Preparation of modified γ-alumina as stationary phase in gas–solid chromatography and its separation performance for hydrogen isotopes

On the basis of impregnation method, several stationary phases were prepared using γ-Al₂O₃ with the solution of transition metal salts and the breakthrough curves of gas chromatograph for H₂ isotopes were analyzed under the temperature of liquid nitrogen. The effects of carrier gas, flow rate and doping concentration on the separation performance for H₂ and D₂ were systematically investigated. The overall results showed that the surface areas and adsorptive capacities of modified γ-Al₂O₃ were slightly lower than unmodified one while the separation performance and symmetry of chromatographic peaks of the former were more excellent. In addition, the chromatographic peaks of ortho- and para-H₂ were no longer separated and the retention time shortened to half on columns of modified γ-Al₂O₃. All the magnetic transition metal ions modified γ-Al₂O₃ did very well for the separation of H₂/D₂ under the conditions of neon as carrier gas with a flow rate of 60 mL/min and column lengths of 1.0 m and injection amounts of 0.1 mL. Especially, the MnCl₂ modified γ-Al₂O₃ exhibited the best performance for separating H₂/D₂ with an optimum doping concentration of 20 wt%.     

Gas separation mechanisms in microporous modified γ-al2o3 membranes

The transport of pure gases and of binary gas mixtures through a microporous composite membrane is discussed. The membrane consists of an alumina support with a mean pore diameter of 160 nm and an alumina top (separation) layer with pores of 2-4 nm. The theory of Knudsen diffusion, laminar flow and surface diffusion is used to describe the transport mechanisms. It appears for the composite membrane that Knudsen diffusion occurs in the toplayer and combined Knudsen diffusion/laminar flow in the support at pressure levels lower than 200 kPa. For the inert gas mixture H₂/N₂ separation factors near 3 could be achieved which is 80% of the theoretical Knudsen separation factor. This value is shown to be the product of the separation factor of the support (1.9) and of the top layer (1.5). The value for the top layer is rather low due to the relatively small pressure drop across this layer. This situation can be improved by using composite membranes consisting of three or more layers resulting in a larger pressure drop across the separation layer.CO₂ surface diffusion occurs on the internal surface of the investigated alumina membranes. At 250-300 K and a pressure of 100 kPa the contribution of surface diffusion flow measured by counterdiffusion is of the same order of magnitude as that resulting from gas diffusion. The adsorption energy amounts —25 kJ/mol and the surface coverage is 20% of a monolayer at 293 K and 100 kPa. The calculated surface diffusion coefficient is estimated to be 2-5 x 10^-9 m²/sec.Modification of the internal pore surface with MgO increases the amount of adsorbed CO₂ by 50-100%.Modifications with finely dispersed silver are performed to achieve O₂ surface diffusion.     

Structural and mechanical characterizations of microporous silica–boron membranes for gas separation

This paper presents structural and mechanical characterizations of microporous silica membranes for gas separation. The membrane separative layer is made of microporous silica–B₂O₃ produced via a sol–gel process. This layer of about 200 nm of thickness is deposited on the internal surface of a tubular asymmetric γ-alumina/α-alumina support. FTIR and Raman analyses indicate the presence of the boron in the silica net and the above methods in conjunction with ¹¹B MAS NMR analyses of the samples indicate that boron is located mainly in the tetrahedral framework position. Such membranes present interesting gas separation properties at temperatures up to 500 °C and transmembrane pressures lower than 8 bar. He permeance values close to 10⁻¹⁰ kmol m⁻² s⁻¹ Pa⁻¹ are obtained, associated with ideal selectivity α(He/CO₂) which can reach 55. Mechanical properties of separative silica-modified layers are measured by nanoindentation and the coefficient of thermal expansion is obtained from pure material.     

Synthesis of highly ordered mesoporous alumina thin films and their framework crystallization to γ-alumina phase

Here we report the preparation of highly ordered mesoporous alumina films existing both as P6(3)/mmc and Fm-3m mesostructures by using triblock copolymer Pluronic P123 as the structure-directing agent. 2D grazing-incidence small-angle X-ray scattering (GI-SAXS) completely proves the existence of two different mesopore structures (i.e., [001]-oriented P6(3)/mmc and [111]-oriented Fm-3m symmetries). After calcination at 1000 °C, the amorphous alumina framework is successfully converted to γ-alumina crystals. During the crystallization process, large uniaxial shrinkage occurs along the direction perpendicular to the substrate with the retention of horizontal mesoscale periodicity, thereby resulting in formation of partially vertical mesoporosity in the film. Through detailed electron microscopic study, we discuss the formation mechanism for the vertical mesoporosity upon calcination. The obtained mesoporous γ-alumina film shows high thermal stability up to 1000 °C, which is highly useful in wide research areas such as catalyst supports and separators.     

