Oxidative esterification of renewable furfural on cobalt dispersed on ordered porous nitrogen-doped carbon

Article abstract of DOI:10.1039/d0ra09945g

A series of highly dispersed cobalt-based catalysts on N-doped ordered porous carbon (Co-NOPC) were synthesized using the sacrificiallate method. MCM-41, ZSM-5 and SBA-15 were employed as hard templates with 2,2′-bipyridine as the ligand. The physical and

Full text of DOI:10.1039/d0ra09945g

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Oxidative esterification of renewable furfural on
cobalt dispersed on ordered porous nitrogen-
doped carbon†
Cite this: RSC Adv., 2021, 11, 3280
Defeng Yin,a Yanxia Zheng,a Lixi Yang,a Shuyue Li,a Daqing Zhu,a Yafei Guo,b
a
a
Haofei Huanga and Ming Wanga
*
*
Cuncun Zuo, Yuchao Li,
A series of highly dispersed cobalt-based catalysts on N-doped ordered porous carbon (Co-NOPC) were
synthesized using the sacrificial-template method. MCM-41, ZSM-5 and SBA-15 were employed as hard
templates with 2,20-bipyridine as the ligand. The physical and chemical properties of the Co-NOPC
catalyst were characterized by Raman, XRD, SEM, TEM, EDX, ICP, BET, XPS. Co-NOPC had been proven
to be a highly efficient catalyst for oxidative esterification of furfural (FUR) to methyl 2-furoate without
alkaline additives. Catalytic performance was correlated to the dispersed cobalt, porous structure and
specific surface area. The relationship between oxygen activation and the strong interaction of cobalt
and pyridine nitrogen were confirmed by XPS. Catalytic performance enhancement mechanisms were
correlated with the redistribution of electrons at the interface between carbon material and cobalt atoms
through the molecular dynamics method and a reaction mechanism was also proposed. The optimized
catalysts showed outstanding catalytic activity and stability and no obvious decrease in activity was found
after 6 cycles with 99.6% FUR conversion and 96% methyl 2-furoate selectivity.
Received 24th November 2020
Accepted 30th December 2020
DOI: 10.1039/d0ra09945g
rsc.li/rsc-advances
Thenfuroic acid was esteried by concentrated H2SO4 in
methanol.10–12 However, KMnO4 and H2SO4 can corrode the
1. Introduction
reaction equipment and the yield of product is low and cannot
be further improved. Recently, supported noble metals, such as
Pd, Ag, Au, and bimetallic catalysts, were synthesized for the
oxidative esterication of FUR to methyl 2-furoate with alkaline
additives, such as CH3ONa, K2CO3.13–15 We have prepared a kind
of Au-supported cerium oxide doped manganese (Au/MnCeOx)
for oxidative esterication of methyl acrolein to methyl meth-
acrylate with highly yield 90.5%.16 Noble metal catalyst has the
advantages of higher activity, higher stability and lower leach-
ing rate, but it suffers excessive price.
More recently, non-noble metal materials have been widely
investigated due to its abundant reserves and equivalent cata-
lytic activity with noble metal catalysts. Among these non-noble
metal catalysts, highly dispersed ones have attracted the most of
attentions due to their maximum atomic utilization and excel-
lent activity. Jagadeesh17,18 synthesized a cobalt-based nano-
catalyst Co3O4/NGr@C. The active site consists of Co3O4 nano-
particles surrounded by N-doped graphene layer. It exhibited
excellent catalytic performance for the activation of molecular
oxygen. Han19 reported a nano-coating N-doped sub-3 nm Co-
based nano-particle catalyst, which showed high catalytic effi-
ciency for the oxidative esterication of aldehydes. Porous
supports are needed to disperse active metal nano-particles.
