Microporous Inorganic Membranes Research Articles

Multivariate Polycrystalline Metal-Organic Framework Membranes for CO2/CH4 Separation.

Membrane technology is attractive for natural gas separation (removing CO2, H2O, and hydrocarbons from CH4) because of membranes' low energy consumption and small environmental footprint. Compared to polymeric membranes, microporous inorganic membranes such as silicoaluminophosphate-34 (SAPO-34) membrane can retain their separation performance under conditions close to industrial requirements. However, moisture and hydrocarbons in natural gas can be strongly adsorbed in the pores of those membranes, thereby reducing the membrane separation performance. Herein, we report the fabrication of a polycrystalline MIL-160 membrane on an Al2O3 substrate by in situ hydrothermal synthesis. The MIL-160 membrane with a thickness of ca. 3 μm shows a remarkable molecular sieving effect in gas separation. Besides, the pore size and environment of the MIL-160 membrane can be precisely controlled using reticular chemistry by regulating the size and functionality of the ligand. Interestingly, the more polar fluorine-functionalized multivariate MIL-160/CAU-10-F membrane exhibits a 10.7% increase in selectivity for CO2/CH4 separation and a 31.2% increase in CO2 permeance compared to those of the MIL-160 membrane. In addition, hydrophobic MIL-160 membranes and MIL-160/CAU-10-F membranes are more resistant to water vapor and hydrocarbons than the hydrophilic SAPO-34 membranes.

Microporous Inorganic Membranes for Gas Separation and Purification

The importance of inorganic membranes for gas separation and purification is analyzed. Although the cost of inorganic membranes is higher than that for polymeric membranes, they have higher permeance, selectivity and better resistance to higher pressure and temperature. The main materials used for porous inorganic membranes are alumina (Al2O3), silica (SiO2), zirconia (ZrO2), zeolite and carbon. Ceramics are compounds of metallic and non-metallic elements. They generally have a macroporous support, an intermediate layer and a small porous top layer. Because the Knudsen gas separation regime has a very low selectivity, various membrane surface modification techniques have started to be experimented with at a number of laboratories. The research focuses on materials that exhibit molecular sieving properties, such as silica, zeolites, MOFs (metal-organic frameworks), graphene and carbon. Finally, gas transport mechanisms through porous membranes are summarized.

Design and fabrication of silica-based nanofiltration membranes for water desalination and detoxification

Inorganic membranes typically have higher mechanical, thermal and chemical stability than their polymeric counterparts. Therefore, in this work a recent transport model was used to investigate the potential of porous inorganic membranes in water desalination. Salt rejection was predicted using the Donnan-steric pore model, in which the extended Nernst-Planck equation was applied to predict ion transport through the pores. The solvent flux was modeled using the Hagen–Poiseuille equation by considering the electroviscous effect. This model showed that inorganic NF membranes cannot achieve the selectivity of the traditional reverse osmosis (RO) membranes, and can only approach the perm-selectivity of the less robust but thinner and more flexible polymeric NF membranes. Nevertheless, inorganic NF membranes with pore size of about 1.5 nm and ζ-potential between 5 and 20 mV allow for a good compromise between water flux and salt rejection. Therefore, silica-based membranes with such properties were fabricated by sol-gel deposition. Since we have recently reported that the chemical and hydrothermal stability of unsupported microporous silica membranes can be highly enhanced by TiO2-doping, two sols were used for membrane deposition: a 5%TiO2-doped silica polymeric sol and a pure silica reference sol. The 5%TiO2-doped silica membrane showed water permeability (2.3 liters per square meter per hour per bar (LMH bar−1)) more than 30 times higher and higher salt selectivity than the pure silica membrane. As predicted by our model, at the 5%TiO2-doped silica membrane had approached without reaching salt rejection of commercial polymeric membranes with similar water flux, but it showed good performances when compared with the already reported inorganic NF membranes. Moreover, the new membrane presented nearly complete retention towards two model micropollutants, namely caffeine and ibuprofen. By investigating limits and potential of microporous inorganic membranes water desalination and detoxification, this work provides new knowledge for their rational design.

