A novel proton-exchange porous silicon membrane production method for μDMFCs

This study introduces a new production method to use as a porous silicon-based proton exchange membrane for μDMFCs. In this respect, EIS, fuel crossover test, and fuel cell performance test at the μDMFC sample cell are performed at room temperature on a porous silicon-based membrane that was produced for passive mode μDMFC as a proton exchange membrane. The reason for performing the fuel crossover test is to ensure the silicon opened pores along the silicon wafer and to examine the fuel permeability of the membrane. The fuel crossover test shows that the fuel cell provides energy for about 60 min with a 50 mL fuel. EIS reveals proton permeability of proton exchange membrane. The calculated value of the conductivity of the membrane is 0.0016 S/cm. OCV of the system is 0.4V, whereas values (with highest power density is 0.1 mW/cm²and with the highest current density is 0.39 mA/cm²) are low. However, porous silicon is not a natural proton conductor. Hence, these values can be increased by different ways such as porous silicon functionalized, or serial connection of fuel cells. On the other hand, the value of OCV is consistent with the previous studies. In sum, this study presents a simple, cost-effective, and short time-consuming method for the production of porous silicon as proton-conducting membrane behavior.     

p-Nitrophenol permeability and temperature characteristics of an acryloyl-L-proline methyl ester-based porous gel membrane

Thermoresponsive porous gel membranes were synthesized by a simultaneously occurring process consisting of radiation-induced polymerization and crosslinking in aqueous solutions at various concentrations of acryloyl-L -proline methyl ester(A-ProOMe) without a crosslinker. Permeation of p-nitrophenol (PNP) through a thermoresponsive porous gel membrane obtained at a monomer concentration of 80% (w/w) drastically reduced around 14°C, the lower critical solution temperature (LCST) of linear poly(A-ProOMe) in water, from 0.60 × 10⁻³ cm/min at 10°C to no permeation at 18°C, accompanied by changes in both size and shape of pores associated with gel shrinkage. Moreover, it was found that porous gel membranes with a porosity of approximately 60% had a greater PNP permeability constant through porous gel membranes with mutually connected pores obtained at a monomer concentration of 50% (w/w) than individually supported pores obtained at a monomer concentration of 70% (w/w). © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1495–1500, 1998     

Controlled growth for synthesizing a compact mordenite membrane

Controlling the growth of zeolite crystals on a porous alumina support is essential for preparing a compact zeolite membarne. First, mordenite seed crystals applied on a nonporous -alumina disk were grown and morphological change of mordenite crystals were observed in the course of growth. Then, mordenite membranes were synthesized on a porous -alumina tube under the same conditions employed in the study using the alumina disks. We found that seed crystal growth was widely controllable by changing water content in reaction solution, which resulted in better control of the morphology of mordenite crystals for synthesizing a thin compact mordenite membrane. Separation properties for mordenite membranes were studied in water–hydrogen binary system at 473 K with 10 kPa of water partial pressure, where no capillary condensation was expected in non-zeolitic pores. Separation factor for a mordenite membrane with a few defects was poor; however, a defect-free mordenite membrane prepared under a suitable condition highly separated steam from hydrogen.     

Preparation and characterization of La<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript>Co<Subscript>0.2</Subscript>Fe<Subscript>0.8</Subscript>O<Subscript>3-δ</Subscript> thin-film membrane on porous support by dip-coating method

Perovskite-type La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ (LSCF6428) thin-film membrane prepared by modified Pechini sol–gel process, was successfully deposited on porous support of similar composition using dip-coating method. Fine grain and crack-free film with perovskite structure was obtained at sintering temperature of 800 °C and dwelling time of 60 min. The cross-sectional image indicated that LSCF6428 thin-film membrane coated on the porous support showed excellent adhesion to the support with uniform thickness. The minimum dense layer thickness obtained by dip-coating method was around 0.5 μm. It was found that the oxygen permeability of the supported thin film was lower than that of the perovskite support, which indicated that the pores of the support were reduced by thin-film deposition on the support surface. The reduction in the pore size led to the more selective permeation mechanism contributes to the overall permeation. Successful deposition of LSCF6428 thin-film membrane on porous support can be considered as a promising technique for the preparation of oxygen separation membrane.     

