Deep Eutectic Solvents as Recyclable Media for Metal Removal from Crude Oil

Crude oil contains trace metal species that impair refinery performance, poison catalysts, and contribute to environmental contamination. Conventional metal-removal processes, such as hydrotreating and solvent extraction, are energy-intensive and costly. In this study, deep eutectic solvents (DESs) were evaluated as alternative media for extracting metal-containing complexes from crude oil. Model experiments using copper, iron, and nickel phthalocyanines dissolved in toluene showed that all complexes were transferred intact into the DES phase. Thermodynamic analysis revealed that extraction is exothermic and driven by protonation of the phthalocyanine ring within acidic DESs, leading to enhanced metal uptake at lower temperatures. Across the formulations tested, more acidic DES systems consistently delivered higher extraction efficiencies. The methodology was then applied to a Nigerian crude oil sample to assess performance under realistic conditions. Acidified DESs effectively extracted metalloporphyrins and other metal species while maintaining good phase separation. A simple filtration-based workflow enabled recovery and reuse of the DES phase with minimal energy input, demonstrating the potential for operationally straightforward and recyclable metal-removal processes. Overall, this work establishes acidified DESs as efficient, mechanistically understood, and potentially low-energy media for the extraction of metal species from crude oil, offering a promising basis for next-generation upgrading technologies.