Enhanced CO2 Adsorption Performance on Hierarchical Porous ZSM

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Enhanced CO2 adsorption performance on hierarchical porous ZSM-5 zeolite Qing Liu, Pingping He, Xingchi Qian, Zhaoyang Fei, Zhuxiu Zhang, Xian Chen, Jihai Tang, Mifen Cui, Xu Qiao, and Yao Shi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02543 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 13, 2017

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Graphic for manuscript

Caption: The CO2 adsorption capacity increased after the introduction of organosilanes, corresponding to increase of the volume of the mesoporous and specific surface area.

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Enhanced CO2 adsorption performance on hierarchical porous ZSM-5 zeolite

Qing Liua, Pingping Hea,b, Xingchi Qiana,b, Zhaoyang Fei*a,b, Zhuxiu Zhanga, Xian Chena, Jihai Tanga,c, Mifen Cuia, Xu Qiao**a,b,c, and Yao Shid

a

College of Chemical Engineering, Nanjing Tech University, Nanjing 210009, PR

China b

State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech

University, Nanjing 210009, PR China c

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM),

Nanjing 210009, PR China d

Key Laboratory of Biomass Chemical Engineering of Ministry of Education,

Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, PR China

*Corresponding author. Tel.: +86 25 83587168; fax: +86 25 83587168, E-mail: [email protected] **Corresponding author. Tel. +86 25 83172298; fax: +86 25 83172298, E-mail: [email protected]


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Abstract: Hierarchical porous ZSM-5 (HP-ZSM-5) was constructed using organosilanes as the growth-inhibitors for CO2 capture. The properties of adsorbents were characterized by XRD, N2 adsorption/desorption, SEM, CO2-TPD, and

27

Al

MAS NMR. It was found that HP-ZSM-5 samples synthesized by organosilanes had a significant effect on the microstructure and morphology. CO2 adsorption capacity of HP-ZSM-5 was up to 58.26 cm3·g-1 at 0 °C and 1 bar, significantly higher than that of ZSM-5 sample. The effective improvement of CO2 adsorption performance mainly originated from the micro/mesoporous composite structure and complex surface morphology, which can provide low-resistant pathways for CO2 through the porous network. Besides, in situ FT-IR was carried out to study the adsorption process on adsorbents, and the results indicated that a faster physical adsorption process was achieved due to the introduction of mesoporous. Keywords: Hierarchical porous ZSM-5; CO2 capture; In situ FT-IR; Adsorption capacity


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1. INTRODUCTION Large anthropogenic CO2 emissions are mainly caused by the burning of the non-renewable fossil fuels, which are outpacing carbon cycling.1 Owing to significant environmental problems, including global warming and ocean acidification, triggered by the rising concentration of CO2 in the atmosphere, the implementation of carbon capture technologies has been suggested to mitigate CO2 emissions.2 Carbon capture can effectively stabilize greenhouse gases concentrations in the atmosphere and make time for developing low-carbon and environment-friendly energy.3 Various CO2 capture technologies, including absorption, adsorption, membrane, and so forth, have been investigated.4 However, the large energy penalty associated with the regeneration of the absorbent are problematic for absorption.5 Membrane separation has been the potential to separate CO2, but most membranes are incapable of extracting CO2 with high purity and recovery from flue gases.6, 7 Recent studies8-10 suggest that adsorption processes using hierarchical porous materials with micropores and mesopores11 are promising alternatives for CO2 capture, including hierarchical porous carbons,12-14 hierarchical porous MOFs,15-17 meso/micro-porous polymer,18, 19 hierarchical porous zeolites and hierarchical porous silica.20-23 The micro/mesoporous zeolites are considered as the promising adsorbents for CO2 capture since they possess a tunable porous structure to overcome molecule diffusion limitations.24 Gong et al.20 provided a biosynthesis route to prepare spherical SAPO-34 constructed by nano-sheets, and such hierarchical SAPO-34 exhibited a high CO2 adsorption capacity of 62 cm3·g-1 at 100 kPa and the CO2/CH4 ideal separation factor of 8:2. Besser et al.25 prepared porous 13X monoliths with a hierarchical pore structure over several length scales using a combination of freeze casting and sacrificial templating technique. Compared with commercially available zeolite 13X beads, the hierarchical porous 13X monoliths featured a high volumetric working capacity of ~1.34 mmol·cm-3 and a fast CO2 uptake showing an adsorption of 50 % within 5~8 s. Ojuva et al.26 synthesized 13X zeolite with a laminar pore structure

