The development of activated carbon from corncob for CO₂ capture

Abstract

The accumulation and incineration of crop waste pollutes the environment and releases a large amount of CO₂. In this study, corncob crop waste was directly activated using solid KOH in an inert atmosphere to prepare porous activated carbon (AC) to capture CO₂, and to introduce N-containing functional groups that favour CO₂ adsorption, urea was mixed with corncob and KOH to prepare N-doped AC. The physical and chemical properties of the AC were characterized, and the effects of the mass ratio of KOH and urea to corncob, the activation temperature and time as well as regeneration were investigated to explore the optimal preparation process. The pores in the AC are mainly micropores, with the specific surface area and pore volume reaching 926.07 m² g⁻¹ and 0.40 cm³ g⁻¹ for KOH-activated corncob and 1096.70 m² g⁻¹ and 0.48 cm³ g⁻¹ after N-doping; the C–O plus O–H ratio and the –NH– ratio, which favour CO₂ adsorption in N-doped AC were 6.04 and 1.92%, respectively. The maximum adsorption capacities for KOH-activated corncob before and after N-doping were 3.49 and 4.58 mmol g⁻¹, respectively, at 20 °C and remained at 3.44 and 4.52 mmol g⁻¹ after ten regenerations. The prepared corncob-based AC showed good application prospects for CO₂ capture.

---

1. Introduction

As a responsible country, despite the great pressure from all aspects, China is still taking positive measures to capture CO₂ to fulfill the double carbon target. Due to the characteristics of the energy structure and the dependence on coal, coal-fired power plants are undoubtedly one of the largest CO₂ emission points, where the CO₂ concentration is 9–15 vol%. Solid sorbents are acknowledged for their potential in capturing CO₂,^1–4 and many researchers have made a substantial contribution in this respect.^5–26

Quyang et al. synthesized SiO₂ nanowires from fibrous sepiolite and impregnated these nanowires with different amines to prepare composite solid sorbents, in which the optimal adsorption capacity of tetraethylenepentamine (TEPA)-functionalized SiO₂ was 3.7 mmol g⁻¹ at 75 °C for 60 vol% CO₂.⁵ Wang et al. impregnated branched polyethylenimine (PEI) onto commercial multiwalled carbon nanotubes and prepared composites to investigate the effect of H₂O and O₂ on CO₂ capture, and the results showed that H₂O had a positive effect, while O₂ had a negative effect on CO₂ adsorption.¹² Chang et al. synthesized the zeolitic imidazolate framework (ZIF-8), in which MgO and ZnO were successfully confined; MgO-confined ZIF-8 showed fast chemisorption kinetics at room temperature, while ZnO-confined ZIF-8 showed physisorption characteristics.²³ The above sorbents all showed good CO₂ adsorption performance, but the expensive supporting materials, active components or complex preparation process caused high CO₂ capture costs, and the development of low-cost CO₂ sorbents is an effective way to reduce CO₂ capture costs.^27–37 Rehman et al. explored nitrogen-containing porous carbons derived from chitosan by a carbonization combining activation process. This material showed a rich microporous structure and N-containing functional groups, and the maximum adsorption capacity reached 6.36 mmol g⁻¹ at 273 K for 100 vol% CO₂.²⁶ Wang et al.²⁹ prepared porous lignite chars from low rank Ordos coal by the activation method, and the CO₂ adsorption capacity was 1.35 mmol g⁻¹ at 20 °C for 15 vol% CO₂.²⁸ Biomass is also used as the raw material to prepare porous carbons as sorbents.^38–41 Ding et al. prepared seaweed-based porous carbons from sargassum and enteromorpha by a one-step KOH activation method, in which the specific surface area and pore volume separately reached 291.8 m² g⁻¹ and 0.24 cm³ g⁻¹, and the maximum CO₂ adsorption capacity was 1.05 mmol g⁻¹ at 25 °C (12% CO₂).³⁸ Pramanik et al. developed the cotton stalk-based porous carbon with the specific surface area of 2695 m² g⁻¹, and the adsorption capacity of 4.24 mmol g⁻¹ at 298 K (100% CO₂) was achieved.⁴⁰

China is a large agricultural country, and every year, millions of crop wastes are stacked or burned, which not only induces serious environmental pollution problems but also releases large amounts of CO₂ and increases the greenhouse gas effect. In this study, corncob, which is common in Shandong Province, China, was used as the raw material, KOH and urea were used as the activator and nitrogen source, respectively, to prepare the AC by a one-step activation method. The pore structure, surface functional groups and morphology of the AC were characterized. The CO₂ adsorption capacities at different KOH or urea to corncob mass ratios, activation temperatures and times, and adsorption temperatures were investigated, and the correlations between adsorption performance and pore properties and functional groups were analysed.

