Acid properties of dealuminated mordenites studied by IR spectroscopy .2. Concentration, acid strength and heterogeneity of OH groups

The concentration of OH groups was determined with two probe molecules: ammonia and pyridine. With all the mordenites, the concentration determined with ammonia was close to calculated values (amount of framework Al minus amount of Na). Quantitative IR studies of pyridine sorption evidenced that in a non-dealuminated mordenite about a half of all the hydroxyls was situated inside large (HF), and a half inside narrow channels (LF). With dealuminated mordenites, the concentration of hydroxyls determined with pyridine and ammonia was close to calculated values. The comparison of the values of OH extinction coefficients both determined with ammonia and pyridine suggested that the acid strength of OH groups increased with the degree of dealumination. The experiments of ammonia desorption evidenced that OH groups in H-Mordenites were heterogeneous. The dealumination removed the less acidic hydroxyls and hydroxyls became more acidic and more homogeneous. In mordenite of Si/Al = 120 the OH groups were practically homogeneous.