Anne Galarneau

Microporosity and connections between pores in SBA-15 mesostructured silicas as a function of the temperature of synthesis

Imaging of the platinum replica of the porous structure and low-pressure argon adsorption allowed us to elucidate the complicated porous structure of SBA-15. These techniques enabled us to draw a coherent picture of the evolution of the SBA-15 precursor mesophase as a function of the synthesis temperature. TEM of the platinum replicas has been unable to show bridges between the structural mesopores of SBA-15 synthesized at low temperature, whereas mesoporous bridges are clearly observed for samples formed at higher temperature. Argon adsorption has evidenced the ultramicroporosity of the materials formed at low temperature, as well as its evolution to secondary porosity with diameters greater than 1.5 nm under more severe hydrothermal treatment.

Textural Control of Micelle-Templated Mesoporous Silicates : the Effects of Co-surfactants and Alkalinity

Abstract The pore size of micelle-templated mesoporous silicates (MTS) is thermodynamically controlled, while their grain size is kinetically controlled. The pore size is affected by all parameters which modify the size of micelles; hydrocarbons can be used as swelling agents and co-surfactants as shrinking agents. In a typical example, alcohol co-surfactants decrease the pore size of amine-templated silica (HMS) by increasing the distance between polar heads in the rim of the micelle. The size of MTS particles depends on the ratio between their nucleation and growth rates. Rapid aggregation of silica and surfactant leads to the formation of small grains, while a low concentration of ionized silica allows the growth of large particles of MTS mesophases. The ionic strength and the pH affect particle size and wall thickness through their effect on the rates of hydrolysis and condensation of the silicate species, as shown by syntheses of MCM-41 in the system cetyltrimethylammonium–tetraethylorthosilicate.

MCM-41 silica monoliths with independent control of meso- and macroporosity

Centimetre sized macroporous silica monoliths consisting of MCM-41 have been prepared by a two-step procedure allowing an independent control of the meso- and macro-porosity. In the first step a monolith with a macroporosity tailored between 2 and 20 μm is prepared under acidic medium by a phase separation, named spinodal decomposition, leading to a bicontinuous structure of a silica/polymer phase and a water phase. The monolith is then reacted in an alkaline solution of cetyltrimethyl ammonium to transform the silica skeleton into MCM-41 under conditions which preserve the original morphology and macroporosity of the material. The combination of spinodal decomposition and pseudomorphic transformation proves very efficient to precisely tune the textural characteristics of macroscopic objects.

Immobilization of lipase on silicas. Relevance of textural and interfacial properties on activity and selectivity

Two lipases from Mucor miehei have been immobilized by adsorption in MCM-41 materials featuring different hydrophilic/hydrophobic surfaces and by encapsulation either in hydrophobic silica sol-gel or in Sponge Mesoporous Silicas (SMS), a new procedure based on the addition of a mixture of lecithin and amines to a sol-gel synthesis to provide pore-size control. The resulting biocatalysts have been evaluated for various ester hydrolysis reactions and compared with commercially available immobilized lipases in silica sol-gel (Sol-gel AK-Fluka) and in ion-exchange resin (Lipozyme-Fluka). Too hydrophilic (pure silica) or too hydrophobic (butyl-grafted silica) supports are not appropriate to develop high activity for lipases. An adequate hydrophobic/hydrophilic balance of the support, such as supported-micelle, provides the best route to enhance lipase activity. The SMS encapsulation procedure enables the highest activity for the lipases. The lecithin/amines mixture structuring the pore network leads to a suitable phospholipids bilayer-like environment, which avoids the necessity to create an interface by substrates assembly. The specificity of the lipase towards middle ester chain length is lost for immobilized lipases, but the activity of lipase towards short ester chain length is considerably increased. This typoselectivity change is more likely related to a strained configuration of the immobilized enzyme.

