Nano/subnano-tuning of porous ceramic membranes for molecular separation

In sol–gel processing, porous ceramic membranes can be prepared by sol-coating porous substrates and drying for gelling, followed by a firing process. Ceramic membranes prepared by sol–gel processing can be categorized into amorphous materials such as silica, and crystalline materials such as alumina and titania. Amorphous silica networks, which can be prepared by the polymeric sol route, have ultra-microporous pores that allow small molecules such as helium and hydrogen to permeate. On the other hand, crystalline materials, which are mostly prepared by the colloidal sol route, have nano-sized pores in the range of one to several nanometers. In this article, sol–gel derived SiO₂ and TiO₂ membranes with controlled pore sizes in the range of sub-nano to nanometers will be reviewed with respect to membrane preparation and to their application in the separation of the gas and liquid phases. Ceramic membranes with high performance can be obtained by precise control of membrane structures (pore size, pore size distribution, thickness, pore shape, etc.) and membrane materials (SiO₂, TiO₂, composite oxide, hybrid materials, etc.). Nano/subnano-tuning of porous ceramic membranes is quite important for the improvement of membrane permeability and selectivity.     

Preparation of water and alkali durable porous glass membrane coated on porous alumina tubing by sol-gel method

Composite porous glass membranes were prepared by the sol-gel method. A thin porous glass layer, about 2 μm thick, was coated on the surface of the porous ceramic tubing (Al₂O₃:99.9 wt.%, pore diameter: 200 nm). The composition of the porous glass layer of the composite membrane was SiO₂-ZrO₂. Considering from the fact that the desalination ratio of the feed aqueous NaCl solution (NaCl 0.5 wt.%) was about 90% by use of these membranes, they were defect-free. The best composition of the porous glass layer was 70 SiO₂-30 ZrO₂ from the standpoint of preparing membranes. These membranes had a large water and alkali durability. These membranes can be expected to apply to recovering dyes and paints from organic solvents and to be used as a gas separation membrane.     

以多孔钛片为载体的氧化铝膜的制备和表征

γ-alumine membranes supported by porous titanium plate were prepared by sol-gel techniqe with alumina isopropoxide. The factors, which influenced membrane per formance, in the membrane preparing process were investigated. A dynamic flow-weighted active pore size distribution apparatus for inorganic membrane were set-up on the basis of Kelvine equation. The results showed that for composite porous membrane the active pore size distribution is about 6-8nm, the most probable pore size is about 7nm and nitrogen permeability is 7.1×10-6mol﹒m-2﹒Pa-1﹒s-2.     

Preparation of ceramic γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-TiO<Subscript>2</Subscript> nanofiltration membranes for desalination

As one of the most recently developed membrane separation processes, nanofiltration (NF) has found a number of industrial applications. Ceramic NF membranes are also regarded as the appropriate choice in many applications, due to their higher chemical and physical stability. In this study, the rejection of the chloride ion is investigated using bi-layered γ-Al₂O₃-TiO₂ NF membranes based on α-alumina supports. Compression is used in preparation of the supports and sol-gel dip-coating for the top-layer formation. SEM micrographs, XRD, and nitrogen adsorption/desorption isotherms are used for membrane characterisation. The results show that the calcination temperature (600°C) results in different crystal structures including the brookite phase of TiO₂, the γ phase of Al₂O₃, and a combined phase of aluminium-titanium oxides. The average pore size of the membrane was identified as 1.6 nm using an adsorption/desorption isotherm. The rejection was also studied for the chloride ion, using a cross-flow filtration module. Filtration tests were carried out under different pressures, pH values, and salt concentrations; these showed a smoother behaviour particularly around the isoelectric points (IEPs) due to the dual-layer structure, with the best rejection at pH of approximately 5.     

Sol-gel processing of inorganic membranes

Basic principles involved in sol-gel processing of ceramic membranes are described. This process has been applied to ceramic ultrafiltration membranes and is now investigated to prepare ceramic nanofilters. In this paper special emphasis is put on new developments concerning microporous zirconia membranes obtained by the polymeric route. A zirconium alkoxide precursor modified with an acetylacetone ligand has been used in order to control particle growth in the sols and pore size distribution in the membranes. N₂ adsorption and X-ray diffraction analysis have been performed on membrane materials showing the influence of process parameters (molar ratio r = acacH/Zr and sintering temperature T) on membrane structural evolution.     

