Microstructural and gas separation properties of CVD modified mesoporous γ-alumina membranes

Chemical vapor deposition (CVD) was used to modify 4 nm pore, sol–gel derived, γ-alumina membranes supported on macroporous α-alumina. Aluminum oxide was deposited in the pores of the γ-alumina membrane by alternating additions of trimethylaluminum (TMA) and water vapor. By reducing the pore size, the permeance of non-condensable gasses was reduced much more than the permeance of condensable gasses due to capillary condensation or preference adsorption of water vapor. The modified membrane that exhibited the best separation properties had a water vapor permeance ranging from 1.5×10⁻⁶ to 3.0×10⁻⁷ mol/m² s Pa, an oxygen permeance ranging from 1.7×10⁻⁷ to 1.5×10⁻⁹ mol/m² s Pa, and a separation factor as high as 140 at room temperature. The microstructure of the pores contained some irregularities which were attributed to an atomic layer CVD (ALCVD) mechanism modified by homogeneous reactions. The effect of the modified ALCVD was higher permeances than would be expected. P-type zeolite membranes were also made and found to have similar separation properties to the more heavily modified γ-alumina membranes.     

Preparation of palladium membranes from the reaction of Pd(C3H3)(C5H5) with H2: wet-impregnated deposition

A new technique to prepare a palladium membrane for high-temperature hydrogen permeation was developed: Pd(C₃H₃)(C₅H₅) an organometallic precursor reacted with hydrogen at room temperature to decompose into Pd crystallites. This reaction together with sintering treatment under hydrogen and nitrogen in sequence resulted in the formation of dense films of pure palladium on the surface of the mesoporous stainless steel (SUS) support. Under H₂ atmosphere the palladium membrane could be sintered at 823 K to form a skin layer inside the support pores. The hydrogen permeance was 5.16×10⁻² cm³ cm⁻² cm Hg⁻¹ s⁻¹ at 723 K. H₂/N₂ selectivity was 1600 at 723 K.     

Fabrication of supported Si3N4 membranes using the pyrolysis of liquid polysilazane precursor

The fabrication process is described of supported microporous Si₃N₄ membranes, prepared by pyrolytically decomposing organo-substituted polysilazane precursor. The membrane had a composite asymmetric structure consisting of a mechanically strong porous Si₃N₄ support which had 42 vol% pores between 0.4 and 0.52 μm, coated with an intermediate and one or two thin active top layers. The individual layers were fabricated by the conventional dip-coating technique.Permeation experiments with He, N₂ and CO₂ have been performed to determine the gas transport characteristics and separation performance of the processed membranes. The permeation is pressure-independent, indicating no viscous flow in the supported top layer. The proposed process has made it possible to prepare membranes with He permeation rates of ≥5.3×10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ and He/N₂ permselectivities of ≥2.0, even in the membrane with one top layer. It is also demonstrated from separation experiments, that the membrane with high quality top layer has the separation factors of 4.7 for He/N₂ and of the theoretical of Knudsen flow for CO₂/N₂.     

Preparation of composite hollow fiber membranes of poly(ethylene oxide)-containing polyimide and their CO2/N2 separation properties

Composite hollow fiber membranes were prepared by a dry-jet wet spinning process using a double layer spinneret. These membranes were composed of a thin and dense outer-layer of poly(ethylene oxide)-containing polyimide and a sponge-like inner layer of other polyimide. The outer layer was responsible for the separation and fabricated as thin as 1 μm. The permeation flux of CO₂, R_CO2, and the CO₂/N₂ selectivity decreased 40% and 10–20%, respectively, in a month after the membrane preparation. The steady performance was still high; for example, R_CO2=69×10⁻⁶cm³ (STP)/(cm² s cm Hg) and the selectivity of 33 at 323 K.     

Proton-conducting membranes based on poly(2,5-benzimidazole) (ABPBI) and phosphoric acid prepared by direct acid casting

We report the preparation of phosphoric acid doped poly(2,5-benzimidazole) (ABPBI) membranes for PEMFC by simultaneously doping and casting from a poly(2,5-benzimidazole)/phosphoric acid/methanesulfonic acid (MSA) solution. The evaporation of MSA yields a very homogeneous membrane having a better controlled composition, avoiding the use of solvent-intensive procedures. Membranes have been prepared with contents of up to 3.0H₃PO₄ molecules per ABPBI repeating unit. These membranes achieve a maximum conductivity of 1.5 × 10⁻² S cm⁻¹ at temperatures as high as 180 °C in dry conditions. These ABPBI membranes are more conveniently prepared than those conventionally formed and doped in separate steps while featuring comparable conductivities (ABPBI × 2.7H₃PO₄ prepared by the soaking method showed a conductivity of 2.5 × 10⁻² S cm⁻¹ at 180 °C in dry conditions).     

