Post-Combustion CO2 Capture Using Solid Sorbents: A Review

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Post-Combustion CO2 Capture Using Solid Sorbents: A Review Arunkumar Samanta,† An Zhao,† George K. H. Shimizu,‡ Partha Sarkar,§ and Rajender Gupta*,† †

Department of Chemical & Materials Engineering, University of Alberta, Edmonton, Alberta, Canada Department of Chemistry, University of Calgary, Calgary, Alberta, Canada § Environment & Carbon Management Division, Alberta Innovates—Technology Futures, Edmonton, Alberta, Canada ‡

ABSTRACT: Post-combustion CO2 capture from the flue gas is one of the key technology options to reduce greenhouse gases, because this can be potentially retrofitted to the existing fleet of coal-fired power stations. Adsorption processes using solid sorbents capable of capturing CO2 from flue gas streams have shown many potential advantages, compared to other conventional CO2 capture using aqueous amine solvents. In view of this, in the past few years, several research groups have been involved in the development of new solid sorbents for CO2 capture from flue gas with superior performance and desired economics. A variety of promising sorbents such as activated carbonaceous materials, microporous/mesoporous silica or zeolites, carbonates, and polymeric resins loaded with or without nitrogen functionality for the removal of CO2 from the flue gas streams have been reviewed. Different methods of impregnating functional groups, including grafting techniques and modifying the support materials, have been discussed to enhance the performance of the sorbents. The performance characteristics of the solid sorbents are assessed in terms of various desired attributes, such as their equilibrium adsorption capacity, selectivity, regeneration, multicycle durability, and adsorption/ desorption kinetics. The potential of metal-organic frameworks (MOFs) is also recognized to determine whether these novel materials provide better CO2 adsorption capacity under low CO2 partial pressure. A comprehensive critical review and analysis of the literature on this subject has been carried out to update the recent progress in this arena. A comparison of different solid sorbents at different stages is made. It also includes a brief review on techno-economic analysis and design aspects of sorbent bed contactor configuration. Finally, a few recommendations have been proposed for further research efforts to progress post-combustion carbon capture.

1. INTRODUCTION Growing environmental concerns in recent times for global warming and climate change have motivated research activities toward developing more-efficient and improved processes for carbon dioxide (CO2) capture from large point sources of CO2. The Intergovernmental Panel on Climate Change (IPCC)’s fourth assessment report1 states that, as a result of anthropogenic CO2 emission, global atmospheric concentration has increased from a preindustrial value of ∼280 ppmv to 379 ppmv in 2005 and ∼390 ppmv currently. Global annual CO2 emissions due to fossil-fuel use have grown by ∼80%, from ∼21 Gt in 1970 to ∼38 Gt in 2004.2 This represented 77% of the total anthropogenic greenhouse gas (GHG) emissions in 2004, and of this, close to 60% was attributed to large (>0.1 Mt CO2 per year) stationary emission sources, such as power plants, gas processing industries, refineries, chemical and petrochemical industries, iron and steel industries, and cement industries. Without the introduction of near-term supportive and effective policy actions by governments, energy-related GHG emissions, mainly from fossil fuel combustion, are projected to rise by over 50%, from 26 Gt CO2 in 2004 to ∼37
40 Gt CO2 by 2030, and possibly even higher.3 Therefore, mitigation has become even more challenging. The global concern for this situation is well-reflected in the deep global engagements that continue from the Rio Earth Summit in 1992 through the 1997 Kyoto Protocol of United Nations Framework Convention on Climate Change (UNFCCC) to the recently concluded UN Climate Change Conference in Cancun, Mexico in December 2010. Capture of CO2 from large point sources, such as r 2011 American Chemical Society

coal-based power plants, natural and synthesis gas processing plants, and cement plants, and its sequestration (CCS) is identified as a major option to address the problem of global warming and climate change. However, the main focus of CCS today is coalbased power plants. CCS includes four primary steps: CO2 capture, compression, transport, and storage. To economically sequester CO2, it is important to have cost-effective capture in a relatively concentrated stream and then compressing the CO2 to high pressures. Three approaches are envisaged for CO2 capture from fossilfuel based power plants: pre-combustion, post-combustion, and oxycombustion.4
6 Pre-combustion capture is applicable to integrated gasification combined cycle (IGCC) plants, while oxy-combustion and post-combustion could be applied to conventional coal- or gas-fired power plants. However, in their present development stage, these approaches are still not ready for implementation on coal-based power plants, because of three primary reasons: (i) they have not been demonstrated in large scale; (ii) the required parasitic loads to supply both power and steam to the CO2 capture plant would reduce power generation capacity by approximately one-third; and (iii) if successfully Special Issue: Nigam Issue Received: April 5, 2011 Accepted: October 27, 2011 Revised: September 17, 2011 Published: October 27, 2011 1438

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Industrial & Engineering Chemistry Research Table 1. Typical Gas Quality in Applications to CO2 Capture CO2 (dry) process

(vol %)4

impurities

natural gas turbine exhaust

3
4

low SOx and NOx levels, O2:

coal-/oil-fired boiler

11
14

high SOx and NOx levels, O2:

IGCC syngas turbine

4.5
6

low SOx and NOx

12%
15% 2%
5% exhaust blast furnace gas (after combustion) cement kiln off-gas

27 14
33

SOx and NOx present SO2 and NOx, trace elements, particulates

scaled up, they would not be cost-effective at their present phase of process development stage.7 Amine-based regenerative chemical absorption processes using aqueous solutions of amine, such as monoethanolamine (MEA), diethanolamine (DEA), diglycol-amine (DGA), N-methyldiethanolamine (MDEA), and 2-amino-2-methyl-1-propanol (AMP) have been widely practiced for several years for CO2 capture from gas streams in natural gas, refinery off-gases and synthesis gas processing.8
10 The gas streams in these processes are at a high pressure. However, the major challenges for CO2 capture from fossil-fueled based thermal power plants are the large volumetric flow rates of flue gas at essentially atmospheric pressure with large amounts of CO2 at low partial pressures and in the temperature range of ∼100
150 °C. The presence of SOx, NOx, and significant oxygen partial pressure in the flue gas add to additional problems for implementation of amine absorption process for CO2 capture from flue gas streams (see Table 1). The state-of-the-art process uses 20
30 wt % aqueous MEA for post-combustion capture of CO2. However, this process is highly energy intensive, because of the high regeneration energy requirement. In addition, thermal and oxidative degradation of the solvents result in large solvent makeup, besides producing products that are corrosive. A techno-economic and environmental assessment study suggests an 80% increase in the cost of electricity when CO2 capture with a 30 wt % aqueous MEA absorption process is implemented with a coal-fired power plant.11 The CO2 capture and compression account for 80% of the total cost, while the balance of the cost (20%) is due to transportation and sequestration. This high cost of CO2 capture reduces the implementation of MEA process for CO2 capture from the flue gas of coal-based power plants.12 Thus, the cost-effective processes for CO2 capture from the flue gas streams of coal-based power plants are essential today in order to mitigate the global warming problems. Among the many process technology options used for CO2 capture from flue gases, there is a growing interest in using adsorption processes as a promising alternative separation technique. Adsorption processes using novel solid sorbents capable of reversibly capturing CO2 from flue gas streams have many potential advantages, compared to other separation techniques for CO2 capture, such as reduced energy for regeneration, greater capacity, selectivity, ease of handling, etc. Adsorption on porous solid media using pressure and/or temperature swing approaches is an emerging alternative that seeks to reduce the costs associated with the capture step. The regeneration energy requirement for CO2

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capture using dry solid sorbent is significantly reduced more than that of the aqueous amine-based process, because of the absence of large amounts of water. Moreover, the heat capacity of solid sorbent is comparatively lower than that of an aqueous amine solvent. The success of such an approach is also dependent on the development of new materials with high adsorption capacity, high CO2 selectivity, durability, and relatively fast kinetics of sorption and desorption. In view of the above, in the past few years, several research groups worldwide have initiated work on the development of new solid sorbents for CO2 capture from flue gas with superior performance and desired economics. The impurities in flue gases such as oxides of nitrogen and sulfur often reduce the performance of the sorbents. Ample proof of this fact can be found in the open literature. Characterization and understanding the behavior of the sorbents in actual flue gas conditions is important. This paper reviews the recent progress in CO2 capture using solid sorbents with and without functionalized groups. A comprehensive literature review and analysis on this subject has been carried out to update the recent progress in this field. It also includes a brief review on techno-economic analysis and design aspects of sorbent bed contactors. 1.1. Criteria for Selecting CO2 Sorbent Material. Sorbent selection is a complex problem.13 The sorbent materials must satisfy some important criteria to be both economical and operational for CO2 capture from flue gas. These criteria are listed in this section.14
16 • Adsorption capacity for CO2: The equilibrium adsorption capacity of a sorbent material represented by its equilibrium adsorption isotherm is of paramount importance to the capital cost of the capture system, because it dictates the amount of adsorbent required, which also fixes the volume of the adsorber vessels. That is, the high adsorption capacity for CO2 reduces both sorbent quantity and process equipment size. In practice, working capacity, defined for a short adsorption time, is preferred to be used in place of equilibrium capacity. In order for solid sorbents to be competitive with existing MEA scrubbing system, the CO2 working capacity must be in the range of 3
4 mmol g
1 of sorbent (hereafter shortened to mmol g
1 in this work).17 • Selectivity for CO2: The selectivity, which is defined by the ratio of the CO2 capacity to that of another component (for example, N2) at a given flue gas composition, has a direct impact on the purity of CO2 captured. The purity of CO2 has impacts on transportation and sequestration and, therefore, plays an important role in CO2 sequestration economics. Flue gas streams from fossil-fuel fired power plants contain N2 and O2. Good sorbent materials should exhibit high CO2 selectivity over these other bulk gas components. It is also important that solid sorbents also show high capacity for CO2 in the presence of significant amounts of water vapor in flue gas. • Adsorption/desorption kinetics: It is essential to have fast adsorption/desorption kinetics for CO2 under the operating conditions. The kinetics of adsorption and desorption controls the cycle time of a fixed-bed adsorption system. Fast kinetics yields a sharp CO2 breakthrough curve in which effluent CO2 concentration changes are measured as a function of time, while slow kinetics provides a distended breakthrough curve. Both of these impacts on the amount of sorbent required. The overall kinetics of CO2 adsorption on a functionalized solid sorbent is influenced by the intrinsic reaction kinetics of CO2 with the functional group present, 1439

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Table 2. Structural Properties of the Adsorbents pore size

pore volume

specific surface

(nm)

