pubs.acs.org/Langmuir © 2010 American Chemical Society
Enhancement of CO2 Sorption Uptake on Hydrotalcite by Impregnation with K2CO3 Jung Moo Lee,† Yoon Jae Min,† Ki Bong Lee,*,† Sang Goo Jeon,‡ Jeong Geol Na,‡ and Ho Jung Ryu‡ †
‡
Department of Chemical and Biological Engineering, Korea University, Seoul 136-713, Korea, and Climate Change Technology Research Division, Korea Institute of Energy Research, Daejeon 305-343, Korea Received July 27, 2010. Revised Manuscript Received November 3, 2010
The awareness of symptoms of global warming and its seriousness urges the development of technologies to reduce greenhouse gas emissions. Carbon dioxide (CO2) is a representative greenhouse gas, and numerous methods to capture and storage CO2 have been considered. Recently, the technology to remove high-temperature CO2 by sorption has received lots of attention. In this study, hydrotalcite, which has been known to have CO2 sorption capability at high temperature, was impregnated with K2CO3 to enhance CO2 sorption uptake, and the mechanism of CO2 sorption enhancement on K2CO3promoted hydrotalcite was investigated. Thermogravimetric analysis was used to measure equilibrium CO2 sorption uptake and to estimate CO2 sorption kinetics. The analyses based on N2 gas physisorption, X-ray diffractometry, Fourier transform infrared spectrometry, Raman spectrometry, transmission electron microscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy were carried out to elucidate the characteristics of sorbents and the mechanism of enhanced CO2 sorption. The equilibrium CO2 sorption uptake on hydrotalcite could be increased up to 10 times by impregnation with K2CO3, and there was an optimal amount of K2CO3 for a maximum equilibrium CO2 sorption uptake. In the K2CO3-promoted hydrotalcite, K2CO3 was incorporated without changing the structure of hydrotalcite and it was thermally stabilized, resulting in the enhanced equilibrium CO2 sorption uptake and fast CO2 sorption kinetics.
1. Introduction Hydrotalcite-like compounds, also known as double-layered hydroxides, anionic clays, or Feitknecht compounds, are peculiar inorganic materials which can capture and exchange anions.1,2 Hydrotalcite-like compounds are versatile, cheap, and easily adjustable and therefore have been used as antacids, anion exchangers, polymer stabilizers, anion scavengers, catalysts, catalyst supports, and sorbents.3 Hydrotalcite-like compounds have double-layered structure consisting of positively charged metal hydroxide layers and negatively charged interlayers containing anions and water molecules.4 The net positive charge in the metal ion layer is compensated with the charge of the anion layer. The general formula of hydrotalcite-like compounds is ½M1 - x II Mx III ðOHÞ2 ½Ax=n n - mH2 O where MII and MIII are respectively bivalent and trivalent metal ions, A is the interlayer anion, and x has values in the range 0.2-0.33. Among various hydrotalcite-like compounds, MgAl-hydrotalcite (or simply hydrotalcite), where MII is magnesium (Mg) and MIII is aluminum (Al), has received great attention due to its sorption capability of CO2 at high temperatures.5,6 The sorption of high-temperature CO2 can be applied to the direct CO2 removal from flue gases without cooling process and to the CO2-sorption *Corresponding author: Tel þ82 2 3290 4851; Fax þ82 2 926 6102; e-mail
[email protected]. (1) Othman, M. R.; Helwani, Z.; Martunus; Fernando, W. J. N. Appl. Organometal. Chem. 2009, 23, 335. (2) Cavani, F.; Trifiro, F.; Vaccari, A. Catal. Today 1991, 11, 173. (3) Bejoy, N. Resonance 2001, 6, 57. (4) Kannan, S.; Kishore, D.; Hadjiivanov, K.; Kn€ozinger, H. Langmuir 2003, 19, 5742. (5) Oliveira, E. L. G.; Grande, C. A.; Rodrigues, A. E. Sep. Purif. Technol. 2008, 62, 137. (6) Nataraj, S.; Carvill, B. T.; Hufton, J. R.; Mayorga, S. G.; Gaffney, T. R.; Brzozowski, J. R. European Patent 1006079 A1, 2000. (7) Lee, K. B.; Sircar, S. AIChE J. 2008, 54, 2293.
