CO2 Adsorption at Elevated Pressure and Temperature on Mg–Al

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CO2 Adsorption at Elevated Pressure and Temperature on Mg−Al Layered Double Hydroxide Margarita J. Ramírez-Moreno,†,‡ Issis C. Romero-Ibarra,† M.A. Hernández-Pérez,‡ and Heriberto Pfeiffer*,† †

Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Circuito exterior s/n, Ciudad Universitaria, Del. Coyoacán, CP 04510, México DF, Mexico ‡ Departamento de Ingeniería en Metalurgia y de Materiales, Escuela Superior de Ingeniería Química e Industrias Extractivas, IPN, UPALM, Av. Instituto Politécnico Nacional s/n, CP 07738, México DF, Mexico ABSTRACT: CO2 adsorption at elevated pressure was studied in a Mg−Al (Mg/Al = 3) layered double hydroxide (LDH). The double-layered structure was prepared via a coprecipitation method. The sample’s structure and microstructure evolutions were characterized using X-ray diffraction, scanning electron microscopy, N2 adsorption, and thermogravimetric and calorimetric analyses. The CO2 adsorption experiments were performed between 5 and 4350 kPa at different temperatures (30−350 °C). Elevated pressure experiments showed that this material was able to adsorb different quantities of CO2 depending on the thermal evolution of its structure and microstructure. The highest CO2 adsorption (5.7 mmol/g) was produced at 300 °C before the layered structure had completely collapsed. At these specific conditions the interlayer space was reduced from 7.78 to 4.39 Å. This interlayer change was attributed to the onset of LDH structural collapse. However, at this temperature the adsorption process must be favored over the adsorption−desorption equilibrium, allowing the maximum CO2 capture.

1. INTRODUCTION In the last 20 years, the removal and recovery of CO2 from hot gas streams have been identified as being among the most important environmental issues to be solved.1−3 Therefore, different types of materials, such as organic materials, polymers, minerals, zeolites, layered double hydroxides, oxides, and ceramics, among others, have been tested as CO2 sorbents through physical or chemical mechanisms.1,4−8 Among these materials, layered double hydroxides (LDHs) are suitable as CO2 sorbents through two different mechanisms: adsorption at low temperatures or absorption at moderate temperatures. LDH materials are mixed metal hydroxides represented by the general chemical composition [M(1−x)2+Mx3+(OH)2]Ax/nn−· mH2O,9−11 where M2+ and M3+ stand for divalent and trivalent cations, respectively, occupying octahedral sites within the hydroxyl layers forming brucite-type layers; x is equal to the M3+/(M2+ + M3+) ratio and includes values over the range of 0.20 to 0.50; and An− is an exchangeable interlayer anion. The most common interlayer anion is the carbonate anion, CO32−. These materials have received considerable attention in recent years because they have a wide range of applications, primarily as catalysts, catalyst supports, and as electrodes,12−17 as well as because they present very interesting thermal structural evolutions.18,19 In particular, Mg−Al−CO3 LDH, known as hydrotalcite-like compounds or anionic clays, has been the most widely studied material. In recent years, different reports have been published regarding the use of LDHs as CO2 sorbents at two different temperature ranges: low (30−200 °C) and moderate (200−600 °C).1,7,8,20−27 In the low-temperature range, the CO2 is usually adsorbed. Moreover, at moderate temperatures, the original layered structure collapses and the Mg(Al)O periclase-like mixed oxide is crystallized, producing a chemical CO2 capture, © 2014 American Chemical Society

