Synergistic enhancement of CO2 adsorption capacity and kinetics in

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Environmental and Carbon Dioxide Issues

Synergistic enhancement of CO2 adsorption capacity and kinetics in triethylenetetrammonium nitrate protic ionic liquid functionalized SBA-15 Wei Zhang, Erhao Gao, Yu Li, Matthew T. Bernards, Younan Li, Guanghan Cao, Yi He, and Yao Shi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01872 • Publication Date (Web): 15 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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Synergistic enhancement of CO2 adsorption capacity and kinetics in triethylenetetrammonium nitrate protic ionic liquid functionalized SBA-15

Wei Zhanga,b, Erhao Gaoa,b, Yu Lia,b, Matthew T. Bernardsc, Younan Lia,b, Guanghan Caoa,b, Yi Hea,d*, and Yao Shia,b* a College of Chemical and Biological Engineering, Zhejiang University, Hangzhou, 310027,

PR China b

Key Laboratory of Biomass Chemical Engineering of Ministry of Education, Zhejiang

University, Hangzhou, PR China c

Department of Chemical and Materials Engineering, University of Idaho, Moscow 83844,

USA d

Department of Chemical Engineering, University of Washington, Seattle, Washington

98195, USA

*Corresponding author. E-mail: [email protected] (Y. He); [email protected] (Y. Shi);

ACS Paragon Plus Environment

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Abstract A series of triethylenetetrammonium nitrate protic ionic liquid (TNPIL) functionalized, highly ordered mesoporous SBA-15 molecular sieves with different loadings have been synthesized via impregnation. The CO2 capture performance of the hybrid sorbents was evaluated under conditions mimicking a combustion flue gas (15% CO2) over the temperature range of 298-348 K. The breakthrough experiments revealed that the sieves with 66% mass loadings of TNPIL (S15-66TNPIL) exhibited the highest CO2 adsorption capacity, 2.12 mmol/g, at 333 K, representing a dramatic enhancement compared to the bare support (883%). The intra-particle diffusion model analysis of the hybrid sorbents demonstrated that the S15-66TNPIL had the fastest CO2 uptake rate, 131×10-3 mmol g-1 s-0.5, in the rate-controlling stage. This was almost five times higher than the bare support alone and is a significant improvement over other IL-functionalized and amine-modified support systems. This can be attributed to the synergistic effects of the high affinity between the TNPIL and CO2 and the fast diffusion rate from distributing the TNPIL across the support with a large surface area. In addition, the S15-66TNPIL exhibited great regeneration capacity. FT-IR analysis coupled with isosteric heat and density functional theory simulations revealed that the adsorption state is dominated by chemisorption and the CO2 preferentially interacts with the primary amine –N(3)H2 to form carbamate, based on the high binding energy. Therefore, this novel TNPIL supported


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system represents a promising candidate for CO2 capture and recovery applications.

Keywords: CO2 adsorption; polyammonium protic ionic liquids; mesoporous silica; Nanopore confinement; DFT simulation

1. Introduction The development of cost-effective and high-efficiency purification processes for CO2 capture from flue gas deriving from the large-scale burning of fossil fuels represents one of the most significant scientific and technological challenges since the industrial revolution, in order to address global warming and anthropogenic climate change

1, 2.

Compared with pre-combustion capture and oxyfuel combustion3, post-combustion capture is a promising process with the advantages of low-cost and ease of retrofitting old facilities4. Numerous post-combustion technologies, including solvents, solid sorbents, and membranes5, have been developed not only in the laboratory, but also on an industrial scale. Aqueous amine solvents, such as monoethanolamine (MEA)6 and diethanolamine (DEA)7, are widely used for CO2 purification. However, this technology suffers from serious defects including corrosion8, high energy penalties9 and amine volatility10. To circumvent these challenges, functional solid adsorbents have been proposed as promising alternatives associated with high efficiency and versatility, low costs, and easy handing11. Over the past few years, a series of amine-functionalized solid sorbents have been


