Manipulating catalytic pathways: Deoxygenation of palmitic acid on multifunctional catalysts

摘要

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_2O_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?￡34;C?￡34;O) on the surface of ZrO_2 during the reaction with palmitic acid at 260 °C in the presence or absence of H_2. Biomass to biofuels: The conversion of palmitic acid to n-pentadecane over ZrO_2 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 ZrO_2 support (see scheme). In the absence of H_2, ketonization is the dominant reaction catalyzed by ZrO _2.