Adsorption of CO2 on Mixed Oxides Derived from Ca–Al–ClO4

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Adsorption of CO2 on Mixed Oxides Derived from Ca−Al− ClO4‑Layered Double Hydroxide Shengping Wang,* Chun Li, Suli Yan, Yujun Zhao, and Xinbin Ma Key Laboratory for Green Chemical Technology, School of Chemical Engineering and Technology, Tianjin University; Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, People’s Republic of China ABSTRACT: In this paper, we studied the CO2 adsorption performances of well-mixed oxides with Ca/Al ratios of 2:1, 2.5:1, 3:1, 3.5:1, and 4:1, which were prepared through the thermal decomposition of co-precipitated hydrotalcite-like Ca−Al−ClO4 precursors. The mixed oxides with a Ca/Al ratio of 3 exhibited the best performance at the adsorption temperature of 600 °C, and approximately 87% of the initial CO2 capture capacity was retained after 50 cycles of multiple carbonation/calcination tests. The favorable cyclical performance of the adsorbent is attributed to the homogeneous distribution of Al2O3 among CaO particles, which effectively prevented the aggregation and sintering of CaO crystallite. Furthermore, a smaller particle size (about 30 nm) of CaO allowed for as much carbonation as possible to take place at the rapid reaction-controlled regime rather than at the diffusion-controlled regime, resulting in the favorable long-term adsorption performance.

1. INTRODUCTION In recent years, CO2 emissions have been the cause of great public concern because of their significant adverse effect on global warming and climate change. To alleviate the greenhouse gas impact, one potential solution is to implement sorptionbased technologies for CO2 capture, storage, and utilization technology (CSUT). Additionally, CO2 is a byproduct of the coal-to-synthetic natural gas (SNG) process, which proceeds in the temperature range of 350−600 °C. CO2 removal in the cycling gas is beneficial to the methanation reaction because a high concentration of CO2 has a negative effect on the catalytic performance of the catalyst. Thus, CO2 removal and capture are also indispensable in the coal-to-SNG process. Among the alternative sorbents, solid adsorbents are considered suitable candidates for CO2 capture because of their many advantages, such as wide operating temperature range, elimination of the generation of liquid wastes, and disposition of spent solid adsorbents without excessive environmental precautions.1 At present, CaO-based solid adsorbents are widely used for high-temperature CO2 capture from flue gases and sorptionenhanced reaction processes, such as coal-to-SNG, as a result of their high adsorption capacity [the maximum theoretical amount of CO2 adsorption is 78.6% (44 g of CO2/56 g of CaO)] and wide material sources. However, they show poor stability with increasing cycles, resulting from the severe sintering of CaO particles and pore structure collapse at high temperatures.2−8 For instance, natural limestone went through a drop to half of the initial fast adsorption capacity after just 10 cycles when tested on thermogravimetric analysis (TGA).9 To reduce this loss in capacity, many methods have been developed, which can be classified into three different categories: (i) synthesis of CaO from different precursors (e.g., calcium hydroxide, calcium acetate hydrate, and calcium citrate tetrahydrate),10 (ii) development of porous structures of CaO/CaCO3-based sorbents,11,12 and (iii) incorporation of inert materials with high-temperature resistance (e.g., Ca12Al14O33, MgAl2O4, Al2O3, and MgO) into sorbents.13−19 © XXXX American Chemical Society

