desorption step. A representative process for n-paraffin separation from naphtha and kerosene is UOP's. Molex process that employs a simulated moving bed.
Sorbent Technology Shuguang Deng Chemical Engineering Department, New Mexico State University, Las Cruces, New Mexico, U.S.A.
INTRODUCTION This article covers the fundamentals, status, and future developments of sorbent materials and their applications in adsorptive separation and purification processes. A sorbent is usually a solid substance that adsorbs or absorbs another type of substance. It is the sorbent that makes a sorption process a unique and different separation and purification process from others. With the rapid development in novel sorbent materials and innovative cyclic adsorption processes, sorption has become a key separation process in many process industries including chemical, petrochemical, environmental, pharmaceutical, and electronic gases. A brief review of the fundamentals of adsorption and the basic requirements for sorbent materials is presented, followed with a summary of the status of commercial sorbents and their applications. The focus of this article is placed on recent advances in novel sorbent materials including oxide molecular sieves, sol–gel derived xerogels and aerogels, metal organic framework, hydrogen storage media, p-complexation and composite sorbents, and high-temperature sorbents for oxygen or carbon dioxide sorption. A concluding section outlines the future research needs and opportunities in sorbent technology development for new energy and environmental applications.
ADSORPTION MECHANISMS AND SORBENT MATERIALS According to King, a mass separating agent is needed to facilitate separation for many separation processes.[1] The mass separating agent for adsorption process is the adsorbent, or the sorbent. Therefore, the characteristic of the sorbent directly decides the performance of any adsorptive separation or purification process. The basic definitions of adsorption-related terminologies are given in the following to clarify and standardize these widely used terms in this field. Adsorption: The adhesion of molecules (as of gases, solutes, or liquids) to the surfaces of solid bodies or liquids with which they are in contact. Encyclopedia of Chemical Processing DOI: 10.1081/E-ECHP-120007963 Copyright # 2006 by Taylor & Francis. All rights reserved.
Absorption: The absorbing of molecules (as of gases, solutes, or liquids) into the solid bodies or liquids with which they are in contact. Sorption: Formation from adsorption and absorption. Adsorbent: A usually solid substance that adsorbs another substance on its surface. Sorbent: A usually solid substance that adsorbs and absorbs another substance. Adsorbate: Molecules (as of gases, solutes, or liquids) that are adsorbed on adsorbent surfaces. ˚. Microporous: Pore size smaller than 20 A ˚. Mesoporous: Pore size between 20 and 500 A ˚. Macroporous: Pore size larger than 500 A Adsorptive separation can be achieved through one of the following mechanisms. Understanding the fundamentals of adsorptive separation mechanisms will allow us to better design or modify sorbent materials to achieve their best possible separation performance.[2–4] Adsorption equilibrium effect is because of the difference in the thermodynamic equilibria for each adsorbate–adsorbent interaction. The majority of adsorptive separation and purification processes are based on equilibrium effect. One example is to generate oxygen-enriched air or relatively pure oxygen (95%) from air using a zeolite molecular sieve 5A or 13X in either a pressure swing adsorption (PSA) or a vacuum swing adsorption (VSA) process. In this case, nitrogen is selectively adsorbed by the zeolite adsorbent, and oxygen is collected from the adsorption effluent stream. Adsorption kinetics effect arises because of the difference of rates at which different adsorbate molecules travel into the internal structure of the adsorbent. There are only a few commercial successes using adsorption kinetic difference to achieve adsorptive separation of gases. The typical example is separation of nitrogen from air using a carbon molecular sieve (CMS). The CMS adsorbent has a similar adsorption equilibrium capacity for both nitrogen and oxygen, but the diffusivity of oxygen in CMS is at least 30 times larger than that of nitrogen in CMS.[5] High-purity 2825
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nitrogen can be recovered from the adsorption effluent stream in a PSA process because oxygen moves much faster than nitrogen into the micropores of CMS adsorbent. However, the cycle time of this CMS-based PSA process is much shorter than that of a typical PSA process based on adsorption equilibrium effect. This is because there will be no separation if both nitrogen and oxygen are allowed to reach adsorption equilibrium with the CMS adsorbent. Molecular sieving effect, also called steric effect, is derived from the molecular sieving properties of some adsorbents with a microporous structure. In this case, the pore openings of the adsorbent structure are small enough to exclude large adsorbate molecules from penetrating the micropores of the adsorbent. This is the extreme case of the kinetic effect. There are several commercial applications based on this mechanism in adsorptive separation processes. One typical example is separating normal paraffin from iso-paraffin and aromatics in an adsorption process using zeolite 5A as an adsorbent. n-Paraffin, with a long straight chain, has a smaller effective diameter than the well-defined aperture of zeolite 5A. Therefore it adsorbs in the micropores of the adsorbent during the adsorption step, and is recovered from the adsorbed phase in the desorption step. A representative process for n-paraffin separation from naphtha and kerosene is UOP’s Molex process that employs a simulated moving bed with binderless zeolite 5A as an adsorbent and light paraffin as a desorbent.[6] We can define separation factor and selectivity as the ability of an adsorbent to separate molecule A from molecule B as:[7] Separation factor : aAB ¼
XA =YA XB =YB
ð1Þ
Here XA, YA are strictly equilibrium mole fractions for component A in the adsorbed phase and adsorbate (fluid) phase, respectively; as are XB, YB for component B. For equilibrium-based adsorptive separation process, the adsorbent selectivity is the same as the separation factor as defined in Eq. (1). Apparently, this definition is not applicable to other processes based on kinetic and steric effects. In a kinetically controlled adsorption process, the adsorbent selectivity depends on both equilibrium and kinetic effects. A simplified definition for adsorbent separation factor is given by Ruthven et al.:[8]
SAB
KA ¼ KB
rffiffiffiffiffiffiffi DA DB
ð2Þ
where SAB is the adsorbent selectivity, K is the adsorption equilibrium constant or isotherm slope, and D is the
effective diffusivity. Although the above equation is strictly valid under the assumptions that components A and B have independent linear adsorption isotherms and independent diffusion process, it provides a good estimate of adsorbent selectivity for kinetically controlled processes. Theoretically speaking, selectivity for adsorbents with a molecular sieving effect should be infinitely large because the larger molecules are excluded from getting into the adsorbent micropores. In reality, the adsorbent selectivity for steric effect is somewhat reduced by combining with the equilibrium effect from adsorption on the surface of large pores. So adsorption processes based on molecular sieving are usually considered as adsorption equilibrium effect. Another very important adsorbent property affecting the adsorption process is the adsorption capacity because it determines the size of an adsorbent vessel, the amount of adsorbents required, and the related capital and operating costs. The requirements for commercial sorbents are discussed briefly as follows.