2. Experimental section

2.1 Materials

Corncob was collected from rural Weifang city, Shandong Province, China. KOH (GR, 85%) and urea (AR, 99%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd, Shanghai, China. HCl (CR, 36–38%) was purchased from Laiyang Fine Chemical Factory, Shandong Province, China. N₂ and CO₂ with a purity of 99.999% and a mixed gas of 85 vol% N₂/15 vol% CO₂ were provided by Weiyang Gas Co., Ltd, Shandong Province, China.

2.2 Preparation of the corncob-based AC

Corncob was washed with running water and deionized water, dried in an oven at 80 °C, and then crushed into powder below 100 meshes for use. KOH and urea were also pulverized to a powder for use. The AC was prepared with a simple one-step activation method.

Corncob and KOH were separately weighed according to a certain mass ratio and uniformly mixed. The mixture was tiled in a tube furnace, and N₂ was passed through for 30 minutes to remove the adsorbed air. Then, the temperature of the tube furnace was elevated at a rate of 5 °C min⁻¹ to the activation temperature and remained at this temperature for a certain time, which is the activation time. Subsequently, the system was cooled to 100 °C, and N₂ was discontinued. The black powder collected from the tube furnace was ground, was washed repeatedly with diluted HCl and deionized water until the pH was close to 7, and was then dried in a vacuum oven at 85 °C for 24 h. The dried black powder was the AC, which were named CKa-T(b), where C represents corncob; K represents KOH; T is the activation temperature, °C; a is the KOH to corncob mass ratio; and b is the activation time, h.

The N-doped AC was also prepared with the one-step method. Corncob, KOH and urea were separately weighed according to a certain mass ratio and uniformly mixed, and then the above process was repeated. What is different is that the temperature was first raised to 450 °C for 0.5 h and then to the activation temperature, and the N-doped AC was named CNcKa-T(b), where N represents urea, and c is the mass ratio of urea to corncob.

2.3 Characterization

The N₂ adsorption–desorption isotherms for corncob-based AC were collected on an ASAP 2460 (Micromeritics, USA) by the physical adsorption of N₂ at the critical temperature of 77 K. The BET surface area (S_BET) was obtained according to the Brunauer–Emmett–Teller (BET) equation, the total pore volume (V_t) was calculated according to the adsorption capacity of N₂ when the relative pressure (P/P₀) reached 0.995, the micropore volume (V_micro) was confirmed from the t-plot curve, and the pore size distribution curves were obtained from the desorption branch according to the density functional theory (DFT) method.

The X-ray photoelectron spectroscopy (XPS) was applied to the corncob-based AC using an EscaLab 250Xi (Thermo Scientific, USA). The surface morphology was collected from a JSM-7500F scanning electron microscope (JEOL, Japan).

2.4 CO₂ adsorption and regeneration

The adsorption and regeneration of the prepared AC were performed on an online fixed bed adsorption device. A total of 0.4 g of the sample was fixed in the centre of the quartz tube, in which the inner diameter was 0.8 cm and length was 50 cm, and N₂ was passed through while the temperature was raised to 100 °C and held for 60 min to drive away preadsorbed air. Then, the temperature was reduced to the desired adsorption temperature, and inlet N₂ was converted to the mixed gas. Here, the CO₂ adsorption process began, and C₀ and C, which were recorded from the online gas chromatograph, represent the CO₂ concentration in the inlet and outlet of the reactor, respectively. As adsorption proceeded, the CO₂ concentration in the outlet continuously increased, and when C equaled C₀, the adsorption process was completed, and the sorbent reached a saturated adsorption state; here, the CO₂ adsorption capacity was the saturated adsorption capacity. The inlet was then converted to N₂, and the temperature was raised to 100 °C to desorb the adsorbed CO₂. As desorption proceeded, C continuously decreased; when C equaled 0, the desorption process finished, and the sorbent was regenerated. Ten adsorption–desorption regenerations were performed on the chosen samples.

