Α-alumina Membrane Research Articles

Nanoporous α-Alumina Membranes with Pore Diameters Tunable over Wide Range of 30-350 nm

Nanoporous and α-alumina membranes with pore diameters tunable over a wide range of approximately 30–350 nm were successfully fabricated by optimizing the conditions for anodizing, subsequent detachment and heat treatment. Flat α-alumina membranes, each with a diameter of 25 mm and a thickness of approximately 50 μm, were obtained without thermal deformation. The present process is promising for fabricating free-standing and flat α-alumina membranes with various pore diameters; such membranes have potential technological and scientific applications in various fields.

Nanoporous α-Alumina Membranes with Pore Diameters Tunable over Wide Range of 30-350 nm

Porous anodic oxide film, which is formed by anodizing of aluminum, is a typical self-ordered nanoporous material. Over the past several decades, studies have been carried out on the protection or design of aluminum surfaces for commercial applications. Recently, anodic alumina films have attracted considerable attention as a key material due to their potential technological applications in area such as filters, catalyst supports, biological applications, electronic, magnetic and optical devices. However, commercially available anodic porous alumina membranes have insufficient pore arrangement and low chemical resistance because the porous alumina films obtained by typical anodizing processes are amorphous. To expand the application of anodic porous alumina with ordered pore arrays, it is necessary to control the cell dimensions (e.g., pore interval, pore diameter, and pore depth) and to enhance the chemical resistance of alumina membranes in extreme environments such as high temperature, high vapor pressure, and high-concentration acid/base solutions. Several fundamental studies have been reported on the fabrication of crystalline anodic porous alumina aimed at improving the chemical properties of α-alumina membranes using a combination of anodizing and heat treatment processes. Since the 2000s, the crystallization of free-standing alumina membranes to the α-phase has also been attempted; however, curling and cracking as a result of thermal deformation caused by the change in density and decomposition and desorption of electrolyte anions from the outer anion-incorporated layer in the cell wall were unavoidable during the transition to crystalline alumina in addition to the desorption of bound water at lower temperatures. In this study, we investigated the optimum conditions for the fabrication of α-alumina membranes with pore diameters tunable over a wide range of approximately 30–350 nm by the anodizing of aluminum and subsequent heat treatment. The morphological and structural change of the porous alumina membrane during heat treatment was also evaluated by scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetry-differential thermal analysis (TG-DTA), and thermogravimetry-mass spectrometry (TG-MS). Because the pore diameter could be adjusted by changing the anodizing conditions including the electrolyte species and formation voltage, anodizing of high-purity aluminum sheets was conducted in sulfuric acid, oxalic acid and phosphoric acid at each specific voltage. Here, we applied two-step anodizing process to improve the regularity of the pore arrangement of the anodic film. As a result, the deformation of alumina membrane such as warp and cracking during heat treatment for crystallization was effectively suppressed [1, 2]. By optimizing the conditions for anodizing, subsequent detachment, and heat treatment, nanoporous and single phase α-alumina membranes with pore diameters tunable over a wide range of approximately 30–350 nm were successfully fabricated. Even in the case of anodic porous film with short pore interval of approximately 60 nm, which was formed at 25 V in sulfuric acid, α-alumina membrane that maintained straight channels was obtained. The α-alumina membranes exhibited high chemical resistance in various concentrated acidic and alkaline solutions as well as when exposed to high temperature steam under pressure [2]. These results open a new route to technological and scientific applications.[1] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Electrochemistry, 82, 448-455 (2014)[2] T. Masuda, H. Asoh, S. Haraguchi, S. Ono, Materials, 8, 1350-1368 (2015)

Fabrication and Characterization of Single Phase α-Alumina Membranes with Tunable Pore Diameters.

