A CuMn2O4spinel oxide as a superior catalyst for the aerobic oxidation of 5-hydroxymethylfurfural toward 2,5-furandicarboxylic acid in aqueous solvent

Article abstract of DOI:10.1039/d0cy01649g

A CuMn₂O₄spinel oxide was preparedviaa freezing-assisted sol-gel method and used in the aerobic oxidation of 5-hydroxylmethylfurfural (HMF) toward 2,5-furandicarboxylic acid (FDCA) in aqueous solvent. A highest FDCA yield of 92.1% over the CuMn₂O₄spinel oxide was achieved and the catalyst could be regenerated by calcination in air after the sixth consecutive run, outperforming several other Mn-based spinel and single oxide catalysts. Kinetic studies reveal that HMF → 2,5-diformylfuran → 5-formylfuran-2-carboxylic acid (FFCA) → FDCA is the primary reaction route of the reaction and that the oxidation of FFCA is the rate-determining step over the CuMn₂O₄spinel. Characterization measurements show that Mn species enrichment and proper Mn⁴⁺/Mn³⁺, Cu²⁺/Cu⁺and Cu/Mn ratios on the surface of the catalyst led to an appropriate O_latt./O_ads.ratio, which facilitated oxygen mobility between the O_latt.consumption and the O_latt.generationviathe refilling of oxygen vacancies. Synergistic effects between Mn and Cu in the CuMn₂O₄spinel inhibit the secondary reaction and accelerate the rate-determining step rate to enhance FDCA formation.

Full text of DOI:10.1039/d0cy01649g

Catalysis
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PAPER
A CuMn O spinel oxide as a superior catalyst for
2
4
the aerobic oxidation of 5-hydroxymethylfurfural
toward 2,5-furandicarboxylic acid in aqueous
solvent†
Cite this: Catal. Sci. Technol., 2021,
11, 1497
a
a
a
a
a
Xiaoyue Wan,* Nannan Tang, Qi Xie, Shuangyan Zhao, Chunmei Zhou,
a
ab
Yihu Dai
and Yanhui Yang
*
A CuMn
oxidation of 5-hydroxylmethylfurfural (HMF) toward 2,5-furandicarboxylic acid (FDCA) in aqueous solvent.
A highest FDCA yield of 92.1% over the CuMn spinel oxide was achieved and the catalyst could be
2 4
O spinel oxide was prepared via a freezing-assisted sol–gel method and used in the aerobic
2 4
O
regenerated by calcination in air after the sixth consecutive run, outperforming several other Mn-based
spinel and single oxide catalysts. Kinetic studies reveal that HMF → 2,5-diformylfuran → 5-formylfuran-2-
carboxylic acid (FFCA) → FDCA is the primary reaction route of the reaction and that the oxidation of FFCA
is the rate-determining step over the CuMn O spinel. Characterization measurements show that Mn
Received 21st August 2020,
Accepted 25th November 2020
2
4
4+
3+
2+
+
species enrichment and proper Mn /Mn , Cu /Cu and Cu/Mn ratios on the surface of the catalyst led
to an appropriate Olatt./Oads. ratio, which facilitated oxygen mobility between the Olatt. consumption and
the Olatt. generation via the refilling of oxygen vacancies. Synergistic effects between Mn and Cu in the
DOI: 10.1039/d0cy01649g
rsc.li/catalysis
2 4
CuMn O spinel inhibit the secondary reaction and accelerate the rate-determining step rate to enhance
FDCA formation.
6
acid and its derivatives. The primary synthesis process of
1
. Introduction
FDCA is the selective oxidation of HMF, which is one of the
key top 12 platform chemicals derived from C6 sugars
through acid-catalyzed cellulose dehydration. Great efforts
have been made over the past few decades in exploring active
Biomass, a renewable carbon-based resource, represents the
most readily implemented, low-cost, and sustainable ‘drop-in’
supplement for fuel and chemicals that are currently derived
1
,2
2
from fossil fuel resources. The production of value-added
chemicals from biomass has thus attracted great attention.
Among these chemicals, 2,5-furandicarboxylic acid (FDCA)
has been identified as one of the top 10 bio-based valued-
added chemicals for establishing the ‘bridge’ between fossil
fuels and biomass resources because it is a key near-market
green monomer substitute for petrochemically produced
terephthalic acid that is used in a number of important bio-
degradable polyesters, fine chemicals, pharmaceuticals and
catalysts for the conversion of HMF into FDCA. In early
studies, stoichiometric and homogenous catalysts such as
7
8
9
10
HNO
3
,
KMnO
4
,
Co/Mn/Br, Co(OAc)
2
/Zn(OAc)
2
/Br,
and
CuCl/t-BuOOH (ref. 11) were explored to transform HMF into
FDCA and the maximum FDCA yield was 85%, catalyzed by
KMnO . Substantial progress has been made in attempting to
4
produce a heterogeneous catalyst for the aerobic oxidation of
HMF to FDCA using O2 as a low-cost and environmentally
friendly oxidant in aqueous media. Supported noble metal
3–5
12–16
13,16–20
21–25
agrochemicals.
nanoparticle catalysts, such as Pt,
Pd,
Au,
and
2
9–32
FDCA can be produced from furfural derivatives such as
-hydroxylmethylfurfural (HMF), 5-methylfurfural, 2-furoic
Ru (ref. 26–28) and their bimetallic catalysts
have been
5
reported as superior catalysts for HMF oxidation in the
presence of base additives. The effect of the support, metal/
bimetal morphology and state, kinetic character and its
influencing factors, reaction mechanism and so on have been
investigated carefully. Our group has reported that Au–Pd
alloy nanoparticles supported on carbon nanotubes (CNTs)
are highly efficient and recyclable heterogeneous catalysts for
the aerobic oxidation of HMF to FDCA in water under base-
a
Institute of Advanced Synthesis, School of Chemistry and Molecular Engineering,
Jiangsu National Synergetic Innovation Center for Advanced Materials, Nanjing
Tech University, Nanjing 211816, China. E-mail: ias_xywan@njtech.edu.cn,
yhyang@njtech.edu.cn
b
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute
of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
3
1
d0cy01649g
free conditions. Moreover, the catalytic performances of Pt
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and Fe O -decorated Pt catalysts for HMF aerobic oxidation
toward FDCA have also been explored in our group.
In view of the high cost and insufficient supply of noble
metals, non-noble catalysts with a unique ability for oxygen
activation, such as transition metal oxides, have been also
developed for the selective conversion of HMF toward FDCA
the O p-band center relative to the Fermi level, proposed to
3
4
1
6,33
be influential parameters for CO oxidation on ZnMn Co O4
x 2−x
5
0
spinel oxides.
