Quaternary ammonium salt grafted nanoporous covalent organic polymer for atmospheric CO2 fixation and cyclic carbonate formation

Article abstract of DOI:10.1016/j.cattod.2020.03.050

Non-redox carbon dioxide utilization through cycloaddition of CO₂ to epoxides offers great promise but suffers from lack of heterogeneous catalysts that don't need additives or pressure. Here we report a systematic post-synthetic modification p

Full text of DOI:10.1016/j.cattod.2020.03.050

Journal Pre-proof
Quaternary ammonium Salt grafted nanoporous covalent organic
polymer for atmospheric CO2 fixation and cyclic carbonate formation
Doyun Kim (Methodology) (Validation) (Investigation) (Formal
analysis), Saravanan Subramanian (Investigation), Damien Thirion
(Formal analysis), Youngdong Song (Formal analysis), Aqil Jamal
(Funding acquisition), Mona S. Otaibi (Formal analysis), Cafer T.
Yavuz (Conceptualization) (Validation) (Supervision) (Project
administration) (Funding acquisition)
PII:
S0920-5861(20)30171-1
DOI:
https://doi.org/10.1016/j.cattod.2020.03.050
CATTOD 12763
Reference:
To appear in:
Catalysis Today
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Revised Date:
Accepted Date:
31 July 2019
13 March 2020
25 March 2020
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Quaternary Ammonium Salt Grafted Nanoporous Covalent
Organic Polymer for Atmospheric CO2 Fixation and Cyclic
Carbonate Formation
1
2
2
1
3,4
¶
Doyun Kim , Saravanan Subramanian , Damien Thirion , Youngdong Song , Aqil Jamal ,
Mona S. Otaibi3, Cafer T. Yavuz 1,2,4*
1Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science
and Technology (KAIST), Daejeon 34141, Republic of Korea
2Graduate School of Energy, Environment, Water and Sustainability (EEWS), KAIST, Daejeon
34141, Republic of Korea
3Research and Development Center, Saudi Aramco, Dhahran, 31311 Saudi Arabia
4Saudi Aramco – KAIST CO2 Management Center, KAIST, Daejeon 34141, Republic of Korea
¶Present address: Inorganic Materials and Catalysis Division, CSIR-Central Salt and Marine
Chemicals Research Institute, G. B. Marg, Bhavnagar - 364 002, Gujarat, India.
*Correspondence and request for materials should be addressed to C. T. Y. (email:
yavuz@kaist.ac.kr)
1

GRAPHICAL ABSTRACT
Highlights
.
Nanoporous polymers retained porous structure after post-modification with quaternary ammonium
functionalities
.
.
Cycloaddition of CO2 to epoxides was achieved with metal free and cost-effective porous catalyst
The nanoporous catalyst worked under atmospheric pressure and without use of any additives or co-
catalysts and showed wide substrate scope
.
Steric hindrance of grafted ammonium salts changed microporosity, CO2 binding energy (Qst) and led
to varying kinetics in cycloaddition reactions
2

Abstract:
Non-redox carbon dioxide utilization through cycloaddition of CO2 to epoxides offer great
promise but suffer from lack of heterogeneous catalysts that don’t need additives or pressure.
Here we report a systematic post-synthetic modification procedure on a highly porous
hydrocarbon framework for efficient grafting of quaternary ammonium salts. The active sites
were tuned and characterized while maintaining the porous structure. The metal free and cost-
effective catalysts showed quantitative selectivity and very high conversion yields in
atmospheric pressure catalysis for cyclic carbonate formation from CO2 and epoxides. The
reaction proceeded without additives or co-catalysts, and tolerant for a wide substrate scope.
We found that the steric hindrance of the alkyl units on ammonium salts affect microporosity,
CO2 binding and the kinetics of cycloaddition reactions. The catalysts were also recyclable, an
attractive prerequisite for industrial implementation.
3

