Conjugated microporous polytriphenylamine networks

Article abstract of DOI:10.1039/c4cc03026e

Conjugated microporous polytriphenylamine networks with surface areas of 530 m² g^-1 were synthesized via Buchwald-Hartwig coupling, resulting in high CO₂ uptake (up to 6.5 wt%) and CO ₂-N₂ selectivity

Full text of DOI:10.1039/c4cc03026e

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Conjugated microporous polytriphenylamine
networks†
Yaozu Liao,a Jens Weberb and Charl F. J. Faul*a
Cite this: Chem. Commun., 2014,
50, 8002
Received 23rd April 2014,
Accepted 4th June 2014
DOI: 10.1039/c4cc03026e
www.rsc.org/chemcomm
Conjugated microporous polytriphenylamine networks with surface
areas of 530 m2 gꢀ1 were synthesized via Buchwald–Hartwig coupling,
resulting in high CO2 uptake (up to 6.5 wt%) and CO2–N2 selectivity
(75) at 1 bar and 303 K.
Conjugated microporous polymers (CMPs) with intrinsic properties
including small pore sizes (o2 nm), large specific surface areas
(46000 m2 gꢀ1),1 high chemical stability, low skeleton density and
reversible redox properties exhibit great promise for gas storage,
separation, sensing, catalysis and battery applications.2 Versatile
CMPs have been obtained readily through template-free chemical
processes by careful selection of building blocks and, commonly,
Scheme 1 Synthetic route to conjugated microporous PTPAs.
suitable C–C or C–N coupling reactions, which showed efficient 530 m2 gꢀ1 using BH coupling (Scheme 1). Our target was to
preparation and high flexibility in the molecular design. Typically, design and synthesise intrinsically microporous materials
Suzuki,3 Sonogashira–Hagihara4 and Yamamoto coupling,5 Schiff- based on conjugated polymer networks from a triphenylamine
base chemistry,6 cyclotrimerization7 and oxidative polymerization8 bromide core and selected aryl amine linkers. This approach,
reactions have been used to synthesize CMPs. Previous studies exploiting careful choice of strut lengths, rigidities of the linkers
demonstrated that control over average micropore size, surface and additional included functionalities, would allow formation of
area, and gas uptake of the CMPs was enabled by alternating the nitrogen-containing CMPs with fine-tuned porosities and CO2
strut lengths, rigidities, and functionalities of the building blocks.2 adsorption/separation capabilities.
Buchwald–Hartwig (BH) coupling is utilized in organic synthesis for
Verification of this approach was achieved by BH cross-
the generation of C–N bonds via the palladium-catalysed cross- coupling of aniline with tris(4-bromophenyl)amine. This resulted
coupling of amines with aryl halides. The BH coupling approach in C–N bond formation, affording the fully reduced molecular
allows for expansion of the repertoire of possible C–N bond formation analogue of the PTPAs, 4,40,400-tris(phenylamino)triphenylamine
through the facile synthesis of aromatic amines. This useful method (analogous to the leucoemeraldine base structure found for OANIs,
also provides a simple route to nitrogen-containing redox-active see ESI,† Scheme S1). Once verified, BH coupling of diamine
systems, as shown in recent work9from our laboratories on the synthesis species afforded black, blue, brown, and yellow insoluble PTPA-1,
of well-defined symmetrical oligo(aniline)s, OANIs. It is noteworthy 2, 3, 4 powders (Fig. S1, ESI†) with yields of 65–80%. The chemical
that BH coupling has rarely been used for CMP preparation.10
structures of the PTPAs were confirmed by Fourier transform
Here we report the synthesis of conjugated microporous infrared (FT-IR) and solid-state 13C cross-polarization magic angle
polytriphenylamine (PTPA) networks with a surface area up to spinning nuclear magnetic resonance (CP/MAS NMR) investiga-
tions. Bands of the primary amine group of the linkers at 3470 and
a School of Chemistry, University of Bristol, Bristol, England BS8 1TS, UK.
