CO2 Adsorption Behavior of Graphite Oxide Modified with

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CO2 Adsorption Behavior of Graphite Oxide Modified with Tetraethylenepentamine Yi Zhang, Yuan Chi,* Changzhong Zhao, Yu Liu, Yuechao Zhao, Lanlan Jiang, and Yongchen Song* Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, School of Energy and Power Engineering, Dalian University of Technology, Dalian 116024, China ABSTRACT: Adsorption with solid sorbents is one of the most promising options for carbon dioxide (CO2) capture. In this study, we developed novel adsorbents of graphite oxide (GO) modified by tetraethylenepentamine (TEPA). Samples were prepared with raw TEPA/GO ratios of 3, 5, 10, and 50 wt %, which we referred to as 3, 5, 10, and 50% aminated graphite oxide (AGO), respectively. The CO2 capture capacities of the different AGO samples were investigated at 25, 40, 70, and 90 °C over a wide range of pressures (0.1−5 MPa) using a high-pressure volumetric analyzer. With increasing pressure, the excess adsorbed amounts of both the pristine and modified GO samples increased significantly, with a peak near 3 MPa followed by a sequential decrease. In addition, both temperature and the amount of TEPA loaded on the surface markedly affected the adsorption capacity. The 3 and 5% AGO samples showed adsorptivity better than that of the pristine GO sample, while the 10 and 50% AGO samples exhibited worse performance. Furthermore, for the 3 and 5% AGO samples, the adsorption capacity was markedly improved at 70 °C in comparison with the pristine GO sample, suggesting that these are appropriate temperatures and TEPA/GO ratios for CO2 capture.

1. INTRODUCTION In recent years, there has been widespread concern about increasing carbon dioxide (CO2) emissions, owing to their major contribution to global warming and abnormal climate change. Developing effective and efficient methods to control and reduce the concentration of atmospheric CO2 has become one of the most challenging issues in environmental conservation. Extensive investigations have been carried out to develop potential capture and separation approaches such as absorption, adsorption, membranes, and cryogenic distillation for curtailing CO2 emissions.1−3 Among these technologies, the most used CO2 capture method is absorption using an amine as the absorbent. However, there still exist some problems, for example, a significant amount of solvent is lost by evaporation, and it may promote the corrosion of the equipment. In comparison to the traditional absorption, adsorption with amine-modified solid sorbents is one of the most promising options as it consumes only a small amount of energy in the regeneration step and does not produce undesirable byproducts or contaminants.4 However, it is difficult to widely apply to industry due to the poor performance and high cost of adsorbents. Researchers try to prepare more efficient and economical adsorbents with high surface area and pore volume, but just a few studies investigated the CO2 adsorption performance at relatively high pressures.5,6 The CO2 adsorption experiments at high pressures can provide theoretical guidance and relevant data for the precombustion CO2 capture, which is characterized by relatively high pressure (up to 4 MPa).7 © XXXX American Chemical Society

Recently, several studies have investigated the amine modification of carbon-based adsorbents such as activated carbon, carbon molecular sieves, and carbon nanotubes to enhance their CO2 adsorption performance.8−10 Graphite oxide (GO) is a commercially available CO2 adsorbent, which can be modified with amine groups. Its use in CO2 capture has been investigated in several previous studies. Zhao et al.11 prepared aminated GO samples modified by excess ethylenediamine (EDA), diethylene triamine (DETA), and triethylenetetramine (TETA), respectively. In CO2 dynamic adsorption experiments, GO/EDA performed well in comparison with the other two samples. Hong et al.12 synthesized GO modified with 3-aminopropyltriethoxysilane and demonstrated its potential for capturing CO2 gas. Mishra and Ramaprabhu5 developed a polyaniline−graphene nanocomposite and CO2 adsorption capacities at a pressure of 11 bar, and different temperatures of 25, 50, and 100 °C were investigated. The results indicated that the CO2 adsorption capacity increased significantly with the increasing pressure. In the current study, for the first time, we synthesized GO modified with tetraethylenepentamine (TEPA) as a CO2 capture candidate, and the CO2 adsorption performance at high pressures was investigated. GO samples modified by different amounts of TEPA (TEPA/GO raw ratios of 3, 5, 10, and 50 wt %) were prepared in ethanol. The CO2 adsorption performance of each of these samples was investigated at Received: September 15, 2017 Accepted: December 15, 2017

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DOI: 10.1021/acs.jced.7b00824 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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temperatures of 25, 40, 70, and 90 °C and pressures of 0.1−5 MPa using a high-pressure volumetric analyzer (HPVA) to determine the optimal conditions for CO2 capture.

