flue gas treatment via co2 adsorption.pdf

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Chemical Engineering Journal 171 (2011) 760–774

Contents lists available at ScienceDirect

Chemical Engineering Journal

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Flue gas treatment via CO2 adsorption

Abdelhamid Sayari a,b,∗, Youssef Belmabkhouta, Rodrigo Serna-Guerrero b

a Department of Chemistry, University of Ottawa, 10Marie Curie, Ottawa, ON,Canada K1N6N5b Department of Chemical andBiological Engineering, University of Ottawa, 161 Louis Pasteur, Ottawa, ON,Canada K1N6N5

a r t i c l e i n f o

Article history:

Received 20 June 2010

Received in revised form 14 January 2011

Accepted 5 February 2011

Keywords:

CO2 adsorption

Zeolite

Carbon

MOFs

Supported amines

a b s t r a c t

Adsorption separation has gained considerable attention as a viable alternative to the currently used,

high energy-demanding aqueous amine scrubbing technologies. This review is a summary of the main

contributions regarding the development of new adsorbents for post-combustion CO2 capture. Emphasishas been placed on materials evaluated at representative flue gas conditions of CO2 partial pressure (i.e.,

0.05–0.2 bar) and temperature (25–75 ◦C). Whenever possible, the effect of moisture on the adsorbent

stability and CO2 uptake is included, although relatively few studies in the literature have focused on this

issue. This review includes adsorbents produced by modification of existing commercial materials as well

as newly developed materials. These adsorbents were separated in two major classes, namely (i) physical

adsorbents including carbons, zeolites and metal-organic frameworks and (ii) chemical adsorbents, i.e.,

amine-functionalized materials. A critical analysis of the literature is provided with the aim of tracing

the main paths currently pursued toward the development of suitable CO2 adsorbents and to provide a

general overview of the advantages and limitations of each family of adsorbents.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

As the concentration of carbondioxide (CO2) in the atmospherekeeps increasing, serious concerns have been raised with respect

to its impact on the environment. Since it started being monitored

in 1958, the increase of CO2 concentration in the atmosphere has

accelerated from less than 1ppm/yr prior to 1970 to more than

2 ppm/yr in recent years [1]. As a result, the atmospheric level of

CO2 increased from 315ppm in 1958 to 385ppm in 2009 [1,2].

CO2 is considered to be the main anthropogenic contributor to

the greenhouse gas effect, as it is allegedly responsible for 60% of

the increase in atmospheric temperature, commonly referred to as

global warming [2,3]. Among the various sources of CO2, approxi-

mately 30% is generated by fossil fuel power plants, making them

major contributors to global warming [4]. Despite their impact on

theenvironment, it is acknowledgedthat fossil fuelswill remain the

leading source of energy for years to come, for both power gener-ation and vehicle transportation. Therefore, it is critical to develop

effective methods for the capture and sequestration of CO2 from

post-combustion effluents, such as flue gas. Some reviews dealing

with the main sources of CO2 and potential strategies to prevent

theirrelease to theenvironment areavailable in the literature [5,6].

Gasabsorption using alkanolaminesolutions has been used forCO2

∗ Corresponding author at: Department of Chemistry, University of Ottawa,

10 Marie Curie, Ottawa,ON, Canada K1N 6N5.

E-mail address: [email protected] (A. Sayari).

scrubbing on industrial scalefor decades. However,this process has

a number of shortcomings. For example, it generates severe cor-

rosion of the equipment, and the regeneration of amine solutionsis highly energy intensive [7]. These drawbacks have been widely

documented, prompting a search for alternative technologies. One

viable route is adsorptionwhich, comparedto other separationpro-

cesses, is recognized to be attractive to complement or replace the

current absorption technology due to its low energy requirement

[4,8]. Therefore, the use of appropriate adsorbents may potentially

reduce thecost associatedwith CO2 separationin theoverallcarbon

capture and storage (CCS) strategy.

Suitable adsorbents for CO2 removal from flue gas should com-

bine several attributes, including:

(i) High CO2 adsorption capacity: CO2 equilibrium adsorp-

tion capacity is one of the main properties used to screen

new adsorbents. Knowledge of the equilibrium adsorption

isothermsisofprimeimportanceforearlyevaluationofpoten-

tialadsorbents.Whenever possible,this review willbe focused

on adsorption properties measured under conditions relevant

to flue gas treatment, i.e., less than 0.4 bar CO2 partial pres-

sure with a total gas pressure of 1–2bar and temperature

below 70–80 ◦C.As a ruleof thumb,Hoet al. [9] suggested that

an optimum adsorbent for CO2 capture from flue gas, should

exhibit a CO2 adsorption capacity of 2–4mmol/g. It is well

established that from the slope of the adsorption isotherm at

low pressure, it is possible to estimate the adsorbate affin-

ity for a given adsorbent. Thus, in terms of CO2 uptake, the

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A. Sayari et al. / Chemical Engineering Journal 171 (2011) 760–774 761

ideal materials shouldexhibit a CO2 adsorption isotherm with

steep slope (favorable CO2 adsorption isotherm) correspond-

ing to high uptake at low CO2 partial pressure. A less steep

slope (unfavorable CO2 adsorption isotherm) is indicative of a

lower affinity toward CO2.

(ii) Fast kinetics: Adsorption kinetics affects primarily the work-

ing adsorption capacity in dynamic processes such as

adsorption in a fixed bed column. A suitable CO2 adsorbent

will have a high rate of adsorption, resulting in a working

capacity close to equilibrium capacity over a wide range of

operatingconditions.However, determinationof kinetic prop-

erties such as diffusionis one of themost challengingissues in

adsorption science, as it involves parameters not always read-

ily available, such as particle size of the adsorbent and use of

adequate experimental set-ups and conditions.

(iii) High CO2 selectivity: The adsorbentselectivitytoward CO2 has

a direct impact on the degree of purity of the product. This in

turn, plays a major role in the economics of the CO2 adsorption

process [9]. Ideally,anadsorbentforfluegastreatmentwillnot

adsorb any nitrogen.

(iv) Mild conditions for regeneration: The ease of regeneration of

the adsorbent is a key property in the selection of materials

for CO2 separation. Depending on the structural and chemi-

calproperties of the adsorbent, adsorption–desorption cyclingmay be achieved via temperature, pressure (or vacuum), con-

centration swing adsorption or a combination thereof. In

practice, incorporation of functional groups can be used to

modify the adsorbent–adsorbate interactions (e.g., Van der

Waals, electrostatic, hydrogen bonding or acid–base interac-

tions) and affect the CO2 uptake and selectivity. Optimum

interactions should be neither too weak nor too strong. Too

weak bonding results in low CO2 adsorption capacity at low

pressure, but easy regeneration. Conversely, strong bonding

induces high adsorption capacity but desorption will be diffi-

cult and costly.

(v) Stabilityduring extensive adsorption–desorption cycling: The

lifetime of adsorbents, which determines the frequency of

theirreplacement, is a critical property of equal importance asthe CO2 adsorption capacity, selectivity and kinetics, because

of its direct impact on the economics of any commercial scale

operation.

