CH4

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Cobalt Incorporated Porous Aromatic Framework for CO2/CH4 Separation Weichao Zhang, Yuanhui Cheng,* Chunshuai Guo, Chengpeng Xie, and Zhonghua Xiang* State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing, 100029, PR China

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S Supporting Information *

ABSTRACT: Natural gas decarburization is an important issue as CO2 reduces its energy density and corrodes equipment with humidity. Here, we used PAF-1 as pours platform and derived PAF-1-SO3Co as an efficient and stable CO2/CH4 separation material by introducing cobalt sulfonate groups. Particularly, transition metal cobalt owns rich d-electron orbit and interacts with polar molecules evidently, which is suitable for improving the interaction between polar adsorbate and adsorbent. The experiment results further demonstrate this rule and show that the selectivity of PAF-1-SO3Co reaches 14.27 at 298 K and 1 bar, which has increased almost 5 times than PAF-1. Importantly, PAF-1-SO3Co exhibits remarkably high regenerability (75%) and sorbent selection parameter (S, 85.8). Moreover, PAF-1-SO3Co has dramatically high working stability, whose adsorption performance shows ignorable decrease after soaking in simulated acidic environment even for 1 week. Accordingly, PAF-1SO3Co is a promising adsorbent for separation and purification of CO2 from natural gas.

1. INTRODUCTION In recent years, because of the massive consumption of fossil fuels, the concentration of carbon dioxide (CO2) in the atmosphere has been sharply rising, which leads to global warming and subsequent a series of natural disasters.1−4 In order to curb atmospheric CO2 concentration, numerous manpower and finance has been devoted to the field of developing low-carbon green energy with high calorific value.5 Although natural gas (NG) still emits CO2 after combustion, since its lowest ration of C/H among the fossil fuel, NG is still considered to be one of the most attractive energy carrier for resolving energy related problems at the current stage of development relying on the fossil fuel.6,7 The main component of NG is methane, typically 80−95%, but also containing some other gases such as carbon dioxide, nitrogen, and heavier hydrocarbons, depending on the source of the gas.8,9 Moreover, methane in landfill gas is also a significant source of NG.10 However, it often contains unacceptable level of contaminants which degrade the purity and calorific value of the NG. Generally, a typical industrial or municipal landfill gas contains approximately 40−60% of carbon dioxide.11 For enabling NG to be an alternative energy source to conventional fuels, efficiently capturing CO2 from methane is essential for the upgrading of NG and the treatment of landfill gas to improve purity and reduce pipeline corrosion induced by acid CO2 gas in the presence of water.12 Consequently, plenty of approaches have been developed to separate CO2 from methane, including chemical conversion,13 solvent absorption,14 membrane separation,15,16 and adsorption separation.17 © XXXX American Chemical Society

Among these strategies, adsorption separation using pours materials has shown much advantage than other methods in terms of economy and technology.18,19 Porous materials with a wide variety of topologies, pore sizes, and functionalities have been explored for CO2 separation and storage,20 including porous carbons,21 zeolites,22 metal−organic frameworks (MOFs),23−25 and covalent organic frameworks (COFs).26,27 For example, Yaghi et al. studied CO2 uptake capacity in IRMOF-74-III and reported that the CO2 adsorption was up to 67 cm3 g−1 at 800 Torr and 298 K. Zhong et al.28 studied CO2 and CH4 adsorption performance in MIL-101(Cr) and reported that the CO2 adsorption was up to 2.44 mmol g−1 at 298 K and 1 bar, the adsorption selectivity (CO2/CH4 = 50:50) reached 4.5 at ambient conditions. However, MOFs are organic−inorganic hybrid materials and formed by selfassembly of organic ligands and metal ions or clusters through coordination bonds, which limited their physicochemical stability in practical conditions especially in the presence of humidity, and their commercial application in gas separation.29,30 Porous aromatic frameworks (PAFs), are new classes of porous organic materials linked by robust covalent bonds. They have large specific surface area, pore volume, rigid skeleton structure as well as ultrahigh hydrothermal stability.31 Received: Revised: Accepted: Published: A

