Highly hydrothermally stable microporous silica membranes for hydrogen separation

Fluorocarbon-modified silica membranes were deposited on gamma-Al2O3/alpha-Al2O3 supports by the sol-gel technique for hydrogen separation. The hydrophobic property, pore structure, gas transport and separation performance, and hydrothermal stability of the modified membranes were investigated. It is observed that the water contact angle increases from 27.2+/-1.5 degrees for the pure silica membranes to 115.0+/-1.2 degrees for the modified ones with a (trifluoropropyl)triethoxysilane (TFPTES)/tetraethyl orthosilicate (TEOS) molar ratio of 0.6. The modified membranes preserve a microporous structure with a micropore volume of 0.14 cm3/g and a pore size of approximately 0.5 nm. A single gas permeation of H2 and CO2 through the modified membranes presents small positive apparent thermal activation energies, indicating a dominant microporous membrane transport. At 200 degrees C, a single H2 permeance of 3.1x10(-6) mol m(-2) s(-1) Pa(-1) and a H2/CO2 permselectivity of 15.2 were obtained after proper correction for the support resistance and the contribution from the defects. In the gas mixture measurement, the H2 permeance and the H2/CO2 separation factor almost remain constant at 200 degrees C with a water vapor pressure of 1.2x10(4) Pa for at least 220 h, indicating that the modified membranes are hydrothermally stable, benefiting from the integrity of the microporous structure due to the fluorocarbon modification.     

Structure of hybrid organic–inorganic sols for the preparation of hydrothermally stable membranes

A procedure for the preparation of hybrid sols for the synthesis of organic–inorganic microporous materials and thin film membranes is reported. We describe silane reactivity and sol structure for acid-catalysed colloidal sols from mixtures of either tetraethylorthosilicate (TEOS) and methyltriethoxysilane (MTES), or bis(triethoxysilyl)ethane (BTESE) and MTES. Early-stage hydrolysis and condensation rates of the individual silane precursors were followed with ²⁹Si liquid NMR and structural characteristics of more developed sols were studied with Dynamic Light Scattering. Condensation was found to proceed at more or less similar rates for the different precursors. Homogeneously mixed hybrid colloids can therefore be formed from precursor mixtures. The conditions of preparation under which clear sols with low viscosity could be formed from BTESE/MTES were determined. These sols were synthesised at moderate water/silane and acid/silane ratios and could be applied for the coating of defect-free microporous membranes for molecular separations under hydrothermal conditions.     

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.     

Long-term pervaporation performance of microporous methylated silica membranes

The incorporation of methyl groups in microporous silica membranes was proven to enhance the service time in the dehydration of a butanol-water mixture at 95 degrees C from a few weeks to more than 18 months with a water flux of about 4 kg m(-2) h(-1) and a selectivity between 500 and 20 000.     

Properties of aluminated silica sols

The properties of colloidal dispersions of “Positive Sol 130M” (E. I. du Pont de Nemours and Co.), which contains silica particles coated with alumina, have been studied in detail. Drifts of pH, leaching of aluminum species, and changes of electrokinetic mobility occurred during aging at room temperature for periods of up to a month, and were attributed to surface reactions. Despite its name, the sol has a point of zero charge (PZC) at pH 9, and consequently the particles carry a positive charge only below this pH. The alumina of the coating dissolves in acidic environments; the extent of leaching of aluminum species is dependent upon pH and length of aging at room temperature. The effect of different anions on this process was examined by using various inorganic acids; it was shown that phosphate and sulfate ions exhibit the strongest effects. The removal of alumina changed the surface characteristics as well as the stability of the sol 130M. The mobilities of the particles were found: to be directly related to the composition of the particle surface, regardless of the time of aging or the pH.     

Formation of hollow silica spheres from molecular silica sols

1. Download : Download high-res image (117KB)
2. Download : Download full-size image

     

Mesoporous silica hybrid membranes for precise size-exclusive separation of silver nanoparticles

