Adsorbents for the post-combustion capture of CO2 using rapid temperature swing or vacuum swing...

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Adsorbents for the post-combustion capture of CO2using rapid temperatureswing or vacuum swing adsorption

Niklas Hedin a,, Linna Andersson a, Lennart Bergstrm a, Jinyue Yan b,c

a Department of Materials and Environmental Chemistry and The Berzeli Center EXSELENT, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Swedenb Chemical Engineering and Technology/Energy Processes, Royal Institute of Technology, Teknikringen 50, SE-100 44 Stockholm, Swedenc Sustainable Development of Society and Technology, Mlardalen University, SE-721 23 Vsters, Sweden

a r t i c l e i n f o

Article history:

Received 4 September 2012Received in revised form 11 November 2012Accepted 15 November 2012Available online 20 December 2012

Keywords:

ZeolitesMetal organic frameworksAmine-modified silicaCarbon capture and storageTemperature swing adsorptionVacuum swing adsorption

a b s t r a c t

In general, the post-combustion capture of CO2 is costly; however, swing adsorption processes can reducethese costs under certain conditions. This review highlights the issues related to adsorption-based pro-cesses for the capture of CO2from flue gas. In particular, we consider studies that investigate CO 2adsor-bents for vacuum swing or temperature swing adsorption processes. Zeolites, carbon molecular sieves,metal organic frameworks, microporous polymers, and amine-modified sorbents are relevant for suchprocesses. The large-volume gas flows in the gas flue stacks of power plants limit the possibilities of usingregular swing adsorption processes, whose cycles are relatively slow. The structuring of CO2adsorbents iscrucial for the rapid swing cycles needed to capture CO2at large point sources. We review the literatureon such structured CO2adsorbents. Impurities may impact the function of the sorbents, and could affectthe overall thermodynamics of power plants, when combined with carbon capture and storage. The heatintegration of the adsorption-driven processes with the power plant is crucial in ensuring the economy ofthe capture of CO2, and impacts the design of both the adsorbents and the processes. The development ofadsorbents with high capacity, high selectivity, rapid uptake, easy recycling, and suitable thermal andmechanical properties is a challenging task. These tasks call for interdisciplinary studies addressing this

delicate optimization process, including integration with the overall thermodynamics of power plants.2012 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4192. Adsorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 420

2.1. Microporous physisorbents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4212.1.1. Zeolites and aluminum phosphates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4212.1.2. Carbon molecular sieves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4242.1.3. Metal organic frameworks, covalent organic frameworks, and microporous polymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424

2.2. Mesoporous physisorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4242.3. Amine-modified mesoporous silica chemisorbents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 425

3. Swing adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 426

4. Structured and supported adsorbents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4274.1. Structured adsorbents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4284.2. Supported adsorbents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429

5. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 430

0306-2619/$ - see front matter 2012 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.apenergy.2012.11.034

Corresponding author. Tel.: +46 8 16 24 17; fax: +46 8 15 21 87.

E-mail address:[email protected](N. Hedin).

Applied Energy 104 (2013) 418433

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1. Introduction

The combustion of fossil fuel supplies the world with 81% of itscommercial energy, and releases 30 1012 kg of CO2annually[1].This release of CO2to the atmosphere has triggered changes in theclimate that call for urgent cuts in the emission of greenhousegases. Carbon capture and storage (CCS) is one of the viable ap-

proaches to reducing such emissions. In contrast with many otherapproaches, CCS can be implemented in the existing energy infra-structure. The difficulties associated with its introduction lie in thehigh cost of investment, and the high penalties from energy defi-ciencies. These costs are large compared with the economic bene-fits from emission trading, as the cost of emitting one tonne of CO2(as of July 17, 2012) is 7.66, according the spot price of the Euro-pean Union Emission Trading System.

CCS is conceptually straightforward: CO2 is collected, trans-ported, and finally stored for a long time. The capture of CO2 isprobably the most expensive part of CCS, and its cost must be re-duced significantly if it is to be implemented in the current energysystem[1]. Established technologies exist for the transportation ofCO2, originally developed for enhanced oil recovery. The long-termstorage of CO2 does not appear to be particularly expensive, buteventual leaks or catastrophic failures are delicate issues that havebeen only partly researched. On a global scale, CCS could reducethe overall cost of mitigating climate change and increase the flex-ibility of approaches used to reduce the emission of greenhousegases (GHG)[2]. According to a joint report by the Organisationfor Economic Co-operation and Development (OECD) and the Inter-national Energy Agency (IEA), in a predicted scenario for 2050, CCSwill be the second-largest contributor to the mitigation of green-house gas emissions, after energy efficiency improvements [3](Fig. 1).

CCS can be implemented in various ways, using methodsincluding post-combustion, pre-combustion, and oxyfuel capture[2,4]. The fact that the current energy infrastructure is based onthe combustionof fossil fuels means that the post-combustion cap-

ture of CO2 represents a straightforward approach to CCS in theshort-term. Pre-combustion and oxyfuel capture have distinctadvantages in terms of the ease and cost of CO2capture; however,it is unlikely that these methods can be introduced on a globalscale with the speed necessary to replace the introduction ofpost-combustion capture. End-of-pipe solutions seem to be themost viable in the short-term. The structure of the global energysystem has a massive inertia, and it cannot be changed rapidly.However, it could be argued that oxyfuel retrofits for supercriticalpulverized coal power plants have a significantly lower capital costthan post-combustion retrofits[5].

A number of large-scale demonstration plants (equipped with acapture capacity of 1 MtCO2/year) for the post-combustion captureof CO2from coal-fired power plants are under development, and it

is planned that they will be in operation within the next decade

[4]. Large demonstration plants are also planned for the post-combustion capture of CO2 from natural gas-fired power plants.The overall technical challenges stem from the additional energyconsumption associated with CCS. The estimated costs for thepost-combustion capture of CO2from coal-fired power plants vary,and are not reviewed here, but the cost of reducing CO2emissionsusing CCS is expected to reduce significantly over time[4]. Currentdevelopments in the post-combustion capture of CO

2have focused

mainly on the power and gas industry sector, but CCS technologiescould potentially be applied in other industrial processes. Forexample, the cement and steel industries emit CO2 on a globallysignificant scale. These other processes produce gas emissions withvarying partial pressures of CO2, and the costs associated with mit-igating the release of CO2vary widely[6].

Liquid amine-based processes for the post-combustion captureof CO2 are closest to being commercialized. These absorption-reaction processes have large thermal losses, which add largeparasitic contributions to CCS. Most of todays technologies forseparating CO2from N2-rich gases rely on absorption in aqueoussolutions of alkanolamines [7]. Alkanolamines have a saturatedhydrocarbon back bond with amine and hydroxyl groups. In theabsorption processes,CO2 acts asa soluteandreacts chemicallywithalkanolamines at low temperatures, whereas the N2does not. Rela-tivelypure CO2 is recovered when the temperatureis raised[8]. Thealkanolaminesolutions are corrosiveandmustbe diluted [9], whichleads to a large thermal loss on heating and cooling of the solvent.The well-researched (and for other CO2separation processes, com-mercialized) solventmonoethanolamine (MEA) has among the bestproperties[10], but we refrain from reviewing the extensive litera-ture on liquid amines for CCS, and instead focus on other reviews.A technological breakthrough will depend on innovative methodsfor CO2-over-N2 separation, which could involve solvents, sorbents,and membranes[2,11], and will involve system integration. Baxteret al.[12]identified technologies that might lower the cost of CCS.Many of these technologies involve functions and structures onthe nanometer length scale[12]. For CCS, they argued for an in-

creased focus (within 5 years) on developingnewliquid absorbentsthatcould reduce costsand energy penalties. They concluded thatinthe mid-term (510 years), new nanostructured solids (adsorbentsor membranes) must be developed to reduce the cost of capturingCO2 from flue gaseson a large scale [12]. Swingadsorption processesor membrane separation processes could be used. Ho et al. [13]showed that adsorption-based processes can be used to separateCO2from flue gases at a reduced cost compared with current tech-nologies, if more CO2-selective sorbents could be developed.

Flue gases have various amounts and types of impurities thatmay affect the separation of CO2 over N2. The concentration ofthe impurity components in the flue gas depends on the varioustrade-offs that were made during the system design and optimiza-tion, and are consequences of technical standards. The understand-

ing of how such impurities affect CO2 capture, transport, and

Fig. 1. Contributions to greenhouse gas mitigation for six different scenarios according Philibert et al. [3].

N. Hedin et al./ Applied Energy 104 (2013) 418433 419


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storage is indispensable[14]. Li et al.[1518]discussed and ana-lyzed the effects of impurities by calculating the thermodynamicproperties of CO2 in the presence of impurities. We expect thepresence of impurities to be at least equally as important inadsorption processes as they are in absorption processes.

This review focuses on sorbents for the post-combustion cap-ture of CO2, and the related swing adsorption, materials engineer-ing, and system integration aspects. Other reviews presentinformation on the additional engineering aspects [7,19,1921].We review the chemistry-related literature on adsorbents for theyears 20102011, and parts of 2012; for older studies, see Choiet al.[22]and Hedin et al.[23].

