Robust selenium-doped carbon nitride nanotubes for selective electrocatalytic oxidation of furan compounds to maleic acid

Article abstract of DOI:10.1039/d1sc01231b

Selective oxidation of biomass-derived furan compounds to maleic acid (MA), an important bulk chemical, is a very attractive strategy for biomass transformation. However, achieving a high MA selectivity remains a great challenge. Herein, we for the first time successfully designed and fabricated Se-doped graphitic carbon nitride nanotubes with a chemical formula of C3.0N-Se0.03. The prepared C3.0N-Se0.03 was highly efficient for electrocatalytic oxidation of various biomass-derived furan compounds to generate MA. At ambient conditions, the MA yield could reach 84.2% from the electro-oxidation of furfural. Notably, the substituents on the furan ring significantly affected the selectivity to MA, following the order: carboxyl group > aldehyde group > hydroxyl group. Detailed investigation revealed that Se doping could tune the chemical structure of the materials (e.g., C3.0N-Se0.03 and g-C3N4), thus resulting in the change in catalytic mechanism. The excellent performance of C3.0N-Se0.03 originated from the suitable amount of graphitic N and its better electrochemical properties, which significantly boosted the oxidation pathway to MA. This work provides a robust and selective metal-free electrocatalyst for the sustainable synthesis of MA from oxidation of biomass-derived furan compounds. This journal is

Full text of DOI:10.1039/d1sc01231b

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Robust selenium-doped carbon nitride nanotubes
for selective electrocatalytic oxidation of furan
compounds to maleic acid†
Cite this: Chem. Sci., 2021, 12, 6342
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
ac
a
Xin Huang,ab Jinliang Song,
Manli Hua,ab Bingfeng Chen,
Zhenbing Xie,ab
*
abc
ac
abc
Huizhen Liu,
Zhanrong Zhang,a Qinglei Meng
and Buxing Han
*
Selective oxidation of biomass-derived furan compounds to maleic acid (MA), an important bulk chemical, is
a very attractive strategy for biomass transformation. However, achieving a high MA selectivity remains
a great challenge. Herein, we for the first time successfully designed and fabricated Se-doped graphitic
carbon nitride nanotubes with a chemical formula of C3.0N-Se0.03. The prepared C3.0N-Se0.03 was highly
efficient for electrocatalytic oxidation of various biomass-derived furan compounds to generate MA. At
ambient conditions, the MA yield could reach 84.2% from the electro-oxidation of furfural. Notably, the
substituents on the furan ring significantly affected the selectivity to MA, following the order: carboxyl
group > aldehyde group > hydroxyl group. Detailed investigation revealed that Se doping could tune the
chemical structure of the materials (e.g., C3.0N-Se0.03 and g-C3N4), thus resulting in the change in
catalytic mechanism. The excellent performance of C3.0N-Se0.03 originated from the suitable amount of
graphitic N and its better electrochemical properties, which significantly boosted the oxidation pathway
to MA. This work provides a robust and selective metal-free electrocatalyst for the sustainable synthesis
of MA from oxidation of biomass-derived furan compounds.
Received 2nd March 2021
Accepted 31st March 2021
DOI: 10.1039/d1sc01231b
rsc.li/chemical-science
At the early stage, MA could be produced with the yield of
Introduction
about 70% via the vapor-phase oxidation of furfural.7 However,
ꢀ
the elevated temperature (above 200 C) was the major limita-
tion of this process. Lately, researchers developed the liquid-
phase oxidation of furfural and 5-hydroxymethylfurfural for
the synthesis of MA in various catalytic systems.8 In spite of the
achieved progress, the MA yield in most of the developed liquid-
phase systems could hardly exceed 60% because of the unstable
nature of furfural and 5-hydroxymethylfurfural, and compli-
cated and harsh conditions were generally needed, including
high temperature and O2 pressure, the employment of chemical
oxidants, or the use of organic solvents (especially organic
carboxylic acid).
Maleic acid (MA) is a versatile bulk chemical, and can be used as
the critical raw material or intermediate to manufacture
unsaturated polyester resins, plasticizers, surface coatings, and
pharmaceuticals.1 Currently, the majority of MA is produced by
the catalytic oxidation of petroleum-based chemicals (i.e., n-
butane,2 butenes,3 benzene and its derivatives4) with the annual
output of millions of tons. To alleviate excessive reliance on the
diminishing fossil resources, the production of useful
commercial chemicals from renewable and abundantly avail-
able lignocellulose has triggered signicant interest in the past
decade.5 In this regard, partial oxidation of lignocellulose-
derived furan compounds (e.g., furfural and 5-hydrox-
ymethylfurfural) has been considered as a promising and
sustainable strategy for the production of MA.6
It is known that electrocatalysis is a promising route to reach
mild and selective organic reactions.9 Occurring at ambient
conditions and excluding the use of chemical oxidants, elec-
trocatalytic synthesis of MA from lignocellulose-derived furan
compounds is expected to become a competitive strategy for
future MA production. It has been reported that MA could be
produced from electrocatalytic oxidation, but toxic PbO2 was
employed as the electrode and the MA yield was only 65.1%.10
Thus, robust and low-cost electrode materials are still highly
desirable for the synthesis of MA via electrocatalytic oxidation of
lignocellulose-derived furan compounds into MA.
aBeijing National Laboratory for Molecular Science, CAS Key Laboratory of Colloid and
Interface and Thermodynamics, CAS Research/Education Center for Excellence in
Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing
100190, China. E-mail: songjl@iccas.ac.cn; hanbx@icas.ac.cn
bSchool of Chemistry and Chemical Engineering, University of Chinese Academy of
Sciences, Beijing 100049, China
cPhysical Science Laboratory, Huairou National Comprehensive Science Center, Beijing
With the advantages of low cost, high electrical conductivity,
and easy turnability in catalytic activity, metal-free carbon-
based materials have been employed as advanced electrode
101400, China
† Electronic Supplementary Information (ESI) available. See DOI:
10.1039/d1sc01231b
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materials in different electrochemical processes.11 However, the
utilization of metal-free carbon-based electrocatalysts for
biomass conversion is still largely limited owing to the high
overpotential, low catalytic activity and product selectivity.
