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Article 2

Water-enhancement in CO Capture by Amines: An Insight into CO-HO Interactions on Amine Films and Sorbents 2

2

Jie Yu, Yuxin Zhai, and Steven S. C. Chuang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05114 • Publication Date (Web): 01 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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Industrial & Engineering Chemistry Research

Water-enhancement in CO2 Capture by Amines: An Insight into CO2-H2O Interactions on Amine Films and Sorbents Jie Yu1†, Yuxin Zhai1†, and Steven S. C. Chuang1* 1

Department of Polymer Science, the University of Akron, 170 University Avenue, Ohio 44325, United States.

ABSTRACT Water, a component in flue gas, plays a significant role in CO2 capture through a complex interaction between water molecule and adsorbed CO2 on amine sorbents. To determine how the H2O-CO2-amine interactions affect amine efficiency and binding energy of adsorbed CO2, we used in situ infrared spectroscopy (IR) to determine the structure of adsorbed CO2 and H2O as well as their relations to adsorption/desorption kinetics and CO2 capture capacity on tetraethylenepentamine (TEPA) films and Class I amine (i.e., impregnated) sorbents.

H2 O

enhanced amine efficiency of TEPA films and sorbents by increasing the accessibility of secondary amine sites to CO2 and promoting the formation of hydronium carbamate and carbamic acid. CO2 adsorbed on the surface of TEPA film as a weakly adsorbed CO2 in the form of hydronium and ammonium – carbamate with a low IR intensity of hydrogen bonding (-OH···OOC or -NH···-OOC) between hydronium/ammonium ions and carbamate ions. CO2 adsorbed on the middle layers (i.e., 0.2 – 0.4 µm below the surface) of TEPA films produced a strongly

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adsorbed species which exhibit an intensive hydrogen bonding band of ammonium – water – carbamate desorbing at temperatures above 120 °C. Comparison of IR spectra shows that the kinetic behaviors of adsorbed CO2 on amine films are correlated well with those of adsorbed CO2 on Class I amine sorbents. Thick amine films and high amine loading sorbents contain high density amine sites which produce mainly strongly adsorbed CO2.

Adsorbed H2O further

increased amine efficiency and the binding energy of strongly adsorbed CO2 through the formation of hydronium carbamate. INTRODUCTION Amine sorbents and solvents have been extensively studied for thermal swing CO2 capture processes because of their potentials for a significant reduction in the cost of CO2 capture for the large-scale coal-fired power plants. These CO2 capture processes have been developed and/or under development on the basis of the acid-base chemistry of CO2-amines. Many studies in liquid and aqueous amines have shown that the interaction of CO2/H2O with amines follows a zwitterion and ammonium carbamate pathway in Scheme 1, forming bicarbonate with a CO2/amine stoichiometry of 1.1-4 Our previous studies provided the infrared results supporting the formation of zwitterions which undergo deprotonation to produce ammonium carbamate.5-8 Bicarbonate, which has been commonly observed in aqueous amine solutions, i.e., alkanolamines, amine-functionalized ionic liquid, and piperazine, but has seldom been found on amine sorbents with primary and/or secondary amines in large quantities.3, 9-18 One factor governing the selection of amine sorbents/solvents for CO2 capture processes is amine efficiency, i.e., the number of CO2 molecule captured by each amine functional group. The amine efficiency of an amine sorbent is dictated by the form of adsorbed CO2 species in


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Scheme 2: (i) ammonium carbamate, which is produced through pathway I-a and I-b, giving an amine efficiency of 0.5, (ii) carbamic acid from pathway II, giving an amine efficiency of 1.7-8, 1924

Achieving a CO2/amine stoichiometry (i.e., amine efficiency) of 1 allows an efficient use of

amine, resulting in reduction of the overall cost of CO2 capture processes. Scheme 1. CO2/H2O adsorption on aqueous amines

Scheme 2. Pathways for the adsorption of CO2 and CO2/H2O on amines O C CO2

H

HN O H

(II)

NH O

NH N C OH ammonium carbamate carbamic acid

H2N

O H2N

CO2

C H2N

(I-a)

O C

NH

CO2/H2O

H3N OH NH

HN

(I-c)

CO2

H

HN

O O H

O

C HN OH

H

(III-b)

O NH

(III-c)

H

O

(I-d)

N H

carbamic acid - water


N O O H C O H N O O H H N C O H

HN

O C HN O

H

NH

H

C

ammonium carbamate hydronium carbamate carbamic acid - water

H

NH

NH hydronium carbamate

CO2/H2O

ammonium carbamate hydronium carbamate

ammonium carbamate

C

C

O H O H NH OH N C O H

NH NH

O

(III-a)

NH

(I-b)

zwitterion

H2O

H

HN O H

NH

NH

H

O

O

H H N H O H H NH

(III-d) NH

hydrogen bonded ammonium-water-carbamate

H O H


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H2O vapor constitutes 4 – 10% of coal-derived flue gas.25 A fundamental issue remains to be addressed is how H2O molecules control the amine efficiency of the amine sorbent and binding energy of adsorbed CO2 at the molecular level. It is important to note that amine sorbents process a number of desirable features: fast CO2 adsorption/desorption kinetics, low binding energy of adsorbed CO2, and minimal corrosion to equipment.7-8, 20, resulted from the solvent-free and/or -deficient environment.

