Solvent-free oxidation of ethylbenzene over hierarchical flower-like core-shell structured Co-based mixed metal oxides with significantly enhanced catalytic performance

Article abstract of DOI:10.1039/c4cy00744a

Herein, we reported the development of new and cost-effective cobalt-based metal oxide catalysts for the oxidation of ethylbenzene, which is considered to be of much importance for the production of high value-added raw materials. The heterogeneous Co-based catalyst system, hierarchical flower-like core-shell structured Co-Zn-Al mixed metal oxides supported on alumina (CoZnAl-MMO/Al₂O₃), was reproducibly prepared by a two-step process, which involved in situ growth of a two-dimensional Co-Zn-Al layered double hydroxide precursor on amorphous alumina microspheres followed by calcination. The materials were characterized by XRD, SEM, TEM, HRTEM, TPR, XPS and nitrogen adsorption-desorption measurement. The results revealed that CoZnAl-MMO/Al₂O₃ catalysts exhibited high dispersion of cobalt species due to well-developed three-dimensional flower-like CoZnAl-MMO platelets as well as the separating effect of the resulting ZnO phase. As-synthesized CoZnAl-MMO/Al₂O₃ catalysts were studied in the oxidation of ethylbenzene without the addition of any solvent and additive using tert-butyl hydroperoxide as the oxygen source and showed much higher catalytic activity and selectivity for acetophenone compared with the conventional supported Co-based catalyst prepared by incipient impregnation. Furthermore, such cost-effective CoZnAl-MMO/Al₂O₃ catalysts possessed high stability and could be reused at least three times without remarkable loss of the catalytic activity.

Full text of DOI:10.1039/c4cy00744a

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Solvent-free oxidation of ethylbenzene over
hierarchical flower-like core–shell structured
Co-based mixed metal oxides with significantly
enhanced catalytic performance†
Cite this: DOI: 10.1039/c4cy00744a
*
Renfeng Xie, Guoli Fan, Lan Yang and Feng Li
Herein, we reported the development of new and cost-effective cobalt-based metal oxide catalysts for the
oxidation of ethylbenzene, which is considered to be of much importance for the production of high
value-added raw materials. The heterogeneous Co-based catalyst system, hierarchical flower-like core–shell
structured Co–Zn–Al mixed metal oxides supported on alumina IJCoZnAl-MMO/Al2O3), was reproducibly
prepared by a two-step process, which involved in situ growth of a two-dimensional Co–Zn–Al layered
double hydroxide precursor on amorphous alumina microspheres followed by calcination. The materials
were characterized by XRD, SEM, TEM, HRTEM, TPR, XPS and nitrogen adsorption–desorption measurement.
The results revealed that CoZnAl-MMO/Al2O3 catalysts exhibited high dispersion of cobalt species due to
well-developed three-dimensional flower-like CoZnAl-MMO platelets as well as the separating effect of
the resulting ZnO phase. As-synthesized CoZnAl-MMO/Al2O3 catalysts were studied in the oxidation of
ethylbenzene without the addition of any solvent and additive using tert-butyl hydroperoxide as the
oxygen source and showed much higher catalytic activity and selectivity for acetophenone compared
with the conventional supported Co-based catalyst prepared by incipient impregnation. Furthermore,
such cost-effective CoZnAl-MMO/Al2O3 catalysts possessed high stability and could be reused at least
three times without remarkable loss of the catalytic activity.
Received 6th June 2014,
Accepted 16th September 2014
DOI: 10.1039/c4cy00744a
www.rsc.org/catalysis
as well as high cost and easy deactivation, limits their large-scale
practical industrial applications.
Introduction
The controlled partial oxidation of alkanes to produce desired
products including alcohols, ketones and carboxylic acids,
which are key building blocks in organic synthesis, is one of
the most important industrial reactions. For instance, selec-
tive oxidation of ethylbenzene (EB) is of much importance for
the production of high value-added acetophenone (AP),1,2
which is a useful raw material for the production of perfumes,
pharmaceuticals, resins and alcohols. In this attractive field,
numerous methods have been developed by using different
oxidizing agents (e.g., molecular oxygen,3–5 hydrogen peroxide,6
tert-butyl hydroperoxide (TBHP)7–9) and a variety of metal cata-
lysts. As for the oxidation of EB, some homogeneous catalysts,
such as metal acetylacetonates1 and metalloporphyrins,10,11
give excellent activity and selectivity.12 Despite their advantages,
the problem of separation and recovery of homogenous catalysts,
Over the past decade, considerable attention has been
drawn to the exploration of clean and environmentally friendly
routes for selective oxidation of alkanes relying on a wide
variety of heterogeneous catalyst systems, which can make
the catalysts easy to recycle and regenerate. Recently, several
heterogeneous catalysts, such as Mn-containing complexes
supported on alumina/silica,13–15 supported Co-containing
complexes,16–20 Mn-MCM-41,21,22 Cr-MCM-41,4 Co-HMS,23
M-APO-11 (M = Co, Mn and V),24 CeAPO-5 molecular sieves,25
hybrid metalloporphyrin@silica nanocomposite microspheres26
and Mn-, Cu-containing hydrotalcites,3,5,6,27 were reported
for the oxidation of EB to AP. Among them, some catalysts
have showed either good selectivity or good activity in the
reaction, but the ability to achieve both excellent activity and
high selectivity is more limited. Meanwhile, the practical
application of some catalysts containing metal complexes is
severely limited by their poor thermal stability under oxida-
tive conditions.
