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Sorbent Technology

Shuguang DengChemical Engineering Department, New Mexico State University,Las Cruces, New Mexico, U.S.A.

INTRODUCTION

This article covers the fundamentals, status, and futuredevelopments of sorbent materials and their appli-cations in adsorptive separation and purification pro-cesses. A sorbent is usually a solid substance thatadsorbs or absorbs another type of substance. It isthe sorbent that makes a sorption process a uniqueand different separation and purification process fromothers. With the rapid development in novel sorbent

materials and innovative cyclic adsorption processes,sorption has become a key separation process in manyprocess industries including chemical, petrochemical,environmental, pharmaceutical, and electronic gases.A brief review of the fundamentals of adsorptionand the basic requirements for sorbent materials ispresented, followed with a summary of the status ofcommercial sorbents and their applications. The focusof this article is placed on recent advances in novel sor-bent materials including oxide molecular sieves, solgelderived xerogels and aerogels, metal organic frame-work, hydrogen storage media, p-complexation andcomposite sorbents, and high-temperature sorbents

for oxygen or carbon dioxide sorption. A concludingsection outlines the future research needs and opportu-nities in sorbent technology development for newenergy and environmental applications.

ADSORPTION MECHANISMS ANDSORBENT MATERIALS

According to King, a mass separating agent is neededto facilitate separation for many separation processes.[1]

The mass separating agent for adsorption process is

the adsorbent, or the sorbent. Therefore, the character-istic of the sorbent directly decides the performance ofany adsorptive separation or purification process. Thebasic definitions of adsorption-related terminologiesaregiveninthefollowingtoclarifyandstandardizethesewidely used terms in this field.

Adsorption: The adhesion of molecules (as of gases,solutes, or liquids) to the surfaces of solid bodiesor liquids with which they are in contact.

Absorption: The absorbing of molecules (as ofgases, solutes, or liquids) into the solid bodiesor liquids with which they are in contact.

Sorption: Formationfrom adsorption and absorption.

Adsorbent: A usually solid substance that adsorbsanother substance on its surface.

Sorbent: A usually solid substance that adsorbs andabsorbs another substance.

Adsorbate: Molecules (as of gases, solutes, orliquids) 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 500A.

Adsorptive separation can be achieved through oneof the following mechanisms. Understanding thefundamentals of adsorptive separation mechanismswill allow us to better design or modify sorbentmaterials to achieve their best possible separation

performance.[24]

Adsorption equilibrium effect is because of the dif-ference in the thermodynamic equilibria for each adsor-bateadsorbent interaction. The majority of adsorptiveseparation and purification processes are based on equili-brium effect. One example is to generate oxygen-enrichedair or relatively pure oxygen (95%) from air using a zeo-lite molecular sieve 5A or 13X in either a pressure swingadsorption (PSA) or a vacuum swing adsorption (VSA)process. In this case, nitrogen is selectively adsorbed bythe zeolite adsorbent, and oxygen is collected from theadsorption effluent stream.

Adsorption kinetics effect arises because of the

difference of rates at which different adsorbate mole-cules travel into the internal structure of the adsorbent.There are only a few commercial successes usingadsorption kinetic difference to achieve adsorptiveseparation of gases. The typical example is separationof nitrogen from air using a carbon molecular sieve(CMS). The CMS adsorbent has a similar adsorptionequilibrium capacity for both nitrogen and oxygen,but the diffusivity of oxygen in CMS is at least 30 timeslarger than that of nitrogen in CMS.[5] High-purity

Encyclopedia of Chemical Processing DOI: 10.1081/E-ECHP-120007963Copyright # 2006 by Taylor & Francis. All rights reserved. 2825


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nitrogen can be recovered from the adsorption effluentstream in a PSA process because oxygen moves muchfaster than nitrogen into the micropores of CMSadsorbent. However, the cycle time of this CMS-basedPSA process is much shorter than that of a typical PSAprocess based on adsorption equilibrium effect. This isbecause there will be no separation if both nitrogenand oxygen are allowed to reach adsorption equilib-rium with the CMS adsorbent.

Molecular sieving effect, also called steric effect, isderived from the molecular sieving properties of someadsorbents with a microporous structure. In this case,the pore openings of the adsorbent structure are smallenough to exclude large adsorbate molecules frompenetrating the micropores of the adsorbent. This isthe extreme case of the kinetic effect. There are severalcommercial applications based on this mechanism inadsorptive separation processes. One typical exampleis separating normal paraffin from iso-paraffin andaromatics 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-definedaperture of zeolite 5A. Therefore it adsorbs in themicropores of the adsorbent during the adsorptionstep, and is recovered from the adsorbed phase in thedesorption step. A representative process for n-paraffinseparation from naphtha and kerosene is UOPsMolex process that employs a simulated moving bedwith binderless zeolite 5A as an adsorbent and lightparaffin as a desorbent.[6]

We can define separation factor and selectivity asthe ability of an adsorbent to separate molecule Afrom molecule B as:[7]

Separation factor : aAB XA=YAXB=YB

1

Here XA, YA are strictly equilibrium mole fractions forcomponent A in the adsorbed phase and adsorbate(fluid) phase, respectively; as are XB, YB for componentB. For equilibrium-based adsorptive separation process,the adsorbent selectivity is the same as the separation fac-tor as defined in Eq. (1). Apparently, this definition is notapplicable to other processes based on kinetic and stericeffects. In a kinetically controlled adsorption process, the

adsorbent selectivity depends on both equilibrium andkinetic effects. A simplified definition for adsorbentseparation factor is given by Ruthven et al.:[8]

SAB KA

KB

ffiffiffiffiffiffiffiDA

DB

r2

where SAB is the adsorbent selectivity, Kis the adsorptionequilibrium constant or isotherm slope, and D is the

effective diffusivity. Although the above equation isstrictly valid under the assumptions that components Aand B have independent linear adsorption isothermsand independent diffusion process, it provides a goodestimate of adsorbent selectivity for kinetically controlledprocesses.

Theoretically speaking, selectivity for adsorbentswith a molecular sieving effect should be infinitelylarge because the larger molecules are excluded fromgetting into the adsorbent micropores. In reality, theadsorbent selectivity for steric effect is somewhatreduced by combining with the equilibrium effect fromadsorption on the surface of large pores. So adsorptionprocesses based on molecular sieving are usually con-sidered as adsorption equilibrium effect.

Another very important adsorbent property affect-ing the adsorption process is the adsorption capacitybecause it determines the size of an adsorbent vessel,the amount of adsorbents required, and the relatedcapital and operating costs. The requirements for

commercial sorbents are discussed briefly as follows.

