Efficacy of modified carbon molecular sieve with iron oxides or choline chloride-based deep eutectic solvent for the separation of CO₂/CH₄

Abstract

It is necessary to separate CO₂ from biogas to improve its quality for the production of biomethane. Herein, an improvement in the separation of CO₂/CH₄ via adsorption was achieved by modifying the surface of CMS. The surface modification of CMS was performed by impregnation with metal oxide (Fe₃O₄) and N-doping (DES–[ChCl:Gly]). Subsequently, the efficacy of the surface-modified CMS was investigated. This involved CMS modification, material characterization, and performance analysis. The uptake of CO₂ by CMS–DES–[ChCl:Gly] and CMS–Fe₃O₄ was comparable; however, their performance for the separation of CO₂/CH₄ was different. Consequently, CMS–DES–[ChCl:Gly] and CMS–Fe₃O₄ exhibited ca. 1.6 times enhanced CO₂ uptake capacity and ca. 1.70 times and 1.55 times enhanced CO₂/CH₄ separation, respectively. Also, both materials exhibited similar repeatability. However, CMS–DES–[ChCl:Gly] was more difficult to regenerate than CMS–Fe₃O₄, which is due to the higher adsorption heat value of the former (59.5 kJ).

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1 Introduction

Presently, biogas and biomethane are considered to be alternative energy sources to fossil fuels and natural gas, which are derived from renewable resources. However, the composition of biogas is generally characterized by the presence of CH₄ (50–70%) and CO₂ (30–50%),^1,2 and thus the CO₂/CH₄ gas mixture must be separated to obtain a more energy-dense product, given that biomethane is a very calorific gas. In contrast, CO₂ has no calorific value.³ Currently, membrane separation, absorption, and cryogenic separation are accessible technologies for eliminating carbon dioxide.^4,5 However, they are expensive and consume a large amount of energy. In the case of CO₂/CH₄ separation, adsorption-based separation has been proposed due to its high purity (>98% vol), ease of operation, and high energy efficiency.⁶ There is a difference in the affinities and diffusivities of CO₂ and CH₄, resulting in separation through adsorption.⁷ Molecular sieves, such as metal–organic frameworks (MOFs),^8–12 silica-derived molecular sieves (ZMSs),^13–17 and carbon-derived molecular sieves (CMSs)^18–22 are frequently utilized for adsorption-based separation. Generally, carbon-derived molecular sieves are durable at low adsorption temperatures and may be synthesized using coal,^23,24 polymers,^25–27 and biomass.^18,28–30 Furthermore, it can be readily regenerated.

CMS can be modified by altering their pores (pyrolysis, partial gasification, and pore constriction) and their surfaces (functionalization and metal impregnation). Furthermore, CMS can be homogeneous regarding their pore size by tuning their pores. Nevertheless, it is quite challenging to achieve a uniform pore size. In contrast, surface modification aims to enhance the affinity of CO₂ for carbon surfaces. In addition to the uniformity of the pores in CMS, the diffusivity of molecules in their pores is also affected by their affinity with the surface of CMS. Increasing the affinity of a molecule for the surface of CMS inhibits the mobility (diffusion) of the molecule in the pores.

In CO₂/CH₄ separation with surface modification, functional groups such as nitrogen, amines, and oxygen are usually added to CMS to increase their affinity for CO₂.³¹ Also, their surface can be modified by impregnating them with metal oxides. In this case, several metal oxides have been used as CO₂ adsorbents, such as MgO, CaO, Fe₃O₄, CuO and NiO. The surface modification is based on the presence of CO₂, which has an affinity for basic metal oxides, whereas CH₄ does not. As a result of these differences, CO₂ can be removed from gas mixtures by utilizing the interaction between the adsorbate and adsorbent. Thus, the presence of basicity may enhance the surface affinity of CMS for CO₂.³² In general, the presence of metal oxides³³ and functional groups such as oxygen and nitrogen³¹ is considered to increase the affinity of CMS for CO₂. CO₂ adsorption may also occur on materials with open metal oxide sites, specific functional groups, and charge species. As a result of the higher polarizability of CO₂, higher amounts of CO₂ are adsorbed than CH₄ at ambient temperatures. Therefore, it can be separated from gas mixtures at this temperature.³⁴

