Hierarchical CeO2@N-C Ultrathin Nanosheets for Efficient Selective Oxidation of Benzylic Alcohols in Water

Article abstract of DOI:10.1021/acs.inorgchem.1c00076

A monodisperse CeO2@N-C ultrathin nanosheet self-assembled hierarchical structure (USHR) has been prepared by metal-organic framework template methods. The uniform coating of nitrogen-doped carbon (N-C) layers could play an important role in the adsorption and activation of benzylic alcohol. The unique 3D hierarchical structure self-assembled by ultrathin nanosheets provided enough active sites for the catalytic reaction. Therefore, the CeO2@N-C USHR can afford excellent catalytic performance for selective oxidation of benzylic alcohols in water.

Full text of DOI:10.1021/acs.inorgchem.1c00076

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Hierarchical CeO2@N−C Ultrathin Nanosheets for Efficient Selective
Oxidation of Benzylic Alcohols in Water
Juan Hao,‡ Zhouyang Long,‡ Liming Sun,* Wenwen Zhan, Xiaojun Wang, and Xiguang Han*
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ABSTRACT: A monodisperse CeO2@N−C ultrathin nanosheet self-assembled
hierarchical structure (USHR) has been prepared by metal−organic framework
template methods. The uniform coating of nitrogen-doped carbon (N−C) layers
could play an important role in the adsorption and activation of benzylic alcohol. The
unique 3D hierarchical structure self-assembled by ultrathin nanosheets provided
enough active sites for the catalytic reaction. Therefore, the CeO2@N−C USHR can
afford excellent catalytic performance for selective oxidation of benzylic alcohols in
water.
proportion of surface active sites. The nanosheets with a two-
dimensional (2D) structure usually can expose numerous
accessible surface active sites in the catalytic process due to
the large surface area.24−27 However, 2D nanosheets with high
surface energy are easy to aggregate, which reduces surface
active sites and catalytic activity. Three-dimensional (3D)
structures inserted by nanosheets with integrated morphology
can prevent the aggregation, which can not only avoid the
reduction of active sites but also promote mass transfer for the
catalytic reaction.28,29 Therefore, the fabrication of N-doped C
layer-coated CeO2 with a nanosheet-interpreted 3D structure
could be beneficial to improve the catalytic activity and
selectivity. Nevertheless, the synthesis of such a material is still
a big challenge.
INTRODUCTION
■
In the field of organic synthesis, selective oxidation of aromatic
alcohols to relevant carbonyl compounds (e.g., aldehydes and
ketones) without overoxidation to carboxylic acids and carbon
dioxide remains challenging and fundamentally important.1,2
Recently, great efforts have been paid to development of high
activity, selectivity, and stable catalysts. In order to achieve good
catalytic performance, some homogeneous metal complexes of
Ru,3 Pd,4 and Cu5 had been used as the co-catalysts; however,
they had suffered from the limited reserves and high price, which
greatly suppress their practical application. Furthermore, most
of the reported catalytic reaction systems require organic
solvents, such as toluene,6,7 ethanol,8 etc., which are usually toxic
and expensive. Therefore, it is urgent to construct a cheap, easily
available, green, and high activity catalyst.
Herein, a monodisperse 3D hierarchical architecture self-
assembled by ultrathin CeO2 nanosheets encapsulated by N-
doped carbon layers (CeO2@N−C USHR) was synthesized by
the thermal treatment of Ce-based metal−organic frameworks
(Ce-MOFs). Due to the unique structure of MOFs, the obtained
CeO2@N−C USHR was uniformly coated by N-doped carbon
layers, which had an important influence on the adsorption and
activation of benzylic alcohol. The unique 3D hierarchical
architecture effectively prevented the agglomeration of ultrathin
CeO2@N−C nanosheets and provided enough active sites for
the catalytic reaction. Based on these two structure character-
istics, the CeO2@N−C USHR exhibits excellent catalytic
activity for selective oxidation of benzylic alcohols in water.
