Highly efficient and selective oxidation of 5-hydroxymethylfurfural by molecular oxygen in the presence of Cu-MnO2 catalyst

Article abstract of DOI:10.1016/j.catcom.2016.11.024

The selective oxidation of 5-hydroxymethylfurfural (HMF) to produce value-added chemicals is a hot topic in the utilization of biomass, in which the development of heterogeneous catalyst and the use of O₂ as terminal oxidant is desired. In this work, a series of metal ion-doped MnO₂ materials are successfully prepared using liquid precipitation method, and employed as catalysts for the aerobic oxidation of HMF in the alcoholic solvents. As a result, it is found that 86.0% conversion of HMF and 96.1% selectivity of 2,5-diformylfuran (DFF) was obtained under suitable conditions. Moreover, the Cu-MnO₂ catalyst can keep a high activity after being recycled five times.

Full text of DOI:10.1016/j.catcom.2016.11.024

Accepted Manuscript
Highly
efficient
and
selective
oxidation
of
5-hydroxymethylfurfural by molecular oxygen in the presence of
Cu-MnO2 catalyst
Xinli Tong, Linhao Yu, Hui Chen, Xuli Zhuang, Shengyun Liao,
Hongge Cui
PII:
DOI:
Reference:
S1566-7367(16)30441-1
doi: 10.1016/j.catcom.2016.11.024
CATCOM 4865
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
27 July 2016
24 November 2016
29 November 2016
Please cite this article as: Xinli Tong, Linhao Yu, Hui Chen, Xuli Zhuang, Shengyun
Liao, Hongge Cui , Highly efficient and selective oxidation of 5-hydroxymethylfurfural by
molecular oxygen in the presence of Cu-MnO2 catalyst. The address for the corresponding
author was captured as affiliation for all authors. Please check if appropriate.
Catcom(2016), doi: 10.1016/j.catcom.2016.11.024
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ACCEPTED MANUSCRIPT
Highly Efficient and Selective Oxidation of 5-Hydroxymethylfurfural by
Molecular Oxygen in the Presence of Cu-MnO2 Catalyst
Xinli Tong*, Linhao Yu, Hui Chen, Xuli Zhuang, Shengyun Liao* and Hongge Cui
Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, School of Chemistry
and Chemical Engineering, Tianjin University of Technology, Tianjin 300384, P. R. China;
Tel.: +86-22-60214259; Fax: +86-22-60214259;
E-mail address: tongxinli@tju.edu.cn; tongxli@sohu.com (X. T.) or mengyunliao@sina.com (S. L.).
Abstract
The selective oxidation of 5-hydroxymethylfurfural (HMF) to produce value-added chemicals is a
hot topic in the utilization of biomass, in which the development of heterogeneous catalyst and the
use of O2 as terminal oxidant is desired. In this work, a series of metal ion-doped MnO2 materials are
successfully prepared using liquid precipitation method, and employed as catalysts for the aerobic
oxidation of HMF in the alcoholic solvents. As a result, it is found that 86.0% conversion of HMF
and 96.1% selectivity of 2, 5-formyl furan (DFF) was obtained under suitable conditions. Moreover,
the Cu-MnO2 catalyst can keep a high activity after being recycled five times.
Keywords: Oxidation; 5-hydroxymethylfurfural; 2, 5-diformylfuran; Manganese oxide; Biomass
transformation
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1. Introduction
Along with the diminishing fossil fuels and the increasing environmental pollution, the efficient
utilization of the renewable biomass has become an attractive hot topic, in which the catalytic
synthesis of fine chemicals and liquid fuels from biomass feedstock is being broadly studied [1-4].