26-28

These features are

Amine functional groups are

surrounding by neighboring amine and alkyl (C-H) groups instead of by H2O molecules in an aqueous amine system.

Aqueous solvent and aqueous solvent-deficient (i.e., sorbent)

environments differ in the numbers of H2O molecules which interact with amine functional groups, adsorbed intermediates, and adsorbed CO2. Voluminous studies on various classes of amine adsorbents, shown in Scheme 3, provide scattering results on the effect of water on their amine efficiencies.18, 24, 29-39 Variation in water effects could be attributed to the difference in the amine density/structure as well as the structures of sorbent particles and substrates. We have used amine thin film as a model to emulate layers of amine molecules in Class I sorbents, illustrated in Scheme 3.

Class I

impregnated amine sorbents which contain high amine loading in general exhibited higher CO2 capture capacity than those in other Class sorbents.18, 28, 40-52 High loading amines in Class I sorbent are closely packed, giving density amine sites than those of Class II grafted amine sorbent.


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Scheme 3. Illustration of supported amine sorbents and deposited amine thin films

0.8 µm TEPA

Stainless steel disk Impregnated amine sorbent (low amine loading) 3.5 µm TEPA

Class I HN N H

NH H2 N

NH

10 µm TEPA

NH2

H2N H O

Double impregnated sorbent (high amine loading)

NH2

HN

H N

OH

OH O H

Porous SiO2

H 2N

N

H

H

H N

O

NH2

Si OR Si OR O OH O O Porous SiO2

Class II

Class III

Class IV

Our recent study showed that under an aqueous environment where amine functional groups are surrounded by water molecules, that is, TEPA/H2O (5:1), CO2 adsorption follows pathway III of Scheme 2, producing a hydronium carbamate which has a stoichiometry of CO2/amine = 1.53 It is worth noting that hydronium carbamate was first proposed by Caplow for interpreting kinetics of carbamate formation and dissociation.54

A recent DFT study proposed that hydronium

carbamate as a major intermediate formed in humid conditions on PEI impregnated silica sorbent during CO2 adsorption.55 The lack of bicarbonate species in the TEPA/H2O (5:1) film may be 5 ACS Paragon Plus Environment


explained by the results of simulation studies that shows neutral or alkaline hydrolysis of carbamate to bicarbonate in Scheme 1, is an unfavorable pathway because of its high activation energy.1, 56-59 In this work, we investigated the effect of 3% H2O vapor on CO2 capture by TEPA thin films with varying thickness: 0.8, 3.5, and 10 µm as well as by Class I amine sorbents. Amine thin film serves as a model for impregnated sorbents and double-impregnated amine sorbents, illustrated in Scheme 3. The amine thin film emulates those immobilized amines on the top and middle layers of a practical amine sorbent for CO2 capture process without complication of porous support. The porous support of amine sorbent is comprised of tortuous structures which could introduce diffusion effects and interfere the infrared spectra of adsorbed species. In situ infrared studies showed that H2O preferentially interacts with secondary amine, disrupting ammonium carbamate pairs, and facilitating the formation of secondary carbamate/hydronium pairs on amine this film. Comparison of IR results revealed that amine thin films and sorbents operated on the same water-enhancement mechanism in CO2 capture. Combining the results of this study with our previous studies of TEPA/H2O (5:1) mixture could provide a clear picture of how molecular interactions of amines-water-CO2 control the formation of adsorbed species and provide an insight into the role of water in controlling amine efficiency and binding energy of adsorbed CO2 in CO2 capture process by amine sorbents.


