Application of Amine-Tethered Solid Sorbents for Direct CO2 Capture

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Application of Amine-Tethered Solid Sorbents for Direct CO2 Capture from the Ambient Air Sunho Choi,† Jeffrey H. Drese,† Peter M. Eisenberger,‡ and Christopher W. Jones*,† † ‡

School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States Global Thermostat LLC, 335 Riverside Drive, New York, New York 10025, United States

bS Supporting Information ABSTRACT: While current carbon capture and sequestration (CCS) technologies for large point sources can help address the impact of CO2 buildup on global climate change, these technologies can at best slow the rate of increase of the atmospheric CO2 concentration. In contrast, the direct CO2 capture from ambient air offers the potential to be a truly carbon negative technology. We propose here that amine-based solid adsorbents have significant promise as key components of a hypothetical air capture process. Specifically, the CO2 capture characteristics of hyperbranched aminosilica (HAS) materials are evaluated here using CO2 mixtures that simulate ambient atmospheric concentrations (400 ppm CO2 = “air capture”) as well as more traditional conditions simulating flue gas (10% CO2). The air capture experiments demonstrate that the adsorption capacity of HAS adsorbents are only marginally influenced even with a significant dilution of the CO2 concentration by a factor of 250, while capturing CO2 reversibly without significant degradation of performance in multicyclic operation. These results suggest that solid amine-based air capture processes have the potential to be an effective approach to extracting CO2 from the ambient air.

’ INTRODUCTION Growing concern about climate change induced by anthropogenic CO2 accumulation has led to interest in technologies that slow the growth of CO2 output or in some of the most ideal scenarios, halt the increase in the atmospheric CO2 concentration. It will be many decades before renewable fuels can bear the majority of the burden of powering our planet. In the meantime, fossil energy will continue to supply the bulk of the energy demand. In such a scenario, the atmospheric CO2 level will continue to climb well above the current concentration of ∼390 ppm.1 When considering the possible ways of removing CO2 from a gas mixture, several approaches can be considered, including cryogenic distillation, membrane purification, absorption with liquids, and adsorption using solids.2 Typically, these methods have targeted relatively concentrated CO2 sources, such as flue gases generated from large point sources including electricity generating power plants. However, roughly one-third of global carbon emissions are associated with distributed sources such as transportation vehicles. Thus, large-scale deployment of carbon capture and sequestration (CCS) technologies at point sources can at best slow the rate of increase of the atmospheric CO2 concentration. In light of this, direct CO2 capture from the ambient air (“air capture”) shows promise. Lackner proposed the use of air capture as a climate mitigation option in 1999.3,4 Since then, a number of scientists have demonstrated methods to extract CO2 from the ambient air and evaluated, to different extents, various process designs that might be used, including use of novel approaches or methodologies previously evaluated prior to 1999 before the consideration of air extraction as a mode of combating climate change.5-25 For ultradilute sources such as the ambient air, many classical CO2 separation processes such as cryogenic distillation and membrane separation are not expected to be cost competitive. r 2011 American Chemical Society

The most well-studied approaches utilize metal hydroxides or oxides (Ca, Na, etc.) and their conversion into metal carbonates, either in solution or as solids. In general, the regeneration of the carbonate into the original oxide or hydroxide has proven to be quite energy intensive, resulting in high projected energy use and process costs.5,11,15,16,21 Nonetheless, continued improvements of air capture designs using this approach are slowly reducing the projected energy costs.18,22 More recently, we,1,26 Lackner,20 and others24 have focused on the use of solid adsorbents for air capture. The array of classical CO2 adsorbents has been primarily evaluated for separation of moderately diluted CO2 from gas mixtures, such as flue gases, as reviewed recently,27 and not the extremely low CO2 partial pressures found in the ambient air. This constraint substantially narrows the list of potential adsorbents for the air capture. For example, physisorbents such as activated carbons and zeolites are expected to have low CO2 adsorption capacities in air capture because the heat of adsorption is quite low for these materials, leading to shallow adsorption isotherms with low adsorption capacities at ultralow pressures.27,28 In addition, these types of adsorbents suffer from overwhelming competitive adsorption of water over CO2 in most cases, so process designs that use them must practice careful humidity control.27 Considering the array of CO2 chemisorbents, calcium13,15 and sodium based oxides12 or lithium zirconates29 are expected to offer considerable adsorption capacities under high temperature conditions, but air capture processes are expected to be most economically favorable when operating near ambient conditions. Indeed, researchers that have studied high temperature air extraction Received: August 15, 2010 Accepted: January 13, 2011 Revised: January 4, 2011 Published: February 16, 2011 2420

