Mesoporous Membranes Research Articles

A general model for estimating the ordering of mesoporous film by grazing incidence small angle X-ray scattering

The ability to quantifiably characterize the structure of mesoporous membranes over a large sample surface area is inherently crucial to the preparation of well-controlled and functional mesoporous silica films. To estimate the degree of ordering in silica films on a statistical level, grazing incidence small angle x-ray scattering (GISAXS) was used to analyze the nanostructure of silica films with high-ordered mesochannel arrays, and a general analysis model was developed for interpreting the resulting GISAXS patterns. Structural information on the films, including the mesochannel size, orientation, positional distribution factor, subunit number in coherent region, and mesochannel length distribution, were obtained by fitting the model to the experimental data. The GISAXS analysis results showed the gradually decreasing ordering of silica film, which can be attributed to two aspects: the positional deviation from ideal lattice sites in the plane perpendicular to the substrate surface, and the wide length distribution of the mesochannels. The proposed model is expected to be successfully extendable to the analysis of several other types of mesoporous film with different pore morphologies.

Identification of Molecular Transport Mechanisms in Micro-Porous Hydrotalcite–Silica Membrane

Hydrotalcite (HT) materials have been known to be able to adsorb $$\hbox {CO}_{2}$$ even at high temperature. However, HT has not been made into a micro-porous membrane because of its meso-porous nature. In order to form a micro-porous HT membrane, silica was selected as a host matrix due to its ability to retain its micro-porosity. In this paper, a micro-porous hydrotalcite–silica membrane was formed on a meso-porous $$\upgamma $$ -alumina layer supported by a macro-porous $$\upalpha $$ -alumina substrate. Most of the micro-porosity determined from nitrogen adsorption measurement was found to be either closed or open but not interconnected, whereas most of the meso-porosity (at ca. 11.2 nm) in the micro-porous membrane was open and interconnected, thereby promoting gas flow. Viscous flow mechanism was observed to dominate transport of gases in macro-porous membrane. Knudsen diffusion dominated transport of gases in meso-porous membrane. On the other hand, surface affinity influenced the transport of carbon dioxide through the micro-porous membrane rather significantly. While permeability of pure hydrogen and carbon dioxide were independent of pressure, the permeability of the gases in the binary mixtures decreased with increasing pressure. Both experiment and simulation demonstrated consistent results.

Silica Colloidal Membranes with Enantioselective Permeability

AbstractRobust mesoporous membranes composed of silica spheres were surface‐modified with chiral selector moieties, including small molecules, macrocycles, and polymers. Diffusion rates of enantiomers of a chiral dye through the resulting asymmetrically modified colloidal membranes were measured and the corresponding permselectivities were calculated. The membranes showed enantioselectivities in the range of 1.2–1.8, which were not significantly affected by the structure of the surface‐immobilized chiral electors. This selectivity is on par with most reported polymer‐based solid membranes and bulk liquid membranes. The enantioselectivity results from the surface‐facilitated mechanism of transport of enantiomers through the mesopores.

Ordered mesoporous carbon membrane prepared from liquefied larch by a soft method

Carbon membranes having ordered mesoporous structure were prepared via liquefaction, resinification, assembly and carbonization steps using natural renewable larch sawdust as starting material. The mesopores of the carbon membranes were formed by assembly of larch-based resin with F127 (PEO–PPO–PEO). The mesoporosity was controllable from disordered to ordered by varying the carbonization temperature. Mesoporous carbon membrane with an ordered two-dimensional hexagonal porous structure having P6mm symmetry was obtained at 700°C. The unique ordered mesoporous structure resulted in the carbon membranes exhibiting high efficiency (permselectivity factor =1.97) for the separation of CO2 from a N2/CO2 mixture.

Insight into Proton Transfer in Phosphotungstic Acid Functionalized Mesoporous Silica-Based Proton Exchange Membrane Fuel Cells

We have developed for fuel cells a novel proton exchange membrane (PEM) using inorganic phosphotungstic acid (HPW) as proton carrier and mesoporous silica as matrix (HPW-meso-silica) . The proton conductivity measured by electrochemical impedance spectroscopy is 0.11 S cm(-1) at 90 °C and 100% relative humidity (RH) with a low activation energy of ∼14 kJ mol(-1). In order to determine the energetics associated with proton migration within the HPW-meso-silica PEM and to determine the mechanism of proton hopping, we report density functional theory (DFT) calculations using the generalized gradient approximation (GGA). These DFT calculations revealed that the proton transfer process involves both intramolecular and intermolecular proton transfer pathways. When the adjacent HPWs are close (less than 17.0 Å apart), the calculated activation energy for intramolecular proton transfer within a HPW molecule is higher (29.1-18.8 kJ/mol) than the barrier for intermolecular proton transfer along the hydrogen bond. We find that the overall barrier for proton movement within the HPW-meso-silica membranes is determined by the intramolecular proton transfer pathway, which explains why the proton conductivity remains unchanged when the weight percentage of HPW on meso-silica is above 67 wt %. In contrast, the activation energy of proton transfer on a clean SiO2 (111) surface is computed to be as high as ∼40 kJ mol(-1), confirming the very low proton conductivity on clean silica surfaces observed experimentally.

