Nanoporous benzoxazole networks by silylated monomers, their exceptional thermal stability, and carbon dioxide capture capacity

Article abstract of DOI:10.1021/cm503627p

The pursuit of synthetic routes for design and preparation of nanoporous polymeric networks with inherent permanent microporosity and functionality through bottom-up methodologies remains a driving force in developing CO₂-philic materials. We report nanoporous, processable, benzoxazole-linked covalent organic polymers (Box-COPs) that show exceptional thermal stability up to 576 °C. Box-COPs can be formed into films thanks to the silylation that is used to guide polymeric network formation. Surface areas of up to 606 m² g^-1 and narrow pore sizes of 4.36 ? were observed with a CO₂ uptake capacity of 139.6 mg g^-1 at 273 K and 1 bar. Box-COPs were stable in boiling water for a week without deteriorating CO₂ capture capacity.

Full text of DOI:10.1021/cm503627p

Article
pubs.acs.org/cm
Nanoporous Benzoxazole Networks by Silylated Monomers, Their
Exceptional Thermal Stability, and Carbon Dioxide Capture Capacity
Hasmukh A. Patel, Dongah Ko, and Cafer T. Yavuz*
Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea
S
* Supporting Information
ABSTRACT: The pursuit of synthetic routes for design and preparation of
nanoporous polymeric networks with inherent permanent microporosity and
functionality through bottom-up methodologies remains a driving force in
developing CO2-philic materials. We report nanoporous, processable, benzox-
azole-linked covalent organic polymers (Box-COPs) that show exceptional
thermal stability up to 576 °C. Box-COPs can be formed into films thanks to the
silylation that is used to guide polymeric network formation. Surface areas of up to
606 m2 g−1 and narrow pore sizes of 4.36 Å were observed with a CO2 uptake
capacity of 139.6 mg g−1 at 273 K and 1 bar. Box-COPs were stable in boiling
water for a week without deteriorating CO2 capture capacity.
however, their porous, network polymers are not yet reported.
Few examples of linear chains were prepared in polyphosphoric
acid (PPA) with excess phosphorus pentoxide as catalyst,44
which has low solubility in organic solvents and removal of PPA
becomes very complicated. This hinders the preparation of
benzoxazole network polymers. Fukumaru et al.45 demon-
strated an alternative route for the synthesis of linear
benzoxazole polymers by functionalization of the monomers,
leading to an improved processability.
INTRODUCTION
■
Carbon dioxide (CO2) emissions from burning fossil fuels
exacerbate global warming and contribute greatly to habitat
change in the oceans and environment.1,2 On the basis of the
most recent data, low price of fossil fuels, if coupled with
untaxed CO2 emissions, could severely impact our way of
living.3 Measurements of atmospheric CO2 levels at Mauna
Loa, Hawaii, show that the greenhouse gas has accumulated
steadily, and spiked above 400 ppm several times in April
2013.4 A wide spectrum of materials that includes zeolites,5
mesoporous silica,6 carbonaceous materials,7,8 metal−organic
frameworks,9,10 and polymeric membranes11−15 have been
considered for CO2 capture by absorption, adsorption, and/or
separation, yet, none of these materials is competitive enough
to replace already established aqueous amine-based CO2
capture, although the latter is an energy inefficient technology.
