Al-Stabilized Double-Shelled Hollow CaO-Based Microspheres with

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Environmental and Carbon Dioxide Issues

Al-stabilized double-shelled hollow CaO-based microspheres with superior CO2 adsorption performance Shan Li, Jiaqi Feng, XiaoChen Kou, Yujun Zhao, Xinbin Ma, and Shengping Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01855 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 27, 2018

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Energy & Fuels

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Al-stabilized double-shelled hollow CaO-based microspheres

2

with superior CO2 adsorption performance

3

Shan Li, Jiaqi Feng, Xiaochen Kou, Yujun Zhao, Xinbin Ma, Shengping Wang*

4

Key Laboratory for Green Chemical Technology, School of Chemical Engineering

5

and Technology, Tianjin University; Collaborative Innovation Center of Chemical

6

Science and Engineering (Tianjin), Tianjin 300072, China

7

*Corresponding author: [email protected] (S. Wang)

8 9

ABSTRACT

10

Novel,

double-shelled

hollow,

Al-stabilized

CaO-based

11

successfully prepared through a facile, green method by using carbonaceous

12

microspheres as sacrificial templates. Both the morphological and adsorption

13

performance differences, between employing organic and inorganic calcium salt as

14

calcium precursors, to fabricate the double-shelled hollow microspheres were

15

exhibited. Derived from calcium acetate, the Al-stabilized double-shelled hollow

16

microspheres with denser shells showed excellent stability. This was ascribed to the

17

shells that were composed of small CaCO3 nano-crystallite (grain size < 50nm) and

18

homogeneously distributed stabilizer, Al2O3 . Well spherical, porous hollow structures

19

facilitated CO2 diffusion and buffered mechanical stresses generated by volume

20

change during the calcium looping. The sorbent, CaA40Al1, exhibited a favorable

21

high-temperature CO2 adsorption performance with the capacity of 0.40 g-CO2 /g-ads

22

over 60 carbonation/calcination cycles. 1

ACS Paragon Plus Environment

microspheres

were

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1. INTRODUCTION

2

In recent years, it has been noted that the excessive use of fossil fuels has brought

3

about a tremendous amount of CO2 emission, which is likely the main contributor to

4

global warming.1 To alleviate the impact of greenhouse gas emissions, CO2 capture

5

and sequestration (CCS) has become a viable solution to constrain CO2 release and

6

mitigate global climate changes.2 Nowadays, calcium looping process is considered to

7

be economically applicable for CO2 emission mitigation at high temperature, which is

8

based on the multi-cycle reversible reaction between CaO and CaCO3 to capture and

9

release CO2.3,4And calcium-based sorbents become the research focus for

10

post-combustion, high temperature CO2 capture, owing to the high theoretical CO2

11

adsorption capacity (44 g of CO2/56 g of CaO), their wide availability, the low cost of

12

the raw materials and their fast adsorption kinetics.5 However, the studies indicated

13

that during the carbonation/calcination of calcium looping, the performance of

14

CaO-based sorbents declined rapidly as a result of the blocking of pores and the

15

severe sintering of CaO particles.6 It is therefore critical to maintain the high

16

reactivity of CaO and mitigate the loss of sorption capacity for developing new

17

sorbents.

18

Hollow microsphere is a kind of microsphere with a cavity, which has attracted

19

intensive attention owing to the advantages such as the well-defined structure and

20

high specific surface area.7 CaO-based sorbents with hollow microsphere structure are

21

promising materials，of which the porous surface can facilitate the CO2 diffusion to

2


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enhance the capacity and kinetics during carbonation. In addition, the hollow structure

2

provides extra space to accommodate the molar volume change associated with the

3

repeated carbonation/calcination cycles. It is noted that multi-shelled hollow

4

microspheres possess a higher packing density compared with mono-shelled hollow

5

microspheres,7 as well as a much shortened diffusion path for CO2, which are

6

favorable for the improvement of the CO2 uptake performance of the CaO-based

7

hollow microspheres. Nevertheless, previous studies showed that CaO-based hollow

8

microspheres, prepared without any dopants, tended to collapse readily during

9

calcium looping, due to the unstable cake-like shell structure.8 Hence it is necessary to

10

introduce an inert refractory binder to enhance the sintering resistance of the shells,

11

further ensuring the overall structural stability of the sorbent.

12

Tammann temperature is the empirical temperature when materials start to sinter.

13

The Tammann temperature of CaCO3 (533℃) is always lower than the working

14

temperature of calcium-based sorbents, which is also the main reason that leads to the

15

rapid deactivating of calcium-based sorbents.9 The sintering-resistant performance of

16

CaO-based sorbents was improved by the incorporation of various inert materials with

17

high Tammann temperature into such sorbents (Al2O3,10-13 MgO,14,

18

TiO218). Among substantial refractory dopants researched, the incorporation of Al has

19

contributed to excellent CO2 adsorption durability of the sorbents. This is due to the

20

formation of thermodynamically and structurally stable inert alumina or calcium

21

aluminate binders.10-13 The inert Al2O3 or calcium aluminate binders can effectively

3


15

ZrO2,16,

17

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separate the CaO particles and thus act as a physical barrier to prevent the sintering

2

and aggregation of the CaO particles composing the shells. Significantly, both the

3

adsorption performance and the existing form of inert support of Al-modified

4

CaO-based sorbents were found to be influenced by the choice of calcium and

5

aluminum precursors.13

6

The research conducted herein developed double-shelled hollow, Al-stabilized,

7

CaO-based microsphere sorbents with outstanding CO2 adsorption stability and a high

8

CO2 capture capacity. The effect of the Ca/Al molar ratio and the precursor of calcium

9

salt on the CO2 adsorption capacity and stability, was evaluated. Thermogravimetric

10

analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM) and

11

transmission eletron microscopy (TEM) were all employed to identify critical

12

material factors that determine the sorption capacity and stability of the sorbents.

