CO 2 Adsorption at Elevated Pressure and Temperature on Mg–Al Layered Double Hydroxide

CO2 Adsorption at Elevated Pressure and Temperature on Mg−AlLayered Double HydroxideMargarita J. Ramírez-Moreno,†,‡ Issis C. Romero-Ibarra,† M.A. Hernandez-Perez,‡

and Heriberto Pfeiffer*,†

†Instituto de Investigaciones en Materiales, Universidad Nacional Autonoma de Mexico, Circuito exterior s/n, Ciudad Universitaria,Del. Coyoacan, CP 04510, Mexico DF, Mexico‡Departamento de Ingeniería en Metalurgia y de Materiales, Escuela Superior de Ingeniería Química e Industrias Extractivas, IPN,UPALM, Av. Instituto Politecnico Nacional s/n, CP 07738, Mexico DF, Mexico

ABSTRACT: CO2 adsorption at elevated pressure was studied in a Mg−Al (Mg/Al = 3) layered double hydroxide (LDH). Thedouble-layered structure was prepared via a coprecipitation method. The sample’s structure and microstructure evolutions werecharacterized using X-ray diffraction, scanning electron microscopy, N2 adsorption, and thermogravimetric and calorimetricanalyses. 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 thermalevolution of its structure and microstructure. The highest CO2 adsorption (5.7 mmol/g) was produced at 300 °C before thelayered 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 adsorptionprocess 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 hotgas streams have been identified as being among the mostimportant environmental issues to be solved.1−3 Therefore,different types of materials, such as organic materials, polymers,minerals, zeolites, layered double hydroxides, oxides, andceramics, among others, have been tested as CO2 sorbentsthrough physical or chemical mechanisms.1,4−8 Among thesematerials, layered double hydroxides (LDHs) are suitable asCO2 sorbents through two different mechanisms: adsorption atlow temperatures or absorption at moderate temperatures.LDH materials are mixed metal hydroxides represented by

the general chemical composition [M(1−x)2+Mx

3+(OH)2]Ax/nn−·

mH2O,9−11 where M2+ and M3+ stand for divalent and trivalent

cations, respectively, occupying octahedral sites within thehydroxyl layers forming brucite-type layers; x is equal to theM3+/(M2+ + M3+) ratio and includes values over the range of0.20 to 0.50; and An− is an exchangeable interlayer anion. Themost common interlayer anion is the carbonate anion, CO3

2−.These materials have received considerable attention in recentyears because they have a wide range of applications, primarilyas catalysts, catalyst supports, and as electrodes,12−17 as well asbecause they present very interesting thermal structuralevolutions.18,19 In particular, Mg−Al−CO3 LDH, known ashydrotalcite-like compounds or anionic clays, has been themost widely studied material.In recent years, different reports have been published

regarding the use of LDHs as CO2 sorbents at two differenttemperature ranges: low (30−200 °C) and moderate (200−600°C).1,7,8,20−27 In the low-temperature range, the CO2 is usuallyadsorbed. Moreover, at moderate temperatures, the originallayered structure collapses and the Mg(Al)O periclase-likemixed oxide is crystallized, producing a chemical CO2 capture,

forming carbonates or bicarbonates. In both temperatureranges, this type of material has not presented the best CO2capture capacities compared to other materials used in thisfield. Consequently, to improve the CO2 sorption capacity ofLDH materials, different alternatives have been proposed,including the addition of alkaline metals to increasebasicity,21,22 the use of several MII and/or MIII structuralmetals to modify the chemical surface properties and/or somemicrostructural characteristics,28,29 and the addition of watervapor on the CO2 flow to modify the particle surfacereactivity30 or a pressure increase.20,24,25,27,31

It must also be noted that there are few studies related toCO2 high-pressure capture on LDH structures. Alpay and co-workers gathered CO2 experimental adsorption isotherm dataon K-promoted LDH samples at different temperatures andvarying pressures, up to 120 kPa, obtaining CO2 adsorptions≤1.0 mmol/g.20,31 Apart from this work, only a few othertheoretical works have been published.24,25,27 Therefore, theaim of the present work was to evaluate the CO2 adsorption atelevated pressures, using the Mg−Al−CO3 LDH (Mg/Al =3.0) as a starting material. The correlation between the LDHthermal structure evolution, with the corresponding chemicalspecies, and the amount of CO2 adsorbed was investigated. Thesample was characterized using several techniques, including X-ray diffraction (XRD), N2 adsorption, scanning electronmicroscopy (SEM), and thermogravimetric (TG-DTG) andcalorimetric (DSC) analyses, before and after the CO2adsorption experiments, for monitoring the structure evolution.

