Permselectivity Values Research Articles

Binary iron cobalt oxide silica membrane for gas separation

This work investigates the preparation, characterisation and performance of binary iron/cobalt oxide silica membranes by sol–gel synthesis using tetraethyl orthosilicate as the silica precursor, and cobalt and iron nitrates. It was found that cobalt and iron oxides were generally dispersed homogeneously in the silica structure, with the exception of a few minor patches rich in cobalt oxide. The sol–gel synthesis affected the micro-structural formation of binary metal oxide silica matrices. Increasing the iron content favoured condensation reactions and the formation of siloxane bridges, and consequently larger average pore sizes which lead to low He/N2 permselectivity values below 20. In the case of high cobalt content, a higher silanol to siloxane ratio was observed with tighter pore size tailoring, as evidenced by higher He/N2 permselectivities reaching 170. The binary metal oxide and silica interfaces proved to follow a molecular sieving mechanism characterised by activated transport where the permeance of the smaller gas molecules (He and H2) increased with temperature up to 500°C, whilst the permeance of larger gas molecules (CO2 and N2) decreased.

Estimating gas permeability and permselectivity of microporous polymers

Experimental and computational investigations have shown that seemingly minor changes in the chemical structure can have a profound effect on the gas adsorption and separation properties of a polymeric membrane. However, the vast number of possible polymer functionalities makes the evaluation of candidate structures a daunting challenge. This study presents the first systematic screening for multiple series of amorphous porous polymers and elucidates several design principles, which will be useful in the design of new intrinsically microporous polymeric membranes for gas separation applications. Specifically, an efficient method for estimating the gas permeability and permselectivity of a polymeric sample by means of free volume theory, grand canonical Monte Carlo simulations, and the solution diffusion model is presented. This method for calculating permeability is orders of magnitude faster than other techniques involving molecular dynamics simulations and is shown to accurately calculate permeability and permselectivity values when compared to available experimental values. As an example, the method is applied to screen four series of polymers of intrinsic microporosity (PIMs) for CO2, CH4, and N2 permeability and CO2/CH4, and CO2/N2 permselectivity. Outstandingly, the gas separation performance of these PIMs was shown not to move parallel to the upper bound for each gas pair. Several porous polymers with high permeabilities and high solubilities were identified, which highlights the fact that permeable polymers with high solubilities coefficients are feasible and a promising route toward achieving industrially applicable gas separation materials.

Cross-linked ionic resins and gels from epoxide-functionalized imidazolium ionic liquid monomers

A novel bis(epoxide)-functionalized ionic liquid (IL) monomer has been reacted with a commercially available amine monomer to produce cross-linked, epoxy-amine-based poly(ionic liquid) (PIL) resins and PIL/IL ion-gels via step-growth (S-G) polymerization. The degree of chemical functionality was controlled by manipulating the monomer stoichiometric ratios. The S-G PIL resins were investigated for CO2 sorption and have a relatively high equilibrium loading of 1.0 mmol CO2/(g resin). PIL/IL ion-gel membranes were subsequently prepared and tested for CO2/light gas permeation. These materials have CO2/N2 and CO2/CH4 permselectivity values near the Robeson upper bound. Analysis of their gas permeation data revealed that these materials exhibit a rare case of inverse CO2/CH4 diffusion selectivity (DCO2/DCH4<1). This phenomenon was attributed to the interaction of CO2 with residual and formed amine groups in the S-G PIL, resulting in a low CO2 diffusivity compared to that of non-interacting CH4.

Preparation of Cellulose acetate membrane by phase inversion technique and modification of surface by Copper Ferro cyanide multilayers

