Carbon Dioxide Adsorption onto Polyethylenimine-Functionalized

Article pubs.acs.org/EF

Carbon Dioxide Adsorption onto Polyethylenimine-Functionalized Porous Chitosan Beads Junpei Fujiki and Katsunori Yogo* Chemical Research Group, Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawadai, Kizugawa, Kyoto 619-0292, Japan S Supporting Information *

ABSTRACT: Polyethylenimine-functionalized porous chitosan (PEI−CS) beads were prepared and their CO2 adsorption performance was evaluated. The CO2 adsorption capacity of PEI−CS was dependent upon both the amine content and surface area of the functionalized beads. PEI−CS showed a CO2 adsorption capacity of 2.3 mmol/g at 313 K and 15 kPa of CO2 in the absence of water vapor that considerably increased to 3.6 mmol/g in the presence of water vapor. To rationalize this phenomenon, the CO2 adsorption mechanisms in the absence and presence of water vapor were investigated by diffuse reflectance infrared Fourier transform spectroscopy. The results indicated that the mechanism of CO2 adsorption onto PEI−CS, in both the absence and presence of water vapor, involved the formation of carbamate. Therefore, the higher CO2 adsorption capacity in the presence of water vapor was attributed to the increased accessibility to amino groups of PEI−CS, owing to swelling of the polyethylenimine chain and/or chitosan framework upon adsorption of water. The herein reported chitosan-based material displays high CO2 adsorption capacity as well as excellent regenerability and, thereby, shows potential as an adsorbent for CO2 capture.

1. INTRODUCTION Global warming has become a major concern during recent decades, and many attempts have been made to prevent increases in the atmospheric concentration of CO2, which is considered to be one of the major causes of climate change.1−7 The separation and recovery of CO2 from large point sources, such as coal-fired power plants, assist in reducing CO2 emissions. For example, liquid amine absorption, adsorption, and membrane separation have been proposed for CO2 recovery.1−8 More specifically, CO 2 capture based on absorption using aqueous amine solvents is the most promising technique. However, application of such a technique has limitations, owing to the high regeneration energy and large equipment requirements as well as problems associated with solvent degradation and equipment corrosion. One of the alternative techniques being considered to overcome such drawbacks is adsorptive separation. Many adsorbents, including zeolites, carbonaceous materials, silica materials, metal organic frameworks (MOFs), and polymers, have been investigated for efficient CO2 capture.1−7 Recently, amine functionalization of porous supports has been considered to be one of the most outstanding approaches for CO2 capture because of the following advantages: high CO2 adsorption capacities in both the absence and presence of water vapor, fast CO2 adsorption and desorption, and easy regeneration.2−7,9−12 To develop efficient amine-modified sorbents, appropriate support properties, including high surface area, large pore volume, and large pores, are required. Mesoporous silica materials are ideal supports because their properties can be tailored easily. Consequently, amine-modified mesoporous silica materials (e.g., amine-grafted and amine-impregnated materials) have been widely investigated as potential adsorbents for CO2 capture.2−7 Drese et al. reported an amine-grafted-type © 2014 American Chemical Society

adsorbent with a high amine content coined as hyperbranched aminosilica. The material displayed a considerably high CO2 adsorption capacity ranging from 2.0 to 5.6 mmol/g toward prehumidified 10% CO2 at 298 K.13 Qi et al. prepared tetraethylenepentamine- and polyethylenimine (PEI)-impregnated mesoporous capsules.14 These sorbents exhibited adsorption capacities of approximately 4.5−7.9 mmol/g toward prehumidified 10% CO2 at 348 K. Additionally, our group reported the synthesis of a solid sorbent loaded with a mixture of amine molecules that displayed a high adsorption capacity (5.9 mmol/g) toward pure CO2 at 323 K.15 Recently, we reported the synthesis and application of polyethylenimine-functionalized chitosan adsorbent (PEI−CS) beads for CO2 capture.16 PEI−CS displayed relatively high CO2 adsorption capacities of 2.3 mmol/g in the absence of water vapor and 3.6 mmol/g at 313 K and 15 kPa in the presence of water vapor. This chitosan (CS)-based adsorbent is a potentially low-cost material because CS is a biomass waste and available in large quantities. Furthermore, PEI−CS can be shaped accordingly to macroscopic-sized beads (∼2 mm in diameter) that can overcome issues typically encountered in current processes, such as plugging, clogging, and packing problems. To date, most CO2 adsorption reports in the literature have examined the use of mesoporous silica-based sorbents in powder form. However, for practical application, pellet- or granule-type sorbents are preferred to avoid packing and handling problems. Accordingly, CS-based materials are potential candidates as adsorbents for effective CO2 capture. CS-based adsorbents have commonly been applied in liquidReceived: April 30, 2014 Revised: September 9, 2014 Published: September 11, 2014 6467

