Oxidative Stability in

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Feasibility of CO Capture from O-Containing Flue Gas Using a Polyethylenimine–Functionalized Sorbent: Oxidative Stability in Long-Term Operation. Yuan Meng, Jianguo Jiang, Aikelaimu Aihemaiti, Tongyao Ju, Yuchen Gao, Jiwei Liu, and Siyu Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b08048 • Publication Date (Web): 27 Aug 2019 Downloaded from pubs.acs.org on August 27, 2019

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ACS Applied Materials & Interfaces

Feasibility of CO2 Capture from O2-Containing Flue Gas Using a Polyethylenimine– Functionalized Sorbent: Oxidative Stability in Long-Term Operation.

Yuan Meng a, Jianguo Jiang a, *, Aikelaimu Aihemaiti a, Tongyao Ju a, Yuchen Gao a, Jiwei Liu a,

and Siyu Han a

a School

of Environment, Tsinghua University, Beijing 100084, China

*Corresponding

author: Email: [email protected]; Tel: (+86) 010 62783548

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 Amine–functionalized sorbents are investigated widely for CO2 capture from flue gas, to mitigate the crisis of global CO2 emission, with the advantages of excellent adsorption and regeneration performance. However, the presence of O2 in flue gas (3–10%) would induce the degradation of the sorbents, and some previous works proposed the strategies at the sacrifice of partial CO2 adsorption capacity. Herein, we focused on the oxidation behavior of PEI– functionalized silica in the long–term operation, and analyzed the degradation mechanism by characterize the oxidized sorbents. The sorbent proved to be oxidative–stable under a lower temperature of air exposure, but the oxidative degradation would indeed occur at more harsh temperatures (above 90 °C). This study demonstrated that CO2 capture from O2–containing flue gas was feasible by controlling the operating temperature (below 75 °C), and the effective capacity of above 135 mg/g could be maintained in the cyclic CO2 capture.

Keywords: amine–functionalized sorbent; CO2 capture; flue gas; long–term stability; oxidative degradation.


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Introduction The massive consumption of fossil fuels has contributed to the excessive anthropogenic carbon dioxide (CO2) emission in the recent years, which has caused great concerns about climate change and global warming. According to the latest data of International Energy Agency (IEA), 33.1 Gt CO2 from fuel combustion was emitted around the world in 2018, and the global atmospheric CO2 concentration climbed to a historic high of averaged 407.4 ppm. 1 The halting of the growth rate of global CO2 emission from fossil fuels is imperative for mitigating global greenhouse effect. For one perspective, carbon capture and storage (CCS) seems to be the most deployable strategy to alleviate CO2 release from large stationary sites, like the power and industrial sectors.

2, 3

Many researchers have a great effort in developing

CO2 capture technologies containing absorption, adsorption, membrane, cryogenic and the hybrid processes. 4, 5 Among the various methods to separate CO2 from dilute streams including flue gas and raw biogas, chemical absorption systems, i.e. amine scrubbing, are considered as the dominant technology for the mitigation of CO2 emissions, due to its technical maturity. 6, 7 However, this technology is accompanied by the intensive–energy consumption resulting from endothermic regeneration of liquid amine, and suffers from the problems of equipment corrosion, amine leaching and environmental pollution. 8-10 By contrast, the selective adsorption of CO2 from flue gas on a solid sorbent might reduce the dependence of energy.

11

The

development of various porous materials is investigated widely for CO2 capture, especially that potassium–based sorbents

12,

nitrogen–doped carbons

11, 13, 14,

metal–organic frameworks

15,

porous organic polymers 16, and other novel materials have attracted enormous interest recently.



Page 4 of 38

Similarly, the fundamental reactions between CO2 and amine can be applied into the adsorption process by amine–functionalization in solid porous media, which can also overcome efficiently the drawbacks of the benchmark technology using aqueous solutions and have higher selectivity than physical adsorbents. 17, 18 As an energy–saving and risk–lower CO2 capture technology, amine–functionalized sorbents for CO2 adsorption from relatively concentrated industrial streams have been widely investigated in the recent years 19. Researches have been conducted in detail on several classes of solid porous materials with abundant pore structures, including silica 20, alumina 21, alkali metal oxide 22, carbon 23, and adsorption resin 24. Immobilization of amine groups into porous supports can be achieved by impregnation of organic amines such as diethanolamine (DEA), tetraethylenepentamine (TEPA) and polyethylenimine (PEI), post–synthesis grafting of aminosilanes, or in–situ polymerization of amine–containing monomers. 25-28 Numerous studies up to this point have mainly concentrated on developing the hybrid sorbents for improving CO2 capture performance, especially by screening amine molecules or adjusting the substrate pore structures.

29

For instance, Li et al. compared the capture performance of PEI–silica sorbents

with different PEI types and molecular weights, and found that the branched PEI with a molecular weight of 800 Da could achieve a highest adsorption capacity of 202 mg /g sorbent. 30

Similarity, Zeleňák et al. reported that SAB–15 silica impregnated with diamine showed

higher adsorption capacities at 273 K and 293 K, in contrast to mono– and triamine, but when the temperature further elevated, the triamine modifier got the best.

31

Thereinto, CO2

adsorption capacity is normally considered as the imperative metric for assessing the sorbents,


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but the high CO2/N2 selectivity in a typical flue gas with a dilute concentration of CO2 (less than 15%) is also required for the application. Zhu et al. noticed the modified PEI could promote the CO2/N2 selectivity of MOF–polymer support from 9.5 to 67.9–73.3.

32

In addition,

adsorbed–CO2 could be efficiently desorbed from the amine–functionalized sorbents via a temperature or pressure swing method, thanks to the reversible chemisorption of CO2, like the amine–grafted microporous silica synthesized by Liu et al. achieved the CO2 capture processes from simulated flue gas and air to be sustainable.

