Fed-Batch Catalytic Deoxygenation of Free Fatty Acids

Fed-batch catalytic deoxygenation of C18 free fatty acids (FFAs) to n -heptadecane was demonstrated in a 50 mL stirred autoclave reactor with continuous FFA injection via a high-pressure syringe pump. High selectivity to the hydrogen-neutral decarboxylation pathway was achieved using a 5 wt % Pd/C catalyst at 300 °C under 5% H 2 (15 atm total pressure); the maximum quasi-steady-state deoxygenation rate under these conditions was 0.46 mmol g cat −1 min −1 [0.083 s −1 turnover frequency (TOF)] with 95% CO 2 selectivity. In a separate experiment, quasi-steady-state SA deoxygenation activity was maintained for >24 h at 0.29 mmol g cat −1 min −1 (0.053 s −1 TOF) with 92% CO 2 selectivity. When higher H 2 partial pressures were employed, an abrupt switchover in reaction pathway and product selectivity from decarboxylation (CO 2 ) to decarbonylation (CO) was observed. Higher CO selectivity leads to increased H 2 consumption because of hydrogenation of heptadecenes, the primary products of the decarbonylation pathway. We infer that the switchover phenomenon results from inhibition of the decarboxylation pathway by H 2 and endogenous CO. Subsequent FFA accumulation increases the decarbonylation rate, producing additional CO. Parametric studies involving FFA feed rate, H 2 partial pressure, and exogenous CO partial pressure support the proposed switchover mechanism. Inhibition of decarboxylation activity was reversible in the short term by reducing the H 2 or CO partial pressure or stopping FFA injection; however, when catalysts were aged >10 h under reaction conditions that favor decarbonylation, decarboxylation activity could not be recovered.