Polypyrrolones for membrane gas separations. I. Structural comparison of gas transport and sorption properties

Despite efforts by the membrane community to develop polymeric materials with improved O₂/N₂ separation performance, limited progress has occurred for almost a decade. Molecular sieving media, which can exhibit gas separation properties superior to polymers, tend to be brittle and uneconomical to produce for large‐scale membrane separation processes. Considering this, the polymer structures investigated in this work were designed to mimic aspects of the structure of molecular sieving media such as zeolites and carbon molecular sieves while maintaining the processability associated with polymers. Significantly attractive gas separation material properties were obtained using hyper rigid polypyrrolone copolymers with controlled packing disruptions between flat, packable segments. The gas transport properties in the materials changed dramatically as a result of different average interchain spacing. Moreover, all of the polypyrrolones studied in this work exhibited performance lying on or above the existing O₂/N₂ upper bound trade‐off line between permselectivity and permeability. These results, therefore, may point the way to a new cycle of membrane materials improvements for gas separations. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1235–1249, 1999     

Finite element assessment of the potential of platelet-filled polymers for membrane gas separations

We use the finite element method to analyze the role of filler aspect ratio and volume loading on the effective permeability and selectivity of gas-separation membranes consisting of a polymer matrix filled with platelet-shaped molecular sieving particles. On the basis of direct 3D finite element estimates, we develop and validate a quick arithmetic procedure for predicting the effective permeability and selectivity of platelet-filled systems. We use this procedure to show that it is feasible to obtain mixed matrix platelet-filled membranes with effective permselectivities considerably exceeding the upper Robeson's bounds.     

Optimization diagram for membrane separations

A novel methodology has been developed which enables optimization of membrane separations. In multi-component separation processes, sieving coefficients for the individual solutes, defined as the ratio of the filtrate and feed concentrations, tend to reach optimum values under different process conditions. It is not possible to determine a priori the pair of sieving coefficients which will give the best combination of product yield and purification for a given application. A purification factor-yield diagram for such an optimization has been developed which utilizes a family of curves representing two dimensionless numbers plotted on yield versus purification-factor coordinates. Analysis can be performed with knowledge of only three experimental variables: the filtrate flux and the two solute sieving coefficients. Complete optimization of membrane processes can be achieved by combining these variables with membrane area, process time, and retentate-volume constraints. The methodology should be applicable to ultrafiltration, microfiltration, and high-performance tangential flow (selective) filtration processes.     

Optimal design of affinity membrane chromatographic columns

A method for the optimal affinity membrane column design, based in the solution of the Thomas kinetic model for frontal analysis in membrane column adsorption, is presented. The method permits to choose suitable membrane operating conditions, column dimensions and processing time, to maximize the throughput when an operating capacity restriction in the range of 80–95% of the column capacity is used. Two basic design charts were obtained by computer simulation, for residence and processing time calculation, respectively. These charts can be used and manipulated in a wide range of operational conditions, provided that four design specifications related to column axial and radial Peclet numbers, length and pressure drop, are fulfilled. The application of the method was illustrated using experimental data and a simple analytical procedure. The implications of the method and results on the design and optimization of affinity membrane chromatographic columns are discussed.     

Optimizing gas chromatographic separations

While separations based on the use of stationary phases specifically designed to maximize solute alphas can, in the strictest sense, be precisely optimized only under specific isothermal conditions, the approach has previously been used even with temperature programmed separations. Selectivity can by further “tuned” by (1) differential variations in the carrier gas velocity through dissimilarly coated coupled columns, and (2) temperature variations in columns of at least moderate polarity. This paper explores the latter approach.     

Recycle and multimembrane permeators for gas separations

The extent of gas separation achievable in a permeator module can be substantially increased by (a) recycling a fraction of the permeated stream, and (b) using more than one type of membrane for a given separation process. The present paper discusses the development and operation of recycle and multimembrane permeator systems and summarizes the results of parametric calculations.     

Liquid membranes for gas/vapor separations

A review on developments of liquid membranes (LMs) in the field of gas and vapor separation of the last 16 years is presented. Liquid membrane configurations employing supports, i.e. immobilized, supported and contained liquid membranes are focussed and detailed information on the respective materials, i.e. supports (supplier, type, thickness, pore width, porosity, tortuosity), liquids and carriers, are presented together with their specific separation tasks. Performance of different LMs in terms of permeability and selectivity as well as stability (duration of testing, applied differential pressures) are compared and discussed. Finally, different preparation methods of LMs are illustrated.     

Continuous gas phase chemical separations

Gas-phase chemical separations have been used for the study of short-lived fission products. Often the chemical separation is achieved in a few seconds. The articles reviews the techniques used in fast, gas-phase separations.     

