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Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX
Adsorption Properties of N2, CH4, and CO2 on Sulfur-Doped Microporous Carbons Wei Su,† Lan Yao,† Meng Ran,† Yan Sun,*,‡ Jia Liu,§ and Xiaojing Wang*,† †
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Tianjin Key Laboratory of Membrane and Desalination Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350, P. R. China ‡ Department of Chemistry, School of Science, Tianjin University, Tianjin 300350, P. R. China § Nanyang Technological University, School of Physics and Mathematics, CBC, Singapore ABSTRACT: A series of sulfur-doped microporous carbon materials were prepared by directly using potassium hydroxide as the activating agent. The specific surface area and pore volume of the sample CKS-5 (activated at 800 °C for 180 min according to an alkali/carbon ratio of 4:1) reached 2088 m2/g and 1.240 cm3/g, respectively. The adsorption isotherms of N2, CH4, and CO2 on five samples were determined by a volumetric method to obtain insight into the relationship between adsorption performance and porosity. CSK-5 with a developed pore structure exhibited a higher adsorption amount of CO2 and CH4 at high pressure. For the selectivity, the CS sample presented the highest selectivity for CO2/CH4, of which the selectivity of separation reached as high as 5.86. The highest selectivity of separation of CH4/N2 (3.644) was present on CSK-7.
1. INTRODUCTION Activated carbon is widely used in pharmaceutical, chemical, and environmental fields due to its well-developed porosity, good thermal stability, low cost, and easy modification.1−4 The physicochemical properties of carbon materials with heteroatoms (N/B/P/S) have been well investigated.5,6 Doping with nitrogen not only improves the adsorption selectivity of carbon material to CO27 but also enhances the storage properties of electrode materials in supercapacitors.8 N-doping of carbon also improves catalyst service life and catalytic performance.9 Bdoped carbon presents an excellent hydrogen storage capacity (up to 5.9 wt %),10 improved oxidation resistance, and loss rate decrease of 20%.11 Compared with nitrogen and boron, sulfur atoms are difficult to introduce into the activated carbon skeleton due to their larger radii.12 Sulfur-doped carbon can be obtained directly by carbonization of sulfur-containing precursors.13 However, the surface area of carbon is relatively low. To improve the pore volume and surface area, sulfur-doped carbon is often synthesized via a hard template, such as hexagonally packed mesoporous silica,14 EMC-2,15 SBA-15,16 and so on. Sulfurdoped carbon always shows outstanding performance for gas adsorption and separation. For example, by comparing the adsorption properties of activated carbon doped with different heteroatoms (N/P/S), Li et al.14 discovered that CO2 has a high affinity for sulfur. By utilizing zeolite EMC-2 as a template and chemical vapor deposition, sulfur-doped microporous activated carbon (SMAC) was prepared by Xia et al.,15 and the carbons present a high adsorption capacity for H2 and CO2. © XXXX American Chemical Society
S-doped mesoporous activated carbon with a high specific surface area and strong adsorption capacity for mercury were synthesized by Shin et al.16 by using sulfur-containing thiophene as a precursor and SBA-15 as a template. The hard template method was complex and costly.13 Recently, Saha et al.17 reported a simple method to synthesize sulfur-doped porous carbons. Additionally, the method was easy to scale up and the sulfur-doped carbon presented a high selectivity of separation for CO2/CH4 due to its high sulfur content (12.9%). Furthermore, the isoteric heat of CO2 at low coverage was 60− 65 kJ/mol, which was the highest in porous carbon. In the present work, SMAC was prepared by KOH activation of sulfur-containing precursors (sodium 4-styrenesulfonate), which is simpler and less expensive than the hard template method. A series of activated carbon samples was prepared under different activation conditions to investigate their adsorption properties for carbon dioxide, methane, and nitrogen.
