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This book shows the promising future and essential issues on the storage of the supercritical gases, including hydrogen, methane and carbon dioxide, by adsorption with controlling the gas-solid interaction by use of designed nanoporous materials. It explains the reason why the storage of these gases with adsorption is difficult from the fundamentals in terms of gas-solid interaction. It consists of 14 chapters which describe fundamentals, application, key nanoporous materials (nanoporous carbon, metal organic frame works, zeolites) and their storage performance for hydrogen, methane, and carbon dioxide.

Thus, this book appeals to a wide readership of the academic and industrial researchers and it can also be used in the classroom for graduate students focusing on clean energy technology, green chemistry, energy conversion and storage, chemical engineering, nanomaterials science and technology, surface and interface science, adsorption science and technology, carbon science and technology, metal organic framework science, zeolite science, nanoporous materials science, nanotechnology, environmental protection, and gas sensors.



Chapter 1. Introduction

The depletion of the global oil reserves and the problem of possible climate changes due to increasing levels of carbon dioxide in the atmosphere have led to an increasing search for both new alternative clean energy sources and, simultaneously, for finding a good way to safely remove the large amount of CO2 being produced with the actual energy systems. In the particular case of transportation, the main energy-consuming sector in developed countries, it accounted in the European Union (UE) in the early 2000s for over 30% of energy use and almost 70% of the oil-derived fuels demand, these figures being thought to continue increasing in the next few years [1]. A relatively recent report (2011), also from the UE, indicated that the CO2 emissions from transportation represented 42% of the total emissions from consumers, emissions from cars meaning over half of the total emissions from the transportation sector [2]. The International Energy Agency has reported that the average CO2 concentration has been recorded to be in 2016 above 400 ppm and that it is growing at a rate of more than 2 ppm per year, with the subsequently expected global climate change [3]. This is shown in Fig. 1.1 [4, 5], where the sharp increase in CO2 concentration in air since 1960 is clearly noted.
Francisco Rodríguez-Reinoso, Katsumi Kaneko

Chapter 2. Fundamental Aspects of Supercritical Gas Adsorption

Understanding of the explicit difference between supercritical gas and vapor is necessary for a better design of nanoporous materials for gas storage; we show the difference between supercritical gas and vapor with the van der Waals equation which leads to the critical point and the second virial coefficient. Detailed explanation on intermolecular interactions such as dispersion interaction and electrostatic interaction and the gas-solid interaction is given for enhancing the physical adsorption of supercritical gas by nanoporous materials. Plausible routes of quasi-vaporization for enhancement of supercritical gas adsorption on nanoporous materials are suggested. The effectiveness of the quasi-vaporization of supercritical gas with the strong interaction potential field of nanopores and the specific surface interaction is shown for supercritical nitric oxide, nitrogen and methane. The simple analytical method of supercritical gas adsorption isotherms with the aid of the quasi-vaporization can provide the quasi-saturated vapor pressure and isosteric heat of adsorption, being useful to design the better nanoporous materials for gas storage.
Fernando Vallejos-Burgos, Tomonori Ohba, Katsumi Kaneko

Chapter 3. Fundamental Science of Gas Storage

High-pressure adsorption measurement of supercritical gas needs accurate particle density which should be obtained by high-pressure He buoyancy measurement. As the surface excess mass adsorption is not greatly larger than the bulk gas contribution in the adsorbed layer, the absolute adsorption amount containing the bulk gas contribution in the adsorbed layer must be used for thermodynamic analysis and evaluation of the storage amount. The plot of the compression factor of adsorbed layer against the inverse of the average adsorbed layer density provides the Henry, virial, and cooperative types, giving information on the strength of the gas-solid interaction. The nanoporous material showing the cooperative type is promising for the storage of the target gas. Two factors of the strength of the gas-solid interaction and the surface area predict that nanopores consisting of narrow belt walls are promising for gas storage. Molecular simulation of methane in the graphitic pore over the wide temperature range from 120 to 300 K indicates an upward shift of the critical temperature of methane adsorbed in the graphitic pore. The heat of adsorption of methane in the graphitic pore without the heat-releasing mechanism elevates the temperature of the graphitic carbon by 70 K, decreasing the adsorption amount of methane by 30%; an efficient heat releasing mechanism must be installed in the storage device.
Tomonori Ohba, Fernando Vallejos-Burgos, Katsumi Kaneko

Chapter 4. Physical Chemistry and Engineering for Adsorptive Gas Storage in Nanoporous Solids

Adsorption gas storage is examined from physical chemistry point of view. Net, excess, and absolute adsorption are defined, and their relation to gas storage capacity is examined. Experimental techniques for measuring adsorption isotherms are detailed. Net adsorption particularly stands out among possible thermodynamic choices since it directly shows the advantage of having the adsorbent in a storage cylinder. In addition to storage capacity, engineering implications of Henry’s law constant, heat of adsorption, and multicomponent adsorption are examined with examples to inform material scientists who develop materials.
Orhan Talu