3. Results and discussion

3.1 Characterization

3.1.1 BET. The N₂ adsorption–desorption isotherms, pore size distribution curves and textural pore properties for KOH-activated corncob before and after N-doping are shown in Fig. 1(a), (b) and Table 1, respectively. In Fig. 1(a), the N₂ adsorption–desorption isotherms showed a sharp rise and type I curve at the initial adsorption stage and then a mild increase with a narrow type H4 hysteresis loop, suggesting that the pores in corncob-based AC were mainly micropores with partial mesopores,^15,27 which could also be proven from the pore size distribution curves in Fig. 1(b). As seen from Table 1, for KOH-activated corncob, as the activation temperature increased from 600 to 800 °C, the specific surface area and pore volume increased from 120.72 to 926.07 m² g⁻¹ and 0.07 to 0.48 cm³ g⁻¹, respectively. For CN1K0.3-700(1), the specific surface area and pore volume were 1096.70 m² g⁻¹ and 0.48 cm³ g⁻¹, respectively, which were significantly higher than the corresponding values of 796.96 cm² g⁻¹ and 0.34 cm³ g⁻¹ for CK0.3-700(1).

Fig. 1 The (a) N₂ adsorption–desorption isotherms and (b) pore size distribution curves for KOH-activated corncob before and after N-doping.

Table 1 The textural pore properties for KOH-activated corncob before and after N-doping

Sample	St (m^2 g^−1)	Smicro (m^2 g^−1)	Vt (cm^3 g^−1)	Vmicro (cm^3 g^−1)	Micro-porosity (%)	Average pore diameter (nm)
CK0.3-600(1)	120.72	108.34	0.07	0.04	57.14	2.38
CK0.3-700(1)	796.96	763.36	0.34	0.30	88.24	1.73
CK0.3-800(1)	926.07	872.08	0.40	0.34	85.00	1.72
CN1K0.3-700(1)	1096.70	1032.29	0.48	0.40	83.33	1.74

---

3.1.2 XPS. The O 1S and N 1S spectra for the KOH-activated corncob before and after N-doping are shown in Fig. 2(a–d) and 3(a–d), and the corresponding peak area ratio for every functional group is listed in Table 2. As shown in Fig. 2 and 3, the O 1S spectra in KOH-activated corncob before and after N-doping were all categorized into four peaks at 530.07, 530.80, 531.67 and 532.84 eV, which are attributed to hydroxyl group (–OH), carbonyl, lactone or ketone group (C=O), ether or alcohol group (C–O) and carboxyl group (–COOH), respectively.^38,42,43 The N 1S spectra in KOH-activated corncob before and after N-doping were all divided into two peaks at 398.31 and 400.21 eV, which are attributed to pyridinic nitrogen (C=N) and pyrrolic nitrogen (–NH), respectively.^37,38,42 Generally, –OH, C–O and –NH promote CO₂ adsorption,⁴² which is further discussed.

Fig. 2 The O 1S spectra for KOH-activated corncob before and after N-doping.

Fig. 3 The N 1S spectra for KOH-activated corncob before and after N-doping.

Table 2 The peak area ratios of different O species for KOH-activated corncob before and after N-doping^a

Sample	O (%)	O (%)				–OH (%)	–C=O (%)	C–O (%)	–COOH (%)
		–OH	–C=O	C–O	–COOH
a 1, 2, 3 and 4 represents CK0.3-600(1), CK0.3-700(1), CK0.3-800(1) and CN1K0.3-700(1), respectively.
1	27.26	18.20	28.83	26.06	26.90	4.96	7.86	7.10	7.33
2	21.55	11.04	26.65	24.08	38.23	2.38	5.74	5.19	8.24
3	20.43	22.67	28.52	21.05	27.76	4.63	5.83	4.30	5.67
4	13.12	15.95	31.16	30.11	22.79	2.09	4.09	3.95	2.99

---

As shown in Table 2, for KOH-activated corncob, as the activation temperature increased from 600 to 800 °C, the peak area ratio for –NH decreased from 1.62 to 0.86%, that for C–O decreased from 7.10 to 4.30%, that for –OH first decreased from 4.96 to 2.38 and then increased to 4.63%, and that for –OH plus C–O first decreased from 12.07 to 7.57 and then increased to 8.93% (Table 3).