3420 (–NH2 stretching) and 1650 cmꢀ1 (–NH2 deformation) as well
as the aromatic C–Br groups of the tris(4-bromophenylamine)
at 1178 cmꢀ1 (aromatic C–Br stretching) are absent or strongly
attenuated in the spectra of the PTPAs (Fig. S2, ESI†). The distinct
quinoid (Q) and benzenoid (B) bands at 1598 and 1498 cmꢀ1 as
well as the aryl C–H band at 820 cmꢀ1 are present in the spectra of
E-mail: charl.faul@bristol.ac.uk
b
¨
Hochschule Zittau/Gorlitz (University of Applied Science), Department of Chemistry,
¨
Theodor-Korner-Allee 16, D-02763 Zittau, Germany
† Electronic supplementary information (ESI) available: Studies on the synthesis,
characterization, morphology, and gas adsorption–desorption of the PTPAs. See
DOI: 10.1039/c4cc03026e
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Fig. 1 Solid-state UV/Vis-NIR spectra of PTPAs.
Fig. 3 (left) N2 adsorption–desorption isotherms (77.4 K) and (right) CO2
adsorption–desorption isotherm at 273 K.
the resulting materials. Solid-state 13C CP/MAS NMR spectra of
all polymers showed three main resonances at B141, B127 and
B118 ppm, originating from the aryl carbons of both the starting
materials (Fig. S3, ESI†). Three additional resonances at 17, 116 and
35 ppm are attributed to the methyl, alkenyl and methylene groups
of PTPA-2, 3 and 4, respectively. The ultra-violet/visible near infra-
red (UV/Vis-NIR) spectra of fully conjugated PTPA-1, PTPA-2, and
PTPA-3 exhibited two peaks at 380 and 600 nm, typically attributed
to the p–p* transition of benzenoid and quinoid rings (Fig. 1),
respectively, as confirmed by recent calculations.9 Specifically,
PTPA-3 displayed a broad peak around 1100 nm, attributed to
the electron and hole migration found in trans-stilbene.11
These results confirm the creation of extended conjugated
PTPA networks with amine linkages.
The excellent thermal stability of PTPAs is ascribed to the
nature of their cross-linked networks.
The porosity parameters of the polymers were initially studied
by gas adsorption analysis using nitrogen as the adsorbate.
Nitrogen adsorption–desorption isotherms of PTPAs measured
at 77.4 K are shown in Fig. 3. All materials showed some outer
surface area, most likely originating from interparticulate porosity
associated with the meso- and macrostructures of the samples
and interparticular voids. This was evidenced by the steep
increase of N2 uptake at relative pressure above 0.9 observed
for all materials. It is plausible to expect that precipitation of
the growing polymer networks is responsible for the formation
of observed macro/mesoporosity.
As expected, PTPAs displayed amorphous structures, as deter-
mined by powder X-ray diffraction (XRD) measurements (broad
peaks at 2y = B12.21, Fig. S4, ESI†). No p-stacking was evident
from these investigations. Scanning electron microscope (SEM)
images showed the morphologies of PTPAs consisting of aggre-
gated nanoparticles with diameters of 200–500 nm (Fig. 2). This
leads to some outer surface area (large meso- and macropores due
to interstitial voids), which is highest for PTPA-2 and PTPA-3 as
evidenced by cryogenic gas adsorption (see below). The transmis-
sion electron microscope (TEM) images indicate the microporous
structures of the PTPA nanoparticles (see Fig. S5, ESI†), which are
typically found in amorphous CMPs. The materials are thermally
stable in N2 up to 500 1C, as revealed by thermal gravimetric
analysis (TGA) (see Fig. S6, ESI†). In particular, for PTPA-3
and PTPA-4, B60 wt% remained after heating to 1000 1C.
PTPA-4 does not show any other signs of porosity and con-
sequently shows the lowest N2 uptake. In contrast, PTPA-1 does
show some broadly distributed mesoporosity in conjunction
with macroporosity, as derived from the hysteresis between the
adsorption and desorption curves, which close around p/p0 B 0.45.
PTPA-2 and PTPA-3 show a low-pressure hysteresis (the isotherm
curves do not close even at very low p/p0), which is often observed for
microporous polymers.12 The hysteresis could result from restricted
access of the adsorbate to the some of the micropores of the
material, especially to those that are blocked by narrow openings.
