Summary
Separation science plays a critical role in our society. Chemical separations are critical in providing the foods and services that are needed to maintain our standard of living and quality of life. Without separations, access to such necessities as chemicals, medicines, clean water, safe food, and energy sources would not be possible. Although chemical separations are integral to numerous industrial processes, they are not always considered during product development. That inattention results in chemical processes that are inefficient or have adverse effects or results. A focus on separation science is needed to overcome those issues and ultimately to improve human health, to reduce the adverse effects of industrial activities, and to develop a sustainable chemical enterprise that can drive the economy.
Given the importance of separation science, the U.S. Department of Energy, the National Science Foundation, and the National Institute of Standards and Technology asked the National Academies of Sciences, Engineering, and Medicine (the National Academies) to develop a research agenda for fundamental research in separations to transform the field and provide opportunities for a paradigm shift.1 As a result of that request, the National Academies convened the Committee on a Research Agenda for a New Era in Separation Science, which prepared this report.
The committee’s report highlights the status of separations today and describes major advances that have occurred in the last 30 years in the field of separations and intersecting disciplines that have had or could have major effects on the field. The committee emphasizes that its report does not provide a comprehensive review of all the advances that have occurred; rather, it describes the advances that have affected separations, outlines remaining gaps and challenges, and suggests key research directions. The identified research directions serve as a foundation for the committee’s research agenda. The committee emphasizes that it was specifically tasked with developing a fundamental research agenda as opposed to undertaking a focused empirical investigation of particular separation processes. Understanding the fundamentals will lead to increased knowledge of how complex mixtures can be separated in a controlled manner and, therefore, advance separation science.
The research agenda presented here focuses on two major themes: designing separation systems that have high selectivity, capacity, and throughput; and understanding temporal changes that occur in separation systems. In addition to the two broad research themes, two cross-cutting topics are explored: establishing standards to enhance reproducibility and adapting data-science methods to accelerate the development of separation systems. Progress on the research directions outlined for each theme will provide a strong foundation for transforming separation science, which could reduce global energy use, improve human and environmental health, and advance more efficient practices in industry.
SEPARATION SCIENCE
The United States has historically held a leadership role in separations and more broadly in science and technology. However, recent advances and investments globally threaten its leadership role. The last 15 years have witnessed a tremendous growth in publications focused on separations from China and in implementation of technology in other areas of the world more rapidly than in the United States. Maintaining U.S. leadership in separation science will require a dedicated effort and commitment by U.S. researchers. As highlighted by the 1987 National Academies report Separation and Purification: Critical Needs and Opportunities, “if a greater portion of the scientific base and common concepts that underlie separations had been known, progress could have been made more rapidly and efficiently.” Although the field of separations has changed in the last 30 years, fundamental sci-
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entific research is still needed to move the field forward to make progress in national needs and opportunities.
Since the 1987 report, the separations community has contributed to methodological and technical advances, including advances in material synthesis, instrumentation for characterization, and molecular modeling and simulation. With those advances, however, the complexity of chemical separations that are needed today and that can be imagined for the future has expanded considerably. And many of the high-priority research needs and opportunities identified by the 1987 report are still relevant.
Thematically, the key challenges identified by this committee can be grouped into topics associated with selectivity, capacity and throughput and topics associated with how separation systems change over time in operating environments. Although selectivity, capacity, and throughput are intrinsic characteristics of separation systems, their relative importance in a particular system will depend on its eventual application. Many of the emerging fundamental questions regarding selectivity, capacity, and throughput are related to the measurement and performance of separation materials in complex and highly variable environments in contrast with the simplified test cases that form the basis of most current fundamental research. Furthermore, there are large gaps in the current understanding of the fundamental chemical processes that control changes in separation systems that occur with time, especially in complex and varying environments. To address the key challenges in separations, the committee developed the research agenda that is summarized below.
RESEARCH AGENDA
As noted above, the research agenda is divided into two themes. Each has several proposed directions. Progress in all the research directions is needed if separation science is to be truly transformed. Focused efforts on improving selectivity, capacity, and throughput and on controlling how separation systems change will lead to better insights into fundamental principles and guide technological innovation. The research directions are presented briefly here; more detailed discussion is provided in Chapter 5. The committee views each research direction as a recommendation for study and exploration. The committee chose not to set priorities for the various research directions because although advances in each will have major effects, advances in no single research direction will be sufficient to transform the field as a whole.
