Nanoscale center-hollowed hexagon MnCo2O4 spinel catalyzed aerobic oxidation of 5-hydroxymethylfurfural to 2,5-furandicarboxylic acid

Article abstract of DOI:10.1016/j.catcom.2018.05.004

A series of bimetallic Mn-Co-O catalysts were synthesized by a simple hydrothermal method, and the catalytic performance was evaluated. Among these catalysts, center-hollowed hexagon MnCo₂O₄ exhibited excellent catalytic effect in aerobic oxidation of 5-hydroxymethylfurfural (5-HMF) to 2,5-furandicarboxylic acid (FDCA). A 99.5% 5-HMF conversion and 70.9% FDCA yield can be obtained under molecular oxygen and weak base condition. The efficient catalytic performance was attributed to the Mn³⁺ ions on the surface of MnCo₂O₄ catalyst, and its high oxygen mobility and reducibility. Furthermore, this simple synthesis process of non-precious metal catalyst is beneficial to the production of FDCA.

Full text of DOI:10.1016/j.catcom.2018.05.004

Accepted Manuscript
Nanoscale center-hollowed hexagon MnCo2O4 spinel catalyzed
aerobic
oxidation
of
5-hydroxymethylfurfural
to
2,5-furandicarboxylic acid
Shuang Zhang, Xiaozhu Sun, Zaihang Zheng, Long Zhang
PII:
S1566-7367(18)30171-7
DOI:
Reference:
doi:10.1016/j.catcom.2018.05.004
CATCOM 5398
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
11 February 2018
7 May 2018
9 May 2018
Please cite this article as: Shuang Zhang, Xiaozhu Sun, Zaihang Zheng, Long Zhang
, Nanoscale center-hollowed hexagon MnCo2O4 spinel catalyzed aerobic oxidation of
5-hydroxymethylfurfural to 2,5-furandicarboxylic acid. The address for the corresponding
author was captured as affiliation for all authors. Please check if appropriate.
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ACCEPTED MANUSCRIPT
Nanoscale center-hollowed hexagon MnCo2O4 spinel catalyzed aerobic oxidation of
5-hydroxymethylfurfural to 2,5-furandicarboxylic acid
Shuang Zhanga,b, Xiaozhu Sunb, Zaihang Zhenga, Long Zhanga,* zhanglongzhl@163.com
aSchool of Chemical Engineering, Changchun University of Technology, No. 2055 Yanan Street, Changchun
130012, P.R. China
bInstitute of Petrochemical Technology, Jilin Institute of Chemical Technology, No. 45 Chengde Street, Jilin
132022, P.R. China
*Corresponding author.

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Abstract
A series of bimetallic Mn-Co-O catalysts were synthesized by a simple hydrothermal
method, and the catalytic performance was evaluated. Among these catalysts,
center-hollowed hexagon MnCo2O4 exhibited excellent catalytic effect in aerobic
oxidation of 5-hydroxymethylfurfural (5-HMF) to 2,5-furandicarboxylic acid (FDCA).
A 99.5% 5-HMF conversion and 70.9% FDCA yield can be obtained under molecular
oxygen and weak base condition. The efficient catalytic performance was attributed to
the Mn3+ ions on the surface of MnCo2O4 catalyst, and its high oxygen mobility and
reducibility. Furthermore, this simple synthesis process of non-precious metal catalyst
is beneficial to the production of FDCA.
Keywords: 5-hydroxymethylfurfural, 2,5-furandicarboxylic acid, non-noble metal,
oxidation

