New nitrogen-rich azo-bridged porphyrin-conjugated microporous networks for high performance of gas capture and storage

Article abstract of DOI:10.1039/c6ra04077b

A series of new conjugated microporous polymers (Azo-1, Azo-2 and Azo-3) based on a nitrogen-rich porphyrin building unit and an azo bond linkage were synthesized by KOH assisted condensation. These materials were characterized by Fourier transform infrar

Full text of DOI:10.1039/c6ra04077b

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New nitrogen-rich azo-bridged porphyrin-
conjugated microporous networks for high
Cite this: RSC Adv., 2016, 6, 30048
performance of gas capture and storage†
a
b
b
c
c
Yanfei Xu,ad Zhi Li, Fan Zhang, Xiaodong Zhuang, Zheng Zeng and Jianjun Wei
*
*
*
A series of new conjugated microporous polymers (Azo-1, Azo-2 and Azo-3) based on a nitrogen-rich
porphyrin building unit and an azo bond linkage were synthesized by KOH assisted condensation. These
materials were characterized by Fourier transform infrared spectroscopy (FT-IR), solid-state 13C NMR,
XPS, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (TEM), X-ray
diffraction (XRD), thermogravimetric analysis (TGA), and tested for gas (N2, CO2 and H2) adsorption. It
was revealed that the azos presented the formation of porous polymer networks affording amorphous
particles with rough surfaces and irregular morphology with excellent thermal stability under nitrogen
conditions. The Brunauer–Emmett–Teller (BET) model of the N2 adsorption gave apparent surface area
ranges of 520–675 m2 ꢀ1. The results from non-local density functional theory (NL-DFT) calculations
g
suggested a pore size distribution between 1.6 and 4.0 nm. The gas (CO2, H2) adsorption isotherms
demonstrated outstanding CO2 uptake up to 17.5 wt% (3.98 mmol gꢀ1 for Azo-2) and moderate H2
storage. The isosteric heats of adsorption (Qst) are high, with values of 36–37 kJ molꢀ1 for the azo
polymers. Moreover, the azo-polymer networks exhibited excellent selectivity with CO2/N2 up to 64.3
for Azo-2 at 273 K/1 bar. It was suggested that the nitrogen-rich active sites of the polymers play an
important role for CO2 capture and storage.
Received 15th February 2016
Accepted 15th March 2016
DOI: 10.1039/c6ra04077b
www.rsc.org/advances
In order to enhance the adsorption interactions and selec-
tivity of CO2 capture, nitrogen has been doped into many
Introduction
porous materials, such as carbon-based materials,15–19 micro-
porous polymer networks.20 Great efforts have been taken to
introduce functional amines into the carbon networks via
organic graing or direct heat treatment, however, these tech-
nologies by post-introduction of amines suffer from some
drawbacks of time consuming, high-temperature need, and/or
high cost. Recently, N-rich porous polymer networks demon-
strate excellence in CO2 caption and conversion. Coskun
group21 reported a nanoporous polymers incorporating N-
heterocyclic carbenes for CO2 capture and conversion. The
resulting polymer networks possess a Brunauer–Emmett–Teller
(BET) surface area of 475 m2 gꢀ1 and CO2 uptake of ꢁ9.55 wt%
(2.1 mmol gꢀ1) at 298 K/1 bar.
Porous polymers1–3 generally possess permanent porosity, low
mass densities, synthetic diversication, and high physico-
chemical stability, which make them highly competitive in gas
capture, storage and separation applications.4–6 Especially for
H2 storage7–9 and CO2 storage and separation,10–12 porous poly-
mers are considered as promising materials. Increasing CO2
concentration in the atmosphere mainly caused by the rapid
consumption of fossil fuels13 has partly led to the global climate
change and some environmental issues,14 which have drawn
great attentions and concerns. Consequently, CO2 capture and
storage is of great interest to meet the energy and environ-
mental demands.
Conjugated micro- and mesoporous polymers (CMPs) that
permit the linking of building blocks in a p-conjugated fashion
are a class of porous materials with 3D or 2D networks and high
surface areas,22 which have attracted much attention. CMPs is
an interesting platform for the molecular design of porous
skeletons because they allow to manipulate their networks and
to tune the porous parameters. One of the remarkable charac-
teristics of CMPs is their crosslinked 3D molecular skeletons.23
These structural features provide capability of mass transport of
the gases into the strictly microporous insides.
aInstitute of Nuclear Technology Application, School of Science, East China University
of Science and Technology, Shanghai 200237, China. E-mail: lizhi@ecust.edu.cn
bInstitute of Advanced Organic Materials, School of Chemistry and Chemical
Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. E-mail:
zhuang@sjtu.edu.cn
cDepartment of Nanoscience, Joint School of Nanoscience and Nanoengineering,
University of North Carolina at Greensboro, USA, 336-285-2859. E-mail: j_wei@
uncg.edu
dShaoxing University, Shaoxing 312099, Zhengjiang Province, China
† Electronic supplementary information (ESI) available: Additional experimental
data for materials. See DOI: 10.1039/c6ra04077b
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Up to date, one of the most characteristic features of CMPs is from Sinopharm Chemical Reagent Co. Ltd. p-Nitro-
the rather broad diversity of p-units.22 Molecular building blocks, benzaldehyde (99%) was obtained from Adamas. p-Phenyl-
ranging from simple phenyl units to extended arenes, heterocy- enediamine (99%) was purchased from Sigma-Aldrich.
clic aromatic units and large macrocycles, have been successfully Melamine (99%) and N,N-dimethylformamide (DMF) (99.8%)
exploited for the synthesis of porous polymers. Recently, were obtained from J&K Chemical. Pyrrole (99%) and benzidine
porphyrin and its derivatives have become one of the major (98%) were purchased from Energy Chemical. Pyrrole was
building blocks24–26 for porous polymer because they possess freshly distilled before used, and the other chemicals were used
large planar p-conjugated structure, peculiar electronic proper- as received.
ties, good photochemical effect, thermal stability,27,28 and so on.