Characteristics of Sorbent Materials Commercial sorbents used in cyclic adsorption processes should ideally meet the following requirements: Large selectivity derived from equilibrium, kinetic, or steric effect; Large adsorption capacity; Fast adsorption kinetics; Easily regenerable; Good mechanical strength; Low cost. The above adsorbent performance requirements can simply transfer to adsorbent characteristic requirements as follows: Large internal pore volume; Large internal surface area; Controlled surface properties through selected functional groups; Controlled pore size distribution, preferably in micropore range; Weak interactions between adsorbate and adsorbent (mostly on physical sorbents); Inorganic or ceramic materials to enhance chemical and mechanical stability; Low-cost raw materials. These basic requirements are usually proposed for adsorbents used in cyclic adsorption processes that are based on physical adsorption. There is an increasing demand for strong chemical adsorbents used in
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purification processes to remove trace contaminants from main stream fluids such as the removal of very toxic contaminants from electronic process gas streams, and the removal of toxic, or radioactive species from contaminated water. In these cases, the sorbents are used as getter materials; no regeneration is needed, and instead, the spent sorbent materials are disposed of in designated areas regulated by government environmental policies.
COMMERCIAL SORBENTS AND APPLICATIONS An excellent review and detailed coverage on commercial adsorbents and new adsorbent materials has been presented by Yang in his newly published monograph on adsorbents.[2] A very brief overview of existing commercial adsorbents is given here. Commercial sorbents that have been used in large-scale adsorptive separation and purification processes include activated carbon, zeolites, activated alumina, silica gel, and polymeric adsorbents. Although the worldwide sales of sorbent materials are relatively small as compared with other chemical commodities, sorbents and adsorption processes play a very important role in many process industries. The estimated worldwide sales of these sorbents are as follows:[2] Activated carbon: $1 billion Zeolite: $1.07 billion Activated alumina: $63 million
Cumulative pore volume, cm3/100 gm
60
Silica gel: $71 million Polymeric adsorbents: $50 million Activated Carbon Activated carbons are unique and versatile adsorbents because of their large surface area, microporous and mesoporous structure, universal adsorption effect, high adsorption capacity for many nonpolar molecules including organic molecules, and high degree of surface reactivity. They are used widely in industrial applications that include decolorizing sugar solutions, personnel protection, solvent recovery, volatile organic compound removal from air and water, water treatment, hydrogen and synthesis gas separation, and natural gas storage.[4,9,10] Activated carbons are produced in two main steps: carbonization of the carbonaceous raw materials at temperatures below 800 C in the absence of oxygen, and activation of the carbonized products.[10] The properties of activated carbon depend largely on the nature of the raw materials, the activating agents and activation conditions. For gas-phase applications, activated carbons are usually made in pellets with mostly micropores; while for liquid-phase applications, activated carbon is produced in powder form with relatively large mesopores to enhance mass transfer rate in the carbons. Fig. 1 compares the pore size distributions of major commercial adsorbents discussed in this section. Activated carbons have a broad pore size distribution like activated alumina and silica gel. Although activated carbon is thought to be ‘‘hydrophobic,’’ it does adsorb
Activated carbon
50 Silica gel 40
30 Zeolite 5A 20 Activated alumina 10
0
MSC MSC 2
5
10
20
Pore diameter, Å
50
Fig. 1 Pore size distributions for activated carbon, silica gel, activated alumina, two molecular sieve carbons (MSCs), and zeolite 5A. (From Ref.[3].)
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quite significant amount of water (>30 wt%) when relative humidity is higher than 50%. An example isotherm of water on activated carbon, along with water isotherms on other commercial adsorbents, is plotted in Fig. 2. The change from ‘‘hydrophobic’’ to ‘‘hydrophilic’’ on the activated carbon surface is attributed to the initial adsorbed water film on the carbon surface. This occurs because when the carbon surface is fully covered with a layer of water molecules, the adsorbed water molecules exhibit strong affinity to other polar molecules including water. Carbon molecular sieve (CMS) is a specially made carbonaceous material with ˚ ). The major very narrow pore size distribution (4–9 A application of CMS is in the generation of high-purity nitrogen from air in a PSA process. The representative physical properties of commercial adsorbents and their major applications are summarized in Tables 1 and 2, respectively.
Zeolites Zeolites are porous crystalline aluminosilicates that are made of assemblies of SiO4 and AlO4 tetrahedra joined together through shared oxygen atoms. The general chemical formula for zeolites is: Mx=n ½ðAlO2 Þx ðSiO2 Þy zH2 O
ð3Þ
where x and y are integers with y=x (Si=Al ratio) equal or larger than 1; n is the valance of cation M, and z is the number of water molecules in each unit cell. The tetrahedra can be arranged in many different ways to form different crystalline structures. Some zeolites
exist as minerals in nature, but all commercially important zeolites are synthetic. Zeolites are unique adsorbents owing to their special surface chemistries and crystalline pore structures. It should be pointed out that probably only 10% of $1 billion worldwide sales of zeolite is used as adsorbents; the majority of commercial zeolites are used as detergent additives (zeolite 4A), animal food additives (zeolite 4A), ion exchange, and catalyst supports. Among all commercial sorbents zeolites are probably the most extensively investigated and documented. Many excellent monographs and review articles are available.[2,11–13] Please refer to Tables 1 and 2 for properties and major applications of zeolites.
Activated Alumina Activated alumina is a porous high-surface area form of aluminum oxide with the formula of Al2O3nH2O. Commercially, it is prepared either from thermal dehydration of aluminum trihydrate, Al(OH)3, or directly from bauxite (Al2O33H2O), as a by-product of the Bayer process for alumina extraction from bauxite. Its surface is more polar than that of silica gel and, reflecting the amphoteric nature of aluminum, has both acidic and basic characteristics. Surface areas are in the range 250–350 m2=g depending on the activation temperature and the source of raw materials. Because activated alumina has a higher capacity for water than silica gel at elevated temperatures it is used mainly as a desiccant for warm gases including air, but in many commercial applications it has now been replaced by zeolitic materials in a thermal swing
Adsorption, kg H2O/100 kg adsorbent
40
E 30
20
D C B
10
A
0
0
20
40 60 Relative humidity, %
80
100
Fig. 2 Equilibrium sorption of water vapor from atmospheric air at 25 C on: (A) alumina (granular), (B) alumina (spherical), (C) silica gel, (D) 5A zeolite, and (E) activated carbon. The vapor pressure at 100% relative humidity is 23.6 torr. (From Ref.[3].)
