Permselectivity Values Research Articles

Nano-sized MIL-101(Cr) MOF for CO2 separation: Crystal downsizing effects on adsorption and mixed matrix membranes

In this paper, nano-sized MIL-101(Cr) metal–organic framework (MOF) and the micro-sized one were synthesized. To reduce the average crystal size, a surfactant namely cetyltrimethylammonium bromide (CTAB) in its optimum amount (Cr3+:CTAB = 1:0.3) along with microwave heating (600 W, 30 min) were applied. The prepared MOFs were characterized by FESEM, EDS, PXRD, BET, FTIR, and TGA analyses. Moreover, CO2 and CH4 adsorption isotherms and kinetics were investigated at 298 K and equilibrium pressures up to 1 bar. The results revealed that the CO2/CH4 ideal adsorption selectivity was improved by 27.15 % for the downsized MOF, so-called MIL-101(Cr)/CTAB, compared to the micro-sized one. Furthermore, by incorporation of the MOFs into the Ultem® 1000 polymer, mixed matrix membranes (MMMs) were fabricated. The morphological analysis of the prepared membranes displayed proper dispersion and compatibility between the polymer and MOFs. The best separation performance was obtained for the MMM containing 20 wt% of the mesoporous MIL-101(Cr)/CTAB. For this MMM, the CO2 permeability and CO2/CH4 ideal perm-selectivity values were enhanced by ∼ 136 % and ∼ 18 %, respectively at 298 K and 4 bar, compared to the neat membrane.

Hydroxyl-Functionalized Polymers of Intrinsic Microporosity and Dual-Functionalized Blends for High-Performance Membrane-Based Gas Separations.

Membrane technology has shown significant growth during the past two decades in the gas separation industry due to its energy-savings, compact and modular design, continuous operation, and environmentally benign nature. Robust materials with higher permeability and selectivity are key to reduce capital and operational cost, pushing it forward to replace or debottleneck conventional energy-intensive unit operations such as distillation. Recently designed ladder polymers of intrinsic microporosity (PIM) and polyimides of intrinsic microporosity (PIM-PI) with pores <20 Å have demonstrated excellent gas permeation performance. Here, a series of plasticization-resistant PIM-based membrane materials is reported, including the first example of a hydroxyl-functionalized triptycene- and Tröger's base-derived ladder PIM and two PIM-PI homopolymers and a series of dual-functionalized polyimide blends containing hydroxyl- and carboxyl-functionalized groups. Specifically, 4,4'-(hexafluoroisopropylidene)diphthalic anhydride (6FDA)-based PIM-PI blends demonstrated extremely high selectivity for a variety of industrially important applications. An optimized polyimide blend containing ─OH and ─COOH groups showed permselectivity values of 136 for CO2/CH4, 11.4 for O2/N2 and 636 for H2/CH4. Such extreme size-sieving capabilities are attributed to physical crosslinking induced by strong hydrogen bonding forming tightly structured polymer networks. The study provides a new general strategy for developing plasticization resistant, robust, and highly-selective PIM-based membrane materials.

On the selective transport of mixtures of organic and inorganic anions through anion-exchange membranes: A case study about the separation of nitrates and citric acid by electrodialysis

Electrodialysis is finding an increasing number of applications, recently also in the separation and purification of organic acids. This technology involves reducing the use of chemicals and the generation of wastes, as compared to conventionally used processes. Gaining insights about the competitive transport between organic and inorganic ions in ED systems is key to achieving efficient separation processes in the bioresource industry. In the present study, the competitive transport of organic (citrates) and inorganic anions (nitrates) through anion-exchange membranes is investigated, under varying pH conditions, ion concentrations, and applied currents, by means of chronopotentiometry and ED experiments. In the absence of nitrate ions and under acidic conditions (pH 2), the resistance of the membrane system is very high because citric acid is mainly present in its undissociated and uncharged form. In the case of sodium citrate (pH 8), the membrane resistance decreases, especially at high current densities, which promote dissociation reactions and the subsequent concentration increase in ionic species near the membrane. Such phenomenon is identified both in chronopotentiometric curves and in long-term ED experiments by a gradual drop in membrane voltage with time. Although being less concentrated than citrates, the selective removal of nitrates is the most effective choice for the separation of both types of species. The selectivity factor of nitrates over citrates reaches the highest values at low applied current densities (below the limiting current density), with stable permselectivity values higher than 20. Such conditions also lead to the least specific energy consumption per nitrates removed (<0.5 kW·hr·kgNO3--1). Therefore, ED can be used under such favorable operating conditions after a clarification step in order to remove inorganic ions from fermentation broths and increase the purity of organic acids. At higher current densities, the enhanced transport of citrate anions produces a significant drop in selectivity towards nitrates and increases the specific energy consumption.