Fabrication and characterization of single layer and multi-layer anodic alumina membranes

Porous alumina films containing parallel capillary pores of uniform size were fabricated by anodically oxidizing high purity aluminum films in phosphoric acid and sulfuric acid solutions. These films were formed into membranes by post-oxidation processing that removes unoxidized aluminum as well as a barrier layer of alumina from the base of the pores. Symmetric membranes were made by oxidizing at constant current density conditions. Two layer composite membranes were made by changing current density during the oxidation process. The thickness, pore density and porosity of each membrane were predicted from the relationships between structural characteristics and processing conditions that were developed in previously reported kinetic studies of anodic oxidation of aluminum.Each membrane was then characterized using permeability measurements. The hydraulic permeability of membranes formed in phosphoric acid and the diffusive permeability of membranes formed in sulfuric acid were measured. A comparison of the measured permeability values to those predicted using the structural characteristics calculated using relationships developed in the kinetic studies shows excellent agreement. These results illustrate that porous alumina membranes can be fabricated with transport characteristics that can be predicted from the processing conditions used during membrane formation.     

Binding of dl-tryptophan to BSA adsorbed in multilayers by polymer chains grafted onto a porous hollow-fiber membrane in a permeation mode

An anion-exchange-group-containing porous membrane in the form of a hollow fiber was prepared to immobilize bovine serum albumin (BSA) as a chiral selector. First, an epoxy-group-containing polymer chain was grafted onto the pore surface of the polyethylene porous hollow-fiber membrane pre-irradiated with an electron beam. Second, the epoxy group was converted to diethylamino and 2-hydroxyethylamino groups. Third, a BSA solution was forced to permeate through the pores of the membrane. As a result, 190 mg BSA per gram of membrane, which amounted to a degree of multilayer binding of about four, were immobilized. Subsequently, a racemic solution of tryptophan (0.02 mM) was forced to permeate through the BSA-multilayered porous membrane at a flow rate ranging from 10 to 80 ml/h. A two-stage stepwise concentration change of tryptophan in the effluent was observed due to independent chiral recognition of d- and l-tryptophan by BSA adsorbed in multilayers within the graft chains.     

Characterization of ultrafiltration membranes by drying

Since the drying rate curves of membranes are comparable to those of porous solid materials, it should be possible to obtain information on membrane structure and the state of water within the membrane from membrane drying experiments and the theory of drying. When membranes are dried on one side either through the active skin or the skin of the porous sublayer, a drying rate curve is obtained which characterizes the membrane. This paper describes how the drying rate curve can give information on the asymmetry of the membranes, on the degree of water uptake after drying, and on the effect of surfactants on rewetting. The drying curve can also be used to examine the effect of membrane manufacture on membrane structure. It has also been shown that partial drying may result in loss of permeability due to loss of the larger pores in the active skin. For the investigations reported in this paper, polyamide and polysulfone ultrafiltration membranes were used.     

The nature of permeability anisotropy and catalytic activity

The phenomena of permeability anisotropy and an increase in the rates of catalytic reactions in porous membranes modified with highly dispersed catalytic systems were analyzed. A model of stochastic gas motions was proposed; this model is based on the hypothesis of the specific interaction of molecules with the inner surface of pores resulting in a nonisotropic distribution of molecules over traveling directions. The effects of asymmetric gas transfer in porous and gradient-porous membranes were considered to explain differences in the rates of heterogeneous catalytic reactions in a nanoporous membrane reactor under changes in the direction of supplying a reaction mixture. From the model proposed, it follows that the transversal diffusion of gas molecules is most probable in the porous medium of a ceramic membrane with a pore-size distribution gradient from large to small pores along the flow direction. This diffusion results in an increase in the frequency of molecular collisions with the wall of a microchannel and, correspondingly, in an increase in the contact time. The model proposed explains the intensification of a number of heterogeneous catalytic reactions performed in the porous media of catalytic porous membranes.     

