Oxidation of biomass-derived furans to maleic acid over nitrogen-doped carbon catalysts under acid-free conditions

Article abstract of DOI:10.1039/c9cy02364j

Maleic acid (MA) is a platform chemical used for synthesizing high-value polymers. Although MA can be produced from biomass-derived furfural, so far, only acidic catalysts have been proposed. Herein, for the first time, we report an acid-free furfural-to-MA conversion using nitrogen-doped carbon (NC) as an effective catalyst with the assistance of hydrogen peroxide (H₂O₂) as an oxidant. A high MA yield (61%) can be achieved over the NC-900 catalyst at 80 °C. The effects of reaction conditions, including H₂O₂ concentration, solvent type, reaction temperature, and time were systematically studied in detail and optimized. The ZIF-8-derived NC catalyst exhibited high activity owing to the existence of high graphitic nitrogen (N-Q) content in the carbon framework. The kinetics study indicates that the oxidation of furfural to MA over the NC-900 catalyst is via the formation of 5-hydroxy-furan-2(5H)-one as the main intermediate. Further, NC-900 can convert a variety of biomass-derived furan compounds to MA under acid-free conditions, indicating the versatility and applicability of the present acid-free, NC-based catalytic system.

Full text of DOI:10.1039/c9cy02364j

View Article Online
View Journal
Catalysis
Science &
Technology
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: C. V. Nguyen, J. R.
Boo , C. Liu, T. Ahamad, S. M. Alshehri, B. M. Matsagar and K. C. W. Wu, Catal. Sci. Technol., 2020, DOI:
10.1039/C9CY02364J.
Volume
Number
21 January 2017
Pages 313-534
7
2
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Catalysis
Science &
Technology
rsc.li/catalysis
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 2044-4761
PAPER
Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
nanotubes produced by atomic layer deposition for hydrogen
generation from hydrolysis of ammonia borane
rsc.li/catalysis

Please do not adjust margins
Catalysis Science & Technology
Page 1 of 10
View Article Online
DOI: 10.1039/C9CY02364J
ARTICLE
Oxidation of biomass-derived furans to maleic acid over nitrogen-
doped carbon catalysts under the acid-free condition
Chi Van Nguyen,ab Jing Rou Boo,a Chia-Hung Liu,c,d Tansir Ahamad,e Saad M Alshehri,e Babasaheb
Received 00th January 20xx,
Accepted 00th January 20xx
M. Matsagar *a and Kevin C.-W. Wu *a
DOI: 10.1039/x0xx00000x
Maleic acid (MA) is a platform chemical used for synthesizing high-value polymers. Although MA can be produced from
biomass-derived furfural, so far, only acidic catalysts have been proposed. Herein, for the first time, we report an acid-free
furfural-to-MA conversion using nitrogen-doped carbon (NC) as an effective catalyst with the assistance of hydrogen
peroxide (H2O2) oxidant. High MA yield (61 %) can be achieved over the NC-900 catalyst at 80 ºC. The effects of reaction
conditions, including H2O2 concentration, solvent type, reaction temperature, and time were systematically studied in detail
and optimized. The ZIF-8-derived NC catalyst exhibited high activity owing to the existence of high graphitic nitrogen (N-Q)
content in the carbon framework. The kinetics study indicates that the oxidation of furfural to MA over the NC-900 catalyst
is via the formation of the 5-hydroxy-furan-2(5H)-one as the main intermediate. Further, the NC-900 can convert a variety
of biomass-derived furan compounds to MA under acid-free conditions, indicating the versatility and applicability of the
present acid-free, NC-based catalytic system.
derived furans into C4 di-acids or anhydride has become an
important research topic.9, 20
Introduction
MA has the highest annual production (1.8 million tons per
To reduce the dependence on fossil resources, the use of
sustainable and renewable resources for producing high-value
chemicals and liquid hydrocarbons have been constantly
increasing in the past decade.1-4 Biomass-derived furan
compounds, generally produced via hydrolysis followed by
dehydration reactions of cellulose and hemicellulose, are
considered as one of the most important renewable
chemicals.5-12 Many high-value products or starting chemicals in
industrial manufacture can be synthesized from the furan
compounds such as tetrahydrofurfuryl alcohol, 2-methyl furan,
2,5-dimethyl furan, etc. by different catalytic reactions.9, 13-15
Among them, the C4 di-acids or anhydride (succinic acid (SA),
maleic acid (MA), and maleic anhydride (MAN)) are important
year), compared to fumaric acid (0.09 million tons per year) and
succinic acid (0.03 million tons per year),21 and the synthesis of
MA generally based on the products of crude oil refinery (i.e.
benzene, butane), which is an exhaustible and non-renewable
feedstock. Therefore, an alternative pathway for producing MA
from a renewable resource is highly demanded.
Recently many reports showed the MA production using
furfural oxidation in either gas or liquid phase media.21-25 Gou
et al. reported the use of phosphomolybdic acid (homogeneous
catalyst) in a biphasic solvent system at 110 ºC (2 MPa O2
pressure) for the aerobic oxidation of furfural to MA.22
Nevertheless, a harsh condition such as high O2 pressure (2 MPa)
and a relatively low yield of MA (34.5%) are the drawbacks of
the proposed system.22 Recently, the catalytic oxidation of
furfural to MA can be carried out under mild conditions using
liquid oxidant (H2O2), which also significantly improved the MA
yield. However, these methods require either betaine
hydrochloride (BHC) or formic acid (4 mL for 1 mmol of furfural)
as a catalyst in large concentration.21, 24 Araji et al. and co-
workers investigated the possibility of using betaine
hydrochloride (BHC) as the catalyst for furfural oxidation with
the assistance of H2O2 as an oxidant to produce MA (61%) and
fumaric acid (FA 31%) within 30 min at 100 ºC.21 Further, the
use of a solid acid catalyst such as titanium silicate is also
reported for the oxidation of furfural to MA.23 In this case, the
highest yield of 78% MA was obtained at 50 ºC within 24 h.23
Although these homogeneous and heterogeneous acidic
catalysts showing good catalytic activity for the furfural
oxidation, these acid catalysts have their own drawbacks such
as metal leaching during the reaction and product purification
starting chemicals in polymer, pharmaceutical, and chemical
16-19
industries.9,
These chemicals were used to produce
polymers, unsaturated polyester resins, vinyl copolymers,
lubricant additives, plastics, and solvents, including γ-
butyrolactone (GBL), 1,4-butanediol (1,4-BDO) and
tetrahydrofuran (THF).18 Thus, the conversion of biomass-
a. Department of Chemical Engineering, National Taiwan University. No. 1, Sec. 4,
Roosevelt Road, Taipei 10607, Taiwan.
