Porous cobalt@N-doped carbon derived from chitosan for oxidative esterification of 5-Hydroxymethylfurfural: The roles of zinc in the synthetic and catalytic process

Article abstract of DOI:10.1016/j.mcat.2019.110695

A newly developed, facile and sustainable self-sacrifice template strategy, in which zinc and chitosan were used as the sacrificial template and carbon source respectively, has been disclosed for the synthesis of a hollow Co-embedded in nitrogen-doped graphite (Co@CN) structure. This material exhibits excellent catalytic efficiency in the oxidative esterification of 5-hydroxymethylfurfural to 2,5-furandicarboxylicacid dimethyl ester that is a significant raw material for polymer synthesis. According to the experimental and calculation results, zinc as the self-sacrificial template and acid-base site regulator has significantly improved the performance of the catalyst. The high specific surface area owing to the partial evaporation of zinc and the optimization of basic and acid sites in the catalyst prove to be the main reasons for its high activity.

Full text of DOI:10.1016/j.mcat.2019.110695

MolecularCatalysisxxx(xxxx)xxxx
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Porous cobalt@N-doped carbon derived from chitosan for oxidative
esterification of 5-Hydroxymethylfurfural: The roles of zinc in the synthetic
and catalytic process
Yamei Lina, Guo-Ping Lua,b, Xin Zhaob, Xun Caob, Lele Yanga, Baojing Zhoua, Qin Zhonga,
Zhong Chenb,*
a School of Chemical Engineering, Nanjing University of Science & Technology, Xiaolingwei 200, Nanjing 210094, China
b School of Materials Science & Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
A R T I C L E I N F O
A B S T R A C T
Keywords:
Cobalt nanoparticles
Zinc
Porous N-doped carbon
Chitosan
Oxidative esterification of
hydroxymethylfurfural
A newly developed, facile and sustainable self-sacrifice template strategy, in which zinc and chitosan were used
as the sacrificial template and carbon source respectively, has been disclosed for the synthesis of a hollow Co-
embedded in nitrogen-doped graphite (Co@CN) structure. This material exhibits excellent catalytic efficiency in
the oxidative esterification of 5-hydroxymethylfurfural to 2,5-furandicarboxylicacid dimethyl ester that is a
significant raw material for polymer synthesis. According to the experimental and calculation results, zinc as the
self-sacrificial template and acid-base site regulator has significantly improved the performance of the catalyst.
The high specific surface area owing to the partial evaporation of zinc and the optimization of basic and acid
sites in the catalyst prove to be the main reasons for its high activity.
Introduction
(Au, Pd) with the help of high pressure or excess bases [20–26]. A few
earth-abundant-metal catalysts have also been explored, but high
The utilization of biomass to produce valuable chemicals and ma-
terials has become a key point in the modern sustainable industry due
to its renewability, safety and low cost [1–4]. Among the platform
chemicals derived from biomass, 5-hydroxymethylfurfural (HMF) ac-
cessible from six-carbons sugars and their polymers, has been used to
bridge the gap between lignocellulosic biomass and important bio-
chemicals [2,5,6]. The oxidation of HMF to 2,5-furandicarboxylic acid
(FDCA), one of the top-10 biobased products from biorefinery carbo-
hydrates, is one of the most significant reaction steps for the conversion
of HMF. FDCA can be used in the production of poly(ethylene-2,5-
furandicarboxylate) (PEF) [7]. However, there is still a dearth of
straightforward and ecofriendly methods for the purification of FDCA
owing to its poor solubility and high boiling point [5]. The production
of 2,5-furandicarboxylicacid dimethyl ester (FDMC) may eliminate the
issue due to its good solubility. Moreover, FDMC can also be used di-
rectly to synthesize polymers through transesterification reaction [8].
