Energy & Environmental Science - For Review Only

High Efficiency Nanocomposite Sorbents for CO2 Capture based on Amine-functionalized Mesoporous Capsules

Journal: Energy & Environmental Science

Manuscript ID: EE-ART-07-2010-000213.R2

Article Type: Paper

Date Submitted by the Author:

n/a

Complete List of Authors:

Qi, Genggeng; Cornell University, Material Science and Engineering Wang, Yanbing; Cornell Univeristy, Material Science and Engineering Estevez, Luis; Cornell Univeristy, Material Science and Engineering Duan, Xiaonan; Cornell Univeristy, Material Science and Engineering Anako, Nkechi; Columbia University, Department of Earth and Environmental Engineering and Department of Chemical Engineering Park, Ah-Hyung; Columbia University, Department of Earth and Environmental Engineering and Department of Chemical Engineering Li, Wen; Georgia Institute of Technology, School of Chemical & Biomolecular Engineering Jones, Christopher; Georgia Institute of Technology, Schol of Chemical and Biomolecular Engineering Giannelis, Emmanuel P.; Cornell Univeristy, Material Science and Engineering

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CREATED USINEGnTeHrEgRySC&ARETICnLvEiTrEoMnPLmATeEn(VtEaRl. 3S.1c) -iSeEnEcWeWW- .FRSoCr.ORRGe/EvLiEeCwTROONInCFlIyLES FOR DETAILS

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Supporting information
High Efficiency Nanocomposite Sorbents for CO2 Capture based on Amine-functionalized Mesoporous Capsules
5 Genggeng Qi, a Yanbing Wang, a Luis Estevez, a Xiaonan Duan, a Nkechi Anako, b Ah-Hyung Alissa Park, b Wen Li, c Christopher W. Jones c and Emmanuel P. Giannelis*a

10 Fig. S1 Breakthrough plots of the sorbents for CO2 capture at 75 oC under
a pre-humidified test gas (10% CO2 balanced with Ar). (a) MC400/10PEI%83 = 74 mg, gas flow rate = 20.22 ml min-1. (b) MC400/10TEPA%83 = 68 mg, gas flow rate = 20.98 ml min-1.

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CREATED USINGETHnEeRrSgCyAR&TICELEnTvEiMrPoLnATmE (eVEnRt.a3.l0)S- ScEiEeWnWcWe.R-SFC.OoRrGR/ELeEvCiTeRwONIOCFnILElyS FOR DETAILS

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Fig. S2 Sorbents based on various supports with different amine loadings. (a) MC400/10PEI%83; (b) MCM-41PEI%83 (c) SBA-15PEI%83; (d) SiO2-400PEI%83.
5

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Fig. S3 Nitrogen adsorption-desorption isotherms of mesoporous silica capsule MC400/10 and PEI impregnated sorbent MC400/10PEI%75.

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Fig. S4 Viscosity difference of polyethylenimine before (a) and after CO2 capture (b). A viscous thin film was observed when the CO2 gas flowed over the amine in the vials at 75 oC. After CO2 capture, the four vials 5 were cooled down to room temperature and inverted for 10 s.

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MANUSCRIPT ID: EE-ART-07-2010-000213.R1
TITLE: High Efficiency Nanocomposite Sorbents for CO2 Capture based on Aminefunctionalized Mesoporous Capsules
Broader context
Increases in atmospheric carbon dioxide gas have been linked to increasing use of fossil fuels over the past century. Post-combustion capture has the greatest near-term potential for reducing CO2 emissions. Solid sorbents provide a promising alternative to conventional amine solutions for CO2 capture. However, practical CO2 capture applications have been impeded primarily by limited sorbent capacity and recyclability. Here we present a novel CO2 capture platform based on oligomeric amines supported on specially engineered mesoporous hollow particles (mesoporous capsules). This new design leads to an exceptional capture capacity of up to 7.9 mmol g-1 under simulated flue gas conditions outperforming both conventional monoethanolamine solutions and other current solid amine impregnated sorbents. In addition to their outstanding CO2 capture capacity, the sorbents are readily regenerated at relatively low temperature and exhibit good stability and recyclability.

CREATED USIENGnTeHrEgRySC&ARETICnLvEiTrEoMnPLmATeEn(VtEaRl. 3S.1c) -ieSEnEcWeWW- .FRSoCr.ORRGe/EvLiEeCwTROONInCFlyILES FOR DETAILS

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High Efficiency Nanocomposite Sorbents for CO2 Capture based on Amine-functionalized Mesoporous Capsules

Genggeng Qi, a Yanbing Wang, a Luis Estevez, a Xiaonan Duan, a Nkechi Anako, b Ah-Hyung Alissa Park, b Wen Li, c Christopher W. Jones c and Emmanuel P. Giannelis*a
5 Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X First published on the web Xth XXXXXXXXX 200X DOI: 10.1039/b000000x

A novel high efficiency nanocomposite sorbent for CO2 capture has been developed based on oligomeric amines (polyethylenimine, PEI, and tetraethylenepentamine, TEPA) functionalized
10 mesoporous silica capsules. The newly synthesized sorbents exhibit extraordinary capture capacity up to 7.9 mmol g-1 under simulated flue gas conditions (pre-humidified 10% CO2). The CO2 capture kinetics were found to be fast and reached 90% of the total capacities within the first few
minutes. The effects of the mesoporous capsule features such as particle size and shell thickness on
CO2 capture capacity were investigated. Larger particle size, higher interior void volume and 15 thinner mesoporous shell thickness all improved the CO2 capacity of the sorbents. PEI impregnated
sorbents showed good reversibility and stability during cyclic adsorption-regeneration tests (50
cycles).

