Recent progress in microporous polymers from thermally rearranged polymers and polymers of intrinsic microporosity for membrane gas separation: Pushing performance limits and revisiting trade-off lines

Polymers are promising materials for gas separation membranes. However, the trade-off relationship between gas permeability and selectivity remains an obstacle for achieving polymer membranes that exhibit high gas permeation with desirable separation efficiency. Improving polymer microporosity is of interest in gas separation membranes to enhance gas transport behavior. Polymer modifications by (a) incorporating intrinsically microporous units and/or (b) increasing chain rigidity can enhance microporosity in conventional polymer membrane materials such as polyimides. These strategies are adopted for new classes of microporous polymers, thermally rearranged (TR) polymers, and polymers with intrinsic microporosity (PIMs), to maximize gas transport properties. Their outstanding gas separation performances have redefined the traditional trade-off lines. This review aims to explore the advances in microporous polymers for gas separation applications. The approaches on TR polymers and PIMs to enhance their microporosity are listed, and their developments are evaluated in the context of revisiting performance limits for industrially relevant gas separation applications.     

Development and characterization of homogeneous membranes de from high molecular weight sulfonated polyphenylene oxide

The high molecular weight polyphenylene oxide (PPO) was sulfonated to different ion exchange capacity (IEC) values using chlorosulfonic acid. The physico-chemical properties along with the gas transport properties of the membranes prepared from sulfonated PPO (SPPO) were evaluated. Sulfonation of PPO results in a linear increase of density with the IEC value while the average d-spacing in polymer remains constant. Sulfonic groups attached to the aromatic ring in the PPO backbone are not thermally stable. On the other hand, when tested with CO₂ at room temperature, the SPPO membranes maintained a constant permeability over the period of 60 days. An increase in IEC value of SPPO results in an increase in O₂/N₂ and CO₂/CH₄ ideal selectivities and a decrease in O₂ and CO₂ permeabilities. The combination of permeability and ideal selectivity for both gas pairs places the SPPO membranes below the respective upper-bound lines for polymeric membranes. However, an increase in the IEC value brings the permeability versus ideal selectivity relationship closer to the upper-bound line, especially for the O₂/N₂ gas pair.     

Polymers of intrinsic microporosity (PIMs): bridging the void between microporous and polymeric materials

Novel types of microporous material are required for chemoselective adsorptions, separations and heterogeneous catalysis. This concept article describes recent research directed towards the synthesis of polymeric materials that possess microporosity that is intrinsic to their molecular structures. These polymers (PIMs) can exhibit analogous behaviour to that of conventional microporous materials, but, in addition, may be processed into convenient forms for use as membranes. The excellent performance of these membranes for gas separation and pervaporation illustrates the unique character of PIMs and suggests immediate technological applications.     

Unconventional, highly selective CO2 adsorption in zeolite SSZ-13

Low-pressure adsorption of carbon dioxide and nitrogen was studied in both acidic and copper-exchanged forms of SSZ-13, a zeolite containing an 8-ring window. Under ideal conditions for industrial separations of CO(2) from N(2), the ideal adsorbed solution theory selectivity is >70 in each compound. For low gas coverage, the isosteric heat of adsorption for CO(2) was found to be 33.1 and 34.0 kJ/mol for Cu- and H-SSZ-13, respectively. From in situ neutron powder diffraction measurements, we ascribe the CO(2) over N(2) selectivity to differences in binding sites for the two gases, where the primary CO(2) binding site is located in the center of the 8-membered-ring pore window. This CO(2) binding mode, which has important implications for use of zeolites in separations, has not been observed before and is rationalized and discussed relative to the high selectivity for CO(2) over N(2) in SSZ-13 and other zeolites containing 8-ring windows.     