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Environmental Science & Technology

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Table 1. HAS Adsorbents of Varied Aminopolymer Loadinga sample

organic

amine loading

aziridine to

pore diameter

BET surface

pore volume

name

contents (wt%)

(mmol/g)

SBA-15 ratio (w/w)

(nm)

area (m2/g)

(cc/g)

SBA

0

0

0

6.2

840

0.78

HAS1

9.9

2.3

0.13

6.3

579

0.69

HAS2

12.3

2.9

0.33

6.4

314

0.51

HAS3

15.9

3.7

0.53

5.9

278

0.49

HAS4

22.9

5.3

5.01

5.4

234

0.39

HAS5

36.1

8.4

2.03

5.0

71

0.15

HAS6

42.5

9.9

2.65

4.9

45

0.11

a

The organic, combustible fraction was measured by TGA and the amine loading was stoichiometrically estimated from this value. The pore diameter, pore volume, and the Brunauer-Emmett-Teller (BET) surface area were determined by the N2 physisorption experiments.

have relied on large, low cost heat sources8,9,30 or novel reactor designs using renewable energy as ways to reduce costs.13,14 Unlike the metal oxide based chemisorbents described above, supported amine adsorbents based on primary, secondary, or tertiary amines operate near ambient conditions, are tolerant to water vapor, and can be regenerated by mild temperature swings.27 Considering these materials, a potentially useful air capture adsorbent would have a relatively high heat of adsorption with CO2, yielding a steep adsorption isotherm (giving high adsorption capacity at low partial pressures), and operate near ambient temperature and pressure in the presence or absence of water. There are three classes of supported amine adsorbents.31 Class 1 adsorbents are composed of polymeric or oxides supports (typically silica) that are physically loaded with amine-containing small molecules or polymers.32-34 Thus, the amine species are physisorbed on the support surface and in the support pores. Class 2 adsorbents are based on amine species that are covalently bound to the surface of the solid support, such as via the use or organosilanes.35-38 These two classes of materials represent the most well-studied classes of amine adsorbents for the application of the flue gas treatment, but there has been very limited research evaluating these adsorbents for capturing CO2 at very low partial pressures. For instance, tetraethylenepentamine39 or diethanolamine40 impregnated MCM-41 silica (class 1 adsorbents) and mono- or triamine functionalized MCM-41,35,36,41 MCM48,42 and SBA-15 43 (class 2 adsorbents) were tested at a room temperature using CO2 mixtures with compositions ranging from 1000 ppm to 5%. Very recently, the CO2 capture capacity of the triamine-grafted pore-expanded MCM-41 (class 2) was measured at an extremely low CO2 partial pressure simulating ambient air, using ∼400 ppm CO2 gas balanced with N2 and O2.44 The adsorption isotherms reported in that work revealed that the class 2 adsorbents outperformed other solid adsorbents such as zeolites, mesoporous silica, MOFs, and carbon-based materials at extremely low CO2 partial pressures (below ∼0.05 bar), as would be expected based on the heat of adsorption of these materials.44 This suggests that other classes of supported amine adsorbents that can accommodate higher amine loadings may offer advantageous CO2 capture properties for air capture applications. Recently, our group reported a new class of amine-based solid adsorbents, described as hyperbranched aminosilica (HAS) materials, which are synthesized via in situ ring-opening polymerization of aziridine off of porous solid supports.45 Materials of this type, that integrate covalently bound polymeric amines in typically porous substrates, are deemed Class 3 supported amine adsorbents.31 These materials have recently been explored for adsorbing CO2 from dilute gas streams, such as simulated flue gas

