One of the most promising strategies for the conversion of non-food, lignocellulosic biomass to renewable fuels and chemicals is through fast pyrolysis. This produces a wide variety of oxygenated products, ranging from light carboxylic acids to heavy phenolic species that must undergo catalytic upgrading to produce fungible fuels. Two families of catalysts that have shown considerable promise for the conversion of many of these oxygenated species to useful products are metals supported on reducible oxides and zeolites. The nature of the catalytically active site on metals supported on reducible oxides, for example Ru/TiO2, is not well understood. By systematically varying catalyst properties, we demonstrate that the active sites responsible for the conversion of methoxy groups of phenolic species are due to surface defects on the reducible oxide, rather than the perimeter surrounding the metal. The importance of the metal on enhancing the activity of the reducible oxide for this reaction is dependent on the reducibility of the oxide itself. The introduction of additional compounds present in the pyrolysis vapors, such as furanics and light oxygenates, compete for adsorption sites on both the metal and reducible oxides to varying extents resulting in significant shifts in product selectivity.
We then demonstrate the conversion of corrosive acetic acid to the chemical building block acetone over zeolites with high selectivity. The mechanism of conversion of light carboxylic acids over zeolites is contrasted to the mechanism observed over reducible oxides. The role of the proximity of active sites on the mechanism and product selectivity will be discussed. These findings will then be extended to help explain products and deactivation rates zeolites in the presence of real pyrolysis oil vapors.