ArticlePDF Available

Abstract and Figures

Membrane-based processes are taking a more and more prominent position in the search for sustainable and energy-efficient gas separation applications. It is known that the separation performance of pure polymers may significantly be improved by the dispersion of suitable filler materials in the polymer matrix, to produce so-called mixed matrix membranes. In the present work, four different organic cages were dispersed in the poly(ether ether ketone) with cardo group, PEEKWC. The m-xylyl imine and furanyl imine-based fillers yielded mechanically robust and selective films after silicone coating. Instead, poor dispersion of p-xylyl imine and diphenyl imine cages did not allow the formation of selective films. The H2, He, O2, N2, CH4, and CO2 pure gas permeability of the neat polymer and the MMMs were measured, and the effect of filler was compared with the maximum limits expected for infinitely permeable and impermeable fillers, according to the Maxwell model. Time lag measurements allowed the calculation of the diffusion coefficient and demonstrated that 20 wt % of furanyl imine cage strongly increased the diffusion coefficient of the bulkier gases and decreased the diffusion selectivity, whereas the m-xylyl imine cage slightly increased the diffusion coefficient and improved the size-selectivity. The performance and properties of the membranes were discussed in relation to their composition and morphology.
Content may be subject to copyright.
molecules
Article
PEEK–WC-Based Mixed Matrix Membranes Containing
Polyimine Cages for Gas Separation
Marcello Monteleone 1, Riccardo Mobili 2, Chiara Milanese 2, Elisa Esposito 1, Alessio Fuoco 1, * ,
Sonia La Cognata 2, * , Valeria Amendola 2and Johannes C. Jansen 1


Citation: Monteleone, M.; Mobili, R.;
Milanese, C.; Esposito, E.; Fuoco, A.;
La Cognata, S.; Amendola, V.; Jansen,
J.C. PEEK–WC-Based Mixed Matrix
Membranes Containing Polyimine
Cages for Gas Separation. Molecules
2021,26, 5557. https://doi.org/
10.3390/molecules26185557
Academic Editor: Jean St-Pierre
Received: 6 August 2021
Accepted: 7 September 2021
Published: 13 September 2021
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
1Institute on Membrane Technology, National Research Council of Italy (CNR-ITM), Via P. Bucci 17/C,
87036 Rende, Italy; m.monteleone@itm.cnr.it (M.M.); e.esposito@itm.cnr.it (E.E.); jc.jansen@itm.cnr.it (J.C.J.)
2Dipartimento di Chimica, Universitàdegli Studi di Pavia, Via Taramelli 12, 27100 Pavia, Italy;
riccardo.mobili01@universitadipavia.it (R.M.); chiara.milanese@unipv.it (C.M.);
valeria.amendola@unipv.it (V.A.)
*Correspondence: a.fuoco@itm.cnr.it (A.F.); sonia.lacognata01@universitadipavia.it (S.L.C.)
Abstract:
Membrane-based processes are taking a more and more prominent position in the search
for sustainable and energy-efficient gas separation applications. It is known that the separation
performance of pure polymers may significantly be improved by the dispersion of suitable filler
materials in the polymer matrix, to produce so-called mixed matrix membranes. In the present work,
four different organic cages were dispersed in the poly(ether ether ketone) with cardo group, PEEK-
WC. The m-xylyl imine and furanyl imine-based fillers yielded mechanically robust and selective
films after silicone coating. Instead, poor dispersion of p-xylyl imine and diphenyl imine cages did
not allow the formation of selective films. The H
2
, He, O
2
, N
2
, CH
4
, and CO
2
pure gas permeability
of the neat polymer and the MMMs were measured, and the effect of filler was compared with the
maximum limits expected for infinitely permeable and impermeable fillers, according to the Maxwell
model. Time lag measurements allowed the calculation of the diffusion coefficient and demonstrated
that 20 wt % of furanyl imine cage strongly increased the diffusion coefficient of the bulkier gases and
decreased the diffusion selectivity, whereas the m-xylyl imine cage slightly increased the diffusion
coefficient and improved the size-selectivity. The performance and properties of the membranes
were discussed in relation to their composition and morphology.
Keywords:
organic cage; mixed matrix membrane; gas separation; Maxwell model; gas permeability;
gas diffusion
1. Introduction
Gas separation by membranes is an application in continuous growth. Because of low
operating costs compared to traditional techniques, it has spread its use in several processes,
including biogas [
1
] and natural gas separation [
2
], H
2
recovery [
3
], post-combustion CO
2
capture, and oxygen enriched air production [
4
,
5
]. The development of new materials that
can improve separation performances represents a continuous request for the research field.
Nowadays, the availability of materials, which are above the upper bound in the Robeson
plot and combine a high permeability with a high selectivity, is very low. In general,
polymers with high permeability (usually rubbers or high free volume polymers) show
a low selectivity, while highly selective materials (usually glassy polymers) are generally
characterized by a low permeability. The addition of fillers to a polymer is a route for
the preparation of mixed matrix membranes (MMMs), in which the final performances
are obtained by exploiting the synergic action of the easy processability of polymers with
the high gas separation performance of the filler. Several additives such as zeolites [
6
],
porous metal-organic frameworks (MOFs) [
7
9
], and organic cages [
10
,
11
] were used for
this purpose. However, compatibility between the polymer and the filler is essential for
the formation of defect-free membranes. Agglomeration and sedimentation of particles, as
Molecules 2021,26, 5557. https://doi.org/10.3390/molecules26185557 https://www.mdpi.com/journal/molecules
Molecules 2021,26, 5557 2 of 16
well as the bad polymer/filler adhesion, are problems which can often occur, inducing the
formation of defects in the membranes [9].
In this work, we study the effect of four different polyimine cages (m-xylyl, p-xylyl,
diphenyl and furanyl-based systems) on the gas transport properties of a poly(ether ether
ketone) with cardo group (PEEK-WC, Figure 1). The latter is a glassy polymer that is
soluble in several organic solvents due to the presence of the Cardo-group in the backbone,
unlike the classical poly (ether ether ketone) (PEEK) and poly (ether ketone) (PEK) [
12
], and
its good solubility in low boiling solvents like chloroform is useful for the preparation of
dense gas separation membranes by solvent evaporation. Recently, PEEK-WC was loaded
with MOF, improving the transport properties in gas separation [13].
Molecules 2021, 26, x FOR PEER REVIEW 2 of 16
cages [10,11] were used for this purpose. However, compatibility between the polymer
and the filler is essential for the formation of defect-free membranes. Agglomeration and
sedimentation of particles, as well as the bad polymer/filler adhesion, are problems which
can often occur, inducing the formation of defects in the membranes [9].
In this work, we study the effect of four different polyimine cages (m-xylyl, p-xylyl,
diphenyl and furanyl-based systems) on the gas transport properties of a poly(ether ether
ketone) with cardo group (PEEK-WC, Figure 1). The latter is a glassy polymer that is
soluble in several organic solvents due to the presence of the Cardo-group in the
backbone, unlike the classical poly (ether ether ketone) (PEEK) and poly (ether ketone)
(PEK) [12], and its good solubility in low boiling solvents like chloroform is useful for the
preparation of dense gas separation membranes by solvent evaporation. Recently, PEEK-
WC was loaded with MOF, improving the transport properties in gas separation [13].
Figure 1. Structure of the cages (1 = furanyl, 2 = m-xylyl, 3 = p-xylyl, 4 = diphenyl) and the polymer
PEEK-WC used in the present work.
Cage-like organic molecules [14,15], cryptands and cryptates in particular [16], are
molecular systems with tridimensional cavities that are well known in the literature for
many applications in solution, including host–guest chemistry, catalysis, and drug
delivery [17–19]. After the pioneering work by Cooper et al. [20], porous organic cages
have also raised attention in solid state as molecular materials for gas separation processes
[21–23]. The cages proposed for these applications are generally characterized by a large
tridimensional cavity, and their porosity is permanent in the solid state, thanks to the rigid
molecular structure. Compared to other porous materials employed in gas separation
(such as zeolites, MOFs, covalent organic frameworks) [24], organic cages are generally
more soluble in organic solvents [25], which makes them more attractive during the
preparation of MMMs, since they can be more easily dispersed in the casting solution
[10,11,26]. Among organic cages, bistren azacryptands have been extensively described in
the literature and they are easily obtained in high yields, starting from a flexible
commercial polyamine (i.e., tris(2-ethylamino)amine, tren) and a properly chosen
dialdehyde [27,28]. The rigidity and size of the polyimine cage framework can be
modulated by changing the spacers connecting the tren subunits. These features, which
are also maintained after reduction to the amino form, are employed by several groups to
enhance the cage selectivity towards guest molecules in solution. A number of bistren
Figure 1.
Structure of the cages (
1
= furanyl,
2
=m-xylyl,
3
=p-xylyl,
4
= diphenyl) and the polymer
PEEK-WC used in the present work.
Cage-like organic molecules [
14
,
15
], cryptands and cryptates in particular [
16
], are
molecular systems with tridimensional cavities that are well known in the literature for
many applications in solution, including host–guest chemistry, catalysis, and drug deliv-
ery [
17
19
]. After the pioneering work by Cooper et al. [
20
], porous organic cages have also
raised attention in solid state as molecular materials for gas separation processes [
21
23
].
