Optimizing the performance of porous pyridinium frameworks for carbon dioxide transformation

Article abstract of DOI:10.1016/j.cattod.2020.01.031

Multifunctional catalysts derived from the integration of discrete catalytic partners in a confined space represent an important approach to emulate some of the design philosophies of enzymes. In an effort to design concepts for highly active catalysts for CO₂ transformations, we synthesize and contrast the performance of two porous pyridinium frameworks. The activity is found to be significantly amplified by the introduction of the amine group on the ortho position of the pyridinium moieties. The resulting catalyst is capable of highly active and selective cycloaddition of aziridines with CO₂ to 5-substituted-2-oxazolidinone, even under ambient conditions (1 bar, 22 °C). Its high activity originates from CO₂ activation by the pendant amine group in the vicinity of the active species, which facilitates the subsequent catalytic steps.

Full text of DOI:10.1016/j.cattod.2020.01.031

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Optimizing the Performance of Porous Pyridinium Frameworks for
Carbon Dioxide Transformation
Yanpei Song (Data curation) (Investigation) (Writing - original draft)
(
(
Validation), Qi Sun (Writing - review and editing) (Methodology)
Formal analysis), Briana Aguila (Writing - review and editing),
Shengqian Ma (Conceptualization) (Supervision) (Resources)
Project administration)
(
PII:
S0920-5861(20)30035-3
DOI:
https://doi.org/10.1016/j.cattod.2020.01.031
CATTOD 12650
Reference:
To appear in:
Catalysis Today
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Revised Date:
Accepted Date:
20 July 2019
16 January 2020
24 January 2020
Please cite this article as: Song Y, Sun Q, Aguila B, Ma S, Optimizing the Performance of
Porous Pyridinium Frameworks for Carbon Dioxide Transformation, Catalysis Today (2020),
doi: https://doi.org/10.1016/j.cattod.2020.01.031
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Optimizing the Performance of Porous Pyridinium Frameworks for Carbon Dioxide
Transformation
1
2
1
1
Yanpei Song , Qi Sun *, Briana Aguila , and Shengqian Ma *
1Department of Chemistry, University of South Florida, 4202 E. Fowler Avenue, Tampa, Florida
3
3620, United States
2College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China
*
Email: sunqichs@zju.edu.cn; sqma@usf.edu
Graphical abstract
Highlights


Boosted activity by introducing the amine group in a porous pyridinium framework
CO
2
activated by the amine group facilitating the subsequent catalytic steps.
1


Superior catalytic performances even under ambient conditions
Abstract
Multifunctional catalysts derived from the integration of discrete catalytic partners in a confined
space represent an important approach to emulate some of the design philosophies of enzymes. In
an effort to design concepts for highly active catalysts for CO transformations, we synthesize and
2
contrast the performance of two porous pyridinium frameworks. The activity is found to be
significantly amplified by the introduction of the amine group on the ortho position of the
pyridinium moieties. The resulting catalyst is capable of highly active and selective cycloaddition
of aziridines with CO
2 °C). Its high activity originates from CO
of the active species, which facilitates the subsequent catalytic steps.
2
to 5-substituted-2-oxazolidinone, even under ambient conditions (1 bar,
2
2
activation by the pendant amine group in the vicinity
Keyword: CO
2
transformation, CO activation, porous organic polymer, organocatalysis, porous
2
ionic polymer.
1
. Introduction
Carbon dioxide is a globally available and nontoxic carbon source in the chemists’ toolbox
1-9]. The utilization of CO as a starting material to integrate into value-added products is of
[
2
practical significance and also supports a carbon-neutral cycle that mitigates the intermittency of
renewable energy sources [10-13]. Anumber of pioneering studies on the catalytic transformations
2

