Synthesis of a Pyridine–Zinc-Based Porous Organic Polymer for the Co-catalyst-Free Cycloaddition of Epoxides

Article abstract of DOI:10.1002/asia.201700258

The synthesis of solid catalysts for the co-catalyst-free cycloaddition of CO₂ has attracted much attention. Herein, we report a hierarchical porous organic polymer, Py-Zn@MA, that is able to catalyze the cycloaddition reaction of epoxides and CO₂ without using any additives or co-catalyst to afford turnover frequency (TOF) values as high as 250 and 97 h^?1 at 130 °C by using pure and diluted CO₂ (simulating flue gas), respectively. These results are superior to those obtained from previously reported heterogeneous co-catalyst-free systems. The high activity of Py-Zn@MA is mainly attributed to its bifunctional nature with ZnBr₂ and pyridine activating the epoxide in a cooperative way. Notably, Py-Zn@MA can be easily prepared on a large scale without using any catalyst and the chemicals are cost effective. Moreover, Py-Zn@MA shows good substrate universality for the cycloaddition reactions of epoxides. Our designed porous organic polymer Py-Zn@MA material has the potential to serve as an efficient catalyst for the direct conversion of flue gas with epoxides into value-added cyclic carbonates.

Full text of DOI:10.1002/asia.201700258

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Synthesis of pyridine-zinc-based porous organic polymer for
cocatalyst-free cycloaddition of epoxides
Authors: He Li, Chunzhi Li, Jian Chen, Lina Liu, and Qihua Yang
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Chemistry - An Asian Journal
10.1002/asia.201700258
FULL PAPER
Synthesis of pyridine-zinc-based porous organic polymer for
cocatalyst-free cycloaddition of epoxides
[
a]
[a,b]
Jian Chen,[a,b] Lina Liu,[a,b] Qihua Yang*[a]
He Li, Chunzhi Li,
Dedication ((optional))
Abstract: Synthesis of solid catalysts for co-catalyst free CO
2
catalysts such as metalloporphyrins and metal-salen complexes
exhibit high activity and selectivity for synthesizing cyclic
carbonates.[14-16] Recently, Ema and co-workers reported a
serious of functional zinc-porphyrin complexes which showed
cycloaddition reaction has attracted much research attention. Herein
we report a hierarchical porous organic polymer, Py-Zn@MA which
could catalyze cycloaddition reaction with epoxides and CO
2
without
-1
using any additives or co-catalyst to afford turnover frequency (TOF)
high activity (TOF up to 31500 h ) for CO
2
cycloaddition
-
1
o
reaction.[16] While for most of homogeneous catalysts, the
difficulty in catalyst reuse and product purification impedes their
wide applications in industry.
2
as high as 250 and 97 h at 130 C using pure and diluted CO (a
simulating flue gas), respectively. These results are superior to most
ever reported heterogeneous co-catalyst free systems. The high
activity of Py-Zn@MA is mainly attributed to its bifunctional nature with
Up to now, several types of heterogeneous catalysts have
been developed, including metal oxides,[17] functional graphene
oxide,[18] supported catalysts,[19] zeolitic imidazolate frameworks
2
ZnBr and pyridine activating epoxide in a cooperative way. It is worth
mentioning that Py-Zn@MA could be easily prepared in large-scale
without using any catalyst and the chemicals are cost-effective.
Meanwhile, Py-Zn@MA shows good substrate universality for
cycloaddition reactions of epoxides. Our designed POP material Py-
Zn@MA has the potential to serve as an efficient catalyst in direct
conversion of flue gas with epoxides into value-added cyclic
carbonates.
[
20-27]
(ZIFs) & metal-organic frameworks (MOFs),
porous organic
polymers (POPs),[
28-34]
etc. Recently, porous organic polymers
(POPs) have demonstrated potential applications in gas storage
and separation,[35-38] energy transfer and conversions,[39,40]
sensing,[41,42] heterogeneous catalysis[28-34,43-47] and so on. POPs
are emerging as burgeoning catalysts due to their high surface
areas, controllable pore structures, physical and chemical stability.
Furthermore, the porosity of POPs enables to overcome diffusion
limitation. To date, several research groups have developed
Introduction
POPs for catalyzing CO
2
cycloaddition reactions. Deng’s group
reported several salen-Co/Zn/Al coordinated POPs which exhibit
The average global temperature rise is closely related to
cumulative emissions of greenhouse gases caused by the
utilization of fossil fuels such as coal, oil and nature gas.[1-3] Thus
carbon capture and storage (CCS) and further usage have
attracted worldwide interest and should be a major global task of
the twenty-first century.[4,5] As an abundant, nontoxic and
good activities for catalyzing CO
atmospheric pressure and room temperature.
