Synthesis of a ZIF-derived hollow yolk-shell Co@CN catalyst for the oxidative esterification of 5-hydroxymethylfurfural

Article abstract of DOI:10.1039/c8gc03868f

A newly developed template protection-sacrifice (TPS) strategy is developed for the synthesis of hollow yolk-shell Co@CN with a large surface area and high pore volume. The catalyst exhibits excellent catalytic efficiency in base-free oxidative esterification of 5-hydroxymethylfurfural under mild conditions with excellent selectivity at a high concentration (2 M) of the reactant.

Full text of DOI:10.1039/c8gc03868f

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DOI: 10.1039/C8GC03868F
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Synthesis of ZIF-Derived Hollow Yolk-Shell Co@CN Catalyst for the
Oxidative Esterification of 5-Hydroxymethylfurfural
Kang-kang Sun,a Shu-jie Chen,b Ze-lin Li,a Guo-ping Lu*a and Chun Cai*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A newly developed template protection-sacrifice (TPS) strategy is were employed in this process, the selectivity of FDMC was
8
disclosed for the synthesis of hollow yolk-shell Co@CN with a still poor (53%). After that, the CoxOy−N@C/α-MnO2 and
9
large surface area and high pore volume. The catalyst exhibits Cu/MnO2 systems were explored respectively, but the
excellent catalytic efficiency on base-free oxidative esterification problems of harsh conditions and poor selectivity have not
of 5-hydroxymethylfurfural under mild conditions with excellent been solved yet. Besides, most of the previous reports on HMF
selectivity at high concentration (2 M) of reactant.
oxidation took place in dilute solutions (0.05-0.25 M) due to
the highly reactive groups in HMF. Side reactions, such as
polymerization and condensation,10 make it impossible for the
reaction to be carried out on a higher concentration. Guided
by the principles of green chemistry, seeking efficient earth-
abundant metal catalysts for the reaction with both high
concentration of reactant and selectivity under alkali-free, low
temperature and atmospheric pressure conditions is an
appealing, yet difficult, task.
Currently, much attention has been placed on convert
renewable biomass resources into liquid fuels and fine
chemicals due to the shortage of fossil resources.1 5-
Hydroxymethylfurfural (HMF) produced from carbohydrates or
cellulose by acid-catalyzed dehydration method,2 can be
oxidized to 2,5-furandicarboxylic acid (FDCA) or its ester furan-
2,5-dimethylcarboxylate (FDMC), which are important building
units for the synthesis of bio-polyethylene furanoate (PEF) to
replace polyethylene terephthalate (PET) plastic.3 However,
due to the large polarity and practically insoluble in water and
most organic solvents, the purification of FDCA is difficult
which limits the application scale of FDCA.4 In contrast, FDMC
is soluble in the most common solvents and can be easily
separated and purified through low-temperature sublimation.
Moreover, FDMC can be directly used in the synthesis of
polymers by transesterification reaction, so it is more suitable
for the replacement of terephthalic acid in plastics than FDCA.5
In order to achieve an excellent reactivity, the reaction is
generally carried out over noble metals (such as Au and Pd)
with the help of high pressure or excess base (MeONa,
K2CO3).6 Considering the high price, toxicity and limited
availability of these noble metals, the oxidative esterification
of HMF to FDMC over earth-abundant-metal catalysts was
investigated recently. Corma and co-workers7 have reported
that FDMC can be prepared by using the transition metal
CoxOy-N@C catalyst. Although K-OMS-2 and high pressure
a.. Chemical Engineering College, Nanjing University of Science & Technology,
Xiaolingwei 200, Nanjing 210094, P. R. China. E-mail:
glu@njust.edu.cn,c.cai@njust.edu.cn
Fig. 1 Formation schematic diagram for the Co@CN nanocomposites.
b. School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou
510006, P. R. China.
