Preparation and Characterization of High-Temperature Thermally Stable Alumina Composite Membrane

A crack- and pinhole-free composite membrane consisting of an α-alumina support and a modified γ-alumina top layer which is thermally stable up to 1100°C was prepared by the sol–gel method. The supported thermally stable top layer was made by dipcoating the support with a boehmite sol doped with lanthanum nitrate. The temperature effects on the microstructure of the (supported and unsupported) La-doped top layers were compared with those of a common γ-alumina membrane (without doping with lanthanum), using the gas permeability and nitrogen adsorption porosimetry data. After sintering at 1100°C for 30 h, the average pore diameter of the La-doped alumina top layer was 17 nm, compared to 109 nm for the common alumina top layer. Addition of poly(vinyl alcohol) to the colloid boehmite precursor solution prevented formation of defects in the γ-alumina top layer. After sintering at temperatures higher than 900°C, the common alumina top layer with addition of poly(vinyl alcohol) exhibits a bimodal pore distribution. The La-doped alumina top layer (also with addition of poly(vinyl alcohol)) retains a monopore distribution after sintering at 1200°C.     

Influence of the intermediate layer on the hydrothermal stability of sol-gel derived hybrid silica membranes

The hydrothermal stability of microporous silica hybrid sol-gel derived membranes is often only tested for either the mesoporous intermediate membrane layer or the microporous separation layer. In this work an investigation is done on the interaction between the intermediate γ-alumina layer and the hybrid (BTESE-derived) silica separation layer during hydrothermal treatment. Although bare γ-alumina is degraded during a hydrothermal treatment, a coating of hydrophobic BTESE on γ-alumina retains its gas separation performance, albeit with a lower mechanical adhesion between the hybrid silica separation layer and the γ-alumina intermediate layer. Applying a monoaluminumphosphate (MAP) coating between the α-alumina support and the γ-alumina layer stabilizes the γ-alumina membrane. A BTESE coating on a MAP modified γ-alumina membrane did not show any signs of delamination after hydrothermal testing. Moreover, a significant increase in the H₂^/N₂ (perm)selectivity, factor 3, was observed for these membranes.     

Preparation Method of Submicrometer-Sized α-Alumina by Surface Modification of γ-Alumina with Alumina Sol

A method is introduced to prepare almost-spherical submicrometer-sized α-alumina via surface modification of γ-alumina with an alumina sol. Milled γ-alumina, in the presence of 3 wt% of α-alumina with a median particle size ( d ₅₀) of 0.32 μm (AKP-30), produced irregularly shaped α-alumina with d ₅₀∼0.3 μm after heat treatment at 1100°C for 1 h. γ-alumina that had been surface-modified by milling in the presence of 3 wt% of the alumina sol resulted in almost-monosized, spherical α-alumina ∼0.3 μm in size after heat treatment at 1100°C for 1 h. Furthermore, almost-spherical α-alumina 0.1—0.2 μm in size was obtained by milling γ-alumina with 3 wt% of AKP-30 alumina in the presence of 3 wt% of the alumina sol, followed by heat treatment at 1100°C for 1 h. The alumina sol that has been introduced in this work seems to act as a dispersant, in addition to helping to form a spherical shape.     

Preparation of Silica/γ-Alumina Membrane with Bimodal Porous Layer for Improved Permeation in Ions Separation

Creating secondary pores in the intermediate layer of hierarchical ceramic membranes successfully increases the permeability of bi-layered membranes by reducing the density of the separating layer. With the optimum secondary pore volume, the permeability of the silica/γ-alumina membrane with low secondary volume is enhanced with a satisfactory retention of organic ions and inorganic ions. However, the silica layer is not well formed when excessive secondary pores were generated in the intermediate layer. This is likely because the bimodal porous structure of γ-alumina with high secondary pore volume is inadequate to prevent the penetration of silica sol into the α-alumina support during dip coating. Thus, the bi-layered membrane with high secondary pore volume shows insufficient retention of Reactive Orange 16 dye and NaCl at low pH.     