(vi) Tolerance to the presence of moisture and other impurities in

the feed: In addition to CO2 and N2, flue gas contains water

vapor and other impurities such as O2 and SO2. The degree

of tolerance and the affinity of the adsorbent to such impu-

rities may affect significantly the strategy to be used, with

direct impact on the overall economics of the CO2 separa-

tion process. Moisture is knownto adverselyaffect CO2 uptake

in a variety of physical adsorbents such as zeolites and acti-

vated carbon. Consequently, the strategies proposed for CO2adsorption from flue gas is likely to include an upstream dry-

ing step. As a result, the overwhelming majority of publishedreports dealing with physical adsorbents have not examined

moisture effects. Whenever possible, this review will provide

a general picture on the behavior of the adsorbents in the

presence of moisture. It is also generally established that CO2adsorbents have high affinity to SO2 and even some affinity

toward NOX, which may adversely affect the CO2 adsorp-

tion capability of the material. Thus, abatement of SO2 and

NOX from flue gas prior to CO2 capture is required in most

cases.

(vii) Low cost: This is another important parameter to be con-

sidered in the development of any potential adsorbent. At

this stage, information on adsorbent cost and other economic

considerations are rather scarce in the literature. Thus, cost-

related issues will not be discussed in this review.

Because flue gas is generally cooled down to ca. 55◦C, to allow

appropriate conditions for SO2 and NOX abatement [10,11], when-

ever possible, this review will be focused on literature reports

dealing with CO2 adsorption using 5–20% CO2-containing mixtures

with a total pressure of 1–2 bar, and temperature between 25 and

70 ◦C. Fora more comprehensiveaccounton CO2 adsorbents in gen-

eral, the reader may refer to an excellent review by Choi et al. [12].

Similarly, the field of high temperature CO2 capture has also been

reviewed by Lee et al. [13].

Adsorbents for CO2 capture can be categorized in many ways,

based on their chemical composition, structural characteristics or

according to the adsorption mechanism involved, i.e., physical vs.

chemical.Physical adsorbents for CO2 capture include carbon mate-

rials, alumino-silicas such as zeolites, alumino-phosphates (AlPOs)

and alumino-silico-phosphates (SAPOs), and more recently metal

organic frameworks (MOFs). The CO2 chemical adsorbents dis-

cussed in this review refer to those obtained through incorporation

of amine groups into solid supports such as mesoporous silica.

Consequently, in this review we distinguished two classes of CO2adsorbents for stack gas treatment, namely physical and amine-

functionalized adsorbents.

2. Physical adsorbents

2.1. Carbons

Because of their wide availability, low cost and high ther-

mal stability, it is largely established that activated carbons have

advantages over other CO2 adsorbents. Among the carbon based

adsorbents reported in the literature, activated carbons (ACs) and

carbon nanotubes (CNTs) are the most investigated materials. CO2adsorption on activated carbons has been studied experimentally

and theoretically for a long time [14] and has found commercial

applications[15,16]. There is a wide rangeof activated carbons with

a large variety of microporous and mesoporous structures. Acti-

vated carbon may be produced from many raw materials such as

coal, coke pitch, wood or biomass sources (e.g., saw dust, coconutshells, olive stones), often via two steps: carbonization and acti-

vation [17]. Carbon molecular sieves (CMS), which are a sub-class

of activated carbon with narrow pore size distribution (PSD), are

kinetic-based adsorbents. They have been commercialized mainly

for the separation of air and the production of high purity N2[18,19]. However, at low CO2 partial pressure, activated carbons

exhibit lower adsorption capacity and selectivity than zeolites due

mainly to their less favorable adsorption isotherms. In spite of

the hydrophobic character of carbon-based adsorbents, their CO2adsorption ability is adversely affected by the presence of water

vapor [20].

Table 1 shows literature data on CO2 adsorption capacity and

selectivity of activated carbons and carbon nanotubes in the par-

tial pressure range of 0.1–0.4 bar at 298–333 K. Considering 1 and2 bar as thelowest andhighesttotalpressureof fluegas, the0.1 and

0.4 bar were chosen arbitrarily as the lowest and the highest CO2partial pressure relevant to flue gas treatment. Notice that most

studies dealing with CO2 adsorption on activated carbons were

undertaken at high pressure and room temperature.

It is important to notice that, although adsorption capacity

varies considerably for different activated carbons at high pressure

[24,25], the adsorption capacity at low pressure is less sensitive to

the nature of carbons. As seen in Table 1, the typical CO2 equilib-

rium adsorption capacityfor activatedcarbons at a partial pressure

of 0.1bar is 1.1mmol/g at room temperature but decreases rapidly

to 0.25mmol/g at 328K. In terms of CO2 adsorption capacity, acti-

vated carbons may be particularly interesting for CO2 removal but

only at high pressure. For example, Himeno et al. [24] showed


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Table 2

Literature survey on CO2 adsorption properties of some zeolites and zeolite-like materials at low pressure.

Zeolites/Si/Al ratio CO2 adsorption

temperature (K)

Adsorption capacity at

0.1–0.4 bar (mmol/g)

N2 adsorption capacity

at 0.9–1.6 bar (mmol/g)

CO2 /N2 capacity molar

ratio

Reference

NaX/1 298 2.8–3.9 0.264–0.46 11–8.5 [48]

NaX/1 323 1.43–2.49 – – [48]

LiX/1 303 3.1–4.6 – – [44]

NaY/2.4 323 0.45–1.17 – – [49]

CsY/2.4 333 0.86–1.2 – – [46]

KY/2.4 333 0.75–1.6 – – [46]Silicalite/∞ 334 0.16–0.45 0.1 1.6 [50]

H-ZSM-5/30 313 0.7–1.5 0.23 3 [51]

Li-MCM-22/15 333 0.68–1 – – [52]

–, notavailable.

favorable adsorption isotherm. This was explained by the domi-

nantacid–base (CO2-framework oxygen atom) interaction over the

polarizing effect in the case of CsY and KY faujasites (particularly

for CsY and to a lesser extent for KY), in contrast to LiY, NaY and X

faujasites.

Table 2 shows the CO2 adsorption properties of different zeo-

lites and zeolite-like materials. As seen, the adsorption capacity

decreased drastically when the temperature increased from 298

to 323K. Akten et al. [47] showed that the CO2

/N2

selectivity for

Na-4A type zeolite also decreased at increased temperature.

In terms of CO2 adsorption kinetics, zeolites are ranked among

the fastest adsorbents, reaching equilibrium capacity within min-

utes [12]. Moreover, a large number of studies were devoted to

NaX faujasite using different recycling configurations, including

temperature swing and pressure swing adsorption [9,40,41,53].

Although the CO2 adsorption enthalpy on X and Y zeolites was

found to be dependent on the nature of extraframework cations,

within the range of 30–50 kJ/mol, it is low enough to allow

reversible CO2 adsorption. Zeolites generally operate without any

loss in performance, provided that the feed stream is strictly dry.

Although low silica materials exhibit high adsorption capacity and

selectivity at low pressure with favorable isotherms, they are very

sensitive to the presence of water, which strongly inhibits the

adsorption of CO2 [54]. This prompted some investigations on theability of hydrophobic high silica zeolites such as MWW zeotype

[52] and NaZSM-5 [55] to remove CO2. However, because high

silica microporous materials contain less extraframework cations

than faujasite zeolites, they exhibit lower adsorption capacity and

adsorption enthalpy [45]. Moreover, similarly to X and Y zeolites,

theyshow decreasing selectivity at increasing temperature [47,56].

Fig. 2. Adsorption isotherms of CO2

on cation-exchanged Y faujasites [46].

