Porous 3D polymers for high pressure methane storage and carbon dioxide capture

Article abstract of DOI:10.1039/c7ta00934h

High surface area 3D polymers represent one of the most promising classes of porous materials because of their high gas uptake and stability to thermal and chemical degradation. A series of porous organic polymers with aromatic building units have been synthesized and compared to explore their high-pressure performance as adsorbents of gases of relevant importance for energy and the environment. Particular attention was paid to methane storage up to pressures as high as 180 bar at ambient temperature. Porous polymers were prepared starting from a wide choice of spatially expanded aromatic monomers: a systematic change in the number of rings, variable size and shape was taken under consideration. The high number of rings (up to 6), which act as multiple reactive sites and form a number of connections between the multi-dentate nodes, result in an extensive cross-linked framework. Condensation was obtained by two alternative synthetic routes, viz., Yamamoto cross-coupling and Friedel-Crafts alkylation reactions. The structural characteristics and high stability of the porous polymers, even to mechanical compression, were carefully determined by several methods, including 1D and 2D solid state NMR, FT IR and thermal analyses. The CH₄ uptake in the porous polymers allowed an understanding of the incremental response to pressure, up to extremely high values, and the exploitation of the extensive pressure range to customize the gas adsorption/desorption cycles for storage and transportation. Owing to the notable presence of large mesopores and network flexibility, combined with high surface area, a remarkable gain at high pressure was achieved, ensuring a highly competitive uptake/delivery efficiency. At 180 bar, adsorption values up to 445 cm³ STP g^-1 were measured for porous organic polymers such as carbazolyl- and triptycene-based materials. The benchmark of these materials PAF1 reaches the value of 916 cm³ STP g^-1 of adsorbed CH₄, exceeding the performance of most of the best performing MOFs, COFs and activated carbons. CO₂ and N₂ adsorption isotherms collected at room temperature enabled the assessment of the suitability of such polymer networks for CO₂ selective separation and capture. In summary, the in-depth and extensive comparative screening within this class of materials up to high pressures provides the necessary parameters for further synthetic and applicative work.

Full text of DOI:10.1039/c7ta00934h

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS nanocages as a lithium ion battery anode material
2
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Journal Name
ARTICLE
Porous 3D Polymers for High Pressure Methane Storage and
Carbon Dioxide Capture
Silvia Bracco, Daniele Piga, Irene Bassanetti, Jacopo Perego, Angiolina Comotti* and Piero
Sozzani*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
High surface area 3D polymers represent one of the most promising classes of porous materials for their high gas uptake
and stability to thermal and chemical degradation. A series of porous organic polymers with aromatic building units have
been synthetized and compared to explore their high pressure performance as absorbents of gases of relevant importance
for energy and the environment. Particular attention was paid to methane storage up to pressures as high as 180 bar and
ambient temperature. Porous polymers were prepared starting from a wide choice of spatially expanded aromatic
monomers: a systematic change in the number of rings, variable size and shape were taken under consideration. The high
number of rings (up to 6), which act as multiple reactive sites and form a number of connections between the multi-dentate
nodes, result in an extensive cross-linked framework. Condensation was obtained by two alternative synthetic routes, viz.,
Yamamoto cross-coupling and Friedel Crafts alkylation reactions. The structural characteristics and high stability of the
porous polymers, even to mechanical compression, were carefully determined by several methods, including 1D and 2D
www.rsc.org/
solid state NMR, FT IR and thermal analyses. The CH uptake in the porous polymers, allowed an understanding of the
4
incremental response to pressure, up to extremely high values, and the exploitation of the extensive pressure range to
customize the gas absorption/desorption cycles for storage and transportation. Owing to the notable presence of large
mesopores and network flexibility, combined with high surface area, a remarkable gain at high pressure was achieved,
3
ensuring a highly competitive uptake/delivery efficiency. At 180 bar, adsorption values up to 445 cm STP/g were measured
for porous organic polymers such as carbazolyl- and triptycene-based materials. The benchmark of these materials PAF1
3
reaches the value of 916 cm STP/g of adsorbed CH
4
, overcoming most of best performing MOFs, COFs and activated
carbons. CO
2
and N
2
adsorption isotherms collected at room temperature enabled the assessment of the suitability of such
selective separation and capture. In summary, the in-depth and extensive comparative screening
polymer networks for CO
2
within this class of materials up to high pressures provides the necessary parameters for further synthetic and applicative
work.
compressed up to high pressures for storage and transportation
inside appropriate CNG containment systems. CNG is also applied in
several other industrial sectors including the wide automotive field.
