Despite the widespread use of amorphous aluminosilicates (ASA) in various industrial catalysts, the nature of the interface between silica and alumina and the atomic structure of the catalytically active sites are still subject to debate. Here, by the use of dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) and density functional theory (DFT) calculations, we show that on silica and alumina surfaces, molecular aluminum and silicon precursors are, respectively, preferentially grafted on sites that enable the formation of Al(IV) and Si(IV) interfacial sites. We also link the genesis of Brønsted acidity to the surface coverage of aluminum and silicon on silica and alumina, respectively.

Brønsted acid sites of similar strength to those in zeolites are found in ASAs prepared by cogelation, as shown by thermogravimetric study of the dehydration of ethanol to ethylene. ASAs prepared by grafting and a commercial ASA (Siralox 30) show Brønsted acid sites that differ from those of zeolites. CO respectively ethanol adsorption followed by infrared spectroscopy identified the OH stretching frequency of the Brønsted acid sites specific to Si/Al 2 O 3 and Al/SiO 2 materials obtained by grafting to be ca. 3600 cm-1 and 3640 cm-1 , respectively, although the intensity of the corresponding band is weak. These sites, contrary to those of a H-USY zeolite with very few extraframework aluminium, are highly sensitive to the surface hydration state: Their number decreases with increasing temperature. The surface structure of Siralox 30 resembles that of Si/Al 2 O 3 prepared in anhydrous conditions, showing similar isosurface acidic properties and distribution of silicon species.