In the present work, four Mn-based oxide catalysts were
synthesized to exclusively investigate their catalytic activity
for HMF oxidation using oxygen as an oxidant in aqueous
solutions. The objective is to disclose the effects that the
preparation method, structural, redox and morphological
properties of the Mn-based spinel catalysts have on HMF
oxidation and to elucidate the synergistic effects between the
Mn and Cu and the role of the oxygen mobility in the CuMn2-
in thermo-, electro- and photocatalytic processes over the
2
past few years. Oxide catalysts (MO ) have been proposed in
x
which the oxidation proceeds via a Mars van Krevelen (MvK)
mechanism consisting of two steps: (1) MOx oxidizes
substrates to form a partially reduced metal species (MO
)
x−δ
along with oxygen vacancy formation, (2) MOx−δ is rapidly
4
O spinel catalysts in achieving high catalytic efficiency in
oxidized by O . Therefore, a lower vacancy formation energy
the aerobic oxidation of HMF to FDCA. A plausible reaction
mechanism is also established and discussed.
2
has been accepted as the descriptor of the oxidizing
3
4
capability of a good oxide catalyst. Hayashi et al. studied
the effect of a MnO2 crystal structure on the aerobic
oxidation of HMF to FDCA, and their density functional
theory (DFT) calculations clarified that the vacancy formation
energies of MnO2 were largely dependent on the local
environment around the oxygen atoms and crystal
2
. Experimental
2.1. Materials and reagents
Cu(NO ) ·3H O (>99%) from Aladdin, Mn(CH COO) ·4H O
3
2
2
3
2
2
(
>98%) from Energy Chemical, and citric acid and ethanol
3
5
structure.
DFT and experimental results revealed that
from Sinopharm Chemical Co. Ltd. were used for
β-MnO , with a low vacancy formation energy, was a good
2
synthesizing the Mn–Cu spinel oxide (CuMn O ).
2
4
3
5
candidate as an oxidation catalyst. Besides this, several
mixed oxides (MnO –CeO , Fe–Zr–O, CuO·MnO ·CeO , Mn–
Co oxides and Mn–Fe mixed oxides) have also been
5
2
5
5
2
-Hydroxymethylfurfural (HMF, 98%) from Sigma-Aldrich,
,5-diformylfuran (DFF, 98%) from Energy Chemical, and
-hydroxymethyl-2-furancarboxylic acid (HMFCA, 98%),
x
2
2
2
3
6–43
reported to transform HMF into FDCA, and higher FDCA
yields have been obtained over mixed oxides than the
corresponding mono-metal oxides. Intriguingly, Gawade et al.
-formylfuran-2-carboxylic
acid
(FFCA,
98%)
and
,5-furandicarboxylic acid (FDCA, 98%) from J&K Chemicals
were used for the catalytic reactions and related
quantification. All the purchased chemicals were used as
received without further purification or treatment.
2 4
successfully synthesized a MnFe O spinel oxide for the
oxidation of HMF to FDCA, and obtained an 85% yield of
44
FDCA at 100 °C in 5 h using TBHP as the oxidant. Zhang
et al. synthesized a series of Mn–Co–O catalysts via a simple
2.2. Catalysts preparation
2 4
hydrothermal method and found that a MnCo O spinel
exhibited the highest activity for the aerobic oxidation of
HMF to FDCA, affording a 70.9% yield of FDCA at 99.5%
2.2.1 Freezing-assisted sol–gel method (CuMn O ).
2
4
CuMn O spinel oxides were synthesized via a sol–gel method
2
4
HMF conversion with 3 equivalents KHCO
3
, which was
using citric acid as the chelating agent. Cu(NO
3 2 2
) ·3H O,
significantly better than the performances of single oxides
Mn(CH COO) ·4H O and citric acid mixed in a 1 : 2 : 2 molar
3
2
2
4
5
(Mn
3
O
4
, Co
3
O
4
) and Mn–Co oxides.
ratio were dissolved in an ethanol and water mixed solution
by stirring. The resulting solution was continuously stirred
for a further 3 h. The solution was then placed in a freeze
dryer overnight until the sample temperature was below −20
°C and a blue colored solid was formed. The sample was
vacuum dried for three days, and then ground into a powder.
The powder was calcined in air at 500 °C for 3 h (heating
Spinels, as one class of unique mixed oxides, appear to be
a promising alternative to precious metal catalysts for
oxidation reactions, owing to the abundance of their
4
6–48
elements and superior catalytic ability.
Also, it is easy to
improve the activity by tuning the type and compositions of
4
9,50
the metal ions in spinel oxides.
investigated the composition effect that ZnMn
containing Mn (x = 0–2.0) spinel oxides has on an oxidation
In our previous work, we
−
1
x
Co2−x
O
4
rate: 4 °C min ) to afford the spinel CuMn
2 4
O powder
sample. NiMn O , MnCu O and CuMn O spinels were
2
4
2
4
2 4
5
0
reaction. The catalytic performance of the ZnMn
x
Co2−x
O
4
synthesized using this method.
2 4
2.2.2 Conventional sol–gel method (CuMn O _CSM). The
spinel oxides indicated that with an increase in the amount
of Mn from 0.2 to 1.8, the activity followed the variation of
the nominal oxidation state of Mn instead of Co. The co-
mixed solution obtained according to the above method was
dried at 60 °C for 5 h under magnetic stirring. Programmed
heating was carried out in an air atmosphere, and the
4
+
3+
4+
presence of Mn and Mn with a ratio of slightly >1 (Mn
:
3
+
−1
Mn ratio of 1.58) resulting in moderate oxygen adsorption
strength, was the key to achieving high activity. This
facilitated the rate-determining step (oxygen adsorption and
O vacancy refilling) during CO oxidation. DFT calculations
have been used to further verify the e filling state of Mn and
temperature ramping rate was 4 °C min . When the
temperature reached 500 °C, the sample was calcined at this
temperature for 3 h, and finally the spinel structure was
5
0
obtained. CuMn O _CSM and LiMn O
2 4
spinels were
2
4
g
synthesized using this method.
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.2.3 Coprecipitation method (CuMn O _CM). Cu
Paper
2
(λ = 1.5418 Å), operated at 40 kV and 40 mA. Scanning
electron microscopy (SEM) morphology characterization was
conducted using a Quanta FEG 450 microscope operated in
high vacuum mode. Transmission electron microscopy (TEM)
measurements were carried out on a JEOL JEM 2100F
microscope at an acceleration voltage of 300 kV. The molar
ratio of Cu/Mn was determined by ICP-OES using ICAP7400.
X-ray photoelectron spectroscopy (XPS) measurements
were carried out using a US Thermo Fisher ESCALAB 250XI
2
4
(
NO ) ·3H O (10 mmol) and Mn(NO ) (20 mmol) were
3 2 2 3 2
dissolved in 100 mL of deionized water to prepare solution I.
Anhydrous Na CO (20 mmol) and NaOH (20 mmol) were
2
3
dissolved in 60 mL of deionized water to prepare solution II.
Solution I was then added dropwise to solution II under
vigorous stirring. The resulting suspension was stirred and
crystallized at 65 °C for 18 h, cooled to room temperature,
and filtered to obtain a white powder. The sample was
thoroughly washed with deionized water, and dried at 120 °C
overnight, and calcined at 500 °C in air for 4 h. Finally,
−
10
spectrometer. The analysis chamber vacuum was 8 × 10
Pa, the excitation source was Al Kα (hv = 1486.6 eV), the
working voltage was 12.5 kV, the filament current was 16 mA,
and the constant energy of the analyzer was 20 eV. Charge
correction was performed using the C 1s 284.80 eV combined
energy as the energy standard. Ultraviolet-visible diffuse
reflectance spectroscopy (UV-vis DRS) measurements were
performed on a Lambda 950 spectrometer (PerkinElmer)
equipped with a 150 mm integrating sphere diffuse reflection
accessory at room temperature in the 200–800 nm wavelength
range. Fourier-transform infrared (FTIR) spectroscopy
analysis was carried out on a Thermo Fisher Scientific
spectrometer using the KBr and sample mixed tableting
method, in the recording wavelength range of 400–4000
2 4
CuMn O _CM spinel was obtained.