Cellulose acetate (CA) membranes were prepared by wet phase inversion technique from acetone/formamide (2/1 wt/wt). The casting solution using a 20 weight-percentage polymer concentration was used for phase inversion. The solution of Potassium Ferro cyanide and Copper sulphate having molar concentration 0.1m, 0.01m, 0.001m, 0.2m, 0.02m, 0.002m were used for deposition on the surface of Cellulose acetate (CA) membrane by alternate changing the two solutions in a special cell. Values of membrane potential were measured and the transport number, fixed charged density and the permselectivity values were calculated for the electrolytes NaCl and KCl. Scanning electron micrographs (SEM) were taken for simple Cellulose acetate (CA) membrane and surface modified multilayers Cellulose acetate (CA) membrane which showed the fine deposition of multilayers of Copper Ferro Cyanide which reduced the pore size ofCellulose acetate(CA) membrane up to nano order. The values of membrane potential (with and without deposition of inorganic multilayers) slightly decreased with time reaching to a constant value, which showed initial accumulation of solution fluid within transmembrane region and saturation thereafter. The values of membrane potential increased with the deposition of multilayers showing decrease in pore size of membrane which has become of the order of nanosize. The values of transport number of cations of (K+ and Na+) were calculated using membrane potential data, which was foundin good resemblance of the ionic size of trace metal ions.

Electrochemical studies on zirconium phosphoborate based heterogeneous membranes

Electrode potential measurements have been applied to study electrical characteristics like transport numbers, permselectivity & fixed charged density of zirconium phosphoborate ion exchange membranes. The potential measurements were made across the cation exchange membrane maintained at 27±0.1 °C, using halide and nitrate salts of alkali and alkaline earth metals as electrolytes. The membrane potentials, transport numbers and permselectivity values increase with increase in average concentration from 0.0055 M to 0.0495 M for 1:1 and 1:2 electrolytes. With the increase in concentration of the electrolyte, the number of counter ions interacting with the membrane surface increases leading to enhanced Donnan exclusion responsible for the increase of transport numbers. Fixed charge density of the membrane (X) for 1:2 electrolytes is higher in magnitude than for 1:1 electrolytes indicating that the cation exchange is taking place as hydrated species. This hypothesis is supported by higher transport numbers for alkaline earth metal ions than alkali metal ions throughout the range of concentration.

Influence of monovalent electrolytes on the electrochemical studies of newly synthesized thermally stable inorganic–organic nanocomposite membrane

A nanostructured inorganic–organic composite membrane has been synthesized by cost effective sol–gel method under acidic conditions. Membrane potential measurements have been carried out in various monovalent electrolytes (KCl (aq), NaCl (aq), etc.) at different concentrations to scrutinize the relationship between fixed charge concentration and electrochemical properties of the membrane. Effective fixed charge densities are found to follow the order: Li+>NH4+>K+>Na+. The calculated values of permselectivity, ionic mobility ratio and counter-ion transport number reveal that the inorganic–organic nanocomposite membrane is more cation-selective towards Li+ ions; however the selectivity decreases with the increase in concentration for all monovalent electrolytes used. These membranes were comprehensively characterized for their physicochemical properties, morphology, molecular interactions, crystallinity and thermal stability by various analyses. The ion-exchange capacity, volume void porosity and water uptake of the membrane are found to be highly dependent on polystyrene content in the membrane phase and these properties decrease with the increase in the amount of polystyrene. Furthermore, membrane with 25% blend ratio with polystyrene is found to exhibit good selectivity along with moderate ion-exchange capacity, which may be used for their application in electro-driven separation at high temperatures or for other electrochemical processes.

Nanostructuring and Microstructuring of Materials from a Single Agropolymer for Sustainable MAP Preservation of Fresh Food

The main objective of the present work was to determine whether a single agropolymer [wheat gluten (WG)] could fit the modified atmosphere packaging (MAP) requirements of a range of six different fresh produce in key terms of oxygen permeation (PeO2) and CO2/O2 permselectivity (S) values. The required properties for optimal packaging of fresh fruits and vegetables were first evaluated using the Tailorpack MAP modelling software (UMR IATE, Montpellier, France) with packaging dimensions and respiratory and optimal atmosphere data as input parameters. Then, the modelled values obtained were compared with the properties of a range of WG composite films: monolayer self‐supported or multilayer at microscale or nanoscale, cast or thermoplasticised, with different formulations (percentage of plasticisers or nanofillers). The experimental gas transfer properties that could be covered by these materials ranged from 0.05 × 10−10 to 2.00 × 10−10 mol/m2 s Pa for PeO2 and up to 18.0 for S. These ranges are much larger than conventional plastics that exhibit PeO2 from 0.10 × 10−10 to 0.20 × 10−10 mol/m2 s Pa and S up to 4.5. It was demonstrated from a food‐requirements‐driven (Tailorpack modelling) and a multiscale film structuring (WG‐based composites) approaches, that transfer properties of WG‐based films would fit the requirements of the six selected fruits and vegetables better than conventional plastics. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Performance of passive direct methanol fuel cell with poly(vinyl alcohol)-based polymer electrolyte membranes

