Modulating the Acidic and Basic Site Concentration of Metal-Organic Framework Derivatives to Promote the Carbon Dioxide Epoxidation Reaction

Article abstract of DOI:10.1002/chem.202100430

Metal-organic framework (MOF) is an ideal precursor/template for porous carbon, and its active components are uniformly doped, which can be used in energy storage and catalytic conversion fields. Metal-organic framework PCN-224 with carboxylporphyrin as t

Full text of DOI:10.1002/chem.202100430

Accepted Article
Accepted Article
Title: Modulation the acidic and basic site concentration of the metal-
organic framework derivatives to promote the carbon dioxide
epoxidation reaction
Authors: Jinhua Ji, Hao Liu, Zewei Chen, Yajun Fu, Weijun Yang, and
Shuang-Feng Yin
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.202100430
Link to VoR: https://doi.org/10.1002/chem.202100430
01/2020

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Modulation the Acidic and Basic Site Concentration of the Metal-
organic Framework Derivatives to Promote the Carbon Dioxide
Epoxidation Reaction
Jinhua Ji,[a] Hao Liu,[a] Zewei Chen,[a] Yajun Fu,[a] Weijun Yang*[a], Shuang-Feng Yin[a]
[a]
J.-H. Ji, H. Liu, Z.-W. Chen, Y.-J. Fu, Prof. W.-J. Yang, Prof. S.-F. Yin
College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China
*E-mail: wjyang@ hnu.edu.cn
Supporting information for this article is given via a link at the end of the document.
Abstract: Metal-organic Framework (MOF) is an ideal
To promote the adsorption of carbon dioxide, the MOF can be
precursor/template for porous carbon, and its active components are designed with different metal ions/clusters and different organic
uniformly doped, which can be used in energy storage and catalytic linking groups to provide advanced features,[7] such as high CO2
conversion fields. Metal-organic framework PCN-224 with
absorption,[8] permanent porosity and active catalytic sites.[9]
carboxylporphyrin as ligand was synthesized, and then Zn2+ and Zinc-based ZIF MOF (ZIF-8, ZIF-90, ZIF-68, etc.) and its
Co2+ ions were coordinated in the center of the porphyrin ring by derivatives are first used as MOF catalysts for the preparation of
post-modification. Here, PCN-224-ZnCo with different ratios of cyclic carbonates by CO2 conversion from epoxides for the first
bimetallic Zn2+/Co2+ ions were used as the precursor, and the metal- time.[10] However, the pore size of this type of MOF is extremely
nitrogen-carbon(M-N-C) material of PCN-224-ZnCo-950 was
small, the ability to adsorb CO2 is extremely weak, and the small
obtained by pyrolyzing the precursor at 950 °C in Ar. Because Zn is particles of ZIF require complicated purification steps to be
easy to volatilize at 950 °C, the formed M-N-C materials can reflect separated from the reactants.[11]
different Co contents and different basic site concentrations. The
formed material still maintains the original basic framework. With the
increase of Zn2+/Co2+ ratio in precursor, the concentration of N-
MOF catalysts based on porphyrin structural units, such as
containing alkaline sites in pyrolysis products gradually increase.
Compared with the precursor, PCN-224-ZnCo1-950 with
Zn2+/Co2+=1:1 has greatly improved basicity and suitable acidic/
alkaline site concentration. It can be efficiently used to carbon
dioxide absorption and catalyze the cycloaddition of CO2 with
epoxide. More importantly, the current method of adjusting the
acidic/basic sites in M-N-C materials through volatilization of volatile
metals can provide an effective strategy for adjusting the catalysis of
MOF derivatives with porphyrin structure.
