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.     

Anodic alumina supported dual-layer microporous silica membranes

We present a new processing scheme for the deposition of microporous, sol–gel derived silica membranes on inexpensive, commercially available anodic alumina (Anodisk™) supports. In a first step, a surfactant-templated mesoporous silica sublayer (pore size 2–6 nm) is deposited on the Anodisk support by dip-coating, in order to provide a smooth transition from the pore size of the support (20 or 100 nm) to that of the membrane (3–4 Å). Subsequently, the microporous gas separation membrane layer is deposited by spin-coating, resulting in a defect-free dual-layer micro-/mesoporous silica membrane exhibiting high permeance and high selectivity for size selective gas separations. For example, in the case of CO₂:N₂ separation, the CO₂ permeance reached 3.0 MPU (1 MPU = 10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹) coupled with a CO₂:N₂ separation factor in excess of 80 at 25 °C. This processing scheme can be utilized for laboratory-scale development of other types of microporous or dense inorganic membranes, taking advantage of the availability, low cost and low permeation resistance of anodic alumina (or other metal oxide) meso- and macroporous supports.     

Electroviscous Effects in Ceramic Nanofiltration Membranes

Membrane permeability and salt rejection of a γ‐alumina nanofiltration membrane were studied and modeled for different salt solutions. Salt rejection was predicted by 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 by using the Hagen–Poiseuille equation by introducing electroviscosity instead of bulk viscosity. γ‐Alumina particles were used for ζ‐potential measurements. The ζ‐potential measurements show that monovalent ions did not adsorb on the γ‐alumina surface, whereas divalent ions were highly adsorbed. Thus, for divalent ions, the model was modified, owing to pore shrinkage caused by ion adsorption. The ζ‐potential lowered the membrane permeability, especially for membranes with a pore radius lower than 3 nm, a ζ‐potential higher than 20 mV, and an ionic strength lower than 0.01 m . The rejection model showed that, for a pore radius lower than 3 nm and for solutions with ionic strengths lower than 0.01 m , there is an optimum ζ‐potential for rejection, because of the concurrent effects of electromigration and convection. Hence, the model can be used as a prediction tool to optimize membrane perm‐selectivity by designing a specific pore size and surface charge for application at specific ionic strengths and pH levels.     

Synthesis and characterization of MCM-48 tubular membranes

MCM-48 membranes have been prepared on alumina supports of different pore sizes. A battery of characterization techniques has been used to study the physical properties and the quality of the membranes prepared. The highest quality membranes were prepared on supports with pore size of up to 60 nm. The MCM-48 membranes were tested in the separation of gas phase mixtures and a cyclohexane/O₂ selectivity higher than 270 was obtained. The selective separation of organic compounds from inert components is a result of the cooperative effects of capillary condensation in MCM-48 pores and of the specific interactions of the permeating compounds and the membrane material.     

Poly(N-isopropylacrylamide) grafted into nanopores: Thermo-responsive behaviour in the presence of different salts

Surface initiated polymerization of N(isopropylacrylamide) (NIPAM) was performed by controlled radical polymerization on PET track-etched membranes presenting two different pore diameters (narrow pores: ∼80 nm and large pores: ∼330 nm). The opening and closing characteristics of the resulting PNIPAM-g-PET membranes were investigated by conductometric measurements carried out at different temperatures below and above the LCST of PNIPAM and in the presence of different salts. Depending on the membrane pore size, two types of permeation control mechanisms are observed. In large pore membranes, expanded PNIPAM chains conformations result in reduced effective pore size and therefore lower permeabilities relative to collapsed macromolecules chain conformations. In contrast, in narrow pore membranes, the expanded PNIPAM brush presents greater degrees of hydration in the surface layer and therefore gives rise to higher permeabilities than the collapsed conformation. In this situation, the overall permeability is thus comparable to that of a hydrogel membrane.     

Influence of polypropylene membrane surface porosity on the performance of membrane distillation process

Two kinds of polypropylene capillary membranes were used in the membrane distillation (MD). These membranes exhibited a similar morphology, but one of them has an additional low porosity layer on the internal surface of capillaries. The changes of membrane performance during MD process of tap water were investigated. The presence of low porosity layer (thickness below 1 μm) caused that the air permeability was reduced from 1.365 to 0.863 dm³/m² s kPa, whereas the MD permeate flux was decreased only by 15%. A significantly larger decline of the flux was caused by CaCO₃ deposit formed during distillation of tap water. This deposit was removed every 30–70 h by rinsing the modules with a 2–5 wt.% HCl. Unfortunately, a repetition of this operation several times resulted in a gradual decline of the maximum permeate flux (distilled water as a feed). However, the module efficiency with the membranes covered by a surface layer of low porosity was found to decreases twice as slowly. The investigations revealed that a low surface porosity does not limit the possibility of surface wetting of polypropylene membranes, but hindered the scale formation inside the pores.     