Separation of hydrogen from steam using a SiC-based membrane formed by chemical vapor deposition of triisopropylsilane

A silicon carbide-based membrane was formed in the macropores of an α-alumina support tube by chemical vapor deposition of triisopropylsilane at 700–800°C with a forced cross-flow through the porous wall. The membrane permeated gases except H₂O mainly by the Knudsen diffusion mechanism at permeation temperatures of 50–400°C. The H₂/H₂O selectivity was near or below unity because of the hydrophilic nature of the membrane. After a heat-treatment in Ar at 1000°C for 1 h, however, the membrane formed at a final evacuation pressure of 1 kPa exhibited a H₂/H₂O selectivity of 3–5, for a mixed feed of H₂–H₂O–HBr system, associated in a thermochemical water-splitting process. The H₂ permeance was (5–6)×10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ at 50–400°C. The membrane maintained the H₂/H₂O selectivity for more than 100 h in the H₂–H₂O–HBr mixture at 400°C.     

Separation performance of polyimide composite membrane prepared by dip coating process

The effects of the preparation conditions in a dip coating process on polyimide composite membranes have been investigated. Polyimide precursor obtained from pyromellitic dianhidride (PMDA) and 4,4′-oxydianiline (ODA) was mixed with triethylamine and poly(amic acid)tri-ethylamine salt (PAA salt) was made. An asymmetric polyimide membrane (PI-2080) as a supporting membrane was dipped in a PAA salt (concentration 0–5 wt.%) methanol solution. The coating layers of PAA salt were converted to these of polyimide by annealing at 200°C for 3 h in an ordinary vacuum oven.The performance of the polyimide composite membrane was evaluated by gas permeation (N₂, O₂, CO₂, at 1 kg/cm²) and pervaporation (feed: a 95 vol.% ethanol aqueous solution at 30–60°C). The composite membranes prepared using a coating solution of 5 wt.% PAA salt showed the CO₂/N₂ selectivity of over 25 on gas permeation, and separation factor α (H₂O/EtOH) of over 800 with a total flux of 0.21 kg/m² h on pervaporation.     

Preparation of a palladium alloy composite membrane supported in a porous stainless steel by vacuum electrodeposition

Pinhole-free palladium/nickel (Pd/Ni) alloy membranes deposited on a porous stainless steel (SUS) support have been fabricated. The deposition was made by vacuum electrodeposition technique which could produce the alloy film less than 1 μm thick. This technique allows for the Pd/Ni alloy by employing Pd/Ni complex reagent, and typical Pd/Ni plating had compositions of 78% Pd and 22% Ni. In order to make the surface smooth and enhance the adhesive bond between the top layer and the substrate, a nascent porous SUS disk was treated sequently with submicron nickel powder and CuCN solution. The important parameters that can affect deposition were pore size, defects, and surface roughness of substrate. The membranes were characterized by permeation experiments with hydrogen and nitrogen at temperatures ranging from 623 to 823 K and pressures from 10.3 to 51.7 cmHg. The composite membranes prepared in this technique yielded excellent separation performance for hydrogen: hydrogen permeance of 5.79×10⁻² cm³/cm² cmHg s and hydrogen/nitrogen (H₂/N₂) selectivity was 4700 at 823 K.     

Facilitated transport of CO2 through polyethylenimine/poly(vinyl alcohol) blend membrane

Polyethylenimine (PEI)/poly(vinyl alcohol) (PVA) blend membranes were prepared for the facilitated transport of CO₂. The polymeric carrier PEI was retained in the blend membrane by the entanglement with PVA chains. The CO₂ permeance decreased with an increase in CO₂ partial pressure in feed gas, whereas the N₂ permeance was nearly constant. This result clearly showed that only CO₂ was transported by the facilitated transport mechanism. The CO₂ and N₂ permeabilities increased monotonically with the PEI weight percent in the blend membrane, whereas the selectivity of CO₂ over N₂ showed a maximum. The selectivity increased remarkably with increasing heat-treatment temperature of the membrane. The highest selectivity obtained reached more than 230 when the CO₂ partial pressure was 0.065 atm. The prepared membrane was stable.     

