Poly(allylamine)–Mesoporous Silica Composite Materials for CO2

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Poly(allylamine) Mesoporous Silica Composite Materials for CO2 Capture from Simulated Flue Gas or Ambient Air Watcharop Chaikittisilp,†,§ Ratayakorn Khunsupat,‡,§ Thomas T. Chen,† and Christopher W. Jones*,†,‡ †

School of Chemical & Biomolecular Engineering and ‡School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0100, United States

bS Supporting Information ABSTRACT: Low-molecular-weight poly(allylamine) is prepared via free-radical polymerization, and the resulting polymer is impregnated into mesocellular silica foams at different amine loadings. The resulting poly(allylamine) silica composites are demonstrated as effective adsorbents for the extraction of carbon dioxide from dilute (simulated flue gas) and ultradilute (simulated ambient air) gas streams. The composite adsorbents are shown to have comparable adsorption capacities to more-conventional poly(ethyleneimine) silica adsorbents. Potential advantages of poly(allylamine)-derived adsorbents are discussed.

1. INTRODUCTION Oxide-supported amine materials represent an important, emerging class of solid adsorbents that is being considered for a variety of carbon dioxide (CO2) separation technologies.1 3 In particular, supported amine materials are being developed as components of adsorption technologies that might one day give an alternative to the well-established, benchmark aqueous amine absorption technology for CO2 capture from dilute CO2 streams, such as flue gas from electricity-generating power plants. Following CO2 capture, the gas may be used as a carbon source for fuel or chemical synthesis,4,5 or sequestered underground.6 10 These supported amine materials have been previously categorized into three classes.2,11,12 Class 1 adsorbent materials are typically based on porous oxides, such as silica, that are impregnated with presynthesized amine polymers or amine-containing small molecules (e.g., monoethanol amine). By far, the most commonly used polymer is low-molecular-weight (ca. 500 800 Da), branched poly(ethyleneimine).13 42 This polymer has proven to be very efficient, with adsorbents containing this polymer having large adsorption capacities and high amine efficiencies (defined as the ratio of the number of moles of CO2 captured per mole of amine). Whereas the amines in Class 1 materials are bound to the support through the force of many weak interactions (e.g., hydrogen bonds, electrostatic interactions, van der Waals interactions), Class 2 materials contain covalent bonds between the small molecule amines and the oxide support; these materials are routinely made via the reaction of amine-containing silanes with the oxide support.43 65 Class 3 materials contain covalently bound polymeric amines on the support, and are prepared via the in situ polymerization of amine-containing monomers on or in the support.11,66 71 The most-efficient adsorbents are generally rich in primary and secondary amine groups, as these groups can capture CO2 under both dry and humid conditions, with primary amines generally having higher heats of adsorption than secondary amines.1,2 However, recently we have shown that secondary amines can be susceptible to oxidation under conditions relevant to CO2 capture from flue gas or ambient air55,72 (for a brief review r 2011 American Chemical Society

Scheme 1. Chemical Structures of Aminopolymers Used

of CO2 capture from ambient air, or “air capture”, see ref 11). In contrast, primary aminopropyl groups seemed to be stable to oxidative degradation under the same conditions. Furthermore, because of the ultradilute conditions required in direct air capture, the large heat of adsorption associated with primary amines is viewed to be advantageous, as it is likely that only these amines play a significant role in CO2 adsorption under these conditions. Poly(ethyleneimine) (PEI) is widely used in adsorbent formulations, because of its wide availability and efficient characteristics in CO2 capture. PEI contains amines in the polymer backbone, with tertiary amines at branch points, secondary amines in the main chain, and primary amines displayed at each chain end (see Scheme 1). It is hypothesized here that the high accessibility of the primary amines at each chain end makes PEI an especially effective active phase for CO2 extraction from ultradilute gas streams such as ambient air. However, because Received: July 21, 2011 Accepted: November 17, 2011 Revised: November 8, 2011 Published: November 17, 2011 14203

