Infiltrating molecular gatekeepers with coexisting molecular solubility and 3D-intrinsic porosity into a microporous polymer scaffold for gas separation

Ji Wu a, Susilo Japip b and Tai-Shung Chung *ab
aNUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore 117456, Singapore. E-mail: chencts@nus.edu.sg
bDepartment of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117576, Singapore

Received 1st November 2019 , Accepted 9th February 2020

First published on 11th February 2020


The inherently broad pore-size distribution in polymer membranes endows them with fewer size-selective microporous regions which could impair their performance and would require challenging size control at the angstrom level for enabling energy-efficient gas separation. Here, we successfully remodeled the non-selective microporous regions in polymer membranes with sub-angstrom size-sensitivity via an unconventional post-(membrane) fabrication infiltration (PFI) method based on a water-soluble member of the highly tunable and expandable organic macrocyclic family, namely, 4-sulfocalix[4]arene (SCA4). The small molecular size and attached multiple sulfonic groups of SCA4 molecules have enabled their complete solvation in common membrane-treating protic solvents, like methanol, such that they could molecularly infiltrate the entire microporous structure of already fabricated polymer membranes which plays the role of an interactive scaffold with extensive hydrogen or ionic bonding sites. Meanwhile, bearing an intrinsic size-sieving 3D open cavity, SCA4 molecules could act as molecular gatekeepers that effectively retard size-indiscriminative gas transport for realizing exceptional molecular-sieving properties towards efficient separation of multiple important gas pairs. This ultra-facile yet unconventional PFI design is free from the longstanding issues of interfacial nano-defects and pore blockage and is also potentially diversifiable by using a pool of other water-soluble and functionalizable macrocyclic counterparts for uncovering new composite-membrane design possibilities.


1. Introduction

Polymer membranes present a promising energy-efficient technology for many important gas separation processes, such as hydrogen recovery (H2/N2, H2/CH4) and carbon capture from syngas, coal gasifiers (H2/CO2) or natural gas (CO2/CH4),1–5 which are all critical for clean energy and environmental development. However, the natural random packing of polymers causes broadly distributed pore sizes in these membranes, which unfortunately include non-size-selective microporous regions that could impair their gas separation efficiency and hence industrial competitiveness. The facile control of such non-selective regions with the required size-sensitivity on the sub-angstrom level still remains a key material challenge.6,7

Utilizing the intrinsic molecular-sieving nanopores of an external nanoporous agent, like 2D or 3D inorganic, carbon-based or extended framework-type materials, in polymer gas separation membranes has attracted broad research interest because of their potential promise for realizing advantageous composite properties. However, their poor solution-processability and challenging compatibility with organic polymers often lead to detrimental issues of particle agglomeration, interfacial voids or pore blockage which conventionally require delicate interfacial design and more aggressive physical mixing or dispersing techniques to be alleviated.8–11 Even for purely organic nanoporous fillers with better polymer affinity and organic-solvent processability, such as covalent organic frameworks (COFs) or porous organic cages (POCs), true molecular mixing or dispersion between individual filler molecules and polymer chains remains difficult due to their inherent tendency to crystallize and self-assemble which perpetuates the threat of nanoscopic interfacial defects.12–14 In this respect, for most of the studied filler choices, only a conventional mixed-matrix structure comprising a discretely dispersed filler phase and a continuous polymer phase has been achieved (Fig. 1). Besides, physical mixing typically carried out before membrane fabrication often creates an uncontrollable window for this crystallization/self-assembly propensity to be displayed as well as for the polymer chains to block the fillers' nanopores, partly due to the high migration and chain mobility in the solution-mixture state.9,15,16


image file: c9ta12028a-f1.tif
Fig. 1 Schematics of the SCA4 infiltration via the PFI strategy. Conventional physically dispersed frameworks and polymers before membrane fabrication tend to form nanoscopic defects due to crystallization propensity, whereas protic-solvent-soluble macrocyclic molecules enable a molecular infiltration route into the already fabricated membrane.

Conversely, if a nanoporous agent could be incorporated into polymer membranes on a molecularly homogeneous basis after the polymers are fabricated into films or selective layers such that the need of undergoing any potentially problematic pre-fabrication mixing stages could be eliminated (i.e. the direct insertion of entire fillers rather than via the pre-seeding process or in situ syntheses), these above issues would be safely avoided. As good as it sounds, this bold conjecture still remains an underexplored, technically challenging gap because it could proceed most possibly via first the complete solvation of the nanoporous agents in certain solvents so as to be able to diffuse deep into the fabricated polymer films. Yet, while inorganic fillers generally lack good solubility, the organic ones tend to be soluble in solvents that dissolve the polymers too.

Here, we propose three key criteria for the choice of material that could possibly enable this design: (1) being water-soluble such that it can be molecularly solvated by common membrane-treating protic solvents, like methanol or ethanol, which can diffuse through the entire polymer microporous structure without dissolving the polymers;17 (2) being capable of forming extensive hydrogen and ionic bonds with the polymer backbones which are stronger than both the solvation forces and the self-aggregation tendencies in order to be homogeneously distributed and firmly anchored into the membranes (possible to be coincided with criterion (1) because water-soluble functionalities tend to form ionic or hydrogen-bond interactions with the N- or O-containing groups not uncommon to find in many polymers); (3) having intrinsic size-sieving pores or open cavities to act as molecular-sieving windows for discriminatory gas passage, which will require such materials to have multi-dimensional architectures, but at the same time also possessing a small molecular size for effectively infiltrating the fabricated films without encountering too much mass transfer resistance. Coexistence of these seemingly contradictory criteria had hardly been considered before because the conventionally perceived multi-dimensional nanoporous materials like the various framework-type particles generally lack protic solvent solubility which is, however, more commonly found in small, non-extended molecules. Yet, these small molecules tend not to possess intrinsic pores because they are often 0- or 1-dimensionally structured.