CeO2 has unique oxygen storage capacity, low cost,
nontoxicity, and chemical and physical stability, which make it
an outstanding support or catalyst for catalytic oxidation.9−13 In
order to achieve the catalytic properties comparable to noble
metals, noble metal materials or peroxides as additives or
oxidants have been employed to improve the catalytic
performance of cerium-based catalysts, which leads to the
sharp rise in cost and environmental pollution.14,15 Recently,
lots of research showed that carbon materials as the co-catalysts
could enhance the catalytic activity and selectivity due to their
low cost and high mobility of charge carriers.16−22 The density
functional theory (DFT) indicated that nitrogen doping can
further modulate the spin density, energy bandgap, and charge
density,23 which can result in a material with a higher catalytic
performance than pure carbon-based catalysts by reducing the
activation energy. Therefore, nitrogen-doped carbon materials
may replace noble metals as the co-catalysts with enhanced
performance for CeO2-based catalysts.
Received: January 10, 2021
Published: May 25, 2021
To reduce the activation energy, more active sites of CeO2 are
needed to react with the activated molecules. Therefore, another
factor for limiting cerium-based catalytic efficiency is the low
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CeO2@N−C. The second stage starting from 400 °C could be
assigned to collapse of the Ce-MOF precursor.31 Thus, we have
chosen the annealing temperature as 550 °C for the carbon-
ization of the Ce-MOF precursor. The product after annealing
treatment was characterized by XRD measurements (Figure
S3), and the XRD pattern exhibited the diffraction peaks
assigned to the cubic CeO2 phase (PDF No. 00-034-0394),
indicating that the Ce-MOF precursors were transformed into
CeO2 during the pyrolysis process. The SEM image (Figure 1c)
showed that the product has preserved the overall nanosheet
self-assembled flower shape of the Ce-MOF precursors. The
magnified observations (inset of Figure 1c) further revealed the
2D ultrathin nanosheets after annealing treatment presented a
rough surface. The thickness of the ultrathin nanosheets was
further characterized by atomic force microscopy (AFM)
analysis (Figure 1d,e), which indicated that the nanosheet had
a thickness of about 4−5 nm. By means of the SEM
measurement, the thickness statistics of the nanosheet is
shown in Figure S4. It could be seen that the thickness of the
nanosheet is mainly concentrated at 4.5 1.1 nm, which was
consistent with the AFM results.
RESULTS AND DISCUSSION
■
A facile MOF as a precursor strategy involving the thermal
annealing of Ce-MOF precursors under an Ar atmosphere was
developed to engineer the monodisperse CeO2@N−C ultrathin
nanosheet self-assembled hierarchical architecture (USHR).
Figure 1a shows the detail synthesis process of USHR. First,
In order to further understand the inner structural
information and components, the CeO2@N−C USHR was
characterized by transmission electron microscopy (TEM).
From Figure 2a, we could further confirm the 2D nanosheet
interconnected 3D hierarchical architecture. A high−magnifi-
cation survey (Figure 2b) detected from a sampling area of the
Figure 1. (a) Schematic image of the synthetic strategy of
monodisperse CeO2@N−C ultrathin nanosheet self-assembled hier-
archical architecture (USHR). Typical SEM images of (b) Ce-MOF
precursor and (c) CeO2@N−C USHR, (d) AFM image of the single
nanosheet exfoliated from CeO2@N−C USHR, and (e) corresponding
height profiles of lines in Figure 1d.
based on the introducing regulator strategy, a monodisperse Ce-
MOF precursor with nanosheet self-assembled flowers had been
obtained by the solvothermal method in an N,N-dimethylfor-
mamide and ethanol mixed solvent (see the experimental details
in the Supporting Information). X-ray powder diffraction
(XRD) measurement has been used to characterize the crystal
structure of Ce-MOFs. The diffraction peaks of the as-
synthesized Ce-MOFs (Figure S1) matched well with published
XRD patterns.30
The scanning electron microscopy (SEM) images of the
precursors have been shown in Figure 1b, which indicated that
these precursor particles possessed high monodisperse and
regular hierarchical architecture morphology interpreted by 2D
nanosheets. The higher-magnification SEM image showed that
these nanosheets possessed smooth surfaces. According to
thermogravimetric analysis (TGA) results of Ce-MOFs (Figure
S2), there were two major stages of rapid weight loss, indicating
that the Ce-MOF precursor decomposes in two steps to produce
Figure 2. (a) Typical TEM image of a monodisperse CeO2@N−C
USHR nanoparticle, (b) high-magnification TEM image of CeO2@N−
C USHR, (c) high-resolution TEM image, and (d) corresponding
SAED pattern of CeO2@N−C USHR; (e, f) STEM image and the
corresponding elemental line profiles of CeO2@N−C USHR nano-
particles, (g−k) STEM image and the corresponding elemental
mapping of elements Ce, O, C, and N, and (l) N2 sorption isotherm
and the corresponding BJH pore size distribution curve of CeO2@N−C
USHR nanoparticles.