Nanoporous amorphous polymers exhibiting permanent
microporosity emerge to fulfill promise for applications in gas
storage and separation,16 charge carrier,17 sensors,18 and
catalysis.19 Thanks to the wide variety of bond-forming
techniques accessible through synthetic organic chemistry, the
nanoporous polymeric networks with diverse porosity,
thermomechanical stability and functionality have been
established.20−22 The most promising nanoporous polymers
are those constructed by rigid monomers through various
organic reactions―Yamamoto, Ullmann, Suzuki, Sonoga-
shira−Hagihara couplings, “click” chemistry, and various
condensation polymerizations,23−25 yielding cross-linked poly-
meric networks with high surface areas, such as COFs, PIMs,
HCPs, CMPs, CTFs, TzFs, BILP, POF, COP, CPOP, PCPs,
POPs, PPF, SNW, PAFs, PPNs, and several others.20,26−43
Thermal stability of CO2 scrubbing materials (e.g., membranes)
is desired as exhaust gas mixtures discharge at high temper-
atures.11 Polybenzoxazoles are known for their superior thermal
stability, and thus their potential in flue gas separations;
In an attempt to produce nanoporous benzoxazole network
polymers, hereafter called benzoxazole bridged nanoporous
covalent organic polymers (Box-COPs), we coupled trivalent
trimesoyl chloride (core) with 4,6-diaminoresorcinol (linker)
while screening a wide variety of conditions that include base
catalysts, solvents, ratios, temperature and pressure of the
reaction. None of the direct pathways has produced the oxazole
ring, which needs a ring closure step after amidic adduct is
formed. We suspect that the rotational freedom of the linker
substituent groups is significantly hindered when network is
formed in a random fashion, hence the inability to execute the
ring closure. We therefore used silylation, where bulky silyl
groups would repel the rest of the linker, which then positions
suitable for ring closing (Figure 1a). A subsequent annealing
step would unravel the nucleophilic hydroxyls before they
attack the amidic carbonyl. In this strategy, the intermediate
would also be prone to be soluble in certain solvents. That was
exactly what we discovered, in that the silylated prepolymer was
soluble in solvents like NMP, leading to a highly sought after
property of processability. Briefly, we produced two Box-COPs
(COP-93 and COP-94) that were synthesized in three steps:
(i) reaction of tert-butyldimethylsilyl chloride with 4,6-
diaminoresorcinol dihydrochloride and 3,3′-dihydroxybenzi-
Received: August 12, 2014
© XXXX American Chemical Society
A
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Chemistry of Materials
Article
further penetration since we believe that even hexagons are
randomly oriented (spirals, helixes). In an ideal ordered
structure, we ought to see a major peak of pore widths similar
to the basic hexagon. Hexagons are interpenetrated, even twice
on the same macrocycle (Figure 1d) due to lack of large pores
within polymeric network which results pore sizes below 5 Å.
In reality, we believe that possible formation of both
interpenetrated networks and/or solid-state packing of the
polymeric networks could contribute to the observed narrow
pore size of COP-93.
RESULTS AND DISCUSSION
■
The structures of silylated monomers, pre−polymers and Box-
COPs were analyzed by FT−IR, 1H NMR, CP/MAS 13C
NMR, and elemental analysis. 1H NMR show chemical shifts at
6.24−6.27 ppm (Ar−H), 3.65 ppm (N−H), 1.00−0.97 ppm
((CH3)3), 0.24−0.2 ppm (Si(CH3)2), confirming the silylation
of the monomers, which is also supported by FT−IR frequency
at 3397 cm−1 (N−H), 2953 cm−1 (Ar−H), and 2862 cm−1
(CH3).46 Pre−polymers reveal characteristic IR peaks of the
amide bonding, 1658 cm−1 (CO) and 1526 (N−H) cm−1,
which clearly indicates the formation of prepolymers (see
Figure S2a in the Supporting Information). A peak at 1366
cm−1 (CH3) also suggesting that the hydroxyl group is
protected by silylated functionality which provide excellent
solubility of prepolymers in NMP. Pre−polymers were also
characterized by solid-state CP/MAS 13C NMR (see Figure S3a
in the Supporting Information), which show chemical shifts at
around 171.6 (CO), 153.4 (Ph−O), 127.8 (Ph−NH), and
32.3−2.8 ppm (Si(CH3)2C(CH3)3). The benzoxazole ring
formation after annealing prepolymers at 400 °C for 3 h is
confirmed from the characteristic peaks at around 1618 cm−1
(CN) and 1425, 1054 cm−1 (benzoxazole ring) (see Figure
S2b in the Supporting Information). Indeed, CP/MAS 13C
NMR spectrum (see Figure S3b in the Supporting
Information) of Box-COPs show chemical shifts that are
assigned for each spectrum with respective structure. 168.4,
155.2, and 145.9 (benzoxazole ring), 134.7 and 118.1 ppm
(Ph), supporting the formation of the benzoxazole linkages.47
The experimental elemental analysis of Box-COPs is also in
accordance with the theoretical values (see Table S1 in the
Supporting Information). The cyclization of benzoxazole ring
in Box-COPs from prepolymers was also monitored through
TGA in N2 environment (Figure 2a). The mass loss observed
for prepolymers in the range of 150−500 °C was attributed to
the deprotection of hydroxyl functionality, i.e. ,removal of silyl
moiety and subsequent benzoxazole ring formation by
dehydration. TGA study revealed that Box-COPs show
unprecedented high thermal stability and almost no mass loss
up to 550 and 576 °C for COP-93 and COP-94, respectively,
under a N2 atmosphere. In an oxidative environment, COP-93
and COP-94 are stable up to 378 and 450 °C, respectively (see
Figure S4 in the Supporting Information).
