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Spectroscopic Investigation of the Mechanisms Responsible for the Superior Stability of Hybrid Class 1/Class 2 CO2 Sorbents: A New Class 4 Category Walter Christopher Wilfong,*,†,‡ Brian W. Kail,§ Christopher W. Jones,# Carlos Pacheco,∥ and McMahan L. Gray*,† †

U.S. Department of Energy, National Energy Technology Laboratory, 626 Cochrans Mill Road, Pittsburgh, Pennsylvania 15236, United States ‡ Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831, United States § AECOM, 626 Cochrans Mill Road, Pittsburgh, Pennsylvania 15236, United States ∥ Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802, United States # School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332, United States S Supporting Information *

ABSTRACT: Hybrid Class 1/Class 2 supported amine CO2 sorbents demonstrate superior performance under practical steam conditions, yet their amine immobilization and stabilization mechanisms are unclear. Uncovering the interactions responsible for the sorbents’ robust features is critical for further improvements and can facilitate practical applications. We employ solid state 29Si CP-MAS and 2-D FSLG 1H−13C CP HETCOR NMR spectroscopies to probe the overall molecular interactions of aminosilane/silica, polyamine [poly(ethylenimine), PEI]/silica, and hybrid aminosilane/PEI/silica sorbents. A unique, sequential impregnation sorbent preparation method is executed in a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) setup to decouple amine binding mechanisms at the amine−silica interface from those within bulk amine layers. These mechanisms are correlated with each sorbents’ resistance to accelerated liquid H2O and TGA steam treatments (H2O stability) and to oxidative degradation (thermal stability). High percentages of CO2 capture retained (PCR) and organic content retained (OCR) values after H2O testing of N-(3-(trimethoxysilyl)propyl)ethylenediamine (TMPED)/PEI and (3-aminopropyl)trimethoxysilane (APTMS)/PEI hybrid sorbents are associated with a synergistic stabilizing effect of the amine species observed during oxidative degradation (thermal gravimetric analysis-differential scanning calorimetry, TGA-DSC). Solid state NMR spectroscopy reveals that the synergistic effect of the TMPED/PEI mixture is manifested by the formation of hydrogen-bonded PEI−NH2···NH2−TMPED and PEI−NH2···HO−Si/O−Si−O (TMPED, T2) linkages within the sorbent. DRIFTS further determines that PEI enhances the grafting of TMPED to silica and that PEI is dispersed among a stable network of polymerized TMPED in the bulk, utilizing H-bonded linkages. These findings provide the scientific basis for establishing a Class 4 category for aminosilane/polyamine/silica hybrid sorbents. KEYWORDS: CO2 capture, hybrid, Class 4, amine sorbent, infrared spectroscopy, NMR

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(BIAS) formulations4−6 to enhance their stabilities and CO2 capacities and sorbent gas contactor designs7−17 make the overall BIAS process a prime candidate for reducing the CO2 emissions. BIAS sorbents are primarily supported by different amorphous or crystalline silica grades due to their high surface area and pore volume, as well as surface Si−OH (silanol)

INTRODUCTION Despite shifts in government policies toward the use of renewable or cleaner energy sources,1 experts predict that coal combustion will continue to play an integral role in fulfilling the global energy demand for decades to come.2 As a result, the continuing need for postcombustion CO2 capture technologies3 will make postcombustion CO2 capture a vital research area in parallel with the pursuit for cleaner/renewable energy sources to cooperatively mitigate climate change. Technological advancements in both basic immobilized amine/silica sorbent © XXXX American Chemical Society

Received: February 18, 2016 Accepted: May 4, 2016

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DOI: 10.1021/acsami.6b02062 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces groups that serve as binding sites for the amines. These sorbents have been organized into three classes (1−3),18 according to their preparation procedure and amine-to-support molecular binding mechanisms. Class 1 sorbents are generally prepared by dry or wet impregnation of a silica support with a polyamine/hydrophilic solvent (methanol, ethanol, etc.) mixture. Principal polyamines employed are tetraethylenepentamine (TEPA)19−23 and poly(ethylenimine) (PEI),24−28 which possess different ratios of −NH2 (primary)/−NH (secondary)/−N (tertiary) amine groups that can potentially adsorb CO2. Infrared spectroscopy (IR) studies of Class 1 TEPA/silica and PEI/silica sorbents indicated that these amine species were immobilized to the silica surface through Si−OH···−NH 2 /−NH hydrogen bonds.29−31 Additional IR studies revealed that ethylenediamine32 (ED) and propylamine33 (PA), both possessing a single type of −NH2 group, adsorbed to a silica thin film via hydrogen-bonding Si−OH···−NH2 interactions. These data further support polyamine immobilization within BIAS sorbents by a similar mechanism. Depending upon the strength of the acid−base interactions of the amine−silica system, protonation of −NH2/−NH groups by surface Si−OH may occur, which forms SiO−···−NH3+/−NH2+ ionic species. These species were observed by Wang et al. for PEI functionalized to SBA-1534 and reveal that ionic amine−silica interactions may be a second mechanism of polyamine immobilization to supports in BIAS sorbents. Once a monolayer of amine species is functionalized to the silica surface of the Class 1 sorbents, it has been postulated from infrared spectroscopy (IR) and energy dispersive X-ray spectroscopy (EDS) data that multilayers form within the sorbent pore and on the external particle surface.30 These bulk species would be immobilized mostly by amine···amine interactions. Primarily using small-angle neutron scattering (SANS) coupled with porosity analysis, Holewinski et al. further revealed that for a Class 1 BIAS sorbent (PEI/SBA-15), bulk amine domains aggregate above the monolayer and accumulate along the length of the pore.35 Class 2 sorbents are typically prepared by wet impregnation of a mixture of a reactive aminosilane and anhydrous hydrophobic solvent, usually toluene, onto a dry, pretreated silica support. Strict control of the H2O content within the system is maintained to manipulate the subsequent grafting reaction between the aminosilane and the silica support. A general aminosilane-silica grafting reaction pathway is shown in Scheme 1 (a). In step 1, the impregnated aminosilane first undergoes nucleophilic addition of 1 mol of its Si−OCH3 groups (OR, R = −CH3, −CH2CH3, etc.) by 1 mol of silica Si− OH groups, which are hydrogen bonded to a second aminosilane molecule. The postulated Si−OH···−NH2/−NH hydrogen bonding interactions were reported to enhance the nucleophilicity of the silanol.32,36 The addition generates 1 mol of a pentacoordinated intermediate, which then loses 1 mol of an anionic alkoxy leaving group in step 2. The aminosilane assists this leaving group by providing a proton, which generates an alcohol (ROH) byproduct.36 The resulting deprotonated amine group is then reprotonated by 1 mol of its hydrogen-bonded Si−OH groups, which regenerates the aminosilane (not shown) and forms 1 mol of grafted aminosilane via Si−O−Si linkages. FTIR and NMR spectroscopies have confirmed the grafting of different aminosilanes, possessing reactive methoxy and ethoxy groups, to silica for different Class 2 BIAS sorbents.37−40