Table 3 The peak area ratios of different N species for KOH-activated corncob before and after N-doping^a

Sample	N (%)	N (%)		C=N (%)	–NH– (%)
		C=N	–NH–
a 1, 2, 3 and 4 represents CK0.3-600(1), CK0.3-700(1), CK0.3-800(1) and CN1K0.3-700(1), respectively.
1	2.86	43.53	56.47	1.24	1.62
2	2.97	47.84	52.16	1.42	1.55
3	2.00	57.23	42.77	1.14	0.86
4	2.89	33.71	66.29	0.97	1.92

---

After N-doping, the ratio for –OH, C–O, –OH plus C–O, and –NH in CN1K0.3-700(1) was 2.09, 3.95, 6.04 and 1.92%, respectively, which all obviously increased compared with the corresponding value for CK0.3-700(1), suggesting that N-doping increased the functional groups that promote CO₂ adsorption.

3.1.3 SEM. Fig. 4 shows the SEM micrographs of KOH-activated corncob before and after N-doping, and the activated corncob exhibited a rough and porous surface. As the activation temperature increased from 700 to 800 °C, the porous nature of the KOH-activated corncob was more obvious, and became much more developed after N-doping. The above phenomenon is consistent with the results of BET characterization.

Fig. 4 SEM micrographs of KOH-activated corncob before and after N-doping.

3.2 The CO₂ adsorption performance of KOH-activated corncob

3.2.1 The effect of the KOH to corncob mass ratio. The amount of activator influences the activation degree, pore development and surface functional group formation of the biomass, so the effect of the KOH to corncob mass ratio on CO₂ adsorption was investigated. The breakthrough adsorption curves and saturated adsorption capacity for KOH-activated corncob at different KOH to corncob mass ratios are displayed in Fig. 5(a) and (b). Here, the activation temperature, activation time and adsorption temperature were fixed at 700 °C, 1 h and 20 °C, respectively.

Fig. 5 The (a) breakthrough adsorption curves and (b) saturated adsorption capacity for KOH-activated corncob at different KOH to corncob mass ratios.

With the mass ratio increasing from 0 to 1, the breakthrough adsorption curve first moved towards the right and then left, so the saturated adsorption capacity first increased and then decreased, and the optimal adsorption capacity of 3.49 mmol g⁻¹ appeared when the mass ratio was 0.3. More KOH promoted more corncob to be pyrolysed and to react, so more developed pores were formed, but too much KOH overreacted the corncob, etched the formed pores and collapsed the formed framework,^29,38 so the adsorption capacity displayed an increasing trend first and then a decreasing trend.

In view of the optimal adsorption capacity appearing at a mass ratio of 0.3, the mass ratio was fixed at 0.3 in the following investigation.

3.2.2 The effect of activation temperature and time on CO₂ adsorption. The activation temperature and time also play important roles in pore development and functional group formation during biomass activation. The mass ratio was fixed as 0.3, and the activation time was determined to be 1 h to investigate the effect of activation temperature on CO₂ adsorption of KOH-activated corncob at 20 °C.

The breakthrough adsorption curves and saturated adsorption capacity for KOH-activated corncob at different activation temperatures are shown in Fig. 6(a) and (b), respectively. As the activation temperature increased from 600 to 800 °C, the breakthrough curve in Fig. 6(a) first moved right and then moved left, and the corresponding saturated adsorption capacity first increased and then decreased, as shown in Fig. 6(b), with the optimal adsorption capacity appearing at 700 °C and being 3.49 mmol g⁻¹ for CK0.3-700(1).

Fig. 6 The (a) breakthrough adsorption curves and (b) saturated adsorption capacity for KOH-activated corncob at different activation temperatures.

