High CO2 uptake and selectivity by triptycene-derived benzimidazole-linked polymers

Article abstract of DOI:10.1039/c2cc16986j

Successful incorporation of triptycene into benzimidazole-linked polymers leads to the highest CO₂ uptake (5.12 mmol g^-1, 273 K and 1 bar) by porous organic polymers and results in high CO₂/N ₂ (63) and CO₂

Full text of DOI:10.1039/c2cc16986j

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COMMUNICATION
High CO uptake and selectivity by triptycene-derived benzimidazole-linked
2
polymersw
Mohammad Gulam Rabbani, Thomas E. Reich, Refaie M. Kassab, Karl T. Jackson and
Hani M. El-Kaderi*
Received 10th November 2011, Accepted 29th November 2011
DOI: 10.1039/c2cc16986j
Successful incorporation of triptycene into benzimidazole-linked
À1
a result of integrating metal-free imidazole rings into the
backbone of this polymer. In addition to pore functionalization,
beneficial adsorptive properties can also be attained through the
use of rigid organic linkers as in the case of triptycene which
allows for a high degree of internal molecular free volume
polymers leads to the highest CO
2
uptake (5.12 mmol g , 273 K
and 1 bar) by porous organic polymers and results in high CO
2 2
/N
(
63) and CO /CH (8.4) selectivities.
2
4
11,12
(IMFV).
Thus, triptycene-based porous materials possess
The continuous and increasing release of carbon dioxide to the
atmosphere, especially from coal-fired plants, has initiated
considerable interest in the development of new materials
enhanced adsorptive gas uptake capacities as demonstrated for
1
3
triptycene-based polymers of intrinsic microporosity (PIMs),
14
15
shape-persistent case molecules, metal salphens, and MOFs.
16
2
and technologies for CO capture and separation due to its
1
drastic impact on the environment. Of similar concern is the
In this study we report on the synthesis of triptycene-derived
BILPs and their performance in small gas (H , CO , CH ) uptake
removal of CO2 from natural gas for efficient storage and
2
transportation of the latter. Among the emerging methods for
2
2
4
and selectivity. The synthesis of 2,3,6,7,14,15-hexaaminotriptycene
HATT) was accomplished by adopting a similar route reported
earlier for the 2,3,6,7,10,11-hexaaminotriphenylene analog
(
addressing these environmental and economical challenges is
the use of porous materials such as metal organic frameworks
1
7
3
4
(
Scheme S2, ESIw). The preparation of triptycene-derived
BILPs (Scheme 1) was conducted by using the condensation
(
MOFs) and nanoporous organic polymer networks as CO
2
adsorbents in which the precise control over the material’s
chemical composition and textural properties can lead to a
significant enhancement in gas storage and separation. In
9
method we reported for BILP-1. A homogeneous solution of
HATT (HCl salt) in DMF was treated with tetrakis(4-formyl-
phenyl)methane (TFPM) or terephthalaldehyde (TPAL) dissolved
in DMF to afford BILP-3 (77%) and BILP-6 (75%), respectively,
as insoluble yellow powders (ESIw). The slow addition of
aldehydes at low temperature under nitrogen is essential for
contrast to the current technology used in post-combustion
5
capture which employs amine solutions (i.e. MEA) that
CO
2
require energy input in the form of heating for their regeneration,
porous architectures bind CO2 less strongly, and therefore
materials regeneration can be accomplished more rapidly
under ambient conditions. In order to attain higher CO2
uptake in porous materials, pore functionalization has been
instrumental. For example, introducing amine functionality,
accessible nitrogen, or open metal sites into the pore wall
1
8
enhanced pore formation and the overall porosity whereas
higher temperature and exposure to molecular oxygen is
1
9
needed to facilitate imidazole ring formation. BILPs are
insoluble in common organic solvents and they remain intact
upon washing with a 2 M solution of HCl or NaOH which
reflects their high chemical stability. Thermogravimetric analysis
(TGA) showed initial weight loss up to around 100 1C that
corresponds to water removal followed by decomposition at
B400 1C (Fig. S1, ESIw) in a similar fashion of porous
6
of metal organic frameworks (MOFs), zeolitic imidazolate
7
8,9
frameworks (ZIFs), or microporous organic polymers can
considerably impact their gas uptake and selectivity. In particular,
2 4 2
the selective uptake of CO over CH or N is believed to arise
9
,20
polybenzimidazole polymers.
Scanning electron microscopy
from enhanced CO -framework interactions through hydrogen
2
1
0
bonding and/or dipole–quadrupole interactions.
We have recently reported the synthesis of BILP-1 and
À1
showed that it stores 4.3 mmol g of CO at 273 K/1 bar
2
2 2 2 4
and exhibits high CO /N (70) and CO /CH (10) selectivity as
Department of Chemistry, Virginia Commonwealth University,
Richmond, VA 23284-2006, USA. E-mail: helkaderi@vcu.edu;
Fax: +1 804 828 8599; Tel: +1 804 828 7505
w Electronic supplementary information (ESI) available: Experimental
procedures, characterization methods, and gas sorption studies. See
DOI: 10.1039/c2cc16986j
Scheme 1 Synthesis of BILP-3 and BILP-6.
Chem. Commun., 2012, 48, 1141–1143 1141
This journal is c The Royal Society of Chemistry 2012