Scheme 1. Proposed Mechanisms for (a) Grafting of a General Aminosilane to a Silica Surface and (b) H2OCatalyzed Polymerization of a General Aminosilane That May Occur within a Silica Pore

The presence of H2O either adsorbed on the surface of nonpretreated silica or introduced during wet impregnation can polymerize the aminosilane, according to Scheme 1 (b). These species can still be attached to silica through a grafted Si−O−Si bond41 or contained within the bulk of the sorbent as a network. Shimizu et al. achieved an increased degree of liquid 3-aminopropyltriethoxysilane (APTES) polymerization by varying the APTES/H2O ratio. These data were verified by FTIR and 29Si NMR spectra and support aminosilane polymerization within the pores of the BIAS sorbents. Because Class 1 sorbents involve relatively weak hydrogen bonds to immobilize the polyamines, these sorbents can be susceptible to degradation during CO2 adsorption−desorption cycling under practical conditions with steam or humidity. This degradation was attributed to leaching and rearrangement of the amines,42−45 as well as structural changes46 in the sorbent. Furthermore, it was shown that some Class 2 sorbents also degraded in the presence of steam,43,47 likely due to leaching of nongrafted species from the sorbent, hydrolysis of Si−O−Si bonds of some grafted species, or collapse of the sorbent pore structure. To improve BIAS sorbent stability, researchers recently developed different Class 1/Class 2 (polyamine/ aminosilane) hybrid sorbent materials.4,31,43,48−50 A hybrid TEPA/APTMS/MCM-41 particle sorbent synthesized by Wang et al. exhibited higher CO2 uptake and both better thermal (TGA decomposition) and CO2 cycling stabilities than a Class 1 TEPA/MCM-41 sorbent.48 These findings were attributed to a synergistic effect between the aminosilane and polyamine. A key innovation to BIAS technology developed by some of us was a hybrid sorbent that incorporated an N-[3-(trimethoxysily)propyl]ethylenediamine (TMPED)/PEI800 (2.3/1) mixture immobilized on silica. Previously reported as NETL 32D51 and A28/ PL12,43 this patented6 sorbent exhibited good CO2 capture B


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ACS Applied Materials & Interfaces

higher PCR values reflected sorbents that would be more stable under steam conditions.43 The sorbents’ organic contents (amine loading) before and after H2O treatment were determined by gradually heating pretreated sorbent from 105 to 1200 °C in flowing air for thermal degradation, while also assessing oxidative stability. Organic content retained (OCR) values were calculated from these amine loadings to further ascertain the sorbents’ stabilities. Solid State NMR Characterization. Cross-polarization-magic angle spinning (CP-MAS) and 1H−13C cross-polarization (CP) heteronuclear correlation (HETCOR) nuclear magnetic resonance (NMR) spectra were acquired with a Bruker AVANCE 300 (Bruker Biospin, Billerica, MA) with operational frequencies of 300.43 MHz for 1H, 75.55 MHz for 13C, and 59.68 MHz for 29Si and equipped with a 4 mm wide-bore H/X CP-MAS probe. Samples were packed into 4 mm zirconia rotors with Kel-F caps and spun at 5 kHz. 13C spectra were referenced to glycine’s carbonyl carbon at 176.5 ppm, 29Si spectra were referenced to tetrakis(trimethylsilyl)silane] at −9.8 ppm, and 1 H−13C HETCOR were referenced with tyrosine·HCl at 2.5 ppm on the most high-field correlation for the 1H axis. Typical acquisition parameters for 29Si CP-MAS spectra were the following: Bruker’s pulse program “cp”, 2k points with acquisition time of 20.5 ms and spectral width of 50 kHz, relaxation delay of 5 s, 10k− 20k scans, contact time of 1 ms, CP ramp.100 with RF field of 31 kHz. Typical acquisition parameters for 13C CP-MAS spectra were the following: Bruker’s pulse program “cp”, 2k points with acquisition time of 20.5 ms and spectral width of 50 kHz, relaxation delay of 3 s, 5k− 27k scans, contact time of 1 ms, CP ramp.100 with RF field of 31 kHz. All 29Si and 13C NMR data were acquired at room temperature (RT) with 1H decoupling (RF field of 62.5 kHz, SPINAL128). Bruker Topspin 1.3 software was used to acquire the spectra. MNova 10.0 software (http://mestrelab.com/) was used for processing the spectra. The NMR data (FID) were zero-filled to 128k points and multiplied by the exponential window function with a line broadening (LB) of 30−50 Hz prior to FT. The procedure for collecting frequency-switched Lee−Goldburg (FSLG) 1H−13C CP HETCOR spectra is shown in Scheme 2, where the spectra were measured according to the method of van Rossum et al.52 at a 5 kHz spinning rate.

(1.8−2.1 mmol CO2/g) and excellent H2O and thermal stabilities during 100 adsorption−desorption cycles (>282 h) under practical conditions.4 Despite the robust characteristics of these novel hybrid Class 1/Class 2 sorbents, the link between their fundamental chemical structure and their superior stability remains elusive. Our objective in this study was to investigate the amine binding mechanisms of Class 1, Class 2, and hybrid Class 1/Class 2 sorbents at the molecular level using diffuse reflectance infrared Fourier transform (DRIFT) and solid state nuclear magnetic resonance (SS NMR) spectroscopies. We compared the DRIFTS spectra of PEI/silica (Class 1), TMPED/silica (Class 2), and TMPED/PEI/silica (Class 1/Class 2) sorbents at 0 to ∼40 wt % sequentially impregnated amine loadings. This approach enabled us to distinguish between the sorbents’ amine binding mechanisms near the amine−silica interface and also within the bulk layers. Solid state NMR spectra of each sorbent class confirmed these mechanisms and their associated chemical structures, which were correlated to each sorbents’ CO2 capture performance and H2O stability. Our results reveal that PEI both catalyzed the grafting of TMPED to silica at the amine−silica interface and enhanced polymerization within the bulk layers of the hybrid sorbent. Interaction of PEI with the bulk TMPED−TMPED network via PEI−NH2···O−Si−O/ OH−Si−TMPED hydrogen bonds strengthened the sorbent and contributed to its superior H2O (liquid, steam) and oxidative stabilities.

