Pt Nanoparticles Supported on Nitrogen-Doped-Carbon-Decorated CeO2 for Base-Free Aerobic Oxidation of 5-Hydroxymethylfurfural

Article abstract of DOI:10.1002/asia.201800738

Currently, the base-free aerobic oxidation of biomass-derived 5-hydroxymethylfurfural (HMF) to produce 2,5-furandicarboxylic acid (FDCA) is attracting intense interest due to its prospects for the green, sustainable, and promising production of biomass-based aromatic polymers. Herein, we have developed a new Pt catalyst supported on nitrogen-doped-carbon-decorated CeO₂ (NC-CeO₂) for the aerobic oxidation of HMF in water without the addition of any homogeneous base. It was demonstrated that the small-sized Pt particles could be well dispersed on the surface of the hybrid NC-CeO₂ support, and the activity of the supported Pt catalyst depended strongly on the surface structure and properties of the catalysts. The as-fabricated Pt/NC-CeO₂ catalyst, with abundant surface defects, enhanced basicity, and favorable electron-deficient metallic Pt species, enabled an almost 100 % yield of FDCA in water with molecular oxygen (0.4 MPa) at 110 °C for 8 h without the addition of any homogeneous base, which is indicative of exceptional catalytic performance. Furthermore, this Pt/NC-CeO₂ catalyst also showed good stability and reusability owing to strong metal–support interactions. An understanding of the role of surface structural defects and basicity of the hybrid NC-CeO₂ support provides a basis for the rational design of high-performance and stable supported metal catalysts with practical applications in various transformations of biomass-derived compounds.

Full text of DOI:10.1002/asia.201800738

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Nitrogen-Doped Carbon-Decorated CeO₂ Supported
Pt Nanoparticles for Base-Free Aerobic Oxidation of 5-
hydroxymethylfurfural
Authors: Changxuan Ke, Mengyuan Li, Guoli Fan, Lan Yang, and Feng
Li
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Nitrogen-Doped
Carbon-Decorated
CeO
2
Supported
Pt
Nanoparticles for Base-Free Aerobic Oxidation of 5-
hydroxymethylfurfural
+
+
Changxuan Ke , Mengyuan Li , Guoli Fan*, Lan Yang and Feng Li*
Abstract: Currently, the base-free aerobic oxidation of biomass-
derived 5-hydroxymethylfurfural (HMF) to produce 2,5-
catalyst with abundant surface defects, enhanced basicity, and
favourable electron-deficient metallic Pt species enabled an almost
100 % yield of FDCA in water using molecular oxygen of 0.4 MPa at
furandicarboxylic acid (FDCA) is attracting intensive interest due to its
prospects for the green, sustainable and promising production of bio-
based aromatic polymers. In this work, we developed a new nitrogen-
°
110 C for 8 h without the addition of any homogeneous base additive,
indicative of an exceptional catalytic performance. What’s more, the
doped carbon-decorated CeO
2
(NC-CeO
2
) supported Pt catalyst for
present Pt/NC-CeO
reusability, owing to strong metal-support interactions. Understanding
the role of surface structural defects and basicity of hybrid NC-CeO
2
catalyst also possessed good stability and
aerobic oxidation of HMF in water without addition of any
homogeneous base. It was demonstrated that small-sized Pt particles
2
could be well dispersed on the surface of the hybrid NC-CeO
2
support,
support provides a basis for rationally designing high performing yet
stable supported metal catalysts applied practically in various
transformations of biomass-derived compounds.
and the activity of supported Pt catalysts depended strongly on
surface structure and property of catalysts. As-fabricated Pt/NC-CeO
2
Introduction
Nowadays, the emission of greenhouse gases caused by using
non-renewable energy sources such as fossil fuels is becoming
increasingly serious.[ Currently, biomass resource is the most
promising alternative to fossil fuels and petrochemicals. However,
the application of biomass has led to some controversy, such as
the production of bioethanol derived from corn and sugar cane.[2]
For these reasons, today's biomass research is moving towards
non-edible raw materials, such as lignocelluloses.[3] Especially,
bio-based furfural derivatives with several functional groups
become important platform compounds to produce chemicals.[4] It
is particularly noteworthy that 2,5-furan dicarboxylic acid (FDCA)
In the past decade, selective oxidation of HMF to produce
FDCA has experienced from the initial use of stoichiometric
oxidants to the current metal-catalyzed aerobic oxidation
1]
processes.[ For the latter, noble metals (e.g. Au,
6]
[7-12]
Pt,[11,13-16]
Au−Cu,[9] Au−Pd,[17-19] Ru[20]) are especially active at appropriate
oxygen pressures and reaction temperatures. In most cases,
however, homogeneous base additives (e.g. Na CO or NaOH)
2 3
still are required to promote this tandem reaction, especially the
hydroxyl group oxidation. So far, the pathways available for the
industrial production of FDCA through catalytic oxidation
processes have not been applied. The main limiting factor is that
the catalyst stability and processing efficiency still are not very
satisfactory. Moreover, excess amounts of base additives added
obtained
by
the
oxidation
of
cellulose-derived
5-
hydroxymethylfurfural (HMF) can be used as a synthetic biomass-
based polymer monomer replacing traditional fossil-derived
terephthalic acid in the economically viable production of value-
added polyethylene terephthalate, polybutylene terephthalate and
other polymers,[5] as well as a building block for constructing
metal-organic framework materials, which can solve harmful
effects of terephthalic acid on living organisms due to its
bioaccumulation.
