Palladium Catalysts for Fatty Acid Deoxygenation: Influence of the Support and Fatty Acid Chain Length on Decarboxylation Kinetics

Supported metal catalysts containing 5?wt% Pd on silica, alumina, and activated carbon were evaluated for liquid-phase deoxygenation of stearic (octadecanoic), lauric (dodecanoic), and capric (decanoic) acids under 5?% H₂ at 300?°C and 15?atm. On-line quadrupole mass spectrometry (QMS) was used to measure CO?+?CO₂ yield, CO₂ selectivity, H₂ consumption, and initial decarboxylation rate. Post-reaction analysis of liquid products by gas chromatography was used to determine n-alkane yields. The Pd/C catalyst was highly active and selective for stearic acid (SA) decarboxylation under these conditions. In contrast, SA deoxygenation over Pd/SiO₂ occurred primarily via decarbonylation and at a much slower rate. Pd/Al₂O₃ exhibited high initial SA decarboxylation activity but deactivated under the test conditions. 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 these substrates. The influence of alkyl chain length on deoxygenation kinetics was investigated for a homologous series of C₁₀?CC₁₈ fatty acids using the Pd/C catalyst. As fatty acid carbon number decreases, reaction time and H₂ consumption increase, and CO₂ selectivity and initial decarboxylation rate decrease. The increase in initial decarboxylation rates for longer chain fatty acids is attributed to their greater propensity for adsorption on the activated carbon support.