13

2. EXPERIMENTAL SECTION

14

2.1.

Preparation

of

Al-stabilized

double-shelled

hollow

15

microspheres. All reagents were analytical grade, purchased from Tianjin Kelmel

16

and used without further purification. The carbonaceous microspheres (CMSs) were

17

synthesized through the emulsion polymerization of sucrose under hydrothermal

18

conditions as described elsewhere.19 The diameters of the carbonaceous microspheres

19

were 1.5 µm. Taking the double-shelled hollow microsphere with a Ca/Al molar ratio

20

of 80:1 as an example, Ca(CH3COO)2 and Al(NO3)3 with a molar ratio of 80:1 were

21

dissolved in the solvent of 30 mL(water/ethanol=2:1, v/v) to obtain a precursor

4


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solution with the Ca2+ concentration of 2.0 M. 0.6 g of newly prepared CMSs were

2

dispersed in 30 mL of the precursor solution with an ultrasonic treatment of 20 min.

3

After a water bath for 8 hours at 30 °C, the resulting suspension was filtered, washed

4

once with water, and then dried at 80 °C overnight in an oven. The resultant brown

5

microspheres were heated in air to 400 °C at the rate of 2 °C/min and then the

6

temperature was raised to 500 °C at the rate of 1 °C/min and kept at 500 °C for 2 h.

7

Other products were synthesized by following a similar procedure. Particularly, taking

8

the samples with a Ca/Al molar ratio of 80:1 as examples, the samples prepared from

9

calcium acetate and Ca(NO3)2 were named as CaA80Al1 and CaN80Al, respectively.

10

2.2. Material characterization. 2.2.1 SEM. The surface morphology of the

11

double-shelled hollow microspheres was characterized using SEM with a Hatachi

12

S4800 field-emission microscope at 3.0 kV. The powder samples were sprayed with

13

gold for 120 s to ensure conductivity.

14

2.2.2 TEM. The detailed double-shelled hollow microsphere structure was observed

15

through TEM and a high-resolution transmission electron microscope (HRTEM)

16

operated at an accelerating voltage of 200 kV. The samples were ultrasonically

17

dispersed in ethanol and dropped onto a copper grid, and then the ethanol rapidly

18

evaporated.

19

2.2.3. XRD. XRD patterns were recorded on a Ragaku D/max-2500 diffractometer

20

with a Cu Kα (40 kV, 100 mA) radiation in the diffraction angle (2θ) range of 10-90°

21

at a scanning rate of 8°/min.

5


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2.2.4 Inductively Coupled Plasma−Optical Emission Spectroscopy (ICP−OES).

2

Elemental analysis of sorbents was performed on ICP−OES (Vista-MPX, Varian) at a

3

high-frequency emission power of 1.5 k W and a plasma airﬂow of 15.0 L/min (λCa=

4

396.847 nm, and λAl= 396.152 nm).

5

2.2.5 Nitrogen Adsorption−Desorption. Porosity characterization was determined

6

from N2 adsorption and desorption isotherms on a Micromeritics Tristar volumetric

7

adsorption analyzer, measured with N2 at −196 °C. The surface area was gained from

8

the Brunauer−Emmett−Teller (BET) equations. The total pore volume was measured

9

at the relative pressure (P/P0) of 0.995 and the pore size distribution measurement

10

was determined using the Barrett−Joyner−Halenda (BJH) method.

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2.3 Sorbent Performance tests. The CO2 adsorption and desorption tests of

12

the double-shelled hollow microspheres were conducted using NETZCH STA 449F3,

13

with the precision of 10-6 g. The test process was described as follows: approximately

14

10 mg of the sorbents were placed in an alumina sample pan. The carbonation process

15

was performed at 600 °C for 45 min in an atmosphere of 50 vol % CO2 (N2 balance,

16

50 mL/min), and a calcination process was carried out at 700 °C for 20 min in an

17

atmosphere of N2 (50 mL/min). The heating and cooling processes are conducted in

18

an atmosphere of N2 (50 mL/min) with a rate of 10 °/min. The above

19

carbonation/calcination process was repeated for 30 cycles and the corresponding

20

multi-cycle results were obtained. The CO2 carbonation capacities of the sorbents in

21

TGA tests were calculated on the basis of the mass change in the following equation:

6


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Carbonation capacity (%)=

× 100%

2 3

3. RESULTS AND DISCUSSION

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3.1 Hollow microspheres derived from inorganic salt. 3.1.1. SEM and

