CO₂ Capture by Carbon Aerogel–Potassium Carbonate Nanocomposites

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 [5–8]. 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.

(a)

(b)

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.

(a)

(b)

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).

(a)

(b)

(c)

(d)

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.

Supplementary Materials

Potassium carbonate contents in carbon-aerogels and CO₂ sorption behaviors of carbon aerogels under a moist condition.

1. Supplementary Material