One-dimensional (1D) nanomaterials have unique applications due to their inherent physical properties. In this study, hexagonally ordered mesoporous silica hybrid anodic alumina membranes (AAM) were synthesized using template-guided synthesis with a number of nonionic n-alkyl-oligo(ethylene oxide), Brij-type (C(x)EO(y)), which are surfactants that have different molecular sizes and characteristics. The hexagonal mesoporous silicas are vertically aligned in the AAM channels with a predominantly columnar orientation. The hollow mesostructured silicas had tunable pore diameters varying from 3.7 to 5.1 nm. In this synthesis protocol, the surfactant molecular natures (corona/core features) are important for the controlled generation of ordered structures throughout AAM channels. The development of ultrafiltration membranes composed of silica mesostructures could be used effectively in separating silver nanoparticles (Ag NPs) in both aqueous and organic solution phases. This would be relevant to the production of well-defined Ag NPs with unique properties. To create a size-exclusive separation system of Ag NPs, we grafted hydrophobic trimethylsilyl (TMS) groups onto the inner pores of the mesoporous silica hybrid AAM. The immobilization of the TMS groups allowed the columnar mesoporous silica inside AAM to retain this inner pore order without distortion during the separation of solution-phase Ag NPs in organic solvents that may cause tortuous-pore membranes. Mesoporous TMS-silicas inside 1D AAM channels were applicable as a size-exclusive separation system to isolate organic solution-phase Ag NPs of uniform morphology and size.     

Synthesis and characterization of uniform alumina-mesoporous silica hybrid membranes

The synthesis and characterization of alumina-mesoporous silica (alumina-MS) hybrid membranes are reported. The hybrids are formed using a variation of the evaporative-induced self-assembly (EISA) process reported by Hayward et al. (Langmuir 2004, 20, 5998) based on dip coating of an Anopore 200 nm membrane with a Brij-56/TEOS/HCl/H2O solution. Numerous analytical methods are used to probe both the hybrid material and the silica phase after dissolution of the Anopore substrate. Most importantly, He/N2 permeation measurements show that the effective pore size of the membrane can be tuned from 20 to 5 nm based on the number of dip-coating cycles used. The observed He/N2 permselectivity of 2.7 +/- 0.11 is nearly identical to the theoretical value obtained (2.65) assuming Knudsen diffusion dominates. The selectivity of these membranes is higher than that of most commercial "5 nm" membranes (2.29), which is ascribed to the lack of pinhole defects in the materials reported here. The hybrid membranes as well as the silica obtained after dissolution of the Anopore substrate have been characterized using scanning and transmission electron microscopy and X-ray diffraction. Those results indicate that the silica deposited in the Anopore membrane possesses uniform pores approximately 5 nm in size, consistent with the permeation studies. The current work presents an alternative approach to materials that possess many of the properties of mesoporous silica thin films (i.e., pores of controlled size and topology) without the difficulty of growing mesoporous silica thin films on porous supports.     

Synthesis of vinyl polymer-silica colloidal nanocomposites prepared using commercial alcoholic silica sols

The surfactant-free synthesis of vinyl polymer-silica nanocomposite particles has been achieved in aqueous alcoholic media at ambient temperature in the absence of auxiliary comonomers. Styrene, methyl methacrylate, methyl acrylate, n-butyl acrylate, and 2-hydroxypropyl methacrylate were homopolymerized in turn in the presence of three commercially available ultrafine alcoholic silica sols. Stable colloidal dispersions with reasonably narrow size distributions were obtained, with silica contents of up to 58% by mass indicated by thermogravimetric analysis. Particle size distributions were assessed using both dynamic light scattering and disk centrifuge photosedimentometry. The former technique indicated that the particle size increased for the first 1-2 h at 25 degrees C and thereafter remained constant. Particle morphologies were studied using electron microscopy. Most of the colloidal nanocomposites comprised approximately spherical particles with relatively narrow size distributions, but in some cases more polydisperse or nonspherical particles were obtained. Selected acrylate-based nanocomposites were examined in terms of their film formation behavior. Scanning electron microscopy studies indicated relatively smooth films were obtained on drying at 20 degrees C, with complete loss of the original particle morphology. The optical clarity of solution-cast 10 microm nanocomposite films was assessed using visible absorption spectrophotometry, with 93-98% transmission being obtained from 400 to 800 nm; the effect of long-term immersion of such films in aqueous solutions was also examined. X-ray photoelectron spectroscopy studies indicated that the surface compositions of these nanocomposite particles are invariably silica-rich, which is consistent with their long-term colloidal stability and also with aqueous electrophoresis measurements. FT-IR studies suggested that in the case of the poly(methyl methacrylate)-silica nanocomposite particles, the carbonyl ester groups in the polymer are hydrogen-bonded to the surface silanol groups. According to differential scanning calorimetry studies, the glass transition temperatures of several poly(methyl methacrylate)-silica and polystyrene-silica nanocomposites can be either higher or lower than those of the corresponding homopolymers, depending on the nature of the silica sol.     

Adsorption properties of microporous silica

The study of CCl₄ adsorption shows that the increase of dispersion potential can increase the specific adsorption in pores up to 3 nm in size.

---

CCl₄ , 3 .