(cm3 g
1)

areaa (m2 g
1)

ref

Carbonaceous Sorbents AC

1.5
2.2

1300

33

AC-hb

2.19

1.55

2829

44

granular AC

2

0.48

954

36

0.7

1100

38

BPL carbon

0.6
0.8

CNT

8.9

0.91

394

36

CNT

23.54f

0.446

407

45

Zeolites NaX

1

0.201e

508

46

NaY

1.54

0.46

811

47

0.14e

404

48

CsX CsY

1.58

0.21

473

47

Zeolite 13X

1.1

0.454

515

49

chabozite

0.43

4.2

485

52

clintopilec

0.44

4.1

18.4

52

clintopiled erionite

0.44

4.5 0.22

13.3 426

52 53

mordenite

0.19

266

53

clinoptilolite

0.066

23

53

0.94

256

50

0.35

750

51

and be resistant to common flue gas contaminants. It is likely that contaminants such as SOx, NOx and heavy metals18 will also require removal from flue gas, because they unfavorably affect the CO2 adsorption capacity of the sorbent materials. • Regeneration of sorbents: The heat of adsorption, which is a measure of the energy required for regeneration, should be substantially low. Heats of adsorption are generally in the range of
25 to
50 kJ mol
1 for physisorption cases and
60 to
90 kJ mol
1 for chemisorptions cases. Moreover, the sorbent should be regenerable through a suitable pathway while maintaining efficient CO2 sorption capacity during repeated adsorption
desorption cycles in a powerplant flue-gas environment. The ease of regeneration of the CO2 sorbent will also help to reduce the cost of capture. • Sorbent costs: These represent perhaps the most subtle characteristics. Tarka et al.,19 have used a baseline of approximately $10/kg for sorbents in their sensitivity analysis for economic performance. According to them, a cost of $5/kg sorbent results in a very good scenario and a cost of $15/kg sorbent is not economical. Therefore, to be more economical, the cost of the CO2 capture sorbent should be in the range of approximately $10/kg. The above-mentioned attributes are desirable for an ideal adsorbent. Rarely will a single adsorbent be optimal in all these attributes. However, useful adsorbents will be those that effectively and economically capture CO2 from flue gas streams.

Silica Support silica gel silica gel KC

2

MCM-41

3.6

1.03

1138

50

PE-MCM-41

9.7

2.03

917

50

200
230

54

470

55

SBA-15

21

PMMA

17

Polymer 1.2

BET surface area. b Activated carbon with high surface area. c Sodium aluminosilicate Type 4A zeolite. d Potassium calcium sodium aluminosilicate. e Microporous pore volume. f Average pore diameter of 1.7
100 nm.

2. PHYSICAL SORBENTS CO2 from flue gas may be removed using a variety of solid physisorbents materials, including porous carbonaceous materials, zeolites, alumina, silica gels, and metal-organic frameworks (MOFs). For physisorption, such as adsorption of CO2 by zeolite or activated carbon, the mechanism for CO2 capture can be visualized as CO2 þ surface S ðCO2 Þ 3 ðsurfaceÞ

a

as well as the mass transfer or diffusional resistance of the gas phase through the sorbent structures. The porous support structures of functionalized solid sorbents also can be tailored, to minimize the diffusional resistance. The faster an adsorbent can adsorb CO2 and be desorbed, the less of it will be needed to capture a given volume of flue gas. • Mechanical strength of sorbent particles: The sorbent must demonstrate microstructure and morphological stability and retain the CO2 capture capacity during multicycling between the absorption and regeneration steps. Microstructure and morphological stability of tailored regenerable sorbents in multicycle operation is critical to maintain high kinetics. Operating conditions, such as high volumetric flow rate of flue gas, vibration, and temperature should not cause appreciable disintegration of the sorbent particles. This could also happen via abrasion or crushing. Therefore, adequate mechanical strength of sorbent particles is critical to minimize the sorbent makeup rate and to keep CO2 capture process cost-effective. • Chemical stability/tolerance to impurities: Solid CO2 capture sorbents—in particular, amine-functionalized sorbents— should be stable in an oxidizing environment of flue gas

where the selective adsorption of CO2, in comparison with N2, is caused by van der Walls attraction between the CO2 molecule and adsorbent surface, as well as by pole
ion and pole
pole interactions between the quadruple of CO2 and the ionic and polar sites of the solid adsorbent surface.20 This section summarizes the recent progress in CO2 capture using solid physisorbents, namely activated carbons, carbon nanotubes, carbon molecular sieves, zeolites, and MOFs. 2.1. Activated-Carbon-Based Solid Sorbents. Inorganic carbon is available in various forms, such as porous activated carbons (ACs), graphenes, and carbon nanotubes (CNTs). ACs are widely used as adsorbents in various industrial applications, e. g., gas purification, water treatment, etc., because of the low cost of raw materials and wide availability.20,21 Sorption behavior of CO2 on commercial and synthesized ACs derived from different natural organic materials, as well as carbon molecular sieves, have been widely studied experimentally and theoretically by various research groups at low to very high pressure.22
43 The structural properties and adsorption capacities of carbon-based adsorbents are summarized in Tables 2 and 3, respectively. Applications of the pressure swing adsorption (PSA) process have been reported to remove CO2 from gas streams using ACs.22,23,25,37,38 Yang and co-workers23 studied the feasibility of the PSA process for the first time, to separate and concentrate CO2 from flue gas using BPL ACs and carbon molecular sieves 1440

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Table 3. CO2 Adsorption Capacity of Carbonaceous Solid Sorbents sorbent

temperature, T (K)

pressure, pCO2 (atm)

capacity (mmol g
1)

process

ref 23

AC

298

1

2.07

PSA

AC

288

1

2.45

PSA

AC

298

0.2

0.75

AC

298

1

3.23

AC

298

1

2.61

PSA

AC

298

0.1

0.57

GC-TCD

ACd

298

1

2.25

ACe AC-A35/4

298 293

1 1

1.53 2.00

flow desorption

AC-F30/470

288

1

2.86

volumetric analysis

AC-F30/470

297.3

0.16

0.65

25 28 33 37 36 43 43 29 51 26

AC-RB

303

1

1.22

AC Norit RB1

294.2

1

2.456

gravimetric analysis

38 32

AC Norit RB1

313.1

0.15

0.50

TPD

24

AC-Norit R1 Extra

298

0.15

0.538

bamboo-derived AC bamboo-derived AC

275 275

2 2

3.00 0.54a

volumetric analysis volumetric analysis

35 35

anthracite-based ACb

303

1

1.38

TGA

42

anthracite-based ACc

303

1

1.33

TGA

SWNT

308

1

2.07

56

graphene

195

1

0.80

57

CMS

303

1

2.43

59

30

42

a

Wet bamboo activated carbon with water loading in the range of 1.35
2.36. Water loading refers to the weight ratio of water to dry carbon. b Activated at 850 °C for 3 h. c Activated at 850 °C for 2 h. d Activated carbon without acid treatment. e Activated carbon with acid treatment.

(CMSs) as sorbents. They achieved a CO2 recovery of ∼68.4% from a flue gas composed of 17% CO2/4%O2/79%N2, using ACs at 298 K, while the amount of CO2 contaminant in the adsorption product was ∼6.3%. They also concluded that the equilibrium separation of CO2 from flue gas using ACs is better than the kinetics separation by CMS. In another work, this group reported that, in the case of equilibrium separation, zeolite 13X is better than the ACs.25 The heat of adsorption (ΔHad) of activated carbon (ΔHad=
30 kJ mol
1) is lower than that of zeolites (ΔHad=
36 kJ mol
1), because of its weaker interaction with CO2. Na et al.33 also demonstrated the PSA process using commercial AC to separate CO2 from the flue gas of a power plant. The CO2 adsorption capacities decreased significantly, e.g., from ca. 3.2 to ca. 1.6 mmol g
1, when temperature was increased from 288 to 328 K at 1 atm. The maximum recovery of 34% with a purity of 99.8% was obtained from PSA process from a flue gas composed of 17% CO2/4% O2/79% N2 at 328 K. In 1998, Do et al.28 presented theoretical and experimental results on CO2 adsorption isotherm at three different temperatures and at a pressure up to 20 kPa on Ajax activated carbon. They observed that the CO2 adsorption capacity decreases significantly as the temperature increases. The reported adsorption capacity was ca. 0.75 and 0.11 mmol g
1 at 298 and 373 K, respectively. Siriwardane et al.37 compared the volumetric adsorption isotherms of CO2 on G-32H activated carbon and molecular sieves 13X and 14A. They established that the surface affinity of molecular sieve 13X for CO2 was relatively better than that of activated carbon and the CO2 adsorption capacity of three sorbents was not adversely affected by the adsorption of other gases such as N2, H2. From their competitive adsorption rate studies, they concluded that it was possible to obtain excellent

separation of CO2 from a gas mixture containing 14.8% CO2 and 85.2% N2 with both types of sorbents. Maroto-Valer and his group39
42 found that anthracite coal with 2 h of activation at 890 °C achieved a CO2 adsorption capacity of 1.49 mmol g
1, measured using a thermogravimetric analyzer (TGA) in pure CO2. Wang et al.31 conducted a comparative study of CO2 sorption on ACs prepared from bamboo chips and coconut shell, in the presence of water, and concluded that water exhibited a detrimental effect on CO2 sorption at low pressure. Recently, Radosz et al.43 proposed a low-cost low-pressure carbon-filter process to capture CO2 from flue gas. All carbonaceous materials studied in this work exhibited a rapid sorption rate and a short sorption cycle. The carbon filter process proposed in this work could recover at least 90% of the flue gas CO2 of 90% purity. ACs are low cost with fast adsorption kinetics and require low regeneration energy. However, the predominance of the porous media of ACs still fails to offset the drawbacks of physical adsorption processes, such as selectivity. The CO2 adsorption capacity using porous activated carbon decreases as the temperature increases (see Figure 1).26,28 At low partial pressures of CO2, ACs exhibit lower adsorption capacity and selectivity than zeolites, because of their unfavorable adsorption isotherms.25 The existence of water may negatively affect the capacity of ACs, because water gets adsorbed competitively.35 In addition, other components or contaminants in flue gas also have a detrimental impact on the CO2 adsorption capacity. 2.2. Carbon Molecular Sieves. Carbon molecular sieves (CMSs), which represent a special class of microporous carbon materials whose unique textural characteristics permit the kinetic separation of gas mixtures, are also studied for CO2 capture. For a CMS to be useful for gas separation, it must possess a narrow 1441

