Ultrasound-Assisted Method for the Preparation

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Surfactant-Template/Ultrasound-Assisted Method for the Preparation of Porous Nanoparticle Lithium Zirconate Hamid R. Radfarnia and Maria C. Iliuta* Chemical Engineering Department, Laval University, 1065 avenue de la M
edecine, Qu
ebec, Canada G1 V 0A6 ABSTRACT: Porous nanoparticle lithium zirconate (Li2ZrO3) was prepared using an ultrasound-assisted surfactant-template method in the liquid-state reaction. The CO2 adsorption performance of the prepared materials was tested under various conditions and compared with that of Li2ZrO3 prepared by the simple surfactant-template method (porous, without sonication) and the conventional soft-chemistry route. The results indicated a better adsorption rate and capacity of porous nanopowders, whether assisted with ultrasound or not, in comparison with the traditional sample. This behavior is mainly due to a less aggregated powder structure and porous framework, facilitating gas and ion diffusion to and from the particle layers. However, the porous adsorbent prepared without sonication exhibited instability during cyclic operation, limiting its application for long-time use. Sonication time and surfactant concentration were found to be key parameters for controlling the crystallite size and the BET surface area. The porous Li2ZrO3 sample prepared with less surfactant and a shorter irradiation time (sample A) had the most favorable sorption kinetics and capacity among all studied samples. The maximum uptake capacity of 22 wt % for sample A compared to 15.2 wt % for the conventional sample (sample J, fabricated by the soft-chemistry method), obtained under a 100% CO2 stream, suggested a noticeable improvement in sorption behavior of the proposed adsorbents compared with traditional Li2ZrO3. Moreover, the adequate cyclic stability of porous powders prepared by sonication identify these materials as promising CO2 acceptors, particularly for integrated sorbent/catalyst systems such as that used for sorption-enhanced steam methane reforming (SESMR). CO2 adsorption experimental data for sample A were successfully modeled at various CO2 partial pressures using a double-exponential equation.

1. INTRODUCTION The increase in the concentration of anthropogenic greenhouse gases (GHGs) in the atmosphere, particularly of CO2, is considered to make a major contribution to global climate change. About 60% of the current CO2 emissions comes from power plants and industry sectors such as oil refineries, cement kilns, and iron, steel, and aluminum production plants.1 Among various options for CO2 emissions mitigation, CO2 capture and sequestration has recently attracted considerable interest. Potential processes for CO2 capture include physical and chemical absorption, gas
solid adsorption, cryogenic separation, and membrane separation. This work concerns CO2 capture by gas
solid adsorption, when the adsorbents operate at high temperatures (above 400 °C) and separate CO2 directly from high-temperature gases. For example, application of high-temperature solid adsorbents in power plants to remove CO2 from hot flue gases as an alternative to conventional absorbers eliminates the necessity of cooling the flue gases to temperatures where the absorption processes become efficient.2,3 The sorption-enhanced steam methane reforming (SESMR) process, an alternative to the traditional steam methane reforming (SMR) process, is another example of a high-temperature adsorbent application that allows for the production of high-purity hydrogen, a green energy source, along with in-situ CO2 capture.3,4 The application of regenerable solid adsorbents for CO2 capture has been largely discussed in the literature, although severe operating conditions are still a deterrent for industrial application. Chemical stability, ease of the regeneration, mechanical and thermal stability, ability to work in long-term multicycle operation, and r 2011 American Chemical Society

feasible production costs are important parameters that should be addressed in developing adequate CO2 acceptors.3 Commonly used high-temperature CO2 adsorbents are generally classified as synthetic (e.g., lithium zirconate,5 lithium silicate,6 and calcium oxide/mayenite composite7) or natural (e.g., CaObased8 and hydrotalcite9). CaO-based materials, the most well-known natural CO2 acceptors, can favorably adsorb CO2. However, their loss of adsorption capacity loss during cyclic operation, caused by particle sintering during high-temperature calcination, is one of the main reasons restricting their application in gas sweeping processes.10
13 Hydrotalcite, another natural adsorbent, typically suffers from low adsorption capacity, which makes it inappropriate for use as a commercial CO2 scavenger.9,14 In 1998, lithium zirconate (Li2ZrO3) was proposed by Nakagawa et al.5 as a novel high-temperature adsorbent candidate with a theoretical CO2 uptake capacity of 0.28 g/g of acceptor in the temperature range of 450
650 °C. The high-temperature stability and proper kinetics are the factors that have recently attracted more attention to lithium zirconate-based CO2 acceptors.15
21 The reversible exothermic carbonation reaction of lithium zirconate is described by the eq 1. The forward chemisorption reaction of CO2 and lithium zirconate is accelerated in the temperature Received: December 1, 2010 Accepted: June 7, 2011 Revised: June 3, 2011 Published: June 07, 2011 9295

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range of 450
650 °C while the regeneration reaction is normally initiated at temperatures above 650 °C.5,22 Li2 ZrO3 + CO2 T Li2 CO3 + ZrO2