Zhou20 employed ZIF-67 as self-template to prepare N-doped
porous carbon, which showed high catalytic activity for oxida-
tive esterication of alcohols to esters. Li21 synthesized an Co-
Renewable biomass resources are promising alternatives for the
sustainable supply of chemical intermediates and liquid fuels.1
The non-precious metal approach for production of chemicals
from biomass resources is attracting immense research interest
globally, which is very important from the perspectives of green
chemistry and sustainable development.2,3 Furfural (FUR) is one
of the most common platform chemicals derived from ligno-
cellulosic biomass, with an annual production volume of more
than 200 000 tons.4 FUR with an aldehyde group and furan ring
is an a,b-unsaturated carbonyl compound which can be applied
for oxidative esterication,5 oxidation-condensation,6 hydroge-
nation reduction,7 decarbonization, and reduction amination
under certain conditions.8
Oxidative esterication of FUR to methyl 2-furoate is an
important reaction in the ne chemical industry. Methyl 2-
furoate was used to produce alkyl furoates which oen were
used as fragrance components and avors.9 Traditionally,
methyl 2-furoate is synthesized by two steps: (i) FUR was
oxidized to through strong oxidants such as KMnO4. (ii)
aSchool of Chemistry and Chemical Engineering, Institute of Clean Chemical Industry,
Shandong University of Technology, Zibo 255049, P. R. China
bSchool of Chemical Engineering and Pharmacy, Henan University of Science and
Technology, Luoyang 471000, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra09945g
3280 | RSC Adv., 2021, 11, 3280–3287
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
based N-doped ordered porous carbon catalyst (ISAS-Co/OPNC) 4.68 g of 2,20-bipyridine were dissolved in 20 ml mixture of water
ꢀ
by hard template method with SBA-15 as sacricial template. and ethanol (v : v ¼ 1 : 1) under robust stirring at 80 C. 2.37 g
The catalyst has high specic surface area, ordered pores and CoCl2$6H2O were dissolved in 50 ml ethanol. Then the two
highly dispersed cobalt nano-particles. It suggests that the solutions were mixed to form a yellow solution and kept stirring
performance of catalyst is largely determined by the supports. for 12 h at 80 ꢀC. The solvent was removed at 80 ꢀC overnight. The
The ideal carrier not only shortens the diffusion pathway but obtained solid was pyrolyzed at 800 ꢀC with a heating rate of
also provides high specic surface area and suitable pores 4 ꢀC minꢁ1 under the protection of N2 in a tube furnace for 2 h.
structure.22,23
Then the black carbon material was etched in 6 M NaOH solution
In order to prepare highly efficient non-noble metal cata- for 24 h to remove template and bulk Co nano-particles. The
lysts, pore structure of the support has been adjusted to material was collected by centrifugation, and washed with warm
increase the accessibility of the Co–Nx active sites in this work. A mixed solution of ethanol and water. Finally, the obtained cata-
ꢀ
series of highly dispersed cobalt-based N-doped on ordered lyst was dried at 70 C under vacuum overnight. Co-NOPC-SBA-
porous carbon catalyst (Co-NOPC) were used for oxidative 15, Co-NOPC-ZSM-5 and Co-NOPC-MCM-41 were abbreviated to
esterication of FUR to methyl 2-furoate. The catalysts were Co-NOPC-S, Co-NOPC-Z and Co-NOPC-M, respectively.
synthesized by hard template method. The complex of cobalt
chloride and 2,20-bipyridine were lled into the pore channel of
2.3 Characterization of the catalyst
sacrice template (using MCM-41, ZSM-5, SBA-15). Due to the
Powder X-ray diffraction (XRD) data of supports and catalysts
constraint effect of the sacricial template and the strong
were collected by a Philips PW-1830 X-ray diffractometer with
coordination effect of the cobalt chloride and 2,20-bipyridine
ꢀ
Cu Ka (l ¼ 1.5406 A). The wide-angle X-ray diffraction data were
ligand, highly dispersed cobalt-based N-doped ordered porous
collected with a scan speed at 7ꢀ minꢁ1 from 5ꢀ to 80ꢀ. The
carbon material was obtained. The catalysts were characterized
small-angleꢁX1-ray diffraction data was analyzed with scan speed
via XRD, Raman, SEM, TEM, BET, ICP–MS, and XPS. Catalyst
at 0.5ꢀ min from 0.5ꢀ to 10ꢀ. The Raman spectra were tested
activity and the inuence of pore structure were investigated
on a Raman spectrometer (Lab RAM HR800) which equipped
and the stability of the catalyst was also tested.
with a CCD detector at 514 nm. The scanning electron micro-
scope (SEM) data was acquired by a Sirion 200 scanning elec-
2. Materials and methods
2.1 Chemicals and materials
2,20-Bipyridine, ethanol, cobalt(II) chloride hexahydrate (CoCl2-
$6H2O), hexadecyltrimethyl ammonium bromide (CTAB), tet-
raethyl orthosilicate (TEOS), sodium hydroxide (NaOH), triblock
copolymer EO20PO70EO20 (P123) and ZSM-5 molecular sieve
tron microscope with the accelerating voltage at 10 kV. The
morphology patterns of Co-NOPC catalyst were collected by
a transmission electron microscopy (TEM) with Tecnai G2F20
transmission electron microscope equipped with a Super-X EDS
detector. The pore size distribution and surface area were per-
formed by N2 physical adsorption and desorption (Micro-
meritics ASAP 2460). The specic surface area and pore size
were purchased from Aladdin. MCM-41 and SBA-15 molecular
distribution were simulated by BET and BJH method, respec-
sieve were self-prepared. All the chemicals are analytical grade
tively. X-ray photoelectron spectroscopy (XPS) were collected by
and were not puried before use.
