Selective Hydrogenation of Methyl Palmitate to Cetyl Alcohol via Ternary Synergistic Catalysis of Ni, Oxygen Vacancies, and Lewis Acid Sites under Mild Reaction Conditions

Abstract

Selective hydrogenation of fatty esters into high-value-added fatty alcohols over non-noble-metal (e.g., Ni and Co) catalysts is highly desirable for the utilization of natural oils. Although these catalysts offer high hydrogenation activity and are reasonably economical, they suffer from low hydrodeoxygenation activity at low temperatures while promoting certain side reactions (e.g., C–C bond hydrogenolysis and decarbonylation/decarboxylation) at high temperatures, which are undesirable for the production of fatty alcohols. In this study, we tested and optimized the performance of Ni metal catalysts with various metal–oxide modifiers and supports for the selective hydrogenation of methyl palmitate to cetyl alcohol. The most efficient catalytic system (Ni-VOx/TiO2, Ni:V ≈ 1:1, mol/mol) achieved almost quantitative conversion with 90% selectivity for cetyl alcohol under mild conditions (220 °C and 4.0 MPa of H2), which was ascribed to the ternary synergistic action of Ni, oxygen vacancies, and Lewis acid sites. The catalytic synergy of the Ni metal and oxygen vacancies (formed by the reduction of TiO2) was primarily responsible for C–O bond cleavage in methyl palmitate to form palmitic aldehyde as the main intermediate. Furthermore, the V species engaged in metal–Lewis acid cooperation at the metal–oxide interface to efficiently promote the C═O bond hydrogenation of palmitic aldehyde and suppress the undesired C–C bond cleavage, thereby facilitating the highly selective production of cetyl alcohol. The obtained insights into the active phase of Ni-VOx/TiO2 catalysts are expected to facilitate the development of highly efficient catalysts for the selective hydrogenation of fatty esters and other biomass-derived compounds containing C═O/C–O groups.