Brønsted acidity of silanol groups on silica immersed in carbon tetrachloride

Abstract

Infrared spectra of n-butylamine, di-n-butylamine and tri-n-butylamine adsorbed from the vapour phase onto the surface of silica are compared with corresponding spectra of the amines on silica immersed in carbon tetrachloride. Adsorption at the solid/vapour interface involved hydrogen-bonding interactions between surface silanol groups and adsorbed amine molecules. The assignment of infrared bands in the spectral range 2400–2800 cm^–1 to vibrations of substituted ammonium ions adsorbed at the solid/liquid interface is confirmed by the presence of similar bands in spectra of silica which had been treated with amine and subsequently exposed to hydrogen chloride gas. Isolated surface silanol groups on silica immersed in carbon tetrachloride exhibit Brønsted acidity in the presence of mono-, di- or tri-n-butylamine molecules which become protonated to form the corresponding quaternary ammonium ions.