Although a large number of attempts have been developed for the
oxidation of HMF into FDCA, using methods such as homogeneous and
heterogeneous catalysis, [9–15] electrocatalysis [16–18] and biocata-
lysis [19], only a few reports have mentioned the oxidative esterifica-
tion of HMF. In general, this transformation is catalysed by noble metals
temperature and O2 pressure are needed, and the selectivity is still poor
in some cases (see more details in Table S1). [27–30] Meanwhile, most
of the previous reports on HMF oxidation took place in dilute solutions
(0.05–0.3 M) due to the highly reactive groups in HMF, which sig-
nificantly hampers both FDMC production on an industrial scale. Fur-
thermore, Co-embedded in nitrogen-doped graphite (Co@CN) prove to
be an efficient catalyst for base-free oxidation of alcohols to esters
[27,28,31]. Therefore, we envisage that a novel Co@CN might be sui-
table for the base-free oxidative esterification of HMF under mild
conditions with higher concentration of the substrate.
Shell biorefinery, referring to the fractionation of crustacean shells
into their major components and the transformation of these compo-
nents into value-added chemicals and materials, has attracted growing
attention in recent years [32–35]. One of these major compounds is
chitosan with lots of amine groups, making its binding affinity for
transition-metal ions higher than other biomass [36]. Thus, chitosan
should be an ideal precursor for the generation of metal@N-doped
carbon materials. More recently, several metal@N-doped carbon ma-
terials derived from chitosan and its derivatives have been developed
for electrocatalysis [37,38], organic synthesis [39–43] and pollutant
degradation [44]. Based on the low boil point of zinc, Jiang and his co-
⁎ Corresponding authors at: School of Materials Science & Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore.
E-mail addresses: glu@njust.edu.cn (G.-P. Lu), ASZChen@ntu.edu.sg (Z. Chen).
https://doi.org/10.1016/j.mcat.2019.110695
Received 22 August 2019; Received in revised form 17 October 2019; Accepted 20 October 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:YameiLin,etal.,MolecularCatalysis,https://doi.org/10.1016/j.mcat.2019.110695

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Fig. 1. The procedures for the preparation of Co@CN-Zn12-5.
workers have introduced a sacrificial template strategy for the synthesis
of porous Co@CN from bimetallic Co-Zn ZIFs [45]. Along this line, we
reasoned that both Co2+ and Zn2+ could coordinate with -NH2 or eOH
groups of chitosan to form CoZn@chitosan, which can then be pyr-
olyzed at a temperature (e.g., 900 °C) to allow evaporation or partial
evaporation of Zn to generate a porous Co@CN (illustrated in Fig. 1).
Ideally, the biomass conversion would be catalyzed by the materials
based on biomass too. Herein, we demonstrate the first example on the
synthesis of porous Co@CN derived from biomass for the oxidative
esterification of HMF. Zinc plays crucial roles both in the synthetic and
catalytic process. In the synthetic process, zinc as the self-sacrificed
template can achieve the formation of porous Co@CN with high spe-
cific surface area (up to 658 m2/g). Moreover, the residual zinc in the
catalyst can increase Lewis basic sites and decrease the strong Lewis
acidic sites. These features may enhance the activation the OeH bonds
[28] and αCH bonds of HMF [ee46], and improve the interaction
between catalyst and reactants [46].
heating rate and the ratio of Zn/Co have significant influences on the
BET surface areas, which affect the yield of FDMC (Fig. 2). In general,
increasing the amount of zinc results in larger BET surface area due to
the evaporation of Zn. More zinc remain in the material after pyrolysis
when the ratio of Zn/Co increases to 16 which results in lower BET
surface areas.
Meanwhile, a moderate heating rate is also necessary for guaran-
teeing the high BET surface area. The reasons may be explained that
low heating rate keep the liquid Zn at a longer time which is harmful to
form meso- or micro pores, and high heating rate may destroy the
structure of the carbon material. Higher BET surface area can enhance
the yield of FDMC owing to the improvement of the interaction between
catalyst and reactant. Compared with other combinations, Co@CN-
Zn12-5 (X = 15 and Y = 5) emerged as the optimum. Other cobalt
catalysts including Co@AC (active carbon), CoZn@AC, Co@MgO, Co@
ZrO2 and Co@CeO2 were employed as comparison, however the yields
of FDMC were poor in all cases (Table S2).