1. Introduction
Global warming of 1.5oC since the pre-industrial levels is believed largely due to the excessive
carbon dioxide (CO2) emissions, through its dominant role as a greenhouse gas.[1] With the
rise in CO2 levels, issues emerged as global climatic change, worsening economies and
concerns for general health of the world population.[2] In order to curb the potential harms
further, Carbon Capture and Utilization (CCU) strategies were developed with the hope that
CCU could provide reduction in carbon emissions and at the same time producing
economically viable products by utilizing CO2.[3-5] For this strategy to be feasible, one should
consider the carbon footprint, thus conversion must take absolute minimal amount of energy.[6,
7]
The formation of cyclic carbonates from epoxides and CO2 has gained attention due to its
exceptional advantages, such as no reduction of carbon necessary, green process using CO2
directly as a feedstock, and exothermic reaction (∆Hr = -144 kJ/mol).[8-10] Also, cyclic
carbonates are used in various applications as polar aprotic solvents, precursor for biomedical
applications, raw materials for engineering plastics and lastly, electrolytes in batteries. To date,
there has been considerable research efforts for catalyzed cycloaddition of CO2 to epoxides.
Homogeneous catalysts of organometallic complexes are found highly active.[11] For example,
an aluminum complex with amino triphenolate ligand shows initial TOFs up to 36,000 h-1 at
90 oC and 10 bar pressure of CO2.[12] Magnesium(II) porphyrin trimer complex based on m-
phenylene-bridged shows 46,000 h-1 at 160 oC and 17 bar pressure of CO2.[13] Chromium(III)
salophen bromide complex shows excellent catalytic activity at room temperature.[14] Often,
high pressure requirements and costly ingredients are proving unattractive for homogenous
catalysis. The main challenge, however, is the tedious separation of catalyst from cyclic
4

carbonate and the left-over metals, even in trace amounts. Alternatively, heterogeneous
catalysts are suggested since they are easily separated. For example, ionic liquids[15-17],
transition metal complexes[18-20], metalated porous polymers[21-24], metal-organic
frameworks (MOFs)[25-28], and porous organic polymers (POPs)[29-33] were studied. The
problem with heterogeneous catalysts is that they are not as active as homogeneous
counterparts. To solve the issue, additives such as aprotic solvents, transition metals, and co-
catalysts such as tetraalkyl ammonium salts, potassium iodide (KI), triethanolamine are
employed.[34-39] In most cases, even process modifications were needed, such that high
pressure and high catalyst loading with increased temperatures became common. Clearly an
active heterogeneous catalyst that does not need any extreme conditions, additives or co-
catalysts is needed. There are some encouraging recent developments, but the search for an
optimum catalyst is still ongoing.[10, 30, 40]
In this work, we designed a series of novel porous polymeric quaternary ammonium salts
as catalytic sites through grafting methodology. After post-synthetic modification, the catalysts
retained materials properties such as, high surface area, porosity, and metal-free structure that
are feasible to conduct atmospheric CO2 uptake and remarkable catalytic activity under
additive-free and 1 bar of CO2 pressure. Moreover, catalytic ammonium loading ratio and steric
hindrance effect has been investigated using four variations of catalysts with three different
alkyl chain lengths of quaternary ammonium salts. The higher loading of ammonium salts and
the less sterically hindered catalytic sites showed higher CO2 uptake and affinity, leading to
enhanced kinetics in cycloaddition reactions. To the best of our knowledge, this is the first
study of nano-porous networks with grafted quaternary ammonium salts and their application
for CO2 capture and subsequent utilization in ambient conditions without additive or co-
5

catalyst.
2. Experimental section
2.1 Materials
4,4'-bis-(chloromethyl)-1,1'-biphenyl (BCMBP, 95%) was purchased from Sigma-Aldrich,
USA. Chloromethyl methyl ether (MOMCl) was provided by Kanto, Japan. Trimethylamine
(TMA, 33% in ethanol) was supplied by Alfa Aesar, USA. Triethylamine (TEA, 99%), N,N-
diisopropylethyl amine (DIPEA, 99%), anhydrous iron(III) chloride (FeCl3, 98%),
tetrahydrofuran (THF), methanol, ethanol, and 1,2-dichloroethane (DCE, 99%) were
supplied by SAMCHUN, South Korea.
+
Scheme 1. The synthetic route for COP-114-CH2NR3 Cl- through post-synthetic modification
2.2 Synthesis of COP-114
To make the porous network, we used Friedel-Crafts reaction (Scheme 1). Starting polymer,
namely COP-114, was synthesized through a direct one-step self-polymerization and formed a
microporous framework.[41] In a typical synthesis of COP-114, we treated BCMBP (5.34 g,
o
0.02 mol) with FeCl3 (3.50 g, 0.02 mol) in dry DCE (25 mL) at 80 C under a nitrogen
6

