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Experimental investigation of CO2 capture capacity: exploring mesoporous silica SBA-15 material impregnated with monoethanolamine and diethanolamine Hongwei Chen, Zhanwei Liang, Xin Yang, Ze Zhang, and Zhiyuan Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01298 • Publication Date (Web): 04 Oct 2016 Downloaded from http://pubs.acs.org on October 7, 2016

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Experimental investigation of CO2 capture capacity: exploring mesoporous silica SBA-15 material impregnated with monoethanolamine and diethanolamine Hongwei Chen, Zhanwei Liang*, Xin Yang, Ze Zhang, Zhiyuan Zhang Key Laboratory of Condition Monitoring and Control for Power Plant Equipment Ministry of Education, North China Electric Power University, Baoding City, Hebei Province, People’s Republic of China. Abstract: A series of efficient adsorbents were prepared by impregnating mesoporous silica SBA-15 with different amounts of monoethanolamine (MEA) and diethanolamine (DEA) in order to improve CO2 capture capacity. The textural properties of pure and modified mesoporous SBA-15 materials were characterized by X-ray diffraction characterization (XRD), transmission electron microscopy (TEM), N2 adsorption-desorption test and thermogravimetric analysis (TGA). When the ratio of DEA to SBA-15 is below 2, these molecules loaded on the support are relatively far away from saturating and allow the accessibility of CO2 molecules to the inner adsorption sites. Further increasing of the amine loading would reduce opportunities of CO2 to contact with internal amino sites, because the amine was covered in a multilayer form or even caking form on the SBA-15 pore surface. The similar performance was observed for MEA. Therefore, the adsorption capacity of CO2 increase with the amount DEA or MEA content; but further increasing of MEA or DEA loaded on the mesoporous SBA-15, the CO2 capture is influenced by the packing effect on the mesoporous hexagonal silica. Temperature effect on adsorption

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was also studied in the range 30 to 90 °C, showing that with the increase of temperature, the adsorptive amounts of adsorbents lessened gradually from highest values at 30 °C since the thermodynamic controlled process. The mesoporous SBA-15 material impregnated with MEA or DEA can provide a perspective to further explore effective adsorbents for CO2 capture. Keywords: CO2 capture; SBA-15; impregnation; adsorption; monoethanolamine; diethanolamine 1 Introduction The anthropogenic emission of greenhouse gases is considered to cause global climate changes, such as global temperature increase and sea level rise. Carbon dioxide (CO2) as one of the main greenhouse gases, which mainly comes from fossil fuel combustion by thermal power stations and cement plant, has been focused on developing cost-effective and energy-efficient processes to capture and storage1. CO2 capture strategies can be divided into pre-combustion, post-combustion and oxy-combustion categories according to the removed position. Among the three ways, post-combustion approaches are generally favored considering the retrofit of existing power plants2,3 and more studies have been carried out on solid sorbents4–6 and chemical solvent2,7–12 for post-combustion CO2 capture. Amine-based regenerative chemical absorption using organic amine solutions such as monoethanolamine (MEA) and diethanolamine (DEA) have been widely applied as the business-like methods7–11. CO2 absorption process using amine aqueous is a typical chemical reaction accompanied with a gas-liquid mass transfer process. To


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enhance the mass transfer of diffusion controlled processes, Jiang et al., 2014, prepared nanofluids based on MEA and MDEA solutions to study the influence of nanoparticles on the CO2 absorption in bubbling reactor, and found that the tested nanoparticles could improve gas-liquid mass transfer during reaction process12. The method of absorbing CO2 in organic amine solutions requires significant energy expenditure. To reduce these energy requirements and enhance energy efficiency, Oh et al., 2016, constructed a superstructure including the conventional MEA-based configuration and structural modifications for optimizing show that the minimum energy costs can be achieved without considering the capital costs2. Despite the improved efficiency in capturing CO2 using amine solvent, the absorption processes still have not solved the most important problems, such as the corrosive properties of equipments, the solvent degradation in the presence of oxygen, the amine waste during the operation and the energy consumption in the regeneration process13,14. Therefore, intense research has been currently focused on the development of alternative procedures for CO2 capture, such as cryogenic technologies, adsorption or membrane based techniques 15 . Comparing with the other two approaches, solid adsorption processes to capture CO2 has gradually become an economically attractive alternative to substitute amine absorption system due to low energy cost, stable performance, benign solid wastes as well as avoiding equipment corrosion 16–19 . Consequently, in recent years there has been an increasing interest in improving efficient solid adsorbents for CO2 capture4,5,17–23.