can cause
a
strong corrosion to the equipment and
simultaneously lead to serious environmental pollution. To
improve the catalytic performances of supported catalysts,
numerous researchers have focused on the study of different
[
14,21]
catalyst supports (e.g. carbon nanotubes,
layered double
hydroxides (LDHs),[
10,12,20-23]
zinc hydroxycarbonate,[19] carbon
materials,[15,17,24,25] metal oxides[7-9,20,25]). Commonly, a strongly
alkaline aqueous medium or adding extra homogeneous base is
required to achieve high yields of FDCA.[8,26] Meanwhile, due to
the formation of acidic FDCA in the absence of homogeneous
base in reaction media, the easy leaching of active metal
C. Ke +, M. Li +, Dr. G. Fan, Prof. L. Yang, Prof. F. Li
State Key Laboratory of Chemical Resource Engineering, Beijing
Advanced Innovation Center for Soft Matter Science and Engineering,
Beijing University of Chemical Technology, P. O. BOX 98, Beijing 100029,
P. R. China.
components can result in a rapid deactivation in the case of LDHs-
supported catalysts.[21,23] Although covalent triazine frameworks
supported Ru clusters exhibited good stability in the above
reaction,[24,27] low FDCA yields (< 90%) were obtained. Therefore,
designing new stable support materials applied in the catalytic
base-free aerobic oxidation of HMF is greatly desirable in terms
of highly efficient production and environmental protection.
As one of the most widely applied metal oxides, ceria has been
used as the wonderful catalyst support and actual catalyst,
because of its unique physicochemical properties including
E-mail: fangl@mail.buct.edu.cn; lifeng@mail.buct.edu.cn
Fax: +8610-64425385; Tel.: +8610-64451226
[+
] These authors contribute equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1016/j.apcata.xxxx.
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tunable acid-base property, good thermal/chemical stability,
favorable structural defects, and storage capacity of oxygen.[
On the other side, it is generally accepted that for supported
catalyst systems, suitable metal-support interactions can
significantly heighten their catalytic performances. Accordingly,
manipulating surface structure and property of catalyst supports
is of vital importance. Recently, nitrogen-doped carbon (NC)
character of Pt species on the surface. For Pt/NC-CeO
it is difficult to find the characteristic (002) plane of crystalline g-
phase, probably due to the formation of a relatively small
amount of amorphous NC uniformly dispersing on the surface
CeO . Elemental analysis by ICP-AES reveals a Pt loading of
about 0.6 wt% on different samples (Table 1), indicating that most
of the active metal has been loaded on the surface of different
supports.
2
sample,
28,29]
3 4
C N
[
30]
2
materials, typically graphitic carbon nitride (g-C
3
[31]
N
4
) possessing
2
the graphite-like sp -bonded C–N structure,
considerable attention in the field of heterogeneous catalysis,
are attracting
[
32-
3
5]
due to their unique structures and properties.[36,37] Specially,
the electron-rich character of nitrogen atoms in the structure of
NC frameworks can induce the generation of surface basic
sites,[38–40] while the catalyst stability can be largely improved by
the interacting of active species with the NC support.
In this contribution, we reported a new nitrogen-doped carbon-
2 2
decorated CeO (NC-CeO ) supported Pt nanoparticles (NPs) and
investigated its catalytic performance in the base-free oxidation of
HMF in water using molecular oxygen. It was demonstrated that
2
as-fabricated Pt/NC-CeO catalyst afforded an exceptionally high
FDCA yield of 100% under mild reaction conditions (i.e. oxygen
o
of 0.4 MPa, 110 C, 8 h). The study on the structure−performance
correlation of catalysts revealed that both enhanced surface
basicity originating from NC component and abundant structural
component (e.g. oxygen vacancies and Ce3
+
defects of CeO
2
0
species), as well as favorable electron-deficient Pt species,
played crucial roles in activating reactants and forming FDCA
product.
3 4 2 2
Figure 1 XRD patterns of Pt/C N (a), Pt/CeO (b), Pt/NC-CeO (c) samples.
Results and Discussion
Figure 2 depicts HRTEM images of supported Pt samples. It is
clearly noted that a few irregular black Pt nanoparticles (NPs) of
about 4.5 nm in diameter disperse on the surface of the sheet-like
Structural characterization of supported Pt samples
g-C
3
N
4
support in the Pt/C
3
N
4
sample. For Pt/CeO
2
, it is observed
0
Figure 1 shows the XRD patterns of three different supported
that the characteristic (111) planes for metallic Pt and CeO
2
Pt samples. One can clearly see the coexistence of g-C
3
N
4
and
phases can be preferentially exposed on the surface, respectively.