纳米Hg2Cl2/Al2O3有序阵列体系的制备和表征

纳米结构材料由于其独特的物理化学性质以及在微电子器件、光开关等方面的应用而备受关注. 多孔氧化铝由于具有孔径分布较窄、取向一致和孔密度高等优点而广泛用于模板制备纳米结构材料. 在多孔氧化铝中可以组装金属纳米粒子[1]、半导体纳米粒子[2]、导电高分子[3]以及碳纳米管[4]等.     

Structure-property relationships for novel wholly aromatic polyamide-hydrazides containing various proportions of para-phenylene and meta-phenylene units III. Preparation and properties of semi-permeable membranes for water desalination by reverse osmosis separation performance

Flat sheet asymmetric reverse osmosis membranes were successfully prepared from N,N-dimethylacetamide (DMAc) solutions of a series of novel wholly aromatic polyamide-hydrazides that contained different amounts of para- and meta-phenylene rings. These polyamide-hydrazides were synthesized by a low temperature solution polycondensation reactions of either 4-amino-3-hydroxybenzhydrazide or 3-amino-4-hydroxybenzhydrazide with an equimolar amount of either terephthaloyl dichloride [TCl], isophthaloyl dichloride [ICl] or mixtures of various molar ratios of TCl and ICl in anhydrous DMAc as a solvent. All the polymers have the same structural formula except of the way of linking phenylene units inside the polymer chains. The content of para- to meta-phenylene moieties was varied within these polymers so that the changes in the latter were 10 mol% from polymer to polymer, starting from an overall content of 0-100 mol%. All the membranes were characterized for their salt rejection (%) and water permeability (cm³ cm⁻² day⁻¹) of 0.5 N aqueous sodium chloride feed solution at 3924 kPa operating pressure. The effects of polymers structural variations together with several processing parameters to achieve the best combination of high selectivity and permeability were studied. Effects of various processing parameters of the membranes on their transport properties were investigated by varying the temperature and period of the solvent evaporation of the cast membranes, coagulation temperature of the thermally treated membranes, annealing of the coagulated membranes, casting solution composition, membrane thickness and the operating pressure. During the thermal treatment step, the asymmetric structure of the membranes with a thin dense skin surface layer supported on a more porous layer was established. The former layer seems to be responsible for the separation performance. The results obtained showed that membrane performance was very much influenced by all of the examined processing variables and that membranes with considerably different properties could be obtained from the same polymer sample by using different processing parameters. Thus, the use of higher temperatures and longer exposure times in the protomembrane forming thermal treatment step would result in a membrane of lower solvent content and with a thicker skin layer and consequently led to higher salt rejection at lower water permeability. Most significantly, the membrane properties clearly depended on the polymer structure. Under identical processing condition, substitution para-phenylene rings for meta-phenylene ones within the polymer series resulted in an increase in salt rejection capability of the membranes. This may be attributed to an increase in their chain symmetry associated with increased molecular packing and rigidity through enhanced intermolecular hydrogen bonding. This produces a barrier with much smaller pores that would efficiently prevent the solute particles from penetration. Coagulation temperature controls the structure (porosity) of the membrane particularly its supported layer and consequently its water permeability. Moreover, annealing of the prepared membranes in deionized water at 100 °C was found essential for useful properties in the single-stage separation applications, which required optimum membrane selectivity. Upon annealing, the membrane shrinks resulting in reducing its pore size particularly in the skin layer and consequently improving the salt rejection. Addition of lithium chloride to the casting solution produced a membrane with increased porosity and improved water permeability. Salt rejection capability of the membranes is clearly affected by the applied pressure, reaching its maximum at nearly 4000 kPa. Furthermore, the water permeability is inversely proportional to the membrane thickness, while the salt rejection is not substantially influenced.     

Heterogeneity within order in crystals of a porous metal-organic framework

Generally, crystals of synthetic porous materials such as metal-organic frameworks (MOFs) are commonly made up from one kind of repeating pore structure which predominates the whole material. Surprisingly, little is known about how to introduce heterogeneously arranged pores within a crystal of homogeneous pores without losing the crystalline nature of the material. Here, we outline a strategy for producing crystals of MOF-5 in which a system of meso- and macropores either permeates the whole crystal to make sponge-like crystals or is entirely enclosed by a thick crystalline microporous MOF-5 sheath to make pomegranate-like crystals. These new forms of crystals represent a new class of materials in which micro-, meso-, and macroporosity are juxtaposed and are directly linked unique arrangements known to be useful in natural systems but heretofore unknown in synthetic crystals.     