Received: March 11, 2014Revised: April 24, 2014Accepted: April 25, 2014Published: April 25, 2014

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© 2014 American Chemical Society 8087 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), magne-sium nitrate (Mg(NO3)2·6H2O, Sigma-Aldrich), as well aspotassium hydroxide and carbonate (KOH and K2CO3, bothfrom Sigma-Aldrich) were used to prepare a hydrotalcite-likematerial by a coprecipitation method at low supersaturationconditions. Appropriate amounts of Al(NO3)3·9H2O andMg(NO3)2·6H2O were dissolved in deionized water to preparea 0.8 M solution for which the Mg/Al molar ratio was selectedto 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 andmaintained at 80 °C, followed by the dropwise addition of thesolution containing the Mg and Al cations into the reactor.Simultaneously, the KOH solution was added to adjust the pHto ∼9.5. The resultant slurry was aged under vigorous stirring at80 °C for 24 h. The precipitate was filtered and washed severaltimes with warm deionized water to remove nitrate ions andpotassium excess. The precipitate was then dried in an oven for24 h at 80 °C.The LDH powders, before and after CO2 sorption, were

characterized by X-ray diffraction, N2 adsorption, scanningelectron microscopy, and thermogravimetric and calorimetricanalyses. XRD patterns were obtained using a D8 Focus byBruker 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 thecorresponding JCPDS files. The microstructural characteristicsof these samples and CO2 elevated pressure products weredetermined using scanning electron microscopy and N2adsorption. The particle size and morphology were determinedby SEM in a JEOL JSM-6701F instrument. Additionally, thetextural characteristics (BET surface area, pore size, and porevolume) were determined by N2 adsorption−desorption at 77K using a Minisorp II instrument from Bel-Japan, employing amultipoint technique. All samples were outgassed at roomtemperature under high vacuum for 24 h before the N2adsorption−desorption tests. Finally, a thermogravimetricanalysis was performed using a Q500HR thermobalance fromTA Instruments under a N2 atmosphere.The CO2 capture capacity at elevated pressure on the

hydrotalcite-like material was determined using a volumetricBelsorp-HP instrument from Bel-Japan. The nonideal behaviorof the CO2 gas was corrected by application of virial equationsemploying four virial coefficients. The virial coefficients werecalculated from NIST data at the respective temperature in theheater at a maximum pressure of 5.0 MPa. Powders wereinitially activated before CO2 sorption tests. First, the samplewas introduced in the adsorption cell and outgassed at 80 °Cfor 0.5 h before testing because the materials are sensitive to thepresence of moisture and environmental CO2. The adsorptionexperiments were performed at temperatures between 30 and350 °C. At each temperature, the pressure was increased up to4350 kPa, establishing equilibrium times between a few secondsand 60 min because the LDH samples usually present slowequilibrium processes.To establish the influence of the pressure and CO2

atmosphere on the fresh LDH thermal evolution, pressuredifferential scanning calorimetry (DSC) experiments werecarried out using a Pressure DSC instrument from InstrumentSpecialist Incorporated. The samples were heated from 30 to400 °C at a rate of 10 °C/min. The experiments were

performed under atmospheric and 4000 kPa pressures in bothN2 and CO2 atmospheres.It is well-known that the original layered-structure LDH can

be reconstituted from the periclase mixed oxides that areobtained from its thermal treatment by the adsorption ofanions. To observe this process, the elevated pressure CO2capture products were rehydrated. Different rehydrationexperiments in a water vapor environment were performed ina temperature-controlled thermobalance TA Instrumentsmodel Q5000SA equipped with a humidity-controlled chamber.The water vapor sorption/desorption isotherms were generatedat 60 °C, varying the relative humidity (RH) from 0 to 80 andthe back to 0 RH%. The experiments were carried out at a rateof 0.5 RH%/min under a N2 flow of 100 mL/min, which arethe same conditions reported in previous works.12,30 Finally, toidentify the layered reconstruction, each rehydrated sample wasimmediately analyzed by thermogravimetric analysis (TGA)under conditions that were the same as those of the thermaldecomposition of fresh LDH.

