Promoting effect of CaO-MgO mixed oxide on Ni/γ-Al₂O₃ catalyst for selective catalytic deoxygenation of palm oil

The study presented herein examines, for the first time in the literature, the role of CaO-MgO as a modifier of γ-Αl₂O₃ for Ni catalysts for the production of green diesel through the deoxygenation of palm oil. The characteristics of the catalytic samples were examined by N₂ adsorption/desorption, XRD, NH₃-TPD, CO₂-TPD, H₂-TPR, XPS and TEM analysis. The carbon deposited on the catalytic surfaces was characterized by TPO, Raman and TEM/HR-TEM. Experiments were conducted between 300 and 400 °C, at 30 bar. Maximum triglyceride conversion and the yield of the target n–C₁₅–n–C₁₈ paraffins increased with temperature up to 375 °C for both catalysts. Both samples promoted deCO₂ and deCO deoxygenation reactions much more extensively than HDO. However, although both catalysts exhibited similar activity at the optimal temperature of 375 °C, the Ni/modAl was more active at lower reaction temperatures, which can be probably understood on the basis of the increased dispersion of Ni on its surface and its lower acidity, which suppressed hydrocracking reactions. Time-on-stream experiments carried out for 20 h showed that the Ni/modAl catalyst was considerably more stable than the Ni/Al, which was attributed to the lower amount and lower crystallinity of the carbon deposits and to the suppression of sintering due to the presence of the CaO-MgO modifiers.