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EXPERIMENTAL SECTION

Immobilized Amine Sorbent Preparation. Preparation of an array of different Class 1, Class 2, and hybrid Class 1/Class 2 immobilized amine CO2 sorbents was accomplished using the procedure reported in our previous work.43 For each sorbent, 4.0 g of a lower molecular weight poly(ethylenimine) Mw = 800 (PEI800, Sigma-Aldrich) (Class 1), N-(3-(trimethoxysilyl)propyl)ethylenediamine (TMPED, Sigma-Aldrich) (Class 2), (3aminopropyl)trimethoxysilane (APTMS, Sigma-Aldrich) (Class 2), or a combination of an aminosilane with PEI (hybrid) were dissolved in 100.0 g of anhydrous methanol (99.8%, Sigma-Aldrich). The amine/methanol solutions were added to 6.0 g of amorphous silica (PQ CS 2129, PQ Corp., Davg = 500 μm) in a 500 mL round-bottom flask, which was placed in a rotary-evaporator and heated at 80 °C while gradually pulling a vacuum of 200 to 720 mmHg for 60 min to evaporate methanol. Successful aminosilane grafting under similar conditions was previously confirmed elsewhere by solid state 29Si NMR.37 A total of seven sorbents (Class 1 = 1, Class 2 = 2, hybrid Class 1/Class 2 = 4) contained 15−40 wt % total organic amines, where the aminosilane/PEI weight ratios of the hybrid sorbents were varied between 0.43 and 2.3. Sorbent H2O Stability and CO2 Capture Testing. Rapid screening for sorbent CO2 capture stability under practical conditions was achieved using our previously published accelerated liquid H2O method.43 Here, 20 mL of room temperature deionized H2O were passed through a bed of 0.5 g of each sorbent within a glass column at a rate of 0.5 mL/min. Conventional TGA steam treatment was accomplished as previously reported,43 where exposure to a 105 °C ∼5% steam/N2 flow was carried out for 10 h. CO2 capture capacities of fresh and H2O-treated sorbents were accomplished in a thermogravimetric analyzer-differential scanning calorimetry system (TGA-DSC, Mettler-Toledo-1 STARe). The sorbents were first pretreated at 105 °C for 60 min in 100 mL/min of flowing N2 to desorb preadsorbed CO2 (atmosphere) and H2O (atmosphere, H2O tests). The sorbents were then cooled to 40 °C, and the flow was switched to 85% CO2/N2 for 60 min for CO2 adsorption. The percentage of CO2 capture retained (PCR) values were calculated by dividing the CO2 capture capacities, H2O-treated/fresh × 100%, where

Scheme 2. Procedure for Collecting FSLG 1H−13C CP HETCOR Spectra of the BIAS Sorbents

Typical acquisition parameters were the following: Bruker’s pulse program “lghetfq”, 2kx64 points with SW2/SW1 of 50/9.5 kHz in F2 and F1, respectively, t1 increment of 52.87 μs, relaxation delay of 3 s, 256−1k scans, contact time of 500 μs, CP ramp.100 with RF field of 31 kHz. Proton 2π during the FSLG decoupling was set to 11.4 μs, and the magic angle pulse length for protons was set at 2.13 μs. Quadrature detection was achieved by using the States-TPPI method. All HETCOR NMR data were acquired at RT with 1H decoupling (RF field of 71 kHz, SPINAL128). Bruker Topspin 1.3 software was used to acquire the spectra, and Bruker Topspin 3.2 software was used for processing. The 2D NMR data set was zero-filled to 2kx2k points and multiplied by the exponential window function with a line broadening (LB) of 100−150 Hz prior to FT in both dimensions. DRIFTS Sequential Impregnation. Sequential impregnations of silica with incremental loadings of PEI, TMPED, and TMPED/PEI were performed in a diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS, Thermoscientific) setup. Approximately 13.0 mg of ground silica were loaded and well packed into a single DRIFTS cell cup, which was sealed with a dome equipped with a ZnSe window, C


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Table 1. Amine Content and CO2 Capture Capacities of Fresh Class 1, Class 2, and Hybrid Class 1/Class 2 Sorbents

a

sorbent

organic/amine loading (wt %)

PL40 T12/PL28

39.0 37.5

T20/PL20

32.5

T28/PL12

26.7

T40 A28/PL12

26 26.8

A40

15.7

aminosilane/PEI800 weight ratio only PEI800 TMPED/PEI80012/28 TMPED/PEI80020/20 TMPED/PEI80028/12 only TMPED APTMS/PEI80028/12 only APTMS

calculated −OCH3 conversion (%)a

CO2 ads., fresh (mmol CO2/g-sorb.)b

amine eff., fresh (mol CO2/mol NH+NH2)

15

2.81 2.81

0.45 0.50

68

2.50

0.50

90

2.1

0.46

48 59

2.0 1.8

0.56 0.51

74

1.0

0.43

Calculated according to the procedure in the Supporting Information. bCO2 adsorption was performed at 40 °C with 83% CO2/N2.

then pretreated at 105 °C for 10 min in flowing N2 to partially remove preadsorbed H2O, and then cooled to 40 °C. The removal of some physisorbed H2O (∼3.8 wt %, TGA) was necessary to clearly observe the amine−silica binding mechanisms from the IR absorbance spectra. Once cooled, the dome was removed, and 30 μL of amine/MeOH solutions were injected onto the surface of the silica beds to saturate the particles. A total of seven injections (I) were used to sequentially deposit the amines: (I1) 0.3 wt % (soln. conc.)/∼0.6 wt % (sorbent loading), (I2) 0.6/∼1.7, (I3) 1.0/∼3.5, (I4) 2.0/∼6.9, (I5) 4.0/∼13.1, (I6) 12.0/∼27.8, (I7) 12.0/∼38.2. After each injection, the resulting amine/MeOH/silica mixtures were aged about 1 min as the dome was replaced. Following, the wet mixtures were heated at 105 °C in flowing N2 for 10−15 min to evaporate the solvent and then cooled. IR single beam spectra, I, of pretreated silica and of the associated sequentially impregnated sorbent samples with different amine loadings were collected at a 4 cm−1 resolution at 40 °C, where each spectrum was averaged from 25 scans collected over 15 s.

