Control of CO2 Adsorption and Desorption Using Polyethylene Glycol

Article pubs.acs.org/JPCC

Control of CO2 Adsorption and Desorption Using Polyethylene Glycol in a Tetraethylenepentamine Thin Film: An In Situ ATR and Theoretical Study Duane D. Miller*,† and Steven S.C. Chuang*,‡ †

AECOM, 3610 Collins Ferry Road, Morgantown, West Virginia 26507-0880, United States First Energy Advanced Energy Research Center, Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, United States

‡

S Supporting Information *

ABSTRACT: The reversible adsorption of CO2 on tetraethylenepentamine (TEPA) and the polyethylene glycol (PEG)modified amine sites were investigated using attenuated total reflection infrared (ATR-IR) spectroscopy, mass spectrometry (MS), and density functional theory (DFT). The presence of PEG at the amine site increased the rate of formation of adsorbed CO2, enhanced the formation of weakly adsorbed CO2 which can be removed from flowing inert gas at room temperature, and decreased CO2 desorption peak (i.e., sorbent regeneration) temperature. The calculated CO2 binding energy (BE) and optimized structures suggest CO2 adsorbed on TEPA primarily in the form of ammonium carbamate. The presence of PEG promoted the formation of a species which exhibited an experimental IR spectrum resembling the simulated spectrum of a low BE zwitterion species. The observation suggests PEG controlled the formation of the adsorbed intermediate species. Modeling of the transient CO2 adsorption profiles further showed PEG accelerated the rate of diffusion of adsorbed CO2 species in TEPA film by decreasing viscosity. The IR spectra of TEPA-PEG revealed PEG could assist in breaking up hydrogen bonding between amine sites, further supporting the promotion of diffusion of adsorbed CO2 through decreasing TEPA viscosity. This study unravelled the mechanism of the effects of PEG on CO2 capture kinetics and capacity on amine sites. bonate species.8−12 Recently, ionic liquids (ILs) have been proposed as promising sorbents due to their unique properties, such as negligible vapor pressures, wide liquid ranges, high thermal stability, and chemical tunability,13−16 but suffer from high viscosity and high costs.17,18 It has been reported the N− H···O hydrogen bonding on CO2 interaction at the IL active site is reflected in the viscosity increase and transformation to a gelatinous substance which restricts mass transfer and limits the application for carbon capture.18,19 A novel approach to improve the CO2 capture in ILs was to incorporate an anion which has an effect on the intermolecular hydrogen bonding network and electron density at the adsorption site.20,21 Studies have shown it is possible to decrease the viscosity of ionic liquids using alcohol22 where the introduction of an O atom as a hydrogen acceptor into the amino-functionalized IL affects the CO2 reactivity and the viscosity during CO2 capture through intermolecular hydrogen bonding.23 It has also been shown that the addition of anions may improve the solubility of CO2 in ILs by adjusting hydrogen bonding.24 Hydrogen

1.0. INTRODUCTION The development of CO2 capture technologies from various emission sources remains a significant industrial challenge in light of the high energy demands and high capital costs that make use of these systems. The carbon capture process based on the aqueous amine is a proven mature technology for the removal of CO2.1 The solid amine-based sorbent has been considered as the preferred material having high absorption capacity, rapid kinetics, long-term regeneration capacity, and low regeneration energy requirement for the improvement of plant operation costs. Thus, the characterization of the amine active site on the molecular scale of the structure, strength, and reactivity of the adsorbing species is of great importance as it is related to the preservation and regeneration of the amine active site during multicycle CO2 removal processes. It has been reported that primary and secondary amines are capable of reacting with CO2 to produce thermally reversible alkylcarbamates, as both the weakly and strongly bound CO2.2 The interactions of CO2 with amines supported on solids have been studied by FTIR spectroscopy.3−7 It is generally accepted that the adsorption of CO2 on supported amines follows the same mechanism as the absorption in aqueous amine processes, which produces ammonium−carbamate ion pairs and bicar© XXXX American Chemical Society

Received: September 20, 2016 Revised: October 15, 2016

A

DOI: 10.1021/acs.jpcc.6b09506 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Experimental apparatus.

thin film was allowed to set stagnant at 50 °C for 30 min. The ZnSe window was heated to 100 °C at 10 °C/min under 100 sccm N2 flow, to evaporate the ethanol solvent, cleaning and drying of the thin film, and then cooled to 50 °C for the adsorption study. The ATR-IR trough volume was 2.89 cm3 at a total flow rate of 100 cm3/min resulting in a residence time of 0.028 min (1.734 s) for the gas flow. This is within the resolution of the FTIR data collection rate of 2.16 s. 2.2. Attenuated Total Reflection Infrared Spectroscopy. The infrared absorbance spectrum was collected using the experimental apparatus, shown in Figure 1, consisting of (i) Brooks Instrument 5850 mass flow controllers, (ii) a gas sampling system with a 4-port valve, (iii) an attenuated total reflectance accessory (ATR-IR, Harrick Scientific) flow cell with a custom reactor manifold mounted to the ATR-IR top plate placed inside of a Fourier transform infrared (FTS6700 FTIR, Thermo-Nicolet) spectrometer equipped with a liquid N2-cooled MCT detector, and (iv) a mass spectrometer (MS, Pfeiffer Omnistar). The 4-port valve allows switching the inlet flow from 100% N2 to 10% CO2 while maintaining a total flow rate of 100 cm3/min over the sorbent layer. Changes in the concentration of IR-observable complexes were monitored by the ATR-IR technique. The spectrometer was operated at a resolution of 4 cm−1, aperture set to automatic, and 32 coadded scans, and the infrared absorbance spectrum of absorbed and gaseous species was obtained by A = −log(Io/I),31 where Io is the background IR single beam spectrum of the TEPA and TEPA−PEG thin films and I is the IR single beam spectrum collected during the CO2 reaction studies. The MS responses corresponding to N2 (m/z = 28), O2 (m/z = 16, 32), and CO2 (m/z = 44) were monitored for the changes in the ATR-IR reactor effluent concentrations. 2.3. Theoretical Method. The carbon dioxide (CO2), TEPA, and TEPA−PEG structures, their complexes, and the calculated harmonic normal-mode frequencies were determined using density functional theory (DFT) in the Spartan 16 (Wave function Inc., USA) software package. DFT was utilized to study the most stable conformations for the amines and their complexes with CO2 (i.e., adsorbed CO2). The optimized molecular geometries and binding energies (BE) were calculated with Becke’s three-parameter exchange functional and gradient corrected functional of Lee, Yang, and Parr (B3LYP),32−34 using the 6-31G** and 6-31+G* basis sets. The calculated infrared normal vibrational frequencies were

bonding is known to play a major role in CO2 adsorption over amine-based sorbents in both the aqueous and solid amines.25,26 Motivated by these studies, polyethylene glycol (PEG) was blended with TEPA on the ZnSe ATR-IR window at various PEG loadings to study the effect of PEG on the amine active site and the hydrogen bonding network. The interesting issues to be addressed are how amine viscosity affects CO2 adsorption, i.e., the formation rate of adsorbed species and its adsorption capacity. TEPA was selected for this study because it has been widely used for immobilized amine sorbents.2,26−30 In this work, we found PEG was able to modify the amine active sites and the formation rates of the adsorbing carbamate species by decreasing the viscosity of the TEPA thin film. The amine−PEG interaction through the intermolecular hydrogen bond improved the rate of CO2 adsorption and increased the rate of adsorbed CO2 formation over the TEPA−PEG system as compared to the TEPA-only system. These results show that a fundamental understanding of the amine site interactions can help in guiding the preparation of future amine-based sorbents for the acid gas separation processes.

