N2 selectivity in amidoxime-modified porous carbon

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Available at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/carbon

Enhanced CO2/N2 selectivity in amidoxime-modified porous carbon Shannon M. Mahurin Sheng Dai a,b,* a b

a,* ,

Joanna Go´rka a, Kimberly M. Nelson b, Richard T. Mayes a,

Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States Dept. of Chemistry, University of Tennessee, Knoxville, TN 37996, United States

A R T I C L E I N F O

A B S T R A C T

Article history:

In this work, we examine the use of the amidoxime functional group grafted onto a hierar-

Received 19 July 2013

chical porous carbon framework for the selective capture and removal of carbon dioxide

Accepted 9 October 2013

from combustion streams. Measured CO2/N2 ideal selectivity values for the amidoxime-

Available online 19 October 2013

grafted carbon were significantly higher than the pristine porous carbon with improvements of 65%. Though the overall CO2 capacity decreased slightly for the activated carbon from 4.97 mmol g1 to 4.24 mmol g1 after surface modification due to a reduction in the total surface area, the isosteric heats of adsorption increased after amidoxime incorporation indicating an increased interaction of CO2 with the sorbent. Total capacity was reproducible and stable after multiple adsorption/desorption cycles with no loss of capacity suggesting that modification with the amidoxime group is a potential method to enhance carbon capture. Ó 2013 Elsevier Ltd. All rights reserved.

1.

Introduction

The persistent rise of atmospheric CO2 levels continues to be a principal cause for concern because of potential environmental consequences, thus motivating considerable research to find innovative means for removing CO2 from combustion products. Though a range of carbon capture and removal strategies have been proposed and developed, the use of liquid absorbents with incorporated amine groups that selectively bind and capture CO2 through chemical absorption processes remains the primary industrial approach. However, because of several inherent drawbacks such as amine degradation, evaporative losses and relatively high regeneration energy, there has been significant interest in developing novel sorbents to replace these amine-based materials. Porous solids have been proposed as one alternative to liquid-based absorbents for the selective removal of CO2 from

combustion streams because they avoid the issues of corrosion and sorbent volatility. Materials such as metal–organic frameworks (MOFs), zeolitic imidazolate frameworks (ZIFs), covalent organic frameworks (COFs), and activated carbons have all been considered for CO2 separation and are in varying stages of development [1–5]. While materials such as MOFs have received much attention recently, porous carbon continues to be a particularly intriguing alternative sorbent material because of its high surface area, excellent thermal stability and chemical resistance, potential for surface functionalization, and wide availability [6]. Though the CO2 adsorption capacity of activated carbon often falls below 3 mmol g1 (25 °C, 1 atm), a number of recent reports have shown that significant improvements in CO2 uptake can be achieved through either optimization of the pore architecture, which can be tuned through appropriate synthesis parameters, or the addition of surface functionality [7,8]. For example, Sevilla

* Corresponding authors. E-mail addresses: [email protected] (S.M. Mahurin), [email protected] (S. Dai). 0008-6223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2013.10.018

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et al. recently highlighted the importance of both microporosity and the presence of polar, basic functional groups in improving CO2 adsorption [9]. The ability to modify the carbon surface is especially important because it offers a means to incorporate surface functional groups with enhanced affinity for CO2 to increase the adsorption capacity. In particular, integrating nitrogencontaining functional groups into the framework of porous carbon has become popular because of the demonstrated improvements in the adsorption capacity of various carbon materials with the addition of these nitrogen groups [10–12]. Though nitrogen functionality has been the most studied, other functional groups such as oxygen-containing ether and hydroxyl groups can also interact with CO2 via electrostatic interactions leading to enhanced CO2 adsorption capacity [13]. For example, Plaza et al. investigated the impact of oxidation on the CO2 capacity of phenolic resin carbon and observed an enhancement in uptake in the oxidized carbon as compared to the parent carbon [14]. In this work, we show that the amidoxime (AO) group which contains both nitrogen and oxygen functionality (see Fig. 1) sonochemically grafted onto a porous carbon matrix can serve as an effective sorbent for the selective capture of CO2. Though it has been used for uranium adsorption [15,16], the amidoxime group has received almost no attention for CO2 separation despite being structurally similar to the amine while at the same time possessing a hydroxyl group which offers the potential for multiple interaction sites for CO2 binding. Zulfiqar et al. recently reported one of the few studies of amidoxime, presenting both theoretical and experimental work that demonstrated the potential of amidoxime for CO2 separation [17]. Patel modified polymers of intrinsic microporosity with amidoxime and observed an increase in CO2 adsorption capacity [18]. In our work, we grafted the amidoxime group onto the surface of porous carbon synthesized by a soft-templating method and measured CO2 adsorption parameters. The addition of the amidoxime group provided a significant enhancement in the CO2/N2 selectivity due to increased CO2 interaction while the overall capacity was reduced due to a decrease in the surface area. The AOmodified carbon was also very stable with no loss of adsorption capacity after five adsorption/desorption cycles suggesting that surface modification using the amidoxime group is a viable approach to enhance CO2 adsorption performance.