3.2. SEM Analysis. As seen in Figure 1a, the pristine GO sample shows folded shape with flocculent structure on the surface, which is in favor of good CO2 adsorption capacity. Figures 1b−e show the SEM morphologies of 3, 5, 10, and 50% AGO samples, respectively. Compared with the pristine GO sample, 3 and 5% AGO exhibit similar morphologies without noticeable variation, and the open structures were intact, while for 10 and 50% AGO, the morphologies change significantly. The flocculent structure disappears, and the surface becomes agglomerate after the amine modification. 3.3. FT-IR Analysis. The IR spectra of the samples are shown in Figure 2. GO shows transmittance bands at 1044, 1224, 1623, 1718, and 3419 cm−1. The bands at 1044 and 1224 cm−1 are associated with C−OH functional groups; the peak at 1623 cm−1 arises from ketone groups,14 and the band at 1718 cm−1 can be attributed to stretching of the CO bond of carbonyl or carboxyl groups. The O−H bond is observed at 3419 cm−1. The spectra of 3, 5, 10, and 50% AGO show a new bend at 1568 cm−1, which can be assigned to the C−N bond.15 Furthermore, the band at 3432 cm−1 is associated with both N−H and O−H bonds.16,17 All of these results further confirm that amine groups were successfully loaded on to the surface of the GO samples. 3.4. BET Analysis. Figure 3a shows the N2 adsorption/ desorption isotherms for the pristine GO sample and modified GO samples at a temperature of 77 K. The BET surface areas of pristine GO and 3, 5, 10, and 50% AGO are 65.42, 42.68, 41.46, 34.08, and 30.49 m2/g, respectively. The loading of TEPA on the surface leads to a decrease in the specific surface area. Figure 3b shows the pore size distribution for different samples. Compared with the pristine GO sample, some of the micropores of modified GO samples disappeared, which was believed to be caused by the TEPA treatment. The GO sample used in this study consisted of GO powders rather than the exfoliated graphene sheets with extremely high BET surface areas that have been used in previous studies.18 3.5. CO2 Adsorption. Figure 4 shows the adsorption isotherms for the pristine GO sample. The experiment was replicated and our measurements of the excess adsorbed amounts had good reproducibility. With increasing temperature, the excess adsorbed capacity decreased, as physical adsorption is an exothermic process.19 As the pressure rose, the excess adsorbed amount at each temperature increased gradually to reach its maximum at 3 MPa before decreasing again. The maximum excess adsorbed amounts were 78.43 mg/g at 25 °C, 64.90 mg/g at 40 °C, 49.22 mg/g at 70 °C, and 44.05 mg/g at 90 °C. These trends can be attributed to the relationship between the excess adsorption capacity nex and the absolute adsorption capacity na:

2. MATERIALS AND METHODS 2.1. Preparation of Samples. The GO sample (particle sizes 10−50 μm) used in this study was produced by Suzhou Tan Feng Co. Ltd., China, using a modified version of Hummers’ method.13 Absolute ethanol and TEPA (>99%) were obtained from Shen Lian Chemical Reagent Co. Ltd., Dalian, China, and pure CO2 (99.999%) was purchased from Dalian Special Gas Co. Ltd., China. Different amounts of TEPA (TEPA/GO raw ratios of 3, 5, 10, and 50 wt %, respectively) were dispersed in 80 mL of ethanol, and each solution was stirred for 30 min. After this, 2 g of GO was added into each solution and stirred vigorously for 4 h. Following this amine modification, the samples were centrifuged at 3000 rpm for 8 min and washed twice with ethanol. The precipitate was dried at 90 °C for 5 h. The final products were obtained as black, dried powders, which we referred to as 3, 5, 10, and 50% aminated graphite oxide (AGO). 2.2. Characterization of Samples. Elemental (C, H, and N) content analysis of samples was carried out using an elemental analyzer (Vario EL III). The morphologies of adsorbents were analyzed using scanning electron microscopy (SEM, NOVA NanoSEM 450). Functional groups were identified using Fourier transform infrared (FT-IR) spectroscopy (6700, Thermo Fisher Scientific Inc.). The samples were measured in the form of KBr pellets. The interfacial features of samples were analyzed by measuring N2 adsorption/desorption isotherms using a JW-BK222 adsorption analyzer (Beijing JWGB Co., Ltd.) at 77 K. Specific surface areas were calculated using the Brunauer−Emmette−Teller (BET) equation. CO2 adsorption isotherms were measured at 25, 40, 70, and 90 °C and 0.1−5 MPa using an HPVA (HPVAII-200, Micromeritics Instrument Co.), and desorption of CO2 was performed at 100 °C under high vacuum (∼10−9 bar) after each cycle of adsorption. The high-pressure transducer provided a reading accuracy of ±0.04%, while the low-pressure transducer provided a reading accuracy of ±0.15%. A static volumetric method was used to calculate the CO2 adsorption capacity. Dual free-space measurements and correction for nonideality of the analysis gas were used to enhance the accuracy of the isotherm data. 3. RESULTS AND DISCUSSION 3.1. Elemental Analysis. The C, H, and N compositions of the samples are listed in Table 1. Notably, while the content of N in the pristine GO sample was 0.064%, this percentage increased with each respective increment in the TEPA/Go raw ratio (increasing to 1.032% in 3% AGO, 1.896% in 5% AGO, 3.587% in 10% AGO, and 9.288% in 50% AGO), indicating that the expected amounts of TEPA were successfully loaded on to the surfaces of the GO samples.

