CHAPTER 4 |
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Capture of Carbon Dioxide by Modified Multiwalled Carbon Nanotubes
Chungsying Lu*, Bilen Wu*, Wenfa Chen*, Yu-Kuan Lin* and Hsunling Bai**
* Department of Environmental Engineering, National Chung Hsing University, 250 Kuo Kuang, Taichung, Taiwan
** Institute of Environmental Engineering, National Chiao Tung University, Hsinchu, Taiwan
Contents
3.1. Adsorption of CO2 onto Various Modified Adsorbents
3.2. Characterizations of Raw and Modified CNTs
3.7. Comparisons with Literature Results
1. INTRODUCTION
The greenhouse gas carbon dioxide (CO2) capture and storage (CCS) technologies from fossil fuel–fired power plant have received significant attention after the Kyoto Protocol came into force on February 16, 2005. Various CO2 capture technologies including absorption, adsorption, cryogenics, membranes, etc. have been investigated [1, 2]. Among these, the absorption–regeneration technology has been recognized as the most developed process so far, with the amine-based or ammonia-based absorption processes having received the greatest attention [3–5].
However, as the energy penalty of the absorption process is still too high, other technologies are being investigated throughout the world. The Intergovernmental Panel on Climate Change (IPCC) special report concluded that the design of a full-scale adsorption process might be feasible and the development of a new generation of materials that would efficiently adsorb CO2 will undoubtedly enhance the competitiveness of adsorptive separation in a flue gas application [6]. Possible adsorbents include activated carbon [7, 8], zeolites [9, 10], silica adsorbents [11, 12], single-walled carbon nanotubes [13], and nanoporous silica-based molecular basket [14, 15].
Carbon nanotubes (CNTs) are unique and one-dimensional macromolecules that have outstanding thermal and chemical stability [16]. These nanomaterials have been proven to possess good potential as superior adsorbents for removing many kinds of organic and inorganic pollutants in air streams [17–19] or in aqueous environments [20–22]. The large adsorption capacity of pollutants by CNTs is mainly attributable to their pore structure and the existence of a wide spectrum of surface functional groups, which can be achieved by chemical modification or thermal treatment to make CNTs that possess optimum performance for particular purposes. Thus, a chemical modification of CNTs would be also expected to have good potential for greenhouse gas CO2 capture. However, such studies are still limited in the literature.
This study investigates the physicochemical properties of raw and 3-aminopropyl-triethoxysilane (APTS)-modified CNTs and their adsorption properties of CO2. Effects of temperature and moisture on the CO2 adsorption in modified CNTs are also given.
2. MATERIALS AND METHODS
2.1. Adsorbents
Commercially available multiwalled CNTs with inner diameter less than 10 nm (L-type, Nanotech Port Co., Shenzhen, China) were selected as adsorbents in this study. The length of CNTs was in the range 5–15 µm and the amorphous carbon content in the CNTs was less than 5 wt%. These data were provided by the manufacturer.
Raw CNTs were thermally pretreated using an oven at 300 °C for 60 min. After thermal treatment, the CNTs were dispersed into flasks containing various kinds of chemical agents, including N-[3-(trimethoxysilyl)propyl]ethylenediamine (EDA), polyethylenimine (PEI; Acros Organics, analytical reagent, NJ, USA), and APTS (Riedel-de Häen, analytical reagent, Seelze, Germany), to determine the optimum modification of CNTs for enhancing CO2 capture. Literature screening indicates that these chemical agents have been used to modify carbon adsorbents [13], zeolites [14, 23], or silica adsorbents [12, 24, 25]. The mixture was then shaken in an ultrasonic cleaning bath (model D400H, Delta Instruments Co., USA) for 20 min and was refluxed at 100 °C for 20 h to remove metal catalysts. After cooling to room temperature, the mixture was filtered through a 0.45-µm mylon fiber filter, and the solid was washed with deionized water until the pH of the filtrate was 7. The filtered solid was then dried at 70 °C for 2 h.