On the other hand, N-doped carbon supported Co
nanoparticles derived from ZIF-67 have been widely used to
Electronic
Supplementary
Information
(ESI)
available:
See
DOI:
10.1039/x0xx00000x
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prepare highly active heterogeneous catalysts due to its (Fig. 3c), and the obtained cobalt nanoparticles wVilielwbAertilcalergOenlinine
abundant Co-Nx active sites.11 However, the Co/CN prepared size and uneven (Fig. 2e). The structure of Co@CN is more
DOI: 10.1039/C8GC03868F
by the pyrolysis of ZIF-67 can neither provide high surface area evident via the STEM (Fig. 4), where the corresponding
nor high nitrogen content, and the metal nanoparticles are element mappings confirm that most of the cobalt elements
also agglomerated, which greatly lowered their catalytic are distributed in the center of the material and surrounded by
activities.12 In addition, Co nanoparticles in Co/CN will porous carbon and nitrogen shells. The small amount of cobalt
gradually leach in the reaction process, which is not conducive elements present in porous
to the reuse of the catalyst. To solve these issues, Zhang and
co-workers reported the hard-templating synthesis of Zn,Co-
ZIF@SiO2 using tetraethyl orthosilicate (TEOS) as a precursor.13
The SiO2 shell played an important role in preventing the
aggregation and fusion of Co,N-CNF nanoparticles during
pyrolysis at high temperature. Cobalt nanoparticles
encapsulated in porous carbons derived from core-shell ZIF-
67@ZIF-8 have also be explored and used in oxygen evolution
reaction (OER) and the oxidation of alcohols.14,25a Moreover,
Cheng and co-workers15 reported a template protection-
sacrifice (TPS) method for the synthesis of Cu2O@ZIF-8 with a
yolk–shell nanostructure. In this method, SiO2 serves as a
protective shell, a hollow-space holder and a sacrificial
template, and ZIF-8 facilitates the transport and diffusion of
substrates and products.
Based on these pervious works, we reasoned that a hollow-
yolk-shell Co@CN material could be synthesized through a
modified TPS method, in which the ZIF-67@SiO2@ZIF-8 was
synthesized by a two-step process at first, then the desired
Co@CN could be obtained following the pyrolysis and etching
Fig. 2 TEM images of (a) ZIF-67@SiO2, (b, c, d) Co@CN, and (e) Co/CN.
(Fig. 1). Compared with traditional core-shell nanostructures,
hollow yolk-shell structure offers many unique features, such
as lower densities, high specific surface area and sufficient
exposed active sites,16 which endow them with potential
applications in heterogeneous catalysis.17 Therefore, we
envisaged the hollow-yolk-shell Co@CN material was an
efficient catalyst for the oxidative esterification of HMF.
To verify the feasibility of our proposed assumption, we
started the investigation by the synthesis of ZIF-67@SiO2 and
ZIF-67@SiO2@ZIF-8. The morphology and micro-structure of
the ZIF-67@SiO2 and ZIF-67@SiO2@ZIF-8 was first
characterized
by TEM and SEM. As shown in Fig. 2a, TEM images of ZIF-
Fig. 3 SEM images of (a) ZIF-67@SiO2@ZIF-8, (b) Co@CN, and (c) Co/CN.
67@SiO2 clearly illustrate that the uniform particle shape of
ZIF-67 is successfully coated with SiO2 shell. The SEM image of
ZIF-67@SiO2@ZIF-8 (fig. 3a) shows that ZIF-67@SiO2
surrounded by ZIF-8 will be formed after the methanol
solution of Zn(NO3)2·6H2O and 2-MeIm is added. The Co@CN
materials were prepared by pyrolysis ZIF-67@SiO2@ZIF-8 at
900 ℃ and NaOH etching treatment. There is almost no
morphological change during the preparation process as
shown in the SEM image (Fig. 3b).
Furthermore, the detailed morphological of Co@CN (Fig. 2c
and 2d) shows that the Co nanoparticles are loaded in the core
section of the nanocomposites homogeneously. Homogeneous
distribution and small size of cobalt nanoparticles have higher
surface/volume ratios, which can maximize the atom
efficiency, and thus contribute to the high yield of FDMC. On
the contrary, if ZIF-67 is calcined directly under the same
conditions, the morphology of ZIF-67 will be severely collapsed
2 | J. Name., 2012, 00, 1-3
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Fig. 4 STEM images and their corresponding element mapping images of Co@CN.
be deconvoluted into four peaks located at 778.6,V7ie8w1A.rt0ic,le7O9n3lin.7e
and 797.2 eV. This indicates that there are metallic Co and
DOI: 10.1039/C8GC03868F
CN shells cannot find through the HRTEM, thus the size of
cobalt in porous CN shells should be sub-nm which may also
exhibit catalytic activity in the oxidative esterification of HMF.
