Metal–Organic Framework-Templated Catalyst: Synergy in Multiple Sites for Catalytic CO2 Fixation

Article abstract of DOI:10.1002/cssc.201700245

The types and quantities of active sites play a critical role in catalysis. Herein, ZnO nanoparticles encapsulated into N-doped porous carbon has been rationally prepared by the pyrolysis of a metal–organic framework (MOF) followed by a moderate oxidation treatment. The resulting catalyst exhibits excellent activity, selectivity, and recyclability in the CO₂ cycloaddtion reactions with epoxides owing to the synergy of multiple sites inherited from the MOF and generated by the oxidation process.

Full text of DOI:10.1002/cssc.201700245

Accepted Article
Title: Metal-Organic Framework-Templated Catalyst: Synergy in
Multiple Sites for Catalytic CO2 Fixation
Authors: Hai-Long Jiang, Meili Ding, Si Chen, Xiao-Qin Liu, Lin-Bing
Sun, and Junling Lu
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the VoR from the journal website shown below when it is published
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To be cited as: ChemSusChem 10.1002/cssc.201700245
Link to VoR: http://dx.doi.org/10.1002/cssc.201700245
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10.1002/cssc.201700245
ChemSusChem
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Metal-Organic Framework-Templated Catalyst: Synergy in
Multiple Sites for Catalytic CO2 Fixation
Meili Ding,[a] Si Chen,[a] Xiao-Qin Liu,[b] Lin-Bing Sun,[b] Junling Lu[a] and Hai-Long Jiang*[a,b]
Dedication ((optional))
Abstract: The types and quantities of active sites play critical roles
in catalysis. Herein, ZnO nanoparticles encapsulated N-doped
porous carbon has been rationally prepared by the pyrolysis of a
metal-organic framework (MOF) followed by moderate oxidation
treatment. The resultant catalyst exhibits excellent activity, selectivity
and recyclability in the CO2 cycloaddtion reactions with epoxides,
due to the synergy of multiple sites inherited from the MOF and
generated by the oxidation process.
conversion.[6] The major reasons behind this should be the lack
of active sites and the weak interactions between CO2 molecules
and carbon materials. To address these issues, three characters
for carbon materials are of great importance: 1) a moderately
high surface area with suitable pore sizes would be beneficial to
the exposure of active sites and transport of reaction
substrates/products; 2) polar functional sites/groups grafted onto
the pore walls are desirable to improve the interaction between
the framework and CO2 molecules;[1a,7] 3) active sites available
for substrates and CO2 are necessary. Based on previous
reports, different acids, such as phenolic hydroxyl and carboxyl
groups, ZnO species, etc., and Lewis base such as pyridinic N,
are effective active sites.[6c,8] Particularly, the integration of acid
and base sites into a single catalyst is favorable to the
conversion of CO2 cycloaddition.[8e]
On the other hand, metal-organic frameworks (MOFs) have
been demonstrated to be ideal templates/precursors to afford
uniform metal (oxide) particles distributed throughout porous
carbon with high surface areas via facile pyrolysis.[6f,9] Among all
MOFs, ZIF-8 (also called MAF-4, Zn(MeIM)2, MeIM = 2-
methylimidazole),[10] as a representative MOF, possesses an
intersecting 3D structure with pore size 11.6 Å, high N content,
ZnN4 node, and large Brunauer–Emmett–Teller (BET) surface
area (1413 m2 g-1), which has been widely used as template and
precursor to offer N-doped porous carbon or carbon-based
composites for diverse applications, particularly in catalysis.[9]
In recent years, ocean acidification, global warming and many
environmental issues caused by greenhouse gases (GHGs)
have attracted growing attention. As the primary source of
GHGs, carbon dioxide (CO2) comes mainly from the burning of
fossil fuels and its concentration was reported to be over 400
ppm in atmospheric air by 2015.[1] On the other hand, CO2 is
actually an economical, abundant, renewable and nontoxic C1
source providing value-added chemicals, such as formic acid,
cyclic carbonate and carbon monoxide.[2] One of the main
challenges for CO2 utilization is the high kinetic and thermo-
dynamic stability of CO2 molecule, which require a large amount
of energy to activate.[2a,2b,3] In this context, the concentrated CO2
molecules around active sites would facilitate its conversion.
