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CO2 gas separation behavior
International Journal of Greenhouse Gas Control 10 (2012) 278–284
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International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
Effects of fluorination on carbon molecular sieves for CH4 /CO2 gas separation behavior Hye-Ryeon Yu a , Seho Cho a , Byong Chol Bai a , Kwang Bok Yi b , Young-Seak Lee a,∗ a b
Department of Fine Chemical Engineering and Applied Chemistry, BK21-E2 M, Chungnam National University, Daejeon 305-764, Republic of Korea Department of Chemical Engineering Education, Chungnam National University, Daejeon 305-764, Republic of Korea
a r t i c l e
i n f o
Article history: Received 15 November 2011 Received in revised form 9 May 2012 Accepted 22 June 2012 Available online 20 July 2012 Keywords: Adsorption Carbon molecular sieve Fluorination Pressure swing adsorption Carbon dioxide gas Separations
a b s t r a c t The surface of carbon molecular sieves (CMSs) was fluorinated to investigate the separation behavior of CH4 /CO2 . The fluorination of CMSs was carried out at various F2 partial pressures to determine the effect of the F2 content. Fluorine functional groups were effectively introduced on the surface of the CMSs, and the volume of pores in the CMSs was increased due to fluorination, especially those with a diameter less ˚ As the fluorine partial pressure was increase, an increase in the CO2 adsorption capacity of CMSs than 8 A. was observed due to Lewis acid–base interactions between the functional groups of CMSs and CO2 . Based on the selective CO2 adsorption results, the CO2 breakthrough with fluorinated CMS occurred at later stages of CH4 /CO2 gas separation process compared to Raw-CMS. Therefore the presence of fluorine on the surfaces of CMSs affects the pore volumes of CMSs. According to the adsorption isotherms, the CO2 adsorption efficiency of CMSs was improved from 1.61 to 2.04 mmol/g at 298 K. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction The concentration of carbon dioxide (CO2 ) and the main source of CO2 emissions is from the combustion of fossil fuels in the atmosphere continue to increase, and serious concerns about the effects of CO2 on the environment have been raised (Hofmann et al., 2009; Bhagiyalakshmi et al., 2011; Anbia et al., 2012). These are considered to be the main anthropogenic contributor to the greenhouse gas effect and are responsible for 60% of the observed increase in the temperature of the atmosphere, which is commonly referred to as global warming (Monastersky, 2009; Yamakasi, 2003). Power generation is a major user of fossil fuels and the demand for electricity is growing steadily throughout the developed world and exponentially in the less developed countries. In order to reduce these environmental concerns, the removal of carbon dioxide from gas/air streams is becoming necessary in many industries. Carbon dioxide can be removed by several methods, including membrane separation, adsorption, and absorption (Suda et al., 1992; Kim et al., 2012; Sarkar and Bose, 1997). Among these methods, adsorption processes present many advantages, such as low operating costs and high selectivity. Like that pressure swing adsorption (PSA) and vacuum swing adsorption (VSA), cryogenic separation, membrane separation techniques (Gomes and Hassan, 2001; Ren et al., 2012).
∗ Corresponding author. Tel.: +82 42 821 7007; fax: +82 42 822 6637. E-mail address: [email protected] (Y.-S. Lee). 1750-5836/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijggc.2012.06.013
In addition, many adsorbents are easy to regenerate and exhibit high adsorption capacities and separation rates (Li et al., 2008). The adsorption kinetics of gas molecules depends on the size, shape, and electrical properties of the adsorbate, which lead to specific interactions with the adsorbent (Reid et al., 1998; Reid and Thomas, 1999; Freitas and Figueiredo, 2001). In general, zeolites, metal-organic framework (MOF) and activated carbons are widely used as selective adsorbents for gas separation because of their molecular sieving properties (Sjostrom and Krutka, 2010; Bai et al., 2011; Barcia et al., 2008; Bastin et al., 2008; Cavenati et al., 2008). Molecular sieves are capable of separating individual components of gaseous mixtures due to their porous structures, which consist ˚ in diameter. In of uniform pores that are several angstroms (A) particulars, the carbon molecular sieves (CMSs) possess specific pore size distributions (PSDs). Unlike conventional activated carbons, CMSs posses narrow PSDs, which increase their selectivity in molecular separations (Juan et al., 1998). Rodrigues et al. (Grande and Rodrigues, 2007a, 2007b) reported about the effective separation characteristic of carbon molecular sieves (CMSs) on CH4 /CO2 separation. Compared with zeolites and MOF, CMSs are more shape-selective for planar molecules, hydrophobic, and stable at high temperatures (Walker et al., 1966). The most important feature of CMSs is their narrow pore size distribution, which can be attained by controlled activation (Hu and Vansant, 1995) or by conducting pore-narrowing techniques on the inherent pore structure. Pore-narrowing procedures have been used on various carbon materials for the production of suitable CMSs, including chemical activation by KOH, benzene pyrolysis, and plasma treatment of
