Step-wise thermal desorption of ammonia from X, Y, and ZSM-5 type zeolites

Abstract

Step-wise thermal desorption (STD) of ammonia chemisorbed at 303 K on Na· Y, CeNa· Y (with different degrees of Ce⁺³ exchange), H· Y, CeNa· X and H· ZSM-5 (with Si/Al ratio varying from 13.6 to 39.7) has been investigated in a number of temperature steps from 303 to 973 K. The chemisorption of ammonia on the zeolites at different temperatures was estimated from the STD data. The chemisorption and STD data have indicated that the zeolite have a broad site energy distribution and differ widely in their acidity. The chemisorption of ammonia has been found to be strongly influenced by the Si/Al ratio and the nature of the cation and its degree of exchange in the zeolite. Good correlations between the chemisorption of ammonia and the catalytic activity of H· ZSM-5 zeolites, (for cumene cracking and o-xylene isomerization), and CeNa· Y zeolites (for cracking of isooctane) have been observed. Ammonia was found to interact strongly with Na⁺ ions at lower temperatures (< 623 K).