2.0. EXPERIMENTAL SECTION 2.1. Preparation and Characterization of the Sorbents. The gases used in this study were purchased from Praxair, N2 (99.999 vol %) and CO2 (99.999 vol %). A 0.0573 M tetraethylenepentamine (TEPA, Aldrich >30.0%) in ethanol (Pharmco-Aaper, 99.98%) solution was added dropwise to an ATR-IR ZnSe window to prepare a thin film for the CO2 reaction study. The thin film was allowed to sit stagnant for 30 min for evaporation of the ethanol solvent, followed by heating to 100 °C at 10 °C/min under 100% N2 flow at 100 sccm to evaporate the remaining solvent and cleaning the TEPA layer, and then cooled to 50 °C for the adsorption study. The single TEPA molecule (length: 15.716 Å, area: 271 Å2, DFT at B3LYP/6-31G**) was added to the ATR ZnSe window (area: 4.35 cm2), and assuming an even distribution of TEPA on the ZnSe window, the evaporated solution produced a 4 μm layer consisting of multiple layers of TEPA on the ZnSe surface. Five TEPA−PEG thin films were also prepared by adding polyethylene glycol (PEG, Aldrich, MW 200) to the TEPA solution at 9.1, 16.7, 23.0, 29.0, and 33.3 wt % PEG loading. The thin film was created by adding dropwise the TEPA− PEG/ethanol solution upon the ZnSe window. The resulting B


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The Journal of Physical Chemistry C determined at the B3LYP/6-31G** level35,36 and were not corrected with scaling factors prior to comparison with the experimental infrared spectra. The binding energies (BEs) for the CO2−TEPA complexes were obtained according to eq 1. BE = E[CO2 − TEPA] − E[CO2 ] − E[TEPA]

C t − Co 4 =1− π C∞ − Co

∞

∑ n=0

( −1)n exp[−D(2n + 1)2 π 2t 2n + 1

/4L2] × cos

(1)

(2n + 1)πz 2L

(5)

where D is the diffusion coefficient; L is the film thickness; and C∞ is the concentration at equilibrium. The Beer−Lambert law relates the concentration of an absorbing species to the IR intensity expressed in differential form dI = −εCIdz, where C is the concentration of the absorbing group and z is the position in the absorbing medium. The IR intensity is related to the absorbance via the expression dI = −IodA. Substitution of eq 5 into eq 4 and integration lead to the following expression in eq 6.30,31

where E[CO2−TEPA] is the total energy of the TEPA with the adsorbed CO2 and E[CO2] and E[TEPA] are the total energies of the CO2 and TEPA molecules, respectively. 2.4. Unsteady State Diffusion Model. CO2 absorption into a TEPA thin film using the ATR-IR technique is analogous to the one-dimensional molecular diffusion with reaction in a semi-infinite slab model (Scheme 1). At the surface of the Scheme 1. TEPA Thin-Film Model

A t − Ao 8γ =1− π[1 − exp(− 2γL)] A∞ − A o ⎡ ⎛ −D (2n + 1)2 π 2t ⎞ (2n + 1)π ⎤ 1 ⎟ ∞ ⎢ exp⎜ exp(− 2γL) + (− 1)n (2γ ) ⎥ 2L ⎝ ⎠ 4L2 ⎥ ∑ ⎢⎢ 2⎞ ⎥ ⎛ 2 (2n + 1)π n=0 ⎜ ⎟ (2n + 1) 4γ + ⎢ ⎥ 2L ⎝ ⎠ ⎣ ⎦

(

)

(6)

where At is the integrated IR absorbance of an absorbing group and A∞ is the intensity at equilibrium. The experimental ATRIR data as a function of time were regressed to At to determine the diffusion coefficient D for the CO2 species in the TEPA thin film. 2.5. Calculating Viscosity of the TEPA Thin Film. The diffusivity of a single particle through a stationary film (TEPA) was used to determine the viscosity of the film assuming the Stokes−Einstein model according to eq 7. The Stokes−Einstein model was used to determine the viscosity from the diffusivity value derived from the ATR-IR intensity signal, allowing us to study the effect of PEG on the viscosity of the TEPA thin film. It was assumed the diffusing CO2 molecule may be analogous to a rigid sphere diffusing through the 4 μm TEPA layer.

TEPA thin film, where z equals the thin-film thickness (L), the concentration of CO2 is constant at CAO, assuming that species A (CO2) reacts by a pseudo-first-order mechanism to form product C. The adsorption kinetics (A + B → C) may be described according to the continuity eq 2. ∂CA + (υ·∇CA) = DAB∇2 CA − RA (2) ∂t Since the TEPA thin film is a 4 μm thin film, contributions from the bulk flow term may be neglected (ν = 0). For this model, the assumption is that one CO2 molecule is freely diffusing while another is immobilized by an instantaneous and irreversible reaction. Therefore, the reaction product of CO2− amine, measured using ATR-IR spectroscopy, may be described by Fick’s second law where the reaction term (RA) may be ignored since the CO2 diffusivity is the limiting step in the adsorption kinetics. It is assumed that Fick’s second law adequately describes the unsteady state diffusion of CO2 into the TEPA thin film; the continuity eq 2 may then be reduced to eq 3.

∂CA ∂ 2C = D2 2A ∂t ∂z

D=

∫0

3.0. RESULTS AND DISCUSSION 3.1. Spectroscopic and Theoretical Characterization of the TEPA Thin Film. Figure 2 shows the simulated infrared spectra for TEPA and TEPA−PEG as compared to the experimental IR spectra during the CO2−TEPA and CO2− TEPA−PEG interaction. The infrared band assignments for the simulated and experimental data are listed in Table 1. The starting geometry for the simulated TEPA molecule was first optimized at the HF/6-31G** level prior to being optimized at the B3LYP/6-31G** level. The simulated infrared intensities, shown in Figure 2a, were normalized by setting the largest intensity value to 1.0. The simulated IR spectra at the DFT level are consistent with those of experimental spectra (Figure 2c): the w(N(2)−H) and w(N(3)−H) wagging vibrations contribute to IR intensity at 799 cm−1, w(N(1)−H), w(N(2)− H) wagging vibrations at 873 and 1503 cm−1, respectively, the δ(N(1)−H) deformation vibration at 1666 cm−1, and the ν(N(1)−CH2) vibration at 1174 cm−1, ν(C−H) vibrations at 2905, 3037, and 3090 cm−1, and the w(C−H) wagging

(3)

L

εC exp( −2γz)dz

(7)

where η is the dynamic viscosity; r is the radius of the spherical particle; kb is the Boltzmann’s constant (1.380658 × 10−23 J/ K); and T is temperature.

The relationship between absorbance and concentration may be described according to eq 4.37 ABS =

k bT 6πηr

(4)

where ε is the molar extinction coefficient; C is the concentration; γ is the evanescent wave decay coefficient; and z is the distance from the reflecting surface. The TEPA thin film has only one face exposed to the gas phase, and the other face is impermeable (i.e., no net mass transport through the ZnSe window). If the initial concentration of penetrant is zero, the concentration (C) at any position within the film is given by eq 5, which had been derived by others.37,38 C


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Table 1. Infrared Experimental and Theoretical Normal Modes of Vibration for the Optimized Structures at the B3LYP/6-31G** Levela wavenumber (cm−1) (theoretical) TEPA 1174 1061, 1323 799, 1503 873 1666 2905

3037 3090 3475, 3484 3569 3478 3481 TEPA-PEG 799 873 1167 1167 1503 1666 2882

Figure 2. ATR-IR infrared spectrum of pure TEPA thin film (a) simulated TEPA IR spectrum at the B3LYP/6-31G** level in vacuum, (b) simulated TEPA IR spectrum at the B3LYP/6-31G** level in propylamine, and (c) experimental ATR-IR absorbance spectrum at 50 °C. Background spectrum taken from the clean ZnSe ATR-IR window at 50 °C and (d) the simulated TEPA-PEG IR spectrum at the B3LYP/6-31G** level.