The mass ratio and activation temperature were fixed at 0.3 and 700 °C, respectively, to investigate the effect of activation time on CO₂ adsorption at 20 °C. The breakthrough adsorption curves and saturated adsorption capacity for KOH-activated corncob at different activation times are shown in Fig. 7(a) and (b), respectively. The adsorption performance also showed an increasing and then decreasing trend, and the best value appeared when the activation time was 1 h.

Fig. 7 The (a) breakthrough adsorption curves and (b) saturated adsorption capacity for KOH-activated corncob at different activation times.

Generally, a higher activation temperature and a longer activation time promote a more adequate reaction between KOH and corncob, so the pore structure and surface functional groups became more developed; however, too high a temperature and too long a time also caused an overreaction between KOH and corncob, which might cause pore framework collapse or favour further decomposition of functional groups.^29,38 Therefore, the adsorption performance for KOH-activated corncob first showed increasing trends and then decreasing trend, and the optimal temperature and time were determined to be 700 °C and 1 h, respectively.

According to the above study, CK0.3-700(1) suggested an optimal adsorption capacity of 3.49 mmol g⁻¹ at 20 °C when the mass ratio of KOH to corncob was 0.3, and the activation temperature and time were 700 °C and 1 h, respectively.

3.3 The effect of N-doping on CO₂ adsorption of CK0.3-700(1)

Urea was mixed with KOH and corncob to introduce more N-containing functional groups to improve CO₂ adsorption. The breakthrough adsorption curves and saturated adsorption capacity for N-doped CK0.3-700(1) at different mass ratios of urea to corncob are shown in Fig. 8(a) and (b). After N-doping, the breakthrough adsorption curve in Fig. 8(a) moved right, suggesting that N-doping improved the adsorption performance of KOH-activated corncob, as shown in Fig. 8(b). After N-doping at mass ratios of 0.5, 1 and 1.5, the saturated adsorption capacity was 4.24, 4.58 and 4.26 mmol g⁻¹, representing increasing of 21.8, 31.4 and 22.2%, respectively, compared with CK0.3-700(1). According to the BET and XPS characterization results comparing CN1K0.3-700(1) and CK0.3-700(1), not only the specific surface area and pore volume but also the ratio of –OH, C–O, and –NH groups that favoured CO₂ adsorption⁴² all increased, which improved CO₂ adsorption.

Fig. 8 The (a) breakthrough adsorption curves and (b) saturated adsorption capacity for N-doped CK0.3-700(1).

3.4 Linear correlation analysis

Linear correlation fitting was conducted on the corncob-based AC data, and the fitting results between the saturated adsorption capacity and S_BET, V_t, V_micro, the N–H group ratio and the O–H plus C–O group ratio are shown in Fig. 9(a–e), respectively. As suggested in Fig. 9, the saturated adsorption capacity did not fit well with any of the above factors, suggesting that the adsorption performance of the corncob-based AC did not completely rely on the pore properties or the surface functional groups but had close relations with both, which could explain why the saturated adsorption capacity did not continuously increase as the activation temperature or time increased.

Fig. 9 The linear correlation analysis between the adsorption performance and (a) S_BET, (b) V_t, (c) V_micro, (d) the N–H group ratio and (e) the O–H plus C–O ratio.

In summary, the optimal preparation process for KOH-activated corncob appeared when the KOH to corncob mass ratio was 0.3, the activation temperature and time were 700 °C and 1 h, respectively, and the optimal urea to corncob mass ratio was 1 for N-doped KOH-activated corncob.

3.5 Regeneration of KOH-activated corncob before and after N-doping

In view of the good adsorption performance, CK0.3-700(1) and CN1K0.3-700(1) were selected to further investigate ten adsorption–desorption regeneration performances. The adsorption and regeneration temperatures were separately set as 20 and 100 °C.

The saturated adsorption capacity for corncob-based AC is shown in Fig. 10. For both CK0.3-700(1) and CN1K0.3-700(1), after ten regenerations, the adsorption capacity all showed an insignificant drop, and the values for both were 3.44 and 4.52 mmol g⁻¹, which are reductions of only 1.43 and 1.31% compared with the corresponding values of the fresh sorbent, which were 3.49 and 4.58 mmol g⁻¹, respectively. Both CK0.3-700(1) and CN1K0.3-700(1) showed good regeneration performance, and the prepared corncob-based AC has potential for CO₂ capture.