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Thereby, zeolites20,21, activated carbon5, porous oxides4, mesoporous silicates21,24–27 and metal-organic frameworks28 have been investigated for the removal of CO2. Among these solid supports, mesostructured silica materials such as SBA families stand out due to their high hydrothermal stability, large surface area and pore volume and uniform hexagonal mesopores that allow anchoring organic groups, also enabling a rapid CO2 diffusion within the pore structure14,29–31. A promising technique is mesostructured SBA-15 functionalized with organic molecules containing amino group, such as amine-grafted6,7,32–36 or impregnated31,37–41method, which is the most studied process because this method shows high acidic CO2 adsorption since they adsorb CO2 through carbamate formation17. Impregnation method has gained wide application due to its simple fabrication technology and high amine loadings38. Thus, Sanz et al., 2010, found that SBA-15 functionalized by impregnation with PEI up to 70% loading obtained a maximum adsorption value close to 90 mg/g at 75 °C and 1 bar, and CO2 adsorption capacity showed an increase with temperature and the main mechanism seemed to be chemisorption of CO2 on amino sites of the modified SBA-1531. Yan et al., 2011, also reported four PEI impregnating SBA-15 materials which were synthesized with different porous properties for CO2 capture, and the results indicated that the CO2 adsorption amount increased with the pore volume of the support, with the maximum adsorption value was 105.2 mg/g achieved at the largest pore volume of 1.14 cm3/g37. Through the study on the different amounts of PEI functionalized SBA-15 as molecular basket sorbents, Wang et al., 2013, conclude that the highest CO2 sorption


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capacity was 154 mg/g at 75 °C over the PEI-60/SBA-15 sorbent, as well as the sorption capacity increased with the raise of temperature before 100 °C and then decreased due to the dominant desorption at further higher temperature40. In the aspect of other amine impregnating SBA-15 adsorbents, Yue et al., 2006, explored TEAP (tetraethylenepentamine) impregnating mesoporous SBA-15 occluded with P123, and estimated that the amount of CO2 capture was as high as 173 mg/g due to the specific interaction of P123 with the amine42. Zhang et al., 2013, illustrated that the adsorption capacity of physical impregnated SBA-15 with TN (acrylonitrile modified TEPA) increased with temperature below 25 °C and reached the maximum of 153 mg/g, and then it slightly decreased to 131.5 mg/g at 80 °C39. Dynamics of CO2 adsorption experiments, conducted by Bollini et al., 2012, were reported for zeolites 13X and APTMS-functionalized SBA-15 with three different amine loadings43,44. The results of this study point to the fact that low and medium amine loadings had better CO2 diffusion properties over zeolites 13X, however, the support material loaded with high APTMS showed adverse effect on CO2 adsorption kinetics. Impregnation of mesoporous silica material with DEA was investigated by Franchi et al. 2005, and also found that the capacity and uptake rate reached maximum with respect to amine loaded and then decreased due to the load of excess amine on the external surface and within the pores45. They compared the CO2 adsorption capacity of DEA loaded on pore-expanded MCM-41 silica (PE-MCM-41), activated carbon, silica gel, and standard MCM-41 silica. It was PE-MCM-41 that could accommodate a larger quantity of amine resulting in higher CO2 adsorption of 104 mg/g ascribe to


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its very large pore volume compared to the other supports. Yue et al., 2008, fabricated a

CO2

capturer

through

impregnating

the

amine

mixture

of

TEPA

(tetraethylenepentamine) and DEA or glycerol on the as-synthesized mesoporous silica SBA-15, and they reported that the maximum CO2 adsorption capacity of the mixed amine-modified SBA-15 sample reached 163 mg/g38. However, they pointed out that the adsorption capacity of silica SBA-15 modified by DEA alone was only 20.8 mg/g. Jadhav et al., 2007, applied impregnation method to modify zeolite 13X with monoethanolamine (MEA), and reported that the adsorption capacity efficiency improved by a factor of ca. 1.6 and 3.5 over the unmodified zeolite at the temperature of 30 °C and 120 °C, respectively20. They pointed out that the highest adsorption capacity of CO2 was about 87 mg/g for zeolite 13X-MEA-10 by comparing with zeolite 13X-MEA-2, 13X-MEA-50 and unmodified zeolite 13X at 30 °C. Summarily, though the MEA and DEA solutions being typical liquid-phase sorbents as well as different types of amine modified mesoporous SBA-15 have been widely applied to capture CO2 from the researches cited above, it could be seen that prior investigation about CO2 adsorption capacity studied separately on SBA-15 functionalized with MEA or DEA. In short, there is a lack of deep understanding about CO2 adsorption by mesoporous silica SBA-15 functionalized with MEA or DEA. We impregnated two different types of amine sources into mesoporous silica SBA-15, and studied systematically the effect of mass loading of MEA or DEA and temperature on the adsorption capacity and the adsorption rate of CO2 gas molecule.


