Ultrafast and Stable CO2 Capture Using Alkali Metal Salt-Promoted

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Ultrafast and Stable CO2 Capture Using Alkali Metal Salt-Promoted MgO-CaCO3 Sorbents Hongjie Cui, Qi-Ming Zhang, Yuanwu Hu, Chong Peng, Xiangchen Fang, Zhen-Min Cheng, Vladimir V. Galvita, and Zhiming Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05829 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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ACS Applied Materials & Interfaces

Ultrafast and Stable CO2 Capture Using Alkali Metal Salt-Promoted MgO-CaCO3 Sorbents

Hongjie Cui,† Qiming Zhang,† Yuanwu Hu,† Chong Peng,‡ Xiangchen Fang,‡ Zhenmin Cheng,† Vladimir V. Galvita§ and Zhiming Zhou†,*

†

State Key Laboratory of Chemical Engineering, East China University of Science and

Technology, Shanghai 200237, China. ‡

§

Dalian Research Institute of Petroleum and Petrochemicals, SINOPEC, Dalian 116045, China. Laboratory for Chemical Technology, Ghent University, Technologiepark 914, B-9052 Gent,

Belgium.

*Corresponding author: [email protected]

1

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

ABSTRACT As a potential candidate for pre-combustion CO2 capture at intermediate temperatures (200-400 °C), MgO-based sorbents usually suffer from low kinetics and poor cyclic stability. Herein, a general and facile approach is proposed for the fabrication of high-performance MgO-based sorbents via incorporation of CaCO3 into MgO followed by deposition of a mixed alkali metal salt (AMS). The AMS-promoted MgO-CaCO3 sorbents are capable of adsorbing CO2 at an ultrafast rate, high capacity and good stability. The CO2 uptake of sorbent can reach as high as above 0.5 gCO2 gsorbent-1 after only 5 min of sorption at 350 °C, accounting for vast majority of the total uptake. In addition, the sorbents are very stable even under severe but more realistic conditions (desorption in CO2 at 500 °C), where the CO2 uptake of the best sorbent is stabilized at 0.58 gCO2 gsorbent-1 in 20 consecutive cycles. The excellent CO2 capture performance of the sorbent is mainly due to the promoting effect of molten AMS, the rapid formation of CaMg(CO3)2 and the plate-like structure of sorbent. The exceptional ultrafast rate and good stability of the AMS-promoted MgO-CaCO3 sorbents promise high potential for practical applications such as pre-combustion CO2 capture from integrated gasification combined cycle plants and sorption-enhanced water gas shift process.

KEYWORDS MgO sorbent, CO2 capture, alkali metal salts (AMS), ultrafast sorption rate, high cyclic stability

2


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1. INTRODUCTION It is widely accepted that the increased atmospheric concentrations of greenhouse gases, especially CO2, contribute greatly to the global warming.1-3 On the one hand, anthropogenic CO2 emissions during 1970-2000 grew with an average rate of 1.72% per year, which increased to 2.6% in the period 2000-2014 and indeed 2.75% during 2010-2014,4 exhibiting a year-on-year increase in this century. On the other hand, fossil fuel fired power plants account for the largest proportion of anthropogenic CO2 emissions.5,6 Therefore, reduction of CO2 emissions from power plants via CO2 capture and storage becomes urgent. Among various CO2 capture techniques applied to power plants, pre-combustion capture has attracted much attention as it can not only generate electricity but also yield high-purity H2 for chemical, refining and other uses.7-9 A typical example of pre-combustion CO2 capture is the integrated gasification combined cycle where CO2 produced through coal gasification followed by water gas shift reaction is needed to be removed. In this case, capture of CO2 at intermediate temperatures (200-400 °C) is preferred over low-temperature aqueous amine systems due to the high energy penalty and capital costs of the latter.10-14 Intermediate temperature CO2 solid sorbents mainly include layered double hydroxides and MgO,12 and the latter can capture CO2 with a very high theoretical capacity (1.1 gCO2 gMgO-1). However, the actual CO2 capture capacity of pure MgO is normally no more than 0.1 gCO2 gMgO-1.12,13 Recent studies on MgO-based sorbents have focused on alkali metal salt-promoted MgO (AMS-MgO).15-18 Alkali metal nitrates such as LiNO3, NaNO3 and KNO3 have been reported to improve the CO2 uptake of MgO via decreasing the energy for activation of the MgO ionic bond and increasing the ion diffusivity that ultimately derive from the ability of molten nitrates to partially dissolve bulk MgO (alkali metal nitrates become molten in the temperature range of 300 to 350 °C commonly used for CO2 sorption on MgO).19,20 3



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Table 1. Comparison of Different AMS-Promoted MgO-Based Sorbentsa Alkali metal salt

MgO content [wt%]

Temperature [°C] / time [min] / atmosphere Sorption

Regeneration

Number of cycles

CO2 uptake after 5 Total CO2 uptake min of sorption (1st/10th/last cycle) st (1 cycle) [gCO2 gsorbent-1] [gCO2 gsorbent-1]