2
.3. Catalytic reaction
The aerobic oxidation of HMF was carried out in a Teflon-
lined stainless-steel autoclave (NS-50-C276, Anhui Kemi
Machinery Technology Co., Ltd. 50 mL). HMF (0.5 mmol) and
a certain amount of catalyst and sodium bicarbonate were
added to the reactor using deionized water (10 mL) as the
solvent. The reaction kettle was installed, purged with oxygen
several times and pressurized with 1 MPa of O , the reaction
2
temperature was 120 °C, and stirring was achieved using a
magnetic stirrer operated at 600 rpm for 18 h. After the
reaction, the autoclave was quickly transferred to ice water.
The reaction solution was filtered and stored in a sample vial.
The oxidation of HMF in the absence of oxygen was also
conducted: (1) a certain amount of HMF, H O and CuMn O
2 4
−
1
cm .
Hydrogen temperature-programmed reduction (H -TPR)
2
measurements were carried out on
AutoChem II 2920 analyzer equipped with
a
Micromeritics
a
mass
2
spectrometry (MS) detector (QGA, Hiden Analytical Ltd.).
Sample (50 mg) contained in a quartz reactor was pretreated
with argon at 300 °C for 1 h. After cooling the sample to 50
C, the argon gas was switched to a 10% H –90% Ar (in vol)
2
gas mixture for hydrogen reduction at a flow rate of 30 mL
min . The temperature was increased from 50 to 900 °C at a
heating rate of 10 °C min . O temperature-programmed
2
desorption (O -TPD) measurements were performed using
the same apparatus. Sample (50 mg) contained in a quartz
reactor was pretreated with argon at 300 °C for 1 h. After
cooling the sample to 50 °C, the argon gas was switched to a
2 2
0% O –90% He (in vol) mixed gas for O adsorption at a
flow rate of 30 mL min , and then purged with argon at this
catalyst were added to the batch reactor; (2), the reactor was
installed and purged with N ; (3) stirred for 10 min then
stirring was stopped and the N vented; (4) the procedure of
2
2
°
2 2
purging with N → stirring → stop stirring → venting with N
was carried out three times to ensure the absence of air in
−
1
the mixture; (5) the reactor was pressurized with N at 1 MPa,
2
−
1
2
and then heated to 120 °C for 3 or 4 h.
The liquid products were analyzed by high performance
liquid chromatography (HPLC, Shimazu LC-20 A) using a
diode-array detector (DAD) and a Shodex SH-1011 sugar
column (8 mm × 300 mm × 6 μm), and the mobile phase was
1
5
mM dilute sulfuric acid solution. At specific wavelengths,
−
1
various products and reactants were calibrated using
standard solutions, 260 nm for HMFCA and FDCA, 285 nm
for HMF, and 290 nm for DFF and FFCA. The conversion of
HMF, the selectivity and yield of the product were calculated
using the following formula.
temperature. The temperature was then raised to 900 °C at a
rate of 10 °C min , and the data was recorded in an argon
−
1
stream.
3
. Results and discussion
nHMF;0 − nHMF;t
Conv: ð%Þ ¼
3.1. Catalytic performances
nHMF;0
3.1.1 Effect of preparation method. Various CuMn O
2 4
nproduct × Ncarbon number of product
spinel catalysts prepared using different methods, including
Yield ð%Þ ¼
nHMF;0 × 6
CuMn O (freezing-assisted sol–gel method), CuMn O _CSM
2
4
2
4
(
conventional
sol–gel
method),
and
CuMn
2
O
4
_CM
(
coprecipitation method), were used in the aqueous-phase
aerobic oxidation of HMF. The performances of these three
catalysts are summarized in Table 1 (entries 1–3). Although
HMF was almost completely converted over these three
CuMn O spinel catalysts (conversions >99.0%), only the
2
.4. Catalyst characterization
X-ray diffraction (XRD) patterns were obtained on a Bruker
AXS D8 Advanced Focus diffractometer using CuKα radiation
2
4
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Table 1 Catalytic behavior of different oxide catalysts in the aerobic oxidation of HMFa
b
Yield (%)
Conv.
c
Entry
Catalyst
(%)
DFF
HMFCA
FFCA
FDCA
Others
1
2
3
4
5
6
7
8
9
CuMn
CuMn
CuMn
2
2
2
O
O
O
4
4
4
_CSM
_CM
99.1
0.0
0.0
0.0
1.3
0.0
0.6
1.0
2.5
0.0
0.9
0.0
0.0
0.0
0.0
0.0
0.0
3.8
1.0
0.0
0.0
0.0
0.0
6.2
2.5
1.4
5.9
7.1
34.3
38.5
9.3
5.8
11.9
3.4
13.4
16.7
90.1
35.1
21.0
25.7
15.3
6.4
79.5
80.8
8.5
50.1
49
27.7
32.9
48.4
58.8
43.3
55.2
100.0
100.0
92.4
77.1
92.1
88.7
66.6
98.7
62.0
100.0
LiMn
NiMn
MnCu
2
O
4
2
O
4
2
O
4
2 3
CuO–Mn O
CuO
Mn
Mn
2
O
O
3
34.1
5.9
41.4
1
1
0
1
3
4
MnO
2
a
Reaction conditions: 0.5 mmol of HMF, 0.84 mmol of Cu-based catalyst or 1.68 mmol of Mn-based catalyst, 10 mL of H
2
O, NaHCO
3
/HMF = 2,
b
1
2
2
.0 MPa of O
-furan-carboxylicacid, and 2,5-furandicarboxylic acid, respectively.
,5-furandimethanol (DHMF) and humin, etc. General situation, humin is the main by-product.