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material at 343 K for 6 h under vacuum. Measurements obtained on BELSORP-BG are based on a gravimetric−volumetric method for determination of the binary component gaseous adsorption isotherms.28−30 The water vapor adsorption isotherms were measured on BELSORP-aqua3 (BEL Japan, Inc.) after pretreatment of the adsorbent material on BELPREP-vacII (BEL Japan, Inc.) at 343 K for 6 h under vacuum. TG curves in He flow following CO2 and/or H2O adsorption were obtained on a thermogravimetric analyzer (Thermo plus TG 8120). After pretreatment of the adsorbent material under He flow, the sample adsorbent was contacted with the simulated flue gas (20% CO2 and 80% N2) in the presence and absence of saturated water vapor at 303 K or humidified He overnight. Desorption of CO2 and/or H2O was performed by heating the CO2- and/or H2O-sorbed material from 303 to 423 K in He stream at a rate of 1 K/min and subsequent He flow for 10 min. The cyclic CO2 adsorption−desorption tests were performed using laboratory-scale equipment (see Figure S1 of the Supporting Information). The adsorbent beads were packed in a stainless-steel column (length, 10 cm; inner diameter, 0.74 cm) and heated at 373 K for 1 h under an Ar flow (50 cm3/min) to remove adsorbed moisture and CO2. After the column was cooled to 313 K under Ar flow, the simulated flue gas (20% CO2 and 80% N2; flow rate, 30 cm3/min) was fed to the column through the water bubbler maintained at 313 K. Following CO2 adsorption for 1 h, the gas flow was switched back to Ar flow (50 cm3/min) and the temperature of the column was raised to 373 K and maintained for 1 h. The effluent concentration of CO2 was analyzed periodically using two online gas chromatographs (GC332 and GC-3200, GL Science, Inc.) equipped with thermal conductivity detectors. The adsorption−desorption cycle was repeated several times to evaluate the regenerability and stability of the adsorbent, and the amount of CO2 adsorbed in each cycle was calculated from the desorption curve. 2.5. In Situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). The CO2 adsorption mechanisms on PEI− CS in both the absence and presence of water vapor were investigated using DRIFTS (Shimadzu Co., Ltd.). Prior to analysis, PEI−CS was ground, then introduced into the DRIFTS cell, and pretreated in flowing He at 343 K for 30 min. The DRIFTS spectra were then collected under a He, humid He, CO2, or humid CO2 atmosphere at 313 K after flowing each gas for 30 min. Humidity was introduced in the chamber by bubbling the respective dry gas in a water reservoir at 313 K. In this study, 20% CO2 (N2 balance) was used as flow gas. Diamond powder was used as the background. The IR resolution was 2.0 cm−1.

phase adsorption toward heavy metals, precious metals, and rare-earth metals recovery.17−21 However, gaseous-phase adsorption onto CS-based adsorbents has hardly been examined. The few reported applications of CS-based adsorbents for CO2 capture show the potential of such materials; however, low adsorption capacities have been observed for gaseous-phase adsorption.22−25 Furthermore, CS has been proposed as a carbon precursor for the preparation of nitrogen-doped porous carbon for CO2 capture.26 In another approach, Alhwaige et al. reported the synthesis and application of calcined hybrid graphene oxide−CS aerogel as an adsorbent that displayed an adsorption capacity of 4.15 mmol/g at 298 K and 100 kPa.27 Thus, CS-based aerogels, including PEI−CS, show potential as highly efficient CO2 adsorbents. To improve the adsorption performance, elucidation of the CO2 adsorption mechanism onto PEI−CS is required. Therefore, this study focused on the adsorption mechanism of CO2 onto PEI−CS and optimization of the preparation conditions.

2. EXPERIMENTAL SECTION 2.1. Materials. CS (low molecular weight) was obtained from Sigma−Aldrich. Lactic acid, sodium hydroxide (NaOH), and isopropyl alcohol (IPA) were obtained from Junsei Chemical Co., Ltd. (Japan). Both ethylene glycol diglycidyl ether (EGDE) and epichlorohydrin (ECH) were obtained from Tokyo Kasei Kogyo Co., Ltd. (Japan). PEIs of different molecular weights (Mw = 10 000, 25 000, and 50 000−100 000) were obtained from Wako Pure Chemical Industries, Ltd. (Japan), Sigma−Aldrich, and Alfa Aesar, respectively. 2.2. Preparation of Amine-Functionalized CS Beads. PEI−CS beads were prepared using a method reported by Kawamura et al.19 Typically, CS powder was dissolved in 5 wt % lactic acid aqueous solution. Then, the CS solution was dropped in 10 wt % NaOH aqueous solution to generate CS beads. After washing with pure water, 100 cm3 of CS was suspended in 100 cm3 pure water and EGDE was added as a cross-linking agent. The cross-link reaction was conducted at 343 K for 3 h. After washing with pure water and IPA, 75 cm3 of cross-linked CS was reacted with ECH (anchoring agent) in IPA at 323 K for 2 h. To increase the content of the amino groups, CS was reacted with 30 wt % PEI aqueous solution at 353 K for 3 h. The obtained PEI−CS beads were freeze-dried using a freeze-dryer (FDU1200, Tokyo Rikakikai Co., Ltd., Japan) after washing with pure water until the pH of filtrate was neutral. 2.3. Characterization. All materials were characterized by N2 adsorption−desorption isotherms measured at 77 K on a Micromeritics ASAP 2420 automatic adsorption system. The specific surface area (SBET) and pore size distribution were calculated using the Brunauer−Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) models, respectively. The pore diameter (DBJH) corresponds to the maxima of the pore size distribution, and the total pore volume was calculated from the amount of N2 adsorbed at a relative pressure P/P0 = 0.99. The nitrogen content (N content) of the materials was determined by elemental analysis on a PE-2400 (PerkinElmer). Infrared (IR) spectra were recorded using the KBr pellet method on a Fourier transform infrared (FTIR) IRPrestige-21 (Shimadzu Co., Japan). The thermal stability of the samples in a He flow was examined by thermogravimetry−differential thermal analysis (TG−DTA) on a Thermo plus TG 8120 (Rigaku Co., Japan). The crystalline structure of the samples was analyzed by X-ray diffraction (XRD) on RINT2200-UltimaIII (Rigaku Co., Japan). 2.4. Adsorption Experiments. The CO2 adsorption isotherms in the absence of water vapor were measured at 313 K on a ChemiSorb HTP chemical adsorption analyzer (Micromeritics, Japan) after pretreatment of the adsorbent material under a He flow at 343 K for 6 h. In contrast, the CO2 adsorption isotherms in the presence of water vapor (moisture pre-adsorption) at 313 K were obtained on BELSORP-BG (BEL Japan, Inc.) after pretreatment of the adsorbent