and

hierarchical

macro/micro-porosity

by


freeze-casting

aqueous

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suspensions of zeolite 13X powder, bentonite, and polyethylene glycol. The zeolite showed a high and rapid CO2 uptake compared to pressed pellets with identical composition and the monoliths displayed a very fast initial uptake where more than 50 % of the maximum uptake was reached within 15 s. ZSM-5 has a high affinity for CO2 molecule, and these interactions occur between the quadrupole moment of CO2 and the electric field of ZSM-5. Besides, desorption of CO2 from ZSM-5 can be completely controlled by the application of mild conditions.

27

What’s more, highly crystalline ZSM-5 with a high surface area

and a three-dimensional pore structure is expected to be a high performance adsorbent for CO2 removal.28-30 In this study, we developed hierarchical porous ZSM-5 utilizing organosilanes to investigate its CO2 adsorption performance. The textural properties and surface morphologies of as-prepared sorbents were characterized by XRD, N2 adsorption/desorption, SEM, CO2-TPD, and

27

Al MAS NMR. CO2 adsorption

behaviour (adsorption capacity, adsorption selectivity and adsorption thermodynamics) were investigated to exhibit the adsorption performance of the adsorbents. What’s more, in situ FT-IR was carried out to study the adsorption process on adsorbents. 2. EXPERIMENTAL SECTION 2.1. Materials and Preparation. All reagents were used without further treatment, as follows: tetraethyl orthosilicate (TEOS, 28.5 wt%) as the silicon source, sodium aluminate solid (NaAlO2,

41.0

wt%

calculated

from

Al2O3)

as

the

aluminum

source,

tetrapropylammonium hydroxide (TPAOH, 25 wt%) and two types of organosilanes as growth-inhibitors, including (3-aminopropyl) triethoxysilane (AMEO, 98 wt%), γ-chloropropyltriethoxysilane (CPTMO, 98 wt%). All HP-ZSM-5 samples were synthesized via a growth-inhibition strategy by using organosilanes as the growth-inhibitors. The HP-ZSM-5 samples were prepared with the following experimental procedures. NaAlO2 was dissolved in distilled water under magnetic stirring, and then TPAOH and organosilane were put into the above


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mixture stepwise. After that, TEOS was added slowly under vigorous and the mixture was stirred for 6 h. The molar composition of the precursor mixtures was as follows: TEOS: NaAlO2: TPAOH: organosilane: H2O=50:2:1.8:0.5:1250. The mixture was transferred into an autoclave for crystallization at 180 °C for 72 h. The sample was collected by filtration, dried at 100 °C and calcined at 550 °C for 5 h to remove the template. The protonated sample was obtained from ion exchange with NH4Cl for four times, following by calcination at 550 °C for 4 h. The HP-ZSM-5 sample developed by CPTMO was labelled as Cl-HP-ZSM-5 and the sample prepared by AMEO was labelled as N-HP-ZSM-5. As comparison, conventional ZSM-5 zeolite was also synthesized under the similar procedures in the absence of organosilanes, and labelled as ZSM-5. 2.2. Material Characterizations. The crystal structure of the adsorbents was checked by powder X-ray diffraction (XRD) on a SarmtLab powder diffractometer using Ni-filtered Cu Kα radiation (λ=0.15406 nm) at a setting of 40 kV and 100 mA. XRD patterns within the range 5°~40° were recorded with a scan rate of 2°·min-1. The identification of the different crystalline phases present was performed by comparison with the corresponding JCPDS cards. The particle size and morphology of the samples were examined by scanning electron microscope (SEM) on a Hitachi S-4800 instrument at an acceleration voltage of 15 kV. N2 physisorption was performed on a BETSORP-II analyzer to gain information about the textural properties of the samples. The samples were outgassed at 200 °C for 2 h prior to the sorption measurements. The Langmuir adsorption isotherm model was used to determine the total surface area in the p/p0 range between 0.05 and 0.20. The mesopore volume and size distribution were calculated from the adsorption branch of the isotherm by the Barrett-JoynerHalenda (BJH) method. The micropore volume was determined by the t-plot method.


