ACIDITY OF SUBSTITUTED MESOPOROUS MOLECULAR-SIEVE MCM-48

The nature, strength and relative concentration of acid sites of silic eous and Al-, Ga-and Fe-substituted MCM-48 have been studied by FTIR s pectroscopy in the region of the nu(OH) vibrations and delta(NH) vibra tions of adsorbed ammonia. The interior surface is strongly hydroxylat ed. Different types of hydroxy groups could be distinguished, vibratin g at 3740 (terminal silanols), 3710 (geminal silanols) and 3500 cm(-1) (associated silanols). The surface is strongly hydrophilic but silano ls are not acidic. Substitution of silicon by Al, Ga and Fe yields Bro nsted acid sites giving rise to an absorbance at 1450 cm(-1) in the sp ectra of adsorbed ammonia. The thermal desorption shows that the acidi c strength of Bronsted sites decreases in the order Al > Ga > Fe as do es the thermal stability. The acidity of sites is weak to medium stron g and Al-MCM-48, especially, contains some strong Bronsted acid sites. Calcination causes the formation of Lewis acidity at the expense of t etrahedrally coordinated Me-III. Two types of Lewis sites absorbing at 1610 (L1) and ca. 1300 cm(-1) (L2) are found on Al-and Ga-MCM-48, whe reas Fe-MCM-48 contains L1 sites only. Lewis acidity decreases in the order Ga approximate to Al much greater than Fe. Al and Ga Lewis sites are very strong and exceed the acidity of Bronsted sites considerably . The catalytic properties in cumene cracking confirm the results of a cidity investigations and show that strong Bronsted acid sites of Al-M CM-48 have an important impact on the activity.