Generally, the electrocatalytic performance of carbon-based
materials can be nely boosted through the construction of
advanced nanostructures or the doping of heteroatoms. It is of
signicant attraction to develop advanced metal-free carbon-
based materials for electrocatalytic conversion of biomass.
To realize efficient synthesis of MA from furan compounds
via electrocatalytic oxidation, herein, we for the rst time
fabricated the selenium (Se)-doped graphitic carbon nitride
nanotubes with the C/N and Se/N atomic ratios of 3.0 and 0.03,
respectively, named as C3.0N-Se0.03. The synthesized C3.0N-Se0.03
as a metal-free catalyst showed superior electrocatalytic activity
on the oxidation of renewable furan compounds into MA. At
ambient conditions, furfural could be selectively converted into
MA with a yield of 84.2% in 0.5 M KHCO3 aqueous solution,
which was much higher than those in the reported typical
heterogeneous systems (Table S1†). Notably, when the furan
compounds became furoic acid and 2,5-furandicarboxylic acid,
which could be easily achieved from electrocatalytic oxidation
of renewable furfural and 5-hydroxymethylfurfural,12 very high
yields (>90%) of MA could be obtained, which has never been
realized previously. The high selectivity at ambient conditions
and the wide applicability conrmed the great potential of the
Fig. 1 Characterizations of the prepared C3.0N-Se0.03. (a) SEM image,
(b) TEM image, (c) HR-TEM image, (d) elemental mappings, (e) XRD
pattern, and (f) N2 adsorption–desorption isotherm.
C
3.0N-Se0.03 on the electrocatalytic oxidation of furan (XPS). Raman spectrum of C3.0N-Se0.03 showed intense D band
compounds to synthesize MA.
at 1340 cmꢁ1 and the G band at 1590 cmꢁ1 (Fig. S1†), indicating
the well-dened graphene-like structure of 3.0N-Se0.03
C
.
Furthermore, the surface chemical composition of the C3.0N-
Se0.03 was determined by XPS technique. XPS survey spectrum of
the C3.0N-Se0.03 (Fig. S2†) could further conrm the presence of
C, N, O, and Se, and the absence of Cl, proving the complete
Results and discussion
Preparation and characterization of the C3.0N-Se0.03
nanotubes
The target material (C3.0N-Se0.03 nanotubes) was prepared by volatilization of NH4Cl. High-resolution XPS spectra of C 1s
pyrolyzing the precursors of urea, NH4Cl, and SeO2, and the indicated that there were three carbon species with the corre-
detail process was described in ESI.† Characterized by SEM sponding binding energies at 284.7, 286.4 and 288.7 eV (Fig. 2a),
(Fig. 1a), the C3.0N-Se0.03 had a rod-like nanostructure with the which attributed to sp2 C–C in the framework, C–NH2 at the
length of 200 to 1000 nm. The nanotube structure could be edges and sp2 C–N–C groups, respectively.15 The signal of N 1s
further veried by TEM (Fig. 1b), and the wall thickness of the could be divided into three peaks at 397.7, 399.8, and 401.5 eV
C
3.0N-Se0.03 nanotubes was about 20 nm with the interior (Fig. 2b), which corresponded to the pyridinic N, graphitic N,
diameter of about 150 nm (Fig. 1c). EDX mapping of C3.0N-Se0.03 and oxidized N, respectively.16 Furthermore, for Se 3d, two peaks
showed that the elements of C, N and Se were distributed at 55.4 and 56.3 eV, which were assigned to Se]C–N and Se–C
uniformly on the wall of the nanotube (Fig. 1d). In the XRD bonds, could be observed (Fig. 2c), implying that Se atoms were
17
pattern of C3.0N-Se0.03 (Fig. 1e), a single broad diffraction peak incorporated into the framework of the C3.0N-Se0.03
.
Addi-
at 24.8ꢀ was found, implying that the C and N in C3.0N-Se0.03 tionally, weak O 1s signal was assigned to surface adsorbed
were arranged in an ordering turbostratic form,13 which was adventitious oxygen-containing species (Fig. 2d). The Fourier-
essential for redox reactions.14 Meanwhile, no peaks for SeO2 or transformed (FT) k3-weighted extended X-ray absorption ne
NH4Cl were found in the pattern, verifying the complete vola- structure (EXAFS) showed that there was only Se–C coordination
˚
tilization of these species during the preparation process of peak (at 1.44 A), and no Se–Se coordination peak (at about 2.08
18
˚
C
3.0N-Se0.03
.
As determined by N2 adsorption-desorption A) was found (Fig. 2e), further verifying that Se element was
method, the C3.0N-Se0.03 possessed a well-dened mesoporous bonded by C and atomically dispersed throughout the C3.0N-
structure (Fig. 1f), and the BET surface area of C3.0N-Se0.03 was Se0.03 framework. Se K-edge X-ray absorption near-edge struc-
67.1 m2 gꢁ1. Additionally, the Se content determined by ICP-MS ture (XANES) showed that the absorption edge of C3.0N-Se0.03
was 2.39 wt% in the C3.0N-Se0.03, while the C/N atomic ratio was located slightly higher energy position than that of Se foil
3.0 based on the element analysis.
