Carbon Nanotube

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was an average of 8 scans with a resolution of 2 cm-1. ... 3100 cm-1 and 3200 cm-1, found between 1650 cm-1 and 1740 cm-1 and 1430 cm-1 and 1530 cm-1.

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ScienceDirect Energy Procedia 114 (2017) 2330 – 2335

13th International Conference on Greenhouse Gas Control Technologies, GHGT-13, 14-18 November 2016, Lausanne, Switzerland

Synthesis and Performance Evaluation of Chitosan/Carbon nanotube (Chitosan/MWCNT) Composite Adsorbent for Postcombustion Carbon Dioxide Capture K. Oslera, N. Twalaa , O. O. Oluwasina b M.O. Daramolaa*, a

School of Chemical and Metallurgical Engineering, Faculty of Engineering and the Built Environment, University of the Witwatersrand, Private Bag X3, Wits 2050, Johannesburg, South Africa. b Chemistry Department, Federal University of Technology, Akure, Ondo State, Nigeria

Abstract

As a preliminary investigation towards obtaining composite carbon nanotube composite adsorbents for CO2 capture, in this study chitosan was impregnated onto the surface of multi-walled carbon nanotubes to enhance the CO2 adsorption capacity of the MWCNT. Surface properties of the CNTs were checked with Raman Spectroscopy. Fourier Transform Infrared Spectroscopy was used to check for functional groups present in the CNTs, pure chitosan and Chitosan/MWCNTs. Thermogravimetric analysis was used to study the CO 2 adsorption performance of the MWCNTs, chitosan and chitosan/MWCNTs. It was found that the CO2 adsorption capacity of the MWCNTs were improved by 650 % through the impregnation of chitosan. © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2017 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. Peer-review under responsibility of the organizing committee of GHGT-13.

Keywords: CCS, Chitosan, Carbon Nanotubes

1. Introduction CO2 capture and storage is a promising technique towards the sustainable use of fossil fuels for electricity generation. This research was aimed at developing a novel adsorbent for the post-combustion capture of CO2. 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of GHGT-13. * Corresponding author. Tel.: +27117177536. E-mail address: [email protected]

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of GHGT-13. doi:10.1016/j.egypro.2017.03.1368