Electrodepositing polydopamine and iron (III) complexed polyacrylic acid for monovalent selective anion exchange membrane fabrication

Monovalent selective anion exchange membranes were efficiently prepared by two-steps electrodeposition. Firstly, a polydopamine (PDA) interlayer was formed onto the surface of a commercial anion exchange membrane (AEM), followed by the deposition of a top layer of Fe3+ complexed polyacrylic acid (Fe-PAA). The PDA interlayer formation was investigated by reversing the polarity of the electric field applied. In particular, modified membrane prepared with PDA layer facing the cathode (labeled as FC-PDA-M) exhibited lower negative charge surface density compared to the membrane prepared with DA solution close to the anode (labeled as FA-PDA-M). This effect resulted in an increased deposition of PAA, due to the diminished electrostatic repulsion, when a solution of Fe3+ complexed polyacrylic acid with a Fe/PAA ratio of 1/4 was electrodeposited on the PDA layer. After eluting the Fe3+ with EDTA, the two manufacture membranes, labeled as FC-PDA/(Fe)PAA-M and FA-PDA/(Fe)PAA-M, were characterized. The former exhibited a higher Cl−/SO42− permselectivity than the latter, although the latter demonstrated better performance compared to both the pristine AEM and a PAA-PDA-M obtained without applying any current during the deposition process. The effect the Fe3+ addition and varied the Fe/PAA ratio was also investigated. The membranes obtained using the Fe/PAA ratio of 1/4 exhibited best separation performance. Finally, the stability of this membrane was assessed by conducting ten consecutive electrodialysis (ED) experiments, revealing consistent Cl−/SO42− permselectivity values within the 5–6 range.

Amine functionalized supported ionic liquid membranes (SILMs) for CO2/N2 separation

Supported ionic liquid membranes (SILMs), containing aprotic N-heterocyclic anion ionic liquids (AHA ILs) in an inorganic inert support, exhibit CO2/N2 permselectivity values as high as 640 at 35.0 °C and 0.03 bar CO2, which represents conditions similar to post-combustion carbon capture (PCCC) from a natural gas power plant. A Fickian model fit to the experimental data estimates CO2 permeability at direct air capture (DAC) conditions of 10,400 barrer and a CO2/N2 permselectivity of 4000 for the best performing IL, triethyl(octyl)phosphonium 4-bromopyrazolide ([P2228][4-BrPyra]). The most important criterion for high selectivity is a large equilibrium constant for binding between the IL and CO2, which results in high CO2 solubility. ILs with smaller molar volumes and with no fluoroalkyl chains enhance N2 rejection. Low viscosity and high IL molar density also enhance CO2/N2 permselectivity and CO2 permeance.

High-Barrier, Biodegradable Films with Polyvinyl Alcohol/Polylactic Acid + Wax Double Coatings: Influence of Relative Humidity on Transport Properties and Suitability for Modified Atmosphere Packaging Applications.

