺ ratio. The Cu₂O/CuO with a molar ratio of NaOH/Cu₂O = 1:1 exhibited the highest catalytic activity with a cyclohexane conversion of 84.3 percent and cyclohexanone selectivity of 77.0 percent. The present work provides a promising strategy to stabilize Cu₂O catalysts in catalytic oxidation systems.

Full text of DOI:10.1016/j.mcat.2020.111134

MolecularCatalysis495(2020)111134
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Enhanced stability and activity for solvent-free selective oxidation of
cyclohexane over Cu2O/CuO fabricated by facile alkali etching method
Congjia Xie, Wei Wang, Yepeng Yang, Liang Jiang*, Yongjuan Chen, Jiao He, Jiaqiang Wang*
School of Chemical Sciences & Technology, National Center for International Research on Photoelectric and Energy Materials, School of Materials and Energy, Yunnan
University, Kunming 650091, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
The easy deactivation of cuprous oxide (Cu2O) have significantly limited their practical applications in catalysis.
Formation of composites with other semiconductors including CuO is an crucial strategy for stability im-
provement of Cu2O. However, this strategy developed so far has often focused on only Cu2O thin film systems.
Herein, Cu2O/CuO composites were fabricated via a simple alkali etching method, for the first time, to stabilize
the powdered Cu2O in liquid-phase selective oxidation of cyclohexane under solvent-free conditions. The etching
method presented remarkable high dispersion of copper species and an appropriate Cu+/Cu2+ ratio on catalyst
surface, which leads to superior catalytic activity and stability. Hot filtration experiment confirmed that Cu2O/
CuO composite was a heterogeneous catalyst which could be reused at least five times without considerable loss
of catalytic activity, whereas the pure Cu2O suffered from severe deactivation caused by oxidative decomposi-
tion of Cu2O and copper leaching. Moreover, the amount of Cu+ and Cu2+ sites on catalyst surface are dra-
matically affected by the amount of alkali during etching process and the catalytic performance can be further
tuned by regulating the Cu+/Cu2+ ratio. The Cu2O/CuO with a molar ratio of NaOH/Cu2O = 1:1 exhibited the
highest catalytic activity with a cyclohexane conversion of 84.3 % and cyclohexanone selectivity of 77.0 %. The
present work provides a promising strategy to stabilize Cu2O catalysts in catalytic oxidation systems.
Cu2O/CuO composites
Cu+/Cu2+ ratio
Selective oxidation of cyclohexane
Cyclohexanone
Solvent-Free conditions
Introduction
incorporation of secondary components [16] has been employed to
enhance the catalytic stability of Cu2O. Especially, a number of sec-
In recent years, copper oxides have attracted considerable attention
due to its prospective application in the fields of gas sensor [1], sodium
ion batteries [2], catalysis [3], and supercapacitor [4]. Among the three
well-established Cu-based oxides: CuO, Cu2O and Cu4O3 [5], CuO is
identified as the most stable copper-oxide phases, while the crystal-
lographic and phases of Cu2O can be easily changed under harsh con-
dition [6]. In particular, Cu2O, as a p-type semiconductor, has gained
considerable research attention for numerous catalytic reactions due to
its tunable bandgap, earth abundancy, and excellent catalytic proper-
ties [7]. Cu2O has been reported to act as an efficient catalyst for sev-
eral selective catalytic reactions such as CO oxidation [8], for-
maldehyde oxidation [9], phenylacetylene oxidation [10], propylene
oxidation [11] and catalytic reduction of 4-nitrophenol [12]. However,
owing to its susceptible chemical stability, self-reduction or self-oxi-
dation is unavoidable when exposed to oxidizing or reducing environ-
ment deactivation [13].
ondary components including carbon quantum dots (CQDs) [17], re-
duced graphene oxide (rGO) [18], ZIF-8 [19], semiconductors [20], and
metal nanoparticles [21] have been extensively studied. Moreover, it
has been demonstrated that Cu2O/CuO nanocomposites could bring
new synergistic catalytic performance, which arise from the construc-
tion of unique Cu-O interfaces [22]. In this regard, incorporation of a
protective layer (CuO nanowires) was shown to be an effective ap-
proach to enhance the photoelectrochemical stability of Cu2O film [23].
Nonetheless, the redox activities of the Cu2O photoelectrode were also
suppressed after introducing the passive outer layer of CuO [23].
Therefore, the exploration of alternative strategies in enhancing both
stability and activity of Cu2O is still highly desired. As a green and
simple modification method [24], chemical etching (acid treatment or
alkali treatment) has been currently paid wide attention for enhance
the catalytic performance of nanoparticles, including Cu2O nano-
particles by controlling their morphologies [25]. Surface treatment by
etching might be a useful approach to stabilize Cu2O because the etched
surfaces will bring about high-activity sites for enhancing chemical
To overcome these drawbacks, various strategies such as en-
gineering crystal facet [14], controlling particle sizes [15], and
⁎ Corresponding authors.
E-mail addresses: jiangliang@ynu.edu.cn (L. Jiang), jqwang@ynu.edu.cn (J. Wang).
https://doi.org/10.1016/j.mcat.2020.111134
Received 18 April 2020; Received in revised form 10 July 2020; Accepted 22 July 2020
2468-8231/©2020ElsevierB.V.Allrightsreserved.

C. Xie, et al.
MolecularCatalysis495(2020)111134
activities [26]. However, the contemporary use of etching for enhanced
Cu2O stability has been largely limited to thin film systems.
centrifuged, washed and finally dried (70 °C, 6 h) in vacuum.