ð1Þ

The high reactivity of lithium zirconate-based adsorbents is extremely important for their widespread application in CO2 scrubbing processes. The main parameters in controlling the reaction rate are generally the reaction limitations, the nature of the precursors, and the material synthesis routes. The bottleneck of CO2 capture using lithium zirconate is mostly related to the formation of solid carbonate and zirconia layers surrounding the unreacted core of lithium zirconate, which consequently limits the chemisorption and diffusion rates.20 Reducing the particle and crystallite sizes,16,22,23 introducing an alkaline salt (e.g., K2CO3),21,22,24 investigating new synthesis routes,15
17,23 and adjusting precursor stoichiometries19,25 are commonly proposed modifications on Li2ZrO3-based materials available in the literature. The solid-state synthesis route consists of mechanical mixing of zirconium oxide and lithium carbonate precursors.5,20 High energy demand and difficulty in controlling both the particle size and the crystal phases are the main drawbacks of this method.16 The initial efforts to modify the traditional solidstate reaction method were based on reducing the primary precursor particle sizes, thus resulting in smaller product grain sizes, or searching for more reactive crystal phases. Xiong et al.22 developed potassium-doped lithium zirconate with different primary ZrO2 particle sizes (1 and 45 μm) using the solid-state reaction. Their results established the important role of the adsorbent particle size: a smaller particle size yields a higher CO2 adsorption rate. Similarly, Nair et al.23 attempted to reduce the primary size of zirconia (ZrO2) using the sol
gel technique. They also investigated the performance of different lithium zirconate crystal phases (tetragonal, t-Li2ZrO3; monoclinic, m-Li2ZrO3) in the carbonation reaction. Their experiments demonstrated the superiority of the lithium zirconate tetragonal phase compared with the monoclinic phase during CO2 adsorption. The recently proposed liquid-state synthesis route for fabricating mainly t-Li2ZrO3 could partly improve the reaction rate and adsorption capacity deficiencies of lithium zirconate traditionally prepared using the solid-state reaction.15,16 The principle of their approach was to use water-soluble precursors such as zirconyl nitrate and lithium acetate or nitrate, resulting in nanoparticle CO2 acceptors with enhanced kinetic rates and capacities. Nevertheless, further application of the proposed tetragonal nanocrystalline lithium zirconate powders in the SESMR process was unsatisfactory because of the poor adsorbent kinetics under real reaction conditions.26,27 Aiming to improve the kinetics of CO2 adsorption, some researchers also attempted to incorporate appropriate dopants into the lithium zirconate crystal structure. For example, Ida et al.20 tried to modify the performance of lithium zirconate adsorbent by adding potassium carbonate promoter, leading to an increase in CO2 adsorption kinetics. The improvement was explained by the formation of a liquid eutectic mixed-salt shell during adsorption, which can accelerate gas diffusion toward the unreacted solid core.20 It is believed that gas diffusion in chemical adsorbents can also be enhanced by controlling the particle sizes and creating porous solid layers.28
30 A well-known method for the preparation of porous nanostructures is the surfactant-template method combined with a conventional synthesis route such as the hydrothermal and sol
gel techniques.28 However, the long reaction time and the tendency for particle aggregation, which results in

multiform-shaped particles, are the main drawbacks of the conventional surfactant-template synthesis.39
30 Recently, the surfactant-template/ultrasound-assisted synthesis has been extensively used as an emerging technique with the advantages of simple reaction steps, greater control of particle size, uniform mixing, less preparation time, and less energy usage.29,31
34 The principal mechanism of sonication is the acoustic cavitation of bubbles including the formation, growth, and implosive collapse of the bubbles, consequently creating localized hot spots. These local spots can produce temperatures as high as 5000 °C, pressures of about 1000 atm, and heating and cooling rates above 1010 K s
1— that is, conditions that are effectively convenient for breaking up the aggregated particles and making nanocrystals.35 Zirconia (ZrO2) nanopowders have recently attracted much interest because of their specific properties for the applications as ceramics, catalyst supports, and anode materials for fuel cells.36
39 In comparison with the conventional synthesis techniques such as the sol
gel, coprecipitation, and hydrothermal techniques, the surfactant-template/ultrasound-assisted method was established as an efficient chemical route, providing high porosity, high surface area, and uniform shapes and sizes of mesoporous zirconia nanoparticles.29,40 This synthesis method could make the product ready for efficient impregnation of active metals. However, to the best of our knowledge, the application of this method for the synthesis of porous mixed active metal oxides/ZrO2 for use in CO2 capture has not yet been proposed in the literature. The objective of this work was, therefore, to develop porous lithium zirconate adsorbents for CO2 capture using a cationic surfactant-template method in the presence and absence of ultrasound treatment. The prepared powders were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD), and Brunauer
Emmett
Teller (BET) analyses and compared with the lithium zirconate prepared in the liquid-state reaction. The material performances in cyclic CO2 sorption and the impact of CO2 concentrations in adsorption and desorption processes were investigated in detail. Furthermore, a doubleexponential model41,42 was used to interpret adsorption experimental data of porous lithium zirconate at various CO2 partial pressures.