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nanosheet, which clearly revealed that numerous small CeO2
particles with uniform size distribution homogeneously laid on
the carbon nanosheet walls. Furthermore, the crystallinity of
small CeO2 particles and the carbon nanosheet was determined
by using HRTEM and selective area electron diffraction (SAED)
patterns. The high-resolution TEM image (Figure 2c) showed
interplanar distance assigned to (111) planes of cubic CeO2
(0.311 nm), revealing that the small particles were CeO2
nanoparticles. The corresponding SAED pattern showed that
polycrystalline rings (Figure 2d) matched well with cubic CeO2.
Scanning transmission electron microscopy (STEM) elemental
profiles and elemental mapping were employed to further study
the inter-structure and composition. As shown in Figure 2e,g,
the STEM image of the exfoliated ultrathin 2D nanosheets and
the corresponding elemental line profiles (Figure 2f) and
mapping profiles (Figure 2h−k) showed the uniform distribu-
tion of elements Ce, O, C, and N in the nanosheet. Therefore,
through the pyrolysis decomposition, the cerium ions in the Ce-
MOFs had been converted into CeO2 nanoparticles, while the
organic ligands (1,2,4,5-benzenetetracarboxylic acid) and
regulator (1,2-benzisothiazolin-3(2H)-one) could be easily in
situ carbonized to form the N-doped carbon layer. Meanwhile,
during the annealing process, the produced gas escaped from the
inside of the Ce-MOF precursor, which resulted in the
formation of a loosely porous structure. The porosity of
CeO2@N−C USHR was further investigated by N2 adsorp-
tion−desorption isotherms. The N2 sorption isotherms showed
that CeO2@N−C USHR had a Brunauer−Emmett−Teller
surface area of about 45.6 m2/g, and the pore distribution was
mainly centered at about 4 nm according to the Barrett−
Joyner−Halenda method (Figure 2l).
respectively.32,33 The high-resolution O 1 s spectrum (Figure
3b) could be split into three peaks, which could be ascribed to
the lattice oxygen (OL) at 529.6 eV, oxygen-deficient regions
(OV) at 530.1 eV, and chemisorbed oxygen (OC) at 531.8 eV.34
The C 1 s spectrum (Figure 3c) indicated the peaks assigned to
binding energy at 284.7 eV (CC/C−C bonds), 285.6 eV (C−
N bonds), and 289.3 eV (C−O bonds), respectively.35 Figure 3d
shows a high-resolution N 1s spectrum, which could be split into
two Gaussian components centered at 399.8 eV corresponding
to N−graphene bonds and 407.2 eV corresponding to N−O
bonds. Therefore, the XPS results further indicated that the
CeO2@N−C USHR nanostructure was composed of CeO2
nanoparticles coated by N-doped C layers.
The uniformly N-doped C-coated CeO2 nanoparticles with
hierarchical architecture provided highly approachable adsorp-
tion and active catalytic sites, leading to the promising
performance of CeO2@N−C USHR as the catalyst. The ability
of CeO2@N−C USHR to catalyze the oxidation reaction of
benzyl alcohol to benzaldehyde in water had been investigated.
The influences of the catalyst amount on the reaction yields
toward oxidation of benzyl alcohol to benzaldehyde had been
studied, and the data are shown in Figure S6. It can be seen that
when the catalyst amount increased from 10 to 50 mg, the
conversion of benzyl alcohol increased from 25.1 to 99.9%.