2.

Experimental

2.1.

Materials

All reagents were used as received without further purification. Poly(ethylene oxide) – poly (propylene oxide) – poly(ethylene oxide) triblock copolymer (EO106PO70EO106, Pluronic F127; BASF), resorcinol, formaldehyde, acrylonitrile, benzoyl peroxide (BPO), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), methanol, potassium hydroxide (KOH) and hydroxylamine were purchased from Sigma Aldrich. Ethylene carbonate (EC) was acquired from Acros Organics, hydrochloric acid (HCl) was purchased from Fisher Scientific, and ethanol (EtOH, 200 proof) was obtained from Decon Laboratories Inc.

2.2.

Preparation of mesoporous carbon supports

The ordered mesoporous carbon (OMC) materials were synthesized using a previously reported, soft-templating method based on the self-assembly of phenolic resin and a block copolymer under highly acidic conditions [19]. In order to improve the specific surface area of the carbon, the OMCs were subjected to a standard base (KOH) activation in which the carbon–KOH mixture (1:3 ratio) was heated at 850 °C for 2 h under flowing nitrogen with a ramp rate of 5 °C/min. The resulting activated OMC, denoted as ‘‘aC,’’ was washed with DI water followed by washing with dilute hydrochloric acid then dried at 80 °C for 24 h. For comparison, a set of the activated carbon was subsequently treated with nitric acid to modify the surface prior to grafting and these samples are denoted by ‘‘oaC.’’

2.3.

Grafting of organic molecules onto carbon support

Sonochemical grafting of polyacrylonitrile (PAN) was performed using two sonication systems of different intensities, a standard ultrasonic cleaning bath (Bransonic 2510, operating at 100 W and 42 kHz) and a sonic probe system supplied by Sonics and Materials Inc. (Vibra Cell VCX750) operating with an adjustable power of 750 W maximum and 25 kHz. Incorporation of the amidoxime group onto the mesoporous carbon consisted of first grafting PAN onto the carbon framework via sonochemical polymerization followed by chemical conversion of the grafted PAN to amidoxime. A detailed description of the synthesis has been previously reported [16]. Briefly, 0.3 g of activated mesoporous carbon was added to 1 g of initiator (BPO) in acetone with subsequent evaporation of the acetone at room temperature. To graft PAN into the pores of the carbon, a round bottom flask equipped with a condenser and a nitrogen purge was placed in a sonication bath and charged with 100 mL of the solvent mixture (DMF/methanol or EC/H2O), BPO-impregnated carbon and 16.6 mL of acrylonitrile. After three cycles of evacuation and refilling with nitrogen, the polymerization was performed under N2 flow and sonication at 60–70 °C for either 5 h. using the bath sonicator or 2 h. at 50% power using the probe sonicator. The final product was collected by centrifugation and washed with DMF, methanol and acetone then dried overnight at 80 °C. The conversion to amidoxime was performed using 10% hydroxylamine in a 50/50 solution of H2O/methanol for 72 h at 80 °C in a closed flask.

Fig. 1 – Structure of the amidoxime functional group.