E-mail: Kevinwu@ntu.edu.tw, matsagar03@ntu.edu.tw
b. Institute of Research and Development, Duy Tan University, Danang 550000,
Vietnam
c. Department of Urology, School of Medicine, College of Medicine, Taipei Medical
University, Taipei, Taiwan
d. Department of Urology, Shuang Ho Hospital, Taipei Medical University, New
Taipei City, Taiwan
e. Department of Chemistry, College of Science, King Saud University, P.O. Box 2455,
Riyadh 11451, Saudi Arabia
†
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 2 of 10
ARTICLE
Journal Name
issues. Therefore, an acid-free environmentally friendly desired temperature under magnetic stirring in a closed system.
View Article Online
DOI: 10.1039/C9CY02364J
catalytic system for furfural-to-MA conversion has been After completion of the reaction, the reaction solution was
scientifically challenging and industrially important.
cooled to room temperature and diluted into 25 mL with
Recently, MOF-derived carbon materials have become an deionized water. The solution was then filtered using a syringe
excellent material for many applications due to their filter (0.22 μm pore size) for HPLC analysis.
extraordinary properties.26-29 Among them, ZIF-8-derived N- Catalyst preparation
doped porous carbon shows significant catalytic applications
since it exhibits extremely high surface area, uniform
Synthesis of ZIF-8-derived N-doped carbon
Nitrogen-doped porous carbons were synthesized analogous to
morphology, higher N content as active sites, etc.30-33 Besides,
the previous literature.14 As-synthesized ZIF-8 powders were
NC is an environmentally benign catalyst and shows superior
calcined under N2 atmosphere at given temperatures (600, 700,
catalytic activity in the oxidation reaction.14 Thus, the NC is
800, and 900 ºC) for 8 h with a heating rate of 5 ºC·min-1. After
considered as a promising catalyst in many catalytic applications.
carbonization, the samples were washed thoroughly with
Therefore, to overcome the drawbacks of present acid-
concentrated HCl (36.5%) to remove the residual zinc
based furfural oxidation to MA and considering significant
components. Finally, the resulting samples were purified by
advantages of NC materials we aim for the oxidation of furfural
washing with deionized water and methanol for several times
to MA using NC catalyst and H2O2 as a liquid oxidant under mild
and activated at 150 ºC for 5 h before each catalytic reaction.
reaction acid-free condition. The synthesis and characterization
The samples were named to be NC-x, where x represents the
of the ZIF-8-derived NC catalyst were described and discussed
calcination temperature of 600, 700, 800, and 900 ºC,
in our previous report (please see Supporting Information, Fig.
respectively.
Synthesis of graphitic carbon nitride
S1-S4, and Table S1).14 In this work, we mainly study the
catalytic performance of NC for the oxidation of furfural to MA.
Graphitic carbon nitride (g-C3N4) was synthesized according to
The effect of various reaction parameters such as calcination
the reported literature.34 The melamine powder (10 g) was
temperature, H2O2 concentrations, solvent, temperatures, and
placed in a crucible and heated under air at 520 ºC to 4 h with a
the reaction time was systematically studied in detail for the
heating rate of 10 ºC min-1. After 4 h, the furnace was cooled to
furfural oxidation to MA. Furthermore, apart from furfural
room temperature. The yellow-colored powder was collected
other furan compounds such as furan, 5-hydroxymethylfurfural
and used for the catalytic oxidation of furfural.
Synthesis of N-doped carbon from biomass
(HMF), furoic acid, and 2,5-furandicarboxylic acid (FDCA) were
also tested for the reaction to extend the application of present
The nitrogen-doped carbon material from biomass was
study for various biomass-derived furan compounds. We also
synthesized according to the reported study with slight
investigated the reaction pathway for the furfural oxidation to
modification.35 First, the urea (12 g) was dissolved in water (10
MA over NC catalyst and further compared the activity of ZIF-8-
mL), then chitosan (1 g) was added slowly to the urea solution
derived NC with graphitic carbon nitride and NC-derived from
under vigorous stirring. Once the complete dispersion of
biomass to investigate the efficiency of ZIF-8-derived NC. The
chitosan is achieved, the acetic acid (500 μL) was added under
role of nitrogen content and type of nitrogen (pyridinic (N-6),
vigorous stirring for 30 min to attain a semi-transparent
pyrrolic (N-5) and graphitic nitrogen (N-Q)) were also
homogeneous paste. Further, the sample was dried at 70 ºC for
investigated using XPS analysis for the oxidation of furfural to
16 h resulted in the formation of white solid. This solid material
MA to find the actual active sites in the NC catalyst.
was then calcined at 900 ºC under N2 atmosphere to 5 h, with a
heating rate of 3 ºC min-1.