In this study, judicious navigation led us to locate one representative and also the most simply structured member, namely 4-sulfocalix[4]arene (SCA4), from the organic macrocyclic calixarene family that perfectly fitted these criteria. SCA4 molecules possess an intrinsic 3-dimensional bowl-shaped cavity (Fig. 1) with a small range of bottom opening sizes around a mean value of 3.0 Å resulting from the partially flexible methylene linkers that give rise to conformational flexibility around the bowl shape.18–20 Therefore, they can act as size-sieving molecular gatekeepers in membranes that selectively pass gases with a size smaller than or similar to that of its mean bottom opening, like H2 (2.89 Å) and CO2 (3.30 Å), while strongly impeding larger gas molecules, like N2 (3.64 Å) and CH4 (3.80 Å). Meanwhile, the multiple water-soluble sulfonic groups on the upper rim of SCA4's molecule-sized body enable its complete solvation in methanol so that the solution can carry SCA4 molecules to molecularly infiltrate the microporous structure of already formed polymer films which serves as a microporous scaffold providing active lodging sites with extensive hydrogen and ionic bonding capability. As such, an ultra-facile but unconventional composite membrane design by incorporating porous agents only after the membrane was fabricated was enabled, named herein the post-fabrication infiltration (PFI) membranes. More importantly, besides SCA4, calixarenes are actually a huge class of tuneable and expandable organic porous molecules with other base cavity sizes built with different numbers of p-phenol subunits, like calix[n]arenes, where n can be, but is not limited to, 4, 5, 6 and 8.21–23 They are also bestowed with dual sites for functionalization.19,20 The upper rim can come with a variety of water-soluble moieties besides sulfonic groups, such as phosphoric or amine groups,24,25 that further broaden the pool of PFI-viable polymers, while the lower rim phenolic hydroxyl could also be subjected to chemical modification to provide additional functionalities or finer tuning of the bottom opening size as the literatures has shown cavity expansion by appending lower rim substituents.26 Therefore, this PFI strategy for utilizing molecules of potentially tuneable intrinsic nanoporosity not only reveals a material advancement by delivering high-performance molecular-sieving composite membranes, but also methodological progress by completely bypassing the ingrained issues of interfacial nano-defects and pore blockage in composite membrane designs.

2. Experimental

2.1 Materials

Monomers for the synthesis of polymers with intrinsic microporosity-1 (PIM-1), namely, 5,5′,6,6′-tetrahydroxy-3,3,3,3′-tetramethyl-1,1′-spirobisindane (TTSBI, 97%) and 2,3,5,6-tetrafluoroterephthalonitrile (TFTPN, 99%), were purchased from Alfa Aesar and Matrix Scientific, respectively. Prior to use, the former was purified via re-crystallization from methanol and the latter was purified via vacuum sublimation. A hydroxylamine solution (50% aqueous) was purchased from Merck and used directly for the functionalization of PIM-1 without any pre-treatment. 4-Sulfocalix[4]arene (SCA4, ≥97%, (C7H6O4S)4, 744.74 g mol−1) and calix[4]arene-25,26,27,28-tetrol (CA4t, 95%, (C7H6O)4, 424.49 g mol−1) were purchased from Merck and dehydrated at 120 °C under vacuum for one night before every use. N,N′-Dimethylformamide (DMF, 99.%) purchased from VWR Chemicals was purified via vacuum distillation at 60 °C before reaction and polymer dissolution. Anhydrous potassium carbonate (K2CO3, >99.5%) and sodium hydroxide pellets (NaOH, >98.5%) from Merck, methanol (MeOH, 99.8%) and ethanol (EtOH, HPLC, 99.8%) from Fisher Chemical, n-hexane (95%) from Tedia, tetrahydrofuran (THF, 99.7%) and hydrochloric acid (HCl, 32% aqueous) from VWR Chemicals, and concentrated sulfuric acid (H2SO4, 95% aqueous) from Avantor® were directly used as received. All purified gases (>99.5%), including H2, O2, N2, CH4, and CO2, and also the mixed gas of 50% H2 and 50% CH4 were supplied by Air Liquide Pte. Ltd. (Singapore).

2.2 Polymer synthesis

The synthesis protocol for the PIM-1 polymers can be found elsewhere.27 The as-synthesized PIM-1 polymer was functionalized with amidoxime to yield amidoxime-functionalized PIM1 (AOPIM1) by a simple one-pot reaction identically adopted from the literature,28 and the ethanol-washed AOPIM1 polymers were dried at 110 °C under vacuum for 20 hours. The respective yields for PIM-1 and AOPIM1 were about 78% and 94%.

2.3 Pristine AOPIM1 membrane fabrication

PIM-1 membranes were fabricated by a common solution-casting method identical to that in the literature.27 AOPIM1 membranes were also prepared via a simple solution-casting method, followed by a solvent exchange process with MeOH to remove trapped solvents, adopted from the literature. Briefly, 0.2 g AOPIM1 polymer was first dissolved in 10 g DMF to prepare a 2 wt% AOPIM1–DMF polymer solution. After stirring overnight and filtering twice using 5 μm PTFE syringe filters, the polymer solution was poured into a Petri dish and then placed in a vented oven at 100 °C and atmospheric pressure for 48 hours to slowly evaporate the solvent. After the film was formed, the Petri dish was removed from the oven and cooled down to room temperature. Deionized water was then added to the Petri dish to immerse the film for 5 min to help delaminate it from the glass surface. The as-cast AOPIM1 film was then cut into even pieces, weighing 65.0 ± 0.5 mg each, and submerged in MeOH contained in a screw-cap bottle with 100 rpm stirring for 24 hours to drive out the occluded DMF. After solvent exchange with MeOH, the moisture on the surface of the films was gently wiped off using tissue paper and then the films were dried under vacuum at 120 °C overnight to obtain the pristine AOPIM1 membrane samples.

The average sample thickness was 25 ± 5 μm, and the average sample weight loss due to the removal of occluded DMF was 7.8 ± 0.4 mg. One batch of the as-cast AOPIM1 membranes was directly dried under vacuum at 150 °C overnight which was an adequate temperature for the complete removal of trapped DMF solvents without immersion into MeOH. They were used as control samples to confirm the effective removal of occluded DMF by solvent exchange.