PCN-224, have large pore size and relatively stable
performance, with a pore size of about 19.1Å.[12] More and more
researchers begun to study the pore size of MOF and tried to
improve the catalytic reaction by increasing the pore size of
MOF.[13]
PCN-224 (Co) is a kind of PCN-224 MOF with Co metal ions
coordinated in the porphyrin center. As an acidic site, Co can
effectively promote the ring opening of the epoxy compounds
and greatly enhance the addition efficiency of CO2 in the
reaction. However, the N base active site in porphyrin is
occupied, which greatly weakens the absorption of CO2 by the N
active site.[14] Therefore, balancing the catalytic activity of acid-
base active centers is an important task to improve porphyrin
MOF catalyst.[15] In addition, the catalyst can be simply and
effectively separated from the reaction mixture by using the
magnetism of cobalt,[16] which is beneficial to the recycling of the
catalyst.[17]
Introduction
As we all know, the global warming caused by CO2 is becoming
more and more serious, so the use of CO2 as carbon source has
been widely concerned in the research.[1] On this premise,
cycloaddition of epoxide and CO2 to form cyclic carbonate is an
atom-economic method to obtain high-yield cyclic carbonate.[2]
Metal-organic framework materials have received extensive
attention in the past decades due to their designable structure
and different physical and chemical properties. Metal-organic
framework materials have attracted extensive attention in the
past decades because of their designable structures and
different physical and chemical properties.[3] Various application
fields including gas storage, separation and catalysis have been
explored and developed, including the adsorption and fixation of
carbon dioxide.[4] The cycloaddition of carbon dioxide and epoxy
compounds to form cyclic carbonates is an atomic economic
process,[5] and cyclic carbonates are used as an electrolytes for
lithium ion batteries and intermediates for the synthesis of fine
chemicals. However, under mild conditions, the kinetic inertia
and stability of CO2 are greatly limited in its utility as a C1
source.[6]
In this study, based on PCN-224 coordinated with two metals,
Zn and Co, the bimetallic PCN-224 is pyrolyzed at a proper
temperature to volatilize Zn, which can not only release part of
basic N sites,[18] but also produce some high-activity defects and
increase the content of coarse pores and mesopores. This can
greatly improve the activity and reaction efficiency of the catalyst.
Pyrolysis of MOF can provide porous carbon with high surface
area and uniform heteroatom distribution. For example, ZIF-
MOF can provide high N content and/or CoNx active sites in
products.[19] After pyrolysis of porphyrin-based MOF, MNx
species of different metals will be stabilized in the carbon matrix
to ensure high dispersion and show synergistic effect in
catalysis.[20]
1
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According to the literature reports,[21] metalloporphyrin is easier
to generate PCN-222 than PCN-224, while metalloporphyrin is
difficult to generate metalized PCN-224. Therefore, in order to
obtain bimetal (Zn and Co) PCN-224-ZnCo, we first synthesized
PCN-224 without metal in the center of porphyrin ring, and then
post modified PCN-224 with ZnCl2 and CoCl2·6H2O to obtain
bimetal (Zn and Co) PCN-224-ZnCo.
Synthesis of PCN-224-ZnCo1-950: PCN-224-ZnCo1 powder
(100mg) was placed in a tubelar furnace and heated to 950 °C
at a rate of 5 °C/min under flowing argon. The porous carbon
composite PCN-224-ZnCo1-950 was obtained by pyrolyzing the
powder at 950 °C for 2h and then naturally cooling to room
temperature. The product was vacuum dried at 80 °C for 24
hours before use.
Experimental Section
Catalytic Performance: The catalyst is used to catalyze the
cycloaddition reaction of CO2 and epoxy compounds, which can
obtain cyclic carbonate with high efficiency. The typical
experimental procedure is as follows: Add PCN-224-ZnCo1-950
(10 mg) and tetra-n-butylammonium bromide (0.216 mmol)
cocatalyst are added into a solvent-free schlenk tube, and a
balloon filled with carbon dioxide is placed at the nozzle.
Evacuate the test tube, and then continue to add epoxides
(propylene oxide, epichlorohydrin, 1,2-epoxy octane, n-butyl
glycidyl ether and epoxy propylene, etc. 5 mmol) to the schlenk
tube. Schlenk tube was putted into an oil bath at 50 °C for 20 h.
