High-Temperature CO2 Capture on Li6Zr2O7: Experimental and

Jun 28, 2010 - High-Temperature CO2 Capture on Li6Zr2O7: Experimental and Modeling Studies ... State Key Lab of Chemical Engineering, College of Chemi...
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Ind. Eng. Chem. Res. 2010, 49, 6593–6598

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High-Temperature CO2 Capture on Li6Zr2O7: Experimental and Modeling Studies Xian-Sheng Yin, Miao Song, Qin-Hui Zhang,* and Jian-Guo Yu State Key Lab of Chemical Engineering, College of Chemical Engineering, East China UniVersity of Science and Technology, Shanghai, 200237, P.R. China

The properties of CO2 adsorption on monoclinic-phase Li6Zr2O7 (m-Li6Zr2O7) in low CO2 concentration stream are studied and compared with tetragonal-phase Li2ZrO3 (t-Li2ZrO3) using thermogravimetric analysis. The results indicate that because of the higher lithium content, about 86.7% capacity can be preserved for m-Li6Zr2O7 (at 1023 K) as the CO2 partial pressure decreases from 1.0 to 0.1 bar, whereas only about 3.5% capacity is preserved for t-Li2ZrO3 (at 848 K). The multicycle test of m-Li6Zr2O7 in 10% CO2 stream exhibits effective performance of CO2 uptake and release, though the capacity reduces gradually. Further, on the basis of the proposed adsorption pathway, a double exponential model is used to simulate the CO2 adsorption processes on m-Li6Zr2O7 with the activation energy of 22.684 and 56.084 kJ/mol for CO2 and Li+ diffusion, respectively, indicating the Li+ diffusion is the limiting step in the adsorption process. 1. Introduction In current times, global warming caused by CO2 emission has become a serious environmental problem.1,2 The capture and storage of CO2 from flue gas is an effective approach for the reduction of CO2 emitted to the atmosphere because the coal-burning power plant is one of the largest sources of CO2 emission.3,4 In light of the fact that the temperature of the flue gas between the turbine and the vent are usually in the range of 625-900 K,4 if CO2 is separated from flue gas at high temperature and further used as feedstock for the synthesis of fuels (e.g., CO and H2), the efficiency and economics of the entire process of the power plant might be improved.5-7 Thus, the problem lies with suitable adsorbents for CO2 capture at high temperature. Zeolites8 and carbon-based materials9,10 exhibit definite CO2 adsorption capacity at relatively low temperature (e673 K), yet these physical adsorbents are not suitable for CO2 separation at higher temperatures. Chemical adsorbents such as hydrotalcite compounds6,11 and metal oxides (e.g., Li2O and CaO)12,13 could adsorb CO2 considerably at higher temperature; however, the former shows poor stability because the capacity decreases significantly after several cycles and the latter requires high energy demanded by regeneration. Lithium-based ceramic is another kind of CO2 adsorbent based on the chemical reaction between lithium and CO2, and it exhibits absolute selectivity, high adsorption capacity, and recycle stability under high temperature. Because Li2ZrO3 was first reported for trapping CO2 as presented by eq 1,14-17 in which the adsorption proceeds at around 773-873 K and desorption takes place above 973 K, a series of lithium-based ceramics, including Li2-xNaxZrO3,18,19 Li4SiO4,20,21 Li4TiO4,22 and Li5AlO4,23 have been studied for CO2 adsorption at high temperature, and it is probable that the CO2 adsorption capacity depends on the lithium content in the ceramics.23,24 Li2ZrO3 + CO2 T Li2CO3 + ZrO2 ; ∆H (298 K) ) -160 kJ/mol (1) Li6Zr2O7 has two crystal phases (triclinic or monoclinic).25 Pfeiffer26 reported that the triclinic-phase Li6Zr2O7 presented a * To whom correspondence should be addressed. Telephone/Fax: 86-21-64252171. E-mail: [email protected].