3. RESULTS AND DISCUSSION 3.1. Characterization of PEI−CS. PEI−CS beads were prepared using the procedure described in section 2.2. Typical IR spectra of raw CS and products at each step of the preparation are shown in Figure S2 of the Supporting Information. After the cross-linking reaction, the intensity of the bands at 2875 and 2920 cm−1, corresponding to CH stretching, increased and the intensity of the band at 1590 cm−1, corresponding to NH bending, decreased slightly. This may be due to the reaction between the amino group of CS and cross-linker EGDE. The bands at 1590, 3300, and 3350 cm−1, corresponding to NH bending and NH asymmetric and symmetric stretching, respectively, disappeared following the reaction between the cross-linked CS and anchor agent ECH, suggesting that ECH reacted with the amino group. Likewise, following PEI modification, bands at 1460, 1590, 3300, and 3350 cm−1, corresponding to CH bending, NH bending, and NH asymmetric and symmetric stretching, respectively, appeared and became more pronounced, indicating successful introduction of PEI on the CS surface. The possible structure 6468

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structure, a freeze-drying method was employed for drying. Furthermore, the pore size distribution shown in Figure 1 showed that PEI−CS additionally featured a few micro- and mesopores. These micro- and mesopores contribute to the relatively high surface area of the material. PEI−CS displayed a high thermal stability up to 550 K, as shown in Figure 3. The

of the prepared PEI−CS based on IR analysis is depicted in Figure S3 of the Supporting Information and is consistent with that proposed in the literature.19 The preparation conditions and physical and chemical properties of PEI−CS are listed in Table S1 of the Supporting Information. To tune the physical and chemical properties of PEI−CS, the added amounts of cross-linker EGDE and anchor agent ECH and the molecular weight of the grafted PEI were controlled. As observed from Table S1 of the Supporting Information, PEI−CS had a relatively high surface area and a high N content. The packing density (ρp) of PEI−CS appeared moderate. A detailed discussion on the influence of such properties on CO2 adsorption is provided in section 3.2. Representative N2 adsorption−desorption isotherm and pore size distribution of the prepared PEI−CS are shown in Figure 1. As observed,

Figure 3. TG curves of PEI−CS, raw CS, and PEI (Mw = 25 000).

degradation temperature of PEI−CS laid between the degradation temperature of CS and that of PEI, suggesting that the thermal stability of the CS framework was enhanced by PEI modification. The particle size of PEI−CS was controlled by the size of the nozzle employed for dropping the CS solution into the NaOH aqueous solution; particles of ∼2 mm in diameter were prepared in this study. 3.2. CO2 Adsorption Characteristics. To investigate the effect of the physical and chemical properties of PEI−CS on the CO2 adsorption performance, several types of PEI−CS were prepared under the different preparation conditions listed in Table S1 of the Supporting Information and the CO 2 adsorption characteristics of the resulting PEI−CS materials were evaluated. Typical CO2 isotherms of PEI−CS and crosslinked CS are shown in Figure S4 of the Supporting Information. As observed, PEI functionalization had a positive influence on CO2 adsorption capacity. With regard to aminemodified materials, it is well-known that the amine content is an important factor influencing the CO 2 adsorption capacity.31−37 The surface area also affects the adsorption performance. Therefore, the respective relationship between these two factors and CO2 adsorption capacity was evaluated as shown in Figures 4 and 5, respectively. As observed from Figure 4, the CO2 adsorption capacity (qCO2) of PEI−CS was dependent upon the amine content (N content); the CO2 adsorption capacity reached a maximum of ∼2.9 mmol/g at 100 kPa. Furthermore, the amine efficiency (qCO2/N content) increased slightly with increasing N contents. This was attributed to the increase in the amine density, similarly observed in amine-modified mesoporous silicas.31−37 The widely accepted mechanism of CO2 adsorption on primary and secondary amines under dry conditions involves the production of carbamate through the formation of carbamic acid as follows:31,36,38−42

Figure 1. Representative nitrogen adsorption−desorption isotherm and pore size distribution of PEI−CS beads.

