Highly efficient hybrid cobalt-copper-aluminum layered double hydroxide/graphene nanocomposites as catalysts for the oxidation of alkylaromatics

Article abstract of DOI:10.1002/cctc.201500890

The selective oxidation of alkylaromatics is of vital importance for the production of high-added-value raw materials. The development of highly efficient heterogeneous catalytic oxidation systems under mild conditions has become an attractive research area. In this work, hybrid Co-Cu-Al layered double hydroxide/graphene (CoCuAl-LDH/graphene) nanocomposites, which were assembled successfully by a one-step coprecipitation route without the use of any additional reducing agents, were used as highly efficient catalysts for the liquid-phase selective oxidation of ethylbenzene using tert-butyl hydroperoxide as the oxidant. A series of characterizations revealed that graphene could stabilize CoCuAl-LDH nanoplatelets effectively in the nanocomposites, and in turn, highly dispersed CoCuAl-LDH could prevent the aggregation of the graphene nanosheets. By fine-tuning the mass ratio of graphene to CoCuAl-LDH, such nanocomposites offered a tunable catalytic oxidation performance. In particular, the nanocomposite with the graphene/CoCuAl-LDH mass ratio of 0.4:1 exhibited a remarkable catalytic performance with a considerable conversion (96.8 %) and selectivity to acetophenone (>95.0 %), which was mainly attributed to the synergism between the active CoCuAl-LDH component and the graphene matrix in the unique hetero-nanostructure. Moreover, the as-assembled nanocomposite catalysts displayed good recyclability and were active for the selective oxidation of other alkylaromatics. Mega results for nanocomposites: The liquid-phase selective oxidation of alkylaromatics is conducted successfully on well-dispersed hybrid Co-Cu-Al layered double hydroxide/graphene nanocomposites, which show an excellent catalytic performance attributable to the synergy between the active Co-Cu-Al layered double hydroxide component and the graphene matrix in the unique hetero-nanostructure of the nanocomposites.

Full text of DOI:10.1002/cctc.201500890

DOI: 10.1002/cctc.201500890
Full Papers
Highly Efficient Hybrid Cobalt–Copper–Aluminum Layered
Double Hydroxide/Graphene Nanocomposites as Catalysts
for the Oxidation of Alkylaromatics
Renfeng Xie, Guoli Fan, Lan Yang, and Feng Li*[a]
The selective oxidation of alkylaromatics is of vital importance
for the production of high-added-value raw materials. The de-
velopment of highly efficient heterogeneous catalytic oxidation
systems under mild conditions has become an attractive re-
search area. In this work, hybrid Co–Cu–Al layered double hy-
droxide/graphene (CoCuAl-LDH/graphene) nanocomposites,
which were assembled successfully by a one-step coprecipita-
tion route without the use of any additional reducing agents,
were used as highly efficient catalysts for the liquid-phase se-
lective oxidation of ethylbenzene using tert-butyl hydroperox-
ide as the oxidant. A series of characterizations revealed that
graphene could stabilize CoCuAl-LDH nanoplatelets effectively
in the nanocomposites, and in turn, highly dispersed CoCuAl-
LDH could prevent the aggregation of the graphene nano-
sheets. By fine-tuning the mass ratio of graphene to CoCuAl-
LDH, such nanocomposites offered a tunable catalytic oxida-
tion performance. In particular, the nanocomposite with the
graphene/CoCuAl-LDH mass ratio of 0.4:1 exhibited a remark-
able catalytic performance with a considerable conversion
(96.8%) and selectivity to acetophenone (>95.0%), which was
mainly attributed to the synergism between the active
CoCuAl-LDH component and the graphene matrix in the
unique hetero-nanostructure. Moreover, the as-assembled
nanocomposite catalysts displayed good recyclability and were
active for the selective oxidation of other alkylaromatics.
Introduction
Nowadays, the selective oxidation of inexpensive alkylaromat-
ics to high-added-value chemicals (e.g., ketones and alde-
hydes) is attracting more and more attention because of its im-
portance both in academic research and in a variety of indus-
trial and fine chemical processes.[1–3] For instance, acetophe-
none (AP), which is commonly produced by the selective oxi-
dation of ethylbenzene (EB), is an important intermediate for
the production of esters, alcohols, aldehydes, pharmaceuticals,
and resins. From both economic and environmental view-
points, efficient and environmentally benign heterogeneous
oxidation processes have become an attractive research
area.[4,5] Over the past decade, considerable endeavors have
been focused on the development of non-noble-metal hetero-
geneous catalysts that contain earth-abundant elements.
These catalysts are cost effective and tolerant to deactivation
in the oxidation of alkylaromatics. For the oxidation of EB to
produce AP, many heterogeneous catalysts, such as supported
metal complexes,[6–14] Mn-MCM-41,[15,16] Co-HMS,[17] M-APO-11
(M=Co, V, and Mn),[18] and CeAPO-5 molecular sieves,[19] have
been explored in recent years. Although several catalysts
showed a good catalytic activity, a high selectivity to AP and
a good stability are not easy to achieve.
Layered double hydroxides (LDHs) with the formula
3+
[M2+
M ]x/n·mH2O are a class of 2D anionic clay
x(OH)2][AnÀ
1Àx
materials.[20] In LDHs, divalent (e.g., Mg2+, Ni2+, Zn2+, Cu2+,
Co2+, Fe2+) and trivalent (e.g., Al3+, Fe3+, Cr3+, Ga3+, Mn3+)
metal cations prearrange in order in the layer lattices at the
atomic level. Recently, these low-cost materials have attracted
tremendous attention because they can be used widely as cat-
alyst precursors, catalysts, and supports.[21] For example, Mn- or
Cu-containing LDHs were found to be effective for the oxida-
tion of EB to AP.[22–25] In most cases, LDHs only showed moder-
ate catalytic activity. In addition, calcined Cr-containing LDHs
were active for the oxidation of EB.[26] However, the high toxici-
ty of Cr6+ poses serious risks to humans and the environment.