Introduction
2
The need for alternative fuels is a key issue in our society, and
methane, or natural gas, is a viable choice as a readily available
option with a low carbon emission. However, current natural gas
delivery methods, such as pipeline and liquid natural gas (LNG)
technologies, can prove unsatisfactory for political and economic
Storage of natural gas in highly porous materials, also known as
Adsorbed Natural Gas (ANG), has been proposed, as an innovative
way to store and transport large amounts of natural gas. Such porous
materials include the widely described zeolites, activated carbons
3
and metal organic frameworks (MOFs) but in some cases there are
1
reasons. This is particularly the case for gas reservoirs which are
4
limitations, in primis thermal and chemical stability. In existing
often located off-shore: natural gas is present in quantities that
typically do not justify capital-intensive infrastructural investment,
such as that necessary to build new pipelines or
liquefaction/regasification facilities. Therefore, keen interest has
nourished Compressed Natural Gas (CNG) technology.2 In CNG
applications, whether terrestrial or marine, natural gas is
5
4
metal-organic compounds, although high CH uptake is observed,
positively charged metal atoms bound to electron-rich organic
ligands can be sensitive in the long term to polar substances, such as
water, that could contaminate and degrade the material during its
6
life-time. In addition, uptake measurements are frequently limited
to moderate pressures, such as 35 or 65 bar, adequate for vehicular
applications. Consequently, there remains a need for high pressure
methane storage materials, which would improve their performance
for general uses.
Porous organic polymers offer a valid option as the carbon-carbon
bond connectivity of such polymers imparts high thermal and
University of Milano Bicocca, Department of Materials Science, Via R. Cozzi 55,
2
0125 Milan, Italy *Email - piero.sozzani@mater.unimib.it.
Electronic Supplementary Information (ESI) available: Experimental details,
thermocalorimetric analysis, MAS NMR spectroscopy, gas adsorption measurements.
The Supporting Information is available free of charge on the ACS Publications website.
See DOI: 10.1039/x0xx00000x
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chemical stability, resistance to contaminants and low water affinity. a double-necked flask equipped with a cooling vapor condenser.
Indeed, a number of them endowed with high surface areas and pore After the reaction, the resulting powder was collected by filtration
volumes, have been proposed in recent years for gas capture and and then washed with methanol several times, until the filtrating
7
storage. For optimal high-pressure performance the contribution of liquor was colorless. The product was purified, by Soxhlet extraction
-
mesopores must be significant, and high deliverable capacity is by methanol for 48 hours and subsequently dried under vacuum (10
3
achieved only if a moderate amount of gas is retained at low pressure
3
torr) at 130 °C for 15 hours. The monomer:FDA:FeCl molar ratios
8
during the discharge step. Porous 3D polymers are endowed with a are as follows, 1:4:4 for trans-stilbene, 1:6:6 for tripticene, 1:8:8 for
complete range of pore size (from subnano to tenths of nanometers) 9,9’-spirobifluorene, rubrene and 4,4'-di(9H-carbazol-9-yl)biphenyl,
and can fulfill these requisites, thus, they are promising candidates 1:12:12 for hexaphenylbenzene, hexaphenyldisilane and
for an efficient storage and delivery. Furthermore, in 3D cross-linked hexaphenylcyclotrisiloxane. The purity of the monomers was > 99%.
polymeric networks with a random number of connections between
General Procedure for the Yamamoto reactions
nodes, high pressure capacity can be improved by framework
The 5,10,15,20-tetrakis(4-bromophenyl)porphyrin monomer was
9
flexibility and expandability. To date, for this family of materials,
purchased by Frontier Scientific. The 1,4-bis(3,6-dibromo-9H-
methane capture has been investigated only in a limited number of
carbazol-9-yl)benzene and the 4,4'-bis(3,6-dibromo-9H-carbazol-9-
1
0
4
reports and CH absorption and storage at high pressures (up to
yl)biphenyl were obtained by bromuration of the precursors with N-
1
80 bar) in porous organic polymers has not yet been explored.
bromosuccinimmide
in
THF
at
40°C
(1:3.5
ratio).
The present research activity focused on the synthesis and methane
uptake/release over a wide range of pressures, up to 180 bar, of high-
surface-area porous materials with carbon-carbon covalent bonds
connecting the aromatic rings. Two synthetic routes were pursued:
Yamamoto-coupling and Friedel-Crafts alkylation (FC) reactions. The
monomers possess a high number of aromatic rings and multiple
reactive sites which connect with other monomers to form multiple
links, resulting in an extensive cross-linked framework in which each
monomeric unit is bound to more than two other monomeric units
Tetraphenylmethane was reacted with elemental Br
2
at room
temperature for 12 hours. The product was crystallized twice to yield
1
tetrakis(4-bromophenyl)methane (purity > 99%, by H NMR).