Nanoporous and single phase α-alumina membranes with pore diameters tunable over a wide range of approximately 60–350 nm were successfully fabricated by optimizing the conditions for anodizing, subsequent detachment, and heat treatment. The pore diameter increased and the cell diameter shrunk upon crystallization to α-alumina by approximately 20% and 3%, respectively, in accordance with the 23% volume shrinkage resulting from the change in density associated with the transformation from the amorphous state to α-alumina. Nevertheless, flat α-alumina membranes, each with a diameter of 25 mm and a thickness of 50 μm, were obtained without thermal deformation. The α-alumina membranes exhibited high chemical resistance in various concentrated acidic and alkaline solutions as well as when exposed to high temperature steam under pressure. The Young’s modulus and hardness of the single phase α-alumina membranes formed by heat treatment at 1250 °C were notably decreased compared to the corresponding amorphous membranes, presumably because of the nodular crystallite structure of the cell walls and the substantial increase in porosity. Furthermore, when used for filtration, the α-alumina membrane exhibited a level of flux higher than that of the commercial ceramic membrane.

Synthesis and Characterization of Nanocomposite Hydroxy-Sodalite/Ceramic Membrane via Pore-Plugging Hydrothermal Synthesis Technique

In this article, synthesis and characterization of nanocomposite hydroxy sodalite (H-SOD)/α-alumina membrane via the so-called “pore-plugging” hydrothermal synthesis (PPH) protocol is reported for the first time. In nanocomposite architecture membranes, zeolite crystals are embedded within the pores of the supports instead of forming thin-film layers of the zeolite crystals on the surface of the supports. The as-prepared membranes were characterized with SEM and FTIR for morphology and purity of the H-SOD crystals. Compared to the conventional in-situ direct hydrothermal synthesis, membranes obtained from PPH possess higher mechanical and thermal stability. In addition, defect control with nanocomposite architecture membrane is possible because the zeolite crystals are embedded within the support pores of the membrane, thereby limiting the maximum defect size to the pore sizes of support. The nanocomposite architecture nature of the membranes safeguards the membrane from shocks or abrasion that could promote defects/inter-crystalline pores formation. These advantages could be helpful in the scale-up process of the preparation procedure of membranes at the commercial level with less-demanding conditions.

Synthesis and characterization of α-Alumina membrane supports and the binding effect of Poly (Vinyl Alcohol)

Ceramic Ultrafiltration membranes are considered as an alternative for the treatment of both stable water-in-oil and oil-in-water emulsions proved to be more effective in comparison with other conventional techniques. In this study, symmetric macro-porous ceramic membranes are prepared through dry pressing of α-alumina powder and the addition of various binders including Poly (vinyl alcohol). The disk is sintered after the organic material is burned away and the porosity can also be adjusted by the variation of sintering temperature. The results show that the fabrication of compact membrane supports can be done by increasing the sintering temperature which leads to grain size reduction. Moreover, application of different binders and additives result in various supports possessing different pore size and size distributions. Scanning electron Microscope (SEM) micrographs illustrate the surface and grain size distributions caused by various binders. These membranes can be used for microfiltration at elevated temperatures and extreme environmental conditions and also as porous supports for the fabrication of composite-asymmetric nanofiltration membranes.

Effect of Process Variables on the Production of Flaxseed Oil Emulsions by Cross-Flow Membrane Emulsification

The aim of this work was to study the effect of process variables on the production of flaxseed oil emulsions by cross-flow membrane emulsification. The process was carried out using an ultrafiltration α-alumina membrane with mean pore size of 0.2 µm, and Tween 20 was used as surfactant. The effect of transmembrane pressure (1.5–4.5 bar), cross-flow velocity (3–8 m/s) and surfactant concentration (1–3 %) on the dispersed phase flux, emulsion stability and droplet size distribution and uniformity was evaluated according to a 23 central composite experimental design. Results showed that all the emulsions presented unimodal droplets distribution, and the average droplet diameter was 1.50 μm. Among all the variables studied, only transmembrane pressure had significant influence on the dispersed phase flux—the increase in transmembrane pressure resulted in higher dispersed phase flux. The other variables did not affect any of the analyzed responses within the studied ranges.