2.4 AOPIM1-SCA4 membrane fabrication by PFI

In screw-cap glass bottles, SCA4 powders, with varied masses of 1.2, 3.1, 5.0 and 10.3 mg, were dissolved into a fixed volume of MeOH to prepare SCA4-in-MeOH solutions with different SCA4 concentrations. Then, the as-cast AOPIM1 films of 65.0 ± 0.5 mg each were submerged in these SCA4-in-MeOH solutions for 24 hours to infiltrate the membranes with SCA4 molecules while exchanging the occluded DMF solvent at the same time. Lastly, these SCA4-infiltrated membrane samples were sent for final drying under vacuum at 120 °C overnight, and the resultant membranes were abbreviated as AOPIM1-SCA4 membranes.

2.5 Quantification of the degree of infiltration by titration

The ideal mole ratio of SCA4 to AOPIM1 polymer is defined as the mole ratio of the total SCA4 dissolved in MeOH to the AOPIM1 polymer content in the as-cast films. Given the average 7.8 mg loss of DMF from 65 mg of the as-cast AOPIM1 films as determined previously, one could then estimate the ideal mole ratios of SCA4 to AOPIM1 polymer to be about 1.5%, 3.8%, 6.2% and 12.7%, respectively. However, not all the SCA4 dissolved in MeOH could be infiltrated into the membrane samples due to the eventual mass transfer equilibrium reached. Thus, the actual mole ratio of SCA4 to AOPIM1 polymer is defined as the mole ratio of SCA4 in the resultant AOPIM1-SCA4 membranes to the AOPIM1 polymer content in the resultant AOPIM1-SCA4 membranes, and it was measured by the titration of residual SCA4 in the used SCA4-in-MeOH solutions.

By using a digital pH meter (Oakton pH 450 Meter Kit), the pH values of initial SCA4-in-MeOH solutions with known SCA4 amounts were measured. Then, these solutions were titrated with a 100 ml MeOH solution containing 20 mg NaOH (the titrant) to the point where their pH values returned to that of pure MeOH. From the titration results, a calibration curve, as shown in Fig. S1, was established between the SCA4 mass in a fixed volume of MeOH and the titrant volume used. By interpolating within the calibration curve, the residual mass of SCA4 in the SCA4-in-MeOH solutions used was determined using the same titrant. As such, the actual mass of SCA4 infiltrated into the membranes could be calculated from the mass difference of SCA4 in MeOH before and after the infiltration process. With the dry mass of the resultant AOPIM1-SCA4 membrane samples being measured, the actual mole ratio of SCA4 to AOPIM1 polymer (526 g mol−1) could be calculated.

Table S1 summarizes the results. According to the actual mole ratios of SCA4 to AOPIM1 polymer, which were found to be 0.99%, 2.44%, 3.48% and 4.96%, the resultant AOPIM1-SCA4 membranes were named AOPIM1-SCA4-1%, AOPIM1-SCA4-2%, AOPIM1-SCA4-3% and AOPIM1-SCA4-5%, respectively, for simplicity. These percentage values were also referred to as the ‘degree of infiltration’ or ‘x% infiltration’ in this study.

2.6 Positron annihilation lifetime spectroscopy (PALS)

Positron annihilation lifetime spectroscopy (PALS) was used to measure the fractional free volume (FFV) and the average pore radii of the AOPIM1-SCA4 membranes by using a conventional bulk PALS instrument under ambient conditions in the laboratory.29 The emission (birth) and the subsequent annihilation (death) of a positron from the 22Na radioactive isotope source were both accompanied by the generation of γ-rays (1.28 MeV and 0.511 MeV, respectively) due to the nuclear decay. As such, the detection of the time difference between the birth and death of the positron allowed for the measurement of the lifetime. The setup included two stacks of polymer film samples (each about 0.5 mm thick and 1 × 1 cm2 in area) sandwiched in between the 22Na source, and the PALS experiments were performed at a counting rate of 210 to 230 counts per s with a total of 5 × 106 counts collected for each spectrum. Four fitted lifetime components, namely, τ1 (para-Positronium (p-Ps)), τ2 (free positrons), τ3 and τ4 (both ortho-Positronium (o-Ps)), were obtained with a small variance from the PALS spectra using the PATFIT program (which assumes a Gaussian distribution of the logarithm of the lifetimes), and the lifetime distribution of these positions was obtained using the MELT program. By approximating the free volume elements as spherical cavities, the mean free-volume radius R (Å) was calculated from o-Ps lifetimes based on an established semi-empirical correlation as shown below,
 
image file: c9ta12028a-t1.tif(1)
where τi is the o-Ps lifetime, τ3 and τ4, in nanoseconds (ns) and ΔR is an empirical constant (1.656 Å). The fractional free volume (FFV) was correlated with R and Ii according to the Williams–Landel–Ferry equation,
 
image file: c9ta12028a-t2.tif(2)
where Ii = I3 or I4 is the intensity (%) of o-Ps and Ri = R3 or R4 refers to the mean free-volume radius (Å).30,31

2.7 Pure- and mixed-gas permeation

Pure-gas permeation tests for each membrane sample were conducted in a variable-pressure constant-volume gas permeation cell with gases being run in the order of H2, O2, N2, and CH4 to CO2. The detailed setup and procedures can be found elsewhere.32 After the sample was mounted into the cell and the lid was properly secured, the system was vacuumed at 35 °C for 20 hours before running the first gas. The testing temperature was maintained at 35 °C throughout all the runs.

Eqn (3) shown below, which is based on the steady-state pressure increment (dp/dt), expresses the gas permeability in terms of several measurable parameters,

 
image file: c9ta12028a-t3.tif(3)
where P is the gas permeability of the membrane in Barrer (1 Barrer = 1 × 10−10 cm3 (STP) cm cm−2 s−1 cmHg−1), V represents the downstream reservoir volume (cm3), A denotes the effective membrane area (cm2), l represents the membrane thickness (cm), T represents the operating temperature (K) and lastly p2 is the upstream pressure (psia). Each gas was tested at least two times to ensure that the deviation is within 1%.

The ideal selectivity of any gas pair is the ratio of the pure gas permeability of one gas to that of the other and was calculated using the following equation,

 
image file: c9ta12028a-t4.tif(4)
where αA/B is the ideal selectivity of the membrane for gas species A over B and PA and PB represent the single gas permeability of gas species A and B, respectively.