Chapter 5. Nanoporous Carbons with Tuned Porosity

The aim of this chapter is to provide an overview on synthetic approaches for the preparation of activated carbons with adequate porosity for the storage and separation of small gases of strategic interest (e.g., H2, CO2, and CH4). Nanoporous carbons are versatile materials that can exist in 3D architectures with high specific surface areas and pores of nanometric dimensions, which can be modulated by the choice of the precursor and the synthesis route. Even though numerous works exist on exploring approaches for the preparation of activated carbons with a controlled pore texture, we limit the discussion here to the control of the microporosity in carbons, since they exhibit the best performance in terms of gas storage, separation, and packing densities. We have summarized most relevant methodologies (covering conventional activations methods as well as new approaches) based on a rational material’s design to control the porosity, emphasizing on the preparation of microporous carbons.
Conchi O. Ania, Encarnacion Raymundo-Piñero

Chapter 6. Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are a relatively young class of porous materials. They consist of inorganic complexes as nodes connected by multifunctional organic molecules (linkers). Highly porous MOFs reach records in terms of storage capacities for gases and vapors. The main characteristics of MOFs responsible for the success of them as adsorbents are crystallinity, modular composition, as well as exceptionally high specific surface areas and pore volumes. The chapter outlines structural building principles of MOFs and describes a few prototypical structures with value for gas storage. Also important characteristics such as pore size distribution and inner surface functionality are controllable in MOFs by the chemistry of the building blocks. As a consequence, MOF materials provide a platform to precisely study the gas adsorption from theoretical and experimental point of view and also to reach ideal material characteristics for the adsorption of the molecules of interest. Moreover, the intrinsic flexibility of the MOFs, leading to structural transformations and unique stepwise adsorption behavior not observed for rigid porous materials, opens new horizons for the design of effective “smart” adsorbents.
V. Bon, I. Senkovska, S. Kaskel

Chapter 7. Zeolites and Other Adsorbents

Zeolites are crystalline nanoporous aluminosilicates, which have been used as selective and efficient catalysts and adsorbents in several industrial applications. Their use as adsorbents since their discovery is briefly reviewed. The main characteristics that render this group of materials and other closely related suitable for adsorptive separation applications are presented. A number of adsorption separation and/or purification processes which either use zeolites or for which zeolites have been proposed and studied as the key adsorbent are reviewed, as well. Amongst them, we find industrial applications, such as drying of gases and liquids, air separation and linear from branched hydrocarbon separations. Other separation processes still under development, such as carbon dioxide removal from post-combustion gases, methane purification, methane storage or olefin/paraffin separation, have been included in this chapter. Despite being a mature research area in adsorption, zeolite-based separation processes are still blooming because of the advent of new zeolite structures and/or compositional variants that could allow for other challenging separations in the near future. Amongst them, pure silica zeolites are found to be outstanding adsorbents since they combine high adsorption capacities and excellent regenerabilities in swing adsorption processes.
Eduardo Pérez-Botella, Miguel Palomino, Susana Valencia, Fernando Rey

Chapter 8. Methane Storage on Nanoporous Carbons

This chapter reviews the requirements for a nanoporous carbon material to be used as an adsorbent for methane storage. Due to the necessity to achieve a large gravimetric and volumetric storage capacity in real applications (e.g., onboard storage for transportation vehicles), a proper carbon material must fulfill certain requirements in terms of porous structure, pore geometry, pore size distribution, and packing density. The effect of these parameters on the methane adsorption/storage capacity will be revised both from a theoretical and experimental perspective. State-of-the-art values for excess and storage capacity for the best performing carbon materials will be summarized. Last but not least, heat and mass transfer limitations will be revised due to the dramatic effect that these scarcely explored variables can have in the performance of the final prototype.
Francisco Rodríguez-Reinoso, Joaquín Silvestre-Albero

Chapter 9. Methane Storage on Metal-Organic Frameworks

High surface area and high porosity metal-organic frameworks (MOF) are an attractive option to store methane at ambient temperature and low pressure (50 bar). More than 40 MOFs have been investigated for their methane storage ability. Like the Chahine rule for cryogenic hydrogen adsorption, it was found that for every 1000 m2 of specific surface area, one can expect 0.06 g/g of maximum methane excess adsorption. With few exceptions, this is largely independent of surface chemistry and geometry. This rule can be broken by materials with optimal pore sizes (∼7 Å) and materials with open metal sites. The thermodynamics of adsorption, as it pertains to applications of methane storage, are discussed. Pelletization of the benchmark MOF, Cu3btc2, was found to have little effect on the storage capacity of the adsorbent. However, when pilot-scale amounts of material were tested, pelletization improved thermal management and increased the permeability. Both were attributed to the extra free space around the pellets. Additionally, on the pilot scale, all MOFs showed improved permeability compared to the benchmark activated carbon.
Anne Dailly, Matthew Beckner

