The synthesis, chain-packing simulation and long-term gas permeability of highly selective spirobifluorene-based polymers of intrinsic microporosity

Article abstract of DOI:10.1039/c8ta02601g

Membranes composed of Polymers of Intrinsic Microporosity (SBF-PIMs) have potential for commercial gas separation. Here we report a combined simulation and experimental study to investigate the effect on polymer microporosity and gas permeability by placing simple substituents such as methyl, t-butyl and fused benzo groups onto PIMs derived from spirobifluorene (PIM-SBFs). It is shown that methyl or t-butyl substituents both cause a large increase in gas permeabilities with four methyl groups enhancing the concentration of ultramicropores (<0.7 nm), which contribute to selective gas transport. The t-butyl substituents lower selectivity by generating a greater concentration of larger, less selective, micropores (>1.0 nm). Long-term ageing studies (>3.5 years) demonstrate the potential of PIM-SBFs as high-performance membrane materials for gas separations. In particular, the data for the PIM derived from tetramethyl substituted SBF reaches the proposed 2015 Robeson upper bound for O₂/N₂ and, hence, hold promise for the oxygen or nitrogen enrichment of air. Mixed gas permeation measurements for CO₂/CH₄ of the aged PIM-SBFs also demonstrate their potential for natural gas or biogas upgrading.

Full text of DOI:10.1039/c8ta02601g

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Materials Chemistry A
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The synthesis, chain-packing simulation and
long-term gas permeability of highly selective
spirobifluorene-based polymers of intrinsic
microporosity†
Cite this: DOI: 10.1039/c8ta02601g
a
b
C. Grazia Bezzu,
Johannes C. Jansen,
Alessio Fuoco,
Mariolino Carta,
Maria-Chiara Ferrari,c
d
d
d
Marcello Monteleone,
Elisa Esposito,
d
f
e
Kyle Hart,
and Neil B. McKeown
T. P. Liyana-Arachchi,e Coray M. Colina
a
*
Membranes composed of Polymers of Intrinsic Microporosity (SBF-PIMs) have potential for commercial gas
separation. Here we report a combined simulation and experimental study to investigate the effect on
polymer microporosity and gas permeability by placing simple substituents such as methyl, t-butyl and
fused benzo groups onto PIMs derived from spirobifluorene (PIM-SBFs). It is shown that methyl or
t-butyl substituents both cause a large increase in gas permeabilities with four methyl groups enhancing
the concentration of ultramicropores (<0.7 nm), which contribute to selective gas transport. The t-butyl
substituents lower selectivity by generating a greater concentration of larger, less selective, micropores
(>1.0 nm). Long-term ageing studies (>3.5 years) demonstrate the potential of PIM-SBFs as high-
performance membrane materials for gas separations. In particular, the data for the PIM derived from
tetramethyl substituted SBF reaches the proposed 2015 Robeson upper bound for O2/N2 and, hence,
hold promise for the oxygen or nitrogen enrichment of air. Mixed gas permeation measurements for
CO2/CH4 of the aged PIM-SBFs also demonstrate their potential for natural gas or biogas upgrading.
Received 20th March 2018
Accepted 10th May 2018
DOI: 10.1039/c8ta02601g
rsc.li/materials-a
scale separations it is necessary to have very high ux (i.e. per-
meance or throughput) to minimise the required size of the
Introduction
membrane system. Unfortunately, known highly permeable
polymers possess poor selectivity and conversely selective
polymers, such as those used presently for commercial appli-
cations, possess low permeability and are hence uncompetitive
for large-scale gas separations with other technologies such as
cryogenic distillation or solvent extraction.7,8 The empirical
trade-off between the polymer properties of permeability (Px)
and ideal selectivity (Px/Py) for a given gas pair (x and y) was
quantied by Robeson in 1991,9 and updated in 2008,10 using
the gas permeability data of a large number of polymers. For
each gas pair, Robeson identied the upper bound in plots of
log Px versus log Px/Py and the position of the gas permeability
data for a new polymer relative to the 1991 and 2008 upper
bounds is used as a universal performance indicator. Theoret-
ical analysis by Freeman suggests that the position of the
Robeson upper bound is a direct consequence of the size-
sieving nature (i.e. diffusivity selectivity) of the highly rigid
glassy polymers, which were used to dene it.11,12 Consequently,
it was suggested that more selective polymers would be ob-
tained by increasing polymer chain rigidity. In addition,
ensuring high free volume through increasing inter-chain
separation would increase gas diffusivity. Polymers of
Polymer membranes are used commercially for the efficient
separation of gases such as oxygen or nitrogen enrichment of
air (i.e. O2 from N2), natural gas and biogas upgrading (i.e.