33

Consequently, amine–functionalized

sorbents have proven to be a promising alternative for efficient CO2–selective capture from flue gas. Nevertheless, some typical impurities in the realistic flue gas like SOx, NOx and O2, which can result in the degradation or deactivation of amines, are of great concern for amine– functionalized sorbents but are frequently overlooked in the CO2 capture studies. 34 The stability of sorbents in the long–term operation can be affected by the interactions with different species, including the urea–linkages formation by high–temperature CO2, oxidative degradation, and irreversible reaction of SO2 or NO2. 35 In contrast to the acid gases, O2 is the most ubiquitous species in flue gas with a relatively high concentration of 3–10%, 36, 37 especially after adequate flue gas desulfurization and denitrification units. It is worth noting that there are almost no specialized facilities for removing O2 from flue gas. The presence of O2 may remain the hurdle for the application of amine–based CO2 capture technologies, thus identifying the oxidation behavior of amine–functionalized sorbents is of critical importance. In some previous studies, the researchers noticed that the oxidative degradation significantly relied on the amine



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structures. Heydari–Gorji et al. examined the oxidative stability of MCM–41 grafted with primary, secondary and tertiary monoamines respectively via long–term exposure to air at 70– 120 °C, and demonstrated that the isolated primary and tertiary amines seemed to be more stable than the secondary amines.

38

But in the case of polyamines, the linear molecules exhibited

better oxidatively–stable performance than the branched polyamines.

34, 39

Therefore, some

strategies were proposed to resist the oxidative degradation by changing the structure of amine groups. For example, the branched polyethylenimine (PEI) was replaced by the linear PEI, polypropylenimine (PPI) or polyallylamine (PAA) to enhance the oxidative stability, but CO2 adsorption capacities were decreased to less than 1 mmol/g.

34, 39, 40

Similarly, Srikanth et al.

and Min et al. further modified the amine–functionalized sorbents via adding additional polyethylene glycol and 1,2–epoxybutane, respectively, and also resulted in a decrease of CO2 adsorption capacity due to the dropped amine density. 35, 41 The reduced adsorption capacity is not conducive to improving the feasibility of CO2 capture. For another perspective, amine–functionalized sorbents undergo rapid oxidative degradation at elevated temperatures, and the oxidation rate can be obviously mitigated by decreasing the temperature. 37, 40 The flue gas temperatures range from 40 to 80 °C, especially that the value will be below 60 °C if flue gas desulfurization (FGD) units are employed. 36, 42, 43

In contrast, the oxidation experiments were performed by exposure to a temperature at above

100 °C in some previous studies, and the oxidation condition were harsh for amine– functionalized sorbents. Therefore, the feasibility of CO2 capture from O2–containing flue gas could be improved by screening to an appropriate temperature range, taking into account the


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realistic flue gas condition. In addition, high CO2 adsorption capacity should also be guaranteed in the O2–containing flue gas. Therefore, in the present work, we prepared the PEI– functionalized silica to examine the oxidative behavior and the stability in the long–term operation, considering the sorbent with obvious advantages of high CO2 adsorption capacity, fast adsorption kinetics and high thermal stability. The sorbent was treated upon exposure to CO2–free air at various temperatures (30–150 °C) for investigating the oxidation behavior of the sorbent, and CO2 adsorption capacities before and after exposure were measured for comparison. The physical and chemical properties of the fresh and treated sorbents were characterized in detail for discerning the oxidative degradation mechanism of amine– functionalized sorbents. Furthermore, we demonstrated the long–term stability over multiple cycles of CO2 capture from simulated O2–containing flue gas, for determining if the sorbent could achieve the oxidatively stable capture while ensuring high CO2 adsorption capacity.

Experimental section Synthesis of the amine–functionalized sorbent The amine–functionalized sorbent was synthesized by wet impregnation following the similar procedure we reported previously.

10

The desired amount of branched PEI with a

molecular weight of 600 Da (Alfa Aesar, 99%) was dissolved into methanol (Aladdin, HPLC grade), and the precipitated silica (Evonik, Sipernat 306) was then added. The mixture was under magnetic stirring vigorously at room temperature for 8 h. After impregnation, the mixture was dried in vacuum oven at 50 °C for 5h to remove the solvent completely. The desired PEI



Page 8 of 38

loading in the sorbent was fixed to roughly 50 wt.%, which had been determined by thermogravimetric analysis and elemental analysis. Hereinafter, the functionalized sorbent obtained is denoted as PEI/SiO2 in the present work. Oxidation of PEI/SiO2 To investigate the oxidation behavior of PEI/SiO2, the sorbent was treated by continuous exposure to CO2–free synthetic air (21% O2 in N2 balance) at different temperatures in a TGA/DSC 2 STARe thermogravimetric analyzer (Mettler Toledo). Hereafter, the sorbent was subjected to oxidation procedure under the atmosphere of CO2–free air at the desired oxidation temperature (30–150 °C) for 3–24 h. In addition, the CO2 adsorption capacities for the fresh and treated sorbents were also measured using TGA. In a typical adsorption–regeneration experiment, approximately 20 mg of PEI/SiO2 was pretreated firstly under pure N2 atmosphere at 120 °C for 30 min to desorb the presented adsorbate, cooled to the adsorption temperature– 75 °C, and then exposed to pure CO2. After 60 min adsorption, the temperature was increased to 90 °C for the regeneration process when the gas flow was switched back to pure N2, and maintained for 30 min. CO2 adsorption capacity (mg CO2/g sorbent) was obtained according to the mass increase determined by TGA. The loss of CO2 adsorption capacity after air exposure was calculated for qualitatively accessing the effect of oxidation conditions on the adsorption performance of PEI/SiO2, expressed as follows:

  (1 

qaft qbef

) 100%

(1)

where qbef and qaft were the CO2 adsorption capacities measured before and after the


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oxidation process, respectively (mg/g), and

 presented the loss of adsorption capacity after

air exposure (%). Characterization methods Elemental compositions (C, H, N) of the pre– and post–oxidized PEI/SiO2 were analyzed by a Vario EL Ⅲ elemental analyzer (Elementar) to reveal the difference of organic compositions. An in–situ diffuse reflectance infrared Fourier transform (DRIFT) spectra analysis of the samples were performed on a Nicolet 6700 spectrometer (Thermo Scientific) equipped with an in–situ reaction cell. DRIFT was employed to verify the change of functional groups in the sorbent during the oxidation process and the spectra were collected with a scanning range of 4000–650 cm–1 and 32 scans at a resolution of 4 cm–1. In addition, to confirm the degradation of PEI/SiO2 induced by O2, the thermogravimetric analysis was conducted under N2 and air, respectively, in a TGA/DSC 1 STARe thermogravimetric analyzer (Mettler Toledo). Typically, the sample was characterized at the temperature range from 30 to 800 °C at a rate of 5 °C/min, and the emissions from PEI/SiO2 decomposed (m/e = 1–50) were monitored online by a Thermostar mass spectrometer (Pfeiffer) using the signal. Some detailed steps of characterizations were given in the supporting information. CO2 capture and cyclic stability measurements CO2 capture from the simulated flue gas (FG, 15% CO2 in N2 balance) and O2–containing flue gas (OFG, 15% CO2 and 10% O2 in N2 balance) was measured in the TGA at different adsorption temperatures (30–120 °C). Furthermore, to evaluate the feasibility of CO2 cyclic



Page 10 of 38

capture from O2–containing flue gas, 50 adsorption–regeneration cycles of PEI/SiO2 were repeated to fully access the long–term stability. Thereinto, CO2 cyclic captures from FG and OFG, respectively, were performed at some temperatures, which were determined based on the oxidation experiments. On the other hand, taking into account air as a low–cost purge gas for regenerating the sorbent proposed by the previous researchers

38, 44,

we briefly attempted the

potential possibility of air stripping by cyclic capture experiments.

Results and discussion Oxidation behavior of PEI/SiO2 After functionalizing silica with PEI by wet impregnation method, the surface area and pore volume of support were obviously decreased due to the filling of PEI into the pore structures, according to the pore characteristics summarized in the supporting information (Table S1 and Figure S1). Correspondingly, the large amount of organic content in PEI/SiO2, indicated by energy dispersive spectroscopy (EDS) mapping (Figure S2), would conduce to provide abundant adsorption sites and ensure a considerable CO2 adsorption capacity.


Page 11 of 38

100

180 150

80

120 60

qbef qaft

90

γ

40

60 20

30 0

Capacity loss (%)

CO2 adsorption capacity (mg·g-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


30

60

90

120

150

0

Oxidation temperature (°C)

Figure 1. CO2 adsorption capacities before (qbef) and after (qaft) air exposure under different oxidation temperatures (30–150 °C) for 12 h and the losses of adsorption capacity (γ). Adsorption was carried out under pure CO2 at 75 °C. The oxidation behavior of the amine–functionalized sorbent was investigated under CO2– free air exposure at different temperatures (30–150 °C), which were deemed as the range of thermal swing operating window in most previous works. CO2 adsorption–regeneration experiments were conducted before and after the treatment, for the purpose of evaluating the oxidative degradation performance in the long–term air exposure. The CO2 adsorption capacities and the corresponding capacity losses after 12 h air exposure are presented in Figure 1. The fresh PEI/SiO2 exhibited a higher adsorption capacity of above 154 mg/g under dry streams at 75 °C. After air exposure, the sorbent suffered barely loss in CO2 adsorption capacity when the oxidation temperature was below 75 °C with a limited loss (less than 0.7%). In addition, the adsorption–regeneration curves before and after the oxidation process (Figure S3) showed almost no difference, indicating there was considerably undiscoverable oxidative



Page 12 of 38

degradation of PEI/SiO2 over this temperature range. However, the sorbent was obviously more sensitive to air when the oxidation temperature was further increased. The CO2 adsorption capacities were reduced drastically by 7.2%, 64.7%, 93.9%, 97.4% and 97.8% when the sorbent was treated by a 12 h air exposure at 90 °C, 105 °C, 120 °C, 135 °C and 150 °C, respectively. The retained adsorption capacities of the oxidized PEI/SiO2 under more harsh conditions (135 °C and 150 °C) were only 4.0 and 3.5 mg/g, which were even worse than the pristine silica 10

and basically indicated that the sorbent was unusable. But it is worth noting that the thermal

stability of amine–functionalized sorbents is also affected by high temperature purging, considered as the physical degradation, due to the evaporation and leaching of amine.

45, 46

Therefore, it is essential to identify the high–temperature degradation of PEI/SiO2 is mainly caused by whether the thermal or oxidative effect, in other words, by the physical or chemical effect. Regarding the thermal stability measurements of PEI/SiO2, similar experiments were conducted upon exposure in flowing N2 instead of air at 90–150 °C. As shown in Figure 2, PEI/SiO2 displayed high thermal stability over the temperature range of 90–150 °C with negligible losses of adsorption capacity from 0.1% to 1.3%, respectively, due to PEI as the polyamine with a higher molecular weight and boiling point in comparison with other simple amines like DEA and TEPA.47 According to the results, the thermal effect was limited and did not significantly destabilize PEI/SiO2, which proved that the progressively degradation at the elevated temperature was induced by oxidation. Not only PEI/SiO2, Yu et al. also reached a similar conclusion and demonstrated that the capacity decrease of amine–functionalized resins under air exposure was mainly ascribed to the oxidative effect.


