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A Research Agenda for Transforming Separation Science (2019)

Chapter: 4 Gaps and Challenges

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Suggested Citation:"4 Gaps and Challenges." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
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Suggested Citation:"4 Gaps and Challenges." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
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Suggested Citation:"4 Gaps and Challenges." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
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Page 51
Suggested Citation:"4 Gaps and Challenges." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
×
Page 52
Suggested Citation:"4 Gaps and Challenges." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
×
Page 53
Suggested Citation:"4 Gaps and Challenges." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
×
Page 54
Suggested Citation:"4 Gaps and Challenges." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
×
Page 55
Suggested Citation:"4 Gaps and Challenges." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
×
Page 56
Suggested Citation:"4 Gaps and Challenges." National Academies of Sciences, Engineering, and Medicine. 2019. A Research Agenda for Transforming Separation Science. Washington, DC: The National Academies Press. doi: 10.17226/25421.
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Page 57

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  4 Gaps and Challenges The preceding chapters gave an overview of the many advances in chemical separations and other fields that have emerged in the last 3 decades. Despite the advances, critical challenges remain for the separations community to confront. This chapter describes the current gaps and challenges, which form the basis of the research agenda that will be defined in Chapter 5. Thematically, key challenges can be grouped into topics associated with selectivity, capacity, and throughput and with the temporal changes in separation systems in operating environments. Selectivity, capacity, and throughput are intrinsic characteristics of a separation system,1 but their relative importance depends on a given system’s eventual application. What those characteristics have in common is that many outstanding, fundamental questions are related to measuring and improving them in complex, highly variable environments—environments that are different from the simplified test cases that form much of the current fundamental-research portfolio. The evolution of separation systems during operation almost always adversely affects system per- formance; degradation and aging are two ubiquitous examples. Far from being an engineering detail di- vorced from fundamental research, the evolution of materials in separation systems poses vital fundamen- tal research challenges. There are substantial gaps in the current understanding of the processes that control the evolution, especially in complex and variable environments. Closing those gaps and develop- ing synthesis or regeneration techniques to mitigate the adverse effects as separation systems evolve are likely to have enormous dividends. In addition to the broad research themes described, two cross-cutting topics that are important in materials chemistry have particular resonance in chemical separations: establishing standards to enhance reproducibility and multimodal characterization of complex materials and adapting data-science methods to accelerate development of separation systems. Those challenges are discussed below. SCIENTIFIC CHALLENGES IN SELECTIVITY, CAPACITY, AND THROUGHPUT Selectivity, capacity, and throughput are arguably the key defining features of separation processes, so developing fundamental approaches that improve performance in these factors lies at the heart of sepa- ration science. The relative importance of the three factors varies in different contexts. In analytical set- tings, for example, the ability to detect trace species with exquisite selectivity in the presence of many confounding analytes might be more critical than the rate at which the separation can be achieved. In con- trast, in large-scale process settings where flow rates might be measured in tons per hour, the capacity and throughput of a process are paramount. In general, however, all three factors contribute to the success of a separation process. Moreover, in many situations, trade-offs exist: improvements in one factor are typically associated with declines in the others. That difficulty has been extensively explored, for example, in membrane- based gas separations (see Box 4-1). Observations have implied that efforts that focus exclusively on one 1 Separation system refers to an entire process or procedure that yields a separation; it can include the separation material, devices, and other components. Prepublication Copy 49

