In Situ Infrared Study of the Effect of Amine Density on the Nature of

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In Situ Infrared Study of the Effect of Amine Density on the Nature of Adsorbed CO2 on Amine-Functionalized Solid Sorbents Uma Tumuluri,† Mathew Isenberg,† Chung-Sung Tan,‡ and Steven S. C. Chuang*,§ †

Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, Ohio 44325-3906, United States Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 30013, P. R. China § Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, United States ‡

S Supporting Information *

ABSTRACT: In situ Fourier transform infrared spectroscopy was used to determine the nature of adsorbed CO2 on class I (amine-impregnated) and class II (amine-grafted) sorbents with different amine densities. Adsorbed CO2 on amine sorbents exists in the form of carbamate−ammonium ion pairs, carbamate−ammonium zwitterions, and carbamic acid. The adsorbed CO2 on high-amine density sorbents showed that the formation of ammonium ions correlates with the suppression of CH stretching intensities. An HCl probing technique was used to resolve the characteristic infrared bands of ammonium ions, clarifying that the band observed around 1498 cm−1 is a combination of the deformation vibration of ammonium ion (NH3+) at 1508 and 1469 cm−1 and the deformation vibration of NH in carbamate (NHCOO−) at 1480 cm−1. Carbamate and carbamic acid on sorbents with low amine density desorbed at a rate faster than those on sorbents with high amine density after switching the flow from CO2 to Ar at 55 °C. Evaluation of the desorption temperature profiles showed that the temperature required to achieve the maximal desorption of CO2 (Tmax. des) increases with amine density. The adsorbed CO2 on sorbents with high amine density is stabilized via hydrogen bonding interactions with adjacent amine sites. These sorbents require higher temperature to desorb CO2 than those with low amine density.

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desorption at 100−120 °C, could be a cost-effective process for removal of CO2 from emissions of coal-fired power plants becasue of the availability of steam for sorbent regeneration. The development of amine sorbents with high CO2 capture capacity has been the primary focus of many studies. Table 1 shows a summary of the physical characteristics and CO2 capture capacities of some sorbents reported in the literature. On the basis of the method of functionalizing the amine on the support, the sorbents are classified into (i) class I (amineimpregnated sorbents, which exhibit weak interations between the amine and the support),4−7 (ii) class II (amine-grafted sorbents, in which the amine molecule is covalentaly bonded to the support),8−12 and (iii) class III (hyperbrached aminosilica (HAS) materials, which are synthesized by in situ ring opening polymerization of aziridine on porous supports).13,14 Class I sorbents have been found to exhibit high capture capacities compared to those of class II and class III sorbents because of their high amine density but exhibit low amine efficiencies compared to those of grafted amines because of diffusion limitations.2,15,16 Class 1 sorbents were also reported to be less stable than class II and III sorbents because of the leaching of

INTRODUCTION The strong correlation between atmospheric CO2 concentration and global climate change has motivated intensive research for the development of cost-effective CO2 capture processes for the control of CO2 emissions from coal-fired power plants, the largest single CO2 source. The conventional liquid absorption process, which uses aqueous amines to remove CO2 from the gas streams in chemical industries, is expensive for the control of CO2 emissions from coal-fired plants because of its high energy requirements, equipment corrosion, and amine degradation. U.S. Department of Energy (DOE) studies have shown that the use of liquid absorption processes for CO2 capture would increase the cost of electricity (COE) of a coal-fired power plant by 80−85%.1 CO2 capture using amine sorbents is a promising technology for CO2 capture from point sources because of (i) its low energy requirements, (ii) the low toxicity of the materials, (iii) its high capture capacity, (iv) the flexibility of the process, and (v) the high stability of the sorbents.2 CO2 capture on the amine sorbents can be operated by the following processes: (1) thermal swing adsorption (TSA), (2) pressure swing adsorption (PSA) or vacumm swing adsorption (VSA), or (3) hybrid pressure and temperature swing adsorption (PTSA).3 Thermal swing adsorption using amine sorbents, which operates with CO2 adsorption at 40−55 °C and © 2014 American Chemical Society

Received: April 7, 2014 Revised: May 29, 2014 Published: June 4, 2014 7405

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Table 1. Summary of CO2 Capture Capacities and Amine Efficiencies of Amine Sorbents Reported in the Literaturea class

surface area of support (m2/g)

N content (wt %)

[CO2] (vol %)

Tads (°C)

Tdes (°C)

CO2 capture capacity (mmol/g)

amine efficiency (mol of CO2/mol of N)

