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International Journal of Chemical Engineering
Volume 2016 (2016), Article ID 4012967, 8 pages
http://dx.doi.org/10.1155/2016/4012967
Research Article

CO2 Capture by Carbon Aerogel–Potassium Carbonate Nanocomposites

Graduate School of Science, Chiba University, Chiba 263-8522, Japan

Received 15 October 2015; Accepted 26 January 2016

Academic Editor: Alírio Rodrigues

Copyright © 2016 Guang Yang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Recently, various composites for reducing CO2 emissions have been extensively studied. Because of their high sorption capacity and low cost, alkali metal carbonates are recognized as a potential candidate to capture CO2 from flue gas under moist conditions. However, undesirable effects and characteristics such as high regeneration temperatures or the formation of byproducts lead to high energy costs associated with the desorption process and impede the application of these materials. In this study, we focused on the regeneration temperature of carbon aerogel–potassium carbonate (CA–KC) nanocomposites, where KC nanocrystals were formed in the mesopores of the CAs. We observed that the nanopore size of the original CA plays an important role in decreasing the regeneration temperature and in enhancing the CO2 capture capacity. In particular, 7CA–KC, which was prepared from a CA with 7 nm pores, exhibited excellent performance, reducing the desorption temperature to 380 K and exhibiting a high CO2 capture capacity of 13.0 mmol/g-K2CO3, which is higher than the theoretical value for K2CO3 under moist conditions.

1. Introduction

Carbon dioxide (CO2) is the principal greenhouse gas. It has been continuously released into the environment through the burning of fossil fuels and has led to global warming and anthropogenic climate change, such as droughts, desertification, permafrost melt, inundation, rising sea levels, and ecosystem disruption, and it is expected to substantially affect the future of mankind. Although alternative energy sources have been extensively investigated, no single alternative source can satisfy global energy demands; fossil fuels are therefore expected to remain the primary energy resource for the next several decades because of their advantages of low cost and high energy density. The atmospheric CO2 concentration was 384 ppm in 2007 and is expected to reach 550 ppm by 2050; hence, mitigating the atmospheric CO2 concentration is critical for protecting our environment [1, 2]. With both high CO2 capture capacity and low cost, alkali metal carbonates (M2CO3, M = K, Na) have been recognized as potential sorbents for CO2 sorption according to the following reaction [3]: The forward reaction is a bicarbonate formation reaction (the theoretical CO2 capture capacity is 7.24 mmol-CO2/g-K2CO3), whereas the reverse reaction is an endothermic regeneration process that begins at 445.3 K and ends at 548.1 K when the heating rate is 20 K/min [4]. Thus, by using alkali metal carbonates, it is possible to selectively sorb CO2 under a moist condition, which usually lowers CO2 capacity of conventional physical adsorbents.

In recent years, to solve problems such as the slow reaction rate of bicarbonate formation and high energy consumption during regeneration, researchers have extensively investigated various composites containing alkali metal carbonates [58]. The regeneration behaviors of alkali metal carbonates change when they are supported on nanoporous structural materials such as activated carbon, Al2O3, or carbon nanofibers or when they are combined with MgO, TiO, or FeOOH, which can themselves capture CO2. Because of the antagonistic relationship between the effects of high regeneration temperatures and low capture capacities, suitable materials have not been developed. Zhao et al. reported that K2CO3 sorbents exhibit better CO2 capture performance than Na2CO3 sorbents and that the selection of a support material with appropriate characteristics is important for carbonation and regeneration [9]. We also previously reported a detailed reaction mechanism for CO2 occlusion by K2CO3 under moist conditions [10, 11]. Therefore, in the present study, we focus on impregnating K2CO3 into the nanopores of carbon aerogels (CAs) prepared by pyrolysis of a dried organic aerogel followed by carbonation, leading to the formation of vitreous black monoliths with highly cross-linked micropores and mesopores [12, 13]. The CAs provide a suitable mesoporous reaction field for forming K2CO3 nanocrystals because the CAs’ surface area, pore structure, and size are easily controlled through manipulation of the molar ratios among reagents or the pH [14, 15]. In the present paper, the CO2 capture ability of K2CO3 nanocrystals incorporated into mesopores of CAs is studied from the viewpoint of lowering the regeneration temperature while achieving high selectivity and high capture capacity.

