Liquid-phase deoxygenation of free fatty acids to hydrocarbons using supported palladium catalysts

Hydrocarbon biofuels that are drop-in replacements for traditional petroleum-derived liquid fuels can be produced from edible and inedible fats and oils (triglyceride sources) via thermocatalytic processes. Liquid-phase deoxygenation of stearic acid (SA) in dodecane at 300°C and 15 atm was employed to screen supported noble metal catalysts for decarboxylation of free fatty acids to hydrocarbons. Commercial samples of Pt/C, Pd/C (4), 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% H 2 (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. Pd/SiO2 and Pt/C catalysts were selective toward decarbonylation (CO production), and Pd/C and Pd/Al2O 3 catalysts were selective toward decarboxylation. Even under H 2, the uniformly impregnated Pd/C catalyst was the most active and selective for the hydrogen-neutral decarboxylation pathway.;Semi-batch deoxygenation of SA employing this 5 wt.% Pd/C catalyst was investigated further using on-line quadrupole mass spectrometry. With fresh catalyst, SA deoxygenation under He occurred rapidly with very high CO 2 selectivity; however, reuse of the catalyst showed an orders of magnitude loss of decarboxylation activity and high decarbonylation selectivity. Experiments employing smaller amounts of fresh catalyst evidenced that decarboxylation activity under He is limited to ∼220 turnovers. Attempts to reactivate the used Pd/C catalyst by H2 treatment were only modestly effective. Increased catalyst lifetime (>2200 turnovers) was achieved by employing a H2-containing purge gas; however, the decarboxylation rate decreases with increasing H2 partial pressure resulting in lower CO 2 selectivity. Increasing the initial SA concentration also inhibited decarboxylation, substantially prolonging the batch time and yielding lower overall CO2 selectivity. The origin of this effect was traced to catalyst poisoning by endogenous CO from the decarbonylation pathway. Catalyst poisoning experiments demonstrated that CO strongly inhibits the decarboxylation pathway and that the inhibitory effects of CO and H2 are additive. Under conditions of strong decarboxylation inhibition, the decarbonylation rate was unaffected, and we infer that decarboxylation occurs over different catalytic sites than decarbonylation. An elementary reaction sequence for Pd-catalyzed decarboxylation is proposed which accounts for our observations.;Fed-batch deoxygenation of SA and oleic acid was demonstrated in a 50-mL stirred autoclave reactor with continuous feeding for run times up to 24 h. The maximum quasisteady state decarboxylation rate observed under 5% H 2 was 0.43 mmol/gcat˙min (0.078 s-1 turnover frequency). When higher H2 partial pressures were employed, an abrupt switchover in product selectivity from CO2 to CO was observed. Higher CO selectivity leads to increased H2 consumption due to hydrogenation of heptadecene, the primary product of the decarbonylation pathway. The on-stream time at which this switchover occurs was found to increase with decreasing H2 pressure. We infer that the switchover phenomenon arises from H2 inhibition of the decarboxylation pathway resulting in SA accumulation. SA accumulation increases the decarbonylation rate leading to further inhibition of the decarboxylation pathway by endogenous CO. Parametric studies involving SA feed rate, H2 partial pressure and exogenous CO partial pressure support the proposed switchover mechanism. Inhibition of decarboxylation activity was reversible at least in the short term by lowering the H2 or CO partial pressure or stopping SA injection; however, if a catalyst was aged >10 h under reaction conditions favoring decarbonylation, decarboxylation activity could not be recovered.