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activated carbon (Kim and Park, 2011). In addition to a narrow pore size distribution, the adsorption capacity is an important parameter of a CMS. Many researchers have conducted surface treatments to increase the CO2 adsorption capacity of a material. Among the surface treatments, fluorination is one of the most effective methods for the treatment of a carbon surface because of the high reactivity of fluorine gas with carbon at room temperature (Tressaud et al., 2004; Lee and Lee, 2002). Furthermore, the nature of the surface of activated carbon can be controlled via fluorination (Lee et al., 2007). In the present study, the surface of CMSs was modified by fluorination, which was performed within a few minutes via a simple procedure. Fluorination changed the surface properties and pore structure of the CMSs and showed significant effects on the CH4 /CO2 gas separation. In addition, a potential mechanism of CO2 adsorption and CH4 /CO2 separation on CMSs is proposed. 2. Material and methods 2.1. Fluorination of CMSs CMSs (CMS FB610, Carbo Tech AC GmbH, Germany) were used in the present study to separate gaseous mixtures. The surface of the CMSs was treated using a fluorination apparatus, which consisted of a reactor, a vacuum pump and a buffer tank connected to gas cylinders. The samples were loaded into the reactor in a nickel boat and were degassed at 323 K for 1 h to remove moisture. Nitrogen gas (99.999%) and fluorine gas (99.8%, Messer Grieheim GmbH) were used during the fluorination process. Fluorination was performed at 1 bar for 10 min at fluorine (F2 ):nitrogen (N2 ) gas volume ratios of 1:9, 3:7, and 5:5. A detailed description of the procedure can be found in our previous study (Im et al., 2009; Yun et al., 2007; Jung et al., 2009). As-received and treated samples were labeled as Raw-CMS, F1N9-CMS, F3N7-CMS, and F5N5-CMS according to the fluorination conditions. 2.2. Characterization of fluorinated CMSs 2.2.1. Chemical component analysis of fluorinated CMSs X-ray photoelectron spectra (XPS) were obtained using a MultiLab 2000 spectrometer (Thermo Electron Corporation, UK) to identify the elements present in the samples. Aluminum K␣ (1485.6 eV) radiation was used as the X-ray source, and an anode voltage of 14.9 keV, a filament current of 4.6 A, and an emission current of a 20 mA was applied. All of the samples were treated at 10−9 mbar to remove impurities. The survey spectra were obtained at a pass energy of 50 eV in increments of 0.5 eV. 2.2.2. Pore characteristics and CO2 adsorption capacity of fluorinated CMSs The pore characteristics and CO2 adsorption capacity of fluorinated CMSs were evaluated by determining the physical adsorption of CO2 in an ASAP2020 (Micromeritics Ins. Corp.). The pore char˚ gas acteristics of the CMSs were investigated using CO2 (3.3 A) because CMS possesses extremely small pore sizes (the diameter ˚ The pore size distribution (PSD) of the CMSs was of N2 gas is 3.8 A). calculated according to density functional theory (DFT) using the DFT Plus software supplied by Micromeritics (Jagiello and Tolles, 1998; Tarazona et al., 1987) at 273 K. The CO2 adsorption capacity was measured at absolute pressures of 0–800 mmHg and at temperatures of 273 and 298 K. The densities of prepared samples were measured by using free space volume and sample weight obtained by basic information during ASAP2020 operation. The densities of Raw-CMS, F1N9-CMS, F3N7-CMS, and F5N5-CMS samples were 0.365, 0.368, 0.367, and 0.360 g/cm3 .
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Fig. 1. Micro-gas chromatography apparatus.
2.2.3. CH4 /CO2 gas separation To investigate the characteristics of CH4 /CO2 gas separation, micro-gas chromatographic analysis was performed with CH4 , CO2 gas and nitrogen. A schematic diagram of the micro-gas chromatography apparatus is shown in Fig. 1. The absorbent bed for micro-gas chromatography has spec with length of 800 mm and diameter of 12 mm. For each CMS, 10 g of the material was loaded onto the adsorption bed and was purged with nitrogen gas (99.999%) at 298 K for 30 min. During this time, the flow rate of the feed gas was maintained at 200 sccm. The adsorption of each gas was carried out using CH4 and CO2 gases (target gas 20 sccm, N2 gas 180 sccm). Using the same apparatus, the characteristics for CH4 /CO2 mixed gas separation was tested with 6:4 (CH4 :CO2 ) gas ratio. Because the landfill gas is comprised of CH4 and CO2 in the approximate proportions of 40%:60% (v/v) (Senior and Kasali, 1990). The CH4 /CO2 (6:4) gas mixture was diluted by 50% with nitrogen gas (mixed gas 100 sccm, N2 gas 100 sccm). 3. Results 3.1. Effects of fluorination on the chemical composition of CMS The functional groups introduced on the surface of the CMSs were identified by examining the C1s and F1s XPS peaks after fluorination, and the results are provided in Table 1 and Fig. 2 (Lee et al., 2009). As shown in Fig. 2, the XPS survey graphs of raw and fluorinated CMSs displayed a distinct carbon peak at 284.5 eV. In the spectra of the fluorinated CMSs, fluorine and oxygen peaks were also observed at 687.7 and 531.0 eV. The atomic ratio of each element on the surface of the CMSs is listed in Table 1. The carbon content of the samples decreased by approximately 27% after fluorination, and an increase in the fluorine content was observed. The C1s peaks were deconvoluted to several pseudo-Voigt functions (the sum of Gaussian and Lorentzian functions) using a peak analysis program (Unipress Co., USA) to study the C F bonds (Wu et al., 2009). The deconvoluted C1s results and the corresponding graphs are provided in Table 2 and Fig. 3, respectively. As shown in Table 2, covalent and semi-covalent C F bonds were present on the Table 1 XPS parameters of fluorinated and raw-CMSs. Component
Raw-CMS F1N9-CMS F3N7-CMS F5N5-CMS
Elemental content (at.%)
F/C (%)
C1s
O1s
F1s
90.8 70.4 66.2 64.3
9.2 7.1 7.9 7.8
0.0 22.5 25.9 27.9
0.00 31.96 39.12 43.39
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Table 2 Assignments and peak parameters of the components of C1s, O1s, and F1s peaks. Component
Assignment
Binding energy (eV)
C(1) C(2) C(3) C(4) C(5) C(6)
C C(sp2 ) C C(sp3 ) C O C O Semi-covalent C F C F
284.5 285.4 286.4 287.4 288.8 290.5
Concentration (%) of the sample Raw-CMS
F1N9-CMS
F3N7-CMS
F5N5-CMS
70.24 18.13 7.49 4.14 0 0
38.21 21.11 10.39 6.05 20.53 3.72
34.21 20.22 12.13 5.74 24.40 3.30
31.72 20.90 12.24 6.40 24.76 3.98
surface of the CMSs. In general, carbon materials form many semicovalent bonds during fluorination at room temperature (Chong et al., 1993). 3.2. Pore characteristics and CO2 adsorption capacity
Fig. 2. XPS survey spectra of Raw-CMS, F1N9-CMS, F3N7-CMS, and F5N5-CMS.
