CF4 Adsorption on Microporous Carbons Prepared by

Article pubs.acs.org/IECR

CF4 Adsorption on Microporous Carbons Prepared by Carbonization of Poly(vinylidene fluoride) Seung Wan Choi,†,§ Seok-Min Hong,†,§ Jong-Ho Park,‡ Hee Tae Beum,‡ and Ki Bong Lee*,† †

Downloaded by TEXAS A&M INTL UNIV on August 24, 2015 | http://pubs.acs.org Publication Date (Web): August 19, 2015 | doi: 10.1021/acs.iecr.5b01228

Department of Chemical and Biological Engineering, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 136-713, Republic of Korea ‡ Oil and Gas Center, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon 305-343, Republic of Korea ABSTRACT: CF4 is a gas with high global warming potential and has an extremely long atmospheric lifetime. This study developed a method for capturing CF4 gas via adsorption using microporous carbons. Microporous carbon adsorbents were synthesized by a facile protocol involving carbonization of poly(vinylidene fluoride) (PVDF) at high temperatures (400−800 °C) without additional activation, and the effects of carbonization on the characteristics and CF4 adsorption of PVDF-based adsorbents were investigated. Increasing the carbonization temperature enhanced the textural properties of the adsorbent, resulting in the increased CF4 adsorption capacity. Above 700 °C, PVDF was fully dehydrofluorinated, and the microporous carbon synthesized at 800 °C exhibited superior textural properties with a maximum CF4 adsorption capacity of 1.85 mol/kg at 25 °C under atmospheric pressure. The PVDF-based microporous carbons also exhibited fast adsorption−desorption kinetics, excellent cyclic stability, and good selectivity for CF4 over N2 at relatively low CF4 pressures. The microporous carbons developed in this study have potential for use as novel adsorbents for CF4 capture. operation, relatively low cost, and easy scale-up.13 Nonflammable and noncorrosive CF4 has a low impact on labor health, and only high concentration causes oxygen-deficient environments. However, its decomposition by pyrolysis produces toxic substances such as carbonyl fluoride (COF2) and hydrogen fluoride (HF).14,15 For this reason, adsorption is more appropriate for CF4 disposal without producing byproducts compared to the pyrolysis methods. Zeolites, activated carbons, metal−organic frameworks, and new materials have been widely used as adsorbents for CO2 separation.16,17 However, adsorption of CF4 has been rarely studied, except for certain adsorption isotherm evaluations using common commercial adsorbents. Ahn et al.18 acquired the adsorption isotherms for CF4 and C2F6 on various zeolites, activated carbons, and silica gel at room temperature using a volumetric adsorption method. Motkuri et al.19 evaluated the efficacy of various metal−organic frameworks for the adsorption of fluorinated carbons. Although the previously tested adsorbents exhibited potential for CF 4 separation, the adsorption capacities did not meet the requirements for commercial operation or the adsorption capacity was the only parameter evaluated among the many requirements for practical application as adsorbents. Moreover, newly synthesized materials have not been applied to CF4 adsorption. CF4 has a stable structure and low reactivity; therefore, a high surface area and pore volume may be the important characteristics for enhancing physisorption. Zeolites, metal−organic frameworks, and activated carbons commonly satisfy these criteria. Carbon adsorbents offer the distinguishing merits of low price, physical and chemical stability, and facile synthesis.20,21

1. INTRODUCTION Reducing the emission of greenhouse gases has been an area of immense concern due to the abnormal climate changes arising from global warming. CO2 is considered a major greenhouse gas, and its capture using several different separation methods has been actively studied.1−4 CO2 accounts for 70% of total greenhouse gas emissions; therefore, CO2 capture and storage technologies have been considered a prospective solution for abating the global warming problems.5 On the other hand, the global warming contribution of CO2 is much lower than that of the same mass of other greenhouse gases such as methane, perfluorocarbons (PFCs), and hydrofluorocarbons (HFCs). The contribution to global warming is quantitatively expressed as the global warming potential (GWP), based on instantaneous radiative force and atmospheric lifetime,6 and the GWP value is represented as a relative measure based on CO2 for which the GWP is standardized to 1. The 100-year GWP values of methane, PFCs, and HFCs are 21, 6500−9200, and 140−11 700, respectively.7 Among the gases with high GWP values, HFCs are conventionally used as refrigerants; however, extensive efforts are being expended to develop alternatives.8 Tetrafluoromethane (CF4), the simplest PFC molecule, has an extremely long lifetime of 50 000 years due to its stable tetrahedral structure, and its 100-year GWP is 6500.9 Generally, CF4 is used for dry etching processes in the semiconductor industry and for chemical vapor decomposition processes in aluminum production.10,11 Thus, the demand for CF4 as an electronic gas has continued to increase due to the rapidly growing semiconductor market, which makes an increase in CF4 emission inevitable.12 In order to mitigate the effect of CF4 on global warming, appropriate steps must be taken for its capture and removal. Compared with other separation technologies, adsorption is particularly effective for gas separation because of its convenient © XXXX American Chemical Society