The catalytic complex was prepared by adding bis(1,5-
cyclooctadiene)nickel(0) (Ni(COD) ) to 2,2’-bipyridyl and cis,cis-1,5-
2
cyclooctadiene (COD) in dry DMF and THF solution. The porous
polymers were obtained by adding dropwise the brominated
monomer, dissolved in THF, to the catalytic mixture, under inert
atmosphere, and the resulting mixture was stirred at 60 °C for 22
hours and at room temperature for 22 hours. The reaction was then
quenched adding concentrated HCl (30 ml), until the solution turned
green with a white suspension. The filtered product was washed with
THF (2 x 100 ml), water (2 x 100 ml) and chloroform (2 x 100 ml) and
(
Figure 1). A systematic variation of the connectivity and geometry
of monomer units allowed the investigation of the influence of
polymer structure on gas adsorption properties up to high pressures.
A number of new monomers and synthetic conditions were explored
to compare methane adsorption for a wide collection of aromatic
porous networks, thus providing efficiency and convenience
parameters for these materials.
-3
dried in vacuum (10 torr) at 200°C. For further details see Tables S1
and S2.
The copolymer was obtained starting from a mixture of tetrakis(4-
bromophenyl)methane and 5,10,15,20-betrakis(4-bromophenyl)
porphyrin (80:20 ratio). The mixture was dissolved in dry THF, and
was added dropwise to the catalytic mixture, under inert
atmosphere, and stirred for 48 hours at 0°C. After the quenching, the
copolymer was recovered as described above.
The porous materials were characterized by several techniques,
including N
D and 2D NMR, in order to extract information about their gas
capacity, thermal stability and structure. CH isotherms show a
2
adsorption isotherms, thermal analysis and solid state
1
4
moderate slope in the initial part, followed by a continuous increase
in uptake at higher pressures, leading to a high storage efficiency. It
is to be noted that the production scalability and the low-cost of the
synthetic procedure using inexpensive catalysts, makes this family of
porous products particularly suitable for practical purposes.
Characterization
Thermal stability was determined from a thermogravimetric analysis
over a temperature range from 30 to 800 °C under air with a heating
rate of 10°C/min. The weight loss was measured at 800°C. FT IR
spectra were collected in transmission on an JASCO 4100
Moreover, evidence of selective CO
temperature and 10 bar over N , enforces the utility of the present
capture and separation for cleaner
2
adsorption, at room
2
spectrometer using KBr disks.
extensive screening, also for CO
combustion processes.
2
1
3C solid state NMR spectra were run at 75.5 MHz on a Bruker Avance
3
00 instrument operating at a static field of 7.04 T equipped with a 4
mm double resonance MAS probe. The samples were spun at magic
angle at a spinning speed of 12.5 kHz, and ramped amplitude cross-
polarization (RAMPCP) transfer of magnetization was applied. The
Experimental
General Procedure for the Friedel-Craft reactions
Formaldehyde dimethyl acetal (FDA) and anhydrous FeCl were
3
added to a solution of monomer in 1,2-dichloroethane, under inert
gas atmosphere. The mixture was then stirred at 80°C for 24 hours in
1
3
9
0° pulse for the proton was 2.9 μs. The C cross polarization (CP)
MAS experiments were run at 298 K using a contact times of 2 ms
13
and a recycle delay of 6 s. C single-pulse excitation (SPE)
experiments with dipolar decoupling from hydrogen were run using
2
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a recycle delay of 60 s. The 90° pulse for carbon was 4.6 μs. slit pore model up to p/p0 = 0.98. The micropore volume was
Crystalline polyethylene was taken as an external reference at 32.8 calculated at p/p° = 0.1. CO and N adsorption isotherms at 298 and
ppm from TMS. 273K and up to 10 bar were collected on a Micromeritics ASAP 2050
2
2
1
13
Phase-modulated Lee−Goldburg (PMLG) heteronuclear H- C instrument. Before the analysis, the samples were left overnight at
correlation (HETCOR) experiments coupled with fast magic angle 130°C under vacuum.
spinning allowed the recording of the 2D spectra with a high
Methane Isotherms
CH sorption measurements at high pressure were performed to test
4
11
resolution in hydrogen and carbon dimensions. Narrow hydrogen
resonances, with line widths on the order of 1−2 ppm, were obtained
with homonuclear decoupling during t1. This resolution permits a
sufficiently accurate determination of the proton species present in
the maximum gas capacity and to determine the isotherm profile
from low to high pressures (up to 180 bar), i.e. in a wider range than
is usual. This yields the 'deliverable gas' potential of the materials.
The experiments were performed using a Micromeritics High
Pressure Volumetric Apparatus (HPAV II), equipped with a pressure-
booster compressor. Typically used were: 1 g samples obtained by
the Friedel Crafts alkylation reaction and, in the case of the
Yamamoto-type reaction, 300 mg samples. Before the analysis, the
samples were activated overnight at 130°C under vacuum, directly in
the steel jar. Each experiment was performed by applying an
adsorption-desorption cycle up to 180 bar at 25°C.
1
13
the system. The 2D PMLG H- C HETCOR spectra were run with an
LG period of 18.9 μs. The efficient transfer of magnetization to the
carbon nuclei was performed by applying the RAMP-CP sequence.