Recent research has focused on enhancing activated carbon via surface modification to increase its affinity for CO₂. Given that CO₂ is a weak Lewis acidic gas, modifying the surface of activated carbon with basic groups or reacting it with N-containing compounds can generate more active functional sites for CO₂, enhancing the affinity for CO₂. For instance, Masruroh et al.²¹ suggested that functionalizing the surface of activated carbon with amine groups such as monoethanolamine (MEA), 2-amino-2-methyl-1-propanol (AMP), and diethanolamine (DEA) is advantageous for improving the surface chemistry, which provides a higher affinity, thereby enhancing its CO₂/CH₄ separation performance. The results indicated that CMS–MEA exhibited the best performance for the separation of CO₂ from CH₄. In the investigation by Mukti et al.,²² an improved iron oxide dispersion on oxidized carbon surfaces could separate CO₂/CH₄. In this respect, Fe₃O₄ is more desirable for separating CO₂/CH₄ than Fe₂O₃. An additional study was conducted by Schott et al.³⁵ on activated mesoporous carbon (AMC), which was modified with metal oxides (CeO₂, CuO, Mn₃O₄, and NiO) to improve its CO₂/N₂ selectivity. Compared to the parent AMC, the selectivity values of the AMC modified with metal oxides were generally higher, with a 72% improvement in the selectivity of the 30% NiO-modified AMC. In addition, Shahkarami et al.³⁶ found that MgO-impregnated activated carbon improved both the CO₂ adsorption and CO₂/N₂ selectivity.

In addition, numerous studies investigated the possibility of transforming activating agents from chemical solvents to green solvents to increase the affinity of carbon for CO₂, such as adding a deep eutectic solvent (DES) to its surface. For example, a choline chloride-based DES exhibited a strong electrostatic interaction with CO₂, contributing to an increase in the affinity for it. Ariyanto et al.²⁰ investigated the possibility of modifying carbon surfaces using a choline chloride-based DES to separate CO₂/CH₄. In their study, three types of alcohol were, i.e., 1-butanol (-ol), ethylene glycol (-diol), and glycerol (-triol), were combined with choline chloride to form a DES as a surface modifier. Among them, the DES containing the triol of glycerol displayed the best performance. Hussin et al.³⁷ compared the impregnation of a deep eutectic solvent (DES) from a mixture of choline hydroxide : urea and choline hydroxide : glycerol on carbon to enhance the CO₂ adsorption capacity. The highest CO₂ adsorption efficiency was observed for the DES-based activated carbon obtained using a mixture of choline hydroxide : urea at a temperature of 25 °C. The modified adsorbent still demonstrated an excellent performance after 11 adsorption cycles and desorption.

In the present study, we synthesized molecular sieves to separate CO₂/CH₄ by impregnating porous carbon surfaces with Fe₃O₄ or DES–[ChCl:Gly]. The procedure for the preparation of CMS involved an oxidation process, followed by an impregnation process using Fe₃O₄ or DES–[ChCl:Gly]. In this study, we investigated different surface modifications and the corresponding separation performance. To the best of our knowledge, this has not been reported to date. The efficacy of DES–[ChCl:Gly] and Fe₃O₄ impregnation was assessed by examining the adsorption isotherms, breakthrough analysis, repeatability performance, and regeneration of the samples. Furthermore, their surface morphology, surface properties, elemental composition, and chemical properties were evaluated using SEM-EDX mapping, FTIR, and nitrogen sorption analysis.

2 Experimental

2.1. Materials

The materials used included porous carbon with a mesh size of 20–25, ferrous(III) nitrate (Merck), H₂O₂ (Merck), choline chloride (Sigma-Aldrich), glycerol (Sigma-Aldrich), methanol (Merck), HP-nitrogen, UHP-carbon dioxide and UHP-methane from PT Aneka Gas Industri Indonesia. A biogas representative comprised of CH₄ and CO₂ (55/45 v/v) was obtained from PT Aneka Gas Industri Indonesia.