A Research Agenda for Transforming Separation Science of the factors (selectivity, capacity or throughput) are likely to have small long-term effects. The follow- ing sections detail multiple aspects of this critical challenge. Advancing the Understanding of Complex Mixtures Samples and process streams encountered in separation problems are almost always complex mix- tures that have many chemical components. Idealized concepts of pure feeds with clean carriers and sin- gle-product chemistries are appropriate for initial consideration in investigating separation options. To solve the problem, the complex mixture must be considered. BOX 4-1 Tradeoffs Between Permeability and Selectivity in Membrane-Based Gas Separations Permeability is used to characterize gas-separation membranes (from commercial gas-separation membranes to contact lenses). For gas separations, permeability is defined as flux per unit time nor- malized by the pressure drop across the membrane (which increases flux) and the membrane thick- ness (which decreases flux). Compilations of permeability (P), a key measure of throughput, and se- lectivity (α, which is defined as the ratio of permeabilities of two species) for hundreds of chemically diverse polymeric gas-separation membranes have consistently shown the existence of tradeoffs be- tween permeability and selectivity. That is, it is possible to produce membranes that have high selec- tivity or membranes with high permeability but not membranes that have both. The figure shows a compilation of data on separations of oxygen from nitrogen (Robeson, 2008). As permeability increas- es (horizontal axis), selectivity (vertical axis) decreases. Work to uncover the fundamental factors that underlie the initially empirical correlations has been instrumental in developing new classes of mem- brane materials that have promise for exceeding the tradeoffs that constrain traditional polymeric membranes. FIGURE 4-1 “Upper bound” plot for the separation of oxygen from an oxygen–nitrogen mixture. For numerous polymer membrane materials, the plot shows the classic trade-off between selectivity (α) and permeability (barrer = 10-10 cm3STP • cm[membrane thickness]/(cm2[area of membrane]) • s • cmHg [transmembrane pressure]). SOURCE: Robeson, 2008. Reprinted with permission; copyright 2008, Elsevier. 50 Prepublication Copy

Gaps and Challenges When components of interest are present at low concentrations, as is the case in many analytical set- tings, understanding and controlling the response of the separation process to the full spectrum of analytes that might confound it are critical. When components of interest are present at high concentrations, as typified by many large-scale chemical processes, nonadditive and nonideal interactions among compo- nents can dramatically influence selectivity, capacity, and throughput. Those effects do not necessarily scale linearly with the concentrations of the components. In the physisorption of molecules in nanopores, for example, the adsorbed concentration can vary roughly exponentially with the molecular weight of the adsorbing species, making the presence of high-molecular-weight contaminants in input streams im- portant. Critical scientific challenges exist in developing a fundamental understanding of the effects of com- plex mixtures on the behavior of separation systems. Tackling those challenges early in considering new approaches can avoid wasteful failures caused by the “show-stopping” complications inherent in complex mixtures. In addition to overcoming expected challenges, studies of mixtures can yield insights that lead to new domains of use. For example, a study of the use of membranes to separate olefins from paraffins (ethylene from ethane and propylene from propane) focused on the main components found in typical steam-cracker separations (Koros et al., 2016). As a part of the study, the more complex actual stream in a manufacturing operation was tested. Use of the actual stream added several components to the study, in- cluding hydrogen, methane, ethyne, propyne, and propadiene. A preference for the permeation of hydro- gen, ethylene, and propylene was confirmed, but the alkynes and diene exhibited even greater permeance than hydrogen. Those observations offered new knowledge of the behavior of the membrane system. More important, the unexpected performance suggested commercial applications of the separation tech- nology that were unimagined before. Exploring the Array of Thermodynamic and Kinetic Mechanisms Because the abstract task of separating chemical mixtures is so broad, an enormous array of physical mechanisms can be used. It can be useful to distinguish between examples in which separations are con- trolled by aspects of thermodynamic equilibrium and examples in which kinetics are crucial. In both cas- es, fundamental challenges limit the ability to achieve high selectivity, capacity, and throughput. For equilibrium-based separations, enthalpic or entropic contributions or a combination thereof can dominate performance. Combining approaches that traditionally focus on just one thermodynamic contri- bution is likely to improve known methods and even lead to the development of fundamentally new sepa- ration mechanisms. Some separations rely on kinetic effects, such as mass-transfer rates through interfaces between liq- uid phases or diffusion rates in porous or structured materials. Accurately characterizing the rates of rele- vant processes presents challenges. Establishing nontrivial structure–property–performance relationships to predict kinetic parameters on multiple relevant length scales is also challenging. Separating Trace Compounds and Using Multistep Processes In more and more new applications, trace compounds must be found and removed. Examples in- clude ultratrace analysis, desalination, nuclear-waste cleanup, and the study of transactinide isotopes. Achieving sufficiently high concentration factors requires high affinity and, because high concentrations of competing species are common, high selectivity. If the only option is to regenerate the separation agent for repeated use, a mechanism is needed to release the bound target species efficiently. Recovery of trace species can be critical because of the need for regulatory compliance or value re- covery. A regulatory example is the picomolar levels of technetium (Tc) in the form of pertechnetate ani- on in groundwater emanating from U.S. Department of Energy nuclear sites, where the radioactivity of the Tc raises a concern. In another example, nonradioactive oxoanions that raise concerns at trace concen- trations in drinking-water sources include perchlorate and arsenate–arsenite. Regarding value recovery, Prepublication Copy 51

