Graphite and PMMA as pore formers for thermoplastic extrusion of porous 3Y-TZP oxygen transport membrane supports

A gas permeable porous support is a crucial part of an asymmetric oxygen transport membrane (OTM). Here, we develop feedstocks for thermoplastic extrusion of tubular, porous 3Y-TZP (partially stabilized zirconia polycrystals, (Y₂O₃)_0.03(ZrO₂)_0.97)) ceramics, using graphite and/or polymethyl methacrylate (PMMA) as pore formers. The influence of pore former content and type, 3Y-TZP particle size and support sintering temperature on the microstructure, porosity and gas permeability were studied. Using at least 40 vol% pore former, consisting of graphite and PMMA in the volume ratio 2:1, tubes with gas permeability exceeding the target of 10⁻¹⁴ m² are obtained. In the temperature range 1250–1400 °C the support gas permeability is insensitive to the sintering temperature, and the feedstocks shrink more than 15% during sintering, making them ideal for co-sintering with functional OTM layers. This demonstrates the suitability of thermoplastic extrusion for fabrication of porous 3Y-TZP OTM supports, or for other technologies requiring porous ceramics.     

Permeability behavior of silicon carbide-based membrane and performance study for oily wastewater treatment

Porous SiC ceramic is considered as a suitable material for hot gas filtration, microfiltration, and many others industrial applications. However, full utilizations of porous SiC ceramics have been limited by high-processing costs. In this study, mullite-bonded porous SiC ceramics membranes were prepared using commercial SiC powder, alumina, clay, and different sacrificial pore formers. The effect of different pore formers on the microstructure, mechanical strength, porosity and pore size distribution, air, and water permeability of porous SiC ceramics were investigated. The average pore diameter, porosities, and flexural strength of the final ceramics varied in the range 3.7-6.5 µm, 38-50 vol. %, and 28-38 MPa, respectively, depending on the characteristics of pore former. The Darcian (k₁) and non-Darcian (k₂) permeability evaluated from air permeation behavior at room temperature was found to vary from 1.48 × 10⁻¹³ to 4.64 × 10⁻¹³ m² and 1.46 × 10⁻⁸ to 6.51 × 10⁻⁸ m, respectively. All membranes showed high oil rejection rate (89%-93%) from feed wastewater with oil concentration of 1557 mg/L. The membrane with porosity ~48 vol% and mechanical strength 31.5 MPa showed and highest pure water permeability of 13 298 Lm⁻²h⁻¹bar⁻¹.     

Improving PVDF Hollow Fiber Membranes for CO2 Gas Capture

Poly(vinylidene fluoride) (PVDF) hollow fiber membranes were obtained by the phase inversion technique. The influence of internal coagulant viscosity (0.001 to 3 Pa s) and air gap (0.6 to 86.4 cm) on the structure and mechanical resistance of the fibers was studied. A “sponge-like” structure free of macrovoids was obtained by using polyvinyl alcohol (PVA) with N-methyl pyrrolidinone and water as internal coagulant (viscosity 3 Pa s). The effect of the air-gap was studied in order to control the structure and obtain mechanically resistant membranes with tensile strength at break between 2.2 and 54.3 N/mm² and pure water permeability ranging from 4 to 199 Lh^?1m^?2bar^?1. CO₂ permeability of these membranes was measured and found to be in the range of 365 to 53200 NLh^?1m^?2bar^?1. The “Dusty Gas” model (DGM) was used to calculate the pore size of the membranes from CO₂ permeability experiments, obtaining pore radius values going from 0.6 to 10.8 µm. Results from modeling were compared with pore sizes observed in SEM images showing that this model can accurately predict pore radius of sponge-like structures; however, pore sizes of membranes presenting sponge-like structures together with finger-like pores were inaccurately predicted by the DGM.     