of CO
heterocycles, such as oxazolidinones from aziridines and CO
due to their prevalence as chiral auxiliaries and as biologically active pharmaceutical agents [14-
8]. Most efforts in catalytic systems design for such transformations have been focused on metal-
2
have been undertaken in recent years. In particular, the formation of five-membered
2
, has received considerable attention
1
based homogeneous catalysts, including metal complexes and alkali metal halides, all of which
exhibit high catalytic activity and selectivity [19-21]. Organocatalysts are gaining traction as they
are often more cost-effective and sustainable than metal-containing complexes, prompting an
ongoing effort for commercialization, despite their molecular nature [22-27]. At large scale,
however, heterogeneous processes are clearly desirable [28-31].
In designing a heterogeneous catalyst for the cycloaddition of aziridines and CO
2
, we focused
) activation,
2) attack by halide anions, and (3) ring closing and extrusion [32-34]. Given the irreplaceable role
our attention on the general proposed catalytic cycle: (1) reactant (aziridines or CO
2
(
of the halide anions in the proposed CO insertion step, we identified the reactant’s activator as the
2
modifiable component. One recurring motif that was found to be effective was the presence of
amine groups, favoring the activation of CO [35]. We surmised that the integration of the amine
2
group within a cationic framework with a halide as counter anion would improve activity. Whereby
the proximity and spatial distribution of the catalytic components to satisfy the requisite geometry
are essential to maximize potential cooperation. To fulfill this task, we were motivated by the
exploration of porous organic polymers (POPs) due to their tunability, allowing one to control,
with high fidelity, the composition of the resulting materials, which are desirable properties for
3

catalytic material design [36-50]. Other innate features of this class of materials that we deemed
favorable for catalysis were its high hydrothermal and chemical stability, as well as large surface
area for ready diffusion of reaction species.
To explore the validity of this concept, herein, we chose pyridinium as the cation building
unit, given its amenable synthesis and easy-to-get properties. To set the proximity between the
catalytic partners, we arrange them on a single molecule to be used as a monomer for material
synthesis. It is shown the performance of the bromine counter anions can be greatly amplified after
the introduction of an amine group on the pyridinium unit, leading to a 7.8-fold acceleration in the
cycloaddition of aziridines and CO
2
over the control material without the amino substitution.
activation and
Moreover, we reveal that the basicity of the amine group is the key to promote CO
2
thereby the overall efficiency. Regardless of the specifics, these findings point to an appealing
approach for modulating the properties of POPs for catalytic transformations and related
applications.
2
. Experimental
2
.1. Materials
All commercially available chemicals were purchased in high quality and used without
further treatment.
.2. Catalyst preparation
Synthesis of PQA-NH Py-Br.
2
2
4

Synthesis of POP-NH
dissolved in dimethylformamide (DMF, 10 mL), followed by the addition of azobisisobutyronitrile
AIBN, 50 mg). After stirring at room temperature to achieve homogeneity, the mixture was
2
Py: As a typical procedure, 3,5-divinylpyridin-2-amine (1.0 g) was
(
transferred into a 20 mL autoclave and heated to 100 C overnight, yielding a monolithic light
yellow solid. The title product was achieved after washing with acetone and drying under vacuum.
3
,5-Divinylpyridin-2-amine was synthesized as follows: 3,5-dibromopyridin-2-amine (2.0 g, 8
mmol), potassium vinyltrifluoroborate (2.64 g, 19.2 mmol), K CO (3.2 g, 24mmol), and
Pd(PPh (0.232 g, 0.2 mmol) were dissolved in a mixture of toluene (25 mL), THF (25 mL), and
O (10 mL), and the resulting mixture was refluxed at 90 °C under N atmosphere for 48 h. The
product was extracted with ethyl acetate, washed with brine, dried over Na SO , and evaporated
2
3
)
3 4
H
2
2
2
4
under reduced pressure, giving the crude compound which was then purified by flash
chromatography with hexane/ethyl acetate (2:1) and 1%v/v triethylamine as eluent to afford the
1
title compound as a yellow powder. H NMR (400 MHz, CDCl
3
, 298K, TMS):  7.98 (d, 1H, J=1.6
Hz), 7.56 (d, 1H, J=2.0 Hz), 6.53-6.64 (m, 2H), 5.56-5.68 (m, 2H), 5.10-5.40 (m, 2H), 4.65 (s,
1
3
2
H). C NMR (100 MHz, CDCl
3
)  155.34, 146.25, 133.38, 131.79, 131.70, 124.84, 118.33,
117.65, 111.72 ppm. 3,5-Dibromopyridin-2-amine was synthesized as follows: 2-aminopyridine
(
(
3.76 g, 40 mmol) and N-bromosuccinimide (14.24 g, 80 mmol) were added to carbon tetrachloride
80 mL) and stirred at room temperature for 24 h. The product was extracted with chloroform,
5