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EXPERIMENTAL Preparation of TEPA thin films Tetraethylenepentamine (TEPA, Sigma-Aldrich, 98%) films were prepared by depositing different concentrations, 1.4 wt%, 3.5 wt%, and 7.1 wt% of TEPA/ethanol solutions on a stainless steel (SAE 304) disk and dried at 100 °C in the presence of Ar to form thin films. The thickness of TEPA films was determined to be 0.8 µm, 3.5 µm, and 10 µm for 1.4 wt%, 3.5 wt%, and 7.1 wt% solution, respectively. The TEPA films were also prepared on a silicon wafer and measured by thin film analyzer (FILMETRICS F20) and InfiniteFocus (Alicona) at room temperature for calibration of the IR intensity of TEPA films with respect to their thickness (Figure S1). Preparation of sorbents The TEPA/SiO2 sorbents were prepared by impregnating TEPA/ethanol solutions on SiO2 support (armophous silica, Rodia chemicals) with amine loadings of 2.4, 5.3, and 9.2 mmol [N]/g [sorbent], corresponding to 9.1, 20.0, and 34.9 wt% of TEPA on 1 g of sorbent, respectively. The SiO2 support has the surface area of 160 m2/g. The morphology of the SiO2 support was reported previously.60 The wet mixture was dried at 100 °C for 30 min in the presence of Ar. The TEPA/SiO2 (2.4) denoted the TEPA/SiO2 with amine loading of 2.4 mmol [N]/g [sorbent], where [N] represents amine function groups. The double impregnated sorbent was prepared by impregnating polyethylenimine (PEI, Mn = 60,000 g/mol, Mw = 750,000 g/mol, 50% aqueous solution, Sigma-Aldrich) and EPON 826 (EPON, Miller–Stephenson) on the sorbent obtained from Aspen Aerogel Co. The total amine loading was 11.3 mmol/g, equivalent to 51 wt % of



amine on the amine sorbent. The double impregnated sorbent possessed high-density amine, resembling those found on the thick TEPA film. In situ IR study of CO2/H2O adsorption/desorption during the thermal swing CO2 capture The metal disk with TEPA film deposited was placed in a DRIFTS cell for in situ IR study of CO2 capture. The experimental setup has been described in details.24 A typical thermal swing CO2 capture cycle, illustrated with IR results of 10 µm TEPA film in Figure 1, consists of the following: (i) pretreatment of the amine thin films or sorbents at 100 °C under 150 cc/min Ar flow (pret.), (ii) adsorption of CO2/H2O with flowing 150 cm3/min 97% CO2/3% H2O for 10 min at 50 °C (ads.), (iii) Ar purge for 10 min at 50 °C under Ar flow (purge), and (iv) temperatureprogrammed desorption (TPD) by heating to 120 °C with 10 °C/min in Ar flow. 3% H2O was brought to CO2 flow by passing a water saturator. The adsorption/desorption of CO2 and CO2/H2O on TEPA films and TEPA/SiO2 were repeated 3 times under the same condition. The IR spectra of TEPA films and TEPA/SiO2 exhibited almost identical contours. Figure 1 (a) presents the IR intensity profiles of gas CO2 (2360 cm-1) and adsorbed H2O (3452 cm-1) as well as temperature profile during a thermal swing CO2 capture cycle. These profiles allow us to confirm the occurrence of each step. The CO2 capture capacity of each sample was obtained by mass spectrometric (MS) measurement of gaseous CO2 desorbed from the sorbents. The area under MS CO2 profile (m/e=44) was converted to the number of mole of CO2 adsorbed by a calibration factor.28 Figure 1 (b) shows the IR absorbance spectra of the sorbent samples during each step, which illustrates the procedures for obtaining the IR spectra of adsorbed species. The IR absorbance spectra of adsorbed species were obtained by log(I0/I), where I is the IR single-beam intensity of the TEPA film at each step and I0 is the IR single-beam intensity of


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atmosphere or pretreated TEPA film. The IR single-beam intensity is obtained by Fourier transforming an interferogram. The IR spectra of those adsorbed species desorbed during Ar purge and TPD can also be obtained by the difference spectrum, e.g., ATPD,120 °C – ATPD, 90°C, which allows distinguishing the structure of weakly and strongly adsorbed species. The spectra of adsorbed species on amine sorbents and thin films were closely examined to elucidate their

purge

ads.

TPD

0.2

o

120

(i)

CO2 10

3452 3363 3288

0 (b)

100

H2O

Int.×5

80

(iv)

(iii)

60

20 Time (min)

30

40

1633 1556 1458

(ii)

2923 2815

Norm. IR Int. (a.u.)

(a)

Temperature ( C)

structures.

ATPD,120 C-ATPD,90

1.0

o

o

C

(iv) TPD ATPD,120 C ATPD,90 C o

o

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(iii) purge AAr,10m AAr,0.5m ACO /H O,10m-APret. 2

2

(ii) ads. ACO /H O,10m ACO /H O,0.5m 2

2

2

2

(i) pret. APret.