0
0
metallic Pt phases in the Pt/C
N
3 4
sample, as evidenced by the
In the case of Pt/NC-CeO
CeO
2
sample, besides Pt NPs, crystalline
appearance of the characteristic (002) diffraction for g-C
3
N
4
2
NPs are uniformly distributed over the surface. Noticeably,
phase and (111), (200), (220) and (311) diffractions for metallic
a few interspersed amorphous NC thin layers are randomly
distributed over the surface. The average size of Pt NPs on
0
Pt phase. The XRD patterns of two CeO
2
-containing Pt samples
present a series of characteristic diffractions of cube fluorite-type
CeO (JCPDS No.43-1002), and no diffractions assignable to
metallic Pt phase can be observed, mirroring the highly dispersive
Pt/NC-CeO
(3.3 nm) and Pt/C
metallic Pt particles. Obviously, in the case of Pt/NC-CeO
2
(3.1 nm) is slightly smaller than those on Pt/CeO
(4.5 nm), reflecting the higher dispersion of
, CeO
2
2
3 4
N
2
2
3 4 2 2
Figure 2 HRTEM images of Pt/C N (a), Pt/CeO (b) and Pt/ NC-CeO (c) samples
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and NC components can be integrated together into the highly
hybridized structure, thus being beneficial to the homogeneous
dispersion of Pt NPs. As shown in Figure 3, HAADF-STEM image
TPD results, it is demonstrated that the introduction of NC
component can strengthen surface basicity of Pt/NC-CeO ,
2
despite the reduced total base sites.
and EDX element mappings of the presentative Pt/NC-CeO
2
To elucidate the surface electronic structure of Pt species,
supported Pt samples were characterized by XPS (Figure 5). In
the fine Pt 4f spectra of samples, there are two Pt 4f7/2 and 4f5/2
sample present the special distributions of C, N, O, Pt and Ce
elements on the surface. Obviously, the distributions of surface C,
N, O and Ce elements based on EDX element mappings are well
consistent with the overall shape of the tested region of the
spin-orbit components at about 70.6 and 73.9 eV, respectively,
indicating the presence of metallic Pt0 species.
[44]
Interestingly,
sample, further mirroring that CeO
2
and NC are evenly distributed
compared with those on the Pt/C
species on Pt/NC-CeO and Pt/CeO
positive shift of ca. 1.0 eV. Such a positive shift is an indication of
3
N
4
, the binding energy of Pt0
2
samples present a large
throughout the sample. ICP-AES results show that Pt content is
2
0
.6 wt%, and Ce content is 79.1 wt%, so the mass ratio of CeO
in the Pt/CN-CeO catalyst is about 98.2%.
In addition, the adsorption-desorption isotherms of Pt/CeO
Pt/NC-CeO
2
2
the strong interactions between Pt species and CeO
2
-containing
0
2
and
supports, thereby resulting in the electron transfer from Pt to the
support and thus the generation of electron-deficient metallic Pt
species. In each case, another two small components at about
71.6 eV and 75.5 eV in Pt samples are assigned to cationic Pt2+
2
samples are characteristic of type IV possessing a
H2-type hysteresis loop (Figure S1), indicative of the typical
mesoporous structure with the interconnected pore system.
Meanwhile, Pt/C
the H1-type hysteresis loop at the P/P
indicative of a mesoporous structure having cylindrical channels.
As listed in Table 1, among three Pt-samples, Pt/CeO sample
has a highest specific surface area, because the surface of CeO
3
N
4
sample shows the isotherms of type IV with
species. As shown in Table 1, compared with that on the Pt/C
the higher relative proportions of surface Pt2+ species on Pt/NC-
CeO and Pt/CeO samples suggest that Pt atoms more easily
interact with surface sites on the CeO
generating more Pt2+ species. The electron-deficient Pt species
are expected to enhance the catalytic performance via the easy
activation of the oxygen molecule in the HMF oxidation.
3 4
N ,
0
range from 0.75 to1.0,
2
2
2
2
component, thereby
0
2
is partially covered by the NC component, thus leading to the
blocking of some mesopores.
Figure 3 HAADF-STEM image (a) and EDX element mappings of C-k (b), N-k
(
c), O-k (d), Ce-L (e) and Pt-L (f) in the Pt/NC-CeO
2
sample.
2 2 2 3 4
Figure 4 CO -TPD profiles of Pt/CeO (a), Pt/NC-CeO (b) and Pt/C N (c)
samples.
Surface structural property of supported Pt samples
CO -TPD experiments were performed to obtain surface
2
basicity of samples. As shown in Figure 4, Pt/CeO
2
sample shows
o
a small desorption at about 100 C and a large and broad
o
desorption in the range of 150-550 C, which are associated with
weak base (WB) sites and medium-strength Lewis base (MLB)
sites, respectively.[ In the case of Pt/C
41]
3 4 2
N -CeO , in addition to
those related to WB and MLB sites, a small desorption appears at
o
the higher temperature of about 630 C, which can be assignable
to strong Lewis base (SLB) sites.[42] Such newly-developed SLB
sites on the Pt/NC-CeO
2
can result from the electron-rich nitrogen
atoms of NC component.[
12,43]
Meanwhile, the density of total
is larger than that for
(0.1803 mmol/g). The reduced total basic sites on
the Pt/NC-CeO is mainly due to the partial overlapping of surface
CeO with NC component in the hybrid NC-CeO support, thus
inhibiting the exposure of MLB sites. Based on the above CO
basic sites (0.2795 mmol/g) for Pt/CeO
Pt/NC-CeO
2
2
2
Figure 5 Pt 4f XPS of different Pt-based samples.
2
2
2
-
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Figure 6 shows the Ce 3d spectra of CeO
-containing samples.