The percent conversion of APTMS methoxy groups for A40 (74%, ∼2.2/1 ratio) was similar to that for APTMS/SBA-15 (∼2/1 ratio), which contained an 8 wt % aminosilane loading.53 These results further indicate the preferential reactivity of aminosilanes toward grafting. The expectedly smaller −OCH3 percent conversion of 59% for A28/PL12 (26.8 wt % amines) than for A40 (15.7 wt % amines) reflects less efficient aminosilane bulk polymerization of the higher-loading sorbent relative to more efficient surface grafting of the lower-loading sorbent. The sorbents possessed good CO2 capture capacities between 2.81 and 1.0 mmol CO2/g-sorb, which decreased with increasing aminosilane content. Increasing the TMPED/ PEI ratio for the hybrid sorbents slightly enhanced their amine efficiencies, presumably due to dispersion of more viscous PEI within the pores by a less viscous “additive”.54 Incorporating these different combinations of PEI with TMPED and APTMS is expected to produce sorbents with varying stabilities under humid CO2 adsorption−desorption conditions. These conditions were simulated by accelerated H2O testing.43 Figure 1 reveals clear differences in the sorbents’ accelerated H2O treatment stabilities, which are ascertained by their PCR and OCR values. Raising the TMPED/PEI ratio from 0 (pure PEI800, PL40) to 2.3 (T28/PL12) in Figure 1 (a) drastically increased both the PCR and OCR values, e.g. sorbent H2O stabilities, which then diminished upon removing PEI from the sorbent (pure TMPED, T40). This trend for the sorbent OCR and PCR values closely mirrors that of the percent −OCH3 conversion for TMPED in Table 1. These data reveal that the extent of aminosilane grafting and polymerization influences the sorbent H2O stability. Sorbents prepared with APTMS in Figure 1 (b) displayed a similar trend in their OCR, PCR, and percent −OCH3 conversion values as those prepared with TMPED. A deviation in this trend for the A40 PCR value might be explained by rearrangement rather than leaching of the grafted APTMS species from within the pores. Our results here clearly reveal the superior H2O stability of hybrid sorbents compared to those with only physisorbed (Class 1) or grafted (Class 2) species. Further corroboration of these results for APTMS/TEPA/MCM-41 sorbents elsewhere48 strengthens our findings. A complementary TGA-DSC investigation into sorbent oxidative degradation resistance is shown in the Supporting Information and reveals a similar stabilizing effect of the aminosilane/PEI mixture on the hybrids as observed for accelerated H2O treatment of these sorbents. Specifically, the

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RESULTS AND DISCUSSION Fresh Sorbent CO2 Capture and H2O Stability Analysis. Table 1 shows the amine loadings and related CO2 capture capacities of the fresh Class 1, Class 2, and hybrid Class 1/Class 2 sorbents. For conciseness in the sorbent labels, lower Mw PEI800, TMPED, and APTMS are abbreviated as PL, T, and A, respectively. A near-optimal loading of 39.0 wt % for PL40 shows effective immobilization of nearly all amine species using the rotary-evaporator method. Decreased sorbent amine loading with increased aminosilane/PEI ratio can be accounted for by the loss of organic species as MeOH, which was produced by reactions involving TMPED, APTMS, and the silica surface. The percent conversion of aminosilane Si−OCH3 groups into stable Si−O−Si and volatile MeOH species via grafting or polymerization was calculated (see the Supporting Information, SI) and is shown in Table 1. Higher percent conversions for hybrid TMPED/PEI sorbents, especially T28/ PL12 (90%), than for pure TMPED (T40, 48%) indicates that PEI catalyzed a combination of the grafting and polymerization reactions. The 48% conversion for T40 (26 wt % loading) corresponds to the reaction of about 1.4 mol of methoxy groups/1 mol of TMPED molecules, which is lower than the ∼2/1 ratio observed elsewhere for a ∼9 wt % loaded TMPED/ SBA-15 sorbent.53 The low loading of the literature sorbent corresponds to, at maximum, a monolayer of coverage that should favor grafting over polymerization due to intimate contact of the aminosilane with the silica surface. In contrast, the high loading of our sorbent contains additional bulk species that likely reacted (polymerized) less efficiently than those at the silica surface. D


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hydrogen bonding, which was observed by the slightly broadened Q3 silanol signal. The spectra of T40 and T28/ PL12 reveal the formation of primarily T2 [R-Si(OSi)2(OH)] and T 3 [R-Si(OSi) 3 ] species along with concomitant consumption of silica Si−OH groups. These data confirm that TMPED was immobilized through the reaction of 2 (T2) and 3 (T3) mol methoxy groups (−OCH3)/1 mol TMPED molecules, according to Scheme 1 (a). More importantly the TMPED/PEI hybrid sorbent displays a measured low Q3/Q4 signal intensity ratio with an associated high percent −OCH3 conversion (Table 1), compared to those values for the Class 2 T40 sorbent. These data confirm the catalytic effect of PEI on the TMPED-silica grafting reaction. The clearly higher T2/T3 signal ratio for T28/PL12 than for T40 implies a greater proportion of grafted TMPED molecules via T2 species, which inherently possess 1 mol −Si−OH/1 mol TMPED. According to Scheme 1 (b), the TMPED T2 Si−OH species likely resulted from partial polymerization of the aminosilane within the sorbent. These Si−OH groups would hydrogen bond with neighboring PEI or TMPED molecules and strengthen the sorbent structure. The 29Si data here support the hypothesis that hybrid sorbents are stabilized by covalent attachment of aminosilanes within the sorbent via Si− O−Si linkages involving T2 moieties. These moieties possess pendant Si−OH groups that can interact with neighboring PEI molecules, providing added resistance to sorbent degradation by H2O (accelerated H2O) and heat (thermal decomposition). The solid state 2-D FSLG 1H−13C CP HETCOR spectra of the sorbents in Figure 3 (a) uncover the molecular interactions between TMPED and PEI by directly probing the environments of their alkyl groups. Although 2-D FSLG 1H−13C CP HETCOR with 500 μs mixing time typically aims at probing long-range intramolecular correlations, we expect this techni-

Figure 1. Percentage of CO2 capture retained (PCR) and organic content retained (OCR, percentage) values after accelerated H2O treatment of fresh (a) TMPED/PEI/silica and (b) APTMS/PEI/silica hybrid sorbents.

data suggest the stabilization of dispersed PEI within a network of TMPED. Solid State NMR Analysis. To elucidate chemical interactions within the sorbents that contribute to their respective stabilities, especially to H2O, solid state 29Si CPMAS and FSLG 1H−13C CP HETCOR NMR spectra were analyzed. Figure 2 compares the 29Si CP-MAS spectra of PL40

Figure 2. 29Si CP-MAS NMR spectra of silica, PL40 (Class 1), T40 (Class 2), and T28/PL12 (hybrid) sorbents.