(10% CO2 in inert gas, fully saturated with water). The work thus far has revealed that the HAS adsorbents can effectively remove CO2 from moderately diluted mixtures simulating flue gases and that the properties of the adsorbent can be tuned by controlling the adsorbent synthesis parameters such as the aziridine monomer to silica ratio.45,46 Here we present in detail an experimental demonstration of the direct CO2 capture from simulated ambient air using supported amine adsorbents containing polymeric amines, mainly focusing on class 3 HAS adsorbents. The HAS materials are able to extract CO2 from dilute gases with CO2 concentrations akin to the ambient air by adsorptive fixation with outstanding efficiency. These adsorbents can also provide a working capacity higher than an example of a conventional class 2 amine-based adsorbent, owing to the large number of amine moieties in the HAS material, while presenting excellent regenerability in successive operations due to the covalent tethering of amine groups onto the sorbents. Changes of the aminopolymer loading lead to structural variations in the adsorbents, making their adsorption capacities and kinetics tunable to some degree. We suggest that the adsorptive CO2 extraction from the ambient air by HAS materials or related solids may become a component of a practical air capture device, because supported amine adsorbents are easy to make, potentially cost-effective, and can offer multicycle stability coupled with the ability to extract CO2 from ultradilute sources.

’ MATERIALS AND METHODS Adsorbent Preparation. As summarized in Table 1, the HAS adsorbents with different aminopolymer loadings were prepared based on a reported method by varying the initial amount of aziridine to that of mesoporous SBA-15.45,46 Details of the silica support, SBA-15,47,48 and aziridine45 preparations can be found in the Supporting Information. Specifically, a desired amount of aziridine was added in the slurry containing a given quantity of SBA-15 in anhydrous toluene (Acros). Subsequently, polymerization of aziridine was carried out by adding a few drops of glacial acetic acid (Aldrich) to initiate the acid-catalyzed ring-opening reaction. The mixture was further reacted overnight at room temperature in a glass pressure vessel. The resulting solid product was collected by successive filtration and washing with copious amounts of toluene and then dried overnight at 50 °C under vacuum before further characterization. A class 2 supported-amine adsorbent was prepared by functionalizing SBA-15 with N-(3-(trimethoxysilyl)propyl)ethane-1,2-diamine (AEAPTMS, Aldrich). Specifically, 2.0 g of AEAPTMS was added to 1.0 g of SBA-15 silica dispersed in anhydrous toluene and then stirred 2421

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Environmental Science & Technology for 24 h at room temperature under Ar atmosphere. The white product was filtered and washed with a copious amount of toluene, and then dried overnight at 50 °C under vacuum before further use, resulting in a supported amine adsorbent containing both primary and secondary amine groups covalently bound to the silica support. Prior to the adsorption experiments, the hybrid adsorbents were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), N2 physisorption, and thermogravimetric analysis (TGA). Details of the characterization methods can be found in the Supporting Information. Adsorption Experiments. The CO2 adsorption characteristics of the HAS materials were assessed under humid conditions using a fixed bed flow system coupled with a mass spectrometer at both 400 ppm and 10% CO2 concentrations. The adsorption system was comprised of a Pfeiffer Vacuum QMS 200 Prisma Quadrupole Mass Spectrometer coupled with a packed bed flow reactor. In a typical adsorption experiment, about 60-80 mg of the adsorbent sample was mixed with ∼300 mg sieved sand (250-425 μm) and loaded in a Pyrex tubular reactor. Before each experiment, the samples were treated at 110 °C for 3 h under Ar flow. The adsorption step was initiated by flowing humidified 400 ppm CO2 with a flow rate of 20 mL/min at ambient conditions. The amount of CO2 captured by the adsorbent was calculated by monitoring the CO2 concentration at the reactor outlet using the mass spectrometer.46 The adsorbent regenerability of the HAS materials was evaluated by multicyclic TGA adsorption experiments using a TA Q500 thermogravimetric analyzer with the following temperature protocol. First, about 30 mg of the adsorbent was loaded in a platinum vessel and subjected to the desorption stage, in which the sample was heated to 110 °C by heating at 5 °C/min under Ar flow at a 100 mL/min rate and sustaining at the upper temperature for 3 h. Then the temperature was lowered to 25 °C and held for 1 h to stabilize the sample weight and temperature before introducing CO2. Adsorption experiments were carried out by exposing the samples to a dry 400 ppm CO2 (balance Ar) at a flow rate of 100 mL/min for 24 h, during which the weight gains were converted to the CO2 capture capacities of the adsorbents. For the adsorbent stability test, these desorption/ adsorption cycles were repeated four times while tracing the changes of the adsorption capacities over multiple cycles. It should be noted that the conditions used for evaluating the adsorbents under laboratory conditions are convenient for scientific testing but do not represent the methodologies (contactor design, regeneration method, gas flow rates, etc.) that would preferentially be used in a practical, large scale process.