Alumino-phosphates (AlPO) and silica-alumino-phosphates

(SAPO) are another class of zeolitic materials that were investi-

gated as potential CO2 adsorbents [57,58]. The overall framework

of AlPOs is neutral and is expected to behave as silicalite or dea-

luminated Y faujasite for CO2 as was shown by Deroche et al. [58]

usinga combinationof molecularsimulation andmicrocalorimetry.

In fact AlPO-18 (with AEI structure) exhibits unfavorable adsorp-

tion isotherm. Similarly to NaX and NaY, the framework of some

SAPOs is negatively charged and the overall charge is balanced by

extraframework cations. In this case, it is expected to obtaina more

favorable CO2 adsorption isotherm with higher adsorption capac-

ity at low pressure of CO2 as reported by Castro et al. [57] f or the

proton form of SAPO-34. However, the CO2 adsorption capacity on

SAPO remains lower than X and Y faujasites.

In conclusion, because of their often highly favorable CO2adsorption isotherms, zeolites and zeolite-like materials with low

Si/Al ratios are among the most promising adsorbents for CO2 cap-

ture from flue gas. However, because of their highly hydrophilic

character, the flue gas needs extensive drying prior to CO2 capture.

Notice that among zeolites, 13X is has been the most investigated

material for the purpose of CO2 capture [9,40,41,53,59]. As pointed

out by Ho et al. [9], further work to develop more selective zeolite

adsorbents toward CO2 vs. N2 and O2 may reduce considerably the

cost of CO2 capture.

2.3. MOFs and zeolite-like MOFs

Although an emerging class of porous materials, metal organic

frameworks (MOFs) have attracted a growing interest, motivating

extensive studies on their CO2 adsorption properties, both theoret-

ically and experimentally. MOFs are porous crystalline materials

composed of self-assembled metallic species and organic linkers

[60,61]. Their pore size and shape can be easily tuned by chang-

ing either the organic ligands or the metallic clusters. They are

typically rigid materials, but some of them exhibit structural flex-

ibility upon adsorption and desorption of gases or liquids [62,63].

The wide range of MOFs combined with their desirable proper-ties such as their remarkably high surface area and controlled

pore size and shape, prompted extensive work on their adsorp-

tive properties, particularly for storage of light gases (H2, CH4) and

storage and separation of CO2. Although the majority of inves-

tigations on CO2 adsorption over MOFs used pure CO2, as well

as CO2-containing mixtures, most measurements and simulations

were carried out at high pressure and often at room or subambient

temperature. Seminal contributions in the synthesis of novel MOFs

andtheir CO2 adsorptionproperties werereportedby Millward and

Yaghi [64]. Their early work was followed by an extensive effort to

develop new types of MOFs for the separation and storage of CO2[65–72]. Millward and Yaghi [64] showed that MOF-117 exhibits

an unprecedented CO2 adsorption capacity at high pressure (e.g.,

ca. 150 wt% at 40 bar), but very small CO2 uptake at subatmo-


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764 A. Sayari et al. / Chemical Engineering Journal 171 (2011) 760–774

Table 3

Literature survey on CO2 adsorption properties of some MOFs and ZMOFs.

MOFS Temperature (K) CO2 adsorption capacity

(mmol/g at 0.1–0.4 bar)

N2 adsorption capacity

(mmol/g at 0.9–1.6 bar)

CO2/N2 selectivity Reference

MOF-508 323 0.1–0.7 0.6–0.9 2 [75]

Cu-BTC 298 0.5–2 0.25 15 [79]

MIL-53 303 0.5–1.15 – – [67]

Ni/DOBDC 296 2.7–4.01 – – [72,73]

CO/BOBDC 296 2.8–5.36 – – [72,73]

Mg/DOBDC (Mg-MOF-74) 296 5.36–6.8 – – [68,72]ZIF-78 298 0.77–1.36 – 50 [70,80]

–, notavailable.

spheric pressure. More recently, Caskey et al. [72] reported a much

higher and reversible adsorption capacity for pure CO2 (23.6wt%,

5.36mmol g−1) at 0.1 bar and room temperature on Mg/DOBDC (or

Mg-MOF-74) (Fig. 3). Although this is an excellent finding, adsorp-

tion of CO2 in mixtures with N2 was not reported. Additional data

regarding adsorption of CO2 at 0.1bar on a number of MOFs may

be found elsewhere [73].

It is important to mention that the majority of MOFs exhibit

unfavorable adsorption isotherms for CO2 in the low pressure

range. Moreover most of these materials adsorb considerable

amounts of N2, leading to low selectivity toward CO2. The highestselectivity toward CO2 vs. N2 was in the range of 5–30 [74–76], and

generally the CO2 adsorption capacity is dramatically reduced at

higher temperature, accompanied by a drop in the CO2 adsorption

selectivity. In terms of kinetics, MOFs are as fast CO2 adsorbents as

zeolites according to somecomputational studies [66–77]. Basedon

the aforementioned observations, MOFs seem to be more suitable

for CO2 storage rather than separation.

Table 3 shows the adsorption properties of different types of

MOFs at low pressure. As seen, the adsorption capacity as well as

the CO2/N2 selectivity formost MOFs, were very lowand decreased

drasticallywhen the temperature increasedfrom 298to 323K. This

has been documented byBarcia etal. [78], Bastinetal.[75], Baeetal.

[74], Yang et al. [79] who showed decreasing CO2/N2 selectivity at

increased temperature for MOF-508b and Cu-BTC.MOFs and ZMOFs structural, chemical and thermal stability has

beenhardlyaddressedin the literature, until recently.It is generally

recognizedthatby farthemostcritical issuefor the stabilityof these

materials is their hydrothermal stability. The behavior of MOFs

and their subfamilies in hydrated conditions varies widely, from

materials that irreversibly degrade even under mild conditions to

materialsthatare highlystable in boiling water. Forexample,MOF-

Fig. 3. CO2 adsorption isotherms(296K, 0–1atm)for M/DOBDC materials. Inset is a

close-up of the low pressure region. Filled and open symbols represent adsorption

and desorption data, respectively [72].

117 and IRMOF-1 were reported to be unstable upon exposure to

air in the presence of humidity [81–83]. The concerns raised by the

stability of MOFs prompted the discovery of a new class of MOFs

referred to as zeolite-like MOFs (ZMOFs) or zeolitic imidazolate

frameworks (ZIF). ZMOFs are crystalline porous materials which

combine the highly desirable properties of zeolites and MOFs,

such as microporosity, high surface areas, and exceptional ther-

mal and chemical stability [69–81]. Because of the strong bonding

between the imidazolate linker and the metal center, many ZMOFs

have high thermal (>673 K) and moisture resistance compared to

other MOF structures [84,85]. Although significant improvementwas observed in terms of CO2 v s. N2 adsorption selectivity, which

increased up to 50 for ZIF-78 [80], theCO2 adsorption capacity was

still low at low CO2 partial pressure (Table 3).

Zeolites, MOFs and ZMOFs are typically hydrophilic and their

application for CO2 capture from flue gas requires partial or com-

plete drying of the gas stream. To circumvent this limitation,

new materials with no hydrophilic adsorption sites referred to

as covalent organic frameworks (COFs) were developed. COFs are

crystalline organic porous materials without metal ions. Furukawa

and Yaghi [86] and Babarao and Jiang [87] reported high CO2adsorption capacities for a series of COFs, but adsorption at low

partial pressure of CO2 appeared to be significantly lower than for

Mg-MOF-74.

