ACID STRENGTH OF SILICA-ALUMINA AND SILICA STUDIED BY MICROCALORIMETRIC MEASUREMENTS OF PYRIDINE ADSORPTION

Microcalorimetric measurements of the differential heat of pyridine adsorption at 473 K were used to probe the distribution of acid strength on silica and silica-supported alumina catalysts. Depositing alumina on silica increased the acid strength of the catalyst. The acid strength distribution curves for the Al2O3/SiO2 sample showed three regions of constant heat of adsorption while silica had an energetically homogeneous surface. The Al2O3/SiO2 catalyst was found to have both Brønsted and Lewis acidity. Incremental adsorption of pyridine on Al2O3/SiO2 indicated that the initial region of constant heat corresponds to strong Lewis acidity while the intermediate region seemed to be Brønsted sites or a combination of weaker Lewis sites and Brønsted sites. The final region observed at higher extents of adsorption showed only H-bonded pyridine which corresponds to adsorption on silica. Estimates of the entropies of adsorption were determined, providing information about the mobility of the adsorbed basic molecules. Surface diffusion over the silica support provides an important pathway for the equilibration of pyridine between acid sites on the Al2O3/SiO2 sample. © 1990.