Theme 1: Designing Separation Systems That Have High Selectivity, Capacity, and Throughput
The appropriateness of a material for a particular separation is most commonly judged by its selectivity for the desired compounds and by the capacity or throughput that it can achieve. Overall, fundamental knowledge on how matter interacts in complex environments is insufficient for the design and manufacture of highly selective separation systems that have high capacity and throughput. Separation scientists consider many concepts when they envision a separation system, such as reversible-reaction chemistry, enthalpic attractions between molecules, and differing diffusivities based on molecular size. A much fuller understanding of such concepts will enable separation systems that are more selective, have higher capacity and throughput, and can be effective for more complex mixtures and in more challenging environments. Development of separation systems with those properties will also enable effective intensification of separation processes.
Advance the Understanding of Complex Systems
The biggest challenge in separation science is to understand and design for complex systems. Tremendous advances have been made in increasing the capacity of a target species and in improving selectivity for removal of one component from a binary mixture. However, separation materials seldom perform as desired when used, for example, with multicomponent mixtures or with highly dilute or concentrated species. The ability to understand and design separation systems for complex mixtures under various realistic conditions will be a turning point for the separations community and is a key to transforming
separation science. Understanding complex systems will require the development of new measurement and simulation techniques, the creation of systems that permit analysis of all analytes without the use of preconcentration methods, and the development of approaches in which the analyte can be completely eluted or removed from a sorbent or stationary phase.
Explore the Entire Array of Thermodynamic and Kinetic Mechanisms
Separation scientists have explored numerous mechanisms to achieve chemical separations with specially designed materials, such as electrostatic interactions, hydrophobic interactions, molecular sieving and size exclusion, and reversible chemical reactions. However, researchers tend to focus on and optimize few mechanisms (usually only one) for any particular separation. In contrast, most separations in nature involve myriad complementary interactions and mechanisms. Following nature’s lead, researchers could explore a wide array of multiple forces, entropic strategies, and cooperative binding mechanisms; a wide range of chemical transformations; and new ways to determine and control differential rates of species transport to facilitate separations.
Characterize the Interface and Understand the Interfacial Forces
Virtually every chemical separation involves the transfer of species from one phase into another phase or the transport of species through a different phase and thus involves an interface or interfacial region.2 However, fundamental science related to interfaces usually focuses on whether transport through an interface is the rate-limiting step, although even this question is not investigated in some cases. With the advent of surface-sensitive experimental spectroscopic tools, researchers now have the ability to probe the structure and dynamics of all types of interfaces that are present in separation systems at the molecular level. That ability opens the possibility of designing and controlling the interface (and the interfacial region that it influences) to facilitate separation. For example, interfaces could be designed to control the concentration of particular species at the surface or to discriminate among molecules selectively on the basis of small differences in size, shape, or chemical functionality. Using and advancing experimental and computation tools to characterize interfaces, especially under operando conditions, will provide information that should enable control of the interfacial phenomena and enhance separations.
Understand the Physical Changes That Result from External Forces
External forces, such as temperature and pressure, can cause physical changes in a separation material, and these changes can affect the affinity of species for that material. Those phenomena, if adequately understood, present the possibility of controlling the structure of separation materials on length scales that allow better control of molecular separations. There are also opportunities to apply nontraditional external driving forces, such as microwaves, optical fields, and magnetic fields.
Theme 2: Understanding Temporal Changes That Occur in Separation Systems
Separation materials can undergo chemical, physical, or structural changes over time with use, which can alter their selectivity, capacity, and throughput. The factors behind and origins of the changes are fundamental to separation science and need to be understood if separation systems that will be robust in the presence of complex environments are to be designed. Although robustness includes the ability to handle a wide array of mixtures and conditions, a key component of robustness is that a separation material’s performance changes little as a function of time.
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2 The interface is the area of contact between a bulk phase and the interfacial region. The interfacial region is the domain between two bulk phases whose structure and extent are determined by the molecular interaction between the bulk phases.