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Characterization

Nitrogen adsorption isotherms were measured at 196 °C using a TriStar 3000 volumetric adsorption analyzer (Micromeritics Instrument Corp.). Prior to the adsorption measurements, the carbon powders were degassed under flowing nitrogen for 2 h at 110 °C to remove any residual water. The specific surface area of the samples was calculated using the Brunauer–Emmett–Teller (BET) method in the relative pressure range of 0.05–0.20. Micropore volume and micropore surface area were obtained from a t-plot using carbon black STPA in the range of 0.45–0.60 nm film thickness. Pore size distributions (PSD) were calculated using the Barrett–Joyner– Halenda (BJH) algorithm for cylindrical pores according to the improved Kruk–Jaroniec–Sayari (KJS) method [20,21] calibrated for pores up to 12 nm. Thermogravimetric measurements were obtained on a TGA 2950 thermogravimetric analyzer (TA Instruments) in the temperature range of 30–800 °C under flowing nitrogen. Transmission electron microscopy (TEM) imaging was performed on a Hitachi HD-2000 scanning transmission electron microscope (STEM) with an accelerating voltage of 200 kV. Gas uptake measurements were obtained using a gravimetric microbalance (Hiden Isochema, IGA). Approximately 60 mg of the carbon–AO was loaded into the sample container and sealed in the stainless steel chamber. The sample was dried and degassed at a temperature of 65 °C and a vacuum pressure of 1 mbar until the mass equilibrated before recording the dry mass. Mass measurements were then acquired at various CO2 pressures up to 1 bar. The temperature of the sample was maintained using a constant-temperature recirculating water bath (VWR).

3.

459

Results and discussion

The soft-templating method used to synthesize the initial porous carbon generates a worm-like pore structure with interconnected pores. Activation effectively creates additional microporosity in the carbon framework which increases the overall surface area compared to the pristine mesoporous carbon. The AO functional group was then grafted onto the activated carbon at two different loadings, 10.6% and 5.4%. We have previously shown that the grafting procedure generates AO attached to the carbon surface. The AO-modified carbon material was examined by X-ray photoelectron spectroscopy (XPS) and the presence of the NH2–C@N–OH functionality was confirmed by both the C1s and N1s spectra [16]. In order to establish the impact of AO grafting on the pore structure of the carbon and to determine the conformation of the deposited AO, SEM images were acquired for both the aC and the AO-modified (aC–AO) carbon (see Fig. 2). Comparison of the microscopy images shows that the addition of the AO group does not alter the primary structure of the carbon. The overall structure of the aC–AO carbon is clearly similar to the worm-like structure of the unmodified aC and the mesopore size appears to be in the range of 9 nm which is comparable to the pore diameter of the unmodified carbon. Furthermore, the deposited AO is not visually apparent which further indicates that grafting occurred primarily within the pores of the carbon as previously described [16] and that the grafted AO surface layer is relatively thin. Though not shown, AO modification of the oaC sample showed similar results indicating that in general the grafting process does not change the morphology of the carbon framework. The textural properties of the aC, oaC and AO-modified carbon materials were further examined using nitrogen

Fig. 2 – Representative SEM images of the (a) activated carbon, (b) aC-AO1 and (c) aC-AO2 showing the similarity in the pore structure of the carbon pre- and post-modification.

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(a)

(b)

Fig. 3 – (a) N2 adsorption isotherms and (b) pore size distributions for the aC, oaC, and AO-modified carbon (for clarity, the N2 adsorption isotherms and pore size distributions for the oaC series were shifted up by 800 and 0.5, respectively). (A color version of this figure can be viewed online.)

Table 1 – Textural properties of the activated and AO-modified carbon. Sample

SBET (m2 g1)

Smi (m2 g1)

Vmi (cm3 g1)

Vme (cm3 g1)

Vt (cm3 g1)

Grafting (%)

aC aC-AO1 aC-AO2 oaC oaC-AO1 oaC-AO2

1857 1288 1512 1400 783 494

1282 866 1026 935 395 254

0.59 0.40 0.47 0.42 0.18 0.11

0.63 0.60 0.59 0.53 0.38 0.39

1.08 1.09 1.27 1.02 0.64 0.52

– 10.6 5.4 – 6.8 6.2

Notation: SBET – BET specific surface area; Smi – micropore surface area obtained by t-plot using carbon black STPA in the range of 0.45–0.60 nm film thickness; Vmi – volume of fine pores (mainly micropores) obtained by t-plot analysis; Vt – single-point pore volume; Vme – volume of mesopores obtained by integration of PSD from 4 nm to 20 nm.

