Multi-walled carbon nanotubes modified with (3-aminopropyl)triethoxysilane for effective carbon dioxide adsorption

International Journal of Greenhouse Gas Control 14 (2013) 65–73

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International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc

Multi-walled carbon nanotubes modified with (3-aminopropyl)triethoxysilane for effective carbon dioxide adsorption Meei Mei Gui a , Yan Xin Yap a , Siang-Piao Chai a,∗ , Abdul Rahman Mohamed b a b

Low Carbon Economy (LCE) Group, Chemical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon Selatan, 46150 Bandar Sunway, Selangor, Malaysia Low Carbon Economy (LCE) Group, School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia

a r t i c l e

i n f o

Article history: Received 12 March 2012 Received in revised form 19 December 2012 Accepted 4 January 2013 Available online 5 February 2013 Keywords: MWCNTs Amine functionalization Adsorbents CO2 adsorption

a b s t r a c t Multi-walled carbon nanotubes (MWCNTs) were modified with (3-aminopropyl)triethoxysilane (APTES) solution in a two-step process. In the first step, the MWCNTs were pre-treated with sulfuric acid and nitric acid (5 molar each mixed at ratio of 3:1, v/v) with the aims to remove metal catalysts impurities and to introduce carboxyl groups on the MWCNT surface. In the second step, the acid pre-treated MWCNTs were functionalized with APTES at 105 ◦ C under various reflux durations. The characteristics of the functionalized MWCNTs were studied by Fourier transform infrared (FT-IR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and Raman spectroscopy. TG analysis shows an increase in APTES loading with increasing reflux duration, giving the maximum loading of 13.75 wt%. This result suggests that as reflux duration increased, more amine groups were attached covalently on the MWCNT surface, forming effective mechanism sites for CO2 adsorption. The highest CO2 uptake of 75.4 mg CO2 adsorbed/g adsorbent was achieved by the amine-functionalized MWCNTs, indicating its superior performance than some other commonly used adsorbents such as SBA-15. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Greenhouse gas (GHG) emission has been dramatically increased within the last 50 years and the rapid acceleration emissions have caused significant impact to the global warming. GHGs mainly consist of 72% of carbon dioxide (CO2 ), 18% of methane and 9% of nitrous oxide (NOx ). Emissions of CO2 are inevitably generated from fossil fuel burning, human activity, rapid development and increase in energy consumption due to escalating growth in population. Owing to these reasons, low carbon economy which involves development of technologies with minimal output of GHG emissions into the environment biosphere has become the main priority to be achieved in the recent growth of global climate awareness. To make CO2 reduction economically feasible, carbon capture and storage has emerged as the best alternative yet essential mitigation strategy for environmental beneficial change (Chunshan, 2006). One of the major priorities for carbon capture and storage is to develop the efficient and cost-effective CO2 capture techniques such as absorption, adsorption and membrane separation (Aaron and Tsouris, 2005; Chunshan, 2006; Davison and Thambimuthu, 2005).

∗ Corresponding author. Tel.: +60 3 55146234; fax: +60 3 55146207. E-mail address: [email protected] (S.-P. Chai). 1750-5836/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijggc.2013.01.004

Carbonaceous materials such as activated carbon, carbon nanotubes (CNTs) and carbon fibre composites have emerged as promising adsorbents due to their good affinity towards CO2 (Lu et al., 2008; Siriwardane et al., 2001). Among many potential adsorbents, CNTs were investigated in this research project attributed to their large surface area, porous structure and excellent thermal stability. Fullerene-related CNTs were first reported via arc-evaporation apparatus by Sumio Iijima back in 1991 (Sumio, 1993). CNTs are nanomaterials in tubular form which made of rolled-up graphene sheets with amazing mechanical properties (Korneva, 2008). These one-dimensional carbon allotropes are of high surface area, high mechanical strength but ultra-light weight, rich electronic properties, and excellent chemical and thermal stability (Ajayan, 1999; Yook et al., 2010). CNTs have been receiving great attention in exploring their potential in chemical, biological and biomedical applications due to their unique structural, electronic, optical and mechanical properties (Amiri et al., 2011). The remarkable physical and chemical properties of CNTs have led to various applications in nanoelectronic devices, polymer composites (Dalton et al., 2000; Ramanathan et al., 2005), chemical sensors or biosensors (Rumiche et al., 2012; Tulliani et al., 2011; Xie et al., 2012) and many more. The major limitation to employ CNTs for the applications restricted to their strong chemical stability behaviour. The nature of CNTs having chemically stable surface causes the difficulty for functional groups to attach on. Thus, CNTs are usually functionalized