Polyvinyl alcohol (PVOH) exhibits outstanding gas-barrier properties, which favor its use as a biodegradable, high-barrier coating on food-packaging films, possibly in combination with modified atmospheres. Nonetheless, its high sensitivity to water can result in a severe loss of barrier properties, significantly limiting its applications with fresh foods and in high-humidity conditions. In this work, the water vapor (PWV) and oxygen permeability (PO2) of high-barrier biodegradable films with PVOH/PLA + wax double coatings were extensively characterized in a wide range of relative humidity (from 30 to 90%), aimed at understanding the extent of the interaction of water with the wax and the polymer matrices and the impact of this on the permeation process. What is more, a mathematical model was applied to the PWV data set in order to assess its potential to predict the permeability of the multilayer films by varying storage/working relative humidity (RH) conditions. The carbon dioxide permeability (PCO2) of the films was further evaluated, and the corresponding permselectivity values were calculated. The study was finally augmented through modified atmosphere packaging (MAP) tests, which were carried out on double-coated films loaded with 0 and 5% wax, and UV-Vis analyses. The results pointed out the efficacy of the PLA + wax coating layer in hampering the permeation of water molecules, thus reducing PVOH swelling, as well as the UV-shielding ability of the multilayer structures. Moreover, the MAP tests underlined the suitability of the double-coated films for being used as a sustainable alternative for the preservation of foods under modified atmospheres.

High monovalent/divalent permselectivity and low ionic resistance of ionene-based anion exchange membranes in electrodialysis

This study presents an investigation on the monovalent/divalent ion permselectivity of anion exchange ionenes, a distinct class of solid polymer polyelectrolytes having cationic fixed charge groups located directly on the polymer backbone, rather than being pendant. These ionenes are commercially available as Aemion® AEMs and are based on hexamethyl-p-terphenyl poly(benzimidazolium) (HMT-PMBI). Monovalent/divalent permselectivity values of four Aemion® membranes of different thickness and ion exchange capacity were determined via electrodialysis in a mixed electrolyte system comprising chloride/sulfate anions. To fully understand the transport phenomena in these materials, ion transport properties were studied in conjunction with water uptake and ionic conductance. When the AEMs were exposed to mixed chloride/sulfate solutions, their water uptake significantly increased compared to purely chloride containing solutions, and the concentration of fixed charge groups in the AEMs consequently decreased. We find that thicker ionenes with lower IEC exhibit the highest permselectivity. The data also reveal that thinner AEMs yield a greater ionic flux loss and decreased permselectivity values, particularly in case of lower IEC AEMs. Surprisingly, despite its low water uptake and high resistance, commercial Selemion AMV possesses lower permselectivity than low IEC ionene-based Aemion® AEMs, which we explain on the basis of the highly tortuous internal morphology of low water content ionenes. Permselectivity-to-resistance ratio values are an order of magnitude higher for low IEC ionenes when compared with high IEC ionenes and Selemion AMV.

Cross-Linked, Monovalent Selective Anion Exchange Membrane: Effect of Prealkylation and Co-ions on Selectivity

Tuning of hydrophobicity, water uptake, and size of the ion channel is very crucial for the monovalent anion-selective membrane (MASM) preparation. In this work, we report the preparation of MASM from the cross-linked polyacrylonitrile-co-polyvinylimidazole (PAN-co-PVI) copolymer. The copolymer was in situ quaternized and cross-linked using the polyacrylonitrile-co-polychloromethylstyrene (PAN-co-PCMSt) copolymer as cross-linker. The benzyl chloride moiety (Ar-CH2Cl) of PAN-co-PCMSt reacts with imidazolium nitrogen of PAN-co-PVI and provides a positive charge on the membrane matrix, whereas the PAN moiety provides greater compatibility with the PAN-co-PVI copolymer matrix. The faster cross-linking without external prealkylation forms a strong network structure with a low ionic charge on the membrane surface. Judicious extent of prealkylation followed by cross-linking forms a controlled network structure and small-sized ionic channel with moderate water uptake and good mechanical stability. The representative AEM0Q and AEM17Q membranes with 0 and 17% prealkylation exhibited a permselectivity (PSO42–Cl–) value of 4.39 and 15.40, respectively, during the separation of 0.01 M NaCl + 0.01 M Na2SO4 by electrodialysis. Upon further increase of prealkylation to 27 and 32%, the water uptake of the membrane increased, which in turn allowed the passage of SO42– along with Cl– and decreased the PSO42–Cl–value to 4.12 and 2.45, respectively. Similar trends were observed during the separation of NaCl + MgSO4, MgCl2 + Na2SO4, and MgCl2 + MgSO4 mixtures of each 0.01 M concentration.