2. EXPERIMENTS 2.1. Preparation of SMAC. Poly(sodium 4-styrenesulfonate) brought from Beijing HWRK Chem. Co., Ltd., was used as the precursor. First, the precursor was heated at 800 °C for 40 min with a N2 flow of 300 mL/min and a heating rate of 10 °C/min. The obtained black powder was washed with 1.2 mol/ Received: March 21, 2018 Accepted: May 28, 2018
A
DOI: 10.1021/acs.jced.8b00227 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Table 1. Preparation Conditions and Pore Structure Characteristics of Activated Carbon conditions of activation
pore structure characteristics
sample
KOH/C ratio
time (min)
temp (°C)
specific surface area (m2·g−1)
pore volume (cm3·g−1)
CS CSK-1 CSK-2 CSK-3 CSK-4 CSK-5 CSK-6 CSK-7 CSK-8
4:1 3:1 5:1 4:1 4:1 4:1 4:1 4:1
120 120 120 60 180 240 180 180
800 800 800 800 800 800 700 850
307 1729 1386 1052 1095 2088 1983 1324 1194
0.280 1.047 0.887 0.651 0.653 1.240 1.207 0.811 0.873
Figure 1. N2 adsorption isotherms at 77 K and pore size distributions of nine samples.
2.2. Characterization and Measurements. The N2 adsorption isotherms of samples at 77 K were determined and used to calculate the Brunauer−Emmett−Teller (BET) surface area. The nitrogen adsorption capacity when P/P0 = 0.99 was used to calculate the pore volume. The pore size distribution (PSD) was calculated by using density functional theory (DFT). Scanning electron microscopy (SEM) (S-4800, Japanese Hitachi Co., Ltd.) and transmission electron
L hydrochloric acid and deionized water, respectively, and then dried at 393 K overnight. The obtained powder was named CS. Then, CS and KOH were mixed, ground, and heated at the activation temperature for some time with a N2 flow of 100 mL/min and a heating rate of 10 °C/min. The obtained mixture was washed and dried with the same method as that of CS. The obtained powder was SMAC named CSK-1−CSK-8. The preparation conditions are shown in Table 1. B
DOI: 10.1021/acs.jced.8b00227 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Figure 2. SEM and TEM photographs of activated carbon samples (parts a and b are SEM and TEM graphs of CS, and parts c and d are SEM and TEM graphs of CSK-5, respectively).
microscopy (TEM) (Tecnai G2 F20, Netherlands FEI, Ltd.) were used to observe the surface morphology of the samples. Xray diffraction (XRD) (D/MAX-2500, Japan Science Co., Ltd.), Fourier transform infrared spectrometry (FT-IR) (TENSOR27, Brooke Germany, Ltd.), elemental analysis (EA) (Vario Micro cube, Alimonta Germany Co., Ltd.), and X-ray photoelectron spectroscopy (XPS) (ESCALAB 250, Thermo Scientific Inc.) were used to characterize the sulfur-doped samples. 2.3. Adsorption Performance Tests. The CO2, CH4, and N2 adsorption isotherms of five samples (CS, CSK-1, CSK-2, CSK-5, and CSK-7) were determined by a volumetric method under 0−0.1 MPa. The high-pressure adsorption isotherms were collected with the high-pressure adsorption devices18,19 at a temperature range from 268 to 308K, and the maximum adsorption pressure was approximately 4.5 MPa.
Figure 4. IR diagram of five samples.
Table 2. Elemental Analysis (EA) of the Sulfur Content of Activated Carbon Samples
3. RESULTS AND DISCUSSION 3.1. Nitrogen Adsorption at 77 K. The nitrogen adsorption isotherms at 77 K for the samples are shown in
sample
CS
CSK-1
CSK-2
CSK-5
CSK-7
S content (wt %)
3.84
0.34
0.48
0.26
0.84
activation temperature, and activation time are three important parameters in the preparation of activated carbon. Both the specific surface area and pore volume increase as the KOH/CS ratio increases from 3 to 4, meaning that more activating agent can develop greater porosity. However, for a KOH/CS ratio of 5, the specific surface area and pore volume decreases, and the porous structure is destroyed by the excessive reaction between KOH and CS. More time is required to achieve a welldeveloped porous structure. The best activation time is approximately 180 min for 800 °C and the KOH/CS ratio of 4. The boiling point of potassium is approximately 762 °C. If the activation temperature is higher than 762 °C, potassium steam will diffuse in the carbon framework, which can produce extra porosity. As a result, the activation reaction at 800 °C is better than that at 700 °C. For 850 °C, the specific surface area and pore volume decrease due to the faster reaction rate. In summary, the porosity of the CSK-5 sample is most developed (with a specific surface area of up to 2088 m2/g and a total pore volume of up to 1.240 cm3/g), which was prepared using a KOH/CS ratio of 4 at 800 °C for 3 h.