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(
SEM) established phase purity of the polymer and revealed
respective isosteric enthalpies of adsorption (Qst) as summarized
aggregated particles B0.10–0.20 mm for BILP-3 and B0.6À0.8 mm
for BILP-6 (Fig. S2, ESIw). The material is amorphous as
evidenced by powder X-ray diffraction analysis (Fig. S3,
ESIw). The chemical connectivity and the formation of the
imidazole ring were confirmed by FT-IR and C solid-state
NMR spectroscopy. The FT-IR spectrum of BILPs (Fig. S4
in Table 1. The CO
2
isotherms are fully reversible and exhibit a
À1
steep rise at low pressures then reach 225 and 145 mg g at 273
and 298 K, respectively, for BILP-3 at 1 bar (Fig. 1B). A slightly
À1
lower uptake by BILP-6 (211 and 121 mg g ) was observed
1
3
under similar conditions. Interestingly, the uptake by BILP-3
À1
(5.12 mmol g ) which is about B19% higher than that of
BILP-1 at 273 K and 1 bar is the highest for porous organic
À1
and 5, ESIw) showed N–H stretching at around 3415 cm
(
À1
8
23
24
polymer networks; including PPN, COFs, functionalized
free N–H) and 3210 cm (hydrogen bonded N–H), while
À1
25
CMPs, functionalized POFs, and triptycene-based cage
26
intense new bands appeared at 1610 cm (CQN), and 1437
and 1492 cm which can be assigned to skeleton vibration of
À1
14
compound. Moreover, BILP-3 outperforms most of MOFs
3,6
9
,21 13
7
and ZIFs. However, its CO
the benzimidazole ring.
C CP-MAS NMR spectra for
2
uptake is still lower than those of
BILP-3 and BILP-6 (Fig. S6, ESIw) exhibit signals around 152
and 151 ppm, respectively, that correspond to NC(Ph)N in
the best performing MOFs (Mg-, Co-, Ni-MOF-74) which can
À1
27
6d
store up to 8.0 mmol g at 296 K/1 atm. We also calculated
2
2
benzimidazole units. These are consistent with the value
the Qst for CO using the virial method (Fig. S15, ESIw).
2
9
observed for BILP-1 (151 ppm) while additional signals in
À1
At zero coverage, the Q values 28.6 kJ mol (BILP-3) and
st
À1
the aromatic range arise from triptycene and tetraphenyl-
methane/phenyl units.
28.4 kJ mol (BILP-6) are higher than the values reported for
6
selective MOFs or
organic polymers, and comparable to CO
2
7
The permanent porosity of BILPs was investigated by argon
uptake measurements. The fully reversible isotherms depicted
in Fig. 1A show a rapid uptake at low pressure (0–0.1 bar)
indicating a permanent microporous nature. The gradual
increase in argon uptake and the minor hysteresis may be
due to the flexible nature of organic polymers. Applying the
Brunauer-Emmett-Teller (BET) model within the pressure
ZIFs which generally feature –NH
pores. The high CO uptake and binding may arise from
enhanced interactions of the polarizable CO molecules
2
or –OH functionalized
2
2
through hydrogen bonding and/or dipole–quadrupole inter-
actions that utilize the protonated and proton-free nitrogen
6
,10
sites of imidazole rings, respectively.
The readily reversible
sorption/desorption behavior and moderate Qst indicate that
2
À1
range of P/P = 0.05–0.15 resulted in SA
o
= 1306 m g
CO interactions with pore walls are weak enough to allow for
2
BET
2
BILP-3) and 1261 m g (BILP-6). These values are compa-
À1
(
material regeneration without applying heat.
2
À1
9
rable to those of BILP-1 (1172 m g ), triptycene-based PIMs
We have also considered BILPs in hydrogen and methane
storage studies because both gases are highly attractive candi-
dates for use in automotive applications as a result of their
abundance and clean aspect. The hydrogen uptakes for BILPs
(2.1–2.2 wt%) at 77 K and 1 bar (Fig. 1C) are higher than any
2
618–1760 m g
À1 13 À1 14
2
(
)
and cage compound (1375 m g ), and
4
,8
other amorphous nanoporous organic polymer networks. Pore
size distribution was estimated from argon isotherms by nonlocal
density functional theory (NLDFT) and found to be centered
4
,13,28
˚
˚
around 7.2 A (BILP-3) and 6.8 A (BILP-6), (Fig. S11, ESIw)
while pore volume was calculated from single point measure-
reported microporous organic polymers.
The Qst for
H
2
was calculated from adsorption data collected at 77 and
À1
ments (P/P
.66 cc/g (BILP-6).
To investigate the impact of microporosity and the amphoteric
o
= 0.95) and found to be 0.65 cc/g (BILP-3) and
87 K. At zero-coverage, the Qst is 8.0 kJ mol for BILP-3 and
À1
0
8.2 kJ mol for BILP-6 (Fig. S16, ESIw). In general, the Qst
values are higher than the values reported for porous organic
4
polymer networks and similar to those of functionalized
pore walls of BILPs on the uptake of small gases and selectively,
we collected CO , CH , and H isotherms and calculated their
À1 26
POFs (8.3 kJ mol ). Additionally, the H uptakes and Q
st
2
4
2
2
compete with the best known MOFs for hydrogen storage
2
9
4
under similar conditions. Similarly, we recorded CH uptakes
at 273 and 298 K up to 1 bar (Fig. 1D). Again, both isotherms
are completely reversible and exhibit maximum uptake of 24
À1
and 17 mg g at 273 and 298 K, respectively, for BILP-3 and
À1
2
7 and 19 mg g at 273 and 298 K, respectively, for BILP-6.
The Qst for CH
4
was calculated from adsorption data collected
À1
at 273 and 298 K and found to be 16.6 kJ mol (BILP-3) and
À1
1
3.2 kJ mol (BILP-6) at zero coverage (Fig. S17, ESIw). The
overall observation from porosity measurements and gas uptake
studies indicates that changing the aldehyde building units from
a linear (terephthalaldehyde) to a tetrahedral (tetrakis(4-formyl-
phenyl)methane) geometry does not appear to have a significant
impact on the surface area or gas sorption properties of BILP-3
and BILP-6. One possible reason is the formation of inter-
penetrated networks as in the case of COF-300 which increases
materials density, lowers the expected surface area, and leads to
3
0
the formation of ultrafine pores.
We have also considered CO /N and CO /CH selectivity
Fig. 1 Gas sorption measurements for BILPs; Ar (A), CO
C), CH (D). Adsorption (filled) and desorption (empty).
2 2
(B), H
2
2
2
4
(
4
studies to evaluate the potential of BILPs in gas separation
1
142 Chem. Commun., 2012, 48, 1141–1143
This journal is c The Royal Society of Chemistry 2012

