Polyurethane-SAPO-34 mixed matrix membrane for CO2/CH4 and CO2/N2 separation

SAPO-34 nanocrystals (inorganic filler) were incorporated in polyurethane membranes and the permeation properties of CO₂, CH_4, and N₂ gases were explored. In this regard, the synthesized PU-SAPO-34 mixed matrix membranes (MMMs) were characterized via SEM, AFM, TGA, XRD and FTIR analyses. Gas permeation properties of PU-SAPO-34 MMMs with SAPO-34 contents of 5 wt%, 10 wt% and 20 wt% were investigated. The permeation results revealed that the presence of 20 wt% SAPO-34 resulted in 4.45%, 18.24% and 40.2% reductions in permeability of CO_2, CH_4, and N₂, respectively, as compared to the permeability of neat polyurethane membrane. Also, the findings showed that at the pressure of 1.2 MPa, the incorporation of 20 wt% SAPO-34 into the polyurethane membranes enhanced the selectivity of CO₂/CH₄ and CO₂/N₂, 14.43 and 37.46%, respectively. In this research, PU containing 20 wt% SAPO-34 showed the best separation performance. For the first time, polynomial regression (PR) as a simple yet accurate tool yielded a mathematical equation for the prediction of permeabilities with high accuracy (R² > 99%).     

Carbon composite membrane derived from a two-dimensional zeolitic imidazolate framework and its gas separation properties

The carbonization of a newly reported two-dimensional zeolitic imidazolate framework (ZIF-L) with leaf-like morphology was investigated by TG, SEM, XRD and XPS. ZIF-L flakes were thermally stable at up to 200 °C, and completely transformed into an amorphous carbonaceous material after heat treatment in nitrogen at 550 °C. A carbon composite membrane was then prepared by deposition of ZIF-L flakes on a porous alumina support and then direct carbonization of ZIF-L film. During the carbonization, the ZIF-L membrane reorganized into a nanoporous carbon composite membrane composed of ZnO nanoparticles and leaf-like carbon flakes. The resulting nanoporous carbon composite membrane exhibited a narrow micropore size distribution, and it had higher BET surface area than the ZIF-L flakes. Gas separation permeation experiments showed that the carbon composite membrane had a high H₂ permeance of 3.5 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹, and moderate H₂/N₂ and H₂/CO₂ ideal selectivities of 6.2 and 4.9, respectively. This work presents a simple and effective method for preparing functional nanoporous carbon composite membranes from ZIFs (or MOFs) for many potential applications.     

Evaluation of a new On-Stream Supercritical Fluid Deposition process for sol–gel preparation of silica-based membranes on tubular supports

A novel “On-Stream Supercritical Fluid Deposition” (OS-SFD) process has been investigated in this work coupling the sol–gel chemistry and a filtration/compression operation in supercritical CO₂ (sc-CO₂), for the production of uniform membranes on/in porous ceramic tubular supports. The versatility of this process allows both the direct formation of thin coatings on porous tubular membrane supports but also their internal modification. An attractive on-line control of the deposition process was operated by recording the transmembrane pressure evolution during membrane formation. Silica membranes were directly deposited on macroporous supports (155 mm long α-Al₂O₃, with 200 nm pore sizes) from TEOS derived sols dissolved in sc-CO₂ and transported to the tubular support where the condensation/gelation and deposition occurred. The deposition mechanism has been correlated with the sol–gel transition in sc-CO₂ conditions and the impact of the deposition temperature, sol formulation and sc-CO₂ flow rate on the membrane characteristics (morphology, weight increase and single gas permeance) have been discussed. Supersaturation and precipitation of transported clusters followed by their condensation and gelation were found as key parameters controlling the silica-based membrane design and microstructure/compacity of the silica network.     

