(571a) Palladium-Carbon Catalysts for Liquid-Phase Deoxygenation of Fatty Acids to Alkanes

Increasing anthropogenic CO₂ generation, limited fossil fuel reserves, and energy security concerns have engendered great interest in transportation fuels derived from bio-renewable resources (biofuels), such as ethanol, biodiesel, and second-generation hydrocarbon biofuels. Catalytic deoxygenation of fats and oils to alkanes can be accomplished by hydrodeoxygenation using conventional NiMo or CoMo catalysts and by decarboxylation (DCX)/decarbonylation (DCN) using supported metals. Palladium-carbon (Pd/C) catalysts are highly active and selective for DCX of fatty acids. Moreover, because oxygen is rejected as CO₂, DCX can occur over Pd/C with minimal net H₂ consumption. In this work, liquid-phase catalytic deoxygenation of stearic acid (SA) to n-heptadecane (n-C₁₇) was investigated over a series of Pd/C catalysts at 300ºC and 15 atm using a semi-batch microreactor with on-line monitoring of CO₂, CO and H₂. The series included several commercial 5% Pd on activated carbon (AC) catalysts, a lab-prepared 5% Pd on carbon black (CB) catalyst, and lab-prepared 5% Pd on AC catalysts. Catalysts were screened for deoxygenation activity under He and 5% H₂/He, and characterized by CO chemisorption, slurry pH measurements, temperature-programmed desorption (TPD), temperature-programmed hydride decomposition (TPHD), and nitrogen porosimetry. Pd/AC catalysts were catalytically active for SA deoxygenation under He. In contrast, Pd/CB was inactive with only 8% SA conversion and high DCN selectivity. Deoxygenation under He occurs primarily via DCX with very high initial activity for Pd/AC catalysts; however, reuse of the same catalyst for additional batches showed an orders of magnitude loss of activity and high DCN selectivity. Additional experiments employing less fresh catalyst evidenced that DCX activity under He is limited to ~220 turnovers for the most active 5% Pd/AC catalyst. Attempts to regenerate the spent Pd/AC catalyst by H₂ treatment were only modestly effective in recovering initial DCX activity. Using TPD experiments, catalytic activity of Pd/C under He was correlated to hydrogen spilled over from Pd onto the AC support during in situ catalyst reduction. Increased catalyst lifetime (>2200 turnovers) was achieved by employing a H₂-containing purge gas, and we found that a 5% H₂ purge gas gave the best results among the conditions investigated. Under 5% H₂, 5 wt.% Pd/AC catalysts exhibited high DCX activity; Pd/CB had high DCN activity under equivalent conditions. Catalysts with basic slurry pH values, large low-temperature (<500°C) CO₂ TPD yields, and significant alkali metals (Na, K) contents had the highest initial DCX activity.