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Using wetness impregnation of MEA and DEA with ethanol, we have claimed to form a multilayer of the amine source onto the hexagonal structure of the support, without losing much of the surface area and pore volume of the silica material. Contrary to other reports in the same field, this work shows an increase of CO2 adsorption capacity, and also shows that the adsorption process follows the traditional thermodynamic behavior and provides a detailed explanation toward the principle governing this process. The objective of this work was to use MEA and DEA modified mesoporous SBA-15 material as adsorbents to capture CO2. The incorporation of MEA and DEA to support was achieved by impregnation technique. The resulting properties of the functionalized SBA-15 adsorbents have been systematically characterized by XRD, TGA, N2 adsorption/desorption, and TEM. Further experimental works were developed to study the effect of amine content on the extent of CO2 adsorption as well as adsorption rate. The influence of temperature on CO2 adsorption capacity of prepared adsorbents has also been investigated. 2 Experimental 2.1. Chemicals Monoethanolamine (MEA, ≥99.7% pure), diethanolamine (DEA, ≥99.7% pure) and ethanol(≥99.7% pure) were bought from Kermel Chemical Regent Tianjin Co. Ltd. and used as received. Mesoporous material SBA-15 was purchased from XFNano Materials Tech. Nanjing Co. Ltd. Carbon dioxide (CO2, 99.99% pure) and nitrogen (N2, 99.99% pure) were supplied by North China Special Gas.


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2.2. Preparation of the adsorbents The functionalized SBA-15 adsorbents were prepared by a wet impregnation method using the following procedure21,46. A desire amount of MEA (M) and DEA (D) were respectively dissolved in 30 ml of ethanol under stirring for 30 min to form fully mixed solution. The required amount of calcined SBA-15 was added to the resulting solution under stirring. The slurry was continuously stirred at room temperature for 60 min, followed by drying at 80 °C for 10 h until the ethanol was evaporated. The obtained adsorbents with different loading were denoted as R-S-n, where R represents MEA and DEA, S and n are the SBA-15 and the weight ratio of R to SBA-15 in the adsorbents, respectively. The amounts of added SBA-15, R and ethanol are specified in Table 1. Table 1. Amounts of SBA-15, R and ethanol used for impregnation. sample

R:SBA-15

sample

R:SBA-15

Ethanol(ml)

M-S-0.5 M-S-1 M-S-2 —

0.5:1 1:1 2:1 —

D-S-0.5 D-S-1 D-S-2 D-S-3

0.5:1 1:1 2:1 3:1

30 30 30 30

2.3 Characterization of adsorbents The structures of SBA-15 and R-S-n samples were characterized by X ray diffraction (XRD), N2 physisorption, thermogravimetric analysis (TGA) and transmission electron microscopy (TEM). Powder XRD pattern was recorded on a Bruker AXS D8 Advance machine using Cu Kα radiation (λ=0.1543nm) in the 2θ range from 0.7° to 5°. N2 physisorption isotherms were measured at -196 °C on a Micromeritics TristarⅡ3020 surface area and porosity analyzer, from which the


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surface area, average pore size and pore volume were obtained simultaneously. Before each adsorption-desorption test, the samples were outgassed at 100 °C for 2 h under a high vacuum condition. The specific surface area values was calculated using Brunauer-Emmett-Teller(BET) equation, the average pore size distributions were estimated

from

the

desorption

branch

of

isotherms

based

on

the

Barrett-Joyner-Halenda (BJH) algorithm assuming a cylindrical geometry of the pores and the total pore volume was taken at the volume of liquid N2 adsorbed at a relative pressure of P/P0 ≥ 0.987. TEM measurements were acquired on a Tecnai G2 F20 S-TWIN electronic microscope working at 200 kV. Thermal chemical and physical properties of samples were characterized by TA Q-600 TGA. About 3 mg of the samples were heated at 10 °C min-1 under nitrogen flow of 100 ml min-1 up to 600 °C. 2.4 CO2 adsorption test CO2 adsorption was measured by TGA (TA Q-600). Approximately 5 mg sample was placed and then heated in the nitrogen (99.99% pure) flow of 100 ml min-1 at the rate of 10 °C min-1 to 105 °C and kept for 2 h to remove preadsorbed moisture and CO2. After cooled to 30 °C, the input gas was switched from nitrogen to CO2 (99.99% pure) and continuously injected for 50 min, which was determined to be sufficient by the sample mass change within ±0.0005 mg. The CO2 adsorption capacity was estimated from the sample mass change in CO2. The adsorption capacities of each sample were tested following above procedures at different temperature from 30 °C to 90 °C.


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3 Results and discussion 3.1 Characterization of adsorbents 3.1.1 X-ray diffraction The low angle XRD patterns of pure SBA-15 and SBA-15 impregnated with different amounts of MEA or DEA are shown in Figure 1. The mesoporous SBA-15 pattern appeared as a predominant signal at 2θ=0.8°and two weak diffraction peaks at higher angles corresponding to the (100), (110), and (200) pore planes respectively, indicating the existence of a well define hexagonal porous structure47. (100)

(a)

(b) (100) (110)(200)

Intensity [a.u.]

(110) (200)

Intensity [a.u.]