washed with brine, dried over Na
2
SO , and evaporated under reduced pressure, giving the crude
4
compound which was then purified by flash chromatography with hexane/ethyl acetate (2:1) and
1
1
%v/v triethylamine as eluent to afford 3,5-dibromopyridin-2-amine as a yellow powder. H NMR
(
400 MHz, d
ppm. The purity of the synthesized compounds was confirmed by NMR analyses (Figure S1).
Synthesis of PQA-NH Py-Br: POP-NH Py (0.5 g) was swelled in acetonitrile (40 mL),
6
-DMSO, 298K, TMS):  7.27 (d, 1H, J=8.4 Hz), 7.02 (d, 1H, J=8.8 Hz), 5.69 (s, 2H)
2
2
followed by the addition of iodomethane (1.0 g). The mixture was then stirred and heated to 80 C
for 72 h. The resulting powder was washed with ethanol and then exchanged with 1 M NaBr
solution three times to afford the title product as a light yellow powder. Elemental analyses reveal
that the content of Br species in PQA-NH Py-Br is 30.3 wt.%.
2
Synthesis of PQA-Py-Br.
POP-Py (0.5 g), which was synthesized according to the previous literature [51], was swelled
in acetonitrile (40 mL), followed by addition of iodomethane (1.0 g). The mixture was then stirred
and heated to 80 C for 72 h. The resulting powder was washed with ethanol and then exchanged
with 1 M NaBr solution three times to afford the title product as a light yellow powder. The
2
-1
Brunauer-Emmett-Teller (BET) surface areas of POP-Py and PQA-Py-I are 979 and 465 m g ,
respectively. Elemental analyses reveal that the content of Br species in PQA-Py-Br is 33.4 wt.%.
2
.3. Catalytic tests
6

In a typical experimental procedure for coupling of aziridines with CO
and 5 mg of catalyst were added into a 7 mL vial. After that, the vial was transferred into a 100
mL stainless steel autoclave, sealed, and purged with CO . Following this, the pressure of CO was
2
, 1 mmol of aziridine
2
2
adjusted to the desired value and the autoclave was placed in a preheated oil bath with stirring for
the mentioned time period. On completion of the reaction, the reactor was cooled to room
temperature and CO was ejected slowly. The conversion and selectivity of the reactions were
2
1
determined by HNMR using terephthalaldehyde as an internal standard. The aziridine compounds
were synthesized according to the literature [32].
For recycling the catalyst, the polymer was separated by centrifugation after the reaction,
washed with DMF, H
2
O, methanol, and hexane in sequence and dried under vacuum, which was
used directly for the next catalytic reaction.
2
.4. Characterization
Liquid NMR spectra were recorded on a Bruker Avance-400 (400 MHz) spectrometer in
which the chemical shifts are expressed in ppm downfield from TMS at =0 ppm, and J values are
1
3
given in Hz. C (100.5 MHz) cross-polarization magic-angle spinning (CP-MAS) solid-state
NMR experiments were recorded on a Varian infinity plus 400 spectrometer equipped with a
magic-angle spin probe in a 4-mm ZrO rotor. Nitrogen sorption isotherms were measured using
2
Micromeritics ASAP 2020M and Tristar system, and before the measurements, the samples were
degassed at 100 °C for 10 h. IR spectra were recorded on a Nicolet Impact 410 FTIR spectrometer.
X-ray photoelectron spectroscopy (XPS) spectra were performed on a Thermo ESCALAB 250
7