(a)

aC aC-AO1 aC-AO2 oaC oaC-AO1 oaC-AO2

-1

-1

CO2 Absorbed (mmol g )

4

CO2 Absorbed (mmol g )

aC aC-AO1 aC-AO2 oaC oaC-AO1 oaC-AO2

5

(b)

4

3 2 1

3

2

1

0

0 0

200

400

600

800

1000

Pressure (mbar)

0

200

400

600

800

1000

Pressure (mbar)

Fig. 4 – CO2 adsorption isotherms for the aC, oaC, and AO-modified material acquired at (a) 0 °C and (b) 25 °C. (A color version of this figure can be viewed online.) adsorption isotherms. Fig. 3 shows the N2 adsorption isotherms and corresponding pore size distributions for the activated carbon, the oxidized-activated carbon and the

AO-modified carbon. All the samples exhibit Type IV isotherms and H1-type hysteresis loops which indicate the presence of a uniform array of cylindrical mesopores. Grafting of

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Table 2 – Adsorption properties of activated and AO-modified carbon. Sample

aC aC-AO1 aC-AO2 oaC oaC-AO1 oaC-AO2

CO2 adsorption (1 bar, mmol g1)

CO2/N2 selectivity

0 °C

25 °C

0 °C

25 °C

4.970 4.237 4.304 3.979 3.101 2.388

2.871 2.489 2.498 2.647 2.068 1.470

14.4 23.8 21.6 18.1 24.4 20.8

11.2 22.4 20.4 18.1 24.1 20.4

5.5

CO2 Adsorption (mmol/g)

5.0

aC oaC

4.5 4.0 3.5 3.0 2.5 2.0 200

400

600

800

1000

1200

1400

2

Smicro (m /g) Fig. 5 – CO2 adsorption as a function of micropore surface area showing a linear relationship. The line is present to guide the eye.

the amidoxime onto both the aC and oaC samples caused a decrease in the adsorption parameters with minor variation in the shape of the isotherm most likely the result of the different sonication conditions. As shown in Table 1, which includes the surface areas and pore volumes for the aC, oaC and AO-modified carbon, the grafting process resulted in significant reductions in the BET surface area (from 1857 m2/g to 1288 m2/g for aC-AO1 and 1512 m2/g for aC-AO2) and micropore volume (from 0.59 cm3/g to 0.40 cm3/g for aC-AO1 and 0.47 cm3/g for aC-AO2). As the grafting yield decreased, there was a greater reduction in micropore volume than mesopore volume. In addition, the size distributions show a slight narrowing of the micropore peak and a reduction in the height while the mesopore peak shifted only slightly. The large reduction in micropore volume and surface area coupled with the relatively small change in the mesopore diameter suggests that the amidoxime primarily deposited onto the micropores and small mesopores.

3.1.

CO2 adsorption capacity

The CO2 absorption isotherms shown in Fig. 4 were first measured for the aC and aC-AO samples up to a maximum pressure of 1 atm and at two different temperatures, 0 °C and 25 °C noting that the measurements were performed in the

DH (kJ/mol)