2.2. Adsorption Experiment
The experimental setup for CO2 capture by CNTs is shown in Fig. 4.1. The adsorption column was made of Pyrex tube of length 20 cm and internal diameter 1.5 cm. The adsorption column was filled with 1.0 g of CNTs (packing height = 3.5 cm) and was placed within a temperature control box (model CH-502, Chin Hsin, Taipei, Taiwan) to maintain a temperature at 25 °C with the exception of temperature effect study, in which the temperature range 5–45 °C (in a 10 °C increment) was tested.
Figure 4.1 A diagram of the experimental setup.
Compressed air was passed through a silica-gel air dryer to remove moisture and oil and then was passed through a high-efficiency particulate air (HEPA) filter (Gelman Science, Ann Arbor, MI, USA) to remove particulates. The clean air was then served as a diluting gas and was mixed with pure CO2 gas that was obtained from a pure CO2 cylinder (99.9% purity) before entering the absorption column. The influent CO2 concentration and the system flow rate were controlled using mass flow controllers (model 247C four-channel readout and model 1179A, MKS instruments Inc., MA, USA). The mixed air was then passed downward into the adsorption column. The influent and effluent air streams were flowed into a gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) by an auto sampling system. The variations in the influent CO2 concentration were below 0.2%, and the system flow rate was controlled at 0.08 lpm that is equal to an empty-bed retention time of 4.6 s.
The relative humidity was kept at 0% with the exception of moisture effect study, in which the relative humidity range 0–100% was tested. Moisture was introduced into the air stream by dispersing the diluting gas through a water bath before being mixed with pure CO2 gas.
2.3. Adsorption Capacity
The amount of CO2 adsorbed on CNTs (q, mg/g) was calculated as
where m is the mass of virgin adsorbents (g), t is the contact time (min), Q is the influent flow rate (lpm), and Cin and Ceff are the influent and effluent CO2 concentrations (mg/l).
2.4. Analytical Methods
CO2 concentration in the air stream was determined using a GC-TCD (model GC-2010, Shimadzu Instruments, Japan). A 30-m fused silica capillary column with inner diameter 0.32 mm and film thickness 5.0 µm (AB-PLOT GasPro, USA) was used for CO2 analysis. The GC-TCD was operated at an injection temperature of 50 °C, a detector temperature of 100 °C, and an oven temperature of 55 °C.
The structural information of CNTs was evaluated by a Raman spectrometer (model Nanofinder 30 R., Tokyo Instruments Inc., Japan). The carbon content in CNTs was determined using a thermogravimetric analyzer (model TG209 F1 Iris, NETZSCH, Bavaria, Germany).
The physical properties of CNTs were determined by N2 adsorption at 77 K using ASAP 2010 surface area and porosimetry analyzer (Micromeritics Inc., Norcross, GA, USA). N2 adsorption isotherms were measured at a relative pressure range 0.0001–0.99. The adsorption data were then used to determine the surface area of CNTs using the Brunauer–Emmett–Teller (BET) equation, whereas the pore size distributions (PSDs) of CNTs were determined from the N2 adsorption data using the Barrett–Johner–Halenda (BJH) equation.
The chemical properties of CNTs were determined using a Fourier transform infrared spectroscopy equipped with an attenuated total reflectance (FTIR/ATR; model FTIR-SP-1 Spectrum One, Perkin Elmer, Japan).
3. RESULTS AND DISCUSSION
3.1. Adsorption of CO2 onto Various Modified Adsorbents
Figure 4.2 shows the qe of 10% CO2 with various modified CNTs. The qe increased after the CNTs were modified by EDA, PEI, and APTS solutions. Because these solutions are polymers with many amine groups, which can react with CO2 to form carbamate in the absence of water [15], and therefore the qe increases. The APTS-modified CNTs have greater enhancement in qe than the EDA- and PEI-modified CNTs. The raw and APTS-modified CNTs were thus selected as adsorbents to study their physicochemical characterizations and adsorption properties of CO2.
Figure 4.2 Equilibrium amount of 10% CO2 adsorbed on raw and various modified CNTs.