Thus, the catalyst amount was set at 50 mg in the following
catalytic tests. The blank experiments indicated that no
oxidation occurs in the absence of heating or catalysts, indicating
that the usage of an appropriate catalyst and heating were the
two key elements for this reaction (Table S1). Therefore, the
catalytic oxidation of benzyl alcohol was performed in glass
pressure bottles in the water at different temperatures (Figure
4a). To thoroughly investigate the catalytic performance of
CeO2@N−C USHR (CeO2 nanoparticles coated by N-doped
C), CeO2@C USHR (CeO2 nanoparticles coated by C, Figure
S7), pure CeO2 USHR (CeO2 nanoparticles, Figure S8), and
commercial CeO2 (Figure S9) are used as reference samples.
The 13C NMR spectra of benzyl alcohol and benzaldehyde are
shown in Figure S10. As shown in Figure 4b, CeO2@N−C
USHR exhibited production yield reaching above 99.9% after 48
h, which was the highest catalytic activity.7,36−38 The catalytic
activity of four samples followed the trend CeO2@N−C USHR
> CeO2@C USHR > CeO2 USHR > commercial CeO2. The
above catalytic experiments indicated that both the morphology
(hierarchical architecture assembled by ultrathin nanosheets)
and the composite (N-doped C layer) have important effects on
their catalytic activity. The nanosheet self-assembled hierarch-
ical architecture increased the specific surface area of the catalyst
and exposed more active sites. The N-doped C layer might act as
the medium to strengthen the adsorption of reactants (benzyl
alcohol) on CeO2 catalysts and reduced the activation energy of
the catalytic reaction. To investigate the origin and reveal the
activity of CeO2@N−C USHR for the oxidation of benzyl
alcohol to benzaldehyde, the kinetic analysis for catalytic
oxidation was performed. Figure 4c shows the curves of benzyl
alcohol conversion versus reaction time at different reaction
temperatures. It appeared that the conversion of benzyl alcohol
increased when the reaction temperature increased from 353 to
423 K. The trend of yields for different catalysts at various
temperatures matched well with the yield at 150 °C (Figure S11
and Figure S12). Accordingly, we plotted ln k versus 1/T (Figure
4d), where k is the rate constant obtained by the linearly fitting
relationship between the yield of benzyl alcohol and the reaction
time according to the integral rate equation. As shown in Figure
The surface chemical composition and valence states of
CeO2@N−C USHR were further revealed by X-ray photo-
electron spectroscopy (XPS). The survey spectrum (Figure S5)
showed the existence of Ce, O, C, and N elements in the CeO2@
N−C USHR sample. Figure 3a shows the high-resolution Ce 3d
spectrum with two kinds of splitted peaks (Ce 3d3/2 labeled with
U and Ce 3d3/2 labeled with V). U1, U0, V1, and V0 reflected the
Ce3+ characteristic peaks, whereas U3, U2, U, V3, V2, and V
corresponded with Ce4+ characteristic peaks. In addition, the
Ce3+ and Ce4+ portions can be calculated to be 28.7 and 71.3%,
Figure 3. High-resolution XPS spectra of the CeO2@N−C USHR
nanoparticle, (a) Ce 3d, (b) O 1s, (c) C 1s, and (d) N 1s.
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To demonstrate the universality of this catalytic system, the
oxidation of a variety of alcohol substrates was investigated over
the CeO2@N−C USHR catalysts (Figure 4f, Table S2, and
Figure S14). The oxidation of 4-hydroxybenzyl alcohol, 4-
methylbenzyl alcohol, 4-methoxybenzyl alcohol, 2-aminobenzyl
alcohol, cinnamyl alcohol, and indole-3-carbinol proceeds
efficiently over the target catalyst CeO2@N−C USHR with
high yields of their corresponding products; however, the
attachment of electron-withdrawing groups (like −F, −Cl, and
−Br) to the para-position of the phenyl ring largely decreased
the efficiency of the oxidation reaction. The corresponding 1H
NMR spectra of products and reactants are shown in Figure S14.