     

Permeation of gases through microporous silica hollow-fiber membranes: Application of the transition-site model

In a previous paper [P. Molyneux, “Transition-site” model for the permeation of gases and vapors through compact films of polymers, J. Appl. Polym. Sci. 79 (2001) 981–1024] a transition-site model (TSM) for the activated permeation of gases through compact amorphous solids was developed and applied to organic polymers; the present paper examines the applicability of the TSM to permeation through microporous silica. The basis of the TSM theory for amorphous solids in general is outlined; the present extension to inorganic glasses has revealed that the transition sites (TS) of this theory, which are the three-dimensional saddle-points critical in the molecular sieving action, equate to the doorways long recognized in permeation through amorphous silica and other inorganic glasses. The TSM, which views permeation as a primary process, is contrasted with the conventional sorption–diffusion model (SDM) for permeation. It is pointed out that in the SDM, the widely accepted analysis into two apparently distinct factors – sorption (equilibrium) and diffusion (kinetic) – has the fundamental flaw that these factors are not independent, since both involve the sorbed state. By contrast, the TSM focuses on the permeant molecule in only two states: as the free gas, and as inserted in a doorway D; hence the characteristics of these doorways – (unperturbed) diameter σ_D, spacing λ, and the thermodynamic parameters θ (force constant) and ν (entropy increment) for the insertion process – can be evaluated. The theory is applied to literature data [J.D. Way, D.L. Roberts, Hollow fiber inorganic membranes for gas separations, Sep. Sci. Technol. 27 (1992) 29–41; J.D. Way, A mechanistic study of molecular sieving inorganic membranes for gas separations, Final Report submitted to U.S. Department of Energy under contract DE-FG06-92-ER14290, Colorado School of Mines, Golden, CO, 1993, www.osti.gov/bridge/servlets/purl/10118702-ZAx4Au/native/1011872.pdf; M.H. Hassan, J.D. Way, P.M. Thoen, A.C. Dillon, Single component and mixed gas transport in silica hollow fiber membrane, J. Membr. Sci. 104 (1995) 27–42] on the permeation through microporous silica hollow-fiber membranes (developed by PPG Industries Inc.) of the nine gases: Ar, He, H₂, N₂, O₂, CO, CO₂, CH₄ and C₂H₄, over the temperature range 25–200 °C. The derived Arrhenius parameters for the permeation of these gases (excepting He) lead to estimates of the four doorway-parameters: σ_D, 125 pm; λ, ca. 30 nm; θ, 0.43 nN; ν, 1.7 pN K⁻¹; these values lie within the ranges of those obtained with the glassy organic polymers. Some “secondary effects”, shown particularly by CO and CO₂, are interpreted as host–guest interactions at the doorway. The behavior of He is anomalous, the permeation rising linearly with temperature. This study confirms that the TSM may be applied to gas permeation by activated molecular sieving for this type of inorganic membrane.     

Prediction of biological activity for high-throughput screening using binary kernel discrimination

High-throughput screening has made a significant impact on drug discovery, but there is an acknowledged need for quantitative methods to analyze screening results and predict the activity of further compounds. In this paper we introduce one such method, binary kernel discrimination, and investigate its performance on two datasets; the first is a set of 1650 monoamine oxidase inhibitors, and the second a set of 101 437 compounds from an in-house enzyme assay. We compare the performance of binary kernel discrimination with a simple procedure which we call "merged similarity search", and also with a feedforward neural network. Binary kernel discrimination is shown to perform robustly with varying quantities of training data and also in the presence of noisy data. We conclude by highlighting the importance of the judicious use of general pattern recognition techniques for compound selection.     

New hybrid membranes based on phosphonic acid functionalized silica particles for PEMFC

This work concerns the development of hybrid organic/inorganic membranes from styrenic phosphonic polymers. The phosphonic charge, composed phosphonic polymers grafted onto silica nanoparticles, was obtained by “grafting onto” method. It consists of synthesizing first the polymer, and then the terminal functions of the latter react with silanol groups of silica. The phosphonated polymer was isolated in two steps, that is, an ATRP polymerization of 4‐chloromethylstyrene followed by Mickaelïs‐Arbusov reaction. After the grafting onto silica, membranes are prepared through formulation containing the charge and the polymer matrix PVDF‐HFP, which are dispersed in DMF. The acid form is obtained by hydrolysis in chlorydric acid. The membrane possessing a 40 wt % charge ratio (IEC = 1.08 meq g^?1) was selected as reference. A proton conductivity of 65 mS cm^?1 at 80 °C was measured in immersed conditions. When the membrane is no more immersed, the value decreases drastically (0.21 mS cm^?1 at 120 °C and 25% RH). © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Optimizing piezoelectric inkjet printing of silica sols for biosensor production

