(2bd) Designing Advanced Separation Processes for Critical Materials and Discovering Porous Membrane Materials for Sustainable Energy Applications

My current and previous research has been focusing on the developing and applying multi-scale molecular modeling to investigate the materials, such as ionic liquids (ILs), lanthanides (Ln), actinides (An), zeolites, MOFs and polymers, and to explore the potential of using them in separations processes. Some highlights of my research are as follows:

1. In the liquid-liquid extraction (LLE), amphiphilic extractant molecules are used to create an organic/aqueous interface and absorb Ln/An metal ions. Due to the difficulty of probing the biphasic interface experimentally, molecule-level understandings of ion assembly at the interface are often missing. Our X-ray fluorescence spectroscopy has shown the enhancement of Ln-ion absorption at the interface with the increasing atomic number in Ln series. Using MD simulations, we have shown that the ion-adsorption enhancement at the interface induced by Ln-ion speciation, which is key to design better separation processes.

2. While amphiphilic extractants are used as chelating agent to transfer metal ions from aqueous to organic phase, the understanding of chelating functionality is key to design better LLE processes. More importantly, free and unbound extractant molecules normally exhibit trans conformation, while the energetically unfavorable cis conformation is most relevant to the chelation. By using MD with Metadynamics (MTD, an advanced sampling methodology), we map out the conformational energetics of several extractant molecules. Such energetic information potentially explains how separation performance and distribution ratios change under different conditions, and provides a different perspective on ligand design for better LLE processes.

3. To make synthetic zeolitic porous materials, organic structure-directing agents (OSDAs) are often used to control pore size and aluminum distribution. While the aluminum distribution controls catalytic properties of zeolites, its relationship with the OSDA is unclear. We develop a model, which use density functional theory (DFT) and classical model to describe interactions between the framework and OSDA. Lattice energies, computed from the model, successfully predict the aluminum proximity and are consistent with experimental titrations. Furthermore, the model elucidates the relationship between OSDA and aluminum (Al) distribution, which will potentially guide better design of catalytic zeolite materials.

With my past research experience, I propose three areas in which my expertise and knowledge will be leveraged.

1. On account of the necessity to replace the inefficient and hazardous organic molecular solvents in the LLE and Ln/An separation processes, IL will be investigated using a combined molecular modeling and experimental method to provide molecular understanding, and to guide the design of better separation processes using ILs.

2. To address the problems with existing rare earth elements (REEs) separation technologies, I will investigate the potential of using zeolite-based membranes, by applying it as both absorbent and ion-exchange platform to efficiently recover REEs from aqueous streams.

3. I will extend my research topics to investigate using zeolites membranes in energy storage applications though molecular modeling, as well as collaborating with experimentalists. Topics will include providing insights of ion transport behaviors through zeolite, triggering discoveries of new membrane materials, and making breakthroughs in designing redox flow batteries for energy storage applications.

I. Developing IL-based separations of Ln and An

Ln are widely used in electronic devices and battery manufacturing; An recycling is of significant importance in fission product treatment and nuclear engineering. The current industrial separation of Ln and An relies on liquid-liquid extraction (LLE) with the use of organic solvents and extractants which are neither environmentally friendly nor efficient. One possibility that has recently drawn lots of attention is the IL. Usually, the LLE process relies on small free energies to derive the separation and ensure reversibility for metal reprocessing. In my preliminary study, I have found that the nano-structured IL phase has greatly stabilized the energetically unfavorable extractant conformation through IL-extractant interactions. Such a finding provides a distinctive insight into the potential energy driver, which underpins the notable separation improvements reported with ILs and indicates a new approach to designing effective separations using solvent effects. It is my belief that, using a combined molecular modeling and experimental method, investigation of ILs in the LLE and Ln/An separations has a broad appeal in the development of renewable energy, advanced manufacturing and nuclear engineering.

II. Molecular Modeling of REE Separation Using Zeolite Membranes

Similar to Ln, the current predominant technology for separating REEs is solvent extraction, requiring large volumes of acidic dissolution, and organic solvents with expensive extractants. Membrane-Based Separations have been identified as a promising sustainable and efficient separation technology. To this end, zeolites, with their characteristics of molecular sieves and tunable Al distribution, can serve as absorbent, ion-exchange platform, and membrane material to separate and recover REEs. I will use the following three hypotheses to design my research plan: (1) tunable REE ion speciation; (2) REE-ion-framework compatibility; (3) tunable zeolite chemical property. I will build up more layers of complexity throughout the research plan, and each proposed study will be built around the three hypotheses listed above. My research will provide molecular-level understanding, which will potentially trigger future research on exploring the application of zeolites in the REE processing technology. Successful outcomes of our proposed research plans will also provide insight into rare earth ion chemistry in zeolites, benefiting other fields, such as REE-exchange-zeolite catalysis.

III. Investigating ion exchange in different zeolites

Since environmental issues and global warming are becoming increasingly problematic for society, researchers from academia and industry are exploring using batteries as an alternative to fossil fuels. Among energy storage technologies, the redox flow battery (RFB) has been identified as a potential solution, due to their safety, high capacity, efficiency and small environmental footprint. In the RFB, one of the key factors that influences the fuel cell performance is the design of ion exchange membrane (IEM). Traditional, organic membranes, such as sulfonated fluoropolymer-copolymers, suffer from low-efficiency and degradation over time, which inhibit the use of RFBs particularly for those operating at elevated temperatures. Zeolites offer us another choice because of their many advantages over organic membranes. They are known for endurance in acidic atmosphere and high temperatures. The molecular sieves formed in zeolites only allow the permeation of certain types of ions, while hindering other species. Such features enable the application of zeolites for membrane materials in the redox flow battery, which requires the permeation of protons and the inhibition of redox active species. In this topic, I plan to use MD simulations to investigate ion exchange behavior for a wide range of zeolites. Moreover, a wide variety of electrolytes, including water, ILs and deep eutectic solvents along with different compositions can also be studied. I believe my theoretical studies will offer insights on the membrane materials, which will guide the design of better ion exchange membranes for RFBs.

Teaching Interests

Education is a lifelong experience with the roles of teacher and student repeatedly shifting. Choosing teaching as my permanent career will be one of the most satisfying decisions of my life. In my opinion, no profession other than teaching offers a greater potential to contribute to and positively impact society and the lives of all those involved in the learning process, including the instructor. I also strongly feel that learning is a two-way street, and both students and teachers learn a lot from each other during this process. Advances in science are handed down from generation to generation. It is an important job for a professor to pass his or her skills and knowledge to the next generation.

Looking back into my experience when I sat in the classroom, I found out that asking and answering questions between the lecturer and students is of vital importance for both roles. To this end, I plan to use problem-based learning techniques in my classes. By using the problem-based teaching strategy, more communications between the instructor and attendees can also be expected. This way, it will be easier for me to get to know better of students with different backgrounds and learning curves. It is also a useful strategy to let me adjust my lecturing based on feedbacks from students.

As a non-native speaker, I found group projects helped me a lot by improving my self-learning, teamwork and communicating skills. I will assign group projects and ask students at all levels to give presentations based on the assigned materials to help them improve their understanding of the subject material. For graduate students, I will provide several topics for them to write a mini proposal based on the knowledge they acquired in the class. This will help the students to learn how to analyze ideas and manage projects. For those project-based courses, I believe excelling at both research and teaching requires careful planning. Ambitious projects, such as overhauling a class, should be gradually and thoughtfully executed while maximizing material reuse and constantly looking for ways to improve the class.