In summary, MOFs, ZMOFs and COFs may be promising materi-als for CO2 removal provided that more favorable CO2 adsorption

isotherms are obtained. Their selectivity and capacity at low par-

tial pressure of CO2 in gas mixtures are quite low and more likely

to be suitable for CO2 storage rather than CO2 separation from flue

gas. Although in their early stages of development, MOFs, ZMOFs

and COFs are promisingmaterials for CO2 adsorption showing very

interesting and adjustable properties.

3. Amine-functionalized adsorbents

The technology currently used in industry for CO2 capture is

absorption with liquid amine solutions. The removal of CO2 by

amines occurs viathe widely accepted formation of carbamate and

bicarbonate species, as represented in Scheme 1 [88]. These are

reversible reactions that permit the regeneration of amines, typi-

cally by heating the CO2-rich solution.

2(RNH2)+ CO2 ↔ RNHCO2−RNH3

+

carbamate

RNH2 + CO2 +H2O ↔ RNH3+HCO3

−

bicarbonate

RNH2←→(RNH3

+)2CO32−

carbonate

The liquid amine absorption process inspired researchers to use

amine-modified solid materials as adsorbents for CO2 capture. As

far as flue gas treatment is concerned, it was anticipated that sup-

ported amines will maintain a high selectivity toward CO2 with a

negligible uptake of other components, particularly N2, but with-

out the aforementioneddrawbacks associatedwith aqueous amine

solutions. According to a study on amine-functional adsorbents by

Gray et al. [89], a capacity of at least 3 mmol/g is required for this


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Scheme 1. Typical reaction pathway between CO2 and amines [88].

process to be competitive against absorption technologies, corrob-

orating the previously mentioned range of 2–4 mmol/g proposed

by Ho et al. [9]. Although the early efforts to produce amine-

functionalized adsorbents were not particularly successful in terms

of adsorption capacity, the collective effort of several research

groups resulted in significant performance improvements, leading

to increasing interest in this subject matter. Based on ISI Web of

Knowledge, Fig. 4 illustrates the remarkable increase in the number

of publications related to CO2 adsorption on amine-functionalized

materials, with ca. 70% of these contributions published in the last

6years.

We have broadly organized the present section according to

the type of interactions between amine groups and the support,

namely (i) amine-impregnatedmaterials where mostly weakinter-

actions occur, and (ii)covalently bonded amine-containing species,obtained typically via surface-grafting of aminosilanes. The ratio-

nale behind such classification is that materials with either strong

or weak interactions exhibit a number of common characteristics.

An example is that grafted materials offer comparatively higher

rate of adsorption than amine-impregnated adsorbents [91] and,

in some cases even higher than commercial adsorbents such as

13X [90]. However, the organic content of amine-grafted adsor-

bents depends on the surface density of hydroxyl groups, needed

to anchor the aminosilane. As for impregnated amines, higher load-

ingsmay be achieved, butoften accompaniedby increasinglystrong

diffusion limitations.

3.1. Amine-impregnatedmaterials

3.1.1. Ordered mesoporous supports

Xu et al. [92] were first to report on polyethyleneimine (PEI)-

impregnated mesoporous materials for CO2 adsorption, coining

the term “molecular basket”. It was found that the adsorption

capacity of PEI-impregnated MCM-41 improved at increased load-

ing. The highest value of adsorption capacity, corresponding to

3.02mmol/g was obtained under a stream of pure CO2 at 75◦C

using a sample with 75w t% PEI. However, the maximum effi-

ciency, i.e., CO2/PEI molar ratio was obtained in the presence of

Fig.4. Number of publications relatedto CO2 capture onamine-functional materials

according to ISI-Web of Science database.

a material containing 50wt% PEI, and decreased steadily at higher

loadings. As discussed later, this behavior was confirmed by other

researchers. Further work under conditions relevant to flue gas

treatment showed that the 50% PEI on MCM-41 silica exhibits an

adsorption capacityof ca.2.1mmol/g inthe presence of 10% CO2/N2at 75 ◦C. A particularly interesting behavior of PEI-impregnated

MCM-41 materials was the fact that, unlike other adsorbents,

adsorptioncapacityimproved as temperature increased from 25 to

75 ◦C. Since theactual adsorption event is exothermic in nature, the

increasing adsorption capacity with temperature was attributed

to the occurrence of a bulk-like state of PEI inside the meso-

pores, withaminegroups notreadilyaccessible at lowtemperature,

resulting in a diffusion-limited process. Since then, other authors

working on PEI-impregnated materials reported similar findings

and the idea of a diffusion-limited process has been generallyaccepted.

Following their early study on PEI-impregnated MCM-41, Xu

et al. [93] analyzed theadsorption of CO2 inhumidstreams. A posi-

tive effectof moisture was observed in terms of increased capacity,

particularly when the molar concentration of water was equal or

lower to that of CO2, providing support to the bicarbonate forma-

tion mechanism. No further increase in adsorption capacity was

observed for streams with higher moisture content. Accordingly,

itscapacitywas enhanced from 2.01 mmol/g in a simulateddry flue

gas containing 15% CO2 to 2.84 mmol/g in a stream containing 10%

moisture and13% CO2, balance air. In a later contribution, thesame

group [94] impregnated PEI on SBA-15 under the assumption that

the structural characteristics of the support would affect the per-

formance of the aminated adsorbents. Allegedly, due to the largerpore size and volume of SBA-15 compared to MCM-41, PEI-SBA-15

used the amine groups more efficiently under the same loading of

50wt% PEI. Indeed, as mentionedabove, while PEI-MCM-41 exhib-

ited an adsorption capacity of 2.1 mmol/g, PEI-SBA-15 showed a

capacity of 3.18 mmol g−1 under a flow of 15% CO2 balance air at

75 ◦C.

Ahn’s group [95,96] has also investigated the adsorptive proper-

ties of PEI-impregnated on a variety of ordered mesoporous silicas.

It was found that at constant PEI loading, the use of various sup-

ports afforded different adsorption capacities, and that supported

PEI had a higher capacity than its pure liquid counterpart. Interest-

ingly, under otherwisethe sameconditions,the adsorption capacity

appeared tobe dependenton pore diameter (dp).WhenPEI wasdis-

persed on a KIT-6 type silica with 6nm dp at a loading of 50%, thematerial adsorbed 3.07mmol/g in a stream of pure CO2 at 75

◦C,

vs. 2.52mmol/g when using MCM-41 with 2.8nm pores as sup-

port. Theadsorption capacityof 50%PEI-loaded KIT-6 in conditions

closer to flue gas was 1.95mmol/g in the presence of 5% CO2/N2at 75 ◦C. The pore size can also affect the rate of adsorption as the

time required toachieve90%of the total capacitywasin theorder of

KIT-6 < SBA-16= SBA-15< MCM-48< MCM-41. Following the ratio-

nale that using large pore sizes afford better adsorption capacity,

PEI and tetraethylenepentamine (TEPA) were impregnated on a sil-

ica monolith with hierarchical pore structure [96]. Due to its largerdp, with mean values at 3, 17 and 120n m, the optimum loading

of PEI was 65wt%, with a capacity of 3.75mmol/g for a stream of

5% CO2 in N2 at 75◦C, a capacity much higher than that obtained

using conventional MCM-41 mesoporous silica as support. TEPA-


7/15


loaded samples were not particularly attractive as their adsorption

capacity deteriorated after only 5 adsorption–desorption cycles.