The cages proposed for these applications are generally characterized by a large tridi-
mensional cavity, and their porosity is permanent in the solid state, thanks to the rigid
molecular structure. Compared to other porous materials employed in gas separation (such
as zeolites, MOFs, covalent organic frameworks) [
24
], organic cages are generally more
soluble in organic solvents [
25
], which makes them more attractive during the preparation
of MMMs, since they can be more easily dispersed in the casting solution [
10
,
11
,
26
]. Among
organic cages, bistren azacryptands have been extensively described in the literature and
they are easily obtained in high yields, starting from a flexible commercial polyamine
(i.e., tris(2-ethylamino)amine, tren) and a properly chosen dialdehyde [
27
,
28
]. The rigidity
and size of the polyimine cage framework can be modulated by changing the spacers
connecting the tren subunits. These features, which are also maintained after reduction to
the amino form, are employed by several groups to enhance the cage selectivity towards
guest molecules in solution. A number of bistren azacryptands (in particular polyimine
systems) has also been reported as solid materials for the electrochemical reduction of
CO
2
to CO and for CO
2
sequestration processes from multicomponent gas streams [
29
31
].
However, as far as we know, this particular class of molecules has never been tested in
Molecules 2021,26, 5557 3 of 16
MMMs. This work is aimed to fill this gap by developing a series of MMMs, with improved
performance in terms of gas permeability and selectivity, containing polyimine bistren
cryptands as fillers. To achieve this goal, we considered four cages featuring different
spacers between the tren subunits (Figure 1). The choice of spacers was based on their
length (e.g., 1,3-xylyl vs. 1,4-xylyl; 4,4
0
-diphenyl vs. 1,4-xylyl), rigidity and presence of
heteroatoms (xylyl vs. furanyl).
The most commonly used model to describe the gas transport in polymeric membranes
is the solution–diffusion model. According to this model, the permeation occurs in three
steps. In the first step, the penetrating gas molecules dissolve in the membrane on the
feed side. In the second step, they diffuse through polymeric matrix from the feed side to
the opposite side (permeate). In the third and final step, the gas molecules desorb from
the permeate side of the membrane into the permeate reservoir. The diffusion process,
which is the rate-determining step and is slower than the other two, depends on the size of
the penetrating gas and on the space available between the polymer chains (free volume)
and on their rigidity. According to the solution–diffusion model, the permeability (P) of
a gas through a membrane is expressed as the product between the diffusivity (D) and
solubility (S):
P=D×S(1)
Permeability is a fundamental parameter that expresses the ability of the membrane
material to be permeated by a specific gas. This value represents the flux of permeate
generated by a pressure difference between the two sides of the membrane, normalized for
the surface area and the thickness of the membrane, and it is commonly given in the unit
Barrer (1 Barrer = 1010 cm3STP cm cm2s1cmHg1).
For a pair of gases aand b, the selectivity (
αa,b
) is determined by the ratio of the
permeability of gas aand the permeability of gas b:
αa,b=Pa
Pb
(2)
In mixed matrix membranes, the presence of a filler influences the transport properties
with respect to that of the pure polymer. Several models were proposed to describe
this effect. One of the simplest and most applied model is the Maxwell model [
32
,
33
],
valid when spherical fillers are present in the membrane at low concentration, up to
approximately 30 vol %. The permeability of a MMM, P
MMM
, is given by the following
equation:
PMMM =PCPd+2Pc2Φd(PcPd)
Pd+2Pc+Φd(PcPd)(3)
where P
c
and P
d
are the permeability of the continuous and dispersed phase, respectively,
while
Φd
is the volume fraction of the dispersed phase. Thus, if the dispersed phase is more
permeable than the continuous phase, the overall permeability increases, with a maximum
theoretical limit given by Pd=:
PMMM,max =PC1+2Φd
1Φd(4)
The overall permeability decreases if the permeability of the dispersed phase is lower
than that of the bulk polymer, with a minimum limit for Pd= 0:
PMMM,min =PC1Φd
1+0.5Φd(5)
More complex situations occur for non-spherical particles, or when the polymer/filler
interaction affects the properties of the bulk and creates a stiffer interface or an interface
with higher free volume.
Molecules 2021,26, 5557 4 of 16
2. Results and Discussion
2.1. Cages Preparation and Characterization
The polyimine bistren cages
1
4
(Figure 1) were obtained in high yields (>70%) fol-
lowing the procedures described in the literature [
34
], which consist in the condensation
between tren and the chosen dialdehyde, mixed in a 2:3 molar ratio in acetonitrile. The
pure products, precipitated from the reaction mixture, were isolated by filtration and dried
under vacuum at room temperature for 48 h. Fura precipitated as brownish microcrystals
from the reaction mixture, while m-xy formed agglomerates of a white microcrystalline
powder. On the other hand, p-xy and diphen precipitated as colorless solids, which tended
to compact under drying conditions. The
1
H-NMR spectra of cages
1
4
(Figures S1–S4)
in CDCl
3
correspond to those reported in the literature [
34
]; FTIR-ATR spectra further
confirm the presence of imine bonds (Figures S5–S8) as it results from the typical peaks
(between 1629 and 1641 cm
1
) due to the stretching of the C=N bond. The solubility of
the compounds was determined experimentally at room temperature in various solvents.
The results are listed in Table 1. These tests showed that Fura and m-xy cages are generally
more soluble than p-xy and diphen in common solvents used in membrane preparation.
Table 1. Solubilities of the prepared cages.
Solubility (wt/vol %) in the Given Solvents
No. Code Polyimine Cages MeOH EtOH CHCl3CH2Cl2THF
1 Fura furanyl-based 0.10 0.08 0.13 0.10 X
2m-xy m-xylyl-based 0.13 0.10 0.40 0.40 0.20
3p-xy p-xylyl-based X X 0.20 0.13 X
4 diphen diphenyl-based X X 0.10 0.07 X
X = completely insoluble in the chosen solvent.
Fura and m-xy were found to be the most crystalline samples among the investigated
systems, as indicated by the XRPD patterns (Figure 2a,b). Instead, an amorphous halo is
evident in both p-xy and the diphenyl cage (Figure 2c,d), where few peaks are superimposed
on a very broad signal in the 10–30
angular range. Notably, the experimental patterns
of Fura and m-xy cages are in very good agreement with the patterns predicted by the
Mercury software [
35
] on the basis of single crystal data reported in the literature [
36
,
37
].
In the rhombohedral crystals of the m-xy cage, reported by Nelson et al. [37], no evidence
of intermolecular interactions or of H-bonded solvent molecules was found. On the other
hand, the orthorhombic Fura crystals [
36
] presented one crystallization water molecule per
cage, H-bonded to the imine bonds of two adjacent cryptand molecules.
TGA (Figure 3) and DSC (Figures S9–S12) measurements reveal that the m-xy and
diphenyl cages are the thermally most stable species among the investigated samples
(Figure 3b,d, Figures S10 and S12). Apart from a slow step of mass loss, attributable to the
release of the crystal water in Fura or of adsorbed solvent in all cages, these samples started
to decompose at temperatures of 200
C and higher. The confirmation of the irreversible
nature of these processes, due to the cage decomposition, is given by the cooling curves in
the DSC profiles that do not show any relevant signal in the considered temperature range.
The TGA curve of p-xy (Figure 3c) shows two small mass losses between room temperature
and 100
C, attributable to solvent release, while decomposition starts at about 180
C. In
the case of Fura, beside the solvent release, decomposition starts at about 200
C, as evident
by the derivative of the TGA curve (DTG, dm/dT trace in Figure 3a) and by the events in
the DSC profile (Figure S9).
Molecules 2021,26, 5557 5 of 16
Molecules 2021, 26, x FOR PEER REVIEW 5 of 16
(a) (b)
(c) (d)
Figure 2. XRPD patterns of (a) Fura and (b) m-xy, (c) p-xy and (d) diphen samples. Black lines:
experimental patterns, and red line: patterns predicted by the Mercury software [35] on the basis
of single crystal data reported in the literature [36,37].
(a) (b)
(c) (d)
Figure 3. Thermogravimetric (solid line) and DTG curves (dashed line) for (a) Fura, (b) m-xy, (c) p-
xy, and (d) diphen cages.
Figure 2.
XRPD patterns of (
a
) Fura and (
b
)m-xy, (
c
)p-xy and (
d
) diphen samples. Black lines:
experimental patterns, and red line: patterns predicted by the Mercury software [
35
] on the basis of
single crystal data reported in the literature [36,37].
Molecules 2021, 26, x FOR PEER REVIEW 5 of 16
(a) (b)
(c) (d)
Figure 2. XRPD patterns of (a) Fura and (b) m-xy, (c) p-xy and (d) diphen samples. Black lines:
experimental patterns, and red line: patterns predicted by the Mercury software [35] on the basis
of single crystal data reported in the literature [36,37].
(a) (b)
(c) (d)
Figure 3. Thermogravimetric (solid line) and DTG curves (dashed line) for (a) Fura, (b) m-xy, (c) p-
xy, and (d) diphen cages.
Figure 3.
Thermogravimetric (solid line) and DTG curves (dashed line) for (
a
) Fura, (
b
)m-xy, (
c
)p-xy,
and (d) diphen cages.
