CeO2@N/C@TiO2 Core-shell Nanosphere Catalyst for the Aerobic Oxidation of 5-Hydroxymethylfurfural to 5-Hydroxymethyl-2-Furancarboxylic Acid

Article abstract of DOI:10.1002/cctc.202100091

Defective D-CeO₂@N/C@TiO₂ nanospheres, each comprising a spherical CeO₂ core coated with shells of N-doped carbon and TiO₂, were successfully synthesized then evaluated for the aerobic oxidation of 5-hydroxymethylfurfural (HMF) to 5-hydroxymethyl-2-furancarboxylic acid (HMFCA). Detailed catalyst characterization studies using XRD, SEM, TEM, TG-DTA, XPS, N₂ physisorption confirmed the hierarchical core-shell structure of the D-CeO₂@N/C@TiO₂ nanospheres, with the defective surface structures created through a thermal hydrogenation process using NaBH₄ promoting HMF conversion. The effect of various reaction parameters, including the reaction time, temperature, oxygen pressure, type of alkali co-reactant and the amount of catalyst, on HMF oxidation to HMFCA over the D-CeO₂@N/C@TiO₂ nanospheres were studied. Under the optimized reaction conditions (temperature 80 °C, reaction time 30 min, O₂ pressure 1 MPa), a high HMF conversion of 87.8 % and a remarkable HMFCA selectivity of 100 % were obtained. In addition, the D-CeO₂@N/C@TiO₂ nanosphere catalyst showed great stability over four consecutive HMF oxidation tests, implying good catalyst stability. Experimental findings were used to develop a plausible reaction mechanism for the selective oxidation of HMF on the D-CeO₂@N/C@TiO₂ nanospheres.

Full text of DOI:10.1002/cctc.202100091

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: CeO2@N/C@TiO2 Core-shell Nanosphere Catalyst for
the Aerobic Oxidation of 5-Hydroxymethylfurfural to 5-
Hydroxymethyl-2-Furancarboxylic Acid
Authors: yong song, Geoffrey I.N. Waterhouse, Feng Han, Yan Li, and
Shiyun Ai
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To be cited as: ChemCatChem 10.1002/cctc.202100091
Link to VoR: https://doi.org/10.1002/cctc.202100091
01/2020

10.1002/cctc.202100091
ChemCatChem
CeO2@N/C@TiO2 Core-shell Nanosphere Catalyst for
the Aerobic Oxidation of 5-Hydroxymethylfurfural to
5-Hydroxymethyl-2-Furancarboxylic Acid
Yong Song[a], Geoffrey I.N. Waterhouse[b], Feng Han[a], Yan Li*[a], and Shiyun Ai*[a]
[a] Yong Song, Prof. Dr. Feng Han, Prof. Dr. Yan Li and Prof. Dr. Shiyun Ai
College of Chemistry and Material Science, Shandong Agricultural University, Taian, Shandong,
271018, China
Email: ashy@sdau.edu.cn
Email: liyan2010@sdau.edu.cn
[b] Prof. Dr. Geoffrey I.N. Waterhouse
School of Chemical Sciences, The University of Auckland Private Bag 92019, Auckland, New Zealand
ABSTRACT
time, temperature, oxygen pressure, type of alkali
co-reactant and the amount of catalyst, on HMF
Defective D-CeO2@N/C@TiO2 nanospheres, each
comprising a spherical CeO2 core coated with
shells of N-doped carbon and TiO2, were
successfully synthesized then evaluated for the
aerobic oxidation of 5-hydroxymethylfurfural
(HMF) to 5-hydroxymethyl-2-furancarboxylic acid
(HMFCA). Detailed catalyst characterization
studies using XRD, SEM, TEM, TG-DTA, XPS, N2
physisorption confirmed the hierarchical core-shell
structure of the D-CeO2@N/C@TiO2 nanospheres,
with the defective surface structures created
through a thermal hydrogenation process using
NaBH4 promoting HMF conversion. The effect of
various reaction parameters, including the reaction
oxidation
to
HMFCA
over
the
D-
CeO2@N/C@TiO2 nanospheres were studied.
Under the optimized reaction conditions
(temperature 80 °C, reaction time 30 min, O2
pressure 1 MPa), a high HMF conversion of 87.8%
and a remarkable HMFCA selectivity of 100%
were
obtained.
In
addition,
the
D-
CeO2@N/C@TiO2 nanosphere catalyst showed
great stability over four consecutive HMF
oxidation tests, implying good catalyst stability.
Experimental findings were used to develop a
plausible reaction mechanism for the selective
oxidation of HMF on the D-CeO2@N/C@TiO2
nanospheres.
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INTRODUCTION
Dwindling fossil fuel reserves and escalating global
energy consumption, coupled the need to curb
anthropogenic CO2 emissions, motivates the
development of renewable energy technologies and
the search for alternative fuels.[1] Biomass
resources represent promising alternative to fossil
fuels, both as an energy source and as a feedstock
for the manufacture of important commodity
chemicals.[2] Particularly important in this regard
are catalytic technologies which can create
valuable chemicals from simple biomass-derived
platform molecule, is obtained by dehydration of
glucose or fructose.[4] HMF can then be
transformed into a variety of high value chemicals
and fuels,[5] with some of the important oxidation
products of HMF including 2,5-diformethylfuran
(DFF), 5-hydroxymethyl-2-furancarboxylic acid
(HMFCA),
5-formyl-2-furancarboxylic
acid
(FFCA), and 2,5-furandicarboxylic acid (FDCA)
(Scheme 1).[6] Amongst these products, FDCA and
HMFCA are considered to be prized targets owing
to their potential use as monomers in the synthesis
of bio-based polymers.[7] HMFCA is formed by the
selective oxidation of the aldehyde group in HMF,
and represents a novel monomer for the synthesis
of polyesters.[8] Further, HMFCA displays
antitumor activity[9] and is an intermediate in the
synthesis of a promising interleukin inhibitor.[10]
platform
chemicals
such
as
5-
hydroxymethylfurfural (HMF).
