Facile Preparation of Porous Carbon Derived from Industrial Biomass Waste as an Efficient CO₂ Adsorbent

Abstract

A porous carbon CO₂ adsorbent based on soybean cake (industrial biomass waste) has been successfully prepared by direct carbonation, following KOH activation. The prepared porous carbon adsorbent exhibits efficient CO₂ capture performance with the highest adsorption capacity of 4.19 and 6.61 mmol/g at 298 and 273 K under atmospheric pressure, respectively. Moreover, the porous carbon adsorbent also shows good static CO₂ adsorption capacity at a low pressure (0.15 bar) with an uptake of 1.26 mmol/g and an equally ideal dynamic CO₂ capture capability with an uptake of 1.28 mmol/g (15% CO₂) at 298 K. Additionally, the ideal adsorbed solution theory (IAST) model has been used to measure the selectivity of the porous carbon, and the IAST factors of CO₂/N₂ (15/85, fuel gas), CO₂/CH₄ (40/60, biogas), and CH₄/N₂ (50/50, coalbed gas) are about 27, 6, and 6, respectively. The dynamic breakthrough test reveals the strong interaction between the porous carbon and CO₂, which also verifies the considerable selective capture ability of this material for CO₂. Furthermore, the soybean cake-based CO₂ adsorbent also presents prominent cyclic regeneration capacity (a five-time cyclic test) with lower isosteric heats (34–18 kJ/mmol) of CO₂ adsorption.

1. Introduction

Greenhouse effect is the initiator of climate issues involving global warming and ocean acidification. Excessive CO₂ emissions from the widespread use of fossil fuels are believed to be the primary contributor to the climate changes.¹ Although new energy sources such as nuclear energy, solar energy, and wind energy have been vigorously promoted globally, it is unlikely to completely abandon the use of fossil fuels in a short period of time.² Currently, chemical adsorption using aqueous amines as a large-scale CO₂ adsorbent in industry has the advantages of great adsorption capacity and fast uptake speed.³ However, the aqueous amines require high energy consumption for regeneration, and prolonged usage can erode equipment and even cause secondary contamination.⁴⁻⁶ In comparison, solid-state adsorbents, such as zeolite,⁷ metal–organic frameworks,^8,9 covalent–organic frameworks,¹⁰ porous organic polymers,¹¹ porous carbon,¹²⁻¹⁴ and so forth, which have the advantages of simple operation, low regeneration energy consumption, and environmental friendliness, are considered as an alternative for CO₂ capture.^15,16

From a practical application point of view, high CO₂ adsorption capacity is not the only criterion for evaluating adsorbents. In addition to pursuing relatively high adsorption performance, efforts should also be made to reduce production costs, including the costs on raw material, operation, and recovery. Biomass resources are widely distributed in nature, and the porous carbon prepared by using biomass as the carbon precursor has renewable, cost-effective, and environment-friendly characteristics, which has attracted great interest in recent years.¹⁷ Yang and co-workers successfully synthesized nitrogen-doped porous coconut shell-based CO₂ adsorbents by combining ammoxidation with potassium hydroxide (KOH) activation, and the CO₂ adsorption capacity at 298 and 273 K (1 bar) were 3.44–4.26 and 4.77–6.52 mmol/g, respectively.¹⁸ Li et al. prepared porous carbons for CO₂ capture by the KOH activation of rice husk char, with the maximum CO₂ uptake of 4.16 and 6.24 mmol/g at 298 and 273 K (1 bar), respectively.¹⁹ Hu’s group achieved efficient CO₂ adsorbents derived from lotus stalk by a facile one-step KOH activation procedure, with the uptake of 3.68 and 5.11 mmol/g at 1 bar, at 298 and 273 K, respectively.²⁰ Zhang and co-workers prepared nitrogen-rich porous carbon CO₂ adsorbents from waste oil-tea seed shells by solid NaNH₂ activation, with an excellent CO₂ uptake of 3.5 mmol/g and an outstanding CO₂/N₂ selectivity of 77.9 at 298 K and 1 bar.²¹ There is no doubt that various types of biomass resources have been widely employed in facilely preparing porous carbon materials for efficiently capturing CO₂.

