N-doped carbon supported Pt catalyst for base-free oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid

Article abstract of DOI:10.1016/j.apcata.2016.07.011

A new kind of N-doped carbon supported Pt catalyst (Pt/C) has been prepared for the selective oxidation of 5-Hydroxymethylfurfural (HMF) to 2,5-Furandicarboxylic Acid (FDCA) in base-free conditions. The catalyst (Pt/C-EDA-x) prepared by using ethylenediamine (EDA) as nitrogen source showed higher activity than those prepared by N,N-dimethylaniline (DMA), ammonia (NH₃) or acetonitrile (ACN) as nitrogen sources. The Pt/C-EDA-4.1 catalyst showed the highest activity in the oxidation of HMF to FDCA and as high as 96.0% FDCA was obtained under optimal reaction conditions (110?°C, 1.0?MPa O₂, 12?h). The samples were characterized by XRD, XPS, CO₂-TPD, TEM, SEM, and elemental analysis. XPS results showed that the pyridine-type nitrogen (N-6) played a key role in the selective oxidation of HMF, which can be attributed to the basicity of N-6 site. CO₂-TPD measurements also showed that the involving of N elements in catalyst preparation introduced a new kind of medium basic site on the support surface. The influence of reaction time, catalyst dosage, and temperature on the HMF oxidation to FDCA catalyzed by Pt/C-EDA-4.1 was studied.

Full text of DOI:10.1016/j.apcata.2016.07.011

Accepted Manuscript
Title: N-doped carbon supported Pt catalyst for base-free
oxidation of 5-hydroxymethylfurfural to
2
,5-furandicarboxylic acid
Author: Xuewang Han Chaoqun Li Yong Guo Xiaohui Liu
Yongguang Zhang Yanqin Wang
PII:
S0926-860X(16)30366-0
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.07.011
APCATA 15948
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
12-4-2016
13-7-2016
17-7-2016
Please cite this article as: Xuewang Han, Chaoqun Li, Yong Guo, Xiaohui Liu,
Yongguang Zhang, Yanqin Wang, N-doped carbon supported Pt catalyst for base-free
oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid, Applied Catalysis
A, General http://dx.doi.org/10.1016/j.apcata.2016.07.011
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<AT>N-doped carbon supported Pt catalyst for base-free oxidation of
5
-hydroxymethylfurfural to 2,5-furandicarboxylic acid
a
a
b**
<
AU>Xuewang Han , Chaoqun Li , Yong Guo
a
b
#
#Email##guoyong.ripp@sinopec.com##/Email##, Xiaohui Liu , Yongguang Zhang , Yanqin
a*
Wang ##Email##wangyanqin@ecust.edu.cn##/Email##
<AU>
a
<
AFF> Shanghai Key Laboratory of Functional Materials Chemistry, Research Institute of
Industrial Catalysis, East China University of Science and Technology, Meilong Road130,
Shanghai, 200237, PR China
b
<
AFF> Research Institute of Petroleum Processing (RIPP), Sinopec, 18 Xueyuan Road 18,
P.O. Box 914-12, Beijing 100083, PR China
<PA>Tel.: 0086-21-64253824; Fax: 0086-21-64253824.
<PA>**Corresponding authors: Tel.: 0086-10- 82368692
<
<
<
ABS-Head><ABS-HEAD>Graphical abstract
ABS-P>
ABS-P><xps:span class="xps_Image">fx1</xps:span>
<ABS-HEAD>Highlights► N-doped carbon supported Pt catalyst was prepared using
hydrothermal and impregnation methods. ► Pt/C-EDA-x exhibited high FDCA selectivity
under base-free conditions. ► Nature of N-doped carbon materials were identified by XPS and
CO -TPD.
2
<ABS-HEAD>Abstract
<ABS-P>A new kind of N-doped carbon supported Pt catalyst (Pt/C) has been prepared for
the selective oxidation of 5-Hydroxymethylfurfural (HMF) to 2,5-Furandicarboxylic Acid
FDCA) in base-free conditions. The catalyst (Pt/C-EDA-x) prepared by using
ethylenediamine (EDA) as nitrogen source showed higher activity than those prepared by
N,N-dimethylaniline (DMA), ammonia (NH ) or acetonitrile (ACN) as nitrogen sources. The
Pt/C-EDA-4.1 catalyst showed the highest activity in the oxidation of HMF to FDCA and as
high as 96.0% FDCA was obtained under optimal reaction conditions (110 °C, 1.0 MPa O
2 h). The samples were characterized by XRD, XPS, CO -TPD, TEM, SEM, and elemental
(
3
2
,
1
2
analysis. XPS results showed that the pyridine-type nitrogen (N-6) played a key role in the
1

