Analysis of the CO2−H2O Chemisorption in Lithium Silicates at LowTemperatures (30−80 °C)Brenda Alcantar-Vazquez,*,† Pablo R. Díaz Herrera,† Alejandro Barrera Gonzalez,† Yuhua Duan,‡

and Heriberto Pfeiffer†

†Instituto de Investigaciones en Materiales, Universidad Nacional Autonoma de Mexico, Circuito Exterior s/n, Cd. Universitaria, Del.Coyoacan, Mexico DF, CP 04510, Mexico‡National Energy Technology Laboratory, United States Department of Energy, 626 Cochrans Mill Road, Pittsburgh, Pennsylvania15236, United States

ABSTRACT: Li8SiO6 and Li4SiO4 were synthesized by a solid-state reaction. Then, dynamic and isothermal thermogravimetricwater vapor sorption experiments were performed using two carrier gases: N2 and CO2. Initially, the Li8SiO6−H2O−(N2 or CO2)systems were analyzed. It was evidenced that Li8SiO6 can trap water physically and chemically, producing Li−OH superficialspecies. When CO2 was used as the carrier gas, Li8SiO6 continued chemisorbing H2O but CO2 was trapped as well, formingLi2CO3 and Li4SiO4 as secondary phases. On the basis of these results, the Li4SiO4−H2O−CO2 system was also analyzed. Theexperimental results and different theoretical thermodynamic calculations confirmed that Li8SiO6 can chemisorb 2 moles of CO2per mole of ceramic at a low temperature range (30−80 °C). Finally, different CO2 chemisorption kinetic experiments wereperformed to analyze and quantify the trapped CO2.

■ INTRODUCTION

The greenhouse effect and global warming have continuouslyincreased in recent decades. Some of the most feasible evidencefor these phenomena that has been reported is that theconcentration of atmospheric carbon dioxide (CO2) reached400 ppm, something not previously observed.1−4

To solve this threatening problem, different materials havebeen proposed and studied as CO2 captors in a widetemperature range (low (30−100 °C), moderate (100−400°C), or high (T ≥ 400 °C)). Specifically, at low temperatures(30−100 °C), different types of materials have been tested aspossible CO2 captors, including zeolites, other organic/inorganic cage structures, activated carbons, and hydro-talcites.5,6 However, the only current commercial materialsare a few amines, which are used in aqueous CO2 absorptionprocesses.7 Nevertheless, alkaline ceramics are another group ofmaterials that can trap CO2 at similar low temperatures: someof these ceramics are CaO, Li5AlO4, Na2ZrO3, andNa2SiO3.

8−49 All of these ceramics have shown their capabilityof trapping CO2 chemically at low temperatures (30−80 °C) inthe presence of water vapor,37,41,42 producing lithium carbonate(Li2CO3) or sodium acid carbonate (NaHCO3). However,among lithium silicates only lithium metasilicate (Li2SiO3) hasbeen tested under similar conditions as a CO2 captor. In thiswork Li2SiO3 showed the ability to capture CO2 under veryspecific conditions (high surface area).12

In recent years, lithium silicates have been studied for severalapplications,7−31,49−56 such as CO2 capture at moderate andhigh temperatures.7−31 The theoretical data suggest that CO2chemisorption in lithium orthosilicate (Li4SiO4) and lithiumoxosilicate (Li8SiO6) can occur at low and high temperatures.23

However, some works have shown that the best CO2chemisorption properties are present between 400 and 650°C; below this temperature range slow and low CO2 capture is

observed. The following reactions show that lithium silicatescan react with different quantities of CO2, producing Li2CO3and different lithium secondary phases, such as Li4SiO4,Li2SiO3, and SiO2 (the most favorable temperature ranges foreach reaction are shown).8−31

+ ⎯ →⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ +− °

Li SiO 2CO 2Li CO Li SiO8 6 2300 700 C

2 3 4 4 (1)

+ ⎯ →⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ +− °

Li SiO 3CO 3Li CO Li SiO8 6 2300 580 C

2 3 2 3 (2)