4000

3000 2000 -1 Wavenumber (cm )


1000


Figure 1. (a) Temperature profile and normalized IR intensity (Norm. IR Int.) profiles of gaseous CO2 (2360 cm-1) and adsorbed H2O (3470 cm-1), (b) IR absorbance spectra of 10 µm TEPA film were collected during a CO2/H2O adsorption/desorption cycle. Insets are the IR difference spectra of TEPA film after 10 min adsorption of CO2/H2O and desorbed species during TPD. RESULTS AND DISCUSSION TEPA thin films and amine sorbents in Figure 2 showed the IR characteristics: asymmetric and symmetric N−H stretching bands at 3363 cm-1 and 3288 cm-1, respectively, and N−H deformation band at 1600 cm-1; the asymmetric and symmetric C−H stretching bands at 2923 cm-1 and 2815 cm-1, respectively, and C−H deformation band at 1458 cm-1. Amine on SiO2 suppressed the intensity of isolated hydroxyl groups at 3731 cm-1 in Figure 2 (b), resulting from the formation of hydrogen bonding between hydroxyl groups and amine groups.7, 19-20, 28 The IR spectra showed a significant variation in their intensities with different film thickness and amine loadings on SiO2. The 0.8 µm film has a lower intensity ratio of N−H/C-H stretching bands at 3288 cm-1/2923 cm-1 than that of 10 µm TEPA film. The increase in these ratios with film thickness can be explained by the increased hydrogen bonding interactions between -NH2/NH in TEPA film, as well as the diminishing effect of the metal surface on TEPA on the top and middle layers of the thicker films.5 The inset in Figure 2 further showed that the N-H···N-H hydrogen bonding interaction broadened the N-H stretching peaks on the 10 µm TEPA film in comparison with the 0.8 µm film. The morphology of TEPA films in Figure S2 exhibited the same roughness as the stainless steel disk on which the TEPA film was distributed (Scheme 3). Thus, we proposed that the low extent of hydrogen bonding on the thinner amine film can be attributed to the lower packing density of amine molecules of which amine functional groups separate with 10 ACS Paragon Plus Environment

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a larger distance than those on the thicker amine film. In other words, the thinner film contains a larger fraction of low density amine sites than the thicker amine film. The IR spectrum of amine sorbent resembles those of TEPA films but exhibits the low intensity ratio of N−H/C−H stretching bands at 3288 cm-1/2923 cm-1 on low amine loading sorbents, which could be attributed to the low amine density on SiO2 support and the hydrogen bonding between hydroxyl and amine groups. Increasing the amine loading and density on the sorbents resulted in the increased intensity ratio of N−H/C−H in Figure 2 (b).


0.5 Absorbance (a.u.)

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NH/CH 3288/2923 0.69

1600 1458

3363 3288

(a)

0.54

TEPA film 10 µm

0.47 3363 3288

3.5 µm 10 µm

0.8 µm 0.8 µm

0.5

1000 NH/CH 3288/2923 0.54

1645 1600 1458

2923 2815

(b)

3363 3288

3000 2000 -1 Wavenumber (cm ) 3731 3675

4000

1/Single Beam (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

2923 2815


double impregnate 11.3

0.49 0.45

TEPA/SiO 2 9.2

0.38

5.3 2.4

SiO2 4000

3000 2000 Wavenumber (cm-1)

1000

Figure 2. IR absorbance spectra of (a) TEPA films with different thicknesses and (b) amine sorbents with subtraction of ambient. Inset: IR absorbance spectra of 0.8 and 10 µm TEPA films. The spectra were obtained after pretreatment at 100 °C. The N−H/C−H ratios were obtained from the IR intensity ratios at specific wavenumbers. Figure 3 shows the IR absorbance spectra of adsorbed species produced from adsorption of CO2, CO2/H2O, H2O, and HCl/H2O on 10 µm TEPA films at 50 °C. Adsorbed CO2 is mainly in the 12 ACS Paragon Plus Environment

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form of primary ammonium carbamate of which ammonium ion bands can be identified by HCl protonation through HCl/H2O adsorption, shown in Figure S3. Following the assignment of ammonium ion bands for HCl/H2O on TEPA film, the IR bands at 1531 and 1556 cm-1 can be unambiguously assigned to carbamate. Co-adsorption of CO2/H2O shifted the carbamate band to 1531 cm-1 along with suppression of secondary amine at 3288 cm-1. Thus, we can conclude that this carbamate species at 1531 cm-1 is associated with the secondary amine site. Comparison of spectra of adsorbed CO2, H2O, and CO2/H2O showed that co-adsorbed CO2/H2O decreased the intensity of the secondary amine (-NH) at 3288 cm-1 at least two times more than adsorbed H2O alone did. A number of our previous TPD study showed that the intensity of carbamate (-NCOO- and -NHCOO-) bands correlated with the amount of CO2 desorbed from the amine sorbents and films.7,

28, 53, 60

The enhanced CO2 capture capacity is manifested by a

significant higher integrated carbamate intensity in CO2/H2O than in CO2 alone.

