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Thermal stability-enhanced tetraethylenepentamine/ silica adsorbents for high performance CO capture 2

Sunghyun Park, Keunsu Choi, Hyun Jung Yu, Young-June Won, Chaehoon Kim, Minkee Choi, So-Hye Cho, Jung-Hyun Lee, Seung Yong Lee, and Jong Suk Lee Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04912 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

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Industrial & Engineering Chemistry Research

Thermal stability-enhanced tetraethylenepentamine/silica adsorbents for high performance CO2 capture Sunghyun Park1,2,+, Keunsu Choi3,+, Hyun Jung Yu4,+, Young-June Won4, Chaehoon Kim5, Minkee Choi5, So-Hye Cho1, Jung-Hyun Lee2,*, Seung Yong Lee1,*, and Jong Suk Lee4,* 1

Center for Materials Architecturing, Korea Institute of Science and Technology, Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea 2

Department of Chemical and Biological Engineering, Korea University, 5-1 Anam-dong, Seongbuk-gu, Seoul 136-791, Republic of Korea 3

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology, Ulsan 44919, Republic of Korea

4

Department of Chemical and Biomolecular Engineering, Sogang University, Seoul 121-742, Republic of Korea

5

Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea

ACS Paragon Plus Environment

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Industrial & Engineering Chemistry Research 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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ABSTRACT

Tetraethylenepentamine (TEPA), consisting mainly of primary and secondary amines, exhibits a high CO2 sorption capacity; however, its poor thermal stability hampers practical utilization in the temperature swing adsorption process for CO2 capture. Here, a facile functionalization of TEPA with 1,2-epoxybutane (EB) substantially enhanced its thermal stability as well as the CO2 adsorption kinetics. Our careful analysis on the liquid-state

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C NMR disclosed the amine state

distribution of EB-functionalized TEPA (EB-TEPA). Although the increase in tertiary amine portion induced by EB functionalization reduced CO2 sorption capacity, the 0.64EB-TEPA (i.e., TEPA functionalized with EB with a TEPA/EB molar ratio of 1:3)/SiO2 showed an excellent long-term stability over the 10 consecutive cycles of adsorption/desorption processes with a CO2 swing capacity of 2.0 mmol CO2 g-1 under dry CO2/N2 (15/85 mol/mol) feed conditions. Also, the first principles calculation identified the configuration of modified TEPA molecules with XRD measurements, supporting an easy access of CO2 into amine moieties of our modified TEPA molecules.

1. INTRODUCTION Serious environmental concerns associated with global warming have motivated many research groups to focus on developing more efficient materials and processes to capture CO2, considered the major cause of global warming. Atmospheric concentrations of CO2 have been continually increasing from a preindustrial value of ca. 280 ppmv to 379 ppmv in 2005, and they are expected to increase even up to 570 ppmv in 2100, possibly increasing the mean global temperature by ca. 1.9 oC1. Among various large point sources for CO2 emissions including coalfired power plants, natural gas processing plants, steel and iron industry, and cement plants etc,


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the coal-fired power plants have been a main focus to mitigate anthropogenic CO2 emissions.2 More specifically, aqueous-amine based absorption is being considered the most mature and industrially developed technology due to the lowest CO2 capture cost. It, however, still requires a considerable amount of regeneration energy due to large amounts of water and also involves adverse environmental impacts due to the corrosive nature of amine. A post-combustion CO2 capture retrofit with solid sorbents may be more attractive over the current state-of-the-art amine-based absorption process since it requires reduced energy for regeneration due to the absence of large amounts of water. Especially, a variety of mesoporous materials loaded with basic nitrogen functionality, so-called supported amine sorbents are promising due to their high CO2 adsorption capacity as well as high selectivity for CO2 over other gases.3,4 Typically, mesoporous silica is used as solid support materials since it requires no treatment with costly surfactants like structure directing agents for the application of CO2 adsorbents and also it is thermally stable compared to other porous materials such as metal organic frameworks (MOFs). Many different types of amines can be impregnated into solid supports

including

diethylenetriamine.