Asymmetric supported dense lanthanum tungstate membranes

In this work, a colloidal processing route for dense asymmetric La_28−xW_4+xO_54+3x/2 membranes for hydrogen gas separation applications was established. Dip coating process conditions were optimized to obtain ≈20 μm thick dense layer supported on a porous substrate of the same composition. Surfactants based on electrosteric stabilization were evaluated to obtain stable suspensions in ethanol. The effect of the quantity and type (rice starch and carbon black) of sacrificial pore formers was evaluated for the porous substrates. Based on our results, samples made with 35–45 vol.% carbon black are the best choice to obtain highly porous supports with the optimum characteristics for the fabrication of asymmetric membranes.     

Silver-Doped Organic-Inorganic Hybrid Silica Membranes by Sol-Gel Method: Preparation and Hydrothermal Stability

Silver-doped methyl-modified silica membranes (Ag/M-SiO₂) have been prepared using the sol-gel method by adding AgNO₃ solution to a methyl-modified silica sol. The influence of silver-doping on the physical and chemical structures, thermal stability of –CH₃ groups, and gas permeation performance for the silica membranes were investigated. The metallic silver results from the reduction of AgNO₃ which can be completely transformed after calcined above 200°C. The Si–CH₃ vibrational bands disappear completely when the calcination temperature is increased to 600°C, which mineralized when the calcination temperature is further increased to 750°C. The doping of silver nanoparticles has nearly no influence on the chemical structure of the methyl-modified silica materials and the thermal stability of –CH₃ groups, but can make the mean pore size, total pore volume, H₂ permeability, and H₂/CO₂ selectivities of the silica membranes increase. When operated at 200°C and a pressure difference of 0.35 MPa, the H₂ permeance and H₂/CO₂ selectivity of Ag/M-SiO₂ membrane with the AgNO₃/tetraethylorthosilicate molar ratio of 0.08 is 8.99 × 10^?6 mol · m^?2 · Pa^?1 · s^?1 and 10.22, respectively. After hydrothermal treatment and regeneration, the Ag/M-SiO₂ membranes show a smaller change in gas permeances and H₂/CO₂ permselectivities than the methyl-modified silica membranes without silver-doping.     

High CO2 resistance of indium-doped cobalt-free 60 wt.%Ce0.9Pr0.1O2-δ-40 wt.%Pr0.6Sr0.4Fe1-xInxO3-δ oxygen transport membranes

The oxygen transport membrane (OTM) has huge application prospects in gas separation and carbon neutralization based on oxygen enriched combustion. In this paper, the family 60 wt.%Ce_0.9Pr_0.1O_2-δ-40 wt.%Pr_0.6Sr_0.4Fe_1-xIn_xO_3-δ (CPO-PSF_1-xI_xO, x = 0.01, 0.025, 0.05, 0.075, 0.1) cobalt-free dual-phase MIEC OTMs doped with indium have been successfully prepared by Pechini method. The phase structure, surface morphology, element distribution, oxygen permeability, and long-term operation stability of these OTMs are systematically explored. Among these OTMs, the champion oxygen permeable flux of CPO-PSF_0.99I_0.01O reaches 1.07 mL min^?1·cm^?2 and 0.80 mL min^?1·cm^?2 at 1000 °C under air/He gradient and air/CO₂ gradient. Meanwhile, CPO-PSF_0.99I_0.01O maintains the value of 0.80 mL min^?1·cm^?2 steadily at 1000 °C for 100 h when pure CO₂ as the sweep gas. The surface element distribution and phase structure of the OTMs after long-term oxygen permeability reaction are investigated by XRD, SEM combining with EDS, where the spent membranes retain the same structure and component as the fresh membranes, demonstrating that the In-doped OTMs have an excellent CO₂ tolerance. Suitable indium substitution for iron of these OTMs not only improves the oxygen permeability, but also maintains the long-term reaction stability of the material.     