a Thermo ESCALAB 250Xi spectrometer with Al Ka radiation (Al
Ka ¼ 1846.6 eV) as the excitation source. Inductive coupled
2.2 Preparation of the catalysts
plasma (ICP, PerkinElmer ICP-OES 7300DV) measurement was
applied to analysis the elemental content of Co-NOPC catalysts.
Organic Element Analyzer measurement was applied to analysis
the elemental content of Co-NOPC catalysts on an element
analyzer (ElementarVario EL cube).
The process of preparing Co-NOPC catalyst was reported in
literature with slightly change in Fig. 1.24 Firstly, 5 g the template,
2.4 Procedure for the oxidative esterication
Co-NOPC catalyst (0.5 g), FUR (50 mmol), methanol (500 mmol)
were mixed in a 50 ml autoclave at room temperature. The reaction
was carried out at 80 ꢀC with magnetic stirring and the autoclave
was charged with 0.5 Mpa O2 with the gas ow at 15 ml minꢁ1
.
Aer two hours, the autoclave was rapidly cooled down to room
temperature and the O2 were discharged. Then, the products were
quantitative by GC using n-heptane as internal standard.
3. Results and discussion
3.1 Materials characterization
In order to explore the crystalline and pore structures of cata-
lysts, XRD was performed. The small-angle XRD patterns of
Fig. 1 Preparation process of Co-NOPC catalyst.
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 3280–3287 | 3281

View Article Online
RSC Advances
Paper
MCM-41 (Fig. S1†) exhibited three peak located at 2.2ꢀ, 3.4ꢀ and
4.2ꢀ, which were assigned to (100), (110), and (200) planes,
corresponding to highly ordered MCM-41 type materials with
a hexagonal structure.25 The small-angle XRD pattern of SBA-15
(Fig. S1†) exhibited similar three peaks located at 0.9ꢀ, 1.4ꢀ and
1.7ꢀ, which were assigned to (100), (110) and (200) planes, cor-
responding to highly ordered SBA-15 type materials with
a hexagonal structure.26 The d spacing values were calculated
according to the Bragg's law. The obtained a value from
different planes for each sample are fairly consistent, conrm-
ing the successful formation of the hexagonal structure of SBA-
15 and MCM-41 in Table S1.† The average cell parameters were
calculated to be 4.1 nm and 9.8 nm for MCM-41 and SBA-15,
respectively.
The pattern of Co-NOPC-Z in Fig. 2 showed three peaks at
44.2ꢀ, 51.6ꢀ and 75.8ꢀ which can be corresponded to (111), (200)
and (220) planes of metallic Co (JCPDS 15-0806). However, no
prominent characteristic peak of metallic Co and oxidative Co
were observed in Co-NOPC-S and Co-NOPC-M, indicating the
higher dispersion of Co nano-particles than Co-NOPC-Z. It is
probably due to the connement effect of sacricial-template
and the strong coordination effect of Co with 2,20-bipyridine,
which limited the polymerization of Co nano-particles.27
Raman spectroscopy was applied to investigate the graphi-
tization degrees of carbon (Fig. 3). All the Raman spectra of Co-
NOPC catalysts showed D and G peaks at 1349.6 cmꢁ1 and
1593.1 cmꢁ1, respectively.28 The strength of the D band is
related to the structural defects, while G band is corresponding
to the in-plane vibrations of the sp2-C.29 The calculated ID/IG
peak area ratio of Co-NOPC-S, Co-NOPC-Z and Co-NOPC-M were
1.06, 1.20 and 1.23, respectively, indicating that Co-NOPC-M
has the most of the structural defects and contains amor-
phous carbon.
Fig. 3 Raman spectra of Co-NOPC catalysts.
ordered aky structures. The fragments of Co-NOPC-S and Co-
NOPC-M were more dispersed than Co-NOPC-Z. Size of the
sheet for Co-NOPC-M is smaller than Co-NOPC-S. The TEM of
Co-NOPC-M (Fig. 4d) revealed that it displayed well-dened
ordered mesoporous structure, which remained the ordered
pore structure of MCM-41 aer the etching of hard template.30
The cobalt particle size of Co-NOPC-M (2.8 ꢂ 0.5 nm) was
smaller than that of Co-NOPC-S (3.5 ꢂ 0.7 nm) and Co-NOPC-Z
(5.7 ꢂ 1.4 nm) catalysts (Fig. 5). The cobalt particle size of Co-
NOPC-M and Co-NOPC-S catalysts were small and relatively
concentrated which may due to the differences in the structure
of molecular sieves. Furthermore, the EDX mappings of Co-
NOPC-M suggested O, N and Co were evenly distributed (Fig. 6).