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synthesized for CO2 adsorption12. Zhang et al.13 impregnated tetraethylenepentamine into SBA-15 molecular sieves with the template P123 and researched the effect of the template and the amine loadings on CO2 uptake capacity. Ren et al.14 synthesized a series of copper silicate nanospheres grafted with mono-, di- and tri-aminosilanes. The results demonstrated that tri-aminosilane grafted nanospheres possessed a high CO2 adsorption capacity and great cyclic regenerability. However, amine-impregnated sorbents suffer from the disadvantages of amine degradation and evaporative loss15-17. Although amine-grafted materials present higher thermal and mechanical stability, the relatively low adsorption capacity of the sorbents are restricted to practical application11. In this framework, using ionic liquids (ILs)18, 19, which have unique physicochemical properties including negligible vapor pressure, high thermal stability, recyclability, and reasonable CO2 solubility, as a loading solvent is a promising option for post-combustion CO2 capture. In fact, the affinity of ILs towards CO2 can be improved by incorporating active chemisorption sites in the cation or anion structures20. However, the high cost, tedious synthetic procedures, and low absorption capacity of conventional ILs still limit their industrial applications. Compared to conventional ILs, polyammonium protic ionic liquids (PILs) not only have the advantages that conventional ILs possess, but also have the merits of low-cost, easy synthesis, and high absorption capacity. Ilioudis et al. synthesized a polyammonium PIL by one-step acid-base neutralization of diethylenetriamine and nitric acid 21. After that, Doyle and coworkers examined its CO2 absorption capability and the results exhibited that


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it has a strong affinity for CO2 22. Hu et al. synthesized a series of polyammonium PILs by neutralization of the low-cost triethylenetetramine with various inorganic acids, and the CO2 absorption capacity of these PILs followed the anion order of [NO3] > [BF4] > [SO4] ≈ [HSO4]. In addition, a 40 wt% triethylenetetrammonium nitrate aqueous solution showed the best absorption capacity with 1.49 mol CO2 per mol PIL at 288 K and 1 bar23. Although the polyammonium PILs have many merits, their CO2 absorption rates are quite low due to their high viscosities. Supporting PILs on a porous matrix can be an effective approach to overcome this challenge. Numerous studies have investigated supported ionic liquid phase (SILP) for CO2 adsorption. The mesoporous molecular sieve SBA-15 is a popular support that is widely used due to its highly ordered 2D hexagonal (p6mm) pore path, uniform pore size, high hydrothermal stability, high specific surface area, and good compatibility with various ILs. Arellano et al.24 synthesized several SBA15 supported, zinc-functionalized IL sorbents. The S15-50 containing 50 wt% ILs exhibited high CO2 adsorption capacity and rapid uptake rates compared with bare support and bulk ILs. Hiremath et al.25 prepared four different ordered mesoporous silica grafted amino acid ionic liquids for CO2 adsorption-desorption. The resulting hybrid sorbents demonstrated fast adsorption kinetics, moderate adsorption capacity, and easy desorption. Cheng et al.26 introduced trihexyl(tetradecyl)phosphonium 1,2,4-triazole into four kinds of SBA-15 with different pore structures and the obtained hybrid sorbent of SBA-15(4.3)-50% exhibited the fastest CO2 adsorption rate. This was attributed to the fact that this hybrid


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sorbent had the smallest pore size and highest total pore cross-sectional area. In order to further lower the cost and enhance the CO2 uptake rate and capacity, we confined triethylenetetrammonium nitrate into highly ordered mesoporous silica SBA-15 with small pore size. The relationships among TNPIL loading, adsorbent textural properties, and dynamic capture performances are evaluated by coupling small-angle x-ray scattering analysis, N2 porosimetry, thermogravimetric analysis, FT-IR spectroscopy, and breakthrough experiments at 298-348 K with a 15% vol. CO2 stream in a fixed-bed column. The CO2 adsorption kinetics and mechanisms are characterized with a Weber-Morris intraparticle diffusion model and density functional theory (DFT) calculations, respectively.

2. Materials and methods 2.1 Materials SBA-15 with a pore diameter of 4.3 nm was purchased from Nanjing JCNANO Technology Co., LTD. (Nanjing, Jiangsu, China). Hydrochloric acid (HCl, 36-38%), nitric acid (HNO3, 65.0-68.0%), triethylenetetramine (TETA, 68%), and methyl alcohol (EtOH, 99.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Carbon dioxide (99.999%), nitrogen (99.999%), hydrogen (99.999%), and mixed CO2/N2 = 15/75 were purchased from Hangzhou Jingong Specual Gas Co., Ltd. (Hangzhou, Zhejiang, China). All chemicals were commercially available and directly used without


