CO2 Capture by Carbon Aerogel–Potassium Carbonate Nanocomposites

Recently, various composites for reducing CO 2 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 CO 2 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 CO 2 capture capacity. In particular, 7CA–KC, which was prepared from a CA with 7 nm pores, exhibited excellent performance, reducing the desorption temperature to 380K and exhibiting a high CO 2 capture capacity of 13.0mmol/g-K 2 CO 3 , which is higher than the theoretical value for K 2 CO 3 under moist conditions.


Introduction
Carbon dioxide (CO 2 ) 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 CO 2 concentration was 384 ppm in 2007 and is expected to reach 550 ppm by 2050; hence, mitigating the atmospheric CO 2 concentration is critical for protecting our environment [1,2].With both high CO 2 capture capacity and low cost, alkali metal carbonates (M 2 CO 3 , M = K, Na) have been recognized as potential sorbents for CO 2 sorption according to the following reaction [3]: The forward reaction is a bicarbonate formation reaction (the theoretical CO 2 capture capacity is 7.24 mmol-CO 2 /g-K 2 CO 3 ), 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 CO 2 under a moist condition, which usually lowers CO 2 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][6][7][8].The regeneration behaviors of alkali metal carbonates change when they are supported on nanoporous structural materials such as activated carbon, Al 2 O 3 , or carbon nanofibers or when they are combined with MgO, TiO, or FeOOH, which can themselves capture CO 2 .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 K 2 CO 3 sorbents exhibit better CO 2 capture performance than Na 2 CO 3 sorbents and that the selection of a support material with appropriate characteristics is important 2 International Journal of Chemical Engineering for carbonation and regeneration [9].We also previously reported a detailed reaction mechanism for CO 2 occlusion by K 2 CO 3 under moist conditions [10,11].Therefore, in the present study, we focus on impregnating K 2 CO 3 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 crosslinked micropores and mesopores [12,13].The CAs provide a suitable mesoporous reaction field for forming K 2 CO 3 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 CO 2 capture ability of K 2 CO 3 nanocrystals incorporated into mesopores of CAs is studied from the viewpoint of lowering the regeneration temperature while achieving high selectivity and high capture capacity.

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 CO 2 was slowly introduced to bleed the air from the chamber.CO 2 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 cm 3 /min.As described later, the porosities of the three different CAs were characterized by N 2 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.

Preparation of CA-Potassium Carbonate (KC)
Nanocomposites.CA-KC nanocomposites were prepared by impregnating the nanopores of the CAs with a 0.15 mol/dm 3 K 2 CO 3 (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.

Measurements and Characterization.
The porosities of the CAs and the xCA-KC nanocomposites were characterized by N 2 adsorption at 77 K using an Autosorb-MP1 (Quantachrome Instruments).CO 2 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 K 2 CO 3 is partially converted into KHCO 3 under an ambient atmosphere containing CO 2 and H 2 O. 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 ( BET ) on the basis of linear plots over the relative pressure (/ 0 ) range 0.05-0.35for the N 2 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 ( mic ).The mesopore diameters ( mes ) were estimated by the Barrett-Joyner-Halenda method.
The amount of K 2 CO 3 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 K 2 CO 3 , the impregnated amounts of K 2 CO 3 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 N 2 atmosphere and maintained at 473 K for 5 min to ensure complete formation of K 2 CO 3 , the temperature was decreased to 313 K, and CO 2 and H 2 O were introduced into the sample chamber of the TG-DTA apparatus for 2 h by flowing CO 2 through distilled water.The xCA-KC nanocomposites reacted with CO 2 and H 2 O to form xCA-KHCO 3 .The crystal structures before and after the CO 2 capture were measured through ex situ Xray diffraction (XRD) on an X-ray diffractometer (MAC Science, M03XHF) equipped with a Cu K radiation source (40 kV, 25 mA, and  = 0.15406 nm).We heated the xCA-KHCO 3 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 CO 2 captured by the xCA-KC samples.The CaO solution was prepared by dissolving 0.6 g of CaO in 100 cm 3 distilled water.Any insoluble precipitate was filtered to yield a saturated solution.The clear solution was bubbled with N 2 gas to remove any dissolved CO 2 from air.The nanocomposites, which captured CO 2 under moist conditions in the TG-DTA chamber at 313 K, were heated at 573 K, and the desorbed CO 2 was recaptured by the CaO solution, leading to the immediate formation of a white precipitate.XRD analysis indicated that the precipitate was CaCO 3 .Before the xCA-KC experiments, the reliability of the CaO solution-precipitation method was evaluated using analytical grade KHCO 3 powder, which indicated 92% precision with respect to the theoretical value.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 N 2 adsorption isotherms, which indicate a partial filling or blocking of the pores by impregnated K 2 CO 3 [6].The results in Table 1 indicate that 16CA possessed the highest surface area and exhibited the greatest decrease in surface area after the K 2 CO 3 impregnation.This is in agreement with the result that 16CA-KC contained a larger amount of impregnated K 2 CO 3 compared with the other xCA-KCs.