PEI−CS displayed a type II isotherm that is indicative of a macroporous or non-porous material. However, scanning electron microscopy (SEM) analysis confirmed that PEI−CS was macroporous, owing to the three-dimensional macroporous network, as observed in Figure 2. To maintain the porous

2R1R2NH + CO2 ↔ R1R2NH 2+ + R1R2NCOO−

Figure 2. SEM image of PEI−CS. 6469

(1)

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amine efficiency of PEI 3

∑ {(molar ratio of n° amine)

=

n=1

× (molar ratio of n° amine)}/ 3

∑ (molar ratio of n° amine) n=1

= (0.5 × 1 + (0.3 + 0.4)/2 × 1 + 0 × 1)/3 or (0.5 × 1 + (0.3 + 0.4)/2 × 2 + 0 × 1)/4 (2)

The experimentally obtained amine efficiency in this study was comparable to the estimated value. This agreement suggests that CO2 adsorption onto the amine-functionalized porous CS beads proceeds via a similar adsorption mechanism to that described in eq 1. Likewise, as observed from Figure 5, CO2 adsorption capacity was influenced by the surface area of the material. For materials with the same N content, the material featuring a higher surface area adsorbed CO2 more effectively because a higher number of amino groups was exposed on the PEI−CS surface. These results indicate that an adsorbent featuring a high N content and a large surface area is more suited to achieving high CO2 adsorption. On the basis of the above results, optimization of the preparation conditions of PEI−CS to achieve high N contents was performed. Because the efficient introduction of PEI on the CS surface is highly dependent upon the cross-linker EGDE and anchor agent ECH, the relationship between the N content and these two factors was investigated. As shown in Table S1 of the Supporting Information, the N content decreased with an increase in the added amount of cross-linker. Because the crosslinker reacts with the amino groups in CS, higher amounts of the anchor agent can be introduced, thereby requiring a small amount of the cross-linker. In contrast, an optimum amount of the anchor agent was determined. This may be attributed to the increase in the density of the anchor sites on the CS surface. Excessive amounts of anchor sites on the CS surface lead to reactions between the anchor sites and the amino groups in PEI. Thus, on the basis of the results obtained, the preparation of a material with a high N content requires a small amount of cross-linker and a moderate optimum amount of anchor agent. However, it should be noted that the addition of considerably too low amounts of cross-linker would result in materials with a low resistance to acidic conditions, as reported.19,49 3.3. CO2 Adsorption in the Presence of Water Vapor. As described earlier, studying the influence of water vapor on CO2 adsorption is important for practical application of adsorbents. With regard to amine-grafted and amine-impregnated materials, in general, the presence of water vapor has a positive influence on CO2 adsorption.3,4,14,31,35,50 In contrast, reports on CO2 adsorption onto CS-based materials are rare, 22−27 and among these studies, only one report demonstrates the increased CO2 adsorption capacity of CS beads in the presence of water vapor. However, the resulting CO2 adsorption capacity was too low to accurately determine the effect of water vapor.25 To investigate CO2 adsorption in the presence of water vapor, concurrent CO2 and H2O adsorption studies on PEI−CS were conducted. Figure 6 shows the resulting CO2 and H2O adsorption isotherms of PEI−CS following pre-adsorption of water vapor at 313 K and

Figure 4. Effect of the N content on CO2 adsorption capacity and amine efficiency.

Figure 5. Effect of the surface area on CO2 adsorption capacity.

where R1 = H (for primary amines) and R1 and R2 = alkyl and aryl, respectively (for secondary amines). Accordingly, the theoretical amine efficiency of primary and secondary amines under dry conditions is 0.5 because the formation of carbamate requires an amine pair site. It was confirmed that primary amines feature high amine efficiencies up to 0.5 provided that sufficient amounts of amino groups are introduced on the support surface, owing to the high reactivity.36,37,43,44 In contrast, it was experimentally determined that secondary amines exhibit reduced reactivity (amine efficiency of ∼0.3− 0.4),36,43−45 owing to steric hindrance. According to the adsorption mechanism described above, CO2 does not react with tertiary amines, as confirmed experimentally.36,43,44 Accordingly, the ratio of the primary (1°), secondary (2°), and tertiary (3°) amino groups of the most available branched PEI, ranging from 1:1:1 to 1:2:1,46−48 affords an estimated maximum amine efficiency of PEI of ∼0.28−0.3 as deduced. 6470

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Figure 7. IR spectra of PEI−CS in different gas atmospheres.

Figure 6. CO2 and H2O adsorption isotherms of PEI−CS following pre-adsorption of water vapor (closed symbols) and CO2 adsorption isotherm of PEI−CS in the absence of water vapor (open symbols) at 313 K.

observed at ∼2300−2400 and 3600−3800 cm−1 in the IR spectrum measured in a CO2 atmosphere.41 The broad band at ∼3000−3500 cm−1 was attributed to OH vibrations of the hydrogen-bonded hydroxyl groups of CS.55 Under wet conditions, the band at 3000−3500 cm−1 could not be distinguished from the broad band at ∼3700 cm−1 that was attributed to OH stretching of adsorbed water38,40,41,51,54 because of overlapping of the bands. The broad band at ∼3700 cm−1 was observed in the IR spectrum of PEI−CS following CO2 adsorption under wet conditions but was not observed in the IR spectrum of PEI−CS following CO2 adsorption under dry conditions. Therefore, the effect of adsorbed water on the CO2 adsorption mechanism under dry conditions could be eliminated. For a better understanding of the adsorption states of CO2 on PEI−CS, the difference in the spectra of PEI−CS in the region of 1100−1800 cm−1 measured before and after CO2 adsorption under wet and dry conditions is presented in Figure 8. The most prominent differences in CO2 chemisorption are represented by the bands in the region of ∼1200−1800 cm−1. To date, accurate assignment of bands in this region remains controversial. Some researchers base their assignments on the belief that the CO2 adsorption mechanism under wet conditions involves the formation of carbamate.38−42,55 Other