The selective oxidation of cyclohexane is an extraordinary sig-
nificant process in petrochemical industry since its products (cyclo-
hexanone and cyclohexanol) are the key intermediate for industrial
production of nylon-66 and nylon-6 [27]. Various copper-based cata-
lysts (such as Cu(I) complexs [28], Cu-Zn-BTC MOF [29], CuCrOx [30],
Cu(I)-ZSM-5 [31]) have been proved to be active for oxidation of cy-
clohexane. However, these catalysts offer either limited conversion
(< 50 %) or low selectivity (< 60 %). Additionally, fiberglass sup-
ported Cu(I)-Cu(0) compounds has been found a remarkable deactiva-
tion in cyclohexane oxidation due to the oxidation of Cu species in the
presence of H2O2 and O2 [32]. On the other hand, most of cyclohexane
oxidation systems are still environmentally hazardous owing to the use
of large amount of organic solvents, leading to the problems of high
disposal wastes and severe copper leaching [33,34]. Thus, developing
an efficient and environmentally friendly heterogeneous catalyst for
cyclohexane oxidation under solvent-free is highly desirable.
Herein, for the first time, we report Cu2O/CuO composites fabri-
cated by alkali (NaOH) etching Cu2O sphere as efficient and stable
heterogeneous catalyst for liquid-phase selective cyclohexane oxidation
to cyclohexanone without any solvents. The effect of alkali concentra-
tion, reaction times, and temperature on catalytic performance of
Cu2O/CuO composites were also investigated. The samples before and
after oxidation reactions were comprehensively characterized re-
garding catalytic stability.
Catalytic oxidation of cyclohexane
Typically, cyclohexane (1 mL, 9.27 mmol), catalyst (20 mg) and
oxidant tert-butyl hydroperoxide (TBHP, 3 mL) were introduced into a
round-bottom flask (50 mL) connected with a condenser with magnetic
stirring at 343 K for 18 h. When the reaction completed and cooled
down, the catalyst were separated via filtration. The products were
quantified using gas chromatography (GC, Shimadzu GC-2014, Japan)
equipped with a flame ionization detector (FID) and AE.OV-624 column
(30 m ×0.25 mm ×0.5 μm).
Recyclability tests and thermal filtration experiments
Thermal filtration experiments were carried out based on a modified
process [37]. 3 mL cyclohexane, 0.1 g catalyst, and 3 mL TBHP was
stirred at 373 K for 1 h, then the catalysts were quickly filtrated off and
the filtration was further reacted with cyclohexane. For the recycl-
ability tests, the catalysts after finishing reaction were filtrated, washed
with ultrapure water and ethanol, dried in vacuum (70 °C, 5 h), and
then was employed in the next tests. In order to check the leaching of
copper species, copper content of the fresh and recovered catalysts were
determined by ICP-MS analyzer (Agilent 7700x, USA). To confirm the
structural stability of catalysts, the recovered catalysts were further
examined by XRD and XPS for comparison with the fresh catalyst.
Results and discussion
Experimental
Synthesis and structural characterization
Preparation of Cu2O
The Cu2O spheres were synthesized by a wet-chemical solution
method, and the Cu2O/CuO composites were eventually generated after
etching process at the room temperature. Initially, Cu(NO3)2 was de-
composed and reduced by glucose and Na2SO3 at room temperature to
fabricate the Cu2O spheres. After separation and drying, the Cu2O
spheres were etched by alkali solution. The final chemical composition
and crystal structure of copper oxide can be adjusted by tuning the
molar ratio of NaOH/Cu2O (i.e. the concentration of NaOH), which
were described in the experimental section. As demonstrated in Scheme
1, this procedure can be explained by the process of dissolution and
precipitation [38] where the unique structures were ascribed to the
synergetic effect of oxidation and etching. Oxygen behaves as an oxi-
dant owing to the reaction was run and expose in air. By the addition of
NaOH, the surface of Cu2O sphere will be firstly oxidized and trans-
formed into Cu(OH)2. A strong alkali concentration further results in
generation of tetrahydroxo-cuprate(II) anions with Jahn-Teller stabili-
zation and finally production of stable CuO precipitates. It can be
concluded that alkali concentrations are responsible for chemical
compositions of particles and reaction kinetics. The mechanism of
oxidation etching can be summarized as following equations [38]:
Cu2O sphere was prepared by a wet-chemical solution method ac-
cording to a reported process with some modifications [35]. 20 mL of
NaOH (0.05 M) solution was dispersed in 10 mL of Cu(NO3)2 (0.01 M)
solution under stirring with a copper hydroxide suspension formed, and
6 mL glucose (0.1 M) solution was added dropwise into the above
mixture. Subsequently, 4 mL of Na2SO3 (0.02 M) solution was added
into the mixture under vigorous stirring. The reaction was stopped after
2 h stirring and aged for 100 min. Finally, the Cu2O samples were ob-
tained from centrifugation (8000 rpm, 10 min), washing with ultrapure
water, and drying (80 °C, 6 h) in vacuum.