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Carbon nanotubes (CNTs) have proven to possess a good adsorption capacity towards the removal of organic and inorganic pollutants because of the hollow and layered structures that have a large surface area; however, their application is hindered by the limited manipulation of CNTs owing to their insolubility in most organic and inorganic aqueous solutions [1]. For this reason, impregnating surfactant onto the surface of CNTs in order to make them chemically active has been studied and this can also improve their application potential for CO2 capture. It has been shown that grafting or impregnating a surfactant onto the surface of a carbon nanotube (CNT) enhances the adsorption capacity of the CNT. Lee et al. reported that impregnating polyethyleneimine onto multi-walled carbon nanotubes (MWCNTs) increased their CO 2 adsorption capacity by 200% [2]. Furthermore, Ngoy et al. reported that grafting a polyaspartamine surfactantant onto MWCNTs increased the CO 2 adsorption capacity by nearly 500 % [3]. Su et al. demonstrated that MWCNT/3-aminopropyltriethoxysilane (APTS) shows good CO 2 adsorption performance when compared to amine-functionalized activated carbon, and also possesses a low theoretical energy of regeneration and superior cyclic stability [4]. However, many of the amine based polymers that have been studied are toxic. Chitosan is an alternative amine based polymer, derived from chitin, a natural biopolymer that when combined with CNTs could provide a material with improved mechanical properties good enough for a wide range of applications including CO2 adsorption, water treatment and so on. Chitin is the main component in the exoskeletons of crustaceans and anthropods, which is considered as a waste material in coastal region, thus the project has the waste beneficiation potential. 2. Materials and method 2.1 Materials and synthesis Multi-walled CNTs (MWCNTs) were purchased from Sigma Aldrich (Pty) SA. The purity, outer diameter and length of the MWCNTs was given as >90 %, 110-170 nm and 5–9 μm, respectively. Chitosan was synthesized from chitin (purchased from Lagos Beach Bar, Nigeria and pulverized to >150 μm) in three stages – demineralisation (6 % HCl, 60 ⁰C and 2 hours), deproteinisation (6 % NaOH, 60 ⁰C and 2 hours) and deacetylation (30 % NaOH, 80 ⁰C and 40 min). 2 g of the MWCNTs were mixed with 100 ml of 30 % nitric acid and stirred under reflux for 4 hours at 110 ⁰C; the mixture was then continually rinsed with distilled water using a vacuum pump until the pH was around 7. The filtrate was dried at 60 ⁰C for 24 hours. 20 ml of 2 % acetic acid was used to dissolve 0.4 g of chitosan which was then mixed with the functionalized MWCNTs prepared; the mixture was stirred for an hour at 80 ⁰C under reflux, washed several times with distilled water, filtered and dried at 60 ⁰C for 24 hours. 2.2 Characterisation and performance evaluation Surface properties of the MWCNTs were checked with Raman Spectroscopy. Raman spectra were acquired with the 514.5 nm line of an argon ion laser and a Horiba Jobin-Yvon LabRAM HR Raman spectrometer equipped with an Olympus BX41 microscope attachment. The incident beam was focused onto the sample with a 100x objective and the power at the sample was kept low (~ 0.6mW) to prevent localized heating by the laser. The backscattered light was dispersed via a 600 lines/mm grating onto a liquid nitrogen cooled CCD detector and the data was collected with LabSpec v5 software. Fourier transform infrared spectroscopy (FTIR) was used to check for functional groups present in the MWCNTs, pure chitosan and Chitosan/MWCNTs. A Perken Elmer Frontier FTIR

spectrometer was used and the spectra were obtained in the frequency range 650 – 4000 cm-1, each spectrum was an average of 8 scans with a resolution of 2 cm-1. Thermogravimetric analysis was used to study the CO2 adsorption performance of the MWCNTs, chitosan and chitosan/MWCNTs. CO2 adsorption performance experiments were conducted by sweeping the samples with N2 gas (flowrate = 60 mL/min) at 1.1 bar and a temperature of 110 ⁰C for 30 minutes to desorb water and other gases present on the surface. After cooling, the samples were exposed to pure CO2 (flowrate = 60 mL/min) under pressure of 1.1 bar and a temperature of 45 ⁰ for 120 minutes. The mass of the sample changed throughout the adsorption experiments and the change in mass of the sample was used to calculate the amount of CO2 adsorbed/g of adsorbent.

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3. Results and Discussion 3.1. Raman Spectroscopy Raman spectroscopy is widely used in the characterisation of CNTs due to the good spatial resolution and sensitivity as well as there is minimal need for sample preparation. Raman spectra results are given in Figure 1. A band can be see between 1000 and 1200 cm-1, this band was caused by the overlight used and is not caused by the sample. Radial breathing modes (RBM) peakes are found between 75 cm-1- and 300 cm-1 and are specific to the raman spectra of single-walled CNTs [6]. The RBM peak is not visible which indicates that the CNTs are in fact multiwalled. The D-peak, viewed between 1300 and 1400 cm-1 and the G-peak, viewed at approximately 1600 cm-1 was used to quantify the structural quality of the MWCNTs. The D-peak originates from impurities (such as catalyst remains) present, but can also be caused by structural defects [5]. A large D-peak would thus indicate CNTs with more impurities and structural defects. But as show in Figure 1 the D-peak is small. The ratio of the D-peak to the G-peak for the MWCNTs is small, this indicates that the CNTs are of excellent structural quality as it means that there is very little presence of amorphous carbon [6]. 1400 1200

G-peak

Intensity (a.u.)