Liu, et al.29 reported a class of metal functional microporous
Synthetic procedures
covalent triazine network prepared using a metalloporphyrin for
Synthesis of 5,10,15,20-tetrakis(4-nitrophenyl)-21H,23H-
enhanced CO2 uptake. It presented a very high BET specic area
porphine (TNPP). TNNP synthesis was followed the experi-
up to ꢁ1500 m2 gꢀ1, and excellent CO2 uptake capability of 3.17
mental procedure in literature.34 p-Nitrobenzaldehyde (22.0 g,
mmol gꢀ1, (13.9 wt%). More recently, a porphyrin polymer with
145 mmol) and acetic anhydride (24.0 mL, 254 mmol) were
benzimidazole linkages30 was synthesized to form a rigid amor-
dissolved in propionic acid (600 mL). The solution was then
phous network that exhibited selective CO2 adsorption with
reuxed, to which freshly distilled pyrrole (10.0 mL, 144 mmol)
capability of 12.1 wt% (2.76 mmol gꢀ1) and a CO2/N2 selectivity of
was slowly added. Aer reuxing for 30 min, the resulting
72 at 273 K/1 bar operation, evident the nanoporous porphyrin
mixture was cooled to the room temperature. Then the precip-
networks for gas capture and storage.
itate was collected by ltration, washed with distilled water and
Furthermore, some azo-bridged porous polymers have been
methanol, and dried under vacuum. The resulting powder was
developed for CO2 capturing properties. Azo-linked porous
dissolved in pyridine (160 mL) which was reuxed for 1 h. Aer
organic framework (a 2D layered Azo-POF-2) exhibited high
cooling, the precipitate was collected by ltration and washed
CO2/N2 selectivity of 76 at 273 K.31 Arab et al. reported32 a series
with acetone to give 4.5 g TNPP as a purple crystal in 15.6%
of new azo-linked polymers (ALPs), with surface area of 412–801
yield. Elem. anal. calcd for C44H26N8O8 (%): C, 66.50; N, 16.11.
m2 gꢀ1, demonstrated CO2 uptake capacities of up to 2.94 mmol
Found: C, 65.62; N, 13.38. HRMS (EI-MS): C44H26N8O8 for [M +
gꢀ1 at 298 K/1 bar and good CO2/N2 selectivity (56). Another
H]+, calculated 795.1874; found 795.1947.
class of ALPs, synthesized by homocoupling of aniline-like
Synthesis of Azo-1. TNPP (0.3176 g, 0.4 mmol), p-phenyl-
building units in the presence of copper(I) bromide and pyri-
enediamine (0.0865 g, 0.8 mmol) and KOH (0.22 g, 3.94 mmol)
dine, possessing higher surface area of 862–1235 m2 gꢀ1, pre-
were dissolved in DMF (25 mL). The reaction mixture was
sented higher CO2 uptake capacities of up to 5.37 mmol gꢀ1 at
ꢂ
heated to 150 C and stirred for 24 h under a nitrogen atmo-
237 K/1 bar.33 The investigation shows that the porosity
sphere. The reaction mixture was cooled down to room
parameters (surface area, pore volume and pore size) play key
temperature, added with 150 mL of distilled water and stirred
roles in CO2 uptake capacity and CO2 selectivity.
for 1 h. Black precipitate was ltered off and washed several
In this paper, combining the advantages of CMPs with the
times with distilled water, acetone and THF in order to remove
porphyrin units, a series of unprecedented, electron-rich
any unreacted monomers and KOH. Subsequently, black
conjugated microporous polymers (Azo-1, Azo-2, and Azo-3)
precipitates were dried at 60 ꢂC under vacuum overnight to yield
based on nitrogen-rich porphyrin building units and azo bond
Azo-1: 0.26 g. Yield was 64% with elemental analysis of C: 72.08;
linkage was synthesized by KOH assisted condensation. The
H: 4.50; N: 14.92 (%).
azo-polymer networks were characterized by solid-state NMR
Synthesis of Azo-2 and Azo-3. The procedure is similar to the
spectroscopy, infrared spectroscopy, and elemental analysis to
synthetic route of Azo-1. The details of the procedure and
obtain their structural and chemical composition. Thermogra-
product data are provided in ESI.†
vimetric analysis showed that all azos networks exhibit a high
thermal stability under N2. Despite their moderate specic BET
Characterization methods and testing of gas sorption
equivalent surface areas (520–675 m2 gꢀ1), they featured
remarkable CO2 uptakes up to 17.52 wt% (3.98 mmol gꢀ1) and
Characterization methods. Fourier transform infrared spec-
excellent selectivity (CO2/N2 ratio 15/1) at 273 K/1 bar, and troscopy (FT-IR) was performed on a Spectrum 100 (Perkin
moderate capability for H2 storage (1.15 wt%, 5.75 mmol gꢀ1) at Elmer, Inc., USA) spectrometer with a scan range of 4000–400
77 K/1 bar. High CO2 adsorption is expected to occur because of cmꢀ1. The sample powders were pulverized with KBr, and
the Lewis acid–base interactions between the active electron- pressed into disks. Solid-state 13C cross polarization/magic
rich aromatic constituents and the electron-poor CO2 angle spinning nuclear magnetic resonance (CP/MAS NMR)
molecules.