The cross-polarization times of 50 μs, 500 μs, 1 ms and 2 ms were
applied. Quadrature detection in t
proportional phase increments method (TPPI). The carbon signals
were acquired during t under proton decoupling by applying the
1
was achieved by the time
2
two-pulse phase modulation scheme (TPPM). The 2D experiments
were conducted at 298 K under magic-angle spinning (MAS)
conditions at 12.5 kHz.
Results and discussion
Synthesis and structure
A series of porous organic materials were prepared by linking
monomers which typically contain two or more aromatic rings linked
in a -covalent system e.g. porphyrin and hexaphenylbenzene, or
fused aromatic rings such as carbazole groups and polyacenes.
Nitrogen and carbon dioxide adsorption isotherms
N adsorption isotherms at 77K were collected on a Micromeritics
2
ASAP 2020 HD instrument. Before the analysis, the samples were left
overnight at 130°C under vacuum. The surface area (m /g) was
calculated from the nitrogen adsorption branch of the nitrogen
adsorption isotherm at 77K, according to Brunauer–Emmett–Teller
2
3
(
BET) and Langmuir models. The total pore volume Vtot (cm /g) was
calculated from the nitrogen isotherms by NLDFT method and carbon
Figure 1. Chemical structures of the microporous 3D polymers. The acronyms of the polymers are reported below the monomer units
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The multiple reaction sites on the aromatic rings provide a
tendency for cross-linking. Moreover, the 3D geometry of the
monomers, combined with the formation of carbon-carbon cross-
links, favoured poorly-packed arrangements of the framework. In
fact, the molecular shape of the monomers, which is generally
retained within the individual monomeric units in the cross-linked
polymeric framework, contribute to the formation of low density
structures. Figure
1
shows the cross-linked polymeric
frameworks: they include polymers of monomeric units, which
were mainly linked by a methylene linker between aromatic rings
by the Friedel-Crafts reaction, as well as homo- and co-polymers
connected by covalent C-C direct links between aromatic groups.
7,12
In the case of Friedel-Crafts alkylation reaction, the use of FDA
and the maintenance of a constant stoichiometry of both FDA and
catalyst with respect to each aromatic ring in the monomer unit
was applied (1 phenyl ring : 2 FDA : 2 catalyst), resulting in an
advantage for a systematic comparison of the results.
Elemental analysis of the porous polymers by the Yamamoto-type
reaction indicated the absence of residual bromine. In the case of
copolymers of porphyrin and tetraphenylmethane monomers
(
PAFPORP), the analysis enabled us to determine the
stoichiometry of resulting network (23/77 mole/mole,
respectively). FTIR spectra of the porous polymers obtained by
Friedel-Craft alkylation reaction revealed the structural features:
in particular, owing to the effect of cross-linking, vibrations at
-1
2
800-3000 cm characteristic of asymmetric and symmetric C-H
stretching are observed, revealing that the networks are linked by
CH groups (Figures S14-S23). The spectra show a band at about
700 cm , typical of hypercross-linked aromatic systems.
2
-1
1
Thermogravimetric analysis demonstrated the stability of these
porous materials up to or more than 500°C, while powder X-ray
diffraction patterns show no defined peak, suggesting the
structural disorder of the networks (Figures S1-S13).
The structural investigation at the molecular level was performed
13
1
13
by 1D C and 2D H- C NMR spectroscopy. The spectra of the
porous polymers obtained by both synthetic routes show
resonances between 110 and 150 ppm assigned to the aromatic
1
3
moieties of monomeric units (Figure 2). No resonances were
present in the alifatic region for the frameworks obtained by
Yamamoto-type reaction, except in the case of PAF1, which
Figure 2. 13C CP MAS NMR spectra of a) HPSiO2, b) HPSiSi, c) STIL,
d) HPPh, e) RUB, f) TRIP, g) CBZ1, h) PAF1, i) PAFPROP and l) PORP.
3
possesses the quaternary sp carbon at the core of the
monomeric unit (Figure 2g-l).
On the opposite, additional signals appear in the aliphatic region
for Friedel-Crafts reaction polymers, indipendently of the
monomer. The pattern is complex, because of multiple alkylation
reactions of the aromatic rings. The connectivity in the
framework was inferred by 2D NMR spectra, which through
nuclear dipole-dipole interactions, highlighted the correlation
between carbon and hydrogen nuclei in close spatial proximity,
providing for the first time in this category of compounds,
evidence of the insertion of connecting bridges and pendant
groups.14 Figure 3 reports the 2D MAS spectrum of triptycene-
based porous polymer (TRIP): the aromatic hydrogens of the main
architecture 
the methylene groups (
paddles of connected monomer units and, vice versa, the benzylic
CH -bridge hydrogens at  = 4.9 ppm, communicate with the
aromatic substituted carbons, unambiguously showing the
phenylene groups to be covalently bonded to the CH linkers.