The Raman spectra (Fig. S3) shows that both Co@CN-Zn0-5 and
Co@CN-Zn12-5 have a considerable graphitization [47,48]. The gra-
phitization of Co@CN-Zn0-5 is better than Co@CN-Zn12-5, which
agrees with the powder XRD results (Fig. S4), and can explain that the
decomposition temperature of Co@CN-Zn0-5 is higher than Co@CN-
Zn12-5 (Fig. S5). The peaks at 44.2° and 51.5° can be seen in powder
XRD patterns of Co@CN-ZnX-5 (X = 0, 4, 12) (Fig. S4), corresponding
to the characteristic diffractions of (110), (200) lattice planes of me-
tallic Co [47]. The surface morphology of Co@CN-ZnX-5 were also
investigated by SEM (Fig. 3). As expected, the number of pores on the
surface of catalysts are increased with increasing the Zn/Co ratio. This
Results and discussion
Initially, the amount of zinc salts and heating rate during the pyr-
olysis were investigated to optimize the catalytic activity of Co@CN
using the oxidative esterification of HMF as the model reaction. The N2
absorption/desorption isotherms and pore size distribution curves of
various Co@CN-ZnX-Y (X for the weight ratio of Zn/Co, Y for the
heating rate of pyrolysis) catalysts are shown in Figs. S1, S2 respec-
tively. All of them are type IV patterns, which confirms these N-doped
carbon materials have a mesoporous structure [41]. Notably, both
Fig. 2. The relationship between yields (yellow
points) of FDMC (or BET surfaces areas (blue
square)) with (a) The heating rate of pyrolysis;
(b) the weight ratio of Zn/Co. Conditions: HMF
0.3 mmol, Co@CN 15 mol% of Co, MeOH 3 mL,
O2 1 atm, 50 °C, 24 h; GC yields with ni-
trobenzene as an internal standard. (For inter-
pretation of the references to colour in this
figure legend, the reader is referred to the web
version of this article).
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Fig. 3. SEM images of Co@CN-ZnX-Y. (a) Co@CN-Zn0-5; (b) Co@CN-Zn4-5; (c) Co@CN-Zn8-5; (d) Co@CN-Zn12-5; (e), (f) SEM mappings of Co@CN-Zn12-5.
observation from SEM is consistent with the BET results. Further SEM
energy dispersive X-ray spectroscopy (EDS) mapping images of Co@CN-
Zn12-5 indicate that most of metallic Co exists in the form of nano-
particles, and the rest is doped in the surface of carbon (Figs. 3e, 3 f, 4 a,
4 b, S6).
As shown in Fig. 4, the Co nanoparticles are in the size range of
100−400 nm and load on the carbon material. The clear lattice fringes
of 0.205 nm and 0.215 nm were observed from Co@CN-Zn12-5 corre-
sponding to Co(111) and Co(100) respectively, [49] and the d-spacings
of 0.250 nm is assigned to CoO(220) (Fig. 4d). [50] The HR-TEM image
displayed in Fig. 4f shows two lattice fringes, corresponding to
Co3O4(220). It should be noted that a small amount of zinc still exists in
Co nanoparticles according to the TEM element mapping images
(Fig. 5). ICP was used to measure the content of Co and Zn in Co@CN-
ZnX-5 (Table S3). There are 0.05 wt% of Zn and 11.9 wt.% of Co in
Co@CN-Zn4-5, while 1.27 wt.% of Zn and 11.7 wt.% of Co were de-
tected in Co@CN-Zn12-5.
can reduce the electronic density of Co0 owing to inductive effects
[49,51]. In addition, the peak intensity of Co 2p in Co@CN-Zn12-5 is
much stronger than Co@CN-Zn0-5, proving that the evaporation of Zn
during the synthetic process leads to more catalytic sites. The peaks at
397.7 eV and 400.3 eV belong to pyrrolic N (or Co-Nx) and pyridinic N
respectively [40,47,48]. Both pyridinic N and Co-Nx could enhance the
oxidation esterification of HMF by providing Lewis basic sites and
catalytic sites [28].
