CO2 adsorption by amine-functionalized MCM-41: a comparison

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CO2 adsorption by amine-functionalized MCM-41: a comparison between impregnation and grafting modification methods Na Rao, Mei Wang, Ziming Shang, Yanwen Hou, Guozhi Fan, and Jianfen Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02906 • Publication Date (Web): 17 Dec 2017 Downloaded from http://pubs.acs.org on December 17, 2017

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CO2 adsorption by amine-functionalized MCM-41: a comparison between impregnation and grafting modification methods Na Rao 1, Mei Wang1,*, Ziming Shang1, Yanwen Hou1, Guozhi Fan1, Jianfen Li1,* 1

School of Chemical and Environmental Engineering, Wuhan Polytechnic University, Wuhan, China

Abstract: Modifications by impregnation and grafting are commonly used for the preparation of amine-functionalized MCM-41. A comprehensive evaluation of the advantages and disadvantages of the two methods was performed in this work. MCM-41 was synthesized by hydrothermal method, setting the amine-loading mass fraction at 40, 50 and 60 wt.%. Three amine-modified adsorbents were prepared by impregnating polyethyleneimine (PEI), and the three other adsorbents were prepared by grafting 3-aminopropyltriethoxysilane (APTS) onto MCM-41. The as-prepared adsorbents were characterized by the XRD, FT-IR, SEM, TGA and N2 adsorption-desorption techniques. CO2 adsorption capacities were measured, and the experimental data were fitted with adsorption kinetic models. The cyclic stability of the adsorbents prepared by the two kinds of amine-modified methods was compared using the cyclic adsorption-desorption experiments. The characterization results showed that the target adsorbents were prepared successfully. The thermal stability of the adsorbents modified by grafting was better than the thermal stability of the adsorbents modified by the impregnation. Maximum CO2

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adsorption

capacities

of

3.53

mmolg-1 (50%-PEI-MCM-41),

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2.41

mmolg-1

(50%-APTS-MCM-41) could be reached at 25 °C and 1 atm, which were 4.7 times and 3.2 times greater than that of MCM-41. The Avrami model fitted the experimental data well, indicating a variety of interactions between the adsorbents and CO2. CO2 adsorption capacity after five adsorption-desorption cycles decreased by 14.22% and 5.19% for the adsorbents prepared by impregnation and grafting, respectively. It was concluded that MCM-41 modified by the impregnation and grafting followed the same kinetic model. The absorbents modified by impregnation showed higher CO2 adsorption capacity and amine-loading efficiency, while those prepared by the grafting had better thermal and cyclic stability. Keywords: CO2 adsorption, amine-functionalized, MCM-41, impregnation, grafting

1. Introduction The greenhouse effect caused by the anthropogenic CO2 emissions is currently attracting intense attention1, 2. Since fossil fuels will still make the main contribution to the production of energy over the next few decades, capture of CO2 from flue gas is one of the most effective approaches for the control of CO2 emissions3. Therefore, carbon capture and storage (CCS) technology has been proposed to alleviate the CO2 threat. Solid sorbents such as zeolite4, active carbon5, carbon coke6, metal organic frameworks7 and mesoporous silica8-10 have been studied for CO2 capture in recent research.


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Mesoporous silica1, 11, 12 was found to be an excellent candidate material for CO2 capture due to its large specific surface area, adjustable pore size and abundant silicon hydroxyl groups that are favorable for surface modification. Among the mesoporous silica, MCM-41 can be obtained by a simple synthesis and shows outstanding structural properties with the high specific surface area of ca. 900 cm2 and a narrow pore size distribution between 35 and 38 Å. However, CO2 capture on MCM-41 without specific active adsorption sites can only rely on physical forces (van der Waals force), resulting in low adsorption capacity and low selectivity13. Therefore, amine modification methods have been proposed to enhance CO2 adsorption performance14-16. There are two methods of modification for solid adsorbents: impregnation and grafting methods. In the former, amine is immobilized in the channel of MCM-41 by van der Waals forces17. In the latter, CO2-philic sites are immobilized by chemical bonds through the condensation of alkanolamine and silanol groups18. Both impregnation and grafting are effective methods for improving the adsorption capacity, as verified by the work of Xu19, Harlick20 and other groups21. In this work, MCM-41 was used as the supporting material, and the performance characteristics of the two kinds of amine-modified adsorbents were comprehensively compared. First, PEI-modified adsorbents and APTS-modified adsorbents were prepared by the impregnation method and the grafting method, and the amine modification fraction was set at 40, 50, and 60 wt.%. Structural properties such as the specific surface area,


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pore size and surface morphology were characterized by the XRD, FT-IR, SEM, TGA and N2 adsorption-desorption techniques. The amine loading efficiency was calculated using the TGA data. Then, the effect of the amine amount on CO2 adsorption performance was studied. In addition, the CO2 adsorption data were fitted by three different adsorption kinetics models. Finally, thermal stability and cycling stability of the two kinds of amine-modified adsorbents were discussed.

