Synthesis of Porous Covalent Quinazoline Networks (CQNs) and Their Gas Sorption Properties

Article abstract of DOI:10.1002/anie.201813075

The development of different classes of porous polymers by linking organic molecules using new chemistries still remains a great challenge. Herein, we introduce for the first time the synthesis of covalent quinazoline networks (CQNs) using an ionothermal

Full text of DOI:10.1002/anie.201813075

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Accepted Article
Title: Synthesis of Porous Covalent Quinazoline Networks (CQNs) and
Their Gas Sorption Properties
Authors: ONUR BUYUKCAKIR, Recep Yuksel, Yi Jiang, Sun Hwa Lee,
Won Kyung Seong, Xiong Chen, and Rodney S. Ruoff
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201813075
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10.1002/anie.201813075
Angewandte Chemie International Edition
COMMUNICATION
Synthesis of Porous Covalent Quinazoline Networks (CQNs) and
Their Gas Sorption Properties
Onur Buyukcakir, Recep Yuksel, Yi Jiang, Sun Hwa Lee, Won Kyung Seong, Xiong Chen and Rodney
S. Ruoff*
Abstract: The development of different classes of porous polymers
by linking organic molecules using new chemistries still remains a
great challenge. Here we introduce for the first time the synthesis of
covalent quinazoline networks (CQNs) using an ionothermal
synthesis protocol. Zinc chloride (ZnCl2) was used as the solvent
and catalyst for the condensation of aromatic ortho-aminonitriles to
produce tricycloquinazoline linkages. The resulting CQNs show a
high porosity with a surface area up to 1870 m2 g-1. Varying the
temperature and the amount of catalyst enables us to control the
surface area as well as the pore size distribution of the CQNs.
Furthermore, their high nitrogen content and significant
microporosity make them a promising CO2 adsorbent with a CO2
uptake capacity of 7.16 mmol g-1 (31.5 wt %) at 273 K and 1 bar.
Because of their exceptional CO2 sorption properties, they are
promising candidates as an adsorbent for the selective capture of
Figure 1. The synthesis route for covalent quinazoline networks (CQNs). a)
Preparation of model compound, tricycloquinazoline (mCQN), b) The synthesis
of CQN-1.
CO2 from flue gas.
The importance of porous materials has recently increased
as well as a large surface area, they suffer from chemical and
physical instability due to boron-containing linkages. The
introduction of imine-linked COFs improved the stability
considerably.[6] In contrast, CTFs, which are a relatively new
class of POPs reported by Thomas et al., were synthesized by
an ionothermal reaction.[7] The trimerization of aromatic nitriles in
the presence of ZnCl2 at high temperatures gives triazine-linked
porous frameworks.[8] Moreover, several other synthesis
methods have been recently reported.[9] The triazine linkage
provides not only a high nitrogen content but also chemically
and physically more stable porous polymers compared to COFs.
Owing to their distinctive properties, they have been tested in
diverse applications such as gas storage, separation, as
catalysts or catalyst supports, and in energy storage.[10] Although
significant progress has been achieved for the preparation of
POPs, the synthesis of new porous polymers is still of great
interest. In this respect, the development of new linkages and
synthesis strategies to produce polymers showing permanent
porosity and large surface area, together with chemical and
physical stability is greatly challenging but desirable. Here we
introduce, for the first time, porous covalent quinazoline
networks (CQNs) prepared by heating aromatic ortho-
aminonitriles under ionothermal reaction conditions using ZnCl2
as both a solvent and trimerization catalyst. This strategy also
allows us to control the surface area as well as the pore size
distribution of CQNs by simply changing the reaction
temperature and the amount of ZnCl2. The tricycloquinazoline
linkages serve not only as a nitrogen-rich polymeric network but
also increase the physicochemical stability due to the presence
of strong C-C and C-N bonds. The CQNs showed a high specific
surface area up to 1870 m2 g-1. In addition to their microporous
texture, the high nitrogen content makes them promising CO2
adsorption materials showing outstanding CO2 uptake capacity
at low pressures (up to 7.16 mmol g-1, at 273 K and 1 bar) with a
high CO2/N2 selectivity (up to 74.7).