Nevertheless, if the industrial biomass waste resources generated in the production process can be reused, the utilization rate of the original production materials can be greatly improved. Furthermore, it can not only achieve sustainable development but also reduce the recovery cost and enrich the production line of the original industrial production process, which can improve economic efficiency. Soybean is a widely cultivated crop and also one of the main raw materials for squeezing vegetable oil. As is known to all, soybean is rich in plant protein with a certain amount of nitrogen, which has been proven to have a positive effect on CO₂ adsorption capacity and selectivity.²²⁻²⁵ Moreover, the downstream products of oil extraction (soybean cakes) are mainly used as feed and fertilizer, and no research on the application of CO₂ adsorbent has been found now.

In this work, the biomass-based porous carbon CO₂ adsorbent with high surface area and controllable pore size was facilely prepared from soybean cake by regulating the KOH activation process. The as-prepared porous carbon CO₂ adsorbent has been measured through related adsorption performance tests, including static pure gas adsorption tests and dynamic mixed gas adsorption tests under specific conditions. The relationship between the physical and chemical properties of the adsorbent and the adsorption performance is related through the corresponding characterization, and the key factors that determine the capture performance of the material are clarified. This study not only provides a reliable choice for CO₂ adsorbents but also states a new strategy for the reuse of biomass waste from industrial production.

2. Results and Discussion

2.1. Morphology and Phase Structure

The surface morphologies of the as-prepared CSC and CSCX-Y were characterized by scanning electron microscopy (SEM). As shown in Figures 1a–c and S3, both the materials before and after activation present smooth blocks without obvious pore structures. For further observation, high-resolution transmission electron microscopy (HR-TEM) images of CSCX-Y (Figures 1d and S4) show abundant amorphous worm-like micropores rather than mesopores, which can be attributed to the activation of highly dispersed potassium ions.²⁶

Figure 1.

Microscopic morphology. SEM images: (a) CSC, (b,c) CSC2-750; HR-TEM image: (d) CSC2-750.

The XRD patterns of the porous carbon prepared in different activation conditions have been presented in Figure 2a. There are two broad and weak peaks centered around 2θ of 21 and 43°, which can be ascribed to the (002) and (100) diffraction patterns of amorphous graphite carbon, respectively.^27,28 This finding further verifies that the prepared soybean cake-based carbon materials have poor crystallinity and an amorphous structure, which is highly consistent with the TEM results. Raman spectra are displayed in Figures 2b and S5b, which are used to study the crystal structure and composition of the as-prepared samples. The peak at 1345 cm^–1 is assigned to the D-band, which is associated with the disorder and imperfect frameworks of carbon atoms caused by vacancies or other defects.³ The G-band at approximately 1591 cm^–1 is characterized by the planar vibrations of the sp²-hybridized carbon in the graphite layer.²⁹ The intensity ratio of the D- and G-bands (I_D/I_G) was also used as a parameter to measure the defect degree of materials.³⁰ In comparison, the I_D/I_G ratio increases from 0.836 to 0.987, indicating that more defects exist in the material after the KOH activation process.

Figure 2.

(a) XRD patterns of CSC and CSCX-Y; (b) Raman spectra of CSC and CSCX-Y.

2.2. Chemical Composition and Surface Properties

The element contents of these materials are revealed by elemental analysis (see Table 1). The contents of C, H, and N in all samples vary in the range of 73.89–81.22, 2.03–0.47, and 3.86–0.52%, respectively. With the increase of the activation temperature and the mass of KOH, the N content of the prepared samples showed a downward trend, which can be attributed to the decomposition or consumption of certain thermally unstable nitrogen species in CSC, owing to the ascending activation intensity.^31,32 Furthermore, the N-containing functional groups which can provide nitrogen source in the materials are inferred by Fourier transform infrared (FT-IR) spectra (see Figure S6). Typically, the peaks at 3446, 2919, 1635, and 1039 cm^–1 can be classified as N–H or O–H stretching vibrations, C–H stretching vibrations, N–H in-plane deformation vibrations, and C–O stretching vibrations (S=O symmetry stretching vibrations), respectively.^24,33