Also, CH -O carbons of the pendant groups reside at short
distance with respect to the aromatic hydrogens ( =6.6- =72.8
H
= 6.6 ppm are in correlation with the carbons of
1
8
C
= 36.3 ppm) that bridge aromatic
2
H
2
2
H
C
4
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ppm cross-peak), demonstrating they are directly substituted to
the aromatic rings through a carbon–carbon bond. By a
quantitative analysis of the MAS spectrum obtained with a long
recycle delay of 60 s, we could extrapolate the abundance of the
linking connectors created by our synthetic procedure using
Friedel-Crafts reaction.
during sorption: an indication of a flexible structure with
expandable pores, because capillary condensation in the
mesopores causes some strain in the network, as systematically
9
observed in soft polymeric materials.
1
13
Figure 3. 2D H- C HETCOR NMR spectra with Lee–Goldbourg
decouling of TRIP. The cross-peaks between hydrogens and
13
carbons are highlighted. In C dimension the MAS NMR spectrum
with recycle delay of 60 s is reported.
N
2
adsorption at 77K
The porosity of the frameworks was tested by N
isotherms at 77 K, which exhibited BET surface areas ranging
2
adsorption
2
2
typically from 1000 to 1700 m /g and up to 4800 m /g for
structures containing tetraphenylmethane as the monomer unit
(
2
Figure 4 and Figures S27-S30). The N adsorption isotherms
exhibit, in most cases, a steeply sloping gas uptake at relatively
low pressures and a continuous rise at higher pressures,
reflecting the presence of micro/mesopore distribution (Figure
2
Figure 4. N isotherms collected at 77K of a selection of porous
polymers: a) CBZ1, b) TRIP, c) PAF1, d) PAFPORP, e) PORP, f)
HPSiSi, g) RUB, h) STIL, i) HPPh and l) HPSiO2.
4
). Pore size distribution was calculated by NLDFT method and
carbon slit pore model (Figures S31 and S32): Table 1 displays the
total pore volumes. The limit cases are the carbazolyl-based
compound (CBZCH2) obtained by Friedel-Craft alkylation
reaction, which displays a main peak with a diameter smaller than
1
0 Å, and the stilbene-based (STIL) and hexaphenyldisilane-based
(
HPSiSi) polymers with pores mainly centered on larger diameters
from 10 to 100 Å.
A large hysteresis is observed in many cases between the
adsorption and desorption branches, as is mostly evident in CBZ1,
CBZ2, TRIP, RUB, STIL, HPSiSi and PAFPORP. The desorption curve
closes only at partial pressures around null point. The hysteresis
loop in the isotherm is consistent with the swelling of the network
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Table 1. Brunauer-Emmet-Teller and Langmuir
surface areas, total pore volume (T.P.V.), micro/total
pore volume ratio (M./T. ratio) and decomposition
temperature (Dec.T.).
triptycene-based (TRIP) materials (Table 2). Such uptake value
is far above the HKUST-1 performance at the same pressure.
Since these high pressure conditions above 100 bar are not
frequently reported in the literature, we compared our results
at lower pressures with those published. Under the conditions
of 65 bar and 298 K the porous polymers CBZ1, CBZ2 and TRIP
3
adsorb a CH
4
amount of 255.4, 252.8 and 240.4 cm STP/g,
Sample
BET
m /g
Langmuir T.P.V. M./T.
Dec.T.
K
corresponding to 0.18, 0.18 and 0.17 g/g, respectively. Such
high values are comparable with or beyond the CH uptake of
COF-5 (0.11 g/g), COF-8 (0.11 g/g), PPN-2 (0.14 g/g), PPN-3
2
2
3
m /g
cm /g
ratio
0.63
0.42
0.46
0.44
0.50
0.50
0.53
0.52
0.56
0.58
0.79
0.58
0.32
4
PAF1a)
4784
2194
1698
1622
1592
1494
1418
1254
1258
1082
1090
1054
872
5485
2492
1942
1834
1895
1703
1612
1525
1428
1284
1250
1256
1004
2.70
2.02
1.42
1.40
1.20
1.13
0.93
0.92
0.85
0.72
0.53
0.69
1.15
500
400
550
550
500
400
450
500
500
450
450
450
500
20,21
PAFPORP
(0.19 g/g), PPN-13 (0.18 g/g) and NiMOF-74 (0.15 g/g).
At
b)
CBZ2
100 bar the amount of methane adsorbed of CBZ1 (0.23 g/g),
CBZ1
CBZ2 (0.23 g/g) and TRIP (0.22 g/g) overcome MOF-5 (0.28
c)
3
TRIP
g/g), Mg (dobdc) (0.19 g/g) and PCN-14 (0.22 g/g).