For any particular separation material, the first challenge is to identify the factors that cause it to change with time. Three of the most common factors are the evolution from a nonequilibrium or metastable state toward an equilibrium state, various chemical reactions that affect the separation system, and interactions of the separation system with unwanted species in the mixture. Suggested research directions for those three factors and a research direction related to the development of adaptive systems are described below.
Determine Changes from Nonequilibrium States That Affect the Chemical and Physical Properties of Separation Materials
Some of the separation materials developed in recent years with the most attractive selectivities, capacities, and throughput are not in their lowest free-energy thermodynamic state; that is, they are nonequilibrium or metastable materials. Glassy polymers are a relevant example, and some porous zeolitic and metal organic framework solids can also be in a nonequilibrium state. Experimental studies, coupled with theory and computation, could be performed to identify aging mechanisms and to understand what controls the rate of change toward lower-energy configurations so that separation scientists can achieve control over the free-energy landscape in such a way that nonequilibrium phases can be purposefully synthesized and their evolution controlled.
Determine the Identity and Rates of Fundamental Chemical Reactions That Can Change Separation Materials and How the Reactions Are Influenced by Operating Conditions
In many cases, chemical reactions that irreversibly alter materials (such as hydrolysis, radiolysis, oxidation, thermal decomposition, oligomerization, and depolymerization) can degrade separation systems. The degradation rates can vary dramatically with temperature, analyte, impurity concentrations, and other physical conditions. There is a pressing need to understand the fundamental reaction pathways that control changes in the performance of separation materials with time. Researchers need to identify reactions and reaction products and determine reaction rates. That information then needs to be combined with modeling and simulation to predict degradation from unwanted chemical reactions so that preventive strategies can be identified. A long-term challenge for experimentalists and computational modelers in this field will be to describe the long-term cumulative effects of degradation that takes place as separation materials evolve during extended use.
Understand the Fate of Unwanted Products
A variety of reaction products can appear in separation systems over long periods and ultimately degrade the performance of separation materials. For example, solids can form through agglomeration or precipitation and block the pores of solid absorbents and membranes. Furthermore, the buildup of degradation products and impurities on a porous surface that has high surface area can provoke additional unforeseen catalytic reactions. Therefore, it is important to understand interactions of the degradation products with separation materials, the relevant phase equilibria, and the fundamentals of nucleation and growth, especially in flow fields. Overall, the fundamental chemistry and phase equilibria of decomposition needs to be better understood and incorporated into the design of separation materials at an early stage of discovery and testing. If the generation of degradation products cannot be inhibited, perhaps the separation system can be tailored to result in products that do not reduce its robustness.
Explore Alternative Strategies to Address Temporal Changes in Separation Systems
A full understanding of degradation mechanisms and rates and of the fate of degradation products will suggest strategies for dealing with temporal changes. One strategy is to reduce the rate of change of the separation system, for example, through adjustments in operating conditions or the use of additives. Another strategy that could be explored especially in situations in which degradation cannot be avoided is planned degradation in which conditions and chemistries are controlled to produce relatively benign degradation products that do not adversely affect the performance of the system. Another strategy for addressing changes in a separation material is to design the materials to be self-healing.
Research in this field will need to combine modeling and simulation with experimental measurements to understand and predict material degradation and the system response to dynamic conditions and possible process disturbances. Only in that way will researchers be able to develop effective and robust (long-lasting and widely applicable) separation systems.
CROSS-CUTTING CONCEPTS
Three concepts emerged throughout the development of each research theme regardless of specific research direction: the importance of synthesis science; the need for standard systems, samples, and methods; and the need to incorporate data science, molecular modeling, and simulation. Synthesis science has been heavily researched and reported on by several U.S. agency efforts and is not covered in detail in this report.
Standard Systems, Samples, and Methods
New separation materials continue to be developed. Often, the separation materials are designed, synthesized, formed, and characterized in individual laboratories with different sets of characterization equipment and different experimental testing protocols. The lack of standards makes it challenging to compare findings easily and to assess reproducibility quickly. Standard systems, samples, and methods for separation materials are critically needed. The lack of standard systems, samples, and methods for separation materials is much more than inconvenient: it is keeping the separations community from moving forward. This deficiency must be addressed if the research agenda to transform separation science is to be successful.
Recommendation: The National Institute of Standards and Technology, in cooperation with the research community, should identify materials and testing protocols for each type of separation material or system that can be used as reference standards.