6FDA-containing polyimide-ionene + ionic liquid gas separation membranes

Polyimides (PI) synthesized from 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) with various diamines have been frequently studied as gas separation membranes. The use of 6FDA in polyimides creates a bent structure than can increase fractional free volume (FFV) and gas permeability. Here, we demonstrate that 6FDA is also a useful building block for PI-ionene materials, which contain cations directly within the polymer backbone. These new 6FDA-containing PI-ionenes were combined with several different imidazolium ionic liquids (ILs) to form thin membranes. The thermal properties of all the derivatives were investigated to determine the relationship between regiochemistry and degradation as well as the intermolecular forces that are present within these structures. The gas separation properties of these 6FDA-containing PI-ionene + IL materials were investigated, showing modest CO₂ permeabilities similar to other polyimide-ionenes and CO₂/CH₄ and CO₂/N₂ permselectivities that were relatively higher than other polyimide-ionenes.     

Polyimide-silica sol–gel membranes from a novel alkoxysilane functionalized polyimide: preparation,characterization and gas separation properties

Novel polyimide-organosilicate hybrid films were prepared by sol–gel process from a novel functionalized polyimide with alkoxysilanes as pendant groups that increase the affinity between inorganic and organic phases. The synthesis of this functionalized polyimide was carried out by an esterification reaction of a copolyimide containing carboxylic acid groups with allyl alcohol and subsequent hydrosililation. Tetraethoxysilane was used as precursor of silica in different amounts to obtain hybrid membranes with a silica content of 5, 10 and 20 %. The polymers and hybrid membranes were structural, mechanical and thermally characterized. The ²⁹Si-NMR solid state spectroscopy confirmed that silica was covalently bonded to the polyimide. SEM pictures showed a good dispersion of the silica particles and an amorphous morphology was observed by WAXS. DSC analyses revealed an increase in rigidity with the increase in silica content. The mechanical strength of the hybrid membranes decreased with the silica amount, exhibiting a brittle behavior. The evaluation of the gas permeation properties revealed that the film with the lowest silica content showed the highest permeability coefficients for O₂, N₂, CH₄, and CO₂ with 34, 8, 6, and 128 barrers respectively, while all hybrid membranes showed similar permselectivities around 4 and 22 for O₂/N₂ and CO₂/CH₄ respectively. The fractional free volume of hybrid membranes determined by positron annihilation lifetime spectroscopy followed the same trend that permeability coefficients, confirming that the gas transport properties are mainly governed by the free volume elements.     

Characteristics and retention properties of a mesoporous γ-Al2O3 membrane for nanofiltration

A mesoporous γ-Al₂O₃ membrane was produced by the sol gel dipping technique, followed by a thermal treatment (calcination and sintering). Different sintering temperatures were applied, which led to membranes with an average pore diameter ranging from 8.7 to 3.4 nm, the latter one corresponding to a MWCO of 900 Da.Salt retention was very much dependent on the pH of the solution as such membranes have an amphoteric character. Minimal salt retention was found at the isoelectric point (pH 7.5). Experiments were carried out with NaCl, MgCl₂ and LaCl₃ at different concentrations and in both single salt solutions and mixtures. The results are interpreted in terms of Donnan exclusion and in terms of the formation of an electrical double layer in the pores.Dynamic corrosion tests showed that some corrosion occurs at a pH of 2 or lower.     

Synthesis of mesoporous γ-alumina by sol–gel process and its characterization and application for sorption of Pu(IV)

Mesoporous γ-alumina samples were prepared by the sol–gel process from the boehmite sol having different template solutions. Copper doped material was also prepared from sol containing template solution along with copper nitrate. Studies were performed to understand the influence of templates on the morphology of the synthesized samples particularly with respect to specific surface area and porosity. Synthesized samples were used to study sorption of Pu(IV) from nitric acid–oxalic acid solutions. Distribution ratios (D) for Pu(IV) were determined using the γ-alumina samples with an objective to employ these for the recovery of Pu.     

Sol–gel synthesis,characterization and catalytic activity of mesoporous γ-alumina prepared from boehmite sol by different methods

In this study, boehmite sols were used as alumina precursors for preparing mesoporous γ-aluminas by two different methods. In one case polyethylenimine was used as a structure-directing agent, and in another case ultrasound treatment was applied. Nitrogen physisorption showed that aluminas that had been prepared by these methods demonstrated different porous structures. The sample obtained without additional treatment had closely packed spherical particles and pores had ink-bottle neck morphology. Ultrasound treatment led to the transformation of ink-bottle pores into cylindrical form and to the increase in surface area and pore volume. Aluminas prepared using polyethylenimine as a template showed larger cylindrical wormhole-like mesopores with a broader pore size distribution, high surface area and pore volume. Catalytic tests showed that textural properties as well as crystallite size were very important parameters of synthesized samples which affected the catalytic activity in the methanol dehydration reaction. It was found that γ-Al₂O₃ prepared by ultrasound treatment had large crystallite size and demonstrated high catalytic activity.     