CO2-tolerant Ni-La5.5WO11.25-δ dual-phase membranes with enhanced H2 permeability

Enhancing the ambi-polar conductivity of the ceramic hydrogen permeable membrane by introducing an electron conductive metallic phase is quite effective, which is helpful for the hydrogen permeation flux improvement. To develop CO₂-tolerant hydrogen permeable membranes with better hydrogen permeability, Ni-La_5.5WO_11.25-δ (Ni-LWO) cermet membranes are fabricated. The alkaline earth metal-free ceramic LWO is used as the main proton-conductive phase and Ni is used as the main electron-conductive phase. The Ni-LWO membrane exhibits good chemical stability in CO₂-containing atmosphere since its hydrogen permeability maintains well in the measurement for about 180 h. Compared with the LWO ceramic membrane, the hydrogen permeability of the Ni-LWO membrane has been improved significantly. The Ni/LWO ratio has great impact on the performance of the cermet membrane. Meanwhile, among all the dual-phase Ni-LWO membranes with different Ni/LWO volume ratios, the membrane with 60 vol% Ni shows the highest hydrogen permeation flux of 0.18 ml min⁻¹ cm⁻² at 1000 °C when the feed gas contains 50% H₂.     

Carbon molecular sieve gas separation membranes based on an intrinsically microporous polyimide precursor

We report the physical characteristics and gas transport properties for a series of pyrolyzed membranes derived from an intrinsically microporous polyimide containing spiro-centers (PIM-6FDA-OH) by step-wise heat treatment to 440, 530, 600, 630 and 800 °C, respectively. At 440 °C, the PIM-6FDA-OH was converted to a polybenzoxazole and exhibited a 3-fold increase in CO₂ permeability (from 251 to 683 Barrer) with a 50% reduction in selectivity over CH₄ (from 28 to 14). At 530 °C, a distinct intermediate amorphous carbon structure with superior gas separation properties was formed. A 56% increase in CO₂-probed surface area accompanied a 16-fold increase in CO₂ permeability (4110 Barrer) over the pristine polymer. The graphitic carbon membrane, obtained by heat treatment at 600 °C, exhibited excellent gas separation properties, including a remarkable CO₂ permeability of 5040 Barrer with a high selectivity over CH₄ of 38. Above 600 °C, the strong emergence of ultramicroporosity (<7 Å) as evidenced by WAXD and CO₂ adsorption studies elicits a prominent molecular sieving effect, yielding gas separation performance well above the permeability-selectivity trade-off curves of polymeric membranes.     

A hydrogen-permselective amorphous silica membrane derived from polysilazane

A microporous amorphous silica membrane has been synthesized by thermal conversion in air of polysilazane on a silicon nitride (Si₃N₄) porous substrate. The porous substrate near the surface layer was penetrated by polysilazane, and converted into mesoporous amorphous silica/Si₃N₄ composite layer. Then, an active molecular sieving microporous amorphous silica thin layer was synthesized on the surface of the mesoporous composite layer. The polysilazane-derived amorphous silica membrane exhibited H₂ permeance of 1.3 × 10⁻⁸ mol/m² s Pa at 573 K, and the permeability ratio of H₂/N₂ was measured to be 141. The effects of heat treatment condition on the meso/microporous structure development of the polysilazane-derived amorphous silica within the Si₃N₄ porous substrate are discussed from a viewpoint of fabricating hydrogen-permselective amorphous silica membranes through polymeric precursor route.     

Highly enhanced ammonia decomposition in a bimodal catalytic membrane reactor for COx-free hydrogen production

Ammonia decomposition in a bimodal catalytic membrane reactor (BCMR) consisting of a Ru/γ-Al₂O₃/α-Al₂O₃ bimodal catalytic support and a hydrogen-selective silica membrane in a single unit was proposed for CO_x-free hydrogen production in the present study. The bimodal catalytic membrane showed a H₂ permeance of 6.2 × 10^-7 mol/(m² s Pa) at 500 °C, with H₂/NH₃ and H₂/N₂ permeance ratios of 200 and 720, respectively. Ammonia conversion was surprisingly enhanced form 45 to 95% at 450 °C in the BCMR after selective H₂ extraction. The BCMR showed excellent stability with respect to both gas permeation properties and catalytic activities.     