forming carbonates or bicarbonates. In both temperature ranges, this type of material has not presented the best CO2 capture capacities compared to other materials used in this field. Consequently, to improve the CO2 sorption capacity of LDH materials, different alternatives have been proposed, including the addition of alkaline metals to increase basicity,21,22 the use of several MII and/or MIII structural metals to modify the chemical surface properties and/or some microstructural characteristics,28,29 and the addition of water vapor on the CO2 flow to modify the particle surface reactivity30 or a pressure increase.20,24,25,27,31 It must also be noted that there are few studies related to CO2 high-pressure capture on LDH structures. Alpay and coworkers gathered CO2 experimental adsorption isotherm data on K-promoted LDH samples at different temperatures and varying pressures, up to 120 kPa, obtaining CO2 adsorptions ≤1.0 mmol/g.20,31 Apart from this work, only a few other theoretical works have been published.24,25,27 Therefore, the aim of the present work was to evaluate the CO2 adsorption at elevated pressures, using the Mg−Al−CO3 LDH (Mg/Al = 3.0) as a starting material. The correlation between the LDH thermal structure evolution, with the corresponding chemical species, and the amount of CO2 adsorbed was investigated. The sample was characterized using several techniques, including Xray diffraction (XRD), N2 adsorption, scanning electron microscopy (SEM), and thermogravimetric (TG-DTG) and calorimetric (DSC) analyses, before and after the CO2 adsorption experiments, for monitoring the structure evolution. Received: Revised: Accepted: Published: 8087

March 11, 2014 April 24, 2014 April 25, 2014 April 25, 2014 dx.doi.org/10.1021/ie5010515 | Ind. Eng. Chem. Res. 2014, 53, 8087−8094

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2. EXPERIMENTAL SECTION Aluminum nitrate (Al(NO3)3·9H2O, Sigma-Aldrich), magnesium nitrate (Mg(NO3)2·6H2O, Sigma-Aldrich), as well as potassium hydroxide and carbonate (KOH and K2CO3, both from Sigma-Aldrich) were used to prepare a hydrotalcite-like material by a coprecipitation method at low supersaturation conditions. Appropriate amounts of Al(NO3)3·9H2O and Mg(NO3)2·6H2O were dissolved in deionized water to prepare a 0.8 M solution for which the Mg/Al molar ratio was selected to be 3:1. Separately, two different solutions of K2CO3 (0.2 M) and KOH (1.5 M) were prepared.10−12,17 The K2CO3 solution was poured into a glass reactor and maintained at 80 °C, followed by the dropwise addition of the solution containing the Mg and Al cations into the reactor. Simultaneously, the KOH solution was added to adjust the pH to ∼9.5. The resultant slurry was aged under vigorous stirring at 80 °C for 24 h. The precipitate was filtered and washed several times with warm deionized water to remove nitrate ions and potassium excess. The precipitate was then dried in an oven for 24 h at 80 °C. The LDH powders, before and after CO2 sorption, were characterized by X-ray diffraction, N2 adsorption, scanning electron microscopy, and thermogravimetric and calorimetric analyses. XRD patterns were obtained using a D8 Focus by Bruker AXS, with a Cu Kα1 radiation at 35 kV and 25 mA. Diffraction patterns were recorded within the 2θ range of 5− 85° with a step size of 0.02° and were correlated to the corresponding JCPDS files. The microstructural characteristics of these samples and CO2 elevated pressure products were determined using scanning electron microscopy and N2 adsorption. The particle size and morphology were determined by SEM in a JEOL JSM-6701F instrument. Additionally, the textural characteristics (BET surface area, pore size, and pore volume) were determined by N2 adsorption−desorption at 77 K using a Minisorp II instrument from Bel-Japan, employing a multipoint technique. All samples were outgassed at room temperature under high vacuum for 24 h before the N2 adsorption−desorption tests. Finally, a thermogravimetric analysis was performed using a Q500HR thermobalance from TA Instruments under a N2 atmosphere. The CO2 capture capacity at elevated pressure on the hydrotalcite-like material was determined using a volumetric Belsorp-HP instrument from Bel-Japan. The nonideal behavior of the CO2 gas was corrected by application of virial equations employing four virial coefficients. The virial coefficients were calculated from NIST data at the respective temperature in the heater at a maximum pressure of 5.0 MPa. Powders were initially activated before CO2 sorption tests. First, the sample was introduced in the adsorption cell and outgassed at 80 °C for 0.5 h before testing because the materials are sensitive to the presence of moisture and environmental CO2. The adsorption experiments were performed at temperatures between 30 and 350 °C. At each temperature, the pressure was increased up to 4350 kPa, establishing equilibrium times between a few seconds and 60 min because the LDH samples usually present slow equilibrium processes. To establish the influence of the pressure and CO2 atmosphere on the fresh LDH thermal evolution, pressure differential scanning calorimetry (DSC) experiments were carried out using a Pressure DSC instrument from Instrument Specialist Incorporated. The samples were heated from 30 to 400 °C at a rate of 10 °C/min. The experiments were