Introduction
Carbon dioxide has drawn significant attention as one of the 20 main anthropogenic contributors to climate change.1 A wide
range of approaches, including solvents,2-5 cryogenic techniques,6 membranes,7, 8 and solid sorbents,9-11 have been proposed to capture CO2. Among these approaches, amine scrubbing is the current state-of-the-art technology for CO2 25 capture on industrial scale.12 Although amines, in particular aqueous solutions of monoethanolamine (MEA), are effective at capturing CO2, they suffer from several disadvantages. They are corrosive and, therefore, increase construction cost. In addition, they do gradually volatilize and degrade, 30 especially in the presence of oxygen and/or sulfur dioxide, both of which necessitate the periodic injection of fresh solution. Furthermore, when MEA is applied to flue gas purification in conventional absorber/stripper systems in power plants, the parasitic energy consumption required for 35 regeneration is considerable (up to 30%). The combined costs of CO2 capture and compression raise the price of generating electrical power by over 60%. Reducing that percentage is a primary goal of R&D activity, much of which has been exploring the performance of alternative sorbents including 40 amines other than MEA. 13
Porous solids such as zeolites or activated carbon are good candidates for capturing CO2 from flue gas through physical adsorption.14 High porosities endow activated carbon and zeolites with CO2 capture capacities of 2-3.5 mmol g-1. 45 However, the CO2/N2 selectivity of activated carbon is relatively low (~10), which makes carbon-based systems practical only for CO2-rich flue gas.15 Although Zeolites offer better CO2/N2 selectivities than those of carbonaceous materials, their CO2 capacities degrade significantly, when 50 water vapor is present in the flue gas.14
CO2 capture by solid sorbents based on amines immobilized in porous solids has been an increasingly active

area of research.14 A variety of amines, supports and immobilizing techniques have been tested and the results have 55 been quite promising. 11, 16, 17 In contrast to amine scrubbers,
solid-supported amine sorbents offer significant advantages
for CO2 capture, including potential elimination of corrosion problems and lower energy cost for sorbent regeneration.
Solid-supported amine sorbents exhibit high selectivity and 60 reversibility for CO2 capture due to the specific CO2-amine
chemistry. Unlike zeolites and activated carbon, amine-
functionalized sorbents have been proved to be tolerant, and
even positive to moisture during CO2 capture, thus eliminating the need for strict humidity control prior to CO2 65 capture.18, 19
Because of their numerous advantages, intensive efforts
have been directed towards the development of solid-
supported amine sorbents. Essentially, two strategies have
been used: covalently tethered or physically impregnated
70 amines to solid supports with large surface area such as mesoporous materials.9, 11, 17, 20-28 Jones et al. developed a hyperbranched aminosilicate material by a one-step reaction
between aziridine and SBA-15, and achieved a capacity about 3.1 mmol g-1 at 25 oC in simulated flue gas conditions.11 The 75 material showed a capacity up to 5.5 mmol g-1 at 25 oC and 4.2 mmol g-1 at 75 oC with higher amine grafting.29 Song et
al. reported the first polyethylenimine impregnated MCM-41 sorbent with a capacity around 3 mmol g-1 at 75 oC in 1 atm dried CO2. The capacity was further improved to 3.2 mmol g-1 80 using SBA-15 as support in simulated flue gas conditions.9, 28
As-synthesized MCM-41 particles was also tested as support
for tetraethylenepentamine and a high CO2 capacity of up to 5.4 mmol g-1 was obtained at 75 oC in 1 atm dried CO2. However, this sorbent showed limited recyclability.23
85 Although solid-supported amine sorbents provide a
promising alternative for CO2 capture, high capture efficiency with good recyclability are required in order to develop an
economically feasible industrial-scale process. Schuette et al.