(1)

When the pressure is low, compared with the adsorbed phase density ρa, the free phase density ρg is relatively small, and nex is nearly the same as na. As the pressure increases, ρg and na initially increase; however, as more and more CO2 molecules occupy the adsorptive sites on the surface of GO sample, na continues to gradually increase to its maximum value, while ρg increases rapidly as the pressure increases and CO2 enters the supercritical or liquid state. At this point, ρg increases at a faster rate than na; thus, nex exhibits a downward trend.

Table 1. Element Composition of Samples element (wt %)

GO

3% AGO

5% AGO

10% AGO

50% AGO

C H N

45.12 2.24 0.064

49.96 1.991 1.032

59.62 1.939 1.896

48.2 3.154 3.587

62.84 3.117 9.288 B

DOI: 10.1021/acs.jced.7b00824 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Figure 1. SEM images of (a) pristine GO and (b) 3, (c) 5, (d) 10, and (e) 50% AGO.

adsorbed amounts at 3 MPa are shown in Table 2. Compared with the original GO sample, the 3 and 5% AGO samples show better absorptivity, while the 10 and 50% AGO samples exhibit worse performance. These results indicate that a limited amount of loaded TEPA will enhance the CO2 adsorption capacity, but too much will have the opposite effect. This is mainly because the specific surface area decreases markedly with excess TEPA; in addition, the loaded TEPA cannot function effectively and efficiently, and the enhancement that would normally result from the presence of amine groups is reduced. Temperature plays an important part in the adsorption process, affecting both physical and chemical adsorption. With increasing temperature, the exothermic physical adsorption process is inhibited. Chemical adsorption is mainly determined by the activity of the amine groups on the surface, which is also dependent on temperature. The adsorption isotherms at different temperatures are depicted in Figure 6. Compared with the pristine GO sample, the 3 and 5% AGO samples had better absorptivity. When the temperature rose to 70 °C, the physical adsorption of both 3 and 5% AGO was reduced; however, the chemical adsorption by the amine groups loaded on the surface became stronger, so the overall adsorption capacity improved. At 90 °C, the excess adsorbed amounts of 3

Figure 2. FT-IR spectra of GO sample and modified GO samples.

The adsorption isotherms of different modified GO samples are shown in Figure 5. Because of the amine groups loaded on the surface, the overall adsorption process includes contributions from both physical and chemical adsorption.20 As in the case of the pristine GO sample, as the pressure rises, the excess adsorbed amounts increase gradually and peak near 3 MPa, followed by a sequential decrease. The maximum excess C

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Figure 3. (a) Nitrogen isotherms and (b) pore size distribution curves.

Table 2. Maximum Excess Adsorbed Amounts at 3 MPa T (°C)

GO (mg/g)

3% AGO (mg/g)

5% AGO (mg/g)

10% AGO (mg/g)

50% AGO (mg/g)

25 40 70 90

78.43 64.90 49.22 44.05

91.46 75.29 63.00 48.32

85.77 72.47 60.03 45.30

53.24 44.43 36.40 35.97

43.41 34.13 32.33 30.98

(2)

where nex−AGO is the adsorbed amount of the modified GO sample and nex−GO is the adsorbed amount of the pristine GO sample. The values of Δn at different temperatures and pressures are shown in Table 3. At 25, 40, 70, and 90 °C, the Δn values of the 3% AGO and pristine GO at 3 MPa were 16.61, 16.01, 28.00, and 9.68%, respectively, and the Δn values of 5% AGO and pristine GO at 3 MPa were 9.36, 11.67, 21.97, and 2.82%, respectively. The greatest increase in the adsorption capacity of both 3 and 5% AGO compared with pristine GO was found at 70 °C. In this study, the CO2 adsorption capacities for 3% AGO at 25 °C are 29.79 and 59.63 mg/g at 0.64 and 5.30 bar, which are similar to other modified GO materials in previous papers. Zhao et al.6 reported a kind of metal−organic framework