After the reaction, the mixture was cooled to room temperature
and separate the catalyst from the mixture with a high-speed
centrifugation. Chloroform was added to small amount of
supernatant to gain the reaction mixture. Use 1H NMR (CDCl3,
TMS as internal standard) to analyze and calculate the
conversion rates of the epoxy carbonates. The separated
catalyst was washed several times with acetone, and then dried
in a vacuum drying oven for 10 hours before being stored for the
next reaction.
All chemicals are purchased from Shanghai Aladdin Bio-Chem
Technology Co, LTD and used without any further purification.
Synthesis of organic ligands: According to previous reports,[22]
tetrakis(4-carboxyphenyl) porphyrin (H2TCPP) was synthesized
with minor modifications according to previous reports.
Generally, pyrrole (3.0 g, 0.043 mol) and methyl p-
formylbenzoate (6.9 g, 0.042 mol) are added into a three-necked
flask, and then 100 mL of propionic acid is slowly added to the
three-necked flask in a fume hood. The solution is heated to
160°C, refluxed for 12 hours, and then cooled to room
temperature. The resulting solution is filtered in a fume hood.
The solid is washed with ethanol, ethyl acetate and
tetrahydrofuran respectively, and dried in a vacuum oven for 10
h to obtain (TPPCOOMe). Then put the obtained solid (1.95 g)
into a three-necked flask, dissolve it with tetrahydrofuran (60 mL)
and methanol (60 mL), and then add a solution of KOH (6.82 g)
in H2O (60 mL). The mixture solution is heated to 80 °C, refluxed
for 12 hours, and then cooled to room temperature. H2O is
added to ensure that the solid is completely dissolved, and then
the solution is adjusted to Ph=3 with 1 mol/L HCl, and no more
purple precipitate is produced. The purple solid is filtered,
washed with 100 mL of water, and dried in a vacuum drying
oven to obtain a blue-purple solid (H2TCPP).[23]
Results and Discussion
Morphology and Structure Characterization
With reference to Zhou’s literature and many improvements,[25]
Zr-based porphyrin MOF (PCN-224)with high stability was
synthesized, based on six-linked Zr6 clusters and tetra(4-
carboxyphenyl) porphyrin (TCPP).Porphyrin ligands form
mesoporous MOF, and then
active Zn2+ and Co2+ are
Synthesis of PCN-224: A few modifications have been made to
the synthesis of PCN-224 according to the latest reports.[24]
Typically, ZrCl4(120 mg), H2TCPP (50 mg) and benzoic acid
(1200 mg) are dissolved ultrasonically in DMF (8 mL) in a 20 mL
Pyrex vial. The mixture is heated in an oven at 120 °C for 24 h.
After cooling to room temperature, cubic dark purple crystals are
collected by filtration.
introduced into the center of the porphyrin ring. The molar ratio
of the two metals is 1:1, so the bimetallic PCN-224 in the text is
named as PCN-224-ZnCo1 (Scheme 1). Finally, after pyrolysis at
950 °C, the structure with highly dispersed ZnNX and CoNx is
transformed into porous carbon nanocubes (Figure 1).The
image of the electron microscope shows that under pyrolysis at
a high temperature of 950 °C, the original cube frame did not
collapse, and some crystals even appeared hollow structures.
With the increase of mesopores and macropores, the active
sites of the catalyst are fully contacted with reactants, which
reduces the mass transfer resistance and greatly shortens the
reaction time. Moreover, there is a synergistic effect between Co
Synthesis of PCN-224-ZnCo1: Putting 1.125mmol of ZnCl2 and
1.125mmol of CoCl2·6H2O (the molar ratio of Zn to Co is 1:1)
into a three-necked flask filled with 30mL DMF, then add 200mg
of PCN- 224, heat to 120°C to react for 12 hours, and then
centrifuge the mixture. Pour out the liquid, wash the remaining
solid twice with fresh DMF and twice acetone with low-boiling
point, and finally put it into a vacuum drying oven and dried at 70
C for 8h to obtain PCN-224-ZnCo1.
and exposed
N
active sites. Calcination increases the
concentration active sites of basic N and promotes the degree of
graphitization, thereby increasing the activity and stability of the
catalyst.