According to the solution-diffusion model, the permeability of polymer membranes for a particular gas is defined as the product of its diffusivity (D) and solubility (S) for that gas. By manipulating eqn (4), the ideal permeability selectivity, αP, can then be defined as the product of diffusivity selectivity, αD, and solubility selectivity, αS, which is expressed by eqn (5) here,

 
image file: c9ta12028a-t5.tif(5)
where DA and DB are the diffusivity coefficients (cm2 s−1) and SA and SB are the solubility coefficients (cm3 (STP) per cm3 membrane bar) of gas species A and B, respectively.

A binary gas mixture of 50% H2 and 50% CH4 (equimolar) was used for mixed gas permeation tests. The operating temperature was again maintained at 35 °C with a continuously supplied feed at 7 bar. The cell was also slowly vented at the upstream side to maintain constant gas composition there. The composition of the permeate was analyzed using an Agilent 7890 gas chromatography (GC) system. The mixed gas permeability of H2 and CH4 was determined using the following equations,

 
image file: c9ta12028a-t6.tif(6)
 
image file: c9ta12028a-t7.tif(7)
where PCO2 and PCH4 denote the permeability of CO2 and CH4, respectively, p2 is the upstream feed pressure (psia) and x and y represent the mole fractions of the gases in the feed and permeate, respectively. Other symbols have the same meanings as previously described, and the mixed-gas selectivity was calculated as the ratio of their permeability.

2.8 Gas sorption measurements

The gas sorption properties of AOPIM1-SCA4 membranes were investigated using an XEMIS-series static sorption microbalance system (UK) using a gravimetric method. Each membrane sample, weighing about 30 to 35 mg, was loaded into the microbalance chamber and the system was first stabilized under <10−6 vacuum for 12 hours at 35 °C (maintained using a thermally controlled water bath) after each gas being admitted into the system. The pressure was then gradually increased from 50 mbar to a maximum of 10 bar followed by slow desorption, and the concentration of adsorbed gas (C, cm3 (STP) per cm3 membrane) as a function of the system pressure was computed using the built-in Hisorp software. The three tested gases were run in the order of N2 and CH4 to CO2 for each sample and all isotherm data points were obtained with a standard deviation of maximally ±10%.

The solubility coefficient (S, cm3 (STP) per cm3 membrane bar) was calculated according to eqn (8) below,

 
image file: c9ta12028a-t8.tif(8)
where p denotes the feed pressure (bar).

The obtained sorption isotherms of AOPIM1 and AOPIM1-SCA4 membranes were fitted to the dual-mode sorption model for glassy polymer as expressed in the equation below,

 
image file: c9ta12028a-t9.tif(9)
where C is the gas concentration (cm3 (STP) per cm3 membrane), kD represents the Henry's law coefficient (cm3 (STP) per cm3 membrane bar), and image file: c9ta12028a-t10.tif and b are the Langmuir capacity parameter (cm3 (STP) per cm3 membrane) and affinity parameter (bar−1), respectively.33

3. Results and discussion

3.1 Molecularly homogeneous SCA4 infiltration into the microporous polymer scaffold

In this work, we strategically selected amidoxime-functionalized PIM-1 (AOPIM) polymers to provide a microporous scaffold for infiltrating SCA4 molecules because of several key advantages. Firstly, AOPIM polymers possess both N-containing amide and O-containing hydroxyl groups which are prone to forming hydrogen-bond or ionic interactions. Also, these functional groups can easily find highly similar counterparts, interaction-wise, in many other polymer backbones containing or easily functionalizable with N- or O-groups. Secondly, the highly permeable but insufficiently selective nature of PIM-1 membranes could allow better visualization of the size-sieving effect of SCA4 and reduce the resulting permeability measuring errors for slow larger gases. Also, multiple important gas pairs have been shown to be separable using PIM-based membranes so that we can identify the breadth of applications of our PFI design.34

By immersing the as-cast AOPIM1 films in methanol solutions of SCA4 molecules with varied concentrations, both the infiltration of SCA4 molecules into the microporous structure and the solvent exchange between methanol and the occluded casting solvent could occur simultaneously (Fig. 1), which yielded the resultant SCA4-infiltrated AOPIM1 (AOPIM1-SCA4) membranes. The degree of infiltration, defined as the actual mole ratio of SCA4 molecules to the polymer content in the membrane, was quantified to be 0.99%, 2.44%, 3.48% and 4.96%, respectively (simplified in the abbreviated sample name as 1%, 2%, 3% and 5%). Fig. 1 shows a schematic for this facile infiltration process in addition to photos of the as-fabricated AOPIM1-SCA4 membranes. From the physical appearances shown in the photos of the samples (Fig. 2a), there were no visible changes of the color, texture, transparency or bendability found in the samples after SCA4 infiltration, indicating minimal disruption to the polymer chain packing. As shown in the imaging results from a scanning electron microscope coupled with an energy-dispersive X-ray spectrometer (SEM-EDX), both the surface and cross-section of the representative AOPIM1-SCA4-3% sample (Fig. 2b and c) appeared homogeneous and smooth and the elemental scans revealed homogeneous distribution of SCA4 molecules across the entire membrane containing the microporous scaffold. The line scan for the sulfur (S) element reflecting a relatively even distribution of the SCA4 concentration across the central region of the membrane (Fig. S10) also proved the ability of SCA4 to infiltrate deeply into the center of a 26 μm-thick dense film. In addition, the SCA4 molecules tend to repel one another (i.e. low self-assembly/aggregation propensity) after the proton-transfer interactions with AO groups on AOPIM1 because they bear similar negative charges, and their distribution tends to follow the inherently fine distribution of AO groups in the membrane because of the strong interactions between them, which would promote the achievement of SCA4 infiltration with molecular homogeneity.


image file: c9ta12028a-f2.tif
Fig. 2 Confirmation of the molecularly homogeneous infiltration of SCA4 molecules. (a) Photos of all AOPIM1-SCA4 samples showing no visible change in the physical appearances and apparent bendability; scanning electron microscopy (SEM) images and the sulfur element (red markers) distribution across (b) the surface and (c) the cross-section of the representative AOPIM1-SCA4-3% sample.