April 29, 2018 July 3, 2018 July 19, 2018 July 19, 2018 DOI: 10.1021/acs.iecr.8b01874 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

drofuran, chloroform, and deionized water; and activated in vacuum at 150 °C for 24 h; we then obtained white powderPAF-1. Synthesis of PAF-1-SO3H. PAF-1 (150 mg) was immersed in a mixed solution of dichloromethane (15 mL) and chlorosulfonic acid (1.0 mL), and the reaction was carried out in an ice−water bath for 1 day. The solid was then collected, washed, filtered, and dried at 150 °C under vacuum for 24 h, and finally PAF-1-SO3H was obtained. Synthesis of PAF-1-SO3Co. PAF-1-SO3H (100 mg) and Cobaltous chloride (800 mg) were immersed in a mixed solution of acetonitrile (20 mL) and water (20 mL), and the reaction was carried out at room temperature for 24 h. The solid was then collected, washed by acetonitrile and water, filtered, and dried at 120 °C under vacuum for 24 h, and finally PAF-1-SO3Co was obtained. 2.3. Characterization. FT-IR was performed on a AC-80 MHz (Bruker) instrument with the wave range of 4000−400 cm−1. Elemental analyses were performed on a Thermo Fisher Scientific Elemental Analyzer (Ea1112, Beijing Research Institute of Chemical Industry, SINOPEC). XPS analysis was obtained on ESCALAB 250 operated at 150 W and 200 eV with monochromated Al Kα radiation. 2.4. Gas Adsorption Measurements. N2 adsorption/ desorption isotherms were measured at 77 K with a Micromeritics ASAP 2460. The samples of 100 mg were degassed at 150 °C for 12 h. The BET specific surface area of products was calculated in the region of P/P0 = 0.05−0.35. Pore size distribution data was calculated from the N2 sorption isotherms based on the DFT model in the Micromeritics ASAP 2460 software package (assuming slit pore geometry). Ultrahigh-purity grade He (99.999%) and N2 (99.9992%) were used for all adsorption measurements. CH4 and CO2 isotherms were recorded with a Micromeritics ASAP 2020. The samples were outgassed at 150 °C for 12 h in vacuum before each measurement. Ultrahigh-purity grade CH4 (99.999%) and CO2 (99.999%) were used for all adsorption measurements.

Its pore structure and surface functional groups can be regulated and modified, which endows them good potential in selective gas adsorption.32 CO2 molecules own dipole moment and CH4 is a nonpolar molecule, so increasing the polar functional groups on the pore wall surface could improve the CO2 adsorption and CO2/CH4 selectivity. Zhou et al.33 synthesized a series of porous polymer networks (PPNs) and studied CO2 adsorption performance in sulfonate-functionalized PPN-6 which shows high CO2 uptake reaching 13 wt % at 298 K and 1 bar. Babarao et al.34 investigated PAFs functionalized with polar organic groups for CO2 adsorption and separation, and found that the functionalized PAF-1 structures have high adsorption capacity for CO2 at ambient conditions. Because the intrinsic quadrupole moment exists in CO2 molecules, whereas none exists in CH4 molecules, any polarizated activated sites or special structures may enhance the separation of CO2/CH4 mixture.35 As metal irons have rich d-electron orbit with dramatic adsorption effect on polar molecules, metal sites in the materials would increase the CO2 adsorption performance, which is also the reason why many MOFs carbon dioxide adsorption performance is higher than many porous organic materials.36−39 Given the stable structure of PAFs framework, it would be interesting and effective after the introduction of metal functional groups to improve CO2 separation over CH4. In this work, PAF-1 was prepared by Yamomato crosscoupling reaction. The introduction of sulfonate function and doping of non-noble metal elements were subsequently carried by the postsynthetic modification as shown in Scheme S1. Intermediate product and final product are named as PAF-1SO3H and PAF-1-SO3Co, respectively. The results from the Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and elemental analysis indicate that sulfonate function and metal has been successfully introduced into PAF-1. Single-component isotherms of CO2 and CH4 were experimentally measured. Ideal adsorbed solution theory (IAST) along with adsorption heat and the systematic evaluation of applicability were employed to predict CO2/CH4 adsorptive selectivity.