2.2. Modification of CMS

Pristine carbon with a mesh size of 20–25 was pre-treated with 10% H₂O₂ to increase its surface wettability. In the next step, pre-treated carbon was impregnated with Fe₃O₄ and DES–[ChCl:Gly]. The metal oxides (Fe₃O₄) were dispersed on pre-treated carbon using the incipient wetness method and calcined. Before impregnation, the metal salt (Fe(NO₃)₃·9H₂O) was dissolved in methanol. The metal salt solution in methanol was gradually added to the carbon pores in the following stages. The use of carbon and methanol in a w/v ratio of 1 : 1 was proposed to achieve a metal oxide content of 5 wt% (Fe). In the subsequent step, the carbon was impregnated with metal salt, and then calcined at 773 K for 6 h. Information regarding the impregnation procedures can be found in the literature.²²

DES–[ChCl:Gly] impregnation on pretreated carbon was performed in accordance with ref. 20. Choline chloride and glycerol were mixed in distilled water in a mol ratio of 0.5 : 0.5 to prepare a 5 wt% DES–[ChCl : Gly] solution. In the following step, the DES–[ChCl:Gly] solution was gradually added to the carbon pores, and then dried at 105 °C overnight. Consequently, the carbon that was impregnated with the metal oxide and DES–[ChCl:Gly] was labeled CMS–Fe₃O₄ and CMS–DES–[ChCl:Gly], respectively. The modification of CMS is schematically illustrated in Fig. 1.

Fig. 1 Schematic of CMS modification procedure.

2.3. Characterization of CMS

The functional groups present in the CMS materials were characterized by infrared Fourier transform analysis (FTIR) using a Nicolet Avatar 360 IR in the wavelength range of 400–4000 cm⁻¹. The morphology of CMS was examined using SEM-EDX imaging with a JEOL JSM6510 LA at 10 kV. A Quantachrome NOVA 2000 was used to record the nitrogen sorption isotherms at 77 K. The Brunauer–Emmett–Teller (BET) method was utilized to determine the surface area in the relative pressure range of 0.05 to 0.25. The total pore volume, denoted by V_t, was computed with the help of the nitrogen sorption isotherm at a relative pressure of P/P₀ = 0.99. The volume of micropores (V_m) and volume of the total pore system (V_t) were calculated using the T-plots. The pore size distributions (PSDs) in this study were determined by density functional theory (DFT) analysis.

2.4. Measurement of adsorption isotherms

The adsorption capacity measurements for CO₂ and CH₄ were conducted at 303 K using the volumetric method. A Swagelok® VCR valve and fitting were used to assemble an ultrahigh vacuum adsorption apparatus. As a precondition for the adsorption measurements, the CMS sample was degassed at a temperature of 423 K for 6 h under vacuum. This was conducted to eliminate any remaining water and surface species. The adsorption isotherms were measured in the range of 0 to 5.0 atm and the adsorption isotherm curves were depicted using the obtained information. Detailed schematic diagrams of the adsorption isotherms and procedures for their measurements are described in the literature.³⁸

2.5. Evaluation of breakthrough curves

Gas mixtures of CH₄ and CO₂ were detected using a portable biogas analyzer (Gas Board 3200 plus, Hubei Cubic Ruiyi Instrument Co., Ltd.). The CMS material was loaded in a fixed bed column (d = 0.95 cm and length = 30 cm), and 0.2 L min⁻¹ of nitrogen gas was injected to remove CH₄, CO₂, and O₂ from the bed. The CH₄–CO₂ gas mixture entered the system at a rate of 0.05 L min⁻¹ at a temperature of 303 K and pressure of 1.2 bar. After completing the separation process, the gas composition returned to its initial concentration. The detailed procedure and breakthrough system for the separation of CO₂/CH₄ are described in the literature.^20,22

3 Results and discussion

3.1. Structural and surface characterization of CMSs

Surface morphologies and functional groups. Fig. 2(a, b and d) display the SEM images of C, CMS–DES–[ChCl:Gly], and CMS–Fe₃O₄, and Fig. 2(c and e) show the elemental mappings of CMS–DES–[ChCl:Gly] and CMS–Fe₃O₄, respectively, where differences in the surface morphology between the pristine carbon and CMSs can be observed. According to Fig. 2(a, b, and d), CMSs have more cavities than pristine carbon. It is also evident from the elemental mapping displayed in Fig. 2(c and e) that Fe₃O₄ and DES–[ChCl:Gly] were evenly distributed on the surface of the carbon. An analysis of the SEM-EDX results was performed to assess the composition of the CMSs. Table 1 provides a detailed analysis of the results.

Fig. 2 SEM images and their elemental mapping of C (a), CMS–DES–[ChCl]:Gly (b and c) and CMS–Fe₃O₄ (d and e).