2
cycloaddition reaction at
[28,48]
Recently, Liu
and co-workers reported that a mesoporous polymer Zn/HAzo-
POP shows very high TOF of 2888 h-1 for cycloaddition of CO
2
and propylene oxide.[29] In most cases, co-catalysts are generally
needed, which is not beneficial for product purification and
practical application. Han and co-workers reported an ionic liquid
2
recyclable carbon source, CO can be used as a C1 building block
for synthesizing lots of value-added chemicals such as methanol,
formic acid, carbonates, methylamines, carboxylic acids, etc.[6-11]
Among them, synthesis of cyclic carbonates and polycarbonates
(
IL) based cross-linked polymer for the cycloaddition of CO
epoxides without using any co-catalysts at high pressure of CO
6 MPa).[49] Ding’s group reported that porous organic polymers
co-functionalized quaternary phosphonium salt and ZnBr -PPh
could efficiently catalyse the CO cycloaddition reactions without
2
with
2
(
2
from epoxides and CO is all-important and has been already
2
3
industrialized since this reaction is of 100% atom economic and
the product is of high economic worth.[12,13] Numerous
homogeneous and heterogeneous catalysts have been designed
2
any co-catalyst.[31] Recently, we reported the synthesis of
polymers functionalized with ILs and hydroxyl group for co-
for catalyzing CO
2
cycloaddition reaction.[14-34] Homogeneous
catalyst free cycloaddtion of CO
2
and epoxides via a cooperative
activation pathway.[50]
Inspired by the previous report that pyridine-zinc complex
[
a]
H. Li, C. Li, J. Chen, L. Liu, Prof. Q. Yang
State Key Laboratory of Catalysis
iChEM
2
could efficiently catalyze the cycloaddition of CO with epoxides
without using any co-catalysts,[
15]
herein, we report the
Dalian Institute of Chemical Physics
Chinese Academy of Science
Dalian 116023, China
E-mail: yangqh@dicp.ac.cn
C. Li, J. Chen, L. Liu
construction of hierarchical POPs Py-Zn@MA, which was
synthesized via a catalyst-free one-pot polycondensation of (4-
pyridinecarboxaldehyde)
efficiently catalyze CO cycloaddition reactions using pure CO
diluted CO (a simulating industrial flue gas: 20% CO and 80%
) without using any additives or co-catalysts.
2
ZnBr
2
and melamine. Py-Zn@MA could
[
b]
or
2
2
University of Chinese Academy of Sciences
Beijing 100049, China
2
2
N
2
Supporting information for this article is given via a link at the end of
the document.
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Chemistry - An Asian Journal
10.1002/asia.201700258
FULL PAPER
Results and Discussion
appear in the range of 40-65 ppm. Additionally, two signals at
around 123 and 149 ppm could be assigned to the carbons in
pyridine units. The results of FT-IR and solid 13C CP-MAS NMR
confirm the successful formation of the aminal linkage for Py@MA
and Py-Zn@MA via one-step polycondensation of melamine with
the corresponding aldehyde.
Zinc pyridine bromide based polyaminal network Py-Zn@MA, and
non-metal coordinated pyridine based polyaminal network
Py@MA, were synthesized via heating the corresponding
o
aldehyde with melamine in DMSO at 170 C for three days.
Synthetic routes for these polyaminal networks are depicted in
Scheme 1. Using this method, metal can be easily fixed in the
skeleton of the polymer in a one-pot polycondensation approach.
Generally, constructing of porous organic polymer (POP) based
heterogeneous catalysts requires multiple synthesis steps or
using costly precious metal catalyst, which may limit their large-
scale synthesis. The synthesis of POP based catalysts in one-
step without using any catalyst has been rarely reported. Py-
Zn@MA and Py@MA are insoluble in common organic solvents
such as methylene dichloride, hexane, toluene, ethanol and
tetrahydrofuran. They are stable in nitrogen atmosphere up to
B
triazine ring (166)
pyridine
A
-HN-C-NH-
(40-65)
Py@MA
Py-Zn@MA
(123,149)
*
*
*
* *
Py-Zn@MA
Py@MA
*
*
reused Py-Zn@MA
4
000
3000
2000
1000
-1
200
150
100
50
300 °C, as revealed by thermogravimetric analysis (TGA) (Figure
Wavenumber (cm )
Chemical Shift (ppm)
S1).
Figure 1. (A) FT-IR and (B) 13C CP-MAS NMR spectra of Py@MA and Py-
NH2
N
Zn@MA (* refers to sideband).
N
N
Br
N Zn N
Br
OHC
CHO
Br
H2N
N
NH2
CHO
X-ray photoelectron spectroscopy (XPS) studies were
conducted to characterize the coordination environment of Zn
N
Br
N
N
N
N
N
N Zn
Br
Zn
N
N
(Figure
pyridinecarboxaldehyde)
2
lower than that of ZnBr (1023.5 eV), due to the variation of the
S2).