23.3 24.0 25.1 16.5 26.1 22.7

absence of water. While the AO group is stable in water, there might be some competition between CO2 and water for the AO absorption sites or trapping of water in the micropores which could modify the CO2 capacity in real flue gas. With this in mind, the CO2 and N2 adsorption capacities are given in Table 2. For the activated carbon, the CO2 adsorption capacity was measured to be 2.87 mmol/g at 1 bar, 298 K and 4.970 mmol/g at 1 bar, 273 K. Though the CO2 uptake values for activated carbon can vary extensively depending on the nature of the carbon and the exact activating parameters, the capacity values measured for the aC are comparable to other studies on activated carbon [7,14,22]. After AO grafting, the CO2 capacities for both loadings were similar at 4.2 mmol g1 for aC-AO1 and 4.3 mmol g1 for aC-AO2, despite the differences in the surface area and AO grafting yields. This indicates that the adsorption capacity does not exclusively correlate with grafting yield; the relationship is more complex. The capacities for both grafting yields were also lower than the original activated carbon which, at first glance, creates the impression that the addition of the AO group was ineffective. However, the reduction in CO2 capacity after AO-modification can be attributed to the decrease in the surface area and micropore volume for the AO-carbon. Fig. 5 demonstrates this effect by showing the linear relationship between micropore surface area and CO2 adsorption capacity. This is similar to previous reports for amine-modified carbon where the addition of the amine produced significant reductions in the overall capacity [23]. It is important to note that when the CO2 capacities are normalized to the total surface area, the AO-carbon materials showed a higher capacity than the activated carbon suggesting enhanced CO2 interaction. To explore the effect of surface treatment prior to AO grafting, we also measured the CO2 adsorption capacity of the oaC and oaC–AO carbon where the activated carbon was oxidized using nitric acid prior to incorporation of the AO group. Grafting yields for the two oaC–AO samples were similar at 6.8% and 6.2%. From Table 1, the oxidation process decreased both the surface area and pore volume of the oaC compared to the activated carbon (aC) with an additional reduction in surface area resulting from subsequent AO grafting. Consequently, the available surface area for CO2 uptake was significantly reduced in all of the oaC and oaC– AO samples compared to the aC and aC–AO samples. Despite the similarity in grafting yields for the two oaC–AO samples, the measured surface areas were very different possibly due to minor variations in the grafting procedure. Furthermore, the grafting yield for the oaC carbon was comparable to

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3.2.

Fig. 6 – Isosteric heats of adsorption calculated using the Clausius–Clapeyron equation. (A color version of this figure can be viewed online.)

the grafting yield for the aC carbon indicating that the oxidation process did not improve grafting yield which agrees with previous reports [16]. It is difficult to make a direct comparison of the effect of grafting yield on the adsorption capacity for the aC versus the oaC because of the influence of both the oxidation process and available surface area. However, we can make a general comparison and note that the CO2 adsorption capacity for the oaC–AO carbon was lower than the corresponding aC– AO material. More specifically, oaC-AO2 exhibited a lower CO2 capacity than oaC–AO1 despite a similar AO grafting yield while aC-AO2 had a slightly higher capacity than aC-AO1 despite a much lower grafting yield. These results can again be attributed to differences in the surface area of the oxidized carbon samples. For both materials, a higher AO grafting yield produced a higher CO2/N2 selectivity as will be discussed in more detail in the Section 3.2. Not surprisingly, the CO2 adsorption capacity of the oaC carbon was reduced after AO modification because of the decreased surface area again highlighting the importance of available surface area in determining the total CO2 capacity. Interestingly, the capacities of the oaC material are higher than the aC material when normalized to the total surface area.

CO2 Adsorption (mmol/g)

4.0 aC aC-AO1 aC-AO2

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 1

2

3

4

5

Cycle

Fig. 7 – Reproducibility of CO2 uptake at 25 °C.