5

TEM Images of Hollow Microspheres Derived From Inorganic Salt. The hollow

6

Al-stabilized CaO-based microsphere sorbents were synthesized via a sacrificial

7

template method. CMSs were prepared as templates via the hydrothermal reaction of

8

sucrose. The mixed metal ions, Ca2+ and Al3+, using inorganic salt Ca(NO3)2 and

9

Al(NO3)3 as precursors, were absorbed and penetrated into the carbon spheres

10

template through the electrostatic force and the pore passages of CMSs. Subsequently,

11

the mixed metal ions, encapsulated in the carbon microspheres, underwent a

12

calcination treatment in air to disintegrate the CMSs. Due to the temperature gradient

13

along the radical direction and the different rates between the shrinkage of

14

carbonaceous particles and the metal precursors oxidation, the double-shelled

15

Al-stabilized CaO-based hollow microspheres structure is formed shell by shell as the

16

template gradually degrades. The graphical model of the formation process of the

17

Al-stabilized double-shelled hollow CaO-based microspheres was shown in Figure 1.

18

The Al-stabilized CaO-based hollow microspheres derived from Ca(NO3)2 with

19

different molar ratios of Ca2+/Al3+ (30:1, 40:1, 80:1) were labeled as CaN30Al1,

20

CaN40Al1, CaN80Al1, respectively. The SEM images of the fresh sorbents displayed

7


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in Figure 2 show that all the sorbents formed homogeneous hollow microspheres with

2

a diameter range of 800-1000 nm (based on above 100 microspheres).

3

The wall of the shells consisted of nano-sized grains and pores. The pores of

4

shells were advantageous for CO2 diffusion and further promoted the reaction

5

between CO2 and CaO grains inside the spheres. There were no wrinkles on the

6

surfaces of the shells, and all the microspheres had well spherical hollow structures,

7

which could buffer the mechanical stresses and further improve the durability of the

8

sorbents. The TEM images of all fresh sorbents derived from Ca(NO3)2 are shown in

9

Figure 3a-c. A double-shelled hollow microsphere structure is observed clearly for all

10

sorbents, and the thickness of such shells was about 80 nm. However, some hollow

11

microspheres were broken into pieces, meaning that the double-shelled hollow

12

structure derived from Ca(NO3)2 was not stable enough.

13

3.1.2. ICP-OES. The actual Ca/Al molar ratios of CaN30Al1, CaN40Al1 and

14

CaN80Al1 were listed in Table 1. The actual Ca/Al molar ratios were 7.2, 8.6 and

15

12.0 corresponding to CaN30Al1, CaN40Al1 and CaN80Al1, respectively. From the

16

result, it can be seen that the actual Ca/Al molar ratios were greater than the

17

theoretical values, indicating that Ca2+ was more readily absorbed by the CMSs than

18

Al3+.

19

3.1.3. XRD Patterns of Sorbents. The crystal identities of both fresh and spent

20

CaN30Al1 sorbents were determined by XRD analysis. As displayed in Figure 4, the

21

XRD pattern of fresh CaN30Al1 indicated that the CMSs templates were removed

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thoroughly after calcination, and that the hollow microspheres were composed of

2

CaCO3 and CaAl4O7. The formation process of crystalline phase of CaCO3 was that

3

during the calcination in the muffle oven, the initially formed CaO from the oxidation

4

of Ca2+ reacted with the CO2 produced by the burning of CMSs template. Since the

5

calcination temperature is 500 °C, CaCO3 would not decompose into CaO below this

6

temperature under atmospheric conditions. The formation of CaAl4O7 indicated that

7

CaO had reacted with Al2O3 during the calcination process. Furthermore, the

8

crystallite size of the sorbents was 67.1 nm which was calculated from the plane 104

9

at an angle of 29° using the Debye-scherrer equation. The grain size was within the

10

same range of the shells thickness, which indicated the shells were comprised of a

11

single layer of numerous nanoparticles. The XRD pattern of spent CaN30Al1

12

suggested that the inert stabilizer phase of the sorbents after 30 cycles was still

13

CaAl4O7. Furthermore, the crystalline size of the CaAl4O7 in the fresh and spent

14

CaN30Al1 sorbents were both calculated based on the Debye-scherrer equation,

15

respectively, which were 61.1 nm for fresh CaN30Al1 sorbent and 72.4 nm for the

16

sorbent after 30 cycles. So the crystalline size of CaAl4O7 after 30 cycles increased a

17

little compared with the fresh one. This is probably ascribed to the agglomeration of

18

particles along with the reaction proceeding.

19

3.1.4. Nitrogen Adsorption−Desorption. The N2 adsorption/desorption isotherms of

20

sorbents CaN30Al1, CaN40Al1 and CaN80Al1 were plotted in Figure 5, showing

21

the mixing feature of typeⅡand IV curves that unravel the coexistence of mesopores

9


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1

and macropores inside the shells. This is in line with the pore size distributions of

2

the three sorbents in Figure 6. Table 2 shows the textural properties such as the BET

3

specific surface area, pore volume and pore size of sorbents CaN30Al1, CaN40Al1

4

and CaN80Al1. From Table 2, it can be seen that CaN30Al1 and CaN40Al1 possess

5

similar specific surface areas and pore volumes, which were both greater than those

6

of CaN80Al1.

