Enhanced CO 2 Adsorption over Silica-Supported ... - ACS Publications

Subscriber access provided by University of Rhode Island | University Libraries

Materials and Interfaces

Enhanced CO2 adsorption over silica-supported tetraethylenepentamine sorbents doped with alkanolamines or alcohols Yuan Zhao, Yidi Zhu, Tianle Zhu, Guiping Lin, Jiangbo Huo, Dong Lv, Haining Wang, and Ye Sun Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03556 • Publication Date (Web): 11 Dec 2018 Downloaded from http://pubs.acs.org on December 16, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 24 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

Industrial & Engineering Chemistry Research

Enhanced CO2 adsorption over silica-supported tetraethylenepentamine sorbents doped with alkanolamines or alcohols Yuan Zhaoa, Yidi Zhub, Tianle Zhua,*, Guiping Linc, Jiangbo Huod, Dong Lva, Haining Wanga,e, Ye Suna a

School of Space and Environment, Beijing Key Laboratory of Bio-Inspired Energy Materials and Devices,

Beihang University, Beijing 100191, China. b Institute c School d

of electrical engineering, Chinese academy of sciences, Beijing 100190, China.

of Aeronautic Science and Engineering, Beihang University, Beijing 100191, China.

Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China.

e Department

of Chemistry, University of Central Florida, Orlando, FL 32816, USA.

Abstract Silica-supported tetraethylenepentamine sorbents (TEPA/SiO2s) for CO2 adsorption were prepared by incipient impregnation method, in which porous silica was loaded with TEPA and alkanolamines or alcohols as dopants. Both CO2 adsorption capacity and amine utilization efficiency of these sorbents were evaluated in a self-assembled fixed bed reactor. The results show that the introduction of alkanolamines or alcohols, especially those with higher amine and hydroxyl densities, into TEPA/SiO2s notably improve their CO2 adsorption performance. The maximum CO2 adsorption capacity (4.14 mmol/g) was obtained over a doped TEPA/SiO2 loaded with 30% TEPA and 30% diethanolamine (DEA). The interaction between hydroxyl and amino groups, leading to the improved dispersion of TEPA phase on the surface of SiO2, was investigated

by

N2

adsorption-desorption

measurements,

scanning

electron

microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). And the calculation based on density functional theory (DFT) shows the existence of hydroxyl group in the doped TEPA/SiO2s increases the CO2 adsorption energy, which is the key factor to achieve the optimized CO2 adsorption capacity and amine utilization efficiency. *Corresponding

author: Prof. Tianle Zhu, E-mail: [email protected]; Tel: +86 10 61716086

ACS Paragon Plus Environment

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

Keywords: CO2 adsorption; amine; sorbent; TEPA; alkanolamine; alcohol. 1. Introduction In the closed environment of an inhabited spacecraft, the high level of CO2 concentration accumulated by human metabolism would severely endanger the health and even lives of astronauts onboard. Hence the effective CO2 adsorption inside the spacecraft plays a crucial role in ensuring perfect air quality for crew members. So far, a variety of the molecular sieve systems have been adopted as the preferred sorbents for such CO2 sequestration.1-3 Despite the stable performance and the excellent safety, the intrinsic hydrophilicity of most molecular sieves generally renders the adsorption system used in this regard much complicated, e.g., the inlet gas dehumidification must precede the CO2 adsorption process. Since the first CO2 adsorption systems containing solid amines, were proposed,4, 5 many researchers have paid intense attention to the development of amine-based sorbents because such kind of materials can get rid of CO₂ from humid air and be regenerated by the pressure-swing desorption.6-12 At present, supported amine sorbents are mainly yielded by means of two preparative techniques, i.e., grafting and wet impregnation.13-16 The former evokes the formation of aminosilane-functionalized mesoporous silica sorbents while the latter affords mesoporous silica ones surfacely coated with amines. In view of a relatively large loading of amine obtained from wet impregnation, very many supported amine sorbents reported hitherto tend to be prepared by this alternative technique. However, the facile agglomeration of amine coated on the support under adsorption conditions


Page 2 of 24

Page 3 of 24 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


inevitably resulted in the poor distribution of amine and the low utilization efficiency of amino groups. For that reason, much effort has recently been devoted to optimizing the amine phase with other additives in order to acquire the sorbents with higher CO2 adsorption capacity.17-22 Introduction of the CO2-neutral surfactants, such as alkylammonium bromides, sodium dodecylbenzenesulfonate, phosphatidylcholine, etc., into polyethyleneimine (PEI) constitutes a relevant example of how to elevate the amine utilization efficiency.17 Due to these added surfactants, extra transfer pathways were found to be created for CO2 diffusion into the deeper PEI films, thus providing more available amine-sites for CO2 adsorption. In addition, some alcohols (e.g., 2-amino-2-methyl-l-propanol and polyethylene glycol) as dopants also exert a remarkable promoting influence which has been attributed to the direct involvement of hydroxyl group in CO2 adsorption over the amine-based sorbents.18-22 To gain an insight into the nature of hydroxyl group function on CO2 adsorption, more detailed investigations should be carried out to develop the amine-based sorbents with a potential possibility of application in this field. Herein, we present the results of preparation, characterization and CO2 adsorption performance of the series of TEPA/SiO2s doped with various alkanolamines or alcohols. And it has been shown that when compared with the undoped TEPA/SiO2s, the doped TEPA/SiO2s exhibit a pronounced increase in CO2 adsorption capacity, largely owing to the interaction between the amino group in TEPA and the hydroxyl group in these dopants. Such a