Catalyst characterization and Products analysis
Experimental
Powder X-ray diffraction (XRD) analysis was carried out with a
Rigaku-Ultima IV instrument using Cu Kα (λ=1.5406 Å) radiation
(40 kV voltage). Diffraction patterns were recorded within a 2θ
range of 10-80º with a 20° min-1 scanning rate. The chemical
state of nitrogen and carbon (N 1s and C 1s) was analyzed using
X-ray photoelectron spectroscopy (XPS), Thermo Scientific,
Theta Probe. To compensate for the surface charge effect,
binding energies were calibrated using the C 1s hydrocarbon
peak at 284.60 eV. The elemental composition of samples
(carbon and nitrogen) was analyzed using the Elemental Vario
EL cube instrument. The N2-adsorption-desorption analysis was
performed using the BELSORP series (II) instrument. Detail
characterization of ZIF-8 and ZIF-8-derived NC was discussed in
our previous study 14.
Materials and methods
Zinc nitrate hexahydrate (98%), 2-methylimidazole (99%), 5-
hydroxymethylfurfural (HMF; 99%), 2,5-furandicarboxylic acid
(FDCA; 99%) Maleic acid (MA; 99%) succinic acid (SA; 99%), 2-
furoic acid (99%), furan (99%), melamine (99), and chitosan
were purchased from Sigma-Aldrich, while furfural (99%),
hydrogen peroxide (H2O2; 35%), and fumaric acid were
procured from Acros Organics. The polyvinylpyrrolidone K-30
(PVP) received from TCI, and all chemicals were used without
any further purification.
The liquid-phase oxidation of furfural to MA was carried out
in a round bottom flask connected with a reflux condenser.
Typically, an 83 L furfural (1 mmol) and 50 mg catalyst were
added into a 25 mL three-neck flask containing 5 mL of water.
The mixture was stirred for 2 min and then 5 mL H2O2 (35 wt%)
was added to it. Further, the reaction mixture was heated to the
The high-pressure liquid chromatography (HPLC) equipped
with a refractive index detector and ICE-Coregel 87H3 column
was used for the analysis of reaction mixture. External standard
2 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 3 of 10
Catalysis Science & Technology
Please do not adjust margins
Journal Name
ARTICLE
calibration curves were utilized to calculate the concentration do not show any relation to the MA yield, and the MA yield
View Article Online
of furfural and all other products. The HPLC operated using the decreases with increase in the concentratDioOnI:o1f0.N10-369s/pCe9CciYe0s2(3F6i4gJ.
aqueous solution of 8 mM sulfuric acid as a mobile phase with 1c and 1d), suggesting that the N-Q species is important active
a flow rate of 0.6 mL min-1 and the column oven temperature site for the selective oxidation of furfural to MA. Besides, the
was kept at 35 ºC during analysis.
surface area of NC-x catalysts could contribute to the catalytic
The conversion and product yield are calculated by the oxidation of furfural to MA. Fig. S5 (ESI) shows the relationship
following formulae.
between the surface area of the NC-x catalyst and the MA yield.
The results indicate that the surface area and MA yield were not
linear with the MA yield. Although NC-700 catalyst and NC-900
catalyst has almost similar BET surface area (NC-700 = 795 m2
g-1; NC-900 = 813 m2 g-1), the NC-700 showed lower MA yield
(53%) compared to NC-900 (61%). Further, the ZIF-8 has a very
high BET specific surface area (1428 m2 g-1) compared to the NC-
900 catalyst. However, it showed lower MA yield (25%) than NC-
900 catalyst (61%), suggesting that the surface area of the NC-x
catalyst does not play an important role in catalytic oxidation of
furfural to MA
Moles of furfural reacted
Initial moles of furfural
Furfural conversion (%) =
X 100
Moles of product formed
Product yield (%) =
X 100
Initial moles of furfural
Results and discussion
Initially, the catalyst evaluation study was carried out with
various catalysts for the oxidation of furfural to MA under the
assistance of liquid oxygen (H2O2) as oxidant. As shown in Table
1, a high furfural conversion (from 98 to 100%) achieved at all
tested catalysts. However, the desired product yield (MA) is
significantly lower (9-34%) without catalyst and with acid
catalysts (Table 1, entry 1-7). Besides, succinic acid (SA) is
produced along with MA, showing that the catalytic oxidation
of furfural using acid catalyst has less selectivity for the MA.36, 37
In contrast, a high MA yield is achieved over the NC-x catalysts,
which was in the range of 40 to 61% within 5 h reaction time
(Table 1, entry 8-11). Moreover, with NC catalysts, the yield of
SA is extremely low (<9%), confirming that NC-x catalysts are
highly active in converting furfural to MA. For comparison, we
used activated carbon (AC) and graphite as a catalyst for furfural
oxidation to MA under similar reaction conditions. The results
showed only 16% and 17% MA yield in the presence of AC and
graphite catalysts without nitrogen content, respectively (Table
1, entry 12-13). Furthermore, the ZIF-8 catalyst showed a 25%
MA yield along with an 18% SA yield (Table 1, entry 14). These
results suggest that nitrogen content in porous carbon played
an important role in converting furfural to MA with higher
selectivity. The activity of furfural oxidation using ZIF-8 and HCl
showed 25 and 34% MA yield, while the NC-900 catalyst showed
61% MA yield, suggesting that NC-900 catalyst can show an
almost two-fold increase in MA yield under acid-free and metal-
free condition.