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1. Introduction
Biomass is ubiquitously available in nature, and can replace non-renewable fossil
resources for the production of renewable value-added chemicals [1]. Therefore,
massive efforts has been done to the conversion of biomass into renewable chemical
raw materials to reduce the overuse of fossil resources [2]. The biodegradable
polyethylene furanoate (PEF) has similar chemical structure and properties with
nondegradable polyethylene terephthalate (PET), which can be applied in the plastics
industry for the production of films and bottles [3]. PEF has excellent performance
compared with PET, such as good thermal stability, low-temperature formability and
excellent gas barrier properties [4,5]. A 100% bio-based 2,5-furandicarboxylic acid
(FDCA) as an crucial monomer for PEF production, could be obtained by the
oxidation of bio-based 5-hydroxymethylfurfural (5-HMF) [6].
Thus, extensive research was conducted to synthesize FDCA for relieving the
excessive pressure on the resources and environment [7]. Even though most of noble
metal catalysts, such as Au [8], Pt [9], Pd [10], Ru [11] and their alloys [12], showed
excellent performance for the oxidation of 5-HMF, the expensive prices imposed a
restriction for the further industrial production. In order to reduce the cost of the
catalyst, homogeneous Co/Mn/Zr/Br [13], [Co(OAc)2/Zn(OAc)2/NaBr] [14] and
[Co(OAc)2/Mn(OAc)2/NaBr] [15] non-noble metal catalysts were applied for the
oxidation of 5-HMF. Because homogeneous catalysts is difficult to reuse, the
oxidation of 5-HMF by heterogeneous non-noble metal catalysts become an attractive
green route. Recently, it was reported that 5-HMF was oxidized using Fe3O4-CoOx

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catalyst or Merrifield resin-Co-Py catalyst, t-BuOOH as oxidant, DMSO or
acetonitrile as solvent, the yield of FDCA could reach 68.6% and 90.4%,
respectively[16,17]. Another research group [18] reported MnxFey mixed oxide as
catalyst in the presence of NaOH for direct oxidation of 5-HMF, while the yield of
FDCA was low. Others have showed Co-Mn modified nanospherical mesoporous
silica catalyst with t-BuOOH as oxidant, 100% 5-HMF conversion could be achieved
with FDCA yield of 72.4% [19]. Some improvements have been reported in the
literature, for instance, MnO2 [4] and MnOx-CeO2 composite oxides [20] were also
applied for the oxidation of 5-HMF, and exhibited favorable catalytic performance in
the weak base condition. In addition, most heterogeneous non-noble metal catalytic
systems often employ harmful oxidant, additive or solvent.
The aim of the present work was preparation of environment-friendly and simple
green catalytic system to efficiently convert 5-HMF into FDCA over heterogeneous
non-noble metal catalysts. In this paper, a series of novel， inexpensive and
heterogeneous Mn-Co-O catalysts were prepared with different Mn/Co molar ratios
by simple hydrothermal method, and applied for the production of FDCA using green
molecular oxygen under weak base condition. The activity of the catalysts for the
oxidation of 5-HMF were investigated. Furthermore, XRD, Raman, N2 physisorption,
H2-TPR, XPS and TEM were used to investigate the nature of catalysts. To the best of
our knowledge, the use of Mn-Co-O bimetallic oxides as catalyst for the aerobic
oxidation of 5-HMF into FDCA has not been reported previously.
2. Experimental Section

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2.1. Catalyst preparation
A series of Mn-Co-O bimetallic oxides were prepared by hydrothermal method.
First, a total of 6 mmol Manganese(II) nitrate aqueous solution and cobalt(II) nitrate
hexahydrate with an appropriate Mn/Co molar ratio (Mn:Co= 1:0; 3:1; 2:1; 1:1; 1:2;
1:3; 0:1) were dissolved in 60 mL distilled water. Subsequently, the NaOH solution
was slowly dropped with vigorous stirring until pH was approximate to 10. The mixed
solution was vigorously stirred for 10 min at room temperature and loaded in
Teflon-lined stainless steel autoclaves. After treated at 180 °C for 4 h, the samples
were washed with distilled water until neutral before drying at 60 °C for 12 h. The
obtained precursor was calcined in a furnace at 400 °C in air for 4 h, followed by
cooling naturally to room temperature. Thus, the Mn-Co-O bimetallic oxides were
prepared.
2.2. 5-HMF oxidation
The oxidation of 5-HMF was studied in a stainless-steel reactor equipped with
magnetic stirring and electrical heating device. In a typical experiment, 0.2 mmol
5-HMF and 0.06 g KHCO3 were dissolved in 5 ml distilled water and added into the
reactor together with 0.1 g catalyst and a magnetic rotor. The reactor was sealed and
filled with 1 MPa O2, and heated to a desired reaction temperature. After a specific
reaction time, the autoclave was naturally cooled down to room temperature, and the
catalyst filtered off prior to the analysis. The reaction mixtures were analyzed by high
performance liquid chromatography (HPLC, Agilent 1260 HPLC system) equipped
with a UV detector and a extend-C18 column at 30 °C. The mobile phase was 0.1 wt%

