Holey Lamellar High-Entropy Oxide as an Ultra-High-Activity Heterogeneous Catalyst for Solvent-free Aerobic Oxidation of Benzyl Alcohol

Article abstract of DOI:10.1002/ange.202004892

The development of noble-metal-free heterogeneous catalysts is promising for selective oxidation of aromatic alcohols; however, the relatively low conversion of non-noble metal catalysts under solvent-free atmospheric conditions hinders their industrial application. Now, a holey lamellar high entropy oxide (HEO) Co_0.2Ni_0.2Cu_0.2Mg_0.2Zn_0.2O material with mesoporous structure is prepared by an anchoring and merging process. The HEO has ultra-high catalytic activity for the solvent-free aerobic oxidation of benzyl alcohol. Up to 98 % conversion can be achieved in only 2 h, to our knowledge, the highest conversion of benzyl alcohol by oxidation to date. By regulating the catalytic reaction parameters, benzoic acid or benzaldehyde can be selectively optimized as the main product. Analytical characterizations and calculations provide a deeper insight into the catalysis mechanism, revealing abundant oxygen vacancies and holey lamellar framework contribute to the ultra-high catalytic activity.

Full text of DOI:10.1002/ange.202004892

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Titel: Holey Lamellar High Entropy Oxide as Ultra-Highly Active
Heterogeneous Catalyst for Solvent-free Aerobic Oxidation of
Benzyl Alcohol
Autoren: Danyang Feng, Yangbo Dong, Liangliang Zhang, Xin Ge, Wei
Zhang, Sheng Dai, and Zhen-An Qiao
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Zitierweise: Angew. Chem. Int. Ed. 10.1002/anie.202004892
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10.1002/ange.202004892
Angewandte Chemie
RESEARCH ARTICLE
Holey Lamellar High Entropy Oxide as Ultra-Highly Active
Heterogeneous Catalyst for Solvent-free Aerobic Oxidation of
Benzyl Alcohol
Danyang Feng,[a] Yangbo Dong,[a] Liangliang Zhang,[a] Xin Ge,[b] Wei Zhang,[b] Sheng Dai,[c] and Zhen-
An Qiao*[a]
Abstract: The development of noble metal free heterogeneous
catalysts is promising for selective oxidation of aromatic alcohols;
however, the relatively low conversion of non-noble metal catalysts
under solvent-free atmospheric condition greatly restricts their
industrial application. Herein, we designed synthesize a novel holey
lamellar high entropy oxide (HEO) Co0.2Ni0.2Cu0.2Mg0.2Zn0.2O material
with mesoporous structure via a simple anchoring and merging
process. The prepared holey lamellar HEO as heterogeneous catalyst
exhibits ultra-high catalytic activity for the solvent-free aerobic
oxidation of benzyl alcohol. Up to 98% conversion can be achieved in
only 2 hours, to our knowledge, it is the highest conversion of benzyl
alcohol oxidation reaction so far. By rational regulating the catalytic
reaction parameters, benzoic acid or benzaldehyde can be selectively
optimized as the main product. Multiple analytical characterizations
and theory calculation provide a deeper insight into the catalysis
mechanism, revealing the synergistic effect of the unique HEO
material, abundant oxygen vacancies and holey lamellar framework
leads to the ultra-high catalytic activity.
prone to safety risks. The ideal catalysts are supposed to be
workable without any solvents or oxidizing agents in atmospheric
conditions.
Compared
with
homogeneous
catalysts,
heterogeneous catalysts have the merits of environmental benign,
facilitating separation and recycle. Therefore, the construction of
green and highly active heterogeneous catalysts for the solvent-
free aerobic oxidation of benzyl alcohol is highly demanded.
Recently, tremendous efforts have been made to study noble
metals heterogeneous catalysts, such as Pt- and Pd-based
catalysts, due to their satisfactory catalytic activity.[5-9]
Nevertheless, taking into account the exorbitant price and finite
reserves of noble metals, non-noble metals heterogeneous
catalysts have a broader application prospect in the catalytic field.
Transition metal oxides, as a class of low-cost and resource-
rich materials, have been widely employed as catalysts in various
types of catalytic processes, such as the aerobic oxidation of
primary carbon hydrogen bonds, alcohols and amines.[10-12] For
instance, Chen et al. reported that Co nanoparticles encapsulated
inside nitrogen-rich carbon exhibited extraordinary efficiency in
catalytic oxidation of alcohols to esters.[13] Zhao et al. found that
magnetic mesoporous TiO2 showed excellent catalytic
performance as a sonocatalyst for the degradation of bisphenol-
A.[14] Compared with simple metal oxides, the synergistic
interactions among multiple types of catalytic active sites in
complex metal oxides make them exhibit higher catalytic activity.