DEA/PE-MCM PEI/SiO2 PEI/PE-SBA TEPA/PE-SBA PEI/SBA PEI20/SBA-15

I I I I I I

917 − 599 599 853 650

− − 13.2 14.1 5.99 6.25

5 0.042 100 100 100 100

25 25 45 45 0 25

75 85 110 110 100

2.36 1.75 3.14 3.72 1.34 0.76

0.37 0.15 0.33 0.37 0.31 0.21

TEPA35/SiO2

I

160

12.83

100

25

100

2.39

0.25

APTMS/SBA-ex AEAPS/SBA-ex TA/SBA- ex APTES/MCM TEPA/MSF APS/MCF MAPS/MCF APTES39/MCM

II II II II II II II II

750 750 750 1045 634 648 648 1012

25 25 25 30 75 25 25 25

100 100 100

− 3.75 2.41 2.51

10 10 10 99 15 0.04 0.04 100

100

1.07 0.99 1.19 1.2 1.5 1.00 0.25 0.47

0.44 0.29 0.25 0.54 − 0.4 0.20 0.26

APTES49/MCM

II

1012

3.97

100

25

100

0.75

0.27

II and I III III

428 840

15.3 14 9.8

10

45 25 25

110

4.88 3.75 3.10

0.45 0.37 0.44

sorbent

APTES-TEPA/PE-SBA azeridine/SBA azeridine/SBA

ref 4 5 6 6 7 this work this work 8 8 8 10 12 19 19 this work this work 11 13 14

a

Abbreviations: Tads, CO2 adsorption temperature; Tdes, CO2 desorption temperature; PE, pore-expanded; ex, ethanol-extracted; MSF, mesocellular silica foam; DEA, diethanolamine; PEI, polyethyleneimine; TEPA, tetraethylenepentamine; APTMS, 3-aminopropyltrimethoxysilane; AEAPS, N-[3(trimethoxysilyl)propyl]ethylenediamine; TA, (3-trimethoxysilylpropyl)diethlenetriamine; APTES, 3-aminopropyltriethoxysilane; MAPS, (Nmethylaminopropyl)trimethoxysilane. prepared by (i) mixing 6.4 g of silica with an ethanol solution containing 3.6 g of TEPA (tetraethylenepentamine, Sigma-Aldrich) and (ii) drying the mixture at 100 °C for 30 min. Class II Sorbents. APTES39/MCM and APTES49/MCM sorbents were prepared by the grafting method. Subscripts 39 and 49 denote the amine loading in weight percent, calculated by elemental analysis. APTES39/MCM sorbent was prepared by (i) dehydrating 10 g of calcined MCM 41 particles under vacuum at 150 °C for 12 h, (ii) dispersing the dehydrated MCM 41 particles in 50 mL of anhydrous toluene followed by stirring the mixture for 30 min at 25 °C, (iii) adding 10 mL of APTES (3-aminopropyltriethoxysilane, SigmaAldrich) to the mixture, (iv) heating the wet mixture at 100 °C for 16 h and recirculating the condensed vapors, and (v) filtering the suspended solid and washing it with 500 mL of anhydrous ethanol and drying the resultant product overnight in air. APTES49/MCM sorbent was prepared by (i) mixing 5 g of APTES39/MCM with 5 mL of APTES and 15 mL of toluene and (ii) heating the wet mixture to 100 °C for 16 h and recirculating the condensed vapors. The resultant product was filtered, washed with ethanol, and dried overnight. The preparation of the mesoporous silica supports is discussed in detail in the Supporting Information. CO2 Capture Capacity Measurement by the Weight Change Method. The CO2 capture capacity of the sorbents was measured by a weight change method that consisted of (i) pretreating 1 g of the sorbents by heating them at 100 °C for 7 min, (ii) flowing 100% CO2 over the sorbents at 25 °C for 10 min for CO2 adsorption, and (iii) desorbing the adsorbed CO2 by heating the samples at 100 °C for 10 min. The change in the weight of the sorbents measured before and after CO2 adsorption was used to calculate the CO2 capture capacity of the sorbents. In Situ CO2 Capture Studies. Figure S1 of the Supporting Information shows the experimental setup consisting of (i) a gas flow manifold that includes mass flow meters and controllers to monitor and control the flow of gases, a four-port valve for switching the inlet gases between Ar and a 15% CO2/air mixture, and a six-port valve for CO2 pulse calibration, (ii) a DRIFT (diffuse reflectance infrared

the amine during multiple adsorption and desorption cycles.14,17,18 The design of amine sorbents for TSA processes requires consideration of the adsorption and desorption kinetics, and of the strength of binding between CO2 and the amine sorbent. A low binding strength at the adsorption temperature could lead to low adsorption equilibrium constants causing the premature breakthrough of CO2. An excessively high binding strength at the desorption temperature could limit the regenerability of the sorbent for subsequent CO2 capture cycles. The objective of this study is to determine the nature of the interactions of CO2 with class I and class II sorbents having different amine densities. An in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technique was used to study the interaction of CO2 with the amine sorbents. We discuss the nature of adsorbed species, with experimental evidence that CO2 adsorbs in the form of carbamate− ammonium pairs and carbamic acid on these two classes of sorbents. The adsorbed species on sorbents with high amine density desorbed at temperatures higher than those of the sorbents with low amine density, revealing the strong dependence of the CO2 binding strength on the amine density of the sorbents.

