Green diesel production via deoxygenation of used cooking oil over metal-support γ-Al₂O₃ catalysts: A comparative study of Ni and Co

The depletion of fossil fuels necessitates efficient catalytic routes for converting waste lipids into renewable diesel-range hydrocarbons. This work highlights the novelty of tailoring metal–support synergy in γ-Al₂O₃-based catalysts to direct deoxygenation pathways without external hydrogen, offering a cost-effective strategy for sustainable green diesel production from waste oils. The deoxygenation reaction was conducted under an inert atmosphere at elevated temperature, and the resulting liquid products were characterized using gas chromatography–mass spectrometry analysis. The results demonstrate that metal modification markedly enhances deoxygenation performance compared to pristine γ-Al₂O₃. Ni/γ-Al₂O₃, possessing higher acidity, promotes intensified deoxygenation accompanied by excessive cracking, leading to an increased formation of lighter hydrocarbons, particularly C₉ (9.21%). In contrast, Co/γ-Al₂O₃ exhibits a more balanced catalytic behavior, favoring decarboxylation and decarbonylation pathways while preserving the carbon backbone of fatty acid derivatives. Consequently, Co/γ-Al₂O₃ produces a liquid product predominantly composed of C₁₅–C₁₈ paraffinic hydrocarbons (40.32%), characteristic of green diesel, with a significantly reduced fraction of oxygenated compounds. Compared to previously reported hydrogen-free alumina-based catalysts, the Co-modified system demonstrates improved carbon-chain retention and enhanced diesel selectivity without noble metals or external H₂. This study provides new insight into the structure–pathway–selectivity relationship in bifunctional catalysts and identifies Co/γ-Al₂O₃ as a promising non-noble catalyst for sustainable green diesel production.