Sugars in the form of monosaccharides and
disaccharides can readily be obtained from biomass
sources via
enzymatic hydrolysis.[3] 5-
Hydroxymethylfurfural, an extremely versatile
Scheme 1. Pathways and products for the selective oxidation of 5-hydroxymethylfurfural (HMF).
However, HMFCA is an intermediate product in
the selective oxidation of HMF, and undergoes
further oxidation to FDCA. As such, is often
difficult to obtain HMFCA with a high selectivity
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during HMF oxidation. Synthetic methods for the
selective oxidation of HMF to HMFCA are
urgently needed. To date, a variety of chemical
approaches, mainly based on the use of noble metal
catalysts, have been explored for converting HMF
into HMFCA.[8b, 11] For example, Gorbanev et al.
evaluated the performance of an Au/TiO2
catalyst for the oxidation of HMF.5 They found that
low O2 pressures or low concentrations of base
favored the intermediate oxidation product
HMFCA relative to FDCA. Davis et al. prepared
Au/C and Au/TiO2 catalysts for the oxidation of
HMFCA over non-noble metal catalysts, though
the studies reported show excellent promise. Zhang
et al. prepared a heterogeneous catalyst (K-10 clay-
Mo) for the oxidation of HMF, obtaining a HMF
conversion of 100% with a HMFCA yield of 86.9%
[16]
over 3 h in toluene.
Using a heterogeneous
triazolium pre-catalyst, iron(II) phthalocyanine and
air, Brandolese et al. obtained HMFCA yield of
87%. [10a] However, these reports have limitations,
as they require complex organic compounds for the
catalyst synthesis and use organic solvents for
HMF oxidation tests. Maria Ventura et al. reported
that CeO2 [17], MgO·CeO2 [18]and mixed oxide
systems based on cerium[19] offer good catalytic
activity for the oxidation of HMF. However, the
main products are FDCA and DFF, thus preventing
the isolation of HMFCA as a stable intermediate.
Highly selective, inexpensive and stable catalysts
must be found for the direct synthesis of HMFCA.
HMF,[8a] obtaining a 92% selectivity to HMFCA
[12]
with 100% conversion of HMF. Wang
et al.
applied a Ru/CsPW catalyst for the oxidation of
HMF, obtaining a HMF conversion of 97.2% with
[13]
a HMFCA yield of 72.9%. Zhao
et al. used
commercially available Ag2O as a catalyst,
obtaining a HMFCA yield of 98% (60 min, 90 °C).
These works demonstrate that various noble metal
catalysts (Ru, Au and Ag) show good initial
selectivity for the oxidation of HMF to HMFCA.[14]
However, these noble metal-based catalysts have a
number of drawbacks, including the high cost of the
noble metals and poor recycling performance,[15]
motivating the search for non-precious metal
catalysts for this reaction.
In this work, we report the successful design of a
novel core-shell structured CeO2@N/C@TiO2
nanosphere catalyst. The catalyst was mixed with
NaBH4 and heated to prepare
a
D-
CeO2@N/C@TiO2 catalyst with a defective surface
structure (Scheme2), which offered outstanding
performance for the selective catalytic oxidation of
HMF to HMFCA. The oxidation of HMF tests were
carried out in a sodium tetraborate solution (pH
9.18) at 80 °C and 1 MPa O2, resulting in a HMF
To date, a few studies have been reported
examining the selective oxidation of HMF to
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10.1002/cctc.202100091
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conversion of 89% and HMFCA selectivity of 100% performance after four cycles, with supporting
after only 30 min. Catalyst recycling tests showed
that the catalyst retained excellent catalytic
catalysts characterization studies used to pinpoint
the origins of the excellent catalytic performance.
Scheme 2. Procedure used to synthesize the D-CeO2@N/C@TiO2 catalyst for the selective oxidation
of HMF to HMFCA.
MATERIALS AND METHODS
CeO2 nanospheres were synthesized using a
Materials.
Cerous
nitrate
hexahydrate
hydrothermal method. Briefly, 0.585 g of
Ce(NO3)3·6H2O, 2.252 g of KBr and 0.6 g of L-
asparagine were dissolved in 50 mL of deionized
water, then stirred for 1 h at room temperature. The
resulting solution was then transferred to a Teflon-
lined stainless-steel autoclave (volume 100 mL)
and heated at 140 °C for 24 h. The CeO2
nanospheres product was collected by vacuum
filtration and washed successively with deionized
water and ethanol (each 5 times), after which the
(Ce(NO3)2·6H2O, >98%) was purchased from
Shanghai Dibo Biotechnology Co.
Ltd. Potassium bromide (KBr) and tetrabutyl
titanate (TBOT) were obtained from BASF
Chemical Trading Co. Ltd. (TianJin, China). L-
asparagine and dopamine hydrochloride were
purchased from Shanghai Alighting Reagent Co.
Ltd. Sodium borohydride (NaBH4) was purchased
from Macklin Chemical Reagent Co. Ltd. Sodium
tetraborate (Na2B4O7) buffer solution was
purchased from Shanghai Hongbei Reagent Co. Ltd.
Distilled deionized water was used in the
experiments. Oxygen gas (99.9%) was obtained
from Shandong Yingchun Gas Co. LTD.
nanospheres were dried at 70 °C for 24 h.