(Fig. 2f), indicating the valence state of Se atom in framework is
The ne structure of C3.0N-Se0.03 was further characterized higher than 0.19
by Raman spectrum and X-ray photoelectron spectroscopy
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Fig. 2 Chemical structure characterization of the C3.0N-Se0.03. (a)
High-resolution XPS spectra of C 1s, (b) high-resolution XPS spectra
of N 1s, (c) XPS spectra of Se 3d, (d) XPS spectra of O 1s, (e) Se K-edge
Fourier transformed EXAFS spectra in the R space, and (f) Se K-edge
XANES.
Fig. 3 (a) LSV curves of C3.0N-Se0.03 at a scan rate of 50 mV sꢁ1 in
0.5 M aqueous KHCO3 solution, (b) Tafel plots of C3.0N-Se0.03 with and
without furfural, (c) the furfural conversion, MA yield and FE of MA at
different applied potentials over C3.0N-Se0.03 electrode, (d) HPLC
chromatogram at different electrolysis times (A, B, C and D represent
for MA, 2(5H)-furanone, FA and furfural, respectively), (e) concentra-
tions of furfural and the oxidation products at various electrolysis
times, and (f) reusability of C3.0N-Se0.03 electrode. Reaction condi-
tions: applied potential, 1.7 V vs. Ag/AgCl; furfural concentration,
10 mM; 0.5 M aqueous KHCO3 electrolyte, 15 mL; reaction time, 6 h.
In comparison, several other CxN-Sey materials were
synthesized at mass ratios of SeO2 to urea of 0.1, 0.25, 0.5, and
1.5, respectively. Based on the C/N and Se/N ratios, these
materials were named as C0.7N-Se0.002, C0.9N-Se0.003, C2.1N-
Se0.01, and C4.2N-Se0.05. Meanwhile, g-C3N4 (synthesized from
urea) and g-C3N4-AC (synthesized from urea and NH4Cl) were
also prepared. The characterizations (TEM, SEM, XRD, Raman
spectra, and XPS) of these materials were described in the ESI
(Fig. S3–S11†). From the characterizations, the SeO2 amount
could signicantly affect the morphology and chemical struc-
ture of the obtained materials. With the increase of SeO2
amount, the morphology changed from nanosheets (g-C3N4, g-
C3N4-AC, C0.7N-Se0.002, and C0.9N-Se0.003) to nanotubes (C3.0N-
Se0.03 and C4.2N-Se0.05), while the chemical structure changed
from the tri-s-triazine structure into graphitic carbon nitride.
These results implied that SeO2 played the template role for the
formation of graphitic carbon nitride nanotubes.
furfural oxidation on the C3.0N-Se0.03 electrode. Furthermore,
the Tafel slope signicantly decreased from 376.3 mV decꢁ1 in
the absence of furfural to 108.8 mV decꢁ1 in the presence of
furfural (Fig. 3b), indicating a much faster catalytic kinetics
aer adding furfural.
As is well-known, the applied potentials generally have
signicant impact on the results. As shown in Fig. 3c, the yield
and faradaic efficiency of MA increased with the increase of the
applied potentials at rst (1.4–1.7 V vs. Ag/AgCl) and then
decreased at the applied potentials higher than 1.7 V vs. Ag/
AgCl. Meanwhile, although the conversions of furfural were
high at the applied potentials below 1.7 V vs. Ag/AgCl, the yields
of MA were low with low yields of 2(5H)-furanone and furoic
acid (Fig. S12a†), which was caused by the undesired side-
reactions of furfural. These results indicated that the optimal
potential was 1.7 V vs. Ag/AgCl over C3.0N-Se0.03 in 0.5 M KHCO3
electrolyte. Moreover, as monitored by high-performance liquid
chromatography (HPLC), the added furfural could be
completely consumed in 6 h at the applied potential of 1.7 V vs.
Ag/AgCl (Fig. 3d and e), while the yield of MA continually
increased and reached the maximum value of 84.2% in 6 h with
Electrocatalytic performances for furfural oxidation
The electrocatalytic performance of the C3.0N-Se0.03 material on
furfural oxidation was conducted in a three-electrode electrol-
ysis cell employing aqueous KHCO3 solution (0.5 M) as the
electrolyte (both anolyte and catholyte). As shown in Fig. 3a, the
water oxidation occurred at an onset potential of about 1.3 V vs.
Ag/AgCl without furfural. Aer adding furfural (10 mM), the
onset potential shied to 1.2 V vs. Ag/AgCl accompanied by
a rapid increase of current density, implying the occurrence of
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a faradaic efficiency of 45.9%. Besides, the current density the reaction time was 12 h over g-C3N4 when the furfural was
slightly decreased during the reaction process because the completely converted at an optimized applied potential of 1.8 V
added furfural was consumed (Fig. S12b†). More importantly, vs. Ag/AgCl, and the yield of MA was only 55.2% (Fig. S16†).
the C3.0N-Se0.03 electrode could be reused for ve cycles with no
Subsequently, the electrocatalytic performance of various
obvious decrease in the MA yield and the faradaic efficiency CxN-Sey materials was studied in 0.5 M KHCO3 electrolyte.
(Fig. 4f). No notable difference for XPS spectra was observed for Similarly, furfural oxidation could occur over these materials
the fresh and recovered C3.0N-Se0.03 (Fig. S13†), suggesting the because the current density was increased and the onset
high stability of C3.0N-Se0.03 electrode under the reaction potential was lowered when furfural was added (Fig. 4a and
conditions.