analysis was conducted on a Bruker AVANCE III 300 Spec-
trometer. Meanwhile, the cross polarization/magic angle spin-
ning nuclear magnetic resonance (CP/MAS NMR) experiments
were achieved by incorporating the total side-band suppression
(TOSS) pulse. Samples were spun at 5 kHz in 4 mm rotor within
Experimental
Materials
Pyridine (99.5%), acetic anhydride (98.5%), potassium a MAS probe. An acquisition time of 20 ms, a contact time of 1
hydroxide (90%), and propionic acid (99%) were purchased ms and a 6.5 ms pre-scan delay were used. The recycle time was 2
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s to obtain fully relaxed spectra. Chemical shis were externally
referenced to adamantane at 38.48 ppm. X-ray photoelectron
spectroscopy (XPS) was performed using an AXIS Ultra DLD
system (Kratos Co., Japan) with Al Ka radiation as the X-ray
source. Thermogravimetric analysis (TGA) of the samples was
undertaken using a Q5000IR (TA Instruments, USA) thermog-
ravimetric analyzer with a heating rate of 20 ꢂC minꢀ1 from
ꢂ
room temperature up to 800 C under nitrogen ow. Scanning
electron microscopy (SEM) was performed using an FEI Sirion-
200 (FEI Co., USA) eld emission scanning electron microscope.
Transmission electron microscopy (TEM) observations were
carried out with a JEOL-2100 (JEOL Ltd., Japan) electron
microscope at an operating voltage of 200 kV. X-ray diffraction
(XRD) measurements were performed using a Rigaku D/Max
Fig. 1 FT-IR spectra of Azo-1, Azo-2, Azo-3, and TNPP.
ꢀ
2500 X-ray diffractometer with Cu Ka radiation (k ¼ 1.54 A) at
the bands located in TNPP, the bands of azo series are much
weaker, indicating that the degree of polymerization is high.
The bands located at 1510 and 1344 cmꢀ1 correspond to N–O
stretching mode,35 suggesting most terminal nitro groups were
reacted.
a generator voltage of 40 kV andꢀa1generator current of 50 mA
with a scanning speed of 2ꢂ min from 2ꢂ to 35ꢂ. HRMS was
performed on Q-Tof Premier (Waters, USA).
Testing of gas sorption. The gas sorption isotherms were
measured via an Autosorb-iQA3200-4 (ASIQA3200-4) sorption
analyzer (Quantatech Co., USA). Nitrogen adsorption/
desorption isotherms were measured at 77 K using a Quan-
tachrome ASIQA3200-4 gas sorption analyser. Carbon dioxide
(CO2) measurements were measured at 273 and 298 K using
a Quantachrome ASIQA3200-4 extended sorption analyser tted
with a chiller circulator dewar. Hydrogen (H2) sorption isotherm
measurements were analyzer at 77 and 87 K. Prior to the
measurement, the samples were degassed in vacuum 150 ꢂC for
4 h. The isosteric heats of adsorption were calculated using the
standard calculation routines provided with the ASIQA3200-4
soware. The pore size distribution was calculated from non-
local density functional theory (NL-DFT).
In the solid-state 13C CP/MAS NMR spectra (Fig. 2), Azo-1,
Azo-2, and Azo-3 gave signals of chemical shi at 151.6, 144.2,
131.2, and 118.1 ppm, which were consistent with those of azo-
linked aromatic polymers,37 attributing to the carbon atoms of
the porphyrin rings and phenyl linkers.38,39 The carbons bonded
to the azine nitrogen (–N]C–) appear at 151.6 ppm whereas
those bonded to the imino nitrogen (–NH–) appear at 118.1
ppm, suggesting that the downeld carbon resonance should
be assigned to the a-pyrrolenine-like ring and the high-eld
resonance to the (1H)-pyrrole-like ring.40 As expected, a typical
peak at around 151.6 ppm, attributed to the azo-linked aromatic
carbons (–Car–N]N–Car–),41 is found for Azo-1, Azo-2, and
Azo-3.
XPS was performed to analyze the elemental composition
and nitrogen bonding congurations in azo series. The XPS
survey spectrum shows the presence of carbon, nitrogen, and
oxygen distinctly compared with the theoretical vales of 850 eV,
399 eV, and 533 eV,42 respectively (see Fig. S1†). The bonding
congurations of nitrogen in azos are revealed by the N 1s core
level spectra (Fig. 3a–c). All of the nitrogen responses of Azo-1
and Azo-2, giving 398.0, 399.9, and 401.0 eV, correspond to
the imine nitrogen43 (–C]N–) (N1), azo nitrogen (Ph–N]N–Ph)
Results and discussion
Synthesis
The strategy for the synthesis of azo-bridged porphyrin-based
conjugated microporous polymers (Azo-1, Azo-2, and Azo-3) is
presented in Scheme 1S.† From previous report,35 azo
compounds can be prepared by simple synthetic protocols.
