(781a) Surface Chemistry of Polyols On Metal Oxides

Surface catalyzed reactions of biomass-derived oxygenates in water are of great interest for the development of biorefining processes. However, little is known about how these compounds interact with the surfaces of different catalysts and supports. The present contribution uses a combination of adsorption isotherms, IR and NMR spectroscopy, and DFT calculations to analyze the surface interactions of polyols with alumina, titania, zirconia, magnesia, and ceria in an aqueous environment and vacuum. While dispersive interactions tend to dominate adsorption on microporous sorbents (e.g. zeolites) directed interactions control adsorption on mesoporous sorbents, such as amorphous oxides. On the surface of the latter, oxygenates compete with water for adsorption sites. Even in the presence of water, polyols with sufficient spatial separation of their alcohol groups (e.g. glycerol) can form stable multidentate surface species on some metal oxides (e.g. alumina). These sorbates are chemisorbed via at least one alkoxy bond while another alcohol group strongly interacts with same surface site. Polyols that only contain vicinal alcohol groups only chemisorb when the co-adsorbed water is removed in vacuum, and the formation of some of certain types of bonds is reversible. Lewis acid sites play a critical role in the formation of these surface species. Consequently, no chemisorbed species are observed on magnesia, which contains next to no acid sites.