Catalytic Deoxygenation of Octanoic Acid Over Supported Pd

Catalytic deoxygenation of octanoic acid (OA) to n-heptane was investigated over a series of supported Pd catalysts at 260ºC and 1 atm using a fixed-bed micro-reactor with on-line gas chromatography. Under equivalent experimental conditions, a commercial 5 wt.% Pd/C catalyst was found to be more active and exhibit greater CO₂ selectivity than Pd/SiO₂, indicating a strong support effect for this reaction [1]. Particle size effects were investigated using Pd/SiO₂ catalysts prepared by two different methods: a 2 wt.% Pd/SiO₂ with an average Pd particle size of 7.5 nm and a broad size distribution prepared by incipient wetness, and a 0.6 wt.% Pd/SiO₂ catalyst containing 1.5-nm particles with a narrow size distribution 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, as evidenced by HRTEM, and adsorbed CO preferentially at 2-fold bridging sites, as evidenced by DRIFTS. The smaller supported nanoparticles exhibited strong linear and bridging n_CO bands. The more highly dispersed Pd/SiO₂ catalyst deactivated rapidly and irreversibly in OA deoxygenation; whereas, the 2 wt.% Pd/SiO₂ catalyst deactivated less rapidly with time-on-stream and could be regenerated by heating in H₂ to remove OA residues. The catalyst deactivation mechanism is inferred to be self-poisoning by OA and CO.  Addition of Au to Pd/SiO₂ was found to improve catalyst stability without significantly reducing catalytic activity. A PdAu/SiO₂ catalyst containing 0.47 wt.% Pd and 1.2 wt.% Au was prepared using a procedure previously reported to produce supported PdAu alloy nanoparticles [2]. The geometric and electronic effects of alloying Pd with Au reduce the CO adsorption energy and mitigate self poisoning by OA (and related species).

References:

[1] J. P. Ford, J. G. Immer, and H. H. Lamb, Top. Catal. 55, 175-184 (2012).

[2] S. N. Reifsnyder, H. H. Lamb, J. Phys. Chem. B 103, 321-329 (1999).