The rheology of Torlon solutions and its role in the formation of ultra-thin defect-free Torlon hollow fiber membranes for gas separation

Fundamental understanding of the material science and rheological engineering to fabricate Torlon^® 4000T-MV and 4000TF hollow fiber membranes with an ultra-thin and defect-free dense-selective layer for gas separation has been revealed. We have firstly investigated the rheology of Torlon^® 4000T-MV and 4000TF dope solutions, and then determined the effect of temperature-correlated shear and elongational viscosities on the formation of Torlon^® fibers for gas separation. Interestingly, Torlon^® 4000T-MV and 4000TF possess different rheological characteristics: the elongational viscosity of Torlon^® 4000T-MV/NMP solution shows strain thinning, while Torlon^® 4000TF/NMP solution shows strain hardening. The balanced viscoelastic properties of dope solutions, which are strongly dependent on the spinning temperature, have been found to be crucial for the formation of a defect-free dense layer. The optimum rheological properties to fabricate Torlon^® 4000T-MV/NMP hollow fibers appear at about 48–50 °C, and the resultant fibers have an O₂/N₂ selectivity of 8.37 and an apparent dense layer thickness of 781 Å. By comparison, the best Torlon^® 4000TF fibers were spun at 24 °C with an O₂/N₂ selectivity of 8.96 and a dense layer of 1116 Å. The CO₂/CH₄ selectivity of the above two Torlon^® variants is 47 and 53.5, respectively.     

Gas–liquid membrane contactors for CO2 removal

In the present work we use a membrane contactor for the separation of CO₂ from CH₄ and we systematically investigate the influence of both the type of membrane and the different process parameters on the overall process performance (permeability and selectivity). This work is important because it reports real process performance data (permeances and selectivities) for the total process consisting of absorption and desorption under practical conditions using feed mixtures. Commercially available porous PP hollow fiber membranes and asymmetric PPO hollow fiber membranes have been applied and MEA was used as absorption liquid in the membrane contactor. The proposed approach allows us to identify the operating window and potential of the process. Although the performance of the PP membranes outperforms the performance of the PPO membranes in terms of productivity and selectivity, the PP fibers are extremely sensitive to only small variations in the feed pressure, resulting in severe performance loss. In addition to that, extremely high liquid losses are observed for the PP fibers especially at elevated temperatures. Factors that are significantly reduced when asymmetric PPO membranes with a dense, ultrathin top layer are used, which thus improves the performance and significantly increases the operating window and potential of the membrane contactor process.     

The role of additives on dope rheology and membrane formation of defect-free Torlon hollow fibers for gas separation

The high demands on high performance membranes for energy, water and life science usages provide the impetus for membrane scientists to search for a comprehensive understanding of membrane formation from molecular level to design membranes with desirable configuration and separation performance. This pioneering work is to elaborate the importance of polymer rheology on hollow fiber formation and reveal the integrated science bridging polymer fundamentals such as polymer cluster size, shear and elongational viscosities, molecular orientation, stress relaxation to membrane microstructure and separation performance for gas separation. Torlon^® poly(amide imide) was employed in this study with various solvent/nonsolvent additives. The effects of additives on polymeric cluster size, hydrogen bonding and dope rheology during the phase inversion have been examined. It is found that hydrogen bonding and strain-hardening characteristics play very important roles in dope rheology and membrane separation performance. Torlon^® possesses strong hydrogen bonds with NMP/water mixtures, the addition of a small amount of water enlarges polymer cluster size, strengthen molecular network (i.e., strain hardening) and facilitate macrovoid-free morphology. However, strong hydrogen bonding may retard chain unfolding during spinning, induce faster relaxation for highly oriented dense-selective skin, and thus reduce gas-pair selectivity. By adjusting dope chemistry and spinning conditions with balanced solubility parameters and dope rheology, we have developed defect-free Torlon^® hollow fiber membranes with an O₂/N₂ selectivity of 8.55 and an ultra-thin layer of 488 Å simply using water as the additive. Fibers spun from dopes containing other additives have the optimal O₂/N₂ selectivity varying from 7.69 to 9.97 at 25 ± 2 °C, and the dense layer thickness varying from 500 Å to 2000 Å. Their corresponding mixed-gas O₂/N₂ selectivity for compressed air varies from 7.12 to 9.00.     