2
, 120 °C, and 18 h. DFF, HMFCA, FFCA, and FDCA denotes 2,5-diformylfuran, 5-hydroxymethyl-2-furancarboxylic acid, 5-formyl-
c
Others denote succinic acid, glycolic acid, formic acid,
CuMn
FDCA yields were observed over the CuMn
CuMn O _CM catalysts, indicating that side reactions
2
O
4
catalyst afforded ≥90% FDCA yield. Unsatisfactory
4
corresponding to mixed phases of Cu1.5Mn1.5O (JCPDS #70-
0262) and CuO (JCPDS #44-0706) (Fig. S3†). The FDCA yield
2
O
4
_CSM and
over MnCu O (entry 6, Table 1) is 25.7%, remarkably lower
2
4
2 4
occurred to a great extent over these two catalysts. The XRD
patterns in Fig. S1† show that CuMn possesses a pure
cubic CuMn O spinel phase (JCPDS #84-0543), whereas
than that over CuMn
such as CuO, Mn , Mn
yield (entries 8–11, Table 1). A mechanical mixture of CuO
and Mn (CuO–Mn ) shows 15.3% FDCA yield (entry 7,
Table 1). These results demonstrate that the CuMn spinel
2
O
4
. Furthermore, single oxide catalysts
2
O
4
2
O
3
3
O
4
, and MnO exhibit <42% FDCA
2
2
4
CuMn
2
O
4
_CSM and CuMn
2
O
4
_CM consist of CuMn
2
O
4
spinel
2
O
3
2 3
O
and Mn
2
O
3
(JCPDS #89-4836). The preparation of the pure
2 4
O
spinel phase benefits from the sol–gel method using citric acid
oxide efficiently catalyzes the aerobic oxidation of HMF in
comparison to other Mn-based spinel oxides and single metal
oxides (entries 3–11, Table 1), affording a high FDCA yield
(90.1%, entry 3 in Table 1).
as a chelating agent, which has been widely used to prepare
5
1
multicomponent oxides such as CoAl
2
O
4
,
2 4
AMn O (A = Cu, Ni
5
2
53
54
and Zn), copper–manganese oxides, and Li Ti O . Zhao
4
5 12
3
+
et al. first prepared OM–Ti –Li
4
Ti
5
O
12 via a modificative sol–
3.1.3 Effect of base additives. The nature and
gel method and reported the stoichiometric cationic
concentration of base additives have a great impact on the
4
+
+
36–40
coordination assembly process of Ti /Li -citrate chelate and
catalytic HMF oxidation performance.
Fig. 1a shows the
5
4
PEO (polyethylene oxide) segments in Pluronic F127. In this
effect that added base has on the aerobic oxidation of HMF
over the CuMn spinel catalyst. Potassium/sodium
2
+
work, we believe that citric acid coordinates with Mn and
2 4
O
2
+
Cu to form a three-dimensional Mn/Cu-citrate chelate gel;
after the vacuum freeze-drying, the pyrolysis of this gel then
bicarbonate/carbonate/hydroxide show noticeable differences.
The HMF conversions reach nearly 100% in the presence of
different bases, significantly higher than under base-free
conditions. The addition of weak bases (NaHCO or KHCO ,
results in a homogeneous mixed CuMn
2 4
O spinel oxide that
has a pure cubic CuMn O spinel phase (JCPDS #84-0543).
2
4
3
3
3
.1.2 Effect of composition on Mn-based spinel oxides and
2 equiv.) leads to higher FDCA yield. However, less than 50%
FDCA yield was observed using medium and strong bases
such as Na CO , K CO , NaOH, and KOH. These results are
single oxides. LiMn and NiMn oxides with a pure
cubic spinel phase were also prepared, and their structures
were confirmed by XRD characterization (JCPDS #35-0782 for
LiMn
S2.† Catalytic performances of the LiMn O and NiMn O
2 4
oxides are shown in Table 1 (entries 4–5). The FDCA yields
over the LiMn and NiMn spinel oxides are only 35.1%
and 21%, respectively. CuMn O displays a superior catalytic
2
O
4
2 4
O
2
3
2
3
in good agreement with the experimental observations made
by Li et al. on the promotion effect of NaHCO and KHCO
suggesting that base additives might facilitate the activation
2
O
4
and JCPDS #71-0852 for NiMn
2
O
4
), as shown in Fig.
3
3
,
2
4
3
6–40
of HMF.
Relatively stronger bases could lead to the
2
O
4
2
O
4
degradation of HMF into undetectable humin or other
intermediates in side reactions. In this work, NaHCO3 was
found to be more effective at promoting the catalytic
2
4
performance in comparison with LiMn
2 4 2 4
O and NiMn O ,
even though they possess the same cubic spinel structure
performances in comparison with KHCO
We further explored the optimal amount of NaHCO3 for
the aerobic oxidation of HMF over CuMn spinel oxides.
3
.
(entries 3–5, Table 1).
Considering that composition is a critical factor with
2 4
O
respect to the catalytic performances of spinel oxides, MnCu
2
-
Identical reaction conditions were used, except that the
O was also prepared via a freezing-assisted sol–gel method.
4
amount of NaHCO was altered from 0 to 3 equivalents. As
3
The XRD pattern of MnCu
2
O
4
shows diffraction peaks
shown in Fig. 1b, the base amount has a remarkable effect
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Paper
Fig. 1 Effect of (a) the base type and (b) the NaHCO
3
equivalent on the aerobic oxidation of HMF over the CuMn
2
O
4
spinel catalyst. Reaction
conditions: 0.5 mmol of HMF; 0.84 mmol of CuMn
2
O
4
catalyst; O
2
, 1 MPa; H
2
O, 10 ml; temperature, 120 °C; reaction time, 18 h; n(base)/n(HMF) = 2.
on the selective oxidation of HMF to FDCA. The HMF
conversion and FDCA yield were 36.3% and 10.9% in the
has on the catalytic behavior of CuMn O in the aerobic
2
4
oxidation of HMF. Upon increasing the reaction temperature
from 90 to 100 °C, the main product was FFCA while the
FDCA yield was slowly increased from 5.4 to 11.5%.
Intriguingly, both the activity and FDCA yield were
significantly increased to 98.2 and 60.0%, respectively, when
the reaction temperature was increased to 110 °C. Increasing
the reaction temperature to 120 °C, the FDCA yield reached
90.1% at ∼100% HMF conversion. However, at a higher
temperature of 130 °C, the FDCA yield decreased to 59.2%,
suggesting that the high reaction temperature resulted in
absence of NaHCO , respectively. In the presence of 1
3
equivalent of NaHCO
and 35.1%, respectively. Notably, the FDCA yield reached
0.1% when the NaHCO /HMF ratio was increased to 2 : 1.
3
, both increased significantly to 81.8%
9
3
3
Further increasing the NaHCO equivalents to 3, the FDCA
yield slightly decreased to 88.9%. Therefore, 2 equivalents of
NaHCO was adopted in the following studies.
3
3
.1.4 Effect of reaction temperature, O
2
pressure, and
catalyst dosage. Fig. 2a shows the influence that temperature
Fig. 2 Effect of the (a) temperature, (b) O
Reaction conditions: 0.5 mmol of HMF; 0.84 mmol of the CuMn
display, the changing trends of DFF, HMFCA, FFCA yield with reaction time are shown in (d), respectively.
2
pressure, and (c and d) time course on the aerobic oxidation of HMF over the CuMn
2 4
O spinel catalyst.
2
O
4
catalyst; H O, 10 ml; n(NaHCO )/n(HMF) = 2. Note that for a clear visual
2
3
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HMF decomposition and the occurrence of other side
reactions. Although full conversion of HMF could be
accomplished at 110 °C, the transformation of FFCA to FDCA
was still incomplete and required higher temperatures.