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Figure 1. Isotherm for adsorption of CO2 on activated carbon.26

pore size distribution (PSD), consisting of pore mouths of molecular sizes and a relatively high micropore volume; these features result in selectivity and capacity, respectively.58 A porous monolithic carbon fiber composite molecular sieve was developed and characterized by Burchell et al.59 The CO2 isotherm of this CMS showed a CO2 uptake of >2.27 mmol g
1 at 30 °C and atmospheric pressure but this uptake is reduced at elevated temperature. However, analysis of the equilibrium and kinetics of batch adsorption of CO2 with a Takeda 5A CMS indicated no molecular sieving action; instead, micropore diffusion was shown to be rate-limiting in the niz-Monge et al.61 process of adsorption dynamics.60 Recently, Alca~ reported that microporous carbon monoliths prepared from nitrated coal tar pitch showed faster CO2 kinetics than commercially available Takeda 3A CMS. 2.3. Carbon Nanotube-Based Solid Sorbents. The use of new generation materials, such as carbon nanotubes (CNTs) and graphene, has also become an active area of research for separation of gas mixtures over the last several years. Considerable theoretical modeling and experimental research efforts are being devoted to investigate the adsorption of CO2 and N2 and their mixtures on CNTs (see Table 3).36,45,56,62
72 By choosing the appropriate pore size and shape and optimum conditions, CNT can act as a suitable candidate for CO2 separation and sequestration.67,71 Cinke et al.56 reported adsorption of CO2 on purified single-walled carbon nanotubes (SWCNT) in the temperature range of 0
200 °C (see Figure 2). SWCNTs exhibited a Brunauer
Emmett
Teller (BET) surface area of 1587 m2 g
1 and a total pore volume of 1.55 cm3 g
1, and the micropore volume was 0.28 cm3 g
1. The CO2 adsorption capacity of SWCNTs was twice that of AC. However, Lu and his group36,45 observed that equilibrium CO2 adsorption capacity of raw CNT was relatively lower than that of granular AC at 25 °C. Skoulidas et al.66 carried out simulations to examine the adsorption and transport diffusion of CO2 and N2 in SWCNTs at room temperature, as a function of nanotube diameter. They reported that transport diffusivities for CO2 in nanotubes with diameters ranging from ∼1 nm to ∼5 nm are roughly independent of pressure. The observed diffusion mechanism is not Knudsen-like diffusion. Based on Monte Carlo simulations, Huang et al.67 showed that CO2 adsorption in the range of 4
9 mmol g
1 is an increasing function of the diameter of the CNT, and CNTs demonstrated a higher selectivity toward CO2 than other sorbents, such as ACs, zeolite 13X, and MOFs that have reported in the literature. Razavi et al.71 also concluded that CNTs exhibited a higher selectivity of CO2 over nitrogen, compared to

Figure 2. Comparison of CO2 adsorption capacities of high-pressure CO conversion (HiPco) single-walled nanotubes (SWNTs) and activated carbon (AC) at 35 °C. (Reproduced with permission from ref 56. Copyright Elsevier, 2003).

other carbon-based materials, for the separation of CO2/N2 mixture. Lithoxoos et al.72 obtained Type I (Langmuir) behavior of the adsorption isotherm for CO2 for SWCNTs. 2.4. Zeolite Sorbents. Microporous crystalline framework materials such as synthetic and natural zeolites are widely used in the field of gas separation and purification. Conventional zeolites are based on silicate frameworks in which substitution of some of the Si with Al (or other metals) leads to a negative charge on the framework, with cations (usually Na or other alkaline or alkaline-earth metals) within the pore structure. These cations can be exchanged to fine-tune the pore size or the adsorption characteristics. Because of their defined crystalline structures, these sorbents have uniform pore sizes in the interval of 0.5 nm to 1.2 nm,73 which is a property that allows them to separate molecules by means of the molecular sieving effect. Separation of gases in zeolites can also occur through the mechanism of selective adsorption of those molecules that have a relatively large energetic dipole and quadruple. The gases that have high quadrupole moment, such as CO2 (
14.29  10
40 C m2),74 interact strongly with the electric field created by the structural cations of zeolites, and this favors their adsorption. Separation of gases by the zeolite adsorbents depends on various factors: structure and composition of the framework, cationic form, purity, size and shape of the molecules, and molecular polarity. The typical physical properties of the zeolites are listed in Table 2. Zeolites have shown promising results for separation of CO2 from gas streams. There is much published literature concerning CO2 adsorption over different types of zeolites such as zeolite A, X, Y, and other natural zeolites, such as chabazites, clintopiles, ferrierites, mordenites, etc.22,37,52,53,75
96 Table 4 summarizes the various zeolites that have been explored to date for application of CO2 separation from gas mixtures. Earlier, a study of adsorption of CO2 on various natural zeolites (namely, mordenite, ferrierite, clinoptilolite, and chabazite) and synthetic zeolites (namely, 4A, 5A, and 13X) showed chabazite and 13X, among the natural and synthetic zeolites, respectively, to be better adsorbents for CO2 separation from N2.79 The work by Siriwardane et al.52 indicated that natural zeolite with the highest sodium content and highest surface area showed the highest CO2 adsorption capacity and highest rates of adsorption. 1442

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Table 4. CO2 Adsorption Capacity of Zeolites temperature, T (K)

pressure, pCO2 (atm)

adsorption capacity (mmol g
1)

Zeolite 13X

293

0.15

2.63

80

Zeolite 13X

295

1

4.61

91

Zeolite 13X

298

1

4.66

NaX

305

1

5.71

gravimetric analysis

NaY

305

1

5.5

gravimetric analysis

NaY

295

1

4.06

NaM

298

1

2.95

gravimetric analysis

83

silicalite Na-ZSM-5

303 303

0.15 1

0.48 0.75

calorimeter-volumetric apparatus GC

104 87

molecular sieve 13X

298

1

2.8
3.6a

PSA

37

molecular sieve 4A

298

1

2.3
3.1a

PSA

molecular sieve 13X

293

0.15

2.18

molecular sieve 4A

293

sorbent

a

experimental procedure

ref

92 83 83 91

37 105

0.15

1.65

molecular sieve 13X

0.1

2.33

fluidized bed

101

molecular sieve 5A

0.1

2.35

fluidized bed

101

erionite (ZAPS) mordenite (ZNT)

290 290

1 1

2.8 1.8

clinoptilolite (ZN-19)

290

1

1.7

ZSM-5b

313

0.1

0.32c

105

53 53 53 GC

90

HZSM-5
30

295

1

1.9

91

HiSiv 3000

295

1

1.44

91

HY-5

295

1

1.13

91

Cyclic tests. b SiO2/Al2O3 ratio = 280. c Experimental data.

An extensive screening study of ∼13 synthetic zeolites, including 5A, 13X, NaY, ZSM-5, HiSiV-3000 (based on ZSM-5 structure with a silica-to-alumina ratio (Si/Al > 1000) was conducted by Harlick and Tezel91 for the removal of CO2 from flue gas. From pure component isotherm data, it was observed that adsorption capacities of the adsorbents increased in the following order (in the pressure range of ∼0
2 atm): 13X ðSi=Al ¼ 2:2Þ > NaY ðSi=Al ¼ 5:1Þ > H
ZSM
5
30 ðSi=Al ¼ 30Þ > HiSiv3000 > HY
5 ðSi=Al ¼ 5Þ (see Figure 3). This might be due to a low Si/Al ratio with cations (sodium) in the structure that show strong interactions with CO2. Zukal et al.102 performed investigation on CO2 adsorption on six high silica zeolites (SiO2/Al2O3 > 60), TNU-9, IM-5, SSZ74, ferrierite, ZSM-5, ZSM-11. The highest CO2 adsorption capacity was found with TNU-9 and IM-5 attaining 2.61 and 2.42 mmol g
1 at the pressure of 100 kPa, respectively. Recent studies also focused on the modification of zeolites via the introduction of large and electropositive, polyvalent cations to enhance the adsorption of CO2. Khelifa et al.94 demonstrated that NaX (Si/Al = 1.21) zeolite exchanged with Ni2+ and Cr3+ had shown a decrease in CO2 adsorption capacity, compared to that of the parent NaX zeolite, because of a weak CO2
sorbent interaction. NaX and NaY and those resulting from ions exchanged with Cs, since it is the most electropositive metal of the periodic table, were tested with regard to the adsorption of CO2 by Diaz et al.47,48 Cs-treated zeolites performed better and were very active for adsorption at higher temperatures (100 °C). Zhang et al.98 prepared chabazite (CHA) zeolites (Si/Al < 2.5) and exchanged them with alkali cations (e.g., Li, Na, and K) and

Figure 3. Comparison of CO2 adsorption isotherms for fresh zeolites at 295 K. (Legend: (—9—) 13X; (—b—) NaY; (—2—) HZSM-5-30; (—(—) HiSiv 3000; and (—1—) HY-5.) (Reproduced with permission from Harlick and Tezel.91 Copyright Elsevier, Amsterdam, 2004.)

alkaline-earth cations (e.g., Mg, Ca, Ba) to assess their potential for CO2 capture from flue gas by vacuum swing adsorption (VSA) for temperatures below 120 °C. From the adsorption isotherm, it was found that NaCHA and CaCHA hold comparative advantages for high temperature (>100 °C) CO2 separation, while the NaX zeolite shows superior performance at relatively low temperatures. According to Katoh et al.87 the selectivity of ion-exchanged ZSM-5 zeolites, M-ZSM-5 (M = Li, Na, K, Rb, and Cs) might be due to the fact that almost all CO2 molecules strongly adsorbed on the cation sites, while N2 interacted with the wall of the H-ZMS-5. 1443