For example, Zn4Co1Ox@carbon hollow particles showed
superior performance for the hydrogenation of nitrobenzene to
aniline.[15] Multi-metallic catalyst CeO2–Co3O4 hybrids exhibited
remarkable synergistic effect in the chemical reduction of NO by
CO.[16] Thus, rational designing transition metal oxide
heterogeneous catalysts equipped with multiple catalytic sites are
of great value in applied catalysis area, but it still suffers from the
much lower catalytic activity than that of noble ones at present.
Among the majority of complex metal oxides, high entropy
oxides (HEO), defined as five or more near-equimolar metal
elements arranged deliberately in a single-phase lattice with
randomized distribution, have attracted ever-increasing attention
because of their diverse redox behavior, abundant cation
compositions, exceptional chemical and thermal stability.[17-19]
The multi-electron redox properties in HEO make it a potential
high-performance catalyst for organic reaction. The most
common approaches to synthesize HEO are arc-melting and
spark plasma sintering methods under ultrahigh temperature.[20]
However, limited by the current high temperature synthetic
conditions, the obtained micron-sized HEO grains expose only a
small portion of the effective catalytic sites, which greatly restricts
their catalytic performance. From this point of view, the synthesis
of HEO catalyst with porous framework composed of self-linked
HEO small nanoparticles will satisfy the requirement of catalytic
reaction and provide opportunities for boosted catalytic activity.
Introduction
Selective oxidation of benzyl alcohol to produce benzaldehyde,
benzoic acid and benzyl benzoate is of great significance in both
laboratory research and industrial applications. Benzaldehyde
and benzoic acid are valuable intermediates in industrial
manufacture, and benzyl benzoate are key commercially raw
materials for fine chemical engineering. Recently, homogeneous
catalysts have been confirmed to be effective in selective
oxidation of alcohols in the presence of organic solvents or under
high pressures.[1-4] However, in the viewpoint of sustainable and
green chemistry, the hazardous organic solvents or
hydroperoxides agents used in homogeneous catalysis easily
lead to environmental pollution problems. Besides, the high-
pressure condition needed in homogeneous catalysis place high
demands on catalytic equipment, consumes more energy and are
[a]
D. Feng, Y. Dong, L. Zhang, Prof. Z.-A. Qiao
State Key Laboratory of Inorganic Synthesis and Preparative
Chemistry, Jilin University, Changchun, Jilin 130012 (China)
E-mail: qiaozhenan@jlu.edu.cn
[b]
[c]
Dr. X. Ge, Prof. W. Zhang
Electron Microscopy Center
Jilin University, Changchun, Jilin 130012 (China)
Prof. S. Dai
Chemical Sciences Division, Oak Ridge National Laboratory, Oak
Ridge, TN, 37831, USA
Supporting information for this article is given via a link at the end of
the document
This article is protected by copyright. All rights reserved.

10.1002/ange.202004892
Angewandte Chemie
RESEARCH ARTICLE
Herein, holey lamellar HEO material Co0.2Ni0.2Cu0.2Mg0.2Zn0.2
O
forces between metal precursors and GO hinder the diffusion and
agglomeration of these nanoparticles.[21] Meanwhile, the
amorphous precursors transform into crystalline mixed metal
oxides nanocrystals and these nanocrystals self-link with each
other to form holey lamellar structure. Further treating the sample
in an air atmosphere at 900 °C, the GO template is completely
burned off and the mixed metal oxides merge into single crystal
phase with high entropy structure, simultaneously the holey
structure can be well maintained. As shown in Figure S2, the more
the metal source is added, the larger the HEO particle size is
obtained.
The transmission electron microscopy (TEM) image of the
holey lamellar HEO was shown in Figure 1a. The interconnected
HEO nanoparticles forms the wall of the holey structure and the
average particle size is estimated to be 8-12 nm, as illustrated by
aberration-corrected scanning transmission electron microscopy
(STEM) image (Figure 1b). The atomistic crystal structure of the
holey lamellar HEO was shown in Figure 1c, the interlayer
spacing of 0.241 nm in HRTEM was indexed to the (111) crystal
plane of HEO phase. To characterize the spatial distribution of
elements in holey lamellar HEO, energy-dispersive spectroscopy
(EDS) elemental mapping (Figure 1d and 1e) and ICP analysis
(Table S1) were conducted, which evidenced that Co, Ni, Cu, Mg,
Zn and O were uniformly distributed over the whole detection
range.
X-ray diffraction (XRD) characterization of holey lamellar HEO
was conducted to verify the structural information. As shown in
Figure 1f, a series of multiphase mixture peaks are presented
after pyrolyzed at 500 °C, among which an obvious peak of
graphitic carbon is observed at 26° (2θ), belonging to the
incomplete combustion of GO substrate. Subsequently further
increasing the calcination temperature to 900 °C, the carbon is
completely removed and the multiphase mixture transform to
face-centered cubic (FCC) single-phase, demonstrating the
multiphase metal oxides transform to HEO phase. The average
HEO particle size is about 11.2 nm calculated by the Scherrer
formula according to the XRD pattern, which is in accordance with
the TEM observation. The diffraction peaks at 2θ values of 36.7,
42.6, 61.9, 74.2 and 78.1o are attributed to the (111), (200), (220),
(311) and (222) lattice planes, respectively.[22] Then the HEO
material is re-calcined under 750 °C, the single-phase transforms
back to multiphase, and this kind of structural reversibility is a key
feature of entropy-driven transition, further confirming the high
entropy structure.[17] The specific surface area and pore volume
of holey lamellar HEO are 42 m2 · g-1 and 0.106 cm3·g -1,
respectively. As shown in Figure 1g, holey lamellar HEO exhibits
a typical IV curve with a H1-type hysteresis loop. The increase in
the adsorption band at P/P0 = 0.7−0.9 confirms the formation of
mesoporous architecture, which comes from the self-linked small
HEO nanoparticles.