However, we choose azo condensation reaction to connect the
building blocks and synthesis of conjugated polymers for the
following reasons: (1) azo condensation is an efficient, well-
established process and does not require expensive catalysts;
(2) it can introduce azo bonds into the conjugated structure that
can produce electron-rich conjugated networks; (3) it promotes
the binding between the electron-rich azo series and the
electron-poorer CO2.
Structural and chemical characterization
FT-IR spectra of Azo-1, Azo-2, and Azo-3 show the characteristic
–N]N– stretching band at around 1598, 1597 and 1598 cmꢀ1
(lN]N: 1630–1580 cmꢀ1
)
along with the respective bands for
36
aromatic rings (see Fig. 1), which is not appearing in the
building block TNPP. Furthermore, several weak bands are
Fig. 2 Solid-state 13C NMR spectra of Azo-1 (black), Azo-3 (red) and
observed in the region from 1300 to 1520 cmꢀ1. Compared with Azo-3 (blue). Asterisks represent spinning sidebands.
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Fig. 4 A typical (a) SEM image and (b) TEM image of Azo-1 polymer.
of loose agglomerates of tiny particles with rough surface and
irregular shapes. For the sake of clarity, an enlarged TEM
images are presented in Fig. 4b. The white region corresponds
to the nanopore channels, while the dark region is attributed to
the polymer networks skeleton. It clearly shows that the pores in
the polymer networks are quite uniform and the polymer
networks demonstrate typical microporous materials.
Fig. 3 N 1s XPS spectra (a–c) and different nitrogen species for azos
(a: Azo-1, b: Azo-2, c: Azo-3), and (d) different structures containing
nitrogen (N1–4).
In order to elucidate the crystallinity of the azo series, these
polymer networks were also characterized by XRD (see Fig. S4†),
intense peaks in the XRD patterns of Azo-1, Azo-2 and Azo-3
were not found except some minor peaks, indicating that Azo-
1, Azo-2 and Azo-3 are mostly amorphous, which was consis-
tent with previous SEM and TEM images showing litter crys-
tallinity within the polymer networks. Indeed, conjugated
microporous polymers are known to be amorphous solids.14
(N2), and pyrrolic nitrogen (–NH–) (N3), respectively.44,45 The
nitrogen responses of Azo-3 can be deconvoluted into ve types:
398.0, 399.9, 401.0, and 405.7, corresponding to the imine
nitrogen (–C]N–) (N1), azo nitrogen (Ph–N]N–Ph) (N2),
pyrrolic nitrogen (–NH–) (N3), and nitro nitrogen (Ph–NO2) (N4),
respectively (Fig. 3d). The integral areas of these ve peaks for
azos were used to calculate the content of nitrogen species (N1–
N4, Fig. S1†), from which we can nd the ratio of N4 for Azo-3 is
the highest, suggesting that the degree of polymerization is
lower than the other two polymer networks.
Studies of gas sorption and storage
Gas sorption. The permanent porosity in the networks was
investigated by sorption analysis using N2, CO2, and H2 as the
adsorbate molecules. Fig. 5a shows the N2 adsorption and
desorption isotherms for Azo-1/2/3, while Table 1 summarizes
the porous properties of a few comparable polymer networks.
Azo-1, Azo-2, and Azo-3 displayed type II isotherms with type I
character at higher relative pressures according to IUPAC clas-
sication. In Fig. 5a, one can nd all of the desorption
isotherms of the three networks exhibit some extent hysteresis,
suggesting the presence of some mesoporous within the mate-
rials. The calculated Brunauer–Emmett–Teller (BET) specic
surface areas of the azo series are 571 m2 gꢀ1, 675 m2 gꢀ1, and
520 m2 gꢀ1, and the Langmuir surface areas 668 m2 gꢀ1, 730 m2
gꢀ1, and 610 m2 gꢀ1 for Azo-1, Azo-2, and Azo-3, respectively.
From the robust porous features of the three materials pre-
sented in Table 1, we found that total pore volumes of the azo
TGA was performed on Azo-1, Azo-2, and Azo-3 to determine
the thermal stability and to conrm the absence of guest
molecules inside the pores (see Fig. S2†). Azo-1 shows mass loss
ꢂ
around 0.30% at temperature 132 C, which is possibly caused
by loss of volatiles (such as moisture) or trapped solvent mole-
cules in the polymer networks when compared to that of TNPP.
Similarly, Azo-2 shows mass loss around 2.41% at temperature
100 ꢂC, and Azo-3 shows mass loss around 0.99% at 133 ꢂC. Due
to the decomposition of the network, a gradual weight loss of
5% for Azo-1, Azo-2, Azo-3 was observed aer 317 ꢂC, 135 ꢂC, 305
ꢂC, respectively and about 10% weight loss at 500 ꢂC for all.
Overall, Azo-1, Azo-2, and Azo-3 retain 65.63%, 68.05%, 66.97%
ꢂ
of their mass respectively at 800 C as revealed by TGA experi-
ments, similar to that of TNNP. However, azo series presents
less weight loss between 400 and 500 ꢂC, which may be
ascribable to the high degree of polymerization. These results
highlight that Azo-1/2/3 have excellent thermal stability in
a nitrogen atmosphere.