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further purification. The triethylenetetrammonium nitrate ([TETA][NO3]) PIL was prepared by a one-step acid-base neutralization of TETA with HNO3 as reported before23: Aqueous nitrate solution (0.5 mol, 40 wt%) was added dropwise into an aqueous TETA solution (0.5 mol, 60 wt%) in an ice-water bath with stirring and N2 protection. To ensure complete proton exchange, the mixture was continually stirred at 293 K for 4 h. Then, the remaining water in the mixture was removed by heating at 373 K for 24 h under vacuum. Finally, the light-yellow [TETA][NO3] was obtained. The hybrid materials were obtained through a wet impregnation method. The impregnation was carried out as follows: a desired amount of [TETA][NO3] was dissolved in 30g of methanol and the mixture was stirred for 30 min at room temperature to promote the complete dissolution. Then, 1g of SBA-15 was added in the solution and the system was stirred for 6 h at room temperature. The methanol was removed at 353 K for 3 h with reflux. The residue was further dried at 373 K under vacuum for 12 h. The obtained samples were denoted as S15-xTNPIL, where TNPIL represents [TETA][ NO3] and x refers to the mass percentage of the TNPIL to the support.

2.2 Characterization of hybrid materials The sorbents textural properties were analyzed by nitrogen physisorption measurements at 77 K with an ASAP 2460 apparatus (Micromeritics Inc., USA). Prior to


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nitrogen adsorption-desorption measurements, the samples were degassed at 373 K in high vacuum for 12 h. The specific surface area (SSA) of sorbents was calculated with the Brunauer-Emmett-Teller (BET) model over a relative pressure (P/P0) range of 0.05-0.30. The elemental analysis was executed with an elemental analyzer (Flash EA 1112, ThermoFinnigan, Italy). Thermogravimetric analysis was conducted with an SDT Q600 (TA Instruments, Inc., New Castle, DE) over the temperature range of 303-773 K with a 10 K/min heating rate, under N2. The small-angle x-ray scattering (SAXS) analysis was obtained with a SAXS/WAXS SYSTEM instrument (XENOCS Co., Ltd., France) using Cu Kα radiation (λ = 0.15418 nm) at 50 kV and 0.6 mA, and a 2θ range of 0.5°-3.5°. The FT-IR spectroscopy (NICOLET 6700, thermal scientific, USA) was carried out between 4000 and 400 cm-1 to identify the surface functional groups of the sorbents.

2.3 CO2 breakthrough experiments and adsorption isotherms The CO2 breakthrough experiments at different temperatures (T = 298, 313, 333 and 348 K) were carried out at atmospheric pressure in a fixed-bed column (6 mm inner diameter and 14 cm length). The column was packed with 0.5 g of the sorbent and fed with a simulated flue gas (CO2 15 vol %, N2 85 vol %) at a flow rate of 20 mL/min. Prior to CO2 adsorption, the sorbents were degassed under a nitrogen flow of 50 mL/min at 423 K for 1.5 h. The temperature was controlled by electrical heaters connected to a PID controller and a K-type thermocouple. The dynamic CO2 concentration of the outlet gas was detected


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by a gas chromatograph with a TCD detector. The CO2 uptake capacity of the sorbents were obtained by the sample CO2 balance, over the column at saturation ωsat (mmol/g): 𝑠𝑎𝑡

ω

1 = 𝑀

[∫

𝑡

𝑄 0

]

𝐶0 ― 𝐶𝑡 𝑇0 1 𝑑𝑡 1 ― 𝐶𝑡 𝑇 𝑉𝑚

(1)

where Q represents the simulated flue gas flow rate (mL/min); 𝐶0 and 𝐶𝑡 represent CO2 content (vol %) of the influent and effluent, respectively; M represents the weight of sorbent (g); 𝑇0 is 273 K; 𝑇 refers to the setting temperature (K); 𝑉𝑚 is 22.4 mL/mmol. The adsorption isotherms of pure CO2 on the sorbents at 298, 323, and 348 K from 0-1 bar were measured with a CO2 physisorption apparatus (ASAP 2460, Micromeritics Inc., USA). Before the isotherm measurements, the samples were activated at 373 K for 12 h under vacuum conditions.

2.4 Density functional theory calculations The geometry optimization and binding energy (BE) between different active sites and CO2 were calculated by density functional theory (DFT) simulations using DMol3 code in Material Studio 2017 R2. The calculations were carried out with Becke’s threeparameter exchange functional and gradient-corrected functional of Lee, Yang, and Parr (B3LYP)27, 28, coupled with the grimme method29 and DNP basis sets. The binding energy of TNPIL-CO2 complexes was obtained with the following equation: BE = E[𝑇𝑁𝑃𝐼𝐿 ― 𝐶𝑂2] ―𝐸[𝑇𝑁𝑃𝐼𝐿] ―𝐸[𝐶𝑂2]


(2)

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where E[TNPIL - CO2] represents total energy of the optimized TNPIL-CO2 complexes, and E[TNPIL] and E[CO2] represent total energy of the optimized TNPIL and CO2 molecules, respectively.