2. Experimental 2.1. Materials

Cetyltrimethylammonium bromide (CTAB), 25% aqueous ammonia, anhydrous ethanol and tetraethyl orthosilicate (TEOS) were obtained from Sinopharm Chemical Reagent Co., Ltd. Polyethyleneimine (PEI) (Mn=600) and 3-aminopropyltriethoxysilane (APTS) were obtained from Aladdin Chemistry Co. Highly pure CO2 (99.99%) gas was provided by Minhui Gas (Wuhan, China).

2.2. Preparation of MCM-41 and amine-modified adsorbents MCM-41 was synthesized by a procedure previously reported in the literature22. Following

a

typical

synthesis

procedure,

25%

aqueous

ammonia,

cetyltrimethylammonium bromide (CTAB) and distilled water were added into a polytetrafluoroethylene bottle. The solution was stirred until it was dispersed


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homogeneously and then tetraethyl orthosilicate (TEOs) was added dropwise. The molar proportions of the overall mixtures were TEOS：CTAB：H2O：NH4OH=1:0.12:130:7.9. Then, the solution was treated at 80 ℃ for 4 days. After cooling to room temperature, the solution was filtrated and washed until no foam was observed. Then, white MCM-41 product was obtained after calcination at 560 ℃ for 5 h. MCM-41 was modified with PEI by the impregnation method using the following procedure19. The desired amount of PEI was dissolved into 20 ml of anhydrous ethanol under stirring for 10 min. Then, 1.0 g MCM-41 was added to the solution. The slurry was continuously stirred for 8 h at room temperature, and the PEI-modified adsorbent (x-PEI-MCM-41) was obtained by drying at 80 ℃ for 12 h (x represents the PEI mass fraction). In this work, x was set at 30%, 40%, and 50wt.%, with x calculated according to Eq. (1). x=

mPEI

mPEI × 100% + mMCM − 41

(1)

MCM-41 was modified with APTS by the grafting method using the procedure previously reported in the literature23. One g of MCM-41 was added into 100 ml of anhydrous ethanol under stirring for 10 min. Then, 1 ml of distilled water was added under continuous stirring for 30 min. The temperature was raised rapidly to 70 ℃, and the desired amount of APTS was added dropwise. The slurry was stirred and refluxed for 10 h. The APTS-modified adsorbent (y-APTS-MCM-41) was obtained after washing with anhydrous ethanol repeatedly and drying at 80 ℃ for 12 h (y represents the APTS mass


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fraction). The value of y (30%, 40%, and 50%) was calculated according to Eq. 2. y=

m APTS × 100% mAPTS + mMCM − 41

(2)

2.3. Experimental method

The structure of MCM-41 was characterized by X-ray diffraction (ASAP 2020, USA); the characterization of the functional groups was carried out using FT-IR (NICOLETis10, USA); thermogravimetric analysis was performed to characterize thermal stability using a simultaneous thermal analyzer (Q600, USA); the morphology of MCM-41 and amine-modified adsorbents was observed by scanning electron microscopy (S-3000N, Japan); N2 adsorption-desorption was conducted on an automatic ratio of the surface and pore size distribution analyzer (Autosorb-6B, USA) at 77 K, and the specific surface area and pore size distribution were calculated by the BET and DFT models. Fig. 1 shows a schematic diagram of the CO2 adsorption apparatus consisting mainly of a mass flow meter (MFM) (Masstrak 810, USA), an infrared gas analyzer (JNYQ-I-41C, China) and a PC recorder. The error of the CO2 adsorption device is no more than 5.0%.


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Fig. 1. Schematic diagram of CO2 adsorption apparatus

Prior to the experiment, the apparatus should be preheated for half an hour to exclude the air. The inlet flow rate of CO2 was set as 50 mlmin-1. The adsorption experiment was conducted at 298 K and 1 bar after the inlet flow was equal to the outlet flow. The CO2 flow was then pulled first into the MFM and then into the U-type tube. The un-adsorbed gas flowed into another MFM and then into the infrared gas analyzer, and ultimately into the exhaust gas adsorption device. CO2 desorption was carried out in a vacuum oven (XMTD-8222, China) with the desorption conditions of 373 K and -0.1 MPa for 1 h. The volume of the gas unabsorbed in the CO2 adsorption process (recorded as V1, V2......Vn) was measured using a mass flow meter (MFM, 820 type, SIERRA Flow Measurement and Control Technology Company, USA). The volume percentage of CO2 for the gas unabsorbed in the CO2 adsorption process (recorded as η1, η2 ......ηi) was determined using a CO2 analyzer (JNYQ-I-41C Infrared Gas Analyzer, Xi’an Juneng Instrument Co., LTD, China). All data were recorded by a computer every 30 s. The CO2 adsorption rate at time “i” (unit: mmol·s-1) was calculated by Eq. (3).