Page 4 of 24

conclusion was confirmed by the observations made with N2 adsorption-desorption measurements,

scanning

electron

microscopy

(SEM),

X-ray

photoelectron

spectroscopy (XPS) and the calculation based on density functional theory (DFT). 2. Experimental 2.1. Preparation of TEPA/SiO2s All chemicals were reagent grade and used as received. Each of five alkanolamines and three alcohols was individually used here as dopant, namely ethanolamine (MEA), diethanolamine (DEA), trolamine (TEA), 2-amino-2-methyl-1-propanol (AMP), 2-(2-Aminoethylamino)ethanol (AEEA), ethanediol (EG), glycerol (GL) and polyethylene glycol (PEG). To prepare the TEPA/SiO2 sorbents, the desired amounts of TEPA and alkanolamine or alcohol were dissolved in 40 ml methanol at room temperature. Then, 2 g porous silica (J&K Scientific LTD, SBET = 184 m2/g) was added into the above solution while vigorous stirring was maintained until methanol evaporated away completely. Finally, the resultant TEPA/SiO2s were pretreated at 80 oC

for 5 h in an argon atmosphere. Nomenclature for these TEPA/SiO2s is as follows:

xTEPA + yalkanolamine or alcohol represents the weight percentages of TEPA and alkanolamine or alcohol in the doped TEPA/SiO2s in order (both alkanolamine and alcohol in abbreviation); x and y range from 10 to 50%, and 0 to 50%, respectively. 2.2. Characterization of TEPA/SiO2s N2

adsorption-desorption

measurements

were

performed

with

the

Brunauer-Emmett-Teller (BET) method on a Micromeritics Tristar II 3020 instrument. SEM images were obtained on a Hitachi SU8010 scanning electron


Page 5 of 24 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


microscope. XPS data were collected on a PHI Quantro SXM ULVAC-PHI, using an Al Kα (1486.7 eV) X-ray source. Thermogravimetric analyses (TGA) were conducted on a Netzsch STA 449F5 apparatus. As for DFT calculation, the Gaussian 09 program23 was adopted and all the related molecular structures were optimized at the B3LYP/6-311++G (d, p) level. The undoped TEPA/SiO2 was modelled into two methylamines whereas the doped TEPA/SiO2 into a methylamine and a methanol. The CO2 adsorption energy was rectified with the basis set superposition error (BSSE)24 and calculated according to Eq. (1) 25, ΔEads = Etotal  E1  E2

(1)

where Etotal is the total energy of a combination of TEPA/SiO2 and an adsorbed CO2 molecule, E1 the energy of free TEPA/SiO2 and E2 the energy of a free CO2 molecule, respectively. 2.3. Evaluation for CO2 adsorption over TEPA/SiO2s CO2 adsorption over TEPA/SiO2s was investigated on a fixed-bed sorption system, as shown in Figure1. In a typical sorption run, 0.8 g TEPA/SiO2 (sieved to −40/+60 mesh) was packed in the U-shaped quartz tube reactor which was placed in a programmed furnace to achieve the desired temperature. Prior to each measurement, the TEPA/SiO2 sample was heated at 80 oC for 60 min to eliminate its physically adsorbed H2O and CO2 and then cooled down to 25 oC, both in a 100 mL/min highly pure Ar flow rate (> 99.999%). After that, the gas stream of 2% CO2 in air with a 30% relative humidity was introduced into the reactor in a 40 mL/min flow rate. The flow rates of CO2, water vapor and air were adjusted by three mass flow controllers



Page 6 of 24

(Beijing Sevenstar Electronics Co., Ltd., D07-7 type) to gain the given concentration of CO2 in the gas phase. The inlet and outlet CO2 concentrations of the reactor were monitored by an online gas chromatograph (Techcomp, GC-7890Ⅱ) equipped with a methane converter and a flame ionization detector. The CO2 adsorption capacity of a TEPA/SiO2 sorbent was determined from Eq. (2) as below,

qs 

t 1  C C  To 1   Q  0 dt    m 0 1  C  T Vm

(2)

where qs is the CO2 adsorption capacity of the TEPA/SiO2 (mmol/g), m the weight of the TEPA/SiO2 (g), Q the gas flow rate (mL/min), C0 the inlet CO2 concentration (vol%), C the outlet CO2 concentration (vol%), t the CO2 adsorption duration (min), To the standard temperature (273 K). T the CO2 adsorption temperature and Vm standard molar volume (22.4 mL/mmol). MFC

Reactor Tube furnace CO2

Distilled Air water

Mixing chamber PV SV

Thermostat

A

GC

Vent

Temperature-controlled instrument

Figure 1. Schematic of the experimental setup.