(ATR-IR)

description ν(N(1)−CH2) w(C−H) w(N(2)−H) w(N(1)−H) δ(N(1)−H) scissor ν(C(2)−H) ν(C(3)−H) ν(C(4)−H) ν(C(5)−H) ν(C(7)−H) ν(C(1)−H) νs(N(1)−H) νa(N(1)−H) ν(N(2)−H) ν(N(3)−H)

1100 1308 1454 1666, 1594 2806

2887 2936 3184 3365

w(N(2)−H) w(N(1)−H) ν(C−O) ν(N(1)−CH2) W(N(2)−H) δ(N(1)−H) scissor ν(C(4)−H)T ν(C(5)−H)T ν(C(3)−H)T ν(C(4)−H)P ν(C(1)−H)T ν(O−H)···NH2 ν(O−H)

980 1119 1119 1590 1672 2806

2928 3042 2863 3071 2936 3457 3180−3365 3841 3180−3500 Carbamate−N(1)H3+−TEPA 1120 1391 1300 1503 1477 1509 1666 1573 1802 2902 2559 2184 3417

vibrations at 1061 and 1323 cm−1, respectively. As illustrated in Figure 2a, the infrared C−H stretching region of the single TEPA molecule is composed of many overlapping bands at the 2905 cm−1. The IR bands due to CH2 groups at 2905 cm−1 are difficult to discern as a result of the overlapping ν(C(3)−H), ν(C(4)−H), and ν(C(5)−H) vibrational modes in the aliphatic backbone of TEPA. In addition, two IR bands at 3037 and 3090 cm−1 correspond to the ν(C(7)−H) and ν(C(1)−H) vibrational modes, respectively. Note that the ν(C(7)−H) is synonymous to the ν(C(2)−H) vibration considering the N3 location as a mode of symmetry. In the experimental data, there are three intense bands in the TEPA spectrum at 2806, 2887, and 2936 cm−1 which may be correlated to the simulated spectra for the ν(C(2)−H), ν(C(3)−H), ν(C(4)−H), and ν(C(5)−H), the ν(C(7)−H), and the ν(C(1)−H) vibrational modes, respectively. The addition of PEG to TEPA produced an IR band at 2863 cm−1 for the ν(C(4)−H)P vibration which is correlated to the simulated spectra at 3042 cm−1. In addition, the simulated spectra for the TEPA−PEG (Figure 2d) show the ν(O−H)··· NH2 IR peak at 3547 cm−1 indicating PEG hydrogen bonding at the amine active site. The experimental data support the theoretical results indicated by the increase in IR intensity and broadening of the IR band at 3180 cm−1 on increasing PEG loading.

a

w(N(1)−H···OOC) ν(N(1)−CO2) νs(OCO) w(N(2)−H) νa(OCO) ν(CO) ν(+N−H···OCO) ν(NH2+···COO−) ν(N(5)−H···NH2)

N1: primary amine; N2: secondary amine.

Comparison of the simulated spectra at the DFT/6-31G** level (Figure 2a) to the experimental spectra in Figure 2c further revealed the DFT method underestimated the ν(N(1)− H) IR intensities. The dropwise addition of TEPA on the ZnSe window, assuming the even distribution of TEPA, produced a 4 μm thin film consisting of multiple layers of TEPA on the ZnSe window. The DFT vacuum calculation does not adequately represent the frequency dispersion of the multilayered TEPA molecules. The presence of ν(N−H) vibrations in the experimental IR spectra may be due to the individual amine groups being affected by external versus internal hydrogen bonding of the neighboring layers of TEPA, which were absent in the single-molecule vacuum DFT calculation. The effect of the external multilayered TEPA may have a significant impact on the spectral shapes; therefore, in an attempt to study this D


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The Journal of Physical Chemistry C phenomenon, we modeled the multilayered TEPA using the Spartan nonaqueous SM8 solvation model39 using propylamine as the solvent molecule.39 The propylamine was chosen as the solvent molecule as its structure and behavior are similar to the TEPA molecule and was available in the Spartan 16 computational chemistry software database for solvation molecules. The strategy of solvation correction by the presence of an external electrostatic field due to the dipolar character of an amine solvent molecule has the effect of increasing the IR intensities for the normal modes corresponding to the νa(N(1)− H) vibration at 3507 cm−1 and the ν(N(3)−H) stretching vibration at 3441 cm−1, shown in Figure 2b. In addition, the presence of an external electrostatic field on the simulated amine group intensities produced a slight shift in wavenumbers for the calculated normal modes as compared to the vacuum calculation. These theoretical results are more consistent with the experimental ATR-IR observations and highlight the limitations of the single-molecule vacuum DFT infrared calculations and the complexity of the TEPA−TEPA network in the thin film. 3.2. IR and Theoretical Studies during CO2 Adsorption over the TEPA Thin Film. The ATR-IR spectra are an average of the IR spectra along the length of the ATR window and the penetration depth of the IR beam. The IR penetration depth is a function of the angle of incidence and the refractive indexes of the crystal and the sample.40 The Safety Data Sheet (SDS) for TEPA lists a refractive index for TEPA of 1.534 at 25 °C. The IR penetration depth was calculated at 1573 cm−1 to be approximately 3.5 μm; therefore, the IR beam penetrates 88% of the TEPA thin film during the CO2 adsorption study. Figure 3a shows the first 6 s (0.1 min) of the ATR-IR absorbance spectra during the CO2 adsorption over TEPA at 50 °C. The absorbance spectrum was obtained by subtracting the background spectra from the clean TEPA layers prior to CO2 exposure at 50 °C. The adsorption of CO2 in the TEPA thin film produced a prominent and broad IR band at 2559 cm−1. This band and its associated broadband may be assigned to the stabilized ammonium ion ν(NH3+···COO−), similar to what we have observed in our previous work during CO2 adsorption,41−43 during SO2 adsorption over aromatic amines,44 and during H2S adsorption over aliphatic amine.27 The broadness of the IR band is a manifestation of hydrogen bonding of the adsorbing species. The broadband is a combination of the N− H deformation and stretching vibrations for the NH3+ and for those produced at the secondary amines (NH2+). Studies have shown the IR bands at secondary ammonium ions produce lower wavenumber bands than those for the NH3+.45 The assignment for the ν(NH2+···COO−) vibration is to the broadband at 2184 cm−1.46 The increase in IR intensity at 1634 cm−1 for the δ(+N(1)−H) vibration is a result of the CO2 interaction at the ammonium ion site. The growth of the band was accompanied by the suppression of the C−H intensity at 2905 cm−1, where the decrease in IR intensity illustrates the effects both the change in geometry and formation of the ammonium ion have on the C−H chain.47 The decrease in relative intensities in the infrared spectrum for the ν(C−H) vibrations, shown in Figure 3a, could suggest a reorientation or disordering in the TEPA layer introduced by the formation of the ammonium carbamate species. CO2 adsorption also resulted in the increase of IR intensities at 1573 cm−1 for the νa(OCO) vibration, at 1477 cm−1 for the νs(OCO) vibration, and at 1300 cm−1 for the ν(N(1)−CO2) vibration, corresponding to the formation of the carbamate

Figure 3. ATR-IR absorbance spectrum: (a) during CO2 adsorption over TEPA at 50 °C, the background spectrum was taken prior to CO2 adsorption; (b) weakly adsorbed CO2 which was removed during flowing N2, and the background spectrum was taken prior to flowing N2, and (c) strongly adsorbed CO2 which was removed during temperature-programmed desorption at 50−100 °C, the background spectrum taken from the CO2-saturated TEPA layer.

