Low-H2 Used Cooking Oil Deoxygenation over a Multifunctional Catalyst: Product-Window Redistribution under H2/Formic Acid Assistance Product-Window Redistribution under H2/Formic Acid Assistance

Abstract

The catalytic conversion of used cooking oil (UCO) to green diesel is not only a conversion problem but also a pathway-control and product-window optimization problem. UCO-derived triglycerides and fatty acids can undergo hydrodeoxygenation (HDO), decarbonylation (DCO), decarboxylation (DCO₂), cracking, and secondary redistribution reactions. In this study, a multifunctional Ni–MgO–Ce_0.6Zr_0.4O₂/γ-Al₂O₃ catalyst was applied for UCO deoxygenation under four hydrogen-supply modes: Catalyst (C), C+H₂, C+FA (formic acid), and combined C+H₂+FA operation. The reactions were compared at 320°C and 350°C, and the products were analyzed by GC-TCD, GC-FID, and DGC/SimDist. The 320°C condition served as a lower-severity reference, where diesel-range products were generally better preserved, whereas 350°C promoted stronger liquid-phase upgrading, higher distillable-product formation, and more effective residue suppression. At 350°C, the distillable fraction below 370°C increased from 78.3% in the catalyst (C) mode to 98.6% in the C+H₂+FA mode, while residue decreased from 21.7% to 1.4%. However, this improvement was accompanied by product lightening, with the diesel-range fraction decreasing from 42.3% to 22.4% and the naphtha-range fraction increasing from 23.8% to 56.8%. Gas analysis showed CO-rich products in the catalyst (C) mode, supporting a strong DCO tendency even without external H₂, while FA-containing modes increased CO_X formation due to both UCO deoxygenation and possible FA decomposition. In the closed reactor, FA-derived gas formation may also have increased internal pressure and contributed to higher reaction severity and product lightening. Overall, the catalyst shows promise for controlled low-H₂ deoxygenation, but secondary cracking and excessive light-product formation must be controlled.