Mechanically stable flat anodic titania membranes for gas transport applications

Anodic titanium oxide (ATO) membranes were produced by two-step anodic oxidation of titanium foil in ethylene glycol electrolyte containing NH₄F at the anodization voltage of 60?V. To provide the mechanical strength necessary for applying tubular anodic films as gas membranes, we utilized the formation of protective continuous TiO₂ layer at the top film surface prior to second anodization. As compared to conventional two-step anodic oxidation this technique decreases dissolution rates of titanium oxide phases with oxidation states lower than +4 (Ti₂O₃, Ti₃O₅), which are forming between titania nanotubes during anodization. The structural parameters of anodic titania films were determined by small-angle X-ray scattering and scanning electron microscopy techniques. According to SEM the proposed method resulted in growth of ATO films with a flat surface without nanotube endings, which enabled to use the films as gas separation membranes. The permeance of individual gases through ATO membranes were found to depend on gas molecular weight (M^?0.5), with absolute values twice exceeding theoretical permeabilities as it was predicted by Knudsen diffusion (up to 63?m³/(m²?×?bar?×?h) for nitrogen at 298?K). Here we ascribe this phenomenon to diffusion according to Knudsen-Smoluchoski mechanism (diffusion with slip, involving specular reflections of molecules), which is appropriate for membranes with straight pores and smooth internal pore surfaces.     

Fabrication of super flux and high thermal shock resistance ceramic membrane support

Ceramic membranes play an important role in high temperature gas-solid filtration. However, the thermal stability of the ceramic support at high temperatures has always been a problem. In this study, porous fused silica ceramic supports were fabricated with hexagonal boron nitride as a sintering aid. The results shown that hexagonal boron nitride could inhibit the crystallization of fused silica ceramic particles at high temperature and act as a sintering addictive to reduce firing temperature. The obtained supports have an average pore size of 72?µm, an open porosity of 42%, a bending strength of 16.5?MPa, a Weibull modulus of 8.67 and a gas permeability of 4.23?×?10⁵ m³/(m² h bar). The bending strength of the support remained 16?MPa after 30 cold-hot cycles, exhibiting high thermal shock resistance. After corrosion in 20?vol% H₂SO₄ solution for 8?h, the weight and the bending strength of the support were diminished by 0.6% and 24.32%, respectively. So, the ceramic support showed good acid corrosion resistance.     

Preparation of fused silica membrane with high performance for high temperature gas filtration by sealing and spray coating

In order to improve the gas permeability and thermal shock resistance of the ceramic membranes applied in high temperature gas-solid separation techniques, fused silica and graphite particles were used as the primary raw material and pore-former agent, and the spray coating based-on PVA sealing was applied to prepare the separation membrane. These approaches remarkably decreases filtration resistance by increasing support permeability and reducing the intrusion of ceramic membrane forming particles into the support as well as the thickness of the membrane. The fabricated membrane had an average pore diameter of 9.85?μm and a gas permeability value of 8.2?×?10⁴?m³/(m² h bar), its dust removal efficiency reached 98.6%.     

Evaluation and fabrication of pore‐size‐tuned silica membranes with tetraethoxydimethyl disiloxane for gas separation

Organic/inorganic hybrid silica membranes were prepared from 1,1,3,3‐tetraethoxy‐1,3‐dimethyl disiloxane (TEDMDS) by the sol‐gel technique with firing at 300–550°C in N₂. TEDMDS‐derived silica membranes showed high H₂ permeance (0.3–1.1 × 10^?6 mol m^?2 s^?1 Pa^?1) with low H₂/N₂ (～10) and high H₂/SF₆ (～1200) perm‐selectivity, confirming successful tuning of micropore sizes larger than TEOS‐derived silica membranes. TEDMDS‐derived silica membranes prepared at 550°C in N₂ increased gas permeances as well as pore sizes after air exposure at 450°C. TEDMDS had an advantage in tuning pore size by the “template” and “spacer” techniques, due to the pyrolysis of methyl groups in air and Si? O? Si bonding, respectively. For pore size evaluation of microporous membranes, normalized Knudsen‐based permeance, which was proposed based on the gas translation model and verified with permeance of zeolite membranes, reveals that pore sizes of TEDMDS membranes were successfully tuned in the range of 0.6–1.0 nm. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

