Creating values from wastes: Producing biofuels from waste cooking oil via a tandem vapor-phase hydrotreating process

•A tandem vapor-phase hydrotreating process was employed to upcycle waste cooking oil.•A 100% hydrodeoxygenation efficiency was observed.•A 7.4-fold yield increase in C5-C19 n-alkanes was obtained over Ni/Al2O3-SiO2 compared with that of non-catalytic.•The hydrodeoxygenation mechanism induced by Ni/Al2O3-SiO2 in the tandem conversion process was proposed.•The flexibility and practicability of this tandem hydrotreating process was evaluated.