Computational Investigation of Immobilized Molecular Extractants for Critical Mineral Recovery

The transition from an energy landscape driven mainly by the combustion of fossil fuels to one driven by renewable resources will require improved extraction and separation techniques for the critical minerals that are essential to many clean energy applications. Molecular extractants provide a promising opportunity for developing efficient and selective strategies for recovering critical elements from both ores and end-of-life materials through alteration of the steric and electronic properties of the molecule. However, despite these opportunities, implementation of molecular extractants for the recovery of critical elements suffers from the frequent need for large volumes of organic solvents in liquid–liquid extractions, representing an environmental hazard and generating significant waste. Immobilizing molecular extractants onto solid materials to create molecule–material hybrids would obviate the need for an organic solvent allowing extraction to proceed at the solid–liquid interface. An in-depth understanding of both the adsorption process of molecular extractants and the resultant effect on the geometric and electronic structure of the molecule towards critical element binding, as well as the influence of the substrate on the extraction process, are necessary for designing robust, efficient, and selective hybrid materials. Towards this end we have investigated computationally the adsorption properties of a series of aldoxime molecular extractants on a variety of substrates using density functional theory. The evaluation of appropriate computational methodologies for representing these systems using thorough benchmarking studies, including solvation effects and dispersion interactions, are described. Next, the thermodynamics and structures of aldoximes immobilized either through physisorption or chemisorption on carbon, silicon, and metal-based substrates were compared to identify strategies for developing materials with high-stability under extraction conditions at the solid–liquid interface. Furthermore, the thermodynamics of critical element extraction were evaluated for the immobilized molecules at the solid–liquid interface to evaluate the efficiency and selectivity of these hybrid materials. These values were compared against the same molecules under typical liquid–liquid extraction conditions to identify the benefits and shortcomings of the immobilization strategy. Finally, initial experimental efforts towards synthesizing and characterizing aldoxime–material hybrids and their performance for the extraction of critical elements are described.