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Scheme 1. The main synthesis process of PCN-224-ZnCo1-950.
highest content of pyrrole N is generated after the volatilization
of Zn in the ZnNx structure of the protoporphyrin unit. It has the
dual characteristics of basic sites and defects formed in PCN-
224-ZnCo1-950 with pyrolysis, which is also an important reason
for the high catalytic activity.
We use pyrolysis to remove Zn from PCN-224-ZnCo1, so it
thereby releases thereby releasing higher alkaline active sites.
The concentration of alkaline active center formed in the raw
material with ratio of Zn/Co=1:1 is reasonably matched with the
concentration of acidic sites formed by Co and Zr in the same
raw material (PCN-224-ZnCo1-950), which produces the best
catalytic effect in the epoxidation of carbon dioxide (Table 2).
Peak of PCN-224-ZnCo1-950 at 789.5eV corresponds to 3p3/2 of
Co element. In the high-resolution 2p spectrum of Co, 781.5eV
indicates that there are Co-N species in the sample, which
proves the interaction between Co and N. In the C 1s spectrum,
it shows that there are four types of C atoms, including C=C, C-
N, C-O and C=N.PCN-224 is chosen because it has 3D porous
structure, very high surface area, and most importantly, the
hollow porphyrin center is easy to metallize. Through controlled
the process of pyrolysis,[26] PCN-224-ZnCo1-950 was obtained. It
can be seen from the electron micrograph that the morphology
of PCN-224-ZnCo1-950 is similar to that of PCN-224 before
modification, showing a concave cubic shape (Figure 1).
Fourier transform infrared (FTIR) spectrum is shown in Figure 2.
Compared with H2TCPP ligand, the asymmetric vibrational
absorption intensity of the C-OH and C=O groups is greatly
reduced after the Zr4+ coordinated with the -COOH group in
PCN-224. By post-modifying PCN-224 with Zn and Co ions, it
can be observed that the synthesized PCN-224-ZnCo1 has no N-
H bond absorption peak. In addition, PCN-224-ZnCo1 has Co-N
and Zn-N absorption peaks at 1004cm-1 and 1008cm-1 (Figure
2b, Figure 2c), which further reflects the metallization of
porphyrin ring with Zn and Co, and then PCN-224-ZnCo1 is
pyrolyzed at a temperature of 950 °C, the disappearance of the
Zn-N bond (Figure 2c) is sufficient to prove the disappearance of
Zn.
From the pore size and specific surface analysis results, it can
be seen that PCN-224-ZnCo1-950 shows a high BET surface
area (1240 m-2g-1), and the pore size is mesopores (S5-S7)
concentrated in about 3 nm. However, from the comparison of
pore sizes before and after pyrolysis, it can be seen that after
pyrolysis (Figure S6,7), a relatively large pore size of about 10-
20 nm has been appeared, indicating that pyrolysis has formed
a graded pore distribution, which is conducive to the diffusion of
substances in the catalytic process, thus promoting the
cycloaddition efficiency.[27] During the pyrolysis process, the Co
species of PCN-224-ZnCo1-950 has a better diffusion coefficient
which can better diffuse to the outer surface, and then the
pyrolysis product of organic ligands are graphitized around the
outer Co.[28] When the carbon shell exceeds the critical thickness
that can stabilize it, the structural collapse, thus forming a hollow
structure of porous carbon (Figure 1f).