Surface modification of ZIF-90 with triptycene for enhanced interfacial interaction in mixed-matrix membranes for gas separation

Zeolite imidazole framework (ZIF-90) nanoparticles were chemically modified by grafting triptycene moieties. The modified nanoparticles were introduced into a triptycene-based polyimide as fillers to generate mixed matrix membranes (MMMs) for gas separation. The incorporation of “hook-like” triptycene moieties in both dispersed and continuous phases led to intimate contact between the two phases and thus defect-free interfacial morphology, due to the supramolecular interlocking and π–π stacking interaction between triptycene units presented in both phases. The filler/polymer solution showed shear thickening behavior due to such strong interfacial interaction. The separation performance of the prepared composite membranes was investigated as a function of filler loading and particle surface grafting density. Pure-gas permeation results showed that the gas permeabilities increased expectedly as the filler loading increased, with stable or improved selectivities. The reduced permeability relative to pristine polyimide film is likely due to the pore blockage of ZIF-90 upon grafting triptycene moieties on the particle surface. Reducing the grafting density of triptycene moieties led to improved permeability and selectivity suggesting good tunability of this series of new composite membranes. Overall, modification of nanofiller with hierarchical triptycene moieties offers a fundamentally new avenue for creation of composite membranes with unique properties in gas separations.     

聚合物/无机纳米粒子复合膜在气体分离中的研究进展

由于聚合物膜具有可高度设计、机械性能好、易于加工 等优点,是理想的气体分离材料。然而,聚合物膜在气体选择性和渗透性方面存在平衡限制,在聚合物中引入纳米粒子,是提高气体分离性能的一种有效手段。本文基于聚合物/无机纳米粒子复合膜在气体分离领域的研究现状,重点阐述了零维纳米粒子（二氧化硅、二氧化钛）、一维纳米粒子（碳纳米管）、二维纳米粒子（氧化石墨烯、二维过渡金属氧化物）、三维纳米粒子（金属有机框架、沸石）对气体分离性能的影响,并展望了聚合物复合分离膜的发展趋势,为未来高效分离膜的研发提供了参考。     

Investigation of gas transport and other physical properties in relation to the bromination degree of polymers of intrinsic microporosity

In this work, the synthesis of novel polymers of intrinsic microporosity (PIMs) with different degrees of bromine substitution by a free-radical substitution reaction was performed. The synthesized polymers were thoroughly characterized and their bromination degree was verified via nuclear magnetic resonance. The brominated PIMs were investigated by infrared spectroscopy, X-ray diffraction, and density measurements and correlated with their gas transport properties. It was found that with an increase in the bromination degree, the synthesized PIMs exhibited a significant increase in polymer chain packing density which led to reduced fractional free volume and consequent decrease in gas diffusion and permeability coefficients. The change in permeability coefficients caused an improvement in the CO₂/N₂, CO₂/CH₄, and O₂/N₂ ideal permeability selectivities. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2752–2761     

Gas separation properties of aromatic poly(amide-imide) membranes

Aromatic poly(amide-imide)s were synthesized using direct 2,2-bis[N-(4-carboxyphenyl)-phthalimidyl] hexafluoropropane (6FDIA) polycondensation with various diamines containing flexible ether groups and bulky substituents. The oxygen and nitrogen gas transport in the poly(amide-imide) membranes was investigate at 35 °C with the pressure between the interval at 2-10 atm. The proposed method is expected to promote the gas permeability of the poly(amide-imide) membrane and maintain the gas selectivity. It was found that both gas permeability and selectivity of poly(amide-imide) membranes increased with increasing fractional free volume and d-spacing. The gas permeability had good correlation with the γ-transition temperature. The bulky pendent group introduced into diamine moiety of poly(amide-imide) could efficiently promote the gas permeability. For the behaviors of gas separation, the gas diffusivity coefficient and solubility selectivity controlled the gas permeability and selectivity, respectively. The sorption behavior of the aromatic poly(amide-imide) membranes can be well explained using the dual mode sorption model. The Langmuir capacity constant and Henry’s law constant increase with FFV increasing. 6F-TBAPS has the best O₂/N₂ separation performance among the poly(amide-imide) membranes.     

Microstructure Manipulation of Covalent Organic Frameworks (COFs)-based Membrane for Efficient Separations

Covalent organic framework(COF) membranes have exhibited great potential to become the next-generation membranes for efficient separations due to the diverse structures, ordered framework pores, tunable functionality and excellent stability. This review presents the microstructure manipulation strategies for separation performance enhancement of COF membranes in recent years. Based on the three mechanisms of molecular sieving, surface diffusion, and facilitated transport, the structural modulation methods to enhance the selectivity of COF membranes are analyzed in detail. Next, strategies of realizing ultrashort mass transfer pathways and ultralow mass transfer resistance for the permeability enhancement are elaborated. Furthermore, the framework stability in COFs, interlayer stability between COF nanosheets and interfacial stability between COF layer and substrate are discussed. Finally, we discuss the existing challenges and perspectives on the future development of COF membranes, targeting at identifying the most promising strategies and directions for the engineering of COF membranes.     