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Industrial & Engineering Chemistry Research of the lack of oxidative stability of the ubiquitous secondary primary amine pair found in PEI (RNHCH2CH2NH2),14,55,72 development of alternative amine-containing polymers is needed. In particular, two key attributes in a polymer would be (i) a large mass fraction of basic amine sites (minimizing the thermal mass of “wasted” material that must be repeatedly heated and cooled in a typical temperature swing cycle), and (ii) a large proportion of primary amine groups, potentially allowing for good oxidative stability and also potential utility under ultradilute gas conditions. Here, we demonstrate the use of poly(allylamine) as a representative aminopolymer that fulfills the above requirements. Poly(allylamine) (PAA) contains all of its amine sites on the side chain of the hydrocarbon backbone, unlike PEI (see Scheme 1). In addition, all the amine sites are primary amines, whereas a typical low-molecular-weight branched PEI contains a distribution of amine types (e.g., 44% primary, 33% secondary, 23% tertiary).68 Furthermore, neglecting end groups from the radical initiator, PAA contains 24% (by weight) basic nitrogen groups, having only one additional methylene group per repeat unit compared to PEI, which is 32% (by weight) basic nitrogen.73 Thus, PAA represents an alternative to PEI for use in amine-rich composite CO2 adsorbents.

2. EXPERIMENTAL SECTION 2.1. Materials. The following chemicals were used as received from the suppliers: allylamine hydrochloride (AAHCl, TCI), anhydrous isopropanol (IPA, 99.5%, Alfa Aesar), methanol (MeOH, 99.5%, Sigma Aldrich), ethanol (EtOH, 99.5%, ACROS), 2,2-azobisisobutyric acid dimethyl ester (MAIB, 98%, AK Scientific), strongly basic ion-exchange resin (Ambersep 900 OH form, Sigma Aldrich), poly(acrylamide) GPC standards (PAM2950, PAM15K, PAM100K, American Polymer Standards), water for GPC (TraceSelect, Sigma Aldrich), Pluronic P123 EOPO-EO triblock copolymer (Sigma Aldrich), 1,3,5-trimethylbenzene (TMB, 97%, Sigma Aldrich), tetraethyl orthosilicate (TEOS, 98%, Sigma Aldrich), ammonium fluoride (NH4F, >96%, Alfa Aesar), hydrochloric acid (HCl, conc. 37%, J.T. Baker), branched poly(ethyleneimine) (branched PEI, Mw = 800 Da, Sigma Aldrich), linear poly(ethyleneimine) (linear PEI, Mw = 2500 Da, Polysciences). 2.2. Synthesis of Poly(allylamine) (PAA). The solution of AAHCl (6.00 g, 0.06 mol), isopropanol (3.99 g), and MAIB (0.79 g, 3.43 mmol) was deoxygenated by argon purging for 1 h. The polymerization was carried out at a constant temperature of 60 °C for 48 h. The resulting precipitate was collected and washed with excess methanol under stirring several times to remove unreacted monomer. PAA-HCl was recovered by filtration and dried under vacuum at room temperature for 24 h to give 4.5 g of white powder (75% yield). 1H NMR (400 MHz, D2O, ppm): 1.37 ( CH2 ; Figure S1 in the Supporting Information, proton (a)), 1.90 ( CH ; Figure S1 in the Supporting Information, proton (b)), 2.91 ( CH2 ; Figure S1 in the Supporting Information, proton (c)). PAA was obtained by using the OH form of a strongly basic ion-exchange resin to remove the HCl. Degassed deionized water (∼60 mL) and strongly basic ion-exchange resin (∼16 g) were added to the obtained PAA HCl and stirred for 1 h. The resulting polymer solution at pH 12 was filtered and the solvent was removed under vacuum. Finally, the polymer was dried under vacuum for 24 h to give 2.7 g of product (60% yield). 1H NMR (400 MHz, D2O, ppm): 1.14 ( CH2 ; Figure S2 in the

