Graphene oxide membranes for gas separation

Recently, the carbon based materials1 attracted attention of the scientific community for applications in membrane gas separation. Tremendous number of carbon based materials such as carbon nanotubes, chemically modified graphene, graphene oxide (GO) or graphite (nano)particles in mixed matrix membrane (MMM) composites was tested within past decade2. It was found that even small loading of graphene and (GO) additives, i.e. 0.05 to 1 wt.%3, led to considerable changes of composite materials properties (elastic modulus, tensile strength, electrical conductivity, and thermal stability) compared to neat polymers. Please click Additional Files below to see the full abstract

Aging of polymers of intrinsic microporosity studied by sorption and permeation

Polymers of intrinsic microporosity (PIMs)1 seem to be effective materials for gas and vapor separations.2 However, gas separation efficiency of PIMs can be strongly influenced by the material aging process connected with the changes of PIMs inner structure.3 With respect to potential industrial applications, the investigation of such changes and their effect on gas and vapor transport is necessary. In this work, we present a detailed study of i) CO2 sorption in PIM-1 via momentary measurements during four years and ii) methanol permeation in PIM-11 and EA-TB-PIM2 via continuous and momentary experiments. Sorption experiments were performed gravimetrically using a self-developed apparatus equipped with McBain’s spiral balances. In this case, PIM-1 membranes were pre-treated (soaking in ethanol with consequent drying at different temperatures) in order to study the influence of temperature on PIM-1 aging. Methanol permeation experiments were performed using a differential flow permeameter with H2 and He as carrier gases. Permeation experiment were performed with PIM-1 and PIM-EA-TB methanol treated membranes. CO2 sorption measurements revealed that, assuming the validity of the solution-diffusion model, the decrease of permeability during aging can be attributed directly to the decrease of diffusivity, whereas solubility is time independent in the studied period of four years. Although higher preparation temperature led to the initial drop of diffusivity, this process stabilized separation performance of PIMs over time (Figure 1). MeOH permeation experiments confirmed previous findings from CO2 tests, that the permeability decrease during the aging is a diffusivity controlled process. Moreover, it was found that the momentary permeation data can be mathematically transferred to continuous data, which are more relevant for applications but more difficult to measure. The nature of aging process was studied by infrared spectroscopy. We have found that aging of PIMs does not influence their chemical structure and; therefore, they undergo only the so called physical-aging. Please click Additional Files below to see the full abstract

A Review on Ionic Liquid Gas Separation Membranes

Ionic liquids have attracted the attention of the industry and research community as versatile solvents with unique properties, such as ionic conductivity, low volatility, high solubility of gases and vapors, thermal stability, and the possibility to combine anions and cations to yield an almost endless list of different structures. These features open perspectives for numerous applications, such as the reaction medium for chemical synthesis, electrolytes for batteries, solvent for gas sorption processes, and also membranes for gas separation. In the search for better-performing membrane materials and membranes for gas and vapor separation, ionic liquids have been investigated extensively in the last decade and a half. This review gives a complete overview of the main developments in the field of ionic liquid membranes since their first introduction. It covers all different materials, membrane types, their preparation, pure and mixed gas transport properties, and examples of potential gas separation applications. Special systems will also be discussed, including facilitated transport membranes and mixed matrix membranes. The main strengths and weaknesses of the different membrane types will be discussed, subdividing them into supported ionic liquid membranes (SILMs), poly(ionic liquids) or polymerized ionic liquids (PILs), polymer/ionic liquid blends (physically or chemically cross-linked ‘ion-gels’), and PIL/IL blends. Since membrane processes are advancing as an energy-efficient alternative to traditional separation processes, having shown promising results for complex new separation challenges like carbon capture as well, they may be the key to developing a more sustainable future society. In this light, this review presents the state-of-the-art of ionic liquid membranes, to analyze their potential in the gas separation processes of the future

Poly[3-ethyl-1-vinyl-imidazolium] diethyl phosphate/Pebax® 1657 Composite Membranes and Their Gas Separation Performance