Characteristics of Sorbent Materials

Commercial sorbents used in cyclic adsorption pro-cesses 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 cansimply transfer to adsorbent characteristic require-ments 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 chemicaland mechanical stability;

Low-cost raw materials.

These basic requirements are usually proposed foradsorbents used in cyclic adsorption processes thatare based on physical adsorption. There is an increas-ing demand for strong chemical adsorbents used in

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purification processes to remove trace contaminantsfrom main stream fluids such as the removal of verytoxic contaminants from electronic process gas streams,and the removal of toxic, or radioactive species fromcontaminated water. In these cases, the sorbents areused as getter materials; no regeneration is needed,and instead, the spent sorbent materials are disposedof in designated areas regulated by government envi-ronmental policies.

COMMERCIAL SORBENTS ANDAPPLICATIONS

An excellent review and detailed coverage on commer-cial adsorbents and new adsorbent materials has beenpresented by Yang in his newly published monographon adsorbents.[2] A very brief overview of existing com-mercial adsorbents is given here. Commercial sorbentsthat have been used in large-scale adsorptive separa-

tionand purification processes include activated carbon,zeolites, activated alumina, silica gel, and polymericadsorbents. Although the worldwide sales of sorbentmaterials are relatively small as compared with otherchemical commodities, sorbents and adsorption pro-cesses play a very important role in many process indus-tries. The estimated worldwide sales of these sorbentsare as follows:[2]

Activated carbon: $1 billion Zeolite: $1.07 billion Activated alumina: $63 million

Silica gel: $71 million Polymeric adsorbents: $50 million

Activated Carbon

Activated carbons are unique and versatile adsorbentsbecause of their large surface area, microporous and

mesoporous structure, universal adsorption effect,high adsorption capacity for many nonpolar moleculesincluding organic molecules, and high degree of sur-face reactivity. They are used widely in industrialapplications that include decolorizing sugar solutions,personnel protection, solvent recovery, volatile organiccompound removal from air and water, water treat-ment, hydrogen and synthesis gas separation, andnatural gas storage.[4,9,10] Activated carbons areproduced in two main steps: carbonization of thecarbonaceous raw materials at temperatures below800 C in the absence of oxygen, and activation ofthe carbonized products.[10] The properties of activated

carbon depend largely on the nature of the raw materi-als, the activating agents and activation conditions.For gas-phase applications, activated carbons areusually made in pellets with mostly micropores; whilefor liquid-phase applications, activated carbon is pro-duced in powder form with relatively large mesoporesto enhance mass transfer rate in the carbons.

Fig. 1 compares the pore size distributions of majorcommercial adsorbents discussed in this section. Acti-vated carbons have a broad pore size distribution likeactivated alumina and silica gel. Although activatedcarbon is thought to be hydrophobic, it does adsorb

Activated alumina

Zeolite 5A

MSCMSC

60

50

40

30

20

10

02 5 10 20 50

Pore diameter,

Cumulativeporevolume,cm3/100gm

Activated carbon

Silica gel

Fig. 1 Pore size distributions for activatedcarbon, silica gel, activated alumina, two mole-cular sieve carbons (MSCs), and zeolite 5A.(From Ref.[3].)

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quite significant amount of water (>30wt%) whenrelative humidity is higher than 50%. An example iso-therm of water on activated carbon, along with waterisotherms on other commercial adsorbents, is plottedin Fig. 2. The change from hydrophobic to hydro-philic on the activated carbon surface is attributed tothe initial adsorbed water film on the carbon surface.This occurs because when the carbon surface is fullycovered with a layer of water molecules, the adsorbedwater molecules exhibit strong affinity to other polarmolecules including water. Carbon molecular sieve(CMS) is a specially made carbonaceous material withvery narrow pore size distribution (49 A). The majorapplication of CMS is in the generation of high-puritynitrogen from air in a PSA process. The representativephysical properties of commercial adsorbents and theirmajor applications are summarized in Tables 1 and 2,respectively.

Zeolites

Zeolites are porous crystalline aluminosilicates that aremade of assemblies of SiO4 and AlO4 tetrahedra joinedtogether through shared oxygen atoms. The generalchemical formula for zeolites is:

Mx=nAlO2xSiO2yzH2O 3

where x and y are integers with y=x (Si=Al ratio) equalor larger than 1; n is the valance of cation M, and z isthe number of water molecules in each unit cell. Thetetrahedra can be arranged in many different ways to

form different crystalline structures. Some zeolites

exist as minerals in nature, but all commerciallyimportant zeolites are synthetic. Zeolites are uniqueadsorbents owing to their special surface chemistriesand crystalline pore structures. It should be pointedout that probably only 10% of $1 billion worldwidesales of zeolite is used as adsorbents; the majority ofcommercial zeolites are used as detergent additives(zeolite 4A), animal food additives (zeolite 4A), ionexchange, and catalyst supports. Among all commer-cial sorbents zeolites are probably the most extensivelyinvestigated and documented. Many excellent mono-graphs and review articles are available.[2,1113] Pleaserefer to Tables 1 and 2 for properties and major appli-cations of zeolites.

Activated Alumina

Activated alumina is a porous high-surface area formof aluminum oxide with the formula of Al2O3nH2O.

Commercially, it is prepared either from thermaldehydration of aluminum trihydrate, Al(OH)3, ordirectly from bauxite (Al2O33H2O), as a by-productof the Bayer process for alumina extraction from baux-ite. Its surface is more polar than that of silica gel and,reflecting the amphoteric nature of aluminum, hasboth acidic and basic characteristics. Surface areasare in the range 250350 m2=g depending on the activa-tion temperature and the source of raw materials.Because activated alumina has a higher capacity forwater than silica gel at elevated temperatures it is usedmainly as a desiccant for warm gases including air, butin many commercial applications it has now been

replaced by zeolitic materials in a thermal swing

40

30

20

10

00 20 40 60 80 100

Relative humidity, %

Adsorption,kgH2O/100kgadsorbent

E

D

C

B

A

Fig. 2 Equilibrium sorption of water vapor fromatmospheric air at 25 C on: (A) alumina (granular),(B) alumina (spherical), (C) silica gel, (D) 5Azeolite, and (E) activated carbon. The vapor pressureat 100% relative humidity is 23.6 torr. (From Ref.[3].)