atmosphere for 24 hours, following a slightly modified one-step self-polymerization
procedure.[42] After cooling to room temperature, water was then added to the mixture. The
precipitate was filtered and rinsed with water and methanol until the filtrate became clear. The
solid was redispersed in 4M hydrochloric acid through sonication until well suspended in
solution. The mixture was once again filtered through vacuum suction then, washed with water,
methanol and Soxhlet extracted with THF for 24 hours.
2.3 Synthesis of COP-114-CH2Cl
To introduce anchoring points, we used chloromethylation where MOMCl (4 mL) was added
on COP-114 (2 g) along with FeCl3 (4.5 g) in dry DCE (50 mL) at room temperature for 72
hours. Water was then added and the precipitate was filtered using vacuum suction. The
obtained solid was rinsed with water, methanol and THF. The obtained dark brown powder was
dried under vacuum at 100 oC overnight. To the best of our knowledge, this is the first Friedel
Crafted based porous polymers that were modified by chloromethylation.
+
2.4 Synthesis of COP-114-CH2NR3 Cl-
Finally, we loaded tertiary amines to form quaternary ammonium salts. In order to make a
+
series of COP-114-CH2NR3 Cl-, COP-114-CH2Cl (1 g) was dispersed in a solution of either
TMA (50 mL, 0.210 mol), TEA (30 mL, 0.211 mol), or DIPEA (37 mL, 0.212 mol). For
catalytic site loading comparison, one of the TMA solutions were stirred at room temperature
o
for 48 h and another TMA mixture was heated up to 70 C under a condenser and constantly
stirred for 48 h. For steric hindrance comparison, the TEA mixture was stirred at 70 oC for 48
o
h. The DIPEA mixture was stirred at 125 C for 48 hours. All reaction mixtures were cooled
down to room temperature and filtered using vacuum suction. While filtering each mixture, the
obtained solid was washed extensively with ethanol and THF. The obtained bright brown
7

powders were dried under vacuum at 50 oC overnight.
2.5 Characterizations
Chemical compositions of the structures were determined by X-ray photoelectron spectroscopy
(XPS) analysis (K-alpha, Thermo VG scientific) and elemental analysis by Thermo Finnigan
FlashEA 1112 (C, H, N) and Flash 2000 series Thermo Scientific Thermo Scientific (C, H, N,
S). In order to prevent halogen contamination, all samples were pretreated with vanadium(V)
oxide. Quantitative amount of each elements was observed using dynamic flash combustion
oxidation method with gas chromatography column thermal conductivity detector. Fourier
transform infrared spectroscopy (FT-IR) was measured with a Nicolet iS50, Thermo Fisher
Scientific Instrument. Simple hydrophobicity test was carried out by dispersing the precursor,
+
COP-114 and final product, COP-114-CH2NR3 Cl- in a mixture of n- hexane and water.
Surface area and porosity analysis was performed with N2 adsorption at 77K (3-flex,
Micromeritics) using Brunauer-Emmett-Teller (BET) method and non-local density functional
o
theory (NLDFT). All samples were degassed at 70 C overnight before analysis for more
accurate measurement. Surface area in micropores, pore size distribution and pore volumes
were calculated using NLDFT. Surface textural properties and morphologies were analyzed by
using a field emission scanning electron microscope (FESEM, Magellan 400) with osmium
coating to reduce the surface charging on the samples. Thermogravimetric analysis (TGA) was
performed with a Shimadzu DTG-60A. The samples were treated by increasing temperature to
800 oC with a 10 oC/min ramping rate under a N2 flow (50 mL/min).
8