More recently, our group reported Co-containing mixed metal
oxides derived from LDH precursors that exhibited a good cat-
alytic performance in the oxidation of EB with a conversion of
ꢀ69% and an AP selectivity of ꢀ80%.[27] Therefore, the further
improvement of the catalytic performance of LDHs-based cata-
lysts remains a challenging task.
As the newest carbon nanomaterial, graphene with a 2D
honeycomb sp2 carbon lattice possesses fascinating physio-
chemical properties, which make it an excellent candidate for
the construction of high-performance graphene-based compo-
site nanomaterials.[28–30] As a catalyst support, graphene dis-
plays a variety of new functionalities in photocatalysis[31–34] and
electrocatalysis[35–38] thanks to its large specific surface area, ex-
cellent conductivity for electron capture and transport, and
[a] Dr. R. Xie, Dr. G. Fan, Prof. L. Yang, Prof. F. Li
State Key Laboratory of Chemical Resource Engineering
Beijing University of Chemical Technology
P. O. BOX 98, Beijing, 100029 (P.R. China)
Fax: (+86)10-64425385
E-mail: lifeng@mail.buct.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500890.
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electron interaction with catalytically active particles.[39] Over
the past five years, graphene-based nanocomposites that con-
tain metal nanoparticles and metal oxides have stimulated
more and more interest[40–45] because of the potential synergy
between the components and superior functionalities for vari-
ous applications as catalysts,[46,47] electrodes,[48,49] sensors,[50,51]
and so forth. Particularly, the dispersion of Ni- and Cr-contain-
ing LDH nanoparticles on graphene sheets can potentially pro-
vide a new way to develop dispersion-enhanced photocata-
lysts and electrode materials.[52–54]
and (113) diffractions of LDH materials. No (001) plane of GO
can be observed, which suggests the effective conversion of
GO to graphene in the final CoCuAl-LDH/graphene nanocom-
posites.
Raman spectroscopy was used to evaluate the graphitic mi-
crostructure of the samples. LDH/G-0.4 presents two main
Raman peaks (referred to as G and D bands) at n˜ =1596 and
1350 cmÀ1 (Figure 2), which correspond to the ordered graphit-
Herein, with the continuous aim to develop new, highly effi-
cient catalysts for the selective oxidation of alkylaromatics,
hybrid nanocomposites of CoCuAl-LDH and graphene, which
were constructed directly through a simple one-step coprecipi-
tation route without the use of any additional reducing
agents, were explored as catalysts in the liquid-phase selective
oxidation of EB to AP with tert-butyl hydroperoxide (TBHP) as
the oxidant because of their unique nanostructures and espe-
cially because of the high dispersion of the active CoCuAl-LDH
component on the graphene sheets and strong interaction be-
tween them. The as-assembled hybrid CoCuAl-LDH/graphene
nanocomposites exhibited a significantly enhanced activity
and selectivity to AP. Moreover, the relationship between the
nanostructure of the nanocomposites and their catalytic per-
formance was investigated.
Figure 2. Raman spectra of LDH/G-0.4 and GO.
Results and Discussion
Characterization of graphene-based nanocomposites
ic lattice vibration mode with E2g symmetry and the breathing
mode of K-point phonons of A1g symmetry for disordered
graphite, respectively.[57] In the case of GO, the Raman peak of
the G band appears at a lower wavenumber of 1580 cmÀ1, sug-
gestive of the conversion of graphite to graphene.[58] Notably,
the intensity ratio value of the D to G bands (ID/IG) increases
slightly from 0.98 for GO to 1.04 for LDH/G-0.4 because of the
reduction of GO that results in restored domains of conjugated
carbon atoms with more structural defects.[59] Furthermore,
FTIR spectra confirm the reduction of GO and the formation of
CoCuAl-LDH/graphene nanocomposites (Figure S1). Notably,
for LDH/G-0.4, no adsorption bands ascribed to the C=O and
CÀO vibrations (nC and nCÀO, respectively) are observed at
The XRD patterns of pure graphene oxide (GO) and CoCuAl-
LDH/graphene nanocomposites are shown in Figure 1. The
=
O
n˜ =1724 and 1421 cmÀ1,[60] which indicates the removal of
most of the O-containing functional groups from GO. These re-
sults demonstrate the successful assembly and hybridization of
CoCuAl-LDH with graphene by a facile coprecipitation route.
We also investigated the morphology and microstructure of
CoCuAl-LDH/graphene nanocomposites by SEM and TEM
measurements. GO shows a typical sheetlike morphology with
a wrinkle-like thin sheet and smooth surface (Figure 3). In the
representative LDH/G-0.4 sample, a large number of hexagonal
platelet-like LDH crystallites with a mean crystal domain of
ꢀ30–40 nm are decorated uniformly on the surface of trans-
parent and flexible graphene, which reflects the good affinity
between them.
Figure 1. XRD patterns of a) LDH/G-0.8, b) LDH/G-0.6, c) LDH/G-0.4, d) LDH/
G-0.3, e) LDH/G-0.2, and f) CoCuAl-LDH. The inset shows the XRD pattern of
pure GO.
XRD pattern of GO shows an intense (001) peak at 2qꢀ10.58,
which indicates the oxidation of graphite that involves the in-
troduction of O-containing functional groups between the
layers of graphite.[55,56] The XRD patterns of the CoCuAl-LDH/
graphene nanocomposites show the (003), (006), (012), (110),
Moreover, the thickness of the graphene sheets estimated
by AFM (Figure S2) is in the range of ꢀ0.74–1.08 nm, which is
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Figure 4. a) HAADF-STEM image of LDH/G-0.4, and element mapping
images of b) C-k, c) O-k, d) Co-k, e) Cu-k, and f) Al-k in LDH/G-0.4.
images of C, O, Co, Cu, and Al in LDH/G-0.4 depict the uniform
distribution of Co, Cu, and Al elements in the LDH nanoflakes
(Figure 4b–f).