higher CO2 adsorption capacity than Li2ZrO3, but the adsorbed sample could not be regenerated. Thus, in this work, the CO2 adsorption properties on monoclinic-phase Li6Zr2O7 in low CO2 concentration stream are investigated and compared with that of Li2ZrO3. In addition, on the basis of the proposed adsorption-desorption pathway, a double exponential model simulating the CO2 adsorption process is discussed. 2. Experimental Section LiOH · H2O, NH3 · H2O, and Zr(NO3)4 · 5H2O (Shanghai Chemical Co. Ltd., China) were used as the reactants to synthesize the adsorbents including monoclinic-phase Li6Zr2O7 (mLi6Zr2O7) and tetragonal-phase Li2ZrO3 (t-Li2ZrO3), and the initial Li/Zr molar ratios are 4.5 and 2.1, respectively, in view of the volatility of Li2O under high temperature.27,28 Appropriate amounts of Zr(NO3)4 · 5H2O and LiOH · H2O were dissolved in deionized water and NH4OH solution (2.5 wt % NH3), respectively, and then mixed by adding the Zr(NO3)4 · 5H2O solution into the lithium hydroxide solution with vigorous stirring; after the mixtures were heated in an oil bath at 363 K for 12 h and further dried at 393 K for 12 h in the oven, the achieved samples were calcinated under the conditions of m-Li6Zr2O7 at 1223 K for 24 h and t-Li2ZrO3 at 873 K for 12 h. The crystalline phases of the samples were identified by powder X-ray diffraction (XRD, Rigaku, D/max-RB using Cu KR Ni-filtered radiation with λ ) 1.5406 Å), operating at 40 kV, 100 mA and scanning in the 2θ range of 15°-80°. The surface morphology of the samples was analyzed by scanning electron microscopy (SEM, JSM-6360LV) with goldsfilled to increase the electron conductivity. The surface areas were measured by N2 adsorption-desorption at 77 K using a Micromeritics ASAP-2010C instrument and calculated by Brunauer-Emmett-Teller (BET) model. Prior to the measurement, all the samples were degassed under vacuum at 473 K for 3 h. A set of curves of CO2 adsorption was tested under defined conditions using a thermogravimetric analyzer (TGA, SDTQ600). About 15 mg of adsorbents was put into the sample pan and heated to the working temperature with 20 K min-1 in N2 flow. Then N2 flow was switched to the testing gas to start the adsorption test. The multicycle test was conducted over continu-

10.1021/ie100710x  2010 American Chemical Society Published on Web 06/28/2010

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Figure 1. XRD patterns of lithium zirconate with different lithium content. (a) and (b) are from JCPDS cards for monoclinic-phase Li6Zr2O7 and tetragonal-phase Li2ZrO3, respectively; (c) and (d) correspond to the samples of m-Li6Zr2O7 and t-Li2ZrO3, respectively; m(hkl), monoclinic-phase Li6Zr2O7; t(hkl), tetragonal-phase Li2ZrO3; 0, monoclinic-phase ZrO2.