State Key Laboratory of Chemical Resource Engineering, Beijing University of
Chemical Technology, P. O. BOX 98, Beijing, 100029, PR China.
E-mail: lifeng@mail.buct.edu.cn; Fax: +8610 64425385; Tel: +8610 64451226
† Electronic supplementary information (ESI) available: Additional experimental
results. See DOI: 10.1039/c4cy00744a
Layered double hydroxides (LDHs, ijM1−x2+Mx3+(OH)2]x+
[Ax/n]
n−·mH2O) belong to a class of highly ordered two-
dimensional (2D) anionic clay materials.28,29 Their structure
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is based on M2+(OH)6 octahedral units sharing edges to build
M(OH)2 brucite-like layers in which trivalent cations can
replace some of the divalent ions giving positively charged
sheets, and the charge is balanced by intercalated anions in
the hydrated interlayer regions.30,31 After calcination at inter-
mediate temperatures (450–600 °C), LDHs are converted into
highly active mixed metal oxides (MMO) with good thermal
stability, large surface area, and high metal dispersion.32–35
Although some LDHs could catalyze the oxidation of EB,3,5,27
they exhibited moderate catalytic activity. Also, MMO derived
from CuNiCr-LDHs could effectively catalyze the oxidation of
EB with 60% conversion and 65% selectivity for AP at 70 °C
for 8 h.9 The toxicity of such a type of Cr-containing catalysts,
however, may cause severe environmental pollution.
Fig.
1
XRD patterns of CoZnAl-LDH/Al2O3 precursors (A) and
CoZnAl-MMO/Al2O3 (B).
Because of its excellent stability, good accessibility, high
specific surface area, and excellent physical strength, alumina
has been extensively used as a catalyst support. Although the
dispersion of active metal components on alumina is often dis-
cussed in the literature, effective techniques to enhance metal
dispersion are rarely reported. On the other hand, increasing
environmental concerns with chemical production require
a greener and energy-efficient process.36 Correspondingly,
the use of neat substrates without any solvent and additive in
the field of heterogeneous catalysis, which can save resources
and energy, particularly is a subject of considerable interest
from the standpoint of environmentally benign production.37,38
Therefore, in the present work, we synthesized a new hetero-
geneous catalyst system consisting of hierarchical flower-like
core–shell structured CoZnAl-MMO supported on amorphous
alumina IJCoZnAl-MMO/Al2O3) by a two-step process, which
involved in situ growth of 2D CoZnAl-LDH platelets on alumina
microspheres and the following calcination. The catalytic per-
formance of as-synthesized CoZnAl-MMO/Al2O3 catalysts was
investigated in liquid-phase oxidation of EB using TBHP as
an oxidant without the addition of any solvent and additive.
To the best of our knowledge, there has been no report about
cost-effective cobalt-based metal oxides with hierarchical
flower-like microstructure for the oxidation of aromatic alkanes
until now.
However, the diffractions related to the crystalline Al2O3 phase
cannot be found in the patterns of samples, suggestive of its
amorphous nature. The above results demonstrate that LDHs
are successfully formed on the surface of Al2O3.
Fig. 1B shows the XRD patterns of calcined CoZnAl-LDH/Al2O3
samples. In the case of MMO-0/A sample, crystalline spinel
phases, such as Co3O4 and/or CoAl2O4, can be observed. However,
it is rather difficult to distinguish between these spinels due
to their almost identical diffraction positions. Furthermore,
some Co3+ cations can be replaced by Al3+ in spinels,40 and
consequently, another type of spinel-like Co2+(Co3+, Al)2O4 phase
may be present. With the increase in [Zn2+]/([Co2+] + [Zn2+])
atomic ratio to 0.5, the ZnO phase is obviously observed. In
the case of MMO-0.75/A, only the ZnO phase is detected and
no spinel phases are found, probably due to the high disper-
sion nature of cobalt-containing spinels. As for Im-MMO/A
and MMO-0.5 comparison samples, the only indexed crystal-
line phases are Co-containing spinels (Fig. S1†).