Morphological and crystallographic characterization
The morphology and microstructure of the Azo-1, Azo-2, and
Azo-3 were investigated by SEM and TEM. All of the polymer
networks illustrated similar morphology; Fig. 4 shows the
representative SEM and TEM images of Azo-1 (more see
Fig. S3†). Similar to other microporous polymers,46,47 Fig. 4a
Fig. 5 (a) N2 adsorption (solid) and desorption (hollow) isotherms of
shows the azo polymer networks are wrinkles, and composing Azo-1, Azo-2 and Azo-3 at 77 K; (b) pore size distribution of azo series.
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Table 1 Surface properties and gas uptake for Azo-1/2/3
H2 uptake
[wt%]
CO2 uptake
[wt%]
a
b
Polymer
network
SBET
SLangmuir
dAV
SMICRO
VMICRO
VTOT
VMICRO
/VTOT
[m2 gꢀ1
]
[m2 gꢀ1
]
[nm]
[m2 gꢀ1
]
[cm3 gꢀ1
]
[cm3 gꢀ1
]
77 K
87 K
273 K
298 K
Azo-1
Azo-2
Azo-3
571
675
520
668
730
610
3.47
2.72
2.89
338
437
376
0.156
0.186
0.164
1.342
1.686
1.383
0.12
0.11
0.12
0.86
1.15
0.97
0.76
0.80
0.71
9.42
17.52
12.63
7.11
8.97
6.60
a
Note: average pore diameter. b Note: total pore volume at P/P0 ¼ 0.99.
series are 1.342 cm3 gꢀ1, 1.686 cm3 gꢀ1, 1.383 cm3 gꢀ1 for Azo-1,
Azo-2, and Azo-3, respectively. These values are comparable or
higher than that of previously reported microporous materials
constructed through azo condensation.37
Fig. 5b shows the pore size distribution (PSD) curves for the
three polymer networks as calculated using NL-DFT. Azo-1
exhibits average pore diameter centering at around 3.47 nm
with a major peak at 1.6 nm, and a minor peak in the 3.8–4.0 nm
region. Azo-2 presents average pore diameter centering at
around 2.72 nm with a major peak at 1.6 nm, and a minor in the
3.8–4.0 nm region. Azo-3 gives average pore diameter centering
at around 2.89 nm with a major peak at 1.6 nm, and a minor in
the 3.8–4.0 nm region. In addition, all of the networks show
weak peaks in the size range of 2–4 nm. The PSD analysis agrees
with the results of the N2 isotherms (Fig. 5a), suggesting that the
azo polymers feature some mesopores within the micropore
structures. Since types I isotherm corresponds to microporous
materials, and type IV is attributed for mesoporous materials.
So the nitrogen isotherms of azos show type I with type IV
character.48 The contribution of microporosity to the networks
Fig. 6 Gas adsorption studies. (a) CO2 adsorption isotherms of Azo-1,
Azo-2, and Azo-3 at 273 K (a) and 298 K (c). H2 adsorption isotherms of
Azo-1, Azo-2, and Azo-3 at 77 K (b) and 87 K (d). Isosteric heats of
adsorption of Azo-1, Azo-2, and Azo-3 for CO2 (e) and H2 (f).
can be calculated as the ratio of the micropore volume, VMICRO
,
over the total pore volume, VTOTAL. The ratios indicate networks
Azo-1/2/3 possess a similar ratio of 0.11–0.12. The results along
with TEM images indicate that Azo-1/2/3 are typical micropo-
rous materials. The ratio of Azo-2 is similar to that of Azo-1/3,
but the absolute micropore volume per g of Azo-2 is larger
than that of Azo-1/3, which may explain its higher N2 adsorp-
tion, and the higher BET specic surface area.
volume as shown in previous BET values. Additionally, it was
found that Azo-3 exhibits a higher CO2 adsorption capacity
than Azo-1 even though its apparent BET surface area is lower.
This difference may arise from pore size distribution effect of
azo and the difference in micropore volume (VMICRO for Azo-1,
Azo-3 is 0.156, 0.164 cm3 gꢀ1, respectively). It has been re-
ported that the micropore volume has a stronger effect on the
gas uptakes of a material than the total pore volume and the
specic surface area.49 On the other hand, it is expected that
the CO2 uptake of the azo networks decrease at 298 K (which
are 71.1 mg gꢀ1 for Azo-1, 89.7 mg gꢀ1 for Azo-2, and 66.0 mg
gꢀ1 for Azo-3, see Fig. 5c). These values are still comparable to
or better than that of very high surface area porous polymers
(Table S1 in ESI†).