Figure 3. SEM and TEM images of a, c) GO and b, d) LDH/G-0.4.
consistent with the bi- and trilayer structure of graphene layers
in view of the thickness of a single graphite layer (ꢀ0.34 nm).
Additionally, the CoCuAl-LDH nanoplatelets with an average
thickness of ꢀ9.0 nm are distributed over the surface of gra-
phene. The low-temperature N2 adsorption–desorption meas-
urements (Figure S3) reveal noticeable hysteresis loops, which
demonstrates the mesoporous nature of these nanocompo-
sites. The BET surface areas of all nanocomposites are smaller
than that of pure CoCuAl-LDH (Table S1) because of the curly
structure of the graphene nanosheets in the composites.
In the course of the assembly of the CoCuAl-LDH/graphene
nanocomposites, metal cations adsorbed electrostatically on
the GO surface firstly precipitate to form CoCuAl-LDH nuclei,
which is accompanied simultaneously by the reduction of GO
to graphene in the alkali medium. Subsequently, the new
CoCuAl-LDH nuclei grow gradually into larger platelet-like LDH
crystals with during the aging time. Finally, CoCuAl-LDH nano-
platelets are dispersed uniformly on the graphene surface. In-
terestingly, the introduction of CoCuAl-LDH nanoplatelets does
not change the morphology of the initial GO nanosheets.
Therefore, CoCuAl-LDH nanoplatelets may play a significant
role in the prevention of the restacking of the graphene
sheets, and graphene in turn provides sites of negative charge
to anchor the LDH nanoplatelets through electrostatic interac-
tion, which thus inhibits the agglomeration of the LDH nano-
platelets to a certain extent and facilitates the dispersion of re-
action sites.
XPS results confirm the existence of C, O, Co, Cu, and Al
without impurities in LDH/G-0.4 (Figure 5a). The C1s spectrum
of GO can be deconvoluted into four peaks centered at bind-
ing energies (BEs) of 289.2, 288.0, 286.9, and 284.5 eV (Fig-
ure 5b), which are assigned to carbon atoms in different func-
tional groups: OÀC=O, C=O, CÀO in CÀOÀC or CÀOÀH, and
CÀC (graphite; including CÀC, C=C, and CÀH), respectively.[61,62]
As a comparison, LDH/G-0.4 shows significantly reduced peak
intensities of the carbon atoms associated with O-containing
groups, especially epoxide and carbonyl functional groups, in-
dicative of the deoxygenation of GO. This observation demon-
strates the reduction of GO in the course of the assembly, and
a small amount of O-containing groups still remain on the gra-
phene surface. These remaining surface O-containing function-
al groups can provide anchoring sites for the LDH nanoplate-
lets and thus realize the stabilization of CoCuAl-LDH. Here,
NaOH can act as both the precipitant for the formation of
CoCuAl-LDH and the reactant for the deoxygenation of O-con-
taining groups in GO.[63]
In the Co2p XPS spectrum of CoCuAl-LDH (Figure 5c), two
peaks at BE=780.5 and 785.3 eV are attributed to Co2p3/2 and
a shake-up satellite peak for Co2+ species,[64] whereas the two
peaks at BE=796.6 and 802.6 eV are assigned to Co2p1/2 and
its satellite peak because paramagnetic Co2+ possesses
a strong satellite peak. In the case of CoCuAl-LDH, the fine
spectrum of Cu2p (Figure 5d) shows two broad peaks cen-
tered at BEꢀ933.4 and 942.2 eV, which are related to Cu2p3/2
and a shake-up satellite peak for Cu2+ ions, respectively. Com-
pared to those for pure CoCuAl-LDH, the BE values of Co2p
and Cu2p for LDH/G-0.4 shift slightly to higher values. It is
known that the BE shifts of elements are mainly associated
with the change in the chemical microenvironments of the ele-
ments.[65,66] The present positive shifts in the BEs of Cu2+ and
Co2+ species in the spectra of LDH/G-0.4 imply the existence of
interactions between positively charged brucite-like layers of
CoCuAl-LDH crystallites and partial negatively charged O-con-
Surface compositions and chemical states
The surface compositions and chemical states of the hybrid
nanocomposites were investigated by high-angle annular dark-
field scanning transmission electron microscopy (HAADF-STEM)
and X-ray photoelectron spectroscopy (XPS). HAADF-STEM elu-
cidates a high dispersion of CoCuAl-LDH nanoplatelets on the
graphene surface (Figure 4a), and the elemental mapping
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Figure 5. a) XPS survey spectrum of LDH/G-0.4, and b) C1s, c) Co2p, and d) Cu2p spectra of LDH/G-0.4 and CoCuAl-LDH.
taining groups of the graphene support because of
the highly hybridized nanostructure of the CoCuAl-
LDH/graphene composites. Such positive shifts of the
BEs of metal elements also can be found in other
graphene-based composites reported previously.[67–69]
Table 1. The results of the EB oxidation over different catalysts.[a]
Co[b]
[wt%] [wt%] [%]
Cu[b]
Conversion
Selectivity
[%]
TON
[mol/mol]
Selective oxidation of EB
Entry Catalyst
The selective oxidation of EB was chosen as a model
reaction to study the catalytic performance of hybrid
CoCuAl-LDH/graphene nanocomposites, and the cat-
alytic results using TBHP as the oxidant are summar-
ized in Table 1. The main products of the oxidation of
EB are AP, benzaldehyde (BA), and 1-phenylethanol
(1-PA), and no byproducts related to oxidation of the
aromatic ring are detected by GC–MS for all the reac-
tions. In the absence of catalyst, a low EB conversion
of 7.1% is obtained. Bare GO and graphene also
show a low activity for the oxidation of EB (13.5 and
9.0% conversion, respectively). In contrast, pristine
CoCuAl-LDH exhibits a much better catalytic per-
formance with an EB conversion of ꢀ68.8% and AP
selectivity of ꢀ88.1%. These results imply that Co2+
AP
BA
1-PA
1
2
3
4
5
6
7
8
9
10
blank
GO
–
–
–
10.9
9.0
8.4
7.7
6.9
6.1
–
–
–
–
4.8
4.0
3.7
3.5
3.1
2.8
–
7.1
13.5
9.0
68.8
69.7
92.2
96.8
87.8
76.9
52.4
40.5 38.2 21.3
16.3 69.5 14.2
29.0 54.3 16.7
88.1 10.3 1.6 23.2
86.0 12.1 1.9 27.8
94.7 3.8 1.3 43.5
95.4 4.2 0.4 50.1
87.7 10.7 1.6 46.9
78.1 19.7 2.2 41.1
–
–
–
graphene
CoCuAl-LDH
LDH/G-0.2
LDH/G-0.3
LDH/G-0.4
LDH/G-0.6
LDH/G-0.8
LDH+graphene[c]
81.6 11.8 6.6
–
[a] Reaction conditions: EB 10 mmol; TBHP 40 mmol; catalyst 0.1 g; reaction tempera-
ture 1208C; reaction time 12 h. [b] Determined by ICP-AES. [c] CoCuAl-LDH mixed
physically with pure graphene.