1000 800 600 400

D-peak RBM

200 0 0

200

400

600

800

1000

Raman Shift

1200

1400

1600

1800

2000

(cm-1)

Figure 1: Raman Spectra of the MWCNTs 3.2. Fourier Transform Infared Spectroscopy

FTIR is commonly used to analyze the chemical bonds and functional groups that are present. FTIR can be used in the application of CNTs to study the surface chemistry and provide for a means to understand the interactions occurring at the surface during CO2 adsorption. Figure 2, Figure 3 and Figure 4 show the FTIR analysis of the MWCNTs, pure chitosan and chitosan/MWCNTs, respectively, and depict various characteristic bands. Figure 2 showed a lot of noise so the curve needed to be smoothened. The presence of noise could be attributed to the presence of carbon and impurities in the MWCNTs. The C-C found at approximately 1000 cm-1 is a characteristic of the MWCNTs [7]. Figure 3 and Figure 4 show that the expected functional groups were obtained. The -NH2 found between 3300 cm-1 and 3500 cm-1 shows the presence of a primary amine, which is beneficial for CO2 capture for the formation of carbamate. The N-H, C=O and C-N found between 3100 cm-1 and 3200 cm-1, found between 1650 cm-1 and 1740 cm-1 and 1430 cm-1 and 1530 cm-1 , respectively form the amide bond, which confirms the biodegradability of the adsorbent by forming a bio-fission bond at which the molecule will break [3].

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Figure 2: FTIR spectrum of MWCNTs

NH2

Impurities from synthesis NH C=O C-N

600

1250

1900

2550

wavenumber (cm-1)

3200

3850

Figure 3: FTIR spectrum of pure chitosan

NH C=O C-N

600

NH2 NH2

C-C

1250

1900

2550

wavenumber (cm-1)

Figure 4: FTIR spectrum of chitosan/MWCNTs composite adsorbent

3200

3850

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3.3. Thermogravimetric analysis TGA (TGA TA STD Q6000) analysis was used to evaluate the CO2 adsorption performance of the MWCNTs, chitosan and chitosan/MWCNTs. Table 1 shows the CO2 adsorbed by each adsorbent at 45 ͼC and 60 mL/min. The MWCNTs showed an extremely low CO2 adsorption capacity – 0.4 mg/g. This adsorption capacity is much lower than what has previously been reported in literature, which range from 12.03 – 21.5 mg/g (see Table 1). Thus the adsorption capacity of the MWCNTs used in this study is not comparable with literature and this could be attributed to the operating conditions used in this study when compared to the reports obtained from literature. Adsorption of gases onto solid materials decreases with increasing temperature. However, the amount of CO2 adsorbed by the pure chitosan was 9 mg/g. The amount of CO2 adsorbed by the chitosan/MWCNTs was 3 mg/g. This is a 66.67 % decrease in the amount of CO2 adsorbed by the pure chitosan and a 650 % increase in the amount of CO2 adsorbed by the MWCNTs. This is a very substantial increase. Lee et al. reported that impregnating polyethyleneimine onto multi-walled carbon nanotubes (MWCNTs) increased their CO 2 adsorption capacity by 200% [2]. Likewise, Ngoy et al. reported that grafting a polyaspartamine surfactantant onto MWCNTs increased the CO2 adsorption capacity by nearly 500 % [3]. In this study, it could be speculated that impregnating chitosan onto MWCNTs enhances the CO2 adsorption capacity of MWCNTs more than what has been reported in literature. However, the CO2 adsorption capacity of the chitosan/MWCNTs is very low. This can be attributed to the low CO2 adsorption capacity of the MWCNTs that were used and the higher temperature at which the adsorption was carried out. The use of a MWCNTs with a higher CO 2 adsorption capacity should yield a more suitable adsorbent. Table 1: CO2 adsorption capacity of MWCNTs from literature Adsorbent

MWCNTs MWCNTs MWCNTs Polysuccinide Polyaspartamide Chitosan MWCNTs Chitosan/MWCNTs

Operating Conditions (Temperature (ͼͼC), Pressure (bar) 25, 1.1 50,1.01 25, 1.1 25, 1.1 25,1.1 45,1.1 45,1.1 45,1.1

Average CO2 adsorbed (mg CO2/ g adsorbent)

Ref.

