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 effect s accompanying one adsorption-desorption cycle of pure ammonia in helium ca rrier gas at 373 K have been carried out on powdered porous materials belon ging to three series: acid-treated clays (K5, K30, KSF), HY zeolites with v arying RAI ratio (HY30, HY20, HY9, HY6) and surfactant-templated aluminosil icates (MSA20, MSA10, MSA5). These data were subsequently used to determine the number and mean strength of surface acid sites. The same experiments w ere performed on three selected samples pre-saturated with water vapour the end of the clause. The non-negligible influence of vicinal water on the re sults of surface acidity tests was demonstrated. The comparison of the flow -microcalorimetry (FMC) method with two-cycle ammonia adsorption and thermo programmed ammonia desorption revealed some differences in the number of su rface acid sites which could be ascribed to various experimental conditions used during desorption or pre-treatment stages in each of these methods. ( C) 2001 Elsevier Science B.V. All rights reserved.