and Cu2+ species in the CoCuAl-LDH are the catalytically active
sites.
increases to 0.3:1, both the conversion and the selectivity to
AP are improved sharply. In particular, a remarkably high EB
conversion of 96.8% with a high AP selectivity of 95.4% is ach-
ieved over LDH/G-0.4. If the graphene/LDH mass ratio exceeds
0.4, the catalytic activity, however, begins to decrease rapidly,
as indicated by the decreased conversion from 96.8 over LDH/
G-0.4 to 76.9% over LDH/G-0.8, mainly because the excess gra-
Notably, the catalytic activity of hybrid CoCuAl-LDH/gra-
phene nanocomposites can be optimized by adjusting the
mass ratio of graphene to CoCuAl-LDH. With the gradual intro-
duction of graphene, the EB conversion is improved slightly in
the case of LDH/G-0.2. As the graphene/LDH mass ratio further
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phene inhibits the exposure of the active CoCuAl-LDH phase
and because of the decreased amounts of total active Co2+
and Cu2+ species. Compared with LDH/G-0.4, the physical mix-
ture of CoCuAl-LDH and graphene with the same graphene/
LDH mass ratio as that of LDH/G-0.4 shows a much lower activ-
ity. In addition, the turnover number (TON) for LDH/G-0.4 is
higher than that of the other composites (Table 1). These re-
sults demonstrate strongly that the synergistic effect between
the graphene matrix and the CoCuAl-LDH component in the
nanocomposites plays an important role to govern the catalyt-
ic performance. In addition, the reaction of entry 7 in Table 1
was further performed in a filtered solution. After 2 h reaction,
the catalyst was removed by hot filtration. Subsequently,
under identical conditions, the reaction continues, and the EB
conversion is increased by less than 3% after 10 h reaction.
This demonstrates that the reaction takes place mostly on the
heterogeneous surface of the CoCuAl-LDH/graphene nano-
composite.
p–p interactions and the close contact of EB with the adjacent
active Co and Cu species in the lattice structure of LDH anch-
ored onto the graphene are favorable for the oxidation of EB;
(ii) the nature of the good dispersion and small size of the
CoCuAl-LDH nanoplatelets is associated with the high catalytic
performance, in spite of a little aggregation of LDH particles;
(iii) the graphene support not only restricts the aggregation or
migration of CoCuAl-LDH nanoplatelets but also interacts
strongly with the active Co2+ and Cu2+ species to thus pro-
mote the activation of TBHP; (iv) in the selective oxidation of
EB, a certain amount of water can be produced. Fortunately,
the graphene nanosheets in the composites are beneficial for
the adsorption of weakly polar EB because of its hydrophobic
properties[75] but not for the adsorption of polar water. This is
a positive factor in the enhancement of the EB conversion.
Therefore, the synergistic effect between CoCuAl-LDH and gra-
phene in the nanocomposites can improve the catalytic per-
formance of hybrid nanocomposites in EB oxidation greatly.
In view of these experimental results and the studies of pio-
neers,[6,15,18,76] a plausible mechanism for the liquid-phase oxi-
dation of EB over as-assembled CuCoAl-LDH/graphene compo-
sites is proposed (Scheme 1). It is well known that the oxida-
tion of EB is supposed to occur through a free radical mecha-
nism to yield primarily the ethylbenzene hydroperoxide inter-
mediate.[76] Co2+ and Cu2+ species have been reported widely
as efficient catalytically active sites for a variety of oxidation re-
actions. In this work, these active electron-deficient metal spe-
cies in nanocomposites, as evidenced by the XPS results,
would be favorable for the adsorption of TBHP and thus pro-
mote the activation of TBHP to form ethylbenzene hydroperox-
ide. Firstly, TBHP can be activated by coordination to the im-
mobilized active Co2+ and Cu2+ species in LDHs to generate
tert-butyl oxygen radicals and hydroxyl radicals.[6] Secondly, the
a-H of EB reacts with the formed radicals to yield a-ethylben-
zene radicals. Subsequently, a-ethylbenzene radicals capture
the hydroxyl groups of TBHP to yield 1-PA.[15] Thirdly, the hy-
drogen atom of the hydroxyl group in 1-PA is captured more
easily by tert-butyl oxygen radicals than the hydrogen atom in
EB, and then 1-ethylbenzene oxygen radicals further capture
For the catalytic oxidation of EB using TBHP as the oxidant,
the catalytic performance of as-assembled LDH/G-0.4 is superi-
or to that of other catalysts reported previously
(Table S2),[12,27,70–74] despite the different reaction conditions.
