Infrared Study of the Adsorption of Diketones on Silica Immersed in Carbon Tetrachloride

Abstract

The adsorption of pentan-3-one and four diketones onto silica immersed in carbon tetrachloride has been studied by infrared spectroscopy. The primary surface–adsorbate interaction for all five ketones involved the formation of hydrogen bonds between surface silanol groups and ketone groups in adsorbate molecules. At low surface coverages of silica by hexan-2,5-dione two isolated silanol groups formed hydrogen bonds to the two ketone groups in each adsorbed hexan-2,5-dione molecule. In contrast, a single silanol group formed a hydrogen bond to one of the two ketone groups in each hexan-2,3-dione molecule adsorbed at the solid/liquid interface. The second ketone group in each molecule was apparently unperturbed by the oxide surface. The mode of adsorption of pentan-2,4-dione on silica was complicated by the stability of the enol form of the diketone. Spectra of pentan-2,4-dione adsorbed from the gas phase and from carbon tetrachloride solution have established that both the keto and enol forms of the diketone existed on silica at the solid/vapour and the solid/liquid interfaces. Corresponding enolization could not occur for 3,3-dichloropentan-2,4-dione which was adsorbed on silica as the diketone. The electronic effect of 3,3-dichloro substitution weakened the surface–adsorbate interaction.