Heterogeneous membranes based on a composite of a hypercrosslinked microparticle adsorbent and polyimide binder

This paper deals with the preparation and characterization of heterogeneous membranes based on microparticle hypercrosslinked polymeric adsorbents with a polyimide binder. The polyimide based membrane extension is hindered by their low permeability. We enhanced the permeability of polyimide membranes by changed chemical structure and by adding of the new type fillers. Hypercrosslinked polystyrene microparticles of diameter 1–5 μm were prepared by SnCl₄-catalyzed Friedel–Crafts reaction of polystyrene with chloromethyl methyl ether in 1,2-dichlorethane solution. The precursor polyamic acid (PAA) was synthesized by the reaction of equimolar amounts of 4,4′-oxy(bis(phthalic anhydride)) (ODPA) and bis(4,4′-oxydianiline) (ODA) or 4,4′-[(1,4-phenylene)dipropane-2,2-diyl]dianiline (BIS P) in N-methylpyrrolidone (content 10 wt.%). A PAA solution in N-methylpyrrolidone with the adsorbent was spread onto a glass substrate and kept at 60–240 °C for 12 h. Heterogeneous membranes were characterized by thermal, mechanical and separation measurements. The permeability for membrane ODPA–BIS P filled with 10 wt.% of hypercrosslinked adsorbent was 3.5 × 10⁻¹¹ cm³(STP) cm cm⁻² s⁻¹ cmHg⁻¹ for nitrogen and 4 × 10⁻⁹ cm³(STP) cm cm⁻² s⁻¹ cmHg⁻¹ for hydrogen. The permeability of homogeneous polyimide membranes is up to one order of magnitude lower. The diffusion coefficient of heterogeneous membranes increased in the order CH₄ < N₂ < O₂ < CO₂ < H₂. The selectivity of hydrogen–nitrogen separation with the amount of adsorbent decreased from 164 to 69. The prepared membranes are intended for separation of gases and low organic molecules even at enhanced temperature. The present paper aims at giving information on the influence of hypercrosslinked adsorbents and polyimide binding materials on the gas separation properties of membranes.     

Thin-film composite crosslinked polythiosemicarbazide membranes for organic solvent nanofiltration (OSN)

In this work we report a new class of solvent stable thin-film composite (TFC) membrane fabricated on crosslinked polythiosemicarbazide (PTSC) as substrate that exhibits superior stability compared with other solvent stable polymeric membranes reported up to now. Integrally skinned asymmetric PTSC membranes were prepared by the phase inversion process and crosslinked with an aromatic bifunctional crosslinker to improve the solvent stability. TFC membranes were obtained via interfacial polymerization using trimesoyl chloride (TMC) and diaminopiperazine (DAP) monomers. The membranes were characterized by Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and contact angle measurement.The membranes exhibited high fluxes toward solvents like tetrahydrofuran (THF), dimethylformamide (DMF) and dimethylsulfoxide (DMSO) ranging around 20 L/m² h at 5 bar with a molecular weight cut off (MWCO) of around 1000 g/mol. The PTSC-based thin-film composite membranes are very stable toward polar aprotic solvents and they have potential applications in the petrochemical and pharmaceutical industry.     

Oxygen permeation through dense La0.1Sr0.9Co0.8Fe0.2O3−δ perovskite membranes: Catalytic effect of porous La0.1Sr0.9Co0.8Fe0.2O3−δ layers

The oxygen permeation performance of a number of La_0.1Sr_0.9Co_0.8Fe_0.2O_3−δ (LSCF1982)-based membranes, consisting of dense LSCF1982 layer with/without porous LSCF1982 layer, was analyzed on the basis of the thickness of the dense layer and catalytic effect of the porous layer. A 0.27 mm thick dense membrane gives oxygen permeation flux ($J_{O_2}$) of 2.33 sccm min⁻¹ cm⁻² at 900 °C, which is increased to 3.55 sccm min⁻¹ cm⁻² on applying a porous layer of LSCF1982 onto the dense membrane. The membrane gives a stable flux for 300 h. The flux was further improved by reducing the thickness of the dense LSCF1982 layer and at 950 °C a flux of 4.47 sccm min⁻¹ cm⁻² is obtained with 0.012 mm thick membrane.     

