Thermotreatment and chemical resistance of porous alumina membrane prepared by anodic oxidation

Porous alumina membranes were prepared by anodic oxidation of aluminum in an aqueous solution of oxalic acid. The thermal and chemical stabilities of porous alumina membranes were investigated by thermal analysis and corrosion test. Alumina membranes prepared in oxalic electrolyte had fairly good thermal stability, but they reacted with boiling water to form boehmite, which blocked pores on the surface of the membrane. For acid and alkali resistance, the membranes were estimated to have chemical stability in aqueous acidic and alkaline solution at 1< pH < 12. On the other hand, pore size and its distribution were not changed at the time of thermal treatment. Polycrystalline alumina membrane obtained by thermal treatment for 3 h at 900 ‡C could suppress corrosion reaction and enhance their resistance in acid and alkaline solution.     

Gas permeation characteristics of silica/alumina composite membrane prepared by chemical vapor deposition

Amorphous silica membranes were deposited by thermal decomposition of tetraethoxysilane at 600–650 ‡C on a porous α-alumina tube with pore size of 110–180 nm or γ-alumina coated α-alumina tube with pore size of 6–8 nm.The forced cross-flow through the porous wall of the support was very effective in plugging macropores. The membranes formed on γ-alumina coated oc-alumina tube showed H₂ permeances much higher than the SiO₂ membranes formed on the α-alumina tube. This indicated that the γ-alumina film was effective in improving the H₂ permeance and H₂/N₂ selectivity. The permeation tests with CO₂, N₂, CH₄, C₃H₈ and i-C₄H₁₀ showed that a very small number of mesopores remained unplugged by the CVD. Permeation of hydrogen was explained by activated diffusion, and that of the other gases by Knudsen diffusion through the unplugged pores. Thus, the total permeance was composed of permeances due to the activated and Knudsen diffusion mechanisms. The contribution of Knudsen diffusion pores decreased to 0.02 when the γ-alumina film was modified at 650 ‡C until P_fe=50 Pa.     

Gas permeation properties in a composite mesoporous alumina ceramic membrane

In this study, the effect of different chemical interactions on the gas permeation properties were investigated in the composite mesoporous ceramic membrane prepared with γ-alumina on the surface of a macroporous ceramic membrane. In the permeation results, the gas permeance of the strongly adsorbing gas species increased in the mesoporous ceramic membranes. It is considered that the permeation of the adsorbing gas species increased through preferential adsorption on the membrane pore surface. It was shown in this study that the modified mesoporous ceramic membrane could increase the permeation performance in the presence of the adsorbing gas species due to the surface diffusion mechanism.     

Strengthened porous alumina membrane tube prepared by means of internal anodic oxidation

A method for preparing an anodic aluminum oxide tube with a much higher mechanical strength than before was developed, where anodic oxidation was carried out from the inside of an aluminum tube (internal anodizing method). It was considered that the internal stress remaining in the alumina layer, which was formed in the course of the internal anodizing, resulted in a considerable improvement of compressive strength against external pressure. As a matter of fact, the tube could withstand at least up to 0.98 MPa of transmembrane pressure, which corresponded to 32.6 MPa in terms of breaking pressure. The tubular alumina membrane obtained had straight mesopores of 20–25 nm in diameter and, therefore, showed the Knudsen permselectivity for inorganic gases. Such a strengthened alumina tube is expected to be used as a porous membrane or as a support for composite membranes.     

Fabrication and characterization of alumina fiber by anodic oxidation and chemical dissolution processes

Alumina fiber was fabricated by an anodic oxidation process from pure aluminum or a chemical dissolution process from porous alumina. In the experiments, porous alumina layer was firstly formed on the surface of pure aluminum by anodic oxidation process in phosphoric acid electrolyte. The alumina fiber was obtained by either further anodic oxidation process or a chemical dissolution process from the porous alumina layer. The thickness of the porous alumina layer, the diameter and wall thickness of the pores in the porous alumina layer, and the length and diameter of the obtained alumina fiber were examined. The formation mechanism of the alumina fiber was discussed.     