Element contents of catalysts measured by ICP-OES and
Organic Element Analyzer were shown in Table 1. The content
of Si for all the samples was lower than 4%, suggesting the
efficient etching by NaOH. The contents of Co for Co-NOPC-M,
Co-NOPC-S and Co-NOPC-Z were 2.7 wt%, 2.3 wt% and 1.7 wt%,
respectively. The Co-NOPC-M also presented the higher content
of N (9.2 wt%) than Co-NOPC-S (7.9 wt%) and Co-NOPC-Z
(8.3 wt%).
SEM and TEM measurements were performed to explore the
micro morphology and pore structure of catalysts. TEM images
showed that the mesoporous molecular sieve MCM-41 and SBA-
15 were synthesized successfully (Fig. S2†). Co-NOPC-S, Co-
NOPC-Z and Co-NOPC-M showed the similar lamellar struc-
ture (Fig. 4a–c). All these three catalysts showed multilayered
Fig. 4 SEM images of o-NOPC-S (a), Co-NOPC-Z, (b) Co-NOPC-M
Fig. 2 XRD patterns of catalysts.
(c), TEM image of Co-NOPC-M (d).
3282 | RSC Adv., 2021, 11, 3280–3287
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
big packing pores generated during the etching process. The
BET surface area of Co-NOPC-M, Co-NOPC-S and Co-NOPC-Z
were 532 m2 gꢁ1, 395 m2 gꢁ1 and 258 m2 gꢁ1, respectively
(Fig. 5b). The pore size of catalysts exhibited inverse trend
around 1.8 nm, 2.5 nm and 3.4 nm, respectively (Table 1). The
different pore size between Co-NOPC-M and Co-NOPC-S was
mainly due to the difference of wall thickness and pore size of
sacrice template. The wall of MCM-41 and SBA-15 were
removed aer etching by 6 M NaOH solution, and then the pore
structure of catalysts formed (Fig. 1). The porous and ordered
pore structure is crucial for the transport of reactants, products
and high surface area is favorable for accessible property of
active sites.
XPS was applied to explore the chemical state of Co-NOPC.
As expected, all the XPS spectra of prepared catalysts revealed
the presence of C, N, Co, Si and O peaks, indicating the co-
doping of N and Co on ordered carbon (Fig. S4†). The occur-
rence of Si peak possibly arises from sacrice template which
was not totally cleaned. The C 1s peaks in Co-NOPC catalysts
were tted into four peaks around 284.4 eV, 285.7 eV, 286.8 eV
and 288.8 eV corresponding to sp3 C–C, C–N & C]N, C–O and
O]C–O, respectively (Fig. 6a, d and g).31,32 The N 1s peaks of Co-
NOPC catalysts was deconvoluted into four peaks: pyridinic N,
pyrrolic N, graphitic N, and nitric oxide, corresponding to the
binding energies at 398.3 eV, 399.1 eV, 400.8 eV and 404.8 eV,
respectively (Fig. 8b, e and h).33 The relative content of
pyridinic N (Table 1) was higher in Co-NOPC-M (around 1.24%)
than Co-NOPC-S (0.82%) and Co-NOPC-Z (0.43%). The
pyridinic N was assigned to the Co–Nx, which is crucial to
activate the oxygen to active species.34,35 In the Co 2p spectrum
of Co-NOPC (Fig. 8c, f and i), three catalysts presented similar
spectra with the electron binding energy of Co2+ 2p3/2 and Co3+
2p1/2 locating around 779.3 eV and 796.3 eV, respectively, indi-
cating that Co centers had a higher electron density. These Co
centers can be ascribed to the fact that Co particles bonded
with N in the graphitic sheets, which agrees with previous
research.36,37 The peaks located at 785.7, 803.3 eV are assigned
to be the satellite peaks. The metallic Co species (BE ¼ 777.9 eV)
were not detected by XPS for Co-NOPC-M and Co-NOPC-S. Co-
NOPC-Z solely unveiled the existence of Co0 2p3/2 band at
778.10 eV, which is agreed well with the XRD results showing
a pure metallic Co phase for Co-NOPC-Z. It is most likely due to
that the pore size of ZSM-5 is larger and cannot form a strong
constraint on cobalt, which leads to the growth of cobalt nano-
particles. Thus, the similar chemical valence state of Co-NOPC
Fig. 5 TEM images and particle size distribution of Co-NOPC-M (a),
Co-NOPC-S (b), Co-NOPC-Z (c).