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adsorption (TSA) process. However, activated aluminahas a low adsorption heat for water and other polarmolecules as compared with zeolite; it is possible toregenerate activated alumina under PSA conditions.Activated alumina also demonstrates moderateadsorption affinity for carbon dioxide, which makesit a suitable sorbent for removing water and carbondioxide from air in a PSA process. These adsorptionproperties of activated alumina have been exploredextensively for air purification applications by indus-trial gas companies.[1417] This is a perfect example todemonstrate the importance of sorbent regenerabilityover sorption capacity and selectivity in pressure swing

adsorption processes. Activated alumina is also anexcellent catalyst support. More applications andrepresentative properties of activated alumina arelisted in Tables 1 and 2.

Silica Gels

Silica gel is the most widely used desiccant because ofits large adsorption capacity for water (40wt%), asshown in Fig. 2, and easy for regeneration ($150C,compared with 350C for zeolites). Silica is a partially

dehydrated polymeric form of colloidal silicic acid withthe formula of SiO2nH2O. Its water content, which istypically about 5 wt%, is presented in the chemicallybonded hydroxyl groups. Silica is an amorphous mate-rial comprising spherical particles of 20200 A in size,which aggregate to form the sorbent with pore sizesin the range of 60250A and surface areas of 100850m2=g, depending on gel density. Its surface hasmainly SiOH and SiOSi polar groups; this is whyit can be used to adsorb water, alcohols, phenols,

amines, etc. by hydrogen bonding mechanisms. Othercommercial applications include the separation of aro-matics from paraffins, the chromatographic separationof organic molecules, and modified silica in chromato-graphy columns.[2,1820]

Polymeric Adsorbents

A wide range of synthetic, nonionic polymers are avail-able for use as sorbents, ion-exchange resins, and par-ticularly for analytical chromatography applications.Commercially available resins in bead form (typically

0.5 mm in diameter) are based usually on copolymersof styrene=divinyl benzene (DVB) and acrylic acidesters=divinyl benzene, and have a wide range of sur-face polarities, porosities, and macropore sizes. Theporosities can be built through emulsion polymeriza-tion of relevant monomers in the presence of a solventthat dissolves the monomers and serves as a poor swell-ing agent for the polymer. This creates a polymermatrix with surface areas ranging up to 1100m2=g.[2,4]

The major application of polymeric adsorbents is inwater treatment. The macroporous polymeric resins canbe modified by attaching different functional groups tomimic activated carbon, and to replace activated carbons

for certain specific applications in food and pharmaceu-tical industries where color contamination by the blackcarbons of the final products is a major concern.

NEW DEVELOPMENTS IN SORBENTMATERIALS AND APPLICATIONS

The past two decades have witnessed major advancesin new nanostructured sorbent materials including

Table 1 Representative physical properties of commercial adsorbents

Adsorbent Nature

Specific surface

area (m2/g)

Pore

diameter (A) Porosity

Particle

density (g/cm3)

Activated carbon Hydrophobic amorphous

Small pore 4001200 1025 0.40.6 0.50.9

Large pore 200600 > 30 $0.5 0.60.8

Zeolite Hydrophilic=hydrophobic

crystalline

600700 310 0.6 1.0

Activated alumina Hydrophilic crystalline=x-rayamorphous

200350 1075 0.5 1.25

Silica gel Hydrophilic=hydrophobicamorphous

Small pore 750850 2226 0.47 1.09

Large pore 300350 100150 0.71 1.62

Polymeric adsorbent Hydrophilic=hydrophobic 4501100 2590 0.5 1.25

Carbon molecular sieve Hydrophilic $400 39 0.5 1.0



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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 processesEthylene from methane and hydrogenVinyl chloride monomer (VCM) from airRemoval of odors from gasesRecovery of solvent vaporsRemoval of SOx, and NOxPurification of heliumClean-up of nuclear off-gasesDecolorizing of syrups, sugars, and molassesWater purification, including removal of phenol, halogenated compounds,pesticides, caprolactam, chlorine

Carbon molecular sieve Nitrogen separation from air

Zeolite Oxygen from airDrying of gasesRemoving water from azeotropesSweetening sour gases and liquidsPurification of hydrogenSeparation of ammonia and hydrogenRecovery of carbon dioxide

Separation of oxygen and argonRemoval of acetylene, propane, and butane from airSeparation of xylenes and ethyl benzeneSeparation of normal from branched paraffinsSeparation of olefins and aromatics from paraffinsRecovery of carbon monoxide from methane and hydrogenPurification of nuclear off-gasesSeparation of cresolsDrying of refrigerants and organic liquidsSeparation of solvent systemsPollution control, including removal of Hg, NOx, and SOx from gasesRecovery of fructose from corn syrup

Activated alumina Drying of gases, organic solvents, transformer oilsRemoval of HCl from hydrogenRemoval of fluorine and boronfluorine compounds in alkylation processesRemoving of water and carbon dioxide from air in a PSA process

Silica gel Drying of gases, refrigerants, organic solvents, transformer oilsDesiccant in packings and double glazingDew 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 radionuclidesRecovery and purification of steroids, amino acids and polypeptidesSeparation of fatty acids from water and tolueneSeparation of aromatics from aliphaticsSeparation of hydroquinone from monomers

Recovery of proteins and enzymesRemoval of colors from syrupsRemoval of organics from hydrogen peroxide

Clays (acid treated and pillared) Removal of organic pigmentsRefining of mineral oilsRemoval of PCBs

(From Ref.[4].)



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mesoporous molecular sieves, solgel-derived metaloxide xerogels and aerogels, metal organic framework,p-complexation and composite adsorbents, new carbo-naceous materials (carbon nanotubes, carbon fibers,superactivated carbons), high-temperature ceramic sor-bents, and strong chemical sorbent materials. Althoughthese new sorbent materials have demonstrated promis-ing sorption properties for many existing and newapplications, systematic studies on synthesis methodsand characterization of these new materials are necessaryto fully explore and realize their potential as commercialsorbents. The review that follows aims at attracting moreresearch efforts to develop novel sorbent materials tomeet 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 surfacearea and pore volume are ideal candidates for use assorbent materials and catalyst supports of many hetero-geneous catalysts. Oxide molecular sieves are generallysynthesized by hydrothermal methods that involve bothchemical and physical transformations within anamorphous oxide gel, often in the presence of a tem-plate species. The gel eventually converts to a crystal-line material in which the template species and=orsolvent molecules are guests within the channels andcages of an oxide host framework. A porous materialis obtained upon removal of the guest molecules fromthe oxide framework. By manipulating the synthesis

parameters, including starting precursors, synthesistemperature, pH, template species, drying, and calcina-tion conditions, it is possible to tailor the pore size andshape of these porous materials for different applica-tions. However, tailoring of porosity in oxide molecu-lar sieves in terms of a priori structural design isextremely difficult because of the inherent complexityof the synthetic procedures employed.[21]