Anotherapproachusedby Yueet al.[97] consistedof impregnat-

ingTEPA on as-synthesizedSBA-15,as opposed to calcinedsupport.

The reported adsorption capacity for a sample with 50wt% load-

ing in the presence of 10% CO2 in N2 was ca. 3.25mmol/g at 75◦C.

The materials prepared with as-synthesized support consistently

performed better thanthose prepared usingthe correspondingcal-

cined supports, with up to 10% higher capacity with the added

advantage that no steps are required to remove the organic tem-

plate. The proposed explanation was that the polymeric template

in the pores of as-synthesized supports interacts with the TEPA,

forming a more even distributionof the functional groups, andpre-

venting TEPA from aggregating into a micellar-like form, which

is believed to be its naturally occurring form. The use of as-

synthesized supports was further explored by Yue et al. [98] using

MCM-41-type silica. A TEPA-MCM-41 sample loaded with 50 wt%

PEI had a capacity of 4.54mmol/g for 5% CO2/N2 at 75◦C, outper-

forming the above-mentioned TEPA-SBA-15.

In addition, Yue et al. [99] impregnated as-synthesized SBA-15

using a mixture of TEPA anddiethanolamine(DEA). In this case, the

maximum CO2/N ratio (adsorption efficiency) was found to be ca.

0.4 at a loading of ca. 30% TEPA and 20% DEA. In the case of TEPA-

DEA-SBA-15 at 75◦C, its adsorption capacity ranged from 3.77 to3.61mmol/gthroughout 6 adsorption–desorptioncycles for 5% CO2in N2. Thegoodperformanceof this adsorbentwas attributed to the

hydroxyl groups present in DEA. This is analogous to the effect of

water vapor, which is associated with a more favorable CO2 to N

stoichiometry, as shown in Scheme 1.

Franchi et al. [100] impregnated DEA on a variety of sup-

ports whose adsorption capacity and stability were compared to

a benchmark adsorbent, i.e., 13X zeolite. The most promising sup-

port used in this work consisted of MCM-41 silica with pores

enlarged by post-synthesis treatment (PE-MCM-41, dp =9.7nm),

which afforded an adsorption capacity of 3 mmol/g at 25◦C in the

presence of 10% CO2 in N2, a value comparatively higher than

that reported for 13X (i.e., 2.8 mmolg−1) under the same condi-

tions. Similarly to supported PEI, the adsorption capacity increasedwith DEAloading, butthe adsorption efficiency (CO2/N) decreased,

suggesting that loading more than 6mmol of DEA per gram of

adsorbent is unattractive.

A series of amine-impregnated mesoporous aluminas (MA),

using diisopropanolamine, triethanolamine, 2-amino-2-methyl-

1,3-propanediol, diethylenetriamine (DETA) and PEI, were inves-

tigated by Plaza et al. [101]. The most attractive materials were

those containing PEI and DETA with 40wt% loading. Not only did

these adsorbents offer higher adsorption capacity at room temper-

ature but, unlike the other materials in this study, their capacity

increased at higher temperature. DETA-MA exhibited a capacity of

ca. 1mmol/g at 25 ◦C for pure CO2, increasing to ca. 1.4 mmol/g

at 57 ◦C. For PEI-MA, the adsorption capacity at 57 ◦C was ca.

1.14mmol/g in the presence of pure CO2. It is worth noting thatthe adsorption capacity was enhanced at higher temperature only

for samples with high amine loadings, supporting the hypothesis

that this behavior is associated with diffusion limitations within

the amine phase at low temperature.

A number of literaturereports explored the regeneration behav-

ior of PEI-impregnated mesoporous materials. Drage et al. [11] used

a proprietary mesoporous silica impregnated with PEI (40 wt%).

The adsorbent showed a capacity of 2.4mmol/g at 70 ◦C in the

presence of 15%CO2 in N2. This work analyzed the effectof regener-

ation temperature using pure CO2 as stripping gas. It was observed

that desorption at a temperature of less than 140 ◦C resulted in

an incomplete regeneration. However, some concerns were raised

regarding the use of such high temperatures, mainly because of

the following problems: (i) evaporation of PEI may occur and (ii)

a secondary reaction between CO2 and amine groups formed a

stable product, most likely urea, resulting in a decreasing num-

ber of adsorption sites. The suggested alternative was to use a

different stripping gas or lower desorption temperature although

sacrificing some working adsorption capacity. As discussed later, a

strategy to prevent the formation of urea during extensive cycling

even at high temperature has been proposed recently by Sayari

and Belmabkhout [102]. A PSA strategy was explored by Dasgupta

etal. [103] using50% PEI-impregnatedSBA-15. The highestcapacity

reported at 75◦C was 1.36 mmol/g for 12% CO2 in N2. A steadystate

was obtained after 15–20 cycles, and the productivity was better

compared to similar PSA procedure using 13X zeolite at 75◦C.

3.1.2. Ordered microporous supports

In addition to mesoporous materials, zeolites have also been

used as supports. Jadhav et al. [104] dispersed monoethanolamine

(MEA) on 13X zeolite producing materials with different loadings.

Quite interestingly, the adsorbent with the highest capacity at low

temperature (i.e., 35◦C), with 1.96mmolg−1 for 15% CO2 in N2,

contained only 2.9wt% MEA, while the best capacity at 75◦C (i.e.,

0.45mmol/g) was obtained on a sample with the highest load-

ing (i.e., 25wt%). These capacities were comparatively higher than

unmodified 13X, which adsorbed 0.64 and 0.36mmol/g at 35 and

75 ◦C, respectively. An interesting advantage of amine-containing13X was a significant improvement in its tolerance to moisture.

While it is generally accepted that preferential adsorption of water

on 13X results in a drastic reduction of CO2 uptake, the adsorption

capacity in the presence of 100% RH decreased by only ca.13% with

respect to dry conditions.

Another type of zeolite, namely beta-zeolite, wasused by Fisher

et al. [105] to support TEPA and compared with TEPA-impregnated

on amorphous alumina and silica. The results clearly showed the

advantages of usinga support withgood structural propertiessince

beta-zeolite was loaded with up to 38.4wt% compared to only 14.6

and 8.3wt% for SiO2 and Al2O3, respectively, most likely as a result

of a comparatively higherpore volume. Such loading translated in a

significantly higher adsorption capacity for TEPA-beta zeolite over

the other samples, being 2.08 mmol/g for 10% CO2 balance nitrogenat 30 ◦C, while it was 0.19 and 0.68mmol/g for TEPA-Al2O3 and

TEPA-SiO2 , respectively.

3.1.3. Other supports

While ordered mesoporous supports are suitable substrates

for the dispersion of amines, other supports were also explored.