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27

Al magic angle spinning (MAS) nuclear magnetic reso-nance (NMR)

measurements were performed on a 600 MHz BrukerAvance III equipped with a 4 mm MAS probe.

27

Al MAS NMR spectra was recorded using one pulse sequence

with a spinning rate of 12 kHz. 100 scans were accumulated with a π/8 pulse width of 0.75 µs and a 2 s recycle delay. Chemical shifts were referenced to 1 M aqueous aluminium nitrate solution. CO2-temperature-programmed desorption (CO2-TPD) was performed on a Micromeritics AutoChem 2920 II instrument. 50 mg sample was activated at 200 °C for 1 h prior to the adsorption CO2 at 50 °C for 1 h. After CO2 was adsorbed on the sample to be saturated, the weak physical adsorption of CO2 was removed using helium flow for 0.5 h to achieve the monolayer layer of CO2 coverage. Then the sample was heated to 300 °C at a rate of 10 °C·min-1 under helium flow of 50 mL·min-1 to get the CO2-TPD results. 2.3. CO2 Adsorption Measurements. Static adsorption experiments of CO2 and N2 can be measured by a BELSORP (BEL, Japan) adsorption apparatus. The dead volume was determined using high-purity helium, assuming that helium can not be adsorbed at the temperatures investigated. The adsorption temperature was controlled by a constant temperature water tank. The isotherms were fitted with the Langmuir model as shown in eq. 1.31

q =

(1)

Where qc and kc are the Langmuir model parameters with the subscripts c denoting the channels, respectively. pt is the system pressure, and q is the adsorption amount. The ideal adsorption solution theory (IAST) has been reported for predicting binary gas mixture adsorption in solid adsorbent. The adsorption selectivity has been defined according to eq. 2. 32, 33 S = ( ⁄ )/( / )


(2)

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Where xi and yi (xj and yj) are the molar fractions of component 1 (component 2) in the adsorbed and bulk phases, respectively. In the calculation, the ratio of CO2/N2 is 15/85, which are typical composition of flue gas and natural gas, respectively. To further understand the interaction between CO2 molecules and adsorbents, the isosteric heats of CO2 adsorption (Qst) were calculated using the Clausius-Clapeyron equation from CO2 adsorption isotherms collected at 0, 12.5 and 25 °C; Qst is determined using eq. 3. 31 = − (

)

(3)

The CO2 breakthrough cures were measured with a packed-bed column (length=10 cm, inner diameter=1.0 cm) connected to a Hidden mass spectrometer in the presence of 15 % CO2 with N2 gas. The complete removal of solvent traces, moisture and other adsorbed species from the adsorbent was achieved through thermal activation at 200 oC under a purge flow of nitrogen. In order to determine the effect of water vapor on CO2 adsorption, the N2 gas passed through a water saturator (30 oC) located in a temperature-controlled water bath. The in situ FT-IR experiments were performed on a iSO 50 IR spectrometer (Thermo Nicolet Corporation, USA) with an in situ diffuse reflectance pool and high-sensitivity MCT detector which was cooled by liquid N2. The adsorbent was loaded in the Harrick IR cell and heated to 200 ºC under N2 flow 40 cm3·min-1 for 120 min to remove adsorbed impurities and then cooled to the desired adsorption temperature. The sample spectra were collected by subtracting the background spectrum that was collected under N2 atmosphere. The in situ FT-IR spectra was recorded by accumulating 32 scans with a resolution of 0.5 cm-1. The flow rate of CO2 was kept at 10 cm3·min-1 at 30 °C. When the CO2 adsorption step was finished, the temperature was increased to 50, 70 and 90 °C with a gas flow of pure N2 (10 cm3·min-1) to degas the samples. 3. RESULTS AND DISCUSSION