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with organic functional groups to improve their solubility and dispersion before they are being used (Harris, 2009). Covalent functionalization schemes allow persistent alteration of the electronic properties of the tubes, as well as to chemically tailor their surface properties (Balasubramanian and Burghard, 2005). In recent years, there are also raising interests in utilizing CNTs for CO2 adsorption due to their reversible nature where adsorption of CO2 can be reversed via desorption with increasing temperature (Schobert et al., 2000). Amine-functionalized CNTs are potential adsorbent for capturing CO2 . Amine groups can react with CO2 to form carbamate ion and bicarbonate ion (Gray et al., 2005; Lu et al., 2008; Su et al., 2010; Xu et al., 2005). CNT is a suitable support of adsorbents because of its high surface area and chemically stable property, which can minimize the possibility of denature of the adsorbents during the CO2 adsorption process. Various methods and different organic functional groups used in the functionalization of CNTs have been discussed (Amiri et al., 2011; Lu et al., 2008, 2010; Murugesan et al., 2011; Ramanathan et al., 2005; Stevens et al., 2003; Su et al., 2009; Wang et al., 2005). The recent development in methodology to functionalize MWCNTs has enlightened the exciting applications and potential improvement in solubility and dispersion of the CNTs. Functionalization of CNTs can be done by attaching certain bioactive species and organic functional groups physically or chemically on the sidewalls of CNTs without modifying the desirable properties of CNTs significantly (Balasubramanian and Burghard, 2005). Many methodologies on modification of CNTs through covalent and non-covalent functionalization can be employed on the surface of CNTs to overcome the hydrophobic nature of carbon. In covalent functionalization, functional groups can be introduced at the bent parts, the part with defects or on the sidewall of CNTs structure. According to Yang et al. (2007), sidewall modification methods can minimize damage to the CNT structure and at the same time it allows incorporation of other reactive groups. Meanwhile, non-covalent functionalization can be done by employing relatively small molecules which contain planar groups that irreversibly adsorb to the nanotube surfaces by ␲-stacking forces (Chen et al., 2002; Dalton et al., 2000; O‘Connell et al., 2001; Star et al., 2001). Non-covalent functionalization includes ultrasonication, addition of surfactants, polymer wrapping, etc. These approaches neither affect the electronic structures of CNTs nor produce stable functionalized nanotubes (Korneva, 2008; Yang et al., 2007). Functionalization of CNTs with amine functional groups can be performed through carboxylation, acylation and amidation. By firstly introducing the carboxylic acid groups, a series of chemical reactions can then be introduced along the interface. For example, the formation of peptide bonds via reaction between carboxyl and amine groups in the amine functionalization process (Li et al., 2005). Pillai et al. (2010) described a two-step chemical functionalization of MWCNTs involving carboxylation and amidation with hexadecyl amine (HDA). The amine content in MWCNTs was successfully controlled by varying the reaction time of the amidation step. Mild acid was selected for the oxidation process to keep the MWCNT structure intact. Amine functional precursors such as ethylenediamine (EDA), (3-aminopropyl)triethoxysilane (APTES) and polyethylenimine (PEI) were frequently reported as effective amine precursors (Gil et al., 2011; Huang et al., 2002; Mergler et al., 2011; Yan et al., 2011). Damian et al. (2010) reported on EDA functionalization with carboxylated MWCNTs (MWCNTs COOH) in their recent publication. In a three-step approach proposed by this research group, carboxyl group ( COOH) was first introduced on the MWCNT surface by reflux in an acid solution mixtures comprising sulfuric acid and nitric acid, followed by acylation, which can be done by adding thionyl chloride (SOCl2 ) to the MWCNTs COOH