Preparation of TiO2 nanospheres.
2 mL of tetrabutyl titanate and 60 mL of
anhydrous acetone were mixed and stirred for 30
min at room temperature. The resulting mixture
Preparation of CeO2 nanospheres. The
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10.1002/cctc.202100091
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was then transferred to the 100 mL hydrothermal
reactor. The hydrothermal reactor was then placed
in an oven at 120 °C for 24 h. The TiO2 nanospheres
were collected by filtration, washed with deionized
water 5 times, then dried at 80 °C for 12 h.
porcelain boat, then heated at 600 °C for 2 h under
a N2 atmosphere. The black product obtained was
denoted herein as CeO2@N/C. TiO2@N/C was
prepared by a similar method, except that the CeO2
nanospheres were replaced by TiO2 nanospheres.
Preparation
TiO2@N/C.
of
CeO2@N/C
and
Preparation of CeO2@TiO2.
CeO2 nanospheres (0.5 g) were ultrasonically
dispersed in 100 mL of ethanol for 1 h, after which
a further 50 mL of ethanol was added to the
dispersion. After stirring for 30 min, tetrabutyl
titanate (1.42 mL) was slowly added dropwise into
the stirred dispersion. The resulting dispersion was
then stirred for 12 h at room temperature, after
which 10 mL of deionized water was added at a rate
of 2 mL/min, with the dispersion then being stirred
for a further 1 h. The solid product obtained was
collected by centrifugation and washed twice with
For the synthesis of CeO2@N/C, 20 mL of an
aqueous 0.1 M tris(hydroxymethyl)aminomethane
(TRIS) solution was mixed with 8 mL of an
aqueous 0.1 M hydrochloric acid solution. The
resulting solution then diluted to 200 mL with
deionized water under continuous stirring to obtain
a Tris-HCl (pH = 8.5) solution. Next, 0.5 g of the
CeO2 nanospheres prepared above was added to the
Tris-HCl buffer solution, which was then subjected
to five alternate cycles of stirring and ultrasonic
treatment at room temperature (each of 30 min
duration) to achieve a good CeO2 nanosphere
ethanol, then dried at 80 °C for 12 h. Next, the
solid product was transferred to a porcelain boat
and heated at 500 °C for 4 h under a N2 atmosphere.
dispersion. Then, 0.03
g
of dopamine
hydrochloride was added, and the resulting
dispersion stirred at room temperature for 24 h. The
grey precipitate obtained was collected by
centrifugation and washed 3 times with deionized
water (300 mL each time), and subsequently dried
The black product obtained is denoted herein as
CeO2@TiO2.
Preparation
of
CeO2@N/C@TiO2.
CeO2@N/C (0.5 g) was ultrasonically dispersed in
100 mL of ethanol for 1 h, after which a further 50
mL ethanol was added to the dispersion. After
stirring for 30 min, tetrabutyl titanate (1.42 mL)
under vacuum at 60 °C for 12 h. After drying, the
sample (polydopamine coated CeO2 nanospheres,
i.e. CeO2@PDA) was transferred to
a
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10.1002/cctc.202100091
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was slowly added dropwise into the stirred
dispersion. The resulting dispersion was then
stirred for 12 h at room temperature, after which 10
mL of deionized water was added at a rate of 2
mL/min, with the dispersion then being stirred for
a further 1 h. The solid product obtained was
collected by centrifugation and washed twice with
CeO2@N/C@TiO2 catalyst (0.1 g), then
pressurized to 1 MPa with oxygen. The oxidation
reaction was carried out at 80 °C for 0.5 h using a
stirring speed of 500 rpm.
Product Analysis. The products formed in the
catalytic oxidation of HMF tests were analyzed
using a high-performance liquid chromatograph
(Shimadzu, Tokyo, Japan) equipped with an
ultraviolet-visible detector. Product separations
were achieved on a C-18 column (Diamosil, 5 µm,
ethanol, then dried at 80 °C for 12 h. Next, the
solid product was transferred to a porcelain boat
and heated at 500 °C for 4 h under a N2 atmosphere.
The black product was denoted herein as
CeO2@N/C@TiO2.
4.6 mm × 200 mm) using an acetonitrile-water
mixture (5:95, v/v) as the mobile phase. The
injection volume was 10 μL, the column
temperature 30 °C, and the mobile phase flow rate
1.0 mL/min. The appearance times of 2,5-
furandicarboxylic acid (FDCA), 5-formyl-2-
furancarboxylic acid (FFCA), 5-hydroxymethyl-2-
Preparation of D-CeO2@N/C@TiO2.
CeO2@N/C@TiO2 (0.5 g) and NaBH4 (1 g) were
mixed and ground to a homogeneous powder in a
mortar and pestle. The powder blend was then
heated at 500 °C for 5 h under a N2 atmosphere.
After the heat treatment, the product was washed 5
times with deionized water, then dried at 80 °C for
8 h. Finally, the product was ground to a fine
furancarboxylic
acid
(HMFCA),
(HMF) and
5-
hydroxymethylfurfural
2,5-
diformethylfuran (DFF) were 4.1, 4.9, 5.5, 7.0 and
8.1 min. The detected wavelength for all products
was 268 nm. Regression coefficients of the linear
regression equations from the standard curves for
all products were greater than 0.999.
powder,
and
denoted
herein
as
D-
CeO2@N/C@TiO2.
Catalytic oxidation of HMF. The catalytic
The HMF conversion (Eq. 1) and HMFCA
selectivity (Eq. 2) were defined as follows:
moles of HMF converted
oxidation of 5-HMF tests were carried out in a
stainless reactor containing a 50 mL Teflon vessel.