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Supporting Information, proton (a)), 1.52 ( CH ; Figure S2 in the Supporting Information, proton (b)), 2.59 ( CH2 ; Figure S2 in the Supporting Information, proton (c)). 2.3. Synthesis of Silica Mesocellular Foam (MCF). A solution of P123 copolymer (16.0 g), water (260 g), and concentrated HCl (47.4 g) was stirred at room temperature for 24 h to complete copolymer dissolution. The flask was then transferred to a 40 °C oil bath and TMB (1.6 g) was added. The mixture was stirred at 40 °C for 2 h; then TEOS (34.6 g) was added. The solution was stirred for an additional 5 min and then left quiescent for 20 h at 40 °C. A solution of NH4F (184 mg) in deionized water (20 mL) was added as a mineralization agent and the mixture was swirled for 5 min before aging at 100 °C for 24 h. The resulting precipitate was filtered, washed with excess water, and dried at 70 °C. A typical silica MCF was then obtained after calcination in air at 550 °C for 6 h (1.2 °C/min ramp). 2.4. Impregnation of Polymers in MCF. The aminopolymerloaded MCF samples with different loadings were prepared via a wet impregnation method. In a typical preparation, the desired amount of amine polymer was dissolved in ethanol under stirring while purging the mixture with argon gas, until the polymer dissolved completely. The necessary amount of calcined MCF then was added to the mixture. The resulting suspension was stirred for 16 h under an argon atmosphere. The EtOH:MCF mass ratio was always maintained constant at 31:1 for each sample, while the MCF:polymer ratio was varied in each case. The resulting solid was recovered via removal of the solvent under vacuum using the Schlenk line, followed by drying under vacuum at ambient temperature for 24 h. The as-prepared adsorbents were denoted as X_MCF_Y, where X represents the amine polymer and Y represents the polymer weight percentage in the sample. Branched PEI, linear PEI, and PAA are referenced as PEIBr, PEILn, and PAA, respectively. 2.5. Material Characterization. The structure of the synthesized PAA polymer was characterized using solution 1H NMR. The measurements were performed using a Mercury Vx 400 MHz with D2O as a solvent. Molecular weights of the polymers were estimated by gel permeation chromatography (GPC) at 30 °C. The GPC system was comprised of a Shimadzu Model LC-20AD pump, a Shimadzu Model RID-10A RI detector, a Shimadzu Model SPD-20A UV detector, a Shimadzu Model CTO-20A column oven, and Viscotek Model TSK Viscogel PWXL Guard, G3000, G4000, and G6000 columns mounted in series. The mobile phase consisted of 0.05 N NaNO3 and the flow rate was maintained at 0.4 mL/min. Poly(acrylamide) standards were used (Mw = 3350, 15500, 99000; Mn = 2765, 12800, 45600). The organic loading of the materials was characterized by combustion using a Netzsch STA409 TGA under a flowing nitrogen-diluted air stream. About 10 mg of the sample was heated to 830 °C at a rate of 10 °C/min. 2.6. CO2 Adsorption. The CO2 adsorption characteristics of aminopolymer-loaded MCF materials were characterized using a TA Instruments Model Q500 thermogravimetric analyzer. A sample weight of ∼20 mg of sorbent was loaded in a platinum vessel for CO2 adsorption performance measurement. First, the initial activation of the sample was carried out at 120 °C by heating at a rate of 5 °C/min under an argon flow at a rate of 100 mL/min and sustained at that temperature for 3 h. The temperature was then decreased to the desired adsorption temperature (25, 50, or 75 °C) and held for 1 h before CO2 was introduced. Adsorption was then initiated by 14204

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exposing the samples to the dry target gas of desired concentration (400 ppm CO2 or 10% CO2 balanced with Ar) at a flow rate of 100 mL/min. Adsorption performance of aminopolymer-loaded MCF materials was compared by measuring their pseudoequilibrium capacities, measured for 3 h at flue gas conditions (10% CO2) and for 12 h under air capture conditions (400 ppm CO2), respectively. Temperature-programmed desorption (TPD) was also carried out after the CO2 adsorption at 25 °C using 10% CO2. The desorption was performed by slowly heating at a rate of 0.1 °C/min under an argon flow (100 mL/min).