tremendously.[1] Besides the well-studied zeolites and activated
carbons, the metal-organic frameworks (MOFs) and porous
organic polymers (POPs) have made a big impact on science
and technology.[2] There have been important advances in POPs
and they have become an extensive class of porous materials.[3]
The adaptation of the synthesis strategies used in the synthesis
of small organic molecules or conventional polymers has played
an important role for the development of several porous
polymers including covalent organic frameworks (COFs),
conjugated microporous polymers (CMPs), porous aromatic
frameworks (PAFs), hyper-crosslinked porous polymers (HCPs),
polymers of intrinsic microporosity (PIMs), and covalent triazine
frameworks (CTFs).[4]
One of the important advances is the discovery of COFs
by Yaghi and co-workers.[5] COFs take advantage of dynamic
covalent chemistry (DCC) by linking organic molecules by more
strong covalent bonds than their metal-organic analogues.
Although the first examples of COFs revealed a high crystallinity
Dr. O. Buyukcakir, Dr. R. Yuksel, Dr. Y. Jiang, Dr. S. H. Lee, Dr. W.
K. Seong, Dr. X. Chen and Prof. Dr. R. S. Ruoff
Center for Multidimensional Carbon Materials (CMCM)
Institute for Basic Science (IBS)
Ulsan 44919 (Republic of Korea)
E-mail: rsruoff@ibs.re.kr, ruofflab@gmail.com
Prof. Dr. R. S. Ruoff
Department of Chemistry
Ulsan National Institute of Science and Technology (UNIST)
Ulsan 44919 (Republic of Korea)
Prof. Dr. R. S. Ruoff
School of Materials Science and Engineering
Ulsan National Institute of Science and Technology (UNIST)
Ulsan 44919 (Republic of Korea)
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201813075
Angewandte Chemie International Edition
COMMUNICATION
observed at approximately 163 ppm can be assigned to the
carbon atoms connected to nitrogen atoms in the
tricycloquinazoline core (Figure S15). The other broad signals at
143, 125 and 108 ppm are assigned to aryl carbons. Like the
FTIR analysis, the observed resonance signals belonging to
carbon atoms attached to nitrile and amino moieties support the
incomplete polymerization of CQN-1a and -1b.
The chemical bonding nature of CQNs was examined by
X-ray photoelectron spectroscopy (XPS). The survey spectrum
of CQNs revealed the presence of C 1s, N 1s and O 1s (Figure
S16). The high-resolution C 1s spectrum of CQN-1c can be
deconvoluted into three major peaks with binding energies 284.3,
285.3 and 286.8 eV (Figure 2b). The peak located at 284.3 eV
can be assigned to carbon atoms in the aryl ring present in the
skeleton (C=C). The peaks at 285.3 and 286.8 eV were
attributed to the carbon atoms presents in the tricycloquinazoline
core with C=N-C and N-C=N bonding, respectively. The N 1s
spectrum denotes that there are two different nitrogen species
with binding energies 398.3 and 400.4 eV, which can be
respectively assigned to the nitrogen atoms located at the
periphery (C=N-C) and the nitrogen atom located at the center
(N-C=N) of the tricyloquinazoline core (Figure 2c).
The powder X-ray diffraction patterns of the CQN-1c
showed two broad diffraction peaks at around 7o and 26o,
signifying the existence of partial crystallinity but a lack of long-
range ordering of the polymeric networks (Figure S17a).
Because of the broadness of the peaks, it is difficult to acquire
detailed information on structural order at the atomic level.
However, the peak at 26o corresponds to the vertical spacing
between stacked layers with a distance of about 3.4 Å. The
peaks become broader with increasing ZnCl2 concentration
indicating the formation of less ordered structures (Figure S17b).