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formic acid solution and acetonitrile (v/v =70:30) at a flow rate of 1.0 mL/min. The
detection wavelengths of 5-HMF and FDCA were 284 and 264 nm, respectively. The
retention time of 5-HMF and FDCA were 2.6 and 3.0 min, respectively.
3. Results and Discussion
Fig.1. XRD patterns (a) and partial enlarged XRD patterns (b) of Mn3O4, Co3O4 and Mn-Co
bimetallic oxides with different Mn/Co molar ratios.
In order to confirm the crystal structure of catalysts, XRD patterns of Mn3O4,
Co3O4, and varying proportions of Mn-Co bimetallic oxides are presented in Fig.1(a).
The characteristic peaks of Mn3O4 appears in 29.1, 31.1, 32.6, 36.2, 38.4, 44.5, 51.2,
58.6, 60.1 and 64.7º, which can be indexed to (112), (200), (103), (211), (004), (220),
(105), (321), (224) and (400) lattice planes of the tetragonal Mn3O4 (JCPDS 24-0734)
[21]. And the diffraction peak of MnOOH appears in 26.6 º indicated that Mn3O4
sample contains a small amount of MnOOH [4]. The diffraction peaks of Mn3-Co1-O,
Mn2-Co1-O and Mn1-Co1-O was similar to that of Mn3O4, but gradually became
broader and weaker with the decrease of Mn ions [20]. The characteristic peaks of
Co3O4 appears in 31.4, 36.9, 38.8, 44.9, 55.8, 59.5 and 65.4º matched with (220),

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(311), (222), (400), (422), (511) and (440) crystallographic planes of the cubic phase
Co3O4 (JCPDS 74–1656) [22]. The diffraction peaks of the Mn1-Co3-O were similar
to that of Co3O4. However, compared to Co3O4, the peaks of Mn1-Co2-O became
broader and weaker. Partial enlarged XRD patterns clearly indicated that the
crystallinity of binary oxide was lower than that of the single metal oxide. Moreover,
it is noteworthy that the values of Mn1-Co2-O at 30.8, 36.4, 44.8, 59.3 and 65.2º
could be indexed to (220), (311), (400), (511) and (440) lattice planes of MnCo2O4
with cubic spinel structure (JCPDS 23-1237) [11]. According to JCPDS for
(Co/Mn)2+(Co/Mn)23+O4, manganese and cobalt ions are adopt mixed valence states of
+3 and +2 in the MnCo2O4 spinel structure, in which Mn3+ and Co3+ distributed over
the octahedral sites, while Mn2+ and Co2+ are located at tetrahedral stacking interstices
[23], which is consistent with XPS results as discussed Fig.S2.(b, c).
Fig.2. Raman spectra of Mn3O4, Co3O4 and Mn-Co bimetallic oxides with different Mn/Co molar
ratios.
As shown in Fig.2, the Raman spectrum of Mn3O4 indicated the characteristic
-1
diffraction peaks at 279, 341 and 631 cm , which were in good agreement with the

