A highly rigid and gas selective methanopentacene-based polymer of intrinsic microporosity derived from Tr?ger's base polymerization

Article abstract of DOI:10.1039/c8ta00509e

Polymers of intrinsic microporosity (PIMs) have been identified as potential next generation membrane materials for the separation of gas mixtures of industrial and environmental relevance. Based on the exceptionally rigid methanopentacene (MP) structural unit, a Polymer of Intrinsic Microporosity (PIM-MP-TB) was designed to demonstrate high selectivity for gas separations. PIM-MP-TB was prepared using a polymerisation reaction involving the formation of Tr?ger's base linking groups and demonstrated an apparent BET surface area of 743 m² g^-1 as a powder. The microporosity of PIM-MP-TB was also characterized by chain packing simulations. PIM-MP-TB proved soluble in chlorinated solvents and was cast as a robust, free-standing film suitable for gas permeation measurements. Despite lower gas permeability as compared to previously reported PIMs, high selectivities for industrially relevant gas pairs were obtained, surpassing the 2008 Robeson upper bound for H₂/CH₄ and O₂/N₂, (e.g., PO₂ = 999 Barrer; αO₂/N₂ = 5.0) and demonstrating a clear link between polymer rigidity and selectivity. Upon aging, the permeability data move parallel to the Robeson upper bounds with a decrease of permeability, compensated by a related increase in selectivity. Mixed gas permeation measurement for CO₂/CH₄ and CO₂/N₂ mixtures confirmed the excellent selectivity of PIM-MP-TB for potentially relevant separations such as biogas upgrading and CO₂ capture from flue gas. Importantly, unlike other high performing PIMs, PIM-MP-TB is prepared in four simple steps from a cheap starting material.

Full text of DOI:10.1039/c8ta00509e

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Materials Chemistry A
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A highly rigid and gas selective methanopentacene-
based polymer of intrinsic microporosity derived
Cite this: DOI: 10.1039/c8ta00509e
from Troger's base polymerization†
¨
Rhodri Williams, Luke. A. Burt, Elisa Esposito, Johannes C. Jansen, *b
a
a
b
b
b
c
Mariolino Carta *d
Elena Tocci,
Carmen Rizzuto, Marek Lanc,
ˇ
a
and Neil. B. McKeown
*
Polymers of intrinsic microporosity (PIMs) have been identified as potential next generation membrane
materials for the separation of gas mixtures of industrial and environmental relevance. Based on the
exceptionally rigid methanopentacene (MP) structural unit, a Polymer of Intrinsic Microporosity (PIM-MP-
TB) was designed to demonstrate high selectivity for gas separations. PIM-MP-TB was prepared using
a polymerisation reaction involving the formation of Tr o¨ ger's base linking groups and demonstrated an
2
ꢀ1
apparent BET surface area of 743 m
g
as a powder. The microporosity of PIM-MP-TB was also
characterized by chain packing simulations. PIM-MP-TB proved soluble in chlorinated solvents and was
cast as a robust, free-standing film suitable for gas permeation measurements. Despite lower gas
permeability as compared to previously reported PIMs, high selectivities for industrially relevant gas pairs
were obtained, surpassing the 2008 Robeson upper bound for H
2
/CH
4
and O
2
/N
2
, (e.g., PO2 ¼ 999
Barrer; aO /N ¼ 5.0) and demonstrating a clear link between polymer rigidity and selectivity. Upon aging,
2
2
the permeability data move parallel to the Robeson upper bounds with a decrease of permeability,
compensated by a related increase in selectivity. Mixed gas permeation measurement for CO /CH and
CO /N mixtures confirmed the excellent selectivity of PIM-MP-TB for potentially relevant separations
such as biogas upgrading and CO capture from flue gas. Importantly, unlike other high performing
PIMs, PIM-MP-TB is prepared in four simple steps from a cheap starting material.
Received 16th January 2018
Accepted 27th February 2018
2
4
2
2
DOI: 10.1039/c8ta00509e
2
rsc.li/materials-a
permeability, while the lack of conformational freedom and
rigidity, due to their fused-ring structures, ensures good
1
. Introduction
Since Polymers of Intrinsic Microporosity (PIMs) were rst re- permselectivity. The design concept of PIMs has produced
1
ported in 2004, a range of potential applications have been polymers whose gas permeability data have redened the
2
–5
6
17–20
identied including catalysis, chiral separations, and as the Robeson upper bounds,
which quantify the state-of-the-art
In particular, trade-off between the desirable properties of permeability and
PIMs with the ability to form exible, robust, self-standing lms selectivity. Hence, PIMs have potential for various industrial gas
7,8
9,10
active material in sensors or ionic diodes.