Preparation and characterization of porous conducting poly(dl-lactide) composite membranes

Solid conducting biodegradable composite membranes have shown to enhance nerve regeneration. However, few efforts have been directed toward porous conducting biodegradable composite membranes for the same purpose. In this study, we have fabricated some porous conducting poly(dl-lactide) composite membranes which can be used for the biodegradable nerve conduits. The porous poly(dl-lactide) membranes were first prepared through a phase separation method, and then they were incorporated with polypyrrole to produce porous conducting composite membranes by polymerizing pyrrole monomer in gas phase using FeCl₃ as oxidant. The preparation conditions were optimized to obtain membranes with controlled pore size and porosity. The direct current conductivity of composite membrane was investigated using standard four-point technique. The effects of polymerization time and the concentration of oxidant on the conductivity of the composite membrane were examined. Under optimized polymerization conditions, some composite membranes showed a conductivity close to 10⁻³ S cm⁻¹ with a lower polypyrrole loading between 2 and 3 wt.%. A consecutive degradation in Ringer's solution at 37 °C indicated that the conductivity of composite membrane did not exhibit significant changes until 9 weeks although a noticeable weight loss of the composite membrane could be seen since the end of the second week.     

Pore Modification in Porous Ceramic Membranes With Sol-Gel Process and Determination of Gas Permeability and Selectivity

Summary: The aim of the study was to investigate the variation in total surface area, porosity, pore size, Knudsen and surface diffusion coefficients, gas permeability and selectivity before and after the application of sol-gel process to porous ceramic membrane in order to determine the effect of pore modification. In this study, three different sol-gel process were applied to the ceramic support separately; one was the silica sol-gel process which was applied to increase porosity, others were silica-sol dip coating and silica-sol processing methods which were applied to decrease pore size. As a result of this, total surface area, pore size and porosity of ceramic support and membranes were determined by using BET instrument. In addition to this, Knudsen and surface diffusion coefficients were also calculated. After then, ceramic support and membranes were exposed to gas permeation experiments by using the CO₂ gas with different flow rates. Gas permeability and selectivity of those membranes were measured according to the data obtained. Thus, pore surface area, porosity, pore size and Knudsen diffusion coefficient of membrane treated with silica sol-gel process increased while total surface area was decreasing. Therefore, permeability of ceramic support and membrane treated with silica sol-gel process increased, and selectivity decreased with increasing the gas flow rate. Also, surface area, porosity, pore size, permeability, selectivity, Knudsen and surface diffusion coefficients of membranes treated with silica-sol dip coating and silica-sol processing methods were determined. As a result of this, porosity, pore size, Knudsen and surface diffusion coefficients decreased, total surface area increased in both methods. However, viscous flow and Knudsen flow permeability were detected as a consequence of gas permeability test and Knudsen flow was found to be a dominant transport mechanism in addition to surface diffusive flow owing to the small pore diameter in both methods. It was observed that silica-sol processing method had lower pore diameter and higher surface diffusion coefficient than silica-sol dip coating method.     

Electro-Optical Properties of Inorganic Oxide/Liquid Crystal Composite Film by the Sol-Gel Process

The Electro-optical properties of a composite ceramic/liquid crystal membrane, with the dispersed liquid crystals (LCs) in the high dielectric constant inorganic oxides, were investigated. The composite membranes having bicontinuous micron structure of matrices and LCs were made by the sol-gel process via the syntheses of multicomponent sols using tetra-n-butyltitanate, barium diethoxyethoxide, methyltriethoxsilane and polyethylene glycol (PEG) for phase separation. The switching behaviors of the composite films have been observed by applying AC voltages at 1 kHz. It was also confirmed that the driving voltage of the composite film depended on the composition of the inorganic oxide matrix. The composite films of high dielectric constant were driven at an electric field under 10 kV/cm, much less than that of existing PDLC.     