Therefore, it is highly desired to develop cost-effective and
efficient strategies to integrate CO2 capture and conversion (C3).
Among various strategies developed for the catalytic CO2
transformation, the cycloaddition of CO2 and epoxides to cyclic
carbonates is very promising, not only because of its atom
economy and green chemistry but also due to the high-valued
products.[4] So far, different types of catalysts have been
reported for the CO2 cycloaddition reaction, for examples, metal
complexes, ionic liquids, and alkali metal salts, etc.[2d,4] In
addition, compared with traditional homogeneous catalysts, the
heterogeneous catalysts, such as metal-organic frameworks
(MOFs) and porous polymers, are attracting great interest on
account of the bifunction of CO2 capture and simultaneous
catalytic conversion.[2c,2g,5] In contrast, porous carbons,
possessing high surface area, excellent stability and low density,
are promising candidates but rarely reported for catalytic CO2
Scheme 1. Schematic illustration showing the preparation of ZIF-8-templated
ZnO@NPC-Ox toward CO2 cycloaddition with epoxides.
[a]
M. Ding, S. Chen, Prof. Dr. J. Lu, Prof. Dr. H.-L. Jiang
Hefei National Laboratory for Physical Sciences at the Microscale
CAS Key Laboratory of Soft Matter Chemistry
Collaborative Innovation Center of Suzhou Nano Science and
Technology, School of Chemistry and Materials Science,
University of Science and Technology of China
Hefei, Anhui 230026 (P.R. China)
Bearing the above points in mind, the generation of
multicomponents in the resultant composite by ZIF-8 pyrolysis
might evoke the synergetic effect among the components and
endow the catalyst with superior catalytic performance for the
CO2 fixation reaction. In this work, the ZIF-8 precursor was
pyrolyzed and subsequently oxidized by sodium hypochlorite to
afford ZnO nanoparticles (NPs) encapsulated into N-doped
porous carbon (ZnO@NPC-Ox) (Scheme 1), which includes
carboxylic acid, phenol and lactone functional groups on the
surface of the porous carbon, given the moderate oxidation
ability of sodium hypochlorite.[11] Remarkably, the resultant
E-mail: jianglab@ustc.edu.cn
Homepage: http://staff.ustc.edu.cn/~jianglab/
Prof. Dr. X.-Q. Liu, Prof. Dr. L.-B. Sun, Prof. Dr. H.-L. Jiang
State Key Laboratory of Materials-Oriented Chemical Engineering
College of Chemistry and Chemical Engineering
Nanjing Tech University
[b]
Nanjing, Jiangsu 210009 (P.R. China)
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10.1002/cssc.201700245
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composite exhibits efficient conversion toward CO2 cycloaddition
under mild conditions (60 °C, 1 bar CO2), on account of the
stretching vibration of the O-H bonds from hydroxyl and carboxyl
groups,[13a,b] which are absent in the spectrum of ZnO@NPC-
synergetic effect of multiple catalytic sites involving ZnO species, 700. The C=O stretching band of lactone is ~1820 cm-1 and the
pyridinic N, hydroxyl and carboxyl groups in the oxidative porous
carbon (Scheme 1).
strong peak at 1650 cm-1 is also attributed to the vibration of
C=O.[13b,c] The band at 1600 cm-1 is assigned to the vibration of
carbon skeleton.[13a,b,d] In addition, the characteristic peaks at
1420, 1368, 1100 and 1048 cm-1 of ZnO@NPC-Ox-700 come
from the C-O stretching (carboxyl), O-H deformation, C-O-C
stretching (lactone) and C-O stretching (alkoxy), respectively.[13d-
The ZIF-8 nanocrystals were synthesized by simply stirring a
mixture of Zn(NO3)2·6H2O and 2-methylimidazole in methanol at
room temperature. The obtained ZIF-8 as both a template and a
precursor underwent pyrolysis at different temperatures in N2
atmosphere to afford the ZnO NPs encapsulated into N-doped
porous carbon (denoted ZnO@NPC-T, T represents pyrolysis
temperature, T = 700, 800, 900 or 1000 °C), which were further
oxidized by NaClO to give ZnO@NPC-Ox-T.