Typically, the reactor was loaded with HMF (0.1 g),
Na2B4O7 buffer solution (10 mL) and D-
HMF conversion =
×100%
moles of added HMF
(1)
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10.1002/cctc.202100091
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moles of HMFCA
HMFCA selectivity =
×100%
(2)
moles of all products
RESULTS AND DISCUSSION
Figure 1. XRD patterns for CeO2, CeO2@N/C, TiO2 and CeO2@N/C@TiO2.
22.5 o, which we associate with an amorphous N-
doped carbon shell on the CeO2 nanospheres. The
XRD pattern of the CeO2@N/C@TiO2 sample
showed additional peaks due to anatase TiO2, the
most intense of which was the (101) reflection at
25.30°. For the D-CeO2@N/C@TiO2 sample,
prepared by heat treatment of CeO2@N/C@TiO2
with NaBH4, the XRD pattern contained many
new and weak peaks. This indicated that the D-
CeO2@N/C@TiO2 sample had undergone
serious reduction during the mild hydrogenation
treatment using NaBH4, to produce TiO2-x, CeO2-
x, Ce(OH)3 and Ce2O3 and other phases.[20] This
indicates that core in the CeO2@N/C@TiO2 was
reducible, and accessible to H2 generated by the
XRD Analysis. XRD patterns for the CeO2
nanospheres, anatase TiO2, CeO2@N/C and
CeO2@N/C@TiO2, and D-CeO2@N/C@TiO2
are displayed in Figure 1. The XRD pattern for
the CeO2 nanospheres showed peaks at 28.54,
33.07, 47.47 and 56.33°, which could readily be
assigned to the (111), (200), (220) and (311)
crystal planes of cubic CeO2. The XRD pattern
of CeO2@N/C was similar to that of the CeO2
nanospheres (i.e. dominated by reflections
associated with CeO2). Interestingly, the CeO2-
related peaks were slightly broader in the
CeO2@N/C sample compared to the CeO2
nanospheres. In addition, the CeO2@N/C sample
showed a very broad feature centered around
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10.1002/cctc.202100091
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thermal decomposition of NaBH4.
Figure 2. TG–DTA curves for CeO2@PDA in N2. The heating rate was 10 °C /min, with the analysis
being performed under a N2 flow.
mass loss was observed in 250-380 °C region,
TG–DTA Analysis. Figure 2 shows TG–DTA
which is due to the conversion of the PDA coating
curves for the thermal conversion of the PDA
into a nitrogen-doped carbon (N/C coating).[22]
coating on the CeO2 nanospheres into a N/C
Coinciding with this mass loss was a large
coating (i.e. the procedure used to form
exothermic peak at 310 °C in the DTA curve.
Above 380 °C, no further mass loss was observed.
CeO2@N/C). The TGA curve shows a small mass
loss in the range 85-200 °C, which can be attributed
to the loss of surface physisorbed water.[21] A large
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Figure 3. SEM images of (a) CeO2, (b) CeO2@N/C, (c) CeO2@N/C@TiO2, and (d) D-CeO2@N/C@TiO2.
CeO2@N/C. After thermal reaction with NaBH4,
SEM Analysis. The stepwise synthesis of the D-
the obtained D-CeO2@N/C@TiO2 possessed a
CeO2@N/C@TiO2 catalyst from the CeO2
very rough surface and open network structure
nanospheres, as described in Scheme 2, is capture
(Figure 3d), which likely resulted from both the
in the SEM images of Figure 3. The CeO2
surface reduction of the TiO2 shell to form
nanospheres were spherical with diameters ranging
amorphous black TiO2 (TiO2-x) and also reduction
from 1.5-2 m (Figure 3a). For the CeO2@N/C
of the CeO2 core to CeO2-x and other Ce(III) phases.
sample (Figure 3b), many small particles can be
Oxygen vacancies created by the reduction
seen coated on the surface of CeO2 nanospheres,
treatment, associated with Ti3+ and Ce3+ states,
which are a N-doped carbon arising from the
were shown below to be beneficial for the selective
thermal decomposition of PDA. Figure 3c shows a
oxidation of HMF.[23]
SEM image of the CeO2@N/C@TiO2 specimen.
The image shows that the TiO2 layer formed a
porous and adherent coating over the underlying
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Figure 4. TEM images of (a) CeO2@N/C@TiO2 and (b) D-CeO2@N/C@TiO2. EDS element maps for
CeO2@N/C@TiO2 are shown below the TEM images.
a similar core-shell morphology, though the shell
TEM Analysis. Figure 4a and b show TEM
around the nanosphere core was more irregular, and
images for the CeO2@N/C@TiO2 and D-
composed of larger particles (as was also seen by
CeO2@N/C@TiO2 catalysts, respectively. The
SEM). Heat treatment of TiO2 powders with
CeO2@N/C@TiO2
catalyst
possessed
a
NaBH4 generally leads to the formation of an
amorphous TiO2-x shell on the surface of the TiO2
particles, which was also expected to occur here
and contribute to the excellent catalytic activity of
D-CeO2@N/C@TiO2 for HMF oxidation.
spherical core-shell structure, with the TiO2 shell
being highly porous and composed of small anatase
TiO2 crystallites. EDS elemental mapping images
revealed a very homogeneous dispersion of Ti, O,
N over the Ce-rich core. After reduction with
NaBH4, the D-CeO2@N/C@TiO2 catalyst showed
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10.1002/cctc.202100091
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Figure 5. High resolution XPS spectra for CeO2@N/C@TiO2 and D-CeO2@N/C@TiO2. (a) Ce 3d
region, (b) Ti 2p region, (c) N 1s region, (d) O 1s region, (e) C 1s region.