Chelate membrane from poly(vinyl alcohol)/poly(n-salicylidene allyl amine) blend. II. Effect of co(II) content on oxygen/nitrogen separation

Facilitated transport of oxygen was performed through chelate membranes containing cobalt with selective oxygen binding ability as a fixed oxygen carrier. Chelate membranes were obtained from Schiff base membranes after treating a poly(allyl amine) (PAAm) and poly(vinyl alcohol) (PVA) blend with salicylaldehyde. It is confirmed that the O? O stretching peak through a frequency change in FTIR could be seen at 1150 cm^?1 between cobalt in the membrane and incoming oxygen. The permeability of oxygen through Schiff base membranes was 2.01?2.98 × 10^?13 [cm³ (STP) cm²/cm s cmHg] and oxygen permselectivity was in the range of 1.83?3.27. For chelate membranes, both the permeability of oxygen and oxygen selectivity increased to 2.15?2.82 × 10^?12 [cm³ (STP) cm²/cm s cmHg] and around 8, respectively. Permselectivity of chelate increased as a result of facilitation of O₂ and inhibition of N₂ transport. Detailed results and the mechanism of facilitation of oxygen are discussed on the basis of molecular interactions. © 1995 John Wiley & Sons, Inc.     

Synthesis of Highly Porous Yttria-Stabilized Zirconia by Tape-Casting Methods

Porous ceramics of Y₂O₃-stabilized ZrO₂ (YSZ) were prepared by tape-casting methods using both pyrolyzable pore formers and NiO followed by acid leaching. The porosity of YSZ wafers increased in a regular manner with the mass of graphite or polymethyl methacrylate (PMMA) to between 60% and 75% porosity. SEM indicated that the shape of the pores in the final ceramic was related to the shape of the pore formers, so that the pore size and microstructure of YSZ wafers could be controlled by the choice of pore former. Dilatometry measurements showed that measurable shrinkage started at 1300 K, and a total shrinkage of 26% was observed, independent of the amount or type of pore former used. Temperature-programmed oxidation (TPO) measurements on the green tapes demonstrated that the binders and dispersants were combusted between 550 and 750 K, that PMMA decomposed to methyl methacrylate between 500 and 700 K, and that graphite combusted above 900 K. The porosity of YSZ ceramics prepared by acid leaching of nickel from NiO–YSZ, with 50 wt% NiO, was studied as a function of NiO and YSZ particle size. Significant changes in pore dimension were found when NiO particle size was changed.     

Evaluation of hydrogen separation performance of Ni-BaCe0.85Fe0.15O3-δ cermet membranes

Adding metal phase as the electronic transport channels in mixed protonic and electronic conductors is an effective method to enhance the conductivity and hydrogen permeability. Ceramic-metal (cermet) Ni-BaCe_0.85Fe_0.15O_3-δ (Ni-BCF8515) dual-phase membranes were successfully fabricated in this article, where BaCe_0.85Fe_0.15O_3-δ (BCF8515) served as the protonic conductor and Ni acted as the electronic conductor. The hydrogen permeability of Ni-BCF8515 dual-phase membrane was improved apparently, reaching 0.325?mL?min^?1?cm^?2 at 1000?°C, which is four times higher than that of the single phase BCF8515 membrane. Hydrogen permeability of Ni-BCF8515 dual-phase membrane under different hydration conditions was studied here. Moreover, Ni-BCF8515 dual-phase membrane showed stable hydrogen permeability under the reducing atmosphere for 50?h.     