(Class 1), T40 (Class 2), and T28/PL12 (hybrid) sorbents, along with the spectrum of silica as a reference. Note, these CPMAS spectra are not quantitative and are discussed qualitatively according to their relative peak intensity ratios. Immobilization of PEI onto silica’s free isolated hydroxyl groups at ∼103 ppm (Si−OH, Q3) and geminal hydroxyl groups at ∼92 ppm (OH−Si−OH, Q2) diminished the PL40 sorbent’s Si−OH (Q3,Q2)/Si−O−Si (Q4, ∼115 ppm) signal intensity ratios compared to those of the pure silica support.37 PEI immobilization to silica was achieved presumably by

Figure 3. Solid state 2-D FSLG 1H−13C CP HETCOR NMR spectra of (a) PL40, T40, and T28/PL12 sorbents and (b) hybrid sorbents with different TMPED/PEI weight ratios. F1 and F2 denote the 1H and 13C axes, respectively, and hollow arrows indicate cross-peak shifts. E


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Figure 4. (a) IR absorbance spectra of PL40 (PEI only), T40 (TMPED only), and T28/PL12 (TMPED/PEI hybrid) sorbents during sequential impregnation of dilute amine/MeOH solutions onto amorphous silica at 40 °C after removing MeOH. Absorbance = log(I0/I), where I0 was the normalized (0 to 1) single beam spectrum of pretreated silica and I was the normalized single beam spectra of amine/SiO2, both at 40 °C. (b) Normalized IR absorbance intensity profiles of key species during sequential impregnation, where 0 and 1 represent the weakest and strongest intensities, respectively.

Figure 3 (b) compares the 2-D FSLG 1H−13C CP HETCOR spectra for hybrid sorbents with different TMPED/PEI ratios. The spectra show slight perturbations in the ∼19 ppm and ∼37−40 ppm cross-peaks upon reducing the aminosilane/ polyamine ratio of the sorbents from 2.8/1.2 (T28/PL12) to 2/ 2 (T20/PL20), followed by negligible shifts for 1.2/2.8 (T12/ PL28). This trend for decreasing extent of chemical shifts correlates with that of diminishing sorbent H2O stability shown in Figure 1 (a). These findings demonstrate that the TMPEDPEI hydrogen bonded species dictate, in part, the stability of hybrid Class 1/Class 2 sorbents. DRIFTS Sequential Impregnation Study. The IR absorbance was calculated as, Absorbance = log (I0/I), where I was the normalized (0 to 1) single beam spectrum of the amine/silica sorbents at different amine loadings and I0 was the normalized background (reference) spectrum, both at 40 °C. Although KBr has been used as a reference/background for

que can be used to reflect intermolecular interactions between PEI and TMPED. The spectrum of the hybrid T28/PL12 reveals noticeable shifts in grafted or polymerized TMPED’s −Si−CH2−R alkyl cross peak at 7.5 ppm along both the F1 (1H, vertical) and the F2 (13C, vertical) axes, relative to the peak positions of the T40 spectrum. Note, changes in the T2 rather than T3 TMPED structure should be favored (Figure 2). Concomitantly, cross peaks at ∼35−40 ppm for branched PEI’s terminal R−CH2−NH2 segment and for TMPED’s terminal R−CH2−NH2 segment were also displaced from their original position for PL40 and T40, respectively. Jointly, these perturbations for T28/PL12 signify the formation of a mixture of hydrogen bonded PEI−NH2···NH2−TMPED species and PEI−NH2···HO−Si/O−Si−O (T2/T2, T3 TMPED) linkages. No clear features for interactions involving −NH groups was observed, although they are likely. F


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ACS Applied Materials & Interfaces CO2 sorbent IR absorbance spectra,34 others favored the silica support as the background during in situ vapor phase amine impregnation to prepare an amine/silica sorbent.39 We similarly chose to use silica as the background for our sequential impregnation studies, because it provides the most direct information about chemical changes occurring within the sample at each amine loading. Figure 4 (a) shows the IR absorbance spectra of (i) PL40, (ii) T40, and (iii) T28/PL12 sorbents at each sequentially deposited wt % amine loading. These spectra were collected at 40 °C after evaporating the impregnation solvent and could represent the chemical structures of the sorbents near the amine−silica interface (interfacial amine layers) and throughout the silica pore (bulk amine layers). Depositing a 0.7 wt % PEI loading onto silica for (i) PL40 produced the characteristic −CH2 stretching bands between 2990 and 2770 cm−1 and the −CH2 bending band at 1464 cm−1 for the PEI alkyl groups.19,55 The amine groups of PEI were evidenced by the −NH2 (primary amine) and −NH (secondary amine) stretching bands at 3367 (asymmetric) and 3294 cm−1 (symmetric), respectively, and by the −NH2 bending band at 1593 cm−1.19,55 The corresponding negative bands for free (Si−OH) and hydrogenbonded (Si−OH···OH−Si) silanol groups of silica at 3741 and 3546 cm−1, respectively,19,30 were accompanied by the broad band between 3500 and 2400 cm−1 for perturbed Si−OH and together indicate interactions between the hydroxyl groups of silica and the amine groups of impregnated PEI. Although the IR band for H-bonded Si−OH groups could be overlapped with that for adsorbed H2O, the small reduction in the 1630 cm−1 O−H bending band for water (region not shown) confirms that 3546 cm−1 is primarily associated with the Si− OH groups. H-bonding of triethylamine (TEA) and propylamine (PA) to a silica film was shown to weaken the surface Si−OH bonds, which shifted their corresponding IR stretching bands to 2677/2750 cm−1 (TEA/PA) and 1000/996 cm−1, respectively.33,56 Formation of the Si−OH bands at 2746 and 1008 cm−1 for our PEI/silica sorbent confirms the presence of the newly generated, H-bonded Si−OH···amine species. The 1286 cm−1 band also observed here could be attributed to a structure-sensitive Si−O−Si vibration57 near the surface of the silica pore,58,59 in other words at the PEI-silica interface. Specifically, it was postulated that this band could represent a strained Si−O−Si bond.58 Overall, the presence of these species for our sorbent suggests rearrangement of the interfacial Si−O−Si structure upon chemical interaction of silica with PEI. Further depositing up to 13.1 wt % PEI onto the silica particles expectedly decreased the band intensities of the surface Si−OH groups and increased the band intensities of the amine, alkyl, perturbed Si−OH, and more importantly Si− OH···amine species. Concomitant growth of the 1286 cm−1 band shows continued disruption of the Si−O−Si network as more PEI is hydrogen bonded to the silica surface. Interestingly, the IR features at 13.1 wt % reveal broadened amine stretching bands, slight red-shifting of the 3294 cm−1 amine stretching band to 3272 cm−1, and formation of a clear broad shoulder band at 3170 cm−1. These features have been observed, in part, for other amine/silica sorbents with higher amine density 19 and liquid amine films with greater thicknesses60 and can be attributed to enhanced amine··· amine hydrogen bonding. Together these IR features indicate initial formation of amine multilayers within the silica pore. Interestingly a near-monolayer coverage of SBA-15 mesopores