The HRTEM image shows the crystal lattice of the Co particle is
0.214 nm, corresponding to the zero-valent cobalt (Fig. S4).
The powder XRD patterns of the ZIF-67@SiO2 and ZIF-
67@SiO2@ZIF-8 (Fig. S1a) match well with the simulated XRD
patterns of ZIF-67 and ZIF-8, suggesting that the introduction
of SiO2 shell in the process of MOF synthesis do not affect the
crystallographic structures of ZIF-67 and ZIF-8. After pyrolysis
the composite at 900 °C and NaOH etching treatment, the XRD
patterns of obtained Co@CN (Fig. S1b) do not show any peaks
assignable to cobalt metal or its compounds, although Co@CN
contains Co actually. The peaks are assignable to cobalt metal
or its compounds were also not detected in Co-CN (Co-CN is
obtained by pyrolysis of ZIF-67@ZIF-8) and Co@CN-1 (Co@CN-
1 is obtained by pyrolysis of ZIF-67@SiO2@ZIF-8 but without
etching). This may be due to the undetectable amount of Co or
the high dispersion of Co.18 On the contrary, due to the large
size and high content of cobalt nanoparticles in Co/CN and
Co@CN-2 (Co@CN-2 is obtained by calcining and etching ZIF-
67@SiO2.), the peaks at 44.2° and 51.5° can be seen in XRD
patterns (Fig. S1c), corresponding to the characteristic
diffractions of (111) and (200) lattice planes of metallic Co,
respectively.22a ICP-MS was used to measure the content of Co
and Zn in Co@CN (Table S1). The results reveal that the total
Co content of Co@CN is about 2.22 wt% and the Zn residue is
almost negligible (0.04 wt%), which is due to the evaporation
of Zn during 900 °C pyrolysis.19 With the gradual evaporation
of zinc at the temperature of 900 °C, the ZIF-8 shell will be
transformed into highly porous carbon shells, thus protecting
the cobalt nanoparticles in the center.
mixed valence state of Co in Co/CN, which is not conducive to
the stability of the catalyst. The Co 2p and N 1s XPS spectra of
Co-CN (Fig. S2c, S2e) is similar to Co@CN, indicating that the
addition of TEOS and etching of SiO2 during the preparation of
the catalyst does not affect the form of cobalt in the Co@CN.
Raman spectra of Co@CN nanocomposites were shown in Fig.
S5. There are two typical peaks are observed at 1352 and 1578
cm−1, indicating the existence of graphite sp2-C (G-band) and
disordered sp3-C (D-band), respectively.24 The ID/IG value of
Co@CN is 0.98, which indicates that Co@CN has
a
considerable graphitization and therefore has high electronic
conductivity.
Further, the porous characteristics of the calcined samples
were investigated by nitrogen adsorption–desorption
isotherms. The BET surface area, pore volume and average
diameter of all synthesized materials were listed in Table S2.