(DETA),

tetraethylenepentamine

(TEPA),

polyethylenimine (PEI) etc.5–16 Even though PEI is most commonly used in amine impregnation due to its excellent thermal stability, TEPA is still a promising candidate due to its high CO2 sorption capacity provided its cyclic stability is secured. For instance, Lu et al.17 demonstrated an excellent CO2 adsorption capacity of 4.27 mmol CO2 g-1 sorbents by impregnating TEPA into Ytype zeolite with a Si/Al molar ratio of 60. Nevertheless, a desorption temperature of 80 oC already started to induce decrease in cyclic swing capacity due to poor thermal stability. Many other research groups also observed drastic reduction in CO2 swing capacity of TEPA-containing


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solid sorbents over the consecutive adsorption/desorption cycles even though the initial adsorption capacity was quite attractive.4,18–23 Various chemical functionalizations of primary to secondary amines have been proposed to enhance the thermal stability of TEPA.21–23 Noteworthy is that the primary amine tends to suffer from oxidation-induced degradation as well as relatively high volatility-induced loss even though it shows a larger adsorption capacity.23 Kim et al.21 functionalized TEPA with acrylonitrile (AN) via Michael addition reaction and observed that the thermal stability of TEPA was enhanced with increasing degree of AN-functionalization. Wu et al.23 also observed acrylamide-functionalized TEPA containing adsorbents exhibited enhanced cyclic stability compared to that of the pristine TEPA-containing counterparts. However, none of them quantitatively analyzed the amine state distribution of their modified TEPA due to the presence of various impurities in the commercial TEPA. Neither, their CO2 adsorption kinetics was investigated. More recently, Choi et al.24 suggested a facile and effective methodology of regenerating the PEI/SiO2 sorbents under 100% CO2 atmosphere at 120 oC without the addition of steam by functionalizing PEI with 1,2epoxybutane (EB). The chemical functionalization of PEI with EB substantially improved the long-term stability of PEI by suppressing undesired urea formation as well as oxidative amine degradation. Although it was associated with PEI modification, we presumed that the functionalization of TEPA with EB would be beneficial for the thermal stability enhancement of TEPA by inducing both high molecular weight and hydrogen bonding due to the addition of 2hyroxybutyl moieties. In this work, the effect of EB-functionalization of TEPA on its thermal stability as well as cyclic swing capacity was investigated by varying the degree of EBfunctionalization. Both CO2 adsorption kinetics and equilibrium uptake in the associated


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TEPA/SiO2 adsorbents were evaluated. Also, the corresponding possible chemical structure of functionalized TEPA was identified based on the first principles calculations. 2. EXPERIMENTAL SECTION 2.1. Materials.

All chemicals were used as received without further purification.

Tetraethylenepentamine (TEPA) (Acros, Technical grade), 1,2-epoxybutane (EB) (Sigma Aldrich, 99%), and methanol (Merck, analytical grade) were used as received. Highly pure N2 (a purity 99.999%) and mixed gas (15/85% CO2/N2) were prepared by Shin Yang Oxygen Co. (Seoul, South Korea). 2.2. Synthesis of Spray-Dried Silica.

Silica microspheres having large porosity were

synthesized by spray-drying of a water slurry containing 1 wt. % fumed silica (OCI, KONASIL K-300) and 0.33 wt. % carbon black (Columbian Chemical, Raven H2O) as a macroporogen. In a typical synthesis, 30 g fumed silica, 10 g of carbon black, 10 g Triton X-100 (Sigma-Aldrich), and 2.95 kg water were mixed. Triton X-100 was added to facilitate the dispersion of carbon black in the aqueous slurry. The resultant slurry was injected to the spray dryer (Shanghai Bilon Instrument Co., Ltd, Bilon-6000y) at the delivery rate of 15 rpm. The air blowing inlet temperature was 185 oC and the outlet temperature was 90 oC. The silica microspheres were collected by cyclone separator at a frequency of 60 Hz and then calcined in dry air at 800 oC to remove the organics and sinter the fumed silica into a 3D porous network. 2.3. Preparation of Adsorbents. Chemical functionalization of 10 g of TEPA was performed by adding different amounts of EB dropwise in TEPA/methanol (17/83 wt/wt) solution followed by stirring reaction solutions for 12 h at room temperature. The reaction of TEPA and EB proceeded at different molar ratios of TEPA to EB (i.e., 1:2, 1:3, 1:4), respectively. Impregnation of TEPA or EB-functionalized TEPA (EB-TEPA) into bare silica was carried out by following