Preparation of Cu2O/CuO composites and CuO
We refer to some previous studies and provide a facile synthesis
procedure for Cu2O/CuO here [36]. Cu2O/CuO composites were pre-
pared by treating 0.5 g Cu2O with alkali (NaOH) solution with a molar
ratio of NaOH: Cu2O = 1:4, 1:2, 1:1, 2:1, respectively. The mixture was
stirred constantly and kept at room temperature (25 °C) for 24 h. The
solution gradually turned black, implying the Cu2O/CuO composites
were obtained. The Cu2O/CuO composites were centrifuged, washed
and finally dried (70 °C, 6 h) in vacuum. The samples synthesized via
etching Cu2O with different ratios of NaOH/Cu2O: 1:4, 1:2, 1:1, and 2:1
were designated as Cu2O/CuO-E (1–4), respectively. For comparison,
0.5 g Cu2O was impregnated in 50 mL NaOH solution with a molar ratio
of NaOH: Cu2O = 50:1 for 4 days to obtain pure CuO. The CuO were
2Cu2O + O2 + 4H2O = 4Cu(OH)2
Cu(OH)2 + 2OH−=[Cu(OH)4]2-
[Cu(OH)4]2−=CuO + 2OH- + H2O
(1)
(2)
(3)
Scheme 1. Schematic illustration of the for-
mation of Cu2O/CuO composites.
2

C. Xie, et al.
MolecularCatalysis495(2020)111134
Fig. 1. N2 sorption isotherm of pure Cu2O, pure CuO, Cu2O/CuO-E (1) and
Cu2O/CuO-E (3).
The N2 adsorption-desorption isotherms of unetched Cu2O and
Cu2O/CuO composites prepared by etching in different concentration of
NaOH solution are displayed in Figs. 1 and S1. All samples exhibited the
characteristic features of type IV isotherm with H1 hysteresis loops
according to IUPAC classification. The corresponding BJH pore size
distributions (Fig. S2) reveals the presence of the mesoporous in com-
posites. Both the pore size and the pore size distribution range of Cu2O/
CuO samples were larger than that of unetched Cu2O and they declined
first and then increased with increasing the NaOH concentration. In
addition, the textural properties of all as-prepared catalysts were
summarized in Table 1. A larger BET surface area and pore volume is
observed for all Cu2O/CuO composites in relation to the pure Cu2O
(0.8 m2 g−1, 0.0035 m3 g−1). When Cu2O was etched in low NaOH
concentration (NaOH/Cu2O molar ratio of 1/2), the surface area de-
creases to 7.1 m2 g−1 and the pore volume of the sample decreased to
0.049 m3 g−1. The further raising alkali concentration (NaOH/Cu2O
molar ratio of 2) results in an increase in the BET surface area and pore
volume to 12.5 m2 g−1 and 0.08 m3 g−1, respectively. When the sample
completely transformed into CuO, it displays the highest BET surface
area (29.8 m2 g−1) and pore volume (0.17 m3 g−1). This variation may
be related to the decreased amount of the larger mesopores as deduced
from Fig. S2 and the gradual formation of CuO nanoparticles with
smaller particle size.
Fig. 2. a) XRD patterns and b) Raman spectra of (a) Cu2O, (b) Cu2O/CuO-E (1),
(c) Cu2O/CuO-E (2), (d) Cu2O/CuO-E (3), (e) Cu2O/CuO-E (4), (f) CuO.
2076), respectively, indicating the successful synthesis of Cu2O in our
work. For the sample of Cu2O/CuO composites, the characteristic peaks
of Cu2O/CuO composites are correspondingly assigned to CuO (JCPDS
no. 80-0076) and Cu2O (JCPDS no. 78-2076), which revealed the si-
multaneous existence of monoclinic CuO phase and cubic Cu2O phase
in synthesized catalysts. No other possible phases such as Cu or Cu
(OH)2 were detected. These results imply that the mixed phase Cu2O/
CuO composite can be successfully obtained by etching pure Cu2O
crystal. Meanwhile, with increasing in NaOH/Cu2O molar ratio, the
The crystal structure and composition of Cu2O/CuO composites
were examined and evaluated by XRD measurement. As observed from
Fig. 2a, the diffraction peaks of Cu2O at 2θ values of 29.6°, 36.4°, 42.3°,
61.3°, 73.5°, 77.3° are observed, which correspond to the lattice planes
of (110), (111), (200), (220), (311), (222) of cubic Cu2O (JCPDS no. 78-
Table 1
Physicochemical properties of the Cu2O, Cu2O/CuO composites, and CuO.
Catalyst
Weight percentagea
Cu+/Cu2+ ratioa
Grain size (nm)a
Cu content
(wt%)b
SBET
Vpore
dpore
c
c
(m2 g−1
)
(m3 g−1
)
(nm)c
Cu2O
CuO
Cu2O
CuO
Total (Theoretical)
Total
(ICP)
External
(XPS)
Cu2O
100
83.2
65.0
44.5
26.4
0
0
—
> 100
94.1
91.7
92.5
96.2
—
—
89.4
88.0
85.0
84.4
82.8
80.4
88.4
86.5
82.8
81.4
80.1
77.6
65.7
61.3
59.4
60.9
62.2
65.8
0.8
7.6
7.1
12.1
12.5
29.8
0.0035
0.064
0.049
0.062
0.080
0.17
18.6
24.1
16.4
18.5
22.2
23.8
Cu2O/CuO-E (1)
Cu2O/CuO-E (2)
Cu2O/CuO-E (3)
Cu2O/CuO-E (4)
CuO
16.8
35.0
55.5
73.6
100
5.5:1
2.1:1
1:1.1
1:2.5
0
15.8
15.2
14.0
17.4
21.9
a
Weight percentage of Cu2O and CuO, Cu+/Cu2+ molar ratio, and grain size of Cu2O and CuO calculated from the XRD data.