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Table 1 Representative physical properties of commercial adsorbents Adsorbent Activated carbon
Nature
Specific surface area (m2/g)
Pore ˚) diameter (A
Porosity
Particle density (g/cm3)
Hydrophobic amorphous
Small pore
400–1200
10–25
0.4–0.6
0.5–0.9
Large pore
200–600
> 30
0.5
0.6–0.8
Zeolite
Hydrophilic=hydrophobic crystalline
600–700
3–10
0.6
1.0
Activated alumina
Hydrophilic crystalline=x-ray amorphous
200–350
10–75
0.5
1.25
Silica gel
Hydrophilic=hydrophobic amorphous 750–850
22–26
0.47
1.09
Small pore Large pore
300–350
100–150
0.71
1.62
Polymeric adsorbent
Hydrophilic=hydrophobic
450–1100
25–90
0.5
1.25
Carbon molecular sieve
Hydrophilic
400
3–9
0.5
1.0
adsorption (TSA) process. However, activated alumina has a low adsorption heat for water and other polar molecules as compared with zeolite; it is possible to regenerate activated alumina under PSA conditions. Activated alumina also demonstrates moderate adsorption affinity for carbon dioxide, which makes it a suitable sorbent for removing water and carbon dioxide from air in a PSA process. These adsorption properties of activated alumina have been explored extensively for air purification applications by industrial gas companies.[14–17] This is a perfect example to demonstrate the importance of sorbent regenerability over sorption capacity and selectivity in pressure swing adsorption processes. Activated alumina is also an excellent catalyst support. More applications and representative properties of activated alumina are listed in Tables 1 and 2.
Silica Gels Silica gel is the most widely used desiccant because of its large adsorption capacity for water (40 wt%), as shown in Fig. 2, and easy for regeneration ( 150 C, compared with 350 C for zeolites). Silica is a partially dehydrated polymeric form of colloidal silicic acid with the formula of SiO2nH2O. Its water content, which is typically about 5 wt%, is presented in the chemically bonded hydroxyl groups. Silica is an amorphous mate˚ in size, rial comprising spherical particles of 20–200 A which aggregate to form the sorbent with pore sizes ˚ and surface areas of 100– in the range of 60–250 A 850 m2=g, depending on gel density. Its surface has mainly Si–OH and Si–O–Si polar groups; this is why it can be used to adsorb water, alcohols, phenols,
amines, etc. by hydrogen bonding mechanisms. Other commercial applications include the separation of aromatics from paraffins, the chromatographic separation of organic molecules, and modified silica in chromatography columns.[2,18–20] Polymeric Adsorbents A wide range of synthetic, nonionic polymers are available for use as sorbents, ion-exchange resins, and particularly for analytical chromatography applications. Commercially available resins in bead form (typically 0.5 mm in diameter) are based usually on copolymers of styrene=divinyl benzene (DVB) and acrylic acid esters=divinyl benzene, and have a wide range of surface polarities, porosities, and macropore sizes. The porosities can be built through emulsion polymerization of relevant monomers in the presence of a solvent that dissolves the monomers and serves as a poor swelling agent for the polymer. This creates a polymer matrix with surface areas ranging up to 1100 m2=g.[2,4] The major application of polymeric adsorbents is in water treatment. The macroporous polymeric resins can be modified by attaching different functional groups to mimic activated carbon, and to replace activated carbons for certain specific applications in food and pharmaceutical industries where color contamination by the black carbons of the final products is a major concern.
NEW DEVELOPMENTS IN SORBENT MATERIALS AND APPLICATIONS The past two decades have witnessed major advances in new nanostructured sorbent materials including
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Table 2 Selected applications of commercial sorbents Adsorbent
Applications (the first molecule is the product)
Activated carbon
Hydrogen separation from syngas and hydrogenation processes Ethylene from methane and hydrogen Vinyl chloride monomer (VCM) from air Removal of odors from gases Recovery of solvent vapors Removal of SOx, and NOx Purification of helium Clean-up of nuclear off-gases Decolorizing of syrups, sugars, and molasses Water purification, including removal of phenol, halogenated compounds, pesticides, caprolactam, chlorine
Carbon molecular sieve
Nitrogen separation from air
Zeolite
Oxygen from air Drying of gases Removing water from azeotropes Sweetening sour gases and liquids Purification of hydrogen Separation of ammonia and hydrogen Recovery of carbon dioxide Separation of oxygen and argon Removal of acetylene, propane, and butane from air Separation of xylenes and ethyl benzene Separation of normal from branched paraffins Separation of olefins and aromatics from paraffins Recovery of carbon monoxide from methane and hydrogen Purification of nuclear off-gases Separation of cresols Drying of refrigerants and organic liquids Separation of solvent systems Pollution control, including removal of Hg, NOx, and SOx from gases Recovery of fructose from corn syrup
Activated alumina
Drying of gases, organic solvents, transformer oils Removal of HCl from hydrogen Removal of fluorine and boron–fluorine compounds in alkylation processes Removing of water and carbon dioxide from air in a PSA process
Silica gel
Drying of gases, refrigerants, organic solvents, transformer oils Desiccant in packings and double glazing Dew point control of natural gas
Polymeric adsorbents
Water purification, including removal of phenol, chlorophenols, ketones, alcohols, aromatics, aniline, indene, polynuclear aromatics, nitro- and chlor-aromatics, polychlorinated biphenyls (PCBs), pesticides, antibiotics, detergents, emulsifiers, wetting agents, kraftmill effluents, dyestuffs, and radionuclides Recovery and purification of steroids, amino acids and polypeptides Separation of fatty acids from water and toluene Separation of aromatics from aliphatics Separation of hydroquinone from monomers Recovery of proteins and enzymes Removal of colors from syrups Removal of organics from hydrogen peroxide
Clays (acid treated and pillared)
Removal of organic pigments Refining of mineral oils Removal of PCBs
(From Ref.[4].)
Sorbent Technology
mesoporous molecular sieves, sol–gel-derived metal oxide xerogels and aerogels, metal organic framework, p-complexation and composite adsorbents, new carbonaceous materials (carbon nanotubes, carbon fibers, superactivated carbons), high-temperature ceramic sorbents, and strong chemical sorbent materials. Although these new sorbent materials have demonstrated promising sorption properties for many existing and new applications, systematic studies on synthesis methods and characterization of these new materials are necessary to fully explore and realize their potential as commercial sorbents. The review that follows aims at attracting more research efforts to develop novel sorbent materials to meet the increasing needs of new energy, environmental, and other emerging technologies.