2.5 Mathematical model of adsorption isotherms and kinetics 2.5.1 Model for adsorption isotherms The dual-site Langmuir model30,

31

is used to describe the adsorption behavior

involving two different adsorption sites. In this system, one is chemisorption and the other one is physisorption. The equation is defined as: 𝑞𝑒 =

𝑞𝑚1.𝑏1𝑝1 1 + 𝑏1𝑝1

+

𝑞𝑚2.𝑏2𝑝2 1 + 𝑏2𝑝2

(3)

where qm1, qm2 and b1, b2 are Langmuir parameters representing the maximum adsorption capacity and adsorption affinity constant for sites 1 and 2, respectively; qe is the equilibrium uptake capacity; p1 and p2 represent the pressure of CO2 in site 1 and 2.

2.5.2 Isosteric heat calculation Isosteric heat (ΔHiso) is calculated from adsorption data at different temperatures using the Clausius-Clapeyron equation as follows 32, 33: ∆𝐻𝑖𝑠𝑜 = 𝑅

𝑑𝑙𝑛𝑝 1 𝑑 𝑇

()

(4)

where p is pressure (bar) at equilibrium uptake, T is the temperature (K), and R is the


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universal gas constant, 8.3145 J/(mol·K).

2.5.3 Model for adsorption kinetics In order to understand the kinetic behavior and rate-controlling step of adsorption, the CO2 adsorption capacity was plotted against t0.5 using the intra-particle diffusion model (IPDM) proposed by Weber and Morris34: 𝑞𝑡 = 𝑘𝑡0.5 +𝐶

(5)

where qt (mmol g-1) represents CO2 uptake capacity at time t (s); k (mmol g-1 s-0.5) refers to the intra-particle diffusion rate constant, and C (mmol g-1) is the intercept.

3. Results and discussion 3.1 Nature of the PILs [TETA][NO3] is a typical representative of the subcollection of PILs, which are usually synthesized through proton transfer from acids to organic bases35. The energy minimized structure and molecular dimensions of the TNPIL are shown in Figure 1, based on an evaluation of the total energy of the structure for NO3- binding with four different amine sites. The structure of [TETA][NO3] is similar to both theoretical and experimental structures of ethylammonium nitrate as reported by Emel’yanenko et al.36. The contact-ion pairs indicate that one of the secondary amines in TETA is protonated by the addition of nitric acid, to form a quaternary ammonium center that interacts with the nitrate anion via


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a strong electrostatic attraction.

Figure 1. Energy minimized structure and molecule dimension of [TETA][NO3]. Color code: gray, C; red, O; blue, N; white, H.

3.2 Physicochemical properties of hybrid sorbents As reported in the literature13, 26, 37, the addition of the TNPIL has a negative effect on the textural properties of the support. To better understand the effect of the TNPIL content on the support, N2 porosimetry analysis and small angle X-ray diffraction were used to obtain the structural properties of the hybrid sorbents. N2 adsorption-desorption isotherms, specific surface area, and SAXS spectra for the bare support and S15-xTNPIL are depicted in Figure 2. The adsorption results in Figure 2a demonstrate that each sample exhibits a


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type Ⅳ adsorption isotherm with a hysteresis loop of type H1 over the relative pressure range of 0.7 ≤ P/P0 ≤ 0.9. This indicates the sorbents have a significant contribution from mesopores. Furthermore, nearly 81.5 cm3 g-1 of N2 adsorbed to SBA-15 at a P/P0 < 10-2, which indicates the SBA-15 has a negligible contribution from micropores. The crystal phase data of the sorbents are shown in Figure 2c. It can be seen that the diffraction peaks are at 2θ = 0.83°, 1.44° and 1.67°, corresponding to the (100), (110), and (200) crystallographic planes in 2D hexagonal (p6mm) mesoporous silica38, 39. When the TNPIL content exceeds 50%, the intensities of the three diffraction peaks are significantly decreased, which indicates the excess TNPIL resides in the support, leading to a decrease in the order of the crystal phase. In addition, the specific surface area of the sorbents gradually decreases as the TNPIL loading increases, with good correlation (SSA R2 > 0.96, Figure 2b). The dp value of S15-33TNPIL listed in Table 1 is slightly increased, with respect to the bare support, which can be attributed to a greater occlusion of micropores. On the contrary, the more TNPIL impregnated into the support, the lower the diameter of the sorbent, due to the pore filling of wider mesopores. The actual loading of the TNPIL in SBA-15 was detected by elemental analysis and the results are listed in Table 2. The results show that nitrogen and carbon levels in S15xTNPIL increase with increases in the ratio of the TNPIL to the support and the average actual TNPIL loading had a corresponding increase from 22.02% to 52.70%. These results are lower than the theoretical weight percentages due to the weight loss of the TNPIL