We can see that over an excellent supported PdII nanoparticle
catalyst,[70] the EB conversion reaches 92.3% with a high AP se-
lectivity of 93.5% at 1308C for 12 h. Other Mn- and Co-contain-
ing heterogeneous catalysts only exhibit a moderate catalytic
activity,[12,71–74] and our Co-based catalyst reported previously is
less active than LDH/G-0.4.[27]
As a result, the CoCuAl-LDH/graphene nanocomposites are
highly efficient to catalyze the oxidation of EB to AP in terms
of both the activity and the selectivity to AP. In particular, LDH/
G-0.4 is more active than other composites. The BET surface
area of LDH/G-0.4 is similar to that of the other composites,
but the CoCuAl-LDH content is lower than that in LDH/G-0.2.
Therefore, the amount of the CoCuAl-LDH component in the
composites does not play a crucial role to achieve a high cata-
lytic performance. The excellent catalytic efficiency could result
from the following four factors: (i) the effective chemical ad-
sorption of EB molecules on the graphene surface through
Scheme 1. Proposed mechanism for the oxidation of EB over the CoCuAl-LDH/graphene nanocomposite with TBHP as the oxidant.
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the hydroxyl groups of fresh TBHP to yield ethylbenzene hy-
droperoxide.[18] Finally, the ethylbenzene hydroperoxide inter-
mediate may react in two different ways to form two final
products, AP and BA.
excess active sites are prone to adsorb AP in the presence of
a higher amount of catalyst, and thus AP may be converted
easily to BA.
Usually, heterogeneous nanoparticle catalysts suffer from
a slightly reduced catalytic activity caused by the aggregation
of nanoparticles during reactions. Consequently, the reusability
of the CoCuAl-LDH/graphene composites was investigated.
After the oxidation, the CoCuAl-LDH/graphene composites
could be separated easily by centrifugation. LDH/G-0.4 can
maintain its excellent catalytic activity without a clear loss of
catalytic performance after four consecutive cycles (Figure 7).
TEM of the used LDH/G-0.4 confirms that active CoCuAl-LDH
The effects of different reaction conditions on the selective
oxidation of EB over the LDH/G-0.4 composite are shown in
Figure 6. The reaction profile shows a rapid increase in both
the EB conversion and the AP selectivity in the initial 3 h reac-
tion period at 1208C, and the selectivities to BA and 1-PA de-
crease gradually (Figure 6a). A high EB conversion of 96.8%
with an AP selectivity of 95.4% is achieved at 12 h. Further-
more, if the reaction time is prolonged to 24 h, a slight reduc-
tion of the AP selectivity is observed. If the reaction tempera-
ture is elevated from 100 to 1208C, the EB conversion and AP
selectivity are improved because of the increased reaction rate
(Figure 6b). If the reaction temperature is increased to 1408C,
both the conversion and selectivity to AP decline. This can be
attributed to a faster decomposition of TBHP and enhanced
cleavage of the CÀC bond at elevated temperatures. If the EB/
TBHP molar ratio is increased from 1:3 to 1:4, the EB conver-
sion and AP selectivity increase from 85.1 and 80.6 to 96.8 and
95.4% (Figure 6c). A further increase in the molar ratio of EB/
TBHP only leads to a slight increase in the conversion and a de-
crease in the selectivity. In contrast, the selectivity to AP de-
creases to ꢀ91.7% because of the further conversion of AP to
BA in the presence of excess THBP. Both the EB conversion and
the AP selectivity over the LDH/G-0.4 are improved with an in-
creased catalyst loading from 0.005 to 0.1 g (Figure 6d). A fur-
ther increase in the catalyst loading from 0.1 to 0.2 g results in
a slight decrease of the AP selectivity. This may be because the
Figure 7. Recyclability of LDH/G-0.4 in the oxidation of EB. Reaction condi-
tions: EB 10 mmol, TBHP 40 mmol, catalyst 0.1 g, reaction temperature
1208C, reaction time 12 h.
Figure 6. Effects of the reaction parameters on the oxidation of EB over LDH/G-0.4: a) reaction time, b) reaction temperature, c) EB/TBHP molar ratio, and
d) catalyst loading.
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crystals still are stabilized on the graphene surface and no
clear agglomeration of LDH particles on the graphene surface
occurs (Figure S4). The high stability of LDH/G-0.4 originates
mainly from the strong interaction between the active LDH
component and the graphene matrix.
Conclusions
Hybrid Co–Cu–Al layered double hydroxide (CoCuAl-LDH)/gra-
phene nanocomposites were assembled by a facile one-step
coprecipitation approach. A series of characterizations revealed
that small CoCuAl-LDH nanoplatelets were well dispersed on
the surface of the graphene nanosheets. The as-assembled
CoCuAl-LDH/graphene composite catalyst with a graphene/
CoCuAl-LDH mass ratio of 0.4:1 exhibited the best catalytic ac-
tivity towards the selective oxidation of ethylbenzene with a re-
markably high selectivity to acetophenone (95.4%) at 96.8%
conversion. The synergy between the CoCuAl-LDH and gra-
phene in the hetero-nanostructured composites, which could
lead to the high dispersion of active Co2+ and Cu2+ species on
the graphene surface and strong interactions between them,
was proved to be responsible for the significantly enhanced
catalytic performance. The as-assembled CoCuAl-LDH/gra-
phene composite catalyst displayed a good recyclability and
could be reused without a remarkable loss of either activity or
selectivity at least four times. Such a robust CoCuAl-LDH/gra-
phene composite with an excellent catalytic performance and
high stability might become a promising candidate for poten-
tial commercial applications in the liquid-phase oxidation of al-
kylaromatics to produce industrially important intermediate
chemicals.
