Manipulating catalytic pathways: Deoxygenation of palmitic acid on multifunctional catalysts

The mechanism of the catalytic reduction of palmitic acid to n-pentadecane at 260 °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 α-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 H₂ 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)}≈r_{(Pd black)}>r_{(Raney Ni)} in the absence of H₂. In situ IR spectroscopy unequivocally shows the presence of adsorbed ketene (Cï£34;Cï£34;O) on the surface of ZrO₂ during the reaction with palmitic acid at 260 °C in the presence or absence of H₂. Biomass to biofuels: The conversion of palmitic acid to n-pentadecane over ZrO₂ mainly proceeds by hydrogenation of the carboxylic acid group to give hexadecanal (rate-determining step), which is catalyzed either solely by Ni sites or synergistically by Ni sites and sites on the ZrO₂ support (see scheme). In the absence of H₂, ketonization is the dominant reaction catalyzed by ZrO ₂.