In order to illuminate the mechanism of CeO2@N−C USHR
for oxidation of benzyl alcohol to benzaldehyde with enhanced
catalytic activity, the adsorption and activation effects of
(111)CeO2 (i.e., CeO2 (111) surfaces) and (001)N‑graphite (i.e.,
N-doped graphite (001) surfaces) on benzyl alcohol were
investigated, respectively. We have used the DFT to research the
adsorptions of benzyl alcohol on (111)CeO2 and (001)N‑graphite
,
and the corresponding calculated adsorption energies were 0.15
and −3.40 eV, respectively. The optimized configurations for the
adsorption of benzyl alcohol on (111)CeO2 and (001)N‑graphite are
shown in Figure S15, which indicated that the coating of the N-
doped carbon layer on the CeO2 surface was beneficial to the
adsorption of benzyl alcohol, attributing to the π−π stacking
interaction between the graphite ring and the benzene ring of
benzyl alcohol. Activation of benzyl alcohol by the N-doped
carbon layer was studied by electron density difference maps
(Figure 5) and Mulliken population (Table S3). From Figure
5a,b, it can be found that the charge exchange between O and
HO atoms of benzyl alcohol became weaker after adsorption on
the (001)N‑graphite. A similar situation also occurred between C
and H1 atoms; that is, the charge exchange between C and H1
Figure 4. (a) Catalytic equation and reaction conditions for selective
oxidation of benzyl alcohol; (b) reaction yields of benzaldehyde from
the oxidation of benzyl alcohol using CeO2@N−C USHR, CeO2@C
USHR, CeO2 USHR, and commercial CeO2 as catalysts at 150 °C for
48 h in water; (c) benzyl alcohol conversion profiles obtained at the
indicated reaction temperature using the CeO2@N−C USHR catalyst;
(d) Arrhenius plot of the oxidation of benzyl alcohol with molecular
oxygen; (e) catalytic durability of CeO2@N,S-C USHR; (f) catalyzed
oxidation of various types of alcohols using CeO2@N−C USHR.
4d, all the plots of ln k versus 1/T were satisfactorily linear. For
the CeO2@N−C USHR catalyst, the slope and intercept were
−4819 and 8.28, respectively. According to the slope and
intercept values and the Arrhenius equation, i.e., ln k = ln A −
Ea/RT, the apparent activation energy (Ea) for the CeO2@N−C
USHR catalyst calculated by the Arrhenius equation was 40.0
1.6 kJ·mol−1, which was the lowest among these catalysts
(CeO2@C USHR (47.0 3.5 kJ·mol−1); CeO2 USHR (56.8
1.9 kJ·mol−1)) (Table 1). The results indicated that the external
Table 1. Apparent Activation Energy (Ea) and Pre-
Exponential Factor (a) of Catalyzed Oxidation of Benzyl
Alcohol to Benzaldehyde Using Various Catalysts
Ea
pre-exponential factor
catalysts
(kJ/mol)
(A/mol−1 dm3 s−1
)
CeO2@N−C USHR 40.0 1.6
21,162
18,769
3944
CeO2@C USHR
47.0 3.5
CeO2 USHR
56.8 1.9
C layer could reduce the Ea of oxidation of benzyl alcohol to
benzaldehyde, and N-doped C can further strengthen this effect.
Impressively, CeO2@N−C USHR nanoparticles could also
exhibit good stability with no obvious decrease in activity during
5 catalytic cycles (Figure 4e), and their original morphology and
crystal structure had been well maintained (Figure S13).39−44
Figure 5. Electron density difference maps of benzyl alcohol from
different angles: (a, c) before adsorption and (b, d) after adsorption on
the N-doped graphite (001) surface.
