Facile synthesis of microporous carbon xerogels for highly selective CO2 adsorption

Journal of Cleaner Production 253 (2020) 120023

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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro

Facile synthesis of microporous carbon xerogels for highly selective CO2 adsorption Shasha Wang a, b, d, Yuelong Xu b, d, Junfeng Miao a, Mengshuai Liu c, Bin Ren b, d, Lihui Zhang b, d, Zhenfa Liu a, b, d, * a

School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin, 300000, China Institute of Energy Resources, Hebei Academy of Sciences, Shijiazhuang, 050081, China State Key Laboratory Base of Eco-chemical Engineering, College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao, 266042, PR China d Hebei Engineering Research Center for Water Saving in Industry, Shijiazhuang, 050081, China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 September 2019 Received in revised form 15 December 2019 Accepted 5 January 2020 Available online 8 January 2020

Microporous carbon xerogels (CXs) were successfully fabricated via a facile one-pot strategy using eutectic salt mixture (KCl and ZnCl2) as dual activator and catalyst. Their pore structures, morphology, and selective CO2 adsorption behaviors were thoroughly studied by varying the contents of eutectic salt mixture. The presented CXs herein showed excellent CO2 adsorption selectivities, and the CX-4 with optimum salt concentration (12.9%) exhibited the highest CO2 adsorption capacity of 4.39 mmol g
1 at 273 K, which was ascribed to their special microscopic pore structure. Compared with the reported carbon materials, the optimum CX-4 exhibits a comparable or superior CO2 adsorption performance under similar conditions, showing great potential for efficient CO2 capture and separation from flue gas and natural gas. © 2020 Published by Elsevier Ltd.

Handling editor: Hua Cai Keywords: Microporous xerogel CO2 capture agent Adsorption selectivity

1. Introduction Over the past decades, worsen climate change has received widespread public attention, which is primarily caused by global warming (Figueroa et al., 2008). The emission of carbon dioxide (CO2) represents the main contributor to global warming, as a result from fossil fuel combustion and other chemical industrial processes (Monastersky, 2013; Yang et al., 2015). There is no doubt that CO2 emission is getting worse and more acute. These trends are expected to continue in the next decades, as other carbon-neutral energy sources are yet to fully substitute fossil fuels. Hence, there is an urgent need to develop new strategies for capturing CO2, which has received increasing attention among scientists and public. Previous studies have suggested that porous adsorbents may offer great potential to capture CO2 via adsorption and separation (Yang et al., 2015; Zhang et al., 2015; Cao and Li, 2015). To date, numerous porous adsorbents such as metal-organic

* Corresponding author. School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin, 300000, China. E-mail address: [email protected] (Z. Liu). https://doi.org/10.1016/j.jclepro.2020.120023 0959-6526/© 2020 Published by Elsevier Ltd.

frameworks (Sumida et al., 2012; Liu et al., 2012), functionalized mesoporous silica (Ren et al., 2012; Yu et al., 2012), porous polymers (Rabbani and El-Kaderi, 2012) and porous carbon materials (Zhou et al., 2012), have been reported for CO2 adsorption. Among these adsorbents, porous carbon materials are recognized as the next-generation absorbents for CO2 capture (Bae and Snurr, 2011; Lu and Hao, 2013) due to their low cost, facile preparation, high specific surface area and pore volume, strong adsorption capacity, excellent chemical stability and tunable pore structure (Wang et al., 2011; Wu and Zhao, 2011; Xia et al., 2011). Many efforts have been devoted to enhance their porosity. For example, Baumann et al. (2008) reported carbon xerogels (CXs) with large surface area (3125 m2 g
1) and high pore volume (1.88 cm3 g
1) via CO2 activation. Moreover, Liu et al. (Liu and Shen, 2013) developed CXs with hierarchically porous structures from resorcinol-formaldehyde gel through KOH and CO2 activation processes. They found that the high specific surface area (2119 m2 g
1) and pore volume (2.73 cm3 g
1) are mainly attributed to large amounts of small mesopores, micropores and 3D macro-structured channels within the CXs. More recently, a series of novel Zn-promoted hierarchically nanoporous carbons (ZNCs) have been prepared from cation exchange resin D113, which demonstrates a high specific surface area