The morphology of the representative MMO-0.5/A sample
was characterized by FE-SEM. As shown in Fig. 2a, amorphous
Results and discussion
Catalyst characterization
Fig. 1A depicts the powder XRD patterns of CoZnAl-LDH/Al2O3
catalyst precursors. It can be seen that all samples mainly
present the characteristic (003), (006), and (012) peaks corre-
sponding to the basal spacing and higher-order diffractions
of NO3−-type hydrotalcite-like materials, similar to those typi-
cally reported in the literature.28 With the incorporation of
Zn ions, the intensities of characteristic (003) and (006) diffrac-
tion peaks for the LDH phase are weakened, indicative of the
decrease in the integrity of the layered structure. Meanwhile,
when the [Zn2+]/([Co2+] + [Zn2+]) atomic ratio increases to 0.5,
the impurity, zinc carbonate hydroxide (Zn4(CO3)(OH)6·H2O,
JCPDS no. 03-0787), is also observed, as indicated by its
characteristic diffraction peak at a 2θ value of about 13.0°.39
Fig. 2 FE-SEM images of samples: (a) Al2O3 microspheres; (b, c) MMO-0.5/A;
(d) Im-MMO-0.5/A; (e) MMO-0.5. The inset in (c) shows a typical
flower-like core–shell structure of cracked MMO-0.5/A.
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Al2O3 presents a uniform spherical morphology with an even
surface and a mean diameter of about 3.0 μm. In the case of
MMO-0.5/A, a large quantity of densely packed and interlaced
nanoplatelets with visible edges and almost no ab-faces is dis-
tributed homogeneously over the Al2O3 support surface, and
the crystallite size of nanoplatelets in the c-direction is about
10–20 nm. No agglomeration or sintering of adjacent platelets
is observed. Moreover, it is interesting to note that the dia-
meter of the resulting MMO-0.5/A increases to about 4.5 μm
(Fig. 2b and c). A typical flower-like core–shell structure of
the cracked MMO-0.5/A sample can be seen from the inset
in Fig. 2c. Obviously the formed Al2O3 core is smaller than
the pure Al2O3 microsphere due to the formation of the
Al-containing CoZnAl-MMO shell. Elemental mapping indi-
cates the homogeneous distributions of Co, Zn and Al species
in the MMO-0.5/A sample (Fig. 3).
In addition, as for the anisotropic CoZnAl-MMO platelet
in MMO-0.5/A, the size in the ab-direction is obviously larger
than that in the c-direction. This is due to the fact that
the initial CoZnAl-LDH crystallites all tend to grow in their
ab-direction perpendicular to the Al2O3 substrate through
a homogeneous nucleation mechanism41 in order to opti-
mize their growth and avoid collisions between growing
crystallites. It suggests that direct in situ growth of LDHs on
the surface of Al2O3 facilitates attachment and stabilization
of LDH particles, leading to the uniform distribution of
CoZnAl-MMO platelets on the support and thus the forma-
tion of the hierarchical flower-like core–shell microstructure.
In comparison, the FE-SEM image of Im-MMO-0.5/A depicts
that some irregular aggregates composed of small parti-
cles are distributed randomly on the support (Fig. 2d). Also,
unsupported MMO-0.5 presents the morphology of uniform
hexagonal platelets with a mean lateral size of about 200 nm
and a thickness of about 50 nm (Fig. 2e).
Fig. 4 TEM (a, b, c) and HRTEM (d) images of the representative
MMO-0.5/A sample.
spacings of about 2.86 Å and 2.45 Å (Fig. 4d), corresponding to
the (220) and (311) planes of the Co3O4 spinel phase, respectively.
To investigate the textural properties of Al2O3 caused by
loading CoZnAl-MMO, N2 adsorption–desorption isotherms
of samples were measured (Fig. 5A). All of the isotherms
exhibit intermediate isotherms between type II (absence of a
plateau at high P/P0) and type IV category (low N2 adsorption
at low P/P0),42,43 with a large hysteresis loop in the relative
pressure range of 0.45 to 1.0, indicating the presence of
capillary condensation in the mesopore structure. According
to the IUPAC classification,44 all samples present a mixture
hysteresis loop of H2 and H3 types with an abrupt closure at
a relative pressure of 0.5 due to the tensile strength effect,
suggesting the presence of slit-shaped and ink-bottle pores in
the whole mesopore network associated with the aggregates
of plate-like particles on the surface of hierarchical core–shell
microstructures. Moreover, CoZnAl-MMO/Al2O3 samples exhibit
a narrow pore size distribution ranging from 2 to 5 nm
(Fig. 5B). Compared with Zn-free MMO-0/A, Zn-containing
Further, the TEM and HRTEM images of representative
MMO-0.5/A (Fig. 4a–c) undoubtedly confirm a well-defined
flower-like core–shell microstructure, where the amorphous
Al2O3 core is coated by the vertically oriented and interlaced
platelet-like MMO thin flakes. In addition, an HRTEM image
of a single nanoflake depicts the lattice fringes with interplanar
Fig. 5 Nitrogen adsorption–desorption isotherms (A) and the pore size
distributions (B) of MMO-0/A (a), MMO-0.25/A (b), MMO-0.5/A (c), and
MMO-0.75/A (d).