2. EXPERIMENTAL SECTION 2.1. Material Preparation. The reagent chemical precursors used in the experiments were as follows: zirconoxy nitrate, ZrO(NO3)2 3 xH2O (Labmat), lithium acetate (Labmat), and cetyltrimethylammonium bromide (CTAB, Fisher). The ultrasound irradiation was produced with a high-intensity ultrasonic processor (Hielscher UP400S, 400 W; frequency 24 kHz, Ti horn, 3-mm tip diameter; Hielscher Ultrasonics GmbH, Teltow, Germany). The porous lithium zirconate was synthesized by the ultrasoundirradiation/surfactant-template route in the aqueous phase. Certain amounts of CTAB above the critical micelle concentration (CMC), measured as explained below, were initially mixed into distilled water, and then zirconoxy nitrate and lithium acetate solutions were added, in order to obtain 50 mL of a solution containing a Li/Zr molar ratio of 2:1. The lithium zirconate complex was then treated by ultrasonic irradiation for a defined time. The reaction temperature was kept constant at 70
75 °C. Finally, the complex solution was dried in the temperature range of 80
85 °C overnight and then calcined at 690 °C for 6 h at a heating/cooling rate of 5 °C/min. The performance of the prepared material was compared with those of porous lithium zirconate prepared without 9296

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Table 1. Material Composition and Characterization BET surface area (m2/g) Li/Zr irradiation crystallite CTABb sample (mmol) (mmol/mmol) time (h) size (nm) a

fresh

used

A

0.31

2:1

1

20

12.0

27.4

B

0.31

2:1

2

20

12.4

20.3

C

0.31

2:1

3

23

10.4

24.3

D

0.78

2:1

1

21

10.4

15.0

E

0.78

2:1

2

24

12.9

14.4

F G

0.78 1.56

2:1 2:1

3 1

24 22

10.4 10.5

14.8 14.1

H

1.56

2:1

2

24

10.2

15.3

I

1.56

2:1

3

26

15.5

18.4

J




2.2:1




21

9.9

11.3

K

0.31

2:1




20

11.3

18.6

a

Sample J refers to powder prepared by the simple liquid-state reaction (soft chemistry16). Porous sample K was prepared using the surfactanttemplate method without sonication. The rest are porous materials prepared using the surfactant-template/ultrasound-assisted method. All adsorbents were synthesized in 50 mL of solution at 70
75 °C. b The measured CMC was 4.55  10
4 mol/L.

sonication and conventional lithium zirconate prepared according to the soft-chemistry route16 as a simple liquid-state reaction. Compositions and synthesis conditions for the lithium zirconate samples are summarized in Table 1. To measure the CMCs of the solutions, different amounts of CTAB were added to an aqueous solution containing zirconoxy nitrate and lithium acetate precursors. Sample conductivities were measured using a conductivity meter (Qcond 2200, Merck) to determine the surfactant concentration where a sharp transition in the conductivity occurred. 2.2. Material Characterization. X-ray diffraction (XRD) measurements were used for the identification of the crystal phases of the synthesized adsorbents. The XRD analyses were carried out on a Siemens powder X-ray diffractometer (D5000) with Nifiltered Cu KR radiation (wavelength of 1.5406 Å). The crystallite sizes were estimated using the Scherrer equation.43 The BET surface area, total mesopore volume, and mean mesopore size were obtained from nitrogen adsorption isotherms at
196 °C using an Intelligent Gravimetric Analyzer (IGA-003; Hiden Isochema , Ltd., Warrington, U.K.). The samples were initially degassed at 500 °C for 3 h before measurement of the nitrogen adsorption isotherm was started. Pore size measurements were also performed using an Auto Pore IV (Micromeritics) mercury porosimeter. Each sample was outgassed under full vacuum prior to analysis. The morphologies of the prepared samples were studied using the images obtained from JEOL JSM-840A scanning electron microscopy (SEM) instrument. 2.3. CO2 Adsorption/Desorption Reaction. CO2 adsorption/desorption experiments were conducted on the IGA-003 instrument. All carbonation/decarbonation experiments, including temperature profiles, and adjustments of the CO2 (99.999%, Praxair) and Ar (argon) (99.999%, Praxair) flows were programmed. About 40 mg of sample was loaded into a quartz container and heated initially at 690 °C for 10 min at a heating rate of 9 °C/min under a pure Ar flow (150 mL/min) to complete regeneration. The temperature was then decreased to the reaction

Figure 1. X-ray diffraction patterns of samples calcined at 690 °C. The symbols correspond to the phases (Δ) ZrO2, (O) t-Li2ZrO3, (2) Li2CO3, and (b) m-Li2ZrO3.

temperature of 575 °C at a cooling rate of 5 °C/min. Once the temperature had stabilized, the argon flow was automatically switched to pure CO2 (150 mL/min). After 30 min of adsorption, the temperature was increased at a ramp rate of 9 °C/min to start the regeneration step. The sample was held for 30 min at 690 °C to achieve complete regeneration. The next cycle was then started by repeating the above-mentioned steps. During the entire process, the sorbent weight and the temperature were continuously recorded. Adsorption experiments at various CO2 partial pressures were initiated once the consecutive sorption cycles under pure CO2 had been completed. Different CO2 concentrations were obtained by regulating the flow rates of CO2 and Ar at a constant total flow rate of 150 mL/min.

