Effects of Synthesis Temperature and Support Material on CO2 and CH4 Permeation through SAPO-34 Membranes

In this research, continuous SAPO-34 membranes were synthesized via secondary growth method onto both α-Al₂O₃ and mullite supports at three levels of synthesis temperature: 185, 195, and 220°C for 24 h. The synthesized membranes were characterized using XRD and SEM analysis and single gas permeation experiments. It was found out that support material and synthesis temperature both have significant effects on the membrane performance. At higher synthesis temperature, SAPO-34 crystals grown over the mullite support become more uniform and smaller in size but those grown on the α-Al₂O₃ support become larger. Effect of synthesis temperature on single gas permeation properties of the synthesized SAPO-34 membranes was also studied. For the mullite supported membranes, the CH₄ and CO₂ permeances decrease as synthesis temperature increases; but in the case of the alumina supported membranes, by increasing synthesis temperature, CH₄ and CO₂ permeances first decrease up to 195°C and then increase up to 220°C. Even in equal membrane thicknesses, the mullite supported membrane shows lower gas permenaces. Increasing synthesis temperature decreases CO₂/CH₄ ideal selectivity for the α-Al₂O₃ supported membranes, while increases for the mullite supported membranes. Under optimum synthesis conditions, at room temperature and 2 bar feed pressure, the CO₂ permeance through the α-Al₂O₃ and the mullite supported SAPO-34 membranes are 8.2 × 10^?7 and 8.5 × 10^?8 (mol/m² · s · Pa), respectively, and CO₂/CH₄ ideal selectivities are 51 and 61, respectively.     

Preparation of Crack-Free Nanocomposite Ceramic Membrane Intermediate Layers on α-alumina Tubular Supports

The objective of this work was to develop a crack-free membrane intermediate layer on tubular ceramic supports via dip-coating. TiO₂ submicron powder, Boehmite as a coupling agent, and SiO2 nanopowder as low melting-point sintering aid were used to deposit a thin and crack-free nanocomposite layer onto α-Al₂O₃ supports. Effects of key parameters such as solid content, number of coated layers, the presence of coupling agent, and a low melting point nanopowder on physicochemical properties like the thickness, microstructure, pore size, pure water flux, and gas permeance of the final modified ceramic supports were investigated. The obtained results showed that after twice coating of support in 3 wt.% bidispersed TiO₂-Boehmite suspension, the pore size of the alumina substrate, ～0.6 micron, was reduced to ～0.1 by the uniform membrane intermediate layers with low permeation resistance could be prepared. Moreover, by adding SiO₂ nanopowder, sintering temperature of intermediate layers decreases considerably (1000 to 700°C).     

Critical zeta potential and the Hamaker constant of oxides in water

The critical zeta potential characterises the flocculated-dispersed state transition of a colloidal dispersion. For many colloidal dispersions, yield stress displayed a linear relationship with the square of zeta potential, indicating that they obeyed the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. From this relationship, the critical zeta potential is obtained from the intercept at the zeta potential axis at a yield stress of zero. The critical zeta potential is a measure of the repulsive potential required to exactly counter the maximum attractive potential between particles in dispersion in the flocculated state. When the forces of interaction between particles in the dispersion are only the van der Waals and electrostatic forces, then the critical zeta potential is indirectly a measure of the van der Waals attractive potential and, hence, it may be used to determine the Hamaker constant of solids in water. This potential is proportional to the square root of the solids Hamaker constant in water. At present, only the ratio of Hamaker constant between two oxides was obtained and compared with that obtained by other techniques. These oxides were ultrapure anatase TiO₂ and γ-Al₂O₃, and they displayed a linear relationship between yield stress and the square of zeta potential. At the conductivity (or ionic strength) of about 3000 μS/cm, the critical zeta potential for both TiO₂ and Al₂O₃ is ∼47 and ∼32 mV, respectively. These critical zeta potential data give a value of 2.2 for the ratio of Hamaker constant of anatase TiO₂/H₂O/TiO₂ to γ-Al₂O₃/H₂O/γ-Al₂O₃. This ratio compares well with a value ranging from 1.0 to 2.18 for rutile TiO₂/H₂O/TiO₂ to α-Al₂O₃/H₂O/α-Al₂O₃ where their Hamaker constants were calculated from the Lifshitz theory using full optical spectral data.     