Formation of integrally skinned asymmetric polysulfone gas separation membranes by supercritical CO2

Integrally skinned asymmetric polysulfone membranes were prepared from originally dense films inducing asymmetry by the formation of the porous layer adding to one side of the membranes chloroform and supercritical CO₂ (SCCO₂), and then allowing the SCCO₂ expansion to occur. The influence of the chloroform/polysulfone mass ratio (g CH₃Cl/g PSF), SCCO₂ density and depressurization rate over the thickness of both the porous and the dense skin layers, the morphology of the porous support and the pure O₂ and N₂ permeability and selectivity performance were studied.The results show that it is possible to induce a very-controlled asymmetry in a dense film following the procedure described in this work and as expected, the thickness of the porous layer increases while the dense skin layer decreases as the chloroform/polysulfone mass ratio increases. Images of the porous layer show that the average-pore size decreases at high SCCO₂ densities and slightly decreases with increasing the CO₂ depressurization rates. The O₂ and N₂ permeability coefficients, measured at 35 °C and 2 bar, for the polysulfone asymmetric membranes are practically the same of those determined in dense films, suggesting that the dense skins are essentially defect-free of pinholes.     

Supercritical CO2 improved phosphine imide reaction on peracetylated β-cyclodextrin

Supercritical CO₂ (sCO₂) was successfully used as solvent and reagent in the ‘one-pot’ phosphine imide reaction. In sCO₂ smooth conditions and with or without assistance of triphenylphosphine-bounded polymer, the reaction efficiently leads to the desired urea β-Cd compounds in a short time, bringing an interesting alternative for solving problems of OVC and hazardous reagent substitution in the synthesis of ureido-Cds and, by extension, of urea derivatives.     

A parametric study of CO2/N2 gas separation membrane processes for post-combustion capture

Capture of CO₂ from flue gases produced by the combustion of fossil fuels and biomass in air is referred to as post-combustion capture. Chemisorbent processes are considered to be the most feasible method and are already at an advanced stage of development, but gas separation membranes are attracting more and more attention as a possible alternative. This paper describes a detailed parametric study of mass and energy balances for a simulated single membrane process. Typical operating conditions (CO₂ concentration in the flue gas, pressure and temperature, etc.) together with the influence of the membrane quality (permeability, selectivity) and membrane area on membrane performance (CO₂ separation degree and CO₂ purity) are simulated over a wide range of parameters.     

Composite hollow fiber membranes for CO2 capture

Composite hollow fibers membranes were prepared by coating poly(phenylene oxide) (PPO) and polysulfone (PSf) hollow fibers with high molecular polyvinylamine (PVAm). Two procedures of coating hollow fibers outside and respective inside were investigated with respect to intrinsic PVAm solution properties and hollow fibers geometry and material.The influence of operating mode (sweep or vacuum) on the performances of membranes was investigated. Vacuum operating mode gave better results than using sweep because part of the sweep gas permeated into feed and induced an extra resistance to the most permeable gas the CO₂. The composite PVAm/PSf HF membranes having a 0.7–1.5 μm PVAm selective layer, showed CO₂/N₂ selectivity between 100 and 230. The selectivity was attributed to the CO₂ facilitated transport imposed by PVAm selective layer. The CO₂ permeance changed from 0.006 to 0.022 m³(STP)/(m² bar h) in direct correlation with CO₂ permeance and separation mechanism of the individual porous supports used for membrane fabrication. The multilayer PVAm/PPO membrane using as support PPO hollow fibers with a 40 nm PPO dense skin layer, surprisingly presented an increase in selectivity with the increase in CO₂ partial pressure. This trend was opposite to the facilitated transport characteristic behaviour of PVAm/porous PSf. This indicated that PVAm/PPO membrane represents a new membrane, with new properties and a hybrid mechanism, extremely stable at high pressure ratios. The CO₂/N₂ selectivity ranged between 20 and 500 and the CO₂ permeance from 0.11 to 2.3 m³(STP)/(m² bar h) depending on the operating conditions.For both PVAm/PSf and PVAm/PPO membranes, the CO₂ permeance was similar with the CO₂ permeance of uncoated hollow fiber supports, confirming that the CO₂ diffusion rate limiting step resides in the properties of the relatively thick support, not at the level of 1.2 μm thin and water swollen PVAm selective layer. A dynamic transfer of the CO₂ diffusion rate limiting step between PVAm top layer and PPO support was observed by changing the feed relative humidity (RH%). The CO₂ diffusion rate was controlled by the PPO support when using humid feed. At low feed humidity the 1.2 μm PVAm top layer becomes the CO₂ diffusion rate limiting step.     