3. RESULTS AND DISCUSSION 3.1. Material Characterization. Porous nanoparticle lithium zirconate was prepared using a surfactant template (CTAB) irradiated with ultrasound in a liquid-state reaction. The effects of the main process parameters in material synthesis (i.e., surfactant concentration and sonication time) were studied in detail. The material prepared using the surfactant-template/ultrasoundassisted method was also compared to (i) the porous adsorbent prepared by the simple surfactant template and to (ii) the conventional lithium zirconate fabricated by the soft-chemistry route. Figure 1 presents the XRD patterns of the nanoscale lithium zirconate for the calcined samples prepared under various operating conditions. It clearly shows the preferential formation of the tetragonal lithium zirconate phase at 690 °C, as formerly addressed in the literature.44 A minor reflection at 30°, corresponding to Li2CO3, can be detected for samples A
I and K (initial Li/Zr molar ratio 2:1), which might be due to either incomplete calcination or low-temperature reaction between the adsorbent and CO2 present in the atmosphere.17 The presence of some ZrO2 reflections in samples A
I and K might be partially due to lithium sublimation during the high-temperature calcination step, leading to incomplete precursor reaction. In addition, small reflections 9297

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Figure 2. X-ray diffraction patterns of samples A, J, and K at various times: A1 and K1, before primary calcination; A2, J1, and K2, after primary calcination; A3, J2, and K3, after CO2 adsorption; A4, J3, and K4, after sample regeneration. The symbols correspond to the phases (Δ) ZrO2, (O) t-Li2ZrO3, (2) Li2CO3, (b) m-Li2ZrO3, (]) LiNO3, and (*) unknown.

at ca. 20° and 27°, detected only for porous samples E
I, could be attributed to the m-Li2ZrO3 phase. Regarding the initial Li/Zr molar ratio (2.2:1) in sample J (conventional lithium zirconate), the appearances of some reflections corresponding to ZrO2 might be on account of either imperfect reaction of Li2CO3 and ZrO2 in the calcination step or low-temperature CO2 adsorption at ambient conditions during material storage. The ratio of Li to Zr in the conventional adsorbent was chosen as 2.2:1 to follow the literature data and to highlight the role of surfactant/ultrasound in material synthesis, because it has been suggested that the adsorbent doped with more lithium shows higher activity than its stoichiometric counterpart.25 To better characterize the proposed materials, the powder diffraction patterns of typical samples A and K before and after primary calcination, as well as after CO2 adsorption and desorption, are presented in Figure 2. It can be seen that patterns A1 and K1 mainly correspond to an amorphous solid, with only lithium nitrate appearing as a crystalline phase. The pattern reflections for samples calcined at 690 °C (A2 and K2) are mainly assigned to t-Li2ZrO3, which disappears upon reaction with CO2, as indicated in patterns A3 and K3. However, a number of peaks for unreacted t-Li2ZrO3 phases can be distinguished for sample K in pattern K3, indicating its imperfect reaction (ca. 42° and 62°).

The appearance of Li2CO3 and ZrO2 crystalline phases, along with the disappearance of t-/m-Li2ZrO3 phases after reaction with carbon dioxide, confirms that the adsorption was effectively performed. The reproduction of t-Li2ZrO3 crystalline phases upon material regeneration at 690 °C (patterns A4 and K4) after 11 successive cycles indicates that the desorption process effectively proceeded through the release of CO2. It is interesting to mention that the minor reflection at 30° assigned to Li2CO3 (observed in pattern A2 and K2) disappeared upon material regeneration after 11 successive cycles. This behavior might be due to rearrangement of the material matrix, leading to better access of unreacted lithium carbonate sites to zirconia crystallites to initiate reaction. Moreover, the cyclic operation of sample K results in the production of some undesirable reflections corresponding to the m-Li2ZrO3 phase, which likely decreases the material activity. Sample J shows a rather different behavior upon adsorption and desorption of CO2. More peaks assigned to Li2CO3 can be observed in the fresh sample J, because of the presence of more Li2CO3 (pattern J1). As expected, upon CO2 adsorption, the peaks assigned to t-Li2ZrO3 disappear (pattern J2), although incomplete reaction of some t-Li2ZrO3 phases can be also recognized (e.g., at ca. 42° and 62°), likely because of the dense structure of sample J. Moreover, the 9298

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Figure 3. Typical SEM images of fresh proposed porous and conventional lithium zirconate samples: (a) A, (b) B, (c) C, (d) E, (e) H, (f) J, and (g) K.