3.2 SCA4-polymer interactions via hydrogen and ionic bonding

Bearing an amide ([double bond, length as m-dash]CR–NH2) and a hydroxyl-containing oxime (–CR[double bond, length as m-dash]N–OH) functional group, the AO moieties could actually engage with the upper rim sulfonic pendant groups of SCA4 via different interactions, namely hydrogen-bonding and proton-transfer interactions (i.e. ionic bonding). In the enlarged 1H NMR spectra (Fig. 3a), the triplet signal assigned to the hydroxyl proton on oxime groups experienced an appreciable decrease in both peak intensity and sharpness as the degree of infiltration increased. This was most probably caused by these protons' involvement in extensive hydrogen bonding with the numerous oxygen atoms on sulfonic groups.35 In contrast, for a control sample incorporated with 5 mol% calix[4]arene-25,26,27,28-tetrol (referred to as CA4t and its membrane sample was abbreviated as AOPIM1-M-CA4t-5%), the aforementioned triplet signal remained largely unaffected. Since CA4t is a SCA4 analog without sulfonic or any pendant groups, this spectral difference verified the necessity of sulfonic groups in forming extensive hydrogen bonding.
image file: c9ta12028a-f3.tif
Fig. 3 Identification of polymer–SCA4 interactions and the bridging effect on polymer chains. (a) 1H NMR spectra enlarged at the proton signals of the –OH on oxime groups; X-ray photoelectron spectra (XPS) for the N 1s element of (b) AOPIM1 and (c) AOPIM-SCA4-3% membranes; (d) X-ray diffraction (XRD) spectra to illustrate the evolution of interchain spacing (dotted arrows to show the trend, 0–5% indicated the degree of SCA4 infiltration); (e) distributions of PALS lifetimes and the corresponding free volume radii.

On the other hand, the amide group with a highly proton-accepting nitrogen (N) tended to undergo proton-transfer interactions with the sulfonic moieties with high protonating tendency. This was evidenced by a much stronger N+ signal (402.25 eV) in the N 1s X-ray photoelectron spectra (XPS) of an AOPIM1-SCA4-3% membrane sample as compared with a pristine AOPIM1 sample (Fig. 3b and c). In addition, the signal peak of [double bond, length as m-dash]CR–NH2 (amide) shifted and increased from a binding energy of 400.50 eV to 400.90 eV, which could be attributed to the electron withdrawal from its N atom during the formation of ionic bonds. Interestingly, previous studies have actually shown the doping of inorganic sulfuric acid into polybenzimidazole (PBI) using a similar post-fabrication solution-mediated route with similar extensive hydrogen-bond and ionic interactions being identified between sulfonic and imidazole groups,36 which implied the potential applicability of SCA4 for other polymers.

3.3 The bridging effect of SCA4 on polymer packing

With four sulfonic groups on the upper rim pointing outwards in four directions, each SCA4 molecule could simultaneously interact with multiple nearby chains or segments on the polymer backbones via extensive hydrogen and ionic bonding, resulting in a bridging effect within the membrane's microporous scaffold. As revealed by X-ray diffraction (XRD) patterns of AOPIM1 membranes (Fig. 3d), two broad amorphous peaks with respective d-spacings of 6.39 Å and 4.90 Å were observed, signifying the characteristic existence of two major types of microporous structures in PIM-1 membranes with different average inter-chain (or inter-segmental) distance. The former accounted for the less efficiently packed regions arising from the contorted ladder-type polymer backbones that formed larger micropores,37 while the latter represented the more densely packed regions from which the ultrafine micropores were derived. Due to the interaction-induced bridging effect, both d-spacing values appeared to experience a slight decreasing trend after the membrane was infiltrated with SCA4 molecules, but the largely rigid 3D structure of SCA4 could help limit the tightening or contractive effect on the overall polymer chain and inter-chain spacing, which was most probably the reason why these d-spacing changes were relatively very small.

It was also observed that the larger microporous regions tended to undergo slightly more contraction (i.e. from 6.39 to 6.20 Å) than the ultrafine ones (i.e. from 4.90 to 4.86 Å) as the degree of infiltration increased, and the fact that the former consisted of more spacious and readily available micropores for accommodating the infiltrating SCA4 molecules than the latter should provide a probable explanation for that. Interestingly, both contractions seemed to slow down significantly after 2% infiltration as reflected by the observation that the average d-spacing gradually stopped evolving after this, which could occur after all spacious regions have been internally bridged by an identical ‘space holder’.

3.4 Enhanced mechanical stability from SCA4's conformational flexibility

The bridging effect between SCA4 molecules and nearby polymer chains helped strengthen the chain-to-chain adhesion forces, hence improving the overall elastic modulus of AOPIM1-SCA4 membranes. As shown in Table S2, 1% to 3% infiltration of SCA4 endowed the membranes with greater tensile strength than that of the pristine one. These reinforced mechanical properties may also arise from the fact that SCA4 could distort its 3D conformation, to a certain extent, from a more bowl-like to a more cup-like conformation (later illustrated by the distorted SCA4 bowl-shapes in Fig. 5) to adapt to the compressive forces exerted by strained micropores during sample deformation. As a closed-loop organic polymer molecule with methylene bridges between the phenol sub-units, SCA4 molecules actually possessed some conformational flexibility on top of their overall structural rigidity, which also resulted in their bottom opening exhibiting a small range of sizes around a mean value of 3.0 Å. The key benefit of such conformational flexibility is that it could help minimize any localized mechanical weak spots that could compromise the membrane's pliability. Although the overall improving trend of the mechanical properties halted at 5% infiltration most likely due to the excessive occupancy of micropores that deprived SCA4 molecules of enough space to adaptively maneuver their shapes, the mechanical robustness of AOPIM1-SCA4-5% was still as good as that of the pristine membrane, revealing the potential of our approach for tackling vigorous practical applications.