Realizing high performance gas filters through nano-particle deposition.

We have studied the separation of a mixture of hydrogen and methane in equal proportions, using a thin film comprised of 10 layers of nanoparticles deposited layer-wise using our "two-point sticking algorithm" which simulates controlled agglomeration of such nanoparticles. We simulate the process of gas separation using LAMMPS. We have studied the scenario where nanoparticles act like hard spheres, maintaining their shape and size, similar to what has been demonstrated by experiments involving self-assembled nanoparticle thin films. We consider the pressure dependence of the results by working at 3 different initial pressures, 0.1 × P0, 0.5 × P0 and P0, where P0 is the atmospheric pressure. Three different diameters of the nanoparticles, namely 3 nm, 6 nm and 9 nm, are considered, and therefore the overall thickness of the membranes considered ranges from 30 nm to 90 nm. We obtained perm-selectivity values that are significantly higher than the Robeson line for hydrogen-methane gas separation, indicating the novelty and therefore the significant applications of this work. We find that while the permeance of hydrogen remains more or less steady with a ten-fold increase of pressure, the corresponding fall in methane's permeance is very sharp. The fall in methane's permeance with increasing pressure is more pronounced the smaller the nanoparticles of the membrane being used. This results in an even higher selectivity at higher pressure for smaller nanoparticle based membranes.

Improved permselectivity and mechanical properties of sulfonated poly dimethyl phenylene oxide cation exchange membrane using MXene nanosheets

In the current research, sulfonated poly (phenylene oxide) (SPPO)/MXene nanosheets cation exchange membranes (CEM) were prepared by solution casting technique. SPPO with different sulfonation degrees (DS) were synthesized, and MXene nanosheets were etched from the MAX phase. The effect of sulfonation degrees (31, 36, and 43 %), polymer concentration (19, 22, and 25 wt%), and MXene loading (0.5, 1, and 2 wt%) on mechanical properties, electrochemical characteristics, and permselectivity of the CEMs were studied. The results showed that an increase in DS, the presence of hydrophilic MXene nanosheets, and a decrease in polymer concentration had synergistic effects on enhancing the hydrophilicity and conductivity of the fabricated CEMs. The ion exchange capacity (IEC) increased with enhancement of DS and MXene loading due to more attaching the functional groups to the polymer structure. On the other hand, an increase in polymer concentration resulted in the reduction of IEC. The tensile strength of CEMs had a reduction trend when the sulfonation degree increased, and polymer concentration decreased. Presence of MXene nanosheets significantly improved the values of permselectivity and tensile strength; For example, permselectivity increased from 80.92 % to 92.30 % by adding 2 % of MXene to the casting solution containing 25 wt% of SPPO with a sulfonation degree of 36 %.

Spirobisindane-Containing Imidazolium Polyimide Ionene: Structural Design and Gas Separation Performance of “Ionic PIMs”

Polymers of intrinsic microporosity (PIMs) have been explored extensively for challenging membrane separation applications. Manipulating microporosity and functionalities of PIM structures has played a major role in the design of membrane materials that can lead to cost-effective CO2 separation processes. Here we have developed a new class of ionenes, in which imidazolium cations are attached (via imides) to spirobisindane groups commonly associated with PIMs yielding a novel ionic-mediated polymer of intrinsic microporosity or “PIM-polyimide ionene” for high-performance CO2 separations. The PIM-polyimide ionene was prepared via Menshutkin reactions between a newly designed spirobisindane-containing bisimidazole monomer and α,α′-dichloro-p-xylene. The resulting thermally and mechanically stable transparent membrane was further studied for its potential utility for CO2 separation. The newly developed PIM-polyimide ionene demonstrated excellent CO2 separation performance relative to other light gases including H2, N2, and CH4. For CO2/CH4 separation, the results surpass the 2008 Robeson upper bound with CO2 permeability of 356.2 barrer and permselectivity of 55. Most notably, the PIM-polyimide ionene membrane outperformed all our previously developed imidazolium ionenes due to the superior CO2 permeability imparted by spirobisindane groups while achieving a comparable performance within the general range of other functionalized PIMs reported in the literature. This unprecedented result of PIM-polyimide ionenes arises from the inclusion of both highly contorted “spiro” centers and the imidazolium cations which precisely alternate in the main chain polymer backbones, resulting in both high diffusivity selectivity and solubility selectivity. It is also noteworthy that PIM-polyimide ionenes exhibited high tolerance against physical aging, as all the gases exhibited very stable permeability and permselectivity values even after a period of 12 months. We hypothesize that the resistance to aging is due to the interactions between ionic groups stabilizing the structure of the polymer matrix, and as such, the ionene microstructure is already close to equilibrium when the membrane is formed. Overall, this is the first example for the integration of PIM structures within the ionene configuration, providing access to potentially numerous novel PIM-ionene materials that can be used in the development of CO2 selective membranes for applications in flue gas, natural gas, and syngas separations.