View Article Online
Table 1 Gas uptake and selectivity for BILP-3 and BILP-6
b
b
b
b
c
H
2
at 1 bar
CO
2
at 1 bar
CH
4
at 1 bar
N
2
at 1 bar
Selectivity
a
Polymer SABET/SALang
77 K
87 K
Q
st 273 K 298 K
Q
st
273 K 298 K
Q
st
273 K 298 K CO
2
/N
2
CO
2
/CH
4
BILP-3
BILP-6
1306/1715
1261/1654
21
22
15
16
8.0 225
8.2 211
145
121
28.6 24
28.4 27
17
19
16.6 3.3
13.2 6.8
2.4
6.7
59 (31)
63 (39)
8.1 (5.4)
8.4 (5.3)
a
2
À1
b
Surface area (m g ) was calculated from Ar isotherm. Gas uptake in mg g and the isosteric enthalpies of adsorption (Qst) in kJ mol .
À1
À1
c
Selectivity (mol/mol) was calculated from initial slope calculations at 273 K and (298 K).
applications. The selectivity of BILPs toward CO
CH was investigated by collecting gas uptake isotherms at
73 and 298 K (Fig. S18, ESIw).
The initial steep rise in CO compared to N
illustrated in Fig. S19, ESIw is most likely due to the high
affinity of CO to the accessible nitrogen sites of imidazole
2
over N
2
and
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(
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2 4
and CH as
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2
moieties. At 273 K and 0.1 bar, which is a typical partial
À1
pressure of CO in flue gases, the CO uptake is 1.26 mmol g
2
2
À1
whereas that of N is only 0.031 mmol g for BILP-3 (Fig. S19,
2
10 B. Zheng, J. Bai, J. Duan, L. Wojtas and M. J. Zaworotko, J. Am.
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À1
ESIw); the corresponding values are 1.39 mmol g
and
1
1
À1
0
.038 mmol g
for BILP-6. Thus, based on initial slope
calculations in the pressure range of 0 to 0.1 bar, the estimated
adsorption selectivity for CO over N is 59 (BILP-3) and 63
BILP-6), Fig. S19, ESIw. This selectivity surpasses BPL
1
2
2
(
7
carbon and ZIFs, and are comparable to Bio-MOF-11
1
1
1
6a
31
81), noncovalent porous materials (NPMs) (74) and
(
9
BILP-1 (70). Recently, organic cage molecules featuring N–H
functionality have been reported to have CO /N selectivity in
2
2
3
2
the range of 36 to 138. However, such high selectivity is
associated with very modest gas uptakes that are very low
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2
1
was calculated using initial slope calculations and found to be
very similar for both polymers; 8.1–8.4 (273 K) and 5.4–5.3
(298 K), as listed in Table 1. Again, these values exceed
7
reported values for BPL carbon and ZIFs.
2
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In conclusion, we have demonstrated that triptycene-derived
BILPs exhibit high CO
their chemical and thermal stability which make them very
attractive in post-combustion CO capture studies.
2
uptake and selectivity in addition to
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2
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¨
¨
We are grateful to the U. S. Department of Energy, Office of
Basic Energy Sciences (DE-SC0002576) for generous support
of this project.
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This journal is c The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 1141–1143 1143