Formation of high-performance 6FDA-2,6-DAT asymmetric composite hollow fiber membranes for CO2/CH4 separation

We report that 6FDA-2,6-DAT polyimide can be used to fabricate hollow fiber membranes with excellent performances for CO₂/CH₄ separation. In order to simplify the hollow fiber fabrication process and verify the feasibility of 6FDA-2,6-DAT hollow fiber membranes for CO₂/CH₄ separation, a new one-polymer and one-solvent spinning system (6FDA-2,6-DAT/N-methyl-pyrrolidone (NMP)) with much simpler processing conditions has been developed and the separation performance of newly developed 6FDA-2,6-DAT hollow fiber membranes has been further studied under the pure and mixed gas systems.Experimental results reveal that 6FDA-2,6-DAT asymmetric composite hollow fiber membranes have a strong tendency to be plasticized by CO₂ and suffer severely physical aging with an initial CO₂ permeance of 300 GPU drifting to 76 GPU at the steady state. However, the 6FDA-2,6-DAT asymmetric composite hollow fibers still present impressive ultimate stabilized performance with a CO₂/CH₄ selectivity of 40 and a CO₂ permeance of 59 GPU under mixed gas tests. These results manifest that 6FDA-2,6-DAT polyimide is one of promising membrane material candidates for CO₂/CH₄ separation application.     

Dehydration of tetrahydrofuran by pervaporation using a composite membrane

Tetrahydrofuran (THF) is a strong aprotic solvent, commonly used in the pharmaceuticals industry due to its broad solvency for both polar and non-polar compounds. THF and water form a homogeneous azeotrope at 5.3 wt.% water thus simple distillation is not feasible to dehydrate THF below this concentration. Pervaporation offers a solution since it is not governed by vapour–liquid equilibria. However many polymer-based pervaporation membranes are cast utilizing THF as the casting solvent and so these membranes have a tendency to swell excessively in its presence. This results in poor separation performance and poor long-term stability and thus renders these membranes unsuitable for THF dehydration.In this study, a new membrane available from CM Celfa, CMC-VP-31 has been tested for the dehydration of THF. The membrane shows excellent performance when dehydrating THF with a flux of over 4 kg m⁻² h⁻¹ when dehydrating THF containing 10 wt.% water at 55 °C dropping to 0.12 kg m⁻² h⁻¹ at a water content of 0.3 wt.%. The permeances of water and THF in the membrane were calculated to be 11.76 × 10⁻⁶ and 7.36 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹, respectively, at 25 °C and found to decrease in the membrane with increasing temperature to values of 6.71 × 10⁻⁶ and 1.63 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ at 55 °C. The flux and separation factor were both found to increase with an increase in temperature thus favouring the operation of CMC-VP-31 at high temperatures to optimize separation performance.     

Separation of benzene/cyclohexane mixtures using polyurethane–silica hybrid membranes

Mixed sols were prepared by dissolving polyurethane (a 30 wt% solution in n-propanol, PU) and tetraethylorthosilicate (TEOS) in ethanol at PU:TEOS mass ratios of 1:2, 1:1, 2:1 and 3:1. Each of the sols was coated on a porous α-alumina support tube by the dipping method, and green membranes were heat-treated at 200°C for 1 h in an atmosphere of nitrogen. A PU membrane was also prepared with PU alone. The membranes were 5–6 μm thick. The polyurethane–silica membranes were swollen in benzene but only slightly in cyclohexane at room temperature. The degree of swelling in benzene decreased with increasing fractions of TEOS in the hybrid sols. The selectivity of benzene to cyclohexane was improved due to the suppression of swelling as a result of hybridization with TEOS. The total permeation flux and benzene/cyclohexane selectivity in the membrane prepared with a sol of PU:TEOS=1:1 were 3×10⁻⁵ kg m⁻² s⁻¹ and 19, respectively.     

Studies on polypyrrole modified nafion membrane for vanadium redox flow battery

In order to prevent the vanadium crossover and preferential water transfer in all-vanadium redox flow battery (VRFB), three methods – electrolyte soaking, oxidation polymerisation and Electrodeposition, were used to modify Nafion 117 membranes using pyrrole. The surface of the modified membranes was uniform and even, and the membranes were characterised in terms of morphology, membrane area resistance, vanadium permeability and water transfer property. The properties of all the modified membranes were improved greatly. The membranes modified by Electrodeposition showed a best combination of the membrane resistance, vanadium permeability and water transfer property, the experimental results showed that the V(IV) ion permeability of polypyrrole modified Nafion membranes by Electrodeposition at the conditions of 0.025 mA cm⁻² and 0 ^°C for 60 min reduced more than 5 times from 2.87 × 10⁻⁶ cm² min⁻¹ to 5.0 × 10⁻⁷cm² min⁻¹, and the water transfer property decreased more than 3 times from 0.72 ml/72 h cm² to 0.22 ml/72 h cm². All above properties made the modified Nafion membranes more applicative in the VRFB system. This paper also reported other methods for Nafion membrane modification and the influences of the deposition conditions on the properties of the membrane selectivity and water transfer.     