predominantly CO2 from CH4) and for hydrogen recovery
during ammonia preparation (H2 from N2).1–4 Recently, their
potential for post-combustion carbon-capture, requiring the
separation of CO2, predominantly from N2 but on a massive
scale, has also been suggested.5,6 For membranes it is desirable
to have good selectivity for one gas over another and for large-
aEaStCHEM, School of Chemistry, University of Edinburgh, David Brewster Road,
Edinburgh, EH9 3FJ, UK. E-mail: neil.mckeown@ed.ac.uk
bDepartment of Chemistry, Swansea University, College of Science, Grove Building,
Singleton Park, Swansea, SA2 8PP, UK
cInstitute for Materials and Processes, School of Engineering, The University of
Edinburgh, Mayeld Road, Edinburgh EH9 3JL, UK
dInstitute on Membrane Technology, ITM-CNR, Via P. Bucci 17/C, 87036 Rende, CS,
Italy
eCenter for Macromolecular Science and Engineering, University of Florida, 318 Leigh
Hall, P. O. Box 117200, Gainesville, FL 32611-7200, USA
fDepartment of Materials Science and Engineering, The Pennsylvania State University,
University Park, PA 16802, USA
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ta02601g
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intrinsic microporosity, such as the archetypal PIM-1 (Fig. 1),
Following the original report on PIM-SBF-1, a macromolec-
follow this design concept by using a wholly fused-ring struc- ular packing simulation study predicted that adding four
ture to provide rigidity and spiro-centres to ensure a contorted methyl substituents to the SBF unit would result in greater
chain structure that cannot pack space efficiently.13,14 Gas intrinsic microporosity.33 Here we report a combined chain-
permeability data for PIMs generally lie over the 1991 upper packing simulation and experimental study to conrm this
bounds for most important gas pairs,15–17 with data for PIM-1 prediction and to investigate further the effect on microporosity
and PIM-7 being used by Robeson to position 2008 upper and gas permeability by placing simple substituents such as
bounds for several important gas pairs.10 In recent years methyl, t-butyl and fused benzo units onto the SBF unit of PIM-
permeability data for an increasing number of PIMs, particu- SBF. In addition, data from lms aged over prolonged periods
larly those which incorporate rigid bridged bicyclic units such of time (>3.5 years) are reported, which suggest that SBF-based
as ethanoanthracene,18–20 triptycene,21–25 methanopentacene26 PIMs have potential for gas separations involving enhanced
and Trogers base,18,21 have been shown to exceed the 2008 upper diffusivity selectivity via a molecular sieving effect.
¨
bounds due to their enhanced selectivity. Indeed, in 2015 Pin-
nau et al. proposed that the O2/N2, H2/N2 and H2/CH4 upper
bounds should be updated using permeability data from aged
lms of triptycene-based PIMs.27
Results and discussion
Synthesis
In 2012, we reported a preliminary account of a novel PIM in
A series of seven PIMs, designated PIM-SBF-1 to PIM-SBF-7, was
which the spirobisindane (SBI) unit of PIM-1 was replaced with
prepared by the aromatic nucleophilic substitution reaction
the spirobiuorene (SBF) unit (PIM-SBF-1; Fig. 1).28 PIM-SBF-1
between their respective 2,20,3,30-tetrahydroxy-9,9-spirobiuorene
demonstrated greater selectivity as compared to PIM-1,
precursors (1a–g) and 2,3,5,6-tetrauoroterephthalonitrile as
without loss of permeability, which placed the data above the
summarised in Scheme 1.13,28 The synthesis of the SBF monomers
2008 upper bound for the O2/N2, H2/N2, H2/CH4 and CO2/CH4
was based on simple adaptations of the well-established route to
gas pairs. The improved performance of PIM-SBF-1 compared to
spirobiuorenes,34 involving the addition of the 2-lithio-anion of
biphenyl to uorenone followed by acid-mediated cyclisation.