2. Adsorbents

Gas molecules that are close to a surface achieve a reduced freeenergy, as they are attracted to the electronic environment of thesurface[24]. The attractive gassolid interactions and the associ-ated loss of entropy lead to an increase in the number density ofgas molecules close to the surface. For a gassolid system, gas mol-ecules spend a much longer time on the surface than in the gas.This surface excess of molecules is called adsorption. The thermo-dynamic behavior of the gassolid system is described by adsorp-tion isotherms (p,T), which capture how the surface excessdepends on temperature and pressure (seeFig. 2for a representa-tion of two such adsorption isotherms). The adsorption of CO2canoccur with or without the formation of chemical bonds. Moleculesphysisorb via a range of physical interactions. For the physisorp-tion of CO2, the electric quadrupole moment-electric field gradientinteraction often dominates the interactions of CO2 with a solidsurface. The magnitude of the electric quadrupole moment ofCO2 is roughly three times larger than that for N2 [30]; hence, itinteracts more strongly than N2with the electrical field gradientsin porous materials. The partitioning of CO2on surfaces is higherthan that of N2in a gas mixture of CO2and N2[7].

The amount of molecules adsorbed increases with increasingpressure, up to a maximum capacity. Because of the exothermicnature of adsorption, the amount of gas adsorbed increases withdecreasing temperature. Since the uptake of gas depends stronglyon both temperature and pressure, gas separation processes basedon cyclic temperature or pressure variations have been devised. Intemperature swing adsorption (TSA), the adsorbent is regeneratedby raising the temperature. In pressure swing adsorption (PSA), thegas components are captured at somewhat elevated pressures, andthe adsorbent is regenerated by lowering the pressure (it should be

noted that the pressures used are always higher than atmosphericpressure, even after the pressure is decreased). Vacuum swingadsorption (VSA) is similar to PSA, and is another non-cryogenicgas separation process. The regeneration is conducted at reducedpressures, and the adsorption occurs at ambient or near-ambientpressures. In conventional PSA, the gas mixture is pressurized dur-ing certain periods in the adsorption cycle. However, the mass

flows of N2and CO2are enormous in the flue gas stacks of powerplants, and it is difficult to imagine how such flows could be com-pressed. In addition to the technical difficulties with compression,Chaffee et al. [25] indicated that regular PSA might also be tooexpensive for the post-combustion capture of CO2. A 1000 MWcoal-fired plant produces 1000 tonnes of dilute-stream CO2 perhour. Hence, we imagine that either TSA or VSA, or combinationsthereof, would be the optimal choice for the capture of CO2undersuch conditions.

Adsorbents are divided into classes defined by their pore sizes.According to the International Union of Pure and Applied Chemis-try (IUPAC), micropores are smaller than 2 nm, mesopores are be-tween 2 and 50 nm, and mesopores are larger than 50 nm. A highcapacity for the adsorption of CO2 is essential for CO2-capture

adsorbents. In this review, we describe the microporous sorbentsas adsorbents, but for such microporous solids the difference be-tween adsorption and absorption is somewhat arbitrary. Derouane[26]discussed how the cases of volume-filling and surface cover-age overlap when the pores become extremely small. Adsorbentsmay have pores in the form of slits, cylinders, or cages intercon-nected by pore windows. When the sizes of the pores, slits, or porewindows are molecule-sized they can separate gas molecules byequilibrium, kinetic, or molecular sieving mechanisms (Fig. 3).Gas molecules have different effective kinetic diameters withinsolids and in gases. CO2appears to have a smaller kinetic diameterthan N2 in microporous solids. In the gas phase, CO2has a largerkinetic diameter than N2[27]. The exact values appear to be sub-strate-dependent, but values of 3.3 and 3.8 have been suggested

for CO2 and N2 in zeolites, respectively [28,29]. When the pore win-dows are much larger than the sizes of CO2 and N2, the potential for

A

B

C

Fig. 3. When the sizes of the pores, slits, or pore windows are molecule-sized theycan separate CO2 from N2 gas molecules by: (A) equilibrium, (B) kinetic, or (C)molecular sieving mechanisms. Here, blue represent N2molecules and yellow CO2.(For interpretation of the references to colour in this figure legend, the reader isreferred to the web version of this article.)

Low T

High T

n1

n2

High pLow p Pressure

Amount

Fig. 2. Two typical adsorption isotherms, which illustrate how the thermodynamicbehavior of the gassolid system depends on temperature and pressure.

420 N. Hedin et al./ Applied Energy 104 (2013) 418433


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the separation of CO2 and N2 is determined by the differential equi-librium partitioning of these molecules, as illustrated in Fig. 3A.When the aperture of the pore openings approaches the size ofthe somewhat larger N2, the CO2-over-N2selectivity of the adsor-bent is enhanced by the difference between the diffusivities. Thediffusivity of N2 is reduced significantly when the dimensions ofthe pore window aperture approach 3.8 ; in contrast, the diffusiv-ity of CO2still matches well with the pore window (Fig. 3B). If thepore window is smaller than 3.8 , the mixture of CO2and N2canbe molecularly sieved (Fig. 3C).

For VSA, the working capacity of an adsorbent is largely defined

by the differential between the uptake at a pressure at 0.1 bar andthe uptake under vacuum conditions.Table 1gives details for dif-ferent sorbents, with the temperature at which adsorption occurs,the amount of CO2adsorbed at a partial pressure of 0.1 bar of CO2,and the selection mechanism for the adsorption. To compare theuptake levels with a hypothetical working capacity, one must as-sume that the uptake under dynamic vacuumconditions is negligi-ble. The capture of CO2 would most likely be implemented afterflue gas condensation, when the temperature of the gas mixtureis 330 K. The adsorption of CO2is seldom studied at this temper-ature: most of the values documented in the literature were deter-mined at temperatures of 273298 K. The uptake of CO2is higherat lowtemperatures than at high temperatures, due to the exother-mic nature of adsorption. The low temperature and the absence of

a subtraction of the uptake under dynamic vacuum conditionmeans that the values inTable 1are overestimates of the workingcapacity. However, the table should still be helpful for a compari-son of various sorbents.

In addition to a working capacity for CO2 sorption, there is arange of other physical, chemical, chemical engineering, and solidmechanics criteria that must be fulfilled for a successful adsor-bent/desorbent. Composition, particle size, pore size, pore volume,and pore connectivity are all essential characteristics that affectthe function of a CO2sorbent. It appears that the heat of adsorptionfor CO2should, ideally, have an intermediate value. If it is too high,the sorbent will be difficult to regenerate economically, and if it istoo low the capacity for adsorbing CO2and the CO2-over-N2selec-tivity will be too small[47]. An ideal CO2adsorbent should have a

high capacity for the uptake of CO2and a high CO2-over-N2selec-tivity, and it should be easy to regenerate, have excellent mass

transport mechanical properties, be tolerant to water vapor, andbe robust to the presence of other gases.

2.1. Microporous physisorbents

Physical adsorption (physisorption) denotes cases with rela-tively weak interactions at the gassolid interfaces. These interac-tions are similar to those appearing in real gases, in terms of theirphysical mechanisms. When adsorbed molecules undergo elec-tronic reconfigurations at the surface, similar to a chemical bond,we typically refer to the process as chemical adsorption (chemi-

sorption); chemisorption is dealt with in Section2.2.Microporous adsorbents have interconnected pores with mole-

cule-sized pore windows, and can be used to separate gas mole-cules via equilibrium, kinetic, or molecular sieving mechanisms.These adsorbents can be crystalline or amorphous. Crystallineadsorbents have remarkably narrow pore-size distributions, dueto their well-defined atomic positions, whereas amorphous adsor-bents have broader pore-size distributions, due to the more ran-dom distribution of bond angles and bond lengths among theiratoms. Zeolites and metal organic frameworks (MOFs) are crystal-line microporous adsorbents, whereas carbon molecular sieves(CMS) and microporous polymers are amorphous microporousadsorbents. Both classes are being studied extensively for theirproperties as CO2 adsorbents, and for their potential use as CO2

sorbents in adsorption-based gas separation.

2.1.1. Zeolites and aluminum phosphates

Zeolites, which are crystalline microporous aluminosilicateswith large internal specific surface areas and volumes, have net-works of interconnecting channels, or cages. Zeolites are describedby the number of oxygen atoms encircling their pore-windowapertures or channels (which form a percolating structure), andthey are referred to as 8-, 10-, and 12-membered ring zeolites; typ-ical such zeolites are displayed in Fig. 4AC, respectively.

2.1.1.1. Hydrophilic zeolites. The zeolite NaX, which is the moststudied adsorbent for the capture of CO2 [22,23], has a structuralcode of FAU [48]. The cages of zeolite X are interconnected by large

windows, seeFig. 4C. The adsorption of CO2on zeolite NaX is largeeven at relatively small pressures of CO2. Its large cages are con-

Table 1

Different sorbents, temperature at which adsorption occurs, amount of CO2 adsorbed at a partial pressure of 0.1 bar of CO2, and the selection mechanism of the adsorption.

Sorbent T(K) n(mmol/g) Selection Ref.

Zeolites

NaX 273 4.0 Physisorption [31]NaA 298 3.1 Physisorption [32]Zeoltie-rho 303 3 Physisorption [33]AlPO4-53 273 0.7 Physisorption [34]

SAPO4-56 273 2.5 Physisorption [31]MOFs

Mg-MOF-74 273 5.8 Chem (?)/Phys [35]rht-MOF-7 273 1.2 Physisorption [36]CD-MOF-2 273 1.6 Physisorption [37]SUMOF-3 273 1.1 Physisorption [38]

Microporous carbons and polymers

Carbon molecular sieve: VR-93 298 1.3 Physisorption [39]Microporous polymer: PI-1 273 0.7 Physisorption [40]Microporous polymer: BILP-.4 273 1.3 Physisorption [41]Microporous polymer: Cs-N1 273 1.2 Physisorption [42]COF-103 273 0.8 Physisorption [43]

Amine-based sorbents

Triamine modified silica (PE-MCM-41) 293 2.3 Chemisorption (Phys) [44]Propylamine modified silica (AMS-6) 273 1.6 Chemisorption (Phys) [45]Amine-modified MOF (CAU-1) 273 1.6 Chem (?)/Phys [46]

Note: For abbreviations see the text or the references.