Similarly, we analyzed PCN-224-ZnCo1-950 by X-ray
photoelectron spectroscopy (XPS), and further determined the
chemical state of each element. As shown in Figure 3a, XPS
spectrum clearly depicts the presence of C, N, O, Co, while the
existence of Zn is not detected, which illustrates the absence of
Zn. In order to further verify the lack of Zn, the true contents of
zinc and cobalt were determined by ICP-AES. It can be seen
that the actual Zn and Co contents in PCN-224-ZnCo1-950 are
0.03 and 0.59wt%, respectively (Table S1). The high-resolution
N 1s XPS spectrum can fit four sub-peaks, indicating that there
are four different types of N, namely pyridine N, graphitized N,
pyrrole N, and oxide N. The peak positions are 398eV and
400.9eV, 399.3eV and 404.5eV. It can be seen from Figure 3b
that the
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Figure 1. SEM images of (a), (b) PCN-224, (c), (d) PCN-224-ZnCo1 and (e), (f) PCN-224-ZnCo1-950.
Figure 2. (a) FTIR spectra of PCN-224ZnCo1-950, PCN-224-ZnCo1 and PCN-224; (b), (c) The magnified FTIR spectra of PCN- 224-
ZnCo1-950, PCN-224-ZnCo1 and PCN-224.
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The formation of pyrolysis products with partial hollow structure
is very beneficial to promote catalytic reaction kinetics. During
pyrolysis process, the Zn/Co species can catalyze graphitization
of organic structures. The HRTEM images clearly show the
lattice stripes of graphitized carbon and Co2O3 metal clusters.
The interplanar spacing is 0.34 nm, corresponding to the (002)
carbon plane of graphite carbon (Figure 4b),which is consistent
with the peak value of θ=25.8°in PXRD pattern (Figure S4).
After pyrolysis, cobalt exists in the form of Co2O3 nanoclusters
with a diameter of 2-3nm and is uniformly embedded in the
network of nitrogen-carbon materials. From the HTEM image,
the lattice fringe spacing of Co2O3 is 0.28, corresponding to the
(225) crystal plane, which is consistent with the peak at θ=44.2°
in the PXRD image. The nano-ZrO2 structure appeared in the
material was formed after the pyrolysis of the node ZrO6 cluster.
The lattice fringe spacing of ZrO2 is 0.3nm, corresponding to the
(101) crystal plane, which is consistent with the peak at θ=43° in
the PXRD pattern (Figure S4).
The degree of graphitization of PCN-224-ZnCo1-950 can be
further characterized by Raman scattering spectrum (Figure S9).
The D and G band at 1322 and 1560 cm-1 correspond to
disordered and sp2 hybrid graphitized carbon, respectively. The
smaller the ratio, the higher the degree of graphitization in the
carbon materials. According to the test, the ID/IG of the material
is 1.06. The graphitization process further adjusts the electronic
structure of the PCN-224-ZnCo1-950 material, which also
promotes the catalytic reaction.
Figure 3. (a) PCN-224-ZnCo1-950 XPS total spectrum. The
high-resolution XPS spectra of (b) N 1s, (c) Co 2p and (d) C 2s
in the PCN-224-ZnCo1-950 sample.
Catalytic Performances for CO2 cycloaddition
The acidity and alkalinity of the catalysts play a very important
role in catalyzing the addition reaction of carbon dioxide and
epoxide to form cyclic acid esters. In this paper, the acidity and
alkalinity characteristics of catalytic materials are measured by
temperature program desorption (TPD) technology, using NH3
(NH3-TPD) and CO2 (CO2-TPD) as probe gases. Tests of CO2-
TPD and NH3-TPD clarified the concentration of alkalinity and
acidity in pyrolyzed PCN-224-ZnCo1-950 material (Figure 5,
Table 1).MOF composed of zinc porphyrin units is pyrolyzed at
high temperature, and the N-site generated by the zinc metal
volatilization process becomes a non-coordinating basic site,
which plays
a very important role in the adsorption and
activation of CO2 and can interact synergistic catalysis occurs at
acidic sites. It can be seen from Figure 5 and Table 1 that the
PCN-224-ZnCo1-950 obtained after pyrolysis has higher
alkalinity and lower acidity than the precursor PCN-224-ZnCo1. It
can be known from the subsequent catalytic results (Figure 6,
Table 2) that PCN-224-ZnCo1-950 has excellent catalytic
performance for the cycloaddition reaction of carbon dioxide and
epoxy compounds. Before pyrolysis, the ratio of alkalinity/acidity
in PCN-224-ZnCo1 is 0.0092. After pyrolysis, the ratio of
alkalinity/acidity in PCN-224-ZnCo1-950 is 0.0267, which has
increased by nearly 3 times.