3.2. Characterizations of Raw and Modified CNTs
Figure 4.3 shows the adsorption and desorption isotherms of N2 onto raw and modified CNTs. It is apparent that the raw CNTs have a more adsorption capacity of N2 than the modified CNTs, indicating that a less amount of porosity within modified CNTs due to attachment of APTS solutions on the surface of modified CNTs. The adsorption isotherms for raw and modified CNTs is type IV, showing an increase in N2 adsorption capacity with increasing relative pressure. This reflects that the raw and modified CNTs have a broad pore size range.
Figure 4.4 shows the PSDs of raw and modified CNTs. It is noted that the PSDs of raw CNTs display a bimodal distribution, including a fine fraction (2–4 nm width range) and a coarse fraction (10–30 nm width range). The pores in the fine fraction are the CNT inner cavities, close to the inner CNT diameter, while the pores in the coarse fraction are likely to be contributed by aggregated pores that are formed within the confined space among the isolated CNTs. The PSDs of modified CNTs only show a coarse fraction in the 7–12 nm width range, and the pore volumes of CNTs become small after modification. The low detection limit for the pore size of employed BET analyzer is approximately 2 nm. Therefore, a quantification of the pore size of adsorbents less than 2 nm is not possible in this study.
Figure 4.3 Adsorption (solid line) and desorption (dash line) isotherms of N2 on raw and APTS-modified CNTs.
Figure 4.4 PSDs of raw and APTS-modified CNTs.
The physical properties of adsorbents are given in Table 4.1. The modified CNTs have smaller BET surface area and pore volume but larger average pore diameter than the raw adsorbents. Because the entrance of small pores of modified CNTs is covered with APTS solutions and thus leads to a smaller surface and pore volume but a larger average pore diameter. Most pore volumes of modified CNTs are in the 5–20 nm pore size range.
Figure 4.5 shows the Raman spectra of raw and modified CNTs. It is clear that there are two sharp peaks. The peak located between 1330 and 1360 cm−1 is the D band that is related to disordered sp2-hybridized carbon atoms of nanotubes containing vacancies, impurities, or other symmetry-breaking defects. The peak near 1580 cm−1 is the G band that is related to graphite E2g symmetry of the interlayer mode, reflecting structural integrity of sp2-hybridized carbon atoms of the nanotubes. Hence, the extent of carbon-containing defects of adsorbents can be evaluated by intensity ratios of D band to G band (ID/IG) [26]. The ID/IG ratios of raw and modified CNTs are 0.461 and 0.402, respectively. The ID/IG ratio of raw CNTs is slightly lower than that of modified CNTs, indicating that the CNTs possess less graphitized structures and more carbon-containing defects after modification.
Table 4.1 Physical properties of adsorbents
Figure 4.5 Raman spectra of raw and APTS-modified CNTs.
Figure 4.6 shows the TG curves of raw and modified CNTs. It is obvious that the TG curve of raw CNTs is considerably stable and shows a little weight loss close to 1% below 450 °C. After that, a significant weight loss begins and ends at 670 °C, in which 4.29% remaining weight was found. The modified CNTs have a broader temperature range for weight loss and exhibit three main weight loss regions. The first weight loss region (<550 °C) can be attributed to the loss of various kinds of surface functional groups. The rapid weight loss region (550–620 °C) can be attributed to the decomposition of CNTs. The third region only shows a very little weight loss close to 1%, in which 17.4% remaining weight was observed. These results showed that after modification, the carbon content in CNTs decreased from 95.71 to 82.6% due to attachment of APTS solutions on the surface of CNTs.
Figure 4.6 TG curves of raw and APTS-modified CNTs.
Figure 4.7 IR spectra of raw and APTS-modified CNTs.
Figure 4.7 shows the IR spectra of raw and modified CNTs. It is observed that the IR spectra of raw CNTs show no significant peaks. In contrast, the IR spectra of modified CNTs show several significant bands at 3674–3702, 3101–3305, 2900–2971, 1530–1560, 1240 and 1020–1081 cm−1, which are associated with bridge Si–OH acidic groups [27], asymmetric and symmetric NH2 stretching –OH, CH stretching from CH2CH2CH2–NH2 groups, NH2 deformation of hydrogen-bonded amino group, CH3 asymmetric deformation of Si–CH3, and Si–O–Si lattice vibrations [28], respectively. These functional groups are abundant on the surface of adsorbents, which can provide many chemical adsorption sites for CO2 capture.