(including Ce3+ species and oxygen vacancies) on the Pt/NC-
2
There are two types of V and U signals for Ce species related to
5/2 and 3d 3/2 spin-orbit components, which may be finely fitted
into four pairs of doublets. Among them, Ce species mainly
CeO
component. Further, the fine O 1s spectra are deconvoluted into
two components: one at 529.3 eV (O ) and another at 531.2 eV
2
should be attributable to the surface covering of NC
3d
[
45]
4+
I
correspond to three (V
0
, U
0
), (V’’, U’’) and (V’’’, U’’’) doublets with
(OII), which are correlated with the lattice oxygen and defective
oxide (i.e. oxygen vacancies) or surface hydroxyl groups.
different unoccupied 4f configurations, while Ce3+ species with
0
[46]
[48]
f1 states contribute to another pair of doubles (V’, U’).
[47]
Especially, Pt/CeO
4
2
I
presents a higher surface OII/(O+OII) atomic
3
+
3+
Interestingly, it is noted from Table 1 that surface Ce /(Ce +
Ce4+ ) atomic ratio on the Pt/CeO
(0.26) is larger than that on the
Pt/NC-CeO (0.20), indicating that Pt/CeO possesses more
surface Ce species. The presence of Ce3+ species is associated
with the formation of surface oxygen vacancies (O ). In
comparison to Pt/CeO , the reduced amount of surface defects
ratio (0.61) than Pt/NC-CeO
2
(0.43), suggestive of the formation
2
of more oxygen vacancies. The result is in good agreement with
the Ce 3d XPS results. As a result, it is speculated that in the
2
2
3
+
present Pt/CeO
2 2
and Pt/NC-CeO samples, Pt species may
v
strongly interact with surface defects of CeO
giving rise to electron-deficient Pt species.
2
or NC-CeO
2
, thus
0
2
2 2
Figure 6 Ce 3d and O 1s XPS of Pt/CeO and Pt/NC-CeO samples.
Table 1 The compositional and structural properties of samples.
Content (wt.%)
SBET [c]
Vp [d]
Dp [e]
Pt /(Pt + Pt2+)
0
0
Ce3+/ (Ce3++ Ce4+
ratio [f]
)
O /(O + O )
ratio [f]
II
II
I
Samples
Pt [a]
C[b]
--
N [b]
--
2
3
ratio [f]
(m /g)
(cm /g)
(nm)
24.6
Pt/C
Pt/CeO
Pt/NC-CeO
3
N
4
0.60
13.8
0.085
0.82
--
--
2
0.61
0.60
0
0
207.7
153.5
0.167
0.125
3.9
3.5
0.74
0.70
0.26
0.20
0.61
0.43
2
7.4
2.0
[a] Determined by ICP-AES; [b] Determined by elemental microanalysis. [c] Specific surface area calculated by the BET method; [d] Pore volume; [e] Mean pore
dimetersize; [f] Determined by XPS analysis.
Raman analysis of CeO
identify the arrangement of metal-oxygen bonds in samples
2
-containing samples was conducted to
Raman results further confirm the strong metal-support
interactions and the presence of surface defects in the cases of
(
4
Figure S2). Clearly, pristine CeO
2
show a strong Raman peak at
CeO
To obtain the information on the surface electronic property of
C and N species, C1s and N 1s spectra of Pt/NC-CeO sample
2
-containing supported Pt samples.
60 cm-1 and two weak peaks at 600 and 1180 cm , which are
-1
correlated with fluorite structure of CeO
respectively. [49] In this context, Ce3+ ions in the structure of CeO
may induce the shift of neighboring oxygen atoms towards Ce ,
thus leading to the oxygen distortion. Accordingly, the lattice
2
and the lattice distortion,
2
2
were analyzed (Figure 7). In the fine C 1s spectrum, three
contributions are assignable to three different carbon-containing
3
+
2
functional groups: carbon with sp hybridized structure (284.6 eV),
distortion of CeO
Ce3+ species. When Pt species are loaded on the surface of CeO
or NC-CeO , the Raman peak related to the lattice distortion is
obviously reduced, probably due to the interactions between Pt
species and surface defects of supports. Meanwhile, there are
two weak Raman peaks at about 567 and 655 cm-1 in the cases
2
implies the formation of oxygen vacancies and
C=N or C-OH group (286.5 eV), and C-N or carboxyl group (288.5
eV).[50] At the same time, the broad N 1s spectrum is fitted into
four components, which are associated with pyridinic N (399.2 eV),
pyrrolic N (399.7 eV), graphitic N (400.8 eV), and oxidized N
(402.5 eV), respectively.[51] This result further illustrates the
2
2
formation of NC component in the Pt/NC-CeO
2
sample.
of Pt/CeO
2
and Pt/NC-CeO
2
, respectively, which are correlated
Accordingly, as-formed surface electron-rich N sites in the NC
component, especially, pyridine-type nitrogen species,[12] may
[
49]
with the vibrations of Pt-O-Ce bond and Pt-O bond. The above
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generate new SLB sites, which can promote the selective
oxidation of HMF. Moreover, it is believed that pyrrole-type
nitrogen species are beneficial to the dispersion of active
metals,[52] thus governing the catalytic performance of the present
Pt/NC-CeO catalyst to some extent.
2
2
Figure 7 C 1s XPS (a), N 1s XPS (b) of Pt/NC-CeO sample.
Catalytic HMF oxidation over supported Pt catalysts
The aerobic HMF oxidation in water was conducted over
different Pt-based catalysts using molecular oxygen (0.4 MPa)
within 4 h at 150 C without the addition of any homogeneous
o
As it is known, during the HMF oxidation using molecular
oxygen, HMF can be first oxidized into 2,5-diformylfuran (DFF) or
base, and the reaction results are summarized in Table 2.