These

observations indicated that adsorbed H2O promoted the accessibility of secondary amines to CO2, generating more secondary carbamate at 1531 cm-1 than primary carbamate at 1556 cm-1. The increased intensity of carbamate in the CO2/H2O spectrum in Figure 3 is also accompanied with an emergence of the broad band at 3081 cm-1 which can be assigned to hydronium ions. Clusters of hydronium ions have been shown to give peaks in the region of 3000 – 3200 cm-1 and 2500 – 2900 cm-1 in Figure S4.61 The broadness of these bands can be attributed to hydrogen bonding interactions between hydronium ion.


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1639 1511 1411

2163

4

2582

10 µm TEPA

HCl/H2O

1531

H 2O 2

1556

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3452 3363 3288 3104 3081 3000


CO2/H2O

O

CO2 0 4000

C

H

HN O H

3500

3000 2500 2000 Wavenumber (cm-1)

1500

NH

1000

Figure 3. IR absorbance spectra of CO2, CO2/H2O, H2O, and HCl/H2O adsorption on 10 µm TEPA film. The structures of adsorbed species were located next to the IR spectra. Figure 4 (a) shows the evolution of IR spectra of adsorbed species during CO2/H2O adsorption on 10 µm TEPA film. The intensities of -NH3+, -NHCOO-, and -NCOO-, as well as -OH of adsorbed H2O, were plotted as a function of time in Figure 4 (b). The first spectrum obtained at 0.01 min is shown in Figure 4 (c) exhibited positive bands in 1200 – 1800 cm-1, which could be assigned to the symmetric and asymmetric vibration of C=O and deformation of N−H in zwitterion. The ammonium carbamate emerged from 0.02 min to 0.05 min with low intensity of the broad bands at 2163, 2582, and those over 3000 cm-1, which are associated with hydrogen bonding between ammonium ions and carbamate ions. 14 ACS Paragon Plus Environment

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It should be noted that the adsorption of H2O lagged that of CO2, shown in Figure 4 (b). IR bands of adsorbed H2O emerged at 0.2 min in Figure 4 (c). The saturated H2O vapor pressure in the CO2 stream was achieved by bubbling CO2 through the water saturator for 30 min prior to the experiment. The delay in H2O adsorption is because H2O vapor travelled at a slower rate than gaseous CO2 in the dry inlet line where H2O vapor underwent re-adsorption/desorption on the wall of the inlet tube. A significant increase in the intensity of adsorbed CO2 occurred at 0.2 min while the bands associated with water at 3570 and 3452 cm-1 emerged. The low wavenumber of O−H at 3452 cm-1 resulted from hydrogen bonding with amine and neighboring water molecules. The difference spectrum of 0.3-0.2 in the inset of Figure 4 (c) gave a band at 3081 cm-1, which can be assigned to hydronium ions. The appearance of hydronium ions was also accompanied with a negative secondary amine (-NH) at 3288 cm-1 and a shift in the carbamate from 1556 to 1531 cm-1, further suggesting that hydronium is associated with a secondary carbamate. Thus, we can conclude that a fraction of secondary amines, which were not accessible by CO2, was able to coordinate with hydronium carbamate during CO2/H2O co-adsorption, resulting in an increase in the amine efficiency. The difference spectrum at 10-1 in Figure 4 (c) showed a carbamic acid band at 1677 cm-1 emerged after 1 min of CO2/H2O adsorption. A number of our previous IR studies have shown that carbamic acid is formed on a secondary amine site.7-8, 24 A high wavenumber of -OH at 3570 cm-1 is assigned to water molecules with a low extent of hydrogen bonding.24

The

adsorption of CO2/H2O on TEPA/SiO2 (9.2) in Figure S5 presented a similar pathway as 10 µm TEPA film. We summarized the process of CO2/H2O co-adsorption in pathway I in Scheme 2, which proceeds through zwitterion ammonium carbamate hydronium carbamate carbamic acid – water.



Figure 4 (a) 3D waterfall IR spectra, (b) IR normalized intensity profiles, and (c) IR absorbance spectra of 10 µm TEPA film during CO2/H2O adsorption. Insets are the difference spectra of adsorbed species on 10 µm TEPA film during CO2/H2O adsorption.5 16 ACS Paragon Plus Environment

