Surface Interactions of C2 and C3 Polyols with γ-Al2O3 and the Role of Coadsorbed Water

The formation of surface species from two- and three-carbon polyols on gamma-Al2O3 in the presence and absence of coadsorbed water is investigated. Aqueous-phase adsorption isotherms indicate that competitive adsorption between water and polyol inhibits the uptake of the polyol molecules on gamma-Al2O3 and that the polyol with the most hydroxyl groups, glycerol, experienced the greatest uptake. Deuterium solid echo pulse NMR measurements support the fact that glycerol strongly interacts with gamma-Al2O3 in the presence of physisorbed water and that ethylene glycol interacts with gamma-Al2O3 only after the physisorbed water has been removed. In situ high-vacuum FT-IR analysis combined with DFT simulations demonstrate that glycerol readily forms a multidentate alkoxy species through its primary hydroxyl groups with coordinatively unsaturated Al atoms of gamma-Al2O3 in the presence of physisorbed water. This surface species exhibits a bridging alkoxy bond from one of its primary hydroxyl groups and a strong interaction with the remaining primary hydroxyl group. FT-IR analysis of 1,3-propanediol on gamma-Al2O3 also demonstrates the formation of a multidentate alkoxy species in the presence of coadsorbed water. In contrast, polyols with hydroxyl groups only on the one- and two-carbon atoms, ethylene glycol, and 1,2-propanediol do not form alkoxy bonds with the gamma-Al2O3 surface when coadsorbed water is present. These polyols will form alkoxy bonds to gamma-Al2O3 when coadsorbed water is removed, and these alkoxy species are removed when water is readsorbed on the sample. The formation of strongly bound, stable multidentate alkoxy species by ethylene glycol and 1,2-propanediol on gamma-Al2O3 is prevented by steric limitations of vicinal alcohol groups.