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Industrial & Engineering Chemistry Research Adsorption kinetics is an important parameter to evaluate the adsorption performance of an adsorbent. Kinetic studies of CO2 adsorption on zeolite are rarely reported. According to the results shown by Hernandez-Huesca et al.,53 the adsorption of CO2 on erionite (ZAPS) occurred very fast and ∼70% of the total capacity was achieved at both 273 and 293 K. Zhang et al.44 suggested that the adsorption kinetics could be described by the linear driving force model and reported that the adsorption activation energy for CO2 on 13X decreases as the pressure increases. Most of the research in adsorption using zeolites has focused on PSA/VSA adsorption for separating CO2 from flue gas in power stations that use coal as fuel.22,46,52,88,93,99 The temperature swing adsorption (TSA) process has also been suggested by various research groups for the separation of CO2 from flue gas using zeolites.97,100 Using the TSA process, Merel et al.97 obtained the best performance with zeolite 5A, in terms of the CO2 capture rate (18%) and volumetric productivity (+23%), when a simulated 10% CO2/90% N2 gas was used. Konduru et al.100 employed the TSA process to capture CO2 from a 1.5% CO2/98.5% N2, using zeolite 13X. Based on an average CO2 recovery of 84% after five consecutive cycles, they concluded that zeolite 13X showed promise as an adsorbent with substantial CO2 uptake capacity. Lee et al.101 pointed out attrition as one potential problem with the zeolite sorbents when used in fluidized beds. Zeolites 5A and 13X presented attrition that was 2
4-fold higher than that of AC and activated alumina. This would possibly cause high maintenance costs for a dry sorbent and problems in the operation of the fluidized-bed process. It has been reported that there is a detrimental effect of water on CO2 adsorption, because it gets preferentially adsorbed from the gas mixture.106,107 Small amounts of water could significantly decrease the CO2 adsorption capacity, because it gets competitively adsorbed on the zeolite surface and blocks the access for CO2.106 In another study, CO2 and water vapor adsorption on zeolite 13X has also indicated that the adsorption of CO2 is considerably inhibited by H2O.107 In summary, zeolites have been studied extensively to determine the effect of type of ions incorporated into the structure on the adsorption equilibrium and energetics. In general, adsorption kinetics of CO2 on zeolites is comparatively fast and achieved equilibrium capacity within a few minutes. However, CO2 adsorption on zeolites is strongly influenced by the temperature and pressure. The adsorption of CO2 decreases as the temperature increases and increases as the gas-phase partial pressure of CO2 increases. The presence of water vapor may limit the application of zeolite sorbents by decreasing its capacity. It is therefore obvious, that by carefully considering and optimizing different important factors, such as basicity, pore size of zeolites, electric field strength caused by the presence of exchangeable cations in their cavities may significantly influence the CO2 adsorption capacities of zeolites.103 2.5. Metal-organic Frameworks. A relatively new class of solid sorbent materials is metal-organic frameworks (MOFs).108 MOFs are network solids composed of metal ion or metal cluster vertices linked by organic spacers. The ability to readily incorporate and vary organic linkers in these materials translates to abundant options to control pore size, pore shape, and the chemical potential of the adsorbing surfaces and, consequently, their selectivity, kinetics, and capacity. Two important features of MOF materials are, as implied by the previous statement, (i) their syntheses can be modular (i.e., entire segments can be replaced

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by a functionalized or longer analogue) and (ii) the solids are crystalline. The modular synthesis leads to the fact that one can develop isoreticular families of solids (based on the same topological net), which is a tremendous advantage for rationally tailoring pore sizes.109 The crystalline nature of MOFs makes them amenable to complete structural characterization via X-ray diffraction (XRD) methods. With atomic coordinates of the framework in hand, designing materials is greatly facilitated. With the nature of the bonding (coordinate covalent) being weaker than that in metal oxides, it is not a given that solvated pores which are observed in crystal structures will necessarily persist upon solvent removal. Indeed, this fact has led to these compounds being classified into three generations: those that collapse irreversibly and are not porous (first generation); those that retain their structures and show reversible gas sorption isotherms (second generation); and a final category where the material behaves more like a sponge and changes structure reversibly with guest sorption (third generation).110 While the permanently porous solids are most akin to zeolitic solids and have the broadest application in gas sorption (and form the bulk of the remaining discussion), the third-generation compounds represent a unique opportunity with MOF materials and, in such systems, gas sorption has been coupled with properties such as magnetism,111 gate opening,112 or hydration-controlled release under ambient conditions.113 With respect to the ability to tune pore surfaces, the most common approach is to generate a bare-metal site lining the pore by liberation of a coordinated solvent (typically water) molecule. This leads to the high heat of adsorption that is observed in MOFs such as HKUST-1,114 MIL-100/101,115 and the MOF-74 family.116 Much effort has also been made on amine-modified MOFs. The amine group can be selected to interact via a physisorptive mechanism,117
119 where an arylamine serves a polarizing function, or a likely chemisorptive interaction with an alkylamine.120 MOF-210, which was synthesized by Yaghi’s group,121 currently holds the CO2 storage record with a saturated CO2 uptake of 2400 mg g
1 at room temperature and ∼50 atm. This MOF has an estimated bulk density of 0.25 g cm
3, a measured specific pore volume of 3.60 cm3 g
1, and a BET surface area of 6240 m2 g
1, which is the highest value reported for any crystalline material. For CO2 capture, efficient sorption at low partial pressures, such as those in post-combustion flue gas streams (∼0.1 atm), would be required. Yazaydin et al.122 have shown that the lowpressure sorption of CO2 in MOFs best correlates with the heat of adsorption of CO2, rather than any structural parameter, such as surface area or pore volume. Heats of adsorption ranging up to 90 kJ mol
1 have been observed in MOFs.120 Higher heats of adsorption are not necessarily more favorable, because a stronger binding of CO2 requires more energy to release the gas. Some 12 reported MOFs have heats of adsorption over 40 kJ mol
1 for CO2 (see Table 5). While a high heat of adsorption would favor CO2 uptake at lower pressures, that fact alone would not necessarily translate to good capture from flue gas in the presence of competing adsorbates, particularly water vapor. A prerequisite for sustainable capture in the presence of moisture would be high moisture stability. Increasingly, water-stable MOFs are being reported. ZIF-8 and ZIF-69 are particularly interesting for CO2 capture in industrial settings, because they have been demonstrated to maintain their crystal structure in environments such as boiling water, boiling benzene, and supercritical CO2.128,129 In 2009, 1444

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Table 5. Heat of Adsorptions of Selected MOFs heat of adsorption MOF

(kJ mol
1)

ref

Cu-BTTri (en)

90

120

Zn2([4-(carboxyphenyl)oxamethyl]

84

123

MIL-100

62

115

USO-1-Al-A

50

124

[Cu(PF6)2(bpetha)2]

50

125

MOF-74 (Mg) bio-MOF-11

47 45

116 117

methane)(4,40 -bipyridine)

MIL-101

44

115

[Pd(m-H-pymo)2 3 (H2O)x]

44

126

MOF-74 (Ni)

41

116

Zn2(Ox)(Atz)2 3 (H2O)0.5

40.8

127

[Pd(m-F-pymo)2 3 (H2O)x]

40

126

Snurr et al.130 reported that, HKUST-1, which is a MOF, exhibited enhanced CO2 uptake in the presence of 4% water. It was recognized that this did not translate to better uptake of CO2 in the presence of higher degrees of hydration. Work by LeVan et al.131 in 2010 examined the Ni-analogue of MOF74 and HKUST-1 for CO2 sorption in humid atmospheres. For the Ni-MOF74 material, high CO2 capacities (3.28 mmol g
1) were found at a point of interest for flue gas application (25 °C, 0.1 atm CO2 partial pressure). Ni-MOF74 has a higher CO2 capacity than benchmark zeolites (5A and NaX) under these conditions. Adsorbed water vapor impacted CO2 adsorption in the MOFs, but not nearly as greatly on 5A and NaX zeolites. Importantly, adsorbed water was more easily removed from the MOFs by regeneration than in the zeolites, not surprising given the less-hydrophilic nature of the MOFs. Most importantly, Ni-MOF74 was found to retain substantial CO2 capacity with moderate H2O loadings. This team also studied the effect of hydration on the stability of the MOF. The HKUST-1 sample showed increasing degradation over 7 runs; however, the Ni-MOF74 material showed higher stability over 10 runs. The authors concluded that “Considering the less intensive regeneration processes compared with benchmark zeolites and its hydrothermal stability, Ni-MOF74 may have a promising future for capturing CO2 from flue gases.” Certainly for low-moisture coals or natural gas, there is potential. This report131 represents the first thorough study of CO2 capture by MOFs with water as a coadsorbent, and it appeared in late 2010. Studies on other MOFs performed in the absence of moisture have reported selectivities for CO2 over CH4 and N2 as high as 30132 and ∼80,117 respectively, under ambient conditions. Given that the field of MOFs is still emerging, a general statement on the prospects for MOF materials is still largely based on their potential. A search for publications on “CO2 adsorption and/or separation in MOFs” per year, on the ISI Web of Knowledge, shows as few as 10 publications in 2006, increasing to 42 in 2008 and 90 in 2010. MOFs have very high capacity at high pressures, however, at atmospheric pressures their capacity is lower as compared to other physical sorbents. Further research is needed to develop MOFs targeting key material properties such as stability, multicycle applicability and competitive sorption.

Figure 4. Effect of temperature on the efficiency of CO2 capture by alkali-metal carbonates. Feed gas composition: 13.8% CO2, 10% H2O, and balance He.134

3. CHEMISORBENTS Most of the conventional physisorbents described above (such as zeolites, ACs, CMSs, and CNTs) suffer from low CO2 adsorption capacities at relatively low CO2 partial pressure and lower selectivity toward CO2. Recently, modifications in the surface chemistry of the porous materials by incorporating basic sites capable of interacting strongly with acidic CO2 in order to increase CO2 adsorption capacity and to keep high selectivity for CO2 are considered very promising. The common modifying functional groups include alkaline carbonates and various amine groups. 3.1. Regenerable Alkali-Metal Carbonate-Based Sorbents. Chemical adsorption of CO2 with a dry regenerable alkali-metal carbonate-based solid sorbent (M2CO3, where M = K, Na, Li) is being considered for a period of time for CO2 capture from flue gas, since its operating temperature could be below 200 °C. This class of sorbents has been investigated for their commercial feasibility by Hoffman et al.133 In the adsorption process (carbonation, reaction 1), CO2 and moisture react with the carbonate sorbent at 60
110 °C and form alkali-metal bicarbonate. Heat treatment of the bicarbonate at 100
200 °C regenerates alkali-metal carbonate, releasing the CO2 (this process in known as decarbonation; see reaction 2). The maximum theoretical capacity of Na2CO3 is 9.43 mmol g
1.