5
-hydroxymethyl-2-furancarboxylic acid (HMFCA) intermediate.
Noticeably, pristine CeO
species only affords a very low HMF conversion of ∼0.3 % (Entry
1), indicating that it is not active for the HMF oxidation. Pt/C
catalyst exhibits a high HMF conversion of 71.9 %, along with a
2
support in the absence of active Pt
Subsequently, DFF or HMFCA can be further converted into 5-
formyl-2-furancarboxylic acid (FFCA). At last, FDCA is produced
by a deep oxidation of FFCA.
3
N
4
Table 2 Catalytic performance for base-free oxidation of HMF over different catalysts [a]
Selectivity (%)
Entry
Catalysts
HMF/Pt molar ratio
Conv. (%)
FDCA
FFCA
DFF
HMFCA
1
2
3
4
5
6
7
CeO
Pt/C
Pt/CeO
2
--
0.3
7.7
22.8
43.4
24.9
0.1
69.5
38.2
4.1
0
0.3
0
3
N
4
163
160
163
163
326
19
71.9
97.8
100
18.1
71.0
99.9
85.2
20.3
65.1
2
Pt/NC-CeO
2
0
0
[
b]
Pt/CeO
Pt/C + CeO
5 wt% Pt/C
2
98.6
42.9
55.6
13.6
45.7
22.4
1.2
0
[
c]
3
N
4
2
33.6
12.3
0.4
0.2
[
5
a] Reaction conditions: catalyst, 0.1g; HMF, 0.5 mmol; O
2
pressure, 0.4 MPa; reaction time, 4 h; reaction temperature, 150 °C. [b] CeO
2
was reduced at
o
00 C for 2 h in 10% (v/v) H
2
/Ar atmosphere. [c] 0.05 g Pt/C and 0.05 g CeO
3
N
4
2
low FDCA selectivity of 18.1 % (Entry 2), while Pt/CeO
gives higher HMF conversion of 97.8 % and FDCA selectivity of
1.0 % (Entry 3). In the above cases, DFF and FFCA can be
formed as main oxidation intermediates during the oxidation of
HMF. It demonstrates that the employment of Pt/C and
Pt/CeO as catalysts leads to the formation of incomplete
2
catalyst
oxidation products within 4 h. The HMF conversion and FDCA
selectivity over commercial Pt/C catalyst with high Pt loading (5
wt%) are only 55.6 % and 65.1 %, respectively, far from
satisfactory. Notably, when Pt/NC-CeO is used as the catalyst,
2
the conversion and the FDCA selectivity can reach 100 % and
99.9 % after a reaction time of 4 h even under a high HMF/Pt
7
3
N
4
2
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atomic ratio of 163 (Entry 4), respectively, indicative of an
of HMF can be detected, indicative of no Pt leaching in the present
exceptional catalytic performance of Pt/NC-CeO
Therefore, hybrid NC-CeO support is apparently superior to the
sole C or CeO support in the aerobic oxidation of HMF into
FDCA. And, it is interestingly noted that compared to untreated
CeO support, H -treated CeO support for loading active Pt
2
catalyst.
Pt/NC-CeO
To obtain the information on the reaction pathways over the
Pt/NC-CeO catalyst, we monitored the change in the HMF
2
.
2
3
N
4
2
2
conversion and product selectivities with the reaction time at a
o
2
2
2
lower reaction temperature of 110 C. As shown in Figure 8a, the
species promotes the formation of FDCA product, along with a
higher FDCA selectivity of 85.2 % (Entry 5), despite a slightly
increased conversion of HMF. The catalytic performance over a
physical mixture of Pt/C N and CeO (Pt/C N + CeO ) was also
3 4 2 3 4 2
studied. For the purpose of facilitating the comparison, the
catalyst loading is maintained at 0.1 g. it can be seen that the
reaction time can significantly influence the distributions of
products. Noticeably, both the HMF conversion and the selectivity
to FDCA gradually increase with increasing of reaction time.
Within the initial 30 min, HMF conversion rapidly reaches as high
as 46.8 %, along with the selectivity of DFF, FFCA and FDCA
accumulating to 48.7, 41.2 and 9.9 %, respectively. DFF
intermediate diminishes gradually with the prolonged reaction
time, while the selectivity to FFCA slowly increases in the initial 1
h and starts to decline gradually with the further increase in the
reaction time. Finally, HMF completely converts after a reaction
time of 5 h. Whereas an almost 100 % yield of FDCA is achieved
after a reaction time of 8 h. It is clearly mentioned that no any
aldehyde group oxidized HMFCA intermediate can be detected at
initial reaction period. Therefore, FFCA should be produced via
molar ratio of HMF/Pt of Pt/C
twice as that of Pt/NC-CeO
Pt/C + CeO catalyst mixture (8.7 %) is a little higher than half
that of Pt/C (13.0%) resulting from the addition CeO with
abundant surface base sites, but far lower than half that of Pt/NC-
3
N
4
+ CeO
2
catalyst mixture is about
2
3 4
or Pt/C N . The yield of FDCA over
3
N
4
2
3
N
4
2
CeO
2
(~50%), probably attribute to the lack of electron-deficient
0
Pt species. In addition, when we increase the amount of HMF
from 0.5 to 1, 1.5, and 2 mmol, a nearly 100% FDCA yield can be
[
54]
achieved over Pt/NC-CeO
5 h under the similar reaction conditions, respectively, further
reflecting the excellent catalytic performance of the Pt/NC-CeO
2
catalyst after reacting for 7, 11, and
DFF intermediate, like Ru-catalyzed HMF oxidation.