18788 DOI: 10.1021/la102974s
enhanced reactions for the production of high-purity hydrogen.7,8 The high-temperature CO2 sorbents can be largely divided into three groups: calcium oxides (dolomite, limestone, and composite CaO such as CaO/Ca12Al14O33); lithium-containing oxides (Li2ZrO3 and Li4SiO4); and hydrotalcites. Table 1 compares equilibrium sorption uptake and sorption properties of representative high-temperature CO2 sorbents. The calcium oxide sorbents have high equilibrium CO2 sorption uptake at 600700 °C. However, they require high-temperature regeneration at above 900 °C, and their equilibrium CO2 sorption uptake keeps decreasing with repeated carbonation/calcination cycles.9-11 The lithium-containing oxides show relatively high equilibrium CO2 sorption uptake which is maintained moderately stable with cycles. However, mass transfer rate is slow at the sorption process, and a relatively high temperature above 700 °C is needed at the regeneration process.12-15 Compared to other high-temperature CO2 sorbents, hydrotalcites have good thermal stability, relatively fast CO2 sorption kinetics, and moderate regeneration temperature.16-20 (8) Hufton, J. R.; Mayorga, S.; Sircar, S. AIChE J. 1999, 45, 248. (9) Senthoorselvan, S.; Hartmut, S. G. S.; Hupa, P. Y. M. J. Eng. Gas Turbine Power 2009, 131, 011801–1. (10) Fennell, P. S.; Davidson, J. F.; Dennis, J. S.; Hayhurst, A. N. J. Energy Inst. 2007, 80, 116. (11) Zhen-Shan, L.; Ning-Sheng, C.; Croiset, E. AIChE J. 2008, 54, 1912. (12) Iwan, A.; Stephenson, H.; Ketchie, W. C.; Lapkin, A. A. Chem. Eng. J. 2009, 146, 249. (13) Venegas, M. J.; Fregoso-Israel, E.; Escamilla, R.; Pfeiffer, H. Ind. Eng. Chem. Res. 2007, 46, 2407. (14) Kato, M.; Nakagawa, K.; Essaki, K.; Maezawa, Y.; Takeda, S.; Kogo, R.; Hagiwara, Y. Int. J. Appl. Ceram. Technol. 2005, 2, 467. (15) Ochoa-Fernandez, E.; Rusten, H. K.; Jakobsen, H. A.; Rønning, M.; Holmen, A.; Chen, D. Catal. Today 2005, 106, 41. (16) Lee, K. B.; Verdooren, A.; Caram, H. S.; Sircar, S. J. Colloid Interface Sci. 2007, 308, 30. (17) Ding, Y.; Alpay, E. Chem. Eng. Sci. 2000, 55, 3461. (18) Yong, Z.; Mata, V.; Rodrigues, A. E. Ind. Eng. Chem. Res. 2001, 40, 204. (19) Reddy, M. K. R.; Xu, Z. P.; Lu, G. Q.; da Costa, J. C. D. Sep. Purif. Technol. 2006, 45, 7504. (20) Hutson, N. D.; Attwood, B. C. Adsorption 2008, 14, 781.
Published on Web 11/19/2010
Langmuir 2010, 26(24), 18788–18797
Lee et al.
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Table 1. Comparison of High-Temperature CO2 Sorbents
sorbent
equilibrium sorption uptakea (mol CO2/kg sorbent) stability
regeneration temperature kinetics (°C)
dolomite 9.3 poor good/fair limestone 10.5 poor fair 6.1 fair CaO/ Ca12Al14O33 4.6 fair fair/poor Li2ZrO3 5.0 fair fair Li4SiO4 1.0 good/fair good K2CO3hydrotalcite a When the partial pressure of CO2 is ∼1 atm.
>900 >900 >900 >700 >700 ∼500
Recently, it has been known that the CO2 sorption capacity of hydrotalcite can be enhanced by impregnation with K2CO3.6,21,22 The impregnation with K2CO3 presumably increases the basicity of sorbents, which is favorable for the sorption of acidic CO2.22 Ebner et al. synthesized and tested K2CO3-promoted hydrotalcite for repeated sorption and desorption of CO2 at between 250 and 500 °C.23 They suggested a nonequilibrium dynamic isotherm model for reversible sorption and desorption of CO2 on the K2CO3-promoted hydrotalcite. Lee et al. measured equilibrium and column dynamic data for CO2 sorption on K2CO3-promoted hydrotalcite at 400 and 520 °C.16 A new chemisorption equilibrium model was developed to describe the CO2 sorption data, and the linear driving force model was used for mass-transfer mechanism to simulate sorption kinetics. Oliveira et al. showed that CO2 sorption capacity of K2CO3-promoted hydrotalcite can be 5-8 times higher than that of unpromoted hydrotalcite.5 They measured CO2 sorption equilibrium isotherms at 579, 676, and 783 K and used the bi-Langmuir model to fit the experimental data. However, the previous studies focused on the demonstration of CO2 sorption enhancement on hydrotalcite by K2CO3 impregnation. The mechanism of CO2 sorption enhancement by K2CO3 impregnation was not clearly understood. This study aimed to elucidate the effect of K2CO3 impregnation in hydrotalcite and the mechanism of CO2 sorption enhancement by K2CO3 impregnation. Thermogravimetric analyses were performed to measure equilibrium CO2 sorption uptake and to estimate CO2 sorption kinetics. In addition, N2 gas physisorption analysis, X-ray diffractometry (XRD), Fourier transform infrared (FT-IR) spectrometry, Raman spectrometry, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX) spectroscopy were used to characterize the properties, structure, and morphology of sorbents. The experimental results showed that the equilibrium CO2 sorption uptake of hydrotalcite could be increased even up to 10 times by impregnation with K2CO3. Also, there was an optimal amount of K2CO3 for a maximum equilibrium CO2 sorption uptake, which was caused by the two conflicting effects: enhanced basicity and reduced surface area with increasing K2CO3 amount. The various analyses showed that the incorporation of hydrotalcite and K2CO3 increased the thermal stability of K2CO3 without changing the structure of hydrotalcite, resulting in both enhanced equilibrium CO2 sorption uptake and fast CO2 sorption kinetics.