CO2/N2 selectivity and interaction energy

Ideal selectivity values were obtained by applying a linear fit to the adsorption isotherms at low pressure (below 200 mbar) and calculating the ratios of the slopes. This provides a suitable measure of the selectivity at pressures appropriate for flue gas applications where the CO2 partial pressure will be much less than 1 bar. Despite the reduction in overall CO2 capacity, AO modification led to significant improvements in the ideal CO2/N2 selectivity for both the aC and oaC carbon. For example, the selectivity for aC-AO1 was 65% higher than the selectivity capacity for aC increasing from 14.4 to 23.8. The selectivity values obtained for the AO-carbon represents a marked enhancement in selectivity compared to previous reports for a variety of porous carbon materials [7,8]. Moreover, this improvement in selectivity is likely due to an increased interaction between CO2 and the AO functional group located on the carbon surface and illustrates the potential of AO modification as a method to enhance CO2 capture. The isosteric heats of adsorption for the aC, oaC and AOmodified carbon were measured as a means to explore the interaction of CO2 with the carbon materials and further elucidate the origin of the enhanced selectivity. This was accomplished by obtaining adsorption isotherms at two different temperatures (0 °C and 25 °C) and fitting the data to the Dual Langmuir Model. Results from the fitting were then used with a variant of the Clausius–Clapeyron equation shown below [24,25]:     P1 T2  T1 ln ¼ DHads ð1Þ P2 R  T1  T2 where P is the pressure, T is the temperature and R is the ideal gas constant to obtain the isosteric heats of adsorption. The curves for the calculated heats of adsorption are shown in Fig. 6. For the aC and aC–AO carbon, the heats of adsorption decreased as the CO2 surface coverage increased until reaching a value in the range of 23–25 kJ/mol which is typical for porous materials such as activated carbon [26] and indicates relatively strong interaction between the CO2 molecule and the heterogeneous pore walls of the carbon. The aC-AO2 material exhibited a higher initial heat of adsorption than both the aC and aC-AO1 at low CO2 surface coverage which is due to the presence of the AO group and the increased surface area of aC-AO2 compared to aC-AO1. These results indicate a stronger interaction between CO2 and the AO functionality on the carbon surface. In comparison to the aC carbon, the heats of adsorption for oaC and oaC–AO carbon were generally lower but there was a much greater relative increase after AO-modification for the oaC carbon with the heat of adsorption increasing from 16.5 kJ/ mol for oaC to 26.1 kJ/mol for oaC-AO1. The oaC and oaC– AO carbon also exhibited an unexpected rise in the heats of adsorption at low surface coverage which is contrary to typical porous materials. In general, the interaction energies for the oxidized materials initially increased with CO2 sorption before reaching a stable heat of adsorption. This could indicate some type of cooperative binding effect which has been observed for amine-functionalized MOFs [27], though the precise reason for these results is currently unclear.

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Overall, the addition of the AO group to the carbon framework resulted in an increase in the heat of adsorption indicative of enhanced CO2 interactions with surface functional groups and all of the values were in the range of physical adsorption which will facilitate the regeneration of the sorbent.

3.3.

Stability of AO-modified carbon

One of the important properties of an adsorbent material is the reproducibility of the CO2 adsorption capacity over a number of adsorption/desorption cycles. The stability of the CO2 adsorption was examined for the aC and aC-AO carbon using a TGA where the carbon was repeatedly exposed to CO2 at 25 °C followed by heating to 80 °C to remove the CO2. Though flue gas contains a variety of components such as H2O, SOx, NOx and residual O2 at low concentrations that could affect the stability of the AO functional group through hydrolysis, particularly at elevated temperature, we focus on the reproducibility of the AO-modified carbon under ideal conditions to obtain a baseline measure of the stability of the material. Fig. 7 shows five adsorption/desorption cycles demonstrating the excellent reproducibility of the AO-modified carbon with no loss of CO2 capacity. Full adsorption capacity was achieved in approximately 30 min and desorption was accomplished with mild heating conditions. This is in agreement with the heats of adsorption which showed that the interaction was based on physical adsorption. These results show that the AO-modified carbon can be repeatedly cycled with no loss in adsorption capacity and rapid uptake kinetics.

4.

Conclusion

We report for the first time the incorporation of the amidoxime functional group into a porous carbon framework to selectively bind CO2. The presence of the functional group causes a reduction of the surface area and a slight decrease in the total CO2 adsorption capacity though the AO modification increased the adsorption capacity when normalized to available surface area. AO modification of the carbon also resulted in a substantial increase in ideal CO2/N2 selectivity due to an enhanced interaction of CO2 with the nitrogen-containing amidoxime functional group. The AO-modified carbon shows excellent stability with no loss in adsorption capacity after five cycles. Further improvements are expected if the grafting yield can be optimized to minimize the reduction in surface area while simultaneous providing CO2 binding sites. These results highlight the importance of both porosity and functionality for improving the selective absorption of CO2 and demonstrate that AO functionalization is a viable means to enhance CO2 capture performance for gas separation applications.

Acknowledgements This work was fully sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy.

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