Received: April 1, 2015 Revised: July 10, 2015 Accepted: August 12, 2015

A

DOI: 10.1021/acs.iecr.5b01228 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Carbon adsorbents are generally prepared by two steps: carbonization and activation. Carbonizable precursors having high carbon content are carbonized at high temperatures in an inert gas. The carbonized intermediate materials are partially gasified in a mildly oxidizing gas such as CO2 and steam to develop the desired porosity and surface area.22 To overcome the difficulty in obtaining a uniform pore size in carbon adsorbents using ordinary activation processes, various templates based on zeolites or silica materials have been used, but additional procedures are required to remove the template from the final carbon adsorbent.23,24 Recently, Xu et al.25 successfully prepared carbon with uniform micropores by carbonization of a poly(vinylidene fluoride) (PVDF) without additional tedious activation processes. In this study, PVDF-based porous carbons are applied to the adsorption of CF4 for the first time. Porous carbons are synthesized herein by simple carbonization of PVDF at high temperatures, and the effects of varying the carbonization temperature on the characteristics and CF4 adsorption behavior of the porous carbons are studied. The characteristics of PVDF and carbonized PVDFs are analyzed using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy. The textural properties of the porous carbons are evaluated based on N2 physisorption; CF4 adsorption is quantified by volumetric adsorption analysis; and cyclic adsorption−desorption is conducted by using a thermogravimetric analyzer.

adsorption at 30 °C under atmospheric pressure for 1 h and desorption under N2 purge for 1.5 h.

2. EXPERIMENTAL SECTION 2.1. Synthesis of PVDF-Based Porous Carbons. PVDF powder (Kynar 761, Elf AtoChem) was uniformly scattered in a boat-shaped alumina disk, and the disk was then placed in a horizontal tubular furnace. The PVDF sample was heated at a rate of 3 °C/min to a certain temperature in the range of 400− 800 °C under Ar, and the sample was maintained at the final temperature for 2 h. Herein, PVDF-based porous carbon is notated as PBPCx, where x indicates the carbonization temperature. 2.2. Characterization of Samples. The morphologies of the pristine PVDF and the PBPC samples were analyzed by using a scanning electron microscope (SEM, S-4300, Hitachi) and a high-resolution transmission electron microscope (HR-TEM, G2 F30ST, Tecnai). Energy dispersive X-ray spectroscope (EDX, EX-200, Horiba) was used to analyze the elemental composition of pristine PVDF and the PBPC samples. The textural properties of the samples were estimated using the Brunauer−Emmett−Teller (BET) equation based on the N2 adsorption data acquired at −196 °C using a volumetric adsorption analyzer (ASAP2020, Micromeritics). Functional group modifications during carbonization were evaluated using Fourier transform infrared spectroscope (FT-IR, Nicolet iS10, Thermo Scientific). The structure of the samples was analyzed using an X-ray diffractometer (XRD, X’Pert MPD, Philips) with Cu Kα radiation in the 2θ range of 5−50°. The weight change of pristine PVDF was measured with a thermogravimetric analyzer (TGA, Q50, TA Instruments) at a heating rate of 3 °C/min under 100 mL/min Ar gas flow. CF4 adsorption tests were conducted using a volumetric adsorption analyzer (BELSORP-mini II, BEL Japan). Prior to the adsorption tests, samples were degassed at 150 °C under vacuum for 12 h, and the adsorption isotherms were obtained at three different temperatures of 25, 40, and 60 °C. The cyclic adsorption stability was evaluated using TGA along with CF4