The pore characteristics of the fluorinated CMSs were investigated, and the results are presented in Table 3. The volume of the ˚ increased by approximately 14.7% after fluorinapores (<4.02 A) tion, and the following trend in the pore volume was observed: F5N5-CMS F3N7-CMS F1N9-CMS. The total pore volume of F1N9CMS and F3N7-CMS increased due to fluorination, whereas the total pore volume of F5N5-CMS decreased slightly. As shown in Fig. 4, low fluorine ratios promoted the formation of small pores, and high fluorine ratios destroyed the pore structure. Figs. 4 and 5 show the pore size distributions (PSDs) of the CMSs calculated using density functional theory (DFT). The volume of the pores in fluorinated ˚ was greater CMSs (especially those with a diameter less than 8 A) than that of Raw-CMS, which increased the CO2 adsorption capacity of fluorinated CMSs. The CO2 adsorption capacities of the samples were measured at 273 and 298 K, and the results are presented in Fig. 6 and Table 4.
Fig. 3. Deconvolution of core-level C1s spectra: Raw-CMS, F1N9-CMS, F3N7-CMS, and F5N5-CMS.
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Table 3 Changes in the pore characteristics of fluorinated CMSs. Sample
Raw-CMS
F1N9-CMS
F3N7-CMS
F5N5-CMS
˚ Pore volume (cm3 /g) (<4.02 A) ˚ Total volume of pores (cm3 /g) (10.83 A)
0.0034 0.1008
0.0042 0.1053
0.0039 0.1067
0.0036 0.1003
Fig. 4. PSDs showing the development of porosity in CMSs as a function of the fluorine ratio.
Compared to that of Raw-CMS, the CO2 adsorption capacity of fluorinated CMSs increased by approximately 2.7% because of an increase in the cumulative PSD (Fig. 5). In addition, the CO2 adsorption capacity decreased with an increase in temperature. Specifically, the CO2 adsorption capacity of Raw-CMS decreased by 38.1% as the temperature increased. The observed reduction in the adsorption capacity of Raw-CMS was greater than that of fluorinated CMSs because fluorination had a stronger effect at higher temperatures (Chamssedine et al., 2011). A possible mechanism of the interaction between the functionalized surfaces of the CMSs and the CO2 gas is provided in Section 4.2.
3.3. CH4 /CO2 gas separation behavior
Fig. 5. Cumulative PSD of Raw-CMS, F1N9-CMS, F3N7-CMS, and F5N5-CMS.
The CO2 adsorption capacities of fluorinated CMSs were greater than that of Raw-CMS. At 273 K, the CO2 adsorption capacities of fluorinated CMSs were 2.67 (F1N9-CMS), 2.66 (F3N7-CMS), and 2.67 (F5N5-CMS) mmol/g.
The characteristics of CH4 and CO2 gas adsorption showed in Fig. 7. In Fig. 7(a), the behavior of CH4 adsorption can be observed. All Samples are rapidly saturated as similar type due to very weak interaction between the CH4 gas and samples. However, the CO2 adsorption behavior of untreated and fluorinated CMS showed a different saturation pattern (Fig. 7(b)). Furthermore the CH4 /CO2 gas separation behavior of the samples was determined, and the results are presented in Fig. 8. The maximum concentration of saturated gas was one-half of the initial value because the mixed gas was diluted by 50% with nitrogen gas. In particular, CO2 separation behavior of samples had a similar pattern like as Fig. 7(b). As shown in Fig. 8, compared to Raw-CMS, saturation occurred slowly in the fluorinated samples because the mass transfer area was broad. The slow saturation kinetics
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Table 4 CO2 adsorption kinetics and CO2 uptake as a function of temperature. Sample
Raw-CMS
F1N9-CMS
F3N7-CMS
F5N5-CMS
CO2 uptake (at 273 K) (mmol/g) CO2 uptake (at 298 K) (mmol/g) The decrement of CO2 uptake (%)
2.60 1.61 0.99 (38.1%)
2.67 1.96 0.71 (26.4%)
2.66 2.03 0.63 (23.7%)
2.67 2.04 0.63 (23.6%)
was attributed to the effects of fluorination; specifically, fluorine present on the surface of the fluorinated CMSs interacts continuously with the CO2 gas for attracting CO2 molecules. Fluorination also enhanced the CO2 adsorption capacity by increasing the volume of the pores with diameters less than 8 A˚ (see Section 3.2 for more detail). In contrast, at adsorption times of less than 5 min, the saturated CH4 concentration increased slightly due to the decrement concentration of CO2 . In F5N5-CMS, the concentration of CH4 was lower than that in Raw-CMS because F5N5-CMS adsorbed small amounts of CH4 and CO2 gas because of its large total pore volume (see Table 3). Therefore, fluorination affected the CO2 adsorption capacity, which altered the behavior of the CH4 /CO2 gas separation. 4. Discussion 4.1. Effects of fluorine on the CO2 adsorption capacity The CO2 adsorption capacity was decreased with an increase in the reaction temperature. In general, the adsorption capacity decreases at higher temperatures because of an increase in the activity of the gas and the adsorption heat of material studied by the Clausius–Clapeyron equation and result of Chen’s group
Fig. 7. Gas adsorption of Raw-CMS, F1N9-CMS, F3N7-CMS, and F5N5-CMS using GC: (a) CH4 adsorption and (b) CO2 adsorption.