Development of a metallic/ceramic composite for the deposition of thin-film oxygen transport membrane

Asymmetric perovskite membranes have an attractive potential in the application of O₂/N₂ gas separation for future membrane-based power plants using oxyfuel technology. In this study – a metal-supported membrane structure with a thin-film perovskite layer and porous ceramic interlayers was developed. Porous NiCoCrAlY sintered at 1225 °C in H₂ was selected as the substrate based on a sufficient permeability and corrosion resistance in co-firing conditions. According to the oxidation behaviour of NiCoCrAlY, the temperature for co-firing of the substrate and the interlayers was defined as 1100 °C for 5 h in air. Two interlayers of La_0.58Sr_0.4Co_0.2Fe_0.8O_3?δ were applied by screen printing. The top layer was deposited by magnetron sputtering with a thickness of 3.8 μm. While gas-tightness was improved considerably, significant air-leakage was still detected. In summary, the successful development of a metal-perovskite-composite is shown, which acts as a basis for further development of a gas-tight metal-supported oxygen transport membrane structure.     

Cross-linked PAN-based thin-film composite membranes for non-aqueous nanofiltration

A new approach on the development of cross-linked PAN based thin film composite (TFC) membranes for non-aqueous application is presented in this work. Polypropylene backed neat PAN membranes fabricated by phase inversion process were cross-linked with hydrazine to get excellent solvent stability toward dimethylformamide (DMF). By interfacial polymerization a selective polyamide active layer was coated over the cross-linked PAN using N,N′-diamino piperazine (DAP) and trimesoyl chloride (TMC) as monomers. Permeation and molecular weight cut off (MWCO) experiments using various dyes were done to evaluate the performance of the membranes. Membranes developed by such method show excellent solvent stability toward DMF with a permeance of 1.7 L/m² h bar and a molecular weight cut-off of less than 600 Da.     

Surface hierarchical porosity in poly (ɛ-caprolactone) membranes with potential applications in tissue engineering prepared by foaming in supercritical carbon dioxide

This article describes the preparation of porous poly (ɛ-caprolactone), PCL, membranes by supercritical CO₂ (SCCO₂) foaming, displaying surface hierarchical macroporosity which could be tailored by careful control of the pressure, in the range of 150–250 bar, and depressurization processes in several steps, showing also pore interconnectivity between both membrane faces. The membranes exhibited two distinct types of surface macroporosity, the larger with diameter sizes of 300–500 μm were surrounded by and also composed of smaller pores of 15–50 μm (same size as inner pores). Membranes were prepared by solvent casting and submitted to different SCCO₂ foaming. Parameters such as membrane thickness, CO₂ flow, foaming time, pressure, temperature and the depressurization processes (rate and profiles), were varied to determine their influence on final porosity and to decipher which parameters were the most critical ones in terms of surface hierarchical pore organization. No remarkable changes in PCL crystallinity were found when membranes were processed under SCCO₂. Finally, biological evaluation of the porous membranes was achieved by seeding human skin fibroblasts on the prepared membranes. The results, in terms of cell adhesion, spreading, proliferation and metabolic activity indicate that these membranes could hold promise for the fabrication of meshes with controlled porosity for tissue engineering applications.     

Selective separation of low concentration CO2 using hydrogel immobilized CA enzyme based hollow fiber membrane reactors