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have evaluated recently post-combustion CO2 capture options based on a plant producing 1,060 ton CO2/day.30 They compared the cost of capturing 90% CO2 using 20% MEA solution vs. a solid sorbent based on polyethylenimine 5 impregnated mesoporous silica. According to their calculations, if the CO2 capture capacity is less than 3 mmol g-1 the economics will favor MEA. However, the solid sorbent starts becoming more favorable compared to MEA as the capture capacity increases to more than 4 mmol g-1. 10 Almost all of the work to date has been focused on the immobilization of amines on as-synthesized or calcined mesoporous materials. Further improvement of the CO2 capture capacity of the sorbents has become challenging because of limitations with the amount of amine loaded and 15 the availability of active sites for CO2 capture in the sorbents. To overcome these limitations, we have been focusing on the development of a novel CO2 capture platform based on oligomeric amines supported on specially engineered mesoporous hollow particles (mesoporous capsules).31 This 20 new design greatly enhances amine incorporation and facilitates transport of CO2 inside the sorbent leading to exceptional capture capacity of 6.6 mmol g-1 under 1 atm dried CO2 gas at 75oC. This capacity represents an improvement of at least 400 and 25% over that of 25 conventional monoethanolamine solution (15% MEA) and other current solid amine impregnated sorbents, respectively, under similar CO2 capture conditions. More importantly the composite sorbents show an extraordinary capacity up to 7.9 mmol g-1 at 75 oC under simulated flue gas conditions (pre30 humidified 10% CO2). In addition to their outstanding CO2 capture capacity, the sorbents are readily regenerated at relatively low temperature (< 100 oC) and exhibit good stability over repetitive adsorption-desorption cycling (50 cycles).
35 Experimental
All chemicals were purchased from Aldrich unless otherwise stated. 2,2’-azobis(2-methylpropionamidine) dihydrochloride (V-50, >97.0%), polyvinylpyrrolidone (K-30, >99.5%), hexadecyltrimethylammonium bromide (CTAB, >99.0%), [240 (acryloyloxy)ethyl] trimethylammonium chloride (AETAC, 80 wt % in water), ammonium hydroxide solution (~30.0%), glycerol diglycidyl ether (GDE), ethylenimine oligomer mixture (PEI, average Mn~423), and tetraethylenepentamine (TEPA) were used without further purification. Styrene (St, 45 >99.0%) was washed through an inhibitor remover column to remove tert-butylcatechol and then distilled under reduced pressure prior to use. Tetraethoxysilane (TEOS, >99.9%, Gelest) and ethanol (80%, VWR) were used as received. Deionized water was generated with a Milli-Q integral pure 50 and ultrapure water purification system.
Synthesis of mesoporous silica supports
Mesoporous silica capsules with different sizes and shell thickness, denoted as MCx/y, where x and y represent the outside diameter and the shell thickness of the mesoporous 55 capsules in nanometers, respectively, were prepared as we described previously.31 Briefly, for MC160/20, CTAB (0.8 g)

was dissolved in a mixture of water (29.0 g), ethanol (12.0 g) and ammonium hydroxide solution (1.0 ml). Polystyrene latex with an average size around 130 nm (9.3 wt%, 10.0 g) was 60 added dropwise to the above CTAB solution at room temperature under vigorous stirring, followed by sonication for 10 min. The derived milky mixture was then magnetically stirred for 30 min before adding dropwise TEOS (4.0 g). The molar ratio of TEOS/CTAB/ethanol/H2O/NH3 was 65 1.0:0.11:13:87:0.83, and the TEOS/polystyrene weight ratio was 4.3. The mixture was kept at room temperature for 48 h before the mesoporous silica coated latex was harvested by centrifugation at 7000 rpm for 40 min. The precipitate was washed with copious amounts of ethanol and then dried at 70 room temperature. Finally the material was calcined in air at 600 oC for 8 h using a heating rate at 3 oC min-1. For MC400/20, polystyrene latex with an average size around 400 nm (9.3 wt%, 25 g) was added dropwise to a mixture of CTAB (0.80 g), water (9.6 g), ethanol (11.0g) and ammonium 75 hydroxide solution (2.0 ml) under vigorous stirring. The derived mixture was sonicated for 10 min and then magnetically stirred for 30 min before adding dropwise TEOS (1.5 g). The TEOS/CTAB/ethanol/H2O/NH3 molar ratio was 1:0.30:32:88:4.4, and the TEOS/polystyrene weight ratio was 80 0.66. The mixture was kept at room temperature for 48 h before the mesoporous silica coated latex was recovered by centrifugation at 7000 rpm for 15 min, followed by copious washings with ethanol. The precipitate was dried in the air overnight and calcined at 600 oC for 8 h in air. MC400/10 and 85 MC400/50 were prepared in the same manner as MC400/20 except that 0.75 g and 3.0 g of TEOS were used, respectively.
To investigate the effect of the support on the capacity of the sorbents, three other supports were synthesized. Silica coated polystyrene nonporous solid nanoparticles with a 90 diameter around 400nm, SiO2-400, was synthesized in the same manner as MC400/10 but without adding CTAB. The derived SiO2-400 nanoparticles were washed with ethanol and dried in air at room temperature. Normal, non-hollow MCM41 mesoporous particles were prepared similarly to MC400/10 95 except no polystyrene latex was used. Mesoporous SBA-15 particles were synthesized as described previously.32
Synthesis of Amine-functionalized Sorbents
The nanocompsite sorbents were prepared via wet impregnation. In a typical preparation, a given amount of PEI 100 or TEPA in ethanol (10 wt %) was added to 50 mg of mesoporous capsules or other supports. The resultant mixture was continuously stirred for about 30 min and then dried at 40 oC for 24 h under reduced pressure (700 mmHg). The thus formed sorbents are denoted as MCx/yPEI%z, where x and y 105 represent the diameter and the shell thickness of the mesoporous capsules in nanometers, respectively, and z the weight percentage of amine in the sorbent.
CO2 Capture and Regeneration of Sorbents
CO2 adsorption/desorption measurements under dry 110 conditions were performed on a TA Instruments Q5000
thermal graphic analyzer. Dried pure CO2 (99.99%) or CO2 (19.98%) balanced with N2 at 1 atm was used for the