Determined by ICP-MS and XPS analysis.
BET specific surface area, pore volume, and pore diameter determined by N2 sorption isotherm.
b
c
3

C. Xie, et al.
MolecularCatalysis495(2020)111134
intensity of Cu2O (111) planes gradually decreased while the intensity
of CuO (002) planes and (111) planes both increased, indicating that
the Cu2O with largely exposed (111) planes was oxidized gradually to
form CuO during etching process. With a further increase in NaOH/
Cu2O ratio to 50:1 and reaction time to 4 days, all the diffraction peaks
of Cu2O were disappeared, implying that Cu2O can be completely
converted into CuO. Furthermore, the weight percentage and the grain
size of CuO and Cu2O for each sample were calculated based on the CuO
(111) and Cu2O (111) planes, respectively, and compiled in Table 1. It
can be seen that the grain size of pure Cu2O and pure CuO are much
larger than that of all CuO/Cu2O samples. With the increase of NaOH/
Cu2O molar ratio, the grain size of CuO decreases gradually from 15.8
to 14.0 nm and then increased to 17.4 nm, while the mass ratio of CuO
increased from 16.8%–73.6%. For Cu2O/CuO-E (3), the crystalline sizes
of CuO and Cu2O were 14.0 nm and 92.5 nm, respectively, and the
weight percentage of CuO and Cu2O were 55.5 % and 45.5 %, respec-
tively. Moreover, the actual total Cu content detected by ICP-MS and
the theoretical total Cu content in these samples were listed in Table 1.
The external Cu contents on the surface of samples roughly calculated
by XPS have also been summarized in Table 1. The actual Cu contents in
all samples were similar to theoretical values with the error less than
4%, indicating that the copper species completely precipitated. The Cu
content detected by ICP were higher than those detected by XPS, at-
tributing to the number of Cu sites exposed on the sample surface was
lower than the total number of Cu sites in samples. It is worth noting
that the Cu content in Cu2O/CuO composites gradually decreases with
the amount of alkali increasing, mainly due to the gradually formation
of CuO.
The Raman spectra also confirmed the similar results about the
composition of samples. The Raman spectra of pure Cu2O, pure CuO,
and Cu2O/CuO composites were shown in Fig. 2b. In detail, these peaks
are related to three modes (Ag, B1 g, and B2 g) reflecting the vibration
of copper metal with oxygen. The bands at 300, 351 and 635 cm−1 are
belonged to CuO and the bands at 151, 215 cm−1 are indexed to Cu2O.
Furthermore, the strength of two bands at 300 cm−1 and 351 cm−1
increases with increasing amount of alkali during synthetic process. The
results indicate that the product change from Cu2O through Cu2O/CuO
to CuO.
SEM was applied to observe the morphology evolution of Cu2O
etched in different concentrations of alkali solution. The corresponding
SEM images were demonstrated in Fig. 3a-j. It was found that the Cu2O
exhibits a typical spherical in shape and the average size was estimated
(∼500 nm) in Fig. 3a,b. Cu2O morphology changed from spherical
structure to approximately spherical flower-like microstructures as-
sembled by numerous nanoplates with the increase of NaOH/Cu2O
molar ratio, clearly implying the process of crystal etching and growth.
Under low concentration of NaOH solution (i.e. NaOH/Cu2O molar
ratio = 1:4, Fig. 3c,d), the surface of original spherical Cu2O with large
exposed (111) planes (Fig. 3i) was firstly etched and oxidized into CuO
nanoflakes and there are only some Cu2O/CuO microstructures being
produced. With the increase of proportion of alkali (Fig. 3e–h), nu-
merous Cu2O/CuO microstructures with nanoflakes were further
formed on the surface of samples. When the molar ratio of NaOH/Cu2O
is 1:1 (Cu2O/CuO-E (3), Fig. 3e–f), the raw material was the most
uniformly etched, exhibiting the most well-distributed morphology. A
further increase in the concentration of NaOH (Fig. 3I,j) results in deep
etching and the formation of non-regular agglomerates.
The detailed structural analysis of Cu2O/CuO-E (3) were further
revealed by TEM. As shown in Fig. 4a–c, TEM images of the re-
presentative Cu2O/CuO-E (3) verify the expected microstructure com-
posed of a large quantity of nanoflakes. The enlarged HRTEM image in
Fig. 4d indicates that the particles were composed of a bi-component of
Cu2O and CuO. The lattice distance of 0.301 nm and 0.246 nm are as-
signed to the (110) plane and (111) plane of Cu2O, respectively, while
the lattice spacing of 0.186 nm and 0.232 nm are corresponding to
(200) plane and (111) plane of CuO, respectively. The particle size of
CuO was estimated to be about 10 nm. The TEM results are in good
consistent with those of XRD, and indicate that Cu2O and CuO were
well uniformly dispersed.