with Al K irradiation at θ=90 for X-ray sources, and the binding energies were calibrated using
the C1s peak at 284.9 eV. Scanning electron microscopy (SEM) images were collected on a Hitachi
SU 8000. Elemental analyses were performed via flask combustion followed by ion
chromatography.
3
. Results and discussion
3
.1. Catalyst synthesis
To target the desired materials, we first constructed various pyridine moieties into porous
frameworks by polymerization of the corresponding vinyl-functionalized monomers under
solvothermal conditions in DMF at 100 C with the assistance of AIBN. Further treatment of the
obtained material (POP-Py and POP-NH
days gave the porous quaternary ammonium (PQA) salt PQA-Py-I and PQA-NH
Here, POP-NH Py and the corresponding ionic product PQA-NH Py-I were selected for thorough
2
Py) with methyl iodide (CH
3
I), heated to 80 C for three
2
Py-I, respectively.
2
2
illustration. The local structures of these materials were elaborated by solid-state 13C NMR
spectroscopy. The successful transformation from the vinyl-functionalized monomers into the
highly polymerized polymer is verified by the disappearance of the characteristic vinyl 13C
resonance located in the range of 100.0 to 110.0 ppm with the concomitant emergence of an intense
peak at 36.7 ppm attributable to the polymerized vinyl groups. In addition, the appearance of a
peak at 43.2 ppm, ascribed to the methyl group in the pyridinium moiety, is indicative of the
occurrence of the quaternization reaction (Figure 1a) [51]. The morphology of POP-NH
2
Py and
PQA-NH Py-I was examined by SEM technique, revealing no noticeable changes after the
2
8

postsynthetic modification. Both materials feature aggregates comprising interconnected irregular
nanoparticles (Figure S2). Nitrogen sorption measurements collected at 77 K indicate that POP-
2
-1
NH
respectively (Figure 1b). The decreased surface area can be reasonably ascribed to the increased
mass after the functionality addition. The full accessibility of ionic sites within PQA-NH Py-I is
indicated by a complete anion-exchange process between I and Br , which is supported by the
XPS analyses (Figure 3c). To quantify the content of bromine species in PQA-NH Py-Br,
elemental analyses were performed, revealing that the weight percentage of bromine species in
PQA-NH Py-Br was 30.3 wt.%, which corresponds to 92% of the pyridine moieties participated
in the quaternization reaction.
2
Py and PQA-NH
2
Py-I are highly porous with BET surface areas of 555 and 310 m g ,
2
-
-
2
2
13
Figure 1. (a) Solid-state C NMR spectra; (b) N
2
sorption isotherms collected at -196 °C; (c) Br3d
and I3d XPS spectra.
3
.2. Catalytic evaluation
The performance of various catalysts were tested for the conversion of 1-butyl-2-phenyl
aziridine into 2-oxazolidinones at 50 °C and 2 MPa CO
9
2
pressure under solvent-free condition

with the results presented in Table 1. 5-Aryl-2-oxazolidinone (1) was preferentially formed with
high regioselectivities in all cases, indicative of the insertion of CO at the more sterically hindered
side of the aziridine. PQA-NH Py-Br exhibited much higher reactivity than PQA-Py-Br, inducing
2
2
a 1-butyl-2-phenyl aziridine conversion of 87% after 5 h, whereby, PQA-Py-Br only afforded a
conversion of 26% under otherwise identical conditions (Table 1, entries 1 and 2), thus suggestive
of an important role of the amine group in catalysis.
Table 1. Cycloaddition of 1-butyl-2-phenyl aziridine with CO into oxazolidinone over various
2
catalysts.a
Entry Catalyst
Time
h)
T
P
Conv.
Select. Regioselect.
(
(°C) (MPa) (%)b
(%)b
>99
>99
>99
83
(%)/1:2b
98:2
98:2
>99
1
PQA-NH
2
Py-Br
5
5
4
4
4
2
2
2
2
50
50
70
70
70
50
50
50
50
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
87
26
9
2
PQA-Py-Br
3
POP-NH
POP-Py
aniline
2
Py
4
6
>99
5c
6
7
>99
>99
>99
>99
>99
>99
PQA-Py-Br
PQA-NH Py-Br
11
37
11
11
>99
7
2
>99
8c
9c
PQA-Py-Br + aniline
PQA-Py-Br +
>99
>99
2
-aminopyridine
1
0
PQA-NH Py-Br + HBr
2
2
50
2.0
10
>99
>99
1
0