Ref

LiNO3-(Na,K)NO2

72

340/60/CO2

450/30/N2

20

0.06

0.61/0.53/0.53

15

(Li,K)NO3

66

350/60/CO2

500/10/N2

20

0.18

0.46/0.37/0.32

17

(Li,Na)NO3-Na2CO3

75

325/10/CO2

425/5/N2

30

0.25

0.40/0.33/0.26

18b

(Li,Na)NO3-Na2CO3

75

300/10/29% CO2

450/5/CO2

30

—

0.27/0.18/0.13

18b

NaNO3-Na2CO3

77

360/90/CO2

400/60/N2

30

0.11

0.63/0.37/0.28

21

NaNO3-Na2CO3

59

325/60/CO2

450/10/N2

14

0.03

0.45/0.30/0.26

22

(Li,Na,K)NO3

83

300/30/CO2

350/30/N2

20

0.13

0.38/0.18/0.14

23

(Li,K)NO3-(Na,K)2CO3

73

350/20/CO2

400/15/N2

30

0.23

0.70/0.48/0.40

24

NaNO3-NaNO2

82

350/30/85% CO2

400/20/N2

15

0.21

0.81/0.39/0.38

25

NaNO3-NaNO2

82

350/30/85% CO2

450/20/CO2

15

—

0.81/0.32/0.32

25


73

350/45/CO2

400/15/N2

50

0.28

0.72/0.69/0.66

this work


70

350/45/CO2

400/15/N2

50

0.64

0.70/0.68/0.65

this workc


59

350/45/CO2

400/15/N2

50

0.54

0.57/057/0.57

this workd


66

350/45/CO2

500/5/CO2

20

0.57

0.65/0.61/0.58

this worke

a

All the sorbents presented here had an initial CO2 capture capacity higher than 0.25 gCO2/gsorbent and experienced

as least 10 sorption-regeneration cycles. b-e CaCO3 was doped into MgO and its content was 9.4, 3.6, 16.4 and 8.7 wt% for the sorbents presented in b, c, d and e, respectively.

Compared to pure MgO, AMS-MgO has much higher capacity, usually above 0.25 gCO2 gsorbent-1.15,17,18,21-25 Very recently, a surprisingly high capacity of 0.87 gCO2 gsorbent-1 (about 96% conversion of MgO) was claimed over a (NaNO3-NaNO2)-MgO.25 Unfortunately, however, most of the AMS-MgO sorbents reported so far suffer from slow kinetics and/or poor stability, which limit their practical applications. The slow kinetics is manifested by the slow sorption rate during the initial several minutes. For example, Harada and Hatton15 prepared a [LiNO3-(Na,K)NO2]-MgO sorbent whose capacity was decreased from 0.61 to 0.53 gCO2 gsorbent-1 over 20 cycles, indicating high capacity, but its sorption rate was slow, with less than 0.1 gCO2 gsorbent-1 after 5 min of sorption. To date, the CO2 uptake of the AMS-MgO sorbents after 5 min of sorption is in the range of 0.03 to 4


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0.25 gCO2 gsorbent-1, as summarized in Table 1. On the other hand, the AMS-MgO sorbents with relatively high CO2 capture capacity are usually subject to a rapid decay in capacity in cyclic operation. For instance, the capacity of the aforementioned (NaNO3-NaNO2)-MgO dramatically dropped to 0.38 gCO2 gsorbent-1 after 15 cycles.25 Indeed, only after 10 cycles under mild conditions (the desorption step was carried out in N2) did the capacities of these sorbents decrease by 13-53 % relative to the first cycle (Table 1, entries 1-3 and 5-9). When severe conditions (the desorption step was conducted in CO2) were applied in few studies, the capacity of sorbent after 10 cycles was reduced by 33% (Table 1, entry 4)18 and 60% (Table 1, entry 10)25. It is therefore still a challenge to develop novel AMS-MgO sorbents with high capacity, rapid sorption rate and good cyclic stability. Herein, we present a simple but general strategy to prepare AMS-promoted MgO-based sorbents that possess ultrafast rate, high capacity and good stability for CO2 capture at intermediate temperatures, wherein CaCO3 is incorporated with MgO followed by deposition of AMS. This strategy is first applied to a delicate flower-like MgO synthesized by a sophisticated procedure, and then extended to a plate-like MgO prepared by a facile method using a cheap magnesium source. In addition, by probing the structure-property interplay of the sorbents, the mechanism for the improved CO2 capture performance is examined.

2. EXPERIMENTAL SECTION 2.1. Materials. LiNO3 (≥ 96.7%), KNO3 (≥ 99.0%), Na2CO3 (≥ 99.8%), K2CO3 (≥ 99.0%), and NH3·H2O (25-28 wt%) were purchased from Sinopharm Group Chemical Reagent Co., Ltd. Magnesium acetate tetrahydrate (Mg(Ac)2·4H2O, ≥ 99.0%), methanol (≥ 99.5%), ethanol (≥ 99.7%), and ethylene glycol (≥ 99.0%) were obtained from Shanghai Titan Scientific Co., Ltd. Calcium lactate pentahydrate (C6H10O6Ca·5H2O, ≥ 99%) and polyvinylpyrrolidone (PVP, average molecular weight of 58000) were purchased from Aladdin Industrial Co., Ltd. Basic magnesium carbonate (> 5



