Green diesel production over nickel-alumina nanostructured catalysts promoted by zinc

Following the one step co-precipitation technique we have synthesized a series of Ni-Zn catalysts supported on alumina. The series involves one non-promoted sample containing 60 wt % Ni and three zinc promoted samples containing 1, 2 and 5 wt % Zn and 59, 58 and 55 wt % Ni, respectively. The specimens were characterized using nitrogen sorption isotherms, H-2-TPR, NH3-TPD, XRD, SEM-EDX, TEM and XPS. The catalytic performance of the samples prepared was evaluated in the selective deoxygenation of sunflower oil using a semi-batch reactor (310 degrees C, 40 bar of hydrogen, hydrogen flow rate equal to 100 mL/min and reactant to catalyst ratio equal to 100 mL/g). The control of the co-precipitation procedure and the direct reduction of dried precipitate resulted to mesoporous nano-structured catalysts with high surface area (247-290 m(2) g(-1)). The non-promoted sample was mainly comprised from NiAl2O4 particles and the promoted ones from Ni-Zn alloys and intermetallic oxo-phases (IMPs) supported on largely amorphous Al2O3 nano-grains. Zinc increased the total yield of alkanes in the diesel range (n-15, n-16, n-17, n-18) though it does not affect the mechanistic scheme of the selective deoxygenation of sunflower oil which is largely realized via decarbonylation/decarboxylation. The promoting action of zinc was attributed to the favoring of the Ni-Zn alloys and IMPs formation. It is more pronounced in the sample with the medium zinc content (58 wt % Ni, 2 wt % Zn, 40 wt % Al2O3) in which an increase of 18% in the total yield of hydrocarbons has been obtained. The maximization of the zinc promoting action in this sample reflects the good compromise between the extent of alloying and surface acidity.