Table 1: Aerobic oxidation furfural to maleic acid under various catalysts
Conversion Maleic acid Succinic acid
Entry
Catalysts
(%)
100
100
100
98
Yield (%)
Yield (%)
1
2
3
4
5
6
-
15
13
9
15
Zn(NO3)2
ZnO
18
26
H2SO4
HCl
17
34
17
10
98.5
100
22
CH3COOH
17
Amberlyst-
15
7
98
16
13
8
NC-900
NC-800
NC-700
NC-600
AC
100
100
100
100
98.6
99
61
58
53
40
16
17
25
3
2
9
10
11
12
13
14
3
9
11
14
18
Graphite
ZIF-8
100
As shown in Table 1 (entry 8-11), the effect of nitrogen
content in the NC-x catalysts and calcination temperature was
investigated for the oxidation of furfural to MA. The gradual
increase of MA yield was observed by increasing the calcination
temperature from 600 to 900 ºC for the ZIF-8 derived NC
catalyst (NC-600, 40%; NC-700, 53%; NC-800, 58%; and NC-900,
61%) (Table 1, entry 8-11). However, the increase of MA yield
was not consistent with the total amount of nitrogen content in
the catalysts (Fig 1b). Further, we investigate the relationship
between the concentration of N-Q, N-5, and N-6 species in the
NC catalysts with a yield of MA (Fig. 1a, 1c, and 1d) to
understand the effect of type of N species on the furfural
oxidation. It is found that the concentration of N-Q species is in
linear proportion to the MA yield (Fig. 1a), whereas, N-5 species
Reaction condition: Furfural (1 mmol), H2O (5 mL), catalysts (50 mg),
H2O2 (5 mL; 35 wt%), time 5 h, temperature 80 oC.
Gao et al. demonstrated that the graphitic N atom acts as a
promotor to stimulate the chemical reactivity of its adjacent
carbon atoms and favors the formation of oxygen radicles after
reacting with H2O2, which is active sites for the furfural
oxidation.38 To confirm this reactive oxygen species in our
reaction system, we examined the chemical state of C 1s in the
NC-900 catalyst using XPS analysis. Fig. 1e and 1f represent the
bonding types of NC-900 catalysts with and without H2O2
treatment (NC-900-H2O2 and fresh NC-900). Based on high-
resolution XPS characterization, the formation of new chemical
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 4 of 10
ARTICLE
Journal Name
bonding (C-O-OH) in case of C 1s chemical status of NC-900-
View Article Online
H2O2 was observed.39-41 It is reported that the peroxide-like
DOI: 10.1039/C9CY02364J
species (-O-OH) exhibited higher catalytic activity for the
oxidation reaction.24, 38 The formation of the reactive oxygen
Fig. 1 Relationship of MA yield with a) N-Q (wt%), b) total amount of N, c) N-5 (wt%) and d) N-6 (wt%). The chemical bonding of C 1s of e) fresh NC-900 catalyst, and f)
species due to the presence of graphitic nitrogen in NC catalyst
is assisting in converting furfural into MA. This result confirms
NC-900 treated with H2O2 oxidant (denoted as NC-900-H2O2), respectively.
that the presence of N-Q species in NC-x catalysts is important
Effect of reaction conditions on catalytic oxidation of furfural
with increasing the furfural concentration from 1 mmol to 5
catalytic active sites for furfural oxidation into MA.
mmol. The result showed almost similar MA yield (56%) without
increasing the H2O2 concentration, suggesting that the use of
higher substrate concentration can help to decrease the oxidant
(H2O2) to substrate ratio to some extent.
The liquid oxidant H2O2 could be decomposed in the presence
of NC catalyst that promoted the formation of reactive oxygen
species because of the presence of graphitic N, as shown in Fig.
42
1f.23,
Thus, the effect of H2O2 concentration on furfural
Maleic acid
Succinic acid
Others
oxidation has been considered. A different H2O2 concentration
(15%, 20%, 25%, and 30%) was tested for the reaction along
with the NC-900 catalyst. The reaction carried out without the
addition of H2O2 is not able to show the formation of MA.
Besides, MA formation was not observed when furfural
oxidation was carried out in the presence of air and pure oxygen
(flow rate 40 mL min-1) at 80 ºC. As shown in Fig. 2, the MA yield
strongly dependent on the H2O2 concentration. The gradual
increase of H2O2 concentration from 15% to 35% results in the
increase in MA yield from 41% to 61%, respectively.
Interestingly, the yield of SA co-product remains unchanged
with increasing H2O2 concentration to be ca. 2%. Besides, a 38%
residual formed for a reaction carried out with 35% H2O2 could
be intermediate products or side products due to the
decomposition and polymerization of furfural. Some
intermediates identified using HPLC analysis of the reaction
mixture by injecting standards of possible products such as
fumaric acid (FA), furoic acid, 5-Hydroxy-2(5H)-furanone
(hydroxyfuranone), and 2-(H)-furanone, respectively (Fig. S6;
ESI). Further, we have carried out the furfural oxidation into MA
100
80
60
40
20
0
15
20
25
30
35
H2O2 Concentration (%)
Fig. 2 Influence of H2O2 concentration on the furfural oxidation over NC-900 as
catalyst; Reaction condition: furfural (1 mmol), catalyst (50 mg), water 5 mL, time
5 h, temperature 80 ºC.
Further, we investigated the effect of various solvent on the
furfural oxidation including water (H2O), dimethylformamide
4 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 5 of 10
Catalysis Science & Technology
Please do not adjust margins
Journal Name
ARTICLE
(DMF), dimethyl sulfoxide (DMSO), dimethylamine (DMA),
diethylene glycol dimethyl ether (Diglyme), and ethanol. As
shown in Fig. 3, low MA yield obtained in the range from 2-17%
in organic solvents. On the other hand, the water solvent
showed a surprisingly high yield of MA (61%) and low yield of
SA (3%). These results indicate that the organic solvent systems
are not efficient for the furfural oxidation, and water is the
superior solvent system than organic solvents as it promotes
the higher MA yield.22, 23 Since the water media can provide a
suitable environment for the decomposition of H2O2 and
promote the formation of peroxide like species (-OOH), further,
H2O2 in water form water and oxygen gas which is more stable
than original H2O2.
View Article Online
DOI: 10.1039/C9CY02364J
Maleic acid
Succinic acid
Others
100
80
60
40
20
0
40
60
80
90
Conversion
Maleic acid
Succinic acid
Temperature (oC)
100
80
60
40
20
0
Fig. 4 Effect of reaction temperature on the furfural oxidation; Reaction condition:
furfural (1 mmol), NC-900 catalyst (50 mg), water 5 mL, H2O2 5 mL (35 wt%), time
5 h.