to obtain MWCNTs COCl, and lastly the amidation process which was carried out with the addition of EDA (Damian et al., 2010). Further experimental works on the modification of CNTs specified for CO2 capture have been reported by researchers using different types of amine precursors, including APTES, PEI and EDA with methanol, toluene or acetone being used as solvent (Hsu et al., 2010; Lu et al., 2008, 2010; Su et al., 2009). The chemical functionalization of pristine CNTs without losing the structural integrity yet still achieving good dispersion in liquid matrix requires extensive research and comprehensive development. Nevertheless, there is growing interest in the application of CNTs for CO2 capture due to the unique characteristics exhibited by CNTs. However, there are yet limited research work reported on the effect of functionalization conditions on the quality of the adsorbents developed. This research work was conducted with the aim at developing CO2 adsorbents based on MWCNTs through amine functionalization under various reflux conditions. In this study, the adsorbents were developed from a two-step amine functionalization process: (i) pre-treatment of MWCNTs with a mixture of nitric acid and sulfuric acid; (ii) amine functionalization with APTES precursor. The effects of acid pre-treatment and the process of functionalization on the quality of the adsorbents were investigated from the aspects of physical and chemical characteristics as well as the CO2 adsorption capacity over the developed adsorbents. 2. Materials and methodology 2.1. Materials and chemicals The MWCNTs used in this work were produced from Co Mo/MgO catalyst using a rotary reactor with the carbon purity of around 66.82%. Sulfuric acid (Riendemanna Schmidt Chemical, 95–97%) and nitric acid (R&M Chemicals, 70%) were used for the pre-treatment of MWCNTs prior to functionalization. For the functionalization of MWCNTs with amine groups, (3-aminopropyl)triethoxysilane (APTES) (purity: >98%, Sigma–Aldrich) was used as the functional precursor whereas toluene (95% purity, PC Laboratory Reagent) was used as solvent in the process. 2.2. Pre-treatment and amine functionalization of MWCNTs Acid pre-treatment of MWCNTs was performed to achieve two aims: (i) removal of metal catalyst particles from the pristine MWCNTs and (ii) carboxylation of the MWCNTs to introduce carboxyl groups on the MWCNT surface prior to amine functionalization. 500 mg of pristine MWCNTs was stirred with a mixture of HNO3 and H2 SO4 (1:3, v/v) at the concentration of 5 molar each, temperature of 100 ◦ C for 3 h under a reflux condition. The amine functionalization was then carried out on the pretreated MWCNTs using APTES. The pre-treated MWCNTs were continuously stirred with APTES and toluene (APTES: toluene of 1:9, v/v) at 105 ◦ C under various reflux durations (1, 5, and 10 h). The mixture was then filtered in a vacuum filtration unit with nylon membrane filters of pore size 0.2 ␮m (Whatman Cat. No 7402004) and washed repeatedly with acetone to remove any APTES remnant. The solid sample was collected and dried in oven at a temperature of 105 ◦ C for 6 h to remove the excess solvent from the solid samples. The synthesized adsorbents were then characterized for their physical and chemical properties. Amine functionalization was also conducted using pristine MWCNTs without acid pretreatment to investigate the effect of acid pre-treatment on the amine functionalization process. The performance of the adsorbents synthesized in this work was then investigated by CO2 adsorption in a gas adsorption test rig.