Oxidation of other alkylaromatics and alkanes
We explored the possibility to use CoCuAl-LDH/graphene com-
posites for the oxidation of other alkylaromatics and alkanes
that contain primary, secondary, or tertiary CÀH bonds under
the similar reaction conditions that we used for the oxidation
of EB. Toluene and cumene, which have primary or tertiary
CÀH bonds, can be oxidized to the corresponding aldehydes,
ketones, or acids with moderate to high conversions over
LDH/G-0.4 (Table 2, entries 1 and 2). LDH/G-0.4 also promotes
the oxidation of alkylaromatics that contain secondary CÀH
bonds (1,2,3,4-tetrahydronaphthalene and diphenylmethane)
with high conversions and selectivities to ketones (Table 2, en-
tries 3 and 4). Moreover, LDH/G-0.4 is active for the oxidation
of nonaromatic methylcyclohexane (Table 2, entry 5). These re-
sults indicate the wide applicability of the nanocomposites for
the oxidation of alkylaromatics and nonaromatic alkanes. In
future, the reaction conditions will be optimized to attain
higher yields of the target products.
Experimental Section
Table 2. Selective oxidation of other alkylaromatics and alkanes over LDH/G-0.4.[a]
Assembly of CoCuAl-LDH/graphene nano-
composites
Entry Substrate
Conversion [%] Selectivity [%]
GO was synthesized from natural graphite
powder by
a
modified Hummers method.[77]
CoCuAl-LDH/graphene nanocomposites with dif-
ferent graphene contents were assembled direct-
ly by the coprecipitation method. Typically,
1
19.4
Co(NO3)2·6H2O
(3.75 mmol),
Cu(NO3)2·9H2O
(1.25 mmol), and Al(NO3)3·9H2O (1.25 mmol) were
added to deionized water (25 mL) with a certain
amount of GO and then dissolved ultrasonically
for 30 min. Subsequently, a base solution of
NaOH (0.2m) and Na2CO3 (0.05m) were added
dropwise into the above salt solution under vigo-
rous stirring until the solution became pH 10.0.
The resulting suspension was aged at 908C for
6 h, and the solid was collected by filtration,
washed with deionized water, and dried at 608C
for 24 h in vacuum oven. The obtained products
are denoted as LDH/G-x (x=0.2, 0.3, 0.4, 0.6, 0.8),
in which x is the mass ratio of graphene to LDH
in the nanocomposites. For comparison, pure
CoCuAl-LDH (or graphene) was synthesized with-
out the addition of GO (or metal salts) under the
above experimental conditions.
2
3
84.8
98.1
4
5
97.3
37.0
Characterization
XRD patterns of samples were collected by using
a Rigaku D/Max-RB diffractometer with a graph-
ite-filtered CuKa source (l=0.15418 nm).
[a] Reaction conditions: substrate 10 mmol, TBHP 40 mmol, catalyst 0.1 g, reaction tempera-
ture 1208C, reaction time 12 h.
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Elemental analysis was performed by using a Shimadzu ICPS-7500
inductively coupled plasma atomic emission spectrometer (ICP-
AES) after the samples were dissolved in dilute HCl (6m).
and the Specialized Research Fund for the Doctoral Program of
Higher Education (20120010110012).
The morphology of the samples was investigated by using a field-
emission scanning electron microscope (FE-SEM, Zeiss Supra 55).
TEM was performed by using a JEM-2100 Electron microscope
(JEOL, Japan) operated at an accelerating voltage of 200 kV.
HAADF-STEM images were recorded by using a JEOL 2010F instru-
ment.
Keywords: cobalt
oxidation · supported catalysts
·
graphene
·
layered compounds
·
[1] A. E. Shilov, G. B. Shul’pin, Chem. Rev. 1997, 97, 2879–2932.
[2] M. S. Chen, M. C. White, Science 2010, 327, 566–571.
[3] X.-H. Li, J.-S. Chen, X. Wang, J. Sun, A. Markus, J. Am. Chem. Soc. 2011,
133, 8074–8077.
RT FTIR spectra were recorded by using a Vector 22 spectrometer
from Bruker.
Raman spectroscopy was conducted by using a Jobin Yvon Horiba
HR800 spectrometer using a 532 nm line of Ar+ ion laser as the ex-
citation source.
[4] T. Mallat, A. Baiker, Chem. Rev. 2004, 104, 3037–3058.
[5] V. R. Choudhary, P. A. Chaudhari, V. S. Narkhede, Catal. Commun. 2003,
4, 171–175.
[6] M. Arshadi, M. Ghiaci, A. A. Ensafi, H. Karimi-Maleh, S. L. Suib, J. Mol.
Catal. A 2011, 338, 71–83.
AFM was performed by using a DI nanoscope IV using a tapping
mode.
[7] D. Habibi, A. R. Faraji, M. Arshadi, S. Heydari, A. Gil, Appl. Catal. A 2013,
466, 282–292.
Low-temperature N2 adsorption–desorption isotherms of the sam-
ples were obtained by using a Micromeritics ASAP 2020 sorptome-
ter apparatus at À1968C. The total specific surface areas were eval-
uated by the multipoint BET method.
[8] R. Wang, B. Gao, W. Jiao, Appl. Surf. Sci. 2009, 255, 4109–4113.
[9] K. O. Xavier, J. Chacko, K. K. M. Yusuff, Appl. Catal. A 2004, 258, 251–
259.
[10] K. O. Xavier, J. Chacko, K. K. M. Yusuff, J. Mol. Catal. A 2002, 178, 275–
281.
XPS was recorded by using a Thermo VG ESCALAB2201-XL X-ray
photoelectron spectrometer using AlKa X-rays (1486.6 eV) as the
excitation source.
[11] Supported Reagents and Catalysts in Chemistry (Eds.: J. H. Clark, S. Evans,
J. R. L. Smith, B. K. Hodnett, A. P. Keybett, J. H. Clark, K. Smith), Royal So-
ciety of Chemistry, Cambridge, 1998, p. 216.
[12] M. Arshadi, M. Ghiaci, Appl. Catal. A 2011, 399, 75–86.
[13] D. Habibi, A. R. Faraji, Appl. Surf. Sci. 2013, 276, 487–496.
[14] D. Habibi, A. R. Faraji, M. Arshadi, H. Veisi, A. Gil, J. Mol. Catal. A 2014,
382, 41–54.