the resulting CO2 adsorption isotherm of PEI−CS in the absence of water vapor at 313 K. As observed, the presence of water vapor enhanced CO2 adsorption capacity. The CO2 adsorption capacity at 100 kPa increased to 4.3 mmol/g, and the amine efficiency also increased from 0.22 (under dry conditions) to 0.37 (under humid conditions). On the basis of the elemental analysis, the N content increased from 7.3 wt % (raw CS) to 16.4 wt % (PEI−CS). Assuming that the error involving the masses of cross-linker and anchor agent is negligible, the N content associated with PEI (NPEI content) was evaluated as 6.6 mmol of NPEI/g of adsorbent. The amine efficiency of the introduced PEI was calculated as 0.40 (under dry conditions) and 0.65 (under humid conditions). This was attributed to an increase in the accessibility to the amino groups upon water adsorption or the difference between the adsorption mechanisms of CO2 in the presence and absence of H2O. Furthermore, H2O adsorption only decreased slightly, owing to the adsorption of CO2, suggesting that the adsorption sites of CO2 and H2O are different. A detailed discussion on this adsorption-enhancing phenomenon is presented in section 3.4. 3.4. CO 2 Adsorption Mechanisms. As described previously, the CO2 adsorption mechanism on primary and secondary amines under dry conditions involves the formation of carbamate. In contrast, various CO2 adsorption mechanisms under wet conditions have been proposed.36,38−42,51−53 Some reports concluded that the CO2 adsorption mechanism onto amino groups under wet conditions also involves the formation of carbamate,38−42,54 whereas other reports ascribed the adsorption mechanism to the formation of bicarbonate and/ or carbonate.51−53 The formation of bicarbonate reported in the literature is depicted as follows: R1R 2NH + CO2 + H 2O ↔ R1R 2NH 2+ + HCO3−

(3)

To determine the mechanism of enhanced CO2 adsorption in the presence of water vapor, the CO 2 adsorption mechanisms of PEI−CS in both the absence and presence of water vapor were investigated using DRIFTS. All experiments were performed following drying at 343 K in He flow. Figure 7 shows the IR spectra of PEI−CS under different gas atmospheres. Bands corresponding to gas-phase CO2 were

Figure 8. Difference in the spectra of PEI−CS measured before and after CO2 adsorption under wet and dry conditions. 6471

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Table 1. IR Band Assignment for CO2 Adsorption onto PEI−CS

a

wavenumber (cm−1)

assignmenta

group

reference

3700 3000−3500 1705 1630 1560 1500 1410 1380 1315

O−H str O−H str CO str NH3+ asym def or H−O−H bend COO− asym str NH3+ asym def NCOO− skeletal vibration or C−N str COO− sym str and/or O−H def NCOO− skeletal vibration

adsorbed water CS carbamate and/or carbamic acid NH3+ or adsorbed water carbamate NH3+ carbamate carbamate and/or carbamic acid carbamate

39, 40, and 54 55 38−41 31, 38−42, and 54 31 and 38−41 38−40 and 42 38, 41, 42, and 54 39 and 40 41, 42, and 54

str, stretching; asym, asymmetric; sym, symmetric; and def, deformation.

adsorption state of CO2, and under humid conditions, there are at least two adsorption states of CO2. Weakly adsorbed CO2 under humid conditions may be due to the slight deformation of carbamate and carbamic acid upon hydrogen bonding with adsorbed water and/or intermediates to carbamate formation, such as diffusive intermediates, as proposed by Mebane et al.56 The authors investigated the role of water for PEI-impregnated mesoporous silica using density functional theory calculations. It was found that the diffusive intermediates involving amine, water, and CO2 related to the enhanced CO2 capacity under humid conditions. Because the energies of formation for the intermediates are in the range of physical adsorption, an initial decrease in the DTG curve for CO2- and H2O-adsorbed PEI− CS may be correlated to the desorption of such intermediates and water. Accordingly, the effect of water adsorption onto PEI−CS (porous and non-porous samples) was evaluated. An adsorption−desorption hysteresis was observed in both water vapor isotherms at 313 K (Figure 9). This may be due to