Fig. 6 EDX mappings of N, O and Co of the catalyst Co-NOPC-M.
N2 adsorption–desorption isotherms measurement was
performed to investigate the pore structural. MCM-41 showed
the highest BET surface area (860 m2 gꢁ1) than SBA-15 (567 m2
gꢁ1) and ZSM-5 (274 m2 gꢁ1) (Table S2†). The pore wall thick-
ness is dened as the difference between the average unit cell
parameter and the pore size (Table S2†). MCM-41 showed
a thinner pore wall thickness (1.7 nm) than SBA-15 (7.1 nm),
which demonstrates that the Co-NOPC-M catalyst would have
smaller pore size than Co-NOPC-S (Table S2†). All the catalysts
(Fig. 7a) exhibited H4 hysteresis loop, which indicates a lot of
Table 1 Hole structure data and elemental analysis of Co-NOPC catalysts
Element contents
Sample
Surface areaa (m2 gꢁ1
)
Pore sizeb (nm)
Coc (wt%)
Nd (wt%)
Sic (wt%)
Pyridinic Ne (wt%)
Co-NOPC-M
Co-NOPC-S
Co-NOPC-Z
532
395
258
1.8
2.5
3.4
2.7
2.3
1.7
9.2
7.9
8.3
1.3
0.3
3.6
1.24
0.82
0.43
a
b
c
d
Surface area evaluated by using the BET model. Pore size evaluated by using the BJH model. Co and Si content measured by ICP–MS.
N
content measured by Organic Element Analyzer. e Pyridinic N content measured by XPS.
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 3280–3287 | 3283

View Article Online
RSC Advances
Paper
Fig. 7 (a) N2 adsorption isotherm linear plot of Co-NOPC catalysts, (b) pore size distribution for Co-NOPC catalysts.
catalysts proved that Co and N were doped into the ordered bonds between Co–N and the carbon skeleton not only have
carbon material and formed Co–Nx active sites by the electron synergistic effect, but also improve the structural stability of the
transfer of surface Co to electronegative N atoms.38 The covalent Co-NOPC catalyst.
Fig. 8 XPS spectra of Co-NOPC catalysts. High resolution XPS spectra of C 1s (a), N 1s (b) and Co 2p (c) regions for Co-NOPC-M, high resolution
XPS spectra of C 1s (d), N 1s (e) and Co 2p (f) regions for Co-NOPC-S and high resolution XPS spectra of C 1s (g), N 1s (h) and Co 2p (i) regions for
Co-NOPC-Z.
3284 | RSC Adv., 2021, 11, 3280–3287
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
3.2 Catalytic performance
The catalytic performance of Co-NOPC was evaluated for the
oxidative esterication of FUR to methyl 2-furoate under mild
condition at presence of oxygen without additional base (Fig. 9).
No methyl 2-furoate was found without catalysts, which indi-
cates that catalyst plays the key role in the reaction system. The
effect of cobalt in catalytic processes was also investigated.
NOPC-M was applied for oxidative esterication of FUR only
with yield around 0.2%, suggesting that cobalt is the main
active sites. The catalytic performance of Co-NOPC-S, Co-NOPC-
M, Co-NOPC-Z was also investigated and the results were
summarized. Co-NOPC-M showed the highest conversion
(99.6%) and methyl-2-furoate selectivity (96%). 65% of FUR with
the selectivity of methyl-2-furoate 46.3% were obtained over Co-
NOPC-S. Compared to other catalysts, Co-NOPC-Z showed the
lowest conversion of FUR (43%), the selectivity of methyl-2-
furoate is also poor (38%). The Co-NOPC-M catalyst also
showed superior stability. The catalytic performance showed no
signicant decrease aer 6 cycles for oxidative esterication of
FUR (Fig. 10).
Analysis of the Co-NOPC structure was further conducted to
explain why nitrogen rich carbon support activates the Co nano-
particles for the selective oxidation of furfural. Comparing all
three Co-NOPC catalysts prepared, the relative content of
pyridinic N and Co and specic surface area are associated with
catalytic performance. The relative content of pyridinic N in Co-
NOPC catalysts had a direct relationship with the conversion
and selectivity of the oxidative esterication (Fig. S7a and b†).
