François Fajula

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.

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.

Textural control of MCM-41 aluminosilicates

The thickness of the walls between mesopores in MCM-41 sieves can be controlled by modifying the solubility of silica and aluminosilicate species, notably by changing the, alkalinity of the synthesis system. This effect strongly influences the thermal stability of aluminosilicate MCM-41 in activation conditions.

Engineering of mesostructured silicas by pseudomorphism

Pseudomorphic transformation is introduced as an effective and versatile method to independently optimize the morphology of the grains and the structural properties of mesostructured silicas. In this account a short overview is given of the preparation of MCM-41 and MCM-48 mesophases as non-aggregated and homogeneously distributed particles with sizes ranging from 5 to 800 microm.

Influence of the source of sulfur on the hydroconversion of 1-methylnaphthalene over a Pt–Pd/USY catalyst

Abstract Hydroconversion of 1-methylnaphthalene was performed over a Pt–Pd/USY catalyst in a batch reactor at 310 °C and 5 MPa of hydrogen pressure in cyclohexane as the solvent and in the presence of 800 ppm of sulfur resulting from different sources, hydrogen sulfide, thiophene and dibenzothiophene. In a general manner, hydrogenation of 1-methylnaphthalene into the corresponding mixture of methyltetralines is not significantly affected by the nature of the sulfur species present in the starting feed. On the contrary, going from hydrogen sulfide to thiophene and finally to dibenzothiophene, hydrogenation of methyltetralines into methyldecalines is lowered and ring-opening of methyltetralines to alkylbenzenes is enhanced. This would agree with the expected sequence of appearance of hydrogen sulfide in the feed. These results are in agreement with dissociation of hydrogen into protonic and hydride species, as already proposed in the presence of sulfided catalysts, i.e., protonic species would be involved for the hydrogenation steps and hydride species for the ring-opening steps. Hydrogen sulfide present as such or resulting from the transformation of thiophene or dibenzothiophene would then reduce the hydrogenation route, and, as a consequence, increase the hydrogenolysis route.

Mechanical Strength of Micelle-Templated Silicas (MTS)

Abstract The mechanical properties of Micelle-Templated Silicas (MTS) are very sensitive items for industrial process applications which might submit catalysts or adsorbents to relevant pressure levels, either in the shaping of the solid or in the working conditions of catalysis or separation vessels. First studies about compression of these highly porous materials have shown a very low stability against pressure. These results concern these specific materials tested. In this study, we show very stable MTS with only a loss of 25% of the pore volume at 3 kbar. The effects of several synthesis parameters on the mechanical strength are discussed.

Proton affinity differences in zeolite: A DFT study

In this study we examine the influence of the chemical composition (aluminum content) on structure and proton affinity of zeolite models using first principle density functional procedures. It is shown that changes in the zeolites aluminum content have a very significant effect on the proton affinity of neighboring protons, making low aluminum zeolites more acidic than high aluminum zeolites. The proton location is also studied.

The influence of the geometric parameters on the electronic properties of faujasite cluster models as derived from density functional theory and Hartree Fock–self consistent field methods

Abstract We report here the results of our density functional theory calculations at local density and nonlocal density approximation levels and Hartree Fock–self consistent field calculations on disilicic acid type cluster models representing faujasite zeolite lattice. The geometric parameters such as Si–O and Al–O bond distances and Si–O–Si, Si–O–Al, O–Si–O and O–Al–O bond angles are derived from the geometry optimization calculations. The predicted geometric parameters are reasonably close to the experimentally reported values, wherever available, except for the slightly overestimated Si–O bond distances. The influence of the geometric parameters on the electronic properties such as total energy, dissociation energy of charge compensating cations and the net charge on bridging oxygen are studied in detail.