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(2178.0 m2 g
1), multilevel pore structure and optimal oxygendoping (Gao et al., 1016). The contents of Zn in precursors are essential contributors that vary the surface area of carbon (Yue et al., 2002). Such approach paved the way for effectively regulation of the hierarchical porous structure molecular level. A number of experimental and computer modeling studies (Casco et al., 2014a) have shown that CO2 adsorption capacity is mainly determined by the volume of micropores, and the micropores can provide more active sites (Zhou et al., 2016). Presseret (Presser et al., 2011) and Casco et al. (2014a) reported that the CO2 uptake on microporous carbons is closely dependent on the micropores that are smaller than a specific diameter. To obtained controllable pore, the techniques of hard and soft templating (Talapaneni et al., 2012; Müllner et al., 2012) or other activation methods (Zhai et al., 2011) have been widely used over the past few decades. The hard template approach is capable of fabricating a wide range of materials, but it is relatively complex and time consuming (To et al., 2016). Despite that the soft templates can be easily removed by heating, the interaction between the surfactants and guest molecules is critical for the production of porous structures (Shi et al., 2015; Luo et al., 2016), as the desired porosity can be influenced by minor variations in synthesis conditions (Li et al., 2016). These approaches may result in a significant mass loss and engrave the pores into the carbon materials. Therefore, the development of simple, green and sustainable synthetic techniques of carbon materials with high microporosity remain desirable for highly-efficient CO2 capture. Hence, this research aims to prepare porous carbon materials (CXs) by a facile, one-step molten salt technique and without any activation for efficient adsorption of CO2. The obtained CXs with spherical-like morphology, large specific surface area (1148 m2 g
1), and high micropore volume (0.52 cm3 g
1) were rarely reported for without activation. The effective adsorption of CO2 is closely related to microporous structure of the generated carbon materials. These materials displayed high adsorption capacity for CO2, they were also capable of separating CO2eN2 and CO2eCH4 mixtures, due to the molecular sieving effects exerted by their micropores. 2. Experimental section 2.1. Material synthesis One-pot synthesis approach was used to prepare the desired CXs, the schematic illustration of synthesizing CXs via salt templating method is shown in Fig. S1. Phloroglucinol, resorcinol, formaldehyde (36.5% in H2O), deionized H2O and eutectic salt mixture [KCl and ZnCl2 (52 mol% ZnCl2)] were dissolved in distilled H2O. All the chemicals were purchased from Aladdin, Shanghai, China. To ensure homogeneity, the mixture was ultrasonicated at room temperature for 30 min. Subsequently, the mixture was transferred into a 40 mL glass vial and properly sealed. The mixture was heated at 50  C in a water bath for 72 h, and then at 80  C for 24 h in order to obtain wet gels. After that, the wet gels were treated by freeze-drying for 24 h. The resultant organic xerogels were carbonized up to 900  C in a tube furnace under N2 flow (100 cm3 min
1) at a heating rate of 2  C min
1 and held at 900  C for 3 h. In order to remove the residual salt porogen, the carbon xerogels materials were grinded in ball mill (400 rpm) for 20 min, then washed using 1 M HCl solution and rinsed with deionized until neutral pH by vacuum filtration. Finally, the samples were dried in the oven at 120  C for 4 h. The molar ratios of phloroglucinol:resorcinol:formaldehyde:deionized H2O:eutectic salt mixture were1:7:16:30:1, 1:7:16:30:2, 1:7:16:30:4, 1:7:16:30:8, and

