Acidity of the silica–alumina/BX3 superacids

Abstract

Acid properties of the catalysts obtained by the reaction of boron halides solutions with silica–alumina containing 87% of SiO₂ were studied. It was found that the strength of the acid sites formed depends on boron halide used and the nature of BX₃ solvent. The stronger acceptor properties of BX₃ the higher acid strength of SiO₂(87%)-Al₂O₃/BX₃ catalysts was observed. For the preparations obtained by the action of BX₃ 1.0 M solutions in CH₂Cl₂ the following order of superacid properties, conformable with halides acceptor strength, was found: SiO₂–Al₂O₃/BI₃ > SiO₂–Al₂O₃/BBr₃ ≫ SiO₂–Al₂O₃/BCl₃ ∼ 0. The BX₃ solvent basicity influences the acid strength of acid centres formed. The less basic is the solvent used, the higher is the strength of the acid sites formed. The experiments with selective poison of Brønsted acid centres (2,6-di-tert-butylpyridine) proved that superacid and strong acid sites of SiO₂(87%)–Al₂O₃/BX₃ catalysts are of protic nature.