Surface Interactions of Glycerol with Acidic and Basic Metal Oxides

The surface species formed by glycerol on γ-Al 2 O 3, TiO 2 anatase, ZrO 2, MgO, and CeO 2 both in the presence and in the absence of bulk water are investigated with infrared spectroscopy. The acid–base properties of the metal oxides are characterized by pyridine and CO 2 adsorption/temperature-programmed desorption. The metal oxides studied provide a distribution of strengths of Lewis acid sites as well as strengths and types of basic sites which afford insight into the role of these various sites in polyol/metal oxide surface interactions. Even in the presence of bulk water, glycerol forms a bridging alkoxy bond through a primary alcohol group to two coordinatively unsaturated metal atoms and participates in a Lewis acid/base interaction between the oxygen atom of the other primary alcohol and a coordinatively unsaturated metal atom that is also involved in the alkoxy bond. These interactions only occur with metal oxides which contain strong Lewis acid sites. A quantitative correlation between the C–O stretching frequencies of the chemisorbed groups and the electronegativity of the metal atoms is established. Glycerol experiences an additional surface interaction via a hydrogen-bonding interaction between its secondary alcohol group and a relatively weak basic surface oxygen atom. Stronger base sites are blocked by adsorbed water or CO 2 . In the absence of strong Lewis acid sites, in the case of MgO, hydrogen-bonding interactions between glycerol and surface hydroxyls are the dominant means of interaction.