Catalytic Deoxygenation of Fatty Acid to Hydrocarbon Using Supported Palladium and Palladium-Gol

Catalytic deoxygenation of fatty acids (FAs) to hydrocarbons using supported Pd catalysts is a key enabling technology in lipid biomass conversion to "drop-in" replacements for petroleum-derived transportation fuels; however, the kinetics and mechanism(s) of FA deoxygenation and the catalyst deactivation mechanism are not well understood. In this work, vapor-phase catalytic deoxygenation of octanoic acid (OA) was investigated over silica- and carbon-supported Pd catalysts at 245-300°C and 1 atm using a fixed-bed micro-reactor with on-line product analysis. A commercial 5 wt. % Pd/SiO2 catalyst was found to be active for OA decarbonylation via a reductive pathway involving octanal as an intermediate. OA conversion and selectivity to n-heptane increased with increasing temperature and hydrogen partial pressure. Under equivalent experimental conditions, a commercial 5 wt. % Pd/C catalyst was found to be more active (>95% OA conversion) and exhibit much greater CO 2 selectivity (65%). With this catalyst, increasing H2 partial pressure inhibited OA decarboxylation and promoted decarbonylation; octanal and octanol were detected only under hydrogen-rich conditions. Moreover, this catalyst had water-gas-shift (WGS) activity under OA deoxygenation conditions. Plausible reaction pathways for OA deoxygenation are proposed based on the experimental results.;OA deoxygenation was investigated further using lab-prepared Pd/SiO 2 and PdAu/SiO2 catalysts. A 1.78 wt. % Pd/SiO2 catalyst with an average Pd particle size of 7.5 nm and a broad size distribution was prepared by incipient wetness, and a 0.6 wt. % Pd/SiO2 catalyst containing 1.5-nm particles with a narrow size distribution was prepared by ion exchange. The catalysts were characterized by CO chemisorption, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and high-resolution transmission electron microscopy (HRTEM). The larger supported nanoparticles exhibited well-defined surface facets that adsorbed CO preferentially at 2-fold bridging sites. The smaller supported nanoparticles exhibited strong linear and bridging nuCO bands. The 0.6 wt. % Pd/SiO2 catalyst had initial OA deoxygenation activity, but deactivated rapidly and irreversibly with time on stream. In contrast, the 1.78 wt. % Pd/SiO2 catalyst deactivated less rapidly and could be regenerated by heating in H2 to remove OA residues. The catalyst deactivation mechanism was inferred to be self-poisoning by OA and CO. PdAu/SiO2 catalysts were prepared that contained 4-5 nm supported alloy nanoparticles. Alloying Pd with Au was found to improve catalyst stability without significantly reducing deoxygenation activity by reducing the CO adsorption energy and mitigating self-poisoning by OA (and related species).;Liquid-phase catalytic deoxygenation of stearic acid (SA) to n-heptadecane (n-C17) was investigated over a series of carbon-supported Pd catalysts at 300ºC and 15 atm using a semi-batch stirred autoclave reactor with on-line gas analysis by quadrupole mass spectrometry. Commercial 5% Pd/C catalysts and in-house prepared 5% Pd/C (activated carbon, AC) and 5% Pd/CB (carbon black, CB) catalysts were screened under He and 5% H2 (balance He). The Pd/C catalysts were characterized by CO chemisorption, slurry pH measurements, temperature-programmed desorption (TPD), and elemental analysis. All the commercial Pd/C catalysts and the lab-prepared Pd/AC catalyst exhibited catalytic activity under He; however, Na- and K-rich catalysts with basic slurry pH values and higher CO2 TPD peak areas had higher n-heptadecane yields and CO2 selectivities. In contrast, the lab-prepared Pd/CB catalyst had negligible activity under He. SA deoxygenation activity under He was inferred to be related to spillover hydrogen generated by in situ reduction of Pd/AC catalysts. Under flowing 5% H2, the Pd/AC catalysts exhibited high SA conversions and high CO2 selectivities; Pd/CB was active for SA decarbonylation under these conditions.