As shown in Fig. 6, there are micropores, mesoporous and
macropores in the materials, as confirmed by the
corresponding pore distribution curve. When the SiO2 shell
was etched by NaOH, the BET surface area of Co@CN
increased from 801 to 857 m2/g, and the pore volume
increased from 1.0 to 1.5 mL/g (Table S2), undoubtedly
manifesting that the presence of SiO2 shell in the process of
making Co@CN could not only suppress the agglomeration of
Co during carbonization but also promote the formation of
unique hollow cavity. The hollow cavity surrounded by a
porous outer shell is considered to be of great
The electronic structure and chemical states of Co@CN were
further studied by XPS spectra (Fig. 5 and Fig. S2a). The survey
spectra show that Co@CN is mainly composed of C, N, O and
Co. No Zn is found which is explained by evaporation of Zn
during the pyrolysis step, which is consistent with the ICP-MS
result. In the Co 2p spectra (Fig. 5a), no metallic Co signal
(778.5 eV and 793.5eV) is found,20 and the peak at 780.2 eV
can be assigned to CoCxNy or cobalt oxides.13,21 The
appearance of CoCxNy can be evidenced by the N 1s spectra of
the Co@CN. There are mainly five types of the nitrogen
species: oxidized N (403.6 eV), graphitic N (401.2 eV), pyrrolic
N (400.1 eV), Co-Nx (398.7 eV) and pyridinic N (398.3 eV).22
These results indicate that N atoms have been successfully
embedded in carbon matrix, and some of them are associated
with Co species to form Co–N sites (Co–Nx), which play an
important role in this reaction. Due to the presence of
pyridinic N, π electrons interact strongly with cobalt, which
makes Co nucleate and grow preferentially at sp2-N site. And it
is advantageous to the immobilization of cobalt and the
reduction of the agglomeration of cobalt nanoparticles.23
On the other hand, graphite nitrogen in Co@CN increases
the conductivity of the material, thus increasing its catalytic
performance. The Co 2p XPS spectrum of Co/CN in Fig. S2b can
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catalysts, only a small number of product 3a were formed
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DOI: 10.1039/C8GC03868F
(Entry 6). It could be concluded that cobalt was inevitable for
this oxidative esterification transformation.
The effects of reaction temperature and time on product
distribution were investigated. The results are shown in Fig. 7.
When the reaction temperature was below 60 °C, the
conversion of HMF was poor. With the increase of reaction
temperature, the yield of FDMC increased gradually (Fig. 7a).
Subsequently, different reaction times were also tested under
optimized conditions, respectively (Fig. 7b). The yield of FDMC
was low and 11% of MFF, and a small amount of methyl 5-
hydroxymethyl-2-furoate were detected after 6 h. When the
reaction time was extended to 10 h, the content of MFF
decreased to 3%. After 12 h, MFF disappeared indicating that
MFF was the intermediate of the reaction.
Fig. 5 XPS spectra of (a) Co 2p region (b) N 1s region.
importance for the oxygen absorption and exposure of the
maximum active sites in the oxidative esterification of HMF.25
Fig. 7 Product distributions in different reaction temperature and times. Reaction
conditions: HMF (0.2 mmol), catalyst (3.8 mol% Co), MeOH (1 mL), 0.1 MPa O2, 80 °C,
12 h. Yields were determined by GC with nitrobenzene as an internal standard. MFF:
methyl 5-formylfuran-2-carboxylate.
Fig. 6 N2 adsorption/desorption isotherms of Co@CN and the corresponding pore-size
distribution curves. After ZIF-67@SiO2@ZIF-8 was calcined, the SiO2 shell was not
etched.
Table 1 Optimization of the conditions for oxidative
esterification of HMF 1a.a
a
O
HO
OH
O
O
O
O
O
OHC
O
The as-prepared Co-based catalysts were employed for the
oxidative esterification of HMF. The reactions were performed
under base-free conditions using atmospheric oxygen as the
green oxidant. Co@CN was first used to the oxidative
esterification of HMF in methanol at 60 °C, while the yield of
FDMC was relatively low (Table 1, Entry 1). When the reaction
temperature was raised from 60 °C to 80 °C, the yield of FDMC
increased from 49 % to 95 % (Entries 1-3). A blank test showed
that no FDMC was produced in the absence of catalyst (Entry
7). Next we tried to carry out the reaction in an air
atmosphere, but the yield was only half that of the reaction in
oxygen atmosphere (Entry 4). When nitrogen doped porous
carbon materials derived from ZIF-8 (CN) were used as
O
O
3a
1a
2a
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Yields (%)b
aldehyde is the rate-limiting step.27b Then the intermediate
MFF is converted into oxidative esterification product FDMC in
the catalytic system.