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during the impregnation process. In addition, the actual PIL loading of S15-66TNPIL was also evaluated through thermogravimetric analysis and the content was 47.87%, which is in close agreement to the elemental analysis result of 47.75%. Moreover, the thermogravimetric analysis was also carried out to assess the thermostability of the sorbents and the results demonstrate that the thermostability of S15-66TNPIL is comparable to other ILs-functional materials 25, 40. Figure 3 shows that the weight loss of SBA-15 exhibits two stages: the first stage occurs below 373 K, and the weight loss is due to the evaporation of physisorbed moisture; the weight loss in the second stage, from 373 K to 773 K, is only 4.25%, and it is attributed to the decomposition of residual template P123 and the dehydration of hydroxyl groups on the pore surfaces. This indicates the support is thermally stable over the temperature range of 273–773 K. In contrast, the weight loss of S1566TNPIL is divided into three regions: the first stage occurs below 353 K, where the weight loss is 5.74% due to moisture evaporation and CO2 desorption as verified by FT-IR analysis (results presented below); the second stage, from 398-538 K, there is a weight loss of 20.32%, which may be due to the decomposition of the TNPIL aggregated in the hexagonal pore path; the third region, when temperatures are greater than 538 K, demonstrates a weight loss of 27.55% which is possibly caused by the disintegration of TNPIL residue on the silicon wall surfaces, as well as in silica rings via hydrogen bonding. The successfully impregnation of TNPIL into the support was further confirmed by FT-IR analysis performed on the bare support and S15-66TNPIL. A comparison of


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transmittance spectra and absorption peaks are reported in Figure 4. The strong characteristic absorption peaks at 1092, 800, and 463 cm-1 can be assigned to asymmetric and symmetric stretching vibration peaks, and the bending vibration peak of Si-O-Si bonds, respectively41. After the TNPIL was impregnated, the following emerging characteristic vibration modes could be recognized: i) the broad absorption peaks at around 2958 and 1460 cm-1 are attributed to the stretching and deformation vibrations of the C-H bond in imine groups, respectively42; ii) the peak observed at 1563 cm-1 is related to the bending vibration peaks of N-H from the primary amine in the TNPIL43; iii) the functionalized sorbent exhibits two peaks at 1383 and 833 cm-1 which are assigned to NO3- vibration. In addition, the broad absorption peak at 2368 cm-1 is the stretching vibration of CO2 molecules in the gas phase due to physisorption. A comparison of transmittance spectra of S15-66TNPIL and S15-66TNPIL-CO2, which represents S15-66TNPIL following a CO2 adsorption experiment, demonstrates that CO2 molecules are adsorbed by the amine groups in the TNPILs to form carbamate. This was further verified by the isosteric heat analysis and DFT calculations below. In case of S15-66TNPIL-CO2, the emerging characteristic absorption peak at 1598 cm-1 is related to the stretching vibration peak of COO- in the NHCOO- groups44, which indicates that the CO2 adsorption is dominated by chemisorption.


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Volume adsorbed(cm3/g)

1000

(a) S15- 0 TNPIL S15-33TNPIL S15-50TNPIL S15-66TNPIL S15-75TNPIL

800

600

400

200

0 0.0

0.2

0.4

0.6

0.8

1.0

Relative Pressure(P/P0)

600

BET specific surface area(m2/g)

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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(b)

500 400 300 200 100 0 0

10

20

30

40

50

TNPIL loading(wt%)


60

70

80

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(c)

(100)

S15- 0 TNPIL S15-33TNPIL S15-50TNPIL S15-66TNPIL S15-75TNPIL (110)

0.5

1.0

1.5

(200)

2.0

2(degree)

2.5

3.0

3.5

Figure 2. Representative (a) N2 adsorption-desorption isotherms at 77 K, (b) specific surface area of S15-xTNPIL as a function of the TNPIL loading, and (c) representative SAXS spectra.