Table 1. Elemental Composition of As-Prepared Materials.

samples	element contents (wt %)
	C	H	N
CSC	73.89	2.03	3.86
CSC2-700	81.22	0.86	1.57
CSC1-750	80.32	0.85	1.60
CSC2-750	78.50	0.67	0.77
CSC3-750	79.14	0.68	0.52
CSC2-800	81.15	0.47	0.55

X-ray photoelectron spectroscopy (XPS) was used to further clarify the elemental composition and chemical states of C and N elements in the shallow. In the XPS survey spectra (Figure S7), there are three characteristic peaks around 285, 398, and 530 eV, which represent the signatures of the orbitals of C 1s, N 1s, and O 1s, respectively. The elemental composition of all samples obtained by XPS analysis is listed in Table S2. Moreover, with the aggravated activation process, the intensity of N peak decreases, indicating the loss of unstable N species on the surface, which is consistent with the results of the elemental analysis. As shown in Figure 3a, the highly analytical C 1s spectra of CSC contain three peaks, including 284.6 (C–C/C=C), 285.5 (C–N/C–O), and 288.1 eV (C=O).^2,33,34 The highly analytical N 1s spectra of CSCX-Y were deconvoluted into two peaks around 398.3 eV (pyridinic-N) and 400.5 eV (pyrrolic-/pyridonic-N) (see Figure 3b and S7), which had been proven to be beneficial for CO₂ capture.³⁵⁻³⁸

Figure 3.

(a) C 1s XPS spectra and (b) N 1s XPS spectra of CSC.

2.3. Pore Structure Analysis

In order to investigate the physical pore characteristic thoroughly, the nitrogen adsorption–desorption isotherms of the as-prepared samples were measured at 77 K. As displayed in Figure 4a, the isotherm of CSC can be classified as typical type III by the IUPAC classification standard, meaning that CSC is a nonporous material. The isotherms of CSCX-Y exhibit the obvious adsorption characteristic of type I, with significant N₂ adsorption at a relatively low pressure (P/P₀ < 0.01). Meanwhile, it can be clearly seen from the pore size distribution (PSD) presented in Figure 4c,d that almost all pore sizes are concentrated below 2.0 nm. All these results indicate that the activated samples (CSCX-Y) are mainly composed of micropores, which are inseparable from the KOH activation process (see the Supporting Information). Simultaneously, a slight hysteresis loop begins to appear from P/P₀ = 0.50, suggesting the existence of mesopores, whose pore sizes are mainly concentrated around 3.5–4.0 nm (Figure 4c,d). The existence of an appropriate amount of mesopores is beneficial to the mass transfer of CO₂ to the adsorption point, so as to improve the utilization rate of micropores.³⁹ For showing the structural properties of the as-prepared materials more intuitively, the obtained physical adsorption data are displayed in Table 2. From the results, it is observed that the S_BET, S_micro, V_t, and V_micro values of samples improved significantly after the KOH activation process. With the activation temperature and the mass ratio of CSC to KOH increasing, the S_BET, V_t, and V_micro values increased from 3 to 1712 m²/g, from 0.49 to 0.95, and from 0 to 0.81 cm³/g, respectively. Beyond this, narrow micropores (<1 nm) have been proven to be advantageous for CO₂ adsorption because of the fact that the pore size close to the size of the adsorbate molecule is more favorable for adsorption (the dynamic diameter of CO₂ is 0.33 nm).^40,41 Thus, a narrow micropore volume (V_n) was obtained by using the Dubinin–Radushkevich (D–R) equation to calculate the CO₂ adsorption data at 273 K. The V_n values of the prepared samples ranged from 0.38 to 0.57 cm³/g. Such abundant microporous properties will provide sufficient space for CO₂ capture.