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ni (mmol·s −1 ) =

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(50 − Vi ) ×ηi 22.4

(3)

The CO2 adsorption capacity (xCO2, unit: mol CO2 ·g-1 adsorbent, expressed as mol·g-1) and the average of CO2 adsorption rate at time “t” could be determined using Equations (4) and (5). t

∑n

i

xCO 2 ( mmol·g −1 ) =

i =1

mabsorbent

(4) t

∑n

i

−1

vCO 2 (mmol·s ) =

i =1

t

where madsorbent is the mass of the adsorbents.

3. Results and discussion 3.1. Characterization of MCM-41 and amine-modified adsorbents

3.1.1 X-ray diffraction

The XRD pattern of all samples were shown in Fig. 2.


(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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Intensity（ a.u.）

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a b1 b2 b3

c1 c2 c3 1

2

3

4

5

2 Theta(°)

Fig. 2. Small-angle XRD pattern of adsorbents. (a: MCM-41, b1-b3: 40%, 50%, 60%-PEI-MCM-41, c1-c3: 40%, 50%, 60%-APTS-MCM-41)

Examination of Fig. 2 (a) shows three peaks at 2θ of 2.28°, 3.92° and 4.53°, which are consistent with the (100), (110) and (200) facets, respectively. The crystallization time and hydrothermal temperature affect the peak positions24. The experimental data were almost in agreement with the data in the reported literature25. It could be concluded that the sample was regular hexagonal stacked MCM-41 mesoporous molecular sieves. After modified with PEI or APTS, all sample retain the character of MCM-41. However, the peaks gradually shift to a higher diffraction angle and the intensity of the peak reduced or even disappeared due to the smaller pore size, which was consistence with the trend that the more amine loaded in MCM-41, the smaller the pore size will be.


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3.1.2. FT-IR spectra

The FT-IR spectra of MCM-41, x-PEI-MCM-41 and y-APTS-MCM-41 are shown in Fig. 3. a

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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b1 b2 b3 2816

16631455

c1 c2 c3 1410 1560

2985 3630

780

591

1025

4500

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber(cm )

Fig. 3. FT-IR spectra of adsorbents. (a: MCM-41, b1-b3: 40%, 50%, 60%-PEI-MCM-41, c1-c3: 40%, 50%, 60%-APTS-MCM-41)

Examination of Figs. 3 shows that all adsorbents exhibited the 1025 cm-1, 780 cm-1 and 591 cm-1 bands which were attributed to the bending and stretching Si-O-Si vibrations. These bands appeared in all six samples, indicating that the MCM-41 skeleton remained unchanged after amine-modification. The broad band at 3630 cm-1 was attributed to the O-H stretching vibration26. For x-PEI-MCM-41 (b1-b3), the new band at 2816 cm-1 was attributed to the C-H symmetrical stretching vibration. The bands at 1663 cm-1 and 1455 cm-1 were ascribed to the N-H deformation in RNH3+ and the bending vibration of C-H in PEI. For y-APTS-MCM-41 (c1-c3), the bands at 2985 cm-1 was attributed to the


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asymmetrical C-H stretching vibrations27. The band at 1560 cm-1 could be assigned to the N-H deformation vibration in the secondary amine. The band at 1410 cm-1 was assigned to the NCOO skeletal vibration28. These results showed that both PEI and APTS were successfully immobilized onto the MCM-41.

3.1.3. TGA weight loss

The TGA profiles of MCM-41, 50%-PEI-MCM-41 and 50%-APTS-MCM-41 are shown in Fig. 4. 100 90 100℃

80

Mass(%)

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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a

150℃

c2

70 60 50

400℃

40 0

100

200

300

400

b2

600℃ 500

600

700

800

900

Temperature(°C)

Fig. 4. TGA curves of (a) MCM-41, (b2) 50%-PEI -MCM-41, (c2) 50%-APTS-MCM-41.

