Inorganic Microporous Membranes Research Articles

Inorganic microporous membranes for hydrogen separation: Challenges and solutions

Porous membrane separation is a competitive hydrogen purification technology due to the advantages of environmental friendliness, energy-saving, simple operation, and low cost. Benefiting from the booming development of materials science and chemical science, great progress has been made in H₂ separation with porous membranes. This review focuses on the latest advances in the design and fabrication of H₂ separation inorganic microporous membranes, with emphasis on the synthetic strategies to achieve structural integrity, continuity and stability. This review starts with a brief introduction to the membrane separation mechanisms, followed by an elaboration on the synthetic challenges and corresponding solutions of various high-performance inorganic microporous membranes based on zeolites, silica, carbon, and metal-organic frameworks (MOFs). At last, by highlighting the prospects of ultrathin two-dimensional (2D) porous membranes, we wish to shed some light on the further development of new materials and membranes for highly efficient hydrogen separation.

Improved H2/CO2 separation performance on mixed-linker ZIF-7 polycrystalline membranes

Membrane-based hydrogen purification has been considered to be a promising alternative because of its low energy consumption, ease of operation, and cost effectiveness. Amino-functionalized ZIF-7 (ZIF-7-NH2) is an attractive candidate for membrane gas separation due to the maintenance of pore-opening structure after total removal of guest molecules (DMF). In this work, ZIF-7-NH2 membrane with improved both H2 permeance and H2/CO2 selectivity than reported mono-ligand ZIF-7 membrane was first prepared. Several activation process for ZIF-7-NH2 membranes to opening the pores were investigated. It was found that the coating of a Pebax®1657 layer on the surface of ZIF-7-NH2 membrane is crucial for preventing the formation of cracks during pore-activation procedure. The permeance of H2 and separation factor for H2/CO2 binary mixtures were improved to ∼ 1 × 10−7 mol/m2 s Pa and ∼19, respectively. The separation performance was very close to the upper bound for inorganic microporous membranes. Furthermore, the improved hydrothermal stability on ZIF-7-NH2 materials endows the membrane is highly potential for hydrogen purification.

無機膜による気体分離

Last decade, much progress has been seen in research in inorganic microporous membranes such as silica, carbon and zeolite based membranes. Microporous membranes are referred to those porous membranes with pore diameter smaller than 2 nm. Silica based membranes are prepared by the sol-gel or CVD method. Carbon membranes are prepared by the pyrolysis of a polymer precursor. Zeolite membranes are prepared by the hydrothermal synthesis. This review gives a summary of recent advances in synthesis, properties and potential applications of these membranes for gas separation.

Hybrid Organic–Inorganic Microporous Membranes with High Hydrothermal Stability for the Separation of Carbon Dioxide

Membranes only: A hybrid organic–inorganic membrane for CO2 separation is prepared by sol–gel deposition of a microporous ethylene-bridged silsesquioxane layer onto a multilayer support. Low CO2 permeance is imparted by doping acidic niobium centers into the hybrid silica network. The membrane exhibits superior hydrothermal stability with He/CO2 selectivities as high as 3700.

Confocal laser scanning microscope analysis of organic–inorganic microporous membranes

The casting solution of an organic–inorganic microporous membrane was formed by dispersing a small quantity of fine alumina particles in a solution of poly(vinylidene fluoride) (PVDF) and dimethylacetamide (DMAC). The modified membranes were prepared by a phase inversion process and their characteristics (surface morphology and structures, surface roughness, porous dispersion, and the distribution of alumina particles) were compared to those of unmodified membranes (without alumina particles) using confocal laser scanning microscopy (CLSM). In order to analyze the membranes using CLSM, it was necessary to add trace amounts of fluorescein to all the samples to enable operation in the fluorescence mode. The inorganic alumina particles were found to be dispersed uniformly in the membranes. The results indicate not only that the organic-inorganic microporous membrane surface pore morphology changes but also that the pore dimensions and the surface roughness increase due to the addition of the alumina particles. At the same time, the study also shows that CLSM is an effective tool for investigating microporous membrane characteristics.