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adsorption runs and ultra high purity N2 (99.995%) was used as a purging gas for CO2 desorption. In a typical adsorption process, about 10 mg of the sorbent was placed in a platinum sample pan. After the sorbent was heated to 100 oC in a N2 5 stream (25 ml min-1) for 60 min to remove all the moisture and CO2 adsorbed from the air, the temperature was decreased to 75 oC at a rate of 20 oC min-1. The gas was then switched from N2 to CO2 (25 ml min-1) and the sorbent was kept at 75 oC for 120 min for the adsorption study. The CO2 capturing 10 capacity of the sorbent in mmol g-1 was calculated from the weight gain of the sample in the adsorption process.
CO2 adsorption/desorption in simulated flue gas was measured with a packed bed flow reactor with an online mass spectrometer as described previously.11, 29 Approximately 70 15 mg of adsorbent was dispersed in 300mg of sieved sand (250 to 425 µm) and loaded into a Pyrex tubular reactor (1/4 in. outer diameter). 10% CO2 balanced with Ar was prehumidified to a steady state by flowing through two glass water bubblers at room temperature. After the samples were 20 completely purged with dry Ar for 1 h at 110 oC, the reactor was switched to the test gas at a flow rate of about 20 ml min1. Adsorption was then performed for 1 h at 75 oC. The CO2 concentration at the reactor outlet was measured with a Pfeiffer Vacuum QMS 200 Prisma Quadrupole Mass 25 Spectrometer (MS). The CO2 breakthrough plot is shown in Fig. S1. The amount of CO2 adsorbed was calculated from the MS output as reported before.11, 29
Two different methods, temperature swing and concentration sweep (altered CO2 partial pressure in 30 adsorption and desorption), were used for CO2 desorption. In the temperature swing the sorbent was first exposed to CO2 at 75 oC for 10 min. The gas was then switched from CO2 to pure N2 (25 ml min-1) and the temperature was increased to 100 oC at a rate of 20 oC min-1 and held at that temperature for 35 10 min to regenerate the sorbent. For the concentration sweep the gas was switched to pure N2 (25 ml min-1) at 75 oC after 10 min CO2 adsorption. The sorbent was then kept at 75 oC in N2 for 25 min for sorbent regeneration.
Characterization
40 Bright-field TEM images were obtained by a FEI Tecnai T12 Spirit Twin TEM/STEM operated at 120kV. Samples were prepared by dispersing the material in ethanol using sonication. A few drops of the dispersion were loaded onto a carbon coated copper microgrid and dried in air. To
45 investigate the morphology of amine impregnated sorbents, a given amount of polyethylenimine and glycerol diglycidyl ether (v:v=1:1) ethanol solution (21.4 wt %) was impregnated into the silica supports. These composites were dried at 20 oC for 24 h under reduced pressure (700 mmHg) and then cured
50 at 80 oC for 6 h. The derived composites using MC400/10, MCM-41 and SBA-15 are denoted as MC400/10PEI-GDE%m, MCM-41PEI-GDE%m and SBA-15PEI-GDE%m, respectively, where m is the polymer loading equivalent to z in pure amine impregnated capsules MCx/yPEI%z. Since
55 MCM-41PEI-GDE%83 and SBA-15PEI-GDE%83 are gels, they were cut by a Leica UC7 cryomicrotome into 100nm slices at -60 oC and loaded onto carbon coated copper

microgrids via dry pickup. For MC400/10PEI-GDE%83, a small amount of the powder was carefully adhered onto quick60 drying glue and then cut under the same conditions as MCM41PEI-GDE%83 and SBA-15PEI-GDE%83 for TEM specimen preparation.
XRD patterns of the mesoporous materials were obtained on a Scintag diffractometor using CuKα (λ=1.54 Å) radiation. 65 FT-IR spectra of the samples were recorded on a Nicolet Smart iTRTM iZ10 spectrometer. Nitrogen adsorptiondesorption isotherms were obtained on a Quantachrome Nova 3000 BET specific surface area analyzer at liquid nitrogen temperature. The specific surface areas of the samples were 70 calculated by the Brunauer-Emmett-Teller (BET) method. The pore size distributions were calculated from the adsorption isotherm using the Barrett–Joyner–Halenda (BJH) model.
Results and Discussion
Design and characterization of CO2 sorbents
75 The new sorbents are based on polyethylenimine, PEI, and tetraethylenepantamine, TEPA, supported on specially designed mesoporous SiO2 hollow capsules. The silica capsules are readily prepared from tetraethyl orthosilicate (TEOS) in the presence of a polystyrene latex template
80 (Scheme 1). A mesoporous shell with tunable thickness is grown around the polystyrene particles with the assistance of CTAB acting as mesopore directing agent. After synthesis the templates are removed by calcination to yield a capsule with a mesoporous SiO2 shell.31
85 The structure of the support plays an important role in the performance of the new sorbents. The main requirement for developing more efficient amine-functionalized CO2 sorbents is the use of a support with well-defined structure, in particular high surface area and pore volume, as well as
90 proper pore size. Simple porous materials such as amorphous SiO2 gel with random pore sizes and shapes are poor candidates, since only a limited amount of the pore area and pore volume can be accessible.33, 34 Mesoporous silica with pore diameters in the nanometer range greatly increases the
95 accessible sorption sites on/in the sorbent and improves the mass transfer during the sorption/desorption process. Unfortunately, not all the pores are available for CO2 capture. When a large amount of amines is loaded, the pore channels are significantly constricted and even blocked. In addition, the
100 amino groups may be unevenly distributed in the mesoporous materials.23, 24 The pressure resistance increases substantially as the free spaces between mesoporous particles are filled with amine molecules. As a result, an amine loading at 50-75 wt % was found to be an optimal range for most mesoporous
105 supports.9, 23, 35 Fig. 1a-d shows TEM images for several MCx/y supports.
Recall x and y correspond to the outside diameter and the shell thickness of the capsules in nanometers, respectively. Uniform particles with a hollow center and a thin shell can be 110 seen. For comparison, Fig. 1e shows solid, ~400 nm SiO2 particles. In this case the polystyrene latex particles were coated with silica similarly to the mesoporous capsules but in the absence of structure directing agent, CTAB. Two widely