Preparation of “pore-fill” type Pd–YSZ–γ-Al2O3 composite membrane supported on α-Al2O3 tube for hydrogen separation

Mesoporous YSZ–γ-Al₂O₃ membranes were coated on α-Al₂O₃ (Ø2 mm) tube by dipping the α-Al₂O₃ support tube into mixed sol consists of nano-size YSZ and bohemite particles followed by drying and calcination at 600 °C. Addition of bohemite in YSZ sol helped a good adhesion and uniform coating of the membrane film onto α-Al₂O₃ support. The quality of the mesoporous YSZ–γ-Al₂O₃ membranes was evaluated by the gas permeability experiments. The number of defects was minimized when the γ-Al₂O₃ content became more than 40%. Addition of γ-Al₂O₃ inhibited the crystal growth of YSZ, sintering shrinkage and distortion stress. Increase of calcination temperature and time results in the increase of pore size and N₂ permeance. A hydrogen perm-selective membrane was prepared by filling palladium into the nano-pores of YSZ–γ-Al₂O₃ layer by vacuum-assisted electroless plating. Crystal growth of palladium was observed by thermal annealing of the membrane at 600 °C for 40 h. The Pd–YSZ–γ-Al₂O₃ composite membrane revealed improved thermal stability allowing long-term operation at elevated temperature (>500 °C). This has been attributed to the improved fracture toughness of YSZ–γ-Al₂O₃ layer and matching of thermal expansion coefficient between palladium and YSZ. Although fracture of the membrane did not occur, decline of H₂ flux was observed when the membrane was exposed in 600 °C. This has been attributed to the agglomeration of palladium particles by crystal growth and dense packing into the pore networks of YSZ–γ-Al₂O₃ by elevation of temperature.     

PEG modified poly(amide-b-ethylene oxide) membranes for CO2 separation

In the present work, membranes from commercially available Pebax^® MH 1657 and its blends with low molecular weight poly(ethylene glycol) PEG were prepared by using a simple binary solvent (ethanol/water). Dense film membranes show excellent compatibility with PEG system up to 50 wt.% of content. Gas transport properties have been determined for four gases (H₂, N₂, CH₄, CO₂) and the obtained permeabilities were correlated with polymer properties and morphology of the membranes. The permeability of CO₂ in Pebax^®/PEG membrane (50 wt.% of PEG) was increased two fold regarding to the pristine Pebax^®. Although CO₂/N₂ and CO₂/CH₄ selectivity remained constant, an enhancement of CO₂/H₂ selectivity (∼11) was observed. These results were attributed to the presence of EO units which increases CO₂ permeability, and to a probable increase of fractional free-volume. Furthermore, for free-volume discussion and permeability of gases, additive and Maxwell models were used.     

PAMAM dendrimer composite membrane for CO2 separation: Formation of a chitosan gutter layer