Relationship between the Mesoporous Intermediate Layer Structure and the Gas Permeation Property of an Amorphous Silica Membrane Synthesized by Counter Diffusion Chemical Vapor Deposition

An amorphous silica membrane with an excellent hydrogen/nitrogen (H₂/N₂) permselectivity of >10 000 and a He/H₂ permselectivity of 11 was successfully synthesized on a γ-alumina (γ-Al₂O₃)-coated α-alumina (α-Al₂O₃) porous support by counter diffusion chemical vapor deposition using tetramethylorthosilicate and oxygen at 873 K. An amorphous silica membrane possessed a high H₂ permeance of >1.0 × 10⁻⁷ mol·(m²·s·Pa)⁻¹ at ≥773 K. The dominant permeation mechanism for He and H₂ at 373–873 K was activated diffusion. On the other hand, that for CO₂, Ar, and N₂ at 373–673 K was a viscous flow. At ≥673 K, that for CO₂, Ar, and N₂ was activated diffusion. H₂ permselectivity was markedly affected by the permeation temperature, thickness, and pore size of a γ-Al₂O₃ mesoporous intermediate layer.     

The Effect of Hydrotalcite Content in Microporous Composite Membrane on Gas Permeability and Permselectivity

Microporous silica membrane containing hydrotalcite (HT) was prepared successfully without losing the former’s molecular sieving property. The microporous HT-silica membrane (200 nm in thickness) was formed on the surface of mesoporous γ-alumina layer (9 μm in thickness) and supported by macroporous α-alumina substrate (ca. 2 mm in thickness). The pore size of the microporous HT-silica membrane was 8.5Å, slightly larger than the pristine silica membrane (5Å). The composite membrane was found to enhance the permeability of gases and permselectivity of carbon dioxide from gas mixture comprising methane, hydrogen or nitrogen. Microporous HT-silica membrane with 15 vol.% HT displayed the highest permselectivities in the order of CO₂/CH₄ > CO₂/N₂ > CO₂/H₂ and the permselectivities decreased with increasing HT content.     

Efficient Preparation of Submicrometer α-Alumina Powders by Calcining Carbon-Covered Alumina

A simple method is described to prepare submicrometer α-alumina by burning carbon supported on the surface of γ-alumina in oxygen flow at a temperature of 800°C. The burning of carbon generates a large amount of heat and leads to a rapid increase in the local temperature inside the pores of alumina. When the temperature is high enough for the phase transformation, α-alumina is obtained in a very short time. It was found that, for carbon contents between 6 and 10 wt%, all the γ-alumina could transform into α-alumina after burning of carbon in oxygen for a short time, and the transformed particle sizes of α-alumina were mostly no more than 1 μm.     

Interreaction Between Al2O3 and a CaO-Al2O3 Melt

Poly crystalline and single-crystalline α-alumina were reacted with a eutectic CaO-Al₂O₃ melt at 1530°C. A reaction zone develops in which a strongly textured CA₆ layer, as well as a CA₂ layer, forms, with a remaining layer of unreacted CaO-AI₂O₃ melt. Silica, an impurity in the α-alumina, is rejected by the advancing CA₆ phase and accumulates as calcium alumino-silicates in channels that assist in the reaction as fast transport paths. Reaction mechanisms and welding are briefly discussed.     

Topotactic Growth of β-Alumina on α-Alumina

Thin foils of polycrystalline α-alumina were reacted with a potassium-rich vapor at ≤900°C. Potassium β-alumina formed along α-alumina grain boundaries and protruded from holes in the foils. Conventional transmission electron microscopy was used to analyze the α-alumina/β-alumina phase boundary for possible orientation relations.     

Microstructure,strength and CO2 separation characteristics of α-alumina supported γ-alumina thin film membrane

Abstract

Abstract

In this work, a mesoporous γ-Al₂O₃ membrane was synthesised on an α-Al₂O₃ substrate by a sol–gel dip coating process. The membranes were characterised using SEM, field emission SEM, X-ray diffractometry and N₂ adsorption/desorption measurements (Brunauer–Emmett–Teller analyses). The characterisation results revealed that a flawless γ-alumina membrane layer with 3·5 μm thickness and 2·1 nm average pore size was achieved. Subsequent separation tests indicated that CO₂ could be separated from N₂ by the mesoporous γ-Al₂O₃ membrane. In spite of the difficulties of the separation, the optimised microstructure achieved for the prepared membrane led to desirable and regular permeation behaviour. It was also observed that separation could be more efficient in high pressure permeation conditions. The prepared substrate fulfilled the required strength and permeability for the thin γ-Al₂O₃ membrane film under these conditions. Accordingly, an optimised completely ceramic membrane structure, applicable for CO₂ separation applications, was achieved.     