Table 1. Physical properties of S15-xTNPIL Sample

SBET (m2/g)

Vp (cm3/g)

dp (nm)

S15- 0 TNPIL

553.93

1.44

5.26

S15-33TNPIL

262.89

0.78

5.38

S15-50TNPIL

114.73

0.36

4.51

S15-66TNPIL

24.39

0.10

3.65

S15-75TNPIL

2.99

0.005

2.59


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Table 2. Elemental composition and average actual TNPIL loading of S15-xTNPIL

Sample

S15- 0 TNPIL S1533TNPIL S1550TNPIL S1566TNPIL S1575TNPIL

N (%)

C (%)

H (%)

N amount

C amount

(mmol/g)

(mmol/g)

Average actual TNPIL loading (%)

0.07

0.07

0.60

0.05

0.06

0.00

8.25

9.54

3.29

5.89

7.95

22.02

8.78

10.03

3.35

6.27

8.36

23.30

18.03

20.49

5.96

12.88

17.08

47.75

19.77

22.79

6.94

14.12

18.99

52.70


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100

2.53% 373

8.99%

90

4.25%

353

398

80

Mass(%)

20.32% 538

70 60

SBA-15 S15-66TNPIL

27.55%

50 613

40 30

273

373

473

573

773

673

Temperature(K)

Figure 3. Representative TGA curves of bare support and S15-66TNPIL.

SBA-15 S15-66TNPIL S15-66TNPIL-CO2

Transmittance(%)

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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2368

1598

1460

800 3438

2958 2852

1629

833

1563

4000

3000

2000

463

1383 1092

-1

Wave number(cm )

1000

0

Figure 4. Representative transmittance spectra and absorption peak identification for bare


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

support, S15-66TNPIL, and S15-66TNPIL-CO2.

3.3 CO2 adsorption to hybrid sorbents The CO2 breakthrough curves, dynamic uptake, adsorption capacities, and amine efficiency of S15-xTNPIL at 313 K and 0.15 bar are shown in Figure 5a, 5b, and 5c, respectively. As a control, the adsorption capacity of bare support was measured to be only 0.24 mmol/g. As the TNPIL loading is increased, the breakthrough time and CO2 adsorption capacity initially increase due to the presence of more amine groups, and then decline, due to the blockage of the pores and the interior of the support from excess TNPIL. The maximum adsorption capacity measured was 1.93 mmol/g for S15-66TNPIL, which is a significant enhancement compared to bare support (804%). However, the highest adsorption efficiency (mol CO2/ mol N) found among these sorbents was on the S1550TNPIL. The IPDM model was utilized to understand the adsorption kinetics and ratecontrolling step of the sorbents. Compared with some previously reported aminefunctionalized45 and ILs-modified24,

26, 46

support systems under same experimental

conditions, the adsorption rate of S15-66TNPIL in the rate-controlling step (131×10-3 mmol g-1 s-0.5) shows significant improvement over the bare support and these previously reported systems, as shown in Figure 5f. The IPDM kinetic plots for S15-xTNPIL are shown in Figure 5d. The kinetic process


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Energy & Fuels

of S15-xTNPIL is divided into three distinct stages: Stage Ⅰ is the initial step dominated by external surface adsorption; Stage Ⅱ is the adsorption region related to intra-particle diffusion; Stage Ⅲ is the equilibrium step. The adsorption rate constants for these three stages are expressed as kⅠ, kⅡ, and kIII, respectively, and they are equal to the slope of a linear fit to the uptake data. In case of SBA-15, the rate constants value of the initial step and the second adsorption region are similar, and this is attributed to the homogeneous physicochemical properties of the internal and external surfaces. Compared with bare support, the TNPIL functionalized sorbents exhibit an apparent 3-step mechanism. Moreover, the adsorption rate of kⅡ for S15-66TNPIL is approximately five times higher than the bare support. The basic nature of the amine groups in the TNPIL provides chemisorptive sites for CO2. As the loading of the TNPIL increases, the adsorption rate of kⅡ increases at first and then declines, as shown in Figure 5e. This is related to the diffusion limitation resulting from excess TNPIL. In addition, the adsorption rate of stage Ⅱ in these hybrid sorbents is significantly higher than the other two stages, thus the second stage of the adsorption process is the rate-controlling step. Overall, the high adsorption capacity and fast uptake rate of the sorbents are an effect of spreading the TNPIL across a large surface area to increase the number of available adsorption sites while eliminating the mass transfer viscosity limitation of bulk TNPIL.