This paper reported the results of developing a novel hollow fiber membrane reactor contained immobilized enzyme for selective separation of low concentration CO₂ from mixed gas streams. In the reactor, two bundles of poly(vinylidene fluoride) (PVDF) hollow fiber membranes were aligned staggered parallel in a tube module, and lab-made poly(acrylic acid-co-acrylamide)/hydrotalcite (PAA-AAm/HT) nanocomposite hydrogel was filled between fibers, in which carbonic anhydrase (CA enzyme) was immobilized. The effects of CA concentration, buffer concentration, the flow rate of sweep gas, operational temperature, and CO₂ concentration on separation performance were investigated in detail. The results showed that the transport resistance was mainly from the hydrogel layer, and decreased greatly with immobilization of carbonic anhydrase in hydrogel. Moreover, immobilized CA could retain over 76% enzymatic activity and thermal stability was also improved. The data showed that this enzyme-based membrane reactor could effectively separate CO₂ at low concentration from mixed gas streams. For the feed with 0.1% (v/v) of CO₂, the selectivity of CO₂/N₂ was 820, CO₂/O₂ was 330, and CO₂ permeance was 1.65×10^?8 mol/m² s Pa. Prolonged runs lasting 30 h showed that separation performances of the membrane reactor were quite stable.     

Effect of BN content on microstructures,mechanical and dielectric properties of porous BN/Si3N4 composite ceramics prepared by gel casting

Porous BN/Si₃N₄ composite ceramics with different BN contents have been fabricated by gel casting. The rheological behaviors of the suspensions, microstructure, mechanical properties, dielectric properties and critical temperature difference of thermal shock (ΔT_C) of porous BN/Si₃N₄ composite ceramics with different BN contents were investigated. With BN contents increasing, the mechanical properties of the porous BN/Si₃N₄ composite ceramics were partially declined, but the dielectric properties and thermal shock resistances were enhanced at the same time. For the porous Si₃N₄ ceramic without BN addition, the porosity, flexural strength, dielectric constant and critical temperature difference were 48.1%, 128 MPa, 4.1 and 395 °C, while for the 10 vol% BN/Si₃N₄ porous composite ceramics, they were 49.4%, 106.6 MPa, 3.8, and 445 °C, respectively. The overall performance of the obtained porous BN/Si₃N₄ composite ceramics indicated that it could be one of the ideal candidates for high-temperature wave-transparent applications.     

An architectural approach to the oxygen permeability of a La0.6Sr0.4Fe0.9Ga0.1O3−δ perovskite membrane

Multilayer membranes based on La_0.6Sr_0.4Fe_0.9Ga_0.1O_3−δ (LSFG) and La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (LSCF) perovskite materials were fabricated to study the impact of membrane architecture on the oxygen permeability. Thick dense membrane and asymmetric membranes were shaped by tape casting and stacked to reach the desired architecture. Asymmetric membranes composed of a thin dense LSFG layer (120 μm) and a thick porous support layer (820 μm) of the same material were co-sintered to obtain crack-free and flat membranes. The use of large corn-starch particles (14 μm) as pore forming agent to the tape-casting slurries resulted in a connected porosity in the sintered support layer with low gas diffusion resistance. Oxygen permeation measurements in an air/argon gradient between 800 and 925 °C showed that the thickness of self-supported LSFG membranes was not the determining factor in the membrane performance for our testing conditions. A catalytic layer of La_0.6Sr_0.4Co_0.8Fe_0.2O_3−δ (LSCF), deposited on the membrane surfaces to catalyze the oxygen exchange reactions, leads to a significant increase of oxygen permeation rates. As the membrane thickness had no effect even if a catalyst coating was used, surface-exchange reactions were thought to be still limiting for the oxygen permeation fluxes. Thus, the improvement of surface activity of LSFG membrane was found to be a key point to reach higher oxygen permeation fluxes.     

Low-temperature preparation of Si3N4 whiskers bonded/reinforced SiC porous ceramics via foam-gelcasting combined with catalytic nitridation

Porous α-Si₃N₄ whiskers bonded/reinforced SiC (Si₃N_4(w)/SiC) ceramics were successfully prepared at as low as 1473 K for 2 h, via a combined foam-gelcasting and catalytic nitridation route using commercial Si and SiC powders containing some Fe impurity as the main raw materials. Small pores (0.03–5 μm) left by the packing of raw material particles and interlocking of in-situ formed Si₃N₄ whiskers coexisted with large ones (8–400 μm) resultant mainly from the foaming process. The impurity Fe from the raw materials Si and SiC acted as an internal catalyst, accelerating the nitridation of Si by increasing the bond length and weakening the bond strength in the N₂ molecules adsorbed on it. As-prepared Si₃N_4(w)/SiC porous ceramics contained 71.53% porosity and had flexural and compressive strengths of 5.60 ± 0.69 MPa and 12.37 ± 1.05 MPa, respectively.     