is designed and synthesized for the first time by an anchoring-
merging process using graphene oxide (GO) as a sacrificial
template. The metal ions are firstly absorbed on the surface of GO
by electrostatic interaction and the pinning forces between metal
precursors and GO template effectively prevent the
agglomeration of the small precursor nanoparticles in the heating
process, then the metal precursors merge into high-entropy
structure with single phase under high temperature. The obtained
holey lamellar HEO as heterogeneous catalyst exhibits ultra-high
catalytic activity for the solvent-free aerobic oxidation of benzyl
alcohol. Remarkably, up to 98% conversion is achieved in only 2
hours, which is the highest conversion of benzyl alcohol reported
so far. The unique structure of elements randomly distributed on
the atomic scale endows the HEO material unexpected catalytic
properties, and the holey lamellar structure composed of
numerous HEO nanoparticles greatly increases surface exposure
of the catalytic sites and makes the catalyst rich in oxygen
vacancies. Density functional theory (DFT) calculations declare
that the presence of oxygen vacancies significantly increases the
adsorption energies of holey lamellar HEO material for benzyl
alcohol. What’s more, the effect of various key parameters in the
catalyzing process are further explored, and the main product can
be selectively optimized as benzoic acid or benzaldehyde by
rational regulating the component of the catalyst and the catalytic
reaction time. These findings suggest that HEO materials have
broad application prospects in the field of industrial catalysis.
Results and Discussion
Scheme1. Schematic illustration of the formation process for holey lamellar
HEO material (Co0.2Ni0.2Cu0.2Mg0.2Zn0.2O).
As shown in Scheme 1, the liquid phase system using ethylene
glycol as a solvent ensures that the metal ions are highly
dispersed at molecular level. GO modified with negatively
charged oxygen-containing functional groups is employed as two-
dimensional substrate, and the electrostatic interaction promotes
the five individual positively charged metal ions (Co2+, Ni2+, Cu2+,
Mg2+, Zn2+) to adsorb and stabilize uniformly onto the surface of
GO template (Figure S1). As the calcination temperature rises,
the precursor nanoparticles are firmly fixed to GO and the pinning
Aerobic oxidation of aromatic alcohols by using efficient and
economic complex transition metal oxides catalysts has great
potential in industrial catalytic applications.[23-25] Herein, the as-
prepared holey lamellar HEO was employed as heterogeneous
catalyst for solvent-free aerobic oxidation of benzyl alcohol
(Figure 2a). The catalytic activity of the catalyst is evaluated in the
oxidation of neat benzyl alcohol under atmospheric condition. In
the absence of catalyst, the conversion of benzyl alcohol is lower
than 1%. Encouragingly, with the addition of the holey lamellar
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10.1002/ange.202004892
Angewandte Chemie
RESEARCH ARTICLE
HEO catalyst, the catalytic conversion increases exponentially
with increasing reaction temperature from 100 °C to 120 °C (Table
S2), and up to 98% conversion can be achieved after reaction at
atmospheric pressure, but also reaches the equilibrium of the
catalytic reaction in less time. There are three products in this
oxidation process: benzaldehyde, benzoic acid and benzyl
120 °C for only 2 hours under solvent-free atmospheric conditions. benzoate. Benzoic acid is the dominant product in the catalytic
To our best knowledge, this is the highest conversion among
previous reported single atom, clusters, noble metal and alloy
catalysts in literatures (Figure 2b and Table S3).[26-33] It should be
pointed out that the TOF numbers in literature were calculated on
the basis of loading amount of noble metals (loading rate=0.5
wt%-5 wt%), and the metal oxides are just work as supports not
catalytic active materials. However, noble metals do not exist in
this catalytic system, and the TOF value can only be calculated to
be 994 in terms of the amount of HEO. Au/Pd-TiO2 catalyst,
whose conversion is 91.6% in 8 hours under 0.2 MPa of O2, is the
most advanced catalyst ever reported.[26] Compared with Au/Pd-
TiO2 catalyst, the holey lamellar HEO catalyst not only works at
reaction, benzaldehyde and benzyl benzoate are identified as the
major byproducts. In the reaction process, the benzyl benzoate is
formed by the esterification of benzyl alcohol with benzoic acid.
