Insight into the Roles of Basic Sites and Oxygen Vacancies in Stearyl Alcohol Hydrodecarbonylation to Paraffins over Ni/ZnAl2O4 Catalysts

The development of highly efficient, sulfur-free catalysts for the production of second-generation biodiesel is crucial to its sustainable production. Ni-based catalysts with good stability were prepared using ZnAl 2 O 4 spinel as the support for the hydrodecarbonylation of stearyl alcohol to paraffins. The effects of the atomic coordination environment, surface basicity distribution, and oxygen vacancy content of the support on the deoxygenation process were investigated by varying the support calcination temperature. The dehydrogenation capability of the catalysts was found to be closely related to the presence of medium-strength basic sites, which influence not only the selectivity of the decarbonylation pathway but also the hydrogen consumption of the reaction. Oxygen vacancies promote the decarbonylation activity of metallic Ni through an electron-enrichment effect, and their content is directly associated with the support structure. Increasing the support calcination temperature led to the transformation of medium-strength basic sites into strong basic sites (coordinatively unsaturated O 2− ions), which was detrimental to the dehydrogenation reaction. Activity evaluations and theoretical calculations demonstrated that the conversion of stearyl alcohol to n-heptadecane proceeds through a continuous dehydrogenation–decarbonylation–hydrogenation sequence, in which decarbonylation is the rate-determining step. Furthermore, transition-state analyses of different reaction pathways indicate that, compared with 1-octadecenone, 1-octadecenal is likely the key intermediate in the synthesis of n-heptadecane from stearyl alcohol because of its more favorable thermodynamic profile.