species, consistent with previous studies.41 The IR intensities at 1138 and 1000 cm−1 indicate the skeletal ν(N(1)−CH2) vibrations for the N1 and N2 locations suggesting CO2 is interacting at these locations. The decrease in IR intensity at 920 and 845 cm−1 corresponds to the w(N1−H) and w(N5− H) wagging vibrations,48 respectively, indicating CO2 is interacting at the primary N1 and N5 amine locations. The differences in the IR bands and intensities for the w(N1−H) and w(N5−H) wagging vibrations clearly indicate the differences in the chemical environments surrounding the amine sites during the CO2 adsorption. The intramolecular proton transfer of an R-NH2+COO−-intermediate could produce NHCOOH (carbamic acid) which has been observed within 9 nm of a CO2−ionic liquid (NH2) film interface by XPS.49 At the top layers of the TEPA film, carbamic acid may be formed; however, the species was not observed at the ZnSe−TEPA interface, similar to a previous study using ATR-IR spectroscopy.46 Flowing nitrogen over the CO2-saturated TEPA layer (Figure 3b) at 50 °C resulted in a decrease in the IR intensity of the bands in the 1000−1600 cm−1 region, corresponding to the removal of a small fraction of weakly adsorbed CO2 (Table 2). This suggests a large amount of CO2 remained strongly bonded at the amine site. During the removal of the weakly adsorbed CO2, there was no decrease in IR intensity corresponding to the stabilized ammonium ion ν(NH3+···COO−) at 2559 cm−1 indicating the ammonium carbamate species was strongly E


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The Journal of Physical Chemistry C Table 2. Literature Comparison for CO2 Capacity and Amine Efficiency CO2 support

PEG loading (wt %)

CO2 Strong Ads. ZnSe ZnSe ZnSe ZnSe ZnSe ZnSe CO2 Weakly Ads. ZnSe ZnSe ZnSe ZnSe ZnSe ZnSe Literature SIO2 SiO2 SBA-15 SiO2 MSU-F beta zeolite SiO2 Al2O3 MCM-48 MCM-48 SBA-15 SBA-15 HMSd

amine type

PEG (mmol)

0 9.1 16.7 23 29 33.3

TEPA TEPA TEPA TEPA TEPA TEPA

0.00 1.15 2.29 3.44 4.58 5.73

× × × × ×

0 9.1 16.7 23 29 33.3

TEPA TEPA TEPA TEPA TEPA TEPA

0.00 1.15 2.29 3.44 4.58 5.73

× × × × ×

TEPA TEPA PEI PEI TEPA TEPA TEPA TEPA APTES APTES APTES aziridin APTS

6.27 27.84 1.78 0.4 × 10−03 -

amine (mmol)

exptl cond.

ads. cap. (mmol/gsorbent)

p* (atm)

ads. T (°C)

peak T (oC)

amine efficiency (CO2/amine)

10 10−04 10−04 10−04 10−04

1.09 1.09 1.09 1.09 1.09 1.09

× × × × × ×

10−03 10−03 10−03 10−03 10−03 10−03

1.37 1.54 1.73 1.56 0.80 0.75

0.15 0.15 0.15 0.15 0.15 0.15

50 50 50 50 50 50

100 97 85 75 75 72

0.26 0.29 0.32 0.29 0.14 0.14

10−04 10−04 10−04 10−04 10−04

1.09 1.09 1.09 1.09 1.09 1.09

× × × × × ×

10−03 10−03 10−03 10−03 10−03 10−03

0.05 0.12 0.21 0.84 0.92 0.94

0.15 0.15 0.15 0.15 0.15 0.15

50 50 50 50 50 50

50 50 50 50 50 50

0.01 0.02 0.04 0.15 0.17 0.17

2.436 445.14 0.79 1.53 5.39 0.82 0.40 0.07 2.05 1.14 1.53 5.55 1.59

1 0.22 0.04 1 1 0.25 0.25 0.25 1 0.05 0.1 0.1 0.9

25 55 30 45 40 30 30 30 25 25 25 25 20

100 115 110 45 225 104 135 125 90 90 110 115 150

0.08 0.93 0.14 0.08 0.12 0.16 0.05 0.89 0.50 0.56 0.57 0.69

−04

6.27 62.12 5.75 0.4 × 10−03 14.16 1.32 0.50 0.29 2.3 2.3 2.72 9.78 2.29

adsorbed. In addition, there was a slight increase in the ν(C− H) vibrations indicating the weakly adsorbed CO2 had a slight impact on the reorganization of the aliphatic backbone of the TEPA on its removal. Figure 3c shows that the thermal regeneration of the TEPA thin film resulted in a decrease in IR intensities for the asymmetric νa(OCO) vibration at 1573 cm−1, the symmetric νs(OCO) vibration at 1477 cm−1, the ν(N(1)−CO2) vibration at 1300 cm−1, the ν(N(1)−CH2) and ν(N(2)−CH2) at 1138 cm−1, and ν(N1−CH2) vibration at 1000 cm−1 corresponding to the removal of the strongly adsorbed CO2. Thermal regeneration also resulted in an increase in IR intensity at 845 and 920 cm−1 for the w(N(5)−H) and w(N(1)−H) wagging vibrations, respectively, corresponding to the removal of the adsorbed CO2 from the primary amine sites. In addition, there was a decrease in the broadband IR intensity centered at 2559 cm−1 corresponding to the removal of the ammonium carbamate ν(NH3+···COO−) species. The boiling point temperature for TEPA is reported in its SDS as 340 °C; therefore, the TEPA thin film is thermally stable at the regeneration temperature of 100 °C. It took approximately 2 min to reach more than 80% of saturated coverage. The thermal regeneration of the 4 μm TEPA thin film revealed 1.37 mmol/g of sorbent CO2 was adsorbed, resulting in an amine efficiency of 0.26. This is consistent with previous studies for CO2 capture over TEPA (Table 2). The amine efficiencies were not calculated as a function of depth. The efficiencies were determined following 5 min CO2 exposure time and thermal regeneration of the thin film. A survey of the literature revealed

ref

2 26 51 28 52 52 52 42 42 53 54 55

some TEPA-based sorbents produced higher CO2 absorption capacities than those found in this work. Studies have shown the CO2 adsorption capacity on amine-based sorbents is a function of amine loading and the surface area of the support material.50 In this work, TEPA was added to the ATR-IR ZnSe window having a low surface area (4.35 cm2) and produced a 4 μm thin film with an amine loading of 1.09 × 10−3 mmol TEPA which resulted in a lower amount of CO2 as compared to these studies. Plotting the ATR-IR transient response of the adsorbing species during the first 24 s of CO2 adsorption, shown in Figure 4, helped elucidate the molecular interactions of CO2 taking place at the amine site. Exposing the TEPA film to CO2 for 6 s resulted in a rapid increase in the intensity for the ν(N(1)− CO2) vibration at 1300 cm−1, followed by the ν(N(1)−CH2) and ν(N(2)−CH2) vibrations at 1138 cm−1. This sequence of the bond formation is illustrated in Scheme 2. The rise of IR intensities for the ν(N(1)−CH2) and ν(N(2)−CH2) vibrations for CO2 adsorbed at the amine site led to the growth of the intensity for the ν(NH3+···COO−) species at 2550 cm−1. The CO2 interaction mechanism based upon the IR intensity data suggests a reaction mechanism at the N1 and N2 locations as shown in Scheme 2, R1 and R2. In addition, the IR intensity data (Figure 4) reveal that between 6 and 10 s CO2 exposure time there was an increase in the rate of formation of the ν(NH3+···COO−) species, i.e., ammonium carbamate. The IR studies of CO2 adsorption and temperatureprogrammed desorption over the TEPA thin film (Figure 3) reveal the structures of both weakly and strongly bonded CO2, F


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stable structures were determined by varying the starting geometries in the computational study. The CO2 molecule was oriented at various locations around the TEPA, and the optimized structures show that CO2 interacts at the three amine sites N1, N2, and N3, relative to the respective starting geometry. The CO2 BEs at the amine site for the various structures were compared at the B3LPY/6-31G** and 631+G* levels and are reported in Table 3. The DFT study revealed a decrease in BE when switching from the 6-31G** to the 6-31+G* basis set, resulting from the charge transfer contribution to the BE decreasing as the size of the basis set increases.56 This phenomenon was also observed in our previous theoretical studies for SO2 adsorption over the aromatic amine44 and for H2S adsorption over the aliphatic amine.27 Increasing basis-set size does not always improve the accuracy of the DFT models. The 6-31+G* basis set was included to improve the accuracy for the hydrogen-bonded structures containing significant electron density at larger distances from the nuclei.57 Comparison of the BEs of the 631G** to the 6-31+G* basis set reveals the same general trends on the formation of the weakly and strongly adsorbed CO2 species. The strategy for studying the CO2 adsorption at the TEPA amine site involved the simulation of the CO2−TEPA structures for the unprotonated amine site and the CO2 NH3+−TEPA structures at the ammonium ion site. The