+ ⎯ →⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯ +− °

Li SiO CO Li CO Li SiO4 4 2400 630 C

2 3 2 3 (3)

+ ⎯ →⎯⎯⎯⎯⎯⎯⎯⎯ +≤ °

Li SiO 2CO 2Li CO SiOT

4 4 2377 C

2 3 2 (4)

+ ⎯ →⎯⎯⎯⎯⎯⎯⎯⎯ +≤ °

Li SiO CO 2Li CO SiOT

2 3 2235 C

2 3 2 (5)

Theoretically, lithium silicates are able to completely reactwith CO2 at low temperatures (30−100 °C),23 where thepresence of water vapor may improve the CO2 total reactivityon these lithium silicates. Consequently, the objective of thiswork was to analyze the CO2−H2O capture process on Li8SiO6and Li4SiO4 at temperatures between 30 and 80 °C.

■ EXPERIMENTAL SECTIONLi8SiO6 and Li4SiO4 were synthesized using a solid-statereaction. The reagents employed were lithium oxide (Li2O) andsilicon oxide (SiO2), both from Aldrich. The powders weremechanically mixed and then heat treated at 800 °C for 8 h.

Received: March 24, 2015Revised: June 2, 2015Accepted: June 16, 2015Published: June 16, 2015

Article

pubs.acs.org/IECR

© 2015 American Chemical Society 6884 DOI: 10.1021/acs.iecr.5b01110Ind. Eng. Chem. Res. 2015, 54, 6884−6892

The corresponding lithium silicate was pulverized. To obtainpure Li8SiO6 and Li4SiO4, 15 and 10 wt % excess of lithiumwere used, respectively.Both lithium silicates, after the synthesis and after the CO2

capture processes, were characterized structurally and micro-structurally using X-ray diffraction (XRD), N2 adsorption,thermogravimetric analysis (TG), and scanning electronmicroscopy (SEM). An AXS D8-Advance diffractometer(Bruker) was used for the XRD characterization with a copperanode X-ray tube. The N2 adsorption−desorption isothermswere acquired on a Bel-Japan Minisorp II instrument at 77 K.Previously, the samples were degassed at room temperature for24 h under vacuum. The TG experiments were obtained usinga Q500HR thermobalance (TA Instruments). Finally, secon-dary electron images were obtained from a JEOL JMS-7600Finstrument. After the complete characterization, dynamic andisothermal thermogravimetric experiments were performed inLi8SiO6 and Li4SiO4 using a Q5000SA thermobalance from TAInstruments. These experiments were performed between 30and 80 °C and at varying relative humidity (RH). Theexperiments were performed using 100 mL/min of N2 or CO2.The methodology of these experiments has been previouslydescribed.42

Because the thermodynamic properties of lithium silicatereactions capturing CO2 and/or H2O are not available in theliterature, ab initio thermodynamic calculations were performedcombining density functional theory (DFT) with latticephonon dynamics. The detailed description of the calculationmethod can be found in previous studies.23,57−60 Allcalculations were performed using the Vienna ab initiosimulation package (VASP)61−63 and PHONON package64

with PAW pseudopotentials and PW91 exchange−correlationfunctionals.

■ RESULTS

Initially, the Li8SiO6 sample was characterized by XRD (Figure1). The diffraction pattern indicated the presence of Li8SiO6 asthe main phase (JCPDS diffraction file 42-0403), with thepresence of Li4SiO4, Li2SiO3, and Li2CO3 in low quantities. Thesquare inset of Figure 1 shows the morphology of Li8SiO6. TheLi8SiO6 particle size was approximately 5−10 μm, and the

particle surface was corrugated. In addition, the N2 adsorption−desorption isotherm for the Li8SiO6 sample corresponded to atype II isotherm, corresponding to a nonporous material.65

Additionally, the surface area of the sample was estimated to be3.0 m2 g−1 using the BET model. These microstructuralanalyses are in good agreement with the solid-state synthesismethod.Previously, Duran-Munoz et al. reported that CO2 capture on

the Li8SiO6 sample was not higher than 1 wt % (<1 mmol g−1)in dry conditions within a temperature range of 30−80 °C.24