Polysulfone — sulfonated poly(ether ether) ketone blend membranes: systematic synthesis and characterisation

The effect of sulfonated poly(ether ether ketone) (SPEEK) in membrane formation and separation properties has been investigated in polysulfone(PSU)/SPEEK/N-methyl-2-pyrrolidinone (NMP) systems. Charged ultrafiltration/nanofiltration membranes were obtained reliably in the range of 0.5–5 wt.% SPEEK in the polymer blend. All PSU/SPEEK blend membranes had substantially higher water flux, salt rejection, porosity and greatly reduced particle adhesion compared to the PSU base membrane. Further, all of these properties varied systematically with variation of SPEEK content. Reproducibility and stability of the membrane properties was excellent. Pore sizes determined from dextran retention data and AFM measurements showed reasonable agreement. Membranes with 5 wt.% SPEEK demonstrated excellent overall properties. Such membranes had very high permeability, 22.6±1.6×10⁻¹¹ m³ s⁻¹ N⁻¹, 0.999 fractional rejection of 4000 Da dextran, 0.65 rejection of 0.001 M NaCl, and only 0.75 mN m⁻¹ adhesion of a 4 μm silica particle. Such membranes are very promising for scale-up of production and testing on real process streams.     

Natural gas permeation in polyimide membranes

Polyimide membranes derived from 6FDA-DAM:DABA and 6FDA-6FpDA:DABA copolymers have been used to separate 50/50 CO₂/CH₄ mixtures and multicomponent synthetic natural gas mixtures at 35 °C and feed pressures up to 55 atm. For 6FDA-DAM:DABA 2:1 membranes the effects of thermal annealing and covalent crosslinking are decoupled with respect to effects on permeabilities and selectivity. Crosslinking at 295 °C with 1,4-butylene glycol and 1,4-cyclohexanedimethanol increases CO₂ permeabilities by factors of 4.1 and 2.4, respectively, at 20 atm feed pressure, without a loss in selectivity, relative to crosslinking at 220 °C. Thermal annealing and crosslinking also reduce CO₂ plasticization effects. Crosslinking of DABA-containing copolymers, therefore, can produce membranes with tunable transport properties that offer significantly higher performance with better plasticization-resistance than that reported in the literature for the commercial polymers Matrimid^® and cellulose acetate for CO₂ removal from natural gas mixtures. Separation of complex mixtures containing CO₂, CH₄, C₂H₆, C₃H₈, and C₄H₁₀ or toluene results in a significant decrease of the CO₂ permeability, but only a moderate decrease in the CO₂/CH₄ selectivity.     

Thermal degradation and evolved gas analysis: A polymeric blend of urea formaldehyde (UF) and epoxy (DGEBA) resin

A polymeric blend has been prepared using urea formaldehyde (UF) and epoxy (DGEBA) resin in 1:1 mass ratio. The thermal degradation of UF/epoxy resin blend (UFE) was investigated by using thermogravimetric analyses (TGA), coupled with FTIR and MS. The results of TGA revealed that the pyrolysis process can be divided into three stages: drying process, fast thermal decomposition and cracking of the sample. There were no solid products except ash content for UFE during combustion at high temperature. The total mass loss during pyrolysis at 775 °C is found to be 97.32%, while 54.14% of the original mass was lost in the second stage between 225 °C and 400 °C. It is observed that the activation energy of the second stage degradation during combustion (6.23 × 10⁻⁴ J mol⁻¹) is more than that of pyrolysis (5.89 × 10⁻⁴ J mol⁻¹). The emissions of CO₂, CO, H₂O, HCN, HNCO, and NH₃ are identified during thermal degradation of UFE.     

