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

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 zeolitesʹ, HMordenite and NaHX, showed that ammonia molecules adsorbed up to 480 K in HZSM5, reacted `layer-after-layerʹ with all the acid sites in a `layerʹ 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.