New ceramic membranes from natural Moroccan phosphate for microfiltration application

This paper is devoted to preparation of low-cost microfiltration (MF) membranes using lixiviated phosphate powder crushed at 50 μm. The filtering layer was coated on phosphate tubular support with 10μm pore diameter and 43% porosity. The preparation of this ceramic layer was performed by the slip-casting method. A deflocculated slip was obtained by mixing mineral powder, polyvinyl alcohol, and water with dispersant. After drying at room temperature for 24 h, the MF layer was heated to 800°C for consolidation. Scanning electron microscopy observation showed homogeneous layers without cracks with an average pore diameter of 0.35μm for the active layer, the thickness is approximately 10 μm. Water permeability obtained is about 700 L/h m2 bar. The membranes have been tested to treat wastewater of phosphate industry, to clarify synthetic solutions of lime and aluminum hydroxide, and to remove bacteriological pollution present in water wells. The experiments carried out show that elaborated MF membrane is very efficient for the reduction of turbidity, total phosphorus, and number of bacterial germs with almost the same performances than that obtained with a commercial α-alumina membrane.

Fabrication of Nanoporous Crystalline Alumina Membrane by Anodization of Aluminum

Self-ordered nanoporous α-alumina membrane having uniformly sized straight pore arrays was fabricated by anodization of aluminum and subsequent through-hole and heat treatment. The detachment of thick anodic oxide film formed in oxalic acid was performed by combining preliminary loaded heating before stepwise voltage drop and cathodic polarization in phosphoric acid to prevent excess dissolution at the through-hole process. Suppression of excess dissolution of the upper layer of anodic alumina membrane acted an important role to provide symmetrical evenness of top and bottom pore density for the prevention of deformation such as warp and cracking during subsequent heating for crystallization. Thus, a thick, robust and chemically resistant nanoporous crystalline α-alumina membrane was successfully obtained by optimizing preparation conditions.

Effects of sintering temperature and initial compaction load on alpha-alumina membrane support quality

This work reports the effect of sintering temperature and compaction load on the quality of α-alumina (~particle size<1µm) membrane support for dry compaction method. We analyzed the membrane synthesis process to get good quality membrane support in a more economical way by reducing the holding time (4h) at different sintering temperatures. The effect of initial compaction load on porosity, pore size, flexural strength and shrinkage during synthesis of α-alumina support at various sintering temperatures was investigated. It was found that at 1500°C sintering temperature and 350kN compaction load, the support exhibited least porosity of ~1.4% and least pore sizes of ~67nm, whereas at 1200°C and 50kN compaction load, the support showed the highest porosity of ~55% and highest pore size of ~505nm.Maximum flexural strength of ~200MPa was obtained for the support synthesized at 1500°C sintering temperature and 350kN load.

Performance Analysis of a Porous Packed Bed Membrane Reactor for Oxidative Coupling of Methane: Structural and Operational Characteristics

The performance of the Oxidative Coupling of Methane (OCM) reactions in a porous ceramic packed bed membrane reactor was experimentally investigated using an Mn–Na2WO4/SiO2 catalyst. A novel practical method was applied to modify the available commercial α-alumina membrane and shape it to the form of an inert fine oxygen distributor in an OCM membrane reactor. The characteristics of such modified membrane and the performance of the resultant OCM membrane reactor are reviewed in this paper. It was observed that establishing a 2 bar pressure gradient across the modified membrane ensures a safe and efficient oxygen-dosing along the OCM membrane reactor. Moreover, using a modified membrane with a descending permeation profile instead of a uniformed permeation profile improved the observed C2 selectivity (ethylene and ethane) by 10% in average. The efficient design of the membrane reactor setup and the stability of the prepared catalyst provided a robust operation and replicable results. In this experimental analysis, a very promising 25.5% C2 yield and 20.3% ethylene yield with 66% C2 selectivity were achieved under very low (20%) diluted reaction atmosphere for the methane-to-oxygen ratio 2. By proper exploiting the carbon dioxide instead of nitrogen dilution, the C2 yield was improved by 1–2% in average.

Nanoporous α-Alumina Membrane with High Chemical Resistance Prepared by Anodizing

not Available.

Sol–Gel Synthesis of Alumina–Titania Nanostructure Membrane: Preparation and Characterization

In the present paper, the preparation and characterization of porous alumina–titania nanostructure membrane is described. The membrane layers consisted of an α-alumina membrane support, which was dip-coated with a composite alumina–titania top layer prepared in a sol–gel process. Thermogravimetric analysis showed that drying progress of the membrane layer is completed at about 560 °C. Scanning electron microscopy analysis displayed a smooth and defect-free layer of alumina–titania membrane after calcination at 600 °C. X-ray diffraction analysis showed the formation of gamma and orthorhombic alumina, brookite titania, and aluminium titanate (Al2TiO5) after the calcination step. Transmission electron microscopy analysis showed that the alumina–titania composite particle size was almost below 40 nm. The Brunauer–Emmet–Teller (BET) surface area, average pore size, and thickness of the membrane top layer were determined to be 292 m2/g, 2.2 nm, and 4 μm, respectively.

