This edition first published 2020
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Fossil fuels are a relatively inexpensive source of energy, and the combustion of these fuels has enabled significant technological advances, has fostered prosperity, and largely powers the global economy of today. Fossil fuels are used over a broad range of sectors including transportation, the industrial sector, and the generation of electricity. The combustion of fossil fuels, however, results in the emission of carbon dioxide into the atmosphere, which leads to negative environmental impacts. Despite the development and growth of renewable energy sources, fossil fuels will continue to play a key part in the energy landscape for the foreseeable future as the global demand for energy continues to grow at an unprecedented rate. Carbon capture, which is a process where carbon dioxide is separated from power plant effluents or industrial processes, offers a technological solution to reduce carbon dioxide emissions while enabling the continued use of fossil fuels. Though carbon capture technologies currently exist, new materials and processes are needed to drive technological advances for more energy‐efficient and cost‐effective separation of carbon dioxide from a mixed gas stream. The importance of this topic is surely reflected in the heightened interest across many sectors including industry.
This book aims to highlight the current state of the art in materials for carbon capture, providing a comprehensive understanding of separations ranging from solid sorbents to liquid sorbents and membranes. The knowledge, expertise, and dedication of the diverse group of contributors have made this book a reality. We feel this book will be helpful to those new to the area of carbon capture, affording an overview of the novel materials currently being explored. Graduate students will find this book useful both as an introduction to the various materials that are on the cutting edge of separations and as a way to expand their fundamental understanding of the separations process. Hopefully, it will also inspire these graduate students and spark their imagination to go beyond the novel materials highlighted in this book and develop new materials with enhanced separations properties. Even experts in the field, experimentalists and theorists alike, will benefit from the diverse and unconventional topics covered in this book. The combined efforts of experts and those new to the field, experimentalists and theorists, scientists and engineers will foster discovery and innovation in carbon capture as well as storage and utilization. We hope that readers of all levels will enjoy this book and discover the wonder of this separations process while also being inspired to contribute their knowledge to a global challenge that affects us all.
We are supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. We are grateful to Sarah Higginbotham and Emma Strickland of Wiley for working with us on the book from proposal to production and to Aruna Pragasam and Adalfin Jayasingh of Wiley for helping us deliver this book. Jianbo Xu helped index the book. We thank all the contributors to this book for their collaboration, time, and patience.
De‐en Jiang1, Shannon M. Mahurin2, and Sheng Dai2, 3
1 Department of Chemistry, University of California, Riverside, CA, USA
2 Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA
3 Department of Chemistry, University of Tennessee, Knoxville, TN, USA
Burning fossil fuels for electricity and transportation has led to steadily increasing CO2 levels in the atmosphere, as recorded in the Keeling curve [1], and, consequently, global warming. This concern has become a major driving force for a larger share of renewable energy in power generation and for electrifying transportation. However, coal‐fired and natural gas‐fired power plants have a long lifetime, which makes post‐combustion carbon capture necessary. In addition, pre‐combustion carbon capture will be an important part of clean‐coal technology. Removal of CO2 from natural gas is also important, especially given the shale‐gas boom. Moreover, direct air capture of CO2 has also been explored by many, since there is already a large amount of emitted CO2 in the air. Hence, carbon capture and storage (CCS) is important for mitigating global warming and climate change [2].
Novel materials hold the key to energy‐efficient carbon capture. As a frontier research area, carbon capture has been a major driving force behind many materials technologies. This book aims to present an overview of the advances in materials research for carbon capture, beyond the commercial amine‐based solvent‐sorption technologies. Broadly speaking, carbon‐capture materials can be divided into two categories: sorbents and membranes. Common sorbents are high‐surface‐area porous materials, such as zeolites, metal‐organic frameworks (MOFs), covalent‐organic frameworks (COFs), and amorphous porous carbonaceous materials. Membranes are mainly of the polymeric type, while inorganic, carbonaceous, and mixed‐matrix membranes (MMMs) are being actively explored.
MOFs are promising large‐capacity adsorbents for CO2 due to their great chemical tunability in controlling the pore size, pore shape and topology, metal‐site chemistry, and linker functional groups [3]. In Chapter 2, Ge and Ma present an overview of the MOF materials for carbon capture, focusing on the correlation between MOF structure and CO2 uptake and tabulating the best‐performing MOFs; they also briefly discuss pure MOF membranes and MOF‐containing mixed‐matrix‐membranes.