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used normal mesoporous supports, MCM-41 and SBA-15 solid particles, were also prepared in the absence of polystyrene template and are shown in Fig. 1f-g for comparison. 5 In an effort to characterize the distribution of the amine in the support the TEM characterization was extended to the composite sorbents as well. Since both PEI and TEPA are viscous liquids at room temperature, half of the polyethylenimine was substituted with the same volume of 10 glycerol diglycidyl ether in order to prepare samples appropriate for TEM imaging. The amine-epoxy mixture was impregnated into the silica supports in the same manner as those sorbents impregnated with pure amine except that they were dried at lower temperature ~20 oC to minimize the 15 crosslinking during the solvent removal. The polymer distribution in the sorbent was then “fixed” by crosslinking the amine-epoxy mixture at 80oC for 6 h.
TEM images of these composites are shown in Fig. 2. In contrast to the normal mesoporous supports,9, 11, 17, 23-26, 35 the 20 exceptionally large pore volume in the hollow cores of the capsules can accommodate much higher amount of amine molecules while the textured mesoporous shell can facilitate access to the interior of the capsules. As shown in Fig. 2a, the amine-epoxy polymer shows a more uniform distribution 25 throughout the support in MC400/10PEI-GDE%83 compared with MCM-41PEI-GDE%83 (Fig. 2c). In the latter sample the support particles are submerged in a sea of amine-epoxy polymer (grey areas) even though the polymer loading is the same in both samples, suggesting that a large amount of the 30 polymer is accommodated in the interior area of the capsules in the former. The higher magnification TEM image of MC400/10PEI-GDE%83 (Fig. 2b) further confirms the interiors of the capsules are coated with the amine. The large excess of polymer outside the normal, non-hollow 35 mesoporous particles results in sorbents that are gel-like when high loadings of amine are used. (i.e. MCM-41PEI%83, Fig. S2b). In contrast, sorbents based on mesoporous capsules containing the same amount of amine tend to be powders (Fig. S2a). 40 Similar results are observed for SBA-15PEI-GDE%83, where a significant amount of the polymer is present outside the particles (blurry grey area, Fig. 2d) (Fig S2c). The ability of the mesoporous capsules to accommodate uniformly higher amounts of amines is a critical feature with regard to CO2 45 capture performance (vide infra). A higher amine loading, uniform amine distribution with less blocked pores/channels all favor more active sites available for CO2 uptake, and therefore, a higher capacity of the sorbent.
The FT-IR spectra of composite sorbents are shown in Fig. 50 3. The spectra of PEI and the mesoporous silica capsules are
also included for comparison. The SiO2 capsules show a strong absorption band near 1100 cm-1 due to the Si-O-Si asymmetric stretching vibrations. In the pure PEI bands at 3368, 3302 and 1595 cm-1 correspond to the asymmetric and 55 symmetric NH2 stretching vibrations while bands at 1457 and 1346 cm-1 are due to the CH2 vibrations. As expected the composite sorbent shows peaks characteristic of both the mesoporous capsules and PEI. When the composite sorbent is