3. RESULTS AND DISCUSSION The synthesis of the PAA HCl was carried out via a one-step free-radical polymerization and PAA was obtained after HCl removal using a basic ion-exchange resin.74 Figure S1 in the Supporting Information shows the 400 MHz 1H NMR spectrum of PAA HCl in D2O. The peak integrations shown a 2:1:2 ratio of (CH2; hydrocarbon backbone):(CH):(CH2; side chain) agree with the expected molecular structure. The NH2 peak was not observed for PAA HCl in D2O, because of rapid proton exchange between NH2 and any H2O in D2O.75 Figure S2 in the Supporting Information illustrates the 1H NMR spectrum of PAA in D2O, showing the original proton resonance peak at 2.91 ppm assigned for the protons in methylene groups between the amines and the hydrocarbon backbone (CH2CH CH2 NH2; proton (c) in Figure S1 in the Supporting Information) of PAA HCl was shifted to 2.59 ppm in PAA (proton (c) in Figure S2 in the Supporting Information), indicating that HCl was successfully removed. The molecular weight distribution of PAA was measured by GPC, using poly(acrylamide) as the polymer standard.76 The PAA prepared in this work was estimated to have an Mn of 1130 Da. The thermochemical and physical properties of MCF and the organic loading in the composite adsorbents were assessed by thermogravimetric analysis (TGA). Figure S3 in the Supporting Information shows the TGA profiles of the synthesized MCF and PAA MCF materials. For the bare MCF material after template removal through calcination, TGA showed a negligible mass loss of 1.0%, which was attributable to the loss of traces of adsorbed water and a small amount of silanol condensation. The PAAloaded MCF samples displayed a mass loss of ∼10% over the 27 160 °C range, which can be attributed to desorption of adsorbed moisture and carbon dioxide. No obvious mass loss occurred from 160 230 °C until the PAA began to decompose above 230 °C in all samples. At ∼830 °C, the PAA was completely decomposed and fully removed as volatile species. These results indicate the maximum stability temperature of these samples under these conditions is ∼230 °C. Three samples of PAA MCF materials with different PAA loadings were prepared, along with the samples of control materials containing the benchmark branched, low-molecular-weight PEI polymer, or linear, low-molecular-weight PEI polymer. Figure 1 presents a comparison of the CO2 capture performance using 10% CO2 over branched PEI-(PEIBr_MCF samples), linear PEI- (PEILn_MCF samples) and PAA-loaded MCF materials (PAA_MCF samples) with different polymer loadings. For branched PEI samples, the adsorption capacities at 28, 39, and 46 wt % polymer loadings were 1.21, 1.83, and 2.40 mmol CO2/g sorbent, respectively. At loadings of 32, 42, and 49 wt %, the CO2 sorption capacities of linear PEI samples were 1.46, 2.28, and 2.51 mmol CO2/g sorbent, respectively.

Figure 1. (a) CO2 adsorption capacity and (b) amine efficiency of branched PEI-, linear PEI-, and linear PAA-loaded samples at different organic loadings, using a 10% CO2 gas stream at 25 °C.