selective oxidation of HMF, which can be attributed to the basicity of N-6 site. CO
2
-TPD
measurements also showed that the involving of N elements in catalyst preparation
introduced a new kind of medium basic site on the support surface. The influence of reaction
time, catalyst dosage, and temperature on the HMF oxidation to FDCA catalyzed by
Pt/C-EDA-4.1 was studied.
<
KWD>Keywords: N-doped carbon; base-free oxidation; 2,5-Furandicarboxylic acid
H1>1. Introduction
<
The strong demands for alternate resources to obtain chemicals, which are produced from
fossil feedstocks, attracted researchers to work on the renewable resource [1, 2]. Biomass, as a
renewable resource, has been demonstrated as promising feedstock for the production of
chemicals in the future, which can be converted to bulk products, such as liquid fuels, or high
value-added chemicals, including glucose, 5-hydroxymethylfurfural (HMF), levulinic acid and
glycerine through bio-refinery processes [3-5]. Among the variety of biomass-derived platform
molecules, HMF, produced by glucose or fructose dehydration, is seen as a key precursor for
bio-renewable chemicals and materials production [6-8] and its selective oxidation to
2
,5-furandicarboxylic acid (FDCA) is an attractive route, as these furan products could be used
as pharmaceuticals and polymer precursors [9, 10]. The molecule of FDCA contain furyl ring
and carboxyl groups in the basic structure, which structure is similar to that of terephthalic acid,
thus it would be a possible replacement monomer for pure terephthalic acid (PTA) used to
produce polyethylene terephthalate (PET) [11, 12].
Traditionally, the aerobic oxidation of HMF to FDCA was carried out by using potassium
permanganate as stoichiometric oxidant which have toxicity issues and produce a large amount
of pollutants [13]. There are also some studies reported that homogeneous catalysts such as
Co/Mn/Br in corrosive solvents (e.g., acetic acid) can be utilized for the oxidation of HMF,
which is widely used for the efficient oxidation of paraxylene (PX) to PTA [14]. However, the
homogeneous catalysts showed some disadvantages, like the difficulty in catalyst recycling
and the pollution to environment. On the contrary, it has more advantages by using
heterogeneous catalyst, since it overcomes the drawbacks of homogeneous catalysts.
Recently, there have been several reports on the aerobic oxidation of HMF to FDCA over
supported transition metal catalysts, including Pt, Pd, Au, Ru, and Cu loaded on various
2

supports, like carbon, hydrotalcite, TiO
Casanova et al. [21] reported that HMF could be selectively converted into FDCA over gold
nanoparticles supported on CeO and TiO in NaOH solution under mild conditions (65-130 °C,
0 bar air). Davis et al. [22] reported the catalytic activity of supported Pt, Pd and Au catalysts
2
, Al
2
O
3
, etc in basic solution [15-20]. For example,
2
2
1
at 22 °C and high pH in a semi-batch reactor and found that FDCA was formed over Pt/C and
Pd/C, but 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) was formed over Au/C and
Au/TiO . In addition, there are some reports on the use of alloy catalysts in the oxidation
2
reaction of HMF, to obtain good results. Some interesting works demonstrated that alloy
catalysts such as Au-Cu, Au-Pd supported on various carriers showed excellent reactivity [20,
2
3-25].
Similar to Au, Pd and alloy catalysts, supported Pt catalysts were also found to be active
toward the aerobic oxidation of HMF into FDCA in the earliest, and are still widely
investigated now. Verdeguer et al. [26] reported that full HMF conversion was achieved by
using Pt/C and Pt-Pb/C catalysts in a strongly alkaline aqueous medium with FDCA yields of
8
1 and 99 %, respectively. A mechanistic investigation involving kinetic modeling has been
partly elucidated using ionic polymer stabilizer afforded Pt nanoparticles for the oxidation of
HMF under mild conditions by Siankevich and co-workers and found that the IP-modified
1
8
surface led to a higher conversion rate at an early phase of the reaction and O atoms come
from H 18O involved in the reaction and incorporated in FDCA [27]. Moreover, Besson et al.
28] reported the oxidation of HMF to FDCA in moderately basic aqueous solutions over
2
[
activated carbon supported Pt and Pt-Bi catalysts and found that Bi can enhance the activity
and stability of the Pt/C catalyst, as well as the selectivity to FDCA. It can be seen that most
studies focused on adding extra homogeneous base to promote the oxidation reaction.
Recently, the development of new solid base catalysts for HMF oxidation without adding
homogeneous base in reaction medium has received widely attention, because it may
effectively reduce the risk of equipment corrosion [29, 30]. Gupta and co-workers developed a
hydrotalcite-supported gold nanoparticle (Au/HT) catalyst for selective oxidation of HMF into
FDCA using molecular oxygen in water under homogeneous base-free condition [31]. The
catalyst showed excellent activity at 95 °C, but the stability of the catalyst decreased gradually,
3