11
have potential as membrane materials for anionic transport,
the separation of gases, vapours or for nanoltration.
exceptional potential for gas separation
free volume generated from the inability of their contorted capture (CO
separation applications such as the recovery of hydrogen (e.g.,
/N ), oxygen- or nitrogen-enrichment of air (O /N ), bio-
is based on the high methane and natural gas purication (CO
1
2
13,14
Their
H
2
2
2
2
15,16
2
/CH
4
1,22
) and carbon
have demon-
2
2
2
/N ). Previously, we and others
polymer chains to pack efficiently, which provides high strated the benet of using bridged bicyclic units in place of the
less rigid spirobisindane and benzodioxin structures of
23
conventional PIMs (e.g. PIM-1). For example, PIM-EA-TB
a
EaStCHEM, School of Chemistry, University of Edinburgh, Joseph Black Building,
23
(Fig. 1a), which is composed of ethanoanthracene (EA) struc-
David Brewster Road, Edinburgh, Scotland EH93FJ, UK. E-mail: neil.mckeown@ed.
tural units linked by Tr ¨o ger's base (TB i.e. methanodiazocene)
Institute on Membrane Technology, ITM-CNR, Via P. Bucci 17/C, 87036 Rende (CS), demonstrates exceptional selectivity and subsequent work
ac.uk
b
Italy. E-mail: johannescarolus.jansen@cnr.it
showed that the replacement of EA with triptycene to give PIM-
c
24
25
Department of Physical Chemistry, University of Chemistry and Technology, Prague,
Trip-TB (Fig. 1b) or benzotriptycene to give PIM-BTrip-TB
Fig. 1c) improves further both gas selectivity and permeability.
Evaluation of a range of potential structural building units
Technick a´ 5, 166 28 Prague 6, Czech Republic
(
d
Department of Chemistry, College of Science, Swansea University, Grove Building,
Singleton Park, Swansea, SA2 8PP, UK. E-mail: mariolino.carta@swansea.ac.uk
for PIMs, assisted by molecular modeling, suggested two
adjustments that could be made to the bridged bicyclic
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ta00509e
1
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Fig. 1 PIMs based on TB linkages between (a) ethanoanthracene (b)
23–25
triptycene or (c) benzotriptycene structural units.
Scheme
1
Synthetic route to PIM-MP-TB (DMM ¼ dimethoxy-
methane; TFA ¼ trifluoroacetic acid).
TB is that shown in Scheme 1, albeit with no control over
regiochemistry due to the mixture of two isomers of the
monomer and the relative orientations of methylene bridges of
the MP building unit and TB linking group (Fig. 3).
Isothermal nitrogen adsorption conrmed the polymer to be
Fig. 2 (a) The structure of methanopentacene (MP); (b) XRD solid state signicantly microporous with a BET surface area (SABET) of
27
2
ꢀ1
crystal structure of MP.
7
43 m g . This is somewhat lower than the values obtained for
2
ꢀ1 23
24–26,33
PIM-EA-TB (1030 m g
)
and other TB PIMs.
Thermog-
ravimetric analysis (Fig. ESI3†) shows that the polymer is stable
ꢁ
structural unit that might enhance further the rigidity of a TB- up to 400 C (<3% loss), much higher than the EA–TB analogue,
23
based PIM. Firstly, the use of a shorter methylene bridge and which undergoes a reverse Diels–Alder reaction to release an
ꢁ
secondly, the incorporation of a greater proportion of aromatic ethylene fragment at around 260 C.
26
rings within the polymer chain. The readily prepared 6,13-
27
dihydro-6,13-methanopentacene (MP) hydrocarbon satised
both of these design criteria (Fig. 2).
2
.2. Simulation of molecular structure and chain packing
28
To assess the relative exibility of the PIM-MP-TB, the
mechanical deformation of the MP structural unit was
2
4
23
2
. Results and discussion
compared with that of triptycene, ethanoanthracene and TB.