Mechanism of catalytic oxidation of furfural over NC-900
To study the kinetics of the catalytic oxidation of furfural over
NC-900 catalyst under acid-free condition using H2O2 as oxidant,
the time-dependence of the reaction was studied at 80 ºC. The
reaction pathway was studied by analyzing intermediate
products at the variation of reaction time combined with the
previous literature and our obtained results. The changes in the
product distribution show that 96% of furfural was consumed in
1 h, as shown in Fig. 5. At the same time, the MA and 5-hydroxy-
furan-2(5H)-one were produced with the dominant yield of 26%
and 41% compared to SA (ca. 1%) and 2-(5H)-furanone (0.5%),
respectively. Further, an increase in reaction time leads to a
decrease of 5-hydroxyl-furan-2(5H)-one yield and dramatic
improvement in the MA yield (Fig. 5), suggesting that the 5-
hydroxy-furan-2(5H)-one was converted to MA. After 5 h
reaction, the 5-hydroxyl-furan-2(5H)-one intermediate is
completely consumed while the MA yield approaches the
highest value (61%). On the other hand, the formation of SA (2%)
and 2-(5H)-furanone (5%) was extremely slow under similar
reaction conditions, suggesting that the reaction proceeds
mainly through 5-hydroxy-furan-2(5H)-one intermediate. The
residue of 33% yield contributed to side products such as small
amounts of fumaric acid (FA), furoic acid, formic acid, furan-
2(5H)-one and maybe some unknown side products due to the
polymerization and decomposition reactions of furfural, as
shown in Fig. S6.
H2O
DMF
DMSO
DMA
Diglyme EtOH
Solvent
Fig. 3 The furfural oxidation under various solvents; Reaction condition: furfural
(1 mmol), NC-900 catalyst (50 mg), solvent 5 mL, H2O2 5 mL (35 wt%), time 5 h,
Temperature 80 oC,
In the kinetic point of view, reaction temperature extremely
affects the selectivity of the desired product in various
reactions.43, 44 Hence, the furfural oxidation was carried out at
different temperatures such as 40 ºC, 60 ºC, 80 ºC, and 90 ºC
using NC-900 catalyst in aqueous media (Fig. 4). The results
showed that the selective catalytic furfural oxidation to MA
over the ZIF-8-derived NC-900 catalyst extensively depends on
the reaction temperature. Although the furfural conversion is
100% at all tested reaction temperatures, the selectivity of MA
changes significantly in various reaction conditions. As shown in
Fig. 4, only 23% MA yield could be obtained within 5 h at 40 ºC.
The highest MA yield (61%) was achieved when the reaction
was carried out at 80 ºC. Besides, further increasing reaction
temperature to 90 ºC results in the decrease of MA yield to 54%,
suggesting the decomposition of furfural due to the formation
of side products at high temperature. Thus, the optimal reaction
temperature for furfural oxidation to MA catalysed by the NC-
900 catalyst is 80 ºC which also offers higher MA selectivity.
It was reported that the furfural oxidation to MA could be
via the formation of 2-(5H)-furanone as an intermediate
product, which is further converted to MA.23, 45 However, this
pathway can be eliminated in the present reaction system
because of the very low yield/selectivity of 2-(5H)-furanone
intermediate over NC-900 catalyst (Fig. 5). Additionally, we
have carried out oxidation using the 2-(5H)-furanone
(intermediate) as a reactant under similar reaction conditions,
instead of furfural, the result showed a small amount of SA
production and no MA yield in 5 h. On the other hand, the 5-
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 6 of 10
ARTICLE
Journal Name
hydroxy-furan-2(5H)-one is the dominant product in 1 h pathway B, in which the furfural undergoes a serieVsieowfArreticalectOionlnines
DOI: 10.1039/C9CY02364J
reaction, which is then converted to MA. Thus, we speculate including the oxidative ring-opening reaction, Baeyer-Villiger
that the major reaction pathway of furfural oxidation using NC- oxidation, rearrangement steps, and hydrolysis to form 5-
900 as catalyst and H2O2 as oxidant should be via the formation hydroxy-furan-2(5H)-one as the main intermediate.
5-hydroxy-furan-2(5H)-one as a main intermediate product Subsequently, the 5-hydroxy-furan-2(5H)-one was oxidized to
(Scheme 1), as proposed by Granados’s and Jacobs’s group.23, 46 MA, as shown in Fig. 5.
The inferior yield of 2-(5H)-furanone intermediate and lower
As above demonstrated, the adjacent carbon atoms to N-Q
possibility of its conversion to MA, suggests the reaction A sites in the nitrogen-doped carbon catalyst could form the
pathway is less significant (Scheme 1).
reactive oxygen species, which served as active sites for
converting furfural to MA. It is reported that N-doped graphitic
catalysts can generate reactive oxygen species in presence of
H2O2 oxidant.38, 42, 47-49 To address this point and free radical
mechanism, we carried out the reaction with and without the
addition of Gallic acid as scavenging agent.50 As shown in Fig.
S7; ESI, a lower MA yield of 34% could be obtained in the
presence of Gallic acid as a scavenging agent, compared to 61%
MA in the absence of the scavenging agent. This result confirms
that the furfural oxidation into MA using H2O2 as an oxidant
follows the radical mechanism. This observation is in good
agreement with the above-obtained results. Overall, hydrogen
peroxide decomposed to radicals which coordinate with
adjacent carbon atoms to graphitic-type nitrogen dopant in NC
to form reactive oxygen species as catalytic active sites, as
confirmed with XPS analysis (Fig 1f). Subsequently, the furfural
interacted with the reactive oxygen species and oxidized into
MA via oxidative ring-opening, Baeyer-Villiger oxidation, and
hydrolysis. Besides, the kinetic study showed that the furfural
oxidation into MA over the N-doped carbon catalyst was
proceeding through the formation of 5-hydroxy-furan-2(5H)-
one as the main intermediate product, this showed that
reaction happens through B reaction pathway (Scheme 1).