M.M. Gui et al. / International Journal of Greenhouse Gas Control 14 (2013) 65–73

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Fig. 1. Schematic diagram of CO2 adsorption test rig.

2.3. CO2 adsorption test CO2 adsorption was conducted in a custom-fabricated CO2 adsorption test rig as shown in Fig. 1. The synthesized adsorbent was packed into the quartz tube adsorption column with glass wool. The temperature of the adsorption process was controlled by a low temperature furnace coupled with a temperature controller system. Pure CO2 with N2 as carrier gas was used as CO2 feedstock. The CO2 adsorption test was carried out with 0.3 g of adsorbent at the temperature of 60 ◦ C and CO2 N2 gas flowrate of 200 ml/min (CO2 :N2 of 5:100, v/v). The gas outlet from the downstream of the adsorption column was continuously collected using gas sampling bag and then transferred to Gas Chromatography (GC) (Agilent 7890A) equipped with thermal conductivity detector (TCD) for analysis. The concentration of CO2 in the gas samples was determined and CO2 breakthrough curve was plotted. The adsorption tests were repeated three times to ensure consistency of the results obtained. The performance of the adsorbents was then evaluated by calculating the total CO2 uptake capacity which includes physisorption and chemisorptions for the individual adsorbent (q, mg/g) using Eq. (1).

q=

1 m



t

Q × (Cin − Ceff )dt 0

(1)

where m is the mass of adsorbents used in the adsorption study (g), t is the contact time (min), Q is the influent flow rate (l/min), Cin and Ceff are the influent and effluent CO2 concentrations (mg/l), respectively (Hiyoshi et al., 2005; Lu et al., 2008; Su et al., 2009, 2011; Su et al., 2010; Yan et al., 2011). 2.4. Characterizations of the adsorbents The characteristics of the developed adsorbents were studied with Fourier transformed infra-red (FT-IR), themogravimetric analysis (TGA), scanning electron microscopy (SEM) and Raman spectroscopy. FT-IR spectra were obtained from Nicolet iS10 FTIR spectrometer using Kbr Die Model 129 to study the types of functional groups attached on MWCNTs after the amine functionalization. Meanwhile, the quantitative analysis for the functional groups was performed using TGA (TA Q50) at a ramping rate of 10 ◦ C/min from ambient temperature to 800 ◦ C in nitrogen flow. Surface morphology of the adsorbents was characterized with FESEM (Hitachi, SU8000). Besides, Raman spectroscopy (Renishaw inVia Raman microscope) was conducted to study the bonding, structure and properties of carbon-based compounds based on the inelastic scattering of monochromatic light. The surface area of the adsorbents were characterized by N2 adsorption at −196 ◦ C using an adsorption apparatus (Micromeritics, ASAP 2010 V3.04E), the surface area was determined from the Brunauer–Emmett–Teller (BET) equation from the total amount of N2 adsorbed.

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(a) Weight Percent (wt %)

3448

% Transmittance

(b)

(a)

100

95

90

85

80 4000

3500

3000

2500

2000

1500

1000

0

500

Fig. 2. FT-IR spectra of (a) pristine MWCNTs and (b) acid pre-treated MWCNTs.

3. Results and discussion 3.1. Adsorbents characterizations i. FT-IR analysis FT-IR analysis of the adsorbents developed from various functionalization conditions was carried out to investigate the types of functional groups attached on the surface of the MWCNTs. The effect of acid pre-treatment on the MWCNTs was first investigated using FT-IR analysis. Fig. 2 compares the FT-IR spectra of pristine MWCNTs and acid pre-treated MWCNTs. From the FT-IR spectra, one can observe that there are several changes of the % transmittance at the wavenumbers of 3448 cm−1 and 1710–1680 cm−1 . Peak observed at 1380 cm−1 can be explained by H C O bend frequency meanwhile the broadening of the 3448 cm−1 peak shows the O H contribution from the carboxylic functional group (Murugesan et al., 2011). In addition, peaks observed at 1710–1680 cm−1 indicate the presence of COO groups in the structure of MWCNTs which were formed from the oxidation by H2 SO4 and HNO3 in the acid pre-treatment process. The FT-IR result indicates that the introduction of the carboxylic groups could be done via oxidation with H2 SO4 and HNO3 mixture which may then create defects on the hexagonal or pentagonal structures of the graphene sheet which is believed would ease the amine functionalization (Damian et al., 2010). FT-IR analysis was also conducted on the adsorbents developed from amine functionalization at the reflux durations of 1, 5 and 10 h as to investigate the types of functional groups introduced. The adsorbents were analyzed with FT-IR and the spectra obtained are shown in Fig. 3. From the FT-IR spectra, 3448