3. Results and discussion
3.1 Catalyst characterization
+
A series of COP-114-CH2NR3 Cl- structures were prepared by self-polymerization,
chloromethylation via Friedel Crafts, and quaternization as shown in Scheme 1. The COP-114
structure was chosen particularly because of the lack of nitrogens in its framework, leading to
easy characterization after grafting with ammonium salts. For initial chemical characterization,
we carried out XPS analysis, hydrophilicity test, FT-IR, and elemental analysis. In XPS, each
step of the post-synthetic modification was verified by survey scan (Fig. 1a). The first spectra
(black line) corresponds to COP-114 of which the network contains only C and H atoms proved
by the strong C1s peak and little Cl 2p peak which is from the remaining chloromethyl groups
after reaction. In the second step, COP-114-CH2Cl (blue line) revealed the increasing peaks of
Cl 2s and Cl 2p owing to Cl atoms of chloromethyl group that are attached to the network.
+
Survey spectra of COP-114-(CH2NMe3 )maxCl- (red line) contains a peak from N 1s, which the
first two spectra did not show at all. Additionally, the presence of O 1s is owing to samples
exposed to the moisture, which can be trapped in the pores. State of each element in COP-114-
+
(CH2NMe3 )maxCl- was verified by C 1s, N 1s, Cl 2p. The N 1s spectrum was fitted into two
peaks that indicate C-N+ bond (quaternary ammonium, 402.38 eV) and C-N bond (tertiary
amine, 399.27 eV), respectively (Fig. 1b). According to the peak intensities of N 1s, most of
the nitrogen atoms were in the form of quaternary ammonium salts. Small intensity of tertiary
amine peak might indicate Stevens rearrangement or Sommelet-Hauser rearrangement might
be occurring in the presence of tertiary amines.[43] C 1s spectrum was deconvoluted into five
peaks which corresponds to C=C (sp2 C, 284.34eV), C-C/C-H (sp3 C, 284.73 eV), C-N+
(quaternary, 286.03 eV), C-Cl (chloromethyl, 286.63 eV), π-π* (π-conjugation, 290.68 eV)
(Fig. 1c). To note, the most intense sp2 C π-π* peak is attributed to the π-π* transition of
electrons with clear evidence for the structure to be made up of cross-linked networks with
aromatic rings. Cl 2p spectrum was fitted to Cl- chloride ion, 2p3/2 and 2p1/2 at 196.63 and
9

198.19 eV; C-Cl chloromethyl 2p3/2 and 2p1/2 at 199.96 and 201.62 eV (Fig. 1d). Existence of
chloride ion after modification showed strong evidence that quaternary step was indeed
successful. The remaining C-Cl bonding peaks indicates that chloromethyl group were not
completely changed to quaternary ammonium due to the size of tertiary amine.
Fig. 1 The XPS spectra; (a) survey scan of COP-114 (black), COP-114-CH2Cl (blue), and COP-
+
114-(CH2NMe3 )maxCl- (red). (b) C 1s, (c) N 1s, and (d) Cl 2p spectra of COP-114-
+
(CH2NMe3 )maxCl- .
In order to quickly verify the post-modification process, a simple hydrophilicity test was
performed by dispersing the original porous polymer (COP-114) and the modified structure
10

+
(COP-114-CH2NR3 Cl-) in a n-hexane and water mixture (Fig. S1). Since COP-114 was
+
hydrophobic, it remained only in hexane phase and because COP-114-CH2NR3 Cl- was
hydrophilic from its quaternary ammonium salts, it readily dispersed in water. FT-IR spectra
also verified existing chemical functional groups (Fig. S2). After chloromethylation, C-Cl
(660cm-1) peak appeared in COP-114-CH2Cl and surface area was decreased (Fig. S3). Then,
+
in COP-114-CH2NMe3 Cl-, a C-N peak has emerged at 1253 cm-1 and C-Cl peak disappeared.
In order to calculate the relative number of catalytic sites, functional group molecular ratio
was determined using the molar percentage of nitrogens to the carbon from the framework
(Table S1). First, temperature had positive correlation with nitrogen loading and quaternary
+
functional group molecular ratio on COP-114-CH2NEt3 Cl- and COP-114-CH2NiPr2Et+Cl-.
+
However, the synthesis of COP-114-CH2NMe3 Cl- was adversely affected by the increased
temperature since the reaction at 70 oC showed that nitrogen loading and functional group ratios
o
were reduced. We suspect that TMA reagent was partially lost at 70 C, even under reflux,
o
owing to the low boiling point of TMA (2.9 C). We also found that homogeneity of the
+
solution by vigorous stirring affected the COP-114-CH2NMe3 Cl- most. Lastly, steric
hindrance of tertiary amines was found to affect quaternization reaction. COP-114-
+
+
CH2NMe3 Cl- always showed higher nitrogen loading than COP-114-CH2NEt3 Cl- and COP-
114- CH2NiPr2Et+Cl-, under same synthetic conditions. As expected, the bigger tertiary amines
could not penetrate deeper and proceed with nucleophilic attack on chloromethyl group as well.
Hence, aliphatic chain length, methyl (Me) < ethyl (Et) < isopropyl (iPr), explained the loading
trend. Thermal stability was tested by TGA and showed degradation predominantly after 400
oC, and the initial weight loss of about 2~3% was due to the captured moisture from air (Fig.
S4).
11