The particle morphology of the CQNs was studied using
scanning electron microscopy (SEM). As shown in Figures 3 and
Figure S18, CQNs showed monolithic morphologies with a size
from several hundred nanometers to several microns. The
formation of a porous network was verified using transmission
electron microscopy (TEM), with the image shown in Figure 3c
clearly showing the layered, microporous structure of CQN-1c.
Elemental mapping by EDX indicated homogeneous
distributions of carbon and nitrogen atoms in the skeletons of the
CQNs (Figures 3e, 3f and S18).
Figure 2. a) FTIR spectra of CQN-1(a-c) and compound 2 (left) and FTIR
spectra of CQN-1c and mCQN (right). b) High resolution XPS C 1s spectrum
of CQN-1c and c) N 1s spectrum of CQN-1c.
The CQNs were synthesized using the ZnCl2 catalyzed
ionothermal trimerization of aromatic ortho-aminonitrile
monomers (Figure 1). In order to test the reaction feasibility and
optimize the reaction conditions, tricycloquinazoline (mCQN)
was selected as a model compound (Figure 1a). The mCQN
was prepared by heating a mixture of compound 1 and one
equivalent ZnCl2 at 300 and 350 oC in flame-sealed ampoules to
produce mCQN with 91 and 97% yields, respectively. In order to
demonstrate the catalytic effect of the ZnCl2 on the trimerization,
a control experiment was conducted by heating the compound 1
in the absence of ZnCl2 to yield c-mCQN. Crude 1H-NMR
analysis of the control samples treated at 300 and 350 oC
showed respective yields of only 6 and 13% (see the Supporting
Information (SI), Figure S10). This control reaction, therefore,
indicates the large catalytic effect of the ZnCl2 on the
trimerization. In the same way, heat treatment of compound 2
(Figure 1b) in the presence of one equivalent ZnCl2 at 300, 350
and 400 oC produced CQN-1a, CQN-1b and CQN-1c,
respectively. In addition to the influence of reaction temperature
on polymerization, the amount of ZnCl2 has a significant effect
on the structural features of the polymers. Hence, the impact of
ZnCl2 concentration was also investigated using five different
o
salt/monomer mole ratios (1.0, 2.5, 5.0, 7.5 and 10) at 400 C
(Table 1). The resulting polymers were initially ground into fine
powder and extensively washed with dilute HCl solution and
water with the goal of removing ZnCl2 residue. After washing, the
CQNs were collected and dried under vacuum to give black
colored fine powders in yields higher than 90 % (Table S1).
Details of the synthesis and purification steps are given in the
supporting information (SI).
The CQNs were investigated by Fourier-transform infrared
spectroscopy (FTIR). The first indication of polymerization is the
disappearance of ortho-aminonitrile related signals. While the
carbonitrile bands around 2250 cm-1 disappeared completely for
CQN-1c, incomplete polymerization was noticed for CQN-1a and
CQN-1b, that were synthesized at lower temperatures of 300
and 350 oC, respectively (Figure 2a, left). As seen in Figure S14,
the increase of salt/monomer mole ratio from 1.0 to 2.5, 5.0, 7.5
or 10 leads to also the complete disappearance of the peaks
associated with the carbonitrile moiety. Furthermore, the
appearance of characteristic tricycloquinazoline vibrations
signals (as observed for mCQN) at around 1625, 1595, 1335
and 1240 cm-1 also indicates the successful trimerization of
ortho-aminonitrile groups into tricycloquinazoline cores (Figure
2a, right). Solid-state cross-polarization magic angle spinning
(CP/MAS) 13C NMR spectra of the CQNs confirmed the
formation of tricycloquinazoline structures, where the resonance
The thermal stability of the polymeric networks was
monitored using thermogravimetric analysis (TGA) by raising the
temperature from room temperature to 900 oC at a rate of 10 oC
min-1 under a nitrogen atmosphere (Figure S19a). CQNs show
excellent thermal stability up to 500 oC. The observed initial
weight loss below 100 oC stemmed from mainly moisture
Figure 3. a), b) and d) SEM images of CQN-1c. c) TEM image of CQN-1c.
e, f) elemental maps of CQN-1c for e) carbon f) nitrogen.