Figure 4.

Nitrogen adsorption–desorption isotherms: (a) CSC and CSC2-Y, (b) CSCX-750; PSDs: (c) CSC2-Y, (d) CSCX-750.

Table 2. Structure Characterization of Soybean Cake-Based Porous Carbon.

samples	SBETa (m2/g)	Smicrob (m2/g)	Vtc (cm3/g)	Vmicrod (cm3/g)	Vne (cm3/g)
CSC	3		0.04
CSC2-700	1092	1022	0.63	0.53	0.38
CSC1-750	809	751	0.49	0.39	0.41
CSC2-750	1390	1308	0.79	0.68	0.57
CSC3-750	1712	1603	0.95	0.81	0.57
CSC2-800	1342	1240	0.76	0.62	0.43

^aSurface area calculated by the BET method at P/P₀ = 0.05–0.3.

^bMicroporous surface area obtained by the T-Plot method.

^cTotal pore volume obtained from the nitrogen adsorption and desorption isotherms at P/P₀ ≈ 0.99.

^dMicropore volume calculated by the T-Plot method.

^eNarrow micropore volume (<1 nm) obtained from the CO₂ adsorption data at 273 K.

2.4. Gas Adsorption Measurement

The gas adsorption tests were performed at 298 and 273 K (1 bar) to measure the adsorption performance of CSCX-Y. As shown in Table 3 and Figures 5 and S8, the CO₂ adsorption capacity of CSCX-Y at 298 and 273 K ranges from 3.29 to 4.19 and 4.35 to 6.61 mmol/g, respectively. In particular, CSC2-750 has the highest adsorption capacity among all samples, with the CO₂ uptake of 4.19 and 6.61 mmol/g at 298 and 273 K, respectively. Compared with the various adsorbents that have been reported (listed in Table S6), this adsorption performance still has commendable features under same conditions. Comparatively, the CO₂ adsorption capacity of CSC3-750 is slightly less than that of CSC2-750, which is 3.97 and 6.34 mmol/g at 298 and 273 K, respectively. It is not difficult to find that CSC2-750 has values of V_n similar with CSC3-750 but with a slightly higher N content of 0.77% than 0.52%. According to the Lewis acid–base interaction between N and CO₂ molecules, nitrogen can promote the process of CO₂ capture.^4,42 In addition, the variation trend of CO₂ adsorption capacity of other samples is basically consistent with that of V_n.

Table 3. Gas Adsorption Uptake^a.

	CO2 uptake (mmol/g)		CH4 uptake (mmol/g)		N2 uptake (mmol/g)
samples	298 K	273 K	298 K	273 K	298 K	273 K
CSC2-700	3.29	4.35	1.94	2.14	0.58	0.86
CSC1-750	3.54	5.38	1.66	2.29	0.60	0.81
CSC2-750	4.19	6.61	1.79	2.61	0.57	0.83
CSC3-750	3.97	6.34	1.98	2.05	0.60	0.82
CSC2-800	3.73	4.57	1.95	2.40	0.49	0.90

^aGas adsorption data measured at 1 bar.

Figure 5.

Gas adsorption isotherms of (a) CSC2-700, (b) CSC1-750, (c) CSC2-750, (d) CSC3-750, and (e) CSC2-800 at 298 K; CO₂ adsorption isotherms of (f) CSCX-Y at 298 K.

In order to determine the degree of each factor influencing the CO₂ adsorption capacity of the samples, a correlation diagram has been drawn, as presented in Figures 6 and S9. Obviously, neither the relation between the surface area (S_BET and S_micro) and CO₂ uptake nor the relation between the N content and CO₂ uptake is a single correlation. The correlation between pore volume (V_t and V_micro) and CO₂ uptake is slightly better, but linear correlation coefficients (R²) are only 0.31 and 0.33 (Figure 6b). Notably, the R² value between V_n and CO₂ uptake is around 0.84 (Figure 6c), suggesting that V_n plays an important role in the CO₂ capture of the samples. The same is true at 273 K (Figure S9). In short, V_n plays a main role in the CO₂ adsorption capacity of this adsorbing material, which can be further modulated by multiple factors of porous textural properties and N content.

