THE EFFECT OF PREHEATING ON THE EVOLUTION OF THE ACIDITY OF PILLARED CLAYS

Thermal studies under vacuum show that the minimum in the DTG curves a scribed to the dehydroxylation of the pillars of Al-PILC and Al-Ga-PIL C samples occurs at 335 and 415-degrees-C respectively. This process p roduces an increase in the concentration of acid centres. This phenome non has been related to the results obtained for pyridine desorption b y IR spectroscopy. The IR spectrum of an Al-PILC sample without therma l treatment prior to pyridine sorption showed a decrease in the intens ities of the typical bands of Lewis-acid centres when pyridine desorpt ion was performed from 200 to 250-degrees-C. However, the intensities of these bands increased when pyridine desorption was performed at 300 -degrees-C. In contrast, for the Al-Ga-PILC sample, there was always a decrease in the intensities of the bands. Nevertheless, when the two samples were treated at 400 or 600-degrees-C prior to pyridine sorptio n, they both showed a continuous decrease in the intensity of the band s of the acid centres in the pyridine desorption process over the whol e temperature range tested.