’ RESULTS AND DISCUSSION Characterization of the Adsorbents. Figure S1 shows the X-ray diffraction patterns of SBA-15 and the HAS adsorbents with different amine loadings. The inset of Figure S1 displays a TEM image of the inorganic substrate, showing the presence of one-dimensional mesoporous channels with a mean pore size of ∼6.5 nm. The diffraction pattern of the pure SBA-15 displays three characteristic peaks around 0.9, 1.5, and 1.8 within the 2θ ranges of 0.6-4.0 degrees, which can be indexed as the (100), (110), and (200) planes of SBA-15, respectively. These peaks are routinely used as an indication of the hexagonal mesoporous structure of SBA-15.49,50 It is known that the intensity of the characteristic peaks decreases with the loading of organic substances into the pores,

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while the position of the peaks do not show significant changes compared to those of the original mesoporous silica support.33,51 Similarly, the characteristic peak positions of pure SBA-15 and the HAS did not show significant differences, which reveals that the long-range ordered structure of SBA-15 was not influenced by incorporating aminopolymers via the hyperbranching polymerization of monomer units. On the other hand, it was observed that the intensity of the low-angle reflection peak decreased as a result of the aminopolymer incorporation onto the mesoporous silica. The series of XRD patterns in Figure S1 illustrates that the (100) peak intensities were reduced as the aminopolymer loadings increased. Xu et al. reported that the poly(ethyleneimine) (PEI) immobilized onto the outside of the MCM-41 pores did not cause considerable intensity changes of the characteristic peaks.33 Thus, at least a portion of the aminopolymer is located in the mesoporous channels of the SBA-15 supports. N2 physisorption was employed to identify the pore characteristics of the HAS adsorbents with varied amine loadings, as summarized in Table 1. N2 physisorption data were interpreted by the Frenkel-Halsey-Hill modified Broekhof-de Boer (BdBFHH) method.52 Figure S2 illustrates the BET surface areas and the BdB-FHH pore volumes of different HAS adsorbents as a function of the aminopolymer loadings. It reveals that both the BET surface areas and pore volumes decreased gradually as the organic loadings in the HAS adsorbents increased. For instance, the HAS1 adsorbent with amine loadings of 2.3 mmol N/g had a BET surface area of 579 m2/g and a pore volume of 0.69 mL/g, whereas those of the unfunctionalized SBA-15 used in this experiment corresponds to 840 m2/g and 0.78 mL/g, respectively. The HAS adsorbents with higher amine loadings, such as HAS6, showed a more significant decrease in surface area and pore volume. Considering the fact that >99% of the surface area of mesoporous silica is in the particle’s interior, these pore characteristics suggest that aminopolymers were mostly grown inside the pore space.46 This result is also consistent with the XRD characterization, which suggests at least a portion of aminopolymers occupies the pore space in the mesoporous channels of SBA-15. On the other hand, the pore diameter data of the HAS adsorbents suggest the occurrence of pore blocking at higher amine loadings, as described in our previous report.46 Figure S3 shows the pore size distributions of different HAS adsorbents estimated by the BdB-FHH method, demonstrating that the pore diameters of the HAS adsorbents decreased as amine loading increased from low to moderate loadings, whereas the pore diameters remained at about 5 nm upon further increases of amine loadings (also see Table 1). For example, compared to the SBA-15 pore diameter of 6.2 nm, the pore size of HAS5 decreased to 5.0 nm with 8.4 mmol N/g amine loading, whereas a further increase of amine loading to 9.9 mmol N/g in HAS6 did not make a discernible change in the pore diameter. This result suggests functionalization occurred in the pores until a critical diameter was reached, but, past this point, additional polymerization occurred with pore blocking, most likely at the pore mouth.46 If organic inclusion occurred via layer by layer growth of the polymers inside the pores, the pore diameters of the HAS materials would be expected to decrease almost linearly, approaching zero. A possible consequence of the pore blocking phenomenon is the increased likelihood of polymerization occurring outside of the pores at higher loadings, leaving the average pore diameter of the remaining unblocked pores unchanged at the critical diameter.46 These observations suggest 2422