Fig. 3 FE-SEM image (a), EDX spectrum (b) of the representative
MMO-0.5/A sample and elemental mapping of Co (c), Zn (d), and Al (e).
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samples have a larger specific surface area and total pore vol-
ume (Table 1). The results demonstrate that part of the pores
of Al2O3 support can be blocked by the in situ formed well-
developed 2D LDH microcrystallines. As for the Im-MMO-0.5/A
comparison sample (Fig. S2†), the much smaller surface
area and pore volume may be the result of the absence of
surface flower-like micro-nanostructures as well as aggregation
of particles.
It is well known that the oxidation states of active metal
species play an important role in catalytic activity in oxida-
tion reactions. H2-TPR measurements were carried out to
determine different forms of oxides in MMO-x/A samples. As
shown in Fig. 6, the reduction of MMO-x/A samples strongly
depends on the Co content. In the case of MMO-0/A, because
the shapes of reduction profiles are not symmetrical and
reduction peaks possess shoulders, the broad peak is fitted
to four peaks by deconvoluting the original profiles with the
elevated temperature. The first small peak at about 340 °C
is assigned to the reduction of surface Co3+ species;45 the
second peak at about 390 °C represents the reduction of Co3+
to Co2+ species in the bulk phase; the third peak at about
458 °C is associated with the further reduction of Co2+ to
Co0 species; the fourth small peak at about 515 °C is ascribed
to the reduction of large cobalt-containing spinel particles.
Interestingly, it is noted that the reduction temperature for
MMO-x/A samples presents an increasing trend with the
introduction of Zn due to the formation of (Co2+, Zn)Al2O4 or
(Co2+, Zn) (Co3+, Al)2O4 spinel-like phases. In the case of
CoZnAl-LDH/Al2O3-derived samples, Co2+ and Co3+ ions in
spinel phases can interact with neighboring groups through
the Co–O bonds polarized by Al3+ or Zn2+ ions in metal oxides
formed through the decomposition of the LDH phase, thus
hindering the reduction of cobalt species. Furthermore, the
interlaced platelet-like microstructure is helpful to resist
aggregation and sintering of MMO particles. For example, in
the case of MMO-0.5/A, the peak temperatures related to
reduction of Co3+ to Co2+ and Co2+ to Co0 increase to about
507 and 590 °C, respectively. Also, the peak areas are much
smaller than those for MMO-0/A in spite of the absence of
small amounts of highly dispersed Co species and large oxide
crystallites. Such increased reduction difficulty for MMO-x/A
samples with a lower cobalt content may be attributed to the
higher dispersion of Co species due to the separating effect of
the ZnO phase around active Co-containing species, which
inhibits H2 approach to Co2+/Co3+ species and thus delays
their reduction.42 Considering the similar aluminum content
in these samples, it appears that the addition of Zn plays
an important role in the improvement of cobalt dispersion.
Meanwhile, Zn present in the LDH-derived samples is respon-
sible for their high surface areas (Table 1).
In comparison, Im-MMO-0.5/A (Fig. S3†) presents a typical
reduction behavior of Co3O4.46 However, the reduction tem-
peratures are higher than those of Co3O4, which is ascribed to
the presence of a smaller amount of Co3O4 particles weakly
interacting with support as well as the aggregation of Co3O4
particles. In contrast, the reduction of unsupported MMO-0.5
involves several complex steps covering a wide range from
250 to 750 °C, which correspond to the reduction of different
superficial Co species. The low-temperature reduction from
250 to 500 °C is associated with the reduction of surface
Co-containing spinel particles, because hydrogen is easily able
to access and reduce these superficial cobalt specials.47,48 The
high-temperature reduction above 500 °C, which occurs at
much higher temperatures than those for MMO-x/A samples,
is assigned to the reduction of large Co-containing spinel
particles. The higher area of high-temperature peaks than
that of low-temperature peaks should be mainly due to severe
aggregation of the spinel particles.
Table 1 Textural parameters of different samples
Catalyst
SBET (m2 g−1 a
)
Vt (cm3 g−1 b
)
Dp (nm)c
MMO-0/A
123
187
172
156
16
0.2746
0.3297
0.3232
0.3190
0.0538
0.1725
8.3
5.7
6.3
6.7
19.1
20.0
MMO-0.25/A
MMO-0.5/A
MMO-0.75/A
Im-MMO-0.5/A
MMO-0.5
35
a
b
c
BET specific surface area. Total pore volume. Mean pore
diameter.