Polymer electrolyte membranes (PEMs) were prepared from poly(vinyl alcohol) (PVA) and a modified PVA polyanion containing 2mol% of 2-methyl-1-propanesulfonic acid (AMPS) groups as a copolymer. The effect of the AMPS content and the crosslinking conditions on the properties of the membranes was investigated in PEMs with various AMPS contents prepared under various crosslinking conditions. The proton conductivity and the permeability of methanol through the PEMs increased with increasing AMPS content, CAMPS, and with decreasing annealing temperature, Ta, because of the increase in the degree of swelling. The permeability coefficient of methanol through the PEM prepared under the conditions of CAMPS = 2.0mol% and Ta190°C was approximately 30 times lower than that of Nafion® 117 under the same measurement conditions. A maximum proton permselectivity of 96 × 103 S cm−3 s, which is defined as the ratio of the proton conductivity to the permeability of methanol, was obtained for this PEM. The permselectivity value is about three times higher than that of Nafion® 117. A passive air-breathing-type DMFC test cell constructed using the PEM delivered 2.4 mW cm−2 of maximum power density, Pmax, at 2M methanol concentration, which is smaller than the value obtained with Nafion® 117. However, at high methanol concentrations (>10M), the Pmax of the PEM decreases slightly to 1.6 mW cm−2 (at a methanol concentration of 20M), whereas the Pmax of Nafion® 117 falls to almost zero.

Gas permeation behavior of CTA polymer inclusion membrane (PIM) containing an acidic carrier for metal recovery (DEHPA)

CTA polymer inclusion membranes (PIMs) containing an acidic carrier (D2EHPA) were prepared and characterized by Fourier transform infrared resonance (FT-IR) spectroscopy and thermogravimetric analyses (TGA). The surface and cross-section morphology of the membrane was inspected by scanning electron microscopy (SEM). The gas transport properties of the membranes were obtained by using pure gases. The permeability and permselectivity coefficients of He, H 2, N 2, O 2, CH 4 and CO 2 in PIMs were determined at two temperature values (25 and 50 °C), feeding single gases (purity level 99.998%) at atmospheric pressure and keeping the permeate side under vacuum (10 −2 mbar). The gas permeability of the pure CTA polymer membrane increases with the temperature for all species considered and the permeation rate order is the following: He > H 2 > CO 2 > O 2 > CH 4 > N 2. The same trend with the temperature was also observed for the samples where the plasticizer (2-NPOE) and D2EHPA were added and a significant increase of the permeation rate of all gases was recorded. In most cases, the ideal permselectivity values, measured through these membranes, result lower than those calculated in neat polymer. However, CO 2/H 2 permselectivity increases during the addition of the plasticizer and extractant.

Evaluation of hybrid silica sols for stable microporous membranes using high-throughput screening

Microporous membranes are a promising option for energy-efficient molecular separations. Long-term hydrothermal stability of the membrane material is of prime importance for several industrial processes. Here, a short overview of silica-based membrane materials and their hydrothermal stability is presented. Following this, the development of a series of organic–inorganic hybrid silica sols is described, based on α,ω-bis(triethoxysilyl)-precursors with bridging methane, ethane, propane, and benzene groups. High-throughput screening was used to scan a range of sol parameters, followed by membrane preparation from the most promising sols. These organic–inorganic hybrid silica (HybSi®) membranes were used in dewatering of lower alcohols by pervaporation. Separation factors up to 200 were found for ethanol/water mixtures, and up to 23 for methanol/water mixtures. Modest permselectivity values for hydrogen over nitrogen were found, ranging up to 20.7 for the shortest bridging group. It was concluded that the length of the organic bridge has a clear effect on the pore size distribution and the selectivity of the membrane.

