Α-alumina Support Research Articles

ZIF-8 membranes with improved reproducibility fabricated from sputter-coated ZnO/alumina supports

Zeolitic imidazolate framework-8 (ZIF-8) membrane has shown great potential for propylene/propane separation based on molecular sieving mechanism. Although diverse synthesis strategies were applied to prepare ZIF-8 membranes, it is still a challenge for reproducible fabrication of high-quality membranes. In this study, high-quality ZIF-8 membranes were prepared through hydrothermal synthesis under the partial self-conversion of sputter-coated ZnO layer on porous α-alumina supports. The reproducibility was significantly improved, compared with that from sol–gel coated ZnO layer, due to the highly controllable sputtering deposition of ZnO precursor. The relationship between the quality of as-synthesized membrane and amount of deposited ZnO was also determined. The effect of pressure drop in C3H6/C3H8 separation on separating performance was also examined.

Effect of the pH Value of Seed Coating Solution on Microstructure of Silicalite-1 Zeolite Separation Layer Grown on α-Alumina Support

Effect of the pH Value of Seed Coating Solution on Microstructure of Silicalite-1 Zeolite Separation Layer Grown on α-Alumina Support

A SAPO-5-coated membrane for nanofiltration of tetraalkyl ammonium hydroxide

Nanofiltration of tetraalkyl ammonium hydroxide (TAAH) was investigated with a membrane coated with SAPO-5, a silicoaluminophosphate with a 12-member ring channel. The SAPO-5-coated membrane was prepared on an α-alumina support by a secondary growth method. Rejection of TAAH by the SAPO-5-coated membrane increased with the molecular size of the TAAH, because of the molecular sieve mechanism imparted by the SAPO-5 coating. However, the rejection decreased with increasing operational pressure because of concentration polarization near the membrane surface. The SAPO-5-coated membrane could be repeatedly used for nanofiltration, after backwashing with a dilute hydrochloric solution between cycles.

Preparation of non-permselective sulfated zirconia catalytic membrane for use in a catalytic membrane reactor

In this work, for the first time, preparation of non-permselective sulfated zirconia catalytic membrane on α-alumina support was investigated. In-house synthesized sulfated zirconia nanopowder calcined at 650°C was used for the preparation of catalytic layers. Aluminum triisopropylate and silica nanopowder were utilized to synthesize appropriate coating sols and facilitate layers fabrication via the sol–gel method. The catalytic layers with gradient composition were coated on the surface modified support. The results showed that the sol with strong acid media (pH<2) could not be used for the preparation of layers. However, the coating sol with pH of 3.5 was completely stable, and sintering of the fabricated layers could be done at low temperatures. The prepared catalytic membrane had suitable porous and crack-free microstructure with desirable crystalline phase, indicating proper sintering operation and successfully preventing the tetragonal to monoclinic phase transformation due to the appropriate applied SZ–Al2O3–SiO2 composite structures.

In-situ aging microwave heating synthesis of LTA zeolite layer on mesoporous TiO2 coated porous alumina support

LTA zeolite layer was successfully grown on a superhydrophilic mesoporous titania layer coated onto porous α-alumina substrate. Mesoporous titania layer was formed as an intermediate bridge in the pore size variation between the macroporous α-alumina support and micro-porous LTA zeolite layer. In-situ aging microwave heating synthesis method was utilized to deposit the LTA zeolite layer. Mesoporous titania layer was pre-treated with UV photons and this was observed to have played a major role in improving the surface hydrophilicity of the substrate leading to formation of increased number of Ti–OH groups on the surface. This increase in Ti–OH groups enhanced the interaction between the synthesis gel and the substrate leading to strong attachment of the amorphous gel on the substrate, thus enhancing coverage of the LTA zeolite layer to almost the entire surface of the 1-inch (25.4mm) diameter membrane. LTA zeolite layer was developed via in-situ aged under microwave irradiation to study the effect of synthesis parameters such as in-situ aging time and synthesis time on the formation of the LTA zeolite layer. Optimized process parameters resulted in the formation of crack-free porous zeolite layer yielding a zeolite–titania–alumina multi-layer membrane with a gradient in porosity.