XPS analyses were performed on the MMO-0.5/A sample
(Fig. 7). Core levels of Co 2p, Zn 2p, Al 2p and O 1s can be
identified from the XPS survey spectra. It can be seen that
the XPS of the Co 2p 3/2 region can be fitted into three
contributions. The first intense peak with a binding energy
(BE) value of about 779.9 eV is assigned to Co 2p3/2 for
Co3+ cations.49,50 The presence of Co2+ species can be con-
firmed by two peaks at 781.6 and 786.8 eV for Co 2p3/2 and
its shake-up satellite, respectively, due to the fact that the low-
spin Co3+ cation only leads to much weaker satellite features
Fig. 6 H2-TPR profiles of MMO-0/A (a), MMO-0.25/A (b), MMO-0.5/A (c),
and MMO-0.75/A (d).
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Fig. 9 Comparison of the catalytic performance of MMO-0.5/A,
Im-MMO-0.5/A, and MMO-0.5 for oxidation of EB. Reaction conditions:
EB, 10 mmol; catalyst, 0.1 g; reaction temperature, 120 °C; EB : TBHP
molar ratio, 1 : 3.
Fig. 7 XPS spectra of the survey, Co 2p, Zn 2p, Al 2p, and O 1s of
MMO-0.5/A sample.
than does high-spin Co2+ with unpaired valence 3d electron
orbitals.51 The BE value of the Zn 2p3/2 peak is about 1021.5 eV,
indicative of the presence of Zn2+ species.52 The fine spectrum
of Al 2p shows a peak at around 74.2 eV, which is related to
the Al3+ species bonded to oxygen.53 From the O 1s spectrum,
it is seen that there are two fitted peaks representing two dif-
ferent types of surface oxygen species. There is general agree-
ment between the literature and the present results such that
the intense peak with a lower BE at around 530.5 eV is charac-
teristic of lattice oxygen bound to metal cations of the spinel
structure, while the small peak with a higher BE at 532.2 eV
belongs most likely to surface oxygen,54 including mainly oxygen
species of hydroxyl groups.55 The aforementioned results are
in good consistency with the XRD and TPR results.
higher activity for selective oxidation of EB than Im-MMO-0.5/A
or MMO-0.5, although MMO-0.5/A has a similar or much lower
amount of Co loading.
The reaction results over different MMO-x/A catalysts
are summarized in Table 2. It is found that MMO-x/A cata-
lysts exhibit a remarkable difference in the catalytic perfor-
mance, and both the conversion of EB and the selectivity
for AP increase with the decrease in Co content when the
ijZn2+]/IJ[Zn2+] + [Co2+]) molar ratio is lower than 0.5. On the con-
trary, both the conversion and the selectivity for AP decrease
with the decrease in Co content when the [Zn2+]/([Zn2+] + [Co2+])
molar ratio is higher than 0.5. For the MMO-0.5/A catalyst,
the maximum conversion of EB reaches 69.5% and the selec-
tivity for AP is about 80.4%. In addition, it is worth noting
that after 12 h of reaction, the turnover number (TON) value
of MMO-0.5/A is 62.0, which is about 5 times higher than
those observed for Im-MMO-0.5/A and MMO-0.5 comparison
catalysts.
Catalytic activity of MMO-x/A catalysts
The aerobic oxidation of EB catalyzed by hierarchical flower-
like core–shell MMO-x/A catalysts with TBHP was investi-
gated. The products are mainly AP, benzaldehyde (BZ) and
benzoic acid (BA), and no oxidation products of the aromatic
ring are found. Fig. 8 presents the reaction pathway that pro-
ceeds during EB oxidation.
The activities of MMO-0.5/A, Im-MMO-0.5/A, MMO-0.5 and
blank for the oxidation of EB were measured. As shown in
Fig. 9, only 5.2% EB conversion is obtained after 12 h in the
absence of the catalyst. In contrast, MMO-0.5/A shows much
In addition, compared with other catalysts reported in the
previous literature,7,8,12,13,18–20,56–62 the as-synthesized MMO-0.5/A
catalyst behaves in a comparable way or even better in most
Table 2 Results of EB oxidation over different catalystsa
Selectivity
(mol%)
AP BZ BA (mol mol−1
)
Cob
(wt%) (%)
Conversion
TON
Catalysts
Blank
—
5.2
52.1
54.8
69.5
22.6
13.6
45.8
43.3 36.9 19.8 —
MMO-0/A
MMO-0.25/A
MMO-0.5/A
MMO-0.75/A
12.8
9.5
6.6
2.1
76.4 18.3 5.3 24.0
79.1 12.9 8.0 34.0
80.4 12.1 7.5 62.0
60.1 29.6 10.3 63.4
23.6 45.6 12.4 12.0
51.6 38.5 9.9 10.4
Im-MMO-0.5/Ac 6.7
MMO-0.5
25.9
a
Reaction conditions: EB, 10 mmol; catalyst, 0.1 g; reaction
temperature, 120 °C; EB : TBHP molar ratio, 1 : 3; reaction time, 12 h.
b
Fig. 8 Reaction pathway for the oxidation of EB.