Activated drying in hydrophobic nanopores and the line tension of water

We study the slow dynamics of water evaporation out of hydrophobic cavities by using model porous silica materials grafted with octylsilanes. The cylindrical pores are monodisperse, with a radius in the range of 1–2 nm. Liquid water penetrates in the nanopores at high pressure and empties the pores when the pressure is lowered. The drying pressure exhibits a logarithmic growth as a function of the driving rate over more than three decades, showing the thermally activated nucleation of vapor bubbles. We find that the slow dynamics and the critical volume of the vapor nucleus are quantitatively described by the classical theory of capillarity without adjustable parameter. However, classical capillarity utterly overestimates the critical bubble energy. We discuss the possible influence of surface heterogeneities, long-range interactions, and high-curvature effects, and we show that a classical theory can describe vapor nucleation provided that a negative line tension is taken into account. The drying pressure then provides a determination of this line tension with much higher precision than currently available methods. We find consistent values of the order of −30 pN in a variety of hydrophobic materials.

New device to measure dynamic intrusion/extrusion cycles of lyophobic heterogeneous systems

Lyophobic heterogeneous systems (LHS) are made of mesoporous materials immersed in a non-wetting liquid. One application of LHS is the nonlinear damping of high frequency vibrations. The behaviour of LHS is characterized by P - ΔV cycles, where P is the pressure applied to the system, and ΔV its volume change due to the intrusion of the liquid into the pores of the material, or its extrusion out of the pores. Very few dynamic studies of LHS have been performed until now. We describe here a new apparatus that allows us to carry out dynamic intrusion/extrusion cycles with various liquid/porous material systems, controlling the temperature from ambient to 120 °C and the frequency from 0.01 to 20 Hz. We show that for two LHS: water/MTS and Galinstan/CPG, the energy dissipated during one cycle depends very weakly on the cycle frequency, in strong contrast to conventional dampers.

Organo-lined alumina surface from covalent attachment of alkylphosphonate chains in aqueous solution

The reaction of octylphosphonic acid with the surface of alumina nanoparticles has been investigated in order to prepare a close packing of grafted-alkyl chains. This goal was attained through a fitting selection of the experimental conditions in terms of pH, reactant amount, reaction time and temperature. DRX, TEM and 31P MAS NMR spectroscopy are all consistent with an efficient covalent anchorage of the alkylphosphonate chains without degradation of the support, demonstrated by the morphology and texture preservation during the modification. In addition to the textural analysis, nitrogen adsorption isotherms provide additional pieces of information on the interaction energy between the nitrogen molecule and the surface. These data, combined with those supplied by the empirical test of floatability of powders (methanol number test), as well as the adsorption properties of the differently functionalized alumina samples using other probes, such as hexane and water, as a function of the chain loading, provided converging information about surface coverage and the hydrophilic/hydrophobic properties of the various materials. Different conformations are proposed to take into account the different results obtained from vapor adsorption measurements.

Best design of heterogenized β-aminoalcohols for improvement of enantioselective addition of diethylzinc to benzaldehyde

Covalent immobilization of (1R,2S)-(−)-ephedrine, used as a model molecule of β-aminoalcohols, on the surface of MCM-41-type mesoporous aluminosilicates, performed by a new sol–gel method, leads to chiral auxiliaries which show greatly enhanced rates and ee’s compared to those reported up to now in the enantioselective addition of diethylzinc to benzaldehyde.

EPR Studies of New Mesostructured Silica Synthesis and Hemoglobin Encapsulation

Enzyme encapsulation in ordered porous silica has been modified by introducing a natural surfactant such as lecithin during the sol-gel process. β-lactose has been used as the enzyme protecting agent and tetraethoxysilane as the source of silica in an hydroalcoholic media. In the present study, the EPR spectroscopy of paramagnetic probes has allowed to monitor the formation of mesophases and the condensation of silica in an isotropic three-dimensional structure, named Sponge Mesoporous Silica, with a pore size of 6 nm and a specific surface area of 600 m2/g. Different techniques of characterization (nitrogen sorption, XRD, TEM, SEM) have been used to study the influence of the different reactants on the structure of the materials. Hemoglobin has been encapsulated in the different materials and its catalytic pseudo peroxydase activity has been evaluated.