The morphological analysis of the cages by SEM shows that most of the powders
have an irregular shape and heterogeneous particles size (Figure 4). In the case of Fura,
Molecules 2021,26, 5557 6 of 16
the SEM images displayed crystals of various size, whose shape is consistent with an
orthorhombic lattice, that could be hypothesized after comparing the experimental XRPD
pattern with that predicted on the basis of published single crystal analysis (Figure 2a) [
36
].
The morphological analysis of m-xy showed large and irregular aggregates; while for p-xy,
fine spherical-shaped particles could be seen even at the highest magnification. The SEM
image of the diphen cage displayed heterogeneous agglomerates of flakes and needle-
like crystals.
Molecules 2021, 26, x FOR PEER REVIEW 6 of 16
The morphological analysis of the cages by SEM shows that most of the powders
have an irregular shape and heterogeneous particles size (Figure 4). In the case of Fura,
the SEM images displayed crystals of various size, whose shape is consistent with an
orthorhombic lattice, that could be hypothesized after comparing the experimental XRPD
pattern with that predicted on the basis of published single crystal analysis (Figure 2a)
[36]. The morphological analysis of m-xy showed large and irregular aggregates; while for
p-xy, fine spherical-shaped particles could be seen even at the highest magnification. The
SEM image of the diphen cage displayed heterogeneous agglomerates of flakes and
needle-like crystals.
1000× 5000× 20,000×
Cages
Fura (1) m-xy (2) p-xy (3) Diphen (4)
Figure 4. SEM images of Cages 14 as collected in their powder form at different magnifications.
2.2. Membrane Preparation and Characterization
The solubility of the cages is not high enough to make a concentrated solution in
chloroform (Table 1); therefore, the dispersion of the cages in the polymer solution was
chosen as the route for membrane preparation in this work. Moreover, only two cages
were suitable for MMMs preparation with the used protocol and polymer, the m-xy and
Fura. Thus, successfully prepared MMMs PEEK-WC/m-xy and PEEK-WC/Fura did not
show any macroscopic defects or inhomogeneities visible to the naked eye, suggesting an
even dispersion of the fillers. They were only slightly opaque due to the different
refractive index of the polymer and the filler materials and had a thickness of 42 μm and
78 μm for PEEK-WC/m-xy and PEEK-WC/Fura, respectively. The membranes were robust
and could be handled without particular attention. Instead, the other two cages, p-xy and
diphen, were not suitable for the preparation of homogeneous and stable membranes due
80 µm 10 µm 3 µm
Figure 4. SEM images of Cages 14as collected in their powder form at different magnifications.
2.2. Membrane Preparation and Characterization
The solubility of the cages is not high enough to make a concentrated solution in
chloroform (Table 1); therefore, the dispersion of the cages in the polymer solution was
chosen as the route for membrane preparation in this work. Moreover, only two cages
were suitable for MMMs preparation with the used protocol and polymer, the m-xy and
Fura. Thus, successfully prepared MMMs PEEK-WC/m-xy and PEEK-WC/Fura did not
show any macroscopic defects or inhomogeneities visible to the naked eye, suggesting an
even dispersion of the fillers. They were only slightly opaque due to the different refractive
index of the polymer and the filler materials and had a thickness of 42
µ
m and 78
µ
m for
PEEK-WC/m-xy and PEEK-WC/Fura, respectively. The membranes were robust and could
be handled without particular attention. Instead, the other two cages, p-xy and diphen,
were not suitable for the preparation of homogeneous and stable membranes due to their
poor dispersion in the polymeric solution. This is confirmed by the SEM images, showing
evident clustering of the p-xy cage that hinders its homogenous dispersion (Figure 4).
Molecules 2021,26, 5557 7 of 16
The MMMs’ morphology was studied by SEM analysis of the top and bottom surfaces,
as well as their cross-section (Figure 5).
Molecules 2021, 26, x FOR PEER REVIEW 7 of 16
to their poor dispersion in the polymeric solution. This is confirmed by the SEM images,
showing evident clustering of the p-xy cage that hinders its homogenous dispersion
(Figure 4).
The MMMs’ morphology was studied by SEM analysis of the top and bottom
surfaces, as well as their cross-section (Figure 5).
1000× 5000× 20,000×
Top surface
PEEK-WC/m-xy
PEEK-WC/Fura
Bottom surface
PEEK-WC/m-xy
PEEK-WC/Fura
Cross Section
PEEK-WC/m-xy
PEEK-WC/Fura
Figure 5. SEM images of the PEEK–WC-based membranes and the cages Fura (1) and m-xy (2) at
different magnifications.
80 µm 10 µm 3 µm
Figure 5.
SEM images of the PEEK–WC-based membranes and the cages Fura (
1
) and m-xy (
2
) at
different magnifications.
It should be noted that the fully organic nature of the polymer and the cages does
not give a strong contrast in the SEM pictures, and thus the cages are not well visible.
The PEEK-WC/m-xy membrane cross-section shows a well-defined dense layer and both
surfaces show no evident pores or defects, confirming the good dispersion of the cage
Molecules 2021,26, 5557 8 of 16
in the membrane. Instead, the PEEK-WC/Fura membrane has irregularities on both
surfaces, such as pores and cracks, and its cross-section is not uniform. The presence of
these defects is a symptom of a membrane with low selectivity that needs to be coated to
perform defect healing. Herein, we have coated the membranes with a dense PDMS layer,
which is a widely used procedure for the correction of pinhole defects in membranes for
gas separation [
38
41
]. The effect of PDMS on the diffusive transport through the dense
membrane is negligible when there is a difference of several orders of magnitude in their
intrinsic permeability coefficients, as is the case for our PEEK–WC-based MMMs. Instead,
PDMS completely blocks the Knudsen diffusion through the pinhole defects (Figure 6),
and the transport through the coated membrane is governed by the solution–diffusion
mechanism.
Molecules 2021, 26, x FOR PEER REVIEW 8 of 16
It should be noted that the fully organic nature of the polymer and the cages does not
give a strong contrast in the SEM pictures, and thus the cages are not well visible. The
PEEK-WC/m-xy membrane cross-section shows a well-defined dense layer and both
surfaces show no evident pores or defects, confirming the good dispersion of the cage in
the membrane. Instead, the PEEK-WC/Fura membrane has irregularities on both surfaces,
such as pores and cracks, and its cross-section is not uniform. The presence of these defects
is a symptom of a membrane with low selectivity that needs to be coated to perform defect
healing. Herein, we have coated the membranes with a dense PDMS layer, which is a
widely used procedure for the correction of pinhole defects in membranes for gas
separation [38–41]. The effect of PDMS on the diffusive transport through the dense
membrane is negligible when there is a difference of several orders of magnitude in their
intrinsic permeability coefficients, as is the case for our PEEK–WC-based MMMs. Instead,
PDMS completely blocks the Knudsen diffusion through the pinhole defects (Figure 6),
and the transport through the coated membrane is governed by the solution–diffusion
mechanism.
Figure 6. Example of CO2 pure gas permeation curves at 25 °C and 1 bar feed pressure of
membrane PEEK-WC/m-xy with and without silicone coating, showing the effective healing of
pinhole defects by the flat baseline (dashed line).
2.3. Pure Gas Transport Properties
An example of two permeation curves, determined by the so-called time lag method
in a fixed-volume pressure-increase setup, and described by (6), is given in Figure 6. The
permeate pressure of CO2 in the PEEK-WC/m-xy membrane is plotted as a function of
time, before and after defect healing with PDMS. From the immediate pressure increase
and the very steep slope of the uncoated sample, it is evident that the PDMS coating is
needed to correct pinhole defects and to obtain a curve where the determination of the
time lag is clear and well defined. This allows the determination of the gas transport
parameters of the MMM, i.e., the permeability and diffusion coefficients of the gases and,
indirectly, the solubility. The resulting flat baseline in the PDMS-coated membrane, i.e.,
the tangent to the very initial part of the curve defined by the term (dp/dt)0 in (Equation 6)
and (Equation (7)), confirms that leak flow through remaining pinhole defects is negligible
for CO2. Wherever this is not the case, a baseline correction was applied via (6) and (7). As
described previously, this procedure allows the correct calculation of the values of P, D,
and S of membranes with few defects [42].
The results of the permeation tests with six pure gases at 25 °C are collected in Table
2. The measurements were performed in the order H2, He, O2, N2, CH4, and finally CO2.
Tests with O2, N2 were repeated at the end of the cycle in order to confirm that there is no
change in the material due to physical aging or plasticization by CO2. The incorporation
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0 500 1000 1500
Permeate Pressure (mbar)
Time (s)
CO
2
Membrane Without PDMS
CO
2
Mem brane Coat ed PDMS
Figure 6.
Example of CO
2
pure gas permeation curves at 25
C and 1 bar feed pressure of membrane
PEEK-WC/m-xy with and without silicone coating, showing the effective healing of pinhole defects
by the flat baseline (dashed line).
2.3. Pure Gas Transport Properties
An example of two permeation curves, determined by the so-called time lag method
in a fixed-volume pressure-increase setup, and described by (6), is given in Figure 6. The
permeate pressure of CO
2
in the PEEK-WC/m-xy membrane is plotted as a function of
time, before and after defect healing with PDMS. From the immediate pressure increase
and the very steep slope of the uncoated sample, it is evident that the PDMS coating is
needed to correct pinhole defects and to obtain a curve where the determination of the time
lag is clear and well defined. This allows the determination of the gas transport parameters
of the MMM, i.e., the permeability and diffusion coefficients of the gases and, indirectly, the
solubility. The resulting flat baseline in the PDMS-coated membrane, i.e., the tangent to the
very initial part of the curve defined by the term (dp/dt)
0
in (Equations (6) and (7)), confirms
that leak flow through remaining pinhole defects is negligible for CO
2
. Wherever this is
not the case, a baseline correction was applied via (6) and (7). As described previously, this
procedure allows the correct calculation of the values of P,D, and Sof membranes with
few defects [42].