The CO2 adsorption capacities increased for all the branched and linear PEI samples as the PEI loading was increased. Samples with higher PEI content allowed for larger CO2 adsorption capacities. The increase in capacity with polymer loading is consistent with the need for two primary or secondary amines to cooperate when capturing CO2 under dry conditions, to form carbamates.1,2 It is noteworthy that, under 10% CO2 conditions, branched PEI yielded a comparable adsorption capacity to the linear PEI. The amine efficiencies for the branched and linear PEI samples ranged from 0.19 mol CO2/mol N to 0.23 mol CO2/ mol N, which was consistent with only moderately efficient use of the amine sites (Figure 1b). The amine efficiencies of the branched PEI sorbents were increased with increasing polymer loadings, while the efficiencies of the linear PEI materials were first increased at low organic loadings and then decreased at high organic loadings. This difference may be due to the structures of PEI (branched versus linear) or the molecular weights of polymers. Under dry simulated flue gas conditions (10% CO2), a maximum amine efficiency of 0.5 is possible for a perfectly linear PEI that contains mostly secondary amines with primary amines on the end of each linear chain. For the branched PEI, if one assumes that only the primary and secondary amines can participate in CO2 capture under dry conditions, the maximum 14205

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Table 1. CO2 Capacity and Amine Efficiency during CO2 Adsorption Experiments Using 10% CO2 and 400 ppm CO2 Feed Gas over PEI- and PAA-Containing Class 1 Adsorbent Samples amine loading sample ID

10% CO2 capacity

(mmol N/g sorbent) (mmol CO2/g sorbent)

10% amine efficiency

400 ppm CO2 capacity 400 ppm amine efficiency Ratio of 10%: 400

(mol CO2/mol N)

(mmol CO2/g sorbent)

(mol CO2/mol N)

ppm CO2 capacity

PEIBr_MCF_28 PEIBr_MCF_39

6.49 8.96

1.21 1.83

0.19 0.20

0.61 1.08

0.09 0.12

2.0 1.7

PEIBr_MCF_46

10.71

2.40

0.22

1.74

0.16

1.4

PEILn_MCF_32

7.37

1.46

0.20

0.44

0.06

3.3

PEILn_MCF_42

9.77

2.28

0.23

0.75

0.08

3.0

PEILn_MCF_49

11.37

2.51

0.22

1.05

0.09

2.4

PAA_MCF_33

5.74

1.35

0.24

0.63

0.11

2.1

PAA_MCF_41

7.24

1.56

0.22

0.86

0.12

1.8

PAA_MCF_54

9.51

1.36

0.14

0.84

0.09

1.6

amine efficiency is 0.385 (77% primary and secondary amines).68 If it is assumed that tertiary amines can accept a proton during the adsorption process, this value becomes 0.5. The experimentally observed values are all below the theoretical maximum. For linear PAA-loaded samples, CO2 sorption capacities of 1.35 and 1.56 mmol CO2/g sorbent were measured for 33 and 41 wt % loading samples, respectively. These capacities are comparable to the PEI-containing samples at similar amine loadings. However, the capacities of the PAA-containing samples are slightly lower than the benchmark PEI. The CO2 sorption capacity decreased as the PAA loading was further increased to 54 wt % (1.36 mmol CO2/g sorbent), suggesting that excess amine polymer can be detrimental to the adsorption performance, consistent with previous observations with PEI-containing samples.2,14,17,18,32,33 Considering that all of the amines in PAA are primary amines, the theoretical maximum amine efficiency is 0.5. The amine efficiencies for PAA samples were decreased with increasing polymer loadings with observed values ranging from 0.14 0.24, being similar to the values obtained with the PEI samples. The adsorption results are also provided in Table 1. Figure 2 and Table 1 present a comparison of the CO2 capture performance and amine efficiency of these same samples using a 400 ppm CO2 gas stream, as a function of the polymer type (branched PEI, linear PEI, and linear PAA) and loading in the MCF support. The adsorption capacity and efficiency of branched PEI increased as the loading increased. The trend of increasing capacity and efficiency with increasing amine loading also held for the samples prepared with linear PEI. However, the adsorption capacity was markedly lower than the branched PEI materials. This is consistent with our hypothesis that adsorbents for CO2 extraction from ultradilute gas streams should maximize the number of primary amines, which have stronger affinities for CO2 (larger heats of adsorption). Linear PEI theoretically only contains primary amines at the end of the linear chains, containing only 3.5% primary amines on average in these samples, with the remaining amines being secondary amines. In contrast, PAA was found to have a larger CO 2 capacity under 400 ppm conditions than the linear PEI samples, but a slightly lower capacity than the branched PEI samples at similar organic loadings. However, if one seeks to compare the materials at similar, low amine loadings, the behavior of the branched PEI and PAA can be considered to be quite similar. For example, in Figure 2b, it can be seen that both the branched PEI and PAA samples had similar amine efficiencies at low polymer