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the previous work.24 Porous silica particles were soaked into TEPA/methanol or EBTEPA/methanol solution (17~34 wt. % solution). The nominal amine loading was fixed as 70 wt. % of adsorbents. The solution was dried at 50 oC for 6 h under vacuum to completely remove methanol. 2.4. CO2 Adsorption/Desorption Experiments. CO2 adsorption/desorption experiment was conducted by using a thermogravimetric analysis (TGA, Scinco N-1500). The samples were dried at 100 oC for 3 h under 50 ml min-1 of 100% N2 to remove solvents and absorbed gas before the test. The samples were cooled to 30 oC and stabilized for 30 min under 50 ml min-1 of 100% N2. Right after stabilization, the feed gas was switched to a mixture gas of CO2/N2 (15/85 mol/mol) with a flow rate of 50 ml min-1 to adsorb CO2 gas. CO2 adsorption capacity was evaluated by mass change of TGA. Desorption was carried out at 90 oC for 30 min under 50 ml min-1 of 100% N2. The adsorption/desorption cycles were repeated 10 times to confirm the cyclic stability of adsorbents. 2.5. Supplementary Characterization. The element analyzer (EA) was performed by Flash 2000 (Thermo Scientific) in order to evaluate change in the O/N molar ratio of EB-TEPA and also to identify the actual concentration of TEPA or EB-TEPA in solid amine sorbents. Also, the bare SiO2 microspheres and TEPA- or different EB-TEPAs-loaded SiO2 microspheres were scanned by using the field emission gun scanning electron microscopy (FEG-SEM, Inspect F (FEI)) at 10 kV.

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C NMR spectra were recorded on a Bruker Avance III 400 to determine the

amine distribution state in TEPA and EB-TEPA. The viscosity was measured by discovery 3 from 30 oC to 90 oC. The density was confirmed by Density Meter (DMA 500, Anton paar) at 20 o

C. The N2 adsorption/desorption isotherms were measured with liquid N2 at 77 K by Belsorp

Max (BEL Japan) volumetric analyzer. The surface area was calculated using the Brunauer-


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Emmett-Teller (BET) method. The total pore volume was determined as the volume of liquid nitrogen adsorbed at a relative pressure of 0.99. Thermal stability was performed by TGA (Scinco N-1500) from 25 to 800 oC under 50 ml min-1 of 100% N2. Heat of adsorption was measured by the simultaneous differential scanning calorimetry and thermo-gravimetric analysis (SDT) (Q600, TA instruments). Initially, the samples were preheated at 100 oC for 3 h under N2 gas with a flow rate of 50 ml min-1. Immediately after the samples were cooled at 30 oC, they were exposed to a mixed gas of CO2/N2 (15/85 mol/mol) with a flow rate of 50 ml min-1 at 30 o

