On the Ambient Temperature Gas-Solid Interaction of Acetone with Some Silica-Supported d° Metal Oxides

The combined use of adsorption microcalorimetry/gas-volumetry and in situ FTIR spectroscopy was employed to investigate qualitative and quantitative/energetic aspects of the ambient (beam-) temperature adsorption of acetone on silica-supported Al, Zr, and Ca oxides. All oxidic systems examined possess strong Lewis acidity, though of different strength (Al > Zr > Ca), whereas only the first two mixed oxides present Brønsted acidity of comparable strength. Acetone uptake yields different types of molecular adsorption: two types of H-bonding interaction with surface silanols, as typical of pure silica systems; coordinative interaction with Lewis acidic sites, part of which is not vacuum reversible at ambient temperature in the case of Al- and Zr-doped systems; strong H-bonding interaction with Brønsted acidic centers (when available), a large fraction of which is not vacuum reversible at ambient temperature. In addition, and only on the Ca-doped system, acetone undergoes the aldol condensation reaction yielding, as main reaction product, mesityl oxide that adsorbs, with different strength, at both surface silanols and Lewis acid sites. The occurrence of the aldol condensation reaction on only one of the three mixed oxides examined (whereas it occurs on all of the corresponding pure oxides) is discussed, with conclusions concerning the sites that catalyze the condensation reaction that do not agree with either of two conflicting interpretations present in the literature.