The results of the permeation tests with six pure gases at 25
C are collected in
Table 2. The measurements were performed in the order H
2
, He, O
2
, N
2
, CH
4
, and finally
CO
2
. Tests with O
2
, N
2
were repeated at the end of the cycle in order to confirm that
there is no change in the material due to physical aging or plasticization by CO
2
. The
incorporation of the cages induced different effects in the transport properties compared
to the neat polymer, depending on the cage type. A decrease in permeability for all gases
was observed in the presence of the m-xy cage, while a gain in selectivity was found for
the gas pairs CO
2
/CH
4
, CO
2
/N
2
, O
2
/N
2
, and He/CH
4
, as displayed in the Robeson
Molecules 2021,26, 5557 9 of 16
diagrams in Figure 7. The remarkable gain in selectivity for the O
2
/N
2
gas pair leads to
an overall improvement of the membrane separation performance that approaches the
1991 upper bound (Figure 7c). The opposite behavior occurs with the Fura cage, where the
permeability of all gases increases, while the selectivity decreases for most gas pairs, except
for CO
2
/N
2
that remains almost constant. Especially with m-xy, the herein proposed MMM
is among the best performing membranes in the class of poly(aryl ether) and poly(aryl
ether ketone)-based systems in terms of selectivity, while with Fura, they are among the
best in terms of permeability.
Table 2.
Pure gas permeability, solubility, and diffusion coefficients and relative selectivity for the neat PEEK–WC, PEEK–
WC/m-xy, and PEEK–WC/Fura membranes.
Membrane Permeability [Barrer] α(Pa/Pb)
N2O2CO2CH4H2He CO2/CH4CO2/N2O2/N2He/CH4
PEEK-WC [13] * 0.24 1.24 6.04 0.25 13.4 12.5 24.2 25.2 5.17 50.0
PEEK-WC/m-xy 0.07 0.68 3.41 0.10 7.75 8.58 34.1 48.7 9.71 85.8
PEEK-WC/Fura 2.25 10.1 52.7 4.27 40.1 30.9 12.3 23.4 4.49 7.24
Membrane Diffusivity [1012 m2s1]α(Da/Db)
N2O2CO2CH4H2He CO2/CH4CO2/N2O2/N2He/CH4
PEEK-WC [13] * 0.45 2.02 0.58 0.14 135 529 4.14 1.29 4.49 3779
PEEK-WC/m-xy 1.02 3.86 0.85 0.20 188 708 4.25 0.83 3.78 3540
PEEK-WC/Fura 28.0 53.0 15.1 10.5 455 776 1.44 0.54 1.89 73.9
Membrane Solubility [cm3STP cm3bar1]α(Sa/Sb)
N2O2CO2CH4H2He CO2/CH4CO2/N2O2/N2He/CH4
PEEK-WC [13] * 0.39 0.46 7.77 1.33 0.07 0.02 5.84 19.9 1.18 0.015
PEEK-WC/m-xy 0.05 0.13 3.02 0.38 0.03 0.009 7.95 60.4 2.60 0.024
PEEK-WC/Fura 0.06 0.14 2.62 0.30 0.07 0.03 8.73 43.7 2.33 0.100
* literature data.
On the basis of the Maxwell Equation (3), the decrease in permeability in the presence
of m-xy, and the increase in permeability for Fura suggests that these fillers have a lower
and higher permeability than the polymer, respectively. Assuming a density of approx.
0.5 g cm
3
for the cages [
29
] and 1.249 g cm
3
for the PEEK-WC [
43
], the volume fraction
of the cages is 38.4% in the MMMs with 20 wt % of cages, and (4) and (5) predict that their
permeability falls in the interval 0.517 P
c
<P
MMM
< 2.87 P
c
. In practice, the decrease for
many gases in the m-xy-based MMM is close to or even slightly larger than predicted by
(5), suggesting that we deal with almost impermeable fillers or that the fillers also affect
the bulk properties of the polymer. Fortunately, along with a decrease in permeability, we
see an increase in selectivity, indicating that the separation performance of the membrane
increases, in spite of its lower productivity. Instead, the increase in permeability with Fura
is much higher than predicted by the Maxwell equation and at the same time there is a
strong reduction in diffusion selectivity with this cage (Figure 8and discussion below).
This is probably related to the presence of additional free volume and might indicate the
formation of nonselective diffusion paths around the cages due to poor adhesion, or it
indicates the presence of voids between poorly dispersed clusters.
A decrease of the solubility coefficient for nearly all bulkier gases was observed in both
mixed matrix membranes. With the m-xy cage, the permeability also decreases, since the
loss in solubility is not balanced by an increase in diffusivity. Instead, with the Fura cage,
the decrease in solubility is overcompensated by a dramatic increase in the gas diffusion
coefficients, leading to an overall increase in permeability.
Molecules 2021,26, 5557 10 of 16
Figure 7.
Robeson diagrams for the (
a
) CO
2
/CH
4
, (
b
) CO
2
/N
2
, (
c
) O
2
/N
2
, and (
d
) He/CH
4
gas pairs
with the upper bounds represented by blue lines for 1991 [
44
], red lines for 2008 [
45
], yellow lines for
2015 (O
2
/N
2
) [
46
], and purple lines for 2019(CO
2
/N
2
) [
47
]. The gas permeability data for PEEK-WC
are reported with red circles
, with pink triangles for PEEK–WC/m-xy
N
, and with green squares for
PEEK–WC/Fura
. Empty symbols are literature data from the database of the Membrane Society
of Australasia (MSA) for Poly(aryl ethers) and Poly(aryl ether ketones)-based membranes (https:
//membrane-australasia.org/msa-activities/polymer-gas-separation-membrane-database/, last
accessed on 29 August 2021).
The logarithm of the diffusion coefficient shows a linear correlation with the square of
the gas diameter, d
2eff
, for all investigated membranes (Figure 8). This trend indicates that
the gas transport in all membranes follows the solution–diffusion model and no anomalies
are taking place [
48
]. However, while the PEEK–WC/m-xy MMM has a similar behaviour
with respect to the neat PEEK–WC membrane, the PEEK–WC/Fura has a much gentler
slope, indicating weaker size-sieving behaviour. This is in agreement with the hypothesis,
formulated during the Maxwell analysis, on the formation of nonselective free volume
elements, for instance in the form of diffusion paths around the cages due to poor adhesion
or due to voids between poorly dispersed clusters.
It should be noted that only in the neat PEEK–WC membrane, the diffusion coefficient
of CO
2
is higher than that of N
2
, as expected on the basis of the effective gas diameters,
while in both MMMs, the CO
2
diffusion coefficient does not increase as much as that of N
2
and the order of the two gases is inverted. An unexpectedly slow transient is usually a sign
of specific interaction of CO
2
with the cage, or simply a higher solubility in internal voids
of the dispersed phase. Further control of the cage size down to nanometer-scale is needed
Molecules 2021,26, 5557 11 of 16
to be able to produce successfully integrally skinned or thin film composite PEEK-WC
membranes that may have a selective layer below 100 nm thick [49].
Molecules 2021, 26, x FOR PEER REVIEW 11 of 16
as that of N
2
and the order of the two gases is inverted. An unexpectedly slow transient is
usually a sign of specific interaction of CO
2
with the cage, or simply a higher solubility in
internal voids of the dispersed phase. Further control of the cage size down to nanometer-
scale is needed to be able to produce successfully integrally skinned or thin film composite
PEEK-WC membranes that may have a selective layer below 100 nm thick [49].
Figure 8. Correlation of the effective diffusion coefficient of six light gases as a function of their
molecular diameter, as defined by Teplyakov and Meares [50], for the PEEK–WC , PEEK–WC/m-
xy , and PEEK–WC/Fura membranes.
3. Materials and Methods
3.1. Materials for Synthesis and Characterization
Solvents and chemicals used for syntheses were HPLC grade. Acetonitrile, diethyl
ether, tris(2-ethylamino) amine 96%, 2,5-furandicarbaldehyde 97%, isophthalaldehyde
97%, terephthalaldehyde reagentPlus 99% and deuterated solvents used for NMR analysis
(CDCl
3
) were purchased from Sigma-Aldrich, Merck Italia (Milano, Italy). Diphenyl-4,4-
dicarboxyaldehyde was synthesized based on a procedure already described by our
group [51]. PEEK-WC was supplied by the Institute of Applied Chemistry, Changchun,
China. PDMS resin ELASTOSIL
®
M 4601 A/B was provided by Wacker Chemie AG.
(Munich, Germany). Chloroform AnalaR NORMAPUR
®
, was supplied by VWR
International srl, Milano, Italy. Pure gases H
2
, He N
2
, O
2
, CH
4
, CO
2
(99.99+%) used for
permeation tests were purchased from SAPIO, Monza, Italy.