Figure 4. N2 adsorption isotherms of sample A before and after cyclic CO2 sorption.

appearance of some new reflections at 49.3° and 50.5° in pattern J3 (regenerated sample J), corresponding to ZrO2, denotes incomplete regeneration of sample J, possibly owing to the agglomerated

structure of this material. Some reflections related to m-Li2ZrO3 phases also appear once the sample has been regenerated after the last sorption cycle. Furthermore, the reduced intensities of 9299

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Industrial & Engineering Chemistry Research ZrO2 and Li2CO3 reflections for regenerated samples A, J, and K after cyclic operation, compared to the fresh ones, are due to sample reactivation during cyclic operation. The BET surface areas of the fresh and used samples measured by the N2 physisorption method, as well as the product crystallite sizes estimated from the Scherrer equation, are presented in Table 1. As can be seen, the powder crystallite sizes increased with increasing surfactant concentration. Generally, micelles formed at surfactant concentrations above the CMC, surrounding the primary particle nuclei within the solution and inhibiting their interaction and growth. It has been reported that surfactants concentration above an optimum concentration in a micellar solution might not result in a further reduction in crystallite sizes, because the multilayers of micelles can begin to enlarge, ultimately resulting in larger particle sizes.46
48 This hypothesis is supported by our results presented in Table 1. The data clearly indicate that the crystallite size grows upon increasing CTAB concentration over 0.31 mmol, which is about 13 times the CMC of our solutions (4.55  10
4 mol/L). It was found that the CMC value is smaller than that of water, 15.57  10
4 mol/L49 at 70 °C. This is in accordance with data reported in the literature for highly ionic nitrate solutions.50 Furthermore, the results indicate that the crystallite sizes increased when the samples were kept for longer times under sonication at a certain CTAB concentration. Samples A and B have the smallest crystallite sizes among all fabricated lithium zirconate samples. Moreover, we could not recognize any apparent correlation

Figure 5. Pore size distributions of sample A before and after cyclic CO2 sorption, measured by mercury porosimetry.

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between surfactant concentration and the BET surface area of the samples, probably because of the low BET values of lithium zirconate chemisorbents, which are of the same order of magnitude. It is well-known that optimal sonication generally results in lower primary particle sizes, greater uniformity in particle size distribution, and marked reductions in the reaction time, compared with the conventional synthesis.29,35,45 However, the creation of compact surfaces is normally inevitable during ultrasonic operation. Employing a surfactant-template method integrated with sonication can significantly reduce the particle aggregation on the material surface. Typical SEM images of fresh porous powders presented in Figure 3a
e illustrate the creation of open porous structures by means of the surfactant-template/sonication method. From a morphological point of view, as indicated in Figure 3a
c, the sonication time does not have a significant effect on the particle shape, although longer irradiation times increase the seed particle sizes to some extent. Meanwhile, the particles get a slightly spherical shape as the surfactant concentration is increased, as illustrated in Figure 3b,d,e. The SEM images of porous sample K (prepared without sonication) and sample J (prepared by simple liquid-state synthesis) are also shown in Figure 3f,g. As indicated, sample J consists of compact layers of large, irregularly shaped aggregates, along with a macropore structure, whereas sample K is porous to some extent, consisting of both large and small aggregates. The powder surface areas of porous samples after 11 successive CO2 adsorption/desorption cycles are also reported in Table 1. From both fresh and used porous sample data, it can be seen that the surface area tends to increase at the end of the cyclic operation. This phenomenon can be ascribed to pore rearrangement during high-temperature adsorption/regeneration operation, which leads to both an increase in porosity and a change in the morphology of the materials. Supporting data are provided in Figure 4, representing nitrogen adsorption isotherms, pore volumes, and mean pore sizes of sample A before CO2 capture and after the last cycle. The results indicate that mesopore the volume and mean pore size increased after the cyclic operation. It should be noted that the N2 isotherms of the fresh and used samples are of type II (IUPAC classification), conforming to macroporous powders. The negligible mesopore volumes determined by N2 isotherm analysis confirm the presence of mostly macropore structures for such low-surface-area chemisorbents. The total pore volumes of meso- to macropores for fresh and used sample A were measured using mercury porosimetry and are presented in Figure 5. According to the pore size distribution curves, the total pore volume of sample A was increased after cyclic operation as a result of the rearrangement and restructuring of pores, which confirms the

Figure 6. Typical SEM images of proposed porous and conventional lithium zirconate samples after 11 cycles for samples (a) A, (b) J, and (c) K. 9300

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Figure 7. Cyclic adsorption/regeneration results for lithium zirconate samples: adsorption under pure CO2 (150 mL/min) at 575 °C, regeneration under pure argon (150 mL/min) at 690 °C. Plots a
k correspond to samples A
K.