98%) was purchased from Adamas Reagent, Ltd. All chemicals were used as received. 2.2. Preparation of MgO. The flower-like MgO support was prepared using a method developed by Song et al.26,27 First, 0.664 g of PVP was dissolved in 80 mL of ethylene glycol by vigorous stirring for 45 min, after which 0.1 mL of ammonium hydroxide was added dropwise to the mixture. Next, 0.856 g of Mg(Ac)2·4H2O was introduced into the above solution and after stirring for another 45 min, the obtained clear solution was transferred to a 100 mL Teflon-lined autoclave and maintained at 180 °C for 8 h. The resulting suspension was centrifuged and the white precipitate was washed four times with ethanol. Finally, the precipitate was dried under vacuum at 60 °C for 5 h, followed by calcination in N2 at 500 °C for 1 h (from room temperature to 500 °C at 2 °C min-1) and in air at 500 °C for another 2 h. The as-prepared MgO was denoted as Mg100. In addition, the plate-like MgO support, i.e. Mg(B)100, was obtained by calcination of basic magnesium carbonate at 500 °C in air for 2 h. 2.3. Preparation of MgO-CaCO3. In a typical synthesis, 0.162 g of C6H10O6Ca·5H2O was first dissolved in 80 mL of methanol by vigorous stirring for 10 min, and then 0.4 g of the above-prepared MgO support was added into the mixture under stirring for 1 h. Next, the above suspension was slowly evaporated under reduced pressure using a rotary evaporator at 60 °C. Finally, the resulting white powder was calcined in air at 600 °C for 1 h (from room temperature to 600 °C at 2 °C min-1). The as-prepared sample was composed of MgO and CaCO3 (evidenced by XRD analysis in the following section), with a MgO/CaCO3 molar ratio of 0.95:0.05. For simplicity, this sample was denoted as Mg95Ca5. By varying the amounts of C6H10O6Ca·5H2O and MgO added during preparation, the molar ratio of MgO to CaCO3 can be adjusted. In this work, three such MgO-CaCO3 supports, i.e., Mg90Ca10, Mg95Ca5 and Mg98Ca2, were prepared. Additionally, Mg(B)95Ca5 and Mg(B)80Ca20 prepared using Mg(B)100 were obtained by the same procedure. 6


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2.4. Preparation of Sorbents. The alkali metal salt (AMS)-promoted MgO-based sorbents were prepared by deposition of alkali metal nitrates and carbonates onto the above-prepared MgO-based supports using a solvent evaporation method.15,24 According to our previous study,24 a mixed alkali metal salt composed of LiNO3, KNO3, Na2CO3 and K2CO3 with a molar ratio of (Li,K)NO3 to (Na,K)2CO3 of 2:1, a molar ratio of LiNO3 to KNO3 of 0.44:0.56, and a molar ratio of Na2CO3 to K2CO3 of 0.5:0.5, can greatly promote the CO2 capture of MgO. In addition, the optimal molar ratio of the mixed alkali metal salt to MgO was found to be 0.15:1. Thus, the above composition of the mixed alkali metal salt and the molar ratio of the salt to the support were still used in this study. In a typical procedure, 0.22 mmol of LiNO3, 0.28 mmol of KNO3, 0.125 of mmol Na2CO3 and 0.125 of mmol K2CO3 were first mixed in 80 mL of methanol and ultrasonicated for 2 h, after which 0.215 g (or 5 mmol) of Mg95Ca5 was added to the solution with vigorously stirring for 6 h. Next, the excess methanol was evaporated in a rotary evaporator under reduced pressure at 60 °C. Finally, the resulting white powder was dried in an oven at 110 °C for 12 h. The as-prepared sorbent was denoted as AMS-Mg95Ca5. 2.5. Evaluation of Sorbents. The CO2 capture performance of sorbent was evaluated using a thermogravimetric analyzer (TGA, WRT-3P, Shanghai Precision & Scientific Instrument Co., Ltd.) at ambient pressure. In each test, about 5 mg of fresh sorbent was placed in a platinum basket and preheated to 400 °C under N2 (50 mL min-1) for 30 min to remove possible adsorbed H2O and CO2. The sorbents were tested in mild and severe conditions. In the mild condition, multiple sorption-desorption cycles were performed alternatively between 350 °C in CO2 for 45 min and 400 °C in N2 for 15 min, with a heating/cooling rate of 10 °C min-1, while in the severe condition, the desorption process was carried out at 500 °C in CO2 for 5 min, with a heating or cooling rate of 30 °C min-1. The CO2 uptake of sorbent and the conversion of MgO are calculated as follows: 7



CO uptake =

Conversion of MgO =

, g /g '()* %

∙'

+*

× 100%

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(1) (2)

where W0 is the initial sorbent weight, Wt is the sorbent weight at time t, x is the weight fraction of MgO in the sorbent, and MMgO and MCO2 are the molar weights of MgO and CO2, respectively. 2.6. Sorption Kinetics. The sorption kinetic experiments were carried out using TGA in CO2 at 260-320 °C (away from the thermodynamic equilibrium). According to the experimental results, the CO2 sorption process over the AMS-promoted MgO-based sorbent is divided into two stages: the surface reaction (SR)-controlled stage and the product layer diffusion (PLD)-controlled stage. This behavior, also observed for other solid sorbents such as Li4SiO428,29 and CaO,30-34 can be described by a well-known double exponential model as follows,35 0 = 12 + 1 exp5−72 89 + 1: exp5−7 89

(3)

where α is the conversion of MgO, t is the time, k1 and k2 are the exponential constants in the SRand PLD-controlled stages, respectively, and A1, A2 and A3 are the pre-exponential factors. The five unknown parameters, i.e., k1, k2, A1, A2 and A3, are estimated by the Levenberg-Marquardt algorithm, which minimizes the sum of squared residuals between the measured and fitted conversions. Next, the activation energies involved in different stages are estimated using the Arrhenius equation, 7 = 7; ?−

@A

BC

D

(4)

where k is the rate constant, k0 is the preexponential factor, Ea is the activation energy, T is the temperature and R is the universal gas constant.