The
Zn
2p3/2
peak
of
(4-
NH
Br
H
N
N
N
N
N
ZnBr
(Py-Zn) is located at 1022.7 eV,
HN
2
2
N
N
N
N
HN
N
N
HN
[51]
N
H
N
H
N
NH
NH
N
N
N
electronic enviroment of Zn after coordination with 4-
pyridinecarboxaldehyde. The Zn 2p3/2 peak of Py-Zn@MA is
about 1022.0 eV, suggesting that the coordination environment of
Zn in Py-Zn@MA is similar to that of Py-Zn.
N
N
NH
N
N
N
H
N
HN
N
Br Zn Br
N
N
N
N
Py@MA
Py-Zn@MA
The porous structure of Py@MA and Py-Zn@MA were
characterized by sorption analysis using nitrogen as sorbent
molecule at 77 K (Figure 2A), and the results are summarized in
Scheme 1. Schematic illustration for the synthesis of porous polyaminal
network, Py@MA and Py-Zn@MA.
Table 1. Py@MA and Py-Zn@MA have Brunauer-Emmett-Teller
2
(
BET) surface areas of 729 and 207 m /g, respectively. Py@MA
and Py-Zn@MA show typical type-I isotherm pattern with a steep
increase at low relative pressure, indicating that the microporous
structure is dominant of two polymers. Furthermore, significant
Py@MA and Py-Zn@MA were characterized by Fourier-
transform infra-red (FT-IR) spectroscopy and solid-state 13C-NMR
technique (Figure 1). The FT-IR spectrum of Py@MA exhibits two
2 0
increases in N uptakes could be found in the P/P range from 0.8
-1
broad peaks at 3410 and 1195 cm , which can be assigned to the
characteristic stretching vibration of the N-H bond of secondary
amine. The peaks at 2970 and 2920 cm-1 are from C-H vibration.
The disappearance of C=O stretching vibration of the aldehyde at
around 1700 cm-1 confirmed the complete polymerization of
melamine and aldehyde for two polyaminals. In addition, the
characteristic vibrations of triazine rings at 1550, 1480 and 1362
cm for Py@MA are clearly observed. Py-Zn@MA shows almost
identical FT-IR spectrum to Py@MA. The slight shifts in
wavenumber of N-H and triazine vibration is possibly due to the
incorporation of Zn in the polymer network. The solid state 13C
cross-polarization magic-angle spinning (CP-MAS) NMR spectra
of Py@MA and Py-Zn@MA are shown in Figure 1B. The strong
resonance at about 166 ppm is assigned to the aromatic carbons
of the triazine ring. Signals for the carbons in the aminal groups
to 1.0, showing the existence of meso/macropores for both
polymers. The TEM and SEM images showed that the two
polymers are composed of unregularly conglomerated small
particles (Figure S3). This suggests that meso/macropores are
mainly formed by the inter-particulate void due to the packing of
the polymer particles. The pore size distribution of Py@MA and
Py-Zn@MA were calculated using nonlocal density functional
theory (NLDFT) method (Figure 2B). Both samples have
micropores centered at 1.3 to 1.6 nm. Meanwhile Py@MA
possesses an additional micropore centered at 0.68 nm, which
can explain the reason that Py@MA has larger BET surface area
and micropore volume than Py-Zn@MA does.
-1
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FULL PAPER
Table 1. Textural properties and CO
2
adsorption performance of Py@MA and Py-Zn@MA.[a]
CO adsorption capacity
2
2 2
Sel. CO /N
S
m /g)
BET
2
V
total
3
V
micro
3
Q
st
(KJ/mol)
Sample
(
(cm /g)
(cm /g)
Henry’s
2
73 K
298 K
1.33
403 K
IAST
25
Law
44
Py@MA
729
1.90
0.10
1.94
1.32
0.08
0.15
34.7
33.2
2
07
0.84
(0.89)
0.01
(-)
Py-Zn@MA
0.91
18
9
(189)
[
a] Data in parentheses refer to the textural properties of Py-Zn@MA after three catalytic cycles.
1
1
1
400
200
000
CO
the Clausius-Clapeyron equation using CO
measured at 273 and 298 K (Figure 2D), and Qst value at the initial
adsorption stage (low CO loading) are provided in Table 1.
