Acidity and catalytic activity of a Nafion-H/silica nanocomposite catalystcompared with a silica-supported Nafion sample

The characterization of acid sites in the recently developed Nafion-H/silic a nanocomposite catalyst was carried out through pyridine adsorption using the surface FT-IR technique. For comparison, an impregnated Nafion-H/SiO2 c atalyst was also studied. A large number of free surface SO3H groups was de tected on the surface of the Nafion-H/silica nanocomposite, in contrast to the impregnated catalyst. Pyridine adsorption demonstrates that Bronsted as well as Lewis acid sites are present on the surface of the samples. There was a large number of available Bronsted sites on the surface of the Nafion -H/silica nanocomposite catalyst. As well as pyridine adsorption, the surfa ce reaction of cyclopropane also revealed that the nanocomposite catalyst h as a high number of acid sites, The impregnated sample exhibits much lower activity. The effect of different types of acidity and of acid strength wer e studied by means of the Friedel-Crafts alkylation (adamantylation of tolu ene) and various rearrangements. In agreement with the results of surface s tudies, the nanocomposite specimen exhibits activity which is several order s of magnitude higher than that observed for the other Nafion samples studi ed. This outstanding activity is attributed to the large surface area and t o the increased accessibility of surface-active sites to reactants in catal ytic transformations. (C) 2000 Academic Press.