Figure 3. XRD diagram of five samples.
Figure 1. The specific surface area and pore volume of the activated carbon samples are summarized in Table 1. All of the isotherms are type I isotherms, indicating a microporous structure, which has been verified by the PSDs of the samples presented in Figure 1. The specific surface area and pore volume of the carbons are significantly increased by KOH activation. The KOH/CS ratio, C
DOI: 10.1021/acs.jced.8b00227 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Figure 5. XPS diagrams of five activated carbon samples (S 2p).
As shown in Figure 2, CS showed an obvious lamellar structure, while CSK-5 showed fluffy and porous amorphous morphology that was not obviously stratified. No sharp diffraction peaks were observed from Figure 3, which indicated the samples were amorphous, agreeing well with the SEM and TEM results. Two wide peaks at 23 and 43° were observed in CS, which demonstrated that the precursor had a low degree of graphitization during carbonization,12,14 while the peaks disappeared in the SMAC samples due to the destruction of graphite microcrystals during chemical activation. The FTIR spectra of all five samples are very similar (see Figure 4), with a slight difference in peak intensities. The band at 3445 cm−1 can be assigned to the stretching vibration of an intermolecular hydrogen bond resulting from the association of water molecules and some oxidized hydrophilic groups in the material. The band at 1626 cm−1 is assigned to the CC vibration. This bond probably appeared due to the recombi-
Table 3. Characteristics of Sulfur Functionality Content in XPS content/functionality (%)
CS
CSK-1
CSK-2
CSK-5
CSK-7
total carbon content total oxygen content total sulfur content CSC CSOx S 2p3/2 S 2p1/2 SO SO
86.1 11.8 2.1 0.91 1.19 33.25 15.82 35.11 15.82
83.5 16.3 0.2 0.03 0.17 9.61 5.12 57.96 27.31
80.5 19.3 0.2 0.05 0.15 14.72 7.76 51.65 25.87
78.0 21.9 0.1 0.01 0.09 3.11 1.48 63.56 31.85
84.5 15.1 0.4 0.12 0.28 20.70 10.34 45.99 22.96
3.2. Characterization. Five representative samples (CS, CSK-1, CSK-2, CSK-5, and CSK-7) were selected for the analysis by SEM, TEM, XRD, FTIR, EA, and XPS.
Figure 6. Adsorption isotherms of CSK-5 to CO2, CH4, and N2 at high pressure (points, experimental values; solid lines, Langmuir−Freundlich equation fitting values). D
DOI: 10.1021/acs.jced.8b00227 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Table 4. Langmuir Equation Parameters for CO2, CH4, and N2 on CSK-5 adsorbate
parameter
268 K
278 K
288 K
298 K
308 K
CO2
nm (mmol·g−1) B (MPa−1) R2 nm (mmol·g−1) B (MPa−1) R2 nm (mmol·g−1) B (MPa−1) R2
21.626 2.353 0.99828 12.319 1.404 0.99709 10.574 0.493 0.99729
21.006 1.795 0.99787 11.684 1.192 0.99793 10.516 0.399 0.99927
20.962 1.265 0.99822 11.742 0.955 0.99825 9.699 0.385 0.99800
18.739 1.152 0.99942 11.363 0.789 0.99811 8.554 0.369 0.99984
17.215 1.021 0.99993 11.649 0.599 0.99836 7.720 0.328 0.99942
CH4
N2
Table 5. Langmuir−Freundlich Equation Parameters for CO2, CH4, and N2 on CSK-5 adsorbate
parameter
268 K
278 K
288 K
298 K
308 K
CO2
nm (mmol·g−1) B (MPa−1) q R2 nm (mmol·g−1) B (MPa−1) q R2 nm (mmol·g−1) B (MPa−1) q R2
23.790 1.689 0.845 0.99992 14.161 0.991 0.794 0.99990 14.868 0.307 0.787 0.99967
23.354 1.303 0.832 0.99983 13.262 0.893 0.823 0.99992 12.233 0.327 0.896 0.99975
23.238 0.983 0.844 0.99984 13.496 0.724 0.832 0.99992 13.812 0.245 0.810 0.99969
19.810 1.009 0.911 0.99990 13.477 0.583 0.823 0.99986 9.192 0.336 0.949 0.99995
17.395 0.997 0.983 0.99994 14.291 0.438 0.828 0.99992 8.533 0.290 0.9367 0.99995
CH4
N2