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Figure 1. Comparison of the equilibrium adsorption capacities measured by the fixed bed system at a room temperature for the HAS hyperbranched aminopolymer adsorbents at 400 ppm and 10% CO2 under humid conditions.

that, although aminopolymers can be grown off of the silica particle exterior under certain conditions, a significant portion of polymers is introduced inside the support pores. CO2 Adsorption Capacity: 400 ppm vs 10% CO2. As expected, changes of the CO2 partial pressure in the target gas can cause significant changes in the adsorption capacity of the amine-based solid adsorbents. For instance, Belmabkhout et al. reported that the equilibrium CO2 adsorption capacity of a triamine-functionalized pore-expanded MCM-41 (TRI-PEMCM-41) decreased from 2.05 mmol CO2/g adsorbent (hereafter, presented in the form of mmol/g) to 1.20 mmol/g when the CO2 concentration decreased from 5% to 1000 ppm.53 More recently, the same group reported that the adsorption capacity of triamine-functionalized MCM-41 corresponds to 2.18, 1.60, and 0.98 mmol/g when the CO2 concentration was 10%, 1%, and 400 ppm CO2, respectively.24,44 These data show that the adsorption capacity of a triamine-functionalized MCM41 decreased by a factor of 2.2 when the CO2 concentration was reduced by a factor of 250 (from 10% to 400 ppm).24 The observations suggest that the adsorption capacity of the aminebased solid adsorbents is not depreciated significantly even with a huge decrease of the CO2 concentration, consistent with the work described by us both here and previously.26 Similarly, the CO2 adsorption capacity of the HAS adsorbents varied along with the change of the CO2 concentration in the feed. A fixed bed flow system coupled with a mass spectrometer was employed to measure the equilibrium adsorption capacities of the HAS adsorbents at ambient temperature under simulated air capture conditions (400 ppm humidified CO2) and flue gas conditions (10% humidified CO2), respectively. The adsorption capacities of the HAS adsorbents from these measurements are illustrated in Figure 1. It shows that, similar to the changes observed in the TRI-PE-MCM-41 adsorbent, the HAS material is subjected to a capacity reduction upon CO2 dilution from 10% to 400 ppm, but the magnitude of the reduction is minor. For instance, the 400 ppm adsorption capacities of the HAS6 adsorbent varied from 3.77 mmol/g to 1.72 mmol/g when the

CO2 concentration was changed from 10% to 400 ppm. This corresponds to the decrease of the adsorption capacity by a factor of 2.2 with a dilution of the CO2 concentration by a factor of 250. It suggests that HAS materials can capture considerable amounts of CO2 from ultradilute CO2 sources, specifically at concentrations similar to those found in the ambient air, and thus can be a promising adsorbent for air capture applications. The air capture capacity of the HAS adsorbents can be tuned by controlling the amine loadings in the adsorbents. Previously, systematic adsorption experiments revealed that the adsorption capacity of the HAS adsorbents at simulated flue gas conditions (10% CO2, 75 °C) can be varied by changing the organic fraction of the hybrids.46 Figure 1 shows that, similar to the results under simulated flue gas conditions, the equilibrium adsorption capacity of the HAS adsorbents at simulated air capture conditions varies along with the aminopolymer content. For example, the CO2 adsorption capacities were elevated from 0.16 mmol/g to 1.72 mmol/g as the organic content increased from 2.3 mmol/g (HAS1) to 9.9 mmol/g (HAS6). It is also noteworthy that the HAS adsorbents with higher amine loadings suffer relatively less impact from inlet CO2 dilution. For instance, the adsorption capacity of the HAS6 adsorbent (amine loading of 9.9 mmol/g) decreased by a factor of 2.2 (from 3.77 mmol/g to 1.72 mmol/g), while that of the HAS1 (amine loading of 2.3 mmol/g) decreased by a factor of 8.4 (from 1.34 mmol/g to 0.16 mmol/g) along with the change of the CO2 concentration from 10% to 400 ppm. It is known that the heat of adsorption of CO2 on solid amine adsorbents increases as the CO2 partial pressure decreases,54 and thus CO2 adsorption onto amine-grafted adsorbents at low CO2 concentrations is dominated by strong chemical adsorption.53 On the other hand, it is also known that, as amine loading decreases, the heat of adsorption of amine-functionalized silica decreases and becomes close to that of the bare silica, due the preponderance of bare, unfunctionalized oxide surfaces in these low-loaded materials.55,56 Thus, for HAS materials with low amine loadings, the properties approach those of the bare silica, 2423