RETRACTED: Preparation and characterization of multilayer mesoporous alumina nano membrane via sol–gel method using new precursors

Abstract In the present study, a mesoporous γ-alumina layer was coated on a macroporous α-alumina support via sol–gel method to produce a multilayer mesoporous alumina membrane with high performance. Different parameters like homogeneity of colloidal solution, additive, working temperature, roughness of the membrane surface, membrane thickness and porosity of the membrane were analyzed. Two types of new different solvents and additives were considered and compared to have a perfect colloidal sol. A crack-free γ-alumina membrane was obtained by adding Poly Vinyl Alcohol to the alumina solution in an optimum conditions. In order to investigate the thermal stability of the membrane, it was dried and heated up to 600 °C. The alumina sols showed particle size distributions in the range of 20–55 nm which was measured by a DLS zeta sizer. The roughness of the sub-layers was measured using a Roughness Checker and density and porosity were measured based on Archimedes principle. X-ray diffraction technique (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to characterize the membrane layer. The thickness of the produced mesoporous membrane was about 4 µm with 16 nm of surface roughness and 5 nm pore size.

Synthesis and performance of B-ZSM-5 zeolite membranes for the separation of volatile organic from water in pervaporation process

ZSM-5 zeolite membrane on porous α-alumina support was prepared by hydrothermal method with B isomorphously substituted into their structure for separating acetone from water by pervaporation. The effects of feed temperature, concentration, flow rate, and permeate pressure on the membrane performance were investigated. The permeate flux increased with the increasing feed concentration, temperature, and flow rate and decreasing permeate pressure. In addition, the results of permeate pressure show that the separation factor is improved with the increasing permeate pressure dramatically. Moreover, this results show an unlike behavior compared to results reported for organics with low volatility. So, it can be concluded that the behavior of pervaporation process is deeply dependent on the volatility of organics when the permeate pressure change.

Stability of ZIF-8 membranes and crystalline powders in water at room temperature

The perceived robust stability of zeolitic imidazolate frameworks (ZIFs) was regarded as an important feature of ZIFs that have intrigued intensive research efforts. However, hydrolysis of ZIF crystals was recently identified under hydrothermal conditions. In this work, the dynamic stability of α-alumina supported ZIF-8 membranes and static stability of both ZIF-8 membranes and powder in water were studied via pervaporation and batch durability tests at room temperature. The sharp increase in water pervaporation flux from 0.04kgm−2h−1 to 1.39kgm−2h−1 and the disappearance of ZIF-8 structure after pervaporation are consistent with morphology and phase structure data showing that ZIF-8 layer on the α-alumina support underwent degradation during water pervaporation. The gradual degradation of ZIF-8 layer was also experimentally observed in batch durability tests in pure water. To better understand the mechanism of stability of ZIF-8 structure, control experiments on hydrolysis of ZIF-8 powder in aqueous conditions were designed and performed. The continuous release of Zn2+ from ZIF-8 structure to the aqueous solution was detected along with the change in pH value of the aqueous system. The presence of 2-methylimidazolate in aqueous solution can stabilize the ZIF-8 structure of both membrane and crystals. These evidences confirm that ZIF-8 crystals can degrade in water due to the dissolution of ZIF-8. The rate and extent of the dissolution depends on the ZIF-8 crystal to water mass ratio.

Synthesis and characterization of ordered mesoporous silica membrane: Role of porous support and gas permeation study

In this work, the supported silica membranes were synthesized on various self-made symmetrical α-alumina supports having different pore sizes by spin coating method. The final synthesized membranes were further characterized by XRD, FESEM and FTIR analysis. Gas separation properties of the resultant supported ordered mesoporous silica membranes were evaluated by single gas permeation experiments. The results showed that the lower pore sized α-alumina supports can effectively reduce the surface defects of the silica layer and enhance the gas permeation properties of the supported silica membranes. The orders of the silica structure were also improved by reducing the pore size of the α-alumina supports. Steady-state single gas permeation using CO2, Ar, He and N2 gases showed Knudsen diffusion mechanism for the membrane synthesized on the α-alumina support having lowest pore size of ∼0.09 μm. The other membranes having higher pore sized supports obeyed both Knudsen as well as viscous diffusion mechanisms. In addition, mixed gas permeation (20% CO2 and 80% N2) studies were carried out using amino functionalized mesoporous silica membrane.

