Highly selective recovery of valuable metals from spent hydrotreating catalysts via deep eutectic solvents

With the increasing trend of heavy and poor-quality petroleum, hydrotreating of heavy oil has rapidly developed, generating large amounts of spent catalysts. Recovering valuable metals from these catalysts is crucial because of their environmental hazards and scarcity of critical metal. This study developed a recovery method using choline chloride-oxalic acid deep eutectic solvent (DES) to efficiently extract vanadium (V) and molybdenum (Mo) while selectively separating nickel (Ni). Under optimized conditions (ChCl:oxalic acid = 1:2, water-to-DES ratio = 0.3 (v/v), 80 °C, 1:20 g/mL, 3 h), the leaching efficiencies reached 97.8 % for V and 95.1 % for Mo, with only 1.8 % for Ni. Kinetic modeling revealed a two-stage leaching mechanism: surface chemical reaction control initially (0–30 min) transitioning to mixed reaction-diffusion control (30–180 min). The activation energies for V and Mo were 18.08 kJ/mol and 18.93 kJ/mol in the initial stage, and 20.63 kJ/mol and 11.16 kJ/mol in the later stage, respectively. The high efficiency of the DES system originates from the synergistic effects of protonation, reduction, and coordination capabilities. Selective separation of Ni was achieved by forming insoluble nickel oxalate dihydrate (NiC₂O₄·2H₂O), while the alumina carrier (Al₂O₃) remained intact. These mechanisms ensure high efficiency and strong selectivity of the multimetal leaching system. This work is the first to demonstrate a highly selective DES-based approach for simultaneous efficient extraction of V and Mo with effective separation of Ni from spent hydrotreating catalysts under mild conditions, providing significant environmental benefits and a sustainable recovery strategy.