Chiral metal-organic frameworks for high-resolution gas chromatographic separations

Chiral metal-organic framework coated open tubular columns are used in the high-resolution gas chromatographic separation of chiral compounds. The columns have excellent selectivity and also possess good recognition ability toward a wide range of organic compounds such as alkanes, alcohols, and isomers.     

Molecularly imprinted sol-gel nanotubes membrane for biochemical separations

In this study, we report a simple procedure for applying molecular imprinting functional groups to the inner surfaces of the template-synthesized sol-gel nanotubes for chemical separation of estrone. The silica nanotubes were synthesized within the pores of nanopore alumina template membranes using a sol-gel method by simultaneous hydrolysis of a silica monomer-imprinted molecule complex and tetraethoxysilane (TEOS). A covalent imprinting strategy was employed by generating a sacrificial spacer through the reaction of the isocyanate group of 3-(triethoxysilyl)propyl isocyanate and a phenol moiety of estrone to form a thermally cleavable urethane bond. This allowed us to remove the imprinted estrone by simple thermal reaction and to simultaneously introduce functional groups into the cavity formed by the silica nanotubes. Experiments indicated that estrone could be bound selectively by such an approach and have a binding affinity of 864 +/- 137 (n = 3).     

A comparison of some recycle permeators for gas separations

The performances of several different recycle permeators are compared by calculating the enrichment, extend of separation, and area requirements for different overall stage cuts and recycle ratios, using the separation of air as an example. Membrane modules investigated included a two-unit series type permeator with permeate recycle, a continuous membrane column with high-pressure feed, and two single-unit countercurrent permeators with permeate recycle (both high-pressure and low-pressure feed). The use of Rony's extent of separation in the analysis greatly aids in the visualization of limiting permeator performance at different recycle ratios. The composition of O₂ in the high-pressure product (reject) stream becomes zero at specific values of the stage cut which depend on recycle ratio for all but the low-pressure feed module. This would require an infinite permeator area, hence stage cuts greater than this limiting value are not possible. For the case of the low-pressure feed unit, the reject stream composition approaches a minimum finite value that depends on pressure ratio in the permeator. This limiting minimum composition would also be reached at specific values of stage cut that depend on recycle ratio and hence a stage cut greater than this limiting value cannot be obtained. Other permeator types (co-current, perfect mixed models) do not behave in this manner. It is concluded that the two-unit series type permeator gives the best separation and requires smaller membrane areas at high recycle ratios. Performance of the four permeators is not greatly different at lower recycle ratios.     

Poly(3-dodecylthiophene) membranes for gas separations

Solution cast membranes of poly(3-dodecylthiophene) (PDDT) were studied for the room temperature separation of N₂, O₂, and CO₂. A procedure for fabricating reproducible, smooth, uniformly thick (∼35 μm), defect-free membranes is described. Permeability values were measured for as-cast PDDT membranes (P_N2=9.4, P_O2=20.2, P_CO2=88.2 Barrers) and selectivity values were calculated (α_O2/N2=2.2, α_CO2/N2=9.4). Chemically induced oxidation (∼23%) with SbCl₅ resulted in a decrease in permeability (P_N2=3.5, P_O2=10.5, P_CO2=48.5 Barrers) and a corresponding increase in permselectivity (α_O2/N2=3.0, α_CO2/N2=14.0). Reduction of the oxidized membrane with hydrazine partially reversed these trends (P_N2=5.4, P_O2=15.1, P_CO2=62.9 Barrers, α_O2/N2=2.8, α_CO2/N2=11.6). The chemical compositions of as-cast, oxidized, and hydrazine-treated PDDT membranes were determined using elemental analysis and energy dispersive X-ray spectrometry. Membrane microstructure was investigated by optical microscopy, TappingMode™ atomic force microscopy and scanning electron microscopy. The composition and microscopy results were correlated with changes in gas-transport properties.     

Crosslinking and stabilization of nanoparticle filled PMP nanocomposite membranes for gas separations

Poly(4-methyl-2-pentyne) (PMP) has been crosslinked using 4,4′-(hexafluoroisopropylidene) diphenyl azide (HFBAA) to improve its chemical and physical stability over time. Crosslinking PMP renders it insoluble in good solvents for the uncrosslinked polymer. Gas permeability and fractional free volume (FFV) decreased as crosslinker content increased, while gas sorption was unaffected by crosslinking. Therefore, the reduction in permeability upon crosslinking PMP was due to decrease in diffusion coefficient. Compared to the pure PMP membrane, the permeability of the crosslinked membrane is initially reduced for all gases tested due to the crosslinking. By adding nanoparticles (FS, TiO₂), the permeability is again increased; permeability reductions due to crosslinking could be offset by adding nanoparticles to the membranes. Increased selectivity is documented for the gas pairs O₂/N₂, H₂/N₂, CO₂/N₂, CO₂/CH₄ and H₂/CH₄ using crosslinking and addition of nanoparticles. Crosslinking is successful in maintaining the permeability and selectivity of PMP membranes and PMP/filler nanocomposites over time.     