3.5 Sharper pore size distribution in AOPIM1-SCA4 membranes

The average pore sizes, their distribution and the total fractional free volume (FFV) of AOPIM1-SCA4 membranes were measured and analyzed by positron annihilation lifetime spectroscopy (PALS) and the numerical results are summarized in Table S3 and the plotted distribution is shown in Fig. 3e. It was observed that the intensity and the mean free volume radii (i.e. pore size) of both the larger (I4, r4) and ultrafine (I3, r3) micropores appeared to experience a mild decrease as the degree of SCA4 infiltration increased, which could be a reasonable trend because hydrogen-bond and ionic bridging effects should be able to mildly tighten the overall polymer packing. The total FFV also appeared to demonstrate a slightly decreasing trend, which could be the result of both these bridging effects as well as the occupancy of micropores by SCA4 molecules. Although the magnitude of these numerical changes seemed rather small, they might still be able to reflect the sufficiently altered nanoporosity within a membrane that could possibly give rise to huge gas transport property changes as corroborated by other studies with small changes in the PALS results.38,39 This was also one of the reasons for employing the PALS technique which could ensure high precision.

It is noteworthy that the pore size distribution illustrated a distinctive difference between the evolutions of these two types of free volume elements. As the degree of infiltration increased, the extent of size heterogeneity was clearly reduced in the larger microporous regions with their pore size distribution becoming sharper and narrower, while the ultrafine pores decreased in their mean size without manifesting obviously narrower distribution. This disparity in the evolution of the pore size distribution revealed the preferential lodging of SCA4 into the larger microporous regions because such an enhancement in size homogeneity was most possibly caused by their favorable accommodation of these identical ‘space holders’ that size-standardized the originally broadly sized micropores. In contrast, the lodging of SCA4 into the ultrafine microporous regions would be more sterically unpreferable so that only a reduction in mean size was observed due to the overall bridging effect being transduced from larger microporous regions via the networked microporous structure of PIM-1 without allowing its distribution to be homogenized.

3.6 Permeation tests for multiple industrially relevant gases

The pure-gas permeation tests on AOPIM1-SCA4 membranes for H2, O2, N2, CH4 and CO2 were carried out at 35 °C and 3.5 bar and the results are summarized in Table S4. By analyzing the gas permeability and selectivity as a function of the degree of SC4 infiltration (Fig. 4a), a general trend of decreasing permeability for all the gases was observed, which was reasonably expected because of the previously observed decreasing trend of the total FFV. However, gases with different sizes displayed vastly different trends. H2 permeability consistently remained within the same order of magnitude with the least extent of reduction experienced throughout the whole studied range, while CO2 permeability experienced a moderately higher decrease due to its relatively larger size than H2 as well as its cylindrical shape which could result in more difficult passage at lateral positions. In contrast, larger-sized gas molecules, like N2 and CH4, experienced a one order of magnitude or higher permeability drop, especially at higher degrees of infiltration, and this asynchronism of permeability changes brought about very interesting evolution of gas selectivity.
image file: c9ta12028a-f4.tif
Fig. 4 Gas transport parameters and sorption behaviors of AOPIM1-SCA4 membranes. (a) Permeability and selectivity changes as a function of the degree of SCA4 infiltration; (b) correlation between the permeabilities of various gases and their kinetic diameters; (c) sorption isotherms with data points fitted to the dual-mode sorption model for N2, CH4 and CO2 pure gases obtained at 35 °C and a pressure up to 10 bar; (d) gas solubility (cm3 (STP) per cm3 membrane bar) and diffusivity (×107 cm2 s−1) as a function of the degree of SCA4 infiltration for N2, CH4 and CO2 at 35 °C and 3.5 bar.

At 1% infiltration, there was about 65% and 73% increase in the H2/N2 and H2/CH4 selectivity, respectively, with almost unaffected H2 permeability because the partial coverage over the entire microporous scaffold by SCA4 cavities started to exhibit some but not complete size-discrimination against the larger N2 and CH4 gas molecules. Once the degree of infiltration reached 2%, the size-sieving effect became drastically more prominent with the H2/N2 and H2/CH4 selectivity becoming 4.6 and 10.6 times, respectively, as high as that of the pristine membrane, but at the expense of merely 15.7% H2 permeability. A 3.3-fold enhancement in CO2/CH4 selectivity was also observed and despite the moderate loss of CO2 permeability, the tremendous improvement in selectivity over CH4 still boosted the overall performance to well transcend the recent upper bound (shown in Fig. 6d). More interestingly, after the SCA4 infiltration, the AOPIM1 membranes even transformed from being originally CO2/H2-selective to increasingly H2/CO2-selective due to the rise of molecular-sieving diffusion within the membranes and a 3.5 times higher H2/CO2 selectivity was obtained even at 2% infiltration.

Being corroborated by the evolution of pore size, SCA4 molecules tended to settle down in the spacious but less- or non-size-selective larger micropores during solution-infiltration. Subsequently, they could act as molecular gatekeepers that guarded these non-selective regions within the membrane from indiscriminate gas passage and also redirected gas transport within the entire interconnected microporous network as elucidated by an illustrative diagram shown in Fig. 5. Since the full coverage of all non-selective regions would require a certain degree of infiltration to be reached, this explained the dramatic performance jump from 1% to 2% infiltration because there should exist a degree of saturated infiltration between 1% and 2% at which all non-selective micropores are remodeled with size-sieving properties. Beyond saturation, further infiltration of SCA4 molecules could result in more unnecessary transport resistance against gas diffusion than additional molecular-sieving effectiveness so that selectivity improvement slowed down from 2% onwards. This phenomenon actually has an analogous explanation in the defect healing process in thin film membranes if we consider non-selective micropores to be nano-defects in the selective layer, and the infiltration of and ionic bridging effect by SCA4 actually ‘healed’ them while bringing in a new gatekeeping cavity. Once all the ‘defects’ were healed, selectivity enhancement slowed down even with additional infiltration.40 This so-called ‘defect-healing’ phenomenon could probably also explain the observation that the evolution of size distribution of the larger micropores became less significant after around 2% infiltration (Fig. 3e) as additional SCA4 would become ineffective in further homogenizing these micropores after they are fully infiltrated with the same SCA4 ‘space holders’.


image file: c9ta12028a-f5.tif
Fig. 5 Illustration of the molecular gatekeeping mechanism in the AOPIM1 microporous scaffold. The size-sieving 3D cavity of SCA4 preferentially grants passage to small H2 and CO2 while strongly impeding large N2 and CH4.

