Article pubs.acs.org/EF
Adsorption of Carbon Dioxide on 3‑Aminopropyl-Triethoxysilane Modified Graphite Oxide Seok-Min Hong, Sung Hyun Kim, and Ki Bong Lee* Department of Chemical and Biological Engineering, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, South Korea S Supporting Information *
ABSTRACT: An awareness of the seriousness and symptoms of global warming is encouraging the development of technologies designed to reduce the emissions of CO2, a representative greenhouse gas, which contributes to global warming. The reduction of CO2 emissions has become a major environmental concern, and various methods such as absorption, adsorption, and membrane separation have been employed in CO2 capture. In recent times, technology designed to capture CO2 by adsorption has received considerable attention due to its low energy consumption and easy regenerability. In this study, 3-aminopropyl-triethoxysilane modified graphite oxide (Gr-APTS) was synthesized via the functionalization of graphite oxide (GO) with amine molecules for the development of a new CO2 adsorbent. GO was prepared to hold extensive oxygen-containing functionalities through oxidizing graphite with highly concentrated acid. Then, the amine molecules were grafted onto the surface of the GO, which provided an enhancement of the CO2 affinity and increased the CO2 adsorption uptake at a temperature of 30 °C and a pressure of ∼1 atm. Gr-APTS also showed a highly stable CO2 adsorption uptake in the adsorption/desorption cycles.
1. INTRODUCTION Carbon dioxide (CO2) emissions are considered to be a significant environmental issue, as they are known to be a major contributor to abnormal climate change. Because fossil fuels are still in high demand as a crucial energy source, the emission of CO2 from fossil fuel combustion is inevitable, and it is therefore essential to design technologies to capture CO2.1,2 Widely applied approaches used for the capture of CO2 are absorption by aqueous solutions,3 adsorption by solid adsorbents,4,5 and membrane separation.6 Among these various technologies, adsorption has the advantage of low energy consumption and easy regeneration, without producing undesirable byproducts or polluted sorbents.4 For commercial adsorbents, activated carbon, zeolite, metal oxides, and mesoporous silica have been used in the CO2 separation processes.4,7−11 Zeolite is a highly ordered microporous crystalline material containing negative framework charges compensated with exchangeable cations, and these structural characteristics enable zeolite to adsorb CO2. Activated carbon is a well-known adsorbent with good regenerability and large diversity in pore size distribution, pore structure, and active surface area.4 Other carbon-based adsorbents, such as carbon molecular sieve,12,13 carbon nanotube,14 and carbon fiber,15,16 have also been studied as promising CO2 adsorbents due to their excellent chemical inertness and thermal stability.17 In order to enhance the CO2 adsorption performance, various functional groups can be attached to the adsorbents by surface modification. This chemical functionalization influences the physicochemical properties of the adsorbent surface, which affects the adsorption behavior.18 Recently, there were studies on amine modification of carbon-based adsorbents such as activated carbon, carbon molecular sieve, and carbon nanotube for enhancement of CO2 adsorption performance.19−21 Maroto-Valer et al. (2008) introduced different amine-enriched © XXXX American Chemical Society
activated carbons and showed that impregnation with amine molecules enhanced the CO2 adsorption capacity.19 Activated carbon impregnated with monoethanolamine had a 64% higher adsorption capacity than untreated adsorbents at a temperature of 30 °C and a pressure of ∼1 atm with a CO2 concentration of 99.8 vol %. Ye et al. (2012) synthesized tetraethylenepentamine (TEPA) modified carbon nanotubes and achieved a CO2 adsorption uptake six times higher than that of untreated carbon nanotubes, under 2.0 vol % of CO2 at 25 °C and atmospheric pressure.21 They also showed that the adsorption uptake increased with an increase in the TEPA loading amount. Graphite has not only properties similar to the abovementioned carboneous adsorbents but also the economic advantage of low cost, and it can therefore be considered as a potential CO2 adsorbent. However, graphite has a weak affinity for CO2 and a low surface area, which limit the CO2 adsorption ability.22 To overcome these limitations, the surface of graphite was modified with amine groups in this study to increase the basicity, which is favorable for adsorption of acidic CO2. 3aminopropyl-triethoxysilane was chosen as an amine functional modifier because it has been widely used in chemical functionalization on porous supports and showed potential to enhance CO2 adsorption.4 To synthesize amine-modified graphite oxide, graphite was first oxidized using concentrated acid and then grafted using 3-aminopropyl-triethoxysilane. In comparison with impregnation, grafting has the advantage of enabling cyclic stability for functional groups and a high adsorptive efficiency, due to the well-distributed amine molecules on the surface.23,24 The CO2 adsorption uptake was measured using thermogravimetric analysis. The structure, morphology, and physical properties of the 3-aminopropylReceived: March 16, 2013 Revised: May 11, 2013
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triethoxysilane modified graphite oxide were characterized using X-ray diffraction, Raman spectroscopy, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), element analysis, and nitrogen physisorption analysis.