Microstructural investigations of tubular α-Al2O3-supported γ-Al2O3 membranes and their hydrothermal improvement

γ-Al₂O₃ meso-porous membranes supported by tubular α-Al₂O₃ substrates were prepared by using the sol-gel method and their nanostructural characterizations were performed for the first time with high-resolution transmission electron microscopy (HRTEM) before and after hydrothermal treatment at 500 °C. The HRTEM images and pore size distribution (PSD) analyses revealed that the morphologies as well as the characteristics of the powder and membrane samples prepared from the same boehmite are not identical. γ-Al₂O₃ and La₂O₃-Ga₂O₃ doped-γ-Al₂O₃ (LGA) membranes supported by α-Al₂O₃ were also fabricated and characterized under thermal and hydrothermal conditions for the purpose of comparisons. Finally, two type α-Al₂O₃/γ-Al₂O₃/SiO₂ (AA-SiO₂) and α-Al₂O₃/La₂O₃-Ga₂O₃-γ-Al₂O₃/SiO₂ (ALGA-SiO₂) membranes have been synthesized and the gas permeance of the membrane were measured in the temperature range 100–500 °C.     

Hydrodesulphurization performance of NiW/TiO2-Al2O3 catalyst for ultra clean diesel

TiO₂-Al₂O₃ binary oxide supports were obtained by sol–gel methods from Tetra-n-butyl-titanate and pseudoboehmite/aluminium chloride resources. The typical physico-chemical properties of NiW/TiO₂-Al₂O₃ catalysts with different TiO₂ loadings and their supports were characterized by means of BET, XRD and UV–vis DRS, etc. The BET results indicated that the specific surface areas of NiW/TiO₂-Al₂O₃ catalysts were as higher as that over pure γ-Al₂O₃ support, and the pore diameters were also large. The XRD and UV–vis DRS analyzing results showed that the Ti-containing supported catalysts existed as anatase TiO₂ species and the incorporation of TiO₂ could adjust the interaction between support and active metal, and impelled the higher reducibility of tungsten. The hydrodesulphurization (HDS) performance of the series catalysts were evaluated with diesel feedstock in a micro-reactor unit, and the HDS results showed that NiW/TiO₂-Al₂O₃ catalysts exhibited higher activities of ultra deep hydrodesulphurization of diesel oil than that of NiW/Al₂O₃ catalyst. The optimal TiO₂ content of NiW/TiO₂-Al₂O₃ catalysts was about 15 m%, and the corresponding HDS efficiency could reach to 100%. The sulphur contents of diesel products over NiW/TiO₂-Al₂O₃ (from pseudoboehmite/AlCl₃) catalysts with suitable TiO₂ content could be less than 15 ppmw, which met the sulphur regulation of Euro IV specification of ultra clean diesel fuel.     

Comparison of Ethylene Glycol Steam Reforming Over Pt and NiPt Catalysts on Various Supports

Steam reforming of ethylene glycol (EG) was studied on Pt and NiPt catalysts supported on γ-Al₂O₃, TiO₂, and carbon. On all supports bimetallic NiPt catalysts show higher activity for H₂ production than the corresponding Pt catalysts as predicted from model surface science studies. The kinetic trends are similar for all catalysts (Pt and NiPt) with the H₂ production rate being zero-order and fractional order with respect to water and ethylene glycol, respectively. Slight differences in selectivity to minor products are observed depending both on active metal and support. On γ-Al₂O₃, NiPt shows higher H₂ and less alkane formation than Pt. TiO₂ supported catalysts show increased water-gas shift activity but also increased selectivity to alkane precursors. NiPt/C is identified as an active and selective catalyst for EG reforming.     