1:7:16:30:10. So, molar concentration of eutectic salt mixture were 1.8%, 3.5%, 6.8%, 12.9%, and 15.6%. The obtained carbon xerogels were named as CX-1, CX-2, CX-3, and CX-4, and CX-5, respectively; while CX was classified as pure carbon xerogel without salts. Table S1 listed the coding of carbon xerogels corresponding to the molar concentration of eutectic salt mixture. 2.2. Material characterization and testing Textural characterization was carried out by nitrogen adsorptionedesorption using an ASAP 2420 adsorption analyzer (Micromeritics Instruments Corporation, Norcross, GA, USA). CXs were degassed for 3 h at 250  C. Specific surface area (SBET) was calculated based on the Brunauer-Emmett-Teller (BET) approach (0.01 < P/P0 < 0.1). The mean pore diameter (D) of all pores was determined. The total pore volume (Vtotal) was measured from the quantity adsorbed at P/P0 ¼ 0.988. The micropore volume (Vmicro) and pore size distribution were assessed using the density functional theory (DFT) approach (Wang et al., 2008). This approach contained the formation of a grand potential functional of the mean local density, and the equilibrium density profile and thermodynamic properties was achieved by minimizing this grand potential functional with regard to the local density. Our proposed method was different from other in terms of the smoothed density approximation (Leofantia et al., 1998). Simultaneous thermogravimetry (TG-DTG) were performed on a TA Instrument SDT Q600 at a heating rate of 10  C min
1 under N2 flow. PY-GC-MS analyses were tested on a system combined with a PY-3030d and a GCMS QP2010. X-ray diffraction (XRD; Rigaku Ultima IV X-ray diffractometer, with a Cu Ka radiation of l ¼ 1.54 Å) were used to examine the structural changes in CXs. The adsorption isotherms of CO2, N2, and CH4 were determined using the ASAP 2420 adsorption analyzer at 0  C and 25  C, respectively. Before conducting each experiment, the samples were degassed at 250  C for 3 h in order to eliminate the guest molecules from the pores of CXs. The dynamic breakthrough curves of the adsorbents were measured using a BSD-MAB Analyzer from Bei Shi De. The separation of CO2 from CO2/N2 or CO2/CH4 binary mixtures was performed on a fixed-bed adsorber (a stainless steel tube with an inner diameter of 6 mm and a length of 100 mm) at about 1 bar and 298 K, which were controlled by a pressure controller and a thermostatic water bath, respectively. Firstly, the bed was heated at 250  C in He at a flow rate of 50 mL min
1 for 2 h. Then, the breakthrough experiment was performed by switching from He to the gas mixture that contained 15% CO2 in N2 (v/v) or 50% CO2 in CH4 (v/v) at the same total flow rate. The effluent gas was monitored online by using an Agilent 7890A gas chromatograph with a thermal conductivity detector (TCD). 3. Results and discussion The porosity of CXs was determined using nitrogen adsorption (77 K). The size distribution and micropore volume of CXs were measured using the DFT method. The textural characteristics of CXs

Table 1 Textural properties of CXs.

CX CX-1 CX-2 CX-3 CX-4 CX-5

SBET (m2$g
1)

Vmicro (cm3$g
1)

Vtotal (cm3$g
1)

D (nm)

656 629 705 751 1148 500

0.17 0.25 0.36 0.39 0.52 0.23

0.39 0.39 0.38 0.41 0.60 0.28

5.76 3.41 2.16 2.08 2.19 2.24

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are presented in Table 1. As shown in Fig. 1, the adsorption amount of CXs increased rapidly and reached a limiting value at extremely low P/P0. This may be due to the adsorbate-adsorbent interactions in small-size micropores at molecular dimension, leading to a very low P/P0 (micropore filling) (Thommes et al., 2015). CX-4 exhibited the greatest adsorption capacity, with the highest corresponding pore volume (0.60 cm3 g
1, Table 1). In Fig. 1, adsorption-desorption curves of CX and CX-1 show type IV(a) isotherms and H4-type hysteresis loops according to the IUPAC classification (Rouquerol et al., 2014), indicating that these two materials consisted of complex structures including micropores, mesopores, and macropores. CX-1 still retained the pore

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structure of CX due to too little salt added. CX-2, CX-3, and CX-5 exhibited type I(a) isotherms. This isotherm pattern is given by microporous materials having mainly narrow micropores (Rouquerol et al., 2014). CX-4 displayed type I(b) isotherm which can be found for materials having pore size distribution over a broader range including wider micropores and narrow mesopores (Rouquerol et al., 2014), consistent with the pore size distribution of CX-4 presented in Fig. 2. Based on the pore size distribution curves, CX-2, CX-3, and CX-5 exhibited microporous characteristics with pore size distribution centered around 0.7e2 nm (Fig. 2). CX and CX-1 possessed micropores, mesopores, and macropores, which were consistent with the attained isotherms. CX-4 showed micropores (<2 nm) and small

Fig. 1. Nitrogen adsorption-desorption isotherms of CXs.