Recent advances and applications of oxide molecu-lar sieves have been summarized in several review arti-cles.[2,2123] Microporous zeolite materials synthesizedwith molecular templates and their applications inhostguest chemistry have been covered elsewhere.[13]

A new class of silicate=aluminosilicate mesoporousmolecular sieves designated as M41S was discoveredin the former Mobil research laboratory by extendingthe concept of zeolite templating with small organicmolecules 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 15100 A,specific surface area of 1040 m2=g, pore volume above0.7cm3=g, and significantly high sorption capacity for

hydrocarbons (49wt% for n-hexane at 40torr and21C, and 67wt% for benzene at 50torr and 25C).[24]

Other significant members of the M41S family includeMCM-48 (cubic phase), MCM-50 (stabilized lamellarphase), SBA-1 (cubic phase), and SBA-2 (cubicphase).[21]

Although M41S type mesoporous oxide molecularsieves have exhibited unique properties of large surfacearea and exceptionally large pore volume (> 0.7 cm3=g),their large pore volume may not be attractive for gassorption because the adsorbateadsorbent interactionsare not enhanced inside the internal pores of these mate-rials.[2] Therefore, M14S type mesoporous oxide molecu-lar sieves without surface modification are rarely used assorbents. Significant research efforts were devoted tosurface modification of M41S materials for differentapplications.[2] An amine-grafted MCM-48 sorbent,synthesized from tetraethoxysilane (TEOS), has beenshown to have a surface area of 1389 m2=g, a silanolnumber of 8, higher thermal stability than MCM-41,

high adsorption selectivity, and high capacity for bothcarbon dioxide and hydrogen sulfide.[25]

SolGel-Derived Xerogels and Aerogels

Solgel processing refers to the fabrication process ofceramic materials by preparation of a sol, gelationof the sol, and removal of the solvent.[26] Sols aredispersions of colloidal particles in a liquid solvent,and a gel is a solid matrix encapsulating a solvent. Ina solgel process, the sol can be formed from a solutionof colloidal powders or hydrolysis and condensation of

alkoxides or salt precursors. In the latter approach,which is much more popular, primary particles of uni-form size are formed and grow in a sol and connect toeach other to form aggregates during gelation. Theseaggregates forming the network of the gel are brokenapart into the primary particles in the drying step.Upon calcination and sintering, these primary particlesare bound together strongly to form a very rigid solidnetwork, and large interparticle space with uniformnanoscale pores is formed. Xerogels are obtained bydrying the gels through evaporation at normal condi-tions under which capillary pressure causes shrinkageof the gel network, while areogels are produced by

drying the wet gels at supercritical conditions wherethe liquidvapor interface is eliminated, and relativelylittle shrinkage of the gel network occurs. Xerogelsand aerogels typically have relatively large surfacearea, high porosity, and internal pore volume, andare ideal candidates as sorbent and catalyst supportmaterials for many applications. The solgel processoffers a very high flexibility to tailor xerogels andareogels for specific applications by manipulating thesynthesis conditions.



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Silica xerogel is probably the most studied anddocumented porous material in the solgel sys-tem.[27,28] Although silica has several crystalline forms,only amorphous silica gel is used as a desiccant (sor-bent). A microporous silica that was synthesized withTEOS as precursor has an average pore size of 6.4 A,pore volume of 0.24 cm3=g, and BrunauerEmmettTeller (BET) surface area of 588m2=g.[29] However,this material lost about 90% of its microporosity whenit was heated at 600C for 30 hr. By doping with 1.5%of alumina, the thermal stability of this microporoussilica was significantly improved.[29]

Crystalline sorbent materials including g-alumina,zirconia, and titania were also synthesized using thesolgel process in Lins group;[29] the representativepore size distribution and pore texture data of xerogelsof g-alumina, zirconia, and titania are summarized inFig. 3 and Table 3, respectively. As shown in Fig. 3,the pore size distributions of these materials are rathernarrow, with an average pore diameter of about 3 nm.

Such narrow size distribution and nanoscale averagepore size are determined by the primary crystalliteparticles. The particles of the solgel-derived alumina,titania, and zirconia, owing to the Ostwald ripeningmechanism,[26] are usually of nanoscale size; the uni-form particle size distributions ofg-alumina crystallitesare plate-shaped with size ranging from about 5 to20 nm. The solgel-derived g-alumina consists ofsuch plate-shaped crystallite particles, which giverise to a relatively large surface area. Crystallites oftetragonal zirconia and rutile are of more sphericalshape, with a crystallite size of about 15 and 11 nm,respectively.[29]

One of the outstanding characteristics of solgel-derived g-alumina xerogel is its excellent mechanicalproperties. Preparation of porous g-alumina granuleswith good mechanical properties and desirable porestructure is of great importance in the developmentof novel catalysts and sorbents for various applica-tions. The superior mechanical properties can bederived from the unique microstructure of the granule,which is defined by compacting small g-alumina crys-tallite particles bound together by the bridges of the

same material formed through coarsening or sintering.Such nanostructured g-alumina can be prepared bycombining the Yoldas process and the oil-dropmethod.[3034] Table 4 compares the crush strengthand attrition rate of solgel-derived g-alumina xerogelgranules with those of several commercial sorbents. Itis clearly shown in Table 4 that the solgel-derived g-alumina xerogel granules have excellent mechanicalproperties as compared with commercial sorbents.The excellent mechanical properties makes solgel-derived alumina granules very suitable for fluidizedbed and other applications including separation andpurification process for food and healthcare products

that have very strict regulations on sorbent powercontamination.

Solgel-derived xeorgel sorbents have been inves-tigated for gas separation, purification, and envi-ronmental applications. g-Alumina sorbents andmembranes doped with cuprous and silver ions havebeen studied for selective adsorption or transfer ofCO and ethylene through p-complexation.[3537]

Significant efforts have been devoted to explore thepossibility of using CuO-doped g-alumina sorbentsfor removing SOx and NOx from flue gas.

[3844]

The solgel-derived CuO=g-alumina sorbents havedemonstrated high sorption capacity, high reactivity

for SO2, and high thermal and chemical stability.The excellent mechanical and desulfurization pro-perties of solgel-derived sorbents make them idealsorbent candidates for fluidized bed desulfurizationprocess. However, the relatively high cost of solgel-derived alumina xerogels may prevent them frombeing used in many large-scale adsorption processes.Research efforts are needed to look for less expen-sive precursors to replace alkaoxides used in theYoldas process.