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hausmannite structure of Mn3O4 [24]. The Raman spectrum of Co3O4 indicated the
-1
characteristic diffraction peaks at 184, 458, 504, 604 and 653 cm ,and it can be
related to cubic phase of Co3O4 [25]. For Mn3-Co1-O, Mn2-Co1-O and Mn1-Co1-O,
the main bands of Mn3O4 become weaker and shifted to lower wavenumbers. For
-1
Mn1-Co2-O, the peaks located at 175, 447, 494, 593 and 641 cm are attributed to the
Raman-active modes (A1g, Eg, and 3F2g) of Co3O4, and the main bands of Co3O4
became weaker and shifted to lower wavenumbers [23]. These phenomenon can be
ascribed to the change in the surface strain or defects owing to the incorporation of
Mn into the Co3O4 crystal structure. Since the defects and strains in the crystal
structure can result in the shifts in Raman peaks [26], which was in accordance with
the XRD results that a new phase state is formed.
Table 1 The catalytic activity of different catalysts for 5-HMF oxidation.[a]
Mn/Co
3/1
Mn/Co
2/1
Mn/Co
1/1
Mn/Co
1/2
Mn/Co
1/3
[b]
a
Blank
-
Mn3O4
24.4
Co3O4
30.2
18.4
1.5
Surface area
(m2 /g)
55.5
95.7
25.3
60.3
98.9
45.0
51.1
99.0
57.2
62.4
99.5
70.9
79.3
95.5
40.9
Conversion
(%)
19.6
1.3
88.3
FDCA Yield
(%)
15.6
[a] 0.2 mmol 5-HMF, 0.1 g catalyst, 0.06 g KHCO3, 5 ml H2O, 1MPa O2, 100 °C, 24 h; [b] no
catalyst
Fig. S1 shows N2 adsorption/desorption isotherm curve of Mn3O4, Co3O4 and
Mn-Co bimetallic oxides with different Mn/Co molar ratios. It can be seen that all the
catalysts belong to type-IV isotherm and typical H2-hysteresis loop appears in relative
pressure (P/P0) range of 0.83-1.0. And the specific surface area of samples calculated
by BET method is the range of 24.4-79.3 m2/g (Table 1). The specific surface area of

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bimetallic oxide was larger than that of the single metal oxide. It may be due to the
lower crystallinity of binary oxide [20]. The specific surface area of 1Mn-3Co-O
(79.3 m2/g) was larger than that of MnCo2O4 (62.4 m2/g), but the FDCA yield of
1Mn-3Co-O was less than that of MnCo2O4. Thus, the specific surface area of
catalysts was not the main factor affecting the catalytic activity.
To determine the optimum catalyst for the oxidation of 5-HMF to FDCA, a series
of tests were carried out over Mn3O4, Co3O4 and Mn-Co bimetallic oxides with
different Mn/Co molar ratios, and results are listed in Table 1. It was observed from
the Table 1, only 19.6% 5-HMF conversion and 1.3% FDCA yield can be obtained
without catalyst. When Mn3O4 was used as catalyst, the conversion of 5-HMF was
88.3% and the yield of FDCA was 15.6%. When Co3O4 was used as catalyst, the
conversion of 5-HMF was 18.4% and the yield of FDCA was only 1.5%. However, it
can be found that 5-HMF conversion and FDCA yield for binary oxide was higher
than that of single metal oxide. Among bimetallic oxides, MnCo2O4 showed optimal
catalytic activity on the selective oxidation of 5-HMF into FDCA, 99.5% 5-HMF
conversion and 70.9% FDCA yield were obtained. Furthermore, the catalytic activity
of MnCo2O4 was compared with other heterogeneous non-noble metal catalysts, and
the results are shown in Table S1.

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Fig.3. H2-TPR of Mn3O4, Co3O4 and Mn-Co bimetallic oxides with different Mn/Co molar ratios.
The relative reducibility of Mn3O4, Co3O4 and Mn-Co bimetallic oxides with
different Mn/Co molar ratios were investigated by H2-TPR as shown in Fig. 3. The
H2-TPR profiles of Mn3O4 shows that two significant reduction peaks at 300 °C and
420 °C, respectively. The first peak can be attributed to the reduction of MnOOH to
Mn3O4, and the second peak can be attributed to the further reduction of Mn3O4 to
MnO [27]. The H2-TPR profile of Co3O4 possessed two adjacent reduction peaks at
296 °C and 372 °C. The first reduction peak was assigned to the reduction of Co3O4 to
CoO, whereas the high temperature peak represents the further reduction of CoO to
Co [28]. Furthermore, the reduction peaks in bimetallic oxides are shifted to a lower
temperature compared with Mn3O4 and Co3O4, implying higher oxygen mobility in
the Mn-Co bimetallic oxides [29]. Importantly, MnCo2O4 can be obtained the lowest
reduction peaks (231°C) among these catalysts, which reveals it have highest oxygen
mobility and reducibility. Therefore, the oxygen mobility and reducibility are the
important factor on catalytic activity of Mn-Co bimetallic oxides.