The large surface area, microporous structure and narrow
pore size distribution of the polymer networks and nitrogen-
enriched networks intrigue us to further explore their appli-
cation in CO2 capture and separation from other gases. The
CO2 sorption of the polymer networks were investigated by
a volumetric method at 273 and 298 K (Fig. 6). For all the three
polymers, the CO2 uptake appears a rapid rise in the initial
stage, implying that CO2 molecule has favorable interaction
with the polymer skeleton. Moreover, it is noted that the
uptake of CO2 continually increases with the relative pressure
in the experimental range, suggesting that a higher adsorption
capacity can be achieved at the high pressure. Fig. 6a shows
We have further studied the hydrogen (H2) sorption
performance of the azo series in powder form. The H2 sorption
of the network polymers were investigated by the volumetric
method at 77 and 88 K. Fig. 6b and d shows the H2 adsorption
isotherms up to a maximum H2 pressure of 1 bar. At 77 K, the
the CO2 uptakes by Azo-1/2/3 are 94.2 mg gꢀ1, 175.2 mg gꢀ1
,
and 126.3 mg gꢀ1 at 273 K and 1 bar, respectively. Azo-2
obtains the highest CO2 uptake (3.98 mmol gꢀ1, 17.52 wt%
at 273 K/1 bar), which may be ascribed to its larger micropore
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H2 uptakes are 8.6 mg gꢀ1 for Azo-1, 11.5 mg gꢀ1 for Azo-2, and
9.7 mg gꢀ1 for Azo-3 (Fig. 6b). Azo-2 exhibits the largest H2
uptake of 5.75 mmol gꢀ1 at 77 K/1 bar (1.15 wt%). Uptakes of
H2 are in the order of Azo-2 > Azo-3 > Azo-1, as same as the
order of CO2 sorption of these networks. This can also be
explained by the pore size distribution effects and the differ-
ence in micropore volume. As expected, the H2 uptakes of
networks decrease at 87 K to 7.6 mg gꢀ1 for Azo-1, 8.0 mg gꢀ1
for Azo-2, and 7.1 mg gꢀ1 for Azo-3 (Fig. 6d). Compared to the
reported porous networks (Table S2 in ESI†), the capability of
H2 uptakes is moderate.
Heats of adsorption. To further understand the pore surface
characteristics of the materials and the adsorption process,
the isosteric heats of adsorption (Qst) for CO2 and H2 were
investigated. Qst of CO2 was calculated using the Clausius–
Clapeyron equation from the sorption data collected at 273
and 298 K. Fig. 6e shows Qst of CO2 for Azo-1/2/3. At the
adsorption onset, the Qst values for Azo-1, Azo-2 and Azo-3 are
37, 36, and 36 kJ molꢀ1, respectively. The values are higher
than that of other porous materials, such as high surface area
MPIs (30.4–34.8 kJ molꢀ1),50 electron-rich PECONFs (29–34 kJ
molꢀ1),51 imine-linked PPFs (21.8–29.2 kJ molꢀ1),52 and
comparable to or higher than microporous polymers with
organic ammonium ions or Lewis basic amine in pores (30–55
kJ molꢀ1).53,54 The heat of adsorption drops by 40% (Azo-1),
30.1% (Azo-2) and 37.5% (Azo-3) during the CO2 uptake
experiments, suggesting that, if the pore volumes of the
materials can be further increased, a signicantly enhanced
CO2 gas uptake can probably be achieved even at low CO2
pressure. The Clausius–Clapeyron equation for heat adsorp-
tion is given as below:55
Fig. 7 Adsorption isotherms of CO2 and N2 gases at 273 K for Azo-1
(a), Azo-2 (b), and Azo-3 (c).
selectivity of Azo-2 for CO2 uptake over N2. The high CO2
adsorption capacity and CO2/N2 adsorption selectivity are the
two key parameters for evaluating the potential of porous
materials in carbon dioxide capture applications. The excellent
selective adsorption of CO2 may pave the way to CO2 capture
and separation, as well as air-cleaning applications by the
microporous materials.
Conclusions
A series of new conjugated microporous polymers, Azo-1, Azo-2,
and Azo-3 was successfully synthesized through azo condensa-
tion of TNPP with p-phenylenediamine, benzidine and mela-
mine, respectively. Their chemical structures were conrmed by
FT-IR and solid-state 13C CP/MAS NMR measurements. TGA
measurements showed excellent thermal stability of these
networks. XRD studies indicated amorphous structure of the
polymers. Pore size distributions revealed a mainly micropo-
rous network structure for Azo-1, Azo-2, and Azo-3 with BET
Qst
ln P ¼
þ C
(1)
RT
where P, T, R and C are the pressure, temperature at the equi-
librium state, the gas constant, and equation constant,
respectively.
From the sorption data collected at 77 and 87 K, Qst of H2
adsorption was calculated using the eqn (1) too. Fig. 6f shows
Qst of Azo-1, Azo-2 and Azo-3 are of 6.2, 8.5 and 8.8 kJ molꢀ1
respectively at the adsorption onset. The heat of adsorption
decreases slightly with increased hydrogen loading.
Compared to the isosteric heats of adsorption for CO2,
adsorption of H2 at the surfaces of the azo networks is much
less, mainly because of little active sites at the surfaces of the
materials. Physisorption of the H2 contributes the most for its
adsorption.
Gas selectivity. To assess the potential applications of these
networks in post combustion CO2 capture under ambient
pressure, single component gas adsorption isotherms were
carried out with CO2 or N2 under 273 K and up to 1 bar (Fig. 7).