According to Fig. 4, the three samples showed two common weight losses in 20-100 °C and above 600 °C. The weight loss in 20-100 °C was due to desorption of water or CO2 in the air. Both PEI and APTS was completely decomposed at 600 °C 29, 30 , thus the weight loss above 600 °C was related to the water losses via condensation of


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Si-OH group31. For 50%-PEI-MCM-41 (b2), a significant weight loss could be observed in the 100-400 °C range with a total weight loss of 44.55%. The weight loss in the 100-400 °C range could be divided into two stages, a slow weight loss stage in the 100-150 °C range and a fast weight loss stage in the 150-400°C range. The two weight loss stages might be caused by the inhomogeneous distribution of PEI, the weight loss in 100-150 °C might due to the decomposition of outer surface PEI, and the weight loss in 150-400 °C might be owing to the decomposition of inner surface PEI. For 50%-APTS-MCM-41 (c2), the total weight loss in 150-600 °C was 11.59%, due to the decomposition of grafted alkylamine. The weight loss of PEI started at 100 °C. The fastest loss started at the temperature of approximately 200 °C and stopped at 400 °C, indicating that the loaded PEI decomposed completely at 400 °C. The weight loss of APTS began at 150 °C, and the loss rate reached the maximum at approximately 500 °C. It was found that the thermal stability of MCM-41 modified by APTS was better than that of the PEI-modified sample, which could be explained as due to the chemical bonding forces being stronger than the van der Waals force. The loading efficiencies of PEI and APTS by impregnation and grafting methods were calculated by Eq. (6). Examination of the TGA data showed that the weight losses caused by amine decomposition of PEI and APTS were 44.55% and 11.59%, respectively. Both PEI and APTS were not fully loaded on the MCM-41 due to the loss of organic amines during the preparation. The amine loading efficiency was 89.10% for 50%-PEI-MCM-41 and 23.18% for 50%-APTS-MCM-41, showing that the loading efficiency of the


Page 12 of 28

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impregnation method was much higher than that of the grafting method.

ηloading =

z ×100% 50%

(6)

where z was the actual amine loading percentage as measured by TGA.

3.1.4. SEM analysis

The surface morphologies of MCM-41, 50%-PEI-MCM-41 and 50%-APTS-MCM-41 samples are shown in Fig. 5. (a)

(b)

(c)

Fig. 5. SEM images of (a) MCM-41, (b) 50%-PEI -MCM-41, (c) 50%-APTSMCM-41.


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The short rod-like stacked morphology and a large external pore can be seen in Fig. 5. For most particles, the diameter was approximately 1 µm, in agreement with the synthetic morphology of MCM-41 prepared by Shio32. Amine-modified MCM-41 still maintained a rod-like structure and external channels, and its surface morphology did not change significantly. The particles dispersed well, and no obvious agglomeration was observed.

3.1.5. N2 adsorption-desorption isotherms

The N2 adsorption-desorption isotherms and pore size distribution of MCM-41, 50%-PEI-MCM-41 and 50%-APTS-MCM-41 are shown in Fig. 6. The structural properties of MCM-41, 50%-PEI-MCM-41 and 50%-APTS-MCM-41 is shown in Table 1. a

(1)

500

Adsorbed amount(cm 3/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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400

300

c2

200

100

b2

0 0.0

0.2

0.4

0.6

0.8

Relative pressure(P/P0）


1.0

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a b2

(2)

2.0

c2

dV(d)(cm3/nm/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

1.0

0.5

0.0 0

1

2

3

4

5

6

7

8

9

10

Pore width(nm)

Fig. 6. Nitrogen adsorption-desorption isotherm (1) and pore size distribution (2) for (a) MCM-41, (b2) 50%-PEI -MCM-41, (c2) 50%-APTS-MCM-41. Table 1. Structural properties of adsorbents SBET (m2/g)

Pore volume (cm3/g)

MCM-41

992

0.691

50%PEI-MCM-41

24

0.012

50%APTS-MCM-41

736

0.369

Sample

Fig. 6 shows IV type isotherms and H4 type hysteresis loop, corresponding the mesoporous structure. The isotherm of MCM-41 showed two hysteresis loops: a sharp pore-filling step below the relative pressure of 0.3 and a small N2 adsorbed step at a relative pressure range 0.3-1.0, indicating a bimodal pore size distribution. With the introduction of 50%APTS and 50%PEI, the hysteresis loop became slimmer and even disappeared, indication the decline of pore volume and specific surface area. According to the data in Table 1, the specific surface area and pore volume of MCM-41 were 992 m2g-1

and

0.691

cm3g-1,

respectively,

whereas


for

50%-PEI-MCM-41

and

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50%-APTS-MCM-41, the specific surface area and pore volume were 24 m2g-1, 0.012 cm3g-1 and 736 m2g-1, 0.369 cm3g-1, respectively. Wang33 conducted an infrared study on CO2 adsorption and showed that PEI was anchored on the surface of the mesoporous material. Therefore, a significant decrease in the specific surface area and pore volume should attribute to the coverage of PEI on the MCM-41. Limited by the grafting site on the MCM-41, only a slight reduction of the specific surface area and pore volume of APTS-modified adsorbents was observed. As shown in Fig.6 (2), the pore size center of MCM-41 was located at 1.2 and 2.7nm, which was consistent with the two hysteresis loops in the isotherm curve. The pore size of 50%-APTS- MCM-41 was centered at 2nm, while the pore size of 50%-PEI-MCM-41 could barely observed. The variation trend of pore size agreed well with the trend of pore volume and specific surface area. The structure properties of the amine-modified adsorbents showed that more amine could be loaded by impregnation method than grafting method.

