Oxidative Degradation of Tetraethylenepentamine-Impregnated Silica

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Environmental and Carbon Dioxide Issues

Oxidative Degradation of TetraethylenepentamineImpregnated Silica Sorbents for CO Capture 2

Quyen Thi Vu, Hidetaka Yamada, and Katsunori Yogo Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04307 • Publication Date (Web): 06 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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Energy & Fuels

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Oxidative Degradation of Tetraethylenepentamine-

2

Impregnated Silica Sorbents for CO2 Capture

3

Quyen T. Vu†, Hidetaka Yamada†‡, and Katsunori Yogo*†‡

4

†

5 6 7 8

Division of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan

‡

Research Institute of Innovative Technology for the Earth, 9-2 Kizugawadai, Kizugawa, Kyoto 6190292, Japan

* Tel.: +81-774-75-2503. Fax: +81-774-75-2318. E-mail: [email protected]

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Abstract: The stability of silica impregnated with commercial tetraethylenepentamine (TEPA) under

11

accelerated oxidizing conditions was evaluated and the changes in the composition of sorbents during

12

oxidative degradation were first reported. The oxidative stability of sorbents depended on the TEPA

13

loading, oxidation duration, temperature, and O2 concentration. The marked loss of the CO2 adsorption

14

capacity of the sorbents in O2-containing environments was mainly caused by changes in the functional

15

groups of TEPA and the sorbent composition, as supported by thermogravimetric, elemental, infrared

16

spectroscopic, and gas chromatography analyses. The results suggested that the hydroxyl groups of silica

17

help to protect TEPA from oxidation. The pore and surface characteristics of sorbent have a significant

18

influence on the O2 diffusion, regulating the oxidation rate. Among isolated components in commercial

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TEPA, 1,4,7,10,13-pentaazatridecane, 4-(2-aminoethyl)-N-(2-aminoethyl)-N´-[2-[(2-

ACS Paragon Plus Environment

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aminoethyl)amino]ethyl]-1,2-ethanediamine and 1-[2-[[2-[(2-aminoethyl)amino]ethyl]-

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amino]ethyl]piperazine were revealed to be sensitive to O2, whereas 1-(2-aminoethyl)-4-[(2-

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aminoethyl)amino]ethyl]piperazine displayed strong O2 resistance.

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Keywords: amine-impregnated silica, CO2 capture, oxidative degradation, solid sorbent,

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tetraethylenepentamine

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1. Introduction

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With an average growth of 2 ppm per year, the atmospheric CO2 level reached 408 ppm in 2018.1 The

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increase in the concentration of atmospheric CO2 is a major cause of human-induced climate change,2,3

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resulting in an increase in the average global temperature. Carbon capture and storage has been

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recognized as a potentially effective strategy to minimize the CO2 emission rate.4–6

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Amine scrubbing using aqueous amine solutions is the current state-of-the-art technology for capturing

13

CO2 from industrial exhaust gases.7,8 However, amine scrubbing suffers from high regeneration cost

14

because the use of water as a solvent consumes considerable energy through sensible heat and vapor

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losses and endothermic reactions during the CO2 desorption process.6,9,10 As well as the extensive energy

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expenditure of the separation, this method has inherent problems including amine loss and equipment

17

corrosion.9,11,12

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One approach to overcome these problems is to immobilize amines on a porous solid carrier. Because

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solid sorbents do not use water as a solvent in the adsorption column, they can lower the energy

20

consumption for regeneration.11,13–15 Additionally, the volatilization of amines can be prevented by

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anchoring amines to a porous support, thereby slowing the rate of equipment corrosion. Among various

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adsorbents including zeolites and activated carbons, amine-impregnated sorbent materials have attracted

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great attention because of their simple preparation procedure and high CO2 adsorption performance.


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With their relatively high boiling points, polyethyleneimine (PEI) and tetraethylenepentamine (TEPA)

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are the most common commercial amines that have been impregnated into porous supports for CO 2

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capture.16–20 Although numerous studies have focused on the adsorption capacity of such materials, 20–24

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their stability has not been thoroughly investigated. The durability of amine sites, which have to maintain

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their structure under practical operating conditions, is important for the development of an effective

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sorbent. The stability of such materials can be affected by both physical and chemical conditions, leading

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to thermal degradation (e.g., evaporation and decomposition)20,25,26 and chemical degradation induced by

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gas species such as O2,16,27,28 SO2,16,20 and NOx.16 The degradation mechanism also depends on the

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presence of CO2.25,29,30 Among these species, O2 is the most pervasive component in flue gas and has been

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identified as the driving force for the oxidative degradation of amine-containing sorbents, shortening the

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lifetime of sorbents.25,31,32 Calleja et al.32 evaluated the exposure of amine-functionalized silicas to air at

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different temperatures and showed that the loss in CO2 capture capacity of sorbent in atmospheric O2 was

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accompanied by an increase in an infrared (IR) band at 1667 cm1. Jones group28 studied the effect of O2

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(5% and 21%) on PEI and poly(allylamine) (PAA) supported materials at two temperatures using Fourier

15

transform (FT)-IR, FT-Raman, and 13C solid-state NMR spectroscopies. They found that secondary-

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amine free, a primary-amine-rich polymer like PAA is stable while PEI is sensitive to oxidation. Heydari-

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Gorji and Sayari25 evaluated the behavior of PEI-impregnated SBA-15 silica in carbon-free air, simulated

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flue gas conditions, and different CO2/O2/N2 mixtures at temperatures up to 120 °C. In their study, PEI-

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based materials degraded rapidly when exposed to carbon-free air at moderate temperatures. Furthermore,

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they noticed that the negative effect of carbon-free air was accompanied by the development of an

21

infrared band at 1665–1680 cm−1, which was associated with the formation of oxime and/or imine species.