2. Experimental Section

2.1. Preparation of CA

All reagents were purchased from Wako Pure Chemical Industries, Ltd. Resorcinol–formaldehyde (RF) solutions were synthesized under the following experimental conditions. The resorcinol-to-sodium carbonate () and resorcinol-to-formaldehyde () molar ratios were fixed at 500 and 0.5, respectively, and the molar ratio of resorcinol to deionized water () was varied among 0.14, 0.28, and 0.7 to produce RF gels with different pore sizes. For a given ratio, the required resorcinol was weighed out and added to deionized water; the resulting mixture was then stirred until the resorcinol was completely dissolved. Formaldehyde and sodium carbonate were added to the mixture, resulting in yellowish homogeneous RF solutions that were subsequently sealed and placed in a thermostated bath at 303 K for 2 weeks for polymerization [12].

Water entrained within the gel network of the polymer was removed through solvent exchange; the gel was successively soaked in mixed solutions of acetone and water at a ratio of 1 : 1 and 3 : 1 and in pure acetone for 15 min each. Finally, the gel was immersed in acetone for 1 day at room temperature. To preserve the structure of the nanoporous material, the wet gel was dried using a supercritical drying process. The wet gel was then placed in a supercritical drying chamber, and CO2 was slowly introduced to bleed the air from the chamber. CO2 was introduced to a pressure of 10 MPa at 318 K and was maintained at this temperature for 3 h. Carbonization of the organic aerogels was conducted at 1173 K for 3 h under Ar flowing at 100 cm3/min. As described later, the porosities of the three different CAs were characterized by N2 gas adsorption measurements at 77 K; the three CAs were observed to have pore widths of 7, 16, and 18 nm; these CAs are denoted as 7CA, 16CA, and 18CA, respectively.

2.2. Preparation of CA–Potassium Carbonate (KC) Nanocomposites

CA–KC nanocomposites were prepared by impregnating the nanopores of the CAs with a 0.15 mol/dm3 K2CO3 (99.5% chemical purity) aqueous solution [3]. The mixture was then stirred with a magnetic stirrer for 24 h at room temperature. The aqueous solution was dried at 378 K in a vacuum evaporator. The samples were subsequently dried again in a furnace under an Ar ambient atmosphere at 573 K for 2 h. The nanocomposites are denoted as xCA–KC, where represents the pore width of the CAs.

2.3. Measurements and Characterization

The porosities of the CAs and the xCA–KC nanocomposites were characterized by N2 adsorption at 77 K using an Autosorb-MP1 (Quantachrome Instruments). CO2 adsorption was measured using a Belsorp-Mini (MicrotracBEL Corp.). The CA and xCA–KC nanocomposites were pretreated by heating to 423 K under vacuum for 2 h before the gas adsorption measurements because K2CO3 is partially converted into KHCO3 under an ambient atmosphere containing CO2 and H2O. Water adsorption isotherms were obtained by a gravimetric method at 303 K; the equilibration time was 2 h.

The Brunauer-Emmett-Teller (BET) method was used to analyze the specific surface areas () on the basis of linear plots over the relative pressure () range 0.05–0.35 for the N2 adsorption isotherm data. The total volume () was obtained at a relative pressure of 0.99, and the Dubinin-Radushkevich equation was used to calculate the micropore volume (). The mesopore diameters () were estimated by the Barrett-Joyner-Halenda method.

The amount of K2CO3 impregnated into the mesopores of CA was determined by thermogravimetry-differential thermal analysis (TG-DTA, Shimadzu DTG-60AH), where the samples were heated to 1073 K. Because the residue consisted of K2CO3, the impregnated amounts of K2CO3 were calculated on the basis of the final TG curves to be 18.5, 21.3, and 18.8 wt% for 7CA–KC, 16CA–KC, and 18CA–KC, respectively, as shown in Figure S-1 (Supporting Information in Supplementary Material available online at http://dx.doi.org/10.1155/2016/4012967).