As a comparison sample, the bulk HEO (Figure S3) catalyst under
the same catalytic condition shows a fairly low catalytic activity
with the conversion of only 6%, confirming the unique holey
lamellar structure plays a crucial role in the excellent catalytic
performance of HEO materials. The recyclability and stability of
holey lamellar HEO catalyst on benzyl alcohol are further
evaluated. After 5 cycles, the selectivity and conversion are
almost the same as that of the fresh catalyst, indicating the
catalyst is highly stable in the catalytic environment.
Figure 1. Characterization of holey lamellar HEO. (a) TEM and (b, c) HRTEM images of holey lamellar HEO. (d) and (e) Elemental mapping images of holey
lamellar HEO. (f) XRD patterns of holey lamellar HEO calcined at different calcination temperature and (g) N2 sorption isotherms of holey lamellar HEO.
To demonstrate the general applicability of our holey lamellar
HEO catalyst, catalytic experiments on benzyl alcohol derivatives
were carried out. As shown in Figure 2c, holey lamellar HEO
shows decent catalytic oxidation performance for the derivatives
compared to other heterogeneous catalysts, though the
conversion is not as high as that of benzyl alcohol. The catalytic
activity is closely related to the substituent group linked with
benzyl alcohol. For electron-donating derivatives, such as
methylbenzyl alcohol or methoxy benzyl alcohol, the selectivity to
corresponding aldehyde is obviously improved (over 50%). The
reduced catalytic conversion is assigned to the high steric
hindrance effect of the substituents impeding the
dehydrogenation in the catalytic reaction. For electron-
withdrawing derivatives, such as nitro-benzyl alcohol, the catalytic
activity is almost the same as the blank test, and this extremely
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10.1002/ange.202004892
Angewandte Chemie
RESEARCH ARTICLE
low conversion is attributed to the synergistic effect of steric
hindrance and catalytic inhibition of the nitro substituents.
Due to HEO is a kind of unique solid solution composed by five
nearly equimolar metal elements, the study on the influence of
each element content on catalytic performance is carried out in
the premise of not changing the holey lamellar structure (Figure
2f). When the content of each metal element is reduced by 10%
separately, except the time reaching chemical equilibrium is
prolonged to nearly 3 hours, there is no obvious change in the
conversion and selectivity under the same catalytic condition
(Figure S4), and the corresponding XRD pattern of the catalysts
does not show any changes either. (Figure S5). Whereas further
reducing the content of each metal element by 30% separately,
the reaction time for reaching chemical equilibrium is dramatically
prolonged to 14 hours, and the catalysts exhibit obviously
increased selectivity of over 50% towards benzaldehyde (Figure
S6). However, compared with holey lamellar HEO catalyst, the
catalytic conversion drops by nearly half, and the peak positions
of the catalysts in the XRD pattern also changes (Figure S7).
These results demonstrate that the specific single-phase HEO
material and the unique holey lamellar structure are both
indispensable for catalytic performance. In light of these results,
by regulating the component of the catalyst and the catalytic
reaction time, we can optimize benzoic acid or benzaldehyde as
the main product of benzyl alcohol oxidation, respectively. The
The FT-IR spectra was used to confirm the oxidation process
of oxidation of benzyl alcohol. As shown in Figure 2d, the peaks
at 1020 cm-1 and 1369 cm-1 is assigned to the stretching vibrations
of (C-OH) and (O-H) in benzyl alcohol, respectively. After reaction
for 0.2 hour, a new peak at 1689 cm-1 belonged to (C=O) group in
benzaldehyde is observed and this peak shifts towards higher
wavenumbers as the benzaldehyde is over oxidized to benzoic
acid and benzyl benzoate in the following oxidation process.
When the catalytic reaction proceeds to 0.5 hour, two additional
peaks located at 2855 cm−1 and 2926 cm−1 appear, which are the
characteristic absorption peaks of saturated methyl and
methylene in benzyl benzoate, demonstrating the esterification
between benzyl alcohol and benzoic acid. The step-by-step-
oxidation process of benzyl alcohol confirmed by infrared is
consistent with our previous speculation. The evolution of
catalytic substrate with reaction time in the oxidation reaction is
shown in Figure 2e. When benzyl alcohol is used as catalytic
substrate, the reaction rate is too fast to monitor, so 4-
methylbenzyl alcohol, whose oxidation process is easier to detect,
is chosen as the substrate. The evolution process of 4-
methylbenzyl alcohol further proves that the catalysis of aromatic
alcohol follows the rule of stepwise oxidation.
corresponding catalysis results are shown in Table 1.
Table 1. Solvent-free aerobic oxidation of benzyl alcohol over various catalysts.