performed under atmospheric and 4000 kPa pressures in both N2 and CO2 atmospheres. It is well-known that the original layered-structure LDH can be reconstituted from the periclase mixed oxides that are obtained from its thermal treatment by the adsorption of anions. To observe this process, the elevated pressure CO2 capture products were rehydrated. Different rehydration experiments in a water vapor environment were performed in a temperature-controlled thermobalance TA Instruments model Q5000SA equipped with a humidity-controlled chamber. The water vapor sorption/desorption isotherms were generated at 60 °C, varying the relative humidity (RH) from 0 to 80 and the back to 0 RH%. The experiments were carried out at a rate of 0.5 RH%/min under a N2 flow of 100 mL/min, which are the same conditions reported in previous works.12,30 Finally, to identify the layered reconstruction, each rehydrated sample was immediately analyzed by thermogravimetric analysis (TGA) under conditions that were the same as those of the thermal decomposition of fresh LDH.

3. RESULTS AND DISCUSSION Figure 1 presents the XRD pattern of the LDH sample. The XRD pattern of this sample was fitted to the 022-0700 JCPDS

Figure 1. XRD pattern of the double-layered hydroxide synthesized via the coprecipitation method at low supersaturation conditions.

file, indicating that the LDH structure was the only phase present in the sample, at least within the XRD detection level (∼5%). After the structural characterization, the sample was microstructurally evaluated using a scanning electron microscope and N2 adsorption. Figure 2 shows the SEM image in which the particle morphology of the sample can be visualized. The hydrotalcite-like material showed a layered structure with platelets or flake-like particles of approximately 500−700 nm in the plane and 5−10 nm in thickness. In addition, the platelets were agglomerated in larger particles of approximately 1 μm. This morphology is in very good agreement with the typical cauliflower-like morphology reported in the literature.32,33 The microstructural characteristics of the sample were complemented by the N2 adsorption−desorption experiment. The sample presented a type IV isotherm (see the inset of Figure 2), according to the IUPAC classification,34 which corresponds to a mesoporous material with a surface area of SBET = 136.6 m2/g. After the structural and microstructural characterization, the material was evaluated in the elevated pressure CO2 adsorption process at different temperatures (30−350 °C). The pressure was increased up to 4350 kPa. Figure 3 shows the adsorbed 8088