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SBA-15 M-S-0.5 M-S-1

D-S-0.5 D-S-1 D-S-2

M-S-2

1

2

3 2θ [degree]

4

D-S-3

5

1

2

3 2θ [degree]

4

5

(a) SBA-15 and MEA impregnated SBA-15. (b) DEA impregnated SBA-15. Figure 1. Powder XRD patterns of samples.

The patterns of MEA or DEA impregnating samples show approximately the same diffraction peak position; despite that their intensities gradually decrease and angles slightly shift high as the amount of MEA or DEA loaded is increased. Especially, two weak peaks appear hardly to distinguish in samples of M-S-2, D-S-2 and D-S-3. This transformation, especially for the highest amounts of MEA or DEA loaded, is probably due to the surfaces covering and the pores stuffing by impregnated materials. However, the position of the diffraction peak (100) remained unchanged for all


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samples, which demonstrated that the SBA-15 mesoporous structure is not damaged even in sample D-S-3. 3.1.2 TGA weight loss The thermal behaviour of all adsorbents as well as the amount of amine loading on mesoporous SBA-15 materials was studied by the thermogravimetric analysis (TGA) and the differential thermogravimetric analysis (DTGA). Figure 2 shows the relationship of the weight loss and its derivate of samples to temperature. As expected, there was about 3% weight loss for SBA-15 before 100 °C due probably to desorption of surface moisture and/or CO2 (Figure 2, (a), (A)). The weight of SBA-15 was almost stained constant up to 600 °C, which indicated that the structure of SBA-15 remained stable even at high temperature as showed by Jing, et al.7. The TGA graphs show two characteristic weight losses for the adsorbents functionalized with MEA or DEA. The initial weight loss region below 120 °C was attributed to desorption of the physically adsorbed water, CO2 or the remaining ethanol. The other weight loss peak occurred in the temperature range from 300 to 500 °C could be primarily due to the decomposition of the amino group and sharp weight loss appeared at 400 and 450 °C for MEA and DEA modified SBA-15, respectively. The temperature of decomposition is obviously higher than that of boiling point of MEA or DEA (170.5 °C and 268.8 °C, respectively). It assumed that the existence of MEA or DEA impregnated in mesoporous of the SBA-15 particles, which can increase the barrier for heat transfer and the decomposed product diffusion40. Consequently, the temperature of weight loss peaks was increased and the


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thermal stability was enhanced simultaneously. Normally, the residual weight was gradually reduced as the ratio of MEA or DEA to SBA-15 increased, which due to more amine loaded on SBA-15. The amine attached to the SBA-15 mesoporous is calculated through initial samples mass subtract residual and impurity (water, CO2 or ethanol) mass which obtained at 110 °C by TGA showed in Table 2. Table 2. Summary of mass ratio. Sample

Residual [%]

Impurity [%]

MEA or DEA [%]

SBA-15 M-S-0.5 M-S-1 M-S-2 D-S-0.5 D-S-1 D-S-2 D-S-3

97 78 63 57 71 62 56 49

3 6 10 12 4 5 7 7

0 16 27 31 25 33 37 44

100

100

0.0

0.0

80

(B) -0.2

70

90

-0.3 (C) 60 (D) 0

100

200 300 400 Temperature [℃]

500

600

-0.1 80

(A) -0.2

70

(B)

60

(C) 50

-0.4

Derivated Weight loss [%/℃]

-0.1

Weight loss [%]

90

Derivated Weight loss [%/℃]

(A)

Weight loss [%]

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

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-0.3

( D) 0

100


500

600

-0.4

(a) (A) SBA-15, (B) M-S-0.5, (C) M-S-1, (D) M-S-2 (b) (A) D-S-0.5, (B) D-S-1, (C) D-S-2, (D) D-S-3 Figure 2. TGA and DTA profiles of the samples

3.1.3 N2 adsorption and desorption isotherms The N2 adsorption/desorption isotherms of SBA-15 and MEA or DEA impregnated SBA-15 along with their BJH pore size distributions are illustrated in Figure 3 and Figure 4, respectively. While the isotherms were showed in Figure 3 (a) and (b), presenting almost a classic type Ⅳ IUPAC isotherm with type H1 hysteresis loops and a progressive change of this typical shape as the different amine is loaded. After MEA