12.03 21.50 21.02 25 47.1 9 0.4 3

[3] [4] [2] [3] [3] This study This study This study

4. Conclusion Chitosan/MWCNTs composite adsorbent was successfully synthesized and evaluated for post-combustion CO2 capture. Raman spectroscopy confirmed that the MWCNTs used in this study contained few defects and were of excellent quality. FTIR results confirmed the presence of the expected functional groups and that the chitosan polymer was successfully impregnated onto the surface of the MWCNTs. The evaluation of the CO2 adsorption performance of the chitosan/MWCNTs adsorbent using TGA shows the amount of CO2 adsorbed by MWCNTs could be enhanced by 650% by impregnating chitosan onto the surface of the MWCNTs. However, the overall CO2 adsorption capacity of the chitosan/MWCNTs adsorbent is too low for use as an adsorbent and chitosan has been shown to be able to increase the CO2 adsorption capacity of CNTs. Thus chitosan could be impregnated onto the surface of a more suitable CNT for CO2 capture and would improve the CO2 adsorption capacity of that CNT.

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Acknowledgements The authors acknowledge the financial support of the EnPe-NORAD agreement between the University of the Witwatersrand (Wits) and the Norwegian University of Science and Technology, Trondheim (NTNU) for the CO 2 capture project. This sub-project was conducted as a 4th year Chemical Engineering research investigation, and forms part of a larger investigation at Wits. Furthermore, the authors would like the thank ABB, South Africa for making the CO2 gas analyser available for the project. The financial assistance of the South African Centre for Carbon Capture and Storage towards this research is hereby acknowledged. Opinions expressed and conclusions arrived at, are those of the author and are not necessarily to be attributed to SACCCS.

References [1] H.A.Shawky, A.H. El-Aassar, & D.E. Abo-Zeid. 2011 Chitosan/Carbon Nanotube Composite Beads: Preparation, Characterization, and Cost Evaluation for Mercury Removal from Wastewater of Some Industrial Cities in Egypt. Journal of Applied Polymer Science, 125, E93-. Retrieved from http://onlinelibrary.wiley.com/doi/10.1002/app.35628/pdf [2] Lee, M., & Park, S. (2015). Silica-Coated Multi-Walled Carbon Nanotubes Impregnated with polyethyleneimine for Carbon Dioxide Capture under Flue Gas Condition. Journal of Solid State Chemistry, 226(2015), 17-23 [3] Ngoy, J., Wagner, N., Riboldi, L., & Bolland, O. (2014). A CO2 capture technology using multi-walled carbon nanotubes. Energy Procedia, 2230 – 2248. [4] Su, F., Lu, C., & Chen, H.-S. (2011). Adsorption, Desorption, and Thermodynamic Studies of CO2 with High-Amine-Loaded Multiwalled Carbon Nanotubes. Langmuir, 27, 8090–8098. [5] Costa, S., Borowiak-Palen, E., Kruszyñska, M., Bachmatiuk, A., & Kalenczuk, R. J. (2008). Characterization of Carbon Nanotubes by Raman Spectroscopy. Poland: Centre of Knowledge Based Nanomaterials and Technologies, Institute of Chemical and Environment Engineering, Szczecin University of Technology. [6] Sethi, R., & Barron, A. R. (2009). Characterization of Single-Walled Carbon Nanotubes by Raman Spectroscopy. OpenStax-CNX. [7] V. Gupta & T.A.Saleh. (2011). Syntheses of Carbon Nanotube-Metal Oxides Composites; Adsorption and Photo-degradation, Carbon Nanotubes - From Research to Applications, Dr. Stefano Bianco (Ed.), InTech, DOI: 10.5772/18009. Available from: http://www.intechopen.com/books/carbon-nanotubes-from-research-to-applications/syntheses-of-carbon-nanotube-metal-oxides-compositesadsorption-and-photo-degradation

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