1
1
PQA-Py-Br
12
12
72
4
22
22
22
70
70
70
0.1
0.1
0.1
2.0
2.0
2.0
4
>99
>99
>99
>99
>99
>99
>99
>99
>99
98:2
98:2
98:2
1
1
1
1
1
2
PQA-NH
PQA-NH
PQA-NH
PQA-NH
PQA-NH
2
2
2
2
2
Py-Br
Py-Br
Py-Br
Py-Br
Py-Br
31
3
92
4
>99
>99
>99
5d
6e
4
4
a
b
Reaction conditions: 1-butyl-2-phenyl aziridine (1 mmol) and catalyst (5 mg). The conversion
1
and selectivity of the reactions were determined by HNMR using terephthalaldehyde as an
internal standard; Select.(%) means the selectivity of oxazolidinone product in the cycloaddition of
c
d
e
aziridines with CO . 5 mg of given additive was introduced. Reuse. Recycled for 5 times.
2
To further prove the efficiency of PQA-NH
2
Py-Br, kinetic studies were performed at 22 °C
and 50 °C (T). As shown in Figure 2a, the conversions at both temperatures steadily increase over
-1
-1
time, following a first-order reaction with rate constants (k) of 0.1592 s and 0.2463 s for 22 °C
and 50 °C, respectively (Figure 2b). By contrast, PQA-Py-Br afforded the corresponding values of
-
1
-1
0
8
.02005 s and 0.0609 s , with rate constants smaller than those of PQA-NH
2
Py-Br by factors of
and 4, respectively (Figures 2c and 2d). To gain more insight into the reaction, the apparent
activation energies were further calculated using Equation (1), with PQA-Py-Br demonstrating an
apparent activation energy (E ) 2.5 times higher than that for PQA-NH Py-Br (31.6 vs. 12.3 kJ
mol ), thus confirming the benefit of incorporating the amine group for catalysis.
A
2
-
1
풌
푬푨
ퟏ
ퟏ
ퟐ
ln ( ) = − ( − ) (1)
풌
ퟏ
푹
푻
푻
ퟏ
ퟐ
11

Figure 2. (a,c) Kinetic rates of 1-butyl-2-phenyl aziridine conversion. (b,d) First order reaction
2
rate plots (R >0.99 for all), in which [A]0 is the initial mole concentration of Aand [A]t is the mole
concentration of A at the time of t .
To ascertain whether the pendant amine group would also be catalytically active, POP-NH Py,
2
POP-Py, and aniline were tested as catalysts. However, all of them were devoid of catalytic activity,
-
under the conditions listed in Table 1, indicating that the Br ions are essential for catalysis and the
primary amine in PQA-NH
2
Py-Br only has a minimal contribution on the reaction outcomes (Table
1
, entries 3-5). To further understand the origin of the enhancement of amine substituted
pyridinium, a set of control experiments was carried out. Adding free aniline or 2-aminopyridine
with PQA-Py-Br gave little improvement in activity, suggesting that the relative position of the
1
2

amine groups is vital for the improvement of the reactivity, with location at the ortho position
within a pyridinium framework being crucial. Treating PQA-NH Py-Br with 1 M of HBr aqueous
2
solution to neutralize the amine groups, the activity was drastically reduced by a factor of 3.7,
close to that of PQA-Py-Br, suggestive of the importance of the basicity of the amine group on the
reaction (Table 1, entries 6-10). The basicity of aromatic amines is not strong enough to activate
CO
due to the enriched CO
adsorption capacity, and the IR spectra of the pristine and CO
2
, but incorporation of them into porous materials increases the capability of CO
2
activation
2
. To shed additional light on this, we compared the volumetric CO
2
2
reacted samples. PQA-NH
2
Py-Br
exhibited a bigger Qst value, particularly at the high pressure region, indicative of its higher
affinity towards CO (Figure 3a and Figure S3). In addition, the CO treated PQA-NH Py-Br
showed an increased intensity at 1650 cm , attributable to the carbamic salt, while, no noticeable
change was observed for PQA-Py-Br before and after CO treatment (Figure 3b and Figure S4)
32,33]. To further probe the pendant base effect on the reaction, we contrasted the performance
of PQA-NH Py-Br and PQA-Py-Br under 1 atm of CO and room temperature, with the aim of
increasing the influence of CO . As expected, their discrepancy in activity was further increased,
affording a 1-butyl-2-phenyl aziridine conversion of 4% and 31% for PQA-Py-Br and PQA-
NH Py-Br, respectively, corresponding to an improvement factor from ca. 3.3 to 7.8 (Table 1,
entries 1, 2, and 11-13).
2
2
2
-1
2
[
2
2
2
2
1
3