Network model for the evolution of the pore structure of silicon-carbide membranes during their fabrication

A pore-network model is developed to study the evolution of the pore spce of a nanoporous silicon-carbide membrane during its fabrication by the chemical-vapor infiltration/chemical-vapor deposition (CVI/CVD) techniques. The pore space of the support is represented by a three-dimensional network of interconnected pores, in which the pores’ effective size is distributed according to a pore size distribution (PSD) that closely mimics the experimental PSD. The chemical reaction that generates the SiC, the various transport mechanisms in the pores, and the evolution of the pore structure during the SiC deposition on the pores’ surface are included in the model. The Maxwell–Stefan equations are used for describing the pore-level transport processes, which include the Knudsen and hindered diffusion, as well as viscous flow. The effect of pore shrinkage and blockage as a result of the deposition of the SiC on the pores’ internal surface is taken into account. The simulator monitors the PSD as the membrane's structure develops by the CVD process and evolves. Also computed are the permeances of argon, the carrier gas, and of helium during the CVD, as well as the membrane's selectivity. Good agreement is found between the simulation results and the experimental data. Thus, the model may be used for determining the optimal conditions under which a membrane may achieve a given value of permselectivity.

A simulation study of the effect of operating and design parameters on the performance of a water gas shift membrane reactor

Abstract The goal of this work is to understand the effect of the relative values of membrane permselectivity, permeation flux and reaction rate on the performance of a water gas shift membrane reactor. This was achieved by simulating the operation of an isothermal tube–shell reactor. Its performance was evaluated based on the CO conversion and H 2 recovery, as well as the permeate and retentate H 2 molar fractions. The maximum enhancement of CO conversion has been observed when the Damkholer number ( Da ) is almost equal to the permeation number ( Pe ). Improvements in CO conversion can be achieved even when membranes with relatively low permselectivity values (∼10) are used. Further increase of permselectivity primarily increased the purity of the H 2 rich stream. The utilization of CO 2 selective instead of H 2 selective membranes could improve CO conversion only if the CO 2 content of the feed is higher than that of H 2 . Finally, simulations using rate expressions that correspond to different detailed reaction mechanisms resulted only in slight differences in reactor performance.

Preparation of new semifluorinated aromatic poly(ether amide)s and evaluation of pervaporation performance for benzene/cyclohexane 50/50 mixture

A series of new solution-processable aromatic poly(ether amide)s (PEAs) were prepared by the phosphorylation polyamidation reaction of terephthalic acid (TA) and four structurally different semifluorinated aromatic diamines (BTAPPI, BTAPPHI, BTAPPF, BTAPPPI). The synthesized polymers (PEA I, PEA II, PEA III and PEA IV) were characterized by FTIR, 1H and 13C NMR, TGA, DSC, DMA and WAXD techniques. The polymers showed good solubility in many organic solvents viz. NMP, DMF, DMAc and pyridine and were insoluble in CHCl 3, benzene (Bz), toluene and cyclohexane (Chx). Membranes of average thickness of 42 μm of these polymers were prepared from their solutions in DMAc. The polymers showed good thermal stability, high glass transition temperatures (233–300 °C) and good mechanical strength. The membranes were investigated for separation of Bz/Chx (50/50 wt%) mixture. PEA IV containing cardo phthalimidine moiety showed the highest permselectivity and pervaporation separation index (PSI) values of 7.6 and 3616 g/m 2 h for Bz/Chx mixture, respectively. PEA III containing cardo fluorene moiety in the main chain showed highest permeation flux of 569 g/m 2 h in the series. The effect of temperature on the pervaporation performance of these membranes was also studied. The activation energies of pervaporation for Bz, Chx and Bz/Chx (50/50 wt%) mixtures through all the PEAs were calculated from the slopes of the respective Arrhenius plots. An attempt has been taken to draw a structure property correlation of the chemical structures of the newly prepared polymers with pervaporation performance.

Separation of benzene/cyclohexane mixture using semifluorinated aromatic poly(ether amide) membranes with and without cardo unit in the main chain

This paper describes the pervaporation study of benzene (Bz)/cyclohexane (Chx) (50/50 wt%) mixture through four structurally different semifluorinated aromatic poly(ether amide) (PEA) membranes (PEA I, PEA II, PEA III and PEA IV). The effects of variation of molecular structure on pervaporation performances towards Bz/Chx (50/50 wt%) mixture at three different temperatures (50, 60 and 70 °C) were studied and the results indicated that the membranes were benzene-selective in nature. The poly(ether amide), PEA IV containing cardo phenolphthalein anilide unit in the main chain showed the highest permselectivity value of 5.9 for a Bz/Chx mixture and the poly(ether amide), PEA III containing cardo fluorene moiety in the main chain showed highest permeation rate of 31.42 kg μm/m 2 h in the series. The activation energies for pervaporation of Bz, Chx and Bz/Chx mixtures through the PEA membranes have been calculated from the Arrhenius equation.