1565 1443 1120

% Transmittance

(d)

(c) (b)

(a)

4000

3500

3000

2500 2000 1500 Wavenumbers (cm-1)

1000

500

Fig. 3. FT-IR spectra of (a) acid pre-treated MWCNTs and amine-functionalized MWCNTs under reflux durations of (b) 1 h, (c) 5 h, and (d) 10 h.

(b)

100

Weight Percent (wt %)

Wavenumbers (cm-1)

95

200

400 Temperature (oC)

600

800

2.5wt% moisture content 2.5wt% volatile compounds

10.5wt% APTES

90

85

80 0

200 400 600 Temperature (oC)

800

Fig. 4. TGA spectra of (a) acid-pretreated MWCNTs and (b) amine-functionalized MWCNTs under reflux duration of 1 h.

several additional peaks were observed at the wavenumbers of 3448, 1565, and 1120 cm−1 . The trends for all three spectra are nearly identical and the intensity of the transmittance of almost all peaks is observed to be more significant with increasing reflux duration. Peaks at 3448 cm−1 observed in all three spectra are expected to be O H stretching caused by the atmospheric moisture. The peak at 1120 cm−1 indicates the presence of C NH2 primary amines groups due to the physical deposition of APTES on MWCNT surface. Meanwhile, N H from the secondary amine group corresponding to the peak at ca. 1565 cm−1 was also observed in all the spectra. The presence of C NH2 and N H groups confirmed the attachment of amine group on the external surface of MWCNTs and the corresponding peaks appeared to be more significant in the adsorbents developed with the reflux for 5 and 10 h. Nevertheless, the peak at ca. 1443 cm−1 , corresponding to amide group ( CO NHR), was also observed in the FT-IR spectra for the adsorbents obtained from the functionalization of 5 and 10 h (Fig. 3c and d). The presence of amide group confirmed the formation of peptide bond between the MWCNTs and the amine group from APTES after the amine functionalization. From the FT-IR spectra, one can know that the functionalization can be improved by increasing the reflux duration, in which significant difference in the FT-IR spectrum was observed from the samples synthesized from 10 h of reflux. The adsorbents were also analyzed by TGA to further investigate the thermal characteristics of the developed adsorbents. ii. TG analysis The quantitative value of the functional groups was identified from the TGA. Pristine MWCNTs are expected to consist of amorphous carbon, moisture, metal particles and volatile carbonaceous impurities. Fig. 4 compares the TGA spectra of the

M.M. Gui et al. / International Journal of Greenhouse Gas Control 14 (2013) 65–73 Table 1 TGA results of the developed adsorbents in various functionalization duration.