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3.1. Material Characterizations. 3.1.1. XRD analysis. Figure 1 shows the XRD patterns of the as-prepared Cl-HP-ZSM-5, N-HP-ZSM-5 and ZSM-5 samples. Diffraction peaks at 2θ of 7.86°, 8.78°, 14.78°, 23.18°, 23.90° and 24.40°, which were in good agreement with those peaks of ZSM-5 zeolite (JCPDS no. 43-0321), were detected for all samples.34 This result suggested that all samples showed the MFI-type framework, indicating that the involvement of organosilanes did not destroy the crystallization process, which will be further proved in next sections.

Cl-HP-ZSM-5 Intensity (a.u.)

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N-HP-ZSM-5

ZSM-5

5

10

15

20

25

30

35

40

45

50

2θ (degree)

Figure 1. XRD patterns of Cl-HP-ZSM-5, N-HP-ZSM-5 and ZSM-5. 3.1.2. SEM analysis. SEM images of Cl-HP-ZSM-5, N-HP-ZSM-5 and ZSM-5 are depicted in Figure 2. It can be seen that the organosilanes remarkably influenced the morphology and particle size of HP-ZSM-5 samples. The ZSM-5 sample exhibited the MFI-typical hexagonal morphology and showed smooth and angular surface, while the Cl-HP-ZSM-5 and N-HP-ZSM-5 samples displayed spherical aggregates of nano-sized crystals with roughen surfaces and relatively small particle sizes (about 1~2 µm in diameter). This result was mainly contributed to the factor that the organosilanes were grafted on the surface of crystal seed through the covalent Si-O-Si linkages and hydrogen bonding interactions,35 which hindered the growth of ZSM-5 during crystallization process and eventually affected the structure and


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morphology.

Figure 2. SEM images of Cl-HP-ZSM-5, N-HP-ZSM-5 and ZSM-5. 3.1.3. N2 adsorption/desorption analysis. Figure 3 illustrates the N2 adsorption/desorption isotherms and mesopore size distributions of Cl-HP-ZSM-5, N-HP-ZSM-5 and ZSM-5 samples. In addition, their textural properties were listed in Table 1. The ZSM-5 showed typical type-I isotherm of microporous materials, while both Cl-HP-ZSM-5 and N-HP-ZSM-5 showed the characteristic of type IV isotherm, which had a hysteresis loop at a p/p0 range of 0.42~1.0. It is reckoned that Cl-HP-ZSM-5 and N-HP-ZSM-5 possessed open pore with the treatment of organosilanes.36-39 Mesopore diameter distributions derived from the isotherms by the BJH method revealed that the ZSM-5 had no mesopores. After organosilanes treatment, Cl-HP-ZSM-5 and N-HP-ZSM-5 had mesopores 2~4 nm in size. The N-HP-ZSM-5 owned small sizes of the mesopores, about 2.4 nm, while the Cl-HP-ZSM-5 held relatively large mesopores, with pore sizes of 3.7 nm. In addition, there was also a big difference among them in terms of mesopore volumes. As listed in Table 1, the BET surface area and total pore volume of ZSM-5 were 383 m2·g-1 and 0.21 cm3·g-1. Nevertheless, Cl-HP-ZSM-5 and N-HP-ZSM-5 possessed larger surface area (maximum 438 m2·g-1) and more pore volume (maximum 0.29 cm3·g-1), mainly attributed to the presence of nanocrystals that increases the external surface area (maximum 80 m2·g-1) and intergranular accumulation of mesopores.40


