(279d) Supported Palladium Catalysts for Decarboxylation of Fatty Acids

Liquid-phase deoxygenation of stearic acid (SA) in dodecane at 300°C and 15 atm was employed to screen supported palladium catalysts for decarboxylation of fatty acids (FAs) to hydrocarbons.  Commercial samples of Pd/C (5), Pd/Al₂O₃, and Pd/SiO₂ catalysts and an in-house prepared Pd/SiO₂ catalyst (each containing 5 wt.% metal) were screened under flowing 0, 5, and 10% H₂ (balance He).  Under flowing He, most of the catalysts studied failed to achieve 100% SA conversion after 4 h under reaction conditions due to rapid deactivation.   The exception was a uniformly impregnated Pd/C catalyst that gave >99% conversion in ~1 h with 99% CO₂ selectivity.  All of the catalysts were far more stable under H₂ yielding nearly complete SA conversion after 4 h; however, they differed markedly in their CO₂ selectivities.  The Pd/C catalysts were found to be highly active and selective for SA decarboxylation under 5% H₂.  In contrast, SA deoxygenation over Pd/SiO₂ catalysts occurred primarily via decarbonylation and at a much slower rates.  Pd/Al₂O₃ exhibited high initial SA decarboxylation activity, but deactivated rapidly under 5% H₂. Similar CO₂ selectivity patterns among the catalysts were observed for deoxygenation of lauric and capric acids; however, the initial decarboxylation rates tended to be lower for Pd/C and Pd/Al₂O₃ with these substrates.  The most active Pd/C catalyst was used to investigate the influence of alkyl chain length on deoxygenation kinetics for C8-C18 FAs.  Generally, as FA carbon number decreases, reaction time and H₂ consumption increase, and CO₂ selectivity and initial decarboxylation rate decrease. We attribute the higher initial decarboxylation rates for longer chain FAs to an increased propensity for FA adsorption on the catalyst support.