Abstract: The effect of fluorination on the surface acidity of γ-Al₂O₃ and the influence of the amount of fluorine on the types strength and number of acid sites have been studied. IR spectra of pyridine(PY) and 2,6-dimethylpyridine(DMPY) adsorbed-desorbed on fluorinated γ-Al₂O₃(F/γ-Al₂O₃) and 31P MAS NMR of trimethylphosphine(TMP) adsorbed on F/γ-Al₂O₃ have been used to measure the acidities of F/γ-Al₂O₃ . It has been found that F/γ-Al₂O₃ presents both Brnsted and Lewis sites. IR spectra of pyridine adsorbed on F/γ-Al₂O₃ pretreated in vacuum at 300 K show four bands at 1 451, 1 491, 1 580 and 1 617 cm^-1, the bands are assigned to PY adsorbed on Lewis acid sites, and two bands at 1 544 and 1 640 cm^-1 are assigned to PY adsorbed on Bronsted acid sites. IR spectra of 2,6-dimethylpyridine adsorbed on F/γ-Al₂O₃ pretreated in vacuum at 300 K shows a major band at 1 637 cm^-1 due to DMPY adsorbed on Bronsted acid sites. ³¹P MAS NMR spectra of trimethylphosphine adsorbed on F/γ-Al₂O₃ pretreated in vacuum at 353 K show three peaks at δ -56 5, -28 2 and -21 9 are assigned to TMP chemisorption to different types of Lewis sites, and a peak at δ -2 8 is assigned to TMP chemisorption to Bronsted acid site. When the fluorine content increases, the total number of Lewis acid sites decreases, whereas their strength increases, a few of new types of Lewis acid sites are created simultaneously. The number and strength of Brnsted acid site presents a maximum for samples with 2%-3% of fluorine content. TMP as a sensitive NMR probe molecule is introduced for acidity measurements on F/γ-Al₂O₃ surfaces in the present study. 31P MAS NMR results agree with that of IR.

Key words: Fluorinated γ-Al₂O₃, Surface acidity, Adsorption, Solid NMR, In situ FTIR