Supramolecular formation via solvent extraction — Metal separation and velocity change using oil phase: Discovered in Japan−France joint research

Solvent extraction has long been used to extract only the desired component from a solution in oil refining, pharmaceutical manufacturing, food processing, and the recycling of useful metals. This is a simple separation method that utilizes the solubility of a substance between two liquids that do not mix, such as water and oil. For example, if an extractant that is soluble only in oil is added to a mixture of oil and water, the target metal ion becomes a metal ion complex through chemical bonding and remains only in the oil. In other words, solvent extraction methods have been used because the relationship between the extractant and the extracted object is important. However, in reality, nanoscale supramolecules are formed through the binding of multiple metal ions and extractant molecules, which is also controlled by the oil phase components.

These findings were reported by an international collaboration of Research Scientist Micheau Cyril, Principal Investigators Yuki Ueda and Ryuhei Motokawa of the Materials Sciences Research Center, Japan Atomic Energy Agency, Deputy Chief Engineer Kazuhiro Akutsu-Suyama of the Comprehensive Research Organization for Science and Society, and Associate Professor Norifumi L. Yamada and Assistant Professor Masako Yamada of the Institute of Materials Structure Science, High Energy Accelerator Research Organization (KEK) with the Institut de Chimie Séparative de Marcoule, ICSM, CEA, CNRS, ENSCM, Univ Montpellier, France. The study was published in the Journal of Molecular Liquids. Motokawa commented on the findings, "Now that a completely different system has been identified, we can separate and extract more efficiently in various fields, such as manufacturing pharmaceuticals."

During their research on reducing the volume of radioactive waste, the group discovered a strange phenomenon in the solvent extraction method using malonamide, which can isolate rare earth and platinum group metals, as an extractant to separate two metals, palladium and neodymium, used for catalysts and permanent magnets. When toluene was used for the oil phase, only palladium was extracted. When heptane was used, both palladium and neodymium were extracted. Furthermore, the extraction rate of palladium becomes very slow when toluene is used. In other words, they found that the extraction rate can be changed by changing the solvent used for the oil phase and that the metals that can be extracted.

To determine the cause of this phenomenon, they performed small-angle X-ray scattering measurements, which can analyze nanoscale structures. As a result, it was found that malonamide forms a small supramolecular assembly of two to three molecules in toluene, only palladium ions fit into the inner pocket, and neodymium ions are not easily incorporated. The size recognition effect of the supramolecular aggregates affected the selectivity of palladium and neodymium. On the contrary, a huge aggregate of about 100 malonamide molecules was formed in heptane. In this case, palladium and neodymium cannot be distinguished. To further investigate the microscopic state of the interface, analysis was performed using a neutron reflectometer (soft-interface analysis system) installed at J-PARC.

The results showed that a dense layer of malonamide formed near the interface when heptane was used for the oil phase. At the same time, when toluene was used for the oil phase, the malonamide was not dense at the interface but formed a dilute layer spread over a region several nanometers away from the interface. This dilute layer reduces the probability of palladium and malonamide colliding at the interface, resulting in slower extraction rates. In this study, the research group determined the structure of the supramolecular assembly during the solvent extraction process and clarified the effect of the structure on the separation of metal ions for the first time. They also showed the possibility of adding a supramolecular assembly perspective to conventional solvent extraction methods, leading to new methods of separating metal ions and the design of new extractants.

Original JAEA press release: https://www.jaea.go.jp/02/press2024/p24041202/

Journal Information
Publication: Journal of Molecular Liquids
Title: Organization of malonamides from the interface to the organic bulk phase
DOI: 10.1016/j.molliq.2024.124372