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

file, indicating that the LDH structure was the only phasepresent in the sample, at least within the XRD detection level(∼5%). After the structural characterization, the sample wasmicrostructurally evaluated using a scanning electron micro-scope and N2 adsorption. Figure 2 shows the SEM image inwhich the particle morphology of the sample can be visualized.The hydrotalcite-like material showed a layered structure withplatelets or flake-like particles of approximately 500−700 nm inthe plane and 5−10 nm in thickness. In addition, the plateletswere agglomerated in larger particles of approximately 1 μm.This morphology is in very good agreement with the typicalcauliflower-like morphology reported in the literature.32,33 Themicrostructural characteristics of the sample were comple-mented by the N2 adsorption−desorption experiment. Thesample presented a type IV isotherm (see the inset of Figure 2),according to the IUPAC classification,34 which corresponds to amesoporous 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 adsorptionprocess at different temperatures (30−350 °C). The pressurewas increased up to 4350 kPa. Figure 3 shows the adsorbed

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

Industrial & Engineering Chemistry Research Article

dx.doi.org/10.1021/ie5010515 | Ind. Eng. Chem. Res. 2014, 53, 8087−80948088

volume at low pressures (between 5 and 100 kPa). In general,the amount of CO2 adsorbed (millimoles per gram) decreasedas a function of the temperature (see the right panel of Figure3). Between 30 and 100 °C, the adsorbed CO2 amountsdecreased from 0.72 to 0.70 mmol/g. However, the final CO2adsorptions observed between 200 and 350 °C decreased to∼0.3 mmol/g. These results agree with previous LDH studiesin which the maximum reported CO2 adsorption was equal toor less than 1.0 mmol/g at similar temperature and pressureranges.25,35 In all cases, the decrease observed in the CO2adsorption process as a function of temperature has beenassociated with the thermal desorption activation as well as apartial destruction of the layered structure. In fact, in allprevious studies the LDH−CO2 capture was performed onoptimized 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 gasstream.The CO2 adsorption process on the LDH samples was

continued at higher pressures (up to 4350 kPa). Figure 4 showsthe CO2 adsorption curves and the maximum CO2 adsorbed(millimoles per gram) at the highest pressure of eachtemperature. At 30 °C, the CO2 adsorption was less than 1mmol/g between 5 and 1100 kPa. However, at higher pressures(P > 1100 kPa), the CO2 adsorption began to increase

exponentially up to 4.73 mmol/g at 4200 kPa. When the CO2adsorption was performed at 100 °C, the exponential growthbegan at a higher pressure, ∼1800 kPa. At this thermalcondition, 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 moredrastically at 200 °C, in which only 1.08 mmol/g of CO2 wasadsorbed. Although the CO2 adsorption trend was similar inthe samples treated at higher temperatures (T > 200 °C), thefinal amount of CO2 adsorbed at 300 °C did not follow thesame trend. Between 200 and 300 °C, the final CO2concentration increased from 1.08 to 5.76 mmol/g, respec-tively. In fact, the quantity of CO2 adsorbed at 300 °C (5.76mmol/g) represented the highest CO2 adsorption produced inthis experimental setup. Finally, at 325 and 350 °C, the totalamount of CO2 adsorbed decreased to 0.08 mmol/g, whichcorresponds to the lowest CO2 adsorption (350 °C).To explain the CO2 adsorption behavior observed at elevated

pressure, the isothermal products were structurally andmicrostructurally recharacterized using XRD, N2 adsorption−desorption, and SEM. Finally, the maximum CO2 adsorptionswere correlated to the thermal stability of the LDH through athermogravimetric analysis. Figure 5 shows the XRD patternsof the elevated pressure isothermal products. The layeredstructure was partially collapsed or destroyed as a function oftemperature from 30 to 300 °C. In this temperature range, the003 peak was right-shifted from 11.5 to 13.65 in 2θ. The d003distance corresponds to the LDH interlayer space, which wasreduced as a function of temperature from 4.78 to 4.39 Å. Thisstructural change must be related to dehydration anddehydroxylation processes of the brucite-type layers. It mustbe noted that the highest CO2 adsorption was obtained at 300°C. Despite the water loss, the change in the interlayer distancewas a minimum; this slight change could be explained as afunction of the CO2 captured. The CO2 adsorbed may haveinhibited the structural collapse and delayed the adsorption−desorption equilibrium. Furthermore, at 325 °C the double-layered structure completely disappeared, resulting in anamorphous structure. Finally, at 350 °C, the periclase-likestructure (Mg(Al)O) was crystallized. These final structuralchanges correspond with the previously described CO2adsorption decrease.After the LDH structural evolution analysis, the samples were

analyzed by N2 adsorption−desorption and SEM to determineany microstructural changes in the samples. Figure 6 shows the

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

Figure 3. CO2 adsorption performed at low pressures (5−100 kPa) at different temperatures (30−350 °C). The right panel shows the maximumCO2 adsorbed as a function of temperature.