Measurement by ICP-AES. c Other product: 1-phenethyl alcohol.
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cases, in spite of different reaction conditions. To the best of
our knowledge, this is the best result for the oxidation of EB
to AP over Co-based metal oxide catalysts.
leads to a relatively lower concentration of EB in the reaction
solution. However, the excess Co active sites are prone to
adsorb AP easily. Therefore, AP may further be converted to
BZ and BA, leading to the decrease in the selectivity for AP.
On increasing the ratio of EB : TBHP from 1 : 2 to 1 : 3 (Table 3,
tests 2 and 4), the conversion of EB increases from 38.8% to
69.5%. However, further increase in the molar ratio of EB :
TBHP does not lead to enhanced conversion (Table 3, test 5).
In contrast, the conversion of EB slightly decreases to 67.6%.
This result is consistent with the observation by Singh et al.24
As the reaction time is increased from 6 to 24 h at 120 °C
(Table 3, tests 2, 6 and 7), the EB conversion and the selectiv-
ity for AP increase from 37.6% to 77.9% and 54.8% to 81.6%,
respectively, while the selectivity for BZ and BA decreases. As
shown in Table 3 (tests 2, 8, 9 and 10), elevating the reaction
temperature from 80 to 120 °C results in a continuous increase
in the conversion of EB and the selectivity for AP because the
rate of reaction can be enhanced with the reaction temperature.
Further increase in the reaction temperature from 120 to 140 °C
slightly improves the conversion of EB. Inversely, the selectivity
for AP displays a decrease. This is because high temperature is
beneficial to heterolytic cleavage of C–C bond, thus enhancing
the rate of the formation of BZ with elevated temperatures.
Recovery and recyclability are two of the most attractive
hallmarks of heterogeneous catalysts. Before reuse of the catalyst,
MMO-0.5/A can be easily recovered from the reaction mixture
by simple centrifugation, washing with ethanol and acetone,
and then drying at 80 °C. Subsequently, the recycling perfor-
mance of MMO-0.5/A was evaluated under identical reaction
conditions. As shown in Fig. 10, MMO-0.5/A can maintain its
excellent catalytic activity in the oxidation of EB (reaching an
AP yield of 53.9%) at 120 °C for 12 h after recycling three
times. The selectivity for AP is close to 80% in all cases, and
the conversion of EB only decreases by about 1.0%. Elemental
analysis by ICP-AES further reveals that the Co leaching loss is
only about 0.8 wt% of the total cobalt after four consecutive
cycles, suggestive of the good stability of the catalyst. This can
be explained by the fact that in situ growth of LDH precursors
on the Al2O3 support is beneficial to form a strong interaction
between active species and support matrix and thus keep
a remarkable stability for the oxidation of EB. In addition,
A number of research studies have revealed that ZnO can
function as a physical spacer among Cu nanoparticles to
achieve a high copper surface area.63,64 However, large ZnO
particles can be formed under high calcination temperatures,
which results in reduced activity.65 In our case, a similar
result is obtained with the addition of zinc. Interestingly,
note that the introduction of an appropriate amount of zinc
into MMO-x/A greatly promotes the oxidation of EB to AP.
Therefore, high conversion and selectivity for AP are achieved
over MMO-0.5/A. In the present heterogeneous catalyst system,
in situ immobilization of Co-containing metal oxides onto the
surface of alumina can greatly enhance the activity and selectivity.
The remarkably enhanced catalytic performance for MMO-x/A
catalysts should be ascribed to the specific 3D core–shell flower-
like architecture with enhanced surface area, suitable mesopore
distribution and high metal dispersion, which can offer more
adsorption sites and active reaction centers and allow sub-
strates to diffuse freely on the MMO platelets. With the increase
in the ijZn2+]/IJ[Zn2+] + [Co2+]) molar ratio to 0.5, the catalytic
activity is enhanced gradually due to the increase in the metal
dispersion in spite of the decrease in the active Co content.
However, in the case of MMO-0.75/A, the less Co loading amount
(about 2.1 wt%) leads to a dramatic decrease in the conver-
sion of EB, although Co species possess higher dispersion as
evidenced by the above TPR results.