With the consideration of downstream chemical processing, the catalytic transformation of
5-hydroxymethl furfural (HMF), a famous platform compound, to the value-added chemical is
important in both science and commerce. Thus, much effort has been devoted to the oxidation of
HMF to 2, 5-diformyl furan (DFF), 5-formyl-2-furancarboxylic acid (FFCA) and 2,
5-furandicarboxylic acid (FDCA), which can be used for the production of drugs, fungicides, and
new functional polymers [5-7]. Thereinto, the oxidation of HMF to DFF is facing to a challenge
owing to the poor selectivity and severe reaction conditions. Although the direct oxidation of HMF
can be successfully performed with stoichiometric oxidants or the assistance of electrophilic agents
[8, 9], the catalytic aerobic oxidation of HMF to prepare DFF is more valuable in the practical
application from the viewpoints of economy and sustainability.
Recently, the use of molecular oxygen or air as oxidant was extensively investigated. In the
homogeneous catalytic process, the catalytic aerobic oxidation of HMF using a mixed Co/Mn/Zr/Br
system has been performed in which a 57% yield of DFF was attained [10]. Ma et al. [11]
investigated that the aerobic oxidation of HMF with the Cu(NO3)2/VOSO4 catalytic system and a
98% yield of DFF was obtained in acetonitrile solvent. Besides, our group also found that the
homogeneous catalytic oxidation of HMF to produce DFF could be achieved using CuI and
1-hydroxybenzotriazole as the catalysts, in which a 93.2% conversion and 99.0% selectivity of DFF
was obtained [12]. However, all these systems have the obvious disadvantage on the separation of
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product and the recycling of catalyst. So, developing the heterogeneous and efficient solid catalyst
for the oxidation of HMF remains an issue of scientific interest at present. In early work, Verdeguer
et al. found that a 19% yield of DFF was obtained under a high temperature and neutral pH when
Pt/C catalyst was used in the aerobic oxidation of HMF [13]. Moreover, Navarro et al. [14] reported
a 50% conversion and 98% selectivity of DFF was obtained for the aerobic oxidation of HMF with
vanadyl complexes supported SBA-15 catalyst. Sádaba et al. [15] detected the performance of
zeolite-supported vanadia catalysts for the aerobic oxidation of HMF, in which a 99% selectivity of
DFF at 84% conversion was obtained. Antonyraj et al. [16] studied the oxidation of HMF with
V2O5/C catalyst, and a 95% conversion in a 96% selectivity of DFF was attained. When
polyanline-grafted vanadyl acetylacetonate was used as catalyst for the aerobic oxidation of HMF, a
99.2% conversion and 86.2% yield of DFF was obtained at 110 ºC for 12 h [17]. In addition, Nie et
al. [18] reported that the Ru/C catalyst could efficiently promote the oxidation of HMF to prepare
DFF with a high yield of about 96% at 110 ºC in toluene. Chen’s group [19] studied the oxidation of
HMF with Ru-PVP/CNT catalyst and a 94% yield of DFF with a full HMF conversion was obtained
under the suitable conditions. Furthermore, the lipophilic nano-ruthenium catalyst was used to the
oxidation of HMF, in which a 94% conversion and 89% selectivity of DFF was attained at 105 ºC for
8 h in methyl isobutyl ketone solvent [20]. Besides, the aerobic oxidation of HMF to DFF was also
investigated with manganese oxide octahedral molecular sieve (OMS-2) as the catalyst in which a
high yield of 97.2% was attained under 0.5 MPa of O2 in N, N-dimethyl formamide (DMF) solvent
[21]. Moreover, the Ag-OMS-2 was found to exhibit the high catalytic activity in the aerobic
oxidation of HMF where a 99% yield of DFF was achieved [22]. Recently, Zhu et al. [23, 24] found
that the Au/MnO2 or Au-Pd/MnO2 efficiently catalyzed the oxidation of HMF, in which about 82%
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conversion of HMF and 99.0% selectivity of DFF was obtained under the optimal conditions. The
above numerous heterogeneous catalysts show good catalytic performance in the aerobic oxidation
of HMF in organic solvents; however, the preparation of catalyst was complicate and expensive
which is disadvantageous for the application.