ous experiments of adsorption (in 10% CO2 flow) and regeneration (in N2 flow) at appropriate temperatures, respectively. 3. Results and Discussion 3.1. Characterization of the Samples. Figure 1 shows the XRD patterns of the prepared m-Li6Zr2O7 and t-Li2ZrO3. The diffraction peaks of m-Li6Zr2O7 match quite well with the peaks of JCPDS card for monoclinic-phase Li6Zr2O7 (JCPDS 34-0312, a ) 10.45 Å, b ) 5.99 Å, c ) 10.21 Å). For sample t-Li2ZrO3, most diffraction peaks could match the standard pattern of tetragonal-phase Li2ZrO3 (JCPDS 20-0647, a ) 9.0 Å, b ) 9.0 Å, c ) 3.43 Å), except the peak at 2θ ) 28° attributed to ZrO2 (JCPDS 37-1484, a ) 5.3129 Å, b ) 5.2125 Å, c ) 5.1471 Å), which resulted from the unreacted Zr(NO3)4, and similar results were reported by the other researchers as well.15 On the other hand, the diffraction peaks of pattern (d) are wider and lower than that of pattern (c), indicating the sample of t-Li2ZrO3 may have a smaller crystalline size than m-Li6Zr2O7; actually, the calculated crystalline sizes using Scherrer equation are 36.6 and 76.1 nm for t-Li2ZrO3 and m-Li6Zr2O7, respectively. The surface morphologies of the prepared m-Li6Zr2O7 and t-Li2ZrO3 are compared in Figure 2. For m-Li6Zr2O7, the product is built up by unregularly polyhedron-shaped and dense particles with sizes ranging within 1.0-5.0 µm, whereas the sample t-Li2ZrO3 is made up of an amount of small particles with sizes ranging within 0.1-0.15 µm, which also validates the speculation of small crystalline size in the previous XRD analysis. Moreover, the sample m-Li6Zr2O7 exhibits more serious agglomeration than t-Li2ZrO3, which may result from the higher calcination temperature and longer calcination time, and the larger initial Li/Zr molar ratio for m-Li6Zr2O7.28 Consequently, the surface area of sample m-Li6Zr2O7 may be very small because of the pore blockage or loss. The N2 adsorption analysis shown in Figure 3 also corroborates this suggestion, in which the shape of the N2 adsorption isotherm for t-Li2ZrO3 shows a very narrow hysteresis loop, while no hysteresis loop appears for m-Li6Zr2O7, and the corresponding surface areas are 13.922 and 1.198 m2/g, respectively. 3.2. Adsorption Properties in Low-Concentration CO2 Gas. To simulate the atmosphere of flue gas from a coal-burning power plant, a mixed gas of CO2 and N2 with CO2 partial pressure at 0.1 bar is prepared as the feed gas for the following tests. Figure 4 shows the CO2 uptake curves on m-Li6Zr2O7 in the mixed gas with different flow rate at 1023 K. The adsorption rate increases obviously with the flow rate switching from 50

Figure 2. Scanning electron micrographs of the (a) m-Li6Zr2O7 and (b) t-Li2ZrO3 samples.

Figure 3. Nitrogen isotherm (adsorption/desorption) of the m-Li6Zr2O7 and t-Li2ZrO3 samples.

to 100 mL/min, but no more increase can be observed with further switching of the flow rate up to 150 mL/min. Therefore, the CO2 adsorption tests in the following parts are always measured at a minimum flow of 100 mL/min to ensure no masstransfer limitations, which is essential for the kinetic studies. The effective temperature for CO2 adsorption on m-Li6Zr2O7 in the prepared feed gas is studied under a set of temperatures (i.e., 873, 923, 973, 1023, and 1073 K). As shown in Figure 5, the adsorption curve tested at 1073 K reaches the plateau within only about 25 min, whereas it costs about 1.0 h to achieve the same adsorption capacity at 1023 K; with further reduction in the working temperature, the adsorption rates decreased significantly and only about 2.4 wt % weight gain can be gained for the case at 873 K within 60 min. It is reported that the CO2

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Figure 4. CO2 adsorption rate on m-Li6Zr2O7 under different flow rates at 1023 K.

Figure 5. Curves of CO2 adsorption on m-Li6Zr2O7 with PCO2 ) 0.1 bar as a function of temperature.

Figure 6. Curves of CO2 adsorption on m-Li6Zr2O7 at 1023 K as a function of CO2 concentration.

diffusion in molten Li2CO3 external shell is faster than that in solid-state Li2CO3 external shell formed on the lithium-based adsorbent during the CO2 adsorption process.7 The testing temperatures of 1023 and 1073 K are above the melting point of Li2CO3 (about 993 K); consequently, the adsorption rates are faster compared with these tested at lower temperatures. In addition, the lithium diffusion in the bulk is greatly activated with the rise in temperature;23 accordingly, the adsorption rate at 1073 K may be accelerated compared with that at 1023 K. Furthermore, the effect of CO2 partial pressure on the adsorption capacity for m-Li6Zr2O7 is presented in Figure 6 with the adsorption processes operated at 1023 K and the CO2 partial pressure varied from 1.0 to 0.1 bar. It shows that about 11.2 wt % weight gain is achieved as tested in pure CO2 flow (PCO2 to

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Figure 7. Curves of CO2 adsorption on t-Li2ZrO3 at 848 K as a function of CO2 concentration.