Table 1 The elements detected by EDX analysis on CMSs

Material	Element, %					pHPZC
	C	O	N	Cl	Fe
C	93.06	6.94	—	—	—	6.93
CMS–DES–[ChCl:Gly]	77.88	16.55	5.25	0.32	—	7.21
CMS–Fe3O4	83.47	10.57	—	—	5.96	7.29

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The modification process involving iron oxide impregnation and DES–[ChCl:Gly] could alter the surface chemistry of CMS, where the addition of DES–[ChCl:Gly] and iron oxide to CMS increased its basicity. This was evidenced by the increase in the pH_PZC value, as shown in Table 1. The pH_PZC value also increased for the composite containing iron oxide and activated carbon nanoparticles.^39,40 This basicity favours CO₂ adsorption. As the basicity increased, the CO₂ affinity increased, while the CH₄ affinity decreased. In Table 1, it is also important to note that N, O, and Cl functional groups were added in the case of DES–[ChCl:Gly] impregnation. The presence of N, O, H, and Cl will increase the affinity for CO₂.

The incorporation of Fe₃O₄ and DES–[ChCl:Gly] in the porous carbon was confirmed by FTIR spectroscopy (Fig. 3). In the pristine carbon and CMSs, the peaks at 1080 cm⁻¹, 1590 cm⁻¹, and 3460 cm⁻¹ are associated with the C–O groups (stretching),⁴¹ carboxyl groups (C=O stretching),^41,42 and hydroxyl groups (O–H stretching),^42,43 respectively. In the spectrum of CMS–DES–[ChCl:Gly], the sharp peak observed at 3440 cm⁻¹ is a result of the overlap in the O–H stretch band and the N–H stretch band.⁴⁴ The FTIR spectrum of chitosan also revealed the overlap of the O–H and N–H peaks.⁴⁵ Also, DES–[ChCl:Gly] was detected on the surface of carbon based on the N–H bending peak at 1640 cm⁻¹.²⁰ CMS–Fe₃O₄ exhibited an additional peak at 590 cm⁻¹, indicating the presence of Fe–O bonds.²²

Fig. 3 FTIR spectra of C, CMS–Fe₃O₄ and CMS–DES–[ChCl]:Gly.

Nitrogen sorption analysis. A nitrogen sorption analysis was conducted at 77 K to characterize the textural properties of pristine carbon and CMSs, and their comparison is illustrated in Fig. 4. Based on the IUPAC classification, carbon and CMS are classed as microporous materials with a type I isotherm (Fig. 4(a)). At 0.1 P/P₀, the pristine carbon exhibited a high nitrogen adsorption volume (ca. 140 cm³ g⁻¹ STP). The volume of nitrogen adsorbed decreased significantly with CMS–DES–[ChCl:Gly], whereas it decreased modestly with CMS–Fe₃O₄. Interestingly, the adsorption–desorption isotherm curve for CMS–DES–[ChCl:Gly] is unclosed, which is probably due to the presence of ink bottle-shaped pores following impregnation.²⁰ Fig. 4(b) shows the results of the pore size distribution evaluated using the QSDFT model based solely on the adsorption data. The pristine carbon content was estimated to be 92 vol% in the micropore region (<2 nm), with a small tail of 4–5 nm. There was a decrease in porosity in the micropore region of CMS–DES–[ChCl:Gly] and CMS–Fe₃O₄. CMS–Fe₃O₄ exhibited a shift to <2 nm with a small tail of 2–3 nm, while CMS–DES–[ChCl:Gly] exhibited a shift to <2 nm region. A few of the small pores appeared to be blocked by DES–[ChCl:Gly] and Fe₃O₄. The pores smaller than 1 nm may become blocked completely, while that wider than 2 nm may experience a reduction in size. This can be attributed to the attachment of DES–[ChCl:Gly] and iron oxide to the carbon surface. Therefore, in the PSDs, it can be seen that the size of <2 nm was still present, while the size of >3 nm was absent.

Fig. 4 Isotherms for nitrogen sorption (a) and distribution of pore sizes (b) for CMSs.

The textural properties of CMSs are presented in Table 2. In comparison to pristine carbon, CMS–DES–[ChCl:Gly] has a lower specific surface area by approximately 64%, whereas that of CMS–Fe₃O₄ decreased by approximately 8%. The impregnation of DES–[ChCl:Gly] resulted in the blockage of the pores and the collapse of the pore walls.^20,46,47 However, regardless of their surface area or pore volume, all the materials are microporous.