Figure 3. (a) Isosteric heat of adsorption (Qst) for the adsorption of CO
2
of PQA-Py-Br and PQA-
NH Py-Br. (b) IR spectra of PQA-NH Py-Br before and after treatment with CO .
2
2
2
On the basis of the results described above and previous reports [32-34], we propose a
tentative mechanism for the PQA-NH Py-Br catalyzed cycloaddition of aziridine with CO (Figure
). Firstly, the substrate and CO are enriched in the porous material, whereby the amine group
coordinates reversibly with CO to afford the carbamate salt, which rationalizes the dependence of
2
2
4
2
2
the reaction rate on CO
2
pressure. The attack of the aziridine substrate on the activated CO carbon
2
-
generates a positive charge center. Subsequently, the nucleophilic attack of the Br ion leads to ring
opening of the aziridine through two different pathways, thus giving rise to different
regioselectivities. Following this, cyclization via an intramolecular nucleophilic attack leading to
oxazolidinones and regeneration of the catalyst. The main product 2-oxazolidinones originates
from ring-opening of the aziridine at the most substituted carbon as this produced the more stable
carbamate salt.
1
4

Figure 4. Proposed mechanism for the PQA-NH
2
Py-Br catalyzed cycloaddition of aziridine with
CO
2
Having proven the efficiency of PQA-NH Py-Br, the reaction conditions were further
2
optimized. 1-Butyl-2-phenyl aziridine was again used as a substrate, while the catalyst loading,
reaction temperature, and CO pressure were varied. When the temperature increased to 70 °C
with 2 MPa of CO , full conversion of 1-butyl-2-phenyl aziridine can be achieved within 4 h (Table
, entry 14). Reducing the catalyst loading to 0.01 mol %, and under otherwise identical conditions
2
2
1
the yield of 5-aryl-2-oxazolidinone exceeded 99% after 48 h, which corresponds to a turnover
number (TON, measured as total moles of product per mole of catalyst) of 10000. When the
pressure and temperature were decreased to 1 bar and 22 °C, respectively, PQA-NH
still efficient, and 92% conversion of 1-butyl-2-phenyl aziridine to oxazolidinone was achieved
after 72 h. Moreover, PQA-NH Py-Br could be filtered and reused without loss of activity after 5
recycles (Table 1, entries 15 and 16, and Table S1).
2
Py-Br was
2
1
5

Encouraged by this, we further investigated its generality in the cycloaddition reactions of
with various aziridines (Table S2). It was found that the reaction of 2-phenylaziridine
CO
2
proceeded smoothly with excellent yield achieved within 4 h. The substrate bearing a branched
alkyl group at the nitrogen atom showed a slower reaction rate and a relatively low selectivity
probably due to the steric interactions as well as the formation of self-oligomers. The rate
dependence on the steric effect of the R group on the nitrogen atom implies that the coordination
of CO
hindered R would be favorable for the coordination with CO
compared with those substrates with more sterically hindered R. These results further support our
2
to the aziridine is a reversible step in the catalytic cycle. The substrates with less sterically
2
, thus resulting in higher reaction rate
catalyst design concept that the introduction of the amine group favorable for the CO
2
coordination
could promote the reaction.
4
. Conclusion
In summary, we have developed two new catalysts for CO
2
transformation, and our results
are significant in demonstrating new design principles to enhance reaction rates under mild
conditions. Incorporating an amine group at the ortho position of the pyridinium framework
increases activity by almost an order of magnitude. Based on experimental studies, the origin of
the rate enhancement is believed to be primarily due to the presence of a pendant base in the
proximity of active sites, promoting the CO activation. Ongoing work in our laboratories is
2
focused on extending such catalyst design concept to other CO
results will be reported separately in due course.
2
related transformations, and the
1
6

CRediT author statement
Yanpei Song: Data curation, Investigation, Writing- Original draft preparation, Validation.
Qi Sun: Writing- Reviewing and Editing, Methodology, Formal analysis.
Briana Aguila: Writing- Reviewing and Editing.
Shengqian Ma: Conceptualization, Supervision, Resources, Project administration.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
ACKNOWLEDGMENT
The authors acknowledge the University of South Florida for financial support of this work.
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