exposed to CO2 new absorption bands at 1650, 1540, and 60 1407 cm-1 appear, which can be assigned to N-H deformation
in RNH3+, C=O stretch, and NCOO skeletal vibration, respectively due to the carbamate formed. The two broad bands at 2460 and 2170 cm-1 after exposure to CO2 are also the result of chemically absorbed CO2 species.36 65 The XRD pattern of the composite sorbents is shown in Fig. 4. The calcined MC400/10 capsules show a relatively broad diffraction peak at 2θ=2.55, corresponding to a d-value of 3.45 nm. Addition of PEI into the capsules to produce MC400/10PEI%75 has little effect on their mesoporous 70 structure, as the diffraction patterns remain virtually unchanged after the introduction of PEI. However, the intensity of the diffraction patterns does decrease especially at higher PEI loadings and in the case of MC400/10PEI%83 the diffraction peak disappears. The diffraction peak intensities 75 can be correlated with the scattering contrast between the silicate walls and the pores. The more amine is incorporated in the pore channels, the lower the peak intensity.37-39 The weaker intensity of the diffraction patterns with increasing PEI loading suggests the filling of the mesoporous pores with 80 PEI.9
A sample of completely degassed mesoporous capsules MC400/10 shows a surface area of 725 m2 g-1 and a type IV isotherm with an H3 hysteresis loop (Fig. S3).40 After filling the capsules with PEI (75 wt %), the surface area decreases to 85 19 m2 g-1 and a type II isotherm is seen (Fig. S3), suggesting that access of nitrogen into the pores at liquid nitrogen temperatures is somewhat restricted. The corresponding pore volumes are 0.73 and 0.069 cm3 g-1 for the empty and amine filled capsules, respectively. The surface area, pore volume 90 and average pore diameter of all samples based on different supports and amine loadings are summarized in Table 1.
CO2 Capture Kinetics
Owing to our interest in developing CO2 sorbents several MC400/10 samples impregnated with different amounts of 95 PEI were evaluated. As shown in Fig. 5, the CO2 capture appears to be a two-stage process. Once the sorbents (both composites and pure amine) were in the CO2 stream, a sharp linear weight gain occurred in the first stage, which was completed in less than 5 min followed by a second, much 100 slower adsorption process. Similar two-stage adsorption kinetics but with a much lower first-stage capacity have been observed in other amine impregnated sorbents.9, 14, 28 It is interesting that the first stage of adsorption showed similar sorption kinetics for all our samples. However, in the case of 105 pure PEI the adsorption reaches a plateau quickly with a corresponding CO2 capture capacity of about 2 mmol g-1. In contrast, the composite sorbents reach a plateau at much higher values, which significantly boosts their CO2 capacity. Note that the capture capacity of the empty capsules is 110 virtually zero under the same CO2 capture conditions. By increasing the amine loading from 67 to 80 wt %, the CO2 capacity in the first stage was improved from 3.2 to 4.2 mmol g-1. Increasing the amine content from 80 to 83 wt % showed no significant capacity enhancement in the first stage. Further 115 increasing the amine loading to 86 wt %, however, resulted in

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a capacity drop to 2.6 mmol g-1. The adsorption capacity increases somewhat during the slow second process. The adsorption kept increasing albeit at a very slow rate even after 120 min indicating that the adsorption didn’t reach its 5 thermodynamic equilibrium. A capacity of up to 5.7 mmol g-1 was obtained after 120 min for MC400/10PEI%83. Note that MC400/10PEI%83 reached more than 5.0 mmol g-1 within the first 10 min of CO2 exposure. The rapid kinetics are very beneficial in shortening the adsorption cycle time for potential 10 practical applications.
The two-stage adsorption seen for all composite sorbents and pure PEI can be attributed to CO2 diffusion resistance developed during the process. Once the sorbent is exposed to CO2, the fresh PEI reacts with CO2 and yields carbamate and 15 other CO2-amine complexes as suggested by FT-IR spectra.41, 42 As more CO2 is adsorbed, the viscosity of polyethylenimine increases substantially, which builds up diffusion resistance for CO2. In fact pure PEI becomes gel-like after CO2 exposure (Fig. S4). This diffusion resistance prevents PEI from 20 achieving its theoretical thermodynamic capture capacity. Dramatic viscosity increases have been reported for other amines or ionic liquids and have been attributed to the formation of salt bridges and/or hydrogen-bonded networks of amine-CO2 zwitterions.43-47 Accessibility of active sites to the 25 CO2 molecules due to the better distribution of the amine in the capsules reduces the diffusion resistance and contributes to the higher capacities of the composites sorbents over the pure PEI.
CO2 capture capacity
30 Several types of mesoporous capsules were synthesized to evaluate the effects of the support structure on the capacity of sorbents (Fig. 6). MC160/20PEI and MC400/20PEI were compared to investigate how particle size of the support affects CO2 capture capacity. MC160/20 and MC400/20 have
35 a similar mesoporous shell structure and thickness but different particle sizes. The maximum capacity of MC400/20PEI (5.25 mmol g-1) is higher than that of MC160/20PEI (4.05 mmol g-1) by 30%. The optimal amine loading (i.e. the amine content in the sorbent at maximum
40 CO2 capacity) increases from 67 wt % for MC160/20PEI to 75 wt % for MC400/20PEI. Thus, larger particle size seems to increase both the optimal amine content as well as CO2 capture capacity. MC400/10PEI, MC400/20PEI and MC400/50PEI were used
45 to study the effects of shell thickness on CO2 capacity. From Fig. 6, MC400/10PEI shows the best optimal amine loading and highest capture capacity suggesting that a thinner shell is more preferable. From Table 1 both the surface area and the pore volume of the composite sorbents decrease as the shell
50 thickness increases. The thicker mesoporous shell layer and the smaller total pore volume may result in more constricted or blocked pores in the sorbents lowering capture capacity. Most of advanced amine solid sorbents to date are based on normal mesoporous supports such as SBA-15, MCM-41.9, 28
55 Therefore, several sorbents using these supports were prepared to compare under identical conditions with the composite sorbents based on the mesoporous capsules. The