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or DEA loading, the S-shape adsorption isotherms were still obtained for all functionalized mesoporous SBA-15, indicating their uniform mesoporous structure. In addition, the N2 uptake was significantly influenced for samples impregnated with DEA comparing with MEA. The BJH pore size distribution curves in Figure 4 showed that uniform pore size distribution was almost retained for amine-functionalized SBA-15 and pore plugging by loaded amine was limited. However, the pore size gradually declined with the increase of MEA or DEA impregnation on support as also showed in Table 3. Textural properties such as surface area, pore volume and mean pore diameter of the samples are summarized in Table 3. The BET fitting plots and parameters of samples were showed in Supplementary Information. MEA or DEA impregnating SBA-15 materials show gradually decreasing values of textural properties while increase amine loading. As observed from Table 3, sample D-S-3, which impregnated with the highest amine content, still presented mesopores properties and available porosity. As well as, it did not present a saturated pore structure due to the excess volume of MEA or DEA impregnated on SBA-15. To confirm this observation, the maximum MEA or DEA content that could be loaded in the SBA-15 was calculated. It can be seen from Table 3 that the pore volume of SBA-15 is 0.9476 cm3/g, and the density of MEA and DEA is about 1 g/cm3. Thus, the maximum amount of MEA or DEA that could be loaded on the SBA-15 pores was 1.04 g, which corresponds to 51% of MEA or DEA in the functioned SBA-15 sample. The percentage of MEA or DEA loaded in the pore volume for all the samples showed in Table 2 presents that the


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porosity of the support is not completely filled, even for D-S-3 sample. 300

(a)

SBA-15 M-S-0.5 M-S-1 M-S-2

600 500

Adsorbed volume [cm3 STP/g]

3 Adsorbed volume [cm STP/g]

700

400 300 200 100 0 0.0

0.2

0.4

P/P0

0.6

0.8

(b)

D-S-0.5 D-S-1 D-S-2 D-S-3

200

100

0 0.0

1.0

0.2

0.4

P/P0

0.6

0.8

1.0

(a) SBA-15 and MEA impregnated SBA-15. (b) DEA impregnated SBA-15. Figure 3. N2 adsorption-desorption isotherms. 0.6

0.4

(a)

D-S-0.5 D-S-1 D-S-2 D-S-3

0.3 3 Pore volume [cm /g·nm]

SBA-15 M-S-0.5 M-S-1 M-S-2

0.5 3 Pore volume [cm /g·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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0.3 0.2 0.1

(b)

0.2

0.1

0.0

0.0 0

10

20 30 Pore diameter[nm]

40

50

0

10

20 30 Pore diameter[nm]

40

50

(a) SBA-15 and MEA impregnated SBA-15. (b) DEA impregnated SBA-15. Figure 4. Pore size distribution. Table 3. Summary of textural parameters. Sample

Surface area [m2/g]

Pore volume [cm3/g]

Pore diameter [nm]

SBA-15 M-S-0.5 M-S-1 M-S-2 D-S-0.5 D-S-1 D-S-2 D-S-3

588.5 367.2 318.7 315.4 196.1 195.3 194.9 186.6

0.9476 0.8292 0.7517 0.7277 0.4099 0.3969 0.3967 0.3869

7.1 6.9 6.5 6.2 6.7 6.6 5.9 5.6

3.1.4 TEM measurements Firstly, the TEM micrographs of pure non-functionalized mesoporous SBA-15 were showed in Figure 5. The image evidences the regular longitudinal arrangement as seen from longitudinal view in Figure 5 (a), and that shows orderly hexagonal arrays from


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transversal view in Figure 5 (b), which are the typical characteristic of SBA-15. The white areas correspond to the pore cavities and darker regions correspond to pore walls in longitudinal view. The TEM micrographs of pure SBA-15 shows that the white areas correspond to pore walls, and the dark cavities correspond to cavities from transversal view which is affirmed by Sanz et al.30.

(a) Longitudinal view (b) Transversal view Figure 5. TEM micrographs of SBA-15.

The TEM images of mesoporous SBA-15 modified by MEA or DEA, M-S-0.5, M-S-2, D-S-0.5, D-S-3, have been compared with non-functionalized ones. The white or dark regions correspond to pore cavities or walls, similarly to pure mesoporous SBA-15. Figure 6 (a), (b) and Figure 7 (a), (b) show micrographs corresponding to low amine loading which appear as discrete regions homogeneously distributed along the sample pores30. Here, the micrographs with low ratio of amine exhibit longitudinal and transversal array of channels where the pure SBA-15 is clearly shown. Figure 6 (c), (d) and Figure 7 (c), (d) displaying images with high amine content present that pores cavities are partially filled up in transversal view, and the outer surface of pore channels are covered from longitudinal view30, as shown by the irregular longitudinal pore channels and vague transversal hexagon.


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Finally, the TEM images confirm that SBA-15 mesostructure impregnated with small amine loading has little change and CO2 diffusion is as well as in pure SBA-15. However, the hexagonal pores have been stuffed by high loaded amine leading to hardly diffusion of CO2 to inner pores.