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exponentially up to 4.73 mmol/g at 4200 kPa. When the CO2 adsorption was performed at 100 °C, the exponential growth began at a higher pressure, ∼1800 kPa. At this thermal condition, the quantity of CO2 adsorbed decreased slightly (3.52 mmol/g) compared to that of the sample treated at 30 °C. The concentration of adsorbed CO2 then decreased more drastically at 200 °C, in which only 1.08 mmol/g of CO2 was adsorbed. Although the CO2 adsorption trend was similar in the samples treated at higher temperatures (T > 200 °C), the final amount of CO2 adsorbed at 300 °C did not follow the same trend. Between 200 and 300 °C, the final CO 2 concentration increased from 1.08 to 5.76 mmol/g, respectively. In fact, the quantity of CO2 adsorbed at 300 °C (5.76 mmol/g) represented the highest CO2 adsorption produced in this experimental setup. Finally, at 325 and 350 °C, the total amount of CO2 adsorbed decreased to 0.08 mmol/g, which corresponds to the lowest CO2 adsorption (350 °C). To explain the CO2 adsorption behavior observed at elevated pressure, the isothermal products were structurally and microstructurally recharacterized using XRD, N2 adsorption− desorption, and SEM. Finally, the maximum CO2 adsorptions were correlated to the thermal stability of the LDH through a thermogravimetric analysis. Figure 5 shows the XRD patterns of the elevated pressure isothermal products. The layered structure was partially collapsed or destroyed as a function of temperature from 30 to 300 °C. In this temperature range, the 003 peak was right-shifted from 11.5 to 13.65 in 2θ. The d003 distance corresponds to the LDH interlayer space, which was reduced as a function of temperature from 4.78 to 4.39 Å. This structural change must be related to dehydration and dehydroxylation processes of the brucite-type layers. It must be noted that the highest CO2 adsorption was obtained at 300 °C. Despite the water loss, the change in the interlayer distance was a minimum; this slight change could be explained as a function of the CO2 captured. The CO2 adsorbed may have inhibited the structural collapse and delayed the adsorption− desorption equilibrium. Furthermore, at 325 °C the doublelayered structure completely disappeared, resulting in an amorphous structure. Finally, at 350 °C, the periclase-like structure (Mg(Al)O) was crystallized. These final structural changes correspond with the previously described CO2 adsorption decrease. After the LDH structural evolution analysis, the samples were analyzed by N2 adsorption−desorption and SEM to determine any microstructural changes in the samples. Figure 6 shows the

Figure 2. SEM image of the LDH sample, showing the agglomerates formed by platelet particles. The inset shows the N2 adsorption− desorption curve.

volume at low pressures (between 5 and 100 kPa). In general, the amount of CO2 adsorbed (millimoles per gram) decreased as a function of the temperature (see the right panel of Figure 3). Between 30 and 100 °C, the adsorbed CO2 amounts decreased from 0.72 to 0.70 mmol/g. However, the final CO2 adsorptions observed between 200 and 350 °C decreased to ∼0.3 mmol/g. These results agree with previous LDH studies in which the maximum reported CO2 adsorption was equal to or less than 1.0 mmol/g at similar temperature and pressure ranges.25,35 In all cases, the decrease observed in the CO2 adsorption process as a function of temperature has been associated with the thermal desorption activation as well as a partial destruction of the layered structure. In fact, in all previous studies the LDH−CO2 capture was performed on optimized sorption conditions such as activated basic sites, improvement of basicity by impregnation with alkaline metals (K-promoted LDH), and addition of water vapor in the gas stream. The CO2 adsorption process on the LDH samples was continued at higher pressures (up to 4350 kPa). Figure 4 shows the CO2 adsorption curves and the maximum CO2 adsorbed (millimoles per gram) at the highest pressure of each temperature. At 30 °C, the CO2 adsorption was less than 1 mmol/g between 5 and 1100 kPa. However, at higher pressures (P > 1100 kPa), the CO2 adsorption began to increase

Figure 3. CO2 adsorption performed at low pressures (5−100 kPa) at different temperatures (30−350 °C). The right panel shows the maximum CO2 adsorbed as a function of temperature. 8089



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Figure 4. CO2 adsorption performed at elevated pressures (up to 4350 kPa) at different temperatures (30−350 °C). The right panel shows the maximum CO2 adsorbed as a function of temperature.