Carbonation: M2 CO3 þ H2 O þ CO2 ¼ 2MHCO3 ( ΔH ¼


135 kJ mol
1
141 kJ mol
1

ð1Þ

when M ¼ Na when M ¼ K

Decarbonation: 2MHCO3 ¼ M2 CO3 þ H2 O þ CO2

ð2Þ

Hayashi et al.134 developed a TSA process and a regeneration cyclic process for CO2 capture using potassium carbonate (K2CO3) supported on AC. For regeneration, pure steam was utilized. As shown in Figure 4, K2CO3 performs the best out of the three carbonates. It has a wide carbonation temperature range where the sorbent efficiency is 100%. They have shown that there was an optimum loading of K2CO3 on AC support and suggest that above the optimum loading, excess K2CO3 blocks micropores of AC, restricting CO2 supply at the active reaction sites, resulting in reduction of adsorption kinetics and working CO2 capture capacity. 1445

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Table 6. Alkali Carbonate Sorbents for CO2 Capture active phase K2CO3

support

temperature of

wt % active

gas

capacity

method of

operation (°C)

phase

composition

(mmol g
1)

testing

AC, activated coke, Ads.: 100 and silica

∼35 wt % in AC simulated flue gas and actual flue

Reg.: 150

∼2.1 (Ads. efficiency ∼80%)

fixed-bed labscale

ref 134

and benchscale

gas in slip stream Na2CO3 ceramic supported Ads.: 60
70 sorbents

10
40 wt %

Reg.: 120
140

simulated flue

∼0.5
3.2

TGA, fixed-bed,

gas and actual

entrained-bed reactor,

flue gas

field testing by cocurrent

151
153

moving bed, limited no. of cycles Na2CO3 ceramic supported Ads.: 50
70 sorbents

35 wt %

Reg.: >135 (in N2)

∼2.6 (∼80%

10% CO2, 12.2% H2O,

efficiency with

and 77.8% N2

a 35% active

bubbling bed

137

TGA

138

fixed-bed, multiple cycle

142
145

coupled fluidized bed

154

phase) Na2CO3 ceramic supported Ads.: 50
70 sorbents

20
50 wt %

Reg.: 120 (in N2)

simulated flue gas:

∼2.3 (>80% sorbent

14.4% CO2, 5.4%

efficiency with a

O2, 10% H2O,

30% active phase)

and 70.2% N2 K2CO3

AC, TiO2, Al2O3,

Reg.: 130
400

CaO, SiO2, and

(moisture up to 9%

zeolites K2CO3

Ads.: 60
100

MgO, ZrO2,

“Sorb KX35”

30 wt %

1% CO2, 0
11% H2O, ∼1.1
2.7 and N2 balance

and balance N2) Ads.:60
100

(proprietary

Reg.:120
220

recipe)

(in N2)

35 wt %

simulated flue gas: dry ∼2.1 (∼96% basis-12% CO2 and

sorbent efficiency)

reactor: adsorber- fast

88% N2; 7
30%

fluidized bed and

moisture

regenerator- bubbling fluidized bed, multiple cycle

K2CO3

“Sorb A” (proprietary

Ads.: 70
90

35 wt %

slip stream-coal-fired flue gas: 7
9%

Reg.: g150 (in N2)

recipe)

CO2 >85% (capacity not available)

coupled fluidized bed

155, 156

reactor: adsorber- fast

CO2 (dry basis),

fluidized bed and

10
19% H2O

regenerator- bubbling fluidized bed, multiple cycle

K2CO3

AC, silica gel, activated Al2O3

K2CO3

modified Al2O3 support-

Ads.: 60

∼25 wt %

Reg.: 200 (in N2) Ads.: 70
90 Reg.: 130 (in N2)

15% CO2, 15% H2O, ∼0.34
1.7 1% CO2, 9% H2O, and N2 balance

149

fluidized bed

and N2 balance ∼28
48 wt %

TGA and bubbling

∼2.9 (∼48 wt % K2CO3 loading)

TGA and fixed-bed,

146

multiple cycle

KAl(CO3)(OH)2

Liang et al.135 suggested that the active sodium compound must be dispersed on a ceramic support such as Al2O3 to satisfy the durability and needs of high kinetics. Research Triangle Institute (RTI, Research Triangle Park, NC) also identified136 that fixed-bed and dense-phase fluidized-bed systems were not optimal reactor schemes for the dry carbonate process, because of its exothermic nature of chemical reaction. In field testing of RTI’s prototype cocurrent downflow gas
solid contactor system was operated for a total of 235 h, using a fossil fuel-derived flue gas. The system has been demonstrated to run continuously for extended periods of time and has achieved >90% capture of CO2 under various process conditions. Seo et al.,137,138 a Korean group, studied the effect of water vapor pretreatment on Na2CO3-based sorbent system in a bubbling fluidized-bed reactor and found that, out of six types of spray-dried sorbent systems, three samples had better CO2

sorption capacity than MEA (33.3 wt % MEA solution was used as the reference and its CO2 absorption capacity was 6 wt %). Okunev et al.139
141 investigated the influence of a porous support matrix on the CO2 sorption of K2CO3. For support, both hydrophilic (silica gel, alumina, vermiculite) and hydrophobic (AC) materials were used. This study found that, initially, the porous alumina/K2CO3 sorbent system possesses the highest dynamic capacity but it decreases immediately after the first cycle. A completely reversible regeneration was observed in case of an AC-impregnated sorbent system. The sorbent system dynamic capacity decreases in the following sequence: alumina > activated carbon ðACÞ > vermiculite > silica gel To identify the best sorbent support system, some Korean researchers142,143 have prepared several K2CO3-based sorbents by impregnating it on various supports, such as AC, TiO2, Al2O3, 1446

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Table 7. Structure of Widely Used Amines for Sorbents Functionalization

MgO, SiO2, and di zeolites. CO2 capture capacities of K2CO3/ AC, K2CO3/TiO2, K2CO3/MgO, and K2CO3/Al2O3 (with an active phase loading of ∼30 wt %) were ∼2.0, 1.9, 2.7, and 1.9 mmol g
1, respectively. However, the CO2 capture capacities of K2CO3/Al2O3 and K2CO3/MgO, after regeneration at e200 °C, decreased, because of the formation of KAl(CO3)2(OH)2, K2Mg(CO3)2, and K2Mg(CO3)2 3 4(H2O) phases during carbonation, which were not completely converted to the original K2CO3 phase. However, regeneration was not a problem in the temperature range of 130°
150 °C in the case of K2CO3/AC and K2CO3/TiO2 sorbent systems. In 2009, a new dry sorbentsystem, called “KZrI30” (30 wt % K2CO3/ZrO2 sorbent system) was developed. The CO2 capture capacity of the sorbent was ∼96% of the theoretical value in the presence of 1% CO2 and 9% H2O at 50 °C, and the capacity was almost same in multicycle operation.144 It is reported that the enhanced CO2 capture capacity can be obtained by converting the entire K2CO3 3 1.5 H2O phase to the KHCO3 phase if the sorbents are fully activated with excess water.145 Recently, Lee et al.146 reported a new regenerable modified-Al2O3 support for K2CO3 sorbent system for CO2 capture below 200 °C. The CO2 capture capacity of the

48 wt % K2CO3 loaded sorbent was ∼2.9 mmol g
1. In addition, it was confirmed that the CO2 capture capacities of new sorbents did not decrease over 5 cycles. Zhao et al.147,148 found that K2CO3 with hexagonal crystals has superior carbonation kinetics over monoclinic K2CO3, because of the crystal structure similarities between K2CO3(hexagonal) and KHCO3. After identifying the crystal structure and carbonation kinetics relationship, the research has focused on studying K2CO3impregnated composite sorbent systems (where the support materials are coconut AC, coal AC, silica gel, activated alumina) and dolomite.149 In 2010, Zhao et al.150 have focused their attention on the effect of carbonation temperature, CO2 concentration, and H2O concentration and operation pressure on carbonation conversion rate and reaction rate of K2CO3 calcined from KHCO3. These basic operational parameters were collected by the group for designing and operating a large-scale CO2 capture process. Table 6 summarizes the literature data on alkali carbonate sorbents for CO2 capture. In summary, the high CO2 capture capacity of Na2CO3 (9.43 mmol g
1) and K2CO3 (7.23 mmol g
1) and favorable carbonation/regeneration temperature between 60 °C and 200 °C suggest them as potential sorbents for post-combustion CO2 1447

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Industrial & Engineering Chemistry Research capture. Besides, dry regenerable alkali-metal-based sorbents have the added advantage of being relatively inexpensive. However, to be commercially viable, the long-term stability and persistence performance of these sorbents under real flue gas conditions of post-combustion applications have yet to be established. 3.2. Amine-Functionalized Solid Sorbents. A variety of microporous/mesoporous materials loaded with basic nitrogen functionality, more specifically, organic amine functionality has been synthesized and characterized to chemisorb CO2 from flue gas streams. Supported amine sorbents have been classified into three classes:157,158 • Class 1: This class of supported sorbents is prepared by physically loading monomeric or polymeric amine species into or onto the porous support (typically, the porous silica by impregnation technique). • Class 2: This class of supported adsorbents is that in which the amine, mainly amine-containing silane, is covalently tethered to a solid support, such as porous silica. This is accomplished by binding amines to oxides via the use of silane chemistry or via preparation of polymeric supports with amine-containing side chains. This provides covalently tethered amine sorbents with the potential to be completely regenerable through multicycle adsorption/desorption uses. • Class 3: These supported adsorbents are based on porous supports upon which amino-polymers are polymerized in situ. This category of supported sorbents can be considered a hybrid of the other two classes.157 The structure of widely used amines for sorbent functionalization is given in Table 7. 3.2.1. Reaction Scheme of CO2 with Amines. Adsorption of CO2 using amine-functionalized sorbents involves chemical reaction and, therefore, it is necessary to know how the nature of amine influences the rate of adsorption and kinetics, in terms of amine efficiency, defined as the number of CO2 molecules adsorbed for each nitrogen atom in the amine functional group present. The zwitterion mechanism originally proposed by Caplow159 and reintroduced by Danckwerts,160 is generally accepted mechanism for the reaction of primary and secondary amines CO2. The mechanism involves two steps: formation of zwitterion (not shown by reaction here), followed by base catalyzed deprotonation of zwitterion. However, overall reaction can be written as

where R2 = H for the primary amine. If we consider only the amine as a base, the equilibrium CO2 loading capacities of primary and secondary amines are limited by the overall stoichiometry of reaction 3 to 0.5 mol (mol of amine)
1. The only reaction of importance between CO2 and the sterically hindered primary or secondary amine161 would be the formation of bicarbonate, as shown by reaction 4, similar to the case of tertiary amines. Therefore, the stoichiometry of reaction 4 allows a CO2 loading of up to 1 mol (mol of amine)
1 for hindered primary or secondary amine.