Oxide Molecular Sieves Microporous and mesoporous oxide molecular sieves that have the characteristics of large internal surface area and pore volume are ideal candidates for use as sorbent materials and catalyst supports of many heterogeneous catalysts. Oxide molecular sieves are generally synthesized by hydrothermal methods that involve both chemical and physical transformations within an amorphous oxide gel, often in the presence of a template species. The gel eventually converts to a crystalline material in which the template species and=or solvent molecules are guests within the channels and cages of an oxide host framework. A porous material is obtained upon removal of the guest molecules from the oxide framework. By manipulating the synthesis parameters, including starting precursors, synthesis temperature, pH, template species, drying, and calcination conditions, it is possible to tailor the pore size and shape of these porous materials for different applications. However, tailoring of porosity in oxide molecular sieves in terms of a priori structural design is extremely difficult because of the inherent complexity of the synthetic procedures employed.[21] Recent advances and applications of oxide molecular sieves have been summarized in several review articles.[2,21–23] Microporous zeolite materials synthesized with molecular templates and their applications in host–guest chemistry have been covered elsewhere.[13] A new class of silicate=aluminosilicate mesoporous molecular sieves designated as M41S was discovered in the former Mobil research laboratory by extending the concept of zeolite templating with small organic molecules to large long-chain surfactant molecules.[24] A representative member of this family is MCM-41, which has a honeycomb-shaped hexagonal arrange˚, ment of uniform mesopores in the range of 15–100 A specific surface area of 1040 m2=g, pore volume above 0.7 cm3=g, and significantly high sorption capacity for
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hydrocarbons (49 wt% for n-hexane at 40 torr and 21 C, and 67 wt% for benzene at 50 torr and 25 C).[24] Other significant members of the M41S family include MCM-48 (cubic phase), MCM-50 (stabilized lamellar phase), SBA-1 (cubic phase), and SBA-2 (cubic phase).[21] Although M41S type mesoporous oxide molecular sieves have exhibited unique properties of large surface area and exceptionally large pore volume (> 0.7 cm3=g), their large pore volume may not be attractive for gas sorption because the adsorbate–adsorbent interactions are not enhanced inside the internal pores of these materials.[2] Therefore, M14S type mesoporous oxide molecular sieves without surface modification are rarely used as sorbents. Significant research efforts were devoted to surface modification of M41S materials for different applications.[2] An amine-grafted MCM-48 sorbent, synthesized from tetraethoxysilane (TEOS), has been shown to have a surface area of 1389 m2=g, a silanol number of 8, higher thermal stability than MCM-41, high adsorption selectivity, and high capacity for both carbon dioxide and hydrogen sulfide.[25]
Sol–Gel-Derived Xerogels and Aerogels Sol–gel processing refers to the fabrication process of ceramic materials by preparation of a sol, gelation of the sol, and removal of the solvent.[26] Sols are dispersions of colloidal particles in a liquid solvent, and a gel is a solid matrix encapsulating a solvent. In a sol–gel process, the sol can be formed from a solution of colloidal powders or hydrolysis and condensation of alkoxides or salt precursors. In the latter approach, which is much more popular, primary particles of uniform size are formed and grow in a sol and connect to each other to form aggregates during gelation. These aggregates forming the network of the gel are broken apart into the primary particles in the drying step. Upon calcination and sintering, these primary particles are bound together strongly to form a very rigid solid network, and large interparticle space with uniform nanoscale pores is formed. Xerogels are obtained by drying the gels through evaporation at normal conditions under which capillary pressure causes shrinkage of the gel network, while areogels are produced by drying the wet gels at supercritical conditions where the liquid–vapor interface is eliminated, and relatively little shrinkage of the gel network occurs. Xerogels and aerogels typically have relatively large surface area, high porosity, and internal pore volume, and are ideal candidates as sorbent and catalyst support materials for many applications. The sol–gel process offers a very high flexibility to tailor xerogels and areogels for specific applications by manipulating the synthesis conditions.
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Silica xerogel is probably the most studied and documented porous material in the sol–gel system.[27,28] Although silica has several crystalline forms, only amorphous silica gel is used as a desiccant (sorbent). A microporous silica that was synthesized with ˚, TEOS as precursor has an average pore size of 6.4 A 3 pore volume of 0.24 cm =g, and Brunauer–Emmett– Teller (BET) surface area of 588 m2=g.[29] However, this material lost about 90% of its microporosity when it was heated at 600 C for 30 hr. By doping with 1.5% of alumina, the thermal stability of this microporous silica was significantly improved.[29] Crystalline sorbent materials including g-alumina, zirconia, and titania were also synthesized using the sol–gel process in Lin’s group;[29] the representative pore size distribution and pore texture data of xerogels of g-alumina, zirconia, and titania are summarized in Fig. 3 and Table 3, respectively. As shown in Fig. 3, the pore size distributions of these materials are rather narrow, with an average pore diameter of about 3 nm. Such narrow size distribution and nanoscale average pore size are determined by the primary crystallite particles. The particles of the sol–gel-derived alumina, titania, and zirconia, owing to the Ostwald ripening mechanism,[26] are usually of nanoscale size; the uniform particle size distributions of g-alumina crystallites are plate-shaped with size ranging from about 5 to 20 nm. The sol–gel-derived g-alumina consists of such plate-shaped crystallite particles, which give rise to a relatively large surface area. Crystallites of tetragonal zirconia and rutile are of more spherical shape, with a crystallite size of about 15 and 11 nm, respectively.[29]
γ-Alumina Titania Zirconia
1.6
dV/dlog(D)
1.2
0.8
0.4
0.0 2
1
3
4
5
6 7 8 9
2
3
10 Pore Diameter (nm)
Fig. 3 Pore size distribution of sol–gel-derived alumina, zirconia, and titania. (From Ref.[29].)
Table 3 Pore texture sol–gel-derived alumina, zirconia, and titania (calcined at 450 C for 3 hr) Xerogel
Average ˚) pore size (A
Pore volume (cm3/g)
BET surface area (m2/g)
g-Al2O3
28
0.33
373
ZrO2
38
0.11
57
TiO2
34
0.21
147
One of the outstanding characteristics of sol– gel-derived g-alumina xerogel is its excellent mechanical properties. Preparation of porous g-alumina granules with good mechanical properties and desirable pore structure is of great importance in the development of novel catalysts and sorbents for various applications. The superior mechanical properties can be derived from the unique microstructure of the granule, which is defined by compacting small g-alumina crystallite particles bound together by the bridges of the same material formed through coarsening or sintering. Such nanostructured g-alumina can be prepared by combining the Yoldas process and the ‘‘oil-drop’’ method.[30–34] Table 4 compares the crush strength and attrition rate of sol–gel-derived g-alumina xerogel granules with those of several commercial sorbents. It is clearly shown in Table 4 that the sol–gel-derived galumina xerogel granules have excellent mechanical properties as compared with commercial sorbents. The excellent mechanical properties makes sol– gel-derived alumina granules very suitable for fluidized bed and other applications including separation and purification process for food and healthcare products that have very strict regulations on sorbent power contamination. Sol–gel-derived xeorgel sorbents have been investigated for gas separation, purification, and environmental applications. g-Alumina sorbents and membranes doped with cuprous and silver ions have been studied for selective adsorption or transfer of CO and ethylene through p-complexation.[35–37] Significant efforts have been devoted to explore the possibility of using CuO-doped g-alumina sorbents for removing SOx and NOx from flue gas.[38–44] The sol–gel-derived CuO=g-alumina sorbents have demonstrated high sorption capacity, high reactivity for SO2, and high thermal and chemical stability. The excellent mechanical and desulfurization properties of sol–gel-derived sorbents make them ideal sorbent candidates for fluidized bed desulfurization process. However, the relatively high cost of sol– gel-derived alumina xerogels may prevent them from being used in many large-scale adsorption processes. Research efforts are needed to look for less expensive precursors to replace alkaoxides used in the Yoldas process.