Yield stress and zeta potential of washed and highly spherical oxide dispersions — Critical zeta potential and Hamaker constant

The linear relationship between yield stress and the square of the zeta potential based on the yield stress-DLVO force model, was used to determine the zeta potential at the point of transition from flocculated to dispersed state of a range of oxide dispersions. The critical zeta potential at this point for washed α-Al₂O₃, TiO₂ and ZrO₂ dispersions was of magnitude 40, 49 and 52 mV respectively. For highly spherical silica and alumina dispersions, this value was 23 and 38 mV respectively. The square of the critical zeta potential is proportional to the Hamaker constant of the oxide in water when the van der Waals force is the only attractive force in play. Thus the critical zeta potential data obtained allowed the Hamaker constant ratio between the three oxide dispersions to be determined. This ratio between rutile TiO₂/water and α-Al₂O₃/water was 1.50. In comparison, a similar value of 1.46 was obtained for the ratio calculated from Hamaker constant value determined via Lifshitz theory. The ratio between rutile TiO₂/water and ZrO₂/water is ∼ 0.90. Using the Hamaker constant of rutile TiO₂/water of 61.2 zJ as the standard, the Hamaker constant determined by our method is 41 zJ for α-Al₂O₃, 68 zJ for ZrO₂ and 13.6 zJ for silica.     

γ-Al2O3中孔陶瓷膜的制备及表征

以粒径为0.3～0.4 μm的α-Al₂O₃为原料,通过悬浮液真空抽吸法,制备出平均孔径约为70 nm的完整无缺陷的片状α-Al₂O₃支撑体;以仲丁醇铝为前驱体,采用颗粒溶胶路线制备出稳定的Boehmite溶胶,以此溶胶采用浸浆法,在制备的α-Al₂O₃支撑体上制备出完整无缺陷的γ-Al₂O₃中孔膜,并考察了烧成温度对γ-Al₂O₃中孔膜性能的影响。结果表明,本文制备出的γ-Al₂ O₃膜的孔径约为3 nm,对PEG的截留分子量为2800～5300,纯水渗透通量为11.5～25.9 L.m^-2.h^-1［7.6×10⁵ Pa,（14±1）℃］。说明在孔径为70 nm左右的载体上直接制备孔径为3 nm的完整的中孔膜是可行的。     

Synthesis of α-Al2O3 at mild temperatures by controlling aluminum precursor,pH, and ethylenediamine chelating additive

This study demonstrates the synthesis of α-Al₂O₃ by sol–gel method according to various reaction parameters. Various Al₂O₃ phases were synthesized by a simple sol–gel method using three different aluminum precursors (aluminum isoporoxide (AIP), Al(OH)₃, and AlO(OH)) and pHs (3, 7, and 9). Thermally treating of the synthesized powders at 1200 °C produced rhombohedral structure α-Al₂O₃. When AIP was used as an aluminum precursor, α-Al₂O₃ was synthesized at all pH levels by calcination at 1200 °C. The structure was easily changed to α-Al₂O₃ by the addition of ethylenediamine as a chelating additive at the lower temperature of 1000 °C. In contrast, no α-Al₂O₃ structure was obtained by using Al(OH)₃ or AlO(OH) precursors at higher pH in spite of thermal treatment at 1200 °C. The specific surface areas were larger in α-Al₂O₃ synthesized using AIP precursor compared with that using Al(OH)₃ and AlO(OH) precursors. Electrophoretic light scattering (ELS) measurement in aqueous solution at pH=7 revealed positive surface charges in the α-Al₂O₃ synthesized using AIP precursor, but negative charges in that synthesized using Al(OH)₃ and AlO(OH) precursors. Most significantly, the α-Al₂O₃ synthesized with the ethylenediamine chelating additive had a negative charge, despite the use of AIP precursor, with a higher mobility and larger aggregated particle diameter.     