The reaction of CO2 with moderately hindered amines includes the formation of a carbamate, as in the case of nonhindered amines

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(see reaction 3), the stoichiometry of which allows a CO2 loading of up to 0.5 mol (mol of amine)
1. Tertiary amines do not react directly with CO2; the primary and secondary amines do react directly with CO2. They lack the free proton needed in the deprotonation step. Instead, the reaction produces protonated amine and bicarbonate ion in the presence of moisture, resulting in a higher capacity for CO2 up to 1.0 mol (mol of amine)
1. The mechanism of this reaction is suggested to be base-catalyzed hydration of CO2, as shown previously in reaction 4.162 Generally, the formation of bicarbonate allows a high equilibrium capacity. However, the kinetics is very slow. 3.2.2. Amine-Functionalized Activated Carbonaceous Materials. It has been shown that, by incorporating certain functional groups into its porous structure, thus enhancing the CO2
adsorbent interactions, the adsorption capacity of carbonaceous materials could be significantly improved. This section involves activated carbonaceous materials such as ACs, biochar, fly ash containing unburnt carbon, CNTs, and solid resins impregnated/grafted with amine functional groups. 3.2.2.1. Amine-Functionalized Activated Carbon Sorbents. Przepiorski et al.163 found significant enhancement in CO2 adsorption in commercial AC treated with ammonia at high temperatures (>200 °C). The sorbent treated with ammonia at 400 °C had shown the adsorption capacity of 1.73 mmol g
1. This enhancement was attributed to the introduction of nitrogen-containing groups to carbon structure. Pevida and his group164 concluded that the CO2 capture capacity is not directly related to the total nitrogen content of sorbents but rather to specific nitrogen functionalities that are responsible for increasing the CO2-adsorbent affinity. Plaza et al.165 reported that amine-treated sorbents prepared from biomass residue and almond shell preactivated with CO2 exhibited significant differences in texture and surface chemistry and, therefore, showed significantly higher capacities than the starting char. Several amine-impregnated solid sorbents were developed by chemical treatment of carbon-enriched fly ash concentrates with various amine groups by Gray et al.,166,167 Maroto-Valer and others,40,168 and Arenillas et al.169 A typical comparison of CO2 adsorption capacities of activated fly ash carbon and its alkanolamine-modified counterparts at various temperatures is shown in Figure 5, based on the maximum possible adsorption capacity data reported by Maroto-Valer and others.168 It was found that activation by steam before impregnation could successfully increase the surface area and pore volume of carbon-enriched fly ash, consequently resulting in increased CO2 capture capacity.40,168 Maroto-Valer et al.40 showed that the impregnation with polyethyleneimine (PEI) could significantly improve the adsorption capacity of this class of sorbents up to 2.13 mmol g
1 at 75 °C, which is much more than that without impregnation (0.22 mmol g
1 at 75 °C). Arenillas and others169 achieved the highest CO2 adsorption capacity of ∼1.02 mmol g
1 at 75 °C using activated fly-ash-derived sorbents impregnated with PEI.40,169 This group169 also impregnated AC derived from fly ash with PEI and its blend with poly(ethylene glycol) (PEG). They confirmed that the addition of PEG into the PEI-loaded sorbents improves the CO2 adsorption capacity and kinetics. This might be due to the bicarbonate formation reaction in the presence of PEG, which attracts more water. The feasibility of a high-surface-area sorbent from low-cost anthracites was also investigated by Maroto-Valer and his group.41,42 They reported adsorption capacity of PEI-impregnated deashed anthracite sorbent was ∼2.13 mmol g
1 at 1448

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Figure 5. CO2 adsorption capacity of activated fly ash carbon and impregnated fly ash at different temperatures, based on the data reported by Maroto-Valer et al.168

75 °C.41 In another study, a decrease in adsorption capacity of activated anthracites impregnated with PEI was observed with increasing adsorption temperature.42 In 2007, Plaza et al.170 also reported similar negative effects on capture capacities of commercial AC (Norit CGP super) impregnated with different alkylamines, such as diethylenetriamine (DETA), pentaethylene hexamine (PEHA), and PEI. More recently, Alesi et al.171 studied CO2 capture and regeneration conditions of tertiary amidine derivatives (Nmethyltetrahydropyrimidine (MTHP)) supported on AC in the temperatures ranging from 29 °C to 50 °C. It was found that CO2 capture on the amidines only occurred in the presence of moisture. Adsorption of moisture on the hydrophilic AC support places a limit on the CO2 capture capacities. The capture capacity of 1,5-diazo-bicyclo[4.3.0]non-5-ene (DBN) impregnated AC was shown to have a higher value up to 0.8 mmol g
1 at an adsorption temperature of 29 °C, compared to those impregnated with 1,8-diazobicyclo[5.4.0]-undec-7-ene (DBU). 3.2.2.2. Amine-Functionalized Carbon Nanotube Sorbents. The potential application of CNT as the support for amineimpregnated sorbents has been studied by Fifield and others.63 Pyrene methyl picolinimide (PMP) was introduced as anchors to increase the affinity of the carbon structure. In 2008, Lu and others36 reported a comparative study of CO2 capture by CNT, granulated AC (GAC), and zeolite modified by 3-aminopropyltriethoxysilane (APTES) (see Table 7). Figure 6 shows the CO2 adsorption capacities of raw and modified adsorbents. After functionalization, CNT showed a significant enhancement in CO2 adsorption capacity, followed by the zeolite. Dillon et al.70 synthesized and characterized PEI-functionalized SWNTs. A maximum absorption of 2.1 mmol g
1 was reported for PEI (25000)-SWNT at 27 °C. CNTs modified by APTES were also tested for their CO2 adsorption potential at multiple temperatures by Su et al.45 The adsorption capacities of CO2 of CNTs and CNTs (APTES) decreased with temperature, indicating the exothermic nature of adsorption process, and increased with water content. They observed that CO2 adsorption capacity of CNT (APTES) was ∼2.59 mmol g
1 at 20 °C and suggested that the CNT (APTES) could be a promising low-temperature adsorbent for CO2 capture. Recently, Hsu et al.68 suggested that a combination of thermal and vacuum desorption of CNT

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Figure 6. CO2 adsorption capacities of raw and modified adsorbents at 25 °C with a [CO2,in] = 50%.36

(APTES) at 120 °C could reduce the regeneration time. The adsorption capacities and other physicochemical properties were preserved for 20 cycles of adsorption/regeneration. 3.2.2.3. Amine-Functionalized Solid Resin Sorbents. Aminefunctionalized solid resins are also being studied for use as sorbents for CO2 capture. Drage et al.172 observed that CO2 adsorption capacity was dependent on both nitrogen functionality and textural properties of precursors of the resins. The best-performing adsorbent in this class was capable of capturing ∼1.86 mmol g
1 at 25 °C under pure CO2. Besides, both categories sorbents showed rapid adsorption halftimes. However, they found a substantial decrease in capacity (∼56%) with increasing the adsorption temperature from 25 °C to 75 °C. In summary, the nitrogen enrichment of carbonaceous materials is found to be effective in enhancing the specific CO2
adsorbate interactions. However, the impregnation of amines resulted in a significant decrease of the surface areas, as well as mesopore or micropore volume.168,170 This is probably due to the pore filling or blockage, because the amine incorporation is assumed to be affected by the molecular size and shape of alkanolamines used, although the mechanism of pore filling is still not well-understood.168 3.2.3. Functionalized Zeolite-Based Sorbents. Zeolites with significant surface area and pore volume present a potential option for CO2 capture. However, it is reported that CO2 adsorption on zeolite sorbents decreases significantly as the temperature increases, and it also shows very low capacity in the presence of moisture in the flue gas. Therefore, there have been a few investigations (Table 8) to synthesize aminated zeolites as alternative sorbents. Zeolite 13X was modified with MEA by Jadhav et al.187 via the impregnation method. The aminated zeolites showed improvement in CO2 adsorption capacity by a factor of ca. 1.6 at 30 °C. A higher capacity at the temperature of 120 °C was obtained with MEA loading of 50 wt %, compared with unmodified zeolite or modified zeolite with low loading level. Unlike at room temperature, where physisorption is dominant, the chemical interaction between CO2 and amine may be playing a significant role in sorption of CO2 at 120 °C, despite reduced pore volume and lower surface area resulted from impregnation. The similar conclusion was also reached by Su et al.188 They dispersed TEPA into commercially available Y-type zeolite (Si/Al = 60). The CO2 1449

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Table 8. CO2 Adsorption Capacity of Amine-Impregnated Solid Sorbents Experimental Conditions adsorption capacity support

amine

amine content (wt %)

(humid) (mmol g
1)

pCO2 (atm)

T (°C)

No. of cycles

ref

MCM-41

PEI

75

3.02

1

75

173

MCM-41

PEI

50

2.05

0.1

75

173

PE-MCM-41

DEA

77

2.93

0.05

25

50

PE-MCM-41

DEA

73

2.81 (2.89)

0.05

25

MCM-41

PEI

50

(3.08)

0.13

75

10

174

MCM-41

TEPA

50

4.54

0.05

75

6

175

SBA-15 SBA-15

TEPA DEA + TEPA

50 50 (30% TEPA,

3.23 3.61

0.05 0.05

75 75

6 6

183 184

SBA-15

PEI

50

3.18

0.15

75

185

SBA-15

PEI

50

1.36

0.12

75

186

SBA-15

APTES

(2.01)

0.10

25

54

KIT-6

PEI

50

1.95

0.05

75

176

monolith

PEI

65

3.75

0.05

75

5

177

mesoporous silica MC400/10

PEI TEPA

40 83

2.4 5.57 (7.93)

1 0.1

75 75

50

178 179

precipitated silica

PEIa

67

50

20% DEA)

b

a

R-IAS

E-100c

PMMA

TEPA

PMMA PMMA

4.55

1

100

180

(4.19)

0.10

25

54

41

(14.03)

0.15

70

181

DBU

29

(3.0)

0.10

25

1

DBU

29

(2.34)

0.10

65

6

PMMA (Diaion)

PEI

40

2.40 (3.53)

0.10

45

182

SiO2 (CARiACT) Zeolite 13X

PEI MEA

40 10

2.55 (3.65) 1.0

0.10 0.15

45 30

182 187

Zeolite Y60

TEPA

50

(4.27)

0.15

60

β-zeolite

TEPA

38

2.08

0.10

30

20

17 17

188 189

Low-molecular-weight PEI (MW ∼ca. 800). b Reformulated immobilized amine sorbent.14 c E-100: ethyleneamine.

