Brønsted acidity of amorphous silica–alumina: The molecular rules of proton transfer

The nature of acid sites on amorphous silica–alumina (ASA) is strongly debated, as well as their infrared signature. We report a combined experimental and computational study to unravel this challenging question at the atomic scale, focusing on proton transfer from ASA to lutidine (2,6-dimethylpyridine), an experimentally widely used molecule for probing Brønsted acid sites. The ASA surface model obtained by density functional theory (DFT) calculations is validated by the comparison of infrared frequencies of OH-groups with experimental spectra. The bands observed are assigned to the various OH-groups present, as a function of their hydrogen-bond donor character and of the proximity of silanols toward aluminum atoms. The affinity of lutidine (2,6-dimethylpyridine) for each site of the ASA surface is then evaluated by sampling the DFT model and varying the experimental pretreatment conditions. A general rule is established for Brønsted acidity of ASA, by comparison with calculations on reference silica, alumina, and mordenite models: the driving force for the proton transfer from OH-groups to lutidine is the stabilization of the conjugated base (after deprotonation) of the hydroxyls, more than the intrinsic acidity of the OH-group. Pseudo-bridging silanols (PBS) are thus found to be capable of proton transfer, thanks to the stabilization of silanolate species by the formation of additional O–Al and O–Si bonds. A prominent role of water molecules adsorbed on Al atoms is also shown: they act as a proton reservoir to express intrinsic acidity and to promote the acidity of neighboring silanols. Finally, we suggest that the ν˜8a and the ν˜8b modes of lutidinium species are inverted with regards to lutidine, contrary to what was previously thought on the basis of empirical data.

Graphical abstractCombined experiments and first-principles calculations unravel the nature and behavior of Brønsted acid sites on amorphous silica–alumina: the key roles of pseudo-bridging silanols and water molecules adsorbed on Al atoms are revealed.Figure optionsDownload full-size imageDownload high-quality image (164 K)Download as PowerPoint slideHighlights► 2,6-Dimethylpyridine is adsorbed on ASA to unravel the nature of acid sites. ► The stability of the conjugated base of the Brønsted acid sites is a key parameter. ► Pseudo-bridging silanols are acidic because of the formation of new M–O bonds. ► H2O adsorbed on Al atoms is a proton reservoir and induces cascade proton transfer.