A(Ce3+)/[A(Ce3+) + A(Ce4+)], where A(Ce3+) and
XPS Analysis. X-ray photoelectron was used to
A(Ce4+) are peak areas for the Ce3+ and Ce4+ signals,
study the chemical changes that occurred during the
respectively, in the Ce 3d spectrum.[25] For D-
reduction
of
CeO2@N/C@TiO2
to
D-
CeO2@N/C@TiO2, the concentration of Ce3+ was
higher, calculated to be ~20%. This implies that the
NaBH4 treatment resulted in the reduction of some
Ce4+ to Ce3+.
CeO2@N/C@TiO2 by thermal treatment with
NaBH4. Figure 5a shows Ce 3d XPS spectra for
CeO2@N/C@TiO2 and D-CeO2@N/C@TiO2. The
CeO2@N/C@TiO2 sample showed peaks at 882.0,
886.8, 897.9, 900.9, 906.4 and 916.3 eV typical for
Ce4+ in CeO2, with weaker peaks at 884.2 and 904.3
eV being due to Ce3+.[24] The D-CeO2@N/C@TiO2
sample also showed peaks for the Ce4+ and Ce3+
states, though the latter were more intense
The Ti 2p XPS spectrum for CeO2@N/C@TiO2
(Figure 5b) showed peaks at 459.1 eV and 464.9
eV in a characteristic 2:1 area ratio, which could
readily be assigned to the Ti 2p3/2 and Ti 2p1/2
signals, respectively, of Ti4+ in TiO2.[26] For D-
CeO2@N/C@TiO2, the Ti 2p3/2 and Ti 2p1/2 signals
were observed at slightly lower binding energies
(458.7 and 464.5 eV, respectively), implying that Ti
cations in D-CeO2@N/C@TiO2 were of a lower
compared
to
the
spectrum
for
the
CeO2@N/C@TiO2 sample. For CeO2@N/C@TiO2,
fraction of cerium present as Ce3+ was calculated to
be ~12% using the formula: Concentration (Ce3+) =
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10.1002/cctc.202100091
ChemCatChem
oxidation state (presumably containing both Ti4+
and some Ti3+).[26a]
for the selective oxidation of HMF over D-
CeO2@N/C@TiO2.
Figure 5c shows N 1s XPS spectra for
CeO2@N/C@TiO2 and D-CeO2@N/C@TiO2. The
spectrum for CeO2@N/C@TiO2 contained peaks at
397.9 and 400.1eV, which can readily be assigned
to pyridinic N and pyrrolic N in the N/C shell. By
comparison, the D-CeO2@N/C@TiO2 sample
The O 1s XPS spectrum for CeO2@N/C@TiO2
(Figure 5d) showed a peak at 529.8 eV due to
lattice
oxygen
in
TiO2.[28]
The
D-
CeO2@N/C@TiO2 sample showed peaks at
529.8,[28] 531.4[28] and 532.9 eV [26a], which are
assigned to lattice oxygen in TiO2,[29] oxygen
vacancies[30] and chemisorbed oxygen, respectively.
The presence of the oxygen vacancies and
chemisorbed oxygen were expected to enhance the
catalytic performance of D-CeO2@N/C@TiO2 for
HMF oxidation, as was confirmed by experiment
below.
showed only a single peak at 398.3eV due to
24a, 27]
pyridinic N.[14,
Further, the amount of
nitrogen (as pyridinic N) appeared to be greatly
enhanced in the D-CeO2@N/C@TiO2 sample. To
explain this, it needs to be remembered that XPS is
a surface analytical technique, probing the top few
nanometers of the sample. For CeO2@N/C@TiO2,
the uniform TiO2 coating meant that not much of
the N-doped carbon shell will be probed during the
experiment. For D-CeO2@N/C@TiO2, the TiO2
shell iswas more disordered and macroporous, thus
allowing more of the underlying N/C shell to be
probed during the XPS analysis. Importantly,
pyridinic N well-known for its oxygen activation
properties, and thus was expected to be beneficial
The C 1s XPS spectra of CeO2@N/C@TiO2 and
D-CeO2@N/C@TiO2 showed peaks at 284.6,
285.7 and 289.5eV which were attributed to C-
C/C=C, C-N/C=N and O-C=O groups,[31]
respectively (Figure 5e). The presence of the C-
N/C=N features in the C 1s spectra, and pyridinic
N in the N 1s spectra, imply that CeO2@N/C@TiO2
and D-CeO2@N/C@TiO2 both contained N atoms
doped into the graphitic carbons domains.[28]
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Table 1. Summarized N2 physisorption data for CeO2, CeO2@N/C, CeO2@ N/C@TiO2 and D-
CeO2@N/C@TiO2.
Catalyst
BET surface area
(m2/g)
BJH Pore diameter (Å)
BJH Pore volume (m3/g)
CeO2
0.78
35.60
61.87
40.38
358.9
355.4
115.2
231.2
0.599
0.151
0.156
0.186
CeO2@N/C
CeO2@N/C@TiO2
D-CeO2@N/C@TiO2
here for achieving a high loading and uniform
distribution of N/C and TiO2. The specific surface
area of the nanospheres progressively increased
after coating first with the N/C shell (CeO2@N/C,
35.60 m2/g) and then the TiO2 shell
(CeO2@N/C@TiO2, 61.87 m2/g). However, the
specific surface area of the D-CeO2@N/C@TiO2
catalyst was only 40.38 m2/g, approximately 35%
smaller than CeO2@N/C@TiO2.