1.0 bar) within 30 min; while tested at PCO2 to 0.1 bar, the adsorption rate is decreased and about 9.71 wt % weight gain is achieved in 30 min. In other words, about 86.7% capacity is preserved for m-Li6Zr2O7 as the PCO2 reduced from 1.0 to 0.1 bar. In comparison, Figure 7 shows a set of adsorption curves of t-Li2ZrO3 as a function of CO2 concentration, and the operation temperature is 848 K according to the other reported results of the optimum adsorption temperature.18 Interestingly, the adsorption behavior is absolutely different from that of m-Li6Zr2O7. It is clear that about 18.5 wt % weight gain could be achieved for the case tested with PCO2 at 1.0 bar in 60 min, yet the achieved capacity is reduced to about 0.65 wt % weight gain with decreasing PCO2 to 0.1 bar, which means only 3.5% capacity could be preserved as the PCO2 decreases from 1.0 to 0.1 bar. The different adsorption behaviors between t-Li2ZrO3 and m-Li6Zr2O7 indicate that the adsorption performance of t-Li2ZrO3 is more dependent on the CO2 partial pressure than that of m-Li6Zr2O7. Chen et al.4 also reported that the adsorption rate of tetragonal-phase Li2ZrO3 was strongly relative to the CO2 partial pressure, and the adsorption reaction rate for tetragonal-phase Li2ZrO3 was very slow under low CO2 partial pressure (PCO2 < 0.1 bar) since the CO2 concentration becomes close to the equilibrium partial pressure of CO2 at the working temperature, which may also be available to explain the CO2 adsorption behaviors of t-Li2ZrO3 observed in this study. For the case of m-Li6Zr2O7, though exhibiting larger particle size and lower surface area as shown previously, the higher lithium content (about 1.5 times higher than that of t-Li2ZrO3) may be favorable for the CO2 capture in the case of low CO2 concentration; moreover, the CO2 diffusion in molten carbonate shell formed on the particle surface of m-Li6Zr2O7 may be faster than that in solid-state carbonate shell formed on t-Li2ZrO3. 3.3. Multicycle Properties in Simulated Flue Gas. The multicycle performance of m-Li6Zr2O7 tested with the conditions of adsorption in 10% CO2 gas and desorption in pure N2 gas is shown in Figure 8. As can be seen, the m-Li6Zr2O7 exhibits a good regenerability though the adsorption capacity is reduced gradually during the cycle processes. It is well-known that the formed Li2CO3 could cause reconstruction and sintering of the adsorbent during the adsorption-desorption processes, which may slow down the CO2 diffusion and subsequently reduce the adsorption capacity within the defined adsorption time; in addition, the losses of Li2O resulting from the sublimation and reaction with the sample pan of Al2O3 at high temperature may also be responsible for the reduced adsorption capacity partially.22 Thus, considering the low CO2 concentration and high temperature of gas from a coal-burning power plant or steam

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Figure 8. Multicycle test of m-Li6Zr2O7 with adsorption at 1023 K in 10% CO2 flow and desorption at 1123 K in N2 flow.