The high resolution Cu 2p and O 1s XPS spectra of Cu2O and Cu2O/
CuO-E (3) catalysts are shown in Fig. 5. The Cu2O species can be easily
identified by its weak satellite from Cu 2p spectra in Fig. 5a. The peaks
fitted at 932.4 eV and 952.2 eV are respectively assigned to Cu 2p3/2
and Cu 2p1/2, indicating that copper belongs to Cu+. The Cu 2p spectra
of Cu2O/CuO-E (3) (Fig. 5a) shows two deconvoluted Cu 2p3/2 peaks at
932.9 and 934.2 eV, corresponding to two components of Cu2O (Cu+
)
and CuO (Cu2+), respectively. It is indicated that the presence of both
CuO and Cu2O in composites. In addition, the main Cu 2p3/2 and Cu
2p1/2 for Cu+ are shifted toward higher binding energies when com-
pared with pure Cu2O, attributed to the incorporation of Cu2+ with a
higher binding energy values in compared with Cu+ [25]. Compared
with weak satellite in XPS Cu 2p spectra of Cu2O, shake-up satellite
peaks pointed at 944.8 and 963.4 eV in Cu 2p spectra of Cu2O/CuO-E
(3) were observed, which can be explained by different disparate or-
bital environment of CuO (3d9) and Cu2O (3d10). There are weak sa-
tellite peaks of Cu2O with full valence orbitals (3d10) because electrons
can not migrate to the higher order bound state. While CuO having
partially filled valence orbitals (3d9), the electronic migration to a
higher bound state can be achieved. The results further indicate that
there is an interaction between Cu2O and CuO. The O 1s XPS spectrum
in Fig. 5b shows that the two peaks of Cu2O at 530.4 and 531.7 eV are
referred to adsorbed oxygen species (Oads) and hydroxyl and carbonate
species, respectively, whereas the O1 s peak of Cu2O/CuO-E (3) is
convoluted into two main peaks centered at about 530.2 and 531.6 eV,
Fig. 3. SEM images of a,b) pure Cu2O; c, d) Cu2O/CuO-E (1); e, f) Cu2O/CuO-E (2); g, h) Cu2O/CuO-E (3); i, g) Cu2O/CuO-E (4).
4

C. Xie, et al.
MolecularCatalysis495(2020)111134
Fig. 4. a-c) TEM images and d) enlarged HRTEM of Cu2O/CuO-E (3).
originating from surface lattice oxygen (Olatt) and adsorbed oxygen
species (Oads), respectively.
TON numbers were defined (Table 2): TONb referred to the total
number of Cu sites detected by ICP-MS and TONs referred to the surface
number of Cu sites detected by XPS [39]. All TONs values were higher
than TONb values, attributing to the number of Cu sites exposed on the
sample surface was lower than the total number of Cu sites in sample.
The turnover frequency (TOF) at at low conversion (< 12 %) was cal-
culated on the basis of the number of Cu sites exposed on the sample
surface detected XPS. It is found that the TON and TOF value of Cu2O/
CuO-E (3) against the catalytic oxidation of cyclohexane are the highest
in comparison to other catalysts. It is well known that the presence of a
porous network in a catalyst increases the contact surface area and
hence enables a better diffusion of pollutants [40]. However, normal-
ized by the BET surface areas, the specific activity of various catalysts
prepared in this work show a significant difference and decrease in the
order Cu2O > Cu2O/CuO-E (1) > Cu2O/CuO-E (2) > Cu2O/CuO-E
(3) > Cu2O/CuO-E (4) > CuO-E, suggesting that the surface areas are
not the active site that controls the reaction rate. Generally speaking,
the cyclohexane oxidation in metal oxides follows a conventional
Haber-Weiss route with the major contribution from coordinatively
unsaturated sites [41]. Therefore, the high surface areas of mesoporous
metal oxides might not be the only reason for their good catalytic
performance, and the nature of copper oxide also plays an important
role.
Catalytic performance
The catalytic performance of the Cu2O, CuO and Cu2O/CuO com-
posites for cyclohexane oxidation reaction using TBHP as oxidizer were
investigated and summarized in Table 2. The blank run exhibited 24.2
% conversion of cyclohexane and negligible selectivity towards cyclo-
hexanone (Table 2, entry 1). It can be seen that Cu2O only obtained a
73.2 % conversion and 62.9 % selectivity towards cyclohexanone
(Table 2, entry 2), which was higher than those of CuO (Table 2, entry
3). The results indicated that both Cu+ and Cu2+ species were active
catalytic components, yet the Cu+ species exhibited higher catalytic
activity. It is noteworthy that the catalytic performance of Cu2O/CuO
grows sharply after introducing of CuO by alkali etching Cu2O. Mean-
while, with a further increase in proportion of CuO, a slightly decline in
both conversion and selectivity was observed. Especially, the highest
conversion (84.3 %) and cyclohexanone selectivity (77.0 %) were
achieved over the Cu2O/CuO-E (3) catalyst (Table 2, entry 4). To in-
vestigate the synergistic effect generated from co-existence of Cu+ and
Cu2+, the turn over number (TON) of catalysts was calculated, as-
suming all copper (Cu+ and Cu2+) species were the active sites. Two
5

C. Xie, et al.
MolecularCatalysis495(2020)111134
Fig. 6. a) Cyclohexane oxidation performance as a function of Cu2+/Cu+, b)
Correlation of the TONs of cyclohexanone with Cu2+/(Cu+ +Cu2+).
To further gain insight into the effect of the Cu valence state on
oxidation reactions, the effect of Cu2+/Cu+ on the catalytic perfor-
Fig. 5. XPS spectra of a) Cu 2p and b) O 1s of Cu2O and Cu2O/CuO-E (3).
mance and TONs of cyclohexanone as a function of Cu2+/(Cu+
+
Cu2+) were investigated (Fig. 6). It is notable that the TONs of
Table 2
Comparison of literature catalysts and our catalyst system for cyclohexane.a.