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Tanthana et al.33 investigated the degradation characteristics of silica-supported TEPA (TEPA/SiO 2) and

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polyethylene glycol (PEG)-modified TEPA/SiO2 (PEG/TEPA/SiO2) using diffuse reflectance FT-IR

24

spectroscopy and mass spectrometry. They reported that the degradation of TEPA/SiO2 in 15 vol% CO2, 4

25

vol% H2O, and the balance air correlated with the accumulation of carboxylate/carbamic species. They

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also showed that the degradation rate of sorbent and the formation of carboxylate species were suppressed


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by addition of PEG to the sorbent. Srikanth et al.34 studied the oxidative degradation of silica-supported

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amine sorbents with varying amounts of TEPA and PEG. They reported that TEPA generated a different

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type of amide under O2 treatment. FT-IR and NMR spectroscopic studies showed that the yellowish color

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of degraded sorbents originated from the formation of imide species. They also found that the addition of

5

PEG to the supported amine sorbents improved their oxidative degradation resistance and slowed down

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the color change. The hydrogen bonds between amine and hydroxyl (OH) groups could change the

7

reactivity of the amines toward O2 molecules and block O2 from accessing TEPA to induce oxidation.

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The oxidative degradation of amine solid sorbents has still not been thoroughly investigated. Clearer

9

understanding of the oxidative degradation process could contribute to the development of more effective

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sorbents. The effect of amine content on the oxidative degradation of amine solid sorbents and the

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changes in the amine composition of solid sorbents during oxidation have not been studied. To address

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this issue and to gain further insight into the oxidative degradation processes of amine solid sorbents, here

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we comprehensively study the stability of mesostructured cellular silica foam (MF) impregnated with

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varying amounts of TEPA under a range of oxidation conditions including different oxidation durations,

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temperatures, and O2 concentrations. To characterize the sorbents, CO2 adsorption measurements,

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elemental analyses, N2 physisorption measurements, diffuse reflectance FT-IR spectroscopy, and gas

17

chromatography (GC) are carried out. The results demonstrate that the degradation of silica-supported

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amine sorbents strongly depends on the oxidative environment and amine content. We find that the

19

degradation of amine solid sorbents is related to the accumulation of C=O or C=N species. The OH

20

groups of the MF support could contribute to the stability of sorbents in oxidative environments.

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Further of this study is to investigate the change in the components of commercial TEPA-impregnated

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silica sorbents during the oxidative degradation. Commercial technical-grade TEPA is a mixture of four

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main ethyleneamine compounds with close boiling points including linear, branched, and two cyclic

24

TEPA products (Figure 1): 1,4,7,10,13-pentaazatridecane (T-LIN), 4-(2-aminoethyl)-N-(2-aminoethyl)-

25

N´-[2-[(2-aminoethyl)amino]ethyl]-1,2-ethanediamine (T-BRN), 1-(2-aminoethyl)-4-[(2-

26

aminoethyl)amino]ethyl]piperazine (T-IPZ), and 1-[2-[[2-[(2-aminoethyl)amino]ethyl]-


4

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amino]ethyl]piperazine (T-EPZ). To the best of our knowledge, however, in almost all the literature on

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TEPA-containing materials for CO2 capture, there is no mention about such components. Recently, we

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first reported that these components were separated from crude TEPA by fractional distillation and then

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the molecular species of each fraction were identified.35 In this study, we reveal that the four main

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components of commercial TEPA show different stabilities depending on their molecular structure.

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Figure 1. Structures of four main ethyleneamine compounds in the commercial technical-grade TEPA.

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2. Experimental Section

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2.1. Materials and synthesis

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MF powder was purchased from Sigma–Aldrich Co., St. Louis, MO, USA. TEPA and methanol were

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purchased from Wako Pure Chemical Industries Co., Tokyo, Japan and used without any further

12

purification. He (99.9999%) and N2 (99.9999%) were supplied by Iwatani Co. (Osaka, Japan). CO2

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(99.995%) and gas mixture (5% O2/balance N2) were purchased from Sumitomo Seika Chemicals Co.,

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Osaka, Japan. O2 (99.995%) and air (99.9%) were provided by Japan Fine Products Co., Tochigi, Japan.

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The preparation of supported amines was described in our previous research. 21 A mixture of TEPA,

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methanol, and MF was agitated, and then the methanol was removed using a rotary evaporator. The

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resulting sorbents are denoted as TEPAx/MF, where x represents the weight percent of TEPA in the

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adsorbent. The TEPA loading used in this work was below 70 wt %, which is nearly the maximum

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allowable amine loading calculated from the density of TEPA (ca. 1 g cm1) and the pore volume of MF

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(1.95 cm3 g1).21

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2.2. Oxidative degradation of adsorbents

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The degradation of adsorbents was conducted in the packed bed reactor (PBR) illustrated in Figure 2.


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Figure 2. Schematic of the packed-bed reactor ((1) I-quartz tube; (2) silica wool; (3) adsorbent; (4) tube

3

fitting union; (5) fitting tee union; (6) thermocouple; (7) oven with PID temperature controller).

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In a typical experiment, a sorbent (0.3 g) was loaded into the PBR, which consisted of a quartz tube

5

column with an inner diameter of 7.0 mm, outer diameter of 9.5 mm, and length of 14 cm. The

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temperature of the PBR was controlled by a proportional–integral–derivative (PID) temperature controller.

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N2 was first flowed through the PBR (40 cm3 min−1) at 100 °C for 1 h to remove the pre-adsorbed gases

8

and moisture. Subsequently, the degradation step was performed at a temperature of 60 to 100 °C under

9

flowing pure O2/air/gas mixture (40 cm3 min−1) for the time of interest (5 h, 18 h and 42 h). After

10

exposing the sample to O2 for the designated time, the sample was rapidly cooled to room temperature in

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flowing pure N2 (40 cm3 min−1). The degraded sample was then collected. To assess the thermal effect

12

independently from that of O2, the fresh sorbents were treated in N2 (flow rate of 40 cm3 min−1) at 100 °C

13

for 19 h before being rapidly cooled to room temperature. The treated TEPA-impregnated MF samples

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are denoted as TEPAx/MF-gas (used in the degradation step)-temperature (°C)-time of oxidation (h). For

15

example, TEPA50/MF-O2-100-5 and TEPA50/MF-5%O2-100-18 represent MF containing 50 wt % of

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TEPA exposed to pure O2 at 100 °C for 5 h and MF containing 50 wt % of TEPA exposed to 5% O2 at

17

100 °C for 18 h, respectively.