g]
Different from ZnO@NPC-Ox-700, the carbon composite
oxidized by HNO3 (ZnO@NPC-700-HNO3) shows an evident
peak around 1725 cm-1 assignable to the stretching vibration of
C=O (carboxyl) (Figure S8).[13a,d] All of these reveal that the
oxidation from sodium hypochlorite results in carboxylic acid,
phenol and lactone functional groups on the pore surface of
ZnO@NPC-Ox-700 and the main functional group in
ZnO@NPC-700-HNO3 is carboxyl, which are consistent with
those in previous reports.[11,14]
The morphology and microstructure of ZIF-8, porous carbon
composites and their oxidized products were observed, with
ZnO@NPC-700 and ZnO@NPC-Ox-700 as the representatives,
respectively, by scanning electron microscopy (SEM) and
transmission electron microscopy (TEM). In comparison with the
polyhedral ZIF-8 nanocrystals in 70-80 nm, ZnO@NPC-700 and
its oxidized product present the retained morphology with slightly
reduced sizes (~50 nm) (Figure S9). The TEM and high-
resolution TEM (HRTEM) images demonstrate that the ZnO NPs
are well dispersed in ZnO@NPC-Ox-700 with average sizes of
~3 nm (Figure 1c and 1d). The lattice fringes with an interplanar
spacing of 0.26 nm correspond to the (002) plane of ZnO, which
is consistent with the PXRD results (Figure 1a).
The phase purity of ZIF-8 was confirmed by powder X-ray
diffraction (PXRD) (Figure S1 in the supporting information).
Upon pyrolysis, the PXRD patterns of all products show a weak
broad peak at ~25 °, which should be assigned to the typical
(002) diffraction of graphitic carbon. The rest of peaks
indentifiable in the pyrolysis products obtained at 700 and
800 °C can be indexed to the wurtzite (ZnO, JCPDS NO. 36-
1451) (Figure S2). In contrast, the corresponding peaks of ZnO
species are not present in the products at 900 and 1000 °C
because ZnO could be reduced by carbon to generate
evaporative Zn at such high temperatures.[9b,9d,9e,12] After the
oxidation treatment with NaClO, the intensity of PXRD peaks
indexed to ZnO species significantly decrease in ZnO@NPC-
Ox-700 and the peaks almost disappear in ZnO@NPC-Ox-800
(Figure 1a). The pore character and surface area of all materials
were investigated by N2 sorption at 77 K. The original ZIF-8
shows relatively high surface area (1235 m2 g-1) and
microporous character (Figure S3). Upon pyrolysis, the resultant
porous carbon composites partially inherit the pore character of
ZIF-8 with relatively high BET surface area (403-1963 m2 g-1)
(Figure S4a). The BET surface area of ZnO@NPC-Ox-T
significantly decreases compared to the above composites, due
to the oxidation of carbon matrix (Figure S5a). Moreover, the
pyrolysis temperature is crucial to the BET surface area of the
composites before and after oxidation treatment. In addition, the
steep rise of sorption curves and the hysteresis loop at relatively
high pressure (P/P0
= 0.9-1.0) manifest the presence of
mesopores and macropores in these materials, which are further
verified by the pore size distribution analysis based on density-
functional theory (DFT) method (Figure S4 and S5). To
demonstrate the reproducibility of these samples, three batches
of ZnO@NPC-Ox-700, as a representative, were investigated
and they displayed almost the same pore structures and
properties (Figure S6). Although the BET surface areas of
ZnO@NPC-Ox-T are not very high (77-150 m2 g-1), the
hierarchical pores, especially macropores, are suited to expose
the catalytic sites and transfer the substrates and products.
Based on the porosity and the suspended polar functional
groups (hydroxyls and carboxyls) on the pore walls, the oxidized
products exhibit moderate CO2 uptake capacity (20-34 mg g-1) at
273 K (Figure S7), which would be important for subsequent
CO2 cycloaddition reaction.
Figure 1. (a) PXRD patterns of ZnO@NPC-Ox-T composites (T = 700, 800,
900 or 1000 °C). (b) FT-IR spectra of ZnO@NPC-700 and ZnO@NPC-Ox-700
catalysts. (c) TEM and (d) HRTEM images of ZnO@NPC-Ox-700. Inset in (c):
enlarged single ZnO@NPC-Ox-700 particle.