Fig. 4a shows the comparison of the energy cost as the dihedral
angles in these units angle deviate from their most stable
conguration, corresponding to the energy minimum as
calculated using the Spartan 10.0 modelling package (Fig. 4b).
The energy proles for the framework distortion about dihedral
angle 1 (Fig. 4a) are very similar for each structural unit (Fig. 4c).
In contrast, those for dihedral angles 2 and 3 are sharper for MP
than for the other units, conrming its greater rigidity (Table
ESI1†). Simulation of chain packing for PIM-MP-TB was per-
formed as described previously for PIM-EA-TB (ESI Section 6†)
2.1. Polymer synthesis and characterization
Unsubstituted methanopentacene is easily prepared by the
double Diels–Alder addition of the diene, dibromoquinodi-
28
methane, to norbornadiene (Scheme 1). The required diene
precursor to dinitromethanopentacene, 1,2-
bis(dibromomethyl)-4-nitrobenzene (2), was synthesised by
a convenient literature procedure in which a dilute solution of
bromine is added slowly to the cheaply available 4-nitro-o-
xylene (1) in chlorobenzene, whilst irradiating the reaction with
a tungsten lamp. This method avoids the use of highly toxic
solvents such as carbon tetrachloride, and the high atom-
inefficiency associated with brominating agents such as N-
29
ꢀ3
and optimized to a polymer density of 1.095 g cm (Fig. 5a).
This is only a small deviation from the experimental density of
.13 g cm for a freshly prepared lm is 3.7% and hence is in
reasonable agreement. The experimental density is higher than
that reported for PIM-1 (1.092 g cm
1.08 g cm ). Free volume as determined by Bondi's
ꢀ3
1
30,31
bromosuccinimide, reported in other procedures.
Treat-
ment of 2 with sodium iodide eliminates bromine to produce
the reactive intermediate dibromoquinodimethane, which
combines with the commercial norbornadiene 3, to give an
ꢀ3
34
)
and of PIM-EA-TB
ꢀ3
35
(
isomeric
mixture
of
2,9(10)-dinitro-6,13-dihydro-6,13-
methanopentacene (4) in 50% yield. This was reduced in the
presence of hydrazine and RANEY® nickel, to obtain the
required 2,9(10)-diamino-6,13-dihydro-6,13-methanopentacene
monomer (5) as a mixture of isomers. The standard TB poly-
23
merisation procedure was followed to afford PIM-MP-TB in
0% yield aer purication by reprecipitation, which is similar
5
to other TB-based PIMs.
It is well established that TB formation using 2-amino-
naphthalene takes place exclusively at the 1-position of the
naphthyl ring so that the expected ideal structure of PIM-MP- methylene bridges of the MP building unit and TB linking group.
Fig. 3 The regiochemistry within the repeat unit of PIM-MP-TB due to
the isomeric mixture of the monomer and the relative orientations of
32
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Fig. 4 (a) The defined dihedral angles in methanopentacene (MP), triptycene (Trip), ethanoanthracene (EA) and TB units; (b) potential energy
surfaces of dihedral angles (DA) 1, 2 and 3 for MP, EA, triptycene and TB. (c) Dihedral angle distribution for the MP (red) and TB (black) units derived
from packing simulations model.
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crystal structure in Fig. 2, but the wide distribution shows that
many monomeric units are either stretched or squeezed into
a conguration far from their thermodynamic equilibrium.
Similar deformations of structural units within packing models
of PIMs have been observed previously for PIM-EA-TB and
34,37,38
PIM-1.
Fig. 5a shows a 3D representation of the free volume
elements and their interconnectivity. Modelling of the pore size
distribution by the method proposed by Hofmann and Heu-
39
chel shows the presence of a majority of pores with an
˚
equivalent sphere radius in the range of 1–5 A (i.e. pore diam-
˚
eter ¼ 2–10 A) (Fig. 6). The free volume element size distribution
was quantied by the Rmax and Vconnect approach, setting the
probe diameter to the size of an o-positronium particle (r ¼
˚
˚
˚
1
.1 A), carbon dioxide (r ¼ 1.65 A), oxygen (r ¼ 1.73 A) and
˚
˚
nitrogen (r ¼ 1.82 A), and using a grid spacing of 0.5 A. The
maximum equivalent sphere radius in PIM-MP-TB is approxi-
˚
mately 7 A, which is smaller than for other other PIMs, such as
PIM-1 (16.5–17 A˚ ) and PIM-EA-TB (15.5 A). This suggests that
34
˚
35
Fig. 5 (a) Molecular model of the polymer structure with a calculated
ꢀ3
density of 1.095 g cm . The free volume elements are indicated as
blue bubbly shapes. Length of the box edges is about 32 A˚ . (b, c)
Structure and “bite” angle distribution of the MP unit in the optimized
polymer chains.