Fig, 5 Reaction kinetics of the furfural oxidation; Reaction condition: furfural (1
Furfural
100
5-Hydroxy-furan-2(5H)-one
Maleic acid
80
Succinic acid
2-(5H)-furanone
60
40
20
0
0
1
2
3
4
5
6
Time (h)
mmol), NC-900 catalyst (50 mg), water 5 mL, H2O2 5 mL (35 wt%), temperature 80
ºC.
As shown in Scheme 1, the furfural oxidation in the presence
of NC-900 as a catalyst and H2O2 as an oxidizing agent could
show two possible reaction pathways at the same time.
Pathway A is the minor pathway to form the side product such
as SA. In contrast, the major reaction pathway is ascribed to the
Scheme 1 The reaction pathway of catalytic furfural oxidation over NC-900 acid-free catalyst.
6 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 7 of 10
Catalysis Science & Technology
Please do not adjust margins
Journal Name
ARTICLE
View Article Online
DOI: 10.1039/C9CY02364J
To understand the reusability of the NC-900 catalyst, we Table 2: The oxidation of biomass-derived furans to MA over NC-900
carried out the recycle experiments with NC-900 catalyst under- catalyst
optimized reaction conditions. As shown in Fig. 6, although the
furfural conversion remains unchanged during the recycling test,
Yield (%)
Conversion
(%)
the MA yield gradually decreased from 61% to 40% after the
fourth run, suggesting that the instability of the catalyst under
the reaction environment. To understand the nature of active
sites of the catalyst after the recycling experiment, we
performed the XPS analysis of fresh and spent catalysts. As
shown in Fig. S2 and Fig. S8, the N 1s of the fresh NC-900 was
deconvoluted into three peaks, corresponding to pyridinic-N (N-
6 at 398.4 eV), pyrrolic-N (N-5 at 399.8 eV), and quaternary (N-
Q at 401.1 eV) (Fig. S2; ESI).14, 51 However, there was a change
in the N 1s of the spent NC-900 catalyst. A new N-O bonding was
formed at 402.8 eV (Fig. S8; ESI).52 Besides, the N-Q species
dramatically decreased to 12.8%, compared to 24.84% in the
fresh catalyst (Table S1) this causes the deactivation of the
Entry
Reactant
MA
41
SA
0
Others
100
100
100
84
59
37
69
43
28
1
2
3
4
5
61
28
38
40
3
5
2
2
catalyst leading to decreasing catalytic activity.
Fig. 6 Recycle test; Reaction condition: furfural (1 mmol), NC-900 catalyst (50 mg),
Conversion
Maleic acid
Succinic acid
100
80
60
40
20
0
72
Reaction condition: Reactant (1 mmol), NC-900 catalyst (50
mg), water 5 mL, H2O2 5. mL (35 wt%), time 5h, temperature
80 ºC.
Conversion
Maleic acid
100
80
60
40
20
0
1
2
3
4
Run
water 5 mL, H2O2 5 mL (35 wt%), time 5 h, temperature 80 ºC.
Effect of biomass-derived furan and catalyst synthesized from the
different carbon and nitrogen sources
To extend the application of this study to various biomass-
derived furan molecules, including furan, furfural, HMF, 2-furoic
acid, and FDCA have been used as a substrate to produce MA
over NC-900 catalyst under acid-free condition. As shown in
Table 2, the furan compounds without a functional group and
with either aldehyde or hydroxyl functional groups exhibited a
100% conversion at 80 ºC within 5 h (Table 2, entry 1-3).
Meanwhile, the furan compounds with an acid functional group
showed lower conversion, which is in the range from 72-84%
(Table 2, entry 4-5). Furthermore, MA yield was observed to be
41%, 28%, 38%, 40%, and 61% for furan, HMF, 2-furoic acid,
FDCA, and furfural, respectively. This result indicates that the
functional groups present on the furan ring significantly affects
the catalyst reactivity for the catalytic oxidation of furan
compounds into MA.
ZIF-8-derived NC Malamine-derived Biomass-derived
g-C3N4
NC
NC and g-C3N4 catalysts
Fig. 7 Effect of various NC catalysts on the furfural oxidation: Reaction condition.
furfural (1 mmol), catalyst (0.05 g), water 5, H2O2 5 mL (35 wt%), time 5 h,
temperature 80 ºC.
Further, we compared the effect of various N-doped carbon
catalysts that were synthesized from the different carbon and
nitrogen sources, such as ZIF-8 (ZIF-8-derived NC), melamine
(melamine-derived g-C3N4), and chitosan-combined urea
(biomass-derived NC). The melamine-derived g-C3N4 and
biomass-derived NC were synthesized similar to the previously
reported methods.34,
35
Further, these catalysts were
characterized using XPS analysis to understand the type of
nitrogen species (Fig. S9; ESI). As shown in Fig. 7, when the
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
Please do not adjust margins

Please do not adjust margins
Catalysis Science & Technology
Page 8 of 10
ARTICLE
Journal Name
catalytic activity of melamine-derived g-C3N4 and biomass-
derived NC sample was tested for furfural oxidation, the results
showed high furfural conversion (98% with biomass-derived NC
and 100 with melamine-derived g-C3N4 catalyst). However, the
melamine-derived g-C3N4 (20% MA) and biomass-derived NC
catalyst (18% MA) exhibited a low MA yield. Whereas, the ZIF-
8-derived NC showed higher MA yield (61%) under similar
reaction conditions, indicating that the ZIF-8-derived NC (NC-
900) is a highly selective and active catalyst for furfural
oxidation into MA due to the higher content of graphitic
nitrogen (N-Q) in the structure of ZIF-8-derived NC catalyst.