S17†). Among these CxN-Sey materials, C3.0N-Se0.03 yielded the
In another aspect, the property of the used electrolytes could highest current density and the lowest onset potential with
affect the electrocatalytic performance of the C3.0N-Se0.03. As furfural (Fig. 4a), indicating that it was more active for the
showed in the LSV curves (Fig. S14†), higher current density and furfural oxidation. When the reaction was conducted at 1.7 V vs.
lower onset potential could be achieved in the experiment with Ag/AgCl (Fig. 4c), the MA yield increased with the increase of the
furfural (10 mM) than those without furfural in all examined C/N atomic ratio and the Se amount, and C3.0N-Se0.03 provided
electrolytes (i.e., 0.5 M (NH4)2SO4, 1 M KOH, 0.5 M H2SO4, and the highest MA yield (84.2%). However, the MA yield decreased
phosphate buffered solution), suggesting that furfural could be when the C/N atomic ratio was increased to 4.2. These results
oxidized in all these electrolytes. From Table S2,† we could nd indicated that the electrocatalytic performance of the CxN-Sey
that the conversions of furfural were high in all the examined materials for furfural oxidation was signicantly affected by the
electrolytes. However, the yield of the desired MA varied with amount of Se and the C/N atomic ratio.
the electrolytes. It was obvious that weak basic electrolytes
In order to compare the kinetics of furfural oxidation among
(0.5 M KHCO3) were benecial for the generation of MA, while various CxN-Sey materials, Tafel and the electrochemical active
the acidic (0.5 M H2SO4 and 0.5 M (NH4)2SO4), neutral (phos- surface area (ECSA) analysis was performed. First, from the LSV
phate buffered solution) and strong basic (1 M KOH) electro- curves, the Tafel slope of C3.0N-Se0.03 (108.8 mV decꢁ1) was
lytes provided very low yields of MA owing to undesired side- much lower than that of g-C3N4 (263.8 mV decꢁ1, Fig. S15b†),
reactions of furfural in these electrolytes. Based on the results and was the lowest among various CxN-Sey materials (Fig. 4b),
in Table S2,† 0.5 M KHCO3 was employed as the optimal elec- suggesting its high intrinsic electrocatalytic activity for furfural
trolyte over C3.0N-Se0.03
.
oxidation into MA. Second, C3.0N-Se0.03 showed much higher
electrochemical surface area (ECSA) than g-C3N4 (Fig. S18a†),
and had the highest ECSA among the CxN-Sey materials
(Fig. 4d), indicating C3.0N-Se0.03 had more catalytically active
sites for furfural oxidation, which favored the reaction. Third,
the charge transfer resistance (Rct) of C3.0N-Se0.03 was lower than
that of g-C3N4 (Fig. S18b†) and other CxN-Sey materials
(Fig. S19†), suggesting a more facile electron transfer process
over C3.0N-Se0.03 to promote the reaction. On the basis of the
above discussions, C3.0N-Se0.03 had the highest electrochemical
surface area and lowest charge transfer resistance among the
examined metal-free carbon-based materials (i.e., g-C3N4 and
CxN-Sey). These advantages could signicantly improve the
oxidation efficiency of furfural to generate MA, and thus
decrease the occurrence of the side-reactions. Thereby, a much
higher selectivity of MA could be achieved over C3.0N-Se0.03 in
comparison with other materials.
Activity of g-C3N4 and various CxN-Sey materials
The activity of C3.0N-Se0.03 and g-C3N4 was initially compared.
Although the current density also increased aer the furfural
was added over g-C3N4 (Fig. S15a†), the increased degree was
lower than that over C3.0N-Se0.03, implying that C3.0N-Se0.03 was
more active for the electrocatalytic oxidation of furfural. Indeed,
Mechanism investigation on the performance of C3.0N-Se0.03
XPS spectra were analyzed deeply to identify the mechanism on
MA selectivity over the prepared CxN-Sey materials. It has been
well accepted that the graphitic N species play signicant role
on oxidations when using N-doped carbon materials as the
catalysts.20 Based on the high-resolution XPS spectra of N 1s
(Fig. S11†), the content of the graphitic N species in the CxN-Sey
materials (Table S3†) decreased with the increase of Se content.
However, although the content of graphitic N in C3.0N-Se0.03 was
lower than those in C0.7N-Se0.002 and C0.9N-Se0.003, the MA
selectivity over C3.0N-Se0.03 was higher than those over C0.7N-
Se0.002 and C0.9N-Se0.003. This is because that more active sites
Fig. 4 (a) LSV curves of a serious of CxN-Sey electrodes at a scan rate
of 50 mV sꢁ1 with 10 mM furfural in 0.5 M KHCO3 solution, (b) Tafel
plots of CxN-Sey electrodes with furfural, (c) the furfural conversion,
MA yield and FE of MA over CxN-Sey electrodes at 1.7 V vs. Ag/AgCl,
and (d) charging current density differences plotted against scan rates
for CxN-Sey electrodes.