A novel sacrificial interlayer-based method for the preparation of silicon carbide membranes

A novel method is presented based on the use of sacrificial interlayers for the preparation of nanoporous silicon carbide membranes. It involves periodic and alternate coatings of polystyrene sacrificial interlayers and silicon carbide pre-ceramic layers on the top of slip-casted tubular silicon carbide supports. Membranes prepared by this technique exhibit single gas ideal separation factors of helium and hydrogen over argon in the ranges 176–465 and 101–258, respectively, with permeances that are typically two to three times higher than those of silicon carbide membranes prepared previously by the more conventional techniques. Mixed-gas experiments with the same membranes indicate separation factors as high as 117 for an equimolar H 2/CH 4 mixture. We speculate that the improved membrane characteristics are due to the sacrificial interlayers filling the pores in the underlying structure and preventing their blockage by the pre-ceramic polymer. The new method has good promise for application to the preparation of a variety of other inorganic microporous membranes.

Molecular Modeling Study of the Permeability−Selectivity Trade-off in Polymeric and Microporous Membranes

A permeability−selectivity trade-off is observed in kinetic (diffusion-based) separations, such as oxygen/nitrogen, on polymeric and inorganic microporous membrane materials. Some materials are inherently better than others in the trade-off, but the molecular-level reasons are not clear. In this paper, we employed a molecular modeling approach to study the oxygen/nitrogen separation and provide insight into optimizing the pore characteristics (geometry, material of construction) for separation performance. The pore models were abstract, with features representative of different microporous or polymeric materials. They comprised three-dimensional atomistic models of unidirectional pores with diameters <5 A; various aspects of the pore geometry and material of construction were varied. Oxygen and nitrogen were modeled as rigid dimers, and the Lennard-Jones (LJ) potential was used to model the gas−pore site interactions. Free energy calculations and transition-state theory were used to predict the permeation...

Membrane reactor for homogeneous catalysis in supercritical carbon dioxide

A membrane reactor is presented for homogeneous catalysis in supercritical carbon dioxide with in situ catalyst separation. This concept offers the advantages of benign high-density gases, i.e., the possibility of achieving a high concentration of gaseous reactants in the same phase as the substrates and catalyst as well as easy catalyst localization by means of a membrane. For the separation of the homogeneous catalyst from the products an inorganic microporous membrane is used. The concept is demonstrated for the hydrogenation of 1-butene using a fluorous derivative of Wilkinson's catalyst [RhCl{P–(C 6H 4- p-SiMe 2CH 2CH 2C 8F 17) 3} 3]. The size of Wilkinson's catalyst, 2–4 nm, is clearly larger than the pore diameter, 0.5–0.8 nm, of the silica membrane. The membrane will, therefore, retain the catalyst, while the substrates and products diffuse through the membrane. Stable operation and continuous production of n-butane has been achieved at a temperature of 353 K and a pressure of 20 MPa. A turnover number of 1.2×10 5 has been obtained during 32 h of reaction. The retention of the catalyst was checked using UV–vis spectroscopy and ICP-AAS; no rhodium or phosphorous species were detected at the permeate side of the membrane.

MICROPOROUS INORGANIC MEMBRANES

Recent development in microporous inorganic membranes represents a significant advance in materials for separation and chemical reaction applications. This paper provides an in-depth review of synthesis and properties of two groups (amorphous and crystalline) of microporous inorganic membranes. Amorphous microporous silica membranes can be prepared by the sol-gel and phase separation methods. Flat sheet, tubular and hollow fiber amorphous carbon membranes have been fabricated by various pyrolysis methods from polymer precursors. A large number of synthesis methods have been developed to prepare good quality polycrystalline zeolite membranes. Several techniques, including vapor and liquid approaches, are reviewed for pore structure modification to prepare microporous inorganic membranes from mesoporous inorganic membranes. Chemical, microstructural and permeation properties of these microporous membranes are summarized and compared among the several microporous membranes discussed in this paper. Theory for gas permeation through microporous membranes is also reviewed, with emphasis on comparison of theoretical with the experimental data. These inorganic microporous membranes offer excellent separation properties by the mechanisms of preferential adsorption, selective configurational diffusion or molecular sieving.