Fig. 10 The saturated adsorption capacity for CK0.3-700(1) and CN1K0.3-700(1) after regeneration.

4. Conclusions

A simple one-step activation method for the preparation corncob-based AC before and after N-doping suggested the development of a microporous structure and abundant O- and N-containing surface functional groups, which promote CO₂ adsorption. The CO₂ adsorption capacity for KOH-activated AC before and after N-doping reached 3.49 and 4.58 mmol g⁻¹, respectively, and remained at 3.44 and 4.52 mmol g⁻¹ after ten regenerations. In addition, the linear correlation results showed that the adsorption was related to both the pore structure and the functional groups and was not dependent on either.

The development of corncob-based AC not only reduces the environmental pollution problem caused by the accumulation and incineration of crop waste and realizes waste resource utilization but also reduces the CO₂ capture costs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The financial support from the National Natural Science Foundation of China (Grant No. 22108208), the State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (Grant No. 2021-K02), and Weifang Science and Technology Development Plan (Grant No. 2021GX004) is gratefully acknowledged.

References

1. M. E. Boot-Handford, J. C. Abanades, E. J. Anthony, M. J. Blunt, S. Brandani, N. Mac Dowell, J. R. Fernandez, M.-C. Ferrari, R. Gross and J. P. Hallett, et al., Carbon capture and storage update, Energy Environ. Sci., 2014, 7(1), 130–189.
2. M. Sevilla and A. B. Fuertes, Sustainable porous carbons with a superior performance for CO₂ capture, Energy Environ. Sci., 2011, 4, 1765–1771.
3. M. Bui, C. S. Adjiman, A. Bardow, E. J. Anthony, A. Boston and S. Brown, et al., Carbon capture and storage (CCS): the way forward, Energy Environ. Sci., 2018, 11, 1062–1176.
4. A. S. Bhown and B. C. Freeman, Analysis and status of post combustion carbon dioxide capture technologies, Environ. Sci. Technol., 2011, 45(20), 8624–8632.
5. J. Quyang, C. Zheng, W. Gu, Y. Zhang, H. Yang and S. L. Suib, Textural properties determined CO₂ capture of tetraethylenepentamine loaded SiO₂ nanowires from α-sepiolite, Chem. Eng. J., 2018, 337, 342–350.
6. J. D. Martell, P. J. Milner, R. L. Siegelman and J. R. Long, Kinetics of cooperative CO₂ adsorption in diamine-appended variants of the metal-organic framework Mg₂(dobpdc), Chem. Sci., 2020, 11, 6457–6471.
7. L. Mafra, T. Čendak, S. Schneider, P. V. Wiper, J. Pires, J. R. B. Gomes and M. L. Pinto, Amine functionalized porous silica for CO₂/CH₄ separation by adsorption: which amine and why, Chem. Eng. J., 2018, 336, 612–621.
8. S. M. Rafigh and A. Heydarinasab, Mesoporous chitosan-SiO₂ nanoparticles: synthesis, characterization, and CO₂ adsorption capacity, ACS Sustainable Chem. Eng., 2017, 5, 10379–10386.
9. J. Yu, Y. Wu and P. B. Balbuena, Response of metal sites toward water effects on post combustion CO₂ capture in metal-organic frameworks, ACS Sustainable Chem. Eng., 2016, 4, 2387–2394.
10. Y. Yang, C. Y. Chuah and T.-H. Bae, Polyamine-appended porous organic polymers for efficient post-combustion CO₂ capture, Chem. Eng. J., 2019, 358, 1227–1234.
11. M. Smyrnioti, C. Tampaxis, T. Steriotis and T. Ioannides, Study of CO₂ adsorption on a commercial CuO/ZnO/Al₂O₃ catalyst, Catal. Today, 2020, 357, 495–502.