Structural characterisation of heat-treated anodic alumina membranes prepared using a simplified fabrication process

A facile method for forming porous anodic alumina membranes based on one-step anodising in sulphuric acid is reported. A flat and well-ordered basal surface incorporating uniformly sized pores was obtained without the need for electrolytic polishing. Excess metallic aluminium was removed from the film using a saturated solution of iodine in methanol. The high-temperature properties of the oxide ceramic membranes were investigated using thermal analysis, mass spectrometry, X-ray diffraction and solid-state nuclear magnetic resonance. At 970 °C the amorphous alumina crystallises to γ-Al₂O₃ with the release of SO₂ and O₂. Finally at 1228 °C the alumina converts into the thermodynamically preferred phase, corundum. The pore structure of the oxide membrane was found to be very stable at elevated temperatures, suggesting applications in materials synthesis, catalysis and gas separation.     

Fabrication of multi-layer composite hollow fiber membranes for gas separation

Using multilayer composite hollow fiber membranes consisting of a sealing layer (silicone rubber), a selective layer (poly(4-vinylpyridine)), and a support substrate (polysulfone), we have determined the key parameters for fabricating high-performance multilayer hollow fiber composite membranes for gas separation. Surface roughness and surface porosity of the support substrate play two crucial roles in successful membrane fabrication. Substrates with smooth surfaces tend to reduce defects in the selective layer to yield composite membranes of better separation performance. Substrates with a high surface porosity can enhance the permeance of composite membranes. However, SEM micrographs show that, when preparing an asymmetric microporous membrane substrate using a phase-inversion process, the higher the surface porosity, the greater the surface roughness. How to optimize and compromise the effect of both factors with respect to permselectivity is a critical issue for the selection of support substrates to fabricate high-performance multilayer composite membranes. For a highly permeable support substrate, pre-wetting shows no significant improvement in membrane performance. Composite hollow fiber membranes made from a composition of silicone rubber/0.1–0.5 wt% poly(4-vinylpyridine)/25 wt% polysulfone show impressive separation performance. Gas permeances of around 100 GPU for H₂, 40 GPU for CO₂, and 8 GPU for O₂ with selectivities of around 100 for H₂/N₂, 50 for CO₂/CH₄, and 7 for O₂/N₂ were obtained.     

Research on the electrochemistry of oxygen ion conductors in the former Soviet Union

This review is focused on the analysis of experimental results on oxygen ion-conducting ceramic materials based on HfO₂, CeO₂, and ThO₂, published in the former Soviet Union. In particular, the physicochemical and transport properties of fluorite-related oxides and the characteristics of electronic conduction in these solid electrolytes are briefly reviewed. Emphasis is given to electrocatalytic and electrochemical properties of cerium-containing oxides, which are promising materials for electrodes of electrochemical cells operating in reducing atmospheres, and mixed-conducting membranes. A comparative analysis of specific features of the solid-electrolyte ceramics based on hafnia, zirconia, ceria, and thoria is performed in order to reveal basic tendencies of oxygen ionic transport in fluorite-type oxides, and to identify the potential applicability of these materials in various high-temperature electrochemical devices. Received: 23 September 1999 / Accepted: 10 October 1999     

Nanotechnological method to control the molecular weight cut-off and/or pore diameter of organic–inorganic composite membrane

Polyimide–alumina composite membranes were fabricated by the nanotechnological copolymerization method of co-polymer which has a constant repeating unit chemically bound by primer on the wall and/or surface of the porous ceramic support. By changing the number of repeating unit (n) in the polymer, the fabricated pyromellitic dianhydride (PMDA)- diaminodiphenylether (ODA) composite membranes have separation factor α_CO2/CH4 in the range 1.0–6.4 and molecular weight cut-off (MWCO) ranging 400–4000. As for the composite membranes of n=20, the separation factor α_CO2/CH4 of the 4,4′-(hexafluoroisopropylidene)-diphathalic anhydride (6FDA)-diaminodiphenylether (ODA) composite membrane was approximately 1.6 times larger than that of the PMDA–ODA composite membranes, and these values were 7.5 and 4.7 at 323 K, respectively. With the increase of temperature, the separation factor decreased, and the value obtained was 4.8 at 423 K. The pure gas permeances through the carbon membrane (PMDA–ODA: n=20) was approximately 75–260 times larger than the values through the PMDA–ODA (n=20) composite membranes. But this membrane did not show any gas separation ability.     