Table 2 Textural properties of CMSs based on nitrogen sorption

Material	SBET, m^2 g^−1	% Smic	V, cm^3 g^−1	Davg, nm
C	708	92.0	0.33	1.88
CMS–Fe3O4	649	92.9	0.31	1.90
CMS–DES–[ChCl:Gly]	258	86.8	0.15	2.33

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3.2. Carbon dioxide and methane adsorption isotherms

As shown in Fig. 5(a and b), CO₂ and CH₄ adsorption isotherm curves were measured at 30 °C. As shown in Fig. 5(a and b), the CO₂ uptake significantly exceeded CH₄ uptake. Similar results were obtained in a previous study.^20,22 Moreover, the impregnation of Fe₃O₄ and DES–[ChCl:Gly] increased the CO₂ uptake, while reducing CH₄ uptake. In Fe₃O₄ and DES–[ChCl:Gly], the active sites are more favourable for the adsorption of CO₂, resulting in an increase in CO₂ capacity. In contrast, the carbon impregnated with Fe₃O₄ and DES–[ChCl:Gly] reduced the affinity for CH₄. Thus, the selectivity for CO₂/CH₄ was enhanced, resulting in the better removal of CO₂ from CO₂–CH₄ mixed gases. The CO₂ uptake by pristine carbon was 1.7 mol kg⁻¹ at 1 atm, whereas that by CMS–Fe₃O₄ and CMS–DES–[ChCl:Gly] was 2.4 and 2.5 mol kg⁻¹, respectively. This drastically improved the CO₂ uptake, which can be attributed to factors, i.e., CO₂ binding Fe₃O₄ or DES–[ChCl:Gly] functional groups in CMS and well-dispersed Fe₃O₄ and DES–[ChCl:Gly] throughout the porous carbon. Meanwhile, the uptake of pristine carbon, CMS–Fe₃O₄, and CMS–DES–[ChCl:Gly] for CH₄ was 1.1 mol kg⁻¹, 0.75 mol kg⁻¹, and 0.76 mol kg⁻¹, respectively.

Fig. 5 Isotherm adsorption of CO₂ (a) and CH₄ (b) on pristine carbon–C, CMS–Fe₃O₄ and CMS–DES–[ChCl]:Gly, adsorption isotherm of CO₂ and CH₄ on pristine carbon–C (c), CMS–Fe₃O₄ (d) and CMS–DES–[ChCl]:Gly (e) (isotherm data fitted with the Toth equilibrium model and IAST model used in simulative adsorption of CO₂–CH₄ (55 : 45) on pristine carbon–C, CMS–Fe₃O₄ and CMS–DES–[ChCl]:Gly), sorption selectivity of CO₂/CH₄ at a temperature of 303 K (f).

The equilibrium parameters were determined using the Toth adsorption model (eqn (1)).

where C_μ is the uptake capacity, C_μs is the maximum uptake capacity, and b and t are the equilibrium constants. Table 3 presents the parameters of the Toth model for CO₂ and CH₄ adsorption in CMSs. Based on the b value (adsorption affinity), it is clear that CO₂ is more favorably adsorbed on the CMSs than CH₄. Further, based on the b value, it can be concluded that the modified CMS exhibited an increased affinity for CO₂. In contrast, the affinity of CH₄ decreased. As shown in Table 3, the b values of CO₂ for pristine carbon, CMS–Fe₃O₄, and CMS–DES–[ChCl:Gly] are 1.53, 2.10, and 3.99, respectively.

Table 3 Equilibrium parameters obtained from the fitting isotherm data using the Toth model

Sample	Adsorbate	Cμs, (mmol g^−1)	b (atm)	t
C	CO2	4.47	1.53	0.62
	CH4	3.96	0.81	0.08
	CO2–CH4	9.09	1.61	0.32
CMS–Fe3O4	CO2	7.70	2.10	0.45
	CH4	4.71	0.33	0.59
	CO2–CH4	7.13	1.13	0.47
CMS–DES–[ChCl:Gly]	CO2	6.73	3.99	0.43
	CH4	5.09	0.41	0.51
	CO2–CH4	6.51	2.02	0.43