Table 1. Porosity parameters, CO2 uptakes and heat of adsorption values of COP-114-
+
CH2NR3 Cl-
CO2 adsorptiong
[mmolg-1]
h
a
b
c
d
Ne
(%)
Functional groupf
molecular ratio
Qst
SBET
Smicro
Vmicro
Vtotal
CO
2
Catalyst
[m2g-1]
[m2g-1]
[cm3g-1]
[cm3g-1]
[kJmol-1]
30.5 (28.0)
29.8 (27.1)
29.5 (27.6)
23.1 (21.6)
273K
298K
323K
+
COP-114-(CH2NMe3
)
maxCl-
621
1066
976
165
401
321
183
0.155
0.296
0.282
0.169
0.766
1.016
0.728
0.905
2.76
2.65
2.29
2.25
2.680
2.362
2.224
1.441
1.629
0.972
0.811
0.732
0.506
C1 (NMe3) 4.01%
C1 (NMe3) 3.48%
C1 (NEt3) 3.35%
COP-114-CH2NMe3+Cl-
COP-114-CH2NEt3+Cl-
COP-114-CH2NiPr2Et+Cl-
1.383
1.285
0.844
C1 (N iPr2Et) 3.65%
686
aSurface areas of COP-114-CH2NR3+Cl- were calculated from N2 isotherms measured at 77K using BET theory, in the range of relative pressure determined by
f
the Rouquerol plots. b,c,dDetermined from non-local density functional theory (NLDFT). eMeasured by dynamic flash combustion method. Calculated from
Equation S1. gCO2 uptake values of COP-114-CH2NR3+Cl- s at 1 bar. hThe isoteric heat of adsorption values (Qst) for CO2 at zero coverage and the values inside
the parentheses refers to heat of adsorption at high loading.
+
Fig. 2 Porosity and gas uptake studies of COP-114-CH2NR3 Cl- derivatives. (a) Nitrogen
adsorption and desorption isotherms at 77K. (b) Corresponding NLDFT pore size distributions.
CO2 adsorption and isotherms collected up to 1bar (c) 273K, (d) 298K, (e) 323K, and (f) the
isosteric heat of adsorption (Qst) plots for CO2.
To measure porosity, we used N2 as a probe gas at 77K. All series of COP-114-
+
CH2NR3 Cl- maintained their porous network properties after post-synthetic modification.
They exhibited hysteretic type IV adsorption isotherms, which feature micro-mesoporous
+
structures (Fig. 2a). The specific surface areas of COP-114-CH2NR3 Cl- were investigated
12

using N2 adsorption isotherms through Brunauer-Emmett-Teller (BET) theory, where the valid
ranges of pressures were calculated from the respective Rouquerol plots. The calculated BET
+
+
surface areas show that COP-114-(CH2NMe3 )maxCl-, COP-114-CH2NMe3 Cl-, COP-114-
+
CH2NEt3 Cl-, COP-114-CH2NiPr2Et+Cl- were found to be 621, 1066, 976, 686 m2g-1,
respectively (Table 1). To investigate further, pore size distributions and micro-porosity were
estimated using non-local density functional theory (NLDFT). All structures showed
hieararchical nature, where micro, meso-porous networks coexist with average pore sizes
below 20 nm (Fig. 2b). SEM images also reflect porous network morphologies corresponding
to the calculated pore distributions (Fig. S5). The more functionalized COP-114-
+
(CH2NMe3 )maxCl- exhibited smaller surface area and porosity compared to COP-114-
+
CH2NMe3 Cl-, because of the additional pore filling during quaternization. On the other hand,
steric hindrance was in play when alkyl groups increased in size, yielding lower surface areas
following the aliphatic length trend of Me < Et < iPr.
3.2 CO2 adsorption
CO2 sorption capacity is predominantly determined by the combination of effects of the anion,
cation, and polymer architecture in ionic liquids (ILs), polymerized ionic liquids (PILs), and
ionic porous organic polymers.[44-50] CO2 adsorption measurements were conducted at
temperatures of 273K, 298K, and 323K to investigate affinity towards CO2 based on the
quaternary ammonium chloride site and the inherent microporosity (Fig. 2c-e). Also, isosteric
heat adsorption values were calculated by using the Clausius-Clapeyron equation to understand
the chemical effect of quaternary ammonium salts on CO2 binding strength (Table S2-S3).
Each sample had similar Qst values according to CO2 loading at 0.1 and 1 mmol/g (Fig. 2f).
+
COP-114-(CH2NMe3 )maxCl- showed high CO2 uptake of 2.680, 1.629, 0.972 mmol/g at 273K,
13