Morphology control of α-Al2O3 platelets by molten salt synthesis

It is of great importance to control the morphology of α-Al₂O₃ plate-like powders since α-Al₂O₃ platelets with different shapes are needed in various applications. This paper was focused on how to control the morphology of α-Al₂O₃ platelets by molten salt synthesis. Results show that the morphology of α-Al₂O₃ platelets is affected by the heating temperature, heating time, the molten salts species, the weight ratio of salt to powders, additives and the addition of nano-sized seeds. Especially, it is very effective to control the morphology of α-Al₂O₃ platelets by adjusting the addition of additives such as Na₃PO₄·12H₂O and TiOSO₄. α-Al₂O₃ flakes with irregular shape are obtained by the addition of Na₃PO₄·12H₂O, while thick α-Al₂O₃ particles with hexagonal shape are obtained by the addition of TiOSO₄. The combination addition of Na₃PO₄·12H₂O and TiOSO₄ makes it possible to obtain thin α-Al₂O₃ platelets with discal shape. A small amount of nano-sized seeds addition also has a strong effect on the size of α-Al₂O₃ platelets. However, if the seeds are added too much, the overlapping and abnormal crystal growth of α-Al₂O₃ platelets occur, and the size distribution becomes nonuniform. The effect mechanism of additives and seeds on the morphology of α-Al₂O₃ platelets was also discussed in this paper.     

Preparation of meso-macroporous α-alumina using carbon nanotube as the template for the mesopore and their application to the preferential oxidation of CO in H2-rich gases

Meso-macroporous α-Al₂O₃ was successfully prepared by using acid-treated carbon nanotube as mesoporous forming agent and polystyrene foam as the template for the macropore. A series of Ru/meso-macroporous α-Al₂O₃ catalysts were prepared by the incipient wetness impregnation method and applied to the preferential oxidation of CO (CO-PROX) in H₂-rich gases. SEM, N₂ adsorption–desorption, H₂-TPR techniques and TEM were employed to characterize the catalysts. The results indicate that the specific surface area was markedly elevated by introducing the mesopores, which led to the higher dispersion of ruthenium nanoparticles on the surface of α-Al₂O₃. The meso-macroporous α-Al₂O₃ supported ruthenium showed very high activity and selectivity for CO-PROX.     

Ti addition effect on α-Al2O3 flakes synthesis using a mixture of boehmite and potassium sulfate

Single-crystal α-Al₂O₃ hexagonal flakes with a diameter of about 200 nm and 20 nm in thickness were obtained by mixing different molar ratios of potassium sulfate to boehmite and heating at 1000 °C. Co-doping 1 mol% TiO₂ can increase the shape anisotropy of α-Al₂O₃ hexagonal flakes, increasing the diameter to 400 nm. The effects of potassium sulfate, Fe₂O₃ and TiO₂ on the phase transformation and morphology development of alumina were investigated using X-ray diffraction analysis (XRD), differential thermal analysis (DTA) and transmission electron microscopy (TEM). The results indicate that co-doping potassium sulfate, Fe³⁺ and Ti⁴⁺ can promote γ → α-Al₂O₃ phase transformation and change the morphology from a vermicular structure into hexagonal platelets. The shape anisotropy of α-Al₂O₃ hexagonal flakes can be increased by adding TiO₂ due to the segregation of Ti⁴⁺ ions onto the surfaces of basal planes of α-Al₂O₃ single crystal particle.     

Synthesis of plate-like α-Al2O3 single-crystal particles in NaCl–KCl flux using Al(OH)3 powders as starting materials

Plate-like α-Al₂O₃ single-crystal particles were successfully synthesized in NaCl–KCl flux using Al(OH)₃ powders as starting materials, and the influence of pre-calcining of Al(OH)₃ powders on the phase formation and morphology of α-Al₂O₃ powders was focused. When Al(OH)₃ powders are used as starting materials, the synthesized product at 900 °C is mainly composed of α-Al₂O₃ and κ-Al₂O₃, and most synthesized particles show alveolate morphology. At 1100 °C, single-phase α-Al₂O₃ powders are developed, in which there are many aggregations of intensively bound plate-like particles. In contrast, using porous amorphous Al₂O₃ powders obtained by pre-calcining Al(OH)₃ powders at 550 °C for 3 h as the starting material, plate-like α-Al₂O₃ single-crystal particles can be well developed above 900 °C. The reason of the influence of pre-calcining of Al(OH)₃ powders on the phase formation and morphology of α-Al₂O₃ powders is also discussed in the paper.     