3.2 Adsorption performance

3.2.1 Adsorption capacity

The adsorption capacities of MCM-41, x-PEI-MCM-41 (x=40%, 50%, 60%) and x-ATPS-MCM-41(x=40%, 50%, 60%) are shown in Fig. 7.


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4.0

Adsorption capacity(mmol/g)

( 1)

40%-PEI-MCM-41 50%-PEI-MCM-41 60%-PEI-MCM-41 MCM-41

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 0

20

40

60

80

100 120 140

160 180 200

Time(s)

( 2)

40%-APTS-MCM-41 50%-APTS-MCM-41 60%-APTS-MCM-41 MCM-41

2.5


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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2.0

1.5

1.0

0.5

0.0 0

50

100

150

200

Time(s)

Fig. 7. CO2 adsorption capacities of MCM-41 and amine-modified MCM-41. (1) MCM-41 and x-PEI-MCM-41, (2) MCM-41 and x-ATPS-MCM-41

Examination of Fig. 7 (1) shows that the adsorption capacity of the adsorbent with 50% PEI loading was the highest and was 4.75 times greater than that of MCM-41. The adsorption capacity increased as the amine amount increased from 40% to 50%, which was due to the introduction of the CO2-affinity sites. However, when the amine loading amount increased from 50% to 60%, PEI aggregated in the channel or on the surface of MCM-41, resulting in greater CO2 diffusion resistance and lower adsorption capacity. According to Fig. 7 (2), 50%-APTS-MCM-41 showed the maximum adsorption


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capacity of 2.41 mmolg-1, which was 3.2 times greater than that of MCM-41. The adsorption capacity increased with the increase of amine loading amount from 40% to 50%, which was due to the increase of the CO2-affinity sites on the MCM-41 surface. However, a further increase in amine loading amount led to a decrease in the adsorption capacity. This finding may indicate that the hydroxy sites on MCM-41 surface were fully occupied when the amine loading amount was smaller than 60%. After repeated washing in preparation of x-APTS-MCM-41, the un-grafted APTS still remained in the pores, clogging the channel and hindering CO2 diffusion.

3.2.2 Adsorption rate

The adsorption rate is also one of the factors that characterize the performance of an adsorbent. The adsorption rate-time curves of MCM-41, x-PEI-MCM-41 (x=40%, 50%, 60%) and x-ATPS-MCM-41(x=40%, 50%, 60%) are shown in Fig. 8. 0.040

40%-PEI-MCM-41 50%-PEI-MCM-41 60%-PEI-MCM-41 MCM-41

( 1) 0.035

Adsorption rate(mmol/s)

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

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0.030 0.025 0.020 0.015 0.010 0.005 0.000 -0.005 0

50

100

150

Time(s)


200

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0.040 40%-APTS-MCM-41 50%-APTS-MMC-41 60%-APTS-MCM-41 MCM-41

( 2) 0.035

Adsorption rate(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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0.030 0.025 0.020 0.015 0.010 0.005 0.000 -0.005 0

50

100

150

200

Time(s)

Fig. 8. CO2 adsorption rates of MCM-41 and amine-modified MCM-41. (1) MCM-41 and x-PEI-MCM-41, (2) MCM-41 and x-ATPS-MCM-41

As seen from Fig. 8, all adsorbents achieved their adsorption equilibrium in 200s. The CO2 adsorption rate of MCM-41 was initially fast, with the fast adsorption stage due to the physical adsorption in the pores. Then, the adsorption rate rapidly decreased with time, mainly due to the decreasing pore volume. The adsorption rate decreased to 0 after the pore was filled, indicating that the adsorption reached equilibrium. The adsorption rate curve of PEI-modified MCM-41 (Fig. 8 (1)) could be divided into two stages: the adsorption rate first increased rapidly to the maximum value and then decreased gradually. Prior to the peak, the adsorption rate of 50%-PEI-MCM-41 was the highest followed by those of 60%-PEI-MCM-41 and 40%PEI-MCM-41. After the peak, the adsorption rate decreased and reached a plateau. Identical observations were reported by Wang34. The rapid adsorption rate stage could be explained as due to PEI adsorbing CO2 on the surface of MCM-41. Then, the CO2 diffused to the multiple layer, and the PEI