3. RESULTS AND DISCUSSION 3.1. Material Characterization. To obtain metalmodified porous adsorbent, we first sulfonated PAF-1 by introducing sulfonic acid groups on the biphenyl structure of the framework, and then reacted with metal ions to obtain cobalt-modified porous materials. As shown in Figure 1, the

2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals were purchased from the commercial resources and used without any further purification. 1,5-cyclooctadiene (cod) and bis (1,5-cyclooctadiene) nickel(0) ([Ni(cod)2]) were purchased from Sigma-Aldrich. N,N-dimethylformamide (DMF) was obtained from Alfa Aesar. Tetrakis (4-bromophenyl) methane (TBM) was purchased from Huangminglong Chem. Ltd. The other reagents were of analytical grade and obtained commercially. 2.2. Material Preparation. Synthesis of PAF-1. PAF-1 was synthesized using a solvothermal method as previous report.40 Typically, 1,5-cyclooctadiene (cod) (0.45 mL, 3.65 mmol) was added to a solution of bis(1,5-cyclooctadiene) nickel(0) ([Ni(cod)2]) (1.03 g, 3.65 mmol) and 2,2′-bipyridyl (0.565 g, 3.65 mmol) in dry DMF (80 mL), and the mixture was stirred until all solids completely dissolved. Then TBM (0.509 g, 0.8 mmol) was added into it. The reaction is performed at 80 °C for 12 h under an argon atmosphere. After the reaction was completed, hydrochloric acid was slowly added to the reaction flask until the solution became a green and transparent solution containing white flocs. The precipitate was collected by filtration; washed with tetrahy-

Figure 1. Infrared spectra of PAF-1, PAF-1-SO3H, and PAF-1-SO3Co. B

DOI: 10.1021/acs.iecr.8b01874 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 2. (a) XPS of PAF-1-SO3Co; (b) enlarged XPS for Co sites.

Figure 3. (a) N2 sorption isotherms at 77 K and (b) pore size distribution for PAF-1, PAF-1-SO3H and PAF-1-SO3Co. Solid and open symbols refer to adsorption and desorption, respectively.

Figure 4. (a) Pure CH4 and (b) CO2 adsorption isotherms on PAF-1, PAF-1-SO3H, and PAF-1-SO3Co at 298 K.

FT-IR characteristic peaks 1086 and 1186 cm−1 are corresponding to sulfonate groups in PAF-1-SO3H and PAF1-SO3Co. The element analysis (Table S1) shows the existence of sulfur element in both modified materials, and which further indicates that both of the adsorbents have been successfully modified by sulfonate groups. Moreover, as shown in Figure 2, two signal peaks at the binding energies of 781.3 and 787.2 eV occurred in XPS spectrum, which corresponded to Co 2p3/2 and Co 2p1/2 of the cobalt metal elements, respectively, indicating the successful preparation of PAF-1-SO3Co. The pore structure of the porous material has a significant effect on the adsorption performance. Hence, we further used the specific surface area and pore size analyzer (ASAP 2460) to characterize the pore structure of these materials. As shown in Figure 2, the N2 adsorption−desorption isotherms at 77 K of these materials belong to the typical I adsorption curve, which rapidly rise in the low pressure zone, and reach the adsorption

saturation after 0.1 bar. Furthermore, the adsorption isotherm of PAF-1 has a small hysteresis loop, indicating the existence of mesoporous structure. As shown in Figure 3b, the pore diameter of PAF-1 is mainly in the region of 1−5 nm, in which 1−2 nm is the most concentrated range, which is also correspond to hysteresis loop of PAF-1. After modification, the N2 uptake dramatically decrease, and the small hysteresis loop in PAF-1 disappears. The pore structures of the both PAF-1SO3H and PAF-1-SO3Co mainly concentrate in the micropore region, unlike the dominated mesopore region in PAF-1 (Figure 3b). The pore size of PAF-1-SO3H and PAF-1-SO3Co is mainly in the range of 0.5−1 and 1−2 nm. It should be mentioned that PAF-1-SO3Co has a certain decrease in specific surface area due to the introduction of cobalt metal comparing with PAF-1-SO3H, but the overall pore size distribution is similar to that of PAF-1-SO3H, indicating that cobalt doping does not destroy the pore structure of PAF-1-SO3H. C