The selectivity for CO2 over N2 was calculated by the ratios of the
initial slopes of CO2 and N2 adsorption isotherms based on the
pressure below 0.10 bar and the ideal adsorbed solution
theory.56 Azo-1, Azo-2 and Azo-3 give CO2/N2 selectivity of 42.1,
64.3 and 44.5, respectively, based on the initial 5 data points
(<0.1 bar) of the isotherm curves, indicating greatly increased
surface areas of 571 m2 gꢀ1, 675 m2 gꢀ1, and 520 m2 gꢀ1
,
respectively, whereas Azo-2 distinguishes by its higher micro-
pore volume. Under the present reaction conditions, pore sizes
between 1.6 and 4 nm were obtained. Owing to the narrow pore
size distribution and nitrogen-rich pore surfaces, azo series
exhibit exceptionally high CO2 uptake. Thanks to the bigger
nano-micropore volume, Azo-2 showed the highest BET surface
area up to 675 m2 gꢀ1 and a remarkable CO2 uptake of 3.98
mmol gꢀ1 (or 17.52 wt%) at 273 K and 1 bar pressure. The heat
of adsorption for CO2 was calculated to be 36–37 kJ molꢀ1
,
which is larger than that of many reported porous materials.
Moreover, azos showed excellent adsorption selectivity with
CO2/N2 up to 64.3 for Azo-2 at 273 K/1 bar. The H2 storage
capability of the Azo-1/2/3 is moderate through physisorption of
the azo polymers. The above results indicate that the reported
azo porous polymer networks are promising candidates as
adsorbents for CO2 capture, separation and storage, and will be
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exploited for CO2 caption in industrial (e.g. combustion gas) 22 Y. Xu, S. Jin, H. Xu, A. Nagai and D. Jiang, Chem. Soc. Rev.,
and environmental air-cleaning.
2013, 42, 7965–8178.
23 A. I. Cooper, Adv. Mater., 2009, 21, 1291–1295.
24 Y. Liu, X. Guo, N. Xiang, B. Zhao, H. Huang, H. Li, P. Shen
and S. Tan, J. Mater. Chem., 2010, 20, 1140–1146.
25 X. Feng, L. Chen, Y. Dong and D. Jiang, Chem. Commun.,
2011, 47, 1979–1981.
Acknowledgements
The authors thank the nancial support from National Natural
Science Foundation of China (51403126). ZZ and JW acknowl-
edge the support from the JSNN at UNCG.
26 N. U. Day, C. C. Wamser and M. G. Walter, Polym. Int., 2015,
64, 833–857.
27 W. M. Campbell, K. W. Jolley, P. Wagner, K. Wagner,
P. J. Walsh, K. C. Gordon, L. Schmidt-Mende,
Notes and references
¨
M. K. Nazeeruddin, Q. Wang and M. Gratzel, J. Phys. Chem.
1 A. Thomas, P. Kuhn, J. Weber, M. M. Titirici and
C, 2007, 111, 11760–11762.
M. Antonietti, Macromol. Rapid Commun., 2009, 30, 221–236. 28 W. Zheng, N. Shan, L. Yu and X. Wang, Dyes Pigm., 2008, 77,
2 D. Wu, F. Xu, B. Sun, R. Fu, H. He and K. Matyjaszewski,
Chem. Rev., 2012, 112, 3959–4015.
3 Z. Xiang and D. Cao, J. Mater. Chem. A, 2013, 1, 2691–2718.
153–157.
29 X. Liu, H. Li, Y. Zhang, B. Xu, S. A, H. Xia and Y. Mu, Polym.
Chem., 2013, 4, 2445–2448.
4 Q. Chen, M. Luo, P. Hammershøj, D. Zhou, Y. Han, 30 V. S. P. K. Neti, J. Wang, S. Deng and L. Echegoyen, RSC Adv.,
B. W. Laursen, C.-G. Yan and B.-H. Han, J. Am. Chem. Soc.,
2012, 134, 6084–6087.
2015, 5, 10960–10963.
31 J. Lu and J. Zhang, J. Mater. Chem. A, 2014, 2, 13831–13834.
5 D. Yuan, W. Lu, D. Zhao and H. C. Zhou, Adv. Mater., 2011, 32 P. Arab, E. Parrish, T. Islamoglu and H. M. El-Kaderi, J.
23, 3723–3725. Mater. Chem. A, 2015, 3, 20586–20594.
6 W. Lu, D. Yuan, J. Sculley, D. Zhao, R. Krishna and 33 P. Arab, M. G. Rabbani, A. K. Sekizkardes, T. Islamoglu and
H.-C. Zhou, J. Am. Chem. Soc., 2011, 133, 18126–18129. H. M. El-Kaderi, Chem. Mater., 2014, 26, 1385–1392.
7 J. Germain, J. M. Frechet and F. Svec, Small, 2009, 5, 1098– 34 M. Yuasa, K. Oyaizu, A. Yamaguchi and M. Kuwakado, J. Am.
1111.
Chem. Soc., 2004, 126, 11128–11129.
8 O. K. Farha, A. O. Yazaydın, I. Eryazici, C. D. Malliakas, 35 H. A. Patel, S. Hyun Je, J. Park, D. P. Chen, Y. Jung,
˙
˘
´
¨
B. G. Hauser, M. G. Kanatzidis, S. T. Nguyen, R. Q. Snurr
and J. T. Hupp, Nat. Chem., 2010, 2, 944–948.
9 M. Pumera, Energy Environ. Sci., 2011, 4, 668–674.
10 D. M. D'Alessandro, B. Smit and J. R. Long, Angew. Chem.,
Int. Ed., 2010, 49, 6058–6082.
C. T. Yavuz and A. Coskun, Nat. Commun., 2013, 4, 1357.
36 P. Narayanan, Essentials Of Biophysics, Anshan, 2000.
37 H. A. Patel, S. H. Je, J. Park, D. P. Chen, Y. Jung, C. T. Yavuz
and A. Coskun, Nat. Commun., 2013, 4, 1357.