The dependency of aerobic oxidation of HMF on the O2
linearly within 12 h, then it further increased slowly from 12
to 18 h. For the intermediate products, such as DFF, HMFCA,
and FFCA, their yields revealed volcano-like curves with an
increase in the reaction time, and their maximum values
reached 2.1% (at 3 h), 9.2% (at 6 h), and 32.2% (at 6 h),
respectively, as shown in Fig. 2d. FFCA served as the main
product in the initial 8 h, and the aerobic oxidation of HMF
to FDCA over the CuMn O spinel catalyst was a series–
pressure was also tested, as shown in Fig. 2b. When the O
2
pressure was 0.5 MPa, HMF conversion was 82.8% and the
main product was FFCA. When the O pressure was 1.0 MPa,
2
2 4
FDCA was produced as the main product, and its yield
reached 90.1%. Further increasing the O2 pressure to 1.5
MPa slightly boosted the FDCA yield. These results suggest
parallel reaction. As illustrated in Scheme 1, two pathways
exist in the reaction, HMF → DFF → FFCA → FDCA and HMF
→ HMFCA → FFCA → FDCA. In particular, the aerobic
oxidations of HMF to DFF/HMFCA and DFF/HMFCA to FFCA
occurred more easily in comparison with the oxidation of
FFCA to FDCA. The latter acts as the rate-limiting step, which
is further discussed in detail in section 3.1.7.
2 4
that the catalytic performance of the CuMn O spinel catalyst
is sensitive to O pressure over a certain range (0.5–1.0 MPa),
2
and that an excessively high pressure (1.5 MPa) was
redundant in terms of promoting the conversion of HMF and
the yield of FDCA.
3.1.6 Reusability of the CuMn O catalyst. Recycling
2
4
The effect of CuMn O spinel catalyst dosage on the
reactions were performed to reveal the stability of the CuMn2-
spinel catalyst. After each reaction, the catalyst was post-
2
4
aerobic oxidation of HMF is shown in Fig. S4.† In a reference
test, 65.4% of HMF conversion was observed in the absence
of catalyst (Table S1†). A remarkably low carbon balance was
obtained at 44.8%, and the yield of FDCA was only 3.3%.
Furthermore, the total yields of DFF, FFCA, and HMFCA were
O
4
treated by centrifuging it, washing it with deionized water
and drying it in an oven at 100 °C. Satisfactory recyclability of
the catalyst was observed over six successive runs, as shown
in Fig. 3. The HMF conversion was maintained at ∼100%.
The FDCA yield remained stable in the initial three cycles,
and then slightly decreased in the next three cycles. The XRD
characterization of the spent catalyst after six reaction runs
exhibited the diffraction peaks of carbon species, which
originated from the deposition of the reaction mixture. After
the regeneration of the spent catalyst by calcination in air at
500 °C for 3 h, the catalyst was still made up of nano-sized
particles (30–200 nm, Fig. S6†) and possessed a pure cubic
spinel phase (JCPDS #84-0543, Fig. S7†), and its catalytic
activity could be successfully recovered to the original level at
90% FDCA yield.
1
0.6%, while the yields of byproducts (e.g., formic acid,
levulinic acid, 2,5-furandimethanol (DHMF), humin, etc.)
were 20%. The color of the liquid after reaction turned to
dark-brown from transparent yellow (Fig. S5a†). In sharp
contrast, the liquid mixture remained unchanged (light
yellow color) during the CuMn O -catalyzed reaction (Fig.
2
4
S5b†). The color change in the HMF oxidation reaction
without catalyst was attributed to the significant degradation
of HMF, e.g., disproportionation via the Cannizzaro reaction
into undetectable by-products and a small quantity of
4
0
oxidation products. As shown in Fig. S4,† when the catalyst
dosage was 0.42 mmol, HMF conversion reached 76.6%,
FFCA was the primary product and the FDCA yield was
3.1.7 Initial reaction rates and rate-limiting step. The key
steps of HMF oxidation were further evaluated. The oxidation
reactions of HMF → DFF, DFF → FFCA, HMF → HMFCA,
HMFCA → FFCA, and FFCA → FDCA were carried out under
<30% conversions to measure the initial reaction rates. As
shown in Table 2, the initial oxidation reaction rates of HMF,
DFF, HMFCA, and FFCA over the CuMn O catalyst were 649,
2
6.9%. When the amount of catalyst was doubled, the HMF
was completely converted, FDCA became the main product
and the FDCA yield was 90.1%. Further increasing the
catalyst dosage maintained the HMF conversion at 100%,
and the FDCA yield slightly improved to 92.1%.
2
4
−
1
−1
3
.1.5 Time course and reaction pathways. Fig. 2c shows
the catalytic performances of CuMn on HMF oxidation as
a function of the reaction time. The conversion of HMF
presents linear increase up until nearly complete
transformation at 12 h. Similarly, the FDCA yield increased
19 820, 299, and 192 μmol g h , respectively. The rate of
DFF → FFCA oxidation was 66 times that of HMFCA → FFCA
oxidation, while the rate of HMF → DFF was 4 times that of
HMF → HMFCA oxidation. In view of the nature of HMF
oxidation as a series–parallel reaction, HMF → DFF → FFCA
2 4
O
a
Scheme 1 Reaction pathway for the selective aerobic reaction of HMF to FDCA over the CuMn
of HMF; 0.84 mmol of the CuMn catalyst; H O, 10 ml; n(NaHCO )/n(HMF) = 2.
2 4
O spinel catalyst. Reaction conditions: 0.5 mmol
2
O
4
2
3
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ratios
performances.
played
significant
In this study, when the Cu/Mn ratio was
roles
in
the
catalytic
4
7,55,56
1
: 2, 100% HMF conversion and 90.1% FDCA yield were
achieved, while when the Cu/Mn ratio was 2 : 1, only 25.7%
FDCA yield was obtained with 92.1% HMF conversion.
Hence, the physical and chemical properties of the CuMn
2 4
O
and MnCu O catalysts were benchmarked using various
2
4
characterization techniques.
.2.1 Bulk compositions and structures of the CuMn
and MnCu O catalysts. The crystal structures of the CuMn -
3
2 4
O
2
4
2
O and MnCu O oxide catalysts were confirmed by XRD (Fig.
4
2 4
S1 and S3†). As mentioned in section 3.1.1, the XRD pattern
of CuMn O oxide shows sharp crystalline peaks
corresponding to pure cubic CuMn O spinel phase. The
2
4
2 4
Fig. 3 The recyclability experiments of the CuMn O spinel catalyst in
the oxidation of HMF. Reaction conditions: HMF (0.5 mmol), catalyst
0.84 mmol), O , 1.0 MPa; H O, 10 ml; temperature, 120 °C; reaction
time, 18 h; n(NaHCO )/n(HMF) = 2.
2 4
(
2
2
2 4 4
pattern of MnCu O oxide includes Cu1.5Mn1.5O and CuO
3
phases. CuMn O and MnCu O oxide catalysts were
2
4
2 4
measured by inductively coupled plasma mass spectrometry
ICP-MS) to calculate the Mn/Cu ratios, as shown in Table
(
→
FDCA was the primary pathway of HMF oxidation with
S5.† The results indicate that the ICP-MS experimentally
measured Cu/Mn ratios in the stoichiometry of the final
oxides matched the theoretical values, which were also the
Cu/Mn ratios used in preparing these oxides. The surface
higher reaction rates, whereas HMF → HMFCA → FFCA →
FDCA was the secondary pathway with relatively lower
reaction rates. It should be noted that the rate of FFCA →
FDCA transformation was 0.37 and 0.0097 times those of the
HMF → DFF and DFF → FFCA steps, respectively, clearly
suggesting that FFCA → FDCA is the rate-limiting step in the
primary pathway. The rate-determining step in the secondary
pathway (HMF → HMFCA → FFCA → FDCA) is HMF →
HMFCA oxidation. Therefore, the transformation of FFCA →
FDCA serves as the rate-determining step in the overall HMF
2 4 2 4
areas of CuMn O and MnCu O were measured as 18.8 and
2
−1
19.5 m g , respectively. The SEM analyses (Fig. S8†) indicate
that both CuMn O and MnCu O comprise nano-sized
2
4
2 4
particles that range in size from 30 to 200 nm.
