NMR and FTIR characterization of surface acidity of fluorinated γ-Al2O3

The effect of fluorination on the surface acidity of gamma-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 gamma-Al2O3(F/gamma-Al2O3) and P-31 MAS NMR of trimethylphosphine (TMP) adsorbed on F/gamma-Al2O3 have been used to measure the acidities of F/gamma-Al2O3,. It has been found that F/gamma-Al2O3 presents both Bronsted and Lewis sites. IR spectra of pyridine adsorbed on F/gamma-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/gamma-Al2O3 pretreated in vacuum at 300 K shows a major band at. 1 637 cm(-1) due to DMPY adsorbed on Bronsted acid sites. P-31 MAS NMR spectra of trimethylphosphine adsorbed on F/gamma-Al2O3 pretreated in vacuum at 353 K show three peaks at delta - 56.5, - 28.2 and -21.9 are assigned to TMP chemisorption to different types of Lewis sites, and a peak at delta - 2.8 is assigned to TMP chemisorption to Bronsted acid site. When the fluorine content increases, the total number of Lewis acid sites decreases, wheriaas their strength increases, a few of new types of Lewis acid sites are created simultaneously. The number and strength of Bronsted 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/gamma-Al2O3 surfaces in the present study. P-31 MAS NMR results agree with that of IR.