Pore structure characterization of supported polycrystalline zeolite membranes by positron annihilation spectroscopy

It is important to characterize the pore structure of supported polycrystalline zeolite membranes but it has remained a major challenge in studying microporous inorganic membranes. This paper reports the use of positron annihilation spectroscopy including positron annihilation lifetime spectroscopy and Doppler broadening energy spectroscopy to non-destructively characterize the pore structure of four MFI zeolite membranes of different microstructures on alumina supports. Positron annihilation lifetime spectroscopy analysis reveals a bimodal pore structure consisting of intracrystalline zeolitic micropores of around 0.6nm in diameter and irregular intercrystalline micropores of 1.4−1.8nm in size for the four MFI zeolite membranes studied. Distributions of the micropores along the membrane thickness direction can be inferred from Doppler broadening energy spectroscopy results, illustrating development of intercrystalline gaps during the growth of the zeolite layer. The amount and size of the intercrystalline micropores of the zeolite membranes vary with the synthesis method, and are the smallest for the randomly oriented MFI zeolite membrane synthesized without template and the largest for the c-oriented MFI zeolite membrane synthesized with the template. The c-oriented membrane has an asymmetrical distribution of intercrystalline pores along the film growth direction as compared to the uniform distribution of the bimodal structure for the other three membranes. The pore structure data obtained by positron annihilation spectroscopy are consistent with the xylene isomer separation performance of these membranes.

多孔性無機膜の分子シミュレーションと気体透過性予測

A non–equilibrium molecular dynamics (MD)technique can be effectively utilized for calculation of permeability ofgases in microporous inorganic membranes as well as adsorption and diffusion simulations combined with the “sorp-tion–diffusion (SD)model”. The effects of microporous structures, such as membrane density and pore size distribu-tion, on gas permeation properties were examined. Analytical model for prediction of permeability and molecularsimulation assisted calculation of permeability were successfully applied to reveal molecular permeation mechanismsin micropores. Microporous structures and physical chemical characteristics of new organosilica materials havebeen also getting clear by using molecular simulations. Gas permeability and selectivity of a BTESE–derivedorganosilica membrane was well predicted from adsorption and diffusion simulations using the SD model. Thedemand of molecular simulation assisted prediction of membrane performance would be of importance especially fordesign of organic–inorganic hybrid microporous membranes.

計算機支援による多孔性無機膜の構造および気体透過性評価

計算機支援による多孔性無機膜の構造および気体透過性評価

Synthesis and performance of microporous inorganic membranes for CO2 separation: a review

Removal of CO2 by membrane technologies is one promising approach as compared to other CO2 capture technologies due to advantages such as simpler operation, higher reliability, lower capital and operating cost, higher energy efficiency, and a cleaner process. In the field of CO2 gas separation, inorganic membranes have been attracting a lot of attention. Three classes of microporous membrane family, i.e. microporous silica membranes, microporous carbon membranes and zeolite membranes, have been widely studied due to their potential in separating CO2 gas molecules, contributed by their distribution of selective micropores which are almost identical to the required molecular sizes for diffusing CO2 gas. This paper review various methods to fabricate the above three types of microporous membranes, at the same time, looking at other researchers employing these methods to fabricate microporous membranes for CO2 separation.

Microporous inorganic membranes for high temperature hydrogen purification

The general mechanisms of gas separation in microporous inorganic membranes are reviewed in this article. Emphasis has been placed on discussing the requirements of membrane pore structure and material properties for high temperature hydrogen separation from other small gases involved in processes of hydrogen production from fossil fuels. The recent research progresses in developing the crystalline zeolite membranes, and amorphous silica-based membranes for high temperature hydrogen separation are critically reviewed. The fundamental issues associated with the zeolite and silica membranes relevant to the practical applications are analyzed based on the relationships between the separation performance and membrane structural and chemical properties.

Use of RO and NF for treatment of copper containing wastewaters in combination with flotation

The process of so-called membrane flotation can be used in combination with NF/RO techniques for treatment of wastewaters containing copper and other heavy metals to decrease the environmental risks. Microporous inorganic membranes are used in membrane flotation process as diffusers for air sparging. Various ways of combination of flotation and membrane filtration in cationic wastewater treatment practice are discussed. With these combined membrane/flotation solutions wastewaters can be treated with high rejection and high pure water recovery. High performance of integrated membrane-electroflotation equipment as well as RO and NF spiralwound systems allows to achieve great economy of freshwater and to diminish wastewater discharge fees.