adsorption capacity reported was 4.27 mmol g
1 at 60 °C in the presence of 15% CO2 and 7% water vapor in gas stream. Besides, Fisher et al.189 employed β-zeolite as a solid support for TEPA impregnation and compared it with TEPA-impregnated silica and alumina. The TEPA-modified β-zeolite exhibited the CO2 adsorption capacity up to 2.08 mmol g
1 at 30 °C under the 10% CO2/90% argon flow, outperforming TEPA/SiO2 and TEPA/Al2O3 sorbents. It was suggested that the higher capacity of TEPA/β-zeolite could be related to zeolite’s high surface area. TEPA/β-zeolite maintained its CO2 capture capacity for more than 10 adsorption/regeneration cycles. 3.2.4. Functionalized Polymer-Based Sorbents. Polymeric amine sorbents have been used for years in closed environments, such as aircraft, submarine, and space shuttles, to capture CO2 for CO2 concentrations of 1000 for CO2/N2 and ∼180 for CO2/ O2. The cyclic adsorption/desorption operation indicated that the adsorbent was stable at 75 °C after 10 cycles of operation. However, it was not stable when the operation temperature was >100 °C. NOx was observed to be adsorbed simultaneously with CO2, indicating the need for preremoval of NOx from flue gas.174,198 In addition, the presence of moisture in the simulated flue gas and flue gas from a natural-gas-fired boiler was shown to enhance the adsorption capacity when the moisture concentration in the feed is lower than that of the CO2. The possible reasons could be the formation of bicarbonate ion (eq 4) during the chemical interaction between PEI and CO2 in the presence of moisture. More recently, a nanoporous SBA-15-supported sorbent with 50 wt % PEI loading was developed by Ma et al.185 This sorbent showed a sorption capacity of 3.18 mmol g
1 at 75 °C under a CO2 partial pressure of 15 kPa. The reported CO2 adsorption capacity was ∼50% higher than that of their previously developed MCM-41-PEI sorbent, possibly due to the higher pore diameter and pore volume of SBA-15. This allows the PEImodified sample prepared from SBA-15 to have a higher surface area for the same PEI loading (50 wt %). A two-stage sorption approach was also proposed by Ma et al.185 to remove CO2 and H2S from gas streams using this sorbent. The sorbent and process has been shown to be capable of removing H2S to a level of SBA
15 ðdp ¼ 5:5Þ≈SBA
16 ðdp ¼ 4:1Þ

> MCM
48 ðdp ¼ 3:1Þ > MCM
41 ðdp ¼ 2:1Þ where dp is the average pore diameter (given in nanometers). The performance was proposed to be influenced by the pore

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diameter and pore arrangement of mesoporous silica materials. Bulky PEI is assumed to be introduced to the pore easily as the pore size in the support increases. The same research group177 also developed PEI-impregnated silica monoliths showing a hierarchical pore structure as a support. For 5% diluted CO2 sorption tests, the amine-impregnated monolith with 65% PEI loading exhibited a maximum adsorption capacity of 3.75 mmol g
1 at 75 °C, outperforming the KIT-6
PEI-50, which has been reported to have an adsorption capacity of 1.95 mmol g
1 under the same conditions.176 Recently, Goeppert et al.180 impregnated nanostructured fumed silica using various organoamines, namely PEI, MEA, DEA, TEPA, and PEHA, as well as 2-amino-2-methyl-1-3-propanediol(AMPD), 2-(2-amino-ethylamino)ethanol (AEAE), etc. They observed that simple amines such as MEA, DEA, AEAE, etc. are not suitable, because of amine leaching problems at higher temperature. More recently, Qi et al.179 proposed a novel high-efficiency CO2 capture platform obtained using PEI and TEPA supported on specially designed mesoporous capsules. The novel composite sorbent showed excellent CO2 capture capacity of 6.6 mmol g
1 under 1 atm of moisture-free CO2 at 75 °C and exceptional CO2 capture capacity of 7.9 mmol g
1 under simulated humid flue gas with 10% CO2 at 75 °C. The CO2 capture kinetics was found to be relatively fast and attained 90% of the total capacity within the first few minutes (see Figure 8). Besides, sorbents could be regenerated below 100 °C and exhibited good cyclic stability over repetitive adsorption/regeneration cycle (∼50 cycles). Further studies took into consideration the mode of regeneration and the lifetime of adsorbents during the evaluations of amine-impregnated adsorbents. Drage et al.178 demonstrated thermal swing desorption for PEI-functionalized proprietary mesoporous silica, using pure CO2, and reported good cyclic regeneration capacities (2 mmol g
1). A successive loss of adsorption capacity of the sorbent—and, therefore, lifetime of sorbents—was observed over numerous regeneration cycles. It was probably due to the secondary reaction between CO2 and PEI above 135 °C to form a stable product (e.g., urea), which leads to the irreversible degradation of the adsorbents. The use of steam as a stripping gas instead of CO2 was suggested in overcoming the problems. In addition to the TSA process, Dasgupta et al.186 investigated a single column five-step PSA option using PEI-impregnated SBA-15 as an adsorbent. A strong adsorptive rinse cycle was suggested for CO2 recovery during PSA process. Pirngruber et al.200 concluded that neither the conventional TSA or VSA modes seem to be a viable option for their amine-immobilized sorbents. In summary, novel amine-impregnated silica supports, as shown in Table 8, are promising and can effectively adsorb CO2 with relatively higher working capacity; even some cases of >4 mmol g
1 stipulated industrial requirements for solid sorbents. The modification of pore size of silica support is shown to enhance the adsorption capacity. Moreover, their adsorption capacities are not impaired by the presence of moisture (in many cases, moisture helps to obtain higher capacity). However, the durability and regeneration kinetics of the amine-impregnated solid sorbents have not been tested adequately under real flue gas conditions. Their desorption kinetics is still slower. Considerable leaching of the amines may be a major drawback for the use of impregnated amine-functionalized sorbents for CO2. 3.2.5.2. Grafted Silica-Supported Sorbents. Many groups have reported the synthesis and characterization of amine-grafted ordered mesoporous silica sorbents (Class 2 category) for CO2 1452

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Figure 9. Modified hexagonal mesoporous silica (HMS) materials.

Figure 8. CO2 capture kinetics of mesoporous silica sorbents with different amine loadings. (Reprinted with permission from ref 179. Copyright The Royal Society of Chemistry, 2011.)

capture. Here amine—mainly, amino-silane—is covalently tethered to the silica support.124 Three methods can be used for the grafting of amine onto a silica support: post-synthesis grafting, direct synthesis by co-condensation, and anionic template synthesis with help of the interaction between the cation head in aminosilane and anionic surfactants.201 The mesoporous nature of the support permits good diffusivity of organic amine into the pore space and, following functionalization, good mass diffusion of CO2 gas molecules into and out of the structure (except when the pores are blocked). In addition, a wide variety of aminosilanes (see Table 2) are being grafted onto the surface of porous silica to investigate the impact of amine type and amine loadings on the CO2 adsorption capacity of resulting sorbent composites. Leal et al.202 first described the chemisorption of CO2 on a APTES grafted surface of silica gel. They confirmed that each molecule of CO2 uses two surface amino groups to form an ammonium carbamate species in the absence of H2O and ammonium bicarbonate surface species in the presence of H2O. However, their sorbent capacity was far below the requirement for industrial application of the sorbents. Afterward, a series of aminopropylfunctionalized (grafted) hexagonal mesoporous silica (HMS) compounds was prepared and characterized by Chaffee’s group203
207 to enhance CO2 adsorption, because of their high porosity. The grafted HMS materials, as shown in Figure 9 were developed by Delaney et al.203 using 3-aminopropyltrimethoxysilane (APTS), aminoethyl-aminopropyl-trimethoxysilane (AEAPTS), and N-[3-(trimethoxysilyl)propyl) diethylenetriamine (DAEAPTS), ethylhydroxyl-aminopropyl-trimethoxysilane (EHAPTS) and diethylhydroxyl-aminopropyl-trimethoxysilane (DEHAPTS) (see Table 7). The modified silica supports have shown very high surface area with varied concentrations of surface bound amine and hydroxyl functional groups. The modified HMS sorbents have also been shown to reversibly adsorb substantially more CO2 than modified silica gel, as reported by Leal et al.202 Another observation reveals that, for HMS-APTS, HMS-AEAPTS, and HMS-DEAPTS, the ratio of CO2 molecules adsorbed per available N atom was ∼0.5. This is consistent with the carbamate formation mechanism, as presented by eq 3. With HMS-DEHAPTS, the ratio was ∼1. Because tertiary amines cannot form stable carbamates (see section 3.2.1), it has been proposed that the hydroxyl groups may serve to stabilize carbamate-type zwitterions.

Based on a systematic investigation on CO2 adsorption on different mesoporous silica substrates and their amine-functionalized hybrid product, Knowles et al.204
207 also pointed out that the extent of surface functionalization is found to be dependent on substrate morphology (e.g., available surface area, pore geometry, and pore volume), diffusion of reagents to the surface, as well as the silanol concentration on the substrate surface. Their results showed that the higher nitrogen content of the tether leads to a higher CO2 capacity on the adsorbent surface. The CO2 adsorption performance of hybrid materials exhibited good adsorption kinetics, reaching equilibrium within 4 min for each sample, and highest CO2 capacity of ∼1.66 mmol g
1 at 20 °C in dry 90% CO2/10% argon mixture.204 As an extension of their previous work on APTS- and AEAPTS-functionalized HMS, and to explore the potential of the longer side chain and greater number of N atoms/tether to achieve still higher CO2 capacities, Knowles et al.207 assessed DAEAPTS-functionalized HMS. The sample with the best CO2 adzsorption performance was determined to have a CO2 adsorption capacity of 1.2 mmol g
1 at 20 °C, which was less than the previously observed capacity for the analogous APTS- and AEAPTS-functionalized silica (1.66 mmol g
1).205 In comparison with APTS- and AEAPTS-functionalized silica sorbents, the DAEAPTS-functionalized sorbents had a greater CO2 adsorption capacity but lower amine efficiencies. It was thought to be due to reduced accessibility of CO2 to the surface-bound amine groups brought about by entanglement (reduced mobility) of the longer hydrocarbon chains within the mesoporous domain and the relative proximity of amine pairs. All functionalized HMS sorbents were found to be thermally stable up to 170 °C in both pure N2 and mildly oxygenated (2%) N2 atmosphere and showed no affinity for N2 and O2. However, DAEAPTS-functionalized sorbents were found to degrade in a highly oxygenated atmosphere. Liang et al.208 synthesized a series of functionalized SBA-15 with melamine-based dendrimers, using the similar stepwise polymerization reaction scheme proposed by Acosta et al.209 It was found that the CO2 adsorption capacities of the dendrimerfunctionalized SBA-15 did not show any improvement over that of the aminopropyl-modified SBA-15. Hiyoshi et al.210,211 demonstrated the potential application of aminosilane-modified mesoporous silica for the separation of CO2 from gas streams containing moisture (see Table 9). The characterization of the adsorbents showed the significant decrease in the surface area or pore volume after grafting, which was close to the predicted values for each adsorbent. In their subsequent research,211 they found that the DAEAPTS
SBA-15 1453

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Table 9. CO2 Adsorption Capacity of Amine Grafted Solid Sorbents Experimental Conditions adsorption capacity support

amine

amine content (mmol g
1)

(humida) (mmol. g
1)

pCO2 (atm)

T (°C)

ref

SBA-15

APTES

2.7

0.52 (0.5)