A Research Agenda for Transforming Separation Science developing chemistry to separate valuable metals from highly dilute sources will address issues of supply security and valorization. Examples are gold from ores, uranium from seawater, and gallium from tail- ings. In all those cases, in keeping with Sherwood's concept (Keller, 1987; Seader et al., 2016), cost in- creases with dilution, and the economics of the processes are poor or marginal at best. Approaches that mitigate this challenge are needed. Trace analysis is important in many scientific fields. Examples are ultratrace analysis of pesticides in fruits and vegetables and similar ultratrace analysis of pharmaceutical compounds, such plasticizers as phthalate esters, and nanoparticles in natural waters. In this context, a multistep process is used in which preconcentration is critical before separation and detection are feasible. Current multistep analytical procedures include different forms of preconcentration or removal of high levels of matrices. Preconcentration methods, such as liquid–liquid extraction or solid-phase extrac- tion followed by highly efficient liquid chromatography, are capable of measuring many target com- pounds of interest in a range from nanograms per liter to micrograms per liter (see Box 4-2). Improve- ments in the extraction, chromatography, and mass-spectrometry portions of those multistep methods have allowed the low detection limits to be achieved. Nevertheless, such detection limits cannot be reached for all compounds of interest, and often the limits are still not low enough. In addition, achieving those limits in the case of many nontarget com- pounds remains challenging. It is important to find means to achieve the highly targeted selectivity re- quired to make progress in this field. Achieving Separations with a Wide Dynamic Range Mixtures usually have multiple species at various concentrations. The ratio of the concentration of the most concentrated species to the least concentrated species of interest is the dynamic range. The need for separations with a wide dynamic range can be illustrated by the detection of the proteins in human blood plasma. Because post-translational modifications of proteins are good biomarkers of disease, this is BOX 4-2 Measuring Dilute Target Compounds Breath analysis is a noninvasive method being developed to profile a person’s physical state. Ana- lytical-scale gas chromatography combined with mass spectrometry (GC–MS) is a conventional meth- od of breath characterization (Wallace and Pleil, 2018). The combination of high-efficiency preconcen- tration with GC–MS detection of substances in human breath makes possible detection of non–small- cell lung cancer, diabetes, liver diseases, breast cancer, chronic obstructive pulmonary disease, and chronic renal failure (Xu et al., 2016; Miekisch and Schubert, 2006). Recently developed microfluidic gas-chromatographic systems allow the separation and analysis of biomarkers of lung cancer at the parts-per-billion level (Gargano et al., 2018). The new methods are still emerging, but continued re- search in the separation techniques of microscale preconcentration and chromatographic separations will facilitate their routine use for field analyses or point-of-care analysis. Liquid chromatography combined with detection techniques, such as mass spectrometry, is in- creasingly used for disease detection and staging. For example, studying human histones and their post-translational modifications can be used to detect and stage a number of cancers. No matter what separation and analysis methods are used to achieve deep proteoform coverage for the analysis of highly modified proteins, such as histones, time-consuming fractionation is undertaken and requires days to complete. Recently, comprehensive nanoflow two-dimensional liquid chromatography com- bined with high-resolution mass spectral detection provided top-down proteoform characterization of HeLa core histones; this method identified nearly 400 proteoforms, twice the number of previously identified proteoforms, from a small sample quantity (1.5 µg). Methods that combine high-resolution chromatography and mass spectrometry are expected to continue to revolutionize disease detection and characterization for point-of-care and clinical analyses in the future. 52 Prepublication Copy

