Session A12: Polymer Membranes for Separations

Lithium Extraction from Brine Using Membrane-based Bipolar Electrodialysis

As the demand for lithium batteries increases, the search for more efficient means of lithium extraction from brine sources has accelerated. However, these brines are complex and contain contaminants such as magnesium, calcium, and sodium, with divalent ions being difficult to remove via conventional ion sorption processes. Selective ion-exchange processes have been developed for upgrading such brines, but these methods typically produce a chloride salt product which is subsequently precipitated as lithium carbonate or electrochemically converted to the more-desirable LiOH in a separate process. Direct one- or few-step upgrading of Li-containing brine to LiOH is desired to avoid unnecessary processing of LiCl. Direct LiOH production from brine mixtures via bipolar membrane electrodialysis (BPED) is being considered. However, most commercial cation exchange membranes (CEMs) used in BPED systems do not possess sufficient monovalent/divalent ion selectivity to prevent precipitation of poorly soluble calcium and magnesium hydroxides in the base product of BPED systems, which foul and damage membranes. Thus, current CEMs are limited to applications where the feed concentration of divalent cations is below 10 ppm. Here, we report synthesis and characterization of Li-selective CEMs for BPED. This process potentially enables the upgrading/conversion of lithium to be practiced on less-processed brines with higher monovalent and divalent ion contamination.

    

Membranes For Clean Energy and Sustainable Environment

In the coming decade, membrane science will play an important role in addressing non thermal based separation needs for various clean energy technologies and helping the world to achieve a sustainable environment. Specific applications like, H₂ utilization and production, CO₂ capture and reuse, plastic deconstructions, chemical process separations and water purifications are noteworthy. On one hand each of these applications has specific performance metrices, on the other hand they share a common fundamental question; relation of membrane structure to transport of various species. In addition to understanding the structure-property relationships, membrane fabrication, and prototype testing are additional research areas for accelerating membrane development. These areas sit at the intersection of engineering, basic science, and science policy and partnerships. There is a need to develop a cross-functional, inter-agency membrane platform initiative to drive such goals. The presentation will focus on key membranes application areas like H2 fuel cells, water purification, waste plastic circularity, and carbon capture, emphasizing recent developments, market and societal needs, and proposed research topics.   

Precise Separations and Ion Transport in Self-Assembled Membranes with 1-nm Scale Pores

Considerable attention has been directed towards the development of membranes based on highly uniform structures formed by molecular self-assembly. Such systems open the possibility of overcoming the selectivity-permeability tradeoff that is intrinsic to the operation of current state-of-the art membranes in use in a wide variety of settings. Here, we examine lyotropic self-assembly of reactive amphiphiles into gyroid and direct hexagonal mesophases, the scalable fabrication of highly-ordered nanostructured polymer thin films from such mesophases, and the performance of the resulting membranes. We consider a variety of scenarios, including molecular filtration in aqueous and non-aqueous media, and ion transport. These membranes are compelling as they circumvent the limitations of pathway tortuosity and size-dispersity of transport-regulating features found in conventional membranes. As such, they enable highly selective, or precise, molecular separations and ion transport, with pore-size tunable in steps as small as 0.1 nm. We present results highlighting how these precisely-defined membranes are enabling the development of new insight regarding molecular transport under nanoscale confinement in the presence of charge, something that is poorly understood from a fundamental perspective.   

Membrane-based fractionation of complex mixtures

The rapid increase in global industrialization necessitates technology shifts in energy production, manufacturing, and carbon management techniques.  Approximately 10-15% of global energy use can be attributed to separation processes, with the vast majority of separations being “thermal” in nature (e.g., distillation). Membranes can augment or potentially disrupt certain incumbent technologies, but issues of perceived risk/reliability, scalability/cost, and performance must be addressed for membranes to move towards world-scale sectors such as hydrocarbon processing. The polymer processing platform for the creation of membrane devices is eminently scalable and has the potential to match the enormous volumes associated with chemical and petrochemical separations. Polymeric organic solvent reverse osmosis (OSRO) and organic solvent nanofiltration (OSN) membranes with the capabilities of fractionating complex mixtures have recently emerged and will be the focus of this talk. Specifically, we will discuss enabling materials for this separation challenge as well as methods to estimate the fluxes of various components in a complex mixture based on a relatively small number of single component experiments or in some cases no experimental data at all. Current challenges in the area of OSRO/OSN will be presented in addition to promising paths forward in this emerging area of membrane science.

    

The role of competitive sorption and plasticization in microporous polymers

Early-stage research in evaluating polymeric materials for membrane-based gas separations frequently relies on pure-gas permeation testing. However, properties predicted from pure-gas measurements rarely match those from measurements of mixtures, precluding simple tests from providing relevant data for many application-specific targets. Differences between pure- and mixed-gas performance are frequently reported for emerging classes of microporous polymers, which often exhibit strong competitive sorption and plasticization effects when materials are tested under complex mixtures. This presentation will introduce several classes of emerging microporous polymers and demonstrate examples of how mixture testing results in separation properties that are significantly different from pure-gas predictions. A detailed discussion will be presented on the role of competitive sorption, and how this phenomenon can be leveraged in certain cases to engineer materials that have mixture property sets that exceed those of pure-gas estimates. In particular, the role of amine functionality will be discussed in emerging microporous polymers, and examples will be presented on how these materials behave in the presence of mixtures containing CO₂, H₂S, CH₄, and other gases common in commercial applications. The role of competitive sorption will then be compared with respect to plasticization to demonstrate how microporous polymers can be designed to achieve stable and enhanced performance under relevant gas-phase compositions.