Figure 1. (a) Synthesis of Box-COPs in three steps: (i) silylation of
monomers, (ii) prepolymer formation with protected hydroxyl group,
and (iii) cyclization at elevated temperature. (b) Hypothetical hexagon
macrocyclic unit for COP-93. (c) Annealed thin film strips of Box-
COPs: COP-93 and 94. (d) Pore size distribution of COP-93 (4.13 Å)
indicates interpenetration of macrocycles, hexagons, and pentagons, or
both.
dine in N,N-dimethylformamide in the presence of triethyl-
amine results in silylated monomers (protection step); (ii)
silylated monomers reacted with 1,3,5-benzenetricarbonyl
trichloride in N-methyl-2-pyrrolidone (NMP) produce prepol-
ymers COP-93-P and COP-94-P (prepolymer formation step);
and (iii) annealing of prepolymers at 400 °C to form Box-
COPs (porous polymer formation step).
Although Box-COPs are formed from rigid monomers, the
lack of order makes it very difficult to accurately predict their
superstructures. Nonetheless, we can attempt to understand
structural motifs based on the possible macrocyclic constructs
(see Figure S1 in the Supporting Information). On the basis of
the most stable bonding angles, there are two most plausible
configurations for macrocycles: hexagon and pentagon. In an
ordered Box-COP, e.g., COP-93, that is formed from simple
hexagons, the pore opening should have been 20 Å, based on
minimizations from both Materials Studio and Avogadro
software that use universal force fields (see Figure 1d and
Figure S1 in the Supporting Information). If pentagons are
formed (similar to the statistical possibility in bucky balls) the
pores have the widest opening of 15 Å. The experimental pore
size distribution of COP-93 is 4.36 Å (Figure 2), indicating a
The porous properties of Box-COPs were evaluated from the
Ar adsorption−desorption isotherms measured at 87 K (Figure
2b), showing typical type I reversible isotherms. The isotherms
displayed rapid Ar uptake at the very low relative pressure
(<0.01), indicative of the characteristics of the permanent
micropores. The specific surface area was determined by the
Brunauer−Emmett−Teller (BET) method at relative pressure
between 0.01−0.25 (see Figure S5 in the Supporting
Information). The BET surface areas of COP-93 and COP-
94 were found to be 606 and 80 m2 g−1, respectively (Table 1).
Figure 2. Thermal stability and porosity of Box-COPs. (a) Mass loss
characteristics with temperature for prepolymers and thermal stability
of Box-COPs, (b) Ar adsorption−desorption isotherms. Inset:
NLDFT pore size distribution.
B
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Chemistry of Materials
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Table 1. Surface Area, Pore Size, CO2 Capture Capacity, CO2/N2 Selectivity, and Isosteric Heat of Adsorption of Box-COPs
CO2 adsorption at 1 bar
d
(mg g−1
)
CO2/N2 selectivity
a
b
c
e
Box-COPs
BETAr (m2 g−1
)
BETCO (m2 g−1
)
pore size (Å)
273 K
298 K
91.1
323 K
273 K
298 K
323 K
Qst (kJ mol−1
)
2
COP-93
COP-94
606
80
336
4.36
20.3
139.6
94.1
60.7
40.5
65.3
49.8
36.4
33.9
35.4
48.0
25.6
26.2
229
59.1
a
b
c
d
BET from Ar adsorption isotherms. BET from CO2 adsorption isotherms 273 K, Median pore size from NLDFT. CO2/N2 selectivity was
e
measured by IAST method for CO2:N2 gas mixture of 15:85. Isosteric heat of adsorption (Qst) at initial loading.
The formation of interpenetrated polymeric network could be
the reason behind the low surface area for COP-94, where the
linker has a mandatory tilting because of its biphenylic
construct. An open hysteresis loop is formed by the irreversible
adsorption and desorption branches for COP-94, which is
attributed to pore network effects. The BET surface areas were
also measured from CO2 isotherms at 273 K, demonstrating
specific surface areas of 336 and 229 m2 g−1 for COP-93 and
COP-94, respectively. The Langmuir surface areas of COP-93
and COP-94 are 960 and 311 m2 g−1, respectively. Nonlocal
density functional theory (NLDFT) pore size distributions
using a slit pore model indicate a significant fraction of the
pores surface originates from micropores with the average pore
sizes of 4.36 and 20.3 Å for COP-93 and COP-94, respectively.