textural analysis of the LDH products. All samples presented type IV isotherms (only a representative isotherm is presented in the figure), according to the IUPAC classification,34 which corresponds to mesoporous materials (Figure 6A). The specific surface was determined using the BET model. Between 30 and 300 °C, the specific surface of the elevated pressure products did not vary significantly (130 ± 10 m2/g) compared to the initial sample (136.6 m2/g). However, the product obtained after the CO2 adsorption at 325 °C presented a higher specific area of 296.3 m2/g. This phenomenon may be explained as a function of the total destruction of the double-layered structure, which induces the formation of a larger area. As previously explained, the LDH structure-collapse process occurs in two steps. First, a dehydration process up to 200 °C occurs, followed by the subsequent destruction of the LDH structure. During the dehydration process, the Al3+ cations migrate to tetrahedral sites in the interlayer region and different modifications occurred in the octahedral brucite-type layer. Similar results have been previously observed by Belloto and co-workers.36 However, this change preserved the LDH structure despite the absence of water. Consequently, a new 3D network partially formed. After the LDH structural destruction, additional trivalent cations (Al3+) changed their octahedral configuration to tetrahedral positions. Thus, the formation of the 3D structure was constituted of closely packed oxygen networks. Finally, at 350 °C, both the total CO2 adsorbed and the specific surface area decreased, which must be attributed to the periclase crystallization process previously observed by XRD. From the textural analysis, it is also worth noting that between 30 and 250 °C, the pore diameter and total pore volume tended to increase, although the specific area did not (Figure 6B). Therefore, the elevated pressure may have induced a reduction in the quantity of pores but an increase in their diameters. This must be related to the dehydration and dehydroxylation processes, previously mentioned. However, at 300 and 325 °C the pore diameters decreased, while the pore volume increased only at 325 °C. These textural changes can be attributed to the total collapse of the double-layered structure and the 3D network formation. The morphology evolution of the LDH powders after the CO2 elevated pressure adsorption experiments was analyzed (Figure 7). When the CO2 adsorption was performed at 30 °C (N-30 in Figure 7A), the particles showed a layered structure similar to platelets, as in the original sample. Nevertheless, the

Figure 5. XRD patterns of the hydrotalcite-like elevated pressure products obtained at the different temperatures.

Figure 6. N2 adsorption−desorption analyses of the hydrotalcite-like elevated pressure products at different temperatures. (A) Typical N2 adsorption−desorption curve of the product obtained at 100 °C; the inset shows the BET specific surface obtained as a function of temperature. (B) Pore diameter and volume tendencies of the hydrotalcite-like elevated pressure products obtained at different temperatures.

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Figure 7. SEM images of the hydrotalcite-like elevated pressure products at different temperatures.

agglomerates (∼1 μm) seemed to be denser than the original agglomerated particles, and the platelet particles became smaller. The platelets decreased their size in the plane from 500−700 nm (in the original sample; see Figure 2) to 150−300 nm, which may be related to the pressure applied to the powders during the CO2 adsorption. These new morphological characteristics became more evident as a function of temperature between 30 and 325 °C. In fact, the platelets presented more ordered distribution when the temperature was increased. The preservation of these platelets is in good agreement with the XRD results. The only significant morphology difference was observed at 350 °C, at which the platelet particles seemed to disappear and collapse. This evident change corresponds to the Mg(Al)O periclase crystallization, which was previously observed by XRD (see Figure 5).

Overall, Figure 8 shows a graphical correlation between the LDH thermal stability (TG experiment) and the maximum CO2 adsorption obtained during the previous isothermal experiments. Between room temperature and 250 °C, the sample lost approximately 16 wt % (TG curve) and the CO2 adsorption tended to decrease. Additionally, in the same temperature interval, the microstructural characteristics did not exhibit any significant changes. Thus, these two parameters must be correlated to the dehydration of superficial and interlayer water molecules, which closes the interlayer space, as was indicated by the XRD results. Between 200 and 300 °C, the thermogram shows a partial stability, as only 2.5 wt % of the sample was lost. This weight loss corresponds to a partial dehydroxylation process, typically associated the Al−OH hydroxides. At this thermal condition, the CO2 adsorption was improved (the highest CO2 adsorption was observed at 8091



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Figure 8. Correlation between the maximum CO2 adsorption (○, low pressures; ■, higher pressures) observed at the different temperatures as a function of the LDH thermal stability described by a thermogravimetric curve. The TG-DTG experiment was performed at atmospheric pressure.