3 Results and Discussion 3.1. CO2 adsorption performance of TEPA/SiO2s The results of CO2 adsorption over the undoped TEPA/SiO2s and the doped TEPA/SiO2s are shown in Figure 2, in which the amine utilization efficiency was expressed in terms of the molar ratio of CO2 to N. For the three undoped TEPA/SiO2s,


Page 7 of 24 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


the CO2 adsorption capacity firstly increases and then decreases with the TEPA loading enhanced (Figure 2a). The attainment of the maximum CO2 adsorption capacity (2.81 mmol/g) and amine utilization efficiency (0.389 CO2/N) was achieved over 40% TEPA/SiO2, viz., the undoped TEPA/SiO2 with a 40% TEPA loading. In consideration of the TEPA loading or the amino group amount alone, 40% TEPA/SiO2 would be expected to bear inferior CO2 adsorption performance compared to 50% TEPA/SiO2. The observation here suggests that other factors may also play an important role in CO2 adsorption over such supported TEPA sorbents. Now, there have been several reports that the CO2 adsorption capacity can be significantly elevated by the introduction of some organic compounds functionalized with hydroxyl group and/or amino group into amine-based sorbents.18-22 So with a 50% total loading limitation of TEPA (30%) plus alkanolamine or alcohol (20%) as dopant, the series of TEPA/SiO2s were prepared to improve their CO2 adsorption performance. In the case of alkanolamines, all the five doped TEPA/SiO2s exhibit a higher CO2 adsorption capacity than 30% TEPA/SiO2. As shown in Figure 2b, 3.75 mmol/g and 3.25 mmol/g of CO2 adsorption capacity were observed over 30% TEPA + 20% DEA/SiO2 and 30% TEPA + 20% TEA/SiO2, respectively, which are even higher than that of 40% TEPA/SiO2 (2.81 mmo/g). Moreover, these two doped TEPA/SiO2s also have a higher amine utilization efficiency than 40% TEPA/SiO2.



a

4

0.4

c

b 3.75

0.3

3.25 3

2.81

2.89

2.74

2.58

2.83 2.5

2.45

1.89

2

0.2

1.78 0.1

1.42 1

0.0

amine utilization efficiency (CO2/N)

5

CO2 adsoroption capacity (mmol/g)

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

Page 8 of 24

0

30%

40%

TEPA/SiO2s

50%

MEA DEA TEA AMP AEEA

EG

GL

PEG(4000) PEG(400)

30%TEPA + 20%Alkanolamine/SiO2s 30%TEPA + 20%Alcohol/SiO2s

Figure 2. CO2 adsorption capacity (column) and amine utilization efficiency (symbol) of TEPA/SiO2s under the conditions: 2 vol.% CO2 in air, 298K adsorption temperature, 30% relative humidity, 40 mL/min total flow rate, 0.8 g sorbent. (a) the undoped TEPA/SiO2s loaded with different amounts of TEPA; (b) the doped TEPA/SiO2s loaded with 30% TEPA and 20% alkanolamines; (c) the doped TEPA/SiO2s loaded with 30% TEPA and 20% alcohols.

As to alcohols as CO2-neutral organic compounds, the improved CO2 adsorption performance was likewise obtained with the alcohol-doped TEPA/SiO2s, but except for 20% PEG(4000)/SiO2 which possesses both a lower CO2 adsorption capacity and an amine utilization efficiency relative to 30% TEPA/SiO2 (Figure 2 c). From the data compiled in Table 1, it can be seen that 30% TEPA + 20% DEA/SiO2 and 30% TEPA + 20% TEA/SiO2 not only own relatively high densities of amino group and hydroxyl group, but also the moderate molar ratio of amino group to hydroxyl group in comparison with the other doped TEPA/SiO2s. This may imply that some kind of synergistic effect on the CO2 adsorption possibly derives from the interaction between amino group and hydroxyl group in the doped TEPA/SiO2s. Table 1 Densities of amino group and hydroxyl group in the doped TEPA/SiO2s. Group density (mmol/g)

Ratio of amino group to

Sorbent hydroxyl group amino group

hydroxyl group


Page 9 of 24 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


30% TEPA + 20% AMP/SiO2

10.17

2.24

4.54

30% TEPA + 20% AEEA/SiO2

11.76

2.32

5.07

30% TEPA + 20 % MEA/SiO2

11.20

3.27

3.43

30% TEPA + 20% DEA/SiO2

9.64

3.81

2.53

30% TEPA + 20% TEA/SiO2

9.27

4.02

2.31

30% TEPA + 20% EG/SiO2

7.92

6.44

1.23

30% TEPA + 20% GL/SiO2

7.92

6.52

1.21

30% TEPA + 20% PEG(400)/SiO2

7.92

1

7.92

30% TEPA + 20% PEG(4000) /SiO2

7.92

0.1

79.2

On account of the highest CO2 adsorption performance acquired over 30% TEPA + 20% DEA/SiO2, DEA can be regarded as the best dopant for the TEPA-based sorbents. Therefore, the various mass ratios of TEPA to DEA were employed to further examine the CO2 adsorption performance of the doped TEPA/SiO2s under the loading conditions: (1) in Figure 3 a, the total loading equal to 30% TEPA plus x% DEA; (2) in Figure 3 b, the total loading of 60% equal to y% TEPA plus z% DEA; (3) x, y and z range from 10 to 35, 10 to 40 and 20 to 50, respectively. As shown Figure 3 a, b, for all the x% TEPA + y% DEA/SiO2s, the CO2 adsorption capacity was almost linearly increased in the initial adsorption stage and ultimately leveled off only if the adsorption process proceeded long enough; the maximum CO2 adsorption capacity (4.14 mmol/g) observed over 30% TEPA + 30% DEA/SiO2 is much higher than those of the TEPA-based sorbents reported to date, such as TEPA/KIT-6 (2.85 mmol/g),18 TEPA/cal-SBA-15 (3.25 mmol/g),19 TEPA/SiO2 (2.09 mmol/g),26 TEPA +



AMP/MCM-41 (3.01 mmol/g)21 and TEPA + APTS /MCM-41 (3.50 mmol/g)27.