Fabrication and H2 flux measurement of asymmetric La27W3.5Mo1.5O55.5-δ − La0.87Sr0.13CrO3-δ membranes

Novel asymmetric hydrogen permeable membranes consist of a dense ceramic–ceramic (cercer) composite layer of La_0.87Sr_0.13CrO_3-δ and La₂₇W_3.5Mo_1.5O_55.5-δ deposited on a tubular porous support of the latter composition. The membranes were produced by extrusion and dip-coating with various thermal cycles required for adjusting the thermal shrinkage of the different layers and obtaining gas tight membrane layers. The produced asymmetric membranes have a dense cercer layer thicknesses ranging from 25 to 50 μm on supports exhibiting a porosity of up to 40 vol%. The effect of processing parameters, such as volume of pore former, coating steps, sintering temperature and soaking time on the microstructure of the membranes is discussed to highlight critical steps in the manufacturing protocol. Hydrogen fluxes were measured as a function of temperature with both wet and dry Ar sweep gas. Results are discussed with respect to membrane architectures and materials properties.     

Gas permeation properties of silica membranes with uniform pore sizes derived from polyhedral oligomeric silsesquioxane

The sol‐gel method was applied in the fabrication of homogenous polyhedral oligomeric silsesquioxane (HOMO‐POSS)‐derived silica membranes. Single gas permeation characteristics in a temperature range of 100–500^°C were examined to discuss the effect of silica precursor on amorphous silica networks. HOMO‐POSS‐derived membranes showed a CO₂ permeance of 1.1 × 10^?7 mol m^?2 s^?1 Pa^?1 with a CO₂/CH₄ permeance ratio of 131 at 100^°C, which is a superior CO₂/CH₄ separation performance by comparison with tetraethoxysilane (TEOS)‐derived silica membranes. Normalized Knudsen‐based permeance (NKP) was applied for quantitative evaluation of pore size. HOMO‐POSS‐derived membranes had loose amorphous silica structures compared to TEOS‐derived membranes and pore size was successfully tuned by changing the calcination temperatures. The activation energy for a HOMO‐POSS‐derived membrane fired at 550^°C with a uniform pore size of ～ 0.42 nm increased linearly with the ratio of the kinetic diameter of the gas molecule to the pore diameter, λ (=d_k/d_p), and showed a trend similar to that of DDR‐type zeolite membranes. © 2011 American Institute of Chemical Engineers AIChE J, 58: 1733–1743, 2012     

Preparation,characterization, and performance evaluation of LTA zeolite–ceramic composite membrane by separation of BSA from aqueous solution

Using low-cost clay supports as substrates, ceramic–LTA zeolite composite membranes (Z1–Z4) were fabricated with hydrothermal crystallization. The composite membranes were achieved with variations in the sequential zeolite depositional steps. For Z1–Z4 membranes, various characterization techniques such as thermogravimetric (TG), particle size distribution (PSD), X-ray diffraction (XRD), and field emission scanning electron microscopic (FE-SEM) analysis were applied. For the Z1–Z4 membranes, the pure water permeability, porosity, and average pore size varied from 1.22 × 10^?7 to 1.19 × 10^?8 m³/m²s kPa, 30–23%, and 215–76 nm, respectively. For the Z4 membrane, ultrafiltration experiments were conducted at a pH of 2.5 and transmembrane pressure differential of 207 kPa using aqueous bovine serum albumin (BSA) solutions. The optimal flux and rejection correspond to 4.54 × 10^?7 m³/m²s and 80%, respectively.     

Characteristics of ammonia permeation through porous silica membranes

A sol–gel method was applied for the preparation of silica membranes with different average pore sizes. Ammonia (NH₃) permeation/separation characteristics of the silica membranes were examined in a wide temperature range (50–400°C) by measurement of both single and binary component separation. The order of gas permeance through the silica membranes, which was independent of membrane average pore size, was as follows: He > H₂ > NH₃ > N₂. These results suggest that, for permeation through silica membranes, the molecular size of NH₃ is larger than that of H₂, despite previous reports that the kinetic diameter of NH₃ is smaller than that of H₂. At high temperatures, there was no effect of NH₃ adsorption on H₂ permeation characteristics, and silica membranes were highly stable in NH₃ at 400°C (i.e., gas permeance remained unchanged). On the other hand, at 50°C NH₃ molecules adsorbed on the silica improved NH₃‐permselectivity by blocking permeation of H₂ molecules without decreasing NH₃ permeance. The maximal NH₃/H₂ permeance ratio obtained during binary component separation was ～30 with an NH₃ permeance of ～10^?7 mol m^?2 s^?1 Pa^?1 at an H₂ permeation activation energy of ～6 kJ mol^?1. © 2009 American Institute of Chemical Engineers AIChE J, 2010     