Catalytic tests
The oxidation reaction was performed in a glass reactor equipped
with a condenser and a magnetic stirrer. Typically, substrate
(10 mmol) and a certain amount of catalyst were added to a 10 mL
one-necked flask, which was immersed in an oil bath, and then
70% aqueous TBHP with a certain substrate/TBHP molar ratio was
added dropwise over 15 min. The reactor was heated to a certain
temperature for a period of time, and the magnetic stirring speed
was set at 900 rpm. After the reaction, the catalyst was separated
by centrifugation, and the reaction mixture was dehydrated with
anhydrous sodium sulfate. The products were analyzed quantita-
tively by GC (Agilent GC-7890B equipped with a DB-WAX capillary
column; 30.0 m”320 mm”0.25 mm) with a flame ionization detec-
tor by the internal standard method using bromobenzene. All of
the mass balances were above 96%. The injector temperature and
detector temperature were set at 2508C. The column temperature
was increased from 100 to 1208C with a ramp rate of 58CminÀ1
and then to 2008C with a ramp rate of 208CminÀ1 and held at
2008C for 1 min. The products were further identified by GC–MS
(Shimadzu QP 2010 with a DB-5 column). The TON was calculated
based on the number of moles of AP product obtained per mole
of Co and Cu. Furthermore, the leaching of the metal during the
course of the reaction was verified by subjecting the filtrate to an-
other reaction under identical conditions. To investigate the reusa-
bility of the catalysts, the catalyst was removed from the reaction
mixtures, washed with ethyl alcohol and acetone, and dried at
708C for 12 h. Then, fresh substrate and the recovered catalyst
were employed in the repeated tests.
[15] K. M. Parida, S. S. Dash, J. Mol. Catal. A 2009, 306, 54–61.
[16] S. Vetrivel, A. Pandurangan, Appl. Catal. A 2004, 264, 243–252.
[17] S. S. Bhoware, S. Shylesh, K. R. Kamble, A. P. Singh, J. Mol. Catal. A 2006,
255, 123–130.
[18] P. S. Singh, K. Kosuge, V. Ramaswamy, B. S. Rao, Appl. Catal. A 1999, 177,
149–159.
[19] S. Devika, M. Palanichamy, V. Murugesan, Appl. Catal. A 2011, 407, 76–
84.
[20] D. G. Evans, R. C. T. Slade, Struct. Bonding 2006, 119, 1–87.
[21] G. Fan, F. Li, D. G. Evans, X. Duan, Chem. Soc. Rev. 2014, 43, 7040–7066.
[22] V. R. Choudhary, J. R. Indurkar, V. S. Narkhede, J. Catal. 2004, 227, 257–
261.
[23] S. K. Jana, Y. Kubota, T. J. Tatsumi, J. Catal. 2007, 247, 214–222.
[24] K. Bahranowski, R. Dula, M. Gasior, Appl. Clay Sci. 2001, 18, 93–101.
[25] B. Monteiro, S. Gago, S. S. Balula, A. A. Valente, I. S. Goncalves, M. Pillin-
ger, J. Mol. Catal. A 2009, 312, 23–30.
[26] S. Vetrivel, A. Pandurangan, Ind. Eng. Chem. Res. 2005, 44, 692–701.
[27] R. Xie, G. Fan, L. Yang, F. Li, Catal. Sci. Technol. 2015, 5, 540–548.
[28] X. Huang, X. Qi, F. Boey, H. Zhang, Chem. Soc. Rev. 2012, 41, 666–686.
[29] Y. Zhu, S. Murali, W. Cai, X. Li, J. W. Suk, J. R. Potts, R. S. Ruoff, Adv.
Mater. 2010, 22, 3906–3924.
[30] X. Huang, Z. Yin, S. Wu, X. Qi, Q. He, Q. Zhang, Q. Yan, F. Boey, H.
Zhang, Small 2011, 7, 1876–1902.
[31] W. Tu, Y. Zhou, Q. Liu, Z. Tian, J. Gao, X. Chen, H. Zhang, J. Liu, Z. Zou,
Adv. Funct. Mater. 2012, 22, 1215–1221.
[32] W. Fan, Q. Zhang, Y. Wang, Phys. Chem. Chem. Phys. 2013, 15, 2632–
2649.
[33] N. Zhang, Y. Zhang, Y-J. Xu, Nanoscale 2012, 4, 5792–5813.
[34] P. Madhusudan, J. Yu, W. Wang, B. Cheng, G. Liu, Dalton Trans. 2012, 41,
14345–14353.
[35] Q. Xiang, J. Yu, M. Jaroniec, Chem. Soc. Rev. 2012, 41, 782–796.
[36] Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, H. Dai, Nat. Mater.
2011, 10, 780–786.
[37] X. Cao, Z. Yin, H. Zhang, Energy Environ. Sci. 2014, 7, 1850–1865.
[38] Z. Yin, J. Zhu, Q. He, X. Cao, C. Tan, H. Chen, Q. Yan, H. Zhang, Adv.
Energy Mater. 2014, 4, 1300574.
Acknowledgements
We are grateful for financial support from the 973 Program
(No.2011CBA00506), National Natural Science Foundation of
China, Fundamental Research Funds for the Central Universities,
[39] E. J. Yoo, T. Okata, T. Akita, M. Kohyama, J. Nakamura, I. Honma, Nano
Lett. 2009, 9, 2255–2259.
ChemCatChem 2016, 8, 363 – 371
www.chemcatchem.org
370
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
[40] R. Kou, Y. Shao, D. Mei, Z. Nie, D. Wang, C. Wang, V. V. Viswanathan, S.
Park, I. A. Aksay, Y. Lin, Y. Wang, J. Liu, J. Am. Chem. Soc. 2011, 133,
2541–2547.
[41] E. Bertolucci, R. Bacsa, A. Benyounes, A. M. Raspolli-Galletti, M. R. Axet,
P. Serp, ChemCatChem 2015, 7, 2971–2978.
[58] S. D. Perera, R. G. Mariano, K. Vu, N. Nour, O. Seitz, Y. Chabal, K. J. Bal-
kus, Jr., ACS Catal. 2012, 2, 949–956.