Surface Area and Porosity of the
Different Catalysts. N2 physisorption
isotherms were collected at 77 K for the CeO2,
CeO2@N/C,
CeO2@N/C@TiO2
and
D-
CeO2@N/C@TiO2 catalysts. BET specific surface
areas, BJH pore volumes and BJH average pore
diameters extracted from the physisorption data is
summarized in Table 1. The CeO2 nanospheres had
a specific surface area of only 0.78 m2/g. The
specific surface area of spherical particles is
inversely proportional to the diameter of the
particles.[32] SEM characterization showed that the
CeO2 particles used in this work were spherical
with a diameter of ~1600 nm. Accordingly, the
specific surface of the spherical CeO2 particles was
low. A larger CeO2 particle diameter was useful
ICP-OES analysis showed that the Ti and Ce
contents in the D-CeO2@N/C@TiO2 catalyst were
24.36 wt.% and 26.94 wt.%, respectively (Table
S1). This corresponds to a CeO2:TiO2 molar ratio
of ~1:2.65. EDS mapping analyses showed the N
and C contents in the D-CeO2@N/C@TiO2 catalyst
to be 12.37 wt.% and 4.58 wt.%, respectively.
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Table 2. Performance comparison of different catalysts for the selective oxidation of HMF.
Product selectivity (%)
Catalyst
CeO2 (S=67 m2/g)
CeO2 (S = 0.78 m2/g)
CeO2@N/C
HMF conv. (%)
49.6
HMFCA
37.3
FFCA
62.7
57.1
43.7
73.2
56.1
44.2
31.7
19.3
-
DFF
-
-
-
-
47.6
42.8
54.0
56.3
TiO2
35.5
26.8
TiO2@N/C
43.2
43.8
CeO2-TiO2
61.5
55.8
-
CeO2@TiO2
68.9
68.3
CeO2@N/C@TiO2
D-CeO2@N/C@TiO2
79.7
72.6
8.0
-
87.8
100.0
Reaction conditions: HMF (0.1 g), Na2B4O7 buffer solution (10 mL), catalyst (0.1 g), T = 80 °C, P(O2) = 1 MPa, reaction
time = 30 min.
offered a slightly higher conversion of HMF and
Catalyst Screening. An initial set of HMF
improved selectivity of HMFCA. Not surprisingly
oxygenation tests were conducted over the various
therefore, the CeO2@N/C@TiO2 catalyst offered a
catalysts at a temperature of 80 °C and an O2
higher conversion of HMF (79.7%) and also
pressure of 1.0 MPa. The reaction time was 30 min,
improved selectivity of HMFCA (72.6%). Results
with the results being summarized in Table 2.
suggest that the N/C coating was beneficial for the
CeO2 and TiO2 showed modest activities for HMF
adsorption and transformation of molecular O2,
conversion, and also modest selectivities towards
thus boosting HMF oxidation to HMFCA. The D-
HMFCA. CeO2 surface area had little influence on
CeO2@N/C@TiO2 showed the highest conversion
HMF oxidation activity or product selectivities.
of HMF (87.8%), with a remarkable selectivity to
The CeO2@N/C and TiO2@N/C catalysts, and the
HMFCA of 100%. Clearly, chemical and structural
mixed oxide catalysts (CeO2-TiO2, CeO2@TiO2),
changes introduced during CeO2@N/C@TiO2
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10.1002/cctc.202100091
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heating with NaBH4 contributed enormously to the
approximately equal to the molar amount of HMF
used in the tests, indicating negligible amounts of
HMF remained adsorbed on the catalysts (thus the
HMF conversions reported here were not
exaggerated simply by reactant adsorption on the
catalysts).
improved
catalytic
properties
of
D-
CeO2@N/C@TiO2.
We calculated the C, H, O mass balance when
CeO2, CeO2@N/C, CeO2@TiO2,
CeO2@N/C@TiO2 and D-CeO2@N/C@TiO2 were
used as catalysts. As shown in Table S2, the total
molar amount of HMF (unreacted + converted) was
Figure 6. O2-TPD curves for CeO2@N/C@TiO2 and D-CeO2@N/C@TiO2
due to the release of lattice oxygen (Olatt, O2-) as
O2,[33a] and are termed β-O species. At temperatures
higher than 650 °C, -O species may be seen
resulting from the migration of lattice oxygen in the
structure of the catalyst[34]. CeO2@N/C@TiO2
showed peaks due α-O and -O species, along with
some weak β-O features. Conversely, D-
O2-TPD Analysis. Since the oxidation of HMF
to HMFCA requires O2 adsorption and activation,
O2-TPD
curves
were
collected
for
CeO2@N/C@TiO2 and D-CeO2@N/C@TiO2
(Figure 6). Oxygen desorption signals in the range
of 50-350 °C can generally be ascribed to weakly
bound molecular physisorbed and/or chemisorbed
CeO2@N/C@TiO2 showed predominantly β-O
-
oxygen (O2 , O-),[33] sometimes termed α-O species.
[35]
species.
This may be due to the fact that D-
Desorption peaks in the range of 350-650 °C are
CeO2@N/C@TiO2 was rich in surface defects such
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10.1002/cctc.202100091
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as oxygen vacancies, resulting in very strong
binding of molecular oxygen and/or good oxygen
storage ability (e.g. CeO2-x + x/2O2 ↔ CeO2).