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Fig. 2. Pore size distribution of CXs and particularly enlarged inset.

mesopores (2e3.4 nm). Moreover, the largest specific surface area (1148 m2 g
1) and the highest pore volume (0.60 cm3 g
1) were obtained among all the as-prepared materials. The increased surface area and pore volume could be due to the assisted carbonization of salts (KCl and ZnCl2) with optimum concentration (12.9%). These salts not only act as chemical activators that promote micropore formation but also function as hard templates to inhibit the framework destruction. (KCleZnCl2)-assisted carbonization represented an effective approach to enhance formation of pores within the carbon framework. TG-DTG analysis was carried out to assess the decomposition performance of organic xerogel, Fig. S2 shows the TG-DTG curve. Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) was used to analyze the volatilized gas at 300e900  C. Mass spectrum of volatile gas are shown in Fig. S3. The mass loss (Fig. S2) between

100 and 250  C could be due to the evaporation of water from the internal surfaces of organic xerogel and decarbonylation of free formaldehyde. The mass loss between 300 and 900  C was accounted for approximately 41.8%. During this process, the organic xerogel in solid state may undergo a decomposition reaction, where the molecular chain was gradually cracked, and carbon dioxide, methanol, toluene, 1-ethyl-3-methyl-benzene, phenol, etc. were generated. The volatilized gas was confirmed by mass spectrometry (Fig. S3). Finally, approximately 48.1 wt% carbon residues were left after pyrolysis at 900  C. The SEM images confirmed clear changes in morphology (Fig. 3). CX salt-free sample displayed a honeycomb-like hole with uneven aperture. CX-1 with a little salt, retained its pore structure similar to CX, but the pore size was more uniform. When adding more salt, the pore structure of CX-2 was changed from honeycomb-like to

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Fig. 3. SEM images of CX(a) and (b), CX1(c) and (d), CX-2(e) and (f), CX-3(g) and (h), CX-4(i) and (j), CX-5(k) and (l).

carbon particles. As the amount of salt was further increased, the carbon-particle structure of CX-3, CX-4, and CX-5 was more pronounced and a larger diameter was observed. These irregular morphologies may result from the salt addition, which affected precipitation, polymerization, and condensation rates (AlMuhtaseb and Ritter, 2003). Moreover a carbon layer like a tulle appeared between the carbon spheres, which may be due to the molten state of salt at a high temperature, and carbon was formed in the direction of salt flow. To further investigate the microstructure of as-prepared CXs, XRD measurements were carried out (Fig. 4). It was found that all samples exhibited two wide (002) and (101) diffraction peaks of amorphous carbon at 24 and 44 , respectively (Liu, Wu, Feng, Mllen). These results indicated that the as-synthesized CXs have low crystallinity. Furthermore, it can be found that for all the samples including salts, the (002) diffraction peak was shifted

Fig. 4. XRD pattern of CXs.

slightly toward the higher angle with enhancing concentrations of the added salts. The slight shift toward a higher angle for the 002 reflection proved decreasing of the interlayer spacing of d002 (Wohlgemuth et al., 2012), this may be due to the development of stacking structure towards graphite structure (Takagi et al., 2007; Binoy et al., 2009). More importantly, CXs may retain their CO2 adsorption and selectivity. The adsorption capability of CXs was tested at 273 K and 298 K using CO2 or N2, and the resulting isotherms are presented in Fig. 5. The physical adsorption of CO2 relies on the van der Waals interaction between adsorbent and adsorbate, and thus is closely associated with the structural parameters, such as specific surface area (Ma et al., 2013, 2014), pore size (especially narrow micropores) (Chang et al., 2019; Chen et al., 2018), and total volume of micropores (Casco et al., 2014b; Chen et al., 2013). The steep increase in the isotherms, especially at lower pressures, reveals a strong binding interaction with CO2. In the present study, the synthesized CXs showed outstanding CO2 absorption capacities (ranged from 3.21 to 4.39 mmol g
1) at 273 K. The amount of adsorption was decreased to 2.92 mmol g
1 (CX-4) and 2.18 mmol g
1 (CX) at 298 K, which maybe due to as temperature increases (and so the kinetic energy of the gas molecules) only the narrower pores are useful for the adsorption of CO2 (Plaza et al., 2012). The significantly enhanced CO2 capture is undoubtedly caused by the extra micropores. Among all samples, CX-4 exhibited the highest CO2 capture capacity, mainly relied on the multi-level pore structure including small mesopores (2e3.5 nm) and further micropores as well as large specific surface area. The multi-stage mesoporous pore structure facilitated the transportation of CO2 molecules inside the material, and the extended micropore volume led to further storage of CO2. The surface area of an adsorbent can provide more adsorption sites for CO2. To examine the structure-property relationship between micropore volume and adsorption capacity, the CO2 capture capacities at 273 K of CXs were plotted against their micropore volume (Fig. S4). It was noted that the CO2 capture capacities displayed positive relationship with the micropore volume, suggesting that

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Fig. 5. CO2 and N2 adsorption curves of CXs at 298 K (a) and 273 K (b).