Figure 2. (a) CO2 adsorption capacity and (b) amine efficiency of branched PEI-, linear PEI-, and linear PAA-loaded samples at different organic loadings, using a 400 ppm CO2 gas stream at 25 °C.

loadings, suggesting that both polymers are similarly efficient for extraction of CO2 from simulated air under these conditions. However, at higher polymer loadings, the branched PEI samples were more effective. As shown in Figure 2, at high polymer loadings, the capacity and amine efficiency for the PAA samples plateau and decrease, respectively. This is hypothesized to be associated with the favorable display of the primary amines sites 14206

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Figure 3. CO2 adsorption capacity at different temperatures using 10% CO2 gas mixtures over PEIBr_MCF_39 samples (circles) and PAA_MCF_41 samples (triangles).

at the end of each chain in highly branched PEI, making these important sites highly accessible. In contrast, PAA is a linear polymer and the primary amines on the side chains of each repeat unit of the polymer are likely not as accessible as in the branched PEI case. Further studies are needed to better characterize the conformations of the polymers in the composites to support or refute these hypotheses. The very small capacity reduction found when comparing the adsorption results from 10% CO2 to 400 ppm CO2 streams with the branched PEI samples (Table 1) is consistent with our past work on PEI and PEI-related materials in air capture studies.13,14,67,77 For example, in the best case, using hyperbranched aminosilica (HAS) adsorbents, the capacity was reduced by a factor of 2.2 when switching from 10% to 400 ppm CO2 (3.77 mmol CO2/g to 1.72 mmol CO2/g).67 A similar decrease by a factor of 2.2 was observed by Sayari and co-workers when evaluating triamine-modified PE-MCM-41 samples.78 Overall, the HAS capacities were decreased by a factor varying from 2 8, depending on the structure of the adsorbent.67 In contrast, the Class 1 sorbents prepared here, based on branched PEI and PAA, seem to be more effective under air capture conditions, relative to their flue gas performance. The branched PEI sample capacities were reduced only by a factor of 1.4 2.0 when reducing the CO2 concentration in the inlet gas by a factor of 250.79 Similarly, the PAA samples were of similar performance, with capacity reductions of 1.6 2.1. These results demonstrate that PAA is a promising new aminopolymer for CO2 capture from air, of similar performance to the best-known sorbents, based on branched low-molecular-weight PEI, at low to moderate amine loadings. As the CO2 adsorption by amine-based sorbents is highly exothermic, the adsorption capacity would thermodynamically be expected to decrease with increasing adsorption temperature. However, the adsorption of CO 2 over many Class 1 materials reported thus far exhibits a different trend; the capacities increased with increasing temperature within a specific temperature range.2,15,18,32 This temperature dependence has been explained to be due to the diffusional effects that are associated with pores almost filled with dense polymer. As the temperature is increased, the polymer chains are suggested to have more mobility,

Figure 4. Histograms of the amount of desorbed CO2 at different temperatures during CO2 temperature-programmed desorption (TPD) after adsorption, using 10% CO2 at 25 °C of (a) PEIBr_MCF_39 samples and (b) PAA_MCF_41 samples.