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Figure 5. IAST-predicted selectivities of CO2 over CH4 in PAF-1, PAF-1-SO3H, and PAF-1-SO3Co for molar ratios of (a) CO2/CH4 = 50/50 and (b) CO2/CH4 = 15/85 mixtures of CO2 and CH4 at 298 K.

3.2. CO2 and CH4 Isotherms. To investigate the gas adsorption ability of these materials, the methane and carbon dioxide adsorption isotherms were measured in ASAP 2020. As shown in Figure 4a, although PAF-1 has the largest specific surface area (3111 m2 g−1) among the three samples, its methane adsorption capacity (0.48 mmol g−1) is lower than that of PAF-1-SO3H (0.62 mmol g−1) and PAF-1-SO3Co (0.55 mmol g−1) at 298 K and 1 bar. This phenomenon could be explained by the factor that PAF-1-SO3H and PAF-1-SO3Co has a pore size distribution of 0.5−1 nm, which makes it have a strong electric field effect on the gas molecules and suits for methane molecules adsorption.41 Unlike methane adsorption, the CO2 adsorption isotherms exhibit a remarkably difference due to the existence of quadrupole moment in CO2 molecular. As shown in Figure 4b, the CO2 adsorption isotherms growth trend of PAF-1-SO3H and PAF-1-SO3Co is dramatically higher than PAF-1, which proves that sulfonation treatment and subsequent metal doping are conducive to improve PAF-1 carbon dioxide adsorption capacity. Moreover, PAF-1-SO3Co has better carbon dioxide adsorption capacity than PAF-1SO3H at medium and low pressure (P < 0.5 bar), which indicates that PAF-1-SO3Co shows a stronger van der Waals forces and could “catch” more CO2 molecules with the presence of cobalt. Moreover, in the low pressure region(P < 0.5 bar), the pore wall of pours materials is not fully occupied by adsorbed molecules. The amount of molecular adsorbed depends mainly on the interaction between adsorbents and adsorbates. The stronger interaction occurred between adsorbents and adsorbates, the greater the amount of molecular adsorbed. As Co ions were introduced into the PAF-1-SO3H, which makes the interaction of PAF-1-SO3Co and CO2 stronger than the interaction of PAF-1-SO3H and CO2 and resulting in more CO2 adsorption amounts in the low pressure region (P < 0.5 bar). The CO2 adsorption capacity of PAF-1-SO3Co is slightly lower than that of PAF-1-SO3H at higher pressure (P > 0.5 bar), but still more than double amount of PAF-1 adsorbed. At 298 K and 1 bar, PAF-1-SO3Co has a CO2 adsorption capacity of 2.52 mmol g−1, which is lower than some famous MOFs,42−44 but still higher than many other similar pours organic materials such as, PPN-645 (1.4 mmol g−1), COF-146 (2.23 mmol g−1), COF-3003 (1.64 mmol g−1) and so on, and further suggesting that PAF-1SO3Co could be a promising candidate for CO2 adsorption storage. 3.3. Adsorption Selectivity for CO2/CH4. The dual-site Langmuir Freundlich (DSLF) adsorption model-based IAST has been applied to explore the adsorption selectivity of porous materials according to our previous method.47 The binary gas

mixing process is carried out at constant spreading pressure and indicated by

∫0

f1°

N1°(f1 )d ln f1 =

∫0

f 2°

N2°(f2 )d ln f2

(1)

where the single-component adsorption amounts and selectivity are further obtained from the above equation by numerical integration and root exploration. To investigate the separation of CO2−CH4, binary mixtures, the adsorption selectivity is defined by Si / j =