[59] T. N. Lambert, C. A. Chavez, B. Hernandez-Sanchez, P. Lu, N. S. Bell, A.
Ambrosini, T. Friedman, T. J. Boyle, D. R. Wheeler, D. L. Huber, J. Phys.
Chem. C 2009, 113, 19812–19823.
[60] W. Zhang, W. He, X. Jing, J. Phys. Chem. B 2010, 114, 10368–10373.
[61] T. N. Narayanan, Z. Liu, P. R. Lakshmy, W. Gao, Y. Nagaoka, D. S. Kumar, J.
Lou, R. Vajtai, P. M. Ajayan, Carbon 2012, 50, 1338–1345.
[62] D. C. Marcano, D. V. Kosynkin, J. M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev,
L. B. Alemany, W. Lu, J. M. Tour, ACS Nano 2010, 4, 4806–4814.
[63] X. Fan, W. Peng, Y. Li, X. Li, S. Wang, G. Zhang, F. Zhang, Adv. Mater.
2008, 20, 4490–4493.
[64] C. Altavilla, E. Ciliberto, Appl. Phys. A 2004, 79, 309–314.
[65] K. Y. Lee, M. Kim, J. Hahn, J. S. Suh, I. Lee, K. Kim, S. W. Han, Langmuir
2006, 22, 1817–1821.
[42] S. Deng, V. Tjoa, H. M. Fan, H. R. Tan, D. C. Sayle, M. Olivo, S. Mhaisalkar,
J. Wei, C. H. Sow, J. Am. Chem. Soc. 2012, 134, 4905–4917.
[43] X. Ji, X. Niu, B. Li, Q. Han, F. Yuan, F. Zaera, Y. Zhu, H. Fu, ChemCatChem
2014, 6, 3246–3253.
[44] B. Sun, Z. Jiang, D. Fang, K. Xu, Y. Pei, S. Yan, M. Qiao, K. Fan, B. Zong,
ChemCatChem 2013, 5, 714–719.
[45] X. Cao, B. Zheng, X. Rui, W. Shi, Q. Yan, H. Zhang, Angew. Chem. 2014,
126, 1428–1433.
[46] M. Petroselli, P. Franchi, M. Lucarini, C. Punta, L. Melone, ChemSusChem
2014, 7, 2695–2703.
[66] Y. T. Kim, T. Mitani, J. Catal. 2006, 238, 394–401.
[67] Y. Tian, Y. Liu, F. Pang, F. Wang, X. Zhang, Colloids Surf. A 2015, 464, 96–
103.
[68] P. Shi, X. Dai, H. Zheng, D. Li, W. Yao, C. Hu, Chem. Eng. J. 2014, 240,
264–270.
[69] Q. Zhang, C. Tian, A. Wu, T. Tan, L. Sun, L. Wang, H. Fu, J. Mater. Chem.
2012, 22, 11778–11784.
[70] M. Ghiaci, Z. Sadeghi, M. E. Sedaghat, H. Karimi-Maleh, J. Safaei, A. Gil,
Appl. Catal. A 2010, 381, 121–131.
[71] S. Vetrivel, A. Pandurangan, J. Mol. Catal. A 2004, 217, 165–174.
[72] S. S. Bhoware, A. P. Singh, J. Mol. Catal. A 2007, 266, 118–130.
[73] G. Satish Kumar, M. Palanichamy, M. Hartmann, V. Murugesan, Catal.
Commun. 2007, 8, 493–497.
[74] K. George, S. Sugunan, Catal. Commun. 2008, 9, 2149–2153.
[75] Y. J. Shin, Y. J. Wang, H. Huang, G. Kalon, A. T. S. Wee, Z. Shen, C. S.
Bhatia, H. Yang, Langmuir 2010, 26, 3798–3802.
[47] H. Qrareya, D. Ravelli, M. Fagnoni, A. Albini, Adv. Synth. Catal. 2013, 355,
2891–2899.
[48] W. Zhou, X. Cao, Z. Zeng, W. Shi, Y. Zhu, Q. Yan, H. Liu, J. Wang, H.
Zhang, Energy Environ. Sci. 2013, 6, 2216–2221.
[49] X. Cao, Y. Shi, W. Shi, G. Lu, X. Huang, Q. Yan, Q. Zhang, H. Zhang, small
2011, 7, 3163–3168.
[50] G. Lu, H. Li, C. Liusman, Z. Yin, S. Wu, H. Zhang, Chem. Sci. 2011, 2,
1817–1821.
[51] X. Cao, Z. Zeng, W. Shi, P. Yep, Q. Yan, H. Zhang, Small 2013, 9, 1703–
1707.
[52] M. Li, J. Zhu, L. Zhang, X. Chen, H. Zhang, F. Zhang, S. Xu, D. G. Evans,
Nanoscale 2011, 3, 4240–4246.
[53] M. Lan, G. Fan, L. Yang, F. Li, Ind. Eng. Chem. Res. 2014, 53, 12943–
12952.
[54] B. Li, Y. Zhao, S. Zhang, W. Gao, M. Wei, ACS Appl. Mater. Interfaces
2013, 5, 10233–10239.
[55] B. F. Machado, P. Serp, Catal. Sci. Technol. 2012, 2, 54–75.
[56] X. Huo, J. Liu, B. Wang, H. Zhang, Z. Yang, X. She, P. Xia, J. Mater. Chem.
A 2013, 1, 651–656.
[76] J. D. Chen, R. A. Sheldon, J. Catal. 1995, 153, 1–8.
[77] W. S. Hummers, R. E. Offeman, J. Am. Chem. Soc. 1958, 80, 1339–1339.
[57] X. J. Zhou, J. L. Zhang, H. X. Wu, H. J. Yang, J. Y. Zhang, S. W. Guo, J.
Phys. Chem. C 2011, 115, 11957–11961.
Received: August 9, 2015
Published online on November 24, 2015
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