Chapter 10. Storage of Hydrogen on Nanoporous Adsorbents

The adsorption of hydrogen has extensively been studied on various nanoporous adsorbents with the driving force being the need to safely store this increasingly important energy vector. This chapter explores the research avenues that have been taken for the storage of hydrogen with zeolites, carbon-based materials, and metal-organic frameworks. Many studies have been devoted to characterization at 77 K and 1 bar.
This chapter highlights that few materials meet the accepted requirements for vehicular hydrogen storage at 77 K and that no material seems to be of interest for hydrogen storage at room temperature. A general need to store the hydrogen under significant pressure is evident. It is clear that there is general necessity for nanoporous materials to stimulate stronger interactions with hydrogen for an adsorptive-based solution to be envisaged, and several strategies are described to this end.
Philip L. Llewellyn

Chapter 11. CO2 Storage on Nanoporous Carbons

Porous carbons are promising adsorbents for CO2 adsorption owing to their large surface area, easy-to-design pore structure, ready availability, tunable surface chemistry, and low energy requirements for regeneration. In this chapter, the potential of porous carbons for carbon capture in different scenarios will be shown. Firstly, the fundamentals of CO2 adsorption by porous carbons will be thoroughly described. Afterward, the synthesis strategies developed so far for the synthesis and control of the textural and chemical characteristics of porous carbons from a variety of precursors will be reviewed in combination with a comprehensive analysis of the performance of the materials produced in CO2 capture at low and high pressure. Traditional activation approaches will be discussed, with reference to recent advances that have allowed a wider control over the textural properties of the materials produced. More recent developments in carbon synthesis will also be discussed, such as nanocasting approaches, carbide-derived carbons, MOF-/COF-derived carbons, and novel polymerization approaches such as the use of deep eutectic solvents (DES).
Marta Sevilla, Guillermo A. Ferrero, Antonio B. Fuertes

Chapter 12. CO2 Storage on Metal-Organic Frameworks

Metal-organic frameworks (MOFs) have received continuous attention because of their highly porous, diversified structures and potential applications as novel adsorbents. In this chapter, the recent progress in developing MOFs for CO2 capture is reviewed. Several strategies being used to improve CO2 adsorption uptake at low pressure are highlighted. These strategies are generation of interaction sites in the pores as well as controlling of pore size of MOFs, building of flexible MOFs, and construction of water-stable MOFs. In addition, the high-pressure CO2 adsorption in highly porous MOFs is reviewed. At last, the studies of CO2-loaded structures with crystallographic technique are summarized.
Yunsheng Ma, Hideki Tanaka, Ryotaro Matsuda

Chapter 13. CO2 Storage on Zeolites and Other Adsorbents

Historically carbon dioxide removal from gaseous streams has been done by applying physical or chemical absorption. However, diverse factors lead to the search for alternative technologies for CO2 separation, such as adsorption technologies. Nanoporous adsorbents are a subset of porous materials typically with porosities larger than 0.4 and porous diameter of up to 100 nm, as defined by International Union of Pure and Applied Chemistry (IUPAC) since 2015. This class spans through the three classic classes of pores, micropores, mesopores and macropores. Comparing with other types of materials, nanoporous ones possess unique properties, which make them the most desired adsorbents for this application, such as, high CO2 adsorption capacity, high selectivity, full regeneration capacity, favourable adsorption kinetics, good mechanical properties, durability and stability.
In this chapter, one of the main classes of materials applied in adsorption processes, zeolite adsorbents, is considered. In addition, other important inorganic materials for CO2 adsorption and storage, such as silicoaluminophosphates (SAPOs) and aluminophosphates (AlPOs), hydrotalcites and metal oxides, are reviewed.
Maria João Regufe, Ana Mafalda Ribeiro, Alexandre F. P. Ferreira, Alírio Rodrigues

Chapter 14. Clathrate-Mediated Gas Storage in Nanoporous Materials

Gas clathrates are solid structures constituted by water molecules forming 3D networks through hydrogen bonding and isolated gas molecules trapped in their inner cavities. These structures have been well known for the last 30–40 years due to their abundance in nature (preferentially methane hydrates) and their relevance as a future fuel source. Similar hydrate structures have been reproduced at lab scale either in bulk conditions (pure water) or in the presence of additives, including the presence of nanoporous materials. It is highly accepted in the literature that the presence of confinement effects in the cavities of these nanoporous materials must exert an important influence in the gas hydrate nucleation and growth process. This chapter summarizes some of the last achievements in the gas hydrate formation process (mainly focusing in methane hydrates) in the presence of high-surface area nanoporous materials (e.g., activated carbons, metal-organic frameworks, zeolites, clays, and silicas) and the effect of the confined space in the formation/dissociation process.
J. Silvestre-Albero
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