氟化γ-Al_2O_3表面酸性的NMR和FTIR表征

NMR and FTIR Characterization of Surface Acidity of Fluorinated γ-Asl_2O_3

用红外光谱和核磁共振技术研究了氟化作用对 γ-Al2 O3的表面酸性 (酸中心类型、强度及数量 )的影响 .吡啶和二甲基吡啶在氟化γ-Al2 O3(F/γ-Al2 O3)表面上吸 -脱附的 IR光谱和三甲基膦在 F/γ-Al2 O3表面上吸附的 31 P MAS NMR结果表明 ,F/γ-Al2 O3表面上既存在 Lewis酸位又存在 Bronsted酸位 . L酸的总量随氟含量的增加而减少 ,但其强度随之增加 ,且同时产生几种新的 L酸位 . B酸的数量和强度在样品含氟量为2 %～ 3 %间存在极大值 .研究还表明 ,三甲基膦是一种测量 F/γ-Al2 O3表面酸性的灵敏的 NMR探针分子 ,31 P MAS NMR和红外光谱的测试结果基本一致 .同时 。

The effect of fluorination on the surface acidity of γ-Al 2O 3 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-Al 2O 3(F/γ-Al 2O 3) and 31P MAS NMR of trimethylphosphine(TMP) adsorbed on F/γ-Al 2O 3 have been used to measure the acidities of F/γ-Al 2O 3 . It has been found that F/γ-Al 2O 3 presents both Brnsted and Lewis sites. IR spectra of pyridine adsorbed on F/γ-Al 2O 3 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-Al 2O 3 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-Al 2O 3 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-Al 2O 3 surfaces in the present study. 31P MAS NMR results agree with that of IR.