C. Heat of adsorption was calculated by dividing the area under the heat-flow curve (W/g-

sorbent·s) by CO2 adsorption capacity (molCO2/g-sorbent). The d-spacing values of amine was measured by wide angle X-ray diffraction (WAXD, Dmax2500/PC (Rigaku)) with Cu Kα radiation (λ=1.5406 Å). Fourier Transform Infrared spectroscopy (FT-IR) spectra were obtained by using a Nicolet iS10 FTIR spectrometer (Thermo Scientific) in a range of 600 – 4000 cm-1 with an attenuated total reflectance (ATR) mode. 2.6. Computational Methods. We performed the first-principles calculations based on the density functional theory using the Vienna Ab-initio Simulation Package (VASP).25,26 The generalized gradient approximation (GGA) was adopted for describing the exchange correlation functional, and the pseudopotential was parameterized under projector augmented wave (PAW) scheme by Perdew-Burke-Emzerhof (PBE).27–29 The energy cut-off on a planewave basis was 500 eV, and the force criteria for optimizing the structure was 0.01 eVÅ-1. We used the DFT-D3 method with Becke-Jonson damping to include the van der Waals interaction between molecules.30,31 Assuming that the TEPA molecules form a crystal structure based on our XRD results, the shape and volume of the periodic cell was fully relaxed during optimization. VESTA code was used to draw the optimized structures obtained from theoretical calculations.32


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3. RESULTS AND DISCUSSION 3.1. Epoxide-Functionalized TEPA. The Figure 1 depicts the chemical functionalization of TEPA by EB with three different molar ratios of EB/TEPA (i.e., 2:1, 3:1, and 4:1). As shown in Figure 1, it is expected that the 2-hydroxybutyl groups are formed through the EB-based chemical functionalization, possibly increasing the molecular weight as well as viscosity. As the degree of EB-functionalization increased, the O/N elemental ratio increased based on our elemental analysis since more hydroxyl groups were formed in the functionalized TEPA (See Table 1). It should be noted that the theoretical O/N elemental ratios of EB-functionalized TEPA (EB-TEPA) samples are almost same as the experimental O/N counterparts, implying that the chemical functionalization of TEPA was completed due to the high ring-opening reactivity of an epoxide. Also, the formation of 2-hydroxybutyl moieties enhanced hydrogen bonding in the EBTEPA sorbents, significantly increasing their viscosity by as much as 35 times (See 0.04 vs. 1.39 Pa·s in Table 1 and Figure. S1). It should be noted that the pH of the EB-TEPA decreased gradually with increasing degree of functionalization. All the EB-TEPA compounds used in this work, however, still exhibited basic characteristic sufficient enough to react with CO2.21 The hydroxyl groups, resulting from the reaction of the amine in TEPA with the epoxide in EB, may further react with other epoxide groups due to their nucleophilic characteristics. Such etherification reaction between the hydroxyl and the epoxide groups, however, only occurs at temperatures higher than 100 oC, which is, therefore, not considered in Figure 1.33


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Figure 1. Schematic representation of EB-based chemical functionalization of TEPA with different EB/TEPA molar ratios. Table 1. Characterization of TEPA and EB-TEPAs. O/N molar ratioa amine

amine distribution (%)b o

o

o

viscosity (Pa·s)c

pHd

exp.

theo.

1 :2 :3

TEPA

-

0.00

46:42:12

0.04

12.03

0.42EB-TEPA

0.42

0.40

31:48:21

0.39

11.85

0.64EB-TEPA

0.64

0.60

15:59:26

1.11

11.70

0.82EB-TEPA

0.82

0.80

7:61:32

1.39

11.46

a

The O/N molar ratio was determined by EA. bThe amine distribution was determined by using 13 C NMR analysis. cThe viscosity was measured at 30 oC, dThe pH was measured at 25 oC for 25 wt. % aqueous solutions. With analysis on functionalization-induced change in the O/N molar ratio, for the first time, the amine state distribution of TEPA and EB-TEPA was determined by using the liquid-state 13C NMR (See Figure 2). Most previous work associated with TEPA focused on the CO2 sorption capacity without characterizing its amine state distribution due to the presence of its various impurities.5,6,8–13,16,18–23 Very recently, Yogo et al.34 identified the chemical structure of three different impurities in the commercial TEPA including 4-(2-aminoethyl)-N-(2-aminoethyl)-N’-