Influence of the composition/texture of Mn–Na–W catalysts on the oxidative coupling of methane

Mn–Na–W silica supported catalysts are highly selective toward ethylene and ethane in the oxidative coupling of methane (OCM) process. However, they present lower activities than La/Sr/CaO catalysts and subsequently a lower ethylene productivity. Optimisation of the Mn–Na–W catalysts has been focused within the range of 2–8wt% of active elements relative to the support, while silica is partially converted into inert cristobalite. This study focused on the increase of the activity of Mn–Na–W catalysts while maintaining their intrinsically high C2 selectivity. The increase in the concentration of the active elements with respect to the support revealed that the improvement of the catalyst activity was possible but limited. A high concentration of Mn–Na–W favours the formation of a smooth cristobalite phase and strongly decreased the number of active sites.The silica support was replaced by silica-free supports to limit the formation of cristobalite. The use of SiC as a support led to the formation of cristobalite near the surface of the catalysts but the porous SiC skeleton, which remained non-oxidised at the core of the catalysts, allowed maintaining a high surface area. SiC supported catalysts thus presented higher activities than silica catalysts. The use of α-alumina support led to catalysts with lower performances and revealed that exposed alumina was detrimental for the catalyst performance.

Helium permeation through a silicalite-1 tubular membrane

A silicalite-1 tubular membrane was prepared on the inner surface of a porous α-alumina support. Helium permeation at different feed volumetric flows (11–41 mL/min) with different sweep flow rates (9–90 mL/min) at STP conditions was measured. The molar fraction was obtained as a function of the residence time ratio. The influences of the geometric parameters of the tubular system and the feed flow rates on the permeation through the membrane were investigated. The dependence of the permeances with the residence time ratio was experimentally obtained and we propose that this dependence is a useful design criterion for tubular membrane permeation systems. The best results in this work were obtained for Q He,in /Q N2,in = 0.22 for V SS /V TS = 7.3. Also, the data showed that an appropriate combination of the flows and the area sections of the system resulted in an optimum value for the Peclet number of 0.3. The experimental data were reproduced by numerically solving the Maxwell–Stefan equations under the assumption that transport across the membrane can be modeled in terms of a Robin-type boundary condition.

Synthesis and characterization of ceramic/carbon nanotubes composite adsorptive membrane for copper ion removal from water

We prepared a novel adsorptive membrane by implanting carbon nanotubes (CNTs) in pore channels of ceramic (α-alumina) support via chemical vapor deposition (CVD) method using cyclohexanol and ferrocene as carbon precursor and catalyst, respectively. Optimization of CNTs growth conditions resulted in uniform distribution of the CNTs in the pore channels of the support. The optimized CNTs-ceramic membrane was oxidized with concentrated nitric acid, and chitosan was employed for filling intertube-CNT gaps. The modified CNTs-ceramic membrane was used for copper ion removal from water, and the effects of the modification steps (oxidation and filling intertube- CNT gaps with chitosan) and pH on permeation flux and rejection of the prepared adsorptive membrane were investigated. Moreover, static adsorption was also investigated and Langmuir and Freundlich isotherms and two kinetics models were used to describe adsorption behavior of copper ions by the prepared adsorptive membrane.