To further understand the existing form of the Zn and N
species in ZnO@NPC-Ox-T composites, X-ray photoelectron
spectroscopy (XPS) has been investigated. The high-resolution
XPS spectra for N 1s reveal two types of N species in
The successful introduction of oxygen surface groups on
ZnO@NPC-Ox was approved by Fourier transform infrared (FT-
IR) spectroscopy, with ZnO@NPC-Ox-700 as the representative
(Figure 1b). The broad peak at 3400 cm-1 is corresponding to the
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10.1002/cssc.201700245
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ZnO@NPC-Ox-T (T = 700, 800): pyridinic N (398.7 eV) and
pyrrolic N (400.3 eV) (Figure 2a and S10a).[6a,15] When the
calcination temperature was further elevated, the most stable
nitrogen (graphitic N) appears and increases, accompanying
with the decrease of the overall N content (from 11.18 wt% in
ZnO@NPC-Ox-700 to 4.19 wt% in ZnO@NPC-Ox-1000) (Figure
S10b, S10c and Table S1). The Zn 2p spectra in all ZnO@NPC-
Ox samples show similar peaks, suggesting their similar
chemical states (Figure 2b and S11). The two broad peaks at
experiments were conducted. As expected, when ZnO@NPC-
Ox-700 was replaced by ZnO@NPC-700 or ZnO pyrolyzed from
ZIF-8 in air, the conversion of substrates sharply lowered to 77%
and 43%, respectively (entries 9-10, Figure S13). Since only two
types of active sites in ZnO@NPC-700 (pyridinic N and ZnO)
and the only active site in pure ZnO, these results reflect the
contribution of hydroxyl, carboxyl and pyridinic N sites to the
CO2 fixation. In addtion, the benefits of ZnO obtained by
porolysis of ZIF-8 over concentional ZnO encapusulated to
porous carbon have been further illustrated by control
experiments with ZnO@NPC-Ox and ZnO@AC (AC = activated
carbon) catalysts (see Experimental Section; Figure S14). These
two catalysts present 85% and 59% yields (entries 11-12),
respectively, in reference to the higher yield of ZnO@NPC-Ox-
700 (entry 1). Therefore, the ZnO obtained by pyrolysis of ZIF-8
possesses superior activity to the conventional ZnO
encapsulated to porous carbon, possibly due to the small sizes,
good dispersity and interactions between ZnO and diverse
functional groups on the surface of ZnO@NPC-Ox-700.
Interestingly, the activity of ZnO@AC (entry 12) is slightly higher
than ZnO (entry 10), which is probably due to the aggregation of
ZnO NPs in the latter. When NPC-Ox-700 was employed as a
catalyst, ~50% substrate can be converted to the target product,
demonstrating that the NPC-Ox-700 support is also active for
the reaction (entry 13, Figure S15).
1021.8 and 1044.5 eV can be assigned to Zn 2p3/2 and 2p1/2
,
respectively.[9i] The peak distance of 22.7 eV is similar to that
reported for ZnO and the content of Zn is dramatically
decreasing with the increased temperatures of pyrolysis, which
are also supported by the aforementioned PXRD results (Figure
1a).
Figure 2. The XPS spectra of high-resolution (a) N 1s and (b) Zn 2p for
ZnO@NPC-Ox-700.
Encouraged by the multiple catalytic sites and hierarchical
pores in ZnO@NPC-Ox, CO2 cycloaddition with styrene oxide
was first chosen as a model catalytic reaction to evaluate their
catalytic properties (Table 1). Among all ZnO@NPC-Ox samples,
ZnO@NPC-Ox-700 possesses the highest conversion (98%)
and selectivity (100%), in the presence of tetrabutyl ammonium
bromide (TBAB), to give the target product phenylethylene
carbonate under mild conditions (60 °C, 1 bar CO2) (entries 1-4).