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2.3. CO2 adsorption studies

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Pure CO2 adsorption isotherms for all fresh and degraded samples were monitored using a surface area

20

and porosimetry measurement system (ASAP 2020, Micromeritics Instrument Co., Norcross, GA, USA)


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under pressures ranging from 2.4 Pa to 100 kPa. Prior to the measurements, samples (~100 mg) were

2

pretreated to remove any moisture and pre-adsorbed gas under vacuum at 40 °C for 6 h. The degradation

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of each sample was evaluated by calculating its CO2 adsorption capacity retention (%) from the

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adsorption isotherms measured before and after the degradation test.

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2.4. Materials characterization

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All materials were characterized by N2 physisorption isotherms measured at −196 °C on an ASAP 2420

7

automatic adsorption system (Micromeritics Instrument Co.). Before measurement, solid sorbents were

8

degassed under vacuum at 40 °C for 6 h to remove water and pre-adsorbed gases. The specific surface

9

area (SBET) and pore size distribution of each sample were calculated using the Brunauer–Emmett–Teller

10

(BET) and Barrett–Joyner–Halenda methods, respectively. SBET was evaluated from adsorption data in the

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relative pressure (P/Po) range of 0.05 to 0.3. The total pore volume was determined from the amount of

12

adsorbed N2 at P/Po = 0.97. The organic loading of each silica support was determined by TG analysis,

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which was conducted on a Thermo Plus TG-DTA 8120 analyzer (Rigaku, Tokyo, Japan). The samples

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were heated to 1000 °C at a rate of 5 °C min−1 under a He atmosphere.

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2.5. Elemental analysis, FT-IR spectroscopy, and GC

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Elemental analyses were performed on a PerkinElmer 2400II CHNS/O instrument (Waltham, MA,

17 18 19 20

USA). The FT-IR spectra of original and degraded sorbents were collected using a Prestige-21 FT-IR spectrometer (Shimadzu, Tokyo, Japan) with 4 cm−1 resolution at room temperature. For GC analyses, samples were extracted from solid sorbents by stirring them in ethanol (99.5%, 6 mL)

21

for 18 h at room temperature. After filtration, the extracted samples were quantified by GC using a flame

22

ionization detector (FID). Analyses were performed on a Shimadzu GC 2010 instrument with a split

23

injector and FID. Compounds were separated on a Restek column Rtx-5-amine (30 m x 0.25 m i.d x 0.1

24

μm-thick film). The column temperature was 100 °C for 10 min, heated to 280 °C at a rate of 20 °C min −1,

25

and then held at 280 °C for 10 min. The total time was 29 min. The split injector was held at 250 °C with

26

a split ratio of 50:1 at a flow rate of 3 mL min−1. The FID temperature was maintained at 300 °C, and the


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injected volume was 1 μL. Ultrapure He (99.9999 %, Air Liquide Kogyo Gas Ltd., Japan) was used as a

2

carrier gas at a constant linear velocity of 30.0 cm s−1. Samples were diluted with ultrapure ethanol

3

depending on the amine content of the sorbent.

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3. Results and Discussion

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3.1. CO2 adsorption behavior of adsorbents

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The stabilities of MF impregnated with varying amounts of TEPA from 10 to 70 wt % in the presence

7

of pure O2, air, or 5% O2 for different times under different temperature conditions were assessed. To

8

evaluate the stability of adsorbents against oxidative degradation, their CO2 adsorption capacities were

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measured.

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3.1.1. Effect of oxidation duration on sorbent performance

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The effect of oxidation duration on the stabilities of MF impregnated with varying amounts of TEPA is

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presented in Figure 3. Initial CO2 adsorption capacities of 0.82, 2.28, 3.61, 4.00, and 4.17 mmol g−1 at

13

40 °C and 100 kPa were observed for MF impregnated with 10, 30, 50, 60, and 70 wt % of TEPA,

14

respectively. The amine solid sorbents exhibited a dramatic decrease in CO2 adsorption capacity under

15

oxidative conditions and the degradation rate increased with oxidation duration. After 5 h of O 2 exposure,

16

TEPA10/MF retained 95% of its original CO2 adsorption capacity, whereas the adsorption capacities of

17

TEPA30/MF, TEPA50/MF, TEPA60/MF, and TEPA70/MF decreased to 33%, 41%, 43%, and 32% of

18

their initial values, respectively. After being treated in pure O2 at 100 °C for 18 h, CO2 adsorption

19

capacities decreased by 13%, 88%, 95%, 95%, and 96% for TEPA10/MF, TEPA30/MF, TEPA50/MF,

20

TEPA60/MF, and TEPA70/MF, respectively. Under these degradation conditions, MF loaded with 30–70

21

wt % TEPA exhibited CO2 adsorption capacities of less than 0.28 mmol g−1, which is the CO2 adsorption

22

capacity measured for the MF support (Table 1). This indicates that amino groups in these adsorbents

23

were totally inactivated and could not capture CO2. The degradation rate of amine solid sorbents

24

depended on the amine loading. Among the sorbents, TEPA10/MF showed the highest stability toward O 2.

25

After exposure to pure O2 at 100 °C for 42 h, the CO2 adsorption capacity of TEPA10/MF remained at


8

Page 9 of 30

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0.53 mmol g−1 (65% of its original capacity), whereas those of TEPA30–70/MF were less than 8% of

2

their original capacities.

CO2 adsorption capacity / mmol g-1

5

Fresh N₂-100°C-18h

4

O₂-100°C-5h O₂-100°C-18h O₂-100°C-42h

3

2

1

0 10

3

30

a)

CO2 adsorption capacity retention / %

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

Energy & Fuels

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60

70

TEPA loading / wt %

100

N₂-100°C-18h

O₂-100°C-5h

O₂-100°C-18h

O₂-100°C-42h

80 60 40 20 0 10

b)

50

30

50

60

70

TEPA loading / wt %

5

Figure 3. Effect of oxidation time on the CO2 adsorption capacities of MF impregnated with 10 to 70

6

wt % of TEPA at 40 °C and 100 kPa: (a) absolute values and (b) capacity retention.