Figure 9. DSC curves of the LDH sample performed at different pressures (atmospheric and 4000 kPa) in N2 and CO2 atmospheres.

by the following reaction equilibrium displacements: Mg(OH)2 ↔ MgO + H2O and Al(OH)2+ ↔ Al2O3 + H2O.36 The same result was observed when the analysis was performed in a CO2 atmosphere. However, the water evaporation and dehydroxylation processes occurred at temperatures lower than those in the N2 atmosphere, independent of the pressure conditions. The water evaporation occurred at 179 °C, while the decomposition of the LDH structure began immediately after the first process, at 240 °C. At 4000 kPa under the same CO2 atmosphere, the peaks were shifted to higher temperatures (at ∼200 °C).37−40 Thus, the P-DSC analysis showed that the layered double hydroxide evolution to the periclase-like structure could occur at lower temperatures based on the influence of pressure and CO2 atmosphere. In addition, to demonstrate the LDH regeneration capacity, different water vapor sorption−desorption isotherms were measured on the CO2 pressure products, using N2 as the carrier gas.12,30 Figure 10 shows the isotherms of different LDH

elevated pressure and 300 °C), which may be correlated to the microstructural changes produced by the aluminum changes and, consequently, to the pore morphology changes and surface reactivity. Finally, when T > 300 °C, the thermogram presented a weight loss of 20 wt %, which corresponds to complete dehydroxylation and may include the beginning of the decarbonation process. In the same temperature range, the CO2 adsorption diminished where the dehydroxylation process induced some structural and microstructural changes. Thus, the double-layered structure was completely collapsed and the textural properties were optimized. Therefore, as the elevated pressure CO2 adsorption experiment was produced in a closed system, the desorbed gases may influence the gas adsorption equilibrium. A second experiment was performed at 325 °C, using the same CO2 pressure product with a second degassing process. The result showed a better CO2 adsorption, and it was improved to ∼6 mmol/g (second cycle). These results strongly agree with the higher BET surface area determined previously. Finally, at temperatures above 325 °C, the periclase-like structure was crystallized, and the decarbonation process must have occurred. These factors reduced the textural properties and, consequently, the CO2 adsorption. To explain the evolution from the amorphous phase to the periclase-like structure at temperatures lower than those reported, a pressure DSC (P-DSC) analysis was performed. Figure 9 shows the P-DSC curves obtained from LDH samples that were subjected to both N2 and CO2 atmospheres between atmospheric and 4000 kPa. The samples exposed to a N2 atmosphere showed two endothermic peaks. The first peak was centered at 195 °C, and it corresponds to the H2O interlayer vaporization. Then, a double process started at 265 °C with a broad shoulder extending to 350 and 387 °C. These peaks correspond to the dehydroxylation of the Mg−Al−O hydroxides, forming brucite-type layers of LDH.35 When the pressure was increased to 4000 kPa, it promoted the endothermic peaks shifting to higher temperatures. Consequently, the H2O interlayer vaporization peak was depicted at 243 °C (an increase of 48 °C in comparison with that of the atmospheric sample). Additionally, the onset temperature of the double dehydroxylation process was shifted to 308 °C. Consequently, the dehydroxylation peak was not observed in this temperature range. This thermal evolution can be explained

Figure 10. Rehydration thermogravimetric analysis of the LDH elevated pressure products obtained between 300 and 350 °C.

product samples. Curves presented type III isotherms with different quantities of final weight gain. These weight increases have been attributed to reversible structural modifications because of adsorbed water molecules. The samples labeled as N-300 reg, N-325 reg, and N-350 reg correspond to the materials evaluated in the CO2 pressure adsorptions at 300, 325, and 350 °C, respectively. The initial LDH structure of the N-300 reg sample was not completely destroyed (see Figure 5); thus, the weight increase corresponds to interlayer water 8092



