CAREER: The solution structure of lanthanide-extractant complexes across liquid-liquid interfaces

稀土元素，包括稀土元素，对清洁能源、国防和消费技术至关重要，如智能手机、风力涡轮机、激光、制导系统和医疗造影剂。对于大多数应用，特定的稀土元素需要近乎纯净的形式。提纯过程需要将元素彼此分离。由于稀土元素的化学相似，分离特别具有挑战性，因此需要大量的溶剂和能量。需要高效的稀土分离工艺，以确保美国稀土元素的可靠、可持续供应。液-液或溶剂萃取法是分离大多数稀土元素的过程。溶剂萃取分离稀土元素的有效性在于萃取剂或配体与特定稀土元素结合的选择性。提高配体选择性和对特定稀土元素的萃取能力将提高分离效率。本项目旨在解析稀土-配体络合物在溶液中的结构，并确定在溶剂萃取中，配体将稀土元素从水相转移到有机相的机理。将确定水-有机界面的分子结构和机理与稀土溶剂萃取的程度和速度之间的关系。这个职业项目将把稀土分离研究纳入本科生的化学工程分离课程，并将扩大在内华达州高中推广化学工程的努力。该项目的教育和推广部分旨在搭建研究生研究和化学分离教学之间的桥梁，向本科生介绍稀土分离的概念，并加强化学工程劳动力发展管道，以支持内华达州向技术型就业的持续扩展。在界面工程计划和既定的刺激竞争研究计划(EPSCoR)的共同支持下，该项目旨在产生基础知识，以确定稀土-配体在溶液中的复杂结构与其相对水-有机溶解度(分离程度)之间的联系，并解决配体结构如何影响稀土离子的传输机制(分离率)。目前大多数稀土萃取剂的选择性都可以提高，因为它只基于尺寸排斥，而且由于收缩而导致的稀土离子尺寸差异很小。这个项目将研究多酸配体作为稀土特定的萃取剂，利用它们由多个质子化中心产生的可调结合强度。将从头算分子动力学模拟与光谱测量相结合，将在不同质子化状态下解析水相和有机相中的稀土-配体络合物结构。有效提取稀土元素的结合专一性是必要的，但还不够，因此，还将研究稀土离子在水-有机界面上的传输。采用稀有事件模拟技术的经典分子动力学模拟技术将产生稀土-配体络合物在水-有机界面上传输的自由能分布，这将被用来确定相对水-有机溶解度和配体介导的稀土在水-有机界面上传输的机理。还将进行提取测量，以验证预测。稀土溶剂萃取的结构和机理知识有限，特别是对于质子化状态随酸度变化的多酸配体。预计该项目中的工作将确定可以使多个酸性配体选择性地结合特定的稀土离子并将其传输到有机相的分子特征。更广泛地说，溶液中的结构在液-液分离中没有得到充分的研究，结构和机械洞察力可以从根本上理解溶剂提取中的结构-功能联系，这是一种需要更高选择性以提高效率和可持续性的重要单元操作。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Rare earth elements, including lanthanides, are vital to clean energy, defense, and consumer technologies, such as smart phones, wind turbines, lasers, guidance systems, and medical contrast agents. For most applications, specific lanthanides are needed in near-purified form. The purification process requires separating the elements from each other. The separation is particularly challenging due to the chemical similarity of the rare earth elements and thus requires substantial amounts of solvent and energy. Efficient rare earth separation processes are needed to ensure a reliable, sustainable supply of rare earth elements for the United States. Liquid-liquid, or solvent, extraction is the separation process by which most rare earth elements are obtained. The effectiveness of solvent extraction to separate rare earth elements lies in the selectivity of extractants, or ligands, to bind a particular rare earth element. Enhancing ligand selectivity and extraction capacity for particular lanthanides will improve the efficiency of the separation. This project seeks to resolve the structure of lanthanide-ligand complexes in solution and determine the mechanism by which ligands transport lanthanides from the aqueous to organic phase in solvent extraction. The connection between molecular structures and mechanisms at aqueous-organic interfaces with the extent and rate of rare earth solvent extraction will be determined. This CAREER project will incorporate rare earth separation research into an undergraduate chemical engineering separation course and will expand efforts to promote chemical engineering in Nevada high schools. The educational and outreach components of the project aim to bridge graduate student research and chemical separations instruction, introduce undergraduate students to the concepts of rare earth separations, and strengthen the chemical engineering workforce development pipeline in support of Nevada’s continued expansion toward technology-based employment.With joint support from the Interfacial Engineering program and the Established Program to Stimulate Competitive Research (EPSCoR), this project intends to generate fundamental knowledge to determine the connection between lanthanide-ligand complex structures in solution with their relative aqueous-organic solubility (separation extent), as well as to resolve how ligand structure affects the transport mechanisms of lanthanide ions (separation rate). The selectivity of most current lanthanide extractants can be improved because it is based on size exclusion alone, and lanthanide ion size differences due to contraction are small. This project will investigate multi-acidic ligands as lanthanide-specific extractants, leveraging their tunable binding strength arising from multiple protonation sites. A combination of ab initio molecular dynamics simulations with spectroscopy measurements will resolve lanthanide-ligand complex structures in aqueous and organic phases at varying protonation states. Binding specificity is necessary but not sufficient for effective lanthanide extraction; therefore, the transport of lanthanide ions across the aqueous-organic interfaces will be investigated as well. Classical molecular dynamics simulations with rare event simulation techniques will generate free energy profiles of lanthanide-ligand complex transport across aqueous-organic interfaces, which will be used to determine relative aqueous-organic solubilities and mechanisms of ligand-mediated lanthanide transport across aqueous-organic interfaces. Extraction measurements will be performed as well to verify predictions. Structural, mechanistic knowledge of lanthanide solvent extraction is limited, especially for multi-acidic ligands whose protonation state change with acidity. It is expected that the work in this project will identify the molecular characteristics that could make multi-acidic ligands selectively bind particular lanthanide ions and transport them to the organic phase. More broadly, structures in solution are understudied in liquid-liquid separations, and structural and mechanistic insights can lead to fundamentally understanding structure-function connections in solvent extraction, an important unit operation with a need for higher selectivity to improve efficiency and sustainability.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.