because of the instability of the hydrotalcite in water. Wang et al. [30] demonstrated the
base-free aerobic oxidation of HMF into FDCA by using carbon nanotube (CNT) supported
Au-Pd alloy catalyst in water and found that the carbon nanotube-enhanced adsorption and the
alloying effect are mainly contributed to the realization of the base-free oxidation reaction. Han
et al. [32] developed an efficient Pt/C-O-Mg catalyst for the oxidation of HMF to FDCA in
aqueous solution without adding homogeneous base and found that it exhibited excellent
selectivity and stability. The yield of FDCA reached 97 %. The excellent activity may come
from the formation of C-O-Mg bond, which was a new basic site and promoted the activation
of hydroxyl and the formation of an intermediate, hemiacetal in the aerobic oxidation reaction.
There are also some other interesting catalysts being reported for the aerobic
oxidation of HMF to FDCA in base-free condition. For Example, Corma et al. reported
the oxidative esterification of HMF to 2,5-dimethylfuroate by using Au-CeO under
2
mild conditions in the absence of any base [16]. In addition, Ru-K-OMS-2 has also been
demonstrated to exhibit catalytic activity in base-free conditions, which gave a
maximum yield of 66 % of FDCA [33]. However, the metal oxide catalyst was unstable
during the reaction in high temperature aqueous solution. Therefore, the development of
stable heterogeneous catalysts for the oxidation of HMF under base-free conditions
remains challenging. As compared with metal oxide, carbon materials exhibit very high
hydrothermal stability. Recently, Ru/C and Ru clusters supported on covalent triazine
frameworks (Ru/CTFs) were reported as stable catalysts for the oxidation of HMF under
base-free conditions [34,35]. But these two catalysts did not exhibit excellent catalytic
activity, and the yield of FDCA was lower than 90 %.
N-doped carbons are well recognized and used in photocatalysis, anode materials in
lithium battery, supercapacitor and catalyst, but never used as support in the selective oxidation
of HMF to FDCA under base-free condition [36-38]. Herein, we designed a simple nitrogen
doped carbon catalyst via hydrothermal method for the base-free oxidation of HMF into FDCA
under mild conditions (110 °C, O 1.0 MPa, 12 h). Then these prepared catalysts were
2
characterized with XPS, TPD, XRD, SEM, TEM and BET in order to understand the effects
of the catalyst properties on the catalytic activities. The influence of reaction time, catalyst
4

dosage, and temperature on the HMF oxidation to FDCA catalyzed by Pt/C-EDA-4.1 was
also studied.
<H1>2. Experimental Section
<H2>2.1 Materials
5-Hydroxymethylfurfural (HMF) (>98%) was purchased from DEMO Medical,
2
2
,5-Diformylfuran (DFF) (>98%), 5-formyl-2-furancarboxylic acid (FFCA) (>98%),
,5-furandicarboxylic (FDCA) (>97%) were purchased from TCI. Chloroplatinic acid
[
(H
Ethylenediamine, Resorcinol, etc., were obtained from SCRC.
H2>2.2 Catalyst preparation
A series of nitrogen doped carbon materials were prepared by hydrothermal
method. A typical synthesis procedure is as following: 4g of glucose, 4g of resorcinol,
g of ethylenediamine (EDA) and 10g of H O were mixed in a 100 mL flask and stirred
2
PtCl
6
)•6H
2
O] was obtained from Alfa-Aesar. Other chemical reagents, such as Glucose,
<
1
2
for 1 h. Then, the mixture was sealed into an autoclave and put into an oven at 180 C
for 24 h for hydrothermal treatment. After reaction, the autoclave was cooled down in a
cold water bath and a black solid was obtained. Then the obtained black solid was
filtered and washed with distilled water for several times until the filtrate was colorless.
3
-1
Finally, the black solid was carbonated in nitrogen flow of 30 cm min at 800 °C for 3
h after drying. Thus-obtained nitrogen-containing carbons were named as C-EDA.
Afterwards, we also synthesized a series of C-EDA materials with different nitrogen
content by changing the dosage of ethylenediamine, and then named as C-EDA-x. As a
contrast, other nitrogen-doped carbons were also synthesized in the same way as
C-EDA by using N,N-dimethylaniline (DMA), ammonia (NH ) or acetonitrile (ACN)
3
as nitrogen source, respectively. As a comparison, hydrothermal carbon (HTC) catalyst
was also synthesized by a hydrothermal method and further carbonization, by using
glucose and resorcinol as carbon sources.
Platinum was loaded by impregnation with an aqueous solution of H
2
PtCl
6
followed by reduction. An aqueous solution of H PtCl containing the required amount
2
6
of Pt (5 wt%) was added dropwise under stirring to an aqueous suspension of support.
5