The effects of selected process conditions for the sol-gel encapsulation of laccase enzymatic extract, obtained from Coriolus hirsutus, were investigated. Screening trials were carried out to identify the parameters having the most pertinent effects on the encapsulation efficiency (EE) and the residual laccase activity. These parameters included water/silane molar ratio (r), HCl content and protein loading, for the pre-gel silica sol as well as the required time for gel drying and for aging, for the sol-gel process. The experimental findings indicated that a sol-gel drying time of over 6?h resulted in a complete loss of laccase catalytic activity, while an increase in the gel aging time led to an enhancement of the residual enzyme activity. Except for r, the investigated parameters demonstrated no significant effect on the EE of the sol-gel encapsulated enzymatic extract. Overall, the encapsulation of laccase extract in the sol-gel matrix resulted in an enhancement of its catalytic activity, where its highest residual activity (349%) was obtained with an r-value of 4, an HCl content of 4?µmol and a protein loading of 1?mg/mL, using 6 and 24?h of drying and aging times, respectively.

Open image in new window

     

Microwave-hydrothermal synthesis of stable nanocrystalline ceria sols for biomedical uses

A new microwave-hydrothermal process has been developed for preparing stable aqueous sols of nanocrystalline ceria. Transmission electron microscopy, UV-Vis spectroscopy, and dynamic light scattering shows that CeO₂ nanoparticle size remains unchanged during hydrothermal treatments. Sodium adenosine triphosphate and sodium citrate are proposed for additional stabilization of the sols.     

A practical high-throughput screening system for enantioselectivity by using FTIR spectroscopy

For the first time FTIR spectroscopy has been applied to the measurement of enantiomeric purity. The underlying concept is based on the use of pseudo-enantiomers that are (13)C-labeled at appropriate positions. Upon applying Lambert-Beer's law in the determination of the concentrations of both enantiomers, the ee values are accessible, accuracy to within +/-5 % of the true values being possible. The application of a commercially available high-throughput FTIR system results in a slightly decreased accuracy (+/-7% for the ee values), but this allows a throughput of up to 10000 samples per day. The method is of interest in the area of combinatorial symmetric catalysis and directed evolution of enantioselective enzymes.     

Evaluation of liquid-liquid microextraction using polypropylene microporous membranes for the determination of organophosphorus flame retardants and plasticizers in water samples

In this work, the suitability of the microporous membrane liquid–liquid extraction (MMLLE) technique for the concentration of several organophosphate esters (OPs) in water samples is assessed. Analytes were first extracted into a few microlitres of an organic solvent, immobilized in the pores of a hollow polypropylene membrane, and then determined by gas chromatography with nitrogen–phosphorus detection (GC–NPD). Main parameters controlling the efficiency of the extraction step were identified and their effects on the performance of the technique discussed. Under final working conditions, 2 cm long polypropylene membranes, containing about 7 μL of octanol in the pores, were dipped in a glass vial filled with 115 mL of water with a 30% of sodium chloride. Extractions were carried out for 12 h, at room temperature, under magnetic stirring. After that, analytes were recovered from the membrane with 0.2 mL of ethyl acetate. This extract was mixed with the internal standard (50 μL of a tripentyl phosphate solution in the same solvent) and finally reduced to ca. 50 μL. Overall enrichment factors for the optimized method ranged from 35 to 1400 times, and the achieved limits of quantification from 0.008 to 0.12 ng mL⁻¹, depending on the considered compound. Globally, the method showed an acceptable linearity and precision for all species, except for tris(2-ethylhexyl) phosphate (TEHP). Performance of the MMLLE approach is compared with that reported for other solid- and liquid-phase microextraction techniques and its suitability for the analysis of real water samples discussed.     

Surface ATRP of hydrophilic monomers from ultrafine aqueous silica sols using anionic polyelectrolytic macroinitiators

A convenient two-step route was developed to prepare new anionic ATRP macroinitiators from near-monodisperse poly(2-hydroxyethyl methacrylate) precursors by partial esterification with 2-bromoisobutyryl bromide, followed by esterification of the remaining hydroxyl groups using excess 2-sulfobenzoic acid cyclic anhydride. These new macroinitiators can be electrostatically adsorbed onto ultrafine cationic Ludox CL silica sols; subsequent surface polymerization of various hydrophilic monomers in aqueous solution at room temperature afforded a range of polymer-grafted ultrafine silica sols. The resulting sterically stabilized particles were characterized by dynamic light scattering, transmission electron microscopy, aqueous electrophoresis, FTIR spectroscopy, and elemental microanalyses.