Gaps and Challenges a topic of considerable interest. The dynamic range of protein concentrations in human blood plasma is expected to be about 1010–1012 (Anderson and Anderson, 2002). No current multistep process that in- volves preconcentration, separation, and detection can analyze proteins across such a large dynamic range. Various methods, such as prefractionation of high-concentration proteins, have been attempted, but some of the lower-concentration proteins are also removed with these methods (Rassi and Puangpila, 2017; Wu et al., 2016). If a particular biomarker is identified, a method of selectively preconcentrating and detecting it is possible. However, nontarget analysis is still difficult or impossible. Similar problems of dynamic range are encountered with metals. For example, traces of plutonium must be measured at sub-parts-per-trillion in biological samples, and reprocessing of used nuclear fuel is performed at kilogram quantities. New ide- as for addressing the challenges associated with effective separations of complex mixtures that have wide dynamic ranges must be developed. In addition to developing separation systems with wide dynamic ranges, there is a gap in develop- ment of models that can reliably predict the performance of separation systems over a much wider range of conditions than that on which direct experimental data are available. Models that could identify the tradeoffs that inevitably appear when one is considering separations over disparate conditions would be useful in addressing questions related to how effective particular separations need to be for utility in a given setting. Understanding and Controlling Interfaces Physical interfaces play a decisive role in the performance of many separation methods. Fundamen- tal advances in the ability to characterize the structure and dynamics of the interfaces will achieve better separations. The detailed properties of interfaces often vary in critical ways between “clean” environ- ments and the more relevant environments associated with complex mixtures. Impurities and other noni- dealities in complex mixtures are likely to affect the long-term structure and properties of interfaces. Achieving more refined control of interfacial properties through synthesis, treatment, and regeneration methods will yield substantial dividends. Adaptation of current characterization techniques or develop- ment of new techniques that can generate molecular-level insight into interfaces in environments relevant to operating conditions would potentially have a large favorable effect. This concept is closely related to the ideas of operando techniques that have made enormous changes in chemical catalysis (Topsøe, 2003) and in contaminant transport in the environment (Henderson, 2002). Understanding Physical Changes in Response to External Forces Most chemical separations are controlled by a small number of driving forces (such as changes in concentration, temperature, pressure, and pH), but many other external stimuli can also play a decisive role. Those stimuli include electromagnetic effects (such as microwaves, optical fields, and magnetic fields) and mass-transport effects associated with complex or locally driven fluid-flow fields. They can act on chemical species that are being separated or on a separation material itself. In many instances that involve what might be termed nontraditional driving forces, it can be difficult to establish physical upper bounds on the performance that might be achievable and to demonstrate specific examples that create fundamental knowledge that can drive development. Determining the appropriate metrics with which to compare the performance of nontraditional approaches with established techniques can be difficult, but these metrics have great value. SCIENTIFIC CHALLENGES IN UNDERSTANDING TEMPORAL CHANGES THAT OCCUR IN SEPARATION SYSTEMS To maximize their scientific or practical value, separation systems must be reliable when exposed to streams of broad and variable composition. The systems might encounter harsh work environments and Prepublication Copy 53

