A NEW METHOD OF PREPARATION OF AEROGEL-LIKE MATERIALS USING A FREEZE-DRYING PROCESS

Pour conserver les propriétés texturales des gels encore imbibés de solvant au moment de leur séchage il existe la possibilité d'évacuer le solvant dans les conditions hypercritiques (procédé à haute température conduisant aux aérogels) et aussi celle de sublimer ledit solvant qui conduit alors à l'obtention de cryogels (procédé à basse température). Une série de cryogels d'oxydes mixtes de nickel et d'aluminium ont été ainsi élaborés. Leurs propriétés texturales ont été déterminées par adsorption-désorption de N2, par la méthode BET, la porosimétrie au mercure tandis que leur structure a été étudiée par les rayons X. Ces cryogels sont comparés aux aérogels de formulation chimique identique. Finalement un échantillon de cryogel a été testé dans la réaction catalytique d'hydrogénation du toluène en méthylcyclohexane qui constitue un procédé potentiel de stockage de l'hydrogène pour les systèmes de transport.In order to preserve the textural properties of a wet gel one possibility is to evacuate the solvent under hypercritical temperature conditions (high temperature process leading to the well known aerogel materials) and another one is to sublimate the solvent frozen at low temperature conditions (low temperature process leading to cryogels). The preparation of a series of several nickel oxide on alumina cryogels is described in details. Their surface and textural properties are measured by physical adsorption-desorption of N2, by the BET method and also by mercury porometry. The corresponding data are compared to those belonging to aerogels of same chemical compositions. Cryogels (as aerogels) appeared to be amorphous from their XRD patterns. Finally one sample of a nickel on alumina cryogel is tested as a potential catalyst in a flow reactor for the reaction of hydrogenation of toluene into methylcyclohexane which is one possible means to store hydrogen in transport systems

Evolution of the porous volume during the aerogel-glass transformation

The evolution of the texture of a silica aerogel during sintering is studied by thermoporometry. During the first stages of the densification the macroporous volume drops strongly while the mesoporous volume seems to be constant. This behavior is in contradiction with sintering models which predict that the sintering rate is faster for bodies with smaller particles or pores. In fact the analysis of the micropore size distribution shows that the sintering of the mesopores is responsible for macropore decrease