Extensive work performed by Filburn’s group [106–108], dealt

with impregnation of a variety of amines, such as PEI,

monoethanolamine, diethanolamine, triethanolamine, and TEPA

on high surface area polymeric supports, mainly polymethyl-

methacrylate (PMMA). Although their original purpose was to

produce adsorbents for air purification in confined environments,

the results have proven to be of interest for other applications

such as flue gas treatment as shown in a later contribution [108],where a TEPA-impregnated PMMA exhibited capacities of 21.45

and 13.88mmol/g at 20 and 70◦C, respectively in the presence of

15% CO2 and 2.6% H2O balance N2. It is also worth noting that the

reported adsorption capacities were remarkably higher than any

other data reported in the literature. Moreover, contrary to amine-

impregnated mesoporous inorganic supports, adsorption capacity

decreased at higher temperature for the polymer-based adsor-

bents. In the same work, TEPA was reacted with acrylonitrile to

selectively transformprimaryaminesinto secondaryaminesbefore

impregnation, under the premise thatsecondary amines are advan-

tageous. However, this wasnot thecase,since the sampleproduced

byimpregnation of themodifiedTEPA, referred to as TEPAN,under-

performed TEPA-PMMA in terms of adsorption capacity and rate

with adsorption capacity valuesof 14.22 and 4.01 mmol/g at 20 and


8/15


9/15


Fig. 5. Amine loading (left) and adsorption capacity (right) vs. TRI/SiO2 ratio on MCM-41 (TRI-M41C)and PE-MCM-41 (TRI-M41EC)[117].

favorable stoichiometry is expected in the presence of moisture,

which was corroborated by the experimental value of adsorption

capacity, increasing from0.41 to 0.89mmol/gin a wet stream (100%

RH). For several years thereafter, no further CO2 adsorption studies

on amine-grafted materials appeared, most likely because of the

lack of interest in this topic. However, with the rapid development

of ordered mesoporous materials, and the increasing awareness

of the greenhouse gas effect, new studies began to appear in the

literature starting in 2003.

In a comparative investigation on propylamine-grafted MCM-

48 and silica xerogel, Huang et al. [114] provided evidence for the

advantages of ordered mesoporous supports. Using 10% CO2 in N2,

a significantly higher capacity of ca. 1.42 mmol/g was obtained forAP-MCM-48 at room temperature vs. ca. 0.58mmol/g for amine-

grafted silica xerogel under the same conditions. Since the amine

loading was 2.3 and 1.7mmol/g for MCM-48 and silica xerogel,

respectively, it is inferred that the significantly higher capacity

of AP-MCM-48 was accompanied by an improved efficiency. Fur-

ther, using a CO2-containing stream with 100% RH gave rise to an

adsorption capacity twice as high with a CO2/N = 1 corresponding

to quantitative transformation of amine groups into ammonium

bicarbonate.

Further evidence for the suitability of periodic mesoporous sup-

ports was reported by Knowles et al. [115] using AP-grafted hexag-

onal mesoporous silica (HMS) and amorphous silica gel. A higher

amine loading of 2.3mmol/g was obtained vs. only 1.1mmol/g

for amorphous silica. These loadings mirrored the difference insurface areas of HMS (1198 m2/g) compared to amorphous silica

(567m2/g). The adsorption capacity at 20◦C inthe presenceof 90%

CO2/Ar was 1.59mmol/g for AP-HMS compared to 0.68mmol/g

for AP-grafted amorphous support. In further work, Knowles

et al.[116] used (3-trimethoxysilylpropyl)diethylenetriamine (TRI)

grafted on HMS and obtained a capacity of 1.34mmol/g in the

presence of 90% CO2 balance Ar at 20◦C. They also found that the

material is thermally stable up to 170 ◦C under pure N2 or mildly

oxygenated environments, a comparatively higher temperature

than amine-impregnated adsorbents.

Sayari’s group made significant contributions to the area of CO2capture by amine-containing nanoporous materials. They demon-

strated the beneficial effect of using materials with larger pore

diameterand porevolume thantypical MCM-41 silica [90,117,118].

To do so, they used the post-synthesis pore-expansion method

developed earlier [119,120]. Based on as-synthesized MCM-41 as

starting material, they generated PE-MCM-41 with pore size and

pore volume up to 20n m and 3.5c m3/g, vs. typically ca. 3–4nm

and ca. 0.7–1cm3/g for regular MCM-41, with hardly any change

in surface area. As shown in Fig. 5, grafting MCM-41 and PE-MCM-

41 with TRI led to comparable amine loadings, because of similar

surface areas. However, as shown in Fig. 5, using 5% CO2 in N2at 25 ◦C, the CO2 uptake was ca. 50% higher for TRI-PE-MCM-41

than TRI-MCM-41, at all amine loadings. Moreover, TRI-PE-MCM-

41 adsorbed CO2 about 30% faster than MCM-41-based material,

showing the importance of pore size and volume.

Another contribution of Sayari’s group was the optimizationof the grafting conditions, leading to dramatic improvement of

amine loading and adsorptive properties. Grafting is traditionally

practiced under reflux, in dry solvent (typically toluene at 110 ◦C)

with large excess of silane. Harlick and Sayari [90] found that

the optimum grafting conditions of TRI on PE-MCM-41 in toluene

were as follows: T = 85 ◦C; water added: 0.3mL per gram of sup-

port; aminosilane added: 3 mL per gram of support. Under such

conditions, the amine content increased by ca. 30% (i.e., 7.98 vs.

6.11mmol/g for conventional dry grafting), whereas the adsorp-

tion capacity using 5% CO2/N2 at 25◦C increased by ca. 70% from

1.55mmol/g for conventional dry grafting to 2.65mmol/g. Thus,

under these CO2 adsorption conditions, the combination of pore

expansion and optimization of grafting conditions improved the

adsorption capacity by close to 300% compared to the adsorbentproducedvia anhydrousgrafting on conventional MCM-41, in addi-

tionto a significant increasein therate of adsorption. The advantage

of using amine-functionalized mesoporous materials was further

evidenced when a stream of humid CO2 w as used. In the pres-

ence of 5% CO2 in N2 with 27% RH, the adsorption capacity for

TRI-PE-MCM-41 increasedto 2.94 mmol/gin contrast to a dramatic

decrease observed for 13X, down to 0.09 mmol g−1. This work also

provided evidence of the advantage of amine-grafted adsorbents

in terms of adsorption kinetics, as the CO2 rate of adsorption on

TRI-PE-MCM-41 was found to be higher than 13X zeolite. Later, it

would be corroborated that TRI-PE-MCM-41 is also comparatively

faster than its PEI-impregnated counterpart [91].

Further studies on TRI-PE-MCM-41 [118] demonstrated that

enhanced capacity was not the only advantage of TRI-PE-MCM-


10/15


Fig. 6. Working adsorption capacity of TRI-PE-MCM-41 over various adsorption–desorption cycles in dry (TRI-70/70-d) and humid (TRI-70/70-h) streams with adsorption

and desorption at 70 ◦C [102].

41, since incorporation of amines significantly increased the

selectivity toward CO2 over N2. Using conditions directly related

to flue gas, i.e., 10% CO2 balance N2 at 50◦C, Serna-Guerrero

et al. [121] obtained a stable capacity of 1.59mmol/g over 100adsorption–desorption cycles with regeneration under vacuum at

90 ◦C.