44

Oxidative degradation at

Page 13 of 38

elevated temperature was serious for the long–term operation, but on the other hand, the stability of PEI/SiO2 under air exposure at a milder temperature made it potentially possible to capture CO2 from O2–containing flue gas. 100

180 150

80

120 60 90 40

qbef

60

qaft γ

20

30 0

90

Capacity loss (%)


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


105

120

135

150

0

N2 exposure temperature (°C)

Figure 2. CO2 adsorption capacities before and after N2 exposure under different temperatures (90–150 °C) for 12 h and the corresponding losses of adsorption capacity. Adsorption was carried out under pure CO2 at 75 °C. In a further experiment, we also investigated the oxidation behavior of PEI/SiO2 under different times (3–24 h) of air exposure at 105 °C, where the sorbent was obviously oxidized, and the results are collected in Figure 3. After continuous exposure in air, the CO2 adsorption capacities were gradually decreased with the oxidation time, and the capacity losses of PEI/SiO2 were increased from 8.5% to 87.6%. In addition, the rates of adsorption capacity losses seemed to be decreased slightly with the oxidation time, especially that the adsorption capacity lost 64.7% in the first 12 h, but only increased another 22.9% in a prolonged exposure. This was consistent with some previous results, for example, Min et al. examined the oxidative stability of the PEI–



based sorbent at 110 °C and noticed that the attenuation of capacity flattened out with oxidation time, moreover, it could be fitted with the first–order kinetic model. 35 Taking into account the degree of capacity losses at different oxidation temperatures, the elevated temperature would accelerate the oxidative degradation. The stability at below 75 °C might also be ascribed to the kinetic effect, from a different perspective, which was considered as a dominant factor in other study. 44 100

180 150

80

120 60

qbef qaft

90

γ

40

60 20

30 0

Capacity loss (%)


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

Page 14 of 38

3

6

9

12

15

18

21

24

0

Oxidation time (h)

Figure 3. CO2 adsorption capacities before and after air exposure at 105 °C for different times (3–24 h) and the losses of adsorption capacity. Adsorption was carried out under pure CO2 at 75 °C. It was noteworthy that the oxidation behavior of amine–functionalized sorbents was different under various materials and conditions as mentioned before, but most researches evaluated the effect of O2 by a similar method via comparing the CO2 adsorption performance before and after exposure. Therefore, Table 1 provides a summary of the oxidative degradation of some amine–functionalized


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solid sorbents reported previously. It was obvious that the oxidation behavior of amine– functionalized sorbents was affected by multiple factors, and the losses of CO2 adsorption capacity varied with the oxidation conditions. As mentioned before, the elevated temperature would simulate the oxidative degradation of the sorbents and the loss of adsorption capacity also was increased with the duration of exposure. In addition, the amine–functionalized sorbents were degraded substantially under a high O2 concentration of 21%, akin to be existed in ambient air. But the effect would be attenuated effectively by the reduction in O2 concentration. This meant that it might be more advantageous to capture CO2 from flue gas with a lower O2 concentration under the same conditions. Furthermore, the amine– functionalized sorbents prepared in different works displayed dissimilar tolerance to O2. The capacity losses of PPI, linear PEI–impregnated oxides and the grafted primary and secondary monoamines were relatively lower under a similar oxidation condition, but the problem of oxidative degradation could not be completely inhibited by these sorbents. On the whole, it was worth noting that the PEI/SiO2 in this work was almost impervious to air exposure at below 75 °C, and could retain a high proportion of CO2 adsorption capacity after oxidation. Table 1 Comparison of the oxidation behavior of different amine–functionalized sorbents under various conditions. Adsorption condition Sample [a]

Oxidation condition

qbef

qaft

γ

(h)

(mg/g)

(mg/g)

(%)

21

12

154.7

154.6

0.04

this work

90

21

12

154.3

143.2

7.2

this work

60

105

21

12

156.1

55.1

64.7

this work

60

105

21

24

153.2

19.1

87.6

this work

Tad [b]

CCO2 [c]

tad [d]

Tox [b]

CO2 [c]

tox [d]

(°C)

(%)

(min)

(°C)

(%)

600BPEI/SiO2

75

100

60

75

600BPEI/SiO2

75

100

60

600BPEI/SiO2

75

100

600BPEI/SiO2

75

100


Ref.


Page 16 of 38

600BPEI/SiO2

75

100

60

120

21

12

156.4

9.5

93.9

this work

1200BPEI/SiO2

60

15

30

110

3

24

87.1

41.8

52

35

600BPEI/SBA–15

75

100

30

75

21

30

n.m. [e]

n.m.

6

48

600BPEI/SBA–15

75

100

30

100

10.5

30

n.m.

n.m.

50

600BPEI/SBA–15

75

100

30

100

21

30

n.m.

n.m.

70

800BPEI/Al2O3

50

10

360

110

5

20

82.3

76.1

7.5

800BPEI/Al2O3

50

10

360

110

21

20

82.3

24.6

70.1

PAA/Al2O3

50

10

360

70

21

20

47.1

44.0

7.0

PAA/Al2O3

50

10

360

110

21

20

47.1

42.2

10.9

600BPEI/SBA–15

75

100

60

120

21

24

90

0.15

98

2500LPEI/SBA–15

75

100

60

110

21

24

74

34

54

IER

40

15

n.m.

80

12

72

n.m.

n.m.

9

IER

40

15

n.m.

80

21

72

n.m.

n.m.

30.2

IER

40

15

n.m.

120

21

72

94.6

18.5

80.5

IER

50

10

n.m.