7

3.1.5. The CO2 uptake performance of sorbents. The multi-cycle CO2 sorption

8

capacities of CaN30Al1, CaN40Al1, CaN80Al1 sorbents were evaluated using a

9

thermogravimetric analyzer, and the results were illustrated in Figure 7. As displayed

10

in Figure 7, CaN80Al1 possessed the highest theoretical adsorption capacity, but

11

showed the lowest practical initial CO2 adsorption capacity (0.40 g-CO2/g-ads). This

12

resulted from the lowest specific surface area and pore volume relative to the other

13

two sorbents, based on the results listed in Table 1. CaN30Al1 and CaN40Al1 both

14

showed a higher initial CO2 capacity of 0.48 g-CO2/g-ads owing to the larger specific

15

surface areas and pore volumes compared with CaN80Al1. The sorbents with higher

16

specific surface area and pore volume can expose more active sites for the CO2

17

reaction and facilitate the diffusion of CO2, finally exhibited a higher initial CO2

18

uptake. It was also observed that the CO2 adsorption stability improved significantly

19

as the doping amount of inert Al increased. After 30 carbonation/calcination cycles,

20

sorbent CaN30Al1 still maintained 0.29 g-CO2/g-ads, which was almost the same

21

capacity as that of the second cycle of CaN80Al1. The lowest content of the inert

10


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stabilizer in sorbent CaN80Al1 resulted in the poorest structure stability and further

2

brought about the sharp decrease of CO2 adsorption capacity. Nevertheless, sorbent

3

CaN30Al1, still showed relatively inferior capacity retention due to the partial

4

sintering of CaO. A TEM image of the spent sorbent CaN30Al1 shown in Figure 3

5

reveals, after 30 cycles the double-shelled hollow microspheres have collapsed into

6

solid spheres and become agglomerated. From the HRTEM images of CaN30Al1

7

(Figure 8) that provided further insight into the structure, it could be observed that the

8

stabilizer was uniformly dispersed in the CaO particles, which avoided further

9

agglomeration of particles and structural collapsing. And the distance between the

10

adjacent planes of the stabilizer is measured to be about 0.207 nm, corresponding to

11

the (330) plane of CaAl4O7, which agrees with the XRD results. It is suggested by

12

some research that calcium aluminates could be further formed by the solid-solid

13

reactions between the stabilizer and CaO during carbonation/calcination cycles, which

14

could further decrease the adsorption capacity of the sorbents.13 These results

15

evidenced that the stabilizer was still CaAl4O7 and any solid-state reaction between

16

CaAl4O7 and CaO absolutely did not occur. Thus, the favorable CO2 uptake was

17

maintained after 30 cycles. As the stabilizer was well dispersed and there was no

18

occurrence of any further solid-state reaction, the inferior durability of sorbents

19

should be attributed to the unsteady double-shelled hollow microsphere structure.

20

Thus, being prepared from an inorganic salt, calcium nitrate, a relatively unstable

21

shell structure was formed due to the large grain size of CaO, thus contributing to the

11


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1

easy collapse of the structure.

2

3.2 Hollow microspheres derived from organic salt. It was found that

3

sorbent originated from Ca(CH3COO)2 exhibited higher CO2 uptake ability, which

4

was attributed to a low tortuosity in its pore system. This resulted in the easy

5

approach of CO2 to the active sites of the sorbent.20

6

In this work, double-shelled hollow, microsphere sorbents, stemmed from organic

7

salt calcium acetate were prepared, characterized and tested. As the solubility of

8

calcium acetate is lower than that of calcium nitrate, the concentration of the

9

precursor solution was decreased from 2M to 1M while keeping the Ca2+/Al3+ molar

10

ratios unchanged (80:1, 40:1, 30:1). They were named CaA30Al1, CaA40Al1,

11

CaA80Al1, respectively.

12

3.2.1. ICP-OES. The actual Ca/Al molar ratios of CaA30Al1, CaA40Al1,

13

CaA80Al1 were listed in Table 1. The actual Ca/Al molar ratios were 6.1, 7.5 and

14

10.7 corresponding to CaA30Al1, CaA40Al1, CaA80Al1, respectively. Comparing

15

the results with those of sorbents derived from Ca(NO3)2, it can be concluded that

16

under a same theoretical Ca/Al molar ratio, the actual Ca/Al molar ratios of the

17

sorbent derived from Ca(CH3COO)2 were a little lower than those from Ca(NO3)2.

18

3.2.2. SEM and TEM Images of Hollow Microspheres Derived From organic Salt.

19

The morphology and structure of the as-synthesized sorbents derived from calcium

20

acetate were investigated by SEM and TEM, by which several important features can

21

be discerned. As shown in SEM images (Figure 9), all the products were spherical in

12


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shape with a microsphere size range of approximately 600-800 nm. The shells of

2

sorbent CaA30Al1 were smoother than those of CaA40Al1 and CaA80Al1, while the

3

latter two had wrinkles and pores on the shells. In addition, the TEM images (Figure

4

10a-c) indicated that all the products exhibited an exclusive characteristic of a

5

double-shelled hollow microsphere structure with the shell thickness of approximately

6

80-90 nm.