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Figure 2. The CO2 adsorption capacity and adsorption half time of the HAS adsorbents with different amine loadings, compared to those of other air capture sorbents published in the literature. The HAS adsorbents are shown as discrete points, while the literature sorbents are drawn as dotted lines because the kinetic data for these adsorbents under air capture conditions are not available.

and the effect of the CO2 dilution is more pronounced.35 Likewise, the relatively minor influence of the CO2 dilution on the HAS materials of high amine loading may be attributed to the dominant impact of the chemical adsorption at extremely low CO2 partial pressures by the large number of hyperbranched amines. In effect, this suggests that the adsorption capacity at the 10% CO2 conditions is elevated due to the presence of some physisorption under these conditions. This suggests that the HAS adsorbent with higher amine loadings may be a more appropriate adsorbent for the air capture applications, from a capacity perspective. Adsorption Kinetics. The kinetics of adsorption and desorption are another important characteristic in assessing the performance of adsorbents for CO2 capture. It is known that the adsorption kinetics of the supported amine adsorbents are influenced by CO2 diffusion to the bulk materials (boundary layer diffusion), mass transfer into and out of the pores, the type and density of the grafted amines, accessibility to the amine moieties, and intrinsic chemical reaction rates, among other factors.46 Thus, changes of the aminopolymer configuration with amine loading may affect the apparent adsorption kinetics of the HAS adsorbents as well. As a preliminary assessment of the adsorption kinetics of these materials under air capture conditions, the adsorption half time (the time when the adsorbent reaches half of its capacity) was measured using the fixed bed system and used as characteristic figure associated with the adsorption kinetics over the various solids. Figure 2 illustrates adsorption capacities and adsorption half times of the HAS adsorbents employed in this work under simulated air capture conditions (humid, 400 ppm CO2), along with those of other air capture sorbents reported in the literature to date. It shows that the adsorption half time of the HAS adsorbents under air capture conditions (except HAS2 which shows a slight decrease in adsorption half time), increases as the adsorption capacity increases i.e., at high amine loadings. For instance, the adsorption capacity of HAS adsorbents can be elevated up to 1.72 mmol CO2/g sorbent by increasing the amine loading to 9.9 mmol/g, but this is accompanied by an increase of adsorption half time to ∼167 min. Slowing of the adsorption

kinetics over high amine loading HAS materials was reported previously under simulated flue gas conditions (10% CO2, 75 °C) and was attributed to pore blocking that occurs as the amine loading is increased.46 It appears that these mass transfer limitations observed over highly loaded HAS materials also yield a detrimental effect on the adsorption kinetics under air capture conditions. This observation suggests that these HAS materials may have a key aminopolymer concentration over which the adsorption kinetics are significantly aggravated, and thus the structure of this hybrid should be carefully controlled to match the desired adsorption capacity and kinetics for air capture applications. Figure 2 also shows the adsorption capacities and kinetics of the low temperature air capture sorbents reported in the literature to date. It should be mentioned that the number of publications reporting air capture capacities is still very limited, and furthermore, their kinetic data are scarcely available. Only a few papers describe the kinetic behavior of the sorbents in terms of half conversion time13 or breakthrough time.24 For instance, an air capture study using TRI-PE-MCM-41 reported its breakthrough time of 167 min along with the adsorption capacity of 0.98 mmol/g.24 In the absence of kinetic data in the form of adsorption half-times, the results for air capture sorbents from the literature are placed on the figure as dotted lines, instead of a data point. It should be noted that a CaO-based adsorbent reporting an adsorption capacity using 500 ppm CO2 along with its half conversion time13 is not shown on this plot since this sorbent was tested under different conditions (high temperature (380 °C, 500 ppm CO2), rather than at ambient air capture conditions. Despite limited availability of kinetic data for most of the reported sorbents, this plot suggests that HAS materials may provide higher air capture capacities than other types of sorbents, including liquid absorbents, ion exchange resins, and zeolites. We suggest that plots of this type, correlating adsorption capacities and adsorption kinetics (e.g., adsorption half time), can be a useful way of visualizing the array of adsorption properties of a number of adsorbents and for identifying an appropriate adsorbent for practical CO2 capture applications. For instance, 2424