2. EXPERIMENTAL SECTION 2.1. Materials. Graphite (particle size 98%), sodium chlorate (NaClO3, > 99%), and 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide methiodide (EDC) were purchased from Sigma-Aldrich. Fuming nitric acid (HNO3, 93%) was provided by Matsunden Chemicals. 2.2. Synthesis of APTS-modified Graphite Oxide. Pristine graphite is not very reactive with chemical moieties due to its stable graphitic structure. Therefore, graphite oxide (GO) was prepared by oxidizing graphite powder with NaClO3 in fuming nitric acid according to the Brodie’s method, which can introduce several oxygen-containing functional groups bringing free electrons not only at the edge of graphitic layer but also on the surface of the basal plane.25 Pure graphite (2 g) and NaClO3 (17.5 g) were stirred in fuming nitric acid (40 mL) at room temperature for 24 h. The resulting solution was washed several times with distilled water and dried at 60 °C overnight. The synthesized GO (200 mg) was mixed with EDC (120 mg) in APTS (2 mL), and the solution was sonicated in a bath-type ultrasonicator for 5 min. The solution was then stirred at room temperature for 2 h. After this amine modification, samples were washed several times with distilled water and precipitated in a centrifuge at 8000 rpm for 10 min. The precipitate was dried at 60 °C overnight and 3-aminopropyl-triethoxylsilane-modified graphite oxide, (abbreviated as Gr-APTS), was obtained. 2.3. Characterization of Samples. The morphologies of adsorbents were analyzed using scanning electron microscopy (SEM, S-4300, Hitachi) and high resolution transmittance electron microscopy (TEM, G2 F30ST, Tecnai). Energy dispersive X-ray spectroscopy (EDX, EX-200, Horiba) was coupled with SEM to determine the silica composition in the samples. An element analyzer (Flash EA 1112 series, CE Instruments) was used to determine the C, H, O, and N compositions of adsorbents. X-ray diffraction (XRD) patterns were obtained with an X-ray diffractometer (X′ Pert MPD, Philips) using Cu Kα radiation with the 2θ range 5−60°. Fourier transform infrared spectroscopy (FTIR, Nicolet iS10, Thermo Scientific) was used to identify the presence of certain functional groups in adsorbents with a range of 900−4000°. A Raman spectrometer (LabRam ARAMIS, Horiba Jobin-Yvon) using an Arion laser with an excitation energy of 514 nm was used to confirm possible defect formation. The surface area was estimated using the Brunauer, Emmett, and Teller (BET) equation26 based on information obtained by N2 adsorption and desorption at 77 K using a volumetric sorption analyzer (Autosorb iQ Station 2, Quantachorme). CO2 adsorption uptake on the adsorbents was measured using thermogravimetric analysis (TGA, Q50, TA Instruments). Before the adsorption tests were performed, moisture and CO2 were removed from the samples by the flow of N2 gas at 100 °C for 2 h. After the pretreatment, the change in sample weight was recorded under a pure CO2 gas flow at a temperature of 30 °C and a pressure of ∼1 atm. The adsorption isotherms of CO2 were additionally measured at 0, 30, and 50 °C to calculate the heat of adsorption using a static volumetric analyzer (ASAP2020, Micromeritics).
Figure 1. SEM images of (a) graphite, (b) graphite oxide, and (c) APTS-modified graphite oxide.
morphologies could be found in the TEM images (Figure S1, Supporting Information). Figure 2 shows the XRD patterns of graphite, GO, and GrAPTS. Graphite exhibited a sharp (002) peak centered at near
3. RESULTS AND DISCUSSION 3.1. Characterization of APTS-Modified Graphite Oxide. Figure 1 shows the morphologies of graphite, GO, and Gr-APTS obtained by SEM. Graphite showed an agglomeration of several thick sheets and the formation of large lamellas. GO showed the exfoliation of the layers of graphitic sheets due to oxidation by the highly concentrated acid. Gr-APTS had similar morphology to GO without noticeable change after the amine modification. Similar
Figure 2. XRD patterns of graphite, graphite oxide, and APTSmodified graphite oxide. B
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Figure 3. Raman spectra of (a) graphite and (b) APTS-modified graphite oxide.