As the pyridinic nitrogen content increased, the conversion and
selectivity increased correlatively. Meanwhile, the relative
content of cobalt content is related to the conversion of furfural,
but has a weak correlation with selectivity (Fig. S7c and d†). The
pyridinic N and cobalt were the effective sites forming Co–Nx as
reported.38,39 EDX mappings of N, O and Co of the catalyst Co-
NOPC-M showed that Co and N had high dispersion. At the
same time, XPS data showed that Co mainly existed as Co2+ 2p3/
2 and Co3+ 2p1/2, which could prove that the catalyst formed the
CO–Nx active site. The electronegativity of doped nitrogen may
stabilize deposited metal and increase the binding energy of
cobalt–carbon, which would enhance the catalyst stability.40 At
Fig. 10 The recyclability of Co-NOPC-M catalyst for oxidative ester-
ification of FUR.
the same time, the doped nitrogen atoms can increase the
electron density of the active metal and promote oxidative
ability of Co atoms.41 Co-NOPC-M catalyst has higher content of
pyridine nitrogen and Co, which can form more active sites.
During the catalyst preparation process, the hard template is
removed by 6 M NaOH solution, and the framework of the hard
template changes into ordered mesoporous channels. N2
adsorption–desorption isotherms and TEM revealed that the
Co-NOPC-M catalyst had order mesoporous structure with the
highest surface area of 532 m2 gꢁ1. As Fig. S7e and f† revealed,
the specic surface area is also closely related to the conversion
of oxidative esterication. Meanwhile, the active metal particle
size of Co-NOPC-M catalyst is smaller, which is also conducive
to improve the activity of the catalyst. High specic surface area,
ordered mesoporous structure and smaller particle size are
crucial for the transport of reactants and the accessible property
of active sites. In summary, the highest activity of Co-NOPC-M
catalyst is mainly due to the following advantages: high active
site content, high dispersion of active site, high specic surface
area, ordered pore structure, and smaller particle size.
The incorporate of nitrogen into NOPC materials could
control the electronic structure of cobalt which would reduce
valence band and increase conduction band.42–44 Nitrogen-rich
carbon with a relatively higher at band potential (or work
function) than that of metallic Co materials, will accept elec-
trons from Co nano-particles until their Fermi level reaches
equilibrium. The Mott–Schottky effect leads to the redistribu-
tion of electrons at the interface between carbon material and
cobalt atom and more positive charges accumulation in the
cobalt atoms.45 The more oxidative area thus forms on the side
of the Co nano-particles accordingly. In addition, the nitrogen
atom doped in the electron-rich carbon material can be used as
an anchor group to bond with the metal center to form complex
centers with homogeneous coordination compounds. And the
phenomenon can improve the catalytic performance, make the
loaded metal nano-particles more dispersed, avoiding the
aggregation of metal particles.46 In previous research, alkaline
additives, such as CH3ONa, K2CO3, were used to speed up the
deprotonation process, which is important step to form ester. In
Fig. 9 Oxidative esterification of FUR with Co-NOPC catalysts.
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 3280–3287 | 3285

View Article Online
RSC Advances
Paper
Fig. 11 Reaction mechanism of oxidative esterification on Co-N/PCN catalysts.
this reaction, the electron-poor cobalt has sufficient strength to
Acknowledgements
attract and activate O2 to eliminate protons from H-cobalt.
Based on the above discussion, the mechanism for oxidative
esterication of FUR with methanol to methyl 2-furoate was
speculated. As shown in Fig. 11, oxygens were activated to
superoxide radical anion at the Co–Nx active sites,34,45 then
reactive oxygen (cO2ꢁ) attacks methanol and converts methanol
to methoxy in step A. Then the doped nitrogen atoms continued
to absorb substrates. Methoxy reacts with FUR to generate the
intermediate hemiacetal in step B. Finally, intermediate hemi-
acetal remove b-H to generate methyl 2-furoate and H2O.16,19,47
These nding will be helpful in the application of Co–N/PCN
catalysts for oxidative esterication.
This work was supported by the Natural Science Foundation of
China (21706149 & 21908132). We would also thank SDUT &
Zibo City Integration Development Project (2018ZBXC387) and
SDUT & Zhangdian District, Zibo City Integration Development
Project (2018).
References
1 C.-H. Zhou, X. Xia, C.-X. Lin, D.-S. Tong and J. Beltramini,
Chem. Soc. Rev., 2011, 40, 5588–5617.