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Table 1 Electrochemical oxidation of other reactants over the C3.0N-
Se0.03
could not only promote the oxidation of furfural to generate MA
but also enhance the excessive oxidation of furfural. Meanwhile,
a
C
4.2N-Se0.05 had the lowest content of active graphitic N, which
Entry
Reactant
C (%)
Y (%)
S (%)
may result in some intermediates being unoxidized over C4.2N-
Se0.05, and thus, the selectivity of MA over C4.2N-Se0.05 was lower
than that over C3.0N-Se0.03. On the basis of the above discussion,
1
2
99.5
93.6
84.2
21.2
84.0
22.6
C
3.0N-Se0.03 with suitable content of graphitic N provided the
highest MA selectivity. In another aspect, the binding energy of
graphitic N in the C3.0N-Se0.03 was higher than those in other
four CxN-Sey materials, indicating that the graphitic N in the
3
4
100
95.2
49.3
95.2
53.1
92.8
C
3.0N-Se0.03 was more positively charged. The more positively
charged graphitic N would result in the ortho-carbon of the
graphitic N being more positive, which was helpful for the
formation of active oxygen species on this carbon.21 This result
further explained the higher catalytic activity of C3.0N-Se0.03
among the synthesized ve CxN-Sey.
5
6
7
96.3
100
90.7
42.1
67.8
94.2
42.1
68.0
99.7
Besides, all the CxN-Sey materials showed better catalytic
performance than g-C3N4 because the different chemical
structures of CxN-Sey and g-C3N4 (owing to the doping of Se)
resulted in the different mechanism. Generally, g-C3N4 acted as
the electron donor to activate O2 to form O2ꢂꢁ.22 However, the rate
of this process was slow, and the generated O2ꢂꢁ was not
a selective species. In contrast, over CxN-Sey, the oxygen radical
was directly formed on the materials, which was much faster
than that of g-C3N4, and the obtained oxygen radical was more
selective due to the satirical effect of the solid materials. Thus,
both the activity and selectivity over g-C3N4 were higher than
those over g-C3N4.
Additionally, Se doping was indispensable for the excellent
performance of C3.0N-Se0.03 nanotubes. Although Se was not the
direct catalytic site, the Se doping played the role of adjusting
the amount and charge delocalization of the active site
(graphitic N), which could contribute to the activity and selec-
tivity for the electro-oxidation of furfural into MA. Thereby, all
CxN-Sey materials showed better performance than g-C3N4, and
8
99.3
56.9
57.3
a
Reaction conditions: applied potential, 1.7 V vs. Ag/AgCl; reactant
concentration, 10 mM; 0.5 M aqueous KHCO3 electrolyte, 15 mL;
reaction time, 6 h.
the synthesis of MA from the electrocatalytic oxidation of
biomass-derived furan compounds. From the view point of
practical application, more works should be conducted, such as
the inuence of impurity on the reaction.
Conclusions
In conclusion, metal-free graphitic carbon nitride nanotubes,
C
3.0N-Se0.03, have been fabricated by pyrolysis of urea, NH4Cl,
and SeO2. As the metal-free catalyst, C3.0N-Se0.03 showed excel-
lent performance for the electrocatalytic oxidation of various
biomass-derived furan compounds into MA. The yield of MA
could reach 84.2% from furfural oxidation at an applied
potential of 1.7 V vs. Ag/AgCl. Moreover, it was observed that the
substituent groups on the furan ring signicantly affected the
reaction selectivity. The substrates with COOH group generally
provided higher yields of MA, and the yield of MA from FA and
2,5-furandicarboxylic acid could reach 95.2% and 90.7%,
respectively. More importantly, the performance of C3.0N-Se0.03
was much better than that of g-C3N4 and other CxN-Sey mate-
rials, which was caused by the higher electrochemical surface
area and lower charge transfer resistance. This work provides an
efficient and selective route to synthesize MA from biomass-
derived furan compounds using metal-free electrocatalyst.
C
3.0N-Se0.03 with suitable amount and higher charge delocal-
ization was the best among all the examined materials.
Scope of the substrates
Inspired by the excellent performance of C3.0N-Se0.03 with
furfural as the substrate, we explored the possibility of MA
synthesis from electrochemical oxidation of other biomass-
derived furan compounds over C3.0N-Se0.03 (Table 1). It was
found that C3.0N-Se0.03 could catalyze the electro-oxidation of
various furan compounds to MA. More importantly, the
substituent groups signicantly affected the yield of MA (Table
1, entries 1–3), which increased following the order: furfuryl
alcohol (21.1%) < furfuryl (84.2) < furoic acid (95.2%). This
might be resulted from that furfuryl alcohol needed most steps
to obtain MA, which increased the possibility of side-reactions.
Experimental
Materials
A
similar tendency was observed when using 5-hydrox-
ymethylfurfural and its derivatives as the substrates (Table 1,
entries 4–8). The yields of MA could reach 95.2% and 90.7% Furfural (99%), furan (98%) and 5-formyl-2-furancarboxylic acid
when furoic acid and 2,5-furandicarboxylic acid were employed (FFCA, >98%) were purchased from Aladdin. 5-Hydrox-
as the reactants (Table 1, entries 3 and 5), respectively. These ymethylfurfural (HMF, 98%) was purchased from Sigma Aldrich
results indicated the general applicability of the C3.0N-Se0.03 on Co. Ltd. 2,5-Furandicarboxylic acid (FDCA, 97%) and furfuryl
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alcohol (98%) were purchased from Alfa Aesar China Co., Ltd. 284.8 eV from adventitious carbon was used for energy refer-
2,5-Diformylfuran (DFF, 98%) and 5-hydroxymethyl-2- encing. The morphologies of materials were characterized by
furancarboxylic acid (HMFCA, 98%) were purchased from J&K a HITACHI S-4800 scanning electron microscope (SEM) and
Scientic Ltd. SeO2 (99%), urea (99%), 2-furoic acid (98%) and
a JEOL JEM-2100F high-resolution transmission electron
2(5H)-furanone (98%) were purchased from Innochem Scien- microscopy (HR-TEM). The samples were dispersed in ethanol
tic Ltd. NH4Cl ($99.5%) was purchased from Beijing Chemical using an ultrasonic bath, and the nal suspensions were
Works. Toray carbon paper (CP, TGP-H-60, 19 ꢃ 19 cm), Naon transferred to TEM grids and dried in ambient air before elec-
D-521 dispersion (5% w/w in water and 1-propanol, $ 0.92 meg tron microscopy analysis. N2 adsorption/desorption isotherms
gꢁ1 exchange capacity) and Naon N-117 membrane (0.180 mm of the materials were measured on a Quadrasorb SI-MP system
thick, $0.90 meg gꢁ1 exchange capacity) were purchased from at 77 K. Before analysis, samples were allowed to outgas at
Alfa Aesar China Co., Ltd. All reagents purchased from 180 ꢀC under turbomolecular vacuum pumping for a minimum
commercial sources were used as obtained without further of 5 h. Raman spectroscopy was carried out using LabRAM HR
purication.