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Figure 5 compares the IR spectra of adsorbed species and their intensities during CO2/H2O coadsorption on TEPA films. Although variations in thickness of TEPA films and in loading of TEPA/SiO2 sorbents resulted in different intensity ratios of 3288/2923 and exhibited different overall integrated intensities in Figure 2, the initial contour of IR spectra of adsorbed species are nearly identical at 0.02 min – 0 min and 0.1 min – 0 min for amine thin films in Figure 5 and for amine sorbents in Figure S6. The similarity in these initial IR spectra of adsorbed species indicates the density of the amine functional groups and their states are nearly identical on the surface of these thin films and sorbents. The observed differences for IR spectra of TEPA films and TEPA/SiO2 sorbents in Figure 2, Figure 5, and Figure S6 are mainly from those amines below the surface. The difference spectra obtained at 1 min – 0.02 min represented those adsorbed species located below the surface of the TEPA film or sorbent but far above the surface of supports, that is, metal disk or SiO2. The difference IR spectra at 0.02 min – 0 min, 1 min – 0.02 min, and 10 min – 1 min correspond to the spectra of adsorbed species located top (0.2 – 0.4 µm from the surface), middle (0.5 – 4.5 µm), and bottom (0.1 – 0.2 µm) layers of TEPA films, listed in Table S2, which were estimated by the molar absorption coefficient in Figure S7.5 The variations in the contour of theses spectra showed that the film thickness and amine loading have a significant effect on adsorbed species. Thicker films and higher amine loading sorbents, which gave a greater extent of hydrogen bonding interactions in NH/NH2, also produced adsorbed species with a higher intensity ratio of a broad ammonium ion band at 2582 cm-1 to the carbamate band at 1531 cm-1. It is reasonable to infer that those films and sorbents, which gave a higher extent of hydrogen bonding, have a higher amine density and more amine functional groups packed in a specific volume.



(a) 0.8 um TEPA film 0 min 0.02 min 1

(b) 3.5 um TEPA film

1 min

10 min

0 min0.02 min

10 min 0 min0.02 min

1 min

NCOO-/1531 cm-1

NCOO-/1531 cm-1 NCOO-/1531 cm-1

10 min

ads. H2O/3452 cm-1

ads. H2O/3452 cm-1

ads. H2O/3452 cm

IR intensity

(c) 10 um TEPA film

1 min

-1

NCOOH/1677 cm-1

NCOOH/1677 cm-1

10 min - 1 min

1


9

10

0.0

3569 3452 3291

10 min - 1 min


1.0 Time (min)

9

10

10 min - 1 min 1 min - 0.02 min

0.02 min - 0 min

0.02 min - 0 min 1000 4000

0.5

1677 1639 1531 1411

1.0 Time (min)

1 min - 0.02 min

1 min - 0.02 min

0.02 min - 0 min 0 4000

0.5

1677 1639 1531 1411

0.0

2925 2815 2582

10

3569 3452 3291

9

1639 1531 1411

1.0 Time (min)

2925 2815 2582

2

0.5

3452 3291

0.0

2925 2815 2582

0

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

1000 4000


1000

Figure 5. IR intensity profiles and IR absorbance spectra of (a) 0.8 µm, (b) 3.5 µm, and (c) 10 µm TEPA films during CO2/H2O adsorption for 10 min. The rising intensity profiles in Figure 5 and Figure S8 showed that the carbamate intensity profile at 1531 cm-1 continue to grow with increasing in water intensity and then levelled off. These profiles are in contrast to a sharp rise and the immediate level-off carbamate profile from the dry CO2 adsorption in Figure S8. These observations further confirm the role of adsorbed H2O in enhancing CO2 capture capacity. The rising intensity profiles in Figure 5, Figure S6 and Figure S8 are equivalent to the breakthrough curves, unraveling kinetics of CO2 adsorption. We further quantified the adsorption kinetics by the adsorption half time, t1/2, the time required for reaching 50% of the final intensity of the breakthrough curve.

The adsorption half times

obtained are listed in Table 1 which shows H2O vapor slowed down CO2 adsorption. We proposed that the adsorption of H2O followed two steps: (i) diffusion of adsorbed H2O into the TEPA films leading to a formation of a hydrogen bonding between H2O and -NH, giving a low