The total pore volume, estimated from the amount adsorbed at
relative pressure = 0.95, is 0.28 and 0.08 cm3 g−1 for COP-93
and COP-94. Although the microporosity in Box-COPs are
permanent and demonstrated similar surface area for repeated
synthesis, the polymeric network lacks ordered structure as
depicted in the XRD patterns (see Figure S6 in the Supporting
Information), indicating formation of amorphous polymers.
We investigated the CO2 and N2 sorption isotherms of Box-
COPs at 273, 298, and 323 K (Figure 3). CO2 sorption
isotherms show CO2 uptake is rapid in the initial stage,
implying that CO2 molecule has favorable interaction with the
Box-COPs network. The reversibility of adsorption−desorption
curves with almost no hysteresis suggest feasibility of these
structures for the practical application in CO2 capture and
regeneration. CO2 capture capacity of COP-93 and COP-94 at
1 bar is 139.6 and 94.1 mg g−1 at 273 K, 91.1 and 59.1 mg g−1
at 298 K, and 60.7 and 40.5 mg g−1 at 323 K, respectively. It
should be noted that the CO2 capacity of COP-93 persistently
increases with a rise in the pressure, and has not reached
saturation at 1 bar, indicating that higher CO2 sorption capacity
can be achieved at even higher pressures. The high CO2
uptakes are mainly attributed to the nanoporous nitrogen-
and oxygen-rich polymeric network of COPs. It has been
reported that the electron-rich networks can yield strong
dipole−quadrupole interaction with CO2, leading to a
significant increase in the CO2 adsorption capacity.48 The
leading CO2 adsorbents, especially nanoporous polymeric
networks with nitrogen and/or oxygen-rich frameworks,
include CPOP-1 (212 mg g−1),28 PPF-1 (267 mg g−1),43
BILP-4 (235 mg g−1),48 ALP-1 (236 mg g−1),49 PPN-6-
CH2DETA (189 mg g−1),50 and TB-COP-1 (228 mg g−1).51
High selectivity of CO2 over N2 is one of the necessary
characteristics for an adsorbent to be used for CO2 capture.
CO2 and N2 isotherms were fitted by Langmuir−Freundlich
dual sites or Langmuir single site models to obtain CO2/N2
selectivity through ideal adsorbed solution theory (IAST) for
the mixture of CO2 and N2 gas in the ratio of 15:85 (see Figure
S7 in the Supporting Information). Researchers have been
predicting and calculating IAST selectivity through various
Figure 3. CO2 adsorption−desorption isotherms for Box-COPs at (a)
273, (b) 298, and (c) 323 K. N2 adsorption−desorption isotherms at
(d) 273, (e) 298, and (f) 323 K.
̇
models. Islamog
lu et al.52 compared CO2/N2 selectivities
obtained from three methods and showed that values depend
on the method used. Here, we calculated CO2/N2 selectivities
of Box-COPs by considering partial pressures of CO2 and N2 in
a flue gas mixture as well as competition between gas molecules
for binding sites.52 The CO2/N2 selectivity of COP-93 and
COP-94 at 1 bar is 65.3 and 49.8 at 273 K, 36.4 and 33.9 at 298
K, and 35.4 and 48 at 323 K, respectively. The CO2/N2
selectivity decreased at 298 K; however, it enhanced at 323
K. The reason behind this behavior is that the CO2 binding
affinity toward electron-rich network did not deteriorate as
much as for N2 binding. CO2/N2 selectivity is also substantially
decreased with the rise in pressure, mainly because of a
pressure-driven pore-filling mechanism irrespective of CO2-
philic sites (Figure 4). CO2/N2 selectivity values are
comparable to other porous polymer networks and carbona-
ceous materials (see Table S2 in the Supporting Informa-
tion).43,48,52,53
The isosteric heat (Qst) of CO2 adsorption is a central
parameter that dictates the affinity of the porous sorbent
toward CO2, which in turn plays a major role in determining
the adsorption selectivity and the necessary energy to release
the CO2 during the regeneration step. Qst of CO2 adsorption
C
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Chemistry of Materials
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sized by reaction of tert-butyldimethylsilyl chloride with 4,6-
diaminoresorcinol dihydrochloride and 3,3′-dihydroxybenzidine in
N,N-dimethylformamide in the presence of triethylamine at 25 °C for
24 h. The silylated monomers were then thoroughly washed with
water and dried at 80 °C under a vacuum overnight. These
functionalized monomers reacted with 1,3,5-benzenetricarbonyl
trichloride in N-methyl-2-pyrrolidone (NMP) at 5−25 °C for 24 h,
which results in prepolymers. The prepolymers were separated from
NMP by evaporating 80% of solvents under vacuum in rotary
evaporator and remaining mixture was added to water, which
produced precipitates. These precipitates were washed with water
and dried. The prepolymers were heated to 400 °C at 2 °C min−1
under vacuum for 3 h to obtain Box-COPs.