xi /xj yi /yj

(2)

where the selectivity refers to the first component over the second one, and the xi, xj and yi, yj denote the molar fractions of species i, j in the adsorbed and bulk phases, respectively. The calculation details and parameters are shown in Table S3. As shown in Figure S1, the coincidence between experimental data and simulation results is very well, and which reflects the accuracy of our computational simulation. Figure 5 shows the CO2/CH4 adsorption selectivity of the three adsorbents in (a) equimolar (b) CO2/CH4 = 15/85 gas mixture based on IAST theory. The material adsorption selectivity under two gas mixture conditions both shows this order: PAF-1-SO3Co > PAF-1-SO3H > PAF-1. In addition, the PAF-1-SO3Co exhibits similar CO2 uptake capacity with PAF-1-SO3H, but shows remarkable enhanced CO2/CH4 adsorption selectivity, far exceeding those without metal modified PAF-1 and PAF-1SO3H, which indicates that the incorporation of metal could improve the adsorption selectivity of the porous adsorbents for carbon dioxide. Compared with other porous organic materials, PAF-1-SO3Co has a high CO2 adsorption selectivity. As clearly shown in Figure 6, at atmospheric pressure, the selectivity of PAF-1-SO3Co for CO2/CH4 mixture is much higher than many other famous porous materials, such as MIL100(Cr),48 BPL-AC,49 ZIF,50 UTSA-5.51 Although the adsorption capacity of PAF-1-SO3Co is not in the forefront among the state-of-the-art porous materials due to the limited specific surface area, it still shows great promising potential for selective adsorption of CO2 over CH4 by considering both adsorption capacity and selectivity. 3.4. Adsorption Heat and Applicability. To evaluate the actual applicability of PAF-1-SO3Co, we further investigated the CO2 adsorption heat, adsorbent evaluation criteria, and working stability of PAF-1-SO3Co. The CO2 adsorption isosteric heat (Qst) was obtained by using the following virial D


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In addition to adsorption heat, we further assess the applicability of these three materials in CO2 separation processes using other four adsorbent evaluation criteria, i.e., CO2 uptake under adsorption conditions, working CO2 capacity, regenerability, and sorbent selection parameter.52 As shown in Table S5, because of introduction of functional groups, PAF-1-SO3H and PAF-1-SO3Co show higher Nads1 and ΔN1 values than PAF-1, and both of which exceed 1.5 mmol g−1, outperforming the monoethanolamine (MEA)-based carbon dioxide capture system and meeting the industrial requirements. The regeneration performance of the three samples follows the order of PAF-1> PAF-1-SO3H> PAF-1SO3Co, which showed a negative correlation with selective order. Owning to introduction of new functional groups and decrease of pore size, the modified materials have a stronger adsorption effect on CO2 molecules at low pressure, and which lead to the result that regeneration performance of modified PAF-1 samples are lower than that of PAF-1. In addition, PAF1-SO3Co has a slightly lower R value (75%) than PAF-1 and PAF-1-SO3H, which could be explained by the fact that the presence of bare cobalt metal elements on the skeleton significantly enhances the adsorption of CO2 at low pressure. Moreover, the relatively high selectivity and working capacity ratios (ΔN1/ΔN2) of PAF-1-SO3Co contribute to the highest S value of 85.8 among the three sorbents, and the S value of PAF-SO3Co is also much higher than many other materials, indicating it is a promising cyclic candidate for carbon capture and separation. Moreover, as shown in Figure 8, the amount of CO2 adsorbed shows ignorable decrease after the recycle adsorption−desorption test, indicating that this material has good application stability.

