ABSTRACT. The estimation of the nature of active surfaces for catalysts of such complex strnctures as silica-aluminas is difficult both from experimental and theoretical aspects. Not only does the particular type of acid (protonic or non-protonic) involved in the hydrocarbon conversion process remain disputable, but it is not clear whether the concept of acidic properties of catalyst surfaces has any justifiable basis at ail. This joint report discusses new experimental data obtained by scientists working at different scientific centres and advocating different concepts of the nature of surface activity of silicaalumina catalysts. Thus, the dual nature of acidity is considered by two investigation methods combined with a thorough study of reaction kinetics on catalysts with intentionally modified concentrations of acidic centres. The role played by the hydrogen of silica-alumina catalysts in hydrocarbon reactions was investigated using deuterium-hydrogen exchange with the catalyst and the extent of the reaction product deuteration. Data on the number of active centres are presented. The relation between the origin of active centres of silica-alumina catalysts and the interaction of the surface hydroxyl groups was studied by vacuum dehydration. Surface non-uniformity and acidity were examined using catalyst poisoning by cations of alkaline metals, labelled with radioactive isotopes. Low activity silica-alumina catalysts show a significant increase of activity at low surface coverage with alkaline metal cations. A special section centres attention on the study of catalyst structure in the process of catalyst formation. One of the authors suggests that the high catalytic activity and stability of catalysts under operating conditions is due to the high coordination number, the charge of the complexing agent cation and the possible coordination variations at elevated temperatures.
RESUME. II a été assez difficile tant du point de vue expérimental que théorique de déterminer la nature de la surface active des catalyseurs ayant une structure aussi complexe que celle des silicaaluminas. Non seulement il reste à discuter la question du type de l'acide (protonique ou nonpretonique) utilisé dans le procédé de conversion des hydrocarbures, mais il n'a pas encore été prouvé si le concept sur les propriétés d'acidité de la surface du catalyseur repose sur une base justifiable. Ce rapport discute les nouvelles données expérimentales obtenues dans divers centres scientifiques par des chercheurs qui défendent des concepts très différents sur la nature de l'activité de la surface des catalyseurs de silica-alumina. Ainsi la double nature de l'acidité a été étudiée par deux méthodes d'investigation, accompagnées d'