Heat of Adsorption and Surface Acidity of Alumino-Silicate and Ferrisilicate Catalysts

Amorphous silica-alumina characterized by their surface acidity are good cracking catalysts. The active sites of these catalysts are considered to be the acid centres. Amorphous silica-alumina were prepared by the coprecipitation and gelation method and iron(III) dispersed on silica was prepared by the incipient wetness method. The samples were calcined and their surface acidity was measured by the base exchange method using ammonium acetate as an adsorbate. The integral heats of preferential adsorption of pyridine from n-heptane solution on the silica-alumina (amorphous aluminosilicate) and iron dispersed on silica (ferrisilicate) were measured by a flow microcalorimeter. For comparison, surface acidities and integral heats of adsorption of a standard sample of HZSM-5 and chromatographic alumina and silica have also been measured. From the heat of adsorption data it has been found that HZSM-5 gives BET type I (monolayer adsorption) and silica-alumina samples give BET type II (multilayer adsorption) isotherms. Comparison of the integral heat of adsorption with heat of desorption showed that pyridine molecules are mostly physisorped on both HZSM-5 and amorphous siliea-alumina samples. It has also been found that there is a good correlation between the surface acidity and integral heat of adsorption of the silica-alumina samples.