After the xCA–KC nanocomposites were heated to 473 K at 10 K/min under an N2 atmosphere and maintained at 473 K for 5 min to ensure complete formation of K2CO3, the temperature was decreased to 313 K, and CO2 and H2O were introduced into the sample chamber of the TG–DTA apparatus for 2 h by flowing CO2 through distilled water. The xCA–KC nanocomposites reacted with CO2 and H2O to form xCA–KHCO3. The crystal structures before and after the CO2 capture were measured through ex situ X-ray diffraction (XRD) on an X-ray diffractometer (MAC Science, M03XHF) equipped with a Cu Kα radiation source (40 kV, 25 mA, and  nm). We heated the xCA–KHCO3 nanocomposites to 473 K in order to study the regeneration process under the same conditions used in the aforementioned experiments.

We used the CaO solution-precipitation method to accurately determine the amount of CO2 captured by the xCA–KC samples. The CaO solution was prepared by dissolving 0.6 g of CaO in 100 cm3 distilled water. Any insoluble precipitate was filtered to yield a saturated solution. The clear solution was bubbled with N2 gas to remove any dissolved CO2 from air. The nanocomposites, which captured CO2 under moist conditions in the TG–DTA chamber at 313 K, were heated at 573 K, and the desorbed CO2 was recaptured by the CaO solution, leading to the immediate formation of a white precipitate. XRD analysis indicated that the precipitate was CaCO3. Before the xCA–KC experiments, the reliability of the CaO solution-precipitation method was evaluated using analytical grade KHCO3 powder, which indicated 92% precision with respect to the theoretical value.

3. Results and Discussion

3.1. BET Surface Area and Pore Size Distributions (PSDs) of the CAs and xCA–KCs

The pore structures were characterized using N2 adsorption isotherms volumetrically measured at 77 K after pretreatment at 423 K for 2 h; the obtained isotherms are presented in Figure 1. The isotherms of the composites are categorized as type IV (IUPAC classification) with hysteresis between the adsorption and desorption branches. These isotherms indicate the presence of mesopores, although the increased adsorption at low relative pressures also indicates the presence of micropores. Zhao et al. examined the effects of the pore structure of Al2O3 supports on the ability of K2CO3 to capture CO2 and observed that K2CO3 could quickly convert to K2CO3·1.5H2O because of the mesoporous structure of the Al2O3 support [8]; they also observed that the micropores of the Al2O3 support facilitated the rapid conversion of K2CO3·1.5H2O to KHCO3. Thus, mesoporous structures can enhance CO2 adsorption [8]. In the present study, the N2 adsorption amount decreased for all samples after they were subjected to the K2CO3 impregnation treatment, indicating that K2CO3 was successfully incorporated into the micropores and mesopores of the CAs.

Figure 1: N2 adsorption isotherms of (a) CAs and (b) xCA–KC samples at 77 K. Pore widths: black, 7 nm; green, 16 nm; red, 18 nm.

The pore structure parameters are compiled in Table 1, which shows that the surface area and pore volume of the xCA–KC nanocomposites decreased compared with those of the original CAs; this is consistent with the results of N2 adsorption isotherms, which indicate a partial filling or blocking of the pores by impregnated K2CO3 [6]. The results in Table 1 indicate that 16CA possessed the highest surface area and exhibited the greatest decrease in surface area after the K2CO3 impregnation. This is in agreement with the result that 16CA–KC contained a larger amount of impregnated K2CO3 compared with the other xCA–KCs.