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Table 4 Comparison of crush strength and attrition rate of sol–gel-derived g-alumina xerogel granules with commercial sorbents Sorbents Sol–gel alumina
Granular shape
Granular size (mm)
Average crush strength (N/granule)
Attrition rate (wt%/hr)
2.0–2.5
160
0.033
Spherical
Sol–gel alumina
Spherical
2.6–2.8
190
Alcoa alumina (LD-350)
Spherical
4.0–4.6
42
0.177
UOP silicalite
Cylindrical
1.4–1.6
16
0.575
Degussa DAY zeolite
Cylindrical
3.5–3.5
40
0.073
Sol–gel-derived metal oxide xerogels were also investigated for water adsorption because most of these metal oxides are good sorbent candidates for desiccant applications.[45–47] Significant research works have been carried out to study the adsorption= complexation properties of heavy metal ions including mercury, Cu(II), CdCl2, etc. in waste water on different sol–gel-derived xerogels.[48–54] The sol–gel-derived xerogels seem to be promising sorbent candidates for waste water treatment. Modified xerogel sorbents also showed promising adsorption properties for removing acid gas CO2 and H2S from natural gas, or as CO2 storage sorbents.[25,55] There are several advantages of using xerogels for enzyme immobilization, including the opportunity to produce them in defined shapes or thin films and the ability to manipulate their physical characteristics including porosity, hydrophobicity, and optical properties.[56,57] Metal oxide composite xerogels can also adsorb methyl orange.[58] There are also reports on microporous and mesoporous carbon xerogels for gas separation and purification.[59,60] As compared with xerogels, aerogels have larger surface area, larger pore volume, and higher porosity.[61–64] Alumina aerogels with a specific surface area as high as 1000 m2=g, and a pore volume as high as 17.3 cm3=g have been synthesized by supercritical carbon dioxide drying, but a very limited information on their adsorption properties was found.[61,62] A super water adsorbent consisting of 17–30% of CaCl2 doped on SiO2 aerogel showed an effective reversible adsorption capacity of 100 wt%; the adsorption capacity of hydrophilic silica aerogels can be fully recovered after regeneration.[64–66] CaO- and MgO-modified SiO2 aerogel sorbents can be used to capture pollution gases including CO2, SO2, CO, and NOx emitted from power plants based on fossil fuels.[67] Several studies reported the use of aerogels as destructive sorbents for toxic gases and radionuclide removal from contaminated environments.[64,68–70] Carbon aerogels can also be made from carbon materials under supercritical carbon dioxide drying conditions; these carbon aerogels were studied for removing uranium and other inorganic ions from contaminated water.[71–73] Aerogels are special sorbent candidates with excellent pore texture, which may play a
major role in environmental protection. However, more studies on their synthesis and adsorption properties are needed.
Metal Organic Framework (MOF) Recently, Yaghi’s group reported a novel crystalline nanoporous material that consists of metal atoms occupying the vertices of a lattice, with the lattice size, porosity, and chemical environment defined by the organic linker molecules that bind the metal atoms into a robust periodic structure.[74–76] These so-called metal organic framework (MOF) materials have been demonstrated to have an exceptionally high specific surface area of 4526 m2=g, and find use as adsorbents for methane and as hydrogen storage materials.[74,77–80] A reticular synthesis method was developed to realize the bottom-up synthesis through top-down design logic by using inorganic, metal organic, and organic molecules to build frameworks and large molecules.[81] Well-defined molecular building blocks that will maintain their structural integrity throughout the construction process were used to build the MOF molecules. It allows remarkable control over composition and structure of the material formed and employs the full range of the molecular synthetic methods and compounds in the preparation of this new type of porous sorbent materials. The ability to molecularly engineer the lattice size, chemical environment, and possibly structure by careful choice of the metal centers and organic linkers offers the opportunity for the development of new types of sorbents that could potentially meet the Department of Energy (DOE) target for hydrogen storage and that can be used for other applications in separation and purification. It is reported that metal organic framework-5 (MOF-5) of composition Zn4O(BDC)3 (BDC: 1,4benzenedicarboxylate) with a cubic three-dimensional extended porous structure and octahedral Zn–O–C clusters with benzene links can adsorb hydrogen up to 4.5 wt% at 78 K, and 1.0 wt% at room temperature and pressure of 20 bar.[74,79] It is identified by inelastic neutron scattering spectroscopy of the rotational
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transitions of the adsorbed hydrogen molecules that zinc and the BDC linker in MOF-5 are the two hydrogen binding sites responsible for hydrogen adsorption on this material. Higher hydrogen adsorption capacity at ambient temperature and 10 bar were observed on similar isoreticular metal organic framework-6 and -8 (IRMOF-6 and -8) having cyclobutylbenzene and naphthalene linkers.[79] A different microporous MOF sorbent [microporous metal coordination materials (MMOM)] was reported to have hydrogen sorption capacities ( 1.0 wt% at room temperature and 48 bar) similar to those of the best single-wall carbon nanotubes.[80] The adsorbed hydrogen can be released when the gas pressure is reduced. MOF sorbents have also been investigated for methane adsorption.[77] The reported methane storage capacity of MOF-6 is 155 cm3 (STP)=cm3 at 298 K and 36 atm, which is significantly higher than that of zeolite 5A (87 cm3 (STP)=cm3) and other coordination framework (213 cm3 (STP)=cm3).[77] Adsorption and desorption of carbon dioxide, nitrogen, and argon on a microporous manganese-based MOF sorbent has been reported.[78] Another interesting porous MOF sorbent, Cu-BTC (polymeric copper(II) benzene-1,3,5tricarboxylate) with molecular sieve character, was studied for its sorption properties of various adsorbates including nitrogen, oxygen, carbon monoxide, carbon dioxide, nitrous oxide, methane, ethylene, ethane, and n-dodecane.[82,83] A detailed investigation of sorption thermodynamics was performed for carbon dioxide by a sorption-isosteric method. It was demonstrated that Cu-BTC sorbent can be used for the separation of carbon dioxide–carbon monoxide, carbon dioxide– methane, and ethylene–ethane mixtures. In addition, this sorbent can also be used to remove carbon dioxide, nitrous oxide, high molecular weight hydrocarbons, and moisture from ambient air before cryogenic separation to produce oxygen and nitrogen.[82]
Sorbent Technology
Table 5 USDOE FreedomCAR hydrogen storage system targets Year Target factor
2005
2010
2015
Specific energy (MJ=kg)
5.4
7.2
10.8
Hydrogen (wt%)
4.5
6.0
9.0
Energy density (MJ=L)
4.3
5.4
9.72
System cost ($=kg=system)
9
6
3
Operating temperature ( C)
20=50
20=50
20=50
Cycle life-time (adsorption= desorption cycles)
500
1000
1500
Flow rate (g=sec)
3
4
5
Delivery pressure (bar)
2.5
2.5
2.5
Transient response (sec)
0.5
0.5
0.5
Refueling rate (kg H2=min)
0.5
1.5
2.0
(From Ref.[84].)
