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

摘要

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 C_4-C_6 ketones, alcohols, carboxylic acids and heterocyclics) formed by the catalytic conversion of polyols over a Pt-Re/C catalyst. Characterization by XRD, TPR and NH_3/CO_2-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 C_(12) condensation product (7-methyl-5-undecanone), the CeZrO_x-based catalysts produce C_(18) and C_9 secondary species, along with light alkanes (≤C_7). Low loadings of Pd (e.g., 0.25 wt%) lead to optimal activity and selectivity for the production of C_(12) species. The high apparent activation energy of the formation of C_9 (140 kJ/mol) compared to the formation of C_(12) and C_(18) 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 CO_2 to the feed was found to reversibly inhibit the self-coupling activity of 2-hexanone. This inhibition is strongest in the presence of CO_2, and TPSR studies indicate that CO_2 is removed from the surface by conversion to CO in the presence of reduced ceria species.