Carbon–Carbon Bond Scission Pathways in the Deoxygenation of Fatty Acids on Transition-Metal Sulfides

The mechanism of the deoxygenation of fatty acids on transition-metal sulfides was determined on the basis of kinetic data obtained with fatty acids, their reaction intermediates (aldehyde and alcohol), and reactants of restricted reactivity (adamantanyl-substituted carboxylic acids). Deoxygenation on MoS₂ proceeds exclusively via hydrogenolysis to aldehyde, followed by hydrogenation to the corresponding alcohol, consecutive dehydration to the olefin, and hydrogenation to the alkane. In contrast, the selectivity on Ni-MoS₂ and on Ni₃S₂ substantially shifts toward carbon oxide elimination routes: i.e., direct production of C_n–1 olefins and alkanes. The carbon losses occur by decarbonylation of a ketene intermediate, which forms only on sites associated with Ni. The rate determining steps are the cleavage of the C–C bond and the removal of oxygen from the surface below and above, respectively, 2.5 MPa of H₂. The different reaction pathways catalyzed by MoS₂ and Ni-MoS₂ are attributed to a preferred deprotonation of a surface acyl intermediate formed upon the adsorption of the fatty acid on Ni-MoS₂. The shift in mechanism is concluded to originate from the higher basicity of sulfur induced by nickel.