allowing CO2 better access to the interior of the solid adsorbents, yielding increased adsorption capacity. Figure 3 shows the temperature dependence of CO2 capture capacities over PAA_MCF_41, in comparison with PEIBr_MCF_39. As expected, the capacities of the PEI sample increased as temperature increased, with the maximum capacity at 50 °C. In contrast, the capacities of PAA samples decreased with increasing temperature, being consistent with the thermodynamic intuition. The difference in the effects of temperature suggests that conformation or mobility of linear PAA on the MCF support differs from that of branched PEI. The branched PEI and PAA samples were also compared by temperature-programmed desorption (TPD) experiments. Figure 4 displays CO2 TPD profiles of PEIBr_MCF_39 and PAA_MCF_41 after adsorption using 10% CO2 at 25 °C for 6 h. In the case of PEIBr_MCF_39, approximately half of the adsorbed CO2 was desorbed at the temperature below 40 °C with another half desorbed in the temperature range of 40 60 °C. In contrast, CO2 adsorbed over PAA_MCF_41 was desorbed at temperatures of 60 °C, in fractions of ca. 65%, 25%, and 10%, respectively. These results suggest that, although the majority of CO2 adsorbed over PAA_MCF_41 is weakly bound to the sorbent, there still exists some CO2 binding to the PAA sample more strongly than the branched PEI sample. 14207

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’ ASSOCIATED CONTENT

bS

Supporting Information. Solution NMR spectra of the synthesized polymers and TGA characterization of the PAA_MCF composite materials. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Author Contributions §

Figure 5. Regenerability of PEIBr_MCF_39 and PAA_MCF_41 via temperature swing adsorption desorption cyclic tests. CO2 capture capacity was obtained using 10% CO2 at 25 °C, and regeneration was performed under an argon flow at 110 °C.

Cyclic stability of the CO2 adsorbents is one of the key parameters for practical capture applications, because the adsorbents must be reused over many cycles. A preliminary assessment of the regenerability of PAA_MCF_41 was determined by application of simple, short multicyclic temperature swing operations using 10% CO2 and a pure argon purge during adsorption and desorption, respectively, with the results compared with PEIBr_MCF_39. As shown in Figure 5, PAA_MCF_41 and PEIBr_MCF_39 show almost-constant CO2 capture capacities, indicating that both materials are comparably stable over these admittedly short multicyclic TSA operations. It is noteworthy that more practical desorption processes such as humid CO 2 -purge TSA and steam stripping would have to be used for larger-scale, more-practical operation.2,12

4. CONCLUSIONS Poly(allylamine) (PAA) was impregnated into porous, mesocellular silica foams at different loadings and the materials were shown to be effective adsorbents for capturing CO2 from gas mixtures with CO2 partial pressures that were consistent with capture from flue gas (10% CO2) or the ambient air (400 ppm CO2), respectively. Class 1 PAA adsorbents with low organic loadings were shown to be of similar utility to branched PEI-containing Class 1 adsorbents, which are currently the benchmark adsorbents for CO2 capture from ambient air and are among the mostpromising adsorbents for flue gas applications. At higher loadings of PAA, the amine sites do not seem to be as accessible, as the CO2 capacities and amine efficiencies decrease. It is hypothesized that this difference in behavior from branched PEI is associated, at least in part, with their different backbone structures, with PAA being a linear polymer with a hydrocarbon backbone. Compared to linear PEI, PAA and branched PEI are much more efficient under the simulated ambient air conditions used here, and this is hypothesized to be due to the very small primary amine loading in linear PEI. It is suggested that there are many opportunities for the design and synthesis of new polymers that might offer improved adsorption performance, relative to the branched PEI benchmark that is used most often in CO2 adsorption studies.

These authors contributed equally to this work.