A poly(amidoamine) (PAMAM) dendrimer composite membrane with an excellent CO₂/N₂ separation factor was developed in-situ. The In-situ Modification (IM) method was used to modify the surface of commercial porous membranes, such as ultrafiltration membranes, to produce a gas selective layer by controlling the interface precipitation of the membrane materials in the state of a received membrane module. Using the IM method, a chitosan layer was prepared on the inner surface of a commercially available ultrafiltration membrane as a gutter layer, in order to affix PAMAM dendrimer molecules on the porous substrate. After chitosan treatment, the PAMAM dendrimer was impregnated into the gutter layer to form a PAMAM/chitosan hybrid layer. The CO₂ separation performance of the resulting composite membrane was tested at a pressure difference of 100 kPa and a temperature of 40 °C, using a mixed CO₂ (5 vol%)/N₂ (95 vol%) feed gas. The PAMAM dendrimer composite membrane, with a gutter layer prepared from ethylene glycol diglycidyl ether and a 0.5 wt% chitosan solution of two different molecular weight chitosans, revealed an excellent CO₂/N₂ separation factor and a CO₂ permeance of 400 and 1.6 × 10⁻⁷ m³ (STP) m⁻² s⁻¹ kPa⁻¹, respectively. SEM observations revealed a defect-free chitosan layer (thickness 200 nm) positioned directly beneath the top surface of the UF membrane substrate. After PAMAM dendrimer treatment, the hybrid chitosan/PAMAM dendrimer layer was observed with a thickness of 300 nm. XPS analysis indicated that the hybrid layer contained about 20–40% PAMAM dendrimer.     

Osmium catalyzed asymmetric dihydroxylation of methyl trans-cinnamate in ionic liquids, followed by supercritical CO2 product recovery

In this work, osmium-catalyzed asymmetric dihydroxylation (AD) of methyl trans-cinnamate was studied. Osmium and chiral ligand catalysts were immobilized in ionic liquid only, without any other reaction solvents, while the recovery of the product was performed by extraction with supercritical CO₂, and compared with results obtained by extractions with organic solvents such as hexane and diethyl ether. In supercritical CO₂ extraction experiments, optimal extraction pressure was found and ionic liquid chosen, so that the highest reaction yields coupled with lowest osmium content in the crude product can be achieved. Finally, recycle experiments of the same (ionic liquid + catalytic system) mixture were successfully conducted. Application of ionic liquids and supercritical CO₂ in osmium catalyzed AD allows for the isolation of the diol basically without contamination with osmium, in high yield and enantiomeric excess, and it makes possible the efficient reuse of ionic liquid solvent and the catalytic system.     

Thermal transformation of quaternary compounds in NaF–CaF2–AlF3 system

Details of quaternary compounds formation in the system NaF–CaF₂–AlF₃ are specified. To achieve this aim, the samples of phases NaCaAlF₆ and Na₂Ca₃Al₂F₁₄ have been obtained by high-temperature solid-phase synthesis. Their thermal behavior when heated up to 800 °C has been studied using the methods of high-temperature X-ray diffraction (XRD) and thermal analysis (TA). The system under consideration can be regarded as a quasibinary section CaF₂–NaAlF₄, where at T=745–750 °C invariant equilibrium is implemented with the phases CaF₂–NaCaAlF₆–Na₂Ca₃Al₂F₁₄–(liquid melt)–(NaAlF₄). The peculiarity of the equilibrium is NaAlF₄ metastability at normal pressure. Below the equilibrium temperature the quaternary phase Na₂Ca₃Al₂F₁₄ is stable and NaCaAlF₆ above this temperature. The phase NaCaAlF₆ fixed by rapid quenching from high temperatures and when heated up to 640 °C decomposes, yielding Na₂Ca₃Al₂F₁₄. Further heating in vacuum at temperature up to 740 °C results in decomposition of Na₂Ca₃Al₂F₁₄ into CaF₂ and Na₃AlF₆. The expected reverse transformation of Na₂Ca₃Al₂F₁₄ into NaCaAlF₆ has not been observed under experimental conditions. Transformations in bulk samples reveal direct and reverse transformation of quaternary phases.

Synopsis

Thermal transformation of the quaternary compounds in system (NaF–CaF₂–AlF₃) was investigated using high-temperature X-ray diffraction (XRD) and thermal analysis (TA). In the system the invariant equilibrium is implemented with the phases CaF₂–NaCaAlF₆–Na₂Ca₃Al₂F₁₄–(liquid melt)–(NaAlF₄) at T=745–750 °C.     

