Acid–base properties of silica–alumina samples derived from NaX zeolites. Part 1.—Physical characterization and an infrared study of the adsorption of acetone

Abstract

Amorphous SiO₂/Al₂O₃ samples have been prepared from NaX zeolite and have been compared with partially ammonium-exchanged NaX zeolites. The specific surfaces and the composition of the oxides have been determined. An increasing TO antisymmetric stretching vibration wavenumber has been found for decreasing Na⁺ and Al³⁺ content in the oxide. This is interpreted as being due to an increasing degree of covalency of the cation–oxygen bond and an increasing ‘maximum acceptor strength’. The latter is defined as the electron-pair acceptor strength a fully accessible cation would exhibit towards an electron-pair donor molecule. Adsorption of acetone caused perturbed OH bands, with their wavenumbers decreasing with higher relative content of silica. Zeolitic oxides were able to form highly unsaturated surface species from acetone. All the oxides exhibited weakly bound, perpendicularly adsorbed acetone as well as acetone adsorbed parallel to the surface. This adsorbate is thought to be a precursor of carboxylate formation. Carboxylate groups were only found with one of the acid-treated samples, ASX3.