120

21

168

61.2

12.3

79

49

TEPA/SiO2

25

100

10

100

21

12

54.3

20.0

63.4

41

pMono/MCM–41

55

100

30

120

21

30

93

86

7.5

38

sMono/MCM–41

55

100

30

120

21

30

56

38

32

TRI/MCM–41

55

100

30

120

21

30

101

6

94

[a] The samples are all named as ‘n x/y’, where ‘x’, ‘y’ and ‘n’ represent the amine type, support type and the molecular weight of polyamine, separately, and other abbreviations are written with the following notation: BPEI–the branched PEI; PAA–polyallylamine, LPEI–the linear PEI, IER–a primary amine– functionalized ion exchange resin, pMono–the primary monoamine, sMono–the secondary monoamine and TRI–the triamine. [b] Tad and Tox stand for the adsorption temperature and oxidation temperature (°C), respectively. [c] CCO2 and CO2 stand for the CO2 and O2 concentration (vol.%) in the adsorption and oxidation atmosphere, respectively. [d] tad and tox stand for the adsorption and oxidation time (h), respectively. [e] n.m. means that the data were not mentioned.

Characterization for oxidative degradation


40

34

44

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Taking into account the complicated degradation pathways of amine–functionalized sorbents, several characterizations were applied for identifying the negative effect of O2 on PEI/SiO2. Thereinto, the microstructures of PEI/SiO2 before and after air exposure were observed by field emission scanning electron microscope (FE–SEM) in Figure S5. But the change of structural morphology was not visible to the loss of CO2 adsorption capacity, which was resemblance with some previous studies. 44 Furthermore, the nitrogen contents of the fresh and treated sorbents were detected by elemental analysis (EA), which were corresponding to the PEI losses, as shown in Table S2. As seen, the contents of C, H and N were reduced in general after high temperature exposure, and theoretically it indicated that some of the organic compounds were escaped from the sorbent during the long–term exposure. Besides, in comparison with the elemental contents of PEI/SiO2 exposed to N2 at the same conditions, air would lead to a slight increase of PEI leaching. However, the N contents were dropped by 21.7% and 24.8% under 135 °C and 150 °C air exposure, respectively, which was still much less than the losses of CO2 adsorption capacities. As mentioned above, the leaching of amine by thermal effect was not the main contributor to the capacity loss. According to the results of last section, the sorbent exhibited a completely different behavior when exposed to CO2–free air or N2. To further analyze the oxidative degradation and thermal degradation of PEI/SiO2, the thermogravimetric analysis ranging of 30–800 °C was employed under the two atmospheres, which was named as PEI/SiO2–N2 and PEI/SiO2–air, respectively, as shown in Figure 4(a). In the both purge gases, PEI/SiO2 was observed a mass loss of approximate 10% below 140 °C due to the removal of CO2 and moisture pre–adsorbed



Page 18 of 38

from ambient air. With a further increase in temperature, N2 and air had completely different effects on the TG and DTG curves of the sorbent. As for PEI/SiO2–N2, the evaporation and destruction of PEI contributed to a characteristic bimodal DTG curve at the temperature ranging from 210 to 550 °C, which were associated with the different positions of PEI loaded.

50

But

under CO2–free air, a rapid mass loss occurred at around 170 °C and a significant peak reflected in the DTG curve. In view of the difference of the components in the atmosphere, O2 was considered to be the main factor inducing the weight loss, and on the other hand, the sorbent was significantly less resistant to air. As the temperature continued to rise under air, the organic components in the oxidized sorbent were further evaporated and decomposed. Since the organic components were totally decomposed at above 680 °C under N2 or air, the residue, namely SiO2, accounted for about 40%. In general, according to TG and DTG curves, the weight loss of PEI/SiO2 could be divided into four stages: stage Ⅰ as the desorption of adsorbates (30–140 °C), stage Ⅱ as the weight loss by oxidative effect (140–200 °C), stage Ⅲ as the thermal evaporation and decomposition (200–680 °C) and stage Ⅳ as the stabilization of the residue (680–800 °C).

In addition, the emissions from PEI/SiO2 decomposed in four stages were

recorded online by a MS detector, when the sample was heated to 100, 170, 320 and 750 °C. It could be seen from the DTG curves that the weight losses of PEI/SiO2 were representative in the corresponding stages when these temperatures were reached. Thereinto, the intensities of mass 14, 16, 28 and 32 were much higher than that of other masses, due to the main components of N2 and O2, so they were listed separately in the inserted figure, as indicated in Figure 4(b). Aside from the N2 and O2, the 27 and 29 fragments resulted the highest peaks, which were


Page 19 of 38 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


considered as the isotope peaks of N2, and other identifiable fragments were those with mass 44 (CO2), 18 (H2O), 17 (NH3) and 1 (H), etc. The formed NH3 could be attributed to the decomposition of NH2 groups and the hydrolysis of the oxidation products.35, 51 In addition, it was noteworthy that O2 was significantly consumed in stage Ⅱ of PEI/SiO2–air, as shown by the arrow, and it was reasonable to think that O2 was reacted for the oxidative degradation. Moreover, the oxidized sorbents exhibited different colors after air exposure at the elevated temperatures, as shown in Figure S6. After air exposure at 75 °C, the sorbent still maintained white like the fresh one. However, with the increasing exposure temperature, the color of the oxidized sorbent was converted from white to yellow and even brown, and the extent of the color change might be associated with the degree of degradation.41 Therefore, the foregoing discussion provided evidence that the oxidative degradation of PEI/SiO2 was more related to several chemical reactions by O2 inducing, and the chemical structures of the oxidation products need to be further analyzed.