As can be seen from Figures 3 and 4, after PCN-224-ZnCo1
pyrolyzed, it showed well-dispersed Co2O3 nanoclusters in the
N-doped porous carbon matrix. The mechanism of
heterogeneous catalyst to catalyze the cycloaddition of CO2 in
an N-doped carbon matrix can be attributed to the synergistic
effects of basic sites derived from the activated N compounds
(N-doped carbon matrix) with Co sites. The acidic site properties
of Co will activate the ring opening of the epoxide (Figure 6). In
addition, the existence of well-dispersed Co species surrounded
by protective carbon walls may contribute to binding and
Figure 4. (a), (b), TEM of PCN-224-ZnCo1-950. (c) TEM of
PCN-224-ZnCo1-950 and corresponding C, N, Co, and Zr
elemental mappings for PCN-224-ZnCo1-950.
5
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activating the epoxides (substrates). The alkalinity of N-
compound formed by the volatilization of zinc can not only
activate the carbon dioxide, but also enhance its adsorption,
thus promoting the catalytic reaction. From the above-mentioned
alkalinity to acidity concentration ratio, it can be seen that in a
good cycloaddition catalyst, the concentration of acidic sites is
still much higher than the alkali site concentration, reaching
more than 37 times (Table 1), which is not a simple one-to-one
relationship.
programmed desorption (TPD) on all samples. The sample was
pretreated at 250 °C for 1 h under carrier gas helium, and then
cooled to the adsorption temperature of 25 °C. NH3 and CO2 are
introduced into the sample as detection gases by continuous
flow. The probe gas is adsorbed to saturation at an adsorption
temperature of 25 °C, which can be observed from the stable
signal of the TCD detector. Then the sample is flushed with
helium until a stable transcranial Doppler signal is obtained, and
the physically adsorbed gas on the sample is removed and use
the TCD detector to measure the desorption temperature of the
program, and the temperature will rise to 270 °C at a rate of
10 °C / min.
To test TPD, we used Micromeritics Chemisorb 2750 automation
system and ChemiSoft TPx software to perform temperature
Figure 5. Temperature program desorption curve of PCN-224-ZnCo1-950: (a) NH3-TPD. (b) CO2-TPD.
content of Zn/Co metal salt (mol%, Zn:Co=1:2, Zn:Co=2:1,
Zn:Co=1:1) and reacted
Table 1 Concentration of acidic and basic sites in PCN material a
with PCN-224 to obtain several kinds of metalized PCN-224 with
different Zn2+/Co2+ contents. Then calcine these samples in a
ꢀamples
ꢁcidic* (mmol/g)
ꢂasic* (μmol/g)
tube furnace at 950 °C in an argon atmosphere respectively.