Additionally, when Li8SiO6 was tested as a CO2 captor at highpressure (1 MPa), the maximum CO2 captured was 2.45 mmolg−1 between 30 and 100 °C.66 Therefore, Li8SiO6 does notcapture high quantities of CO2 in this temperature range.Conversely, some reports have shown that H2O(v) enhances thekinetic CO2 chemisorption of lithium or sodium ceramics in thesame temperature range.37,41,42

Li8SiO6 was exposed to water vapor between 30 and 80 °Cusing N2 as a carrier gas. Figure 2A shows the water sorption−desorption isotherms. These sorption isotherms correspondedto type III, indicating a relatively weak adsorbate−absorbentinteraction.65 Additionally, the water sorption varied as afunction of temperature. Samples presented large hysteresisloops, which did not close. The sorption−desorption curvespresented large weight increments, suggesting that Li8SiO6

Figure 1. XRD diffraction pattern of lithium oxosilicate (Li8SiO6)synthesized by solid state reaction. The inset shows the SEM image.

Figure 2. Water vapor sorption−desorption isotherms (A) andderivative thermogravimetric curves (B) of Li8SiO6 sample attemperatures between 30 and 80 °C, using N2 as carrier gas.

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reacted with water vapor. At 30 and 40 °C, the Li8SiO6 samplepresented similar water sorptions of approximately 7.5 wt %,but at 80 °C, the final weight increment increased to 29.8 wt %.Thus, the final weight gain increased as a function oftemperature. Additionally, the loss of mass began at higherRH when the temperature was increased. On the other hand,the final water trapped varied between 7.3, 4.5, and 12.6 wt % at30, 65, and 80 °C, respectively. The DTG curves of the weightchange as a function of time are shown in Figure 2B. Thesecond part of these curves (t ≥ 160 min) shows theevaporation process, where it is indicated as a change in thesign of the derivate. The evaporation time decreased from 307.8to 235.6 min when the isotherms were performed at 30 and 80°C, respectively. Moreover, the evaporation rate was larger as afunction of temperature. According to literature,37,41,42 theweight gained in this type of experiment is attributed to H2Osuperficially adsorbed and/or chemisorbed as Li−OH and Si−OH species.Additionally, Figure 3 exhibits the change in mass as a

function of the temperature at different humidity. The amountof water absorbed is a function of the relative humidity presentin the atmosphere (Figure 3A). The trapped water increasedfrom 0.7 to 33.3 wt % at 20% and 80% RH, respectively.Additionally, there was different behavior in the curve with 40%relative humidity (RH). To explain this behavior, additionalexperiments with 35% and 45% RH were performed. As shownin Figure 3B, at 35% and 40% RH two inflection points areevident. The first corresponds to the start of desorption,whereas the second is the beginning of another adsorptionprocess. The first inflection point was observed at 58 and 66 °Cat 35% and 40% RH, respectively. The second inflection pointwas observed at 66.8 and 73.7 °C at 35% RH and 40% RH,respectively. As expected, at higher humidity, there was a highertemperature for the desorption process. Nevertheless, at 45%RH, only the first inflection point was present; the secondinflection point most likely required a temperature higher than80 °C.The hydration and hydroxylation reactions during the

humidity ramps (Figure 2) were confirmed by the thermaldecomposition of products (data not shown). These resultsshow that Li8SiO6 chemisorbs water vapor, through thefollowing reaction:

+ → +12

Li SiO H O12

Li SiO 2LiOH8 6 2 4 4 (6)

No further hydroxylation was assumed because it has beenreported that Li4SiO4 does not present significant hydroxylationat similar thermal and humidity conditions.67 On the basis ofreaction 6, a total reaction conversion should correspond to a20% weight increment. In the relative humidity curves (seeFigure 2) the final weight increments were between 5 and 12.5wt %, which correspond to 25% and 62.5% efficiency, notconsidering the possible presence of adsorbed water. If thiswere the case, the CO2 chemisorption reaction would beproduced by any of the reactions 1−5 or by the followingreaction:

+ → +2LiOH CO Li CO H O2 2 3 2 (7)

Figure 4 shows the Li8SiO6−CO2−H2O sorption−desorp-tion curves, where important increments were registered as afunction of temperature. As with Li8SiO6−N2−H2O, thehysteresis loops did not close. These results indicated H2Oand/or CO2 chemisorption. During the sorption process, the

Figure 3. Water vapor sorption−desorption on Li8SiO6 in a dynamic process at different relative humidity conditions: (A) 20%, 40%, 60%, and 80%RH and (B) 20%, 35%, 40%, and 45% RH.

Figure 4. Water vapor sorption−desorption isotherms of the Li8SiO6sample, generated at temperatures between 30 and 80 °C, using CO2as carrier gas.

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weight gains increased as a function of the temperature and thefinal weights; after the desorption process, the increase wasfrom 11.5 wt % at 30 °C to 40.1 wt % at 80 °C. Therefore, theLi8SiO6−CO2−H2O system always presented higher weightincrements than those observed in the Li8SiO6−N2−H2Osystem, at any temperature (Figure 5). Therefore, the Li8SiO6−CO2−H2O system must have different processes of hydrationor hydroxylation and not only carbonation.

Nevertheless, CO2 carbonation in lithium silicates (reactions1−5) is not active between 30 and 80 °C.7−31 Thus, if CO2 ischemisorbed, it is mainly associated with the presence of LiOHor at least with the presence of superficial hydroxides over theLi8SiO6 particles. These results are interesting because it hasbeen described in the literature that Li8SiO6 chemisorbsconsiderably small amounts of CO2 (≤1% wt) at dry conditionsin the same temperature range.27 As previously described, watervapor reacts with Li8SiO6 superficially, producing Li−OH andSi−OH species or even LiOH. These new hydroxyl speciesmust be more reactive to CO2.Figures 6 and 7 show the calculated thermodynamic

properties of H2O reactions with three lithium silicates (seereactions 6, 8, and 9) because lithium hydroxide is responsiblefor the CO2 capture under humid conditions.

+ → +Li SiO H O Li SiO 2LiOH4 4 2 2 3 (8)

+ → +Li SiO H O SiO 2LiOH2 3 2 3 (9)

Figure 6A shows the calculated heat of reactions (ΔH)involved in the hydroxylation process of the lithium silicates,and it is evident that the Li8SiO6 and Li4SiO4 hydroxylations areexothermic reactions, whereas the Li2SiO3 process isendothermic. Additionally, the ΔG values (Figure 6B) indicatethat lithium silicate hydroxylation reactions are stable for onlyLi8SiO6 and Li4SiO4. Thus, the ΔG values indicate that theLi8SiO6−N2−H2O reaction system is stabilized by LiOH andLi4SiO4, where Li4SiO4 may continue reacting with water,although the experimental results have proved that this processis almost negligible.67

Moreover, Figure 7 shows the graphs describing where theGibbs free energy change is equal to zero at a given PH2O and Tfor the reactions 6, 8, and 9, and the Li8SiO6−H2O and

Li4SiO4−H2O chemisorption and desorption regions can bedetermined. These two reaction systems are thermodynamically

Figure 5. Comparison of water vapor isotherms of the Li8SiO6 usingCO2 and N2 as carrier gases performed at 30 and 80 °C.

Figure 6. Calculated thermodynamic data of different reactions ofH2O with three different lithium silicates: (A) heat of reaction (ΔH)and (B) free energy (ΔG).

Figure 7. Calculated van’t Hoff plots of the relationships among thefree energy (ΔG), temperature (T), and gas pressure (P in logarithmicscale).