Synthesis of a novel freestanding conjugated triazine-based microporous membrane through superacid-catalyzed polymerization for superior CO2 separation

This study aims at developing a novel freestanding conjugated triazine-based membrane (ETM-1) with permanent porosity via a low-temperature superacid-promoted polymerization reaction. In this venue, we used the bifunctional 4-ethynylbenzonitrile (4-EBN) monomer featuring ethynyl (CCH) and nitrile (CN) functionalities. At the same time, trifluoromethanesulfonic acid (CF3SO3H) served as both the catalyst and solvent. The cross-linked Polymer's physicochemical properties were systematically examined using TGA, BET, solid-state 13C CP-MAS NMR, ATR-FT-IR, and XPS spectroscopy techniques. With conjugated s-triazine (as a result of cyclotrimerization of nitrile) and CCH (as a result of cationic polymerization of ethynyl), the microporous ETM-1 membrane displayed a superior CO2 uptake capacity of up to 3.87 mmol g−1 (170.3 mg g−1) and enhanced ideal CO2/N2 and CO2/CH4 permselectivity values of 47 ± 2 and 21 ± 1 at 298 K and 1 bar, respectively. The notable selectivity of ETM-1 towards CO2 can be stated in terms of the molecular sieving (i.e., kinetic selection) characteristic of the microporous membrane, its electron-rich π-conjugated skeleton, and enhanced basicity, suitable for interacting with CO2via dipole-quadrupole and acid-base interactions. Moreover, ETM-1 exhibited the isosteric heat of CO2 adsorption ranging from 28.1 to 34.3 kJ mol−1, implying strong physisorption of CO2 upon the high-affinity adsorption sites of the membrane.

Polysulfone Mixed-Matrix Membranes Comprising Poly(ethylene glycol)-Grafted Carbon Nanotubes: Mechanical Properties and CO2Separation Performance

Polysulfone (PSF)-based mixed-matrix membranes (MMMs) were developed through incorporation by solvent casting of poly(ethylene glycol)-grafted carbon nanotubes (PEG-g-CNTs) into the polymer matrix. Single-gas permeability and the respective perm-selectivity values of the resulting MMMs for CO2, CH4, and N2 along with mixed-gas selectivities were determined at various temperatures and pressures. The performance was further analyzed by obtaining gravimetrically gas adsorption isotherms and interpreting the results to determine the solubility and diffusivity coefficients of the prepared membranes. Simultaneous improvement in CO2 permeability, selectivity, and mechanical robustness was achieved. Indicatively, an increase in CO2 permeability by 52.4% at 1.5 bar was discerned for the MMM containing 5 wt % PEG-g-CNTs along with enhancements of 81 and 74% in CO2/N2 and CO2/CH4 perm-selectivities, respectively, while an up to 43.4% increase in tensile modulus and a 12.5% increase in tensile strength were achieved as well. This performance, in conjunction with the stability of the fillers, and the low cost, structural stability, and commercial availability of the polymer make the PSF/PEG-g-CNT MMM a promising base case for developing industrially relevant CO2 separation membranes.