Role of metal ion impurities in generation of oxygen gas within anodic alumina

The generation of oxygen gas within an amorphous anodic alumina film is reported. The film was formed by anodizing aluminum, which was first electropolished and then chemically polished in CrO₃-H₃PO₄ solution, in sodium tungstate electrolyte. The procedure results in incorporation of mobile Cr³⁺ species, from the chemical polishing film, and mobile W⁶⁺ species, from the electrolyte, into the amorphous structure. The tungsten species are present in the outer 27% of the film thickness, while Cr⁶⁺ species occupy a thin layer within the tungsten-containing region. Above the Cr³⁺ containing layer, a band develops that contains oxygen bubbles of a few nanometres size. The oxygen is generated by oxidation of O²⁻ ions of the alumina. A mechanism of oxygen generation within the alumina is proposed based on the electronic band structure of the oxide, modified by the Cr³⁺ and W⁶⁺ species, and on the ionic transport processes during oxide growth.     

Microstructural characterization and gas permeation performance of freeze-cast alumina supports

One of the major challenges for obtaining asymmetric membranes has been the preparation of ceramic supports with tailored pore structures. The methods commonly used for processing these materials do not allow a fine control of the support structure, which may decrease their mass transport capacity. As a consequence, freeze-casting is a promising technique for obtaining supports with tailored pore structures and permeation behaviors. This work deals with the preparation of freeze-cast alumina macroporous supports. Different freezing routes and alumina powders were used in this study. It was correlated the effect of these parameters on the pore structure and permeation capacity of the obtained supports. It was shown that the isotropic layer observed at the samples bottom side plays a key role in their non-Darcian permeability. As far as we know, this is the first time that this approach is reported in the literature for freeze-cast samples.     

Highly selective modification of silicon oxide structures fabricated by an AFM anodic oxidation

Anodic oxidation via atomic force microscopy is a promising method for creating submicron-sized silicon dioxide patterns on a local surface. The area patterned by AFM anodic oxidation (AAO) has different chemical properties from the non-patterned area, and thus site-selective modification of patterned surfaces is quite possible. In this study, we combined the AAO with self-assembly method and/or wet chemical etching method for the fabrication of positive and/or negative structures. These locally modified surfaces could be used to the site-selective arrangement and integration of various materials based on a pre-described pattern.      

Evaluation of hydrogen permselective separation from “synthesis gas” components based on single gas permeability measurements on anodic alumina membranes

Anodic alumina membranes were prepared by anodizing aluminum followed by chemical and hydrothermal treatments. Anodization was performed on both tube surfaces. External anodization was restricted to selected spots leaving an aluminum mechanically strong frame. Nitrogen sorption data analysed with the CPSM method (Corrugated Pore Structure Model) detected a mesopore structure (i.e. D_mean;15–20) and surface areas of 2–20 m²/g. SEM microscopy images revealed a regular pore structure of independent pores with densities of N_pore ~ 5.6 × 10¹⁴ pores/m². Single gas permeances (Π) for X: H₂, CH₄, CO and CO₂ were measured on a Wicke–Kallenbach apparatus at varying mean transmembrane pressure P_m by the “dead-end” method. The observed slight dependence of Π on Ρ_m is indicative of strong Knudsen diffusion and weak viscous flow contributions. By correlating the experimental data with a linear Π vs Ρ_m relationship, Knudsen contribution evaluation was enabled, and found to vary in the range K_C ≈ 0.7–1.0. The Knudsen number criterion for flow regime discrimination is critically discussed and a realistic dual Knudsen number approach is proposed. Experimental permselectivities α_Η2Χ = Π_H2/Π_X (X ≠ H₂) approach by 70–100% their Knudsen selectivity counterparts. Anodic alumina membranes exhibit pore structure and gas permeability characteristics useful in designing integrated gas separation and catalytic membrane reactor systems.     