Figure 4. Adsorption isotherms of the pristine GO sample.

and 5% AGO were close to those of the pristine GO sample, resulting in weaker chemical adsorption than was observed at 70 °C. The difference between the excess adsorbed amount (Δn) of a modified GO sample and that of the pristine GO sample is given by the following equation:

Figure 5. Adsorption isotherms of the modified GO samples. D

DOI: 10.1021/acs.jced.7b00824 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Figure 6. Adsorption isotherms at different temperatures.

Table 3. Increments of Adsorbed Amounts between Modified GO Samples and Pristine GO T (°C) 25 40 70 90

sample 3% 5% 3% 5% 3% 5% 3% 5%

AGO AGO AGO AGO AGO AGO AGO AGO

0.1 MPa

0.5 MPa

1 MPa

2 MPa

3 MPa

4 MPa

5 MPa

11.56 12.12 6.513 4.85 17.40 10.85 4.72 4.47

13.00 12.16 6.81 4.66 13.44 10.90 4.57 4.08

13.61 10.36 8.93 6.50 15.21 14.33 4.16 2.90

14.02 8.84 11.72 8.65 16.16 18.66 4.80 2.08

16.61 9.36 16.01 11.67 28.00 21.97 9.68 2.82

17.81 7.16 21.14 16.87 43.02 34.87 25.03 7.12

33.58 15.18 40.12 36.14 187.53 141.38 164.14 14.01

decrease. The excess adsorbed capacity decreased with increasing temperature. The 3 and 5% AGO samples had absorptivity better than that of pristine GO, while the 10 and 50% AGO samples exhibited worse performance. It appears that an appropriate amount of loaded TEPA will enhance the CO2 adsorption capacity; however, too much loaded TEPA will lead to the opposite result, owing to both a decrease in the specific area and reduced availability of amine groups. Furthermore, as the temperature rose to 70 °C, although the physical adsorption of both 3 and 5% AGO became weaker, the chemical adsorption of the amine groups loaded on the surface became stronger, leading to an overall improvement in the adsorption capacity. Future work should focus on the development of new materials with large specific surface areas as well as determination of the optimal temperature and amount of loaded amine for absorption.

(MOF-5) and AGO composite. When the pressure reached 4 bar, the CO2 uptakes of MOF-5, MOF-5/AGO, and MOF-5/ GO at 298 K were 28.16, 23.76, and 46.64 mg/g, respectively. Shin et al.21 reported new CO2 adsorbents based on graphite oxide (GO) modified with amine groups from polyethyleniminethe (PEI), at 298 K and 1 bar, the CO2 adsorption capacities for PEI-GO 40, PEI-GO 50, PEI-GO 60, and PEIGO 70 were 18.25, 21.21, 32.89, and 11.88 mg/g, respectively. Hong et al.12 reported the CO2 uptake of 3-aminopropyltriethoxysilane modified graphite oxide was 51.04 mg/g at 30 °C under atmospheric pressure.

4. CONCLUSIONS In this work, novel adsorbents of GO modified by 3, 5, 10, and 50 wt % TEPA were prepared, and CO2 adsorption experiments were conducted across a wide range of pressures and temperatures. In pristine GO and the modified GO samples, the excess adsorbed amounts increased gradually with increasing pressure, peaking near 3 MPa, with a subsequent E

DOI: 10.1021/acs.jced.7b00824 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.S.). *E-mail: [email protected] (Y.C.). ORCID

Yuan Chi: 0000-0003-1530-1503 Yu Liu: 0000-0002-6003-9121 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This paper was supported by the National Key R&D Program of China (Grant 2016YFB0600804) and National Natural Science Foundation of China (Grants 51576031 and 51436003).

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ABBREVIATIONS GO, graphite oxide TEPA, tetraethylenepentamine EDA, ethylenediamine DETA, diethylenetriamine TETA, triethylenetetramine PEI, polyethyleniminethe AGO, aminated graphite oxide MOF-5, metal−organic framework BET, Brunauer−Emmette−Teller

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DOI: 10.1021/acs.jced.7b00824 J. Chem. Eng. Data XXXX, XXX, XXX−XXX