Electrochemical effect and permselectivity of monovalent ions in polystyrene-bismuth oxyiodide composite membrane

Membrane potential and permselectivity are critical parameters for variety of electrochemically-driven separation. Our goal in this work was to better understand the ion transport in membrane. The membrane potential followed the trend NaCl > KCl > NaNO3 > KNO3 attributed to the different electrolyte size. The fixed charge density of the membrane was evaluated using theoretical models: (a) Nagasawa, (b) Kobatake and (c) Teorell Meyer Sievers. K+ counter-ion shows low value of permselectivity because of its strong binding affinity for the polymer fixed charge groups. NO3− with higher hydrated radii and lower charge density also tended to result in low membrane permselectivity.

Systematic Analysis Reveals Thermal Separations Are Not Necessarily Most Energy Intensive

Summary At the industrial level, separation of a variety of mixtures is predominantly carried out by thermally driven processes such as distillation. Nevertheless, the current literature seems to suggest that much is to be gained by replacing these processes with alternative non-thermal methods, which will potentially require a fraction of the energy used by distillation. This is primarily due to the widespread perception that thermal separation processes, particularly those that involve vaporization, are highly energy intensive. However, this belief is not well founded, because it is based on extrapolation from comparisons in the literature, many of which make ad-hoc assumptions and result in incorrect conclusions. Our analysis shows that this confusion arises because the processes utilize different types of energy: heat and electricity. Here, we present a consistent framework that enables a fair comparison between different processes that consume different forms of energies. Using this framework, we refute the general perception that thermal separation processes are inherently the most energy intensive and conclusively show through the examples of two challenging mixtures constituting components with low relative volatilities, that distillation processes, when run properly, can consume remarkably lower fuel than non-thermal membrane alternatives, which have often been touted as more energy efficient.

Gas permeation properties of highly cross-linked castor oil-based polyurethane membranes synthesized through thiol-yne click polymerization

In this paper, a new chemistry approach was developed for synthesis of castor oil (CO)-based polyurethane (PU) membranes to attain improved structural properties for gas separation applications. For this purpose, propyne-terminated CO-based PU prepolymer (PTPU) was synthesized by the reaction of CO and isophorone diisocyanate (IPDI), followed by propargyl alcohol (PrAl) and isocyanate (NCO) terminated prepolymer reaction. The ultimate membranes were prepared through thiol-yne cross-linking reaction of PTPU and pentaerythritol tetrakis (3-mercaptopropionate) (PETMP), in presence of azobisisobutyronitrile (AIBN), as a reaction initiator. It was revealed that mechanical and thermal properties of the prepared membranes were improved owing to the formation of flexible thioether linkages through cross-linking reaction. The gas permeation measurements were carried out in the temperature and pressure ranges of 298–338 K and 200–1200 kPa, respectively. The results exhibited reverse size selective performance, as a typical transport behavior of rubbery membranes. The infinite dilution permeability coefficient of CO2 at 298 K was found identical to 2.78 Barrer, along with the infinite dilution perm-selectivity values for CO2/CH4 and CO2/H2 obtained equal to 25.27 and 7.94, respectively. Furthermore, higher permeation activation energies were found in this work compared to a number of cross-linked rubbery membranes in the literature, such as poly (dimethylsiloxane) (PDMS) and poly(ethylene glycol) diacrylate (PEGDA).

Two-dimensional materials modified layered double hydroxides: A series of fillers for improving gas barrier and permselectivity of poly(vinyl alcohol)

A series of modified 2D nanosheets (graphene oxide-layered double hydroxide, GO-LDH; boron nitride-LDH, BN-LDH; and molybdenum disulfide-LDH, MoS2-LDH) are prepared via a cost-effective, environmentally-friendly, and effortless method based on the concept of electrostatic interaction. The modified 2D nanosheet/Poly(vinyl alcohol) (PVA) nanocomposite films show a comprehensive improvement of physicochemical properties. Among the synthesized films, the O2 barrier performance of the 1% GO-LDH/PVA nanocomposite film (1.44 × 10−13 cm3 cm/cm2·s·cmHg) is 25-fold higher than that of the pure PVA film (3.68 × 10−12 cm3 cm/cm2·s·cmHg). The 1% GO-LDH/PVA shows tremendous improvement in tensile strength (84.7 MPa) and modulus (3032 MPa) values than the PVA film (46.7 and 371 MPa). Furthermore, the 2% BN-LDH/PVA nanocomposite shows the H2/CO2 permselectivity value of 102.5, which is superior to the Robeson upper bound and most reported separation films. In addition to the improved gas barrier and permselectivity performances, an exceptional combination of the thermal stability, tensile strength/modulus, flexibility, and transparency of the nanocomposite film could lead to applications in food packaging, H2 purification, and CO2 capture.