Table 1: Pore parameters of the CA and CA–KC samples.
3.2. Structural Changes Induced by CO2 Capture and Regeneration

The structural changes of the xCA–KCs that accompanied CO2 capture and regeneration (the reverse reaction), which are represented as reaction (1) in Introduction, were examined by XRD analysis, as shown in Figure 2. Figure 2(a) shows the XRD patterns of the xCA–KC nanocomposites after the CO2 capture under moist conditions. The main diffraction peaks of KHCO3 are present, verifying that the KHCO3 nanocrystals were introduced into the mesopores of the xCAs through the impregnation process. Although peaks attributable to K4H2(CO3)3·1.5H2O as well as to KHCO3 were observed in the patterns of 16CA–KC and 18CA–KC, all the peaks in the pattern of 7CA–KC were assigned to KHCO3. The patterns in Figure 2(b) indicate almost complete regeneration because most of the main peaks were assigned to K2CO3·1.5H2O instead of KHCO3. This suggests that K2CO3 was formed by heat treatment at 473 K. Although K2CO3·1.5H2O could be formed because of the deliquescent nature of K2CO3 under ambient conditions during the ex situ XRD experiment, K2CO3 impregnated into the mesopores of xCAs was exposed to the ambient atmosphere and more easily reacted with atmospheric water.

Figure 2: XRD patterns after CO2 capture and regeneration: (a) xCA–KC nanocomposite after CO2 capture under moist conditions; (b) xCA–KC nanocomposites placed under ambient atmosphere after being regenerated at 473 K for 2 h. Pore width: black, 7 nm; green, 16 nm; red, 18 nm. ▼: KHCO3, □: K4H2(CO3)3·1.5H2O, ▽: K2CO3, and ◆: K2CO3⋅1.5H2O.
3.3. Decomposition of xCA–KC Nanocomposites

The xCA–KC nanocomposites were regenerated by heating to 473 K under an N2 atmosphere. The changes in weight and temperature are shown in Figure 3; these results reflect characteristic thermal decomposition. In the TG traces of the xCA–KCs, the first weight loss is attributed to the desorption of water and the second is attributed to the decomposition of KHCO3 to K2CO3, which is accompanied by the evolution of CO2 and H2O. The blue line shows the decomposition process of bulk KHCO3, which began to decompose at >423 K. By contrast, the xCA–KCs clearly decomposed at lower temperatures with decreasing pore size of the original CAs: the decomposition onset temperature decreased to 420, 390, and 380 K for 18CA–KC, 16CA–KC, and 7CA–KC, respectively. Thus, 7CA–KC exhibits an effective decrease in the required regeneration temperature. We concluded that nanocrystals of K2CO3 impregnated into the nanopores of CA exhibited high reactivity, resulting in easier regeneration at lower temperatures. The regeneration temperature for 7CA–KC is lower than that for other potassium-based sorbents, and the regeneration behaviors are changed when K2CO3 is loaded onto different sorbents, mainly depending on the properties of the support material [16].

Figure 3: TG curves for the nanocomposites. Blue: KHCO3, black: 7CA–KC, green: 16CA–KC, and red: 18CA–KC.
3.4. Water Adsorption

Water adsorption analysis is important for the xCA–KC nanocomposites because K2CO3 sorbs water from the ambient atmosphere. Zhao et al. reported that even though K2CO3–silica gel (SG) exhibits a relatively high pore volume, the total CO2 sorption is only 34.5% because of the strong hygroscopicity of SG, easily leading to hydration of K2CO3 to K2CO3·1.5H2O [8, 17]. This hydration should influence the CO2-sorption amount of K2CO3 under moist conditions [18]. Because the CO2 capture process of xCA–KCs involves both occlusion and physical adsorption, understanding the effects of water on CO2 sorption of the original CA and the xCA–KCs is important. Figure 4 shows water sorption isotherms of K2CO3 at 303 K. Almost no water was adsorbed below a relative pressure of 0.2, whereas the sorption amount increased with increasing pressure and reached a maximum value of 9.6 mmol/g, corresponding to the water content in K2CO3·1.5H2O (10.9 mmol/g). The desorption did not return to the original point because K2CO3·1.5H2O hardly releases water molecules at ambient temperature, as demonstrated in the XRD experiments (Figure 2). Such a large hysteresis is likely observed because the hydrate formation from K2CO3 to K2CO3·1.5H2O is extremely slow.