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The XPS spectra of MnCo2O4 is shown in Fig. S2. The survey spectra is shown in
Fig. S2 (a), it gives signals mainly associated with Co 3p, O 1s, Mn2p and Co 2p
species, indicating that MnCo2O4 was composed of Co, O and Mn elements. As
shown in Fig. S2 (b), the binding energy of Co 2p3/2 and Co 2p1/2 are located at
780.1 eV and 796.1 eV, respectively, which are attributed to the coexistence of Co(II)
and Co(III) [11]. In addition, two peaks of Mn 2p3/2 and Mn 2p1/2 are located at
641.9 eV and 653.7 eV in Fig. S2 (c) are responsible for the Mn(II) and Mn(III) [11].
In order to further determine the oxidation states of Mn ions, we can deconvolute the
Mn 2p peak into four peaks by refined fitting. The peaks at 641.7 eV and 653.2 eV
can be assigned to the presence of Mn(II), while the other two peaks at 643.5 and
654.9 eV are characteristic of the Mn(III) [30]. According to literature [26], surface
oxygen species have high mobility, and easier to be activated. So, the types of surface
oxygen species of MnCo2O4 were also identified by the O 1s XPS spectra as
displayed in Fig. S2 (d). The O 1s curves of MnCo2O4 can be fitted into three peaks at
530.2 eV, 531.3 eV and 532.4 eV, denoted as OI, OII and OIII respectively. The main
peak OI was associated with lattice oxygen (O2-); a shoulder peak OII was assigned to
oxygen defects or surface adsorbed oxygen (O22- and O-), another shoulder peak OIII
was likely to be ascribed to adsorbed molecular water. The 5-HMF oxidation to
FDCA may be proceed via the Mn3+ /Mn2+ redox cycles of MnCo2O4 catalyst. Firstly,
Mn2+ was oxidized by O2 to Mn3+ forming Mn3+O2- species, and the Mn3+O2- could
oxidize 5-HMF to FDCA while reduced to Mn2+, which could be reoxidized by O2.
Thus, Mn3+ is the active site of MnCo2O4 catalyst.

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TEM images of Mn3O4, Co3O4 and Mn-Co bimetallic oxides are shown in Fig S3.
In addition to the irregular Mn3O4 and Mn3-Co1-O, Co3O4 and Mn2-Co1-O are close
rectangle, Mn1-Co1-O forms a solid hexagonal structure of about 100 nm. When
Mn/Co molar ratios is 1/2, it was observed that MnCo2O4 is about 100 nm
hexagon with a hole at the centre. And this structure was broken along with the
reduction of manganese content. It can be seen the morphology of catalyst can be
changed by controlling the molar ratio of manganese and cobalt.
4. Conclusions
Mn-Co bimetallic oxides with different Mn/Co molar ratios have been synthesized
via simple hydrothermal method, and a nanoscale hexagon MnCo2O4 with a hole at
the centre was formed when Mn/Co molar ratios was 1/2. MnCo2O4 was proved to
have high activity for the aerobic oxidation of 5-HMF to FDCA, and can be obtained
99.5% the conversion of 5-HMF and 70.9% the yield of FDCA under optimal reaction
conditions, which was significantly better than Mn3O4, Co3O4 and other bimetallic
oxides. XPS analysis identified that Mn3+ is the active site of MnCo2O4 catalyst.
MnCo2O4 has the lowest reduction peaks illustrated by the H2-TPR, which can
improve oxygen mobility of catalyst and benefit the oxidation of 5-HMF. This work
provided a green and facile method for developing the aerobic oxidation of 5-HMF to
FDCA using non-precious metal catalysts.
Acknowledgements
This work was supported by Jilin Provincial Scientific and Technologic
Development Project (No. 20140309016GX) and Science and Technology Project of

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the Education Department of Jilin Province (No. JJKH20170552KJ).

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Graphical abstract

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Highlights
• Nanoscale center-hollowed hexagon MnCo2O4 was synthesized by a simple
hydrothermal method.
• An 70.9% yield of FDCA was obtained under mild conditions.
• This work provides a green and economical route for FDCA production.