Structural Characterization and Gas Sorption. Solid-state
cross-polarization magic angle spinning (CP/MAS) 13C NMR spectra
collected by a Bruker Avence III 400 WB NMR spectrometer. FT−IR
spectra were recorded on KBr pellets. Ar adsorption−desorption
isotherms were obtained with a Micromeritics 3Flex accelerated
surface area and porosimetry analyzer at 87 K, after the samples had
been degassed at 150 °C for 5 h under vacuum. CO2 and N2
adsorption−desorption isotherms for Box-COPs were measured at
273, 298, and 323 K using a static volumetric system. The temperature
during adsorption and desorption was kept constant using a
temperature controller. Prior to the adsorption measurements, the
samples were dried at 150 °C for 5 h. All the adsorption−desorption
experiments were carried out twice to ensure the reproducibility.
There were no noticeable differences in the isotherm points obtained
from both experiments.
Figure 4. IAST CO2/N2 selectivities for CO2:N2 gas mixture (15:85)
at (a) 273, (b) 298, and (c) 323 K. (d) Isosteric heat of CO2
adsorption for Box-COPs.
for Box-COPs is around 25.6−26.2 kJ mol−1 at initial loading
(Figure 4d), indicating CO2 adsorption−desorption cycles can
be achievable through pressure/vacuum swing sorption
techniques. We studied the effect of boiling water on the
stability of Box-COPs to mimic the harsh conditions sorbent
may endure, since the exhaust gas contains water vapor and the
water stability of CO2 sorbents is one of the key requirements
for practical CCS applications. The CO2 capture capacities of
Box-COPs after this treatment was also measured. Interestingly,
Box-COPs showed no loss in CO2 uptake efficiency after being
boiled for a week, suggesting their robust nature (see Figure S8
in the Supporting Information). For the effect of moisture in
CO2 uptake (as industrial flue gases always contain), we
anticipate that the capacities will certainly differ in humid
conditions, although it may or may not be as significant.
ASSOCIATED CONTENT
■
S
* Supporting Information
Synthesis, FT−IR, CP/MAS 13C NMR, elemental analysis,
TGA. IAST fitting curves, Qst, and boiling water stability. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: yavuz@kaist.ac.kr. Fax: +82 42 350 2248. Tel: +81 42
350 1718.
■
Notes
CONCLUSIONS
■
The authors declare no competing financial interest.
In summary, we reported the first set of processable, electron-
rich, and highly thermally stable Box-COPs with permanent
microporosity. Silylation of building blocks facilitate control
over network formation and processability, pointing the
method’s potential in other heterocyclic porous networks,
such as imidazoles, oxadiazoles, triazoles, tetrazines. The
synthesis also benefits from not needing to use rare earth
catalysts, a significant issue when scale up is considered. An
excellent CO2 capture capacity (139.6 mg g−1 at 273 K and 1
bar) with low Qst (25.6−26.2 kJ mol−1) was achieved in an
amorphous nanoporous polymeric network. Box-COPs showed
unprecedented thermal stability up to 576 °C and CO2 capture
capacity did not hinder after boiling in water for a week.
Because of facile adsorption−desorption characteristics, these
sorbents provide effective regeneration capability without the
use of addition energy and also show moderate CO2/N2
selectivities at elevated temperatures. Using our strategies
presented here, a series of benzoxazole polymeric networks can
easily be synthesized for physisorption- or membrane-based
CO2 capture technologies.
ACKNOWLEDGMENTS
■
We acknowledge the financial support by grants from Korea
CCS R&D Centre, Basic Science Research Program
(2013R1A1A1012998), and IWT (NRF-2012-C1AAA001-
M1A2A2026588), funded by National Research Foundation
of Korea (NRF) under the Ministry of Science, ICT & Future
Planning of Korean government.
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