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(a)

3

Quantity adsorbed (cm /g)

150

dVp/dW (cm3/nm)

100

50

0

1

2

3

4

45

50

Pore diameter (nm) 0 0.0

0.2

0.4

0.6

0.8

Relative pressure (p/p0)

1.0

(b)

3


150

3

d Vp/d W (cm /nm)

100

50

0

1

2

3

4

45

50

Pore diameter (nm ) 0 0.0

0.2

0.4

0.6

0.8


1.0

(c) 150 3


100 dVp/dW (cm3/nm)

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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50

0

1

2

3

4

Pore diameter (nm)

45

50

0 0.0

0.2

0.4

0.6


0.8

1.0

Figure 3. N2 adsorption/desorption isotherms and pore size distributions of (a) Cl-HP-ZSM-5, (b) N-HP-ZSM-5 and (c) ZSM-5. Table 1. Physico-chemical parameters of Cl-HP-ZSM-5, N-HP-ZSM-5 and ZSM-5.


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Sample

S/(m2·g-1) BET

Vp/(cm3·g-1) [a]

External

Micropore

[a]

Mesopore[b]

Dp[b]/(nm)

Total

CO2 adsorption

SiO2/Al2O3[c]

sites[d]/(µmol·g-1)

Cl-HP-ZSM-5

438

80

0.17

0.12

0.29

3.8

26

1505

N-HP-ZSM-5

405

56

0.18

0.07

0.25

2.4

25.5

846

ZSM-5

383

13

0.19

0.02

0.21

-

25

832

[a] Calculated by the t-plot method; [b] Calculated by the BJH method; [c] Determined by XRF analysis; [d] Determined by CO2-TPD analysis.

3.1.4.

27

Al MAS NMR analysis. 27Al MAS NMR was employed to study the

environment of Al over Cl-HP-ZSM-5, N-HP-ZSM-5 and ZSM-5 samples. As shown in Figure 4, all samples can be observed two peaks in the spectra. The strong resonance peak centered at ~56 ppm corresponded to AlO4 tetrahedra of framework Al species. The aluminum spectra also exhibited a very weak peak at ~3 ppm, indicating the existence of few octahedral extra framework aluminum species.41 The signals of HP-ZSM-5 samples hardly increased at 3 ppm compared to ZSM-5 sample, further indicating their perfect framework crystallinity without extra framework aluminium species.

Cl-HP-ZSM-5

Intensity (a.u.)

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

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N-HP-ZSM-5

ZSM-5

100

80

60

40

20

0

-20

-40

27

Al MAS NMR (ppm)

Figure 4. 27Al MAS NMR spectra of three samples. 3.1.5. CO2-TPD analysis. CO2-TPD analysis was carried out to determine the CO2 adsorption sites over Cl-HP-ZSM-5, N-HP-ZSM-5 and ZSM-5 samples. As shown in Figure 5, CO2 desorption started at about 50 °C, reached the maximum rate at about 120 °C, and desorbed thoroughly at 200 °C. Compared with ZSM-5, the number of CO2 adsorption sites of Cl-HP-ZSM-5 (1505 µmol·g-1) increased significantly after the addition of organosilane (Table 1). These CO2 adsorption sites


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were responsible for the high extent of capture of CO2 molecules due to the formation of weak interaction energy between CO2 molecules and the CO2 adsorption sites.42, 43

TCD signal (a.u.)