To further address the long-term stability of amine-

functionalized adsorbents for CO2 capture,Sayari andBelmabkhout

[102] carried an in-depth investigation using extensive

adsorption–desorption cycling under different conditions. As

shown in Fig. 6, they found that under dry conditions, the adsor-

bent will ultimately deactivate even under mild conditions. The

degree of deactivation depended on the nature of the adsorbent

and the adsorption–desorption conditions. The adsorbent deacti-

vation was clearly associated with the formation of urea groups,

which are stable under the desorption conditions. To prevent the

formation of urea and drastically improve the stability of amine-

functionalized adsorbents, the use of humid streams was proposed.As illustrated in Fig. 6, the adsorbent underwent more than 700

cycles without any loss in adsorption capacitywhen the adsorption

anddesorption gases contained7% RH at 70◦C. Another interesting

finding was that by treating deactivated AP-grafted PE-MCM-41

in the presence of water vapor at ca. 200 ◦C, it was possible to

hydrolyze the urea groups and fully regenerate the adsorbent.

The advantages of using TRI were also discussed by Hiyoshi

et al. [122] in a thorough comparative analysis of monoamine,

diamine and triamine-bearing molecules grafted on SBA-15. The

higher amine density achieved through the use of TRI resulted in

the best performing adsorbent. The reported adsorption capacity

for TRI-SBA-15 at 60◦C and 15% CO2 was of 1.58mmol/g under dry

conditions and 1.80mmol/g in a stream containing 60% RH. Fur-

thermore, they reported that TRI-SBA-15 was stable over 50 cyclesof adsorption at 60◦C and desorption at 100 ◦C.

Kimetal. [123] made a comparative studybetweenmesoporous

silica (MS) functionalized with molecules containing 1–3 amine

groups produced by anhydrous grafting and co-condensation.

In general, samples prepared by co-condensation presented

higher amine contents, that reportedly promoted a better amine-

efficiency and adsorption capacity. In line with the observations

mentioned above, the adsorbent with the highest capacity was

TRI-MS, with 1.74mmol/g for pure CO2 at 25◦C, while the most

efficient under the same conditions was the AP-MS sample with

a CO2/N ratio of 0.43 and a capacity of 1.14mmol/g. This is con-

sistent with findings by Serna-Guerrero et al. [124] who obtained

the maximum efficiency of CO2/N= 0.5 using AP-grafted PE-MCM-

41, whereas the CO2

/N ratio for TRI-PE-MCM-41 never exceeded

0.34 [121]. In addition, Kim et al. [123] compared their amine-

grafted materials with PEI-impregnated KIT-6 silica. Although a

higher capacity was obtained on the PEI-containing sample (i.e.,

1.79mmol/g), its CO2/N efficiency at room temperature was only0.1.

The use of diamine-bearing molecules was investigated under

the hypothesis that the occurrence of two amine groups in

close proximity will lead to enhanced formation of carbamate,

thus higher CO2/N efficiency. Knofel et al. [125] grafted N -[3-

(trimethoxysilyl)propyl] ethylenediamine (EDA) on SBA-16 silica.

Although this work was mainly focused on CO2 adsorption at high

pressure, it clearly showed that incorporation of amine groups

resulted in an improved capacity at CO2 partial pressures below

1 bar. Thereportedcapacityfor pure CO2 at 1barwas of 1.4mmol/g

at27 ◦C for the best performing EDA-SBA-16. It was observed how-

ever, that at high pressure (ca. 4 bar or more), the non-aminated

samples exhibited higher adsorption capacity. A possible explana-

tion was that physical adsorption predominates at high pressureand so, the higher pore volume of the unmodified support offers a

comparative advantage in terms of adsorption capacity.

In recent years, efforts to further improve the grafting pro-

cess have been pursued, with the aim of improving the efficiency

and capacity of aminated silicas. Wang et al. [126] incorporated

AP-functionality by simultaneous extraction of structure direct-

ing agent and grafting on as-synthesized SBA-15. The adsorbent

obtained by the proposed approach outperformed a sample syn-

thesized using the typical grafting procedure on calcined SBA-15.

The sample usingas-synthesized support produced a material with

an adsorption capacity of ca. 0.45mmol/g at 65◦C a t a C O2 partial

pressure of 0.1bar, representinga CO2/N efficiency of 0.44, close to

thestoichiometricratio of 0.5. It was suggestedthat, unlike calcina-

tion, the extraction of surfactant template performed with ethanolpreserved the surface silanol groups, whichtranslated into a better

distribution of surface amines with a subsequent improvement of

adsorption capacity.

The drawback of surface silanol groups removal during calci-

nation of the support was also addressed by Wei et al. [127]. They

proposed rehydrating SBA-15by soaking it in water at 97◦C, before

grafting with EDA. The obtained material had an amine content

of 3.06mmol/g, and a capacity of 0.73mmolg−1 for 0.15bar CO2at 60 ◦C. A similar material prepared using non-hydrated SBA-15

hadan amine loading of 2.59 mmol/g and an adsorption capacity of

0.59mmol/g.

Zelenak et al. reported on the effect of pore size [128] and

the basicity of the functional groups [129] on the performance of

amine-functionalized adsorbents for CO2

capture. It was suggested


11/15


Fig. 7. Schematic representation of thesynthesis of hyperbranched aminosilica according to Jones et al. [136].

that large pore sizes are associated with an efficient use of amine

groups. Indeed, when grafting AP on MCM-41 with a pore size of

3.3nm, a high amine content was obtained (i.e., 3 mmol/g), but theadsorption capacity was only 0.57mmol/g for 10% CO2 at 25

◦C.

In contrast, a capacity of 1.54mmol/g was obtained when SBA-15

with a pore size of 7.1nm was used as support, despite a slightly

lower amine loading of 2.7 mmol/g. The lower efficiency of MCM-

41-basedmaterial, however,may notbe only a resultof a difference

in pore sizes. The amine surface density reported for AP-MCM-

41 was lower, having only 1.1 amine group per nm2 compared to

2.4 amines per nm2 for AP-SBA-15. Since admittedly two amine

molecules in close proximity are required for reaction with CO2,

this would be a disadvantage for the lower amine density sample.

Some interesting studies were devoted to the effect of the sup-

ports on the adsorbent performance. Knofel et al. [130] compared

AP-grafted mesoporous silica (MS) and mesoporous titania (MT).

The highest adsorption capacity of ca. 0.24mmol/g for 10% CO2at 30 ◦C was obtained with AP-MS. This is a low capacity com-

pared to other materials reported in the literature, but the main

finding of this work was that the properties of the support may

influence the behavior of the functionalized adsorbent. While no

interactions were detected between CO2 and the silica support,

interactions occurred in the presence of MT. This is reflected in

a higher capacity when expressed in terms of surface area, i.e.,

1mol/m2 and 0.6mol/m2 for AP-MT and AP-MS, respectively.

Another approachexploredby Lu et al. [131] wasthe useof particles

with defined geometry, by grafting EDA on mesoporous spherical

particles. The adsorption capacity at 60◦C in the presence of 10%

CO2 in air was of 0.73 mmol/g. These adsorbents showed a remark-

able stability when regenerated using a TSA procedure at 120 ◦C

or under VSA, although their adsorption capacity decreased in thefirst VSA cycle. In addition, it was suggested that the combina-

tion of heat and vacuum resulted in an improvement in desorption

rate.

Looking for an inexpensive source of silica, Bhagiyalakshmi

et al. [132] grafted tris(2-aminoethyl)amine (TREN) and TEPA onto

chloropropyl-modified mesoporous supports produced from rice

husk. The highest capacities were obtained with TREN-grafted

MCM-48 with values of ca. 1.59 and 1.36mmol/g at 25 and 50◦C,

respectively in the presence of pure CO2.

