Acidity and Catalytic Activity of a Nafion-H/Silica Nanocomposite Catalyst Compared with a Silica-Supported Nafion Sample

The characterization of acid sites in the recently developed Nafion-H/silica nanocomposite catalyst was carried out through pyridine adsorption using the surface FT-IR technique. For comparison, an impregnated Nafion-H/SiO2 catalyst was also studied. A large number of free surface SO3H groups was detected on the surface of the Nafion-H/silica nanocomposite, in contrast to the impregnated catalyst. Pyridine adsorption demonstrates that Brønsted as well as Lewis acid sites are present on the surface of the samples. There was a large number of available Brønsted sites on the surface of the Nafion-H/silica nanocomposite catalyst. As well as pyridine adsorption, the surface reaction of cyclopropane also revealed that the nanocomposite catalyst has a high number of acid sites. The impregnated sample exhibits much lower activity. The effect of different types of acidity and of acid strength were studied by means of the Friedel–Crafts alkylation (adamantylation of toluene) and various rearrangements. In agreement with the results of surface studies, the nanocomposite specimen exhibits activity which is several orders of magnitude higher than that observed for the other Nafion samples studied. This outstanding activity is attributed to the large surface area and to the increased accessibility of surface-active sites to reactants in catalytic transformations.