Acidity and catalytic activity of a nafion-H/silica nanocomposite catalyst compared with a silica-supported nafion sample

Acid sites were characterized in the Nafion-H/silica nanocomposite catalyst through pyridine adsorption using the surface FTIR method. An impregnated Nafion-H/SiO₂ catalyst was also studied for comparison. The ring-opening reaction of cyclopropane was used to characterize surface acidity. The 13% Nafion-H/silica nanocomposite sample had a large number of surface SO₃H groups compared to the silica-supported Nafion-H; this guarantees excellent performance in catalytic transformations. This excellent activity was due to the increased accessibility of surface-active sites to reactants, made certain by the large surface area and the porous structure of the silica matrix. Pyridine adsorption data showed that the surface concentration of Broensted sites of the nanocomposite material was ~ 1 order of magnitude higher than that of the silica-supported sample. However, the difference in catalytic activity was more pronounced when the initial rates of the catalytic reactions were compared, indicating that the concentration of surface-active sites was not the sole factor determining catalytic activity. The higher accessibility of protic sites on the surface in the Fries rearrangement was possibly due to the experimental conditions and the favorable steric demand of the reacting molecule.