Enhancing Cetane Index in Biodiesel Hydrogenation: Insights from NiO-MoO₃/TiO₂ and NiO/TiO₂ Catalysts

As a renewable, biodegradable, and nontoxic fuel, biodiesel represents a greener alternative to fossil diesel. Its high cetane and flash point, combined with its intrinsic oxygen content, result in more efficient and rapid combustion. Moreover, biodiesel generates substantially lower levels of sulfur, aromatic hydrocarbons, and other harmful emissions. Because it can be used in existing engines without modification, biodiesel can be seamlessly blended with conventional diesel or utilized in its pure form. This study reports the catalytic hydrogenation of biodiesel to enhance its cetane indexa critical parameter for improving combustion quality and engine performancethereby facilitating its transition toward green diesel. Biodiesel was first synthesized via transesterification using a heterogeneous MgO catalyst, achieving a 98% conversion. Subsequent hydrogenation was conducted using NiO/TiO₂ and NiO-MoO₃/TiO₂ catalysts with 15 mol % and 25 mol % metal loadings, respectively. The catalysts were characterized by atomic force microscopy (AFM), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and Raman spectroscopy, confirming their structural and morphological features. Crystalline phases were observed in all samples, while nickel oxide was detected exclusively in the anatase TiO₂ phase, attributed to high dispersion as evidenced by the absence of NiO reflections in the XRD and Raman spectra. AFM imaging revealed distinct phase separation and spherical particle morphology in the NiO/TiO₂ 25 mol % catalyst, indicating efficient metal dispersion. The hydrogenated biodiesel exhibited significant improvements in fuel properties: cetane index values increased by approximately 50% relative to the original biodiesel, reaching 59.6-60.5, which complies with both ASTM D6751 (≥40) and EN 14214 (≥51) standards. Additionally, the viscosity (4.5 mm²/s at 40 °C) and density (0.873 g/cm³ at 20 °C) remained within the prescribed limits. These findings demonstrate that customized Ni-based catalysts can effectively upgrade biodiesel to high-quality renewable diesel, aligning its fuel properties with international specifications and providing valuable insights for the design of efficient catalytic systems for sustainable energy applications.