A Research Agenda for Transforming Separation Science are expected to operate stably for long periods. During operation, separation media generally undergo aging or degradation that results in loss of selectivity or capacity. Such changes can be caused by physical (thermal) and chemical aging of the materials themselves or can occur as a loss in surface characteristics due to fouling caused by accumulation, surface-catalyzed reactions, or other conditions. Many pressing fundamental research issues are associated with the robustness of separation systems, including the achievement of detailed descriptions of the mechanisms of aging and fouling and the devel- opment of novel regeneration or “healing” procedures. A thorough understanding of aging through many separation cycles is needed with consideration of the influence of temperature and pressure and of the diversity of chemical effects that might occur because of impurities or components of the streams under consideration. In operating environments, such deviations from “typical” operating conditions are often associated with unplanned but unavoidable upsets. Because aging and degradation often occur over weeks or months, effort should be placed on developing accelerated aging concepts and techniques that are strongly grounded in the fundamental understanding of robustness. Determining Changes from Nonequilibrium That Affect Chemical and Physical Properties of Separation Materials In some classes of separation materials, materials in a nonequilibrium or metastable state are used. For example, glassy polymers are examples of nonequilibrium materials that have lower free energy states to which kinetic access might be extraordinarily slow. Polymorphs (often nonporous) with lower free energy can exist in crystalline porous materials, such as zeolites, but access to these more stable structures involves very high free-energy barriers, so the thermodynamically metastable crystals are stable from a practical viewpoint. However, events associated with chemical separations can catalyze or accelerate changes in nonequilibrium or metastable materials. The events include excursions in temperature or pressure and chemical effects of molecules that are the target of the separation or of contaminants in mixtures that are being separated. Such events can dominate the viability of the separation process, so the ability to control the evolution of the separation materials via these mechanisms is critical. A central challenge is to under- stand the fundamental mechanisms that control the evolution of nonequilibrium states, particularly in complex environments. Without such understanding, efforts to implement accelerated testing protocols that speed the cycle of materials development or work to synthesize materials with intrinsically improved stability are likely to remain empirical and inefficient. Determining Identities and Rates of Fundamental Chemical Reactions That Result in Changes in Separation Media and How the Reactions Are Influenced by Operating Conditions The temporal evolution of nonequilibrium materials discussed above involves processes in which no change in the chemical stoichiometry of the underlying material occurs. An equally important way in which separation media can evolve is through chemical reactions, which are often irreversible. The deg- radation of materials in acidic environments is one of many examples. The influence of operating condi- tions and the difference between “clean” and “complex” conditions can be dramatic. Fundamental in- sights into the nature of the underlying chemical processes are needed if approaches to mitigate or avoid them are to be discovered. Understanding the Fate of Unwanted Products Chemical and physical interactions between separation materials and the mixtures to which they are exposed can generate products that are not readily removed from the system. When that happens, the identity and fate of the products can dominate the time scale on which the separation system can be used. Fouling of membranes during water purification is a dramatic example: foulant layers of chemical and physical complexity form and require aggressive regeneration procedures that in turn place strong con- 54 Prepublication Copy

Gaps and Challenges straints on the types of membrane media that can be used. Another example is the blockage of pores in activated-carbon adsorbents used in the removal of dyes and other contaminants from water, which causes a decrease in performance and capacity. Advancing the ability to characterize the location, chemistry, and structure of unwanted products during chemical separations, particularly in the presence of the complex mixture characteristic of operating conditions, is critical for expanding the scope of current separation systems and developing new systems. Exploring Alternative Strategies for Addressing Temporal Changes in Separation Systems The three topics described above highlight different routes by which separation systems can evolve during use: by leaving a nonequilibrium state, through chemical reactions, and through the buildup of by- products. Those processes typically reduce separation performance. However, there are ways to use pro- cesses that would usually be viewed as damaging. For example, considerable activity has occurred in the field of self-healing materials (Cordier et al., 2008; Wu et al., 2008), and some of the ideas in this field might have considerable value for separation materials. Controlled degradation can be used as a tool to enable the synthesis of materials not readily made by direct means (Jayachandrababu et al., 2017). Be- cause temporal changes in separation systems are of such importance, there is great value in exploring strategies to reverse or avoid the changes in ways that go beyond simply slowing down or passivating ma- terials against apparently inevitable processes. SCIENTIFIC CHALLENGES IN DEFINING STANDARD SYSTEMS, SAMPLES, AND METHODS The diversity and complexity of materials and processes that can be used in chemical separations are both signs of the opportunities and a challenge to the research community. To maximize the effectiveness of research on chemical separations and to drive the development of advanced techniques, community efforts to enhance the reliability and reproducibility of data and to create well-defined model systems that can be characterized by an array of orthogonal tools are needed. The associated challenges cut across all aspects of chemical separations. Validating Data The importance of data reproducibility is relevant in all fields of scientific research; separation sci- ence is no exception. There are already examples in which well-defined material standards have been es- tablished for specific applications through interlaboratory studies (Nguyen et al., 2018). Finding effective ways to define standards of that kind that are widely adopted and that address key potential sources of variance among experiments would have great value. The challenge intersects with the challenges already listed above. For example, efforts to define “standard” mixtures that represent key aspects of the complex mixtures in a particular field of interest or to define protocols for studying aging and degradation would be valuable. Efforts to use meta-analysis of the large amount of existing literature on many classes of sep- aration materials might also point to efficient strategies to improve the reliability of available data (Park et al., 2017). Developing Well-Characterized Model Systems The opportunities associated with developing well-characterized model systems are distinct from those associated with data reliability. Such approaches as computational modeling can potentially be used to explore a wide array of materials or operating conditions. A key challenge is to validate the underlying computational methods and delineate the limits of their accuracy. In that and similar circumstances, enormous value comes from using well-defined materials that can be deeply characterized with a broad variety of experimental techniques. Model systems can also be fertile testing grounds for fundamental Prepublication Copy 55

