Heterogeneity of OH groups in NaH-mordenites: Effect of Na/H exchange degree

Abstract

Our earlier results and the results of other authors gave evidence that OH groups in NaH-mordenites may be localized both in the 12- and 8-ring channels. In this study we determined (by quantitative i.r. studies of pyridine and ammonia sorption) the concentration and acid strength of both kinds of hydroxyls. The first protons introduced into mordenite (at low exchange degrees) form HF hydroxyls in the 12-ring channels. Their concentration was close to the theoretical values (amount of AI minus amount of Na). At an exchange degree of about 50% the concentration of these hydroxyls reached the maximal value, about 4 OH/u.c), and the next protons introduced form LF hydroxyls in the 8-ring channels. This limit value of 4 OH/u.c. may be related to the number of 4.1 Na+ ions in D and E sites in the 12-ring channels in dehydrated Na-mordenites. The acid strength of HF and LF hydroxyls was studied by comparing the values of stretching frequencies and extinction coefficients of OH bands as well as in pyridine (or ammonia) thermodesorption experiments. It has been found that the acid strength of both kinds of hydroxyls increased with the exchange degree. It is discussed by assuming the heterogeneity of OH groups in NaH-mordenites, and assuming that in each population of hydroxyls (HF and LF) there are more and less acidic ones. It may be assumed that in each population of hydroxyls the first protons introduced form less acidic hydroxyls and the next protons more acidic ones. It increases the average acid strength of each of these two populations of hydroxyls. It has been found also that the formation of LF hydroxyls did not influence the acid strength of HF hydroxyls. Opposite results were obtained earlier with NaHY zeolites in which the formation of O3H in hexagonal prisms increased distinctly the acid strength of O1H inside supercavities.