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loaded in the pores began to adsorb CO2. With the active adsorption-site gradually saturating, the adsorption process then reached equilibrium. The adsorption rate of APTS-modified MCM-41 (Fig. 8 (2)) was roughly L-shaped. Prior to the turning point, the adsorption rate of modified adsorbents was slightly smaller than that of MCM-41. Therefore, it was presumed that physical adsorption was the main adsorption mechanism at this stage. The slower adsorption rate of the adsorbents was due to the reduced pore structure after modification. After the turning point, the adsorption rate increased slightly, reached a constant stage, and finally achieved adsorption equilibrium, while the adsorption rate of MCM-41 was nearly decreased to a smaller value after the turning point. It was concluded that the modified-adsorbents depended largely on chemical adsorption at this stage. Among the modified adsorbents, the 50%-PEI-MCM-41 and 50%-APTS-MCM-41 showed the optimal average adsorption rates of 0.1141 mmols-1 and 0.0062 mmols-1, respectively, 36 times and twice larger than that of MCM-41.

3.3 Adsorption-kinetic

The pseudo-first-order, pseudo-second-order and Avrami models were used to fit the adsorption data of 50%-PEI-MCM-41 and 50%-APTS-MCM-41 at 298 K, and 1 bar. The equations for the three adsorption kinetics models were as follows: Q = Q 1 − exp− t


（7）

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（8）

=

= 1 − exp −

（9）

where k1 (s-1), k2 (g/mmolmin) and ka (s-1) are adsorption constants; Qt (mmolg-1) is the adsorption capacity at time t; Qe (mmolg-1) is the equilibrium adsorption capacity; and nA was the kinetic index of the Avrami model. The fits of the adsorption kinetic curves for 50%-PEI-MCM-41 and 50%-APTS-MCM-41 by the above three models are shown in Fig. 9. Table 2 shows the parameters of the kinetic models for CO2 capture. 4.0

( 1)


3.5 3.0 2.5 2.0

Experimental Data Pseudo-first-order Pseudo-second-order Avrami

1.5 1.0 0.5 0.0 0

20

40

60

80

100

120

140

160

180

Time(s)

( 2)

2.5


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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2.0

Experimental data Pseudo-first-order Pseudo-second-order Avrami

1.5

1.0

0.5

0.0 0

50

100

150

200

Time(s)

Fig. 9. Corresponding fits of (1) 50%-PEI-MCM-41 and (2) 50%-ATPS-MCM-41 using three kinetic models


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Table 2. Parameters of kinetic models for CO2 capture over 50%-PEI-MCM-41 and 50%-APTS-MCM-41 Kinetic model

Pseudo-first-order

Pseudo-second-order

Avrami

Parameter

50%-PEI-MCM-41

50%-APTS-MCM-41

Qe (mmol·g-1)

5.7528

3.3054

k1 (s-1)

0.0067

0.0079

R2

0.9739

0.9738

Qe (mmol·g-1)

9.8157

5.2154

k2 (g·(mmol min)-1)

4.10E-04

9.96E-04

R2

0.9716

0.96916

Qe (mmol·g-1)

3.6856

2.5576

ka (s-1)

0.0139

0.0121

nA

1.58

1.49

R2

0.9984

0.9880

As shown in Fig. 9, both the pseudo-first-order and pseudo-second-order models overestimated the adsorption capacity at the initial stage and equilibrium stage, while underestimating

the

adsorption

capacity

near

the

equilibrium

stage.

The

pseudo-first-order model was based on the assumption that the adsorption capacity was proportional to the number of active sites on the adsorbent and had a higher degree of fitting for the adsorbent with a lower surface coverage35. The first-order model predicted a Qe value that was too high, indicating that not all active sites participated in CO2 adsorption during the actual adsorption process. The pseudo-second-order kinetic model was based on the assumption that chemical adsorption was the rate controlling step36. The adsorption rate curve showed that the last stage was the rate-controlling step and the last


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stage should include both chemical and physical adsorption. According to Fig. 9 and Table 2, the Avrami model shows a good fit for 50%-PEI-MCM-41 and 50%-APTS-MCM-41, with the R2 values of approximately 0.99 and the Qe values that are close to the experimental data. nA values were 1.58 and 1.49 for 50%-PEI-MCM-41 and 50%-APTS-MCM-41, respectively, representing the growth dimension of the adsorption sites. Thus, the Avrami model could be used to predict the adsorption process of modified-MCM-41 adsorbents.