Catalyst
Substrate
Product Sel.(%)
29
t(h)
2
Conv.(%)
98
Holey HEO
50
58
19
Holey HEO
Holey HEO
bulk HEO
1
73
49
33
8
0.5
67
28
3
24
2
6
99
Holey HEO
(0.9Ni)
89
28
62
9
Figure 2. (a) The reaction pathway of the oxidation of benzyl alcohol over holey
lamellar HEO catalyst. (b) Comparison of the catalytic conversion of holey
lamellar HEO catalyst with those reported in the literatures (2.5%Au–
2.5%Pd/TiO2,[26] Pd/GC,[27] Au/MnO2-R,[28] Au1/CeO2,[29] Pd/mHCPs,[30] Au(8
wt%)/U3O8,[31] 1% (Au+Pd)/C,[32] 0.5%Pd@C-GluA-550,[33]). (c) Catalytic
performance of holey HEO in benzyl alcohol, methylbenzyl alcohol, methoxy
benzyl alcohol and nitro-benzyl alcohol oxidation. (d) FT-IR spectra of the benzyl
alcohol evolution as a function of reaction time. (e) The time–activity profile for
4-methoxy benzyl alcohol oxidation under aerobic conditions. (f) Catalytic
performance of Co element is reduced by 10%, 20% and 30%, separately.
Holey HEO
(0.7Ni)
14
24
48
58
18
23
Blank
\
\
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10.1002/ange.202004892
Angewandte Chemie
RESEARCH ARTICLE
reported and validated that the energy required for the formation
of an oxygen vacancy in small nanoparticles is much lower than
for the regular surface, and the oxides with smaller particle size(＞
10 nm) tend to generate more oxygen vacancy defects than that
of bulk materials. (ii) The random substitutional doping and
arrangement of the five metal atoms in HEO structure facilitate
the formation of oxygen vacancies. (iii) The grab and consumption
of surrounding lattice oxygen atoms from oxides during the rGO
combustion process produce an oxygen deficiency atmosphere
and results in more oxygen vacancies. However, for bulk HEO,
the relative peak strength of defective oxygen is much weaker
than that of the holey ones. The difference in O1s spectra
indicates that there are abundant oxygen vacancies in holey
lamellar HEO. The presence of oxygen vacancies can enhance
the dehydrogenation rate of benzyl alcohol substrate, which
greatly improved the catalytic activity of the catalyst.[36] The high-
resolution XPS spectra of metal elements were shown in Figure
S9.
[a] Reaction conditions: 15 ml of substrate, 10 mg of catalysts, O2 1 atm，
120 °C.
Comparative tests of lacking arbitrary one component in the
material synthesis process were performed and the
corresponding catalytic performances were shown in Table S4. It
can be observed that the quaternate catalysts only show a
conversion of about 20% after 24 hours of catalytic reaction and
the lack of arbitrary one element has the similar influence on the
catalytic activity. Furthermore, the single component and two,
three components catalysts exhibit poorer catalytic performance,
and the conversions are as low as the bulk HEO. The above
catalytic results indicate that there is no obvious difference in the
role of the five metal elements in the catalysis reaction, and each
metal element could simultaneously serve as benzyl alcohol
adsorbents and active sites, which greatly promotes the
improvement of catalytic efficiency and provides potentiality for
the ultra-high catalytic activity.
The density of oxygen vacancies of holey HEO can be
controlled by adjusting the calcination temperature (Figure S10).
As the calcination temperature increases to 1000 °C , the HEO
particles show obvious sintering phenomenon, and the content of
oxygen vacancies drops by nearly half to 22%. Further increasing
the calcination temperature to 1100 °C , the corresponding
morphology and oxygen vacancies content are similar to that of
the bulk HEO. Both of the above two catalysts exhibit low catalytic
activity similar to bulk HEO. The presence of oxygen vacancies
can be further evidenced by X-band electron paramagnetic
resonance (EPR). As shown in Figure 3b, a weak EPR signal of
holey lamellar HEO at g=2.003 can be observed, which is
characteristic of single-electron trapped oxygen vacancies.[37]
The O2-TPD experiment was carried out to investigate the
oxygen species over the holey lamellar HEO. As shown in Figure
3c, the desorption peaks located at about 100 °C, 150-500 °C and
600-800 °C are attributed to molecular oxygen species adsorbed
on oxygen vacancies, surface labile oxygen and crystal lattice
oxygen, respectively. Compared with bulk HEO, the holey
lamellar HEO possesses broader peak at about 100 °C,
demonstrating higher oxygen vacancies contents, which agrees
with the O 1s XPS result. H2-TPR test was employed to gain
insights into the effect of oxygen vacancies on the reducibility of
the oxygen species on holey lamellar HEO. As shown in Figure
3d, compared with bulk HEO, the intensity of hydrogen
consumption peaks for the holey lamellar HEO are much stronger
and the three peaks dramatically shift to lower temperatures in the
reduction process, demonstrating the better oxygen delivery
capacity in oxygen vacancy-abundant holey lamellar HEO.[38]
Figure 3. (a) Oxygen XPS spectrum comparison of holey and bulk HEO. (b)
EPR spectra of holey and bulk HEO. (c) O2-TPD and (d) H2-TPR spectra of
holey and bulk HEO.