Selective Transport of Cesium and Strontium Ions Through Polymer Inclusion Membranes Containing Calixarenes as Carriers

Abstract

Cs⁺ and Sr²⁺ are selectively removed over Na⁺ from acidic aqueous solutions with high Na⁺ concentrations by using membranes designed to selectively transport one of the two cations. To this end, calix[4]arene derivatives were used as carriers in polymer inclusion membranes (PIMs). The synthesis and characterization of new calix[4]arene derivatives (a bisamide (2) and three bisesters (3, 5 and 6)) used for the separation of Sr²⁺ are described. Another bisester (4) was employed for the same separation. In addition, a calix[4]arene-crown-6 (7) was incorporated into the membrane for Cs⁺ extraction. The concentration of each membrane component (polymer, carrier and counter-ion) was optimized and the permeability coefficients (P, m sec^?1) of Cs⁺, Sr²⁺ and Na⁺ were determined. A synergistic effect between the calixarenes and dinonylnaphtalenesulfonic acid, used as counterion, (DNNS, 8) was observed. High selectivity of Cs⁺ over Na⁺ and of Sr²⁺ over Na⁺ were obtained with compounds 7 and 3, respectively. The best P for Sr²⁺ was obtained with compound 4. A long-term experiment was carried out to demonstrate the durability of PIMs. PIMs are compared to classical supported liquid membranes.     

Electropolymerization of Molecular‐Sieving Polythiophene Membranes for H2 Separation

Membrane technologies that do not rely on heat for industrial gas separation would lower global energy cost. While polymeric, inorganic, and mixed‐matrix separation membranes have been rapidly developed, the bottleneck is balancing the processability, selectivity, and permeability. Reported here is a softness adjustment of rigid networks (SARs) strategy to produce flexible, stand‐alone, and molecular‐sieving membranes by electropolymerization. Here, 14 membranes were rationally designed and synthesized and their gas separation ability and mechanical performance were studied. The separation performance of the membranes for H₂/CO₂, H₂/N₂, and H₂/CH₄ can exceed the Robeson upper bound, among which, H₂/CO₂ separation selectivity reaches 50 with 626 Barrer of H₂ permeability. The long‐term and chemical stability tests demonstrate their potential for industrial applications. This simple, scalable, and cost‐effective strategy holds promise for the design other polymers for key energy‐intensive separations.     

Recent developments in polyetherimide membrane for gas separation

Polyetherimide (PEI) is an extraordinary type of polyimide with excellent thermal and mechanical properties. The polymer is also gas permeable and is considered one of the best membranes for gas separation. Despite the high selectivity, PEI suffers from low permeability due to the trade‐off between phenomena in polymers. To overcome this limitation, fillers are added during the membrane preparation to create voids for better gas transport. In this paper, permeability and selectivity data of PEI membranes for the separation of oxygen, carbon dioxide, and helium are discussed. The paper also summarizes the reported studies for adding fillers to improve the membrane performance.     

Olefin/paraffin gas separations with 6FDA-based polyimide membranes

Pure gas permeation and sorption experiments were carried out for the gases ethylene, ethane, propylene and propane using polyimides based on 4,4′-(hexafluoroisopropylidene) diphthalic anhydride (6FDA). Composite membranes and free films were used. Experiments were performed at 308 K and feed pressures up to 17 atm for ethylene and ethane and 9 atm for propylene and propane. Mixed gas permeation experiments were carried out with 50 : 50 olefin/paraffin feed mixtures. For all investigated polyimides, the ideal ethylene/ethane separation factor ranged between 3.3 and 4.4 and the ideal propylene/propane separation factor ranged between 10 and 16 at a feed pressure of 3.8 atm and 308 K. In mixed gas permeation experiments, up to 20% lower selectivity was found for the ethylene/ethane separation and up to 50% reduced selectivity for the propylene/propane separation compared to the ideal selectivity. The influence of feed temperature on separation and permeation properties will be discussed based on pure gas permeability data at 298 and 308 K.     