Characterization of highly selective microporous carbon hollow fiber membranes prepared from a commercial co-polyimide precursor

In this work, the preparation of gas separating carbon hollow fiber membranes based on a 3,3′4,4′- benzophenone tetracarboxylic dianhydride and 80% methylphenylene-diamine + 20% methylene diamine co-polyimide precursor (BTDA-TDI/MDI, Ρ84 Lenzing GmbH), their permselectivity properties as well as details of the carbon nanostructure are reported. Hollow fibers were initially prepared by the dry/wet phase inversion process in a spinning set-up, while the spinning dope consisted of P84 as polymer and NMP as solvent. The developed polymer hollow fibers were further carbonized in nitrogen at temperatures up to 1173 K. Thermogravimetric analysis was used to investigate the weight loss during the carbonization process. The nitrogen, methane and carbon dioxide adsorption capacity of the prepared materials was determined gravimetrically at 273 and 298 K and hydrogen adsorption experiments were performed at 77 K up to 1 bar. Scanning electron microscopy was used to elucidate the morphological characteristics and the nanostructure while H2 sorption at 77 K was applied to evaluate the microporosity of the developed carbon hollow fiber membranes. In all cases, the permeability (Barrer) of He, H2, CH4, CO2, O2 and N2 were measured at atmospheric pressure and temperatures 313, 333 and 373 K and were found higher than those of the precursor. Moreover, the calculated permselectivity values were significantly improved. The developed carbon fibers exhibit rather low H2 permeance values (8.2 GPU or 2.74 × 10−9 mol/m2·s·Pa) with a highest H2/CH4 selectivity coefficient of 843 at 373 K.

Constant concentration method of measuring hydrogen permeation from a hydrogen mixture with Pd membrane

The conventional flow method of measuring hydrogen permeation flux was found to be inaccurate and inadequate to obtain a consistent value of hydrogen flux and permeance because of changing hydrogen concentration along the palladium membrane tube in a hydrogen mixture. We designed a new method in which the hydrogen concentration was kept constant in the retentate. This constant concentration method was a more accurate measurement of hydrogen permeation flux in all of the possible hydrogen mixtures: H 2 + Y with Y = Ar, N 2 and CH 4 and various hydrogen concentrations under different pressure. Permselectivity of the hydrogen mixture was measured under this constant concentration method and was compared with both the conventional flow-through method and separate flow measurement of pure component gases. All three methods gave a different value of permselectivity for the same composite mixture. This method enables us to measure hydrogen flux and permeance accurately in the corresponding composition of the mixture. We found that even with the same partial hydrogen pressure differential for Sieverts’ equation, the hydrogen flux and permeance decreased dramatically with the lowering of hydrogen concentration in the feed.

Polysiloxanes with pendent bulky groups having amino-hydroxy functionality: Structure–permeability correlation

A series of poly(dimethylsiloxane) (PDMS) films substituted with different bulky groups in the pendent chain having amino and hydroxy functional groups have been prepared. FT-IR, 1H- and 13C NMR, GPC, TGA, DSC and WAXD techniques were used to characterize the products. The gas transport properties, viz., permeability, solubility and diffusivity, of the functionalized PDMS films were determined for N 2, O 2, CH 4 and CO 2 gases. With the incorporation of increasingly bulkier substituents the permeability coefficient was found to decrease. The decrease in value was observed to be low compared to other reported bulky substituents. The glass transition temperature increases with increasing bulkiness of the substituent. The fractional free volume (FFV) and d-spacing values were also determined and correlated with the permeability coefficients. The permselectivity of these films for O 2/N 2 and CO 2/CH 4 gas pairs increases due to increase in the solubility selectivity value. Among these, the PDMS film functionalized with 2,6-diisopropyl phenyl amino-hydroxy groups shows high permselectivity value for the studied gas pairs without much reduction in the permeability value.