PIM-1 was attributed to the enhanced rigidity of the SBF unit29
over that of SBI as demonstrated by molecular dynamics
Hence, using a series of substituted 2-bromobiphenyl derivatives
modelling. Work by the group of Pinnau subsequently
demonstrated that polyimides of intrinsic microporosity (PIM-
PIs) incorporating the SBF building unit also have enhanced
performance for gas separation relative to SBI-based PIM-
PIs.30–32
(2a–d) as precursors, several 2,3-dimethoxyuorenones (4a–e)
were prepared via a three-step procedure involving treatment with
n-butyl lithium and addition of CO2 to give the respective
carboxylic acids (3a–d), followed by an acid mediated ring closure
(i.e. intramolecular acylation). The 2-lithio-anions derived from
the same 2-bromobiphenyl derivatives (2a–e) were combined with
uorenones 4a–e to give, aer acid treatment, a series of tetra-
methoxyspirobiuorenes (5a–g). Demethylation using boron tri-
bromide gave the SBF monomers 1a–g, which were used in the
polymerisation reactions immediately due to their tendency to
oxidise on standing.
With the exception of those containing fused benzo-
substituents (i.e. PIM-SBFs 6 and 7), the resulting PIM-SBFs
all proved soluble in chloroform, facilitating purication by
reprecipitation into a non-solvent (methanol) to remove oligo-
meric impurities. Gel Permeation Chromatography (GPC) of the
soluble PIM-SBFs 1–5 conrmed that high molecular mass
polymer was achieved in each case with a range of weight-
average molecular mass (Mw) of 75–110 ꢀ 103 g molꢁ1 relative
to polystyrene standards (Table 1). The ESI† gives full experi-
mental details and spectroscopic data for all precursors,
monomers and polymers.
Simulation of solid-state packing and prediction of porosity
Molecular dynamics (MD) simulations were conducted using
the LAMMPS soware package.35 PIM-SBF samples were con-
structed using the Polymatic simulated polymerization algo-
rithm36 and the compression/decompression scheme.37 The
PIM-SBF structure generation consists of three steps: (1)
random packing of respective PIM-SBF 100 repeat monomers
into a simulation box under periodic boundary conditions at
Fig. 1 The structures of the spirobisindane-based PIM-1 and the
spirobifluorene-based PIM-SBF series.
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Scheme 1 The synthesis of PIM-SBFs 1–7 (see Section 2 of the ESI† for details).
a low arbitrary density of 0.3–0.4 g cmꢁ3, (2) polymerization via apparent Brunauer–Emmett–Teller (BET) surface areas (SABET
)
Polymatic simulated polymerization algorithm and (3) within the range of 727–938 m2 gꢁ1 (Table 1), with the value for
compression/decompression of the polymerized sample, ‘21- PIM-SBF-2 being similar to that from PIMs possessing the
steps’ to generate a nal simulation box at relevant experi- highest SABET for solution processable polymers.18,24 It is
mental conditions.37 Details of the methodology of the chain notable that the BET value for PIM-SBF-2 calculated from the
packing simulations, structural analysis and gas adsorption simulation study is very similar to the apparent value measured
simulation are given in Section 1 of the ESI.†38
directly from N2 isotherms and supports the prediction that
methyl substituents will enhance intrinsic microporosity by
increasing the inter-chain distance.33 Both simulation and
experimental analysis of PIM-SBF-5 conrm that microporosity
is also enhanced by the use of t-butyl substituents. Overall, there
is good correlation between the predicted and experimentally
Gas adsorption and gas transport properties
The powdered form of all of the PIM-SBFs, obtained from
precipitation, adsorb a large amount of nitrogen (N2) at 77 K.
Analysis of the N2 adsorption isotherms (Fig. S5–10†) gives
Table 1 Molecular mass and gas adsorption properties of PIM-SBFs 1–7
CO2 uptaked
CO2 uptake modele
a
b
c
Mw
SABET
SABET model
VTotal
VM
PIM-SBF
(g molꢁ1
)
Mw/Mn
(m2 gꢁ1
)
(m2 gꢁ1
)
(ml gꢁ1
)
(ml gꢁ1
)
(mmol gꢁ1
)
(mmol gꢁ1
)
1
2
3
4
5
6
7
89 000
95 000
76 000
75 000
110 000
—
2.3
2.3
1.8
1.9
2.4
—
803
938
794
752
882
768
727
760
950
—
0.71
0.69
0.54
0.54
0.70
0.45
0.48
0.28
0.33
0.28
0.28
0.31
0.26
0.26
2.4
3.0
2.6
2.8
2.5
2.8
2.5
3.8
4.2
—
—
—
980
800
820
3.6
4.1
4.3
—
—
a
b
BET surface area calculated from N2 adsorption isotherm obtained at 77 K. Total pore volume estimated from N2 uptake at P/Po ¼ 0.98.
c
d
e
Micropore volume estimated from N2 uptake at P/Po ¼ 0.05. CO2 adsorption at 1 bar and 273 K. CO2 uptake calculated from packing model
appears to overestimate experimental uptake by a factor of 1.5 as reported for previous studies.