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EXPERIMENTAL SECTION

Sorbent Preparation. Class I Sorbents. PEI20/SBA-15 and TEPA35/SiO2 sorbents were prepared by the wet impregnation method. Subscripts 20 and 35 denote the amine loading in weight percent, calculated by elemental analysis. PEI20/SBA-15 was prepared by (i) mixing 7 g of SBA-15 with an ethanol solution containing 6 g of PEI (polyethylenimine, 50%, Mw ∼ 750000, Sigma-Aldrich) and (ii) drying the resultant mixture at 100 °C for 30 min. High-molecular weight PEI was selected because of its low cost. TEPA36/SiO2 was 7406

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Table 2. Physical Characteristics of APTES39/MCM, APTES49/MCM, PEI20/SBA-15, and TEPA35/SiO2 Sorbents sorbent

N content (wt %)

amine density (no. of N atoms/nm2)

no. of amine layersa

%N coverageb

CO2 capture capacity (mmol/g)c

CO2 capture capacity (mmol/g)d

amine efficiency (mol of CO2/mol of N)c

APTES39/MCM APTES49/MCM PEI20/SBA-15 TEPA35/SiO2

2.51 3.97 6.25 12.83

1.07 1.33 4.14 34.50

0.32 0.40 0.15 2.50

66.67 87.00 28.00 99.92

0.47 0.75 0.76 2.39

0.06 0.07 0.08 1.57

0.26 0.27 0.21 0.25

a c

Calculated using the projected area of an amine molecule (Supporting Information). bEstimated from the percentage decrease in Si-OH. Calculated by the weight change method. dCalculated from MS TPD.

Figure 1. (a) Absorbance spectra, (b) optical microscope images, (c) SEM images, (d) EDS mapping of nitrogen (N), and (e) TEM images of APTES39/MCM, APTES49/MCM, PEI20/SBA, and TEPA35/SiO2 sorbents. Absorbance = log(1/ISB), where ISB is a single beam of the sorbents after pretreatment. Absorbance spectra of MCM, SBA-15, and SiO2 supports are included for comparison. Fourier transform, Spectra-Tech) cell filled with 50 mg of sorbent placed in a Nicolet 6700 Fourier transform infrared spectroscopy (FTIR) bench (IR), (iii) a Pfeiffer QMS 200 quadruple mass spectrometer (MS), and (iv) a Labview module, which monitors and controls the temperature of the DRIFT cell, the heating rate, and the position of the four-port and six-port valves continuously throughout the CO2 capture cycles. Before the CO2 capture cycles, the sorbent was pretreated by being heated to 130 °C at a rate of 10 °C/min for 10 min in the presence of Ar flowing at a rate of 150 cm3/min. CO2 calibration was subsequently performed by pulsing 1 and 3 cm3 of 100% CO2 through the DRIFTS cell at 130 °C via the six-port valve and then cooling the sample to 55

°C. A typical CO2 capture cycle was performed by (i) exposing the sorbent to Ar flowing at a rate of 150 cm3/min for 1 min, (ii) exposing the sorbent to 15% CO2 flowing at a rate of 150 cm3/min for 5 min for CO2 adsorption, (iii) purging the residual CO2 gas in the DRIFTS cell with Ar flowing at a rate of 150 cm3/min for 10 min, and (iv) regenerating the sorbent by performing temperature-programmed desorption (TPD), where the sorbent was heated to 130 °C at a rate of 10 °C/min in the presence of Ar, held at 130 °C for 5 min, and then cooled to 55 °C. The amount of CO2 desorbed during TPD was obtained by converting the area under the CO2 MS profile (m/e 44) during TPD into volume using the CO2 pulse calibration. The adsorbed CO2 that is removed from the surface during Ar purging is 7407

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considered weakly adsorbed CO2, and the adsorbed CO2 remaining on the sorbent surface is considered strongly adsorbed CO2 that can be removed only by TPD.20