Layered double hydroxides (LDHs), also known as hydrotalcite-like compounds (HTlcs), are a new type of threedimensional (3D) basic solid.20 These materials are widely used as catalysts, precursors, and CO2 adsorbents,21−24 owing to their well-defined layered structure with nanometer (0.3−3 nm) interlayer distances. The general formula for LDHs is [M2+1−xM3+x(OH)2]An−x/n, in which M2+ represents a divalent cation (e.g., Mg2+, Zn2+, and Ca2+), M3+ is a trivalent cation (e.g., Al3+, Cr3+, and Fe3+), An− acts as an interlayer anion, such as CO32−, NO3−, and ClO4−, and the value of x is normally between 0.17 and 0.33.25 LDHs have been considered promising CO2 solid adsorbents because of their relatively fast adsorption/desorption rate and favorable cyclic stability.26 Upon thermal treatment above 400 °C, the layered structures of LDHs are disrupted to form an amorphous mixed solid oxide with a larger surface area and pore volume as well as good dispersion and stability at high temperatures, which is favorable for CO2 sorption.1,27 GarciaGallastegui et al. suggested that the CO2 adsorption capacity could be greatly enhanced by employing multi-walled carbon nanotubes and graphene oxide as support for Mg−AlLDHs.28,29 Wang et al. developed a facile isoelectric point (IEP) method to successfully prepare a nanosized spherical Mg3Al−CO3-LDH. The CO2 adsorption capacity of the mixed oxides achieved by calcinating Mg3Al−CO3-LDH at 400 °C is 2.55 wt % at 200 °C.30 On the other hand, Hutson et al. first used ClO4− as a substitute for CO32− and found that the Mg− Al−ClO4-LDHs pretreated at 400 °C exhibited a surprising performance with a 15.6 wt % adsorption uptake at 330 °C in the CO2 equilibrium adsorption isotherms.31 Nevertheless, the adsorption capacities of the Mg-based Mg−Al-LDH powders are still relatively low because of their lower adsorption temperature (200−400 °C), which limits the extent of their Received: October 23, 2015 Revised: January 4, 2016

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DOI: 10.1021/acs.energyfuels.5b02506 Energy Fuels XXXX, XXX, XXX−XXX

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examined by XRD analysis. As shown in Figure 1, all of the LDH samples showed the same reflection peaks corresponding

applications. Therefore, there is a large need to synthesize hydrotalcite-derived mixed oxides with high CO2 adsorption capacities for high-temperature CO2 separation from flue gases. Although Ca-based LDHs have been considered promising precursors for capturing CO2, owing to their high reactivity with CO2,32−35 little research has focused on developing Cabased LDH nanoparticles using ClO4− as the interlayer anion. In this paper, Ca−Al−ClO4-LDHs were synthesized through a conventional co-precipitation method and used as precusors for high-temperature CO2 adsorption for the first time. Structural characteristics and morphology were evaluated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) measurements. A detailed investigation on the adsorption capacities and cyclic stabilities of the mixed oxides derived from Ca−Al−ClO4-LDHs was conducted based on TGA.

2. EXPERIMENTAL SECTION 2.1. Sorbent Synthesis. Ca−Al−ClO4 hydrotalcite-like precursors were prepared using a co-precipitation method. In a typical preparation, a mixed nitrate metal solution (0.5 M) with a Ca2+/Al3+ ratio of 2, 2.5, 3, 3.5, and 4 was quickly added to 0.5 M NaOH solution containing a proper amount of NaClO4 and then the mixture solution was held at room temperature under vigorous stirring for 30 min. Next, the slurry was transferred to autoclave for hydrothermal treatment at 100 °C for 10 h. The solid precipitate was obtained through filtering and washing the slurry, followed by drying the product at 80 °C overnight. Finally, the resulting hydrotalcites were calcined at 600 °C for 4 h in a muffle furnace to obtain the derived mixed oxides as CO2 sorbents. 2.2. Material Characterization. 2.2.1. XRD. The crystalline structure of the hydrotalcites and the derived mixed oxides were characterized by XRD using a Bruker D8 Focus operating at 40 kV and 40 mA equipped with nickel-filtered Cu Kα radiation (λ = 1.540 56 Å) and operating in a 2θ range of 5−90° at a scanning rate of 0.02°/s. 2.2.2. Inductively Coupled Plasma−Optical Emission Spectroscopy (ICP−OES). Elemental analysis of adsorbents was performed on ICP−OES (Vista-MPX, Varian) at a high-frequency emission power of 1.5 kW and a plasma airflow of 15.0 L/min (λCa = 396.847 nm, and λAl = 396.152 nm). 2.2.3. SEM. The microstructure of the CaO sorbents during multicycle carbonation/regeneration was chosen for SEM analysis. SEM/energy-dispersive X-ray (EDX) analysis was conducted on a Hitachi S4800 field emission microscope at 10.0 kV. 2.3. Sorbent Performance Measurement. N2, used as purge gas during the desorption period and as dilution gas during the carbonation period, was 99.99% pure. CO2 used for adsorption experiments had a purity of 99.99%. The concentration of carbon dioxide used for sorption tests was 50 vol % (50 vol % N2). Thermogravimetric adsorption of CO2 on mixed oxide and desorption were measured using STA449F3, Netzsch. A small amount of sorbent (∼10 mg) was heated to the carbonation temperature of 600 °C at a rate of 10 °C/min in the presence of a N2 atmosphere before the adsorption−desorption experiments and then kept at the same temperature for 10 min to remove all impurities and pre-adsorbed CO2. During the adsorption process, 50 mL/min CO2 reactant gas along with 50 mL/min N2 purge gas was passed over the sample pan. During the desorption process, 50 mL/min pure N2 was passed over the pan. The adsorption and desorption temperatures were 600 and 700 °C, respectively. The adsorption and desorption times were set at 45 and 20 min to allow for the sorbent to adsorb and desorb completely.