Structural Changes Induced by CO 2 Capture and Regeneration.
The structural changes of the xCA-KCs that accompanied CO 2 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 CO 2 capture under moist conditions.The main diffraction peaks of KHCO 3 are present, verifying that the KHCO 3 nanocrystals were introduced into the mesopores of the xCAs through the impregnation process.Although peaks attributable to K 4 H 2 (CO 3 ) 3 ⋅1.5H 2 O as well as to KHCO 3 were observed in the patterns of 16CA-KC and 18CA-KC, all the peaks in the pattern of 7CA-KC were assigned to KHCO 3 .The patterns in Figure 2(b) indicate almost complete regeneration because most of the main peaks were assigned to K 2 CO 3 ⋅1.5H 2 O instead of KHCO 3 .This suggests that K 2 CO 3 was formed by heat treatment at 473 K.Although K 2 CO 3 ⋅1.5H 2 O could be formed because of the deliquescent nature of K 2 CO 3 under ambient conditions during the ex situ XRD experiment, K 2 CO 3 impregnated into the mesopores of xCAs was exposed to the ambient atmosphere and more easily reacted with atmospheric water.under an N 2 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 KHCO 3 to K 2 CO 3 , which is accompanied by the evolution of CO 2 and H 2 O.The blue line shows the decomposition process of bulk KHCO 3 , 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 K 2 CO 3 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 K 2 CO 3 is loaded onto different sorbents, mainly depending on the properties of the support material [16].

Water Adsorption.
Water adsorption analysis is important for the xCA-KC nanocomposites because K 2 CO 3 sorbs water from the ambient atmosphere.Zhao et al. reported that even though K 2 CO 3 -silica gel (SG) exhibits a relatively high pore volume, the total CO 2 sorption is only 34.5% because of the strong hygroscopicity of SG, easily leading to hydration of K 2 CO 3 to K 2 CO 3 ⋅1.5H 2 O [8,17].This hydration should influence the CO 2 -sorption amount of K 2 CO 3 under moist conditions [18].Because the CO 2 capture process of xCA-KCs involves both occlusion and physical adsorption, understanding the effects of water on CO 2 sorption of the original CA and the xCA-KCs is important.Figure 4 shows water sorption isotherms of K 2 CO 3 at 303 K. Almost no water was adsorbed below a relative pressure / 0 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 K 2 CO 3 ⋅1.5H 2 O (10.9 mmol/g).The desorption did not return to the original point because K 2 CO 3 ⋅1.5H 2 O 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 The water sorption isotherms of the CAs and xCA-KCs differ from those of K 2 CO 3 , as shown in Figure 5.Because the CAs are less hydrophilic than K 2 CO 3 , 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).
By contrast, the water sorption behavior of the xCA-KCs was more complicated because both hydration of K 2 CO 3 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 K 2 CO 3 and CAs.This phenomenon may be attributable to the nanocrystals of K 2 CO 3 incorporated into nanopores of the CAs being more reactive and deliquescent under high humidity conditions than bulk K 2 CO 3 .The CO 2 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 CO 2 uptake after K 2 CO 3 impregnation, and all the xCA-KCs exhibited a CO 2 capture capacity of approximately 2.4 mmol CO 2 /g-sorbent at 0.1 MPa.Thus, the xCA-KCs do not exhibit good CO 2 capture ability under dry conditions.

CO 2 Capture
Ability under Moist Conditions.The CO 2 sorption capacity of the xCA-KCs was calculated on the basis of their mass change resulting from reaction (1): one mole of K 2 CO 3 occludes a stoichiometric amount of one mole of each of CO 2 and H 2 O, and the two values were calculated as the total CO 2 sorption capacity (  ; mmol-CO 2 /g-sorbent) and the CO 2 occlusion capacity of K 2 CO 3 (  ; mmol-CO 2 /g-K 2 CO 3 ).They are denoted as where  (mmol) is the total amount of CO 2 captured, as obtained from TG data;  CO 2 (g/mol) is the molar mass of CO 2 ;  (g) is the mass of the sorbent; and  is the impregnation rate of K 2 CO 3 into the nanocomposites.CO 2 sorption was conducted at 313 K at a flow rate of 100 cm 3 /min in a saturated mixed gas of CO 2 and water, as shown in Figure 6.The increase in weight (%) corresponds to the conversion of K 2 CO 3 to 2KHCO 3 .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 K 2 CO 3 exhibits 45% CO 2 sorption in 100 min, and Luo et al. [11] verified that the carbonation of K 2 CO 3 is low.The reaction rate of the xCA-KC nanocomposites was faster than that of bulk K 2 CO 3 , indicating the effect of nanostructured K 2 CO 3 .
The CO 2 capture capacity results are summarized in Table 2; the results indicate that 7CA-KC exhibits the highest CO 2 sorption capacity,   = 2.68 mmol/g-sorbent (  = 14.5 mmol/g-K 2 CO 3 ), which is substantially higher than the theoretical amount of 7.24 mmol/g-K 2 CO 3 .The capture capacity results reported in Table 2 were estimated simply from the weight increase attributed to the sorption of CO 2 and H 2 O.However, the mechanism is complicated and still under study.To more precisely determine the amount of CO 2 captured by the xCA-KC nanocomposites, we used another method to measure the net amount of CO 2 captured, as described in Section 3.6.