Highly permeable mesoporous silica membranes synthesized by vapor infiltration of tetraethoxysilane into non-ionic alkyl poly(oxyethylene) surfactant films

Mesoporous silica membranes were prepared on porous alumina substrates by a vapor infiltration of tetraethoxysilane (TEOS) into a non-ionic poly(oxyethylene) (Brij56) surfactant film. Periodic mesostructured silica membranes were formed on both α- and γ-alumina substrates pre-treated with polystyrene. The polystyrene polymer plugged the pores of the alumina substrates and inhibited the deposition of silica in the alumina pores, resulting in the formation of a very thin silica membrane without a silica/alumina composite layer at the interface between mesoporous silica and the alumina substrates. The calcined mesoporous silica membrane showed very high nitrogen permeance (>10⁻⁶ mol m⁻² s⁻¹ Pa⁻¹). The single gas permeation was governed by the Knudsen diffusion mechanism. The durability of the mesoporous silica membrane against moisture in air was improved by a silylation with trimethylethoxysiliane.     

Solubility of CF4 in lower alcohols

This work is inserted in a research program that consists mainly in the experimental and theoretical study of the effect of association/solvation between solute and solvent molecules in the solubility of gases in liquids. The solubilities of tetrafluoromethane in lower alcohols (methanol, ethanol, 1-propanol, 1-butanol) have been determined in the temperature range of 288–308 K, at atmospheric pressure, using an automated apparatus recently assembled in our laboratory. An accuracy of 0.6% and a precision of the same order of magnitude was achieved. The solubility is the lowest in methanol which is the highest associated alcohol and increases with the C-content of the alcohol. The molar Gibbs energy, enthalpy and entropy of solution were calculated from the experimental results (at 1 atm partial pressure of the gas and 298 K). A semiempirical correlation between the solubility of CF₄ in alcohols at 298 K and the Gutmann acceptor number (AN) of the solvents has been developed.     

Properties of PWA/ZrO2-doped phosphosilicate glass composite membranes for low-temperature H2/O2 fuel cell applications

Phosphosilicate doped with a mixture of phosphotungstic acid and zirconium oxide (PWA/ZrO₂–P₂O₂–SiO₂) was investigated as potential glass composite membranes for use as H₂/O₂ fuel cell electrolytes. The glass membranes were studied with respect to their structural and thermal properties, proton conductivity, pore characteristics, hydrogen permeability, and performance in fuel cell tests. Thermal analysis including TG and DTA confirmed that the glass was thermally stable up to 400 °C. The dependence of the conductivity on the humidity was discussed based on the PWA content in the glass composite membranes. The proton transfer in the nanopores of the PWA/ZrO₂–P₂O₅–SiO₂ glasses was investigated and it was found that a glass with a pore size of ∼3 nm diameters was more appropriate for fast proton conduction. The hydrogen permeability rate was calculated at various temperatures, and was found to be comparatively higher than for membranes based on Nafion^®. The performance of a membrane electrolyte assembly (MEA) was influenced by its PWA content; a power density of 43 mW/cm² was obtained at 27 °C and 30% relative humidity for a PWA/ZrO₂–P₂O₅–SiO₂ glass membrane with a composition of 6–2–5–87 mol% and 0.2 mg/cm² of Pt/C loaded on the electrode.     