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10.1002/anie.201813075
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trapped inside the pores of CQNs. As also is typically observed
for the CTFs prepared in ZnCl2 salt melts,[7,8a,b,e] some residual
Zn metal salts (ZnCl2/ZnOx) also remained inside the pores of
our CQNs. In order to quantify any residual Zn metal salt,
thermogravimetric analysis (TGA) was performed in air and less
than 5.3 wt % residue was found (Figure S19b). The EDX
spectra also verified the presence of Zn and Cl atoms (Figure
S20) trapped inside the pores of CQNs. Elemental mapping by
EDX showed an essentially homogeneous distribution of Zn and
Cl atoms throughout the materials (Figure S18).
The elemental composition of each CQN was analyzed
using X-ray photoelectron spectroscopy (XPS) survey analysis
and elemental analysis by combustion (Table S1). Elemental
analysis revealed a small deviation from the ideal C, H, and N
values and this is primarily due to trapped water molecules and
trapped Zn metal salt, as also shown by thermogravimetric and
EDX analysis. The C/N ratio of CQNs also shows a slight
deviation from the ideal C/N ratio, which is likely due to partial
carbonization as well as the presence of unreacted terminal
groups due to incomplete polymerization. Note that the
increasing ZnCl2/monomer ratio does not lead to a notable
change in the C/N ratio (that ranges from 3.12 to 3.19, close to
the ideal value of 3.00).
The porosity of the CQNs was determined using nitrogen
adsorption-desorption isotherms measured at 77 K (Figure 4a)
which showed type I reversible sorption profiles. The steep
nitrogen uptake at low relative pressures (below 0.01) indicated
their microporous nature. The specific surface areas and pore
size distributions were calculated using the Brunauer-Emmett-
Teller (BET) and Non-Local Density Functional Theory (NLDFT),
respectively. The BET specific surface areas of CQN-1a and -1b
were measured as only 229 m2 g-1 and 328 m2 g-1, respectively.
On the other hand, increasing the temperature to 400 oC
increases the surface area of CQN-1c to 664 m2 g-1. This
observed substantial increase implies the formation of more
extended networks compared to the incomplete polymerization
of CQN-1a and 1b. Furthermore, to gain insight into the impact
of the salt to monomer ratio on the porosity of the CQNs, five
different ZnCl2/monomer mole ratios (1.0, 2.5, 5.0, 7.5 and 10)
were also analyzed. The increasing concentration of ZnCl2 leads
to an increase in the surface area of the CQNs. CQN-1g had the
highest surface area 1870 m2 g-1 along with a pore volume of
0.93 cm3 g-1. As previously observed for the CTFs,[11] the use of
higher amounts of ZnCl2 up to a certain ratio leads to an
increase in both surface area and pore volume by inducing the
formation of more amorphous structures having a lower density
than the long-range ordered networks. The pore size
distributions (Figure 4b) show that all CQNs are predominantly
microporous and the increasing ZnCl2 concentration leads to not
Figure 4. Gas sorption properties of the CQNs. a) N2 adsorption-desorption
isotherms of CQNs measured at 77K b) Pore size distributions of CQNs
calculated from N2 isotherms using NLDFT. CO2 adsorption isotherms of
CQNs collected up to 1 bar at c) 273 K, and d) 298 K. e) The calculated Qst of
CQNs for CO2. f) The calculated CO2/N2 selectivity of CQNs at 298 K under
flue gas conditions (CO2/N2:10/90 v/v) using IAST.
penalties, degradation, and corrosive effects. Therefore,
physisorptive CO2 adsorbents like POPs are seen as
a
promising alternative by virtue of their low heat of adsorption and
light elemental composition (C, H, N, O, S, and B).[8b-d, 14] Even
though many POPs have been tested for CO2 capture and
separation by simulating flue gas conditions, the exploration of
new structures is needed. To this end, we have investigated the
CO2 capture and separation capability of CQNs.