Page 19 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


wavenumber of -OH at 3452 cm-1, shown in Figure 5 and (ii) accumulation of H2O on the surface of TEPA films leading to the formation of H2O with a less extent of hydrogen bonding giving a high wavenumber of -OH at 3570 cm-1. Figure 6 compares IR spectra of adsorbed species during desorption from amine thin films. Reducing CO2 partial pressure by switching the flow from 15% CO2/H2O to Ar at 50 °C removed part of adsorbed species, which can be classified as weakly adsorbed species. The IR spectrum at Ar, 0 min – Ar, 3 min in Figure 6 (a) and (b), which is the difference spectrum between 0 min with 3 min during Ar purge, represented those species adsorbed at and near the surface of TEPA films. These species include carbamic acid (-NCOOH) at 1677 cm-1 and H2O at 3452 and 3569 cm-1. The H2O band, which has a higher wavenumber at 3569 cm-1, is a manifestation of a less extent of hydrogen bonding interactions than those exhibiting lower wavenumber at 3452 cm-1. The 0.8 µm film begin to desorb CO2 at 50 °C under flowing Ar by removing weakly adsorbed ammonium carbamate and H2O, as evidenced by Ar, 0 min – Ar, 3 min and Ar, 5 min – Ar, 10 min in Figure 6 (a).24, 62 Adsorbed CO2 on 3.5 µm and 10 µm TEPA films required a higher temperature to desorb than those on 0.8 µm TEPA film. Adsorbed H2O accumulated on the surface of 10 µm TEPA and the carbamic acid were desorbed at temperatures above to 65 °C. The desorption of hydronium and ammonium carbamate (i.e., strongly adsorbed CO2) on the 10 µm film began at temperatures above 70 °C and 90 °C, respectively. Although hydronium, ammonium, and water gave the stretching band in the same region, hydronium ion in Figure 6 (c) does exhibit a different contour from those of water in Figure 6 (a). These results showed that the binding energy of adsorbed species increased in the order: surface H2O < carbamic acid < hydronium carbamate< ammonium carbamate. Desorption of adsorbed species from TEPA/SiO2



in Figure S9 followed the same trend as TEPA films but gave low desorption temperatures, attributed to the low amine density on SiO2 support. (a) 0.8 um TEPA film 0 min3 min 5 min 10 min o 60 C Ar purge NCOO /153 1 cm -1

TPD

ads. H

o

100 C

2

ads. H

O/34 52

2 O/ 3 4 52 c -1 m

o

7 cm -1 120 C

0 min

10 min 70 oC Ar purge

o

TPD

90 C o 100 C

-

o

-1

NCOO /1531 cm

120 C

m 7c 67 -1 H/1 OO cm NC 452

NCOOH/167

o

90 C

(c) 10 um TEPA film

O/ 3 .H2

Norm. IR Int. (a.u.)

1

(b) 3.5 um TEPA film 0 min 3 min 5 min 10 mino 70 C TPD Ar purge o -1 90 C o NCOO /1531 cm 100 C

ads

cm -1

-1

gaseous CO2/2360 cm

-1

-1

gaseous CO2/2360 cm

gaseous CO2/2360 cm

-1

0

o

0.3

Ar,3 min-Ar,5 min

25

30

o

o

o

o

o

o

TPD,70 C-TPD,75 C

Ar,3 min-Ar,5 min Ar,0 min-Ar,3 min

Ar,0 min-Ar,3 min

20 Time (min)

TPD,90 C-TPD,95 C

o

TPD,88 C-TPD,90 C

Ar,5 min-Ar,10 min

15

1677 1639 1531 1411

30 10

2582

25

3569 3452 3291 3081

20 Time (min)

1677 1639 1569 1411

15

2582

30 10

3569 3480 3291 3081

25 1639 1531 1411

20 Time (min) 2582

15 3452 3291 3081

10

Absorbance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

TPD,65 C-TPD,70 C

0.0 4000


1000 4000


1000 4000


1000

Figure 6. Normalized IR intensity profiles and IR absorbance spectra of (a) 0.8 µm, (b) 3.5 µm, and (c) 10 µm TEPA films during Ar purging and TPD after exposure to CO2/H2O flow for 10 min. Time scale is Figure 6 is extended from Figure 5. At t=10 min, the flow is switched from CO2/H2O to Ar. Ar, 0 min and Ar, 3 min indicates that the sorbent had been under Ar flow for 0 and 3 min, respectively.


1677 NCOOH + 1639 NH3 1556 NHCOO1531 NCOO1411 NH2+

2163 NH3+

2582 NH2+/NH3+

3000 NH3+

double impregnate (11.3)

5 TEPA/SiO2 (9.2)

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3479 -OH 3363 NH2 3288 NH/NH2 3081 H3O+

Page 21 of 31

TEPA/SiO2 (5.3)

TEPA/SiO2 (2.4)

0 4000

3000

2000

1000

-1

Wavenumber (cm )

Figure 7. IR absorbance spectra of TEPA/SiO2 and double impregnated sorbents after exposure to 15% CO2 and 15% CO2/H2O flowing for 10 min. Insets are the difference spectra of sorbents obtained by ICO2/H2O – ICO2. Figure 7 compares the IR spectra of adsorbed CO2 as well as adsorbed CO2/H2O on amine sorbents. The difference spectra between adsorbed CO2 and adsorbed CO2/H2O in Figure 7 inset, showed adsorbed H2O enhanced the formation of carbamic acid (-NCOOH) and



Page 22 of 31

hydronium carbamate on secondary amine on sorbents with high amine loadings.