3.4 Stability of the modified-adsorbents during CO2 adsorption-desorption cycles

Cyclic stability is an important factor in the evaluation of the suitability of the adsorbent for long-term practical application. Fig. 10 shows the adsorption capacities of MMC-41 (1), 50%-PEI-MCM-41 (2) and 50%-APTS-MCM-41 (3) in five consecutive CO2 adsorption-desorption cycles. (1)

0.8


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0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 1

2

3

4

Cycle number


5

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4.0

(2) Adsorption capacity(mmol/g)

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 1

2

3

4

5

Cycle number

(3)

2.5


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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2.0

1.5

1.0

0.5

0.0 1

2

3

4

5

Cycle number

Fig. 10. Cyclic CO2 adsorption capacities of (1) MCM-41, (2) 50%PEI-MCM-41 and (3) 50%APTS-MCM-41.

According to Fig. 10, pure MCM-41 showed great cycle stability with only a 4.22% decrease on adsorption capacity after five adsorption-desorption cycles. For 50%-PEI-MCM-41, the adsorption capacity decreased by 12.65% after the first cycle and a small decline was observed for the next four cycles, which may be due to the PEI on the outer surface of MCM-41 volatilizing in the first adsorption-desorption cycle and the relatively higher stability of the PEI on the inner surface. The cyclic adsorption capacity


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trend was consistent with the PEI decomposition stage in TGA results. After five adsorption-desorption cycles, the adsorption capacity decreased by 14.22%. For 50%-APTS-MCM-41, no dramatic decrease in CO2 adsorption capacity was observed. CO2 adsorption capacity decreased by 5.19% in five adsorption-desorption cycles. In conclusion, the pure MCM-41 showed better cycle stability than the amine-modified MCM-41, and the performance of grafted MCM-41 was more stable than that of impregnated MCM-41, which was consistent with the TGA characterization results.

4. Conclusion Amine-functionalized mesoporous silica material (such as MCM-41) is becoming the focus of research because of its excellent CO2 adsorption performance and great stability. Impregnation and grafting are the two main approaches for amine-modification of this material. The merits and demerits of the two amine-functionalized methods were systematically compared in this paper. MCM-41 was synthesized by the hydrothermal method, and amine-functionalized MCM-41 was prepared by impregnating and grafting method, setting the amine-loading mass fraction at 40, 50 and 60 wt.%, respectively. MCM-41 and amine-modified MCM-41 samples were characterized by the XRD, FT-IR, TGA, N2 adsorption-desorption and SEM techniques. Then, the adsorption performance and stability of the adsorbents were evaluated by CO2 adsorption-desorption experiments. Finally, the CO2 adsorption experimental data were fitted with kinetic models.


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The results showed the following. (1) Amine-functionalized adsorbents could be prepared successfully using impregnation or grafting. (2) The 50%-PEI-MCM-41 sample showed the optimum adsorption capacity and adsorption rate of 3.53 mmolg-1 and 0.1141 mmols-1, respectively, at 25 °C under 1 atm, whereas the optimum adsorption capacity and adsorption rate of 2.41 mmolg-1 and 0.0062 mmols-1, respectively, were obtained for 50%-APTS-MCM-41. In addition, TGA results showed that the adsorbents loaded with PEI began to decompose at approximately 100 °C, while decomposition occurred at approximately 150 °C for the adsorbents grafted with APTS. The adsorption capacity of PEI-modified adsorbents decreased by 14.22%, while that of the APTS-modified adsorbents decreased by 5.19% after five CO2 adsorption-desorption cycles. (3) CO2 adsorption curve of 50%-PEI-MCM-41 showed the presence of a two-stage adsorption process, and the 50%-APTS-MCM-41 samples exhibits a roughly L-shaped adsorption curve. The Avrami model showed great fitting accuracy for both 50%-PEI-MCM-41 and 50%-APTS-MCM-41 and could be used to predict the adsorption process for amine-modified adsorbents. In conclusion, adsorbents prepared by grafting showed higher stability but lower adsorption capacity than those prepared by the impregnation method. The combination of impregnation and grafting will be investigated in future work and is likely to obtain some effective adsorbents with high amine loading, superior CO2 adsorption performance and great stability.


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Authors Information Corresponding Authors *

Phone: +8-27-83943856. E-mail: [email protected]; [email protected].

Notes The authors declare no competing financial interest.