After that, the mixture was ultrasonically treated for 0.5 h to ensure the solid being
uniformly dispersed. Then excess amount of NaBH was slowly added to the mixture
4
with stirring under room temperature. The solid phase were finally separated by
filtration and washed with deionized water for several times, and then dried in a
vacuum desiccator at 60 °C.
<H2>2.3 Catalyst characterization methods
Elemental analyses were performed on Elementar's Vario EL III microanalyzer.
C, H, and N values were calculated as per atomic weight of C = 12.01, H = 1.008, N
=14.007, and O = 15.999. The values obtained for each element were expressed as a
percentage of the total molecular weight of the compound. The synthesized catalysts
were fully characterized by the following methods: Powder X-ray diffraction (XRD)
patterns were collected in the θ-2θ mode using a Bruker D8 Focus diffractometer
(CuKa1 radiation, k= 1.5406 Å), operated at 40 kV and 40 mA with scattering angles
(2θ) of 8-60°. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo
ESCA LAB-250 spectrometer with monochromatic Al Ka radiation, and the XPS
results were calibrated using the C 1s peak at 284.8 eV. XPS Peak 4.1 was employed to
deconvolve the N 1s peaks using the Shirley-type baseline and an iterative
least-squared optimization algorithm. Nitrogen sorption isotherms were measured at
-196 °C with a NOVA 4200e sorption analyzer. Before the measurements, the samples
were degassed at 100 °C in vacuum for 8 h. The Brunauer-Emmett-Teller (BET)
method was utilized to calculate the apparent surface area.
Temperature programmed desorption of carbon dioxide (CO -TPD) was carried
2
out on Micromeritics AutoChem 2920 equipped with mass spectrometry using
quadrupole spectrometer (MS OmniStar). 100 mg of catalyst was placed in quartz
sample tube. Before TPD experiments, the catalysts were outgassed at 500 °C for 2 h in
a flow of helium. Subsequently the catalysts were cooled down to 50 °C and treated
with a CO
2
flow for 60 min. Weak adsorbed CO
2
was removed by flushing with He at
5
5
0 °C for 60 min. The desorption of CO was measured by heating the catalyst from
2
-1
0 °C to 600 °C at a heating rate of 10 °C min in He flow. The desorbed products were
6

analyzed by mass spectrometry. The number of basic sites was calculated from the CO
peaks (the molecular ion, m/z = 44).
2
Scanning electronic microscope (SEM) images were obtained from JEOL
JSM-63602V and Hitachi S-3400N microscope operated at 5 kV or 15 kV,
respectively. Transmission electron microscopy (TEM) images were recorded on a FEI
TECNAI F20 s-TWIN instrument, and the electron beam accelerating voltage was 200
kV. Chemical analysis of the samples was performed by using inductively coupled
plasma atomic emission spectrometry (ICP-AES).
<H1>2. 4 Reaction procedure and product analysis
The oxidation reaction of HMF was performed in a batch-type Teflon-lined
stainless-steel autoclave (100 mL), equipped with an internal thermo-controller and
pressure gauge. In a typical catalytic experiment, 0.04 g of catalysts, 10 mL of aqueous
solution of HMF (0.05 M) were added to a stirred autoclave reactor. Initially, the
system was filled with 1.0 MPa of oxygen at room temperature, and then it begins to
heat up. After the system reaches the required temperature, the reaction starts by
stirring with a magnetic stirrer.
When the reaction was finished, the mixture was made alkaline by adding a certain
amount of sodium hydrate so that the crystalline FDCA can be completely dissolved for
accurate analysis, followed by analysis using HPLC (Agilent 1200 series, Bio-Rad
Aminex HPX-87H column, 300 mm × 7.8 mm × 9 μm). A 0.004 M H
2
SO mobile
4
-1
phase was employed as mobile phase at 55 °C with a flow rate of 0.45 mL min . The
yields of FDCA and intermediate products were based on conversion of HMF
and confirmed by calibration of standard solutions of the reactants and products.
H1>3. Results and Discussion
<
<
H2>3.1 Characterization of catalysts
The textural structures of catalysts are measured and summarized in Table 1. The
2
-1
calculated specific surface area of HTC is 434 m g , while the specific surface area of
2
-1
nitrogen-doped carbons decreased gradually from 67 to 20 m g with increasing N content
from 1.0 to 8.3 wt%. The lower specific surface area could be attributed to higher
7