3. RESULTS AND DISCUSSION 3.1. Flower-Like AMS-Promoted MgO-CaCO3 Sorbent. Figure 1 displays the FESEM and HRTEM images of hierarchical MgO (Mg100) and MgO-CaCO3 supports (Mg100-xCax). The porous Mg100 (Figure 1a-d) has a flower-like microsphere structure comprised of nanoflakes with a 8


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thickness of several nanometers, which after doping with CaCO3, i.e., Mg100-xCax, loses the spherical morphology but the nanoflake-assembled flower-like structure is still retained (Figure 1e-l). The flower-like architecture is mainly formed by an Ostwald ripening process.26,27 XRD patterns of Mg100 and Mg100-xCax samples (Figure 2a) indicate that Mg100 only contains MgO (JCPDS No. 75-0447), while Mg100-xCax consists of MgO and CaCO3 (JCPDS No. 47-1743). The specific surface area and pore volume of Mg100-xCax, which decrease with increasing the amount of CaCO3, are much smaller than those of Mg100 (Table S1). Furthermore, different from Mg100 with a unimodal pore-size distribution (PSD) centered at about 4 nm of pore diameter, the Mg100-xCax supports exhibit bimodal PSD curves with one peak at 3-4 nm and the other at 10-30 nm (Figure S1). The change in textural property after doping with CaCO3 is in line with the relatively larger particles and pores appeared in the nanoflakes of Mg100-xCax, indicating that CaCO3 incorporates with MgO.

Figure 1. (a, b, e, f, i-l) FESEM and (c, d, g, h) HRTEM images of supports: (a-d) Mg100; (e-h) Mg95Ca5; (i, j) Mg98Ca2; (k, l) Mg90Ca10. 9



a

1- MgO; 2- CaCO3

1

1

Mg100

1

Mg98Ca2

Intensity (a.u.)

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

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1

1

2

Mg95Ca5 Mg90Ca10

2

2

22

b

1 5

3- Na2CO3; 4- K2CO3; 5- KNO3

3,5 55 1 3 5

53 5

1

5

3 3

2

AMS-Mg100 AMS-Mg98Ca2 AMS-Mg95Ca5

4

10

20

30

2

2,3

22

40 50 60 2θ (degree)

AMS-Mg90Ca10

70

80

Figure 2. XRD patterns of supports (Mg100, Mg98Ca2, Mg95Ca5, and Mg90Ca10), and sorbents (AMS-Mg100, AMS-Mg98Ca2, AMS-Mg95Ca5, and AMS-Mg90Ca10).

Figure 3. (a, b, e, f, i-l) FESEM, (c, g) HRTEM and (d, h) EDS elemental mapping images of sorbents: (a-d) AMS-Mg100; (e-h) AMS-Mg95Ca5; (i, j) AMS-Mg98Ca2; (k, l) AMS-Mg90Ca10.

The flower-like architecture is well maintained in the AMS-promoted MgO-based sorbents, i.e., AMS-Mg100 (Figure 3a-d), AMS-Mg95Ca5 (Figure 3e-h), AMS-Mg98Ca2 (Figure 3i,j) and AMS-Mg90Ca10 (Figure 3k,l), but with more smooth surfaces than the supports, implying the deposition of AMS on the support surface. The presence of the AMS species such as Na2CO3 10


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(JCPDS No. 37-0451), K2CO3 (JCPDS No. 49-1093) and KNO3 (JCPDS No. 70-0205) on the sorbents are evidenced by the XRD analysis (Figure 2b), but LiNO3 is not observed probably due to its high dispersion on the support. Similar to the PSD of supports, all AMS-Mg100-xCax are characteristic of bimodal PSD, whereas AMS-Mg100 displays a unimodal PSD (Figure S2). The specific surface area and pore volume of sorbents (Table 2) are smaller than those of supports, confirming the successful deposition of AMS. The EDS mapping of various metals in AMS-Mg100 (Figure 3d) and AMS-Mg95Ca5 (Figure 3h) indicates the homogeneous distribution of the AMS species on the support. The above analyses clearly demonstrate the reliability of the preparation method for AMS-promoted MgO-CaCO3 sorbents. Table 2. Textural Properties of the AMS-Promoted MgO-Based Sorbents MgO contenta [wt%]

CaCO3 contenta [wt%]

BET surface area [m2 g-1]

Pore volumeb [cm3 g-1]

Average pore diameterb [nm]

AMS-Mg100

73.0

−

84.9

0.34

15.9

AMS-Mg98Ca2

70.0

3.6

43.3

0.21

24.3

AMS-Mg95Ca5

65.7

8.7

38.0

0.21

25.0

AMS-Mg90Ca10

59.2

16.4

36.3

0.18

20.4

AMS-Mg(B)95Ca5

65.7

8.7

29.9

0.16

26.1

Sorbent

a

Referring to the nominal content, but indeed also representing the actual content because no operations that may result in the loss of species were performed during the preparation process. b Determined from the adsorption branch using the Barrett-Joyner-Halenda (BJH) method.