Cobalt-doped silica membranes for gas separation

Cobalt-doped silica membranes were synthesized using tetraethyl orthosilicate-derived sol mixed with cobalt nitrate hexahydrate. The cobalt-doped silica structural characterization showed the formation of crystalline Co₃O₄ and silanol groups upon calcination. The metal oxide phase was sequentially reduced at high temperature in rich hydrogen atmosphere resulting in the production of high quality membranes. The cobalt concentration was almost constant throughout the film depth, though the silica to cobalt ratio changed from 33:1 at the surface to 7:1 at the interface with the alumina layer. It is possible that cobalt has more affinity to alumina, thus forming CoOAl₂O₃. The He/N₂ selectivities reached 350 and 570 at 160 °C for dry and 100 °C wet gas testing, respectively. Subsequent exposure to water vapour, the membranes was regenerated under dry gas condition and He/N₂ selectivities significantly improved to 1100. The permeation of gases generally followed a temperature dependency flux or activated transport, with best helium permeation and activation energy results of 9.5 × 10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ and 15 kJ mol⁻¹. Exposure of the membranes to water vapour led to a reduction in the permeation of nitrogen, attributed to water adsorption and structural changes of the silica matrix. However, the overall integrity of the cobalt-doped silica membrane was retained, given an indication that cobalt was able to counteract to some extent the effect of water on the silica matrix. These results show the potential for metal doping to create membranes suited for industrial gas separation.     

The diffusion of methane in inorganic membrane pores under Knudsen conditions: the kinetics and depth of pyrocarbon deposition and its influence on transport properties

The kinetics of the topochemical reaction of methane dehydrogenation was studied using TRUMEM ultrafiltration membranes (TiO₂ + Cr₂O₃ on porous steel, the size of transport pores 50 nm). The depth of the deposition of pyrocarbon nanocrystallites (PNC) into pores was determined. The depth of PNC deposition was estimated using scanning electron microscopy and high-resolution energy-dispersion spectrometry. The deposition of PNC at a 4.9 kPa methane pressure in the reaction zone created Knudsen conditions for methane diffusion in pores. The deposition of PNC therefore occurred over the whole area of pore surfaces. Studies of the kinetics of PNC formation on the surface of pores showed that reaction rate V and its constant k substantially depended on reaction duration. The influence of PNC deposition on the electrosurface properties and permeability of the membranes to ethanol and dodecane was studied. After the deposition of PNC with L _c = 1.0–1.1 nm on the surface of pores, the ζ-potential and surface charge density (σ) decreased; simultaneously, the efficiency with respect to ethanol increased.     

The effects of electropolishing on the nanochannel ordering of the porous anodic alumina prepared in oxalic acid

A series of porous anodic alumina has been prepared by anodizing aluminum surface in 0.3 M oxalic acid at different voltages. Prior to anodizing, the surface was pretreated in two different electropolishing electrolytes. One was Brytal solution (15% Na₂CO₃ and 5% Na₃PO₄) at 80 °C in which the electropolishing was performed at 2 V. This resulted in about 100–150 nm apart random features of 4–5 nm height. The other was the commonly employed perchloric acid–alcohol solution (1:4 ratio by volume), in which the electropolishing was performed at 20 V. The resulting surface comprised nanostripes of 1–2 nm amplitude with a wavelength of about 50 nm. The former pretreatment proved better for self-ordering of the pores at the anodizing voltage of 50–60 V, while the latter pretreatment was found better at the anodizing voltage of 40 V. The improved pore ordering at a given voltage was attributed to the higher pore density as associated with greater repulsive interactions among the pores.     

Modelling of the pore structure variation with pH for pore-filled pH-sensitive poly(vinylidene fluoride)–poly(acrylic acid) membranes

Pore structure variation as a function of pH was investigated for the pore-filled pH-sensitive poly(acrylic acid)-poly(vinylidene fluoride) membranes. The pore radius reduced drastically as the poly(acrylic acid) gel incorporated inside the nascent substrate, which is from 113 nm of nascent substrate to as low as 7.0 nm of pore-filled membranes at pH acidic. For the membranes, the pore radii at pH neutral estimated by the extend Nernst–Planck equation (2.76–4.20 nm) and by the Spiegler–Kedem model with the steric-hindrance pore model (3.4–4.1 nm) are close to each other and comparable with that calculated from the poly(acrylic acid) gel correlation length (1.79–2.93 nm). The calculated pore density at pH neutral (49–258 × 10¹⁴ m⁻²) is much higher than that at pH acidic (2.8–39.8 × 10¹⁴ m⁻²). The results are interpreted in terms of the gel structure in the pore-filled membranes.     

