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 tha t Si.-OH-Al groups in HZSM-5 zeolites were heterogeneous. The experiments o f ammonia adsorption in HZSM-5 zeolite and in two 'reference zeolites', HMo rdenite and NaHX, showed that ammonia molecules adsorbed up to 480 K in HZS M-5, reacted 'layer-after-layer' with all the acid sites in a 'layer' witho ut selecting the most acidic centres and without redistribution. Redistribu tion which includes desorption step (activated reaction) may be too slow un der the experimental conditions. The adsorption of ammonia layer-after-laye r, without selecting the most acidic sites, explains the constant value of adsorption heat of ammonia in microcalorimetric experiments. (C) 2001 Elsev ier Science BN. All rights reserved.