Very recently, due to its easy preparation and high electrocatalytic activity, metal ion-doped
MnO2 catalyst has been extensively studied [25]. In this work, considering the probable synergistic
effect of Cu and MnO2, we employ the Cu-doped MnO2 as heterogeneous catalyst for the oxidation
of HMF with molecular oxygen as oxidant. It is found that 86.0% conversion and 96.1% selectivity
of DFF was obtained in the presence of Cu(4.8 mol%)-MnO2 catalyst at 140 ºC for 5 h.
2. Experimental
2.1 The preparation of nano-structured MnO2 catalyst
The aqueous solution (100 mL) of KMnO4 (5.89 g) was added into an aqueous solution (30 mL)
of MnSO4·H2O (8.8 g) and concentrated HNO3 (3 mL). Then, the resulting mixture was refluxed at
100 °C for 24 h. Finally, the dark brown solid formed was filtered off, washed with a large amount
of water, and dried under open air at 150 °C, affording 7-8 g of nano MnO2 material.
2. 2 The preparation of metal ion doped MnO2 catalysts
For the preparation of Cu-doped MnO2 catalyst: firstly, an aqueous solution (100 mL) of KMnO4
(5.89 g) was added into an aqueous solution which contains MnSO4·H2O (8.8 g), a certain amount of
CuSO4 and concentrated HNO3 (3 mL). Then, the resulting mixture was refluxed at 100 °C for 24 h.
Finally, the dark brown solid formed was filtered off, washed with a large amount of water, and dried
under open air at 150 °C, affording Cu-doped MnO2 material which is signified by Cu (x
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mol%)-MnO2 (x mol% refers the molar ratio of Cu2+ and MnO2; if x mol%= 2.4 mol%, x mol% can
be left out). Similarly, Cr-MnO2, V-MnO2, Mo-MnO2, Mg-MnO2, Ca-MnO2 or Al-MnO2 catalysts
are prepared through Cr(NO3)3, NaVO3, K2MoO4, Mg(NO3)2, Ca(NO3)2 and Al(NO3)3 replacing
CuSO4 as the doping metal salts.
2.3 General procedure for the aerobic oxidation of 5-hydroxymethylfurfural (HMF)
All oxidation experiments are performed in a 120 mL autoclave equipped with the magnetic stirring
and automatic temperature control. A typical procedure for the oxidation of HMF is as follows: a
methanol (15 mL) solution of HMF (0.252 g, 2.0 mmol) and Cu-MnO2 catalyst (0.05g) is charged
into the reactor, and the atmosphere inside is replaced with the pure oxygen after the reactor is sealed.
Under stirring, oxygen is charged to 0.3 MPa at room temperature and the autoclave is preheated to
140 ºC, and then kept for 5 h. After reaction, the autoclave was cooled and the obtained mixture is
analyzed by HPLC and GC-MS instruments after the excess gas is purged (the detection of product is
presented in 1.2 and Figure S1 of supporting information).
3. Results and Discussion
3.1 TEM images
Figure 1. TEM images of different catalytic materials [a. MnO2; b and c. Cu (4.8mol%)-MnO2].
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In order to reveal the structure and morphology of catalytic materials, the TEM characterization
was performed. As shown in Figure 1a, the pure MnO2 material possesses a fibrous morphology. The
average length and width of the fibers were 500 ± 100 nm and 20 ± 5 nm, respectively. While, the
fibrous morphology of Cu (4.8 mol%)-MnO2 material is more regular which trends to the formation
of nanowire structure (Figures 1b and 1c). Correspondingly, the average length and width of the
fibers were 90 ± 50 nm and 25 ± 2 nm, respectively. These results indicate that the structure and
morphology becomes well-defined after being doped by 4.8 mol% Cu2+ ion. Moreover, it can be
inferred that the substitutional doping of Cu ion into the MnO2 lattice, combined with the results of
the XRD detection (Figure S2), which is consistent with the conclusion of the reference [25].