BET characterization data given in Table 1. The total pore volumes of sample A before and after CO2 cyclic sorption were measured as 0.97 and 1.38 mL/g, respectively. In addition, the total pore volume of fresh sample A (0.97 mL/g) was higher than that of conventional Li2ZrO3, reported in the literature to be 0.6 mL/g,16 showing the superiority of the proposed porous material compared with the conventional Li2ZrO3 adsorbents. SEM images of typical samples after long-term operation (11 successive cycles) presented in Figure 6a
c can also support the aforementioned observation. According to Figure 6a, the spherical grains of porous sample A were reshaped into dumbbell-like particles, which can possibly provide more active sites for CO2

adsorption. The macroporosity of sample A was also apparently enhanced at the end of the cyclic operation. Similar particle reconfigurations and reconstructions of pore matrixes were also found for all other porous samples. As indicated in Table 1, the most significant increase in the specific surface area at the end of 11th cycle occurred in sample A, prepared with the lowest surfactant concentration. In other words, the stability of the pore structure is highly influenced by the amount of surfactant used during material preparation. Sample A, followed by samples C and B, shows significantly more stability in pore structure than the other samples, under severe cyclic operating conditions. The large agglomerates in sample K seem to be cleaved into smaller sizes as 9301

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Industrial & Engineering Chemistry Research the porosity grows, as presented in Figure 6b. Similarly, the structure of sample J changed into a macropore matrix with smaller aggregates, although the particles were still quite large in comparison with those of all other porous samples (Figure 6c). 3.2. CO2 Adsorption/Desorption. The adsorption and desorption cycles were carried out at 575 and 690 °C, under pure CO2 and Ar flows, respectively. The reaction time was set at 30 min for both adsorption and regeneration steps. The impact of CO2 partial pressure in the sorption behavior was subsequently investigated in detail. Figure 7 presents the cyclic CO2 adsorption and regeneration behaviors for the fabricated samples. Concerning the experimental data for the porous lithium zirconate samples (A
I and K), a self-reactivation phenomenon (i.e., growth of CO2 uptake) was observed as the number of cycles increased. This phenomenon is likely due to the restructuring of the pore framework during successive adsorption and desorption operations, facilitating CO2 gas diffusion and chemisorption reaction on active sites of the powders. This behavior is in agreement with the material characterization data. This phenomenon was more significant for the porous samples synthesized with higher surfactant concentrations and irradiation times, most likely because of their more complex pore structures. As can be seen from Figure 7, samples A and C exhibited the highest CO2 uptake (22 wt %), followed by samples I (21.2 wt %), D (20.9 wt %), B (20.8 wt %), F (20.3 wt %), E (20.1 wt %), G (19.1 wt %), and H (17.5 wt %). It is worth mentioning that, by increasing the number of cycles, the regeneration of porous samples E, F, G, and I, synthesized with higher surfactant concentrations, did not become completely reversible. The exact reason for this behavior does not seem very clear; however, it might be due to kinetic constraints coming from (i) the collapse of fragile pores during restructuring of pore arrays or (ii) the corrugated pore structure (high tortuosity) of the material, which limits CO2 desorption from part of the sorbent. Regarding samples J and K, a specific sorption trend cannot be established because the sample uptake capacity either increased or decreased irregularly, as shown in Figure 7j,k, which makes these materials unreliable for practical applications. The initial drop of the uptake capacity can be explained by the primary sintering of particles during the initial sorption cycles. The uptake capacity was partly recovered by further reconstruction of macropores and cleavage of large aggregates (Figure 6b,c). The maximum adsorbate uptake capacity of sample K, which is more porous to some extent than sample J, was as much as 19.2 wt %, markedly improved in comparison with sample J, which yielded a CO2 uptake of just 15.2 wt %. However, sample K showed more instability during 11 successive cycles than did sample J. In addition, the conventional sample J cannot be fully regenerated, which partly results in the inactivation of the adsorbent, in agreement with the powder XRD data discussed previously. Concerning the reaction kinetics, the maximum sorption rates for all samples in cyclic operation are shown in Figure 8a,b. As can be seen, the maximum rate of adsorption for porous samples generally increased with the number of cycles, whereas the maximum desorption rates were relatively constant during cyclic operation. However, sample K showed a completely different behavior: its adsorption and desorption rates decreased constantly with increasing number of cycles, probably because of its weak pore structure and particle sintering. The reason for the increase in the adsorption rates of porous samples A
I is the enhancement of the surface area and porosity, as well as the morphology reconstruction during the reaction. Among the porous materials, sample A

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Figure 8. Maximum reaction rates of lithium zirconate samples versus successive operation cycles: (a) adsorption, (b) regeneration.

showed, on average, the most favorable and steady rates of adsorption, likely because of its adequate surface area and stable pore structure. It is worth pointing out that the maximum adsorption rate of sample A was approximately 3 times greater than that of conventional lithium zirconate (sample J). The lowest adsorption kinetics was exhibited by sample J, characterized by an aggregated surface, preventing the free access of CO2 molecules into the deeper layers of active sites. Concerning the rate of regeneration, the most adequate reaction rate was exhibited by porous samples A, C, and E, whereas sample J had the worst regeneration rate. The performances of the synthesized adsorbents at various CO2 concentrations were also studied in detail. According to Figure 9, the uptake capacity and kinetics of the samples fell off as the CO2 partial pressure decreased. This behavior was much more pronounced when the CO2 partial pressure decreased below 0.5 bar, as previously addressed in other works.16,17 It should be pointed out that sample J did not present desirable sorption behavior up to a carbon dioxide partial pressure of 0.5 bar; thereafter, it approached the behavior of the porous samples. The maximum sorption rates of all samples, estimated from the experimental data at different CO2 partial pressures, are given in Table 2. The reaction rates for all samples were quite low at low adsorbate concentrations, mainly because of diffusion and 9302

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Figure 9. Adsorption/regeneration experimental results at different CO2 partial pressures: PCO2 = (a) 1, (b) 0.75, (c) 0.5, (d) 0.25, and (e) 0.1 bar. Adsorption under a total gas flow of 150 mL/min at 575 °C, regeneration under pure argon (150 mL/min) at 690 °C.