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Subsequently, it was possible to predict the simultaneous adsorption of CO₂ and CH₄ on CMSs using the equilibrium adsorption of the single components of CO₂ and CH₄ on CMSs (written as CO₂–CH₄). The ideal adsorption solution theory (IAST) approximates the equilibrium equations for multicomponent systems by applying eqn (3) and (4), where the total amount absorbed can be determined. Fig. 5(c–e) present the calculation results for CO₂–CH₄ multicomponent adsorption based on the IAST model. The composition of the CO₂–CH₄ adsorption is 45% : 55%. Based on the IAST equation for CO₂–CH₄ adsorption, the maximum adsorption capacity was smaller than that for single CO₂ components. The results indicate competition for the interaction between CH₄ and CO₂ on CMSs. CO₂ is more readily adsorbable than CH₄, depending on the b values of the single components in each CMS. This is also evident from the CO₂–CH₄ curve, which tends to be more closely related to CO₂.

C⁰_μ = f⁰(P⁰_j) (2)

The separation of CO₂/CH₄ depends heavily on the selectivity of CO₂/CH₄. Fig. 5(f) illustrates the selectivity of CO₂/CH₄. In accordance with the studies in the literature, the selectivity of pristine carbon decreased with an increase in pressure, as evidenced by the data.^20,48 CO₂/CH₄ had a selectivity of ca. 2.3–0.5 on the pristine carbon in the pressure range studied. Compared to pristine carbon, CMS DES–[ChCl:Gly] and CMS–Fe₃O₄ exhibited improved selectivity for CO₂/CH₄. The CO₂/CH₄ selectivity increased by 3.3 times on CMS–Fe₃O₄, while that on CMS–DES–[ChCl:Gly] increased by 3.9 times. CMS–DES–[ChCl:Gly] provides almost as much CO₂ uptake as CMS–Fe₃O₄ despite its lower surface area. Additionally, CMS–DES–[ChCl:Gly] also provided higher CO₂/CH₄ selectivity. In addition to surface area, it appears that the affinity for the active site (DES–[ChCl:Gly] or Fe₃O₄) influenced the CO₂ uptake. A more favorable affinity is the basis for improving the separation of CO₂/CH₄, given that it will inhibit CO₂ diffusion in the pores, but not CH₄ diffusion. Thus, as the diffusivity ratio increases, the separation of CO₂/CH₄ will be improved.

Adsorption studies on gases consider the heat of adsorption (ΔH_ads) as a significant variable. It explains how adsorbents interact with their adsorbates. Employing the Van't Hoff equation (eqn (4)), it is possible to determine the heat of adsorption (ΔH_ads) of CO₂ and CH₄ on pristine carbon and both CMSs. The heat of adsorption was calculated in Henry's region, provided at a zero-coverage limit.

where ΔH_ads is the heat of adsorption (J mol⁻¹), K is the Henry constant (dimensionless), T is the temperature (K), and R is the ideal gas constant (J (mol K)⁻¹). A comparison of the CO₂ and CH₄ heat adsorption on pristine carbon, CMS–DES–[ChCl:Gly], and CMS–Fe₃O₄ is shown in Table 4. The results show a lower CO₂ adsorption energy in pristine carbon than in CMS–DES–[ChCl:Gly] and CMS–Fe₃O₄. In terms of heat of adsorption, the mechanism on pristine carbon and CMS–Fe₃O₄ is physisorption, while that on CMS–DES–[ChCl:Gly] is chemisorption. Hence, the CO₂ interactions with CMS–DES–[ChCl:Gly] are stronger than that between the pristine carbon and CMS–Fe₃O₄. This may be attributed to the fact that electrostatic interactions are responsible for the strong interactions between CMS–DES–[ChCl:Gly] and CO₂. The interactions that may occur include those involving oxygen in CO₂ with N⁺ in choline, oxygen in CO₂ with H bonds from glycerol, and carbon in CO₂ with Cl⁻. The presence of n-functionalization on carbon has also been shown to interact strongly with CO₂ in other studies.⁴⁹ Table 5 provides a comparison of the adsorption heat values from various sources.