Adapting Theory and Data Science for Separations
The vision for the future is to develop predictive models for radically advancing the design of separation materials and systems. Computational methods could ultimately allow the tailoring of materials for targeted separations with a desired performance and define synthesis strategies to create tailored materials. To reach that state, the complexity and realism of models for key aspects of separations need to change substantially. Fields in which data science and predictive modeling and simulation can help to advance separation science include solvation effects, solution structuring, and interfacial structure; thermodynamic separation mechanisms and molecular transport properties; materials evolution in complex environments; identification of synthesis paths and strategies; model validation for disordered and responsive media; and the effect of external stimuli on separation media.
Recommendation: The research community should use data science, modeling, and simulation with experimental measurements to develop a fundamental understanding of separation materials in complex environments and at multiple scales.
IMPLEMENTATION OF THE RESEARCH AGENDA
Implementing the research agenda will require the reinvigoration of a vibrant separation-science and engineering community of people who work together to train a new generation of separation scientists and take advantage of advanced experimental and computational tools. The revitalization will require reversing the trend in the number of separation-science faculty in the United States (a 40% decrease in top analytical-chemistry programs and a 30% decrease in top chemical-engineering departments since 1987). In addition, undergraduate and graduate separation-science courses in chemistry and chemical-engineering departments should be carefully evaluated. A survey conducted by the American Institutes of Chemical Engineers found that most undergraduate chemical-engineering separation-science courses still spend the majority of class time on distillation;3 this might not convey the excitement of new advances in separation technology or the potential for alternative or hybrid separation methods that are less energy-intensive or solvent-intensive.
Recommendation: Because separation science is integral to chemistry and chemical engineering, these academic departments should provide high-quality training in separations.
Researchers in chemistry and chemical engineering are among the most likely to identify themselves as separation scientists, but communication and collaboration between the two groups—which have strongly complementary skills, knowledge, and approaches—is not sufficient. Every opportunity to improve such interactions—whether through conferences, workshops, joint courses, contact at national user facilities, or research funding opportunities—should be pursued.
Recommendation: The chemistry and chemical engineering separation communities should seek opportunities for substantive interactions that will inspire creativity and the integration of concepts in research and education.
The need to leverage discoveries and tools from a wide variety of disciplines beyond chemistry and chemical engineering will continue to be critical in the new era of separation science with support by funding agencies. Multidisciplinary, multi-investigator collaborations with principal investigators drawn from different disciplines with diverse expertise will be needed to perform all the theoretical, computational, and experimental research needed to advance the frontiers. Intersecting disciplines include material science, physics, biochemistry, environmental engineering, information science, molecular biology, and such subfields as catalysis, surface science, nanoscience, spectroscopy, modeling and simulation, and transport phenomena.
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3 The survey was conducted by the AIChE Education Division. Data were solicited from 166 chemical-engineering programs in the United States that are accredited by the Accreditation Board for Engineering and Technology, Inc. The response rate was 36%, and the data are posted on the AIChE web site (see https://www.aiche.org/community/sites/divisions-forums/education-division/how-we-teach-surveys).
Recommendation: Federal agencies should promote cross-fertilization of separation researchers from intersecting disciplines and encourage collaborative projects as a key priority.
CONCLUDING REMARKS
The key research directions identified in this report will transform the way in which separation systems are designed. To implement the research agenda, cross-fertilization among chemists, chemical engineers, and researchers in many other disciplines and subfields could reinvigorate the community and improve training of the future workforce. Advances in data science and machine learning will facilitate the design and synthesis of robust separation materials through improved modeling and simulation. Newly developed experimental tools, from laboratory-scale instruments to large-scale capabilities available in U.S. national laboratory user facilities, will provide critical connections between fundamental properties and macroscopic-separation performance. Clearly defining reference materials and testing protocols will help to improve reproducibility and measurement quality.
Support that ranges from education and resources to steady funding of individual and collaborative research will be essential for scientific progress. If successfully implemented, the transformation of separation science could affect analytical-scale and process-scale industries and lead to greater U.S. economic competitiveness, a more sustainable chemical-manufacturing ecosystem, and improved human health and environment. The transformation could also affect affiliated fields of science and engineering and lead the way toward design of hierarchical materials for numerous other applications.
