Acidity of substituted mesoporous molecular sieve MCM-48

Abstract

The nature, strength and relative concentration of acid sites of siliceous and Al-, Ga- and Fe-substituted MCM-48 have been studied by FTIR spectroscopy in the region of the ν_OH vibrations and δ_NH vibrations of adsorbed ammonia. The interior surface is strongly hydroxylated. Different types of hydroxy groups could be distinguished, vibrating at 3740 (terminal silanols), 3710 (geminal silanols) and 3500 cm⁻¹ (associated silanols). The surface is strongly hydrophilic but silanols are not acidic. Substitution of silicon by Al, Ga and Fe yields Brønsted acid sites giving rise to an absorbance at 1450 cm⁻¹ in the spectra of adsorbed ammonia. The thermal desorption shows that the acidic strength of Brønsted sites decreases in the order Al > Ga > Fe as does the thermal stability. The acidity of sites is weak to medium strong and Al-MCM-48, especially, contains some strong Brønsted acid sites. Calcination causes the formation of Lewis acidity at the expense of tetrahedrally 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, whereas Fe-MCM-48 contains L1 sites only. Lewis acidity decreases in the order Ga ≈ AlFe. Al and Ga Lewis sites are very strong and exceed the acidity of Brønsted sites considerably. The catalytic properties in cumene cracking confirm the results of acidity investigations and show that strong Brønsted acid sites of Al-MCM-48 have an important impact on the activity.