0.15

60

210

SBA-15

AEAPSf

4.2

0.87 (0.9)

0.15

60

210

SBA-15

DAEAPTS

5.1

1.1 (1.21)

0.15

60

210

SBA-15b

APTES

2.61

0.66 (0.65)

0.15

60

211

SBA-15b

AEAPSf

4.61

1.36 (1.51)

0.15

60

211

SBA-15

DAEAPTS

5.8

1.58 (1.80)

0.15

60

211

SBA-15 SBA-15

AEAPTS AEAPTS

0.45 1.95

0.15 1

25 22

218 218

SBA-15c

AEAPTS

0.91

0.15

25

217

SBA-15

APTES

0.4

0.04

25

213

SBA-15

APTES

2.72

1.53

0.1

25

232

SBA-15

aziridine polymer

9.78

(5.55)

0.1

25

230

SBA-15

aziridine polymer

9.78

0.1

75

230

SBA-15

aziridine polymer

7

(1.98)

0.1

75

229

SBA-15 SBA-16

aziridine polymer AEAPTS

7 0.76

(3.11) 1.4

0.1 1

25 27

229 231

SBA-12

APTES

2.76

1.04

0.1

25

232

MCM-41

APTES

3

0.57

0.1

25

232

PE-MCM-41

DAEAPTS

7.95

2.65

0.05

25

220

PE-MCM-41

DAEAPTS

7.8

2.28

0.05

70

223

MCM-48

APTES

2.3

2.05

1

25

212

MCM-48

APTES

2.3

1.14

0.05

25

212

HMSd HMSd

APTS DAEAPTS

2.29 4.57

1.59 1.34

0.9 0.9

20 20

205 207

0.73

0.1

60

234

1.26

0.89

1

50

202

b

(4)

MSPe

AEAPTS

silica gel

APTES

CNTs

APTES

1.32

0.15

20

68

CNTs

AEAPTS

2.59

0.5

20

45

a

CO2 adsorption capacity within parentheses indicates humid condition result. b SBA-15 support is boiled in water for 2 h followed by grafting of aminosilanes. c EDA-SBA-15 enhanced by backfilling with propylsilane (C3) in supercritical fluid propane. d Hexagonal mesoporous silica. e Mesoporous spherical-silica particles. f N-(2-aminoethyl)-3-aminopropyltriethoxysilane.

showed improved CO2 adsorption capacity after SBA-15 was boiled in water for 2 h, followed by the grafting of aminosilanes. The amount of CO2 adsorbed reached 1.58 and 1.80 mmol g
1 in the absence and presence of water vapor, respectively, under the same experimental conditions. The efficiencies of the aminosilanes at identical amine surface density, were found to be in the following order: APTES > AEAPTS > DAEAPTS In an effort to develop selective sorbents for the removal of CO2 and H2S from natural gas mixtures, 3-aminopropyl-functionalized silica xerogel and MCM-48 silica sorbents were also synthesized and studied by Huang et al.212 The CO2 uptake capacity of amine-grafted MCM-48 sorbent was always higher than that of amine-grafted xerogel. With the respect to pure CO2, the amount of CO2 adsorbed reached 2.05 mmol g
1 at 25 °C. In the presence of water, the CO2 adsorption capacity was doubled as the adsorption mechanism changed from carbamate formation to bicarbonate formation. Gray and co-workers54,213
216 prepared a series of aminegrafted SBA-15 sorbents for CO2 adsorption. They reported that

APTES-grafted SBA-15 could adsorb up to 0.4 mmol g
1, whereas SBA-15 grafted with AEAPTS215 could adsorb 0.79 mmol g
1 at 25 °C.213 CO2 was found to adsorb on the amine sites in the form of bicarbonate and carbonates. Therefore, enhanced CO2 adsorption capacity was reported in the presence of H2O, because it helps to form carbonate and bicarbonate,213 which was confirmed by Khatri et al.216 Khatri et al.216 and Zheng et al.217,218 investigated the thermal stability of several grafted SBA-15 and found these to be stable up to 250 °C. In addition, the SO2 adsorption on APTES-SBA-15 resulted in a negligible CO2 adsorption capacity, indicating the necessity of SO2 removal before amine-based CO2 adsorption.216 As a continuous effort to develop high-capacity, water-tolerant amine-grafted silica-based sorbents with large amine loadings, as well as with large pore volume and pore size, Sayari and coworkers219
228 developed pore-expanded MCM-41 mesoporous silica (PE-MCM-41) grafted with amine, such as DAEAPTS. The sorbent was prepared through a post-synthesis hydrothermal treatment of the as-synthesized MCM-41.221 The DAEAPTSgrafted PE-MCM-41 support showed an adsorption capacity of 2.05 mmol g
1 at 25 °C and 1.0 atm for a dry 5% CO2 in N2 feed 1454

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Figure 10. Relationship between amine, CO2, carbamate, and urea species during CO2 adsorption
desorption under dry and humid conditions.228

mixture with amino-silane loading of 5.98 mmol (N) g
1.219 They showed that the effect of amine surface density of the sorbent has a strong impact on the adsorption efficiency. However, the presence of moisture did not significantly enhance the performance of the amine-impregnated PE-MCM-41 sorbents as expected toward the bicarbonate formation pathway. Subsequently, Harlick and Sayari220 also focused on optimizing the grafting conditions and concluded that, in comparison to the dry grafting procedure, wet grafting via the co-addition of water at 85 °C showed an increase in the total amine content, resulting in a 90% overall improvement. They found that, through regeneration under a vacuum at 70 °C, the sorbent showed good stability over 100 cycles with an average working adsorption capacity of 2.28 mmol g
1 for pure CO2,223 while the temperature swing regeneration process was suitable only above 120 °C.224 In addition to thermal stability, it also showed infinitely high selectivity for CO2 over N2 and O2.224
227 It was also confirmed by Belmabkhout and Sayari227 that SO2 has an adverse effect on CO2 removal, which has also been demonstrated by Khatri et al.216 Recently, they228 reported that the deactivation of the amine-grafted MCM-41 silica sorbents took place through the urea formation reaction under dry conditions (Figure 10), even under mild conditions of 20 °C, and deactivation was dependent on the adsorption
desorption conditions and the nature of the adsorbent. Moreover, this research group noted that their sorbent underwent over 700 cycles without any loss of sorption capacity when adsorption and regeneration was carried out using a humid gas with ∼7.5% relative humidity at 70 °C. Therefore, they suggested that the stability of sorbent could be enhanced significantly by inhibiting the formation of urea, using moisture-containing gases. A novel covalently tethered hyperbranched aminosilica (HAS) sorbent (Figure 11) with high amine content capable of capturing CO2 reversibly from flue gas was developed and compared with other covalently supported solid sorbents by Jones and his research group.229,230 HAS was synthesized via a one-step surface polymerization reaction of aziridine monomer inside SBA-15 pores230 The HAS sorbent had an amine loading of 7.0 mmol N g
1 and CO2 adsorption capacity of 3.08 mmol g
1 when tested in a packed-bed reactor under a flow of 10% CO2/90% argon saturated with water at 25 °C. It was stable over 12 cycles with regeneration at 130 °C. Following their early research work, Drese et al.230 proposed modification of the HAS synthesis conditions, such as the aziridine-to-silica ratio and the solvent to further tune the sorbent’s composition, adsobent capacity, and kintetics. They found that higher amine loadings—and, therefore, higher potential active adsorption sites—contributed to a better adsorption capacity. Knofel et al.231 functionalized SBA-16 silica with AEAPTS. The functionalization of SBA-16, which has a three-dimensional (3D) cubic pore matrix with interconnected pores, allows good accessibility for the grafting species, as well as the gas molecules during adsorption. The sorbent capacity was not found to be

Figure 11. Hyperbranched amino silica.

higher than ∼1.4 mmol g
1 at 27 °C with high enthalpies for adsorption for CO2 (
100 kJ/mol). The comparison of three APTES-grafted mesoporous silica materials—MCM-41 (dp = 3.3), SBA-12 (dp = 3.8), and SBA-15 (dp = 7.1)—was made by Zelenak et al.232 The sorbent capacity was in accordance with the order of pore size as well as amine surface density, similar to that observed in the amine-impregnated mesoporous silica sorbents. Kim et al.233 developed and tested a series of amine-functionalized mesoporous silica sorbents via anionic surfactant-mediated synthesis method for CO2 adsorption at room temperature. Higher amine loading on the mesoporous structure was determined to be the governing factor to achieve high CO2 adsorption, as expected. Table 9 summarized the CO2 adsorption capacity of aminegrafted adsorbents. Although the functionalization of mesoporous silicas with amine functional groups significantly improves the CO2 adsorption capacity of silica substrate, the reported equilibrium CO2 adsorption capacities are not as high as those reported with amine-impregnated mesoporous silicas. Moreover, the low thermal stability of mesoporous silicas, in the presence of water vapor at elevated temperature, is still one of the major concerns. 3.2.6. Impregnated Alumina-Supported Sorbents. Because of its high resistance to steam and good mechanical and thermal stability properties, porous alumina was also considered for impregnation.235 Plaza et al.236 synthesized and tested a series of solid sorbents by impregnating six different amines (DETA, DIPA, TEA, AMPD, PEHA, and PEI) on the surface of a mesoporous alumina. Alumina sorbent impregnated with 40 wt % DETA (A-DETA) presented the highest CO2 adsorption capacities throughout the temperature range of 298
373 K (ca. 1.82 mmol g
1 at 373 K). A-DETA sorbent doubled the CO2 capture capacity of the raw alumina at 298 K (i.e., 1.36 mmol g
1 vs 0.68 mmol g
1). It is also worth noting that alkanolamine-impregnated samples (A-TEA, A-AMPD, and A-DIPA) showed a gradual decrease in CO2 adsorption with 1455

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Industrial & Engineering Chemistry Research increasing temperature, whereas the alkylamine-impregnated samples (A-DETA, A-PEHA, and A-PEI) presented a gradual increase in CO2 capture capacity. The probable reason for this could be an improvement of CO2 mass diffusivity through the amine film with temperature, because this would allow more amine groups to react.173 The above discussion clearly indicates that chemisorbents hold great potential to overcome the energy penalty in the base case: the MEA process. Several chemisorbents, such as aminefunctionalized sorbent (both impregnated and grafted), have shown promise to meet the desired working capacity target in simulated flue gas conditions. Mostly, chemisorbents are found to have higher CO2 selectivity. Amine-functionalized polymerbased sorbents showed very high adsorption capacity, but there is not adequate information regarding other selection criteria. Impregnated mesoporous silica sorbents showed improved capacity in the presence of water vapor, whereas grafted silica showed good thermal stability at high temperature (