7

During the calcium looping, CaCO3 and CaO mutually convert repeatedly. These

8

processes cause a molar volume change from 36.9 to 16.7 cm3/mol or conversely.21

9

This phenomenon causes the collapse of hollow microspheres and the sintering of

10

particles. To investigate the effect of volume change on the stability of the

11

double-shelled hollow structure, SEM images of fresh CaA30Al1 and the sorbents

12

after the first carbonation/calcination cycle were compared in Figure 11. Both

13

microspheres have a similar morphology and structure, but the hollow microspheres

14

didn’t show any collapse after calcination at 700 °C and only had a little decrease in

15

diameter. The diameters of CaO and CaCO3 hollow microspheres were 640 nm and

16

690 nm, respectively, which were analyzed based on 100 samples from the SEM

17

images. It indicated that the double shelled hollow microspheres had flexible shells

18

which could change diameter size during the calcium looping and buffer the

19

mechanical stresses in three dimensions.

20

3.2.3. XRD Patterns of Sorbents. The XRD patterns (Figure 12) confirmed that all

21

the sorbents were composed of CaCO3 without another peak occurring, implying that

13


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1

the CMSs templates have been removed completely by the calcination treatment in air.

2

Besides this, peak reflections belonging to the calcium aluminates or alumina were

3

not observed in the XRD patterns (Figure 12), indicating that the inert aluminum

4

component was well-dispersed or had formed an amorphous phase in the mixed

5

oxides.2 The HRTEM image (Figure 10 d, e) further disclosed that the nanoparticles

6

composing the shells were highly crystalline with interplanar spacing of 0.304 nm and

7

0.240 nm. The interplanar spacing corresponded to the (104) plane of CaCO3 and the

8

(311) plane of Al2O3, respectively. This evidenced that the formation of the

9

highly-dispersed stabilizer Al2O3, of which the particles might be too small to give any

10

good coherent X ray diffraction. Further, the CaCO3 crystallite sizes of the sorbents

11

were 42.6, 27.2, 41.9 nm, corresponding to CaA30Al1, CaA40Al1, CaA80Al1,

12

respectively, which was calculated from the plane (104) using the Debye-Scherrer

13

equation. It can be concluded that the crystalline sizes of CaCO3 derived from

14

calcium acetate were smaller than that of CaCO3 derived from calcium nitrate. The

15

nanoparticles presented uniform grain sizes far smaller than the shell thickness and

16

cross-linked with each other, which led to the formation of relatively more compact

17

and tougher shells. CaA40Al1 had the smallest grain size, which suggested that the

18

sorbents could expose the most active sites for the carbonation reaction.

19

With the addition of ethanol in the preparation process of Al-stabilized CaO-based

20

sorbents, using organic calcium salt as a precursor, some calcium acetate hydrolyzed

21

gradually with aluminum nitrate on the electronegative surface of CMSs. This formed

14


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1

slightly soluble aluminum acetate. Calcium acetate itself would hydrolyze as well.

2

Both hydrolyzation could favor the access of ions into the CMSs and store even more

3

ions to form more compact shells. Anions with large radii of the organic salt might be

4

responsible for the high dispersion of inert aluminum salt in the CMSs template,

5

which contributed to the formation of grains with smaller size subsequent to the

6

calcination process .12 Fine particle sizes and high dispersion of the stabilizer phase in

7

the CaO structure were beneficial to eliminate any solid reaction between Al2O3 and

8

CaO, which guaranteed the amount of active CaO.

9

3.2.4. Nitrogen Adsorption−Desorption. The N2 adsorption/desorption isotherms of

10

all sorbents from Ca(CH3COO)2 are exhibited in Figure 5, which also appear to be the

11

mixing of type Ⅱand IV curves that reveal the existence of both mesopores and

12

macropores inside the shells. Based on the pore size distributions of the three sorbents

13

presented in Figure 6, the pores of the above sorbents consisted of mesopores and a

14

small quantity of macropores. Compared with the pore distributions of the sorbents

15

from Ca(NO3)2, the sizes of the mesopores and the quantity of macropores of sorbents

16

derived from Ca(CH3COO)2 were smaller and fewer. The textural properties of

17

CaA30Al1, CaA40Al1, CaA80Al1 were revealed by N2 adsorption and desorption, of

18

which the results were shown in Table 2. It can be seen that the specific surface area

19

and pore volume of CaA40Al1 is the largest among the sorbents stemmed from

20

Ca(CH3COO)2, while those of CaA80Al1 is the smallest.

21

3.2.5. The CO2 uptake performance of sorbents. The CO2 adsorption performance

15


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

of sorbents CaA30Al1, CaA40Al1, CaA80Al1 over 30 cycles under a mild condition

2

(carbonation at 600 °C, calcination at 700 °C) was shown in Figure 13. It was

3

observed that the adsorption stability of hollow microspheres derived from organic

4

salt calcium acetate obviously surpassed those derived from inorganic salt precursor.