The only contribution dealing with amine-grafted zeolites used

ITQ-6[133], which offers attractive characteristics suchas highcon-

centration of surface silanol groups anda poresize in thenanometer

range. The mostpromising adsorbenthad a capacityof 0.67mmol/g

for 12% CO2 at 20

◦

C.

3.3. Hyperbranched aminosilicas

A different method of functionalization, introduced recently,consisted in iterative building of amine-containing dendrimers

inside the porous supports. Liang et al. [134,135] produced highly

branched dendrimers by step-wise reaction between diisopropy-

lethylamineand cyanuric chloride inside the pores of SBA-15 [134]

or mesocellular siliceous foams [135]. The optimum adsorbent pro-

duced with this approach was obtained after 3 reaction steps, with

a capacity of ca. 1 mmol/g for 90% CO2 in Ar at 20◦C. However, as

a larger number of reaction steps were performed to obtain higher

generation dendrimers, the adsorbent lost its structural properties,

allegedly due to space limitations, which negatively impacted the

adsorptive properties.

Jones’ group [136,137] proposed an innovative amine polymer-

ization approach inside SBA-15 channels with promising results.

In this case, aziridine was polymerized by ring opening inside thepores of SBA-15 producing a “covalently tethered hyperbranched

aminosilica”, as represented in Fig. 7. This material exhibited a

capacityof 3.11 mmol/g under a flowof water saturated10%CO2/Ar

at 25 ◦C. The CO2/N efficiency was as high as 0.44 at room tem-

perature, close to the theoretical value of 0.5. With respect to its

performance at 75◦C, and 10% CO2/Ar, the hyperbranched-SBA-

15 was stable, presenting an average adsorption capacity of ca.

1.98mmol/g over 12 cycles with regeneration at 130 ◦C. In a later

contribution [138], it was shown that higher loading of hyper-

branched amines afforded a better capacity. The best reported

adsorbent had an amine loading of 9.78mmol/g and adsorbed ca.

4mmol/g at 10% CO2/N2 at75◦C in the presence of humidity.

As summarized in Table 5, Similarly to amine-impregnated

adsorbents, the covalently bonded aminated adsorbents spanmaterials with a wide variety of characteristics and performances.

However, a number of common advantages and limitations of

amine-graftedmaterials canbe outlined. Onlysupportsthat exhibit

surface hydroxyl groups can be used to produce amine-grafted

materials. It was observed that high amine loading is a result

of high surface area and availability of surface silanol groups,

but the efficient use of functional groups is observed mainly

in supports with large pores. While the equilibrium adsorption

capacities are certainly not as high as those reported with some

amine-impregnated adsorbents, properly designed amine-grafted

materials do not exhibit the strong diffusion limitations observed

in impregnated adsorbents. Therefore highadsorption ratesare not

restricted upon operating at high temperature. A particular advan-

tageoffered by amine-graftedadsorbents is theirhigh stabilityover


12/15


Table 5

Literature data on CO2 adsorption capacity of amine-grafted adsorbents.

Support Amine Capacity

(mmol/g)

Amine

loading

(mmol/g)

CO2/N Experimental conditions Reference

CO2 concentration T (◦C)

Silica gel AP 0.89 1.26 0.71 100% (100% RH) 50 [113]

MCM-48 AP 2.3 2.3 1 10% (100% RH) 25 [114]

HMS AP 1.59 2.29 0.69 90% 20 [115]

HMS TRI 1.34 4.57 0.29 90% 20 [116]

PE-MCM-41 TRI 1.59 7.9 0.20 10% 50 [121]

SBA-15 TRI 1.80 5.80 0.31 15% (humid) 60 [122]

MS TRI (co-cond) 1.74 5.18 0.34 100% 25 [123]

SBA-16 EDA 1.4 0.76 1.84 100% 27 [125]

SBA-15 AP 0.45 2.56 0.18 10% 65 [126]

SBA-16 EDA 0.727 3.06 0.24 15% 60 [127]

SBA-15 AP 1.54 2.72 0.57 10% 25 [128]

SBA-12 AP 1.04 2.13 0.49 10% 25 [129]

MS AP 0.24 1.6 0.15 10% 30 [130]

MSP EDA 0.73 0.99 0.73 10% 60 [131]

MCM-48 TREN 1.36 4 0.34 100% 50 [132]

ITQ-6 AP 0.67 1.26 0.53 12% 20 [133]

SBA-15 Amine-dendrimers 1 1.25 0.40 90% 20 [134]

SBA-15 Aziridine polymer 4 9.78 0.41 10% (humid) 75 [138]

hundreds, most likely thousands, of adsorption–desorption cycles

[102].

Although this review focused on CO2 capture, as mentioned in

the Introduction, there are other impuritiesin flue gas. Of particular

concern with respect to amine-functional materials is the pres-

ence of SO2, as it was found that it affects negatively their cyclic

performance. For example, it was recently reported thatafter expo-

sure to SO2, the working adsorption capacity of TRI-PE-MCM-41

decreased from 1.57mmol/g to 0.89mmol/g for a mixture of 10%

CO2/N2 at50◦C [139]. Supportedby gravimetric measurementsand

FTIRspectroscopy,it wasproposed thatSO2 reacts irreversibly with

the primary amines of the triamine functional molecules. Conse-

quently, it might be necessary to engineer processes to remove

SO2 prior to CO2 capture to preventing its contact with amine-

functionalized adsorbents.

4. Conclusions

Major advances have been achieved towardthe development of

a CO2 capture technology basedon adsorption. Physical adsorbents

suchas zeolites,carbon-basedmaterials andMOFs werefound to be

suitable,mostlyat lowtemperatureand highpressure. Theseadsor-

bents, however, often adsorb water vapor preferentially over CO2 ,

andtheir CO2 adsorption capacityat low pressure is not sufficiently

high. Although these materials may provide elegant solutions for

CO2 sequestration and storage, theyare not particularly suitable for

post-combustion gas treatment. Nevertheless, a continuous effort

is being deployed to circumvent such drawbacks. The strategies

beingused include surface modificationto enhancethe interactions

with CO2, thus increasing the adsorption capacity at low pressure.Another route is to design completely newmaterials such as ZMOFs

andCOFs with increased tolerance to moisture in the gas feed, thus

improved CO2 selectivity.

Likewise, tremendous progress has been achieved in the devel-

opment of novel chemical adsorbents such as amine-modified

materials with large surface area. By optimizing the synthesis con-

ditions and using supports with adequate structural properties, it

was possible to develop materials with superior CO2 adsorptive

properties, particularly suitable for flue gas treatment. Typically,

these materials exhibit large CO2 adsorption capacity even at low

pressure, high rate of adsorption anddesorption, andexcellent tol-

erance to moisture in the feed. Furthermore, contrary to physical

adsorbents, the selectivity of amine-functionalized materials is not

significantly affected by temperature, at least within the range of

interest for flue gas treatment. While the stability of this kind of

adsorbents has been questioned, it was recently demonstrated that

their stability may be dramatically enhanced during thousands of

adsorption–desorption cycles, provided that the feed and purge

gases contains moisture. The role of moisture is to prevent the

formation of urea linkages, which is the main source of material

deactivation.

This review clearly showed a steady improvement in the CO2adsorptive properties of novel materials. The course followed so

far has resulted in major achievements that may well pave the way

for an alternative CO2 capture technology in the near future.

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