Figure 4. ATR-IR intensity data during the first 24 s of CO2 exposure over TEPA at 50 °C.

which were further studied at the B3LYP/6-31G** and B3LYP/6-31+G* levels. The starting geometries for the CO2−TEPA systems, shown in Scheme 2 and Scheme S1, were optimized at the HF/6-31G** level prior to proceeding to the B3LYP/6-31G** and B3LYP/6-31+G* levels. The most

Scheme 2. CO2 Interaction and Reaction Mechanisms over the TEPA Amine Active Sites Based upon the in Situ IR Data and the Optimized Structures at the B3LYP/6-31G** Level for CO2 Adsorption over TEPA

G


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electrostatic attraction between the oppositely charged ions could lead to stronger BE for the adsorbed CO2 as compared to the CO2 adsorption at the neutral amine site. The simulated ammonium carbamate (carbamate− N(1)H3+−TEPA2) structure (Scheme 2c) resulted in an increase in the BE to −15.48 kcal/mol at the N1 location. These theoretical results are in good agreement with other theoretical58 and experimental results showing CO2 adsorption BE between 70 and 100 kJ/mol.59,60 The simulated IR spectra for the optimized structures (Scheme 3) for CO2 adsorbed at the amine site are shown in Figure 5a−d. The theoretical vibrational modes for the TEPA, CO2−TEPA (N1), CO2 N(1)H3+−TEPA (N1), and carbamate−N(1)H3+−TEPA are summarized in Table 1. Comparison of the IR spectra for the optimized structures reveals that the CO2 adsorption did not significantly alter the simulated IR bands for N1 (Figure 5b) as opposed to the N2 and N3 amine sites (not shown). The simulated spectra for CO2 adsorption at the primary amine site (Scheme 2a and 2b, Figure 5b and 5c) are in agreement with the experimental IR intensities for adsorbed CO2 on TEPA shown in Figure 5e. The presence of the ammonium ion (Figure 5c and 5d), the stronger interactions, and the type of adsorption mode did not significantly alter the simulated IR bands of the amine adsorption sites. The simulated carbamate structure, however, produced the ν(+N(1)−H···−OCO) vibration at 2902 cm−1, assigned to the 2559 cm−1 broadband in the experimental data, indicating the amine reactivity for CO2 uptake, the ammonium ion formation, and CO2 hydrogen bonding interaction at the amine sites are consistent with what was observed based on the IR absorbance spectra of strongly adsorbed CO2 (Figure 3c) and intensity data shown in Figure 4. Previous simulation studies have shown that the amine interactions with the CO2 in 1-(3-aminopropyl)-3-n-butylimidazonium tetrafluoroborate IL were the source of the unusually high viscosity.19,61 The theoretical ν(+N(1)−H···−OCO) vibration at 2902 cm−1 (Figure 5d) and the broadband centered at 2559 cm−1 (Figure 5e) indicate the presence of a hydrogen bonding network that would facilitate the formation of an interconnected network of ammonium carbamate ion pairs. The hydrogen bonding near the surface may produce a higher viscosity than the bulk, affecting the diffusion of ion pairs. This may be indicated by the lead−lag relationship in Figure 4 which showed that the initial formation of the ν(NH3+···COO−) species at 2550 cm−1 lagged the emergence of the bands at ν(N(1)−CH2) and ν(N(2)−CH2) vibrations at 1138 cm−1 and ν(H2N−CH2) at 1000 cm−1. 3.3. IR and Theoretical Studies during CO2 Adsorption over the TEPA−PEG Thin Film. Figure 6a shows the first 6 s of the ATR-IR absorbance spectra during the CO2 adsorption over TEPA−PEG at 16.7 wt % loading PEG and 50 °C. The 16.7 wt % PEG loading on TEPA was chosen for the FTIR study because this loading resulted in the highest increase in CO2 adsorption capacity as compared to the other PEG loadings (Table 2). The absorbance spectrum was obtained by subtracting the background spectra from the clean TEPA−PEG layer prior to CO2 exposure at 50 °C. CO2 exposure to the TEPA−PEG thin films with PEG loading at 16.7 wt % (Figure 6a) produced similar IR intensities for the stabilized ammonium ion ν(NH 3 + ···COO − ) at 2559 cm −1 , the νa(OCO) vibration at 1573 cm−1, the νs(OCO) vibration at 1477 cm−1, and the ν(N(1)−CO2) vibration at 1300 cm−1. The assignment of these bands to adsorbed CO2 species will be further discussed in light of simulated spectra in Figure 9.

Table 3. Binding Energies Calculated at the B3LYP/631G** and 6-31+G* Level BE

BE

B3LYP/6-31G**

B3LYP/6-31+G*

(kcal/mol)

(kcal/mol)

CO2−TEPA (N1) CO2−TEPA (N2) CO2−TEPA (N3) CO2 N(1)H3+−TEPA CO2 N(2)H3+−TEPA CO2 N(3)H3+−TEPA carbamate−N(1)H3+−TEPA2 carbamate−N(2)H3+−TEPA2 carbamate−N(3)H3+−TEPA2

−3.79 −3.55 −3.50 −8.25 −6.93 −6.83 −15.48 −15.94 −10.94

−2.58 −1.84 −1.86 −6.80 −5.75 −5.78 −11.56 −11.86 −9.32

TEPA-PEG CO2−PEG CO2−TEPA−PEG (N1) CO2−TEPA−PEG (N2) CO2−TEPA−PEG (N3) CO2−OH−PEG−TEPA (N1) CO2−OH−PEG−TEPA (N2) CO2−OH−PEG−TEPA (N3) CO2 N(1)H3+−TEPA−PEG CO2 N(2)H3+−TEPA−PEG CO2 N(3)H3+−TEPA−PEG carbamate−N(1)H3+−TEPA2−PEG carbamate−N(2)H3+−TEPA2−PEG carbamate−N(3)H3+−TEPA2−PEG

−3.53 −4.39 −4.34 −4.16 −4.71 −4.06 −5.01 −10.45 −9.94 −5.13 −17.11 −22.93 −20.55

−1.68 −2.96 −2.39 −2.16 −3.60 −2.22 −3.58 −8.76 −8.07 −4.14 −12.74 −19.95 −18.47

structure TEPA

TEPA structures containing the ammonium ion (protonated amine site) are indicated by the label NH3+−TEPA. The optimized structures for CO2 adsorption at these amine active sites are summarized in Scheme 2a (CO2−TEPA species) and 2b (CO2 N(1)3+−TEPA species) for the weakly adsorbed CO2 and in 2c (carbamate−N(1)H3+−TEPA) for the strongly adsorbed CO2. The theoretical BE for the weakly adsorbed CO2 was −3.79 kcal/mol at the N1 location, −3.55 kcal/mol at the N2 location, and −3.50 kcal/mol at the N3 location. These theoretical results show a decreasing trend in the BE. We would expect secondary amines to be more basic due to the electrondonating effect of the methyl group to the cation. However, due to solvation effects and steric hindrance, there is a balancing effect which causes the N1, N2, and N3 amine sites to have similar basicities and corresponding similar CO2 BE, as shown in Table 3 where the BE varied by −0.29 kcal/mol. Each amine site is capable of capturing a CO2 molecule, which is consistent with the experimental IR results showing a small amount of weakly adsorbed CO2 (Figure 3b). The effect of the ammonium ion on the BE for the weakly adsorbed CO2 N(1)H3+−TEPA species resulted in an increase in BE to −8.25 kcal/mol for the N1 location as compared to −3.79 kcal/ mol for the unprotonated site. The increase in BE may be caused by the electrostatic interactions resulting from the induced partial positive charge on the nitrogen and the partial negative charge on the oxygen atom. Comparing the optimized structures in Scheme 2a and 2b revealed that the interaction of CO2 species at the ammonium ion site (+N−H)···OCO produced the intermolecular distances of 2.919 Å (N+···O) as compared to the unprotonated (N−H)···OCO distance of 3.393 Å indicating a stronger hydrogen bond was formed at the ammonium site. The hydrogen bonding coupled with the H