The TEM, high-resolution TEM (HRTEM), and high-angle
annular
microscopy (HAADF-STEM) images of the CuMn
MnCu catalysts are presented in Fig. 4 and S5.† The lattice
dark-field
scanning
transmission
electron
2 4
O
and
oxidation reaction catalyzed by CuMn
2
O
4
spinel oxides.
2 4
O
The initial oxidation reaction rates over the MnCu O
spacings of 0.25, 0.29 and 0.48 nm can be observed clearly
from the HRTEM image (Fig. 4b), corresponding well to the
2
4
catalyst were also studied, as shown in Table 2. Although the
initial oxidation rate of HMF oxidation over MnCu was
slightly faster than that over CuMn O , the rates of DFF and
2
O
4
(311), (220), and (111) standard planes of the CuMn
2 4
O
spinel. For the MnCu O oxides, cubic lattices with distances
2
4
2 4
FFCA oxidations over the MnCu
of the rates over the CuMn catalyst, respectively. Note that
the yield of by-products such as HCOOH, levulinic acid (LA)
and humin reached 15% over MnCu
2
O
4
catalyst were 1/2 and 2/3
between the planes of 0.21, 0.25, 0.29, and 0.48 nm can be
identified, as shown in Fig. S9b.† These values are identical
to the distances of the (400), (311), (220), and (111) standard
2 4
O
2
O
4
.
planes of the Cu1.5Mn1.5
the (002) planes of CuO. Energy-dispersive X-ray spectroscopy
EDS mapping) was used to directly observe the distribution
4
O spinel. 0.25 nm corresponds to
(
3
.2. Catalyst characterization
characteristics of the spinel target elements. Cu, Mn, and O
elements were found to be evenly distributed and highly
dispersed in CuMn O (Fig. 4d–f). For MnCu O , the Cu, Mn,
2 4 2 4
3
+
In view of the presence of two Jahn–Teller ions (Mn and
2
+
Cu ) in the Cu
x
Mn3−x
O
4
spinel materials, different Cu/Mn
Table 2 Initial reaction rates of HMF, DFF, HMFCA, and FFCA oxidation over various catalystsa
−1
−1
Initial reaction rate (μmol g
h )
b
Catalyst
HMF oxidation Step 1 HMF → DFF Step 1′ HMF → HMFCA Step 2 DFF → FFCA Step 2′ HMFCA → FFCA Step 3 FFCA → FDCA
CuMn
MnCu
CuO
2
O
O
4
649
604
475
1309
3317
519
604
475
1309
3317
130
—
—
—
—
19 820
9870
30 543
51 988
32 168
299
—
—
—
—
192
127
9
1250
705
2
4
Mn O
2 3
MnO
2
a
b
Reaction conditions: 84 mg of NaHCO
3
, 10 mL of H
2
O, 1 MPa of O
2
, 120 °C. The details are referred to in Tables S2–S4.† The initial reaction
rate of HMF oxidation is equal to the rate of step 1 plus the rate of step 1′.
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(
Fig. 5). The H -TPR profile of CuMn O shows one main
2
2 4
peak at 252 °C and one shoulder peak at 187 °C. There is one
broad peak at 217 °C in the MnCu -TPR profile. Note that
2 4 2
O H
the reduction profile of pure CuO is characterized by a main
peak at 268 °C, and a two-step reduction profile was observed
for Mn
2 3
O at 370 and 500 °C, attributed to the reduction of
46,47,55
Mn O to Mn O and Mn O to MnO.
Hence, the
reduction temperatures of the CuMn O and MnCu O spinel
2
3
3
4
3 4
2
4
2 4
2 3
samples were lower than those of pure CuO and Mn O ,
implying enhanced reducibility and oxygen mobility owing to
the Jahn–Teller effect on Cu–Mn spinel oxide and the
δ+
δ+
synergistic interaction between Mn and Cu ions.
O -TPD profiles of the CuMn O and MnCu O4 catalysts
2
2
4
2
were analyzed to determine the oxygen species, as presented
in Fig. 6. The desorption of reactive surface oxygen species
and lattice oxygen species occurred at moderate temperatures
from 300 to 600 °C and relatively higher temperatures above
2
8
6
00 °C, respectively. The desorption temperature of the
surface oxygen species of CuMn O was 416.1 °C, ∼35 °C
2
4
lower than that of the MnCu
oxygen mobility of the CuMn
2
O
4
oxide, indicating the higher
2 4
O in comparison with that of
MnCu O . The desorption of the lattice oxygen species at
2
4
above 600 °C included two peaks, at 710.6 and 844.6 °C, for
MnCu and 719.4 and 772.7 °C for CuMn . The distinct
desorption behavior of the lattice oxygen species (e.g., easier
desorption from CuMn ) reflects the different degrees of
2
O
4
2 4
O
2 4
O
δ+
δ+
the Jahn Teller effect of the Cu and Mn ions and Cu–O–
Mn interaction strengths due to the various location and
ratio of these two ions in the catalysts.
3
.2.3 Surface properties of the CuMn
2
O
4
and MnCu
2
O
4
Fig.
corresponding EDS mappings of (d) Cu, (e) Mn and (f) O elements,
respectively, of the CuMn catalysts.
4 (a) TEM, (b) HRTEM, (c) HAADF-STEM images and
catalysts. The FTIR spectra of the CuMn O and MnCu O
samples display peaks from 800 to 400 cm in low-frequency
2
4
2
4
−
1
2
O
4
mode, assigned to M–O lattice vibrations. The bands above
−
1
5
80 cm can be readily indexed to the vibration of atoms in
O elements were found to be evenly distributed and Cu was
densely distributed in some areas (Fig. S9d–f†), confirming
the presence of copper oxide in the MnCu O spinel.
tetrahedral oxygen environment corresponding to A–O of the
5
6–60
AMn
2
O
4
in the spinels.
The bands between 500 and 570
−
1
cm correspond to the vibration of atoms in the octahedral
oxygen environment mainly as a result of the manganese in
the Mn-base spinel (Fig. S10†). The bands at 439.0 cm
belong to the vibration mode of Cu–O–Mn in the Cu–Mn
2
4
3
.2.2 Bulk redox properties of the CuMn
2
O
4
and MnCu
2
O
4
−
1
catalysts. H
2
-TPR experiments were performed to determine
the reducibility of the CuMn O4 and MnCu O samples
2
2 4
5
6–60
spinel.
Fig. 5
2 2 4 2 4 2 3
H -TPR profiles of the CuMn O , MnCu O , CuO, and Mn O
catalysts.
