Heterogeneity of OH groups in HZSM-5 zeolites. IR studies of ammonia adsorption and desorption

Abstract

The studies of ammonia desorption followed by IR spectroscopy suggested that Si–OH–Al groups in HZSM-5 zeolites were heterogeneous. The experiments of ammonia adsorption in HZSM-5 zeolite and in two ‘reference zeolites’, HMordenite and NaHX, showed that ammonia molecules adsorbed up to 480K in HZSM-5, reacted ‘layer-after-layer’ with all the acid sites in a ‘layer’ without selecting the most acidic centres and without redistribution. Redistribution which includes desorption step (activated reaction) may be too slow under the experimental conditions. The adsorption of ammonia layer-after-layer, without selecting the most acidic sites, explains the constant value of adsorption heat of ammonia in microcalorimetric experiments.