and PBPC800, respectively. Pristine PVDF is characterized by a uniform spherical morphology, whereas carbonization led to the development of an irregular shape. No noticeable morphological differences were apparent for the PBPC samples carbonized at different temperatures. The HR-TEM image of PBPC800 shown in Figure 1d demonstrates the well-distributed pores with a size less than 1 nm, suggesting that well-developed micropores were formed during the carbonization process.26,27 Figure 2a shows the N2 adsorption isotherms of the PBPC samples, acquired at −196 °C. All samples exhibit type I isotherms according to the IUPAC classification, which indicates probable dominance of micropores.28 The total N2 adsorbed by PBPC increased as the carbonization temperature increased. Table 1 summarizes the textural properties of the PBPC samples estimated from the N2 adsorption data. The specific surface area of the PBPCs was assessed using the standard BET method, and the micropore volume was calculated by the Dubinin−Astakhov method. The surface area and pore volume increased with increasing carbonization temperature, and the maximum values were obtained with PBPC800. PBPC700 and PBPC800 exhibited similar textural properties because both samples were fully carbonized at sufficiently high temperatures. Figure 2b shows the pore size distribution of PBPCs carbonized at different temperatures. All PBPCs show a very narrow peak at less than 1 nm. As the carbonization temperature increased, the dominant pore size of the PBPCs decreased from ∼0.85 nm to ∼0.7 nm. Figure 3a indicates the weight change of pristine PVDF with increasing temperature under Ar flow. An appreciable weight decrease was initiated at 380 °C, and a steep weight change was observed up to 420 °C followed by a continuous, slow weight loss up to 720 °C. Carbonization of PVDF is expected to occur most significantly around 400 °C and to be complete at 720 °C. The elemental composition of pristine PVDF and the PBPCs was measured using EDX (Figure 3b). Fluorine accounts for 32% of the total weight of pristine PVDF; the fluorine content

3. RESULTS AND DISCUSSION 3.1. Characteristics of PVDF-Based Porous Carbons. Figure 1a−c shows the SEM images of pristine PVDF, PBPC400,

Figure 1. SEM images of (a) pristine PVDF, (b) PBPC400, and (c) PBPC800. (d) HR-TEM image of PBPC800.

B


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Industrial & Engineering Chemistry Research

Figure 3. (a) Weight change of pristine PVDF with increasing temperature under Ar gas flow and (b) elemental composition of pristine PVDF and PBPCs.

Figure 2. (a) N2 adsorption (solid symbols) and desorption (open symbols) at −196 °C and (b) DFT pore size distributions of PBPCs.

Table 1. Textural Properties Estimated from N2 Adsorption Data at −196 °C and CF4 Adsorption Capacity at 25 °C for PVDF-Based Porous Carbons sample

SBET (m2/g)

VTa (cm3/g)

VMb (cm3/g)

CF4 adsorption capacityc (mol/kg)

PBPC400 PBPC500 PBPC600 PBPC700 PBPC800

586 903 955 971 991

0.268 0.373 0.412 0.437 0.434

0.240 0.365 0.386 0.396 0.403

0.57 1.27 1.68 1.84 1.85

The XRD diffraction patterns of pristine PVDF and the PBPCs are presented in Figure 4a. Three main peaks were observed at 18.4°, 20.0°, and 26.6°, which are characteristic of the α-phase of PVDF and represent the (020), (110), and (021) reflections of pristine PVDF, respectively.29 However, after carbonization, no specific peaks were evident. The FT-IR spectrum of pristine PVDF (Figure 4b) shows peaks characteristic of the α-phase at 796, 855, 880, 976, and 1074 cm−1 and of the β-phase at 840 and 1409 cm−1.30−32 No notable peaks were detected in the FT-IR profile of the carbonized PVDF samples. These results imply that the crystalline phase of PVDF was degraded and an amorphous carbon phase was generated during the carbonization step. 3.2. CF4 Adsorption on PVDF-Based Porous Carbons. Figure 5a shows the CF4 adsorption isotherms acquired at 25 °C for PBPCs synthesized at various carbonization temperatures in the range of 400−800 °C. Increasing the carbonization temperature increased the CF4 adsorption capacity of PBPC, which is attributed to the enhanced surface area and micropore volume that resulted from the increased carbonization temperature. CF4 has a very stable nonpolar tetrahedral structure, and its permanent dipole and quadrupole moments are zero.33 In this light, CF4 adsorption is probably controlled by physisorption rather than chemisorption, and a large surface area and pore volume may be more crucial for CF4 adsorption than any other properties of the adsorbent. Among the PBPCs evaluated, the highest CF4 adsorption capacity of 1.85 mol/kg at 25 °C and 1