Single gas permeation through compositionally different zeolite NaA membranes: Observations on the intercrystalline porosity in an unconventional, semicrystalline layer

The successful application of zeolite A membranes in the industrial market has thus far been restricted to the pervaporative dehydration of solvent streams in the pharmaceutical and engineering industries. Their application in gas separation processes remains elusive, largely due to the existence of uncontrollable, intercrystalline diffusion pathways in the boundary regions of neighbouring crystals. In this study the gas permeation characteristics of an unconventional, semicrystalline (70%) zeolite NaA layer, where the boundary phase is filled with amorphous aluminosilicate, was examined and compared to a traditional polycrystalline membrane. To this end, we used single gas permeation of H 2, N 2 and SF 6 at two temperatures. In general, the permeances of all gases through the conventional layer increased in response to temperature, indicating a governing contribution of activated (gaseous) diffusion over the temperature interval tested. Typical permeances of H 2 were in the order of 4.8 × 10 −7 (23 °C) to 6.5 × 10 −7 mol m −2 s −1 Pa −1 (107 °C). For the semicrystalline layer, the same trends were observed for N 2 and SF 6, while the permeance of H 2 decreased from 8.7 × 10 −7 (23 °C) to about 3.9 × 10 −7 mol m −2 s −1 Pa −1 at 107 °C. This was attributed to the expansion of the crystal/amorphous interface at higher temperature. By comparing the permselectivity values with the respective Knudsen factors, it was shown that diffusion through the semicrystalline layer, at lower temperature, was predominantly based on molecular sieving (PS H 2/SF 6 = 63.8), which yielded higher selectivities than the traditional membrane under the same conditions (PS H 2/SF 6 = 11.4). However, at higher temperature the H 2/SF 6 permselectivity of the crystalline layer (5.7) was somewhat higher than that of the semicrystalline membrane (5.2). Based on theoretical considerations, it was concluded that the crystal/amorphous interface in the semicrystalline membrane constituted a denser closure of the boundary interface, which could be attributed to a lower charge barrier presented by the amorphous phase (Si/Al > 1), but this integrity was lost at higher temperature due to the thermal instability of the amorphous material. The results therefore suggested that future research should focus on lowering the intercrystalline charge barrier during synthesis, which could reduce the intercrystalline porosity and improve the gas separation properties of NaA membranes in general.

Perfluoroacylated Ethyl Cellulose: Synthesis, Characterization, and Gas Permeation Properties

Perfluoroacylated derivatives of ethyl cellulose [R = CF 3 CO (2a), C 2 F 5 CO (2b), C 3 F 7 CO (2c), C 7 F 15 CO (2d), C 6 F 5 CO (2e)] were synthesized in good yields by the reaction of various perfluoroacylating agents with residual hydroxy groups of ethyl cellulose (1; DS Et , 2.69). FTIR spectra of the resulting polymers 2a-d furnished the evidence for complete substitution of hydroxy protons by the perfluoroacyl groups. All the derivatives (2a-e) were soluble in common organic solvents and displayed enhanced solubility in moderately polar aprotic and nonpolar solvents. The onset temperatures of weight loss of 2a-d in air were higher than 270 °C, indicating fair thermal stability. Free-standing membranes of 1 and 2a-e were fabricated, and 2a-d exhibited large contact angle with water and enhanced gas permeability (P) as compared to 1. An optimum increment in the size of the perfluoroacyl group led to the largest increment in the gas permeability of the polymers; i.e., 2c exhibited the highest P values (e.g., P CO2 284 barrers; cf. P CO2 of 1 110 barrers). The P CO2 /P N2 and P CO2 /P CH4 permselectivity values of the polymers (2a-e) were in the range of 14-22 and 8-11, respectively.

Heterogeneous ion-exchange membranes based on sulfonated poly(1,4-phenylene sulfide)

Poly(1,4-phenylene sulfide) was sulfonated with fuming sulfuric acid to different degrees. The products were ground and sieved (mesh size 50 μm) to obtain small particles. The particles and linear polyethylene were mixed in various ratios and the resulting blends were press-molded at 150°C to obtain the membranes. Membranes containing up to 66 wt.% of sulfonated particles could be prepared without any problem with the required mechanical strength. The membranes with high concentrations of highly sulfonated particles were as conductive as Nafion 117 membranes and had high values of permselectivity in HCl solutions.