Preparation and characterization of lotus ceramics with different pore sizes and their implication for the generation of microbubbles for CO2 sequestration applications

Four kinds of porous mullite ceramics, named lotus ceramics because of the similarity of their microstructure with lotus roots, were prepared by an extrusion method using rayon fibers of four different diameters (8.1, 9.6, 16.8 and 37.6 μm) as the pore formers. The physicochemical properties of these samples were characterized to test their applicability for the generation of microbubbles. The lotus ceramic samples contained pores of 9.4, 10, 15.6 and 30 μm size and porosities of 45–48%. SEM micrographs confirmed that the cylindrical pores were oriented unidirectionally along the extrusion direction and the degree of alignment was greater with larger fiber diameter. The permeability for gaseous CO₂ increased with increasing pore size from 3×10^?13 to 8×10^?13 m². The four lotus ceramic samples, a commercial air stone (72 μm) and two simple tubes (1000 and 3500 μm) were used to generate microbubbles in water under ambient conditions from a gas mixture of CO₂ and air. It was found that the bubble size could be decreased with bubblers of smaller pore size. In the bubble size measurements for pure CO₂ and air, the air bubbles were larger than the CO₂ bubbles due to partial dissolution of CO₂ into the water during bubbling. In order to generate smaller size bubbles using porous ceramic bubblers, the liquid must penetrate through the pores of the lotus ceramics before the gas is introduced into the system.     

Preferential CO oxidation over supported Pt catalysts

Preferential CO oxidation reaction has been carried out at a gas hourly space velocity of 46,129 h^?1 over supported Pt catalysts prepared by an incipient wetness impregnation method. Al2O3, MgO-Al₂O₃ (MgO=30 wt% and 70 wt%) and MgO were employed as supports for the target reaction. 1 wt% Pt/Al₂O₃ catalyst exhibited very high performance (X _CO >90% at 175 ^°C for 100 h) in the reformate gases containing CO₂ under severe conditions. This result is mainly due to the highest Pt dispersion, easier reducibility of PtO _x , and easier electron transfer of metallic Pt. In addition, 1 wt% Pt/Al₂O₃ catalyst was also tested in the reformate gases with both CO₂ and H₂O to evaluate under realistic condition.     

Pore size of microporous polymer membranes

Pore sizes of microporous polymer membranes were determined by the calculation based on the gas permeability of porous media. The gas permeability coefficient K (given by J = K Δp/l, where J is the steady-state gas flux, Δp is the pressure, difference, and l = the thickness of a membrane) for porous membrane can be given generally by where K₀ is the Knudsen permeability coefficient, η is the viscosity of the permeant gas, B₀ is the geometric factor of a membrane, and Δp? is the mean pressure of the gas on both sides of a membrane. From gas permeability measurements which yield the pressure dependence of gas permeability coefficient (expressed as above equation), the mean pore size of the porous membrane can be estimated as where M is the molecular weight of the permeant gas. The validity of this method was examined with various Millipore filters of which nominal pore sizes are known. It was confirmed that the method provided a simple and reliable means of estimating mean pore size of microporous membranes. The method was applied to investigate the influence of factors involved in preparation of microporous polysulfone membranes by coagulation procedure. It was found that the mean pore size of porous polysulfone membrane increases with (1) increasing with casting thickness, (2) increasing temperature of coagulation bath, and (3) decreasing concentration of polymer in casting solution (DMF as solvent). Water flux and water flux decline due to compaction are also examined as a faction of pore size, porosity, and the thickness of membranes.