Tin oxide surfaces. Part 15.—Infrared study of the adsorption of propene on tin(IV) oxide, tin(IV) oxide–silica and tin(IV) oxide–palladium oxide

Abstract

The adsorption of propene on tin(IV) oxide, tin(IV) oxide–silica and tin(IV) oxide–palladium oxide has been studied by infrared spectroscopy. Tin(IV) oxide–silica were found to chemisorb propene as a surface acetate species when the oxides were outgassed at ambient temperatures, while only tin(IV) oxide–silica reacted with propene after outgassing at 716 K, again to form a surface acetate species. Tin(IV) oxide–palladium oxide outgassed at ambient temperatures was found to chemisorb propene both as a surface acetate and acrylate, while after outgassing at 570 K reaction with propene leads only to a surface acetate species. The most probable mechanism for the formation of the surface acetate involves initial electrophilic addition of acidic hydroxy groups to the CC double bond, giving a surface isopropoxide which undergoes oxidation to the acetate via an intermediate, coordinated acetone. Consistent with this hypothesis, only tin(IV) oxide–silica is active at higher temperatures of pretreatment, where Brönsted acidity for this sample remains high but is very low for the other two oxides. The surface acrylate is formed by a similar process, although the initial process appears to involve a palladium-induced C—H bond fission of the methyl group of propene, generating a surface allyloxide species which undergoes oxidation as before.