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Scheme 3. CO2 Interaction and Reaction (i.e., Adsorption) Mechanisms over TEPA−PEG Amine Active Sites Based upon the in Situ IR Data and the Optimized Structures at the B3LYP/6-31G** Level for CO2 Adsorption over TEPA-PEG

the removal of a weakly adsorbed CO2. The amount of weakly adsorbed CO2 was determined by MS analysis, as shown in Table 2, revealing that the increase in PEG loading resulted in an increase in the amount of weakly bound CO2. Comparison of the absorbance spectrum for the CO2−TEPA (Figure 3b) to the CO2−TEPA−PEG (Figure 6b) reveal a larger amount of CO2 was weakly adsorbed corresponding to a CO2 fraction of 0.24 (0.21 mmol/g of sorbent), which may be the result of the presence of the hydroxyl group in the TEPA layer, indicated by the decrease in IR intensity at 1425 cm−1 for the ν(HO···COO) vibration. This suggests PEG has the effect of increasing the amount of weakly bonded CO2 over the TEPA−PEG sorbent. In addition, the removal of the weakly bound CO2 produced an increase in the IR intensities for the ν(C−H) vibrations, as compared to the TEPA only (Figure 3b). These spectral features are different as compared to the TEPA only (Figure 3b). It is interesting to observe when increasing the PEG loading from 16.7 to 23 wt % there was a significant increase in the amount of the weakly adsorbed CO2 from 0.21 to 0.84 mmol CO2/g of sorbent. The increase in weakly bound CO2 also corresponded to an increase in strongly adsorbed CO2 for the 9.1 and 16.7 wt % loaded PEG and to a decrease in strongly adsorbed CO2 for the 23, 29, and 33.3 wt % loadings. The

The presence of PEG also resulted in the increase in IR intensity at 1425 cm−1 for the ν(HO···COO) vibration indicating CO2 interacting with the hydroxyl group of PEG. Studies have reported the oxygen atom of alcohols may become nucleophiles by removing the hydrogen with, for example, a basic amine. The resulting oxygen nucleophile may have an affinity to adsorb CO2.62−65 The appearance of these IR bands coupled with the IR intensity at 1425 cm−1 for the ν(HO··· COO) vibration reveals PEG may affect the CO2−amine interaction mechanism but did not modify the carbamate species indicated by the similar spectral features and broadband for the ammonium ion at 2559 cm−1, shown in Figure 6a. The addition of PEG to TEPA did not shift the ammonium ion ν(NH3+···COO−) band at 2559 cm−1, suggesting that the effect of both the change in geometry and formation of the ammonium ion was independent of PEG up to 16.7 wt % loading. Further increasing the PEG loading to 33.3 wt % loading however did broaden the spectral features for the ν(NH3+···COO−) ion band, shown in Figure S1, indicating PEG may also interfere with the formation of hydrogen bonding within the ammonium−carbamate network. Flowing nitrogen over the CO2-saturated TEPA−PEG layer (Figure 6b) resulted in a decrease in the IR intensity indicating I


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Figure 6. ATR-IR absorbance spectrum during (a) CO2 adsorption over TEPA−PEG (16.7%) sorbent at 50 °C, where the background spectrum was taken prior to CO2 adsorption, (b) flowing N2, where the background spectrum was taken prior to flowing N2, and (c) temperature-programmed desorption at 50−100 °C, where the background spectrum taken from the CO2 saturated TEPA-PEG layer.

by the weight of TEPA and PEG loaded onto the ZnSe window. Plotting the IR intensities of the adsorbing species during the first 24 s of CO2 exposure, shown in Figure 7, reveals the molecular interactions of CO2 taking place at the amine site in the presence of PEG. Comparison of the IR intensities for the CO2−TEPA system (Figure 4) to those shown in Figure 7a reveals PEG increased the rate of CO2 uptake at the amine active site. The rapid increase in IR intensity for the ν(N(1)− CO2) vibration at 1300 cm−1 corresponded to the improvement in CO2 capacity due to PEG. The IR intensity data indicated that the increase in CO2 capacity also effected the formation rate of the NH3+ ion shifting forward the intensity profile to within the first 6 s of CO2 exposure, where the crossover point for the ν(NH3+···COO−) species occurred at 5 s, as compared to 10 s for the TEPA-only system in Figure 4. The sequence of steps based upon the IR intensity data is shown in Scheme 3, TEPA−PEG IR interaction. The IR data suggest PEG plays a role in the adsorption capacity of the TEPA−PEG amine-based sorbent, the formation rate of NH3+, and the enhancement in the CO2 uptake rate. The amine efficiency of the TEPA compared to the 16 wt % PEG−TEPA was 26 and 32%, respectively. PEG produced a 20% increase in adsorption capacity for the thermally regenerated CO2. The lower efficiency and delay in the growth of the ν(NH3+···COO−) band for TEPA suggest TEPA restricted CO2 access to some of the available bulk amine

Figure 5. Simulated infrared spectrum for the optimized structures at the B3LYP/6-31G** level for (a) CO2−TEPA (N1), (b) TEPA, (c) CO2−TEPA−TEPA (N1), (d) CO2−NH3+−TEPA (N1), and (e) experimental IR absorbance spectrum during CO2 adsorption over TEPA at 50 °C. The background spectrum taken from the clean TEPA layer prior to CO2 exposure at 50 °C.

higher PEG loading further increases the ν(HO···COO) vibrational intensity in Figures S1(d), S1(e), and S1(f). The HO···COO interaction at high loadings resulted in a decrease in CO2 access to the bulk amine sites. The regeneration of the CO2-saturated TEPA−PEG layer was facilitated by heating the ATR-IR window to 100 °C (Figure 6c). Heating the thin film produced a greater decrease in IR intensities in the 1100−1600 cm−1 region as compared to the pure TEPA (Figure 3c), indicating the ability of PEG to improve the sorbent capacity and regeneration of the amine site, as shown in Table 2. MS analysis of the reactor effluent during thermal regeneration revealed that the positive effect of PEG on increasing the CO2 adsorption capacity is similar to what has been shown in other studies during CO 2 adsorption26,66,67 and during H2S adsorption27 using PEG blended with the aliphatic amine.27 Further increasing PEG loading beyond 16.7 wt % led to a decrease in the IR intensities of adsorbed CO2 and amine efficiency (Table 2). The amine− sorbent capacity (CO2/g of sorbent) was obtained by dividing the amount of CO2 in the reactor effluent during regeneration J


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Figure 7. ATR-IR intensity data during the first 24 s of CO2 exposure over TEPA−PEG at 50 °C.