Poly(ionic liquid)s are an innovative class of materials with promising properties in gas separation processes that can be used to boost the neat polymer performances. Nevertheless, some of their properties such as stability and mechanical strength have to be improved to render them suitable as materials for industrial applications. This work explored, on the one hand, the possibility to improve gas transport and separation properties of the block copolymer Pebax® 1657 by blending it with poly[3-ethyl-1-vinyl-imidazolium] diethyl phosphate (PEVI-DEP). On the other hand, Pebax® 1657 served as a support for the PIL and provided mechanical resistance to the samples. Pebax® 1657/PEVI-DEP composite membranes containing 20, 40, and 60 wt.% of PEVI-DEP were cast from solutions of the right proportion of the two polymers in a water/ethanol mixture. The PEVI-DEP content affected both the morphology of the dense membranes and gas transport through the membranes. These changes were revealed by scanning electron microscopy (SEM), time-lag, and gravimetric sorption measurements. Pebax® 1657 and PEVI-DEP showed similar affinity towards CO2, and its uptake or solubility was not influenced by the amount of PIL in the membrane. Therefore, the addition of the PIL did not lead to improvements in the separation of CO2 from other gases. Importantly, PEVI-DEP (40 wt.%) incorporation affected and improved permeability and selectivity by more than 50% especially for the separation of light gases, e.g., H2/CH4 and H2/CO2, but higher PEVI-DEP concentrations lead to a decline in the transport properties

Comparison of pure and mixed gas permeation of the highly fluorinated polymer of intrinsic microporosity PIM-2 under dry and humid conditions: Experiment and modelling

This manuscript describes the gas separation performance of PIM-2, a partially fluorinated linear copolymer synthesized from 5,5',6,6'-tetrahydroxy-3,3,3',3'-tetramethylspirobisindane (TTSBI) and decafluorobiphenyl (DFBP). As one of the early members of the family of polymers of intrinsic microporosity, it had never been tested as a gas separation membrane because of insufficient mechanical resistance. This has been solved only recently, allowing the preparation of robust self-standing films. Molecular modelling studies demonstrated a high fractional free volume (34%) and an elevated surface area (642 m(2) g(-1)), and the latter is in good agreement with experimental BET results. Pure gas permeabilities measured on a fixed-volume time-lag instrument at 1 bar compare well with the results of mixed separation tests on a variable volume setup from 1-6 bar(a). Molecular modelling and independent sorption measurements on a gravimetric sorption balance both show strong dual-mode sorption behaviour, especially for CO2 and to a lesser extent for CH4. Temperature-dependent pure gas permeation measurements show typical Arrhenius behaviour, with a clear increase in the activation energy for diffusion with the increasing molecular size of the gas, indicating high size-selectivity. This is in agreement with the highly rigid PIM structure, determined by AFM force spectroscopy measurements. The dual-mode behaviour results in a moderate pressure dependence of the CO2 permeability and the CO2/N-2 and CO2/CH4 selectivity, all slightly decreasing with increasing pressure. The presence of humidity in the gas stream has a remarkable small effect on the membrane performance, which is probably due to the high fluorine content and the consequently low water vapour solubility in the polymer, as confirmed by gravimetric sorption measurements. The manuscript describes an extensive study on the structure-property relationships in PIM-2.Web of Science594art. no. 11746

Effect of physical aging on the gas transport and sorption in PIM-1 membranes

Understanding of the properties over long time scales is a key requirement for the successful application of novel polymers as membrane materials. In this light, the physical aging of dense PIM-1 films with different previous histories was monitored for more than 4 years via parallel gas sorption and permeability measurements. The effect of aging on the individual transport parameters, permeability, solubility and diffusivity, was studied on alcohol treated membranes with high excess free volume. Thermal conditioning of these membranes led to accelerated aging and a reduction of the initial gas permeability and diffusivity of the membranes. A long-term CO2 sorption analysis showed aging affected the sorption kinetics much more than the total equilibrium sorption. This was confirmed by permeation studies with six different gases, showing that the reduction of the permeability coefficient of the samples as a function of time is almost entirely due to a reduction of the diffusion coefficient. A renewed alcohol treatment of the aged membrane led to significant rejuvenation of the membrane. To the best of our knowledge, this is the first systematic long term aging study on PIM-1 via simultaneous analysis of sorption and permeation kinetics. Mixed gas permeation measurements with a CO2/CH4 mixture and an N-2/O-2/CO2 mixture confirm the excellent permselective properties of the PIM-1 membranes even after long aging