Preparation and characterization of novel macroporous cellulose beads regenerated from ionic liquid for fast chromatography

Macroporous cellulose beads (MCB) used as anion exchangers were successfully prepared from cellulose solution in ionic liquid by double emulsification followed by cross-linking and modification with diethylaminoethyl. The pore structure and properties of the MCB were investigated and the results were compared with homogeneous cellulose beads (HCB). The MCB in size of about 71 μm is characterized by two sets of pores, i.e., diffusion pores (10–20 nm) and macropores (800–2000 nm), determined by mercury porosimeter. In addition, the bed permeability and effective porosity for BSA of MCB-packed column are 58% and 25% higher than those of HCB-packed column, respectively. The adsorption properties of MCB were evaluated, and compared with HCB and commercial absorbent (Sepharose 6 Fast Flow, CSFF). It is found that the pore diffusivity of BSA in MCB is over 7.9 times higher than HCB, and 6.7 times higher than CSFF, respectively. While the equilibrium adsorption capacity (q_m) of BSA on MCB is obviously lower than that on HCB and CSFF, the dynamic binding capacity (DBC) on MCB at 10% breakthrough reaches 47.7 mg/mL, higher than HCB (40.3 mg/mL) and CSFF (46.2 mg/mL) at flow rate of 360 cm/h. In addition, the MCB-packed column showed better column efficiency over the HCB packed one. Therefore, we demonstrated that the MCB possessed more advantages than other ones, like HCB and CSFF, and was expected as an ideal material for fast chromatography.     

Needle-like calcium carbonate assisted self-assembly of mesostructured hollow silica nanotubes

Mesostructured hollow silica nanotubes (MHSNTs) were successfully produced via the self-assembly of C₁₆TMABr and silica species on the surface of needle-like calcium carbonate nanoparticles in an alkaline medium at room temperature. The characterization of MHSNTs by transmission electron microscopy (TEM), scanning electron microscopy (SEM), pore size distribution (PSD) and Brunauer-Emmett-Teller (BET) indicated that MHSNTs had uniform tubular hollow structures with big openings, a length of 1.5-2.0 μm, an inner diameter of 150-200 nm at the open end and 50-60 nm at the closed end, and a wall thickness of 20-30 nm, as well as a narrow PSD around 2.3 nm in the shells and a BET surface area as high as ∼975.3 m²/g. By small-angle X-ray diffraction (XRD), BET and pore structure analysis, it was found that more uniformly structured mesopores could be formed by the method of removing the double templates simultaneously through a solvent extraction process, as compared to the separate removal of the double templates by calcinating and then etching in an acidic solution, and the amount of C₁₆TMABr affected the mesoporous structures of MHSNTs greatly. The formation processes of MHSNTs were also studied with XRD and FTIR.     

Application of hydroxy sodalite films as novel water selective membranes

Supported hydroxy sodalite (H-SOD) membranes were prepared on α-alumina disks using direct hydrothermal synthesis at 413 K for 3.5 h. The continuity of the membranes was verified using single gas permeation of He and N₂ at ambient conditions. The membranes were impermeable to N₂ and He, which validated absence of defects in the membrane structure. The membranes were used in dewatering several organic alcohol/water mixtures (organic alcohol being: methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 1-pentanol, and 2-pentanol). The influence of feed temperature (303–473 K), feed concentration (0–100 mol% alcohol in the feed), and absolute feed pressure (1.6–2.4 MPa) on the water flux were analyzed. The absolute feed pressure had no effect on the water permeance. The membrane exhibited a water/alcohol separation factor larger than 10⁶ and showed excellent thermal, mechanical, and operation stability in continuously dehydrating a water/ethanol mixture (72 mol% water) by pervaporation at 473 K and 2.2 MPa for 125 h. The normalized water flux correlated well with the water feed concentration for the primary alcohols. Below 40 mol% water in mixtures with secondary alcohols the water flux was three orders of magnitude lower. The water mobility through the membrane had an activation energy of ∼15 kJ/mol.     

The influence of pyrocarbon modification on the physicochemical characteristics of the surface of pores and transport properties of inorganic membranes

The structure of pyrocarbon crystallites deposited on the surface of the pore channels of TRUMEM composite inorganic membranes (TiO₂ on porous steel) was studied by X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. According to the X-ray data, the lattice of pyrocarbon crystallites of size L _c=40.0 nm has hexagonal symmetry with d ₀₀₂=3.368 Å. The deposition of pyrocarbon crystallites with L _c up to 1.5 nm was identified by X-ray photoelectron spectroscopy and scanning electron microscopy. Coating the pore channels of membranes with pyrocarbon decreased the density of the electric charge on their surface by a factor of ～5.5. The temperature dependences of the hydrodynamic permeability coefficient were obtained for the initial and pyrocarbon-modified membranes when polar and nonpolar fluids flowed through them. The electrostatic force and energy of the interaction of ethanol molecules with each other and the surface of pores were calculated; the results were compared with the O-H···O H-bond energy. The main reason for the formation of alcohol adsorption layers was the formation of H-bonds between its molecules and O-H···O atoms on the surface of pores.     