TitaniaZirconia

-Alumina

1 102 3 4 5 6 7 8 9 2 3

Pore Diameter (nm)

dV/dlog(D)

1.6

1.2

0.8

0.4

0.0

Fig. 3 Pore size distribution of solgel-derived alumina,zirconia, and titania. (From Ref.[29].)

Table 3 Pore texture solgel-derived alumina, zirconia, andtitania (calcined at 450C for 3hr)

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



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Solgel-derived metal oxide xerogels were alsoinvestigated for water adsorption because most ofthese metal oxides are good sorbent candidates fordesiccant applications.[4547] Significant research workshave been carried out to study the adsorption=complexation properties of heavy metal ions includingmercury, Cu(II), CdCl2, etc. in waste water on differentsolgel-derived xerogels.[4854] The solgel-derived

xerogels seem to be promising sorbent candidates forwaste water treatment. Modified xerogel sorbents alsoshowed promising adsorption properties for removingacid gas CO2 and H2S from natural gas, or as CO2storage sorbents.[25,55] There are several advantagesof using xerogels for enzyme immobilization, includingthe opportunity to produce them in defined shapes orthin films and the ability to manipulate their physicalcharacteristics including porosity, hydrophobicity,and optical properties.[56,57] Metal oxide compositexerogels can also adsorb methyl orange.[58] There arealso reports on microporous and mesoporous carbonxerogels for gas separation and purification.[59,60]

As compared with xerogels, aerogels have largersurface area, larger pore volume, and higher poros-ity.[6164] Alumina aerogels with a specific surface areaas high as 1000m2=g, and a pore volume as high as17.3cm3=g have been synthesized by supercritical carbondioxide drying, but a very limited information on theiradsorption properties was found.[61,62] A super wateradsorbent consisting of 1730% of CaCl2 doped onSiO2 aerogel showed an effective reversible adsorptioncapacity of 100 wt%; the adsorption capacity of hydro-philic silica aerogels can be fully recovered after regen-eration.[6466] CaO- and MgO-modified SiO2 aerogelsorbents can be used to capture pollution gases includ-

ing CO2, SO2, CO, and NOx emitted from power plantsbased on fossil fuels.[67] Several studies reported the useof aerogels as destructive sorbents for toxic gases andradionuclide removal from contaminated environ-ments.[64,6870] Carbon aerogels can also be made fromcarbon materials under supercritical carbon dioxidedrying conditions; these carbon aerogels were studiedfor removing uranium and other inorganic ions fromcontaminated water.[7173] Aerogels are special sorbentcandidates with excellent pore texture, which may play a

major role in environmental protection. However, morestudies on their synthesis and adsorption properties areneeded.

Metal Organic Framework (MOF)

Recently, Yaghis group reported a novel crystalline

nanoporous material that consists of metal atomsoccupying the vertices of a lattice, with the lattice size,porosity, and chemical environment defined by theorganic linker molecules that bind the metal atoms intoa robust periodic structure.[7476] These so-called metalorganic framework (MOF) materials have beendemonstrated to have an exceptionally high specificsurface area of 4526m2=g, and find use as adsorbentsfor methane and as hydrogen storage materials.[74,7780]

A reticular synthesis method was developed to realizethe bottom-up synthesis through top-down designlogic by using inorganic, metal organic, and organicmolecules to build frameworks and large molecules.[81]

Well-defined molecular building blocks that willmaintain their structural integrity throughout theconstruction process were used to build the MOFmolecules. It allows remarkable control over composi-tion and structure of the material formed and employsthe full range of the molecular synthetic methods andcompounds in the preparation of this new type ofporous sorbent materials. The ability to molecularlyengineer the lattice size, chemical environment, andpossibly structure by careful choice of the metal cen-ters and organic linkers offers the opportunity for thedevelopment of new types of sorbents that couldpotentially meet the Department of Energy (DOE)

target for hydrogen storage and that can be usedfor other applications in separation and purification.

It is reported that metal organic framework-5(MOF-5) of composition Zn4O(BDC)3 (BDC: 1,4-benzenedicarboxylate) with a cubic three-dimensionalextended porous structure and octahedral ZnOCclusters with benzene links can adsorb hydrogen upto 4.5wt% at 78K, and 1.0wt% at room temperatureand pressure of 20 bar.[74,79] It is identified by inelasticneutron scattering spectroscopy of the rotational

Table 4 Comparison of crush strength and attrition rate of solgel-derived g-alumina xerogel granules with commercial sorbents

Sorbents Granular shape

Granular size

(mm)

Average crush strength

(N/granule)

Attrition rate

(wt%/hr)

Solgel alumina Spherical 2.02.5 160 0.033

Solgel alumina Spherical 2.62.8 190

Alcoa alumina (LD-350) Spherical 4.04.6 42 0.177

UOP silicalite Cylindrical 1.41.6 16 0.575

Degussa DAY zeolite Cylindrical 3.53.5 40 0.073



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transitions of the adsorbed hydrogen molecules thatzinc and the BDC linker in MOF-5 are the two hydro-gen binding sites responsible for hydrogen adsorptionon this material. Higher hydrogen adsorption capacityat ambient temperature and 10 bar were observed onsimilar isoreticular metal organic framework-6 and -8(IRMOF-6 and -8) having cyclobutylbenzene andnaphthalene linkers.[79] A different microporous MOFsorbent [microporous metal coordination materials(MMOM)] was reported to have hydrogen sorptioncapacities ($1.0wt% at room temperature and 48 bar)similar to those of the best single-wall carbon nano-tubes.[80] The adsorbed hydrogen can be released whenthe gas pressure is reduced.