2 N. Huo, H. Ma, X. Wang, T. Wang, G. Wang, T. Wang, L. Hou,
J. Gao and J. Xu, Chin. J. Catal., 2017, 38, 1148–1154.
3 J. Cai, H. Ma, J. Zhang, Z. Du, Y. Huang, J. Gao and J. Xu,
Chin. J. Catal., 2014, 35, 1653–1660.
4 B. Kamm, P. R. Gruber and M. Kamm, Bioreneries-industrial
processes and products, Wiley-VCH, Weinheim, 2006.
5 H. Zhou, S. Hong, H. Zhang, Y. Chen, H. Xu, X. Wang,
Z. Jiang, S. Chen and Y. Liu, Appl. Catal., B, 2019, 117767.
4. Conclusions
A series of highly dispersed cobalt-based N-doped ordered
porous carbon was prepared via hard template method. The
pore structure of the support was adjusted by using different
sacrice templates (SBA-15, MCM-41, ZSM-5), which revealed
that hard template has a signicant effect on the pore structure
and catalytic performance. The Co-NOPC-M catalyst showed the
best catalytic performance than the catalysts for the oxidative
esterication with high conversion of FUR (99.6%) and selec-
tivity of methyl 2-furoate (96%) under mild condition. The good
catalytic performance and recyclable characteristics were due to
the highly dispersed cobalt, order mesoporous structure and
large specic surface area.
´
6 I. Sadaba, M. Ojeda, R. Mariscal, J. L. G. Fierro and
M. L. Granados, Appl. Catal., B, 2011, 101, 638–648.
7 X. Zhang, M. Han, G. Liu, G. Wang, Y. Zhang, H. Zhang and
H. Zhao, Appl. Catal., B, 2019, 244, 899–908.
8 J. Q. Li, Chem. Eng. J., 2001, 81, 338–339.
´
´
´
´
9 E. Angelica, P. Mıriam, S. angel, G. Monica, P. Analia,
R. Gustavo and B. Guillermo, J. Coat. Technol. Res., 2019,
16, 159–166.
10 M. Manzoli, F. Menegazzo, M. Signoretto and D. Marchese,
Catalysts, 2016, 6, 107.
11 Q.-Y. Zhou, S.-h. Wang and W.-y. Shi, Chem. Eng., 2008, 8,
57–59.
Conflicts of interest
There are no conicts to declare.
3286 | RSC Adv., 2021, 11, 3280–3287
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
12 R. Radhakrishnan, K. Kannan, S. Kumaravel and 29 J. Li, G. Liu, X. Long, G. Gao, J. Wu and F. Li, J. Catal., 2017,
S. Thiripuranthagan, RSC Adv., 2016, 6, 45907–45922. 355, 53–62.
13 F. Menegazzo, M. Signoretto, F. Pinna, M. Manzoli, V. Aina, 30 S. Molaei, T. Tamoradi, M. Ghadermazi and A. Ghorbani-
G. Cerrato and F. Boccuzzi, J. Catal., 2014, 309, 241–247.
Choghamarani, Microporous Mesoporous Mater., 2018,
14 M. Signoretto, F. Menegazzo, L. Contessotto, F. Pinna,
S1387181118303585.
M. Manzoli and F. Boccuzzi, Appl. Catal., B, 2013, 129, 31 Z. K. Yang, C.-Z. Yuan and A.-W. Xu, ACS Energy Lett., 2018, 3,
287–293. 2383–2389.
15 M. J. Taylor, L. J. Durndell, M. A. Isaacs, C. M. Parlett, 32 X. Ma, Y.-X. Zhou, H. Liu, Y. Li and H.-L. Jiang, Chem.
K. Wilson, A. F. Lee and G. Kyriakou, Appl. Catal., B, 2016,
180, 580–585.
16 Y. Tian, Y. Li, Y. Zheng, M. Wang, C. Zuo, H. Huang, D. Yin,
Commun., 2016, 52, 7719–7722.
33 R. V. Jagadeesh, K. Natte, H. Junge and M. Beller, ACS Catal.,
2015, 5, 1526–1529.
Z. Fu, J. Tan and Z. Zhou, Ind. Eng. Chem. Res., 2019, 58, 34 R. V. Jagadeesh, H. Junge, M.-M. Pohl, J. R. Radnik,
¨
19397–19405.
A. Bruckner and M. Beller, J. Am. Chem. Soc., 2013, 135,
17 R. V. Jagadeesh, T. Stemmler, A. E. Surkus, M. Bauer and
M. Beller, Nat. Protoc., 2015, 10, 916–926.
18 F. A. Westerhaus, R. V. Jagadeesh, G. Wienhfer, M.-M. Pohl,
10776–10782.
