Adsorption and Subsequent Surface Reactions of Bifunctional Compounds From Bio-Oil Vapors On Oxide Supports

Vapor phase upgrading of short oxygenates in bio-oil mixture needs development of chemoselective catalysts that convert individual molecules or functional groups while leaving others untouched. The development of chemoselective catalysts requires thorough understanding of selective adsorption modes and reactivity of molecules on catalyst surfaces. Likely, some of these catalysts will be supported metals, and the question arises whether oxygenates will solely interact with the metal if the support surface is polar and hydrophilic. Other questions concern the competition of various molecules in a mixture for sites or the preferential mode of adsorption of molecules with more than one functional group.

Hydroxyacetone is an excellent candidate for a fundamental investigation because it has two functional groups and is contained in bio-oil vapor in significant concentrations. We investigated the adsorption modes of hydroxyacetone on conventional oxide supports, represented by silica, alumina, titania, ceria and zirconia, using in-situ diffuse reflectance IR spectroscopy (DRIFTS) as the primary method. Monofunctional reference compounds, specifically acetone and n-propanol, were also adsorbed on these oxides to assess the role of each functionality for reactivity and generate spectra for comparison. Low temperature adsorption studies and pulse thermal analysis were carried out to estimate the temperatures at which surface reactions occur and the heat of adsorption of hydroxyacetone on the oxide supports. Spectra recorded during temperature-programmed experiments evidence strong interaction of hydroxyacetone with the oxide surfaces and incomplete desorption even after heating to 400^oC. In conclusion, hydrophilic supports may be unsuitable for upgrading highly reactive short oxygenates.