Enhanced compactness of oriented MFI zeolite films by heat treatment of seed layer

The achievement of compact and defect-free film structure is crucial for the application of b-oriented MFI zeolite film. In this work, a novel heat treatment technique was used to treat zeolite seeded substrates prior to secondary growth. The influence of the heat treatment parameters such as treat temperature and time on the final film morphology were systematically investigated. The relationship between film compactness and the parameters was established. Under the optimized treat temperature of 120?°C and treat time of 1?h, compact and uniform b-oriented MFI zeolite film was achieved. The applicability of the optimum heat treatment condition was validated by employing various film substrates.     

Morphological Forms of α-Alumina Particles Synthesized in 1,4-Butanediol Solution

Phase-pure, monodispersed, hexagonal plates of single-crystal α-alumina (∼ 2 μm wide and ∼0.5 μm thick) have been prepared via precipitation by treating an aluminum hydrous oxide precursor in 1,4-butanediol at 300°C under autogenous vapor pressure. Present work shows that KOH is the only reagent that precipitates an aluminum hydrous oxide precursor suitable to synthesize α-alumina in 1,4-butanediol solution. In contrast, the use of NaOH or NH₄OH as the precipitating reagent for the precursor material does not yield the alpha phase. The solution pH at which the precursor materials are precipitated is also a critical factor for the formation of α-Al₂O₃. Phase-pure α-alumina powders were also only synthesized from the aluminum hydrous oxide precursors precipitated in the pH range from 10 to 10.5. The results of X-ray diffraction and scanning electron microscopy indicate that longer reaction times promote the phase transformation from the intermediate boehmite phase to α-alumina. The complete transformation from boehmite to α-alumina requires reaction times of about 12 h.     

Phase Transformation in Nanometer-Sized γ-Alumina by Mechanical Milling

Nanocrystalline γ-alumina powders of 50-nm size were milled by high-energy ball milling. It was found that the pure γ-alumina phase showed great stability and did not transform to any other phase even after a long milling time. On the other hand, γ-alumina, which contained a small amount of the α-alumina phase, showed a gradual phase transformation from γ- to α-alumina on milling. The phase transformation mechanism during milling appears to be nucleation and growth type and promoted by the α-alumina seed.     

Hydrothermal Synthesis of Submicrometer α-Alumina from Seeded Tetraethylammonium Hydroxide-Peptized Aluminum Hydroxide

Submicrometer α-alumina powder was successfully synthesized from seeded aluminum hydroxide peptized with tetraethylammonium hydroxide (TENOH) and hydrothermally treated at 200°C, using α-alumina particles as seeds. The powders were characterized by XRD, SEM, DTA-TG, and BET analyses. Results showed that seeding could greatly enhance the transition to α-alumina at 200°C without formation of other transient alumina phases. α-Alumina with some amount of boehmite formed in the seeded samples, whereas boehmite was the exclusive phase formed in the nonseeded sample. The morphology of α-alumina embedded in the boehmite matrix for the seeded samples suggests a direct transition from aluminum hydroxide to α-alumina without the formation of transient alumina phases. The formation of α-alumina in the seeded samples at temperatures as low as 200°C could be attributed to a favored nucleation in the TENOH-peptized aluminum hydroxide and to the subsequent hydrothermal treatment that supplies the necessary activation energy for crystal growth. Transition of boehmite to α-alumina in the hydrothermally treated samples with low-seed contents was significantly promoted by heat-treating the samples at 500°C.     