Creating and Modulating Microdomains in Pore‐Spanning Membranes

The architecture of the plasma membrane is not only determined by the lipid and protein composition, but is also influenced by its attachment to the underlying cytoskeleton. Herein, we show that microscopic phase separation of “raft‐like” lipid mixtures in pore‐spanning bilayers is strongly determined by the underlying highly ordered porous substrate. In detail, lipid membranes composed of DOPC/sphingomyelin/cholesterol/Gb₃ were prepared on ordered pore arrays in silicon with pore diameters of 0.8, 1.2 and 2 μm, respectively, by spreading and fusion of giant unilamellar vesicles. The upper part of the silicon substrate was first coated with gold and then functionalized with a thiol‐bearing cholesterol derivative rendering the surface hydrophobic, which is prerequisite for membrane formation. Confocal laser scanning fluorescence microscopy was used to investigate the phase behavior of the obtained pore‐spanning membranes. Coexisting liquid‐ordered‐ (l_o) and liquid‐disordered (l_d) domains were visualized for DOPC/sphingomyelin/cholesterol/Gb₃ (40:35:20:5) membranes. The size of the l_o‐phase domains was strongly affected by the underlying pore size of the silicon substrate and could be controlled by temperature, and the cholesterol content in the membrane, which was modulated by the addition of methyl‐β‐cyclodextrin. Binding of Shiga toxin B‐pentamers to the Gb₃‐doped membranes increased the l_o‐phase considerably and even induced l_o‐phase domains in non‐phase separated bilayers composed of DOPC/sphingomyelin/cholesterol/Gb₃ (65:10:20:5).     

Preparation and characterization of NiP amorphous alloy/ceramic composite membrane

The NiP amorphous alloy/ceramic composite membrane of high selectivity and permeability for hydrogen was prepared by a new technique of partial electroless plating. Its permeability and ideal separation factor for H₂/Ar were investigated. The results demonstrated that the permeability of the NiP amorphous alloy membrane was almost the same as that of a porous inorganic membrane, but the separation factor for H₂/Ar through the NiP membrane was obviously higher than that through a porous inorganic one. The morphology and microstructure of NiP amorphous alloy/ceramic composite membrane were characterized by SEM and XRD.     

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.     

Nano oxides incorporated sulfonated poly(ether ether ketone) membranes with improved selectivity and stability for vanadium redox flow battery

Three kinds of sulfonated poly(ether ether ketone) (SPEEK)/nano oxide (Al₂O₃, SiO₂, and TiO₂) composite membranes are fabricated for vanadium redox flow battery (VRFB) application. The composite membranes with 5 wt% of Al₂O₃, SiO₂, and TiO₂ (S/A-5 %, S/S-5 %, and S/T-5 %) exhibit excellent cell performance in VRFB. Incorporation of nano oxides (Al₂O₃, SiO₂, and TiO₂) in SPEEK membrane improves in aspect of thermal, mechanical, and chemical stabilities due to the hydrogen bonds’ interaction between SPEEK matrix and nano oxides. The energy efficiencies (EEs) of composite membranes are higher than that of Nafion 117 membrane, owing to the good balance between proton conductivity and vanadium ion permeability. The discharge–capacity retentions of composite membranes also overwhelm that of Nafion 117 membrane after 200 cycles, indicating their good stability in VRFB system. These low-cost SPEEK/nano oxide composite membranes exhibit great potential for the application in VRFB.     

A MOF Glass Membrane for Gas Separation

Metal–organic framework (MOF) glasses are promising candidates for membrane fabrication due to their significant porosity, the ease of processing, and most notably, the potential to eliminate the grain boundary that is unavoidable for polycrystalline MOF membranes. Herein, we developed a ZIF‐62 MOF glass membrane and exploited its intrinsic gas‐separation properties. The MOF glass membrane was fabricated by melt‐quenching treatment of an in situ solvothermally synthesized polycrystalline ZIF‐62 MOF membrane on a porous ceramic alumina support. The molten ZIF‐62 phase penetrated into the nanopores of the support and eliminated the formation of intercrystalline defects in the resultant glass membrane. The molecular sieving ability of the MOF membrane is remarkably enhanced via vitrification. The separation factors of the MOF glass membrane for H₂/CH₄, CO₂/N₂ and CO₂/CH₄ mixtures are 50.7, 34.5, and 36.6, respectively, far exceeding the Robeson upper bounds.     