The catalytic efficiency of different batches of ZnO@NPC-Ox-
700 is almost the same, suggesting the reproducible activity
(Table S2). Evidently, the presence of ZnO and high-content
pyridinic N in ZnO@NPC-Ox-700 are responsible for the efficient
conversion (Figure 2 and Table S1), while pyrrolic N and
graphitic N are almost inessential because of the lack of Lewis
basicity.[6a] It needs to be stressed that both the catalyst and co-
catalyst (TBAB) are indispensable for the efficient conversion
(entries 5-6). In reference to TBAB, tetrabutyl ammonium
chloride (TBAC) only leads to 34% conversion, probably
because the strong constraint of the quaternary ammonium ion
for the chloride ion with small radius (entry 7).[16] Unexpectedly,
when ZIF-8 and TBAB was used, the conversion (16%) was
almost similar to the that of pure TBAB (14%), indicating the low
reactivity of ZIF-8 under such conditions (entry 8), possibly due
to the lack of sufficient active sites and/or the small apertures of
ZIF-8 (3.4 Å). It is worth noting that, although ZIF-8 might be
able to host larger molecules than its pore aperture due to the
mobility of linkers, the reaction would be considerably restricted
due to the low diffusion rates.[17] To demonstrate the significance
of multiple catalytic sites in ZnO@NPC-Ox-700, control
Figure 3. Conversion (blue column) and selectivity (red column) of
ZnO@NPC-Ox-700 toward CO2 cycloaddition with styrene oxide during ten
consecutive runs.
To understand the critical step of the mild oxidation,
ZnO@NPC-700 was oxidized in 1 M HNO3 solution to give
ZnO@NPC-700-HNO3. This strong oxidation treatment mainly
results in carboxylic acids on the pore surface of carbon and
most of ZnO in the carbon would be etched off, supported by the
abovementioned FT-IR results, the disappearance of PXRD
peaks for ZnO and the result of inductively coupled plasma
atomic emission spectroscopy (ICP-AES) (Figure S8, S16 and
Table S3).[11,14] Unfortunately, the resultant product showed only
56% conversion after 2 days under similar conditions (entry 14).
For further comparison, the ZnO@NPC-Ox-700 was also treated
in 1M HNO3 solution to give ZnO@NPC-Ox-700-HNO3. Such
treatment results in not only the disappearance of the PXRD
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characteristic peaks of ZnO but also the reduced conversion of
58% (entry 15, Figure S16 and Table S3). All above results
imply that the great activity of ZnO@NPC-Ox-700 is ascribed to
the synergy of multiple catalytic sites involved and NaClO is an
ideal oxidant for the post-synthetic oxidation compared to the
traditional strong oxidant such as HNO3. In addition to the
activity, as a heterogeneous catalyst, ZnO@NPC-Ox-700 can be
readily separated from the reaction system by simple filtration.
Remarkably, both conversion and selectivity of the catalyst
remain almost unchanged during the ten runs of circulation
(Figure 3). Moreover, the PXRD pattern of ZnO@NPC-Ox-700
demonstrates the retained structural integrity after recycling
(Figure S17).
cycloaddtion reaction. In consideration of the low boiling point of
1,2-epoxypropane and 1,2-epoxybutane, both of them were
converted at relatively low temperatures. To our delight, all these
epoxides with different substituent groups gave good to
excellent conversion and high selectivity (Table 2). Due to the
low reaction temperature, the catalytic efficiency is a bit lower
than styrene oxide (entries 1-2). The cycloaddtion of all the
epoxides with electron withdrawing substituents (-Cl, -Br and -
OPh) proceeds more efficiently than styrene oxide (entries 3-5).
That is probably because that the C-O bonds of these substrates
are weaker than that of styrene oxide caused by the conjugation
between epoxy group and benzene ring.
Table 2. The CO2 cycloaddition with epoxides substituted with different
functional groups over ZnO@NPC-Ox-700.[a]
Table 1. The catalytic CO2 cycloaddition reaction of styrene oxide under
different conditions.[a]
Entry
Substrate
Temperature
(°C)
Time
(d)
Con.[b]
(%)
Sel.[b]
(%)
Entry
1
Catalyst
Con.[b] (%)
98
Sel.[b] (%)
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
1
2
25
45
3
3
85
90
100
100
ZnO@NPC-Ox-700 + TBAB
ZnO@NPC-Ox-800 + TBAB
ZnO@NPC-Ox-900 + TBAB
ZnO@NPC-Ox-1000 + TBAB
TBAB
2
94
3
89
3
4
5
60
60
60
1
1
2
94
>99
100
100
4
87
O
95
5
14
Br
6
ZnO@NPC-Ox-700
<1
>99
7[c]
8
ZnO@NPC-Ox-700 + TBAC
ZIF-8 + TBAB
34
[a] Reaction conditions: 1 mmol substrate, 50 mg ZnO@NPC-Ox-700, 0.2
mmol TBAB, 2 mL acetonitrile, 1 bar CO2. [b] Determined by GC and using n-
dodecane as a standard.