2. Experimental Section 2.1. Samples and Reagents. Three kinds of hydrotalcites (MG30, MG50, and MG70; the number represents the ratio of MgO) were provided by Sasol Germany GmbH and used after (21) Reijers, H. Th. J.; Valster-Schiermeier, S. E. A.; Cobden, P. D.; van den Brink, R. W. Ind. Eng. Chem. Res. 2006, 45, 2522. (22) Yang, J. I.; Kim, J. N. Korean J. Chem. Eng. 2006, 23, 77. (23) Ebner, A. D.; Reynolds, S. P.; Ritter, J. A. Ind. Eng. Chem. Res. 2006, 45, 6387.
Langmuir 2010, 26(24), 18788–18797
Figure 1. Equilibrium CO2 sorption uptake on unpromoted hydrotalcites (gray) and K2CO3-promoted hydrotalcites (black) at the temperature of 400 °C.
Figure 2. Effect of K2CO3 amount on the equilibrium CO2 sorption uptake of hydrotalcites at the temperature of 400 °C.
Figure 3. Effect of temperature on the equilibrium CO2 sorption uptake of K2CO3-promoted hydrotalcites. calcination under static air at 550 °C for 6 h. The calcined hydrotalcites were tested without any other modification (represented as unpromoted hydrotalcites) or further impregnated with potassium carbonate (K2CO3, g99%, Sigma-Aldrich). In the procedure of impregnation, hydrotalcite was soaked with K2CO3 solution and then dried at 110 °C under vacuum. The soaking and drying procedure was repeated several times to facilitate the distribution of K2CO3 in the hydrotalcite. Finally, the samples were calcined again in air at 550 °C for 6 h.24 The ratio of K2CO3 and hydrotalcite denoted below is the ratio of materials added at the stage of sample preparation. Pristine hydrotalcites that were not calcined did not (24) Lee, K. B.; Lee, J. M.; Jeon, S. G.; Na, J. G.; Ryu, H. J. Proceedings of the 20th International Offshore and Polar Engineering Conference, ISOPE, Beijing, 2010.
DOI: 10.1021/la102974s
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Lee et al. Table 2. Surface Area and Porosity of Unpromoted Hydrotalcites and K2CO3-Promoted Hydrotalcites unpromoted hydrotalcite
BET surface area (m2/g) average pore diameter (nm) total pore volume (cm3/g) mesopore volume fraction (%)
K2CO3-promoted hydrotalcite
MG30
MG50
MG70
MG30
MG50
MG70
260.9 8.3 0.542 90.8
186.5 5.2 0.243 80.1
215.2 5.1 0.273 83.3
45.8 17.0 0.194 93.8
36.9 13.4 0.123 96.2
46.2 10.6 0.122 96.2
Figure 4. SEM images and EDX analysis of (a) unpromoted MG70 hydrotalcite and (b) K2CO3-promoted MG70 hydrotalcite. show any noticeable CO2 sorption uptake; therefore, the only calcined hydrotalcites were tested in this study. 2.2. CO2 Sorption. Equilibrium CO2 sorption uptake on hydrotalcites was measured using a thermogravimetric analysis (TGA, Q50, TA Instruments). Before CO2 sorption experiments, moisture and CO2 on samples were removed by flowing N2 gas at 600 °C for 10 h. After regeneration, the change of sample weight was recorded in the condition of pure CO2 gas flow at the pressure of ∼1 atm and at the temperature of 50, 200, 300, 400, or 600 °C. The experiments for equilibrium CO2 sorption uptake were repeated three times, and the average value was used for comparison. 2.3. Characterization. BET surface area, pore size, and pore volume of samples were determined from the information on N2 physisorption at -196 °C using a BELSORP-max system (Bel Japan, Inc.). Each sample was degassed under vacuum (
