CHARACTERIZATION OF ACID CATALYSTS BY MEANS OF MODEL REACTIONS .1. ACID STRENGTH NECESSARY FOR THE CATALYSIS OF VARIOUS HYDROCARBON REACTIONS

A series of hydrocarbon reactions, such as 3,3-dimethyl-1-butene isomerization at 200°C, isomerization and disproportionation of o-xylene and of 1,2,4-trimethylbenzene at 350°C and isomerization and cracking of various alkanes (1,2,4-trimethylpentane and 2,4-dimethylpentane at 350°C; 2-methylpentane and n-hexane at 400°C) were carried out in a flow reactor on an acid zeolite (HY) whose acid sites are of widely different strengths. The minimum strength that the acid sites must have for catalysing each reaction is estimated by comparing the change in the rate as a function of the temperature of pyridine desorption. There are significant differences between the reactions: the slower is the reaction, the stronger are the acid sites required for its catalysis. Hence the isomerization of 3,3-dimethyl-1-butene, which occurs very rapidly, requires weak acid sites just able to retain pyridine adsorbed above 220°C, whereas n-hexane transformation, which is 100 times slower, requires very strong acid sites capable of retaining pyridine adsorbed up to 520°C. This series of reactions can therefore be used to characterize the strength of acid catalysts. © 1990.