Lewis and Bronsted Acid Sites on Silica-Alumina

Abstract

SOME years ago, Mapes and Eischens¹, Nicholson² and Roev, Filimonov and Terenin³ studied the infra-red spectrum of ammonia chemisorbed on silica–alumina cracking catalysts. Mapes and Eischens¹ reported absorption bands at 3,330 cm⁻¹ and 1,640 cm⁻¹ which were assigned to ammonia adsorbed on Lewis (electron acceptor) acid sites. In addition weak bands were observed at 3,030 cm⁻¹ and 1,450 cm⁻¹, which were attributed to NH₄⁺ ions formed by chemisorption of ammonia on Bronsted {proton donor) sites. From the relative intensities of the bands, Lewis sites were thought to predominate on dehydrated silica–alumina although adsorbed water was observed to cause conversion of Lewis to Bronsted sites. Nicholson² only investigated the spectrum in the region of NH stretching vibrations and reported approximately equal numbers of Lewis and Bronsted sites. However, Roev et al.³ observed only ammonia on Lewis acid sites for their sample of silica–alumina.