The products were applied to catalyze the reaction of CO2 with
1,2-epoxyoctane, the reaction time 8h. The measured catalysts
were PCN-224-ZnCo3-950 (Zn:Co=1:2), PCN-224-ZnCo2-950
(Zn:Co=2:1), PCN-224-ZnCo1-950 (Zn:Co= 1:1). The product is
cyclic octane carbonate, and the conversion rates of 1,2-
epoxyoctane are 43.7%, 40.5%, and 55.8% respectively (see
Table 2). It can be seen that the Zn/Co ratio is higher than 1 or
less than 1, the catalytic activity is not as good as the catalyst
PCN-224-ZnCo1-950 with the Zn/Co ratio equal to 1. In addition,
PCN-224 containing no metal and PCN-224-Co containing only
cobalt were further calcined at 950 °C. After strict heat treatment,
two calcination catalysts PCN-224-950 and PCN-224-Co-950
were obtained, and their catalytic performance was tested. The
CO2 and 1,2-epoxyoctane were used in the ring reaction, and
the reaction was carried out at 45 °C for 8h. The catalytic
conversion rates of PCN-224-950, PCN-224-Co-950, and PCN-
224-ZnCo1-950 to 1,2-epoxyoctane were 36.2%, 48.1%, and
55.8%, respectively (Table 2). It can be seen that the catalytic
effect of metal-free catalyst (equivalent to full zinc content and
then all volatilization) and catalyst containing total cobalt acidic
sites is not as good as PCN-224-ZnCo1-950 with a ratio of
Zn/Co=1:1. The substrate for catalyzing the reaction is enlarged.
Different substrates were used to test the catalytic performance
of PCN-224-ZnCo1-950 (Table 3). The reaction conditions:
NH3-TPD
CO2-TPD
PCN-224-ZnCo1
0.429
0.264
3.96
7.06
PCN-224-ZnCo1-
950
[a]: Acidic/basic concentrations were obtained by area
integration of the peak area of CO2-TPD and NH3-TPD,
respectively
It can be seen from the above discussion that the ratio of the
basic sites of N to acidic sites of cobalt will affect the conversion
rate of the CO2 reaction. If the basic sites of N increase greatly
and the acidic sites are reduced, will the catalytic performance
continue to improve? Although there are many basic sites that
can greatly absorb the acidic gas CO2, not enough acid sites will
also affect the progress of the reaction. We mixed different
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catalysts 10 mg, 5 mmol of epoxy compound, 0.216 mmol of
cocatalyst nBu4NBr,[29] 50 °C, 1 bar CO2 and 16h. The results
are shown in Table 3. It can be seen from Table 3 that the
catalyst also has a very significant catalytic epoxidation effect on
epoxy compounds with long carbon chains, which benefits (for
forming) from its larger pore structure and graded pore
characteristics. Compared with the catalysts reported in
literature (Figure S8), PCN-224-ZnCo1-950 has obvious
advantages. After the reaction was completed, the used catalyst
was washed three times with methanol, and then dried in
vacuum at 80 °C for 6h. The results of repeated use
experiments showed (Figure S10) that after 8 times of repeated
use, the catalytic performance has changed little.
Table 2: Catalytic activities of different catalysts a
Entry
Catal.
Sub.
Temp.(°C)
Cyclic Carbonate
Yield (%)
1
2
3
4
5
6
7
8
PCN-224
Epoxy octane 45
Epoxy octane 45
Epoxy octane 45
Epoxy octane 45
Epoxy octane 45
Epoxy octane 45
Epoxy octane 45
Epoxy octane 45
26.4
32.8
33.6
43.7
40.5
36.2
48.1
55.8
PCN-224-Co
PCN-224-ZnCo1
PCN-224-ZnCo3-950
PCN-224-ZnCo2-950
PCN-224-950
PCN-224-Co-950
PCN-224-ZnCo1-950
[a]:Reaction conditions: Epoxy octane(5 mmol), catalyst (10 mg), nBu4NBr (0.216 mmol), 45 °C,8 h, 1 atm CO2.
Table 3 PCN-224-ZnCo1-950 catalyzed the cycloaddition reaction of various substrates with carbon dioxide
Entry
Epoxide
Product
Yield
（%）
1
100
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2
3
98
67
4
5
62
59
Reaction condition: expoxides (5 mmol), PCN-224-ZnCo1-950 catalyst (10 mg), nBu4NBr (0.216 mmol),
50 °C, 16h, 1 bar CO2. Conversion was determined by 1H NMR spectroscopy.
Figure 6． Diagram of the reaction process of carbon dioxide cycloaddition.
which also have an important role in promoting the cycloaddition
reaction of carbon dioxide.