CS, SS, and CS, respectively,15,20 and are consistent with the SEM and XRD results. The content of sulfur measured by elemental analysis was summarized in Table 2. CS had the highest sulfur content. During the activation reaction, the sulfur atom was more active in the reaction. As a result, the sulfur content of SMAC significantly decreased. XPS was used to determine the chemical state of sulfur. The 2p orbital spectrogram of S is shown in Figure 5, and the results are shown in Table 3. Two fitting peaks with a 2:1 intensity ratio at 163.7 eV (2p3/2) and 164.9 eV (2p1/2) were observed,12,15 corresponding to CSC. The peak at 168.3 eV corresponds to oxidized sulfur in the form of SO, and the peak at 169.5 eV is attributed to SO. Thus, all samples were successfully incorporated with sulfur in the nonoxidized state (CSC) and in the oxidized state (CSOx). The amount of sulfur on CS was highest, which decreased after activation especially on CSK-5. On CSK-5, the peak intensity at
Figure 7. Isosteric adsorption heat divergence diagram of CSK-5 to CO2, CH4, and N2.
nation of chemical bonds during the carbonation and activation processes. Bands at 1120, 910, and 775 cm−1 correspond to
Figure 8. Adsorption isotherms of CO2, CH4, and N2 on five samples at 298 K. E
DOI: 10.1021/acs.jced.8b00227 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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Figure 9. Selectivity separation of five samples for CO2/CH4 and CH4/N2 (298 K).
agrees better with the Langmuir−Freundlich equation than with the Langmuir equation. According to the Gibbs−Helmholtz equation, the data of the five different temperatures of 268−308 K were calculated as the isosteric heat of adsorption. The results are shown in Figure 7. As the adsorption capacity increased from 0.6 to 14 mmol/g, the isosteric heat of CH4 on CSK-5 decreased from 18.55 to 16.34 kJ/mol, and then gradually leveled off. The isosteric heat of CO2 and N2 fluctuated between 21.55 and 25.40 kJ/mol and 14.83 and 17.85 kJ/mol, respectively, decreasing first and then increasing with the increase in adsorption capacity. The isosteric heat of CO2 for CSK-5 at high coverage was similar to that of sulfur-doped carbon reported by Xia15 and was higher than that for pure carbon and N-doped carbon.23 Both sulfur content and PSD play an important role in determining the interaction between CO2 and carbon.15 3.4. Low-Pressure Adsorption Performance. The adsorption data of five samples (CS, CSK-1, CSK-2, CSK-5, and CSK-7) at low pressure were measured, and the adsorption isotherms are shown in Figure 8. The adsorption capacity of CS is lower than that of the other four samples. The PSDs of the samples are similar. Therefore, the adsorption capacity was mainly determined by the pore volume and surface property. The sulfur content of CS was the highest, which could improve the adsorption capacity of CO2. However, the pore volume of CS was much lower than that of the other four samples. As a result, CS presented the lowest adsorption capacity. The adsorption selectivity S1,2 was used to evaluate the separation effect of adsorbents on the mixed system. The definition of binary mixture adsorption selectivity S1,2 is
164 eV was very weak, which confirmed that the activation broke the CS bond and introduced oxidized sulfur.12 At lower activation temperatures, the fewer carbonsulfur bonds in CSK-7 were observed. The temperature condition is an important factor determining the chemical state of sulfur in the material. By controlling the temperature, the ratio of sulfur chemical states can be adjusted. 