7

Sanz-Pérez et al.36 reported that the contribution of CO2 physically adsorbed was significant in the

8

adsorbent containing a low amount of amino groups. In the present results (Table 1), the CO 2 adsorption

9

capacity of MF accounted for 34% of that of TEPA10/MF. This percentage dropped to 7–12% for

10

TEPA30–70/MF. The high contribution of physisorption may partially explains why the TEPA10/MF

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showed the highest stability. However, assuming that the CO2 physisorption of support remained in both


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fresh and degraded TEPA10/MF, a decrease in CO2 chemisorption capacty can be calculated to be 20%

2

after being treated in pure O2 at 100 °C for 18 h, which is still the smallest among all the tested sorbents at

3

the same conditions. The effect of amine concentration on the oxidative degradation of amine solid

4

sorbents is discussed in detail later.

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To evaluate the thermal effect independently from that of O2, the sorbents were treated at 100 °C for 18

6

h in the presence of pure N2. Figure 3 reveals that the CO2 adsorption capacities of all sorbents remained

7

above 96% of their original values when treated in pure N2 at 100 °C for 18 h, indicating that these

8

adsorbents displayed high thermal stability. Therefore, the decrease of the CO2 uptake of sorbents caused

9

by the thermal effect is negligible. This means that oxidative degradation was the main reason for the

10

observed loss in CO2 adsorption capacity of the amine solid sorbents under oxidizing conditions.

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3.1.2. Effect of oxidation temperature on sorbent performance

12

To investigate the effect of temperature on the oxidative degradation of amine solid sorbents, all

13

sorbents were exposed to pure O2 at a flow rate of 40 cc min−1 for 18 h in the temperature range from 60

14

to 100 °C. Figure 4 illustrates the temperature dependence of the CO2 adsorption behavior of the sorbents

15

during oxidation. The degree of deactivation of the amine solid sorbents increased with temperature.

16

None of the amine solid sorbents were influenced by oxidative treatment at 60 °C, retaining above 98% of

17

their original CO2 adsorption capacities. However, with elevating temperature, the sorbents showed

18

marked decreases in CO2 adsorption capacities, which were caused by oxidative degradation rather than

19

thermal degradation effects, as illustrated above. A small decrease of capacity during oxidative treatment

20

was observed for TEPA10/MF. This sorbent retained 98% of its original CO 2 adsorption capacity after O2

21

exposure at 80 °C for 18 h and 87% of its original uptake after O2 exposure at 100 °C for 18 h. In contrast,

22

TEPA30/MF, TEPA50/MF, TEPA60/MF, and TEPA70/MF decreased by 66%, 52%, 38%, and 43% of

23

their initial CO2 uptakes, respectively, after exposure to O2 at 80 °C for 18 h. The CO2 adsorption

24

capacities of these samples decreased by more than 88% of their initial values after exposure to O 2 at

25

100 °C for 18 h.


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CO2 adsorption capacity / mmol g-1

5 O₂-60°C-18h

O₂-80°C-18h

O₂-100°C-18h

4

3

2

1

0 10

1

a)

30

b)

60

70

O₂-80°C-18h

O₂-100°C-18h

100 80 60 40 20 0 10

2

50

TEPA loading / wt %

O₂-60°C-18h


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

Energy & Fuels

30

50

60

70

TEPA loading / wt %

3

Figure 4. Effect of oxidation temperature on the CO2 adsorption capacity of samples at 40 °C and 100

4

kPa: (a) absolute values and (b) capacity retention.

5

3.1.3. Effect of O2 concentration on sorbent performance

6

The effect of the O2 concentration on the degradation rate of the sorbents was then investigated; the

7

results are shown in Figure 5. 5% O2, air and pure O2 exposure lowered the CO2 capture ability of the

8

sorbents. The decreases in the adsorption capacities of the adsorbents were associated with O 2 molecules

9

inducing oxidation. As mentioned above, the CO2 adsorption capacity of sorbents impregnated with 10,

10

30, 50, 60 and 70 wt % of TEPA in pure O2 at 100 °C for 18 h retained 87%, 12%, 5%, 5%, and 4% of

11

their original CO2 uptakes. The degradation proceeded slowly at the low concentrations of O2. Treatment


11

Energy & Fuels

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in air, which decreased the O2 concentration by five times compared with that of O2 treatment, lowered

2

the degradation rate of these sorbents. Compared with treatment in pure O2 under the same conditions, the

3

degradation rates of samples containing 30–70 wt % of TEPA in air were decreased by 2–3 times.

4

Specifically, TEPA10/MF, TEPA30/MF, TEPA50/MF, TEPA60/MF, and TEPA70/MF retained 87%,

5

21%, 15%, 15%, and 14% of their original adsorption capacities, respectively, after treatment in air. The

6

corresponding values after treatment in 5% O2 were even higher: 91%, 42%, 46%, 73%, and 61%,

7

respectively. These results suggest that the oxidative degradation of TEPA-impregnated sorbents is

8

difficult to avoid if O2 is present in the feed gas. CO2 adsorption capacity / mmol g-1

3 O₂-100°C-18h 2.5

Air-100°C-18h 5%O₂-100°C-18h

2 1.5 1 0.5 0 10

9

a)


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 12 of 30

30

50

60

70

50 60 TEPA loading / wt %

70

TEPA loading / wt % 100

O₂-100°C-18h Air-100°C-18h

80

5%O₂-100°C-18h

60

40

20

0 10

10

b)

30

11

Figure 5. Effect of O2 concentration on the CO2 adsorption capacity of samples at 40 °C and 100 kPa: (a)

12

absolute values and (b) capacity retention.