Volatile Organic Compound (VOC) Removal by Vapor Permeation at Low VOC Concentrations: Laboratory Scale Results and Modeling for Scale Up.

Petroleum transformation industries have applied membrane processes for solvent and hydrocarbon recovery as an economic alternative to reduce their emissions and reuse evaporated components. Separation of the volatile organic compounds (VOCs) (toluene-propylene-butadiene) from air was performed using a poly dimethyl siloxane (PDMS)/α-alumina membrane. The experimental set-up followed the constant pressure/variable flow set-up and was operated at ∼21 °C. The membrane is held in a stainless steel module and has a separation area of 55 × 10−4 m2. Feed stream was set to atmospheric pressure and permeate side to vacuum between 3 and 5 mbar. To determine the performance of the module, the removed fraction of VOC was analyzed by Gas Chromatography/Flame Ionization Detector (GC/FID). The separation of the binary, ternary and quaternary hydrocarbon mixtures from air was performed at different flow rates and more especially at low concentrations. The permeate flux, permeance, enrichment factor, separation efficiency and the recovery extent of the membrane were determined as a function of these operating conditions. The permeability coefficients and the permeate flux through the composite PDMS-alumina membrane follow the order given by the Hildebrand parameter: toluene > 1,3-butadiene > propylene. The simulated data for the binary VOC/air mixtures showed fairly good agreement with the experimental results in the case of 1,3-butadiene and propylene. The discrepancies observed for toluene permeation could be minimized by taking into account the effects of the porous support and an influence of the concentration polarization. Finally, the installation of a 0.02 m2 membrane module would reduce 95% of the VOC content introduced at real concentration conditions used in the oil industry.

Preparation of pH-responsive ceramic composite membranes by grafting acrylic acid onto α-alumina membranes

A pH-responsive ceramic composite membrane was prepared by chemical graft polymerization of acrylic acid (AA) onto the KH-570 modified μ-alumina membrane. The influence of monomer concentration on the gating characteristics of the pH-responsive membrane was investigated. The FT-IR spectrum, contact angle and water filtration rate of the membrane were measured. The monomer concentration was found to have a remarkable effect on the pH-response coefficient and the water filtration rate. In addition, the grafted membrane exhibited fast and reversible response to the pH change in the external solution.

Partial Pore Blocking in Microfiltration Recycling of a Semisynthetic Metalworking Fluid

In this paper, the fouling of sintered α-alumina membranes by an uncontaminated semisynthetic metalworking fluid (MWF) is addressed. Experimental evidence of the form of flux reduction curves, scanning electron microscope images of the membranes, and MWF particle size measurements is used to identify two fouling mechanisms, pore blocking and partial pore blocking, as the major contributors to flux decline. A probability-based mechanistic model is developed based on the time-dependent particle size distribution and membrane pore sizes. The model is fitted to experimental data from two commonly used membrane pore sizes with good agreement. Partial blocking is shown to be a predominant first step in the pore blocking mechanism in microfiltration of semisynthetic MWFs due to the tortuous nature of the pores present in sintered ceramic membranes.

Co-sintering synthesis of tubular bilayer α-alumina membrane

In order to reduce production time and costs, a co-sintering process was proposed to prepare tubular bilayer α-alumina membrane. The shrinkage mismatch between two membrane layers during co-sintering was the first problem to be solved. The selection of the co-sintering temperature was the main issue addressed in this paper. Experimental results showed that too large mismatch in the sintering shrinkage of two membrane layers could cause defects such as cracks and big pores while moderate shrinkage mismatch between two membrane layers during co-sintering was beneficial. SEM images of the membrane prepared by co-sintering at 1300 °C showed that the membrane surface was defect-free and the interfacial bonding was good. Ultrasonic treatment did not damage the microstructure and the permeability of the membrane synthesized by co-sintering at 1300 °C. The filtration of the CaCO 3 suspension showed that the membrane treated by co-sintering had a higher steady permeate flux than the membrane prepared by conventional process and can withstand a high back-flushing pressure.