Figure 4: Water adsorption isotherms for K2CO3. Filled symbols: sorption; open symbols: desorption.

The water sorption isotherms of the CAs and xCA–KCs differ from those of K2CO3, as shown in Figure 5. Because the CAs are less hydrophilic than K2CO3, the physical adsorption of water is attributed to the micropores of the CAs [19]; thus, the three CAs should exhibit water uptake amounts corresponding to their micropore volume, as indicated in Figures 1(a) and 5(a) and in Table 1. The desorption isotherms did not exhibit adsorption hysteresis; they returned to the starting point reversibly, as shown in Figure 5(a).

Figure 5: Water sorption and CO2 sorption isotherms. Water sorption isotherms at 303 K for CAs (a) and xCA–KCs (b). CO2 adsorption isotherms at 273 K under dry conditions for CA (c) and for xCA–KCs (d). Pore widths: black □, 7 nm; green △, 16 nm; red ○, 18 nm.

By contrast, the water sorption behavior of the xCA–KCs was more complicated because both hydration of K2CO3 and physical adsorption by the micropores of the nanocomposites contributed to water sorption. Figure 5(b) shows remarkable increases in the water sorption uptakes of the xCA–KCs in the high relative pressure region. Notably, the amounts of sorbed water were greater than the sum of the water sorption amounts of K2CO3 and CAs. This phenomenon may be attributable to the nanocrystals of K2CO3 incorporated into nanopores of the CAs being more reactive and deliquescent under high humidity conditions than bulk K2CO3. The CO2 sorption isotherms of the CAs and xCA–KCs are shown in Figures 5(c) and 5(d), respectively. Because of their diminished pore volumes, all the nanocomposites exhibited smaller CO2 uptake after K2CO3 impregnation, and all the xCA–KCs exhibited a CO2 capture capacity of approximately 2.4 mmol CO2/g-sorbent at 0.1 MPa. Thus, the xCA–KCs do not exhibit good CO2 capture ability under dry conditions.

3.5. CO2 Capture Ability under Moist Conditions

The CO2 sorption capacity of the xCA–KCs was calculated on the basis of their mass change resulting from reaction (1): one mole of K2CO3 occludes a stoichiometric amount of one mole of each of CO2 and H2O, and the two values were calculated as the total CO2 sorption capacity (; mmol-CO2/g-sorbent) and the CO2 occlusion capacity of K2CO3 (; mmol-CO2/g-K2CO3). They are denoted aswhere (mmol) is the total amount of CO2 captured, as obtained from TG data; (g/mol) is the molar mass of CO2; (g) is the mass of the sorbent; and is the impregnation rate of K2CO3 into the nanocomposites.

CO2 sorption was conducted at 313 K at a flow rate of 100 cm3/min in a saturated mixed gas of CO2 and water, as shown in Figure 6. The increase in weight (%) corresponds to the conversion of K2CO3 to 2KHCO3. Sample 7CA–KC exhibited the greatest weight change (~17%), and bicarbonate formation reached equilibrium after 30 min for all the xCA–KC nanocomposites. Green et al. [20] reported that K2CO3 exhibits 45% CO2 sorption in 100 min, and Luo et al. [11] verified that the carbonation of K2CO3 is low. The reaction rate of the xCA–KC nanocomposites was faster than that of bulk K2CO3, indicating the effect of nanostructured K2CO3.

Figure 6: TG curves of carbonation reaction for xCA–KCs. Pore widths: black 7 nm; green 16 nm; red 18 nm.

The CO2 capture capacity results are summarized in Table 2; the results indicate that 7CA–KC exhibits the highest CO2 sorption capacity,  mmol/g-sorbent ( mmol/g-K2CO3), which is substantially higher than the theoretical amount of 7.24 mmol/g-K2CO3. The capture capacity results reported in Table 2 were estimated simply from the weight increase attributed to the sorption of CO2 and H2O. However, the mechanism is complicated and still under study. To more precisely determine the amount of CO2 captured by the xCA–KC nanocomposites, we used another method to measure the net amount of CO2 captured, as described in Section 3.6.