Py@MA and Py-Zn@MA have similar Qst (34.7 vs 33.2 kJ/mol),
suggesting the physical adsorption nature for CO . For both
samples, Qst decreases as the CO uptake capacity increases,
indicating CO is preferentially adsorbed on the strong binding
sites (the polyaminal skeleton) at first, and weak binding sites and
the CO molecules aggregation are dominant at higher CO
loading. The Qst for Py@MA is lower than that of Py-Zn@MA at
CO loading higher than 0.2 mmol/g, showing that the interaction
of CO with Py@MA is weaker than that with Py-Zn@MA. This is
possibly due to the existence of Zn in Py-Zn@MA enhances the
2
, isosteric heats of adsorbtio n (Qst) were calculated based on
A
Py@MA
Py-Zn@MA
B
2
uptake isotherms
Py@MA
8
6
4
2
00
00
00
00
0
2
2
Py-Zn@MA
2
2
0
.0
0.2
0.4
0.6
0.8
1.0
1
10
100
Relative Pressure (P/P0)
Pore Size (nm)
40
35
30
25
20
2
2
2
.0 C Py@MA:
,
,
D
Py@MA
Py-Zn@MA
Py-Zn@MA:
,
,
2
1.5
1.0
0.5
0.0
2
2
+
[
52,53]
interaction for CO
2
.
The Qst of both polymers is higher than
that of most reported POPs, such as azo-COPs (24.8-32.1
kJ/mol),[54,55] carbazole-based conjugated microporous polymers
56,57]
(
27.1-30.8 kJ/mol),[
hyper-cross linked polymers (21.0-23.5
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
[58]
0
.0
0.2
0.4
0.6
0.8
1.0
kJ/mol), etc. The above results indicate that the abundant N
CO2 Uptake (mmol/g)
Pressure (bar)
atoms in the polyaminal skeleton could produce high surface
polarity, leading to enhanced binding affinity for CO
CO /N selectivity is an important parameter for adsorption-
driven separation, thus CO /N selectivity of Py@MA and Py-
2
.
Figure 2. (A) N
2
sorption isotherms, (B) NLDFT pore-size distribution curves,
uptake isotherms measured at 273 K (circle), 298 K (triangle), 403 K
square) and (D) the isosteric heat of adsorption for Py@MA and Py-Zn@MA.
(C) CO
2
2
2
[
35]
(
2
2
Zn@MA were calculated based on Henry’ law and ideal
adsorption solution theory (IAST) at 298 K (Table 1, Figure S4
and S5). IAST CO /N selectivity at 298 K with composition of gas
The CO
are shown in Figure 2C. No saturation of the uptake could be
found with pressure of CO in the range of 0 to 1 bar. Py@MA
affords moderate CO uptakes of 1.94 and 1.33 mmol/g at 273
and 298 K, respectively. While Py-Zn@MA gives relatively lower
CO uptake capacity than Py@MA, which may be due to its less
amount of micropores and lower BET surface area. Meanwhile,
the CO uptake isotherms for two samples at high temperature
403 K) were also investigated (Figure 2C). To our surprise, Py-
Zn@MA could also afford CO capture capacity of 0.15 mmol/g at
2
uptake isotherms of two polymers at 273 and 298 K
2
2
mixture similar to the flue gas (CO /N : 0.15/0.85) for Py@MA and
2
2
2
Py-Zn@MA is calculated to be 25 and 9, and the values of CO /N2
2
selectivity calculated using Henry’s law are 44 and 18,
respectively. The difference in CO /N selectivity using Henry’s
2
2
2
2
law and IAST theory was believed to be related with the
adsorption site heterogeneity as literatures reported previously.[
It is known that CO /N selectivity strongly depends on the amount
59]
2
2
2
(
of ultramicropore volume and N species. Thus the CO /N2
2
2
selectivity of Py@MA is higher than that of Py-Zn@MA, since
such high temperature, higher than that of Py@MA. Interestingly,
no nitrogen could be adsorbed under such high temperature. This
indicates that Py-Zn@MA has a potential to serve as catalyst
Py@MA possesses higher amount of nitrogen and micropores.
To assess the catalytic performance of Py@MA and Py-
Zn@MA for CO2 cycloaddition reaction, we employ propylene
oxide (PO) as a model substrate and the results are summarized
using flue gas or diluted CO
2
under high temperature. To
determine the strength of the interactions between polymers and
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Chemistry - An Asian Journal
10.1002/asia.201700258
FULL PAPER
1
00
100
80
60
40
20
0
in Table 2. Py@MA only showed low conversion of 5.6%
conversion after 6 h. Also the selectivity to carbonate was rather
low on Py@MA due to the formation of the side product 1,2-
propanediol. To our delight, zinc intercalated polyaminal polymer
A
B
8
0
60
Py-Zn@MA could serve as an excellent catalyst for CO
2
4
0
0
0
cycloaddition reaction with PO with 96% conversion and 99%
selectivity. According to the kinetic curves of the cycloaddition
reaction between CO and PO catalyzed by Py-Zn@MA, the
2
reaction rate was faster in the initial 30 minutes and after that the
reaction rate gradually slows down, possibly due to the
2
Py@MA+ZnBr2
Py-Zn@MA
0
1
2
3
4
5
6
0
1
2
3
4
5
6
7
8
Reaction Time (h)
Reaction Time (h)
2
consumption of CO and PO (Figure 3A). Turnover frequency of
-
1
Py-Zn@MA was calculated to be 250 h , which is higher than
those of heterogeneous co-catalyst free catalytic systems, such
as MOFs, POPs, ZIFs and multi-functional grapheme oxide
materials (Table S1). For instance, salen-Zn based conjugated
microporous polymer Zn-CMP affords TOF of 51 h-1 at 120 C and
Figure 3. Kinetic curves for PO cycloaddition reactions using (A) pure CO
catalyzed by Py-Zn@MA and physical mixture of ZnBr and Py@MA; (B) diluted
CO (20% CO and 80% N ) catalyzed by Py-Zn@MA.