The emergence of effective molecular-sieving structures in these membranes consequently resulted in a strong negative correlation between the pure-gas permeability and the gas kinetic diameter (Fig. 4b), especially at and beyond the degree of saturated infiltration. In comparison, those of PIM-1 and AOPIM1 membranes exhibited a much weaker size dependence because of the presence of non-selective microporous regions without being guarded by molecular gatekeepers. It is important to note that the SCA4 cavity possesses a small range of size around 3.0 Å owing to its conformational flexibility so that it might not perfectly refuse the passage of gas molecules with a kinetic diameter bigger than 3.0 Å. Nevertheless, a strong molecular-sieving nature was still achieved as both the likelihood and the rate of diffusion of larger gases were tremendously lowered as their size increased. For example, CO2 (3.3 Å) was granted passage much more preferentially than CH4 (3.8 Å) for enabling high CO2/CH4 selectivity.

3.7 Dual-mode sorption analyses and gas transport properties

From the sorption isotherms of AOPIM1-SCA4 (Fig. 4c) samples obtained with N2, CH4 and CO2 respectively at 35 °C over a range of pressures from 0 to 10 bar, there were no significant changes found in the overall gas solubility after the infiltration of SCA4 for these three gases, which reflected that the chemistry and microporous structure of AOPIM1 polymers remained largely unaltered. These isotherms displayed a good fit to the dual-mode sorption model of glassy polymers,33 and those of the solid SCA4 powder also resembled dual-mode behaviors (Fig. S5), demonstrating SCA4's inherent affinity for gases.

By examining the fitted dual-mode capacity parameters summarized in Table S5, the change of sorption sites for each probe gas was explored so that the proposed guarded microporous network structure within AOPIM1-SCA4 membranes could be further elucidated. For N2 and CH4 that have larger sizes than CO2, the number of Langmuir sites decreased while the corresponding Henry's law sites increased as signified by the corresponding lower image file: c9ta12028a-t11.tif but higher kD constant values, respectively, after the infiltration of SCA4. These opposing changes in the two sorption sites revealed that the original Langmuir sites, contributed by the excess free volume elements in the AOPIM1 membranes, became less accessible to N2 and CH4 after they were guarded by the size-discriminative SCA4 gatekeepers. As a result, these sites started to resemble the Henry's law sites for N2 and CH4. In addition, the capture ability of SCA4 cavities also contributed to the increased affinity parameter, b, for N2 and CH4. In contrast, being the smallest and most condensable gas among the three tested gases, CO2 experienced minimal changes in the nature of its corresponding sorption sites as the image file: c9ta12028a-t12.tif constants did not show any significant variation after SCA4 infiltration and only some capture effect was observed as the parameter b slightly increased, which also supported the preferential passage of CO2 through SCA4 cavities over larger gases. Interestingly, image file: c9ta12028a-t13.tif values were found to remain roughly unchanged from a certain degree of infiltration onwards which was again between 1% and 2%, similar to the degree of saturated infiltration. A probable explanation is that additional SCA4 molecules could only generate a marginal impact on the overall affinity of these sites towards N2 and CH4. However, the mechanistic understanding of this phenomenon might require a separate study and was not further explored here.

From the sorption isotherms, the solubility (S) of each gas at 3.5 bar was obtained and is summarized in Table S6 together with their diffusivity (D) determined from the solution-diffusion correlation (permeability (P) = D × S).41 As illustrated by the evolution of these two gas transport parameters (Fig. 4d), the solubility of all three gases (N2, CH4 and CO2) remained mostly constant as the degree of SCA4 infiltration increased, whereas their diffusivity dropped more significantly with CH4 experiencing the most drastic reduction. This showed that the solubility selectivity of H2 against each of these three gases as well as that of CO2 against CH4 remained almost unchanged while their diffusivity selectivity was greatly enhanced. Therefore, the dramatic enhancements in H2/N2, H2/CH4 and CO2/CH4 selectivity were contributed primarily by the diffusivity selectivity increase arising from the size-sieving SCA4 cavities that favored the diffusion of smaller gases. The previously discussed CO2/H2-to-H2/CO2 selectivity transition was also a result of this domination of diffusivity selectivity over solubility selectivity after the introduction of SCA4 molecular gatekeepers as the originally non-selective microporous regions gradually evolved to a highly size-sieving state.

3.8 Control experiments illustrating SCA4's gatekeeping role and unblocked cavities

Two important control experiments were carried out to reveal the gas passage through and the unblocking of SCA4 cavities. For the former, AOPIM1 membranes were infiltrated with inorganic sulfuric acid via a similar procedure to crosslink their microporous structure, which yielded AOPIM1-H2SO4 membranes.36 Under the same infiltration conditions, AOPIM1-H2SO4 membranes showed a higher degree of crosslinking as reflected by the larger amount of insoluble content in their samples (Table S2) as well as the slightly higher permeability but much lower selectivity than those of the analogous AOPIM1-SCA4 membranes (Table S8). If the passage of smaller gas molecules did not occur through SCA4 cavities and whatever permeability and selectivity changes were solely due to the bridging effect on the polymer packing, by virtue of SCA4's larger size than that of sulfuric acid, the AOPIM1-SCA4 membranes logically should have displayed higher permeability but lower selectivity than AOPIM1-H2SO4, especially when H2SO4 molecules were actually able to produce a higher degree of crosslinking among the polymers. Yet, the opposite was observed and exceptionally high selectivity for multiple gas separations was obtained only using AOPIM1-SCA4 membranes, indicating that the gas passage occurred through a highly size-sieving window which in our control cases could only be the SCA4 open cavity. The molecular gatekeeping role of the SCA4 open cavity in the polymer microporous scaffold was also evidenced from this. As for the latter, SCA4 molecules were physically blended with AOPIM1 polymers to fabricate the AOPIM1-M-SCA4 membranes as a method control sample and all of their separation performances fell far below those of the AOPIM1-SCA4 membranes with no strong molecular-sieving nature being manifested. This was most possibly caused by the blockage or filling of the SCA4's single cavity by polymer chains during the intense physical mixing process. In contrast, solution-infiltrating SCA4 in the already solidified membranes where polymer chains exhibited much less mobility than in a solution state could minimize the likelihood of cavity blockage, which is one key advantage of the PFI design that is not realizable by conventional inorganic or framework-type nanomaterials.