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Cl-HP-ZSM-5

N-HP-ZSM-5

ZSM-5

50

100

150

200

250

300

o

Temperature ( C)

Figure 5. CO2-TPD profiles of adsorbents at a rate of 10 °C·min-1 from 50 to 300 °C. 3.2. CO2 Adsorption. 3.2.1. CO2 adsorption capacity. Figure 6 shows the CO2 adsorption isotherms of three samples at different temperature and 1 bar. The corresponding CO2 uptake values were listed in Table 2. It was found that Cl-HP-ZSM-5 and N-HP-ZSM-5 samples showed higher adsorption capacity (58.26 and 56.77 cm3·g-1 at 0 °C) compared with ZSM-5 sample (53.16 cm3·g-1 at 0 °C). The possible reasons are as follows. Firstly, CO2 adsorption capacity was influenced by porous structure and specific surface area.44 Cl-HP-ZSM-5 and N-HP-ZSM-5 samples owned higher surface area (438 and 405 m2·g-1) and larger pore volume (0.29 and 0.25 cm3·g-1) compared to ZSM-5 (383 m2·g-1 and 0.21 cm3·g-1) in Table 2. Secondly, it can be concluded from SEM images that ZSM-5 sample exhibited smooth and angular surface, while the N-HP-ZSM-5 and Cl-HP-ZSM-5 samples displayed spherical aggregates of nano-sized crystals with roughen surfaces and relatively small particle sizes, which may short the CO2 transfer diffusion path. Lastly, the higher physical adsorption capacity of HP-ZSM-5 samples most likely caused by larger amount of


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CO2 adsorption sites from the result of CO2-TPD in Table 1. Therefore, the effective improvement of CO2 adsorption performance mainly originates from the micro/mesoporous composite structure, complex surface morphology and CO2 adsorption sites. Table 2. CO2 adsorption capacity over three samples at different temperature and 1 bar. Qe/(cm3·g-1) Samples

0 °C[a]

12.5 °C[a]

25 °C[a]

50 °C (humid)[b]

Cl-HP-ZSM-5

58.26

51.38

43.78

17.54 (11.34)

N-HP-ZSM-5

56.77

50.36

42.86

13.57

ZSM-5

53.16

47.02

40.44

11.12

[a] Determined by CO2 adsorption isotherms at 1 bar; [b] Determined by CO2 breakthrough cures in the presence of 15% CO2 with N2.

60

(a) o

3

Adsorption capacity (cm /g)

50

25 C o 12.5 C o 0 C

40

30

20

10

0 0.2

0.4

0.6

0.8

1.0

Pressure (bar) 60

(b) 50

3


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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40

o

25 C o 12.5 C o 0 C

30

20

10

0 0.2

0.4

0.6

0.8

Pressure (bar)


1.0

Page 15 of 26

60

(c) 50

3


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40

o

25 C o 12.5 C o 0 C

30

20

10

0 0.2

0.4

0.6

0.8

1.0

Pressure (bar)

Figure 6. Adsorption isotherms of CO2 over (a) Cl-HP-ZSM-5, (b) N-HP-ZSM-5, (c) ZSM-5 at different temperature. 3.2.2. CO2 adsorption selectivity. In order to compare the adsorption selectivity on three samples, the adsorption isotherms of N2 and CO2 were measured at 0 and 25 °C (Figure S1 and S2 in the Supporting Information). The N2 uptake on Cl-HP-ZSM-5 was just 6.92 cm3·g-1 at 0 °C and 1 bar, much lower than the uptake of CO2 (58.02 cm3·g-1). Similar results were also observed on N-HP-ZSM-5 and ZSM-5 samples. IAST has been widely used to predict adsorption selectivity of gas mixtures. In this calculation, the ratio of CO2/N2 is 15/85, which is the typical component of flue gas. Fitting parameters of Langmuir model were listed in Table S1 (Table S1 in the Supporting Information), and the IAST selectivity results at 0 °C and 25 °C were shown in Figure 7, respectively. It was found that the CO2/N2 selectivity at 0 °C is higher than that at 25 °C. The adsorption selectivity of ZSM-5 sample was similar to that of HP-ZSM-5 samples at low pressure, but the former surpassed the later at high pressure. This was caused by the different active adsorption sites on micropores and mesopores at different pressures.45 The adsorption potential of micropore is much higher than that of mesopore at low pressure. When the pore size of micropore is close to the size of adsorbate molecules (CO2 0.33 nm and N2 0.364 nm), the small differences of the adsorbate molecules can lead to the huge difference of adsorption rate,46 so the CO2 molecules are easier to enter micropores compare to N2. And the volume and diameter of micropore for three samples (0.17, 0.18 and 0.19 cm3/g) are