CaO Solution-Precipitation Method.
To accurately determine the amount of CO 2 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 CaCO 3 precipitate to estimate the CO 2 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-K 2 CO 3 for 7CA-K 2 CO 3 , 16CA-K 2 CO 3 , and 18CA-K 2 CO 3 , respectively, which is also reported in Table 2.Although the amounts of CO 2 uptake are lower than those obtained from the TG measurements, these are net values of CO 2 capture capacity.Because the impregnated amount is insufficient,   is relatively low.If the impregnation of K 2 CO 3 into the mesopores of CA can be improved, more CO 2 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 CO 2 occlusion reaction with water vapor.
The CO 2 capture amounts are higher than the theoretical values in all xCA-KCs, as shown in Table 2.This can be The CO 2 capture amounts in the present study are excellent compared with those reported for other K 2 CO 3 -loaded composites [16,21,22].Although dry potassium-based sorbents such as K 2 CO 3 -MgO exhibit excellent CO 2 capacities (9.0-14.9mmol-CO 2 /g-K 2 CO 3 ) 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 Al 2 O 3 -K 2 CO 3 generates KAl(CO 3 ) 2 (OH) 2 during the synthesis process and this byproduct does not completely convert to K 2 CO 3 at temperatures below 563 K and observed that K 2 CO 3 -AC and K 2 CO 3 -TiO 2 can be regenerated at relatively low temperatures of 473 and 403 K, respectively.However, these two composites exhibit lower CO 2 capture capacities of 6.5 and 6.3 mmol-CO 2 /g-K 2 CO 3 , respectively.AC-K 2 CO 3 is considered a promising CO 2 capture sorbent because of its low regeneration energy requirement and high CO 2 capture capacity.However, to maintain these features, H 2 O activation is required to convert K 2 CO 3 to an activated form (K 2 CO 3 ⋅1.5H 2 O) [18].ZrO 2 -K 2 CO 3 has a capacity of 6.2-6.9 mmol-CO 2 /g-K 2 CO 3 when the reaction temperature is within 323-333 K under an ambient atmosphere of 9% H 2 O, 1% CO 2 , and balance N 2 [5,23].

Conclusion
CA-K 2 CO 3 nanocomposites (xCA-KC) were prepared by impregnation of K 2 CO 3 nanocrystals into the mesopores of three CAs with different pore sizes of 7, 16, and 18 nm for the development of an excellent CO 2 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 CO 2 capture.The xCA-KCs can be completely regenerated at temperatures below 423 K; 7CA-K 2 CO 3 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 K 2 CO 3 , which rendered the K 2 CO 3 crystals unstable and reduced the regeneration temperature.These xCA-KC nanocomposites exhibited excellent CO 2 capture capacity and can be considered a promising material for CO 2 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.

Table 1 :
[8]e parameters of the CA and CA-KC samples.BET (m 2 /g)   (cm 3 /g)  mic (cm 3 /g)  mes (nm) CO 3 could quickly convert to K 2 CO 3 ⋅1.5H 2 O because of the mesoporous structure of the Al 2 O 3 support[8]; they also observed that the micropores of the Al 2 O 3 support facilitated the rapid conversion of K 2 CO 3 ⋅1.5H 2 O to KHCO 3 .Thus, mesoporous structures can enhance CO 2 adsorption[8].In the present study, the N 2 adsorption amount decreased for all samples after they were subjected to the K 2 CO 3 impregnation treatment, indicating that K 2 CO 3 was successfully incorporated into the micropores and mesopores of the CAs.The pore structure parameters are compiled in Table tion) 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 Al 2 O 3 supports on the ability of K 2 CO 3 to capture CO 2 and observed that Nanocomposites.The xCA-KC nanocomposites were regenerated by heating to 473 K Figure 2: XRD patterns after CO 2 capture and regeneration: (a) xCA-KC nanocomposite after CO 2 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.: KHCO 3 , ◻: K 4 H 2 (CO 3 ) 3 ⋅1.5H 2 O, : K 2 CO 3 , and X: K 2 CO 3 ⋅1.5H 2 O.