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favorable over a certain range of temperatures and PH2O, whichmeans that under such conditions of H2O pressure, thereactions are thermodynamically favored. Therefore, Li8SiO6hydroxylation is more favored than the Li4SiO4 hydroxylationprocess under temperature and H2O pressure conditions usedhere.On the other hand, in a previous paper,23 the ΔH and ΔG

values involved in the carbonation process of these lithiumsilicates were investigated. According to this study and the datashown above, in the 30−100 °C range, the Li8SiO6, Li4SiO4,and Li2SiO3 carbonation and hydroxylation energy values variedsignificantly (Table 1). Lithium silicates have smaller ΔH and

ΔG hydroxylation values than those of carbonation. Thus, theenergy required for the hydroxylation processes is smaller,although these products are less stable than those obtainedduring carbonation. In addition, the ΔH value required for theLiOH carbonation is smaller than those observed for thelithium silicates. On the basis of all of the theoretical andexperimental results, the most feasible reaction mechanism is asfollows: Li8SiO6 reacts initially with water, producing ahydroxylation process, according to reaction 6. Then, LiOHsubsequently reacts with CO2, producing the final carbonationprocess. Thus, H2O acts as a catalytic intermediate species,decreasing the activation energy of the whole CO2chemisorption process.Different kinetic experiments of the Li8SiO6−CO2−H2O

system are presented in Figure 8. These isotherms wereperformed at 40, 60, and 80 °C, varying the RH between 20%and 80%. The RH significantly modifies the reaction rateprocesses during the first minutes. At 80 °C, the Li8SiO6−CO2−H2O system treated with 20% RH increased its weight byonly 4.8 wt %, whereas the same Li8SiO6−CO2−H2O system at80% RH increased its weight more than 10 times (61.2 wt %).The improvement in the activity of water sorption−desorptionwith temperature, and consequently in the CO2 chemisorption,is due to merely kinetic issues.All of the isothermal products were characterized using TG

to quantify the superficial water adsorbed, as well as thehydroxyl and carbonate species present on Li8SiO6. Figure 9shows the TG and DTG curves of the Li8SiO6−CO2−H2Osystem at 80 °C with different RH (20%, 40%, 60%, and 80%),where three well-defined temperature ranges are depicted: (1)Between room temperature and 279 °C, the samples lost aslight quantity of weight, which could be attributed to thedehydration process. It must be noted that the samplesindicated minimal adsorbed water (≤3 wt %). (2) Thedehydroxylation process was observed between 279 and 600

°C, and (3) the decarbonation process was indicated at T ≥600 °C and was attributed to the Li2CO3 decompositionprocess. The decarbonation process increased as a function ofthe RH. Finally, in the DTG curves, two peaks were observed at

Table 1. Enthalpy and Gibbs Energies of the Hydroxylationand Carbonation Reactions of Different Lithium Silicates,between 30 and 100 °C

lithiumsilicate

ΔHcarba

(kJ/mol) ΔGcarba (kJ/mol)

ΔHhydrox(kJ/mol)

ΔGhydrox(kJ/mol)

Li8SiO6 −175 between −130and −110

−122 between −76and −60

Li4SiO4 −145 between −90 and−70

−71 between −25and −5

Li2SiO3 −60 between −20and 0

3 between 50 and60

LiOH −98 between −88 and−85

aFrom ref 23.

Figure 8. Kinetic isotherms performed at different temperatures (40,60, and 80 °C) and RH (20%, 40%, 60%, and 80%) with a CO2 flow.

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924 and 943 °C, which may be associated with the thermaldecomposition of lithium silicates because it has been reportedthat lithium ceramics begin to decompose because of lithiumoxide sublimation at T ≥ 900 °C.68

On the basis of the decomposition TG experimentsdescribed above, the amounts of CO2 chemisorbed byLi8SiO6 were determined and are plotted in Figure 10. From

these curves, it is evident that if the RH was increased, the totalamount of CO2 chemisorbed increased. In fact, 80 °C and 80%of RH were the best thermal and humidity conditions at whichthe highest carbonation was produced (35.1 wt %). This weightincrement corresponds to a 71.6% total efficiency, namely, 7.97mmol of CO2 per gram of Li8SiO6 (mmol/g). The maximumCO2 chemisorption possible is 11.13 mmol/g. Therefore, thepresence of water vapor, at low temperatures (30−80 °C),enhances the CO2 chemisorption in Li8SiO6 in comparison todry conditions, where only ∼1 mmol/g is chemically trapped inthis temperature range.24 These results are comparable to thoseshown by other alkaline ceramics, such as Li5AlO4 and Na2SiO3,which are able to trap 8.4 and 4.6 mmol of CO2 per gram ofceramic, respectively, under similar conditions.37,41,42