Effect of Seed Coating Layer on the Microstructure of NaA Zeolite Separation Layer Grown on α-alumina Support

Effect of Seed Coating Layer on the Microstructure of NaA Zeolite Separation Layer Grown on α-alumina Support

Surface modification of α-alumina support in synthesis of silica membrane for hydrogen purification

In this work, an experimental study was carried out on the synthesis of silica membrane for hydrogen purification, in which synthesis of γ-alumina intermediate layer using cheaper and safer source was investigated. For this purpose, aluminum hydroxide was selected and the bohmite sols were prepared by acid or base catalyzed hydrolysis of the different salts for comparing with alkoxide source. The SEM micrographs showed no distinct γ-alumina layer on the substrate coated by base catalyzed sol of salt (sample 1), while a homogeneous γ-alumina layer was formed by acid catalyzed sol of salt (sample 2). After γ-alumina layer formation, the gas permeance mechanism was approximately changed. These results were similar to SEM results and N2 permeance experiments of sample 3 in which substrate was coated with alkoxide sol. However, the γ-alumina layer of sample 2 had no good adhesion to the substrate. Nevertheless, use of aluminum hydroxide can be promised to synthesis of γ-alumina layer; the membrane was synthesized on the modified support with aluminum tri-sec-butylate sol. In particular, in the synthesized silica membrane as the temperature increases, permselectivity of H2/CO2 and H2/N2 increases from 4.7 and 7.3 at room temperature to 9.4 and 11.6 at 100 °C and to 23.4 and 31.3 at 200 °C, respectively.

Development of a hybrid membrane through coupling of high selectivity zeolite T on ZIF-8 intermediate layer and its performance in carbon dioxide and methane gas separation

This work demonstrated a novel hybrid membrane development through coupling of zeolite T membrane on ZIF-8 intermediate layer. ZIF-8 which acted as an intermediate layer was firstly grown on a macroporous α-alumina support via vacuum seeding. Pure phase zeolite T membrane was then synthesized on ZIF-8 which was pre-grown on the α-alumina support. Single gas permeation of standing free zeolite T membrane showed a CO2/CH4 selectivity of 68 with CO2 and CH4 gas permeances of 6.08×10−8 and 0.09×10−8mol/m2.s.Pa, respectively. On the other hand, the CO2/CH4 selectivity of the hybrid membrane was 3 folds as compared to standing free zeolite T membrane, giving the highest CO2/CH4 selectivity of 182 and the CO2 and CH4 gas permeances of 5.46×10−8 and 0.030×10−8mol/m2.s.Pa, respectively. Both gas permeances of CO2 and CH4 increased with increasing feed pressure from 1 to 4bar with a slight decrease in the selectivity observed at the feed pressure of 4bar. While for the effect under increasing temperature, CO2 showed an increasing trend of its gas permeability from 5.46×10−8 to 6.12×10−8mol/m2.s.Pa while CH4 gas permeability increased slightly from 0.03×10−8 to 0.042×10−8mol/m2.s.Pa due to greater mobility of gas molecules promoted by higher temperature.

Low temperature synthesis of NaA zeolite membranes: The effect of primary and secondary crystallizations

NaA zeolite membranes were prepared on porous α-alumina support tubes (inner side) by primary (in-situ) and secondary (ex-situ) crystallization processes at 65°C for 6h, 12h and 24h each. For secondary crystallization process, poly(ethyleneimine) (PEI) was used as a buffer layer for proper attachment of the NaA seed crystals (prepared at 65°C/2h) onto the alumina support surface. The membranes prepared by primary crystallization process showed thicknesses of about 10μm, 20μm and 30μm for growth times of 6h, 12h and 24h, respectively, while the membranes prepared by secondary crystallization process rendered the corresponding thicknesses of 25μm, 35μm and 70μm, respectively. PEI coating rendered good quality of membrane for secondary crystallization process with lower growth time (6h). The better quality of membrane which had interlocked dense structure with minimum defects was obtained for primary crystallization at 65°C for 12h growth time. Pervaporation study for ethanol/water mixture was performed with the prepared membranes.

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.