CO2 adsorption capacity (mmol/g)

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

Page 10 of 24

4

a

b

3

2

1 30% TEPA+10% DEA/SiO2 30% TEPA+20% DEA/SiO2 30% TEPA+30% DEA/SiO2 30% TEPA+35% DEA/SiO2

0 0

30

60

90

120

150 0

10% TEPA+50% DEA/SiO2 20% TEPA+40% DEA/SiO2 30% TEPA+30% DEA/SiO2 40% TEPA+20% DEA/SiO2

30

60

90

120

150

Time (min) Figure 3. Effects of the mass ratio of TEPA to DEA on CO2 adsorption over TEPA + DEA/SiO2s. (a) 30% TEPA loading and the DEA loading range of from 10 to 35%; (b) 60% total loading equal to the TEPA loading percentage (from 10 to 40%) plus the DEA loading percentage (from 20 to 50%).

3.2. Characterization of TEPA/SiO2s To validate the existence of interaction between the amino group and the hydroxyl group in these sorbent materials, the three undoped TEPA/SiO2s and the two DEAdoped TEPA/SiO2s of choice (30% TEPA + 20% DEA/SiO2 and 30% TEPA + 30% DEA/SiO2) were investigated by N2 adsorption-desorption, SEM and XPS techniques. Table 2 summarizes the textural characteristics of porous silica and the above five representative sorbents of TEPA/SiO2s. As shown in Table 2, the BET specific surface areas of all the TEPA/SiO2s decrease in value compared with that of the porous silica used as support, which apparently stems from the pore blockage of the support due to the formation of TEPA or TEPA plus DEA sorbent phase on the exterior surface of the porous silica. However, two interesting observations were made as follows: (1) for 30% TEPA/SiO2 and 40% TEPA/SiO2, the pore volumes of these two undoped TEPA/SiO2s did not diminish as expected but increased from 0.54 cm3/g (SiO2) to 0.97 cm3/g and 0.61 cm3/g, respectively; (2) for the five TEPA/SiO2s,


Page 11 of 24 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


the new pore voids with the pore sizes from 15 to 70 nm were found to be yielded (Figure 4), which are larger than that of the porous silica. The abnormal phenomena here can be well explained by the supposition that the TEPA or TEPA plus DEA sorbent phase might germinate on the surface of the porous silica and form additional three-dimensional net frameworks with mesopore or macropore openings. And net framework structures as such would afford more abundant channels for CO2 diffusion and facilitate the full contact of CO2 with amino group sites. More than that, it should be noted that in despite of the same total mass amine loading, 30% TEPA + 20% DEA/SiO2 presents a higher BET surface area and larger pore volume than 50% TEPA/SiO2. This may be good evidence that DEA as dopant can promote more homogeneous dispersion of amine sorbent phase on the surface of the porous silica. Table 2 Textural characteristics of porous silica and five representative sorbents of TEPA/SiO2s. BET surface area

Pore volume

Average pore

(cm3/g)

size (nm)

Sorbent (m2/g)

SiO2

184

0.54

14.4

30% TEPA/SiO2

69

0.97

37.5

40% TEPA/SiO2

42

0.61

36.7

50% TEPA/SiO2

20

0.32

42.5


28

0.37

40.4


15

0.23

50.5



0.030

0.025

dV/dD (cm3g-1nm-1)

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

a

0.020

0.015

0.010

b

0.005

c d

0.000 0

10

20

30

40

50

f

e

60

70

80

Pore Size (nm)

Figure 4. Pore size distributions of porous silica and five representative sorbents of TEPA/SiO2s. (a) SiO2; (b) 30% TEPA/SiO2; (c) 40% TEPA/SiO2; (d) 50% TEPA/SiO2; (e) 30% TEPA + 20% DEA/SiO2; (f) 30% TEPA + 30% DEA/SiO2.

The further examination of amine dispersion on the surface of the porous silica was carried out using SEM analysis in conjunction with EDX nitrogen-mapping. The scanning electron micrographs of the five TEPA/SiO2s are shown together with their images of EDX nitrogen-mapping in Figure 5. Although no significant distinction was found in the morphologies, the characteristic images of EDX nitrogen-mapping provide some valuable information about the amine dispersion in these TEPA/SiO2s. For example, the green nitrogen-related light spots, which are able to trace the position of amino group in the submicroscopic scale, uniformly distribute in the images of EDX nitrogen-mapping of both 30% TEPA/SiO2 and 40% TEPA/SiO2 (Figure 5 a, b). By contrast, such dispersion uniformity of green light spots is not the case in the image of EDX nitrogen-mapping of 50% TEPA/SiO2 (Figure 5 c). The results obtained here demonstrate that without DEA, the excessive agglomeration of amine sorbent phase inevitably occurred on the surface of the porous silica on the condition of the amine loading equal to 50%. In the presence of DEA, the uniform dispersion of amine sorbent phase, nevertheless, could be kept until the total amine