Thin Films: Polymer-Modified Mesoporous Silica Thin Films for Redox-Mediated Selective Membrane Gating (Adv. Funct. Mater. 11/2014)

The control of ionic transport through nanometersized pores is described by M. Gallei, A. Brunsen, and co-workers. On page 1591, the charge of mesoporous silica membranes is controlled by the oxidation or reduction of redox-responsive ferrocene-containing polymers. Depending on the surface charge, mainly determined by the polymer oxidation state, positively or negatively charged test-molecules are able to diffuse into the pores or are excluded. Cover-Artwork by Criss Hohmann, Nanosystems Initiative Munich (NIM).

Studies on the meso-sized selectivity of a novel organic/inorganic hybrid mesoporous silica membrane

A novel free-standing organic/inorganic hybrid mesoporous silica membrane (HMSM) composed of mesoporous silica rods inside the channels of porous polyethylene terephthalate (PET) membrane was synthesized by employing the aspiration-induced infiltration method combined with solvent extraction. As-synthesized HMSM, in short PET-templated HMSM, was characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), N2 adsorption–desorption, thermal gravimetric analysis (TG) and molecule permeation measurements. The results indicate that the PET-templated HMSM presents the structural integrity of membrane material, the mesoporosity of hybrid mesoporous material, and the size-selectivity of nanofilter between small molecule and biomacromolecule. Moreover, the ion transfer behaviors occurring at the liquid/liquid (water/1, 2-dichloroethane) interface supported by PET-templated HMSM were investigated. It is found that such a HMSM-support liquid/liquid interface presents the asymmetric diffusion field with meso-sized selectivity between small ion and biomacromolecule. Both of the molecule permeation and the ion transfer experiment results demonstrate that the PET-templated HMSM can be applied as the nanofiltration membrane not only in aqueous solution but also at the membrane-supported liquid/liquid interface due to the meso-sized selectivity of mesopores formed in the channels of PET. This work firstly extends the applications of HMSMs with unique structure of pores-in-pores to the membrane-supported liquid/liquid interface electrochemistry and the size-selective ion transfer at the liquid/liquid interface.

Roll-to-Roll Preparation of Mesoporous Membranes by Nanoparticle Template Removal

Mechanistic and kinetic insights into the removal of soluble nanoparticles as templates for mesopores now permit scale-up of mesoporous polymer membranes. Investigations on the effect of pH and nanoparticle dissolution time showed that pH levels of 0 or 1 are necessary to enable fast pore template dissolution. Approximately 5 wt % of the originally applied nanoparticles remained in the membrane due to complete encapsulation by the polymer matrix but did not contaminate the permeate during prolonged use. We demonstrated continuous production of 17 m2 (100 m length) of membrane using a commercial roll-to-roll coating unit. These membranes were successfully applied in gravitation driven water filtration. The rates of rejection of bacteria from heavily contaminated natural pond water were higher than 99.99%.

Electrochemical enzymatic biosensor with long-term stability using hybrid mesoporous membrane

An acetylcholinesterase-immobilized sensor unit was successfully prepared by encapsulating the enzyme within hybrid mesoporous silica membranes (F127-MST). Through a novel combination with tetracyanoquinodimethane, both acetylcholine and organophosphorus pesticides were successfully detected with high sensitivity. Furthermore, we manufactured the working prototype of an enzyme sensor with this sensor unit for detecting dichlorvos, aldicarb and parathion. At present, the detection limit in this working prototype either equaled or surpassed that of others. Also, we have the advantage of increased stability of the enzyme against the outer environment by encapsulation of the enzymes into a silica nanospace. Consequently, acetylcholinesterase immobilized in F127-MST is a practical sensor with high sensitivity, reusability, and storage stability.

Functionalized mesoporous structured inorganic materials as high temperature proton exchange membranes for fuel cells

High temperature proton exchange membrane fuel cells based on functionalized mesoporous silica nanocomposite membranes.