Table 3 shows the effects of different reaction conditions
on the solvent-free oxidation of EB over the representative
MMO-0.5/A catalyst. It is well known that increasing the
amount of the catalyst in batch reaction processing will
increase the conversion due to more active sites provided by
catalysts. However, the above positive effect is not always to
be utilized if the selectivity for the product is considered. As
shown in Table 3, (tests 1, 2 and 3), the conversion of EB over
MMO-0.5/A improves with a catalyst dosage of 0.05 to 0.2 g.
After an increase in selectivity for AP in the cases of lower
catalyst dosages, the selectivity decreases from 80.4% to 69.6%
with increasing catalyst dosage from 0.1 to 0.2 g. This is because
with the increasing amount of active sites, higher conversion
Table 3 Effects of different reaction conditions on the solvent-free EB oxidationa
Selectivity (mol%)
Mass of
catalyst (g)
EB : TBHP
(mol per mol)
Reaction
time (h)
Temperature
(°C)
Conversion
(%)
Test
AP
BZ
BA
9.2
7.5
10.3
4.8
6.2
12.8
4.4
1
2
3
4
5
6
7
8
9
10
0.05
0.1
0.2
0.1
0.1
0.1
0.1
0.1
0.1
0.1
1 : 3
1 : 3
1 : 3
1 : 2
1 : 4
1 : 3
1 : 3
1 : 3
1 : 3
1 : 3
12
12
12
12
12
6
24
12
12
12
120
120
120
120
120
120
120
80
39.5
69.5
72.3
38.8
67.6
37.6
77.9
20.7
41.3
70.7
63.5
80.4
69.6
66.4
76.5
54.8
81.6
73.1
73.5
64.4
27.3
12.1
20.1
28.8
17.3
32.4
14.0
14.6
19.7
29.6
12.3
8.8
6.0
100
140
a
Reaction conditions: EB, 10 mmol; catalyst, MMO-0.5/A.
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and catalyst dosage could improve the conversion of EB by
accelerating the rate of oxidation, but a higher reaction tempera-
ture and catalyst dosage could lead to the decrease in the selec-
tivity for AP due to the enhanced deep oxidation. Moreover, this
catalyst possessed attractive stability and could retain high
catalytic activity after recycling three times. Owing to their
intrinsic properties such as high surface area, chemical stability
and low cost, it is expected that such a type of highly-dispersed
and stable hierarchical flower-like core–shell Co-based metal
oxide catalysts may work as an alternative, which are probably
appropriate in the practical oxidation of alkylaromatics.
Experimental
Preparation of amorphous Al2O3 microspheres
Fig. 10 Reproducibility of MMO-0.5/A in the oxidation of EB. Reaction
conditions: EB, 10 mmol; catalyst, 0.1 g; reaction temperature, 120 °C;
EB : TBHP molar ratio, 1 : 3; reaction time, 12 h.
In a typical synthesis, Al2IJSO4)3·18H2O (0.014 mol), CO(NH2)2
(0.042 mol), and sodium tartrate (0.0035 mol) were dissolved
in distilled water (200 mL) and ethanol (80 mL) to form a
mixed solution and then the solution was transferred into an
autoclave, followed by hydrothermal treatment at 165 °C for
1 h. The white precipitate was separated by centrifugation and
washed with distilled water and alcohol several times. The
product was dried in a vacuum oven at 80 °C for 8 h. Finally,
amorphous Al2O3 microspheres were obtained by calcination
of the above precipitation at 500 °C for 6 h.
a further reaction of test 2 in Table 3 was performed. After a
reaction time of 2 h, the reaction was hot filtered to remove
the solid catalyst. Then, the reaction was allowed to continue
under the same conditions. Only a trace amount of the desired
product could be identified under the same conditions after
another reaction for 12 h. The above results strongly suggest
that the present reaction proceeds mostly on the heteroge-
neous surface.
Preparation of CoZnAl-MMO/Al2O3 samples
In general, the current study can be more advantageous
over previous studies because of the two following merits.
Firstly, the synthesis of CoZnAl-LDH precursors was performed
by an in situ growth route, which resulted in excellent sta-
bility of the resultant core–shell MMO-x/A catalysts due to
the strong adhesion of flower-like Co-containing metal oxide
micro/nanostructures on the support. Secondly, a uniform dis-
tribution of multi-metal species in the platelet-like CoZnAl-LDH
precursor facilitated high dispersion of active Co species, as
well as strong metal–support interactions, thus leading to sig-
nificantly enhanced catalytic activity in the oxidation of EB.