It is well known that the extensive use of organic solvents is
an important factor in resource waste and environmental
pollution.28 In order to test the industrial and practical value of
this catalytic system, we tried to perform the reaction under
scale-up conditions. When the concentration of HMF was
increased to 2 M, the isolated yield of FDMC was 84% after 24
hours of reaction (Entry 11).
View Article Online
DOI: 10.1039/C8GC03868F
Entry
Catalyst
T(℃) Conversion (%)
2a
49 (40)
70 (62)
95 (89)
55 (50)
0
3a
0
0
0
0
55
15
11
11
14
5
1
2
3
4c
5
6
7
8
9
Co@CN
Co@CN
Co@CN
Co@CN
CN (ZIF-8)
-
Co/AC
Co/MgO
Co/ZrO2
60
70
80
80
80
80
80
80
80
53
74
100
69
55
15
24
22
25
0
13 (8)
11 (7)
11 (7)
89
10d
Co@CN
80
94
(84)g
a
Reaction conditions: 1a (0.2 mmol), catalyst (3.8 mol% of Co), MeOH (1
b
mL), 0.1 MPa O2, 80 °C, 12 h. Yields were determined by GC with
nitrobenzene as an internal standard. Air was used instead of O2. 1a (2
mmol), catalyst (3.8 mol% of Co), MeOH (1 mL), 0.1 MPa O2, 80 °C, 24 h.
c
d
g
Isolated yield.
A series of Co-based materials with different supports such
as active carbon (AC), MgO and ZrO2 were prepared by
classical precipitation method and also tested as reference
catalysts in this reaction under the optimized conditions
(Entries 8-10). The yield and selectivity of 2a were quite low
compared to the reaction was carried out under the catalysis
of Co@CN. In addition, many by-products such as 5-
Fig. 8 The BET surface area of different catalysts and the corresponding yield of FDMC.
Co/CN, Co-CN and Co@CN-1 are obtained by the pyrolysis of ZIF-67, ZIF-67@ZIF-8 and
(dimethoxymethyl)-2-furanmethanol, are produced during the ZIF-67@SiO2@ZIF-8 respectively; Co@CN-2 is obtained by calcining and etching ZIF-
67@SiO2.
reaction.
O
O
OH
OMe
Co@CN
O
O
O
CHO
Co@CN, O2
CH3OH
O
CHO
HO
HO
OHC
O
HO
O
MeOH, O2
80 oC
O
A
HMF
1a
2a
-H2O
organic waste (kg)
product (kg)
E Factor =
= 2.5
OMe
OMe
OMe
OMe
O
O
O
O
HO
HO
OMe
Scheme 2 E Factor studies
C
B
HMMF
slow
HO
MeO
O
O
O
O
O
O
OHC
MeO
OMe
OMe
OMe
MFF
FDMC
D
Scheme 1 Proposed oxidation pathway of HMF to FDMC.
On the basis of the above results and previous reports,6c,6e,8
we proposed a plausible mechanism for the present process
shown in Scheme 1. First, the hemiacetal intermediate, A (non-
detectable through GC-MS), is obtained by the reaction of
aldehyde group over HMF. Because the oxidation of aldehyde
function is always faster than the oxidation of alcohol function,
there is no 2,5-furandicarboxaldehyde formation in the
system.27a The structure of active hemiacetal makes it easier to
convert the intermediate A into methyl 5-hydroxymethyl-2-
furoate (HMMF) intermediate in the presence of O2. With the
increase of reaction temperature or reaction time, the
hydroxymethyl group in HMMF is further oxidized to the
aldehyde, to form MFF. And the oxidation of hydroxyl to
Fig. 9 Re-uses of Co@CN for the oxidative esterification of HMF. Reaction conditions:
HMF (0.2 mmol), catalyst (3.8 mol% Co), MeOH (1 mL), 0.1 MPa O2, 80 °C, 12 h.
This greatly reduces the use of solvent in the synthesis process
and is conducive to the realization of industrial production.