Figure 9. XRD patterns of m-Li6Zr2O7 by CO2 adsorption at 1023 K for 40 min (a) and desorption at 1123 K for 40 min (b). 2, triclinic-phase Li6Zr2O7; O, monoclinic-phase Li2ZrO3; 1, tetragonal-phase Li2ZrO3; b, monoclinic-phase Li2CO3; 0, unknown phase.

methane reformer (SMR), m-Li6Zr2O7 may be a new option for CO2 adsorption.16,29 To reveal the reaction pathway for m-Li6Zr2O7 during the processes of CO2 adsorption and desorption, the microstructure changes of samples after the first cycle are characterized by means of XRD and SEM analysis. Figure 9 shows the XRD patterns of (a) and (b) corresponding to the adsorbed and desorbed samples, respectively. As can be seen, the diffraction peaks of pattern (a) are mainly comprised of Li2CO3 (JCPDS 22-1141, a ) 8.36, b ) 4.98, c ) 6.19) and Li2ZrO3 (JCPDS of monoclinic-phase 33-0843, a ) 5.43, b ) 9.03, c ) 5.42), and the pattern of desorbed sample shows only peaks assigned to triclinic-phase Li6Zr2O7 (JCPDS 36-0122). This means that, after CO2 adsorption at 1023 K, the sample m-Li6Zr2O7 is converted to Li2CO3 and Li2ZrO3 completely, and then with desorption at 1123 K, Li2CO3 and Li2ZrO3 react to produce triclinic-phase Li6Zr2O7 but not monoclinic-phase structure. It is reported that the triclinic-phase Li6Zr2O7 is a metastable structure and may be transformed to stable monoclinic-phase Li6Zr2O7 under high temperature.20 In addition, our previous research found that the process of the structure transformation of Li6Zr2O7 from triclinic phase to monoclinic phase is slow, even above 1173 K.28 Thereby, the adsorbent may be composed of triclinic-phase Li6Zr2O7 indeed after the first cycle due to the temperature of desorption being too low to transform Li6Zr2O7 from triclinic to monoclinic phase, and the produced triclinic-phase Li6Zr2O7 is able to adsorb and release CO2 effectively for the following cycles, whereas as mentioned previously, Pfeiffer and Bosch26 pointed out that triclinic-phase Li6Zr2O7 could not be regenerated after the CO2 desorption. This opinion is obviously not consistent with the experimental results observed here. By comparing the detailed testing

Figure 10. SEM images of m-Li6Zr2O7 by CO2 adsorption (a) and desorption (b).

conditions, we find that the desorption temperature in Pfeiffer and Bosch’s report might be too low to regenerate the adsorbed Li6Zr2O7, which may be the main reason for the different results observed by the two studies. Figure 10 shows the SEM images of the adsorbed and desorbed samples corresponding to images of (a) and (b), respectively. Comparing with the SEM image of the initial adsorbent (image (b) in Figure 2), we notice that the morphology of the adsorbed sample is quite different, and it seems that the product is made up of two different phases since a loose layer covers the surface of the particles. Combing the previous XRD analysis, the loose layer might be the quenched liquid carbonate formed on the particle surface of the adsorbent and the uncovered part of the particles exhibiting dense surface may correspond to the produced Li2ZrO3 during the adsorption process.17 Furthermore, the loose layer disappears in the SEM image of the desorbed sample, indicating the Li2CO3 has reacted with Li2ZrO3 completely. On the basis of the above analyses, the pathway of the CO2 adsorption-desorption on m-Li6Zr2O7 could be depicted as Figure 11. There are two different sections of adsorptiondesorption on m-Li6Zr2O7 during the multicycle process. Section (A) is the first cycle of CO2 adsorption (from (a) to (c)) and regeneration (from (d) to (f)) on monoclinic-phase Li6Zr2O7. As presented, at the initial stage of adsorption, the Li2CO3 external shell is formed on the surface of the adsorbent, under which the Li2ZrO3 internal shell is produced accordingly, and then the CO2 and the ions (Li+ and O2-) should diffuse through the formed shells of Li2CO3 and Li2ZrO3, respectively, to the reaction interface for further adsorption reaction (corresponding to images of (a) and (b)) until the whole adsorption process

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Figure 12. Comparison of experimental data tested in 10% CO2 atmosphere (point) and simulated CO2 adsorption curves (solid curves) on m-Li6Zr2O7 at different temperatures.