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can be further tuned using the extra-framework cations that bal-ance the charge of the framework, as certain cation sites are pres-ent near the pore window. Hence, zeolite A is a suitable molecularsieve for small gas molecules. As discussed above, CO2appears tohave a smaller kinetic diameter than N2in microporous solids, andvalues of 3.3 and 3.8 have been suggested for CO2and N2in zeo-lites, respectively [28,29]. Zeolites CaA, NaA, and KA are calledzeolite 5A, 4A, and 3A, respectively, with the number denotingthe approximate size of the pore window aperture. Breck et al.[59] showed that the capacity for the adsorption of CO2 variedwith the Na+-to-K+ ratio on zeolite NaKA, and Yeh and Yang [60]extended this study to report enhanced CO2-over-N2 selectivityof CO2on zeolite NaKA, using percolation theory. Previous studiesexamined aspects of this kinetic selection of CO2 over N2 onzeolite NaKA [32,61], and Liu et al. [32] showed a high CO2-over-N2selectivity by tuning the Na

+-to-K+ ratio in zeolite NaKA.For zeolite NaA, the pore window is large enough for both N2and CO2 to adsorb; however, this is not the case for zeolite KA.For a carefully selected Na+-to-K+ ratio, zeolite NaKA shows asignificantly enhanced CO2-over-N2 selectivity, compared withzeolite NaA. K+ preferably replaces Na+ in the site close to thepore-window aperture; hence, K+ affects the size of the pore win-dow even at small levels of exchange. The uptake kinetics of CO2on structured versions of zeolite NaKA have been examined previ-ously, and the zeolite showed high selectivity for CO2-over-N2selection, and a rapid uptake of CO2 [61]. It seems that the CO2selectivity of zeolite NaKA sorbents can be significantly enhancedby creating a differential between the CO2and N2diffusion rates,without significantly affecting the uptake kinetics, and that thezeolite can be structured into monoliths that can be used in rapidswing cycles. Zeolite NaKA exhibits advantageous properties forthe removal of CO2 from dry mixtures of CO2and N2. It is likelythat an additional water-removal step would have to be imple-mented for this adsorbent to be used for the removal of CO2fromflue gases. Of note, in the zeolite NaX family, zeolite NaKA is anextremely hydrophilic sorbent.

Adsorbents for CO2 are also researched and used for the pro-cessing of natural gas and biogas. We do not provide an extensivereview of the literature related to this work, but we do mentionsome studies that are important for the post-combustion captureof CO2 from flue gases. In the context of the upgrading of biogasfrom landfills, Montanari et al. [62] studied the coadsorption ofCO2with methane on the zeolites X and 4A. Similarly as for meth-ane, the capacity for the sorption of CO2 was lower on zeolite Athan on zeolite 13X; however, the regeneration with N2was fasterfor zeolite 4A than for 13X (zeolite 13X is another name for zeoliteNaX). They concluded that the capacity for the adsorption of CO2was enhanced in the presence of water on zeolite 13X, but noton zeolite NaA. Such enhancement of the adsorption of CO2contra-dicts the earlier findings discussed above. It would be advanta-

geous if the effects of water on the adsorption of CO2on zeolitesNaX and Na(K)A could be clarified further, as it would clearly beadvantageous if water enhanced the adsorption of CO2. Tomadakiset al.[63]studied the separation of H2S from CO2on zeolites 4A,5A, and 13X with PSA, and concluded that almost pure CO 2couldbe produced using zeolites 5A and 13X.

Bulnek et al. [64] studied the combined physisorption andchemisorption of CO2 on zeolite ferrierite, where the charge-balancing ions on the zeolite were exchanged for alkali metalcations (FER) (Li+, Na+, K+). They concluded that the energy ofadsorption with a low coverage of CO2 was much higher for theversions of zeolite ferrierite with a low Si-to-Al ratio than for thosewith a high ratio. Ridha et al.[6567] investigated the feasibility ofthe low-pressure encapsulation of CO2 within zeolite NaKCHA,

arguing that the hysteresis loop observed in the low-pressure re-gime could allow for the encapsulation of CO2. They studied the

adsorption of CO2 and N2 on zeolites LiCHA, NaCHA, and KCHA.We believe that zeolite NaKCHA could potentially be applied forthe kinetic separation of CO2over N2. It would also be interestingto compare the properties of zeolite NaKA and NaKCHA. Hudsonet al.[68]recently documented an enhanced CO2-over-N2selectiv-ity in zeolite SSZ-13 with counterions exchanged for Cu2+; this zeo-lite has a CHA structure and molecule-sized pore windowapertures.

In this subsection, we highlighted a study that investigated amicroporous titanium silicate[69].This material is not strictly azeolite, per definition, but it is hydrophilic and related to zeolites.Park et al. [69] studied the adsorptionof CO2 on a microporous tita-nium silicate, ETS-10, which had its counterions exchanged for al-kali cations. Among the studied forms, Na+-ETS-10 showed thehighest specific surface area, while Li+-ETS-10 showed the highestuptake of CO2 at 298 K. Li-ETS-10 showed the highest basestrength, and Cs+-ETS-10 showed the lowest.

2.1.1.2. Hydrophobic zeolites. Zeolites can be rather hydrophobic,and such zeolites could be relevant for the capture of CO2 in thepresence of H2O. Zeolites with a high Si-to-Al ratio are more hydro-phobic than those with a low ratio. Lively et al. [70]showed thatsuch hydrophobic zeolites could be introduced as an adsorbentwithin hollow fibers. They showed that these fiber-adsorbent con-structs could, in principle, separate CO2 at an energy cost of0.13 GJ/ton-CO2 captured, when certain heat integration prob-lems were solved. In another study, Lively et al.[71] examined hol-low polymeric fibers with embedded adsorbent particles, andshowed how this composite could be used for rapid TSA processes.Such hollow fibers with adsorbents appear to be relevant for rapidTSA, which could be regenerated by steam. Membranes that arecombined with sorbents are called mixed matrix membranes(MMMs), and many studies have investigated these materials.Chung et al. and Bernardo et al. reviewed this literature in detail[72,73]. Yang et al. [74]studied ion-exchanged zeolite beta, andtheir capacities as adsorbents for CO2. Zeolite beta typically has a

large Si-to-Al ratio, and is consequently a relatively hydrophobiczeolite. They concluded that zeolite beta with K+ ions showed thebest performance. Zukal et al. [75] measured and compared the up-take of CO2on six high-silica zeolites: TNU-9, IM-5, SSZ-74, ferrie-rite, ZSM-5, and ZSM-11. The channels of ZSM-5 (Fig. 4B) areencircled by 10-membered rings. IM-5 had the highest capacityto adsorption of CO2and could be highly relevant for further stud-ies. Further developments in hydrophobicadsorbents with a signif-icant capacity for CO2adsorption are expected.

2.1.1.3. Aluminum phosphates. The structures of microporous alu-minophosphates (AlPO or AlPO4) and silicoaluminophosphates(SAPO or SAPO4) are closely related to those of zeolites, and veryclosely related to those of microporous silicates [76,77]. Liu[34]

studied a range of AlPO4 structures with small pore apertures,and concluded that certain structures exhibited an enhancedCO2-over-N2 selectivity. This enhancement was hypothesized tobe related to a kinetic effect. The overall uptake of CO2was some-what lower than expected, possibly due to a non-perfect calcina-tion. In SAPO4, the framework carries a negative charge, which isbalanced by cations. SAPO4-34 is an interesting adsorbent thathas the structural character of chabazite (CHA). Zhang et al. [78]prepared Sr-SAPO4-34 and showed that it adsorbed more CO2thandid Na-SAPO4-34. In the context of inorganic membranes forCO2-over-N2 separation, Li and Fan [79] showed that SAPO4-34membranes exceeded what was previously held to be the upperbound for CO2-over-N2separation with membranes. This sorbentcould also be relevant to the study of adsorption-driven separation,

and could potentiallybe appliedas the sorbent component in MMMconstructs. Cheung et al.[31]showed that the H-form of SAPO4-56



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exhibited a large capacity for CO2adsorption, similar to the uptakeon zeolite NaX. This large capacity for CO2 adsorption wascombined with an uptake of H2O lower than that on zeolite X atlowvapor pressures of water. These findings suggest that thecapac-ity for the adsorption of CO2 is significantly higher on SAPO4-56than on SAPO4-34.

2.1.2. Carbon molecular sievesCarbon molecular sieves (CMSs) are microporous carbon-basedsorbents with molecule-sized pores. They are prepared in a multi-step procedure that uses the carbonization of biomass (coconutshell granules or similar), activation, deposition (typically usingchemical vapor deposition (CVD)), and the subsequent carboniza-tion of aromatic molecules. These adsorbents have found applica-tions in the production of N2 from air. Ultrapure N2 can beproduced via kinetically enhanced separation, using the differencesin the diffusion coefficients for N2and O2in a certain CMS[28,80].Ruthven and Reyes[81]showed that in the kinetically enhancedseparation of gases, the particle size distribution (PSD) affects theselectivity.

Alcaniz-Monge et al. [82,83]studied the adsorption of CO2 onCMSs in the form of particles and monoliths, and made a detailedinvestigation of the effects of pore blockage. Bikshapathi et al.[84] prepared CMSs from carbon fibers, and studied the break-through of adsorbed CO2. Silvestre-Albero et al.[39]studied CMSs,and concluded that they are at least equivalent to, if not betterthan, certain MOFs (zeolites 13X and 5A) in terms of several as-pects relating to the adsorption of CO2. Wahby et al.[85]studiedsimilar CMSs and their potential use as selective adsorbents forCO2. We believe that the possibilities for a kinetic selection ofCO2-over-N2on CMSs are worthy of more detailed study.