Hydrogen can be stored both physically and chemically in a confined vessel with or without the assistance of a storage media. The most commonly used methods for hydrogen storage are: gaseous and liquid hydrogen storage, solid state storage in complex metal hydrides, chemical storage materials, and in nanostructured materials.[2,85] The representative hydrogen storage capacities, hydrogen storage, and release conditions in various materials are summarized in Table 6. Carbon nanotubes are probably the most investigated and documented hydrogen storage sorbent materials. Several excellent reviews on carbon nanotubes for hydrogen storage are available.[2,86] As shown in Table 6, the hydrogen storage capacities on representative carbon nanotubes are below 6 wt%, the most referred DOE target for 2010.[84,87,88] The following concerns about carbon nanotubes as hydrogen storage materials have driven research in this area to other directions:[85]
Hydrogen Storage Media The development of hydrogen-fueled transportation system and portable electronics will demand new materials that can store large amounts of hydrogen at ambient temperature and relatively low pressures with small volume, light weight, fast charging and discharging time, cyclic stability, and low cost. Table 5 summarizes the targets for hydrogen storage system for automotive applications set by USDOE. The hydrogen storage capacities are calculated as both weight and volume percentage of the storage system.[84] To achieve these goals, the hydrogen storage media (sorbent) should have a high reversible hydrogen sorption capacity, low weight and high packing density as well as fast sorption=desorption kinetics, and low cost.
1. Difficult to meet the DOE’s long-term target (9 wt%); 2. Mechanisms for hydrogen sorption in carbon nanotubes are not well understood; 3. Part of the adsorbed hydrogen can only be recovered at high temperatures; 4. Preparation and purification of carbon nanotubes involve complicated and expensive processes, which leads to high cost of carbon nanotubes; 5. Hydrogen storage capacity is quite sensitive to sorbent preparation conditions; 6. Mixed results on hydrogen adsorption capacity have been reported.
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Table 6 Summary of hydrogen storage capacity of various nanostructured materials Materials
H2 storage capacity (wt%)
H2 storage conditions
H2 release conditions
Reference
Carbon nanotubes Single-walled
4.2
10 MPa, 300 K
1 bar, 300 K
[87]
Multi-walled
3.6
7 MPa, 298 K
1 bar, 298 K
[88]
Non-carbonaceous nanotubes BN
4.2
108 bar, 298 K
1 bar, 298 K
[89]
TiS2
2.5
40 bar, 298 K
1 bar, 298 K
[90]
4.5
0.75 bar, 77 K
1 bar, > 77 K
[74]
1.0
48 bar, 298 K
1 bar, 298 K
[80]
Mg2NiH4
3.6
1 bar, 500 K
1 bar, > 528 K
[91]
NaAlH4
8.0
90 bar, 403 K
Microporous MOF MOF-5 MMOM Metal hydrides
Mg(AlH4)2 LiBH4
[92]
6.6
1 bar, > 436 K
[93]
13.3
1 bar, > 473 K
[94]
Nitrides 9.3
1 bar, 443–473 K
1 bar, > 700 K
[95]
H2(H2O)2
5.3
1 bar, 77 K
1 bar, > 77 K
[97]
H2(H2O)
10.0
6000 bar, 190 K
< 6000 bar, > 190 K
[97]
(H2)4(CH4)
33.3
2000 bar, 77 K
< 2000 bar, > 77 K
[97]
Li3N Clathrate=molecular compounds
Noncarbonaceous nanotubes including boron nitride (BN) and titanium sulfide (TiS2) have been prepared and studied for hydrogen sorption.[89,90] Hydrogen storage capacity (2.5–4.5 wt%) similar to those for carbon nanotubes have been obtained on these noncarbonaceous materials. MOF-based sorbents for hydrogen sorption was discussed in the previous section. As suggested in Table 6, the hydrogen sorption capacities on MOF-5 and MMOM are lower than those on carbon nanotubes. However, MOF sorbents look more promising than carbon nanotubes as hydrogen storage media for the following reasons: 1. MOF is easy to make and is less expensive; 2. Sorption sites for hydrogen on MOF are better defined; 3. MOF sorbents may have extremely high specific surface area (> 4000 m2=g); 4. It is possible to tailor the interaction between hydrogen and MOF by manipulating synthesis parameters including different building blocks. Metal hydrides were widely investigated for hydrogen storage, and are believed to be ideal hydrogen
storage system because they have the following characteristics:[2,84,91–94] 1. Relatively high hydrogen storage capacity at modest pressures as indicated in Table 6; 2. Fast hydrogen charging and discharging rates; and 3. Moderate temperature for hydrogen desorption. However, metal hydrides also suffer from the following disadvantages as hydrogen storage materials: 1. High sensitivity to impurities in hydrogen (CO, H2O, O2, CO2, and H2S); 2. Storage capacity and rates decay with hydrogen charge–discharge cycles; and 3. Relatively high cost as compared with gaseous and liquid hydrogen storage methods. Another interesting hydrogen storage material is lithium nitride (Li3N), which shows 9.3 wt% useful hydrogen storage capacity between thermal swing cycles (473–700 K).[95] The requirement for high-temperature desorption will greatly limit its applications. Most recently, hydrogen clathrate hydrate and other
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Sorbent Technology
molecular compounds were found to have hydrogen storage capacities as high as 33.3 wt%.[96–98] This is a very innovative way to store hydrogen with exceptionally high capacity to meet the DOE long-term target. However, these clathrate and hydrogen storage compounds were synthesized at extremely high pressures and at liquid nitrogen temperature. It is unlikely these clathrate hydrates will be used for hydrogen storage until we find new clathrate hydrate compounds that can be synthesized and are stable at much lower pressures.
p-Complexation Sorbents and Composite Sorbents A very good review article based on a panel study of status, future research needs, and opportunities for porous sorbent materials was published several years ago.[99] It was pointed out that very significant advances have been made in tailoring the porosity of porous sorbent materials in terms of size and shape selectivity. Relatively little progress has been achieved in terms of chemoselectivity of sorbents based on specific interactions between adsorbate molecules and functional groups in the sorbents. Incorporation of active sites into sorbents is of high priority in the development of sorbents. The p-complexation bond is a weak chemical bond that is slightly stronger than van der Waals interaction, which governs physical sorption processes. Sorbents with p-complexation capability tend to have higher selectivity than other physical sorbents for certain adsorbate molecules. Several different types of p-complexation sorbents with Cuþ or Agþ ions supported on different supports (SiO2, g-Al2O3, TiO2, variety of zeolites, polymer resin, and activated carbon) were synthesized using different methods including thermal dispersion, wet-impregnation, sol–gel, microwave heating, ionexchange zeolite, and ion-exchange resin.[34–36,99–105] It was found that the CO adsorption capacity increases with Cuþ loading in an activated alumina supported sorbent.[100,101] To achieve the highest sorption capacity, the active species should be dispersed as a monolayer form.[99] The potential applications of these p-complexation sorbents include:[2] 1. 2. 3. 4. 5. 6.