1
Meanwhile, to elucidate the catalytic ability of Pt-based catalysts
for the hydroxyl group oxidation, the oxidation of furfuryl alcohol
(FOL) as the reaction substrate was further carried out over
different supported Pt catalysts (Table 3). Notably, the FOL
2
catalyst. However, due to the low solubility of FDCA, when the
amount of HMF higher than 0.5 mmol, FDCA precipitation can be
formed under our experimental condition, causing problems for
the separation of the catalyst. Hence, in view of the solubility of
FDCA at room temperature, the concertation of HMF was set at
oxidation over the Pt/NC-CeO
Pt/C and Pt/CeO , thereby giving rise to both the high
conversion of 99.9 % and the high furoic acid selectivity of 98.6 %.
2
is much faster than those over
3
N
4
2
0
.5 mmol in the following tests. [53] Furthermore, after one reaction, It means that like HMF, FOL also can be first oxidized to
ICP-AES result indicates that Ce concentration in the reaction
filtrate is only about 8.2 ppm, i.e., the molar ratio of FDCA /Ce is
about 2.1 × 10 , hence the influence of the formation of sault of
furaldehyde via the alcoholic hydroxyl group oxidation and then to
furoic acid in the present cases of Pt-based catalysts.
5
FDCA on the catalytic performance can be neglected. The above
results illustrate that the surface area of catalysts is not a crucial
factor in improving the catalytic performances, because Pt/NC-
Table 3 Catalytic performance for base-free oxidation of FOL over different
catalysts.[
a]
Selectivity (%)
Entry
Catalysts
Conv. (%)
CeO
2
has a lower surface area than Pt/CeO
2
(Table 1). Moreover,
Furaldehyde
76.1
2-furoic acid
23.9
1
2
3
Pt/C
Pt/CeO
Pt/NC-CeO
3
N
4
56.2
83.6
99.9
the leaching of Pt/NC-CeO
2
catalyst was investigated under the
2
23.8
1.4
76.2
98.6
same reaction conditions. First, the catalyst was added into the
2
o
water without reactants at 150 C with O
2
(0.4 MPa). After 4 h, the
[a] Reaction conditions: catalyst, 0.1 g; FOL, 0.5 mmol; O
2
pressure, 0.4
catalyst was filtered and HMF was added into the solution to carry
out another oxidation reaction. One can note that no conversion
MPa; reaction time, 4 h; reaction temperature, 150 °C; CH
= 50/50), 40 mL.
3
CN: water (v/v
2
Figure 8 Effects of reaction time (a), reaction temperature (b) and reaction cycles (c) on HMF oxidation over the Pt/NC-CeO sample. Reaction conditions: (a)
reaction temperature, 110 °C; (b) reaction time, 4 h; (c) reaction temperature, 150 °C and reaction time, 1 h.
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Based on the product distributions, HMF can be rapidly oxidized
o
2
Pt/NC-CeO at 110 C after 8 h reaches as high as 163, which is
to produce DFF intermediate over the Pt/NC-CeO
2
catalyst, which
higher than those over other metal-based catalysts previously
reported in the absence of the additional homogeneous base
(Table S1). In addition, contrast experiments were performed to
address the necessity of a base under high dilution conditions in
our case. As listed in Table 4, though with same HMF/Pt molar
is further oxidized to FFCA and then FDCA. Throughout the
reaction, the selectivity of FDCA gradually increases, indicating
that this product is quite stable under the present reaction
conditions. Conversely, previous results revealed that the
oxidation with base additive over Au/TiO
2
, Au/CeO
2
, and Au/LDH
ratio (163), when NaHCO
increased from 13.6 % to 43.2%. Similarly, after addition of
support with abundant base sites (CeO , C ), the yield of FDCA
3
was added, the yield of FDCA
could first proceed by the oxidation of aldehyde group in HMF.
[
7,8,20]
It suggests that support structure of the present Pt/NC-CeO
2
2
3 4
N
catalyst should have a preferential effect on the activation and
oxidation of the hydroxymethyl group in HMF, thus leading to the
effective oxidation from alcohol to aldehyde and further carboxylic
acid products. More importantly, despite different reaction
conditions, the turnover number (TON) for FDCA formed over the
increased slightly under the similar reaction condition. The above
results indicate that base sites in our catalyst indeed has a
positive effect on the yield of FDCA and a base is necessary even
under high dilute conditions.
Table 4 Catalytic performance for oxidation of HMF over different catalysts [a]
Selectivity (%)
Entry
Catalysts
HMF/Pt molar ratio
Conv. (%)
FDCA
FFCA
DFF
HMFCA
1
2
3
4
Pt/NC-CeO
2
163
163
163
163
68.9
89.7
74.2
71.8
19.8
48.2
24.6
22.1
45.9
35.3
46.2
46.8
34.1
16.3
29.1
31.1
0.1
0.2
0.1
0
2 3
Pt/NC-CeO + 0.1g NaHCO
2 2
Pt/NC-CeO +0.1g CeO
2 3 4
Pt/NC-CeO +0.1g C N
2
[a] Reaction conditions: catalyst, 0.1g; HMF, 0.5 mmol; O pressure, 0.4 MPa; reaction time, 1 h; reaction temperature, 110 °C.