Fig. 8. CH4 /CO2 gas separation of Raw-CMS, F1N9-CMS, F3N7-CMS, and F5N5-CMS. Fig. 6. CO2 adsorption kinetics of Raw-CMS, F1N9-CMS, F3N7-CMS, and F5N5-CMS at 273 and 298 K.
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Fig. 9. Proposed mechanism of the interactions between the functionalized surface of CMSs and CO2 gas.
(Chen et al., 2012). In the present study, as the CO2 adsorption temperature increased, the adsorption capacity of Raw-CMS and the fluorinated CMSs was decreased by 38.1 and 25 vol.%, respectively. The observed reduction in the adsorption capacity was lower for the fluorinated CMSs because interactions between fluorine groups on the surface of CMS and CO2 are stronger at 298 K more than interactions at 273 K (Chamssedine et al., 2011). It is well known that the CO2 adsorption capacity strongly was related to the micorporosity at 273 K (Chen et al., 2012). In the other hand, interactions between fluorine groups on carbon surface and CO2 molecular was increased under 298 K due to enhancement of functional group activity. Therefore, CO2 adsorption capacity of modified CMSs was increased compared to untreated CMSs at 298 K. Fluorination introduced numerous defects and strongly basic sites (Chamssedine et al., 2011; Wu et al., 2007; Choudary et al., 2001; Zhou et al., 2006). Many papers have been presented to support this result considering the bond length, strength and polarity. The bond length of C F (0.1332 nm) is also longer than that of C H (0.1068 nm), the bond strength of C F (111 kcal mol−1 ) is larger than that of C H, and the polarity of C F is thus very strong (Ma et al., 2006). Moreover the adsorption of acidic CO2 alternatively may be enhanced by the presence of other basic functionalities on the carbon surface: CO2 is a weak Lewis acid (electron-acceptor) that can interact with electron-donors (Plaz et al., 2011). Therefore, the strength of Lewis acid–base interactions between the functionalized surfaces of the CMSs and CO2 gas was increased (Fig. 9(a)). In addition, the reactivity of fluorine was increased as the temperature increased from 273 to 298 K (Allayarov et al., 1999). Thus, the CO2 adsorption capacity was increased because fluorinated CMSs possess strongly basic groups, which were introduced under different fluorination conditions (Zhang et al., 2010).
of the fluorine which has high electric negativity. The amorphous carbon with weak carbon/carbon bonding was removed by fluorine gas reaction under circumstances such as fluorinated carbon gases (CF4 and C2 F6 ) (Im et al., 2011; Yoshiyuki et al., 2007; Pehrsson et al., 2003). The enlarged pore width affected gas separation by promoting CH4 adsorption in the early stages of the process. Therefore, the extent of the surface modifications and the pore structure of the material must be controlled to effectively separate gases and adsorb CO2 .
