Catalytic behaviors of Ni/γ-Al2O3 and Co/γ-Al2O3 during the hydrodeoxygenation of palm oil

The deactivation and regeneration behaviors of Ni/gamma-Al2O3 and Co/gamma-Al2O3 catalysts in the hydrodeoxygenation (HDO) of palm oil were investigated at 573 K and 5 MPa in a trickle bed reactor. The catalysts were prereduced at 773 K for 3 h before the HDO experiments. The catalysts exhibited good catalytic activity (>90% yield) and remained stable for 100 h on-stream. Nevertheless, after 150 h onstream, the product yield gradually decreased from 92.2 to 76.2% over the Ni catalyst and dramatically declined from 88.6% to ca. 56.6% over the Co catalyst. The combined decarbonylation and/or decarboxylation (DCOx) reactions were dominant over the HDO reaction when the reaction was catalyzed by the Ni catalyst. Meanwhile, the contribution of DCOx and HDO reactions was nearly comparable over the Co catalyst. The XRD and XANES analyses confirmed the partial formation of metallic nickel or cobalt after prereduction and the further in situ reduction during the HDO experiments, suggesting the coexistence of the metal and metal oxides on the catalyst surfaces. The XRD and TEM analyses revealed some sintering of the nickel and cobalt particles during the time course of reaction. Based on the TPO analysis, the carbon deposition rate on the cobalt catalyst was faster than that on the nickel catalyst and would be the major reason for the catalyst deactivation, and the sintering was the minor one. Additionally, the regeneration under air at 773 K followed by reduction in H-2 at 773 K can completely restore the catalytic activity.