Further, the effect of the reaction temperature on the HMF
oxidation catalyst was investigated over the Pt/NC-CeO (Figure
b). It is obvious that the HMF conversion and the FDCA
The above CO
2
-TPD results reveal that Pt/NC-CeO
2
possesses
2
a small amount of newly-developed SLB sites, despite a smaller
amount of total basic sites than Pt/CeO . In this regard, surface
8
2
selectivity are remarkably improved as the reaction temperature
is elevated from 90 to 150 °C. When the reaction temperature is
raised to 150 °C, an almost 100 % yield of FDCA is achieved with
the complete conversion of DFF and FFCA intermediates. It
illustrates that high reaction temperatures can promote the
oxidations of alcoholic hydroxyl and carbonyl groups in reactants
and intermediates to achieve desired FDCA product.
strong basic sites are beneficial to the generation of negatively
charged alkoxide intermediates through the interaction with the
alcoholic hydroxyl group in HMF and thus the generation of β-
0
[55,56]
hydride on Pt species.
Accordingly, the surface basicity of
Pt/NC-CeO catalyst, especially surface strong basic sites, can
2
play an important role in improving the oxidation of alcoholic
hydroxyl group in HMF. Meanwhile, the XPS results show the
In addition, we investigated the recyclability of Pt/NC-CeO
2
presence of more surface Pt2+ species in the case of Pt/NC-CeO
besides electron-deficient Pt species. These electron-deficient
2
,
0
catalyst (Figure 8c). After the catalyst was recovered by
centrifugation and the following washing, the recovered catalyst
was applied to the next reaction. After six consecutive cycles, the
FDCA yield is decreased only by less than 1.0 %, indicating that
0
2+
Pt and cationic Pt species may facilitate the of C−H bond
δ+
dissociation and the molecular oxygen activation, like Au
species,[57,58] thereby benefiting for the enhanced catalytic
the Pt/NC-CeO
2
may be recycled without a remarkable activity
2
behavior of Pt/NC-CeO catalyst.
Moreover, surface Ce3+ species adjacent to metallic Pt species
can coordinate oxygen in the alcoholic hydroxyl group or carbonyl
group in HMF or DFF and FFCA, while surface oxygen vacancies
may fix molecular oxygen and facilitate the decomposition of O-O
bond in oxygen molecule, thus affording active oxygen species at
the metal–support perimeter interface to accelerate the
0
loss. The ICP-AES results of the reaction filtrate show a negligible
leaching loss of Pt species, the N content of the spent catalyst
determined by elemental analysis does not change after six runs,
while the Ce leaching loss is only 1.6 wt% after the recycling test,
mirroring the nearly unchanged composition of the catalyst after
six runs. Moreover, the XRD patterns and TEM images reveal that
the spent catalyst remains its initial structure and morphology
conversion of reactants. In this regard, compared with Pt/CeO
2
,
(
Figure S3), and no aggregation or growth of Pt NPs can be found.
the enhanced catalytic activity of H -treated CeO supported Pt
2
2
It illustrates that the present Pt/NC-CeO catalyst possesses an
excellent stability and recyclability, mainly attributable to strong
metal-support interactions.
2
one with more surface defects (Table 2, entry 5) further illustrates
a promotional role of surface defects (Ce3+ or oxygen vacancies)
in the HMF oxidation. Accordingly, surface defects of supports
also should be a key factor in governing the oxidation of HMF.
In general, in the present catalytic system, the catalytic
performance of supported Pt catalysts strongly depends on
surface structure and property of catalysts. Correspondingly,
Mechanism of catalytic HMF oxidation
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Pt/NC-CeO
2
catalyst with stronger basic sites, favorable surface
Powder X-ray diffraction (XRD) patterns of different samples
were collected on a Shimadzu XRD-6000 diffractometer with Cu
Kα source (λ= 0.15418 nm). The Pt content in supported catalysts
samples was analyzed on a Shimadzu ICPS-7500 inductively
coupled plasma atomic emission spectroscope (ICP-AES).
Elemental microanalysis (Elementar Vario analyzer) was applied
to analyze the contents of nitrogen and carbon elements. Low-
temperature nitrogen adsorption–desorption experiments were
carried out on a sorptometer apparatus (Micromeritics ASAP
2020) at 77 K. Raman spectra were collected on Princeton
MontaVista Raman microscope equipped using a 532 nm
excitation laser. Transmission electron microscopy (TEM) and
high-resolution TEM (HRTEM) were performed using a JEOL
JEM-2100 microscope. High-angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM) was
conducted using a JEOL2010F instrument with energy-dispersive
X-ray spectroscopy (EDX). X-ray photoelectron spectroscopy
3
+
oxygen vacancies/Ce species, and unique electron-deficient Pt
species shows a superior catalytic activity to Pt/CeO and Pt/C
catalysts in the HMF oxidation to produce FDCA. Nevertheless,
the detailed roles of NC-CeO and Pt species in the aerobic
2
3 4
N
2
oxidation still need to be further determined by theoretical and
experimental investigations accurately and systemically.
