Oxidative denitrogenation of quinoline using cobalt ferrite catalysts

The presence of nitrogenated compounds in fossil fuels leads to NOx formation during combustion; these are harmful pollutants that pose significant environmental and health challenges. Hydrotreatment, the conventional strategy for denitrogenation, requires severe conditions, which motivates the search for more environmentally friendly alternatives. Oxidative denitrogenation (ODN) requires milder operating conditions and explores the oxidative reactivity of nitrogen compounds. In this study, cobalt ferrite (CoFe₂O₄) catalysts were synthesized and coated with silica (SiO₂) or carbon (C), and applied for the ODN of quinoline, common in fuels, using hydrogen peroxide as the oxidant.

The superparamagnetic CoFe₂O₄ (core) was synthesized using the sol–gel method and, using an adaptation of the Stöber method, further coated with SiO₂ or C (shell). Characterization techniques, such as XRD, FTIR, and contact angle measurements, confirmed the core–shell structure of the developed catalyst and showed a remarkable change in the ferrite’s hydrophobic surface properties upon coating (a decrease from 130° to 40° with silica). Crystallite sizes in the range of 19-20 nm were obtained.

Quinoline adsorption tests showed the low adsorption capacity of the materials, in accordance with the low surface area and pore volume determined by N₂ adsorption isotherms at 77 K (S_BET = 9-10 m² g^-1). In oxidation reactions, CoFe₂O₄@SiO₂ showed the best catalytic performance (X_QN = 74%, 8 h), likely ascribed to its hydrophilic surface, favorable to the generation of oxygen reactive species through the decomposition of H₂O₂ and consequent higher quinoline removal. Quinoline degradation was verified by GC-MS, which indicated the opening of the pyridine ring.

The results highlight that cobalt ferrite-based catalysts employed in oxidative denitrogenation are capable of degrading and mineralizing quinoline under mild conditions, as confirmed by TOC analyses. The surface properties of the coatings significantly increased the catalytic efficiency, emphasizing their potential as environmentally friendly and efficient candidates for the removal of nitrogen compounds from fossil fuels.