A Research Agenda for Transforming Separation Science descriptions of the phenomena that control a material’s performance. Strong connections between funda- mental and applied researchers and among experts able to apply and use diverse scientific tools and well- defined model systems will create many opportunities to accelerate development of fundamental under- standing and application-inspired insights. SCIENTIFIC CHALLENGES IN ACCELERATING CHEMICAL SEPARATIONS WITH DATA SCIENCE The recent emergence of powerful and readily accessible machine learning and data-science meth- ods has created interest in diverse research disciplines. In many ways, chemical separations are ripe for advances based on those methods inasmuch as the number of separation materials and operating environ- ments and the span of chemical challenges are far larger than can be systematically addressed through direct experiments. In principle, data science has the potential to accelerate progress in all aspects of chemical separations. Initial efforts have been made, but the use of data-science methods in chemical sep- arations is nascent at best. The gaps that exist in applying these methods to chemical separations are not in the existence and availability of computational methods—appropriate methods are widely accessible—but rather in the availability of appropriate data. Early experience in using data-science methods in various chemical fields suggests that the insightful engagement of people who have deep domain expertise and can curate the available data will be critical for success. If data-science efforts are to lead to long-term successes in chemical separations, approaches that mesh these efforts with the domain-specific challenges outlined earlier in this chapter will be required. CONCLUSIONS Chemical separations are entering an era in which fundamental advances will be possible. In broad terms, this fertile intellectual landscape will include challenges associated with the intrinsic characteristics of separations—selectivity, capacity, and throughput—and challenges associated with the evolution of separation systems during use. There are large gaps in both areas, and progress in both will be critical for major advances in the field. A factor that is common to both is the need to consider complex environ- ments rather than clean, model separations. The difficulties of understanding and controlling the phenom- ena that appear in complex chemical environments and in situations that span large dynamic ranges should become defining features of fundamental work in chemical separations. The research community’s ability to address the challenges described above will be greatly en- hanced if efforts are made to adopt approaches that maximize the utility of new work. Seeking communi- ty standards that enhance data reproducibility is an example that will have value throughout the field. In a similar vein, carefully selected model materials or separation systems that are thoroughly characterized by multimodal experiments and can provide a firm basis for modeling and simulation or development of en- hanced experimental methods will have great value. Finally, the tools of data science are likely to be val- uable in chemical separations, provided that they are thoughtfully combined with a domain-driven formu- lation of research questions and extensive efforts to curate data that can provide confidence in the applicability of the models that will emerge. REFERENCES Anderson, N. L., and N. G. Anderson. 2002. The human plasma proteome: history, character, and diagnostic pro- spects. Molecular & cellular proteomics: MCP 1 (11):845-867. doi: 10.1074/mcp.R200007-MCP200. Cordier, Philippe, François Tournilhac, Corinne Soulié-Ziakovic, and Ludwik Leibler. 2008. Self-healing and thermoreversible rubber from supramolecular assembly. Nature 451:977. doi: 10.1038/nature06669. 56 Prepublication Copy