Gas Permeation and Separation Mechanisms through Microporous Inorganic Membranes studied with Molecular Simulations

Gas Permeation and Separation Mechanisms through Microporous Inorganic Membranes studied with Molecular Simulations

Performance evaluation of microporous inorganic membranes in the dehydration of industrial solvents

The separation potential of five different commercial tubular inorganic membranes consisting of A-, T- and Y-type zeolites from Mitsui and microporous silica from ECN and Pervatech has been examined in more than 30 dehydration applications. These separations include alcohols, glycols, carboxylic acids, esters, ethers, ketones, amines, nitriles, and halogenated hydrocarbons. The membranes combine excellent permeate flux with very good selectivity up to values of several thousand. Water permeances valuing between 10 and 20 kg/m 2 h bar partial pressure difference represent a new dimension in solvent dehydration. The pervaporation separation index (PSI) of standard polymeric membranes is around 100 while for inorganic materials index values of up to 50,000 are achieved. The performance ranking of the membranes in term of selectivity decreases in the sequence zeolite A > zeolite T > Pervatech silica > ECN silica. Regarding the flux the order is reversed with ECN silica > Pervatech silica > zeolite A > zeolite T. The reason for this behavior is founded in the different membrane structure and adsorption characteristics. The separation layer of the zeolite membranes is at least 10 μm thick whereas the thickness of the silica membranes is below 200 nm. On the other hand, zeolite crystals have a strictly defined pore size allowing a sharper separation in comparison to the amorphous silica with a pore size distribution. The water adsorption on zeolites follows a Langmuir isotherm while for silica it is in the Henry region. Thus, concentration polarization effects are more severe for the latter material and viscosity of the systems has a major impact on the transport resistances in the boundary layer. These conclusions were confirmed by a comparison of single component to mixture permeation data. A simple transport model based on normalized permeate flux is used to calculate the influence of operation parameters, such as concentration, temperature and permeate pressure, on the separation characteristic from a single measurement. Water can be separated with high efficiency from complex solvent and reaction mixtures including methanol by A-type zeolite membranes. According to process needs the cut-off can be shifted to the separation of both water and methanol by use of a Y-type zeolite or amorphous silica membrane. The chemical stability of all tested membranes in aprotic solvents is excellent while their acid and base resistance is limited. For the A-type zeolite an acidic environment has to be avoided by all means, whereas the T- and Y-type zeolite can withstand a lower pH to some extent. Standard silica is stable down to a pH value of 3. Durability can be further improved by chemical modifications of the sol. In basic environment, the γ-alumina support is affected at a pH greater than 11. Membranes show no signs of thermal degradation up to 150 °C. In pervaporation, micro-cracks have been detected at high permeation rates above 30 kg/m 2 h. A temperature difference is necessary to transfer the heat for the evaporation of the permeating components from feed to permeate side. Hence, thermal stress is a likely cause for the observed destruction of the membrane. Tubular membranes can withstand normal operation pressures of up to 30 bar without any problem. Contact of the membrane with solid particles should be avoided to guarantee mechanical stability. This can be achieved by introducing a feed filter in the range of 10–100 μm into the feed stream.

Molecular Simulation Studies of Gas Permeation through Microporous Inorganic membranes

Molecular Simulation Studies of Gas Permeation through Microporous Inorganic membranes

Gas Permeation Properties through Ultra Microporous Inorganic Membranes

Gas Permeation Properties through Ultra Microporous Inorganic Membranes

A unified approach of gas, liquid and supercritical solvent transport through microporous membranes

The main objective of this study is the determination and description of mass transfer mechanisms of fluids under various states—gas, liquid and supercritical—through microporous inorganic membranes. The work was focused on the permeation of CO 2 through membranes whose selective layer is constituted by MFI zeolite, a material with particular properties of porosity, crystalline structure and adsorption. To describe the evolution that experiences the fluid inside the microporous medium as a function of pressure and temperature, models based on the formulation of Maxwell–Stefan were selected. As a whole, it appears whatever the state of the fluid the transfer mechanisms are the same. The transfer of fluid results from a combination of diffusive mechanism and interaction with the membrane surface. It is well in agreement with the fact that the proximity between the pore size and molecules enables one to foresee.

Recovery of Liquid CO2 from Cleaning Solutions Without Phase Change Using Ultrafiltration and Microfiltration Membranes