TOF (h−1
)
Specific activity (mmol m−2 h-1
)
Con. (%)d
Sele. (%)e
Cyone
Yield of Cyone (%)f
Ref.
b
c
Entry
Catalysts
TONb
TONsb
CyOH
Others
1
2
3
4
5
6
7
8
Blank test
Cu2O
CuO
—
—
—
—
24.2
73.2
61.4
70.4
84.1
84.3
78.1
47.0
40.0
28.0
67.3
78.0
—
13.3
14.6
17.1
14.4
13.4
3.5
86.7
22.5
30.5
23.6
20.7
19.5
23.0
—
—
6.3
7.9
—
—
This work
This work
This work
This work
This work
This work
This work
[28]
[29]
[31]
[42]
[43]
18.7
15.5
15.2
23.6
24.4
22.0
—
—
—
—
—
19.0
18.9
25.7
32.0
32.6
28.3
—
—
—
—
—
0.76
0.94
0.72
0.64
1.06
0.69
—
—
—
—
—
9.7
0.3
0.97
0.85
0.83
0.54
—
—
—
—
—
62.9
52.4
62.0
65.9
77.0
70.3
73.0
51.0
27.7
38.5
74.0
46.0
32.2
43.6
55.4
64.9
54.9
34.3
20.4
7.6
Cu2O/CuO-E (1)
Cu2O/CuO-E (2)
Cu2O/CuO-E (3)
Cu2O/CuO-E (4)
Cu(I) complex
Cu-Zn-BTC
Cu(I)-ZMS-5
CuCrOs
6.7
27.0
49.0
66.0
53.9
26.0
9
10
11
12
25.9
57.7
Cu-Pd/TiO2
Con., conversion; Sele., selectivity.
a
Typical reaction conditions: substrate (cyclohexane), 1 mL; catalyst, 0.02 g; cyclohexane: TBHP, 3 mL; reaction temperature, 70 °C; time, 18 h.
b
c
TONb = Moles of product yield/moles of Cu sites (ICP values) in catalyst. TONs = Moles of product yield/moles of external Cu sites (XPS values) in catalyst.
TOF = moles of product yield per mole of catalyst per hour) values under a kinetically controlled regime at 70 °C for 1 h.
Cyclohexane conversion = Moles of cyclohexane reacted/initial moles of cyclohexane×100 %.
d
e
Product selectivity = Moles of this product/Moles of each product×100 %, CyOH: cyclohexanol, Cyone: cyclohexanone, others: 2-Butanone, 3-methoxy-3-
methyl and1-Pentene, 4,4-dimethyl.
f
Yield of Cyone = Cyclohexane conversion × selectivity of Cyone/100.
6

C. Xie, et al.
MolecularCatalysis495(2020)111134
cyclohexanone increased steadily at low Cu2+/(Cu+ + Cu2+), reached
a maximum at Cu2+/(Cu+ + Cu2+) = 0.5, and decreased with further
increases in the ratio of Cu2+ (Fig. 6b). Therefore, the optimal TONs of
cyclohexanone on the Cu2O/CuO-E (3) catalyst lies in the high disperse
of Cu2+ site (Table 1) and the cooperative effect of Cu2+ and Cu+. In
addition, the excessive Cu2+ incorporated is unfavorable to improve
the catalytic activities including conversion and selectivity to cyclo-
hexanone. Therefore, an appropriate Cu+/Cu2+ ratio is a determined
factor for effective enhancement in catalytic properties of Cu2O for
cyclohexane oxidation. The catalytic performance of our Cu2O/CuO
composites catalysts were further compared with previous literature
reports on Cu based catalysts (Table 2, entries 8–12). These results give
a strong evidence that the active Cu+/Cu2+ species play a vital role in
the presence oxidation reaction. Compared with the reported Cu-based
catalysts, the Cu2O/CuO composites catalysts exhibits superior catalytic
properties for cyclohexane oxidation. Consequently, the Cu2O/CuO
composite is an excellent candidate for liquid phase cyclohexane oxi-
dation in the solvent-free system.
Fig. 8. Effect of reaction temperature on the oxidation of cyclohexane perfor-
mance over Cu2O/CuO-E (3).
The reusability and hot filtration experiment
Influence of reaction time
To identify whether the CuO/CuO-E (3) catalyst is a heterogeneous
catalyst, hot filtration testing was operated based on a modified process
[37]. No further reaction of filtrate after hot filtration was detected
compared to before the filtration, indicating that the active species
(copper) were barely leached out and the CuO/CuO-E (3) did act as a
heterogeneous catalyst.
The effect of the reaction time on cyclohexane oxidation over Cu2O/
CuO-E (3) was studied from 2 h–24 h and shown in Fig. 7. It is found
that the increasing reaction time from 2 h to 18 h generated a con-
tinuous and considerable increase of cyclohexane conversion from
50.8%–84.3% and cyclohexanone selectivity from 22.5%–77.0%, re-
spectively. In contrast, the selectivity towards cyclohexanol clearly
decreased from 27.5 % to 3.5 %. Additionally, with further increasing
reaction time, little change of cyclohexane conversion and a significant
decrease of cyclohexanone selectivity from 77.0%–69.4% were ob-
served, along with a decrease of cyclohexanol from 3.5 % to 2%. This is
due to the formation of deep oxidation products.
