Acid properties of NaH-mordenites: Infrared spectroscopic studies of ammonia sorption

Abstract

Ammonia was used as a probe molecule to study the acid properties of NaH-mordenites. Ammonia reaches all acid sites in both large and narrow channels. The concentration of Bronsted sites (Si OH Al groups) and of Lewis acid sites was followed as a function of the Na/H exchange degree and the calcination temperature (extent of dehydroxylation). In NaH-mordenites of exchange degrees lower than 80%, the concentration of Bronsted sites was found to be practically the same as the theoretical value (the amount of Al minus the amount of Na). In H-mordenites of exchange degrees of 100%, the experimental value was 20% lower than the theoretical one. This difference was explained by a dehydroxylation that occurs during activation at 820 K. Studies of dehydroxylation have shown that bulk dehydroxylation starts at about 900 K. At 1050 K, most hydroxyls are lost, and the concentration of Lewis acid sites attains the maximal value, 4.5 sites/u.c. This value is comparable to the theoretical one, half of the Al content in H-mordenite (3.6 sites/u.c.). Variation of the O H stretching frequency with the Na/H exchange degree and with the extent of dehydroxylation suggests variation of the acid strength. This is discussed in the terms of the heterogeneity of Si OH Al groups in NaH-mordenites.