sites. The adsorbed species are stabilized by hydrogen bonding with neighboring amines in the form of an ionic interconnected network68 which may have slowed CO2 diffusion into the bulk amine groups. The formation of the interconnected hydrogenbonded network could increase the viscosity of the TEPA layer or even form a gel at the TEPA surface, inhibiting CO2 adsorption, similar to findings for ILs.18−21 During the CO2 adsorption studies, the FTIR spectrometer was configured with a spectral resolution of 4 cm−1, aperture set to automatic, and 32 coadded scans, resulting in time resolution of 2.16 s. Comparison of the first absorbance spectrum collected in the FTIR spectrometer is shown in Figure 8a for TEPA and Figure 8b and 8c for the TEPA−PEG system with a PEG loading of 9.1 and 16.7 wt %, respectively. The IR bands for CO2 adsorption in the TEPA system did not appear until 4.38 s CO2 exposure, suggesting the presence of the weakly adsorbed CO2. These species are believed to be those below −5 kcal/mol which are less than half of the latent heat of water (−10.5 kcal/mol) and are difficult to remain bonded to the amine site at room temperature. The IR data indicate that CO2 interaction in the TEPA−PEG system was occurring at the N1 and N2 amine locations indicated by the IR intensities at 1573 cm−1 for νa(OCO) vibration, at 1477 cm−1 for νs(OCO) vibration, at 1300 cm−1 for ν(N(1)−CO2) vibration, at 1138 cm−1 for the ν(N(1)−CH2) and ν(N(2)−CH2) vibrations, and at 920 cm−1 for the w(N(1)−H) wagging vibration. In addition, the IR intensities at 1425, 1100, and 1058 cm−1 for the ν(HO··· COO), ν(HO−CH2), and w(O−H) vibrations, respectively, appear simultaneously, indicating these weakly adsorbed CO2 interacting in the hydrogen bonding TEPA−PEG network. These results are also consistent with the PEG-modified amine site shown in Figure 2c. The differences in the rate enhancement on addition of PEG might be explained by the presence of PEG and its effect on the hydrogen bonding network of the ion pairs associated with the neighboring primary and secondary amine groups. These interactions were studied further using DFT. The starting geometries for the CO2−TEPA−PEG systems, shown in Scheme 3 and Scheme S2, were optimized at the HF/631G** level prior to proceeding to the B3LYP/6-31G** and

Figure 8. ATR-IR absorbance spectrum at 0.036 min, during CO2 adsorption at 50 °C over (a) TEPA, (b) 9.1 wt % PEG/TEPA, and (c) 16.7 wt % PEG/TEPA sorbents. The background spectrum taken from the clean TEPA and TEPA−PEG layers prior to CO2 exposure at 50 °C.

B3LYP/6-31+G* levels, and the CO2 BEs are reported in Table 3. The weakly adsorbed CO2 species was assigned to the CO2− TEPA−PEG and CO2 N(1)H3+−TEPA−PEG zwitterionic species. The calculated BEs for the weakly adsorbed CO2− TEPA show a similar trend in decreasing BE as TEPA for the N1 location (−4.39 kcal/mol), the N2 (−4.34 kcal/mol), and N3 (−4.16 kcal/mol); however, PEG has the effect of increasing the BE at the amine site, as compared to the CO2−TEPA (−3.79 kcal/mol, N1) system. PEG had the effect of stabilizing the CO2 N(1)H3+−TEPA−PEG zwitterionic species indicated by an increase in BE for the N1 (−10.45 kcal/ mol) and the N2 (−9.94 kcal/mol) locations. The theoretical BE for the strongly adsorbed carbamate−N(1)H3+−TEPA− PEG species also showed PEG has the effect of increasing the BE at N1 (−17.11 kcal/mol), N2 (−22.93 kcal/mol), and N3 (−20.55 kcal/mol) locations. The stabilizing effect of PEG on the amine site is consistent with the qualitative trend in the calculated BE for adsorbed CO2. The DFT results show that the BE at the secondary amine was higher than for the primary amine. These theoretical results may indicate PEG influence on improving the BE and reactivity for CO2 at the amine site. The higher BE for these adsorbed CO2 at the TEPA−PEG amine site may also have influenced the rate of CO2 uptake during the first 6 s of exposure (Figure 8a) and equilibrium constant of the CO2−amine reactions (i.e., CO2 adsorption on amine sites). Figure 9 shows the simulated infrared spectra for the CO2− TEPA−PEG interactions, optimized at the B3LYP/6-31G** level and the experimental IR data for the 16.7 wt % PEG loading at 50 °C. The theoretical vibrational modes are summarized in Table 4. The optimized structures for CO2 adsorption at the amine site reveal that the weakly adsorbed K


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Table 4. Infrared Experimental and Theoretical Normal Modes of Vibration for the Optimized Structures at the B3LYP/6-31G** Levela wavenumber (cm−1) (theoretical) 2420 2896 2918 2957 2993 3489 3779 2911 2967 2996 3321 3510 1582 2454 2891 2993 3009 3063 3420 3462 3498C 1241 1498 1667 1744 2434 2925 2986 3357 3456 a

(experimental)

description

CO2−TEPA−PEG 2350 2806

ν(COC) ν(C(5)−H)T ν(C(6)−H)T ν(C(2)−H)P ν(C(8)−H)P ν(O−H)···N(1)H2 ν(O−H···OOC) CO2−OH−PEG−TEPA (N1) ν(C(2)−H)T ν(C(1)−H)P 2936 ν(C(1)−H)P ν(O−H···N(2)−H) ν(O−H···NH2) CO2 N(1)H3+−TEPA−PEG 1634 w(+N(1)−H) 2350 ν(COC) ν(N(1)−H)···OH 2806 ν(C(5)−H)P ν(C(2)−H)P 2806 ν(C(3)−H)P νs(+N(1)−H) ν(O−H)···NH2 νa(+N(1)−H) + carbamate−N(1)H3 −TEPA−PEG ν(+N−H···OOCN) 1477 νs(OCO) 1573 νa(OCO) ν(CO) 2559 ν(+N(1)−H···OCO) ν(C(2)−H)T ν(C(1)−H)P ν(O−H)···NH2 νs(O−H···OOC)

T = TEPA, P = PEG. N1: primary amine; N2: secondary amine.

H···OOC) at 3456 cm−1 (Figure 9e) resulted in a bond length d(O−H···O) of 2.72 Å.69 The presence of PEG in the hydrogen bond network produced weaker hydrogen bonds indicated by the longer bond lengths. The transfer of electron density due to the PEG hydrogen bonding resulted in a shift to lower wavenumber for the ν(NH3+···COO−) vibration at 2434 cm−1 (Figure 9e) on the formation of the carbamate species as compared to the TEPA only (Figure 5d). The PEG insertion into the bonding scheme comprised the new hydrogen-bonded network in the TEPA−PEG layer. These results are consistent with the experimental IR data during the first 6 s of CO2 exposure to the TEPA−PEG thin films (Figure 8b and 8c). These theoretical results support the experimentally observed ability of PEG to modify the adsorption capacity of the TEPA thin film by providing access to a larger number of bulk amine groups. The CO2 diffusion into the TEPA thin film was monitored using the IR intensity at 1573 cm−1 for the carbamate νa(OCO) vibration. The representative time response for the normalized integrated absorbance is shown in Figure 10a for the CO2− TEPA, revealing a rapid increase in absorbance within 0.5 min

Figure 9. Simulated infrared spectrum for the optimized structures at the B3LYP/6-31G** level for (a) CO2−TEPA−PEG (N1), (b) TEPA−PEG, (c) CO2−OH−PEG−TEPA (N1), (d) CO2 NH3+− TEPA−PEG (N1), and (e) carbamate−N(1)H3+−TEPA−PEG and (f) experimental IR absorbance spectrum during CO2 adsorption, desorption under N2 flow, and during TPD over TEPA−PEG at 50 °C. The IR spectrum for desorption and TPD were inverted for comparison to the adsorption spectrum.