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Figure 3. Cyclic regenerability of the HAS4 adsorbent compared to a representative class 2 adsorbent. The working capacities of the HAS4 and the diamine-functionalized SBA-15 were measured by TGA over four adsorption/desorption cycles using dry 400 ppm CO2 at ambient temperature. An example of the multicycle TGA adsorption curve of the HAS adsorbent is shown in the inset.

sorbents located close to the bottom right of the plot can be regarded as more favorable CO2 capture adsorbents comprising both high adsorption capacity and fast adsorption kinetics. This plot also suggests that the HAS adsorbents that do not exhibit pore blocking, which retards adsorption kinetics, are needed for the air capture performance to be enhanced further. Adsorbent Regenerability. Regenerability of the adsorbents is another important parameter for practical air capture applications, as the adsorbent should retain stable CO2 adsorption performance for prolonged cyclic operation. The regenerability of the HAS adsorbents for flue gas capture was previously demonstrated over 10 cycles.45 Here, the HAS4 adsorbent was assessed by measuring its working capacity over four adsorption (25 °C) and desorption (110 °C) cycles using a TGA instrument. Recurring measurement of the CO2 uptake curves (Figure 3, inset) illustrated that the adsorptive performance of the HAS adsorbent was quite stable under the simulated air capture conditions, preserving its initial capacity without significant degradation. The regenerability of the HAS adsorbent was also compared with that of the class 2 diamine-functionalized solid made by reacting N-(3-(trimethoxysilyl)propyl)ethane-1,2-diamine (APAEPTMS) with SBA-15.45 Figure 3 shows that the HAS adsorbent reveals outstanding regenerability comparable to the class 2 adsorbent in short, multicycle operations. The material stability of the HAS adsorbent can be attributed in part to the fact that the amine components in the HAS are covalently bound to the SBA-15 support like those in the diamine-functionalized SBA-15.45,46 These stability test results suggest that the HAS material is robust enough to allow repeated temperature swing cycles without significant loss of amine content and thus can provide a stable air capture capacity during multiple adsorption/desorption operations. The stability can be further enhanced by running under humidified conditions.31,57 In this work, the potential of HAS adsorbents, which incorporate hyperbranched aminopolymers on and in porous silica supports, to be effective materials for extracting CO2 from the ambient air was demonstrated. In comparison to the conventional CO2 capture techniques targeting flue gases generated from large point sources, adsorptive CO2 fixation from the

ambient air provides a unique opportunity for a carbon negative technology, in that it can reduce the actual CO2 level in the atmosphere, in principle, if applied on a large scale.5,20 The adsorption characteristics of the HAS adsorbents, including the adsorption capacity, apparent adsorption kinetics, and adsorbent regenerability, were investigated using ultradilute CO2-containing gas mixtures that simulate the current atmospheric CO2 concentration. The experimental results showed that the HAS materials provide the ability to capture CO2 directly from simulated air efficiently, both under bone dry conditions and fully humidified conditions. In particular, HAS materials could be prepared to provide a higher adsorption capacity than a conventional class 2 adsorbent incorporating covalently bound amines, while capturing CO2 reversibly without significant degradation of performance in short, multicyclic operations. The structure of the HAS adsorbents can be tuned to present favorable adsorption properties simply by controlling the amount of initial monomer during the adsorbent synthesis. Materials of this type, if incorporated into appropriate, scalable processes, may one day be important components of practical air capture technologies. Shorter term applications might include production of mildly concentrated CO2 for industrial use (e.g., carbon feedstock for algae farms utilizing enclosed bioreactors), while on a grander scale, air extraction can be considered as a means of addressing the rising global atmospheric CO2 concentration.5,20

’ ASSOCIATED CONTENT

bS

Supporting Information. Figures S1-S3 and text. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Fax: (404)894-2866. E-mail: [email protected].

’ ACKNOWLEDGMENT Funding for the work on air capture was provided by Global Thermostat, LLC. 2425



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