was observed at 10 wt % PEI,35 while others observed a similar coverage at 12 wt % TEPA/SiO2 and PEI/SiO2.30 Increasing the amine loading of the PL40 sorbent to 38.2 wt % primarily formed these bulk amine multilayers. A smaller red shift of −197 cm−1 in the band of amine···amine species compared to a larger −995 cm−1 shift in the band of the Si− OH···amine species shows weak interactions among the multilayers. Their low binding strength suggests that amines comprising the bulk might be easily removed by steam during CO2 adsorption−desorption cycling or by liquid water during accelerated H2O testing. Significant disruption of the silica structure at high PEI wt % was evidenced by the prominent 1286 cm−1 band, which also red-shifted by 40 cm−1 to 1246 cm−1. Molecular orbital calculations revealed red-shifting in a similar Si−O stretching IR band with decreasing ring size of the model silica structure.61 The IR features here suggest that at higher amine density, the Si−O−Si structure of the silica support partially broke down into smaller units via a basecatalyzed mechanism. Preparing sorbents by sequential impregnation may introduce a slight concentration gradient of deposited amine species vertically within the silica bed. This issue is addressed in the SI (Figures S2 and S3) and does not significantly influence our results. Furthermore, significant physical disturbance of the sorbent bed by sequentially impregnating the amine species could disrupt the particle packing or bed height62 and produce false IR features in the bulk SiO2 region below 1300 cm−1. These features would likely be manifested, in part, by the formation and shifting of meaningless IR bands. As a control test, we performed two sequential injections of pure MeOH solvent on fresh ground silica to discern any effects of bed disturbance from those of chemical changes during amine/ MeOH sequential impregnations observed below 1300 cm−1. Figure S4 in the SI reveals a near flat baseline below 1300 cm−1, with no SiO2-related IR bands formed after both MeOH injections. These results confirm that changes in the absorbance spectra after the amine/MeOH sequential impregnations arise primarily from chemical interactions involving the amine species and silica. Figure 4 (b) shows the IR absorbance intensity profiles of total silica hydroxyl groups (free plus H-bonded) and Hbonded amine species during sequential impregnation of PL40. These band intensities were measured according to specific baselines, which are shown in Figure S5 of the SI. Worth noting, the largest standard deviation expected for all measured IR band intensities is 2.5%, based on three measurements of the 1008 cm−1 band at its weakest intensity. Sharp consumption of surface Si−OH groups and corresponding generation of Si− OH···amine species up to 13.1 wt % were followed by gradual changes in both moieties up to 38.2 wt %. These results, along with the sharp increase in amine···amine hydrogen bonding beginning at 6.9−13.1 wt %, further confirm the formation of a near-monolayer amine coverage on the available Si−OH groups followed by the formation of bulk amine multilayers.30,35 It is important to note that the free and H-bonded Si−OH groups comprising the total Si−OH profile likely have different IR extinction coefficients, which precludes exact quantification of the number of each type of −OH group that is interacting with the amines. However, because the total Si−OH profiles are internally compared for sorbents having the same silica support, and consequently similar free and H-bonded Si−OH content, relative comparison of these profiles within our study is justified. G


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O−Si band intensity within the bulk up to 38.2 wt % provides evidence for a polymerized (interconnected) TMPED network. A corresponding −46 cm−1 red-shift in the strained Si−O−Si band, which is −6 cm−1 more than that for PEI/silica, suggests the TMPED−TMPED Si−O−Si network is unstable and may be susceptible to degradation (hydrolysis) under humid CO2 capture conditions. It is unclear if TMPED was immobilized via ionic −NH3+···−OSi− species, which were observed elsewhere.64,65 Similar trends in the (ii) T40 IR intensity profiles (Figure 4 (b)) as those for PL40 support the formation of interfacial and bulk aminosilane regions. A more drastic change in the 1045 cm−1 Si−O−Si profile (0.52 normalized abs. int.) than in the 3741 + 3546 cm−1 total Si−OH profile (0.18 normalized abs. int.) after the near monolayer coverage strengthens the hypothesis for TMPED−TMPED polymerization within the bulk, according to Scheme 1 (b). The source of H2O for this reaction would be primarily from the impregnation solution or potentially water displaced from the silica surface by H-bonded TMPED. Because the complete polymerization of aminosilanes to form Si−O−Si involves first the formation of Si−OH groups (Scheme 1 (b)), it is possible that some of these species did not further convert into the final product. A gradually increasing 1008 cm−1 profile for the bulk layers could suggest the formation of pendant Si−OH groups attached to the TMPED− TMPED network that subsequently interacted with neighboring TMPED molecules. The formation of TMPED Si−OH groups is supported by the observed T2 species in the 29Si CPMAS NMR spectra of the rotary evaporator-prepared sorbents (Figure 2). Notably, it has been shown spectroscopically that H2O content affects the polymerization of aminosilane (aminopropyltriethoxysilane) species both in solution and within silica.64,66 These results suggest that the inherently different amounts of preadsorbed H2O on our silica before synthesizing T40 and T28/PL12 sorbents in the rotary-evaporator (3.9 wt %, H2O, not pretreated) and in the DRIFTS (0.9 wt %, pretreated at 105 °C) may generate different amine binding mechanisms. However, because of the similarly low total water concentrations in both of the wet amine/solvent/silica systems, 0.26 wt % (rotary-evaporator) and 0.41 wt % H2O (DRIFTS), we expect that H2O played a similar role in all reactions involving TMPED. Three key differences in the IR features of the nearmonolayer (up to 13.1 wt %) amine binding mechanisms for (iii) T28/PL12 compared to the other sorbents include the following: (1) greater relative formation of Si−OH···amine species [1008 cm−1 intensity profile, 2746/3100 cm−1 intensity ratio profile (Figure S6 in the SI)], (2) enhanced grafting of TMPED to silica via Si−O−Si bonds in the presence of PEI compared to T40 (1045 cm−1 intensity profile), and (3) greater consumption of silica’s surface hydroxyl groups compared to T40 (3741 + 3546 cm−1 intensity profile). Because of the high TMPED/PEI ratio, the aminosilane dispersed the polymeric amine and facilitated the formation of PEI−NH2/−NH···Hbonded silanol (Si−OH···OH−Si) moieties. Figure S7 in the SI supports this hypothesis by a more drastic reduction in the number of Si−OH···OH−Si species (3546 cm−1) for T28/PL12 than for T40. It was shown that preadsorbed ethylenediamine (ED) on SiO2 catalyzed the reaction of (3-aminopropyl)dimethoxysilane (APDMES) with silica, largely by interactions of immobilized ED with incoming APDMES.32 We postulate that for T28/