CO2 (Figure 9b) produced the ν(O−H)···NH2 band at 3489 cm−1 indicating the ability of PEG to break up the TEPA− TEPA hydrogen bonding network in the TEPA thin film (Figure 2c). The simulated spectra also reveal PEG hydrogen bonding with CO2 produced the ν(O−H···OOC) at 3779 cm−1. These results suggest PEG ability to break up the hydrogen bonding network, through the primary (3462 cm−1) and secondary (3321 cm−1) amine O−H···N−H and O−H··· OOC interactions. In the TEPA system the ν(+N1−H··· OCO−) at 2902 cm−1 (Figure 5d) hydrogen bond length d(+N−H···O) was 2.68 Å, whereas the PEG interaction ν(O− L


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Figure 10. Comparison between experimental data points and curve generated by the equation that describes Fickian diffusion for (a) 4 μm TEPA, (b) various TEPA film thicknesses, (c) 4 μm TEPA + H2O, and (d) 4 μm TEPA with 9.1 and 16.7% PEG.

of PEG to TEPA at 9.1 and 16.7 wt % loading, shown in Figure 10d, resulted in a further increase in effective diffusivity and decrease in viscosity to 18.4 and 15.7 cP, respectively, shown in Table 5. The experimental density and molar volume of TEPA are 0.911 g/mL and 191.0 mL/mol, respectively, and for PEG they are 1.124 g/mL and 186.8 mL/mol. Thus, the addition of PEG to TEPA resulted in an increase in the density and molar volume of the blended TEPA−PEG. The high free volume in the TEPA−PEG system may be partially responsible for the decrease in viscosity of TEPA−PEG mixtures and possibly the increase in CO2 capacity as compared to the pure TEPA case. The effect of the decrease in viscosity on the TEPA thin film is consistent with the results of MS analysis of the reactor effluent during the temperature-programmed desorption (TPD) studies, shown in Figure 11. Adding PEG to TEPA increased the amount of CO2 adsorbed and decreased the thermal regeneration temperature of the sorbent. Based on these results,

followed by the slower increase until reaching a steady state at 3.0 min. The integrated absorbance data were modeled using the Fickian diffusion model, where the effective diffusivity was determined experimentally to be 3.97 × 10−07 cm2/s for the TEPA thin film, listed in Table 5. The dynamic viscosity, Table 5. Effective Diffusivity and Calculated Viscosity of TEPA, TEPA + H2O, and 9.1 and 16.7 wt % Loading PEG species TEPA TEPA + H2O 9.1% PEG 16.7% PEG Experimental TEPA viscosity, 20 °C

Deff (cm2/s) 3.97 5.01 5.55 6.49

× × × ×

calculated η (cP)

10−07 10−07 10−07 10−07

25.7 20.4 18.4 15.7 23.4 cP

according to the Stokes−Einstein equation, was determined to be 25.7 cP. The spectroscopic derived viscosity was in relatively good agreement with the experimentally determined viscosity for TEPA at 20 C of 23.4 cP. Variation in the thin-film thicknesses, 130.7, 261.4, 522.8, and 784.4 μm, altered the permeation rate of the CO2 indicated by longer time required to reach the steady state IR intensity, shown in Figure 10b, but did not alter the adsorption mechanism of the TEPA. The IR intensity data shown in Figure 10b overlay as a result of the normalization of the data. A H2O saturator was used to increase the relative humidity of the carrier gas at 20 °C, where the humidified carrier gas was allowed to flow over the TEPA thin film for 15 min in order to study the effect of added moisture on CO2 adsorption and diffusion. The H2O present in the TEPA thin film produced a more rapid increase in IR intensity (Figure 10c) and corresponding increase in the effective diffusivity to 5.01 × 10−07 cm2/s and decrease in viscosity to 20.4 cP. The addition

Figure 11. MS intensity profiles during CO2 regeneration at 100 °C over various PEG loadings on TEPA. M


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adsorbed CO2 with a low BE species. Comparison of simulated and experimental IR spectra suggests that this low BE adsorbed CO2 could be in the form of a zwitterionic species, (CO2 N(1)H3+−TEPA−PEG; CO2 N(2)H3+−TEPA−PEG). By varying the PEG loading in the TEPA layer, it was possible to modify the viscosity of the TEPA thin film and to adjust the BE of adsorbed CO2, controlling CO2 capture capacity and adsorption/desorption kinetics.

the peak desorption temperatures as a function of PEG loading are similar to studies in tunable basic ILs where the CO2 peak desorption temperatures varied between 182 and 263 °C.70 Hydrogen bonding between molecules and polymer chains is one of the factors governing viscosity.71 The interaction between PEG’s OH and TEPA’s NH3+ and NH2+ active sites led to the disruption of the hydrogen bonding network, thus decreasing the viscosity of the TEPA film. Molecular simulations reveal the viscosity is a function of the hydrogen bonding networks between the anion and cation species formed during the reaction of CO2 with the amine tailored IL. The strongest evidence on PEG breaking up the TEPA−TEPA hydrogen bonding network was demonstrated in the formation of the hydrogen bonding between TEPA and PEG indicated by the increase in IR intensity and broadening of the IR band for the νs(N(1)−H) vibration at 3180 cm−1 (Figure 2c). PEG increased the BE for adsorbed CO2 at 9.1 and 16.7 wt % PEG but decreased viscosity of TEPA by breaking up TEPA−TEPA hydrogen bonding as indicated by the formation of the ν(O−H)···NH2 band at 3457 cm−1, shown in Figure 2d. This enabled PEG to both improve the reaction rate of CO2 with TEPA and decrease the viscosity of the thin film, thereby modifying the peak desorption temperature. The CO2 MS profile for 16 wt % PEG loading (Figure 11) reveals a high fraction of adsorbed CO2 on the TEPA−PEG desorbed at lower temperature than those on TEPA. PEG could stabilize adsorbed CO2 in the form of CO2 N(1)H3+− TEPA−PEG (−10.45 kcal/mol) and CO2 N(2)H3+−TEPA− PEG (−9.94 kcal/mol). It is important to note that the CO2 N(1)H3+−TEPA−PEG and N(2)H3+−TEPA−PEG are zwitterionic species. This is evidenced by a higher IR intensity of the simulated bands in the 1100−1600 cm−1 region in Figure 9d, where further conversion of the CO2 to ammonium carbamate (carbamate−N(1)H3+−TEPA−PEG) led to a decrease in the IR intensities for these bands shown in Figure 9e. The experimental IR spectra in Figure 9f for adsorbed CO2 on TEPA−PEG show the IR characteristics of a zwitterion species which gave a high intensity ratio of the bands in the 1100−1600 cm−1 region to those bands in the 2200−3300 cm−1 region. Desorption of these CO2 species from TEPA− PEG at peak temperatures of 70−80 °C shown in Figure 11 strongly suggests the majority of these CO2 species are in the form of zwitterion [CO2 N(1)H3+−TEPA−PEG (−10.45 kcal/ mol) and CO2 N(2)H3+−TEPA−PEG (−9.94 kcal/mol)]. Increasing PEG concentration further increased the concentration of these zwitterion species, shifting regeneration temperature to lower temperature. In addition, these zwitterion species have a stoichiometry of CO2/amine of 1:1. Thus, raising PEG concentration also led to increases in amine efficiency. In contrast, adsorbed CO2 on TEPA with a desorption peak temperature of 100 °C was likely in the form of the carbamate− N(1)H 3 + −TEPA 2 (−15.48 kcal/mol) and carbamate− N(2)H3+−TEPA2 (−15.94 kcal/mol) which have a stoichiometry of CO2/amine of 1:2.

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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.jpcc.6b09506. Schemes S1 and S2 show the optimized structures at the B3LYP/6-31G** level for CO2 adsorption over TEPA and TEPA−PEG, respectively. Figure S1 shows the IR absorbance spectra, effect of PEG on the 4 μm TEPA thin film at 50 °C, and the PEG loading (a) 0 wt %, (b) 9.1 wt %, (c) 16.7 wt %, (d) 23.0 wt %, (e) 29.0 wt %, and (f) 33.3 wt % PEG/TEPA (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: (+1) 304-285-5292. Fax: (+1) 304-285-4403. E-mail: [email protected]. *Tel.: (+1) 330-972-6993. Fax: (+1) 330-972-5856. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the U.S. Department of Energy under Grant DE-FE0001780 and DE-FC2607NT43086.

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REFERENCES

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