Methanol plays a crucial role in this catalytic system, which
protects the aldehyde group by the formation of the
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corresponding acetal, thus reducing the occurrence of side structure Co@CN catalyst could obtained after NaOH etching
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reactions.2d
treatment, which proves to facilitate mass transfer of the
DOI: 10.1039/C8GC03868F
In order to investigate the roles of the hollow yolk-shell active component and protect the catalytically active sites. In
structure on the reaction, we next conducted another set of this architecture, the porous shell derived from ZIF-8 is
experiments by replacing Co@CN with similar catalysts (Fig. 8). believed to facilitate the transportation and adsorption of
In general, larger BET surface areas results in higher yields of substrate and oxygen, and it can also protect the active cobalt
FDMC. The yield of FDMC catalyzed by ZIF-67-derived Co/CN nanoparticles core from leaching effectively. Methanol was
was obviously much lower as compared with Co@CN. The used as a solvent, an esterification and aldehyde protecting
activity of Co-CN derived from ZIF-67@ZIF-8 was slightly higher agent in this system. The good selectivity, high efficiency,
than Co/CN, but lower than Co@CN. If the silica shell was not alkali-free and recyclability (low E factor) as well as mild
etched after calcination (Co@CN-1), its catalytic effect was still reaction conditions, may enhance their potential in the
lower than Co-CN even with larger BET surface areas, chemical industry application.
indicating the silica shell could inhibit the reaction obviously
owing to the suppression of the interaction between the
reactant and the active site. The catalytic activity of Co@CN-2
Conflicts of interest
obtained by calcining and etching ZIF-67@SiO2 was also not
comparable to Co@CN. These results indicated that the hollow
cavity between the metal core and the porous carbon shell
plays a key role in the catalyst process: (1) the hollow cavity
provides sufficient space for confined catalysis; (2) the
permeable shell also facilitates mass transfer of the active
component and protects the catalytically active sites from
leaching even under harsh reaction conditions, thereby
increasing the activity and stability of the hollow yolk-shell
material.26
The E Factors was used to illustrate the problem of waste
generation in chemicals manufacture.29 A lower E factor means
more efficient and therefore less pollution to the
environment.30 Based on 14% organic solvent loss in the
solvent recovery process of the model system (Scheme 2), the
E factor of the system (reaction concentration is 2.0 M) was
only determined to be 2.5 (For detailed procedures, see ESI).
The recycling of catalysts is also an important issue in the
development of sustainable chemicals. After reaction, Co@CN
can be easily recovered and the yield of FDMC was 87% after
five reuses (Fig. 9). As shown in Figure S1 and Figure S2d, the
XRD and XPS spectra of recovery Co@CN (Co@CN-3) are not
obvious different from the fresh samples. Meanwhile, it can be
seen from the SEM image (Fig. S7a) and the TEM image (Fig.
S7b) of Co@CN-3 that there is no obvious change in the
morphology and the Co nanoparticles did not agglomerate.
Moreover, ICP-MS analysis revealed that the content of cobalt
in Co@CN was almost no change after five cycles (Table S1).
These results evidently suggest that the catalytic activity was
stable and the porous carbon shell holds the cobalt metal very
tightly, thus reducing the metal from leaching, and promoting
the catalyst recycling effectively.
There are no conflicts to declare.
ACKNOWLEDGEMENTS
We gratefully acknowledge Chinese Postdoctoral Science
Foundation (2015M571761, 2016T90465) for financial support.
This work was a project funded by the Priority Academic
Program development of Jiangsu Higher Education Institution.
We also thank Wan-ying Tang at Analysis and Test Center
Nanjing University & Technology for the help in obtaining the
Raman data. .
Notes and references
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5
6
Conclusions
In summary, we have demonstrated a novel and efficient
TPS method to synthesize a hollow yolk-shell structure Co@CN
catalyst that can utilize atmospheric oxygen, an eco-friendly
and natural abundance oxidant, for a highly efficient base-free
oxidative esterification of HMF in high concentration (2 M).
The SiO2 shell acts as a protective shell to prevent the
agglomeration of ZIF-67 during pyrolysis. A hollow yolk-shell
7
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Green Chemistry
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Green Chemistry
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DOI: 10.1039/C8GC03868F
A ZIF-derived hollow yolk-shell Co@CN catalyst is synthesized for the oxidative
esterification of 5-hydroxymethylfurfural.