From the results of the XPS analysis of g-C3N4 and biomass-
derived NC catalysts (Fig. S9; ESI), it can be seen that the N-Q
species (graphitic N) present in g-C3N4 and biomass-derived NC
catalyst is only 1.85% and 4.71%, which are lower than the N-Q
species in ZIF-8-derived NC-900 catalyst (24.84%), this again
confirms that the N-Q species of NC-900 catalyst playing an
important role in the selective oxidation of furfural to MA. This
comparison study showed that the ZIF-8-derived NC exhibited
the high catalytic performance in the oxidation of furfural to MA
compared to the g-C3N4 and biomass-derived NC. Further, The
NC-900 catalyst is capable of converting a variety of biomass-
derived furan compounds to MA under acid-free condition,
indicating the versatility and excellent applicability of the NC-
based catalytic system.
Acknowledgements
View Article Online
DOI: 10.1039/C9CY02364J
The authors would like to thank the Ministry of Science and
Technology (MOST), Taiwan (104-2628-E-002-008-MY3; 105-
2221-E-002-227-MY3; 105-2218-E-155-007; 105-2221-E-002-
003-MY3; 105-2622-E155-003-CC2) and the National Taiwan
University (105R7706) for the funding support. The authors also
thank to Researchers Supporting Project number (RSP-2019/6),
King Saud University, Riyadh, Saudi Arabia.
References
1
2
P. Gallezot, Chem. Soc. Rev., 2012, 41, 1538-1558.
A. Corma, S. Iborra and A. Velty, Chem. Rev., 2007, 107, 2411-
2502.
R. J. van Putten, J. C. van der Waal, E. de Jong, C. B. Rasrendra,
H. J. Heeres and J. G. de Vries, Chem. Rev., 2013, 113, 1499-
1597.
3
4
5
6
7
8
9
Y.-T. Liao, B. M. Matsagar and K. C. W. Wu, ACS Sustainable
Chem. Eng., 2018, 6, 13628-13643.
M. E. Zakrzewska, E. Bogel-Lukasik and R. Bogel-Lukasik,
Chem. Rev., 2011, 111, 397-417.
R. Mariscal, P. Maireles-Torres, M. Ojeda, I. Sadaba and M.
Lopez Granados, Energy Environ. Sci., 2016, 9, 1144-1189.
B. M. Matsagar and P. L. Dhepe, Catal. Sci. Technol., 2015, 5,
531-539.
B. M. Matsagar and P. L. Dhepe, New J. Chem., 2017, 41, 6137-
6144.
X. Li, P. Jia and T. Wang, ACS Catal., 2016, 6, 7621-7640.
10 N. Brun, P. Hesemann and D. Esposito, Chem. Sci., 2017, 8,
4724-4738.
11 B. M. Matsagar, S. A. Hossain, T. Islam, H. R. Alamri, Z. A.
Alothman, Y. Yamauchi, P. L. Dhepe and K. C. W. Wu, Sci. Rep.,
2017, 7, 13508.
12 B. M. Matsagar, C. Van Nguyen, M. S. A. Hossain, M. T. Islam,
Y. Yamauchi, P. L. Dhepe and K. C. W. Wu, Sustainable Energy
Fuels, 2018, 2, 2148-2153.
13 J. Zhang and J. Chen, ACS Sustainable Chem. Eng., 2017, 5,
5982-5993.
14 C. V. Nguyen, Y. T. Liao, T. C. Kang, J. E. Chen, T. Yoshikawa, Y.
Nakasaka, T. Masuda and K. C. W. Wu, Green Chem., 2016, 18,
5957-5961.
15 B. M. Matsagar, M. K. Munshi, A. A. Kelkar and P. L. Dhepe,
Catal. Sci. Technol., 2015, 5, 5086-5090.
16 C. Delhomme, D. Weuster-Botz and F. E. Kühn, Green Chem.,
2009, 11, 13-26.
17 S. Shi, H. Guo and G. Yin, Catal. Commun., 2011, 12, 731-733.
18 J. H. Carpenter, J. Ind. Eng. Chem., 1921, 13, 410-413.
19 S. Dutta, L. Wu and M. Mascal, Green Chem., 2015, 17, 2335-
2338.
Conclusions
Herein we report the catalytic oxidation of furfural into MA
using an N-doped carbon catalyst in H2O2 liquid oxidant under
the acid-free condition for the first time. The synthesized ZIF-8-
derived NC-900 catalyst shows a high percentage of graphitic
nitrogen (24.84%), which exhibited high selectivity in the
furfural oxidation to MA. The effect of various reaction
parameters such as H2O2 concentrations, solvent effect,
temperature and time effect was systematically studied in
detail. A high yield of MA (61%) could be achieved at 80 ºC in 5
h in the presence of NC-900 as catalyst and H2O2 as oxidant,
respectively. We demonstrated with the XPS characterization
that the graphitic nitrogen (N-Q species) is key active sites for
catalytic oxidation of furfural into MA. Besides, the kinetic study
showed that the furfural oxidation into MA over N-doped
carbon catalyst was proceeding through the formation of 5-
hydroxy-furan-2(5H)-one as the main intermediate product,
this shows that reaction happens through B pathway. A
comparison of catalytic activity of N-doped carbon catalysts,
including g-C3N4, biomass-derived NC, and ZIF-8-derived NC-
900, suggested that the ZIF-8-derived NC exhibited the highest
catalytic performance in the oxidation of furfural to MA,
compared to the g-C3N4 and biomass-derived NC. We believe
that this work can find potential applications for several
biomass conversions under acid-free conditions.
20 H. Choudhary, S. Nishimura and K. Ebitani, Appl. Catal., A,
2013, 458, 55-62.