researchers base the bands assignment on the belief that the CO2 adsorption mechanism involves the formation of bicarbonate instead.51−53 In this work, as observed, both CO2 adsorption spectra obtained under dry and wet conditions were similar, except for the band/peak intensity. These findings indicate that the adsorption mechanisms of CO2 onto PEI−CS are the same in both the presence and absence of water vapor. Furthermore, as described in the previous section, the amine efficiency of PEI−CS in the presence of water vapor was below 0.5. These results indicate that bicarbonate formation may not take place. Accordingly, the band assignment in this region was based on this consideration and are listed in Table 1; details can be found elsewhere.38−42,55 The CO stretching vibration at 1705 cm−1 was observed in both spectra. The bands at 1630 and 1500 cm−1 were assigned to NH3+ in ammonium carbamate ion pairs, and the bands at 1560 and 1380 cm−1 (shoulder) were attributed to COO − asymmetric and symmetric stretching, respectively. The band at 1630 cm−1 was assigned to the H−O−H bending vibration of adsorbed water, and the bands at 1410 and 1315 cm−1 were attributed to NCOO− skeletal vibration. Assignment of the band at 1280 cm−1 was not determined in this work because of the discrepancy in literature reports. For instance, Khatri et al. assigned the band to COO− stretching of carbamic acid,51 whereas Knöfel et al. assigned the band to COO− stretching of bidentate carbonate.40 The latter authors indicated the onset of bidentate carbonate formation for CO2 adsorption onto surface silanol with an amino group. With regard to PEI−CS, the formation of bidentate carbonate is likely because of the presence of several types of hydroxyl groups. Although the vibration of adsorbed water additionally contributes to the significant increase in the absorbance intensity under wet conditions, the increase in the intensity primarily relates to the increase in the amount of adsorbed CO2 that is consistent with the equilibrium data and is likely due to the onset of water adsorption. To elucidate the CO2 adsorption states on PEI−CS, TG analyses of CO2- and/or H2O-adsorbed PEI−CS were conducted, and the results are shown in Figure S5 of the Supporting Information. On the basis of the differential thermogravimetry (DTG) curve of H2O-adsorbed PEI−CS, some of the adsorbed H2O molecules were desorbed at 303 K and most the adsorbed H2O molecules were desorbed by 343 K. With regard to CO2-adsorbed PEI−CS, only a DTG peak at ∼343 K was observed and CO2 was desorbed below 353 K. With regard to CO2- and H2O-adsorbed PEI−CS, although most of the CO2 molecules desorbed below 323 K, some CO2 molecules remained strongly adsorbed onto PEI−CS. These results suggest that, under dry conditions, there is one

Figure 9. Water vapor adsorption−desorption isotherms of porous and non-porous PEI−CS with the same N content at 313 K.

swelling of the CS framework and/or the grafted PEI chain upon water adsorption. Furthermore, oven-dried non-porous PEI−CS displayed a comparable adsorption−desorption behavior of water vapor with that of porous PEI−CS, indicating the onset of considerable swelling in the high P/P0 region. Minor structural changes of PEI−CS upon water ad(b)sorption were additionally observed by XRD analysis. In the XRD patterns of both samples shown in Figure S6 of the Supporting Information, the diffraction peak associated with CS was 6472

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observed at ∼2θ = 20°.57,58 With regard to the wet sample, the broad peak corresponding to liquid water was observed at ∼2θ = 26°59 and the peak at 2θ = 20° shifted slightly, suggesting the likely onset of swelling. According to the results of the IR study, it was determined that the CO2 adsorption mechanisms onto PEI−CS in the presence and absence of water vapor were the same. However, a slight decrease in H2O adsorption, owing to the adsorption of CO2, was observed for the sample pretreated with water vapor (Figure 6). However, as noted, the amount of H2O desorbed (≈0.5 mmol/g) was smaller than the amount of CO2 adsorbed (≈4 mmol/g), indicating that the adsorption sites for CO2 and H2O are different. These results strongly suggest that water adsorption onto PEI−CS enhances accessibility to the inner amino groups in PEI−CS. This may be the main factor resulting in increased amounts of adsorbed CO2. 3.5. Regenerability and Stability Tests. For practical application, high regenerability and stability of the adsorbent over numerous adsorption−desorption cycles are required. Accordingly, cyclic adsorption−desorption tests were performed to evaluate the regenerability and stability of PEI−CS using a fixed-bed reactor. Adsorption was conducted at 313 K in a humidified simulated flue gas stream, and the adsorbent was regenerated at 373 K in an argon stream. Figure 10 shows

CO2 adsorption mechanisms in the absence and presence of water vapor were investigated. The prepared PEI−CS beads are macroporous and feature a relatively high surface area and a high amine content. The CO2 adsorption capacity of PEI−CS was dependent upon both the amine content and surface area, and both properties were controlled by adjusting the preparation conditions employed (i.e., amount of cross-linker and anchor agent). DRIFTS studies revealed that the CO2 adsorption mechanism in both the absence and presence of water vapor was the same (i.e., involving the formation of ammonium carbamate). The enhanced CO2 adsorption was attributed to the swelling of the PEI chain and/or CS framework in PEI−CS upon water adsorption, as indicated by the structural changes of PEI−CS observed in the XRD analysis upon water ad(b)sorption. Moreover, PEI−CS featured high CO2 adsorption capacity (3.6 mmol/g at 313 K and 15 kPa) in the presence of water vapor, high thermal stability, and good regenerability. Herein, the reported CSbased material shows potential as an adsorbent for effective CO2 capture.

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ASSOCIATED CONTENT

S Supporting Information *

Preparation conditions and physical and chemical properties of PEI−CS (Table S1), schematic diagram of the experimental setup for the cyclic adsorption−desorption test (Figure S1), IR spectra of raw chemical and products (Figure S2), estimated structure of the PEI−CS adsorbent (Figure S3), typical CO2 isotherms of PEI−CS and cross-linked CS (Figure S4), TG and DTG curves for H2O-adsorbed PEI−CS, CO2-adsorbed PEI− CS, and CO2- and H2O-adsorbed PEI−CS (Figure S5), XRD patterns of dried and wetted PEI−CS (Figure S6) and adsorption−desorption curves on cyclic tests (Figure S7). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +81-774-75-2305. Fax: +81-774-75-2318. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

Figure 10. Cyclic CO2 adsorption−desorption tests over PEI−CS.