pores in PIM-MP-TB are both smaller and less interconnected
relative to those in PIM-1 or PIM-EA-TB but similar to, for
example, Hyon®AD.
The pore size distribution was calculated via NLDFT and
H–K analysis of the CO uptake at 273 K (Fig. ESI1†). Fig. 6
40
2
shows the pore size distribution of PIM-MP-TB. Qualitatively,
PIM-MP-TB has a lower total pore volume and fewer pores over
method was calculated to be 25.8 ꢂ 0.5% which is slightly
35
smaller than that of PIM-EA-TB (27.6%) and of PIM-1
˚
35
36
5 A, compared to PIM-EA-TB, which correlates with its lower
BET surface area and pore volume measured by gas adsorption.
(
28.8%). The calculated fractional free volume conrms the
poor polymer packing that is typical for all PIMs. The dihedral
angle distribution for the MP structural unit observed within
the packing simulation correlate well with the energy potential
of deformation calculated by molecular dynamics (Fig. 5). The
2.3. Pure gas transport
Before permeability measurements, the lms were soaked in
“bite” angle of the MP units in the optimized chains of PIM-MP-
methanol and dried to remove traces of residual casting solvent
TB (Fig. 5b), shows a very wide distribution (Fig. 5c). The
average angle is close to the value found for the XRD solid state
37,38,41
and to reverse the effect of physical aging.
The gas
permeability, diffusivity and solubility coefficients, with the
respective ideal selectivity, for the methanol-treated, aged and
thermally treated lms of PIM-MP-TB are given in Table 1. The
precise sample history is given in Table ESI4.† The data are
compared with those of the archetypal PIM-1 and PIM-EA-TB.
The order of permeability for the PIM-MP-TB is H
He > O > CH > N
2 4 2
2
> CO
, similar to that of PIM-EA-TB but different
to that of more permeable PIMs (PIM-Trip-TB and PIM-BTrip-
2
>
12
24,25
TB),
2 2 2
in which CO has a higher permeability than H , and O
has a higher permeability than He. This behavior indicates
greater size selectivity for PIM-MP-TB as demonstrated by very
high diffusivity selectivity (D /D ) with respect to PIM-1, PIM-EA-
x
y
TB and PIM-Trip-TB (Table 1). The high size selectivity is best
seen from the very steep correlation between the diffusion
coefficient and the effective diameter of the gas (Fig. 8). The
excellent performance of PIM-MP-TB can be attributed to the
superior rigidity of the MP bridged bicyclic unit, as discussed
above. The high permselectivity, compared with that of many
other PIMs, combined with moderately high permeabilities,
places it close to the upper bound for the CO /CH and CO /N
2
4
2
2
gas pair (Fig. 7a and c) and well above the 2008 upper bound for
gas pairs involving H /CH ; a ¼ 15.3
(PH2 ¼ 4050 Barrer) with H
(Fig. 7b) and H /N
(a ¼ 20.3). Furthermore, PIM-MP-TB
Fig. 6 Averaged size distribution functions using O-positronium (r ¼
1
1
.1 A˚ ), carbon dioxide (r ¼ 1.65 A˚ ), oxygen (r ¼ 1.73 A˚ ) and nitrogen (r ¼
2
2
4
39
.82 A˚ ) as probes using the Hofmann–Heuchel method.