2
2
2
2
2
o
MPa without using co-catalyst.[48] Multi-functional graphene
oxide material GO-DMEDA-I could obtain TOF with 46.4 h under
With zinc bromide as catalyst, the conversion was only 9.1%.
The physical mixture of ZnBr and Py@MA polymer affords much
higher conversion of 95%. This suggests that the active site could
be generated smoothly by mixing the ZnBr and Py@MA, and
Py@MA could serve as a cocatalyst. Thus, Py-Zn@MA should be
a bifunctional catalyst with ZnBr and pyridine interacted in the
3
-1
2
o
pressure.[18]
1
40 C and 2 MPa CO
2
2
2
Table 2. The performance of various catalysts for the cycloaddition reaction
[a]
network acting as active site and co-catalyst, respectively. Based
on the previous work reported by Kim and co-workers,[15,60] we
proposed the mechanism of cycloaddition reaction of PO
catalyzed by Py-Zn@MA (Scheme 2). First, PO was reacted with
of PO and CO
2
.
CO
purity
%)
2
T
(oC)
Conv.
(%)
Sel.
(%)
TOF
(h )
Cat.
[b]
-1 [c]
(
2
Py-Zn@MA to form a dimer, and then CO was inserted to form a
carbonate-bridged intermediate. After that, the coordination of an
additional PO causes a nucleophilic attack of the carbonate
followed by cyclization to afford a cyclic carbonate and the
Py@MA
100
100
100
130
130
130
5.6
96
75
99
-
Py-Zn@MA
ZnBr
Py@MA+
250
4.8
2
catalyst is regenerated. TOF of the mixture of ZnBr and Py@MA
2
9.1
>99
is much lower than that of Py-Zn@MA. The higher activity of Py-
Zn@MA is attributed to its close contact between Zn and pyridine,
which could make the formation of intermediate more easily.
100
130
95
99
67
[d]
ZnBr
2
2
Meanwhile, Py-Zn@MA could adsorb more CO than Py@MA
Py@MA-Zn[e]
Py-Zn@MA
Py-Zn@MA
Py-Zn@MA
Py-Zn@MA[f]
100
100
100
100
20
130
100
120
150
130
92
63
81
96
95
99
98
99
99
99
101
51
does under reaction conditions (0.15 vs 0.08 mmol/g), which also
contributed to its higher activity by facilitating the formation of
intermediate. A controlled sample Py@MA-Zn synthesized from
112
260
97
post-synthetic metalation of Py@MA with ZnBr
2
was prepared.
Though high conversion of 92% could be achieved, Py@MA-Zn
-1
affords much lower TOF than Py-Zn@MA (101 vs 250 h ). This
result revealed the superiority of Py-Zn@MA synthesized via a
one-pot incorporation.
[
a] Reaction conditions: S/C = 360, PO (6.11 mmol), polymer (20 mg), 2
O
N
CO2
MPa, 6 h. [b] Conversion and selectivity were determined by GC using butyl
acetate as the internal standard. [c] TOF was calculated at the conversion
below 30%. [d] Reaction conditions: S/C = 360, PO (6.45 mmol), Py@MA
O
O
Br
Br
Br
O
O
Zn
Zn
Br
N
N
(
16 mg), ZnBr
2
(4 mg), 2 MPa, 6 h. [e] Reaction conditions: S/C = 360, PO
(20% CO
, a simulating component of flue gas), the initial pressure is 3
O
O
O
O
O
(
9.16 mmol), Py@MA-Zn (30 mg), 2 MPa, 6 h. [f] For diluted CO
2
2
N
O
O
and 80% N
MPa.
2
O
O
Br
Br
Br
Br
Br
Br
Zn
Zn
Zn
Zn
Br
Br
N
N
O
O
N
O
O
Br
Br
Br
O
Zn
Zn
Br
O
N
Scheme 2. Proposed reaction mechanism for the cycloaddition reaction of
CO with PO over bifunctionalized Py-Zn@MA.