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approximately the same, thus the selectivity at low pressure was less different between these samples. With the increase of pressure, the mesopores provided more adsorption sites at high pressure and these sites adsorbed both CO2 and N2, which reduced the adsorption selective slightly for HP-ZSM-5 samples.47 Although HP-ZSM-5 samples displayed a slight lower selectivity of CO2/N2, the selectivity was still higher than 60 at 0 °C and 1 bar.

75

Cl-HP-ZSM-5 N-HP-ZSM-5 ZSM-5

70

Selectivity

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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65 60 55 50 45 0.2

0.4

0.6

0.8

1.0

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Figure 7. IAST selectivity of CO2/N2 over Cl-HP-ZSM-5, N-HP-ZSM-5 and ZSM-5 at 0 °C. 3.2.3. CO2 adsorption under flue gas. In order to test the adsorbents performance in practice, the CO2 breakthrough curves of three samples were measured at 50 °C and under 0.15 bar CO2 partial pressure in Figure 8a. As shown in Table 2, the CO2 adsorption capacities of Cl-HP-ZSM-5, N-HP-ZSM-5 and ZSM-5 were 17.54, 13.57, 11.12 cm3·g-1, respectively. The Cl-HP-ZSM-5 and N-HP-ZSM-5 showed higher adsorption capacity compared with ZSM-5 sample. Wirawan et al.29 explored the CO2 adsorption capacity on H-ZSM-5 at 50 ºC and 0.15 bar, and the adsorption capacity was up to 11.2 cm3·g-1. Masala et al.30 discovered that H-ZSM-5 displayed about 8.96 cm3·g-1 CO2 at 60 ºC and 0.15 bar. As the flue gas may contain water vapor, it was important to evalute the effect of moisture on CO2 adsorption performance of the current adsorbents. Cl-HP-ZSM-5 was selected to investigate the effect of water vapor (13.74%) on CO2 uptake at 50 ºC in the 15/85 of CO2/N2


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mixture in Figure 8b. Typical CO2 breakthrough curves in the presence of dry and humid gas feed showed that the water vapore had a negative effect on the CO2 adsorption. As discussed previously, CO2 molecule was prevented to enter the pores in the presence of water during the CO2 adsorption.48 1.0

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0.4

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Figure 8. CO2 breakthrough curves of (a) three samples and (b) Cl-HP-ZSM-5 under dry and humid conditions. 3.2.4. CO2 adsorption heat. Figure 9 shows the estimated isosteric heats of adsorption for CO2 on Cl-HP-ZSM-5, N-HP-ZSM-5 and ZSM-5 samples. In order to understand the adsorbate-adsorbent interaction, the heat of adsorption (Qst) was calculated by using the Clausius-Clapeyron equation from the adsorption isotherms collected at 0, 12.5 and 25 °C. As shown in Figure 9, all the estimated isosteric heat of


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adsorption for CO2 on three samples are about 26 kJ/mol. Choudhary et al. found that ZSM-5 had adsorption heat of 26.1 kJ/mol and Dunne et al. explored that heat of adsorption on H-ZSM-5 was 38 kJ/mol.48, 49 At the beginning of adsorption process, large number of vacant pores were available on the sorbents surface that caused stronger forces between adsorbate and adsorbent, so the values of Qst were maximum at low surface loading. As the adsorption process went on, the active sites of samples became occupied, thus causing weaker interaction between adsorbent and adsorbate, as a consequence the heat of adsorption slightly decreased50 and then achieved an equilibrium, which confirmed the fast-physical adsorption process. 40

