Vapour-phase C-C coupling reactions of biomass-derived oxygenates over Pd/CeZrOx catalysts

Article abstract of DOI:10.1016/j.jcat.2009.06.014

Studies of aldol condensation/hydrogenation reactions of 2-hexanone were carried out over Pd/CeZrO_x and CeZrO_x catalysts at temperatures between 573 and 673 K, and pressures of 5-26 bar. These studies were formulated to address the catalytic upgrading to transportation fuels of the mono-functional oxygenated compounds (consisting primarily of C₄-C₆ ketones, alcohols, carboxylic acids and heterocyclics) formed by the catalytic conversion of polyols over a Pt-Re/C catalyst. Characterization by XRD, TPR and NH₃/CO₂-TPD showed that Pd/CeZrO_x catalyst consists of a partially reducible solid solution of cerium and zirconium oxides, and possesses both acidic and basic functionalities. Reaction kinetics studies show that in addition to the expected C₁₂ condensation product (7-methyl-5-undecanone), the CeZrO_x-based catalysts produce C₁₈ and C₉ secondary species, along with light alkanes (≤C₇). Low loadings of Pd (e.g., 0.25 wt%) lead to optimal activity and selectivity for the production of C₁₂ species. The high apparent activation energy of the formation of C₉ (140 kJ/mol) compared to the formation of C₁₂ and C₁₈ species (15 and 28 kJ/mol, respectively) indicates that these species may be formed as a result of the decomposition of heavier condensation products. The self-coupling of 2-hexanone was found to be positive order in both 2-hexanone and hydrogen. The addition of primary alcohols and carboxylic acids as well as water and CO₂ to the feed was found to reversibly inhibit the self-coupling activity of 2-hexanone. This inhibition is strongest in the presence of CO₂, and TPSR studies indicate that CO₂ is removed from the surface by conversion to CO in the presence of reduced ceria species.