Water Enhancement in CO2 Capture by Amines: An Insight into CO2

Mar 1, 2018 - Water, a component in flue gas, plays a significant role in CO2 capture through a complex interaction between water molecules and adsorb...
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Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 4052−4062

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Water Enhancement in CO2 Capture by Amines: An Insight into CO2− H2O Interactions on Amine Films and Sorbents Jie Yu,† Yuxin Zhai,† and Steven S. C. Chuang* Department of Polymer Science, The University of Akron, 170 University Avenue, Akron, Ohio 44325, United States S Supporting Information *

ABSTRACT: Water, a component in flue gas, plays a significant role in CO2 capture through a complex interaction between water molecules and adsorbed CO2 on amine sorbents. To determine how the H2O−CO2−amine interactions affect amine efficiency and the 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. H2O 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 the 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 adsorbed species that exhibits 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 that 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.

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INTRODUCTION Amine sorbents and solvents have been extensively studied for thermal swing CO2 capture processes because of their potential for a significant reduction in the cost of CO2 capture for largescale coal-fired power plants. These CO2 capture processes have been developed and/or are 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 the 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 that undergo deprotonation to produce ammonium carbamate.5−8 Bicarbonate 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 Scheme 2: (i) ammonium carbamate, which is produced through pathways I-a and I-b, © 2018 American Chemical Society

giving an amine efficiency of 0.5 and (ii) carbamic acid from pathway II, giving an amine efficiency of 1.7,8,19−24 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. H2O vapor constitutes 4−10% of coal-derived flue gas.25 A fundamental issue that 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.7,8,20,26−28 These features result from the solventfree and/or -deficient environment. Amine functional groups are surrounded 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 that Received: Revised: Accepted: Published: 4052

December 11, 2017 February 20, 2018 March 1, 2018 March 1, 2018 DOI: 10.1021/acs.iecr.7b05114 Ind. Eng. Chem. Res. 2018, 57, 4052−4062

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Industrial & Engineering Chemistry Research Scheme 1. CO2/H2O Adsorption on Aqueous Amines

Scheme 2. Pathways for the Adsorption of CO2 and CO2/H2O on Amines

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 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 and doubleimpregnated 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 composed of tortuous structures that could introduce diffusion effects and interfere with 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

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 other Class sorbents.18,28,40−52 High-loading amines in Class I sorbent are closely packed, giving higher amine density than those in Class II grafted amine sorbent. 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 that 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 4053

DOI: 10.1021/acs.iecr.7b05114 Ind. Eng. Chem. Res. 2018, 57, 4052−4062

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Industrial & Engineering Chemistry Research Scheme 3. Illustration of Supported Amine Sorbents and Deposited Amine Thin Films

TEPA/SiO2 with amine loading of 2.4 mmol [N]/g [sorbent], where [N] represents amine function groups. The doubleimpregnated 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 detail.24 A typical thermal swing CO2 capture cycle, illustrated with IR results of the 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) temperature-programmed 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 three times under the same condition. The IR spectra of TEPA films and TEPA/SiO2 exhibited almost identical contours. Figure 1a presents the IR intensity profiles of gas CO2 (2360 cm−1) and adsorbed H2O (3452 cm−1) as well as the

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 SECTION Preparation of TEPA Thin Films. Tetraethylenepentamine (TEPA, Sigma-Aldrich, 98%) films were prepared by depositing different concentrations, 1.4, 3.5, and 7.1 wt %, of TEPA/ ethanol solutions on a stainless steel (SAE 304) disk and drying at 100 °C in the presence of Ar to form thin films. The thickness of TEPA films was determined to be 0.8, 3.5, and 10 μm for 1.4, 3.5, 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 a 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. TEPA/SiO2 (2.4) denoted the 4054

DOI: 10.1021/acs.iecr.7b05114 Ind. Eng. Chem. Res. 2018, 57, 4052−4062

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Industrial & Engineering Chemistry Research

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: IR difference spectra of the TEPA film after 10 min adsorption of CO2/ H2O and desorbed species during TPD.

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 the MS CO2 profile (m/e = 44) was converted to the number of moles of CO2 adsorbed by a calibration factor.28 Figure 1b 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 singlebeam intensity of the TEPA film at each step and I0 is the IR single-beam intensity of 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 structures.

Figure 2. IR absorbance spectra of (a) TEPA films with different thicknesses and (b) amine sorbents with subtraction of ambient air. 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.

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 the 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 the 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 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.

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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 and 3288 cm−1, respectively, and N−H deformation band at 1600 cm−1; the asymmetric and symmetric C−H stretching bands at 2923 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 2b, resulting from the formation of hydrogen 4055

DOI: 10.1021/acs.iecr.7b05114 Ind. Eng. Chem. Res. 2018, 57, 4052−4062

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Industrial & Engineering Chemistry Research

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.

hydronium ions have been shown to give peaks in the regions 3000−3200 and 2500−2900 cm−1 in Figure S4.61 The broadness of these bands can be attributed to hydrogen bonding interactions between hydronium ion. Figure 4a 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 4b. The first spectrum obtained at 0.01 min is shown in Figure 4c and exhibited positive bands in the 1200−1800 cm−1 range, which could be assigned to the symmetric and asymmetric vibration of CO and deformation of NH in the zwitterion. The ammonium carbamate emerged from 0.02 to 0.05 min with low intensity of the broad bands at 2163 and 2582 cm−1, and of those over 3000 cm−1, which are associated with hydrogen bonding between ammonium ions and carbamate ions. It should be noted that the adsorption of H2O lagged that of CO2, shown in Figure 4b. IR bands of adsorbed H2O emerged at 0.2 min in Figure 4c. 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 traveled at a slower rate than gaseous CO2 in the dry inlet line where H2O vapor underwent readsorption/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 0.30.2 in the inset of Figure 4c gave a band at 3081 cm−1, which can be assigned to hydronium ions. The appearance of hydronium ions was also accompanied by 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