Cl-HP-ZSM-5 N-HP-ZSM-5 ZSM-5 30

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Figure 9. CO2 isosteric heat of adsorption over Cl-HP-ZSM-5, N-HP-ZSM-5 and ZSM-5. 3.2.5. In situ FTIR analysis. In situ FTIR was carried out to elucidate the CO2 adsorption and desorption process. Figure 10 shows the FTIR spectra for CO2 adsorption (a, b, c) and CO2 desorption (d, e, f) on three samples. The most obvious band in the spectra was found at 2330 cm-1, which was related to physisorbed CO2.51 Meanwhile, 3580 and 3688 cm-1 (combination bands) also corresponded to physisorbed CO2 molecules.52 Chemisorbed CO2 was not detected,53 indicating that no formation of carbonate species occurred in our study. As shown in Figure 10 (a, b, c), CO2 adsorption rate had an obvious improvement after the introduction of mesoporous. The peak appeared at 2330 cm-1 after 1 minute and 3 minutes of CO2


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exposure for Cl-HP-ZSM-5 and N-HP-ZSM-5, respectively. As for ZSM-5, it took 6 minutes for the appearance of the peak. In addition, the saturated adsorption time reduced from 15 minutes for ZSM-5 to 7 minutes for Cl-HP-ZSM-5, suggesting a fast-physical adsorption process. Figure 10 (d, e, f) showed the vast majority of CO2 was desorbed at 50 °C and completed at 90 °C. Low temperature desorption further proved that CO2 was mainly physisorbed onto three samples.54 The introduction of mesoporous may short the CO2 transfer diffusion path,55 which was beneficial to increasing the physical adsorption performance of ZSM-5 adsorbents. (d) Cl-HP-ZSM-5

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Figure 10. In situ FTIR spectra for CO2 adsorption (a, b, c) and CO2 desorption (d, e, f) on three samples.


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4. CONCLUSIONS In this research, we prepared HP-ZSM-5 zeolites combining micro- and mesopous structure by utilizing organosilanes. Two as-synthesized HP-ZSM-5 samples exhibited higher and faster CO2 adsorption performance compared to ZSM-5 sample. Besides, the maximum adsorption capacity of HP-ZSM-5 was up to 58.26 cm3·g-1 at 0 °C and 1 bar. The HP-ZSM-5 samples exhibited a faster physical adsorption process. Simultaneously, we verified that the microstructure and morphology were the main factors for CO2 adsorption performance on HP-ZSM-5 samples. The introduction of a large number of mesopores in ZSM-5 facilitated CO2 diffusion and transfer inside the adsorbents and provided the larger specific surface area, which promoted the CO2 adsorption performance. ACKNOWLEDGEMENTS This study was supported by National Natural Science Foundation of China (Grant No. 21606130, 21306089), Science and Technology Department of Jiangsu (Grant No. BY2015005-02), State Key Laboratory of Materials-Oriented Chemical Engineering (Grant No. ZK201610) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) REFERENCES (1) Andres, R. J.; Boden, T. A.; Breon, F. M.; Ciais, P.; Davis, S.; Erickson, D.; Gregg, J. S.; Jacobson, A.; Marland, G.; Miller, J.; Oda, T.; Olivier, J. G. J.; Raupach, M. R.; Rayner, P.; Treanton, K., A synthesis of carbon dioxide emissions from fossil-fuel combustion. Biogeosciences 2012, 9(5), 1845-1871. (2) Jacobson, M. Z., Review of solutions to global warming, air pollution, and energy security. Energy & Environmental Science 2009, 2(2), 148-173. (3) Liu, Q.; Xiong, B.; Shi, J.; Tao, M.; He, Y.; Shi, Y., Enhanced tolerance to flue gas contaminants on carbon dioxide capture using amine-functionalized multiwalled carbon nanotubes. Energy & Fuels 2014, 28(10), 6494-6501. (4) Aaron, D.; Tsouris, C., Separation of CO2 from flue gas: A review. Separation


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Enhanced CO2 Adsorption Performance on Hierarchical Porous ZSM

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