12

Page 13 of 30 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

Energy & Fuels

1

3.1.4. Effect of amine concentration on sorbent performance

2

Comparison of Figure 3, 4, and 5 reveals that the oxidative stability of MF-supported TEPA depends on

3

the amine loading of the support. Among the sorbents, TEPA10/MF exhibited the highest stability under

4

oxidative degradation conditions. The stability of TEPA10/MF against oxidation suggests that amino

5

groups on the silica surface resist oxidation more effectively than those of the amine bulk. This may be

6

attributed to the hydrogen bonds formed between the OH groups of the MF support and amino groups of

7

TEPA, which is consistent with previous results. 33,34,37,38 Chuang’s group33,34 reported that hydrogen-

8

bonding interactions between OH groups of PEG and amino groups of TEPA suppressed the oxidative

9

degradation of supported TEPA sorbents by blocking the amine sites from the approach of O 2, making

10

them less sensitive to the oxidative conditions. The same group37 also reported that the hydrogen bonding

11

between OH groups of poly(vinyl alcohol) and secondary amines of branched PEI could protect PEI from

12

oxidative degradation. Very recently, Min et al.38 noted that the increased stability of macroporous silica

13

impregnated with 1,2-epoxybutane-functionalized PEI was caused by the hydrogen bonds formed

14

between the abundant OH groups generated after 1,2-epoxybutane functionalization and nearby amines.

15

The rate of TEPA oxidation may depends on the concentrations of TEPA and O2, which can be

16

expressed as shown in equation (1). Amine oxidation requires the presence of dissolved O 2; therefore, it

17

depends on the diffusion of O2.

18

r = -k[O2]m[TEPA]n

19

(1)

where k is the reaction rate constant, m and n are the reaction orders with respect to O2 and TEPA,

20

respectively. The m and n exponents should be determined experimentally. However, they have not been

21

considered in this work.

22

Oxidative degradation of an amine can proceed via free radical formation through the reaction of O 2

23

and the amine at elevated temperature.39–42 This type of free radical chain reaction is initiated by a trigger

24

such as thermal fluctuation, resulting in abstraction of a hydrogen atom to form a free radical. Further

25

reactions involve free radical propagation and formation of hydroperoxides, which also decompose to


13

Energy & Fuels 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 14 of 30

1

produce free radicals. The chain reaction is terminated when two radicals combine to produce an inactive

2

species.

3

Initiation:

4

RH  R• + H •

5

Propagation:

6

R• + O  ROO•

(3)

7

ROO• + RH  ROOH + R•

(4)

8

ROOH  RO• + OH •

(5)

9

ROOH + ROOH  RO• + ROO• + H O

(6)

10

RH + RO•  ROH + R•

(7)

11

Termination:

12

R• , RO• , ROO• , etc + R• , RO• , ROO• , etc  Non − radical products

13

The oxidation rate is slow in the induction period. However, the reaction accelerates until almost all

14

reactants have been consumed. Many types of compounds that exhibit different CO 2 capture abilities can

15

be formed by TEPA oxidation.

16

(2)

(8)

Except for TEPA10/MF, TEPA60/MF is the most stable of the samples in pure O2 at 80 °C for 18 h.

17

Under these conditions, the oxidation rate may mainly depend on the rate of O 2 diffusion to amine

18

molecules. The pores and surfaces of the sorbent could influence the O2 diffusion, regulating the

19

oxidation rate. The diffusion rate of O2 to amines decreased with increasing amine content because of the

20

agglomeration of amine molecules in the pores of the support (Figure 6). Degradation of supported amine

21

sorbents was directly proportional to the diffusion rate of O2; therefore, the oxidation rate decreased as the

22

amine content increased. For this reason, the degree of oxidative degradation of samples increased from

23

TEPA30/MF to TEPA60/MF. As for TEPA70/MF, the surface of the support was almost completely

24

covered by amine molecules. Thus, the diffusion rate of O2 into the pores was low. However, O2 could

25

easily react with amines on the surface of the support, leading to an increase in the level of free radicals


14

Page 15 of 30 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

Energy & Fuels

1

that promoted amine degradation. Therefore, TEPA70/MF was more sensitive to oxidative conditions

2

than TEPA60/MF.

3 4

Figure 6. Illustration of the agglomeration of amine molecules in the pores and on the surface of silica

5

with amine loadings from 10 wt % to 70 wt %.

6

When the temperature was raised to 100 °C, the reaction rate increased and the O2 diffusion was

7

enhanced compared with those at lower temperature. Consequently, the degree of degradation of TEPA

8

was affected depending on oxidation time, amine concentration, O2 concentration, and the number of free

9

radicals. The oxidation of TEPA possibly released many types of compounds that exhibited different CO 2

10

adsorption capacities. The behaviors of newly formed species also contributed to the stability of sorbents.

11

The changes in the composition of sorbents during oxidation affected their adsorption capacities as well

12

as their stability.

13

After treatment in pure O2 at 100 °C for 18–42 h, the stability of sorbents was as follows:

14

TEPA30/MF>TEPA50/MF≈TEPA60/MF≈TEPA70/MF. The sorbents retained less than 12% of their

15

original CO2 adsorption capacities and lower CO2 adsorption capacities than that of the support without

16

TEPA, suggesting that the oxidation of TEPA in the sorbents was in the termination stages. Almost all of

17

the TEPA was consumed to form new products that were incapable of capturing CO 2, therefore

18

decreasing the ability of the sorbents to adsorb CO2. Moreover, the agglomeration of these new inactive

19

products on the surfaces and in the pores of the support may decrease the CO 2 adsorption capacity of the

20

amine-supported sorbents below that of the support without TEPA.

21

3.2. Characterization of sorbents before and after oxidative degradation


15

Energy & Fuels

1

All the initial sorbents were white. After heating in O2 at 80 °C for 18 h, all the sorbents remained

2

white. After oxidation under more severe conditions, TEPA10/MF remained white, whereas the other

3

sorbents varied from pale to intense yellow. The extent of the color changes of the sorbents was

4

proportional to their degree of degradation.

