Acidity of the silica–alumina/BX3 superacids
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.