2
d)
PORP
Carbazolyl-based porous polymers required Nickel-catalyzed
coupling reactions (Yamamoto-type reaction) limiting the
synthesis of these materials to small scales while the
triptycene-based porous polymer was obtained with the
e)
SPBF
STIL
RUB
HPPh
CBZCH2
HPSiO2
HPSiSi
cheap catalysts such as FeCl
3
and in high yields, making this
f)
material suitable for applications.
a) ref. n. 15. b) ref. n. 16. c) ref. 9. d) ref. 17. e) ref. 18. f) ref. 19.
We can observe that the best performances were obtained by
rigid structures in which 3 or 4 aromatic rings connected to a
node protrude at different angles, such as tripticene,
spirobifluorene and tetraphenylmethane, because these
structures are favorable to ensure expansion of the
framework in all directions. Contrarily, a number of aromatic
rings exceeding 4 did not provide advantage in both surface
area and pore capacity owing to the overcrowded
arrangement on the same monomer. As for the shape factor,
elongated structures (such carbazole-based molecules), which
are most favorable in MOF design, are not outstanding in the
present analysis, because they require the cooperation of
geometrical constraints provided by coordinative bonds on a
metal center.
Gravimetric CH adsorption isotherms
4
Thanks to the large surface area, pore volume and large
contribution of the mesopores, porous organic polymers were
selected as potentially adsorptive materials for methane
storage, especially at high pressures. The polymers were
tested experimentally in a wide range of high pressure
conditions up to 180 bar, all measurements being taken three
times to check the reproducibility of the data (Figure 5a). In
CH isotherms it is possible to observe an increasing uptake
4
over the whole range of pressures, although the slope
diminishes at high pressures. At 180 bar adsorption values up
3
to 445 cm STP/g (0.32 g/g) were measured for porous organic
polymers endowed with surface areas of more than 1600
2
m /g, such as carbazolyl-based (CBZ1 and CBZ2) and
6
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Fig. 5 CH
4
adsorption isotherms collected at 298K of a) porous
is stored in PAF1 at high pressures, proving the enormous
relevance to explore high pressure range. Already at 100 bar
polymers up to 180 bar and b) PAF1, PAFPOR and PORP as
compared to HKUST-1.
3
PAF1 adsorbs 643 cm STP/g (0.46 g/g) overcoming most of
3
best performing MOFs (HKUST-1, PCN14, MOF-5), COFs
21
4,22
In general, the maximum CH
4
absorption values are related to
(COF-102 and COF-103) and carbon (AX-21 and LMA738)
.
the surface area and pore capacity obtained by nitrogen at
The tetraphenyl-porphyrin-based porous polymer shows 294
3
3
7
7K, in fact, lower adsorption values of 322 and 242 cm STP/g
cm STP/g of adsorbed CH
4
at 110 bar, while the copolymer
tetraphenylmethane and
were observed in the porous materials with surface areas of
containing
both
2
about 1250 and 1050 m /g, respectively. The only exception is
tetraphenylporphyrin monomeric units, PAFPORP, displays an
3
hexaphenyldisilane-based (HPSiSi) compound which, despite
intermediate absorption value (464 cm STP/g), which is a
2
3
the low surface area of 872 m /g, absorbs up to 332 cm STP/g
owing to the large mesoporosity/macroporosity and the
competitive value and is in agreement with the pore capacity
3
of the material (2.02 cm /g). The isotherms are fully
3
relatively large total pore capacity of 1.15 cm /g. This is mainly
reversible, as observed by the coincidence of adsorption and
desorption branches, and are reproducible over several cycles
(Figure S34), thanks to the high stability of the materials.
The isosteric heat of adsorption, as measured by Clausius-
Clapeyron equation and based on the adsorption values of
273K and 298K, is from 19 up to 21 kJ/mol at low coverage for
the various samples (Figure S43). These values are quite
notable and match, or go beyond, the performance of MOF:
i.e. Ni-MOF-74 (21.4 kJ/mol), PCN-14 (18.7 kJ/mol) and
derived by the geometry of the monomer in which six p-
phenyl groups protrude from the Si-Si moiety, producing 3D
nodes that form a complex branching system. The carbazolyl-
based porous polymer as synthesized by Friedel Craft
alkylation (CBZCH2), exhibits a behavior different from that of
the homologue porous polymers obtained by Yamamoto-type
reaction, in fact in CBZCH2 a type-I profile is observed,
3
reaching a limit value of 168 cm STP/g at 180 bar: the CH
4
3,20
isotherm shape mirrors the Langmuir profile of N
2
adsorption
HKUST-1 (17 kJ/mol).
Such high values indicate that the
isotherm and the presence of an intense peak in the
ultramicropore region.
electron rich aromatic rings play a decisive role in methane
adsorption even in the absence of active metal sites. They
confirm values found in other porous aromatic frameworks
7
3
-1
Table 2. Amount of methane adsorbed Q (cm STP g ) by
and are attributed to multiple CH -  interactions of CH with
4
23
porous polymers at 100 bar and 180 bar.
aromatic rings, densely populated in this family of polymer
frameworks.
