Identification of the active acid sites of fluorinated alumina catalysts dedicated to n-butene/isobutane alkylation Original Research Article

The acid–base properties of pure and fluorinated alumina (0.6–22 wt.%) were determined by temperature-programmed desorption of NH3 and by infrared spectroscopy (IR) of adsorbed pyridine and 2,6-dimethylpyridine. No direct correlation between total acidity of materials and catalytic activity in the alkylation of n-butene by isobutane was observed. However, a good correlation between Brønsted acidity and the catalytic activity was obtained. Indeed, the higher the overall Brønsted acidity is the higher their activity in the alkylation reaction increases. Moreover, inactive catalysts in alkylation were obtained by neutralizing selectively the Brønsted sites created by F-addition with sodium ions. From physical characterizations such as FTIR, it was proposed that the active Brønsted sites are associated to aluminium hydroxyfluorides formed during the catalysts preparation on low F-containing catalysts.