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ACKNOWLEDGMENTS This work was supported by the Ministry of Economy, Trade and Industry (METI), Japan. The authors express special thanks to A. Asamoma (Graduate School of Materials Science, Nara Institute of Science and Technology) for technical assistance with the elemental analysis.

the CO2 adsorption capacities over five adsorption−desorption cycles. As observed, in general, PEI−CS maintained a high CO2 adsorption performance across all cycles, thereby suggesting that this material is stable in cyclic temperature swing adsorption operations. Furthermore, the adsorption and desorption rates remained constant because the adsorption− desorption curves measured over five cycles were mostly identical, as shown in Figure S7 of the Supporting Information. In the desorption step, two peaks were observed. The first peak was due to heating effects, and the other peak was attributed to the shrinkage of PEI−CS upon desorption of water. These findings demonstrate that the CS-based adsorbent developed herein is a feasible candidate as a CO2 adsorbent, owing to the high CO2 adsorption−desorption performance.

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REFERENCES

(1) Markewitz, P.; Kuckshinrichs, W.; Leitner, W.; Linssen, J.; Zapp, P.; Bongartz, R.; Schreiber, A.; Müller, T. E. Energy Environ. Sci. 2012, 5, 7281−7305. (2) Yu, C. H.; Huang, C. H.; Tan, C. S. Aerosol Air Qual. Res. 2012, 12, 745−769. (3) Samanta, A.; Zhao, A.; Shimizu, G. K. H.; Sarkar, P.; Gupta, R. Ind. Eng. Chem. Res. 2012, 51, 1438−1463. (4) Choi, S.; Drese, J. H.; Jones, C. W. ChemSusChem 2009, 2, 796− 854. (5) Songolzadeh, M.; Ravanchi, M. T.; Soleimani, M. World Acad. Sci., Eng. Technol. 2012, 70, 225−232.

4. CONCLUSION PEI−CS beads were prepared for CO2 capture. The CO2 adsorption performance of PEI−CS was evaluated, and the 6473

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(6) D’Alessandro, D. M.; Smit, B.; Long, J. R. Angew. Chem., Int. Ed. 2010, 49, 6058−6082. (7) Song, C. Catal. Today 2006, 115, 2−32. (8) Merkel, T. C.; Lin, H.; Wei, X.; Baker, R. J. Membr. Sci. 2010, 359, 126−139. (9) Wang, W.; Wang, X.; Song, C.; Wei, X.; Ding, J.; Xiao, J. Energy Fuels 2013, 27, 1538−1546. (10) Fauth, D. J.; Gray, M. L.; Pennline, H. W.; Krutka, H. M.; Sjostrom, S.; Ault, A. M. Energy Fuels 2012, 26, 2483−2496. (11) Zhao, L.; Bacsik, Z.; Hedin, N.; Wei, W.; Sun, Y.; Antonietti, M.; Titirici, M. M. ChemSusChem 2010, 3, 840−845. (12) Alesi, R. W., Jr.; Kitchin, J. R. Ind. Eng. Chem. Res. 2012, 51, 6907−6915. (13) Drese, J. H.; Choi, S.; Lively, R. P.; Koros, W. J.; Fauth, D. J.; Gray, M. L.; Jones, C. W. Adv. Funct. Mater. 2009, 19, 3821−3832. (14) Qi, G.; Wang, Y.; Estevez, L.; Duan, X.; Anako, N.; Park, A. H. A.; Li, W.; Jones, C. W.; Giannelis, E. P. Energy Environ. Sci. 2011, 4, 444−452. (15) Dao, D. S.; Yamada, H.; Yogo, K. Ind. Eng. Chem. Res. 2013, 52, 13810−13817. (16) Fujiki, J.; Yogo, K. Chem. Lett. 2013, 42, 1484−1486. (17) Kanai, Y.; Oshima, T.; Baba, Y. Ind. Eng. Chem. Res. 2008, 47, 3114−3120. (18) Inoue, K.; Yoshizuka, K.; Ohto, K. Anal. Chim. Acta 1999, 388, 209−218. (19) Kawamura, Y.; Mitsuhashi, M.; Tanibe, H.; Yoshida, H. Ind. Eng. Chem. Res. 1993, 32, 386−391. (20) Ramesh, A.; Hasegawa, H.; Sugimoto, W.; Maki, T.; Ueda, K. Bioresour. Technol. 2008, 99, 3801−3809. (21) Onsøyen, E.; Skaugrud, Ø. J. Chem. Technol. Biotechnol. 1990, 49, 395−404. (22) Phalakornkule, C.; Foungchuen, J.; Pitakchon, T. J. Sustainable Energy Environ. 2012, 3, 153−157. (23) Huang, C. C.; Shen, S. C. J. Taiwan Inst. Chem. Eng. 2013, 44, 89−94. (24) Yoshida, H.; Oehlenschlaeger, S.; Minami, Y.; Terashima, M. J. Chem. Eng. Jpn. 2002, 35, 32−39. (25) Valechha, A.; Thote, J.; Labhsetwar, N.; Rayalu, S. Int. J. Knowl. Eng. 2012, 3, 103−106. (26) Fan, X.; Zhang, L.; Zhang, G.; Shu, Z.; Shi, J. Carbon 2013, 61, 423−430. (27) Alhwaige, A. A.; Agag, T.; Ishida, H.; Qutubuddin, S. RSC Adv. 2013, 3, 16011−16020. (28) Dreisbach, F.; Lösch, H. W.; Nakai, K. Chem. Eng. Technol. 2001, 24, 1001−1005. (29) Nakai, K. J. Colloid Interface Sci. 2001, 240, 17−23. (30) Miyamoto, M.; Fujioka, Y.; Yogo, K. J. Mater. Chem. 2012, 22, 20186−20189. (31) Hiyoshi, N.; Yogo, K.; Yashima, T. Microporous Mesoporous Mater. 2005, 84, 357−365. (32) Sanz, R.; Calleja, G.; Arencibia, A.; Sanz-Pérez, E. S. J. Mater. Chem. A 2013, 1, 1956−1962. (33) Xu, X.; Song, C.; Andrésen, J. M.; Miller, B. G.; Scaroni, A. W. Microporous Mesoporous Mater. 2003, 62, 29−45. (34) Harlick, P. J. E.; Sayari, A. Ind. Eng. Chem. Res. 2007, 46, 446− 458. (35) Harlick, P. J. E.; Sayari, A. Ind. Eng. Chem. Res. 2006, 45, 3248− 3255. (36) Sayari, A.; Belmabkhout, Y.; Da’na, E. Langmuir 2012, 28, 4241−4247. (37) Young, P. D.; Notestein, J. M. ChemSusChem 2011, 4, 1671− 1678. (38) Aziz, B.; Hedin, N.; Bacsik, Z. Microporous Mesoporous Mater. 2012, 159, 42−49. (39) Bacsik, Z.; Ahlsten, N.; Ziadi, A.; Zhao, G.; Garcia-Bennett, A. E.; Martín-Matute, B.; Hedin, N. Langmuir 2011, 27, 11118−11128. (40) Knöfel, C.; Martin, C.; Hornebecq, V.; Llewellyn, P. L. J. Phys. Chem. C 2009, 113, 21726−21734.