Synthesis of ultrafine α-Al2O3 powder by two-step hydrolysis

The ultrafine α-Al₂O₃ powder has been successfully synthesized via two-step hydrolysis of aluminum isopropoxide. The effects of synthesis parameters on the α-Al₂O₃ were investigated. The experimental results indicated that the concentration of aluminum isopropoxide, water bath temperature, the molar ratio of aluminum isopropoxide to isopropanol and aluminum isopropoxide to deionized water greatly affect the amount of nuclei and the microstructure of α-Al₂O₃. Additionally, the water bath temperature affects the rate of nucleation and growth of crystals. Transmission electron microscopy (TEM) images display that the well-dispersed α-Al₂O₃ powder with particle size about 100 nm was obtained via the method after calcination at 1200 °C for 1 h. The coalescence of crystal particles led to the formation of vermicular α-Al₂O₃ particles. The X-ray diffraction (XRD) and Fourier transform infrared spectra (FTIR) analysis demonstrated the possible phase transition process of α-Al₂O₃. The scanning electron microscopy (SEM) images of samples calcined at various temperatures showed the microstructure transformation. Brunauer-Emmett-Teller (BET) surface area and mean pore size of the samples varied with the calcination temperature. The addition of cetylrimethylammonium bromide (CTAB) has impacts on the phase transformation of α-Al₂O₃.     

A model for ceramic membrane formation by dip-coating

The wet ceramic membrane formation on a porous substrate in the dip-coating process plays a key role for high quality separation layers of the asymmetric membranes. A model to quantitatively describe the membrane growth under a driving force of the capillary filtration is derived on the basis of the slip-casting model. As an example, α-Al₂O₃ microfiltration membranes were prepared by dip-coating. The influence of the dipping time on the membrane thickness was examined. The data for α-Al₂O₃ microfiltration membranes fit the predictions made by the model well. In addition, the results of γ-Al₂O₃ membranes demonstrate that this model is also valid for ultrafiltration membrane formation by dipping sols.     

Effect of carbon and α-Al2O3 on controlled synthesis of AlON powder

The α-Al₂O₃/C mixtures were prepared by ball milling, and then AlON powders were synthesized by carbothermal reduction and nitridation (CRN). The effects of α-Al₂O₃ particle size, carbon powders morphology and particle size on the morphology and particle size of the synthesized AlON powders were studied. The results showed that as the particle size of α-Al₂O₃ increases, the particle size of the synthesized AlON powders will also increase, but the surface morphology will not be affected. The increase of the carbon particle size will increase the particle size of the synthesized AlON powders. The pore size and number of pores of the synthetic AlON powders were very similar to the morphological characteristics of the carbon powders. In addition, the mechanism of controlling the synthesis of AlON powder with α-Al₂O₃ and carbon as raw materials was also discussed.     

NiO/Al2O3 oxygen carriers for chemical-looping combustion prepared by impregnation and deposition-precipitation methods

Ni-based oxygen carriers (OC) with different NiO content were prepared by incipient wet impregnation, at ambient (AI), and hot conditions (HI) and by deposition-precipitation (DP) methods using γ-Al₂O₃ and α-Al₂O₃ as supports. The OC were characterized by BET, Hg porosimetry, mechanical strength, TPR, XRD and SEM/EDX techniques. Reactivity of the OC was measured in a thermogravimetric analyzer and methane combustion selectivity towards CO₂ and H₂O, attrition rate, and agglomeration behavior were analyzed in a batch fluidized bed reactor during multicycle reduction-oxidation tests.XRD and TPR analysis showed the presence of both free NiO and NiAl₂O₄ phases in most of the OC. The interaction of the NiO with the alumina during OC preparation formed NiAl₂O₄ that affected negatively to the OC reactivity and methane combustion selectivity towards CO₂ and H₂O during the reduction reaction. The NiO-alumina interaction was more affected by the support type than by the preparation method used. The NiO-alumina interaction was stronger in the OC prepared on γ-Al₂O₃.The OC were evaluated in the fluidized bed reactor with respect to the agglomeration process. OC prepared by the AI and HI methods with NiO contents up to 25 wt%, OC prepared by the DP method on γ-Al₂O₃ with NiO content lower than 30 wt%, and OC prepared by the DP method on α-Al₂O₃ with a NiO content lower than 26 wt% did not agglomerated. OC that agglomerated showed an external layer of NiO over the particles. It seems that the most important factor affecting to the formation of the external NiO layer on the OC, and so to the agglomeration process, was the metal content of the OC. The attrition rates of the OC prepared using γ-Al₂O₃ as support were higher than the ones prepared using α-Al₂O₃ as support, and in general the attrition rates of all the OC were low.The OC prepared by AI, HI or DP methods on α-Al₂O₃ as support had appropriated characteristics to be used in the chemical-looping combustion process.     