Recovery of liquid and supercritical fluid (SCF) CO2 in the application of CO2 as processing solvent in extraction, chemical, and biochemical reactions is an important issue. Membrane-based separation may provide an option in recovering CO2 in liquid and/or SCF phase without expensive, energy intensive recompression. The potential application of microporous inorganic membranes in separating liquid CO2 without phase change was investigated. A high-pressure membrane filtration unit was designed, built, and tested. Microporous stainless steel (0.2 μm) and ceramic (0.02 μm, and 1000 Dalton) tubular elements were retrofitted into in-house designed, high-pressure housings to test the functionality of the system using Triton X-100 solute in liquid CO2 as feed. In-house designed fiberoptic cells coupled with a UV–VIS spectrophotometer was used for on-line measurement of solute Triton X-100 in the feed and permeate streams. The fiberoptic cell coupled with UV–VIS spectrophotometer was capable of providing on-line concentration measurement of Triton X-100 at operating pressures up to 100 bar. Two different path-length fiberoptic cells, 5.0 mm and 0.5 mm, were designed and built to cover a wide range of solute concentrations. Membrane filtration experiments with ceramic membranes showed that 1000-Dalton membrane offered a solute rejectivity of about 80%. The coarse ceramic membrane (0.02 μm) had a lower rejectivity, which was about 55%. The 0.2-μm stainless steel membrane provided very little solute rejectivity.

Integrated System Design for Dewatering of Solvents with Microporous Inorganic Membranes

Integrated System Design for Dewatering of Solvents with Microporous Inorganic Membranes

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.

Microporous Inorganic Membranes for Gas Separation

Microporous inorganic membranes are potentially useful in gas separation in emerging areas such as catalytic reactors, gasification of coal, molten-carbonate and solid-electrolyte fuel cells, and water decomposition by thermochemical reactions. If the feed or product gases can be separated at elevated temperatures specific to each process, the energy required for purification could be greatly reduced. Advances in the development of inorganic membranes have been quite rapid in recent years. For example, in 1991 the reported CO2/N2 selectivity at ambient temperature was less than 10, but by 1997 it had improved to approximately 100.The permeation rate and permselectivity of porous inorganic membranes are dependent on the microstructures of membrane/support composites such as pore size and distribution, porosity, tortuosity, and the affinity between permeating species and pore walls. Figure 1 shows the relationship between molecular weight and kinetic diameter (calculated from minimum equilibrium cross-sectional diameter) for a selected series of molecules. Hydrogen and helium are smaller and lighter than the others. Structural isomers such as n-C4H10 and i-C4H10 have the same mass but quite different sizes. Therefore, the control of micropores is of critical importance in these cases. However, the molecular masses and sizes of CO2 and N2 are not greatly different; thus the difference in affinity is important for separation of these molecules.In order to achieve effective separation of small-molecule gases, the membrane pores should be smaller than 2 nm. In the case of mesopores or macropores, gases permeate with low selectivities through these pores. In this article, preparation processes and permeation properties of porous inorganic membranes are reviewed, and permeation mechanisms are discussed.

Characterization of the transport properties of microporous inorganic membranes

The development of microporous inorganic membranes for gas separation depends crucially on the accurate characterization of gas transport through such membranes. Particularly in the initial development stages, in which low membrane selectivities are common, measurement of pure gas permeance as a function of mean pressure can provide a useful characterization tool. For example, this method can be used to obtain information regarding pore size reduction in microporous membranes. Unfortunately, due to demanding limits on experimental error and misinterpretation of data, this method has been often applied incorrectly. In this paper, a theoretical basis for describing gas transport through both monolithic and multilayer porous systems is described. Implications of this formalism for practical data acquisition and analysis are discussed. Experimental equipment and methods are described for collecting data satisfying these strict criteria for data precision. The reproducibility of the experimental methods is illustrated by data obtained for a microporous membrane layer deposited on a porous support. Application of the present formalism to the prediction of transport characteristics for different gases, as well as to the detailed modeling of the pore structure of multilayer systems, is also illustrated by permeation data for several supported thin film microporous membranes.

Organic “template” approach to molecular sieving silica membranes

We demonstrate a “template” approach to prepare microporous inorganic membranes exhibiting high flux combined with high selectivity, overcoming limitations inherent to both conventional inorganic (sol-gel, CVD) and organic membrane approaches. Hybrid organic-inorganic polymers prepared by co-polymerization of tetraethoxysilane (TEOS) and methyltrie-thoxysilane (MTES) were deposited on commercial asymmetric alumina supports. Heat treatments were employed to densify the inorganic matrix and pyrolyze the methyl ligands, creating a continous network of micropores. Resulting membranes exhibited very high CO 2 permeance values (2.57×10 −3 cm 3/cm 2 s cm Hg) combined with moderate CO 2 CH 4 selectivities (12.2). Subsequent derivatization of the pore surfaces with monomeric TEOS significantly increased CO 2 CH 4 separation factors (71.5) with only a modest reduction in CO 2 permeance (2.04×10 −4 cm 3/cm 2 s cm Hg). Combined CO 2 CH 4 selectivity factors and CO 2 fluxes exceeded those of all known organic membranes.