16
9
ZnO@NPC-700 + TBAB
ZnO + TBAB
77
10[d]
11[e]
12[f]
13[g]
14[h]
15[i]
43
ZnO@NPC-Ox + TBAB
ZnO@AC + TBAB
85
As mentioned above, extensive efforts have been devoted to
CO2 cycloaddition reaction.[2c,2d,2g,5] Therefore, it would be
naturally to compare the catalytic performance of ZnO@NPC-
Ox-700 with the corresponding systems involving catalysts of
porous carbons, ZIFs and MOFs, etc. As shown in Table S4, the
reactions over porous carbons, ZIFs and MOFs mostly require
harsh conditions and high energy (relatively high temparature
and/or CO2 pressure). Moreover, some catalysts like MOF-5 are
sensitive to moisture/water or/and a few ligands of MOF
catalysts are expensive, which are unfavorable to practical
applications. In contrast, ZnO@NPC-Ox-700 based on
inexpensive raw materials requires mild catalytic reaction
conditions and exhibits high catalytic performance and superb
stability (Figure 3 and S17).
59
NPC-Ox-700 + TBAB
ZnO@NPC-700-HNO3 + TBAB
ZnO@NPC-Ox-700-HNO3 + TBAB
50
56
58
[a] Reaction conditions: 1 mmol styrene oxide, 50 mg catalyst, 0.2 mmol
TBAB, 2 mL acetonitrile, 1 bar CO2, 60 °C, 2 d. [b] Determined by GC and
using n-dodecane as standard. [c] 0.2 mmol TBAC. [d] Catalyst: 6.9 mg ZnO
pyrolyzed from ZIF-8 in air (calculated by the actual content of Zn in
ZnO@NPC-Ox-700). [e] Catalyst: 50 mg ZnO@NPC-Ox. [f] Catalyst: 50 mg
ZnO@AC. [g] Catalyst: 43.1 mg NPC-Ox-700. [h] Catalyst: 50 mg
ZnO@NPC-700-HNO3. [i] Catalyst: 50 mg ZnO@NPC-Ox-700-HNO3.
On the basis of the above-mentioned catalytic results, we
proposed the reaction mechanism as follows: As displayed in
Scheme S1 to S3, the C-O bond of epoxide can be firstly
polarized by the phenolic hydroxyl groups, carboxylic acid or the
To further examine the general application of ZnO@NPC-
Ox-700 catalyst, various epoxides were investigated for CO2
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Zn2+ of ZnO (different acid sites).[8] Meanwhile, CO2 can be
activated by pyridinic N or the O2- of ZnO (Lewis base sites).[6c,8d]
Then, the heterolysis of the C-O bond occurs by the nucleophilic
attack of halide anion on the less sterically hindered carbon
atom of the epoxide. Subsequently, the carbon atom of CO2 is
nucleophilically attacked by the ring-open intermediate. Finally,
the internal nucleophilic attack of the ring-open intermediate
results in the formation of cyclic carbonate with the regeneration
of the catalyst and co-catalyst.
bottom flask and stirred at 105 °C for 1.5 h. After cooling down to room
temperature, the solid was obtained by filtration, washing with methanol
for several times and drying at 60 °C in vacuum.
Synthesis of ZnO derived from ZIF-8: The activated ZIF-8 powder (500
mg) was pyrolyzed at 700 °C for 2 h at a heating rate of 5 °C/min in air.
Finally, ZnO powder was obtained by cooling down to room temperature
naturally.
Synthesis of NPC-Ox-700: Typically, ZnO@NPC-Ox-700 (100 mg) was
introduced and stirred in HCl aqueous solution (1 M, 20 mL) at room
In summary, the ZnO@NPC-Ox composites have been
rationally synthesized by the pyrolysis of ZIF-8 at different
temperatures in N2 atmosphere and subsequent oxidation with a
moderate oxidant, sodium hypochlorite. In contrast to the
traditional oxidant like HNO3, the moderate oxidation not only
results in different functional groups on the surface of carbon but
also avoids to etch off important components like ZnO,
eventually offering the oxidized porous carbon with multiple
catalytic sites (ZnO, pyridinic N, hydroxyl and carboxyl, etc.).