Development of a Novel Metalworking Fluid Engineered for Use With Microfiltration Recycling

Membrane microfiltration is a promising technology that has been shown to extend metalworking fluid (MWF) life by eliminating contaminants while allowing the fluid to stay in use. However, the efficacy of this technology is compromised by the clogging of the filter pores in a process known as membrane fouling. In this paper the fouling issue is addressed by the development of a semi-synthetic MWF specifically designed to not foul microfiltration membranes. The composition of the designed MWF is discussed and compared with a commercial MWF. Cross-flow microfiltration fouling tests were carried out in low-pressure, high-velocity conditions on ceramic α-alumina membranes. Several common MWF components are shown not to be factors of membrane fouling on these membranes. The flux of the designed fluid was found to reach an immediate steady state at about twice the value of the steady-state flux of the tested commercial fluid. Scanning electron microscope imaging was used to further evaluate membrane fouling by each fluid. The machining capabilities of the designed fluid were examined in terms of cutting forces and machining temperature.

Towards single step production of multi-layer inorganic hollow fibers

In this work we propose a generic synthesis route for the single step production of multi-layer inorganic hollow fibers, based on polymer wet spinning combined with a heat treatment. With this new method, membranes with a high surface area per unit volume ratio can be produced, while production time and costs are dramatically reduced. The proof-of-principle of the concept will be demonstrated with the production of double layer α-alumina hollow fibers. Although various problems were anticipated at the interface of the layers, the adhesion between the two layers is surprisingly good, both in the precursor and the sintered fiber. Produced fibers show an asymmetric structure with a porosity between 37 and 45%. The macrostructure of the sintered fiber is largely determined by the macrostructure of the precursor fiber, while differences in microstructure disappear during the heat treatment step. The proposed method is not limited to α-alumina membranes; in principle many ceramic or metallic powders may be used. This means that this method can open up the way for a new generation of membranes.

Preparing and testing Pd films of thickness 1–2 micrometer with high selectivity and high hydrogen permeance

Thin Pd films with high selectivity are advantageous both from a cost point of view as well as for achieving higher hydrogen fluxes through the film. Pd films of thickness down to 1 μm were deposited on the inside of asymmetric α-alumina membranes from the Societé des Céramiques Techniques (SCT) (200 nm pore size) by a modified electroless plating process. Pre-treatment, the plating rate during electroless plating and membrane post-plating treatment must be optimized individually to obtain very high quality films. Membrane defects develop when the substrate is not catalyzed properly before plating, when the plating rate is too fast and/or when the membrane is not cleaned thoroughly after plating. Hydrogen permeances varied between 6 and 15 μmol/m2 Pa sec for temperatures from 330 to 450°C and Pd films from 1.0 to 1.5 μm thickness. Hydrogen to nitrogen selectivity was above 100 for all membranes tested and above 400 for all but two membranes (thickness 1.0–1.5 μm).

Comparing and Modeling the Dehydrogenation of Ethanol in a Plug-Flow Reactor and a Pd−Ag Membrane Reactor

In this study the performance of a membrane reactor was compared to that of a plug-flow reactor for the dehydrogenation of ethanol. The membrane consisted of a 2.2 μm Pd-Ag film (23 wt% Ag) deposited on the inside of an asymmetric α-alumina membrane tube (SCT). The membrane tube was packed with a 14.5 wt% Cu on SiO 2 catalyst. The effects of the sweep gas flow rate, the ethanol feed flow rate, and the temperature on acetaldehyde yield and selectivity were investigated. A model was developed using measured kinetic data and membrane permeance data to predict the performance of the membrane reactor. The membrane reactor performed significantly better than the plug-flow reactor at all temperatures tested. The best results were obtained at 275°C, where the total ethanol exit conversion increased from 45% (plug-flow reactor) to 60% at low feed flow rates and from 36% to 46% at high feed flow rates. The acetaldehyde selectivity for the membrane reactor increased from the lower 80% range to above 90% at 275°C. The model underpredicted the total ethanol conversion, indicating that the measured reaction rate parameters for a differential reactor were lower than those for the membrane reactor.