2
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Chemistry - An Asian Journal
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FULL PAPER
The effect of reaction temperature on the catalytic
shows excellent activity for catalyzing cycloaddition reaction of
performance of Py-Zn@MA was investigated (Table 2, Figure 4A). PO without using any co-catalyst. To the best of our knowledge,
As temperature increases from 100 to 130 oC, PO conversion
increases monotonously from 63 to 96%, while selectivity
maintains at a high level of about 99%. TOF also increased from
there are few of POPs or MOFs could serve as a catalyst in co-
catalyst free system using flue gas or diluted CO as a feedstock.
2
Functionalized five-membered alkylene carbonates are
-1
[62]
51 to 230 h , indicating that temperature has a big influence on
attractive in a variety of applications. Apart from PO, we also
the catalytic activity of Py-Zn@MA. Further increasing
investigated substrate scope of epoxides and was summarized in
Scheme 3. The result showed that Py-Zn@MA was active for all
the substrates employed. The TOF increases in the order of
cyclohexene oxide < butyl glycidyl ether < styrene oxide < 1,2-
epoxybutane < epichlorohydrin < propylene oxide. The activity of
epoxides decreased as the alkyl length increased. 1,2-
Epoxybutane and butyl glycidyl ether with long alkyl length could
be smoothly converted to the corresponding cyclic carbonates
with high conversion. Styrene oxide with large molecular size
o
-1
temperature to 150 C, TOF increases slightly to 260 h and high
conversion and selectivity could be maintained, indicating the
good stability of Py-Zn@MA at high temperature. The pressure of
CO
Zn@MA as shown in Figure 4B. In the pressure range from 0.5 to
MPa, both of the PO convertion and PC selectivity increase with
the increase in CO pressure. At PO pressure less than 2 MPa,
only a few CO could be dissolved in PO solution which could limit
2
also has a big influence on the catalytic activity of Py-
2
2
2
the reaction equilibrium. Thus, the activity is low. At the same time, could also be efficiently converted to corresponding cyclic
-
1
PO could be hydrolysed to 1,2-propanediol. Although the formed
,2-propanediol has the possibility to be convert to the same
carbonate product, the efficiency is lower than one-step direct
cycloaddition. Thus, the selectivity at low CO pressure is
relatively low. If further increasing CO pressure to 3 MPa, both of
carbonate with high conversion of 97% and TOF of 42 h . The
above results show that Py-Zn@MA with hierarchical pore
structure could accommodate large molecules and efficiently
convert them to carbonates. However, when the internal epoxide,
cyclohexene oxide (CHO) was used, the activity was lower
presumably due to its high steric hindrance leading to the difficulty
in diffusion of CHO to the active sites located in micropores of Py-
1
2
2
the PO convertion and PC selectivity decrease slightly, possibly
due to the low concentration of PO in the vicinity of active sites of
pressure.[
32]
Zn@MA. From the discussion above we can conclude that the
[32]
Py-Zn@MA at high CO
2
steric effect plays an important role in the cycloaddition reaction
3
2
2
1
1
00
50
00
50
00
100
100
A
B
of epoxides and CO
investigated using diluted CO
could be converted to the corresponding cyclic carbonates using
2
diluted CO , though TOFs were lower than those using pure CO .
2
. Furthermore, substrate scope was also
2
. To our delight, all of the epoxides
90
2
80
80
This result further confirms that Py-Zn@MA has the capacity of
conversion of flue gas to carbonates.
TOF
70
Conversion
Selectivity
5
0
Conversion
Selectivity
0
60
60
100
110
120
130
140
150
0.5
1.0
1.5
2.0
2.5
3.0
o
Pressure (MPa)
Temperature ( C)
Time (h):
6
12
96
61
12
96
38
12
32
12
99
34
12
98
29
12
97
42
12
90
30
Pure
CO2
Figure 4. Influnce of (A) reaction temperature and (B) CO
2
pressure on the
Conv. (%):
96
catalytic performance of Py-Zn@MA.
-1
TOF (h ) : 125
8.8
12
Time (h):
8
Diluted
CO2 Conv. (%):
98
90
21
2
Owing to the great increase in CO emission by the power
-
1
TOF (h ) :
5.8
2
plant, many systems have been developed for CO capture from
flue gas.[
61]
2
However, CO capture processes still face the
drawbacks such as high energy consumption, large cost,
equipment corrosion, etc. Thus direct conversion of flue gas to
value-added product is more attractive. Inspired of the good
activity of Py-Zn@MA in the cycloaddition reaction, we further
investigated its catalytic performance in cycloaddition reaction
with diluted CO (20% CO and 80% N , a simulating component
2 2 2
of flue gas). As shown in Figure 3B, Py-Zn@MA could also
efficiently catalyze cycloaddition reaction of PO to obtain 95%
Scheme 3. Cycloaddition reactions of different types of epoxides with pure or
diluted CO catalyzed by Py-Zn@MA.