Sample
Name
PAF1
Q (100 bar )
(cm STP g )
643
Q (180 bar)
(cm STP g )
916
3
-1
3
-1
Volumetric CH adsorption isotherms
4
There is also a general desire to store or transport the largest
amount of gas per tank available volume. Obviously, medium-
high pressures help increase storage, but there has been little
experimental testing of the practical gain obtained with an
efficient absorbent at high pressures, nor of the appropriate
gas compression needed to obtain the maximum gain for the
various materials. These desiderata are valid for any to-be-
stored or transported gas, from the automotive area to CNG.
The volumetric uptake allowed us to compare the results
directly with compressed natural gas technology for naval
transportation where volume, and not weight, is the critical
issue.
PAFPORP
CBZ1
CBZ2
TRIP
PORP
RUB
SPBF
HPSiSi
STIL
CBZCH2
HPPh
HPSiO2
443
318
328
300
280
247
240
240
230
228
189
175
n.d.
436
452
400
n.d.
290
289
332
322
167
242
230
The density of porous polymers plays a key role in the
determination of volumetric gas uptake. Since the porous
polymers presented here do not show crystalline order, a
valuable method to determine the specific volume occupied
by the material is based on pore volume (known by N2
adsorption at 77K), combined with direct density
measurement of the framework walls by He picnometry.
The benchmark of these materials PAF1 reaches the value of
3
9
16 cm STP/g of adsorbed CH at 180 bar (Figure 5b), owing
4
2
to its high surface area (4800 m /g) and pore volume (2.7
cm /g). The extremely high methane uptake, corresponding to
3
6
5% by weight, represents one of the top values in gravimetric
absorptive materials at room temperature. The comparison
with HKUST-1 allowed us to show that HKUST-1 saturates at
Indeed, the CH uptake values per matrix volume (Figure 6a)
4
reveal a behaviour that, in most cases, attenuates the
performance differences previously observed.
30-40 bar: at such pressure it shows comparable
performances, but much more absolute amount of methane
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Most of MOFs exhibit a large absorption capacity up to about
0 bar, after which saturation occurs. Instead, the CH
The total volumetric uptake of PAF1 measured at 110 bar is of
3
3
8
4
198 cm STP/cm and is comparable with the values measured
for MOF-5 and MOF-210, although the latter exhibits a higher
isotherms of the studied porous polymers reveal that on
increasing the pressure to higher values than 100 bar there is
a continuous gain in the absorbed amounts. This feature
results in a high total volumetric uptake capacity, due to the
remarkable contribution of mesoporosity at pressures as high
as 180 bar. Furthermore, in such porous polymers, the slope
2
4
surface area (Figure 6a). At higher pressure (180 bar) the CH
4
3
uptake reaches the notable value of 266 cm STP/g. On
comparing the total volumetric uptake of CH in the presence
4
of PAF1 there is, with respect to pure compressed methane, a
maximum gain of an extra 110% at 70 bar. Collectively, most
compounds of the family showed considerable volumetric
uptake, although less capacitive, than PAF1, consistently with
their density, which is a critical parameter for volumetric
uptake (Figure 6b).
The porous aromatic polymers are shape-persistent networks
as shown by their high mechanical stability. This was
demonstrated experimentally by mechanical compression, at
a pressure of 9000 psi (620 bar), of the said porous materials
of CH adsorption isotherms is moderate at low pressure,
4
which is a great advantage for a high working capacity, e.g. the
amount of deliverable methane taken into account the
practical discharge pressure, which can be of several
atmospheres to feed the pipelines.
and the registration of the CH isotherms after release of
4
mechanical compression. The reduction of storage capacity
was less than 8%, consistent with the reduction of pore
capacity measured by N adsorption isotherm at 77K (Figure
2
S33). This result demonstrates the scare tendency to close-
packing, owing to the intrinsic shape factor of the monomeric
units that, had they been able to close-pack, would have
resulted in reduced efficiency. The bridges realized to link the
monomers through C-C or C-CH -C covalent bond contributed
2
to the creation of cross-linked polymeric frameworks, that
preserve their porosity after releasing compression.
The present paper is the first report on the experimental
screening of CH uptake in the wide pressure range up to 180
4
bar in investigating the behaviour of this class of materials.
Indeed, the absorption properties of these porous polymers,
together with their thermal stability up to 450-500°C, their
chemical inertia, especially towards acidic components and
water, their scalable synthesis and low-cost catalysts, make
these materials highly competitive for gas storage.