To verify the recycling performance of pure Cu2O and Cu2O/CuO-E
(3) catalyst, recycling test were carried out under optimized conditions
(i.e. 70 °C, 18 h, cyclohexane 1 mL, TBHP 3 mL) and the results are
shown in Fig. 9a,b. Notably, pure Cu2O catalyst suffered from marked
deactivation with a selectivity of the cyclohexanone decline from
68.6%–11.4% after the third run (Fig. 9b). In order to investigate the
reasons for the decrease of the catalytic activity of Cu2O, elemental
analysis of the catalyst before and after reaction were investigated by
ICP-MS, with the results as listed in Table 3. The Cu content in Cu2O
decreased by 20 % after the first run, suggesting that copper leaching
did occur, hence, affecting the reactivity of the catalyst upon sub-
sequent reuse. In contrast, Cu2O/CuO composites exhibited a very high
catalytic stability with no significant loss of the catalytic properties
after five cycles. Accordingly, the leaching loss of Cu is less than 0.6 wt.
% (Table 3), indicating that Cu active sites loss was effectively inhibited
in Cu2O/CuO-E (3) catalyst during reactions. This result was consistent
with that of fast hot catalyst filtration.
Effect of reaction temperature
Effect of the reaction temperature on oxidation reaction of cyclo-
hexane over Cu2O/CuO-E (3) was investigated in the range of
30−70 °C. The results in Fig. 8 showed strong dependent on reaction
temperature. When the reaction temperature was low (≤ 40 °C), the
conversion of cyclohexane increased slightly and there was almost no
target product (cyclohexanol or cyclohexanone) produced. With the
reaction temperature raising from 40 to 70 °C, both substrate conver-
sion and product (cyclohexanone) selectivity increased sharply and the
maximum values are obtained at 70 °C.
In order to confirm the structural stability of the catalysts, the re-
cycled catalysts were characterized by XRD and XPS and the results are
shown in Fig. 9c–h. The XPS composition analyses of fresh and utilized
samples are compiled in Table 3. Clearly, the XRD pattern of utilized
Cu2O (Fig. 9c) showed the characteristic diffraction peaks of Cu(II)
carbonate dihydroxide. The Cu 2p XPS spectra of utilized Cu2O (Fig. 9e)
show that the two peaks of Cu 2p3/2 and Cu 2p1/2 located at 934.8 eV
and 954.3 eV, respectively, were attributed to the Cu(II) carbonate
dihydroxide. Correspondingly, the O 1s peak of utilized Cu2O (Fig. S3)
located at 532.2 eV was attributed to hydroxyl and carbonate species.
According to the peaks area, the molar percentage of Cu+ and Cu2+
were calculated and listed in Table 3. It noted that a very high Cu2+
/
Cu+ molar ratio of 28 on the surface of utilized Cu2O was estimated.
The results suggested that the pure Cu2O was unstable in the reaction
system and large amount of Cu2O transformed into Cu(II) carbonate
dihydroxide in the first catalytic activity test. However, The XRD pat-
terns (Fig. 9d) and XPS spectra (Figs. 9g and S4) of the recycled Cu2O/
CuO-E (3) were similar to the fresh one, demonstrating that the Cu2O/
CuO composite has its compositions unchanged after reaction. Ac-
cording to Table 3, the utilized Cu2O/CuO-E (3) possesses a relatively
lower Cu+ content (19.0 %) compared with that in fresh catalysts (21.3
Fig. 7. Effect of reaction time on cyclohexane oxidation performance over
Cu2O/CuO-E (3).
7

C. Xie, et al.
MolecularCatalysis495(2020)111134
Fig. 9. Recyclability performance of a) pure Cu2O and b) Cu2O/CuO-E (3); c-d) XRD patterns and e-f) Cu 2p XPS spectra of catalysts (Cu2O and Cu2O/CuO-E (3))
before and after reaction.
%). 2% increase of Cu2+ content indicated that the transition from Cu+
to Cu2+ during the reaction stage can be effectively inhibited. These
results indicated that the leaching of Cu species and the oxidation
transformation of Cu+ into Cu2+ were responsible for the deactivation
of Cu2O during the catalytic reaction. It is also disclosed that the Cu2O/
CuO catalyst was stable and could be reused after the reaction.
radicals (Eqs. (6) and (7)), in a sequence that finally yields cyclohexanol
(CyOH) and cyclohexanone (Cy = O) (Eqs. (8) and (9)). Alternatively,
cyclohexanone and cyclohexanol can originate from the recombination
of two CyOOH (Eq. (10)). Cyclohexanol can also derive from insertion
of lattice oxygen from the metal oxide to the Cy% adsorbed over the
catalyst surface [41].
Overall, according to the report by Conte et al., the catalytic per-
formance of the metal oxides for cyclohexane oxidation mainly depends
on the coordination to unsaturated metal centers, adsorbed oxygen and
lattice oxygen from the metal oxides [41]. Here, owing to CuI co-
ordinatively unsaturated sites, the oxidation reactions are initiated
much faster over Cu2O and Cu2O/CuO in comparison to CuO, leading to
the rapid production of tBuO% radicals and decomposition of CyOOH.
This results in little accumulation of CyOOH in reaction system and
rapid progression all the way to CyOH (Eqs. (6) and (8)). In this regard,
we believe that pure Cu2O is more active than pure CuO for cyclo-
hexane oxidation. This result is full agreement with the experimental
results in catalytic performance.