Figure 7. Effect of (a) reaction time, (b) temperature and (c) O2 pressure on the selective oxidation of HMF
over D-CeO2@N/C@TiO2. Reaction conditions: (a) 0.1 g catalyst, 1 MPa O2, 0.1g HMF, 110 °C (b) 0.1 g
catalyst, 1 MPa O2, 30 min, 0.1g HMF (c) 0.1 g catalyst, 0.1g HMF, 30 min, 80 °C.
conversion of HMF reached ~89.2% after 10 min
Effect of Reaction Time, Temperature
and then remained relatively constant as the
and O2 Pressure on HMF Oxidation. In
reaction time was increased to 4 h. As the reaction
order to determine the optimal conditions for the
time increased, the selectivity to FFCA and FDCA
catalytic oxidation of HMF to HMFCA over D-
progressively increased, as expected based on
CeO2@N/C@TiO2, the effects of reaction time,
Scheme 1. A reaction time of 30 min thus afforded
reaction temperature and oxygen pressure were
the highest HMFCA yield, and was used in
investigated in detail. Figure 7a shows the effect of
subsequent experiments.
reaction time on the oxidation of HMF. The
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10.1002/cctc.202100091
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Figure 7b shows the effect of reaction
temperature on the oxidation of HMF. It can be seen
that the conversion of HMF slightly increased with
of oxygen, the conversion of HMF was only 15.5%
with a selectivity to HMFCA of 100%. When the
O2 pressure was increased to 1 MPa, the conversion
temperature in the range 80-120 °C, reaching 90.3% of HMF increased to 87.8% with no change in the
at 120 °C. The selectivity of HMFCA was ~100%
in the temperature range of 80-100 °C, but
decreased at temperatures above 100 °C due to
FFCA formation. Hence, a temperature of
80 °Cwas used in the subsequent experiments.
selectivity to HMFCA (still 100%). However, on
raising the pressure of O2 to 1.5 MPa, the selectivity
to HMFCA decreasesd to 83% due to the formation
of FFCA. Accordingly, 1 MPa of O2 was identified
as the optimal oxygen gas pressure for the selective
oxidation of HMF to HMFCA over the D-
CeO2@N/C@TiO2 catalyst (at a reaction time of 30
min and a reaction temperature of 80 °C).
Figure 7c shows the effect of O2 pressure on the
oxidation of HMF. It can be seen that in the absence
Figure 8. Effect of different bases on the HMF selective oxidation performance of D-CeO2@N/C@TiO2.
Reaction conditions: 0.1 g catalyst, 1 MPa O2, 0.1g HMF, 80 °C, 30 min. The molar ratio of the added base
amount to HMF was 1:1 in all experiments.
studied the effect of different bases on the catalytic
Effect of Different Bases on HMF
oxidation of HMF,[13] with both HMF conversion
Oxidation. In a previous work, Zhao et al.
and product selectivities depending on the base
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10.1002/cctc.202100091
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used. Accordingly, we explored the effect of
different alkali solutions on the catalytic oxidation
of HMF by D-CeO2@N/C@TiO2. As shown in the
Figure 8, the conversion of HMF in water was 78%,
with a HMFCA selectivity of 77%. We then
examined the influence of the different alkali
additives (NaOH, Na2CO3, NaHCO3, Na2B4O7) at a
1:1 ratio to HMF. The pH of the reaction solutions
were 12.73, 11.47, 8.45, and 9.18, respectively. As
shown in Figure 8, when the pH of the catalytic
reaction is high, the selectivity to DFF increased.[36]
When the pH is low, the selectivity to HMFCA was
variable. When the pH of the reaction solution was
weakly alkaline, as in the case of aqueous Na2B4O7
(pH = 9.18), the selectivity to HMFCA was
optimized (~ 100%).
Figure 9. Effect of catalyst amount on the HMF selective oxidation performance of D-CeO2@N/C@TiO2.
Reaction conditions: 1 MPa O2, 0.1g HMF, 80 °C, 30 min, Na2B4O7 buffer solution (pH 9.18).
selectivity of HMFCA both improved to 63% and
Effect of Catalyst amount on HMF
55%, respectively. On increasing the amount of
Oxidation. Figure 9 shows the effect of the
catalyst further, the conversion of HMF and the
amount of D-CeO2@N/C@TiO2 catalyst on the
selectivity to HMFCA continued to increase.
oxidation of HMF. In the absence of catalyst, the
When the amount of catalyst was 100 mg, the
HMF conversion was only 40% with a HMFCA
conversion of HMF and the selectivity to HMFCA
selectivity of only 35%. On increasing the catalyst
were optimized at 87.8% and 100%, respectively.
amount to 20 mg, the conversion of HMF and the
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10.1002/cctc.202100091
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While the selectivity to DFF decreased as the
amount of catalyst increased, due to DFF
conversion to FFCA. The selectivity to HMFCA
decreased when the catalyst amount exceeded 100
mg, thus 100 mg of the D-CeO2@N/C@TiO2
catalyst was identified as the optimal amount for
the selective oxidation of HMF to HMFCA. When
the amount of catalyst was 120 mg, peroxidation
to form FDCA occurred.
Figure 10. Effect of catalytic test run on the HMF selective oxidation performance of D-CeO2@N/C@TiO2.
Reaction conditions: 1 MPa O2, 0.1g HMF, 80 °C, 30 min, Na2B4O7 buffer solution (pH 9.18).
The reason for this sudden drop in selectivity to
Catalyst Recycling Tests. The recyclability
HMFCA needs more investigation, though is
of the D-CeO2@N/C@TiO2 catalyst for the
likely associated with a chemical change on the
selective oxidation of HMF to HMFCA was also
surface of the D-CeO2@N/C@TiO2 catalyst (such
investigated. Figure 10 shows that D-
as full oxidation of the initially defective CeO2-x
CeO2@N/C@TiO2
retained
outstanding
core to stoichiometric CeO2).
performance over the first four runs, with
negligible change in the conversion of HMF
(~87 %) or the selectivity of HMFCA (~100 %).