3.3. High-Pressure Adsorption Performance. The sample with the most developed pore structure, CSK-5, was selected for high-pressure adsorption tests (see Figure 6). The adsorption isotherms of CSK-5 for CO2, N2, and CH4 are type I isotherms. The adsorption amount of CO2 (14.07−18.97 mmol/g) was more than the adsorption amount of CH4 (8.63− 10.72 mmol/g). The N2 adsorption amount showed the lowest value: 4.65−7.48 mmol/g. However, the adsorption mechanisms for these three gases were different. The critical temperature of methane is 126 K, which is lower than the experimental temperature. Thus, the methane adsorption mechanism is monolayer supercritical adsorption. The critical temperature of CO2 (equal to 304.3 K) is closer to the experimental range. Therefore, the adsorption process included supercritical adsorption and subcritical adsorption. Micropores below 2 nm were filled at subcritical temperature, and the adsorption isotherm was of the type I. The Langmuir and Langmuir−Freundlich equations were used for the fitting analysis.21,22 The Langmuir equation is given by ⎛ n ⎞ B·P θ=⎜ ⎟= ⎝ nm ⎠ 1 + B · P
(1)
⎛ x1/y ⎞ 1⎟ S1,2 = ⎜⎜ ⎟ x / y ⎝ 2 2⎠
where θ is the ratio of surface coverage, nm is the ratio of the adsorbent monolayer adsorption capacity to the adsorbent mass (constant value), B is the parameter of the equation, and p is the gas pressure at the adsorption equilibrium. The Langmuir−Freundlich equation is given by ⎛ n ⎞ B·P q θ=⎜ ⎟= q ⎝ nm ⎠ 1 + B · P
(3)
where xi and yi are the mole fractions of the adsorption phase and the gas phase, respectively. The ideal adsorption solution model (IAST) was used to calculate the selectivity,24 and the results were presented in Figure 9. The CO2/CH4 selectivity of CS was higher than that of the other four samples. The main reason for such a high value is attributed to its PSD and functional groups on the surface. The nonactivated CS samples possessed most of the nonoxidized sulfur functional groups (C−S−C). The interaction between the quadrupole moment of the CO2 molecule and the polar interpole of the sulfur functional group, such as the acidic CO2 molecule and the basic
(2)
where q is the nonuniformity parameter of the adsorbent surface. The coefficient of determination R2 and the parameters are listed in Table 4 and Table 5. All R2 values in Table 4 are above 0.997, while those in Table 5 are all above 0.999. R2 values in Table 5 are closer to 1, indicating that the experimental data F
DOI: 10.1021/acs.jced.8b00227 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
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C−S,13,14,25 resulted in CS exhibiting a high selectivity of CO2.26 However, the interaction between methane molecules and sulfur-containing functional groups was weaker, resulting in the carbon pore sizes mainly determining the selectivity of activated carbon. Therefore, CSK-7 had the best CH4/N2 selectivity (3.644).
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4. CONCLUSIONS Porous sulfur-doped activated carbon materials were successfully achieved by carbonizing sulfur-containing precursors and activation by potassium hydroxide. By optimizing the synthesis condition, the specific surface area and pore volume can reach 2088 m2/g and 1.240 cm3/g, respectively. Evidently, S-doped activated carbon is an amorphous material with a fluffy porous structure. Sulfur exists in two forms: oxidation state (C−S−C) and non-oxidation state (C−SOx−). CSK-5 with a high specific surface area showed a high adsorption capacity for carbon dioxide and methane. Effectively restraining the loss of S atoms is a key problem to be explored in the future when expanding the pore structure of carbon materials. The selectivity of samples to CH4/N2 was mainly affected by the pore structure. The best selectivity of CH4/N2 reached 3.644 on CSK-7.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Jia Liu: 0000-0002-0892-1539 Xiaojing Wang: 0000-0002-1493-3333 Funding
The support from Natural Science Foundation of Tianjin City (No. 14JCYBJC21200) is greatly appreciated. Notes
The authors declare no competing financial interest.
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REFERENCES
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DOI: 10.1021/acs.jced.8b00227 J. Chem. Eng. Data XXXX, XXX, XXX−XXX