In the synthesis of Al2O3-supported CoZnAl-LDHs, Co(NO3)2·6H2O,
Zn(NO3)2·3H2O, and NH4NO3 (0.0375 mol) with different
[Zn2+]/([Zn2+] + [Co2+]) molar ratios of x (x = 0, 0.25, 0.5 and
0.75) were dissolved in 280 mL of deionized water to obtain
a clear solution with a total metal cation concentration of
0.3 M. The resulting solution was added to a four-necked flask
with 0.4 g of Al2O3, and the resulting slurry was transferred to
an autoclave and aged at 100 °C for 12 h, thoroughly washed
with deionized water until the pH reached 7, and dried at
60 °C for 12 h in air. The obtained CoZnAl-LDH/Al2O3 precur-
sors were denoted as L-x/A. L-x/A was calcined in static air at
500 °C for 6 h, and the calcined product was denoted as
MMO-x/A.
Conclusions
For comparison, Al2O3 (0.4 g) was added to 1.0 mL of
Co(NO3)2·6H2O and Zn(NO3)2·3H2O solution (0.7 M) and then
impregnated for 12 h. The resulting supported catalyst pre-
cursor was obtained after drying at 60 °C for 12 h, and cal-
cined in static air at 500 °C for 6 h, and the calcined product
was denoted as Im-MMO-0.5/A. In addition, CoZnAl-LDH with
a Co/Zn/Al molar ratio of 2 : 2 : 1 was prepared by our previ-
ously reported separate nucleation and aging step method.66
The CoZnAl-LDH precursor was calcined in static air at 500 °C
for 6 h, and the calcined product is denoted as MMO-0.5.
In summary, Co-containing LDH microcrystallites were in situ
grown on the surface of microspherical alumina. The resulting
Al2O3-supported CoZnAl-MMO presented a hierarchical flower-
like core–shell microstructure due to well-developed 2D platelet-
like LDHs grown on the support surface. Compared with the
conventional catalyst prepared by incipient impregnation and
the bulk CoZnAl-MMO catalyst, Al2O3-supported CoZnAl-MMO
possessed higher metal dispersion due to the homogeneous
distribution of MMO nanoplatelets as well as the stronger
interaction between active species and matrix. In particular,
the introduction of an appropriate amount of Zn into LDH
precursors could greatly improve the dispersion of active
Co species and thus catalytic performance in solvent-free
liquid-phase oxidation of EB with TBHP as an oxidant to
produce AP. Increasing the reaction temperature, reaction time
Characterization
X-ray diffraction (XRD) data of the samples were collected at
room temperature using a Rigaku D/Max-RB diffractometer
with a graphite-filtered Cu Kα source (λ = 0.15418 nm).
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Elemental analysis was performed using a Shimadzu
Acknowledgements
ICPS-7500 inductively coupled plasma atomic emission spec-
troscope (ICP-AES) after the samples were dissolved in dilute
hydrochloric acid (1 : 1).
The morphology of the samples was investigated using a
field emission scanning electron microscope (FE-SEM, Zeiss
Supra 55 with an accelerating voltage of 20 kV) which was
combined with energy-dispersive X-ray spectroscopy (EDX)
performed using an Oxford Instruments INCA analyzer for
determination of the metal composition.
Transmission electron microscopy (TEM) and high-resolution
TEM (HRTEM) were carried out using a JEM-2100F electron
microscope (JEOL, Japan) operated at an accelerating voltage
of 200 kV.
N2 adsorption–desorption isotherms of the samples were
obtained with a Micromeritics ASAP 2020 sorptometer appa-
ratus at −196 °C. All samples were outgassed prior to analysis
at 200 °C before adsorption measurements. The total specific
surface areas were evaluated by the multipoint Brunauer–
Emmett–Teller (BET) method. The mesopore size distribution
and average pore diameter were determined by the Barrett–
Joyner–Halenda (BJH) method applied to adsorption isotherms.
X-ray photoelectron spectroscopy (XPS) spectra were recorded
using a VG ESCALAB 2201 XL spectrometer with monochro-
matic Al Kα X-ray radiation (1486.6 eV photons). Binding
energies were calibrated based on the graphite C 1s peak at
284.5 eV.
The reduction behavior of calcined samples was studied
by hydrogen temperature-programmed reduction (H2-TPR)
using a Micromeritics ChemiSorb 2720 instrument. A 0.1 g
sample, which was placed in a quartz U-tube reactor, was
degassed at 200 °C for 2 h under argon flow (40 mL min−1).
Then TPR was conducted in a stream of 10% (v/v) H2/Ar
(50 mL min−1) with a heating rate of 5 °C min−1 up to 1000
°C. The effluent gas was analyzed by a thermal conductivity
detector (TCD).
We gratefully acknowledge the financial support from the
973 Program (2011CBA00506), the National Natural Science
Foundation of China, the Program for Changjiang Scholars
and Innovative Research Team in University (IRT1205), and a
project from the Beijing Engineering Center for Hierarchical
Catalysts.
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