Energy & Fuels

1.0

(a)

0.8

S15- 0 TNPIL S15-33TNPIL S15-50TNPIL S15-66TNPIL S15-75TNPIL S15-83TNPIL S15-86TNPIL

C/C0

0.6 0.4 0.2 0.0 0

2 CO2 uptake(mmol/g)

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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2

4

6

8

Time(min)

10

12

14

(b)

S15- 0 TNPIL S15-33TNPIL S15-50TNPIL S15-66TNPIL S15-75TNPIL S15-83TNPIL S15-86TNPIL

1

0 0

500

1000

1500

2000

t(s)


2500

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0.5

(c)

Adsorption efficiency(mol CO2/mol N)

CO2 adsorption capacity(mmol/g)

2.0 1.8

0.4

1.6 1.4

0.3

1.2 1.0

0.2

0.8 0.6

0.1

0.4 0.2 0

20

40

60

0.0

80

100

TNPIL loading content(%) 2

(d) S15- 0 TNPIL S15-33TNPIL S15-50TNPIL S15-66TNPIL S15-75TNPIL S15-83TNPIL S15-86TNPIL Stage Ⅰ Stage Ⅰ Stage III

CO2 uptake(mmol/g)

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

Energy & Fuels

1

0 0

10

20

0.5

0.5

t (s )

30


40

50

Energy & Fuels

160

(e)

140

kⅠ(*10-3, mmol g-1 s-0.5)

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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120 100 80 60 40 20 0

20

40

60

80

100

TNPIL loading(wt%)

Figure 5. Representative (a) CO2 breakthrough curves, (b) dynamic uptake, (c) adsorption capacity and amine efficiency, (d) IPDM kinetic plots, and (e) adsorption rate during stage Ⅱ of S15-xTNPIL at 313 K and 0.15 bar. (f) Comparison of the adsorption rate kⅡ for S15-66TNPIL to previously reported amine and ILs functionalized support systems24, 26, 45,


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Energy & Fuels

46.

3.4 Effect of temperature on adsorption performance Adsorption temperature is an important parameter in CO2 adsorption kinetics. Thus, the effect of temperature on the CO2 uptake performance of S15-66TNPIL was evaluated at temperatures of 298, 313, 333, and 348 K, as depicted in Figure 6. The CO2 adsorption process is influenced by thermodynamics and dynamics47. As the temperature increases from 298 to 333 K, the adsorption capacity of S15-66TNPIL has a corresponding increase from 1.86 to 2.12 mmol/g. The increase is due to the impact of the higher temperature on the diffusion of CO2, thus more CO2 adsorbs with the TNPIL. Therefore, the adsorption process is dominated by dynamics at temperatures below 333 K. However, when the temperature rises to 348 K, the adsorption capacity sharply reduces to 1.49 mmol/g because the adsorption process of CO2

to amine groups is an exothermic reaction, and therefore

less favorable at high temperature. In addition, the CO2 desorption rate is faster than the adsorption rate at higher temperatures. Therefore, the operation temperature for CO2 adsorption from a combustion flue gas should be at or below 333 K.


Energy & Fuels

2.5

2.0 CO2 uptake (mmol/g)

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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1.5 298 K 313 K 333 K 348 K

1.0

0.5

0.0 0

10

20

30

Time (min) Figure 6. Representative CO2 dynamic uptake of S15-66TNPIL at different temperatures.

3.5 Adsorption isotherms and isosteric heat Figures 7a and 7b exhibit the CO2 adsorption isotherms, dual-site Langmuir model plots, and isosteric heat of adsorption for CO2 on S15-66TNPIL, respectively. The isotherms present a sharp rise at low pressure due to the chemisorption between CO2 and the amine groups in the TNPIL. This is followed by a slow increase from 0.15 to 1.0 bar because the adsorption turns to a physisorption process. Moreover, the dual-site Langmuir model closely fits the adsorption isotherms across this range of pressures at all three temperatures, which indicates the heterogeneous nature of the surface48. The isosteric heat is calculated by the Clausius-Clapeyron equation and the obtained value for S15-66TNPIL


Page 27 of 36

is significantly higher than 40 KJ/mol. This indicates that the adsorption process on these sorbents is dominated by chemisorption49. In addition, the isosteric heat curve for the sorbent decreases linearly, further confirming the heterogeneous nature of the surface. 70

(a)

298 K 323 K 348 K Dual-site Langmuir model

Adsorbed Amount(cm3/g)

60 50 40 30 20 10 0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Pressure(bar) 90

(b)

80 70

ΔH (KJ/mol)

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

Energy & Fuels

60 50 40 30 20 10 0

15

20

25

30

35

40

45

50

Adsorption capacity(cm-3/g)

Figure 7. Representative (a) adsorption isotherms and dual-site Langmuir model plots at


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298 K, 323 K, and 348 K and (b) isosteric heat of adsorption for CO2 on S15-66TNPIL.