Comprehensive strategy to design highly ordered mesoporous Nafion membranes for fuel cells under low humidity conditions

Highly-ordered mesoporous-structured Nafion membranes are synthesized by a new soft template method assisted with silica colloidal mediator for fuel cells.

Anodic alumina-templated synthesis of mesostructured silica membranes – current status and challenges

Hierarchically structured membranes of mesoporous silica in anodic alumina: status and challenges.

Membranas Poliméricas de PE-g-MA-Al/AlPO4 Impregnado com Fe, Al Utilização na Recuperação de Efluentes de Galvanoplastia

Neste trabalho foram desenvolvidas membranas filtrantes de materiais hibridos de polietileno de alta densidade modificado com anidrido maleico e ionizado com hidroxido de aluminio (PE-g-MA-Al) (matriz orgânica) e fosfato de aluminio amorfo puro (AlPO4) ou impregnado com ferro e/ou aluminio (fase dispersa inorgânica). Essas membranas foram obtidas nas composicoes de PE-g-MA-Al (M1), PE-g-MA-Al/AlPO4 (M2), PE-g-MA-Al/AlPO4-Al (M3), PE-g-MA-Al/AlPO4-Fe (M4) e PE-g-MA-Al/AlPO4-Fe,Al (M5), na proporcao de 97/3% em massa, pelo metodo de casting, usando xileno a quente e sob refluxo, como solvente. As tecnicas de espectroscopia na regiao do infravermelho (FTIR), difracao de raios X (DRX), analise termogravimetrica (TGA), analise textural, tensao superficial e microscopia eletronica de varredura (MEV) serviram para comprovar a influencia da natureza dos materiais e da metodologia usada na confeccao das membranas na incorporacao da fase inorgânica na matriz polimerica e nas propriedades dos materiais hibridos resultantes. A aplicacao das membranas na recuperacao de metais presentes no efluente de galvanoplastia mostrou que a presenca do ferro e/ou aluminio influenciou na morfologia e nas propriedades texturais desses materiais, e consequentemente na utilizacao como membranas mesoporosas de ultrafiltracao, com os melhores resultados apresentados por M1 e M3 na recuperacao de ferro (32 e 35%) e zinco (62 e 60%).

Controlling Polymerization Initiator Concentration in Mesoporous Silica Thin Films

We present a strategy toward controlled polymer density in mesopores by specifically adjusting the local amount of polymerization initiator at the pore wall. The polymerization initiator concentration as well as the polymer functionalization has a direct impact on mesoporous membrane properties such as ionic permselectivity. Mesoporous silica-based thin films were prepared with specifically adjusted amount of polymerization initiator (4-(3-triethoxysilyl)propoxybenzophenone (BPSilane)) or initiator binding functions ((3-aminopropyl)triethoxysilane (APTES)), directly and homogeneously incorporated into the silica wall pursuing a sol-gel-based co-condensation approach. The amount of polymerization initiator was adjusted by varying its concentration in the sol-gel precursor solution. The surface chemistry, porosity, pore accessibility, and reactivity of the surface functional groups were investigated by using infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray reflectometry, ellipsometry, atomic force microscopy, and transmission electron microscopy. We could gradually modify the amount of reactive polymerization initiators in these mesoporous membranes. Mesopores were maintained for APTES containing films for all tested ratios up to 25 mol % and for BPSilane containing films up to 15 mol %. These films showed accessible and charge-dependent ionic permselectivity and an increasing degree of functionalization with increasing precursor ratio. This approach can directly result in control of polymer grafting density in mesoporous films and thus has a direct impact on applications such as the control of ionic transport through mesoporous silica membranes.

Composite PET Membrane with Nanostructured Ag/AgTCNQ Schottky Junctions: Electrochemical Nanofabrication and Charge-Transfer Properties

Large-area nanostructured Ag/Ag-tetracyanoquinodimethane (TCNQ) Schottky junctions are fabricated electrochemically on a mesoporous polyethylene terephthalate (PET) membrane-supported water/1, 2-dichloroethane (DCE) interface. When the interface is polarized, Ag(+) ions transfer across the PET membrane from the aqueous phase and are reduced to form metallic Ag on the PET membrane, which reacts further with tetracyanoquinodimethane (TCNQ) in the DCE phase to form nanostructured Ag/AgTCNQ Schottky junctions. Once the mesoporous membrane is blocked by metallic Ag, a bipolar mechanism is proposed to explain the successive growth of AgTCNQ nanorods and Ag film on each side of the PET membrane. Due to the well-formed nanostructure of Ag/AgTCNQ Schottky junctions, the direct electrochemical behavior is observed, which is essential to explain the physicochemical mechanism of its electric performance. Moreover, the composite PET membrane with nanostructured Ag/AgTCNQ Schottky junctions is tailorable and can be assembled directly into electric devices without any pretreatment.