5

The decrease in the CO2 adsorption capacity of the amine-impregnated sorbents in an oxidative

6

environment can be caused by three factors. First, the amine content may be decreased during oxidation

7

because of thermal evaporation. To analyze this, TG and GC analyses were carried out. Second, new

8

products may accumulate in the pores and on the surfaces of the support, probably leading to blocked

9

pores in the degraded sorbents, which has a negative effect on the CO2 uptake. To investigate this, the

10

surface area and pore volume of the sorbents were measured. Third and most importantly, the amino

11

functional group is modified by oxidation into new species that have no ability to capture CO 2. Some of

12

the newly formed species may evaporate during the oxidation. FT-IR spectroscopic analysis was

13

conducted to examine the changes in the functional groups and changes in the composition of sorbents

14

were observed by GC. Elemental analysis was also performed to confirm the change in the composition

15

of sorbents. Fresh N₂-100°C-18h O₂-100°C-5h O₂-100°C-18h O₂-100°C-42h Air-100°C-18h O₂-80°C-18h O₂-60°C-18h

100

80

Weight change / %

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 16 of 30

60

40

20

0 0

16 17

200

400

600

800

1000

Temperature / °C

Figure 7. TG curves for TEPA70/MF before and after exposure to different oxidation treatments.

18

TG analysis was carried out on representative samples; the results are shown in Figure 7. The TG

19

results indicated that after oxidative degradation, TEPA70/MF still retained at least 64 wt % of its total


16

Page 17 of 30

1

organic content. Compared with the original sorbent, the loss in total organic content was about 9 wt %.

2

After being treated in N2 at 100 °C for 18 h or in pure O2 at 60 °C for 18 h, treated TEPA70/MF retained

3

the same TG profile as that of the original sorbent. The sorbents lost almost all of their organic

4

compounds (>94 wt %) when heated to 300 °C. Comparison with the TG data for the original sorbent

5

revealed that the sorbent lost just 2 wt % of TEPA after thermal treatment in N2, ruling out the possibility

6

that thermal evaporation of the amine is the main reason for the decrease in CO2 uptake. In contrast, when

7

the oxidation temperature was raised to 80 and 100 °C, the TG profiles for the oxidatively degraded

8

sorbents differed from that of the original sorbent, indicating that the degraded sorbents contain

9

degradation products with a higher boiling point than that of TEPA. Heating to 300 ºC only removed part

10

of the organic content (49–77 wt %) from the supports. The organic content remaining in the support was

11

directly related to the degree of degradation. Almost all of the organic compounds were removed when

12

the temperature was raised to 900 ºC. The changes in the TG profiles of the sorbents reflected the

13

decrease in the CO2 adsorption capacities of the sorbents during oxidation. The TEPA70/MF sorbent after

14

exposure to N2 at 100 °C for 18 h or O2 at 60 °C for 18 h, which exhibited the same TG profiles as that of

15

the fresh one, retained above 99% of their initial CO2 uptakes. By contrast, under the other oxidation

16

conditions used in this work, the treated TEPA70/MF sorbents displayed changes in their TG profiles and

17

decreases in CO2 uptake of at least 43%. 400

Fresh N₂-100°C-18h O₂-100°C-5h O₂-100°C-18h O₂-100°C-42h Air-100°C-18h O₂-80°C-18h O₂-60°C-18h

300

SBET / m2 g-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

Energy & Fuels

200

100

0 10

18 19

30

50

60

70

TEPA loading / wt %

Figure 8. Surface areas (SBET) of original and degraded sorbents.


17

Energy & Fuels

Fresh N₂-100°C-18h O₂-100°C-5h O₂-100°C-18h O₂-100°C-42h Air-100°C-18h O₂-80°C-18h O₂-60°C-18h

2

Pore volume / cm3 g-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

Page 18 of 30

1.5

1

0.5

0 10

1 2 3

30

50

60

70

TEPA loading / wt %

Figure 9. Pore volumes of original and degraded sorbents. The residues of impregnated amines in the mesopores of the MF support were confirmed by N 2

4

physisorption experiments. The results are presented in Figure 8 and 9. Gradual decreases in the surface

5

area and pore volume of the MF support were accompanied with a gradual increase in the amount of

6

TEPA loading on/in the support. The surface area and pore volume of MF were 448 m2 g−1 and 1.95 cm3

7

g−1, respectively. When MF was loaded with 70 wt % of TEPA, the surface area of the support decreased

8

to 7 m2 g−1 and its pore volume fell to 0.01 cm3 g−1, indicating that the surface and mesopores of MF were

9

nearly completely covered and filled by amines, respectively. This result supported the agglomeration of

10

amine molecules in/on the amine solid sorbents (see Figure 6), which can affect the diffusion rate of O 2 as

11

well as that of CO2. After thermal treatment in pure N2 at 100 °C for 18 h, the surface area and pore

12

volume of sorbents increased slightly, suggesting that a small amount of amine evaporated from the

13

sorbents. This result is consistent with the change in the total amount of organic compounds determined

14

by TG analysis. Both the pore volume and surface area of sorbents increased after oxidation under severe

15

conditions, suggesting that the accumulation of new products in the pores and on the surfaces of the

16

support did not block the pores in the oxidized sorbents. These changes can be explained by both the

17

slight evaporation of TEPA and the release of degradation products with low molar mass during oxidation.

18


18

Page 19 of 30 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

1

Energy & Fuels

Table 1. Element contents and CO2 uptakes of representative sorbents. Sorbent

C

H

N

CO2 adsorption capacity

[%]

[%]

[%]

[mmol g-1]

MF

a

a

a

0.28

TEPA10/MF

6.25

1.50 3.81

0.82

TEPA10/MF-O2-100-18 5.17

1.17 3.32

0.71

15.12 3.74 9.90

2.38

TEPA30/MF-O2-100-18 13.40 1.75 7.18

0.28

TEPA30/MF

TEPA50/MF

26.55 6.47 17.55 3.74

TEPA50/MF-O2-100-18 23.31 3.19 12.51 0.19 TEPA60/MF

30.48 7.30 22.22 3.99

TEPA60/MF-O2-100-18 28.44 4.00 15.21 0.21 TEPA70/MF

35.53 8.51 25.91 4.17

TEPA70/MF-O2-100-18 31.89 4.84 17.41 0.18 a

Not measured.