APTES 1 h APTES 5 h APTES 10 h

Moisture content (wt%)

Volatile compounds (wt%)

APTES loading (wt%)

2.50 3.75 3.75

2.50 3.75 3.75

10.50 12.50 13.75

acid pre-treated MWCNTs and adsorbent prepared from 5 h of reflux. As observed from the TGA spectra in Fig. 4, there were no functional groups detected in the acid pre-treated MWCNTs (Fig. 4a). On the other hand, the weight loss for the adsorbent developed from 1 h of reflux (Fig. 4b) was observed and it can be generally categorized into three regions. Approximately 2.5 wt% of weight loss observed in the first region (100–200 ◦ C) corresponded to the moisture loss. The second weight loss (2.5 wt%) observed in the temperature range of 200–400 ◦ C was attributed to the decomposition of volatile compounds such as APTES that physically deposited on the surface of MWCNTs. The major weight loss region was observed in the temperature range of 400–800 ◦ C. This weight loss was due to the decomposition of APTES that attached via covalent bonding. The actual APTES loading can be calculated from the percent weight loss from the TGA spectra. It was estimated that about 10.5 wt% of APTES was introduced on the MWCNTs for 1 h of reflux. Table 1 summarizes the TGA results of the adsorbents obtained from the reflux durations of 1, 5, and 10 h. The amount of ATPES was found to increase proportionally to 12.5 and 13.75 wt% for the adsorbents prepared from the reflux durations of 5 and 10 h, respectively. From this observation, one can conclude that the reflux duration is a positive manipulated response in which the loading of amine groups increased with increasing the reflux duration. TGA was also conducted on the adsorbents obtained from the amine functionalization using the pristine MWCNTs with and without acid pre-treatment for 10 h of reflux (Fig. 5). As we can observe from the TGA spectra, the APTES loading on the pristine MWCNTs was approximately 10 wt%, which is only equivalent to the ATPES loading on the adsorbent obtained from 1 h of reflux using the acid pre-treated MWCNTs (Fig. 4b). In addition, the TGA spectra showed that the APTES loaded on the MWCNTs without acid pre-treatment exhibited lower thermal

100

Weight Percent (wt %)

Sample name

69

95

90

85 with acid pretreatment

80

without acid pretreatment

75 0

200

400 600 Temperature (oC)

800

1000

Fig. 5. Comparisons between the adsorbents obtained from amine functionalization for 10 h with and without acid pre-treatment.

stability as compared to that of the acid pre-treated MWCNTs. As shown in Fig. 5, the APTES loaded on the MWCNTs without acid pre-treatment started to decompose at the temperature of approximately 330 ◦ C, meanwhile the decomposition temperature of the APTES on the acid pre-treated MWCNTs was at approximately 400 ◦ C. This observation suggests a relatively stronger bonding of amine groups (contained in APTES) with carboxyl groups pre-introduced on the MWCNTs during the acid pre-treatment stage. Summarizing the results obtained from the FT-IR and TGA as discussed above, the overall process for the two-step acid pretreatment and amine functionalization can be represented by a schematic molecular structure as shown in Fig. 6. In the proposed schematic in Fig. 6b, pristine MWCNTs were firstly treated with H2 SO4 and HNO3 , carboxyl groups were introduced on the defect sites of the MWCNTs from the acid oxidation. In the second stage of functionalization with APTES, peptide bond was formed as a result of carboxyl group reacted with the amine group from the APTES, and thus grafting the APTES on the MWCNTs. iii. SEM analysis

Fig. 6. (a) Molecular structure of APTES and (b) schematic representation of the two-step amine functionalization process.

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Fig. 7. SEM images of (a) acid pre-treated MWCNTs, amine-functionalized MWCNTs from the reflux for 5 h (b and c) and 10 h (d and e) at both low magnification and high magnification scans.

SEM characterization was carried out for the aminefunctionalized MWCNTs after 5 and 10 h of reflux. Fig. 7 shows the SEM images of acid pre-treated MWCNTs (a) and MWCNTs after the functionalization for 5 h (b and c) and 10 h (d and e) at low and high magnification scans, respectively. Fig. 7a shows a clean MWCNT surface resulted from the acid pre-treatment process. The SEM images show that the adsorbent developed from the amine functionalization for 5 h (Fig. 7b and c) contained APTES with more uniform distribution on the MWCNT surface as compared to the adsorbent obtained from 10 h of reflux (Fig. 7d and e). Referring to Fig. 7c, high magnification