Comprehensive study of the templating effect on the ZnO nanostructure formation within porous hard membranes

ZnO nanowires were synthesized by combining the template-assisted method with three different ZnO growth approaches, i.e. sol–gel, aqueous chemical growth, and electrodeposition. We obtained nanostructures of 200, 50, and even 5 nm diameter in porous alumina and ordered mesoporous silica membranes, showing effective filling of the template channels and the formation of oriented ZnO nanostructures.

Polymer‐Modified Mesoporous Silica Thin Films for Redox‐Mediated Selective Membrane Gating

Controlling structure and function to switch ionic transport through synthetic membranes is a major challenge in the fabrication of functional nanodevices. Here we describe the combination of mesoporous silica thin films as structural unit, functionalized with two different redox‐responsive ferrocene‐containing polymers, polyvinylferrocene (PVFc) and poly(2‐(methacryloyloxy)ethyl ferrocenecarboxylate) (PFcMA), by using either a grafting to, or a grafting from approach. Both mesoporous film functionalization strategies are investigated in terms of polymer effect on ionic permselectivity. A significantly different ionic permselective behavior can be observed. This is attributed to different polymer location within the mesoporous film, depending on the functionalization strategies used. Additionally, the influence of chemical oxidation on the ionic permselective behavior is studied by cyclic voltammetry showing a redox‐controlled membrane gating as function of polymer location and the pH value. This study is a first step of combining redox‐responsive ferrocene‐containing polymers and mesoporous membranes, and thus towards redox‐controlled ionic transport through nanopores.

Defect-sealing synthesis of vertically oriented ordered mesoporous silica membranes

Vertically oriented ordered mesoporous silica membranes have been successfully synthesized in our laboratory in the form of silica plugs filling the macron-sized straight pores of hydrophobic track-etched polycarbonate membrane support. However, these membranes have shown gaps between the plugs and support pore wall which make the membranes unfeasible for use. This paper reports on techniques of synthesis of defect-free ordered mesoporous silica membranes by filling the gaps with microporous silica. Here, the elimination of defects is achieved by filling the membrane gaps with an alkoxysilane followed by exposure to humid air to allow controlled hydrolysis and condensation resulting in the formation of microporous silica within the gaps. Molecular probing gas permeation and helium/nitrogen (or oxygen) binary separation tests, coupled with surface characterization methods, show that the final membranes contain ordered mesopores of about 2.7 nm pore diameter, running through the membrane, with gaps sealed by microporous silica having a pore size <0.55 nm.

Fabrication of phosphotungstic acid functionalized mesoporous silica composite membrane by alternative tape-casting incorporating phase inversion technique

Meso-porous silica (MCM-41) membranes functionalized by phosphotungstic acid (HPW) for high temperature proton exchange membrane fuel cells (HT-PEMFCs) are successfully developed by a cost-effective tape-casting incorporating phase inversion and vacuum assisted wet impregnation techniques. The microstructure of the membrane is characterized by field emission scanning electron microscopy (FESEM). The effect of MCM-41 content on the tensile strength, ultimate elongation, and weight gain ratio and swelling ratio in water/methanol of the membranes are investigated in detail. The thermal stability of MCM-41 membrane with/without HPW is analyzed by thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) techniques. In particular, the effects of HPW loading and MCM-41 content on the proton conductivity of HPW/MCM-41 membranes are studied comprehensively. The results on the swelling ratio and tensile tension show that the developed membranes can be applied as an electrolyte membrane for HT-PEMFCs. The developed MCM-41 membrane, in which polyethersulfone (PESf) is used as the supporting backbone, is able to operate up to 200 °C. The single cell assembled from HPW/MCM-41 membrane with 70 wt.% HPW loading gives a peak output power of ∼230 mW cm−2 and ∼125 mW cm−2 in H2/air at 90 °C and in methanol/air at 150 °C without any humidification, respectively.

Study on Electrospun-Membrane-Based Filter to Rapidly Remove Reactive Yellow 2 from Wastewater

Uniform highly ordered mesoporous silica nanofiber membrane was successfully prepared onto copper mesh via coaxial electrospinning combining with the solvent evaporation and extraction induced surfactant assembly process. After a high temperature treatment and amination, it was employed to remove reactive yellow 2 (RY2) in wastewater showing remarkable adsorption capacity. The maximum adsorption capacity can reach up to 371.7 mg/g. The structural properties of synthesized mesoporous silica were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier transform infrared spectrum (FTIR). And the adsorption behavior of prepared material was analyzed through three kinds of isotherm models.