Effects of molecular structure and thermal annealing on gas transport in two tetramethyl bisphenol-A polymers

The effects of molecular structure and thermal history on the gas transport properties of tetramethyl bisphenol-A polycarbonate (TMPC) and tetramethyl bisphenol-A polysulfone (TMPSF) are reported. The presence of the bulky methyl side groups on the aromatic rings of TMPC and TMPSF inhibits both chain packing and molecular motions in these polymers as compared with standard polycarbonate and polysulfone. The methyl substitution results in significant increases in permeability with little or no loss of selectivity, making these materials promising candidates for membrane-based gas separations applications. Thermal annealing produces a densification of the materials, resulting in decreased permeabilities. Selectivities remained roughly the same or increased slightly upon annealing at temperatures slightly below the glass transition temperature. Thus, thermal annealing may be useful for “fine-tuning” a membrane's properties for applications where improved selectivity is desirable, even at the expense of decreased permeability.     

多孔材料表面修饰聚酰亚胺非对称混合基质膜对CO2/N2和CO2/CH4的气体分离

兼具高通量和高选择性的气体分离膜是研究膜分离材料的目标.采用相转化法制备了聚酰亚胺非对称膜,并将其作为基底膜材料,分别在其表面修饰掺有金属有机框架材料Cu₃(BTC)₂ (1, 3, 5-均苯三甲酸合铜),沸石咪唑酯骨架材料ZIF-8以及镁铝水滑石MgAl-LDHs的聚酰胺酸溶液,经热亚胺化后制成非对称混合基质膜.研究了该系列非对称混合基质膜的结构特性和对CO₂、CH₄和N₂气体分离性能;考察了ZIF-8的掺杂量对非对称混合基质膜透气性能的影响.结果表明非对称聚酰亚胺膜的表面修饰可有效地改变膜的表面性质,掺杂ZIF-8的非对称混合基质膜气体的透气性能和选择性都增加,且掺杂量为5% (w)时CO₂/N₂和CO₂/CH₄的理想选择性分别高达24和83,为合成高效的CO₂分离膜提供了借鉴.     

Continuous Porous Aromatic Framework Membranes with Modifiable Sites for Optimized Gas Separation

Continuous microporous membranes are widely studied for gas separation, due to their low energy premium and strong molecular specificity. Porous aromatic frameworks (PAFs) with their exceptional stability and structural flexibility are suited to a wide range of separations. Main-stream PAF-based membranes are usually prepared with polymeric matrices, but their discrete entities and boundary defects weaken their selectivity and permeability. The synthesis of continuous PAF membranes is still a major challenge because PAFs are insoluble. Herein, we successfully synthesized a continuous PAF membrane for gas separation. Both pore size and chemistry of the PAF membrane were modified by ion-exchange, resulting in good selectivity and permeance for the gas mixtures H₂/N₂ and CO₂/N₂. The membrane with Br^? as a counter ion in the framework exhibited a H₂/N₂ selectivity of 72.7 with a H₂ permeance of 51844 gas permeation units (GPU). When the counter ions were replaced by BF₄^?, the membrane showed a CO₂ permeance of 23058 GPU, and an optimized CO₂/N₂ selectivity of 60.0. Our results show that continuous PAF membranes with modifiable pores are promising for various gas separation situations.     

Design criteria for dense permeation-selective membranes for enantiomer separations

Dense enantioselective membranes can distinguish between two enantiomers by different mechanisms. At this moment, it is not clear which mechanism provides the best membranes for large-scale enantiomer separations. Therefore, we studied the design criteria for permeation-selective membranes combining literature data, experiments and model calculations. Literature data on dense permeation-selective membranes for enantiomer separation show that these membranes could be divided into two different classes: diffusion selective and sorption selective. Reviewing the literature on diffusion-selective membranes shows that these membranes have one main disadvantage: the inverse proportionality relation between the permeability and selectivity. This disadvantage is absent for sorption-selective membranes. As a model system, the diffusion of phenylalanine through a packed bed of polypropylene beads coated with N-dodecyl-l-hydroxyproline:Cu(II) was studied. The experiments showed that the material could selectively adsorb phenylalanine (Phe) with a selectivity (d/l) of 1.25. However, no permeation selectivity could be detected. With a dual sorption model these results could be interpreted. These model calculations showed that the permeation selectivity only approaches the intrinsic selectivity of the selector if the selectively adsorbed population is mobile and the non-selective permeation is minimized. Therefore, to our opinion more emphasis should be put on the development of sorption selective membranes.     