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[2-[(2-aminoethyl)amino]ethyl]-1,2-ethanediamine (branched TEPA; T-BRN), 1-(2-aminoethyl)4-[[(2-aminoethyl)amino]ethyl]-piperazine (TEPA with an inner piperazine ring; T-IPZ), and 1[2-[[2-[(2-aminoethyl)amino]ethyl]-amino]ethyl]-piperazine (TEPA with a piperazine ring at the edge; T-EPZ). Nevertheless, those impurities still contribute to high CO2 sorption capacity due to their amine functional groups.34 As shown in all the chemical structures in Figure 2, the primary, secondary, and tertiary amines in each chemical molecule involve 1, 2, and 3 neighboring carbon atoms respectively, regardless of its chemical structure. With that in mind, the relative amount of different amine states was determined by integrating the area under the characteristic peaks in the liquid-phase

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C NMR corresponding to each carbon atom. Noteworthy is that the value of the

integral associated with the carbon atoms next to the secondary and the tertiary amines should be divided by two and three respectively, in order to avoid the overestimation of the number of corresponding amines. The main characteristic peaks of c, f, and g in Figure 2 correspond to the primary and secondary amines in chemical compounds, reflecting that the TEPA is the predominant substance. The remaining peaks were exactly overlapped with those characteristic peaks corresponding to other three aforementioned impurities.26 Also, all three cases of EB-based reactions generated new characteristic peaks of α, α’, β, γ, and δ corresponding to the 2hydroxybutyl carbon atoms in Figure 2, supporting that the chemical modification of TEPA proceeded. The characteristic peak of α’ is associated with the tertiary amine while that of α is related to the secondary amine. Also, the characteristic peaks of β, γ, and δ occurred at the certain chemical shift positions regardless of the reaction sites. The relative intensity of the primary amine peak (peak c) compared to other peaks decreased monotonically with increasing degree of EB-functionalization, implying that the composition of primary amines significantly


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decreased. A closer inspection revealed that the relative area of the α’ peak increased as the degree of EB-functionalization increased, implying that more tertiary amines were formed. The relative amounts of the secondary amines also continually increased with an increasing degree of EB-functionalization, but at a relatively smaller increasing rate compared to the increasing rate for the tertiary amines.


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Figure 2. 13C NMR spectra of (a) TEPA (b) 0.42EB-TEPA (c) 0.64EB-TEPA (d) 0.82EB-TEPA with chemical structures of all TEPA compounds including all three impurities. The chemical structure of the EB-TEPA represents the chemical modification where both primary and secondary amines react with EB. The quantitative amine distribution was determined by using


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the

following

equation;

1o:

2o:

3o

amine

=

(Aa+Ab+Ac):

(Ae+Af+Ag+Aα)/2:

(Ad+Ah+Ai+Aj+Ak+Aα’)/3, where Ai is the integrated peak area for i species. 3.2. Characterization of TEPA- or EB-TEPA/SiO2 Adsorbents. The pristine TEPA or each EB-TEPA was soaked into porous silica with large pore volume of ca. 2.75 cm3 g-1 (See Table 2 and Table S1) to prepare for the solid-supported CO2 adsorbents. Such large pore volume allows for a high concentration of TEPA impregnation, leading to a high CO2 adsorption capacity. Since the pore volume of silica and the density of TEPA are 2.75 cm3 g-1 and 0.99 g cm-3 respectively, a mixture of 30 wt. % silica and 70 wt. % TEPA or EB-TEPA was added to the methanol solution, followed by the complete evaporation of methanol in order to fully fill the entire pores of our house-made silica with TEPA or each EB-TEPA (See Figure S2). As shown in Figure S2, the bare SiO2 microspheres seem to be hollow and the surface seems to be quite rough, while those TEPA or EB-TEPA containing SiO2 particles seem to be filled and their surface became relatively smoother. Nevertheless, there was no change in the size of SiO2 particles regardless of the impregnation of TEPA or EB-TEPA. It should be also noted that the difference between the density of pristine TEPA and that of each E-TEPA was negligible. For convenience’s sake, samples were named TEPA/SiO2 or xEB-TEPA/SiO2, where x is the O/N molar ratio. As shown in Table 2, both the surface area and pore volume of TEPA or EB-TEPA-containing SiO2 particles were substantially reduced since the SiO2 particles were fully filled with amines. Also, the EA results revealed that the number of N per g-sorbent for EB-TEPA containing SiO2 adsorbents continually decreased as the degree of EB-functionalization increased since the molecular weight increased with the EB-based functionalization of TEPA while the number of N per mole of TEPA was constant (i.e., 5). It was 18.1, 10.1, 8.6, and 7.4 mmolN g-1 for TEPA,