In order to further determine the acid-base properties of Co@CN-
ZnX-5, NH3-temperature-programmed desorption (NH3-TPD) and CO2-
TPD were tested (Fig. 7). In these catalysts, the Lewis acidic sites are
corresponding to Co and Zn sites, and the Lewis basic sites are ascribing
to N and O sites (CoOx, CoNx, pyridinic-N etc.). Co@CN-Zn12-5 has a
similar amount of Lewis acidic sites with Co@CN-Zn4-5, and both of
them have more acidic sites than Co@CN-Zn0-5 (Fig. 7c). Higher sur-
face areas of Co@CN-Zn4-5 and Co@CN-Zn12-5 expose more Co and Zn
sites, thus resulting more acidic sites. Co@CN-Zn12-5 have much more
Lewis basic sites than the other two materials (Fig. 7d). Moreover,
Co@CN-Zn4-5 has stronger acidic sites than Co@CN-Zn12-5 (Fig. 7c). It
can be concluded that the presence of zinc in Co@CN-Zn12-5 increases
the Lewis basic sites and decreases the Lewis acids sites [52,53].
To gain preliminary insights into the reaction mechanism, the
The chemical states of Co@CN-ZnX-5 (X = 0, 12) were further
studied by XPS spectra (Fig. 6). Compared with Co@CN-Zn0-5, the Co
peak of Co@CN-Zn12-5 shifts by 0.4 eV (779.1–779.5 eV) to the higher
binding energy (Fig. 6a) owing to the presence of Co2+, Co3+ and Zn2+
(The peaks at 1021.3 and 1044.6 are assigned to Zn2+ in Fig. S7b) that
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Fig. 4. (a), (b), (c), (e) TEM images of Co@CN-Zn5-12; (d), (f) HR-TEM images of Co@CN-Zn5-12.
oxidative esterification of HMF to FDMC was carried out using Co@CN-
Zn12-5 as the catalyst tracked by GC and GC–MS. As shown in Fig. 8,
HMF was gradually converted into FDMC with time. Compounds 2–4
generated when the reaction started, and all of them was consumed as
the reaction proceeded. 4 had a higher yield than 2 and 3 at the be-
ginning of the reaction. Based on these results and previous reports
[30,54,55], a proposed oxidative esterification pathway of HMF to
FDMC was illustrated in Scheme 1. It can be conclusion that (1) 2, 3 and
4 are the intermeidates of FDMC; (2) the major pathway is HMF-A-4-
MFF-D-FDMC; (3) The step of 4 to MFF is the rate determination step.
(4) The Lewis acid-base sites (AB) can enhance the hemiacetalization
(HMF to A, MFF to D). (5) The Co and Lewis basic sites (B) promote the
oxidation of hemiacetal to the corresponding ester (A to 4, 4 to MFF, D
to FDMC).
There is a thin layer of CoOx on the surface of Co NPs based on TEM
and XPS results, which may be the main catalytic sites of Co@CN-Zn12-
5. Therefore, to further demonstrate the roles of residual zinc in the
catalyst, the charge density maps of CoO-Zn(110) and Co3O4-Zn(110)
were simulated based on density function theory (DFT). The results
show that zinc has much low charge density than cobalt (Fig. 9). This
indicates that residual zinc can increase the charge density of CoOx,
which explains why zinc doping in the catalyst increases Lewis basic
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Fig. 5. TEM images and their corresponding element mapping images of Co@CN-Zn12-5.
sites and decreases the Lewis acids sites. The α-C-H dissociation of al-
cohol is the rate determination step in the oxidation (or oxidative es-
terification) of alcohol to carboxyl acid (or carboxyl ester) [54,55]. The
Gibbs free energy profiles of the α-C-H dissociation suggest that CoO-Zn
(110) has higher catalytic activity than CoO(110) (Table S4).
of zinc during the synthetic process results in the high specific surface
area that increases the number of Lewis acidic sites; (2) the residual
zinc in the catalyst leads to the increase of Lewis basic sites and de-
crease of the strong Lewis acidic sites; (3) the optimization of basic and
acid sites not only improves the interaction between the catalyst and
reactants (or intermediates, products), but also accelerates the depro-
tonation and αeCH dissociation processes [e38,45–47]. For example,
Based on the experimental and calculation results, there are several
roles of zinc in the synthetic and catalytic process. (1) The evaporation
Fig. 6. XPS spectra: (a) Co 2p3/2 of Co@CN-ZnX-5 (X = 0, 12), (b) N 1s of Co@CN-Zn12-5.