12. Y. Wang, X. Hu, T. Guo, W. Tian, J. Hao and Q. Guo, The competitive adsorption mechanism of CO₂, H₂O and O₂ on a solid amine adsorbent, Chem. Eng. J., 2021, 416, 129007–129017.
13. M. A. O. Lourenço, M. Fontana, P. Jagdale, C. F. Pirri and S. Bocchini, Improved CO₂ adsorption properties through amine functionalization of multi-walled carbon nanotubes, Chem. Eng. J., 2021, 414, 128763–128775.
14. F. Liu, W. Fu and S. Chen, Synthesis, CO₂ characterization and adsorption performance of a thermosensitive solid amine adsorbent, J. CO₂ Util., 2019, 31, 98–105.
15. X. Wang, W. Zeng, M. Song, F. Wang, X. Hu, Q. Guo and Y. Liu, Polyetheramine improves the CO₂ adsorption behavior of tetraethylenepentamine-functionalized sorbents, Chem. Eng. J., 2019, 364, 475–484.
16. Y. Liu and J. Hou, Selective adsorption of CO₂/CH₄ mixture on clay-rich shale using molecular simulations, J. CO₂ Util., 2020, 39, 101143–101152.
17. G. N. Muriithi, L. F. Petrik and F. J. Doucet, Synthesis, characterization and CO₂ adsorption potential of NaA and NaX zeolites and hydrotalcite obtained from the same coal fly ash, J. CO₂ Util., 2020, 36, 220–230.
18. J. Chang, C. Hou, D. Wan, X. Zhang, B. Xu, H. Tian, X. Wang and Q. Guo, Enhanced CO₂ adsorption capacity of bi-amine co-tethered flue gas desulfurization gypsum with water of hydration, J. CO₂ Util., 2020, 35, 115–125.
19. A. Rehman and S.-J. Park, Environmental remediation by microporous carbon: an efficient contender for CO₂ and methylene blue adsorption, J. CO₂ Util., 2019, 34, 656–667.
20. Y. Kuwahara, A. Hanaki and H. Yamashita, A direct conversion of blast furnace slag to a mesoporous silica-calcium oxide composite and its application in CO₂ captures, Green Chem., 2020, 22, 3759–3768.
21. G. Tuci, A. Iemhoff, A. Rossin, D. Yakhvarov, M. F. Gatto, R. Balderas-Xicohténcatl, L. Zhang, M. Hirscher, R. Palkovits, C. Pham-Huu and G. Giambastiani, Tailoring morphological and chemical properties of covalent triazine frameworks for dual CO₂ and H₂ adsorption, Int. J. Hydrogen Energy, 2022, 47, 8434–8445.
22. C. H. Lee, S. W. Choi, H. J. Yoon, H. J. Kwon, H. C. Lee, S. G. Jeon and K. B. Lee, Na₂CO₃-doped CaO-based high-temperature CO₂ sorbent and its sorption kinetics, Chem. Eng. J., 2018, 352, 103–109.
23. C.-W. Chang, Y.-H. Kao, P.-H. Shen, P.-C. Kang and C.-Y. Wang, Nanoconfinement of metal oxide MgO and ZnO in zeolitic imidazolate framework ZIF-8 for CO₂ adsorption and regeneration, J. Hazard. Mater., 2020, 400, 122974–122986.
24. W.-T. Zhen, K. Huang and S. Dai, Solvothermal and template-free synthesis of N-functionalized mesoporous polymer for amine impregnation and CO₂ adsorption, Microporous Mesoporous Mater., 2019, 290, 109653–109659.
25. A. Jayakumar, A. Gomez and N. Mahinpey, Post-combustion CO₂ capture using solid K₂CO₃: discovering the carbonation reaction mechanism, Appl. Energy, 2016, 179, 531–543.
26. A. Rehman, Y.-J. Heo, G. Nazir and S.-J. Park, Solvent-free, one-pot synthesis of nitrogen-tailored alkali-activated microporous carbons with an efficient CO₂ adsorption, Carbon, 2021, 172, 71–82.
27. A. Rehman and S.-J. Park, From chitosan to urea-modified carbons: tailoring the ultra-microporosity for enhanced CO₂ adsorption, Carbon, 2020, 159, 625–637.