Based on the synergy of the different types of catalytic sites and
the porosity inherited from ZIF-8, the optimized catalyst,
ZnO@NPC-Ox-700, exhibits excellent activity, selectivity,
stability and recyclability toward the CO2 cycloadditon reactions
with diverse epoxides under mild conditions. Given the structural
diversity and tailorability of MOFs, this synthetic approach would
open up an avenue to the design of advanced porous
composites involving multiple sites for diverse applications,
especially in catalysis.
temperature for
6 h. The product (NPC-Ox-700) was obtained by
centrifugation, washing with water and methanol for several times and
drying at 60 °C in vacuum.
Synthesis of ZnO@NPC-Ox and ZnO@AC: Typically, the mixture of
NPC-Ox-700 or activated carbon (AC, 200 mg), Zn(NO3)2·6H2O (118 mg,
0.396 mmol) (the same Zn2+ content as that in ZnO@NPC-Ox-700) and
ethanol (1 mL) was stirred at room temperature for 1 h. Then, ethanol
was evaporated under vacuum at 70 °C for 1 h and the dried powder was
subsequently pyrolyzed at 400 °C for 3 h at a heating rate of 5 °C/min
under a N2 flow of ~60 mL/min. The product was obtained by cooling
down to room temperature naturally.
Catalytic Performance Evaluation
The cycloaddtion of CO2 and epoxide: Typically, A mixture of catalyst
(50 mg), TBAB (65 mg, 0.2 mmol), acetonitrile (2 mL) and epoxide (1
mmol) was added into a 5 mL round-bottom flask, which was connected
to a CO2 balloon. Then, the mixture was stirring at 60 °C for 2 days.
Recyclability investigation for ZnO@NPC-Ox-700 catalyst: After the
cycloaddtion reaction of CO2 and styrene oxide for 2 days, the catalyst
(ZnO@NPC-Ox-700) was separated by filtration, washed with methanol
for several times and dried overnight at 60 °C in vacuum. After that, the
catalyst was used by another cycloaddtion of CO2 and styrene oxide.
This procedure was conducted up to ten times to examine the
recyclability of ZnO@NPC-Ox-700.
Experimental Section
Preparation of catalysts
Synthesis of ZIF-8 nanocrystals: ZIF-8 nanocrystals were prepared
according to the previous literature with minor modifications.[18] Typically,
Zn(NO3)2·6H2O (1.68 g, 5.65 mmol) was dissolved in 80 mL methanol.
Then a mixture of 2-methylimidazole (3.70 g, 45 mmol) and 80 mL
methanol was added to the above solution with stirring for 24 h at room
temperature. The precipitate was separated by centrifugation, washing
with methanol for three times and soaking in fresh methanol for one day.
Finally, the ZIF-8 powder was obtained by centrifugation, washing with
methanol for three times and drying overnight at 60 °C in vacuum.
Acknowledgements
This work was supported by the 973 program (2014CB931803),
the NSFC (21371162, 21673213 and 21521001), the State Key
Lab of Materials-Oriented Chemical Engineering (KL15-02) and
the Recruitment Program of Global Youth Experts.
Synthesis of ZnO@NPC: The activated ZIF-8 powder (500 mg) was
pyrolyzed at different temperatures (700, 800, 900, 1000 °C) for 2 h at a
heating rate of 5 °C/min under a nitrogen flow of about 60 mL/min. Finally,
the resulting product was obtained by cooling down to room temperature
naturally.
Keywords: carbon • metal–organic frameworks • doping • CO2
fixation • heterogeneous catalysis
[1]
[2]
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Meili Ding, Si Chen, Xiao-Qin Liu, Lin-
Bing Sun, Junling Lu and Hai-Long
Jiang*
Page No. – Page No.
Metal-Organic Framework-Templated
Catalyst: Synergy in Multiple Sites for
Catalytic CO2 Fixation
The ZnO nanoparticles encapsulated N-doped porous carbon with multiple catalytic
sites has been rationally synthesized by pyrolysis of a metal-organic framework
followed by moderate oxidation treatment. Based on the synergy of multiple sites
(ZnO, pyridinic N, hydroxyl and carboxyl groups), the resultant catalyst exhibits
excellent activity, selectivity and recyclability in the CO2 cycloaddtion reactions with
epoxides under mild conditions.
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