5

Albeit with the highest CaO content, CaA80Al1 showed the lowest initial capacity

6

analogous to CaA30Al1, ascribed to the smallest specific surface area and pore

7

volumes among the three sorbents based on the textural properties shown in Table 2.

8

The capacity of the sorbent CaA80Al1 decreased sharply because there were not

9

enough stabilizers to form a supporting skeleton to prevent the sintering of

10

nanoparticles. It is noteworthy that there exists an obviously long-term self-activation

11

process for CaA30Al1. CaA40Al1 demonstrated the highest initial adsorption

12

capacity and still maintained 0.42 g-CO2/g-ads after 30 cycles, which was almost

13

twice as high as that of CaA30Al1 (0.27 g-CO2/g-ads). The grain size (27.2 nm) of

14

sorbent CaA40Al1, was smaller than that of CaA30Al1 (42.6 nm) and CaA80Al1

15

(41.9 nm). This contributed to more exposal of active sites for the reaction with CO2.

16

Besides, according to the results shown in Table 2, CaA40Al1 possesses the largest

17

specific surface area and pore volume among the three sorbents originated from

18

Ca(CH3COO)2. This enables CaA40Al1 to provide the most active sites for CO2 to

19

react with CaO and promote the CO2 diffusion, leading to a higher conversion of the

20

carbonation reaction. Sorbents of CaA40Al1 were chosen to test the long-term CO2

21

adsorption durability of the double-shelled hollow microsphere derived from organic

16


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salt. After 60 carbonation/calcination cycles, CaA40Al1 still maintained a carbonation

2

capacity of 0.40 g-CO2/g-ads (Figure 14), showing that the CaA40Al1 owned both a

3

well-defined structure to accommodate the volume change and an appropriate amount

4

of stabilizer to prevent sintering.

5

Figure 15 displays the SEM images of sorbent CaA40Al after calcium looping

6

under the mild condition. As shown in Figure 15, the hollow microsphere structures of

7

sorbent CaA40Al1 were still maintained after 60 cycles, accompanied by trivial

8

collapse and agglomeration. Yet, the shells still maintained the appearance of being

9

porous and fluffy. From the broken shells in the image shown in Figure 15, a clear

10

double shell structure can be seen, which manifested the free volume of the hollow

11

microspheres could well buffer the mechanical strain during the cycles to maintain the

12

structure and a favorable capacity retention. Ascribed to such stable double-shelled

13

hollow microsphere structure with denser shells, the high CO2 capture capacities of

14

the sorbents were maintained over a multitude of carbonation/calcination cycles.

15

It was reported that the CO2 capture performance deteriorated when the carbonation

16

or calcination temperature increased.10 Calcination conditions have a major effect on

17

durability. Increasing the calcination temperature can aggravate the sintering and

18

lower the CO2 uptake.22 Due to the excellent stability of sorbents CaA30Al1 and

19

CaA40Al1, the sorbents were tested under a severe condition which were carbonated

20

at 700 °C, and regenerated at 850 °C, while keeping other adsorption performance

21

evaluation conditions constant. The CO2 adsorption performance of sorbents

17


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

CaA30Al1 and CaA40Al1 over 30 cycles under severe condition were shown in

2

Figure 16. Sorbent CaA30Al1 displayed an obvious self-activation phenomenon at the

3

initial 4 calcium looping cycles. As the reaction temperature elevated, the initial CO2

4

uptake of sorbents CaA30Al1 and CaA40Al1 (in cycle 4) increased to 0.43 and 0.48,

5

respectively. In addition, sorbents still showed excellent adsorption durability under

6

the severe condition, which indicated that double-shelled hollow microspheres

7

derived from organic salt with denser shells could well handle the mechanical stresses

8

brought by the volume changes, which gave rise to high structural stability and

9

enhanced CO2 adsorption performance.

10

Furthermore, CaA40Al1 was also evaluated under a more realistic condition. The

11

CO2 proportion in the carbonation atmosphere was lowered to 15% in N2 and the

12

calcinations was carried out in pure CO2 under 900 ℃, which were similar to the

13

realistic condition. As shown in the Figure 17, the initial capacity of CaA40Al1

14

decreased to 0.35 g-CO2/g-ads under the realistic condition, while under mild

15

condition the initial capacity is 0.45 g-CO2/g-ads. And after the second cycle, the

16

capacity decreased sharply over time. It can be seen that reaction condition of calcium

17

looping has greater effect on the capture performance on sorbents. In addition,

18

diameters of the double-shelled hollow spheres sorbents were less than 1 µm and their

19

mechanical strength was not guaranteed, both of which may retard their direct use in

20

the circulating fluidized beds. Consequently, the research work on how to directly

21

bring the sorbents into application in practical industrial process remains to be carried

18


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Energy & Fuels

1

on.

2

4. CONCLUSIONS

3

A series of Al-stabilized double-shelled hollow CaO-based microspheres were

4

synthesized by using different calcium precursors via a sacrificial templating method.

5

Both the existing form of inert support materials that could be either Al2O3 or

6

CaAl4O7 and the crystallize size of CaCO3 depended on calcium precursors used.