Figure 11. Schematic illustration for CO2 adsorption (e1073 K) and desorption (g1123 K) on m-Li6Zr2O7. Section (A) is the first cycle processes including the adsorption process from (a) to (c) in CO2 flow and desorption process from (d) to (f) in N2 flow, for monoclinic-phase Li6Zr2O7; section (B) is the following multicycle process for triclinic-phase Li6Zr2O7 with the adsorption process from (g) to (i) in CO2 flow and the regeneration process from (d) to (f) in N2 flow.

was completed (corresponding to image (c)). The desorption process is the reverse process of adsorption but the produced internal shell and the final product are composed of the triclinicphase Li6Zr2O7. Section (B) is the following multicycle process of CO2 adsorption-regeneration on triclinic-phase Li6Zr2O7 with similar diffusion behaviors of CO2 and ions (Li+ and O2-) described in section (A). Accordingly, the reaction of CO2 adsorption-regeneration could be proposed as eq 2 with the theoretical CO2 adsorption capacity of about 13 wt % weight gain, which is consistent with the experimental result. Li6Zr2O7 + CO2 T 2Li2ZrO3 + Li2CO3

(2)

4. Model and Analysis of CO2 Adsorption Kinetics Several mathematical models for CO2 adsorption on lithiumbased adsorbents have been proposed previously.17,20,30 Among them, a double exponential model presented in eq 3 was successfully used to simulate the CO2 adsorption on lithiumbased adsorbents such as Li4SiO4,20 Li4-xNaxSiO4,31 and Li5AlO4,23 where Qt represents the weight change of CO2 adsorbent, t is the adsorption time, kCO2 and kLi are the rate constants corresponding to the CO2 diffusion from the surface of the particle and the Li+ diffusion from the core to the reaction interface, respectively; A, B, and C are the pre-exponential factors. On the basis of the depicted adsorption process in Figure 11 and the similar adsorption pathway for lithium ceramic adsorbents, this model may also be appropriable to the m-Li6Zr2O7. Qt ) A exp-kCO2t + B exp-kLit + C

(3)

Figure 12 presents the comparisons between the experimental data and the simulated uptake curves for m-Li6Zr2O7 in 10% CO2 atmosphere at different temperatures. As expected, all of the curves can fit to the double exponential model. In addition,

Figure 13. Plots of ln k (i.e., kCO2 or kLi) vs 1/T for the diffusion processes of CO2 and Li+ on m-Li6Zr2O7. Table 1. Kinetic Parameters Obtained from Uptake Curves of m-Li6Zr2O7 Fitted to a Double Exponential Model temperature (K) kCO2 (1/s) kLi (1/s) 873 923 973 1023 1073

16.7800 18.1247 20.8244 26.0561 29.1563

0.0304 0.0319 0.0419 0.0619 0.1382

A

B

C

-6.5000 -2.9979 2.9186 -1.6369 -4.529 4.4176 -1.7113 -10.2031 9.3769 -27.0841 -12.6063 11.4414 -48.9501 -13.8872 11.4821

the values including kCO2, kLi, A, B, and C obtained at each temperature are shown in Table 1. As can be seen, the kCO2 is 2 orders of magnitude larger than kLi, indicating the limiting step of the whole process is the lithium diffusion process. Furthermore, Figure 13 shows the well-fitting linear trends for the plots of ln k versus 1/T. It is thought that if the kinetic constant values (i.e., kCO2 or kLi) are linear-dependent with the corresponding temperatures (1/T), the gradients of these best-fit lines may follow an Arrhenius-type behavior as presented in eq 4:20 k ) k0 exp(-Ea/RT)

(4)

in which k0 is the reaction rate constant, Ea is the activation energy of the CO2 or Li+ diffusion processes, R is the gas constant, and T is the absolute temperature, respectively. Consequently, the activation energies for the diffusion processes of CO2 and Li+ in m-Li6Zr2O7 can be estimated to be 22.684 and 56.084 kJ/mol, respectively. This result further confirms that the lithium diffusion is the limiting step in the whole adsorption process. 5. Conclusions The CO2 adsorption properties of monoclinic-phase Li6Zr2O7 (m-Li6Zr2O7) in low CO2 concentration stream are investigated