The decrease in the NH stretching frequencies resulted from an increase in the extent of hydrogen bonding between amine molecules. The characteristic features of CH 2 asymmetric and symmetric stretching are observed at 2938 and 2865 cm−1, respectively. The symmetric stretching of CH2 next to the nitrogen atom in the sorbents with secondary or tertiary amines (TEPA35/SiO2 and PEI20/SBA-15) is observed at lower frequencies (2800 cm−1),25 and its intensity is higher than those of the sorbents with only a primary amine (APTES39/ MCM and APTES49/MCM). NH2 and CH2 deformation bands are observed at 1598 and 1456 cm−1 for all the sorbents,8,22,26 while the Si−CH2−R stretching vibration band at 1240 cm−1 is observed for the grafted sorbents.25 The absorbance peak at 1670 cm−1 for the TEPA35/SiO2 sorbent can be assigned to CO, suggesting oxidative degradation of this sorbent during drying. Our previous studies have shown that the sorbents with a high amine density are susceptible to oxidative degradation.27 Amine density also influences the hydrophilicity of the sorbents. Comparison of the absorbance spectra of the preadsorbed H2O and CO2 on the sorbents before pretreatment shown in Figure S4 of the Supporting Information shows that the intensity ratio of adsorbed H2O to adsorbed CO2 decreased as follows: APS39/MCM > APS49/MCM and PEI20/ SBA-15 > TEPA35/SiO2 (indicating that the sorbents with a higher surface OH density are more hydrophilic). It has been reported that the CH stretching intensity increases with the increase in the temperature due to the stretching PEI chains inside the pore channels of the support at high temperatures.28 Such a change in the CH intensity was not observed on the sorbents reported in this study (Figure S5 of the Supporting Information). It is expected that temperature has no effect on the NH and CH intensities of class II sorbents because the covalent bonding between the amine and the silica surface restricts the stretching of the APTES molecule at high temperatures. Although TEPA molecules are in the form of multilayers in the TEPA35/SiO2 sorbent, the low viscosity of TEPA compared with that of PEI indicates that these multilayers of TEPA molecules are mobile on the SiO2 surface and its CH stretching is not confined. The absence of such a change in CH intensity for PEI/SBA-15 appears to be due to its molecular weight being higher than those reported for PEI. SEM images and EDS mapping in Figure 2b show the

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RESULTS AND DISCUSSION Table 2 shows the physical characteristics of APTES39/MCM, APTES49/MCM, PEI20/SBA-15, and TEPA35/SiO2 sorbents. The amine density of the sorbents, defined as the number of N atoms per square nanometer, was calculated from the N content measured by a carbon hydrogen nitrogen (CHN) analyzer, and the surface area of the supports was measured by BET. The amine efficiency defined as the molar ratio of adsorbed CO2 to the amine functional groups varied from 0.21 to 0.26. The reactions between CO2 and amine functional groups on the surface of solid sorbents have been shown to resemble those in the liquid amine process. 2R1RNH + CO2 → R1R 2NH 2+ + R1R 2NCOO− (carbamate)

(1) +

−

2RNH 2 + CO2 → RNH3 + RNHCOO (carbamate) (2)

RNH 2 + CO2 → RNHCOOH (carbamic acid)

(3)

Characterization of Sorbents. Figure 1a shows the absorbance spectra of MCM, APTES39/MCM, APTES49/ MCM, SBA-15, PEI20 /SBA-15, SiO 2 , and TEPA 35 /SiO 2 sorbents after pretreatment. The absorbance spectra of silica supports exhibit the characteristic features of isolated silanol groups at 3743 cm−1 and the hydrogen-bonded OH groups in the region between 3700 and 3550 cm−1.21 The absorbance peak at 3743 cm−1 represents the silanol groups present only on the surface. The silanol clusters embedded in the pore walls could not be detected by DRIFTS. Characteristic bands for the Si−O−Si stretching vibration are observed at 1060 and 800 cm−1,21,22 and the overtone bands for the Si−O−Si lattice vibration are observed at 2000 and 1874 cm−1.23 The OH functional groups are present mainly on the surface of these silica materials, whereas the Si−O−Si groups are present primarily in the bulk. The intensity ratio (Si−OH/Si−O−Si) is expected to increase with the surface area of SiO2-based supports.21 The intensity ratio and surface area data, summarized in Table S3 of the Supporting Information, show the expected trend. The absence of the direct linear relationship could be due to the overlapping of the bands associated with Si−O−Si. The absorbance peak intensities of Si−OH and Si− O−Si vibrations decreased when the amines are grafted (APTES39/MCM and APTES49/MCM) or impregnated (PEI20/SBA-15 and TEPA35/SiO2) on the supports. The absorbance spectrum of the TEPA35/SiO2 sorbent does not exhibit the features of isolated silanol groups because of the presence of the multilayers of TEPA on the SiO2 surface. The absorbance spectra of the amine sorbents exhibit characteristic features of both the functionalized amine and the silica supports. The absorbance spectra of the amines sorbents exhibit the characteristic features of NH2 asymmetric and symmetric stretching at 3376 and 3309 cm−1, respectively. The order of NH stretching vibration frequencies is as follows: APTES39/MCM (3376 and 3309 cm−1) > APTES49/MCM (3365 and 3303 cm−1) > PEI20/SBA-15 (3361 and 3298 cm−1) > TEPA35/SiO2 (3359 and 3294 cm−1). Knöfel et al.22 and Hiyoshi et al.24 observed a similar trend of a decrease in the NH stretching frequencies with an increase in amine density.

Figure 2. Comparison of absorbance spectra of adsorbed CO2 before and after HCl pretreatment on the APTES49/MCM sorbent. 7408

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Figure 3. (a) MS intensity of CO2 (m/e 44) and H2O (m/e 18), (b) temperature profile, (c) absorbance spectra [A = log(1/ISB)], and (d) difference IR spectra (difference = Aaft.pret − A) of the sorbent during a typical CO2 capture cycle on the TPSENa sorbent.