3.6 Adsorption mechanism To get deeper insight into the interactions between amine groups and CO2 at the molecular level, DFT calculations were conducted to calculate interaction configurations and binding energies for CO2 adsorption over different amine groups. The physisorptive and chemisorptive BEs between different active sites and CO2 are listed in Table 3 and the optimal adsorption configuration is presented in Figure 8. In this configuration, the binding interactions of two TNPIL molecules contain intermolecular hydrogen bonds of NH…ONOO- and N-H…NH2. In addition, both non-protonated nitrogen species and the oxygen sites in the nitrate have a physisorption effect on CO2 resulting in the relaxation of specific adsorption sites and CO2. After CO2 adsorbed in an amine group, the proton of the amine is transferred to an adjacent amine from another TNPIL, forming a carbamate zwitterion that is stabilized by a hydrogen bond with a protonated amine site in a neighboring TNPIL. Furthermore, only the high BE of carbamate-N(3)H3+-TNPIL (-64.5 kJ/mol) is consistent with the theoretical chemisorption value50 and our experimental results, so the primary amine -N(3)H2 is the main active site that reacts with CO2. The reaction can be depicted as followed: CO2 + 2RNH2 → RHNCOO- + RNH3+

(6)

These results provide a theoretical basis for the design of highly efficient CO2 adsorbents.


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Energy & Fuels

BE = -22.6 kJ/mol

BE = -64.5 kJ/mol

Carbamate-N(3)H3+-TNPIL

TNPIL(N3)-CO2

Figure 8. Optimal adsorption configurations of CO2 over the –N(3)H2 in TNPIL. Color code: gray, C; red, O; blue, N; white, H.

Table 3. Binding energies calculated at the B3LYP/DNP level BE B3LYP/DNP

Structure

(kJ/mol) TNPIL(N1)-CO2

-29.4

TNPIL(N2)-CO2

-33.2

TNPIL(N3)-CO2

-22.6

TNPIL(O1)-CO2

-13.5

TNPIL(O2)-CO2

-28.3


-15.7


-26.6


-64.5


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

3.7 Regenerability In light of its superior CO2 adsorption capacity and uptake rate, the stability of S1566TNPIL was examined over multiple adsorption/desorption cycles. The results show that almost 92% of the original CO2 adsorption capacity of the sorbent is maintained after 10 cycles. In these cyclic tests, the adsorption step was performed at 333 K and the inlet CO2 concentration was 15 vol%, and the regeneration process was conducted at 373 K in N2 for 30 mins. Figure 9 shows that the adsorption capacity of S15-66TNPIL decreased by 8.0% after the first cycle. The decrease of adsorption capacity can be attributed to the significant increase of viscosity of TNPIL after CO2 capture due to the formation of a carbamate which creates a hydrogen bonds network leading to slower diffusion during adsorbent recovery 20, 51.

However, there are only minor fluctuations for saturated adsorption capacity over the

subsequent 9 cycles. Therefore, S15-66TNPIL has great recycling performance. These preliminary results are encouraging for the industrial applications of the TNPILfunctionalized sorbents in CO2 adsorption.


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Page 31 of 36

2.5

CO2 adsorption capacity(mmol/g)

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

Energy & Fuels

2.0

1.5

1.0

0.5

0.0 0

1

2

3

4

5

6

7

8

9

10

11

Cycle

Figure 9. Regeneration properties of S15-66TNPIL for CO2 adsorption.

4. Conclusions In this work, supported protic ionic liquid sorbents were synthesized by wet impregnation of low-cost triethylenetetrammonium nitrate into a highly ordered mesoporous silica at different loading levels. The breakthrough experiments highlighted that SBA-15 loaded with 66 wt% PIL exhibited the highest CO2 adsorption capacity, reaching 2.12 mmol/g at 333 K. On the other hand, the CO2 uptake rate of the sorbent in the rate-controlling step is 131×10-3 mmol g-1 s-0.5, nearly five times higher than the bare support and significantly improved over other related IL and amine functionalized support systems. In addition, DFT calculations suggested that the primary amine -N(3)H2 in TNPIL acts as an active site for CO2 capture. During cyclic adsorption/regeneration experiment,


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the system was able to restore 92% of the initial CO2 adsorption capacity. Overall, TNPIL impregnated SBA-15 sorbents exhibit the advantages of low-cost, high adsorption capacity, fast adsorption kinetics, and desirable regenerability. Thus, they may serve as promising candidates for CO2 capture in industrial applications.

Acknowledgements The authors would like to thank the National Natural Science Foundation of China (grant number 21676245 and 51750110495) and the National Key Research and Development Program of China (grant number 2018YFC0213806) for financial support.

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Synergistic enhancement of CO2 adsorption capacity and kinetics in

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