TG (wt.%)

(a)

100 80 60 40

II

I

III

IV

DTG (wt.%·min-1)

0 -3

thermal effect SiO2 – N2

-6

SiO2 – Air PEI/SiO2 – N2

-9

PEI/SiO2 – Air

oxidative effect

-12

100

200

300

400

500

600

700

800

o

Temperature ( C)

(b)

PEI/SiO2-N2

2

200 100

Ion current (× 10-11 A)

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

Page 20 of 38

1

0

14

28

0 PEI/SiO2-Air

2

200 100

Stage Stage Stage Stage

1

I II III IV

0

14

16

28

32

0 0

5

10

15

20

25

30

35

40

45

50

m/e (amu)

Figure 4. (a) Thermogravimetric (TG) and derivative thermogravimetric (DTG) curves of SiO2 and PEI/SiO2 under N2 and air. (b) Mass spectra of PEI/SiO2–N2 and PEI/SiO2–air in different stages. In–situ DRIFT spectra of PEI/SiO2 under a flow of CO2–free air were employed to determine the potential structures of the oxidized sorbent, and the spectrum before air exposure was recorded as a background for the correction of the following spectra, as shown in Figure S7. Thereinto, the spectra ranging from 1000 to 2000 cm–1 are exhibited here (Figure 5), where


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the main IR bands are observed. It was worth noting that since the spectrum of the fresh PEI/SiO2 had been deducted from these in–situ DRIFT spectra, the bands after oxidation were considered to be enhanced or weakened, according to whether the absorbances were above or below the baseline. As seen, after air exposure for 12 h at different temperatures (75–150 °C), an obvious decrease in the absorbance of these bands at 2800–3000 cm–1 associated with the – CH2– asymmetric and symmetric stretching in the polyamine. 39, 44 Similarly, it was found that the absorbances of the peaks in the ranges of 1452 and 3300–3400 cm–1 were detected to be weakened than the fresh sorbent, and these bands were assigned to the –CH2– bending vibration and N–H asymmetric and symmetric stretching vibration, respectively, which were existed in the PEI molecules.

41, 44, 48

The obvious reduction of these bands might point to the

transformation or decomposition of the functional groups in the fresh PEI/SiO2, due to the oxidative degradation, and the changes were amplified with the elevated temperatures. On the other hand, a marked band at 1664 cm–1 attributed to C=O or C=N stretching vibration was observed in the oxidized PEI/SiO2, and the intense band was enhanced sharply with the oxidation temperature. Taking into account the foregoing study, the intensity of the band was positively correlated with the degree of amine degradation. This prominent IR band in the range of 1660–1680 cm–1 was consistence with the formation of amide, imine species and/or carboxylic acids, which could be interacted by O2 with the impregnated PEI in the sorbent. 35, 48

The appeared band in this range were also found in other oxidized amines like EDA 52, TEPA

41, PPI 39, and the primary amine–functionalized resin 44, which were considered as the oxidative

species containing C=O or C=N functional groups as well. The new species were additionally



Page 22 of 38

corroborated by a variety of 1D and 2D nuclear magnetic resonance (NMR) spectroscopy techniques in some previous works. 41, 53 Besides, the resolution of N–H bending vibration at 1600 cm–1, the C–N stretching vibration at 1403 cm–1, and the –CH2– deformation at 1274 cm–1 were displayed in the oxidized sorbents and enhanced with the increasing oxidation temperature. The appearance of these bands cross–checked the formation of amide or other similar substances during the oxidative degradation. Notably, as shown in Figure 5(b), the in–situ DRIFT spectra exhibited a similar pattern under air exposure with different times, that the appearance of the bands attributed to C=O or C=N functional groups, was accompanied by an apparent decrease in absorbance of C–H and N–H groups, as summarized in Table 2. The intensity of the characteristic bands was also changed with the increasing oxidation time and the degree of oxidative degradation. As a consequence of oxidative degradation, the amine functional groups in the sorbent were altered and new species like amide or other similar substances were formed by O2 inducing, which resulted in the incapability for CO2 capturing. Table 2. Assignment of the bands in the in–situ DRIFT spectra. Wavenumber (cm–1)

Assignment

Change [a]

Ref.

1274

–CH2– deformation

↑

52

1403

C–N stretching

↑

45

1452

–CH2– bending

↓

41, 54

1600

N–H bending

↑

40

1664

C=O/C=N stretching

↑

35, 40

2794

–CH2– symmetric stretching

↓

39, 52

2886

C–H stretching

↓

41

2925

–CH2– asymmetric stretching

↓

39, 52

3303

N–H symmetric stretching

↓

52, 55


Page 23 of 38

3384

N–H asymmetric stretching

52

↓

[a] The intensity of the characteristic bands is increased (↑) or decreased (↓) after oxidation, compared with the fresh PEI/SiO2. 2.5

1452

1.5

1274

1403

1600

2.0

Absorbance (a.u.)

75ºC 12h 90ºC 12h 105ºC 12h 120ºC 12h 135ºC 12h 150ºC 12h

1664

(a)

1.0

0.5

0.0 2000

1800

1600

1400

1200

1000

-1

Wavenumber (cm ) 0.25

1664

105ºC 1h 105ºC 2h 105ºC 3h 105ºC 6h 105ºC 9h 105ºC 12h

1602

1384

0.20

1452

1276

(b)

Absorbance (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


0.15

0.10

0.05

0.00 2000

1800

1600

1400

1200

1000

-1

Wavenumber (cm )

Figure 5. In–situ DRIFT spectra for PEI/SiO2 before and after air exposure (a) at different temperatures for 12 h and (b) at 105 °C for different times. CO2 capture from O2–containing flue gas After investigating the oxidation behavior of PEI/SiO2 and the O2–induced degradation



Page 24 of 38

mechanism, the elevated temperature was indeed the main factor of the oxidative degradation, and would promote conversion of the amine functional groups into the substances lack of the available CO2 adsorption sites. On the other hand, PEI/SiO2 exhibited an oxidative–stable performance under air exposure condition at below 75 °C, which meant a milder condition would increase the oxidation stability. And more realistically, flue gas with a lower O2 concentration than ambient air also seemed more conducive to the application of PEI/SiO2, so the feasibility of CO2 capture from O2–containing flue gas by PEI/SiO2 could be examined. Firstly, CO2 capture experiments were carried out by TGA at different temperatures under the O2–free or O2–containing simulated flue gas (FG or OFG), and the adsorption–regeneration curves were collected for comparison, as summarized in Figure 6. In terms of CO2 capture from FG, the highest CO2 adsorption capacity of 143.0 mg/g was obtained at 75 °C, which was less than the uptake of 154 mg/g from pure CO2, due to the lower partial pressure. Notably, a different temperature dependency of the adsorption capacity was displayed under the lower and higher adsorption temperatures. With the increasing adsorption temperature from 75 to 120 °C, the CO2 adsorption capacities of PEI/SiO2 were substantially decreased to 30.2 mg/g, due to the exothermic reaction of CO2 and amine functional groups.