Total pore volume at P/P0 ∼ 0.99. bMicropore volume determined from the Dubinin−Astakhov equation. cCF4 adsorption capacity measured by a volumetric method at 25 °C.

a

declined gradually with an increase in the carbonization temperature up to 700 °C. At 700 °C, fluorine was completely removed and only carbon remained, resulting in complete carbonization of PVDF. PBPC700 and PBPC800 contained no fluorine due to the high carbonization temperature and a sufficiently long time for dehydrofluorination. These results agree well with the change of textural properties of materials (Table 1), suggesting that the removal of fluorine from PVDF played a major role in developing micropores, which eventually increased total pore volume and surface area. C


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value for ordinary physisorption. The isosteric heat of adsorption decreased with increasing amount of CF4 adsorbed up to ∼1 mol/kg, implying heterogeneity of adsorbent. It is thought that CF4 molecules are preferentially adsorbed on the highly active sites based on stronger interaction between adsorbent and CF4 molecules at low loading and it causes higher release of heat. Figure 7 shows the kinetic data of CF4 adsorption on PBPC800 obtained using a thermogravimetric analyzer and the fitting with adsorption kinetic models. The CF4 adsorption kinetic was relatively fast, and 80% of the maximum adsorption capacity was achieved within 17 min (Figure 7a). The most widely used kinetic models, pseudo-first-order adsorption kinetic model, and pseudo-second-order adsorption kinetic model, were compared with data, and the models are represented below.39,40 Pseudo-first-order adsorption kinetic model:

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dt

= k1(qe − qt )

(1)

Pseudo-second-order adsorption kinetic model dqt dt

= k 2(qe − qt )2

(2)

where k1 and k2 are the rate constants of pseudo-first-order adsorption kinetic model and pseudo-second-order adsorption kinetic model, respectively, qe is the equilibrium adsorption capacity, and qt is the time-dependent adsorption capacity. Integration of eqs 1 and 2 for the boundary conditions of q = 0 at t = 0 and q = q at t = t gives linear forms of models as follows: log(qe − qt ) = log qe − t 1 t = + qt qe k 2qe 2

Figure 4. (a) XRD patterns and (b) FT-IR spectra of pristine PVDF and PBPCs.

k1t 2.303

(3)

(4)

The linear plots of log(qe − qt) against t and t/qt against t are represented in Figure 7b,c, respectively, and the pseudo-secondorder adsorption kinetic model shows much better correlation than the pseudo-first-order adsorption kinetic model. It was reported that the pseudo-second-order adsorption kinetic model matches experimental data when the concentration of adsorbate in gaseous state is not much higher than that in the adsorbent surface.41 From the plot of qt against t0.5 in Figure 7d, rapid initial adsorption was discerned, and then slow adsorption followed after a few minutes, which was probably caused by intraparticle diffusion.39 The cyclic stability analysis of CF4 adsorption−desorption by PBPC800 using a thermogravimetric analyzer is shown in Figure 8. Adsorption was carried out under a flow of CF4 for 60 min at 30 °C and 1 atm, and N2 purging was performed in the desorption step carried out for 90 min at the same temperature and pressure. For seven repetitive cycles, a stable working capacity of 1.29 mol/kg was well maintained. In addition to the excellent cyclic stability, fast adsorption−desorption kinetics was observed. CF4 is normally emitted with a large amount of N2 gas; therefore, the CF4/N2 selectivity of the adsorbent is an additional factor for CF4 removal processes. In this study, the CF4/N2 selectivity was estimated from the experimental data obtained from single-component isotherm measurements by using the ideal adsorbed solution theory (IAST).42,43 N2 adsorption on PBPC800 was measured using a volumetric adsorption analyzer at 25 °C resulting in a linear isotherm, indicative of weak