Hierarchically structured polyglycolide – a biomaterial mimicking natural bone

Polyglycolide (poly(hydroxyacetic acid)) with wide variability of pore size can be prepared by combination of a solid-state polymerization reaction and addition of sodium chloride crystals. The solid state reaction leads to a composite of polyglycolide and NaCl. The latter can be washed out with water leaving behind porous polyglycolide with interconnected pores in the micrometer range. To achieve a higher porosity and to induce larger pores, additional coarse sodium chloride was added before polymerization. This opens a way to functionally graded polyglycolide resembling natural bone that should have a potential as bone substituting material.     

Controlled release of model drugs through a molecular recognition ion gating membrane in response to a specific ion signal

A controlled-release device that responds to a specific molecular signal is an ideal goal in drug delivery and tissue engineering. A molecular recognition ion gating membrane, in which a copolymer of N-isopropylacrylamide and benzo[18]-crown-6-acrylamide was grafted onto the surface of the porous polyethylene film, was used to control the permeability of vitamin B12 and lysozyme in response to a specific ion signal. The observed response depended on the amount of grafted copolymer. When the grafting ratio was below 15%, the membrane pores opened by Ca2+ and closed by Ba2+. The permeability of model drugs became higher by opening of the pores. On the other hand, when the grafting ratio was above 15%, the properties of the membrane changed. The permeability of model drugs became lower by Ca2+ due to dehydration of the grafted copolymer. The opposite responses were observed at different grafting ratios.     

The effect of gas surface diffusion on the asymmetric permeability of two-layer porous membranes

The permeability is calculated for two-layer membranes composed of porous layers with nano- and microsized pores. It is found that, in nanosized pores, the gas transfer occurs in the free-molecular regime in the pore volume and via diffusion along the adsorption layer. The degree of adsorption layer filling is determined from the Langmuir isotherm. The dependence of the diffusion coefficient in the adsorption layer on the degree of surface coverage is taken into account. The transfer in the microsized pored is described in a hydrodynamic regime. The values of the membrane permeability are determined at different orientations with respect to the direction of a gas flow. It is shown that the difference between the permeability values may be as large as 60%.     

Hierarchically Porous Poly(ionic liquid) – Organic Cage Composite Membrane for Efficient Iodine Capture

The effective capture of iodine with high volatility and poisonousness is significant for reprocessing the spent nuclear fuel. In this article, we report a hierarchically porous poly(ionic liquid)-organic cage composite membrane (PIL@CC3) possessing a gradient content distribution of CC3 cage crystals throughout the membrane to capture iodine vapor. The introduction of microporous CC3 can significantly enhance the uptake capacity of iodine up to 980 mg g⁻¹, which is superior to that of a pristine PIL membrane carrying large meso- and macropores (99 mg g⁻¹), and CC3 crystalline powder (662 mg g⁻¹). Such enhanced performance benefits from the micro-meso-macroporous structure of the PIL@CC3 membrane in which the large meso- and macropores facilitate the mass transfer of iodine molecules from the external environment into the surface of the CC3 crystal, followed by diffusion of iodine molecules from the CC3 surface into the interior and exterior pores of the CC3 crystal. In addition, the asymmetric distribution of CC3 crystals across the PIL@CC3 membrane also displays its advantage in intercepting trace iodine, revealing its great potential for practical application. This study provides an idea for constructing hierarchically porous membrane composites for the removal of toxic vapors.     