2
2
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ꢁ
Table 1 Gas permeabilities P
x
, diffusion coefficients D
x
, solubility coefficients S
x
, and ideal selectivities a (P
x
y
/P ), a (D
x
/D
y
) and a (S
x
/S
y
) at 25 C for
a methanol-treated film of PIM-MP-TB, PIM-EA-TB and PIM-1 with comparable thickness (94 mm, 95 mm and 93 mm, respectively) and for the
aged and thermally treated samples of PIM-MP-TB
Permeability, diffusivity and solubility
CO CH
Selectivity a (P
x
/P
y
) a (D
x
/D
y
) a (S
x
/S
y
)
a
Polymer (state ) Transport parameter
N
2
O
2
2
4
H
2
He
H
2
/N /CH
2
H
2
4
CO
2
/N
2
O
2
/N
2
2 4
CO /CH
PIM-MP-TB (a)
P
D
x
[Barrer]
200
18.3 106
8.21 7.08
125
21.4 134
13.3 88.4
380
40.5 177
7.04 6.91
773
165
3.52 3.54
999
3500
26
101
2340
633
378
264
6
31
158
26
14.7
572
12
4050 1310 20.3
2870 4640 157
15.3
478
0.03
17.7
43
55.3
12.8
419
0.03
3.68
82
17.5
1.42
12.3
18.7
29.6
28.4
13.4
1.01
13.2
16.6
1.21
13.6
5.0
5.8
13.3
4.33
3.26
14.8
24.4
25.7
8.92
3.42
2.61
10
ꢀ12
2 ꢀ1
x
[10
m s
]
3
ꢀ3
ꢀ1
S
P
P
P
P
x
[cm (STP) cm bar
[Barrer]
]
1.06
2790 956
1120 492
0.21
0.13
22.3
52.3
61
0.86
5.18
6.26
6.65
4.29
4.37
0.98
2.77
2.74
1
PIM-MP-TB (b)
PIM-MP-TB (c)
PIM-MP-TB (d)
PIM-EA-TB (a)
x
648
x
x
x
[Barrer]
[Barrer]
[Barrer]
812
368
1630 5100
7310 2720 19.2
5000 6000 123
ꢀ
3
12
2 ꢀ1
D
x
[10
m s
]
41
93.3
ꢀ3
ꢀ1
ꢀ1
S
x
[cm (STP) cm bar
[Barrer]
]
]
35.75 1.10
0.34
0.16
PIM-1 (a)
P
D
x
2140 12 800 1280
452
4710 1830 6.1
5760 7120 35
ꢀ
3
12
2 ꢀ1
x
[10
m s
]
199
48
70
13.7
2.84
3.5
ꢀ3
S
x
[cm (STP) cm bar
0.61
0.19
0.17
0.04
a
ꢁ
State: (a) methanol treated; (b) day 1 aer thermal conditioning under vacuum at 140 C; (c) aer aging of 118 days and intermediate mixed gas
permeation; (d) aer 370 days of aging and intermediate mixed gas permeation. See ESI Table 4 for the complete sample history.
˚
demonstrate a high oxygen permeability (PO2 ¼ 999 Barrer) mechanism, as O
2
has a smaller kinetic diameter (3.46 A for O
2
˚
showing higher O
2
/N
2
selectivity (a ¼ 5.0) with respect to PIM- and 3.64 A for N
2
) whereas both gases have a similar solubility
EA-TB (a ¼ 4.3), which is close to the revised 2015 upper in glassy polymers. Fig. 7 also demonstrates the effect of phys-
19
bound (Fig. 7d). The high O
2 2
/N selectivity conrms that the ical aging of PIM-MP-TB aer different treatments. Upon
separation is strongly dependent on a molecular sieving ageing, the data points move almost parallel to the Robeson
2 4 2 4 2 2 2 2
Fig. 7 Robeson plots of PIM-MP-TB for the CO /CH , H /CH , CO /N and O /N gas pairs after methanol treatment ( ), aged for 1 ( ), 118 ( ),
ꢁ
3
70 ( ) days after the thermal treatment at 140 C under vacuum. Mixed gas data after 110 days aged ( ). The data for PIM-1 ( ) and recent TB-
ꢁ
PIMs like PIM-EA-TB ( ) and PIM-Trip-TB ( ) after MeOH are given for comparison. Feed pressure 1 bar and temperature 25 C. The total aging
time in days for each point is indicated as “xxx d”.
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decrease as a function of the pressure and the selectivity
remains nearly constant. Thus, PIM-MP-TB maintains excellent
gas transport properties under mixed gas conditions for the
CO /N gas pair, close to the ideal selectivity (a ¼ 29.6).
2
2
3
. Conclusions
The MP structural unit proved useful for making a high per-
forming PIM. PIM-MP-TB demonstrated high selectivities for
2 2 2 2 2 4
several important gas pairs such as O /N , H /N and H /CH ,
Fig.