298K, 323K, respectively and the Qst value was 30.5 kJ/mol (Table 1). It is likely to be resulted
from the charged nature of the polymer enhancing the CO2 binding in comparison to the non-
charged network.[29, 51] With aforementioned functional ratio and steric hindrance, the higher
functionalized and the less steric hindered resulted in the highest uptake as expected. Of note,
+
+
surface area of COP-114-(CH2NMe3 )maxCl- is lower than COP-114-CH2NMe3 Cl- resulting in
+
less physisorptive sites compared to COP-114-CH2NMe3 Cl-. However, CO2 capacity and Qst
+
+
value of COP-114-(CH2NMe3 )maxCl- is higher than COP-114-CH2NMe3 Cl- indicating the
contribution of the quaternary ammonium cation and chloride on CO2 binding. On the other
hand, the less sterically hindered quaternary ammonium (iPr < Et < Me) showed the higher
CO2 adsorption and Qst. This behavior could be explained as longer chains blocking the
interactions between CO2 and the functional groups and also more likely of micropore
blocking.[52]
3.3 CO2 conversion
Table 2. Optimization of reaction conditions for the cycloaddition of CO2 to epichlorohydrina
Conversionb
(%)
Selectivityb
(%)
Temperature
(oC)
Amount
(mg)
Time
(h)
Entry
Catalyst
1
2
3
4
5
6
7
8
5
100
100
100
100
100
28
24
24
24
24
24
24
24
24
55
84
89
99.7
>99
2.86
14.0
56.3
99
99
99
99
99
99
99
99
10
15
20
25
20
20
20
COP-114-
+
(CH2NMe3 )maxCl-
40
60
14

9
10
20
20
80
100
24
24
90.7
0
99
-
COP-114
aReaction conditions: catalyst, epichlorohdrin (5 mmol), and CO2 1bar. bDetermined by using
1H-NMR and 13C-NMR.
After verifying the CO2 binding ability at the high end of the physisorptive regime, COP-
+
114-(CH2NMe3 )maxCl- was tested for cycloaddition of CO2 to epoxides. To optimize the
conditions, we chose the most commonly studied substrate, epichlorohydrin and reacted with
atmospheric CO2 under various conditions (Table 2). Catalyst loading was set for 20 mg (4%
w/w with respect to the substrate) after checking that the catalyst loading of 15 mg converts
89% and 25 mg, 99% (entries 3, 5). Increasing the temperature from room temperature to
100 °C had a pronounced positive effect on the yield of the cyclic carbonate (entries 4, 6-9).
Kinetics of cycloaddition reaction was monitored at the optimized conditions (Fig. 3a, Table
S2). As expected, increasing nitrogen ratio is corresponding to higher conversion and catalysts
with longer alkyl chains showed less conversion. The results of kinetic curve are similar to that
+
of Qst. Thereby, a proposed mechanism for cycloaddition reaction using COP-114-CH2NR3 Cl-
as a catalyst could be the activation of epoxide through interaction between its oxygen atom
and the quaternary ammonium center, and a subsequent nucleophilic attack by Cl− anion to
form an oxy anion intermediate (Fig. 3b).[53, 54] This active intermediate then reacts with CO2
interacted quaternary ammonium salt to form the corresponding cyclic carbonates.[55] It is
important to note that polycarbonate has not formed in any of the tested conditions, mainly due
to it being a kinetic product. [56]
15

