The effect of ammonia adsorption on the acidity of silica-alumina and alumina catalysts: The nature of the acid sites

Abstract

Acidic catalysts employed in a number of petroleum refining processes are known to be readily poisoned by basic nitrogen compounds. To throw light on the poisoning process and the nature of the acid sites, the effect of basic nitrogen compounds on the acid strength distribution of silica-alumina and alumina catalysts was determined using H R as well as 0 indicators. Adsorption of NH 3 on M-46 silica-alumina has a pronounced effect on the acid strength distribution with a minor change in the number of sites; on alumina, the situation is reversed. Desorption of NH 3 from silica-alumina at elevated temperatures is accompanied by the loss of a considerable amount of bound water; this should modify the number and strength of the catalyst acid sites. Acid strength distributions derived from high temperature NH 3 desorption data are therefore believed incorrect. The oxidation of triphenylmethane to the trityl ion in the presence of light and of perylene to the cation radical by silica-alumina are not readily poisoned by nitrogen bases, in contrast to the catalytic activity. This suggests the sites involved are weakly electrophilic and of little catalytic importance. The results favor the view that oxygen under the catalytic influence of Bronsted acids leads to oxidation of these hydrocarbons rather than strong Lewis acids as proposed by several investigators. Some suggestions relating to the nature of the acid sites on silica-alumina are made based on these experimental data.