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of ꢂ100 mm thickness, suitable for gas permeability studies. The
pure gas permeabilities of PIM-SBFs 1–5, which had rst been
soaked in methanol and then dried in air at room temperature
for 24 h, are given in Table 2. Methanol treatment has been
shown previously to reverse the effects of physical ageing for
glassy ultra-permeable polymers and it also removes the last
residues of casting solvent.16,39,40 In addition, methanol treat-
ment allows a direct comparison between the gas permeabilities
of different polymers prior to ageing. The equivalent data for
a freshly methanol treated lm of PIM-1 of similar thickness,
measured under identical conditions to those of the PIM-SBFs,
are also provided in Table 2 for comparison.
For each methanol treated PIM-SBF the order of gas
permeabilities is CO2 > H2 > O2 > He > CH4 > N2, which is the
same as that for PIM-1. As noted previously,28 whilst the values
for gas permeabilities are broadly similar for PIM-1 and PIM-
SBF-1 for gases with smaller kinetic diameters (i.e. He ¼ 2.69;
Fig. 2 Pore size distribution (dV/dw) of PIM-SBF-1 (black line), PIM-
SBF-2 (red line) and PIM-SBF-5 (blue line) calculated from the chain
packing simulations plotted together with the DFT cumulative pore
volume (Vcum) of PIM-SBF-1 (black circles), PIM-SBF-2 (red circles) and
PIM-SBF-5 (blue circles) calculated from CO2 adsorption data using
DFT.
˚
H2 ¼ 2.8; CO2 ¼ 3.3; O2 ¼ 3.46 A), those for larger molecules (i.e.
˚
N2 ¼ 3.64; CH4 ¼ 3.87 A) are smaller (>30%) resulting in
a signicant enhancement of ideal selectivities. As noted
previously,28 this places the data for PIM-SBF-1 above the 2008
upper bound for the O2/N2, H2/N2, N2/CH4 and CO2/CH4 gas
pairs and can be directly ascribed to the greater rigidity of the
SBF units as compared to SBI units of PIM-1. It is evident from
the data for PIM-SBFs 2–5 that placing methyl or t-butyl
substituents onto the SBF unit causes a moderate to large
increase in gas permeabilities relative to those of both PIM-1
and PIM-SBF-1. This is consistent with the conclusion derived
from predictive packing simulations33 and experimental gas
adsorption, discussed above, that methyl groups help to
increase the inter-chain separation distance and generate
greater intrinsic microporosity. The order of increasing gas
permeability is PIM-SBF-1 (0 Me) < PIM-SBF-4 z PIM-SBF-3
(both 2 Me) < PIM-SBF-5 (6 Me) < PIM-SBF-2 (4 Me). The gas
permeabilities for PIM-SBF-2 and PIM-SBF-5 are signicantly
higher than for all reported PIMs with the exception of PIM-1,42
which is slightly less permeable than PIM-SBF-2 but more so
than PIM-SBF-5, and the recently reported PIM-TMN-Trip,24 the
latter deriving its exceptionally high permeability from its
determined BET surface areas for all PIM-SBFs (i.e. within
100 m2 gꢁ1), which is within the range of reported experimental
values of the BET surface area of PIM-1 quoted in the literature
(720–870 m2 gꢁ1) caused by the variation in N2 adsorption
isotherms at 77 K.38 Micropore size distributions calculated from
CO2 adsorption at 273 K for PIM-SBF-1, PIM-SBF-2 and PIM-SBF-5
(Fig. S11†) using DFT correlate well with those calculated from
the chain packing simulations (Fig. 2 and S12†).
Signicantly, both PIM-SBF-2 and PIM-SBF-5 possess greater
microporosity than PIM-SBF-1 but with a clear difference in
pore size distribution so that PIM-SBF-2 has a greater contri-
bution from ultramicropores (<0.7 nm) whereas PIM-SBF-5 has
greater pore volume from larger micropores (>1.0 nm).