microporous feature plays a major role in capturing CO2. In view of practical application, the CXs should possess both excellent adsorption capacity and high selectivity towards CO2 compared to other gases (e.g. N2). Fig. 5(a) shows the N2 adsorption isotherms at 298 K and 1.0 bar. Indeed, the CO2 adsorption capacities of CXs were significantly higher than their N2 capture capacities under similar conditions. For instance, the N2 capture capacity of CX-4 was extremely low (0.46 mmol g
1), representing only one sixth of its CO2 capture capacity. Ideal adsorbed solution theory (IAST) has been widely applied to estimate the selectivity of typical flue gas with 85% N2 and 15% CO2 (Plomp et al., 2009). The results of IAST-predicted adsorption selectivity of CXs are presented in Fig. 6. Notably, CXs exhibited a high adsorption selectivity from 26 to 38 for CO2 over N2, under the conditions of 298 K and 1 bar. These findings indicate that both CO2 and N2 can be efficiently separated by CXs. As summarized in Table 2, the CO2 adsorption performance of CX-4 as-obtained was compared to those of previously reported porous carbon materials under the similar conditions. In this work, the CX-4 was prepared without any activation, but its CO2 adsorption and IAST CO2/N2 selectivity can definitely compete with other carbon materials (Yang et al., 2017; Jowita and Mietek, 2015; Alegre et al., 2019), and even activated carbon materials (Chen et al., 2015; Maroto-Valer et al., 2005). This is mainly related to its high micropore volume. Therefore, the excellent CO2 capture capacity along with high CO2/N2 selectivity of CXs makes them outstanding adsorbents for CO2 capture. To assess the strength of interaction between CXs and CO2 molecules, the isosteric heat (Qst) was measured using the CO2 adsorption isotherms derived from the ClausiuseClapeyron equation at 273 and 298 K (Himeno et al., 2005). The plot of Qst is shown as a function of CO2 uptake in Fig. 7. The initial adsorption heat values (15e24 kJ mol
1) of the five CXs were lower than the typical value for chemisorption (>60 kJ mol
1) (Samanta et al., 2012), indicating that the carbon material undergo a physical adsorption process for CO2 capturing. The high values of initial Qst revealed a strong interaction between the pore walls of CXs and CO2 molecules. Adsorption has been recognized as an exothermic process, and the more heat released, the stronger the interaction between adsorbent and adsorbate. It could be seen that as the adsorption amount increased, the heat of adsorption gradually decreased. This is probably due to the fact that the initial amount of adsorption is low, and the corresponding adsorption may approximately be single-layer adsorption. Hence, the interaction force between CXs

Fig. 6. IAST-predicted adsorption selectivity of CO2 and N2 on CXs at 298 K.

and CO2 is characterized by the adsorption heat value at this stage, and the binding of CO2 on CXs pore walls can be relatively uniform. At the end of adsorption stage, the molecular layer of CO2 adsorbed on CXs surface were thicker, reflecting the interaction force between CO2 molecules. Therefore, when the amount of adsorption increases up to a certain extent, the adsorption heat value nearly achieves equilibrium. In addition, our findings suggest the excellent cycle stability and excellent adsorption characteristics of CXs for easy regeneration. On the basis of the above results, we further explored the adsorption and selectivity of CO2 and CH4. As shown in Fig. 8, all the isotherm curves showed type-I according to the IUPAC classification (Rouquerol et al., 2014), and there was a rapid increase of CO2 adsorption at low pressures, wherein CO2 molecules could freely penetrate into the micropores and interact strongly with the CXs. The superior CO2 and CH4 uptakes of CX-4 compared to other CXs at 273 K and 298 K are ascribed to the considerable micropore volume and large specific surface area (Table 1) of the former, as previously revealed (Cai et al., 2014; Feng et al., 2014). The adsorption capacity of CH4 was extremely lower than CO2 for all CXs samples and it can be generalized that CXs are weak adsorbents for CH4, these differences may be due to the smaller molecular size of CO2 (3.3 Å) than CH4 (3.8 Å). Generally, the Fig. 9 also showed the same results.