Fig. 6
2 2 4 2 4
O -TPD profiles of the CuMn O and MnCu O catalysts.
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2 4
Fig. 7 Deconvoluted XPS spectra of the (a) Cu 2p, (b) Mn 2p, and (c) O 1s orbital levels of the CuMn O catalysts.
+
2+
The deconvoluted XPS spectra of the Cu 2p, Mn 2p and O
s electronic energy levels in the CuMn O and MnCu O
The ratio of Cu : Cu on the surface of CuMn
2 4 2
O and MnCu -
1
O was 38.3: 61.7 and 14.2: 85.8, respectively (Table 3).
2
4
2
4
4
samples are displayed in Fig. 7 and S11,† respectively, and
The corresponding O 1s XPS spectra of the CuMn O and
2 4
their corresponding surface atomic ratios of Cu : Mn : O are
2 4
MnCu O samples are shown in Fig. 7 and S8,† respectively.
The deconvolution of the O 1s spectrum clearly shows two
3+
4+
also reported in Table 3. It is shown that the Mn , Mn ,
2
+
+
2−
Cu , Cu , lattice oxygen (O , Olatt.) and reactive surface
surface oxygen species at 529.6 and 530.9 eV, attributed to
−
2−
−
64–68
oxygen (O
2
, O
2
and/or O , Oads.) all coexist on the surface
O
latt. and Oads.
.
The O quantitative analysis of the XPS is
2
+
of the CuMn O sample, while there is also Mn on the
listed in Table 3. On the surface of the CuMn O sample, the
2
4
2 4
surface of the MnCu
The binding energies (BEs) of Mn , Mn and Mn
2p3/2 of the CuMn O and MnCu O samples are
approximately 640.5, 642.1 and 644.0 eV, respectively.
2
O
4
sample.
O
latt. amount was nearly equal to that of Oads., in a ratio of
2
+
3+
4+
50.7 : 49.3. However, on the MnCu surface, the Olatt.
content (44.7%) was slightly lower than the Oads. content
(55.3%).
2 4
O
(
)
2
4
2 4
5
5,60–64
Clearly signal differences can be observed in the Mn 2p
region, and each signal has two different components. Under
the condition that the peak area ratio of 2p3/2 and 2p1/2 is 2 :
To better understand the surface properties together with
the bulk properties, quantitative XPS analysis was carried
out, as shown in Table 3. Comparing the surface Mn molar
fraction determined by XPS to the bulk Mn molar fraction
measured by ICP, the Mn molar fraction on the CuMn O
surface (Cu : Mn, 1 : 3) was higher than the bulk (1 : 2),
indicating the surface enrichment of the Mn species. The Mn
surface enrichment phenomenon was also exhibited on the
1
, and the half-peak widths are nearly identical, the two sets
of peaks were well fitted by a Mn 2p electron curve. The Mn
compositions on the surface of the CuMn sample were
Mn and Mn , and the ratio of Mn : Mn was 59.3 : 40.7
2
4
2
O
4
3
+
4+
3+
4+
(
Table 3). However, there were three compositions on the
2
+
3+
4+
MnCu
2
O
4
surface, Mn , Mn , and Mn , and the ratio of
MnCu
bulk CuMn
(Cu : Mn : O, 1 : 3 : 6). While the Cu : Mn : O ratio of MnCu O
2
O
4
catalyst. In comparison with the Cu : Mn : O ratio of
2
+
3+
4+
Mn : Mn : Mn was 25.9 : 36.6 : 37.5 (Table 3).
2 4
O
(1 : 2 : 4), O species also enriched the surface
The Cu 2p XPS spectra of the CuMn O and MnCu O
2
4
2
4
2
4
4
samples are shown in Fig. 7 and S8 ,† respectively. The Cu 2p
spectrum of both samples can be resolved into Cu 2p1/2 and Cu
surface (1 : 1 : 3) was close to the ratio of bulk Cu1.5Mn1.5
(1 : 1 : 2.7), one of the compositions of MnCu
implied that the composition on the surface of MnCu O is
O
2 4
O , this also
2
p3/2. The signal with low binding energy (930.7 eV) was
2
4
+
attributed to Cu 2p3/2, and the signal at high binding energy
4
mainly Cu1.5Mn1.5O .
2
+
(933.7 eV) was indicative of Cu 2p3/2 along with its satellite
55,60–64
+
from 938.0 to 948.0 eV.
was clearly distinguishable in the CuMn
samples, this Cu phase was not observed in the XRD pattern
of the sample due to the detection limit of the XRD technique.
Even though the presence of Cu
and MnCu
3
.3. Discussion
3.3.1 Synergy promotion effect of the CuMn
HMF oxidation. The mechanical mixture of Mn O and CuO
O
2 4
2 4
O
+
2 4
O spinel on
2
3
2 4 2 4
Table 3 Summary of the XPS spectral results over the CuMn O and MnCu O catalysts
Binding energy (eV)
Mn 2p3/2
Cu 2p3/2
O 1s
Area fraction (%)
Cu : Mn : O ratio
2
+
3+
4+
+
2+
2+
3+
4+
+
2+
a
Catalyst
Mn
Mn
Mn
Cu
Cu
O
ads.
O
latt.
Mn
Mn
Mn
Cu
Cu
O
ads.
O
latt.
Bulk
Surface
CuMn
MnCu
2
O
O
4
—
641.3 644.0 930.8 933.6 530.9 529.6
0
59.3
36.6
40.7
37.5
38.3 61.7
14.2 85.8
49.3
55.3
50.7
44.7
1 : 2 : 4 1 : 3 : 6
2 : 1 : 4 1 : 1 : 3
2
4
640.6 641.6 644.3 930.7 933.7 530.9 529.5 25.9
a
Calculated based on the ICP-OES results.
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showed a higher FDCA yield than the mono oxide (Mn O or
To further clarify the MvK mechanism of the Olatt. over
CuMn O in the HMF oxidation reaction, anaerobic HMF
oxidation reactions were carried out with N under base-free
2
2
3
CuO). When Mn- and Cu-based oxides formed a spinel, a
higher FDCA yield was obtained. Hence, in comparison with
the mono oxide and CuO–Mn O the CuMn O4 spinel
2
4
reaction conditions (O free conditions, experimental details
2
3
2
2
exhibited a synergy promotion effect for HMF oxidation to
FDCA. To better understand the synergy promotion effect of
the CuMn O spinel, the initial oxidation reaction rates of
are given in the ESI†). The results indicate that even though
there was no oxygen in the reactor, HMF could be
transformed into HMFCA, FFCA and FDCA through oxygen
insertion (Oins) processes, where the HMF conversion was
8.4% and the amount of Oins was 34.3 μmol within 3 h (entry
1 in Table S6†). These inserted Oins species came from the
lattice oxygen of the CuMn O catalyst. Upon increasing the
2
4
HMF/DFF/FFCA (Table 2) and the catalytic behavior with
reaction time over mono oxides (Mn ) were investigated
Fig. S12†). As shown in Table 2, the initial rates of HMF,
DFF, and FFCA conversions over the CuMn O spinel, Mn O
2 3
O
(
2
4
2
3
2 4
and CuO catalysts were evaluated from the results of the
oxidation of the substrates at the initial stage. The initial
rates over CuMn O spinel or Mn O or CuO catalysts were in
the order of rstep3 < rstep1 ≪ rstep2, which indicates that the
limiting step of HMF oxidation over the three catalyst is step
reaction time to 4 h, the HMF conversion and Oins amount
slightly increased to 9.4% and 36.2 μmol (entry 2 in Table
S6†), implying that most of the lattice oxygen on the catalyst
surfaces was consumed within 3 h for a slow reaction rate in
2
4
2 3
the Oins process. The autoclave of the 4 h O -free reaction
2
3
(the oxidation of –CHO in FFCA), and the next is step 1 (the
oxidation of –CHO or –CH OH in HMF). The rstep3 and rstep1
steps over the CuMn O spinel were 21 and 1.4 times faster
was cooled to room temperature, O gas was charged into the
reactor, stirring was carried out at room temperature for 2 h,
and then the O2 was released. Then, the anaerobic HMF
2
2
2
4
than over CuO, respectively. Therefore, in comparison with
2
oxidation reaction was performed again under N for 4 h.