Table 2. Maximum Adsorption/Desorption Rates of Reaction (wt %/min) at CO2 Partial Pressures below 1 bar sample

0.75 bar

0.5 bar

0.25 bar

0.1 bar

A

1.794/2.201

0.486/0.417

0.158/0.15

0.102/0.110

B

1.208/1.755

0.171/0.259

0.145/0.115

0.142/0.088

C

1.591/2.129

0.441/0.327

0.141/0.155

0.074/0.095

D E

1.739/2.094 1.010/1.885

0.479/0.455 0.286/0.299

0.128/0.136 0.073/0.084

0.090/0.059 0.057/0.117

F

1.098/1.679

0.330/0.451

0.172/0.089

0.106/0.051

G

1.162/1.789

0.329/0.464

0.089/0.117

0.053/0.069

H

0.702/1.203

0.275/0.195

0.091/0.102

0.063/0.076

I

1.353/1.972

0.298/0.246

0.105/0.140

0.096/0.049

J

0.602/1.203

0.235/0.275

0.161/0.101

0.097/0.117

K

0.769/1.428

0.302/0.436

0.165/0.104

0.076/0.065

thermodynamic constraints near the equilibrium point. However, sample A showed a rather more desirable sorption kinetics compared with the other porous samples. 3.3. CO2 Chemisorption Modeling and Analysis. During CO2 adsorption, CO2 is initially chemisorbed directly on the superficial surfaces of lithium zirconate particles, forming a Li2CO3
ZrO2 external shell that quickly covers the unreacted sites.51 The CO2 chemisorption is then kinetically controlled by the CO2 and ion (Li+ and O2-) diffusion process through the formed shell. Among the numerical models available in the literature, the recent double-exponential model was successfully used to model CO2 chemisorption on ceramic materials, including the diffusion and chemisorptions impacts, as given by the equation41,42,51 y ¼ Ae
k1 t + Be
k2 t + C

ð2Þ

where y represents the weight percentage of CO2 adsorbed; t is the adsorption time; k1 is the rate constant for CO2 direct

Figure 10. Experimental and regressed data for porous sample A for different CO2 partial pressures at 575 °C.

chemisorption; k2 is the rate constant for the diffusion process; and A, B, and C are pre-exponential factors. It was reported that the controlling step is the diffusion process because the superficial carbonate--zirconia product layers, freely developed by the initial gas chemisorption, are responsible for restricting the further ion and gas diffusion. Figure 10 presents a comparison between the calculated uptake curves and the experimental data for sample A at different CO2 concentrations. In addition, the kinetic parameters obtained by fitting the isothermal data to a double-exponential model are reported in Table 3 for different CO2 partial pressures. The reaction rate continuously decreased with time as a result of the formation of the carbonate
zirconia shells, which grew constantly, thus limiting the ion diffusion. Lithium and oxygen ions 9303

dx.doi.org/10.1021/ie102417q |Ind. Eng. Chem. Res. 2011, 50, 9295–9305

Industrial & Engineering Chemistry Research Table 3. Kinetic Parameters for CO2 Chemisorption at 575 °C on Porous Sample A, Fitted to a Double-Exponential Model PCO2(bar)

k1 (1/min)

k2 (1/min)

1

0.4713

0.2953

0.75

0.2523

0.2087

0.5

0.0204

0.00956

0.25

0.0084

0.0084

0.1

0.0051

0.0051

ARTICLE

processes, was successfully applied to interpret the adsorption experimental data. It seems that further detailed studies are necessary to improve the sorption properties of these porous adsorbents and to reduce the gas and ion diffusion limitations at low CO2 partial pressures. Moreover, experimental adsorption tests on the proposed materials under real reaction conditions (e.g., fixed-bed reactor) would also be very useful.

’ AUTHOR INFORMATION Corresponding Author

competed to diffuse through the external layer and reach the surface in order to react with CO2.51 According to Table 3, when the CO2 partial pressure wais reduced from 1 to 0.5 bar, the chemisorption and diffusion rates (k1 and k2, respectively) decreased continuously, because of the constraint on the external CO2 diffusion to the surface of the adsorbent particles, which reduced the chemisorption rate. The chemisorption and diffusion parameters became practically equal at lower pressures. Moreover, the rates of chemisorption (k1) and diffusion (k2) were found to be of the same order of magnitude and at least 1 order of magnitude higher than those reported in other works for conventional lithium-based adsorbents.41,42,51 These results confirm that the diffusion and chemisorption rates can be accelerated by decreasing the grain sizes and creating a porous framework. Smaller grain sizes lead to a thinner carbonate
zirconia layer, enhancing the rate of ion diffusion. In addition, a porous structure facilitates the access of CO2 to the active sites, thereby increasing the CO2 diffusion and chemisorption rates.