2.1.3. Metal organic frameworks, covalent organic frameworks, and

microporous polymers

Metal organic frameworks (MOFs) have been studied as adsor-bents for CO2. MOFs are crystalline compounds with intercon-

nected pores, and are formed by metal ions coordinated withrigid organic linkers. Fig. 4D shows a structural model of thewell-studied MOF-5. MOFs have been studied in some detail in re-cent years for the separation of CO2in the upgrading of natural gas,biogas, and landfill gas, and for the capture of CO2from flue gas.These applications were reviewed recently by Bae and Snurr [86],who evaluated different MOFs in terms of their uptake of CO2, theirworking capacity as a sorbent, their regenerability, their adsorptiveselectivity, and a combined adsorptive and desorptive selectionparameter. In their screening, they did not consider the kineticsof CO2or any heat-related aspects. In their analysis, the followingMOFs displayed various characteristics that would be beneficial inadsorption-driven flue gas capture based on vacuum swing pro-cesses: zeolitic imidazole frameworks (ZIFs) ZIF-78, -79, -81, -82

[87], and Co-carborane MOF-4b[88],Ni-MOF-74[89], ethylenedi-amine-H3[(Cu4Cl)3-(BTTri)8][90], [ZN2(tcpb){p-(CF3)NC5H4}2][91].ZIFs have topologies identical to zeolites, and derive their structurefrom heterocyclic and nitrogen-containing linkers[86]. In additionto the research reviewed by Bae and Snurr[86], we highlight a fewrecent studies. Debatin et al.[92]showed that MOFs based on thein situ synthesis of an imidazolate-4-amide-5-imidate ligand dis-played a significant uptake of CO2. MOFs can form interpenetratedstructures with smaller pore sizes, compared with their non-inter-penetrated equivalent structures. These interpenetrated structuresare promising for the separation of CO2from gas mixtures, becausethe electrical field gradients are larger in small pores than in largepores. Yao[38]studied the uptake of CO2 and N2 on three suchinterpenetrated MOFs based on Zn4O clusters and rigid dicarboxyl-

ates, and McDonald et al.[93]showed remarkably high capacitiesand selectivities for the adsorption of CO2 in an Mg-based MOF

modified with bifunctional amines. Functionalized MOFs can alsoexhibit enhanced interactions with CO2, via either enhanced elec-trical field gradients, or specific interaction patterns induced bythe functionalization. Devic et al.[94,95]indicated that such inter-actions can be of the donoracceptor type between certain hydro-xyl groups and CO2. It is technically difficult to differentiatebetween the contributions from enhanced electrical field gradientsand donoracceptor interactions for functionalized MOFs, com-pared with their non-modified equivalents. Spectroscopy, and inparticular IR spectroscopy, can be helpful for such differentiation.

Mg-MOF-74 displays very promising properties[35]; however,it is unclear if the parasitic contributions for an adsorption-drivenseparation of CO2-over-N2would be lower for MOFs than for zeo-lites or similar adsorbents. A recent study by Lin et al. [47]indi-cated that MOFs might have greater parasitic contributions thancertain zeolites. This indication, together with the potentially highcost of production and the often water-sensitive nature of MOFs,may limit their suitability as adsorbents for the post-combustioncapture of CO2from flue gases.

Recently, covalent organic frameworks and microporous poly-mers have been studied in detail for various applications, includingthe separation of CO2from flue gases. These microporous organicscan exhibit ultrahigh specific surface areas, and other relevantproperties [96]. Studies have shown that certain members of thisclass of solids can adsorb significant amounts of CO2and simulta-neously display a high CO2-over-N2 selectivity, under conditionsrelevant for the post-combustion capture of CO2 from flue gas[4143,97100].

2.2. Mesoporous physisorbents

Mesoporous adsorbents have pores in the size regime of 250 nm, which allows for rapid mass transport. Belmabkhout et al.[101,102] reported the high adsorption capacity of mesoporous sil-ica for CO2, CH4, N2, H2, and O2, andMa et al.[103] showed that theadsorption of CO2on a MCM41 sorbent with both micro- and mes-

opores could be improved by tailoring the mesopores using eitherhexadecyltrimethylammonium bromide (CTAB) as a soft template,or mesoporous carbon as a hard template. These solids displayed ahigh capacity for the adsorption of CO2at pressures of CO2higherthan those used in VSA or TSA. We doubt that mesoporous physi-sorbents could be applied for the capture of CO2from flue gases;however, the inclusion of mesoporosity within microporous sor-bents could be particularly beneficial for the mass transport kinet-ics, and could allow for more rapid swing cycles.

Activated carbons are well-known adsorbents, and have micro-and mesopores. Choi et al.[22]and Hedin et al.[23]compared theperformance of these materials for the separation of CO2, and re-viewed the related literature. Their capacity for the adsorption ofCO2 is typically large only at high pressures of CO2. Siriwardane

et al.[104] studied and compared the adsorption of CO2on zeolitesand activated carbons, revealing that at a CO2pressure of 4 bar and25 C, the activated carbons had adsorbed approximately 50% oftheir equilibrium capacity. These activated carbons showed signif-icantly higher adsorption of CO2 compared with zeolites at highpressures of CO2, but displayed a lower uptake of CO2at pressuresof CO2relevant for the removal of CO2from flue gases using VSAand TSA.

Dantas et al. [105]studied breakthrough curves for CO2, andconcluded that amine-modified active carbons did not display lar-ger capacities than unmodified carbons. An et al.[106]fabricatedactivated carbons fromphenolic resins. These materials had mainlymicropores, and the researchers concluded that small pores aresuperior to large pores when the partial pressure of CO2is low in

the mixture from which the CO2is to be separated. Activated car-bons have a weaker physical interaction with CO2than aluminum-



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rich zeolites such as zeolite NaX and NaA. This means thatalthough the specific surface area of activated carbons can be large,they typically display a rather small uptake of CO2 at low pres-sures, which renders them less relevant as adsorbents for VSA.

2.3. Amine-modified mesoporous silica chemisorbents

Chemisorbents display properties that give them the potentialto be applied for the separation of CO2from flue gases, and as sor-bents for the chemical looping cycle (CLC). Current research fo-cuses on weak chemisorbents such as amines, and strongchemisorbents such as strong bases. For strong chemisorbents thatare relevant for CLC, we refer to the review by Choi et al. [22]. Here,we review recent studies on amine-based silica. Substrates otherthan silica have been used as supports for amines, including car-bons, zeolites, and MOFs[107112].

Alkanolamines in water and chilled ammonia have been studiedfor the capture of CO2from flue gases. In the liquid phase, the CO2-amine chemistry is largely known [113,114]. Amines react withCO2and form various compounds. Masuda et al.[115]showed thatcarbamates were formed in protophobic solvents, carbamic acidswere formed in protophilic solvents, and HCO3 formed in propa-nol/water mixtures. The local chemistry at the silica interface is ex-pected to be somewhat different from that in water, which couldopen up different energetics and kinetics for the sorption of CO2.Additional benefits could result from tethering the amine on solidsubstrates, in terms of the potential environmental and health con-cerns regarding byproducts from the liquid-based processes. Thehigh volatilities of the amines and byproducts could be reducedby tethering the amines. Certain alkanolaminewater mixturesare particularly corrosive, and tethering the amines to substratescould reduce such corrosion.

In analogy with the chemistry of amines and CO2in the liquidphase, amines and polyamines have been tethered to silica sup-ports, or have been introduced simply by coating or filling thepores. While silica-amine sorbents can be loaded with large

amounts of amines, there is a tradeoff between amine loadingand the remaining surface area. When the pores of the substrateare filled with the amine, the capacity for the adsorption of CO2is large, but there is no room for gas transport within the compos-ite sorbents, and the advantage of rapid gas-phase transport is lost.Hence, the amine-filled porous materials act primarily as absor-bents, rather than adsorbents.

Recently, there has been discussion regarding which silica sub-strate should be used. Mesostructured silica with ordered poreshas typically been used as a substrate for amines. Although theadsorption data may be easier to interpret from such well-orderedsubstrates, there are both economic and technical advantages inusing unordered and mesoporous silica supports [116]. However,mesostructured silica supports are commonly rather unstable un-

der steam conditions[117].The pores of the silica should not be too small. It has been

shown that it is advantageous to expand the pores of MCM-41and similar silica substrates before tethering the amines [44,118123]. MCM-48 is a silica substrate with unusually large specificsurface areas, and a three-dimensionally interconnected structure.Bacsik et al.[45]observed a higher uptake of CO2on AMS-6 thanon MCM-48 when both were modified with tethered amine groups.AMS has larger pores, but the same structure as MCM-48.

The chemistry governing the reactions of CO2with amines onsilica supports is not fully understood. Molecular spectroscopytechniquesmost often Fourier transform infrared (FTIR) spectros-copy, but also 13C nuclear magnetic resonance spectroscopyhavebeen used to study the chemistry of these reactions [44,124129].