Study of CuZnMOx oxides (M = Al,Zr, Ce,CeZr) for the catalytic hydrogenation of CO2 into methanol

CuO–ZnO–Al₂O₃ catalysts were synthesized by two methods, sol–gel and co-precipitation syntheses. Al₂O₃ was then substituted with other supports, such as ZrO₂, CeO₂ and CeO₂–ZrO₂ in order to have a better understanding of the support's effect. These catalysts containing 30 wt% of Cu were then tested for CO₂ hydrogenation into methanol. The effect of reaction temperature and GHSV on the catalytic behaviour was also investigated. The best results were obtained with a 30 CuO–ZnO–ZrO₂ catalyst synthesized by co-precipitation and calcined at 400 °C. This catalyst presents a good CO₂ conversion rate (23%) with 33% of methanol selectivity, leading to a methanol productivity of 331 g_MeOH.kg_cata⁻¹·h⁻¹ at 280 °C under 50 bar and a GHSV of 10,000 h⁻¹.     

Highly selective thiocyanate electrode based on bis-bebzoin-semitriethylenetetraamine binuclear copper(II) complex as neutral carrier

A novel selective thiocyanate PVC membrane electrode based on bis-bebzoin-semitriethylenetetraamine binuclear copper(II) [Cu(II)₂–BBSTA] as neutral carrier is reported, which displays an anti-Hofmeister selectivity sequence in following order: SCN⁻ > ClO₄⁻ > I⁻ >Sal⁻ >SO₃²⁻ >NO₃⁻ > H₂PO₄⁻ > Cl⁻ >NO₂⁻ > SO₄²⁻. The electrode exhibits Nernstian potential linear range to thiocyanate from 1.0 × 10⁻¹ to 9.0 × 10⁻⁷ mol/l with a detection limit 7.0 × 10⁻⁷ mol/l and a slope of −57.0 mV/decade in pH 5.0 of phosphorate buffer solution at 25 °C. The response mechanism is discussed in view of the AC impedance technique and the UV spectroscopy technique. From comparison of potentiometric response characteristics between the binuclear metallic complex copper(II) [Cu(II)₂–BBSTA] and mononuclear copper(II) metallic complex [Cu(II)–BBSDA], an enhanced response towards thiocyanate from the electrode based on binuclear metallic complex copper (II) [Cu(II)₂–BBSTA] was observed. The electrode based on binuclear copper(II) compound was used to determine the thiocyanate content in waste water with satisfactory results.     

Hydrogen/oxygen polymer electrolyte membrane fuel cells (PEMFCs) based on alkaline-doped polybenzimidazole (PBI)

When complexed with alkaline such as potassium hydroxide, sodium hydroxide or lithium hydroxide, films (40 μm thick) of polybenzimidazole (PBI) show conductivity in the 5 × 10⁻⁵–10⁻¹ S/cm⁻¹ range, depending on the type of alkali, the time of immersion in the corresponding base bath and the temperature of immersion. It has been shown that PBI has a remarkable capacity to concentrate KOH, even in an alkaline bath of concentration 3 M. The highest conductivity of KOH-doped PBI (9×10⁻² S cm⁻¹) at 25°C obtained in this work is higher than the we had obtained previously as optimum values for H₂SO₄-doped PBI (5 × 10⁻² S cm⁻¹ at 25°C) and H₃PO₄-doped PBI ( 2 × 10⁻³ S cm⁻¹ at 25°C). PEMFCs based on an alkali-doped PBI membrane were demonstrated, and their characteristics exhibited the same performance as those of PEMFCs based on Nafion® 117. Their development is currently under active investigation.     

Pervaporation separation of aromatic/aliphatic hydrocarbons by crosslinked poly(methyl acrylate-co-acrylic acid) membranes

Copolymers of methyl acrylate and acrylic acid were synthesized to fabricate membranes ionically crosslinked using aluminum acetylacetonate for the separation of toluene/i-octane mixtures by pervaporation at high temperatures. The formation of the ionic crosslinking via bare aluminum cations was characterized by UV–VIS spectroscopy and solubility tests. Reproducibility and the reliability of the methodology for membrane formation and crosslinking were confirmed. The effects of acrylic acid content, crosslinking conditions, pervaporation temperature, and feed composition on the normalized flux and the selectivity for toluene/i-octane mixtures were determined. A typical crosslinked membrane showed a normalized flux of 26 kg μm m⁻² h⁻¹ and a selectivity of 13 for a 50/50 wt.% feed mixture at 100°C. The pervaporation properties including solubility selectivity and diffusivity selectivity are discussed in terms of swelling behavior. The performance of the current membranes were benchmarked against other membrane materials reported in the literature.