CO2 include absorption peaks at 1700 cm−1 (carbamic acid, CO stretch) and 1560 cm−1 (carbamate, COO− asymmetric stretch).20,23,29,30,35 A small shoulder at 3220 cm−1 indicates the possibility of formation of zwitterions (NH2+COO−) during CO2 adsorption. The characteristic band at 1498 cm−1 of adsorbed CO2 is a combination of NH3+ deformation vibration at 1508 and 1469 cm−1 and NH deformation vibration in carbamate ion at 1480 cm−1. Danon et al. observed that the absorbance band formed at 1701 cm−1 during CO2 adsorption on densely loaded APTES/ SBA-15 (DAPS) sorbent shifted to higher frequencies (1714 cm−1) after evacuation of CO2, which they assigned to the surface-bound carbamate species instead of carbamic acid.29 The absorbance spectra during the Ar purge of the APTES39/ MCM sorbent shown in Figure 6, which has a low amine density (1.07 N atoms/nm2) compared to that of DAPS (2.77 N atoms/nm2, estimated), did not exhibit any shift in the 1700 cm−1 band during Ar purge, suggesting that the surface-bound carbamate species were not formed during CO2 adsorption. The difference could be attributed to the different nature of OH. The shoulder band at 1700 cm−1 in Figure 2 could be assigned to the carbamic acid for the sorbents on the isolated amine, which does not have neighboring amine to accept H+. This assignment is further confirmed in Figure 4, by a prominent 1702 cm−1 band on TEPA/SiO2 of which TEPA possesses a 2/3 ratio of primary to secondary amine. Note that the secondary amine on the TEPA molecule is more geometrically restricted than the primary amine in contact

heterogeneous nature of these sorbents of which the amine is not uniformly distributed on the support surface. Thus, the calculated number of amine layers in Table 2 serves only as reference for comparison. HCl Probing. Many infrared studies of the interaction of CO2 with amine sorbents suggest that the adsorbed CO2 species could exist in a number of different forms, including carbamate−ammonium pairs, carbamate−ammonium zwitterions, and carbamic acid.22,23,29−34 However, the overlapping IR bands of ammonium ions and carbamate in the 1200−1750 cm−1 region result in ambiguous band assignment. We have further clarified the band assignment in the 1200−1750 cm−1 region by an HCl probing approach in which adsorbed CO2 is exposed to gaseous HCl. Ammonium ions, -NH3+ and NH2+, are expected to form by displacing carbamic acid and carbamate, because HCl is stronger as an acid than CO2. HCl probing of APTES49/MCM, which possesses a single type of primary amine in contrast to TEPA and PEI, which possess both primary and secondary amine sites, should produce only positive -NH3+ bands and a negative carbamate band. Figure 2 shows the results of probing studies on strongly adsorbed CO2 (i.e., after purge) on APTES49/MCM. The positive bands in the difference spectrum that are assigned to NH3+ include the absorption peak at 3005 cm−1 (NH3+ asymmetric stretch), a broad absorption peak between 2800 and 1800 cm−1 (NH3+, hydrogen-bonded), and absorption peaks at 1610−1630 cm−1 (NH3+ deformation), 1508 cm−1 (NH3+), and 1469 cm−1 (NH3+). The negative bands that are assigned to adsorbed 7409

