Deoxygenation of Palmitic Acid on Unsupported Transition-Metal Phosphides

Highly active bulk transition-metal phosphides (WP, MoP, and Ni₂P) were synthesized for the catalytic hydrodeoxygenation of palmitic acid, hexadecanol, hexadecanal, and microalgae oil. The specific activities positively correlated with the concentration of exposed metal sites, although the relative rates changed with temperature due to activation energies varying from 57 kJ mol^-1 for MoP to 142 kJ mol^-1 for WP. The reduction of the fatty acid to the aldehyde occurs through a Langmuir-Hinshelwood mechanism, where the rate-determining step is the addition of the second H to the hydrocarbon. On WP, the conversion of palmitic acid proceeds via R-CH₂COOH → R-CH₂CHO → R-CH₂CH₂OH → R-CHCH₂ → R-CH₂CH₃ (hydrodeoxygenation). Decarbonylation of the intermediate aldehyde (R-CH₂COOH → R-CH₂CHO → R-CH₃) was an important pathway on MoP and Ni₂P. Conversion via dehydration to a ketene, followed by its decarbonylation, occurred only on Ni₂P. The rates of alcohol dehydration (R-CH₂CH₂OH → R-CHCH₂) correlate with the concentrations of Lewis acid sites of the phosphides. (Chemical Equation Presented).