The

enhancement effect of H2O was not observed on low amine loading sorbents (i.e., TEPA/SiO2 (2.4) and TEPA/SiO2 (5.3)), which showed a similar CO2 capture capacity from dry and wet CO2 adsorption, listed in Table 1. The absence of H2O enhancement can be related to the extensive interactions between amine functional groups and hydroxyl groups on SiO2 support. Such an interaction, which is present prior to CO2/H2O co-adsorption, can be observed by the low intensity of the NH/NH2 bands at 3288 and 3363 cm-1 for the low loading amine sorbents in Figure 2 (b). Our previous results (Table S3) revealed that CO2 capture capacity of TEPA films can be enhanced by a wide range of concentrations of H2O vapor and liquid H2O.53

Table 1

summarizes the key performance data obtained from this study. Reducing the thickness of TEPA films and amine loading of TEPA/SiO2 sorbents increased amine efficiency and decreased desorption temperature (Tdes). The fast adsorption kinetics, represented by adsorption halftime (t1/2), can also be achieved by decreasing the thickness of TEPA films and amine loading of amine sorbents. Adsorbed H2O enhanced the CO2 capture capacity and amine efficiency on both TEPA films and high loading amine sorbents but slowed down CO2 adsorption kinetics.


Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. CO2 capture performance on amine films and sorbents

samples

0.8 µm TEPA

3.5 µm TEPA

10 µm TEPA

TEPA/SiO2 (2.4)

TEPA/SiO2 (5.3)

TEPA/SiO2 (9.2)

Double impregnate

CO2 capture capacity. (mmol of CO2/ g of sample)

amine efficiency (mol of CO2/ mol of N)

enhanc ement

CO2

6.13

0.23

84%

CO2/H2O vapor

11.28

0.44

CO2

5.68

0.22

CO2/H2O vapor

10.15

0.39

CO2

5.25

0.20

CO2/H2O vapor

9.00

0.34

CO2

0.86

0.36

CO2/H2O vapor

0.85

0.36

CO2

1.71

0.32

CO2/H2O vapor

1.73

0.32

CO2

2.86

0.31

CO2/H2O vapor

4.03

0.45

CO2

2.68

0.25

CO2/H2O vapor

3.25

0.29

gas steam

79%

71%

-1%

1%

41%

21%


t1/2 (min)

Tdes. (°C)

% of adsorbed CO2 100 °C

120 °C

0.07

105.3

35%

0

0.09

112.4

46%

0

0.08

111.2

52%

0

0.10

118.8

78%

26%

0.15

121.7

79%

48%

0.18

122.0

91%

64%

0.10

59.5

0

0

0.27

58.7

0

0

0.06

61.5

19%

0

0.07

74.0

29%

15%

0.03

99.3

56%

7%

0.05

104.6

60%

18%

0.14

110.5

66%

21%

0.17

116.7

75%

59%


CONCLUSIONS In situ IR study allows observation of adsorbed species evolved from CO2 and CO2/H2O adsorption/desorption on amine thin films and sorbents. Adsorbed CO2 on the surface layer of TEPA films (i.e., within 0.2 – 0.4 µm depth), characterized by low density amine sites, is in the form of zwitterions and weakly bounded ammonium – carbamate with a low IR intensity of hydrogen bonding. These species which can be removed at 50 °C in flowing Ar were classified as weakly adsorbed CO2. As adsorbed CO2 entered the bulk layer (i.e., 0.2 – 0.4 µm below the surface), strongly adsorbed CO2 in the form of ammonium carbamate and hydronium carbamate which exhibit an intense IR intensity of hydrogen bonding between ammonium ions and carbamate ions. The bulk layer is characterized by close proximity of neighboring amine sites, i.e., high density amine sites. Enhancement of H2O in amine efficiency can be attributed to the formation of hydronium carbamate and carbamic acid both of which have a 1:1 stoichiometry of CO2: -NH/NH2. The effect of amine film thickness and amine loading in sorbents on their CO2 capture capacity can be related to the density of amine sites which can be characterized by the IR intensity ratio of -NH/NH2 to -CH3. The density of amine sites and its neighboring sites have to be considered in the design of amine solvent and sorbents for CO2 capture.


Page 24 of 31

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ASSOCIATED CONTENT Supporting Information The supporting Information is available free of charge on the ACS Publications website. Further details about IR spectra of CO2/H2O adsorption on sorbents. AUTHOR INFORMATION Author Contributions †

J.Y. and †Y.Z. contributed equally to this work.

Corresponding Authors *[email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This work was supported by the U.S. Department of Energy under grants DE-FE0013127 and Ohio Coal Development Office under grants R-14-04 and R-15-08. We thank Mr. Tong Liu for his help in the drawing the 3D structures.



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