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with the neighboring amine functional groups. These observations are also consistent with the adsorption of CO2 on a docecylamine (DEC)/SiO2 sorbent with only primary amine and dipentylamine (DIP)/SiO2 sorbent with only secondary amine,20 showing that adsorbed CO2 on secondary amine exhibits a prominent carbamic acid band. In Situ CO2 Capture Cycle. Figure 3 shows a CO2 capture cycle on the TPSENa sorbent, which was selected to illustrate the following stages of the CO2 capture process used in this work: (i) pretreatment, (ii) CO2 adsorption, (iii) Ar purge, and (iv) temperature-programmed desorption (TPD). Pretreatment at 100 °C in the presence of Ar caused desorption of CO2 and partially removed preadsorbed H2O from the sorbent. A 15% CO2/air mixture flowing over the pretreated sorbent resulted in a temperature spike of 30 °C followed by CO2 breakthrough. A step switch from CO2 to Ar allowed the removal of residual CO2 in the DRIFT cell and weakly adsorbed CO2 species from the sorbent. The remaining strongly adsorbed CO2 was further removed by TPD. The desorbed CO2 exhibited as a broad peak of which the peak temperature serves as an index of the binding energy of adsorbed CO2 on the amine sorbents. The oscillations in the temperature profile at 120 °C in Figure 3b are due to the deviation of temperature from the set point in our Labview setup. Figure 3c shows the absorbance spectra taken (i) before pretreatment, (ii) after pretreatment, (iii) during CO 2 adsorption, (iv) after Ar purge, and (v) after TPD during a typical CO2 capture cycle. The absorbance (A) is calculated by log(1/ISB), where ISB is the single-beam spectrum in each stage (i.e., i−v). The changes that occurred on the surface of the sorbent during each stage can be discerned by the difference spectra shown in Figure 3d. The difference spectra were obtained by A − Aaft.pret, where A is the absorbance spectrum taken during CO2 adsorption, after purge, and after TPD and Aaft.pret is the absorbance spectrum after pretreatment. The IR spectra of adsorbed CO2 during CO2 adsorption and after Ar purge exhibit characteristic features of carbamic acid (1702 cm−1), carbamates (1560, 1408, and 1318 cm−1), and ammonium ions (3005, 1635, and 1498 cm−1). After TPD, a nearly flat line was obtained for the difference spectrum, indicating the complete removal of adsorbed CO2, i.e., complete regeneration of the sorbent. Figure 4 compares the absorbance spectra of adsorbed CO2 on these various class I and II sorbents. APTES39/MCM and APTES49/MCM possess exactly the same primary amine on the sorbent surface with different amine densities. A higher amine density of the class II sorbent, i.e., grafted amine, gave a higher intensity of ammonium ions and a lower intensity of CO in the carbamic acid. This observation further confirmed that a neighboring amine is needed for the formation of ammonium ions. The absorbance spectra of adsorbed CO2 on PEI/SBA-15 and TEPA/SiO2 sorbents, which have both primary and secondary amine sites, exhibited features similar to those of the adsorbed CO2 on APTES/MCM sorbents (primary amine sorbent) with different intensities. Despite PEI having a higher fraction of secondary amines than primary amines, its branched structure inhibits the accessibility of most secondary amines for CO2 adsorption as evidenced by the low intensity of carbamic acid at 1702 cm−1. PEI-based sorbents reported in the literature exhibited low CO2 capture capacity below 75 °C.31,36−38 The high viscosity reflects the rigid structure of the branched PEI.

Figure 4. Adsorbed CO2 on APTES39/MCM, APTES41/MCM, PEI20/ SBA-15, and TEPA35/SiO2 sorbents. Absorbance = log(I0/I), where I0 is the single-beam spectrum of the sorbent after pretreatment and I is the single-beam spectrum taken during CO2 adsorption and after Ar purge.

The low amine efficiency of PEI/SBA-15 (at 55 °C) in our study confirms that the major fraction of amine sites was inaccessible to adsorb gaseous CO2 because of the rigid structure of high-molecular weight PEI at low temperatures. A high adsorption temperature is expected to decrease the PEI’s viscosity, resulting in an increase in the accessibility of amine sites for CO2 capture. TEPA35/SiO2, whose amine density is 1 order of magnitude higher than those of APTES39/MCM, APTES49/MCM, and PEI20/SiO2, exhibited nearly equal IR intensities for ammonium ion bands between 2800 and 1800 cm−1 and at 3005 cm−1 as well as carbamates between 1750 and 1200 cm−1. Intense bands in the region between 2800 and 1800 cm−1 are a result of hydrogen bonding of NH, resembling those IR features of amino acids.25 Purging the sorbent with Ar led to a slight decrease in the intensity of adsorbed CO2 bands, which indicates the high CO2 binding strength of TEPA35/SiO2 compared to those of the other sorbents. An appreciable decrease in intensity of the carbamic acid at 1702 cm−1 further confirmed the assignment of this species on a secondary amine, which is known to be less basic than the primary amine. It is interesting to note that the absorbance spectra during CO2 adsorption on the sorbents with low amine density (APTES39/MCM, APTES49/MCM, and PEI20/SBA-15) exhibited an absorbance band at 3220 cm−1 that was assigned to RNH2+COO−,39 whereas the TEPA35/SiO2 sorbent, which has high amine density, did not exhibit this band. This observation suggests the possibility of zwitterion ion formation only on the sorbents with low amine densities. The intensities of ammonium ion (NH3+) at 3005 and 1635 cm−1, hydrogenbonded ammonium ions between 2800 and 1800 cm−1, and 7410

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NCOO skeletal vibrations at 1408 and 1318 cm−1 increase with amine density, suggesting the formation of carbamate− ammonium ion pairs during CO2 adsorption on the sorbents with high amine density. The adsorbed CO2 on high-amine density sorbents showed the increase in the extent of CH suppression with an increase in the extent of formation of ammonium ions. The absorbance spectra, shown in Figure 5a, show the decrease in the intensities of the adsorbed CO2 during Ar

Figure 6. Decay curves of adsorbed species during purge of APTES39/ MCM, APTES49/MCM, PEI20/SBA-15, and TEPA35/SiO2 sorbents.

sorbent. These results suggest that the support of amine sorbents has a minimal effect on the CO2 desorption kinetics compared to the effect of amine density. Figure 7 shows the MS intensity of the CO2 gas phase and the intensity profiles of adsorbed species during TPD for