polymerization degree of carbon and nitrogen sources. The chemical composition of various
carbon samples prepared by using different nitrogen sources were detected by an elemental
analyzer and the results are also shown in Table 1. In order to investigate the effect of nitrogen
content, the C-EDA-x samples, with different nitrogen contents from 1.0 to 8.3 wt% prepared
by adding different amounts of EDA in the hydrothermal process, were also characterized and
summarized in Table 1. The N content of C-NH and C-DMA was 3.8 and 3.2 wt%,
3
respectively. However, the N content of C-ACN (0.3 wt%) was lower than mentioned above
due to only a small amount of nitrogen atoms enter the carbon skeleton.
{
Table 1}
The level of structural order in the carbonaceous materials was checked using XRD
measurements (Fig. 1). For all supports, two broad peaks are observed at around 20°-30°, and
3.4°, assigned to (002) and (100) graphite planes, respectively, which shows the amorphous
4
carbon structure. Furthermore, we can see the obvious shift towards higher angles of (002)
reflection compared with that of HTC, indicating a significant decrease in the interlayer
spacing. The peak shifts gradually to the higher diffraction-angle side with increasing nitrogen
content (23.5° for C-EDA-1, 24.1° for C-EDA-4.1, and 24.2° for C-EDA-8.3 compared to
2
3.1° for HTC), and can be directly related to the much smaller size of the nitrogen atom than
that of carbon atoms. Therefore, nitrogen doping resulted in more defects and disorder in the
structure of supports, which may become structural basis of the active sites [39, 40].
{
Fig. 1}
The catalytic activity was measured over various catalysts summarized in Table 1. It can be
found that the catalyst prepared by using EDA as nitrogen source had the highest activity. In
addition, we also compared the activity of Pt/-C-EDA-4.1 and conventional supported Pt
catalysts in Table 1. It can be found that the catalytic activity of Pt/C-EDA in the absence of
base conditions is comparable to that of the conventional supported Pt catalysts in the presence
of base.
8

It is also necessary to eliminate the effect of nitrogen content of different catalysts on the
catalytic activity. Accordingly, we synthesized Pt/C-ACN-0.6 and Pt/C-NH -4.9 by adjusting
3
the amount of nitrogen sources added, and tested their catalytic activity, which results shown
in Table S1. When using Pt/C-EDA-4.1 as catalyst, the conversion of HMF reached 88.4 %,
while the yields of FDCA and FFCA were 18.3 and 50.7 %, respectively, after reaction for 4
h. However, for the Pt/C-ACN-0.6 and Pt/C-NH
yield of intermediates are relatively low compared to Pt/C-EDA-4.1 after reaction for 4 h. We
extended the reaction time for 18 h using Pt/C-NH -4.9 as catalyst so that the HMF and
3
-4.9, both the conversion of HMF and the
3
intermediate products could completely transformed into FDCA, and the results showed that
the selectivity to FDCA was only 77.4 % (low carbon balance because of partial
polymerization). Therefore, prolonging the reaction time cannot improve the selectivity of
FDCA to reach as high as Pt/C-EDA-4.1, which means that the nature of these catalysts is
obviously different. It is proved once again that the Pt/C-EDA-4.1 catalyst has excellent
catalytic activity and high FDCA selectivity.
In order to study the relationship between the nitrogen species and the reaction
activity, we characterized the samples by X-ray photoelectron spectroscopy (XPS). Fig.
2
and Fig. S1 shows the detailed spectra corresponding to the experimental and curve
fitted N 1s spectra of each catalyst. In the N 1s spectra, three peaks with
electron-binding energies of 401.4 ± 0.5, 400.3 ± 0.3 and 398.6 ± 0.3 eV have been
observed after deconvolution and can be attributed to graphene-type nitrogen (N-Q),
pyrrole-type nitrogen (N-5) and pyridine-type nitrogen (N-6), respectively [41]. Table
2
and Table S2 summarize the binding energies (eV) and relative intensities (%) of each
N species as observed from the respective N 1s spectrum of various samples. For the
sample with 1.0 wt% N (C-EDA-1.0), the relative intensity of N-6, N-5 and N-Q were
2
2.0, 36.8 and 41.2 %, respectively, which has a lowest yield of FDCA (80.4 %). An
interesting phenomenon is that the more nitrogen in the catalyst, the higher the catalytic
activity. Indeed, the 5 wt% Pt/C-EDA-4.1 and Pt/C-EDA-8.3 (5 wt%) catalysts
exhibited better catalytic activity. Therefore, we analyzed the relationship between the
catalytic activity and the relative content of nitrogen species. Fig. 3 is the plot of
9