21 N. Araji, D. D. Madjinza, G. Chatel, A. Moores, F. Jérôme and
K. De Oliveira Vigier, Green Chem., 2017, 19, 98-101.
22 H. Guo and G. Yin, J. Phys. Chem. C, 2011, 115, 17516-17522.
23 N. Alonso-Fagúndez, I. Agirrezabal-Telleria, P. L. Arias, J. L. G.
Fierro, R. Mariscal and M. L. Granados, RSC Adv., 2014, 4,
54960-54972.
24 X. Li, B. Ho, D. S. W. Lim and Y. Zhang, Green Chem., 2017, 19,
914-918.
25 I. Agirre, I. Gandarias, M. L. Granados and P. L. Arias, Biomass
Convers. Biorefin., 2019, DOI: 10.1007/s13399-019-00462-w.
26 S. Dang, Q.-L. Zhu and Q. Xu, Nat. Rev. Mater., 2017, 3, 17075.
27 L. Yang, X. Zeng, W. Wang and D. Cao, Adv. Funct. Mater.,
2018, 28, 1704537.
Conflicts of interest
There are no conflicts of interest.
8 | J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins

Page 9 of 10
Catalysis Science & Technology
Please do not adjust margins
Journal Name
ARTICLE
28 M. H. Yap, K. L. Fow and G. Z. Chen, Green Energy Environ.,
2017, 2, 218-245.
View Article Online
DOI: 10.1039/C9CY02364J
29 C. Wang, Y. V. Kaneti, Y. Bando, J. Lin, C. Liu, J. Li and Y.
Yamauchi, Mater. Horiz., 2018, 5, 394-407.
30 S.-H. Hsu, C.-T. Li, H.-T. Chien, R. R. Salunkhe, N. Suzuki, Y.
Yamauchi, K.-C. Ho and K. C. W. Wu, Sci. Rep., 2014, 4, 6983.
31 R. R. Salunkhe, C. Young, J. Tang, T. Takei, Y. Ide, N. Kobayashi
and Y. Yamauchi, Chem. Commun., 2016, 52, 4764-4767.
32 F. Zheng, Y. Yang and Q. Chen, Nat. Commun., 2014, 5, 5261.
33 H. X. Zhong, J. Wang, Y. W. Zhang, W. L. Xu, W. Xing, D. Xu, Y.
F. Zhang and X. B. Zhang, Angew. Chem. Int. Ed., 2014, 53,
14235-14239.
34 R. C. Pawar, S. Kang, J. H. Park, J.-h. Kim, S. Ahn and C. S. Lee,
Sci. Rep., 2016, 6, 31147.
35 Y. Ren, Z. Yuan, K. Lv, J. Sun, Z. Zhang and Q. Chi, Green Chem.,
2018, 20, 4946-4956.
36 H. Choudhary, S. Nishimura and K. Ebitani, Chem. Lett., 2012,
41, 409-411.
37 N. Alonso-Fagúndez, M. L. Granados, R. Mariscal and M.
Ojeda, ChemSusChem, 2012, 5, 1984-1990.
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.
39 S. Umrao, T. K. Gupta, S. Kumar, V. K. Singh, M. K. Sultania, J.
H. Jung, I.-K. Oh and A. Srivastava, ACS Appl. Mater. Inter.,
2015, 7, 19831-19842.
40 H. Au, N. Rubio and M. S. P. Shaffer, Chem. Sci., 2018, 9, 209-
217.
41 J. Chen, Y. Zhang, M. Zhang, B. Yao, Y. Li, L. Huang, C. Li and G.
Shi, Chem. Sci., 2016, 7, 1874-1881.
42 L. B. Khalil, B. S. Girgis and T. A. M. Tawfik, J. Chem. Technol.
Biotechnol., 2001, 76, 1132-1140.
43 C. V. Nguyen, D. Lewis, W.-H. Chen, H.-W. Huang, Z. A.
Alothman, Y. Yamauchi and K. C. W. Wu, Catal. Today, 2016,
278, Part 2, 344-349.
44 R. Jin, Y. Liu, Z. Wu, H. Wang and T. Gu, Catal. Today, 2010,
153, 84-89.
45 Y. Rodenas, R. Mariscal, J. L. G. Fierro, D. Martín Alonso, J. A.
Dumesic and M. López Granados, Green Chem., 2018, 20,
2845-2856.
46 J. Wahlen, B. Moens, D. E. De Vos, P. L. Alsters and P. A.
Jacobs, Adv. Synth. Catal., 2004, 346, 333-338.
47 X. Wang and Y. Li, J. Mater. Chem. A, 2016, 4, 5247-5257.
48 X. Hu, Z. Zhou, Q. Lin, Y. Wu and Z. Zhang, Chem. Phys. Lett.,
2011, 503, 287-291.
49 D. S. Su, S. Perathoner and G. Centi, Chem. Rev., 2013, 113,
5782-5816.
50 B. Badhani, N. Sharma and R. Kakkar, RSC Adv., 2015, 5,
27540-27557.
51 C. Van Nguyen, S. Lee, Y. G. Chung, W.-H. Chiang and K. C. W.
Wu, Appl. Catal., B, 2019, 257, 117888.
52 C. Maddi, F. Bourquard, V. Barnier, J. Avila, M.-C. Asensio, T.
Tite, C. Donnet and F. Garrelie, Sci. Rep., 2018, 8, 3247.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
Please do not adjust margins

Catalysis Science & Technology
Page 10 of 10
View Article Online
DOI: 10.1039/C9CY02364J
Graphical Abstract
Oxidation of biomass-derived furans to maleic acid over
nitrogen-doped carbon catalysts under the acid-free
condition
Chi Van Nguyen, Jing Rou Boo, Babasaheb M. Matsagar *
and Kevin C.-W. Wu *
We report an acid-free effective furfural-to-MA
conversion system using nitrogen-doped carbon catalyst
and H2O2 oxidant.