Infrared studies of surface acidity and reversible folding in palygorskite

The IR absorption spectra of a palygorskite sample from Spain showed two previously unreported bands in the OH-stretching region. To characterize the nature of these OH groups pyridine was adsorbed on the sample. It appears that at 150 degrees C the palygorskite folded and the OH groups protonated, resulting in the formation of a deformed pyridinium ion between 150 and 290 degrees C. A high concentration of thermally stable Lewis-acid sites on the surface of the palygorskite was also noted.