relative intensity against the percentage of pyridine-type (N-6), pyrrole-type (N-5) and
graphite-type (N-Q) nitrogen species on the surface of C-EDA-x detected by XPS.
With increasing nitrogen content, the proportion of N-6 in all nitrogen atoms gradually
increased, and a significant increase of FDCA yield was detected. It is notable that
when the relative intensity of N-6 and N-5 was equal, the catalyst showed highest
activity. A negative correlation could be observed between the yield of FDCA and the
relative content of graphite-type nitrogen species. The catalyst activity cannot be
correlated with the relative amount of pyrrole-type nitrogen species. Thus, through the
above discussion, we can deduce that the N-6 play a key role in the selective oxidation
of HMF, which can be attributed to the basicity of N-6 site confirmed by CO
2
-TPD
later. Fig. 4 showed the possible positions of N species doped on carbon [42].
{
{
{
{
Fig. 2}
Table 2}
Fig. 3}
Fig. 4}
The basicity of all the supports was detected via CO
Fig. 5. It can be seen that the adsorption of CO
the weak chemical interaction of CO molecules with nitrogen groups at the temperature of
less than 100 °C, suggesting the existence of weak basic sites. On the contrary, the desorption
of CO on C-EDA-x at a temperature higher than 100 °C is due to chemically adsorbed CO
which means that involving N in catalyst preparation introduced a new medium strength
basic site in the carbonaceous materials. Obviously, the CO desorption peak area increased
significantly with the increase of nitrogen content in the C-EDA-x, which may be attributed
2
-TPD and the result was shown in
2
on C-DMA, C-NH , and C-ACN occurs via
3
2
2
2
,
2
to the increase of absolute amount of N-6 species. Previous studies mentioned that basic sites
on the surface of catalyst could promote the activation of hydroxyl and the formation of
intermediate hemiacetal in oxidation reaction of alcohols [29, 31, 43, and 44]. Indeed, this
kind of Pt/C-EDA-4.1 catalyst had a best activity for HMF oxidation under base-free
conditions due to alkalinity from pyridine-type nitrogen.
1
0

{
Fig. 5}
<
H2>3.2 Influence of various reaction conditions on the catalytic activity of Pt/C-EDA-4.1 on
HMF oxidation
The time course of the conversion of HMF catalyzed by Pt/C-EDA-4.1 is shown in Fig.
(a). Yields of FFCA and DFF accumulated to 33.8 and 11.3 % at the first 4 h and then
6
diminished gradually as the reaction processed; this indicates that HMF was oxidized rapidly
to DFF and FFCA, then gradually transferred to FDCA, and finally the yield of FDCA
reached 96 % after reaction for 12 h. This reaction pathway is in agreement with the literature
[
28].
The influence of reaction catalyst dosage, temperature and O pressure on the HMF
2
oxidation to FDCA catalyzed by Pt/C-EDA-4.1 was also studied, and the experiments were
carried out at 10, 20, 40 mg and 70, 90, 110 °C, respectively. As shown in Fig. 6 (b), it is
clear that the catalyst dosage had a significant effect on the oxidation of HMF to FDCA. The
yield of FDCA increased notably when the catalyst dosage increased from 10 to 40 mg. A
good HMF conversion of 99 % and a 49.2% yield of FDCA were obtained after 12 h by the
use 10 mg of Pt/C-EDA-4.1. With a further increase in catalyst dosage from 10 to 40 mg,
FDCA yield gradually increased, reaching 96 % after 12 h. These result means that the
catalytic sites of 40 mg of Pt/C-EDA-4.1were sufficient to absorb the HMF and facilitate the
oxidation reaction. Fig. 6 (c) shows the influence of the temperature with respect to the HMF
conversion and the products yield. Remarkably, the aerobic oxidation of HMF was sensitive
to the reaction temperature. The higher the reaction temperature was, the higher the FDCA
yield was. When the reaction was performed under 70 °C, the yield of FDCA was not more
than 60 % in 12 h. When the temperature was raised to 110 °C, the yield of FFCA gradually
decreased from 38 to 0 % meanwhile the yield of FDCA increased from 55.8 to 96 %.
Therefore, higher reaction temperature promoted the conversion of HMF to FDCA. The high
catalytic activity of Pt/C-EDA-4.1 on the oxidation of HMF was tested with different
oxygen pressure under base-free conditions (Fig. 6 (d)). At a lower pressure (0.25 MPa), the
intermediate product was detected, meaning that the oxidation process was incomplete. On
1
1

