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Synthesis and applications of organophosphonic acid compounds as extractants for rare earth element separation and beyond
Kuvayskaya, Anastasia
Kuvayskaya, Anastasia
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2024
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Abstract
The realization of a shift from traditional energy sources towards more environmentally friendly alternatives has led to an ever-growing need for essential raw materials such as rare earth elements (REEs). Nuclear energy, catalysis, phosphors, superconductors, permanent magnets, and optical materials rely heavily on REEs. Consequently, a crucial area of technology is the extraction and isolation of REEs from complex mixtures. Chemical homogeneity causes various REEs to accumulate in source minerals, making it more challenging for their separation and obtaining commercially feasible pure elements requires multistage extractions and repeated separation techniques. Solvent extraction is a commercially employed technique for separating rare earth metals. Extractants are crucial to the separation process because they can form complexes with the water-soluble REE cations and switch their solubility into the organic phase. Many extractants have been developed, including carboxylic and phosphorous acids, β-diketones, phosphorous esters, phosphine oxides, and various amines. Commercial extractants, however, have low selectivity, which renders processing time-, energy-, and solvent-intensive. Recently, promising REE separation results have been reported on organophosphorus extractants, including asymmetric dialkylphosphinic acids, monoalkylphosphinic acids, and styryl phosphonate monoesters.
In this work, two generations of aryl vinyl phosphonic acid esters were designed and synthesized. The synthesis optimization resulted in a two-step reaction allowing various high-purity aromatic vinyl phosphonic acid monoesters. Utilization of the Heck coupling reaction enabled functionalization of vinyl phosphonic acid with different aromatic and photoswitchable moieties. The employment of Steglich esterification led to the formation of two generations of organic soluble non-symmetric mono-esters. The 1st generation of extractants was complexed with Eu3+ and studied compared to traditional extractants, resulting in an unexpected finding: the order of increasing extraction strength matched the order of decreasing calculated dipole moment of the synthesized ligands rather than pKa. The 2nd generation of extractants included phosphonic acid esters containing photoswitchable moieties that are currently being studied for their extraction properties.
In another application, the employment of a versatile Heck coupling approach resulted in synthesizing 14 diverse anionic monomers capable of modular “cyanostar”-stabilized anion dimerization. The optimized synthetic approach to vinyl phosphonic acids and corresponding vinyl phosphonates allowed access to solubility-tuned ditopic monomers for preparing supramolecular polymers.
Finally, the broad scope of applicability of aryl vinyl phosphonic acids was evident from the evaluation of carbazole-based PA derivatives as self-assembled monolayer (SAM)-based hole transport/extraction layers (HTM) in perovskite solar cells. The introduction of stronger bonding molecules at the buried interface reduced the amorphous phase around perovskite/HTM/ITO interfaces and increased the stability of fabricated devices. The champion minimodule with the hybrid HTM retained operational efficiency of 17.5% after 10 weeks of outdoor testing, the first to achieve this property to our knowledge, as independently measured by the Perovskite PV Accelerator for Commercializing Technologies center.
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