38 X. Feng, L. Liu, Y. Honsho, A. Saeki, S. Seki, S. Irle, Y. Dong,
A. Nagai and D. Jiang, Angew. Chem., 2012, 124, 2672–2676.
´
11 G. Ferey, C. Serre, T. Devic, G. Maurin, H. Jobic,
P. L. Llewellyn, G. De Weireld, A. Vimont, M. Daturi and 39 A. Modak, M. Nandi, J. Mondal and A. Bhaumik, Chem.
J.-S. Chang, Chem. Soc. Rev., 2011, 40, 550–562.
Commun., 2012, 48, 248–250.
12 R. Dawson, E. Stockel, J. R. Holst, D. J. Adams and 40 L. Frydman, A. C. Olivieri, L. E. Diaz, A. Valasinas and
A. I. Cooper, Energy Environ. Sci., 2011, 4, 4239–4245.
B. Frydman, J. Am. Chem. Soc., 1988, 110, 5651–5661.
13 N. Du, H. B. Park, G. P. Robertson, M. M. Dal-Cin, T. Visser, 41 S. Xie, A. Natansohn and P. Rochon, Macromolecules, 1994,
L. Scoles and M. D. Guiver, Nat. Mater., 2011, 10, 372–375. 27, 1489–1492.
14 N. B. McKeown and P. M. Budd, Chem. Soc. Rev., 2006, 35, 42 A. P. Hitchcock and C. E. Brion, J. Electron Spectrosc. Relat.
675–683. Phenom., 1980, 18, 1–21.
15 J. Wang, I. Senkovska, M. Oschatz, M. R. Lohe, L. Borchardt, 43 A. P. Dementjev, A. de Graaf, M. C. M. van de Sanden,
¨
A. Heerwig, Q. Liu and S. Kaskel, ACS Appl. Mater. Interfaces,
2013, 5, 3160–3167.
K. I. Maslakov, A. V. Naumkin and A. A. Serov, Diamond
Relat. Mater., 2000, 9, 1904–1907.
16 G. Sethia and A. Sayari, Carbon, 2015, 93, 68–80.
17 G.-P. Hao, W.-C. Li, D. Qian and A.-H. Lu, Adv. Mater., 2010,
22, 853–857.
44 L. Silipigni, G. De Luca, Q. Quattrone, L. M. Scolaro,
G. Salvato and V. Grasso, J. Phys.: Condens. Matter, 2006,
18, 5759–5772.
18 K. Huang, S.-H. Chai, R. T. Mayes, G. M. Veith, 45 W. Lian, Y. Sun, B. Wang, N. Shan and T. Shi, J. Serb. Chem.
K. L. Browning, M. A. Sakwa-Novak, M. E. Potter, Soc., 2012, 77, 335–348.
C. W. Jones, Y.-T. Wu and S. Dai, Chem. Commun., 2015, 46 Y. Yang, Q. Zhang, Z. Zhang and S. Zhang, J. Mater. Chem. A,
51, 17261–17264.
2013, 1, 10368–10374.
19 G. Sethia and A. Sayari, Energy Fuels, 2014, 28, 2727–2731.
20 N. Popp, T. Homburg, N. Stock and J. Senker, J. Mater. Chem.
A, 2015, 3, 18492–18504.
21 S. N. Talapaneni, O. Buyukcakir, S. H. Je, S. Srinivasan,
Y. Seo, K. Polychronopoulou and A. Coskun, Chem. Mater.,
2015, 27, 6818–6826.
47 M. Mastalerz, M. W. Schneider, I. M. Oppel and O. Presly,
Angew. Chem., Int. Ed., 2011, 50, 1046–1051.
48 G. Crini and P.-M. Badot, Sorption Processes and Pollution:
Conventional and Non-conventional Sorbents for Pollutant
´
Removal from Wastewaters, Presses Univ. Franche-Comte,
2010.
30054 | RSC Adv., 2016, 6, 30048–30055
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49 A. P. Katsoulidis and M. G. Kanatzidis, Chem. Mater., 2012, 54 Y. Fu, D. Sun, Y. Chen, R. Huang, Z. Ding, X. Fu and Z. Li,
24, 471–479.
Angew. Chem., 2012, 124, 3420–3423.
50 G. Li and Z. Wang, Macromolecules, 2013, 46, 3058–3066.
51 P. Mohanty, L. D. Kull and K. Landskron, Nat. Commun.,
2011, 2, 401.
52 Y. Zhu, H. Long and W. Zhang, Chem. Mater., 2013, 25, 1630–
1635.
55 V. Krungleviciute, L. Heroux, A. D. Migone, C. T. Kingston
and B. Simard, J. Phys. Chem. B, 2005, 109, 9317–9320.
56 X. Zhu, C.-L. Do-Thanh, C. R. Murdock, K. M. Nelson,
C. Tian, S. Brown, S. M. Mahurin, D. M. Jenkins, J. Hu,
B. Zhao, H. Liu and S. Dai, ACS Macro Lett., 2013, 2, 660–663.
53 A. Demessence, D. M. D'Alessandro, M. L. Foo and J. R. Long,
J. Am. Chem. Soc., 2009, 131, 8784–8786.
This journal is © The Royal Society of Chemistry 2016
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