Page 12 of 24

Page 13 of 24 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


loading was up to 60% (Figure 5 d, e). Therefore, it can be envisaged that some kind of interaction between amino group and hydroxyl group boost a fully uniform dispersion of amine sorbent phase on the surface of the porous silica even under the conditions of 50% and 60% total amine loadings, thus, resulting in more amine-sites available for CO2 adsorption. Another point should be mentioned that despite relatively low nitrogen content compared to 40% TEPA/SiO2, 30% TEPA + 20% DEA/SiO2 exhibits a better performance for CO2 adsorption, which suggests that DEA as dopant may play an additional role in enhancing amine utilization efficiency (vide infra).



a

500nm

b

500nm

c

500nm

d

500nm

e

500nm Figure 5. Images of SEM and EDX-nitrogen mapping of five representative sorbents of TEPA/SiO2s. (a) 30% TEPA/SiO2; (b) 40% TEPA/SiO2; (c) 50% TEPA/SiO2; (d) 30% TEPA + 20% DEA/SiO2; (e) 30% TEPA + 30% DEA/SiO2.

XPS N 1s spectra measured from the five TEPA/SiO2s are shown in Figure 6. For the three undoped TEPA/SiO2s, one broad asymmetrical XPS peak was resolved into


Page 14 of 24

Page 15 of 24

two sub-peaks centered at 398.3 and 399.6 eV (Figure 6 a, b, c), in agreement with the secondary amino group (-NH-) and the primary amino group (-NH2), respectively.28-30 For the two DEA-doped TEPA/SiO2s, three sub-peaks at 398.3, 399.6 and 401.2 eV in order were obtained from the deconvolution of XPS spectra (Figure 6 d, e). As it is well known, the binding energy of N 1s for an amine molecule generally gains an increment value of 1.5 eV as soon as its amino group is hydrogen-bonded with a hydroxyl group.31-33 As a result, for the two DEA-doped TEPA/SiO2s, the XPS peak at 399.6 eV can be assigned to hydrogen-bonded -NH- and free -NH2 whereas the one at 401.2 eV hydrogen-bonded -NH2. Owing to the fact that no XPS peak of the three undoped TEPA/SiO2s was found at 401.2 eV, the conclusion may be arrived at that for 30% TEPA + 20% DEA/SiO2 and 30% TEPA + 30% DEA/SiO2 (inter alia the latter), the perfect uniformity of amine dispersion on the surface of the porous silica could arise from the formation of hydrogen bond between the amino group in TEPA and the hydroxyl group in DEA, which effectively hinders the amine phase from agglomeration.

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


398.3

b

398.3

c

399.6

399.6

398.3

399.6

405 402 399 396 405 402 399 396 405 402 399 396

d

e

399.6

401.2

405

402

398.3 399

396

399.6

401.2

405

398.3 402

Binding Energy (eV)


399

396


Page 16 of 24

Figure 6. XPS N1s spectra of five representative sorbents of TEPA/SiO2s. (a) 30% TEPA/SiO2; (b) 40% TEPA/SiO2; (c) 50% TEPA/SiO2; (d) 30% TEPA + 20% DEA/SiO2; (e) 30% TEPA + 30% DEA/SiO2.

3.3. Calculation of CO2 adsorption energy The density functional theory (DFT) method was utilized to determine the CO2 adsorption energy (Eads), namely the energy released from the formation of Lewis acid-base complexes involving CO2, methylamine (CH3NH2) and/or methanol (CH3OH), with the same calculation formula and basis set superposition as those adopted in the previous publications.25, 34, 35 The three putative modes of complexation reaction were postulated below: (1) the bimolecular combination of one CO2 and one CH3NH2; (2) the trimolecular combination of one CO2 and two CH3NH2s; (3) the trimolecular combination of one CO2, one CH3NH2 and one CH3OH. The data listed in Table 3 show that the complexation reaction in Mode 3 should be energetically more favorable than the reactions in Modes 1 and 2 by virtue of its maximum absolute value of CO2 adsorption energy (34.33 kJ/mol), relative to those (12.06 kJ/mol and 31.11 kJ/mol) obtained in Modes 1 and 2. That is to say, the complexation reaction more easily takes place on the coexistence of CO2, CH3NH2 and CH3OH, in which the hydroxyl group in methanol must play a crucial role in the formation of acid-base complexes composed of the above three components. Table 3 CO2 adsorption energy for different combinations of CO2, CH3NH2 and CH3OH. Functional group

Combination of CO2, CH3NH2 and CH3OH

Isolated amino group

ΔEads (kJ·mol-1)