It has been suggested that additional methods such as CO2 adsorp-
tion should be used for micropore analysis (see below).13 PTPA-3
shows the highest apparent Brunauer–Emmett–Teller (BET)
surface areas (adsorption branch: 450 m2
g
ꢀ1; desorption
branch: 530 m2 ꢀ1, see Table 1). Further studies analysing
g
the gas separation potential of this material in more detail is
currently under way. PTPA-3 seems to also feature some ill-defined
mesoporosity, indicated by the step of the desorption isotherm at
the characteristic p/p0 of 0.45 (emptying of ink-bottle mesopores by
cavitation). PTPA-2, in contrast, does not show signs of meso-
porosity based on N2 analysis. The specific pore volumes and
surface areas of the PTPA materials are moderate in comparison to
other CMPs. However, the combination of micro-, meso- and
macroporosity (especially for the PTPA-3 with a determined pore
volume of 0.36 cm3 gꢀ1) is of interest for applications that require
good mass transfer through so-called transport pores.
CO2 adsorption was measured for all PTPA materials at 273 K
(see Fig. 3) to gain further information on the microporosity.14
All materials showed significant CO2 uptake. The highest uptake
was found for PTPA-3 (6.5 wt%), followed by PTPA-2 (5.5 wt%),
Fig. 2 SEM images of prepared PTPAs.
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Table 1 Porosity parameters and CO2 separation properties of the microporous PTPAs
CO2 uptake at 1.0 bar (wt%)
SBET (m2 gꢀ1
)
PVc (cm3 gꢀ1
)
MPVd (cm3 gꢀ1
)
m-pore SGCMC (m2 gꢀ1
)
MPVGCMC (cm3 gꢀ1
)
273 K
303 K
a
e
e
PTPA
1
2
3
4
52
0.11
0.06
0.36
0.03
n.a.
0.05
0.18
n.a.
195
292
356
130
0.07
0.11
0.13
0.05
3.7
5.5
6.5
2.2
n.d.
2.6
3.4
62/120b
450/530b
22
n.d.
a
b
Surface area calculated from nitrogen adsorption isotherms using the BET equation. Surface area calculated from the desorption branch of the
c
d
N2 isotherms using the BET equation. Pore volume calculated from nitrogen adsorption at p/p0 = 0.95. Micropore volume calculated from
QSDFT (slit pore model, desorption branch) analysis of N2 data. Micropore surface area determined from GCMC analysis of the CO2 adsorption
e
data at 273 K. n.a. not applicable. n.d. not determined.
and PTPA-1 and PTPA-4 (uptake o4 wt%). The isotherms such as cryogenic spray-freezing and food preservation that
showed only weak hysteresis and were analysed using the com- would benefit from CO2 capture. CO2 adsorption on PTPA-3 at
mercialized Grand-canonical Monte-Carlo (GCMC), method- 30 1C (303 K) was 0.765 mmol gꢀ1 (3.4 wt% at 1.0 bar), lower
ology to extract porosity information (see Table 1).15 All materials compared with some standard materials (zeolites, activated
show similar broad pore size distributions, which is in line with carbon). The N2 adsorption on PTPA-3 at 30 1C was found to
their amorphous nature. The main fraction of pores have sizes be 0.022 mmol gꢀ1 at 1.0 bar, resulting in an equilibrium
between 0.5 and 1 nm (Fig. S7, ESI†).
selectivity of a(CO2/N2)eq B 35. Further analysis was performed
Generally, PTPAs prepared from monomers containing longer by calculating the IAST selectivity at 30 1C based on the assump-
and more rigid struts should show higher apparent specific tion of a gas composition of 15% (v/v) CO2 balanced with N2. The
surface areas if no interdigitating structures are observed. necessary description of the isotherm was achieved by fitting the
Shorter linkers might lead to compact structures of low porosity. experimental data points by either a simple Langmuir (N2) or a
However, the monomer geometry alone is not decisive in deter- dual-site Langmuir fit (CO2). It should however be noted that the
mining final porosity, as the reaction conditions, choice of CO2 data could also be described very well by a simple Langmuir
solvents and solubility of the forming polymeric network phase fit (see Fig. S8, ESI†). The IAST selectivity of CO2 over N2 is
all contribute to the physical properties of the final CMPs. It is calculated to be 75 at ambient pressure (1.0 bar, 30 1C). This
nevertheless of worth to discuss the observed porosities with value compares very favourably with most reported results for
regard to the monomer geometry and rigidity, allowing compar- microporous organic polymers, usually with selectivities in the
ison and conclusions to be drawn for the present systems. The range 20 to 50.17 The predicted adsorption isotherms from the
use of the stilbene monomer (L3), which is significantly more gas mixture are shown in Fig. 4.