4. CONCLUSIONS Porous nanoparticle lithium zirconate was successfully synthesized in this work using a surfactant-template method assisted by ultrasound irradiation and characterized for carbon dioxide cyclic adsorption/desorption. The impact of the main experimental parameters, namely, surfactant (CTAB) concentration and sonication time, on the adsorbent properties was also studied in detail. The experimental results demonstrated the necessity of controlling the irradiation time and surfactant concentration on account of their marked impacts on the adsorbent characteristics. The proposed and conventional lithium zirconate samples were tested on multicycle carbon dioxide sorption, and it was concluded that sample A, the porous lithium zirconate synthesized with the lowest amount of surfactant and the shortest sonication time, showed the most favorable sorption behavior (22 wt % uptake at 100% CO2), whereas sample J, made by the conventional soft-chemistry route, had the poorest performance (15.2 wt % uptake at 100% CO2). Because the long-time stability of a sorbent is a critical parameter for the commercialization of a CO2 scavenger, the multicycle stabilities of the proposed porous lithium zirconate materials were also studied, leading to reasonable stability of the most of sonicated porous samples but questionable longterm stability for samples J and K (porous, nonsonicated). The use of the fabricated adsorbents was also investigated at different CO2 partial pressures, indicating that both the conventional sample and the proposed porous materials still suffered from restricted gas and ion diffusion at low CO2 concentrations, thus reducing the adsorbate uptake capacity and kinetics. A doubleexponential model, considering the chemisorption and diffusion

*Tel.: 1-418-656-2204. Fax: 1-418-656-5993. E-mail: maria-cornelia. [email protected].

’ ACKNOWLEDGMENT Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC), FQRNT Centre in Green Chemistry and Catalysis (CGCC), and Centre de Recherche sur les Propri
et
es des Interfaces et sur la Catalyse (CERPIC), Laval University, is gratefully acknowledged. The authors greatly appreciate the very helpful assistance of specialists from Hiden Isochema, Ltd. (Warrington, U.K.). ’ REFERENCES (1) Metz, B.; Davidson, O.; Coninck, H. D.; Loos, M.; Meyer, L. IPCC Special Reports: Carbon Dioxide Capture and Storage; Cambridge University Press: Cambridge, U.K., 2005. (2) 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. (3) Yong, Z.; Meta, V.; Rodrigues, A. E. Adsorption of Carbon Dioxide at High Temperature—A Review. Sep. Purif. Technol. 2002, 26, 195. (4) Harrison, D. P. Sorption-Enhanced Hydrogen Production: A Review. Ind. Eng. Chem. Res. 2008, 47, 6486. (5) Nakagawa, K.; Ohashi, T. A Novel Method of CO2 Capture from High Temperature Gases. J. Electrochem. Soc. 1998, 145, 1344. (6) Kato, M.; Nakagawa, K. New Series of Lithium Containing Complex Oxides, Lithium Silicates, for Application as a High Temperature CO2 Absorbent. J. Ceram. Soc. Jpn. 2001, 109, 911. (7) Li, Z. S.; Cai, N. S.; Huang, Y. Y.; Han, H. J. Synthesis, Experimental Studies, and Analysis of a New Calcium-Based Carbon Dioxide Sorbent. Energy Fuels 2005, 19, 1447. (8) Silaban, A.; Narcida, M.; Harrison, D. P. Characteristics of the Reversible Reaction Between CO2(g) and Calcined Dolomite. Chem. Eng. Commun. 1996, 146, 149. (9) Ding, Y.; Alpay, E. Equilibria and Kinetics of CO2 Adsorption on Hydrotalcite Adsorbent. Chem. Eng. Sci. 2000, 55, 3461. (10) Bandi, A.; Specht, M.; Sichler, P.; Nicoloso, N. In Situ Gas Conditioning on Fuel Reforming for Hydrogen Generation. Proceedings of the 5th International Symposium on Gas Cleaning, Pittsburgh, PA, September 2002. (11) Abanades, J. C. The Maximum Capture Efficiency of CO2 Using a Carbonation/Calcination Cycle of CaO/CaCO3. Chem. Eng. J. 2002, 90, 303. (12) Grasa, G. S.; Abanades, J. C. CO2 Capture Capacity of CaO in Long Series of Carbonation/Calcination Cycles. Ind. Eng. Chem. Res. 2006, 45, 8846. (13) Sun, P.; Lim, C.; Grace, J. R. Cyclic CO2 Capture of LimestoneDerived Sorbent during Prolonged Calcination/Carbonation Cycling. AIChE J. 2008, 54, 1668. (14) Barelli, L.; Bidini, G.; Gallorini, F.; Servili, S. Hydrogen Production Through Sorption-Enhanced Steam Methane Reforming and Membrane Technology: A Review. Energy 2008, 33, 554. 9304

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’ NOTE ADDED AFTER ASAP PUBLICATION After this paper was published online June 29, 2011, a correction was made to the units in the column headings of Table 3. The revised version was published July 27, 2011.

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