26°, corresponding to the crystal graphite. This indicates that the interlayer distance of graphite (referred to as d-spacing, or interplanar spacing) was about 3.4 Å from calculation performed using the Bragg’s law.27 Oxidation caused the shift of the distinct peak toward the lower angle of 2θ = 14.8° with a d-spacing of 6.0 Å. The distance between the layers within graphite increased owing to the insertion of oxide from residual oxygen functional groups. Gr-APTS exhibited a peak at about 7.8°, representing a d-spacing of 11.4 Å. The amine-modified graphite oxide showed a further increase in d-spacing, which was attributed to the long chains of amine molecules intercalated among the layers.28 These XRD data support the results shown in SEM images; GO and Gr-APTS had widened layers, whereas packed layers were observed in graphite. Figure 3 shows the Raman spectra that imply the occurrence of structural change in the surface modification from graphite to Gr-APTS. Graphite and Gr-APTS had two distinct peaks located at about 1360 and 1580 cm−1. The peak at 1360 cm−1 represents the D band related to the disorderness of graphite due to sp3-hybridized carbons, which reflect either defects or the edge planes of graphite. The peak at 1580 cm−1 represents the G band related to the sp2-hybridized carbon in graphite. The intensity ratio of D band to G band (ID/IG) expresses the sp3/sp2 carbon ratio, indicating the degree of disorderness. GrAPTS had a higher intensity ratio (ID/IG = 1.38) than graphite (ID/IG = 0.41). The change of intensity ratio is attributed to the breakage of the sp2-hybrdizied structure that generates defects and edge planes upon surface modification.29 Figure 4 compares the FTIR spectra of GO and Gr-APTS. The broad band within the range of 3700−3000 cm−1 is attributed to the stretching vibration of the OH bond. GO revealed absorption bands at 1720, 1365, and 1052 cm−1, corresponding to the CO vibration from the ketone group,30 OCO from the carboxyl group,31 and CO deformation from the epoxy group, respectively.32 The band at 1630 cm−1 could be assigned to the skeletal vibrations of unoxidized sp2hybrdized carbons from the graphitic structure. Gr-APTS revealed absorption bands at 2850−2920 cm−1, which were related to the bending vibration of CH2 owing to the presence of APTS chain molecules.33 Furthermore, there were additional bands at 1474 and 1440 cm−1, which are associated with the NH and NH2 deformation of the hydrogen-bonded amine group, respectively.34,35 In comparison with those of GO, the FTIR spectra of Gr-APTS showed not only newly appearing
Figure 4. FTIR spectra of graphite oxide and APTS-modified graphite oxide.
bands but also weakened bands of oxygen functional groups. With increasing amount of APTS used for modification, the characteristic peaks for NH and NH2 as well as the alkyl chain became more apparent (Figure S2, Supporting Information). Table 1 shows the compositions of graphite, GO, and GrAPTS analyzed by EDX. The original graphite contained C in Table 1. EDX Data of Graphite, Graphite Oxide, and APTSModified Graphite Oxide atom
graphite (wt %)
GO (wt %)
Gr-APTS (wt %)
C O Si
96.01 3.99
62.34 37.66
63.54 31.23 5.23
major proportions (96.01 wt %), with a minimal O content (3.99 wt %). Within GO, elements of C (62.34 wt %) and O (37.66 wt %) predominated, due to the massive oxygen functional groups introduced during oxidation. However, after GO was modified with APTS, Gr-APTS contained Si (5.23 wt %), from the silica in the attached 3-aminopropyl-triethoxysilane. From the characteristic bands of APTS in FTIR spectra and remaining Si atoms after washing procedure in EDX C
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to the amine molecules attached to the surface of the graphite. The possible mechanism for the CO2 adsorption is suggested in Scheme 1. In the element analysis, a nitrogen content of 3.64
analysis, it is thought that APTS was successfully chemically bonded on the GO. Figure 5 shows the N2 adsorption isotherm on the graphite and on Gr-APTS at a temperature of 77 K. The BET surface
Scheme 1. Schematic of Possible Mechanism for CO2 Adsorption on APTS-Modified Graphite Oxide
wt % was confirmed in the Gr-APTS. Two primary amine molecules react with a single CO2 molecule, leading to carbamate formation, according to eq 1. 37 From the information derived from the adsorption uptake and nitrogen content, the amount of adsorbed CO2 per amine functional group was calculated at 0.446 mol CO2/mol N. This value is slightly less than the theoretical ratio of 0.5. It is thought that some excessive amine molecules attached to the surface could not be accessed by CO2 molecules due to their complex long chain structures. Figure 5. N2 adsorption (solid line) and desorption (dotted line) at 77 K on graphite and APTS-modified graphite oxide.
