Fed-Batch Catalytic Deoxygenation of Free Fatty Acids

Abstract

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% H2 (15 atm total pressure); the maximum quasi-steady-state deoxygenation rate under these conditions was 0.46 mmol gcat−1 min−1 [0.083 s−1 turnover frequency (TOF)] with 95% CO2 selectivity. In a separate experiment, quasi-steady-state SA deoxygenation activity was maintained for >24 h at 0.29 mmol gcat−1 min−1 (0.053 s−1 TOF) with 92% CO2 selectivity. When higher H2 partial pressures were employed, an abrupt switchover in reaction pathway and product selectivity from decarboxylation (CO2) to decarbonylation (CO) was observed. Higher CO selectivity leads to increased H2 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 H2 and endogenous CO. Subsequent FFA accumulation increases the decarbonylation rate, producing additional CO. Parametric studies involving FFA feed rate, H2 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 H2 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.