(41) Srikanth, C. S.; Chuang, S. S. C. J. Phys. Chem. C 2013, 117, 9196−9205. (42) Bacsik, Z.; Atluri, R.; Garcia-Bennett, A. E.; Hedin, N. Langmuir 2010, 26, 10013−10024. (43) Ko, Y. G.; Lee, H. J.; Oh, H. C.; Choi, U. S. J. Hazard. Mater. 2013, 250, 53−60. (44) Ko, Y. G.; Shin, S. S.; Choi, U. S. J. Colloid Interface Sci. 2011, 361, 594−602. (45) Zelenak, V.; Halamova, D.; Gaberova, L.; Bloch, E.; Llewellyn, P. Microporous Mesoporous Mater. 2008, 116, 358−364. (46) Dick, C. R.; Ham, G. E. J. Macromol. Sci., Part A: Pure Appl. Chem. 1970, 4, 1301−1314. (47) Rivas, B. L.; Geckeler, K. E. Adv. Polym. Sci. 1992, 102, 171− 188. (48) von Harpe, A.; Petersen, H.; Li, Y.; Kissel, T. J. Controlled Release 2000, 69, 309−322. (49) Kawamura, Y.; Yoshida, H.; Asai, S.; Kurahashi, I.; Tanibe, H. Sep. Sci. Technol. 1997, 32, 1959−1974. (50) Gray, M. L.; Hoffman, J. S.; Hreha, D. C.; Fauth, D. J.; Hedges, S. W.; Champagne, K. J.; Pennline, H. W. Energy Fuels 2009, 23, 4840−4844. (51) Khatri, R. A.; Chuang, S. S. C.; Soong, Y.; Gray, M. Ind. Eng. Chem. Res. 2005, 44, 3702−3708. (52) Chang, A. C. C.; Chuang, S. S. C.; Gray, M.; Soong, Y. Energy Fuels 2003, 17, 468−473. (53) Khatri, R. A.; Chuang, S. S. C.; Soong, Y.; Gray, M. Energy Fuels 2006, 20, 1514−1520. (54) Wang, X.; Schwartz, V.; Clark, J. C.; Ma, X.; Overbury, S. H.; Xu, X.; Song, C. J. Phys. Chem. C 2009, 113, 7260−7268. (55) Kumirska, J.; Czerwicka, M.; Kaczyński, Z.; Bychowska, A.; Brzozowski, K.; Thöming, J.; Stepnowski, P. Mar. Drugs 2010, 8, 1567−1636. (56) Mebane, D. S.; Kress, J. D.; Storlie, C. B.; Fauth, D. J.; Gray, M. L.; Li, K. J. Phys. Chem. C 2013, 117, 26617−26627. (57) Zhang, Y.; Xue, C.; Xue, Y.; Gao, R.; Zhang, X. Carbohydr. Res. 2005, 340, 1914−1917. (58) Yen, M.-T.; Yang, J.-H.; Mau, J.-L. Carbohydr. Polym. 2009, 75, 15−21. (59) Fukushima, Y. Clays Clay Miner. 1984, 32, 320−326.

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