R&D activities in Japan on methanol synthesis from CO2 and H2

Methanol synthesis from CO₂ and H₂ can be a useful process for conversion and transportation of hydrogen energy derived from non-fossil energies. More than ten research groups in Japan have extensively investigated the methanol synthesis from both academic and practical points of view. Recent R&D activities in Japan for developing high performance catalysts, for elucidating the reaction mechanism and also for operating a bench scale plant have been reviewed in this paper.     

Syntheses, crystal structures and Raman spectra of Ba(BF4)(PF6), Ba(BF4)(AsF6) and Ba2(BF4)2(AsF6)(H3F4); the first examples of metal salts containing simultaneously tetrahedral BF4 and octahedral AF6 anions

In the system BaF₂/BF₃/PF₅/anhydrous hydrogen fluoride (aHF) a compound Ba(BF₄)(PF₆) was isolated and characterized by Raman spectroscopy and X-ray diffraction on the single crystal. Ba(BF₄)(PF₆) crystallizes in a hexagonal space group with a=10.2251(4) Å, c=6.1535(4) Å, V=557.17(5) Å³ at 200 K, and Z=3. Both crystallographically independent Ba atoms possess coordination polyhedra in the shape of tri-capped trigonal prisms, which include F atoms from BF₄⁻ and PF₆⁻ anions. In the analogous system with AsF₅ instead of PF₅ the compound Ba(BF₄)(AsF₆) was isolated and characterized. It crystallizes in an orthorhombic Pnma space group with a=10.415(2) Å, b=6.325(3) Å, c=11.8297(17) Å, V=779.3(4) Å³ at 200 K, and Z=4. The coordination around Ba atom is in the shape of slightly distorted tri-capped trigonal prism which includes five F atoms from AsF₆⁻ and four F atoms from BF₄⁻ anions. When the system BaF₂/BF₃/AsF₅/aHF is made basic with an extra addition of BaF₂, the compound Ba₂(BF₄)₂(AsF₆)(H₃F₄) was obtained. It crystallizes in a hexagonal P6₃/mmc space group with a=6.8709(9) Å, c=17.327(8) Å, V=708.4(4) Å³ at 200 K, and Z=2. The barium environment in the shape of tetra-capped distorted trigonal prism involves 10 F atoms from four BF₄⁻, three AsF₆⁻ and three H₃F₄⁻ anions. All F atoms, except the central atom in H₃F₄ moiety, act as μ₂-bridges yielding a complex 3-D structural network.     

Preparation of β-cyclodextrin-polyaniline complex in supercritical CO2

A method is proposed to prepare β-cyclodextrin (β-CD)/polyaniline (PANI) inclusion complex. In this route, benzoyl peroxide (BPO, the oxidant) is first encapsulated into the cavity of β-CD. Aniline is then carried into the cavity of β-CD by supercritical (SC) CO₂, which polymerizes in situ to form inclusion complex. The product is characterized by FT-IR, UV-Vis, ¹H NMR and XRD techniques. The results suggest that the columnar inclusion complexes may be formed.     

The solubility of CO2 and N2O in olive oil

The solubility of CO₂ and N₂O in olive oil has been measured at temperatures of about 298, 310, and 323 K with a gravimetric microbalance under pressures up to 2 MPa. The molecular weight of olive oil has been analyzed and found to be about 882 g mol⁻¹ as a mixed oil compound. The observed solubility data have been correlated with a cubic equation of state (EOS) model. N₂O has a larger solubility than CO₂ in olive oil based on either the mole or mass fraction. The present results clarify some ambiguities from the previous N₂O solubility data in the literature.     

Thermodynamic equilibrium study of mineral elements evaporation in O2/CO2 recycle combustion

The facility for the analysis of chemical thermodynamics method (F*A*C*T) based on the Gibbs energy minimization principle, was used to characterize the evaporation of mineral elements of coal in O₂/CO₂ recycle combustion. The effects of atmosphere and temperature on the speciation of mineral species were discussed. The results show that Na(K)Cl(g), FeO(g), and SiO(g) are the dominant gaseous species of the mineral elements. The dominant species of mineral elements in flue gases depend on both the combustion conditions (reducing or oxidizing) and the atmosphere. In O₂/CO₂ mixture combustion, the evaporation rate of mineral elements is much lower than that in air combustion, especially under reducing atmosphere. The total evaporation of mineral elements in O₂/CO₂ atmosphere and air combustion under reducing conditions is 4.46% and 9.65% respectively, up to the temperature of 2400 K. The calculation values are consistent with the experiment values. The decrease in the mineral element evaporation is helpful to suppress the tendency to form fine particle matter and the tendency of initial ash deposition.     