High pressure gas permeability of microporous carbon membranes

Microporous carbon membranes are prepared, characterised structurally and tested in terms of high pressure CO₂ permeability at temperatures around the critical. A maximum in the permeance versus relative pressure curve is observed in close analogy to the case of mesoporous membranes. This weakens considerably as the temperature is increased above the critical. The results offer significant input for an improved understanding and theoretical modelling of the process and may be potentially useful for the identification of the optimal pressure and temperature conditions for efficient gas separations.     

Correlation between Performances of Hollow Fibers and Flat Membranes for Gas Separation

This work presents an attempt at correlating the available permeability/selectivity literature data for hollow fibers and flat membranes. Therefore, this paper gathers the information pertaining to membrane materials for which membrane properties of flat membranes and hollow fibers have both been reported. An overview of the relations between selectivity and permeance of hollow fiber membranes for various gas pairs (O₂/N₂, CO₂/CH₄, CO₂/N₂, H₂/N₂, H₂/CO₂, H₂/CH₄ and He/N₂) is presented first. The upper bound lines are the ones proposed by Robeson, which were calculated by assuming a one-micron-thick skin layer as proposed by Robeson in 2008. From the results obtained, a relation between the selectivity ratio in both kinds of membranes (α_H/α_f) and skin layer thickness (l) calculated from flat membranes and hollow fibers gas permeation data for these pairs of gases is also presented. The skin layer thicknesses measured using seven different experimental techniques for six commercial membranes are compared. The influences of spinning parameters on the morphology and performance of hollow fiber membrane gas separation are discussed. Finally, an analysis is made of the reasons why the dense skin layer thicknesses of a hollow fiber calculated using permeance and permeability data vary for different gases and also differ from direct experimental measurements.     

The effects of polymer chain rigidification, zeolite pore size and pore blockage on polyethersulfone (PES)-zeolite A mixed matrix membranes

The polyethersulfone (PES)-zeolite 3A, 4A and 5A mixed matrix membranes (MMMs) were fabricated with a modified solution-casting procedure at high temperatures close to the glass transition temperatures (T_g) of polymer materials. The effects of membrane preparation methodology, zeolite loading and pore size of zeolite on the gas separation performance of these mixed matrix membranes were studied. SEM results show the interface between polymer and zeolite in MMMs experiencing natural cooling is better (i.e., less defective) than that in MMMs experiencing immediate quenching. The increment of glass transition temperature (T_g) of MMMs with zeolite loading confirms the polymer chain rigidification induced by zeolite. The experimental results indicate that a higher zeolite loading results in a decrease in gas permeability and an increase in gas pair selectivity. The unmodified Maxwell model fails to correctly predict the permeability decrease induced by polymer chain rigidification near the zeolite surface and the partial pore blockage of zeolites by the polymer chains. A new modified Maxwell model is therefore proposed. It takes the combined effects of chain rigidification and partial pore blockage of zeolites into calculation. The new model shows much consistent permeability and selectivity predication with experimental data. Surprisingly, an increase in zeolite pore size from 3 to 5 Å generally not only increase gas permeability, but also gas pair selectivity. The O₂/N₂ selectivity of PES-zeolite 3A and PES-zeolite 4A membranes is very similar, while the O₂/N₂ selectivity of PES-zeolite 5A membranes is much higher. This implies the blockage may narrow a part of zeolite 5A pores to approximately 4 Å, which can discriminate the gas pair of O₂ and N₂, and narrow a part of zeolites 3A and 4A pores to smaller sizes. It is concluded that the partial pore blockage of zeolites by the polymer chains has equivalent or more influence on the separation properties of mixed matrix membranes compared with that of the polymer chain rigidification.