When tethered amines react with CO2 they mainly form carba-mateammonium ion pairs, when the amine density is large en-

ough. Two amine groups react with one CO2 molecule.Carbamateammonium ion pairs are the main control on whetherwater vapor is present. At low amine densities, carbamic acid andsilylcarbamates tend to form in parallel with each other. Such sily-lcarbamates appear to form only under dry conditions. The uptakeof CO2 typically increases under moist conditions [44,130,131].Danon et al. [125] excluded the formation of HCO3

- during theadsorption of CO

2on SBA-15 mesoporous silica modified with (3-

aminopropyl) triethoxysilane, even with water present. Numerousauthors have speculated about the formation of HCO3 , perhaps inanalogy with reactions with liquid amines. The uptake of CO2onamines tethered to sorbents at low pressures of CO2tends to havea critical dependence on the amine density. The low-pressure up-take becomes significant only when the local amine density is highenough for ammoniumcarbamate ion pairs to form. Previousstudies systematically studied this effect for monoamines tetheredto silica[131133].

It is important to quantify the contributions from the physi-sorption and chemisorption of CO2on amines, because this balancedetermines the regeneration conditions that are necessary for therecovery of CO2from the sorbent. Thermodynamic methods (suchas conventional adsorption measurements) are not ideal for thedifferentiation of such contributions, especially for cases whenthe heats of adsorption are similar between physisorption andchemisorption. Aziz et al.[127]quantified these contributions byusing infrared spectroscopy to determine the chemisorption andphysisorption of CO2 on amine-modified silica, and showed thatCO2mainly chemisorbed, even at low amine densities.

Mesoporous silicas have also been modified with amine oramine-like functionalities other than (3-aminopropyl)triethoxysi-lane. For example, Kim et al.[134]recently studied derivatives ofhydroxylated amidine in their solid state for the capture of CO2,Belmabkout et al.[135,136]studied silica modified with triamines,which showed a significant uptake of CO2, and Serna-Guerrero[137,138]modeled the kinetics and thermodynamic of the uptakeof CO2on related sorbents.

Hicks et al.[139]studied tethered and hyperbranched polyeth-yleneimine (PEI) on mesoporous silica; this system displayed effec-tive and reversible capture of CO2 [140]. These compositesdisplayed high amine loading, and chemical anchoring of amines,which made them attractive for the capture of CO2. Drese et al.[141] measured an extraordinary CO2-uptake capacity of5.6 mmol/g for hyperbranched aminosilica (HAS) on SBA-15 sup-ports, and Choi et al. [142] investigated related adsorbents for theirpotential use in the capture of CO2 from dilute mixtures, whichcould be useful for applications in submarines or space shuttles,or, speculatively, for the capture of CO2 from air. Satyapal et al.[143]discussed these hybrid sorbents, which were based on a por-ous polymer, oligomeric amines, and a polymeric modifier that en-hanced the rate of uptake of CO2. Using a similar approach, several

groups have studied mesoporous silica filled or coated withamines, revealing that tetraethylenepentamine (TEPA) in particularshowed significant potential for the capture of CO2[144147]. Insuch composite sorbents, polyethylene glycol enhanced the CO2sorption kinetics of TEPA[148]. Using impregnation from solution,Ebner et al.[149]introduced up to 40 wt.% of polyethylene(imine)into a support of porous silica in the form of beads. During theapplication of up to 76 consecutively applied adsorptiondesorp-tion cycles, the working capacity for the sorption of CO2under iso-thermal conditions ranged from 0.25 to 2.8 mol/kg, with amaximum observed at a temperature of 80 C. Jiang et al. [150]studied the CO2 adsorption capacity of porous spheres of poly-methylmethacrylate covered with multilayers of amine-basedpolyelectrolytes, revealing that up to 10 layers of PEI and polysty-

rene sulfonate (PSS; which were added using a layer-by-layer pro-cess) resulted in a capacity for CO2 adsorption of 1.7 mol/kg



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sorbent. Wang et al. [151] improved the CO2adsorption capacity ofsuch films (containing PEI) by introducing surfactants to createadditional pathways for the CO2 to diffuse into the films. At30 C, the porous film with PEI adsorbed up to 142 mg/g of CO2,and >50% of the amines reacted with CO2. Films with PEI andsurfactants showed improved CO2sorption kinetics compared withthose without surfactants, and the regeneration performanceswere comparable for films with and without surfactants. The PEIfilms were deposited on three types of mesoporous supports: hier-archically porous silica, mesoporous carbon aerogels, and meso-porous silica of the type SBA-15. Qi et al. [152] studied thecapture of CO2 on nanocomposite sorbents based on capsules ofmesoporous silica functionalized with PEI or TEPA. Under simu-lated flue gas conditions, the TEPA-functionalized capsules showeda maximum capacity of 7.9 mmol/g for the adsorption of CO2. Thekinetics for the CO2uptake were fast: within the first few minutes,90% of the total CO2 uptake capacity was filled. The amine-functionalized capsules, with a thin and mesoporous silica shell(12 nm) and a large interior void volume, showed the highestcapacity to adsorb CO2. The use of larger particles increased boththe optimal amine content and the capacity for the adsorption ofCO2. Composite sorbents impregnated or filled with oligomericamines could potentially be used for the large-scale capture ofCO2; however, their longevity has been questioned[140].

Oxidation in the presence of oxygencould degrade amine-basedsorbents. Flue gases can contain significant amounts of oxygen gas,and monoethanolamine undergoes such oxidative degradation.Bollini et al. [153] showed that tethered primary and tertiaryamines were stable in an oxidizing environment with a somewhatelevated temperature, but secondary amines degraded. They con-cluded that it would be difficult to use secondary amines in thepost-combustion capture of CO2. This limits the potential of teth-ered secondary diamines, triamines, and TEPA and PEI supportedon silica for the adsorption-driven post-combustion capture ofCO2in the case of a high oxygen content in the flue gas.

The parasitic losses related to the chemical reactions of amines

with certain impurities have received little attention. Many aminestend to form heat-stable salts with SO2and carbonyl sulfide (COS)[154], and one would expect amine-modified silica to react in asimilar way, possibly with an added parasitic contribution. Wehave not found any studies investigating how irreversible reactionsbetween COS and amine groups could affect amine-based sorbents.

3. Swing adsorption

Swing adsorption processes make use of the differences be-tween the tendencies for adsorption of the individual componentsin multicomponent gaseous mixtures (such as flue gases fromcombustion). The most common swing adsorption methods use acyclic variation of the pressure or temperature. These methodsare known as pressure swing adsorption (PSA), vacuum swingadsorption (VSA), thermal swing adsorption (TSA), and the relatedelectrically induced thermal swing adsorption (ESA)[28,155].

Here, we review the extensive literature on the optimizationand variation of these swing adsorption processes, focusing onthe removal and concentration of a minority component of CO2(heavy) from N2(light), due to its potential use in post-combustioncapture. Industrial PSA processes are closely related to the originalSkarstrom cycle[156], and find uses in, for example, refinery tech-nology[78]. This cycle includes two beds, which are subjected topressurization, adsorption, blow down, and purge, with a patternthat maintains a continuous stream of product. Fig. 5illustratesthese developments for one of the beds in this cycle. The bed ispressurized with CO2-lean N2-rich gas in step A, as illustrated inFig. 5A. The flue gas is fed to the bed in step B, displayed inFig. 5B and CO2-lean gas is produced. CO2-rich gas is produced instep C (blow down) and step D (purge), as illustrated in Fig. 5Cand D.

These PSA processes typically purify weakly adsorbing species(e.g., H2) from gaseous mixtures with strongly adsorbing species.CO2 typically adsorbs more strongly to surfaces than N2, due toits larger electric quadrupolar moment. To recover CO2from a mix-ture with a large proportion of N2, other PSA processes have beendeveloped, modeled, and evaluated. The methods used to recoverheavy components using PSA are known as heavy-reflux PSAprocesses. A rinse step is introduced in the Skarstrom cycle afterthe feed, which equilibrates the beds and flushes out the lightproduct (N2) before the heavy component (CO2) is recovered. Aso-called dual-reflux PSA process has been developed, and models,

descriptions, and experimental demonstrations for this processhave been presented [157159]. In this particular PSA cycle, a rinsestep was introduced to increase the purity of the strongly adsorbedcomponent (CO2) over the weakly adsorbed component (N2);however, Park et al. [160]showed that the rinsing dramaticallyincreased the power consumption. Fiandaca et al.[161]presenteda preliminary investigation of an algorithm for fast-cycle PSAoperation. Although their study focused on the production of N2from air, it was also helpful for the optimization of CO2separation.Thakur et al. [162] recently argued that the duplex PSAproposed by Leavitt[163]could be modified to achieve a compactprocess.

It is likely that conventional PSA processes will be difficult toimplement for the capture of CO2 in flue gases from large point

sources. Indeed, Chaffee et al. [25]indicated that this process istoo expensive [25]. We believe that the technical difficultiesmainly lie in the pressurization steps in PSA, and in the integrationof the enormous flow in the flue gas stack for a normal power sta-tion. However, there are advantages in combusting fuel at some-what elevated pressures [164], and for such power plants theapplication of rapid PSA could be more straightforward. RapidVSA and rapid TSA are more promising for the capture of CO2fromflue gases. Note that the CO2capture step in VSA and TSA must beintegrated with the complex thermodynamics of the power gener-ation plant itself. The separation performance can be challenging,due to the different compositions of the flue gases and the varia-tions in the configuration of the VSA/TSA with the power plant. Re-cent studies have investigated the integration of CCS with the

various components in a power generation system[165172]. Suchintegration may result in a reduction in the energy consumption

N2

CO2

N2

CO2:N2(15:85)

N2

CO2

N2

CO2

CO2

N2

N2

N2

CO2

A B C D

Fig. 5. One of the beds in a Skarstrom PSA cycle, which is subjected to: (A)

pressurization, (B) adsorption of CO2 and production of N2, (C) blow down withproduction of CO2and (D) purge with production of CO2.



