Influence of the cerium content of CeHY catalysts on their physicochemical and catalytic properties

Abstract

The physicochemical and catalytic properties of a stabilized HY zeolite and of CeHY zeolites were compared. The Bro˝nsted acid sites of CeHY zeolites are definitely stronger than those of HY whereas their Lewis acid sites are weaker; the total number of acid sites decreases with the cerium content. The strength of protonic sites and the activity for n-heptane cracking at 350°C are maximal for an exchange rate of about 45%. For all the samples the cracking activity is clearly related to the strong protonic acidity. The rates and selectivities of the transformations of n-heptane and of its isomers (methylhexanes, 2,4-dimethylpentane) were compared. It can be shown that on all the catalysts, n-heptane cracking occurs through the classical carbenium ion chain mechanism in which the formation of nC 7 secondary carbenium ions is the limiting step; cracking products (essentially C 3 and iC 4) result mainly from the scission of 2,4- and 2,2-dimethyl carbenium ions,