Evolution. Fourier transform infrared (FT-IR) spectra were ob-
tained using a Bruker Tensor 27 spectrometer, and the sample
was prepared by KBr pellet method. X-ray absorption ne
structure (XAFS) measurement for Se K-edge was carried out at
room temperature at the 1W1B beamline of the Beijing
Synchrotron Radiation Facility (BSRF). For XAFS analysis, the
extraction of the c(k) function was performed using the Athena
program. Prior to merging, the spectrum was aligned to the rst
and largest peak in the smoothed rst derivative of the
absorption spectrum, background removed, and normalized.
The averaged k3-weight c(k) function was Fourier transformed
with a Rbkg value of 1.0. Normalized m(E) data were obtained
directly from the Athena program of the IFEFFIT package.
Preparation of CxN-Sey
The route to synthesize different CxN-Sey was very similar except
for the different usage of SeO2. Herein, we described the route
for the preparation of C3.0N-Se0.03 as an example. Typically,
NH4Cl, SeO2 and urea with a mass ratio of 10 : 1 : 1 was added
into a polytetrauoroethylene milling jar along with agate balls.
Aerwards, the jar was placed in a vibrating ball miller (300
r minꢁ1) and the mixture was ball-milled for 6 h. The obtained
white product was dried and then carbonized in a tube furnace
ꢀ
ꢀ
at 350 C for 2 h and 550 C for 2 h under Ar atmosphere (at
a heating rate of 5 C minꢁ1). The obtained black powder was
ꢀ
collected aer the temperature was cooled down to room
temperature. The C/N atomic ratio was detected to be 3.0 by
element analysis, and the Se content was 2.39 wt% determined
by ICP-AES.
Other CxN-Sey materials with different mass ratios of SeO2
and urea (0.1, 0.25, 0.5 and 1.5) were synthesized, and the cor-
responding C/N atomic ratio was detected to be 0.7, 0.9, 2.1, and
4.2 by element analysis, respectively. Meanwhile, determined by
ICP-AES, the Se contents in these CxN-Sey materials were
0.68 wt%, 0.82 wt%, 1.26 wt% and 3.46 wt%, respectively. Thus,
the obtained materials were named as C0.7N-Se0.002, C0.9N-
Se0.003, C2.1N-Se0.01, and C4.2N-Se0.05, according to the C/N and
Se/N atomic ratios of the nal products.
Preparation of electrode
To prepare the electrode, 2 mg of the as-prepared catalysts and
10 mL of Naon D-521 dispersion were dispersed in 1 mL
acetone and ultrasonicated to form uniform suspension, and
suitable amount of the suspension was loaded on the 1 cm ꢃ
1 cm CP. Aer being dried in N2 atmosphere, the loading of
catalyst could be calculated as 2 mg cmꢁ2 on the basis of the
CP's weight change.
Linear sweep voltammetry (LSV) measurement
An electrochemical workstation (CHI 660E, Shanghai CH
Instruments Co., China) was employed. Linear sweep voltam-
metry (LSV) measurements were conducted in
a single
Preparation of g-C3N4-AC
compartment cell with three electrodes, including a working
electrode, a platinum gauze auxiliary electrode, and an Ag/AgCl
reference electrode. The LSV measurements in electrolyte
were carried out in a certain potential range at a sweep rate of
50 mV sꢁ1 under slight magnetic stirring. In addition, aqueous
KHCO3, H2SO4, KOH, (NH4)2SO4 solutions and phosphate
buffer solution (PBS, pH ¼ 7) were chose as the electrolyte (both
anolyte and catholyte) to study the optimal reaction conditions
for electrochemical oxidation of furfural to maleic acid.
The route for the synthesis of g-C3N4-AC was the same as that
for the preparation of CxN-Sey except for the absence of SeO2.
Preparation of bulk g-C3N4
The route for the synthesis of bulk g-C3N4 was the same as that
for the preparation of g-C3N4-AC except that only urea was used.
Characterization
Electrochemical oxidation of furfural
Powder X-ray diffraction (XRD) patterns were collected on the X-
ray diffractometer (Model D/MAX2500, Rigaku) with Cu-Ka The electrochemical oxidation of furfural was carried out at
radiation. X-ray photoelectron spectroscopy (XPS) analysis was room temperature using an electrochemical workstation (CHI
performed on the Thermo Scientic ESCA Lab 250Xi using 660E, Shanghai CH Instruments Co., China) with a typical H-
200 W monochromatic Al Ka radiation, and the 500 mm X-ray type cell, which was separated by a Naon membrane. The
spot was used. The base pressure in the analysis chamber was electrochemical cell is congured with a three-electrode system:
about 3 ꢃ 10ꢁ10 mbar. Typically, the hydrocarbon C 1s line at a working electrode as anode, a platinum gauze auxiliary
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electrode as the counter electrode (cathode), and an Ag/AgCl
reference electrode. The best electrochemical oxidation experi-
ment was conducted in 15 mL of a 0.5 M KHCO3 solution in the
presence or absence of 10 mM furfural.