3.2 SEM images
In the following, the catalytic materials are characterized by the SEM technique. Figure S3 shows
the SEM images of MnO2, Cu(2.4 mol%)-MnO2, Cu(4.8 mol%)-MnO2,Cu(6.0 mol%)-MnO2, Cu(7.2
mol%)-MnO2 and Cu(9.6 mol%)-MnO2, respectively. As a result, it is found that there exists the
irregular surface in the obtained pure MnO2 material. Along with the increase of doping amount of
Cu2+ ion from 2.4 mol% to 9.6 mol%, the material surfaces become more and more regular in which
the surface of Cu(4.8 mol%)-MnO2 is loose and similar to nanowire. However, when the doping Cu
amount is larger and larger, the material surface is tighter and tighter although the formation of more
nanowires was achieved. These phenomena also confirm the relationship of the surface area of
materials and the doping Cu amount.
3.3 The oxidation of 5-hydroxymethylfurfural (HMF) with various catalysts
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Scheme 1. The reaction equation for selective oxidation of HMF with molecular oxygen
Table 1. The oxidation of HMF to DFF with different catalysts [a].
Entry Catalysts Conv.(%)[b]
Product distribution (%) [b]
Others
1
2
(1+2)
3
4
1
2
3
4
5
6
7
8
MnO2
57.9
74.8
77.6
67.9
56.1
54.5
51.5
39.5
51.9 18.3 70.2 4.3 14.6 10.9
50.9 17.6 68.5 1.0 18.7 11.8
30.7 20.1 53.8 6.8 21.3 18.1
45.8 12.3 58.1 1.2 26.5 14.2
Cu-MnO2
Cr-MnO2
V-MnO2
Mo-MnO2
Mg-MnO2
Ca-MnO2
Al-MnO2
46.4 16.2 63.6 7.1 24.6
4.7
40.3 8.9 49.2 5.9 19.8 25.1
45.7 12.5 58.2 4.6 16.7 20.5
43.5 13.6 57.1 7.4 26.8
8.7
[a] Reaction conditions: HMF 0.2520 g (2 mmol), the catalyst 0.05 g, in 15 mL of methanol, under 0.3 MPa of O2,
time 5 h, temperature 140 ºC. [b] The results are obtained by GC and HPLC with the internal standard technique.
Initially, the oxidation of HMF, as shown in Scheme 1, is performed in methanol solvent under 0.3
Mpa of O2 using different metal ion-doped maganease oxides as catalysts. The obtained data are
summarized in Table 1. It is found that the conversion of HMF and the selectivity of 1 was
respectively 57.9% and 70.2% with the MnO2 catalyst at 140 ºC for 5 h (entry 1). While, the
conversion was elevated to 74.8%, 77.6% or 67.9% when Cu-MnO2, Cr-MnO2 or V-MnO2 were
used as the catalysts, respectively (entries 2, 3 and 4), in which the selectivity of 1 was 68.5%, 53.8%
or 51.8%, respectively. Moreover, it is also found that the selectivity of 3 was respectively increased
to 18.7%, 21.3% or 26.5% from the original 14.6%. Furthermore, the catalytic performance of
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Mo-MnO2, Mg-MnO2 and Ca-MnO2 were investigated, and the conversion of HMF was respectively
56.1%, 54.5% and 51.5%, in which the selectivity of 1 was 63.6%, 49.2% and 58.3%, respectively.
Besides, a only 39.5% conversion of HMF and 57.1% selectivity of 1 was obtained in the presence of
Al-MnO2 catalyst. These data exhibit that the doping of Cu2+, Cr3+, and V3+ may efficiently promote
the aerobic oxidation of HMF in which the Cu-MnO2 catalyst is more suitable for the production of 1
in methanol solvent (the oxidation states of Mn, Cu, Cr and V have been confirmed by the
representive XPS data in the Figure S4 of supporting information). It can be attributed that the
substitutional doping of Cu ion into the MnO2 lattice is beneficial to the exchange of Mn4+ and M3+
or the shift of lattice oxygen. Besides, related to the characterization of different catalysts, it can be
seen that the activity of catalytic materials is closely associated with the BET surface area (the data
are given in Table S1 in the supporting information). Therein, the doping of Cu2+, Cr3+, and V5+ lead
to the surface area of MnO2 increasing from 88.9 m2·g-1 to 100.9, 102.5 and 99.3 m2·g-1, respectively.