image for the adsorbent obtained from 5 h of functionalization shows that the APTES was quite evenly distributed along the nanotubes. Such distribution of APTES is desired as to achieve high effective surface area of amine for the adsorption of CO2 . On the other hand, we can observe from Fig. 7d and e that agglomeration of the APTES occurred after pro-longed reflux for 10 h. The agglomeration of the APTES is unfavoured as it can lead to the reduction of the effective surface area for CO2 adsorption. This observation readily indicates that 5 h of reflux is the most appropriate condition for the functionalization of amine on the MWCNTs.

M.M. Gui et al. / International Journal of Greenhouse Gas Control 14 (2013) 65–73

71

1.0 Pristine MWCNTs

(b)

Amine Functionalized MWCNTs

0.8

C/C0

Intensity (a.u)

(a)

(b)

0.6

0.4 APTES_1hr APTES_5hrs

(a) 1000

0.2 1200

1400

1600

1800

APTES_10hrs Pristine MWCNTs

2000

Raman Shift (cm-1)

0.0 0

Fig. 8. Raman spectra of pristine MWCNTs and amine-functionalized MWCNTs.

The influence of the amine distribution on the effective surface area of the adsorbents can be further studied with the surface area analysis. BET analysis on the acid pre-treated MWCNTs and the developed adsorbents reveals that the surface area of the acid pre-treated MWCNTs decreased from 180.19 to 146.30 and 128.12 m2 /g after 5 and 10 h of functionalization, respectively. BET surface area analysis indicates a significant reduce in the surface area with increasing functionalization duration. The reduction in the surface area might be due to the attachment of APTES that led to a denser network of the MWCNTs. The result is correspondence with the SEM images, revealing the agglomeration of APTES after 10 h of functionalization. iv. Raman spectroscopy Raman spectroscopy was conducted on the pristine MWCNTs and amine-functionalized MWCNTs. Fig. 8 shows the Raman spectra of the pristine MWCNTs and amine-functionalized MWCNTs prepared from 10 h of reflux. Two sharp peaks were observed at ∼1330 and ∼1580 cm−1 . The peak at ∼1330 cm−1 is the D-band which is related to disordered sp2 -hybridized carbon atoms of nanotubes while the peak at ∼1580 cm−1 is corresponding to the graphitic structure of the interlayer mode of nanotubes. For the pristine MWCNTs, both G- and D-bands were observed to have an almost similar intensity height. The ID /IG ratio for the amine-functionalized MWCNTs was found to increase from 1.032 for pristine MWCNTs to 1.353 after the amine functionalization, explaining that the aminefunctionalized MWCNTs possessed more carbon-containing defects and is less graphitized as compared to the pristine MWCNTs Table 2.

2

4

6

8 10 Time (min)

14

16

Fig. 9. Breakthrough curves for the CO2 adsorption test.

enabled the adsorption of CO2 via physisorption (by the MWCNTs) and chemisorption (by the amine groups). The chemisorption of CO2 occurred through the formation of carbamate from the reaction between the secondary amine groups (R NH R) in the APTES with the CO2 molecules, as shown in Eq. (2); meanwhile, the CO2 adsorption mechanism is illustrated in Fig. 10. It was suggested that CO2 would form weak chemical bonding with secondary amine groups, subsequently lead to the formation of carbamate zwitterions (Pinto et al., 2011; Sayari et al., 2012; Su et al., 2011; Yu et al., 2012). CO2 + 2RNH ↔ 2RNCOOH → RNCOO−+ NH2 R

3.2. CO2 adsorption studies The CO2 adsorption performance of the developed adsorbents was investigated. The downstream gas from the adsorption column was collected and the CO2 concentration was analyzed and calculated from the GC. The adsorption capacity of pristine MWCNTs was examined as well for the reason of comparison. The performance of CO2 adsorption over the adsorbents developed from amine functionalization for 1, 5 and 10 h was evaluated from CO2 breakthrough curves as shown in Fig. 9. Besides, the adsorption capacity of each adsorbent was calculated using Eq. (1) and the finding is summarized in Table 2. From the breakthrough curves, it can be seen that the pristine MWCNTs exhibited weak CO2 adsorption. The CO2 adsorption capacity was found greatly improved after amine functionalization with APTES, suggesting a better adsorption of CO2 with the presence of amine groups on the MWCNTs surface, which

12

Fig. 10. CO2 adsorption mechanism.