Figure 1. XRD spectra of Ca1−xAlx(OH)2(ClO4)x.

to the (003) and (006) planes at angles of 11° and 23°, respectively, indicating the well-formed crystalline-layered structure. The LDH structure, Ca1−xAlx(OH)2(ClO4)x·mH2O, consisted of brucite-like layers, in which Ca and Al cations were in close proximity and shared octahedral positions. The interlayer spacing was occupied by charge-compensating ClO4− anions and weakly bonded water molecules. Also, in the raw materials, there were weak peak reflections at ≈23.3°, 29.8°, 32.0°, 36.3°, 39.8°, 42.8°, 43.8°, 46.4°, 48.2°, 48.7°, 57.4°, and 61.0°, which were attributed to the crystal phase of CaCO3. It was probable that Ca2+ reacted with OH− to form Ca(OH)2, which could easily absorb CO2 in the air, thus forming the product of CaCO3. As the synthesized powders were calcined at 600 °C, the characteristic peaks of LDH disappeared and new phases were identified, as shown in Figure 2. This demonstrated that the layered structure was completely collapsed and finally disappeared after heat treatment at a high temperature. It was

3. RESULTS AND DISCUSSION 3.1. Material Characterization. 3.1.1. XRD Patterns. The crystal identities of the Ca−Al−ClO4-LDHs with different ratios of [Ca2+]/[Al3+] (2:1, 2.5:1, 3:1, 3.5:1, and 4:1) were

Figure 2. XRD spectra of Ca1−xAlx(OH)2(ClO4)x-derived mixed oxides. B

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Energy & Fuels noted that the crystal phase of CaO with diffraction peaks at about 32.4°, 37.6°, 54.3°, and 64.6° was detected. Peak reflections, attributed to CaCO3, were still observed on the mixed oxides with Ca/Al ratios of 2.5 and 3.0, which may have been a result of incomplete decomposition of CaCO3. However, the peak reflections belonging to the aluminum compounds were not observed in the XRD patterns, likely indicating that the aluminum compounds were well-dispersed or had formed an amorphous phase in the mixed oxides. Further, the crystallite size of the mixed oxides was calculated from the plane (200) using the Debye−Scherrer equation, as listed in Table 1. The crystallite sizes of CaO of all of the mixed oxides with different Ca/Al ratios were relatively small (about 30 nm) and did not exhibit notable differences. Table 1. Physical Properties and Grain Particle Size of Ca− Al-LDH-Derived Mixed Oxides Ca/Ala

2

2.5

3

3.5

4

Ca/Alb crystallite size (nm)c

1.96 29.7

2.16 29.4

2.66 29.2

1.94 30

1.95 35.4

a

Theoretical value of the Ca/Al ratio. bActual value of the Ca/Al ratio. Determined from the XRD peak of the CaO (200) plane by the Debye−Scherrer equation. c

