Manipulating Catalytic Pathways: Deoxygenation of Palmitic Acid on Multifunctional Catalysts

The mechanism of the catalytic reduction of palmitic acid to n-pentadecane at 260 degrees C in the presence of hydrogen over catalysts combining multiple functions has been explored. The reaction involves rate-determining reduction of the carboxylic group of palmitic acid to give hexadecanal, which is catalyzed either solely by Ni or synergistically by Ni and the ZrO2 support. The latter route involves adsorption of the carboxylic acid group at an oxygen vacancy of ZrO2 and abstraction of the alpha-H with elimination of O to produce the ketene, which is in turn hydrogenated to the aldehyde over Ni sites. The aldehyde is subsequently decarbonylated to n-pentadecane on Ni. The rate of deoxygenation of palmitic acid is higher on Ni/ZrO2 than that on Ni/SiO2 or Ni/Al2O3, but is slower than that on H-zeolite-supported Ni. As the partial pressure of H-2 is decreased, the overall deoxygenation rate decreases. In the absence of H-2, ketonization catalyzed by ZrO2 is the dominant reaction. Pd/C favors direct decarboxylation (-CO2), while Pt/C and Raney Ni catalyze the direct decarbonylation pathway (-CO). The rate of deoxygenation of palmitic acid (in units of mmol mol(total metal)(-1) h(-1)) decreases in the sequence r((Pt black)) approximate to r((Pd black)) > r((Raney Ni)) in the absence of H-2. In situ IR spectroscopy unequivocally shows the presence of adsorbed ketene (C=C=O) on the surface of ZrO2 during the reaction with palmitic acid at 260 degrees C in the presence or absence of H-2.