Fabrication of high performance Matrimid/polysulfone dual-layer hollow fiber membranes for O2/N2 separation

Matrimid/polysulfone (PSf) dual-layer hollow fiber membranes were fabricated by using co-extrusion and dry-jet wet-spinning phase-inversion techniques. The effects of the spinning dope composition, spinneret dimension, spinneret temperature and the air gap distance on the hollow fiber membranes separation performance were studied. Aging phenomenon was also studied. After coated by 3 wt% silicon solution, the hollow fiber membranes have an O₂/N₂ selectivity of 7.55 at 25 °C, 506.625 kPa which exceeds the intrinsic value of Matrimid. The membranes have an O₂ permeance of 9.36 GPU with an apparent dense-layer thickness of 1421 Å calculated from the O₂ permeability. SEM images show the high porosity underneath the dense skin. It indicates that non-solvent addition is not necessary in the inner spinning dope to induce the macroviod formation. The binodals of the Matrimid/solvent/H₂O and PSf/solvent/H₂O indicate that the composition of the spinning dope plays an important role in the structure and the gas separation performance of the dual-layer hollow fiber membranes. The delayed demixing of the inner spinning dope may fabricate low resistance support layers in the dual-layer hollow fiber membranes.     

Novel carbonyl iron-bismuth clusters - synthesis, structure, CO2 insertion and potential as molecular precursors for BiFeO3

The reaction of Fe₂(CO)₉ with Bi(OSiMe₂^tBu)₃ gave soluble [(CO)₄FeBi(OSiMe₂^tBu)]₂ (1) in moderate yield whereas in case of Bi(O^tBu)₃ used as starting material both [(CO)₄FeBi(O^tBu)]_n (2) and the bismuth-iron cluster [(CO)₃FeBi₃(O^tBu)₄{OCO(O^tBu)}]₂ (3) were isolated. The latter forms upon insertion of CO₂, released during reaction of diiron nonacarbonyl with bismuth tert-butoxide, into a Bi-O^tBu bond. The compounds were characterized by IR and ¹H NMR spectroscopy as well as thermogravimetric analyses. Additionally, the molecular structures of compounds 1 and 3 were elucidated by single crystal X-ray diffraction. The core structure of [(CO)₄FeBi(OSiMe₂^tBu)]₂ (1) is build up by a four-membered Bi₂Fe₂ ring whereas [(CO)₃FeBi₃(O^tBu)₄{OCO(O^tBu)}]₂ (3) is composed of two tetrahedral FeBi₃ cluster cores that dimerise via bridging -OCO(O^tBu) ligands. Analysis of the TGA residues by PXRD revealed that compound 2 is the best precursor for multiferroic BiFeO₃ among the compounds studied here, although Bi₂₅FeO₃₉ was detected as minor impurity.     

Solubility of CO2 in imidazolium-based tetrafluoroborate ionic liquids

The solubility of CO₂ in imidazolium ionic liquids (ILs), 1-butyl-3-methyl imidazolium tetrafluoroborate ([bmim][BF₄]), 1-hexyl-3-methyl imidazolium tetrafluoroborate ([hmim][BF₄]) and 1-octyl-3-methyl imidazolium tetrtafluoroborate ([omim][BF₄]) was determined at 305-25 K and pressures from 1 to 9 MPa. The influence of chain length of alkyl substituents on the imidazolium cation on the solubility of CO₂ was investigated. The differences in solubility with chain length are in the sequence [omim][BF₄] > [hmim][BF₄] > [bmim][BF₄]. The solubility data were correlated by the extended Henry's law, and enthalpy, Gibbs free energy and entropy changes were obtained.