Similar to PL40, immobilization of 0.7 wt % of TMPED on silica for (ii) T40 occurred, in part, via hydrogen bonding of the amine groups to free hydroxyl groups. Additional hydrogen bonding of the Si−OCH3 species, whose broad band is at 1184 cm−1, to silica was shown by the broad Si−OH band between 3500 and 3000 cm−1. A similar broad band centered at 3350 cm−1 was observed for H-bonding of trimethoxymethylsilane to a silica film.33 The broad band in the T40 spectra is overlapped with that for the perturbed hydroxyls. However, comparing the curved IR band shape for the T40 spectra to the more linear shape for the PL40 spectra in this region confirms the presence of the H-bonded Si−OCH3 species on the aminosilane-based sorbent. Because of the close proximity of this Si−OH vibration with those of the amines, we hereafter use 3100 cm−1 to describe its features. Weaker binding strength of the methoxy than amine groups to free hydroxyl groups was evidenced by a smaller red-shift in the associated Si−OH band: −641 cm−1 by −Si−OCH3 and −995 cm−1 by −NH/−NH2. These results are consistent with those in the literature.33,56 Due to their weak binding strength, TMPED molecules attached to silica via Hbonds rather than grafted covalent bonds, via loss of methoxy groups, are relatively unstable and may be easily removed from the sorbent by steam (practical CO2 testing) or by liquid water. In contrast to the spectrum for PL40, the spectrum for T40 exhibits a Si−O−Si stretching vibration at 1045 cm−1. This band is in the 1100−1000 cm−1 range typically observed for different aminosilane/silica systems and confirms covalent attachment of TMPED to silica.19,32,33,37,56 Furthermore, the absence of a negative band at 3546 cm−1 shows that H-bonded Si−OH species did not immobilize TMPED at this low loading. A broad band centered at about 808 cm−1, which is also present in the spectra of PL40 and T28/PL12, is likely an overlap of similar low intensity bands for N−H wagging of amines63 and Si−O vibration of the aminosilane. Alternatively this band could represent a silica support Si−O band, which would be associated with Si−O overtone bands in the 1800 to 2000 cm−1 range. However, the weak 1940 cm−1 IR band intensity for T28/PL12 (containing PEI and TMPED) shown in Figure S5 of the Supporting Information did not change appreciably with amine loading. These negligible IR changes in the 1940 cm−1 band are in contrast to the more pronounced changes in the 808 cm−1 band, which together suggest that the latter band is more associated with IR features of impregnated PEI and TMPED rather than the silica support. Overall, the immobilization of TMPED to silica at the aminosilane-silica interface occurred by a combination of hydrogen bonding via amines and methoxies and grafting to free Si−OH. Impregnating additional TMPED up to a 13.1 wt % loading enhanced the amount of H-bonded and grafted species to silica via both free and H-bonded Si−OH groups. The increasing 2746/3100 cm−1 intensity ratio along with the decreasing intensity of the 3546 cm−1 band (beginning at 3.5 wt %, not shown) shows that H-bonded silanols favored TMPED immobilization primarily through hydrogen-bonded amine··· silanol species. Concomitant sharpening of the 1184 cm−1 Si− OCH3 band, in other words less broadened features, further suggests the negligible methoxy···silanol or methoxy-amine interactions. These newly available methoxy groups could extend from the interface and into the pore, where incoming TMPED could polymerize and form a bulk network. Similar to PL40, formation of the distinct 3170 cm−1 band along with broadened amine features at 13.1 wt % loading signifies initial TMPED bulk multilayers. Continued strengthening of the Si− H


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ACS Applied Materials & Interfaces PL12, the dispersed PEI···H-bonded silanol species interacted with incoming TMPED, which accelerated the subsequent TMPED-silica reaction. The subdued Si−OCH3 IR features for this sorbent compared to those for T40 at similar amine loadings is in-line with the high percent conversion of these groups into Si−O−Si species for the corresponding rotaryevaporator sorbent (Table 1). Overall these results show a synergistic effect between PEI and TMPED, which stabilized the hybrid sorbent at the amine/silica interface. Continued formation of the Si−O−Si species into the bulk layers of T28/ PL12 is evidenced by their strong IR band intensity and indicates TMPED polymerization. Although the strain of the Si−O−Si network was observed throughout the entire sorbent, the 1261 cm−1 band did not red shift. This is in contrast to redshifts observed for both PL40 and T40 and suggests a stabilizing effect of PEI on the polymerized TMPED network within the hybrid sorbent. Overall, the conclusions drawn from the DRIFTS data are corroborated both by the SS NMR findings and by the percent Si−OCH3 conversion calculations presented in Table 1. Based upon these consistent results, Figure 5 illustrates our

Figure 6. Derivative, normalized weight profiles during the first 3 min of CO2 adsorption over fresh, steam-treated, and accelerated liquid H2O-treated (a) PL40, (b) T40, (c) T20/PL20, and (d) T28/PL12 sorbents. These sorbents were all prepared using the rotaryevaporation method. The a.u. label (y-axis) represents arbitrary units for the normalized mass.

regimes, from (i) 0 to 0.57 min, (ii) 0.57 to 1.18 min, and (iii) > 1.18 min. Generally, asymmetric profile features about the time of maximum uptake rate for immobilized amine sorbents reflect rapid adsorption to exposed surface amines followed by diffusion through and subsequent adsorption within bulk amines.25,34,49 For our sorbent, adsorption of 25% of the total adsorbed CO2 to surface amines is observed in regime (i), whereas the remaining 75% is adsorbed to amines in two distinct regions of the sorbent in regimes (ii) and (iii). We hypothesize that regime (i) represents adsorption to amines exposed on the external particle surface as well as amines covering the pore mouths. Regime (ii) highlights continued adsorption from the mouth into the pore, where the adsorbed species form an interconnected network.30 Importantly, the network slows CO2 diffusion through, and adsorption to, bulk amines deeper inside the pore and near the amine−silica interface, which is reflected in regime (iii). Steam-treating the PL40 sorbent for a total of 10 h in the TGA reduced the sorbent organic content from 39.0 to 37.8 wt % (TGA). This reduction shows a loss of external and bulk amines near the pore mouths. Sequential impregnation DRIFTS data indicated that these bulk species were weakly immobilized by amine···amine hydrogen bonds. The remaining bulk amines in the sorbent were likely redistributed somewhat homogeneously inside the pore and exhibited reduced CO2 diffusion limitations. This was inferred by symmetric features of the CO2 uptake profile, where the maximum adsorption rate increased by a factor of ∼2 and was observed at a faster time (0.37 min) than that of the fresh sorbent (0.57 min). Treating the sorbent with flowing liquid water (accelerated H2O) dramatically reduced the sorbent organic content to 4.5 wt %, due to complete removal of the bulk amines and much of the interfacial amines. Rapid CO2 uptake by this treated sorbent supports easy access of CO2 to the remaining amines scattered across the silica surface. Derivative uptake profiles for the (b) fresh T40 sorbent also revealed three CO2 adsorption regimes, which were reduced to