B. Lagrain, D. Verboekend and B. F. Sels, Science, 2020,
367, 1385–1390; (c) Y. Jing, Y. Guo, Q. Xia, X. Liu and
Y. Wang, Chem, 2019, 5, 2520–2546; (d) X. Wu, N. Luo,
S. Xie, H. Zhang, Q. Zhang, F. Wang and Y. Wang, Chem.
Soc. Rev., 2020, 49, 6198–6223; (e) C. Mondelli,
¨ ´ ´
G. Gozaydın, N. Yan and J. Perez-Ramırez, Chem. Soc. Rev.,
HPLC analysis of oxidation products
´
2020, 49, 3764–3782; (f) R. Gerardy, D. P. Debecker,
The conversion of furfural and the yield of the corresponding
products were characterized by HPLC technique. Specically,
100 mL samples were collected from the electrolyte aer the
addition of furfural and at different reaction time and then
diluted with 900 mL water. Aer ltrated by a 0.22 nm lter, the
samples were used for the HPLC analysis. The HPLC analysis
was conducted on a Shimadzu Prominence Liquid Chromato-
graph (LC-20TA) equipped with a C18 column (Ascentis Express,
15 cm ꢃ 2.1 mm, 2.7 mm) and an ultraviolet detector. Mobile
phase A is 0.1% formic acid aqueous solution adjusted to pH ¼
ꢄ3 with ammonium formate, and mobile phase B was aceto-
nitrile. The ow rate is 0.3 mL minꢁ1 (gradient program: 100% A
for 2 min, to 20% A over 32 min, and held for 18 min). The
quantication of furfural and its oxidation products were
calculated based on the calibration curves of those standard
compounds purchased from commercial vendors.
J. Estager, P. Luis and J.-C. M. Monbaliu, Chem. Rev., 2020,
120, 7219–7347; (g) S. Zhou, J. Lai, X. Liu, G. Huang,
G. You, Q. Xu and D. Yin, Green Energy Environ., 2020, DOI:
10.1016/j.gee.2020.09.009.
6 (a) R. Wojcieszak, F. Santarelli, S. Paul, F. Dumeignil,
F. Cavani and R. V. Gonçalves, Sustainable Chem. Processes,
2015, 3, 9; (b) C. Xu, E. Paone, D. Rodrıguez-Padron,
R. Luque and F. Mauriello, Chem. Soc. Rev., 2020, 49, 4273–
4306; (c) X. Li and Y. Zhang, Green Chem., 2016, 18, 643–647.
7 N. Alonso-Fagundez, M. L. Granados, R. Mariscal and
M. Ojeda, ChemSusChem, 2012, 5, 1984–1990.
8 (a) Y. Lou, S. Marinkovic, B. Estrine, W. Qiang and
G. Enderlin, ACS Omega, 2020, 5, 2561–2568; (b) X. Li,
B. Ho, D. S. W. Lim and Y. Zhang, Green Chem., 2017, 19,
´
´
´
´
914–918; (c) A. C. Alba-Rubio, J. L. G. Fierro, L. Leon-Reina,
R. Mariscal, J. A. Dumesic and M. L. Granados, Appl.
´
Catal., B, 2017, 202, 269–280; (d) N. Alonso-Fagundez,
Author contributions
I. Agirrezabal-Telleria, P. L. Arias, J. L. G. Fierro,
R. Mariscal and M. L. Granados, RSC Adv., 2014, 4, 54960–
54972.
Xin Huang, Jinliang Song and Buxing Han proposed the project,
designed and conducted the experiments and wrote the
manuscript. Other authors performed some experiments and
discussed the work.
9 (a) F. Wang and S. S. Stahl, Acc. Chem. Res., 2020, 53, 561–
574; (b) L. Fan, C. Xia, F. Yang, J. Wang, H. Wang and
Y. Lu, Sci. Adv., 2020, 6, eaay3111; (c) L. Zeng, H. Li, J. Hu,
D. Zhang, J. Hu, P. Peng, S. Wang, R. Shi, J. Peng,
C.-W. Pao, J.-L. Chen, J.-F. Lee, H. Zhang, Y.-H. Chen and
A. Lei, Nat. Catal., 2020, 3, 438–445; (d) T. Tang, L. Ding,
Z. Jiang, J.-S. Hu and L.-J. Wan, Sci. China: Chem., 2020, 63,
1517–1542; (e) X. Yang, C. Chen, Z. Zhou and S. Sun, Acta
Phys.-Chim. Sin., 2019, 35, 472–485.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
10 S. R. Kubota and K.-S. Choi, ACS Sustainable Chem. Eng.,
2018, 6, 9596–9600.
The authors thank the National Key Research and Development
Program of China (2018YFB1501602), National Natural Science
Foundation of China (21673249, 21733011), Beijing Municipal
Science & Technology Commission (Z191100007219009), the
Chinese Academy of Sciences (QYZDY-SSW-SLH013), and Youth
Innovation Promotion Association of CAS (2017043).
11 (a) D. Li, C. Li, L. Zhang, H. Li, L. Zhu, D. Yang, Q. Fang,
S. Qiu and X. Yao, J. Am. Chem. Soc., 2020, 142, 8104–8108;
(b) Q. Lai, J. Zheng, Z. Tang, D. Bi, J. Zhao and Y. Liang,
Angew. Chem., Int. Ed., 2020, 59, 11999–12006; (c)
M. A. Bajada, S. Roy, J. Warnan, K. Abdiaziz, A. Wagner,
M. M. Roessler and E. Reisner, Angew. Chem., Int. Ed.,
2020, 59, 15633–15641.