Table 4 Heat of adsorption and Henry constants for CH₄ and CO₂ in CMSs

Adsorbate	Material	ΔHads (kJ mol^−1)	T (K)	Henry constant, Kp^a	Henry constant, K^b
a gmol (g atm)^−1.b Dimensionless, K = (KpρpRT)/(1 − ε).
CO2	C	10.9	303	3.4 × 10^−3	259
			313	8.4 × 10^−4	65
			323	1.9 × 10^−4	16
	CMS–Fe3O4	16.4	303	3.7 × 10^−3	283
			313	1.8 × 10^−3	140
			323	6.5 × 10^−4	52
	CMS–DES–[ChCl:Gly]	59.5	303	5.2 × 10^−3	394
			313	3.8 × 10^−3	293
			323	2.1 × 10^−3	172
CH4	C	9.8	303	8.0 × 10^−4	60
			313	4.1 × 10^−4	33
			323	3.3 × 10^−4	29
	CMS–Fe3O4	10.6	303	9.5 × 10^−4	72
			313	7.9 × 10^−4	62
			323	6.1 × 10^−4	49
	CMS–DES–[ChCl:Gly]	10.3	303	7.7 × 10^−4	58
			313	6.8 × 10^−4	53
			323	5.9 × 10^−4	48

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Table 5 The heat of adsorption and amount of CH₄ and CO₂ adsorbed in other CMSs

Adsorbate	Material	SSABET, (m^2 g^−1)	Mol adsorbed (mmol g^−1) (1 atm, 303 K)	ΔHads (kJ mol^−1)	Ref.
CO2	C	708	1.85	10.9	This study
	CMS–Fe3O4	649	2.94	16.4	This study
	CMS–DES–[ChCl:Gly]	258	2.85	59.5	This study
	AC–Fe2O3	212	1.10	33.2	50
	AC–Co3O4	185	1.60	28.1	50
	Porous carbon–La2O3	715	1.47	33.4	51
	S-doped microporous carbon	729	2.46	22.0	52
	N-doped microporous carbon	614	4.04	59.3	31
	AC–MgO	615	2.19	—	36
CH4	C	708	1.08	9.8	This study
	CMS–Fe3O4	649	0.85	10.6	This study
	CMS–DES–[ChCl:Gly]	258	0.64	10.3	This study
	AC–Fe2O3	212	0.30	20.2	50
	AC–Co3O4	185	0.40	22.0	50

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3.3. CO₂/CH₄ separation: a breakthrough curve analysis

The performance of the materials in separating CO₂/CH₄ was evaluated through breakthrough analysis. Fig. 6(a) depicts the CO₂ breakthrough curves for pristine carbon and CMSs. The breakthrough curves demonstrate the ratio of the outlet concentration at a given time (C_t) to the initial inlet concentration (C₀) over time. It is evident from the data that the CO₂ breakthrough time for CMS–DES–[ChCl:Gly] and CMS–Fe₃O₄ increased compared to pristine carbon. CMSs exhibited an increase in breakthrough time by 40% compared to the pristine carbon. Due to this strong interaction between CO₂ and CMSs, the CO₂ diffusion is slower. Both CMS–DES–[ChCl:Gly] and CMS–Fe₃O₄ showed similar breakthrough times. Interestingly, CMS–Fe₃O₄ had a steeper breakthrough curve than CMS–DES–[ChCl:Gly]. This suggests that CMS–DES–[ChCl:Gly] has a stronger interaction with CO₂ than CMS–Fe₃O₄. Furthermore, other studies suggest that nitrogen-doped carbon exhibits slow adsorption kinetics.³³ In contrast, studies on pristine carbon and CMS–Fe₃O₄ indicated that mass transfer occurs instantaneously.^20,22

Fig. 6 CO₂ breakthrough curves of mixed gas CO₂/CH₄ for CMS (a). Desorption curves for CMS (b) at a temperature of 303 K and atmospheric pressure.

During the separation of CO₂/CH₄, the ability to regenerate the absorbent is an imperative factor. Fig. 6(b) illustrates a regeneration graph for pristine carbon and CMSs. The data indicate that the regeneration time of CMS–DES–[ChCl:Gly] is longer than that of CMS–Fe₃O₄ and pristine carbon. Therefore, CMS–DES–[ChCl:Gly] will be more difficult to regenerate due to its strong interaction with CO₂. Furthermore, this conclusion is supported by the results of the adsorption heat value of CMS–DES–[ChCl:Gly], which is quite high (59.5 kJ), as explained in Section 3.2.