One weakness limiting the application of many MOFs in capturing CO2 from water‐vapor‐saturated flue gas is their sensitivity to moisture. Porous carbonaceous materials, on the other hand, are both chemically and thermally stable. They are usually made from pyrolysis of a carbon‐atom‐containing precursor that can be either a polymer or a small molecule [4]. At the high‐temperature‐treatment end (∼900 °C or higher), the carbon content is high (>90 mol%), and the resulting materials are just called porous carbons. In Chapter 3, Zhang and Lu review the different approaches to make porous carbons, from the perspectives of templates and precursors, and their performances for carbon capture as adsorbents.
Ben, Qiu, and their workers have pioneered the design and synthesis of a different type of porous carbonaceous materials called porous aromatic frameworks (PAFs), which can be visualized by replacing all the CC bonds in the diamond with groups such as the biphenyl, leading to a material with a huge surface area of over 5000 m2 g−1 [5]. PAFs have generated a lot of interest as a material platform for gas storage and separation. In Chapter 4, Ben and Qiu review PAFs for carbon capture and strategies for their further improvement.
Computational modeling and virtual screening are playing an increasingly important role in materials discovery for catalysts, batteries, thermoelectrics, and topological phases, to name a few. So carbon capture is not an exception. In Chapter 5, Jain, Babarao, and Thornton comprehensively review the computational methods, candidate materials, and criteria for virtual screening of materials as membranes and sorbents for carbon capture. Moreover, they show the physical insights that can be gained from computational modeling in understanding the many factors that come into play.
In Chapter 6, Jiang and workers further summarize the advances in using computational modeling to guide the development of ultrathin membranes based on 2D materials such as graphene for gas separations. Interlayer‐spacing tuning exhibits great potential in control of molecular and ionic transport in 2D membranes [6–8]. The field of 2D membranes for gas separations was to a large extent initiated by the original proof of concept of one‐atom‐thin membranes for gas separations by Jiang et al. [9]. In this chapter, they review the progress made both experimentally and computationally in this field since their original work in 2009, focusing on the computational aspects for guiding future experimental developments.
Polymeric membranes are commercially used for gas separations and water desalination [10]. Their performances are limited by a trade‐off between selectivity and permeability called the Robeson upper bound [11]. In Chapter 7, Bara and Horne review the polymeric membranes for CO2 separation for different types of polymers; they also briefly touch upon facilitated transport and membrane contactors. In Chapter 8, Huang and Dai present an overview of carbon‐based membranes for CO2 separation.
Increasing materials complexity has been a key driver in recent advances in membrane separations to leverage both interactions and transport via different components and building blocks of the composite materials [12]. The complexity built in the composite materials supplies a large space of imagination for use‐inspired fundamental studies via mixing and matching of materials, a point emphasized in a special issue of Science (2 November 2018). In the context of gas separations, the best example is the MMMs [13]. Strategies in designing MMMs to overcome the upper bound include the use of nano‐sized or nanosheet‐shaped molecular sieving fillers with a polymer and the elimination of the interfacial gaps [10]. In Chapter 9, Bae and coworkers review composite materials for carbon capture in terms of both adsorbents and membranes. Dendrimers provide a different approach toward materials complexity. In Chapter 10, Taniguchi discusses how poly(amidoamine) dendrimers can be used for carbon capture.
Ionic liquids (ILs) as a nonvolatile but versatile medium have attracted great interest in the areas of separations, energy storage, and catalysis, among others. Advanced ionic systems, such as confined ionic liquids [14], poly(ionic liquid)s [15], and porous ionic polymers [16] offer many opportunities in utilizing the long‐range Coulombic interaction to tune the structure and assembly of the molecular building blocks that impact molecular/ionic transport in either a sorbent [17] or a membrane setup [18,19]. In Chapter 11, Pan and Wang summarize the recent advances in using ionic liquids for chemisorption of CO2, while in Chapter 12, Mahurin and coworkers review IL‐based membranes for CO2 separation.
In sum, this book is aimed at presenting to the reader the latest advances in the materials aspect of carbon capture, drawing from the contributors' expertise. This field is still quickly advancing, driven by the urgent need to mitigate carbon emissions. Although there are still developments that are not covered in the following 11 chapters, we hope that they do present some of the most important classes of materials currently being pursued for carbon capture.