MOF sorbents have also been investigated formethane adsorption.[77] The reported methane storagecapacity of MOF-6 is 155 cm3 (STP)=cm3 at 298 K and36 atm, which is significantly higher than that of zeolite5A (87 cm3 (STP)=cm3) and other coordinationframework (213 cm3 (STP)=cm3).[77] Adsorption and

desorption of carbon dioxide, nitrogen, and argon ona microporous manganese-based MOF sorbent hasbeen reported.[78] Another interesting porous MOFsorbent, Cu-BTC (polymeric copper(II) benzene-1,3,5-tricarboxylate) with molecular sieve character, was stu-died for its sorption properties of various adsorbatesincluding nitrogen, oxygen, carbon monoxide, carbondioxide, nitrous oxide, methane, ethylene, ethane, andn-dodecane.[82,83] A detailed investigation of sorptionthermodynamics was performed for carbon dioxideby a sorption-isosteric method. It was demonstratedthat Cu-BTC sorbent can be used for the separationof carbon dioxidecarbon monoxide, carbon dioxide

methane, and ethyleneethane mixtures. In addition,this sorbent can also be used to remove carbondioxide, nitrous oxide, high molecular weight hydro-carbons, and moisture from ambient air beforecryogenic separation to produce oxygen and nitrogen.[82]

Hydrogen Storage Media

The development of hydrogen-fueled transportationsystem and portable electronics will demand new mate-rials that can store large amounts of hydrogen at ambi-ent temperature and relatively low pressures with small

volume, light weight, fast charging and dischargingtime, cyclic stability, and low cost. Table 5 summarizesthe targets for hydrogen storage system for automotiveapplications set by USDOE. The hydrogen storagecapacities are calculated as both weight and volumepercentage of the storage system.[84] To achieve thesegoals, the hydrogen storage media (sorbent) shouldhave a high reversible hydrogen sorption capacity,low weight and high packing density as well as fastsorption=desorption kinetics, and low cost.

Hydrogen can be stored both physically and chemi-cally in a confined vessel with or without the assistanceof a storage media. The most commonly used methodsfor hydrogen storage are: gaseous and liquid hydrogenstorage, solid state storage in complex metal hydrides,chemical storage materials, and in nanostructuredmaterials.[2,85] The representative hydrogen storagecapacities, hydrogen storage, and release conditionsin various materials are summarized in Table 6.

Carbon nanotubes are probably the most investi-gated and documented hydrogen storage sorbent mate-rials. Several excellent reviews on carbon nanotubes

for hydrogen storage are available.[2,86]

As shown inTable 6, the hydrogen storage capacities on representa-tive carbon nanotubes are below 6 wt%, the mostreferred DOE target for 2010.[84,87,88] The followingconcerns about carbon nanotubes as hydrogen storagematerials have driven research in this area to otherdirections:[85]

1. Difficult to meet the DOEs long-term target(9wt%);

2. Mechanisms for hydrogen sorption in carbonnanotubes are not well understood;

3. Part of the adsorbed hydrogen can only berecovered at high temperatures;

4. Preparation and purification of carbon nano-tubes involve complicated and expensive pro-cesses, which leads to high cost of carbonnanotubes;

5. Hydrogen storage capacity is quite sensitive tosorbent preparation conditions;

6. Mixed results on hydrogen adsorption capacityhave been reported.

Table 5 USDOE FreedomCAR hydrogen storage systemtargets

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 9.72System 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].)



11/22

Noncarbonaceous nanotubes including boronnitride (BN) and titanium sulfide (TiS2) have been pre-pared and studied for hydrogen sorption.[89,90] Hydro-gen storage capacity (2.54.5 wt%) similar to those forcarbon nanotubes have been obtained on these noncar-bonaceous materials. MOF-based sorbents for hydrogensorption was discussed in the previous section. Assuggested in Table 6, the hydrogen sorption capacitieson MOF-5 and MMOM are lower than those on car-bon nanotubes. However, MOF sorbents look morepromising than carbon nanotubes as hydrogen sto-rage 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 (> 4000m2=g);4. It is possible to tailor the interaction between

hydrogen and MOF by manipulating syn-thesis parameters including different buildingblocks.

Metal hydrides were widely investigated for hydro-gen storage, and are believed to be ideal hydrogen

storage system because they have the followingcharacteristics:[2,84,9194]

1. Relatively high hydrogen storage capacity atmodest 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 fol-lowing 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

chargedischarge cycles; and3. Relatively high cost as compared with gaseousand liquid hydrogen storage methods.

Another interesting hydrogen storage material islithium nitride (Li3N), which shows 9.3 wt% usefulhydrogen storage capacity between thermal swingcycles(473700 K).[95] The requirement for high-temperaturedesorption will greatly limit its applications.Most recently, hydrogen clathrate hydrate and other

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]

Microporous MOF

MOF-5 4.5 0.75 bar, 77 K 1 bar, > 77 K [74]

MMOM $1.0 48 bar, 298 K 1 bar, 298 K [80]

Metal hydrides

Mg2NiH4 3.6 1 bar, $500 K 1 bar, > 528 K [91]

NaAlH4 8.0 90 bar, 403 K [92]

Mg(AlH4)2 6.6 1 bar, > 436 K [93]

LiBH4 13.3 1 bar, > 473 K [94]

Nitrides

Li3N 9.3 1 bar, 443473 K 1 bar, > 700 K [95]

Clathrate=molecular compounds

H2(H2O)2 5.3 $1 bar, 77 K 1 bar, > 77 K [97]

H2(H2O) 10.0 $6000 bar, 190K < 6000 bar, > 190 K [97]

(H2)4(CH4) 33.3 $2000bar, 77 K < 2000 bar, > 77 K [97]



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molecular compoundswere found to have hydrogen sto-rage capacities as high as 33.3 wt%.[9698] This is a veryinnovative way to store hydrogen with exceptionallyhigh capacity to meet the DOE long-term target. How-ever, these clathrate and hydrogen storage compoundswere synthesized at extremely high pressures and atliquid nitrogen temperature. It is unlikely these clathratehydrates will be used for hydrogen storage until wefind new clathrate hydrate compounds that can besynthesized and are stable at much lower pressures.

p-Complexation Sorbents andComposite Sorbents

A very good review article based on a panel study ofstatus, future research needs, and opportunities forporous sorbent materials was published several yearsago.[99] It was pointed out that very significantadvances have been made in tailoring the porosity of

porous sorbent materials in terms of size and shapeselectivity. Relatively little progress has been achievedin terms of chemoselectivity of sorbents based on spe-cific interactions between adsorbate molecules andfunctional groups in the sorbents. Incorporation ofactive sites into sorbents is of high priority in the devel-opment of sorbents.

The p-complexation bond is a weak chemical bondthat is slightly stronger than van der Waals interaction,which governs physical sorption processes. Sorbentswith p-complexation capability tend to have higherselectivity than other physical sorbents for certainadsorbate molecules. Several different types ofp-com-

plexation sorbents with Cu

or Ag

ions supported ondifferentsupports(SiO2,g-Al2O3,TiO2, variety ofzeolites,polymer resin, and activated carbon) were synthesizedusing different methods including thermal dispersion,wet-impregnation, solgel, microwave heating, ion-exchange zeolite, and ion-exchange resin.[3436,99105] Itwas found that the CO adsorption capacity increaseswith Cu loading in an activated alumina supportedsorbent.[100,101] To achieve the highest sorption capacity,the active species should be dispersed as a monolayerform.[99] The potential applications of these p-complexa-tion sorbents include:[2]

1. Desulfurization of gasoline and diesel fuels;2. Separation of olefins and paraffins;3. CO separation from synthesis gases;4. CO removal from hydrogen;5. Removal of aromatics; and6. Removal of volatile organic compounds

(VOCs).