-12.06


Page 17 of 24 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


Two amino groups

-31.11

Amino group and hydroxyl group

-34.33

According to the results reported in the publication,36 direct interaction between one CO2 and one amine produces a binary CO2-RNH2 complex under nearly anhydrous conditions via Reaction 1 in Figure 7. Given that one CO2 and two amines are simultaneously implicated in the acid-base complexation reaction, one of two amino groups is considered at least formally to first be deprotonated, which coincides with the protonation of another amino group, and undergo a nucleophilic coordination on CO2, then resulting in a carbamate anion. And the carbamate anion further reacts with the above resultant protonated amine to give a ternary CO2-RH-RH3+ complex (Reaction 2 in Figure 7). The same reaction sequence as that in Reaction 2 also applies to the trimolecular combination of one CO2, one amine and one alcohol, in which nevertheless, the protonation of hydroxyl group rather than amino group occurs exclusively, as depicted in Reaction 3 in Figure 7.37 From a practical view, the CO2 adsorption system of one amine plus one alcohol should be more preferred than that of two amines in that the former obviously bears a higher utilization efficiency of amino group than the latter. In this context, the advantageous functions displayed by the hydroxyl group in the TEPA/SiO2s can be summarized into the two following aspects: (1) promoting homogeneous amine



Page 18 of 24

dispersion on the surface of the porous silica; (2) exerting a synergistic effect on the CO2 adsorption over the amine-based sorbents. R

R

NH2 + CO2

N C

O

O

(1)

O HN:

R

+ R

H

HN: + H

C

R

H 2N

+

H

R

C

H+

O

O-

N

O

(2)

O R

O: + R

HN: +

C

H

H

O

R

HO

+ H+

R

H

O-

N C O

(3)

Figure 7. Adsorption reaction scheme for various combinations of CO2, amine and alcohol: (1) one CO2 and one amine; (2) one CO2 and two amines; (3) one CO2, one amine and one alcohol.

3.4. Stability and reusability of the sorbents The thermal stability of the five representative sorbents of TEPA/SiO2s was examined by TGA Analyses (shown in Figure 8). As reference sample, the porous silica shows neither weight losses nor endothermic peaks throughout the temperature-programmed process. With respect to the five TEPA/SiO2s, the two endothermic peaks (derived from weight losses) are observed in the temperature range from 25 to 600 oC, which were generally attributable to the evolution of CO2 and H2O (at 90 oC) from as well as the pyrolysis of the amine sorbent phase (at 260 oC) in these amine-based sorbents.38 Due to the ambient temperature adopted in this field, the TEPA/SiO2s here will have a sustained stability to remain intact under the practical CO2 adsorption conditions.


Page 19 of 24

As for the reusability of the five TEPA/SiO2s, the CO2 adsorption-desorption experiment was repeatedly conducted up to ten cycles with 30% TEPA + 30% DEA/SiO2 and 30% TEPA/SiO2, respectively. Prior to the next run, the selected sorbents were regenerated in Ar flow at 80 oC for 40 min. As shown in Figure 9, the first five runs afford a 4.35% reduction in the CO2 adsorption capacity from 4.14 to 3.96 mmol/g for 30% TEPA + 30% DEA/SiO2 whereas a 4.23% reduction from 1.89 to 1.81 mmol/g for 30% TEPA/SiO2. Such reductions might be reconciled with the possible heat endurance of the ternary CO2-RH-RH3+ complex and the concomitant incomplete release of adsorbed CO2. And a steady level of CO2 uptake (3.923.97 mmol/g and 1.751.78 mmol/g for the two above sorbents) was reached in the other five subsequent runs, hence indicating that both of the doped and undoped TEPA/SiO2s prepared in this work can be characteristic of excellent reusability.

b

100 90

SiO2

80 30% TEPA/SiO2

70

40% TEPA/SiO2

60

50% TEPA/SiO2

50

30% TEPA+20% DEA/SiO2

40 30% TEPA+30% DEA/SiO2

30 20

0

SiO2

Derivative weight (%/oC)

a

Weight (%)

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


-2 90 oC

-4

30% TEPA/SiO2

40% TEPA/SiO2 50% TEPA/SiO2

-6

30% TEPA+20% DEA/SiO2 30% TEPA+30% DEA/SiO2

-8 100

200

300

400

500

600

260 oC

100

o

Temperature ( C)

200

300

400

500

600

Temperature (oC)

Figure 8. TGA (a) and DTGA (b) profiles of porous silica and five representative sorbents of TEPA/SiO2s.



30% TEPA+30% DEA/SiO2 30% TEPA/SiO2

5

Adsorption capacity(mmol/g)

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

Page 20 of 24

4.14

4.11

4

4.06

4.01

3.96

3.97

3.95

3.92

3.93

3.92

3

2

1.89

1.86

1.83

1.81

1.81

1.78

1.77

1.76

1.75

1.76

1

0 1

2

3

4

5

6

7

8

9

10

Cycle number Figure 9. Alternate CO2 adsorption and desorption over 30% TEPA + 30% DEA/SiO2 and 30% TEPA/SiO2.

4. Conclusions The TEPA/SiO2s doped with alkanolamines or alcohols exhibits an improved CO2 adsorption capacity and amine utilization efficiency, with the two maximum values of 4.14 mmol/g and 0.389 achieved over 30% TEPA + 30% DEA/SiO2, among other things. The formation of hydrogen bond between the amino group and the hydroxyl group in those TEPA/SiO2s inhibits the agglomeration of amine to some extent, which then leads to homogeneous dispersion of the amine sorbent phase and enables more amino group sites available to CO2 adsorption. On the base of results calculated with the DFT method, the added important role of hydroxyl group as the third participant also lies in its direct involvement in the CO2-amine interaction, which makes CO2 adsorption more readily proceed, with one amino group combined with one CO2.