MFI zeolite as a support for automotive catalysts with reduced Pt sintering

The noble metals used as catalysts in automotive exhaust systems are subject to sintering at extreme temperatures, leading to deterioration of catalytic activity. Zeolite with the MFI (ZSM5) structure is examined as a support for Pt particulate catalysts. The MFI structure is composed of agglomerates of single-crystal zeolite with interstitial mesoporosity. Pt fixed within these mesopores is shown through high-temperature aging tests in air to be highly resistant to sintering due to the mechanical constraints on particle size imparted by the mesoporous structure. The deterioration of catalytic activity after aging is significantly lower than that for comparable γ-alumina supported catalyst.     

Sol-Gel Alumina Coating for Improved Cyclic Oxidation Resistance of Silicon Nitride

A 25 nm thick α-alumina layer was deposited on a turbine-grade silicon nitride by sol-gel dip coating and subsequent heat treatment in air at 1200°C. This layer had a nanometer grain structure. Silicon nitride protected by this thin layer showed a significant improvement in oxidation resistance over its uncoated counterpart after 200 cyclic exposures in air at 1250°C. The oxide layer grown on the coated silicon nitride also exhibited superior surface morphology, compared with the uncoated silicon nitride.     

Transitions in Vapor-Deposited Alumina from 300° to 1200°C

The transition of amorphous alumina to α-alumina was studied by X-ray diffraction, electron diffraction, DTA, TGA, and microscopic observation. The amorphous alumina was prepared by condensing vapor from evaporating molten alumina in vacuo onto the glass envelope of the vacuum chamber. The amorphous alumina was transformed to a poorly crystalline material by heating for 16 hr between 570° and 670°C. Between 670° and 1200°C, the poorly crystalline alumina was converted to α-alumina via two parallel series of transition aluminas. The principal series was γ-alumina to δ-alumina to α-alumina. A minor amount of θ-alumina developed from the initial crystallization and persisted throughout the duration of the principal series as a parallel path. Some conversion of δ- to θ-alumina was detected above 900°C. DTA produced an unexplained exothermic peak at 320°C and a second exothermic peak at 860°C which corresponded to formation of metastable aluminas.     

Sintering of Mixtures of Seeded Boehmite and Ultrafine α-Alumina

α-Alumina was fabricated by dry pressing mixtures of seeded boehmite and fine α-alumina (i.e., 0.2 and 0.3 μm diameter) to reduce the large shrinkage of boehmite-derived α-alumina. The maximum green density was obtained with mixtures containing ∼70%α-alumina for both alumina powders. The ∼15% linear shrinkage and microstructures of these samples were comparable to 100% alumina powder samples. Samples with 0.2 μm alumina sintered to densities >95% at 1300°C whereas 1400°C was needed for samples with 0.3 μm alumina. These results indicate that boehmite can be used as a substitute for relatively expensive ultrafine α-alumina powders.     

Synthesis of MFI films on α-alumina at neutral pH

Oriented films of MFI were prepared on α-alumina substrates by a method including substrate seeding and subsequent growth of the seeds in a synthesis gel containing fluoride as mineralizing agent. The films were characterized by scanning electron microscopy and X-ray diffraction. The preparation procedure presented here is promising for the synthesis of highly siliceous MFI films on alumina substrates.     

Effect of Pore Size Distribution of Carbon-Covered Alumina on the Preparation of Submicrometer α-Alumina Powders

Calcining carbon-covered alumina (CCA) samples at 800°C in an oxygen flow is an efficient method to prepare α-alumina powders. It is found that the pore size distribution of CCA samples, which depends on the carbon content and the pore size distribution of the precursor alumina used, is one of the key factors for the total conversion of γ-alumina to α-alumina and the complete combustion of carbon in the pores of alumina. No matter how high the carbon content, total conversion does not occur for CCA samples prepared from alumina possessing the most probable pore size of about 5.2 nm. Using γ-alumina with the most probable pore size of 6.1 nm as the precursor of CCA samples, total transformation occurs when the carbon content of CCA ranges from 11.9 to 17.3 wt%, but the color of as-prepared α-alumina is not pure white but light gray. Polyethylene glycol (PEG 20 000), added to the sucrose/γ-alumina system, can expand the pores of CCA samples after carbonization, and calcining of thus-prepared CCA results in a complete transformation of γ-alumina to pure white α-alumina with a particle size of about 1 μm when the carbon content of CCA is between 6 and 19 wt%.