Catalytic Properties of BF3-treated Aluminas. IV. The Effect of BF3Adsorption on the Acidic Properties of Aluminum Oxides

An account was given of the surface acidic properties of a series of BF3-treated aluminum oxides, ranging in their precalcination temperature up to 1000°C. It was concluded that all aluminum oxides dehydrated over 300°C were converted to solid acids with new strong acid centers of H0≤−8.2 by adsorption with BF3. This new acid centers would be certainly of the Brönsted type. Their catalytic activities in some carbonium ion type reactions are well correlated with the n-butylamine acidity in the presence of Hammett’s indicators, and the maximum number of acid centers was observed in catalyst samples precalcined at 450 and 800°C respectively. The present study also suggests that the acid centers developed on the surface of aluminum oxides are somewhat different in nature depending upon their precalcination temperatures. Since there was a good correlation between the amount of the remaining hydroxyl groups and that of the absorbed BF3, the surface hydroxyl groups, in addition to the aluminum atoms serving as the Lewis centers, which are not fully coordinated, were discussed as the most important factors in the formation of such acid centers.