Effect of Pd promotion and catalyst support on the Ni-catalyzed deoxygenation of tristearin to fuel-like hydrocarbons

The incomplete reduction and poor dispersion of Ni sites restrict the catalytic performance of Ni catalysts in the decarboxylation/decarbonylation (DCX/DCN) of triglycerides to fuel-like hydrocarbons. In this study, by employing Pd as promoter and/or using multifunctional supports, the activity of Ni catalysts in tristearin deoxygenation was greatly improved. Notably, 25%Ni/Al₂O₃ and 0.75%Pd/Al₂O₃ displayed conversion values ≤ 2%. In contrast, 25%Ni-0.75%Pd/Al₂O₃ afforded 100% conversion, 100% yield of diesel-like hydrocarbons and a selectivity to C17 – the main product of DCN/DCX – of 67%. The fact that the performance of the Ni–Pd catalyst is vastly superior to that of the corresponding monometallic formulations is clearly indicative of a promotion effect resulting from the interaction between the two metals. Pd addition promoted Ni reduction, thereby increasing the number of active sites. The effect of different supports (Al₂O₃, SiO₂–Al₂O₃, ZrO₂, and Ce_0.8Pr_0.2O₂) of Ni–Pd catalysts were studied, and the use of SiO₂–Al₂O₃ was observed to enhance catalyst performance by further promoting Ni reduction as well as through its high surface area and strong acidity. Moreover, using SiO₂–Al₂O₃ as the support partially changes the deoxygenation pathway from DCX/DCN to hydrodeoxygenation, which is attributed to the ability of strong acid sites to catalyze the dehydrogenation of an alcohol intermediate to the corresponding alkene.