Nafion/PTFE and zirconium phosphate modified Nafion/PTFE composite membranes for direct methanol fuel cells

We prepared Nafion/PTFE (NF) and zirconium phosphate (ZrP) hybridized Nafion/PTFE composite membranes (NF–ZrP). NF–ZrP composite membranes were prepared via two processes. One is impregnating sub-μm porous PTFE membrane directly in a Nafion/ZrOCl₂ solution (NF–Zr–d). The other is impregnating sub-μm porous PTFE membrane in a Nafion solution to prepare NF composite membrane, and then the NF membrane was impregnated in a ZrOCl₂ aqueous solution via in situ precipitation method (NF–Zr–I). The ZrOCl₂ inserted in NF composite membranes was then reacted with phosphoric acid to form ZrP and thus NF–ZrP–d and NF–ZrP–I composite membranes were obtained. The direct methanol fuel cell (DMFC) performances of membrane electrode assemblies prepared from Nafion-117, NF, NF–ZrP–d, and NF–ZrP–I composite membranes were investigated. The effects of introducing sub-μm porous PTFE film and ZrP particles into Nafion membranes on the DMFC performance were investigated. The influence of ZrP hybridizing process into NF membranes (the process of preparing NF–ZrP–I is inserting ZrOCl₂ into NF membranes after Nafion is annealed and the process of preparing NF–ZrP–d is mixing ZrOCl₂ into a Nafion solution before Nafion is annealed) on the morphology of NF–ZrP composite membranes and thus on the DMFC performance was also discussed.     

有机物单层分散在相关材料制备中的应用

与无机氧化物和盐类在载体表面自发单层分散相类似, 许多有机物也可以在载体表面自发单层分散.有机物在载体表面单层分散行为和分散后的存在状态与有机物分子形状和结构特点及载体表面性质和孔结构有关. 利用有机物在载体表面的单层分散, 可以设计制备具有优异性能的材料. 本文简要综述了近年来这方面研究工作取得的相关进展, 主要介绍了有机物单层分散在碳/氧化物复合物、氧化物和薄壁中孔碳材料的制备和织构调控方面的一些应用实例. 单层分散的有机物热分解后可在载体表面形成均匀的薄碳层, 以无机多孔氧化物为载体可制备出包覆均匀碳薄层的碳/氧化物复合物, 这种碳/氧化物复合物在染料吸附、催化剂载体和光催化方面显现出好的性能. 以溶胶-凝胶法制备氧化物时, 分散的有机物可以隔离溶胶颗粒, 从而制备出高比表面积的氧化物并可对孔容进行调控, 以此方法制备的γ-氧化铝比表面积可达506 m²·g^-1. 在惰性气氛中加热上述碳/氧化物复合物, 碳层可抑制氧化物的相变; 而在氧气中, 碳层燃烧发热会促进相变, 由此可快速制备超细α-氧化铝. 包覆均匀碳薄层的氧化物载体对碳起支撑作用, 在将氧化物溶解去除后, 可便捷制得高比表面积、大孔容、高中孔率的薄壁中孔碳材料, 碳材料的形貌、孔径分布等可通过选用不同织构的氧化物载体进行调控.     

Microstructural Analysis of the Effects of Poly(ethylene glycol) on an Acid Catalyzed Sol-Gel Derived Ceramic Material

Ceramic materials have been derived from an acid catalyzed sol-gel process. The addition of different molecular weights and concentrations of polyethylene glycol (PEG) to the sol mixture modifies the phase behaviour of the sol-gel process. The resulting gel is burned at 973 K to make porous ceramic materials. Nitrogen adsorption-desorption isotherms are used to assess the effects of PEG on the internal structure of the burned ceramic material. These isotherms indicate an extensive pore network exists consisting of micropores and mesopores. In the micropore region of the isotherms, the _S-plot analysis reveals changes in specific primary micropore volumes, specific total pore volumes, specific external surface areas and specific SPE surface area when PEG is added in the sol-gel process. The average pore width and the overall mesopore size distribution curves shift to higher pore size values and ranges on addition of PEG to the sol-gel mixture. The presence of PEG during the sol-gel process leads to an apparent narrowing of the micropore size distribution. The results of this work clearly indicate that the molecular weight and the concentration of a polymer, such as PEG, influences the eventual internal structure of a ceramic after burning.