The CO2 capture performance of the above flower-like sorbents are first evaluated under a mild condition where the CO2 sorption step is carried out in pure CO2 at 350 °C for 45 min while the desorption step in pure N2 at 400 °C for 15 min. As presented in Figure 4, all sorbents have high CO2 capture capacity and excellent stability: for AMS-Mg100, AMS-Mg98Ca2, AMS-Mg95Ca5 and AMS-Mg90Ca10, the initial capacity is 0.72, 0.70, 0.65 and 0.57 gCO2 gsorbent-1, respectively, which after 50 consecutive sorption-desorption cycles stabilizes at 0.66, 0.65, 0.61 and 0.57 gCO2 gsorbent-1, respectively, corresponding to a decay in capacity of 8.3, 7.1, 6.2 and 0%, respectively. It appears that the doping of MgO with CaCO3 improves the cyclic stability of AMS-promoted MgO-based 11



sorbent but decreases the CO2 capture capacity owing to the reduced content of MgO in the sorbent (Table 2). All spent sorbents after multiple cycles show particle agglomeration and sintering, together with shapes different from those of fresh sorbents (Figure 5): irregular shapes for AMS-Mg100 and AMS-Mg98Ca2, and plate shapes for AMS-Mg95Ca5 and AMS-Mg90Ca10. The shape evolution of these sorbents during cyclic operation indicates that the unique flower-like structure is not the prerequisite for high-performance AMS-promoted MgO-based sorbents. 1.0

AMS-Mg100

0.8

1st cycle

0.6

0.5

0.2

0.2

0.0

AMS-Mg98Ca2

0

1

2

0.0

0.5

25th cycle

0.6 0.2

0.0

0.4

0.1

AMS-Mg95Ca5

0.2

0.0

0.5

0

1

2

0.0 50th cycle

0.6

0.0

CO2 uptake (gCO2 gsorbent-1)

0.4

0.1

0.0 CO2 uptake (gCO2 gsorbent-1)

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

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0.2

AMS-Mg90Ca10

0.4

0.1

0.5

0.2

0.0 0

1

2

0.0

0.0 0

1000 2000 3000 0 Time (min)

10

20 30 40 Time (min)

50

Figure 4. CO2 uptake profiles of various sorbents over 50 sorption-desorption cycles under a mild condition (sorption at 350 °C in pure CO2 for 45 min; desorption at 400 °C in pure N2 for 15 min) and at the first, 25th and 50th cycle (inset: initial 2 min of sorption).

Figure 5. FESEM images of spent AMS-Mg100, AMS-Mg98Ca2, AMS-Mg95Ca5, and AMS-Mg90Ca10 under a mild condition (sorption at 350 °C in pure CO2 for 45 min; desorption at 400 °C in pure N2 for 15 min). 12


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Besides the improved stability, another advantage of AMS-Mg100-xCax over AMS-Mg100 is the greatly enhanced sorption rate. As shown in Figure 4, the CO2 uptake for AMS-Mg100 after 5 min of sorption (indicated by the dashed line) at the first cycle is 0.28 gCO2 gsorbent-1, which dramatically increases to 0.64, 0.60 and 0.54 gCO2 gsorbent-1 for AMS-Mg98Ca2, AMS-Mg95Ca5 and AMS-Mg90Ca10, respectively, accounting for about 91, 92 and 95% of the total uptake in 45 min, respectively. The rate enhancement also reflects from spent sorbents, e.g., the 25-cycle or 50-cycle used sorbents. To the best of our knowledge, this is the highest rate reported to date for the MgO-based sorbents (Table 1). The fast kinetics makes this type of sorbents particularly valuable for applications that require CO2 to be removed quickly, e.g., the sorption-enhanced water gas shift process where the in situ CO2 removal must be quick enough to ensure the high-purity hydrogen production.36 The rate enhancement has a close association with the CaCO3 content in the sorbent. As displayed in the inset of Figure 4, the initial rate during the first 2 min of sorption increases with increasing the CaCO3 content from 0 to 16.4 wt%, highlighting the significant role of CaCO3. It is worth of note that the ultrafast rate is achieved at the sacrifice of CO2 capture capacity. However, even then the final capacity of AMS-Mg100-xCax after 50 cycles of repeated sorption and desorption is still higher than that we have found in the literature concerning the AMS-promoted MgO-based sorbents (Table 1). In situ XRD analysis of sorbent during CO2 sorption affords a better understanding of the basics of the enhanced kinetics of CaCO3-dopant sorbents. Figure 6a shows that Na2Mg(CO3)2 (JCPDS No. 24-1227), K2Mg(CO3)2 (JCPDS No. 33-1495) and MgCO3 (JCPDS No. 08-0479) are formed on AMS-Mg100, in accordance with our previous study on nanosheet AMS-MgO.24 By contrast, in addition to the products mentioned above, another double salt, CaMg(CO3)2 (JCPDS No. 73-2409), occurs on AMS-Mg95Ca5 (Figure 6b) and its formation rate appears much faster than the other two 13



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double salts and even than MgCO3 at the initial 2 min of sorption. Thus, it is the rapid formation of CaMg(CO3)2 that leads to the rate enhancement of CaCO3-dopant sorbents during the initial stage of sorption. In addition, the formation of MgCO3 over AMS-Mg95Ca5 is accelerated with an obvious increase in intensity of the (104) peak as compared to AMS-Mg100, especially at 4 and 6 min after the start of sorption (Figure 6c), which is in agreement with the higher rate of AMS-Mg100-xCax observed between 2 and 5 min in the uptake profiles (Figure 4). The formation of double salts is reversible,22,37,38 and all the salts decompose during desorption at elevated temperature (Figure S3). Note that the bimodal PSD of AMS-Mg100-xCax probably also contributes to the rate enhancement as larger pores can facilitate the diffusion of CO2 in the sorbent.18,39,40

2

(a)

2

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3 4

35

1- MgO 2- MgCO3 3- Na2Mg(CO3)2 4- K2Mg(CO3)2 5- CaMg(CO3)2 6- CaCO3

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33 32 2θ (degree)

33

Figure 6. In situ XRD analysis of (a) AMS-Mg100 and (b) AMS-Mg95Ca5 during CO2 sorption at 350 °C in CO2 stream; (c) XRD diffractograms in the range of 31.5-33.5° for 4-20 min of sorption.