3.4 The effect of reaction medium in the aerobic oxidation of HMF with Cu-MnO2 catalyst
Table 2. Oxidation of HMF with Cu-MnO2 catalyst in different solvents [a].
Conversion
(%)[b]
72.8
Product distribution (%) [b]
Entry
Solvents
1[c]
3
Others
7.4
1
2
3
4
5
6
7
8
9
ethanol
n-propanol
n-butanol
89.1
74.5
73.2
62.1
72.5
82.1
64.2
68.3
79.8
3.5
71.6
16.0
14.3
11.7
8.3
9.5
70.9
12.5
26.2
19.2
10.7
10.3
13.2
10.3
1-pentanol
toluene
62.6
50.3
benzonitrile
water
18.5
7.2
30.2
25.5
18.5
9.9
dichloromethane
acetonitrile
42.1
26.7
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[a] Reaction conditions: HMF 0.2520 g (2 mmol), the Cu-MnO2 catalyst 0.05 g, in 15 mL of solvent, under 0.3
MPa of O2, reaction time 5 h, temperature 140 ºC. [b] The results are obtained by GC and HPLC with internal
standard technique. [c] The selectivity of 1 includes the partial acetal compound.
The effects of reaction media on the selective oxidation of HMF were studied in the presence of
the Cu-MnO2 catalyst. As shown in Table 2, it is found that 72.8% and 71.6% conversions as well as
89.1% and 74.5% selectivities of 1 were respectively obtained in ethanol and n-propanol. Besides,
the conversion of HMF was respectively 70.9% and 62.6% 5 in n-butanol and 1-pentanol solvent.
Furthermore, several organic solvents including toluene, benzonitrile, water, CH2Cl2 and CH3CN
have also been employed in the aerobic oxidation of HMF with Cu-MnO2 catalyst. As a result, a
50.3%, 18.5% or 30.2% conversion of HMF was obtained in toluene, benzonitrile or water solvent.
In addition, a 42.1% and 26.7% conversion was obtained when CH2Cl2 and CH3CN were used as the
solvent. It can be seen that the ethanol or methanol as the solvent are superior to other solvents
(toluene, benzonitrile, water, CH2Cl2 and CH3CN, etc.), which is probably due to oxidation reaction
being more easily performed and the produced DFF is more stable in these alcoholic solvents.
Considering the efficiency and enconomy, ethanol is considered as the most promising solvent in
oxidation of HMF by molecular oxygen.
3.5 The influence of the doping copper (Cu) amount
Table 3. The effect of the doping Cu amount on the oxidation of HMF [a]
.
Conv.
(%)[b]
33.6
Product distribution (%) [b]
The doping
Cu amount
0 mol%
Entry
Acetal[c]
1+Acetal
90.9
Ester[d]
7.0
1
3
1
2
3
4
5
49.6
47.5
49.3
51.2
51.5
41.3
41.6
42.5
44.2
44.6
2.1
3.5
2.9
3.2
2.8
2.4 mol%
3.6 mol%
4.8 mol%
6.0 mol%
72.8
89.1
7.4
74.8
91.8
5.3
77.7
95.4
1.4
68.7
96.1
1.1
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6
7
7.2 mol%
9.6 mol%
48.7
34.6
53.2
53.5
45.1
44.3
98.3
97.8
0.9
1.2
0.8
1.0
[a] Reaction conditions: HMF 0.2520 g (2 mmol), the Cu(x mol%)-MnO2 catalyst 0.05 g, in 15 mL of ethanol, under 0.3 MPa of O2,
reaction time 5 h, temperature 140 ºC. [b] The results are obtained by GC and HPLC with internal standard technique. [c] The obtained
acetal refers to 5-(diethoxymethyl)furan-2-carbaldehyde. [d] The generated ester refers to ethyl 5-formylfuran-2-carboxylate.