4.2. CO2 selectivity of fluorinated CMSs
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Many researchers have shown that CO2 has a significant quadrupole moment that induces specific interactions with adsorbents, whereas CH4 does not display this type of behavior (Llewellyn et al., 2006). Therefore, CH4 has weak adsorbate–adsorbent interaction forces compared to CO2 which has the strong dipole moment of carbonyl bonds. Fig. 9(b) presents the effect of fluorination on the selective adsorption of CH4 /CO2 mixed gases and the adsorption capacity of CO2 due to interactions between the fluorinated surface of CMSs and CO2 gas. However, high fluorine partial pressures increased the pore width of F5N5-CMS because of the high chemical reaction
5. Conclusions Surface-modified CMSs were prepared via fluorination at atmosphere pressure and room temperature to investigate the CH4 /CO2 separation behavior. The fluorine content of the surfaces of the CMSs and CO2 adsorption capacity increased to 22.5% and 2.67 mmol/g (273 K, F1N9-CMS) due to fluorination. Lewis acid–base interactions between the surface of fluorinated CMSs and CO2 were affected by the fluorine content, and the pore vol˚ of the CMSs increased ume (especially pores with diameters of 8 A) due to fluorination. The mass-transfer area of the CMSs also increased upon fluorination. Accordingly, compared to Raw-CMS, the CH4 /CO2 gas separation behavior of surface-modified CMSs changed to breakthrough late in the adsorption process. Acknowledgement This work was supported by the Korea Gas Corporation and HYUNDAI Engineering & Construction. References
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International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
Effects of fluorination on carbon molecular sieves for CH4 /CO2 gas separation behavior Hye-Ryeon Yu a , Seho Cho a , Byong Chol Bai a , Kwang Bok Yi b , Young-Seak Lee a,∗ a b
Department of Fine Chemical Engineering and Applied Chemistry, BK21-E2 M, Chungnam National University, Daejeon 305-764, Republic of Korea Department of Chemical Engineering Education, Chungnam National University, Daejeon 305-764, Republic of Korea
a r t i c l e
i n f o
Article history: Received 15 November 2011 Received in revised form 9 May 2012 Accepted 22 June 2012 Available online 20 July 2012 Keywords: Adsorption Carbon molecular sieve Fluorination Pressure swing adsorption Carbon dioxide gas Separations
a b s t r a c t The surface of carbon molecular sieves (CMSs) was fluorinated to investigate the separation behavior of CH4 /CO2 . The fluorination of CMSs was carried out at various F2 partial pressures to determine the effect of the F2 content. Fluorine functional groups were effectively introduced on the surface of the CMSs, and the volume of pores in the CMSs was increased due to fluorination, especially those with a diameter less ˚ As the fluorine partial pressure was increase, an increase in the CO2 adsorption capacity of CMSs than 8 A. was observed due to Lewis acid–base interactions between the functional groups of CMSs and CO2 . Based on the selective CO2 adsorption results, the CO2 breakthrough with fluorinated CMS occurred at later stages of CH4 /CO2 gas separation process compared to Raw-CMS. Therefore the presence of fluorine on the surfaces of CMSs affects the pore volumes of CMSs. According to the adsorption isotherms, the CO2 adsorption efficiency of CMSs was improved from 1.61 to 2.04 mmol/g at 298 K. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction The concentration of carbon dioxide (CO2 ) and the main source of CO2 emissions is from the combustion of fossil fuels in the atmosphere continue to increase, and serious concerns about the effects of CO2 on the environment have been raised (Hofmann et al., 2009; Bhagiyalakshmi et al., 2011; Anbia et al., 2012). These are considered to be the main anthropogenic contributor to the greenhouse gas effect and are responsible for 60% of the observed increase in the temperature of the atmosphere, which is commonly referred to as global warming (Monastersky, 2009; Yamakasi, 2003). Power generation is a major user of fossil fuels and the demand for electricity is growing steadily throughout the developed world and exponentially in the less developed countries. In order to reduce these environmental concerns, the removal of carbon dioxide from gas/air streams is becoming necessary in many industries. Carbon dioxide can be removed by several methods, including membrane separation, adsorption, and absorption (Suda et al., 1992; Kim et al., 2012; Sarkar and Bose, 1997). Among these methods, adsorption processes present many advantages, such as low operating costs and high selectivity. Like that pressure swing adsorption (PSA) and vacuum swing adsorption (VSA), cryogenic separation, membrane separation techniques (Gomes and Hassan, 2001; Ren et al., 2012).
∗ Corresponding author. Tel.: +82 42 821 7007; fax: +82 42 822 6637. E-mail address: [email protected] (Y.-S. Lee). 1750-5836/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijggc.2012.06.013
In addition, many adsorbents are easy to regenerate and exhibit high adsorption capacities and separation rates (Li et al., 2008). The adsorption kinetics of gas molecules depends on the size, shape, and electrical properties of the adsorbate, which lead to specific interactions with the adsorbent (Reid et al., 1998; Reid and Thomas, 1999; Freitas and Figueiredo, 2001). In general, zeolites, metal-organic framework (MOF) and activated carbons are widely used as selective adsorbents for gas separation because of their molecular sieving properties (Sjostrom and Krutka, 2010; Bai et al., 2011; Barcia et al., 2008; Bastin et al., 2008; Cavenati et al., 2008). Molecular sieves are capable of separating individual components of gaseous mixtures due to their porous structures, which consist ˚ in diameter. In of uniform pores that are several angstroms (A) particulars, the carbon molecular sieves (CMSs) possess specific pore size distributions (PSDs). Unlike conventional activated carbons, CMSs posses narrow PSDs, which increase their selectivity in molecular separations (Juan et al., 1998). Rodrigues et al. (Grande and Rodrigues, 2007a, 2007b) reported about the effective separation characteristic of carbon molecular sieves (CMSs) on CH4 /CO2 separation. Compared with zeolites and MOF, CMSs are more shape-selective for planar molecules, hydrophobic, and stable at high temperatures (Walker et al., 1966). The most important feature of CMSs is their narrow pore size distribution, which can be attained by controlled activation (Hu and Vansant, 1995) or by conducting pore-narrowing techniques on the inherent pore structure. Pore-narrowing procedures have been used on various carbon materials for the production of suitable CMSs, including chemical activation by KOH, benzene pyrolysis, and plasma treatment of