Fig. 3 (a) Kinetics curve for the cycloaddition reaction of CO2 to epichlorohydrin. (b) Proposed
mechanism for the observed catalytic reaction.
After optimization of catalyst conditions, we selected a range of epoxides for both chemical
variation and their industrial importance (Table 3). All substrates we tested have near
quantitative selectivity to the corresponding cyclic carbonate and no polycarbonate was
detected. Conversion yields varied based on the reactivity and sterics of the epoxide.[57] As
shown in entries 1-4, conversion ratio was decreasing orderly from epichlorohydrine to
epichlorobromine, and then styrene oxide, to 2-(4-fluorophenyl)oxirane. Epichlorohydrin is
more reactive than bromo derivative due to the strong inductive effect and yields higher
conversions in the same conditions. The delocalization on styrene is rather interrupted by the
strong electron withdrawing effect by fluorine substitution, hence lower stability of the
intermediate. Hexyl epoxide has both lower boiling point and lower resonance than the other
heavier hydrocarbons (entries 5~9) and therefore lower conversion was observed. Among the
ether side groups (entries 7~9), the bis epoxide had harder conversion mainly because of sterics,
but also due to the double drain on the electronic stabilization of the phenyl ring. Interestingly,
ortho substituted version of 1,2-epoxy-3-phenoxypropane has higher conversion yield (entries
7-8). We suspect that it is mainly because of the higher boiling point, although a more thorough
16

understanding is necessary.
Table 3. Cycloaddition reactions of CO2 with epoxides catalyzed by COP-114-
+
(CH2NMe3 )maxCl-.*
Conversion
(%)
Selectivity
(%)
Entry
1
Epoxides
Product
99.7
98.8
95.8
58.2
95.5
86.7
95.0
90.8
28.3
99
99
99
99
99
99
99
99
99
2
3
4
5
6
7
8
9
*Reaction conditions: Catalyst (20mg), epoxides (5mmol), and CO2 1bar.
Lastly, we tested for recyclability under the optimized reaction conditions. COP-114-
+
(CH2NMe3 )maxCl- showed recyclable activity for cycloaddition of CO2 to epichlorohydrin
with high conversion (99%) and selectivity (99%) in each cycle (Fig. 4).
17

Fig. 4 Conversion and selectivity yields of the recycling tests in the cycloaddition of CO2 to
+
epichlorohydrin using COP-114-(CH2NMe3 )maxCl-.
4. Conclusions
In summary, we have shown that the grafted porous polymers of quaternary ammonium salts
from COP-114 could retain porous features and provide increased availability of active sites
per mass of the heterogeneous catalysts. The catalysts were able to capture atmospheric CO2
and subsequently convert epoxides to cyclic carbonates under ambient CO2 pressure without a
co-catalyst, solvent, or additive. They provide outstanding selectivity and very high conversion
yields for a wide variety of epoxides under harsh conditions. Since commercially available
catalysts require costly co-catalyst support or additives, the structures built in this work offer
significant promise in their industrial implementation. In addition, we were able to study
catalyst loading, steric factors and other parameters like stirring rate, thanks to the simple and
nitrogen free construct of the COP-114. The porous polymers reported were the first examples
of quaternary ammonium salt grafting and also the first of such structures in cycloaddition
18

reaction of CO2 to epoxides. The results indicate a feasible route for the CCU methods that are
not energy intensive and could be commercially viable. Further work would be directed in
understanding framework improvements, such as imparting Lewis acid/base sites. Since our
work clearly identified the power of the quaternary ammonium salts in porous materials, the
structural enhancements would be the logical next step.
CRediT statement
Doyun Kim: Methodology, Validation, Investigation, Formal Analysis, Writing
Saravanan Subramanian: Investigation
Damien Thirion: Formal analysis
Youngdong Song: Formal analysis
Aqil Jamal: Funding acquisition
Mona S. Otaibi: Formal Analysis
Cafer T. Yavuz: Conceptualization, Validation, Writing, Supervision, Project administration, Funding
acquisition
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.
Acknowledgements
This work was primarily funded by the Saudi Aramco-KAIST CO2 Management Center. C. T.
Y., S. S. and D. T. also acknowledge support by National Research Foundation of Korea (NRF)
grant funded by the Korean government (MSIP) (No. NRF-2017M3A7B4042140).
19

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