Desulfurization of gasoline and diesel fuels; Separation of olefins and paraffins; CO separation from synthesis gases; CO removal from hydrogen; Removal of aromatics; and Removal of volatile organic compounds (VOCs).
A p-complexation sorbent can also be viewed as a composite sorbent especially when the sorbent support
contributes significantly to the adsorption. Composite sorbents are typically made by physically mixing the powders of constituent sorbents with different sorption properties; they tend to have multiple sorption sites for different adsorbate molecules. One example of a composite sorbent is a mixture of activated alumina and zeolites for removing moisture, carbon dioxide, and other trace components from air in an air-purification process prior to cryogenic air separation.[106–108] Conventionally, moisture is removed by activated alumina, carbon dioxide by zeolite 13X, and hydrocarbons by zeolite 5A.[107,108] Traditional air-purification processes employ multiple layers consisting of activated alumina, zeolite 13X, and optional zeolite 5A sorbents in a single vessel to achieve significant removal of moisture, carbon dioxide, and hydrocarbons from air. The major disadvantages of layered bed are nonuniform sorbent packing for a short sorbent layer, very significant temperature variation (> 30 C, sometimes called cold spots) between the zeolite and the activated alumina sorbent layers. The large temperature difference could upset the sorption process operation if it is designed to be operated isothermally. It is beneficial to have a single sorbent with multiple sorption features for different impurities and eliminate sorbent layering and temperature variations.
High-Temperature Ceramic O2 Sorbents Lin et al. disclosed in a U.S. patent a new group of sorbents for air separation and oxygen removal using oxygen-deficient perovskite-type ceramics as sorbents.[109] Perovskite-type ceramics are a group of metal oxides having the general formula of ABO3. The ideal perovskite structure for ABO3 is shown in Fig. 4. It consists of cubic array of corner-sharing BO6 octahedra, where B is a transition metal ion. The A-site ion, interstitial between the BO6 octahedra, may be occupied by an alkali, an alkaline earth, or a rare earth ion. Alternatively, the perovskite structure may be regarded as a cubic close packing of layers of AO3 with B cations placed in the interlayer octahedral interstices.[110] This group of the sorbents can be viewed as chemisorbents that can selectively adsorb a considerable amount of oxygen at high temperatures (> 300 C), and theoretically has an infinitely high selectivity for oxygen over nitrogen or other nonoxygen species. The presence of other gases has negligible effect on the separation properties of these new sorbents. High-temperature membrane separation of oxygen has also received increasing interest from other industrial gas companies.[111–113] Development of high-temperature oxygen separation technology opens up several high-temperature applications of oxygen including syngas production, hydrogen production, and partial oxidation fuel reforming processes.
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A A
B
0.5 0.45
δ
0.4 0.35 LSCF-1 0.3 0.25 0.0001
The oxygen equilibrium and kinetic properties of perovskite-type ceramics have been extensively studied primarily for applications as fuel cell electrodes and oxygen permeable membranes,[110] and only a few for oxygen sorption.[114–117] Oxygen nonstoichiometry (d) occurs in some perovskite-type ceramics with B-site cations of variable oxidation states and A-site cations partially substituted by another cation with a lower oxidation state. Oxygen nonstoichiometry, or oxygen content, for a perovskite-type ceramic of a given composition is a function of temperature and oxygen partial pressure. Therefore, by changing temperature or oxygen partial pressure, the value of oxygen nonstoichiometry or the degree of oxygen vacancy in the material changes. Within a certain range of temperature and oxygen partial pressure the change of the oxygen nonstoichiometry does not affect the perovskite structure, and the change of the oxygen content in the material is a reversible process. The oxygen nonstoichiometry of the perovskite sorbents can be measured gravimetrically at different temperatures and oxygen partial pressures. Oxygen sorption capacity on the sorbent can then be calculated from the oxygen nonstoichiometry data once the initial state (zero sorption capacity) of the sorbent material is defined.[114] Figs. 5 and 6 are examples of oxygen nonstoichiometry of La1–xSrxCo1–yFeyO3–d perovskite oxide sorbents as a function of oxygen partial pressure or temperature, respectively.[114] The corresponding oxygen sorption isotherm of La1–xSrxCo1–yFeyO3–d perovskite oxide sorbents that were calculated from the oxygen nonstoichiometry data are shown in Fig. 7.[114] From these oxygen isotherms we can conceive a high-temperature vacuum swing sorption or temperature swing sorption process for oxygen separation or oxygen removing applications by using the La1–xSrxCo1–yFeyO3–d perovskite oxide sorbents. Future studies on
0.01 PO2 (atm)
0.1
1
B 0.5 1
2
3
0.45
δ
Fig. 4 Ideal perovskite structure for ABO3 type oxides.
0.001
0.4
0.35
0.3 0.0001
LSCF-2
0.001
0.01 PO2 (atm)
0.1
1
Fig. 5 Change of oxygen nonstoichiometry d with oxygen partial pressure (LSCF-1. La0.1Sr0.9Co0.5Fe0.5O3–d; LSCF-2, La0.1Sr0.9Co0.9Fe0.1O3–d). (From Ref.[114].)
perovskite oxide sorbents are needed to address the issues of slow desorption rate, potential sorbent structure stability in cyclic processes, and effective regeneration methods. High-Temperature CO2 Sorbents Increased awareness of the global warming trend has led to worldwide concerns regarding ‘‘greenhouse gas’’ emissions. Greenhouse gases include CO2, CH4, and N2O and are mostly associated with the production and utilization of fossil fuels, with CO2 being the single greatest contributor to global warming. Significant research efforts are being devoted worldwide on looking for economical ways of mitigating CO2 emission problem.[118–122] Carbon capture and sequestration costs can be considered in terms of four components: capture, compression, transport, and injection. Typically about 75% of this cost is attributable to capture
2838
Sorbent Technology
0.45 0.4
PO2 = 0.209 atm
δ
0.35 0.3 LSCF-2 LSCF-1
0.25 0.2 300
400
500 T(˚C)
600
700
800
Fig. 6 Change of oxygen nonstoichiometry d with temperature (LSCF-1, La0.1Sr0.9Co0.5Fe0.5O3–d; LSCF-2, La0.1Sr0.9 Co0.9Fe0.1O3–d). (From Ref.[114].)