8 Diffusion coefficients and corresponding selectivities vs.
well above the 2008 Robeson upper bounds and approach
commercially useful separation factors for the aged lm. The
CO /N ideal selectivity (a ¼ 17.5) is maintained under mixed
nitrogen as function of the squared effective diameter of the pene-
trant gases for PIM-MP-TB (C), PIM-1 (B), PIM-EA-TB (>) and PIM-
Trip-TB (ꢃ).
42
25,35,37
ꢁ
2
2
Feed pressure 1 bar and temperature 25 C.
2 2
gas conditions (10% CO in N ), up to 6 bar, which offers good
potential for the application of PIM-MP-TB as a high ux rst
pass membrane in a two membrane system for CO capture
2
from ue gas. These results provide further evidence that
molecular rigidity is a major determinant of intrinsic micro-
porosity and high size selectivity. The molecular sieving
performance, combined with the pore size distribution,
suggests that an optimum distribution of small uniform
micropores is achieved by maximising chain rigidity and
internal free volume of the constituent monomer units.
Molecular modelling conrms the high chain rigidity and
shows that PIM-MP-TB has fewer interconnected pores than
other common PIMs, which explains its relatively low perme-
ability in combination with a strongly size sieving nature. Most
Fig. 9 (a) Pressure dependence of the CO
with the corresponding CO /CH selectivity, using the binary mixture
CO /CH (52.1 : 47.9 vol%); (b) pressure dependence of the CO and
permeabilities with the corresponding CO /N
selectivity, using the importantly, gas permeability data of PIM-MP-TB approach
2 4
and CH permeabilities
2
4
2
4
2
N
2
2
2
2
2
binary mixture CO
Measurement temperature 25 C.
2
/N
2
(10 : 90 vol%) 110 days aged PIM-MP-TB.
those obtained from the triptycene-based PIMs KAUST-PI-1,
ꢁ
21
24
TPIM-1 and PIM-Trip-TB, which have been used to dene
the 2015 upper bounds for the H /CH , H /N and O /N gas
2
4
2
2
2
2
19
pairs. However, in contrast to these high performing PIMs, the
synthesis of PIM-MP-TB is achieved in only four simple steps
from a cheap starting material.
upper bound, in the direction of higher selectivity and lower
permeability compared to the methanol treated membrane. The
ꢁ
thermal conditioning at 140 C for 24 h just aer methanol
treatment (1 day aged), produces a similar effect suggesting that
accelerated aging was achieved. Aer one year of ageing, with
intermediate mixed gas permeation measurements, the
performance remains still close to the 2008 Robeson upper
Conflicts of interest
There are no conicts to declare.
bound for the CO
upper bound for the H
2
/CH
4
and CO
/CH , H
2
/N
/N
2
separation, and above the
Acknowledgements
2
4
2
2
and O
2
/N
2
gas pairs.
The work leading to these results has received funding from the
European Union's Seventh Framework Program (FP7/2007–
2.4. Mixed gas permeation
4
2
013) under grant agreement no. 608490, project M CO
2
. Ales-
PIM-MP-TB also demonstrates promising permselectivity for sio Fuoco and Marcello Monteleone are gratefully acknowl-
mixed gas separation. Mixed gas permeability measurements edged for their help in some of the gas permeation experiments.
were carried out using a sample aged for 110 days. Two different
binary gas mixtures were used: CO
CO /N (10/90 mol%), simulating biogas with high CO
and CO
the pressure was increased stepwise from 1 to 6 bar (a) at 25 C.
2 4 2
Fig. 9 shows that for the CO /CH mixture the CO permeability
2 4
/CH (52.1/47.9 mol%) and
Notes and references
2
2
2
content
2
poor ue gas, respectively. During the measurement,
1 P. M. Budd, B. S. Ghanem, S. Makhseed, N. B. McKeown,
K. J. Msayib and C. E. Tattershall, Chem. Commun., 2004,
230–231.
ꢁ
is virtually independent from the pressure, whereas the CH
4
4
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permeability slightly increases and, subsequently, the CO /CH
2
selectivity decreases with pressure. This suggests that the CO2
induces matrix swelling, thus, facilitating CH permeation. For
the CO /N mixture, both CO and N permeability slightly
4
2
2
2
2
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