Figure 6. Comparison of selectivity for CO2/CH4 gas mixtures (CO2/CH4 = 50/50) in PAF-1-SO3Co with some typical adsorbent.

equation47 with the adsorption data (Figure S9) collected at T = 278, 288, 298, 308 K and P < 1 bar. m

ln P = ln N + 1/T ∑ aiN i + i=0

n

∑ biN i i=0

(3)

m

Q st = −R ∑ aiN i i=0

(4)

where P is the pressure expressed in Torr, N is the amount adsorbed in mmol g−1, T is the temperature in K, ai and bi are virial coefficients, and m and n represent the number of coefficients required to adequately describe the isotherms. Qst is the isosteric heat of adsorption, and R is the universal gas constant. As shown in Figure 7, the calculated Qst values for CO2 adsorption on the PAF-1-SO3Co and PAF-SO3H are both fast

Figure 8. Ten cycles of CO2 uptake for PAF-1-SO3Co up to 1 bar at 298 K. Figure 7. Isostreric heat of absorption (Qst) for CO2 in PAF-1, PAF1-SO3H, and PAF-1-SO3Co.

In the actual situation, the gas component often contains a certain amount of water vapor, sulfur dioxide, and other acidic gases, which may lead to the formation of weak acid droplets in the separation process and reduce the performance of the adsorbents. To detect PAF-1-SO3Co adsorption stability in such cases, we configured 0.01 mol L−1 of dilute hydrochloric acid to simulate the adsorption environment and left PAF-1SO3Co in solution for a week at room temperature. Subsequently, the porous adsorbent was removed, washed, and dried, and used to test methane and carbon dioxide adsorption properties. As shown in Figure 9, the CH4 and CO2 adsorption curves of PAF-1-SO3Co after immersing in hydrochloric acid solution were almost coincident with former

decrease with the uptake amounts increase, and the Qst for the PAF-1 keeps stable. Clearly, it could be found that the heat of adsorption for the modified PAF-1 are both higher than PAF-1 whose Qst values is 15.1 kJ mol−1 at zero loading. Owning to introduction of cobalt, the CO2 absorption energies of PAF-1SO3Co are higher than the other two samples, and reaches 31.5 kJ mol−1 close to zero loading, which is indicative of the strong interactions between pore skeleton with the gas molecules and also gives further explanation for the high selectivity of PAF-1-SO3Co. E


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Figure 9. Comparison of (a) CH4 and (b) CO2 adsorption isotherms of PAF-1-SO3Co and treated PAF-1-SO3Co. The treated PAF-1-SO3Co was immersed in diluted hydrochloric acid for a week and then activated at 423 K.

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curves, indicating that PAF-1-SO3Co could maintain good adsorption stability under weak acid conditions and could be an ideal separation material for CO2/CH4 mixture.