3.2. Syntheses and Characterization of Cages 14
Syntheses and characterizations of the investigated cages (see 14 in Figure 1) have
been already reported [34,36,37,52]. For this work, Fura, m-xy, p-xy, and diphen were
obtained following the procedure recently described by Lehn et al. [34], using acetonitrile
as the solvent for the synthesis. The cages were precipitated from the reaction mixtures,
collected by filtration, and dried under vacuum. The obtained polyimine cages were
employed in the preparation of MMMs without further purification.
Cages Characterization
The solubility of cages 14 was tested in various solvents (MeOH, EtOH, CHCl
3
,
CH
2
Cl
2
, and THF at T = 25 °C): about 2 mg of each cage was weighted in a test tube and
solvent was added in small portions until complete dissolution of the powder at room
Figure 8.
Correlation of the effective diffusion coefficient of six light gases as a function of their
molecular diameter, as defined by Teplyakov and Meares [
50
], for the PEEK–WC
, PEEK–WC/m-xy
N, and PEEK–WC/Fura membranes.
3. Materials and Methods
3.1. Materials for Synthesis and Characterization
Solvents and chemicals used for syntheses were HPLC grade. Acetonitrile, diethyl
ether, tris(2-ethylamino) amine 96%, 2,5-furandicarbaldehyde 97%, isophthalaldehyde
97%, terephthalaldehyde reagentPlus 99% and deuterated solvents used for NMR analysis
(CDCl
3
) were purchased from Sigma-Aldrich, Merck Italia (Milano, Italy). Diphenyl-
4,4
0
-dicarboxyaldehyde was synthesized based on a procedure already described by our
group [
51
]. PEEK-WC was supplied by the Institute of Applied Chemistry, Changchun,
China. PDMS resin ELASTOSIL
®
M 4601 A/B was provided by Wacker Chemie AG. (Mu-
nich, Germany). Chloroform AnalaR NORMAPUR
®
, was supplied by VWR International
srl, Milano, Italy. Pure gases H
2
, He N
2
, O
2
, CH
4
, CO
2
(99.99+%) used for permeation tests
were purchased from SAPIO, Monza, Italy.
3.2. Syntheses and Characterization of Cages 14
Syntheses and characterizations of the investigated cages (see
1
4
in Figure 1) have
been already reported [
34
,
36
,
37
,
52
]. For this work, Fura, m-xy, p-xy, and diphen were
obtained following the procedure recently described by Lehn et al. [
34
], using acetonitrile
as the solvent for the synthesis. The cages were precipitated from the reaction mixtures,
collected by filtration, and dried under vacuum. The obtained polyimine cages were
employed in the preparation of MMMs without further purification.
Cages Characterization
The solubility of cages
1
4
was tested in various solvents (MeOH, EtOH, CHCl
3
,
CH
2
Cl
2
, and THF at T = 25
C): about 2 mg of each cage was weighted in a test tube and
solvent was added in small portions until complete dissolution of the powder at room
temperature (upon sonication). The experiment was then repeated for each solvent. The
obtained results are shown in Table 1.
Molecules 2021,26, 5557 12 of 16
1
H-NMR spectra were recorded on a Bruker ADVANCE 400 spectrometer (operating
at 9.37 T, 400 MHz). Chemical shifts are reported in ppm with the residual solvent as
internal reference. NMR spectra were recorded at 25.0 C.
X-ray powder diffraction (XRPD) measurements were performed at room temperature
on the powders of the four cage samples after manual grinding in an agate mortar using a
Bruker D5005 diffractometer (Bruker Corporation, Billerica, MA, USA) with CuKa radiation,
graphite monochromator, and scintillation detector. The measurements were performed
from 3
to 70
with step scan mode: scan step 0.02
, counting time 10 s per step; X-ray tube
working conditions: 40 kV and 40 mA.
For the Fourier Trasform Infrared analysis (FTIR), a Nicolet FTIR iS10 spectrometer (Nicolet,
Madison, WI, USA) equipped with attenuated total reflectance (ATR) sampling accessory (Smart
iTR with diamond plate) was used. Thirty-two scans in the 4000–600 cm
1
range at 4 cm
1
resolution were coadded. Well-ground powder samples were used and spectra were
obtained after pressing the sample onto the ATR diamond crystal at room temperature
(20
C). Peak wavenumbers were attributed by using the “Find peaks” function of the
OMNIC™ Spectra Software.
Thermogravimetric analysis (TGA) was performed by a Q5000 apparatus (TA Instru-
ments, New Castle, DE, USA) interfaced with a TA5000 data station under nitrogen flux
(10 mL min
1
) in a platinum pan by heating about 3 mg of sample from room temperature
up to 500 C (heating rate 5 K min1).
Differential scanning calorimetry (DSC) was performed by a Q2000 apparatus (TA
Instruments, New Castle, DE, USA) interfaced with a TA5000 data station by heating
about 3 mg of powder in an open aluminum crucible from
50
C to 350
C (heating
rate 5 K min
1
) under nitrogen flux (50 mL min
1
). Three independent measurements
on three different samples were performed for each cage. The temperature accuracy of
the instrument is
±
0.1
C, the precision is
±
0.01
C, and the calorimetric reproducibility
is
±
0.05%. TGA and DSC data were analyzed by the Universal Analysis software by TA
Instruments.
Scanning electron microscopy (SEM) analysis of the powder was performed by Phe-
nom Pro X desktop SEM, Phenom-World. The images were acquired with an accelerating
voltage of 10 kV at different magnification: 1000×, 5000×, and 20,000×.
3.3. Membranes Preparation
Mixed matrix membranes in PEEK–WC were prepared with a loading of 20 wt %
of each cage on the basis of polymer mass. PEEK–WC was dissolved in chloroform at
3 wt %
under magnetic stirring for 24 h at room temperature. Then, the obtained solution
was filtered by glass syringe filter of 3.1
µ
m. The cages were dispersed in chloroform
and sonicated for 30 min before their addition to the PEEK–WC solution. The PEEK–
WC/Cage suspension was sonicated for 5 h at room temperature in order to obtain a
homogeneous dispersion. The solutions were casted in a Teflon petri dish and the self-
standing dense membranes were obtained by solvent evaporation at 35
C. Finally, the
obtained membranes were coated with PDMS Elastosil M 4601 to perform the defect
healing process. The two-component PDMS, A and B, were mixed in weight ratio 9:1
according to the instructions of the supplier without the use of a solvent. A film of ca.
25
µ
m of the resin was applied on the surface of the membrane by a casting knife. The
coated membranes were kept at room temperature to complete the crosslinking in 24 h.
3.4. Membranes Characterization
3.4.1. Morphological Characterization Membranes (SEM)
Scanning electron microscopy (SEM) analysis of the membranes was performed by a
Phenom Pro X desktop SEM, Phenom-World. The images were acquired with an accelerat-
ing voltage of 15 kV at different magnifications: 1000×, 5000×, and 20,000×.
Molecules 2021,26, 5557 13 of 16
3.4.2. Single Gas Permeation Method
Single gas permeation measurements were performed on circular membranes (ex-
posed area 13.84 cm
2
) at 25
C and at a feed pressure of 1 bar by a fixed volume/pressure
increase instrument designed by HZG and constructed by EESR (Geesthacht, Germany).
Further details on the measurement protocols and data treatment are described in a previ-
ous paper [
53
]. Before each measurement, the membranes were evacuated in the testing
cell by a turbo-molecular pump until complete desorption of all previously adsorbed gases
and humidity. For the same reason, between two consecutive tests, the membranes were
evacuated for a time equal to 10 times the time lag of the previous gas.
The time lag method was used for the determination of the permeability (P), diffusion
(D), and solubility coefficients (S), which can be obtained from the increase of the permeate
pressure, pt, as a function of time, t, after exposure of the membrane to the gas [54]:
pt=p0+dp
dt 0
·t+RT
VP·Vm
·A·l·pf·S
×D·t
l21
62
π2
n=1
(1)n
n2expD·n2·π2·t
l2(6)
where p
0
and (dp/dt)
0
are the starting pressure and baseline slope, respectively, which should
be negligible in a well-evacuated and leak free membrane and permeability instrument. R
is the universal gas constant, Tthe absolute temperature, V
P
the permeate volume, V
m
the
molar volume of a gas at standard temperature and pressure [22.41
×
10
3
m
3STP
mol
1
at
0
C and 1 atm], Athe exposed membrane area, lits thickness, p
f
the feed pressure, Sthe
gas solubility and Dthe diffusion coefficient. The permeability Pwas obtained from the
permeation curve (7) in the pseudo steady-state:
pt=p0+dp
dt 0
.t+RT A
VpVm.pfP
ltl2
6D(7)
The diffusion coefficient is inversely proportional to time lag (
Θ
) and was calculated
from (8):
Θ=l2
6D(8)
The solubility coefficient (S), was calculated from the solution–diffusion transport
model (9):
S=P/D(9)
4. Conclusions
In this work, four different polyimine bistren cages were studied as fillers for the
preparation of PEEK–WC-based mixed matrix membranes for gas separation. Only the
two cages m-xy and Fura proved to be suitable for obtaining robust dense MMMs with
few enough pinhole defects to be healed by PDMS coating. Instead, inhomogeneous
dispersions were obtained with the two cages diphen and p-xy in the polymer solution,
which led to non-uniform and highly defective membranes also after the PDMS coating.