further increasing the reaction pressure to 0.5 and 1.0 MPa, the activity of catalyst increased
gradually and a higher FDCA yield was obtained without any FFCA was detected. The above
results were consistent with the previous reports on platinum catalyzed oxidation of HMF to
DFF [28].
{
Fig. 6}
The proposed pathways for the oxidation of HMF are shown in Scheme 1. In the initial
stage of the reaction, the substrate quickly transformed to DFF, then subsequently oxidized to
FFCA via hydration of aldehyde to hemiacetal catalyzed by the basic sites on the surface of
Pt/C-EDA-4.1. Similarly, the formed FFCA can be further oxidized to FDCA by the way of the
formation of the intermediate hemiacetal [29, 31, 42, and 43].
{Scheme 1}
<H2>3.3 Catalyst stability and reusability
The stability and reusability of the Pt/C-EDA-4.1 catalyst was investigated by reusing
the catalyst in consecutive catalytic runs. In each run, the catalyst was separated from the
reaction mixture by high-speed centrifugation, washed with water, and was then filtered and
reused without further treatment in the following run. As shown in Fig. 7, HMF conversion
was gradually decreased from 88.4 % in the first run to 75.9 % in the fifth run. FDCA yield
was also decreased from 18.3 % in the first run to 8.1 % in the fifth run. This may be due to
the leaching and oxidation of Pt nanoparticles, as appeared in previous report [28].
{
Fig. 7.}
In order to reveal the intrinsic cause of the catalyst activity decrease, the spent catalyst was
characterized by TEM, SEM, and XPS (Fig. 8 and Fig. S2). As seen in Fig. 8 (a) and (b), a
comparison of SEM images between the fresh and the spent catalyst confirmed that the catalyst
showed no morphological change. However, there was clear growth or aggregation of Pt
particles (from 4.06 to 5.42 nm) after stability tests under base-free conditions based on TEM
image and histogram of the Pt particle size distribution (Fig. 8 c-f). In addition, the spent
catalyst analyzed by ICP and Elemental analysis revealed that the platinum loading decreased
1
2

from 5.1 wt% to 4.3 wt% and the nitrogen content decreased from 4.1 wt% to 3.7 wt%.
Although the precious metal and nitrogen content decreased, there was no obvious change in
the proportion of platinum valence state and three nitrogen species (Fig. S2). Thus, the most
possible reason for the slight decrease of the catalytic activity was due to the slight leaching of
Pt, N and the growth of Pt nanoparticles during the recycling procedures. The leaching and
growth of Pt nanoparticles would be prevented by carbothermal reduction [45], this work is
still in progress.
{
Fig. 8}
<H1>4. Conclusions
In summary, N-doped carbon supported Pt catalyst (Pt/C-EDA-x) was successfully
prepared via hydrothermal method by using glucose and resorcinol as carbon sources and
EDA as nitrogen source. Several types of nitrogen elements, inclulding graphene-type
nitrogen (N-Q), pyrrole-type nitrogen (N-5) and pyridine-type nitrogen (N-6), were
successfully introduced into the carbon structure which was detected by XPS. Meanwhile,
nitrogen doping resulted in more defects and disorder in the structure of supports, which may
become structural basis of the active sites. The alkalinity of C-EDA-x was detected by
CO -TPD, which means that involving N in catalyst preparation introduced a new
2
medium strength basic site on the support surface. Moreover, the increasing of nitrogen
content has an obvious improvement effect, which makes the catalyst has higher N-6
proportion, more basic sites, and higher alkalinity, and facilitates the HMF oxidation to
FDCA under base-free aqueous solution. Several reaction conditions were studied to optimize
the reaction conditions on the HMF oxidation to FDCA catalyzed by Pt/C-EDA-4.1. In
addition, the possible reason for the decrease of the catalytic activity was due to the slight
leaching of Pt, N and the growth of Pt nanoparticles during the recycling procedures.
<ACK>Acknowledgements
This project was supported financially by the National Natural Science Foundation of
China (No. 91545103 and 21273071), the Science and Technology Commission of Shanghai
Municipality (13JC1401902 and 10dz2220500), and Sinopec.
<
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<Figure>Fig. 1. XRD patterns of HTC and C-EDA-x supports with different N content.
<Figure>Fig. 2. XPS spectra of N 1s in Pt/C-EDA-1.0, Pt/C-EDA-2.3, Pt/C-EDA-4.1 and
Pt/C-EDA-8.3 samples.
Figure>Fig. 3. Plot of relative intensity against the percentage of pyridine-type (N-6 ■),
pyrrole-type (N-5 ●), and graphite-type (N-Q ▲) nitrogen species on the surface of C-EDA-x.
<
<
<
<
Figure>Fig. 4. Possible positions of N species doped on carbon.
Figure>Fig. 5. CO -TPD spectra of HTC, C-EDA-x, C-DMA, C-NH
Figure>Fig. 6. (a) Time course of HMF oxidation over Pt/C-EDA-4.1. Reaction conditions:
O (10 ml), Pt/C-EDA-4.1 (0.04 g), molar ratio HMF/Pt = 50, 110 °C, 12 h,
1.0 MPa; (b) Effect of catalyst dosage on HMF oxidation. Reaction conditions: HMF (0.5
mmol), H O (10 ml), 110 °C, 12 h, O 1.0 MPa. (c) Effect of reaction temperature on HMF
oxidation. Reaction conditions: HMF (0.5 mmol), H O (10 ml), Pt/C-EDA-4.1 (0.04 g), molar
ratio HMF/Pt = 50, 12 h, O 1.0 MPa; (d) Effect of oxygen pressure on HMF oxidation.
2
3
, and C-ACN.
HMF (0.5 mmol), H
2
O
2
2
2
2
2
1
5