As shown in Figures 4c and 4d, the CO2 adsorption
isotherms show that CQNs showed outstanding CO2 uptake
capacity at 273 K and 298 K up to 1 bar. Because of its highest
surface area, CQN-1g exhibits the highest CO2 uptakes of 7.16
mmol g-1 (31.5 wt % at 273 K and 1 bar) and 4.57 mmol g-1 (20.1
wt % at 298 K and 1 bar). The CO2 uptakes of COF-1g at 0.10
bar, corresponding to the partial pressure of CO2 in the flue gas,
are 2.04 and 1.03 mmol g-1 at 273 and 298 K, respectively. It is
worth mentioning that to the best of our knowledge CQNs
showed the highest CO2 uptake values compared with
previously reported POPs (Table S2). The CO2 adsorption
efficiency is closely related to pore size and heteroatom
functionalities. Although there are debates underway about the
role of nitrogen functionalities on CO2 uptake capacity, the
interactions between CO2 molecules and N-containing
functionalities also contribute to the CO2 adsorption at some
level.[15] In this sense, this exceptionally high CO2 uptake of
CQNs can be attributed to their affinity to CO2 molecules
stemming from the synergetic effects of a high microporosity
(0.90 -1.10 nm) with the high nitrogen content (up to 24 wt %) of
the networks. In order to evaluate the CO2 affinity, the heat of
adsorption (Qst) of CQNs for CO2 was calculated from CO2
isotherms measured at three different temperatures (273, 283,
only an increase in microporosity but also contributes
a
mesoporosity to the networks. Detailed porosity values are given
in Table 1. The results suggest that the surface area and pore
size distributions of CQNs can be tuned easily by changing the
synthesis temperature and the amount of ZnCl2 used.
The earth’s growing population together with its increasing
energy demands has increased the combustion of fossil fuels,
which is seen as the main source of the anthropogenic CO2
emission.[12] In this respect, the technologies for the selective
capture of CO2 from sources such as flue gas have become
more crucial in reducing this now increasing CO2
concentration.[13] Current CO2 capture methods use amine
scrubbing technologies and suffer from high regeneration energy
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10.1002/anie.201813075
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COMMUNICATION
Table 1. Gas sorption properties of CQNs prepared at different temperatures and ZnCl2 concentrations.
[a]
[b]
[c]
[d]
[f]
[g]
Temp.
[oC]
ZnCl2
Eq.
SABET
SALang.
VTotal
SAMicro
CO2 at 273K[e]
CO2 at 298K[e]
Qst for CO2
[kJ mol-1]
CO2/N2
CQNs
[m2 g-1]
[m2 g-1]
[cm3 g-1]
[m2 g-1]
[mmol g-1] (wt %)
[mmol g-1] (wt %)
Selectivity
CQN-1a
CQN-1b
CQN-1c
CQN-1d
CQN-1e
CQN-1f
CQN-1g
300
350
400
400
400
400
400
1.0
1.0
1.0
2.5
5.0
7.5
10
229
328
279
386
0.13
0.16
0.31
0.60
0.79
0.86
0.93
122
234
-
-
-
-
-
-
-
-
664
779
480
4.10 (18.0)
5.63 (24.8)
6.58 (29.0)
6.50 (28.6)
7.16 (31.5)
2.96 (13.0)
3.78 (16.6)
4.39 (19.3)
4.18 (18.4)
4.57 (20.1)
35.5
34.6
41.7
40.1
40.6
74.7
54.1
44.6
43.5
42.7
1249
1647
1749
1870
1467
1931
2068
2213
891
1190
1183
1264
[a] The calculated BET surface areas of CQNs from N2 adsorption isotherms (77 K) using the relative pressure ranges determined according to Rouquerol
plots (Figure S21and S22). [b] Langmuir surface area of CQNs. [c] Total pore volumes at P/Po=0.99. [d] The micropore surface areas of CQNs calculated
using t-plot analysis. [e] CO2 uptake values of CQNs at 1 bar, the values in parentheses are the CO2 uptakes at weight percent (wt %) at 1 bar. [f] The isosteric
heat of adsorption of CQNs for CO2 obtained from the CO2 isotherms collected at 273, 283 and 298 K. [g] The CO2/N2 selectivity of CQNs at 298 K calculated
using IAST for the gas mixture CO2/N2:10/90 (v/v).
and 298 K) using the Clausius–Clapeyron equation (Figure 4e).