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activated carbon (Kim and Park, 2011). In addition to a narrow pore size distribution, the adsorption capacity is an important parameter of a CMS. Many researchers have conducted surface treatments to increase the CO2 adsorption capacity of a material. Among the surface treatments, fluorination is one of the most effective methods for the treatment of a carbon surface because of the high reactivity of fluorine gas with carbon at room temperature (Tressaud et al., 2004; Lee and Lee, 2002). Furthermore, the nature of the surface of activated carbon can be controlled via fluorination (Lee et al., 2007). In the present study, the surface of CMSs was modified by fluorination, which was performed within a few minutes via a simple procedure. Fluorination changed the surface properties and pore structure of the CMSs and showed significant effects on the CH4 /CO2 gas separation. In addition, a potential mechanism of CO2 adsorption and CH4 /CO2 separation on CMSs is proposed. 2. Material and methods 2.1. Fluorination of CMSs CMSs (CMS FB610, Carbo Tech AC GmbH, Germany) were used in the present study to separate gaseous mixtures. The surface of the CMSs was treated using a fluorination apparatus, which consisted of a reactor, a vacuum pump and a buffer tank connected to gas cylinders. The samples were loaded into the reactor in a nickel boat and were degassed at 323 K for 1 h to remove moisture. Nitrogen gas (99.999%) and fluorine gas (99.8%, Messer Grieheim GmbH) were used during the fluorination process. Fluorination was performed at 1 bar for 10 min at fluorine (F2 ):nitrogen (N2 ) gas volume ratios of 1:9, 3:7, and 5:5. A detailed description of the procedure can be found in our previous study (Im et al., 2009; Yun et al., 2007; Jung et al., 2009). As-received and treated samples were labeled as Raw-CMS, F1N9-CMS, F3N7-CMS, and F5N5-CMS according to the fluorination conditions. 2.2. Characterization of fluorinated CMSs 2.2.1. Chemical component analysis of fluorinated CMSs X-ray photoelectron spectra (XPS) were obtained using a MultiLab 2000 spectrometer (Thermo Electron Corporation, UK) to identify the elements present in the samples. Aluminum K␣ (1485.6 eV) radiation was used as the X-ray source, and an anode voltage of 14.9 keV, a filament current of 4.6 A, and an emission current of a 20 mA was applied. All of the samples were treated at 10−9 mbar to remove impurities. The survey spectra were obtained at a pass energy of 50 eV in increments of 0.5 eV. 2.2.2. Pore characteristics and CO2 adsorption capacity of fluorinated CMSs The pore characteristics and CO2 adsorption capacity of fluorinated CMSs were evaluated by determining the physical adsorption of CO2 in an ASAP2020 (Micromeritics Ins. Corp.). The pore char˚ gas acteristics of the CMSs were investigated using CO2 (3.3 A) because CMS possesses extremely small pore sizes (the diameter ˚ The pore size distribution (PSD) of the CMSs was of N2 gas is 3.8 A). calculated according to density functional theory (DFT) using the DFT Plus software supplied by Micromeritics (Jagiello and Tolles, 1998; Tarazona et al., 1987) at 273 K. The CO2 adsorption capacity was measured at absolute pressures of 0–800 mmHg and at temperatures of 273 and 298 K. The densities of prepared samples were measured by using free space volume and sample weight obtained by basic information during ASAP2020 operation. The densities of Raw-CMS, F1N9-CMS, F3N7-CMS, and F5N5-CMS samples were 0.365, 0.368, 0.367, and 0.360 g/cm3 .
279
Fig. 1. Micro-gas chromatography apparatus.
2.2.3. CH4 /CO2 gas separation To investigate the characteristics of CH4 /CO2 gas separation, micro-gas chromatographic analysis was performed with CH4 , CO2 gas and nitrogen. A schematic diagram of the micro-gas chromatography apparatus is shown in Fig. 1. The absorbent bed for micro-gas chromatography has spec with length of 800 mm and diameter of 12 mm. For each CMS, 10 g of the material was loaded onto the adsorption bed and was purged with nitrogen gas (99.999%) at 298 K for 30 min. During this time, the flow rate of the feed gas was maintained at 200 sccm. The adsorption of each gas was carried out using CH4 and CO2 gases (target gas 20 sccm, N2 gas 180 sccm). Using the same apparatus, the characteristics for CH4 /CO2 mixed gas separation was tested with 6:4 (CH4 :CO2 ) gas ratio. Because the landfill gas is comprised of CH4 and CO2 in the approximate proportions of 40%:60% (v/v) (Senior and Kasali, 1990). The CH4 /CO2 (6:4) gas mixture was diluted by 50% with nitrogen gas (mixed gas 100 sccm, N2 gas 100 sccm). 3. Results 3.1. Effects of fluorination on the chemical composition of CMS The functional groups introduced on the surface of the CMSs were identified by examining the C1s and F1s XPS peaks after fluorination, and the results are provided in Table 1 and Fig. 2 (Lee et al., 2009). As shown in Fig. 2, the XPS survey graphs of raw and fluorinated CMSs displayed a distinct carbon peak at 284.5 eV. In the spectra of the fluorinated CMSs, fluorine and oxygen peaks were also observed at 687.7 and 531.0 eV. The atomic ratio of each element on the surface of the CMSs is listed in Table 1. The carbon content of the samples decreased by approximately 27% after fluorination, and an increase in the fluorine content was observed. The C1s peaks were deconvoluted to several pseudo-Voigt functions (the sum of Gaussian and Lorentzian functions) using a peak analysis program (Unipress Co., USA) to study the C F bonds (Wu et al., 2009). The deconvoluted C1s results and the corresponding graphs are provided in Table 2 and Fig. 3, respectively. As shown in Table 2, covalent and semi-covalent C F bonds were present on the Table 1 XPS parameters of fluorinated and raw-CMSs. Component
Raw-CMS F1N9-CMS F3N7-CMS F5N5-CMS
Elemental content (at.%)
F/C (%)
C1s
O1s
F1s
90.8 70.4 66.2 64.3
9.2 7.1 7.9 7.8
0.0 22.5 25.9 27.9
0.00 31.96 39.12 43.39
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Table 2 Assignments and peak parameters of the components of C1s, O1s, and F1s peaks. Component
Assignment
Binding energy (eV)
C(1) C(2) C(3) C(4) C(5) C(6)
C C(sp2 ) C C(sp3 ) C O C O Semi-covalent C F C F
284.5 285.4 286.4 287.4 288.8 290.5
Concentration (%) of the sample Raw-CMS
F1N9-CMS
F3N7-CMS
F5N5-CMS
70.24 18.13 7.49 4.14 0 0
38.21 21.11 10.39 6.05 20.53 3.72
34.21 20.22 12.13 5.74 24.40 3.30
31.72 20.90 12.24 6.40 24.76 3.98
surface of the CMSs. In general, carbon materials form many semicovalent bonds during fluorination at room temperature (Chong et al., 1993). 3.2. Pore characteristics and CO2 adsorption capacity
Fig. 2. XPS survey spectra of Raw-CMS, F1N9-CMS, F3N7-CMS, and F5N5-CMS.