A p-complexation sorbent can also be viewed as acomposite sorbent especially when the sorbent support

contributes significantly to the adsorption. Compositesorbents are typically made by physically mixing thepowders of constituent sorbents with different sorptionproperties; they tend to have multiple sorption sites fordifferent adsorbate molecules. One example of a com-posite sorbent is a mixture of activated alumina andzeolites for removing moisture, carbon dioxide, andother trace components from air in an air-purificationprocess prior to cryogenic air separation.[106108] Con-ventionally, moisture is removed by activated alumina,carbon dioxide by zeolite 13X, and hydrocarbons byzeolite 5A.[107,108] Traditional air-purification processesemploy multiple layers consisting of activated alumina,zeolite 13X, and optional zeolite 5A sorbents in a sin-gle vessel to achieve significant removal of moisture,carbon dioxide, and hydrocarbons from air. The majordisadvantages of layered bed are nonuniform sorbentpacking for a short sorbent layer, very significant tem-perature variation (> 30C, sometimes called coldspots) between the zeolite and the activated alumina

sorbent layers. The large temperature difference couldupset the sorption process operation if it is designed tobe operated isothermally. It is beneficial to have a sin-gle sorbent with multiple sorption features for differentimpurities and eliminate sorbent layering and tempera-ture variations.

High-Temperature Ceramic O2 Sorbents

Lin et al. disclosed in a U.S. patent a new group of sor-bents for air separation and oxygen removal usingoxygen-deficientperovskite-typeceramics as sorbents.[109]

Perovskite-type ceramics are a group of metal oxideshaving the general formula of ABO3. The ideal perov-skite structure for ABO3 is shown in Fig. 4. It consistsof cubic array of corner-sharing BO6 octahedra, whereB is a transition metal ion. The A-site ion, interstitialbetween the BO6 octahedra, may be occupied by analkali, an alkaline earth, or a rare earth ion. Alternatively,the perovskite structure may be regarded as a cubicclose packing of layers of AO3 with B cations placedin the interlayer octahedral interstices.[110] This groupof the sorbents can be viewed as chemisorbents thatcan selectively adsorb a considerable amount of oxy-gen at high temperatures (> 300C), and theoretically

has an infinitely high selectivity for oxygen over nitrogenor other nonoxygen species. The presence of other gaseshas negligible effect on the separation properties of thesenew sorbents. High-temperature membrane separationof oxygen has also received increasing interest fromother industrial gas companies.[111113] Development ofhigh-temperature oxygen separation technology opensup several high-temperature applications of oxygenincluding syngas production, hydrogen production,and partial oxidation fuel reforming processes.



13/22

The oxygen equilibrium and kinetic properties of

perovskite-type ceramics have been extensively studiedprimarily for applications as fuel cell electrodes andoxygen permeable membranes,[110] and only a few foroxygen sorption.[114117] Oxygen nonstoichiometry (d)occurs in some perovskite-type ceramics with B-sitecations of variable oxidation states and A-site cationspartially substituted by another cation with a loweroxidation state. Oxygen nonstoichiometry, or oxygencontent, for a perovskite-type ceramic of a given com-position is a function of temperature and oxygen par-tial pressure. Therefore, by changing temperature oroxygen partial pressure, the value of oxygen nonstoi-chiometry or the degree of oxygen vacancy in the mate-

rial changes. Within a certain range of temperature andoxygen partial pressure the change of the oxygen non-stoichiometry does not affect the perovskite structure,and the change of the oxygen content in the material isa reversible process. The oxygen nonstoichiometry ofthe perovskite sorbents can be measured gravimetricallyat different temperatures and oxygen partial pressures.Oxygen sorption capacity on the sorbent can then becalculated from the oxygen nonstoichiometry data oncethe initial state (zero sorption capacity) of the sorbentmaterial is defined.[114] Figs. 5 and 6 are examples ofoxygen nonstoichiometry of La1xSrxCo1yFeyO3dperovskite oxide sorbents as a function of oxygen

partial pressure or temperature, respectively.[114]The corresponding oxygen sorption isotherm ofLa1xSrxCo1yFeyO3d perovskite oxide sorbents thatwere calculated from the oxygen nonstoichiometrydata are shown in Fig. 7.[114] From these oxygenisotherms we can conceive a high-temperaturevacuum swing sorption or temperature swing sorptionprocess for oxygen separation or oxygen removingapplications by using the La1xSrxCo1yFeyO3dperovskite oxide sorbents. Future studies on

perovskite oxide sorbents are needed to address theissues of slow desorption rate, potential sorbentstructure stability in cyclic processes, and effectiveregeneration methods.

High-Temperature CO2 Sorbents

Increased awareness of the global warming trend hasled to worldwide concerns regarding greenhouse

gas emissions. Greenhouse gases include CO2, CH4,and N2O and are mostly associated with the productionand utilization of fossil fuels, with CO2 being the sin-gle greatest contributor to global warming. Significantresearch efforts are being devoted worldwide on look-ing for economical ways of mitigating CO2 emissionproblem.[118122] Carbon capture and sequestrationcosts can be considered in terms of four components:capture, compression, transport, and injection. Typi-cally about 75% of this cost is attributable to capture

A B

Fig. 4 Ideal perovskite structure for ABO3 type oxides.B

1 2 3

A

LSCF-2

LSCF-1

0.5

0.45

0.4

0.35

0.3

0.250.0001 0.001 0.01 0.1 1

0.5

0.45

0.4

0.35

0.30.0001 0.001 0.01 0.1 1

PO2 (atm)

PO2 (atm)

Fig. 5 Change of oxygen nonstoichiometry d with oxygen

partial pressure (LSCF-1. La0.1Sr0.9Co0.5Fe0.5O3d; LSCF-2,La0.1Sr0.9Co0.9Fe0.1O3d). (From Ref.[114].)



