Surface chemistry of acetone on metal oxides: IR observation of acetone adsorption and consequent surface reactions on silica-alumina versus silica and alumina

Pathways and generated surface species of adsorption and consequent surface reactions of acetone vapor on characterized silica, alumina, and similar to 5 wt % silica-alumina were examined by in-situ infrared (IR) spectroscopy, following degassing at room (RT) and higher temperatures (100-400 degrees C). For reference and confirmatory purposes, adsorptives of mesityl oxide and acetic acid, and adsorbents of K-modified and pyridine-covered silica-alumina, were employed. In the absence of Lewis and Bronsted acid sites, as well as of basic sites (i.e., on silica), acetone molecules are weakly hydrogen-bonded to surface OHdelta+ groups to desorb completely at 100 degrees C, without involvement in any further surface reactions. The availability of such acid-base sites on alumina and silica-alumina facilitates acetone chemisorption and activation for aldol condensation type surface reactions, leading to formation of surface species of mesityl oxide at RT to 200 degrees C and their oxidative conversion into acetate species at 300-400 degrees C. A more obvious availability of Bronsted acid sites on silica-alumina enhances progression of the surface reactions involved.