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and the costs of the overall system. The composition of the fluegases depends on the fuel and type of combustion used, but it isgenerally dominated by N2, 3%15% CO2 by volume, H2O, O2, Ar,and small amounts of SOx and NOx [5,173]. Many amines tendto form heat-stable salts with SO2and COS [153], and such salts in-crease the parasitic contributions to the CO2 separation anddecrease the lifetime of the sorbent. Besides the thermal perfor-mance, it is essential to optimize the economic performance byconsidering external factors, which could include policy incentivesand regulations devised for the implementation and commerciali-zation of CCS[5].

In VSA, the strongly adsorbed component is recovered at sub-atmospheric pressures. Such vacuum recovery of adsorbed CO2ispromising for the separation of CO2from flue gases in a dual-bedconfiguration [157159,173176]. Ho et al. [13] showed that acombined process using zeolite 13X with both high-pressure andvacuum desorption steps resulted in a predicted cost for CO2re-moval that was comparable to that of CO2 capture using MEAabsorption. The selectivity of Zeolite X is high, but not high enoughto give pure CO2with a single step. However, if the CO2-over-N2selectivity could be enhanced, the purity could be increased, andthe cost of the separation could be reduced. Recently, Delgadoet al. [177] used theoretical modeling to evaluate the feasibilityof CO2recovery using activated carbon in a novel VSA cycle. Theirmodel described the structure of the setup using the bed void frac-tion and an axial dispersion coefficient, and the particle diameterin Erguns equation [177,212]. The theoretical results indicatedthat it would be possible to recover CO2with high purity (>93%)from a mixture containing 13% CO2at 40 C, with higher recovery(>90%) and lower power consumption (95%, which appears to be a specific requirement[178]. Aar-on and Tsouris[7] suggested that PSA and VSA likely cannot beused (with a standard sorbent) as a stand-alone processes for thecapture of CO2; however, they suggested that they could be used

as a pre-step before an absorption-driven process. This adds tothe existing motivation for the further development of adsorbentsdesigned for processes with small parasitic contributions to theseparation of CO2from flue gases.

In TSA, steam can be used for the regeneration of adsorbents,thereby giving TSA an advantage over VSA, which requires a vac-uum. Merel et al. [179]proposed, and demonstrated experimen-tally, the use of TSA for the capture of CO2 from flue gases.Recently, the prospects for the economic implementation of TSAwere strengthened by Grande et al. [180,181]in their evaluationof CO2 capture using an ESA process. They estimated the energyconsumption associated with the separation of CO2to be 2.04 GJ/tonne CO2, for an adsorbent consisting of 70 wt.% zeolite X and30 wt.% of a conducting binder. Moate et al. [182] performed a

numerical study of the material energy balances for the captureof CO2 from N2/O2/CO2 mixtures using a TSA system, concludingthatdetailedheat managementis necessaryto maximize the regen-eration efficiency. Combining vacuum temperature swing cycleswith steam may offer advantages, as argued by Dutcher et al. [183].

The sheer size of typical power plants places certain constraintson the cycle times that can be used in capturing CO 2using swingadsorption processes. The flow of gas through the stack of a powerplant is enormous: a 1000 MW coal-fired plant produces1000 tonnes of CO2/h in a dilute stream. A rough calculation givesa relation between the mass of the sorbent needed and the cycletime as follows: (mass adsorbent) (relative capacity) = (flowrate) (cycle time) (number of beds). A typical cycle of 10 minwould demand 3000 tonnes of adsorbents. These quantities of

adsorbents are impractically large, and would lead to the needfor huge amounts of processing equipment. By reducing the cycle

times, smaller quantities of adsorbents could be used, and the costfor separation could be reduced. Methods to achieve rapid swingcycles, and technologies associated with these processes, are cur-rently under development [184]. We strongly believe that theuse of structured or supported adsorbents will be crucial for suchfast-cycling of VSA, VPSA, or TSA. Section4reviews current devel-opments related to such structured adsorbents.

In an extensive screening study, Harlick and Tezel [185]con-cluded that zeolites NaX or NaY were the best adsorbents for cap-turing CO2 using a PSA process, in terms of the capacity androbustness during cyclic adsorption. The PSA/VSA process hadlow partial pressures of CO2in the feed, and used low regenerationpressures. Gomes et al. [186]investigated a two-bed PSA processusing experiments and calculations, and a range of adsorbentswas tested (zeolite 5A, NaX, one type of CMS, and gamma alumina).A breakthrough test indicated that zeolite NaX was the most suit-able sorbent. Tlili et al. [187] studied the small-scale capture of CO2using zeolite 5A, and suggested that VSA may be more useful on alarge scale, while Yoshida et al. [188]showed that trace compo-nents of a heavy component (CO2or Xe) could be successfully en-riched from air using an enriching reflux PSA process with aparallelized equalization step.

Harlick and Tezel [189] determined the CO2 adsorption iso-therms for NaY at different pressures and temperatures (20200 C, 0.0052.0 bar). The collected data were described well bythe Toth isotherm. From this isotherm, they derived the workingcapacities for a few PSA, TSA, and pressure temperature swingadsorption (PTSA) cycles. For the NaY, they predicted that a PSAor PTSA process would have the highest capacity when operatedunder steep gradient (temperature or pressure) conditions. Phelpsand Ruthven [190] tested the performance of a counter-current ad-sorber (with silicalite) on an endless belt. Although an endless beltadsorber would be volume-efficient, its engineering aspects arecomplex, and this method is unlikely to find widespread usage.

Sorbents with mesopores allow faster gas diffusion than micro-porous sorbents. Mesopores havethe potentialto reduce the limita-

tions associated with mass transport in micropores during loadingand unloading on the sorbents. Chaffee et al. [25]reported on arange of amine-modified sorbents with mesopores for applicationsin PSA/VSA processes, emphasizing the need to study the effects ofimpurities. Of note, it is important in general to study the effects ofimpurities on post-combustion capture processesthis is by nomeans specific to adsorption-driven processes. In addition, Chaffeeet al.[25]presented data for amine-modified adsorbents that ad-sorbed more CO2at higher temperatures than at low temperatures.This dependency occurredbecausethe pores became filledwith thereactive aminosilane during synthesis, and access to the pores wastherefore diffusion-limited at low temperatures. Dasgupta et al.[191]employed PEI-impregnated SBA-15 in a five-step PSA cycle,and concluded that the hybridsorbent was comparable to commer-

cial zeolite adsorbents in terms of productivity (kg-CO2/min). Theadsorbent could be regenerated under typical PSA conditions.

Wurzbacher et al. [192] studied temperature-vacuum swingprocesses using a sorbent based on a commercial silica gel withtethered amine groups, revealing a stable performance over 40adsorption/desorption cycles. Bastin et al.[193] gave the first dem-onstration of the separation and removal of CO2from binary mix-tures of CO2/N2and CO2/CH4, and from ternary mixtures of CO2/N2/CH4, using a fixed bed with a sorbent based on an interpenetratedmetal organic framework MOF-508b.

4. Structured and supported adsorbents

Conventional swing processes with adsorbents rely on the useof beds of adsorbents in the form of packed beads or pellets, with



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the size of the pellets typically being several micrometers [24].Packed beds have a low operational cost and allow for continuousoperation, but the temperature control is poor and heat waves candevelop, thereby reducing the beds abilities to separate CO2fromN2. The cycle times for packed beds are commonly on the order oftenths of minutes, where the mass transfer resistance is not a lim-iting factor. The beds operate under the fluidization threshold,which makes it difficult to reduce the cycle times. For the rapid cy-cles needed for the capture of CO2from flue gases, the contact timebetween adsorbent and adsorbate is likely to be on the order ofseconds. Hence, the timescale of adsorption in the column mustbe significantly reduced to match that of the packed beds, to avoidthe immediate breakthrough of the input gas[194,195]. The time-scale of adsorption in a packed bed can be reduced by using smal-ler beads or pellets, thus creating shorter diffusion paths, but theuse of smaller particles will also lead to unmanageable pressuredrops and low mechanical strength. Structured or supported adsor-bents offer an alternative to packed beds for swing adsorption pro-cesses, and could potentially achieve much faster mass transferkinetics, smaller pressure drops, improved heat transfer, and amore uniform temperature distribution. Such rigid and durableadsorbent structures could also ensure a long lifetime for theassemblies and a correspondingly high number of operating cycles.

4.1. Structured adsorbents

For all swing adsorption processes, the microscopicsorbent par-ticles must be assembled into macroscopic shapes, as a fine pow-der cannot be used directly. For such assemblies, the powder isstructured using different kinds of binders, but binderless methodsare currently being developed. Pavlov et al.[196]reviewed the lit-erature on the binder-free granulation of zeolites NaX and zeoliteA. These methods could enhance the overall capacity, and possiblythe selectivity, by minimizing the use of binders; however, beadsare less suited for rapid swing cycles, as fluidization should beavoided in PSA/VSA/TSA with beds[197].

Adsorbents can also be structured into more advanced macro-scopic shapes, including hierarchically porous structures, lami-nates, and foams that provide enhanced transport and allow thepossibility of using rapid cycle times. Rezaei and Webley [198] pre-sented a methodology to compare different configurations of struc-tured adsorbents, indicating that, for example, laminatedstructures can be superior to pellets if the sheet is thin enough. Re-zaei et al. [199,200] showed through a theoretical study that adsor-bent particles with highly branched and macroporous channelsand an overall macroporosity of 4050% represented a near-opti-mal structure, when the working capacity for cyclic adsorptionprocesses was maximized. Rezaei et al.[201]developed a mathe-matical model to simulate the diffusion and adsorption in a systemof zeolite X films on cordierite supports, and parameterized this

model to fit experimental data. The model indicated that a filmthickness of 10 lm would be optimal for nonporous supports with1200 cpsi. For these dimensions, the capacity for CO2 adsorptionwould approach the adsorption capacity of beads, and the CO2adsorption kinetics would not be compromised. A simplified math-ematical model was employed to describe the (dispersed plug)flow within the monolith, and it was assumed that the uptake ofCO2 within the zeolite film and on the porous support occurredin parallel, due to the presence of open grain boundaries and cracksin the zeolite film.