rigid compared with its ethylene-bridged counterpart L4, results
The fact that the CO2 adsorption isotherm could also be
in the formation of significant microporosity. Interestingly, the represented by a simple Langmuir approach indicates that the
very rigid (but shorter) p-phenylenediamine (L1) yields a moderately surface area of PTPA-3 is very homogenous. This is indeed
microporous material. The low microporosity could result from the supported by first estimation of the isosteric heat of adsorption
formation of a more dense structure (PTPA-1), i.e. the conjugated qst based on the CO2 isotherms measured at 273 and 303 K. The qst
segments are less separated and might adhere to each other, remained almost constant over the whole coverage range (values
favouring pore closure. The methyl-decorated biphenylene-based between 27 and 28 kJ molꢀ1) indicating a very stable operation
linker L2 yields a network of considerable microporosity. The regime. The qst values are somewhat higher than values commonly
strategy to enhance the free volume of, for example, membrane found for activated carbons or other polymers, but low enough for
polymers using such methyl-decorated monomers is well known,16 adsorbent regeneration without too high an energetic penalty.
and is effective here as well. In the investigated systems monomer
molecular structures do impact on the observed porosity, thus
following commonly known trends. It also shows that our chosen
platform is modular and that porosities could be tuned to reach
higher values through further exploration and optimization of the
reaction conditions.
The presence of fairly small pores with a uniform distribution
of polar amine units is expected to result in favourable interactions
between specific adsorbate molecules and the polymeric adsor-
bent. We analysed the potential of PTPA-3 for gas separation,
exemplified by the CO2–N2 gas pair. The separation of CO2
from, for example, flue gas (post-combustion) is of current
interest with regard to the capture of CO2 for environmental
Fig. 4 CO2 and N2 adsorption–desorption isotherms (single gas) for PTPA-3
(greenhouse gas) or economic reasons (CO2 as alternative feed-
stock for new polymers). There are various other applications, 0.15/0.85 v/v gas mixture of CO2 and N2 at 303 K (based on IAST methodology).
at 303 K (symbol + line) with predicted adsorption isotherms (line only) from a
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4 (a) J. Jiang, F. Su, A. Trewin, C. D. Wood, N. L. Campbell, H. Niu,
C. Dickinson, A. Y. Ganin, M. J. Rosseinsky, Y. Z. Khimyak and
A. I. Cooper, Angew. Chem., Int. Ed., 2007, 46, 8574; (b) J.-X. Jiang,
F. Su, A. Trewin, C. D. Wood, H. Niu, J. T. A. Jones, Y. Z. Khimyak
and A. I. Cooper, J. Am. Chem. Soc., 2008, 130, 7710; (c) R. Dawson,
A. Laybourn, R. Clowes, Y. Z. Khimyak, D. J. Adams and A. I. Cooper,
Macromolecules, 2009, 42, 8809; (d) Q. Chen, M. Luo, T. Wang,
J.-X. Wang, D. Zhou, Y. Han, C.-S. Zhang, C.-G. Yan and B.-H.
Han, Macromolecules, 2011, 44, 5573.
In summary, we have successfully synthesized a series of
conjugated microporous polytriphenylamine (PTPAs) based on a
triphenylamine core motif using Buchwald–Hartwig (BH) cross-
coupling. The BET surface area of these conjugated microporous
polymers is up to 530 m2 gꢀ1. Owing to the microporosity and the
homogenously electron-rich and polar pore surface, they exhibit
high CO2 uptakes of up to 6.5 wt% and good CO2–N2 adsorption
ideal selectivities of up to 75 at 303 K and 1.0 bar. The overall
porosity and the corresponding gas adsorption properties strongly
depend on the building block size, molecular architectures and
functionalities. BH coupling presents a promising approach for
large-scale manufacturing of conjugated microporous polymers
and provides a high conversion of functional monomers used. Post-
synthesis treatment of the tertiary and secondary amines
resulting in permanent quarterisation and careful choice of
corresponding counterions may enable tuning of gas uptake and
selectivity. Continued investigation into the design, synthesis and
characterisation of related materials is on-going in our laboratories.
We are grateful to the European Commission Marie Curie
International Incoming Fellowship (FP7-PEOPLE-2012-IIF
TANOGAPPs No. 326385) for generous support of this project.
5 (a) J. Schmidt, M. Werner and A. Thomas, Macromolecules, 2009,
42, 4426; (b) K. Zhang, B. Tieke, J. C. Forgie and P. J. Skabara,
Macromol. Rapid Commun., 2009, 30, 1834.