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Table 2 Comparison of textual properties and CO2 capture performance with other reported carbon materials (1 bar). Adsorbent

Activation

CX-4 Carbon xerogel Carbon carbon from cigarettes CS-500-1.5 Carbon spheres Cellulose xerogels Carbon xerogels Commercial activated carbon CMK-3 a

CO2 adsorption(mmol g
1)

CO2/N2 Selectivity

Micropore Volume (cm3 g
1)

Reference

273 K

298 Ka

No No No Yes Yes

4.39 4.19 2.02 6.20 5.39

2.92 3.31 1.51 4.13 3.23

32.0 NA NA 22.0 NA

0.52 0.31 0.01 0.44 0.56

This work Liu et al. (2019) Yang et al. (2017)

Yes No No Yes No Yes No

2.68 2.8 NA NA 3.34 2.79 NA

1.50 NA 3.00 3.42 NA 0.89 1.70

33 NA NA NA NA 17 NA

NA 0.20 0.32 0.37 NA NA NA

Chen et al. (2015) Jowita and Mietek (2015) Zhuo et al. (2016)

Sun et al. (2017)

Alegre et al. (2019) Maroto-Valer et al. (2005) Liu et al. (2011)

NA means not available.

Fig. 7. CO2 isosteric heat of CXs.

IAST was applied to estimate the selectivity of typical natural gas containing equal amounts of CO2 and CH4 (Fig. 9). The results showed that CX-3 had the highest selectivity to CO2 and CH4, whereas, CX-4 was just moderate. This finding indicated that a material with a suitable pore size can exhibit a high selectivity for CO2 and CH4 adsorption. Therefore, a porous carbon material with high micropore volume, suitable pore size, and high specific surface area may be beneficial for CO2 capture. Using simulated flue gas with a composition of 15% CO2 and 85% N2 as adsorbate, the breakthrough curve experiments were carried out at 283 K to evaluate the separation performance of the CX-4 sample. As shown in Fig. S5, the breakthrough curves included three stages. At the beginning (0e4 s), both CO2 and N2 were captured because the CX-4 was not saturated with CO2 and N2. In the second stage (4e60 s), CO2 was still captured by the CX-4, whereas N2 broke through the column quickly. This may be resulted from the stronger interaction between CO2 and CX-4, some of the preadsorbed N2 molecules were expelled into the effluent stream by CO2 molecules; this expulsion explained why the concentration of N2 at this stage was higher than that of equilibrium concentration. The results strongly demonstrated that CX-4 could selectively capture CO2 from N2. In the last stage (>60 s), CO2 was also detected in the effluent stream, and the bed gradually became saturated.

Fig. 8. CO2 and CH4 adsorption curves of CXs at 298 K (a) and 273 K (b).

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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21376062), the Foundation of Key R&D Program of Hebei Province (18393616D) and Hebei Province Applied Basic Research Program-Key Basic Research Project (18964005D). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.jclepro.2020.120023. References

Fig. 9. IAST-predicted adsorption selectivity of CO2 and CH4 on CXs at 298 K.

Meanwhile, the same breakthrough experiment of the CX-4 was repeated by using a stream of 50% CO2 in CH4 (v/v) (Fig. S6). The separation behavior conducted instantaneously after the process started, indicating a slight adsorption capacity for the CX-4 to CH4. No CO2 was detected during the downstream of the column until a contact time of 38 s, and complete saturation of the packed column occurred after ca. 137 s. The results showed that the CX-4 could provide extremely high adsorption selectivity of CO2 from CH4.

4. Conclusions In this study, various microporous CXs are obtained via a facile one pot method using eutectic salt mixture (KCl and ZnCl2) as activator and catalyst. The as-prepared CXs had rich micropores with a specific surface area high to 1148 m2 g
1 and pore volume high to 0.60 cm3 g
1. The CXs deriving from eutectic salt mixture displayed outstanding CO2 capture capacities (3.21e4.39 mmol g
1) at 273 K and high selectivity of CO2eN2 and CO2eCH4 mixtures, which were relatively higher than other carbon materials. The high CO2 adsorption capacity and selectivity of CXs may be attributed to their high micropore volume. Thus, the CXs developed in this study can serve as prominent CO2 adsorbent materials.

Author contributions section Shasha Wang and Zhenfa Liu designed experiments; Yuelong Xu, Junfeng Miao, and Shasha Wang carried out experiments; Mengshuai Liu and Bin Ren analyzed experimental results. Lihui Zhang, Shasha Wang and Zhenfa Liu wrote the manuscript.

Declaration of competing interest No conflict of interest exits in the submission of this manuscript, and it is approved by all authors for publication. We would like to declare on behalf of all the co-authors that the work described is original research that has not been published previously, and not under consideration for publication elsewhere, in whole or in part. All the authors listed have approved the manuscript that is enclosed.

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