To better understand the mechanism of the ultra-high catalytic
activity, X-ray photoelectron spectroscopy (XPS) spectra of the
holey HEO catalyst were recorded, and the bulk HEO was
employed as a comparative sample. Figure S8 shows the XPS
survey spectra, demonstrating the co-existing of Co 2p, Ni 2p, Cu
2p, Mg 2p, Zn 2p, and O 1s in both bulk and holey HEO. As shown
in Figure 3a, the O 1s spectrum of holey lamellar HEO can be
fitted into three distinct peaks. The main peak located at 530.7 eV
can be ascribed to oxygen ions in the crystal lattice, peak at about
531.6 eV is attributed to oxygen vacancies and peak at 532.9 eV
corresponds to surface labile oxygen.[34,35] Among the various
oxygen species, the content of oxygen vacancies in holey HEO is
up to 39%. The generation of oxygen vacancies is mainly
attributed to the following aspects: (i) Oxygen vacancies formed
by reducing the HEO particle size to the nanoscale level. It is
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10.1002/ange.202004892
Angewandte Chemie
RESEARCH ARTICLE
Figure 4. DFT-calculated conformation of adsorption states and adsorption
energies of a benzyl alcohol molecule on (a) bulk HEO and (b) holey HEO. (c)
Electron localization function maps of bulk HEO (the upper one) and holey HEO
(the lower one).
nanoparticles prepared by the anchoring-merging process form
the holey lamellar framework, and the unique holey lamellar
structure provides high specific surface areas and abundant
exposed catalytic active sites for the catalytic reaction, leading to
exponentially increased oxidation efficiency. Consequently, the
synergistic effect of the unique HEO material, abundant oxygen
vacancies and holey lamellar framework together leads to the
ultra-high catalytic activity.
Oxygen vacancies are one of the fundamental and inherent
defects of oxide materials, which have an important influence on
their catalysis performance. DFT calculations were employed to
demonstrate the role of oxygen vacancies in the adsorption
process of benzyl alcohol molecules.[39,40] Due to the five
elements play the same role in the reaction, the values of
adsorption energies (Eads) calculated by selecting arbitrary
element as adsorption site are close to each other, which is
verified by the similar Eads values obtained via randomly selecting
Cu and Mg as the adsorption sites in bulk HEO modeling (Figure
S11). Figure 4a and 4b reveal that the Eads of holey lamellar HEO
is 1.43 eV, five times larger than that of bulk material (0.27 eV),
indicating that the benzyl alcohol is more easily adsorbed on holey
lamellar catalyst. This result is attributed to the metal catalytic
sites near oxygen vacancies possessing stronger adsorption
capacity towards the hydroxyl groups in benzyl alcohol. As shown
in Figure 4c, for an ideal HEO, the surface electrostatic potential
is dominated by oxygen and shows negative potential. For the
vacancy-abundant holey lamellar HEO, many metal ions are
exposed on the surface of the material, and the electrostatic
potential near these metal ions is positive. When the -OH group
of benzyl alcohol approaches, the H combines with the surface
oxygen by hydrogen bond, and the O combines with the positively
charged metals through metal-oxygen bond, greatly increasing
the adsorption energy.[41,42] However, for bulk HEO, the OH—O
angle is too small to form hydrogen bond (Figure S12). Thus, H in
benzyl alcohol molecule cannot form hydrogen bond with surface
oxygen on bulk HEO as the case in holey HEO.
To elucidate the contribution of HEO in catalytic reactions,
comparative catalytic tests of the quaternary, ternary, binary and
single metal oxide catalysts were performed under the premise of
not changing the holey lamellar structure (Table S5), and their
corresponding oxygen vacancies contents were estimated by O
XPS (Figure S13). The XPS results show that the oxygen
vacancies contents of above catalysts are 22%, 25%, 41% and
37% respectively, showing an increasing trend with metal element
species increases. It's worth noting that oxygen vacancies
contents in quaternary and ternary catalysts are similar with holey
lamellar HEO but these catalysts exhibit much lower catalytic
activity. According to the above, in addition to oxygen vacancies,
the HEO material with unique crystal structure is indispensable to
its ultra-high catalytic properties.
Conclusion
In conclusion, we have demonstrated that the holey lamellar HEO
(Co0.2Ni0.2Cu0.2Mg0.2Zn0.2O) catalyst prepared by
a simple
anchoring-merging process was ultra-highly efficient for the
solvent-free aerobic atmospheric oxidation of benzyl alcohol. The
presence of abundant oxygen vacancies and exposed catalytic
sites in the catalyst significantly enhances their catalytic efficiency,
and up to 98% conversion rate can be achieved at 120 °C in only
2 hours. By rational regulating the catalytic reaction parameters,
we can selectively optimize benzoic acid or benzaldehyde as the
dominant product. This study is significant in the utilization of HEO
material with unique architecture and indeed give a brilliant
perspective in the development of heterogeneous catalysts in
industrial catalysis area.