Figure 5. (a) Absorbance spectra during Ar purge on the APTES39/ MCM sorbent. Absorbance = log(I0/I), where I0 is the single-beam spectrum of the sorbent after pretreatment and I is the single-beam spectrum taken after Ar purge. (b) Decay curves of the adsorbed species.

purge. Weakly adsorbed species are removed from the sorbent during Ar purge. The decay curves of CO2 and adsorbed species are plotted by normalizing the absorbance peak intensities of 2360 cm−1 (CO2), 1702 cm−1, and 1560 cm−1 (COO−) shown in Figure 5b. Figure 6 compares the decay curves of adsorbed CO2 during desorption from APTES39/MCM, APTES49/MCM, PEI20/ SBA-15, and TEPA35/SiO2 sorbents. The rate of removal of adsorbed species from sorbents during Ar purge decreased with an increase in amine density. The stability of adsorbed species on any amine sites will be increased by an adjacent amine site. A high slope of decay curves for the APTES39/MCM, APTES49/MCM, and PEI20/SBA-15 sorbents was observed because of the dispersed amine sites. The removal of adsorbed species at a fast rate from the APTES39/MCM, APTES49/MCM, and PEI20/SBA-15 sorbents could also be attributed to the ordered pore structure of the MCM and SBA-15 supports compared to that of the amorphous SiO2 support. This issue was further investigated by performing a systematic CO2 capture study with the PEI/ SiO2 sorbent with different amine densities. Figure S7 of the Supporting Information shows the decay curves of the adsorbed species on PEI/SiO2 sorbents with different amine densities. The adsorbed species desorbed at a higher rate from the lowamine density sorbent than from the high-amine density

Figure 7. MS intensity of CO2 gas phase (m/e 44), IR intensity profile of CO (1702 cm−1) and COO‑ (1560 cm−1) during TPD for APTES39/MCM, APTES49/MCM, PEI20/SBA-15, and TEPA35/SiO2 sorbents.

APTES 39 /MCM, APTES 49 /MCM, PEI 20 /SBA-15, and TEPA35/SiO2 sorbents. The adsorbed species remaining on the sorbents after Ar purge desorbed rapidly with the increase in temperature. The temperature required to desorb the maximal amount of CO2 (Tmax.des) increases with the increase in the amine density of the sorbents, which confirms that the 7411