Thermally stable polyimide isomers for membrane-based gas separations at elevated temperatures

The temperature dependence of gas sorption and transport properties is examined for two polyimide isomers. The permeabilities and solubilities of five gases in these materials are reported over an extensive temperature range from 35 to 325°C. Also, the activation energies for permeation, the heats of sorption, and the activation energies for diffusion obtained for both polyimides are compared and correlated with physical properties of the polymers and penetrants. The influence of temperature on the selective properties of these membrane materials is discussed for three gas separations; He/N₂, CO₂/CH_4, and O₂/N_2. Thorough analysis of these data provides insight into the influence of the subtle difference in chain structure of the two isomers. The performance of the 6FDA-6F_p DA as a separation membrane at high temperatures suggests that it is an outstanding candidate for use in novel elevated temperature applications. ©1995 John Wiley & Sons, Inc.     

Modulation-induced error in comprehensive two-dimensional gas chromatographic separations

There is a fundamental difference between data collected in comprehensive two-dimensional gas chromatographic (GCxGC) separations and data collected by one-dimensional GC techniques (or heart-cut GC techniques). This difference can be ascribed to the fact that GCxGC generates multiple sub-peaks for each analyte, as opposed to other GC techniques that generate only a single chromatographic peak for each analyte. In order to calculate the total signal for the analyte, the most commonly used approach is to consider the cumulative area that results from the integration of each sub-peak. Alternately, the data may be considered using higher order techniques such as the generalized rank annihilation method (GRAM). Regardless of the approach, the potential errors are expected to be greater for trace analytes where the sub-peaks are close to the limit of detection (LOD). This error is also expected to be compounded with phase-induced error, a phenomenon foreign to the measurement of single peaks. Here these sources of error are investigated for the first time using both the traditional integration-based approach and GRAM analysis. The use of simulated data permits the sources of error to be controlled and independently evaluated in a manner not possible with real data. The results of this study show that the error introduced by the modulation process is at worst 1% for analyte signals with a base peak height of 10xLOD and either approach to quantitation is used. Errors due to phase shifting are shown to be of greater concern, especially for trace analytes with only one or two visible sub-peaks. In this case, the error could be as great as 6.4% for symmetrical peaks when a conventional integration approach is used. This is contrasted by GRAM which provides a much more precise result, at worst 1.8% and 0.6% when the modulation ratio (MR) is 1.5 or 3.0, respectively for symmetrical peaks. The data show that for analyses demanding high precision, a MR of 3 should be targeted as a minimum, especially if multivariate techniques are to be used so as to maintain data density in the primary dimension. For rapid screening techniques where precision is not as critical lower MR values can be tolerated. When integration is used, if there are 4-5 visible sub-peaks included for a symmetrical peak at MR=3.0, the data will be reasonably free from phase-shift-induced errors or a negative bias. At MR=1.5, at least 3 sub-peaks must be included for a symmetrical peak. The proposed guidelines should be equally relevant to LCxLC and other similar techniques.     

Recent advances in the formation of ultrathin polymeric membranes for gas separations

During the past 20 years membrane systems have been applied to a limited number of commercial gas separations. To further advance membrane-based gas separations, current research efforts focus on optimization of (i) membrane materials, (ii) membrane structures and (iii) membrane system design. In this overview, recent developments in the formation of high-performance gas separation membranes are discussed. The gas separation properties of state-of-the-art integrally skinned asymmetric membranes and thin-film composite membranes are summarized. Future directions for the preparation of advanced gas separation membranes are highlighted.     

Polyurethane membrane for gas fractionation

Polyurethanes (PU) elastomeric membranes were synthetized using TDI reacted with bifunctional and polyfunctional polyols. In the case of bifunctional polyols, polypropyleneglycols of different chain length were used.Permeability, diffusion coefficient (D), solubility (S) were experimentally measured and solubility (S) was calculated with different gases (CO, H₂, CO₂, CH₄, C₂H₄, C₂H₆). The results show some possible practical applications. The diffusion coefficient D was found to depend upon the diameter of the permeating gas molecule and on the structure of the polymers, nevertheless no simple relation could be found with the free volume calculated with the W.L.F. theory.The solubility constant of the different gases in all the synthesized polymers could be interpreted on the basis of the regular solution theory founded on the solubility parameters of the gases and of the polymers.