CHARACTERIZATION OF SOME CATALYSTS USING VACUUM BALANCES

A vacuum quartz-spring balance was used to determine the nitrogen adso rption isotherms at -196-degrees C of silica-titania, silica-alumina, silica-magnesia and chromia-alumina catalysts. The surface acidities o f these acid catalysts were determined by following the desorption of pyridine using an electronic vacuum balance. Good agreement was found between the nitrogen adsorption isotherms determined by the vacuum qua rtz-spring balance and those followed by conventional volumetric metho ds. The adsorption of pyridine at 35-degrees C was found to be of the physical type where the sorbed pyridine was completely expelled by pro longed out-gassing at the adsorption temperature. The adsorption of py ridine at 150-degrees C on the catalysts investigated is mainly chemic al and the amount of pyridine chemisorbed determines the number of aci d sites per unit weight of the catalyst or per unit surface area. The thermal desorption of pyridine was found to be related to the strength of the surface acid sites. A fairly good relationship was found betwe en the surface acid density (number of acid sites per cm2) and the cat alytic activity towards isopropanol dehydration.