Figure 6.

Relationship between CO₂ adsorption capacity and (a) surface area, (b) pore volume, (c) V_n, and (d) N content at 298 K and 1 bar.

Meanwhile, other industrial situations such as adsorption at low pressure (0.15 bar, fuel gas treatment) need to be considered. As shown in Table S4 and Figures 5f and S8f, CSCX-Y also exhibits great adsorption capacity at 0.15 bar, with the CO₂ uptake from 0.74 to 1.26 mmol/g and from 1.48 to 1.86 mmol/g at 298 and 273 K, respectively. As expected, among all tested samples, CSC2-750 showed the highest adsorption capacity at 298 K and 0.15 bar, with the uptake of 1.26 mmol/g, which can be rivalled to that of pre-eminent reported materials (listed in Table S6). Interestingly, it can be seen from Figure 5 that the adsorption isotherms of all tested samples are in an unsaturated state, which means that the samples also have a certain potential in higher pressure adsorption.

In practical applications, not only the case of single gas but also the situation of multicomponent gas competition should be considered. As shown in Figure 5, the adsorption capacity of CSCX-Y for CO₂ is much higher than that for N₂, indicating that CSCX-Y has an excellent selective capture capacity for CO₂. In particular, the CO₂ adsorption capacity of CSC2-750 is 4.19 and 6.61 mmol/g, but the N₂ adsorption capacity is just 0.57 and 0.83 mmol/g at 298 and 273 K (1 bar), respectively. Flue gas is treated in a typical industrial waste gas treatment system, which is usually simulated with 15% CO₂ and 85% N₂. In order to evaluate the separation performance of the samples, the ideal adsorbed solution theory (IAST) model was used to obtain the selectivity factor of the tested samples.^43,44 The calculated IAST factor of CSC2-750 is about 27 (CO₂/N₂ = 15/85) at 298 K (Figures 7a and S10), which is similar or even higher than that of previously reported materials (listed in Table S6). The breakthrough curves (CO₂/N₂ = 15:85) were used for the further analysis of dynamic separation capability. As shown in Figure 7b, for CSC2-750, N₂ breaks through the fixed bed much earlier than CO₂, indicating that the interaction between the adsorbent and CO₂ is much stronger than that between the adsorbent and N₂, which may be closely related to N contained in the samples. Cheerfully, the dynamic CO₂ capture capacity (15% CO₂) is 1.28 mmol/g (Figure S11a), which is almost the same as the static low-pressure adsorption (0.15 bar). It is proved that the prepared materials have great potential for the separation of flue gas. For the sake of understanding the extended usability of samples, methane (CH₄) adsorption was also carried out at 298 K (see in Figure 5 and Table 3). It can be clearly seen that the CH₄ adsorption capacity of CSCX-Y varying from 1.66 to 1.98 mmol/g is lower than that of CO₂, but much higher than that of N₂, and the IAST factors of both CO₂/CH₄ (40/60, biogas) and CH₄/N₂ (50/50, coalbed gas) are about 6 at 298 K (Figure 7a). There is no doubt that the as-prepared samples show a high application potential when dealing with different mixed gases.

Figure 7.

(a) IAST selectivity of CSCX-Y; (b) breakthrough curves of CSC2-750 (15% CO₂, 85% N₂); (c) Q_st of CSCX-Y; (d) cyclic regeneration tests of CSC2-750 at 298 K.