2
The recycling stability of Py-Zn@MA was investigated
(Figure 5). Herein, we choose PO conversion less than 50% to
evaluate the stability of Py-Zn@MA due to the fact that it gives
more appropriate results than those obtained at high PO
conversion. The PO conversion slightly decreases from 43.5% to
37% after three cycles. Meanwhile, PO conversion decreased
from 38% to 23% after two cycles when using Py@MA-Zn as
catalyst, suggesting the instability of Py@MA-Zn prepared via the
post-synthetic metalation method. For understanding the
deactivation of Py-Zn@MA, the zinc content was analyzed for the
conversion with the selectivity of 99% under 3 Mpa diltuted CO
at 130 C. The reaction time was prolonged to 8 h to achieve high
2
o
2
conversion. Due to the low CO concentration in simulated flue
gas, the reaction was performed under 3 MPa. If the pressure is
declined to 2 MPa only 70% PO conversion could be obtained
after 12 h. These results further confirm that the carbonation
reaction is a pressure-dependent reaction. Whatever, Py-Zn@MA
nd
rd
reused catalyst. Zinc content of Py-Zn@MA after 2 and 3 cycle
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Chemistry - An Asian Journal
10.1002/asia.201700258
FULL PAPER
analyzed by ICP was 5.5 wt% and 5.3 wt%, respectively.
Comparing to the zinc content of the fresh sample (5.6 wt%), the
leaching of zinc is negligible. Meanwhile, the morphology of Py-
Zn@MA could be well maintained as proved by SEM image
mmol/g, and no nitrogen could be adsorbed under similar
conditions. As a result, Py-Zn@MA shows good catalytic activity
for cycloaddition reactions of epoxides with pure CO
2
(TOF: 250
(TOF: 97 h ) at 130 oC without using any
additives or co-catalyst. The high efficiency of Py-Zn@MA is
mainly attributed to ZnBr and pyridine interacted in polymer
-1
-1
h ) or diluted CO
2
(
Figure S6). The BET surface area and pore structure of the Py-
Zn@MA after three cycles was analyzed by N sorption isotherm
Figure S7). Comparing to fresh Py-Zn@MA, the resued sample
2
2
(
network acting as active site and co-catalyst, respectively.
Moreover, Py-Zn@MA with hierarchical structure could convert
different types of epoxides to carbonates, including butyl glycidyl
ether and styrene oxide. Py-Zn@MA could be easily prepared in
large-scale without using any catalyst and the chemicals are cost-
effective. Using similar method, numerous polymers could be
prepared and utilized in various research areas. Further studies
are now in progress in our laboratory.
2
2
has only slightly lower BET surface area (189 m /g vs 207 m /g).
These results indicate that both leaching of zinc and porous
structure are not responsible to the decrease of the activity. Thus,
the chemical composition of reused Py-Zn@MA was
characterized with FT-IR. The characteristic peaks of Py-Zn@MA
in FT-IR spectrum of the reused sample were identical to the fresh
one (Figure 1A). However, an extra peak near 1640 cm-1 could be
observed in the FT-IR spectrum of reused Py-Zn@MA. This peak
may be assigned to the carbonyl of PC. TG analysis data shows
the weight loss of reused Py-Zn@MA is larger than that of fresh
one, suggesting that some product or solvent were trapped in the
reused catalyst (Figure S1). We washed and dried the catalyst
efficiently during the recycling process, however there still
remains some PC in the reused catalyst. We speculated the C=O
of PC is likely to have some interaction with pyridine N and is more
difficult to be removed during the recycle process. As a result, the
Experimental Section
Chemicals and reagents
All chemicals were used as received without any further purification. 4-
Pyridinecarboxaldehyde, zinc bromide (ZnBr ) and melamine (MA) were
2
obtained from Aladdin Co. (China). Dimethyl sulphoxide (DMSO) was
purchased from Sigma-Aldrich Company, Ltd. (USA). Other reagents were
bought from Shanghai Chemical Reagent, Inc. of the Chinese Medicine
co-catalyst cannot be regenerated. Thus, tetrabutylammonium
bromide (TBAB) was added for the 4th cycle. The activity was
increased to 55%. Under similar conditions, TBAB only affords
Group. (4-Pyridinecarboxaldehyde)
according to literature method.[63]
2
ZnBr
2
(Py-Zn) was synthesized
5.8% PO conversion. This confirms that the interaction of PC with
Py-Zn@MA may be the main reason for the catalyst deactivation.
The PO conversion could be maintained at 55% for the following
cycles with the addition of TBAB. This indicates the zinc
intercalated in the polymer network is very stable.