2

The evaporation of products with low molar mass during the degradation of amino functional groups

3

corresponds to the change in the element content of the sorbents. The composition and CO 2 adsorption

4

capacities of representative sorbents before and after oxidative degradation at 40 °C and 100 kPa are

5

shown in Table 1 and Figure 10. The contents of C, H, and N of the sorbents decreased after oxidative

6

degradation, supporting the evaporation of organic compounds. However, the decreases of the contents of

7

N and other elements cannot fully explain the marked suppression of the CO 2 adsorption capacity of the

8

sorbents. A typical example is TEPA60/MF: after degradation in pure O 2 at 100 °C for 18 h, its C, H, and

9

N contents decreased by 93%, 55%, and 68%, respectively, yet its CO2 adsorption capacity decreased by

10

95%. Therefore, the evaporation of amine-containing compounds is not the main reason for the observed

11

decrease in CO2 uptake after oxidation. The loss in CO2 uptake of oxidized supported amine sorbents is

12

mainly caused by the change of amino functional groups. The minor decrease of C content and the


19

Energy & Fuels

1

marked drops of N and H contents suggest that water and new volatile amine-containing compounds, such

2

as ammonia and urea, form during the oxidative degradation of the sorbent. 100

Element content and CO2 uptake retentions / %

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 30

C

N

H

CO₂ uptake

80 60 40 20 0 10

30

50

60

70

TEPA loading / wt %

3 4

Figure 10. Element content and CO2 uptake retentions of sorbents after degradation in pure O2 at 100 °C

5

for 18 h.

6

In this work, to examine the change in the composition of sorbents in an O2-containing environment, a

7

solvent extraction method was used to extract organic compounds from the silica support. GC analysis

8

was performed on the extracted solutions of representative samples; the results are shown in Figure 11.

9

The total amount of the four main compounds in TEPA60/MF was 92.7 area %, in which T-BRN, T-LIN,

10

T-IPZ, and T-EPZ contributed 13.6, 47.8, 24.7, and 6.6 area %, respectively. The total amount of such

11

compounds remained 90.24 area % after thermal treatment in pure N2 at 100 °C for 18 h, suggesting the

12

evaporation of a small amount of amine from the sorbent. This result is consistent with the changes in the

13

total amount of organic compounds measured by TG analysis as discussed above. The total amount of

14

these compounds in the sorbent decreased to 59.9 and 6.8 area % after exposure to O 2 at 80 °C for 18 h

15

and 100 °C for 18 h, respectively. These results suggest that the rapid decrease in the organic content

16

during the oxidation contributed to the sharp decline in the CO2 capacity of the TEPA-supported materials.

17

The GC results also revealed that during the oxidation, TEPA was modified to form new species, some of

18

which had a low molar mass and were easily evaporated from the oxidized sorbents. However, the newly

19

formed species were not identified in this study. Additionally, there were considerable changes in the


20

Page 21 of 30

1

contents of the four main compounds in commercial TEPA after oxidation. Figure 11 reveals that the

2

intensities of peaks from T-BRN, T-LIN, T-IPZ, and T-EPZ in GC traces decreased after oxidation. The

3

intensities of peaks from T-LIN, T-BRN, and T-EPZ decreased rapidly, whereas the decrease in the

4

intensity of the peak from T-IPZ was the slowest of the four main TEPA components. These results

5

suggest that T-IPZ exhibited the highest O2 resistance among the four main components of TEPA.

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

Energy & Fuels

6 7

Figure 11. GC charts of TEPA60/MF before and after oxidative degradation.

8 9

Scheme 1. Radical mechanism of six-membered ring formation for the oxidative degradation of T-LIN.


21

Energy & Fuels 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

1

Page 22 of 30

The low stability of T-LIN, T-BRN, and T-EPZ can be explained by the formation of a six-membered

2

ring through radical mechanism which was proposed for 1,1,4,7,7-pentamethyldiethylenetriamine by

3

Lepaumier et al.43 This mechanism is illustrated for T-LIN in Scheme 1. First, the amino group is

4

activated into an aminium radical cation through an electron abstraction from a secondary amine. Then,

5

an intramolecular rearrangement leads the [1,7] H migration in the ionized system. Subsequently, a

6

capture of a hydrogen radical leads to the formation of aminium cation with -NH3+, a very good leaving

7

group, which is followed by the formation of an ammonia and a six-membered ring compound. While T-

8

IPZ cannot follow this scheme because of its structure, T-LIN, T-BRN, and T-EPZ can degrade to form

9

ammonia, which is coherent with the losses in N and H contents measured by elemental analysis as

10

mentioned above. The six-membered ring compound in Scheme 1 still contains amino groups reactive

11

with CO2. However, CO2 adsorption capacities after degradation are not higher than those of the bare

12

silica, indicating that the six-membered ring compound is not a final product, but may be an intermediate.

13

Because it has not been identifed in this work, further studies are needed to support the proposed scheme.

14

It is well known that primary and secondary amines react with CO2 whereas tertiary amines cannot

15

react with CO2 in the absence of water.44–46 The proposed reaction mechanism is as follows:

16

2R1R2NH + CO2 ↔ R1R2NCOO− + R1R2NH2⁺

(9)

17

Among the four main compounds in commercial TEPA, T-LIN and T-BRN contain more primary and

18

secondary amino groups than T-IPZ and T-EPZ. Therefore, T-LIN and T-BRN may have more potential

19

to capture CO2 than T-IPZ and T-EPZ. However, we previously reported that T-LIN and T-BRN

20

exhibited lower amine efficiencies, defined as moles of adsorbed CO2 per mole of amino groups in an

21

amine solid sorbent, and lower cyclic capacities than those of T-IPZ and T-EPZ. 35 Moreover, T-LIN and

22

T-BRN displayed higher adsorption heats than those of T-IPZ and T-EPZ. Considering the results in this

23

work, an increase in the T-IPZ content of commercial TEPA should improve the CO 2 adsorption

24

performance and stability of sorbents containing TEPA.


