Active sites of alumina and silica-alumina as observed by temperature programmed desorption*1

The active sites for chemisorption of ethylene on alumina and on a series of mixed gel silica-alumina catalysts have been investigated by the temperature programmed desorption technique as a function of the temperature of activation of alumina and of the composition of silica-alumina. The activity of these catalysts for ethylene hydrogenation and dimerization at low temperatures has also been measured to obtain additional qualitative and quantitative characterization of the active sites. The total number of sites on alumina is increased when the temperature of evacuation (i.e., the activation temperature) is raised but a treatment with moisture is necessary to create the stronger sites, the sites which are responsible for ethylene dimerization. The specific activity (the activity per site) of the weaker sites of alumina, the sites responsible for ethylene hydrogenation at low temperatures, increases with the evacuation temperature while the specific activity of the stronger sites does not change at evacuation temperatures higher than 550 °. The total number of active sites on silica-alumina increases monotonously with the alumina content, but the specific activity per site for ethylene dimerization goes through a maximum (and the activation energy through a minimum) at about 35 mole % alumina. The results show that the composition of silica-alumina determines both the number of sites and their specific activity. The activity for ethylene hydrogenation is highest with pure alumina and drops sharply when small amounts of silica are added. The transition from alumina to silica-alumina is clearly shown by the activity curves for these two reactions, hydrogenation and dimerization of ethylene.