Evaluation of Cryotechniques for Tem Observation of Sols - Application to Boehmite Sols Used in Catalysts Forming

The use of suspensions in the heterogeneous catalysis field being a big issue, it appears important to be able to control this step as much as possible. Size of the objects in the suspension, aggregation state, homogeneity or heterogeneity (all called texture or structure here) are some important parameters which are sometimes difficult to evaluate with traditionalmethods such as X-ray scattering or rheology. The cryotechniques applied to transmission electron microscopy is then an interesting alternative. Three different methods of cryofixation have been tested on low solid content suspensions of boehmite commonly used during the dispersion step of alumina carrier forming. Two different cryoprotectants were tested as well. It is shown that liquid propane immersion cooling (PL) and slam freezing (SF) methods require the use of a cryoprotectant whereas high pressure technique (HP) avoids ice crystal formation without any addition of cryoprotectant in the suspension. It is shown also that DMSO is efficient with PL and SF techniques only when added at a high concentration (50%) to the suspension. Glycerol is preferred because water is vitrified since that 30% of glycerol is added. Comparisons of high pressure preparations with and without glycerol show very little differences of texture on the replica. Explanations of the origin of the small differences of density are tentatively given. More analyses are now under progress to clarify the role of glycerol. Cryomicroscopy is conclude to be a very good technique when the texture of a suspension must be studied for example in the elaboration or in the improvement of a peptisation or a colloid formation process. L'utilisation de suspensions dans la fabrication des catalyseurs hétérogènes étant très prometteuse, il apparaît important de maîtriser cette étape au mieux. La taille des objets en suspension, l'état d'agrégation, l'homogénéité ou l'hétérogénéité (appelés texture ou structure dans cet article) sont autant de paramètres qu'il est parfois difficile de connaître par des techniques traditionnelles telles que la diffusion des rayons X ou la rhéologie. Les techniques cryogéniques utilisées conjointement avec la microscopie électronique par transmission (MET) peuvent alors être une alternative intéressante pour évaluer ces propriétés. Trois méthodes de cryofixation ont été testées sur des suspensions de boehmite peu concentrées en solide du type de celles habituellement utilisées au cours de l'étape de dispersio