3.1.2. ICP Analysis. The actual Ca/Al ratio of the mixed oxides derived from Ca1−xAlx(OH)2(ClO4)x was determined using ICP−OES. As listed in Table 1, the actual value of the 2:1 sample was similar to the theoretical value, while other samples had smaller actual Ca/Al ratios. This could be due to the difference in the precipitation constants of Ca and Al. In general, Al precipitates at around pH of 3.7, and Ca precipitates at pH more than 9. Hence, aluminum precipitates first, and then calcium precipitates.36 In addition, the incompatibility of the ionic sizes of Al and Ca (0.62 versus 1.06) would not make the additional Ca insert onto the layer of LDHs, thus resulting in the lower actual value of the Ca/Al ratio for high CaO content.32 Among them, the 3:1 mixed oxides had the highest Ca percentage content according to the ICP results. 3.1.3. SEM/EDX Measurements. The distribution of calcium and aluminum of the calcined LDHs was determined using SEM/EDX measurements. As presented in Figure 3, the first two diagrams were back-scattered electron images of the whole chemical element and the bright spots in the remaining three diagrams represented distributions of calcium, aluminum, and oxygen elements. It could be observed that the distributions of calcium and aluminum on the surface of the calcined LDHs were homogeneous, implying that CaO and Al2O3 were wellmixed. 3.2. CO2 Adsorption Performances of the Derived Mixed Oxides. 3.2.1. Effect of Ca/Al Ratios. The CO2 absorption capacities of sorbents with different ratios of [Ca2+]/[Al3+] (2:1, 2.5:1, 3:1, 3.5:1, and 4:1) were evaluated using a thermogravimetric analyzer. Figure 4 showed the CO2 adsorption capacity curves in the first cycle of the mixed oxides derived from Ca1−xAlx(OH)2(ClO4)x. It was observed that the 3:1 and 2.5:1 mixed oxides had the higher adsorption capacity among all of the mixed oxides. Notably, the 3:1 mixed oxide had the highest adsorption capacity of 29.0 wt %. The CO2 uptake values followed the trend of 3:1 > 2.5:1 > 2:1 ≈ 4:1 ≈ 3.5:1, suggesting that the CO2 adsorption capacity of the mixed oxides increased with an increasing actual Ca content. The incomplete decomposition of CaCO3 would produce more

Figure 3. SEM image of the Ca−Al-LDH-derived mixed oxides (Ca/ Al = 3) and EDX elementary mapping of Ca, Al, and O.

Figure 4. CO2 adsorption capacities of Ca1−xAlx(OH)2(ClO4)xderived mixed oxides in the first cycle.

active CaO in the process of heating to the adsorption temperature on TGA, thus making the sorbents show higher CO2 uptake. As shown in Figure 5, a slight decline in CO2 adsorption capacity could be observed on the 3:1 mixed oxides in the first 5 cycles, while the sorbents with Ca/Al ratios of 2, 2.5, 3.5, and 4 displayed no apparent capacity decrease. This indicated that high-dispersion Al2O3 played a large role in effectively reducing the sintering of CaO particles at high carbonation and calcination temperatures. When Al2O3 was dispersed into the CaO particles, Al2O3 particles would surround CaO particles and hinder mutual touching of CaO C

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Figure 5. CO2 adsorption capacity of Ca1−xAlx(OH)2(ClO4)x-derived mixed oxides with the number of cycles.

Figure 7. CO2 adsorption capacity of Ca1−xAlx(OH)2(ClO4)x-derived mixed oxides (Ca/Al = 3) with the number of cycles.

particles. Ensuring that the Al2O3 and CaO particles were mixed well would increase the anti-aggregation effect of CaO particles. 3.2.2. Influence of Adsorption and Desorption Temperatures. The CO2 adsorption performances of the mixed oxides derived from Ca1−xAlx(OH)2(ClO4)x (Ca/Al = 3) with different adsorption/desorption temperatures were displayed in Figure 6. It was observed that the adsorption capacities