(1) Lin, Z.; Lv, Z.; Zhou, X.; Xiao, H.; Wu, J.; Li, Z. A post-synthetic strategy to prepare ACN@Cu-BTCs with enhanced water vapor stability and CO2 /CH4 separation selectivity. Ind. Eng. Chem. Res. 2018, 57, 3765−3772. (2) Chen, Y.; Wu, H.; Liu, Z.; Sun, X.; Xia, Q.; Li, Z. Liquid-assisted mechanochemical synthesis of copper based MOF-505 for the separation of CO2 over CH4 or N2. Ind. Eng. Chem. Res. 2018, 57, 703−709. (3) Dawson, R.; Stöckel, E.; Holst, J. R.; Adams, D. J.; Cooper, A. I. Microporous organic polymers for carbon dioxide capture. Energy Environ. Sci. 2011, 4 (10), 4239−4245. (4) Shu, G.; Zhang, C.; Li, Y.; Jiang, J.-X.; Wang, X.; Li, H.; Wang, F. Hypercrosslinked silole-containing microporous organic polymers with N-functionalized pore surfaces for gas storage and separation. J. Appl. Polym. Sci. 2018, 135 (8), 45907. (5) Plasynski, S. I.; Vikara, D. M.; Srivastava, R. D. In progress and new developments in CO2 capture and storage. Proceedings of the 2008 AIChE Spring National Meeting; AIChE, 2008. (6) Liang, C.; Shi, Z.; He, C. T.; Tan, J.; Zhou, H.; Zhou, H. L.; Lee, Y.; Zhang, Y. B. Engineering of pore geometry for ultrahigh capacity methane storage in mesoporous metal-organic frameworks. J. Am. Chem. Soc. 2017, 139 (38), 13300−13003. (7) Lan, J.; Cao, D.; Wang, W. High uptakes of methane in Li-doped 3D covalent organic frameworks. Langmuir 2010, 26 (1), 220−226. (8) Makal, T. A.; Li, J. R.; Lu, W.; Zhou, H. C. Methane storage in advanced porous materials. Chem. Soc. Rev. 2012, 41 (23), 7761. (9) Howarth, R. W.; Santoro, R.; Ingraffea, A. Methane and the greenhouse-gas footprint of natural gas from shale formations. Clim. Change 2011, 106 (4), 679−690. (10) Esfandiari, K.; Ghoreyshi, A. A.; Jahanshahi, M. Using artificial neural network and ideal adsorbed solution theory for predicting the CO2/CH4 selectivities of metal−organic frameworks: A comparative study. Ind. Eng. Chem. Res. 2017, 56, 14610−14622. (11) Bae, Y. S.; Mulfort, K. L.; Frost, H.; Ryan, P.; Punnathanam, S.; Broadbelt, L. J.; Hupp, J. T.; Snurr, R. Q. Separation of CO2 from CH4 using mixed-ligand metal-organic frameworks. Langmuir 2008, 24 (16), 8592−8598. (12) Lee, B.-S.; Lin, S.-T. Screening of ionic liquids for CO2 capture using the COSMO-SAC model. Chem. Eng. Sci. 2015, 121, 157−168. (13) Liu, L.; Nicholson, D.; Bhatia, S. K. Adsorption of CH4 and CH4/CO2 mixtures in carbon nanotubes and disordered carbons: A molecular simulation study. Chem. Eng. Sci. 2015, 121, 268−278. (14) Tamajón, F. J.; Á lvarez, E.; Cerdeira, F.; Gómez-Díaz, D. CO2 absorption into N-methyldiethanolamine aqueous-organic solvents. Chem. Eng. J. 2016, 283, 1069−1080. (15) Chen, X. Y.; Vinh-Thang, H.; Rodrigue, D.; Kaliaguine, S. Amine-functionalized MIL-53 metal−organic framework in polyimide mixed matrix membranes for CO2 /CH4 separation. Ind. Eng. Chem. Res. 2012, 51, 6895−6906. (16) Zou, X.; Zhu, G. Microporous organic materials for membranebased gas separation. Adv. Mater. 2018, 30 (3), 1700750.

4. CONCLUSION In summary, we have successfully prepared PAF-1-SO3Co as a highly efficient CO2/CH4 separation material by introduction sulfonic groups and cobalt metal into PAF-1. Our experimental results show that cobalt-impregnated PAF-1-SO3H exhibits greater CO2/CH4 selectivity than PAF-1-SO3H and unmodified material. Particularly, at T = 298 K, P = 1 bar, the selectivity of PAF-1-SO3Co reach 14.27 (CO2/CH4 = 50/50). Furthermore, PAF-1-SO3Co has a high regenerability and sorbent selection parameter, which are much better than many reported materials such as active carbon, zeolites, and MOFs. Particularly, PAF-1-SO3Co has remarkably high working stability, whose adsorption ability shows ignorable decrease after soaking in a simulated acidic environment even for 1 week. Accordingly, the PAF-1-SO3Co could be a promising cyclic adsorbent with high selectivity for separation and purification of CO2 from natural gas.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.8b01874.

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REFERENCES

Tables S1−S5, Scheme S1, Figures S1−S5 (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yuanhui Cheng: 0000-0003-2308-7375 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSF of China (51502012, 21676020, 21606015, 21620102007); The Start-up fund for talent introduction of Beijing University of Chemical Technology (buctrc201420 and buctrc201714); Talent cultivation of State Key Laboratory of Organic−Inorganic Composites; Distinguished scientist program at BUCT (buctylkxj02) and the ‘‘111” project of China (B14004). F


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