Recycling and separation of critical elements using porous materials

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Critical metallic elements, such as lithium, rare-earth elements (REEs), cobalt, and aluminum, are non-renewable resources that are vital to modern technologies. For example, lithium is used in batteries for electronic devices and electric vehicles, and REEs are used in jet engines and anti-corrosion coatings for metals. Some critical elements, such as lithium and REEs, are not naturally abundant within the United States, and their supply and cost can be affected by shifting geopolitical factors. Understanding how critical elements can be separated and purified will advance the Nation's economy and security through advancements in mining and recycling of domestic critical elements. However, it is challenging to achieve the separation of critical metal elements with high purities by existing methods. Microporous titanosilicate materials contain unique structural features that are suitable for separating critical metal elements with high purities. Therefore, this research project will use a set of experimental and computational methods to design microporous titanosilicate materials for the highly selective separation of critical elements. The project also involves training students of various educational levels as well as outreach activities that disseminate the research findings to a broader audience from diverse backgrounds.The goal of this research project is to elucidate how the spatial arrangements of ion-adsorption sites in microporous titanosilicates facilitate high-resolution separation of metal ions with very similar properties by continuous ion exchange/continuous ion chromatography (CIX/CIC). Titanosilicates have long-range order ion-adsorption sites inside their micropores, where the micropores are channels with sizes similar to hydrated metal ions. Through material design, these ordered ion-adsorption sites could offer tunability toward metal ion selectivity. The investigators will use a set of experimental and computational methods for different length scales (ion adsorption site, unit cell, and particle scales) to design materials for the separation of REEs and other metal ions. The project will first focus on relating the fine structural features inside titanosilicate pores, such as charged site density and distribution, to REE ion adsorption and separation performance. Structures with different charged site densities will be synthesized and simulated. Ion adsorption equilibrium constants will be measured experimentally as well as computed using density functional theory calculations. Computational methods describing the hydration and dehydration of metal ions inside micropores will be developed. These efforts will lead to recommendations for structural features for separating REE cations. The project will then focus on evaluating the effects of material structure (chemical composition, pore size, and pore topology) on metal ion migration rates and activation energies. This outcome will be achieved by studying ion adsorption and migration on microporous materials of various structures (titanosilicates, clays, and zeolites) using experimental and computational methods. Finally, the project will focus on determining the effect of particle size and morphology on REE adsorption capacity, strength, and kinetics, which will be an important step in preparing these materials for practical applications. This project will provide guidelines for designing microporous materials for separating chemically similar metal ions (such as separating REEs from each other or separating lithium and sodium), which can also be customized based on the feedstock composition and impurities.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.