Fabrication and testing of a planar microstructured concept module with integrated palladium membranes

A planar microstructured hydrogen separation module has been fabricated to study the hydrogen permeation through free-standing palladium-based membranes (Pd, PdCu and PdAg) with minimal influence by concentration polarization.The membranes were laser-welded directly between two face-to-face arranged stainless-steel sheets with 10 microchannels each (width × depth × length of the channels: 500 μm × 300 μm × 2 cm).Single gas hydrogen and mixed gas permeation experiments (H₂/N₂) were conducted between 300 and 400 °C. The permeabilities and activation energies of the membranes in this temperature range were calculated.The 12.5 μm thick membrane was successfully tested up to 650 kPa, indicating that the microchannel plates provide a good mechanical support even for very thin membranes. However, settling of the film into the microchannels on the permeate side was observed due to the overpressure on the retentate side suggesting even finer channels and eventually the use of an additional porous support for very high differential pressure.The concentration polarization effects in the membrane module were evaluated in terms of the film effectiveness factor η which is a familiar concept from heterogeneous catalysis. It could be shown that the microchannel configuration effectively decreases concentration polarization.     

Non-metal catalytic synthesis of graphene from a polythiophene monolayer on silicon dioxide

Direct synthesis of graphene without metal catalysts on a dielectric substrate is a major goal in graphene-based electronics and is an increasingly popular nanotechnology alternative to metal oxide semiconductor technology. However, current methods for the synthesis of these graphenes have many limitations, including the use of metal catalyst. Herein, we report a facile approach to the direct synthesis of graphene sheets based on the self-assembled monolayers (SAMs) technique. The new method for metal catalyst-free direct synthesis of a graphene sheet is through a solution-processable, inexpensive, easy, and reproducible cross-linked polythiophene self-assembled monolayer (SAM) that is formed via the [4 + 2] π cycloaddition reaction of π-electron conjugated thiophene layer self-assembled on the dielectric silicon dioxide substrate. The bifunctional molecules were carefully designed to create an SAM via silanization of alkoxy silane groups on the SiO₂ substrate, and at the other end, a thin cross-linked polythiophene layer via a [4 + 2] π-electron cycloaddition reaction of π-electron conjugated thiophene SAM. By heating the cross-linked polythiophene SAM up to 1000 °C under a high vacuum, single-layered or few-layered graphene sheets were successfully prepared on the dielectric silicon oxide substrate.     

Modification of PES/PU membrane by supercritical CO2 to enhance CO2/CH4 selectivity: Fabrication and correlation approach using RSM

Integrally skinned asymmetric gas separation membranes of polyethersulfone (PES)/polyurethane (PU) blend were prepared using supercritical CO₂ (SC-CO₂) as a nonsolvent for the polymer solution. The membrane consisted of a dense and a porous layer, which were conjoined to separate CO₂ from CH₄. The FTIR, DSC, tensile and SEM tests were performed to study and characterize the membranes. The results revealed that an increase in SC-CO₂ temperature causes an increment in permeance and a decrease in membrane selectivity. Furthermore, by raising the pressure, both permeance and selectivity increased. The modified membrane with SC-CO₂ had much higher selectivity, about 5.5 times superior to the non-modified membrane. This higher selectivity performance compared to previous works was obtained by taking the advantages of both using partial miscible blend polymer due to the strong polar–polar interaction between PU PES and SC-CO₂ to fabricate the membrane. The response surface methodology (RSM) was applied to find the relationships between several explanatory variables and CO₂ and CH₄ permeance and CO₂/CH₄ selectivity as responses. Finally, the results were validated with the experimental data, which the model results were in good agreement with the available experimental data.