Reaction conditions: HMF (0.5 mmol), H
HMF/Pt = 50, 110 °C, 12 h.
2
O (10 ml), Pt/C-EDA-4.1 (0.04 g), molar ratio
<
Figure>Fig. 7. The cycle usage tests of Pt/C-EDA-4.1 for the base-free oxidation of HMF.
Reaction conditions: Reaction conditions: HMF (0.5 mmol), H O (10 ml), catalyst (0.04 g),
molar ratio HMF/Pt = 50, 110 °C, 4 h, O 1.0 MPa.
Figure>Fig. 8. (a, b) SEM images of fresh Pt/C-EDA-4.1 and used Pt/C-EDA-4.1; (c, d) TEM
2
2
<
images of fresh Pt/C-EDA-4.1 and used Pt/C-EDA-4.1; (e, f) histogram of Pt particle size
distribution of fresh Pt/C-EDA-4.1 and used Pt/C-EDA-4.1.
<Figure>Scheme 1. Schematic reaction for the oxidation of HMF to FDCA on Pt/C-EDA-4.1.
<Table>Table 1 Surface area, elemental analysis of various carbon samples and catalytic
activity of different carbonaceous materials supported platinum (5 wt%) for the oxidation of
[
a]
HMF in the base-free aqueous phase .
2
-1
Sample
S
BET/m g
From elemental analysis
From XPS
Conv.
Yield (%)
N%
--
C%
H%
N%
--
C%
--
FDCA FFCA
HTC
434
97.2 <0.3
97.3 <0.3
96.6 <0.3
90.5 1.4
86.8 1.4
91.2 1.4
98.4
>99
98.6
>99
>99
97.8
83.5
80.4
82.8
96.0
93.4
84.1
1.6
0.4
0.7
0
0.8
7.8
C-EDA-1.0 67
C-EDA-2.3 36
C-EDA-4.1 28
C-EDA-8.3 20
1.0
2.3
4.1
8.3
3.8
1.2
2.7
3.8
6.3
2.9
86.4
85.2
86.6
80.5
84.4
C-NH3
--
C-DMA
C-ACN
--
--
--
--
--
3.2
0.3
--
--
--
92.7 1.1
95.9 0.7
2.7
--
--
--
--
85.7
--
--
--
--
>99
97.0
100
100
100
47.1
1.8
98
32.1
10.7
1
[
b]
Pt-Bi/C
--
--
--
--
--
--
[
c]
Pt/TiO
2
91
0
[
d]
Pt/ZrO
2
95
0
[
a] Reaction conditions: substrate (0.5 mmol), catalyst (0.04 g), H
2
O (10 mL), O
2
(1.0 MPa),
1
10 °C, 12 h; the conversion and yield were determined by HPLC; [b] Reference 28, reaction
/HMF = 4; [c]
Reference 44, reaction conditions: 100 °C, 4 MPa air, 0.1M HMF, HMF/Pt molar ratio=100,
conditions: 100 °C, 4 MPa air, 0.1 M HMF, HMF/Pt molar ratio=100, NaHCO
3
1
6

Na
2
CO /HMF = 2; [c] Reference 44, reaction conditions: 100 °C, 4 MPa air, 0.1M HMF,
3
HMF/Pt molar ratio=100, NaHCO
Table>Table 2 The binding energies (eV) and relative intensities (%) of each N species
observed from the respective N 1s spectrum of various samples.
3
/HMF = 4.
<
Sample
Species
Binding
Relative
energy of N 1s
intensity (%)
(eV)
C-EDA-1
N-6
N-5
N-Q
398.4
400.3
401.3
398.7
400.7
401.5
22.0
36.8
41.2
27.9
36.9
35.2
C-EDA-2.3 N-6
N-5
N-Q
N-6
N-5
N-Q
N-6
N-5
N-Q
C-EDA-4.1
C-EDA-8.3
398.4
400.4
401.3
398.2
400.2
401.0
29.4
42.0
28.6
40.9
35.6
23.5
TDENDOFDOCTD
1
7