56

But at below 75 °C, a lower

temperature was also not conductive to CO2 capture, and the CO2 adsorption capacity was decreased to 87.1 mg/g when the temperature was declined to 35 °C. This observation was associated with the diffusion resistance for high–loading amine–functionalized sorbents and CO2 performance was influenced by both the thermodynamic effect and the diffusion– controlled effect, which were also reported in many previous studies. 24, 47 In addition, according


Page 25 of 38

to the curves, the strategy of N2 purging at 90 °C could realize the complete regeneration of the CO2–adsorbed PEI/SiO2. 150

30°C

60°C

45°C

100

FG OFG

50

CO2 uptake (mg·g-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


0 150

75°C

105°C

90°C

120°C

100 50 0 0

30

60

90

0

30

60

90

0

30

60

90

0

30

60

90

Time (min)

Figure 6. CO2 adsorption–regeneration curves of PEI/SiO2at different adsorption temperatures. Adsorption was carried out under the simulated flue gas (FG, 15% CO2 in N2 balance) and O2–containing flue gas (OFG, 15% CO2 and 10% O2 in N2 balance) , and regeneration was carried out under pure N2 at 90 °C. On the other hand, the CO2 capture behavior of PEI from O2–containing flue gas was also measured under the same condition, depicted as the red dashed lines in Figure 6. It was found that the CO2 adsorption–regeneration curves were almost identical in comparison of those captured from FG, not to mention the CO2 adsorption capacities, when the adsorption temperature was less than 90 °C. CO2 capture from OFG could also achieve an adsorption capacity of up to 142.6 mg/g, and 10% O2 in OFG seemed not to induce the serious degradation



of PEI/SiO2, which was consistent with the results of air exposure. The adsorption capacity was decreased by 0.96% for CO2 capture from OFG relative to FG at 90 °C. However, there was evidence that as the adsorption temperature was further increased, the difference of the CO2 capture from FG and OFG was generally significant, especially at the highest temperature of 120 °C. The distorted adsorption curves at the elevated temperatures were noteworthy. In terms of 105 °C, CO2 uptake was increased to 86.5 mg/g after around 20 min adsorption from OFG, but the values were reduced slightly during the further prolonged adsorption, whereas the final CO2 uptake after 60 min was 85.1 mg/g. The curve exhibited a trend of increasing first and then decreasing, which was significant diverse from the adsorption curve with a continuous growth. As comparison, 88.8 mg/g of CO2 was captured from FG in the first 20 min, and the sorbent further adsorbed to 88.9 mg/g when the adsorption process was finished. As for 120 °C, the trend was more apparent, especially that the CO2 uptake was even less than zero. Considering the interaction of O2 and PEI/SiO2 at the elevated temperature in the foregoing investigation, this phenomenon was owing to the weight loss during the oxidation degradation, which was even lower than the initial weight of the sorbent before adsorption, and the decreasing CO2 uptake was calculated based on the weight change recorded by TGA. Therefore, these results demonstrated that 10% O2 in OFG posed great difficulties to CO2 capture of PEI/SiO2 at the elevated temperatures (105 °C and 120 °C), but the capture condition in flue gas after FGD was not so harsh and the sorbent could maintain oxidative stability in this single adsorption– regeneration experiment at a lower temperature. Cyclic stability under O2–containing flue gas


Page 26 of 38

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To further access the feasibility of PEI/SiO2 for CO2 capture from O2–containing flue gas with a limited temperature range, taking into account that the oxidative degradation would be amplified in the long–term operation, the 50 consecutive cycles of adsorption–regeneration under different conditions were therefore explored. The CO2 adsorption capacities at each cycle were collected in Figure 7(a–d), and the stability under FG or OFG in the long–term operation was compared. In the case of CO2 capture from FG, PEI/SiO2 exhibited a steady behavior in the consecutive adsorption–regeneration cycles under O2–free atmosphere, when the adsorption temperature was less than 90 °C. The CO2 adsorption capacities in the first capture were 118.7, 138.4 and 131.4 mg/g at 60, 75 and 90 °C, respectively, and after 50 cycles, the corresponding losses of adsorption capacity were 0.4%, 0.8% and 4.5%. But at the adsorption temperature of 105 °C, it was gradually decreased from 89.2 to 59.9 mg/g with the cycle times and underwent a loss of 32.9%, which was owing to the amine loss during the long–term operation at the elevated temperature. 57 By contrast, in terms of 10% O2 introducing into FG, the cyclic stability was affected to varying degrees at different adsorption temperatures. After 50 cycles of CO2 capture from OFG, the CO2 adsorption capacities of PEI/SiO2 were only decreased moderately at 60 °C, 75 °C and 90 °C by 1.1%, 2.1% and 8.8%, respectively. It was noteworthy that the CO2 adsorption capacity could be maintained at above 135 mg/g in the long–term operation of 75 °C capture, which was obviously higher than other oxidation–stable amine–functionalized sorbents at the sacrifice of a part of CO2 adsorption capacity. 35, 39, 41 And the adsorption capacities of 116 mg/g were also guaranteed within 50 cycles at 60 °C and 90 °C. Additionally, the level of oxidative degradation



Page 28 of 38

was obviously lower, compared with other chemical pathways like the urea–linkages formation by high–temperature CO2 and irreversible adsorption of acid gases.