textural analysis of the LDH products. All samples presentedtype IV isotherms (only a representative isotherm is presentedin the figure), according to the IUPAC classification,34 whichcorresponds to mesoporous materials (Figure 6A). The specificsurface was determined using the BET model. Between 30 and300 °C, the specific surface of the elevated pressure productsdid not vary significantly (130 ± 10 m2/g) compared to theinitial sample (136.6 m2/g). However, the product obtainedafter the CO2 adsorption at 325 °C presented a higher specificarea of 296.3 m2/g. This phenomenon may be explained as afunction of the total destruction of the double-layered structure,which induces the formation of a larger area. As previouslyexplained, the LDH structure-collapse process occurs in twosteps. 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 totetrahedral sites in the interlayer region and differentmodifications occurred in the octahedral brucite-type layer.Similar results have been previously observed by Belloto andco-workers.36 However, this change preserved the LDHstructure despite the absence of water. Consequently, a new3D network partially formed. After the LDH structuraldestruction, additional trivalent cations (Al3+) changed theiroctahedral configuration to tetrahedral positions. Thus, theformation of the 3D structure was constituted of closely packedoxygen networks. Finally, at 350 °C, both the total CO2adsorbed and the specific surface area decreased, which mustbe attributed to the periclase crystallization process previouslyobserved by XRD.From the textural analysis, it is also worth noting that

between 30 and 250 °C, the pore diameter and total porevolume tended to increase, although the specific area did not(Figure 6B). Therefore, the elevated pressure may have induceda reduction in the quantity of pores but an increase in theirdiameters. This must be related to the dehydration anddehydroxylation processes, previously mentioned. However, at300 and 325 °C the pore diameters decreased, while the porevolume increased only at 325 °C. These textural changes can beattributed to the total collapse of the double-layered structureand 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 structuresimilar to platelets, as in the original sample. Nevertheless, the

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

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

Figure 6. N2 adsorption−desorption analyses of the hydrotalcite-likeelevated pressure products at different temperatures. (A) Typical N2adsorption−desorption curve of the product obtained at 100 °C; theinset shows the BET specific surface obtained as a function oftemperature. (B) Pore diameter and volume tendencies of thehydrotalcite-like elevated pressure products obtained at differenttemperatures.



agglomerates (∼1 μm) seemed to be denser than the originalagglomerated particles, and the platelet particles becamesmaller. The platelets decreased their size in the plane from500−700 nm (in the original sample; see Figure 2) to 150−300nm, which may be related to the pressure applied to thepowders during the CO2 adsorption. These new morphologicalcharacteristics became more evident as a function of temper-ature between 30 and 325 °C. In fact, the platelets presentedmore ordered distribution when the temperature was increased.The preservation of these platelets is in good agreement withthe XRD results. The only significant morphology differencewas observed at 350 °C, at which the platelet particles seemedto disappear and collapse. This evident change corresponds tothe Mg(Al)O periclase crystallization, which was previouslyobserved by XRD (see Figure 5).

Overall, Figure 8 shows a graphical correlation between theLDH thermal stability (TG experiment) and the maximumCO2 adsorption obtained during the previous isothermalexperiments. Between room temperature and 250 °C, thesample lost approximately 16 wt % (TG curve) and the CO2adsorption tended to decrease. Additionally, in the sametemperature interval, the microstructural characteristics did notexhibit any significant changes. Thus, these two parametersmust be correlated to the dehydration of superficial andinterlayer water molecules, which closes the interlayer space, aswas indicated by the XRD results. Between 200 and 300 °C, thethermogram shows a partial stability, as only 2.5 wt % of thesample was lost. This weight loss corresponds to a partialdehydroxylation process, typically associated the Al−OHhydroxides. At this thermal condition, the CO2 adsorptionwas improved (the highest CO2 adsorption was observed at

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



elevated pressure and 300 °C), which may be correlated to themicrostructural changes produced by the aluminum changesand, consequently, to the pore morphology changes and surfacereactivity.Finally, when T > 300 °C, the thermogram presented a

weight loss of 20 wt %, which corresponds to completedehydroxylation and may include the beginning of thedecarbonation process. In the same temperature range, theCO2 adsorption diminished where the dehydroxylation processinduced some structural and microstructural changes. Thus, thedouble-layered structure was completely collapsed and thetextural properties were optimized. Therefore, as the elevatedpressure CO2 adsorption experiment was produced in a closedsystem, the desorbed gases may influence the gas adsorptionequilibrium. A second experiment was performed at 325 °C,using the same CO2 pressure product with a second degassingprocess. The result showed a better CO2 adsorption, and it wasimproved to ∼6 mmol/g (second cycle). These results stronglyagree with the higher BET surface area determined previously.Finally, at temperatures above 325 °C, the periclase-like