Another phenomenon in Figure 6 is that the diffraction peaks assigned to the AMS species are invisible, which is due to melting of AMS because, on the one hand, these peaks exist in the XRD patterns of AMS-Mg100 and AMS-Mg95Ca5 at temperatures up to 300 °C but disappear at 350-500 °C (Figure S4), and on the other hand, the possibility is excluded that the AMS used here will decompose below 530 °C (Figure S5). Therefore, the AMS species are in the molten state at the working temperatures of this study. Consistent with literature reports,17,19 the molten AMS dissolves some MgO as reflected by the shift of the MgO(200) peak to lower angles with increasing 14


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temperature (Figure S4), which consequently promotes the sorption of CO2 on MgO.

100

100

320 oC

(a) 80

(b)

280 oC

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symbols: measured lines: fitted

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0

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Figure 7. Measured and fitted conversion of MgO at 260-320 °C for (a) AMS-Mg100 and (b) AMS-Mg95Ca5. The sorption behaviors in this temperature range are away from the thermodynamic equilibrium.

It has been reported in previous studies37,41,42 that the double salts have higher thermodynamically stability than MgCO3 and thus raise the operating temperature of MgO in the CO2 capture. Similarly, the formation of CaMg(CO3)2 in addition to Na2Mg(CO3)2 and K2Mg(CO3)2 over AMS-Mg95Ca5 further broadens the CO2 sorption temperature compared to AMS-Mg100. As shown in Figure S6, the optimal sorption temperature of AMS-Mg100 is 325 °C, which is lower than that of AMS-Mg95Ca5 (350 °C). Therefore, the kinetic experiments were performed at 260-320 °C away from the thermodynamic equilibrium. As shown in Figure 7, The CO2 sorption behaviors on AMS-Mg100 and AMS-Mg95Ca5 are well depicted by the double exponential model (Equation 3). The k1 values obtained at different temperatures for AMS-Mg95Ca5 are about 3-4 times those for AMS-Mg100 (Table S2), which agrees well with the results shown in Figures 4 and 6 and demonstrates that the SR-controlled stage is greatly accelerated with the addition of CaCO3. Moreover, the activation energies for AMS-Mg95Ca5 in the SR- and PLD-controlled stages are estimated to be 26.8 and 52.9 kJ mol-1, respectively, which are lower than the corresponding values 15



for AMS-Mg100, 42.9 and 56.8 kJ mol-1 (Figure S7), further revealing the higher activity of AMS-Mg95Ca5. Next, the flower-like sorbents are evaluated under a severe condition where the desorption step is carried out in CO2 instead of N2. From the viewpoint of practical application, the regeneration of saturated sorbents in CO2 flow is wanted considering the CO2 released during regeneration. A high temperature of 500 °C is used for rapid desorption because the starting desorption temperature in CO2 for these sorbents reaches about 470 °C according to temperature-sweep analysis (Figure S8). As shown in Figure 8, as expected, the stability of the sorbents at the severe condition becomes worse compared to the mild condition: for AMS-Mg100, AMS-Mg98Ca2, AMS-Mg95Ca5 and AMS-Mg90Ca10, the CO2 capture capacity at the 10th cycle is 0.51, 0.51, 0.56 and 0.50 gCO2 gsorbent-1, respectively, corresponding to a decay in capacity of 29.2, 27.1, 13.8 and 12.3%, respectively. However, AMS-Mg95Ca5 and AMS-Mg90Ca10 behave much better than the other two in that their capacities stabilize for the last 4 cycles, retaining about 86 and 88% of the initial capacities, respectively.

Figure 8. CO2 uptake profiles of various sorbents over 10 sorption-desorption cycles under a severe condition (sorption at 350 °C in pure CO2 for 45 min; desorption at 500 °C in pure CO2 for 5 min). 16


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The good stability of AMS-Mg95Ca5 and AMS-Mg90Ca10 probably arises from the plate-like structure (Figures 5 and 8) that is formed during the cyclic operation, which, acting as a skeleton, can to some extent prevent the agglomeration of small particles and subsequent sintering. Moreover, this structure seems to be helpful for preserving the uniform distribution of AMS on the support, as no aggregation of AMS on AMS-Mg95Ca5 and AMS-Mg90Ca10 but distinct aggregation on AMS-Mg100 and AMS-Mg98Ca2 (the smooth surfaces indicated by the dashed cycles in Figure 8 signify aggregation of AMS on the sorbent because a similar surface is observed for the thermal-treated AMS (inset in Figure S5). Although AMS-Mg98Ca2 contains CaCO3, the content of CaCO3 is too small (only 3.6 wt%) to allow formation of the plate-like structure. The aggregation of AMS is undoubtedly unfavorable for CO2 sorption because some MgO will, as a result, not be covered by AMS and accordingly lacks the promoting effect that the molten AMS has on the dissolution of MgO, which in turn leads to the decay in sorption capacity.43