A Amount Adsorbed (mmol/g)
0.5 0.4
500˚C 600˚C
0.3 0.2 0.1 0 0.0001
0.001
0.01 PO2 (atm)
0.1
1
B Amount Adsorbed (mmol/g)
0.6 0.5
500˚C 600˚C
and compression processes. Sorption of carbon dioxide on solid sorbents is receiving increased attention in view of the importance of both the removal and the recovery of carbon dioxide from flue gases.[123,124] Physical sorbents for carbon dioxide separation and removal were extensively studied by industrial gas companies.[125–127] Zeolite 13X, activated alumina, and their improved versions are typically used for removing carbon dioxide and moisture from air in either a TSA or a PSA process.[125–128] The sorption temperatures for these applications are usually close to ambient temperature. There are a few studies on adsorption of carbon dioxide at high temperatures. The carbon dioxide adsorption isotherms on two commercial sorbents hydrotalcite-like compounds, EXM911 and activated alumina made by LaRoche Industries, are displayed in Fig. 8.[123,124] As shown in Fig. 8, LaRoche activated alumina has a higher carbon dioxide capacity than the EXM911 at 300 C. However, the adsorption capacities on both sorbents are too low for any practical applications in carbon dioxide sorption at high temperature. Conventional physical sorbents are basically not effective for carbon dioxide capture at flue gas temperature (> 400 C). There is a need to develop effective sorbents that can adsorb carbon dioxide at flue gas temperature to significantly reduce the gas volume to be treated for carbon sequestration. Only a handful of studies on high-temperature carbon dioxide sorbents have been published in the past few years.[123,124,129–133] It is believed that lithium zirconate (Li2ZrO3) is one of the most promising sorbent materials for carbon dioxide separation from flue gas at high temperature because it can absorb a large amount of carbon dioxide at around 400–700 C.[130,131] The carbon dioxide adsorption and desorption uptake curves on lithium zirconate are shown in Fig. 9.[131] As shown in this figure, about 20% carbon dioxide was captured by the lithium zirconate sorbent during sorption step at 500 C based on the following reaction: Li2 ZrO3 þ CO2 ! Li2 CO3 þ ZrO2
ð4Þ
0.4 0.3 0.2 0.1 0 0.001
0.01
0.1
1
PO2 (atm) Fig. 7 Sorption isotherms of: (A) La0.1Sr0.9Co0.5Fe0.5O3–d and (B) La0.1Sr0.9Co0.9Fe0.1O3–d at 500 and 600 C. (From Ref.[114].)
About 80% of adsorbed carbon dioxide can be desorbed with hot air 780 C. Addition of potassium carbonate (K2CO3) and Li2CO3 into Li2ZrO3 remarkably improves the CO2 sorption rate of the Li2ZrO3-based sorbent materials. X-ray diffraction (XRD) analysis for phase and structural changes during the sorption= desorption process shows that the reaction between Li2ZrO3 and CO2 is reversible.[131] Based on this work, a TSA process can be developed for carbon dioxide removal from flue gas using Li2ZrO3-type sorbent materials. High-temperature carbon dioxide sorbents can also find applications in fuel reforming process to enhance fuel to hydrogen conversion efficiency. It was reported
Sorbent Technology
2839
A 0.55 0.50 EXM911
0.45
20˚C
Q (mmol/g)
0.40 0.35 0.30 0.25 0.20 0.15
200˚C
0.10 0.05 0.00 0.0
300˚C 0.1
0.2
0.3
0.6 0.4 0.5 Pressure (bar)
0.7
0.8
0.9
1.0
B 1.1 1.0 Activated Alumina
0.9
20˚C
0.8 0.7 200˚C
0.6 0.5 0.4 0.3
300˚C
0.2 0.1 0.0 0.0
0.1
0.2
0.3
0.4 0.5 0.6 Pressure (bar)
0.7
that sorption of carbon dioxide can enhance the production of hydrogen for a steam–methane reforming process using a mixture of Ni-based reforming catalyst and a Ca-based sorbent. The rates of the reforming, water-gas shift, and carbon dioxide removal reactions are sufficiently fast that combined reaction equilibrium was closely approached, allowing for >95 mol% hydrogen to be produced in a single step.[134]
CONCLUSIONS AND FUTURE DIRECTIONS Existing commercial sorbents including activated carbon, zeolites, activated alumina, and silica gels will continue to play important roles in adsorptive separation and purification for current process industries in the near future. However, they cannot meet the needs
0.8
0.9
1.0
Fig. 8 Adsorption isotherms of carbon dioxide on commercial sorbents. (A) Hydrotalcite-like compound, EXM911; (B) LaRoche Industries activated alumina at 20, 200, and 300 C. (From Refs.[123,124].)
of future technological developments in the new energy economy and the stringent environmental regulations. The newly developed nanostructured sorbent materials have shown some very promising features, but they are basically unexplored and systematic investigations are needed on both synthesis methods and adsorption characteristic studies. The following are the author’s views on future research needs in both sorbent synthesis and applications: 1. Explore entirely new sorbent synthesis routes to better control of both sorbent pore texture and surface property. 2. Design new sorbent materials from basic building blocks and introduce active sorption sites according to sorbent–adsorbate interaction requirements. MOF material syntheses using
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Sorbent Technology
REFERENCES
800
10
600
Temperature ( C)
Weight change (%)
20
0
0
200
400
600
400 800
Time (min) Fig. 9 CO2 sorption and regeneration on the modified Li2ZrO3. Sorption process: 50% CO2 balanced by dry air at 500 C. Desorption process: 50% CO2 balanced by dry air at 780 C ! dry air at 780 C. Gas flow rate: 150 ml=min. (From Ref.[131].)
the isoreticular method and sol–gel technique are two examples of this approach. 8. A better understanding of the relationship between sorbent–adsorbate interaction, sorption equilibrium, and kinetics through molecular simulation, and provide guidance for sorbent synthesis. In terms of applications, new sorbents should be developed to meet the following pressing needs: 1. Deep desulfurization of fossil fuels for fuel cell application. 2. Hydrogen purification (H2S, CO, and CO2 removal). 3. Hydrogen and methane storage sorbents and processes. 4. Water treatment (arsenic, radionuclides and heavy metal ions and anions removal). 5. Air pollution control (SOx, NOx, and other toxic gases removal). 6. Chemisorbents as effective getter materials for toxic process gas and liquid streams. 7. Effective high-temperature carbon dioxide sorbents for carbon dioxide sequestration. ACKNOWLEDGMENT Professor Y.S. Lin is acknowledged for providing his publications and comments on high-temperature sorbents discussed in this entry.
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