7

Hollow microspheres with denser and tougher shells derived from organic acid salt,

8

namely calcium acetate, showed better CO2 adsorption durability than from inorganic

9

salt, namely calcium nitrate. The double-shelled hollow microsphere structure could

10

facilitate CO2 diffusion and buffer mechanical stresses of volume change during the

11

calcium looping in three dimensions. Particularly, the sorbent CaA40Al1 prepared

12

from calcium acetate and aluminum nitrate showed the best performance for CO2 high

13

temperature capture. It had a favorable initial CO2 capacity of 0.45 g-CO2 /g-ads and a

14

final uptake of 0.40 g-CO2 /g-ads over 60 carbonation/ calcination cycles in a mild

15

condition, as well as an initial uptake of 0.50 g-CO2 /g-ads and a final uptake 0.43

16

g-CO2 /g-ads over 30 cycles under a severe condition. The obtained encouraging

17

results would open up a new avenue in developing multi-shelled hollow microspheres

18

sorbents for high temperature CO2 capture.

19 20

ACKNOWLEDGEMENTS

19


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Financial support by National Key R&D Program of China (2017YFB0603300), the

2

Program for New Century Excellent Talents in University (NCET-13-0411) and the

3

Program of Introducing Talents of Discipline to Universities (B06006) is gratefully

4

acknowledged.

5

6

REFERENCES

7

(1) Boothandford, M. E.; Abanades, J. C.; Anthony, E. J. et al. Carbon capture and

8

storage update. Energy Environ. Sci. 2014, 7 (1), 130-189.

9

(2) Wang, S.; Li, C.; Yan, S. et al. Adsorption of CO2 on mixed oxides derived from

10

Ca–Al–ClO4-Layered double hydroxide. Energy Fuels 2016, 30, 1217−1222.

11

(3) Blamey. J.; Anthony. E. J.; Wang. J. et al. The calcium looping cycle for

12

large-scale CO2 capture. Prog. Energy Combust. Sci. 2010, 36(2), 260-279.

13

(4) Perejón. A.; Romeo. L. M.; Lara. Y. et al. The Calcium-Looping technology for

14

CO2 capture: On the important roles of energy integration and sorbent behavior. Appl

15

Energy. 2016, 162, 787-807.

16

(5) Kenarsari, S. D.; Yang, D. L.; Jiang, G. D., et al. Review of recent advances in

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carbon dioxide separation and capture. RSC Adv. 2013, 3 (45), 22739-22773.

18

(6) Dou, B.; Wang, C.; Song, Y. et al. Solid sorbents for in-situ CO2 removal during

19

sorption-enhanced steam reforming process: A review. Renewable Sustainble Energy

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Rev. 2016, 53, 536-546.

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(7) Guan, B. Y.; Kushima, A.; Yu, L. et al. Coordination polymers derived general

2

synthesis of multishelled mixed metal-oxide particles for hybrid supercapacitors. Adv.

3

Mater. 2017, 29(17),1605902.

4

(8) Wang, S. P.; Shen, H.; Fan, S. S. et al. CaO-based meshed hollow spheres for CO2

5

capture. Chem. Eng. Sci. 2015, 135, 532-539.

6

(9) Zhao, M. ; Shi.J. ; Zhong, X. et al. A novel calcium looping absorbent

7

incorporated with polymorphic spacers for hydrogen production and CO2 capture.

8

Energy Environ. Sci. 2014, 7(10), 3291-3295.

9

(10) Stendardo, S.; Andersen, L. K.; Herce, C. Self-activation and effect of

10

regeneration conditions in CO2–carbonate looping with CaO–Ca12Al14O33 sorbent.

11

Chem. Eng. J .2013, 220, 383-394.

12

(11) Qin, C.; Liu, W.; An, H.; Yin, J.; Feng, B. Fabrication of CaO-based sorbents for

13

CO2 capture by a mixing method. Environ. Sci. Technol. 2012, 46(3), 1932-9.

14

(12) Radfarnia, H. R.; Iliuta, M. C. Metal oxide-stabilized calcium oxide CO2 sorbent

15

for multicycle operation. Chem. Eng. J. 2013, 232, 280-289.

16

(13) Zhou, Z.; Qi, Y.; Xie, M. et al. Synthesis of CaO-based sorbents through

17

incorporation of alumina/aluminate and their CO2 capture performance. Chem. Eng.

18

Sci. 2012, 74, 172-180.

19

(14) Wang, S.; Fan, L.; Li C. et al. Porous spherical CaO-based sorbents via

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PSS-assisted fast precipitation for CO2 capture. ACS Appl. Mater. Interfaces 2014, 6

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(20),18072-18077.

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(15) Daud, F. D. M.; Kumaravel, V.; Sreekantan S. et al. Improved CO2 adsorption

2

capacity and cyclic stability of CaO sorbents incorporated with MgO. New J. Chem.

3

2016, 40 (1), 231-237.

4

(16) Wang, Y.; Zhang, W.; Li, R. et al. Design of Stable Cage-like CaO/CaZrO3

5

Hollow Spheres for CO2 Capture. Energy Fuels 2016, 30 (2), 1248-1255.

6

(17) Ping, H. L.; Wu, S. F. CO2 Sorption Durability of Zr-Modified Nano-CaO

7

Sorbents with Cage-like Hollow Sphere Structure. ACS Sustainable Chem. Eng. 2016,

8

4 (4), 2047-2055.