3.2. Plate-Like AMS-Promoted MgO-CaCO3 Sorbent. Considering that the flower-like structure is not responsible for the high performance of the AMS-promoted MgO-CaCO3 sorbents, we try to substitute an easily accessible and scalable MgO for the flower-like MgO that is not readily prepared. The MgO (Mg(B)100) obtained by simple calcination of basic magnesium carbonate exhibits a plate-like architecture with arbitrary arrangement (Figure 9a), which is preserved after addition of CaCO3 (Mg(B)95Ca5, Figure 9b) and subsequent deposition of AMS species (AMS-Mg(B)95Ca5, Figure 9c). The presence of CaCO3 and AMS species in AMS-Mg(B)95Ca5 are confirmed by XRD analysis (Figure S9a). The specific surface area and pore volume of Mg(B)100, Mg(B)95Ca5 and AMS-Mg(B)95Ca5 are smaller but comparable to those of Mg100, Mg95Ca5 and AMS-Mg95Ca5 (Table S1 and Table 2). Different from the bimodal PSD of AMS-Mg95Ca5, AMS-Mg(B)95Ca5 shows a unimodal PSD centered at around 10 nm (Figure S10). 17


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

CO2 uptake (gCO2 gsorbent-1)


0.8

g

1st cycle AMS-Mg Ca 95 5

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500 1000 1500 2000 0 300 Time (min)

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Figure 9. FESEM images of (a) Mg(B)100, (b) Mg(B)95Ca5, (c) AMS-Mg(B)95Ca5, (d) 30-cycle used AMS-Mg(B)95Ca5 under a mild condition (sorption at 350 °C in pure CO2 for 45 min; desorption at 400 °C in pure N2 for 15 min), (e) 20-cycle used AMS-Mg(B)95Ca5 under a severe condition (sorption at 350 °C in pure CO2 for 45 min; desorption at 500 °C in pure CO2 for 5 min), and (f) 30-cycle used AMS-Mg(B)95Ca5 within a short sorption time (sorption at 350 °C in pure CO2 for 10 min; desorption at 400 °C in pure N2 for 10 min); (g-j) CO2 uptake profiles of AMS-Mg(B)95Ca5: (g) comparison with AMS-Mg95Ca5 at the first cycle (inset: initial 2 min of sorption); (h) over 30 cycles under a mild condition; (i) over 20 cycles under a severe condition, and (j) over 30 cycles in a short sorption time.

Like AMS-Mg95Ca5, AMS-Mg(B)95Ca5 possesses ultrafast rate, high capacity and good stability. Compared to AMS-Mg95Ca5, the initial rate of AMS-Mg(B)95Ca5 is only slightly decreased (Figure 9g), with a CO2 uptake of 0.57 gCO2 gsorbent-1 after 5 min of sorption, accounting for about 88% of its capacity, which is comparable to the value for AMS-Mg95Ca5 (92%). Likewise, the used AMS-Mg(B)95Ca5 contains not only MgCO3 but also CaMg(CO3)2, Na2Mg(CO3)2 and K2Mg(CO3)2 (Figure S9b). This result reveals that it is the formation of CaMg(CO3)2 rather than the bimodal PSD that predominantly affects the sorption rate. In addition, the CO2 capture capacity of AMS-Mg(B)95Ca5 remains constant at 0.62 gCO2 gsorbent-1 (about 95% of the initial capacity) after 30 cycles under the mild condition (Figure 9h), which under the severe condition is stabilized at 0.58 gCO2 gsorbent-1 (about 89% of the initial capacity) after 13 cycles during a 20-cycle operation (Figure 9i). By comparison with the sorbent25 evaluated at similar mild and severe conditions (Table 1, 18


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entries 9 and 10), AMS-Mg(B)95Ca5 exhibits much higher stability. AMS-Mg(B)95Ca5 was further tested within a short sorption time (10 min) over 30 cycles. As shown in Figure 9j, the initial CO2 uptake is around 0.6 gCO2 gsorbent-1, which is stabilized at 0.56 gCO2 gsorbent-1 after 30 cycles. By comparison, the sorbent18 tested under a similar condition (Table 1, entry 3) displays a relatively poor stability (from 0.40 to 0.26 gCO2 gsorbent-1 after 30 cycles). Similar to the morphology of spent AMS-Mg95Ca5 (Figures 5 and 8), all spent AMS-Mg(B)95Ca5 sorbents under mild (Figure 9d) and severe (Figure 9e) conditions as well as within a short sorption time (Figure 9f) present plate-like shapes, further underlining the important role of the plate-like structure in stabilizing the sorbent. It is noteworthy that the mount of CaCO3 in the sorbent should be limited to a certain range; otherwise the CO2 capture performance of sorbent will deteriorate. As shown in Figure S11a, AMS-Mg(B)80Ca20 (30 wt% of CaCO3) undergoes about 45% decay in capacity (from 0.46 to 0.25 gCO2 gsorbent-1) over 20 cycles under the severe condition although its capacity is stabilized during the last several cycles; moreover, its initial rate is much lower than that of AMS-Mg(B)95Ca5 (Figure S11b). Such performance is probably ascribed to the relatively large amount of CaCO3 that may, to a certain extent, prevent the contact between AMS and MgO and thus slow down the formation of CaMg(CO3)2. Additionally, the morphology of AMS-Mg(B)80Ca20 evolves from plate-like (Figure S11c) to irregular, non-plate-like shapes together with particle sintering and distinct aggregation of AMS after 20 cycles (Figure S11d), which gives rise to poor stability (especially during the first 15 cycles). On the other hand, the preceding results indicate that AMS-Mg98Ca2 has a too low amount of CaCO3 to enable good stability (Figure 8).

