γ-Alumina and Amorphous Silica–Alumina: Structural Features, Acid Sites and the Role of Adsorbed Water

The adsorption of water, pyridine, CO and CO₂ on a commercial γ-Al₂O₃ and a commercial amorphous silica alumina (ASA) cracking catalyst are reinvestigated by means of FTIR spectroscopy. It is concluded that the real active hydroxyl groups on γ-Al₂O₃ are those absorbing at 3790, 3770 and 3730 cm⁻¹ and that they are significantly acidic. The strongest Lewis acid sites are identified as Nuclear Magnetic Resonance (NMR)-silent (or still undetected) tricoordinated Al³⁺, becoming tetracoordinated when interacting with basic adsorbates. More than one pyridine molecule may adsorb over such highly coordinatively unsaturated sites at high pyridine vapour pressure. It has also been confirmed that the existence of Brønsted acid sites sufficiently strong to protonate pyridine on ASA is its intrinsic property, not due to coadsorbed water. A “stuffed amorphous silica” model is proposed for low-alumina ASA, where part of Al³⁺ ions compensate the charge in-balance due to the substitution of Al for Si in a silica-like tetrahedral framework. Interstitial Al³⁺ occupy 4-, 5- or 6-coordination interstices in the amorphous silica-like framework as do alkali cations in “stuffed silicas” aluminosilicates. Such “interstitial” Al³⁺ ions when located accidentally in opened interstices exposed at the surface play the role of strong Lewis sites. When these ions are located near a silanol group, they increase the Brønsted acidity of the terminal silanols by neutralizing the negative charge produced by their dissociation when interacting with bases.