CO2, N2 gas sorption and permeation behavior of chitosan membrane

The sorption equilibria for CO₂ and N₂ in dry chitosan membrane at 20 and 30 ‡C were measured by a pressure decay method. The steady-state permeation rates for CO₂ and N₂ in dry and wet (swollen with water vapor) chitosan membranes at 20 and 30 ‡C were measured by a variable volume method. The sorption equilibrium for N₂ obeyed Henry’s law, whereas that for CO₂ was described apparently by a dual-mode sorption model. This non-linear sorption equilibrium for CO₂ could be interpreted by the interaction of sorbed CO₂ with the chitosan matrix expressed as a reversible reaction. The logarithm of the mean permeability coefficient for CO₂ in dry chitosan membrane increased linearly with upstream gas pressure. A linear increase of the logarithmic mean permeability coefficient for CO₂ with the pressure could be interpreted in terms of a modified free-volume model. The mean per-meability coefficient for N₂ in dry chitosan membrane only slightly increased with upstream gas pressure. The per-meabilities for CO₂ and N₂ in wet chitosan membrane increased by 15 to 17 times and 11 to 15 times, respectively, as compared to those in the dry membrane.     

工业苯酚丙酮装置氧化尾气的气相色谱分析

采用质量分数2.5%的邻苯二甲酸二壬酯和质量分数2.5%的硅藻土,以chromsorb G·DMCX(150~180μm)为担体的色谱填充柱进行分离,氢火焰离子化检测器进行检测,用峰面积外标法进行定量分析,同时对工业苯酚丙酮装置氧化尾气组成进行气相色谱分析。对色谱柱的选择、操作条件的优化、定性、定量分析,进样量和线性范围、最低检测限等实验项目进行了考察,实验结果表明此方法具有操作简单、分析速度快、分析成本低、重复性与准确性好等特点。     

Influence of oxidation temperature on the gas permeation and separation properties in a microporous carbon membrane

Of thermosetting polymers, polyphenylene oxide (PPO) is considered as one of the promising alternative polymeric precursors for carbon membrane preparation. In this study, the PPO derived carbon membranes were prepared by carbonization and followed by air-oxidation as post-treatment method to modify the membrane pore structures. The characterization of the pore properties showed that air-oxidation enlarged the pore structure for the postoxidized carbon materials. The permeation results for the post-oxidized carbon membranes showed that the extent of the permeation modification was strongly dependent on the oxidation temperature. In the binary mixture gas systems, the permeation performance of the adsorbing gas species increased due to the surface diffusion mechanism. It is considered in the oxidation effect on the permeation modification that the post-oxidation of the carbon membranes increased gas permeation and separation properties.     

Control of the spatial distribution of porous alumina micro-domes formed during anodic oxidation

We found that micro-domes of porous alumina are self-assembled during anodic oxidation of an aluminum plate. We investigated the effects of the morphology of the initial aluminum surfaces on the formation of these micro-domes and found that the formation of micro-domes depends on the initial surface roughness of the substrate. We have also achieved spatial control over the distribution of these micro-domes through the use of artificial scratches on the initial surface. The origin of this control is the fact that micro-domes are preferentially formed inside hollow areas formed by the scratch. We investigated the inner structure of the micro-dome by separating it from the substrate. Inside the micro-domes, we observed nano-pore arrays similar to a porous alumina membrane, though the regularity of these pores is slightly worse than for the nano-pores around the micro-dome. These results indicate that the porous alumina micro-domes can be used as microscale nanoporous components.     