Acknowledgments The authors are grateful for the financial support from Hubei Province Natural Science Foundation of China (2014CFB883), Hubei Province Excellent Science and Technology Innovation Team Project（T201407), Program for the Young and Middle-aged Talent of Hubei Province Department of Education (Q20151707). References (1) Samanta, A.; Zhao, A.; Shimizu, G. K. H.; Sarkar, P.; Gupta, R. Ind. Eng. Chem. Res. 2012, 51, 1438-1463. (2) Wang, Q.; Luo, J.; Zhong, Z.; Borgna, A. Eng. Environ. Sci. 2010, 4, 42-55. (3) Li, W.; Gao, Z.; Yu, J.; Wang, Y. Ind. Eng. Chem. Res. 2013, 52, 14965-14974. (4) Su, F.; Lu, C.; Kuo, S. C.; Zeng, W. Energ. Fuel. 2010, 24, 1441-1448. (5) Gholidoust, A.; Atkinson, J. D.; Hashisho, Z. Energ. Fuel. 2017, 31. (6) Wang, X.; Wang, D.; Song, M.; Xin, C.; Zeng, W. Energ. Fuel. 2017, 31. (7) Salehi, S.; Anbia, M. Energ. Fuel. 2017, 31. (8) Heydarigorji, A.; Yang, Y.; Sayari, A. Energ. Fuel. 2011, 25, 4206-4210. (9) Sanz, R.; Calleja, G.; Arencibia, A.; Sanzpérez, E. S. Energ. Fuel. 2013, 27, 7637-7644. (10) Dao, D. S.; Yamada, H.; Yogo, K. Energ. Fuel. 2015, 29, 985-992. (11) Yu, C. H.; Huang, C. H.; Tan, C. S. Aerosol. Air. Qual. Res. 2012, 12, 745-769. (12) Dutcher, B.; Fan, M.; Russell, A. G. Acs. Appl. Mater. Inter. 2015, 7, 2137-48. (13) Zhao, X. S.; Lu, G. Q.; Millar, G. J. Ind. Eng. Chem. Res. 1996, 35, 2075-2090. (14) Loganathan, S.; Ghoshal, A. K. Chem. Eng. J. 2017, 308, 827-839. (15) Sanz, R.; Calleja, G.; Arencibia, A.; Sanz-Pérez, E. S. Micropor. Mesopor. Mat. 2012, 158, 309-317. (16) Harlick, P. J. E.; Sayari, A. Studies in Surface Science & Catalysis. 2005, 158, 987-994.


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(17) Wang, D.; Ma, X.; Sentorunshalaby, C.; Song, C. Ind. Eng. Chem. Res. 2012, 51, 3048-3057. (18) Waddell, T. G.; Leyden, D. E.; Debello, M. T. J. Am. Chem. Soc. 1981, 103, 5303-5307. (19) Xu, X.; Andresen, J. M.; Song, C.; Miller, B. G.; Scaroni, A. W. 2002. (20) Harlick, P. J. E.; Sayari, A. Ind. Eng. Chem. Res. 2006, 45, 3248-3255. (21) Fujiki, J.; Yogo, K. Energ. Fuel. 2014, 28, 6467. (22) Loganathan, S.; Tikmani, M.; Ghoshal, A. K. Langmuir the Acs Journal of Surfaces & Colloids. 2013, 29, 3491-9. (23) Ko, Y. G.; Shin, S. S.; Choi, U. S. Journal of Colloid & Interface Science. 2011, 361, 594. (24) Khushalani, D.; Kuperman, A.; Ozin, G. A.; Tanaka, K.; Coombs, N.; Olken, M. M.; Garcés, J. Adv. Mater. 2010, 7, 842-846. (25) Kamarudin; K., S. N. Fuel. Process. Technol. 2013, 106, 332-337. (26) Wang, W.; Yang, X.; Fang, Y.; Ding, J. Appl. Energ. 2009, 86, 170-174. (27) Socrates, G., Infrared and Raman characteristic group frequencies table and charts. J. Wiley: UK, 2004; pp 1-347. (28) Alex C. C. Chang; Steven S. C. Chuang, †; McMahan Gray, a.; Soong‡, Y. Energ. Fuel. 2003, 17, 468-473. (29) Xu, X.; Song, C.; Andrésen, J. M.; Miller, B. G.; Scaroni, A. W. Micropor. Mesopor. Mat. 2003, 62, 29-45. (30) Kishor, R.; Ghoshal, A. K. Chem. Eng. J. 2015, 262, 882-890. (31) Chen, C. Y.; Li, H. X.; Davis, M. E. Micropor. Mat. 1993, 2, 17-26. (32) Shio, S.; Kimura, A.; Yamaguchi, M.; Yoshida, K.; Kuroda, K. Cheminform. 1999, 30, 2461-2462. (33) Wang, X.; Ma, X.; Song, C.; Locke, D. R.; Siefert, S.; Winans, R. E.; Möllmer, J.; Lange, M.; Möller, A.; Gläser, R. Micropor. Mesopor. Mat. 2013, 169, 103-111. (34) Wang, X.; Schwartz, V.; Clark, J. C.; Ma, X.; Overbury, S. H.; Xu, X.; Song, C. J. Phys. Chem. 2009, 113. (35) Haerifar, M.; Azizian, S. J. Phys. Chem. C. 2013, 117, 8310–8317. (36) Ho, Y. S.; Mckay, G. Chem. Eng. J. 1998, 70, 115-124.


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CO2 adsorption by amine-functionalized MCM-41: a comparison

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