14/22

and compression processes. Sorption of carbon dioxideon solid sorbents is receiving increased attention in viewof the importance of both the removal and the recoveryof carbon dioxide from flue gases.[123,124]

Physical sorbents for carbon dioxide separation andremoval were extensively studied by industrial gascompanies.[125127] Zeolite 13X, activated alumina,and their improved versions are typically used forremoving carbon dioxide and moisture from air ineither a TSA or a PSA process.[125128] The sorptiontemperatures for these applications are usually closeto ambient temperature. There are a few studies onadsorption of carbon dioxide at high temperatures.The carbon dioxide adsorption isotherms on twocommercial sorbents hydrotalcite-like compounds,EXM911 and activated alumina made by LaRocheIndustries, are displayed in Fig. 8.[123,124] As shown inFig. 8, LaRoche activated alumina has a higher carbondioxide capacity than the EXM911 at 300C. However,the adsorption capacities on both sorbents are too

low for any practical applications in carbon dioxidesorption at high temperature. Conventional physicalsorbents are basically not effective for carbon dioxidecapture at flue gas temperature (> 400C). There is aneed to develop effective sorbents that can adsorbcarbon dioxide at flue gas temperature to significantlyreduce the gas volume to be treated for carbonsequestration.

Only a handful of studies on high-temperature car-bon dioxide sorbents have been published in the pastfew years.[123,124,129133] It is believed that lithium zirco-nate (Li2ZrO3) is one of the most promising sorbentmaterials for carbon dioxide separation from flue gas

at high temperature because it can absorb a large amountof carbon dioxide at around 400700C.[130,131] Thecarbon dioxide adsorption and desorption uptakecurves on lithium zirconate are shown in Fig. 9. [131]

As shown in this figure, about 20% carbon dioxidewas captured by the lithium zirconate sorbent duringsorption step at 500C based on the following reaction:

Li2ZrO3 CO2 ! Li2CO3 ZrO2 4

About 80% of adsorbed carbon dioxide can be des-orbed with hot air 780C. Addition of potassium car-bonate (K2CO3) and Li2CO3 into Li2ZrO3 remarkably

improves the CO2 sorption rate of the Li2ZrO3-basedsorbent materials. X-ray diffraction (XRD) analysis forphase and structural changes during the sorption=desorption process shows that the reaction betweenLi2ZrO3 and CO2 is reversible.

[131] Based on this work, aTSAprocesscan be developed forcarbon dioxide removalfrom flue gas using Li2ZrO3-type sorbent materials.

High-temperature carbon dioxide sorbents can alsofind applications in fuel reforming process to enhancefuel to hydrogen conversion efficiency. It was reported

0.45

0.4

0.35

0.3

0.25

0.2300 400 500 600 700 800

T(C)

LSCF-2LSCF-1

PO2 = 0.209 atm

Fig. 6 Change of oxygen nonstoichiometry d with tempera-ture (LSCF-1, La0.1Sr0.9Co0.5Fe0.5O3d; LSCF-2, La0.1Sr0.9Co0.9Fe0.1O3d). (From Ref.

[114].)

A

500C

600C

0.5

0.4

0.3

0.2

A

mountAdsorbed(mmol/g)

0.1

00.0001 0.001 0.01 0.1 1

PO2 (atm)

B

500C

600C

0.6

0.5

0.4

0.3

0.2

AmountAdsorbed(mmol/g)

0.1

00.001 0.01 0.1 1

PO2 (atm)

Fig. 7 Sorption isotherms of: (A) La0.1Sr0.9Co0.5Fe0.5O3dand (B) La0.1Sr0.9Co0.9Fe0.1O3d at 500 and 600

C. (FromRef.[114].)



15/22

that sorption of carbon dioxide can enhance the pro-duction of hydrogen for a steammethane reformingprocess using a mixture of Ni-based reforming catalystand a Ca-based sorbent. The rates of the reforming,water-gas shift, and carbon dioxide removal reactionsare sufficiently fast that combined reaction equilibriumwas closely approached, allowing for >95 mol% hydro-

gen to be produced in a single step.

[134]

CONCLUSIONS AND FUTURE DIRECTIONS

Existing commercial sorbents including activated car-bon, zeolites, activated alumina, and silica gels willcontinue to play important roles in adsorptive separa-tion and purification for current process industries inthe near future. However, they cannot meet the needs

of future technological developments in the new energyeconomy and the stringent environmental regulations.The newly developed nanostructured sorbent materialshave shown some very promising features, but they arebasically unexplored and systematic investigations areneeded on both synthesis methods and adsorptioncharacteristic studies. The following are the authors

views on future research needs in both sorbent synthe-sis and applications:

1. Explore entirely new sorbent synthesis routes tobetter control of both sorbent pore texture andsurface property.

2. Design new sorbent materials from basic build-ing blocks and introduce active sorption sitesaccording to sorbentadsorbate interactionrequirements. MOF material syntheses using

0.000.0 0.1 0.2 0.3 0.4 0.5

Pressure (bar)

Q(

mmo

l/g)

0.6 0.7

200C

20CEXM911

300C

0.8 0.9 1.0

0.05

0.10

0.15

0.20

0.250.30

0.35

0.40

0.45

0.50

0.55

0.00.0 0.1 0.2 0.3 0.4 0.5

Pressure (bar)

0.6 0.7

200C

20CActivated Alumina

B

A

300C

0.8 0.9 1.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

Fig. 8 Adsorption isotherms of carbondioxide on commercial sorbents. (A)

Hydrotalcite-like compound, EXM911;(B) LaRoche Industries activated aluminaat 20, 200, and 300C. (From Refs.[123,124].)



16/22

the isoreticular method and solgel techniqueare two examples of this approach.

8. A better understanding of the relationshipbetween sorbentadsorbate interaction, sorp-tion equilibrium, and kinetics through molecu-lar simulation, and provide guidance forsorbent synthesis.

In terms of applications, new sorbents should be

developed to meet the following pressing needs:

1. Deep desulfurization of fossil fuels for fuel cellapplication.

2. Hydrogen purification (H2S, CO, and CO2removal).

3. Hydrogen and methane storage sorbents andprocesses.

4. Water treatment (arsenic, radionuclides andheavy metal ions and anions removal).

5. Air pollution control (SOx, NOx, and othertoxic gases removal).

6. Chemisorbents as effective getter materials for

toxic process gas and liquid streams.7. Effective high-temperature carbon dioxide sor-

bents for carbon dioxide sequestration.

ACKNOWLEDGMENT

Professor Y.S. Lin is acknowledged for providing hispublications and comments on high-temperature sor-bents discussed in this entry.

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0 200 400

Time (min)

Temperature(C)

Weightchange(%)

400

0

10

20

600

800

600 800

Fig. 9 CO2 sorption and regeneration on the modifiedLi2ZrO3. Sorption process: 50% CO2 balanced by dry air at500C. 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].)



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