According to the research results above and relevant data in
reference,[30] the process of PCN-224-ZnCo1-950 catalyzing the
cycloaddition reaction of carbon dioxide can be deduced, as
shown in Figure 6. First, acidic sites, such as metallic Co
nanoclusters, coordinate with the O of the epoxide to activate
CONCLUSION
epoxy compounds. At the same time, basic sites, such as
pyridine nitrogen or pyridine nitrogen, can easily capture and
adsorb CO2. The lone pair attacks the carbonyl group of CO2
and activates CO2. Afterwards, negatively charged O in CO2
attacks the C in epoxide and opens the ring. While C-O bond are
formed, the coordination bonds between metal clusters and O
are broken. Finally, the intramolecular O anion attacks the C in
CO2, the C-N bond broken, and the ring closed. Thus, the
corresponding cyclic carbonate is obtained. During the reaction
process, the mobile electrons in nitrogen-carbon structure plays
a significant role in promoting the intramolecular charge transfer
during the acid/base co-catalysis process. The hollow structure,
larger pore size and hierarchical pore structure can promote the
raw material adsorption and the product diffusion; the
volatilization of zinc and the diffusion, migration and fusion of
cobalt particles will produce some unsaturated defect structures,
Metal-organic framework PCN-224 MOF with porphyrin ligand
as structural unit was synthesized. Zn and Co metal ion
solutions were added to metallization in different proportions,
and then pyrolyzed into stable catalyst under argon atmosphere
for CO2 conversion reaction. Due to the low melting point of Zn,
it can be volatilized at 950 °C, thereby releasing abundant basic
sites to improve the conversion rate of the carbon dioxide
cycloaddition. As the ratio of Zn/Co in the precursor increases,
the concentration of N-containing basic sites in the pyrolysis
product gradually increases. Among them, PCN-224-ZnCo1-950
with Zn/Co=1:1, is a catalyst whose basicity is greatly improved
compared with that of the precursor. It has a suitable acid/basic
site concentration ((acid site concentration)/ (basic site
concentration) =37.4), and can efficiently catalyze the
cycloaddition reaction of carbon dioxide and epoxide. Thanks to
its macroporous and hierarchical pore structure, the catalyst has
8
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10.1002/chem.202100430
Chemistry - A European Journal
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an excellent catalytic effect on epoxy compounds with longer
carbon chains. Better than most catalysts reported in the
literature. After PCN-224-ZnCo1 was pyrolyzed, it showed well-
dispersed Co2O3 nanoclusters in the N-doped porous carbon
matrix. The mechanism of the heterogeneous catalyst to
catalyze the cycloaddition of CO2 in an N-doped carbon matrix
can be attributed to the synergistic effects of basic sites derived
from the activated N compounds (N-doped carbon matrix) with
Co sites (Co2O3 nanoclusters).Firstly the acidic sites coordinate
with the O of the epoxide and activate the epoxy compound. At
the same time, the basic N-containing active sites are easy to
capture and adsorb CO2, and its lone pair of electrons attack the
carbonyl group of CO2 to make CO2 activated. Then activated
CO2 reacts with activated epoxy compound to generate
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Acknowledgemengts
The authors would like to give thanks for the financial support
from the National Natural Science Foundation of China (No.
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Keywords: PCN-224 derivative; pyrolysis; metalloporphyrin;
carbon dioxide conversion; cycloaddition
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10.1002/chem.202100430
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Entry for the Table of Contents
We successfully synthesized PCN-244-ZnCo1, which was then pyrolyzed to the target product PCN-224-ZnCo1-950 at 950 °C. The
physical structure of PCN-224-ZnCo1-950 did not change much as judged by SEM and N2 adsorption and desorption. Due to the
volatilization of Zn, PCN-224-ZnCo1-950 has greatly improved basicity and suitable acidic/alkaline site concentration compared with
the precursor. It can be efficiently used to carbon dioxide absorption and catalyze the cycloaddition of CO2 with epoxides.
10
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