The pore characteristics of the fluorinated CMSs were investigated, and the results are presented in Table 3. The volume of the ˚ increased by approximately 14.7% after fluorinapores (<4.02 A) tion, and the following trend in the pore volume was observed: F5N5-CMS F3N7-CMS F1N9-CMS. The total pore volume of F1N9CMS and F3N7-CMS increased due to fluorination, whereas the total pore volume of F5N5-CMS decreased slightly. As shown in Fig. 4, low fluorine ratios promoted the formation of small pores, and high fluorine ratios destroyed the pore structure. Figs. 4 and 5 show the pore size distributions (PSDs) of the CMSs calculated using density functional theory (DFT). The volume of the pores in fluorinated ˚ was greater CMSs (especially those with a diameter less than 8 A) than that of Raw-CMS, which increased the CO2 adsorption capacity of fluorinated CMSs. The CO2 adsorption capacities of the samples were measured at 273 and 298 K, and the results are presented in Fig. 6 and Table 4.
Fig. 3. Deconvolution of core-level C1s spectra: Raw-CMS, F1N9-CMS, F3N7-CMS, and F5N5-CMS.
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Table 3 Changes in the pore characteristics of fluorinated CMSs. Sample
Raw-CMS
F1N9-CMS
F3N7-CMS
F5N5-CMS
˚ Pore volume (cm3 /g) (<4.02 A) ˚ Total volume of pores (cm3 /g) (10.83 A)
0.0034 0.1008
0.0042 0.1053
0.0039 0.1067
0.0036 0.1003
Fig. 4. PSDs showing the development of porosity in CMSs as a function of the fluorine ratio.
Compared to that of Raw-CMS, the CO2 adsorption capacity of fluorinated CMSs increased by approximately 2.7% because of an increase in the cumulative PSD (Fig. 5). In addition, the CO2 adsorption capacity decreased with an increase in temperature. Specifically, the CO2 adsorption capacity of Raw-CMS decreased by 38.1% as the temperature increased. The observed reduction in the adsorption capacity of Raw-CMS was greater than that of fluorinated CMSs because fluorination had a stronger effect at higher temperatures (Chamssedine et al., 2011). A possible mechanism of the interaction between the functionalized surfaces of the CMSs and the CO2 gas is provided in Section 4.2.
3.3. CH4 /CO2 gas separation behavior
Fig. 5. Cumulative PSD of Raw-CMS, F1N9-CMS, F3N7-CMS, and F5N5-CMS.
The CO2 adsorption capacities of fluorinated CMSs were greater than that of Raw-CMS. At 273 K, the CO2 adsorption capacities of fluorinated CMSs were 2.67 (F1N9-CMS), 2.66 (F3N7-CMS), and 2.67 (F5N5-CMS) mmol/g.
The characteristics of CH4 and CO2 gas adsorption showed in Fig. 7. In Fig. 7(a), the behavior of CH4 adsorption can be observed. All Samples are rapidly saturated as similar type due to very weak interaction between the CH4 gas and samples. However, the CO2 adsorption behavior of untreated and fluorinated CMS showed a different saturation pattern (Fig. 7(b)). Furthermore the CH4 /CO2 gas separation behavior of the samples was determined, and the results are presented in Fig. 8. The maximum concentration of saturated gas was one-half of the initial value because the mixed gas was diluted by 50% with nitrogen gas. In particular, CO2 separation behavior of samples had a similar pattern like as Fig. 7(b). As shown in Fig. 8, compared to Raw-CMS, saturation occurred slowly in the fluorinated samples because the mass transfer area was broad. The slow saturation kinetics
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Table 4 CO2 adsorption kinetics and CO2 uptake as a function of temperature. Sample
Raw-CMS
F1N9-CMS
F3N7-CMS
F5N5-CMS
CO2 uptake (at 273 K) (mmol/g) CO2 uptake (at 298 K) (mmol/g) The decrement of CO2 uptake (%)
2.60 1.61 0.99 (38.1%)
2.67 1.96 0.71 (26.4%)
2.66 2.03 0.63 (23.7%)
2.67 2.04 0.63 (23.6%)
was attributed to the effects of fluorination; specifically, fluorine present on the surface of the fluorinated CMSs interacts continuously with the CO2 gas for attracting CO2 molecules. Fluorination also enhanced the CO2 adsorption capacity by increasing the volume of the pores with diameters less than 8 A˚ (see Section 3.2 for more detail). In contrast, at adsorption times of less than 5 min, the saturated CH4 concentration increased slightly due to the decrement concentration of CO2 . In F5N5-CMS, the concentration of CH4 was lower than that in Raw-CMS because F5N5-CMS adsorbed small amounts of CH4 and CO2 gas because of its large total pore volume (see Table 3). Therefore, fluorination affected the CO2 adsorption capacity, which altered the behavior of the CH4 /CO2 gas separation. 4. Discussion 4.1. Effects of fluorine on the CO2 adsorption capacity The CO2 adsorption capacity was decreased with an increase in the reaction temperature. In general, the adsorption capacity decreases at higher temperatures because of an increase in the activity of the gas and the adsorption heat of material studied by the Clausius–Clapeyron equation and result of Chen’s group
Fig. 7. Gas adsorption of Raw-CMS, F1N9-CMS, F3N7-CMS, and F5N5-CMS using GC: (a) CH4 adsorption and (b) CO2 adsorption.