3.9 Overall separation performances and their stability against aging

The drastic enhancements in H2/N2 and H2/CH4 selectivity with small compromises in H2 permeability drove the AOPIM1-SCA4 membranes from 2% onwards to perform much closer to or even well above the state-of-the-art 2015 upper bounds recently updated for these two separations (Fig. 6a and b)42 and to also be comparable with or even outperform many other top-performance polymer-based membranes, including newly synthesized PIMs, thermally treated or crosslinked membranes and some mixed-matrix membranes (MMMs). Similarly, the CO2/CH4 and H2/CO2 performances were also boosted to well transcend 2008 upper bounds43 and became comparable to other state-of-the-art reports (Fig. 6c and d).36,42,45–55
image file: c9ta12028a-f6.tif
Fig. 6 Pure-gas separation performances of AOPIM1-SCA4 membranes compared with recent upper bounds and other high-performance polymer-based membranes. Plots of (a) H2/N2, (b) H2/CH4, (c) H2/CO2 and (d) CO2/CH4 separation performances at 35 °C and 3.5 bar with either 2008 or 2015 or both upper bounds (0–5% denotes the degree of infiltration, the numbers in parentheses indicate days of aging, the numbers at the end of abbreviated names of literature studies indicate the pyrolysis temperature in °C, and arrows are drawn as a guide to the eye).

In addition, the AOPIM1-SCA4 membranes demonstrated good performance stability against physical aging as all these pure-gas separation performances at 2% and 3% infiltration either moved even closer to or consistently stayed well above the 2008 or 2015 upper bounds after 60 days (Fig. 6a–d). During aging, some losses of gas permeability were inevitable because the residual insufficiently selective regions closed down and the ultrafine micropores also shrank in size due to the gradually relaxed polymer chains collapsing excess free volume. Nevertheless, both these changes not only resulted in a more size-discriminative overall pore structure, but might also amplify the prominence of the gatekeeping effect of SCA4 cavities, leading to more than proportionate gains in the H2 selectivity. CO2/CH4 selectivity did not undergo further enhancement after aging due to the collapsed excess free volume that retarded its solubility selectivity, but its overall aged performance remained persistently beyond the upper bound while that of the pristine membrane fell below. Meanwhile, the AOPIM1-SCA4 membranes also showed attractive O2/N2 separation performance which was high above the 2008 upper bound and well-maintained after aging, suggesting the potential of our SCA4-infiltration technology for air purification applications as well. The mixed-gas H2/CH4 performance of AOPIM1-SCA4 membranes tested using an equimolar gas mixture (Table S7 and Fig. S7) was lower than the pure-gas performance with respect to both permeability and selectivity, which is typical because of the competitive sorption among gas species in a mixed permeation environment. Nonetheless, their mixed-gas H2/CH4 selectivity still sharply increased with the degree of infiltration (up to 24 times as high) while the maximal reduction in H2 permeability was only 35%. As a result, the overall mixed-gas separation performance was able to transcend the 2008 pure-gas upper bound easily.

Importantly, to obtain such ultra-efficient, durable and industrially relevant multiple gas separations, our PFI membrane design is highly facile and potentially scalable, and has strategically overcome the longstanding issues in conventional composite membranes, especially those prepared by the often problematic physical-mixing route. Also, huge separation performance enhancement could be achieved with only a small consumption of SCA4 (2–3 mol%), suggesting an extremely high enhancement-to-consumption leverage that was uncommon in conventional composite membranes which typically required 10–30 wt% of incorporated porous agents to exhibit some advantageous effects, if any.44 Lastly, although the illustrated size-sieving around 3.0 Å in this work already successfully targeted a variety of energy and environmentally important gas separations, future efforts in its size and functionality tuning via base size change or lower rim substitution coupled with different types of water-soluble upper rims can possibly bring this technology over to many other polymers that target an even wider range of molecular separations.

4. Conclusions

This work realized the unconventional post-fabrication infiltration (PFI) membrane design to incorporate external intrinsically porous agents into polymer membranes after dense film formation. In this design, issues of interfacial void and pore blockage, which are often exacerbated by the conventional physical-mixing strategy, can be completely overcome by choosing a water-soluble organic macrocyclic molecule, more specifically in this study, 4-sulfocalix[4]arene (SCA4). Its unique feature of coexisting solubility in common membrane-treating protic solvents, like methanol, and the presence of a 3D intrinsic open cavity in its molecule-sized body enabled SCA4 to homogeneously infiltrate the microporous scaffold contained in the amidoxime-functionalized PIM-1 (AOPIM1) membranes via the formation of extensive hydrogen or ionic bonding, which could offer generalizable design principles across other existing polymers. The AOPIM1-SCA4 membranes fabricated based on this ultra-facile PFI design demonstrated drastically enhanced molecular-sieving characteristics with outstanding separation performances for multiple important gas pairs being achieved, including H2/CO2, H2/N2, H2/CH4, CO2/CH4 and O2/N2, which well surpass the 2008 or 2015 upper bounds and are as good as or even better than those of many other polymer-based membranes with outstanding performances. Investigation into the pore size distribution, sorption behavior, gas transport properties and relevant control experiments mechanistically revealed the role of SCA4 molecules as size-sieving molecular gatekeepers guarding non-selective regions within the microporous network in AOPIM1 membranes. Being backed by a large pool of size-tunable and functionalizable water-soluble macrocyclic molecules, this proposed PFI membrane design offers potential applicability in an excitingly wide array of energy-intensive molecular applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National University of Singapore (NUS) under the project titled “Membrane research for CO2 capture phase 2” (grant number R-279-000-505-133) and also the Dean's Office, Faculty of Engineering, NUS, under the project named “Natural Gas Centre” (NUS grant number R-261-508-001-646).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ta12028a

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