In the following, the effect of doping Cu amount on the HMF oxidation was further studied in
ethanol. Seen from the Table 3, the doping amount of Cu2+ ion was changed from 2.4 mol% to 9.6
mol%. It is found that, along with the doping amount of Cu2+ being added from 0 mol% to 4.8 mol%,
the conversion of HMF was increased from 33.6% to 77.7% and the selectivity of 1 was also
elevated from 90.9% to 95.4%. However, when the doping amount of Cu2+ was further elevated to
6.0 mol%, 7.2 mol% and 9.6% mol%, the conversion of HMF was respectively declined to 68.7%,
48.7% and 34.6%, although the selectivity of 1 was still increased a little. Considering to the change
of surface area along with the increase of doping amount, in which the surface area of
Cu(2.4mol%)-MnO2, Cu(4.8mol%)-MnO2, Cu(6.0mol%)-MnO2, Cu(7.2mol%)-MnO2 and
Cu(9.6mol%)-MnO2 is 92.3, 100.9, 83.7, 66.1 and 32.7 m2·g-1, respectively, which further prove that
the surface area of these catalysts play a curial role on their catalytic activities. Accordingly, the best
catalyst is Cu(4.8mol%)-MnO2 material. It should be attributed to the cooperation effect of the
substitutional doping of Cu ion into the MnO2 lattice and the elevation of surface area.
3.6 The effect of the amount of catalyst and the recycling of catalyst
Figure S5 shows the effect of different amounts of catalyst on the oxidation of HMF. It is seen that
both the conversion and selectivity of 1 were gradually increased along with the amount of catalyst
being increased from 0.025 g to 0.100 g. However, the selectivity of HMF was almost kept
unchanged. The maximum data were 86.0% conversion and 96.1% selectivity of 1 where 0.100 g of
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Cu (4.8 mol%)-MnO2 was used. When the amount of catalyst was further increased to 0.15 g or more,
the selectivity of 1 was decreased appreciably although the conversion was further elevated. In
succession, the effects of reaction temperature and time were also studied and the obtained data are
indicated in Figure S6 and Figure S7 of the supporting information.
In addition, the recycling of Cu(4.8 mol%)-MnO2 catalyst was examined to study its stability.
After the oxidation reaction, the solid catalyst was separated, and washed with anhydrous ethanol,
and then dried at 80 ºC for 12 h before being reused in the next run. As shown in Figure S8, it was
seen that the conversion of HMF and the selectivity of 1 kept more than 75% even after the Cu(4.8
mol%)-MnO2 material being recycled five times. These results indicated that the catalyst is
recyclable and stable in the oxidation of HMF using dioxygen as oxidant.
4. Conclusion
In summary, we have developed a new selective and aerobic oxidation of HMF to produce DFF in
the presence of a heterogonous Cu-doped MnO2 catalyst. This process allows efficient and selective
oxidation of HMF under mild conditions, which will provide a promising aerobic oxidation
technique for the sustainable utilization of biomass-derived platform compounds.
Acknowledgements
This work is financially supported from the Natural Science Foundation of China (No. 21003093)
and the State Key Program of National Natural Science of China (No.21336008).
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Graphical Abstract
Highly efficient and selective oxidation of biomass-derived 5-hydroxymethylfurfural with molecular oxygen has
been successfully performed in the presence of a heterogeneous Cu-MnO2 catalyst.
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Highlights
 The efficient oxidation of 5-hydroxymethylfurfural is successfully performed.
 Nano-structured Cu-MnO2 is a preferable catalyst in ethanol solvent.
 An 82.6% yield of 2, 5-diformylfuran is obtained under mild conditions.
 The catalyst still keeps a high activity after being recycled for 5 times.
15