Acidic properties of silica—alumina gels as a function of chemical composition. Titration and catalytic activity measurements

Abstract

The strength and number per unit surface area of acid sites on a series of silica—aluminas of composition ranging from 0 to 100 % in alumina have been investigated. The results of titration by n-butylamine in the presence of six different H_R indicators and of activity and selectivity in the transformation of 4-methylpentan-2-ol are compared.

An overall parallelism between the amount of acidity of different strengths and the activity in the different reactions (i.e., dehydration, cis-trans-isomerization, double-bond shift and isomerization of the hydrocarbon skeleton, which set in successively with increasing acidity of the catalyst) is observed.

Acidity in all samples arises only from strongly acidic (H_R⩽–13.3) and intermediate strength sites (–6.63 ⩽H_R⩽+ 0.82).

Comparison of the results with those of cation exchange capacity measurements is not satisfactory, because of the inherent limitations of the latter method. Results for strong acid sites agree perfectly with those reported in infrared studies using adsorbed pyridine and CD₃CN. Results concerning intermediate strength sites suggest that i.r. studies, titration and catalytic acitivity measurements may complement each other.