Compared to the pure polymer, the permeability of all the tested gases increased in
presence of Fura, and decreased with the m-xy cage. In terms of selectivity, the presence of
m-xy increases the selectivity for the gas pairs CO
2
/CH
4
, CO
2
/N
2
, O
2
/N
2
, and He/CH
4
,
whereas the CO
2
/N
2
selectivity did not significantly change with Fura and it decreased
for the other gas pairs. The behavior can be explained with the Maxwell model, which
indicates that the permeability of m-xy is lower than that of the polymer matrix whereas
that of Fura is much higher. The exceptionally high permeability in the presence of Fura
suggests that there are other permeation pathways, probably in the cage/polymer interface.
The use of these cages as fillers in the polymer matrix generally increases the diffusivity
for all the investigated gases. In particular, the slight increase in diffusivity promoted
by m-xy enhances the size-selectivity of the membrane. The diffusivity of the bulkiest
Molecules 2021,26, 5557 14 of 16
gas molecules increases remarkably with the Fura cage, resulting in an evident loss in
diffusion selectivity. The gas transport follows the solution–diffusion mechanism also
after the dispersion of the cages in the membranes, because it does not change the linear
correlation between the logarithm of the diffusion coefficient and the square of the effective
gas diameter d
2eff
. The obtained results are a good starting point for studying the effect
of cages concentration on the gas transport properties of MMMs in different polymer
materials, which will be evaluated in detail in further studies. Since integrally skinned
or thin film composite PEEK–WC membranes may have a selective layer thickness below
100 nm [
49
], future work should also focus on further control of the cage size down to
nanometer-scale, in order to produce realistic membranes successfully.
Supplementary Materials:
The following are available, Figure S1:
1
H-NMR spectrum of Fura in
CDCl
3,
, Figure S2:
1
H-NMR spectrum of m-xy in CDCl
3
, Figure S3:
1
H-NMR spectrum of p-xy in
CDCl
3
, Figure S4:
1
H-NMR spectrum of the diphenyl cage in CDCl3, Figure S5: FTIR-ATR spectrum
of Fura, Figure S6: FTIR-ATR spectrum of m-xy, Figure S7: FTIR-ATR spectrum of p-xy, Figure S8:
FTIR-ATR spectrum of the diphenyl cage, Figure S9: DSC curve of Fura, Figure S10: DSC curve of
m-xy, Figure S11: DSC curve of p-xy, Figure S12: DSC curve of the diphenyl cage, Figure S13: XRPD
patterns of the p-xy (left) and diphenyl (right) cages samples.
Author Contributions:
Conceptualization, J.C.J. and V.A.; investigation: synthesis and characteriza-
tion of cages: R.M., C.M. and S.L.C.; membrane preparation and characterization: M.M.; permeation
measurements, M.M. and E.E.; data curation, A.F.; writing—original draft preparation, M.M., A.F.,
J.C.J. and S.L.C.; writing—review and editing, A.F., J.C.J. and V.A.; supervision, J.C.J. and V.A.; project
administration, V.A.; funding acquisition, J.C.J. and V.A. All authors have read and agreed to the
published version of the manuscript.
Funding:
Part of the work carried out for this manuscript received financial support from the
Fondazione CARIPLO, programme “Economia Circolare: ricerca per un futuro sostenibile” 2019,
Project code: 2019–2090, MOCA—Metal Organic frameworks and organic CAges for highly selective
gas separation membranes and heavy metal capture devices.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Original data are available from the authors upon request.
Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or
in the decision to publish the results.
Sample Availability: Samples of the compounds are available from the authors upon request.
References
1.
Stanovsky, P.; Karaszova, M.; Petrusova, Z.; Monteleone, M.; Jansen, J.C.; Comesaña-Gándara, B.; McKeown, N.B.; Izak, P.
Upgrading of raw biogas using membranes based on the ultrapermeable polymer of intrinsic microporosity PIM-TMN-Trip. J.
Membr. Sci. 2021,618, 118694. [CrossRef]
2.
Scholes, C.A.; Stevens, G.W.; Kentish, S.E. Membrane gas separation applications in natural gas processing. Fuel
2012
,96, 15–28.
[CrossRef]
3.
Lu, H.T.; Li, W.; Miandoab, E.S.; Kanehashi, S.; Hu, G. The opportunity of membrane technology for hydrogen purification in the
power to hydrogen (P2H) roadmap: A review. Front. Chem. Sci. Eng. 2021,15, 464–482. [CrossRef]
4.
Lin, H.; Zhou, M.; Ly, J.; Vu, J.; Wijmans, J.G.; Merkel, T.C.; Jin, J.; Haldeman, A.; Wagener, E.H.; Rue, D. Membrane-Based
Oxygen-Enriched Combustion. Ind. Eng. Chem. Res. 2013,52, 10820–10834. [CrossRef]
5.
Chawla, M.; Saulat, H.; Masood Khan, M.; Mahmood Khan, M.; Rafiq, S.; Cheng, L.; Iqbal, T.; Rasheed, M.I.; Farooq, M.Z.; Saeed,
M.; et al. Membranes for CO2/CH4and CO2/N2Gas Separation. Chem. Eng. Technol. 2020,43, 184–199. [CrossRef]
6.
Kosinov, N.; Gascon, J.; Kapteijn, F.; Hensen, E.J.M. Recent developments in zeolite membranes for gas separation. J. Membr. Sci.
2016,499, 65–79. [CrossRef]
7.
Khdhayyer, M.R.; Esposito, E.; Fuoco, A.; Monteleone, M.; Giorno, L.; Jansen, J.C.; Attfield, M.P.; Budd, P.M. Mixed matrix
membranes based on UiO-66 MOFs in the polymer of intrinsic microporosity PIM-1. Sep. Purif. Technol.
2017
,173, 304–313.
[CrossRef]
Molecules 2021,26, 5557 15 of 16
8.
Khdhayyer, M.; Bushell, A.F.; Budd, P.M.; Attfield, M.P.; Jiang, D.; Burrows, A.D.; Esposito, E.; Bernardo, P.; Monteleone, M.;
Fuoco, A.; et al. Mixed matrix membranes based on MIL-101 metal–organic frameworks in polymer of intrinsic microporosity
PIM-1. Sep. Purif. Technol. 2019,212, 545–554. [CrossRef]
9.
Kamble, A.R.; Patel, C.M.; Murthy, Z.V.P. A review on the recent advances in mixed matrix membranes for gas separation
processes. Renew. Sustain. Energy Rev. 2021,145, 111062. [CrossRef]
10.
Zhang, Q.; Li, H.; Chen, S.; Duan, J.; Jin, W. Mixed-matrix membranes with soluble porous organic molecular cage for highly
efficient C3H6/C3H8separation. J. Membr. Sci. 2020,611, 118288. [CrossRef]
11.
Bushell, A.F.; Budd, P.M.; Attfield, M.P.; Jones, J.T.A.; Hasell, T.; Cooper, A.I.; Bernardo, P.; Bazzarelli, F.; Clarizia, G.; Jansen, J.C.
Nanoporous organic polymer/cage composite membranes. Angew. Chem. Int. Ed. 2013,52, 1253–1256. [CrossRef] [PubMed]
12.
Jansen, J.C.; Drioli, E. Poly(ether ether ketone) derivative membranes—A review of their preparation, properties and potential.
Polym. Sci. Ser. A 2009,51, 1355. [CrossRef]
13.
Esposito, E.; Bruno, R.; Monteleone, M.; Fuoco, A.; Ferrando Soria, J.; Pardo, E.; Armentano, D.; Jansen, J.C. Glassy PEEK-WC vs.
Rubbery Pebax
®
1657 Polymers: Effect on the Gas Transport in CuNi-MOF Based Mixed Matrix Membranes. Appl. Sci.
2020
,10,
1310. [CrossRef]
14. Steed, J.W.; Atwood, J.L. Supramolecular Chemistry; John Wiley & Sons, Ltd.: Chichester, UK, 2009; ISBN 9780470740880.
15. Sanders, J.K. Supramolecular Chemistry. Concepts and Perspectives. Von J.-M. Lehn. VCH Verlagsgesellschaft, Weinheim, 1995.
271 S., geb. 128.00 DM/Broschur 58.00 DM.-ISBN 3-527-29312-4/3-527-29311-6. Angew. Chem. 1995,107, 2617. [CrossRef]
16. Fabbrizzi, L. Cryptands and Cryptates; World Scientific Publishing Europe Ltd.: London, UK, 2017; ISBN 978-1-78634-369-7.
17.
Kubik, S. Anion Recognition in Aqueous Media by Cyclopeptides and Other Synthetic Receptors. Acc. Chem. Res.
2017
,50,
2870–2878. [CrossRef] [PubMed]
18.
Galan, A.; Ballester, P. Stabilization of reactive species by supramolecular encapsulation. Chem. Soc. Rev.
2016
,45, 1720–1737.
[CrossRef]
19.
McNaughton, D.A.; Fares, M.; Picci, G.; Gale, P.A.; Caltagirone, C. Advances in fluorescent and colorimetric sensors for anionic
species. Coord. Chem. Rev. 2021,427, 213573. [CrossRef]
20.
Tozawa, T.; Jones, J.T.A.; Swamy, S.I.; Jiang, S.; Adams, D.J.; Shakespeare, S.; Clowes, R.; Bradshaw, D.; Hasell, T.; Chong, S.Y.;
et al. Porous organic cages. Nat. Mater. 2009,8, 973. [CrossRef]
21.