To confirm that Li8SiO6 is reacting with 2 moles of CO2(reaction 1, independently of the temperature range) andconsequently assuming that Li4SiO4 is not reacting, Li4SiO4 wassynthesized by a solid-state reaction, and the Li4SiO4−CO2−H2O experiments were performed. Figure 11 shows different

dynamic experiments of varying temperature and relativehumidity in the presence of two gas carriers: CO2 and N2.The total weights gained were much lower than those observedin Li8SiO6 at 30 and 70 °C. At 30 °C, minimal final weightchange was observed for flows of N2−H2O and CO2−H2O,approximately 0.5 wt %. When the temperature increased to 70°C, a slight increase in weight occurred; in CO2−H2O flow, themaximum weight was 3.1 wt %, whereas in N2−H2O, a loss ofweight was observed and the final weight was 1.8 wt %.Moreover, isotherms performed at 60 °C with varying humiditywith CO2 (inset in Figure 11) showed that the amount of CO2chemisorbed in Li4SiO4 was very low (∼3 wt %). Therefore, theLi4SiO4, secondary phase formed during the CO2 chemisorp-tion process on Li8SiO6 did not enhance the CO2 capture

Figure 9. Thermogravimetric (A) and derivative thermogravimetric (B) curves of Li8SiO6 samples isothermally treated at 80 °C and different RH(20%, 60%, and 80%).

Figure 10. Quantification of the CO2 desorbed during thethermogravimetric analyses from Li2CO3 by Li8SiO6 varying temper-ature and RH.

Figure 11. Comparison of water vapor isotherms of the Li4SiO4 usingCO2 and N2 as carrier gases performed at 30 and 70 °C. The insetshows the isotherms performed at 60 °C and relative humiditybetween 40% and 80%, with a CO2 flow.

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under the conditions of this study. Hence, the assumption ofreaction 1 as the main carbonation reaction is correct.

■ CONCLUSIONS

The CO2−H2O sorption process was evaluated on two lithiumsilicates (Li8SiO6 and Li4SiO4) at low temperatures (30−80°C). Lithium silicates were prepared by solid-state reaction andwere characterized by different techniques. The Li8SiO6−N2−H2O and Li8SiO6−CO2−H2O systems were evaluated. WhenN2 was used as carrier gas, water was superficially adsorbed inLi8SiO6 and was chemically trapped. The thermal and humidityconditions determined the amounts of water adsorbed andchemisorbed. Li8SiO6 can be superficially hydroxylated in thepresence of water vapor. When the Li8SiO6−CO2−H2O systemwas analyzed, the results changed. Li8SiO6 continued trappingwater, but CO2 was chemisorbed, producing Li2CO3 andLi4SiO4. The experimental and theoretical results indicated thatLi8SiO6 can react with 2 moles of CO2 in the presence of watervapor, which means that under these thermal and humidityconditions, Li8SiO6 was able to absorb up to 7.97 mmol of CO2per gram of ceramic. This is considerably higher CO2 capturecapacity than that obtained under dry conditions. Finally, tocorroborate these findings, the Li8SiO6 reaction mechanism inthe presence of CO2 and H2O, Li4SiO4 was evaluated; Li4SiO4was able to trap only a minimal amount of CO2 in the samethermal and humidity conditions.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: bcecilia_19@iim.unam.mx.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was financially supported by the Projects SENER-CONACYT (Grant 150358) and PAPIIP-UNAM (Grant IN-102313). The authors thank Adriana Tejeda and Omar Novelofor technical help.

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