CO
The porous materials under investigation exhibit a high CO
capacity, as shown by the CO isotherms at room temperature
2
adsorption isotherms
2
2
and up to 10 bar (Figure 7a). The CO
2
uptake values of 183,
3
1
74, 162 and 150 cm STP/g are found for the porous
polymers CBZ1, CBZ2, TRIP and PORP, respectively,
3
overcoming the performances of zeolite 13X (150 cm /g), ZIF-
3
8
(78.4 cm /g) and HKUST-1 under the same conditions and
3
9,21,25
active carbons (46 cm /g a 22 bar).
Within the present
series of aromatic compounds, the stored amount is
substantially proportional to the surface area and pore
volume. At 10 bar, the slope of the isotherms is still positive,
indicating that the porous materials are not yet saturated by
4
Figure 6. CH adsorption isotherms of a) PAF1, PAFPORP and
PORP and b) porous polymers at room temperature and up to 180
bar. The value of total absorption volume over volume is reported
versus pressure. The dashed lines represent the difference
between the total adsorption value and the pure compressed
CO . PAF1 and its copolymer with porphyrin are far more
2
efficient than the other matrices (Figure 7b). Actually, PAF1
shows a benchmark adsorption value: under the relatively
mild conditions of room temperature and 10 bar it reaches
4
CH .
8
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5
24 mg/g (11.9 mmol/g), such high value outperforms the
the majority of the samples, while a dramatic gain is achieved
at 273K and 10 bar by PAF1 (Figure 7b and c). The high CO
best representatives of MOFs (MOF-177, MOF-205 and MOF-
5
2
21,26
), and is comparable to PPN-4.
adsorption value of 1672 mg/g (38 mmol/g) obtained by PAF1,
under the said pressure/temperature conditions, is far higher
26
than the values of activated carbon AC-Maxsorb. At 195K
and 1 bar PAF1 absorbs 2596 mg/g of CO
2
, equal to 1322
3
cm /g and 59 mmol/g, that is far higher than the best recently
26
proposed CO absorbent materials.
2
The isosteric heat of absorption at low coverage, determined
by the Clausius-Clapeyron equation, is quite high, ranging
from 24.8 to 29.6 kJ/mol. Interestingly, the CBZCH2 sample,
obtained by Friedel Crafts alkylation, exhibits a higher heat of
adsorption of 29.0 kJ/mol, than that of compound CBZ2,
which was derived by condensation of the same monomer
with Yamamoto reaction (25.2 kJ/mol). This was due to the
27
relevant presence of small microporosity in CBZCH2. The CO
interaction with the surfaces could have been augmented by
oxygen–containing pendant groups, such as CH -OH and CH
O-CH groups originated by the FDA reagent and revealed by
MAS NMR.
2
2
2
-
3
Consistently, the TRIP sample, prepared by Friedel Crafts
alkylation reaction and containing the highest number of
pendant groups shows the highest isosteric heat of adsorption
(29.6 kJ/mol). A value of about 30 kJ/mol is considered to be
an optimal balance between an effective uptake and a
28
convenient release by a porous framework. The covalent
organic polymers do not contain bonds susceptible to
hydrolysis. Indeed, polymer stability towards water, water
being a frequent contaminant of CO
advantage over most MOFs, which are water sensitive.
The porous polymers are poor absorbers of N , compared to
CO absorption at the same temperature (see Fig. 7b and c at
98K and 273K, respectively). Indeed, the adsorption
selectivity in favor of CO with respect to other components is
2
streams, is a general
2
2
2
2
influenced by several factors including the geometry of the
pores (size and shape) and energy interaction between the
gases and pore walls. The porous materials exhibit excellent
CO
Adsorbed Solution Theory IAST) ranging from 15 to 25 at room
temperature (starting from a 15:85 CO :N mixture), and the
2
/N
2
selectivity (S) at low pressure (estimated by Ideal
2
2
selectivity values are weakly dependent on pressure. Such
values are higher than those reported for activated carbon
BPL AC (S=20-26 at 1 bar), zeolite 13X (S=18 at 1bar and S=3
25,29
at 10 bar) and ZIF-8 (S=7.6-8.4 at 1 bar),
opening
perspectives for applications to post-combustion treatment of
industrial polluting emissions.
Fig. 7 a) CO
2 2
and N adsorption isotherms (filled and open labels,
respectively) of the porous polymers at 298 K. b) Absolute
CONCLUSIONS
adsorption at 10 bar of CO
and 273K (c).
2
(red) and N
2
(light blue) at 298K (b)
The experimental screening comprises gravimetric and
volumetric gas uptake by the frameworks obtained by 3D
polymerization or condensation of various monomers with
increasing number of phenyl, phenylene, carbazole rings and
acenes in a unique unitary frame. Indeed, a large variety of
Moreover, reducing the temperature by only 25K, from
ambient temperature to 273K, uptake increases by a 50% for
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6
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Journal of Materials Chemistry A
Page 12 of 12
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DOI: 10.1039/C7TA00934H
Table of Contents
Porous 3D polymers, fabricated by multidentate
monomers, adsorb efficiently CO
bar.
2 4
and CH up to 180