Gaps and Challenges El Rassi, Z., and C. Puangpila. 2017. Liquid-phase based separation systems for depletion, prefractionation, and enrichment of proteins in biological fluids and matrices for in-depth proteomics analysis—An update covering the period 2014–2016. Electrophoresis 38 (1):150-161. doi: 10.1002/elps.201600413. Henderson, M. A. 2002. The interaction of water with solid surfaces: fundamental aspects revisited. Surface Science Reports 46(1-8):1-308. Jayachandrababu, Krishna C., Souryadeep Bhattacharyya, Yadong Chiang, David S. Sholl, and Sankar Nair. 2017. Recovery of Acid-Gas-Degraded Zeolitic Imidazolate Frameworks by Solvent-Assisted Crystal Redemption (SACRed). ACS Applied Materials & Interfaces 9 (40):34597-34602. doi: 10.1021/acsami.7b11686. Keller, G.E., II, 1987. Separations: New Directions for an Old Field. AIChE Monograph Series 83(17). Koros, W. J., L. Xu, M. K. Brayden, M. V. Martinez, and B. A. Stears. 2016. Hollow Fiber Carbon Molecular Sieve Membrane and Preparation and Use Thereof. USA: Dow Global Technologies LLC; Georgia Tech Research Corporation. Nguyen, H. G. T., L. Espinal, R. D. van Zee, M. Thommes, B. Toman, M. S. L. Hudson, E. Mangano, S. Brandani, D. P. Broom, M. J. Benham, K. Cychosz, P. Bertier, F. Yang, B. M. Krooss, R. L. Siegelman, M. Hakuman, K. Nakai, A. D. Ebner, L. Erden, J. A. Ritter, A. Moran, O. Talu, Y. Huang, K. S. Walton, P. Billemont, and G. De Weireld. 2018. A reference high-pressure CO2 adsorption isotherm for ammonium ZSM-5 zeolite: re- sults of an interlaboratory study. Adsorption 24 (6):531-539. doi: 10.1007/s10450-018-9958-x. Park, J., J. D. Howe, and D. S. Sholl. 2017. How Reproducible Are Isotherm Measurements in Metal-Organic Frameworks? Chemistry of Materials 29 (24):10487-10495. doi: 10.1021/acs.chemmater.7b04287. Robeson, L. M. 2008. The upper bound revisited. Journal of Membrane Science 320 (1):390-400. doi: 10.1016/ j.memsci.2008.04.030. Seader, J. D., E. J. Henley, and D. K. Roper. 2016. Separation Process Principles: 4th Edition. Wiley. United States. Topsøe, H. 2003. Developments in operando studies and in situ characterization of heterogeneous catalysts. Journal of Catalysis 216 (1):155-164. doi: 10.1016/S0021-9517(02)00133-1. Wu, C., J. Duan, T. Liu, R. D. Smith, and W.-J. Qian. 2016. Contributions of immunoaffinity chromatography to deep proteome profiling of human biofluids. Journal of Chromatography B 1021:57-68. doi: 10.1016/j. jchromb.2016.01.015 Wu, D.Y., S. Meure, and D. Solomon. 2008. Self-healing polymeric materials: A review of recent developments. Progress in Polymer Science 33 (5):479-522. doi: 10.1016/j.progpolymsci.2008.02.001. Prepublication Copy 57

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Separation science plays a critical role in maintaining our standard of living and quality of life. Many industrial processes and general necessities such as chemicals, medicines, clean water, safe food, and energy sources rely on chemical separations. However, the process of chemical separations is often overlooked during product development and this has led to inefficiency, unnecessary waste, and lack of consensus among chemists and engineers. A reevaluation of system design, establishment of standards, and an increased focus on the advancement of separation science are imperative in supporting increased efficiency, continued U.S. manufacturing competitiveness, and public welfare.

A Research Agenda for Transforming Separation Science explores developments in the industry since the 1987 National Academies report, Separation and Purification: Critical Needs and Opportunities. Many needs stated in the original report remain today, in addition to a variety of new challenges due to improved detection limits, advances in medicine, and a recent emphasis on sustainability and environmental stewardship. This report examines emerging chemical separation technologies, relevant developments in intersecting disciplines, and gaps in existing research, and provides recommendations for the application of improved separation science technologies and processes. This research serves as a foundation for transforming separation science, which could reduce global energy use, improve human and environmental health, and advance more efficient practices in various industries.

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