The proposed reaction mechanism
A mechanistic pathway for cyclohexane oxidation over Cu2O/CuO
composites was proposed and depicted in Scheme 2. It is reported that
the reaction pathway follows a free-radical chain reaction mechanism
[44]. According to the Haber-Weiss cycle [41], the first step of the
oxidation is an initiation step and the initiation rate strongly depends
on the coordination to unsaturated metal centers [41]. For Cu2O/CuO
catalyst, the oxidant TBHP could be decomposed into active radicals
tBuO% and tBuOO% (Eqs. (1) and (2)) in the presence of copper species
(CuI/CuII). Subsequently, tBuO% radicals can extract H from cyclo-
hexane (Cy) to produce cyclohexyl radical (Cy%) (Eq. (4)). Afterwards,
Cy% would further interact with O2 which could come from air atmo-
sphere or the adsorbed oxygen on the surface of metal oxide, leading to
the generation of the peroxyl radical (CyOO%). CyOO% further performs
the H-abstraction from Cy to produce CyOOH and Cy% (Eq. (5)). Over
copper (CuI/CuII) sites, the CyOOH will decompose to CyO% and CyOO%
Moreover, according to the above results and discussion, we can
deduce the deactivation mechanism of Cu2O catalysts in cyclohexane
oxidation reaction (Scheme 3). The CuI catalyst is more active and ef-
fective for TBHP decomposition in initiation step. However, a higher
initiation rate over CuI sites will also lead to rapid production of OH−
.
If the consumption rate of OH− is slower than its generation rate, the
Table 3
ICP and XPS composition analyses of fresh and utilized Cu2O and Cu2O/CuO-E (3).
a
Catalyst
Cu Content(μg mL−1
)
Cu molar percentageb
Cu2+/Cu+ ratiob
O-bonding molar percentageb
Olatt/Oads ratiob
Cu2+
Cu+
Olatt
Oads
Fresh Cu2O
Utilized Cu2O
Fresh Cu2O/CuO-E (3)
Utilized Cu2O/CuO-E (3)
236.3
189.2
141.6
140.7
0.0
100.0
3.4
21.3
19.0
0
_
_
_
_
_
_
96.6
78.7
81.0
28:1
3.69:1
4.25:1
50.3
38.8
49.7
60.2
1:1
1:1.5
a
Determined by ICP−MS analysis.
Calculated by XPS analysis.
b
8

C. Xie, et al.
MolecularCatalysis495(2020)111134
Scheme 2. Proposed reaction equations for cyclohexane oxidation with TBHP, catalyzed by Cu2O/CuO catalyst.
excessive OH− would further reacted with as-generated CuII, finally
resulting in the formation of Cu2(OH)2CO3 and the deactivation of
Cu2O. For Cu2O/CuO composites, in initiation step, CuII sites in com-
posites can also facilitate the decomposition of TBHP to give H+ (Eq.
(2)) [45]. The H+ could not only accelerate the the deprotonation of
reactant substrate (Eq. (4)) [31], but also could consume excessive OH−
generated from Eq. (1), thus avoiding generation of Cu2(OH)2CO3 and
further inhibiting the deactivation of Cu2O. Meanwhile, the consump-
tion of OH− and H+ can also promote the reaction equilibrium shifting
toward the right side, leading to further accelerate the initiation rate.
This synergistic effect of CuI and CuII is the main cause of the higher
stability and catalytic performance of Cu2O/CuO than that of pure
Cu2O and pure CuO.
composites was a heterogeneous catalyst which can be reused at least
five times with almost the same activity. The Cu2+ and Cu+ species
were found to be well distributed on the surface of catalyst, leading to
an enhanced synergetic catalysis effect. An appropriate Cu+/Cu2+ ratio
provides a matched reaction speed on Cu2O and CuO nanodomain,
which thereby improved the catalytic performance and prevented the
Cu2O species from oxidative decomposition. The current study provide
a new strategy for designing high stability Cu2O-based catalyst with
potential applications in cyclohexane oxidation.
CRediT authorship contribution statement
Congjia Xie: Conceptualization, Methodology, Writing - original
draft. Wei Wang: Methodology, Supervision. Yepeng Yang:
Validation, Investigation. Liang Jiang: Conceptualization, Writing -
review & editing. Yongjuan Chen: Software. Jiao He: Resources,
Supervision. Jiaqiang Wang: Conceptualization, Writing - review &
editing.
Conclusions
We report a facile and promising method to stabilize Cu2O in liquid
phase cyclohexane oxidation reaction in the presence of TBHP. Cu2O/
CuO composites were successfully synthesized by mild etching of Cu2O
in NaOH solution. The as-prepared Cu2O/CuO-E (3) achieved higher
catalytic performance (84.3 % cyclohexane conversion, 77.0 % cyclo-
hexanone selectivity) than pure Cu2O and many reported Cu-based
catalysts. Hot filtration experiment proved that the Cu2O/CuO
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to
Scheme 3. Initiation step of cyclohexane oxi-
dation over Cu2O catalyst.
9

C. Xie, et al.
MolecularCatalysis495(2020)111134
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This work was supported by the National Natural Science
Foundation of China (21573193) and Key Research and Development
Plan of Yunnan Province (2018BA065). The authors also thank the
Industrialization Cultivation Project (2016CYH04) from Yunnan
Provincial Department of Education, Yunling Scholars of “Ten
Thousand Talents Program” of Yunnan Province, the Program for
Innovation Team of Yunnan Province, and Yunnan Provincial
Collaborative Innovation Center of Green Chemistry for Lignite Energy
for financial support. The authors also thank the Program for Science
and Technology Projects of Kunming (2019-1-N-25318000002603) for
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