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0.42EB-TEPA, 0.64EB-TEPA, and 0.82EB-TEPA, respectively which were almost identical with the respective theoretically calculated counterpart in Table 2. Change in the amine state distribution for each EB-TEPA was confirmed again by evaluating the heat of adsorption in the intact TEPA or different EB-TEPA containing SiO2 adsorbents. It is well known that the primary amines exhibit higher heat of sorption for CO2 than the secondary and tertiary amines do.24,35 Consistent with our liquid-phase

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C NMR results, the heat of

adsorption decreased monotonically with higher degree of EB-functionalized TEPA/SiO2 adsorbents (See Table 2 and Figure S3), supporting that the contents of the primary amines diminished. Table 2. Characterization of Bare SiO2 and TEPA or EB-TEPA Containing SiO2 Adsorbents. amine loading (mmolN g-1) exp. theo.

heat of adsorption (kJ mol-1)

adsorbents

surface area (m2 g-1)

pore volume (cm3 g-1)

bare SiO2

198

2.75

-

0.00

-

TEPA/SiO2

3.6

~0

18.1

18.5

82.3

0.42EB-TEPA/SiO2

2.8

~0

10.1

10.5

72.6

0.64EB-TEPA/SiO2

5.9

~0

8.6

8.6

71.9

0.82EB-TEPA/SiO2

4.3

~0

7.4

7.3

68.6

3.3. Long-Term CO2 Swing Capacity of TEPA- or EB-TEPA/SiO2 Adsorbents. Securing the long-term CO2 swing capacity of the adsorbents is as critical as improving their CO2 adsorption capacity for the practical operation of the temperature swing adsorption (TSA) process. The long-term CO2 swing capacity of all solid-supported CO2 adsorbents used in this work was evaluated by characterizing their cyclic swing capacity over the consecutive 10 cycles


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of adsorption/desorption processes. Each adsorption curve was made under a mixture gas of CO2/N2 (15/85 mol/mol) at 30 oC while each desorption curve was made under N2 at 90 oC. Although the pristine TEPA/SiO2 adsorbents showed the highest CO2 swing capacity of 2.80 mmol g-1 in the very first cycle, they showed continuous and substantial reduction in CO2 adsorption swing capacity over the 10 cycles of adsorption/desorption processes, reflecting that their cyclic stability is poor. Also, such a reduction in CO2 adsorption swing capacity for TEPA/SiO2 adsorbents may be partly associated with the insufficient desorption at 90 oC due to the large amounts of primary amines in TEPA/SiO2 adsorbents.8 As for the EB-TEPA/SiO2 adsorbents, the average CO2 swing capacity continually decreased in the order of 0.42EBTEPA/SiO2 (2.40 ± 0.05) > 0.64EB-TEPA/SiO2 (2.00 ± 0.04) > 0.82EB-TEPA/SiO2 (1.54 ± 0.01) mmol g-1 (See Figure 3a). Nevertheless, both 0.64EB-TEPA/SiO2 and 0.82EB-TEPA/SiO2 adsorbents effectually maintained their initial performance over the 10 cycles of adsorption/desorption processes due to their thermal stability enhancement. A careful analysis showed a slight but continuous reduction in the mass of 0.42EB-TEPA/SiO2 during each desorption over the 10 cycles of adsorption/desorption processes, implying that the thermal stability of 0.42EB-TEPA/SiO2 sorbent is still undesirable (See Figure 3b). The thermal stability of TEPA and each EB-TEPA was also evaluated by using TGA. As shown in Figure 4, the evaporation temperature of EB-functionalized TEPA compounds increased monotonically as the degree of functionalization increased (i.e., TEPA (229 oC) < 0.42EB-TEPA (296 oC) < 0.64EBTEPA (307 oC) < 0.82EB-TEPA (320 oC)), reflecting that their thermal stability was enhanced with increasing degree of functionalization. Such thermal stability enhancement was attributed to a combination of the increase in molecular weight and the hydrogen bonding between amine and hydroxyl moieties. Thus, it can be seen that the higher degree of EB-functionalization induces


