The chemisorption of trimethylamine and pyridine on a commercial silica-alumina catalyst was studied by infrared techniques. Trimethylamine was found to be unsuitable as an agent for differentiating between Lewis and Brpinsted sites because it is dissociatively adsorbed with concomitant generation of protons which form protonated chemisorbed species. Pyridine, on the other hand, is not dissociatively adsorbed and is, therefore, useful for determining the relative numbers of Lewis and Brpinsted acid sites by making use of the differences in the spectra of the pyridinium ion and coordinately bonded pyridine. Approximately equal numbers of Lewis and Brpinsted sites were observed on the highly dehydrated silica-alumina. The perturbing effects of potassium poisoning and irreversible HzO adsorption on the distribution of acid sites were studied. It was found that potassium poisoning weakens the majority of acid sites and eliminates apparent Brgnsted acidity. Irreversibly adsorbed H2O converts Lewis sites to Brpinsted sites; however, the interaction is relatively weak since the Ha0 can be removed by evacuation. These observations provide considerable insight into the nature of the acid sites and have led to the formulation of the following model. It is suggested that all of the primary acid sites on a silicaalumina are of the Lewis type centered on active surface aluminum atoms and that apparent Brpinsted sites are produced by a second-order interaction between the molecule chemisorbed on a Lewis site and a nearby surface hydroxyl group. This model qualitatively fits the experimental observations to date.
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