Energy consumption and regeneration has been one of the key parameters limiting the practical application of many adsorbent materials. In this work, the isosteric heats of adsorption (Q_st) of CSCX-Y have been calculated by applying the Clausius–Clapeyron equation to obtain the CO₂ adsorption data at 298 and 273 K,^5,15 which are presented in Figure 7c. The maximum value of Q_st is 34–18 kJ/mmol, which is well below the energy required to break the O=C=O bond (∼749 kJ/mmol), suggesting that the interaction between CO₂ and the adsorbent is mainly a physisorption process based on van der Waals forces.^30,45 However, the Q_st value of the porous carbon decreases with the descending N content and is higher than that of non-nitrogen samples (∼20 kJ/mmol),⁴⁶ which means that there is also an interaction between N and CO₂ molecules, implying that N also plays a certain role in promoting CO₂ capture. In order to investigate the regeneration of the samples, more than five times of cyclic regeneration tests have been carried out in samples. It can be seen from Figures 7d and S11b that the performance of the tested samples is basically unchanged, suggesting that the sample has excellent application prospects.

3. Conclusions

In summary, the porous carbon CO₂ adsorbent was successfully prepared based on industrial biomass waste (soybean cake) by an easy carbonization–activation step. The porous carbon has high surface area (809–1712 m²/g), abundant microporous volume (0.39–0.81 cm³/g), remarkable narrow microporous volume (0.38–0.57 cm³/g), controllable pore size, and ideal nitrogen content (0.52–3.86 wt %), which can provide a strong support for CO₂ capture. Conspicuously, CSC2-750 with a larger surface area of 1390 m²/g and a higher narrow microporous volume of 0.57 cm³/g exhibits efficient CO₂ adsorption capacity of 4.19 and 6.61 mmol/g at 298 and 273 K (1 bar), respectively. Moreover, CSC2-750 shows a good CO₂ adsorption capacity of 1.26 mmol/g at 0.15 bar, an ideal dynamic CO₂ capture capacity of 1.28 mmol/g (15% CO₂), and also a considerable CO₂ selectivity, with the IAST factors of 27 (CO₂/N₂ = 15/85), 6 (CO₂/CH₄ = 40/60), and 6 (CH₄/N₂ = 50/50), respectively. Additionally, through a five-time cyclic regeneration test, it has been found that CSC2-750 has a stable adsorption performance. Notably, the isosteric heats of adsorption of all samples vary from 34 to 18 kJ/mmol, which are far lower than chemisorption energy consumption, further confirming the fact that the materials can be reused. Combining the adsorption data with the porous textural properties and N content, it was found that the CO₂ adsorption capacity of this material is not affected by a single factor but by an integration of multiple factors, among which the narrow microporous volume plays a dominant role. All in all, the soybean cake-based porous carbon CO₂ adsorbent has a great potential for greenhouse gas adsorption and separation.

4. Experimental Section

4.1. Materials

Soybean cake was obtained from an oil mill in a village in Dalian, China. Anhydrous KOH was bought from Tianjin Damao Chemical Reagent Factory, and concentrated hydrochloric acid (HCl 36–38 wt %) was purchased from Tianjin Chemical Reagent Plant 3.

4.2. Preparation of Porous Carbon CO₂ Adsorbent from Soybean Cake

The digital photos of soybean oil extraction equipment and soybean cakes are shown in Figure S1. First, the soybean cake powder (SCP) was obtained by grinding a certain amount of soybean cakes by a grinder. Then, the SCP was transferred into a tubular furnace for carbonization at 600 °C for 2 h under a nitrogen flow. The obtained carbon samples were designated as CSC.

Typically, 1 g of CSC was added into 10 mL of aqueous solution containing 2 g of KOH, and the mixture was stirred vigorously at room temperature for 7 h. Then, the mixture was transferred to an oven and dried at 105 °C for 10 h until the solvent was completely evaporated. After this, the mixture was activated at 750 °C for 120 min with a heating rate of 5 °C/min under a nitrogen atmosphere. Subsequently, the activated sample was washed to neutrality with 0.5 mmol/L HCl solution, and then it was washed with deionized water three times to remove the salt produced by acid–base neutralization. Finally, the washed sample was dried at 105 °C for 12 h to obtain the porous carbon (see in Scheme 1). For the convenience of discussion, the as-prepared samples were denoted as CSCX-Y, where X represents the mass ratio of KOH to CSC and Y stands for the activation temperature. To illustrate with an example, the above-described sample can be denoted as CSC2-750. Additionally, the yields of the as-prepared samples are shown in Table S1.