CO2 capacity of these sorbents with various amine loadings was measured under 1 atm CO2 at 75 oC and the results are 60 included in Fig. 6. The SBA-15 based sorbents show a moderately higher capacity than those based on MCM-41 at the same amine loading. Both the SBA-15 and MCM-41 based sorbents show an optimal amine loading at 75 wt % and a capacity around 4 mmol g-1. MC400/10PEI%83 has a capacity 65 enhancement by at least 40 % over the sorbents based on these normal mesoporous supports, suggesting that the mesoporous capsules are more advantageous over other porous materials used as support. Under the testing conditions, the stoichiometry of CO2 and amine (CO2/N) for 70 MC400/10PEI%75 and MC400/10PEI%83 are around 0.3, which, although still lower than the theoretical stoichiometry of 0.5,14 is much higher than those of MCM-41PEI%75 (CO2/N=0.22) and SBA-15PEI%75 (CO2/N=0.23). For complete comparison we have prepared and evaluated 75 composite sorbents based on “solid” SiO2 coated particles of similar size as the capsules (i.e. particles prepared in the absence of a structure directing agent and without removing the PS latex, Fig. 1e). The sorbent based on the nonporous nanoparticles, SiO2-400PEI, has poor capacity even lower 80 than that of pure PEI despite that the size of the support is similar to that of MC400/10.
Other than the structure of the support, the effect of amine type on the adsorption capacity was investigated using two different amines, PEI and TEPA. As shown in Fig. 6, the 85 optimal amine loading was not affected by the type of amine and both were about 83 wt %. However, a higher CO2 capacity was obtained from MC400/10TEPA than MC400/10PEI at the same amine loading and the stoichiometry of CO2 and amine (CO2/N) was improved to 90 0.33. We suspect that the higher density of amino groups and less viscous nature of TEPA compared to PEI provides more reactive sites for CO2 and thus a higher overall adsorption capacity.
The performance of the composite sorbents, 95 MC400/10PEI%83 and MC400/10TEPA%83, in a low CO2
concentration dry gas (balanced with N2 or Ar) and simulated flue gas was also investigated and the results are summarized in Table 2. The two sorbents exhibit outstanding capacities above 4.4 mmol g-1 even at 0.1 atm dry CO2. When simulated 100 flue gas (pre-humidified 10% CO2) was used, the capacity of MC400/10PEI%83 increased to 5.58 mmol g-1 and that of MC400/10TEPA%83 to 7.93 mmol g-1. The positive effect of moisture on the capacity of amine functionalized sorbents has been reported before.17, 19, 27, 48 It was claimed that the 105 formation of bicarbonate in the presence of moisture contributes to the increase of the capacity.17, 19, 27, 48
Cyclic adsorption-desorption studies
For potential practical applications, in addition to high CO2 capturing capacity, the sorbent must possess long-term 110 stability and regenerability with a minimum difference in adsorption/desorption temperatures or pressures to lower cost. At the same time, a short cycle time is generally preferred. To this end, two processes, temperature swing and concentration sweep, were used to evaluate the stability and regenerability

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of the mesoporous capsule sorbents in a short cycling time. Fig. 7 depicts the adsorption–desorption cycles of MC400/10PEI%83 and MC400/10TEPA%83 using a temperature swing process. After exposure to dry CO2 at 75 5 oC for 10 min, the temperature was increased to 100 oC. Both sorbents can be regenerated within 10 min in N2. MC400/10PEI%83 was more stable than MC400/10TEPA%83. The former retained ~88% of its capacity whereas MC400/10TEPA%83 dropped to about 60% of its capacity 10 after 50 cycles. The better cyclic performance of MC400/10PEI%83 may be due to the higher boiling point of PEI, which contributes to better temperature stability. Fig. 8 shows the regeneration behavior of MC400/10PEI%83 and MC400/10TEPA%83 using a concentration sweep process. 15 After CO2 capture at 75 oC, N2 was used as the stripping gas to remove the CO2 from the sorbent at the same temperature for 25 min. Both MC400/10PEI%83 and MC400/10TEPA%83 had only a slight decrease in capacity (less than 10%) after 50 cycles.
20 Conclusion
We have synthesized and evaluated a new, highly efficient CO2 sorbent platform based on specially designed mesoporous silica capsules impregnated with polyethylenimine (PEI) and tetraethylenepentamine (TEPA). The specially designed 25 capsules offer increased amount of amine incorporation and reactive sites for CO2 capture leading to exceptional capturing performance of up to 7.93 mmol g-1 in simulated flue gas. In addition to the simple synthesis and their outstanding CO2 capture capacity, the sorbents are readily and fully 30 regenerated at relatively low temperature (< 100oC) and exhibit good stability over repetitive adsorption-desorption cycling.
Acknowledgment
This publication was based on work supported by Award No. 35 KUS-C1-018-02, made by King Abdullah University of
Science and Technology (KAUST).
Notes and References
a Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, USA; E-mail: epg2@cornell.edu 40 b Department of Earth and Environmental Engineering and Department of Chemical Engineering, Columbia University, New York, NY 10027, USA c School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332, USA 45 † Electronic supplementary information (ESI) available: Adsorption isotherms of MC400/10 and MC400/10PEI%75 composite sorbent; Viscosity difference of amines before and after CO2 capture. See DOI: 10.1039/b000000x
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Table 1. Structural features for the mesoporous supports and their composite sorbents.