9

(18) Peng, W.; Xu, Z.; Zhao, H. Batch fluidized bed test of SATS-derived

10

CaO/TiO2-Al2O3 sorbent for calcium looping. Fuel 2016, 170, 226-234.

11

(19) Sun, X.; Li. Y. Colloidal carbon spheres and their core/shell structures with

12

noble-metal nanoparticles. Angew. Chem. 2004, 43(5), 597–601.

13

(20) Martavaltzi, C. S.; Lemonidou, A. A. Development of new CaO based sorbent

14

materials for CO2 removal at high temperature. Microporous Mesoporous Mater. 2008,

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110 (1), 119-127.

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(21) Yancheshmeh M. S.; Radfarnia, H. R.; Iliuta, M. C. High temperature CO2

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sorbents and their application for hydrogen production by sorption enhanced steam

18

reforming process. Chem. Eng. J. 2016, 283, 420-444.

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(22) Zhao, M.; Bilton, M.; Brown, A. P. et al. Durability of CaO–CaZrO3 Sorbents for

20

High-Temperature CO2 Capture Prepared by a Wet Chemical Method. Energy Fuels

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2014, 28 (2), 1275-1283.

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3 4

Figure 1. The graphical model of the formation process of the Al-stabilized

5

double-shelled hollow CaO-based microspheres

6 7 8 9 10 11 12 13 14 15 16 17 18

23


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

Figure 2. SEM images of (a) CaN30Al1 (b) CaN40Al1 ( c) CaN80Al1

3 4

24


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1 2

Figure 3. TEM images of (a) CaN30Al1 (b) CaN40Al1 (c) CaN80Al1 (d) CaN30Al1

3

after 30 cycles

4 5 6 7 8

25


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Page 26 of 41

Table 1. The Ca/Al molar ratios of various sorbents Samples

Theoretical Ca/Al molar ratios

Practical Ca/Al molar ratios

CaN30Al1

30:1

7.2

CaN40Al1

40:1

8.6

CaN80Al1

80:1

12

CaA30Al1

30:1

6.1

CaA40Al1

40:1

7.5

CaA80Al1

80:1

10.7

2 3 4 5 6

26


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Energy & Fuels

1 2 3

Figure 4. XRD patterns of fresh sorbent CaN30Al1 and after 30 calcination / carbonation cycles

4 5 6 7 8 9 10

27


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

Figure 5. The N2 adsorption/desorption isotherms of various sorbents

3 4 5

28


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Figure 6. The pore size distribution of the sorbents

3 4 5 6 7 8 9 10 11 12 13

29


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Page 30 of 41

Table 2. The textural properties of various sorbents Sample CaN30Al1

BET specific surface area Pore volume (m2/g) (cm3/g） 19 0.166

Pore size (nm) 60

CaN40Al1

18

0.118

43

CaN80Al1

11

0.078

58

CaA30Al1

14

0.069

40

CaA40Al1

20

0.098

31

CaA80Al1

12

0.059

35

2 3 4 5 6 7 8

30


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Figure 7. CO2 adsorption capacity of double-shelled microsphere sorbents

3 4 5 6 7 8 9

31


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

Figure 8. HRTEM images of CaN30Al1 after 30 cycles

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

32


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Energy & Fuels

1 2

Figure 9. SEM images of (a) CaA30Al1 (b) CaA40Al1 (c) CaA80Al1

3 4 5 6 7 8 9 10 11 12 13 14 15 16

33


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

Figure 10. TEM images of (a) CaA30Al1 (b) CaA40Al1 (c) CaA80Al1 (d, e)

3

HRTEM images of CaA40Al1

4 5 6 7 8 9 10 11

34


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Energy & Fuels

a

b

1 2

Figure 11. SEM images of CaA30Al1 (a) fresh sorbent composed of CaCO3 (b) after

3

calcination at 700 °C composed of CaO

4 5 6 7 8 9 10 11

35


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

Figure 12. XRD patterns of all sorbent derived from calcium acetate

3 4 5 6 7 8 9 10 11 12 13 14

36


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Energy & Fuels

1 2

Figure 13. CO2 adsorption capacity of double-shelled microsphere sorbents derived

3

from organic salt at mild carbonation/calcination condition

4 5 6 7 8 9 10 11 12 13

37


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

Figure 14. Long term CO2 adsorption capacity of sorbents CaA40Al1 at mild

3

carbonation/calcination condition

4 5 6 7 8 9 10 11 12 13 14

38


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Figure 15. SEM images of CaA40Al1 after 60 cycles in mild condition

3 4 5 6 7 8 9 10 11 12 13 14 15 16

39


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

Figure 16. CO2 adsorption capacity of double-shelled microsphere sorbents derived

3

from organic salt at severe carbonation/calcination condition

4 5 6

40


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Figure 17. CO2 adsorption capacity of CaA40Al1 tested under more realistic

3

condition (carbonation: 600 ℃, 15% CO2 in N2, 45min, calcinations: 900℃, 100% CO2,

4

20min)

41


Al-Stabilized Double-Shelled Hollow CaO-Based Microspheres with

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