35 Y. Pan, G. Wu, Y. He, J. Feng and D. Li, J. Catal., 2019, 369,
222–232.
J. R. Radnik, A.-E. Surkus, J. Rabeah, K. Junge, H. Junge and 36 L. Zhang, A. Wang, W. Wang, Y. Huang, X. Liu, S. Miao, J. Liu
M. Nielsen, Nat. Chem., 2013, 5, 537–543. and T. Zhang, ACS Catal., 2015, 5, 6563–6572.
19 J. Han, F. Gu and Y. Li, Chem.–Asian J., 2016, 11, 2594–2601. 37 T. Zhang, W. Liu, L. Zhang, W. Yan, X. Liu, X. Yang, S. Miao,
20 Y.-H. Lu and H.-T. Chen, J. Phys. Chem. C, 2014, 118, 10043–
10052.
21 J. Ke, J.-W. Xiao, W. Zhu, H. Liu, R. Si, Y.-W. Zhang and
C.-H. Yan, J. Am. Chem. Soc., 2013, 135, 15191–15200.
W. Wang and A. Q. Wang, Chem. Sci., 2016, 10, 1039.
38 Y. Gao, G. Hu, J. Zhong, Z. Shi, Y. Zhu, D. S. Su, J. Wang,
X. Bao and D. Ma, Angew. Chem., Int. Ed., 2013, 52, 2109–
2113.
22 H. Yunhu, Y.-G. Wang, W. Chen, R. Xu, L. R. Zheng, J. Zhang, 39 M. Groves, A. Chan, C. Malardier-Jugroot and M. Jugroot,
J. Luo, R.-A. Shen, Y. Zhu and W.-C. Cheong, J. Am. Chem.
Soc., 2017, 139, 17269–17272.
23 A. Han, W. Chen, S. Zhang, M. Zhang, Y. Han, J. Zhang, S. Ji,
L. Zheng, Y. Wang and L. Gu, Adv. Mater., 2018, 1706508.
24 Y. Han, Z. Wang, R. Xu, W. Zhang, W. Chen, L. Zheng,
Chem. Phys. Lett., 2009, 481, 214–219.
40 X. Xu, Y. Li, Y. Gong, P. Zhang, H. Li and Y. Wang, J. Am.
Chem. Soc., 2012, 134, 16987–16990.
41 P. Zhang, Y. Gong, H. Li, Z. Chen and Y. Wang, Nat.
Commun., 2013, 4, 1593.
J. Zhang, J. Luo, K. Wu, Y. Zhu, C. Chen, Q. Peng, Q. Liu, 42 C. Wang, L. Qiao, C. Qu, W. Zheng and Q. Jiang, J. Phys.
P. Hu, D. Wang and Y. Li, Angew. Chem., Int. Ed., 2018, 57,
11262–11266.
25 A.-M. Brezoiu, M. Deaconu, I. Nicu, E. Vasile, R.-A. Mitran,
Chem. C, 2011, 113, 812–818.
43 H. J. Xiang, B. Huang, Z. Y. Li, S.-H. Wei, J. L. Yang and
X. G. Gong, Phys. Rev. X, 2011, 2(1), 278–279.
C. Matei and D. Berger, Microporous Mesoporous Mater., 44 X. Wang, X. Li, L. Zhang, Y. Yoon, P. K. Weber, H. Wang,
2019, 275, 214–222. J. Guo and H. Dai, Science, 2009, 324, 768–771.
26 J. Du, H. Xu, J. Shen, J. Huang, W. Shen and D. Zhao, Appl. 45 H. Su, K.-X. Zhang, B. Zhang, H.-H. Wang, Q.-Y. Yu, X.-H. Li,
Catal., A, 2005, 296, 186–193.
27 L. Zhang, J. Wang, N. Shang, S. Gao, Y. Gao and C. Wang,
Appl. Surf. Sci., 2019, 491, 544–552.
28 M. Li, F. Xu, H. Li and Y. Wang, Catal. Sci. Technol., 2016, 6,
3670–3693.
M. Antonietti and J.-S. Chen, J. Am. Chem. Soc., 2017, 139,
811–818.
46 X. H. Li and M. Antonietti, Chem. Soc. Rev., 2014, 44, 6593–
6604.
47 L. Zhang, A. Wang, W. Wang, Y. Huang, X. Liu, S. Miao, J. Liu
and T. Zhang, ACS Catal., 2015, 5, 6563–6572.
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 3280–3287 | 3287