Permeation of supercritical fluids through a MFI zeolite membrane

This work involves the joining of two new technologies: supercritical fluids (SCF) and inorganic microporous materials. The objective of this study is the determination and description of transfer mechanisms of supercritical fluids through inorganic microporous membranes whose selective layer is constituted of MFI zeolite, a material with particular properties of porosity, crystalline structure and adsorption. To describe the evolution that experiences the supercritical fluid inside the microporous medium as a function of pressure and temperature, models based on the formulation of Stefan–Maxwell and ordinarily used for gas phase transfer were selected. A statistical analysis of permeation data allow to distinguish between the convective (or Poiseuille viscous) and diffusive (Knudsen flow) contributions, as well as to highlight the effect of adsorption at wall. As a whole it appears that the transfer of SCF results from a combination of diffusive mechanism and interaction with the membrane surface, well in agreement with what the proximity of sizes of pores and gas molecules enables to foresee. Under the assumption that adsorption is characterised by a low occupation rate of available places inside the micropores, the Stefan–Maxwell equations give a good description of the general system behavior.

Enzyme preparation with invertase activity from yeast cell lysate using tangential flow filtration

An experimental model with yeast biomass was designed to apply an inorganic microporous membrane to a separative process and to determine the filtrate parameters. This process included the recovery of a specific intracellular enzyme, invertase, which is constitutive in many yeast species. It was found that the inorganic microporous membrane was effective to separate soluble components and to concentrate cell debris from yeast cell lysate. Also, the wash operation mode (diafiltration) proved to be equally effective. The specific activity of the obtained solid enzyme preparation is comparable to a commercial preparation.

Mechanisms of protein fouling in cross-flow UF through an asymmetric inorganic membrane

Flux decline and retention of aqueous solutions of several proteins (pepsin, BSA, lipase, γ-globulin and invertase) with molecular weights of 36, 67, 80, 150 and 270 kDa are studied when they are tangentially filtered through an inorganic microporous membrane with a nominal pore size of 0.02 μm made by Anopore under a constant applied pressure of 5.0 kPa. The fouling process of this ultrafiltration membrane is investigated as a function of the protein concentration, the tangential velocity at the membrane surface and the nature and size of the protein. Finally, the deposition mechanism on the membrane surface and/or into its porous structure is analyzed in terms of several kinetic models. It is seen that an initial particularly intense flux decline due to external blockage is followed by an internal deposition (partially retained proteins) or the formation of a cake (totally retained proteins). The kinetics of these fouling mechanisms are proved to follow reasonable trends versus feed concentrations, recirculation velocity and the protein molecular weight.

Retention of proteins in cross‐flow UF through asymmetric inorganic membranes

AbstractThe flow and retention of 0.1% w/w aqueous solutions of pepsin, bovine serum albumin (BSA), lipase, γ‐globulin, and invertase with molecular weights of 36,000, 67,000, 80,000, 150,000, and 270,000 dalton (g/mol) are studied when they are tangentially filtered through an inorganic microporous membrane with a nominal pore size of 0.02 × 10−6 m made by Anopore, with transmembrane pressure differences up to 100 kPa. The data were analyzed within the frame of the film layer theory for the concentration polarization phenomenon. This allows the mass‐transfer coefficient to be obtained for the cell used as a function of the feed circulation speed and the molecular weight of the solute. Apparent and true retention curves obtained lead to a size exclusion radius smaller than the nominal one but very similar to that obtained from scanning electronic photographs.