In the fifth cycle, the conversion of HMF
decreased slightly whilst the selectivity to
HMFCA decreased to 64.9%, along with a
concomitant increase in the selectivity to FFCA.
The physicochemical and textural properties of
the used catalysts were studied. XPS, O2-TPD and
SEM studies were performed on catalysts after 4
and 5 cycles of HMF oxidation. The results of these
physicochemical and texture analyses are
summarized below.
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As shown in Fig S1, as the number of cycles of
HMF oxidation tests increased from 4 to 5, the
amount of Ce3+ in the catalyst increased. This may
be due to electron transfer to Ce4+ ions, or
alternatively the formation of CeO2-x phase via loss
of lattice oxygen. Evidence for this is seen in the
O2-TPD data (Fig S2). After 4 cycles, the catalyst
oxygen migration.[33a] After 5 cycles, the amount of
β-O in the catalyst was greatly reduced. Fig S3
shows SEM images of catalysts after 4 and 5 cycles
of HMF oxidation. After 4 cycles, the catalyst
structure had collapsed to some extent, though still
retained a porous structure. However, after 5 cycles
of tests, serious agglomeration and densification of
had a lot of β-O, which can convert lattice oxygen
into surface chemisorbed oxygen via assisting
the catalyst was observed, which obviously played
a key role in the decreased activity of the catalyst.
Scheme 3. Reaction mechanism for HMF oxidation over the D-CeO2@N/C@TiO2
catalyst.
subsequently dehydrogenated to form a carboxylic
Catalytic mechanism. Based on the above
acid.[37] In the initial step of the reaction, the
results, a plausible reaction mechanism for HMF
aldehyde group of HMF is first adsorbed onto the
oxidation to HMFCA on the D-CeO2@N/C@TiO2
strong Lewis acid sites (Ti4+ species) of the TiO2
catalyst was developed (scheme 2). In the presence
outer coating, then being transformed into a gem
of an added base, OH- ions activate the aldehyde
diol intermediate through a hydration reaction.[38]
group of HMF, in the process generating a
Meanwhile, defective Ti3+ species created during
hemiacetal or gem diol intermediate which is
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10.1002/cctc.202100091
ChemCatChem
-
the catalyst synthesis are oxidized to Ti4+ species
via the release an electron, which cause the
reduction Ce4+ in the core to Ce3+.[39] A gaseous O2
molecule then adsorbs on the N-doped carbon (i.e.
N/C) component of the catalyst which is rich in
pyridinic sites, resulting in chemisorbed oxygen
species such as O2 and O-, with the latter becoming
part of the CeO2 lattice (O2-).[37] The gem diol
intermediate
then
undergoes
oxidative
dehydrogenation to HMFCA and one molecule of
water.[40]
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10.1002/cctc.202100091
ChemCatChem
CONCLUSIONS
A D-CeO2@N/C@TiO2 catalyst with a defective
surface was successfully prepared by heat
converting surface adsorbed oxygen into CeO2
lattice oxygen. The components of the D-
CeO2@N/C@TiO2 catalyst, such as Lewis acid Ti4+
sites in the outer shell for HMF adsorption,
pyridinic N sites in the N/C layer for O2 activation,
and the mixed Ce3+/Ce4+ states in the CeO2 core, all
act synergistically to tune the activity and
selectivity of the catalytic reaction. Importantly, D-
CeO2@N/C@TiO2 showed negligible change in
performance over four consecutive catalytic tests,
suggesting that the developed noble-metal free
catalyst has excellent promise for the synthesis of
valuable HMFCA from HMF.
treatment
of
a
core-shell
structured
CeO2@N/C@TiO2 material with NaBH4 at 500 °C
in N2 for 5 h. Catalytic oxidation tests conducted in
a pressurized reactor demonstrated that D-
CeO2@N/C@TiO2
possessed
exceptional
selectivity for the oxidation of HMF to HMFCA in
the presence of O2. Under optimized testing
conditions (100 mg catalyst, 80 °C, 1 MPa O2,
reaction time 30 min, Na2B4O7 buffer), a HMF
conversion of 87.8% and a HMFCA selectivity of
100% was achieved. O2-TPD revealed that the D-
CeO2@N/C@TiO2 catalyst was very effective in
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10.1002/cctc.202100091
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acknowledges funding support from the
MacDiarmid Institute for Advanced Materials and
Nanotechnology, and Shandong Provincial Double
Hundred Talents Programme for Foreign Experts.
Conflicts of interest
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
KEYWORDS: Aerobic oxidation, Core-shell
This work was financially supported by the
Shandong Provincial Natural Science Foundation
of China (No. ZR2017MB035). GINW
nanospheres, Defective, HMF, HMFCA
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10.1002/cctc.202100091
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10.1002/cctc.202100091
ChemCatChem
Chowdhury, Journal of Molecular
Catalysis A: Chemical 2013, 378, 47-56.
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10.1002/cctc.202100091
ChemCatChem
Entry for the Table of Contents
Defective D-CeO2@N/C@TiO2 nanospheres, each comprising a spherical CeO2 core coated with
shells of N-doped carbon and TiO2, were successfully synthesized then evaluated for the aerobic oxidation
of HMF to HMFCA. Under the optimized reaction conditions (temperature 80 °C, reaction time 30 min,
O2 pressure 1 MPa), a high HMF conversion of 87.8% and a remarkable HMFCA selectivity of 100% were
obtained, suggesting that the developed noble-metal free catalyst has excellent promise for the synthesis of
valuable HMFCA from HMF.
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