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reduction in CO2 adsorption capacity, but increase in thermal stability. With that in mind, we found that the 0.64EB-TEPA/SiO2 exhibited the optimum balance between CO2 swing capacity and thermal stability in the current adsorption/desorption cyclic process condition, suggesting that the EB-functionalization is a viable strategy for the application of TEPA in the TSA process.

Figure 3. Long-term CO2 swing capacity of TEPA/SiO2 or EB-TEPA/SiO2 over the 10 cycles of adsorption/desorption

processes.

(a)

Calculated

CO2 swing

capacity

and

(b)

CO2

adsorption/desorption profiles.


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Figure 4. TGA curves from 25 to 800 oC for TEPA and modified TEPA under N2 atmosphere. 3.4. Adsorption Kinetics of TEPA- or EB-TEPA/SiO2 Adsorbents. The CO2 adsorption kinetics is also one of the most important criteria for the evaluation of the efficiency of CO2 adsorbents. The kinetic uptake of CO2 by the pristine TEPA/SiO2 and each EB-TEPA/SiO2 adsorbent at 1 atm of CO2/N2 (15/85 mol/mol) and 30 oC is shown in Figure 5. It is clearly shown that the CO2 adsorption kinetics was improved in the order of TEPA/SiO2 < 0.42EBTEPA/SiO2 < 0.64EB-TEPA/SiO2 < 0.82EB-TEPA/SiO2, reflecting that the functionalization of TEPA with EB promotes CO2 adsorption. For instance, the 0.82 EB-TEPA/SiO2 adsorbent achieved 95% equilibration in 374 sec, while the TEPA/SiO2 counterparts did in 4096 sec.

Figure 5. CO2 adsorption kinetics for TEPA/SiO2 and EB-TEPA/SiO2


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The theoretical calculation on the structure of TEPA and EB-TEPA adsorbents may offer clues about the effect of EB-functionalization on the CO2 adsorption kinetics. We performed the first-principles calculations based on the density functional theory to investigate any change in the d-spacing induced by EB-based functionalization of TEPA. Various configurations of pristine TEPA and 0.82EB-TEPA with four 2-hydroxybutyl moieties were attempted by changing the number and the reaction site of EBs, and their energetically optimized structures are shown in Figure 6. Our calculations suggest that EB functionalization caused the intermolecular distance to increase in two different directions simultaneously. Most EB molecules may react with the primary amines at the end of TEPA molecules, slightly increasing the distance between the TEPA backbones in the (010) direction (i.e., from 4.28 to 4.32 Å) by the steric effect (See Figure 6b). When EB reacts with secondary amine groups, the subsequent 2hydroxybutyl moieties align perpendicular to the main chain of TEPA molecules (See Figure 6b) and the distance between two neighboring EB-TEPA main chains along the (001) direction substantially increases to ca. 8.0 Å. Such enlarged space induced by those 2-hydroxybutyl moieties may also promote CO2 diffusion, enhancing CO2 accessibility to amine groups.