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systematically. The results indicate that, after the first adsorptiondesorption cycle, the m-Li6Zr2O7 is transformed to triclinicphase Li6Zr2O7 actually, and retains effective performance of CO2 adsorption-desorption for the following cycles though the capacity reduced gradually during the multicycle process because of the sintering of adsorbent and the loss of lithia. A double exponential model describing the diffusion processes of the CO2 and Li+ was used successfully to simulate the process of CO2 adsorption on m-Li6Zr2O7, and the activation energy for the two diffusion processes were 22.684 and 56.084 kJ/ mol, respectively, indicating lithium diffusion is the limiting step in the adsorption process. Acknowledgment The research received financial support from the National Science Foundation of China (20976047), Special Nano Science and Technology Project of STCSM (0852 nm02100), and Shanghai International Cooperation Project (08160704000). Literature Cited (1) Daniel, A.; Lashof, A.; Dilip, R.; Ahuja, R. Relative Contributions of Greenhouse Gas Emissions to Global Warming. Nature 1990, 344, 529. (2) Song, C. Global Challenges and Strategies for Control, Conversion and Utilization of CO2 for Sustainable Development Involving Energy, Catalysis, Adsorption and Chemical Processing. Catal. Today 2006, 115, 2. (3) Yang, H.; Xu, Z.; Fan, M.; Gupta, R.; Slimane, R. B.; Bland, A. E.; Wright, I. Progress in Carbon Dioxide Separation and Capture: A Review. J. EnViron. Sci. 2008, 20, 14. (4) Ochoa-Ferna´ndez, E.; Rønning, M.; Grande, T.; Chen, D. Synthesis and CO2 Capture Properties of Nanocrystalline Lithium Zirconate. Chem. Mater. 2006, 18, 6037. (5) Khomane, R. B.; Sharma, B. K.; Saha, S.; Kulkarni, B. D. Reverse Microemulsion Mediated Sol-gel Synthesis of Lithium Silicate Nanoparticles under Ambient Conditions: Scope for CO2 Sequestration. Chem. Eng. Sci. 2006, 61, 3415. (6) Ding, Y.; Alpay, E. High Temperature Recovery of CO2 from Flue Gases Using Hydrotalcite Adsorbent. Process Saf. EnViron. Prot. 2001, 79, 45. (7) Ida, J.; Lin, Y. S. Mechanism of High-Temperature CO2 Sorption on Lithium Zirconate. EnViron. Sci. Technol. 2003, 37, 1999. (8) Siriwardane, R. V.; Shen, M-. S.; Fisher, E. P. Adsorption of CO2 on Zeolites at Moderate Temperatures. Energy Fuels 2005, 19, 1153. (9) Yong, Z.; Mata, V. G.; Rodrigues, A. E. Adsorption of Carbon Dioxide on Chemically Modified High Surface Area Carbon-based Adsorbents at High Temperature. Adsorption 2001, 7, 41. (10) Siriwardane, R. V.; Shen, M.-S.; Fisher, E. P.; Poston, J. A. Adsorption of CO2 on Molecular Sieves and Activation Carbon. Energy Fuels 2001, 15, 279. (11) Yong, Z.; Mata, V. G.; Rodrigues, A. E. Adsorption of Carbon Dioxide onto Hydrotalcite-like Compounds (HT1cs) at High Temperatures. Ind. Eng. Chem. Res. 2001, 40, 204. (12) Mosqueda, H. A.; Vazquez, C.; Bosch, P.; Pfeiffer, H. Chemical Sorption of Carbon Dioxide (CO2) on Lithium Oxide (Li2O). Chem. Mater. 2006, 18, 2307.

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ReceiVed for reView March 23, 2010 ReVised manuscript receiVed May 20, 2010 Accepted May 26, 2010 IE100710X