Monoliths could have significant advantages over packed bedsof adsorbents in, for example, rapid VSA for the post-combustioncapture of CO2. Higher massflows could be used through the mono-lith than through a packed bed, if the monolith was appropriately

structured to reduce the pressure drop. The kinetics for the adsorp-tion of CO2have been studied in monolithic carbons by Brandani

et al.[202]. Burchell et al.[203]developed a composite adsorbentfor usage in TSA/ESA systems, where the monolithic body allowedfor rapid heating. The combination of the monolithic body with anappropriate Ohmic resistance enabled the bed to be heated rapidlyusing a current of 5A. Hao et al. [204]synthesized carbon-basedmonoliths with pore structures on multiple length scales; thesemonoliths were nitrogen-doped, and exhibited a significant CO2uptake capacity and CO

2-over-N

2 selectivity. Akhtar [205,206]

developed a monolithic form of zeolite NaX, using a slip castingprocess followed by a thermal treatment on sacrificially templatedzeolite monoliths, and Schumann et al.[207]reduced the amountof binder used in the granulation of zeolite X via a specializedtreatment. Akhtar[61]showed recently that mechanically strongzeolite NaKA monoliths had the highest estimated figure of meritwith a K+ content of 10 at%. The figure of merit combined theCO2-over-N2 selectivity (in an IAST model) with the time depen-dent capacity to adsorb CO2. Natural extensions of this studywould include structuring such zeolite NaKA monoliths to reducethe pressure drop, or introducing multifunctionalities to enableESA. This could assist in assessing the potential of this techniquefor practical use in the post-combustion capture of CO2from dryflue gases.

The adsorption and desorption of CO2on surfaces are exother-mic and endothermic processes, respectively, and cause a thermalwave to develop within the adsorbent bed [80]. The velocity of thistemperature wave can be many times higher than that of theadsorbing gas [208210]. The different rates of heat and masstransfer within the bed can have detrimental effects on the overallperformance of the adsorbent. The heat transfer in structuredadsorbents can be improved compared with packed beds by incor-porating a thermally conducting support material [204,211].Increasing the heat transfer is of high importance for the achieve-ment of rapid temperature swings, and could be helpful in otherconfigurations, where it could decouple the rates of heat transferfrom the rates of mass transfer; this would make the processesmore robust to variations in the operational parameters. The high

porosity and thin walls of a macroporous structure may also im-prove the transport of heat through convection, resulting in a moreuniform distribution of heat[198,200].

Small pressure drops can be achieved in structured adsorbentsby using rapid cycling, and by increasing the number of accessiblepathways and widening the channels in the adsorbents[198,200].However, it should be noted that the volumetric working capacityof the adsorbent decreases when the size and number of passagesis increased. Introducing transfer paths of random orientation willincrease the average path of transportation for gas molecules,which is typically quantified by the tortuosity (a geometric factorthat gives the measure of a geometrical constraint independentof temperature and the nature of the gaseous species [24]). The tor-tuosity is commonly used in empirical and theoretical models such

as Erguns equation and the KozenyCarman equation, which de-scribe the pressure drop and mass transport in porous media,respectively [212,213]. Increasing the tortuosity of a structuredadsorbent will improve the mixing of gases, but will also increasethe pressure drop. Highly ordered structures such as honeycombsand laminar structures have a tortuosity of 1, which is lower thanthe tortuosity values of 23 commonly assigned to packed bedsand irregularly structured adsorbents. In the most fundamentalsense, macropores are needed in adsorbents to facilitate the con-tact between the adsorbents microparticles and the CO2. Struc-tured adsorbents have certain advantages over packed beds,since their working capacities are typically higher[198,200].

This example highlights the difficulties in quantifying a com-plex macroporous structure for simulation purposes. Such quanti-

fications are needed to describe the detailed mesoscaledependencies in packed beds and structured adsorbents for the



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post-combustion capture of CO2. Dantas et al.[214216] simulatedCO2-over-N2separation in a PSA fixed-bed single column, and as-sessed the separation purity of CO2, the recovery of CO2, and thebreakthrough curves for different temperatures, feed times, andpurge times, to match experimental data. They used a model basedon the linear driving force (LDF) approximation for the masstransfer to consider the energy and momentum balances. Theadsorbents studied included zeolite X, activated carbon, and N-enriched activated carbon. They assumed that the diffusion of N2and CO2into the adsorbent was controlled by molecular diffusionin the macropores, following previous studies by Ruthven et al.[217,218]. The mass transfer in the column was dependent onparameters that were characteristic of the macroporous structure;i.e., the particle diameter, the porosity, the axial dispersion coeffi-cient, and an assumed tortuosity value of 2.2 for zeolite X andactivated carbon, and 1.8 for N-enriched activated carbon. Thepressure drop and velocity changes were described using Ergunsequation, which considers the overall structure of the fixed bedin terms of the porosity and particle diameter. The performanceof the (macroporous) structure was thus only related to material-specific and constant parameters, and not directly to the structureitself. Only the tortuosity could be said to describe the architectureof the structure, but this parameter was chosen more or lessarbitrarily.

4.2. Supported adsorbents

Supported adsorbents consist of an active adsorbent supported(typically as a coating) by a matrix that provides the mechanicalstrength and the desired macroscopic structure; an example ofsuch a matrix is a honeycomb structure. The production of sup-ported adsorbents has strong similarities with that of heteroge-neous catalysts, where the active catalytic material is distributedonto the surface of a ceramic support [219,220]. The manufactur-ing of supported adsorbents typically proceeds via two steps,where the matrix or support is commonly produced using tradi-

tional ceramic processing techniques that involve extrusion[221]followed by sintering[222]. The adsorbent can then be applied tothe surfaces of the supports using various techniques, such asdeposition from solgel or hydrothermal solutions, or powderdeposition. Recent examples include the method used by Moscaet al.[223], who applied NaX coatings to cordierite supports usinga hydrothermal approach originally developed for membranefilms. They showed that the use of these supported adsorbents inPSA systems resulted in smaller pressure drops than when regularadsorbents were used. Rezaei et al.[201]deposited films of zeoliteX of varying thickness on supports based on cordierite monolithswith a cell density of 1200 cells per square inch (cpsi).

Carbon supports are also of interest, as they have low densityand relatively high heat conduction. Wu et al. [224] supported

nanosized calcium oxide particles on mesoporous carbon, andevaluated their capacity for the adsorption of CO2at temperaturesof 200500 C. These CaO/C composites, activated at a temperatureof 700 C for 1 h in an N2atmosphere, showed the capacity to ad-sorb up to 7 mmol/g of CO2when subjected to an atmosphere of100% CO2for 2 h. Unfortunately, the CaO nanocrystallites grew insize after repeated adsorptiondesorption cycles. A grain-growth-related reduced capacity is common for the CaCO3/CaO/CO2 systemused in CLC, which will not be reviewed further here.

5. Conclusions

We have presented an overview of the significant results for thecapture of CO2from flue gases using sorbents. The sorbents were

categorized as microporous and mesoporous, and as chemisor-

bents and physisorbents. Many microporous solids show consider-able potential as sorbents for CO2, especially those with smallpores and a high isosteric heat of physisorption. Certain MOFs alsodisplay advantageous properties, but it is unclear whether theseproperties are significantly better than those of other materials.It is likely that the enormous flue gas flows in power plants cannotbe compressed, which impacts the choice of adsorbent. Physisor-bents with small micropores are likely more suitable for thepost-combustion capture of CO2 than mesoporous sorbents. VSAor TSA with rapid cycles make physisorbents with large pores lessuseful. The study of the kinetic selection of CO2over N2has begun,and we expect many more studies of such adsorbents in the future.

Many of the studied sorbents are sensitive to water in the fluegases, and we expect this to be the subject of further study. VSAor TSA would most likely be used after the water condensationstep, but the amount of water in the flue gases would still be sig-nificant. Previous studies suggest that microporous organic, puremicroporous silica, and aluminumphosphate sorbents are less sen-sitive to water than are traditional zeolites, and most MOFs. We ex-pect more studies to be performed on microporous polymers toinvestigate their suitability as CO2 sorbents. In general, they aremore water-stable than MOFs, and it might be possible to integratethem in polymer membranes or MMMs.

Amine-modified mesoporous solids also show considerable po-tential as chemisorbents for CO2. They show high CO2-over-N2selectivity and are robust towards water. In analogy with liquidamines, supported amines could be sensitive to impurities suchas SO2 and COS. The latter has not been considered in detail bythe research community. Studies have indicated that secondaryamines are sensitive towards oxidative degradation, which couldlimit the possibilities of using supported TEPA and PEI. We expectadditional studies on the eventual degradation of secondaryamines in representative flue gas mixtures at temperatures rele-vant for the post-combustion capture of CO2. We anticipate ongo-ing investigations of both mesostructured silica and unorderedmesoporous silica substrates as substrates for amines. For actual

applications in the flue gas capture of CO2, the inexpensive andunordered silica substrates are likely to be most relevant.

