FT INFRARED STUDY OF BRONSTED ACIDITY OF H-MORDENITES - HETEROGENEITY AND EFFECT OF DEALUMINATION

Pyridine adsorption on the parent mordenite (Si/Al = 10.7) showed that the 3,610 cm(-1) OH band presents two components: one at 3,612 cm(-1), corresponding to OH groups in the main channels; the other, at 3,585 cm(-1), inaccessible to pyridine, attributed to OH groups in the side pockets. CO adsorption showed that the high-frequency groups are more acidic (Delta nu(OH) = 332 cm(-1); nu(CO) 2,177 cm(-1))than the latter(Delta nu(OH) = 260 cm(-1); nu(CO) = 2,169 cm(-1)). Steaming at 930 K followed by acid leaching modified the structure, making the low-frequency OH groups accessible to pyridine. CO adsorption still evidenced the heterogeneity of the Bronsted acid sites whose origin is discussed. This study specifies the assignment of a band occasionally observed at 3,660 cm(-1) It corresponds to water adsorption on acidic OH groups.