Heterogeneity of OH groups in H-mordenites: Effect of dehydroxylation

Abstract

Quantitative i.r. studies of ammonia and pyridine sorption in H-mordenite showed that the amount of Br\/onsted acid sites (acidic hydroxyls) 6.5 H+/u.c. was close to the theoretical value calculated from the chemical analysis (7.2 H+/u.c.). Approximately half of this amount (3.0 H+/u.c.) was situated inside the 12-ring channels (the main channels) and half (3.5 H+/u.c.) inside the 8-ring channels. Hydroxyls in both kinds of channels were found to be prone to dehydroxylation to the same extent. The number of Lewis acid sites (created by dehydroxylation) detected by pyridine was much lower than detected by ammonia. This observation, together with the fact that in dehydroxylated H-mordenite most of the acidic hydroxyls are inaccessible to pyridine, suggests that dehydroxylation results in a narrowing of pores, making them inaccessible to pyridine (also to other bulky reactant molecules). We studied the acid strength of OH groups by comparing the stretching frequencies and extinction coefficients of free OH bands and in ammonia thermodesorption experiments. We found that in the nondehydroxylated H-mordenite, the acid strength of the OH groups inside the 12-ring channels was higher than inside the 8-ring. Dehydroxylation decreases the acid strength of hydroxyls. This concerns both the whole population of OH groups and the population of hydroxyls accessible to pyridine only. These effects are discussed considering the heterogeneity of OH groups in H-mordenite and removal of the most acidic hydroxyls first.