Gas permeability
Simple solvent casting from chloroform solution allowed the
fabrication of optically clear and robust lms of PIM-SBFs 1–5,
Table 2 Ideal gas permeabilities of freshly methanol treated and aged films of PIM-SBFs 1–5 measured at 25 ꢃC with data for PIM-1 39,41 given for
comparison
PIM-SBFa
PHe
PH2
PO2
PN2
PCO2
PCH4
PO2/PN2
PH2/PN2
PCO2/PN2
PCO2/PCH4
1
1560
914
4330
2190
9160
4240
5460
3370
4900
3960
5590
4710
4500
2440e
1420
486
3820
910
1820
965
1760
1260
2750
1870
2200
600e
340
87.5
1150
166
463
219
474
286
1080
550
857
125e
8850
2410
22 300
3870
10 900
4850
10 600
6410
16 400
10 000
13 300
2840d
532
102
2020
184
878
248
834
331
2480
925
4.2
5.6
3.2
5.5
3.9
4.5
3.7
4.4
2.6
3.4
2.6
4.8e
12.7
25.0
8.0
25.6
11.8
15.4
10.3
13.7
5.2
26.0
27.5
19.4
23.3
23.5
22.2
22.3
22.2
15.2
18.0
15.5
22.7d
16.6
23.6
11.0
21.0
12.4
19.6
12.7
19.3
6.6
(2088)b
2
3110
1700
1971
1270
1930
1470
2060
1830
1706
1140d
(1295)b
3
(1294)b
4
(1428)b
5
(1439)b
PIM-1c
(Aged)
8.4
10.8
11.6
17.9d
1150
5.3
159d
19.5e
a
Film thickness: PIM-SBF-1 ¼ 80 mm, PIM-SBF-2 ¼ 115 mm, PIM-SBF-3 ¼ 120 mm, PIM-SBF-4 ¼ 120 mm, PIM-SBF-5 ¼ 98 mm. b Aged for the number
of days given in parentheses. c PIM-1 (freshly MeOH treated)39 ¼ 128 mm. d PIM-1 (aged 1380 days) ¼ 102 mm.41 PIM-1 (aged 1200 days) ¼ 110 mm.39
e
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unique 2D chain structure. In addition, there are several poly- concentration of larger, less selective, micropores (>1.0 nm)
acetylenes with high gas permeabilities, such as the ultra- (Fig. 2). In addition, the t-butyl substituents of PIM-SBF-5 will be
permeable poly(trimethylsilylpropyne) (PTMSP), when freshly relatively free to undergo thermal motions so that they may act
treated with methanol, although with much lower selectivities as molecular turnstiles facilitating the transport of larger gas
relative to the PIMs.43 For PIM-SBFs 2–4, in which the methyl molecules such as N2 and CH4.
groups are attached directly to the SBF unit, the selectivity
enhancement noted for PIM-SBF-1 over PIM-1 is maintained
providing data points, which are of a similar distance above the
2008 Robeson upper bounds for the O2/N2, H2/N2, CO2/N2 and
CO2/CH4 gas pairs (Fig. 3).
This is consistent with the high concentration of ultra-
micropores (<0.7 nm) suggested by chain-packing simulation
and analysis of gas adsorption data for PIM-SBF-2 (Fig. 2).
Ultramicropores, being of similar size to the kinetic diameter of
gas molecules, are responsible for selective gas transport. In
contrast, the relatively low selectivity of PIM-SBF-5 can be
ascribed to a broader pore size distribution with a greater
Extended ageing studies
Physical ageing (i.e. loss of free volume over time) is a general
feature of glassy polymers. For example, data for PIM-1 shows
that aer an extended period of 1360 days the value for PO2 falls
to 317 Barrer but with a corresponding increase in selectivity of
PO2/PN2 ¼ 5.0.41 Ageing studies were conducted on lms of all
of the PIM-SBFs with gas permeability measurements taken
aer at least 1200 days to allow for a direct comparison with the
previous PIM-1 ageing study41 (Table 2). On extended ageing of
PIM-SBF-1–4 the order of gas permeability becomes H2 > CO2 >
He > O2 > CH4 > N2, which is more conventional for glassy
Fig. 3 Robeson plots for the (a) O2/N2, (b) H2/N2, (c) CO2/N2 and (d) CO2/CH4 gas pairs showing the position of the gas permeability data for
PIM-SBFs 1–5 and that of PIM-1. Data from films aged for >3.5 years (e.g. 2a) are joined by an arrow to those of the freshly methanol treated films
to form ageing trend-lines. Upper bounds are represented by black (1991), red (2008) and dotted (2015) lines. Mixed CO2/CH4 data at pressures
of 1–6 bar for the film of PIM-SBF-1 aged for 2088 days are also plotted with red arrow indicating increasing pressure.