’ ACKNOWLEDGMENT This work has been partially supported by the School of Chemical & Biomolecular Engineering at Georgia Tech through the J. Carl & Sheila Pirkle Faculty Fellowship. We thank Drs. K. Venkatasubbaiah and N. A. Brunelli for their experimental help and fruitful discussions. ’ REFERENCES (1) Choi, S.; Drese, J. H.; Jones, C. W. Adsorbent materials for carbon dioxide capture from large anthropogenic point sources. ChemSusChem 2009, 2, 796–854. (2) Bollini, P.; Didas, S. A.; Jones, C. W. Amine-oxide hybrid materials for acid gas separations. J. Mater. Chem. 2011, 21, 15100–15120. (3) D’Alessandro, D. M.; Smit, B.; Long, J. R. Carbon dioxide capture: Prospects for new materials. Angew. Chem., Int. Ed. 2010, 49, 6058–6082. (4) Grabow, L. C.; Mavrikakis, M. Mechanism of methanol synthesis on Cu through CO2 and CO hydrogenation. ACS Catal. 2011, 1, 365–384. (5) Gupta, M.; Smith, M. L.; Spivey, J. J. Heterogeneous catalytic conversion of dry syngas to ethanol and higher alcohols on Cu-based catalysts. ACS Catal. 2011, 1, 641–656. (6) Keskin, S.; van Heest, T. M.; Sholl, D. S. Can metal organic framework materials play a useful role in large-scale carbon dioxide separations? ChemSusChem 2010, 3, 879–891 (7) Jones, C. W.; Maginn, E. J. Materials and processes for carbon capture and sequestration. ChemSusChem 2010, 3, 863–864. (8) Yu, K. M. K.; Curcic, I.; Gabriel, J.; Tsang, S. C. E. Recent advances in CO2 capture and utilization. ChemSusChem 2008, 1, 893–899. (9) Aaron, D.; Tsouris, C. Separation of CO2 from flue gas: A review. Sep. Sci. Technol. 2005, 40, 321–348. (10) Shukla, R.; Ranjith, P.; Haque, A.; Choi, X. A review of studies on CO2 sequestration and caprock integrity. Fuel 2010, 89, 2651–2664. (11) Jones, C. W. CO2 capture from dilute gases as a component of modern global carbon management. Annu. Rev. Chem. Biomol. Eng. 2011, 2, 31–52. (12) Li, W.; Choi, S.; Drese, J. H.; Hornbostel, M.; Krishnan, G.; Eisenberger, P. M.; Jones, C. W. Steam-stripping for regeneration of supported amine-based CO2 adsorbents. ChemSusChem 2010, 3, 899–903. (13) Choi, S.; Gray, M. L.; Jones, C. W. Amine-tethered solid adsorbents coupling high adsorption capacity and regenerability for CO2 capture from ambient air. ChemSusChem 2011, 4, 628–635. (14) Chaikittisilp, W.; Kim, H.-J.; Jones, C. W. Mesoporous aluminasupported amines as potential steam-stable adsorbents for capturing CO2 from simulated flue gas and ambient air. Energy Fuels 2011, 25, 5528–5537. 14208

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(75) Fischer, T.; Heitz, W. Synthesis of polyvinylamine and polymer analogous reactions. Macromol. Chem. Phys. 1994, 195, 679–687. (76) The lowest molecular weight of poly(acrylamide) (PAM) standard used had Mn and Mw of 2765 and 3350 Da, respectively. The PAA samples prepared in this work had lower molecular weights than the standards, and hence, the molecular weight values assigned by GPC are often extrapolated outside the calibration range. Given this fact and that the calibration polymers, PAM, are different from PAA, the molecular weight data are not rigorously quantitative and should be viewed as only an estimate. (77) Choi, S.; Drese, J. H.; Eisenberger, P. M.; Jones, C. W. A new paradigm of anthropogenic CO2 reduction: Adsorptive fixation of CO2 from the ambient air as a carbon negative technology. Presented at the AIChE Annual Meeting, Nashville, TN, 2009. (78) Belmabkhout, Y.; Serna-Guerrero, R.; Sayari, A. Amine-bearing mesoporous silica for CO2 removal from dry and humid air. Chem. Eng. Sci. 2010, 65, 3695–3698. (79) It should be noted that these are pseudoequilibrium capacities, reported here for scientific evaluation. A practical air capture process would not work with equilibrium capacities, but rather with working capacities that may be substantially lower (under otherwise identical, dry conditions) than those reported here. In this context, adsorption kinetics are critical and deserve detailed analysis in future works.

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