Acknowledgements
This work was supported by the Young Thousand Talented
Program and the National Natural Science Foundation of China
(21671073 and 21621001), the “111” Project of the Ministry of
Education of China (B17020), Program for JLU Science and
Technology Innovative Research Team. SD was supported by
U.S. Department of Energy, Office of Science, Basic Energy
Sciences, Materials Sciences and Engineering Division.
Keywords: porous material• anchoring and merging process •
mesoporous structure • high entropy oxide • oxidation of benzyl
alcohol
[1]
[2]
[3]
[4]
[5]
D. J. Cole-Hamilton, Science 2003, 198, 317-341.
C. Parmeggiani, F. Cardona, Green Chem. 2012, 14, 547-564.
R. Reithmeier, C. Bruckmeier, B. Rieger, Catalysts 2012, 2, 544-571.
L. Wu, Y. Zhang, Y. G. Ji, Curr. Org. Chem. 2013, 17, 1288-1302.
On the basis of above findings, the ultra-high catalysis
performances of holey lamellar HEO are mainly attributed to the
following aspects: (i) The unique structure of elements randomly
distributed on the atomic scale endows the HEO material
unexpected catalytic properties for the oxidation of benzyl alcohol.
(ii) When the high-entropy oxide particles get as small as
nanoscale, a large number of oxygen vacancies appear among
the material. The presence of oxygen vacancies boosts the
oxygen delivery capacity and greatly improves the adsorption
energy towards benzyl alcohol, making the oxidation reaction
more easily be triggered. (iii) The interconnected HEO
A. Abad, P. Concepcion, A. Corma, H. Garcia, Angew. Chem. Int. Ed.
2005, 44, 4066–4069.
[6]
[7]
[8]
T. Wang, P. Zhang, Y. Sun, B. Liu, Y. Liu, Z. A Qiao, Q. Huo, S. Dai,
Chem. Mater. 2017, 29, 4044−4051.
Y. Zhao, C. Yu, S. Wu, W. Zhang, W. Xue, Z. Zeng, Catal. Lett. 2018,
148, 3082–3092.
J. Sun, Y. Han, H. Fu, X. Qu, Z. Xu, S. Zheng, Chem. Eng. J. 2017, 313,
1–9.
This article is protected by copyright. All rights reserved.

10.1002/ange.202004892
Angewandte Chemie
RESEARCH ARTICLE
[9]
W. Zhan, J. Wang, H. Wang, J. Zhang, X. Liu, P. Zhang, M. Chi, Y. Guo,
Y. Guo, G. Lu, S. Sun, S. Dai, H. Zhu, J. Am. Chem. Soc. 2017, 139,
8846-8854.
[31] V. R. Choudhary, R. Jha, P. Jana, Green Chem. 2007, 9, 267–272.
[32] N. Dimitratos, Jo. A. Lopez-Sanchez, D. Morgan, A. F. Carley, R.
Tiruvalam, C. J. Kiely, D. Bethella, G. J. Hutchings, Phys. Chem. Chem.
Phys. 2009, 11, 5142–5153.
[10] D. Li, F. Ruan, Y. Jin, Q. Ke, Y. Cao, H. Wang, T. Wang, Y. Song, P. Cui,
Catal. Sci. Technol. 2019, 9, 418-424.
[33] P. Zhang, Y. Gong, H. Li, Z. Chen, Y. Wang, Nat. Commun. 2013, 4,
[11] H. Zhanga, C. Wu, W. Wang, J. Bu, F. Zhou, B. Zhang, Q. Zhang, Appl.
Catal. B: Environ. 2018, 227, 209–217.
1593.
[34] R. Wei, M. Fang, G. Dong, C. Lan, L. Shu, H. Zhang, X. Bu, J. C. Ho,
[12] Y. Du, Q. Wang, X. Liang, Y. He, J. Feng, D. Li, J. Catal. 2015, 331, 154–
ACS Appl. Mater. Interfaces 2018, 10, 7079−7086.
161.
[35] B. Karimi, A. Zamani, J. Iran. Chern. Soc. 2008, 5, Sl-S20.
[13] H. Su, K. X. Zhang, B. Zhang, H. Wang, Q. Yu, X. Li, M. Antonietti, J.
[36] J. Lu, J. Song, H. Niu, L. Pan, X. Zhang, L. Wang, J. J. Zhou, Appl. Surf.
Sci. 2016, 371, 61-66.
Chen, J. Am. Chem. Soc. 2017, 139, 811−818.
[14] P. Liu, W. Li, B. Thokchom, B. Park, M. Cui, D. Zhao, J. Khim, J. Mater.
Chem. A 2015, 3, 6492-6500.
[37] X. Zou, J. Liu, J. Su, F.Zuo, J. Chen,, P. Feng, Chem. Eur. J. 2013, 19,
2866-2873.
[15] H. Tian, X. Liu, L. Dong, X. Ren, H. Liu, C. A. H. Price, Y. Li, G. Wang, J.
Liu, Adv. Sci. 2019, 6, 1900807.