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Figure 6. The optimized structures of (a) TEPA (b) 0.82EB-TEPA obtained from theoretical calculations. Grey, yellow, blue and red balls represent carbon, hydrogen, nitrogen, and oxygen, respectively. Along with our first principles calculations, the XRD measurements were conducted on the pristine TEPA and all EB-TEPA sorbents. In the XRD patterns, the pristine TEPA exhibits a single characteristic peak at ca. 20o, corresponding to a d-spacing of 4.47 Å (See Figure 7) which is similar to our simulation result (i.e., 4.28 Å). All of the E-TEPA compounds also exhibit a common peak at ca. 20o, denoted by Y, while those EB-TEPA compounds show a small shoulder peak at ca. 10o, denoted by X. Closer inspection reflected that as the degree of EBfunctionalization increases, the Y peak shifts towards lower diffraction angle side from 19.85o (~4.47 Å) to 19.73o (~4.50 Å), 19.61o (~4.52 Å), and 19.50o (~4.55 Å), consistent with our simulation result that the side distance between the molecular chains of EB-TEPA was increased (See Figure 6b). The emergence of a shoulder peak X in the lower diffraction angle implies that a larger distance between TEPA molecules was developed by functionalization, possibly enhancing the accessibility of CO2 molecules to amines with better adsorption kinetics as shown in Figure. 5.36,37 Again, it was consistent with our simulation result that the distance between the neighboring EB-TEPA molecules substantially increased in the (001) direction.


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Figure 7. X-ray diffraction pattern for TEPA and EB-TEPAs. 4. CONCLUSION Here, we chemically modified tetraethylenepentamine (TEPA) by a simple reaction with 1,2epoxybutane (EB) to improve the thermal stability of TEPA. As the degree of EBfunctionalization increased, the CO2 swing capacity of the corresponding TEPA/SiO2 sorbents decreased due to increasing tertiary amine contents as verified by the liquid-phase

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C NMR

analysis. Nevertheless, the thermal stability of the associated solid supported sorbents was substantially improved due to the increase in the molecular weight of TEPA as well as the formation of hydrogen bonding. Consequently, the 0.64EB-functionalized TEPA/SiO2 maintained a CO2 swing capacity of 2.0 mmol CO2 g-1 over the 10 adsorption/desorption cycles under the dry CO2/N2 (15/85 mol/mol) feed at ambient condition. In addition to the thermal stability enhancement, our EB-based, chemical modification of TEPA effectively improved CO2 adsorption kinetics probably due to the bulky 2-hydroxybutyl groups-induced enhancement of inter-molecular chain distance, even though they became more viscous with an increasing degree


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of EB-functionalization. Also, our first principles calculations described the possible configurations of the EB-functionalized TEPA structures with enlarged space between neighboring EB-TEPA molecules, further supporting the facile access of CO2 into amine functional groups. The EB-based functionalization of TEPA with considerable thermal stability enhancement is a critical step forward for the practical application in the area of CO2 capture.

ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Characterizations including the viscosity of TEPA and modified TEPA, and the BET surface area, pore volume, pore diameter, and particle size of bare silica (PDF)

AUTHOR INFORMATION Corresponding Author *J.-H.L. E-mail : [email protected] *S.L. E-mail : [email protected] *J.L. E-mail : [email protected] Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This research was supported by the Korea CCS R & D Center (KCRC) (No. 2014M1A8A1049315). This research was supported by “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20174010201150). NOMENCLATURE TEPA = tetraethylenepentamine EB = 1,2-epoxybutane DETA = diethylenetriamine PEI = polyethyleneimine AN = acrylonitrile E-TEPA = EB-functionalized TEPA T-BRN = 4-(2-aminoethyl)-N-(2-aminoethyl)-N’-[2-[(2-aminoethyl)amino]ethyl]-1,2ethanediamine T-IPZ = 1-(2-aminoethyl)-4-[[(2-aminoethyl)amino]ethyl]-piperazine T-EPZ = 1-[2-[[2-[(2-aminoethyl)amino]ethyl]-amino]ethyl]-piperazine


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ABSTRACT GRAPHIC


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Silica

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