(a) Longitudinal view (M-S-0.5)

(b) Transversal view (M-S-0.5)

(c) Longitudinal view (M-S-2) (d) Transversal view (M-S-2) Figure 6. TEM micrographs of M-S-n

(a) Longitudinal view (D-S-0.5)

(b) Transversal view (D-S-0.5)


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(c) Longitudinal view (D-S-3) (d) Transversal view (D-S-3) Figure 7. TEM micrographs of D-S-n

3.2 CO2 adsorption 3.2.1 Adsorption quantity The influence of MEA or DEA content loaded on SBA-15 on CO2 adsorption capacity has been studied by performing experimental using pure CO2 in TGA at the temperature ranging from 30 °C to 90 °C. This temperature range typically corresponds to the combustion gases on the commercial scale after desulfurization in most thermal power plants. The CO2 adsorption performance of different MEA or DEA impregnating SBA-15 samples are shown in Figure 8, where the amount of CO2 captured after equilibration, in terms of milligram CO2 per gram of sorbent, is plotted against temperature ranging from 30 °C to 90 °C. As observed, pure SBA-15 shows a temperature-dependent pattern which is characteristic of a typical physisorption process, with relative high adsorption value at low temperature and a gradually continuous decrease in adsorption capacity at high temperature, as it occurs in many classical solid adsorbents such as activated carbon and zeolites. The CO2 adsorption capacity increases with the amine loading, both for MEA or


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DEA impregnating SBA-15 samples. After achieving the highest adsorption capacity at ratio of MEA to SBA-15 is 1 to 1 and DEA to SBA-15 is 2 to 1, respectively, a small decrease was observed with further increasing of amine loading, as presented in the samples of M-S-2 and D-S-3. M-S-1 and D-S-2 samples, which have not the highest MEA or DEA content, show a highest adsorption capacity from the very beginning. With the further increasing of MEA or DEA loaded on the mesoporous SBA-15, the CO2 capture is influenced by the packing effect on the mesoporous hexagonal silica. Initially, the support material surface is considered to be covered with lower level of MEA or DEA loadings, such as M-S-0.5, D-S-0.5 and D-S-1 samples. These molecules loaded on the support are relatively far away from saturating and allow the accessibility of CO2 molecules to the inner adsorption sites. The observed regular hexagonal structure of the mesoporous silica SBA-15, which was confirmed by XRD analysis, TEM images, N2 adsorption-desorption test and TGA in section 3.1, can further explain the above condition. Thus the diffusion resistance in the ordered hexagonal surface could be ignored, and the CO2 adsorption capacity increased with amine loading38. Later, when MEA or DEA loading is higher, such as M-S-1 and D-S-2 samples, the amount of MEA or DEA molecules within the pore structure of the SBA-15 material is higher, and thus increasing the internal amino sites for CO2 molecules. Moreover, the mesoporous silica SBA-15 was then impregnated with exactly the right amount of MEA or DEA. That is, there is no block in the mesoporous silica material pores. This


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Energy & Fuels

explanation would agree with the highest values of CO2 adsorption observed in samples M-S-1 and D-S-2, respectively. Finally, further increasing of the amine loading, such as M-S-2 and D-S-3 samples, would reduce opportunities of CO2 to contact with internal amino sites, because the amine was covered in a multilayer form or even caking form on the SBA-15 pore surface. The multilayer amine did not completely fill up the pore channels, but stayed on the external or internal surface of particles, which is also reported by Sanz30. This multilayer would limit or prevent the CO2 molecules from accessing to the all active sites of amines, and cause a significant decrease of CO2 adsorption capacity. The mesoporous properties of SBA-15 did not change completely which was confirmed by N2

adsorption/desorption

process,

and

CO2

adsorption

process

was

also

thermodynamically controlled for the adsorption capacity was decreased with temperature. More amine loading as sample M-S-2 or D-S-3 did not exceeded the pore saturation level, and the dynamic adsorption process was not predominantly controlled by diffusion. As the reaction of CO2 adsorption is an exothermic process21,38,48, the adsorption capacity should decrease with the increase of temperature. Therefore, Temperature is a significant factor affecting the adsorption performance. The CO2 capture capacities of samples for either pure or amine modified SBA-15 material decreased at temperature ranging from 30 to 90 °C which were also illustrated in Figure 8. With the increase of temperature, the capture capacity of adsorbents continuously declined significantly from highest values at 30 °C to the lowest values at 90 °C. This trend is followed at


Energy & Fuels

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overall experimental temperature range, thus the adsorption is always a thermodynamic controlled process. Jing et al., 2014, obtained the similar performance for SBA-15 functionalized with melamine-based and acrylate-based amine dendrimers7. The CO2 capture capacities of either MEA or DEA modified SBA-15 silica material changed from a kinetic controlled region at low temperature to a thermodynamic controlled region at overall experimental temperature range. This adsorption performance could be interpreted by the fact that SBA-15 silica material possessed regularly mesoporous structure which has been affirmed by XRD, TEM, N2 adsorption-desorption test and TGA in section 3.1. For this advanced texture, CO2 capture process using MEA or DEA functionalized SBA-15 silica material exhibited easier access to adsorption sites and weaker diffusion resistance, thus gas diffusion did not play a predominant role, even at low temperature7. In figure 8(b), it presents that the D-S-3 sample decreases sharply with the increase of temperature, and the amount of CO2 capture for D-S-3 is lower than that for D-S-1 after 60 °C. This phenomenon is related to both amine loading on the support and exothermic process of reaction. With higher amine loading sample, the CO2 adsorption capacity decrease more rapidly with the increase of reaction temperature because much more amount of amine impregnated on the support carry out the exothermic reaction of CO2 capture, and thus the CO2 adsorption of sample with higher amine loading show more sharply decrease with the increase of temperature than that with lower amine loading.