structure was crystallized, and the decarbonation process musthave occurred. These factors reduced the textural propertiesand, consequently, the CO2 adsorption.To explain the evolution from the amorphous phase to the

periclase-like structure at temperatures lower than thosereported, a pressure DSC (P-DSC) analysis was performed.Figure 9 shows the P-DSC curves obtained from LDH samplesthat were subjected to both N2 and CO2 atmospheres betweenatmospheric and 4000 kPa. The samples exposed to a N2atmosphere showed two endothermic peaks. The first peak wascentered at 195 °C, and it corresponds to the H2O interlayervaporization. Then, a double process started at 265 °C with abroad shoulder extending to 350 and 387 °C. These peakscorrespond to the dehydroxylation of the Mg−Al−Ohydroxides, forming brucite-type layers of LDH.35 When thepressure was increased to 4000 kPa, it promoted theendothermic peaks shifting to higher temperatures. Con-sequently, the H2O interlayer vaporization peak was depictedat 243 °C (an increase of 48 °C in comparison with that of theatmospheric sample). Additionally, the onset temperature ofthe double dehydroxylation process was shifted to 308 °C.Consequently, the dehydroxylation peak was not observed inthis temperature range. This thermal evolution can be explained

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 CO2atmosphere. However, the water evaporation and dehydrox-ylation processes occurred at temperatures lower than those inthe N2 atmosphere, independent of the pressure conditions.The water evaporation occurred at 179 °C, while thedecomposition of the LDH structure began immediately afterthe first process, at 240 °C. At 4000 kPa under the same CO2atmosphere, the peaks were shifted to higher temperatures (at∼200 °C).37−40 Thus, the P-DSC analysis showed that thelayered double hydroxide evolution to the periclase-likestructure could occur at lower temperatures based on theinfluence of pressure and CO2 atmosphere.In addition, to demonstrate the LDH regeneration capacity,

different water vapor sorption−desorption isotherms weremeasured on the CO2 pressure products, using N2 as the carriergas.12,30 Figure 10 shows the isotherms of different LDH

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

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

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

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



molecules responsible for the complete regeneration of theLDH structure. In the other two samples (N-325 reg and N-350 reg), the final weight increases were equal to 26 and 32 wt%, respectively, which corresponds to water moleculesassociated with the LDH regeneration. The amount of wateradsorbed in sample N-325 reg was less than that in sample N-350 reg because the sample is an amorphous phase (see Figure5). Therefore, it is possible that a residual amount of waterand/or hydroxyls remained in the sample. All these results arein accordance with previous works.12,30

4. CONCLUSIONSAn LDH (Mg/Al = 3.0) material was synthesized via acoprecipitation method, and then it was evaluated as a CO2adsorbent at elevated pressures (up to 4350 kPa) at differenttemperatures (30−350 °C). The experiments showed that theLDH sample possessed highly suitable properties as a CO2sorbent at elevated pressures. After the structural andmicrostructural analyses of the initial LDH and the elevatedpressure LDH products, it was found that LDH tends to adsorbCO2 as a function of its structure and microstructure, whichevolves thermally. The double-layered structure was preservedup to 300 °C, showing a d003 peak reduction as a function oftemperature. In this temperature range (30−300 °C), thequantity of CO2 adsorbed decreased as a function oftemperature. However, at 300 °C the highest CO2 adsorptionwas produced. At 325 °C, the double-layered structure wascompletely destroyed and no crystalline phase was detected,while at 350 °C the Mg(Al)O structure was crystallized and theCO2 adsorption decreased. The amorphous and periclase phaseformations were produced at temperatures lower than thoseusually reported because of pressure and thermal conditions.Different microstructural features of the LDH material

evolved as a function of temperature and pressure. Thesestructural and microstructural changes in the interlayer spaceinduced the highest CO2 adsorption of 5.76 mmol/g at 300 °Cand 4350 kPa. However, this temperature is not enough toreach the adsorption−desorption equilibrium that allows themaximum CO2 capture. Once the amorphous phase wasconfirmed by XRD at 325 °C, the specific surface area and porevolume were increased. The best CO2 adsorption (∼6 mmol/g) was obtain during a second adsorption cycle on the samplepreviously evaluated at 325 °C because in this case there wasnot steam present in the closed system.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: [email protected]. Phone: +52 (55) 56224627.Fax: +52 (55) 56161371.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was financially supported by the project SENER-CONACYT 150358. M.J.R.-M. thanks CONACYT and PIFI-IPN for financial support. Authors thank Gerardo GonzalezArenas for technical support.

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CO 2 Adsorption at Elevated Pressure and Temperature on Mg–Al Layered Double Hydroxide