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(2) Balat, H.; Oz, C. Applications of carbon dioxide capture and storage technologies in reducing emissions from fossil-fired power plants. Energy Explor. Exploit. 2009, 25, 357−362. (3) da Silva, E. F.; Booth, A. M. Emissions from postcombustion CO2 capture plants. Environ. Sci. Technol. 2013, 47, 659−660. (4) Du, H.; Ebner, D.; Ritter, J. A. Pressure dependence of the nonequilibrium kinetic model that described the adsorption and desorption behavior of CO2 in K-promoted hydrotalcite like compound. Ind. Eng. Chem. Res. 2011, 50, 412−418. (5) Kizzie, A. K. C.; Wong-Foy, A. G.; Matzger, A. J. Effect of humidity on the performance of microporous coordination polymers as adsorbents for CO2 capture. Langmuir 2011, 27, 6368−6373. (6) Wang, M.; Lawal, A.; Stephenson, P.; Sidders, J.; Ramshaw, C. Post-combustion CO2 capture with chemical absorption: A state-ofthe-art review. Chem. Eng. Res. Des. 2011, 89, 1609−1624. (7) Loganathan, S.; Tikmani, M.; Ghoshal, A. K. Novel poreexpanded MCM-41 for CO2 capture: Synthesis and characterization. Langmuir 2013, 29, 3491−3499. (8) Garcia, S.; Pis, J. J.; Rubiera, F.; Pevida, C. Predicting mixed-gas adsorption equilibria on activated carbon for precombustion CO2 capture. Langmuir 2013, 29, 6042−6052. (9) Cavani, F.; Trifiro, F.; Vaccari, A. Hydrotalcite-type anionic clays: Preparation, properties and applications. Catal. Today 1991, 11, 173− 301. (10) Rives, V. Layered Double Hydroxides: Present and Future; Nova Science Publishers: New York, 2001. (11) Ortiz-Landeros, J.; Zeifert, B.; Hesiquio-Garduño, M.; Vázquez, A.; Salmones, J. Effect of Fe content on structure and surface properties of hydrotalcite-like compounds. J. Metastable Nanocryst. Mater. 2005, 24−25, 253−256. (12) Pfeiffer, H.; Lima, E.; Lara, V.; Valente, J. S. Thermokinetic study of the rehydration process of a calcined MgAl-layered double hydroxide. Langmuir 2010, 26, 4074−4079. (13) Figueras, F. Base catalysis in the synthesis of fine chemicals. Top. Catal. 2004, 29, 189−196. (14) Sels, B. F.; De Vos, D. E.; Jacobs, P. A. Hydrotalcite-like anionic clays in catalytic organic reactions. Catal. Rev.: Sci. Eng. 2001, 43, 443− 488. (15) Rao, K. K.; Gravelle, M.; Valente, J. S.; Figueras, F. Activation of Mg-Al hydrotalcite catalysts for aldol condensation reactions. J. Catal. 1998, 173, 115−121. (16) Kumbhar, P. S.; Valente, J. S.; Millet, J. M.; Figueras, F. Mg-Fe hydrotalcite as a catalyst for the reduction of aromatic nitro compounds with hydrazine hydrate. J. Catal. 2000, 191, 467−473. (17) Valente, J. S.; Pfeiffer, H.; Lima, E.; Prince, J.; Flores, J. Cyanoethylation of alcohols by activated MgAl Layered Double Hydroxides: Influence of rehydration conditions and Mg/Al molar ratio on Brönsted basicity. J. Catal. 2011, 279, 196−204. (18) Yang, W.; Yongman, K.; Liu, P. K. T.; Sahimi, M.; Tsotsis, T. T. A study in situ techniques of the thermal evolution of the structure of a Mg-Al-CO3 layered double hydroxide. Chem. Eng. Sci. 2002, 57, 2945− 2953. (19) Yongman, K.; Yang, W.; Liu, P. K. T.; Sahimi, M.; Tsotsis, T. T. Thermal evolution of the structure of a Mg−Al−CO3 layered double hydroxide: Sorption reversibility aspects. Ind. Eng. Chem. Res. 2004, 43, 4559−4570. (20) Ding, Y.; Alpay, E. Equilibria and kinetics of CO2 adsorption on hydrotalcite adsorbent. Chem. Eng. Sci. 2000, 55, 3461−3474. (21) Reijers, H. T. J.; Valster-Schiermeier, S. E. A.; Cobden, P. D.; van der Brink, R. W. Hydrotalcite as CO2 sorbent for sorptionenhanced steam reforming of methane. Ind. Eng. Chem. Res. 2006, 45, 2522−2530. (22) Sharma, U.; Tyagi, B.; Jasra, R. V. Synthesis and characterization of Mg−Al−CO3 layered double hydroxide for CO2 adsorption. Ind. Eng. Chem. Res. 2008, 47, 9588−9595. (23) Meis, N. N. A. H.; Bitter, J. H.; de Jong, K. P. Support and size effects of activated hydrotalcites for precombustion CO2 capture. Ind. Eng. Chem. Res. 2010, 49, 1229−1235.