atm was achieved with PBPC800. This adsorption capacity is higher than that of certain reported zeolites, MOFs, and activated carbons: 0.47 and 0.78 mol/kg for zeolite 5A and zeolite 13X, respectively, at 30 °C and 1 atm;18 0.99). CF4 adsorption isotherms are also represented in logarithm scale (Figure 5c,d) to clearly see the CF4 adsorption at the low pressure region that is important for practical application. The isosteric heat of adsorption of CF4 on PBPC800 was calculated from the CF4 adsorption isotherm data acquired at three different temperatures by using the Clausius−Clapeyron equation.37,38 The calculated isosteric heat of adsorption was in the range of 26−28 kJ/mol (Figure 6), which corresponds to the D


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Figure 5. CF4 adsorption isotherms of (a) PBPCs at 25 °C and (b) PBPC800 at 25, 40, and 60 °C; log−log plots of CF4 adsorption isotherm data of (c) PBPCs at 25 °C and (d) PBPC800 at 25, 40, and 60 °C.

Table 2. Langmuir Adsorption Isotherm Model Parameters for the CF4 Adsorption on PBPC800 q = qmaxbP/(1 + bP) temperature (°C)

qmax (mol/kg)

b (1/atm)

H (= qmax × b, mol/kg·atm)

R2

25 40 60

2.46 2.21 1.90

2.716 1.756 1.332

6.668 3.872 2.524

0.9968 0.9973 0.9992

adsorbent/adsorbate interaction. The calculated IAST selectivity was in the range of 2−18, as shown in Figure 9. The CF4/N2 selectivity of the adsorbent increased exponentially with decreasing CF4 partial pressure. This result is promising because the concentration of CF4 emitted in industry is very low. It is believed that PBPC800 has stronger interaction with CF4 than N2 at the low pressure region due to the larger dipole polarizability of CF4 (2.824 Å) compared to that of N2 (1.710 Å).44 The decrease of selectivity with increasing CF4 partial pressure is attributed to the heterogeneity of PBPC800.

Figure 6. Isosteric heat of adsorption of CF4 on PBPC800.

PVDF during the carbonization step. Increasing the carbonization temperature from 400 to 800 °C increased the surface area and pore volume of the carbons, resulting in enhanced CF4 adsorption. The crystal structure of PVDF was degraded with generation of an amorphous carbon phase during carbonization; PVDF was fully carbonized above 700 °C. PBPC800 had the

4. CONCLUSIONS Novel porous carbons for CF4 adsorption were synthesized by simple carbonization of PVDF without further activation. Uniform micropores were developed by dehydrofluorination of E


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Figure 7. (a) Kinetic data of CF4 adsorption on PBPC800, (b) fitting with pseudo-first-order adsorption kinetic model, (c) fitting with pseudo-secondorder adsorption kinetic model, and (d) plot for intraparticle adsorption kinetic model.

Figure 8. Cyclic test of CF4 adsorption−desorption using PBPC800.

Figure 9. IAST selectivity of CF4 over N2 at 25 °C for PBPC800.

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highest micropore volume of 0.403 cm3/g and BET surface area of 991 m2/g and exhibited a remarkable CF4 adsorption capacity of 1.85 mol/kg at 25 °C and 1 atm. PBPC800 also exhibited fast adsorption−desorption kinetics, excellent cyclic stability, and high CF4/N2 selectivity at low CF4 pressure. The porous carbons obtained by carbonization of PVDF are considered promising adsorbents for CF4 removal.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +82 2 3290 4851. Fax: +82 2 926 6102. E-mail: [email protected] (K.B. Lee). Author Contributions

§ These authors contributed equally (S.W. Choi and S.-M. Hong).

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Industrial & Engineering Chemistry Research Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by the “R&D Center for Reduction of Non-CO2 Greenhouse Gases (2013001690013)” funded by the Korean Ministry of Environment (MOE) as the “Global Top Environment R&D Program” and by a Human Resources Development Program (20134010200600) grant from the Korean Institute of Energy Technology Evaluation and Planning (KETEP), funded by the Korean Ministry of Trade, Industry, and Energy.


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CF4 Adsorption on Microporous Carbons Prepared by

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