Synthesis and characterization of γ-alumina nanospheres templated by lauric acid

A novel route for the synthesis of alumina nanospheres was reported by a surfactant-governed approach in the presence of lauric acid.The products were characterized using X-ray diffraction (XRD),scanning electron microscope (SEM) and N2 adsorption-desorption techniques.The results show that the produced alumina nanospheres possess uniform nanosphere sizes ranging from 80 120 nm,and high surface area of 550 m2/g.It suggests that the synthesized alumina nanospheres are formed through self-assembly of surfactant/alumina species complex in 1-propanol system.     

Sol–gel derived mesoporous and microporous alumina membranes

Stable polymeric and colloidal boehmite sols were prepared by sol–gel process through controlled hydrolysis/condensation reactions. The particle sizes of the colloidal sols were in the 12–25 nm range depending on the process parameters and about 2 nm for polymeric sols. The presence of a significant increase in the microporosity content of the heat treated polymeric membranes relative to the mesoporous colloidal membranes might make the design of thermally stable microporous alumina membranes with controlled pore structures possible. The phase structure evolution in the 600–800 °C range had shown that the crystallization of the gamma alumina in the amorphous matrix starts at about 800 °C. This indicated that the pore structure stability may be enhanced through processing up to this relatively high temperature in polymeric alumina derived unsupported membranes. The permeance values of the two and three layered colloidal alumina membranes were observed to be independent of pressure which implies that the dominant gas transport mechanism is Knudsen diffusion in these structures. This was also supported by the 2.8 nm BJH pore sizes of the colloidal membranes. The Knudsen diffusion equation derived permeances of the polymeric alumina membranes with thicknesses of about 300 nm were determined to be very close to the experimentally determined permeance values.     

Effect of γ-irradiation on the catalytic activity and selectivity of γ-alumina II. Conversion of isopropanol

The catalytic conversion of isopropanol was conducted over a poorly crystalline -alumina irradiated with different doses of -rays (25–150 Mrad). The catalytic reaction was carried out at 180–400°C in a flow technique under atmospheric pressure. The results showed that the dose of 25 Mrad resulted in a decrease of about 50% of the dehydration activity which suffered a further slight decrease upon irradiation at a dose of 50 Mrad. Increasing the dose in the range of 50–150 Mrad effected an increase in the dehydration activity reaching a maximum limit at 100 Mrad, then decreased abruptly by a dose of 150 Mrad. -irradiation led also to creation of some active sites contributing in dehydrogenation of isopropanol to producing acetone. These results were discussed in terms of removal of Brönsted acidity (25–50 Mrad), responsible for the dehydration reaction and to transformation of Lewis to Brönsted acidity (100 Mrad) by the action of liberated water from the dehydration reaction. The drop in dehydration activity due to irradiation at 150 Mrad might result from an efficient removal of the Brönsted acid sites created. The induced dehydrogenation activity of irradiated aluminas was attributed to creation of some electron-donor centers.     

Metal–organic framework-based mixed-matrix membranes for gas separation: An overview

Mixed-matrix membranes (MMMs) have been studied widely in the field of gas separation due to their potential to overcome performance barriers found in traditional polymeric membranes. Most polymeric membranes exhibit a trade-off between permeation and selectivity, which has limited their development in many challenging separation applications. One solution to this issue utilizes the introduction of fillers into the polymer matrix to produce MMMs. Out of the many different fillers, metal–organic frameworks stand out as a promising candidate due to their highly tunable structure, molecular sieving effect, and superior compatibility with the polymer matrix. This review will provide an in-depth look into the basic mechanisms of MMMs for gas separation and different approaches to model the permeation of gases through the membrane. In addition, challenges facing the field and recent research trends for MMMs will be discussed as well as their many applications for different gas separations. Finally, some insight on the future direction for MMMs will be covered, focusing on many intriguing opportunities and challenges that must be further explored to advance this technology.     

Separation of acetone/water mixtures by a modified γ-alumina membrane via a new method

A new surface deposition method was researched to decrease pore size of ceramic membranes. CaCO₃ was chosen to modify the top layer of γ-alumina membranes prepared by sol–gel processes. Separation of gaseous acetone/water mixtures by vapor permeation was carried out to characterize the membranes. Improvement of membrane separation property after modifications and SEM photographs proved that this new method was effective to reduce the membrane pore size.     

Separation of binary gas mixtures by means of sol–gel modified ceramic membranes. Prediction of membrane performance

Ceramic membranes based on an alumina support with two successive layers of alumina of decreasing pore size and a sol–gel top layer, were characterized by gas permeability experiments and used for separating binary H₂ and N₂ gas mixtures. A mathematical model based on mass balance calculations was developed to predict the composition of permeated gas as a function of the different experimental parameters. No gas diffusion assumption is made, and it allows, after a previous characterization of the membrane, to find the optimal conditions for gas separation.     

Interface and properties of epoxy resin modified by elastomeric nano-particles

Epoxy resin has been widely used as structuralmaterials and adhesives in electronics, aerospace in-dustries and etc. for its impressive overall properties.However, epoxy network is brittle and notch sensitive,which restricts its application scope. As a re…