Acknowledgments The authors would like to thank the National Key R&D Program of China (No.


Page 21 of 24 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


2017YFC0211804 and No. 2016YFC0207103).

Reference (1) Knox, J. C.; Campbell, M.; Murdoch, K.; Miller, L. A.; Jeng, F. Integrated Test and Evaluation of a 4-Bed Molecular Sieve (4BMS) Carbon Dioxide Removal System (CDRA), Mechanical Compressor Engineering Development Unit (EDU), and Sabatier Engineering Development Unit (EDU). SAE International 2005. (2) Jeng, F. F.; Campbell, M.; Lu, S. D.; Smith, F.; Knox, J. Modeling and Analyses of an Integrated Air Revitalization System of a 4-Bed Molecular Sieve Carbon Dioxide Remova System (CDRA), Mechanical Compressor Engineering Development Unit (EDU) and Sabatier Engineering Development Unit. SAE International 2006. (3) Knox, J. C.; Campbell, M.; Miller, L. A.; Mulloth, L.; Varghese, M.; Luna, B. Integrated Test and Evaluation of a 4-Bed Molecular Sieve, Temperature Swing Adsorption Compressor, and Sabatier Engineering Development Unit. SAE International 2006. (4) Eton, T.; Nihei, T.; Otsuji, K.; Satoh, S.; Hatano, S. Experimental Study for Carbon Dioxide Removal System in Space Station. SAE International 1987. (5) Toft, G.; Aitchison, T. An Investigation into the Carbon Dioxide Removal Performance of a Novel Hydrophobic Adsorbent. SAE International 2009. (6) Chee, W. K.; Lim, H. N.; Zainal, Z.; Huang, N. M.; Harrison, I.; Andou, Y. Flexible Graphene-Based Supercapacitors: A Review. J. Phys. Chem. C. 2016, 120, 4153-4172. (7) Yan, X.; Komarneni, S.; Yan, Z. CO2 adsorption on Santa Barbara Amorphous-15 (SBA-15) and amine-modified Santa Barbara Amorphous-15 (SBA-15) with and without controlled microporosity. J. Colloid Interf. Sci. 2013, 390, 217-224. (8) Gebald, C.; Wurzbacher, J. A.; Borgschulte, A.; Zimmermann, T.; Steinfeld A. Single-component and binary CO2 and H2O adsorption of amine-functionalized cellulose. Environ. Sci. Technol. 2014, 48, 2497-2504. (9) Builes, S.; Lópezaranguren, P.; Fraile, J.; Vega, L. F.; Domingo, C. Analysis of CO2 adsorption in amine-functionalized porous silicas by molecular simulations. Energ. Fuel 2015, 29, 150526180632000. (10) Le, Y.; Guo, D.; Cheng, B.; Yu, J. Amine-functionalized monodispersed porous silica microspheres with enhanced CO2 adsorption performance and good cyclic stability. J. Colloid Interf. Sci. 2013, 408, 173-180. (11) Chen, Y. H.; Lu, D. L. Amine modification on kaolinites to enhance CO2 adsorption. J. Colloid Interf. Sci. 2014, 436, 47-51. (12) Bollini, P.; Brunelli, N. A.; Didas, S. A.; Jones, C.W. Dynamics of CO2 adsorption on amine adsorbents. 2. Insights into adsorbent design. Ind. Eng. Chem. Res. 2015, 51, 15153-15162. (13) Sanz-Pérez, E. S.; Olivares-Marín, M.; Arencibia, A.; Sanz, R.; Calleja, G.; Maroto-Valer, M. M. CO2 adsorption performance of amino-functionalized SBA-15 under post-combustion conditions. Int. J. Greenh. Gas Con. 2013, 17, 366-375. (14) Kim, J. Y.; Choi, S. B.; Kim, D. W.; Lee, S.; Jung, H. S.; Lee, J. K.; Hong, K. S. Surfactant-Assisted Shape Evolution of Thermally Synthesized TiO2 Nanocrystals and Their Applications to Efficient Photoelectrodes. Langmuir 2008, 24, 4316-4319. (15) Zhao, X.; Hu, X.; Hu, G.; Bai, R.; Dai, W.; Fan, M.; Luo, M. Enhancement of CO2 adsorption and amine efficiency of titania modified by moderate loading of diethylenetriamine. J. Mater. Chem. A 2013, 1, 6208-6215. (16) Yao, M.; Dong, Y.; Feng, X.; Hu, X.; Jia, A.; Xie, G.; Hu, G.; Lu, J.; Luo, M.; Fan, M. The effect of post-processing conditions on aminosilane functionalizaiton of mesocellular silica foam for post-combustion CO2 capture. Fuel 2014, 123, 66-72.