(2)

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Table 2 Comparison of CO2 uptake for the amine-functionalized MWCNTs and other APTES-modified adsorbents. Reference

Adsorbents

Functionalized chemicals

q (mg CO2 /g adsorbent)

Conditions

This study Chang et al. (2003) Huang et al. (2002) Zelenak et al. (2008) Su et al. (2011) Lu et al. (2008)

MWCNTs SBA-15 Silica xerogel SBA-15 MWCNTS MWCNTS

APTES APTES APTES APTES APTES APTES

75.4 17.6 25.0 66.0 40.0 25.0

Cin :5%, T:60 ◦ C Cin :4%, T:25 ◦ C Cin :5%, T:25 ◦ C Cin :10%, T:25 ◦ C Cin :5%, T:50 ◦ C Cin :5%, T:25 ◦ C

From the adsorption studies, it was found that the maximum CO2 uptake of 75.4 mg CO2 /g adsorbent was achieved in the present work. This finding suggests that the amine-functionalized MWCNTs at the reflux duration of 5 h emerged to be a potential adsorbent for CO2 . Although adsorbent developed from 10 h of reflux contained the highest loading of amine functional groups, the CO2 uptake capacity of this adsorbent was found to be slightly lower than that of the adsorbent developed from the amine functionalization for 5 h. The deviation in result might be due to the agglomeration of the APTES on the surface of MWCNTs that reduced the effective surface area for CO2 adsorption as evidenced by the SEM and BET surface area analysis as discussed in the previous section. Table 2 shows the CO2 uptake for the adsorbents developed in this study as compared with other APTES-modified adsorbents reported in selected journal articles. It was found that the total CO2 uptake of amine-functionalized MWCNTs is superior as compared to other types of adsorbents such as SBA-15.

4. Conclusions Effective adsorbents for CO2 capture were developed from amine functionalization of MWCNTs with APTES in a two-step process; i.e. acid pre-treatment with H2 SO4 and HNO3 , followed by amine functionalization with APTES. During the acid pre-treatment process, metal catalysts in the pristine MWCNTs were removed and COO group was introduced to the MWCNT surface. The presence of COO groups on the MWCNTs surface was the important sites for the amine groups to attach on in the subsequent covalent functionalization process. In the reflux at 105 ◦ C, peptide bond ( CONH ) was formed from the reaction between COO and the amine groups available in the APTES precursor. TGA spectra show that the peptide bond is a strong covalent bonding which could only be decomposed at a temperature of approximately 400 ◦ C, which has made the amine-functionalized MWCNTs a high thermally stable adsorbent. Besides, the amine loading was found to increase with the increase in the reflux duration. A maximum APTES loading of 13.75 wt% was achieved for 10 h of reflux. CO2 adsorption test for the amine-functionalized MWCNTs shows a superior performance, giving the highest CO2 uptake of 75.4 mg CO2 adsorbed/g adsorbent.

Acknowledgements The authors would like to thank the funding provided by the Ministry of Higher Education Malaysia for the Long-term Research Grant Scheme (account number: 2110226-113-00) and Fundamental Research Grant Scheme (reference number: FRGS/2/2010/TK/MUSM/03/4), Monash University for the Monash Internal Seed Fund (account number: E-4-11), and Chemical and Sustainable Process Engineering (CSPE) research group, Monash University Sunway Campus, for the equipment and facilities support.

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