Fig. 8. CH4 /CO2 gas separation of Raw-CMS, F1N9-CMS, F3N7-CMS, and F5N5-CMS. Fig. 6. CO2 adsorption kinetics of Raw-CMS, F1N9-CMS, F3N7-CMS, and F5N5-CMS at 273 and 298 K.
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Fig. 9. Proposed mechanism of the interactions between the functionalized surface of CMSs and CO2 gas.
(Chen et al., 2012). In the present study, as the CO2 adsorption temperature increased, the adsorption capacity of Raw-CMS and the fluorinated CMSs was decreased by 38.1 and 25 vol.%, respectively. The observed reduction in the adsorption capacity was lower for the fluorinated CMSs because interactions between fluorine groups on the surface of CMS and CO2 are stronger at 298 K more than interactions at 273 K (Chamssedine et al., 2011). It is well known that the CO2 adsorption capacity strongly was related to the micorporosity at 273 K (Chen et al., 2012). In the other hand, interactions between fluorine groups on carbon surface and CO2 molecular was increased under 298 K due to enhancement of functional group activity. Therefore, CO2 adsorption capacity of modified CMSs was increased compared to untreated CMSs at 298 K. Fluorination introduced numerous defects and strongly basic sites (Chamssedine et al., 2011; Wu et al., 2007; Choudary et al., 2001; Zhou et al., 2006). Many papers have been presented to support this result considering the bond length, strength and polarity. The bond length of C F (0.1332 nm) is also longer than that of C H (0.1068 nm), the bond strength of C F (111 kcal mol−1 ) is larger than that of C H, and the polarity of C F is thus very strong (Ma et al., 2006). Moreover the adsorption of acidic CO2 alternatively may be enhanced by the presence of other basic functionalities on the carbon surface: CO2 is a weak Lewis acid (electron-acceptor) that can interact with electron-donors (Plaz et al., 2011). Therefore, the strength of Lewis acid–base interactions between the functionalized surfaces of the CMSs and CO2 gas was increased (Fig. 9(a)). In addition, the reactivity of fluorine was increased as the temperature increased from 273 to 298 K (Allayarov et al., 1999). Thus, the CO2 adsorption capacity was increased because fluorinated CMSs possess strongly basic groups, which were introduced under different fluorination conditions (Zhang et al., 2010).
of the fluorine which has high electric negativity. The amorphous carbon with weak carbon/carbon bonding was removed by fluorine gas reaction under circumstances such as fluorinated carbon gases (CF4 and C2 F6 ) (Im et al., 2011; Yoshiyuki et al., 2007; Pehrsson et al., 2003). The enlarged pore width affected gas separation by promoting CH4 adsorption in the early stages of the process. Therefore, the extent of the surface modifications and the pore structure of the material must be controlled to effectively separate gases and adsorb CO2 .
4.2. CO2 selectivity of fluorinated CMSs
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Many researchers have shown that CO2 has a significant quadrupole moment that induces specific interactions with adsorbents, whereas CH4 does not display this type of behavior (Llewellyn et al., 2006). Therefore, CH4 has weak adsorbate–adsorbent interaction forces compared to CO2 which has the strong dipole moment of carbonyl bonds. Fig. 9(b) presents the effect of fluorination on the selective adsorption of CH4 /CO2 mixed gases and the adsorption capacity of CO2 due to interactions between the fluorinated surface of CMSs and CO2 gas. However, high fluorine partial pressures increased the pore width of F5N5-CMS because of the high chemical reaction
5. Conclusions Surface-modified CMSs were prepared via fluorination at atmosphere pressure and room temperature to investigate the CH4 /CO2 separation behavior. The fluorine content of the surfaces of the CMSs and CO2 adsorption capacity increased to 22.5% and 2.67 mmol/g (273 K, F1N9-CMS) due to fluorination. Lewis acid–base interactions between the surface of fluorinated CMSs and CO2 were affected by the fluorine content, and the pore vol˚ of the CMSs increased ume (especially pores with diameters of 8 A) due to fluorination. The mass-transfer area of the CMSs also increased upon fluorination. Accordingly, compared to Raw-CMS, the CH4 /CO2 gas separation behavior of surface-modified CMSs changed to breakthrough late in the adsorption process. Acknowledgement This work was supported by the Korea Gas Corporation and HYUNDAI Engineering & Construction. References
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