Sample ID
MC400/10 MC400/20 MC400/50 MC400/10PEI%75 MC400/10PEI%80 MC400/10PEI%83 MC400/20PEI%67 MC400/20PEI%75 MC400/20PEI%80c MC400/50PEI%67 MC400/50PEI%75c MC400/50PEI%80c MCM-41 MCM-41PEI%75c
SBA-15 SBA-15PEI%75

Inner diametera / nm
345 ± 10 355 ± 10 350 ± 10 345 ± 10 345 ± 10 345 ± 10 355 ± 10 355 ± 10 355 ± 10 350 ± 10 350 ± 10 350 ± 10
N/A N/A N/A N/A

Shell thicknessa / nm
12 ± 3 24 ± 5 51 ± 5 12 ± 3 12 ± 3 12 ± 3 24 ± 5 24 ± 5 24 ± 5 51 ± 5 51 ± 5 51 ± 5 N/A N/A N/A N/A

BET surface areab / m2g-1 7.25×102 1.30×103 1.32×103 19.48 10.64 6.16 12.24 5.53 3.82 9.65 3.15 0.45 1.03×103 1.80 8.28×102 2.38

Pore size b / nm
3.1 3.1 3.1 3.0 3.1 3.6 3.6 3.6 3.1 3.1 3.6 3.6 2.6 N/A 5.6 N/A

a Measured by TEM. b Measured by a nitrogen adsorption BET method. c The pore volume is below the detection limit.

Pore volume b / cm3g-1
0.73 0.48 0.32 0.069 0.031 0.016 0.054 0.011 N/A 0.025 N/A N/A 0.67 N/A 0.89 0.023

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Table 2. Capture capacity of composite sorbents at low CO2 partial Pressure at 75 oC

Sample ID
MC400/10PEI%83 MC400/10PEI%83 MC400/10PEI%83a MC400/10TEPA%83 MC400/10TEPA%83 MC400/10TEPA%83a

CO2 concentration
(%)
20.0 10.0 10.0 20.0 10.0 10.0

Sorbent capacity / mmol g-1
4.91 4.45 5.58 6.39 5.57 7.93

a The test gas was pre-humidified to a steady state by flowing through two glass water bubblers at room temperature.

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Scheme 1. Synthesis of amine impregnated composite sorbents based on mesoporous capsules.

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Fig. 1 Transmission electron micrographs of various silica supports. (a) MC400/10, (b) MC400/20, (c) MC400/50, (d) MC160/20, (e) Silica coated polystyrene particle SiO2-400, (f) MCM-41 and (g) SBA-15. Scale bar = 200 nm.
11

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Fig. 2 Transmission electron micrographs of composites prepared by impregnation of polyethylenimine and glycerol diglycidyl ether (v:v=1:1) into the mesoporous supports. (a) MC400/10PEI-GDE%83, (b) MC400/10PEI-GDE%83 (high magnification, scale bar= 200 nm), (c) MCM-41PEI-GDE%83 and (d) SBA-15PEI-GDE%83.

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Fig. 3 FT-IR spectra of support, PEI (before and after CO2 capture) and the nanocomposite sorbent (before and after CO2 capture).
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Fig. 4 XRD patterns of mesoporous silica capsules with different PEI loadings.

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Fig. 5 CO2 capture kinetics of mesoporous silica sorbents with different amine loadings.

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Fig. 6 CO2 capture capacity of composite sorbents based on various mesoporous silica supports. The capture capacity was measured after 120
.min at 75 oC under 1 atm pure CO2

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Fig. 7 Cyclic adsorption-desorption of sorbents based on mesoporous silica capsules containing 83% amine using a temperature swing. (CO2 capture at 75 oC under 1 atm pure CO2 for 10 min; sorbent desorption at 100 oC under 1 atm pure N2 for 10 min).

17

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Fig. 8 Cyclic adsorption-desorption of sorbents based on mesoporous silica capsules containing 83% amine using a concentration sweep. (CO2 capture at 75 oC under 1 atm pure CO2 for 10 min; sorbent desorption at 75 oC under 1 atm pure N2 for 25 min).

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TABLE OF CONTENTS
High Efficiency Nanocomposite Sorbents for CO2 Capture based on Amine-functionalized Mesoporous Capsules
5
Genggeng Qi, a Yanbing Wang, a Luis Estevez, a Xiaonan Duan, a Nkechi Anako, b Ah-Hyung Alissa Park, b Wen Li, c Christopher W. Jones c and Emmanuel P. Giannelis*a
10 a Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, USA; E-mail: epg2@cornell.edu b Department of Earth and Environmental Engineering and Department of Chemical Engineering, Columbia University, New York, NY 10027, USA
15 c School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332, USA

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20
A novel CO2 capture sorbent platform based on specially structured mesoporous supports was developed. The sorbents show excellent CO2 capture capacity and recyclability over 50 cycles.
25
30

19