Aqueous-phase synthesis of PAA in PVDF membrane pores for nanoparticle synthesis and dichlorobiphenyl degradation

This paper deals with bimetallic (Fe/Pd) nanoparticle synthesis inside the membrane pores and application for catalytic dechlorination of toxic organic compounds form aqueous streams. Membranes have been used as platforms for nanoparticle synthesis in order to reduce the agglomeration, encountered in solution phase synthesis which leads to a dramatic loss of reactivity. The membrane support, polyvinylidene fluoride (PVDF) was modified by in situ polymerization of acrylic acid in aqueous phase. Subsequent steps included ion exchange with Fe²⁺, reduction to Fe⁰ with sodium borohydride and Pd deposition. Various techniques, such as STEM, EDX, FTIR and permeability measurements, were used for membrane characterization and showed that bimetallic (Fe/Pd) nanoparticles with an average size of 20–30 nm have been incorporated inside of the PAA-coated membrane pores. The Fe/Pd-modified membranes showed a high reactivity toward a model compound, 2,2′-dichlorobiphenyl and a strong dependence of degradation on Pd (hydrogenation catalyst) content. The use of convective flow substantially reduces the degradation time: 43% conversion of dichlorobiphenyl to biphenyl can be achieved in less than 40 s residence time. Another important aspect is the ability to regenerate and reuse the Fe/Pd bimetallic systems by washing with a solution of sodium borohydride, because the iron becomes inactivated (corroded) as the dechlorination reaction proceeds.     

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.     

Radiation-induced graft polymerization and sulfonation of glycidyl methacrylate on to porous hollow-fiber membranes with different pore sizes

An epoxy group containing monomer, glycidyl methacrylate, was grafted on to a porous hollow-fiber membrane, made of polyethylene, whose different pore sizes ranged from 0.2 to 0.5 μm. The resulting epoxy group was converted into a sulfonic acid group as a cation-exchange group to capture metal ions. The porous network was retained after grafting and subsequent sulfonation because the poly-GMA chains invaded the polymer matrix. An increase in the SO₃H group density of the graft chain decreased the permeability of pure water because the graft chains expanded toward the pore interior due to their mutual electrostatic repulsion. The ion-exchange adsorption of Pb ions during the permeation of a Pb(NO₃)₂ solution through the pores edged by the cation-exchange graft chains was observed with a negligible diffusional mass-transfer resistance.     

Preparation of glucose-sensitive microcapsules with a porous membrane and functional gates

A glucose-sensitive microcapsule with a porous membrane and with linear-grafted polyacrylic acid (PAAC) chains and covalently bound glucose oxidase (GOD) enzymes in the membrane pores acting as functional gates was successfully prepared. Polyamide microcapsules with a porous membrane were prepared by interfacial polymerization, PAAC chains were grafted into the pores of the microcapsule membrane by plasma-graft pore-filling polymerization, and GOD enzymes were immobilized onto the PAAC-grafted microcapsules by a carbodiimide method. The release rates of model drug solutes from the fabricated microcapsules were significantly sensitive to the existence of glucose in the environmental solution. In solution, the release rate of either sodium chloride or VB(12) molecules from the microcapsules was low but increased dramatically in the presence of 0.2mol/L glucose. The prepared PAAC-grafted and GOD-immobilized microcapsules showed a reversible glucose-sensitive release characteristic. The proposed microcapsules provide a new mode for injection-type self-regulated drug delivery systems having the capability of adapting the release rate of drugs such as insulin in response to changes in glucose concentration, which is highly attractive for diabetes therapy.     

High-speed recovery of antimony using chelating porous hollow-fiber membrane

A porous hollow-fiber membrane containing an iminodiethanol (IDE) group as the chelate-forming group was applied to the recovery of antimony in the permeation mode. An antimony solution was forced to permeate through the pores of the chelating porous hollow-fiber membrane, driven by a transmembrane pressure. The membrane with a thickness of 0.7 mm and a porosity of 70% had an iminodiethanol group of 1.6 mol/kg of the membrane and a water flux of 0.95 m/h at 0.1 MPa and 298 K. The breakthrough curves of antimony overlapped irrespective of the permeation rate of the antimony solution ranging from 2 to 20 ml/min, i.e. the residence time across the membrane thickness ranging from 3.4 to 0.34 s, because of negligible diffusional mass-transfer resistance of the ionic species of antimony to the iminodiethanol group. At antimony concentrations below 10 mg/l (pH 4.0), a linear adsorption isotherm was obtained. The adsorbed antimony was quantitatively eluted by permeation of 2 M hydrochloric acid through the pores of the membrane.