We reviewed the literature on structured adsorbents for CO2.Traditionally, adsorption-driven gas separation has been particu-larly attractive for midsized separation units, because the swingcy-cles are 10 min long; however, such cycle times are notcompatible with the enormous flows of flue gases in large powerplants. Additional developments in rapid swing cycles for the sepa-ration of CO2from flue gases are forthcoming. Such rapid cycles re-quire structured adsorbents that are mechanically robust. It shouldbe noted that structured adsorbents could allow for rapid and morecost effective adsorption-driven gas separation, but such processesmust be analyzed in combination with the full thermodynamics ofthe power plant. Heat integration is expected to be crucial.

We expect more research on the system integration of adsorp-tionseparation units for the post-combustion capture of CO2 inpower plants. Such integration must be analyzed while consideringthe full thermodynamics, including those of the adsorption-drivencapture of CO2. For example, it is essential to study how traceamounts of various components could interfere with the adsorp-tion processes and the overall thermodynamics. The parasitic con-tributions associated with the use of chemisorbents orphysisorbents should be further studied. It seems that the heat ofadsorption should be not too small, nor too large.

The development of adsorbents with high capacity, high selec-tivity, rapid uptake, easy recycling, and suitable thermal andmechanical properties is a challenging task. Truly interdisciplinarystudies could enable this delicate optimization to be performed in

the near future.



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References

[1] Hester RE, Harrision RM, editors. Carbon capture sequestration andstorage. Cambridge, UK: The Royal Society of Chemistry; 2010.

[2] Saundry P, editor. Carbon capture and sequestration program of the USdepartment of energy. Environmental information coalition, national councilfor science and the environment, Washington, DC; 2009.

[3] Philibert C. Technology penetration and capital stock turnover-lessons fromIEA scenario analysis. OECD and IEA information paper: COM/ENV/EPOC/IEA/

SLT. International Energy Agency, Paris; 2007, vol. 4.[4] Haszeldine RS. Carbon capture and storage: how green can black be? Science2009;325:164752.

[5] Chen L, Yong SZ, Ghoniem AF. Oxy-fuel combustion of pulverized coal:Characterization, fundamentals, stabilization and CFD modeling. Prog EnergyCombust Sci 2012;38:156214.

[6] Metz B, Davidson OR, de Conick HC, Loos M, Mayer LA. IPCC special report oncarbon dioxide capture and storage. Cambridge, UK: Cambridge UniversityPress; 2005.

[7] Aaron D, Tsouris C. Separation of CO2from flue gas: a review. Sep Sci Technol2005;40:32148.

[8] Isaacs EE, Otto FD, Mather AE. Solubility of mixtures of H 2S and CO2 in amonoethanolamine solution at low partial pressures. J Chem Eng Data1980;25:11820.

[9] Mago BF, West CW. Corrosion inhibitors for alkanolamine gas treatingsystem. US patent 3,959,170; 1976.

[10] Mamun S, Svendsen HF, Hoff KA, Juliussen O. Selection of new absorbents forcarbon dioxide capture. Energy Convers Manage 2007;48:2518.

[11] Feron PHM. Exploring the potential for improvement of the energy

performance of coal fired power plants with post-combustion capture ofcarbon dioxide. Int J Greenh. Gas Control 2010;4:15260.

[12] Baxter J, Bian Z, Chen G, Danielson D, Dresselhaus MS, Fedorov AG, et al.Nanoscale design to enable the revolution in renewable energy. EnergyEnviron Sci 2009;2:55988.

[13] Ho MT, Allinson GW, Wiley DE. Reducing the cost of CO 2capture from fluegases using pressure swing adsorption. Ind Eng Chem Res 2008;47:488390.

[14] Pipitone G, Bolland O. Power generationwith CO2 capture: technology for CO2purification. Int J Greenh. Gas Control 2009;3:52834.

[15] Li H, Ji X, Yan J. A new modification on RK EOS for gaseous CO 2and gaseousmixtures of CO2and H2O. Int J Energy Res 2006;30:13548.

[16] Li H, Yan J. Impacts of equations of state (EOS) and impurities on the volumecalculation of CO2 mixtures in the applications of CO2 capture and storage(CCS) processes. Appl Energy 2009;86:276070.

[17] Li H, Yan J. Evaluating cubic equations of state for calculation of vaporliquidequilibrium of CO2and CO2-mixtures for CO2capture and storage processes.Appl Energy 2009;86:82636.

[18] Li H, Yan J, Yan J, Anheden M. Impurity impacts on the purification process inoxy-fuel combustion based CO2 capture and storage system. Appl Energy2009;86:20213.

[19] Yong Z, Mata V, Rodrigues AE. Adsorption of carbon dioxide at hightemperature a review. Sep Purif Technol 2002;26:195205.

[20] Lee KB, Beaver MG, Caram HS, Sircar S. Reversible chemisorbents for carbondioxide and their potential applications. Ind Eng Chem Res 2008;47:804862.

[21] White CM, Strazisar BR, Granite EJ, Hoffman JS, Pennline HW. Separation andcapture of CO2from large stationary sources and sequestration in geologicalformations Coalbeds and deep saline aquifers. J Air Waste Manage Assoc2003;53:645715.

[22] Choi S, Drese JH, Jones CW. Adsorbent materials for carbon dioxide capturefrom large anthropogenic point sources. ChemSusChem 2009;2:796854.

[23] Hedin N, Chen LJ, Laaksonen A. Sorbents for CO2 capture from flue gasaspects from materials and theoretical chemistry. Nanoscale 2010:181941.

[24] Ruthven DM. Principles of adsorption and adsorption processes. NewYork: John Wiley & Sons, Inc.; 1984.

[25] Chaffee AL, Knowles GP, Liang Z, Zhang J, Xiao P, Webley PA. CO2capture byadsorption: materials and process development. Int J Greenh Gas Control

2007;1:118.[26] Derouane EG. Zeolites as solid solvents. J Mol Catal A Chem 1998;134:2945.

[27] Reid RC, Prausnitz JM, Sherwood TK. The properties of gases and liquids. NewYork: McGraw-Hill; 1977.

[28] Yang RT. Gas separation by adsorption processes. London: Imperical CollegePress; 1997.

[29] Breck DW.Zeolite molecularsieves. Malabar: Robert E. Krieger Pub. Co.;1974.[30] Graham C, Imrie DA, Raab RE. Measurement of the electric quadrupole

moments of CO2, CO., N2, Cl2and BF3. Mol Phys 1998;93:4956.[31] Cheung O, Liu Q, Bacsik Z, Hedin N. Silicoaluminophosphates as CO2sorbents.

Microporous Mesoporous Mater 2012;156:906.[32] Liu Q, Mace A, BacsikZ, Sun J, Laaksonen A, Hedin N. NaKA sorbents with high

CO2-over-N2 selectivity and high capacity to adsorb CO2. Chem Commun2010;46:45024.

[33] Palomino M, Corma A, Jorda JL, Rey F, Valencia S. Zeolite Rho: a highlyselective adsorbent for CO2/CH4 separation induced by a structural phasemodification. Chem Commun 2012;48:2157.

[34] Liu Q, Cheung NCO, Garcia-Bennett AE, Hedin N. Aluminophosphates for CO2

separation. ChemSusChem 2011;4:917.

[35] Britt D, Furukawa H, Wang B, Glover TG, Yaghi OM. Highly efficientseparation of carbon dioxide by a metal-organic framework replete withopen metal sites. Proc Natl Acad Sci USA 2009;106:2063740.

[36] Luebke R, Eubank JF, Cairns AJ, Belmabkhout Y, Wojtas L, Eddaoudi M. Theunique rht-MOF platform, ideal for pinpointing the functionalization and CO2adsorption relationship. Chem Commun 2012;48:14557.

[37] Gassensmith JJ, Furukawa H, Smaldone RA, Forgan RS, Botros YY, Yaghi OM,et al. Strong andreversible binding of carbon dioxide in a green metal organicframework. J Am Chem Soc 2011;133:153125.

[38] Yao Q, Su J, Cheung O, Liu Q, Hedin N, Zou X. Interpenetrated metal-organic

frameworksand their uptakeof CO2at relatively low pressures. J Mater Chem2012;22:1034551.

[39] Silvestre-Albero J, Wahby A, Seplveda-Escribano A, Martnez-Escandell M,Kaneko K, Rodrguez-Reinoso F. Ultrahigh CO2adsorption capacity on carbonmolecular sieves at room temperature. Chem Commun 2011;47:68402.

[40] Laybourn A, Dawson R, Clowes R, Iggo JA, Cooper AI, Khimyak YZ, et al.Branching out with aminals: microporous organic polymers fromdifunctional monomers. Polym Chem 2012;3:5337.

[41] Rabbani MG, El-Kaderi HM. Synthesis and characterization of porousbenzimidazole-linked polymers and their performance in small gas storageand selective uptake. Chem Mater 2012;24:15117.

[42] Kiskan B, Antonietti M, Weber J. Teaching new tricks to an old indicator: pH-switchable, photoactive microporous polymer networks fromphenolphthalein with tunable CO2 adsorption power. Macromolecules2012;45:135661.

[43] Furukawa H, Yaghi OM. Storage of hydrogen, methane, and carbon dioxide inhighly porous covalent organic frameworks for clean energy applications. JAm Chem Soc 2009;131:887583.

[44] Belmabkhout Y, Sayari A. Effect of pore expansion and aminefunctionalization of mesoporous silica on CO2adsorption over a wide rangeof conditions. Adsorpt 2009;15:31828.

[45] Bacsik Z, Ahlsten N, Ziadi A, Zhao G, Garcia-Bennett AE, Martn-Matute B,et al. Mechanisms and kinetics for sorption of CO2 on bicontinuousmesoporous silica modified with n-propylamine. Langmuir2011;27:1111828.

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