Separation and enrichment of carbon dioxide by capillary membrane module with permeation of carrier solution

A novel facilitated transport membrane for gas separation using a capillary membrane module is proposed in which a carrier solution is forced to permeate the membrane. Both a feed gas and a carrier solution are supplied to the lumen side (high pressure side, feed side) of the capillary ultrafiltration membrane and flow upward. Most of the carrier solution which contains dissolved solute gas, CO₂ in the present case, permeates the membrane to the permeate side (low pressure side, shell side), where the solution liberates dissolved gas to form a lean solution. The lean solution is circulated to the lumen side. This type of capillary membrane module was applied to the separation of CO₂ from model flue gases consisting of CO₂ and N₂. Monoethanolamine (MEA), diethanolamine (DEA) and 2-amino-2-methyl-1-propanol (AMP) were used as carriers or absorbents of CO₂. The feed side pressure was atmospheric and the permeate side was evacuated at about 10 kPa. CO₂ in the feed gas was successfully concentrated from 5–15% to more than 98%. The CO₂ permeance was as high as 2.7×10⁻⁴ mol m⁻² s⁻¹ kPa⁻¹ (8.0×10⁻⁴ cm³ cm⁻² s⁻¹ cmHg⁻¹) when the CO₂ mole fraction in the feed was 0.1 and temperature was 333 K. The selectivity of CO₂ over N₂ was in the range from 430 to 1790. The membrane was very stable over a discontinuous one-month testing period.     

Selective permeation and separation of steam from water–methanol–hydrogen gas mixtures through mordenite membrane

Separation properties of a mordenite membrane for water–methanol–hydrogen mixtures were studied in the temperature range from 423 to 523 K under pressurized conditions. The mordenite membrane was prepared on the outer surface of a porous alumina tubular support by a secondary-growth method. It was found that water was selectively permeated through the membrane. The separation factor of water/hydrogen and water/methanol were 49–156 and 73–101, respectively. Even when only hydrogen was fed at 0.5 MPa, its permeance was as low as 10⁻⁹ mol m⁻² s⁻¹ Pa⁻¹ up to 493 K, possibly suggesting that water pre-adsorbed in the micropores of mordenite hindered the permeation of hydrogen. The hydrogen permeance dramatically increased to 6.5 × 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹ at 503 K and reached to 1.4 × 10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹ at 523 K because of the formation of cracks in the membrane. However, the membrane was thermally stabilized in the presence of steam and/or methanol.     

AgBiVMo oxide catalytic membrane for selective oxidation of propane to acrolein

A metal ions (Ag, Bi, V, Mo) modified sol–gel method was used to prepare a mesoporous Ag_0.01Bi_0.85V_0.54Mo_0.45O₄ catalytic membrane which was used in the selective oxidation of propane to acrolein. By optimizing the preparation parameters, a thin and perfect catalytically active membrane was successfully prepared. SEM results showed that the membrane thickness is 5 μm. XRD results revealed that Ag_0.01Bi_0.85V_0.54Mo_0.45O₄ with a Scheelite structure, which is catalytically active for the selective oxidation of propane to acrolein, was formed in the catalytic membrane only when AgBiVMoO concentrations were higher than 40%. Catalytic reaction results demonstrated that the selective oxidation of propane could be controlled to a certain degree, such as to acrolein, in the catalytic membrane reactor (CMR) compared to the fixed bed reactor (FBR). For example, a selectivity of 54.85% for acrolein in the liquid phase was obtained in the CMR, while only 8.31% was achieved in the FBR.     

Development of biomorphic alumina using egg shell membrane as bio-template

Hierarchical interwoven alumina was developed using egg shell membrane as a bio-template. Fibrous network of the egg shell membrane was replicated to form fibrous alumina networks by soaking the membranes in metal salt solution and by subsequent calcination. The synthesis was systematically studied by varying the calcination temperature (400 ? 1200 °C). Morphological features of the developed alumina networks were characterized using electron microscopes and structural investigations were carried out using X-ray diffractometer. Surface features of the hierarchical structures developed were also studied. Calcium and sulfur based compounds got crystallized from egg shell membranes along with alumina. The developed biomorphic networks by virtue of their high surface area and porous properties will be potential candidates for applications in water purification.     