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-amineloading 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 2b. 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 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 a 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 coadsorbed CO2/H2O decreased the intensity of the secondary amine (−NH) at 3288 cm−1 at least 2 times more than adsorbed H2O alone did. A number of our previous TPD studies 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 significantly 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 by an emergence of the broad band at 3081 cm−1, which can be assigned to hydronium ions. Clusters of 4056

DOI: 10.1021/acs.iecr.7b05114 Ind. Eng. Chem. Res. 2018, 57, 4052−4062

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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

We summarized the process of CO2/H2O coadsorption in pathway I in Scheme 2, which proceeds through zwitterion → ammonium carbamate → hydronium carbamate → carbamic acid−water. 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 contours of IR spectra of adsorbed species are nearly identical at 0.02 min - 0 and 0.1 min - 0 min

by CO2, was able to coordinate with hydronium carbamate during CO2/H2O coadsorption, resulting in an increase in the amine efficiency. The difference spectrum at 10-1 in Figure 4c 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 pathway similar to that of the 10 μm TEPA film. 4057

DOI: 10.1021/acs.iecr.7b05114 Ind. Eng. Chem. Res. 2018, 57, 4052−4062

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Industrial & Engineering Chemistry Research

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.

Table 1. CO2 Capture Performance on Amine Films and Sorbents % of adsorbed CO2 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

gas steam CO2 CO2/H2O CO2 CO2/H2O CO2 CO2/H2O CO2 CO2/H2O CO2 CO2/H2O CO2 CO2/H2O CO2 CO2/H2O

vapor vapor vapor vapor vapor vapor vapor

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

amine efficiency (mol of CO2/mol of N)

6.13 11.28 5.68 10.15 5.25 9.00 0.86 0.85 1.71 1.73 2.86 4.03 2.68 3.25

0.23 0.44 0.22 0.39 0.20 0.34 0.36 0.36 0.32 0.32 0.31 0.45 0.25 0.29

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 densities 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 on 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

% enhancement 84 79 71 −1 1 41 21

t1/2 (min)

Tdes (°C)

0.07 0.09 0.08 0.10 0.15 0.18 0.10 0.27 0.06 0.07 0.03 0.05 0.14 0.17

105.3 112.4 111.2 118.8 121.7 122.0 59.5 58.7 61.5 74.0 99.3 104.6 110.5 116.7

100 °C 120 °C 35 46 52 78 79 91 0 0 19 29 56 60 66 75

0 0 0 26 48 64 0 0 0 15 7 18 21 59

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. The rising intensity profiles in Figure 5 and Figure S8 showed that the carbamate intensity profile at 1531 cm−1 continues to grow with increasing in water intensity and then leveled 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. 4058

DOI: 10.1021/acs.iecr.7b05114 Ind. Eng. Chem. Res. 2018, 57, 4052−4062

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Industrial & Engineering Chemistry Research

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 indicate that the sorbent had been under Ar flow for 0 and 3 min, respectively.

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 and 90 °C, respectively. Although hydronium, ammonium, and water gave the stretching band in the same region, hydronium ion in Figure 6c does exhibit a different contour from those of water in Figure 6a. 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. 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 hydronium carbamate on secondary amine on sorbents with high amine loadings. The enhancement effect of H2O was not observed on low-amineloading sorbents (i.e., TEPA/SiO2 (2.4) and TEPA/SiO2 (5.3)), which showed similar CO2 capture capacities 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 coadsorption, can be observed by the low intensity of the NH/NH2 bands at 3288 and 3363 cm−1 for the low-amineloading sorbents in Figure 2b. 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

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 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 6a,b, which is the difference spectrum between 0 and 3 min during the 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 began 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 6a.24,62 Adsorbed CO2 on 3.5 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 was desorbed at 4059

DOI: 10.1021/acs.iecr.7b05114 Ind. Eng. Chem. Res. 2018, 57, 4052−4062

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Industrial & Engineering Chemistry Research

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 −CH2/CH3. The densities of amine sites and their neighboring sites have to be considered in the design of amine solvent and sorbents for CO2 capture.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b05114. Further details about IR spectra of CO2/H2O adsorption on sorbents, optical microscope images and surface roughness profies, and IR vs thickness, adsorbed CO2, and path length × concentration; tables of surface roughness data, thickness calibration data, and CO2 capture capacity and amine efficiency data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*S. S. C. Chuang. E-mail: [email protected]. ORCID

Steven S. C. Chuang: 0000-0002-6279-0584 Author Contributions †

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

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 drawing the 3D structures.

Figure 7. IR absorbance spectra of TEPA/SiO2 and doubleimpregnated sorbents after exposure to 15% CO2 and 15% CO2/ H2O flowing for 10 min. Insets: difference spectra of sorbents obtained by ICO2/H2O − ICO2.

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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 highamine-loading sorbents but slowed down CO2 adsorption kinetics.

REFERENCES

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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 lowdensity 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 was 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 4060

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