10, 58

However, the cyclic

performance of capture from OFG at 105 °C was not satisfactory, and 88.5% of adsorption capacity was dropped dramatically. In view of the results of the oxidation behavior and the single capture at 105 °C, the oxidative degradation was amplified in the long–term operation as expected. According to the cyclic stability measurements, it was demonstrated that CO2 capture by PEI/SiO2 from O2–containing flue gas was stable when the applied temperature was limited to below 75 °C, which was consistent with the conventional condition. On the whole, PEI/SiO2 would be degraded significantly under OFG at above 105 °C and the adsorption capacity was low after multiple cycles, due to the oxidation of the available amine sites, as illustrated in Scheme 1. However, CO2 capture from OFG could be feasible and efficient with a high adsorption capacity remained, when the adsorption temperature was less than 75 °C. Furthermore, if the O2 concentration in flue gas was lower (< 10%), the sorbent might withstand higher operation temperature based on the concentration effect mentioned above.


Page 29 of 38

150 100 50

(a) 60°C

0 150 100


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


FG/N2

50

OFG/N2

(b) 75°C

0 150 100 50

(c) 90°C

0 150 100 50

(d) 105°C

0 150 100

OFG/N2

50 0

OFG/air

(e) 75°C

0

10

20

30

40

50

Number of cycles

Figure 7. (a–d) CO2 adsorption capacities of PEI/SiO2 over 50 cycles of adsorption– regeneration. Adsorption was carried out under FG or OFG at different temperatures, and regeneration was carried out under pure N2 at 90 °C). (e) CO2 adsorption capacities of PEI/SiO2 over 50 cycles of adsorption–regeneration. Adsorption was carried out under FG at 75 °C, and regeneration was carried out under N2 or air at 90 °C. On the other hand, Heydari–Gorji et al. and Yu et al. proposed that air as a low–cost alternative could even be contemplated as the stripping gas for regenerating the sorbent, instead of the conventional inert gases like N2 or Ar. 38, 44 Considering the oxidative effect of 21% O2



in air on PEI/SiO2, we attempted briefly the possibly of air stripping under appropriate conditions in the first time. Firstly, the regeneration temperature was screened from 75 to 95 °C for verifying whether air could completely regenerate the CO2–adsorbed sorbent, as shown in Figure S8. It was obvious that more CO2 was desorbed with the increasing regeneration temperature and the desorption rates were also elevated, caused by the reversible and exothermic interactions of CO2 and amine functional groups. Thereinto, based on the adsorption–regeneration curves, 90 °C was the lowest temperature for achieving complete regeneration in 30 min, which was same with the screened temperature under N2 stripping in our previous study. Then, a similar cyclic adsorption–regeneration experiment with 50 times was performed under the condition of CO2 capture from OFG at 75 °C and air stripping at 90 °C, as presented in Figure 7(e). The additive air exposure at 90 °C induced a reduction of 24.8% in the CO2 adsorption capacity after 50 cycles, compared with the 2.1% by N2 stripping. This result was consistent with the oxidation behavior of PEI/SiO2 under air exposure at 90 °C, and air conduced to a lower tolerance temperature to oxidative degradation relative to the simulated flue gas containing 10% O2. Nevertheless, if a lower regeneration temperature (like 75 °C) was selected or the oxidative resistance of the sorbent was improved, air stripping was still possible for regeneration.


Page 30 of 38

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Scheme 1. CO2 capture performance of PEI/SiO2 from FG or OFG under different conditions.

Conclusions In this work, we explored the feasibility of CO2 capture from the O2–containing flue gas by a conventional PEI–functionalized silica, and emphasized on the oxidation behavior in the long–term operation under different conditions. PEI/SiO2 exhibited an oxidative–stable performance with a limited 0.7% loss of CO2 adsorption capacity after 12 h air exposure at below 75 °C, but a high temperature would accelerate the significant degradation mainly caused by the oxidative effect. In addition, analysis of the oxidized sorbent via TGA–MS and in–situ DRIFT spectroscopy indicated that the oxidative degradation was associated with the formation



of new species like amide or other similar substances by O2 inducing. It was demonstrated that the temperature could play a determining role in the oxidation behavior, and the long–term stable operation of PEI/SiO2 in the O2–containing atmosphere would be feasible by controlling the applied temperature. Taking into account more realistic flue gas condition, the oxidative stability of the sorbent was examined by 50 times of cyclic CO2 capture from the simulated flue gas with 10% O2. In comparison of capture from O2–free flue gas, no significant changes were observed when the adsorption temperature was below 75 °C, which was consistence with the actual temperature range of flue gas after FGD facility. In particular, the CO2 adsorption capacity could be steady at above 135 mg/g in the long–term operation of 75 °C, and the stable and efficient CO2 capture from O2–containing flue gas was achieved by PEI/SiO2.

Supporting Information Detailed characterization methods; textural properties, SEM images and EDS mappings of SiO2 and PEI/SiO2; CO2 adsorption–regeneration curves before and after air exposure; SEM images, elemental analysis, the color changes, DRIFT spectra and O1s spectra of the fresh and oxidized sorbents; regeneration performance by air purging.

Conflicts of interest There are no conflicts of interest to declare.

Acknowledgements We are thankful to the financial support by the National Natural Science Foundation of China (Grant 21576156 and Grant 21776160) and the Major Science and Technology Program 32 ACS Paragon Plus Environment

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for Water Pollution Control and Treatment (2017ZX07202005).



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