Table 2: CO2 capture capacity, as measured by TG–DTA and CaO soln–ppt method.
3.6. CaO Solution-Precipitation Method

To accurately determine the amount of CO2 captured by the xCA–KC composites, the CaO solution-precipitation method described in experimental Section 2.3 was used. We measured the mass of the CaCO3 precipitate to estimate the CO2 uptake; the results indicated sorption capacities of = 2.45, 2.1, and 1.7 and occlusion capacity of = 13.0, 9.77, and 9.18 mmol/g-K2CO3 for 7CA–K2CO3, 16CA–K2CO3, and 18CA–K2CO3, respectively, which is also reported in Table 2. Although the amounts of CO2 uptake are lower than those obtained from the TG measurements, these are net values of CO2 capture capacity. Because the impregnated amount is insufficient, is relatively low. If the impregnation of K2CO3 into the mesopores of CA can be improved, more CO2 will be captured and the sorbents will be easily regenerated. The values of and exhibit a dependence on the pore size of the original CA. In the case of impregnation of 7 nm mesopores of CA, a higher value of (per g-sorbent) is obtained. It may be because a smaller particle should be more reactive and show a higher efficiency for the CO2 occlusion reaction with water vapor.

The CO2 capture amounts are higher than the theoretical values in all xCA–KCs, as shown in Table 2. This can be because the remaining pores should be efficient for chemical absorption and physical adsorption of CO2 such as the formation of H2CO3 in the pores (Figure S-2).

The CO2 capture amounts in the present study are excellent compared with those reported for other K2CO3-loaded composites [16, 21, 22]. Although dry potassium-based sorbents such as K2CO3–MgO exhibit excellent CO2 capacities (9.0–14.9 mmol-CO2/g-K2CO3) that are substantially higher than the theoretical value, these sorbents produce many other byproducts, leading to a higher temperature of 623 K for regeneration. Lee et al. [5, 21] reported that Al2O3–K2CO3 generates KAl(CO3)2(OH)2 during the synthesis process and this byproduct does not completely convert to K2CO3 at temperatures below 563 K and observed that K2CO3–AC and K2CO3–TiO2 can be regenerated at relatively low temperatures of 473 and 403 K, respectively. However, these two composites exhibit lower CO2 capture capacities of 6.5 and 6.3 mmol-CO2/g-K2CO3, respectively. AC–K2CO3 is considered a promising CO2 capture sorbent because of its low regeneration energy requirement and high CO2 capture capacity. However, to maintain these features, H2O activation is required to convert K2CO3 to an activated form (K2CO3·1.5H2O) [18]. ZrO2–K2CO3 has a capacity of 6.2–6.9 mmol-CO2/g-K2CO3 when the reaction temperature is within 323–333 K under an ambient atmosphere of 9% H2O, 1% CO2, and balance N2 [5, 23].

4. Conclusion

CA–K2CO3 nanocomposites (xCA–KC) were prepared by impregnation of K2CO3 nanocrystals into the mesopores of three CAs with different pore sizes of 7, 16, and 18 nm for the development of an excellent CO2 sorbent with a high capacity, high selectivity, and low energy cost for regeneration. The performance of the nanocomposites is attributed to both chemical and physical capture being involved in the CO2 capture. The xCA–KCs can be completely regenerated at temperatures below 423 K; 7CA–K2CO3 in particular exhibited excellent results, where regeneration began at 380 K and was completed at 420 K. These results were attributed to the high reactivity of nanostructured K2CO3, which rendered the K2CO3 crystals unstable and reduced the regeneration temperature. These xCA–KC nanocomposites exhibited excellent CO2 capture capacity and can be considered a promising material for CO2 capture from the viewpoints of economic effectiveness and energy efficiency. We also concluded that CAs can be used as a porous support for the preparation of nanocomposites with low sorbent regeneration temperatures.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work was supported by the Iwatani Naoji Foundation’s research grant and a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (Japan). The authors would like to thank Enago (http://www.enago.jp/) for the English language review.

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