Notes and references
1 P. Sudarsanam, E. Peeters, E. V. Makshina, V. I. Parvulescu 12 (a) B. You, X. Liu, N. Jiang and Y. Sun, J. Am. Chem. Soc.,
and B. F. Sels, Chem. Soc. Rev., 2019, 48, 2366–2421.
2 G. Centi, F. Triro, J. R. Ebner and V. M. Franchetti, Chem.
Rev., 1988, 88, 55–80.
3 W. A. Skinner and D. Tieszen, Ind. Eng. Chem., 1961, 53, 557–
558.
4 B. Cornils, Angew. Chem., 2004, 116, 2378–2379.
5 (a) J. Yan, Q. Meng, X. Shen, B. Chen, Y. Sun, J. Xiang, H. Liu
and B. Han, Sci. Adv., 2020, 6, eabd1951; (b) Y. Liao,
2016, 138, 13639–13646; (b) Y. Kwon, K. J. P. Schouten,
J. C. van der Waal, E. de Jong and M. T. M. Koper, ACS
Catal., 2016, 6, 6704–6717; (c) A. R. Poerwoprajitno,
L. Gloag, J. Watt, S. Cychy, S. Cheong, P. V. Kumar,
T. M. Benedetti, C. Deng, K.-H. Wu, C. E. Marjo,
D. L. Huber, M. Muhler, J. J. Gooding, W. Schuhmann,
D.-W. Wang and R. D. Tilley, Angew. Chem., Int. Ed., 2020,
59, 15487–15491.
S.-F. Koelewijn, G. Van Den Bossche, J. Van Aelst, S. Van 13 S. Yang, W. Li, C. Ye, G. Wang, H. Tian, C. Zhu, P. He,
Den Bosch, T. Renders, K. Navare, T. Nicola, K. Van Aelst,
M. Maesen, H. Matsushima, J. Thevelein, K. Van Acker,
G. Ding, X. Xie, Y. Liu, Y. Lifshitz, S.-T. Lee, Z. Kang and
M. Jiang, Adv. Mater., 2017, 29, 1605625.
6348 | Chem. Sci., 2021, 12, 6342–6349
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Edge Article
Chemical Science
14 G. Zhang, M. Zhang, X. Ye, X. Qiu, S. Lin and X. Wang, Adv. 18 K. Jiang, B. Liu, M. Luo, S. Ning, M. Peng, Y. Zhao, Y. R. Lu,
Mater., 2014, 26, 805–809.
15 (a) L. Chen, R. Yan, M. Oschatz, L. Jiang, M. Antonietti and
T. S. Chan, F. M. F. de Groot and Y. Tan, Nat. Commun., 2019,
10, 1743.
K. Xiao, Angew. Chem., Int. Ed., 2020, 59, 9067–9073; (b) 19 T. Wang, Q. Wang, Y. Wang, Y. Da, W. Zhou, Y. Shao, D. Li,
S. N. Talapaneni, G. Singh, I. Y. Kim, K. AlBahily,
A. a. H. Al-Muhtaseb, A. S. Karakoti, E. Tavakkoli and
A. Vinu, Adv. Mater., 2020, 32, 1904635.
S. Zhan, J. Yuan and H. Wang, Angew. Chem., Int. Ed., 2019,
58, 13466–13471.
20 (a) H. Fu, K. Huang, G. Yang, Y. Cao, H. Wang, F. Peng,
Q. Wang and H. Yu, ACS Catal., 2020, 10, 129–137; (b)
S. Li, Q. Gu, N. Cao, Q. Jiang, C. Xu, C. Jiang, C. Chen,
C. Pham-Huu and Y. Liu, J. Mater. Chem. A, 2020, 8, 8892–
8902.
16 (a) Y. Wang, P. Du, H. Pan, L. Fu, Y. Zhang, J. Chen, Y. Du,
N. Tang and G. Liu, Adv. Mater., 2019, 31, 1807540; (b)
P. Xia, M. Antonietti, B. Zhu, T. Heil, J. Yu and S. Cao, Adv.
Funct. Mater., 2019, 29, 1900093; (c) D. Zhao, C.-L. Dong,
B. Wang, C. Chen, Y.-C. Huang, Z. Diao, S. Li, L. Guo and 21 O. S. G. P. Soares, R. P. Rocha, A. G. Gonçalves,
´
˜
S. Shen, Adv. Mater., 2019, 31, 1903545.
17 (a) F. Li, T. Li, C. Sun, J. Xia and H. Xu, Angew. Chem., Int. Ed.,
J. L. Figueiredo, J. J. M. Orfao and M. F. R. Pereira, Appl.
Catal., B, 2016, 192, 296–303.
2017, 129, 9910–9914; (b) X. Meng, C. Yu, X. Song, J. Iocozzia, 22 (a) Y. Xiao, G. Tian, W. Li, Y. Xie, B. Jiang, C. Tian, D. Zhao
J. Hong, M. Rager, H. Jin, S. Wang, L. Huang, J. Qiu and
and H. Fu, J. Am. Chem. Soc., 2019, 141, 2508–2515; (b)
¨
Z. Lin, Angew. Chem., Int. Ed., 2018, 57, 4682–4686.
F. Su, S. C. Mathew, L. Mohlmann, M. Antonietti, X. Wang
and S. Blechert, Angew. Chem., Int. Ed., 2011, 50, 657–660.
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