6 (a) M. G. Schw, B. Fassbender, H. W. Spiess, A. Thomas, X. L. Feng
and K. Mu¨llen, J. Am. Chem. Soc., 2009, 131, 7216; (b) G. Y. Li,
B. Zhang, J. Yan and Z. G. Wang, Chem. Commun., 2014, 50, 1897.
7 (a) R. Palkovits, M. Antonietti, P. Kuhn, A. Thomas and F. Schu¨th,
Angew. Chem., 2009, 121, 7042; (b) S. J. Ren, M. J. Bojdys, R. Dawson,
A. Laybourn, Y. Z. Khimyak, D. J. Adams and A. I. Cooper, Adv.
Mater., 2012, 24, 2357; (c) X. Zhu, C. Tian, S. M. Mahurin, S. H. Chai,
C. M. Wang, S. Brown, G. M. Veith, H. M. Luo, H. L. Liu and S. Dai,
J. Am. Chem. Soc., 2012, 134, 10478.
8 (a) J. Schmidt, J. Weber, J. D. Epping, M. Antonietti and A. Thomas,
Adv. Mater., 2009, 21, 702; (b) Y. L. Luo, B. Y. Li, W. Wang, K. B. Wu
and B. E. Tan, Adv. Mater., 2012, 24, 5703.
9 Z. C. Shao, P. Rannou, S. Sadki, N. Fey, D. M. Lindsay and C. F. J. Faul,
Chem. – Eur. J., 2011, 17, 12512.
´
10 J. Germain, F. Svec and J. M. J. Frechet, Chem. Mater., 2008, 20, 7069.
´
11 A. Moissett, R. F. Lobo, H. Vezin, K. A. Al-Majnouni and C. Bremard,
J. Phys. Chem. C, 2010, 114, 10280.
12 P. Kuhn, A. Forget, D. S. Su, A. Thomas and M. Antonietti, J. Am.
Chem. Soc., 2008, 130, 13333.
13 J. Jeromenok and J. Weber, Langmuir, 2013, 29, 12982.
Notes and references
1 D. Q. Yuan, W. G. Lu, D. Zhao and H.-C. Zhou, Adv. Mater., 2011,
23, 3723.
2 (a) J.-X. Jiang, A. Trewin, F. B. Su, C. D. Wood, H. J. Niu, J. T. A. Jones,
Y. Z. Khimyak and A. I. Cooper, Macromolecules, 2009, 42, 2658;
´
´
14 (a) D. Lozano-Castello, D. Cazorla-Amoros and A. Linares-Solano,
Carbon, 2004, 42, 1233; (b) N. Ritter, I. Senkovska, S. Kaskel and
J. Weber, Macromolecules, 2011, 44, 2025.
(b) F. Vilela, K. Zhang and M. Antonietti, Energy Environ. Sci., 2012, 15 P. I. Ravikovitch, A. Vishnyakov, R. Russo and A. V. Neimark,
5, 7819; (c) Y. Xu, S. Jin, H. Xu, A. Nagai and D. Jiang, Chem. Soc.
Rev., 2013, 42, 8012.
3 (a) J. Weber and A. Thomas, J. Am. Chem. Soc., 2008, 130, 6334;
Langmuir, 2000, 16, 2311.
16 K. Tanaka, M. Okano, H. Toshino, H. Kita and K.-I. Okamoto,
J. Polym. Sci., Part B: Polym. Phys., 1992, 30, 907.
¨
(b) L. Chen, Y. Honsho, S. Seki and D. Jiang, J. Am. Chem. Soc., 2010, 17 (a) R. Dawson, E. Stockel, J. R. Holst, D. J. Adams and A. I. Cooper,
132, 6742; (c) D. B. Xiao, Y. Li, L. L. Liu, B. Wen, Z. J. Gu, C. Zhang
and Y. S. Zhao, Chem. Commun., 2012, 48, 9519; (d) Y. H. Xu,
A. Nagai and D. L. Jiang, Chem. Commun., 2013, 49, 1591.
Energy Environ. Sci., 2011, 4, 4239; (b) Q. Chen, M. Luo, P. Hammershøj,
D. Zhou, Y. Han, B. W. Laursen, C.-G. Yan and B.-H. Han, J. Am. Chem.
Soc., 2012, 134, 6084.
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