CO2 adsorption capacity declined slightly at first but gradually stabilized after 6 cycles of carbonation and calcination. Overall, there was a 4 wt % CO2 adsorption capacity decrease after 50 cycles, indicating that the mixed oxides derived from Ca−AlLDHs displayed favorable stability for CO2 capture. According to the SEM/EDX results illustrated in Figure 3, CaO and Al2O3 binary oxides derived from Ca−Al−ClO4LDHs were well-mixed and distributed homogeneously, which was helpful for CO2 adsorption. As shown in Table 1, CaO nanoparticles of the 3:1 mixed oxides with an average diameter of 29.2 nm were synthesized. According to the CO2 adsorption theory, there are two stages of reaction: a fast reaction (chemical-reaction-controlled) stage and a slow reaction (diffusion-controlled) stage.3,4,6 During the initial reaction, the CaCO3 product layer would form and cover the surface. Subsequently, the layer would prevent further diffusion of CO2 into the sorbent to react with inside CaO particles, and the whole process would then be diffusion-controlled.3−5 Lu et al. compared the CO2 uptake capacity of CaO-based sorbents prepared using flame spray pyrolysis (FSP) and calcination (CAL) of selected calcium precursors. As a result of the nanostructure (30−50 nm grain size), the FSP-made sorbents performed better than all of those CAL-made sorbents with a bigger particle size of 400 nm.37 Thus, the sorbent would have increased functionality with a small particle size (e.g., 30−50 nm), because this would allow for carbonation to take place at the rapid reaction-controlled regime in contrast to the diffusion-controlled regime with larger sorbent particles. In conclusion, the mixed oxides derived from the Ca−Al−ClO4LDHs showed excellent stability after 50 cycles, with approximately 87% of the initial CO2 capture capacity retained because of the small particle size of the oxides and the fine dispersion of CaO. 3.2.4. SEM Images of Sorbent Morphology before and after Adsorption. Changes to the microstructure of the 3:1 sample during the multicycle carbonation/calcination process were also investigated. The SEM images of the mixed oxides before CO2 adsorption and after 50 cycles of adsorption were illustrated in Figure 8. It was observed that CaO particles grew and aggregated during the 50 cycles. However, the mixed oxides after multicycles showed a looser and more porous structure, probably resulting from the repeat occurrence of the reversible chemical reaction of CaO + CO2 ↔ CaCO3 as a

Figure 6. CO2 adsorption performances of the Ca1−xAlx(OH)2(ClO4)x-derived mixed oxides with the different adsorption and desorption temperatures (Ca/Al = 3) .

increased from 16.4 to 39.5 wt % as the temperature rose to 800 °C from 500 °C in the first cycle. Additionally, the sorbents showed favorable stability during the first 5 cycles even under the desorption temperature of 900 °C. On account of the coalto-SNG process proceeding in the temperature range of 350− 600 °C as well as saving energy, 600 °C was chosen as the adsorption temperature to investigate the CO2 capture stability of the sorbent. 3.2.3. Stability of the Sorbent. To further investigate the long-term stability of the mixed oxides derived from Ca−AlLDHs, the capacity performances of 50 carbonation/calcination cycles of the 3:1 mixed oxides were shown in Figure 7. The D

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Figure 8. SEM images of fresh adsorbent and adsorbent after 50 absorbent cycles: (A and C) fresh adsorbent and (B and D) adsorbent after 50 cycle runs.

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result of the molar volume difference between CaCO3 (34.1 cm3 mol−1) and CaO (16.9 cm3 mol−1).

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4. CONCLUSION Well-mixed Ca−Al oxide CO2 sorbents were synthesized by applying a co-precipitation method using Ca−Al−ClO4-LDHs as the precursors. SEM/EDX characterization showed that CaO and Al2O3 binary mixed oxides were well-dispersed as a result of the special structure of LDHs. The uniform distribution of CaO among Al2O3 prevented the CaO crystallite from growing and sintering, further resulting in favorable cyclical performances. The 3:1 mixed oxides performed best and showed good stability with a 4 wt % decrease of the CO2 adsorption capacity after 50 cycles at the adsorption and desorption temperatures of 600 and 700 °C, respectively. Its excellent cyclic performance was also attributed to the small particle size (∼30 nm) that allowed for CaO carbonation to take place as much as possible at the rapid reaction-controlled stage. These promising results suggest that the mixed oxides derived from LDHs can be used effectively in CO2 capture from flue gases and sorptionenhanced reaction processes.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-22-87401818. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the National Natural Science Foundation of China (NSFC, Grant U1462122), the Program for New Century Excellent Talents in University (NCET-13-0411), and the Program of Introducing Talents of Discipline to Universities (B06006) is gratefully acknowledged. E

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DOI: 10.1021/acs.energyfuels.5b02506 Energy Fuels XXXX, XXX, XXX−XXX