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(4) Franchi, R. S.; Harlick, P. J. E.; Sayari, A. Applications of PoreExpanded Mesoporous Silica. 2. Development of a High-Capacity, Water-Tolerant Adsorbent for CO2. Ind. Eng. Chem. Res. 2005, 44, 8007−8013. (5) Goeppert, A.; Czaun, M.; May, R. B.; Prakash, G. K. S.; Olah, G. A.; Narayanan, S. R. Carbon Dioxide Capture from the Air Using a Polyamine Based Regenerable Solid Adsorbent. J. Am. Chem. Soc. 2011, 133, 20164−20167. (6) Olea, A.; Sanz-Pérez, E. S.; Arencibia, A.; Sanz, R.; Calleja, G. Amino-functionalized pore-expanded SBA-15 for CO2 adsorption. Adsorption 2013, 19, 589−600. (7) Yan, X.; Komarneni, S.; Yan, Z. CO2 adsorption on Santa Barbara Amorphous-15 (SBA-15) and amine-modified Santa Barbara Amorphous-15 (SBA-15) with and without controlled microporosity. J. Colloid Interface Sci. 2013, 390, 217−224. (8) Li, Y.; Sun, N.; Li, L.; Zhao, N.; Xiao, F.; Wei, W.; Sun, Y.; Huang, W. Grafting of Amines on Ethanol-Extracted SBA-15 for CO2 Adsorption. Materials 2013, 6, 981−999. (9) Bollini, P.; Brunelli, N. A.; Didas, S. A.; Jones, C. W. Dynamics of CO2 Adsorption on Amine Adsorbents. 2. Insights Into Adsorbent Design. Ind. Eng. Chem. Res. 2012, 51, 15153−15162. (10) Loganathan, S.; Tikmani, M.; Ghoshal, A. K. Novel PoreExpanded MCM-41 for CO2 Capture: Synthesis and Characterization. Langmuir 2013, 29, 3491−3499. (11) Sanz, R.; Calleja, G.; Arencibia, A.; Sanz-Perez, E. S. Development of high efficiency adsorbents for CO2 capture based on a double-functionalization method of grafting and impregnation. J. Mater. Chem. A 2013, 1, 1956−1962. (12) Liu, S.-H.; Hsiao, W.-C.; Chiang, C.-C. Synthesis of Stable Tetraethylenepentamine-Functionalized Mesocellular Silica Foams for CO2 Adsorption. J. Chem. 2013, 2013, 6. (13) Choi, S.; Drese, J. H.; Eisenberger, P. M.; Jones, C. W. Application of Amine-Tethered Solid Sorbents for Direct CO2 Capture from the Ambient Air. Environ. Sci. Technol. 2011, 45, 2420−2427. (14) Hicks, J. C.; Drese, J. H.; Fauth, D. J.; Gray, M. L.; Qi, G.; Jones, C. W. Designing Adsorbents for CO2 Capture from Flue Gas: Hyperbranched Aminosilicas Capable of Capturing CO2 Reversibly. J. Am. Chem. Soc. 2008, 130, 2902−2903. (15) Builes, S.; Vega, L. F. Effect of Immobilized Amines on the Sorption Properties of Solid Materials: Impregnation versus Grafting. Langmuir 2013, 29, 199−206. (16) Wilfong, W. C.; Chuang, S. S. C. Probing the Adsorption/ Desorption of CO2 on Amine Sorbents by Transient Infrared Studies of Adsorbed CO2 and C6H6. Ind. Eng. Chem. Res. 2014, 53, 4224− 4231. (17) Choi, S.; Gray, M. L.; Jones, C. W. Amine-Tethered Solid Adsorbents Coupling High Adsorption Capacity and Regenerability for CO2 Capture from Ambient Air. ChemSusChem 2011, 4, 628−635. (18) Goeppert, A.; Meth, S.; Prakash, G. K. S.; Olah, G. A. Nanostructured silica as a support for regenerable high-capacity organoamine-based CO2 sorbents. Energy Environ. Sci. 2010, 3, 1949− 1960. (19) Didas, S. A.; Kulkarni, A. R.; Sholl, D. S.; Jones, C. W. Role of Amine Structure on Carbon Dioxide Adsorption from Ultradilute Gas Streams such as Ambient Air. ChemSusChem 2012, 5, 2058−2064. (20) Srikanth, C. S.; Chuang, S. S. C. Infrared Study of Strongly and Weakly Adsorbed CO2 on Fresh and Oxidatively Degraded Amine Sorbents. J. Phys. Chem. C 2013, 117, 9196−9205. (21) Silva, M. E.; Chakravartula, S. S.; Chuang, S. C. Silica-Supported Amine Catalysts for Carbon−Carbon Addition Reactions. Top. Catal. 2012, 55, 580−586. (22) Knöfel, C.; Martin, C. L.; Hornebecq, V.; Llewellyn, P. L. Study of Carbon Dioxide Adsorption on Mesoporous AminopropylsilaneFunctionalized Silica and Titania Combining Microcalorimetry and in Situ Infrared Spectroscopy. J. Phys. Chem. C 2009, 113, 21726−21734. (23) Hiyoshi, N.; Yogo, K.; Yashima, T. Adsorption characteristics of carbon dioxide on organically functionalized SBA-15. Microporous Mesoporous Mater. 2005, 84, 357−365.

adsorbed species in the high-amine density sorbents are stabilized via hydrogen bonding and require a high temperature to desorb the CO2.

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CONCLUSIONS The effect of amine density on the nature of adsorbed CO2 on class I and class II amine sorbents was investigated by in situ FTIR spectroscopic analysis of the adsorption and desorption of CO2 on sorbents having different amine densities. Adsorbed CO2 on both class I and class II amine sorbents exists in the form of carbamate−ammonium ion pairs, carbamate−ammonium zwitterions, and carbamic acid regardless of the amine density of the sorbent. CO2 capture studies revealed that the CO2 adsorbs mainly (i) on primary amine sites of class I amine sorbents with a low amine density (i.e., PEI20/SBA-15) and (ii) on secondary amine sites of class I amine sorbents with a high amine density (i.e., TEPA35/SiO2). Evaluation of the decay curves of adsorbed CO2 on class I and class II amine sorbents showed that the adsorbed CO2 binds weakly and desorbs at a higher rate on sorbents with a low amine density than the adsorbed CO2 sorbents with a high amine density. The highamine density sorbents require higher temperatures to desorb CO2 than those with a low amine density, because of the nature of adsorbed CO2 on sorbents with a high amine density that is stabilized via hydrogen bonding interactions with adjacent amine sites.

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ASSOCIATED CONTENT

S Supporting Information *

Preparation of mesoporous silica supports, calculation of the numbers of amine layers, IR absorbance spectra of sorbents before pretreatment, and comparison of adsorbed CO2 on mono primary and mono secondary amine sorbents. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: (330) 972-6993. Fax: (330) 972-5290. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Mr. Worasaung Klinthong and Dr. Chih-Hung Huang for the MCM-based adsorbents, Mr. Ernesto Silva Mojica and Mr. Walter Christopher Wilfong for their valuable inputs, Miss Yuxin Zhai for her help in obtaining SEM and TEM images, and Mr. Mahesh Dawadi for his help in Gaussian simulations. This work is supported by U.S. Department of Energy Grant DE-FE0001780 and ROC National Science Council Grant NSC102-3113-P-007-007.

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