Effect of operating parameters on the selective catalytic deoxygenation of palm oil to produce renewable diesel over Ni supported on Al₂O₃, ZrO₂ and SiO₂ catalysts

The present work investigated the production of Green Diesel through the deoxygenation of palm oil over Ni catalysts supported on γ-Αl₂O₃, ZrO₂ and SiO₂ for a continuous flow fixed bed reactor. A comprehensive experimental study was carried out in order to examine the effects of temperature, pressure, LHSV and H₂/oil feed ratio on catalytic activity during short (6 h) and long (20 h) time-on-stream experiments. The catalysts were prepared through the wet impregnation method (8 wt% Ni) and were extensively characterized by N₂ adsorption/desorption, XRD, NH₃-TPD, CO₂-TPD, H₂-TPD, H₂-TPR, XPS, TEM/HR-TEM and Raman. The characterization of the materials prior to reaction revealed that although relatively small Ni nanoparticles were achieved for all catalysts (4.3 ± 1.6 nm, 6.1 ± 1.8 nm and 6.0 ± 1.8 nm for the Ni/Al₂O₃, Ni/ZrO₂ and Ni/SiO₂ catalysts, respectively), NiO was better dispersed on the Ni/ZrO₂ catalyst, while the opposite was true for the Ni/SiO₂ sample. In the case of Ni/Al₂O₃, part of Ni could not participate in the reaction due to its entrapment in the NiAl₂O₄ spinel phase. Regarding performance, although an increase in H₂ pressure led to increases in paraffin conversion, the increase of temperature was beneficial only up to a critical value which differed for each catalytic system under consideration (375 °C, 300 °C and 350 °C for the Ni/Al₂O₃, Ni/ZrO₂ and Ni/SiO₂ catalysts, respectively). All catalysts favored the deCO₂ and deCO deoxygenation paths much more extensively than HDO, irrespective of testing conditions. Time-on-stream experiments showed that all catalysts deactivated after about 6 h, which was attributed to the sintering of the Ni particles and/or their covering by a thin graphitic carbon shell.