Figure 5. Hypothesized structure of a stable, hybrid aminosilane/ polyamine/silica sorbent, T28/PL12.

hypothesized structure of a stable hybrid Class 1/Class 2 sorbent, specifically T28/PL12. Because of the distinct differences in the molecular structures and amine binding mechanisms of hybrid sorbents compared to those for Class 1 and Class 2, we propose a Class 4 category for sorbents comprised of polyamine/aminosilane mixtures immobilized on silica and potentially on other porous inorganic supports. Fresh and H2O-Treated Sorbent CO2 Adsorption Behavior. The different mechanisms for binding TMPED, PEI, and TMPED/PEI to the silica support produced sorbents with different CO2 adsorption behaviors. This was confirmed by the distinctive normalized CO2 uptake profiles for the sorbents prepared by the rotary-evaporator method (Figure S8, SI). Moreover, the associated derivative normalized weight profiles for the sorbents, both fresh and after steam and accelerated H2O treatments (Figure 6), highlight the effects of H2O degradation on the bulk and interfacial amine structures. The profile for (a) fresh PL40 exhibits three unique adsorption I


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ACS Applied Materials & Interfaces two regimes after steam and water treatments. Diminished organic content from 26.0 wt % to 23.8 wt % (steam-treated) and 2.9 wt % (water-treated) coincided with the significant differences among their derivative CO2 profiles, especially the onset times and values of the maximum uptake rates. Considering these CO2 results along with those of the DRIFTS, we hypothesize that the strained Si−O−Si network of the polymerized TMPED−TMPED structure (bulk) and Hbonded TMPED/Si−OH···Si−OCH3 (amine−silica interface) were hydrolyzed and some amines leached from the sorbent. CO2 adsorption over fresh and treated hybrid (c) T20/PL20 and (d) T28/PL12 sorbents revealed a stabilizing effect for the TMPED/PEI mixture, especially for the latter sorbent, which further compliments the NMR and sequential impregnation DRIFTS results. The more limited variations within the hybrid sorbents’ CO2 profiles for different treatment conditions relative to those observed for the Class 1 and Class 2 sorbents clearly demonstrate the stability of both the bulk and interfacial amine layers. The absence of a loss in organic content of T28/ PL12 after steam treatment confirms the strength of the bulk PEI/cross-linked TMPED system under practical H2O vapor conditions. This is in contrast to the unstable bulk species comprising the Class 1 and Class 2 sorbents. In the hybrid sorbent, PEI is distributed among and interacts with the network by PEI−NH2···HO−Si−TMPED and PEI−NH2···O− Si−O links and resists leaching from the pores. We postulate that the dispersed PEI also provides a path for H2O transport through the sorbent particle. Preferential interaction of H2O with hydrophilic PEI prevents the hydrolysis and leaching of the TMPED−TMPED bulk network. An organic content of 11.9 wt % for T28/PL12 after accelerated H2O treatment illustrates near-complete retention of the interfacial amine species and further exemplifies the cooperative effect of PEI and TMPED to stabilize the sorbent.

adsorption profiles compared to those of Class 1 and Class 2 sorbents. Overall our studies revealed critical differences between the amine binding mechanisms of hybrid Class 1/Class 2 CO2 sorbents from those of prototypical Class 1 and Class 2 sorbents at the molecular level. These mechanisms facilitated high stability of the sorbents under liquid H2O and steam conditions, which is required for their practical application. This work provides the scientific basis for establishing a Class 4 category that encompasses these hybrid aminosilane/polyamine/inorganic support based sorbents.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b02062. Procedure for determining the amount of reacted aminosilane methoxy groups; TGA-DSC thermal decomposition study; IR absorbance spectra of sorbents prepared by sequential impregnation and the rotary evaporator method; DRIFTS absorbance profiles of different species during sequential impregnation; DRIFTS sequential MeOH injection control test; CO2 adsorption profiles for sorbents prepared by the rotaryevaporator and sequential impregnation methods (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Funding

This project was funded by the Department of Energy, National Energy Technology Laboratory, an agency of the United States Government, through a support contract with AECOM. This research was supported in part by an appointment to the National Energy Technology Laboratory Research Participation Program, sponsored by the U.S. Department of Energy and administered by the Oak Ridge Institute for Science and Education. A portion of this technical effort was performed in support of the National Energy Technology Laboratory’s ongoing research under the RES contract DE-FE0004000. C.W.J. acknowledges support from NSF via CBET-1403239.

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CONCLUSIONS Solid state 29Si CP-MAS and 2-D FSLG 1H−13C CP HETCOR NMR spectroscopies were utilized to probe the molecular interactions of different amine types with the silica support in Class 1, Class 2, and hybrid Class 1/Class 2 sorbents. These interactions were further investigated by a sorbent sequential impregnation DRIFTS technique, which elucidated aminosilane and polyamine immobilization mechanisms at both the amine− silica interface and within the bulk amine layers. NMR data confirmed that the superior resistance of hybrid sorbents to degradation by liquid/vapor H2O (H2O stability) and by oxidation in air (thermal stability) resulted primarily from the formation of hydrogen bonded PEI−NH2···NH2−TMPED and PEI−NH2···HO−Si/O−Si−O (T2 TMPED) linkages. These species gave higher PCR and OCR values for TMPED/PEI/ silica (T28/PL12) and APTMS/PEI/silica (A28/PL12) sorbents after stability testing than for sorbents comprised of only an aminosilane or PEI plus the support. The DRIFTS data further revealed that hybrid sorbents (T28/PL12) were stabilized at the amine−silica interface via enhanced grafting of the aminosilane (TMPED) to silica, catalyzed by a high density of immobilized PEI species: PEI−NH2/−NH···Hbonded Si−OH. Stability of the bulk layers was ascribed to trapped PEI molecules within a robust, polymerized TMPED network. Resistance of TMPED/PEI sorbents to steam degradation via these binding mechanisms was manifested by the relatively minimal changes in their derivative CO 2

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Vyacheslav Romanov and Lei Hong for use of the DRIFTS system. REFERENCES

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