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Energy & Fuels

As can be seen in Figure 8 (b), the highest amount of CO2 adsorption is 154.5 mg/g at 30 °C corresponding to D-S-2. The relative increase of adsorption at 30 °C from D-S-0.5 to D-S-1 is about 59% and from D-S-1 to D-S-2 is only 3.5%, while the equivalent decrease when passing from D-S-2 to D-S-3 is 1.8%. This small amount of reduction for CO2 capture also conformed that the DEA impregnating on SBA-15 did not stuff in hexagonal pores and might just be coat in a multilayer form as explaining above. A similar behaviour was found with the MEA impregnating SBA-15 materials, Figure 8 (a), which show a maximum CO2 adsorption weight is only 42.5 mg/g for sample M-S-1. Yan et al., 2011, also reported four PEI impregnating SBA-15 materials with the maximum adsorption value of 105.2 mg/g achieved at the largest pore volume of 1.14 cm3/g37. Wang et al., 2013, conclude that the highest CO2 sorption capacity was 154 mg/g at 75 °C over the PEI-60/SBA-15 sorbent40. Yue et al., 2006, explored TEAP (tetraethylenepentamine) impregnating mesoporous SBA-15 occluded with P123, and estimated that the amount of CO2 capture was 173 mg/g due to the specific interaction of P123 with the amine42. Zhang et al., 2013, illustrated that the adsorption capacity of physical impregnated SBA-15 with TN (acrylonitrile modified TEPA) reached the maximum of 153 mg/g, and then it slightly decreased to 131.5 mg/g at 80 °C39. Franchi et al. 2005, and also found that the capacity reached maximum CO2 adsorption of 104 mg/g45. Yue et al., 2008, reported that the maximum CO2 adsorption capacity of the mixed amine-modified SBA-15 sample reached 163 mg/g38. However, they pointed out that the adsorption capacity of silica SBA-15 modified by DEA alone was only 20.8 mg/g. Jadhav et al., 2007, pointed out that the


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highest adsorption capacity of CO2 was about 87 mg/g for zeolite 13X-MEA-10 by comparing with zeolite 13X-MEA-2, 13X-MEA-50 and unmodified zeolite 13X at 30 °C20. It can be seen that the CO2 adsorption capacity of D-S-2 (154.5 mg/g) in Figure 8 (b) is higher by comparison with previous studies presenting above expect of Yue et al. 2006 and Yue et al. 2008. Though there is a slightly lower for CO2 adsorption capacity of D-S-2 than that of reported by Yue et al. 2006 and Yue et al. 2008, it is has advantage with more simple process to load one reagent and thus lower cost of adsorbent. 160

45 SBA-15 M-S-2 M-S-1 M-S-0.5

35

120 100

30 25 20 15

80 60 40 20

10 5

D-S-3 D-S-2 D-S-1 D-S-0.5

140 CO2 adsorption [mg/g]

40 CO2 adsorption [mg/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

30

40


80

90

30

40


80

90

(a) SBA-15 and MEA impregnating samples. (b) DEA impregnating samples. Figure 8. Relationship between the CO2 adsorption capacity and temperature for all samples.

3.2.2 Adsorption rate Besides the adsorption capacity, the adsorption rate is another important parameter in practical applications. Figure 9(a) shows adsorption temperature influence of SBA-15 and same loaded MEA or DEA samples on the adsorption mass ratio in 60 min. The adsorption rate over pure SBA-15 was smallest at the temperature of 50 °C and increased at 30 °C but still lower than that of MEA or DEA impregnating SBA-15, which can be observed in Figure 9(a). It also showed, for each functionalized SBA-15 sample, that the adsorption rate was higher at 30 °C than that at 50 °C, and the highest


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adsorption mass ratio presented in time is later at 50 °C than that at 30 °C. The rate decrease was simultaneously controlled by physical and chemical adsorption processes. On one hand, according to kinetic theory of gas, an increase in temperature corresponded to faster motion of the CO2 molecules and the force due to the motion of molecules tended to keep them apart. When the molecules energy was big enough to detach the CO2 molecules from particles, the attraction force of particles could not bound CO2 molecules to the mesoporous surface, and then resulting in escaping process. Therefore, with increasing the temperature, the physical adsorption of CO2 decreased gradually. On the other hand, the chemical adsorption of CO2 was a thermodynamically controlled procedure as illustrating in section 3.2.1. The chemical reactions between CO2 and NH2 groups are shown in Eq.(1) and (2), which are exothermal reactions and carried out in the opposite direction as increasing the reaction temperature. Thus, the adsorption rate of CO2 based on chemical reaction mechanism decreased when increasing the reaction temperature. In summary, with the increasing of temperature, the adsorption rate decreased due to synchronously dominated by the physical and chemical adsorption. 2HOC2H4NH2 + CO2 ↔ HOC2H4NH 3+ + HOC2H4NHCOO − 2(HOC2H4) 2 NH + CO2 ↔ ( HOC2H4) 2 NH 2+ + ( HOC2H4) 2 NCOO −