molecules responsible for the complete regeneration of the LDH structure. In the other two samples (N-325 reg and N350 reg), the final weight increases were equal to 26 and 32 wt %, respectively, which corresponds to water molecules associated with the LDH regeneration. The amount of water adsorbed in sample N-325 reg was less than that in sample N350 reg because the sample is an amorphous phase (see Figure 5). Therefore, it is possible that a residual amount of water and/or hydroxyls remained in the sample. All these results are in accordance with previous works.12,30

4. CONCLUSIONS An LDH (Mg/Al = 3.0) material was synthesized via a coprecipitation method, and then it was evaluated as a CO2 adsorbent at elevated pressures (up to 4350 kPa) at different temperatures (30−350 °C). The experiments showed that the LDH sample possessed highly suitable properties as a CO2 sorbent at elevated pressures. After the structural and microstructural analyses of the initial LDH and the elevated pressure LDH products, it was found that LDH tends to adsorb CO2 as a function of its structure and microstructure, which evolves thermally. The double-layered structure was preserved up to 300 °C, showing a d003 peak reduction as a function of temperature. In this temperature range (30−300 °C), the quantity of CO2 adsorbed decreased as a function of temperature. However, at 300 °C the highest CO2 adsorption was produced. At 325 °C, the double-layered structure was completely destroyed and no crystalline phase was detected, while at 350 °C the Mg(Al)O structure was crystallized and the CO2 adsorption decreased. The amorphous and periclase phase formations were produced at temperatures lower than those usually reported because of pressure and thermal conditions. Different microstructural features of the LDH material evolved as a function of temperature and pressure. These structural and microstructural changes in the interlayer space induced the highest CO2 adsorption of 5.76 mmol/g at 300 °C and 4350 kPa. However, this temperature is not enough to reach the adsorption−desorption equilibrium that allows the maximum CO2 capture. Once the amorphous phase was confirmed by XRD at 325 °C, the specific surface area and pore volume were increased. The best CO2 adsorption (∼6 mmol/ g) was obtain during a second adsorption cycle on the sample previously evaluated at 325 °C because in this case there was not steam present in the closed system.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: pfeiff[email protected]. Phone: +52 (55) 56224627. Fax: +52 (55) 56161371. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the project SENERCONACYT 150358. M.J.R.-M. thanks CONACYT and PIFIIPN for financial support. Authors thank Gerardo González Arenas for technical support.

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REFERENCES

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CO2 Adsorption at Elevated Pressure and Temperature on Mg–Al

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