28. T. F. Aquino, S. T. Estevam, V. O. Viola, C. R. M. Marques, F. L. Zancan, L. B. Vasconcelos, H. G. Riella, M. J. R. Pires, R. Morales-Ospino, A. E. B. Torres, M. Bastos-Neto and C. L. Cavalcante Jr, CO₂ adsorption capacity of zeolites synthesized from coal fly ashes, Fuel, 2020, 276, 118143–118152.
29. X. Wang, W. Zeng, W. Liu, X. Cao, C. Hou, Q. Ding and Y. Lu, CO₂ adsorption of lignite chars after one-step activation, New J. Chem., 2020, 44, 13755–13763.
30. M. G. Plaza, C. Pevida, B. Arias, J. Fermoso, M. D. Casal, C. F. Martín, F. Rubiera and J. J. Pis, Development of low-cost biomass-based adsorbents for post combustion CO₂ capture, Fuel, 2009, 88, 2442–2447.
31. H. Yi, F. Li, P. Ning, X. Tang, J. Peng, Y. Li and H. Deng, Adsorption separation of CO₂, CH₄, and N₂ on microwave activated carbon, Chem. Eng. J., 2013, 215–216, 635–642.
32. A. Arami-Niya, T. E. Rufford and Z. Zhu, Activated carbon monoliths with hierarchical pore structure from tar pitch and coal powder for the adsorption of CO₂, CH₄ and N₂, Carbon, 2016, 103, 115–124.
33. J. Chen, J. Yang, G. Hu, X. Hu, Z. Li, S. Shen, M. Radosz and M. Fan, Enhanced CO₂ capture capacity of nitrogen-doped biomass-derived porous carbons, ACS Sustainable Chem. Eng., 2016, 4, 1439–1445.
34. D. P. Vargas, M. Balsamo, L. Giraldo, A. Erto, A. Lancia and J. C. Moreno-Pirajań, Equilibrium and dynamic CO₂ adsorption on activated carbon honeycomb monoliths, Ind. Eng. Chem. Res., 2016, 55, 7898–7905.
35. J. Wang and S. Kaskel, KOH activation of carbon-based materials for energy storage, J. Mater. Chem., 2012, 22, 23710–23725.
36. M. Meng, Z. Qiu, R. Zhong, Z. Liu, Y. Liu and P. Chen, Adsorption characteristics of supercritical CO₂/CH₄ on different types of coal and a machine learning approach, Chem. Eng. J., 2019, 368, 847–864.
37. Z. Ma, Z. Yang, H. Zhang and Z. Liu, Nitrogen-doped microporous carbon materials with uniform pore diameters: design and applications in CO₂ and H₂ adsorption, Microporous Mesoporous Mater., 2020, 296, 109992–109998.
38. S. Ding and Y. Liu, Adsorption of CO₂ from flue gas by novel seaweed-based KOH-activated porous biochars, Fuel, 2020, 260, 116382–116391.
39. A. Sarwar, M. Ali, A. H. Khoja, A. Nawar, A. Waqas, R. Liaquat, S. R. Naqvi and M. Asjid, Synthesis and characterization of biomass-derived surface-modified activated carbon for enhanced CO₂ adsorption, J. CO₂ Util., 2021, 46, 101476–101489.
40. P. Pramanik, H. Patel, S. Charola, S. Neogi and S. Maiti, High surface area porous carbon from cotton stalk agro-residue for CO₂ adsorption and study of techno-economic viability of commercial production, J. CO₂ Util., 2021, 45, 101450–101461.
41. G. Huang, Y. Liu, X. Wu and J. Cai, Activated carbons prepared by the KOH activation of a hydrochar from garlic peel and their CO₂ adsorption performance, New Carbon Mater., 2019, 34, 247–257.
42. Y. Xu, Z. Yang, G. Zhang and P. Zhao, Excellent CO₂ adsorption performance of nitrogen-doped waste biocarbon prepared with different activators, J. Cleaner Prod., 2020, 264, 121645–121654.
43. L. Zhang, S. Tang, F. He, Y. Liu, W. Mao and Y. Guan, Highly efficient and selective capture of heavy metals by poly(acrylic acid) grafted chitosan and biochar composite for wastewater treatment, Chem. Eng. J., 2019, 378, 122215–122231.

---