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atoms after adsorption (Figure 5c) was weaker than that before
adsorption (Figure 5d). After benzyl alcohol adsorbed on the
(001)N‑graphite, the calculated Mulliken charge on O and C
increased from −0.78 and −0.36 e to −0.74 and −0.34 e,
respectively; meanwhile, the Mulliken charge on HO and H1
decreased from 0.55 and 0.31 e to 0.53 and 0.28 e, respectively
(Table S3). The bond lengths of corresponding O−HO and C−
H1 stretched from 0.976 and 1.112 Å to 0.983 and 1.114 Å,
respectively. Based on the charge redistribution and the
elongated bond length, it could be inferred that the O−HO
and C−H1 bonds of benzyl alcohol became weak and easy to
break after adsorption on the (001)N‑graphite. In other words, the
N-doped carbon layer could activate O−HO and C−H1 bonds in
benzyl alcohol, which was conducive to the oxidation of benzyl
alcohol to benzaldehyde. The above calculation data exhibited
that coating N-doped carbon layers on CeO2 not only effectively
enhanced the adsorption of benzyl alcohol but also had an
important influence on the activation of benzyl alcohol.
Materials Science, Jiangsu Normal University, Xuzhou
221116, P. R. China
Zhouyang Long − Jiangsu Key Laboratory of Green Synthetic
Chemistry for Functional Materials, School of Chemistry &
Materials Science, Jiangsu Normal University, Xuzhou
221116, P. R. China
Wenwen Zhan − Jiangsu Key Laboratory of Green Synthetic
Chemistry for Functional Materials, School of Chemistry &
Materials Science, Jiangsu Normal University, Xuzhou
221116, P. R. China; orcid.org/0000-0002-7004-9155
Xiaojun Wang − Jiangsu Key Laboratory of Green Synthetic
Chemistry for Functional Materials, School of Chemistry &
Materials Science, Jiangsu Normal University, Xuzhou
221116, P. R. China; orcid.org/0000-0002-1461-4922
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00076
Author Contributions
‡These authors contributed equally.
CONCLUSIONS
■
Notes
In summary, N-doped porous carbon-coated CeO2 with
hierarchical architecture has been engineered via a Ce-MOF
as the hard template. Experimental results about the catalytic
activities toward selective oxidation of benzyl alcohol to
benzaldehyde and the corresponding theoretical calculation
indicated that the N-doped C layer can act as the medium to
strengthen the adsorption of benzyl alcohol reactants on CeO2
catalysts and reduced the activation energy of the catalytic
reaction. The 3D hierarchical architecture can provide lots of
active sites to react with activator molecules. Therefore, CeO2@
N−C USHR exhibits excellent catalytic performance, including
the activity, selectivity, and cycle stability, due to the synergetic
effect of 3D hierarchical architecture and N-doped C layers. The
obtained CeO2@N−C USHR product in this study is promising
for other catalytic systems and may inspire further studies to
develop green synthesis.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Grant No. 22005126), the Natural
Science Foundation of Jiangsu Province (BK20191466) and
Xuzhou City (KC20061), and the Postgraduate Research and
Practical Innovation Program of Jiangsu Province
(KYCX20_2247).
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00076.
■
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Experiment details, XRD and TGA results of the Ce-MOF
precursor (Figures S1, S2), XRD and XPS of the CeO2@
N−C USHR (Figures S3, S4), and characteristics and
catalytic activity of compared samples and recycled
CeO2@N,S-C HN (Figures S5−S10) (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Liming Sun − Jiangsu Key Laboratory of Green Synthetic
Chemistry for Functional Materials, School of Chemistry &
Materials Science, Jiangsu Normal University, Xuzhou
221116, P. R. China; Email: limingsun1226@jsnu.edu.cn
Xiguang Han − Jiangsu Key Laboratory of Green Synthetic
Chemistry for Functional Materials, School of Chemistry &
Materials Science, Jiangsu Normal University, Xuzhou
221116, P. R. China; orcid.org/0000-0003-2469-4572;
Email: xghan@jsnu.edu.cn
(10) Peng, R. S.; Li, S. J.; Sun, X. B.; Ren, Q. M.; Chen, L. M.; Fu, M.
L.; Wu, J. L.; Ye, D. Q. Size effect of Pt nanoparticles on the catalytic
oxidation of toluene over Pt/CeO2 catalysts. Appl. Catal. B-Environ.
2018, 220, 462−470.
Authors
Juan Hao − Jiangsu Key Laboratory of Green Synthetic
Chemistry for Functional Materials, School of Chemistry &
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