4. CONCLUSION In summary, we have developed a facile technique to prepare high-performance MgO-based CO2 sorbents, which can be produced from cheap and available precursors and thus can be readily scaled 19



up for industrial applications. Compared to AMS-promoted MgO sorbents, AMS-promoted MgO-CaCO3 sorbents with a certain amount of CaCO3 can greatly improve the CO2 capture performance of sorbent, mainly due to the rapid formation of CaMg(CO3)2 and the plate-like structure itself: the former results in the ultrafast sorption rate and the latter in the good cyclic stability. The best sorbent developed here has a high CO2 uptake of 0.57 gCO2 gsorbent-1 after only 5 min of sorption, and its capacity after 20 consecutive sorption-desorption cycles under a severe condition (sorption at 350 °C in pure CO2 for 45 min; desorption at 500 °C in pure CO2 for 5 min) is stabilized at 0.58 gCO2 gsorbent-1, indicating excellent CO2 capture performance. Worthy of mention is that the CO2 concentration at the outlet of the water-gas shift reactor in an integrated gasification combined cycle process is about 40%. In addition, the gas stream contains a certain amount of steam, which would probably influence the performance of AMS-promoted MgO-CaCO3 sorbents. Investigations about the effect of steam on the CO2 sorption-desorption process with AMS-promoted MgO-CaCO3 sorbents at relatively low CO2 concentration are under way in our lab.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website. Detailed information about characterization techniques such as N2 physisorption, XRD, FESEM, and HRTEM; textural properties of support materials; estimated kinetic parameters; N2 physisorption isotherms and pore size distribution curves of supports and sorbents; in-situ XRD analysis of sorbents; TGA analysis of AMS; temperature-sweep analysis of sorbents.

AUTHOR INFORMATION Corresponding author *E-mail: [email protected] (Z Zhou) 20


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Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (21776088) and the Fundamental Research Funds for the Central Universities (222201718003).

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Figure Captions Figure 1. (a, b, e, f, i-l) FESEM and (c, d, g, h) HRTEM images of supports: (a-d) Mg100; (e-h) Mg95Ca5; (i, j) Mg98Ca2; (k, l) Mg90Ca10.

Figure 2. XRD patterns of supports (Mg100, Mg98Ca2, Mg95Ca5, and Mg90Ca10), and sorbents (AMS-Mg100, AMS-Mg98Ca2, AMS-Mg95Ca5, and AMS-Mg90Ca10).

Figure 3. (a, b, e, f, i-l) FESEM, (c, g) HRTEM and (d, h) EDS elemental mapping images of sorbents: (a-d) AMS-Mg100; (e-h) AMS-Mg95Ca5; (i, j) AMS-Mg98Ca2; (k, l) AMS-Mg90Ca10.

Figure 4. CO2 uptake profiles of various sorbents over 50 sorption-desorption cycles under a mild condition (sorption at 350 °C in pure CO2 for 45 min; desorption at 400 °C in pure N2 for 15 min) and at the first, 25th and 50th cycle (inset: initial 2 min of sorption).

Figure 5. FESEM images of spent AMS-Mg100, AMS-Mg98Ca2, AMS-Mg95Ca5, and AMS-Mg90Ca10 under a mild condition (sorption at 350 °C in pure CO2 for 45 min; desorption at 400 °C in pure N2 for 15 min).

Figure 6. In situ XRD analysis of (a) AMS-Mg100 and (b) AMS-Mg95Ca5 during CO2 sorption at 350 °C in CO2 stream; (c) XRD diffractograms in the range of 31.5-33.5° for 4-20 min of sorption.

Figure 7. Measured and fitted conversion of MgO at 260-320 °C for (a) AMS-Mg100 and (b) AMS-Mg95Ca5. The sorption behaviors in this temperature range are away from the thermodynamic equilibrium.

Figure 8. CO2 uptake profiles of various sorbents over 10 sorption-desorption cycles under a severe condition (sorption at 350 °C in pure CO2 for 45 min; desorption at 500 °C in pure CO2 for 5 min).

Figure 9. FESEM images of (a) Mg(B)100, (b) Mg(B)95Ca5, (c) AMS-Mg(B)95Ca5, (d) 30-cycle used AMS-Mg(B)95Ca5 under a mild condition (sorption at 350 °C in pure CO2 for 45 min; desorption at 27



400 °C in pure N2 for 15 min), (e) 20-cycle used AMS-Mg(B)95Ca5 under a severe condition (sorption at 350 °C in pure CO2 for 45 min; desorption at 500 °C in pure CO2 for 5 min), and (f) 30-cycle used AMS-Mg(B)95Ca5 within a short sorption time (sorption at 350 °C in pure CO2 for 10 min; desorption at 400 °C in pure N2 for 10 min); (g-j) CO2 uptake profiles of AMS-Mg(B)95Ca5: (g) comparison with AMS-Mg95Ca5 at the first cycle (inset: initial 2 min of sorption); (h) over 30 cycles under a mild condition; (i) over 20 cycles under a severe condition, and (j) over 30 cycles in a short sorption time.

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Ultrafast and Stable CO2 Capture Using Alkali Metal Salt-Promoted

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