Scheme 1. Synthetic Process of Soybean Cake-Based Porous Carbon.

4.3. Characterization of the Synthesized Materials

A Nova NanoSEM 450 scanning electron microscope was used to obtain SEM images with an acceleration voltage of 3 kV. An FEI Tecnai G² F30 transmission electron microscope was used to obtain TEM pictures when the accelerating voltage was 300 kV. A SmartLab 9KW X-ray diffractometer with Cu Kα (0.1542 nm) radiation was used to characterize the crystal structure of the synthesized materials in the scanning range of 10–90° (2θ) at the rate of 5°min^–1. A Thermo VG ESCALAB 250 system was used to perform XPS analysis, with the Al-Kα X-ray source operating at 150 W (15 kV). Elemental analysis of the tested samples was done by an Elementar Vario MACRO EL Cube microanalyzer. Raman spectra were measured through a confocal laser micro-Raman spectrometer (Thermo DXR Microscope, USA). The FT-IR spectra of the as-prepared samples were recorded using a Thermo Scientific Nicolet 6700 FT-IR spectrometer.

Beishide Instrument 3H-2000PM was the main test device used to obtain the physical structure characterization of all samples, and the nitrogen adsorption–desorption isotherms were measured at 77 K. All the tested samples were degassed for 6 h under a vacuum at 423 K and cooled naturally to room temperature before measurement. The multipoint Brunauer–Emmett–Teller (BET) method was used to measure the surface area (S_BET) of all the tested samples, which was calculated from the adsorbed data in the relative pressure (P/P₀) range of 0.05–0.3. At the relative pressure of 0.99, the nitrogen adsorption data of the as-tested samples were used as the basis for calculating the total pore volume (V_total). Both the micropore volume and surface area (V_micro and S_micro, < 2 nm) were gained by using the T-Plot method as the calculation model. PSD was obtained from the nonlocal density functional theory. The D–R equation was used to obtain the narrow micropore volume (V_n, <1 nm) by calculating the CO₂ adsorption data at 273 K.⁴⁷ Both the IAST model selectivity factor and the value of isosteric heats of adsorption (Q_st) were calculated by the software of the Beishide instrument.

4.4. Gas Adsorption Measurements

The CO₂ and N₂ static adsorption data of all as-tested samples were measured by Beishide Instrument 3H-2000PM at 298 and 273 K at atmospheric pressure. The CO₂ and N₂ adsorption capacities were tested at 298 and 273 K at low pressure by the Beishide Instrument 3H-2000PM after changing the instrument test pressure to 0.15 bar. The breakthrough curves (CO₂/N₂ = 15/85) were obtained by a Beishide Instrument Multi-Component Adsorption Breakthrough Curve Analyzer. The CH₄ adsorption capacity was measured by the Beishide Instrument BSD-PS at 298 K. High-purity N₂ (99.999%), CO₂ (99.99%), and CH₄ (99.999%) were used for adsorption measurements.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c04165.

• Pictures of the oil extraction equipment and soybean cakes; rising program of the carbonization process; thermogravimetric analysis of the soybean cake; KOH activation mechanism; prediction of mixed gas selectivity by IAST; isosteric heats of adsorption; multicomponent adsorption breakthrough curves; regeneration cycle test; yield of all samples; SEM and TEM images of CSCX-Y; XRD patterns, Raman spectra of CSC; FT-IR spectra and XPS analysis of CSC and CSCX-Y; gas adsorption isotherms of CSCX-Y at 273 K and 1 bar; relationship between CO₂ adsorption capacity and surface area, pore volume, V_n, and N content at 273 K and 1 bar; CO₂ adsorption uptake at 273 K and 0.15 bar; maximum isosteric heats of adsorption (CH₄ and N₂) of CSCX-Y; and CO₂ adsorption uptake of different adsorbents (PDF)

The authors declare no competing financial interest.