NMR study on reaction processes from aluminum chloride hydroxides to alpha alumina powders

Starting from gelatinous aluminum chloride hydroxide, the transformation process toward α-Al₂O₃ was examined using ²⁷Al NMR, both in the liquid and solid states, as a main analytical tool. By increasing the hydrolysis ratio (h, defined as [OH⁻]/[Al³⁺]) of the starting aqueous precursor up to h = 2.5, the transition temperature to the final product, α-Al₂O₃, decreased to as low as 500°C. In this case, the structural change from amorphous alumina to α-Al₂O₃ took place without intermediate transition Al₂O₃ phases. Examining the process of networking during the transition from aqueous sol–through the state of xerogel–to final anhydrous oxide by nuclear magnetic resonance (NMR) revealed the presence of highly polymeric species mainly ascribed to δ-[Al₂O₈Al₂₈(OH)₅₆(H₂O)₂₄]¹⁸⁺ (δ-Al₃₀). δ-Al₃₀ species were found in the solution phase and became predominant after drying. We conclude that the lower temperature synthesis of α-Al₂O₃ became possible due to preformation of polymerized AlO₆ construction units in the precursor, reducing the energy barrier for the nucleation of the final α-Al₂O₃ phase.     

Novel Au/TiO2/Al2O3 · xH2O catalysts for CO oxidation

Au/Al₂O₃ · xH₂O and Au/TiO₂/Al₂O₃ · xH₂O (x = 0–3) catalysts were prepared by assembling gold nanoparticles on neat and TiO₂-modified Al₂O₃, AlOOH, and Al(OH)₃ supports, and their catalytic activity in CO oxidation was tested either as synthesized or after on-line pretreatment in O₂–He at 500 °C. A promotional effect of TiO₂ on the activity of gold catalysts was observed upon 500 °C-pretreatment. The catalyst stability as a function of time on stream was tested in the absence or presence of H₂, and physiochemical characterization applying BET, ICP-OES, XRD, TEM, and ²⁷Al MAS NMR was conducted.     

Fabrication and microstructural characterization of the novel optical ceramic consisting of α-Al2O3@amorphous alumina nanocomposite core/shell structure

In this study, α-Al₂O₃@amorphous alumina nanocomposite core-shell structure was synthesized from AlCl₃ and the commercial α-Al₂O₃ nanoparticles as the starting materials via a wet chemical route. The results indicated that the shell material mainly comprised of ammonium chloride and boehmite phases. Boehmite was transformed to the amorphous and γ-Al₂O₃ phases after the calcination process and the shell material was completely converted to γ-Al₂O₃ at 1000?°C. However, for the α-Al₂O₃@amorphous alumina core-shell nanoparticles were completely converted to α-Al₂O₃ at 1000?°C. It can be concluded that α-Al₂O₃ core particles, as the seed crystalline, help to transforming of γ-Al₂O₃ phase as the shell material directly without forming transitional phases to α-Al₂O₃. The optical polycrystalline alumina was fabricated using spark plasma sintering of α-Al₂O₃@amorphous alumina core-shell nanocomposite. The body sintered has a final density of ～99.8% and the in-line transmittance value is ～80% within the IR range.