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polymers. When the data from aged lms are placed on the
appropriate Robeson plot (Fig. 3), it can be observed that PIM-
SBF-2 ages at an approximately similar rate relative to both
PIM-1 and PIM-SBF-1 with loss of permeability combined with
a signicant increase in selectivity. Interestingly, for the O2/N2
and H2/N2 gas pairs these trend lines are approximately parallel
and are steeper in gradient than the Robeson upper bounds. For
these two gas pairs selectivity is based on differences in gas
diffusivity, therefore, size-selectivity is being enhanced beyond
that expected by the standard polymer permeability-selectivity
trade-off. The pore structure of PIM-SBF-1–4 translates into
a very steep correlation of the diffusion coefficient with the gas
diameter, highlighting the strong size-selectivity of these poly-
mers particularly aer ageing (Fig. 4a). The slope is highest for
the less permeable and more selective PIM-SBF-1 and lowest for
most permeable and less selective PIM-SBF-5, showing that the
permselectivity is closely related to the diffusion selectivity. The
steeper slope, and thus increased size-sieving behaviour aer
aging, is an indication for a denser packing and lower free
volume of the aged sample with respect to the MeOH treated
sample (Fig. 4b).
Hence, ageing is preferentially reducing larger micropores
within the PIM-SBFs. Notably, the data for PIM-SBF-2 aer
ageing for 1295 days (PO2 ¼ 910 Barrer; PO2/PN2 ¼ 5.5) is very
close to the proposed 2015 upper bound for O2/N2 (Fig. 3a).27
In contrast, for the CO2/N2 gas pair the ageing trend line for
each of PIM-SBF-1–4 are at a much shallower gradient than the
2008 Robeson upper bound (Fig. 3c), which can be explained by
CO2/N2 separation being dominated by solubility selectivity.
CO2 solubility is correlated to the amount of free volume within
the polymer which is lost on ageing. For the CO2/CH4 gas pair
there is a contribution to selectivity from both gas solubility and
diffusivity and the ageing trend lines for PIM-SBF-1–4 are well
above and roughly parallel to the 2008 Robeson upper bound
(Fig. 3d). For this gas pair the data for aged lm of PIM-SBF-1 is
particularly impressive (PCO2 ¼ 2410 Barrer; PCO2/PCH4 ¼ 24).
The ageing of PIM-SBF-5 is anomalously slow relative to that
of the other PIM-SBFs with large values of gas permeability
demonstrated almost four years aer methanol treatment (e.g.
PCO2 > 10 000 Barrer) and the original CO2 > H2 > O2 > He > CH4
> N2 order of gas permeabilities is maintained (Table 2; Fig. 3).
Remarkably, the values of gas permeabilities for this aged lm
of PIM-SBF-5 are greater than those of freshly methanol treated
PIM-SBF-1. Therefore, the bulky t-butyl groups of PIM-SBF-5
appear better than the methyl substituents of PIM-SBFs 2–4 at
maintaining the distance between polymer chains during
ageing.
Fig. 4 Correlation of the diffusion coefficient with the square of the
effective gas diameter for aged samples (A)44 PIM-SBF-1 ( ), PIM-SBF-
2 ( ), PIM-SBF-3 ( ), PIM-SBF-4 ( ), PIM-SBF-5 ( ). Effect of sample
history for PIM-SBF-4 (B) MeOH treated sample ( ) and aged
sample ( ).
PIM-SBF-5, they age similarly to PIM-1 with commensurate
increases in selectivities (Fig. 3). Therefore, for thin lms of
PIM-SBF-2, it can be estimated that the separation of important
gas pairs based on diffusivity selectivity (e.g. O2/N2) would
approach those of commercial polymer membranes (e.g. PO2/
PN2 > 5.5) but with much higher permeance (ꢂthree times that
expected for PIM-1).