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Fig. 7. (a) NH3-TPD profile, (b) CO2-TPD profile, the number of (c) acidic sites and (d) basic sites of Co@CN-ZnX-5.
based on XPS results (Fig. S7f). Meanwhile, an obvious aggregation of
catalyst can be found in SEM images after four runs (Fig. S10), which is
further confirmed by BET results (BET surface areas are reduced from
658 m2/g to 494 m2/g.) Moreover, ICP-MS analysis revealed a slight
loss of the content of cobalt in Co@CN-Zn12-5 (Table S3, 11.7 wt.% to
10.9 wt.%) after recycling. The aggregation of catalyst and the oxida-
tion of the surface Co may be the two main reason for the decrease in
catalyst activity.
Conclusions
In summary, we have demonstrated a novel, efficient and sustain-
able sacrificial template method to synthesize a porous Co@CN catalyst
that can utilize atmospheric oxygen for highly efficient base-free oxi-
dative esterification of HMF under mild conditions (50 °C) at a high
concentration (1 M) of the reactant. The evaporation of zinc during the
synthetic process results in the high specific surface area with increased
number of Lewis acidic sites, meanwhile the residual zinc in the catalyst
leads to an increase of the Lewis basic sites and decrease of the strong
Lewis acidic sites. Based on experimental and quantum calculation re-
sults, both features are beneficial to the catalytic activity of Co@CN-
Zn12-5 owing to the improvement of the interaction between the cat-
alyst and reactants (or intermediates, products) and the acceleration of
the deprotonation and α-C-H dissociation processes. The dual roles of
zinc during the synthetic and catalytic process provide a new idea for
designing Porous transition-metal@CN. In addition, this strategy is
easily scalable, so a more general applicability of such porous transi-
tion-metal@CN for other catalytic reactions are expected.
Fig. 8. Effect of reaction time on HMF conversion and selectivity of FDMC, 2, 3
and 4 over Co@CN-Zn12-5 (Reaction conditions: HMF 0.3 mmol, Co@CN 15
mol% of Co, MeOH 3 mL, O2 1 atm, 50 °C, 24 h).
more Lewis basic sites enhances the deprotonation of alcohol, and more
but weaker Lewis acidic sites (M of MNx and MOx, M = Co, Zn) promote
the absorption of reactants and desorption of products.
The heterogeneous nature of the catalyst was demonstrated by a hot
filtration experiment (Fig. 10). The catalyst was filtered off after 8 h at
50 °C, and the isolated solution was allowed to react for a further 16 h
under identical conditions. No further increase in yield was observed.
Studies were also conducted to assess the potential for recycling of the
catalyst (Fig. 11). After completion of reaction, the catalyst was easily
separated by centrifugation. Then it was washed with methanol to re-
move the absorbed organic chemicals, and dried in a vacuum oven at
60 °C. The crude product was further purified by flash column chro-
matography, and its structure was confirmed by NMR (Fig. S8). The
results reveal that Co@CN-Zn12-5 could be reused at least four times in
subsequent reactions without a significant loss of catalytic activity.
As shown in Fig. S9 the XRD spectra of recovery Co@CN-Zn12-5
indicate the same crystalline peaks with a reduced peak intensity after
four runs. An obvious oxidation of the surface Co in the catalyst is found
Experimental section
General
All chemicals were purchased from commercial suppliers and used
directly without further purification. GC–MS was performed on an
Waters Quattro micro™ GC that is a tandem quadrupole mass spectro-
meter equipped with Electron Impact (EI) and Chemical Ionization (CI
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Scheme 1. Proposed oxidative esterification pathway of HMF to FDMC.