Mechanical properties of hybrid composites prepared by ice-templating of alumina

Ceramic-polymer hybrid composites are often designed for its high strength and low density. Ice-templating (freeze casting) is a promising method for preparation of such composites. However, the most of the reported mechanical properties were gained from a small volume of material. In this work 70 cm³ of the lamellar composite with lamella length, up to 70 mm was prepared by ice-templating followed by polymer infiltration. The volume of ceramic (alumina) in starting suspensions was varied from 25 to 45 vol.% and the same manufacturing process was applied. The fracture toughness and flexural strength were determined on prepared beams from plates by loading in bending. The fractographic analysis conducted on the fracture surfaces and obtained mechanical properties demonstrated that an optimal strength/density ratio lies between 51 and 55% of alumina volume fraction. The density ranking from 2.6 to 2.8 cm⁻³ of these composites results in values of Weibull strength above 110 MPa.     

Rapid evaluation of oxidation catalysis by gas sensor system: total oxidation, oxidative dehydrogenation, and selective oxidation over metal oxide catalysts

The rapid evaluation of catalysis is an indispensable technology for the success of combinatorial chemistry. A small-sized, less expensive, easily operating screening is desirable for parallel settings which dramatically shortens the evaluation time. Recent advances in gas sensors have enabled us to use them for the rapid evaluation of oxidation catalysis. Three typical catalytic oxidations over metal oxide catalysts were evaluated by gas sensor systems optimized for each catalytic system. The first one is the total oxidation of carbon monoxide in air. Five catalytic combustion-type gas sensors were used in a parallel reactor system to shorten the evaluation time. The second one is the oxidative dehydrogenation (ODH) of ethane over the mixed oxide of nickel and iron. The evaluation of the ODH catalysis was performed by a selective olefin sensor which determines the concentration of C₂H₄ in C₂H₆. The third one is the selective oxidation catalysis of propane over alkali modified Fe/SiO₂. The effluents including CO, CO₂, aldehydes and ketones in propane were analyzed by the CO, CO₂ and semiconductor-type gas sensors selective toward aldehydes and ketones. These evaluation results indicated that gas sensors have a good potential for the rapid evaluation of oxidation catalysts.     

Electrochemical destruction of chlorophenoxy herbicides by anodic oxidation and electro-Fenton using a boron-doped diamond electrode

The degradation of herbicides 4-chlorophenoxyacetic acid (4-CPA), 4-chloro-2-methylphenoxyacetic acid (MCPA), 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) in aqueous medium of pH 3.0 has been comparatively studied by anodic oxidation and electro-Fenton using a boron-doped diamond (BDD) anode. All solutions are totally mineralized by electro-Fenton, even at low current, being the process more efficient with 1 mM Fe²⁺ as catalyst. This is due to the production of large amounts of oxidant hydroxyl radical (OH) on the BDD surface by water oxidation and from Fenton’s reaction between added Fe²⁺ and H₂O₂ electrogenerated at the O₂-diffusion cathode. The herbicide solutions are also completely depolluted by anodic oxidation. Although a quicker degradation is found at the first stages of electro-Fenton, similar times are required for achieving overall mineralization in both methods. The decay kinetics of all herbicides always follows a pseudo first-order reaction. Reversed-phase chromatography allows detecting 4-chlorophenol, 4-chloro-o-cresol, 2,4-dichlorophenol and 2,4,5-trichlorophenol as primary aromatic intermediates of 4-CPA, MCPA, 2,4-D and 2,4,5-T, respectively. Dechlorination of these products gives Cl⁻, which is slowly oxidized on BDD. Ion-exclusion chromatography reveals the presence of persistent oxalic acid in electro-Fenton by formation of Fe³⁺-oxalato complexes, which are slowly destroyed by OH adsorbed on BDD. In anodic oxidation, oxalic acid is mineralized practically at the same rate as generated.