Martínez-Ahumada, E.; He, D.; Berryman, V.; López-Olvera, A.; Hernandez, M.; Jancik, V.; Martis, V.; Vera, M.A.; Lima, E.; Parker,
D.J.; et al. SO2Capture Using Porous Organic Cages. Angew. Chem. Int. Ed. 2021,60, 17556–17563. [CrossRef]
22.
Chen, L.; Reiss, P.S.; Chong, S.Y.; Holden, D.; Jelfs, K.E.; Hasell, T.; Little, M.A.; Kewley, A.; Briggs, M.E.; Stephenson, A.; et al.
Separation of rare gases and chiral molecules by selective binding in porous organic cages. Nat. Mater.
2014
,13, 954–960.
[CrossRef]
23. Hasell, T.; Cooper, A.I. Porous organic cages: Soluble, modular and molecular pores. Nat. Rev. Mater. 2016,1, 16053. [CrossRef]
24.
Beuerle, F.; Gole, B. Covalent Organic Frameworks and Cage Compounds: Design and Applications of Polymeric and Discrete
Organic Scaffolds. Angew. Chem. Int. Ed. 2018,57, 4850–4878. [CrossRef] [PubMed]
25.
Little, M.A.; Cooper, A.I. The Chemistry of Porous Organic Molecular Materials. Adv. Funct. Mater.
2020
,30, 1909842. [CrossRef]
26.
Evans, J.D.; Huang, D.M.; Hill, M.R.; Sumby, C.J.; Thornton, A.W.; Doonan, C.J. Feasibility of mixed matrix membrane gas
separations employing porous organic cages. J. Phys. Chem. C 2014,118, 1523–1529. [CrossRef]
27.
Amendola, V.; Bergamaschi, G.; Miljkovic, A. Azacryptands as molecular cages for anions and metal ions. Supramol. Chem.
2018
,
30, 236–242. [CrossRef]
28.
Alibrandi, G.; Amendola, V.; Bergamaschi, G.; Fabbrizzi, L.; Licchelli, M. Bistren cryptands and cryptates: Versatile receptors for
anion inclusion and recognition in water. Org. Biomol. Chem. 2015,13, 3510–3524. [CrossRef]
29.
Gajula, R.K.; Kishor, R.; Prakash, M.J. Imine-Linked Covalent Organic Cage Porous Crystals for CO
2
Adsorption. ChemistrySelect
2019,4, 12547–12555. [CrossRef]
30.
Gayen, K.S.; Das, T.; Chatterjee, N. Recent Advances in Tris-Primary Amine Based Organic Imine Cages and Related Amine
Macrocycles. Eur. J. Org. Chem. 2021,2021, 861–876. [CrossRef]
31.
Wang, F.; Sikma, E.; Duan, Z.; Sarma, T.; Lei, C.; Zhang, Z.; Humphrey, S.M.; Sessler, J.L. Shape-persistent pyrrole-based covalent
organic cages: Synthesis, structure and selective gas adsorption properties. Chem. Commun.
2019
,55, 6185–6188. [CrossRef]
[PubMed]
32.
Shimekit, B.; Mukhtar, H.; Murugesan, T. Prediction of the relative permeability of gases in mixed matrix membranes. J. Membr.
Sci. 2011,373, 152–159. [CrossRef]
33. Pal, R. Permeation models for mixed matrix membranes. J. Colloid Interface Sci. 2008,317, 191–198. [CrossRef] [PubMed]
34.
Kołodziejski, M.; Stefankiewicz, A.R.; Lehn, J.M. Dynamic polyimine macrobicyclic cryptands-self-sorting with component
selection. Chem. Sci. 2019,10, 1836–1843. [CrossRef]
35.
MacRae, C.F.; Sovago, I.; Cottrell, S.J.; Galek, P.T.A.; McCabe, P.; Pidcock, E.; Platings, M.; Shields, G.P.; Stevens, J.S.; Towler, M.;
et al. Mercury 4.0: From visualization to analysis, design and prediction. J. Appl. Crystallogr. 2020,53, 226–235. [CrossRef]
36.
McDowell, D.; Nelson, J.; McKee, V. A furan-derived schiff-base crypt and incorporating the trans, trans dicarbimine link.
Polyhedron 1989,8, 1143–1145. [CrossRef]
37.
McKee, V.; Robinson, W.T.; McDowell, D.; Nelson, J. Solid state structure and solution conformation of a macrobicyclic cyclophane.
Tetrahedron Lett. 1989,30, 7453–7456. [CrossRef]
Molecules 2021,26, 5557 16 of 16
38.
Suleman, M.S.; Lau, K.K.; Yeong, Y.F. Characterization and Performance Evaluation of PDMS/PSF Membrane for CO
2
/CH
4
Separation under the Effect of Swelling. Procedia Eng. 2016,148, 176–183. [CrossRef]
39.
Haider, B.; Dilshad, M.R.; Atiq Ur Rehman, M.; Vargas Schmitz, J.; Kaspereit, M. Highly permeable novel PDMS coated
asymmetric polyethersulfone membranes loaded with SAPO-34 zeoilte for carbon dioxide separation. Sep. Purif. Technol.
2020
,
248, 116899. [CrossRef]
40.
Madaeni, S.S.; Badieh, M.M.S.; Vatanpour, V.; Ghaemi, N. Effect of titanium dioxide nanoparticles on polydimethylsilox-
ane/polyethersulfone composite membranes for gas separation. Polym. Eng. Sci. 2012,52, 2664–2674. [CrossRef]
41.
Madaeni, S.S.; Badieh, M.M.S.; Vatanpour, V. Effect of coating method on gas separation by PDMS/PES membrane. Polym. Eng.
Sci. 2013,53, 1878–1885. [CrossRef]
42.
Jansen, J.C.; Friess, K.; Drioli, E. Organic vapour transport in glassy perfluoropolymer membranes: A simple semi-quantitative
approach to analyze clustering phenomena by time lag measurements. J. Membr. Sci. 2011,367, 141–151. [CrossRef]
43.
Jansen, J.; Macchione, M.; Drioli, E. High flux asymmetric gas separation membranes of modified poly(ether ether ketone)
prepared by the dry phase inversion technique. J. Membr. Sci. 2005,255, 167–180. [CrossRef]
44.
Robeson, L.M. Correlation of separation factor versus permeability for polymeric membranes. J. Membr. Sci.
1991
,62, 165–185.
[CrossRef]
45. Robeson, L.M. The upper bound revisited. J. Membr. Sci. 2008,320, 390–400. [CrossRef]
46. Swaidan, R.; Ghanem, B.; Pinnau, I. Fine-Tuned Intrinsically Ultramicroporous Polymers Redefine the Permeability/Selectivity
Upper Bounds of Membrane-Based Air and Hydrogen Separations. ACS Macro Lett. 2015,4, 947–951. [CrossRef]
47.
Comesaña-Gándara, B.; Chen, J.; Bezzu, C.G.; Carta, M.; Rose, I.; Ferrari, M.-C.; Esposito, E.; Fuoco, A.; Jansen, J.C.; McKeown, N.B.
Redefining the Robeson upper bounds for CO
2
/CH
4
and CO
2
/N
2
separations using a series of ultrapermeable benzotriptycene-
based polymers of intrinsic microporosity. Energy Environ. Sci. 2019,12, 2733–2740. [CrossRef]
48.
Fuoco, A.; Rizzuto, C.; Tocci, E.; Monteleone, M.; Esposito, E.; Budd, P.M.; Carta, M.; Comesaña-Gándara, B.; McKeown, N.B.;
Jansen, J.C. The origin of size-selective gas transport through polymers of intrinsic microporosity. J. Mater. Chem. A
2019
,7,
20121–20126. [CrossRef]
49.
Jansen, J.C.; Buonomenna, M.G.; Figoli, A.; Drioli, E. Asymmetric membranes of modified poly(ether ether ketone) with an
ultra-thin skin for gas and vapour separations. J. Membr. Sci. 2006,272, 188–197. [CrossRef]
50.
Teplyakov, V.; Meares, P. Correlation aspects of the selective gas permeabilities of polymeric materials and membranes. Gas Sep.
Purif. 1990,4, 66–74. [CrossRef]
51.
Miljkovic, A.; La Cognata, S.; Bergamaschi, G.; Freccero, M.; Poggi, A.; Amendola, V. Towards building blocks for supramolecular
architectures based on azacryptates. Molecules 2020,25, 1733. [CrossRef]
52.
Yang, Z.; Lehn, J.M. Dynamic covalent self-sorting and kinetic switching processes in two cyclic orders: Macrocycles and
macrobicyclic cages. J. Am. Chem. Soc. 2020,142, 15137–15145. [CrossRef] [PubMed]
53.
Fraga, S.C.; Monteleone, M.; Lanˇc, M.; Esposito, E.; Fuoco, A.; Giorno, L.; Pilnáˇcek, K.; Friess, K.; Carta, M.; McKeown, N.B.; et al.
A novel time lag method for the analysis of mixed gas diffusion in polymeric membranes by on-line mass spectrometry: Method
development and validation. J. Membr. Sci. 2018,561, 39–58. [CrossRef]
54.