+/CI-) sources. GC analyses are performed on an Agilent 7890A in-
strument (Column: Agilent 19091J-413: 30 m × 320 μm × 0.25 μm,
carrier gas: H2, FID detection). XRD analysis was performed on X-ray
diffraction (XRD, Shimadzu 6000 X-ray diffractometer) with Cu Kα
radiation (λ =0.154 nm). High-resolution transmission electron mi-
croscopy (HR-TEM) and selected area electron diffraction (SAED) pat-
terns of the samples were characterized using a field-emission trans-
mission electron microscope (FE-TEM, JEM-2100 F, JEOL, Japan). The
elemental compositions were determined by energy dispersive X-ray
spectroscopy (EDS) using the genesis microanalysis system (EDAX) at-
tached to the TEM. Scanning electron microscopy (SEM) images were
performed using
a Field emission scanning electron microscopy
(FESEM, JEOL JSM-7600 F). X-ray photoelectron spectroscopy (XPS)
was performed on an X-ray photoelectron spectroscopy (XPS, Thermo
Fisher Scientific Theta Probe system). Inductively coupled plasma mass
spectrometry (ICP-MS) was analyzed on Optima 7300 DV. BET surface
area and pore size measurements were performed with N2 adsorption/
desorption isotherms at 77 K on a Micromeritics ASAP Tri-star II 3020
instrument. Before measurements, the samples were degassed at 150 °C
Fig. 10. Hot filtration experiments.
Fig. 9. The charge density maps of (a) CoO-Zn(110) (one Co is replaced by Zn); (b) Co3O4-Zn(110) (one Co is replaced by Zn) [56].
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The procedure for the oxidative esterification of HMF in high concentration
(1 M)
A mixture of HMF (3 mmol), Co@CN-ZnX-Y (15 mol% of Co) were
added in MeOH (3 mL), which was stirred under 1 atm oxygen at 50 °C
for 30 h. After the reaction was completed, the liquid phase were ana-
lyzed by GC/MS. The yield of the product was determined by GC with
nitrobenzene as an internal standard. The recyclability of the Co@CN
was investigated under the same reaction conditions (3 mmoL HMF in
3 mL of methanol, magnetically stirring, 1 atm oxygen, 50 °C, 30 h) by
using the recovered catalysts. After each cycle, the catalyst was isolated
from the solution by centrifugation, washed three times with methanol
(3 mL × 3), and dried under vacuum to remove the residual solvent and
then reused for another reaction cycle.
Fig. 11. Recycle studies. Conditions: HMF 3 mmol, Co@CN-Zn12-5 15 mol% of
Co, MeOH 3 mL, O2 1 atm, 50 °C, 30 h; GC yields with nitrobenzene as an in-
ternal standard.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
for 12 h. Thermal Gravimetric Analyser (TGA) was detect on a TA
Instruments Q500. Temperature-programmed desorption (TPD)-NH3
and CO2-TPD were conducted on a Quantachrome TPRWin v3.52 in-
strument. The samples were pretreated in He flow at 300 °C with a rate
of 15 mL/min for 30 min and cooled to 50 °C, and then swept in CO2
(NH3) flow with a rate of 15 mL/min for 40 min. After treatment in He
flow for 50 min to remove physical adsorption, the sample were raised
at a heating rate of 10 °C/min to 800 °C, the signals were monitored by
a TCD detector.
Acknowledgements
We gratefully acknowledge Key Laboratory of Biomass Energy and
Material, Jiangsu Province (JSBEM201912), Chinese Postdoctoral
Science Foundation (2015M571761, 2016T90465), and A*STAR
Science and Engineering Research Council (Grant 1528000048) of the
Singapore Government for financial support. This work was a project
funded by the Priority Academic Program development of Jiangsu
Higher Education Institution.
The general procedure for the synthesis of Co@CN-ZnX-Y (X for the weight
ratio of Zn/Co, Y for the heating rate of pyrolysis)
Appendix A. Supplementary data
Chitosan (500 mg), CoCl2·6H2O (5 wt.% of Co in chitosan, 100 mg)
and Zn(OAc)2·2H2O (5X wt.% of Zn in chitosan, 85X mg) were dissolved
in a mixture solvent of water and methanol (v/v = 1/1 30 mL). The
mixture was stirred at room temperature for 24 h, then continuously
stirring at 80 °C for 2 h. Subsequently, the solution was evaporated and
transferred into a solid mixture, which was dried at 80 °C for 12 h. The
obtained powder was pyrolysis in a tubular furnace under an Ar at-
mosphere with a heating rate of Y °C/min up to 900 °C for 2 h, followed
by cooling to ambient temperature to afford Co@CN-ZnX-Y.
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2019.110695.
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