[38] S. Gong, Z. Xie, W. Li, X. Wu,, N. Han, Y. Chen, Appl. Catal. B: Environ.
2019, 241, 578–587.
[16] X. Wang, S. Zhao, Y. Zhang, Z. Wang, J. Feng, S. Song, H. Zhang, Chem.
Sci. 2016, 7, 1109-1114.
[39] Q. Wang, L, Chen, S. Guan, X. Zhang, B. Wang, X. Cao, Z. Yu, Y. He,
D. G. Evans, J. Feng, D. Li, ACS Catal. 2018, 8, 3104−3115.
[17] C. M. Rost, E. Sachet, T. Borman, A. Moballegh, E. C. Dickey, D. Hou,
[40] J. Zheng, X. Chen, X. Zhong, S. Li, T. Liu, G. Zhuang, X. Li, S. Deng, D.
J. L. Jones, S. Curtarolo, J. P. Maria, Nat. Commun. 2015, 6, 8485.
Mei, J. Wang, Adv. Funct. Mater. 2017, 27, 1704169.
[18] J. Zhang, J. Yan, S. Calder, Q. Zheng, M. A. McGuire, D. L. Abernathy,
Y. Ren, S. H. Lapidus, K. Page, H. Zheng, J. W. Freeland, J. D. Budai,
R. P. Hermann, Chem. Mater. 2019, 31, 3705−3711.
[41] Y. Ma, S. Zhang, C. Chang, Z. Huang, J. C. Ho, Y. Qu, Chem. Soc. Rev.
2018, 47, 5541-5553.
[42] Z. Huang, T. Zhang, C. Chang, J. Li, ACS Catal. 2019, 9, 5523−5536.
[19] A. Sarkar, Q. Wang, A. Schiele, M. R. Chellali,S. S. Bhattacharya, D.
Wang, T. Brezesinski, H. Hahn, L. Velasco, B. Breitung, Adv. Mater. 2019,
31, 1806236.
[20] A. Sarkar, R. Djenadic, N. J. Usharani, K. P. Sanghvi, V. S.K.
Chakravadhanula, A. S. Gandhi, H. Hahn, S. S. Bhattacharya, J. Eur.
Ceram. Soc. 2017, 37, 747–754.
[21] H. Wang, J. T. Robinson, G. Diankov, H. Dai, J. Am. Chem. Soc. 2010,
132, 3270–3271.
[22] N. Qiu, H. Chen, Z. Yang, S. Sun, Y. Wang, Y. Cui, J. Alloy. Compd.
2019, 777, 767e774.
[23] Y. Fu, L. Sun, H. Yang, L. Xu, F. Zhang, W. Zhu, Appl. Catal. B: Environ.
2016, 187, 212–217.
[24] Z. Shi, C. Zhang, C. Tang, N. Jiao, Chem. Soc. Rev. 2012, 41, 3381–
3430.
[25] W. Zhong, H. Liu, C. Bai, S. Liao, Y. Li, ACS Catal. 2015, 5, 1850−1856.
[26] D. I. Enache, J. K. Edwards, P. Landon, B. Solsona-Espriu, A. F. Carley,
A. A. Herzing, M. Watanabe, C. J. Kiely, D. W. Knight, G. J Hutchings,
Science 2006, 311, 362-365.
[27] G. Wu, X. Wang, N. Guan, L. Li, Appl. Catal. B: Environ. 2013, 136-137,
177-185.
[28] L. C. Wang, Y. Liu, M. Chen, Y. Cao, H. He, K. Fan, J. Phys. Chem. C
2008, 112, 6981-6987.
[29] T. Li, F. Liu, Y. Tang, L. Li, S. Miao, Y. Su, J. Zhang, J. Huang, H. Sun,
M. Haruta, A. Wang, B. Qiao, J. Li, T. Zhang, Angew. Chem. Int. Ed.
2018, 57, 7795-799.
[30] T. Gao, T. Wang, W. Wu, Y. Liu, Q. Huo, Z. A. Qiao, S. Dai, Adv. Mater.
2019, 31, 1806254.
This article is protected by copyright. All rights reserved.

10.1002/ange.202004892
Angewandte Chemie
RESEARCH ARTICLE
RESEARCH ARTICLE
Danyang Feng[a]Yangbo Dong,[a]
Liangliang Zhang,[a] Xin Ge,[b] Wei
Zhang,[b] Sheng Dai,[d] and Zhen-An
Qiao*[a]
Page No. – Page No.
Holey Lamellar High Entropy Oxide
as Ultra-Highly Active Heterogeneous
Catalyst for Solvent-free Aerobic
Oxidation of Benzyl Alcohol
A novel high entropy oxide material with mesoporous structure is prepared by an
anchoring and merging process, and exhibits ultra-high catalytic activity for the
oxidation of benzyl alcohol. Benzoic acid or benzaldehyde can be selectively
optimized as the main product by rational regulating the catalysis parameters. The
ultra-high catalysis performance is attributed to the synergistic effect of the porous
framework and abundant oxygen vacancies.
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