Wijmans, J.G.; Baker, R.W. The Solution–Diffusion Model: A Unified Approach to Membrane Permeation. In Materials Science
of Membranes for Gas and Vapor Separation; Freeman, B., Yampolskii, Y., Pinnau, I., Eds.; John Wiley & Sons, Ltd.: Hoboken, NJ,
USA, 2006.
ResearchGate has not been able to resolve any citations for this publication.
Article
Full-text available
We report the first experimental investigation of porous organic cages (POCs) for the demanding challenge of SO 2 capture. Three isostructural cage molecular materials were studied. An imine functionalized POC (CC3) showed modest and reversible SO 2 capture, while a secondary amine POC (RCC3) exhibited high but irreversible SO 2 capture. A tertiary amine POC (6FT‐RCC3) demonstrated very high SO 2 capture (13.78 mmol g ‐1 ; 16.4 SO 2 molecules per cage) combined with excellent reversibility for at least 50 adsorption‐desorption cycles. The adsorption behavior was investigated by FTIR spectroscopy, 13 C CP MAS NMR experiments and computational calculations.
Article
Full-text available
The potential of an ultrapermeable benzotriptycene-based polymer of intrinsic microporosity (PIM-TMN-Trip) for the upgrading of biogas is investigated. Permeation experiments were performed using an in-house bespoke permeation unit for pure gases and gas mixtures, and included tests with model mixtures as well as real biogas from a sewage treatment plant, under dry and humid conditions. Permeability and CO2/CH4 selectivity for either pure gases or for real biogas were high and lie close to or on the recently defined 2019 Robeson upper bound based on ideal permselectivities. In addition, a remarkable increase in CO2/CH4 selectivity was observed after two weeks of continuous exposure to CO2 due to a significant decrease of CH4 permeability. The constant CO2 permeability and increased selectivity upon ageing suggest that ageing in the presence of CO2 causes a rearrangement, rather than a reduction of the fractional free volume. The mixed gas permeability experiments were performed with high stage-cut in order to mimic a real separation process, and the results confirmed the potential of PIM-TMN-Trip membranes for biogas upgrading.
Article
Full-text available
In this work, we report the synthesis of a new bis(tris(2-aminoethyl)amine) azacryptand L with triphenyl spacers. The binding properties of its dicopper complex for aromatic dicarboxylate anions (as TBA salts) were investigated, with the aim to obtain potential building blocks for supramolecular structures like rotaxanes and pseudo-rotaxanes. As expected, UV-Vis and emission studies of [Cu2L]4+ in water/acetonitrile mixture (pH = 7) showed a high affinity for biphenyl-4,4′-dicarboxylate (dfc2−), with a binding constant of 5.46 log units, due to the best match of the anion bite with the Cu(II)-Cu(II) distance in the cage’s cavity. Compared to other similar bistren cages, the difference of the affinity of [Cu2L]4+ for the tested anions was not so pronounced: conformational changes of L seem to promote a good interaction with both long (e.g., dfc2−) and short anions (e.g., terephthalate). The good affinity of [Cu2L]4+ for these dicarboxylates, together with hydrophobic interactions within the cage’s cavity, may promote the self-assembly of a stable 1:1 complex in water mixture. These results represent a good starting point for the application of these molecular systems as building units for the design of new supramolecular architectures based on non-covalent interactions, which could be of interest in all fields related to supramolecular devices.
Article
Gas separation processes are amongst the most important operations in the refineries and gas-related industries. Recently, many efforts are being dedicated towards modifying the gas separation properties of existing polymers to further expand their use for extensive industrial gas separation applications. Mixed matrix membranes (MMMs), which are organic–inorganic hybrid membranes, have been proposed as the alternative approach to intensify the comprehensive gas separation performance of the polymeric membranes. In this regard, we analyze and review the recent scientific and technological advances in the development of MMM's, including the emerging class of inorganic fillers like two-dimensional (2D)materials, that have been the focus of much recent work for gas separation. The review also discusses the current issues associated with the filler materials and further provides an outline to overcome the emerging challenges for the future development of high performance MMMs.
Article
The global energy market is in a transition towards low carbon fuel systems to ensure the sustainable development of our society and economy. This can be achieved by converting the surplus renewable energy into hydrogen gas. The injection of hydrogen (⩽10% v/v) in the existing natural gas pipelines is demonstrated to have negligible effects on the pipelines and is a promising solution for hydrogen transportation and storage if the end-user purification technologies for hydrogen recovery from hydrogen enriched natural gas (HENG) are in place. In this review, promising membrane technologies for hydrogen separation is revisited and presented. Dense metallic membranes are highlighted with the ability of producing 99.9999999% (v/v) purity hydrogen product. However, high operating temperature (⩾300 °C) incurs high energy penalty, thus, limits its application to hydrogen purification in the power to hydrogen roadmap. Polymeric membranes are a promising candidate for hydrogen separation with its commercial readiness. However, further investigation in the enhancement of H2/CH4 selectivity is crucial to improve the separation performance. The potential impacts of impurities in HENG on membrane performance are also discussed. The research and development outlook are presented, highlighting the essence of upscaling the membrane separation processes and the integration of membrane technology with pressure swing adsorption technology.
Article
Molecules bearing three primary amine groups are ubiquitous substances in various fields of synthetic chemistry. Some of them are commercially available and the rests are emerged from the designed protocols of synthetic chemists. It has been observed that such aliphatic as well as aromatic triamines are excellent precursors for the design and synthesis of various cage molecules of which organic imine cages (OICs) and related amine macrocycles belong to the category of special interest. These compounds possess unique architectures and already made their marks in the field of supramolecular chemistry, synthetic methodology, and material science. In this review we aimed to consider recent reports that highlighted the syntheses, special features, and applications of primary triamine based OICs and related amine macrocycles. Intramolecular integration of Tris‐primary amine moiety and multiple aldehyde centers often leads to organic imine cages (OICs) and related amine macrocycles which demand commendable position in the field of material science and supramolecular chemistry. In this review we highlighted recent studies regarding the synthesis and special features of such OICs and their saturated counterparts.
Article
In this review we cover recent developments in fluorescent and colorimetric anion sensors. These systems employ a range of different non-covalent interactions including hydrogen- and halogen-bonding, Lewis acidic boron-based sensors, metal-based sensors, charged systems, compounds that use anion-pi interactions to stabilise complexes, photoswitchable systems, the use of excimers and molecular logic gates in sensing. Recent developments in anion-selective chemodosimeters are also surveyed.
Article
Dynamic covalent component self-sorting processes have been investigated for constituents of different cyclic orders, macrocycles and macrobicyclic cages based on multiple reversible imine formation. The progressive assembly of the final structures from dialdehyde and polyamine components involved the generation of kinetic products and mixtures of intermediates which underwent component selection and self-correction to generate the final thermodynamic constituents. Importantly, constitutional dynamic networks (CDNs) of macrocycles and macrobicyclic cages were set up either from separately prepared constituents or by in situ assembly from their components. Over time, these CDNs underwent conversion from a kinetically trapped out-of-equilibrium distribution of constituents to the thermodynamically self-sorted one through component exchange in different dimensional orders.
Article
Gas mixture separation of C3H6/C3H8 has great importance in industry with increasing demand for value-added polypropylene. However, realization of a cost- and energy-efficient separation procedure and its practicability is still a challenge. This work reports a group of mixed-matrix membranes (MMM) by incorporating porous organic molecular cages (POMC) within polymer. Unlike traditional solid filler, POMC filler (CC3) can be dissolved in solvent and mixed with polymer in molecular lever. The uniform distribution of CC3 particles and/or even single CC3 cage in polymer form an attractive hierarchical transport channel, which allows binary mixture permeation test (1/1, v/v) on MMM-20 wt% (0.3 MPa and 20 °C) have very fast C3H6 permeability (390 Barrer) and good C3H6/C3H8 separation factor (12.1). Gas transportation mechanism was evaluated and unveiled by positron annihilation lifetime spectroscopy and molecular dynamic simulation. More importantly, the MMM-20 wt% demonstrates stable C3H6/C3H8 separation during long term test, which is far beyond the Robeson upper bound, indicating high potential for feasible usage.
Article
Novel asymmetric polyethersulfone membranes loaded with SAPO-34 particles were prepared using phase inversion technique and then surface coated with PDMS using spin coating method. The mixed matrix membranes were then characterized by FTIR, TGA, SEM-EDX and gas permeation analysis. Effect of SAPO-34 loading as well as operating pressure was also analyzed on gas permeation properties of both coated and uncoated membranes. SAPO-34 loading resulted in improvement of permeability of all the gases without much decrease in CO2 selectivity with respect to methane and nitrogen whereas PDMS coating resulted in improvement of selectivity of CO2 at the expense of decrease of permeance of all the gases. It was found that PDMS coated PES membrane, loaded with 30 wt. % SAPO-34, having thickness of 45 micron, demonstrated high CO2 permeance of 641.77 GPU, CO2/CH4 ideal selectivity of 4.45 and CO2/N2 ideal selectivity of 12.45, respectively at 20 bar and 25 °C. It was found that the performance of this membrane crossed the Robeson upper bound limit 2008 for CO2/N2 separation whereas for CO2/CH4 separation it crossed the previous upper bound limit 1991. Finally, the performance of this membrane was also analyzed under mixed gas conditions for CO2/CH4 separation at high pressure.