Flow-through pore characteristics of monolithic silicas and their impact on column performance in high-performance liquid chromatography

In order to elucidate the role of the flow-through characteristics with regard to the column performance in high-performance liquid chromatography (HPLC) native and n-octadecyl bonded monolithic silica rods and columns, respectively of 100 mm length and 4.6 mm ID with mesopores in the range between 10 and 25 nm and macropores in the range between 0.7 and 6.0 μm were examined by mercury intrusion/extrusion, scanning electron microscopy, image analysis and permeability. The obtained data of the flow-through pore sizes and porosity values as well as surface-to-volume ratio of the stationary phase skeleton enabled to predict their influence to the chromatographic separation efficiency. Our data demonstrate that mercury porosimetry is a reliable technique to obtain all the characteristic parameters of the flow-through pores of silica monoliths. An important result of our examination was that the surface-to-volume ratio of monolithic silica skeletons had more significant impact to the separation process, rather than the average flow-through pore sizes. We could also show the essential differences between the particulate and monolithic stationary phases based on theoretical computation. The results, obtained from other characterization methods also indicated the structural complexity of monolithic silica samples. Permeability of columns is a generally applicable parameter to characterize all chromatographic phases no matter the chemistry or format. The correlation coefficient obtained for mercury intrusion and permeability of water was 0.998, though our investigation revealed that the surface modification is more likely influencing the obtained results. Further, the assumption of the cylindrical morphology of flow-through pores is not relevant to the investigated monolithic silica columns. These results on the morphology of the flow-through pores and of the skeletons were confirmed by the image analysis as well. Our main finding is that the flow-through pore sizes are not relevant for the estimation of the chromatographic separation efficiency of monolithic silica columns.     

Electrospinning preparation and drug delivery properties of Eu/Tb doped mesoporous bioactive glass nanofibers

Luminescent Eu³⁺/Tb³⁺ doped mesoporous bioactive glass nanofibers (MBGNFs) with average diameter of 100-120 nm were fabricated by electrospinning method. Pluronic P123 and N-cetyltrimethylammonium bromide (CTAB) were used as co-surfactants to generate porous structure of the nanofibers. N₂ adsorption-desorption measurement reveals that the MBGNF:Eu³⁺ have a surface area of 188 m² g⁻¹, a pore volume of 0.246 cm³ g⁻¹ and average pore size of 4.17 nm, and the MBGNF:Tb³⁺ have a surface area of 171 m² g⁻¹, a pore volume of 0.186 cm³ g⁻¹ and average pore size of 3.65 nm. Photoluminescence measurements reveal that the MBGNF:Eu³⁺ show strong red emission dominated by the ⁵D₀ → ⁷F₂ transition of Eu³⁺ at 614 nm with a lifetime of 1.356 ms, and MBGNF:Tb³⁺ show strong green emission dominated by the ⁵D₄ → ⁷F₅ transition of Tb³⁺ at 544 nm with a lifetime of 1.982 ms. The biocompatibility tests on L929 fibroblast cells using MTT assay reveal low cytotoxicity of MBGNF. These luminescent nanofibers show sustained release properties for ibuprofen (IBU) in vitro. The emission intensities of Eu³⁺ in the drug delivery system vary with the released amount of IBU, thus making the drug release be easily tracked and monitored by the change of the luminescence intensity.     

Preparation of porous TiO2/silica composites without any surfactants

TiO₂-SiO₂ composites, with high specific surface area (up to 308 m²/g), large pore volume, and narrow distribution with average pore sizes of 3.2 nm, have been synthesized from wollastonite and titanium sulfate in the absence of any surfactants. Calcium sulfate, a microsolubility salt, plays an important role in the formation of pores in this porous TiO₂/silica composite. The microstructure and chemical composition of composite were characterized by X-ray diffractometry (XRD), transmission electron microscopy (TEM) equipped with energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectrometer (XPS) and N₂ adsorption and desorption analysis. The as-prepared porous titanium dioxide-silicon dioxide composites with high specific surface area and well-crystallized anatase contents were used as an efficient photocatalyst.