2R − NH 2 + CO2 → R − NHCOO− + R − NH3+
(1)
Figure 7 shows the CO2 adsorption isotherms on Gr-APTS at the temperature of 0, 30, and 50 °C. As the adsorption
area of Gr-APTS was approximately 19 m2/g, which is almost twice that of graphite at ∼11 m2/g. The increased surface area of Gr-APTS can be attributed to the breakage of the sp2hybridized structure with widened interlayers from the surface modification. Graphite and Gr-APTS corresponded to Type IIb isotherm with H3 hysteresis, (according to IUPAC classification), which normally represents the characteristic adsorption behavior of nonrigid slit-shaped pores of aggregated powders.36 3.2. CO2 Adsorption on APTS-Modified Graphite Oxide. Figure 6 compares the CO2 adsorption uptake on
Figure 7. CO2 adsorption isotherms at 0, 30, and 50 °C for APTSmodified graphite oxide.
temperature increased, the amount of adsorbed CO2 decreased substantially. The isosteric heat of adsorption, calculated by means of the Clausius−Clapeyron equation is in the range 26.9−28.3 kJ/mol, and it corresponds to the ordinary physisorption. In the adsorption operation process, cyclic stability and ease of regeneration are aspects that are as significant as the enhancement of CO 2 . Figure 8 shows the cyclic CO 2 adsorption stability for Gr-APTS in repeated adsorption and regeneration. Adsorption was carried out at a temperature of 30 °C and a pressure of ∼1 atm, under a pure CO2 purge, followed by regeneration at a temperature of 100 °C under a pure N2 purge. Adsorption and regeneration were continued until an adsorption/desorption equilibrium was reached. Gr-APTS exhibited rapid adsorption/desorption kinetics, which can
Figure 6. CO2 adsorption uptake of graphite, graphite oxide, and APTS-modified graphite oxide at 30 °C under atmospheric pressure.
graphite, GO, and Gr-APTS at 30 °C, which was measured by TGA. Graphite had practically no adsorption ability for CO2 gas, and GO had a very low adsorption uptake of 0.074 mol/kg. However, on Gr-APTS the adsorption uptake significantly increased to 1.16 mol/kg. The increased basicity appears to be a major factor in enhancing the adsorption uptake and is related D
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Center, and the Human Resources Development Program (20114010203050) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government’s Ministry of Trade, Industry and Energy.
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Figure 8. Cyclic CO2 adsorption test for APTS-modified graphite oxide, (adsorption at 30 °C and desorption at 100 °C).
reduce the time required to reach an equilibrium state. After 7 cycles of adsorption/desorption, about 90% of the initial CO2 adsorption uptake was maintained and no further noticeable decrease could be found.
4. CONCLUSIONS This study demonstrates the feasibility of capturing CO2 gas using APTS-modified graphite oxide. Through XRD analysis, it was confirmed that the interlayer distance of GO was increased due to oxidation by the highly concentrated acid (i.e., introduction of various oxygen functional groups). A further increase in the interlayer distance of APTS-modified graphite oxide was attributed to the intercalation of amine molecules. EDX and FTIR results showed that APTS was successfully attached to the surface of graphite oxide by the reaction between the oxygen functional groups and amine molecules (which interact with CO2 to form carbamate). The CO2 adsorption uptake of APTS-modified graphite oxide was significantly enhanced compared to that of graphite and graphite oxide. In repeated adsorption/desorption operations, the APTS-modified graphite oxide showed fast adsorption/ desorption kinetics and a good cyclic stability.
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ASSOCIATED CONTENT
S Supporting Information *
TEM images of graphite and APTS-modified graphite oxide (Figure S1). FTIR spectra of APTS-modified graphite oxide with different amounts of amine used in modification (Figure S2). This information is available free of charge via the Internet at http://pubs.acs.org.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*Telephone: +82-2-3290-4851. Fax: +82-2-926-6102. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a National Research Foundation (NRF) grant funded by the Korean government’s Ministry of Education, Science, and Technology, through the Basic Science Research Program (2012-008941), the Korea CCS R&D E
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