Sulfur resistant Pd and Pd alloy membranes by phosphidation

Novel ultra-thin Pd and Pd-alloy (Pd–Ag, Pd–Cu) membranes, with a phosphorus component, were prepared on a porous hollow fiber α-alumina support. The aim was to study the effect of the presence of a P additive on the behavior of the membranes towards sulfur poisoning. To incorporate P in the structure, the membranes were exposed to PH3 as a gaseous phosphorous source. The performance and sulfur resistance of the membranes with and without P was evaluated by measuring the hydrogen permeance and the H2/N2 selectivity at 673K before and during exposure to a gas mixture of 100ppm H2S in H2, and also after regeneration in H2. The membranes were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), both before exposure to H2S and after exposure/regeneration. XRD shows no detectable amounts of sulfur or phosphorus compounds in any of the exposed or P-treated membranes respectively. XPS indicates that S was located on the surface of the H2S/H2 treated Pd and Pd–Ag membranes. No S was detected on the surface of the Pd–Cu membranes indicating that no irreversible S compounds were formed in the latter. P was also detected on the surface of the P-treated membranes. The presence of phosphorus lowered the hydrogen permeance and H2/N2 selectivity; however it drastically improved the Pd-alloy membranes' capacity of recovery in H2 after H2S poisoning. The Pd–Cu membrane recovered only part of its H2 permeance after regeneration, going from 11.2×10−7molm−2s−1Pa−1 (as-prepared) to 2.3×10−7molm−2s−1Pa−1, while in the presence of P the H2 permeance varied from 5.2×10−7molm−2s−1Pa−1 (as-prepared) to 6.5×10−7molm−2s−1Pa−1 and its H2/N2 selectivity increased from 30,000 to 40,000. Similarly, the Pd–Ag membrane recovered better in the presence of P going from 17×10−7molm−2s−1Pa−1 (as-prepared) to 6.3×10−7molm−2s−1Pa−1 in contrast to 52×10−7molm−2s−1Pa−1 (as-prepared) to 5.9×10−7molm−2s−1Pa−1 in the absence of P. SEM images show that in the absence of P, more cracks and pinholes are visible on the surface of the H2S/H2 exposed membranes. The presence of P produces more homogeneous surfaces and less segregation, conferring a structural integrity to the membrane by suppressing the formation of cracks and pinholes.

Separation of tetramethyl ammonium hydroxide using an MFI-type zeolite-coated membrane

Nanofiltration of tetramethyl ammonium hydroxide (TMAH) was investigated using an MFI-type zeolite-coated membrane. The membrane could be successfully prepared on an α-alumina support by hydrothermal synthesis. The MFI-type zeolite powder possesses a high adsorption capacity for TMAH at all pH values, with maximum adsorption amount determined by Langmuir adsorption mechanism of 0.593mmol/g at pH=11.63, because it presents a negatively charged surface. Nanofiltration of TMAH can thus be achieved through both molecular sieve and adsorption mechanisms by MFI-type zeolite. The MFI-type zeolite-coated membrane can be repeatedly used for nanofiltration, with backwashing with a dilute hydrochloric solution between cycles.

Synthesis of a composite inorganic membrane for the separation of nitrogen, tetrafluoromethane and hexafluoropropylene

The advanced use of inorganic membranes, such as zeolites, in large-scale industrial processes is hindered by the inability to manufacture continuous and defect-free membranes. We therefore aimed to construct such a defect-free membrane. Various zeolites were synthesised on the inner surface of α-alumina support tubes by a hydrothermal process. Gas permeation properties were investigated at 298 K for single component systems of N2, CF4 and C3F6. Ideal selectivities lower than Knudsen selectivities were obtained as a result of defects from intercrystalline slits and crack formation during synthesis and template removal. A composite ceramic membrane consisting of a ceramic support structure, a mordenite framework inverted intermediate zeolite layer and a Teflon AF 2400 top layer was developed to improve separation. The Teflon layer sealed possible defects present in the separation layer forcing the gas molecules to follow the path through the zeolite pores. Ideal selectivities of 88 and 71 were obtained for N2/CF4 and N2/C3F6 respectively. Adsorption experiments performed on materials present in the membrane structure suggested that although adsorption of C3F6 onto Teflon AF 2400 compared to CF4 results in a considerable contribution to permeation for the composite ceramic membrane, the sealing effect of the zeolite layer by the Teflon layer is the reason for the large N2/CF4 and N2/C3F6 selectivities obtained. The Teflon layer effectively sealed intercrystalline areas in-between zeolite crystals, which resulted in high ideal selectivies for N2/CF4 and N2/C3F6.