Nature, Structure and Strength of the Acidic Sites of Amorphous Silica Alumina:  An IR and NMR Study

Abstract

IR spectroscopy of probe molecules (pyridine, 2,6-dimethylpyridine, and CO) as well as high-resolution solid state NMR and especially double-resonance experiments give a new insight into the acidic sites of amorphous silica alumina (ASA). ASA samples are heterogeneous compounds that contain a silica alumina mixed phase as well as aluminum clusters and pure silica zones. The distribution of various forms depends both on the preparation method and on the Si/Al ratio. Formation of mixed phase leads to the creation of acidic hydroxyl groups of various strength, up to that present in dealuminated HY zeolite. Detailed spectroscopic analyses show that these acidic OH groups correspond to the silanol groups located in close vicinity to an Al atom in tetrahedral environment. The strength of the acidity of the OH species of ASA could be modified both by the location of the vicinal Al atom on the surface or in the bulk and by the number of aluminum atoms in the vicinity of silanol group. Cogelification of high silica-containing ASA appears as the best mean to prepare homogeneous amorphous aluminosilicate, which exhibits the strongest Brønsted acidity.

Acidity and Characterization of 12-Tungstophosphoric Acid Supported on Silica-Alumina

“…The characteristics of acid sites on amorphous silica-alumina have been debated for a long time in the literature. 17,[36][37][38] However, different reports using diffuse reflectance Fourier transformed infrared spectroscopy (DRIFTS), density functional theory (DFT) and their combination have clarified it. 17 It has been proposed that silica-alumina surface is formed by isolated silanols and/or hydrogen-bonded bridge silanols.…”

Density functional theory simulations of complex catalytic materials in reactive environments: beyond the ideal surface at low coverage

“…Structural hypotheses are not unanimous between research groups. [96][97][98][99][100][101] New insights have recently been acquired through DFT calculations, 87,[102][103][104] leading to a surface model for silicated alumina, which accounts for the presence of original surface sites at the origin of mild acidity. Note that, to the best of our knowledge, despite significant effort on the simulation of bulk aluminosilicate glasses for geology, 105,106 simulation studies of the surface of ASA have not been undertaken before, except for some earlier attempts to choose "local" models, as aluminosilsesquioxanes.…”

“…Our DFT calculations show that they can exist on the ASA surface, 87 even if the coordination number of their aluminium atom is not systematically equal to four. Silanols bonded to lowcoordinated aluminium atoms by a Si-O-Al bridge were presented as the most acidic Brønsted sites by Crépeau and co-workers, 98 depending on the number and coordination of Al. These sites are related to the silanol-Al proposed within the DFT model.…”

Influence of Metal Oxides Impregnated on Silica—Alumina in the Removal of Sulphur and Nitrogen Compounds from a Hydrotreated Diesel Fuel Stream

“…According to the literature (Crépeau et al 2006;Zaki et al 2001), for alumina and silica-alumina samples, bands at 1615 and 1622 cm -1 were assigned to Al 3+ sites in octahedral and tetrahedral symmetries, respectively. For the Siral sample (Figure 3), it was not possible to clearly identify the bands assigned to Al 3+ sites in octahedral and tetrahedral symmetries, because only a broad band with a small shoulder was observed around 1622 cm -1…”