Deoxygenation of bio-oil over solid base catalysts: From model to realistic feeds

This study investigates the design of mild base catalysts for the deoxygenation of bio-oil via aldol condensation paths. The first part rationalizes the active, selective, and stable performance of supported MgO catalysts in the vapor phase condensation of propanal, which is maximized upon the mechanochemical activation of a siliceous USY zeolite (Si/Al = 405) with 1 wt.% Mg(OH)(2). Infrared spectroscopic studies of the interaction with CO and CO2, reveal that the presence of 4-coordinate Mg likely localized in framework defects on the zeolite surface, and the avoidance of MgO formation are key to moderating the basicity with respect to the bulk oxide. The second part compares the best-performing MgO/USY catalyst with the most promising previously reported catalysts: IC-grafted USY and Ca-hydroxyapatite. A detailed kinetic analysis of the conversion of individual bio-oil constituents (propanal or acetic acid) and binary mixtures thereof (propanal with acetic acid, methanol, or water) provides insights into the reaction network, rate limiting steps, and relative surface coverage of reactants and products over each catalyst. This enables the anticipation of the aldol-condensation performance in simulated and real bio-oil mixtures. A significant inhibitory effect is observed in the presence of acetic acid, but the K-grafted USY zeolite is found to preserve its stability and retain the highest activity due to the weaker adsorption of acidic compounds. The presence of water has no pronounced effect on the observed reaction rates, while methanol is found to selectively poison Ca-hydroxyapatite due to the competitive adsorption on the active sites. The attained results under real bio-oil conditions demonstrate the effectiveness of this simplified approach to bridge the complexity gap between the study of model compounds and real bio-oil. (C) 2015 Elsevier B.V. All rights reserved.