Ultimately, for practical membrane application in gas sepa-
rations, the PIM selective layer would be the thin skin on
Mixed gas permeation
a hollow bre or a thin lm within a composite membrane. It is Mixed gas permeation measurements with CO2/CH4 were
well-established that physical ageing of polymers, particularly carried out both across a total pressure range of 1–6 bar using
those of high free volume, is much more rapid in thin lms.45,46 a 35/65 vol% mixture and a CO2 feed concentration of 10–50%
A recent study on PIM-1 hollow bres47 estimates that the gas in CH4 at 3 bar for two representative aged samples: the most
permeability and selectivity of the thin selective layer (ꢂ3 mm; selective sample, PIM-SBF-1 aged over 2088 days, and the most
PO2 ¼ 200 Barrer; PO2/PN2 ¼ 4.6) are similar to those of a thick permeable sample, PIM-SBF-5 aged for 1439 days (Fig. 5).
lm (102 mm) that had been aged for 1360 days.41 The long-term
For the mixed gas measurements PIM-SBF-1 and PIM-SBF-5
ageing study of the PIM-SBFs suggest that, with the exception of both maintain excellent gas permeability to give values that are
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Journal of Materials Chemistry A
Fig. 5 Mixed gas separation performance of aged PIM-SBF-1 (A) and aged PIM-SBF-5 (C) as a function of the feed pressure for the CO2/CH4
(35/65 vol%) mixture and performance of PIM-SBF-1 (B) and PIM-SBF-5 (D) as a function of the feed composition for the CO2/CH4 mixture at
a pressure of 3 bar(a). Filled symbols for increasing, and empty symbols for decreasing pressure and CO2 concentration steps.
similar to the ideal values measured with pure gases. In both selectivities being obtained. Indeed, the excellent performance
lms the CO2 permeability and CO2/CH4 selectivity decrease of aged PIM-SBF-1 in mixed gas CO2/CH4 separation experi-
about 20% upon a pressure increase from 1 to 6 bar, while the ments (PCO2 >2000 Barrer; PCO2/PCH4 > 25) suggests its use
CH4 permeability remains virtually constant. On the other might allow signicant reductions to the size of current
hand, at a feed pressure of 3 bar the transport parameters of membrane systems for industrially relevant gas separations,
PIM-SBF-5 are independent of the gas composition in the range such as natural gas or biogas upgrading.
of 10–50% CO2, which is good for a stable operation of
a membrane separation plant under variable feed conditions,
for instance in the case of biogas separation. At a pressure of 1
Conflicts of interest
bar, the permselectivity is substantially higher than the ideal
value and the points lie in a more favourable region of the
Robeson diagram (Fig. 3d). At higher pressures, they converge
close to the ideal values measured from pure gases.
There are no conicts of interest to declare.
Acknowledgements
The research leading to these results has received funding from
the EU FP7 Framework Program under grant agreement no.
608490, project M4CO2 and from the EPSRC (UK) grant numbers
Conclusions
Both methyl (PIM-SBF-2-4) and t-butyl substituents (PIM-SBF-5) EP/M01486X/1 and EP/K008102/2. This work was also sup-
attached to the SBF unit increase intrinsic microporosity as ported by the US National Science Foundation (DMR-1604376).
demonstrated by chain packing simulations, and by analysis of High-performance computational resources were provided by
both gas adsorption data and gas permeability of the PIM-SBF the University of Florida Research Computing and the Research
series. However, these two substituents generate a signicant Computing and Cyberinfrastructure unit at Pennsylvania State
difference in pore size distribution, as predicted by packing University. Paola Bernardo and Gabriele Clariza are gratefully
simulation, that has a marked effect on the ideal gas selectivity acknowledged for some preliminary pure gas permeation
ˇ
of these polymers. Furthermore, these differences appear to be measurements and Marek Lanc for his help in developing the
accentuated on ageing over an extended period with good mixed gas separation system.
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Paper
24 I. Rose, C. G. Bezzu, M. Carta, B. Comesana-Gandara,
E. Lasseuguette, M. C. Ferrari, P. Bernardo, G. Clarizia,
A. Fuoco, J. C. Jansen, K. E. Hart, T. P. Liyana-Arachchi,
C. M. Colina and N. B. McKeown, Nat. Mater., 2017, 16, 932–937.
25 I. Rose, M. Carta, R. Malpass-Evans, M.-C. Ferrari,
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C. Rizzuto, M. Lanc, M. Carta and N. B. McKeown,
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