3.3. Breakthrough Curves
Figure 4.8 shows the breakthrough curves of 50% CO2 adsorption on raw and modified CNTs. CO2 gas can be efficiently adsorbed on CNTs. The breakpoint time, which is defined as the time at which the breakthrough curve first begins to rise appreciably, and the breakthrough time of CO2 adsorption, respectively, are 1 and 2 min for the raw CNTs and 2 and 4 min for the modified CNTs. Both breakpoint time and breakthrough time become longer after modification by APTS solutions.
3.4. Adsorption Isotherms
Figure 4.9 shows the adsorption isotherms of 5–50% CO2 with raw and modified CNTs. It is seen that the equilibrium adsorption capacity (qe) increased with an increase in Cin and was greatly enhanced after the CNTs were modified by APTS solutions. With the 50% CO2 inlet, the qe of raw and modified CNTs is 69.2 and 96.3 mg/g, respectively, in which 30.1 mg/g enhancement in CO2 adsorption was obtained.
Figure 4.8 Breakthrough curves of CO2 with raw and APTS-modified CNTs.
Figure 4.9 Adsorption isotherms of CO2 with raw and APTS-modified CNTs.
3.5. Temperature Effect
Figure 4.10 shows the adsorption isotherms of 5–50% CO2 with modified CNTs at various temperatures. The qe decreased with an increase in temperature and increased qe was found at 5 °C, indicating the exothermic nature of adsorption process. As the temperature increased from 5 to 45 °C, the qe decreased from 101 to 80.9 mg/g. The adsorption isotherm curves at 5–25 °C are relatively close, reflecting that the effects of temperature change on the qe are less significant from 5 to 25 °C than from 25 to 45 °C.
3.6. Moisture Effect
Figure 4.11 shows the qe of 50% CO2 with modified CNTs at various relative humidities (RHs). It is apparent that the qe increased with an increase in relative humidity. There are two possible reasons to explain the increase in qe in the presence of moisture [15]. First, with the adsorption of water on the surface of modified CNTs, CO2 gas may dissolve into water. Second, the chemical interaction between CO2 and APTS may be further enhanced to form bicarbonate in the presence of water. As the RH increased from 0 to 100%, the qe increased from 96.3 to 100 mg/g.
Figure 4.10 Effect of temperature on the qe of APTS-modified CNTs.
Figure 4.11 Effect of relative humidity on the qe of APTS-modified CNTs.
3.7. Comparisons with Literature Results
The comparisons of qe with various APTS-modified adsorbents are given in Table 4.2. It is apparent that the qe was greatly enhanced after these adsorbents were modified by APTS solutions. The qe of APTS-modified CNTs is much higher than those of many types of APTS-modified silica adsorbents reported in the literature. These comparisons suggest that the APTS-modified CNTs are promising adsorbents for the capture of CO2 from air streams.
Table 4.2 Comparisons of CO2 adsorption performance with various APTS-modified adsorbents
4. CONCLUSIONS
The raw and APTS-modified CNTs were selected as adsorbents to study their characterizations and adsorption properties of CO2 from air streams. The physicochemical properties of CNTs were improved after modification, including the increase in defective structure and surface functional groups, which made CNTs adsorb more CO2. With the 50% CO2 inlet, the amount of adsorbed CO2 on raw and modified CNTs was 69.2 and 96.3 mg/g, respectively. The CO2 adsorption performance of modified CNTs decreased with an increase in temperature but increased with an increase in relative humidity. The modified CNTs possess higher CO2 adsorption capacity than many types of APTS-modified silica adsorbents reported in the literature, suggesting that the APTS-modified CNTs are promising adsorbents for the capture of CO2 from air streams.
ACKNOWLEDGMENT
This work was supported by the National Science Council, Taiwan, under a contract no. NSC95-2623-7-009-014.
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