22

1668 C=O/C=N

1387 1304

CH stretching 2937 2888 2820

1600 NH2 bending 1458 CH2 bending

NH2 stretching 3360 3290

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

Energy & Fuels

Absorbance / a.u.

Page 23 of 30

O2-100°C-42h O2-100°C-5h N2-100°C-18h Fresh 4000

3500

3000

2500 Wavenumber /

1

2000

1500

1000

cm-1

2

Figure 12. FT-IR single-beam spectra of TEPA70/MF before and after exposure to O2/N2 at 100 °C for

3

different durations.

4

Finally, FT-IR spectroscopic analysis was carried out on representative sorbents to identify the newly

5

formed species in the degraded sorbents. Figure 12 reveals that exposure to O 2 was accompanied by the

6

development of a band at 1668 cm−1 that was correlated with the observed decrease in CO2 capacity. The

7

new intense peak at 1668 cm−1 was assigned to C=O or C=N stretching of urea, amide, nitrite, acid, imine

8

or imide groups, consistent with the oxidation of polyamines.25,28,31,34,47,48 The FT-IR spectra of TEPA-

9

impregnated sorbents after oxidative treatment showed decreases in the intensities of NH vibration

10

(3300–3600 cm−1), CH stretching (2800–3000 cm−1), and CH bending (1304 and 1458 cm−1). The other

11

characteristics of the TEPA60/MF FT-IR spectrum compared with that of oxidized TEPA60/MF were a

12

decrease in the intensity of the vibration at 1600 cm−1, which was the characteristic band of the N-H

13

bending vibration of primary/secondary amines, and a shift of the CH bending vibration from 1458 to

14

1446 cm−1. The changes in the formation of C=O/C=N species and the decreased intensity of bands from

15

N-H and C-H species indicated that TEPA was oxidized to form nitrogen-containing groups with lower

16

basicity and therefore lower affinity for CO2, causing the CO2 adsorption capacity to decrease.


23

Energy & Fuels 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

1

Furthermore, additional bands appeared at 1545, 1493, and 1387 cm−1 in the FT-IR spectrum of the

2

oxidized sample, supporting its exponential decay.

Page 24 of 30

3

4. Conclusions

4

It is impossible to avoid the oxidative degradation of TEPA-impregnated silica sorbents if O 2 is present

5

at moderate temperature. The oxidative stability of TEPA-supported sorbents depends on the amine

6

loading, oxidation duration, temperature, and O2 concentration. The marked loss in the CO2 adsorption

7

capacity of the sorbents was mainly caused by the conversion of basic amines into non-basic compounds

8

and the formation of ammonia. Most importantly, the changes in the composition of commercial TEPA

9

during oxidative degradation were first investigated. The deactivation of amine solid sorbents in O 2-

10

containing environments depended on the amine structure. T-IPZ exhibited the highest oxidative stability

11

of common TEPA components. The CO2 adsorption behavior and stability of sorbents containing TEPA

12

could be improved by increasing the T-IPZ content of commercial TEPA. Amines which are unfavorable

13

to form six-membered rings may be advantages for the development of effective sorbents for CO 2 capture.

14

Acknowledgments

15

This work was financially supported by the Ministry of Economy Trade and Industry, Japan. We thank

16

Mr. Katao Shouhei and Mr. Asanoma Fumio for assistance with elemental analyses.

17

References

18

[1]

National Oceanic & Atmospheric Administration (NOAA) "Trends in Atmospheric Carbon

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dioxide". Available from https://www.esrl.noaa.gov/gmd/ccgg/trends/global.html (accessed November

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2018).

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[2]

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International energy agency IEA, CO2 Emissions from Fuel Combustion - Highlights. Int. Energy

Agency 2017, 9–20. [3]

Intergovernmental Panel on Climate Change IPCC. Climate Change 2014: Mitigation of Climate

Change: Cambridge University Press, 2015, 141.


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[4]

Stenven Chu. Carbon Capture and Sequestration. Science, 2009, 325 (5948), 1599.

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Stuart Haszeldine R. Carbon Capture and Storage: How green can black be? Science, 2009, 325

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D’Alessandro D. M.; Smit B.; Long J. R. Carbon dioxide capture: Prospects for new materials.

Angew. Chem. Int. Ed. 2010, 49, 6058–6082. [7]

Mazari S. A.; Ali B. S.; Jan B. M.; Saeed I. M.; Nizamuddin S. An overview of solvent

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[10] Mandal B. P.; Bandyopadhyay S. S. Absorption of carbon dioxide into aqueous blends of 2-

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Karl T. R.; Trenberth K. E. Modern Global Climate Change. Science 2003, 302 (5651), 1719–

Rochelle G. T. Amine Scrubbing for CO2 Capture. Science 2009, 325 (5948), 1652–1654.

amino-2-methyl-1-propanol and monoethanolamine. Chem. Eng. Sci. 2006, 61 (16), 5440–5447. [11] Dutcher B.; Fan M.; Russell A. G. Amine-based CO2 capture technology development from the beginning of 2013-A review. ACS Appl. Mater. Interfaces 2015, 7 (4), 2137–2148. [12] Goto K.; Okabe H.; Chowdhury F. A.; Shimizu S.; Fujioka Y.; Onoda M. Development of novel absorbents for CO2 capture from blast furnace gas. Int. J. Greenh. Gas Control 2011, 5 (5), 1214–1219. [13]

Samanta A.; Zhao A.; Shimizu G. K. H.; Sarkar P.; Gupta R. Post-combustion CO2 capture using

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CO2 uptake (mmol g-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

Intensity (a.u.)

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Absolute pressure (kPa)

Fresh sorbent Degraded sorbent

Retention time (min)


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Oxidative Degradation of Tetraethylenepentamine-Impregnated Silica

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