Fig. 7 depicts the CO₂ and CH₄ breakthrough curves for the different CMS materials. Initially, neither CH₄ nor CO₂ gas was present, as shown in Fig. 6. CH₄ with a purity of higher than 98% appeared after 200 s. This was followed by CO₂ until the concentration of CO₂ in the outlet equalled the concentration in the inlet. Each material had a similar CH₄ breakthrough time, but a different CO₂ breakthrough time. The CMS surface and gas molecules may interact strongly, causing a delay in CO₂ flow at the outlet. Consequently, CO₂ gas diffused more slowly within the bed. Furthermore, the difference between the CH₄ signal and CO₂ signal appears to be larger, indicating that CH₄ and CO₂ were separated more effectively.

Fig. 7 Breakthrough curves of mixed gas CO₂/CH₄ for pristine carbon (a), CMS–Fe₃O₄ (b), CMS–DES–[ChCl]:Gly (c),²⁰ and repeatability of CO₂/CH₄ separation of CMSs (d).

According to Fig. 7(a), the CO₂–CH₄ breakthrough time difference for pristine carbon is 130 s. CMS–Fe₃O₄ exhibited an approximately 1.5-fold increase in CO₂–CH₄ breakthrough time compared to the pristine carbon (see Fig. 7(b)). Meanwhile, CMS–DES–[ChCl:Gly] experienced a 1.7-fold increase (Fig. 7(c)). In the repeatability study, the sequence of CO₂/CH₄ separation was cycled and the bed was regenerated once it was full of CO₂. Interestingly, CMS–DES–[ChCl:Gly] and CMS–Fe₃O₄ maintained their breakthrough time in the three cycles. These materials performed similarly to the pristine carbon but had a much higher capacity for uptake. The cycling tests indicate that CMS–DES–[ChCl:Gly] and CMS–Fe₃O₄ can be repeatably used in terms of separation. Thus, it is evident that CMS–DES–[ChCl:Gly] has a wider CO₂–CH₄ breakthrough time difference and reasonable repeatability. Nevertheless, it requires a longer period for regeneration. Meanwhile, CMS–Fe₃O₄ has a shorter CO₂–CH₄ breakthrough time than CMS–DES–[ChCl:Gly]. Despite this, it is repeatable and regenerates quickly.

4 Conclusions

A feasibility study was conducted regarding the modification of CMS by impregnating DES–[ChCl:Gly] and Fe₃O₄ in the surface of porous carbon for CO₂/CH₄ separation. Several aspects of CO₂/CH₄ separation were evaluated, including CO₂ breakthrough time, CO₂–CH₄ breakthrough time difference, regeneration capability, and repetition cycle. Additionally, the uptake of CO₂ and CH₄ was examined. The CO₂–CH₄ breakthrough time difference for CMS–DES–[ChCl:Gly] was higher than that for CMS–Fe₃O₄. Consequently, CMS–DES–[ChCl:Gly] and CMS–Fe₃O₄ exhibited an enhanced CO₂ uptake capacity by ca. 60% at 30 °C and 1 atm, while enhancing the separation of CO₂/CH₄ by ca. 69% and 54%, respectively. The adsorption energy of CO₂ in CMS–DES–[ChCl:Gly] (59.5 kJ) was higher than that of CMS–Fe₃O₄ (16.4 kJ), and thus a longer regeneration time was required. According to the cycling tests, CMS–DES–[ChCl:Gly] and CMS–Fe₃O₄ could be repeatably used. Thus, CMS–Fe₃O₄ is more favourable for CO₂/CH₄ separation than CMS–DES–[ChCl:Gly]. This is due to its repeatability and ability to be regenerated quickly.

Author contributions

Nur Indah Fajar Mukti: conceptualization, investigation, methodology, formal analysis, funding acquisition, writing-original draft. Teguh Ariyanto: supervision, funding acquisition, writing-review and editing. Wahyudi Budi Sediawan: supervision, writing-review and editing. Imam Prasetyo: conceptualization, supervision, funding acquisition, writing-review and editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by World Class Research Grant, Ministry of Education, Culture, Research, and Technology of the Republic of Indonesia, grant number 1786/UN1/DITLIT/Dit-Lit/PT.01.03/2022. N. I. F. M. acknowledges doctoral scholarship (Beasiswa Unggulan Dosen Indonesia) through Indonesia Endowment Fund for Education.

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