CuO, the CuMn
step.
2
O
4
spinel improves the rate of the limiting
The HMF conversion and amount of Oins were almost
regained to the original levels (entry 3 in Table S6†). All these
Intriguingly, the initial rates of steps 1, 2 and 3 over
Mn were higher than the rates over the CuMn spinel
Table 2), while the FDCA yield over Mn O was 34.1% with
2 4
results demonstrate that the lattice oxygen of the CuMn O
catalyst was catalytically active in HMF oxidation in the way
of a consuming–resupplying cycle.
2
O
3
2
O
4
(
2
3
9
8.7% HMF conversion (Table 1). Hence, the time course
Taking full account of the materials characterization and
catalytic results, a plausible reaction mechanism for the
for HMF conversion and the corresponding products over
the Mn O mono oxide was investigated. As summarized in
2
3
Fig. S12,† DFF, FFCA, and FDCA were detected as the
products, while no HMFCA was produced in the first step
for HMF oxidation over Mn O . Fig. S12† shows that
2
3
byproducts including HCOOH, LA, and DHMF were
observed in the initial stage of the Mn -catalyzed HMF
oxidation reaction, implying that in addition to the
oxidation, HMF could also be catalyzed by Mn through
2 3
O
2 3
O
Cannizzaro disproportionation or degradation to byproducts.
This could explain why the reaction color became darker as
the reaction time was prolonged. Therefore, in spite of the
high catalytic ability to activate HMF molecules over the
Mn O catalyst, the CuMn O spinel catalyst can suppress
2
3
2 4
the side reactions to produce byproducts and enhance the
production of FDCA.
3
.3.2 Reaction mechanism of the CuMn O -catalyzed HMF
2 4
oxidation. The MvK mechanism is generally accepted in the
catalytic oxidation of organic compounds over spinel oxides.
The MvK mechanism involves surface oxygen mobility as
follows: (1) the Olatt. depletion to form oxygen vacancies
([]vacan.) during the organic molecule being oxidized, Olatt.
(Olatt. → []vacan.), (2) the Olatt. regeneration Ol′att: from Oads.
resulting in corresponding O vacancy generation (Oads.
vacan. → Ol′att: + ½ꢀv′acan:), along with Oads. regeneration through
adsorption and dissociation of O molecules. At
+
[]
the ½ꢀ′
2
vacan:
the same time, the oxygen mobility was accompanied by the
oxidation and re-oxidation of metal with different oxidation
Scheme 2 Plausible reaction mechanism of the aerobic oxidation of
HMF to FDCA over the CuMn O catalyst.
2 4
5
0,65–68
states.
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Paper
4
+
3+
2+
+
aerobic oxidation of HMF to FDCA over the CuMn O catalyst
proper Mn /Mn (40.7 : 59.3), Cu /Cu (61.7 : 38.3), and Cu/
Mn (1 : 3) ratios on the surface enhance the selective catalytic
2
4
is proposed in Scheme 2A. Based on the above-mentioned
3.3.2 Reaction mechanism of CuMn -catalyzed HMF
oxidation) MvK mechanism, the lattice oxygen species on the
(
2
O
4
2 4
behavior of CuMn O . All these conditions provide the
appropriate Olatt./Oads. ratio (50.7 : 49.3), which facilitates
oxygen mobility between the Olatt. consumption and the Olatt.
generation via refilling of the oxygen vacancies, accelerating
the oxidation reaction. Finally, significant synergistic effects
between Mn and Cu in the CuMn O spinel oxide inhibit the
2
−
CuMn O surface ([O ]) were clarified as being catalytically
2
4
active sites. The key reaction progresses include (1) the
oxidation of the –CH OH group in HMF or HMFCA to a
2
–CHO group and (2) the oxidation of the –CHO group in
2
4
HMF, DFF or FFCA to a –COOH group (Scheme 1). As shown
secondary reaction and improve the rate of the limiting step
to enhance FDCA production.
2
−
in Scheme 2A, the R–CH OH was adsorbed by [O ] to form a
2
pentacyclic compound and further converted to R–CHO
through a dehydrogenation oxidation. The R–CHO could be Conflicts of interest
−
generated as a hemiacetal or gem-diol intermediate by OH
2
−
There are no conflicts to declare.
ions, which was then was dehydrogenated by [O ] to form R–
2
−
COOH. In this context, [O ] was critical in driving the
aerobic oxidation of HMF toward FDCA over the CuMn O
Acknowledgements
2
4
2
−
catalyst, suggesting that the formation of [O ] was also
crucial in this oxidation reaction.
We are grateful to the support from the Natural Science
Foundation of the Jiangsu Higher Education Institutions of
China (19KJB150029), the Natural Science Foundation of China
2
−
The generation of [O ] through oxygen mobility on the
CuMn surface could be determined by these factors
Scheme 2B): (1) the amount of high-valent metal ions (Mn ,
2 4
O
(22008116), the National Key Research and Development
4
+
(
Program of Chinaa (2018YFE0122600), the Natural Science
Foundation of China (21802070, 21503187), the Natural Science
Foundation of Jiangsu Province, China (BK20170986), the
Nanjing Tech University, the SICAM Fellowship from Jiangsu
National Synergetic Innovation Center for Advanced Materials
and the Light of West China Program of the Chinese Academy of
Sciences.
2
+
2−
Cu ) available to directly form [O ], (2) the amount of low-
valent metal ions (Mn , Cu ) along with oxygen vacancies,
3) the coexistence of Cu , Mn , Cu and Mn in the
balance of Cu + Mn ⇌ Cu + Mn , (4) the reaction gas
including Only when these four factors are met
simultaneously, can [O ] be continuously generated. In this
work, the Mn /Mn ratio (40.7 : 59.3), Cu /Cu ratio (61.7 :
8.3), Cu/Mn ratio (1 : 3) on the surface enable oxygen species
enrichment and an the appropriate Olatt./Oads. ratio (50.7 :
9.3), further ensuring oxygen mobility balance between the
3
+
+
2
+
3+
+
4+
(
2
+
3+
+
4+
2
O .
2
−
4
+
3+
2+
+
3
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