Novel Horizon: Smart TiO2/Sn(IV)SbP Nanocomposite with Enhanced Electrochemical and Photocatalytic Properties

In the present research work, sol-gel derived smart nanocomposite of tin antimonophosphate mixed with different concentration of TiO2 (0–40%) has been synthesized and further characterized through XRD, SEM, EDS, and HRTEM. The aim of this study is to relate the electrical conductivity dependence behavior with the different doping concentration of TiO2 (0–40%) in the composite. For this, ion exchange capacity (IEC) has been selected as a preliminary criterion for the measurement of other electrochemical properties like; membrane potential, transport number and permselectivity. The composite with 20% TiO2 in tin antimonophosphate possesses a maximum value, i.e. 0.97 meq/g for IEC, and has been further studied as a case for the material characterisation. Membrane potential, transport number, permselectivity values for the nanocomposite membrane have been found to be better than pristine membrane, for both monovalent and bivalent ions. The composite with 20% TiO2 in tin antimonophosphate further used a photocatalyst for the degradation MB dye. It shows 97% removal efficiency with apparent rate constant of 0.01314 min–1. The photocatalytic degradation of MB follows pseudo-first order kinetics.

Effect of Physical Aging on Gas Transport in Asymmetric Polyimide Hollow Fibers Prepared by Triple-Orifice Spinneret

The systematic evaluation of the gas transport properties related to differences in the history of the samples is a useful tool to appropriately design a membrane-based gas separation system. The permeation rate of six pure gases was measured over time in asymmetric hollow-fiber (HF) samples, that were prepared according to the non-solvent-induced phase separation in different operation conditions, in order to identify their response to physical aging. Four types of HFs having a different structure were analyzed, comparing samples spun in a triple-orifice spinneret to HFs prepared using a conventional spinneret. A generalized gas permeance decline, coupled to a maintained permselectivity for the different gas pairs, was observed for all HFs. Instead, H2/N2 permselectivity values were enhanced upon aging. Cross-linked hollow-fiber samples showed a marked size-sieving behavior that favored the separation of small species, e.g., hydrogen, from other larger species and a good stability over time. Some HFs, post-treated using different alcohols, presented a permeance decay independently on the alcohol type and a greater selectivity over time.

Physicochemical interactions of organic acids influencing microstructure and permselectivity of anion exchange membrane

Permselectivity and water uptake behaviors of anion exchange membrane (AEM) were investigated with organic acids. Contribution of the number of carboxyl group (acetic acid: AA, malic acid: MA, citric acid: CA) and hydrocarbon chain lengths (formic acid: FA, AA, propionic acid: PA and n-butyric acid: BA) in water uptake and permselectivity values were separately investigated with five different concentrations 0.025, 0.05, 0.075, 0.1 and 0.125 mol·L−1 of each acid at two different pH 2.0 and 7.5 conditions. Microstructural changes occurring due to interaction of carboxylate anion and NR4+ groups of AEM resulted in permselectivity behavior. Permselectivity trend: CA < MA < AA could be explained by counter-ion condensation resulting out of strong counter-ion interactions with AEM fixed charges. Co-ion mobility values could explain variation in permselectivity at pH 2.0 and 7.5. While, water uptake and ionic size were dominant factors to explain the permselectivity trend: FA > AA > PA > BA. Complex nature of interactions due to properties (size, charge, ionic charge density, mobility, diffusivity, stokes radius etc.) of carboxylate anions were explained using diffusivity ratio (counter/co-ion, D2/D1), adsorption equilibrium and fraction of dissociated species.