(1) (2)

After verifying the effect of temperature on CO2 capture rate, further adsorption experiments were carried out for different MEA or DEA loading samples at temperature of 30 °C. The Figure 9(b), (c) performed the influence of different amount amine impregnated in samples M-S-n and D-S-n on adsorption rate at


Energy & Fuels

constant temperature of 30 °C. As expected, all the samples had almost same start time which confirmed that the regular mesoporous structure of SBA-15 was not filled by amine loading as illustrating in XRD, N2 adsorption/desorption and TEM images. 108

D-S-0.5 ( 30℃)

107

Mass ratio [%]

106 D-S-0.5 ( 50℃)

105 104

M-S-0.5 ( 30℃)

103

M-S-0.5 ( 50℃)

102

SBA-15( 30℃) SBA-15( 50℃)

101 100 0

10

20

30 40 Time [min]

50

60

(a) Comparison of adsorption rate at 30 °C and 50 °C for sample SBA-15, M-S-0.5 and D-S-0.5. 116

105 M-S-1

D-S-2

104

Mass ratio [%]

103

D-S-3

112

M-S-2

Mass ratio [%]

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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M-S-0.5 SBA-15

102

D-S-1

108 D-S-0.5

104

101 100

100 0

10

20

30 40 Time [min]

50

60

0

10

20

30 40 Time [min]

50

60

(b) Adsorption rate of M-S-n and SBA-15 at 30 °C. (c) Adsorption rate of D-S-n and at 30 °C. Figure 9. Mass ratio as a function of adsorption time measured by TGA under pure CO2 flow.

Comparing the adsorption rate curves of M-S-n with D-S-n, it is found that the D-S-n curves are steeper than that of M-S-n samples which lead to the lower adsorption kinetics. The saturation time corresponding to Wt/W0=0.95 (Wt/W0 is CO2 adsorption weight/total adsorption weight) was 30 min for pure SBA-15 and M-S-n samples. Except D-S-2 sample, the saturation time for the rest of DEA functionalized SBA-15 samples were 20 min. Although the saturation time of D-S-2 was almost 50 min, which was not impacted its CO2 adsorption capacity because the adsorption amount was highest after 10 min.


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The cyclic adsorption performance of adsorbents was examined, and the results showed that the CO2 adsorption capacity was 28 and 83 mg/g for M-S-1 and D-S-2, respectively, after three adsorption-desorption cycles. However, Jadhav et al., 2007, found that the MEA-modified adsorbent successfully retained adsorption capacity in three consecutive cycles. The regeneration result also showed that there were some loss of MEA in the first cycle and no further loss was observed in the subsequent cycles20.

Franchi

et

al.,

2005,

showed

that

the

DEA-impregnated

PE-MCM-41exhibited good cyclic stability and maintained its capacity for CO2 under repeated

seven

adsorption-desorption

cycles45.

Therefore,

further

detailed

demonstration on cycle performance is needed for forthcoming research.

4 Conclusions Pure mesoporous silica SBA-15 and MEA or DEA impregnating SBA-15 were tested by XRD, TEM, N2 adsorption-desorption test and TGA, indicating the decrease of the pore size and pore volume with the increase of amine loading. But the mesoporous structure did not completely destroyed when the amount of amine load exceeded a certain value. The CO2 adsorption capacity of functionalized SBA-15 was significantly improved comparing with pure support and the adsorbents modified by DEA showed a better adsorption performance than that by MEA. The influence of amine content and temperature on adsorption properties was also systematically explored, which showed that the adsorption capacity declined with the increase of temperature. The CO2 adsorption capacity increases with the amine loading, both for MEA or DEA impregnating SBA-15 samples. After achieving the highest adsorption


Energy & Fuels

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capacity, a small decrease was observed with further increasing of amine loading. Therefore, the maximum capture amount estimated at 30 °C was 42.5 and 154.5 mg/g for M-S-1 and D-S-2, respectively. Comparing the adsorption rate curves of M-S-n with D-S-n, it is found that the D-S-n curves are steeper than that of M-S-n samples. It also observed, for each functionalized SBA-15 sample, that the adsorption rate was higher at 30 °C than that at 50 °C, and the highest adsorption mass ratio presented in time is later at 50 °C than that at 30 °C.

Acknowledgement This project is supported by The National Nature Science Foundation (China, No. 50876030)

Supporting Information The BET fitting plots and parameters of samples.

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