Manipulating Catalytic Pathways: Deoxygenation of Palmitic Acid on Multifunctional Catalysts

Abstract

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 ZrO 2 support. The latter route involves adsorption of the carboxylic acid group at an oxygen vacancy of ZrO 2 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/ZrO 2 than that on Ni/SiO 2 or Ni/Al 2 O 3, 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 ZrO 2 is the dominant reaction. Pd/C favors direct decarboxylation (−CO 2), 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 H 2. In situ IR spectroscopy unequivocally shows the presence of adsorbed ketene (CCO) on the surface of ZrO 2 during the reaction with palmitic acid at 260 °C in the presence or absence of H 2.