Determination of surface acidity of powdered porous materials based on ammonia chemisorption: comparison of flow-microcalorimetry with batch volumetric method and temperature-programmed desorption

Flow-calorimetric measurements of the transfer of matter and thermal effects accompanying one adsorption–desorption cycle of pure ammonia in helium carrier gas at 373 K have been carried out on powdered porous materials belonging to three series: acid-treated clays (K5, K30, KSF), HY zeolites with varying Si:Al ratio (HY30, HY20, HY9, HY6) and surfactant-templated aluminosilicates (MSA20, MSA10, MSA5). These data were subsequently used to determine the number and mean strength of surface acid sites. The same experiments were performed on three selected samples pre-saturated with water vapour the end of the clause. The non-negligible influence of vicinal water on the results of surface acidity tests was demonstrated. The comparison of the flow-microcalorimetry (FMC) method with two-cycle ammonia adsorption and thermoprogrammed ammonia desorption revealed some differences in the number of surface acid sites which could be ascribed to various experimental conditions used during desorption or pre-treatment stages in each of these methods.