The CQNs show high Qst values in the range of 35 to 43 kJ mol-1
at zero loading and the high loading affinity stays between 21-23
kJ mol-1. In addition to the Qst values for CO2 of CQNs that are in
the range of a physisorptive adsorption mechanism (lower than
43 kJ mol-1), the reversible CO2 isotherms also suggest a
physisorption process (Figure S23).
functional features, we suggest that CQNs will attract great
interest. The simple preparation of variable organic building
blocks and the easy ionothermal polymerization strategy will
help in the synthesis of new CQNs derivatives, which may also
have uses as catalysts and energy storage materials.
Acknowledgements
These findings have motivated us to investigate the
CO2/N2 selectivities of CQNs by simulating flue gas conditions.
The selectivities were calculated according to ideal adsorption
solution theory (IAST) using CO2 and N2 isotherms measured at
298 K for a mixture of CO2 and N2 with a ratio of 10/90 (v/v)
(Figure 4f). The CQN-1c exhibited the highest CO2/N2 selectivity
of 74.7 at 1 bar whereas the COF-1g, which has the highest CO2
uptake capacity, showed the lowest selectivity (40.6) (Table 1).
Two of the parameters that affect the CO2/N2 selectivity is the
higher polarizability and quadrupole moment of CO2 molecules
compared to N2. This polarizability difference helps achieve a
higher interaction between polar adsorbents and CO2 molecules,
and this affinity contributes to the CO2/N2 selectivity. Another
important parameter is the pore size, which plays a dominant
role in the selectivity by a molecular sieving effect. However, the
kinetic diameter difference between CO2 and N2 molecules is
about 0.34 Å and it is highly challenging to achieve such pore
size control, especially in amorphous materials. Although CQNs
are microporous and have a high CO2 affinity, their pore size
distribution does not allow for a perfect sieving mechanism. This
clearly explains the lowering of the selectivity of CQNs with
increasing surface area in which more N2 molecules can be
accommodated. However, the kinetics of adsorption should also
be taken into consideration since the narrow pore size leads to a
slow adsorption rate.
In summary, we have demonstrated for the first time the
design and synthesis of covalent quinazoline networks (CQNs).
In addition to their high chemical and thermal stability, they
showed a surface area greater than 1800 m2 g-1 along with large
pore volumes up to 0.93 cm3 g-1. They also exhibited an
excellent CO2 uptake capacity of up to 7.16 mmol g-1 at 273 K
and 1 bar. Their high CO2 affinity and microporous nature
contribute to their high CO2/N2 selectivity up to 74.7 (298 K and
1 bar) under flue gas conditions. These results suggest that they
are promising materials for gas adsorption and separation
applications. Because of their characteristic structural and
This work is supported by IBS-R019-D1.
Keywords: • microporous • polymer • ionothermal • quinazoline
• CO2 capture
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Entry for the Table of Contents
COMMUNICATION
A new class of porous organic
polymers: Porous covalent
Onur Buyukcakir, Recep Yuksel, Yi
Jiang, Sun Hwa Lee, Won Kyung
Seong, Xiong Chen, and Rodney S.
Ruoff*
quinazoline networks (CQNs) are
designed and synthesized for the first
time. CQNs have high surface area
and microporosity with chemical and
thermal stability. They show
Page No. – Page No.
Synthesis of Porous Covalent
Quinazoline Networks (CQNs) and
Their Gas Sorption Properties
outstanding CO2 adsorption capacity
up to 31.5 wt % at low pressures.
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