Simultaneous determination of Lewis and Bronsted site densities on silica alumina using infrared techniques

Abstract

When a pyridine−covered surface of a silica−alumina catalyst is contacted with water, there is an inversion of the population of coordinately bound pyridine to species bound by proton exchange. We have used this fact along with a standard ’’titration’’ technique to determine in a single series of measurements on the same catalyst the number of Lewis and Bronsted sites both before and after water exposure. This modified titration technique was applied to a series of experimental silica−alumina catalysts. The total number of acid sites and the number of Lewis and Bronsted sites per unit area of catalyst were determined as a function of the silica content of the catalyst. The Bronsted−site concentrations agree excellently with previously published data by Clark and Holm (J. Catal., 1963) on the catalytic isomerization of o−xylene over similar silica−alumina catalysts.