In summary, a highly efficient and stable nitrogen-doped
carbon-decorated CeO
2
supported Pt-based catalyst was
fabricated and applied in the oxidation of HMF to produce FDCA
in water using molecular oxygen without the addition of any
homogenous base. Structural characterization revealed the
unique features of abundant surface defects, enhanced surface
basicity, favorable electron-deficient metallic Pt species and
strong metal-support interactions in the Pt/NC-CeO
developed Pt/NC-CeO catalyst exhibited an exceptional catalytic
efficiency with an almost 100% FDCA under mild reaction
2
catalyst. The
2
conditions, as well as an excellent stability. Moreover, the TON
(XPS) was carried out on a Thermo VG ESCALAB250
spectrometer using the Al Kα X-ray radiation. According to the
o
value for FDCA formed over the Pt/NC-CeO
2
at 110 C after 8h
could reach about 163, which was higher than those over other
metal-based catalysts previously reported in the absence of the
additional homogeneous base. The present study provides a new
approach for designing other high performing and stable
supported metal catalysts based on NC-decorated metal oxide
supports with adjustable active metal species and metal oxide
supports applied in a variety of heterogeneous catalysis.
C1s signal of 284.6 eV, the binding energies were calibrated.
Temperature-programmed desorption of carbon dioxide (CO -
2
TPD) was performed on Micromeritics ChemiSorb 2920. First, the
sample (0.1g) was pretreated under a He flow (20 mL/min) at
100 °C for 1 h, and then exposed to a CO
2
/He mixture flow (20
o
mL/min) at 50 C for 1 h. Afterwards, the sample was purged using
a He flow (20 mL/min) for 30 min and heated up to 800 °C at a
heating rate of 10 °C /min to continuously detect carbon dioxide
concentration by a thermal conductivity detector. Assuming a
single carbon dioxide molecule can be adsorbed at a sole base
site, the surface density of basic sites can be assessed.
Experimental section
Preparation of supported Pt catalysts
Catalytic aerobic HMF oxidation
2
Hybrid NC-CeO support was synthesized by a modified
solution-phase reduction-oxidation method developed by our
The oxidation of HMF was performed at a stainless-steel
autoclave reactor. In a typical reaction, catalyst (0.1 g) and HMF
group.[ First, Ce(NO
59]
)
3
3 2
⋅6H O (0.01 mol) and melamine (0.01
mol) were dispersed and dissolved into 80 mL of deionized water
to obtain solution A, while NaBH (0.1 mol) was dissolved into 80
(
0
0.5 mmol) were added into 40 ml of deionized water. Then, O
.4 MPa was purged into the reactor. At a certain reaction
2
of
4
mL of deionized water to obtain solution B. Subsequently, A and
B solutions were simultaneously put into a colloid mill, mixed
rapidly at a 4000-rpm rotor speed for 3 min and aged at 90 C for
temperature, the reaction was initiated by magnetic stirring at a
stirring speed of 900 rpm. After the oxidation, the liquid products
o
were quantitively analyzed by
chromatography with UV detector and
chromatographic column (mobile phase: dilute H
a Shimadzu LC-20A liquid
2
4 h. The resulting precipitate was filtered, washed with deionized
a
a
SO
HPX-87H
aqueous
o
water, and dried at 70 C for 12 h. At last, the resultant solid was
calcined at 500 C for 3 h under air atmosphere to form hybrid
2
4
o
solution of 10 mM at a flow rate of 0.6 mL/min). According to
external standard curves, HMF conversion and product
selectivities were obtained from three parallel experiments. In all
cases, mass balances were more than 97 %.
2 2
NC-CeO support. Moreover, pure CeO support was synthesized
based on the above procedure in the absence of melamine, and
pristine g-C
3
N
4
support was synthesized by calcining melamine at
o
500 C for 4 h.
Pt/NC-CeO
prepared by using NaBH
sample (0.5 g) was put into 50 mL of aqueous solution containing
PtCl ⋅6H O (0.01 g). Subsequently, 20 mL of NaBH aqueous
solution, where the NaBH
2
with the Pt loading of about 0.6 wt % was
4
as reductive reagent. First, NC-CeO
2
Acknowledgments
H
2
6
2
4
We greatly thank the financial support from National Natural
Science Foundation of China (21776017; 21521005).
4
/Pt molar ratio is set at 15:1, was put
o
into the support-containing suspension and aged at 25 C for 6h
o
at nitrogen atmosphere. At last, the solid was dried at 70 C under
Keywords: Nitrogen-doped carbon • Ceria • Surface basicity •
2 3 4
vacuum. For comparison, CeO or g-C N supported Pt sample
also was prepared according to the above similar process.
Surface defects • Oxidation
Characterization
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Entry for the Table of Contents
FULL PAPER
+
+
Changxuan Ke , Mengyuan Li ,
Nitrogen-doped carbon-decorated
*
*
Guoli Fan , Lan Yang and Feng Li
2
CeO supported Pt-based catalyst
was fabricated and applied in the
oxidation of HMF to produce FDCA in
water using molecular oxygen
without the addition of any
homogenous base. The unique
features such as abundant surface
defects, enhanced surface basicity,
favorable electron-deficient metallic
Pt species, and strong metal-support
interactions endow the prepared
Page No. – Page No.
Nitrogen-Doped Carbon-Decorated
CeO Supported Pt Nanoparticles
for Base-Free Aerobic Oxidation of
-hydroxymethylfurfural
2
5
2
Pt/NC-CeO catalyst with exceptional
catalytic efficiency and stability under
mild reaction conditions
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