Characterization
C,
H,
N
elemental
analysis
was
performed
on
an
Oxygen/Nitrogen/Hydrogen Analyzer (EMGA-930, HORIBA, Japan) and a
Carbon/Sulfur Analyzer (EMIA-8100, HORIBA, Japan). FT-IR spectra in
the range of 400-4000 cm-1 were collected with a Nicolet Nexus 470 IR
spectrometer using KBr pellets. Solid-state NMR spectra were performed
on a Bruker 500 MHz spectrometer equipped with a magic-angle spin
Conversion
Selectivity
100
80
60
40
20
0
probe using a 4-mm ZrO
NO ). The experimental parameters are as follows: 8 kHz spin rate,
rotor. 13C signals were referenced to Glycine
2
(
C H
2 5
2
5
s pulse delay, and 2500 scans. The thermo gravimetric analysis (TGA)
was performed from 25 to 900 oC under nitrogen atmosphere with a
o
heating rate of 5 C/min using a NETZSCH STA 449F3 analyzer. Scanning
electron microscopy (SEM) was undertaken using a FEI Quanta 200F
scanning electron microscope operating at an acceleration voltage of 2-30
kV. Transmission electron microscopy (TEM) was performed on a
HITACHI 7700 at an acceleration voltage of 100 kV. Before measurement,
the samples were fully dispersed in ethanol and then deposited on a holey
carbon film on a Cu grid. X-ray photoelectron spectroscopy (XPS) was
performed on an ESCALAB 250Xi (Thermo Scientific) with
monochromated AlKα radiation (photon energy, 1486.6 eV). The nitrogen
sorption isotherms were performed on a Micromeritics ASAP 2020 system
1
2
3
4
5
6
7
Run
Figure 5. The recycle stability of Py-Zn@MA for the CO
2
cycloaddition
reaction (5 mg of TBAB was added for the 4th to 7 runs).
th
o
volumetric adsorption analyzer at -196 C. All samples were degassed at
o
1
00 C for 6 h prior to the sorption measurements. The BET surface area
Conclusions
0
was calculated from the adsorption data at a relative pressure P/P in the
range of 0.05-0.25. Pore size distributions were determined from the
adsorption branches using nonlocal density functional theory (NLDFT)
In summary, we synthesized two polyaminal porous organic
polymers, Py-Zn@MA and Py@MA via one-pot
polycondensation approach. Both polymers show good CO
uptake capacity and high isosteric heats of adsorbtion due to the
abundant N atoms in the polymers. Even at high temperature (130
oC), Py-Zn@MA could also afford CO
method. CO
2 2
and N sorption experiments were carried out on a
a
Micromeritics ASAP 2050 system volumetric adsorption analyzer. Zinc
content of the materials was determined using a PLASAM-SPEC-II
inductively coupled plasma atomic emission spectrometry (ICP) by
digesting the polymer in HNO /H SO (1:1, v/v).
2
3
2
4
2
uptake capacity of 0.15
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Chemistry - An Asian Journal
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FULL PAPER
Synthesis of Py-Zn@MA and Py@MA
This work was supported by the NSFC (No. 21321002, 21232008)
and the Strategic Priority Research Program of the Chinese
Academy of Sciences Grant No. XDB17020200.
Syntheis of Py-Zn@MA: A dried 25 mL round flask equipped with a stirrer
and a condenser was degassed using three evacuation-nitrogen-backfill
cycles. Under nitrogen flow, (4-pyridinecarboxaldehyde)
2 2
ZnBr (Py-Zn)
Keywords: porous organic polymers • pyridine-zinc catalyst •
(
660 mg, 1.5 mmol), melamine (250 mg, 2.0 mmol) and DMSO (5 mL) were
o
added and heated at 170 C for 72 h. Finally, the system was cooled to
room temperature, the yellow-brown powder was isolated and washed
CO
2
cycloaddition reaction• cocatalyst-free • cyclic carbonates
2 2
successively with dichloromethane (CH Cl ), tetrahydrofuran (THF), N,N-
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5
runs.
[
[
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
FULL PAPER
A bifunctionalized pyridine-zinc-based
porous organic polymer, Py-Zn@MA
was synthesized and used for
He Li, Chunzhi Li, Jian Chen, Lina Liu,
Qihua Yang*
cocatalyst-free cycloaddition of
Page No. – Page No.
epoxides with CO
of Py-Zn@MA is attributed to its
bifunctional nature with ZnBr and
2
. The high activity
O
O
O
Synthesis of pyridine-zinc-based
porous organic polymer for
cocatalyst-free cycloaddition of
epoxides
N
O
O
2
Br
Br
Br
Br
Zn
Zn
pyridine activating epoxide in a
cooperative way. The catalyst could
be easily prepared in large-scale and
shows good substrate universality for
cycloaddition reactions of epoxides.
N
Co-catalyst Free !
Hierarchical porous organic polymer
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