(17) Wang, J.; Long, D.; Zhou, H.; Chen, Q.; Liu, X.; Ling, L. Surfactant promoted solid amine sorbents for CO2 capture. Energ. Environ. Sci. 2012, 5, 5742-5749. (18) Wang, X.; Chen, L.; Guo, Q. Development of hybrid amine-functionalized MCM-41 sorbents for CO2 capture. Chem. Eng. J. 2015, 260, 573-581. (19) Yue, M. B.; Chun, Y.; Cao, Y.; Dong, X.; Zhu, J. H. CO2 Capture by as-prepared SBA-15 with an occluded organic template. Adv. Funct. Mater. 2006, 16, 1717-1722. (20) Yue, M. B.; Sun, L. B.; Cao, Y., Wang, Z. J.; Wang, Y.; Yu, Q.; Zhu, J. H. Promoting the CO2 adsorption in the amine-containing SBA-15 by hydroxyl group. Micropor. Mesopor. Mat. 2008, 114, 74-81. (21) Wang, X.; Guo, Q.; Zhao, J.; Chen, L. Mixed amine-modified MCM-41 sorbents for CO2 capture. Int. J. Greenh. Gas Con. 2015, 37, 90-98. (22) Xu, X.; Song, C.; Andrésen, J. M.; Miller, B. G.; Scaroni, A. W. Preparation and characterization of novel CO2 “molecular basket” adsorbents based on polymer-modified mesoporous molecular sieve MCM-41. Micropor. Mesopor. Mat. 2003, 62, 29-45. (23) Frisch, G. W. T. M. J.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H. T.; Vreven, J. A.; Montgomery, Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A. 02, Gaussian Inc., Wallingford, CT, 2009. (24) Boys, S. F.; Bernardi, F. The calculation of small molecular interactions by the differences of separate total energies. Some procedures with reduced errors. Mol. Phys. 2002, 19, 553-566. (25) Wang, H. Y.; Sun, Y.; Zhu, T. L.; Wang, W. Z.; Deng, H. Adsorption of acetaldehyde onto carbide-derived carbon modified by oxidation. Chem. Eng. J. 2015, 273, 580-587. (26) Fisher, J. C.; Tanthana, J.; Chuang, S. S. C. Oxide-supported tetraethylenepentamine for CO2 capture. Environ. Prog. Sustain. 2010, 28, 589-598. (27) Chen, J.; Shi, Y.; Liu, Y.; Ye, Q.; Shen, M.; Shi, J.; Pan, H. Carbon dioxide capture by functionalized solid amine sorbents with simulated flue gas conditions. Environ. Sci. Technol. 2011, 45, 5710-5716. (28) Hook, D. J.; Vargo, T. G.; Gardella, J. A.; Litwiler, K. S.; Bright, F. V. Silanization of radio frequency glow discharge modified expanded poly(tetrafluoroethylene) using (aminopropyl)triethoxysilane. Langmuir 1991, 7, 142-151. (29) Kallury, K. M. R.; Macdonald, P. M.; Thompson, M. Effect of surface water and base catalysis on the silanization of silica by (aminopropyl) alkoxysilanes studied by X-ray photoelectron spectroscopy and 13C cross-polarization/magic angle spinning nuclear magnetic resonance. Langmuir 1994, 10, 492-499. (30) Moses, P. R.; Wier, L. M.; Lennox, J. C.; Finklea, H. O.; Lenhard, J. R.; Murray, R. W. X-ray photoelectron spectroscopy of alkylaminesilanes bound to metal oxide electrodes. Anal. Chem. 1978, 50, 576-585. (31) Harder, P.; Woell, C.; Grunze, M.; Heid, S.; Effenberger, F.; Bierbaum, K. Induced orientational order in long alkyl chain aminosilane molecules by preadsorbed octadecyltrichlorosilane on hydroxylated Si(100). Langmuir 1997, 13, 445-454. (32) Kong, Y.; Shen, X.; Fan, M.; Yang, M.; Cui, S. Dynamic capture of low-concentration CO2 on amine hybrid silsesquioxane aerogel. Chem. Eng. J. 2016, 283, 1059-1068.


Page 22 of 24

Page 23 of 24 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


(33) Kong, Y.; Shen, X.; Cui, S.; Fan, M. Development of monolithic adsorbent via polymeric sol-gel process for low-concentration CO2 capture. Appl. Energ. 2015, 147, 308-317. (34) Li, K.; Kress, J. D.; Mebane, D. S. The mechanism of CO2 adsorption under dry and humid conditions in mesoporous silica-supported amine sorbents. J. Phys. Chem. C. 2016, 120, 23683−23691. (35) Galindo, V. V.; Matus M. H.; Cabrera M. A. M. Analysis of the CO2 absorption through a series of amines within the integration of a computational chemistry and process simulation scheme. Int. J. Greenh. Gas Con. 2016, 50, 198-205. (36) Arstad, B.; Fjellvåg, H.; Kongshaug, K. O.; Swang, O.; Blom, R. Amine functionalised metal organic frameworks (MOFs) as adsorbents for carbon dioxide. Adsorption 2008, 14, 755-762. (37) Crooks, J. E.; Donnellan, J. P. ChemInform abstract: Kinetics and mechanism of the reaction between carbon dioxide and amines in aqueous solution. Cheminform 1989, 20, 331-333. (38) Yan, X.; Zhang, L.; Zhang, Y.; Qiao, K.; Yan, Z.; Komarneni, S. Amine-modified mesocellular silica foams for CO2 capture. Chem. Eng. J. 2011, 168, 918-924.



TOC graphic


Page 24 of 24

Enhanced CO 2 Adsorption over Silica-Supported ... - ACS Publications

Recommend Documents